65,736 research outputs found
Lattice study of two-dimensional N=(2,2) super Yang-Mills at large-N
We study two-dimensional N=(2,2) SU(N) super Yang-Mills theory on Euclidean
two-torus using Sugino's lattice regularization. We perform the Monte-Carlo
simulation for N=2,3,4,5 and then extrapolate the result to N = infinity. With
the periodic boundary conditions for the fermions along both circles, we
establish the existence of a bound state in which scalar fields clump around
the origin, in spite of the existence of a classical flat direction. In this
phase the global (Z_N)^2 symmetry turns out to be broken. We provide a simple
explanation for this fact and discuss its physical implications.Comment: 24 pages, 13 figure
Electron correlations in MnGaAs as seen by resonant electron spectroscopy and dynamical mean field theory
After two decades from the discovery of ferromagnetism in Mn-doped GaAs, its
origin is still debated, and many doubts are related to the electronic
structure. Here we report an experimental and theoretical study of the valence
electron spectrum of Mn-doped GaAs. The experimental data are obtained through
the differences between off- and on-resonance photo-emission data. The
theoretical spectrum is calculated by means of a combination of
density-functional theory in the local density approximation and dynamical
mean-field theory (LDA+DMFT), using exact diagonalisation as impurity solver.
Theory is found to accurately reproduce measured data, and illustrates the
importance of correlation effects. Our results demonstrate that the Mn states
extend over a broad range of energy, including the top of the valence band, and
that no impurity band splits off from the valence band edge, while the induced
holes seem located primarily around the Mn impurity.Comment: 5 pages, 4 figure
Analytic continuation by averaging Pad\'e approximants
The ill-posed analytic continuation problem for Green's functions and
self-energies is investigated by revisiting the Pad\'{e} approximants
technique. We propose to remedy the well-known problems of the Pad\'{e}
approximants by performing an average of several continuations, obtained by
varying the number of fitted input points and Pad\'{e} coefficients
independently. The suggested approach is then applied to several test cases,
including Sm and Pr atomic self-energies, the Green's functions of the Hubbard
model for a Bethe lattice and of the Haldane model for a nano-ribbon, as well
as two special test functions. The sensitivity to numerical noise and the
dependence on the precision of the numerical libraries are analysed in detail.
The present approach is compared to a number of other techniques, i.e. the
non-negative least-square method, the non-negative Tikhonov method and the
maximum entropy method, and is shown to perform well for the chosen test cases.
This conclusion holds even when the noise on the input data is increased to
reach values typical for quantum Monte Carlo simulations. The ability of the
algorithm to resolve fine structures is finally illustrated for two relevant
test functions.Comment: 10 figure
Entanglement negativity in quantum field theory
We develop a systematic method to extract the negativity in the ground state
of a 1+1 dimensional relativistic quantum field theory, using a path integral
formalism to construct the partial transpose rho_A^{T_2} of the reduced density
matrix of a subsystem A=A1 U A2, and introducing a replica approach to obtain
its trace norm which gives the logarithmic negativity E=ln||\rho_A^{T_2}||.
This is shown to reproduce standard results for a pure state. We then apply
this method to conformal field theories, deriving the result E\sim(c/4) ln(L1
L2/(L1+L2)) for the case of two adjacent intervals of lengths L1, L2 in an
infinite system, where c is the central charge. For two disjoint intervals it
depends only on the harmonic ratio of the four end points and so is manifestly
scale invariant. We check our findings against exact numerical results in the
harmonic chain.Comment: 4 pages, 5 figure
Logarithmic corrections to finite size spectrum of SU(N) symmetric quantum chains
We consider SU(N) symmetric one dimensional quantum chains at finite
temperature. For such systems the correlation lengths, ground state energy, and
excited state energies are investigated in the framework of conformal field
theory. The possibility of different types of excited states are discussed.
Logarithmic corrections to the ground state energy and different types of
excited states in the presence of a marginal opeartor, are calculated. Known
results for SU(2) and SU(4) symmetric systems follow from our general formula.Comment: 5 pages, 1 figure; Typos corrected and minor changes made for clarit
Fast nucleon emission as a probe of the isospin momentum dependence
In this article we investigate the structure of the non-local part of the
symmetry term, that leads to a splitting of the effective masses of protons and
neutrons in asymmetric matter. Based on microscopic transport simulations we
suggest some rather sensitive observables in collisions of neutron-rich
(unstable) ions at intermediate () energies. In particular we focus the
attention on pre-equilibrium nucleon emissions. We discuss interesting
correlations between the N/Z content of the fast emitted particles and their
rapidity or transverse momentum, that show a nice dependence on the
prescription used for the effective mass splitting.Comment: 5 pages, 6 figures, revtex
New twist field couplings from the partition function for multiply wrapped D-branes
We consider toroidal compactifications of bosonic string theory with
particular regard to the phases (cocycles) necessary for a consistent
definition of the vertex operators, the boundary states and the T-duality
rules. We use these ingredients to compute the planar multi-loop partition
function describing the interaction among magnetized or intersecting D-branes,
also in presence of open string moduli. It turns out that unitarity in the open
string channel crucially depends on the presence of the cocycles. We then focus
on the 2-loop case and study the degeneration limit where this partition
function is directly related to the tree-level 3-point correlators between
twist fields. These correlators represent the main ingredient in the
computation of Yukawa couplings and other terms in the effective action for
D-brane phenomenological models. By factorizing the 2-loop partition function
we are able to compute the 3-point couplings for abelian twist fields on
generic non-factorized tori, thus generalizing previous expressions valid for
the 2-torus.Comment: 36 pages, 1 figure; v2: typos corrected, proof in the Appendix
improve
Entanglement entropy and quantum field theory: a non-technical introduction
In these proceedings we give a pedagogical and non-technical introduction to
the Quantum Field Theory approach to entanglement entropy. Particular attention
is devoted to the one space dimensional case, with a linear dispersion
relation, that, at a quantum critical point, can be effectively described by a
two-dimensional Conformal Field Theory.Comment: 10 Pages, 2 figures. Talk given at the conference "Entanglement in
Physical and information sciences", Centro Ennio de Giorgi, Pisa, December
200
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