4,502 research outputs found
A Generalized Ginzburg-Landau Approach to Second Harmonic Generation
We develop a generalized Ginzburg-Landau theory for second harmonic
generation (SHG) in magnets by expanding the free energy in terms of the order
parameter in the magnetic phase and the susceptibility tensor in the
corresponding high-temperature phase. The non-zero components of the SHG
susceptibility in the ordered phase are derived from the symmetries of the
susceptibility tensor in the high-temperature phase and the symmetry of the
order parameter. In this derivation, the dependence of the SHG susceptibility
on the order parameter follows naturally, and therefore its nonreciprocal
optical properties.
We examine this phenomenology for the magnetoelectric compound CrO as
well as for the ferroelectromagnet YMnO.Comment: European Journal of Physics B (accepted
Control of the finite size corrections in exact diagonalization studies
We study the possibility of controlling the finite size corrections in exact
diagonalization studies quantitatively. We consider the one- and two
dimensional Hubbard model. We show that the finite-size corrections can be be
reduced systematically by a grand-canonical integration over boundary
conditions. We find, in general, an improvement of one order of magnitude with
respect to studies with periodic boundary conditions only. We present results
for ground-state properties of the 2D Hubbard model and an evaluation of the
specific heat for the 1D and 2D Hubbard model.Comment: Phys. Rev. B (Brief Report), in pres
Spin-charge separation at small lengthscales in the 2D t-J model
We consider projected wavefunctions for the 2D model. For various
wavefunctions, including correlated Fermi-liquid and Luttinger-type
wavefunctions we present the static charge-charge and spin-spin structure
factors. Comparison with recent results from a high-temperature expansion by
Putikka {\it et al.} indicates spin-charge separation at small lengthscales.Comment: REVTEX, 5 pages, 5 figures hardcopies availabl
Breakdown of the Luttinger sum-rule at the Mott-Hubbard transition in the one-dimensional t1-t2 Hubbard model
We investigate the momentum distribution function near the Mott-Hubbard
transition in the one-dimensional t1-t2 Hubbard model (the zig-zag Hubbard
chain), with the density-matrix renormalization-group technique. We show that
for strong interactions the Mott-Hubbard transition occurs between the
metallic-phase and an insulating dimerized phase with incommensurate spin
excitations, suggesting a decoupling of magnetic and charge excitations not
present in weak coupling. We illustrate the signatures for the Mott-Hubbard
transition and the commensurate-incommensurate transition in the insulating
spin-gapped state in their respective ground-state momentum distribution
functions
Spin-liquid and magnetic phases in the anisotropic triangular lattice: the case of -(ET)X
The two-dimensional Hubbard model on the anisotropic triangular lattice, with
two different hopping amplitudes and , is relevant to describe
the low-energy physics of -(ET)X, a family of organic salts. The
ground-state properties of this model are studied by using Monte Carlo
techniques, on the basis of a recent definition of backflow correlations for
strongly-correlated lattice systems. The results show that there is no magnetic
order for reasonably large values of the electron-electron interaction and
frustrating ratio , suitable to describe the non-magnetic
compound with X=Cu(CN). On the contrary, N\'eel order takes place for
weaker frustrations, i.e., , suitable for
materials with X=Cu(SCN), Cu[N(CN)]Cl, or Cu[N(CN)]Br.Comment: 7 pages, Physical Review B 80, 064419 (2009
A Variational Monte Carlo Study of the Current Carried by a Quasiparticle
With the use of Gutzwiller-projected variational states, we study the
renormalization of the current carried by the quasiparticles in
high-temperature superconductors and of the quasiparticle spectral weight. The
renormalization coefficients are computed by the variational Monte Carlo
technique, under the assumption that quasiparticle excitations may be described
by Gutzwiller-projected BCS quasiparticles. We find that the current
renormalization coefficient decreases with decreasing doping and tends to zero
at zero doping. The quasiparticle spectral weight Z_+ for adding an electron
shows an interesting structure in k space, which corresponds to a depression of
the occupation number k just outside the Fermi surface. The perturbative
corrections to those quantities in the Hubbard model are also discussed.Comment: 9 pages, 9 figure
Pseudogap and antiferromagnetic correlations in the Hubbard model
Using the dynamical cluster approximation and quantum monte carlo we
calculate the single-particle spectra of the Hubbard model with next-nearest
neighbor hopping . In the underdoped region, we find that the pseudogap
along the zone diagonal in the electron doped systems is due to long range
antiferromagnetic correlations. The physics in the proximity of is
dramatically influenced by and determined by the short range correlations.
The effect of on the low energy ARPES spectra is weak except close to the
zone edge. The short range correlations are sufficient to yield a pseudogap
signal in the magnetic susceptibility, produce a concomitant gap in the
single-particle spectra near but not necessarily at a location in
the proximity of Fermi surface.Comment: 5 pages, 4 figure
Pairing and superconductivity driven by strong quasiparticle renormalization in two-dimensional organic charge transfer salts
We introduce and analyze a variational wave function for quasi
two-dimensional kappa-ET organic salts containing strong local and nonlocal
correlation effects. We find an unconventional superconducting ground state for
intermediate charge carrier interaction, sandwiched between a conventional
metal at weak coupling and a spin liquid at larger coupling. Most remarkably,
the excitation spectrum is dramatically renormalized and is found to be the
driving force for the formation of the unusual superconducting state.Comment: 4 pages, 4 figure
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