2D Superconductors Research Articles

Topological superconductivity in monolayer Td−MoTe2

Topological superconductivity has attracted significant attention due to its potential applications in quantum computation, but its experimental realization remains challenging. Recently, monolayer Td−MoTe2 was observed to exhibit gate tunable superconductivity, and its in-plane upper critical field exceeds the Pauli limit. Here, we show that an in-plane magnetic field beyond the Pauli limit can drive the superconducting monolayer Td−MoTe2 into a topological superconductor. The topological superconductivity arises from the interplay between the in-plane Zeeman coupling and the unique Ising plus in-plane spin-orbit coupling (SOC) in the monolayer Td−MoTe2. The Ising plus in-plane SOC plays the essential role to enable the effective px + ipy pairing. As the essential Ising plus in-plane SOC in the monolayer Td−MoTe2 is generated by an in-plane polar field, our proposal demonstrates that applying an in-plane magnetic field to a gate tunable 2D superconductor with an in-plane polar axis is a feasible way to realize topological superconductivity.

Tunable Mirror-Symmetric Type-III Ising Superconductivity in Atomically-Thin Natural Van der Waals Heterostructures.

Van der Waals (vdW) crystals with strong spin-orbit coupling (SOC) provide great opportunities for exploring unconventional 2D superconductors, wherein new pairing states emerge due to the interplay of SOC with crystalline symmetries, electronic correlations, quenched disorders and external modulation forces, etc. Here, a distinct mirror-symmetry protected Ising pairing state with unprecedented Γ- and M-valley symmetries in natural vdW heterostructures (vdWH) of interweaving tetragonal SnSe and trigonal 1H-TaSe2 monolayers is reported, in which the unidirectional lattice interlocking effectively suppresses the K-valley Ising pairing mechanism by incommensurate charge-density-wave (CDW) transitions. In the 2D limit of an TaSe2/SnSe bilayer with intact basal mirror symmetry (Mz), the mirror-symmetric vdWH Ising superconductors show anomalous in-plane magnetic field B‖-controlled enhancements in the critical temperature Tc, which is completely absent for multilayer vdWHs with broken Mz induced by orthorhombic stacking between nearest-neighbour TaSe2 monolayers. The experimental observations consistently reveal a mirror symmetry-protected type-III Ising state in the inversion asymmetric lattice of 1H-TaSe2, which is predicted to be a mixture of spin-singlet and spin-tripletstates.

Two-Dimensional Superconductivity and Anomalous Vortex Dissipation in Newly Discovered Transition Metal Dichalcogenide-Based Superlattices.

Properties of layered superconductors can vary drastically when thinned down from bulk to monolayer owing to the reduced dimensionality and weakened interlayer coupling. In transition metal dichalcogenides (TMDs), the inherent symmetry breaking effect in atomically thin crystals prompts novel states of matter such as Ising superconductivity with an extraordinary in-plane upper critical field. Here, we demonstrate that two-dimensional (2D) superconductivity resembling those in atomic layers but with more fascinating behaviors can be realized in the bulk crystals of two new TMD-based superconductors Ba0.75ClTaS2 and Ba0.75ClTaSe2 with superconducting transition temperatures 2.75 and 1.75 K, respectively. They comprise an alternating stack of H-type TMD layers and Ba-Cl layers. In both materials, intrinsic 2D superconductivity develops below a Berezinskii-Kosterlitz-Thouless transition. The upper critical field along the ab plane () exceeds the Pauli limit (μ0Hp); in particular, Ba0.75ClTaSe2 exhibits an extremely high ≈ 14 μ0Hp and a colossal superconducting anisotropy (/) of ∼150. Moreover, the temperature-field phase diagram of Ba0.75ClTaSe2 under an in-plane magnetic field contains a large phase regime of vortex dissipation, which can be ascribed to the Josephson vortex motion, signifying an unprecedentedly strong fluctuation effect in TMD-based superconductors. Our results provide a new path toward the establishment of 2D superconductivity and novel exotic quantum phases in bulk crystals of TMD-based superconductors.

High Temperature Phonon-Mediated Superconductivity with T c above 100 K in monolayer Na(BC)2 and K(BC)2

Abstract Two-dimensional (2D) materials have demonstrated promising prospects owing to their distinctive electronic properties and exceptional mechanical properties. Among them, 2D superconductors with T c above the boiling point of liquid nitrogen (77 K) will exhibit tremendous applicable value in the future. Here, we design two 2D superconductors Na(BC)2 and K(BC)2 with MgB2-like structure, which are theoretically predicted to host T c as high as 99 and 102 K, respectively. The origin of such high T c is ascribed to the presence of two σ-bonding bands and van Hove singularity at the Fermi level. Furthermore, the T c of Na(BC)2 is boosted up to 153 K with a biaxial strain of 5%, which set a new record among 2D superconductors. The predictions of Na(BC)2 and K(BC)2 open the door to explore 2D high temperature superconductors and provide a potential future for developing new applications in 2D materials.

Inverse-proximity effect and 2D superconductivity of Ta/Au bilayer thin films

This study investigated the inverse-proximity effect (IPE) and two-dimensional (2D) superconductivity in α-phase Ta films and Ta/Au bilayers. These bilayers were deposited onto silicon substrates using a direct current (DC) sputtering system. The findings reveal that higher substrate temperatures and increased film thickness facilitate the formation of α-Ta up to a temperature limit of 300°C, resulting in higher Tc around 3.5–3.7 K. The fundamental superconductivity of α-Ta thin films is similar to that of a conventional s-wave BCS superconductor. The IPE in the Ta/Au bilayer films was observed, where the Tc increases and decreases with the Au overlayer thickness. This behavior was quantitatively described by considering a model accounting for the reduced superconducting fluctuations induced by the highly conductive Au layer. Furthermore, the presence of 2D superconductivity in the Ta film and Ta/Au bilayer was examined through critical field analysis, vortex motions, and Berezinskii-Kosterlitz-Thouless (BKT) transition detection. Finally, the results were summarized in an H-T phase diagram illustrating the presence of IPE and 2D superconductivity in a Ta/Au bilayer. This study provides additional insights into 2D superconductivity within normal/superconductor (N/S) bilayers, laying the groundwork for future research to optimize film properties to align more closely with theoretical expectations.

Strong electron–phonon coupling and predicted high superconducting transition temperature of MXenes revealed in 2H-Mo2N under biaxial stress

Extensive investigations have been conducted on Mo-based MXenes due to their high superconducting temperatures (Tc). This work theoretically reports strong electron–phonon coupling (EPC) and high Tc (≈38 K) of 2H-Mo2N under biaxial stress, with excellent mechanical properties. EPC and Tc are elucidated upon dynamically stable strain range. At 0% strain, EPC constant (λ) and Tc are 1.32 and 22.7 K, respectively. They are improved when subjected to biaxial stresses. Strong EPC with λ over 2.0 occurs at −4%, −2.5%, and 5% strains, yielding enhanced Tc. The finding suggests that the energy levels of electronic bands at the Fermi level are enhanced by strain, which enhances EPC. Moreover, the impact of functional groups on superconductivity has been investigated. The λ’s of Mo2NH2 and Mo2NO2 are reduced, and Tc drops nearly to 20.0 and 0.9 K, respectively. This work provides an example of designing 2D superconductors by tuning atomic recipes and strain-dependent engineering.

What do we learn from impurities and disorder in high-Tc cuprates?

A series of experimental studies established that the differing morphologies of the phase diagrams versus hole doping nh of the various cuprate families are mostly controlled by defects and disorder. In the minimally disordered cuprate Yttrium Baryum Copper Oxide (YBCO) we introduced controlled detfects that allowed us to probe the metallic and superconducting states. We demonstrate that the extent of the spin glass phase and the superconducting dome can be controlled by the concentration of spinless (Zn, Li) impurities substituted on the planar Cu sites. NMR frequency shift measurements establish that these defects induce, in their vicinity, a cloud with a Kondo-like paramagnetic behavior. Its “Kondo” temperature and spatial extent differ markedly between the pseudogap and strange metal regimes. We have performed transport measurements on single crystals with a controlled content of in-plane vacancies introduced by electron irradiation. At high T, the inelastic scattering of the carriers has been found independent of disorder and completely governed by the excitations of the correlated electronic state. The low T upturns in the resistivity associated with single-site Kondo-like scattering are qualitatively in agreement with local magnetism induced by spinless impurities. The apparent metal insulator crossover is only detected for a very large defect content, and part of the large resistivity upturn remains connected with Kondo-like paramagnetism. In the superconducting state, the defect-induced reduction of Tc scales linearly with the increase in residual resistivity induced by disorder. High-field magnetoresistance experiments permit us to determine the paraconductivity due to superconducting fluctuations. The latter vanishes beyond a temperature T’c and a field H’c that both decrease with increasing in-plane defect content. In the pseudogap regime, the weaker decrease of T’c with respect to that of Tc reveals a large loss of superconducting phase coherence in the presence of disorder. In light of our experimental results, we initiate a discussion of its interplay with pair breaking. Our data also permit us to confirm that the differing phase diagrams are due to competing orders or disorders that are family-specific. In the ideal phase diagram of a disorder-free cuprate, 2D superconductivity should persist at low doping. This ensemble of experimental results provides serious challenges for the theoretical understanding of superconductivity in these correlated electron systems.

Novel emergent phases in a two-dimensional superconductor

In this letter, we report our observation of an extraordinarily rich phase diagram of a LaScO3/SrTiO3 heterostructure. Close to the superconducting transition temperature, the system hosts a superconducting critical point of the Infinite-randomness type characterized by an effective dynamical exponent νz that diverges logarithmically. At lower temperatures, we find the emergence of a magnetic field-tuned metallic phase that co-exists with a quantum Griffiths phase (QGP). Our study reveals a previously unobserved phenomenon in 2D superconductors—an unanticipated suppression of the QGP below a crossover temperature in this system. This concealment is accompanied by the destruction of the superconducting quantum critical point (QCP) signaled by a power-law divergence (in temperature) of the effective dynamical exponent. These observations are entirely at odds with the predictions of the infinite-randomness scenario and challenge the very concept of a vanishing energy scale associated with a QCP. We develop and discuss possible scenarios like smearing of the phase transition that could plausibly explain our observations. Our findings challenge the notion that QGP is the ultimate ground state in two-dimensional superconductors.

Higher Order Topological Defects in a Moiré Lattice

AbstractTopological defects are ubiquitous, they manifest in a wide variety of systems such as liquid crystals, magnets or superconductors. The recent quest for non‐abelian anyons in condensed matter physics stimulates the interest for topological defects since they can be hosted in vortices in quantum magnets or topological superconductors. In addition to these vortex defects, this study proposes to investigate edge dislocations in 2D magnets as new building blocks for topological physics since they can be described as vortices in the structural phase field. It demonstrates the existence of higher order topological dislocations within the higher order moiré pattern of the van der Waals 2D magnet CrCl3 deposited on Au(111). Surprisingly, these higher order dislocations arise from ordinary simple edge dislocations in the atomic lattice of CrCl3. This study provide a theoretical framework explaining the higher order dislocations as vortices with a winding Chern number of 2. It is expected that these original defects can stabilize some anyons either in a 2D quantum magnet or within a 2D superconductor coupled to it.

Giant Tunability of Rashba Splitting at Cation-Exchanged Polar Oxide Interfaces by Selective Orbital Hybridization.

The 2D electron gas (2DEG) at oxide interfaces exhibits extraordinary properties, such as 2D superconductivity and ferromagnetism, coupled to strongly correlated electrons in narrow d-bands. In particular, 2DEGs in KTaO3 (KTO) with 5d t2g orbitals exhibit larger atomic spin-orbit coupling and crystal-facet-dependent superconductivity absent for 3d 2DEGs in SrTiO3 (STO). Herein, by tracing the interfacial chemistry, weak anti-localization magneto-transport behavior, and electronic structures of (001), (110), and (111) KTO 2DEGs, unambiguously cation exchange across KTO interfaces is discovered. Therefore, the origin of the 2DEGs at KTO-based interfaces is dramatically different from the electronicreconstruction observed at STO interfaces. More importantly, as the interface polarization grows with the higher order planes in the KTO case, the Rashba spin splitting becomes maximal for the superconducting (111) interfaces approximately twice that of the (001) interface. The larger Rashba spin splitting couples strongly to the asymmetric chiral texture of the orbital angular moment, and results mainly from the enhanced inter-orbital hopping of the t2g bands and more localized wave functions. This finding has profound implications for the search for topological superconductors, as well as the realization of efficient spin-charge interconversion for low-power spin-orbitronics based on (110) and (111) KTO interfaces.

Stoichiometric control of electron mobility and 2D superconductivity at LaAlO3-SrTiO3 interfaces

SrTiO3-based conducting interfaces, which exhibit coexistence of gate-tunable 2D superconductivity and strong Rashba spin-orbit coupling (RSOC), are candidates to host topological superconductive phases. Yet, superconductivity is usually in the dirty limit, which tends to suppress nonconventional pairing and therefore challenges these expectations. Here we report on LaAlO3/SrTiO3 (LAO/STO) interfaces with large mobility and mean free paths comparable to the superconducting coherence length, approaching the clean limit for superconductivity. We further show that the carrier density, mobility, and formation of the superconducting condensate are controlled by the fine-tuning of La/Al chemical ratio in the LAO film. We find a region in the superconducting phase diagram where the critical temperature is not suppressed below the Lifshitz transition, at odds with previous experimental investigations. These findings point out the relevance of achieving a clean-limit regime to enhance the observation of unconventional pairing mechanisms in these systems.

Superconducting Two-Dimensional FeSe Grown on the Fe-Enriched Interface.

Two-dimensional (2D) tetragonal FeSe has sparked extensive research interest owing to its tunable superconductivity, providing valuable insights into the design of high-temperature superconductors. Currently, the intricate Fe-Se phase diagram poses a challenge to the controlled synthesis of superconducting 2D FeSe in a pure tetragonal phase. Here, we exploit the ion-exchange property of fluorophlogopite mica to devise a straightforward approach for the phase-controlled synthesis of tetragonal FeSe on an Fe-enriched mica surface within a molten salt environment. This method successfully produces highly crystalline FeSe in a pure tetragonal phase with adjustable thickness. We investigated the surface composition of the postgrowth mica substrate using various microscopic and spectroscopic characterizations to highlight the importance of the Fe-enriched growth interface in the phase-selective synthesis of 2D tetragonal FeSe. The obtained 2D FeSe exhibited 2D superconductivity, comparable to that of FeSe mechanically exfoliated from bulk crystals, confirming the high quality of our samples. Beyond tetragonal FeSe, 2D antiferromagnetic FeTe and superconducting FeSxSeyTe1-x-y have been phase-selectively synthesized via this approach. Our study elucidates the significance of the growth interface on the phase-selective synthesis of 2D materials and presents potential opportunities for the phase-controlled synthesis of 2D multiphase materials via the rational design of the growth interface.

Beyond Conventional Charge Density Wave for Strongly Enhanced 2D Superconductivity in 1H-TaS2 Superlattices.

Noncentrosymmetric transition metal dichalcogenide (TMD) monolayers offer a fertile platform for exploring unconventional Ising superconductivity (SC) and charge density waves (CDWs). However, the vulnerability of isolated monolayers to structural disorder and environmental oxidation often degrade their electronic coherence. Herein, an alternative approach is reported for fabricating stable and intrinsic monolayers of 1H-TaS2 sandwiched between SnS blocks in a (SnS)1.15TaS2 van der Waals (vdW) superlattice. The SnS block layers not only decouple individual 1H-TaS2 sublayers to endow them with monolayer-like electronic characteristics, but also protect the 1H-TaS2 layers from electronic degradation. The results reveal the characteristic 3 × 3 CDW order in 1H-TaS2 sublayers associated with electronic rearrangement in the low-lying sulfur p band, which uncovers a previously undiscovered CDW mechanism rather than the conventional Fermi surface-related framework. Additionally, the (SnS)1.15TaS2 superlattice exhibits a strongly enhanced Ising-like SC with a layer-independent Tc of ≈3.0 K, comparable to that of the isolated monolayer 1H-TaS2 sample, presumably attributed to their monolayer-like characteristics and retained Fermi states. These results provide new insights into the long-debated CDW order and enhanced SC of monolayer 1H-TaS2, establishing bulk vdW superlattices as promising platforms for investigating exotic collective quantum phases in the 2D limit.

Unveiling intrinsic material and extrinsic pinning dimensionality in superconductors: Why Fe(Se,Te) is able to mimic YBCO

Discovery of iron-based superconductors paved the way to a competitor of high-temperature superconductors, easier to produce, better performing in high fields, and promising to be less expensive. Critical parameters are investigated by resistivity measurements as a function of temperature, field, and angle R(T,H,θ). This work presents a deep analysis of H-θ phase diagram of PLD-processed Fe(Se,Te) superconducting films, thus revealing material and pinning anisotropy at once. By selecting different thresholds along the R(T,H,θ) curves, all possible regimes emerge. Surprisingly, anisotropy arises moving from the upper critical field toward the irreversibility line: gradually a non-monotonous transition from 3D to 2D, and backward to 3D occurs. Although Fe(Se,Te) appears as a 3D superconductor, its anisotropic pinning landscape shows up similarities with an intrinsic layered superconductor and Fe(Se,Te) definitively mimics YBCO. We propose a general method to disentangle, in any other superconductor, material dimensionality and pinning anisotropy that are key constraints for applications.

Steering of Vortices by Magnetic Field Tilting in Open Superconductor Nanotubes.

In planar superconductor thin films, the places of nucleation and arrangements of moving vortices are determined by structural defects. However, various applications of superconductors require reconfigurable steering of fluxons, which is hard to realize with geometrically predefined vortex pinning landscapes. Here, on the basis of the time-dependent Ginzburg-Landau equation, we present an approach for the steering of vortex chains and vortex jets in superconductor nanotubes containing a slit. The idea is based on the tilting of the magnetic field B at an angle α in the plane perpendicular to the axis of a nanotube carrying an azimuthal transport current. Namely, while at α=0∘, vortices move paraxially in opposite directions within each half-tube; an increase in α displaces the areas with the close-to-maximum normal component |Bn| to the close(opposite)-to-slit regions, giving rise to descending (ascending) branches in the induced-voltage frequency spectrum fU(α). At lower B values, upon reaching the critical angle αc, the close-to-slit vortex chains disappear, yielding fU of the nf1 type (n≥1: an integer; f1: the vortex nucleation frequency). At higher B values, fU is largely blurry because of multifurcations of vortex trajectories, leading to the coexistence of a vortex jet with two vortex chains at α=90∘. In addition to prospects for the tuning of GHz-frequency spectra and the steering of vortices as information bits, our findings lay the foundation for on-demand tuning of vortex arrangements in 3D superconductor membranes in tilted magnetic fields.

Quantum Griffiths Singularity in Ordered Artificial Superconducting-Islands-Array on Graphene.

Quantum Griffiths phase (QGP) is a novel quantum phenomenon of quantum phase transition in two-dimensional (2D) superconductors, and the emergence of inhomogeneous superconducting rare regions immersed in a metallic matrix is theoretically related to the quantum Griffiths singularity (QGS). However, the theoretical proposal of superconducting rare regions still lacks intuitive experimental verification. Here, we construct an artificial ordered superconducting-islands-array on monolayer graphene with the aid of an anodic aluminum oxide (AAO) membrane. The QGS under both in-plane and out-of-plane magnetic fields is evidenced by the divergent dynamical critical exponent and is in compliance with the direct activated scaling behavior. The phase diagram clearly shows that the QGP is indeed bred in the rare superconducting regions within isolated superconducting islands with a vanished quantum coherence. Our results reveal the universal features of QGP in artificial heterostructured systems and provide a visualized platform for the theoretical proposal of QGS.

Two-dimensional Rashba superconductivity in Ni/Bi bilayers evidenced by nonreciprocal transport

When two different materials are brought together, a plethora of quantum phenomena and functionalities can emerge. A prominent example is the superconductivity in Ni/Bi bilayers, which arises from the artificial layered structure composed of the non-superconducting ferromagnetic and heavy metals. Although this system has been shown to exhibit unconventional superconducting properties, the underlying mechanism of the superconductivity remains elusive. Here, we provide experimental evidence of the microscopic coexistence of two-dimensional (2D) superconductivity and broken space-inversion symmetry in the Ni/Bi bilayer. The evidence is obtained from nonreciprocal transport around the superconducting transition temperature, where the resistance depends on the direction of an applied current and an external magnetic field. We find that the nonreciprocal superconducting transport is most pronounced around the Berezinskii–Kosterlitz–Thouless transition temperature. These observations support the 2D Rashba superconductivity in the Ni/Bi bilayer, which will serve as a basis for advancing the understanding of unconventional superconductivity.

Identifying Chern numbers of superconductors from local measurements

Fascination in topological materials originates from their remarkable response properties and exotic quasiparticles which can be utilized in quantum technologies. In particular, large-scale efforts are currently focused on realizing topological superconductors and their Majorana excitations. However, determining the topological nature of superconductors with current experimental probes is an outstanding challenge. This shortcoming has become increasingly pressing due to rapidly developing designer platforms which are theorized to display very rich topology and are better accessed by local probes rather than transport experiments. We introduce a robust machine learning protocol for classifying the topological states of two-dimensional (2D) chiral superconductors and insulators from local density of states (LDOS) data. Since the LDOS can be measured with standard experimental techniques, our protocol contributes to overcoming the almost three decades standing problem of identifying the topological phase of 2D superconductors with broken time-reversal symmetry.

Plasma-Induced 2D Electron Transport at Hetero-Phase Titanium Oxide Interface.

Interfaces of metal oxide heterojunctions display a variety of intriguing physical properties that enable novel applications in spintronics, quantum information, neuromorphic computing, and high-temperature superconductivity. One such LaAlO3 /SrTiO3 (LAO/STO) heterojunction hosts a 2D electron liquid (2DEL) presenting remarkable 2D superconductivity and magnetism. However, these remarkable properties emerge only at very low temperatures, while the heterostructure fabrication is challenging even at the laboratory scale, thus impeding practical applications. Here, a novel plasma-enabled fabrication concept is presented to develop the TiO2 /Ti3 O4 hetero-phase bilayer with a 2DEL that exhibits features of a weakly localized Fermi liquid even at room temperature. The hetero-phase bilayer is fabricated by applying a rapid plasma-induced phase transition that transforms a specific portion of anatase TiO2 thin film into vacancy-prone Ti3 O4 in seconds. The underlying mechanism relies on the screening effect of the achieved high-density electron liquid that suppresses the electron-phonon interactions. The achieved "adiabatic" electron transport in the hetero-phase bilayer offers strong potential for low-loss electric or plasmonic circuits and hot electron harvesting and utilization. These findings open new horizons for fabricating diverse multifunctional metal oxide heterostructures as an innovative platform for emerging clean energy, integrated photonics, spintronics, and quantum information technologies.

A Gate-Tunable Ambipolar Quantum Phase Transition in a Topological Excitonic Insulator.

Coulomb interactions among electrons and holes in 2D semimetals with overlapping valence and conduction bands can give rise to a correlated insulating ground state via exciton formation and condensation. One candidate material in which such excitonic state uniquely combines with non-trivial band topology are atomic monolayers of tungsten ditelluride (WTe2 ), in which a 2D topological excitonic insulator (2D TEI) forms. However, the detailed mechanism of the 2D bulk gap formation in WTe2 , in particular with regard to the role of Coulomb interactions, has remained a subject of ongoing debate. Here, it shows that WTe2 is susceptible to a gate-tunable quantum phase transition, evident from an abrupt collapse of its 2D bulk energy gap upon ambipolar field-effect doping. Such gate tunability of a 2D TEI, into either n- and p-type semimetals, promises novel handles of control over non-trivial 2D superconductivity with excitonicpairing.