title: Terahertz-induced acceleration of massive Dirac electrons in semimetal bismuth.
abstract: Dirac-like electrons in solid state have been of great interest since they exhibit many peculiar physical behaviors analogous to relativistic mechanics. Among them, carriers in graphene and surface states of topological insulators are known to behave as massless Dirac fermions with a conical band structure in the two-dimensional momentum space, whereas electrons in semimetal bismuth (Bi) are expected to behave as massive Dirac-like fermions in the three-dimensional momentum space, whose dynamics is of particular interest in comparison with that of the massless Dirac fermions. Here, we demonstrate that an intense terahertz electric field transient accelerates the massive Dirac-like fermions in Bi from classical Newtonian to the relativistic regime; the electrons are accelerated approaching the effective "speed of light" with the "relativistic" beta β = 0.89 along the asymptotic linear band structure. As a result, the effective electron mass is enhanced by a factor of 2.4.
authors.affiliation: Department of Physics, Faculty of Engineering, Yokohama National University, Yokohama 240-8501, Japan.
authors.forename: Yasuo
authors.initials: Y
authors.lastname: Minami
authors.affiliation: Department of Physics, Faculty of Engineering, Yokohama National University, Yokohama 240-8501, Japan.
authors.forename: Kotaro
authors.initials: K
authors.lastname: Araki
authors.affiliation: International Center for Materials Nanoarchitectonics, National Institute for Materials Science, Tsukuba 305-0044, Japan.|CREST, Japan Science and Technology Agency, Kawaguchi 332-0012, Japan.
authors.forename: Thang Duy
authors.initials: TD
authors.lastname: Dao
authors.affiliation: International Center for Materials Nanoarchitectonics, National Institute for Materials Science, Tsukuba 305-0044, Japan.|CREST, Japan Science and Technology Agency, Kawaguchi 332-0012, Japan.|Department of Condensed Matter Physics, Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan.
authors.forename: Tadaaki
authors.initials: T
authors.lastname: Nagao
authors.affiliation: Department of Physics, Faculty of Engineering, Yokohama National University, Yokohama 240-8501, Japan.|International Center for Materials Nanoarchitectonics, National Institute for Materials Science, Tsukuba 305-0044, Japan.|CREST, Japan Science and Technology Agency, Kawaguchi 332-0012, Japan.|LxRay Co., Ltd., Nishinomiya 663-8172, Japan.|Department of Applied Physics, National Defense Academy, Yokosuka 239-8686, Japan.
authors.forename: Masahiro
authors.initials: M
authors.lastname: Kitajima
authors.affiliation: Department of Physics, Faculty of Engineering, Yokohama National University, Yokohama 240-8501, Japan.
authors.forename: Jun
authors.initials: J
authors.lastname: Takeda
authors.affiliation: Department of Physics, Faculty of Engineering, Yokohama National University, Yokohama 240-8501, Japan.
authors.forename: Ikufumi
authors.initials: I
authors.lastname: Katayama