Kramers Weyl semimetals as quantum solenoids and their applications in spin-orbit torque devices

AbstractKramers Weyl semimetals are Weyl semimetals that have Weyl points pinned at the time reversal invariant momenta. Recently it has been discovered that all chiral crystals host Weyl points at time reversal invariant momenta, so metals with chiral lattice symmetry all belong to the category of Kramers Weyl semimetals. In this work, we show that due to the chiral lattice symmetry, Kramers Weyl semimetals have the unique longitudinal magnetoelectric effect in which the charge current induced spin and orbital magnetization is parallel to the direction of the current. This feature allows Kramers Weyl semimetals to act as nanoscale quantum solenoids with both orbital and spin magnetization. As the moving electrons of Kramers Weyl semimetal can generate longitudinal magnetization, Kramers Weyl semimetals can be used for new designs of spin-orbit torque devices with all electric control of magnetization switching for magnets with perpendicular magnetic anisotropy.

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New type of Weyl semimetal with quadratic double Weyl fermions

Proceedings of the National Academy of Sciences ◽

10.1073/pnas.1514581113 ◽

2016 ◽

Vol 113 (5) ◽

pp. 1180-1185 ◽

Cited By ~ 158

Author(s):

Shin-Ming Huang ◽

Su-Yang Xu ◽

Ilya Belopolski ◽

Chi-Cheng Lee ◽

Guoqing Chang ◽

...

Keyword(s):

Experimental Studies ◽

Orbit Coupling ◽

Spin Orbit Coupling ◽

Spin Orbit ◽

Experimental Realization ◽

Weyl Semimetal ◽

Chiral Magnetic Effect ◽

Wide Range ◽

Weyl Semimetals ◽

Weyl Fermions

Weyl semimetals have attracted worldwide attention due to their wide range of exotic properties predicted in theories. The experimental realization had remained elusive for a long time despite much effort. Very recently, the first Weyl semimetal has been discovered in an inversion-breaking, stoichiometric solid TaAs. So far, the TaAs class remains the only Weyl semimetal available in real materials. To facilitate the transition of Weyl semimetals from the realm of purely theoretical interest to the realm of experimental studies and device applications, it is of crucial importance to identify other robust candidates that are experimentally feasible to be realized. In this paper, we propose such a Weyl semimetal candidate in an inversion-breaking, stoichiometric compound strontium silicide, SrSi2, with many new and novel properties that are distinct from TaAs. We show that SrSi2 is a Weyl semimetal even without spin–orbit coupling and that, after the inclusion of spin–orbit coupling, two Weyl fermions stick together forming an exotic double Weyl fermion with quadratic dispersions and a higher chiral charge of ±2. Moreover, we find that the Weyl nodes with opposite charges are located at different energies due to the absence of mirror symmetry in SrSi2, paving the way for the realization of the chiral magnetic effect. Our systematic results not only identify a much-needed robust Weyl semimetal candidate but also open the door to new topological Weyl physics that is not possible in TaAs.

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Fermi-arc diversity on surface terminations of the magnetic Weyl semimetal Co3Sn2S2

Science ◽

10.1126/science.aav2334 ◽

2019 ◽

Vol 365 (6459) ◽

pp. 1286-1291 ◽

Cited By ~ 95

Author(s):

Noam Morali ◽

Rajib Batabyal ◽

Pranab Kumar Nag ◽

Enke Liu ◽

Qiunan Xu ◽

...

Keyword(s):

Electronic Properties ◽

Time Reversal ◽

Surface States ◽

Time Reversal Symmetry ◽

Fermi Arc ◽

Weyl Semimetal ◽

Fermi Arcs ◽

Bulk Surface ◽

Weyl Semimetals ◽

Node Connectivity

Bulk–surface correspondence in Weyl semimetals ensures the formation of topological “Fermi arc” surface bands whose existence is guaranteed by bulk Weyl nodes. By investigating three distinct surface terminations of the ferromagnetic semimetal Co3Sn2S2, we verify spectroscopically its classification as a time-reversal symmetry-broken Weyl semimetal. We show that the distinct surface potentials imposed by three different terminations modify the Fermi-arc contour and Weyl node connectivity. On the tin (Sn) surface, we identify intra–Brillouin zone Weyl node connectivity of Fermi arcs, whereas on cobalt (Co) termination, the connectivity is across adjacent Brillouin zones. On the sulfur (S) surface, Fermi arcs overlap with nontopological bulk and surface states. We thus resolve both topologically protected and nonprotected electronic properties of a Weyl semimetal.

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Kramers nodal line metals

Nature Communications ◽

10.1038/s41467-021-22903-9 ◽

2021 ◽

Vol 12 (1) ◽

Author(s):

Ying-Ming Xie ◽

Xue-Jian Gao ◽

Xiao Yan Xu ◽

Cheng-Ping Zhang ◽

Jin-Xin Hu ◽

...

Keyword(s):

Time Reversal ◽

Nodal Line ◽

Spin Orbit Coupling ◽

Fermi Surfaces ◽

Nodal Lines ◽

New Class ◽

Topological Materials ◽

Weyl Semimetals ◽

Chiral Crystals ◽

Torus Type

AbstractRecently, it was pointed out that all chiral crystals with spin-orbit coupling (SOC) can be Kramers Weyl semimetals (KWSs) which possess Weyl points pinned at time-reversal invariant momenta. In this work, we show that all achiral non-centrosymmetric materials with SOC can be a new class of topological materials, which we term Kramers nodal line metals (KNLMs). In KNLMs, there are doubly degenerate lines, which we call Kramers nodal lines (KNLs), connecting time-reversal invariant momenta. The KNLs create two types of Fermi surfaces, namely, the spindle torus type and the octdong type. Interestingly, all the electrons on octdong Fermi surfaces are described by two-dimensional massless Dirac Hamiltonians. These materials support quantized optical conductance in thin films. We further show that KNLMs can be regarded as parent states of KWSs. Therefore, we conclude that all non-centrosymmetric metals with SOC are topological, as they can be either KWSs or KNLMs.

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Multiorbital singlet pairing and d + d superconductivity

npj Quantum Materials ◽

10.1038/s41535-020-00304-3 ◽

2021 ◽

Vol 6 (1) ◽

Author(s):

Emilian M. Nica ◽

Qimiao Si

Keyword(s):

Time Reversal ◽

Heavy Fermion ◽

Degrees Of Freedom ◽

Spin Orbit Coupling ◽

Spin Orbit ◽

Matrix Structure ◽

Spin Singlet ◽

Low Energies ◽

Triplet Pairing ◽

Orbital Space

AbstractRecent experiments in multiband Fe-based and heavy-fermion superconductors have challenged the long-held dichotomy between simple s- and d-wave spin-singlet pairing states. Here, we advance several time-reversal-invariant irreducible pairings that go beyond the standard singlet functions through a matrix structure in the band/orbital space, and elucidate their naturalness in multiband systems. We consider the sτ3 multiorbital superconducting state for Fe-chalcogenide superconductors. This state, corresponding to a d + d intra- and inter-band pairing, is shown to contrast with the more familiar d + id state in a way analogous to how the B- triplet pairing phase of 3He superfluid differs from its A- phase counterpart. In addition, we construct an analog of the sτ3 pairing for the heavy-fermion superconductor CeCu2Si2, using degrees-of-freedom that incorporate spin-orbit coupling. Our results lead to the proposition that d-wave superconductors in correlated multiband systems will generically have a fully-gapped Fermi surface when they are examined at sufficiently low energies.

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Low-Energy Optical Conductivity of TaP: Comparison of Theory and Experiment

Crystals ◽

10.3390/cryst11050567 ◽

2021 ◽

Vol 11 (5) ◽

pp. 567

Author(s):

Alexander Yaresko ◽

Artem V. Pronin

Keyword(s):

Experimental Data ◽

Optical Conductivity ◽

Low Energy ◽

Spin Orbit Coupling ◽

Spin Orbit ◽

Natural Explanation ◽

Weyl Semimetal ◽

Unintentional Doping ◽

Mott Formula ◽

Theory And Experiment

The ab-plane optical conductivity of the Weyl semimetal TaP is calculated from the band structure and compared to the experimental data. The overall agreement between theory and experiment is found to be best when the Fermi level is slightly (20 to 60 meV) shifted upwards in the calculations. This confirms a small unintentional doping of TaP, reported earlier, and allows a natural explanation of the strong low-energy (50 meV) peak seen in the experimental ab-plane optical conductivity: this peak originates from transitions between the almost parallel non-degenerate electronic bands split by spin-orbit coupling. The temperature evolution of the peak can be reasonably well reproduce by calculations using an analog of the Mott formula.

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