# Two Dimensional Multigap Superconductivity in Bulk 2H-TaSSe

C. Patra  Department of Physics, Indian Institute of Science Education and
Research Bhopal, Bhopal, 462066, India  T. Agarwal  Department of Physics,
Indian Institute of Science Education and Research Bhopal, Bhopal, 462066,
India  Rajeshwari R. Chaudhari  Department of Physics, Indian Institute of
Science Education and Research Bhopal, Bhopal, 462066, India  R. P. Singh
<EMAIL_ADDRESS>Department of Physics, Indian Institute of Science
Education and Research Bhopal, Bhopal, 462066, India

###### Abstract

Superconducting transition metal dichalcogenides emerged as a prime candidate
for topological superconductivity. This work presents a detailed investigation
of superconducting and transport properties on 2H-TaSeS single crystals using
magnetization, transport, and specific heat measurements. These measurements
suggest multigap anisotropic superconductivity with the upper critical field,
breaking Pauli limiting field in both in-plane and out-of-plane directions.
The angle dependence of the upper critical field suggests 2-dimensional
superconducting nature in bulk 2H- TaSeS.

##  I INTRODUCTION

Recently superconductivity in layered transition metal dichalcogenides (TMDs)
CDW ; CDW_in_NbSe2 ; structure ; graphene_structure  have drawn immense
research interest due to its ability to host unique physical properties such
as superconductivity, charge density wave (CDW) fermisurface_nesting ;
polar_charge_symme_TaS2 , electron-electron correlation electronic_corelation
, and topological properties topology_case . TMDs with a general formula MX2
(M = Mo, W, Ta, Nb and X = Te, Se, S) consist of a layer of TM atom sandwiched
between layers of chalcogen atoms and can exist in different polymorphs such
as hexagonal 2H, trigonal 1T, and rhombohedral 3R structures
graphene_structure ; structure_differ . Among the different families of TMDs,
archetypal systems, NbSe2 structure_NbSe2 , NbS2 NbS2_structure , TaSe2
1T_TaSe2_xSx_pressure , TaS2 TaS2_structure  are reported to be intrinsic
superconductors, while superconductivity has been successfully induced by the
application of pressure pressure_SC_MoTe2 ; SC_doping ; SC_doping1  and
chemical doping in semiconducting MoS2 MoS2 , MoSe2 SC_MoSe2  and WTe2 WTe2_SC
.

The Ta-based TMD system, 2H-TaSe2 and 2H-TaS2, is known to host charge density
wave and superconductivity at low temperatures TS2 ; disorder_TaSeS .
Moreover, 2H-TaSe2 shows CDW ordering at 90 K and the onset of
superconductivity at 0.15 K CDW_TaSe2 ; TaSe2_xSx , 2H-TaS2 exhibits a chiral
charge order system with superconductivity below 0.8 K TS2 ; TaS2_chairal .
The enhancement of the superconducting transition in monolayers and
intercalation of elements or organic compounds mono_layer_TaS2 ; CuxTaS2 ;
mono_TaSe2  resulted from the suppression of CDW by doping or disorder.

Recently 4Hb-TaS2 phase consists of alternating stacking of weakly coupled
1T-TaS2 (Mott insulator and proposed gapless spin liquid spin_liquid_TaS2 )
and 1H-TaS2 (2D superconductor with charge density wave) revealed the
signature of time-reversal symmetry breaking (TRSB) TRSB_4H_TaS2  in the
superconducting ground state. Apart from this, the different phase of TaSeS
shows exiting superconducting properties. Scanning tunnelling microscopic
studies on the disorder-driven superconductor 1T-TaSeS suggest a non-trivial
link between superconductivity and charge order STM_TaSeS ; topology_Tas2 .
2H-TaSeS shows an enhanced superconducting transition temperature, resulted
from suppression of CDW due to disorder, as the similar electronic structure
of 2H-TaSe2, 2H-TaSeS and 2H-TaS2 ruled out the role of the dopant
disorder_TaSeS ; electronic_sturcture_of_2H_TaS2 . However, the detailed
superconducting, anisotropic and normal state properties of different phases
of TaSeS are not available, which are crucial to understanding the
superconducting gap/ground state of any superconductors disorder_TaSeS

This work reports single crystal growth and detailed electronic and
superconducting properties of 2H-TaSeS. Comprehensive critical field and heat
capacity measurements suggest anisotropic multigap superconducting behaviour.
The large in-plane and out-of-plane directions upper critical field breaks the
Pauli limiting field. Interestingly, the angle dependence of the upper
critical field well-fit 2D Tinkham model suggests 2D superconductivity in bulk
2H-TaSeS single crystal.

Figure 1:  (a) The microscopic image of single crystal and the inset shows the
Laue image of the single crystal. (b) Single crystal powder X-ray diffraction
pattern indicates crystal oriented along [00l] plane. (c) Refinement of powder
sample for determining lattice parameters. (d) Side view of 2H-TaSeS unit
cell.

##  II EXPERIMENTAL METHODS

2H-TaSeS single crystals were synthesized by the chemical vapour transport
method using iodine as the transport agent. The stoichiometric ratios of Ta,
Se, and S were thoroughly ground and sealed in a quartz ampule with iodine as
the transport agent. The ampule was then placed in a two-zone furnace at a
temperature gradient of 850∘C - 750∘C. After 15 days, shiny single crystals
were grown in the cold zone of the tube. The Laue diffraction pattern was
recorded using a Photonic-Science Laue camera. The orientation of the crystal
plane was also confirmed by X-ray diffraction (XRD) at room temperature on a
PANalytical diffractometer equipped with $CuK\alpha$ radiation ($\lambda$ =
1.54056 Å). Magnetization measurements were done using a Quantum Design
Magnetic Measurement System (MPMS3, Quantum Design). Transport properties were
measured using the four-probe method and specific heat measurements were
performed using the two-tau model in the Physical Property Measurement System
(PPMS, Quantum Design).

##  III RESULTS AND DISCUSSION

###  a Sample characterization

The microscopic image of single-crystal 2H-TaSeS shows crystal in centimetre
order in Fig. 1(a). The inset of Fig. 1(a) represents the Laue pattern of the
crystal. It confirms that the crystal orientation is along the [00l]
direction. It was further confirmed by the powder XRD Fig. 1.(b). To determine
the lattice parameters, powder XRD performed on crushed single crystals and
confirmed a hexagonal structure with lattice parameters, a = b = 3.37(2) Å, c
= 12.38(4) Å, which is in agreement with earlier reports of 2H-TaS2 TaSe2_xSx
. The structure of the unit cell is shown in Fig. 1(d).

Figure 2:  (a) Zero resistivity drop happens at 3.9(1) K and the inset shows
the resistivity data above the superconducting transition temperature is
fitted by the BG model. (b) Magnetization data was collected in ZFCW-FCC mode
that shows the superconducting transition temperature at 3.90(3) K at 1 mT
applied field.

###  b Electrical Resistivity and Magnetization

The temperature-dependent resistivity of 2H-TaSeS above 10 K, which shows
metallic behaviour up to 300 K and below 3.9(1) K, undergoes a superconducting
transition. The resistivity data above the transition temperature was well
fitted by using the Bloch-Grüneisen model resistivity_of_MgB2 ; Bi2Pd ;
theory_parallel_resistor . In accordance with this model resistivity, can be
described as,

$\rho(T)=\rho_{0}+\rho_{BG}(T)$ (1)

where $\rho_{BG}(T)$ is defined as

$\rho_{BG}(T)=r\left(\frac{T}{\Theta_{D}}\right)^{5}\int_{0}^{\frac{\Theta_{D}}{T}}\frac{x^{5}}{(e^{x}-1)(1-e^{-x})}dx$
(2)

where $\rho_{0}$ is residual resistivity, $r$ is the material-dependent
constant, and $\Theta_{D}$ is the Debye constant. The fit using Eq.(1) can be
seen in inset of Fig. 2(a) which give $\rho_{0}$ = 635.42(8) $\mu\ohm$-cm, $r$
= 1.70(2) m$\Omega$-cm, and $\Theta_{D}$ = 115(1) K.

Figure 3:  (a) Low field variation of magnetization data in two different
directions. (b) Lower critical field variation with temperature is well fitted
with GL equations and gives values 4.3(2) and 2.1(1) mT for $H\perp c$ and
$H\parallel c$ directions. Figure 4:  (a) and (b) Shows resistivity variation
with temperature for $H\perp c$ and $H\parallel c$, respectively. (c) The two-
gap fitting of the upper critical field from resistivity data for
perpendicular and parallel directions and the black dash lines show Pauli
limit of upper critical field, i.e. $H_{c2}^{P}$ = 1.86$T_{c}$ =7.25 T.

The superconducting transition of the sample was confirmed by dc magnetization
measurement on a single crystal in zero field cooled worming (ZFCW) and field
cooled cooling (FCC) mode with an applied 1 mT field. The transition
temperature is observed at $T_{c}$ = 3.90(3) K for $H\perp c$ (Fig. 2(b)) and
$T_{c}$ = 3.83(3) K for $H\parallel c$. For both perpendicular and parallel
field directions, $T_{c}$ is almost the same. But lower critical field
($H_{c1}(T)$) and the upper critical field ($H_{c2}(T)$) give a huge
difference in different directions. For the calculation of $H_{c1}(0)$
Two_gap_in_Hc1_in_CaFeCoAsF ; Anisotropies_Hc1_MgB2 , the field value for the
corresponding temperature is extracted from the magnetization curve (Fig.
3(a)), from which the Meissner lines have deviated. The temperature variation
of the lower critical field value is fitted with the Ginzburg-Landau equation
by Eq. (3).

$H_{c1}(T)=H_{c1}(0)\left[1-\left(\frac{T}{T_{c}}\right)^{2}\right]$ (3)

Fig. 3(b) clearly shows anisotropy in a different direction, and the value of
$H_{c1}(0)$ for $H\perp c$ is 4.3(2) mT and for $H\parallel c$ is 2.1 (1) mT.

The upper critical field ($H_{c2}(T)$) of the 2H-TaSeS system was measured
from the transport ($\rho-T$) measurement. The resistivity was measured both
in-plane and out-of-plane directions of the crystal. Fig. 4(a) and (b) shows
the resistivity with temperature in different directions in magnetic field up
to 6.0 T. From the resistivity curves, $H_{c2}(T)$ values were extracted by
taking onset value for $T_{c}$. Fig. 4(c) shows an upturn present near $T_{c}$
in both the $H\perp c$ and $H\parallel c$ directions. It can not be explained
by Ginzburg-Landau theory and Werthamer-Helfand-Hohenberg (WHH) WHH_Hc2
model. However, this type of behaviour was observed in MgB2
MgB2_very_high_upper_critical_field ; MgAlB2_2_gap ; MgBC2_2_gap ,
LaFeAsO0.89F0.11 Two_Band_Sc_LaFeAsOF , and some iron-based superconductors,
which can be fitted using the two-band modelMgB2_two_gap_theory ;
Hole_pocket_driven_superconductivity ; Twoband_of_112_iron ; clean_or_dity .
It can be expressed as

$\begin{split}ln\frac{T}{T_{c}}=\frac{1}{2}\left[U(s)+U(\eta
s)+\frac{\lambda_{0}}{w}\right]-\\\ \\{\frac{1}{4}\left[U(s)-U(\eta
s)-\frac{\lambda_{-}}{w}\right]^{2}+\frac{\lambda_{12}\lambda_{21}}{w}\\}^{\frac{1}{2}}\end{split}$
(4) $H_{c2}=\frac{2\phi_{0}T_{S}}{D_{1}};\eta=\frac{D_{2}}{D_{1}}$

and

$U(s)=\psi(s+\frac{1}{2})-\psi(\frac{1}{2})$

where $\lambda_{-}=\lambda_{11}-\lambda_{22}$,
$\lambda_{0}=(\lambda_{-}^{2}+4\lambda_{12}\lambda_{21})^{1/2}$ and
$w=\lambda_{11}\lambda_{22}-\lambda_{12}\lambda_{21}$. $\lambda_{11}$,
$\lambda_{22}$ are the intraband coupling constants and $\lambda_{12}$,
$\lambda_{21}$ are the interband coupling constants. $D_{1}$ and $D_{2}$ are
diffusivities of two bands, respectively. $\phi_{0}$ is flux quantum and
$\psi(x)$ is the digamma function. Fig. 4(c) represents the multi-gap feature
with anisotropy in $H\perp c$ and $H\parallel c$ directions. The fit using Eq.
(4) gives the upper critical field value $H_{c2}^{\perp}(0)$ 15.97(3) T for
perpendicular and $H_{c2}^{\parallel}(0)$ 9.59(1) T for parallel directions of
field.

In a type-II superconductor, the Cooper pair can break in the applied magnetic
field via the orbital and Pauli paramagnetic limiting. In orbital pair
breaking, the field-induced kinetic energy of a Cooper pair exceeds the
superconducting condensation energy, whereas in Pauli paramagnetic limiting it
is energetically favourable for the electron spins to align with the magnetic
field, thus breaking the Cooper pairs. For the type-II superconductor, Pauli
paramagnetic or Clogston-Chandrasekar limit is defined as $H_{c2}^{P}$ =
1.86$T_{c}$.

Table 1: Comparative superconducting parameters for 2H-TaSeS with some layered compounds Parameter | TaSeS | NbSe2 NbSe2_pauli  | NbS2 NbS2_pauli  | Ba6Nb11Se28 Ba6Nb11Se28
---|---|---|---|---
$H_{c2}^{\perp}$(T) | 15.97 | 17.3 |  | 8.84
$H_{c2}^{\parallel}$(T) | 9.59 | 5.3 | 1.6 | 0.57
$H_{c2}^{P}$(T) | 7.25 | 13.54 | 10.4 | 4.27
$\xi_{\perp c}(\text{\AA}$) | 58.61 | 78.8 | 143 | 240.4
$\xi_{\parallel c}(\text{\AA}$) | 35.19 | 24.1 | 9.7 | 15.6
$\gamma$ | 1.67 | 3.3 | 7.94 | 10.53

The Pauli violation ratio defined as $H_{c2}^{{}^{\parallel}}(0)/H_{c2}^{P}$.
For 2H-TaSeS, PVR in perpendicular and and parallel directions are 2.2 and 1.3
respectively. Violation of Pauli limit of the upper critical field has been
observed in other layered superconductors like NbSe2 NbSe2_pauli , NbS2
NbS2_pauli , and Ba6Nb11Se28 Ba6Nb11Se28 , when applied magnetic field is
perpendicular to the crystallographic c-axis [see Table 1]. However, in case
of 2H-TaSeS this violation has been observed for both in-plane ($H\perp c$)
and out-of-plane ($H\parallel c$) directions with an anisotropy factor
($\gamma=H_{c2}^{\perp}(0)/H_{c2}^{\parallel}(0)$) of 1.67 Nb2PdSe5 ;
Ba6Nb11Se28 ; Fe_based_hc2 . In the layered superconductors Pauli limit
voilation can happen due to strong spin-orbit coupling or finite-momentum
pairing bulk_2D_SC ; 2D_material ; graphene . Strong SOC leads Ising type
superconductivity, which is observed recently in monolayer 2H-NbSe2 MoS2 ;
ising_nbse2 . The finite-momentum pairing can gives rise to the Fulde-Ferrell-
Larkin-Ovchinnikov (FFLO) state. To certain the exact mechanism of Pauli limit
violation, further low temperature angle depend measurements along with
theoretical inputs are required.

To further explore the anisotropy, in-plane and out-of-plane upper critical
fields, the resistivity was measured at different angles. The angle variation
of field dependence of the resistivity at 2.5 K superconductor_CeIr3  is shown
in Fig. 5(a), where $\theta$ is the angle between the magnetic field and the
normal to the sample plane. The angle dependence of resistivity curve shows a
clear anisotropy signature from angles $\theta=0\degree$ ($H\parallel c$) to
$\theta=90\degree$ ($H\perp c$). The upper critical field ($H_{c2}(\theta,T)$)
data in Fig. 5(b) shows a cusplike peak at $\theta\simeq 90\degree$. This
angular dependence of cusp-like feature can be explained by two models, i.e.
3D anisotropic GL (AGL) model and the model for thin film, 2D Tinkham model
3D_GL_model ; Sr2RuO4 ; AuSn4 . The relevant equations for these models are in
Eq. (5) and Eq. (6), respectively.

$\left(\frac{H_{c2}(\theta,T)sin\theta}{H_{c2}^{\perp}}\right)^{2}+\left(\frac{H_{c2}(\theta,T)cos\theta}{H_{c2}^{||}}\right)^{2}=1$
(5)
$\left(\frac{H_{c2}(\theta,T)sin\theta}{H_{c2}^{\perp}}\right)^{2}+\left\arrowvert\frac{H_{c2}(\theta,T)cos\theta}{H_{c2}^{||}}\right\arrowvert=1$
(6)

Figure 5:  (a) The angle dependence resistivity variation indicates anisotropy
in the system. (b) The angle dependence upper critical field from resistivity
measurement 2.5 K fitted with 3D GL model and 2D Tinkham model.

Solid orange and green lines show the 3D GL and 2D Tinkham model fitting
consequently. The 2D Tinkham model gives a better fitting result compared to
others represented in the inset of Fig. 5(b). This implies that
superconductivity is better described as 2D than as 3D for this sample. This
2D kind of superconducting behaviour was observed in 4HbTaS2 TRSB_4H_TaS2 .

The Ginzburg-Landau coherence length ($\xi_{\perp c}$(0) = 58.61(2) Å and
$\xi_{\parallel c}$(0) = 35.19(2) Å) estimate using $H_{c2}^{\parallel
c}(0)=\frac{\phi_{0}}{2\pi\xi_{\perp c}^{2}(0)}$ and $H_{c2}^{\perp
c}(0)=\frac{\phi_{0}}{2\pi\xi_{\parallel c}(0)\xi_{\perp c}(0)}$. Using the
Ginzburg-Landau coherence lengths and the lower critical field values
($H_{c1}^{\perp}(0)$ = 4.3(2) mT and $H_{c1}^{\parallel}(0)$ = 2.1(1) mT), the
penetration depth of Ginzburg-Landau ($\lambda_{\perp c}(0)$ = 6115(13) Å and
$\lambda_{\parallel c}(0)$ = 2821(11) Å) is calculated using Eq. (7)$-$(8)
LiFeAs_Hc1 .

$H_{c1}^{\parallel c}(0)=\frac{\phi_{0}}{4\pi\lambda^{2}_{\perp
c}(0)}ln\left[\frac{\lambda_{\perp c}(0)}{\xi_{\perp c}(0)}+0.12\right]$ (7)

$H_{c1}^{\perp c}(0)=\frac{\phi_{0}}{4\pi\lambda_{\parallel c}\lambda_{\perp
c}(0)}ln\left[\frac{\lambda_{\parallel c}(0)}{\xi_{\parallel
c}(0)}+0.12\right]$ (8)

where $\phi_{0}$ (= 2.07$\times$10-15 T$m^{2}$) is the magnetic flux quantum.
The two characteristic length parameters are used to calculate the Ginzburg-
Landau parameter $\kappa_{\perp c}$ = 104 and $\kappa_{\parallel c}$ = 80$>$
$\frac{1}{\sqrt{2}}$ by Eq. (9), indicating a type II behaviour of the sample.

$\kappa_{\perp c}=\frac{\lambda_{\perp c}(0)}{\xi_{\perp c}(0)}$ (9)

###  c Specific heat

The zero-field low temperature specific heat data show a discontinuity at
3.79(7) K, the same as the reported superconducting transition temperature via
resistivity and magnetization measurements. The zero-field data above the
superconducting transition temperature is fitted with
$C/T=\gamma_{n}+\beta_{3}T^{2}$, where $\gamma_{n}$ is the electronic
contribution, and $\beta_{3}$ is the phononic contribution. The parameters
were 8.13 mJ/mol-K2 and 1.33 mJ/mol-K4, respectively. Furthermore, the Debye
temperature, $\theta_{D}$ = 163.34 K, was calculated using Eq. (10).

$\theta_{D}=\left(\frac{12\pi^{4}RN}{5\beta_{3}}\right)^{\frac{1}{3}}$ (10)

where $R$ is the universal gas constant (=8.314 J mol${}^{-1}K^{-1}$) and $N$
is the number of atoms per formula unit.

To reveal the superconducting gap parameter, the detailed electronic specific
heat ($C_{el}(T)$) in the superconducting state analysed. The $C_{el}(T)$ (Eq.
(11)) in the superconducting state was calculated by subtracting the phononic
contribution from total specific heat $C(T)$.

$C_{el}(T)=C(T)-\beta_{3}T^{3}$ (11)

Figure 6:  (a) low-temperature electronic contribution of specific data is
better fitted with two-gap comparative single gap model. (b) The Sommerfeld
coefficient was calculated from field-dependent of specific heat curve. The
inset shows $\gamma$ depends on the square root of H.

The temperature dependence of the electronic contribution of specific heat in
the superconducting state can be described using the fully gaped model given
below in Eq. (12).

$\frac{S}{\gamma_{n}T_{c}}=-\frac{6}{\pi^{2}}\left(\frac{\Delta(0)}{k_{B}T_{c}}\right)\int_{0}^{\infty}[fln(f)+(1-f)ln(1-f)]\,dy,$
(12)

where $f(\xi)=[exp(E(\xi)/k_{B}T)+1]^{-1}$ is the fermi function
$E(\xi)=\sqrt{\xi^{2}+\Delta(t)^{2}}$, where $\xi$ is energy of normal
electron comparative to Fermi energy, $y=\xi/\Delta(0)$ , $t=T/T_{c}$ and
$\Delta(t)=tanh[1.82(1.018((1/t)-1))^{0.51}]$ is the BCS approximation for the
temperature dependence of the energy gap. The normalized electronic specific
heat is related to entropy by Eq. (13).

$\frac{C_{el}}{\gamma_{n}T_{c}}=t\frac{d(S/{\gamma_{n}T_{c}})}{dt}$ (13)

The temperature dependence of $C_{el}(T)$ was fitted using Eq. (12) and Eq.
(13). In Fig. 6(a), the dotted black line represents the s-wave fitting and
gives the gap value of 2.19 meV. It reproduces the experimental data above T
$\simeq$ 2.38 K. The s-wave fitting deviates at lower temperatures. To explain
the temperature behaviour, the phenomenological two-gap $\alpha$ model (s+s
wave) is used Strong_Coupled_Superconductors ; theory_two_gap . In this model,
each band is characterized by the corresponding Sommerfeld coefficient
$\gamma_{n}$ = $\gamma_{1}$ \+ $\gamma_{2}$, and the total specific heat was
calculated by two gap parameters ($\Delta_{1}$ and $\Delta_{2}$) and their
comparative weights ($\gamma_{1}/\gamma_{n}\equiv$ x and
$\gamma_{2}/\gamma_{n}\equiv$ 1-x). The Fig. 6(a) (red dotted line) exhibits a
better agreement across the whole temperature range, in particular for T <
2.38 K. The gap values are 1.49 meV and 2.27 meV, with a fraction of 0.09. To
obtain the true nature of the superconducting gap, heat-capacity data is to be
analyzed well below $T_{c}$/10.

The superconducting gap symmetry can be further confirmed by the magnetic
field dependence of the Sommerfeld coefficient $\gamma$(H). In conventional
fully gapped type-II superconductor it is proportional to the vortex density.
As we apply more field the vortex density increases due to an increase in the
number of field induced vortices which in turn enhances the quasiparticle
density of states. This gives rise to a linear relation between $\gamma$ and
H, i.e., $\gamma(H)$ $\propto H$ for a nodeless and isotropic s-wave
superconductor Volovik_effect ; Behavior_of_dirty_superconductors ;
Superconductivity_of_Metals_and_Alloys . For a superconductor with nodes in
the gap, Volovik predicted a nonlinear relation given by $\gamma(H)$ $\propto$
$\sqrt{H}$ d_Wave_Superconductors ; YNi2B2C ; N_Nakai_et_al ;
specific_heat_2_gap_NbSe2 . The Sommerfeld coefficient $\gamma$ was calculated
by fitting with Eq. (14) for various fields and extrapolating it to T = 0 K
[see Fig. 6(b)].

$\frac{C_{el}}{T}=\gamma+\frac{a}{T}exp\left(-b\frac{T_{c}}{T}\right)$ (14)

The inset Fig. 6(b) shows that the Sommerfeld coefficient varies with the
square root of H. The linear deviation of $\gamma$ indicates that TaSeS is a
possible multi-gap system.

Figure 7:  The plot of the superconducting transition temperature versus Fermi
temperature for different superconducting families. In between, two solid blue
lines show the unconventional band of superconductors. TaSeS lies close to the
unconventional band.

The superconducting materials can be classified as conventional or
unconventional based on Tc/TF ratio that is given by Uemura et al. 1st_uemra ;
2nd_uemra ; 3rd_uemra . The Chevrel phases, heavy fermions, Fe-based
superconductors, and high Tc superconductors are fallen into the
unconventional category as Tc/TF ratio within 0.01$\leq$Tc/T${{}_{F}}\leq$0.1
range. Fermi temperature for TaSeS is 4825 K, obtained by solving a set of
five equations uemra_equation ; uemra_equation_2 ; uemra_plot_2
simultaneously. The ratio of Tc/TF = 0.0008 place it 2H TaSeS near to the band
of unconventional superconducting materials [see Fig. 7].

##  IV CONCLUSION

In summary, we studied the magnetization, electrical, and magnetotransport
properties of 2H-TaSeS. It confirms multigap superconductivity in 2H-TaSeS
having a superconducting transition temperature of 3.9 K and Pauli limit
breaking in both the in-plane and out-of-plane directions. The angle
dependence of the upper critical field well-fit 2D Tinkham model suggests 2D
superconductivity in bulk 2H-TaSeS single crystals. All results indicate
2H-TaSeS a is a new candidate for unconventional superconductor and can host
Ising or FFLO type superconductivity. Further, low temperature and thickness
dependence measurements along with theoretical inputs are required to
understand the exact superconducting pairing mechanism.

##  V Acknowledgments

R. P. S. acknowledge Science and Engineering Research Board, Government of
India, for the CRG/2019/001028 Core Research Grant.

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