Topological Superconductors Research Articles

Surface superconductivity in the topological Weyl semimetal t-PtBi2.

Topological superconductivity is a promising concept for generating fault-tolerant qubits. Early experimental studies looked at hybrid systems and doped intrinsic topological or superconducting materials at very low temperatures. However, higher critical temperatures are indispensable for technological exploitation. Recent angle-resolved photoemission spectroscopy results have revealed that superconductivity in the type-I Weyl semimetal-trigonal PtBi2 (t-PtBi2)-is located at the Fermi-arc surface states, which renders the material a potential candidate for intrinsic topological superconductivity. Here we show, using scanning tunnelling microscopy and spectroscopy, that t-PtBi2 presents surface superconductivity at elevated temperatures (5 K). The gap magnitude is elusive: it is spatially inhomogeneous and spans from 0 to 20 meV. In particular, the large gap value and the shape of the quasiparticle excitation spectrum resemble the phenomenology of high-Tc superconductors. To our knowledge, this is the largest superconducting gap so far measured in a topological material. Moreover, we show that the superconducting state at 5 K persists in magnetic fields up to 12 T.

Altermagnetic topological insulator with $\mathcal{C}$-paired spin-valley locking

AbstractAltermagnetism emerges as the third fundamental collinear magnetism recently. Associating nontrivial topology and quantum orders to this newly established magnetism is an important physical topic and may lead to interesting physics such as unusual quantum spin Hall effect with time-reversal-symmetry breaking, topological superconductivity and spin-valley locking. Here, we first generalize the spinless Haldane model from the usual honeycomb lattice to the otherwise simpler square lattice. We then restore the doublet spin degree of freedom, the resulted spinful model resembles the Kane-Mele model and is able to describe the unprecedented physics which intertwine $\mathcal{C}$ C -paired spin-valley locking, Dirac physics, nontrivial band topology and altermagnetism. Our works shed lights on future studies and applications in spintronics and valleytronics for altermagnetic materials, the lattice theories developed in this article may also find their broad applications to optical lattice with ultra cold atoms, photonic lattice, etc. other than the usual electronic systems.

High-throughput structural screening and property study of noncentrosymmetric superconductors in Th-based ZrNiAl-like compounds

Noncentrosymmetric superconductors (NCSs) are emerging as potential materials for the investigation of nontraditional phenomena, including both the presence of a spin-singlet pairing configuration and the occurrence of a spin-triplet pairing configuration, as well as the manifestation of topological superconductivity. In this work, we constructed a high-throughput theoretical design and performed a computational screening of Th-based NCSs with ZrNiAl-like structures based on the first principles method. Through systematic stability evaluations, we expanded the range of Th-based ZrNiAl-like compounds with superior stability to 45 materials. In this group of substances, we have pinpointed 17 prospective entities exhibiting notable band separation in the vicinity of the Fermi level. Electron–phonon coupling calculations indicated that 14 of these compounds are superconducting. By performing an analysis of electronic properties, we proposed that ThAlRu and ThAlOs are two promising NCSs in this family. This work may aid in the exploration of the design and synthesis of Th-based ZrNiAl-like materials, leading to the discovery of more NCSs with unique superconducting properties.

Engineering Epitaxial Interfaces for Topological Insulator — Superconductor Hybrid Devices with Al Electrodes

AbstractProximity‐induced superconductivity in hybrid devices of topological insulators and superconductors offers a promising platform for the pursuit of elusive topological superconductivity and its anticipated applications, such as fault‐tolerant quantum computing. To study and harness such hybrid devices, a key challenge is the realization of highly functional material interfaces with a suitable superconductor featuring 2‐periodic parity‐conserving transport to ensure a superconducting hard‐gap free of unpaired electrons, which is important for Majorana physics. A superconductor well‐known for this characteristic is Al, however, its direct integration into devices based on tetradymite topological insulators has so far been found to yield non‐transparent interfaces. By focusing on Bi2Te3‐Al heterostructures, this study identifies detrimental interdiffusion processes at the interface through atomically resolved structural and chemical analysis, and showcases their mitigation by leveraging different interlayers – namely Nb, Ti, Pd, and Pt – between Bi2Te3 and Al. Through structural transformation of the interlayer materials (X) into their respective tellurides (XTe2) atomically‐sharp epitaxial interfaces are engineered and further characterized in low‐temperature transport experiments on Al‐X‐Bi2Te3‐X‐Al Josephson junctions and in complementary density functional theory calculations. By demonstrating functional interfaces between Bi2Te3 and Al, this work provides key insights and paves the way for the next generation of sophisticated topological devices.

Uniform Diffusion of Cooper Pairing Mediated by Hole Carriers in Topological Sb2Te3/Nb.

Spin-helical Dirac Fermions at a doped topological insulator's boundaries can support Majorana quasiparticles when coupled with s-wave superconductors, but in n-doped systems, the requisite induced Cooper pairing in topological states is often buried at heterointerfaces or complicated by degenerate coupling with bulk conduction carriers. Rarely probed are p-doped topological structures with nondegenerate Dirac and bulk valence bands at the Fermi level, which may foster long-range superconductivity without sacrificing Majorana physics. Using ultrahigh-resolution photoemission, we report proximity pairing with a large decay length in p-doped topological Sb2Te3 on superconducting Nb. Despite no momentum-space degeneracy, the topological and bulk states of Sb2Te3/Nb exhibit the same isotropic superconducting gaps at low temperatures. Our results unify principles for realizing accessible pairing in Dirac Fermions relevant to topological superconductivity.

Proximity‐Induced Unconventional Superconducting Quantum Oscillation in WTe2/NbSe2 Heterostructures

AbstractThe electron pairing mechanisms in topological superconductors are pivotal for understanding the emergent topology‐related quantum states and play key roles in condensed matter physics. Theoretically, the quantization of magnetic flux in nano‐hole arrays within topological superconductors can be studied for understanding the Bogoliubov quasiparticles via the Little–Parks oscillation. However, experimental evidence of the quasiparticle states in such structures of topological superconductors remains elusive. Here, the unconventional superconducting quantum oscillation phenomena of the proximity‐induced 2D topological superconductivity (TSC) is demonstrated in a nano‐hole array of WTe2/NbSe2 heterojunction. The unconventional oscillation period in WTe2 is substantially smaller than that in the s‐wave superconductor NbSe2 (corresponds to Cooper pairs), implying the probable presence of novel quasiparticle states associated with TSC. Interestingly, such phenomena of multi‐charge flux quanta might be related to the multi‐particle bound states of TSC. These observations provide a new approach for exploring the unconventional superconducting quantum states in topological superconductors.

Wave Function Engineering on Superconducting Substrates: Chiral Yu-Shiba-Rusinov Molecules.

Magnetic adatoms on superconductors give rise to Yu-Shiba-Rusinov (YSR) states that hold considerable interest for the design of topological superconductivity. Here, we show that YSR states are also an ideal platform to engineer structures with intricate wave function symmetries. We assemble structures of iron atoms on the quasi-two-dimensional superconductor 2H-NbSe2. The Yu-Shiba-Rusinov wave functions of individual atoms extend over several nanometers enabling hybridization even at large adatom spacing. We show that the substrate can be exploited to deliberately break symmetries of the adatom structure leading to hybridized YSR states exhibiting symmetries that cannot be found in orbitals of iso-structural planar molecules in the gas phase. We exploit this potential by designing chiral YSR wave functions of triangular adatom structures. Our results significantly expand the range of interesting quantum states that can be engineered using arrays of magnetic adatoms on superconductors.

Single-gap isotropic $s-$wave superconductivity in single crystals AuSn$_4$

London, \lambda_L (T)λL(T), and Campbell, \lambda_{C} (T)λC(T), penetration depths were measured in single crystals of a topological superconductor candidate AuSn_44. At low temperatures, \lambda_L (T)λL(T) is exponentially attenuated and, if fitted with the power law, \lambda(T) \sim T^nλ(T)∼Tn, gives exponents n>4n>4, indistinguishable from the isotropic single s-s−wave gap Bardeen-Cooper-Schrieffer (BCS) asymptotic. The superfluid density fits perfectly in the entire temperature range to the BCS theory. The superconducting transition temperature, T_c = 2.40 ± 0.05Tc=2.40±0.05 K, does not change after 2.5 MeV electron irradiation, indicating the validity of the Anderson theorem for isotropic s-s−wave superconductors. Campbell penetration depth before and after electron irradiation shows no hysteresis between the zero-field cooling (ZFC) and field cooling (FC) protocols, consistent with the parabolic pinning potential. Interestingly, the critical current density estimated from the original Campbell theory decreases after irradiation, implying that a more sophisticated theory involving collective effects is needed to describe vortex pinning in this system. In general, our thermodynamic measurements strongly suggest that the bulk response of the AuSn_44 crystals is fully consistent with the isotropic s-s−wave weak-coupling BCS superconductivity.

Exploring topological phases in superconducting transition metal (Sc, Ti, V)-carbides

The combination of non trivial band topology and superconductivity, resulting in topological superconductivity, igniting a fervent pursuit in the realm of quantum materials. Through density functional theory and maximally localized Wannier functions, we delve into the electronic and topological properties of transition metal carbides ΛC (with Λ= Sc, Ti, V). Phonon dispersions guarantee the structural stability of these superconductors. Witness the presence of band inversion within the Brillouin Zone of TiC and VC, contrasting the absence of such band inversion in ScC. In addition, the nonzero Z2 topological invariant as well as the occurrence of Dirac cone in the surface spectrum of TiC and VC, unveil their topologically nontrivial character. These transition metal carbides emerge as promising candidates for probing the depths of topological superconductivity and unraveling the associated Majorana bound states.

Quantum geometrical properties of topological materials

The momentum space of topological insulators and topological superconductors is equipped with a quantum metric defined from the overlap of neighboring valence band states or quasihole states. We investigate the quantum geometrical properties of these materials within the framework of Dirac models and differential geometry. Their momentum space is found to be always a maximally symmetric space with a constant Ricci scalar, and the vacuum Einstein equation is satisfied in 3D with a finite cosmological constant. For linear Dirac models, several geometrical properties are found to be independent of the band gap, including a peculiar straight line geodesic, constant volume of the curved momentum space, and the exponential decay form of the nonlocal topological marker, indicating the peculiar yet universal quantum geometrical properties of these models.

Braiding induced by the finite-size effect in one-dimensional topological superconductors

Braiding induced by the finite-size effect in one-dimensional topological superconductors

FIB-fabrication of superconducting devices based on Bi2Se3 junctions

Recent advances in quantum technologies are highly influencing the current technological scenario. Hybrid devices combining superconductors and topological insulators represent an excellent opportunity to study the topological superconducting phase, which offers interesting features that might have significant implications in the development of quantum sensing and quantum computing. Furthermore, focused ion beam techniques, whose versatility enables to create sophisticated devices with high degree of customization, can enhance the creation of complex devices. Here, we develop a novel approach for creating single-crystal devices that is applied to the fabrication of superconducting devices based on topological insulator Bi2Se3 in a geometry characteristic of a superconducting quantum interference device. Characterization of these devices reveals that superconductivity is induced in our crystal and the supercurrent is modulated by applying an external magnetic field. These results open the way to tailoring the response of hybrid devices that combine superconductors and topological insulators by focused ion beam techniques.

Self-consistent study of topological superconductivity in two-dimensional quasicrystals

Self-consistent study of topological superconductivity in two-dimensional quasicrystals

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.

Experimental Detection of Topological Electronic State and Large Linear Magnetoresistance in SrSn4 Superconductor

AbstractWhile recent experiments confirm the existence of hundreds of topological electronic materials, only a few exhibit the coexistence of superconductivity (SC) and a topological electronic state. These compounds attract significant attention in forefront research because of the potential for the existence of topological SC, paving the way for future technological advancements. SrSn4 is known for exhibiting unusual SC below the transition temperature (TC) of 4.8 K. Recent theory predicts a topological electronic state in this compound, which is yet to be confirmed by experiments. Systematic and detailed studies of the magnetotransport properties of SrSn4 and its Fermi surface characterizations are also absent. For the first time, a quantum oscillation study reveals a nontrivial 𝝅‐Berry phase, very light effective mass, and high quantum mobility of charge carriers in SrSn4. Magnetotransport experiment unveils large linear transverse magnetoresistance (TMR) of more than 1200% at 5 K and 14 T. Angle‐dependent transport experiments detect anisotropic and fourfold symmetric TMR, with the maximum value (≈2000%) occurring when the angle between the magnetic field and the crystallographic b‐axis is 45°. The results suggest that SrSn4 is the first topological material with SC above the boiling point of helium that displays such high magnetoresistance.

Topological superconductivity in Fibonacci quasicrystals

We investigate the properties of a Fibonacci quasicrystal (QC) arrangement of a one-dimensional topological superconductor, such as a magnetic atom chain deposited on a superconducting surface. We uncover a general mutually exclusive competition between the QC properties and the topological superconducting phase with Majorana bound states (MBS): there are no MBS inside the QC gaps and the MBS never behave as QC subgap states and, likewise, no critical or winding QC subgap states exist inside the topological superconducting gaps. Surprisingly, despite this competition, we find that the QC is still highly beneficial for realizing topological superconductivity with MBS. It both leads to additional large nontrivial regions with MBS in parameter space, that are topologically trivial in crystalline systems, and increases the topological gap protecting the MBS. We also find that shorter approximants of the Fibonacci QC display the largest benefits. As a consequence, our results promote QCs, and especially their short approximants, as an appealing platform for improved experimental possibilities to realize MBS as well as generally highlight the fundamental interplay between different topologies. Published by the American Physical Society 2024

Enhancement of superconductivity in W5Si3 with Tc ∼ 6.2 K by P-doping

The effects of phosphorus doping on the prototypical transition metal silicide W5Si3, predicted to host a topologically nontrivial band structure and exhibit superconductivity below its critical temperature (Tc) of 2.7 K, have been investigated. Upon P-doping, the lattice parameter a decreases and c increases for W5Si3–xPx up to x = 0.7. Notably, Tc and the upper critical field of W5Si3–xPx are enhanced by over twofold as x increases, achieving 6.2 K and 5.5 T at x = 0.7, respectively. This enhancement in bulk superconductivity is attributed to a shift of the Fermi level toward a local maximum in the density of states upon P-doping, as indicated by the first-principles calculations. Our results suggest an effective doping strategy which significantly enhances the superconductivity in W5Si3. The emerging W5Si3-type superconductors offer a promising platform for realizing possible topological superconductivity.

Josephson coupling across magnetic topological insulator MnBi2Te4

Topological superconductors hosting Majorana zero modes are of great interest for both fundamental physics and potential quantum computing applications. In this work, we investigate the transport properties of the intrinsic magnetic topological insulator MnBi2Te4 (MBT). In normal transport measurements, we observe the presence of chiral edge channels, though with deviations from perfect quantization due to factors such as non-uniform thickness, domain structures, and the presence of quasi-helical edge states. Subsequently, we fabricate superconducting junctions using niobium leads on MBT exfoliated flakes, which show an onset of supercurrent with clear Josephson coupling. The interference patterns in the superconducting junctions reveal interesting asymmetries, suggesting changes in the magnetic ordering of the MBT flakes under small applied magnetic fields. Moreover, the modulation of the critical current by magnetic field reveals a SQUID-like pattern, suggesting the presence of supercurrent through the quasi-helical edge states.

Majorana bound states and phase transitions in first- and second-order topological superconductors

Majorana bound states and phase transitions in first- and second-order topological superconductors

Two-band superconductivity in Pt-intercalated ZrTe2, a Dirac semimetal

This study presents compelling evidence of superconductivity with 3.5 K onset and zero-resistance state at 2.25 K in the Dirac semimetal compound ZrTe2, upon intercalation with platinum in the van der Waals gap. The temperature dependence of the upper critical field and the normalized superfluid density can be modeled effectively using a two-gap model, consistent with the possibility of topological superconductivity.