21,197 research outputs found
Can Frustration Preserve a Quasi-Two-Dimensional Spin Fluid?
Using spin-wave theory, we show that geometric frustration fails to preserve
a two-dimensional spin fluid. Even though frustration can remove the interlayer
coupling in the ground-state of a classical anti-ferromagnet, spin layers
innevitably develop a quantum-mechanical coupling via the mechanism of ``order
from disorder''. We show how the order from disorder coupling mechanism can be
viewed as a result of magnon pair tunneling, a process closely analogous to
pair tunneling in the Josephson effect. In the spin system, the Josephson
coupling manifests itself as a a biquadratic spin coupling between layers, and
for quantum spins, these coupling terms are as large as the inplane coupling.
An alternative mechanism for decoupling spin layers occurs in classical XY
models in which decoupled "sliding phases" of spin fluid can form in certain
finely tuned conditions. Unfortunately, these finely tuned situations appear
equally susceptible to the strong-coupling effects of quantum tunneling,
forcing us to conclude that in general, geometric frustration cannot preserve a
two-dimensional spin fluid.Comment: 12 pages, 3 figure
Light pseudo-Goldstone bosons without explicit symmetry breaking
A mechanism is discussed to obtain light scalar fields from a spontaneously
broken continuous symmetry without explicitly breaking it. If there is a
continuous manifold of classical vacua in orbit space, its tangent directions
describe classically massless fields that may acquire mass from perturbations
of the potential that do not break the symmetry. We consider the simplest
possible example, involving a scalar field in the adjoint representation of
SU(N). We study the scalar mass spectrum and its RG running at one-loop level
including scalar and pseudoscalar Yukawa couplings to a massive Dirac fermion.Comment: minor typographical changes, 12 pages, 4 figure
Diagrammatic perturbation theory and the pseudogap
We study a model of quasiparticles on a two-dimensional square lattice
coupled to Gaussian distributed dynamical fields. The model describes
quasiparticles coupled to spin or charge fluctuations and is solved by a Monte
Carlo sampling of the molecular field distributions. The non-perturbative
solution is compared to various approximations based on diagrammatic
perturbation theory. When the molecular field correlations are sufficiently
weak, the diagrammatic calculations capture the qualitative aspects of the
quasiparticle spectrum. For a range of model parameters near the magnetic
boundary, we find that the quasiparticle spectrum is qualitatively different
from that of a Fermi liquid in that it shows a double peak structure, and that
the diagrammatic approximations we consider fail to reproduce, even
qualitatively, the results of the Monte Carlo calculations. This suggests that
the pseudogap induced by a coupling to antiferromagnetic fluctuations and the
spin-splitting of the quasiparticle peak induced by a coupling to ferromagnetic
spin-fluctuations lie beyond diagrammatic perturbation theory
Magnetic quantum phase transition in an anisotropic Kondo lattice
The quantum phase transition between paramagnetic and antiferromagnetic
phases of the Kondo lattice model with Ising anisotropy in the intersite
exchange is studied within the framework of extended dynamical mean-field
theory. Nonperturbative numerical solutions at zero temperature point to a
continuous transition for both two- and three-dimensional magnetism. In the
former case, the transition is associated with critical local physics,
characterized by a vanishing Kondo scale and by an anomalous exponent in the
dynamics close in value to that measured in heavy-fermion CeCu_{5.9}Au_{0.1}.Comment: 4 pages, 3 figures. Version published in Phys. Rev. Let
A Three-Dimensional Solution of Flows over Wings with Leading-Edge Vortex Separation. Part 2: Program Description Document
For abstract, see N75-32026
Support of the Third Solar Wind conference
The program of invited talks at the Third Solar Wind Conference is provided, with a table of contents of the proceedings
Quantum replica approach to the under-screened Kondo model
We extend the Schwinger boson large N treatment of the underscreened Kondo
model in a way that correctly captures the finite elastic phase shift in the
singular Fermi liquid. The new feature of the approach, is the introduction of
a flavor quantum number with K possible values, associated with the Schwinger
boson representation. The large N limit is taken maintaining the ratio k=K/N
fixed. This approach differs from previous approaches, in that we do not
explicitly enforce a constraint on the spin representation of the Schwinger
bosons. Instead, the energetics of the Kondo model cause the bosonic degrees of
freedom to ``self assemble'' into a ground-state in which the spins of K bosons
and N-K conduction electrons are antisymmetrically arranged into a Kondo
singlet. With this device, the large N limit can be taken, in such a way that a
fraction K/N of the Abrikosov Suhl resonance is immersed inside the Fermi sea.
We show how this method can be used to model the full energy dependence of the
singular Abrikosov Suhl resonance in the underscreened Kondo model and the
field-dependent magnetization.Comment: Revised draft, with plots explicitly showing logarithmic scaling of
inverse coupling constant. Small corrections prior to submission to journa
Apollo particles and fields subsatellite magnetometer experiment
The results of the Apollo 15 subsatellite magnetometer experiment are reported. The magnetometer is described including the operation, and specifications. Orbit plots presented are altitude versus time, selenographic longitude versus latitude, and the ecliptic projection of the earth-moon system. The lunar magnetic field, solar wind interaction with the moon, the transfer function of the moon, and the plasma sheet interaction with the moon are discussed
Spontaneous interlayer coherence in bilayer Kondo systems
Bilayer Kondo systems present interesting models to illustrate the
competition between the Kondo effect and intermoment exchange. Such bilayers
can exhibit two sharply distinct Fermi liquid phases which are distinguished by
whether or not the local moments participate in the Fermi sea. We study these
phases and the evolution from one to the other upon changing Kondo coupling. We
argue that an ordered state with spontaneous interlayer phase coherence
generically intervenes between the two Fermi liquids. Such a condensate phase
breaks a U(1) symmetry and is bounded by a finite-temperature
Kosterlitz-Thouless transition. Based on general arguments and mean-field
calculations we investigate the phase diagram and associated quantum phase
transitions.Comment: 4 pages, 3 figs, (v2) misprints in eqs corrected, final version as
publishe
Schwinger Boson approach to the fully screened Kondo model
We apply the Schwinger boson scheme to the fully screened Kondo model and
generalize the method to include antiferromagnetic interactions between ions.
Our approach captures the Kondo crossover from local moment behavior to a Fermi
liquid with a non-trivial Wilson ratio. When applied to the two impurity model,
the mean-field theory describes the "Varma Jones" quantum phase transition
between a valence bond state and a heavy Fermi liquid.Comment: 4 pages, 4 figures. Changes to references and text in v
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