240 research outputs found

### The BCS gap equation for spin-polarized fermions

We study the BCS gap equation for a Fermi gas with unequal population of
spin-up and spin-down states. For $\cosh(\delta_\mu/T) \leq 2$, with $T$ the
temperature and $\delta_\mu$ the chemical potential difference, the question of
existence of non-trivial solutions can be reduced to spectral properties of a
linear operator, similar to the unpolarized case studied previously in
\cite{FHNS,HHSS,HS}. For $\cosh(\delta_\mu/T) > 2$ the phase diagram is more
complicated, however. We derive upper and lower bounds for the critical
temperature, and study their behavior in the small coupling limit.Comment: 23 pages, 1 figur

### Semiclassics in the lowest Landau band

This paper deals with the comparison between the strong Thomas-Fermi theory
and the quantum mechanical ground state energy of a large atom confined to
lowest Landau band wave functions. Using the tools of microlocal semiclassical
spectral asymptotics we derive precise error estimates. The approach presented
in this paper suggests the definition of a modified strong Thomas-Fermi
functional, where the main modification consists in replacing the integration
over the variables perpendicular to the magnetic field by an expansion in
angular momentum eigenfunctions. The resulting DSTF theory is studied in detail
in the second part of the paper.Comment: Latex2e, 31 page

### The BCS Critical Temperature in a Weak External Electric Field via a Linear Two-Body Operator

We study the critical temperature of a superconductive material in a weak external electric potential via a linear approximation of the BCS functional. We reproduce a similar result as in Frank et al. (Commun Math Phys 342(1):189–216, 2016, [5]) using the strategy introduced in Frank et al. (The BCS critical temperature in a weak homogeneous magnetic field, [2]), where we considered the case of an external constant magnetic field

### The BCS critical temperature in a weak external electric field via a linear two-body operator

We study the critical temperature of a superconductive material in a weak
external electric potential via a linear approximation of the BCS functional.
We reproduce a similar result as in [Frank, Hainzl, Seiringer, Solovej, 2016]
using the strategy introduced in [Frank, Hainzl, Langmann, 2018], where we
considered the case of an external constant magnetic field.Comment: Dedicated to Herbert Spohn on the occasion of his seventieth
birthday; 29 page

### Renormalization and asymptotic expansion of Dirac's polarized vacuum

We perform rigorously the charge renormalization of the so-called reduced
Bogoliubov-Dirac-Fock (rBDF) model. This nonlinear theory, based on the Dirac
operator, describes atoms and molecules while taking into account vacuum
polarization effects. We consider the total physical density including both the
external density of a nucleus and the self-consistent polarization of the Dirac
sea, but no `real' electron. We show that it admits an asymptotic expansion to
any order in powers of the physical coupling constant \alphaph, provided that
the ultraviolet cut-off behaves as \Lambda\sim e^{3\pi(1-Z_3)/2\alphaph}\gg1.
The renormalization parameter $

### Low Density Limit of BCS Theory and Bose-Einstein Condensation of Fermion Pairs

We consider the low density limit of a Fermi gas in the BCS approximation. We
show that if the interaction potential allows for a two-particle bound state,
the system at zero temperature is well approximated by the Gross-Pitaevskii
functional, describing a Bose-Einstein condensate of fermion pairs.Comment: LaTeX2e, 17 page

### Gradient corrections for semiclassical theories of atoms in strong magnetic fields

This paper is divided into two parts. In the first one the von Weizs\"acker
term is introduced to the Magnetic TF theory and the resulting MTFW functional
is mathematically analyzed. In particular, it is shown that the von
Weizs\"acker term produces the Scott correction up to magnetic fields of order
$B \ll Z^2$, in accordance with a result of V. Ivrii on the quantum mechanical
ground state energy. The second part is dedicated to gradient corrections for
semiclassical theories of atoms restricted to electrons in the lowest Landau
band. We consider modifications of the Thomas-Fermi theory for strong magnetic
fields (STF), i.e. for $B \ll Z^3$. The main modification consists in replacing
the integration over the variables perpendicular to the field by an expansion
in angular momentum eigenfunctions in the lowest Landau band. This leads to a
functional (DSTF) depending on a sequence of one-dimensional densities. For a
one-dimensional Fermi gas the analogue of a Weizs\"acker correction has a
negative sign and we discuss the corresponding modification of the DSTF
functional.Comment: Latex2e, 36 page

### Construction of the Pauli-Villars-regulated Dirac vacuum in electromagnetic fields

Using the Pauli-Villars regularization and arguments from convex analysis, we
construct solutions to the classical time-independent Maxwell equations in
Dirac's vacuum, in the presence of small external electromagnetic sources. The
vacuum is not an empty space, but rather a quantum fluctuating medium which
behaves as a nonlinear polarizable material. Its behavior is described by a
Dirac equation involving infinitely many particles. The quantum corrections to
the usual Maxwell equations are nonlinear and nonlocal. Even if photons are
described by a purely classical electromagnetic field, the resulting vacuum
polarization coincides to first order with that of full Quantum
Electrodynamics.Comment: Final version to appear in Arch. Rat. Mech. Analysi

### A new approach to the modelling of local defects in crystals: the reduced Hartree-Fock case

This article is concerned with the derivation and the mathematical study of a
new mean-field model for the description of interacting electrons in crystals
with local defects. We work with a reduced Hartree-Fock model, obtained from
the usual Hartree-Fock model by neglecting the exchange term. First, we recall
the definition of the self-consistent Fermi sea of the perfect crystal, which
is obtained as a minimizer of some periodic problem, as was shown by Catto, Le
Bris and Lions. We also prove some of its properties which were not mentioned
before. Then, we define and study in details a nonlinear model for the
electrons of the crystal in the presence of a defect. We use formal analogies
between the Fermi sea of a perturbed crystal and the Dirac sea in Quantum
Electrodynamics in the presence of an external electrostatic field. The latter
was recently studied by Hainzl, Lewin, S\'er\'e and Solovej, based on ideas
from Chaix and Iracane. This enables us to define the ground state of the
self-consistent Fermi sea in the presence of a defect. We end the paper by
proving that our model is in fact the thermodynamic limit of the so-called
supercell model, widely used in numerical simulations.Comment: Final version, to appear in Comm. Math. Phy

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