Topological Quantum Phenomena in Condensed Matter with Broken Symmetries

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■ Core Research Group A01
 "Novel Edge States in Superconductors with Broken Time-Reversal Symmetry"

Superconductivity is a state of matter with broken gauge invariance in which electrons pair and acquire coherent phase of their wave function. For some superconductors time-reversal symmetry (TRS) may be additionally broken because of the spontaneous alignment of the orbital moment (leading to the "chiral" state) or spins ("non-unitary" state) of the Cooper pairs. The research plans of the group A01 is to investigate topological quantum phenomena in superconductors with broken TRS.

First, we aim to obtain thorough characterization of the "bulk" superconducting properties of the ruthenium-oxide (ruthenate) superconductors that we discovered and for which firm evidence for spin-triplet pairing with broken TRS has been accumulated. Second, we extend the research to microcrystals and junctions using the ruthenate superconductors and pursue emerging phenomena at the surfaces and junction boundaries. Third, we investigate TRS broken pairing states in junctions between a conventional spin-singlet superconductor and a ferromagnetic semiconductor, which itself breaks TRS.

Many of these phenomena can be topologically characterized and are intimately related to the concepts the project members (including those in the Core Research Group D01) have made significant contribution to, such as the chiral edge current, spin supercurrent, odd-frequency pairing, and inverse-proximity effect. These concepts are commonly applicable to a variety of other physical systems. In close collaboration with the Core Research Group B01 investigating charge-neutral systems such as spin-triplet superfluid of liquid helium 3, and the Core Research Group C01 investigating superconductors and insulators with broken inversion-symmetry, we will contribute to the foundation of the topological quantum physics.


■ Core Research Group B01
 "Novel Edge Phenomena in Spin-Triplet Superfluids"

The purpose of this project "Novel Edge Phenomena in Spin-Triplet Superfluids" is to investigate novel quantum phenomena in spin-triplet superfluid 3He. In particular, we are planning to study nature of the topologically protected states at the surface of superfluid itself and at the interface with another materials. Using the nuclear magnetic resonance (NMR) technique in the rotating cryostat at ultra low temperatures, we intend to determine the spatial variation of order parameter in 3He-A phase in a restricted geometry. At the same time, we will fix the long term unsolved issue called 'intrinsic angular momentum of A phase'. Recently the Andreev bound state at a surface of 3He-B phase has been observed by the transverse ultrasonic spectroscopy. According to theoretical predictions, the bound states is described by the Majorana fermion excitation. We will study characteristic features of the Majorana bound state both experimentally and theoretically. Superfluid in slab or thin film is expected to show a novel phase under the external magnetic field. The mass supercurrent flowing along the edge of film may be a characteristic phenomenon in such phase and is related to the electric edge current in the chiral superconductor (Core Research Group A01) and the spin edge current in the NSC superconductor or topological insulators (Core Research Group C01). In collaboration with the Core Research Group D01, this project plans to study new topological phenomena such as a half quantized vortex, the Majorana excitation at the core of a singular vortex, and the odd-frequency pairing state in aerogel.


■ Core Research Group C01
 "Novel Phenomena in Electron Fluids with Broken Inversion Symmetry"

We explore exotic phenomena due to broken symmetries and spin-orbit interaction in systems such as non-centrosymmetric superconductors, surfaces where carrier density can be tuned by the electric field, as well as topological insulators.
By combining various experimental investigations on high-quality samples and theoretical study such as band calculation, we aim to elucidate the novel physics, particularly its topological aspect, and develop new concepts in these systems.


■ Core Research Group D01
 "Theory of Topological Condensates"

The aim of our project "Theory of quantum phenomena in condensed matter physics" is to study novel quantum phenomena in unconventional superconductors, superfluids, Bose Einstein Condensates (BEC), and topological insulators. The states of these materials in bulk are characterized by non-trivial topological numbers, which leads to the appearance of topologically protected edge states at a surface of these materials or at an interface with another materials. We are planning to clarify universal phenomena common to these topological materials and predict unique phenomena peculiar to each material. The final goal of theory group is to propose novel concepts in condensed-matter physics concerning the topological quantum phenomena. The main topics are as follows.

  • Quantum phenomena in edge state and relevant topics in non-centrosymmetric superconductors and superconductors with broken time reversal symmetry.
  • Novel type of Cooper pair like odd-frequency pairing
  • Electronic structure and edge states of topological insulator
  • New quantum phenomena on the topological insulator
  • BEC with internal degree of freedom and its topological excitation
  • Non Abelian vortex in BEC
  • Topological vortex in superfluid
  • Universal and underlying concept in topological quantum phenomena

In collaboration with experimental groups, we aim to develop new concepts in the present research area.