8,477 research outputs found
Ground States in the Spin Boson Model
We prove that the Hamiltonian of the model describing a spin which is
linearly coupled to a field of relativistic and massless bosons, also known as
the spin-boson model, admits a ground state for small values of the coupling
constant lambda. We show that the ground state energy is an analytic function
of lambda and that the corresponding ground state can also be chosen to be an
analytic function of lambda. No infrared regularization is imposed. Our proof
is based on a modified version of the BFS operator theoretic renormalization
analysis. Moreover, using a positivity argument we prove that the ground state
of the spin-boson model is unique. We show that the expansion coefficients of
the ground state and the ground state energy can be calculated using regular
analytic perturbation theory
Field evolution of the magnetic structures in ErTiO through the critical point
We have measured neutron diffraction patterns in a single crystal sample of
the pyrochlore compound ErTiO in the antiferromagnetic phase
(T=0.3\,K), as a function of the magnetic field, up to 6\,T, applied along the
[110] direction. We determine all the characteristics of the magnetic structure
throughout the quantum critical point at =2\,T. As a main result, all Er
moments align along the field at and their values reach a minimum. Using
a four-sublattice self-consistent calculation, we show that the evolution of
the magnetic structure and the value of the critical field are rather well
reproduced using the same anisotropic exchange tensor as that accounting for
the local paramagnetic susceptibility. In contrast, an isotropic exchange
tensor does not match the moment variations through the critical point. The
model also accounts semi-quantitatively for other experimental data previously
measured, such as the field dependence of the heat capacity, energy of the
dispersionless inelastic modes and transition temperature.Comment: 7 pages; 8 figure
Importance of Different Regions of H-2 for MLC Stimulation 1
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/65829/1/j.1399-0039.1973.tb01008.x.pd
Ground State and Resonances in the Standard Model of Non-relativistic QED
We prove existence of a ground state and resonances in the standard model of
the non-relativistic quantum electro-dynamics (QED). To this end we introduce a
new canonical transformation of QED Hamiltonians and use the spectral
renormalization group technique with a new choice of Banach spaces.Comment: 50 pages change
Phase diagrams of the 2D t-t'-U Hubbard model from an extended mean field method
It is well-known from unrestricted Hartree-Fock computations that the 2D
Hubbard model does not have homogeneous mean field states in significant
regions of parameter space away from half filling. This is incompatible with
standard mean field theory. We present a simple extension of the mean field
method that avoids this problem. As in standard mean field theory, we restrict
Hartree-Fock theory to simple translation invariant states describing
antiferromagnetism (AF), ferromagnetism (F) and paramagnetism (P), but we use
an improved method to implement the doping constraint allowing us to detect
when a phase separated state is energetically preferred, e.g. AF and F
coexisting at the same time. We find that such mixed phases occur in
significant parts of the phase diagrams, making them much richer than the ones
from standard mean field theory. Our results for the 2D t-t'-U Hubbard model
demonstrate the importance of band structure effects.Comment: 6 pages, 5 figure
Mean-field evolution of fermions with singular interaction
We consider a system of N fermions in the mean-field regime interacting
though an inverse power law potential , for
. We prove the convergence of a solution of the many-body
Schr\"{o}dinger equation to a solution of the time-dependent Hartree-Fock
equation in the sense of reduced density matrices. We stress the dependence on
the singularity of the potential in the regularity of the initial data. The
proof is an adaptation of [22], where the case is treated.Comment: 16 page
Approach to ground state and time-independent photon bound for massless spin-boson models
It is widely believed that an atom interacting with the electromagnetic field
(with total initial energy well-below the ionization threshold) relaxes to its
ground state while its excess energy is emitted as radiation. Hence, for large
times, the state of the atom+field system should consist of the atom in its
ground state, and a few free photons that travel off to spatial infinity.
Mathematically, this picture is captured by the notion of asymptotic
completeness. Despite some recent progress on the spectral theory of such
systems, a proof of relaxation to the ground state and asymptotic completeness
was/is still missing, except in some special cases (massive photons, small
perturbations of harmonic potentials). In this paper, we partially fill this
gap by proving relaxation to an invariant state in the case where the atom is
modelled by a finite-level system. If the coupling to the field is sufficiently
infrared-regular so that the coupled system admits a ground state, then this
invariant state necessarily corresponds to the ground state. Assuming slightly
more infrared regularity, we show that the number of emitted photons remains
bounded in time. We hope that these results bring a proof of asymptotic
completeness within reach.Comment: 45 pages, published in Annales Henri Poincare. This archived version
differs from the journal version because we corrected an inconsequential
mistake in Section 3.5.1: to do this, a new paragraph was added after Lemma
3.
The tetralogy of Birkhoff theorems
We classify the existent Birkhoff-type theorems into four classes: First, in
field theory, the theorem states the absence of helicity 0- and spin 0-parts of
the gravitational field. Second, in relativistic astrophysics, it is the
statement that the gravitational far-field of a spherically symmetric star
carries, apart from its mass, no information about the star; therefore, a
radially oscillating star has a static gravitational far-field. Third, in
mathematical physics, Birkhoff's theorem reads: up to singular exceptions of
measure zero, the spherically symmetric solutions of Einstein's vacuum field
equation with Lambda = 0 can be expressed by the Schwarzschild metric; for
Lambda unequal 0, it is the Schwarzschild-de Sitter metric instead. Fourth, in
differential geometry, any statement of the type: every member of a family of
pseudo-Riemannian space-times has more isometries than expected from the
original metric ansatz, carries the name Birkhoff-type theorem. Within the
fourth of these classes we present some new results with further values of
dimension and signature of the related spaces; including them are some
counterexamples: families of space-times where no Birkhoff-type theorem is
valid. These counterexamples further confirm the conjecture, that the
Birkhoff-type theorems have their origin in the property, that the two
eigenvalues of the Ricci tensor of two-dimensional pseudo-Riemannian spaces
always coincide, a property not having an analogy in higher dimensions. Hence,
Birkhoff-type theorems exist only for those physical situations which are
reducible to two dimensions.Comment: 26 pages, updated references, minor text changes, accepted by Gen.
Relat. Gra
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