71 research outputs found
Cyclotron resonance and Faraday rotation in graphite
The optical conductivity of graphite in quantizing magnetic fields is
analytically evaluated for frequencies in the range of 10--300 meV, where the
electron relaxation processes can be neglected and the low-energy excitations
at the "Dirac lines" are more essential. The conductivity peaks are explained
in terms of the electron transitions in graphite.
Conductivity calculated per one graphite layer tends on average to the
universal conductivity of graphene while the frequency is larger than the
Landau spacing.
The (semi)metal-insulator transformation is possible under doping in high
magnetic fields.Comment: 4 pages, 5 figure
3D Dirac semimetal Cd3As2: A review of material properties
Cadmium arsenide (Cd3As2) - a time-honored and widely explored material in
solid-state physics - has recently attracted considerable attention. This was
triggered by a theoretical prediction concerning the presence of 3D
symmetry-protected massless Dirac electrons, which could turn Cd3As2 into a 3D
analogue of graphene. Subsequent extended experimental studies have provided us
with compelling experimental evidence of conical bands in this system, and
revealed a number of interesting properties and phenomena. At the same time,
some of the material properties remain the subject of vast discussions despite
recent intensive experimental and theoretical efforts, which may hinder the
progress in understanding and applications of this appealing material. In this
review, we focus on the basic material parameters and properties of Cd3As2, in
particular those which are directly related to the conical features in the
electronic band structure of this material. The outcome of experimental
investigations, performed on Cd3As2 using various spectroscopic and transport
techniques within the past sixty years, is compared with theoretical studies.
These theoretical works gave us not only simplified effective models, but more
recently, also the electronic band structure calculated numerically using ab
initio methods.Comment: 16 pages, 16 figure
Spin resonance in EuTiO3 probed by time-domain GHz ellipsometry
We show an example of a purely magnetic spin resonance in EuTiO3 and the
resulting new record high Faraday rotation of 590 deg/mm at 1.6 T for 1 cm
wavelengths probed by a novel technique of magneto-optical GHz time-domain
ellipsometry. From our transmission measurements of linear polarized light we
map out the complex index of refraction in the GHz to THz range. We observe a
strong resonant absorption by magnetic dipole transitions involving the Zeeman
split S=7/2 magnetic energy levels of the Eu 2+ ions, which causes a very large
dichroism for circular polarized radiation.Comment: 4 pages, 4 figure
Two-dimensional conical dispersion in ZrTe5 evidenced by optical spectroscopy
Zirconium pentatelluride was recently reported to be a 3D Dirac semimetal,
with a single conical band, located at the center of the Brillouin zone. The
cone's lack of protection by the lattice symmetry immediately sparked vast
discussions about the size and topological/trivial nature of a possible gap
opening. Here we report on a combined optical and transport study of ZrTe5,
which reveals an alternative view of electronic bands in this material. We
conclude that the dispersion is approximately linear only in the a-c plane,
while remaining relatively flat and parabolic in the third direction (along the
b axis). Therefore, the electronic states in ZrTe5 cannot be described using
the model of 3D Dirac massless electrons, even when staying at energies well
above the band gap 6 meV found in our experiments at low temperatures.Comment: Physical Review Letters 122, 217402 (2019). Corrected acknowledgment
Multi-component magneto-optical conductivity of multilayer graphene on SiC
Far-infrared diagonal and Hall conductivities of multilayer epitaxial
graphene on the C-face of SiC were measured using magneto-optical absorption
and Faraday rotation in magnetic fields up to 7 T and temperatures between 5
and 300 K. Multiple components are identified in the spectra, which include:
(i) a quasi-classical cyclotron resonance (CR), originating from the highly
doped graphene layer closest to SiC, (ii) transitions between low-index Landau
levels (LLs), which stem from weakly doped layers and (iii) a broad optical
absorption background. Electron and hole type LL transitions are optically
distinguished and shown to coexist. An electron-hole asymmetry of the Fermi
velocity of about 2% was found within one graphene layer, while the Fermi
velocity varies by about 10% across the layers. The optical intensity of the LL
transitions is several times smaller than what is theoretically expected for
isolated graphene monolayers without electron-electron and electron-phonon
interactions.Comment: 9 pages, 6 figure
BiTeCl and BiTeBr: a comparative high-pressure optical study
We here report a detailed high-pressure infrared transmission study of BiTeCl
and BiTeBr. We follow the evolution of two band transitions: the optical
excitation between two Rashba-split conduction bands, and the
absorption across the band gap. In the low pressure range, ~GPa,
for both compounds is approximately constant with pressure and
decreases, in agreement with band structure calculations. In BiTeCl, a clear
pressure-induced phase transition at 6~GPa leads to a different ground state.
For BiTeBr, the pressure evolution is more subtle, and we discuss the
possibility of closing and reopening of the band gap. Our data is consistent
with a Weyl phase in BiTeBr at 56~GPa, followed by the onset of a structural
phase transition at 7~GPa.Comment: are welcom
Infrared spectroscopy study of the in-plane response of YBa2Cu3O6.6 in magnetic fields up to 30 Tesla
With Terahertz and Infrared spectroscopy we studied the in-plane response of
an underdoped YBa2Cu3O6.6 single crystal with Tc=58(1) K in high magnetic
fields up to B=30 Tesla applied along the c-axis. Our goal was to investigate
the field-induced suppression of superconductivity and to observe the
signatures of the three dimensional (3d) incommensurate copper charge density
wave (Cu-CDW) which was previously shown to develop at such high magnetic
fields. Our study confirms that a B-field in excess of 20 Tesla gives rise to a
full suppression of the macroscopic response of the superconducting condensate.
However, it reveals surprisingly weak signatures of the 3d Cu-CDW at high
magnetic fields. At 30 Tesla there is only a weak reduction of the spectral
weight of the Drude-response (by about 3%) that is accompanied by an
enhancement of two narrow electronic modes around 90 and 240 cm-1, that are
interpreted in terms of pinned phase modes of the CDW along the a- and
b-direction, respectively, and of the so-called mid-infrared (MIR) band. The
pinned phased modes and the MIR band are strong features already without
magnetic field which suggests that prominent but short-ranged and slowly
fluctuating (compared to the picosecond IR-time scale) CDW correlations exist
all along, i.e., even at zero magnetic field.Comment: 12 pages, 3 figure
Intrinsic Terahertz Plasmons and Magnetoplasmons in Large Scale Monolayer Graphene
We show that in graphene epitaxially grown on SiC the Drude absorption is
transformed into a strong terahertz plasmonic peak due to natural nanoscale
inhomogeneities, such as substrate terraces and wrinkles. The excitation of the
plasmon modifies dramatically the magneto-optical response and in particular
the Faraday rotation. This makes graphene a unique playground for
plasmon-controlled magneto-optical phenomena thanks to a cyclotron mass 2
orders of magnitude smaller than in conventional plasmonic materials such as
noble metals.Comment: to appear in Nano Letter
Non-uniform carrier density in CdAs evidenced by optical spectroscopy
We report the detailed optical properties of CdAs crystals in a wide
parameter space: temperature, magnetic field, carrier concentration and crystal
orientation. We investigate high-quality crystals synthesized by three
different techniques. In all the studied samples, independently of how they
were prepared and how they were treated before the optical experiments, our
data indicate conspicuous fluctuations in the carrier density (up to 30%).
These charge puddles have a characteristic scale of 100 m, they become
more pronounced at low temperatures, and possibly, they become enhanced by the
presence of crystal twinning. The Drude response is characterized by very small
scattering rates ( meV) for as-grown samples. Mechanical treatment,
such as cutting or polishing, influences the optical properties of single
crystals, by increasing the Drude scattering rate and also modifying the high
frequency optical response. Magneto-reflectivity and Kerr rotation are
consistent with electron-like charge carriers and a spatially non-uniform
carrier density.Comment: Accepted in Physical Review
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