The search for broken time reversal symmetry (TRSB) in unconventional superconductors intensified in the past year as more systems have been predicted to possess such a state. Following our pioneering study of TRSB states in Sr$_2$RuO$_4$ using magneto-optic probes, we embarked on a systematic study of several other of these candidate systems. The primary instrument for our studies is the Sagnac magneto-optic interferometer, which we recently developed. This instrument can measure magneto-optic Faraday or Kerr effects with an unprecedented sensitivity of 10 nanoradians at temperatures as low as 100 mK. In this paper we review our recent studies of TRSB in several systems, emphasizing the study of the pseudogap state of high temperature superconductors and the inverse proximity effect in superconductor/ferromagnet proximity structures.
The Kagome lattice exhibits rich quantum phenomena owing to its unique geometric properties. Appealing realizations are the Kagome metals AV$_3$Sb$_5$ (A = K, Rb, Cs), where unconventional charge density wave (CDW) is intertwined with superconductivity and non-trivial band topology. Several experiments suggest that this CDW is a rare occurrence of chiral CDW characterized by orbital loop current. However, key evidences of loop current, spontaneous time-reversal symmetry-breaking (TRSB) and the coupling of its order parameter with the magnetic field remain elusive. Here, we investigate the CDW …
We report the discovery of Zr$_3$Ir as a new type of unconventional noncentrosymmetric superconductor (with $T_c = 2.3$ K), here investigated mostly via muon-spin rotation/relaxation ($μ$SR) techniques. Its superconductivity was characterized using magnetic susceptibility, electrical resistivity, and heat capacity measurements. The low-temperature superfluid density, determined via transverse-field $μ$SR and electronic specific heat, suggests a fully-gapped superconducting state. The spontaneous magnetic fields, revealed by zero-field $μ$SR below $T_c$, indicate the breaking of time-reversal s…
Recent experiments have pointed to a chiral p-wave-like superconductivity in epitaxial Bi/Ni bilayers that are spontaneously time-reversal symmetry breaking (TRSB), making it a promising platform for exploring physics useful for topologically protected quantum computing. Quite intriguingly, evidence has emerged that in non-epitaxial Bi/Ni bilayers, superconductivity arises due to the formation of NiBi3, which has been reported to host coexisting ferromagnetic and superconducting orders at the surface. We perform high resolution surface magneto-optic Kerr effect (SMOKE) measurements using a Sag…
Chains of magnetic atoms, placed on the surface of s-wave superconductors, have been established as a laboratory for the study of Majorana bound states. In such systems, the breaking of time reversal due to magnetic moments gives rise to the formation of in-gap states, which hybridize to form one-dimensional topological superconductors. However, in unconventional superconductors even non-magnetic impurities induce in-gap states since scattering of Cooper pairs changes their momentum but not their phase. Here, we propose a path for creating topological superconductivity, which is based on an un…
In the recent search for unconventional- and topological superconductivity, noncentrosymmetric superconductors (NCSCs) rank among the most promising candidate materials. Surprisingly, some of them -- especially those containing rhenium -- seem to exhibit also time-reversal symmetry (TRS) breaking in their superconducting state, while TRS is preserved in many other isostructural NCSCs. To date, a satisfactory explanation for such discrepant behavior, albeit crucial for understanding the unconventional superconductivity of these materials, is still missing. Here we review the most recent develop…
Recently, compounds with noncentrosymmetric crystal structure have attracted much attention for providing a rich playground in search for unconventional superconductivity. NbIr$_2$B$_2$ is a new member to this class of materials harboring superconductivity below $T_{\rm c} = 7.3(2)$~K and very high upper critical field that exceeds Pauli limit. Here we report on muon spin rotation ($μ$SR) experiments probing the temperature and field dependence of effective magnetic penetration depth in this compound. Our transverse-field -$μ$SR results suggest a fully gaped $s$-wave superconductvity. Further,…
We calculate the optical conductivity of type-I and type-II multi-Weyl semimetals(m-WSMs) in time-reversal(TR) broken case at finite doping. It is found that both longitudinal and Hall conductivities are renormalized by J(monopole charge) perpendicular to Weyl nodes separation and longitudal conductivity along nodes has non-trivial dependence on J. The Kerr rotations in thin films and bulk have different dependence on system parameters. The Kerr rotation in ultra-thin films is independent of monopole charge J and depends only on Fermi velocity v F while in bulk it dependent on monopole charge …
Unconventional superconductors have been long sought for their potential applications in quantum technologies and devices. A key challenge impeding this effort is the difficulty associated with probing and characterizing candidate materials and establishing their order parameter. In this Letter, we present a platform that allows us to spectroscopically probe unconventional superconductivity in thin-layer materials via the proximity effect. We show that inducing an s-wave gap in a sample with an intrinsic d-wave instability leads to the formation of bound-states of quasiparticle pairs, which ma…
Microwave Kerr/Faraday Resonance in Two-dimensional Chiral Superconductors
The University of Osaka（大阪大学）；Institute for Open and Transdisciplinary Research Initiatives, The University of Osaka（大阪大学开放跨学科研究倡议研究所）；Japan Atomic Energy Agency（日本原子能机构）；The University of Tokyo（东京大学）；Louisiana State University（路易斯安那州立大学）；Kyoto University（京都大学）
Large magneto-optical Kerr effect induced by collinear antiferromagnetic order
University of Tokyo（东京大学）；RIKEN Center for Emergent Matter Science (CEMS)（理化学研究所 新兴物质科学中心）；Hokkaido University（北海道大学）；Tokyo Metropolitan University（东京都立大学）；PRESTO, Japan Science and Technology Agency (JST)（日本科学技术振兴机构 PRESTO）；Research Center for Advanced Science and Technology (RCAST), University of Tokyo（东京大学 先进科学技术研究中心）；Institute for Solid State Physics, University of Tokyo（东京大学 固体物理研究所）；Department of Physics, University of Tokyo（东京大学 物理学系）；Institute of Engineering Innovation, University of Tokyo（东京大学 工学创新研究所）
The collective mode spectrum of a symmetry-breaking state, such as a superconductor, provides crucial insight into the nature of the order parameter. In this context, we present a microscopic weak-coupling theory for the collective modes of a generic multi-component time-reversal symmetry breaking superconductor, and show that fluctuations in the relative amplitude and phase of the two order parameter components are well-defined underdamped collective modes, even in the presence of nodal quasiparticles. We then demonstrate that these "generalized clapping modes" can be detected using a number …
Electrical Control of Dynamic Spin Splitting Induced by Exchange Interaction as Revealed by Time Resolved Kerr Rotation in a Degenerate Spin-Polarized Electron Gas
