Candidate For Topological Superconductivity Research Articles

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.

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.

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.

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.

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.

Signature of superconducting onset in presence of large magnetoresistance in type-II Dirac semimetal candidate Ir2In8S

Since the discovery of type-II Dirac semimetal (DSM) as the potential candidate for topological superconductor, magneto-transport studies on diverse type-II DSMs have been of tremendous research interest. Here we report the structural and magneto-transport properties of type-II DSM candidate Ir2In8S under high pressure. With increasing pressure, this shows dramatic suppression of its characteristic large magneto-resistance, which is however partially regained upon release of pressure. No superconductivity has emerged with increasing pressures up to ∼20 GPa. However, in the pressure-released sample a significant resistivity drop below ∼4 K has been detected. The field dependent resistivity and dc magnetization measurements confirm this as superconducting onset. Ir2In8S thus becomes a unique system exhibiting large MR above the superconducting transition. X-ray diffraction results show that the ambient tetragonal structure (P42/mnm) remains stable up to ∼7 GPa, above which this undergoes a reversible structural transition into an orthorhombic structure (Pnnm). The observed enhanced residual resistivity and concurrent increase in carrier density of the normal metal state of the pressure-cycled sample indicate that the enhanced impurity scattering plays a significant role in the emergence of superconductivity.

Non-trivial band topology in the superconductor AuSn4: a first principle study

Topological semimetals such as Weyl or Dirac semimetal with superconductivity have emerged as a new class of topological materials to realize and study Majorana Fermion. This article reports the density functional theory calculated bulk electronic band structure of recently discovered topological superconductor candidate AuSn4. The study has been performed on AuSn4 considering two space groups symmetries viz. Aea2 and Ccce as reported earlier. This study is further extended to the calculation of Z2 invariants. The Fermi surfaces corresponding to the bands, which are responsible for non-trivial band topology along with the surface states are also mapped. The complete study suggests that AuSn4 is a topological semimetal. On AuSn4, it is the first report in the literature showing the non-trivial band topology based on first-principle calculations.

Probing the topological surface states in superconducting Sn4Au single crystal: a magneto transport study

Materials exhibiting bulk superconductivity along with magnetoresistance (MR) in their normal state have emerged as suitable candidates for topological superconductivity. In this article, we report a flux free method to synthesize single crystal of topological superconductor candidate Sn4Au. The phase purity and single crystalline nature are confirmed through various characterizations viz. x-ray diffraction, field emission scanning electron microscopy, selected area electron diffraction, and transmission electron microscopy. Chemical states of the constituent element viz. Sn and Au are analysed through x-ray photoelectron spectroscopy. Superconductivity in synthesized Sn4Au single crystal is evident form ρ-T plot, for which the critical field (H c) is determined through ρ-H plot at 2 K i.e. just below critical temperature T c. A positive MR is observed in ρ-H measurements at different temperatures above T c, viz. at 3 K, 5 K, 10 K and 20 K. Further, the magnetoconductivity (MC) is analysed by using Hikami–Larkin–Nagaoka formalism, which signifies the presence of weak antilocalization (WAL) effect in Sn4Au. Angle dependent magneto-transport measurement has been performed to detect the origin of observed WAL effect in Sn4Au single crystal. Normalized MC vs Hcosθ plot shows presence of topological surface states in the studied system. It is evident that Sn4Au is a 2.6 K topological superconductor.

Strong-coupling superconductivity with Tc ∼ 10.8 K induced by P doping in the topological semimetal Mo5Si3

By performing P doping on the Si sites in the topological semimetal Mo5Si3, we discover strong-coupling superconductivity in Mo5Si3−xPx (0.5 ≤ x ≤ 2.0). Mo5Si3 crystallizes in the W5Si3-type structure with space group of I4/mcm (No. 140), and is not a superconductor itself. Upon P doping, the lattice parameter a decreases while c increases monotonously. Bulk superconductivity is revealed in Mo5Si3−xPx (0.5 ≤ x ≤ 2.0) from resistivity, magnetization, and heat capacity measurements. Tc in Mo5Si1.5P1.5 reaches as high as 10.8 K, setting a new record among the W5Si3-type superconductors. The upper and lower critical fields for Mo5Si1.5P1.5 are 14.56 T and 105 mT, respectively. Moreover, Mo5Si1.5P1.5 is found to be a fully gapped superconductor with strong electron-phonon coupling. First-principles calculations suggest that the enhancement of electron-phonon coupling is possibly due to the shift of the Fermi level, which is induced by electron doping. The calculations also reveal the nontrivial band topology in Mo5Si3. The Tc and upper critical field in Mo5Si3−xPx are fairly high among pseudobinary compounds. Both of them are higher than those in NbTi, making future applications promising. Our results suggest that the W5Si3-type compounds are ideal platforms to search for new superconductors. By examinations of their band topologies, more candidates for topological superconductors can be expected in this structural family.

Optical study on topological superconductor candidate Sr-doped Bi2Se3

Utilizing infrared spectroscopy, we study the charge dynamics of the topological superconductor candidate Sr x Bi2Se3. The frequency-dependent reflectivity R(ω) demonstrates metallic feature and the scattering rate of the free carriers decreases with temperature decreasing. The plasma edge shows a slight blue shift upon cooling, similar to the behavior of Cu x Bi2Se3. As the carrier concentration n obtained by Hall resistivity increases slightly with the decreasing temperature, the effective mass is proved to increase as well, which is in contrast with that of Cu x Bi2Se3. We also perform the ultrafast pump-probe study on the Sr0.2Bi2Se3 compounds. Resembling its parent compound Bi2Se3, three distinct relaxation processes are found to contribute to the transient reflectivity. However, the deduced relaxation times are quite different. In addition, the electron-optical-phonon coupling constant is identified to be λ = 0.88.

Protonation enhanced superconductivity in PdTe2

Electrochemical ionic liquid gating is an effective way to intercalate ions into layered materials and modulate the properties. Here we report an enhanced superconductivity in a topological superconductor candidate PdTe2 through electrochemical gating procedure. The superconducting transition temperature was increased to approximately 3.2 K by ionic gating induced protonation at room temperature. Moreover, a further enhanced superconductivity of both superconducting transition temperature and superconducting volume fraction was observed after the gated samples were placed in a glove box for 2 months. This may be caused by the diffusion of protons in the gated single crystals, which is rarely reported in electrochemical ionic liquid gating experiments. Our results further the superconducting study of PdTe2 and may reveal a common phenomenon in the electrochemical gating procedure.

Topological superconductivity in Rashba spin-orbital coupling suppressed monolayer β-Bi2Pd

The weak interlayer Van Der Waals material β-Bi2Pd has recently been established as a strong topological superconductor candidate with unconventional spin-triplet pairing and Majorana zero modes at vortices. In this article, we study the topological characters and the superconducting pairing, which are still obscure in monolayer β-Bi2Pd, in light of our effective theoretical model. We find that the non-Rashba spin-orbital coupling plays a critical role in realizing and tuning various novel topological natures. In particular, the spin-triplet p-wave superconducting pairing with Majorana zero mode is revealed in monolayer β-Bi2Pd. Our studies deepen the understanding of topology and superconductivity in monolayer β-Bi2Pd and indicate it is a promising platform for achieving low-dimensional topological superconductivity.

Planar Hall effect and large anisotropic magnetoresistance in a topological superconductor candidate Cu0.05PdTe2

We report the observation of a large anisotropic magnetoresistance (AMR) and planar Hall effect (PHE) in a topological superconducting candidate Cu0.05PdTe2. The AMR and PHE data in Cu0.05PdTe2 can be well explained by the semiclassical theory, confirming that the magneto-transport behaviors of the Cu0.05PdTe2 superconductor are related to its topological nature. The AMR ratio in Cu0.05PdTe2 is one order of magnitude larger than those in traditional ferromagnetic metals. The present results suggest that Cu0.05PdTe2 is a promising material in future magnetoresistive devices with low power consumption.

Thickness dependence of quantum transport in the topological superconductor candidate SnTaS2

We present the thickness dependence of magnetotransport study on the mechanically exfoliated topological superconductor candidate SnTaS2. As the thickness decreases, the superconducting transition temperature Tc is gradually suppressed and ultimately out of detection when the thickness is comparable to the superconducting coherence length. The enhanced disorder with the decrease in thickness is expected to play an important role on the suppressed Tc. Furthermore, the distinct weak antilocalization effect is observed in the SnTaS2 nanoflakes, and the temperature-dependent phase coherence length extracted from weak antilocalization agrees with strong electron–phonon scattering in the sample. Our results provide insight into the electronic properties in the low dimensional limit of topological superconductor candidate SnTaS2.

Superconductivity in doped Weyl semimetal Mo0.9Ir0.1Te2 with broken inversion symmetry

This work presents the emergence of superconductivity in Ir—doped Weyl semimetal T d —MoTe2 with broken inversion symmetry. Chiral anomaly induced planar Hall effect and anisotropic magneto-resistance confirm the topological semimetallic nature of MoIr x Te2. Observation of weak anisotropic, moderately coupled type-II superconductivity in T d -MoIr x Te2 makes it a promising candidate for topological superconductor.

Resonance from antiferromagnetic spin fluctuations for superconductivity in UTe2.

Superconductivity originates from the formation of bound (Cooper) pairs of electrons that can move through the lattice without resistance below the superconducting transition temperature Tc (ref. 1). Electron Cooper pairs in most superconductors form anti-parallel spin singlets with total spin S = 0 (ref. 2), although they can also form parallel spin-triplet Cooper pairs with S = 1 and an odd parity wavefunction3. Spin-triplet pairing is important because it can host topological states and Majorana fermions relevant for quantum computation4,5. Because spin-triplet pairing is usually mediated by ferromagnetic (FM) spin fluctuations3, uranium-based materials near an FM instability are considered to be ideal candidates for realizing spin-triplet superconductivity6. Indeed, UTe2, which has a Tc ≈ 1.6 K (refs. 7,8), has been identified as a candidate for achiral spin-triplet topological superconductor near an FM instability7-14, although it also has antiferromagnetic (AF) spin fluctuations15,16. Here we use inelastic neutron scattering (INS) to show that superconductivity in UTe2 is coupled to a sharp magnetic excitation, termed resonance17-23, at the Brillouin zone boundary near AF order. Because the resonance has only been found in spin-singlet unconventional superconductors near an AF instability17-23, its observation in UTe2 suggests that AF spin fluctuations may also induce spin-triplet pairing24 or that electron pairing in UTe2 has a spin-singlet component.