21 research outputs found
The Spin-SAF transition in NaV2O5 induced by spin-pseudospin coupling
We present microscopic estimates for the spin-spin and spin-speudospin
interactions of the quarter-filled ladder compound NaV2O5, obtained by exactly
diagonalizing appropriate clusters of the underlying generalized Hubbard
Hamiltonian. We present evidence for a substantial interladder spin-pseudospin
interaction term which would allow simultaneously for the
superantiferroelectric (SAF) charge (pseudospin) ordering and spin
dimerization. We discuss the values of the coupling constants appropriate for
NaV2O5 and deduce the absence of a soft antiferroelectric mode
On the Stacking Charge Order in NaV2O5
We propose a mechanism for the observed stacking charge order in the
quarter-filled ladder compound NaV2O5. Via a standard mapping of the charge
degrees of freedom onto Ising spins we explain the stacking order as a result
of competition between couplings of the nearest and next-nearest planes with
the 4-fold degenerate super-antiferroelectric in-plane order.Comment: 4 pages, 5 figure
Renormalization group approach to Fermi Liquid Theory
We show that the renormalization group (RG) approach to interacting fermions
at one-loop order recovers Fermi liquid theory results when the forward
scattering zero sound (ZS) and exchange (ZS) channels are both taken into
account. The Landau parameters are related to the fixed point value of the
``unphysical'' limit of the forward scattering vertex. We specify the
conditions under which the results obtained at one-loop order hold at all order
in a loop expansion. We also emphasize the similarities between our RG approach
and the diagrammatic derivation of Fermi liquid theory.Comment: 4 pages (RevTex) + 1 postcript file, everything in a uuencoded file,
uses epsf (problem with the figure in the first version
Ordering in Two-Dimensional Ising Models with Competing Interactions
We study the 2D Ising model on a square lattice with additional non-equal
diagonal next-nearest neighbor interactions. The cases of classical and quantum
(transverse) models are considered. Possible phases and their locations in the
space of three Ising couplings are analyzed. In particular, incommensurate
phases occurring only at non-equal diagonal couplings, are predicted. We also
analyze a spin-pseudospin model comprised of the quantum Ising model coupled to
XY spin chains in a particular region of interactions, corresponding to the
Ising sector's super-antiferromagnetic (SAF) ground state. The spin-SAF
transition in the coupled Ising-XY model into a phase with co-existent SAF
Ising (pseudospin) long-range order and a spin gap is considered. Along with
destruction of the quantum critical point of the Ising sector, the phase digram
of the Ising-XY model can also demonstrate a re-entrance of the spin-SAF phase.
A detailed study of the latter is presented. The mechanism of the re-entrance,
due to interplay of interactions in the coupled model, and the conditions of
its appearance are established. Applications of the spin-SAF theory for the
transition in the quarter-filled ladder compound NaV2O5 are discussed.Comment: Minor revisions and refs. added; published version of the invited
paper in a special issue of "Low Temp. Physics
Leading Temperature Corrections to Fermi Liquid Theory in Two Dimensions
We calculate the basic parameters of the Fermi Liquid: the scattering vertex,
the Landau interaction function, the effective mass, and physical
susceptibilities for a model of two-dimensional (2D) fermions with a short
ranged interaction at non-zero temperature. The leading temperature dependences
of the spin components of the scattering vertex, the Landau function, and the
spin susceptibility are found to be linear. T-linear terms in the effective
mass and in the ``charge-sector''- quantities are found to cancel to second
order in the interaction, but the cancellation is argued not to be generic. The
connection with previous studies of the 2D Fermi-Liquid parameters is
discussed.Comment: 4 pages, 1 figur
Metamagnetic Quantum Criticality
A renormalization group treatment of metamagnetic quantum criticality in
metals is presented. In clean systems the universality class is found to be of
the overdamped, conserving (dynamical exponent z=3) Ising type. Detailed
results are obtained for the field and temperature dependence of physical
quantities including the differential susceptibility, resistivity and specific
heat near the transition. An application of the theory is made to Sr3Ru2O7,
which appears to exhibit a metamagnetic critical end-point at a very low
temperature and a field of order 5-7T.Comment: 4 pages latex (Revtex 4) and 3 eps figure
The Fermi Liquid as a Renormalization Group Fixed Point: the Role of Interference in the Landau Channel
We apply the finite-temperature renormalization-group (RG) to a model based
on an effective action with a short-range repulsive interaction and a rotation
invariant Fermi surface. The basic quantities of Fermi liquid theory, the
Landau function and the scattering vertex, are calculated as fixed points of
the RG flow in terms of the effective action's interaction function. The
classic derivations of Fermi liquid theory, which apply the Bethe-Salpeter
equation and amount to summing direct particle-hole ladder diagrams, neglect
the zero-angle singularity in the exchange particle-hole loop. As a
consequence, the antisymmetry of the forward scattering vertex is not
guaranteed and the amplitude sum rule must be imposed by hand on the components
of the Landau function. We show that the strong interference of the direct and
exchange processes of particle-hole scattering near zero angle invalidates the
ladder approximation in this region, resulting in temperature-dependent
narrow-angle anomalies in the Landau function and scattering vertex. In this RG
approach the Pauli principle is automatically satisfied. The consequences of
the RG corrections on Fermi liquid theory are discussed. In particular, we show
that the amplitude sum rule is not valid.Comment: 25 pages, RevTeX 3.