5,946 research outputs found
Spin Tunneling, Berry phases and Doped Antiferromagnets
Interference effects between Berry phase factors in spin tunneling systems
have been discussed in recent Letters by Loss, DiVincenzo and Grinstein and von
Delft and Henley. This Comment points out that Berry phases in spin tunneling
are important in another interesting case: the two dimensional doped
antiferromagnet. I show that the dispersion of a single hole in the t-J model
changes sign as where is the size of the spins. This provides
an interpretation of the numerical results for the s=\half model, and a
prediction for other spin sizes.Comment: 5 pages, LaTe
Effects of non-adiabaticity on the voltage generated by a moving domain wall
We determine the voltage generated by a field-driven domain wall, taking into
account non-adiabatic corrections to the motive force induced by the
time-dependent spin Berry phase. Both the diffusive and ballistic transport
regimes are considered. We find that that the non-adiabatic corrections,
together with the contributions due to spin relaxation, determine the voltage
for driving fields smaller than the Walker breakdown limit.Comment: 8 pages, 3 figure
A Path Intergal Approach to Current
Discontinuous initial wave functions or wave functions with discontintuous
derivative and with bounded support arise in a natural way in various
situations in physics, in particular in measurement theory. The propagation of
such initial wave functions is not well described by the Schr\"odinger current
which vanishes on the boundary of the support of the wave function. This
propagation gives rise to a uni-directional current at the boundary of the
support. We use path integrals to define current and uni-directional current
and give a direct derivation of the expression for current from the path
integral formulation for both diffusion and quantum mechanics. Furthermore, we
give an explicit asymptotic expression for the short time propagation of
initial wave function with compact support for both the cases of discontinuous
derivative and discontinuous wave function. We show that in the former case the
probability propagated across the boundary of the support in time is
and the initial uni-directional current is . This recovers the Zeno effect for continuous detection of a particle
in a given domain. For the latter case the probability propagated across the
boundary of the support in time is and the
initial uni-directional current is . This is an anti-Zeno
effect. However, the probability propagated across a point located at a finite
distance from the boundary of the support is . This gives a decay
law.Comment: 17 pages, Late
Spin pumping by a field-driven domain wall
We calculate the charge current in a metallic ferromagnet to first order in
the time derivative of the magnetization direction. Irrespective of the
microscopic details, the result can be expressed in terms of the conductivities
of the majority and minority electrons and the non-adiabatic spin transfer
torque parameter . The general expression is evaluated for the specific
case of a field-driven domain wall and for that case depends strongly on the
ratio of and the Gilbert damping constant. These results may provide an
experimental method to determine this ratio, which plays a crucial role for
current-driven domain-wall motion.Comment: 4 pages, 1 figure v2: some typos corrected v3: published versio
Antiferromagnetic Spinor Condensates are Quantum Rotors
We establish a theoretical correspondence between spin-one antiferromagnetic
spinor condensates in an external magnetic field and quantum rotor models in an
external potential. We show that the rotor model provides a conceptually clear
picture of the possible phases and dynamical regimes of the antiferromagnetic
condensate. We also show that this mapping simplifies calculations of the
condensate's spectrum and wavefunctions. We use the rotor mapping to describe
the different dynamical regimes recently observed in Na condensates. We
also suggest a way to experimentally observe quantum mechanical effects
(collapse and revival) in spinor condensates.Comment: minor revisions. some typos correcte
Vortex Dynamics and Hall Conductivity of Hard Core Bosons
Magneto-transport of hard core bosons (HCB) is studied using an XXZ quantum
spin model representation, appropriately gauged on the torus to allow for an
external magnetic field. We find strong lattice effects near half filling. An
effective quantum mechanical description of the vortex degrees of freedom is
derived. Using semiclassical and numerical analysis we compute the vortex
hopping energy, which at half filling is close to magnitude of the boson
hopping energy. The critical quantum melting density of the vortex lattice is
estimated at 6.5x10-5 vortices per unit cell. The Hall conductance is computed
from the Chern numbers of the low energy eigenstates. At zero temperature, it
reverses sign abruptly at half filling. At precisely half filling, all
eigenstates are doubly degenerate for any odd number of flux quanta. We prove
the exact degeneracies on the torus by constructing an SU(2) algebra of
point-group symmetries, associated with the center of vorticity. This result is
interpreted as if each vortex carries an internal spin-half degree of freedom
('vspin'), which can manifest itself as a charge density modulation in its
core. Our findings suggest interesting experimental implications for vortex
motion of cold atoms in optical lattices, and magnet-transport of short
coherence length superconductors.Comment: 15 pages, 15 figure
Quantum phase transitions in the Fermi-Bose Hubbard model
We propose a multi-band Fermi-Bose Hubbard model with on-site fermion-boson
conversion and general filling factor in three dimensions. Such a Hamiltonian
models an atomic Fermi gas trapped in a lattice potential and subject to a
Feshbach resonance. We solve this model in the two state approximation for
paired fermions at zero temperature. The problem then maps onto a coupled
Heisenberg spin model. In the limit of large positive and negative detuning,
the quantum phase transitions in the Bose Hubbard and Paired-Fermi Hubbard
models are correctly reproduced. Near resonance, the Mott states are given by a
superposition of the paired-fermion and boson fields and the Mott-superfluid
borders go through an avoided crossing in the phase diagram.Comment: 4 pages, 3 figure
Exact Analysis of Entanglement in Gapped Quantum Spin Chains
We investigate the entanglement properties of the valence-bond-solid states
with generic integer-spin . Using the Schwinger boson representation of the
valence-bond-solid states, the entanglement entropy, the von Neumann entropy of
a subsystem, is obtained exactly and its relationship with the usual
correlation function is clarified. The saturation value of the entanglement
entropy, , is derived explicitly and is interpreted in terms of
the edge-state picture. The validity of our analytical results and the
edge-state picture is numerically confirmed. We also propose a novel
application of the edge state as a qubit for quantum computation.Comment: 4 pages, 2 figure
Schwinger-Boson Mean-Field Theory of Mixed-Spin Antiferromagnet
The Schwinger-boson mean-field theory is used to study the three-dimensional
antiferromagnetic ordering and excitations in compounds , a large
family of quasi-one-dimensional mixed-spin antiferromagnet. To investigate
magnetic properties of these compounds, we introduce a three-dimensional
mixed-spin antiferromagnetic Heisenberg model based on experimental results for
the crystal structure of . This model can explain the experimental
discovery of coexistence of Haldane gap and antiferromagnetic long-range order
below N\'{e}el temperature. Properties such as the low-lying excitations,
magnetizations of and rare-earth ions, N\'{e}el temperatures of different
compounds, and the behavior of Haldane gap below the N\'{e}el temperature are
investigated within this model, and the results are in good agreement with
neutron scattering experiments.Comment: 12 pages, 6 figure
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