Candidate For Superconductivity Research Articles

Single-gap isotropic $s-$wave superconductivity in single crystals AuSn$_4$

London, \lambda_L (T)λL(T), and Campbell, \lambda_{C} (T)λC(T), penetration depths were measured in single crystals of a topological superconductor candidate AuSn_44. At low temperatures, \lambda_L (T)λL(T) is exponentially attenuated and, if fitted with the power law, \lambda(T) \sim T^nλ(T)∼Tn, gives exponents n>4n>4, indistinguishable from the isotropic single s-s−wave gap Bardeen-Cooper-Schrieffer (BCS) asymptotic. The superfluid density fits perfectly in the entire temperature range to the BCS theory. The superconducting transition temperature, T_c = 2.40 ± 0.05Tc=2.40±0.05 K, does not change after 2.5 MeV electron irradiation, indicating the validity of the Anderson theorem for isotropic s-s−wave superconductors. Campbell penetration depth before and after electron irradiation shows no hysteresis between the zero-field cooling (ZFC) and field cooling (FC) protocols, consistent with the parabolic pinning potential. Interestingly, the critical current density estimated from the original Campbell theory decreases after irradiation, implying that a more sophisticated theory involving collective effects is needed to describe vortex pinning in this system. In general, our thermodynamic measurements strongly suggest that the bulk response of the AuSn_44 crystals is fully consistent with the isotropic s-s−wave weak-coupling BCS superconductivity.

Visualization of Unconventional Rashba Bands and Vortex Zero Mode in the Topological Superconductor Candidate AuSn4.

The noble metal alloy AuSn4 has recently been identified as an intrinsic surface topological superconductor, promisingly hosting the Majorana zero mode (MZM) for topological quantum computing. However, the atomic visualization of its nontrivial surface states and MZM remains elusive. Here, we report the direct observation of unconventional surface states and vortex zero mode in AuSn4 by scanning tunneling microscopy/spectroscopy. Unlike the trivial metallic bulk states of Sn-terminated surfaces, the Au-terminated surfaces exhibit pronounced surface states near the Fermi level, arising from unconventional Rashba bands characterized by shared helical spin textures. In the superconducting state, the Sn-terminated surfaces exhibit conventional Caroli-de Gennes-Matricon bound states, while the Au-terminated surfaces display sharp zero-energy core states resembling MZMs in a nonquantum-limit condition. This distinction may result from the dominant contribution of unconventional Rashba bands on the Au-terminated surface. Our results provide a new platform for studying termination-dependent topological surface states and MZM in noble-metal-based superconductors.

Stabilization of U 5f2 configuration in UTe2 through U 6d dimers in the presence of Te2 chains

We investigate the topological superconductor candidate UTe2 using high-resolution valence-band resonant inelastic x-ray scattering at the U M4,5 edges. We observe atomiclike low-energy excitations that support the correlated nature of this unconventional superconductor. These excitations originate from the U 5f2 configuration, which is unexpected since the short Te2-Te2 distances exclude Te2 being 2−. By utilizing the photoionization cross-section dependence of the photoemission spectra in combination with band structure calculations, we infer that the stabilization of the U 5f2 configuration is due to the U 6d bonding states in the U dimers acting as a charge reservoir. Our results emphasize that the description of the physical properties should commence with a 5f2 ansatz. Published by the American Physical Society 2024

Evidence of strong and mode-selective electron–phonon coupling in the topological superconductor candidate 2M-WS2

The interaction between lattice vibrations and electrons plays a key role in various aspects of condensed matter physics — including electron hydrodynamics, strange metal behavior, and high-temperature superconductivity. In this study, we present systematic investigations using Raman scattering and angle-resolved photoemission spectroscopy (ARPES) to examine the phononic and electronic subsystems of the topological superconductor candidate 2M-WS2. Raman scattering exhibits an anomalous nonmonotonic temperature dependence of phonon linewidths, indicative of strong phonon–electron scattering over phonon–phonon scattering. The ARPES results demonstrate pronounced dispersion anomalies (kinks) at multiple binding energies within both bulk and topological surface states, indicating a robust and mode-selective coupling between the electronic states and various phonon modes. These experimental findings align with previous calculations of the Eliashberg function, providing a deeper understanding of the highest superconducting transition temperature observed in 2M-WS2 (8.8 K) among all transition metal dichalcogenides as induced by electron–phonon coupling. Furthermore, our results may offer valuable insights into other properties of 2M-WS2 and guide the search for high-temperature topological superconductors.

A recipe for an effective selection of promising candidates for high-temperature superconductors among binary hydrides

Recent research on compressed binary hydrides have unveiled the potential for achieving superconductivity at near-room-temperature. Nevertheless, the available decision-making procedures standing behind the selection of constituent elements that may potentially exhibit high values of critical temperature (Tc) are far from optimal. In other words, a lot of experimental and numerical effort is wasted on exploring unpromising compounds. By conducting a deep study of a database containing over 580 binary hydride superconductors, we were able to observe some interesting relationships between Tc and selected physico-chemical properties of examined compounds. Among studied parameters, the ratio of the sum of the molecular weight of heavier atoms to the total mass of all hydrogen atoms in the chemical formula of hydride (MX/MH) was found to be the most valuable indicator that can help to screen for new promising superconductor candidates. This is because the highest Tc requires the lowest MX/MH ratio. Statistical analysis indicates a 28% chance of finding Tc > 200 K within 0 <MX/MH < 15. It is expected that these findings will not only allow for more efficient use of resources by improving future superconductor candidates selection but also they will accelerate ongoing experimental and numerical research that should bring new exciting discoveries in a much shorter time.

Quantum Interference between Quasi-2D Fermi Surface Sheets in UTe_{2}.

UTe_{2} is a spin-triplet superconductor candidate for which high quality samples with long mean free paths have recently become available, enabling quantum oscillation measurements to probe its Fermi surface and effective carrier masses. It has recently been reported that UTe_{2} possesses a 3D Fermi surface component [Phys. Rev. Lett. 131, 036501 (2023)PRLTAO0031-900710.1103/PhysRevLett.131.036501]. The distinction between 2D and 3D Fermi surface sections in triplet superconductors can have important implications regarding the topological properties of the superconductivity. Here we report the observation of oscillatory components in the magnetoconductance of UTe_{2} at high magnetic fields. We find that these oscillations are well described by quantum interference between quasiparticles traversing semiclassical trajectories spanning magnetic breakdown networks. Our observations are consistent with a quasi-2D model of this material's Fermi surface based on prior dHvA-effect measurements. Our results strongly indicate that UTe_{2}-which exhibits a multitude of complex physical phenomena-possesses a remarkably simple Fermi surface consisting exclusively of two quasi-2D cylindrical sections.

Two-dimensional superconductivity in hexagonal layered Na2B2H structure at ambient pressure

Alkaline metal and alkaline earth metal borohydrides may provide ideal candidates for high-temperature superconductors. By replacing the metal element of the layered hexagonal structure Li2B2H in our earlier work, a novel energetically metastable hexagonal Na2B2H, designated as h-Na2B2H, was proposed at ambient condition. First-principles calculations suggest that h-Na2B2H may be prepared from Na3B20, Na, and H2 or from the pure elements by utilizing high-temperature and high-pressure technology. The h-Na2B2H exhibits the highest tensile stress of 24.2 GPa at a strain of 0.245 along the [1–10] direction among the four directions [100], [001], [110], and [1–10]. Electronic property calculations reveal that h-Na2B2H holds two-dimensional conductivity along B atom layers. Electron–phonon coupling calculation indicates h-Na2B2H has a Tc of 42.4 K with λ = 1.73. The superconductivity in h-Na2B2H primarily depends on the Na and B atomic vibrations. Thus, h-Na2B2H can be a potential superconducting material with 2D conductivity at atmosphere pressure.

The observation of π-shifts in the Little-Parks effect in 4Hb-TaS2

Finding evidence of non-trivial pairing states is one of the greatest experimental challenges in the field of unconventional superconductivity. Such evidence requires phase-sensitive probes susceptible to the internal structure of the order parameter. We report the measurement of the Little-Parks effect in the unconventional superconductor candidate 4Hb-TaS2. In half of our rings, which are fabricated from single-crystals, we find a π-shift in the transition-temperature oscillations. According to theory, such a π-shift is only possible if the order parameter is non-s-wave. In the absence of crystallographic defects, the shift provides evidence of a multi-component order parameter. Thus, this observation increases the likelihood of the two-component order parameter scenario in 4Hb-TaS2. Furthermore, we show that Tc is enhanced as a function of the out-of-plane field when a constant in-plane field is applied, which we explain using a two-component order-parameter.

Inhomogeneous high temperature melting and decoupling of charge density waves in spin-triplet superconductor UTe2

Charge, spin and Cooper-pair density waves have now been widely detected in exotic superconductors. Understanding how these density waves emerge — and become suppressed by external parameters — is a key research direction in condensed matter physics. Here we study the temperature and magnetic-field evolution of charge density waves in the rare spin-triplet superconductor candidate UTe2 using scanning tunneling microscopy/spectroscopy. We reveal that charge modulations composed of three different wave vectors gradually weaken in a spatially inhomogeneous manner, while persisting to surprisingly high temperatures of 10–12 K. We also reveal an unexpected decoupling of the three-component charge density wave state. Our observations match closely to the temperature scale potentially related to short-range magnetic correlations, providing a possible connection between density waves observed by surface probes and intrinsic bulk features. Importantly, charge density wave modulations become suppressed with magnetic field both below and above superconducting Tc in a comparable manner. Our work points towards an intimate connection between hidden magnetic correlations and the origin of the unusual charge density waves in UTe2.

Multilayer neural network models for critical temperature of cuprate superconductors

A multi-layer neural network is used to extract the value of a superconductor’s Tc of cuprate from two models. The first model extracts Tc from the structure’s chemical composition whereas the second model extracts Tc from the structure’s chemical composition and the lattice parameters. The back-propagation algorithm is used to find the empirical equation of Tc. It can calculate error signals and redistribute backward propagation signals. This paper studies four systems of cuprate superconductors: YBaCuO, BiSrCaCuO, TlBaCaCuO, and HgBaCaCuO. In the first model, the Tc of four high-temperature oxide superconductors are calculated as a function of eight parameters and one output, which is Tc. Although the same output is produced in the second model, it is produced as a function of eleven parameters. The eight parameters are superconductor type number (Bi2212, Bi2223, Hg1201, Hg1212, Hg1223, Y123, Y124, Y247, Tl1223, Tl2212, Tl2223), first component composition, second component composition, third component composition, fourth component composition, atomic number of doping type, doping composition, and oxygen composition of the first model. The previous parameters with the three lattice parameters a, b and c are used in the second model. The trained deep learning models have shown a high degree of performance in matching the trained distributions. After analysing the results, we deduce electronegativity plays an important role in increasing Tc of cuprate superconductors. Using the obtained Tc prediction model, the scope is expanded to include the eleven unexplored multi-element materials. Candidates for superconductors with a higher Tc that can be synthesized are proposed. By comparison with other models of machine learning, the suggested models in this paper give the highest Tc for predicting new cuprate superconductors.

Larger in-plane upper critical field and superconducting diode effect observed in topological superconductor candidate InNbS2 nanoribbons

Larger in-plane upper critical field and superconducting diode effect observed in topological superconductor candidate InNbS2 nanoribbons

Three-dimensional electronic structure of the superconductor Sn4Sb3 by angle-resolved photoemission spectroscopy

The layered material Sn4Sb3 exhibits superconductivity with 1.47 K and is proposed to be a topological superconductor candidate. In this study, we investigate the electronic structure of Sn4Sb3 using angle-resolved photoemission spectroscopy and density functional theory (DFT) calculations. Despite its layered structure, the band structure of Sn4Sb3 shows strong kz dependence, leading to the formation of a three-dimensional Fermi surface. The electronic bands exhibit three-fold symmetry at most kz planes and six-fold symmetry at the Γ and Z planes. These observations are consistent with DFT calculations, except for the presence of additional flat-like bands located 500 meV below the Fermi level. The photon energy dependence measurement show noticeable kz dispersion in one of the splitted branches, suggesting a bulk origin of the feature, and negligible kz dispersion in another branch, implying the surface origin of the state.

Superconducting ternary hydrides: progress and challenges.

Since the discovery of the high-temperature superconductors H3S and LaH10 under high pressure, compressed hydrides have received extensive attention as promising candidates for room-temperature superconductors. As a result of current high-pressure theoretical and experimental studies, it is now known that almost all the binary hydrides with a high superconducting transition temperature (T c) require extremely high pressure to remain stable, hindering any practical application. In order to further lower the stable pressure and improve superconductivity, researchers have started exploring ternary hydrides and had many achievements in recent years. Here, we discuss recent progress in ternary hydrides, aiming to deepen the understanding of the key factors regulating the structural stability and superconductivity of ternary hydrides, such as structural motifs, bonding features, electronic structures, electron-phonon coupling, etc. Furthermore, the current issues and challenges of superconducting ternary hydrides are presented, together with the prospects and opportunities for future research.

Point-contact Andreev reflection measurements on ZrRuAs single crystals

ZrRuAs single crystals with hexagonal crystal structure, which is a kind of non-centrosymmetric superconductor with relatively high transition temperature, is predicted to be a topological superconductor candidate. To explore the exotic properties of ZrRuAs, the superconducting gap of ZrRuAs needs to be measured. In this work, point-contact Andreev reflection (PCAR) spectroscopy is used to investigate the superconductivity of ZrRuAs single crystals. The double conductance peaks in the PCAR spectra are observed along different crystal directions. The superconducting gap can be quantitatively analyzed by using the modified Blonder–Tinkham–Klapwijk model based on the s-wave superconductivity. The temperature dependence of the extracted superconducting gap follows the Bardeen–Cooper–Schrieffer theory. Our results suggest the isotropic s-wave superconductivity in ZrRuAs.

Antiferromagnetic Spin Fluctuations and Unconventional Superconductivity in Topological Superconductor Candidate YPtBi Revealed by ^{195}Pt-NMR.

We report ^{195}Pt nuclear magnetic resonance (NMR) measurements on topological superconductor candidate YPtBi, which has broken inversion symmetry and topological nontrivial band structures due to the strong spin-orbit coupling. In the normal state, we find that Knight shift K is field- and temperature independent, suggesting that the contribution from the topological bands is very small at low temperatures. However, the spin-lattice relaxation rate 1/T_{1} divided by temperature (T), 1/T_{1}T, increases with decreasing T, implying the existence of antiferromagnetic spin fluctuations. In the superconducting state, no Hebel-Slichter coherence peak is seen below T_{c} and 1/T_{1} follows T^{3} variation, indicating the unconventional superconductivity. The finite spin susceptibility at zero-temperature limit and the anomalous increase of the NMR linewidth below T_{c} point to a mixed state of spin-singlet and spin-triplet (or spin-septet) pairing.

Differences in the resistive and thermodynamic properties of the single crystalline chiral superconductor candidate SrPtAs

The locally noncentrosymmetric superconductor SrPtAs is proposed to host a topological chiral $d$-wave state, but experimental reports have been limited to polycrystalline samples. Here we report the synthesis of single crystalline SrPtAs grown from Pb flux. SrPtAs crystallizes in the hexagonal space group $P{6}_{3}/mmc$ with lattice parameters $a=4.2445(4)$ \AA{} and $c=8.9513(18)$ \AA{}. Magnetic susceptibility and electrical resistivity measurements reveal a superconducting transition at ${T}_{c}\phantom{\rule{4pt}{0ex}}\ensuremath{\sim}2.2$ K, in agreement with previous reports on polycrystalline samples. Surprisingly, heat capacity data show only a small bulk transition at 0.7 K. We discuss the possible origins of the discrepancy between the various measurements.

Superconducting Gap Structure of the Noncentrosymmetric Topological Superconductor Candidate HfRuP

We investigate the gap symmetry of the topological superconductor candidate HfRuP, which crystallizes in a noncentrosymmetric hexagonal crystal structure, using muon spin rotation/relaxation (μSR) measurements in transverse-field (TF) geometry. The temperature and magnetic field dependencies of the superconducting relaxation rate derived from the TF-μSR spectra can be well described by an isotropic s-wave gap. The superconducting carrier density ns = 1.41(1) × 1026 m−3 and the magnetic penetration depth, λ(0) = 603(2) nm, were calculated from the TF-μSR data. Interestingly, the ratio between the superconducting transition temperature and the superfluid density, Tc/λ−2(0) ∼ 3.3, is very close to those of unconventional superconductors. Further, our zero-field (ZF) μSR results do not show any considerable change in the muon spin relaxation above and below the superconducting transition temperature, suggesting that time-reversal symmetry is preserved in the superconducting state of this superconductor.

Designing ternary superconducting hydrides with A15-type structure at moderate pressures

Ever since hydrogen-based superconductors have been actively considered as candidates for room-temperature superconductors, binary hydrides have been almost completely explored theoretically. Now researchers are turning their interest to ternary hydrides. However, the enormous compositional and structural possibilities in the ternary hydride space limit the search efficiency. Here, using an advanced crystal structure search method combined with first-principles calculations, a series of ternary hydrides AXH6 (MgPH6, MgAsH6, CaSbH6, CaSnH6, NaSbH6 and ScSiH6) with the A15-type structure are predicted to be stable at around one million atmospheres. Among them, CaSnH6 presents the lowest thermodynamically stable pressure of 110 GPa. CaSbH6, CaSnH6 and ScSiH6 may be dynamically stable down to 41 (70), 71 (90) and 86 (70) GPa, respectively, without (with) anharmonic effects and temperature effects. Hydrogen atoms in these A15-type structures are found to form weak covalent bonds to X atoms. More interestingly, we find that ternary hydrides with the AXH6 stoichiometry may assume the A15-type structure under pressure when the sum of the chemical valences of the two non-hydrogen atoms is six or seven. Based on electron-phonon coupling (EPC) calculations, all these ternary hydrides are estimated to be superconductors. CaSbH6, CaSnH6 and ScSiH6 have superconducting transition temperatures (Tcs) of 65, 53 and 116 K at 45, 75 and 90 GPa, respectively, while MgPH6 has the highest Tc of 148 K at 135 GPa. Our study will provide guidance for designing high Tc ternary hydrides near ambient pressures.

The Potential Superconducting Materials in MRI Scanner—Comparison between NbTi and MgB2

Although the technology used in commercial MRI scanners is in a very mature state, there are still many reasons for researchers to keep exploring how it can be optimized, such as the pursuit of higher image quality and greater patient comfort while keeping costs low. Changing the material of the superconducting magnets in MRI scanners is one of these ways. Currently, NbTi is the most commonly used superconducting magnet in MRI scanners, while in situ MgB2 is one of the hottest candidates for MRI superconductors. This paper presents the advantages of NbTi being able to be realistically installed in MRI scanners for many years and also analyzes the match of in situ MgB2 to various requirements of superconducting magnets in MRI scanners. The comparison shows that it is still NbTi that is more advantageous in recent years to come. MgB2, on the other hand, does almost match the requirements of commercial MRI scanners, if cost is not considered. However, this does not mean that MgB2 can be installed in commercial MRI scanners, and a lot of technical modifications and developments are still needed to adapt MgB2 to commercial MRI scanners before that.

Anomalous superconducting proximity effect of planar Pb–RhPb2 heterojunctions in the clean limit

Interest in superconducting proximity effect has been revived by the exploitation of Andreev states and by the possible emergence of Majorana bound states at the interface. Spectroscopy of these states has been so far restricted to just a handful of superconductor-metal systems in the diffusion regime, whereas reports in otherwise clean superconductor-superconductor heterojunctions are scarce. Here, we realize molecular beam epitaxy growth of atomically sharp planar heterojunctions between Pb and a topological superconductor candidate RhPb2 that allows us to spectroscopically image the proximity effect in the clean limit. The measured energy spectra of RhPb2 vary with the spatial separation from proximal Pb, and exhibit unusual modifications in the pairing gap structure and size that extend over a distance far beyond the coherence length. This anomalously long-range proximity (LRP) effect breaks the rotational symmetry of Cooper pair potential in real space and largely deforms the Abrikosov vortex cores. Our work opens promising avenues for fundamental studies of the Andreev physics and extraordinary states in clean superconducting heterojunctions.