5,905 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
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
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
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
Chemical dynamics from the gas-phase to surfaces
The field of gas-phase chemical dynamics has developed superb experimental methods to probe the detailed outcome of gas-phase chemical reactions. These experiments inspired and benchmarked first principles dynamics simulations giving access to an atomic scale picture of the motions that underlie these reactions. This fruitful interplay of experiment and theory is the essence of a dynamical approach perfected on gas-phase reactions, the culmination of which is a standard model of chemical reactivity involving classical trajectories or quantum wave packets moving on a Born–Oppenheimer potential energy surface. Extending the dynamical approach to chemical reactions at surfaces presents challenges of complexity not found in gas-phase study as reactive processes often involve multiple steps, such as inelastic molecule-surface scattering and dissipation, leading to adsorption and subsequent thermal desorption and or bond breaking and making. This paper reviews progress toward understanding the elementary processes involved in surface chemistry using the dynamical approach
Observability of Quantum Criticality and a Continuous Supersolid in Atomic Gases
We analyze the Bose-Hubbard model with a three-body hardcore constraint by
mapping the system to a theory of two coupled bosonic degrees of freedom. We
find striking features that could be observable in experiments, including a
quantum Ising critical point on the transition from atomic to dimer
superfluidity at unit filling, and a continuous supersolid phase for strongly
bound dimers.Comment: 4 pages, 2 figures, published version (Editor's suggestion
Spin Waves in Striped Phases
In many antiferromagnetic, quasi-two-dimensional materials, doping with holes
leads to "stripe" phases, in which the holes congregate along antiphase domain
walls in the otherwise antiferromagnetic texture. Using a suitably parametrized
two-dimensional Heisenberg model on a square lattice, we study the spin wave
spectra of well-ordered spin stripes, comparing bond-centered antiphase domain
walls to site-centered antiphase domain walls for a range of spacings between
the stripes and for stripes both aligned with the lattice ("vertical") and
oriented along the diagonals of the lattice ("diagonal"). Our results establish
that there are qualitative differences between the expected neutron scattering
responses for the bond-centered and site-centered cases. In particular,
bond-centered stripes of odd spacing generically exhibit more elastic peaks
than their site-centered counterparts. For inelastic scattering, we find that
bond-centered stripes produce more spin wave bands than site-centered stripes
of the same spacing and that bond-centered stripes produce rather isotropic low
energy spin wave cones for a large range of parameters, despite local
microscopic anisotropy. We find that extra scattering intensity due to the
crossing of spin wave modes (which may be linked to the "resonance peak" in the
cuprates) is more likely for diagonal stripes, whether site- or bond-centered,
whereas spin wave bands generically repel, rather than cross, when stripes are
vertical.Comment: 12 pages, 12 figures, for some high-res.pics, see
http://physics.bu.edu/~yaodx/spinwave/spinw.htm
Influence of lattice distortions in classical spin systems
We investigate a simple model of a frustrated classical spin chain coupled to
adiabatic phonons under an external magnetic field. A thorough study of the
magnetization properties is carried out both numerically and analytically. We
show that already a moderate coupling with the lattice can stabilize a plateau
at 1/3 of the saturation and discuss the deformation of the underlying lattice
in this phase. We also study the transition to saturation where either a first
or second order transition can occur, depending on the couplings strength.Comment: Submitted to Phys. Rev.
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