A. DECONTO-MACHADO et al

Alice Deconto Machado, Instituto de Astrofísica de Andalucía (IAA-CSIC),
Glorieta de la Astronomía, S/N, 18008 Granada, Spain.

# Optical and UV properties of a radio-loud and a radio-quiet Population A
quasar at high redshift††thanks: Invited talk presented at the 13th Serbian
Conference on spectral line shapes in astrophysics (SCSLSA)

A. Deconto-Machado  A. del Olmo  P. Marziani  J. Perea  G. M. Stirpe
Instituto de Astrofísica de Andalucía (IAA-CSIC), Granada, Spain  INAF,
Osservatorio Astronomico di Padova, Padova, Italy  INAF, Osservatorio di
Astrofisica e Scienza dello Spazio, Bologna, Italy<EMAIL_ADDRESS>
###### Abstract

Different properties of quasars may be observed and analysed through the many
ranges of the electromagnetic spectrum. Pioneering studies showed that an “H-R
diagram” for quasars was needed to organize these data, and that more than two
dimensions were necessary: a four dimensional Eigenvector (4DE1) parameter
space was proposed. The 4DE1 makes use of independent observational properties
obtained from the optical and UV emission lines, as well as from the soft-X
rays. The 4DE1 “optical plane”, also known as the quasar Main Sequence (MS),
identifies different spectral types in order to describe a consistent picture
of QSOs. In this work we present a spectroscopic analysis focused on the
comparison between two sources, one radio-loud (PKS2000-330, $z=3.7899$) and
one radio-quiet (Q1410+096, $z=3.3240$), both showing Population A quasar
spectral properties. Optical spectra were observed in the infrared with
VLT/ESO, and the additional measures in UV were obtained through the fitting
of archive spectra. The analysis was performed through a non-linear multi-
component decomposition of the emission line profiles. Results are shown in
order to highlight the effects of the radio-loudness on their emission line
properties. The two quasars share similar optical spectroscopic properties and
are very close on the MS classification while presenting significant
differences on the UV data. Both sources show significant blueshifts in the UV
lines but important differences in their UV general behaviour. While the
radio-quiet source Q1410+096 shows a typical Pop A UV spectrum with similar
intensities and shapes on both CIV$\lambda$1549 and SiIV$\lambda$1392, the UV
spectrum of the strong radio-loud PKS2000-330 closely resembles the one of
population B of quasars.

###### keywords:

quasars, radio-loudness, high redshift, spectroscopy

††articletype: review Table 1: Main properties of PKS2000-330 and Q1410+096. Source | RA (J2000) | DEC (J2000) | $z$ | $M_{\rm{i}}$ | $m_{\rm{H}}$
---|---|---|---|---|---
(1) | (2) | (3) | (4) | (5) | (6)
Q1410+096 | 14 13 21.05 | +09 22 04.8 | 3.3240 | -29.44 | 15.62
PKS2000-330 | 20 02 24.00 | -32 51 47.0 | 3.7899 | -30.99 | 15.32

##  1 Introduction

In order to organise the spectroscopic diversity observed in low-redshift
quasars, [Sulentic . (2000)] introduced the fourth-dimensional Eigenvector 1
(4DE1), a correlation space that considers several key observational measures
(including optical, UV, and X-ray) as well as physical parameters such as
outflow prominence and accretion mode. One of the most important measures is
the full width at half maximum (FWHM) of the H$\beta$ broad component, thought
to be a measure of virialized motions in the accretion disc and thus crucial
for black hole mass estimations. Also, another fundamental parameter is the
ratio between the intensities of the Fe II blend at 4570Å and H$\beta$
($R_{\rm{Fe\textsc{II}}}=I(\textrm{Fe {II}}\lambda 4570)/I($H$\beta)$), that
is related to the Eddington ratio (Marziani ., 2001) and many of the optical
and UV spectral line measures (Kovačević ., 2010; Marziani ., 2010;
Shapovalova ., 2012), and which could be used to estimate physical parameters
of the Broad Line Region (BLR) as the ionisation state, the electron density
or the column density (Panda ., 2020; Ferland ., 2009).

The optical plane of the 4DE1 (defined by the FWHM(H$\beta$) vs. RFeII) is the
so-called Main Sequence (MS) of quasars and the physical parameters related
with accretion rate and outflowing gas seem to be changing along it (Marziani
., 2018). The MS gave rise to the concept of two populations of quasars, which
present significant spectroscopic differences (Zamfir ., 2010). Along the Main
Sequence, the physical properties vary from sources with low Fe II emission
and high black hole mass “disk-dominated” QSOs (Population B), to the extreme
Population A with strong Fe II emission, narrower Lorentzian line profiles,
lower ionization spectra and evidence of strong outflows, “wind-dominated”
quasars (Martínez-Aldama ., 2018). The optical plane of the 4DE1 is driven by
the Eddington ratio convolved with orientation effects, and a critical
Eddington ratio of $L/L_{\rm{Edd}}\approx 0.2$, associated with an accretion
mode change, may play a key role in the observed MS (Marziani ., 2019;
Sulentic ., 2017).

Only 10% of the known quasars are strong emitters in radio (radio-loud).
Zamfir . (2010) analysed about 500 quasars observed with Sloan Digital Sky
survey (SDSS) and found that radio-quiet sources are distributed equally in
Pop. A and Pop. B. But this is not true for radio-loud sources. In the optical
domain of the 4DE1, the radio-loud sources show a preference of having FWHM of
H$\beta$ higher than 4000 km s-1 and a R${}_{\rm{Fe\textsc{II}}}$ lower than
0.5, which are the properties that characterize Pop. B quasars. However,
recent results have found a relatively high fraction of intermediate radio-
emitters in extreme Pop. A quasars (del Olmo ., 2021; Ganci ., 2019). These
results may support a real dichotomy between radio-loud and radio-quiet QSOs.
This work aims to contribute to that study by analysing the spectroscopic
differences of two Population A quasars at high redshift, one radio-loud and
one radio-quiet. Observational data and properties of the two quasars are
described in Section 2. The performed optical and UV spectral analysis are
presented in Sec. 3, while the main results are detailed in Sec. 4. Comparison
of the spectroscopic properties of both sources and the main conclusions are
presented in sections 5 and 6 respectively.

##  2 Sample & Observational Data

Figure 1: Optical spectra obtained with VLT/ISAAC for Q1410+036 (top panel)
and PKS2000-330 (bottom panel). Grey dashed line indicates the powerlaw
obtained on the specfit fitting to represent the continuum level. Green line
shows the FeII contribution. Vertical dotted lines indicate the rest-frame of
the main emission lines on the spectra. Figure 2: Location of the two sources
(in pink) in the Hubble diagram. Grey dots represent a random subsample of the
SDSS DR-16 catalogue from Lyke . (2020). Galactic extinctions are obtained
from Schlafly Finkbeiner (2011) and $K$-correction through Richards . (2006).

In this work we present measured properties in the optical and UV spectral
regions of two quasars, one radio-loud (PKS200-330) and one radio-quiet
(Q1410+096), selected from our ongoing work of a complete sample of 36 quasars
at high redshift ($z=2-4$), that will be presented in a forthcoming paper. The
infrared observations were performed at the 8-meter VLT telescope, through the
European Southern Observatory (ESO) programmes 083.B-0273(A) for PKS2000-330
and 085.B-0162(A) for the observations of Q1410+096. The ISAAC spectrograph
was operated in service mode with a slit width of 0.6”. The spectroscopic
reduction of the new VLT observations were performed in the standard way using
the routines of the astronomical package IRAF. Fig. 1 shows the obtained rest-
frame optical spectra of the two quasars, with a S/N ratio of $\approx$ 55 in
both spectra. Redshift estimation was performed based on the H$\beta$ emission
line profile fitting, and then the same $z$ was applied for both optical and
UV spectra.

For the UV spectral range (observed in the optical domain at the redshift of
the sample), which includes high ionization lines as C IV$\lambda$1549, He
II$\lambda$1640, Si IV+O IV]$\lambda$1500, we use the archive from the Sloan
Digital Sky Survey to obtain the spectrum of Q1410+096. In the case of
PKS2000-330 (the radio-loud source) the UV spectrum was digitalized from
Barthel . (1990). Fluxes at radio frequencies and the study of radio data of
the sources were collected from the Faint Images of the Radio Sky at Twenty-
Centimeters (FIRST) survey archive, as well as from the NVSS survey.

The selected sources are among the objects with the highest redshift (z) in
our sample. The motivation behind selecting these two QSOs is that they share
a similar MS location and a similar optical spectrum, but are different in
terms of radio emission and UV spectrum. Table LABEL:tab:properties lists the
coordinates (Col. 2 and 3), estimated redshift ($z$, Col. 4), $i$-band
absolute magnitude ($M_{\rm{i}}$, Col. 5), and $H$-band apparent magnitude
($m_{\rm{H}}$, Col. 6) of these two sources. In Fig. 2 is shown the location
of the two sources in the $M_{\rm{i}}$ vs. $z$ plane, where we have also
represented a random QSO subsample from the SDSS DR16 catalogue (Lyke ., 2020)
for a comparison with our data. The two sources present higher absolute
magnitude $M_{\rm{i}}$ and larger redshift when compared with the SDSS data
average.

  * •

Q1410+096: this object has been identified as a Broad Absorption Line (BAL)
QSO (Allen ., 2011) with absorptions seen mainly on the region of the
CIV$\lambda$1549 and Si IV$\lambda$1397 emission lines (see Fig. 3). The UV
spectrum was obtained from the SDSS archive through the DR16 catalog (Ahumada
., 2020). The Q1410+096 redshift is reported in the SDSS database as 3.3405,
which is significantly different (by $\delta z\sim$ 0.0165) from our
estimation in the optical spectrum, $z\approx 3.3240$. We have verified that
there are no calibration problems in the optical spectrum. In addition, our
determination of the redshift is based on the fitting of the H$\beta$ line,
showing a peaked and well-defined profile (see Fig. 4 upper left) in which the
uncertainty in the centroid of the H$\beta$ is about 100 km s-1. We have also
carried out a fitting of the 1900Å blend in the UV SDSS spectrum and we have
verified that (as can be seen in Fig. 3 upper, where rest-frame position of
the emission lines of the 1900Å blend are shown) the redshift determined
through H$\beta$ is in very good agreement with the one we obtained from the
main emission lines of the 1900Å blend (Al III$\lambda 1857$ doublet, Si
III]$\lambda 1892$ and C III]$\lambda 1908$). Probably the different redshift
in the SDSS database could be due to some error in the redshift estimation of
the SDSS DR8 automatic pipeline for this object.

  * •

PKS2000-330: This source is a very powerful radio-loud quasar, presenting a
radio flux of $446.0\pm 15.7$ mJy according to the NVSS catalog (Condon .,
1998), and a corresponding radio-loudness parameter $R_{\rm{k}}=4118$,
computed following the approach introduced by Kellermann . (1989). No emission
information is provided by the FIRST catalog as the position of this source is
not in the sky area covered by the FIRST survey. There is also no available
spectrum for this object in the SDSS database. For doing the UV spectral
analysis, we digitalized the one provided by Barthel . (1990). This spectrum
has a smaller wavelength range than the one of Q1410+096, and it does not
cover 1900Å blend nor HeII$\lambda$1640 emission line. We therefore analysed
only SiIV$\lambda$1397 and C IV$\lambda$1549 regions. Regarding the optical
region, PKS2000-330 present a very peaked and Lorentzian-like H$\beta$
profile, which also allows for a good estimation of the rest-frame.

Figure 3: Spectra for the UV region of Q1410+096 (top panel) and PKS2000-330
(bottom panel). Orange, pink, and purple lines identify the local fitted
continuum by specfit on the corresponding UV regions. Vertical dotted lines
show the rest-frame (obtained from the H$\beta$ fittings) of the main emission
lines on the spectra.

##  3 Spectral Analysis

Figure 4: Fittings and residuals for the optical and UV region of Q1410+096
(top panels) and PKS2000-330 (bottom panels). From left to right: H$\beta$+[O
III]$\lambda\lambda 4959,5007$, Si IV$\lambda 1392$, and C IV$\lambda 1549$
emission lines regions. Pink dashed lines show the final fitting. Broad
components are represented by black lines, while blueshifted components are in
blue. Orange and green lines represent narrow and Fe II components. Brown
lines represent the absorptions seen in the spectra. Residuals are shown in
the bottom of each plot.

The spectral analysis was carried out through the specfit routine of IRAF. By
using this tool, we were able to perform simultaneously a minimum-$\chi^{2}$
fit of the continuum, the FeII pseudo-continuum and the emission/absorption
individual spectral line components. In the regions of interest, we have fit
for the two sources the H$\beta$+[OIII]$\lambda\lambda$4959,5007Å lines in the
optical range, and SiIV$\lambda$1397+OIV$\lambda$1402 and
CIV$\lambda$1549+HeII$\lambda$1640 emissions (only CIV in the case of
PKS2000-330 as its spectrum does not cover the HeII line) in the UV spectra.

###  3.1 Optical range

In the specfit fits of the optical region we adopted a power law to describe
the continuum. For the Fe II emission, located at both sides of H$\beta$, we
used a scalable FeII template as is explained in Marziani . (2009). Since
Q1410+096 and PKS2000-330 are Population A quasars, the full broad profile of
H$\beta$ of the two objects has been modeled including initially three
components (see e.g. Marziani . (2018)): 1) a symmetric and unshifted
Lorentzian-like profile broad component (BC, in order to account for the
virialized subsystem of the BLR); 2) a blueshifted gaussian component (BLUE),
sometimes skewed, as is the case of Q1410+096. This component is mainly
detected in Pop. A quasars and it is related with non-virial motions and/or
outflowing gas. Usually, when its presence is needed, it is fitted with the
same FWHM and shift of the ones observed in the [OIII] lines; and 3) a
narrower gaussian component (NC) consistently modeled with the narrow
component of [OIII].

In the case of the [OIII]$\lambda\lambda$4959,5007 emission lines, each line
of the doublet is fitted by a "narrow" symmetrical gaussian component (NC),
and an added semibroad blueshifted gaussian component (named BLUE) associated
to the presence of outflows. We fitted the doublet considering a fixed ratio
of theoretical intensities of 1:3 (Dimitrijević ., 2007), the same FWHM, and
same line shift. As in H$\beta$, the [O III]$\lambda\lambda$4959,5007 BLUE
component are allowed to be skewed to the blue, indicating the presence of an
outflow contribution. Also, our initial guess is that the blueshifted
components of H$\beta$ and [O III]$\lambda\lambda$4959,5007 share the same
shift, FWHM and asymmetry.

Table 2: Flux intensities of the H$\beta$ and C IV$\lambda$1549 emission
lines.

Source | f(H$\beta$) |  f(C IV$\lambda$1549)
---|---|---
|  [10-15 erg s-1 cm-2 Å-1] |  [10-15 erg s-1 cm-2 Å-1]
(1) | (2) | (3)
Q1410+096 | 5.9 | 2.2
PKS2000-330 | 6.6 | 4.5

###  3.2 UV range

Regarding the fitting of the UV spectra, we divide the spectral range into two
regions centered on the most important emission lines: one in which
CIV$\lambda$1549 dominates the emission, and the other centered in
SiIV$\lambda$1397+OIV$\lambda$1402 lines. In both cases the continuum was
modeled locally by a power-law as it is shown in Fig. 3. Similarly to the
H$\beta$ line, broad UV lines are fitted by using an unshifted Lorentzian
profile (BC), kept fixed at the rest-frame determined with H$\beta$, plus one
or more asymmetric Gaussians to model the blueward excess (BLUE).

In the case of Q1410+096 where both CIV and SiIV are contaminated by the
presence of strong absorption features in the blue side, the fittings include
absorption lines modeled as Gaussians, so that specfit takes into account them
to evaluate the broad emission lines parameters. Nevertheless, the
contamination by strong absorption features in the blue side of both profiles
make the fit less accurate and the fitted parameters affected by larger
uncertainties. Since C IV$\lambda$1549 and He II$\lambda$1640 are expected to
present a similar shape, the spectral analysis of the He II$\lambda$1640
emission line in Q1410+096 is performed in a similar way of C IV$\lambda$1549,
sharing the same FWHM and shifts (Martínez-Aldama ., 2018). In the same way,
broad component of SiIV was modeled with the same emission components of CIV
(BC+BLUE), with only the flux varying freely.

##  4 Results

###  4.1 H$\beta$+[O III]$\lambda\lambda 4959,5507$

Fig. 1 shows the VLT spectra used on the H$\beta$+[O
III]$\lambda\lambda$4959,5007. The two objects show very similar behaviour on
the optical range, specially in terms of Fe II contribution (green lines) and
H$\beta$ profile. Flux intensities of H$\beta$ are shown in Table 2. specfit
models for the optical data of the two sources are shown in the left plots of
Fig. 4. The FWHM of the H$\beta$ broad component of Q1410+096 and PKS2000-330
are also very similar, however the shape of the profile at the rest-frame
wavelength present a small difference making it necessary to include a very
narrow component to account for the narrowed peak seen in the profile of
PKS2000-330.

Table LABEL:tab:measurements lists the measurements we obtained after the
specfit fitting for the optical region. We present FWHM and centroid velocity
at 1/2 intensity (c(1/2)) of the full profile for H$\beta$ and [O
III]$\lambda\lambda$4959,5007 as well as the relative intensity compared to
the total flux (I/Itot), and the FWHM and wavelength shift for the individual
line components. There are no shifts for the broad line components of
H$\beta$. Also, we do not include the blueshifted components of the H$\beta$
profile on the full profile measures because they are too small to present a
significant contribution to the full emission line or are clearly related with
the [O III]$\lambda\lambda$4959,5007 blueshifted components (case in which it
is possible that the components present the same physical origin). In the case
of the [O III]$\lambda\lambda$4959,5007 emission lines, the blueshifted
components are stronger and present a more skewed shape for Q1410+096 than for
PKS2000-330.

After performing the multicomponent fitting, we can locate the two sources on
the optical plane of the MS. Fig. 5 shows the location of Q1410+096 and
PKS2000-300 on the Main Sequence (represented by the grey and green areas).
The red dashed line indicates the A/B Population boundary for the FWHM in the
Main Sequence corresponding to a quasar luminosity of about $10^{48}$ erg s-1,
representative of the luminosity of our sample, according to Marziani .
(2009). As can be seen, the two sources are very similar in terms of
FWHM(H$\beta$) and R${}_{\rm{Fe\textsc{II}}}$ and are classified as Pop. A2
quasars.

###  4.2 UV

UV spectra and fittings of the CIV$\lambda$1549+He II$\lambda$1640 and
SiIV$\lambda$1392 are shown in Figs. 3 and 4, respectively. Table
LABEL:tab:measurements_uv reports the FWHM and c(1/2) of the CIV full profile
as well as the intensities, FWHM, and shift of the individual components. The
blueshifted component of the CIV$\lambda$1549 is usually much stronger than
the ones observed in the optical range, which can be seen as an indicative of
wind activities surrounding the central region. This is also observed in the
two sources, where the full profile of CIV is blueshifted by about 2000 km
s-1. In the case of the RQ BAL Q1410+096 the presence of strong absorption
features could explain the lower blue-shift observed (parametrized by c(1/2))
in the CIV full profile when compared with the RL PKS2000-330. Previous
studies had shown that smaller values of c(1/2) in UV lines in BALs are due
mainly to the presence of deep absorptions in the blue side of the profiles
(Martínez-Aldama . (2018)).

Also, when comparing the two sources in this region, it is clear that their
spectral behaviour is different. While in Q1410+096 the ratio between the
intensities of CIV$\lambda$1594 and SiIV$\lambda$1397 is $\sim$ 0.96 (a value
expected for Pop. A quasars), the situation is the opposite for PKS2000-330.
In this case, there is a very small contribution of SiIV$\lambda$1397, much
smaller than the CIV$\lambda$1549 profile, presenting a Pop. B-like UV
spectrum with a CIV$\lambda$1594/SiIV$\lambda$1397 $\sim$ 1.40.

Figure 5: Location of the two sources on the optical plane of the 4DE1. The
radio-quiet quasar (Q1410+096) is represented by the filled circle, while the
radio-loud quasar (PKS2000-330) is shown as an open circle. Grey and green
regions indicate the distribution of the Main Sequence of quasars. Red
horizontal line shows the Population A/B boundary for high luminosity quasars,
following the FWHM(H$\beta$)-Mi relation from Sulentic . (2004). Table 3:
Measurements for the optical range.

|  |  | Optical |
---|---|---|---|---
Source |  | H$\beta_{\rm{full}}$ |  | H$\beta_{\rm{BC}}$ |  | H$\beta_{\rm{NC}}$ |  | [O III]full |  | [O III]BLUE |  | [O III]NC
|  | FWHM | c(1/2) |  | I/Itot | FWHM |  | I/Itot | FWHM |  | FWHM | c(1/2) |  | I/Itot | FWHM | Shift |  | I/Itot | FWHM | Shift
|  | [km s-1] | [km s-1] |  |  | [km s-1] |  |  | [km s-1] |  | [km s-1] | [km s-1] |  |  | [km s-1] | [km s-1] |  |  | [km s-1] | [km s-1]
(1) |  | (2) | (3) |  | (4) | (5) |  | (6) | (7) |  | (8) | (9) |  | (10) | (11) | (12) |  | (13) | (14) | (15)
Q1410+096 |  | 3394 | 54 |  | 1.0 | 3394 |  | - | - |  | 3363 | -1404 |  | 0.73 | 5573 | -1255 |  | 0.27 | 1379 | -260
PKS2000-330 |  | 3138 | 28 |  | 0.93 | 3138 |  | 0.072 | 1082 |  | 1314 | -425 |  | 0.42 | 1550 | -827 |  | 0.58 | 1082 | -263

Table 4: Measurements for the C IV$\lambda$1549.

| UV
---|---
Source | C IVfull |  | C IVBLUE |  | C IVBC
| FWHM | c(1/2) |  | I/Itot | FWHM | Shift |  | I/Itot | FWHM
| [km s-1] | [km s-1] |  |  | [km s-1] | [km s-1] |  |  | [km s-1]
(1) | (2) | (3) |  | (4) | (5) | (6) |  | (7) | (8)
Q1410+096 | 6311 | -1746 |  | 0.42 | 10882 | -1525 |  | 0.58 | 3293
PKS2000-330 | 4950 | -1923 |  | 0.71 | 7339 | -1219 |  | 0.29 | 3141

##  5 Discussion: Is there any relation between H$\beta$+[O
III]$\lambda\lambda$4959,5007 and C IV$\lambda 1549$?

By analysing Table LABEL:tab:measurements_uv, it is clear that the blueshift
component plays an important role in the full profile of C IV$\lambda 1549$ in
the two sources, with a comparable shift towards lower wavelengths ($\sim$
2000 km s-1). Q1410+096 (radio-quiet) shows a C IV$\lambda 1549$ BLUE with a
FWHM $=10882$ km s-1, while PKS2000-330 (radio-loud) have a C IVBLUE that is
somewhat narrower ($\sim 7300$ km s-1). The fact that radio-quiet sources show
a preference of having wider FWHM(C IVBLUE) than the radio-loud ones is
already known by the recent literature and seems to be the normal behaviour in
this type of sources (Richards ., 2021). Apart from that, these results also
may indicate that there is an important contribution of outflowing gas in the
nuclear activity in the two sources, in agreement with previous results in
other quasar samples (Marziani ., 2017; Sulentic ., 2017).

Another point that we would like to highlight here is that the radio-quiet we
analyse presents a slightly higher shift to blue wavelengths when compared to
the radio-loud. However, this behaviour is not exclusively observed in C
IV$\lambda$1549\. Actually, it is also seen in the blueshifted component of [O
III]$\lambda\lambda$4959,5007 (Ganci ., 2019), although rare radio-loud NLSy1s
do show significant shifts in [O III] (Berton ., 2016). In the two sources, C
IV$\lambda$1549 and [O III]$\lambda\lambda$4959,5007 present similar shifts
towards the blue. As a consequence, the full profiles of these two emission
lines present a negative c(1/2), indicating a strong contribution of the
blueshifted components to the full profile. Also, the FWHM(CIVfull) is much
wider for the radio-quiet source than for the radio-loud source, with a
difference greater than 1000 km s-1 for C IV$\lambda$1549 and $\sim 2000$ km
s-1 for [O III]$\lambda\lambda$4959,5007.

Both [O III]$\lambda\lambda$4959,5007 and C IV$\lambda$1549 blueshifted
components represent a significant fraction of their respective full profiles.
This is an expected behaviour especially at high/intermediate redshift
(Sulentic ., 2004, 2007; Marziani ., 2009). When compared to samples with
lower redshift, sources such as Q1410+096 and PKS2000-330 (and in general
sources at intermediate redshifts) tend to show components in lines like [O
III]$\lambda\lambda$4959,5007 and C IV$\lambda$1549 that are wider and more
blueshifted.

Sulentic . (2017), for instance, analyse samples with different redshifts and
luminosities showing that the higher the luminosity, the stronger the CIV
outflows tend to be. In the high-$z$ context, the emission component related
with the outflowing gas becomes the dominant constituent of the broad profile.
Similar behaviour is observed in [O III] emission lines, which present
significant differences in the full profile when comparing high and low
luminosity sources. Cano-Díaz . (2012) find a massive outflow on kpc scales in
the [O III] for 2QZJ002830.4-281706, which has a $z=2.4$. Similar results were
also found in Marziani . (2016), for a Hamburg ESO sample with intermediate
$z$ and high luminosity. Zakamska . (2016) analyse four extremely red quasars
with $z\sim 2.5$ and discover blueshifted components with very broad FWHM and
strongly shifted towards blue wavelengths.

The FWHM of the broad components of H$\beta$ and C IV$\lambda$1549 also
indicate some correspondence between optical and UV regions. For Q1410+096,
the FWHM of the two lines differ by $\sim 100$ km s-1. And in the case of
PKS2000-330, this difference is smaller than 5 km s-1. When considering the
full profile, it can be well reproduced by almost only the BC component in the
H$\beta$ profile. This is not true for C IV$\lambda$1549, case in which the
FWHM of the broad component represent only 58% for Q1410+096 and 29% for
PKS2000-330.

##  6 Conclusions

We analysed a pair of Pop. A2 quasars, which are similar respect to the
optical spectra and are very close on the MS classification but present
significant differences on the UV data and also on the radio emission: one is
a very powerful radio-loud QSO (PKS2000-330) and the other is a radio quiet
(Q1410+096). The main conclusions are:

  * •

The C IV$\lambda$1549 and [O III]$\lambda\lambda$4959,5007 emission line
profiles present a similar behaviour in Q1410+096 and also in PKS2000-330,
with a more significant shift towards lower wavelengths in the case of the
radio-quiet quasar;

  * •

The [O III]$\lambda\lambda$4959,5007 emission lines in the two sources are
different. In PKS2000-330, we observe a symmetric blueshifted component. On
the other hand, in Q1410+096 it is necessary a very skewed blueshifted
component (also in H$\beta$). This may be an indicative that the two objects
probably have different behaviour in the observed winds;

  * •

The general behaviour of the UV spectra is very different when comparing the
two sources. In the case of the radio-quiet source (Q1410+096) C
IV$\lambda$1549 and Si IV$\lambda 1392$ are very similar in intensity and
shape, which is a characteristic expected for Pop. A quasars. The same is not
true for PKS2000-330, which is a radio-loud quasar and present a more Pop.
B-like spectrum in the UV region.

However, a more detailed study is needed in order to perform a complete
analysis of the clear behaviour of radio-quiet and radio-loud sources along
the Main Sequence. The next step of this work will be to perform an analysis
of the spectroscopic properties of our complete sample of 36 quasars and to
study the differences between radio-loud and radio-quiet sources.

## Acknowledgments

The authors thank the anonymous referee for her/his valuable suggestions that
helped us to significantly improve the present paper. A.D.M. acknowledges the
support of the INPhINIT fellowship from "la Caixa" Foundation (ID 100010434).
The fellowship code is LCF/BQ/DI19/11730018. A.D.M. and A.d.O. acknowledge
financial support from the State Agency for Research of the Spanish MCIU
through the project PID2019-106027GB-C41 and the “Center of Excellence Severo
Ochoa” award to the Instituto de Astrofísica de Andalucía (SEV-2017-0709).

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## Author Biography

Alice Deconto Machado. PhD candidate at the Instituto de Astrofísica de
Andalucía (IAA-CSIC, Granada, Spain), under the supervision of Dr. Ascensión
del Olmo Orozco (IAA-CSIC, Granada, Spain) and Dr. Paola Marziani (INAF-
Padova, Italy).


# Common Electronic Origin of Superconductivity in (Li,Fe)OHFeSe Bulk
Superconductor and Single-Layer FeSe/SrTiO3 Films

Lin Zhao1,♯, Aiji Liang1,♯, Dongna Yuan1,♯, Yong Hu1,♯, Defa Liu1, Jianwei
Huang1, Shaolong He1, Bing Shen1, Yu Xu1, Xu Liu1, Li Yu1, Guodong Liu1,
Huaxue Zhou1, Yulong Huang1, Xiaoli Dong1,∗, Fang Zhou1, Zhongxian Zhao1,2,
Chuangtian Chen3, Zuyan Xu3 and X. J. Zhou1,2,∗
1National Lab for Superconductivity, Beijing National Laboratory for Condensed
Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing
100190, China
2Collaborative Innovation Center of Quantum Matter, Beijing 100871, China
3Technical Institute of Physics and Chemistry, Chinese Academy of Sciences,
Beijing 100190, China
♯These authors contributed equally to the present work.
∗Corresponding author<EMAIL_ADDRESS><EMAIL_ADDRESS>
(May 23, 2015)

The mechanism of high temperature superconductivity in the iron-based
superconductors remains an outstanding issue in condensed matter
physicsStewart ; FWang ; Dagotto . The electronic structure, in particular the
Fermi surface topology, is considered to play an essential role in dictating
the superconductivityKuroki ; Mazin1 ; FWang1 ; AVChubukov ; VStanev ; FWang2
; Mazin2 . Recent revelation of distinct electronic structure and possible
high temperature superconductivity with a transition temperature Tc above 65 K
in the single-layer FeSe films grown on the SrTiO3 substrate provides key
information on the roles of Fermi surface topology and interface in inducing
or enhancing superconductivityQKXue ; DFLiu ; SLHe ; SYTan ; WHZhangCPL ;
JJLee ; XLiu ; JFHe ; RPeng1 ; RPeng2 ; JFGe ; XLReview . Here we report high
resolution angle-resolved photoemission measurement on the electronic
structure and superconducting gap of a novel FeSe-based superconductor,
(Li0.84Fe0.16)OHFe0.98Se, with a Tc at 41 K. We find that this single-phase
bulk superconductor shows remarkably similar electronic behaviors to that of
the superconducting single-layer FeSe/SrTiO3 film in terms of Fermi surface
topology, band structure and nearly isotropic superconducting gap without
nodes. These observations provide significant insights in understanding high
temperature superconductivity in the single-layer FeSe/SrTiO3 film in
particular, and the mechanism of superconductivity in the iron-based
superconductors in general.

The iron-based superconductorsStewart ; FWang ; Dagotto  represent the second
class of high temperature superconductors after the first discovery of high
temperature superconductivity in cupratesBednorz . The electronic structure of
the iron-based compounds usually consists of hole-like bands near the
Brillouin zone center and electron-like bands near the Brillouin zone corners,
leading to proposals that the electron scattering between the hole pockets
around the Brillouin zone center and the electron pockets around the Brillouin
zone corners is a viable mechanism for electron pairing in the iron-based
superconductorsKuroki ; Mazin1 ; FWang1 ; AVChubukov ; VStanev ; FWang2 ;
Mazin2 . With the discovery of the AxFe2-ySe2 (A=K, Cs, Rb, Tl and etc.)
superconductorsJGGuo ; MHFang ; DXReview ; Dagotto , such a Fermi surface
nesting picture is challenged because the superconducting phase in AxFe2-ySe2
superconductors does not contain hole pockets near the Brillouin zone
centerTQian ; DXMouPRL ; YZhang ; LZhaoPRB ; DXReview ; Dagotto . The latest
discovery of possible high temperature superconductivity above 65 K in the
single-layer FeSe film grown on the SrTiO3 substrate (denoted as FeSe/SrTiO3
film hereafter) puts even stronger constraints on the Fermi surface nesting
picture because there is no Fermi surface present near the zone center at
allQKXue ; DFLiu ; SLHe ; SYTan ; XLReview . While the implications of the
Fermi surface topology in the AxFe2-ySe2 superconductors and superconducting
single-layer FeSe/SrTiO3 films are significant, the material-specific
complications make the conclusion on the role of Fermi surface topology
ambiguous. The AxFe2-ySe2 superconductors are well-known to have problems of
inhomogeneity, phase separation and small superconducting volume fraction. The
exact nature of the superconducting phase remains unclear, and its coexistence
with the insulating phase suggests that interface may play an important role
in the superconductivityYJYan ; WBao ; ZWang ; WLi . There are strong
indications that interface in the single-layer FeSe/SrTiO3 films plays an
important role in giving rise to high temperature superconductivityQKXue ;
DFLiu ; SLHe ; SYTan ; WHZhangCPL ; JJLee ; XLiu ; JFHe ; RPeng1 ; RPeng2 ;
JFGe ; XLReview . With the complications of the interface effect and phase
separation problem involved in AxFe2-ySe2 superconductors and superconducting
single-layer FeSe/SrTiO3 films, it becomes less straightforward to conclude
whether the same effect of Fermi surface topology on superconductivity is
operative in all the iron-based superconductors.

In this paper, we report high resolution angle-resolved photoemission (ARPES)
measurements on a new FeSe-based superconductor, (Li1-xFex)OHFe1-ySe, with a
superconducting critical temperature at 41 KXFLuPRB ; XFLuNM ; XLDong ;
XLDong11111 ; BLei . We find that this superconductor contains electron-
pocket(s) near the Brillouin zone conners without hole-pockets near the zone
center. The superconducting gap around the electron-like Fermi surface near
the zone corner is nearly isotropic without nodes, and its temperature
dependence follows the usual BCS form. These behaviors are strikingly similar
to that of the superconducting single-layer FeSe/SrTiO3 films. This is the
first single-phase bulk superconductor with a relatively high Tc$\sim$41 K,
free from the complication of phase separation and interface effect, that
shows electron-like Fermi pockets only. Our observations provide significant
insights on the superconductivity mechanism in the iron-based bulk
superconductors, as well as the origin of high temperature superconductivity
in the single-layer FeSe/SrTiO3 films.

Figure 1a shows the crystal structure of the new FeSe-based superconductor:
(Li1-xFex)OHFe1-ySeXFLuNM ; XLDong11111 . It consists of FeSe layers
sandwiched in between the (LiFe)OH layers along the c direction. The in-plane
lattice constant a or b is 3.78 Å which is close to that (3.76 Å) in the bulk
FeSe superconductor (Tc$\sim$ 8.5 K)FCHsu , but the distance of the two
adjacent FeSe layers in (Li1-xFex)OHFe1-ySe (9.32 Å) is much larger than that
in bulk FeSe (5.5 Å)FCHsu . This indicates a weak interaction between the two
adjacent FeSe layers in (Li1-xFex)OHFe1-ySe and its enhanced two-dimensional
nature of the electronic structure compared with bulk FeSe. In comparison, the
single-layer FeSe/SrTiO3 film consists of a strictly two-dimensional Se-Fe-Se
layer on the top of the SrTiO3 substrate; the thickness of the FeSe layer is
5.5 $\AA$ (Fig. 1b)QKXue . In comparison, the distance between the two
adjacent FeSe layers in the AxFe2-ySe2 is 7.02 $\AA$, however, there is an in-
plane displacement between the two adjacent FeSe layers (Fig. 1c)JGGuo . The
latest success of growing high-quality large-sized (Li1-xFex)OHFe1-ySe single
crystalsXLDong11111  makes it possible to carry out ARPES measurements on the
new superconductors. The sample we have measured in this paper has a
composition of Li0.84Fe0.16OHFe0.98Se (abbreviated as FeSe11111 hereafter)
XLDong11111  with a superconducting transition temperature Tc$\sim$41 K (Fig.
3d).

Figure 1d shows the Fermi surface mapping of FeSe11111 measured at 20 K. The
spectral weight at each momentum is obtained by integrating the EDC (energy
distribution curve) spectral weight within [-10,+10] meV energy window with
respect to the Fermi level (EF). The measured Fermi surface contains a nearly
circular electron-like Fermi surface around M points (Fig. 1d). There is no
indication of Fermi crossing around the $\Gamma$ (0,0) point. Such a Fermi
surface topology is very similar to that found in the superconducting single-
layer FeSe/SrTiO3 film (Fig. 1e) with a superconducting temperature at
$\sim$55 KDFLiu . In comparison, there are electron pockets around the
Brillouin center in the AxFe2-ySe2 superconductor (Fig. 1f)DXMouPRL ; LZhaoPRB
. The area of the electron pocket near M gives a good measure of the electron
doping level for the strongly two-dimensional systems. Considering that the
Fermi surface around M consists of two degenerate Fermi surface sheets, the
estimated electron counting in FeSe11111 (Fig. 1d) is $\sim$0.08 electrons/Fe.
This is smaller than 0.10 electrons/Fe in single-layer FeSe film (Fig. 1e)
with a Tc$\sim$55 KDFLiu  and 0.12 electrons/Fe in vacuum-annealed single-
layer FeSe/SrTiO3 film with a Tc$\sim$ 65 KSLHe . We note that the electron
counting in both FeSe11111 and superconducting single-layer FeSe/SrTiO3 film
is much smaller than 0.18 electrons/Fe in AxFe2-ySe2 superconductor (Fig.
1f)TQian ; DXMouPRL ; YZhang ; LZhaoPRB  when only considering the M point
electron Fermi surface sheet.

Figure 2 shows the band structure (a-c) and photoemission spectra (l-n) of
FeSe11111 measured across three momentum cuts. For comparison, the band
structure measured on a superconducting single-layer FeSe/SrTiO3 filmDFLiu
(Fig. 3(e-g)) and on (Tl,Rb)xFe2-ySe2 superconductorDXMouPRL  (Fig. 3(h-j))
along the same three momentum cuts are also presented. Overall, the band
structure of FeSe11111 is very similar to that of the superconducting single-
layer FeSe/SrTiO3 film with slight quantitative difference in the band
position. Near the $\Gamma$ point, two hole-like bands can be resolved for
FeSe11111 (Fig. 2c) that are well below the Fermi level, as seen more clearly
in the corresponding second-derivative image (Fig. 2d) and the photoemission
spectra (Fig. 2n). We note that there is a very weak signal near the Fermi
level at $\Gamma$ point which may be due to the leakage from a band above the
Fermi level. Similar multiple hole-like bands have also been observed near
$\Gamma$ in the superconducting single-layer FeSe/SrTiO3 filmsSLHe  and in
KxFe2-ySe2 superconductorLZhaoPRB , with slight variation in the band top
position. In comparison, the electronic structure of (Tl,Rb)xFe2-ySe2
superconductor shows two electron-like bands crossing the Fermi level (Fig.
2j)DXMouPRL . Around M2 and M3 points, we observed in FeSe11111 parabolic
electron-like bands crossing the Fermi level (Figs. 2a and 2b), forming the
electron pocket around M. These are very similar to those observed in the
superconducting single-layer FeSe/SrTiO3 film (Figs. 2e and 2f). Careful
examination indicates that the Fermi momentum (kF) for the electron pocket of
FeSe11111 is $\sim$0.22 $\pi$/a (lattice constant a=3.78 $\AA$) (Figs. 2a and
2b) which is slightly smaller than 0.25 $\pi$/a in the superconducting single-
layer FeSe/SrTiO2 film (Figs. 2e and 2f)DFLiu , both being significantly
smaller than 0.35 $\pi$/a in (Tl,Rb)xFe2-ySe2 superconductor ((Figs. 2h and
2i) DXMouPRL . The bottom of the electron-like band in FeSe11111 is $\sim$50
meV below the Fermi level(Fig. 2b), slightly shallower than the 60 meV in the
superconducting single-layer FeSe/SrTiO3 Film (Fig. 2f)DFLiu  but similar to
the $\sim$50 meV in the (Tl,Rb)xFe2-ySe2 superconductor (Fig. 2i)DXMouPRL .
The combined information of the Fermi momentum and the band width gives a good
measure of the electron effective mass that is 2.9me, 2.7me and 6.1me (me is
free electron mass) for FeSe11111, superconducting single-layer FeSe/SrTiO3
film, and the (Tl,Rb)xFe2-ySe2 superconductor, respectivelyDFLiu ; DXMouPRL .
It is interesting to note that electron effective mass for the electron-like
band near M is similar for FeSe11111 and superconducting single-layer
FeSe/SrTiO3 film, but both are much smaller than that in (Tl,Rb)xFe2-ySe2
superconductorDXMouPRL .

The clear identification of electron-like Fermi surface sheet near M point
makes it possible to investigate the superconducting gap in this new FeSe11111
superconductor. We start by examining the temperature dependence of the
superconducting gap. Fig. 3b shows the original photoemission image measured
at a low temperature of 18 K along the momentum cut (its location is shown in
Fig. 3a) crossing the electron-like Fermi surface around M2. The data is
divided by the corresponding Fermi distribution function; the spectral weight
suppression near the Fermi level indicates the opening of the superconducting
gap (Fig. 3b). Following the procedure commonly used in the study of high
temperature cuprate superconductorsMNorman , the symmetrized photoemission
spectra (EDCs) measured on one Fermi momentum at different temperatures are
shown in Fig. 3c. There is a clear gap opening at low temperatures, as
indicated by a dip at the Fermi level in the symmetrized EDCs (Fig. 3c). With
increasing temperature, the dip at EF is gradually filled up and becomes
almost invisible above 40 K. We used two methods to extract the gap size in
the superconducting state. The first method is to pick the peak position in
the symmetrized EDCs while the second one is to fit the symmetrized EDCs with
the phenomenological formula proposed by Norman et al.MNorman . The top-middle
inset of Fig. 3c shows the symmetrized EDC at 18 K and the fitting result.
Both methods give consistent gap size within an experimental uncertainty of 2
meV. Fig. 3e shows the measured gap size at different temperatures from
symmetrized EDCs for both Fermi momenta. The gap size basically follows a BCS
form with a Tc around (42$\pm$2) K and a gap size of $\sim$14 meV at zero
temperature (Fig. 3e). This is in excellent agreement with the Tc$\sim$41 K
from direct magnetic measurement (Fig. 3d). We note that FeSe11111 is very
sensitive to air and processing temperature. To make sure there is no change
on the superconductivity of our sample during the ARPES measurements, we
carried out magnetic measurements on the measured sample before and after the
ARPES experiment. Fig. 3d shows the magnetic measurement before ARPES
measurement; both the field-cooled (FC) and zero-field-cooled (ZFC) modes give
similar magnetic onset Tc $\sim$ 41 K with a sharp transition of $\sim$1.5 K.
The sharp transition and its 100% diamagnetic shielding demonstrate the high
quality of the measured single crystal. After ARPES experiment (not shown in
Fig. 3d), the magnetic measurement on the cleaved sample shows an onset
Tc$\sim$ 40 K with a sharp transition of $\sim$1.5 K. These indicate that
there is negligible superconductivity degradation on the sample during the
sample handling and ARPES measurements.

Now we come to the momentum dependence of the superconducting gap in the
FeSe11111 superconductor. For this purpose we took high-resolution Fermi
surface mapping (energy resolution of 4 meV) of the electron pocket around M2
at 20 K in the superconducting state (Fig. 4a). The corresponding Fermi
momenta are identified, marked by red empty circles, and labelled by numbers
from 1 to 16 in Fig. 4a. Fig. 4b shows symmetrized photoemission spectra
(EDCs) at the Fermi momenta around the Fermi surface. The extracted
superconducting gap is plotted in Fig. 4c. The superconducting gap is nearly
isotropic with a gap size at (13$\pm$2) meV; there is no gap node (zero gap)
around the Fermi surface. For comparison, the gap size around the electron-
like Fermi surface at M2 point for the superconducting single-layer
FeSe/SrTiO3 filmDFLiu  is also plotted (Fig. 4c). It is also nearly isotropic
without gap node although its magnitude ($\sim$15 meV) is slightly larger than
that in FeSe11111. In FeSe11111, the bonding between the FeSe layer and the
adjacent (Li,Fe)OH layer is rather weak (van der Waals bonding). The
separation between the two adjacent FeSe layers is large and their mutual
interaction is expected to be rather weak. These make its behaviors close to
the single-layer FeSe film that shows strong two-dimensionality. It is thus
reasonable to conclude that there is no nodes of the superconducting gap in
the FeSe11111 superconductor.

From the above results, we have found that the FeSe11111 superconductor
exhibits similar behaviors to the superconducting single-layer FeSe/SrTiO3
film, in terms of the Fermi surface topology, band structure and the
superconducting gap symmetry. Amongst all the iron-based superconductors
discovered so far, the majority of them show common electronic structure with
hole-like Fermi surface sheets around the $\Gamma$ point and electron-like
Fermi surface near the M pointStewart . The AxFe2-ySe2 superconductorTQian ;
DXMouPRL ; YZhang ; LZhaoPRB ; DXReview ; Dagotto  and the superconducting
single-layer FeSe/SrTiO3 filmsDFLiu ; SLHe ; SYTan ; XLReview  have been the
only two exceptions that have distinct Fermi surface topology, i.e., no hole-
like Fermi surface at the $\Gamma$ point and there are only electron pockets.
The present work has established the FeSe11111 superconductor as a new third
exception on the list. It has been proposed that the electron scattering
between the hole-like bands near $\Gamma$ and electron-like bands near M is
the cause of electron pairing in the iron-based superconductorsKuroki ; Mazin1
; FWang1 ; AVChubukov ; VStanev ; FWang2 ; Mazin2 . The discovery of only
electron-like Fermi surface in the AxFe2-ySe2 superconductor and the
superconducting single-layer FeSe/SrTiO3 films challenges such a Fermi surface
nesting scenario. However, some material-specific properties prevent us from
making a decisive conclusion. The AxFe2-ySe2 superconductor is well-known for
its phase separation problem; the superconducting phase occupies only a small
fraction of the entire sample volume and the nature of the superconducting
phase remains unclearYJYan ; WBao ; ZWang ; WLi ; DXReview ; Dagotto . The
coexistence of the superconducting phase and the insulating 245 phase makes
people wonder whether the interface between them is responsible for the
observed superconductivity. On the other hand, the surprising discovery of
high temperature superconductivity in the single-layer FeSe/SrTiO3 films
indicates that interface may play a significant role in inducing or enhancing
superconductivityQKXue ; DFLiu ; SLHe ; SYTan ; WHZhangCPL ; JJLee ; XLiu ;
JFHe ; RPeng1 ; RPeng2 ; JFGe ; XLReview . In this sense, the complications of
phase separation and interface effect hinder the direct comparison of the
AxFe2-ySe2 superconductors and superconducting single-layer FeSe/SrTiO3 films
with most other iron-based superconductors. The newly-discovered FeSe11111
superconductor is advantageous in that it is a single-phase bulk
superconductor, free from complications of phase separation and interface
effect, thus making it possible to compare directly with other iron-based bulk
superconductors. The observation of high temperature superconductivity with a
Tc at 41 K in bulk FeSe11111 superconductor that has electron-pocket-only
Fermi surface topology makes a strongest and most decisive case that the hole-
like Fermi surface around $\Gamma$ is not necessary for the superconductivity
in the iron-based superconductors. This rules out Fermi surface nesting
scenario as the common pairing mechanism for the iron-based superconductors
and points to the significant role of the electron pockets near M in governing
high temperature superconductivity in the iron-based superconductors.

The present work also has important implications on the high temperature
superconductivity in the single-layer FeSe/SrTiO3 films. First, our results
indicate that the electron-pocket-near-M-only Fermi surface topology is not
confined to the superconducting single-layer FeSe/SrTiO3 films only; instead,
it can also be present in bulk superconductors. Our experimental results have
shown that this form of Fermi surface topology is favorable for achieving high
temperature superconductivity in the iron-based superconductors. Second, it is
remarkable that Tc as high as 41 K can be achieved in the bulk FeSe11111
superconductor where there is no strain effect, no interface effect, and no
strong electron-phonon coupling effectJJLee  involved as in the
superconducting single-layer FeSe/SrTiO3 films. It is known from the
superconducting single-layer FeSe/SrTiO3 films that Tc can be tuned by varying
the electron doping levelSLHe . Therefore, it is possible that Tc=41 K is
lower in FeSe11111 than that in the superconducting single-layer FeSe films
($\sim$65 K or higher) because the electron doping level in FeSe11111 is low.
This raises a big possibility that Tc of the FeSe11111 superconductor may be
further increased if higher electron-doping is made possible and the
superconductor-insulator transitionBLei  can be avoided. The high Tc and its
possible enhancement with increasing doping in the FeSe11111 superconductor
provide key insight on understanding the superconductivity in the single-layer
FeSe/SrTiO3 films. Third, we note that the in-plane lattice constant a or b of
the FeSe11111 superconductor (3.78 $\AA$)XFLuNM ; XLDong11111  is comparable
to the primary bulk FeSe superconductor (3.76 $\AA$), but significantly
smaller than the single-layer FeSe film epitaxially grown on the SrTiO3
substrate (3.905 $\AA$). Considering high Tc=41 K and its possible enhancement
upon further elecron-doping in the FeSe11111 superconductor, this indicates
that the interface strain does not play primary role in determining Tc. On the
other hand, the distance between the two adjacent FeSe layers seems to have
strong correlation with superconductivity in the FeSe-based superconductors.
As the distance changes from 5.5 $\AA$ for bulk FeSe, to 9.32 $\AA$ in the
FeSe11111 superconductor, and to infinity in the single-layer FeSe/SrTiO3
films, Tc increases from 8.5 K, to 41 K, to 65 K or higher (Fig. 1). In the
meantime, the top of the hole-like band at $\Gamma$ changes its position from
above the Fermi level for bulk FeSeJMaletz , to 50$\sim$70 meV below the Fermi
level for FeSe11111, to 80 meV below the Fermi level for the superconducting
single-layer FeSe films (Fig. 2). These results again indicate that
suppression of the hole-like bands near the $\Gamma$ point is favorable for
achieving high temperature superconductivity in the iron-based
superconductors.

In summary, by performing high resolution ARPES measurements on the new FeSe-
based superconductor, FeSe11111, we have discovered that it exhibits
strikingly similar electronic behaviors to the superconducting single-layer
FeSe/SrTiO3 film, in terms of the Fermi surface topology, band structure and
the superconducting gap symmetry. This is the first single-phase bulk
superconductor with a relatively high Tc$\sim$41 K that shows electron-like-
Fermi-pockets-only Fermi surface topology that offers a direct comparison with
other iron-based bulk superconductors. Our observations provide significant
insights on the superconductivity mechanism in the iron-based bulk
superconductors, as well as in the single-layer FeSe/SrTiO3 films.

METHODS

Single crystal preparation. High quality (Li1-xFex)OHFe1-ySe single crystals
were grown by hydrothermal approachesXLDong11111 . Large crystals of
K0.8Fe1.6Se2 (nominal 245 phase) are specially grown and used as a kind of
_matrix_ for a hydrothermal ionic exchange reaction. The K ions in
K0.8Fe1.6Se2 are completely released into solution after the hydrothermal
reaction process. Single crystal XRD and powder XRD measurements confirm the
high quality and high purity of the FeSe11111 single crystals. In particular,
no trace of K ions in the FeSe11111 crystals is detected by energy dispersive
X-ray spectrometry (EDX) and ICP-AES analyses, confirming a complete release
of K ions after the hydrothermal ionic exchange. The sample is sensitive to
air and treatment temperature due to the existence of hydroxide. All the
samples for ARPES measurements were prepared in glove box and vacuum case
within a short time and then transferred to ARPES measurement chamber to be
kept at low temperature during the ARPES experiments.

High resolution ARPES measurements. High resolution angle-resolved
photoemission measurements were carried out on our lab system equipped with a
Scienta R8000 electron energy analyzerGDLiu . We use helium discharge lamp as
the light source which can provide photon energies of h$\upsilon$= 21.218 eV
(helium I). The energy resolution was set at 4 meV for both the Fermi surface
mapping and band structure measurements (Fig. 1 and Fig. 2) and for the
superconducting gap measurements (Figs. 3 and 4). The angular resolution is
$\sim$0.3 degree. The Fermi level is referenced by measuring on a clean
polycrystalline gold that is electrically connected to the sample. The sample
was cleaved and measured in vacuum with a base pressure better than
5$\times$10-11 Torr.

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Acknowledgement
We thank Dunghai Lee, Tao Xiang, Guangming Zhang and Zhongyi Lu for
discussions. X.J.Z. thanks financial support from the NSFC (11190022, 11334010
and 11374335), the MOST of China (973 program numbers: 2011CB921703,
2015CB921000 and 2011CBA00110) and the Strategic Priority Research Program (B)
of CAS with Grant No. XDB07020300. X.D., F. Z. and Z.X.Z thank financial
support from the NSFC(11274358), the MOST of China (973 program number
2013CB921701) and the Strategic Priority Research Program (B) of CAS with
Grant No. XDB07020100.

Author Contributions
X.J.Z., L.Z., X.L.D. and Z.X.Z. proposed and designed the research. D.N.Y.,
H.X.Z, Y.L.H., X.L.D., F.Z. and Z.X.Z.contributed to single crystal
preparation. L.Z., A.J.L., Y.H., D.F.L., J.W.H., S.L.H., B.S., Y.X., X.L.,
L.Y., G.D.L., X.L.D., Z.Y.X., C.T.C.and X.J.Z. contributed to the development
and maintenance of Laser-ARPES system. L.Z., A.J.L., Y.H. carried out the
experiment with D.F.L., J.W.H., S.L.H., B.S., Y.X., and X.L.. L.Z., A.J.L.,
Y.H. and X.J.Z. analyzed the data. X.J.Z. and L.Z. wrote the paper. All
authors discussed the results and commented on the manuscript.

Additional information
Competing financial interests: The authors declare no competing financial
interests.

Figure 1: Fermi surface of (Li0.84Fe0.16)OHFe0.98Se superconductor and its
comparison with that in the superconducting single-layer FeSe/SrTiO3 film and
(Tl,Rb)xFe2-ySe2 superconductor. (a). The crystal structure of
(Li1-xFex)OHFe1-ySe. The c lattice constant is 9.318 ${\AA}$ for optimally-
doped sample with a Tc at 42 KXFLuNM ; XLDong11111 . (b). Schematic structure
of single-layer FeSe film deposited on a SrTiO3 substrateQKXue . The thickness
of the FeSe layer is $\sim$5.5 ${\AA}$. (c). Schematic structure of
(Tl,Rb)xFe2-ySe2JGGuo . The distance of the adjacent FeSe layers is $\sim$7.02
${\AA}$. (d). Fermi surface mapping of (Li0.84Fe0.16)OHFe0.98Se superconductor
measured at 20 K. The spectral weight distribution was obtained by integrating
the measured photoemission spectra (EDCs) within [-10,+10] meV energy window
with respect to the Fermi level as a function of kx and ky. (e)-(f). Fermi
surface of a superconducting single-layer FeSe/SrTiO3 filmDFLiu  and
(Tl,Rb)xFe2-ySe2 DXMouPRL  . For convenience, the four equivalent M points are
labeled as M1($\pi$,$\pi$), M2(-$\pi$,$\pi$), M3(-$\pi$,-$\pi$) and
M4($\pi$,-$\pi$). The light-grey arrow in (d) marks the main electric field
direction on the sample surface from the light source. In (d) to (f), the data
around M1 and M4 are obtained from the data around M3 and M2, respectively, by
applying the inversion symmetry.  Figure 2: Band structure and photoemission
spectra of (Li0.84Fe0.16)OHFe0.98Se superconductor measured along high-
symmetry momentum cuts. (a) to (c) show photoemission images of
(Li0.84Fe0.16)OHFe0.98Se measured at a temperature of 20 K along three high-
symmetry cuts, $\Gamma$ cut, M2 cut and M3 cut, respectively. (d) shows the
second-derivative image of (c) with respect to the energy in order to reveal
the band structure better. For comparison, (e) to (g) and (h) to (j) show
photoemission images of a superconducting single-layer FeSe/SrTiO3 film and
(Tl,Rb)xFe2-ySe2 measured along the same $\Gamma$ cut, M2 cut and M3 cut,
respectively. The location of the three momentum cuts are shown in (k). The
red dashed lines in (a-j) images are the guide to the eyes for the observed
band structures. (l), (m) and (n) show photoemission spectra (energy
distribution curves, EDCs) of (Li0.84Fe0.16)OHFe0.98Se for the $\Gamma$ cut,
M2 cut and M3 cut, corresponding to (a-c) images, respectively. The
photoemission spectra at $\Gamma$ point are colored as red while the spectra
at the Fermi momenta (kFs) are colored as blue.  Figure 3: Temperature
dependence of the superconducting gap of the (Li0.84Fe0.16)OHFe0.98Se
superconductor. (a) shows schematically the electron-like Fermi surface near
M2 point and the location of the momentum cut (blue line) across the Fermi
surface. (b). Photoemission image measured at 18 K along the momentum cut near
the M2 point (blue line in (a)). The data is divided by the corresponding
Fermi distribution function in order to highlight the opening of an energy gap
around the Fermi level. Two Fermi momenta are marked by arrows as kFL and kFR.
(c). Symmetrized photoemission spectra (EDCs) at the Fermi momentum kFL
measured at different temperatures. The middle-top inset shows the expanded
region of the 18 K symmetrized EDC around the Fermi level and the fitting
result (red line) using the formula from Ref.MNorman  to get the gap size.
(d). Magnetic measurement of the superconducting transition temperature (Tc)
for the (Li0.84Fe0.16)OHFe0.98Se sample we measured. Both field-cooled (FC)
and zero-filed-cooled (ZFC) mode measurements show an onset Tc at $\sim$41 K
with a sharp transition width of $\sim$1.5 K. (e). Temperature dependence of
the measured superconducting gap. The gap size is obtained by picking up the
peak position from the symmetrized EDCs (c) or by fitting the near-EF
symmetrized EDCs using the formula from Ref.MNorman ; both approaches give
similar results within experimental uncertainty. The error bars are defined as
standard deviation. The gap size from EDCs of both the left Fermi momentum
(kFL) and the right Fermi momentum (kFR) are plotted. The red dashed line is a
BCS gap form with a gap size of $\sim$14 meV at zero temperature.  Figure 4:
Momentum dependence of the superconducting gap of (Li0.84Fe0.16)OHFe0.98Se
superconductor. (a). Fermi surface mapping near M2 point. The corresponding
Fermi crossings are marked by red empty circles with numbers from 1 to 16 (for
clarity, the intermittent points are not labeled). (b). Symmetrized EDCs
measured at 20 K from Fermi crossing 1 to 16 along the measured electron-like
Fermi surface around M2. The red dashed lines represent the energy positions
of $\pm$ 13 meV with respect to the Fermi level. (c). Momentum dependence of
the superconducting gap (red solid circles) along the Fermi surface around M2
for (Li0.84Fe0.16)OHFe0.98Se superconductor. For comparison, the gap along the
electron-like Fermi surface around M2 of the superconducting single-layer FeSe
film is also plotted (blue empty circles)DFLiu . The error bars are defined as
standard deviation.


The original training datasets for both simulators are discarded and we now
consider the simulator as the oracle for any required training set and
population functionals. In particular, for each regime, we generate 25,000
samples to obtain a Monte Carlo representation of the ground-truth respective
population function $p(x;\sigma)$.

##### Generating ground-truth outcome processes $Y$.

For outcome variables $Y$ from which we want to obtain
$\mu_{\sigma^{\star}}:=\mathbb{E}[Y;\sigma^{\star}]$ for given test regimes
$\sigma^{\star}$, we consider models of the type
$Y=\tanh(\lambda^{\top}X)+\epsilon_{y}$, with random independent normal
weights $\lambda$ and $\epsilon_{y}\sim\mathcal{N}(0,v_{y})$. $\lambda$ and
$v_{y}$ are scaled such that the ground truth variance of
$\lambda^{\mathsf{T}}X$ is a number $v_{x}$ sampled uniformly at random from
the interval $[0.6,0.8]$, and set $v_{y}:=1-v_{x}$.

For each of the four benchmarks (i.e., based on either the Sachs et al. data
or DREAM data, with either a DAG model-based ground-truth or an IFM-based
ground truth), we generate 100 random vectors $\lambda$. The point of these
100 problems is just to illustrate the ability to learn (noisy) summaries of
$X$, or general downward triggers or markers predictable from $X$ under
different conditions. When generating $Y$ from $X$, _we keep a single sample
for $X$_. We then generate an unique sample for each of the 100 $Y$ variations
given the same $X$ data.

##### Generating training data.

For training our models, we additionally generate 5000 samples for the
observational regime (baseline) and 500 samples for each of the remaining 4
experimental conditions (Sachs) and 10 experimental conditions (DREAM). To map
from $X$ to $Y$, we use the model described above.

##  Appendix H Further Experimental Results

In Table 2 (see also Fig. 12) we present a series of further experimental
results in numerical form based on the following metrics: i) proportional root
mean squared error (pRMSE): the average of the squared difference between the
ground truth $Y$ and estimated $\widehat{Y}$, where each entry is further
divided by the ground truth variance of the corresponding $Y$; and ii) rank
correlation (rCOR): the Spearman’s $\rho$ between the ground truth vector
$\mu_{\sigma^{\star}}$ for all entries in $\Sigma_{\mathrm{test}}$, and the
corresponding estimated vector.121212The IFM-3 results are omitted from Tables
2 and 3, as the method does not convergence in a reasonable time.

In Table 3, we present results from a series of one-sided binomial tests to
determine whether models significantly outperform the black box baseline.

Table 2: Results of our interventional generalization experiments for the
Sachs and DREAM datasets. The values correspond to the average of $100$ $Y$
problems.

| Sachs | DREAM
---|---|---
| Causal-DAG | Causal-IFM | Causal-DAG | Causal-IFM
| pRMSE | pRMSE | pRMSE | pRMSE
Blackbox | $0.043$ | $0.414$ | $0.025$ | $0.174$
Causal-DAG | $0.014$ | $0.408$ | $0.017$ | $1.337$
IFM-1 | $0.105$ | $0.168$ | $0.022$ | $0.185$
IFM-2 | $0.051$ | $0.111$ | $0.107$ | $0.769$
IFM-3 | $0.123$ | $0.175$ | – | –
| rCOR | rCOR | rCOR | rCOR
Blackbox | $0.696$ | $0.701$ | $0.953$ | $0.930$
Causal-DAG | $0.873$ | $0.405$ | $0.972$ | $0.502$
IFM-1 | $0.546$ | $0.835$ | $0.952$ | $0.942$
IFM-2 | $0.673$ | $0.821$ | $0.865$ | $0.737$
IFM-3 | $0.503$ | $0.811$ | – | –

Table 3: P-values from a series of one-sided binomial tests against the null hypothesis that models perform no better on average than the black box model. Significance at $\alpha=0.05$ is indicated with one asterisk, and $\alpha=0.001$ with two. Data | Simulation | DAG | IFM1 | IFM2 | IFM3
---|---|---|---|---|---
DREAM | Causal-DAG | $<0.001^{**}$ | $0.044^{*}$ | $1$ | NA
DREAM | Causal-IFM | $1$ | $0.972$ | $1$ | NA
Sachs | Causal-DAG | $<0.001^{**}$ | $1$ | $0.998$ | $1$
Sachs | Causal-IFM | $0.956$ | $<0.001^{**}$ | $<0.001^{**}$ | $<0.001^{**}$
Figure 12: Overlapping density plots showing average rank correlation between
true treatment effects and those predicted by the black box model and IFM1,
respectively. Ideal performance is a point mass on $1$.