Jean-Marie Poumirol

Integer Quantum Hall Effect in Trilayer Graphene

By using high-magnetic fields (up to 60 T), we observe compelling evidence of the integer quantum Hall effect in trilayer graphene. The magnetotransport fingerprints are similar to those of the graphene monolayer, except for the absence of a plateau at a filling factor of ν=2. At a very low filling factor, the Hall resistance vanishes due to the presence of mixed electron and hole carriers induced by disorder. The measured Hall resistivity plateaus are well reproduced theoretically, using a self-consistent Hartree calculations of the Landau levels and assuming an ABC stacking order of the three layers.

Electron cyclotron effective mass in indium nitride

We report on cyclotron effective mass measurement in indium nitride epilayers grown on c-sapphire, using the thermal damping of Shubnikov-de-Haas oscillations obtained in the temperature range 2–70 K and under magnetic field up to 60 T. We unravel an isotropic electron cyclotron effective mass equal to 0.062±0.002m0 for samples having electron concentration near 1018 cm−3. After nonparabolicity and polaron corrections we estimate a bare mass at the bottom of the band equal to 0.055±0.002m0.

Unveiling the magnetic structure of graphene nanoribbons.

We perform magnetotransport measurements in lithographically patterned graphene nanoribbons down to a 70 nm width. The electronic spectrum fragments into an unusual Landau levels pattern, characteristic of Dirac fermion confinement. The two-terminal magnetoresistance reveals the onset of magnetoelectronic subbands, edge currents and quantized Hall conductance. We bring evidence that the magnetic confinement at the edges unveils the valley degeneracy lifting originating from the electronic confinement. Quantum simulations suggest some disorder threshold at the origin of mixing between chiral magnetic edge states and disappearance of quantum Hall effect.

Edge magnetotransport fingerprints in disordered graphene nanoribbons

We report on (magneto)-transport experiments in chemically derived narrow graphene nanoribbons under high magnetic fields (up to 60 Tesla). Evidences of field-dependent electronic confinement features are given, and allow estimating the possible ribbon edge symmetry. Besides, the measured large positive magnetoconductance indicates a strong suppression of backscattering induced by the magnetic field. Such scenario is supported by quantum simulations which consider different types of underlying disorders (smooth edge disorder and long range Coulomb scatters).

Edge Magneto-Fingerprints in Disordered Graphene Nanoribbons

We report on (magneto)-transport experiments in chemically derived narrow graphene nanoribbons under high magnetic fields (up to 60 Tesla). Evidences of field-dependent electronic confinement features are given, and allow estimating the possible ribbon edge symmetry. Besides, the measured large positive magnetoconductance indicates a strong suppression of backscattering induced by the magnetic field. Such scenario is supported by quantum simulations which consider different types of underlying disorders (smooth edge disorder and long range Coulomb scatters).

Anodic bonded graphene

We show how to prepare graphene samples on a glass substrate with the anodic bonding method. In this method, a graphite precursor in flake form is bonded to a glass substrate with the help of an electrostatic field and then cleaved off to leave few layer graphene on the substrate. Now that several methods are available for producing graphene, the relevance of our method is in its simplicity and practicality for producing graphene samples of about 100 µm lateral dimensions. This method is also extensible to other layered materials. We discuss some detailed aspects of the fabrication and results from Raman spectroscopy, local probe microscopy and transport measurements on these samples.

Electron–hole coexistence in disordered graphene probed by high-field magneto-transport

We report on magneto-transport measurement in disordered graphene under a pulsed magnetic field of up to 57 T. For large electron or hole doping, the system displays the expected anomalous integer quantum Hall effect (IQHE) specific to graphene down to the filling factor ν=2. In the close vicinity of the charge neutrality point, the system breaks up into coexisting puddles of holes and electrons, leading to a vanishing Hall and finite longitudinal resistance with no hint of divergence at very high magnetic field. Large resistance fluctuations are observed near the Dirac point. They are interpreted as the natural consequence of the presence of electron and hole puddles. The magnetic field at which the amplitude of the fluctuations is the largest is directly linked to the mean size of the puddles.

Electrically controlled terahertz magneto-optical phenomena in continuous and patterned graphene

The magnetic circular dichroism and the Faraday rotation are the fundamental phenomena of great practical importance arising from the breaking of the time reversal symmetry by a magnetic field. In most materials, the strength and the sign of these effects can be only controlled by the field value and its orientation. Furthermore, the terahertz range is lacking materials having the ability to affect the polarization state of the light in a non-reciprocal manner. Here we demonstrate, using broadband terahertz magneto-electro-optical spectroscopy, that in graphene both the magnetic circular dichroism and the Faraday rotation can be modulated in intensity, tuned in frequency and, importantly, inverted using only electrostatic doping at a fixed magnetic field. In addition, we observe strong magneto-plasmonic resonances in a patterned array of graphene antidots, which potentially allows exploiting these magneto-optical phenomena in a broad THz range.

Isotope effect in superconducting n-doped SrTiO3

We report the influence on the superconducting critical temperature Tc in doped SrTiO3 of the substitution of the natural 16O atoms by the heavier isotope 18O. We observe that for a wide range of doping this substitution causes a strong (~50%) enhancement of Tc. Also the magnetic critical field Hc2 is increased by a factor ~2. Such a strong impact on Tc and Hc2, with a sign opposite to conventional superconductors, is unprecedented. The observed effect could be the consequence of strong coupling of the doped electrons to lattice vibrations (phonons), a notion which finds support in numerous optical and photo-emission studies. The unusually large size of the observed isotope effect supports a recent model for superconductivity in these materials based on strong coupling to the ferroelectric soft modes of SrTiO3.

High magnetic field induced charge density waves and sign reversal of the Hall coefficient in graphite.

We report on the investigation of magnetic field induced charge density waves and Hall coefficient sign reversal in a quasi-two-dimensional electronic system of highly oriented pyrolytic graphite under very strong magnetic field. The change of Hall sign coefficient from negative to positive occurs at low temperature and high magnetic field just after the charge density wave transition, suggesting the role of hole-like quasi-particles in this effect. Angular dependent measurements show that the charge density wave transition and Hall sign reversal fields follow the magnetic field component along the c-axis of graphite.