113 research outputs found
Decrypting the cyclotron effect in graphite using Kerr rotation spectroscopy
We measure the far-infrared magneto-optical Kerr rotation and reflectivity
spectra in graphite and achieve a highly accurate unified microscopic
description of all data in a broad range of magnetic fields by taking
rigorously the c-axis band dispersion and the trigonal warping into account. We
find that the second- and the forth-order cyclotron harmonics are optically
almost as strong as the fundamental resonance even at high fields. They must
play, therefore, a major role in magneto-optical and magneto-plasmonic
applications based on Bernal stacked graphite and multilayer graphene.Comment: 4 pages, 3 figures + Supplemental Materia
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
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
Fabry-Perot enhanced Faraday rotation in graphene
We demonstrate that giant Faraday rotation in graphene in the terahertz range
due to the cyclotron resonance is further increased by constructive Fabry-Perot
interference in the supporting substrate. Simultaneously, an enhanced total
transmission is achieved, making this effect doubly advantageous for
graphene-based magneto-optical applications. As an example, we present
far-infrared spectra of epitaxial multilayer graphene grown on the C-face of
6H-SiC, where the interference fringes are spectrally resolved and a Faraday
rotation up to 0.15 radians (9{\deg}) is attained. Further, we discuss and
compare other ways to increase the Faraday rotation using the principle of an
optical cavity
Fermi liquid behavior of the in-plane resistivity in the pseudogap state of YBa_2Cu_4O_8
Our knowledge of the ground state of underdoped hole-doped cuprates has
evolved considerably over the last few years. There is now compelling evidence
that inside the pseudogap phase, charge order breaks translational symmetry
leading to a reconstructed Fermi surface made of small pockets. Quantum
oscillations, [Doiron-Leyraud N, et al. (2007) Nature 447:564-568], optical
conductivity [Mirzaei SI, et al. (2013) Proc Natl Acad Sci USA 110:5774-5778]
and the validity of Wiedemann-Franz law [Grissonnache G, et al. (2016) Phys.
Rev. B 93:064513] point to a Fermi liquid regime at low temperature in the
underdoped regime. However, the observation of a quadratic temperature
dependence in the electrical resistivity at low temperatures, the hallmark of a
Fermi liquid regime, is still missing. Here, we report magnetoresistance
measurements in the magnetic-field-induced normal state of underdoped
YBa_2Cu_4O_8 which are consistent with a T^2 resistivity extending down to 1.5
K. The magnitude of the T^2 coefficient, however, is much smaller than expected
for a single pocket of the mass and size observed in quantum oscillations,
implying that the reconstructed Fermi surface must consist of at least one
additional pocket.Comment: Main + SI : published versio
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
Lifshitz critical point in the cuprate superconductor YBa2Cu3Oy from high-field Hall effect measurements
The Hall coefficient R_H of the cuprate superconductor YBa2Cu3Oy was measured
in magnetic fields up to 60 T for a hole concentration p from 0.078 to 0.152,
in the underdoped regime. In fields large enough to suppress superconductivity,
R_H(T) is seen to go from positive at high temperature to negative at low
temperature, for p > 0.08. This change of sign is attributed to the emergence
of an electron pocket in the Fermi surface at low temperature. At p < 0.08, the
normal-state R_H(T) remains positive at all temperatures, increasing
monotonically as T \to 0. We attribute the change of behaviour across p = 0.08
to a Lifshitz transition, namely a change in Fermi-surface topology occurring
at a critical concentration p_L = 0.08, where the electron pocket vanishes. The
loss of the high-mobility electron pocket across p_L coincides with a ten-fold
drop in the conductivity at low temperature, revealed in measurements of the
electrical resistivity at high fields, showing that the so-called
metal-insulator crossover of cuprates is in fact driven by a Lifshitz
transition. It also coincides with a jump in the in-plane anisotropy of ,
showing that without its electron pocket the Fermi surface must have strong
two-fold in-plane anisotropy. These findings are consistent with a
Fermi-surface reconstruction caused by a unidirectional spin-density wave or
stripe order.Comment: 16 pages, 13 figures, see associated Viewpoint: M. Vojta, Physics 4,
12 (2011
de Haas-van Alphen oscillations in the underdoped cuprate YBaCuO
The de Haas-van Alphen effect was observed in the underdoped cuprate
YBaCuO via a torque technique in pulsed magnetic fields up to
59 T. Above an irreversibility field of 30 T, the magnetization exhibits
clear quantum oscillations with a single frequency of 540 T and a cyclotron
mass of 1.76 times the free electron mass, in excellent agreement with
previously observed Shubnikov-de Haas oscillations. The oscillations obey the
standard Lifshitz-Kosevich formula of Fermi-liquid theory. This thermodynamic
observation of quantum oscillations confirms the existence of a well-defined,
close and coherent, Fermi surface in the pseudogap phase of cuprates.Comment: published versio
Shubnikov-de Haas oscillations in YBa_2Cu_4O_8
We report the observation of Shubnikov-de Haas oscillations in the underdoped
cuprate superconductor YBaCuO (Y124). For field aligned along the
c-axis, the frequency of the oscillations is T, which corresponds
to % of the total area of the first Brillouin zone. The effective
mass of the quasiparticles on this orbit is measured to be times
the free electron mass. Both the frequency and mass are comparable to those
recently observed for ortho-II YBaCuO (Y123-II). We show that
although small Fermi surface pockets may be expected from band structure
calculations in Y123-II, no such pockets are predicted for Y124. Our results
therefore imply that these small pockets are a generic feature of the copper
oxide plane in underdoped cuprates.Comment: v2: Version of paper accepted for publication in Physical Review
Letters. Only minor changes to the text and reference
Optical Self Energy in Graphene due to Correlations
In highly correlated systems one can define an optical self energy in analogy
to its quasiparticle (QP) self energy counterpart. This quantity provides
useful information on the nature of the excitations involved in inelastic
scattering processes. Here we calculate the self energy of the intraband
optical transitions in graphene originating in the electron-electron
interaction (EEI) as well as electron-phonon interaction (EPI). Although optics
involves an average over all momenta () of the charge carriers, the
structure in the optical self energy is nevertheless found to mirror mainly
that of the corresponding quasiparticles for equal to or near the Fermi
momentum . Consequently plasmaronic structures which are associated with
momenta near the Dirac point at are not important in the intraband
optical response. While the structure of the electron-phonon interaction (EPI)
reflects the sharp peaks of the phonon density of states, the excitation
spectrum associated with the electron-electron interaction is in comparison
structureless and flat and extends over an energy range which scales linearly
with the value of the chemical potential. Modulations seen on the edge of the
interband optical conductivity as it rises towards its universal background
value are traced to structure in the quasiparticle self energies around
of the lower Dirac cone associated with the occupied states.Comment: 30 pages, 10 figure
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