Model for ferromagnetic Weyl and nodal line semimetals: Topological invariants, surface states, anomalous and spin Hall effect

AbstractSpin Hall effect (SHE) has its special position in spintronics. To gain new insight into SHE and to identify materials with substantial spin Hall conductivity (SHC), we performed high-precision high-throughput ab initio calculations of the intrinsic SHC for over 20,000 nonmagnetic crystals. The calculations revealed a strong relationship between the magnitude of the SHC and the crystalline symmetry, where a large SHC is typically associated with mirror symmetry-protected nodal line band structures. This database includes 11 materials with an SHC comparable to or even larger than that of Pt. Materials with different types of spin currents were additionally identified. Furthermore, we found that different types of spin current can be obtained by rotating applied electrical fields. This improves our understanding and is expected to facilitate the design of new types of spin-orbitronic devices.

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Non-Hermiticity and topological invariants of magnon Bogoliubov–de Gennes systems

Progress of Theoretical and Experimental Physics ◽

10.1093/ptep/ptaa151 ◽

2020 ◽

Vol 2020 (12) ◽

Author(s):

Hiroki Kondo ◽

Yutaka Akagi ◽

Hosho Katsura

Keyword(s):

Hall Effect ◽

Topological Insulators ◽

Three Dimensional ◽

Surface States ◽

Theoretical Models ◽

Topological Invariant ◽

Topological Invariants ◽

Topological Phases ◽

Ideal Candidate ◽

Edge Surface

Abstract Since the theoretical prediction and experimental observation of the magnon thermal Hall effect, a variety of novel phenomena that may occur in magnonic systems have been proposed. We review recent advances in the study of topological phases of magnon Bogoliubov–de Gennes (BdG) systems. After giving an overview of previous works on electronic topological insulators and the magnon thermal Hall effect, we provide the necessary background for bosonic BdG systems, with particular emphasis on their non-Hermiticity arising from the diagonalization of the BdG Hamiltonian. We then introduce definitions of $$ \mathbb{Z}_2 $$ topological invariants for bosonic systems with pseudo-time-reversal symmetry, which ensures the existence of bosonic counterparts of “Kramers pairs.” Because of the intrinsic non-Hermiticity of bosonic BdG systems, these topological invariants have to be defined in terms of the bosonic Berry connection and curvature. We then introduce theoretical models that can be thought of as magnonic analogs of two- and three-dimensional topological insulators in class AII. We demonstrate analytically and numerically that the $$ \mathbb{Z}_2 $$ topological invariants precisely characterize the presence of gapless edge/surface states. We also predict that bilayer CrI$$_3$$ with a particular stacking would be an ideal candidate for the realization of a two-dimensional magnon system characterized by a nontrivial $$ \mathbb{Z}_2 $$ topological invariant. For three-dimensional topological magnon systems, the magnon thermal Hall effect is expected to occur when a magnetic field is applied to the surface.

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Determination of Out‐of‐Plane Spin Polarization of Topological Surface States by Spin Hall Effect Tunneling Spectroscopy

physica status solidi (b) ◽

10.1002/pssb.202000032 ◽

2020 ◽

pp. 2000032

Author(s):

Matthias Götte ◽

Thomas Dahm

Keyword(s):

Hall Effect ◽

Spin Polarization ◽

Surface States ◽

Tunneling Spectroscopy ◽

Spin Hall Effect ◽

Out Of Plane ◽

Topological Surface

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The spin Hall effect of Bi-Sb alloys driven by thermally excited Dirac-like electrons

Science Advances ◽

10.1126/sciadv.aay2324 ◽

2020 ◽

Vol 6 (10) ◽

pp. eaay2324 ◽

Cited By ~ 5

Author(s):

Zhendong Chi ◽

Yong-Chang Lau ◽

Xiandong Xu ◽

Tadakatsu Ohkubo ◽

Kazuhiro Hono ◽

...

Keyword(s):

Hall Effect ◽

Room Temperature ◽

Spin Current ◽

Surface States ◽

Spin Hall Effect ◽

Narrow Gap ◽

Spin Conversion ◽

Topological Surface ◽

Sputter Deposited ◽

Increasing Temperature

We have studied the charge to spin conversion in Bi1−xSbx/CoFeB heterostructures. The spin Hall conductivity (SHC) of the sputter-deposited heterostructures exhibits a high plateau at Bi-rich compositions, corresponding to the topological insulator phase, followed by a decrease of SHC for Sb-richer alloys, in agreement with the calculated intrinsic spin Hall effect of Bi1−xSbx. The SHC increases with increasing Bi1−xSbx thickness before it saturates, indicating that it is the bulk of the alloy that predominantly contributes to the generation of spin current; the topological surface states, if present, play little role. Unexpectedly, the SHC is found to increase with increasing temperature, following the trend of carrier density. These results suggest that the large SHC at room temperature, with a spin Hall efficiency exceeding 1 and an extremely large spin current mobility, is due to increased number of thermally excited Dirac-like electrons in the L valley of the narrow gap Bi1−xSbx alloy.

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