184 research outputs found
On the destruction of the hidden order in URuSi by a strong magnetic field
We present a study of transport properties of the heavy fermion URuSi
in pulsed magnetic field. The large Nernst response of the hidden order state
is found to be suppressed when the magnetic field exceeds 35 T. The combination
of resistivity, Hall and Nernst data outlines the reconstruction of the Fermi
surface in the temperature-field phase diagram. The zero-field ground state is
a compensated heavy-electron semi-metal, which is destroyed by magnetic field
through a cascade of field-induced transitions. Above 40 T, URuSi
appears to be a polarized heavy fermions metal with a large density of carriers
whose effective mass rapidly decreases with increasing magnetic polarization.Comment: published versio
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
Impact of the capping layers on lateral confinement in InAs/InP quantum dots for 1.55 um laser applications srudied by magneto-photoluminescence.
We have used magnetophotoluminescence to study the impact of different capping layer material combinations (InP, GaInAsP quaternary alloy, or both InP and quaternary alloy) on lateral confinement in InAs/InP quantum dots (QDs) grown on (311)B orientated substrates. Exciton effective masses, Bohr radii, and binding energies are measured for these samples. Conclusions regarding the strength of the lateral confinement in the different samples are supported by photoluminescence at high excitation power. Contrary to theoretical predictions, InAs QDs in quaternary alloy are found to have better confinement properties than InAs/InP QDs. This is attributed to a lack of lateral intermixing with the quaternary alloy, which is present when InP is used to (partially) cap the dots. The implications of the results for reducing the temperature sensitivity of QD lasers are discussed. ©2005 American Institute of Physic
Fermi-surface reconstruction and two-carrier model for the Hall effect in YBa2Cu4O8
Pulsed field measurements of the Hall resistivity and magnetoresistance of
underdoped YBa2Cu4O8 are analyzed self-consistently using a simple model based
on coexisting electron and hole carriers. The resultant mobilities and Hall
numbers are found to vary markedly with temperature. The conductivity of the
hole carriers drops by one order of magnitude below 30 K, explaining the
absence of quantum oscillations from these particular pockets. Meanwhile the
Hall coefficient of the electron carriers becomes strongly negative below 50 K.
The overall quality of the fits not only provides strong evidence for
Fermi-surface reconstruction in Y-based cuprates, it also strongly constrains
the type of reconstruction that might be occurring.Comment: 5 pages, 4 figures, updated after publication in Physical Review B
(Rapid Communication
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
Pressure-induced structural transitions triggering dimensional crossover in lithium purple bronze Li0.9M6O17
At ambient pressure, lithium molybdenum purple bronze (Li0.9Mo6O17) is a
quasi-one dimensional solid in which the anisotropic crystal structure and the
linear dispersion of the underlying bands produced by electronic correlations
possibly bring about a rare experimental realization of Tomomaga-Luttinger
liquid physics. It is also the sole member of the broader purple molybdenum
bronzes family where a Peierls instability has not been identified at low
temperatures. The present study reports a pressure-induced series of phase
transitions between 0 and 12 GPa. These transitions are strongly reflected in
infrared spectroscopy, Raman spectroscopy, and x-ray diffraction. The most
dramatic effect seen in optical conductivity is the metallization of the
c-axis, concomitant to the decrease of conductivity along the b-axis. This
indicates that high pressure drives the material away from its quasi-one
dimensional behavior at ambient pressure. While the first pressure-induced
structure of the series is resolved, the identification of the underlying
mechanisms driving the dimensional change in the physics remains a challenge.Comment: 10 pages, 12 figure
Réalisation d’un laser à faible courant de seuil, avec des boites quantiques InAs/InP organisées et couplées latéralement
Nous présentons ici la réalisation d’un laser à faible courant de seuil avec des boites quantiques (QDs) organisées et couplées InAs/InP sur subsstrat (311)B pour une émission à 1.55 m. En effet, pour des hautes densités de QDs, une organisation périodique apparaît dans le plan. Cette organisation renforce le couplage latéral inter-boites. Des expériences de magnéto-photoluminescence permettent de mettre en évidence ces effets de couplage. Ce couplage améliore l’injection des porteurs. Une émission laser avec des faibles courants de seuil est obtenue avec de telles boites
Exposure of young children to household water lead in the Montreal area (Canada): the potential influence of winter-to-summer changes in water lead levels on children's blood lead concentration
Drinking water represents a potential source of lead exposure. The purpose of the present study was to estimate the magnitude of winter-to-summer changes in household water lead levels (WLLs), and to predict the impact of these variations on BLLs in young children. A study was conducted from September, 2009 to March, 2010 in 305 homes, with a follow-up survey carried out from June to September 2011 in a subsample of 100 homes randomly selected. The first 1-L sample was drawn after 5 min of flushing, followed by a further 4 consecutive 1-L samples after 30 min of stagnation. Non-linear regression and general linear mixed models were used for modelling seasonal effects on WLL. The batchrun mode of Integrated Exposure Uptake Biokinetic (IEUBK) model was used to predict the impact of changes in WLL on children's blood lead levels (BLLs). The magnitude of winter-to-summer changes in average concentrations of lead corresponded to 6.55 mug/L in homes served by lead service lines (LSL+ homes) and merely 0.30 mug/L in homes without lead service lines. For stagnant samples, the value reached 10.55 mug/L in 'LSL+ homes' and remained very low (0.36 mug/L) in 'LSL- homes'. The change in the probability of BLLs >/=5 mug/dL due to winter-to-summer changes in WLL was increased from /=5 mug/dL in young children during warm months was reduced by at least 40% by flushing tap-water
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