Giant second-harmonic generation (SHG) in the terahertz (THz) frequency range is observed in a thin film of an s-wave superconductor NbN, where the time-reversal ($\mathcal{T}$-) and space-inversion ($\mathcal{P}$-) symmetries are simultaneously broken by supercurrent injection. We demonstrate that the phase of the second-harmonic (SH) signal flips when the direction of supercurrent is inverted, i.e., the signal is ascribed to the nonreciprocal response that occurs under broken $\mathcal{P}$- and $\mathcal{T}$-symmetries. The temperature dependence of the SH signal exhibits a sharp resonance, which is accounted for by the vortex motion driven by the THz electric field in an anharmonic pinning potential. The maximum conversion ratio $η_{\mathrm{SHG}}$ reaches $\approx10^{-2}$ in a thin film NbN with the thickness of 25 nm after the field cooling with a very small magnetic field of $\approx1$ Oe, for a relatively weak incident THz electric field of 2.8 kV/cm at 0.48 THz.
Higgs mode in superconductors, i.e. the collective amplitude mode of the order parameter does not associate with charge nor spin fluctuations, therefore it does not couple to the electromagnetic field in the linear response regime. On the contrary to this common understanding, here, we demonstrate that, if the dc supercurrent is introduced into the superconductor, the Higgs mode becomes infrared active and is directly observed in the linear optical conductivity measurement. We observed a sharp resonant peak at $ω=2Δ$ in the optical conductivity spectrum of a thin-film NbN in the presence of dc…
Transport is called nonreciprocal when not only the sign, but also the absolute value of the current, depends on the polarity of the applied voltage. It requires simultaneously broken inversion and time-reversal symmetries, e.g., by the interplay of spin-orbit coupling and magnetic field. So far, observation of nonreciprocity was always tied to resistivity, and dissipationless nonreciprocal circuit elements were elusive. Here, we engineer fully superconducting nonreciprocal devices based on highly-transparent Josephson junctions fabricated on InAs quantum wells. We demonstrate supercurrent rec…
CNR-SPIN（意大利国家研究委员会自旋物理研究所）；Okayama University（冈山大学）；Dipartimento di Fisica “E. R. Caianiello”, Università di Salerno（萨莱诺大学E.R.卡亚涅洛物理学系）；NEST Istituto Nanoscienze-CNR and Scuola Normale Superiore（纳米科学研究所和比萨高等师范学院）
Nonreciprocal transport refers to charge transfer processes that are sensitive to the bias polarity. Until recently, nonreciprocal transport was studied only in dissipative systems, where the nonreciprocal quantity is the resistance. Recent experiments have, however, demonstrated nonreciprocal supercurrent leading to the observation of a supercurrent diode effect in Rashba superconductors, opening the vision of dissipationless electronics. Here we report on a supercurrent diode effect in NbSe$_2$ constrictions obtained by patterning NbSe$_2$ flakes with both even and odd layer number. The obse…
When both inversion and time-reversal symmetries are broken, the critical current of a superconductor can be nonreciprocal. In this work we show that in certain classes of two-dimensional superconductors with antisymmetric spin-orbit coupling, Cooper pairs acquire a finite momentum upon the application of an in-plane magnetic field, and as a result, critical currents in the direction parallel and antiparallel to the Cooper pair momentum become unequal. This supercurrent diode effect is also manifested in the polarity-dependence of in-plane critical fields induced by a supercurrent. These nonre…
We report an unconventional and promising route to self-assemble distributed superconductor-ferromagnet-superconductor (S-F-S) Josephson Junctions on single crystal [100] MgO. These structures consist of [110] epitaxial nano-plaquettes of Fe covered with superconducting NbN films of varying thickness. The S-F-S structures are characterized by strong magnetoresistance (MR) anisotropy for the in-plane and out-of-plane magnetic fields. The stronger in-plane MR suggests decoherence of S-F-S junctions whose critical current follows a (1-T/Tc) and (1-T/Tc)1/2 dependence for T Tc and T<<Tc respective…
In a search for a simple proximity system of a topological insulator and a superconductor for studying the role of surface versus bulk effects by gating, we report here on a first step toward this goal, namely the choice of such a system and its characterization. We chose to work with thin film bilayers of grainy 5 nm thick NbN films as the superconductor, overlayed with 20 nm thick topological layer of $\rm Bi_2Se_3$ and compare the transport results to those obtained on a 5 nm thick reference NbN film on the same wafer. Bilayers with ex-situ and in-situ prepared $\rm NbN-Bi_2Se_3$ interfaces…
Nonreciprocity in superconductors is attracting much interest owing to its fundamental importance as well as its potential applicability to engineering. In this paper, we generalize the previous theories of the intrinsic superconducting diode effect (SDE) and microscopically elucidate its relationship with the nonreciprocity of the transition lines under supercurrent. We derive a general formula for the intrinsic SDE by using the phenomenological Ginzburg-Landau theory and thereby show that the SDE is determined by the relative angle between the magnetic field and an effective anti-symmetric s…
A Kapitza Pendulum Route to Supercurrent Tunnel Diodes
Istituto di Struttura della Materia of the National Research Council（国家研究委员会材料结构研究所）；Institute for Theoretical Solid State Physics, IFW Dresden（德累斯顿IFW理论固体质物理研究所）；CNR-SPIN；Istituto Lombardo “Accademia di Scienze e Lettere”（伦巴第科学院与文学院）；NEST Istituto Nanoscienze-CNR and Scuola Normale Superiore（CNR纳米科学研究所NEST与比萨高等师范学院）；Würzburg-Dresden Cluster of Excellence ct.qmat（维尔茨堡-德累斯顿卓越集群ct.qmat）；Universitá di Salerno（萨莱诺大学）
Nonreciprocal phenomena in the normal state are well established and key to many commercial applications. In contrast, superconducting analogs, such as the superconducting diode effect (SDE), are only starting to be experimentally explored and pose significant challenges to their theoretical understanding. In this work we put forth a phenomenological picture of the SDE based on the generalized Ginzburg-Landau free energy, which includes a Lifshitz invariant as the hallmark of noncentrosymmetric helical phase of the finite-momentum Cooper pairs. We reveal that such a Lifshitz invariant drives t…
Geometric Superconducting Diode Effect in an NbN Nanoring
Nanjing University（南京大学）；Purple Mountain Laboratories（紫金山实验室）；Zhejiang University（浙江大学）；State Key Laboratory of Spintronics Devices and Technologies, Nanjing University（南京大学自旋电子学器件与技术国家重点实验室）；Nanjing University of Science and Technology（南京理工大学）
Supercurrent Diode Effect in Josephson Interferometers with Multiband Superconductors
CNR-SPIN（意大利国家研究委员会自旋物理学研究所）；Institute for Theoretical Solid State Physics, Leibniz-Institut für Festkörper- und Werkstoffforschung IFW-Dresden（德累斯顿勒布尼兹固体与材料研究所凝聚态物理理论研究所）；Istituto Lombardo “Accademia di Scienze e Lettere”（伦巴第科学院）；NEST Istituto Nanoscienze-CNR and Scuola Normale Superiore（意大利国家研究委员会纳米科学研究所与比萨高等师范学院）；Universitá di Salerno（萨莱诺大学）
Supercurrent spin Hall effect enabled nanopillar Josephson diodes
School of Physical Sciences, National Institute of Science Education and Research (NISER)（国家科学教育研究中心物理科学学院）；Homi Bhabha National Institute（霍米·巴巴国立研究所）；Center for Interdisciplinary Sciences, National Institute of Science Education and Research (NISER)（国家科学教育研究中心交叉科学中心）
Epitaxial niobium-nitride thin films with a critical temperature of Tc=16K and a thickness of 100nm were fabricated on MgO(100) substrates by pulsed laser deposition. Low-temperature magnetic force microscopy (MFM) images of the supercurrent vortices were measured after field cooling in a magnetic field of 3mT at various temperatures. Temperature dependence of the penetration depth has been evaluated by a two-dimensional fitting of the vortex profiles in the monopole-monopole model. Its subsequent fit to a single s-wave gap function results in the superconducting gap amplitude Delta(0) = 2.9 m…
A scaling relation of vortex-induced rectification effects in a superconducting thin-film heterostructure
Osaka University（大阪大学）；Institute for Open and Transdisciplinary Research Initiatives, Osaka University（大阪大学开放跨学科研究倡议研究所）；Institute for Materials Research, Tohoku University（东北大学材料科学研究所）
Superconducting diode effect, in analogy to the nonreciprocal resistive charge transport in semiconducting diode, is a nonreciprocity of dissipationless supercurrent. Such an exotic phenomenon originates from intertwining between symmetry-constrained supercurrent transport and intrinsic quantum functionalities of helical/chiral superconductors. In this article, research progress of superconducting diode effect including fundamental concepts, material aspects, device prospects, and theoretical/experimental development is reviewed. First, fundamental mechanisms to cause superconducting diode eff…
Ferroelectricity in a magnon Bose-Einstein condensate: Nonreciprocal superfluidity, exceptional points, and Majorana bosons
The University of Osaka（大阪大学）；Kyoto University（京都大学）；Fukushima Medical University（福岛医科大学）；Institute for Solid State Physics, The University of Tokyo（东京大学固体物理研究所）
Pseudo-superconducting-diode effect in ferroelectric Josephson junctions
Kavli Institute of Nanoscience, Delft University of Technology（代尔夫特理工大学卡弗里纳米科学研究所）；Department of Quantum Nanoscience, Delft University of Technology（代尔夫特理工大学量子纳米科学系）；Department of Quantum Technology, Netherlands Organisation for Applied Scientific Research (TNO)（荷兰应用科学研究组织（TNO）量子技术部）
Finite-momentum Cooper plasmons in superconducting terahertz microcavities
Max Planck Institute for the Structure and Dynamics of Matter（马克斯·普朗克物质结构与动力学研究所）；Center for Free-Electron Laser Science (CFEL)（自由电子激光科学中心）；Department of Physics, Columbia University（哥伦比亚大学物理系）；Department of Physics, University of California Santa Barbara（加州大学圣塔芭芭拉分校物理系）
Large Field-Free Superconducting Diode Effect with Nonmonotonic Polarity Reversals in NbSe$_2$/CrBr$_3$ Heterostructures
Southern University of Science and Technology（南方科技大学）；International Quantum Academy, and Shenzhen Branch, Hefei National Laboratory（国际量子科学院，合肥国家实验室深圳分院）；Eastern Institute for Advanced Study, Eastern Institute of Technology（宁波东方理工大学（暂名）先进研究院）；State Key Laboratory of Optoelectronic Materials and Technologies, Sun Yat‑sen University（中山大学光电信息材料与技术国家重点实验室）
Reversible field-free superconducting diode effect controlled by an antiferromagnet
Institute of Physics, Czech Academy of Sciences（捷克科学院物理研究所）；Faculty of Nuclear Sciences and Physical Engineering, Czech Technical University in Prague（布拉格捷克理工大学核科学与物理工程学院）；Niels Bohr Institute, University of Copenhagen（哥本哈根大学尼尔斯·玻尔研究所）；Faculty of Mathematics and Physics, Charles University（查理大学数学与物理学院）
Symmetry Origins of the Field-Free Superconducting Diode Effect in the Kagome Superconductor CsV$_3$Sb$_5$
University of Science and Technology Beijing（北京科技大学）；International Quantum Academy, and Shenzhen Branch, Hefei National Laboratory（国际量子学院合肥国家实验室深圳分院）；Southern University of Science and Technology（南方科技大学）；Hong Kong University of Science and Technology（香港科技大学）；Ningxia University（宁夏大学）；Sun Yat-sen University（中山大学）；Peking University（北京大学）；Anhui University（安徽大学）
