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Nakayama, Katsumasa*; Suzuki, Kei
Physics Letters B, 843, p.138017_1 - 138017_7, 2023/08
Times Cited Count：0 Percentile：0.02(Astronomy & Astrophysics)The Casimir effect is a quantum phenomenon induced by the zero-point energy of relativistic fields confined in a finite-size system. This effect for photon fields has been studied for a long time, while the realization of counterparts for fermion fields in Dirac/Weyl semimetals is an open question. We theoretically demonstrate the typical properties of the Casimir effect for relativistic electron fields in Dirac/Weyl semimetals and show the results from an effective Hamiltonian for realistic materials such as Cd
As
and NaBi. We find an oscillation of the Casimir energy as a function of the thickness of the thin film, which stems from the existence of Dirac/Weyl nodes in momentum space. Experimentally, such an effect can be observed in thin films of semimetals, where the thickness dependence of thermodynamic quantities is affected by the Casimir energy.
Nakayama, Katsumasa*; Suzuki, Kei
Proceedings of Science (Internet), 430, p.379_1 - 379_9, 2023/04
The conventional Casimir effect has been studied in the continuous spacetime, but to elucidate its counterpart in the lattice space is an important subject. Here, we discuss various types of Casimir effects for quantum fields on the lattice. By using a definition of the Casimir energy on the lattice, we show that the Casimir effect for the Wilson fermion is similar to that for the continuous Dirac fermion. We apply our definition to an effective Hamiltonian describing Dirac semimetals, such as CdAs and NaBi, and find an oscillatory behavior of the Casimir energy as a function of film thickness of semimetals. We also study contributions from Landau levels under magnetic fields and the Casimir effect for nonrelativistic particle fields on the lattice.
Hattori, Koichi*; Suenaga, Daiki*; Suzuki, Kei; Yasui, Shigehiro*
EPJ Web of Conferences, 276, p.01015_1 - 01015_5, 2023/03
Times Cited Count：0 Percentile：0.91(Physics, Atomic, Molecular & Chemical)We investigate the QCD phase diagram in strong magnetic fields with heavy-quark impurities and determine the ground state within the mean-field analysis. The ground state is characterized by magnitudes of the pairing not only between the light quark and antiquark, i.e., chiral condensate, but also between the light quark and heavy-quark impurity, dubbed the Kondo condensate. We propose signatures of the interplay and/or competition between those two pairing phenomena reflected in the magnitude of the chiral condensate that is saturated with respect to the magnetic-field strength and anomalously increases with increasing temperature.
Kurebayashi, Daichi*; Araki, Yasufumi; Nomura, Kentaro*
Journal of the Physical Society of Japan, 90(8), p.084702_1 - 084702_9, 2021/08
Times Cited Count：8 Percentile：61.42(Physics, Multidisciplinary)We theoretically study current- and charge-induced spin torque in magnetic Weyl semimetals. In magnetic Weyl semimetals (WSM), topologically nontrivial band structure mediates anomalous coupling between magnetization and transport, making WSMs preferable for spintronics systems. In this paper, we determine all current-induced spin torques including spin-orbit torque and spin-transfer torque, up to first order with respect to spatial and temporal derivatives and electrical currents. We also calculate the charge-induced spin torque microscopically. We find the charge-induced spin torque originates from the chiral anomaly due to the correspondence between spin operators and axial current operators in our model.
Ishikawa, Tsutomu*; Nakayama, Katsumasa*; Suzuki, Kei
Physical Review Research (Internet), 3(2), p.023201_1 - 023201_23, 2021/06
The Casimir effect arises from the zero-point energy of particles in momentum space deformed by the existence of two parallel plates. For degrees of freedom on the lattice, its energy-momentum dispersion is determined so as to keep a periodicity within the Brillouin zone, so that its Casimir effect is modified. We study the properties of Casimir effect for lattice fermions, such as the naive fermion, Wilson fermion, and overlap fermion based on the M
bius domain-wall fermion formulation, in the 
, 
, and 
dimensional spacetime with the periodic or antiperiodic boundary condition. An oscillatory behavior of Casimir energy between odd and even lattice size is induced by the contribution of ultraviolet-momentum (doubler) modes, which realizes in the naive fermion, Wilson fermion in a negative mass, and overlap fermions with a large domain-wall height. Our findings can be experimentally observed in condensed matter systems such as topological insulators and also numerically measured in lattice simulations.
Ishikawa, Tsutomu*; Nakayama, Katsumasa*; Suzuki, Kei
Physics Letters B, 809, p.135713_1 - 135713_7, 2020/10
Times Cited Count：10 Percentile：76.76(Astronomy & Astrophysics)We propose a definition of the Casimir energy for free lattice fermions. From this definition, we study the Casimir effects for the massless or massive naive fermion, Wilson fermion, and (Mbius) domain-wall fermion in 1+1 dimensional spacetime with the spatial periodic or antiperiodic boundary condition. For the naive fermion, we find an oscillatory behavior of the Casimir energy, which is caused by the difference between odd and even lattice sizes. For the Wilson fermion, in the small lattice size of 
, the Casimir energy agrees very well with that of the continuum theory, which suggests that we can control the discretization artifacts for the Casimir effect measured in lattice simulations. We also investigate the dependence on the parameters tunable in Mbius domain-wall fermions. Our findings will be observed both in condensed matter systems and in lattice simulations with a small size.
Suenaga, Daiki*; Suzuki, Kei; Araki, Yasufumi; Yasui, Shigehiro*
Physical Review Research (Internet), 2(2), p.023312_1 - 023312_13, 2020/06
The Kondo effect is induced by the interaction between light fermions near the Fermi surface and heavy impurities, and it affects electric/thermal/transport properties of matter. The chirality (right-handed or left-handed) is one of the unique properties of relativistic (Dirac or Weyl) fermions. In normal matter, the numbers of right- and left-handed particles are equivalent to each other, but environments with a chirality imbalance can also be realized. In this paper, we theoretically propose the Kondo effect driven by a chirality imbalance (or chiral chemical potential) of relativistic light fermions. This effect is caused by the mixing between a right-handed (or left-handed) fermion and a heavy impurity in the chirality imbalanced matter. This is different from the usual Kondo effect induced by finite density (or chemical potential) for light fermions. We construct an effective model with an interaction between a relativistic fermion and a heavy impurity, and we derive the realization of the Kondo effect from both a perturbative calculation and a nonperturbative mean-field approach. We also discuss the temperature dependence, the coupling constant dependence, the susceptibilities, and the order of the phase transition for the Kondo effect. Such a Kondo effect will be tested by future lattice simulations.
Suenaga, Daiki*; Suzuki, Kei; Yasui, Shigehiro*
Physical Review Research (Internet), 2(2), p.023066_1 - 023066_11, 2020/04
The QCD Kondo effect is a quantum phenomenon in which heavy (charm or bottom) quarks exist as impurity particles in quark matter composed of light quarks at extremely high density. In this paper, we theoretically predict the existence of the exciton modes above the ground state of the quark matter governed by the QCD Kondo effect. We construct an effective model based on the mean field approximation and investigate possible quantum numbers (such as spin and parity) of excitons and their dispersion relations. These excitons can be electrically (color) neutral, so that they are observed as the neutral currents in transport phenomena. As a result, they contribute to violation of the Wiedemann-Franz law for the electric (color) and heat conductivities. Such Kondo excitons are an universal phenomenon for relativistic (Dirac or Weyl) fermions, and the same concept will also be applied to Dirac or Weyl electron systems.
Araki, Yasufumi
Annalen der Physik, 532(2), p.1900287_1 - 1900287_16, 2020/02
Times Cited Count：34 Percentile：87.84(Physics, Multidisciplinary)Ishikawa, Tsutomu*; Nakayama, Katsumasa*; Suzuki, Kei
no journal, ,
We propose a definition of the Casimir energy for free lattice fermions. From this definition, we study the Casimir effects for the massless or massive naive fermion, Wilson fermion, and (Mbius) domain-wall fermion in dimensional spacetime with the spatial periodic or antiperiodic boundary condition. For the naive fermion, we find an oscillatory behavior of the Casimir energy, which is caused by the difference between odd and even lattice sizes. For the Wilson fermion, in the small lattice size of , the Casimir energy agrees very well with that of the continuum theory, which suggests that we can control the discretization artifacts for the Casimir effect measured in lattice simulations. We also investigate the dependence on the parameters tunable in Mbius domain-wall fermions. Our findings will be observed both in condensed matter systems and in lattice simulations with a small size.
Hattori, Koichi*; Suenaga, Daiki*; Suzuki, Kei; Yasui, Shigehiro*
no journal, ,
no abstracts in English
Ozawa, Akihiro*; Araki, Yasufumi; Nomura, Kentaro*
no journal, ,
no abstracts in English
Suzuki, Kei; Fujii, Daisuke; Nakayama, Katsumasa*
no journal, ,
The conventional Casimir effect is defined for photon fields in the QED vacuum, whereas various quasiparticle fields realized in condensed matter systems can lead to novel types of Casimir-effect-like phenomena. In QCD and nuclear physics, such a situation is rare, but there are some possibilities in dense-QCD/nuclear matter. For example, the dual chiral density wave (DCDW) phase has been studied as the ground state of finite-density QCD. In this talk, we discuss the typical features of the Casimir effect in a small-size medium in such a ground state. The Casimir effect from quark fields leads to oscillations of physical quantities as a function of system size. A counterpart of this phenomenon is expected to appear also in Weyl semimetals, and we discuss the comparison between quark matter and Weyl semimetals.
Fujii, Daisuke; Suzuki, Kei; Nakayama, Katsumasa*
no journal, ,
Recently, new types of Casimir effects realized in condensed matter systems have been discovered. For example, the photonic Casimir effect in Weyl semimetals was found to exhibit remarkable behavior. This new type of Casimir effect may also be realized in dense quark matter. In this presentation, we discuss typical features of the Casimir effect in finite density QCD using the Nambu-Jona-Lasinio model. In particular, an remarkable behavior of the Casimir effect in the dual chiral density wave phase is revealed.
Ozawa, Akihiro*; Araki, Yasufumi; Nomura, Kentaro*
no journal, ,
