34,709 research outputs found
Spectral correlations in the crossover between GUE and Poisson regularity: on the identification of scales
Motivated by questions of present interest in nuclear and condensed matter
physics we consider the superposition of a diagonal matrix with independent
random entries and a GUE. The relative strength of the two contributions is
determined by a parameter suitably defined on the unfolded scale.
Using results for the spectral two-point correlator of this model obtained in
the framework of the supersymmetry method we focus attention on two different
regimes. For << 1 the correlations are given by Dawson's integral
while for >> 1 we derive a novel analytical formula for the two-point
function. In both cases the energy scales, in units of the mean level spacing,
at which deviations from pure GUE behavior become noticable can be identified.
We also derive an exact expansion of the local level density for finite level
number.Comment: 15 pages, Revtex, no figures, to be published in special issue of J.
Math. Phys. (1997
On the Incommensurate Phase in Modulated Heisenberg Chains
Using the density matrix renormalization group method (DMRG) we calculate the
magnetization of frustrated S=1/2 Heisenberg chains for various modulation
patterns of the nearest neighbour coupling: commensurate, incommensurate with
sinusoidal modulation and incommensurate with solitonic modulation. We focus on
the order of the phase transition from the commensurate dimerized phase (D) to
the incommensurate phase (I). It is shown that the order of the phase
transition depends sensitively on the model. For the solitonic model in
particular, a -dependent elastic energy modifies the order of the
transition. Furthermore, we calculate gaps in the incommensurate phase in
adiabatic approximation.Comment: 8 pages, 9 figure
Systematic Mapping of the Hubbard Model to the Generalized t-J Model
The generalized t-J model conserving the number of double occupancies is
constructed from the Hubbard model at and in the vicinity of half-filling at
strong coupling. The construction is realized by a self-similar continuous
unitary transformation. The flow equation is closed by a truncation scheme
based on the spatial range of processes. We analyze the conditions under which
the t-J model can be set up and we find that it can only be defined for
sufficiently large interaction. There, the parameters of the effective model
are determined.Comment: 16 pages, 13 figures included. v2: Order of sections changed.
Calculation and discussion of apparent gap in Section IV.A correcte
Building block libraries and structural considerations in the self-assembly of polyoxometalate and polyoxothiometalate systems
Inorganic metal-oxide clusters form a class of compounds that are unique in their topological and electronic versatility and are becoming increasingly more important in a variety of applications. Namely, Polyoxometalates (POMs) have shown an unmatched range of physical properties and the ability to form structures that can bridge several length scales. The formation of these molecular clusters is often ambiguous and is governed by self-assembly processes that limit our ability to rationally design such molecules. However, recent years have shown that by considering new building block principles the design and discovery of novel complex clusters is aiding our understanding of this process. Now with current progress in thiometalate chemistry, specifically polyoxothiometalates (POTM), the field of inorganic molecular clusters has further diversified allowing for the targeted development of molecules with specific functionality. This chapter discusses the main differences between POM and POTM systems and how this affects synthetic methodologies and reactivities. We will illustrate how careful structural considerations can lead to the generation of novel building blocks and further deepen our understanding of complex systems
Nonequilibrium Relaxations and Aging Effects in a Two-Dimensional Coulomb Glass
The relaxations of conductivity have been studied in the glassy regime of a
strongly disordered two-dimensional electron system in Si after a temporary
change of carrier density during the waiting time t_w. Two types of response
have been observed: a) monotonic, where relaxations exhibit aging, i.e.
dependence on history, determined by t_w and temperature; b) nonmonotonic,
where a memory of the sample history is lost. The conditions that separate the
two regimes have been also determined.Comment: 4 pages; published versioi
Mott-Hubbard transition in infinite dimensions
We calculate the zero-temperature gap and quasiparticle weight of the
half-filled Hubbard model with a random dispersion relation. After
extrapolation to the thermodynamic limit, we obtain reliable bounds on these
quantities for the Hubbard model in infinite dimensions. Our data indicate that
the Mott-Hubbard transition is continuous, i.e., that the quasiparticle weight
becomes zero at the same critical interaction strength at which the gap opens.Comment: 4 pages, RevTeX, 5 figures included with epsfig Final version for
PRL, includes L=14 dat
- …