354 research outputs found
Temperature-dependent spin gap and singlet ground state in BaCuSi2O6
Bulk magnetic measurements and inelastic neutron scattering were used to
investigate the spin-singlet ground state and magnetic gap excitations in
BaCuSi2O6, a quasi-2-dimensional antiferromagnet with a bilayer structure. The
results are well described by a model based on weakly interacting
antiferromagnetic dimers. A strongly temperature-dependent dispersion in the
gap modes was found. We suggest that the observed excitations are analogous to
magneto-excitons in light rare-earth compounds, but are an intrinsic property
of a simple Heisenberg Hamiltonian for the S=1/2 magnetic bilayer.Comment: 10 pages, 4 figures, REVTeX and PS for text, PS for figures direct
download: http://papillon.phy.bnl.gov/preprints/bacusio.htm
Time-dependent density-matrix functional theory for biexcitonic phenomena
We formulate a time-dependent density-matrix functional theory (TDDMFT)
approach for higher-order correlation effects like biexcitons in optical
processes in solids based on the reduced two-particle density-matrix formalism
within the normal orbital representation. A TDDMFT version of the Schr\"odinger
equation for biexcitons in terms of one- and two-body reduced density matrices
is derived, which leads to finite biexcitonic binding energies already with an
adiabatic approximation. Biexcitonic binding energies for several bulk
semiconductors are calculated using a contact biexciton model
Optical Control of Field-Emission Sites by Femtosecond Laser Pulses
We have investigated field emission patterns from a clean tungsten tip apex
induced by femtosecond laser pulses. Strongly asymmetric modulations of the
field emission intensity distributions are observed depending on the
polarization of the light and the laser incidence direction relative to the
azimuthal orientation of tip apex. In effect, we have realized an ultrafast
pulsed field-emission source with site selectivity on the 10 nm scale.
Simulations of local fields on the tip apex and of electron emission patterns
based on photo-excited nonequilibrium electron distributions explain our
observations quantitatively.Comment: 4 pages, submitted to Physical Review Letter
Glucocorticoid receptor-PPARα axis in fetal mouse liver prepares neonates for milk lipid catabolism.
In mammals, hepatic lipid catabolism is essential for the newborns to efficiently use milk fat as an energy source. However, it is unclear how this critical trait is acquired and regulated. We demonstrate that under the control of PPARα, the genes required for lipid catabolism are transcribed before birth so that the neonatal liver has a prompt capacity to extract energy from milk upon suckling. The mechanism involves a fetal glucocorticoid receptor (GR)-PPARα axis in which GR directly regulates the transcriptional activation of PPARα by binding to its promoter. Certain PPARα target genes such as Fgf21 remain repressed in the fetal liver and become PPARα responsive after birth following an epigenetic switch triggered by ÎČ-hydroxybutyrate-mediated inhibition of HDAC3. This study identifies an endocrine developmental axis in which fetal GR primes the activity of PPARα in anticipation of the sudden shifts in postnatal nutrient source and metabolic demands
Theory of Umklapp-assisted recombination of bound excitons in Si:P
We present the calculations for the oscillator strength of the recombination
of excitons bound to phosphorous donors in silicon. We show that the direct
recombination of the bound exciton cannot account for the experimentally
measured oscillator strength of the no-phonon line. Instead, the recombination
process is assisted by an umklapp process of the donor electron state. We make
use of the empirical pseudopotential method to evaluate the Umklapp-assisted
recombination matrix element in second-order perturbation theory. Our result is
in excellent agreement with the experiment. We also present two methods to
improve the optical resolution of the optical detection of the spin state of a
single nucleus in silicon.Comment: 9 pages, 6 EPS figures, Revtex
Dislocation-induced spin tunneling in Mn-12 acetate
Comprehensive theory of quantum spin relaxation in Mn-12 acetate crystals is
developed, that takes into account imperfections of the crystal structure and
is based upon the generalization of the Landau-Zener effect for incoherent
tunneling from excited energy levels. It is shown that linear dislocations at
plausible concentrations provide the transverse anisotropy which is the main
source of tunneling in Mn-12. Local rotations of the easy axis due to
dislocations result in a transverse magnetic field generated by the field
applied along the c-axis of the crystal, which explains the presence of odd
tunneling resonances. Long-range deformations due to dislocations produce a
broad distribution of tunnel splittings. The theory predicts that at subkelvin
temperatures the relaxation curves for different tunneling resonances can be
scaled onto a single master curve. The magnetic relaxation in the thermally
activated regime follows the stretched-exponential law with the exponent
depending on the field, temperature, and concentration of defects.Comment: 17 pages, 14 figures, 1 table, submitted to PR
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
Spin dynamics of Mn12-acetate in the thermally-activated tunneling regime: ac-susceptibility and magnetization relaxation
In this work, we study the spin dynamics of Mn12-acetate molecules in the
regime of thermally assisted tunneling. In particular, we describe the system
in the presence of a strong transverse magnetic field. Similar to recent
experiments, the relaxation time/rate is found to display a series of
resonances; their Lorentzian shape is found to stem from the tunneling. The
dynamic susceptibility is calculated starting from the microscopic
Hamiltonian and the resonant structure manifests itself also in .
Similar to recent results reported on another molecular magnet, Fe8, we find
oscillations of the relaxation rate as a function of the transverse magnetic
field when the field is directed along a hard axis of the molecules. This
phenomenon is attributed to the interference of the geometrical or Berry phase.
We propose susceptibility experiments to be carried out for strong transverse
magnetic fields to study of these oscillations and for a better resolution of
the sharp satellite peaks in the relaxation rates.Comment: 22 pages, 23 figures; submitted to Phys. Rev. B; citations/references
adde
Nonexponential Relaxation of Magnetization at the Resonant Tunneling Point under a Fluctuating Random Noise
Nonexponential relaxation of magnetization at resonant tunneling points of
nanoscale molecular magnets is interpreted to be an effect of fluctuating
random field around the applied field. We demonstrate such relaxation in
Langevin equation analysis and clarify how the initial relaxation (square-root
time) changes to the exponential decay. The scaling properties of the
relaxation are also discussed.Comment: 4 pages, 4 fgiure
A Distribution of Tunnel Splittings in Mn-Acetate
In magnetic fields applied parallel to the anisotropy axis, the relaxation of
the magnetization of Mn measured for different sweep rates is shown to
collapse onto a single scaled curve. The form of the scaling implies that the
dominant symmetry-breaking process that gives rise to tunneling is a locally
varying second-order anisotropy, forbidden by tetragonal symmetry in the
perfect crystal, which gives rise to a broad distribution of tunnel splittings
in a real crystal of Mn-acetate. Different forms applied to even and
odd-numbered steps provide a distinction between even step resonances
(associated with crystal anisotropy) and odd resonances (which require a
transverse component of magnetic field).Comment: 4 pages, 5 figures. New title; text more clearly writte
- âŠ