††thanks: Corresponding authors††thanks: Corresponding authors

# Development of a Scanning Tunneling Microscope for Variable Temperature
Electron Spin Resonance

Jiyoon Hwang  Center for Quantum Nanoscience, Institute for Basic Science,
Seoul 03760, South Korea  Department of Physics, Ewha Womans University, Seoul
03760, South Korea  Denis Krylov  Center for Quantum Nanoscience, Institute
for Basic Science, Seoul 03760, South Korea  Ewha Womans University, Seoul
03760, South Korea  Robertus J. G. Elbertse  Department of Quantum
Nanoscience, Kavli Institute of Nanoscience, Delft University of Technology,
Lorentzweg 1, Delft 2628 CJ, The Netherlands.  Sangwon Yoon  Center for
Quantum Nanoscience, Institute for Basic Science, Seoul 03760, South Korea
Ewha Womans University, Seoul 03760, South Korea  Taehong Ahn  Center for
Quantum Nanoscience, Institute for Basic Science, Seoul 03760, South Korea
Department of Physics, Ewha Womans University, Seoul 03760, South Korea
Jeongmin Oh  Center for Quantum Nanoscience, Institute for Basic Science,
Seoul 03760, South Korea  Department of Physics, Ewha Womans University, Seoul
03760, South Korea  Lei Fang  Center for Quantum Nanoscience, Institute for
Basic Science, Seoul 03760, South Korea  Ewha Womans University, Seoul 03760,
South Korea  Won-jun Jang  Samsung Advanced Institute of Technology, Suwon
13595, South Korea  Franklin H. Cho  Center for Quantum Nanoscience, Institute
for Basic Science, Seoul 03760, South Korea  Ewha Womans University, Seoul
03760, South Korea  Andreas J. Heinrich  Center for Quantum Nanoscience,
Institute for Basic Science, Seoul 03760, South Korea  Department of Physics,
Ewha Womans University, Seoul 03760, South Korea  Yujeong Bae
<EMAIL_ADDRESS>and<EMAIL_ADDRESS>Center for Quantum
Nanoscience, Institute for Basic Science, Seoul 03760, South Korea  Department
of Physics, Ewha Womans University, Seoul 03760, South Korea

###### Abstract

Recent advances in increasing the spectroscopic energy resolution in scanning
tunneling microscopy (STM) have been achieved by integrating electron spin
resonance (ESR) with STM. Here, we demonstrate the design and performance of a
home-built STM capable of ESR at temperatures ranging from 1 K to 10 K. The
STM is incorporated with a home-built Joule-Thomson refrigerator and a 2-axis
vector magnet. Our STM design allows for the deposition of atoms and molecules
directly into the cold STM, eliminating the need to extract the sample for
deposition. In addition, we adopt two methods to apply radio-frequency (RF)
voltages to the tunnel junction, the early design of wiring to the STM tip
directly, and a more recent idea to use an RF antenna. Direct comparisons of
ESR results measured using the two methods and simulations of electric field
distribution around the tunnel junction show that, despite their different
designs and capacitive couplings to the tunnel junction, there is no
discernible difference in the driving and detection of ESR. Furthermore, at a
magnetic field of $\sim$1.6 T, we observe ESR signals (near 40 GHz) sustained
up to 10 K, which is the highest temperature for ESR-STM measurement reported
to date, to the best of our knowledge. Although the ESR intensity
exponentially decreases with increasing temperature, our ESR-STM system with
low noise at the tunnel junction allows us to measure weak ESR signals with
intensities in the sub-fA range. Our new design of ESR-STM, which is
operational in a large frequency and temperature range, can broaden the use of
ESR spectroscopy in STM and enable the simple modification of existing STM
systems, which will hopefully accelerate a generalized use of ESR-STM.

##  I Introduction

Magnetic single atoms and molecules are the pinnacle of ever-increasing
demands for information storage density Bogani and Wernsdorfer (2008);
Natterer  _et al._ (2017) and quantum information processing Thiele  _et al._
(2014); Yang  _et al._ (2019a). Reaching the ultimate spatial limit requires
the technical capabilities to achieve the single spin sensitivity and control.
The scanning tunneling microscopy (STM) provides a particularly high spatial
resolution that allows it to image single atoms and molecules, characterize
their electronic and magnetic states, and manipulate their positions on a
surface Binnig  _et al._ (1982); Bian  _et al._ (2021). Despite its
exclusively high sensitivity with atomic resolution, the spectral resolution
of STM is often limited by temperature due to the thermal Fermi-Dirac
broadening of the energy of tunneling electrons Lambe and Jaklevic (1968);
Song  _et al._ (2010a); Ast  _et al._ (2016); Delgado and Fernández-Rossier
(2011). Even when the spin systems are cooled down below 1 K temperatures at a
high cost, achieving spectral resolution of tens of nano-eV in the
conventional spectroscopy of STM, such as inelastic electron tunneling
spectroscopy (IETS), is impossible. In contrast, the detection of spin
transitions with a few nano-eV energy resolution is easily attainable in
electron spin resonance (ESR). Conventional ensemble ESR, however, requires at
least $10^{7}$ spins ($10^{12}$ for nuclear spins) to get strong enough
signalsBlank  _et al._ (2003); Assig  _et al._ (2013); Ciobanu, Seeber, and
Pennington (2002), while its spectroscopic resolution is not limited by
temperature. The integration of ESR with STM maximizes the powerful benefits
of both approaches, and the single spin ESR detection with atomic resolution
has been exclusively achieved by ESR spectroscopy in STM Baumann  _et al._
(2015); Chen, Bae, and Heinrich (2021).

Figure 1:  Design of the home-built ESR-STM. (a) Overview of the whole system
including a cryostat, a vector magnet, a mechanical damping system, a cooling
system, an STM, and a preparation chamber. The preparation chamber and the
ultrahigh vacuum (UHV) insert are connected to the top flange of an exchange
gas can (EGC) via a flexible bellows. The UHV insert is supported by three
arms connected to the system via three flexible bellows (labelled as
‘Pendulum’). (b) ESR-STM setup on a massive concrete block. The cryostat rests
on a concrete block floated by four air springs in a sound-proof room. (c)
Velocity spectrum measured on the floor of the sound-proof room (purple, QNS
floor) and on top of the floating concrete block (magenta, QNS block) in
comparison with other precision laboratories (blue, IBM Almaden; green, IBM
Zurich; light blue, MPI Stuttgart). (d) Power spectral density (PSD) of the
tunnel current measured with pendulum on (magenta) and off (blue)
($V_{\mathrm{DC}}$ = 10 mV, $I$ = 1 nA, Integral-gain = 1 nm/s). (e)
Schematics showing pendulum operation. The UHV insert is suspended by three
welded bellows, while schematics shows only two bellows for simplicity. When
the pendulum is in the ‘off’ state (top), the UHV insert is misaligned and
thus in mechanical contact with EGC. In the ‘on’ state (bottom), the pendulum
is aligned inside the EGC without touching the EGC and, thus, mechanically
isolated from the rest of the system.

ESR measurements in STM require the use of specialized cables for the
injection of radio-frequency (RF) electric fields into the tunnel junction
Paul  _et al._ (2016); Seifert  _et al._ (2020a). However, adding RF cables or
replacing existing cables with RF cables to a low-temperature STM for ESR
capability is not so trivial since most STMs are already compactly wired and
semi-rigid RF cables may cause vibration and heat transmissions to the tunnel
junction Natterer  _et al._ (2019); Friedlein  _et al._ (2019); Xu  _et al._
(2019); van Weerdenburg  _et al._ (2021); Drost  _et al._ (2022). Furthermore,
to obtain strong ESR signals from a single atom/molecule, it is important to
sufficiently polarize the spin state, which imposes a strict requirement on
temperature of the system. Given that many existing STMs operate in the 2–4 K
temperature range or above, an external magnetic field greater than 1 T
facilitates the polarization of surface spins, necessitating RF fields with
frequencies higher than 30 GHz at the tunnel junction.

In this work, we introduce an extremely stable, home-built ESR-STM which
operates at temperatures ranging from 1 K to at least 10 K. We use RF cables
for the STM tip and an antenna parallel to the sample plane, which enables us
to examine RF signal transmission to the tunnel junction through two different
cabling schemes. The RF cable going directly to the STM tip allows for a high
transmission to the STM junction, where the STM tip apex is positioned within
a nanometer scale from an atom or a molecule on a surface. However, the
resonance frequency available at the tunnel junction is limited to around 35
GHz due to the requirement for a flexible coaxial cable for the coarse motion
of the STM tip. In contrast, for the antenna placed about 4 mm distant from
the tunnel junction, we observe that a larger frequency window than the direct
wiring to the STM tip is available at the tunnel junction, enabling us to
perform high temperature ESR measurements up to 10 K. While the ESR
intensities exponentially decrease at elevated temperatures, our ESR-STM
enables us to detect weak ESR signals such as sub-fA. Extending the
temperature range accessible for the ESR spectroscopy in STM and providing the
sensitivity without incurring large costs to modify existing STMs for a lower
temperature operation will hasten the spread of this technique for the phase-
sensitive control and detection of spins on a surface.

##  II Instrument design

We present the design of an ultrahigh vacuum (UHV) ESR-STM operational at a
broad range of temperatures with high mechanical stability. The ESR-STM system
comprises a cryostat (MAXesTM 6T/4T vector magnet, American Magnetics Inc.), a
cooling system, a UHV insert with an STM, and a UHV preparation chamber [Fig.
1(a)]. The liquid 4He (LHe) bath cryostat contains a commercially available
two-axis vector magnet (American Magnetics Inc., maximum 6 T in-plane and 4 T
out-of-plane of a sample) and an exchange gas can (EGC) which is filled with
4He exchange gas. The UHV insert, which includes the transfer neck with
baffles, ‘5K-flange’, ‘1K-pot’, and STM, is positioned inside of the EGC and
cooled by 4He exchange gas without direct contact with the LHe reservoir. The
1K-pot contains a home-built Joule-Thomson (JT) refrigerator which serves as
the system’s coldest location. The STM is rigidly attached to the 1K-pot by
materials having high mechanical stiffness and high thermal conductivity.
Samples are prepared in-situ in a separate preparation chamber and transferred
to the STM sample stage using a home-built vertical manipulator with a travel
length of about 2 m. In the following sections, we describe each part in
detail.

###  II.1 Mechanical dampers

The STM measurement relies on the precise placement of a metallic tip a
nanometer apart from the surface Binnig  _et al._ (1982); Bian  _et al._
(2021). The extremely high spatial resolution is predicated on the exponential
sensitivity of tunnel current on the separation between the STM tip and a
sample. For the same reason, ambient vibrations reaching the tunnel junction
are exponentially amplified in the tunnel current signals. Constructing high-
performance STM with atomic-scale spatial precision, thus, necessitates
mitigating the impact of external vibration sources on the tunnel junction.
Ambient vibration sources include mechanical vibrations and acoustic noise.

To avoid such noise sources from being transmitted to the tunnel junction, we
installed the entire STM system on a floating concrete block, which weighs 80
tons, in a sound-proof room. The room is located in a low vibration facility
built on a 1.4 m thick concrete foundation which is separated from the rest of
the building [Fig. 1(a) and (b)]. The massive concrete block is placed in the
center of the sound-proof room, separated by about 1 m from the acoustically
absorptive wall, and lifted up by four air springs (BiAir-ED-HE-MAX, Bilz
Vibration Technology AG) to further isolate the block from floor vibrations.

Figure 1(c) compares the facility’s velocity spectrum to that of other
facilities Lörtscher, Widmer, and Gotsmann (2013); Bian  _et al._ (2021),
which was measured using an accelerometer (731A/P31, Wilcoxon) on the floor of
the sound-proof room and on the concrete block under ambient vibration
conditions. The measured linear acceleration is integrated to obtain linear
velocity, which is then converted to an A-weighted one-third octave band
spectrum after Fourier transform. The floor noises higher than 3 Hz are
significantly suppressed by the massive inertia block, resulting in a one
order of magnitude reduction in noise on the block. Given that the lowest
fundamental acoustic mode is at 11.16 Hz in the 5.69 m $\times$ 7.68 m
$\times$ 6.28 m size room, the feature appearing at around 12 Hz in the noise
spectrum [Fig. 1(c), QNS block] is presumably the result of acoustic
stimulation of the concrete block motionMacLeod  _et al._ (2016). The noise
level we attained in the overall frequency range relevant to STM performance
is comparable to or even better than noise levels measured in the world’s
leading precision laboratories [Fig. 1(c)]. At considerably higher
frequencies, where the STM stage’s resonances are located, little
environmental vibrations are transmitted.

While the entire system is rigidly mounted on the floating concrete block, we
provide one more damping stage for the STM. To reduce vibration transfer and
provide flexibility to adjust alignment, the UHV insert, which includes the
STM unit near its lower end, is connected to the top flange using one central
bellows for the connection of the preparation chamber and three side bellows
for three supporting arms [Fig. 1(a) and (e)]. The UHV insert can be floated
by controlling the pressure in the three side bellows and lifting three
supporting arms, which suspends the STM like a pendulum. The 1.5 m tall UHV
insert has a 1–2 mm gap with the EGC. Mechanical deformation and misalignment,
thus, might cause the mechanical contact of the UHV insert to the EGC.
Controlling the pressure of the three side bellows allows us to adjust the
alignment of the UHV insert with respect to the EGC. The idea for this design
stems from the first low-temperature STM built by Don Eigler at IBM Eigler and
Schweizer (1990). As shown by the power spectral density of the tunnel current
[Fig. 1(d)], which is measured at a setpoint of $V_{\mathrm{DC}}$ = 10 mV and
$I$ = 1 nA using 109 V/A gain preamplifier (SA-606F2, NF corp), we observed
significant noise transfer from the environment to the tunnel junction when
the pendulum is not adequately operational [upper in Fig. 1(e)]. When the
pendulum is well isolated [lower in Fig. 1(e)], the noise level at the tunnel
junction is significantly reduced. The combination of the floating concrete
block and the internal pendulum results in a remarkable mechanical stability
of the system.

###  II.2 Joule-Thomson refrigerator

Figure 2:  Joule-Thomson (JT) refrigerator. (a) Overview of the cooling system
including JT refrigerator, 5K-flange, and 1K-pot. For the JT refrigerator, the
4He gas is cooled through cooling posts and inserted to the capillary at the
1K-pot. The liquefied He is accumulated to the lower Cu part of 1K-pot, where
the whole STM structure is attached by Cu rods. The 1K-pot is pumped through
the EGC. At the pumping port, the cooling post and baffle cover are located,
which cool the He gas and electrical feedthroughs, respectively. (b)
Photographs of the UHV insert with baffle cover on 5K-flange, the UHV chamber
made of stainless steel and BeCu, 5K-flange with feedthroughs and a cooling
post, and 1K-pot. (c) Temperature of STM controlled by a heater installed on
the 1K-pot during the operation of JT refrigerator. By adjusting the heating
power, the system can stably operate at different temperatures. Inset: STM
temperature during the operation of JT refrigerator without heating the STM
stage, which can run continuously at 1.3 K.

The LHe bath cryostat, containing the superconducting magnets, provides a LHe
reservoir (maximum 133 L of LHe) with a hold time of around 140 hours allowing
for the operation of magnets. The EGC is housed in the cryostat and is in
direct contact with LHe. The EGC is composed of three different segments [Fig.
1(a)]. The top part from room temperature (RT) top flange to the end of the
cryostat’s neck is made of stainless steel with a small wall thickness
($\sim$7 mm) to minimize thermal conduction. The choice of materials for the
middle section is oxygen-free, high conductivity (OFHC) copper (Cu) for better
cooling of the UHV insert regardless of the level of LHe in the cryostat. The
last part is the tail of the EGC, which is placed in the magnet’s center. To
avoid the creation of eddy currents while ramping the magnet, the tail is made
of stainless steel. The EGC is filled with He exchange gas (0.01–3 mbar). The
work shown in this manuscript uses 4He as exchange gas, which can be replaced
with 3He to achieve lower base temperatures.

The UHV insert is situated inside the EGC and cooled by the exchange gas
without direct contact with LHe, which lowers the vibration transfer stemming
from the boiling of LHe and external vibration sources. The UHV insert
includes the transfer neck with baffles, 5K-flange, 1K-pot, and STM [Fig. 1(a)
and 2(a)]. The transfer neck is a 750 mm long nipple with DN35CF, made of
stainless steel, with an outer diameter of 40 mm and a wall thickness of 1.4
mm. We installed six baffles outside the transfer neck with a diameter close
to the inner diameter of the EGC, to prevent thermal radiation from reaching
the bottom of the UHV insert and to provide a thermal gradient from the RT top
flange to the 5K-flange. The baffles are made of gold-plated Cu without Ni
plating. All six baffles are located in the top stainless steel part of the
EGC except the last one, which is located in the Cu section of the EGC. The
5K-flange maintains a gap of about 2 mm to the EGC, allowing the exchange gas
to effectively transfer heat between the EGC and the 5K-flange. All electrical
feedthroughs are installed on the 5K-flange to cool electrical cables before
entering the STM stage. We additionally installed a baffle cover made of OFHC
Cu to help cooling all feedthroughs on the 5K-flange [labelled as ‘Baffle
cover’ in Fig. 2(a) and (b)].

To achieve STM operation at temperatures below 5 K, the UHV insert is equipped
with a JT refrigerator in the 1K-pot. The 1K-pot is made of two different
materials: stainless steel for the upper section and Cu for the lower section
[Fig. 2(b) lower right corner]. The 1K-pot with a volume of $\sim$0.1 L is
mounted on the UHV side of the 5K-flange above the magnet and thermally
isolated from the surrounding UHV chamber. The 1K-pot is continuously fed with
cold, compressed He gas through a thin capillary [Fig. 2(a)]. Through the JT
effect, some of this gas is liquefied in the 1K-pot. The liquefied He is
accumulated in the lower Cu part of the 1K-pot, where the STM is rigidly
attached through three rods made of OFHC Cu. The capillary inside the 1K-pot
is 0.3 m long and is coiled three times and partially submerged in the LHe in
the 1K-pot [Fig. 2(a)]. The capillary is composed of Inconel tube with an
inner diameter of 500 $\mu$m. We create a flow impedance by fitting a tungsten
wire with a nominal diameter of 500 $\mu$m inside the Inconel tube. In steady-
state flow, the phase transition of He happens via the JT effect in the
1K-pot, liquefying some of the exchange gas and making the 1K-pot the coldest
place in the entire system Song  _et al._ (2010b). We installed a heater on
the 1K-pot to control STM temperatures (see Table 1 for wiring). As shown in
Fig. 2(c), we can accurately and reliably adjust the STM temperature by
varying the power applied to the heater.

The boil-off from the LHe in the 1K-pot goes through a pumping line (a 0.15 m
long nipple with DN16CF), where it cools the incoming gas in a counter-flow
heat exchanger [Fig. 2(a)]. We used a customized gasket with a reduced inner
diameter (12 mm) to avoid additional noise production caused by pumping
liquefied He at around the $\lambda$-point of He ($\sim$2.18 K). The pumped
gas then exits into the EGC at the 5K-flange. The exchange gas (4He) is pumped
out at the RT side using a turbo pump (HiPace 450, Pfeiffer Vacuum) and a
roots pump (A100L, Pfeiffer Vacuum), compressed using a customized diaphragm
pump (N186.1.2AN/V.12E, KNF), cleaned by a cold trap, and injected back into
the capillary located at the 1K-pot [Fig. 1(a)]. Prior to entering the 1K-pot,
the compressed gas passes two precooling stages, on one of baffles and the
5K-flange, where the 1/8 inch diameter stainless steel tube for the insert gas
is brazed around Cu rods [Fig. 2(a) and (b)]. Continuous operation of the JT
refrigerator has no perceptive effect on the hold time of LHe in the cryostat
or the noise level at the tunnel junction, which allows us to keep the JT
refrigerator running during STM operation. To minimize heat input from thermal
radiation, the 1K-pot and STM are protected by a radiation shutter at the end
of the transfer neck [Fig. 2(a)].

###  II.3 Design of RF antenna

Figure 3:  RF signal transmission of two different antennas. (a) Experimental
setup for RF transmission test of the PT- (upper) and PS-antennas (lower).
Using a network analyzer, RF signals are applied to the antenna and the
transmitted signals to the tip are measured. The PT- and PS-antennas are made
of stripped parts of coaxial cables which run parallel to the tip and to the
sample, respectively. The PS-antenna is terminated with a 50-Ohm load and held
in place with the help of an antenna mount. (b) Signal transmission for the
PT-antenna (blue) and PS-antenna with (magenta) and without (yellow) a 50-Ohm
terminator. (c) Signal transmission for the PT-antenna (blue) and PS-antenna
(magenta) without the sample. While the transmission for the PT-antenna
remains nearly identical, we observed a considerable loss in transmission
between the PS-antenna and the tip when the sample was removed.

The first demonstration of ESR of single atoms on a surface was achieved by
applying RF voltages into the tunnel junction via the STM tip (referred to as
the "original ESR-STM design"), where the RF voltages are combined with the DC
bias voltage at RT using a bias teeBaumann  _et al._ (2015); Paul  _et al._
(2016). Due to the complex modifications to the RF cablings of STM in the
early design, Seifert et al.Seifert  _et al._ (2020a) introduced one simple
antenna capacitively coupled to the tip, where the antenna was installed as
parallel as possible to the tip (PT-antenna). The PT-antenna offers larger
signal transmission to the tunnel junction at high frequency than the original
ESR-STM design. Here, we introduce a new design of an antenna parallel to the
sample (PS-antenna) and compare two different antennas (PT- and PS-antennas)
by measuring and simulating their signal transmission to the tip and
simultaneously to the sample. We provide a universal approach to integrating
RF cables into the STM, while the antenna design was dedicated based on the
geometry of our STM by considering three aspects: i) stable transmission while
allowing for the coarse motion of the tip, ii) mechanical stability, and iii)
suppression of standing waves.

Figure 3(a) shows the experimental setup for the characterization of signal
transmission from two different antennas (PT- and PS-antennas) to the tip-
sample junction using a network analyzer (N5224B, Keysight). We set the tip
and each antenna using semi-rigid coaxial cables (UT-085C-LL, Carlisle
Interconnect Tech.) and the sample using a Cu foil. The PT-antenna and the tip
are made of coaxial cables with unshielded inner conductors at the ends, while
the PS-antenna is made by stripping the outer conductor in the middle of a
coaxial cable and terminating it with a 50-Ohm load [Fig. 3(a)]. We used an
antenna mount to hold two sides of the PS-antenna’s outer conductors to
rigidly support the antenna and thus provide better mechanical stability
[lower in Fig. 3(a)]. Note that the metallic antenna mount may behave like a
signal receiver when it is less than 50 mm away from the antenna. The tip and
each antenna are connected to the network analyzer for transmission (S21)
measurements using flexible cables (FLC-1M-SMSM+, Mini-Circuits), and
calibrations are performed using an electronic calibration module (Ecal N4693A
(4693-60003), Keysight) to eliminate the systematic errors from the flexible
cables.

Despite their different designs, both PT- and PS-antennas exhibit a uniform
decrease over the whole frequency range, with sharp dips at frequencies lower
than 10 GHz. We ascribe such sharp attenuations to interferences of incident
and reflected signals at unshielded parts of the semi-rigid cables. The
transmission of an antenna often has a strong frequency dependence where the
resonance frequency is determined by its size and shape. RF signals with
wavelengths longer than the dimensions of an antenna are difficult to transmit
through capacitive couplingGustrau (2012). Such losses at low frequencies
remained even after we removed the sample [Fig .3(c)], supporting our
hypothesis.

While the signal transmissions through two different antennas are comparable,
we found distinct transmission paths between two antennas. The shape and
dimension of the PT-antenna are chosen to be similar to those of the tip and,
thus, we expect maximum transmission between the two by a capacitive coupling
as the antenna is placed parallel to the tip. In the range of 10 to 30 GHz, we
indeed observe less loss from the PT-antenna compared to the PS-antenna [Fig.
3(b)]. The PS-antenna, on the other hand, is less coupled to the tip since it
is designed to be parallel to the sample and thus practically perpendicular to
the tip. We also note that there is a considerable change in the transmission
for the PS-antenna when the sample was removed, indicating significant
coupling of the PS-antenna to the sample [Fig. 3(b) and (c)]. Despite rather
drastic changes in the design, these results qualify that the PS-antenna is an
adequate alternate to the PT-antenna, particularly at frequencies above 30 GHz
as we see better transmission properties. Note that we have a 20 mm long
unshielded part of the cable for the PT-antenna. Adjusting the stripped area
possibly provides additional room to improve the signal transmission.

We briefly discuss the role of the 50-Ohm terminator in the PS-antenna. A
proper termination of an RF cable is essential to reduce the creation of
standing waves. Ideally, if the load impedance at the cable’s end is matched
to the characteristic impedance of the transmission cable, no reflections
appearGustrau (2012). However, in reality, the impedance mismatch between the
cable (50 Ohm) and the STM junction (typically 1 GOhm) causes significant
reflections and thus the creation of standing waves. To suppress reflections,
we put a 50-Ohm terminator (BM11132, Bracke) rated up to 40 GHz at the end of
the transmission cable [Fig. 3(a)]. Figure 3(b) shows signal transmissions
from the PS-antenna to the tip with and without the 50-Ohm terminator,
designated as PS and PS open, respectively. We observed that adding the 50-Ohm
terminator helps reducing some abrupt features in the transmission.

Figure 4:  COMSOL simulations of electric field distribution. (a) Schematics
of the simulation setup and the simulated images of electric fields around the
antenna, tip, and sample for the PT- and (b) PS-antenna when oscillating
voltage signals are applied through the antennas. Three selected simulation
results are shown when the frequency of the incident oscillating voltage is
set to 14.5, 24, and 30 GHz.

The signal transmissions of each antenna to the tip-sample junction are
further investigated using COMSOL multiphysics to simulate the electric field
in space. In the simulation, we provide an oscillating voltage of 1 V to the
transmission line of the antenna, which is then transmitted to the junction.
Figures 4(a) and (b) show simulated results calculated at incident electric
field frequencies of 14.5, 24, and 30 GHz for the PT- and PS-antennas. The
simulation results reveal that for both PT- and PS-antennas a strong localized
electric field is generated at the tip-sample junction. On average, the PS-
antenna transmits somewhat less to the junction than the PT-antenna across all
frequencies (5–35 GHz). However, we observed that at some frequencies (for
example at 14.5 GHz), the PT-antenna barely transmits signals. Because the tip
and the PT-antenna have similar dimensions, the transmission might change
drastically at particular frequencies where a half of a wavelength ($\sim$10
mm) is comparable to the length of the PT-antenna or the tip. The PS-antenna,
on the other hand, is less frequency-sensitive due to its highly distinct
dimensions from the sample. Furthermore, the PS-antenna partially transmits
signals through the tip, which allows it to transmit the RF signals without
considerable frequency dependence by transmitting the signals through
alternative paths. We note that moving the tip laterally along the antenna
results in no significant change of the localized electric field at the tip-
sample junction. The simulation results support the practical replacement of
the PT-antenna with the PS-antenna, particularly for STMs where the sample is
fixed while the tip moves in large coarse steps. In the later sections, we
will demonstrate that this newly-designed antenna is indeed competitive with
the PT-antenna for ESR-STM measurement.

Figure 5:  STM stage and cabling schemes. (a) Overview of the STM stage
including the tip, antenna, sample stage, and piezo stacks. Coaxial cables
(orange and cyan) are used for connections to the tip and the antenna. The
sample holder is clamped by BeCu springs (light orange) with sapphire wheels
(light gray). A molybdenum (Mo) bolt of the sample holder is in contact with
the BeCu spring (light orange), which is connected to the inner conductor of a
flexible coaxial cable (cyan) as a tunnel current signal line. The PtIr tip is
in a tin plated BeCu pin spring socket (2-330808-7, AMP-TE connectivity) as a
tip holder which is mounted in the Macor tip plate on the sapphire plate
sitting on three piezo stacks for the tip’s coarse and scan motions (below).
Only two stacks are shown in the side view. (b) Wiring schemes for ESR-STM
measurement. The same RF cable schemes are used for the tip and antenna except
for the cables between the 1K-pot to STM stage. For the tip side, a flexible
coaxial cable is used between the semi-rigid Cu cable and the tip. For the
antenna, the semi-rigid Cu cable is partially stripped near the sample, then
returned back to the 1K-pot, and followed by NbTi cable with a 50-Ohm
terminator.

###  II.4 Scanning tunneling microscope with RF cabling

Table 1: Summary of electrical cables used in the system |  RT top flange to 5K-flange |  5K-flange to 1K-pot |  1K-pot to STM
---|---|---|---
Piezo stacks |  CC-SS, Lake Shore Cryotronics |  CC-SS, Lake Shore Cryotronics |  TYP3-2TW-15, Accu-Glass Products
Sample current |  N12-36M-131, New England Wire |  N23-44M-100-0, New England Wire |  CW-2040-3650P, Cooner Wire
Tip bias |  SCA49141-09, Fairview; |  SC-160/50-NbTi-NbTi, Coax Japan |  UT-085C-LL, Carlisle Interconnect Tech;
|  SC-119/50-SSS-SS, Coax Japan |  |  CW2040-3650P, Cooner Wire
Antenna |  SCA49141-09, Fairview; |  SC-160/50-NbTi-NbTi, Coax Japan |  UT-085C-LL, Carlisle Interconnect Tech
|  SC-119/50-SSS-SS, Coax Japan |  |
Temperature |  Silicon diodes / Cernox |  Cernox |
Sensors |  with Quad-LeadTM cryogenic wires, |  with Quad-twistTM cryogenic wire, |  -
|  Lake Shore Cryotronics |  Lake Shore Cryotronics |
Heaters |  WMW-32, Lake Shore Cryotronics |  SS 304 insulated with HNY, |  -
|  |  California Fine Wire company |

Our STM is relatively simple in design [Fig. 5(a)] and comprises the tip,
sample stage, and cables for RF signal transmission, electrical control, and
STM signal. Our sample is vertically loaded and held in place by two BeCu
springs with sapphire wheels [Fig. 5(a)]. The sapphire wheels allow us to
transfer samples without producing excessive friction and hold the sample in
the STM stage without electrical contact to the stage. The sample holder is
electrically isolated from the STM stage made of silicon phosphorus copper
(SiPCu) by sapphire pieces between them. These sapphire pieces are glued to
the rest of STM stage using epoxy (T7110, EPO-TEK). For better thermal
coupling, the bottom part of sample holder is contacted to the SiPCu stage
without sapphire but covered by a Kepton tape for electrical insulation.

While the sample stays in place, we move the tip for both coarse and fine
motion. The coarse motion of the tip is given in two directions, parallel
($x$) and perpendicular ($z$) to the sample surface for lateral coarse
positioning and coarse approaching, respectively [Fig. 5(a)]. For each
direction, we used two shear piezoelectric actuators. The same shear piezo
actuators are used for the fine scanning motions in $x$ and $z$. For fine
motion in the other in-plane direction ($y$), we used four linear piezo
actuators. The total 8 piezo actuators (PAXZ+0063, PI) are thus stacked
together, where an alumina plate is inserted between actuators responsible for
motion in different directions to prevent electrical interference. A sapphire
plate is mounted on top of three sets of these piezo stacks [Fig. 5(a)]. A tip
mount made of Macor is glued to the sapphire plate and at present, the tip
cannot be changed in-situ. The tip is made of PtIr wire. If the tip is
contaminated and, thus, the tunnel current does not show an exponential
dependence on the tip-sample distance, we clean the tip by applying a high
bias voltage ($\sim$100 V) between the tip and the sample as part of field
emission process while using a preamp with low gain ($10^{7}-10^{8}$ V/A). To
protect the STM wire from damage caused by excessively high current flow, we
insert a resistor between the high voltage power supply and the tip during
field emission. A 50 mm long BeCu spring pulls the sapphire plate against the
piezo stacks. The spring is designed in such a way that the normal force
exerted by the spring results in right amount of friction for the coarse
motion of the slip-stick mechanism without mechanical instability during
scanning.

The electrical cables used for STM operation are carefully selected in
consideration of the electrical noise transfer, signal loss, and heat transfer
for each section [Fig. 5(b)]: i) from RT top flange to 5K-flange, ii) from
5K-flange to 1K-pot, and iii) from 1K-pot to STM. The choice of cables for
each component is summarized in Table 1. From the RT flange to the 5K-flange,
all cables are thermalized by the exchange gas in the EGC. For further
thermalization, flexible cables are wrapped around gold-plated Cu posts
located at baffles. All cables are connected to the UHV side using electric
feedthroughs. Since unshielded parts of high voltage cables can cause an
electric arc in the EGC filled with He gas at a pressure of 0.01–3 mbar, we
paid special attention to the high voltage multi-pin feedthroughs (9132004,
MDC) at the RT top flange a nd the 5K-flange. To protect high voltage cables
from the electric arc, we used Teflon tubes to cover the cable ends and
connectors, and a hand-strippable coating compound (TURCO5580-G, AeroBase
Group) to cover the metallic part of feedthroughs between pins.

We used coaxial cables both for the tip bias line and for the antenna to
inject RF signals to the tunnel junction [Fig. 5(b) and Table 1]. We used
stainless steel semi-rigid coaxial cables (SC-119/50-SSS-SS, Coax Japan)
between the RT top flange and the 5K-flange [Fig. 5(b) light gray]. The cables
are connected to the RT top flange using hand-formable coaxial cables
(SCA49141-09, Fairview) [Fig. 5(b) purple] to permit pendulum adjustment.
Through electrical feedthroughs (242-SMAD27G-C16, Allectra), the cables are
connected to semi-rigid NbTi coaxial cables [Fig. 5(b) gray] on the UHV side.
From 1K-pot, we used different cabling schemes for the tip and antenna. For
both, we installed semi-rigid Cu coaxial cables [Fig. 5(b) orange]. For the
tip, the Cu cable is connected to the PtIr tip by a flexible Cu cable
(CW2040-3650p, Cooner Wire), using a tin-plated BeCu pin spring socket
(2-330808-7, AMP-TE connectivity) as an adapter. While the RF cable for the
STM tip terminates with an open end at the STM stage, the RF cable for the
antenna passes the STM stage parallel to the sample surface and terminates
with a 50-Ohm terminator near the 5K-flange [Fig. 5(b)]. Note that in an
earlier work Seifert  _et al._ (2020a), an antenna was positioned parallel to
the tip, which is not ideal in our design due to changing distance between the
antenna and the tunnel junction during coarse motion of the tip. Instead, we
installed an antenna parallel to the sample plane as introduced in the
previous section (Fig. 3). For the antenna, the outer conductor of the coaxial
cable is stripped off by $\sim$20 mm around the tip and the exposed inner
conductor acts as the antenna to transfer RF signals to the STM junction. The
inner conductor is $\sim$ 2 mm above the tip and $\sim$ 3 mm away from the
sample, ensuring that the RF cable does not interrupt the tip’s coarse motion
or sample transfer. Both sides of the stripped cable are rigidly fixed, which
makes the antenna robust against mechanical vibration.

###  II.5 UHV chamber and sample preparation

In our ESR-STM system, the preparation chamber and the UHV insert are in UHV.
The preparation chamber is equipped with several e-beam and thermal
evaporators, a heating stage, a manipulator, a sputter gun (IQE 11/35, SPECS),
an Auger electron spectrometer (ESA 100, STAIB), leak valves for inserting
gases (such as Ar, O2, and N2), a residual gas analyzer (RGA100, Stanford
Research Systems), a turbo molecular pump (HiPace 700 H, Pfeiffer Vacuum), and
an ion pump (VacIon Plus 500 Starcell, Agilent). In the preparation chamber,
we prepare atomically clean substrates and grow ultra-thin insulating layers
on metallic substrates, e.g., MgO(100) on Ag(100). After the sample
preparation, the sample is transferred from the preparation chamber to the STM
stage in the UHV insert using a manipulator with the travel length of $\sim$2
m.

Access to the UHV insert from the preparation chamber is given by shutter
mechanisms. There is no visual access to the scan head during the sample
transfer procedure. To ensure a safe sample transfer, we installed a transfer
guiding ring near the STM stage, which guides the manipulator to reach the STM
stage. furthermore, we are able to adjust the pendulum to align the UHV insert
(thus, the STM stage) with the manipulator. We have indicators which monitor
the movement of the pendulum. During the sample transfer, the indicators show
any change of pendulum positions caused by misalignment between the
manipulator and the STM stage.

Our UHV insert does not require bake-out and is pumped through the preparation
chamber before cooling the system. Once we start cooling the UHV insert and
its pressure becomes lower than the preparation chamber due to cryogenic
pumping effect, we close the gate valve between the preparation chamber and
the UHV insert. We observed no perceptible degradation of the clean sample for
at least 5 months.

Many STMs require samples to be taken out in order to deposit atoms or
molecules on surfaces. This leads to an unavoidable heating of the samples,
which often causes unwanted diffusion of adsorbates. It can also lead to
intrusion of hydrogen into the STM stage. Our STM design allows direct
deposition of atoms and molecules onto the sample surface without removing the
sample from the STM stage. As shown in Fig. 1(a), we installed evaporators on
the rotary platform. While the manipulator is aligned with the UHV insert
during the sample transfer, we rotate the platform to align evaporators with
the STM stage for the evaporation of atoms or molecules. Our sample holder
made of molybdenum (Mo) is machined to have an $\sim 8^{\circ}$ slope. This
ensures that some atoms, which are evaporated in the preparation chamber at a
distance of $\sim$2 m from the sample, land on the sample surface. Together
with a mechanically actuated shutter located at the 5K-flange [Fig. 2(a)],
this permits in-situ deposition of atoms/molecules on our sample which remains
at a temperature of about 10 K during deposition.

##  III System operation and performance characterization

Using this home-built ESR-STM, we implement ESR spectroscopy of hydrogenated
titanium (TiH) atoms on two monolayers (MLs) of MgO on Ag(100) Yang  _et al._
(2019b); Bae  _et al._ (2018). We characterize frequency-dependent
transmission to the tunnel junction through the tip and the newly introduced
antenna (Fig. 5), which allows us to compare available frequency windows and
transmission efficiency. We perform the side-by-side comparison of the two ESR
measurements using the same tip and atomic species and ultimately present ESR
spectra measured at elevated temperature (10 K) without losing energy
resolution.

###  III.1 Transfer function

Figure 6:  Experimental setup for ESR measurement. (a) A constant current STM
image of Fe and TiH on MgO/Ag(100) ($V_{\mathrm{DC}}$ = 100 mV, $I$ = 20 pA, T
= 1.3 K). (b) Characteristic STM d$I$/d$V$ spectra of Fe and TiH. The Fe and
different TiH species (TiHO/TiHB) are easily identified by their distinct
d$I$/d$V$ spectra and apparent heights. Successive curves are shifted by 0.4
and 1.2 nS, respectively, for clarity ($V_{\mathrm{DC}}$ = 100 mV, $I$ = 100
pA, $V_{\mathrm{mod}}$ = 1 mV, $T$ = 1.3 K). (c) Quantifying RF transmission
to the tunnel junction. The IETS step of Fe is broadened while applying RF
voltages, which is used to quantify the effective RF amplitudes at the tunnel
junction ($V_{\mathrm{DC}}$ = 14.3 mV, $I$ = 100 pA, $V_{\mathrm{mod}}$ = 1
mV, $T$ = 1.3 K, $f$ = 16.21 GHz, $P_{\mathrm{RF}}$ = –16 dBm). (d) Transfer
functions measured by applying RF voltages through the tip (blue) and the
antenna (magenta). Both curves are shifted by –50 dB to have 0 dB
correspondent to the zero attenuation of the transmission line.

Figure 6(a) shows a constant current STM image of Fe and TiH on 2 MLs of MgO
on Ag(100). We deposited Fe and Ti in-situ while the sample was kept below 10
K. The Ti tends to be hydrogenated during the deposition by the residual
hydrogen in the UHV chamber. The thickness of the MgO layer was determined
from the point-contact measurement and IETS steps of FePaul  _et al._ (2017).
Spin-polarized tips were made by picking up Fe atoms. TiH, as the simplest
spin systemYang  _et al._ (2019b); Bae  _et al._ (2018), is chosen for the ESR
measurement. While Fe absorbs on top of the oxygen atoms in the MgO lattice,
TiH is found at two different binding sites, a bridge between oxygen atoms
(TiHB) Bae  _et al._ (2018) or atop oxygen atoms (TiHO) Yang  _et al._
(2019b). The d$I$/d$V$ spectra [Fig. 6(b)] and apparent heights in constant
current STM images [Fig. 6(a)] readily identify Fe and TiH on MgO/Ag(100).

As a crucial step towards frequency-sweep ESR measurements, we characterize
the frequency-dependent transfer functionPaul  _et al._ (2016) of both RF
transmission lines (the tip and the antenna) using IETS steps of Fe at around
14.5 meV [Fig. 6(c)]Paul  _et al._ (2017). Note that IETS steps of TiHO are
also availableSeifert  _et al._ (2020a); Drost  _et al._ (2022); van
Weerdenburg  _et al._ (2021). The IETS steps are broadened while applying RF
power to the tunnel junction. The IETS step broadening, which follows an
arcsine distribution, is used as a reference to convert the effective RF
voltage at the tunnel junction to a voltage measured by a lock-in amplifier
(SR865A, SRS). The lock-in amplifier detects variations in effective RF
voltages in the tunnel junction caused by frequency-dependent transmission.
Figure 6(d) shows transfer functions in the frequency range of 5–45 GHz. While
the transfer functions of the two cabling schemes are comparable over a wide
frequency range, the prominent difference is manifested at frequencies higher
than 37 GHz and frequencies lower than 15 GHz. Due to the existence of a
flexible cable (CW2040-3650p, Cooner Wire) for coarse movement of the tip and
a bare wire as the tip, considerable signal loss occurs at frequencies greater
than 37 GHz. This limitation is circumvented by using an antenna, which does
not require the use of lossy lines and thus enables for transmission at
frequencies greater than 37 GHz. At frequencies lower than 15 GHz, however,
the antenna transmits less than the tip, which is presumably related to
resonances of the antenna ($\sim$20 mm in length) and its surroundings, such
as the sample ($\sim$5 mm in diameter) and the tip ($\sim$10 mm in length).

Based on the Fourier analysis of transfer functions in Fig. 6(d), we found no
prominent standing waves for both transmission lines, which indicates all
cables are properly connected. Standing wave features from the Fourier
transform for the antenna are scarcely visible, which corresponds to the
length of the stripped part ($\sim$20 mm). Given the huge impedance mismatch
of the stripped cable, the standing waves created for the antenna’s
transmission line are negligibly small. We ascribe this suppression of
standing waves to the effects of a 50-Ohm terminator, which is further
supported by the antenna test shown in Fig. 3(b). Employing the measured
transfer functions, we are able to apply RF voltages with constant amplitudes
at the tunnel junction throughout a large frequency range (5–40 GHz) by
adjusting the power output of the generator. Once calibrated after an
iterative optimization procedure of transmission measurements, we observed no
significant change in the transfer function for at least several weeks (even
possibly several months if there is no change of cables), which implies that
the RF cables are well thermalized to fixed temperature points, and that the
temperature of these points is not dependent on the LHe level in the cryostat.

###  III.2 Single atom ESR spectroscopy

Figure 7:  Characterization of the TiH and the spin-polarized tip for ESR
measurement. (a) d$I$/d$V$ spectra of TiHB at different magnetic fields
($B_{\mathrm{ext}}=$0, 2, and 6 T in the in-plane direction, $V_{\mathrm{DC}}$
= 4 mV, $I$ = 70 pA, $V_{\mathrm{mod}}$ = 50 $\mu$V, $T$ = 1.3 K). (b)
d$I$/d$V$ spectra of TiHB with different spin polarization of the tip. The
spin-averaging tip corresponds to the Ag-coated STM tip. The STM tip is spin-
polarized by picking up Fe atoms on the surface. Each spectrum was measured
after picking up Fe atom one-by-one (5 spectra measured with 0 (spin-averaging
tip, light blue) to 4 Fe atoms (spin-polarized tip, dark blue) at the tip
apex) ($B_{\mathrm{ext}}=0.9$ T, in-plane direction, $V_{\mathrm{DC}}$ = 10
mV, $I$ = 100 pA, $V_{\mathrm{mod}}$ = 1 mV, $T$ = 1.3 K).

We implement the frequency-sweep ESR spectroscopy on a single TiH by applying
RF voltages through the tip or the antenna. TiH is a well-known spin-1/2
systemBae  _et al._ (2018); Yang  _et al._ (2019b). In a typical spectroscopy
of STM, the energy splitting between two spin states of TiH by Zeeman energy
is only visible if large external magnetic fields are applied [Fig. 7(a)]
since the spectral resolution is restricted by the thermal broadening of the
tunneling electrons’ energy and electrical noises. The spectral resolution can
be significantly enhanced by ESR spectroscopy in STM. In ESR-STM, the energy
is measured with respect to the frequency of an applied RF field and thus the
energy resolution is limited only by the spin dynamics of the objects such as
spin relaxation and coherence timesChen, Bae, and Heinrich (2021). For the ESR
measurement, a spin-polarized tip is prepared by picking up several Fe atoms
to the tip (typically 3–10 Fe atoms) and checking its polarization by
d$I$/d$V$ curves of TiHO or TiHB that show an asymmetric feature near the zero
bias voltage [Fig. 7(b)]. Larger spin polarization facilitates ESR measurement
but does not guarantee ESR signals since detection and driving of ESR require
spin-polarization of the tip and simultaneously inhomogeneous transverse (or
longitudinal depending on transitions) magnetic fields from the tip Bae  _et
al._ (2018); Yang  _et al._ (2019b); Seifert  _et al._ (2020b).

Figure 8:  Comparison of ESR spectra measured using the tip and the antenna
for RF transmission. (a) ESR of TiHB measured using the tip for RF
transmission ($V_{\mathrm{DC}}$ = 50 mV, $I$ = 20 pA, $V_{\mathrm{RF}}$ = 21
mV, $B_{\mathrm{ext}}$ = 1 T, $T$ = 1.3 K). (b) ESR spectrum of the same TiHB
atom measured using the same microtip but the antenna for RF transmission
($V_{\mathrm{DC}}$ = 50 mV, $I$ = 20 pA, $V_{\mathrm{RF}}$ = 22 mV,
$B_{\mathrm{ext}}$ = 1 T, and $T$ = 1.3 K). (c) ESR spectra of TiHB at
different magnetic fields measured using the antenna. Curves are shifted for
clarity ($V_{\mathrm{DC}}$ = 50 mV, $I$ = 20 pA, $V_{\mathrm{RF}}$ = 20 mV,
and $T$ = 1.3 K). (d) Resonance frequencies and (e) intensities of ESR signals
at different magnitudes of magnetic fields. The values are extracted from the
fit to eq. 1 of the data in (c) (Antenna; $V_{\mathrm{DC}}$ = 50 mV, $I$ = 20
pA, $V_{\mathrm{RF}}$ = 20 mV, and $T$ = 1.3 K) (Tip; $V_{\mathrm{DC}}$ = 50
mV, $I$ = 20 pA, $V_{\mathrm{RF}}$ = 21 mV, and $T$ = 1.3 K). The solid lines
in (d) are linear fits, which gives the TiHB magnetic moments, $\mu$. The
solid line in (e) corresponds to the estimated ESR intensity based on the
Boltzman distribution of spin state populations.

We compare the ESR spectra of TiHB measured using the tip or the antenna for
RF voltage transmission. For RF transmission through the tip, the RF voltages
are combined with the DC bias voltage by a bias-tee at RT [Fig. 5(b)]. For the
ESR measurement using the antenna, we just apply DC bias voltages to the tip,
while the RF voltages are applied separately to the antenna. During ESR
measurements, transfer functions calibrated for each RF cabling are used to
compensate for frequency-dependent RF transmissions. Both measurements use the
same signal detection scheme, which detects RF-induced current using a lock-in
amplifier while modulating RF powerPaul  _et al._ (2016). Figures 8(a) and (b)
show ESR spectra obtained by applying RF voltages to the tip and the antenna,
respectively. Note that we used the same spin-polarized tip and atomic species
to measure the change in dc current ($\Delta I$) caused by the population
change of Zeeman states and the precession of the transverse magnetization at
the resonance frequencyBae  _et al._ (2018). The ESR spectra are fit to the
equationBae  _et al._ (2018):

$\Delta
I=I\textsubscript{0}+I\textsubscript{p}\cdot\frac{1+\alpha\delta}{1+{\delta}^{2}}$
(1)

with $\alpha$ asymmetry factor, $I\textsubscript{p}$ ESR intensity, and
$\delta$ normalized frequency ($\delta=\Delta f/{\pi\Gamma}$, where $\Delta f$
is the difference of RF frequency and resonance frequency and $\Gamma$ is the
full width at half-maximum of the signal). As analyzed using eq. 1, with well-
calibrated transfer functions in two different schemes, we observed ESR
spectra with comparable ESR intensity, line-shape, and resonance frequency,
which reflects that the underlying mechanism of ESR driving and detection is
the same for these two different methods of ESR spectroscopy in STM.

The ESR signal intensity increases with increasing the population differences
between two states. The thermal population can be controlled by changing
temperature or energy splitting, where the Zeeman splitting stems from the
applied magnetic fields. As shown in Fig. 8(c), the ESR spectra were measured
at different magnetic fields using the antenna. We observed no spurious
features in a large frequency window (24–40 GHz) which might be caused due to
miscalibrations of the transfer function. With increasing magnetic field, the
resonance frequency of ESR signals linearly increases [Fig. 8(c) and (d)],
where the slope reflects the magnetic moments of TiHB, $\mu=\sim
0.83\mu_{\mathrm{B}}$, in the in-plane fieldKim  _et al._ (2021); Bae  _et
al._ (2018). With increasing resonance frequencies and thus Zeeman energies,
the signal intensity tends to increase (the solid lines in Fig. 8(e) show the
expected ESR intensities based on the Boltzmann distribution).

###  III.3 ESR spectroscopy at variable temperatures

Figure 9:  Temperature dependence of ESR signals. (a) ESR signals of TiHB
measured at different temperatures from 1.3 K to 10 K ($V_{\mathrm{DC}}$ = 40
mV, $I$ = 40 pA, $V_{\mathrm{RF}}$ = 20 mV, and $B_{\mathrm{ext}}$ = 1.59 T).
(b) d$I$/d$V$ spectra measured at different temperatures ($V_{\mathrm{DC}}$ =
10 mV, $I$ = 100 pA, $V_{\mathrm{mod}}$ = 1 mV, and $B_{\mathrm{ext}}$ = 1.59
T). The spectra were measured using the same tip and atom with (a). The
assymetric step at zero bias is observable at 1.3 K, while the feature is
indistinguishable at 9 K. (c) The ESR signal intensities and (d) linewidths
for ESR spectra in (a), as extracted using eq. 1. (e) The smallest ESR signal
measured using our ESR-STM system ($V_{\mathrm{DC}}$ = 32 mV, $I$ = 0.8 pA,
$V_{\mathrm{RF}}$ = 30 mV, and $B_{\mathrm{ext}}$ = 1 T, and $T$ = 1.3 K)

Taking advantage of large transmission through the antenna at high
frequencies, we implement the ESR spectroscopy at higher temperatures which
have not been explored before in ESR-STM. We maximize the population
difference by increasing the Zeeman energy splitting so that the ESR signals
appear at the largest frequency ($\sim$40 GHz) in our frequency window. Figure
9(a) shows the ESR spectra at different temperatures. Our STM enables us to
conduct experiments at elevated temperatures in the range from 1.3 K to at
least 10 K. Using a spin-polarized tip, which shows very strong ESR signals of
3 pA at 1.3 K (out of 40 pA setpoint current), we succeeded in detecting the
ESR signal at 10 K, which is $\sim$10 times weaker than the one at 1.3 K. As
shown in Fig. 9(c), the ESR signal intensity falls exponentially with rising
temperature, which is consistent with the exponential dependence of Boltzmann
distribution on temperature. While the peak intensity varies dramatically with
temperature, there is no discernible trend in the ESR peak broadening [Fig.
9(d)], which indicates that there is no major change in the energy resolution
of ESR in STM with temperature. In comparison, the spin-dependent feature at
zero bias is smeared out in the d$I$/d$V$ curve [Fig. 9(b)] since the spectral
resolution of the d$I$/d$V$ spectroscopy is limited by the thermal broadening
of tunneling electrons’ energy. The strength of ESR spectroscopy in STM, thus,
becomes more evident for experiments at higher temperatures. In addition, the
highly stable operation of ESR-STM allows for the extremely sensitive
measurement of single-spin ESR spectroscopy with the corresponding change in
tunnel current by sub-fA [Fig. 9(e)].

##  IV Conclusion

To summarize, we have presented the design and performance of ESR-STM
operating at temperatures of 1–10 K. We for the first time demonstrated ESR of
a single atom at elevated temperatures up to 10 K, which was realized by
introducing an antenna positioned parallel to the sample and optimizing the
operation of ESR-STM. We demonstrated the characterization of the newly
introduced antenna in comparison with the early design RF cablings, which
provides varieties in designs of RF antennas for integrating ESR in STM and
easy modifications of existing STMs for ESR applications. A planned future
upgrade will replace the 4He gas in the JT refrigerator with 3He gas, which
should enable stable performance below 0.5 K.

###### Acknowledgements.

We thank Alexander F. Otte and Alexandre Artaud for their initiative works and
fruitful discussions on COMSOL simulations. All authors acknowledge support
from the Institute for Basic Science (IBS-R027-D1).

## Data Availability Statement

The data that support the findings of this study are available from the
corresponding author upon reasonable request.

## References

  * Bogani and Wernsdorfer (2008) L. Bogani and W. Wernsdorfer, “Molecular spintronics using single-molecule magnets,” Nat. Mater. 7, 179–186 (2008).
  * Natterer  _et al._ (2017) F. D. Natterer, K. Yang, W. Paul, P. Willke, T. Choi, T. Greber, A. J. Heinrich, and C. P. Lutz, “Reading and writing single-atom magnets,” Nature 543, 226–228 (2017).
  * Thiele  _et al._ (2014) S. Thiele, F. Balestro, R. Ballou, S. Klyatskaya, M. Ruben, and W. Wernsdorfer, “Electrically driven nuclear spin resonance in single-molecule magnets,” Science 344, 1135–1138 (2014).
  * Yang  _et al._ (2019a) K. Yang, W. Paul, S.-H. Phark, P. Willke, Y. Bae, T. Choi, T. Esat, A. Ardavan, A. J. Heinrich, and C. P. Lutz, “Coherent spin manipulation of individual atoms on a surface,” Science 366, 509–512 (2019a).
  * Binnig  _et al._ (1982) G. Binnig, H. Rohrer, C. Gerber, and E. Weibel, “Surface studies by scanning tunneling microscopy,” Phys. Rev. Lett. 49, 57–61 (1982).
  * Bian  _et al._ (2021) K. Bian, C. Gerber, A. J. Heinrich, D. J. Müller, S. Scheuring, and Y. Jiang, “Scanning probe microscopy,” Nat. Rev. Meth. Primers 1, 36 (2021).
  * Lambe and Jaklevic (1968) J. Lambe and R. C. Jaklevic, “Molecular vibration spectra by inelastic electron tunneling,” Phys. Rev. 165, 821–832 (1968).
  * Song  _et al._ (2010a) Y. J. Song, A. F. Otte, Y. Kuk, Y. Hu, D. B. Torrance, P. N. First, W. A. de Heer, H. Min, S. Adam, M. D. Stiles, A. H. MacDonald, and J. A. Stroscio, “High-resolution tunnelling spectroscopy of a graphene quartet,” Nature 467, 185–189 (2010a).
  * Ast  _et al._ (2016) C. R. Ast, B. Jäck, J. Senkpiel, M. Eltschka, M. Etzkorn, J. Ankerhold, and K. Kern, “Sensing the quantum limit in scanning tunnelling spectroscopy,” Nat. Commun. 7, 13009 (2016).
  * Delgado and Fernández-Rossier (2011) F. Delgado and J. Fernández-Rossier, “Inelastic electron tunneling spectroscopy of a single nuclear spin,” Phys. Rev. Lett. 107, 076804 (2011).
  * Blank  _et al._ (2003) A. Blank, C. R. Dunnam, P. P. Borbat, and J. H. Freed, “High resolution electron spin resonance microscopy,” J. Magn. Reson. 165, 116–127 (2003).
  * Assig  _et al._ (2013) M. Assig, M. Etzkorn, A. Enders, W. Stiepany, C. R. Ast, and K. Kern, “A 10 mk scanning tunneling microscope operating in ultra high vacuum and high magnetic fields,” Rev. Sci. Instrum. 84, 033903 (2013).
  * Ciobanu, Seeber, and Pennington (2002) L. Ciobanu, D. Seeber, and C. Pennington, “3d mr microscopy with resolution 3.7$\mu$m by 3.3$\mu$m by 3.3$\mu$m,” J. Magn. Reson. 158, 178–182 (2002).
  * Baumann  _et al._ (2015) S. Baumann, W. Paul, T. Choi, C. P. Lutz, A. Ardavan, and A. J. Heinrich, “Electron paramagnetic resonance of individual atoms on a surface,” Science 350, 417–420 (2015).
  * Chen, Bae, and Heinrich (2021) Y. Chen, Y. Bae, and A. J. Heinrich, “Harnessing the quantum behavior of spins on surfaces,” Adv. Mater. , 2107534 (2021).
  * Paul  _et al._ (2016) W. Paul, S. Baumann, C. P. Lutz, and A. J. Heinrich, “Generation of constant-amplitude radio-frequency sweeps at a tunnel junction for spin resonance stm,” Rev. Sci. Instrum. 87, 074703 (2016).
  * Seifert  _et al._ (2020a) T. S. Seifert, S. Kovarik, C. Nistor, L. Persichetti, S. Stepanow, and P. Gambardella, “Single-atom electron paramagnetic resonance in a scanning tunneling microscope driven by a radio-frequency antenna at 4 k,” Phys. Rev. Res. 2, 013032 (2020a).
  * Natterer  _et al._ (2019) F. D. Natterer, F. Patthey, T. Bilgeri, P. R. Forrester, N. Weiss, and H. Brune, “Upgrade of a low-temperature scanning tunneling microscope fore electron-spin resonance,” Rev. Sci. Instrum. 90, 013706 (2019).
  * Friedlein  _et al._ (2019) J. Friedlein, J. Harm, P. Lindner, L. Bargsten, M. Bazarnik, S. Krause, and R. Wiesendanger, “A radio-frequency spin-polarized scanning tunneling microscope,” Rev. Sci. Instrum. 90, 123705 (2019).
  * Xu  _et al._ (2019) C. Xu, Y. Que, Y. Zhuang, K. Wang, and X. Xiao, “Construction of a gigahertz-bandwidth radio-frequency scanning tunneling microscope based on a commercial low-temperature system,” Rev. Sci. Instrum. 90, 103706 (2019).
  * van Weerdenburg  _et al._ (2021) W. M. J. van Weerdenburg, M. Steinbrecher, N. P. E. van Mullekom, J. W. Gerritsen, H. von Allwörden, F. D. Natterer, and A. A. Khajetoorians, “A scanning tunneling microscope capable of electron spin resonance and pump–probe spectroscopy at mk temperature and in vector magnetic field,” Rev. Sci. Instrum. 92, 033906 (2021).
  * Drost  _et al._ (2022) R. Drost, M. Uhl, P. Kot, J. Siebrecht, A. Schmid, J. Merkt, S. Wünsch, M. Siegel, O. Kieler, R. Kleiner, and C. R. Ast, “Combining electron spin resonance spectroscopy with scanning tunneling microscopy at high magnetic fields,” Rev. Sci. Instrum. 93, 043705 (2022).
  * Lörtscher, Widmer, and Gotsmann (2013) E. Lörtscher, D. Widmer, and B. Gotsmann, “Next-generation nanotechnology laboratories with simultaneous reduction of all relevant disturbances,” Nanoscale 5, 10542–10549 (2013).
  * MacLeod  _et al._ (2016) B. P. MacLeod, J. E. Hoffman, S. A. Burke, and D. A. Bonn, “Acoustic buffeting by infrasound in a low vibration facility,” Rev. Sci. Instrum. 87, 093901 (2016).
  * Eigler and Schweizer (1990) D. M. Eigler and E. K. Schweizer, “Positioning single atoms with a scanning tunneling microscope,” Nature 344, 524–526 (1990).
  * Song  _et al._ (2010b) Y. J. Song, A. F. Otte, V. Shvarts, Z. Zhao, Y. Kuk, S. R. Blankenship, A. Band, F. M. Hess, and J. A. Stroscio, “A 10 mk scanning scanning probe microscopy facility,” Rev. Sci. Instrum. 81, 121101 (2010b).
  * Gustrau (2012) F. Gustrau,  _RF and Microwave Engineering: Fundamentals of wireless communcations_ (A John Wiley $\&$ Sons, Ltd., 2012).
  * Yang  _et al._ (2019b) K. Yang, W. Paul, F. D. Natterer, J. L. Lado, Y. Bae, P. Willke, T. Choi, A. Ferron, J. Fernandez-Rossier, A. J. Heinrich, and C. P. Lutz, “Tuning the exchange bias on a single atom from 1 mt to 10 t,” Phys. Rev. Lett 122 (2019b).
  * Bae  _et al._ (2018) Y. Bae, K. Yang, P. Willke, T. Choi, A. J. Heinrich, and C. P. Lutz, “Enhanced quantum coherence in exchange coupled spins via singlet-triplet transitions,” Sci. Adv. 4, eaau4159 (2018).
  * Paul  _et al._ (2017) W. Paul, K. Yang, S. Baumann, N. Romming, T. Choi, C. P. Lutz, and A. J. Heinrich, “Control of the millisecond spin lifetime of an electrically probed atom,” Nat. Phys. 13, 403–407 (2017).
  * Seifert  _et al._ (2020b) T. S. Seifert, s. Kovarik, D. M. Juraschek, N. A. Spaldin, P. Gambardella, and S. Stepanow, “Longitudinal and transverse electron paramagnetic resonance in a scanning tunneling microscope,” Sci. Adv. 6 (2020b), 10.1126/sciadv.abc5511.
  * Kim  _et al._ (2021) J. Kim, W.-J. Jang, T. H. Bui, D.-J. Choi, C. Wolf, F. Delgado, Y. Chen, D. Krylov, S. Lee, S. Yoon, C. P. Lutz, A. J. Heinrich, and Y. Bae, “Spin resonance amplitude and frequency of a single atom on a surface in a vector magnetic field,” Phys. Rev. B 104, 174408 (2021).