2016 Fiscal Year Annual Research Report
| Project/Area Number |
16J07545
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| Research Institution | The University of Tokyo |
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Principal Investigator |
張 驍驍 東京大学, 大学院工学系研究科(工学部), 特別研究員(DC1)
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| Project Period (FY) |
2016-04-22 – 2019-03-31
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| Keywords | skyrmion / monopole / magnetoresistivity / ultrasonic response / Weyl semimetal / Floquet theory / photoinduced phases / Luttinger liquid |
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| Outline of Annual Research Achievements |
We studied a three-dimensional chiral magnet system inhabited by emergent magnetic monopoles connected by skyrmion tubes, exploring the transport and elastic properties due to the influence from this skyrmion/monopole lattice. In the presence of coupling between spin waves and electrons/phonons, we used Green’s functions or effective models to explore the effects originated from the topologically nontrivial magnetic texture upon which spin waves are generated. The major results are predicting a hump-dip-peak longitudinal magnetoresistivity, reproducing the anisotropic ultrasonic responses experimentally observed, and identifying a new topological phase transition. And we compared the experiments with the theory and discussed their corroboration.
We turned to photoinduced nonequilibrium states to find new topological properties of matter. We analytically and numerically found that, in the presence of a linearly polarized light, a tunable model Hamiltonian can exhibit complex topological Weyl semimetal phases, which is controllable by the frequency of the driving term. We also made use of the Keldysh formalism in order to calculate the physically observable Hall conductivity. We studied its dependence on the periodic drive and the cause of the non-monotonic profile.
We focused on the chiral 1D channels induced by a strong magnetic field in Weyl semimetals. We formulated the low-energy theory of the correlated electrons in terms of Tomonaga-Luttinger liquid with Coulomb interactions. Localization effects due to disorders are to be considered.
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| Current Status of Research Progress |
Current Status of Research Progress
1: Research has progressed more than it was originally planned.
Reason
The progress is beyond the plan made one year ago. The project about electric transport in the skyrmion/monopole lattice in MnGe was completed and published soon after the beginning of the fellowship. The major project in my plan about ultrasonic elastic responses was completed and submitted. The theory we finally obtained is basically as we expected. In all, we predicted a hump-dip-peak longitudinal magnetoresistivity, reproduced the anisotropic ultrasonic responses experimentally observed, and identified a new topological phase transition. Fortunately, there is indispensable input from the experimentalists for us to justify the validity of our theory.
Besides, we further kept an eye on other latest trends in the condensed matter physics community. In the first place, we considered the Floquet bands generated in a graphene-like Dirac system both analytically and numerically. Unfortunately, at the final stage of this study, we noticed a very similar work published half a year earlier. Recently, Weyl fermion has been experimentally observed in condensed matter systems and continues attracting attention because of its unique topological state. Therefore, we tried to produce photoinduced Weyl semimetal phases by applying linearly polarized light to a band insulator. Furthermore, we considered phenomena in Weyl semimetals under a strong external magnetic field.
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| Strategy for Future Research Activity |
Weyl points in solids are actually magnetic monopoles of the Berry phase magnetic fields. They are exactly the counterpart in momentum space of the real-space monopole defects in the skyrmion lattice we studied previously. We reckon it as a promising research topic successive to the real-space ones. And in the light of our experience till now, we indeed hope to think more along this line.
We will finish our Tomonaga-Luttinger liquid study in the first place. Afterwards, we plan for including long-range interaction effects to a Weyl semimetal system in a mean-field study. The basic idea is to develop an iterative routine so as to self-consistently solve the Hamiltonian by taking all possible pairing order parameters into account. We expect to observe, upon increasing the interaction strength, two types of finite interband excitonic pairings, i.e., internode and intranode ones, and gap opening due to internode couplings. In accordance with our plan made one year ago, we as well hope to couple our systems with monopole-like features, i.e., monopole lattice and Weyl semimetal, with other interesting systems such as crystallographic defects and genuine Dirac magnetic monopoles. Specifically, we will consider Weyl/Dirac electrons scattered by Dirac monopoles and other potential profiles and Weyl/Dirac semimetal with spatial confinements. As an estimation in the first place, we are aware of the possible obstacles to analytic solutions. Therefore, we need to learn and employ numerical methods of ordinary/partial differential equations.
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Research Products
(8 results)
