Session U60: Topological Superconductivity: 3D TIs, Weyl Semimetals and TMDs

Valley Hall transport and superconductivity signatures in twisted transition metal dichalcogenides

Selected by Focus Topic Organizer (Peide Ye)   

Topological linkage for thermoelectricity and superconductivity in their nontrivial topological state

Most thermoelectric materials and topological insulators were made from heavy elements and have narrow band gaps and strong spin-orbit couplings. The nontrivial topological insulating states are the essential base for the topological superconductivity. It remains unclear how the thermoelectric figure of merit performs in the topological state, and whether a thermoelectric material can evolve into a topological superconductor and what is the driving force if the latter happens. By choosing an n-type Bi₂Te_2.7Se_0.3, a highly efficient thermoelectric material and topological insulator, we perform measurements of the thermoelectric and superconducting properties at high pressures. We find that the figure of merit increases with pressure and exhibits a maximum at around 1 GPa where the topological phase transition takes place. After that, superconductivity appears and the critical temperature dramatically increases with increasing pressure in the phase with the nontrivial topological state. Its upper critical field exhibits different temperature dependence compared to the other superconducting phases without topological order. The topological phase transition is proposed to serve the linkage for the high-performance thermoelectricity and topological superconductivity.
  

Theory of Giant In-Plane Critical Field Enhancement in Superconducting Films on a Topological Insulator Surface

Majorana zero modes (MZMs) are of interest due to potential applications in topological quantum computing. Topological insulators (TIs), with spin-momentum locked surface states, may enable emergence of MZMs when coupled to an s-wave superconductor (SC) via the proximity effect. We recently observed a giant enhancement of the in-plane H_c in thin Al, a superconductor with minimal intrinsic spin-orbit coupling (SOC), evaporated onto a 3-dimensional strong TI.¹ Here we present a theoretical analysis of this enhancement. Superconductivity at in-plane fields well in excess of the Chanderasekhar-Clogston (CC) limit is understood as a consequence of strong SOC induced in the thin Al films from the topological surface state through the inverse proximity effect. The CC model is combined with the Maki equation to calculate H_c at different temperatures. The calculated H_c(T) shows excellent quantitative agreement with the experimental result. A relatively large value of the induced spin-orbit scattering rate (in comparison to the superconducting energy gap) is derived, indicating a strong proximity coupling in this TI-SC heterostructure which may be a promising platform for observing MZMs.
¹Z. Lin et. al., unpublished.   

Characterizing the Superconducting State of CuxBi2Se3 Through Muon-Spin Relaxation/Rotation

The discovery of topological superconductors has been a major objective in the field since they were first predicted theoretically. The doped topological insulator Cu_xBi₂Se₃ is a promising candidate for topological superconductivity, but there is much conflicting evidence on the subject. Identifying the superconducting symmetry of the material is paramount in determining whether it has a topologically nontrivial ground state. Muon spin relaxation/rotation (SR) experiments provide a heretofore unexplored method of probing the superconducting state of Cu_xBi₂Se₃. Here, we present SR data collected on a single-crystal sample of Cu_0.3Bi₂Se₃. Measurements conducted in zero-field conditions demonstrate that time reversal symmetry is preserved in the superconducting state, while transverse-field measurements reveal superfluid density behavior that is consistent with p-wave pairing. Such a scenario could support topological superconductivity in Cu_xBi₂Se₃. However, a more conventional s-wave pairing scenario also provides a reasonable fit to the data, making an unambiguous conclusion difficult. Nevertheless, these results represent a valuable addition to the body of experimental data available for Cu_xBi₂Se₃.   

In-plane Critical Field far beyond the Paramagnetic Limit for Thin Al Films on Topological Insulator Surface

Heterostructures of an s-wave superconductor and a strong topological insulator are regarded as a viable platform for the creation of Majorana zero mode (MZM). However, the microscopic understanding of the proximity effect between the superconductor and topological surface state has yet to be fully elucidated. Here we report measurements of the in-plane critical field (H_C//) of thin Al, a superconductor with minimal spin-orbit interaction (SOI), on the surface of (Bi_0.5Sb_0.5)₂TeSe₂ (BSTS). H_C// of the Al films are directly compared with those of simultaneously grown control samples of Al films on SiO₂. Giant enhancement of H_C// beyond the paramagnetic limit was observed for the Al on BSTS. The enhancement diminishes with increasing Al thickness. The H_C//(T) of thin Al on SiO₂ is consistent with the Chanderasekha-Clogston theory, while the H_C//(T) of thin Al on BSTS can be well-described by a theory based the Maki equation invoking strong induced SOI in Al. The results provide direct evidence for an electrically transparent interface and strong proximity effect between the Al and BSTS. The Al/BSTS heterostructure may be a promising system for inducing topological superconductivity and controlled generation of MZMs.   

Uniaxial-Strain Control of Nematic Superconductivity in SrxBi2Se3

Recently, nematic superconductivity, in which the superconducting gap spontaneously lifts the rotational symmetry of the lattice, has been discovered [1]. In nematic superconductivity, multiple superconducting domains with different nematic orientations can exist. These domains can be controlled by a conjugate external stimulus, such as the stain induced by a uniaxial stress with a piezoelectric device [2]. Here, we report for the first time control of the nematic superconductivity and their domains of Sr_xBi₂Se₃, through externally-applied uniaxial stress [3]. The suppression of subdomains indicates that it is the Δ_4y state that is most favored under compression along the basal Bi-Bi bonds. These results provide an inevitable step towards microscopic understanding and future utilization of the unique topological nematic superconductivity.
[1] S. Yonezawa et al., Nat. Phys. 13, 123 (2017).
[2] I. Kostylev, S. Yonezawa, and Y. Maeno, J. Appl. Phys. 125, 082535 (2019).
[3] I. Kostylev et al., arXiv:1910.03252.   

Doping and pressure effects on Weyl semimetal Mo1-xWxTe2

Systematic pressure and doping dependent study on Mo_1-xW_xTe₂ was carried out to investigate the correlation among chemical doping, structural transition, and superconductivity. Doping dependent resistivity measurement at ambient pressure shows structural transition temperature (T_s) for MoTe₂ to be 249 K, which continuously increases as doping level (x) increases and reaches a maximum value of 613 K for WTe₂. The structural transition of WTe₂ at ambient pressure is also confirmed by conducting temperature dependent synchrotron X-ray diffraction from 300 K to 673 K. Three samples with different doping (x = 0.10, 0.40, 0.75) were selected for high pressure measurement. The superconducting transition temperature - pressure (T_c - P) phase diagrams have been constructed and will be discussed. Our experimental results revealed a competition between the structural transition and superconductivity in Mo_1-xW_xTe₂. Superconductivity emerges when the T_s is suppressed below 175 K and T_c increases as T_s is suppressed.   

Theory on superconducting chiral crystal and its applications

Chiral crystals are shown to have nontrivial band topology: at the time reversal invariant momenta, Kramers Weyl points and multi-fold Weyl points can emerge due to the chiral lattice symmetry. In the normal metallic state, the chiral lattice symmetry along with the band topology introduces the magnetoelectric effect and circular photogalvanic effect to modify the electromagnetic properties in the mateirals. However, how does the chiral lattice symmetry and the associated band topology affect the superconducting phase in chiral crystals remains unknown. In this work, we classify the possible pairing phases resulted from chiral lattice symmetry and find various topological superconducting phases. We propose the candidate topological superconducting chiral crystal and study its unique superconducting properties.   

Correlation-driven renormalization of Weyl nodes in the Ce-113 series

We study the electronic properties of the inversion-broken CeTX3 (T = Co, Rh, Ir, X=Si,Ge) heavy-fermion materials through a combination of LDA and LDA+Guzwiller methods. In this class of compounds the electronic mass is enhanced, causing narrowing the Ce f-electron bands. Our LDA+Guzwiller procedure captures these renormalization effects, yielding a mass enhancement that is in good agreement with experimental specific heat measurements. We also track the renormalization of Weyl points by the correlations, which move them closer to the Fermi energy. We briefly comment on the interplay of Weyl physics and pressure-induced superconductivity in these compounds.
  

One-dimensional states residing on edges and steps in few-layer WTe2

WTe₂ is a layered material with rich topological properties. The bulk crystal is a type-II Weyl semimetal and the monolayer is a two-dimensional topological insulator. Recently, it has been predicted that higher order topological insulator state can appear in WTe₂. An observation of 1D, highly conductive channels, known in this case as hinge states, is hindered by the bulk conductivity of WTe₂. Here, we employ the Josephson effect to disentangle the contribution of the hinge states from the bulk in electronic transport. We observe 1D current carrying states on edges and steps in few-layer WTe₂. The width of the states is deduced to be below 100 nm. A supercurrent in them can be measured over distances up to 3 µm and in perpendicular magnetic field up to 2 T. Moreover, the dependence of the supercurrent on the magnetic field is compatible with the asymmetric Josephson effect predicted to occur in topological systems with broken inversion symmetry.   

Probing Proximity Effect in Topological Insulator/Superconductor Heterostructures

Topological insulator (TI)/superconductor heterostructures are predicted to host Majorana zero modes, which are building blocks of topological qubit. In this work, we synthesize high-quality large-area TI/graphene (Gr)/gallium heterostructures combining confinement epitaxy and molecular beam epitaxy. Atomically thin Ga film superconducts at T_{c}~4K [B. Bersh, et. al, arXiv:1905.09938], and the growth of TI preserves its superconductivity. We probe the proximity effect in the TI film using four-terminal transport and tunneling spectroscopy. Hints of a proximity induced gap in the TI film are observed in transport measurements. We demonstrate the efficacy of a clean van der Waals tunnel junction in probing the superconducting gap of thin film NbSe_{2} and present tunneling spectroscopy measurements on TI/Gr/Ga heterostructures.   

Topological superconductivity in proximitized transition metal dichalcogenide Josephson junctions

We theoretically investigate the emergence of topological superconductivity in Josephson junctions (JJ) built of a two-dimensional transition metal dichalcogenide (TMD) layer in proximity to an s-wave superconductor and a magnetic insulator. We show that in a certain range of chemical potential and magnetic proximity strength, Majorana bound states form at the ends of the normal region of a TMD-based JJ. The evolution of the topological superconducting phase as the magnetization direction is varied is also investigated. The effect of the interplay between Ising spins, proximity exchange, and Rashba spin-orbit coupling on the system transition to the topological superconducting phase, as well as the corresponding signatures on the ground state phase and critical current, are analyzed in detail for both phase-fixed and phase-unbiased JJs.   

Superconductivity in Cd3As2 thin films

For the first time we observed superconductivity in Cd₃As₂ thin films without applying external pressure. We studied thin (40 nm -100 nm) films of Cd₃As₂ grown on various substrates by two various methods: thermal evaporation and magnetron sputtering from Cd₃As₂ single crystals. Both Raman spectroscopy and X-ray diffraction on the films show results typical for Cd₃As₂ with tetragonal lattice. A zero-resistance state was registered by the four-probe method on Hall bar shaped samples. Also a zero-resistance plateau was observed in dV/dI characteristics at low currents. The superconducting state was suppressed by applying a magnetic field or increasing the current above critical values H_c and I_c, respectively. H_c perpendicular to the film plane was smaller than the in-plane critical field at the same temperature. Analysis of dependences of critical magnetic field H_c on critical temperature T_c reveals a clearly pronounced linear behavior within the intermediate temperature range, similar to that observed for superconducting bulk Cd₃As₂ and Bi₂Se₃ under pressure.