1,717 research outputs found
Reconstructing Generalized Exponential Laws by Self-Similar Exponential Approximants
We apply the technique of self-similar exponential approximants based on
successive truncations of continued exponentials to reconstruct functional laws
of the quasi-exponential class from the knowledge of only a few terms of their
power series. Comparison with the standard Pad\'e approximants shows that, in
general, the self-similar exponential approximants provide significantly better
reconstructions.Comment: Revtex file, 21 pages, 21 figure
Lattice effects on the spin dynamics in antiferromagnetic molecular rings
We investigate spin dynamics in antiferromagnetic (AF) molecular rings at
finite temperature in the presence of spin-phonon (s-p) interaction. We derive
a general expression for the spin susceptibility in the weak s-p coupling limit
and then we focus on the low-frequency behavior, in order to discuss a possible
microscopic mechanism for nuclear relaxation in this class of magnetic
materials. To lowest order in a perturbative expansion, we find that the
susceptibility takes a Lorentzian profile and all spin operators (, ) contribute to spin dynamics at wave vectors . Spin anisotropies
and local s-p coupling play a key role in the proposed mechanism. Our results
prove that small changes in the spatial symmetry of the ring induce qualitative
changes in the spin dynamics at the nuclear frequency, providing a novel
mechanism for nuclear relaxation. Possible experiments are proposed.Comment: 4 pages, 2 figures. to appear in PR
Electron spin relaxation in semiconducting carbon nanotubes: the role of hyperfine interaction
A theory of electron spin relaxation in semiconducting carbon nanotubes is
developed based on the hyperfine interaction with disordered nuclei spins I=1/2
of C isotopes. It is shown that strong radial confinement of electrons
enhances the electron-nuclear overlap and subsequently electron spin relaxation
(via the hyperfine interaction) in the carbon nanotubes. The analysis also
reveals an unusual temperature dependence of longitudinal (spin-flip) and
transversal (dephasing) relaxation times: the relaxation becomes weaker with
the increasing temperature as a consequence of the particularities in the
electron density of states inherent in one-dimensional structures. Numerical
estimations indicate relatively high efficiency of this relaxation mechanism
compared to the similar processes in bulk diamond. However, the anticipated
spin relaxation time of the order of 1 s in CNTs is still much longer than
those found in conventional semiconductor structures.Comment: 11 pages, 2 figure
Low energy electronic states and triplet pairing in layered cobaltates
The structure of the low-energy electronic states in layered cobaltates is
considered starting from the Mott insulating limit. We argue that the coherent
part of the wave-functions and the Fermi-surface topology at low doping are
strongly influenced by spin-orbit coupling of the correlated electrons on the
level. An effective t-J model based on mixed spin-orbital states is
radically different from that for the cuprates, and supports unconventional,
pseudospin-triplet pairing.Comment: 4 pages, 3 figure
Temperature and magnetic field dependent longitudinal spin relaxation in nitrogen-vacancy ensembles in diamond
We present an experimental study of the longitudinal electron-spin relaxation
time (T1) of negatively charged nitrogen-vacancy (NV) ensembles in diamond. T1
was studied as a function of temperature from 5 to 475 K and magnetic field
from 0 to 630 G for several samples with various NV and nitrogen
concentrations. Our studies reveal three processes responsible for T1
relaxation. Above room temperature, a two-phonon Raman process dominates, and
below, we observe an Orbach-type process with an activation energy, 73(4) meV,
which closely matches the local vibrational modes of the NV center. At yet
lower temperatures, sample dependent cross relaxation processes dominate,
resulting in temperature independent values of T1, from ms to minutes. The
value of T1 in this limit depends sensitively on magnetic field and can be
tuned by more than an order of magnitude.Comment: 5 pages, 3 figures, and 3 pages of supplemental material with
additional figure
Spin Transition in the Half-Filled Landau Level
The transition from partial to complete spin polarization of two-dimensional
electrons at half filling of the lowest Landau level has been studied using
resistively-detected nuclear magnetic resonance (RDNMR). The nuclear
spin-lattice relaxation time is observed to be density independent in the
partially polarized phase but to increase sharply at the transition to full
polarization. At low temperatures the RDNMR signal exhibits a strong maximum
near the critical density.Comment: 4 pages, 3 postscript figures. As published in Phys. Rev. Lett. 98,
086801 (2007
NMR and NQR study of pressure-induced superconductivity and the origin of critical-temperature enhancement in the spin-ladder cuprate SrCaCuO
Pressure-induced superconductivity was studied for a spin-ladder cuprate
SrCaCuO using nuclear magnetic resonance (NMR) under
pressures up to the optimal pressure 3.8 GPa. Pressure application leads to a
transitional change from a spin-gapped state to a Fermi-liquid state at
temperatures higher than . The relaxation rate shows
activated-type behavior at an onset pressure, whereas Korringa-like behavior
becomes predominant at the optimal pressure, suggesting that an increase in the
density of states (DOS) at the Fermi energy leads to enhancement of .
Nuclear quadrupole resonance (NQR) spectra suggest that pressure application
causes transfer of holes from the chain to the ladder sites. The transfer of
holes increases DOS below the optimal pressure. A dome-shaped versus
pressure curve arises from naive balance between the transfer of holes and
broadening of the band width
Structural, orbital, and magnetic order in vanadium spinels
Vanadium spinels (ZnV_2O_4, MgV_2O_4, and CdV_2O_4) exhibit a sequence of
structural and magnetic phase transitions, reflecting the interplay of lattice,
orbital, and spin degrees of freedom. We offer a theoretical model taking into
account the relativistic spin-orbit interaction, collective Jahn-Teller effect,
and spin frustration. Below the structural transition, vanadium ions exhibit
ferroorbital order and the magnet is best viewed as two sets of
antiferromagnetic chains with a single-ion Ising anisotropy. Magnetic order,
parametrized by two Ising variables, appears at a tetracritical point.Comment: v3: streamlined introductio
On the hyperfine interaction in rare-earth Van Vleck paramagnets at high magnetic fields
An influence of high magnetic fields on hyperfine interaction in the
rare-earth ions with non-magnetic ground state (Van Vleck ions) is
theoretically investigated for the case of ion in axial symmetrical
crystal electric field (ethylsulphate crystal). It is shown that magnetic-field
induced distortions of -electron shell lead to essential changes in
hyperfine magnetic field at the nucleus. The proposed theoretical model is in
agreement with recent experimental data.Comment: 4 pages, no figures, submitted to J. Phys. : Cond. Mat
Orbital disorder induced by charge fluctuations in vanadium spinels
Motivated by recent experiments on vanadium spinels, VO, that show
an increasing degree of electronic delocalization for smaller cation sizes, we
study the evolution of orbital ordering (OO) between the strong and
intermediate-coupling regimes of a multi-orbital Hubbard Hamiltonian. The
underlying magnetic ordering of the Mott insulating state leads to a rapid
suppression of OO due to enhanced charge fluctuations along ferromagnetic
bonds. Orbital double-occupancy is rather low at the transition point
indicating that the system is in the crossover region between strong and
intermediate-coupling regimes when the orbital degrees of freedom become
disordered
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