3,015 research outputs found
Resonant and crossover phenomena in a multiband superconductor tuning the chemical potential near a band edge
Resonances in the superconducting properties, in a regime of crossover from
BCS to mixed Bose-Fermi superconductivity, are investigated in a two-band
superconductor where the chemical potential is tuned near the band edge of the
second mini-band generated by quantum confinement effects. The shape resonances
at T=0 in the superconducting gaps (belonging to the class of Feshbach-like
resonances) is manifested by interference effects in the superconducting gap at
the first large Fermi surface when the chemical potential is in the proximity
of the band edge of the second mini-band. The case of a superlattice of quantum
wells is considered and the amplification of the superperconducting gaps at the
3D-2D Fermi surface topological transition is clearly shown. The results are
found to be in good agreement with available experimental data on a
superlattice of honeycomb boron layers intercalated by Al and Mg spacer layers.Comment: 13 pages, 9 image
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Revised classification of the soils of Belize
The soil classification of Belize is revised and updated, starting from the system devised for 'Land in British Honduras' by A. C. S. Wright and his colleagues in the 1950s, the only previous country-wide survey of soils and land resources. The revised classification is a three-tiered system consisting of soil suites, subsuites and series. The suites are based on rock type, and the subsuites on soil profile characteristics. The main groups of soils, the soil suites, and the soil subsuites are described separately at progressively increasing levels of detail. There are insufficient data to describe soil series as yet, but guidelines for the definition and naming of new series are indicated. The classes of the revised system are correlated with all of the soil classifications previously used in Belize, and with the two main international systems of soil classification. Important chemical features of the-main soil groups are summarized and compared. The comparisons vindicate the revised classification
Thermal properties of MgB2: the effect of disorder on gap amplitudes and relaxation times of p and s bands
We present thermal conductivity and specific heat measurements on MgB2 and
Mg-AlB2 samples. Thermal properties have been analysed by using a two-gap model
in order to estimate the gap amplitudes, D(0)p and D(0)s and the intra-band
scattering rates, Gss and Gpp. As a function of Al doping and disorder D(0)s
rapidly decreases, while D(0)p is rather constant. Gss and Gpp are increased by
the disorder, being Gpp more affected than Gss.Comment: 2 pages, 3 figures, presented to the conference M2S-HTSC, 25-30 May
2003, Rio de Janeir
Correlation between oxygen isotope effects on the transition temperature and the magnetic penetration depth in high-temperature superconductors close to optimal doping
The oxygen-isotope (^{16}O/^{18}O) effect (OIE) on the in-plane magnetic
penetration depth \lambda_{ab}(0) in optimally-doped YBa_2Cu_3O_{7-\delta} and
La_{1.85}Sr_{0.15}CuO_4, and in slightly underdoped YBa_2Cu_4O_8 and
Y_{0.8}Pr_{0.2}Ba_2Cu_3O_{7-\delta} was studied by means of muon-spin rotation.
A substantial OIE on \lambda_{ab}(0) with an OIE exponent
\beta_O=-d\ln\lambda_{ab}(0)/d\ln M_O\approx - 0.2 (M_O is the mass of the
oxygen isotope), and a small OIE on the transition temperature T_c with an OIE
exponent \alpha_O=-d\ln T_{c}/d \ln M_O\simeq0.02 to 0.1 were observed. The
observation of a substantial isotope effect on \lambda_{ab}(0), even in
cuprates where the OIE on T_c is small, indicates that lattice effects play an
important role in cuprate HTS.Comment: 6 pages, 4 figure
Constraining dark energy with Sunyaev-Zel'dovich cluster surveys
We discuss the prospects of constraining the properties of a dark energy
component, with particular reference to a time varying equation of state, using
future cluster surveys selected by their Sunyaev-Zel'dovich effect. We compute
the number of clusters expected for a given set of cosmological parameters and
propogate the errors expected from a variety of surveys. In the short term they
will constrain dark energy in conjunction with future observations of type Ia
supernovae, but may in time do so in their own right.Comment: 5 pages, 3 figures, 1 table, version accepted for publication in PR
Mixed order parameter symmetries in cuprate superconductors
The recent observation of an inflection point in the temperature dependence
of the in-plane magnetic field dependence (lambda_ab) is investigated within a
two-band model with coupled order parameters of different symmetries. While the
dominant order parameter has d-wave symmetry, the smaller one is of s-wave
symmetry. Superconductivity is robust in the d-wave channel and induced via
interband interactions in the s-wave subsystem.Comment: 10 pages, 4 figure
Oxygen isotope effect on superconductivity and magnetism in Y_1-xPr_xBa_2Cu_3O_7-\delta
Oxygen isotope (^{16}O/^{18}O) effects (OIE's) on the superconducting
transition (T_c), the spin-glass ordering (T_g), and the antiferromagnetic
ordering (T_N) temperatures were studied for Y_1-xPr_xBa_2Cu_3O_7-\delta as a
function of Pr content (0.0\leq x \leq 1.0). The OIE on T_c increases with
increasing x up to x\approx0.55, where superconductivity disappears. For
decreasing x the OIE's on T_N and T_g increase down to x\approx 0.7 where
antiferromagnetic order and down to x\approx0.3 where spin-glass behavior
vanish, respectively. The OIE's on T_g and T_N are found to have {\it opposite
signs} as compared to the OIE on T_c. All OIE's are suggested to arise from the
isotope dependent mobility (kinetic energy) of the charge carriers.Comment: 5 pages, 3 figures, 1 tabl
Electron-Phonon Coupling in High-Temperature Cuprate Superconductors Determined from Electron Relaxation Rates
We determined electronic relaxation times via pump-probe optical spectroscopy
using sub-15 fs pulses for the normal state of two different cuprate
superconductors.We show that the primary relaxation process is the
electron-phonon interaction and extract a measure of its strength, the second
moment of the Eliashberg function\lambda=800\pm200 meV^{2} for
La_{1.85}Sr_{0.15}CuO_{4} and \lambda=400\pm100 meV^{2} for
YBa_{2}Cu_{3}O_{6.5}. These values suggest a possible fundamental role of the
electron-phonon interaction in the superconducting pairing mechanism.Comment: As published in PR
Characterising the frequency response of impedance changes during evoked physiological activity in the rat brain
OBJECTIVE: Electrical impedance tomography (EIT) can image impedance changes associated with evoked physiological activity in the cerebral cortex using an array of epicortical electrodes. An impedance change is observed as the externally applied current, normally confined to the extracellular space is admitted into the conducting intracellular space during neuronal depolarisation. The response is largest at DC and decreases at higher frequencies due to capacitative transfer of current across the membrane. Biophysical modelling has shown that this effect becomes significant above 100 Hz. Recordings at DC, however, are contaminated by physiological endogenous evoked potentials. By moving to 1.7 kHz, images of somatosensory evoked responses have been produced down to 2 mm with a resolution of 2 ms and 200 μm. Hardware limitations have so far restricted impedance measurements to frequencies  2 kHz using improved hardware. APPROACH: Impedance changes were recorded during forepaw somatosensory stimulation in both cerebral cortex and the VPL nucleus of the thalamus in anaesthetised rats using applied currents of 1 kHz to 10 kHz. MAIN RESULTS: In the cortex, impedance changed by -0.04 ± 0.02 % at 1 kHz, reached a peak of -0.13 ± 0.05 % at 1475 Hz and decreased to -0.05 ± 0.02 % at 10 kHz. At these frequencies, changes in the thalamus were -0.26 ± 0.1%, -0.4 ± 0.15 % and -0.08 ± 0.03 % respectively. The signal-to-noise ratio was also highest at 1475 Hz with values of -29.5 ± 8 and -31.6 ±10 recorded from the cortex and thalamus respectively. Signficance: This indicates that the optimal frequency for imaging cortical and thalamic evoked activity using fast neural EIT is 1475 Hz
Onset of dielectric modes at 110K and 60K due to local lattice distortions in non-superconducting YBa_{2}Cu_{3}O_{6.0} crystals
We report the observation of two dielectric transitions at 110K and 60K in
the microwave response of non-superconducting YBa_{2}Cu_{3}O_{6.0} crystals.
The transitions are characterized by a change in polarizability and presence of
loss peaks, associated with overdamped dielectric modes. An explanation is
presented in terms of changes in polarizability of the apical O atoms in the
Ba-O layer, affected by lattice softening at 110K, due to change in buckling of
the Cu-O layer. The onset of another mode at 60K strongly suggests an
additional local lattice change at this temperature. Thus microwave dielectric
measurements are sensitive indicators of lattice softening which may be
relevant to superconductivity.Comment: 5 pages, 3 ps format figure
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