arXiv subjects

Yasumasa Tsutsumi

Publications and source records attributed to Yasumasa Tsutsumi.

28 records · Page 2Linked to original sources

arXiv2013

UPt$_3$ as a Topological Crystalline Superconductor

We investigate the topological aspect of the spin-triplet $f$-wave superconductor UPt$_3$ through microscopic calculations of edge- and vortex-bound states based on the quasiclassical Eilenberger and Bogoliubov-de Gennes theories. It is shown that a gapless and linear dispersion exists at the edge of the $ab$-plane. This forms a Majorana valley, protected by the mirror chiral symmetry. We also demonstrate that, with increasing magnetic field, vortex-bound quasiparticles undergo a topological phase transition from topologically trivial states in the double-core vortex to zero-energy states in the normal-core vortex. As long as the $d$-vector is locked into the $ab$-plane, the mirror symmetry holds the Majorana property of the zero-energy states, and thus UPt$_3$ preserves topological crystalline superconductivity that is robust against the crystal field and spin-orbit interaction.

cond-mat.supr-con↗

arXiv2013

Surface Bound States and Spontaneous Current in Cyclic d-Wave Superconductors

On the basis of Eilenberger theory, surface bound states and spontaneous current are studied in cyclic d-wave superconductors as a broken time-reversal symmetry of superconductivity in cubic lattice symmetry. We discuss how the spontaneous current and the electronic states depend on the orientation of the surface relative to the symmetry of superconductivity. The condition for topological Fermi arcs of zero-energy surface bound states to appear is identified in the complex pairing function of the cyclic d-wave.

cond-mat.supr-con↗

arXiv2012

Edge Current due to Majorana Fermions in Superfluid $^3$He A- and B-Phases

We propose a method utilizing edge current to observe Majorana fermions in the surface Andreev bound state for the superfluid $^3$He A- and B-phases. The proposal is based on self-consistent analytic solutions of quasi-classical Green's function with an edge. The local density of states and edge mass current in the A-phase or edge spin current in the B-phase can be obtained from these solutions. The edge current carried by the Majorana fermions is partially cancelled by quasiparticles (QPs) in the continuum state outside the superfluid gap. QPs contributing to the edge current in the continuum state are distributed in energy even away from the superfluid gap. The effect of Majorana fermions emerges in the depletion of the edge current by temperature within a low-temperature range. The observations that the reduction in the mass current is changed by $T^2$-power in the A-phase and the reduction in the spin current is changed by $T^3$-power in the B-phase establish the existence of Majorana fermions. We also point out another possibility for observing Majorana fermions by controlling surface roughness.

cond-mat.supr-con↗

arXiv2012

A Spin Triplet Superconductor UPt$_3$

Motivated by a recent angle-resolved thermal conductivity experiment that shows a twofold gap symmetry in the high-field and low-temperature C phase in the heavy-fermion superconductor UPt$_3$, we group-theoretically identify the pairing functions as $E_{1u}$ with the $f$-wave character for all the three phases. The pairing functions are consistent with the observation as well as with a variety of existing measurements. By using a microscopic quasi-classical Eilenberger equation with the identified triplet pairing function under applied fields, we performed detailed studies of the vortex structures for three phases, including the vortex lattice symmetry, the local density of states, and the internal field distribution. These quantities are directly measurable experimentally by SANS, STM/STS, and NMR, respectively. It is found that, in the B phase of low $H$ and low $T$, the double-core vortex is stabilized over a singular vortex. In the C phase, thermal conductivity data are analyzed to confirm the gap structure proposed. We also give detailed comparisons of various proposed pair functions, concluding that the present scenario of $E_{1u}$ with the $f$-wave, which is an analogue to the triplet planar state, is better than the $E_{2u}$ or $E_{1g}$ scenario. Finally, we discuss the surface topological aspects of Majorana modes associated with the $E_{1u}^f$ state of planar like features.

cond-mat.supr-con↗

arXiv2012

Half-Quantum Vortices in Thin Film of Superfluid $^3$He

Stability of a half-quantum vortex (HQV) in superfluid $^3$He has been discussed recently by Kawakami, Tsutsumi and Machida in Phys. Rev. B {\bf 79}, 092506 (2009). We further extend this work here and consider the A$_2$ phase of superfluid $^3$He confined in thin slab geometry and analyze the HQV realized in this setting. Solutions of HQV and singly quantized singular vortex are evaluated numerically by solving the Ginzburg-Landau (GL) equation and respective first critical angular velocities are obtained by employing these solutions. We show that the HQV in the A$_2$ phase is stable near the boundary between the A$_2$ and A$_1$ phases. It is found that temperature and magnetic field must be fixed first in the stable region and subsequently the angular velocity of the system should be increased from zero to a sufficiently large value to create a HQV with sufficiently large probability. A HQV does not form if the system starts with a fixed angular velocity and subsequently the temperature is lowered down to the A$_2$ phase. It is estimated that the external magnetic field with strength on the order of 1 T is required to have a sufficiently large domain in the temperature-magnetic field phase diagram to have a stable HQV.

cond-mat.supr-con↗

arXiv2012

Edge mass current and roles of Majorana Fermions in superfluid 3He A-phase

The total angular momentum associated with the edge mass current flowing at the boundary in the superfluid $^3$He A-phase confined in a disk is proved to be $L=N\hbar/2$, consisting of $L^{\rm MJ}=N\hbar$ from the Majorana quasi-particles (QPs) and $L^{\rm cont}=-N\hbar/2$ from the continuum state. We show it based on an analytic solution of the chiral order parameter for quasi-classical Eilenberger equation. Important analytic expressions are obtained for mass current, angular momentum, and density of states (DOS). Notably the DOS of the Majorana QPs is exactly $N_0/2$ ($N_0$: normal state DOS) responsible for the factor 2 difference between $L^{\rm MJ}$ and $L^{\rm cont}$. The current decreases as $E^{-3}$ against the energy $E$, and $L(T) \propto -T^2$. This analytic solution is fully backed up by numerically solving the Eilenberger equation. We touch on the so-called intrinsic angular momentum problem.

cond-mat.supr-con↗

arXiv2012

Superconducting gap structure of CeIrIn5 from field-angle-resolved measurements of its specific heat

In order to identify the gap structure of CeIrIn5, we measured field-angle-resolved specific heat C(phi) by conically rotating the magnetic field H around the c axis at low temperatures down to 80 mK. We revealed that C(phi) exhibits a fourfold angular oscillation, whose amplitude decreases monotonically by tilting H out of the ab plane. Detailed microscopic calculations based on the quasiclassical Eilenberger equation confirm that the observed features are uniquely explained by assuming the dx2-y2-wave gap. These results strongly indicate that CeIrIn5 is a dx2-y2-wave superconductor and suggest the universal pairing mechanism in CeMIn5 (M = Co, Rh, and Ir).

cond-mat.supr-con↗

arXiv2011

Field Evolution of the Fulde-Ferrell-Larkin-Ovchinnikov State in a Superconductor with Strong Pauli Effects

The Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phase in the vortex lattice state is quantitatively studied using the selfconsistent Eilenberger theory in three-dimensional (3D) space. We estimate free energy to determine the FFLO phase diagram in the H-T plane and stable FFLO wave number in the isotropic system with the 3D Fermi sphere and s-wave pairing. To facilitate the experimental identification of the FFLO state, we investigate the field evolution of NMR spectra and flux line lattice form factors obtained in neutron scattering in the FFLO vortex states. Possible applications of our results to experimental data on CeCoIn5 are mentioned.

cond-mat.supr-con↗

arXiv2009

Textures and Vortices in d-Wave Fermi Condensates in Atomic Gases

Fundamental properties of superfluids with d-wave pairing symmetry are investigated theoretically. We consider neutral atomic Fermi gases in a harmonic trap, the Cooper pairing being produced by a Feshbach resonance via a d-wave interaction channel. A Ginzburg-Landau (GL) functional is constructed which is symmetry constrained for five component order parameters (OPs). We find stable OP textures and vortices for all the three phases which are known to be the energy minimum of the GL functional; the ferromagnetic, polar and cyclic phases both at rest and under rotation. In particular, we touch upon the stability conditions for a non-Abelian fractional 1/3-vortex in the cyclic phase under rotation. It is proposed how to create the intriguing 1/3-vortex experimentally in atomic gases via optical means.

cond-mat.supr-con↗

arXiv2009

Singular and Half-Quantum Vortices and Associated Majorana Particles in Superfluid 3He-A between Parallel Plates

Motivated by a recent experiment on superfluid 3He-A confined in narrow parallel plates using a rotating cryostat, we explore possible vortices stable under magnetic field applied to arbitrary angle relative to the plates in order to seek vortices which can accommodate the Majorana zero mode in the core. After proving that the singular vortex with the unit winding number provides the Majorana mode in the spinful situation, we establish the phase diagram in the plane; the rotation frequency $Ω$ vs system size R by finding possible order parameter textures within the Ginzburg-Landau framework. We also analyze the stability for a single and a pair of half-quantum vortices, which possesses the Majorana mode in its core. It is concluded from the above mention that the Majorana zero mode can be found in the present on-going experimental setting at ISSP, Univ. Tokyo.

cond-mat.supr-con↗