6,384 research outputs found
Functional approach for pairing in finite systems: How to define restoration of broken symmetries in Energy Density Functional theory ?
The Multi-Reference Energy Density Functional (MR-EDF) approach (also called
configuration mixing or Generator Coordinate Method), that is commonly used to
treat pairing in finite nuclei and project onto particle number, is
re-analyzed. It is shown that, under certain conditions, the MR-EDF energy can
be interpreted as a functional of the one-body density matrix of the projected
state with good particle number. Based on this observation, we propose a new
approach, called Symmetry-Conserving EDF (SC-EDF), where the breaking and
restoration of symmetry are accounted for simultaneously. We show, that such an
approach is free from pathologies recently observed in MR-EDF and can be used
with a large flexibility on the density dependence of the functional.Comment: proceeding of the conference "Many body correlations from dilute to
dense Nuclear systems", Paris, February 201
A large- PNJL model with explicit Z symmetry
A PNJL model is built, in which the Polyakov-loop potential is explicitly
Z-symmetric in order to mimic a Yang-Mills theory with gauge group
SU(). The physically expected large- and large- behaviours of the
thermodynamic observables computed from the Polyakov-loop potential are used to
constrain its free parameters. The effective potential is eventually
U(1)-symmetric when is infinite. Light quark flavours are added by using
a Nambu-Jona-Lasinio (NJL) model coupled to the Polyakov loop (the PNJL model),
and the different phases of the resulting PNJL model are discussed in 't
Hooft's large- limit. Three phases are found, in agreement with previous
large- studies. When the temperature is larger than some deconfinement
temperature , the system is in a deconfined, chirally symmetric, phase for
any quark chemical potential . When however, the system is in a
confined phase in which chiral symmetry is either broken or not. The critical
line , signalling the restoration of chiral symmetry, has the same
qualitative features than what can be obtained within a standard PNJL
model.Comment: To appear in Phys Rev
Description of Pairing correlation in Many-Body finite systems with density functional theory
Different steps leading to the new functional for pairing based on natural
orbitals and occupancies proposed in ref. [D. Lacroix and G. Hupin,
arXiv:1003.2860] are carefully analyzed. Properties of quasi-particle states
projected onto good particle number are first reviewed. These properties are
used (i) to prove the existence of such a functional (ii) to provide an
explicit functional through a 1/N expansion starting from the BCS approach
(iii) to give a compact form of the functional summing up all orders in the
expansion. The functional is benchmarked in the case of the picked fence
pairing Hamiltonian where even and odd systems, using blocking technique are
studied, at various particle number and coupling strength, with uniform and
random single-particle level spacing. In all cases, a very good agreement is
found with a deviation inferior to 1% compared to the exact energy.Comment: 14 pages, 9 figure
Effect of the Kondo correlation on thermopower in a Quantum Dot
In this paper we study the thermopower of a quantum dot connected to two
leads in the presence of Kondo correlation by employing a modified second-order
perturbation scheme at nonequilibrium. A simple scheme, Ng's ansatz [Phys. Rev.
Lett. {\bf 76}, 487 (1996)], is adopted to calculate nonequilibrium
distribution Green's function and its validity is further checked with regard
to the Onsager relation. Numerical results demonstrate that the sign of the
thermopower can be changed by tuning the energy level of the quantum dot,
leading to a oscillatory behavior with a suppressed magnitude due to the Kondo
effect. We also calculate the thermal conductance of the system, and find that
the Wiedemann-Franz law is obeyed at low temperature but violated with
increasing temperature, corresponding to emerging and quenching of the Kondo
effect.Comment: 6 pages, 4 figures; accepted for publication in J Phys.: Condensed
Matte
Surface induced magnetization reversal of MnP nanoclusters embedded in GaP
We investigate the quasi-static magnetic behavior of ensembles of
non-interacting ferromagnetic nanoparticles consisting of MnP nanoclusters
embedded in GaP(001) epilayers grown at 600, 650 and 700{\deg}C. We use a
phenomenological model, in which surface effects are included, to reproduce the
experimental hysteresis curves measured as a function of temperature (120-260
K) and direction of the applied field. The slope of the hysteresis curve during
magnetization reversal is determined by the MnP nanoclusters size distribution,
which is a function of the growth temperature. Our results show that the
coercive field is very sensitive to the strength of the surface anisotropy,
which reduces the energy barrier between the two states of opposite
magnetization. Notably, this reduction in the energy barrier increases by a
factor of 3 as the sample temperature is lowered from 260 to 120 K.Comment: 7 pages, 5 figure
Out of equilibrium transport through an Anderson impurity: Probing scaling laws within the equation of motion approach
We study non-equilibrium electron transport through a quantum impurity
coupled to metallic leads using the equation of motion technique at finite
temperature T. Assuming that the interactions are taking place solely in the
impurity and focusing in the infinite Hubbard limit, we compute the out of
equilibrium density of states and the differential conductance G_2(T,V) to test
several scaling laws. We find that G_2(T,V)/G_2(T,0) is a universal function of
both eV/T_K and T/T_K, being T_K the Kondo temperature. The effect of an in
plane magnetic field on the splitting of the zero bias anomaly in the
differential conductance is also analyzed. For a Zeeman splitting \Delta, the
computed differential conductance peak splitting depends only on \Delta/T_K,
and for large fields approaches the value of 2\Delta . Besides the traditional
two leads setup, we also consider other configurations that mimics recent
experiments, namely, an impurity embedded in a mesoscopic wire and the presence
of a third weakly coupled lead. In these cases, a double peak structure of the
Kondo resonance is clearly obtained in the differential conductance while the
amplitude of the highest peak is shown to decrease as \ln(eV/T_K). Several
features of these results are in qualitative agreement with recent experimental
observations reported on quantum dots.Comment: 9 pages, 7 figure
Dimensional Evolution of Spin Correlations in the Magnetic Pyrochlore Yb2Ti2O7
The pyrochlore material Yb2Ti2O7 displays unexpected quasi-two-dimensional
(2D) magnetic correlations within a cubic lattice environment at low
temperatures, before entering an exotic disordered ground state below T=265mK.
We report neutron scattering measurements of the thermal evolution of the 2D
spin correlations in space and time. Short range three dimensional (3D) spin
correlations develop below 400 mK, accompanied by a suppression in the
quasi-elastic (QE) scattering below ~ 0.2 meV. These show a slowly fluctuating
ground state with spins correlated over short distances within a
kagome-triangular-kagome (KTK) stack along [111], which evolves to isolated
kagome spin-stars at higher temperatures. Furthermore, low-temperature specific
heat results indicate a sample dependence to the putative transition
temperature that is bounded by 265mK, which we discuss in the context of recent
mean field theoretical analysis.Comment: 5 pages, 6 figure
Kondo effect of an adatom in graphene and its scanning tunneling spectroscopy
We study the Kondo effect of a single magnetic adatom on the surface of
graphene. It was shown that the unique linear dispersion relation near the
Dirac points in graphene makes it more easy to form the local magnetic moment,
which simply means that the Kondo resonance can be observed in a more wider
parameter region than in the metallic host. The result indicates that the Kondo
resonance indeed can form ranged from the Kondo regime, to the mixed valence,
even to the empty orbital regime. While the Kondo resonance displays as a sharp
peak in the first regime, it has a peak-dip structure and/or an anti-resonance
in the remaining two regimes, which result from the Fano resonance due to the
significant background leaded by dramatically broadening of the impurity level
in graphene. We also study the scanning tunneling microscopy (STM) spectra of
the adatom and they show obvious particle-hole asymmetry when the chemical
potential is tuned by the gate voltages applied to the graphene. Finally, we
explore the influence of the direct tunneling channel between the STM tip and
the graphene on the Kondo resonance and find that the lineshape of the Kondo
resonance is unaffected, which can be attributed to unusual large asymmetry
factor in graphene. Our study indicates that the graphene is an ideal platform
to study systematically the Kondo physics and these results are useful to
further stimulate the relevant experimental studies on the system.Comment: 8 pages, 5 figure
Fermi Surface of The One-dimensional Kondo Lattice Model
We show a strong indication of the existence of a large Fermi surface in the
one-dimensional Kondo lattice model. The characteristic wave vector of the
model is found to be , being the density of the
conduction electrons. This result is at first obtained for a variant of the
model that includes an antiferromagnetic Heisenberg interaction between
the local moments. It is then directly observed in the conventional Kondo
lattice , in the narrow range of Kondo couplings where the long
distance properties of the model are numerically accessible.Comment: 11 pages, 6 figure
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