1,674 research outputs found
Inhomogeneous Nuclear Spin Flips
We discuss a feedback mechanism between electronic states in a double quantum
dot and the underlying nuclear spin bath. We analyze two pumping cycles for
which this feedback provides a force for the Overhauser fields of the two dots
to either equilibrate or diverge. Which of these effects is favored depends on
the g-factor and Overhauser coupling constant A of the material. The strength
of the effect increases with A/V_x, where V_x is the exchange matrix element,
and also increases as the external magnetic field B_{ext} decreases.Comment: 5 pages, 4 figures (jpg
A modified triplet-wave expansion method applied to the alternating Heisenberg chain
An alternative triplet-wave expansion formalism for dimerized spin systems is
presented, a modification of the 'bond operator' formalism of Sachdev and
Bhatt. Projection operators are used to confine the system to the physical
subspace, rather than constraint equations. The method is illustrated for the
case of the alternating Heisenberg chain, and comparisons are made with the
results of dimer series expansions and exact diagonalization. Some discussion
is included of the phenomenon of 'quasiparticle breakdown', as it applies to
the two-triplon bound states in this model.Comment: 16 pages, 12 figure
The Trapped Polarized Fermi Gas at Unitarity
We consider population-imbalanced two-component Fermi gases under external
harmonic confinement interacting through short-range two-body potentials with
diverging s-wave scattering length. Using the fixed-node diffusion Monte Carlo
method, the energies of the "normal state" are determined as functions of the
population-imbalance and the number of particles. The energies of the trapped
system follow, to a good approximation, a universal curve even for fairly small
systems. A simple parameterization of the universal curve is presented and
related to the equation of state of the bulk system.Comment: 4 pages, 2 tables, 2 figure
Magnetic structure and phase diagram in a spin-chain system: CaCoO
The low-temperature structure of the frustrated spin-chain compound
CaCoO is determined by the ground state of the 2D Ising model on
the triangular lattice. At high-temperatures it transforms to the honeycomb
magnetic structure. It is shown that the crossover between the two magnetic
structures at 12 K arises from the entropy accumulated in the disordered
chains. This interpretation is in an agreement with the experimental data.
General rules for for the phase diagram of frustrated Ising chain compounds are
formulated.Comment: 4 pages, 2 figure
Anomalous dynamics in two- and three- dimensional Heisenberg-Mattis spin glasses
We investigate the spectral and localization properties of unmagnetized
Heisenberg-Mattis spin glasses, in space dimensionalities and 3, at T=0.
We use numerical transfer-matrix methods combined with finite-size scaling to
calculate Lyapunov exponents, and eigenvalue-counting theorems, coupled with
Gaussian elimination algorithms, to evaluate densities of states. In we
find that all states are localized, with the localization length diverging as
, as energy . Logarithmic corrections to density of
states behave in accordance with theoretical predictions. In the
density-of-states dependence on energy is the same as for spin waves in pure
antiferromagnets, again in agreement with theoretical predictions, though the
corresponding amplitudes differ.Comment: RevTeX4, 9 pages, 9 .eps figure
From Effective Lagrangians, to Chiral Bags, to Skyrmions with the Large-N_c Renormalization Group
We explicitly relate effective meson-baryon Lagrangian models, chiral bags,
and Skyrmions in the following way. First, effective Lagrangians are
constructed in a manner consistent with an underlying large-N_c QCD. An
infinite set of graphs dress the bare Yukawa couplings at *leading* order in
1/N_c, and are summed using semiclassical techniques. What emerges is a picture
of the large-N_c baryon reminiscent of the chiral bag: hedgehog pions for r >
1/\Lambda patched onto bare nucleon degrees of freedom for r < 1/\Lambda, where
the ``bag radius'' 1/\Lambda is the UV cutoff on the graphs. Next, a novel
renormalization group (RG) is derived, in which the bare Yukawa couplings,
baryon masses and hyperfine baryon mass splittings run with \Lambda. Finally,
this RG flow is shown to act as a *filter* on the renormalized Lagrangian
parameters: when they are fine-tuned to obey Skyrme-model relations the
continuum limit \Lambda --> \infty exists and is, in fact, a Skyrme model;
otherwise there is no continuum limit.Comment: Figures included (separate file). This ``replaced'' version corrects
the discussion of backwards-in-time baryon
Spin-spin correlators in Majorana representation
In the Majorana representation of a spin 1/2 we find an identity which
relates spin-spin correlators to one-particle fermionic correlators. This
should be contrasted with the straightforward approach in which two-particle
(four-fermion) correlators need to be calculated. We discuss applications to
the analysis of the dynamics of a spin coupled to a dissipative environment and
of a quantum detector performing a continuous measurement of a qubit's state
Baryons with Many Colors and Flavors
Using recently-developed diagrammatic techniques, I derive some general
results concerning baryons in the expansion, where is the number of
QCD colors. I show that the spin-flavor relations which hold for baryons in the
large- limit, as well as the form of the corrections to these relations at
higher orders in , hold even if , where is the
number of light quark flavors. I also show that the amplitude for a baryon to
emit mesons is , and that meson loops attached to
baryon lines are unsupressed in the large- limit, independent of . For
, there are ambiguities in the extrapolation away from because
the baryon flavor multiplets for a given spin grow with . I argue that the
expansion is valid for baryons with spin and {\it arbitrary}
flavor quantum numbers, including e.g. baryons with isospin and/or strangeness
. This allows the formulation of a large- expansion in which it is not
necessary to identify the physical baryons with particular large- states.
symmetry can be made manifest to all orders in , yet group
theory factors must be evaluated explicitly only for . To
illustrate this expansion, I consider the non-singlet axial currents, baryon
mass splittings, and matrix elements of \mybar ss and \mybar s \gam_\mu
\gam_5 s in the nucleon.Comment: 19 pages, plain TeX, 4 uuencoded postscrip figures, LBL-35539,
NSF-ITP-94-4
Quantum Hall Ferrimagnetism in lateral quantum dot molecules
We demonstrate the existance of ferrimagnetic and ferromagnetic phases in a
spin phase diagram of coupled lateral quantum dot molecules in the quantum Hall
regime. The spin phase diagram is determined from Hartree-Fock Configuration
Interaction method as a function of electron numbers N, magnetic field B,
Zeeman energy, and tunneling barrier height. The quantum Hall ferrimagnetic
phase corresponds to spatially imbalanced spin droplets resulting from strong
inter-dot coupling of identical dots. The quantum Hall ferromagnetic phases
correspond to ferromagnetic coupling of spin polarization at filling factors
between and .Comment: 4 pages, 4 figure
Pion-Nucleon Scattering in a Large-N Sigma Model
We review the large-N_c approach to meson-baryon scattering, including recent
interesting developments. We then study pion-nucleon scattering in a particular
variant of the linear sigma-model, in which the couplings of the sigma and pi
mesons to the nucleon are echoed by couplings to the entire tower of I=J
baryons (including the Delta) as dictated by large-N_c group theory. We sum the
complete set of multi-loop meson-exchange
\pi N --> \pi N and \pi N --> \sigma N Feynman diagrams, to leading order in
1/N_c. The key idea, reviewed in detail, is that large-N_c allows the
approximation of LOOP graphs by TREE graphs, so long as the loops contain at
least one baryon leg; trees, in turn, can be summed by solving classical
equations of motion. We exhibit the resulting partial-wave S-matrix and the
rich nucleon and Delta resonance spectrum of this simple model, comparing not
only to experiment but also to pion-nucleon scattering in the Skyrme model. The
moral is that much of the detailed structure of the meson-baryon S-matrix which
hitherto has been uncovered only with skyrmion methods, can also be described
by models with explicit baryon fields, thanks to the 1/N_c expansion.Comment: This LaTeX file inputs the ReVTeX macropackage; figures accompany i
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