1,288 research outputs found
Magnetoplasmon resonances in polycrystalline bismuth as seen via terahertz spectroscopy
We report the magnetic field-dependent far-infrared reflectivity of
polycrystalline bismuth. We observe four distinct absorptions that we attribute
to magnetoplasmon resonances, which are collective modes of an electron-hole
liquid in magnetic field and become optical and acoustic resonances of the
electron-hole system in the small-field limit. The acoustic mode is expected
only when the masses of distinct components are very different, which is the
case in bismuth. In a polycrystal, where the translational symmetry is broken,
a big shift of spectral weight to acoustic plasmon is possible. This enables us
to detect an associated plasma edge. Although the polycrystal sample has grains
of randomly distributed orientations, our reflectivity results can be explained
by invoking only two, clearly distinct, series of resonances. In the limit of
zero field, the optical modes of these two series converge onto plasma
frequencies measured in monocrystal along the main optical axes.Comment: Accepted in PR
Magneto-optical Kramers-Kronig analysis
We describe a simple magneto-optical experiment and introduce a
magneto-optical Kramers-Kronig analysis (MOKKA) that together allow extracting
the complex dielectric function for left- and right-handed circular
polarizations in a broad range of frequencies without actually generating
circularly polarized light. The experiment consists of measuring reflectivity
and Kerr rotation, or alternatively transmission and Faraday rotation, at
normal incidence using only standard broadband polarizers without retarders or
quarter-wave plates. In a common case, where the magneto-optical rotation is
small (below 0.2 rad), a fast measurement protocol can be realized,
where the polarizers are fixed at 45 with respect to each other. Apart
from the time-effectiveness, the advantage of this protocol is that it can be
implemented at ultra-high magnetic fields and in other situations, where an
\emph{in-situ} polarizer rotation is difficult. Overall, the proposed technique
can be regarded as a magneto-optical generalization of the conventional
Kramers-Kronig analysis of reflectivity on bulk samples and the Kramers-Kronig
constrained variational analysis of more complex types of spectral data. We
demonstrate the application of this method to the textbook semimetals bismuth
and graphite and also use it to obtain handedness-resolved magneto-absorption
spectra of graphene on SiC.Comment: 11 pages, 4 figur
Revealing puddles of electrons and holes in compensated topological insulators
Three-dimensional topological insulators harbour metallic surface states with
exotic properties. In transport or optics, these properties are typically
masked by defect-induced bulk carriers. Compensation of donors and acceptors
reduces the carrier density, but the bulk resistivity remains disappointingly
small. We show that measurements of the optical conductivity in BiSbTeSe
pinpoint the presence of electron-hole puddles in the bulk at low temperatures,
which is essential for understanding DC bulk transport. The puddles arise from
large fluctuations of the Coulomb potential of donors and acceptors, even in
the case of full compensation. Surprisingly, the number of carriers appearing
within puddles drops rapidly with increasing temperature and almost vanishes
around 40 K. Monte Carlo simulations show that a highly non-linear screening
effect arising from thermally activated carriers destroys the puddles at a
temperature scale set by the Coulomb interaction between neighbouring dopants,
explaining the experimental observation semi-quantitatively. This mechanism
remains valid if donors and acceptors do not compensate perfectly.Comment: 11 pages with 7 figures plus supplemental material (3 pages
Optical characterization of BiSe in a magnetic field: infrared evidence for magnetoelectric coupling in a topological insulator material
We present an infrared magneto-optical study of the highly thermoelectric
narrow-gap semiconductor BiSe. Far-infrared and mid-infrared (IR)
reflectance and transmission measurements have been performed in magnetic
fields oriented both parallel and perpendicular to the trigonal axis of
this layered material, and supplemented with UV-visible ellipsometry to obtain
the optical conductivity . With lowering of temperature we
observe narrowing of the Drude conductivity due to reduced quasiparticle
scattering, as well as the increase in the absorption edge due to direct
electronic transitions. Magnetic fields dramatically
renormalize and asymmetrically broaden the strongest far-IR optical phonon,
indicating interaction of the phonon with the continuum free-carrier spectrum
and significant magnetoelectric coupling. For the perpendicular field
orientation, electronic absorption is enhanced, and the plasma edge is slightly
shifted to higher energies. In both cases the direct transition energy is
softened in magnetic field.Comment: Final versio
PHYSICAL PROPERTIES OF TOPOLOGICAL INSULATOR: BISMUTH SELENIDE THIN FILMS
Topological Insulator (TI) is new classes of materials with gapless surface states and insulating bulk. The topological connection can be traced back to the discovery of Integer Quantum Hall Effect in 1980. In the last decade, new categories of topological insulators were predicted and later discovered, that have gained a lot of attraction for room-temperature applications. Since the experimental observation of single Dirac cone on the surface states of Bismuth selenide (Bi2Se3) in 2009, it has emerged as the prototype. Bismuth Selenide has one of the highest bulk band gaps of 0.3 eV among all TI materials. While its single crystal properties are well documented, thin films are producing equally exciting discoveries. In this work, Bi2Se3 thin films were synthesized using magnetron sputtering method and a diverse set of physical properties, such as structural, optical, and electronic, are investigated. In particular, properties of few-layer (ultra-thin) Bi2Se3 thin films are studied. Optical properties of Bi2Se3 was particularly revealing. We observed a sharp increase (blue shift) in the bulk band gap of Bi2Se3 by almost 0.5 eV as it approached the two-dimensional limit. Strong thickness-dependent structural and transport properties were also observed
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