207 research outputs found
Charge carrier density collapse in La_0.67Ca_0.33MnO_3 and La_0.67Sr_0.33MnO_3 epitaxial thin films
We measured the temperature dependence of the linear high field Hall
resistivity of La_0.67Ca_0.33MnO_3 (T_C=232K) and La_0.67Sr_0.33MnO_3
(T_C=345K) thin films in the temperature range from 4K up to 360K in magnetic
fields up to 20T. At low temperatures we find a charge carrier density of 1.3
and 1.4 holes per unit cell for the Ca- and Sr-doped compound, respectively. In
this temperature range electron-magnon scattering contributes to the
longitudinal resistivity. At the ferromagnetic transition temperature T_C a
dramatic drop in the number of current carriers down to 0.6 holes per unit
cell, accompanied by an increase in unit cell volume, is observed. Corrections
of the Hall data due to a non saturated magnetic state will lead a more
pronounced charge carrier density collapse.Comment: 5 pages, 5 EPS figures, submitted to Eur. Phys. J.
Re-entrant spin susceptibility of a superconducting grain
We study the spin susceptibility chi of a small, isolated superconducting
grain. Due to the interplay between parity effects and pairing correlations,
the dependence of chi on temperature T is qualitatively different from the
standard BCS result valid in the bulk limit. If the number of electrons on the
grain is odd, chi shows a re-entrant behavior as a function of temperature.
This behavior persists even in the case of ultrasmall grains where the mean
level spacing is much larger than the BCS gap. If the number of electrons is
even, chi(T) is exponentially small at low temperatures.Comment: 9 pages, 3 figures. To be published in PR
Resonant Magnetization Tunneling in Mn12 Acetate: The Absence of Inhomogeneous Hyperfine Broadening
We present the results of a detailed study of the
thermally-assisted-resonant-tunneling relaxation rate of Mn12 acetate as a
function of an external, longitudinal magnetic field and find that the data can
be fit extremely well to a Lorentzian function. No hint of inhomogeneous
broadening is found, even though some is expected from the Mn nuclear hyperfine
interaction. This inconsistency implies that the tunneling mechanism cannot be
described simply in terms of a random hyperfine field.Comment: Some minor revisions, title changed, updated figures, two added
notes, one added reference. RevTeX, 4 pages, 3 postscript figures. Submitted
to Rapid Communication
Interplay between localized and itinerant d electrons in a frustrated metallic antiferromagnet, 2H-AgNiO2
We report the electronic and magnetic behaviour of the frustrated triangular
metallic antiferromagnet 2H-AgNiO2 in high magnetic fields (54 T) using
thermodynamic and transport measurements. Here localized d electrons are
arranged on an antiferromagnetic triangular lattice nested inside a honeycomb
lattice with itinerant d electrons. When the magnetic field is along the easy
axis we observe a cascade of field-induced transitions, attributed to the
competition between easy-axis anisotropy, geometrical frustration and coupling
of the localized and itinerant system. The quantum oscillations data suggest
that the Fermi surface is reconstructed by the magnetic order but in high
fields magnetic breakdown orbits are possible. The itinerant electrons are
extremely sensitive to scattering by spin fluctuations and a significant mass
enhancement (~ 3) is found.Comment: 4 page
Thermodynamic properties of a small superconducting grain
The reduced BCS Hamiltonian for a metallic grain with a finite number of
electrons is considered. The crossover between the ultrasmall regime, in which
the level spacing, , is larger than the bulk superconducting gap, ,
and the small regime, where , is investigated analytically
and numerically. The condensation energy, spin magnetization and tunneling peak
spectrum are calculated analytically in the ultrasmall regime, using an
approximation controlled by as small parameter, where is the
number of interacting electron pairs. The condensation energy in this regime is
perturbative in the coupling constant , and is proportional to . We find that also in a large regime with
, in which pairing correlations are already rather well developed,
the perturbative part of the condensation energy is larger than the singular,
BCS, part. The condition for the condensation energy to be well approximated by
the BCS result is found to be roughly . We show how
the condensation energy can, in principle, be extracted from a measurement of
the spin magnetization curve, and find a re-entrant susceptibility at zero
temperature as a function of magnetic field, which can serve as a sensitive
probe for the existence of superconducting correlations in ultrasmall grains.
Numerical results are presented which suggest that in the large limit the
1/N correction to the BCS result for the condensation energy is larger than
.Comment: 17 pages, 7 figures, Submitted to Phys. Rev.
Effects of Spin-Orbit Interactions on Tunneling via Discrete Energy Levels in Metal Nanoparticles
The presence of spin-orbit scattering within an aluminum nanoparticle affects
measurements of the discrete energy levels within the particle by (1) reducing
the effective g-factor below the free-electron value of 2, (2) causing avoided
crossings as a function of magnetic field between predominantly-spin-up and
predominantly-spin-down levels, and (3) introducing magnetic-field-dependent
changes in the amount of current transported by the tunneling resonances. All
three effects can be understood in a unified fashion by considering a simple
Hamiltonian. Spin-orbit scattering from 4% gold impurities in superconducting
aluminum nanoparticles produces no dramatic effect on the superconducting gap
at zero magnetic field, but we argue that it does modify the nature of the
superconducting transition in a magnetic field.Comment: 10 pages, 5 figures. Submitted to Phys. Rev.
Magnetization of Mn_12 Ac in a slowly varying magnetic field: an ab initio study
Beginning with a Heisenberg spin Hamiltonian for the manganese ions in the
Mn_12 Ac molecule, we find a number of low-energy states of the system. We use
these states to solve the time-dependent Schrodinger equation and find the
magnetization of the molecule in the presence of a slowly varying magnetic
field. We study the effects of the field sweep rate, fourth order anisotropic
spin interactions and a transverse field on the weights of the different states
as well as the magnetization steps which are known to occur in the hysteresis
plots in this system. We find that the fourth order term and a slow field sweep
rate are crucial for obtaining prominent steps in magnetization in the
hysteresis plots.Comment: LaTeX, 11 pages, 12 eps figure
Novel Phases in the Field Induced Spin Density Wave State in (TMTSF)_2PF_6
Magnetoresistance measurements on the quasi one-dimensional organic conductor
(TMTSF)_2PF_6 performed in magnetic fields B up to 16T, temperatures T down to
0.12K and under pressures P up to 14kbar have revealed new phases on its P-B-T
phase diagram. We found a new boundary which subdivides the field induced spin
density wave (FISDW) phase diagram into two regions. We showed that a
low-temperature region of the FISDW diagram is characterized by a hysteresis
behavior typical for the first order transitions, as observed in a number of
studies. In contrast to the common believe, in high temperature region of the
FISDW phase diagram, the hysteresis and, hence, the first order transitions
were found to disappear. Nevertheless, sharp changes in the resistivity slope
are observed both in the low and high temperature domains indicating that the
cascade of transitions between different subphases exists over all range of the
FISDW state. We also found that the temperature dependence of the resistance
(at a constant B) changes sign at about the same boundary. We compare these
results with recent theoretical models.Comment: LaTex, 4 pages, 4 figure
- …