Spin-Orbit-Parity-Coupled Superconductivity in Topological Monolayer WTe_{2}

Quantum-Geometric Origin of Out-of-Plane Stacking Ferroelectricity

Stacking ferroelectricity (SFE) has been discovered in a wide range of van der Waals materials and holds promise for applications, including photovoltaics and high-density memory devices. We show that the microscopic origin of out-of-plane stacking ferroelectric polarization can be generally understood as a consequence of a nontrivial Berry phase borne out of an effective Su-Schrieffer-Heeger model description with broken sublattice symmetry, thus elucidating the quantum-geometric origin of polarization in the extremely nonperiodic bilayer limit. Our theory applies to known stacking ferroelectrics such as bilayer transition-metal dichalcogenides in 3R and T_{d} phases, as well as general AB-stacked honeycomb bilayers with staggered sublattice potential. Our explanatory and self-consistent framework based on the quantum-geometric perspective establishes quantitative understanding of out-of-plane SFE materials beyond symmetry principles.

Enhanced Critical Field of Superconductivity at an Oxide Interface

The nature of superconductivity and its interplay with strong spin-orbit coupling at the KTaO3(111) interfaces remain a subject of debate. To address this problem, we grew epitaxial LaMnO3/KTaO3(111) heterostructures. We show that superconductivity is robust against the in-plane magnetic field, with the critical field of superconductivity reaching ∼25 T in optimally doped heterostructures. The superconducting order parameter is highly sensitive to the carrier density. We argue that spin-orbit coupling drives the formation of anomalous quasiparticles with vanishing magnetic moment, providing significant condensate immunity against magnetic fields beyond the Pauli paramagnetic limit. These results offer design opportunities for superconductors with extreme resilience against the applied magnetic fields.

Orbital Fulde-Ferrell Pairing State in Moiré Ising Superconductors

In this Letter, we study superconducting moiré homobilayer transition metal dichalcogenides where the Ising spin-orbit coupling (SOC) is much larger than the moiré bandwidth. We call such noncentrosymmetric superconductors, moiré Ising superconductors. Because of the large Ising SOC, the depairing effect caused by the Zeeman field is negligible and the in-plane upper critical field (B_{c2}) is determined by the orbital effects. This allows us to study the effect of large orbital fields. Interestingly, when the applied in-plane field is larger than the conventional orbital B_{c2}, a finite-momentum pairing phase would appear which we call the orbital Fulde-Ferrell (FF) state. In this state, the Cooper pairs acquire a net momentum of 2q_{B}, where 2q_{B}=eBd is the momentum shift caused by the magnetic field B and d denotes the layer separation. This orbital field-driven FF state is different from the conventional FF state driven by Zeeman effects in Rashba superconductors. Remarkably, we predict that the FF pairing would result in a giant superconducting diode effect under electric gating when layer asymmetry is induced. An upturn of the B_{c2} as the temperature is lowered, coupled with the giant superconducting diode effect, would allow the detection of the orbital FF state.

Synthesis of an aqueous, air-stable, superconducting 1T'-WS2 monolayer ink

Liquid-phase chemical exfoliation can achieve industry-scale production of two-dimensional (2D) materials for a wide range of applications. However, many 2D materials with potential applications in quantum technologies often fail to leave the laboratory setting because of their air sensitivity and depreciation of physical performance after chemical processing. We report a simple chemical exfoliation method to create a stable, aqueous, surfactant-free, superconducting ink containing phase-pure 1T'-WS₂ monolayers that are isostructural to the air-sensitive topological insulator 1T'-WTe₂. The printed film is metallic at room temperature and superconducting below 7.3 kelvin, shows strong anisotropic unconventional superconducting behavior with an in-plane and out-of-plane upper critical magnetic field of 30.1 and 5.3 tesla, and is stable at ambient conditions for at least 30 days. Our results show that chemical processing can make nontrivial 2D materials that were formerly only studied in laboratories commercially accessible.

Direct Observation of the Topological Surface State in the Topological Superconductor 2M-WS2

The quantum spin Hall (QSH) effect has attracted extensive research interest because of the potential applications in spintronics and quantum computing, which is attributable to two conducting edge channels with opposite spin polarization and the quantized electronic conductance of 2e²/h. Recently, 2M-WS₂, a new stable phase of transition metal dichalcogenides with a 2M structure showing a layer configuration identical to that of the monolayer 1T' TMDs, was suggested to be a QSH insulator as well as a superconductor with a critical transition temperature of around 8 K. Here, high-resolution angle-resolved photoemission spectroscopy (ARPES) and spin-resolved ARPES are applied to investigate the electronic and spin structure of the topological surface states (TSS) in the superconducting 2M-WS₂. The TSS exhibit characteristic spin-momentum-locking behavior, suggesting the existence of long-sought nontrivial Z2 topological states therein. We expect that 2M-WS₂ with coexisting superconductivity and TSS might host the promising Majorana bound states.

Spin-triplet superconductivity from excitonic effect in doped insulators

SignificanceWe present a mechanism for unconventional superconductivity in doped band insulators, where short-ranged pairing interaction arises from Coulomb repulsion due to virtual interband or excitonic processes. Remarkably, electron pairing is found upon infinitesimal doping, giving rise to Bose-Einstein condensate (BEC)-Bardeen-Cooper-Schrieffer (BCS) crossover at low density. Our theory explains puzzling behaviors of superconductivity and predicts spin-triplet pairing in electron-doped ZrNCl and WTe2.

Ising pairing in atomically thin superconductors

Ising-type pairing in atomically thin superconducting materials has emerged as a novel means of generating devices with resilience to a magnetic field applied parallel to the two-dimensional (2D) plane. In this mini-review, we canvas the state of the field by giving a historical account of 2D superconductors with strongly enhanced in-plane upper critical fields, together with the type-I and type-II Ising pairing mechanisms. We highlight the vital role of spin-orbit coupling in these superconductors and discuss other effects such as symmetry breaking, atomic thicknesses, etc. Finally, we summarize the recent theoretical proposals and highlight the open questions, such as exploring topological superconductivity in these systems and looking for more materials with Ising pairing.

Gate-tunable superconductivity and charge-density wave in monolayer 1T'-MoTe2 and 1T'-WTe2

Using first-principles calculation methods, we reveal a series of phase transitions as a function of gating or electron doping in monolayered quantum spin Hall (QSH) insulators, 1T'-MoTe₂ and 1T'-WTe₂. With increasing electron doping, we show that a phonon mediated superconducting phase is realized first and is followed by a charge density wave (CDW) phase with a nonsymmorphic lattice symmetry. The newly found CDW phase exhibits Weyl energy bands with spin-orbit coupling with fractional band filling, and reforms into a topological nontrivial phase with fully filled bands. The robust resurgence of the QSH state coexisting with the CDW phase is shown to originate from band inversions induced by the nonsymmorphic lattice distortion through the strong electron-phonon interaction, thus suggesting the realization of various interfacial states between superconducting states, and various CDW and topological states on a two-dimensional crystal by inhomogeneous gating or doping.

Enhanced Superconductivity in Monolayer Td-MoTe2

Crystalline two-dimensional (2D) superconductors (SCs) with low carrier density are an exciting new class of materials in which electrostatic gating can tune superconductivity, electronic interactions play a prominent role, and electrical transport properties may directly reflect the topology of the Fermi surface. Here, we report the dramatic enhancement of superconductivity with decreasing thickness in semimetallic T_d-MoTe₂, with critical temperature (T_c) increasing up to 7.6 K for monolayers, a 60-fold increase with respect to the bulk T_c. We show that monolayers possess a similar electronic structure and density of states (DOS) as the bulk, implying that electronic interactions play a strong role in the enhanced superconductivity. Reflecting the low carrier density, the critical temperature, magnetic field, and current density are all tunable by an applied gate voltage. The response to high in-plane magnetic fields is distinct from that of other 2D SCs and reflects the canted spin texture of the electron pockets.