466 research outputs found
Residual meson-meson interaction from lattice gauge simulation in a simple QED model
The residual interaction for a meson-meson system is computed utilizing the cumulant, or cluster, expansion of the momentum-space time correlation matrix. The cumulant expansion serves to define asymptotic, or free, meson-meson operators. The definition of an effective interaction is then based on a comparison of the full (interacting) and the free (noninteracting) time correlation matrices. The proposed method, which may straightforwardly be transcribed to other hadron-hadron systems, here is applied to a simple 2+1 dimensional U(1) lattice gauge model tuned such that it is confining. Fermions are treated in the staggered scheme. The effective interaction exhibits a repulsive core and attraction at intermediate relative distances. These findings are consistent with an earlier study of the same model utilizing L\"{u}scher's method where scattering phase shifts are obtained directly
Homopolar bond formation in ZnVO close to a metal-insulator transition
Electronic structure calculations for spinel vanadate ZnVO show that
partial electronic delocalization in this system leads to structural
instabilities. These are a consequence of the proximity to the
itinerant-electron boundary, not being related to orbital ordering. We discuss
how this mechanism naturally couples charge and lattice degrees of freedom in
magnetic insulators close to such a crossover. For the case of ZnVO,
this leads to the formation of V-V dimers along the [011] and [101] directions
that readily accounts for the intriguing magnetic structure of ZnVO.Comment: 5 pages, 3 figures, 1 tabl
CdV2O4: A rare example of a collinear multiferroic spinel
By studying the dielectric properties of the geometrically frustrated spinel
CdV2O4, we observe ferroelectricity developing at the transition into the
collinear antiferromagnetic ground state. In this multiferroic spinel,
ferroelectricity is driven by local magnetostriction and not by the more common
scenario of spiral magnetism. The experimental findings are corroborated by
ab-initio calculations of the electric polarization and the underlying spin and
orbital order. The results point towards a charge rearrangement due to
dimerization, where electronic correlations and the proximity to the
insulator-metal transition play an important role.Comment: 4+ pages, 3 figure
Enhanced dimerization of TiOCl under pressure: spin-Peierls - to - Peierls transition
We report high-pressure x-ray diffraction and magnetization measurements
combined with ab-initio calculations to demonstrate that the high-pressure
optical and transport transitions recently reported in TiOCl, correspond in
fact to an enhanced Ti3+-Ti3+ dimerization existing already at room
temperature. Our results confirm the formation of a metal-metal bond between
Ti3+ ions along the b-axis of TiOCl, accompanied by a strong reduction of the
electronic gap. The evolution of the dimerization with pressure suggests a
crossover from the spin-Peierls to a conventional Peierls situation at high
pressures.Comment: 9pages, 4 figure
Singularity Structure, Symmetries and Integrability of Generalized Fisher Type Nonlinear Diffusion Equation
In this letter, the integrability aspects of a generalized Fisher type
equation with modified diffusion in (1+1) and (2+1) dimensions are studied by
carrying out a singularity structure and symmetry analysis. It is shown that
the Painlev\'e property exists only for a special choice of the parameter
(). A B\"acklund transformation is shown to give rise to the linearizing
transformation to the linear heat equation for this case (). A Lie
symmetry analysis also picks out the same case () as the only system among
this class as having nontrivial infinite dimensional Lie algebra of symmetries
and that the similarity variables and similarity reductions lead in a natural
way to the linearizing transformation and physically important classes of
solutions (including known ones in the literature), thereby giving a group
theoretical understanding of the system. For nonintegrable cases in (2+1)
dimensions, associated Lie symmetries and similarity reductions are indicated.Comment: 8 page
Pressure-induced metal-insulator transition in MgV_2O_4
On the basis of experimental thermoelectric power results and ab initio
calculations, we propose that a metal-insulator transition takes place at high
pressure (approximately 6 GPa) in MgV_2O_4.Comment: 2 pages, 3 figures, accepted in Physica B (Strongly Correlated
Electron Systems '07
Pressure Dependent Low Temperature Kinetics for CN + CH3CN: Competition between Chemical Reaction and van der Waals Complex Formation
International audienceThe gas phase reaction between the CN radical and acetonitrile CH3CN was investigated experimentally, at low temperatures, with the CRESU apparatus and a slow flow reactor to explore the temperature dependence of its rate coefficient from 354 K down to 23 K. Whereas a standard Arrhenius behavior was found at T > 200 K, indicating the presence of an activation barrier, a dramatic increase in the rate coefficient by a factor of 130 was observed when the temperature was decreased from 168 to 123 K. The reaction was found to be pressure independent at 297 K unlike the experiments carried out at 52 and 132 K. The work was complemented by ab initio transition state theory based master equation calculations using reaction pathways investigated with highly accurate thermochemical protocols. The role of collisional stabilization of a CN⋯CH3CN van der Waals complex and of tunneling induced H atom abstractions were also considered. The experimental pressure dependence at 52 and 132 K is well reproduced by the theoretical calculations provided that an anharmonic state density is considered for the van der Waals complex CH3CN⋯CN and its Lennard-Jones radius is adjusted. Furthermore, these calculations indicate that the experimental observations correspond to the fall-off regime and that tunneling remains small in the low-pressure regime. Hence, the studied reaction is essentially an association process at very low temperature. Implications for the chemistry of interstellar clouds and Titan are discussed
Study of the pressure effects in TiOCl by ab initio calculations
Electronic structure calculations on the low dimensional spin-1/2 compound
TiOCl were performed at several pressures in the orthorhombic phase, finding
that the structure is quasi-one-dimensional. The Ti3+ (d1) ions have one t2g
orbital occupied (dyz) with a large hopping integral along the b direction of
the crystal. The most important magnetic coupling is Ti-Ti along the b axis.
The transition temperature (Tc) has a linear evolution with pressure, and at
about 10 GPa this Tc is close to room temperature, leading to a room
temperature spin-Peierls insulator-insulator transition, with an important
reduction of the charge gap in agreement with the experiment. On the
high-pressure monoclinic phase, TiOCl presents two possible dimerized
structures, with a long or short dimerization. Long dimerized state occurs
above 15 GPa, and below this pressure the short dimerized structure is the more
stable phase.Comment: 3 pages, 3 embedded figures, 1 table. A. Pi\~neiro, et al.,J. Magn.
Magn. Mater. (2009) (accepted
Quantum Discord and entropic measures of quantum correlations: Optimization and behavior in finite spin chains
We discuss a generalization of the conditional entropy and one-way
information deficit in quantum systems, based on general entropic forms. The
formalism allows to consider simple entropic forms for which a closed
evaluation of the associated optimization problem in qudit-qubit systems is
shown to become feasible, allowing to approximate that of the quantum discord.
As application, we examine quantum correlations of spin pairs in the exact
ground state of finite spin chains in a magnetic field through the quantum
discord and information deficit. While these quantities show a similar
behavior, their optimizing measurements exhibit significant differences, which
can be understood and predicted through the previous approximations. The
remarkable behavior of these quantities in the vicinity of transverse and
non-transverse factorizing fields is also discussed.Comment: 10 pages, 3 figure
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