Noncentrosymmetric superconductors, characterized by broken inversion symmetry, represent a distinctive subclass of unconventional superconductors. Here, we report the growth of single crystals of a noncentrosymmetric superconductor h-NbS, and systematical characterization of crystal structure and superconducting properties. Different from o-NbS, h-NbS crystallizes in a NiAs-type hexagonal superstructure with site-selective Nb vacancies. First-principles calculations reveal that the Nb-4d states dominate at the Fermi level (EF), with the density of states near EF manifesting significant sensitivity to the concentration of the Nb vacancies. Nodeless moderately coupled two-gap superconductivity with a critical temperature of 2.6 K is confirmed in this strong type-II superconductor. The anisotropic parameters based on upper and lower critical fields have small values of '1.4 and '1.3, respectively, which demonstrate that the Cooper pairs show almost isotropic resilience to magnetic field. We surprisingly note that the upper critical field along the ab plane surpasses the Pauli limit, largely arising from the two-dimensional Fermi-surface sheet around the K and H points of the Brillouin zone, which is related to the breaking of inversion symmetry. In contrast to quasi-one-dimensional and quasi-two-dimensional Nb-chalcogenide superconductors, the three-dimensional h-NbS featuring Pauli-limit violation provides a unique material for investigating unconventional superconductivity.

The full-Heusler intermetallic compound ScAu2In was previously found to be not superconducting above 2 K, and a much earlier report of superconductivity at 3 K was ascribed to elemental indium. In this paper, ScAu2In samples were prepared by the arc-melting method, and its physical properties were revisited by measurement down to 0.4 K. Bulk superconductivity with Tc similar to 1.3 K of ScAu2In has been revealed. The upper critical field mu 0Hc2 of similar to 54.6 mT is well below the Pauli limit, and the gap ratio 2 Delta(0)/kBTc of 2.27 is evidently lower than the value of 3.52, which together indicate that the superconductivity is in the BCS weak-coupling regime. First-principles calculations present the energy bands and density of states (DOS) of ScAu2In, manifesting that it is a multiband metal because different components contribute to the states at the Fermi level (EF). The nearly flat-band topology around the Gamma and L points, and the EF in the vicinity of a van Hove singularity in DOS may boost the superconductivity. Furthermore, non-zero topological indices & Zopf;4 and & Zopf;8 infer that ScAu2In is a candidate of topological superconductor. Our results gain more insights into the superconductivity in Heusler compounds, and provide a platform for probing the interplay between superconductivity and topological states.

We report a study of crystal and electronic structures, and superconductivity of the Chevrel-phase (CP) compound NaMo6Se8. Na+ insertion results in a lattice expansion relative to the parent Mo6Se8, and substantially enhances the density of states at the Fermi level within a rigid-band shift picture. Bulk superconductivity with a sharp transition at = 9.7 K is evidenced by electrical transport, magnetic, and thermodynamic measurements. The compound displays an exceptionally large upper critical field & micro; 0 H c2(0) = 33.6 T, almost twice the Pauli paramagnetic limit and placing it among selenide CP superconductors with the highest-H c2 values. Strong spin-orbit scattering offsets the Pauli paramagnetic effect, leading to an enhancement of the effective Pauli-limiting field. Specific-heat data reveal a low-lying Einstein phonon mode associated with Na-cation oscillating, and elucidate the intermediate-coupling two-gap BCS superconductivity. The narrow-band electronic structures, enhanced spin-orbit interactions, and Na-induced low-lying Einstein phonons synergistically promote T c and & micro; 0 H c2. Our work establishes NaMo6Se8 as a compelling high-field superconductor and highlights its promising relevance for high-field superconducting magnets.

Superconductivity was observed in the full-Heusler compound TiPd2Ga below 1.4 K. Polycrystalline samples of TiPd2Ga were prepared by the arc-melting method. TiPd2Ga adopts a centrosymmetric cubic structure with the space group Fm-3m. Electrical-resistivity and magnetic-susceptibility measurements reveal that TiPd2Ga is a type-II superconductor with a large Ginzburg-Landau parameter. The upper critical field & micro;0Hc2 of '2.28 T is slightly lower than the Bardeen-Cooper-Schrieffer Pauli limit (& micro;0HP = 2.58 T) in this three-dimensional superconductor with inversion symmetry, suggesting conventional superconducting states. Thermodynamic and transport data evidence the multigap weak-coupling BCS regime of TiPd2Ga. In addition, we note that a charge density wave like transition lies at '90 K. First-principles calculations indicate that the Pd-4d and Ti-3d orbits dominate the states at the Fermi level (EF), showing that different electronic components induce the multiband characters. The flat band topology and van Hove singularity near EF contribute to the superconducting and charge instabilities in this system. The discovery of the first Ti-containing Heusler superconductor TiPd2Ga may benefit to make sense of the pairing mechanisms of Heusler-phase and other related intermetallic superconductors.

Abstract We report the growth and physical characterization of high-quality single crystals of the layered manganese oxypnictide Ba 2 Mn 2 Bi 2 O, which features hexagonal Mn 2 Bi 2 O double layers separated by Ba 2+ ions. Magnetic susceptibility measurements reveal a broad hump around 170 K, characteristic of low-dimensional spin fluctuations, followed by two successive magnetic transitions at T N ≈ 32 K and T C ≈ 28 K. These transitions are confirmed by specific heat measurements, which show two distinct anomalies—unlike the isostructural compound Ba 2 Mn 2 Sb 2 O, which exhibits only one. Isothermal magnetization measurements demonstrate significant magnetic anisotropy, a clear hysteresis loop is observed for the field parallel to the c-axis below T C , providing direct evidence for the weak ferromagnetic component. The small magnetic entropy change (Δ S mag ≈ 0.43 J/mol·K) suggests significant short-range magnetic correlations above T N . Electrical resistivity follows a small-polaron hopping model with an activation energy of 0.04 eV, indicating semiconducting behavior. First-principles calculations including U -dependence checks reveal that Ba 2 Mn 2 Bi 2 O is a narrow-gap semiconductor (0.26 eV) with quasi-two-dimensional electronic structure, where carriers are confined within the Mn 2 Bi 2 O planes. The coexistence of antiferromagnetic order and weak ferromagnetism at low temperatures may arise from Dzyaloshinskii-Moriya interactions induced by strong spin-orbit coupling on Bi sites. This work establishes Ba 2 Mn 2 Bi 2 O as a promising platform for exploring correlated magnetism in heavy pnictide-based layered systems.

We report on the pressure dependence of superconducting transition temperature Tc and upper critical field Bc2(0) through electrical transport of the Ti4Co2O superconductor (eg.,the superconducting transition temperature Tc = 2.5 K and the Bc2(0)=7.2T=2.9Tc). We find that the Tc exhibits non-monotonic pressure dependence:it rises monotonically at first with a pressure coefficient of dTc/dP=0.034 K/GPa, but rapidly decreases around 10-20 GPa, and then increases with the dTc/dP = 0.023 K/GPa, up to= 4.31 K at 69.7 GPa. Concurrently, the Bc2(0)exhibits a dome shaped pressure dependence, with its maximum at 5 GPa of almost twice the value at ambient pressure, exceeding the weak-coupling Pauli paramagnetic limit Bp throughout the whole pressure range. By comparing the normal-state and superconducting properties, we identify two distinct superconducting regimes, with a low-pressure superconducting phase characterized by an enhanced Bc2(0)values and Fermi-liquid normal-state electrical transport (the exponent n = 2), and a high-pressure superconducting phase with a monotonically increased Tc and an enhancement in phonon scatterings (the exponent n = 4). Room-temperature synchrotron X-ray diffraction indicates that there is no structural transition up to 55.8 GPa, which gives a relatively large bulk modulus of 192 GPa in comparison with other alloy superconductors. First-principles calculations suggest that the nonmonotonic Tc maybe closely related to the evolution of the density of states of Ti4Co2O upon compression, which is different from those of isostructural superconductors Ti4Ir2O and Nb4Rh2C. Our results show that even in the Ti4Co2O with weak spin-orbit coupling, superconductivity remains highly sensitive to the external stimuli such as pressure.

We present a comprehensive investigation into the superconducting properties of La3Al, a La-based metal with a kagome structure. La3Al crystallizes in a Ni3Sn-type crystal structure (space group P6(3)/mmc), where the La atoms form a kagome lattice. Resistivity measurements reveal superconducting transition with T-c(onset)=6.37 K and T-c(zero)=6.18 K. In magnetic susceptibility measurements, the superconducting transition is observed at 6.16 K. The lower and upper critical fields are determined to be 22.17 mT and 6.69 T, respectively. Heat capacity measurements confirm the bulk superconductivity, showing a normalized specific heat change of Delta C-e / (gamma T-c) = 2.16 and an electron-phonon coupling strength of lambda(ep) = 0.92. DFT calculations reveal the intricate band structure of La3Al. The notable specific heat jump, coupled with the electron-phonon coupling strength lambda(ep), indicates that La3Al exhibits characteristics of an intermediately coupled type-II superconductor.

The Chevrel phase (CP), characterized by its unique Mo6X8 (X = S, Se, Te) cluster structure, represents a class of promising materials demonstrating exceptional performance in various applications, including battery cathodes, electrocatalysts, and superconductors. However, the exploration of new CP derivatives remains challenging due to the inherent lattice destabilization caused by cation intercalation, particularly evident in selenide and telluride systems. This study reports the successful synthesis of thermodynamically metastable K1+δMo6Se8 (δ ∼ 0.37) and the superconducting properties therein. K1+δMo6Se8 crystallizes in the triclinic space group P1̅ (No. 2), where potassium cations occupy interstitial sites between the Mo6Se8 clusters. Comprehensive characterization through electrical resistivity, magnetization, and specific heat measurements reveals bulk superconductivity at Tc = 8.9 K. Notably, the upper critical field is estimated to be 26.4 T, violating the Pauli paramagnetic limit. Furthermore, low-temperature specific heat analysis indicates possible multigap superconducting behavior. Our findings not only expand the family of high-critical-field superconducting CPs but also demonstrate the potential to synthesize novel CP materials through solid-state reactions at lower temperatures.

Magnetoresistance (MR) is a pivotal transport phenomenon within the realm of condensed matter physics. In recent years, materials exhibiting extremely large unsaturated magnetoresistance (XMR), which are often potential topological materials, have garnered significant attention. In this study, we synthesized single crystals of ZrBi2 and performed electrical and specific heat measurements on them. The resistivity of ZrBi2 displays metallic behavior with a high residual resistance ratio. Notably, the MR of ZrBi2 reaches approximately 2.0 x 103% at 2 K and 16 T without saturation. Weak Shubnikov-de Haas oscillations with two frequencies were observed above 13.5 T, which correspond to 237 T and 663 T. Hall effect fitting yields nearly equal concentrations of electron and hole carriers with concentrations of approximately 1021 cm-3 and mobilities of approximately 5000 cm2 & sdot;V-1 & sdot;s-1 at 2 K. The XMR could be attributed to the electron-hole compensation with high mobility.

We report the structural and superconducting properties of Ge-intercalated 2H-NbSe2 polycrystals. GexNbSe2 samples with nominal 0 <= x <= 0.1 crystallize in the space group P6(3)/mmc with Ge disorderedly occupying the interlayer Se-6 octahedral interstices. Superconducting critical temperature T-c monotonically decreases from 7.2 K in NbSe2 to 4.9 K in Ge0.1NbSe2. Studies on resistivity, magnetization and specific heat derive the superconducting- and normal-state parameters, indicating that the suppression of the two-gap superconductivity is mostly caused by the lowered electron-phonon coupling parameter lambda(e-p) and density of states at the Fermi level N(E-F). Surprisingly, the upper critical field H-c2 and irreversible field H-irr of the low Ge-level samples are enhanced compared with those of the undoped one, which may ascribe to the electron scattering and vortex pinning by nonmagnetic Ge. This study suggests the feasibility for improving high-field performance by slight impurity doping and advances the understanding of superconductivity in transition-metal dichalcogenides.

Electron and hole compensation and ultrahigh mobility of topological semimetals are becoming a guiding principle for designing thermoelectric materials with large Nernst effect, following which several topological semimetals are discovered. Here, beyond the desirable band features, the essential role of Dirac Fermion and phonon coupling is further highlighted for the exceptional Nernst effect. By investigating the topological semimetal of TaSb2 single-crystalline, a high Nernst thermopower of 1200 mu V K-1 and a giant Nernst power factor (PFN) of 8300 mu W cm-1 K-2 are observed, which are record values in the topological XPn2 (X = Ta, Nb; Pn = As, Sb) family. In particular, the Dirac Fermion and phonon coupling presented by the phonon-drag effect amplifies the respective Seebeck coefficients of electrons and holes and therefore enhances the Nernst thermopower. These results provide new insights for future design of novel materials with large Nernst effects and demonstrate the potential of TaSb2 for solid-state cooling and thermo-management at cryogenic temperatures.

Non-centrosymmetric 4H(a)-NbSe2 has recently been reported to exhibit nodeless two-gap s + s-wave pairing and large upper critical field far beyond the Pauli limit. In this paper, we focus on the crystal and electronic structures, and superconductivity (SC) of sulfur-doped 4H(a)-NbSe2. Upon S doping to 4H(a)-NbSe2-xSx (0 <= x <= 0.4), the unit cell is compressed accompanied by decrease in interlayer spacing. Paramagnetic Meissner effect is observed in the S-doped samples. The onset transition temperature T-c is down from similar to 6.4 K in pristine 4H(a)-NbSe2 to similar to 4.9 K in 4H(a)-NbSe1.6S0.4, indicating the breakdown of the multi-band SC. The three-dimensional Fermi surface centered at the & Gcy; point shrinks with doping and vanishes at x = 0.4, accounting for the reduced multi-gap character. We ascribe the drop in T-c to the lowered density of states at the Fermi level N(E-F) and electron-phonon coupling. The critical current density J(c) is enhanced at low S concentration because a slight S as pinning centers strengthens the vortex pinning. J(c) at high doping level, upper critical field mu H-0(c2) and irreversible field mu H-0(irr) decrease with increasing the S content, which is attributed to the prominent impurity scatterings. Pauli-limit-violated mu H-0(c2) is reduced by S doping arising from the weakening of the spin-orbit coupling. The results of this study represent the first report on doped 4H(a)-NbSe2 to our knowledge, we hope that it could motivate investigation in the future on doping effect in 4H(a)-NbSe2 to probe the SC of transition-metal-dichalcogenide superconductors.

4Ha-NbSe2 single crystals were grown by the chemical vapor transport method followed by a quench to room temperature. The temperature-dependent magnetization and in-plane resistance reveal that the sample exhibits both charge-density-wave and superconducting orders, condensing at TCDW 43 K and Tc 6.5 K, respectively. The superconductivity was studied by specific heat and lower critical field (Hc1) down to 0.5 and 0.4 K, respectively. The electronic specific heat at superconducting state could be well described by an isotropic two-gap BCS model. The superconducting gaps were obtained to be of AL = 1.1 meV and AS = 0.3 meV with the weighting factors as 0.938 and 0.062, respectively. The in-plane Hc1//ab(T) and out-of-plane H//c c1 (T ) also support the two-gap s + s-wave scenario, which derives Hc1'/ab(0) and H//c c1 (0) as 78.3 and 149.8 Oe, respectively. The anisotropic parameter at 0 K based on Hc1 [PHc1 (0)] was deduced to be 1.9. Our results reveal nodeless two-gap pairing symmetry in 4Ha-NbSe2, which would gain more insight into the superconductivity of TMDs.

We report on crystal growth and physical properties of the quasi-one-dimensional compound Bi19S27I3 by combining crystal structure, electrical resistivity, magnetic properties, Seebeck coefficient, Hall coefficient as well as hydrostatic pressure effect up to 11.5 GPa. Unlike n-type Bi19S27I3 crystals, the maximum size of high-quality p-type Bi19S27I3 crystals can reach 2-3 mm by optimizing the chemical vapor transport method. The measurement results indicate that Bi19S27I3 is a diamagnetic semiconductor with two thermal activation energies, a large one E-g1 similar to 0.81 eV and a small one E-g2 similar to 0.36 eV, a huge room-temperature Seebeck coefficient of -1000 mu V/K, and improved thermoelectric power factor similar to 2.2 mu W cm(-1) K-2 owing to the enhanced electrical conductivity. Under pressure, Bi19S27I (3)undergoes a semiconductor-to-metal transition, and the thermal activation energy continuously decreases to almost zero near a critical pressure of 4.25 GPa. Accompanying this process, a density-wave-like transition emerges, characterized by the reversible jump observed in the temperature dependence of the resistivity. As the pressure further increases, the resistivity undergoes a crossover from a Fermi metal to a low-temperature upturn below a characteristic temperature, which decreases from 81 K at 4.5 GPa to 37 K at 11.5 GPa. The upturn in resistivity has a linear dependence on the logarithmic temperature, but does not saturate at low temperatures, which basically excludes a Kondo-like state and indicates the possibility of Anderson weak localization. High-pressure synchrotron x-ray diffraction confirms the absence of structural transition for P<12.05 GPa at room temperature, supporting pressure-induced electronic transition. Our density functional theory calculation on the assumption that the Bi1 occupies an average of similar to 1/6 contradicts experimental electron bands, indirectly indicating that Bi1 should be partially ordered and has many vacancies in Bi19S27I3. Our results provide good examples for studying the mechanism of semiconductor metallization and exploring thermoelectric functional properties in low-dimensional materials

The oxyselenide KV2Se2O, featuring an anti-K2NiF4-type layered structure, was successfully synthesized using a self-flux method. Comprehensive structural, electrical, and heat transport, as well as magnetism measurements, were performed on both single crystals and polycrystals. At 105 K, a density-wave (DW)-like anomalous transition is revealed in temperature-dependent resistivity and Hall coefficient, signifying possibly the opening of DW gaps. This transition is also confirmed by the temperature dependence of magnetic susceptibility and specific heat as a second-order transition. No superstructural peaks and peak splitting were observed in the conventional x-ray diffraction spectra, indicating either a rather small distortion amplitude or a more complicated mechanism than charge- or spin-density waves. Additionally, measurements on the magnetotransport properties of KV2Se2O single crystals under a magnetic field up to 16 T have been conducted. Clear Shubnikov-de Haas (SdH) oscillations are observed, and parameters of the Fermi surface (FS), such as Fermi vector and effective mass, are calculated. Multiple FS sheets contribute to the SdH oscillations with frequencies F alpha = 5 T, F delta = 62 T, F beta = 220 T, F gamma = 364 T, and F eta = 586 T, respectively. Among them, F alpha corresponds to a very small FS area and cyclotron effective mass, which is expected for Dirac or Weyl points. Beyond the quantum limit of alpha-FS, an observation of negative magnetoresistance is noted. A linear fit to the Landau level fan diagrams yields values of Berry phase 0.97 pi (1.17 pi) for F alpha (F beta), indicating the existence of nontrivial band structures in KV2Se2O.

We report the growth and physical properties of high-quality needle-shaped Re3Ge7 single crystal with various characterizations. It exhibits a gapped metal-insulator (MI)-like phase transition with evident Shubnikov-de Haas oscillation of low-T electrical resistivity. The superconducting phase diagram was revisited by measuring the electrical resistivity in a cubic anvil cell (CAC) under various hydrostatic pressures up to 12 GPa. Unlike the results of single crystals in a diamond anvil cell (DAC) and polycrystalline in CAC, the gapped MI-like phase transition of single-crystal Re3Ge7 evolves into three charge density wave (CDW)-like ones, which are reduced gradually by applied pressure and then collapse at several critical pressures; approaching the CDW-like quantum criticality, two distinct superconducting phases emerge and transit from one to another in a narrow pressure range; the significant increase in superconducting width indicates their strong competition. By combining density functional theory calculations, the origin of the melting MI-like transition caused by pressure and Ga doping, as well as the competitive CDW-like phase transitions and distinct superconductivity, are discussed.

A series of tetragonal Fe$_{1-x}$Co${_x}$Se single crystals with a complete Co doping range (0$\leq$x$\leq$0.52) up to its solid solubility limit in FeSe have been grown by an eutectic AlCl${_3}$/KCl molten salt method. The typical lateral size of as-grown Fe$_{1-x}$Co${_x}$Se single crystals is 1$-$5 mm. The chemical composition and homogeneity of the crystals was examined by both inductively coupled plasma atomic emission spectroscopy and energy dispersive spectrometer. X-ray diffraction analysis demonstrates that the crystal lattice parameters $a$ and $c$ are both linearly decreased with increasing Co doping level x. In the whole doping range, all the samples show metallic behaviour in contrast to a metal insulator transition of Cu-doped FeSe according to the resistivity measurements

The doping effects of alkaline earth metal Ba on the quasi-one-dimensional superconductor K 2 Mo 3 As 3 have been systematically investigated. A series of polycrystalline samples (K 1– x Ba x ) 2 Mo 3 As 3 with nominal doping level x from 0 to 0.9 were fabricated. Ba 2+ cations were found to selectively substitute K + cations on the 1 c site, forming a new orderly doping phase (K 0.75 Ba 0.25 ) 2 Mo 3 As 3 , and no intermediate phase was discovered. (K 0.75 Ba 0.25 ) 2 Mo 3 As 3 adopts the same crystal structure as K 2 Mo 3 As 3 with the space group P –6 m 2 (No. 187). Neither magnetic susceptibility nor electrical resistivity measurement reveals bulk superconductivity above 2 K. The absence of superconductivity can be explained by the evolution of energy bands with electron doping, as revealed by the calculation of band structures. The site-selective substitution is also revealed to be a generic behavior in the 233-type CrAs/MoAs-based compounds, which may lead to the finding of similar chemical phases.

Unconventional superconductivity in bulk materials under ambient pressure is extremely rare among the 3d transition metal compounds outside the layered cuprates and iron-based family. It is predominantly linked to highly anisotropic electronic properties and quasi-two-dimensional (2D) Fermi surfaces. To date, the only known example of a Co-based exotic superconductor is the hydrated layered cobaltate, NaxCoO2·yH2O, and its superconductivity is realized in the vicinity of a spin-1/2 Mott state. However, the nature of the superconductivity in these materials is still a subject of intense debate, and therefore, finding a new class of superconductors will help unravel the mysteries of their unconventional superconductivity. Here, we report the discovery of superconductivity at ∼6.3 K in our newly synthesized layered compound Na2CoSe2O, in which the edge-shared CoSe6 octahedra form [CoSe2] layers with a perfect triangular lattice of Co ions. It is the first 3d transition metal oxychalcogenide superconductor with distinct structural and chemical characteristics. Despite its relatively low TC, this material exhibits very high superconducting upper critical fields, μ0HC2(0), which far exceeds the Pauli paramagnetic limit by a factor of 3-4. First-principles calculations show that Na2CoSe2O is a rare example of a negative charge transfer superconductor. This cobalt oxychalcogenide with a geometrical frustration among Co spins shows great potential as a highly appealing candidate for the realization of unconventional and/or high-TC superconductivity beyond the well-established Cu- and Fe-based superconductor families and opens a new field in the physics and chemistry of low-dimensional superconductors.