5,038 research outputs found
Electrically driven magnetism on a Pd thin film
Using first-principles density functional calculations we demonstrate that
ferromagnetism can be induced and modulated on an otherwise paramagnetic Pd
metal thin-film surface through application of an external electric field. As
free charges are either accumulated or depleted at the Pd surface to screen the
applied electric field there is a corresponding change in the surface density
of states. This change can be made sufficient for the Fermi-level density of
states to satisfy the Stoner criterion, driving a transition locally at the
surface from a paramagnetic state to an itinerant ferromagnetic state above a
critical applied electric field, Ec. Furthermore, due to the second-order
nature of this transition, the surface magnetization of the ferromagnetic state
just above the transition exhibits a substantial dependence on electric field,
as the result of an enhanced magnetoelectric susceptibility. Using a linearized
Stoner model we explain the occurrence of the itinerant ferromagnetism and
demonstrate that the magnetic moment on the Pd surface follows a square-root
variation with electric field consistent with our first-principles
calculations.Comment: 8 pages, 7 figure
The turbulent spectrum created by non-Abelian plasma instabilities
Recent numerical work on the fate of plasma instabilities in weakly-coupled
non-Abelian gauge theory has shown the development of a cascade of energy from
long to short wavelengths. This cascade has a steady-state spectrum, analogous
to the Kolmogorov spectrum for turbulence in hydrodynamics or for energy
cascades in other systems. In this paper, we theoretically analyze processes
responsible for this cascade and find a steady-state spectrum f_k ~ k^-2, where
f_k is the phase-space density of particles with momentum k. The exponent -2 is
consistent with results from numerical simulations. We also discuss
implications of the emerging picture of instability development on the
"bottom-up" thermalization scenario for (extremely high energy) heavy ion
collisions, emphasizing fundamental questions that remain to be answered.Comment: 17 pages, 5 figure
Many-body effects on adiabatic passage through Feshbach resonances
We theoretically study the dynamics of an adiabatic sweep through a Feshbach
resonance, thereby converting a degenerate quantum gas of fermionic atoms into
a degenerate quantum gas of bosonic dimers. Our analysis relies on a zero
temperature mean-field theory which accurately accounts for initial molecular
quantum fluctuations, triggering the association process. The structure of the
resulting semiclassical phase space is investigated, highlighting the dynamical
instability of the system towards association, for sufficiently small detuning
from resonance. It is shown that this instability significantly modifies the
finite-rate efficiency of the sweep, transforming the single-pair exponential
Landau-Zener behavior of the remnant fraction of atoms Gamma on sweep rate
alpha, into a power-law dependence as the number of atoms increases. The
obtained nonadiabaticity is determined from the interplay of characteristic
time scales for the motion of adiabatic eigenstates and for fast periodic
motion around them. Critical slowing-down of these precessions near the
instability leads to the power-law dependence. A linear power law is obtained when the initial molecular fraction is smaller than the 1/N
quantum fluctuations, and a cubic-root power law is
attained when it is larger. Our mean-field analysis is confirmed by exact
calculations, using Fock-space expansions. Finally, we fit experimental low
temperature Feshbach sweep data with a power-law dependence. While the
agreement with the experimental data is well within experimental error bars,
similar accuracy can be obtained with an exponential fit, making additional
data highly desirable.Comment: 9 pages, 9 figure
Nonlinear adiabatic passage from fermion atoms to boson molecules
We study the dynamics of an adiabatic sweep through a Feshbach resonance in a
quantum gas of fermionic atoms. Analysis of the dynamical equations, supported
by mean-field and many-body numerical results, shows that the dependence of the
remaining atomic fraction on the sweep rate varies from
exponential Landau-Zener behavior for a single pair of particles to a power-law
dependence for large particle number . The power-law is linear, , when the initial molecular fraction is smaller than the 1/N
quantum fluctuations, and when it is larger.
Experimental data agree better with a linear dependence than with an
exponential Landau-Zener fit, indicating that many-body effects are significant
in the atom-molecule conversion process.Comment: 5 pages, 4 figure
On the conversion efficiency of ultracold fermionic atoms to bosonic molecules via Feshbach resonances
We explain why the experimental efficiency observed in the conversion of
ultracold Fermi gases of K and Li atoms into diatomic Bose gases
is limited to 0.5 when the Feshbach resonance sweep rate is sufficiently slow
to pass adiabatically through the Landau Zener transition but faster than ``the
collision rate'' in the gas, and increases beyond 0.5 when it is slower. The
0.5 efficiency limit is due to the preparation of a statistical mixture of two
spin-states, required to enable s-wave scattering. By constructing the
many-body state of the system we show that this preparation yields a mixture of
even and odd parity pair-states, where only even parity can produce molecules.
The odd parity spin-symmetric states must decorrelate before the constituent
atoms can further Feshbach scatter thereby increasing the conversion
efficiency; ``the collision rate'' is the pair decorrelation rate.Comment: 4 pages, 3 figures, final version accepted to Phys. Rev. Let
What can we learn from Dijet suppression at RHIC?
We present a systematic study of the dijet suppression at RHIC using the
VNI/BMS parton cascade. We examine the modification of the dijet asymmetry A_j
and the within-cone transverse energy distribution (jet-shape) along with
partonic fragmentation distributions z and j_t in terms of: qhat; the path
length of leading and sub-leading jets; cuts on the jet energy distributions;
jet cone angle and the jet-medium interaction mechanism. We find that A_j is
most sensitive to qhat and relatively insensitive to the nature of the
jet-medium interaction mechanism. The jet profile is dominated by qhat and the
nature of the interaction mechanism. The partonic fragmentation distributions
clearly show the jet modification and differentiate between elastic and
radiative+elastic modes
Dirac and Normal Fermions in Graphite and Graphene: Implications to the Quantum Hall Effect
Spectral analysis of Shubnikov de Haas (SdH) oscillations of
magnetoresistance and of Quantum Hall Effect (QHE) measured in quasi-2D highly
oriented pyrolytic graphite (HOPG) [Phys. Rev. Lett. 90, 156402 (2003)] reveals
two types of carriers: normal (massive) electrons with Berry phase 0 and
Dirac-like (massless) holes with Berry phase pi. We demonstrate that recently
reported integer- and semi-integer QHE for bi-layer and single-layer graphenes
take place simultaneously in HOPG samples.Comment: 4 page
3-point off-shell vertex in scalar QED in arbitrary gauge and dimension
We calculate the complete one-loop off-shell three-point scalar-photon vertex
in arbitrary gauge and dimension for Scalar Quantum Electrodynamics. Explicit
results are presented for the particular cases of dimensions 3 and 4 both for
massive and massless scalars. We then propose non-perturbative forms of this
vertex that coincide with the perturbative answer to order .Comment: Uses axodra
Quantum Shock Waves - the case for non-linear effects in dynamics of electronic liquids
Using the Calogero model as an example, we show that the transport in
interacting non-dissipative electronic systems is essentially non-linear.
Non-linear effects are due to the curvature of the electronic spectrum near the
Fermi energy. As is typical for non-linear systems, propagating wave packets
are unstable. At finite time shock wave singularities develop, the wave packet
collapses, and oscillatory features arise. They evolve into regularly
structured localized pulses carrying a fractionally quantized charge - {\it
soliton trains}. We briefly discuss perspectives of observation of Quantum
Shock Waves in edge states of Fractional Quantum Hall Effect and a direct
measurement of the fractional charge
Resonance Damping in Ferromagnets and Ferroelectrics
The phenomenological equations of motion for the relaxation of ordered phases
of magnetized and polarized crystal phases can be developed in close analogy
with one another. For the case of magnetized systems, the driving magnetic
field intensity toward relaxation was developed by Gilbert. For the case of
polarized systems, the driving electric field intensity toward relaxation was
developed by Khalatnikov. The transport times for relaxation into thermal
equilibrium can be attributed to viscous sound wave damping via
magnetostriction for the magnetic case and electrostriction for the
polarization case.Comment: 5 pages no figures ReVTeX
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