Edelstein effect and diode effect in noncentrosymmetric superconductors

概要

Summary of thesis:
Edelstein effect and diode effect
in noncentrosymmetric superconductors
Yuhei Ikeda
In this thesis, I report a theoretical investigation of various transport response
phenomena in noncentrosymmetric superconductors. Inversion symmetry breaking
affects electronic states in the form of spin-orbit interactions. As a result, phenomena
called Fermi surface splitting (Rashba splitting) and spin-momentum locking occurs. In
the superconducting state, a peculiar quantum state also appears. This paper aims to
investigate the effect of such a peculiar quantum state on the transport response quantities.
In this thesis, I describe the following two main results.

Discovery of an enhancement of the Edelstein effect (magneto-electric effect)
on the surface of d-wave topological superconductors (Chap.2).
Discovery of sign reversal and efficiency enhancement of the superconducting
diode effect under weak impurity scattering (Chap.3).
In Chapter 2, we discuss the Edelstein effect in superconductors. In
systems with inversion symmetry braking, the spin and momentum of
electrons are coupled due to the spin-orbit coupling and the Fermi surface
splits, as described above. It is known that applying an electric current to
such a system induces spin magnetization. In the field of spintronics, this
phenomenon is called the Edelstein effect. Switching of magnetization
domains by electric current using this phenomenon has been reported and is
attracting particular attention in terms of applications. However, it is known
that domain switching using this effect usually requires a huge current
density, and the Joule heating created by the dissipative current is
problematic. Therefore, spintronics using dissipation-free superconducting
currents is a solution to the problem. On the other hand, in the field of
superconductors, it has been reported that there exists a surface state
protected by topological invariants in d-wave superconductors with spatial
inversion symmetry breaking. This surface state is called the surface
Majorana state due to its similarity. This surface localized state is known to
have a peak near zero energy due to particle-hole symmetry. I have calculated
the Edelstein effect using this surface Majorana state, noting that it is an
ideal stage for obtaining giant transport response phenomena. I found that
the Edelstein effect is enhanced by about two orders of magnitude at the

surface of the d-wave superconductor. Furthermore, it is found that the
enhancement at the surface is caused by the surface Majorana state.
In Chapter 3, we discuss the superconducting diode effect. Nonreciprocal
transport phenomena are one of the central research topics in modern
condensed matter physics. Nonreciprocal transport refers to the
inequivalence of transport response quantities in one and opposite directions.
A concrete example is the rectification effect of an ideal diode, i.e., a
phenomenon with finite resistance in one direction and infinite resistance in
the opposite direction. Recently, a non-equivalence in the superconducting
critical current was discovered in the superconductor Nb/V/Ta. By
appropriately adjusting the magnetic field and applied current, it is possible
to create a situation in which a non-dissipative supercurrent flows in one
direction and a dissipative current flows in the opposite direction. This is
called the superconducting diode effect. Since the diode is one of the
fundamental devices of modern electronics, energy-saving circuits based on it
are expected to be developed, and it is essential to further research and
development. Microscopic theoretical calculations in the deparing mechanism
have revealed that the superconducting diode effect increases beyond the
scaling law near the superconducting transition temperature and that the
sign of the diode effect reverses in the low-temperature high-field region.
Interestingly, the magnetic field region where the sign of the diode effect
reverses roughly coincides with the crossover region of the superconducting
state called helical superconductivity. Most of the above studies assume a
pure superconductor, but impurity effects are an unavoidable problem in real
superconductors. The diode effect at low temperatures exhibits interesting
features, but the impurity effect in this regime has not been clarified. I
formulate a microscopic theory for the intrinsic superconducting diode effect
with disorder and find that the sign reversal of the diode effect is preserved
under weak impurity effects. Furthermore, the peak position of the diode
effect was found to capture the precursor of the helical crossover. This
indicates that the superconducting diode effect has the potential to be an
experimental probe of helical superconductivity. It was also found that the
efficiency of the diode effect can be increased from about 15% to 20%.
Both of the above results clarify transport phenomena unique to
noncentrosymmetric superconductors and are expected to lead to the
development of properties of thin-film superconductors. ...