Julien Levallois

Giant Faraday rotation in single- and multilayer graphene

The rotation of polarized light in certain materials when subject to a magnetic field is known as the Faraday effect. Remarkably, just one atomic layer of graphene exhibits Faraday rotations that would only be measurable in other materials many hundreds of micrometres thick.

Infrared- and Raman-Spectroscopy Measurements of a Transition in the Crystal Structure and a Closing of the Energy Gap of BiTeI under Pressure

BiTeI is a giant Rashba spin splitting system, in which a noncentrosymmetric topological phase has recently been suggested to appear under high pressure. We investigated the optical properties of this compound, reflectivity and transmission, under pressures up to 15xa0GPa. The gap feature in the optical conductivity vanishes above p∼9u2009u2009GPa and does not reappear up to at least 15xa0GPa. The plasma edge, associated with intrinsically doped charge carriers, is smeared out through a phase transition at 9xa0GPa. Using high-pressure Raman spectroscopy, we follow the vibrational modes of BiTeI, providing additional clear evidence that the transition at 9xa0GPa involves a change of crystal structure. This change of crystal structure possibly inhibits the high-pressure topological phase from occurring.

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°) is attained. Further, we discuss and compare other ways to increase the Faraday rotation using the principle of an optical cavity.

Multicomponent 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.

Hybridization gap and anisotropic far-infrared optical conductivity of URu2Si2

We performed far-infrared optical spectroscopy measurements on the heavy fermion compound URu2Si2 as a function of temperature. The lights electric field was applied along the a or c axis of the tetragonal structure. We show that in addition to a pronounced anisotropy, the optical conductivity exhibits for both axis a partial suppression of spectral weight around 12 meV and below 30 K. We attribute these observations to a change in the band structure below 30 K. However, since these changes have no noticeable impact on the entropy nor on the DC transport properties, we suggest that this is a crossover phenomenon rather than a thermodynamic phase transition.

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 to the temperature variation of the 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. An approach to obtain a stronger increase of the THz generation efficiency at cryogenic cooling of the OH1 crystal is discussed.

Versatile setup for optical spectroscopy under high pressure and low temperature

We present an optical setup for spectroscopic measurements in the infrared and of Raman shift under high pressure and at low temperature. Using a membrane-driven diamond anvil cell, the pressure can be tuned in situ up to 20 GPa and the temperatures ranges from room temperature down to 18 K in transmission mode and 13 K in reflection mode. In transmission, the setup is entirely working under vacuum to reduce the water absorption features and obtain a higher spectral stability. Since the infrared throughput obtained with a thermal source is limited, the use of a synchrotron source allowed to enhance the performance, as illustrated with results obtained with various materials. The analysis of the reflectivity is adapted so that it benefits from ambient pressure data and produces quantitative optical conductivity curves that can be easily compared to the results at ambient pressure.

Decrypting the cyclotron effect in graphite using Kerr rotation spectroscopy

Abstract We measure the far-infrared magneto-optical Kerr rotation and reflectivity spectra of 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.

Fermi liquid behavior of the in-plane resistivity in the pseudogap state of YBa2Cu4O8

Significance High-temperature superconductivity evolves out of a metallic state that undergoes profound changes as a function of carrier concentration, changes that are often obscured by the high upper critical fields. In the more disordered cuprate families, field suppression of superconductivity has uncovered an underlying ground state that exhibits unusual localization behavior. Here, we reveal that, in stoichiometric YBa2Cu4O8, the field-induced ground state is both metallic and Fermi liquid-like. The manuscript also demonstrates the potential for using the absolute magnitude of the electrical resistivity to constrain the Fermi surface topology of correlated metals and, in the case of YBa2Cu4O8, reveals that the current picture of the reconstructed Fermi surface in underdoped cuprates as a single, isotropic electron-like pocket may be incomplete. 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(7144):565–568], optical conductivity [Mirzaei SI, et al. (2013) Proc Natl Acad Sci USA 110(15):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 YBa2Cu4O8 that are consistent with a T2 resistivity extending down to 1.5 K. The magnitude of the T2 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.

Temperature-Dependent Ellipsometry Measurements of Partial Coulomb Energy in Superconducting Cuprates

We performed an experimental study of the temperature and doping dependence of the energy-loss function of the bilayer and trilayer Bi-cuprate family. The primary aim is to obtain information on the energy stored in the Coulomb interaction between the conduction electrons, on the temperature dependence thereof, and on the change of Coulomb interaction when Cooper-pairs are formed. We performed temperature-dependent ellipsometry measurements on several Bi