Signatures Of Superconductivity Research Articles

Prediction of s^{±}-Wave Superconductivity Enhanced by Electronic Doping in Trilayer Nickelates La_{4}Ni_{3}O_{10} under Pressure.

Motivated by the recently reported signatures of superconductivity in trilayer La_{4}Ni_{3}O_{10} under pressure, we comprehensively study this system using abinitio and random-phase approximation techniques. Without electronic interactions, the Ni d_{3z^{2}-r^{2}} orbitals show a bonding-antibonding and nonbonding splitting behavior via the O p_{z} orbitals inducing a "trimer" lattice in La_{4}Ni_{3}O_{10}, analogous to the dimers of La_{3}Ni_{2}O_{7}. The Fermi surface consists of three electron sheets with mixed e_{g} orbitals, and a hole and an electron pocket made up of the d_{3z^{2}-r^{2}} orbital, suggesting a Ni two-orbital minimum model. In addition, we find that superconducting pairing is induced in the s^{±}-wave channel due to partial nesting between the M=(π,π) centered pockets and portions of the Fermi surface centered at the Γ=(0,0) point. With changing electronic density n, the s^{±} instability remains leading and its pairing strength shows a domelike behavior with a maximum around n=4.2 (∼6.7% electron doping). The superconducting instability disappears at the same electronic density as that in the new 1313 stacking La_{3}Ni_{2}O_{7}, correlated with the vanishing of the hole pocket that arises from the trilayer sublattice, suggesting that the high-T_{c} superconductivity of La_{3}Ni_{2}O_{7} does not originate from a trilayer and monolayer structure. Furthermore, we confirm the experimentally proposed spin state in La_{4}Ni_{3}O_{10} with an in-plane (π, π) order and antiferromagnetic coupling between the top and bottom Ni layers, and spin zero in the middle layer.

Effective model and electron correlations in trilayer nickelate superconductor La4Ni3O10

Recently, signatures of superconductivity with critical temperature from 20 to 30 K have been reported in pressured trilayer nickelate La4Ni3O10 through a pressure-induced structure transition. Here we explore the evolution of electronic structures and electronic correlations in different phases of La4Ni3O10 under corresponding pressure regions, by using density functional theory (DFT) combined with dynamical mean-field theory (DMFT). Similar to bilayer superconductor La3Ni2O7, the electronic bands in superconducting La4Ni3O10 are dominated by Ni-3 dx2−y2 and 3 dz2 orbits near the Fermi level, in contrast, the inner Ni–O plane in La4Ni3O10 generates a doublet hole-pocket Fermi surfaces around the Brillouin-zone corner, meanwhile one branch of the Ni- 3dz2 bands is pushed very close above the Fermi level, which can induce an electron pocket through small electron doping. The DFT+DMFT simulations suggest that the electronic correlations only give minor modification to the Fermi surfaces, meanwhile the Ni- 3dz2 and 3 dx2−y2 states on outer Ni–O layers have considerable greater mass enhancements than on the inner layer. The sensitiveness of electronic structure under doping and unique layer dependence of correlation suggest a distinct superconducting mechanism with respect to bilayer La3Ni2O7. Based on the DFT and DFT+DMFT simulations, we eventually derive a trilayer effective tight-binding model, which can produce rather precise electronic bands and Fermi surfaces, hence can serve as an appropriate model to further study the superconducting mechanism and paring symmetry in trilayer La4Ni3O10.

Pressure-induced magnetic phase and structural transition in SmSb2

Abstract Motivated by the recent discovery of unconventional superconductivity around a magnetic quantum critical point in pressurized CeSb2, here we present a high-pressure study of an isostructural antiferromagnetic (AFM) SmSb2 through electrical transport and synchrotron x-ray diffraction measurements. At P C ~2.5 GPa, we found a pressure-induced magnetic phase transition accompanied by a Cmca→P4/nmm structural phase transition. In the pristine AFM phase below P C, the AFM transition temperature of SmSb2 is insensitive to pressure; in the emergent magnetic phase above P C, however, the magnetic critical temperature increases rapidly with increasing pressure. In addition, at ambient pressure, the magnetoresistivity (MR) of SmSb2 increases suddenly upon cooling below the AFM transition temperature and presents linear nonsaturating behavior under high field at 2 K. With increasing pressure above P C, the MR behavior remains similar to that observed at ambient pressure, both in terms of temperature- and field-dependent MR. This leads us to argue an AFM-like state for SmSb2 above P C. Within the investigated pressure of up to 45.3 GPa and the temperature of down to1.8 K, we found no signature of superconductivity in SmSb2.

Imaging the Meissner effect in hydride superconductors using quantum sensors.

By directly altering microscopic interactions, pressure provides a powerful tuning knob for the exploration of condensed phases and geophysical phenomena1. The megabar regime represents an interesting frontier, in which recent discoveries include high-temperature superconductors, as well as structural and valence phase transitions2-6. However, at such high pressures, many conventional measurement techniques fail. Here we demonstrate the ability to perform local magnetometry inside a diamond anvil cell with sub-micron spatial resolution at megabar pressures. Our approach uses a shallow layer of nitrogen-vacancy colour centres implanted directly within the anvil7-9; crucially, we choose a crystal cut compatible with the intrinsic symmetries of the nitrogen-vacancy centre to enable functionality at megabar pressures. We apply our technique to characterize a recently discovered hydride superconductor, CeH9 (ref. 10). By performing simultaneous magnetometry and electrical transport measurements, we observe the dual signatures of superconductivity: diamagnetism characteristic of the Meissner effect and a sharp drop of the resistance to near zero. By locally mapping both the diamagnetic response and flux trapping, we directly image the geometry of superconducting regions, showing marked inhomogeneities at the micron scale. Our work brings quantum sensing to the megabar frontier and enables the closed-loop optimization of superhydride materials synthesis.

Polymorphism in the Ruddlesden-Popper Nickelate La3Ni2O7: Discovery of a Hidden Phase with Distinctive Layer Stacking.

We report the discovery of a novel form of Ruddlesden-Popper (RP) nickelate that stands as the first example of long-range, coherent polymorphism in this class of inorganic solids. Rather than the well-known, uniform stacking of perovskite blocks ubiquitously found in RP phases, this newly discovered polymorph of the bilayer RP phase La3Ni2O7 adopts a novel stacking sequence in which single-layer and trilayer blocks of NiO6 octahedra alternate in a "1313" sequence. Crystals of this new polymorph are described in space group Cmmm, although we note evidence for a competing Imam variant. Transport measurements at ambient pressure reveal metallic character with evidence of a charge density wave transition with an onset at T ≈ 134 K. The discovery of such polymorphism could reverberate to the expansive range of science and applications that rely on RP materials, particularly the recently reported signatures of superconductivity in bilayer La3Ni2O7 with Tc as high as 80 K above 14 GPa.

Effects of pressure and doping on Ruddlesden-Popper phases Lan+1NinO3n+1

Recently, the discovery of superconductivity with a critical temperature Tc up to 80 K in Ruddlesden–Popper phases Lan+1NinO3n+1 (n = 2) under pressure has garnered considerable attention. Up to now, the superconductivity was only observed in La3Ni2O7 single crystal grown with the optical-image floating zone furnace under oxygen pressure. It remains to be understood the effect of chemical doping on superconducting La3Ni2O7 as well as other Ruddlesden–Popper phases. Here, we systematically investigate the effect of external pressure and chemical doping on polycrystalline Ruddlesden–Popper phases. Our results demonstrate that the application of pressure and doping effectively tunes the transport properties of Ruddlesden–Popper phases. We find pressure-induced superconductivity up to 86 K in La3Ni2O7 polycrystalline sample, while no signatures of superconductivity are observed in La2NiO4 and La4Ni3O10 polycrystalline samples under high pressure up to 50 GPa. Our study sheds light on the exploration of high-Tc superconductivity in nickelates.

Signature of Superconductivity in Pressurized La4Ni3O10

The discovery of high-temperature superconductivity near 80 K in bilayer nickelate La3Ni2O7 under high pressures has renewed the exploration of superconducting nickelate in bulk materials. The extension of superconductivity in other nickelates in a broader family is also essential. Here, we report the experimental observation of superconducting signature in trilayer nickelate La4Ni3O10 under high pressures. By using a modified sol-gel method and post-annealing treatment under high oxygen pressure, we successfully obtained polycrystalline La4Ni3O10 samples with different transport behaviors at ambient pressure. Then we performed high-pressure electrical resistance measurements on these samples in a diamond-anvil-cell apparatus. Surprisingly, the signature of possible superconducting transition with a maximum transition temperature (T c) of about 20 K under high pressures is observed, as evidenced by a clear drop of resistance and the suppression of resistance drops under magnetic fields. Although the resistance drop is sample-dependent and relatively small, it appears in all of our measured samples. We argue that the observed superconducting signal is most likely to originate from the main phase of La4Ni3O10. Our findings will motivate the exploration of superconductivity in a broader family of nickelates and shed light on the understanding of the underlying mechanisms of high-T c superconductivity in nickelates.

Emergence of High-Temperature Superconducting Phase in Pressurized La3Ni2O7 Crystals

The recent report of pressure-induced structural transition and signature of superconductivity with T c ≈ 80 K above 14 GPa in La3Ni2O7 crystals has garnered considerable attention. To further elaborate this discovery, we carried out comprehensive resistance measurements on La3Ni2O7 crystals grown in an optical-image floating zone furnace under oxygen pressure (15 bar) using a diamond anvil cell (DAC) and cubic anvil cell (CAC), which employ a solid (KBr) and liquid (glycerol) pressure-transmitting medium, respectively. Sample 1 measured in the DAC exhibits a semiconducting-like behavior with large resistance at low pressures and gradually becomes metallic upon compression. At pressures P ⩾ 13.7 GPa we observed the appearance of a resistance drop of as much as ∼ 50% around 70 K, which evolves into a kink-like anomaly at pressures above 40 GPa and shifts to lower temperatures gradually with increasing magnetic field. These observations are consistent with the recent report mentioned above. On the other hand, sample 2 measured in the CAC retains metallic behavior in the investigated pressure range up to 15 GPa. The hump-like anomaly in resistance around ∼ 130 K at ambient pressure disappears at P ⩾ 2 GPa. In the pressure range of 11–15 GPa we observed the gradual development of a shoulder-like anomaly in resistance at low temperatures, which evolves into a pronounced drop of resistance of 98% below 62 K at 15 GPa, reaching a temperature-independent resistance of 20 μΩ below 20 K. Similarly, this resistance anomaly can be progressively shifted to lower temperatures by applying external magnetic fields, resembling a typical superconducting transition. Measurements on sample 3 in the CAC reproduce the resistance drop at pressures above 10 GPa and realize zero resistance below 10 K at 15 GPa even though an unusual semiconducting-like behavior is retained in the normal state. Based on these results, we constructed a dome-shaped superconducting phase diagram and discuss some issues regarding the sample-dependent behaviors on pressure-induced high-temperature superconductivity in the La3Ni2O7 crystals.

Synthesis of possible room temperature superconductor LK-99: Pb9Cu(PO4)6O

The quest for room temperature superconductors has been teasing scientists and physicists, since its inception in 1911 itself. Several assertions have already been made about room temperature superconductivity, but have never been verified or reproduced across the labs. The cuprates were the earliest high transition temperature (T c) superconductors, and it seems that copper has done the magic once again. In July 2023, a Korean group synthesized a lead apatite based compound LK-99, showing a T c of above 400 K (Lee et al 2023 arXiv: 2307.12008; 2023 arXiv: 2307.12037; Lee et al 2023 J. Korean Cryst. Growth Cryst. 33 61). The signatures of superconductivity in the compound are very promising, in terms of resistivity (ρ = 0) and diamagnetism at T c. Although, the heat capacity (C p) did not show the obvious transition at T c. Inspired by the interesting claims of the above room temperature superconductivity in LK-99, in this article, we report the synthesis of polycrystalline samples of LK-99, by following the same heat treatment as reported in Lee et al (2023 arXiv: 2307.12008; 2023 arXiv: 2307.12037) using a two-step precursor method. The phase is confirmed through x-ray diffraction measurements, performed after each heat treatment. The room temperature diamagnetism is not evidenced by the levitation of a permanent magnet over the sample or vice versa. The isothermal magnetization measurement at 280 K shows that as synthesized sample of LK-99 is paramagnetic. Further measurements for the confirmation of bulk superconductivity in variously synthesized samples are underway. Our results on the present LK-99 sample, synthesized at 925 °C, as of now do not confirm the appearance of bulk superconductivity at room temperature. Further studies with different heat treatments are underway.

Signatures of superconductivity near 80 K in a nickelate under high pressure.

Although high-transition-temperature (high-Tc) superconductivity in cuprates has been known for more than three decades, the underlying mechanism remains unknown1-4. Cuprates are the only unconventional superconductors that exhibit bulk superconductivity with Tc above the liquid-nitrogen boiling temperature of 77 K. Here we observe that high-pressure resistance and mutual inductive magnetic susceptibility measurements showed signatures of superconductivity in single crystals of La3Ni2O7 with maximum Tc of 80 K at pressures between 14.0 GPa and 43.5 GPa. The superconducting phase under high pressure has an orthorhombic structure of Fmmm space group with the [Formula: see text] and [Formula: see text] orbitals of Ni cations strongly mixing with oxygen 2p orbitals. Our density functional theory calculations indicate that the superconductivity emerges coincidently with the metallization of the σ-bonding bands under the Fermi level, consisting of the [Formula: see text] orbitals with the apical oxygen ions connecting the Ni-O bilayers. Thus, our discoveries provide not only important clues for the high-Tc superconductivity in this Ruddlesden-Popper double-layered perovskite nickelates but also a previously unknown family of compounds to investigate the high-Tc superconductivity mechanism.

Electrical and thermal transport properties of the kagome metals ATi3Bi5(A=Rb,Cs)

We report electrical and thermal transport properties of single-crystalline kagome metals $A{\text{Ti}}_{3}{\text{Bi}}_{5}\phantom{\rule{4pt}{0ex}}(A=\text{Rb},\phantom{\rule{4pt}{0ex}}\mathrm{Cs})$. Different from the structrually similar kagome superconductors $A{\mathrm{V}}_{3}{\mathrm{Sb}}_{5}$, no charge density wave instabilities are found in $A{\mathrm{Ti}}_{3}{\mathrm{Bi}}_{5}$. At low temperatures below 5 K, signatures of superconductivity appear in $A{\mathrm{Ti}}_{3}{\mathrm{Bi}}_{5}$ as seen in magnetization measurements. However, bulk superconductivity is not evidenced by specific heat results. Similar to $A{\mathrm{V}}_{3}{\mathrm{Sb}}_{5}, A{\mathrm{Ti}}_{3}{\mathrm{Bi}}_{5}$ show a nonlinear magnetic field dependence of the Hall effect below about 70 K, pointing to a multiband nature. Unlike $A{\mathrm{V}}_{3}{\mathrm{Sb}}_{5}$ in which phonons and electron-phonon coupling play important roles in thermal transport, the thermal conductivity in $A{\mathrm{Ti}}_{3}{\mathrm{Bi}}_{5}$ is dominated by electronic contributions. Moreover, our calculated electronic structures of ${\mathrm{ATi}}_{3}{\mathrm{Bi}}_{5}$ suggest that van Hove singularities are sitting well above the Fermi energy. Compared with $A{\mathrm{V}}_{3}{\mathrm{Sb}}_{5}$, the absence of charge orders in $A{\mathrm{Ti}}_{3}{\mathrm{Bi}}_{5}$ is closely associated with minor contributions from electron-phonon coupling and/or van Hove singularities.

Evidence Against Superconductivity in Flux Trapping Experiments on Hydrides Under High Pressure

It has recently been reported that hydrogen-rich materials under high-pressure trap magnetic flux, a tell-tale signature of superconductivity (Minkov et al., Trapped magnetic flux in hydrogen-rich high-temperature superconductors, Ref. 1). Here, we point out that under the protocol used in these experiments the measured results indicate that the materials don’t trap magnetic flux. Instead, the measured results either are experimental artifacts or originate in magnetic properties of the sample or its environment unrelated to superconductivity. Together with other experimental evidence analyzed earlier, this clearly indicates that these materials are not superconductors.

Emergence of correlations in alternating twist quadrilayer graphene.

Alternating twist multilayer graphene (ATMG) has recently emerged as a family of moiré systems that share several fundamental properties with twisted bilayer graphene, and are expected to host similarly strong electron-electron interactions near the magic angle. Here, we study alternating twist quadrilayer graphene (ATQG) samples with twist angles of 1.96° and 1.52°, which are slightly removed from the magic angle of 1.68°. At the larger angle, we find signatures of correlated insulators only when the ATQG is hole doped, and no signatures of superconductivity, and for the smaller angle we find evidence of superconductivity, while signs of the correlated insulators weaken. Our results provide insight into the twist angle dependence of correlated phases in ATMG and shed light on the nature of correlations in the intermediate coupling regime at the edge of the magic angle range where dispersion and interaction are of the same order.

Probing superconducting gap of the high-entropy alloy Ta1/6Nb2/6Hf1/6Zr1/6Ti1/6 via Andreev reflection spectroscopy

High-entropy alloy (HEA) superconductors characterized as having a high mixing entropy have attracted substantial interest because of their remarkable potential for technological applications. However, the superconducting (SC) gap of HEA superconductors, which is one of the important factors for designing SC devices, has scarcely been studied. Herein, we report the direct observation of the fully gapped pairing symmetry of the HEA superconductor ${\mathrm{Ta}}_{1/6}{\mathrm{Nb}}_{2/6}{\mathrm{Hf}}_{1/6}{\mathrm{Zr}}_{1/6}{\mathrm{Ti}}_{1/6}$, probed using Andreev reflection spectroscopy. The Andreev reflection, a pronounced signature of superconductivity, is observed in the differential conductance ($\mathrm{d}I/\mathrm{d}V$) spectra below the SC transition temperature ${T}_{\mathrm{c}}$ of 7.9 K, which is well explained by the modified Blonder-Tinkham-Klapwijk model. The SC energy gap (\ensuremath{\Delta}) as a function of temperature and magnetic field follows the Bardeen-Cooper-Schrieffer (BCS) theory with $\mathrm{\ensuremath{\Delta}}(0)=1.37\phantom{\rule{0.28em}{0ex}}\mathrm{meV}$. However, the gap to ${T}_{\mathrm{c}}$ ratio, $2\mathrm{\ensuremath{\Delta}}(0)/{k}_{B}{T}_{c}=4.03$, is larger than the value of 3.54 predicted by the weak-coupling BCS theory, indicating that ${\mathrm{Ta}}_{1/6}{\mathrm{Nb}}_{2/6}{\mathrm{Hf}}_{1/6}{\mathrm{Zr}}_{1/6}{\mathrm{Ti}}_{1/6}$ belongs to the class of moderately coupled nodeless superconductors.

Evidence for unconventional superconductivity in twisted trilayer graphene.

Magic-angle twisted trilayer graphene (MATTG) has emerged as a moiré material that exhibits strong electronic correlations and unconventional superconductivity1,2. However, local spectroscopic studies of this system are still lacking. Here we perform high-resolution scanning tunnelling microscopy and spectroscopy of MATTG that reveal extensive regions of atomic reconstruction favouring mirror-symmetric stacking. In these regions, we observe symmetry-breaking electronic transitions and doping-dependent band-structure deformations similar to those in magic-angle bilayers, as expected theoretically given the commonality of flat bands3,4. Most notably in a density window spanning two to three holes per moiré unit cell, the spectroscopic signatures of superconductivity are manifest as pronounced dips in the tunnelling conductance at the Fermi level accompanied by coherence peaks that become gradually suppressed at elevated temperatures and magnetic fields. The observed evolution of the conductance with doping is consistent with a gate-tunable transition from a gapped superconductor to a nodal superconductor, which is theoretically compatible with a sharp transition from a Bardeen-Cooper-Schrieffer superconductor to a Bose-Einstein-condensation superconductor with a nodal order parameter. Within this doping window, we also detect peak-dip-hump structures that suggest that superconductivity is driven by strong coupling to bosonic modes of MATTG. Our results will enable further understanding of superconductivity and correlated states in graphene-based moiré structures beyond twisted bilayers5.

Low-Temperature Thermal Conductivity of the Two-Phase Superconductor CeRh2As2

CeRh2As2is a rare unconventional superconductor (Tc= 0.26 K) characterized by two adjacent superconducting phases for a magnetic fieldH‖c-axis of the tetragonal crystal structure. Antiferromagnetic order, quadrupole-density-wave order (T0= 0.4 K) and the proximity of this material to a quantum-critical point have also been reported: The coexistence of these phenomena with superconductivity is currently under discussion. Here, we present thermal conductivity and electrical resistivity measurements on a single crystal of CeRh2As2between 60 mK and 200 K and in magnetic fields (H‖c) up to 8 T. Extrapolation of our normal-state data to zero temperature validates the Wiedemann-Franz law within the error bars. TheTdependence of the thermal conductivityκ(T) shows a pronounced drop belowTcwhich is also field dependent and thus interpreted as the signature of superconductivity. However, the large residual resistivity and the lack of sharp anomalies inκ(T) at the expected transition temperatures clearly indicate that samples of much higher purity are required to gain more information about the superconducting gap structure.

Orbitally selective resonant photodoping to enhance superconductivity

Signatures of superconductivity at elevated temperatures above $T_c$ in high temperature superconductors have been observed near 1/8 hole doping for photoexcitation with infrared or optical light polarized either in the CuO$_2$-plane or along the $c$-axis. While the use of in-plane polarization has been effective for incident energies aligned to specific phonons, $c$-axis laser excitation in a broad range between 5 $\mu$m and 400 nm was found to affect the superconducting dynamics in striped La$_{1.885}$Ba$_{0.115}$CuO$_4$, with a maximum enhancement in the $1/\omega$ dependence to the conductivity observed at 800 nm. This broad energy range, and specifically 800 nm, is not resonant with any phonon modes, yet induced electronic excitations appear to be connected to superconductivity at energy scales well above the typical gap energies in the cuprates. A critical question is what can be responsible for such an effect at 800 nm? Using time-dependent exact diagonalization, we demonstrate that the holes in the CuO$_2$ plane can be photoexcited into the charge reservoir layers at resonant wavelengths within a multi-band Hubbard model. This orbitally selective photoinduced charge transfer effectively changes the in-plane doping level, which can lead to an enhancement of $T_c$ near the 1/8 anomaly.

Correlated Insulating States and Transport Signature of Superconductivity in Twisted Trilayer Graphene Superlattices.

Layers of two-dimensional materials stacked with a small twist angle give rise to beating periodic patterns on a scale much larger than the original lattice, referred to as a "moiré superlattice." Here, we demonstrate a higher-order "moiré of moiré" superlattice in twisted trilayer graphene with two consecutive small twist angles. We report correlated insulating states near the half filling of the moiré of moiré superlattice at an extremely low carrier density (∼10^{10} cm^{-2}), near which we also report a zero-resistance transport behavior typically expected in a 2D superconductor. The full-occupancy (ν=-4 and ν=4) states are semimetallic and gapless, distinct from the twisted bilayer systems.

High-pressure synthesis of Ba2RhO4 , a rhodate analog of the layered perovskite Sr-ruthenate

A new layered perovskite-type oxide Ba$_2$RhO$_4$ was synthesized by a high-pressure technique with the support of convex-hull calculations. The crystal and electronic structure were studied by both experimental and computational tools. Structural refinements for powder x-ray diffraction data showed that Ba$_2$RhO$_4$ crystallizes in a K$_2$NiF$_4$-type structure, isostructural to Sr$_2$RuO$_4$ and Ba$_2$IrO$_4$. Magnetic, resistivity, and specific heat measurements for polycrystalline samples of Ba$_2$RhO$_4$ indicate that the system can be characterized as a correlated metal. Despite the close similarity to its Sr$_2$RuO$_4$ counterpart in the electronic specific heat coefficient and the Wilson ratio, Ba$_2$RhO$_4$ shows no signature of superconductivity down to 0.16 K. Whereas the Fermi surface topology has reminiscent pieces of Sr$_2$RuO$_4$, an electron-like e$_g$-($d_{x^2-y^2}$) band descends below the Fermi level, making of this compound unique also as a metallic counterpart of the spin-orbit-coupled Mott insulator Ba$_2$IrO$_4$.

Two-Dimensional 2M-WS2 Nanolayers for Superconductivity.

Recently, a newly discovered VIB group transition metal dichalcogenide (TMD) material, 2M-WS2, has attracted extensive attention due to its interesting physical properties such as topological superconductivity, nodeless superconductivity, and anisotropic Majorana bound states. However, the techniques to grow high-quality 2M-WS2 bulk crystals and the study of their physical properties at the nanometer scale are still limited. In this work, we report a new route to grow high-quality 2M-WS2 single crystals and the observation of superconductivity in its thin layers. The crystal structure of the as-grown 2M-WS2 crystals was determined by X-ray diffraction (XRD) and scanning tunneling microscopy (STM). The chemical composition of the 2M-WS2 crystals was determined by energy dispersive X-ray spectroscopy (EDS) analysis. At 77 K, we observed the spatial variation of the local tunneling conductance (dI/dV) of the 2M-WS2 thin flakes by scanning tunneling spectroscopy (STS). Our low temperature transport measurements demonstrate clear signatures of superconductivity of a 25 nm-thick 2M-WS2 flake with a critical temperature (TC) of ∼8.5 K and an upper critical field of ∼2.5 T at T = 1.5 K. Our work may pave new opportunities in studying the topological superconductivity at the atomic scale in simple 2D TMD materials.