# Abstract (WS-C, poster)

PS-C1

canceled

PS-C2 Spin Hall Effect and Quantum Spin Vorticity Principle
Fukuda, Masahiro
Department of Micro Engineering, Kyoto University, Japan
Participation: from 11/17 to 11/21

Abstract:
The spin Hall effect (SHE), which refers to the conversion of an electric current into a transverse flow of spin, is one of the most important phenomena in the field of spintronics. In this study, it is proposed that the SHE can be explained as the conversion of kinetic momentum into electronic spin vorticity on the basis of the "quantum spin vorticity principle". Similarly, it is proposed that the inversed SHE can be explained as the conversion of electronic spin vorticity generated by applied spin torque into kinetic momentum.

PS-C3 A density functional study of an interaction-driven Chern insulator realized on optical lattice systems
Kitamura, Sota
Department of Physics, University of Tokyo, Japan
Participation: from 11/14 to 11/21

Abstract:
One of the most remarkable consequences of many-body effects on topological systems is a possibility of interaction-induced spontaneous symmetry breaking toward non-trivial topological insulators, which is called a topological Mott insulator (TMI). While the TMI phases are proposed in various tight-binding models with long-range interactions, condensed-matter realization of the TMI has yet to be achieved. On the other hand, realization and detection of non-trivial topological quantities have been extensively studied in cold atom systems. Motivated by such progresses, we propose here a scheme to realize TMI in cold atoms on optical lattices. One of key ingredient in the TMI phase is the inter-site interaction, which is usually too small in cold atom systems with short-ranged interactions. We resolve this difficulty by employing spin-dependent optical lattices. We then confirm this idea numerically by extending the existing density functional theory for cold atom systems to a formalism with the non-collinear spin density functional. This has enabled us to actually demonstrate a existence of the TMI phase. Namely, the proposed system turns out to exhibit a phase transition from a semimetalic phase to the Chern insulator as the repulsive interaction is increased.

PS-C4

canceled

PS-C5 Effect of Interlayer Spin-flip Tunneling for Interlayer Magnetoresistance in Multilayer Dirac Fermion Systems
Kubo, Kenji
Kyoto University, Japan
Participation: from 11/4 to 12/5

Abstract:
We investigate the effect of the interlayer spin-flip tunneling for the interlayer magnetoresistance under magnetic fields in $$\alpha$$-(BEDT-TTF)$$_2$$I$$_3$$, which is a multilayer massless Dirac fermion system under pressure. The mean field of the spin-flip correlation associated with the interlayer Coulomb interaction enables the interlayer spin-flip tunneling. Assuming the non-vertical interlayer spin-flip tunneling, we calculate the interlayer magnetoresistance using the Kubo formula. The crossover magnetic field, at which the interlayer magnetoresistance changes from positive to negative is shifted by the Zeeman energy and in good agreement with the experiment.

PS-C6 Excitonic mass generation in Dirac electron systems
Matsuno, Genki
Nagoya University, Japan
Participation: from 11/17 to 11/21

Abstract:
It is considered that massless Dirac electrons are realized in an organic conductor $$\alpha$$-(BEDT-TTF)$$_2$$I$$_3$$ in low temperature, under high pressure. But it has been observed that a logarithmic increase of resistivity with decreasing temperature. For the mechanism of this behavior, we consider excitonic mass generation. Excitonic mass generation occurs by longrange coulomb repulsion, which is important in 2-dimensional Dirac electron systems. For simplification, we studied about axcitonic order in honeycomb lattice model(carefully about atomic arrangement) and which order is realized among possible order parameters with different symmetries. In this presentation, we will discuss about the character of the order and effects for transport properties. The case of $$\alpha$$-(BEDT-TTF)$$_2$$I$$_3$$ is also reported.

PS-C7 Meissner Effect in Superconducting Dirac Electrons
Mizoguchi, Tomonari
Department of Physics, The University of Tokyo, Japan
Participation: from 11/17 to 11/21

Abstract:
Dirac electrons in solids, characterized by the linear dispersion relation, attract lots of interests because of their unique transport properties. Although the transport phenomena of Dirac electrons in a normal state are intensively studied, there have not been the transport phenomena in a superconducting state. In particular, it is unclear whether the superconducting Dirac electrons show the Meissner effect, because the diamagnetic part in the current operator is absent.
In this presentation, we discuss the Meissner effect of relativistic Dirac electrons in a superconducting state. We clarify that the Messier kernel becomes finite by use of inter-band contributions.

PS-C8 Study of spin current in three-dimensional Dirac system
Department of Physics, University of Tokyo, Japan
Participation: from 11/17 to 11/21

Abstract:
A spin current in a 3D Dirac system like bismuth has been studied for several years. However, the definition of the proper spin current in this system is still controversial.
We derive the time derivative of spin for the 4x4 effective Dirac Hamiltonian of bismuth in the linear response regime. We define a new spin current in the derivation and construct the spin continuity equation. We also discuss the spin Hall conductivity using our definition of the spin current.

PS-C9 Density-Matrix Renormalization Group Study of Effective Spin Model for Na2IrO3
Shinjo, Kazuya
YITP, Kyoto Univ. , RIKEN, Japan
Participation: from 11/17 to 11/28

Abstract:
We study extended Kitaev-Heisenberg honeycomb lattice model that has recently been proposed to describe magnetic properties in A$$_2$$IrO$$_3$$ (A=Na,Li). The model shows exotic orders including Neel, stripy and zigzag antiferromagnetic orders and quantum spin liquid states. By using two-dimensional density-matrix renormalization group method, we find that the zigzag order appears in a parameter regime relevant to Na$$_2$$IrO$$_3$$. Studying spin correlation functions and entanglement structure, we make phase diagram of the model.

PS-C10 Topology of insulators and superconductors with additional order-two point group symmetries
Shiozaki, Ken
Department of Physics, Kyoto University, Japan
Participation: from 11/4 to 12/5

Abstract:
Topological crystalline insulators and superconductors are gapped free fermion topological phases protected by space group symmetries, besides any of ten classes of symmetries defined by time-reversal symmetry and particle-hole symmetry. In this presentation, we show the classification of topological crystalline insulators and superconductors with an additional order-two point group symmetry. The additional order-two point group symmetry we consider is general and it includes Z$$_2$$ global symmetry, mirror reflection, two-fold rotation, inversion, and their anti-unitary symmetries (such as magnetic point group symmetry), and also their "anti-symmetries" that anticommutes with Hamiltonians. The additional order-two symmetry provides an additional generator of the Clifford algebra and shifts classifying spaces of Hamiltonians. We can construct the dimensional hierarchy of the K-groups with the additional symmetry. We find that the topological periodic table shows a novel periodicity in the number of flipped coordinates under the order-two spatial symmetry, in addition to the Bott-periodicity in the space dimensions. Various symmetry protected topological phases and gapless modes will be identified and discussed in a unified framework. Obtained K-groups suggest that defect zero modes can be considered as boundary states of lower-dimensional crystalline insulators and superconductors. We also present topological classification of symmetry protected Fermi points. The bulk classification and the surface Fermi point classification provide a novel realization of the bulk-boundary correspondence in terms of the K-theory.
[1] Ken Shiozaki and Masatoshi Sato, arXiv:1403.3331.

PS-C11 Topological Thermoelectric Response in Superconductors
Sumiyoshi, Hiroaki
Kyoto University, Japan
Participation: from 11/4 to 12/2

Abstract:
In some classes of superconductors, the Berry phase due to nontrivial $$\boldsymbol{k}$$-space geometry is inherent in the pairing states and causes exotic transport phenomena. We have studied the Nernst effect due to the Berry phase in superconducting fluctuation regime of chiral superconductors, and shown that the new contribution can successfully explain the resent experimental result of URu$$_2$$Si$$_2$$, which can not be understood within any previous theories. In this presentation, we also discuss other thermoelectric transport phenomena due to the Berry phase in superconductors.

PS-C12 Critical fluctuation induced dissipationless anomalous Hall effect
Takashima, Rina
Department of Physics, Kyoto University, Japan
Participation: from 11/17 to 11/21

Abstract:
The intrinsic anomalous Hall effect (AHE) in metal is induced by the Berry curvature of the band, and gives the dissipationless Hall current, which is independent of the relaxation time. So far this effect has been discussed mainly in static systems, such as Rashba spin-orbit coupled system under a uniform exchange field. Here, in our work, we propose a novel scenario of dynamically induced AHE. We consider a system with spin-orbit coupling and a fluctuating exchange field. We demonstrate that even when the berry curvature fluctuates and is averaged to zero in real space, the dissipationless AHE still appears under magnetic field.

PS-C13 Mott physics on topological insulators in one dimension
Yoshida, Tsuneya
RIKEN, Japan
Participation: from 11/10 to 11/28

Abstract:
In these years, topological properties of correlated systems have attracted much interest since such systems host gapless edge states which are source of novel phenomena; for instance, quantization of (spin) Hall conductivity, topological magnetoelectric effects, realization of Majorana fermion etc.. One of the important issues of this field is the correlation effect. Although this issue is extensively studied, there are still open problems to be addressed. For example, a topological Mott insulator whose nontrivial structure is reflected only in gapless spinon excitations at edges is proposed by using a sort of slave-boson theory, while more accurate numerical frameworks, such as quantum Monte Carlo, do not support it. Hence, establishment of this phase by a more accurate approach is desired. In addition to this, properties of topological phase transitions in correlated systems are also left as open issues.
In this paper, we address these two issues in one-dimensional systems by calculating the winding number and entanglement spectrum [1]. Our systematic analysis of bulk and edge behaviors shows a clear evidence of this exotic phase and reveals the relation between symmetry and gapless edge modes in the single-particle excitation spectrum. Furthermore, we propose a new type of continuous topological phase transition accompanied by gap-closing in a collective mode instead of that in the one-particle density of states. Our DMRG analysis demonstrates that such a transition occurs in spin-liquid phases.
[1] T. Yoshida et al, Phys. Rev. Lett. 112, 196404 (2014).