46 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
Oscillations quantiques et magnétotransport dans des systèmes à fortes corrélations électroniques
Dans cette thèse, nous nous sommes intéressés aux propriétés électroniques de deux familles de systèmes à fortes corrélations électroniques, les supraconducteurs à haute température critique (cuprates) et les fermions lourds, lorsqu'ils sont soumis à un fort champ magnétique (60 T). Depuis leur découverte en 1986, le diagramme de phase des supraconducteurs à haute température critique reste énigmatique. L'une des questions fondamentales concerne la nature de l'état normal à basse température. Dans la phase surdopée (p>0.16), on retrouve les caractéristiques d'un métal conventionnel, avec notamment une grande surface de Fermi. Dans la phase sous-dopée, les mesures d'ARPES semblent indiquer que la surface de Fermi n'est pas fermée et est seulement constituée d'arcs de Fermi déconnectés et aucune mesure expérimentale n'a permis jusqu'à présent de mettre en évidence une surface de Fermi fermée. En mesurant la résistance de Hall dans deux oxydes de cuivre de type YBCO, nous avons mis en évidence des oscillations quantiques établissant l'existence, à basse température, d'une surface de Fermi fermée et cohérente pour les cuprates sous-dopés. La faible fréquence d'oscillation mesurée indique que la surface de Fermi est constituée de petites poches, en fort contraste avec le grand cylindre observé du côté surdopé. De plus, l'observation d’un effet Hall négatif dans l'état normal à basse température suggère la présence d'électrons dans la surface de Fermi. On discute alors la possibilité qu'une reconstruction de la surface de Fermi entraîne l'apparition de petites poches d’électrons et de trous. Dans un second temps, nous présentons des mesures de magnétotransport et d'effet Nernst dans le fermion lourd URu2Si2 . Il apparaît que la mystérieuse phase ordre caché apparaissant à T <17.5 K soit caractérisée par des porteurs en faible nombre mais très mobiles, pouvant induire l'émergence d’un effet Nernst important. L'application d'un champ magnétique supérieur à 35 T à basse température déstabilise cet ordre et un état métallique plus conventionnel semble être restauré au dessus de 40 T.In this thesis, we are interested in the properties of two families of strongly correlated electron systems (high-temperature superconducting cuprates and heavy fermions) when a strong magnetic field (60 T) is applied. Since their discovery in 1986, the phase diagram of high-temperature superconductors remains enigmatic. One fundamental question concerns the nature of the normal state at low temperature. In the overdoped regime, the material behaves as a reasonably conventional metal, with a large Fermi surface. In the underdoped regime, however, ARPES measurements seem to indicate that the Fermi surface is not closed and consists of disconnected Fermi arcs. There is, so far, no direct convincing experimental observation of a closed and coherent Fermi surface. By performing Hall resistance measurements in two copper oxides (YBCO), we have observed quantum oscillations establishing the existence, at low temperature, of a closed and consistent Fermi surface for underdoped cuprates. The low oscillation frequency reveals a Fermi surface made of small pockets, in contrast to the large cylinder characteristic of the overdoped regime. Moreover, the negative sign of the Hall coefficient in the normal state at low temperature reveals that these pockets are electron-like rather than hole-like. We discuss the possibility that a reconstruction of the Fermi surface causes the appearance of small electron and hole-like pockets. In a second time, we present magnetotransport and Nernst effect measurements in the heavy fermion URu2Si2 . It appears that the mysterious hidden order phase, which appears when T <17.5 K, is characterized by a low carrier density with a high mobility, that could induce a large Nernst effect. The application of a magnetic field above 35 T at low temperature destabilizes the hidden order phase and a more conventional metal seems to be recovered above 40 T
Generation of broadband THz pulses in organic crystal OH1 at room temperature and 10 K
We studied the effects of cryogenic cooling of a 2-[3-(4-hydroxystyryl)-5,
5-dimethylcyclohex-2-enylidene] malononitrile (OH1) crystal on the generation
of broadband THz pulses via collinear optical rectification of 1350 nm
femtosecond laser pulses. Cooling of the OH1 crystal from room temperature to
10 K leads to a ~10% increase of the pump-to-THz energy conversion efficiency
and a shift of the THz pulse spectra to a higher frequency range. Both effects
are due the temperature variation of THz absorption and the refractive index of
the OH1 crystal. This conclusion has been verified by temperature dependent
measurements of the linear absorption in the THz frequency region
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
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
Quantum oscillations in underdoped YBa_2Cu_3O_6.5
Shubnikov-de Haas and de Haas-van Alphen effects have been measured in the
underdoped high temperature superconductor YBaCuO. Data are in
agreement with the standard Lifshitz-Kosevitch theory, which confirms the
presence of a coherent Fermi surface in the ground state of underdoped
cuprates. A low frequency T is reported in both measurements,
pointing to small Fermi pocket, which corresponds to 2% of the first Brillouin
zone area only. This low value is in sharp contrast with that of overdoped
TlBaCuO, where a high frequency kT has been
recently reported and corresponds to a large hole cylinder in agreement with
band structure calculations. These results point to a radical change in the
topology of the Fermi surface on opposing sides of the cuprate phase diagram.Comment: proceeding of the ECRYS-200
Giant Faraday rotation in single- and multilayer graphene
Optical Faraday rotation is one of the most direct and practically important
manifestations of magnetically broken time-reversal symmetry. The rotation
angle is proportional to the distance traveled by the light, and up to now
sizeable effects were observed only in macroscopically thick samples and in
two-dimensional electron gases with effective thicknesses of several
nanometers. Here we demonstrate that a single atomic layer of carbon - graphene
- turns the polarization by several degrees in modest magnetic fields. The
rotation is found to be strongly enhanced by resonances originating from the
cyclotron effect in the classical regime and the inter-Landau-level transitions
in the quantum regime. Combined with the possibility of ambipolar doping, this
opens pathways to use graphene in fast tunable ultrathin infrared
magneto-optical devices
Quantum oscillations and the Fermi surface in an underdoped high-Tc superconductor
Despite twenty years of research, the phase diagram of high transition-
temperature superconductors remains enigmatic. A central issue is the origin of
the differences in the physical properties of these copper oxides doped to
opposite sides of the superconducting region. In the overdoped regime, the
material behaves as a reasonably conventional metal, with a large Fermi
surface. The underdoped regime, however, is highly anomalous and appears to
have no coherent Fermi surface, but only disconnected "Fermi arcs". The
fundamental question, then, is whether underdoped copper oxides have a Fermi
surface, and if so, whether it is topologically different from that seen in the
overdoped regime. Here we report the observation of quantum oscillations in the
electrical resistance of the oxygen-ordered copper oxide YBa2Cu3O6.5,
establishing the existence of a well-defined Fermi surface in the ground state
of underdoped copper oxides, once superconductivity is suppressed by a magnetic
field. The low oscillation frequency reveals a Fermi surface made of small
pockets, in contrast to the large cylinder characteristic of the overdoped
regime. Two possible interpretations are discussed: either a small pocket is
part of the band structure specific to YBa2Cu3O6.5 or small pockets arise from
a topological change at a critical point in the phase diagram. Our
understanding of high-transition temperature (high-Tc) superconductors will
depend critically on which of these two interpretations proves to be correct