14,050 research outputs found
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
Duality and the Legendre Transform
We define a weak-strong coupling transformation based on the Legendre
transformation of the effective action. In the case of N\es 2 supersymmetric
Yang-Mills theory, this coincides with the duality transform on the low energy
effective action considered by Seiberg and Witten. This Legendre transform
interpretation of duality generalizes directly to the full effective action,
and in principle to other theories.Comment: 6 pages, LaTe
Scaling of excitations in dimerized and frustrated spin-1/2 chains
We study the finite-size behavior of the low-lying excitations of spin-1/2
Heisenberg chains with dimerization and next-to-nearest neighbors interaction,
J_2. The numerical analysis, performed using density-matrix renormalization
group, confirms previous exact diagonalization results, and shows that, for
different values of the dimerization parameter \delta, the elementary triplet
and singlet excitations present a clear scaling behavior in a wide range of
\ell=L/\xi (where L is the length of the chain and \xi is the correlation
length). At J_2=J_2c, where no logarithmic corrections are present, we compare
the numerical results with finite-size predictions for the sine-Gordon model
obtained using Luscher's theory. For small \delta we find a very good agreement
for \ell > 4 or 7 depending on the excitation considered.Comment: 4 pages, 4 eps figures, RevTeX 4 class, same version as in PR
Entanglement Measure for Composite Systems
A general description of entanglement is suggested as an action realized by
an arbitrary operator over given disentangled states. The related entanglement
measure is defined. Because of its generality, this definition can be employed
for any physical systems, pure or mixed, equilibrium or nonequilibrium, and
characterized by any type of operators, whether these are statistical
operators, field operators, spin operators, or anything else. Entanglement of
any number of parts from their total ensemble forming a multiparticle composite
system can be determined. Interplay between entanglement and ordering,
occurring under phase transitions, is analysed by invoking the concept of
operator order indices.Comment: 6 pages, Revte
Vacuum Decay in Theories with Symmetry Breaking by Radiative Corrections
The standard bounce formalism for calculating the decay rate of a metastable
vacuum cannot be applied to theories in which the symmetry breaking is due to
radiative corrections, because in such theories the tree-level action has no
bounce solutions. In this paper I derive a modified formalism to deal with such
cases. As in the usual case, the bubble nucleation rate may be written in the
form . To leading approximation, is the bounce action obtained by
replacing the tree-level potential by the leading one-loop approximation to the
effective potential, in agreement with the generally adopted {\it ad hoc}
remedy. The next correction to (which is proportional to an inverse power
of a small coupling) is given in terms of the next-to-leading term in the
effective potential and the leading correction to the two-derivative term in
the effective action. The corrections beyond these (which may be included in
the prefactor) do not have simple expressions in terms of the effective
potential and the other functions in the effective action. In particular, the
scalar-loop terms which give an imaginary part to the effective potential do
not explicitly appear; the corresponding effects are included in a functional
determinant which gives a manifestly real result for the nucleation rate.Comment: 39 pages, CU-TP-57
Conductance of a spin-1 quantum dot: the two-stage Kondo effect
We discuss the physics of a of a spin-1 quantum dot, coupled to two metallic
leads and develop a simple model for the temperature dependence of its
conductance. Such quantum dots are described by a two-channel Kondo model with
asymmetric coupling constants and the spin screening of the dot by the leads is
expected to proceed via a two-stage process. When the Kondo temperatures of
each channel are widely separated, on cooling, the dot passes through a broad
cross-over regime dominated by underscreened Kondo physics. A singular, or
non-fermi liquid correction to the conductance develops in this regime. At the
lowest temperatures, destructive interference between resonant scattering in
both channels leads to the eventual suppression of the conductance of the dot.
We develop a model to describe the growth, and ultimate suppression of the
conductance in the two channel Kondo model as it is screened successively by
its two channels. Our model is based upon large-N approximation in which the
localized spin degrees of freedom are described using the Schwinger boson
formalism.Comment: 16 pages, 10 figure
Dynamical confinement in bosonized QCD2
In the bosonized version of two dimensional theories non trivial boundary
conditions (topology) play a crucial role. They are inevitable if one wants to
describe non singlet states. In abelian bosonization, color is the charge of a
topological current in terms of a non-linear meson field. We show that
confinement appears as the dynamical collapse of the topology associated with
its non trivial boundary conditions.Comment: 11 pages, figures not included, ftuv/92-
THE DYSON-SCHWINGER EQUATION FOR A MODEL WITH INSTANTONS - THE SCHWINGER MODEL
Using the exact path integral solution of the Schwinger model -- a model
where instantons are present -- the Dyson-Schwinger equation is shown to hold
by explicit computation. It turns out that the Dyson-Schwinger equation
separately holds for every instanton sector. This is due to Theta-invariance of
the Schwinger model.Comment: LATEX file 11 pages, no figure
Black hole formation from colliding bubbles
Some indication of conditions that are necessary for the formation of black
holes from the collision of bubbles during a supercooled phase transition in
the the early universe are explored. Two colliding bubbles can never form a
black hole. Three colliding bubbles can refocus the energy in their walls to
the extent that it becomes infinite.Comment: 12 pages, NCL93-TP13 (RevTeX
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Controls on development and diversity of Early Archean stromatolites
The ≈3,450-million-year-old Strelley Pool Formation in Western Australia contains a reef-like assembly of laminated sedimentary accretion structures (stromatolites) that have macroscale characteristics suggestive of biological influence. However, direct microscale evidence of biology—namely, organic microbial remains or biosedimentary fabrics—has to date eluded discovery in the extensively-recrystallized rocks. Recently-identified outcrops with relatively good textural preservation record microscale evidence of primary sedimentary processes, including some that indicate probable microbial mat formation. Furthermore, we find relict fabrics and organic layers that covary with stromatolite morphology, linking morphologic diversity to changes in sedimentation, seafloor mineral precipitation, and inferred microbial mat development. Thus, the most direct and compelling signatures of life in the Strelley Pool Formation are those observed at the microscopic scale. By examining spatiotemporal changes in microscale characteristics it is possible not only to recognize the presence of probable microbial mats during stromatolite development, but also to infer aspects of the biological inputs to stromatolite morphogenesis. The persistence of an inferred biological signal through changing environmental circumstances and stromatolite types indicates that benthic microbial populations adapted to shifting environmental conditions in early oceans
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