Session Y01: Topological Superconductivity: Intrinsic and Bulk

Controlling two-phase superconductivity with pressure in CeRh2As2

CeRh₂As₂ has a unique superconducting phase diagram with two superconducting states and a critical temperature of T_c = 0.3 K. In our current understanding, a magnetic field drives a transition from a superconducting state of even parity at low fields to one of odd parity at high fields. This phenomenon is likely based on the crystal structure with locally broken inversion symmetry in the Ce layers and two Ce layers related by global inversion symmetry. Even in the case of pure singlet pairing within the layer an odd-parity superconducting gap function can be reached by a sign change of the gap between the two Ce layers. This way of creating odd-parity superconductivity is different from the traditional one - where odd parity comes from the pairing function itself - and might be a direction of finding more candidate materials of odd-parity superconductivity. Remarkably, the odd-parity state is predicted to be a topological crystalline superconducting state. In order to guide the search for new topological superconductors with higher transition temperatures, we need to understand, which material parameters favour is appearance.

Here, I will give an overview on the current experimental progress on CeRh₂As₂ including phase diagrams covering the normal state and the angle dependence. I will also show results where we use pressure to control the superconducting states by tuning the normal state properties. We find that superconductivity is fostered by correlations in proximity to a quantum critical point. Furthermore, pressure effectively shifts the parity transition to lower fields.   

Polar Kerr effect measurements of half-Heusler compound YPtBi

Strong spin-orbit coupling in the half-Heusler compound YPtBi leads to a topologically protected surface state and to unconventional superconductivity in the bulk. It was recently suggested [arXiv:2212.09786] that the surface state in this compound could host a superconducting ground state which breaks time reversal symmetry and is independent from the bulk superconductivity. In my talk, I will show polar Kerr effect and scanning tunneling microscopy measurements in search of this surface superconductivity, its order parameter and its interplay with the bulk superconducting ground state.   

Strain-gradient driven superconductivity in GdAuGe hexagonal Heusler membranes

Recent experiments and theory suggest that the rare earth half-Heusler compounds RPtBi [1] and their hexagonal cousins such as SrPtAs [2] are spin triplet superconductors. However, the low T_C of 0.77 K for YPtBi [1] prohibits their study via spectroscopic probes that would help to understand the pairing, e.g., STM and ARPES. Here, starting from single crystalline membranes of a similar hexagonal Heusler GdAuGe, we found that large strain gradient induces superconductivity in this material when rippled. Unstrained GdAuGe is antiferromagnetic below ~17 K and remains antiferromagnetic with homogeneous strain. When subjected to extreme strain gradients, GdAuGe shows superconductivity below∼4 K, as detected via Meissner effect, and a critical field phase diagram congruent with a type II superconductor. The presence of coexisting antiferromagnetic ordering is consistent with the superconductivity in non-centrosymmetric half-Heuslers YPtBi [1] and HoPdBi [3] and the Fe based superconductors [4] where the ordered state destabilizes into a configuration that support spin fluctuations and leads to cooper pairing. Our results emphasize the potential of strain gradient engineering in membranes of quantum material in accessing new phases which are otherwise inaccessible in the material.

1.T. Takenaka et al, Science Advances, 7, 12, (2021).

2.M. H. Fischer et al, Phys. Rev. B 89,020509(R) (2014)

3.A M Nikitin et al, 2015 J. Phys.: Condens. Matter 27

4.C. Liu et al, Phys. Rev. Lett. 128, 137003 (2022).   

Doping induced superconductivity in topological insulator SnBi2Te4

Topological superconductors (TSCs) can host Majorana bound states and are promising platforms for quantum computation. TSCs can be realized by constructing artificial heterostructures with a topological insulator in proximity to an s-wave superconductor. Another approach is through the synthesis or discovery of materials with both nontrivial topological electronic structures and superconducting properties. A recent report showed that the topological insulator SnBi₂Te₄ becomes a bulk superconductor when substituting Sn with In [1]. The superconducting critical temperature of the indium doped compound was raised to 1.85K as the indium ratio x is increased to 0.61 in Sn_1-xIn_xBi₂Te₄. Here we use scanning tunneling microscope (STM) to investigate in situ cleaved Sn_0.4In_0.6Bi₂Te₄ at 40mK. The Te-terminated surface shows a single U-shaped superconducting gap with size of 0.3meV, as well as Dirac bands outside the gap. The evolution of a vortex lattice with the magnetic field and spatial distribution of zero-bias conductance near a vortex core indicate a weak-coupling s-wave BCS superconductor, with coherence length of 17nm. Adjusting doping is required so that the Dirac point is located near the Fermi level and the topological surface states are properly gapped by the induced bulk superconductivity.

[1] M. A. McGuire et al, Phys. Rev. Materials 7, 034802 (2023)   

Anomalous transverse resistance in the topological superconductor β-Bi2Pd

In topological materials, there are topological surface states with chirality.  Certain topological materials are also topological triplet superconductors, such as β-Bi₂Pd [1,2], which host spin-polarized surface states as reported by angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy studies [3,4].  In this work, we investigate the transverse voltage to the applied magnetic field within the vortex phase to the upper critical field H_C2 [5]. The transverse drop is not odd-symmetric but even-symmetric to the applied magnetic field.  The sign of the transverse voltage is dictated not by the polarity of the magnetic field, but by the broken inversion symmetry at the interfaces. This key feature indicates the intrinsic chirality in the topological surface states in β-Bi₂Pd as revealed by transport measurements.

[1] Y Li, X Xu, M.-H Lee, M.-W Chu, C. L. Chien, Science 366, 238 (2019).

[2] X Xu, Y Li, C. L. Chien, arXiv:2210.08733 (2022).

[2] Y.-F Lv et al., Sci. Bull. 62, 852 (2017).

[3] K. Iwaya et al., Nat. Commun. 8, 976 (2017).

[4] X. Xu, Y. Li, C. L. Chien, Nat. Commun. 13, 5321 (2022).   

Topological superconducting pairing in a Chiral Kramers Weyl semimetal

Topological superconductors present an ideal platform for exploring nontrivial superconductivity and realizing Majorana boundary modes in materials. However, finding a single-phase topological material with nontrivial superconducting states is a challenge. Here, we predict nontrivial superconductivity in the pristine chiral metal RhGe with a transition temperature of 5.8 K. Chiral symmetries in RhGe enforce multifold Weyl fermions at high-symmetry momentum points and spin-polarized Fermi arc states that span the whole surface Brillouin zone. These bulk and surface chiral states support multiple type-II van Hove singularities that enhance superconductivity in RhGe. Our detailed analysis of superconducting pairing symmetries involving Chiral Fermi pockets in RhGe indicates the presence of nontrivial superconducting pairing. Our study establishes RhGe as a promising candidate material for hosting mixed-parity pairing and topological superconductivity.   

Topological surface states host superconductivity induced by the bulk condensate in YRuB2

Although the idea of achieving topological superconductivity (TSC) in hybrid heterostructures of superconductors and systems with topologically nontrivial band structures has been proposed, the realization of TSC in a single stoichiometric material is most desired for fundamental experimental exploration of TSC and its device applications. In this context, based on electronic structure calculations, it was recently proposed that YRuB₂ is a candidate TSC that also hosts special hourglass-type Dirac rings protected by symmetry. Bulk measurements on YRuB₂ detect a single superconducting gap of ∼ 1 meV. We performed surface-sensitive Andreev reflection spectroscopy on YRuB₂ and detected the bulk superconducting gap as well as another superconducting gap of ∼ 0.5 meV. From our analysis of electronic structure, we show that the smaller gap is formed in the topological surface states in YRuB₂ due to the proximity of the bulk superconducting condensate. Therefore, we present YRuB₂ as a unique system where the interaction between topological surface states and bulk superconductivity leads to novel physical insights in understanding the candidate topological superconductors.   

Understanding heterogeneities in topological superconductor FeTe0.55Se0.45

We have investigated the role of heterogeneities in the intrinsic topological superconductor FeTe_0.55Se_0.45 single crystal, using scanning tunneling microscopy. High spatial and spectral resolutions at ultra-low temperature (40 mK) and under high magnetic field (9-2-2 T) allow us to study putative Majorana bound states (MBS) at the vortex core. Isolated MBS are revealed at vortex core and distinguished from complex in-gap states near zero bias. The relatively large superconducting gap of FeTe_0.55Se_0.45 enables us to evaluate how Caroli-de Gennes-Matricon states suppress superconductivity at the surface steps, and how subsurface defects give rise to in-gap states in high magnetic field. The understanding of heterogeneities in intrinsic topological superconductors will facilitate the realization and control of MBS for creating error-tolerant qubits.   

Pressure induced dimensional crossing in a quasi-one-dimensional superconductor

Low-dimensional superconductors have been a fertile playground for the research of exotic superconductivity. In this talk, I will present our recent work on the effect of uniaxial strain and hydrostatic pressure on a quasi-one-dimensional topological superconductor candidate. When the tensile strain up to 1.2% is applied on the one-dimensional chain direction, broadening of the superconductor transition was observed with the superconducting temperature Tc remaining almost the same. When hydrostatic pressure up to 1.65 GPa is applied, the T_c first remains the same and then slowly decreases. Measurements have shown a strong H_c2 anisotropy at ambient pressure while upon the increasing pressure,  the anisotropy was hugely suppressed with an isotropic nature. We will discuss the origin of this dimensional cross and its implication on possible topological superconductivity.    

Searching for ideal topological crystalline insulators and topological superconductors in Pb-Sn-In-Te system

The discovery of 3D topological insulator  materials and topological superconductor  open  up a new research field in the condensed matter physics.  In order to search for the topological superconductor, we have grown a large number of the single crystals of  Pb-system ( Pb-Sn-In-Te) topological crystalline insulator  and their topological superconductor .  We have measured the physical properties on these single crystals by various techniques. We have studied the effect of crystal growth condition, impurity and composition on the bulk electrical conductivity of these single crystals. We try to find out which composition and crystal growth condition is the best for the ideal topological insulator,  topological crystalline insulator and  topological superconductor.  We have got the bulk topological superconductor  with T_c=5K.   

Anomalous Hall effect and two-dimensional Fermi surfaces in the charge-density-wave state of kagome metal RbV3Sb5

Kagome metals AV₃Sb₅ (A = K, Rb, and Cs) are recently discovered platforms featuring an unusual charge-density-wave (CDW) order (T_CDW~80-100K) and superconductivity (T_c~ 0.9-2.5 K). Recently, some evidences show that the time-reversal symmetry is broken in the CDW phase and the anomalous Hall effect (AHE) has been observed in KV₃Sb₅ and CsV₃Sb₅ inside the novel CDW phase. In this talk, I will report a comprehensive study of a high-quality RbV₃Sb₅ single crystal with magnetotransport measurements, which demonstrate the emergence of the AHE in RbV₃Sb₅ when the CDW state develops. The magnitude of the anomalous Hall resistivity in the low temperature limit is comparable to the reported values in KV₃Sb₅ and CsV₃Sb₅. The magnetoresistance channel further reveals a rich spectrum of quantum oscillation frequencies. In particular, a large quantum oscillation frequency (2235 T), which occupies ~ 56% of the Brillouin zone area was revealed, further angle-dependent measurement unequivocally indicates the presence of two-dimensional Fermi surfaces in RbV₃Sb₅. Our results provide indispensable information for understanding the AHE and band structure in kagome metal AV₃Sb₅.   

Chiral Superconductivity from Loop-Current Chern Metal in Kagome Superconductor AV3Sb5 (A=K,Rb,Cs)

Recently, extensive study on the Kagome metals and superconductors AV3Sb5 (A=K,Rb,Cs) have revealed fascinating correlated quantum states. Central to the discovery is the evidence for Time-Reversal Symmetry (TRS) breaking in both the Charge Density Wave (CDW) metal state and the superconducting state. In this work, we study the superconducting state derived from the loop-current CDW metal. The loop current CDW was proposed to account for evidence of TRS breaking and has features in good agreement with experimental observations of small Fermi pockets. Here we carry out instability analysis in the pairing channel using Ginzburg-Landau theory and found loop current drive a chiral topological superconductor. The physical reason behind the TRS breaking pairing is attributed to the complex Josephson scattering among Fermi pockets induced by loop current. The results are further confirmed through self-consistent mean-field calculation.   

Thermal Hall measurements to detect spontaneous thermal Hall effect in kagome superconductor CsV3Sb5

Chiral superconductivity, breaking time-reversal symmetry, has attracted attentions as a topological superconductor characterized by bulk quasiparticle bands with non-trivial topological numbers. Whereas some candidates of chiral superconductors are suggested mainly by µSR and Kerr effect measurements, verification by bulk measurements is required. A prominent feature can be used to confirm chiral superconductivity is to investigate the appearance of the spontaneous thermal Hall effect at the zero field at the superconducting transition. In this talk, I shall introduce our trials to detect the spontaneous thermal Hall effect in the kagome superconductor CsV₃Sb₅.