Topological superconductors are characterized by topological invariants that describe the number and nature of their robust boundary modes. These invariants must also have observable consequences in the bulk of the system, akin to the quantized bulk Hall conductivity in the quantum Hall effect, but such consequences are made elusive by the spontaneous breaking of $U(1)$ symmetry in the superconductor. Here we focus on 2+1 dimensional spin-less $p$-wave superconductors and show that emergent gravity serves as a natural bulk probe for their topological invariant. This emergent gravity is due to the same attractive interaction between fermions that leads to superconductivity, and is therefore built into topological superconductors. The bulk response of a topological superconductor to the emergent gravitational field is encoded in a gravitational Chern-Simons term, and is related to the existence of robust boundary modes via energy-momentum conservation, or gravitational anomaly inflow. The gravitational Chern-Simons term implies a universal relation between variations in the superconducting order parameter and the energy-momentum currents and densities that they induce. The spontaneous breaking of $U(1)$ symmetry in the superconductor leads to additional bulk responses, encoded in a gravitational pseudo Chern-Simons term. Although not of topological nature, these carry surprising similarities to the topological responses of the gravitational Chern- Simons term. We show how these two types of responses can be disentangled.
Recent development in fabrication of hybrid nanostructures allows for creation of quantum interferometers that combine semiconductor and superconductor materials. We show that in those nanostructures the joint phenomena of Aharonov-Bohm effect and Andreev reflections can be used to determine the length on which the electron is retro-reflected as a hole. We propose to exploit this feature for probing of the quasiparticle coherence length in semiconductor-superconductor hybrids by a magnetoconductance measurement.
From stacking to function: emergent states and quantum devices in 2D superconductor heterostructures
Institute of Brain-Inspired Intelligence, National Laboratory of Solid State Microstructures, School of Physics, Collaborative Innovation Center of Advanced Microstructures, Nanjing University（南京大学脑启发智能研究所、固体微结构物理国家重点实验室、物理学院、先进微结构协同创新中心）；Jiangsu Physical Science Research Center（江苏省物理科学研究中心）；Institute of Interdisciplinary Physical Sciences, School of Science, Nanjing University of Science and Technology（南京理工大学理学院交叉物理科学研究院）
Twisted interfaces between stacked van der Waals cuprate crystals enable tunable Josephson coupling between in-plane anisotropic superconducting order parameters. Employing a novel cryogenic assembly technique, we fabricate Josephson junctions with an atomically sharp twisted interface between Bi2Sr2CaCu2O8+x crystals. The Josephson critical current density sensitively depends on the twist angle, reaching the maximum value comparable to that of the intrinsic junctions at small twisting angles, and is suppressed by almost 2 orders of magnitude yet remains finite close to 45 degree twist angle. …
In condensed matter systems, zero-dimensional or one-dimensional Majorana modes can be realized respectively as the end and edge states of one-dimensional and two-dimensional topological superconductors. In this $\textit{top-down}$ approach, $(d-1)$-dimensional Majorana modes are obtained as the boundary states of a topologically nontrivial $d$-dimensional bulk. In a $\textit{bottom-up}$ approach instead, $d$-dimensional Majorana modes in a $d$-dimensional system can be realized as the continuous limit of a periodic lattice of coupled $(d-1)$-dimensional Majorana modes. We illustrate this idea…
Superconductors can be classified as topological or not based on whether time-reversal symmetry (TRS), chiral symmetry, and particle-hole symmetry are preserved or not. Further, topological superconductors can also be classified as chiral or helical. In this paper, using Hanbury-Brown and Twiss (HBT) shot noise correlations and the non-local conductance, we probe metal/2D unconventional superconductor/metal junctions to understand better the pairing topological vs. non-topological or helical vs. chiral or nodal vs. gapful. We see that HBT correlations are asymmetric as a function of bias volta…
Emergent quantum phenomena via phase-coherence engineering in infinite-layer nickelate superconductors
Peking University（北京大学）；University of Electronic Science and Technology of China（电子科技大学）；Renmin University of China（中国人民大学）；ShanghaiTech University（上海科技大学）；Tsinghua University（清华大学）；Institute of Physics Chinese Academy of Sciences（中国科学院物理研究所）；University of Chinese Academy of Sciences（中国科学院大学）；Songshan Lake Materials Laboratory（松山湖材料实验室）；Beijing Normal University（北京师范大学）；Fudan University（复旦大学）；Hefei National Laboratory（合肥国家实验室）
Noncollinear magnetism opens exciting possibilities to generate topological superconductivity. Here, we focus on helical and cycloidal magnetic textures in magnet-superconductor hybrid structures in a background magnetic field. We demonstrate that this system can enter a topological phase which can be understood as a set of parallel topological wires. We explore and confirm this idea in depth with three different approaches: a continuum model, a tight-binding model based on the magnetic unit cell, and exact diagonalization on a finite two-dimensional lattice. The key signature of this topologi…
In noncentrosymmetric superconductors (NCSs), the conversion of a charge current into spin magnetization - the so called magnetoelectric effect - is the direct indicator of the unconventional, mixed-parity order parameter. This paper proposes a scheme to detect the magnetoelectric effect by anomalous, equal-spin Andreev tunneling in NCS/ferromagnet contacts. The proposal relies on the ability to generate spin-polarized triplet pairing by passing an electric current through an NCS. Such an induced triplet pairing bears a similarity to the paradigmatic nonunitary pairing in triplet superfluids w…
Thermopower probing emergent local moments in magic-angle twisted bilayer graphene
Indian Institute of Science（印度科学学院）；Indian Institute of Technology Kanpur（坎普尔印度理工学院）；National Institute for Materials Science（物质材料研究机构）；The Ohio State University（俄亥俄州立大学）
Despite growing interest in them, highly crystalline two-dimensional superconductors derived from exfoliated layered materials are few. Employing the anisotropic Migdal-Eliashberg formalism based on {\it ab initio} calculations, we find monolayer NiTe$_{2}$ to be an intrinsic superconductor with a $T_{\text c}\sim$5.7~K, although the bulk crystal is not known to superconduct. Remarkably, bilayer NiTe$_{2}$ intercalated with lithium is found to display two-gap superconductivity with a critical temperature $T_{\text{c}}\sim 11.3$~K and superconducting gap of $\sim$3.1~meV, arising from a synergy…
We propose a method to probe and control the interactions between an ensemble of magnetic impurities in a superconductor via microwave radiation. Our method relies upon the presence of sub-gap Yu-Shiba-Rusinov (YSR) states associated with the impurities. Depending on the sign of the detuning, radiation generates either a ferro- or antiferromagnetic contribution to the exchange interaction. This contribution can bias the statistics of the random exchange constants stemming from the RKKY interaction. Moreover, by measuring the microwave response at the YSR resonance, one gains information about …
The energy of an Andreev bound state in a clean normal metal in contact with two superconductors disperses with the difference $Δϕ$ in the superconducting phase between the superconductors in much the same way as the energies of electrons in a one-dimensional crystal disperse with the crystal momentum $k$ of the electrons. A normal metal with $n$ superconductors maps on to a $n-1$ dimensional crystal, each dimension corresponding to the phase difference $ϕ_i$ between a specific pair of superconductors. The resulting band structure as a function of the phase differences $\{Δϕ_i\}$ has been prop…
The interplay between superconductivity, magnetic field and spin-orbit coupling can lead to the realization of non--trivial topological phases. Recent experiments have found signatures of such phases in magnetic nanoflakes formed by nanostructures coupled to a superconducting substrate. These heterostructures comprise a topologically non-trivial region surrounded by a trivial one due to the finite magnetic exchange field induced by the magnetic nanoflake. The analysis of the topological phase diagram of such a system shows that a similar phase separation occurs by tuning the chemical potential…
Emergent spin quantum Hall edge states at the boundary of two-dimensional electron gas proximitized by an $s$-wave superconductor
L. D. Landau Institute for Theoretical Physics（列夫·朗道理论物理研究所）；Laboratory for Condensed Matter Physics, HSE University（高等经济大学凝聚态物理实验室）；P. N. Lebedev Physical Institute, Russian Academy of Sciences（俄罗斯科学院列别捷夫物理研究所）；Osipyan Institute of Solid State Physics, Russian Academy of Sciences（俄罗斯科学院奥西普扬固体物理研究所）；HSE University（高等经济大学）
Probing chiral symmetry with a topological domain wall sensor
University of Zurich（苏黎世大学）；Julius-Maximilians-Universität Würzburg（维尔茨堡朱利叶斯-马克西米利安大学）；Institute of Physics, Polish Academy of Sciences（波兰科学院物理研究所）；International Research Centre MagTop, Institute of Physics, Polish Academy of Sciences（波兰科学院物理研究所马格托普国际研究中心）；Physikalisches Institut, Experimentelle Physik II, Julius-Maximilians-Universität Würzburg（维尔茨堡朱利叶斯-马克西米利安大学物理系实验物理II）
We propose to directly probe the chirality of Majorana edge states in 2D topological superconductors using polarization selective photon absorption. When shining circularly polarized light on a 2D topological superconductor in disk geometry, the photons can excite quasiparticles only when the polarization of the light matches the chirality of the Majorana edge states required by the angular momentum conservation. Hence one can obtain the chirality of the Majorana edge states by measuring the photon absorption rate. We show that the polarization selective photon absorption can also serve as a s…
