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Abstract The coherence length ξ is the fundamental length scale of superconductors which governs the sizes of Cooper pairs, vortices, Andreev bound states, and more. In BCS theory, the coherence length is ξ BCS = ℏ v F /Δ, where v F is the Fermi velocity and Δ is the pairing gap. It is clear that increasing Δ will shorten ξ BCS. In this work, we show that the quantum metric, which is the real part of the quantum geometric tensor, gives rise to an anomalous contribution to the coherence length. Specifically, $$\xi =\sqrt{{\xi }_{{{{\rm{BCS}}}}}^{2}+{\ell }_{{{{\rm{qm}}}}}^{2}}$$ ξ = ξ BCS 2 + ℓ qm 2 for a superconductor where ℓ qm is the quantum metric contribution. In the flat-band limit, ξ does not vanish but is bound below by ℓ qm. We demonstrate that under the uniform pairing condition, ℓ qm is controlled by the quantum metric of minimal trace in the flat-band limit. Physically, the Cooper pair size of a superconductor cannot be squeezed down to a size smaller than ℓ qm which is a fundamental length scale determined by the quantum geometry of the wave functions. Lastly, we compute the quantum metric contributions for the family of superconducting moiré graphene materials, demonstrating the significant role played by quantum metric effects in these narrow-band superconductors.

Abstract Materials showing second-order nonlinear transport under time reversal symmetry can be used for Radio Frequency (RF) rectification, but practical application demands room temperature operation and sensitivity to microwatts level RF signals in the ambient. In this study, we demonstrate that BiTeBr exhibits a giant nonlinear response which persists up to 350 K. Through scaling and symmetry analysis, we show that skew scattering is the dominant mechanism. Additionally, the sign of the nonlinear response can be electrically switched by tuning the Fermi energy. Theoretical analysis suggests that the large Rashba spin-orbit interactions (SOI), which gives rise to the chirality of the Bloch electrons, provide the microscopic origin of the observed nonlinear response. Our BiTeBr rectifier is capable of rectifying radiation within the frequency range of 0.2 to 6 gigahertz at room temperature, even at extremely low power levels of −15 dBm, and without the need for external biasing. Our work highlights that materials exhibiting large Rashba SOI have the potential to exhibit nonlinear responses at room temperature, making them promising candidates for harvesting high-frequency and low-power ambient electromagnetic energy.

Abstract An axion insulator is a three-dimensional (3D) topological insulator (TI), in which the bulk maintains the time-reversal symmetry or inversion symmetry but the surface states are gapped by surface magnetization. The axion insulator state has been observed in molecular beam epitaxy (MBE)-grown magnetically doped TI sandwiches and exfoliated intrinsic magnetic TI MnBi2Te4 flakes with an even number layer. All these samples have a thickness of ~ 10 nm, near the 2D-to-3D boundary. The coupling between the top and bottom surface states in thin samples may hinder the observation of quantized topological magnetoelectric response. Here, we employ MBE to synthesize magnetic TI sandwich heterostructures and find that the axion insulator state persists in a 3D sample with a thickness of ~ 106 nm. Our transport results show that the axion insulator state starts to emerge when the thickness of the middle undoped TI layer is greater than ~ 3 nm. The 3D hundred-nanometer-thick axion insulator provides a promising platform for the exploration of the topological magnetoelectric effect and other emergent magnetic topological states, such as the high-order TI phase.

Abstract The coexistence of gate-tunable superconducting, magnetic and topological orders in magic-angle twisted bilayer graphene provides opportunities for the creation of hybrid Josephson junctions. Here we report the fabrication of gate-defined symmetry-broken Josephson junctions in magic-angle twisted bilayer graphene, where the weak link is gate-tuned close to the correlated insulator state with a moiré filling factor of υ = −2. We observe a phase-shifted and asymmetric Fraunhofer pattern with a pronounced magnetic hysteresis. Our theoretical calculations of the junction weak link—with valley polarization and orbital magnetization—explain most of these unconventional features. The effects persist up to the critical temperature of 3.5 K, with magnetic hysteresis observed below 800 mK. We show how the combination of magnetization and its current-induced magnetization switching allows us to realise a programmable zero-field superconducting diode. Our results represent a major advance towards the creation of future superconducting quantum electronic devices.

The nonlinear Hall effect describes a recently discovered phenomenon in which a time-reversal symmetric material with a Berry curvature dipole develops a transverse voltage at double the frequency of an applied current. Here, the authors theoretically explore twisted bilayer WSe2 under strain, and find that it can exhibit a large nonlinear Hall effect that is highly sensitive to the topological properties of the material.

Abstract Recently, multifold fermions characterized by band crossings with multifold degeneracy and Fermi surfaces with higher Chern numbers have been discovered experimentally in AlPt (Schroter et al. in Nat Phys 15:759–765, 2019) and XSi(X = Rh,Co) (Sanchez et al. in Nature 567:500–505, 2019; Rao et al. in Nature 567:496–499, 2019; Takane et al. in Phys Rev Lett 122:076402, 2019). In this work, we largely expand the family of multifold fermion materials by pointing out that several well-studied noncentrosymmetric superconductors are indeed multifold fermion metals. Importantly, their normal state topological properties, which have been ignored in previous studies, play an important role in the superconducting properties. Taking Li 2 Pd 3 B $\mathrm{Li} _{2}\mathrm{Pd} _{3}\mathrm{B}$ and Li 2 Pt 3 B $\mathrm{Li} _{2}\mathrm{Pt} _{3}\mathrm{B}$ as examples, we found a large number of unconventional degenerate points, such as double spin-1, spin-3/2, Weyl and double Weyl topological band crossing points near the Fermi energy, which result in finite Chern numbers on Fermi surfaces. Long Fermi arc states in Li 2 Pd 3 B $\mathrm{Li} _{2}\mathrm{Pd} _{3}\mathrm{B}$ , originating from the nontrivial band topology were found. Importantly, it has been shown experimentally that Li 2 Pd 3 B $\mathrm{Li} _{2}\mathrm{Pd} _{3}\mathrm{B}$ and Li 2 Pt 3 B $\mathrm{Li} _{2}\mathrm{Pt} _{3}\mathrm{B}$ are fully gapped and gapless superconductors, respectively. By analyzing the possible pairing symmetries, we suggest that Li 2 Pd 3 B $\mathrm{Li} _{2}\mathrm{Pd} _{3}\mathrm{B}$ is a conventional s-wave superconductor or DIII class topological superconductor with Majorana surface states. Li 2 Pt 3 B $\mathrm{Li} _{2}\mathrm{Pt} _{3}\mathrm{B}$ , being gapless, is likely to be a nodal topological superconductor with dispersionless surface Majorana modes. We further identified that several noncentrosymmetric superconductors, such as Mo 3 Al 2 C $\mathrm{Mo} _{3}\mathrm{Al} _{2}\mathrm{C}$ , PdBiSe, Y 2 C 3 $\mathrm{Y}_{2}\mathrm{C}_{3}$ and La 2 C 3 $\mathrm{La} _{2}\mathrm{C}_{3}$ , are multifold fermion superconductors. This work calls for a revisit for the study of noncentrosymmtric superconductors which provide platforms for investigating the interplay between superconductivity and topologically nontrivial Fermi surfaces.

Chiral crystals with spin-orbit coupling (SOC) termed Kramers Weyl semimetals possess Weyl points at time-reversal invariant momenta. Here, the authors propose a new class of topological materials with doubly degenerate lines connecting time-reversal invariant momenta in achiral noncentrosymmetric materials with SOC.

A Kramers Weyl semimetal has a chiral crystal structure and is thought to exhibit unique physical properties due to the chiral lattice symmetry. Here, the authors theoretically demonstrate that this class of material can exhibit a strong longitudinal magnetoelectric response that could be used for magnetic switching in a ferromagnetic system.

The mechanism of current-driven magnetization switching in twisted bilayer graphene (TBG) is poorly understood. Here, He et al. show that a small current can generate a large orbital magnetization due to symmetry breaking by the twisting and substrate in TBG, leading to a giant orbital magnetoelectric effect.

Recently, gate-defined Josephson junctions in magic angle twisted bilayer graphene (MATBG) were studied experimentally, and highly unconventional Fraunhofer patterns were observed. In this work, we show that an interaction-driven valley-polarized state connecting two superconducting regions of MATBG would give rise to a long-sought-after purely electric controlled φ_{0}-junction in which the two superconductors acquire a finite phase difference φ_{0} in the ground state. We point out that the emergence of the φ_{0}-junction stems from the valley-polarized state which breaks time-reversal symmetry, and trigonal warping effects which break intravalley inversion symmetry. Importantly, a spatially nonuniform valley-polarization order parameter at the junction can explain the key features of the observed unconventional Fraunhofer patterns. Our work explores the novel transport properties of the valley-polarized state, and we suggest that gate-defined MATBG Josephson junctions could realize the first purely electric controlled φ_{0}-junctions.

The superconducting magnetoelectric effect, which is the current-induced magnetization in a superconductor, mainly focused on spin magnetization in previous studies, but ignored the effect of the orbital magnetic moments carried by the paired Bloch electrons. In this work, we show that orbital magnetic moments in superconductors can induce large orbital magnetization in the presence of a current. We constructed a unified description for the current-induced spin and orbital magnetization across the superconductor-normal metal phase transition. We find that in a superconductor with uniform pairing, the current-induced magnetization at a given current density is the same as that in its normal metal state, while with the nonuniform superconducting pairing, the current-induced magnetization exhibits an abrupt change in magnitude near the superconductor-normal metal phase transition. Importantly, our theory predicts the orbital magnetoelectric effect in superconducting twisted bilayer graphene which has paired Bloch electrons with large orbital magnetic moments and negligible spin-orbit coupling. We propose that the measurement of the current-induced orbital magnetoelectric effect can be used to detect the possible nonuniform pairings in twisted bilayer graphene and other newly discovered superconductors with nontrivial Berry curvatures.

In a recent work [S. Manna, P. Wei, Y. Xie, K. T. Law, P. A. Lee, and J. S. Moodera, Proc. Natl. Acad. Sci. 117, 8775 (2020)10.1073/pnas.1919753117], signatures of a pair of Majorana bound states (MBSs) were found in an experimental platform formed by EuS islands deposited on top of a gold surface which was made superconducting through proximity coupling to a superconductor. In this paper, we provide a theoretical understanding for how MBSs can be formed in EuS/Au/superconductor heterostructures. We focus on the strip geometry where a narrow ferromagnetic strip is deposited on a planar structure. We first explicitly map out the topological phase diagram of the EuS/Au/superconductor heterostructure using the lattice Green's function method. Importantly, we find that the chemical potential step between the region with and without EuS covering is a crucial ingredient for the creation of MBS of this setup. Next, we focus on the Bogoliugov quasiparticles that are bound to the region under the EuS by Andreev reflections from the surrounding superconductors. Moreover, we obtain the topological regimes analytically using the scattering matrix method. Notably, we confirm that the normal reflections induced by the chemical potential step are essential for creating finite topological regimes. Furthermore, the area of the topological regimes shows periodic oscillation as a function of chemical potential as well as the sample width. We conclude by showing that the feromagnetic strip geometry holds a number of advantages over other quasi-one-dimensional schemes that have been proposed.

The magnetoelectric effect (or Edelstein effect) in noncentrosymmetric superconductors states that a supercurrent can induce spin magnetization. This is an intriguing phenomenon which has potential applications in superconducting spintronic devices. However, the original Edelstein effect only applies to superconductors with polar point group symmetry. In recent years, many new noncentrosymmetric superconductors have been discovered, such as superconductors with chiral lattice symmetry and superconducting transition metal dichalcogenides with various lattice structures. In this Rapid Communication, we provide a general framework to describe the supercurrent-induced magnetization in these recently discovered superconductors with gyrotropic point groups.

An external magnetic field is needed to drive a nanowire in proximity to an s-wave superconductor into a topological regime which supports Majorana end states. However, magnetic field generally suppresses the proximity superconducting gap induced on the nanowire. In recent experiments using InSb nanowires coupled to conventional superconductors such as Al and NbTiN, the induced proximity gap vanishes at magnetic fields B∼1 T. This results in a small superconducting gap on the wire and a narrow topological regime which is proportional to the strength of the magnetic field. In this work, we show that by placing nanowires in proximity to recently discovered Ising superconductors such as the atomically thin transition-metal dichalcogenide NbSe_{2}, the topological superconducting gap on the wire can be maintained at a large magnetic field as strong as B∼10 T. This robust topological superconducting gap is induced by the unique equal-spin triplet Cooper pairs of the parent Ising superconductor. The strong magnetic field allows a topological regime ten times larger than those in InSb wires coupled to conventional superconductors. Our work establishes a realistic platform for building robust Majorana-based qubits.

The study of non-Abelian Majorana zero modes advances our understanding of the fundamental physics in quantum matter and pushes the potential applications of such exotic states to topological quantum computation. It has been shown that in two-dimensional (2D) and 1D chiral superconductors, the isolated Majorana fermions obey non-Abelian statistics. However, Majorana modes in a Z_{2} time-reversal-invariant (TRI) topological superconductor come in pairs due to Kramers’s theorem. Therefore, braiding operations in TRI superconductors always exchange two pairs of Majoranas. In this work, we show interestingly that, due to the protection of time-reversal symmetry, non-Abelian statistics can be obtained in 1D TRI topological superconductors and may have advantages in applications to topological quantum computation. Furthermore, we unveil an intriguing phenomenon in the Josephson effect, that the periodicity of Josephson currents depends on the fermion parity of the superconducting state. This effect provides direct measurements of the topological qubit states in such 1D TRI superconductors.

Recently, the veracity of the published chronology for the Pliocene section of North Atlantic Ocean Drilling Program Site 982 was called into question. Here, we examine the robustness of the original age model as well as the proposed age model revision. The proposed revision is predicated on an apparent misidentification of the depth to the Gauss–Matuyama (G/M) polarity chronozone reversal boundary (2.581 Ma) based on preliminary shipboard paleomagnetic data, and offers a new chronology that includes a hiatus between ~3.2 and 3 Ma. However, an even more accurate shore-based, u-channel-derived polarity chronozone stratigraphy for the past ~2.7 Ma supports the shipboard composite stratigraphy and demonstrates that the original estimate of the depth of the G/M reversal in the Site 982 record is correct. Thus, the main justification forwarded to support the revised chronology no longer exists. We demonstrate that the proposed revision results in a pronounced anomaly in sedimentation rates proximal to the proposed hiatus, erroneous assignment of marine-isotope stages in the Site 982 Pliocene benthic stable oxygen isotope stratigraphy, and a markedly worse correlation of proxy records between this site and other regional paleoclimate data. We conclude that the original chronology for Site 982 is a far more accurate age model than that which arises from the published revision. We strongly recommend the use of the original chronology for all future work at Site 982.

We find an optical Raman lattice without spin-orbit coupling showing chiral topological orders for cold atoms. Two incident plane-wave lasers are applied to simultaneously generate a double-well square lattice and periodic Raman couplings, the latter of which drive the nearest-neighbor hopping and create a staggered flux pattern across the lattice. Such a minimal setup can yield the quantum anomalous Hall effect with a large gap-bandwidth ratio in the single particle regime, while in the interacting regime it achieves the J1-J2-K spin model, with the nearest-neighboring (J1) and next-nearest-neightboring (J2) exchange coupling coefficients, and the three three-spin interacting parameter (K) is controllable. We show that the J1-J2-K spin model may support a chiral spin liquid phase. It is interesting that the quantum anomalous Hall state can be detected by only measuring the Bloch states in the two symmetric momentum points of the first Brillouin zone. Further, we also show that heating in the present optical Raman lattice can be essentially reduced compared with the conventional laser-assisted tunneling schemes. This suggests that the predicted topological states be reachable with the current experimental capability. [ABSTRACT FROM AUTHOR]