arXiv subjects

G. Martinez

Publications and source records attributed to G. Martinez.

At least 55 records · Page 3Linked to original sources

arXiv2008

Magneto-transmission as a probe of Dirac fermions in bulk graphite

Far infrared magneto-transmission spectroscopy has been used to probe "relativistic" fermions in highly oriented pyrolytic and natural graphite. Nearly identical transmission spectra of those two materials are obtained, giving the signature of Dirac fermions via absorption lines with an energy that scales as \sqrt{B}. The Fermi velocity is evaluated to be c*=1.02x10^6 m/s and the pseudogap at the H point is estimated to be below 10 meV.

cond-mat.other↗

arXiv2008

High-Energy Limit of Massless Dirac Fermions in Multilayer Graphene using Magneto-Optical Transmission Spectroscopy

We have investigated the absorption spectrum of multilayer graphene in high magnetic fields. The low energy part of the spectrum of electrons in graphene is well described by the relativistic Dirac equation with a linear dispersion relation. However, at higher energies (>500 meV) a deviation from the ideal behavior of Dirac particles is observed. At an energy of 1.25 eV, the deviation from linearity is 40 meV. This result is in good agreement with the theoretical model, which includes trigonal warping of the Fermi surface and higher-order band corrections. Polarization-resolved measurements show no observable electron-hole asymmetry.

cond-mat.mes-hall↗

arXiv2008

Dirac fermions at the H point of graphite: Magneto-transmission studies

We report on far infrared magneto-transmission measurements on a thin graphite sample prepared by exfoliation of highly oriented pyrolytic graphite. In magnetic field, absorption lines exhibiting a blue-shift proportional to sqrtB are observed. This is a fingerprint for massless Dirac holes at the H point in bulk graphite. The Fermi velocity is found to be c*=1.02x10^6 m/s and the pseudogap at the H point is estimated to be below 10 meV. Although the holes behave to a first approximation as a strictly 2D gas of Dirac fermions, the full 3D band structure has to be taken into account to explain all the observed spectral features.

cond-mat.mes-hall↗

arXiv2008

The RICH detector of the AMS-02 experiment: status and physics prospects

The Alpha Magnetic Spectrometer (AMS), whose final version AMS-02 is to be installed on the International Space Station (ISS) for at least 3 years, is a detector designed to measure charged cosmic ray spectra with energies up to the TeV region and with high energy photon detection capability up to a few hundred GeV. It is equipped with several subsystems, one of which is a proximity focusing RICH detector with a dual radiator (aerogel+NaF) that provides reliable measurements for particle velocity and charge. The assembly and testing of the AMS RICH is currently being finished and the full AMS detector is expected to be ready by the end of 2008. The RICH detector of AMS-02 is presented. Physics prospects are briefly discussed.

astro-ph↗

arXiv2007

Magnetoplasmons excitations in graphene for filling factors $ν\leq 6$

In the frame of the Hartree-Fock approximation, the dispersion of magnetoplasmons in Graphene is derived for all types of transitions for filling factors $ν\leq 6$. The optical conductivity components of the magnetoplasmon curves are calculated. It is shown that the electron-electron interactions lead to a strong re-normalization of the apparent Fermi velocity of Graphene which is different for different types of transitions.

cond-mat.str-el↗

arXiv2007

The AMS-RICH velocity and charge reconstruction

The AMS detector, to be installed on the International Space Station, includes a Ring Imaging Cerenkov detector with two different radiators, silica aerogel (n=1.05) and sodium fluoride (n=1.334). This detector is designed to provide very precise measurements of velocity and electric charge in a wide range of cosmic nuclei energies and atomic numbers. The detector geometry, in particular the presence of a reflector for acceptance purposes, leads to complex Cerenkov patterns detected in a pixelized photomultiplier matrix. The results of different reconstruction methods applied to test beam data as well as to simulated samples are presented. To ensure nominal performances throughout the flight, several detector parameters have to be carefully monitored. The algorithms developed to fulfill these requirements are presented. The velocity and charge measurements provided by the RICH detector endow the AMS spectrometer with precise particle identification capabilities in a wide energy range. The expected performances on light isotope separation are discussed.

astro-ph↗

arXiv2007

Selective vulnerability to kainate-induced oxidative damage in different rat brain regions

Some markers of oxidative injury were measured in different rat brain areas (hippocampus, cerebral cortex, striatum, hypothalamus, amygdala/piriform cortex and cerebellum) after the systemic administration of an excitotoxic dose of kainic acid (KA, 9 mg kg(-1) i.p.) at two different sampling times (24 and 48 h). Kainic acid was able to lower markedly (P < 0.05) the glutathione (GSH) levels in hippocampus, cerebellum and amygdala/piriform cortex (maximal reduction at 24 h). In a similar way, lipid peroxidation, as assessed by malonaldehyde and 4-hydroxyalkenal levels, significantly increased (P < 0.05) in hippocampus, cerebellum and amygdala/piriform cortex mainly at 24 h after KA. In addition, hippocampal superoxide dismutase (SOD) activity decreased significantly (P < 0.05) with respect to basal levels by 24 h after KA application. On the other hand, brain areas such as hypothalamus, striatum and cerebral cortex seem to be less susceptible to KA excitotoxicity. According to these findings, the pattern of oxidative injury induced by systemically administered KA seems to be highly region-specific. Further, our results have shown that a lower antioxidant status (GSH and SOD) seems not to play an important role in the selective vulnerability of certain brain regions because it correlates poorly with increases in markers of oxidative damage.

q-bio.TO↗

arXiv2007

Nimesulide limits kainate-induced oxidative damage in the rat hippocampus

Kainate induces a marked expression of cyclooxygenase-2 after its systemic administration. Because cyclooxygenase-2 activity is associated to the production of reactive oxygen species, we investigated the effects of nimesulide, a selective cyclooxygenase-2 inhibitor, on kainate-induced in vivo oxidative damage in the rat hippocampus. A clinically relevant dose of nimesulide (6 mg/kg, i.p.) was administered three times following kainate application (9 mg/kg, i.p.). After 24 h of kainate administration, the drastic decrease in hippocampal glutathione content and the significant increase in lipid peroxidation were attenuated in nimesulide-treated rats, suggesting that the induction of cyclooxygenase-2 is involved in kainate-mediated free radicals formation.

q-bio.TO↗

arXiv2007

Time course of oxidative damage in different brain regions following transient cerebral ischemia in gerbils

The time course of oxidative damage in different brain regions was investigated in the gerbil model of transient cerebral ischemia. Animals were subjected to both common carotid arteries occlusion for 5 min. After the end of ischemia and at different reperfusion times (2, 6, 12, 24, 48, 72, 96 h and 7 days), markers of lipid peroxidation, reduced and oxidized glutathione levels, glutathione peroxidase, glutathione reductase, manganese-dependent superoxide dismutase (MnSOD) and copper/zinc containing SOD (Cu/ZnSOD) activities were measured in hippocampus, cortex and striatum. Oxidative damage in hippocampus was maximal at late stages after ischemia (48-96 h) coincident with a significant impairment in glutathione homeostasis. MnSOD increased in hippocampus at 24, 48 and 72 h after ischemia, coincident with the marked reduction in the activity of glutathione-related enzymes. The late disturbance in oxidant-antioxidant balance corresponds with the time course of delayed neuronal loss in the hippocampal CA1 sector. Cerebral cortex showed early changes in oxidative damage with no significant impairment in antioxidant capacity. Striatal lipid peroxidation significantly increased as early as 2 h after ischemia and persisted until 48 h with respect to the sham-operated group. These results contribute significant information on the timing and factors that influence free radical formation following ischemic brain injury, an essential step in determining effective antioxidant intervention.

q-bio.TO↗

arXiv2007

Measurement of the Fermi Constant by FAST

An initial measurement of the lifetime of the positive muon to a precision of 16 parts per million (ppm) has been performed with the FAST detector at the Paul Scherrer Institute. The result is tau_mu = 2.197083 (32) (15) microsec, where the first error is statistical and the second is systematic. The muon lifetime determines the Fermi constant, G_F = 1.166353 (9) x 10^-5 GeV^-2 (8 ppm).

hep-ex↗

arXiv2007

Thermal Dileptons at LHC

We predict dilepton invariant-mass spectra for central 5.5 ATeV Pb-Pb collisions at LHC. Hadronic emission in the low-mass region is calculated using in-medium spectral functions of light vector mesons within hadronic many-body theory. In the intermediate-mass region thermal radiation from the Quark-Gluon Plasma, evaluated perturbatively with hard-thermal loop corrections, takes over. An important source over the entire mass range are decays of correlated open-charm hadrons, rendering the nuclear modification of charm and bottom spectra a critical ingredient.

hep-ph↗

arXiv2007

Magnetospectroscopy of epitaxial few-layer graphene

The inter-Landau level transitions observed in far-infrared transmission experiments on few-layer graphene samples show a behaviour characteristic of the linear dispersion expected in graphene. This behaviour persists in relatively thick samples, and is qualitatively different from that of thin samples of bulk graphite.

cond-mat.mtrl-sci↗

arXiv2006

Landau level spectroscopy of ultrathin graphite layers

Far infrared transmission experiments are performed on ultrathin epitaxial graphite samples in a magnetic field. The observed cyclotron resonance-like and electron-positron-like transitions are in excellent agreement with the expectations of a single-particle model of Dirac fermions in graphene, with an effective velocity of c* = 1.03 x 10^6 m/s.

cond-mat.mes-hall↗

arXiv2006

Hole dynamics in canted antiferromagnets

We have analyzed the dynamics of a single hole doped in a canted antiferromagnet using the t-J model. Within the self consistent Born approximation we have found that the hole propagates at two different energy scales along the antiferromagnetic and the ferromagnetic components of the canted order, respectively. While the many body quasiparticle excitation has its origin in the coherent coupling of the hole with the magnon excitations of the antiferromagnetic component, the ferromagnetic component gives rise to a free like hole motion at higher energies. We have found a non-trivial behavior of the hole spectral function with the canting angle. In particular, in the strong coupling regime, the quasiparticle weight strongly depends on the momenta, vanishing inside the magnetic Brillouin zone for a canting angle greater than 60 degree.

cond-mat.str-el↗

arXiv2006

The Ring Imaging Cherenkov detector (RICH) of the AMS experiment

The Alpha Magnetic Spectrometer (AMS) experiment to be installed on the International Space Station (ISS) will be equipped with a proximity focusing Ring Imaging Cherenkov (RICH) detector for measuring the electric charge and velocity of the charged cosmic particles. A RICH prototype consisting of 96 photomultiplier units, including a piece of the conical reflector, was built and its performance evaluated with ion beam data. Preliminary results of the in-beam tests performed with ion fragments resulting from collisions of a 158 GeV/c/nuc primary beam of Indium ions (CERN SPS) on a Pb target are reported. The collected data included tests to the final front-end electronics and to different aerogel radiators. Cherenkov rings for a large range of charged nuclei and with reflected photons were observed. The data analysis confirms the design goals. Charge separation up to Fe and velocity resolution of the order of 0.1% for singly charged particles are obtained.

astro-ph↗

arXiv2006

Magnetophonon resonance in high density, high mobility quantum well systems

We have investigated the magnetophonon resonance (MPR) effect in a series of single GaAs quantum well samples which are symmetrically modulation doped in the adjacent short period AlAs/GaAs superlattices. Two distinct MPR series are observed originating from the $Γ$ and X electrons interacting with the GaAs and AlAs longitudinal optic (LO) phonons respectively. This confirms unequivocally the presence of X electrons in the AlAs quantum well of the superlattice previously invoked to explain the high electron mobility in these structures (Friedland et al. Phys. Rev. Lett. 77,4616 (1996).

cond-mat.mes-hall↗

arXiv2006

Frohlich mass in GaAs-based structures

The Frohlich interaction is one of the main electron-phonon intrinsic interactions in polar materials originating from the coupling of one itinerant electron with the macroscopic electric field generated by any longitudinal optical (LO) phonon. Infra-red magneto-absorption measurements of doped GaAs quantum wells structures have been carried out in order to test the concept of Frohlich interaction and polaron mass in such systems. These new experimental results lead to question the validity of this concept in a real system.

cond-mat.mes-hall↗

arXiv2006

Electron-phonon coupling in the two phonon mode ternary alloy $Al_{0.25}In_{0.75}As/Ga_{0.25}In_{0.75}As$ quantum well

We have investigated the infrared transmission of a two-dimensional (2DEG) electron gas confined in a $Al_{0.25}In_{0.75}As/Ga_{0.25}In_{0.75}As$ single quantum well in order to study the electron optical phonon interaction in a two phonon mode system. Infrared transmission experiments have been performed in both the perpendicular Faraday (PF) and tilted Faraday (TF) configurations for which the growth axis of the sample is tilted with respect to the incident light propagation direction and to the magnetic field direction. The experimental results lead to question the validity of the concept of polaron mass in a real material.

cond-mat.mes-hall↗