32 research outputs found
New SDW phases in quasi-one-dimensional systems dimerized in the transverse direction
The spin density wave instabilities in the quasi-one-dimensional metal
(TMTSF)_2ClO_4 are studied in the framework a matrix random phase approximation
for intra-band and inter-band order parameters. Depending on the anion ordering
potential V which measures the lattice doubling in the transverse direction,
two different instabilities are possible. The SDW_0 state at low values of V is
antiferromagnetic in b direction and has the critical temperature that
decreases rapidly with V. The degenerated states SDW_(+-), stable at higher
values of V, are superpositions of two magnetic orders, each one on its
subfamily of chains. As V increases the ratio between two components of
SDW_(+-) tends to zero and the critical temperature increases asymptotically
towards that of SDW instability for a system having perfect nesting and no
anion order. At intermediate V the metallic state can persist down to T=0.Comment: Submitted to Europhysics Letter
Dielectric properties of multiband electron systems: II - Collective modes
Starting from the tight-binding dielectric matrix in the random phase
approximation we examine the collective modes and electron-hole excitations in
a two-band electronic system. For long wavelengths (), for
which most of the analysis is carried out, the properties of the collective
modes are closely related to the symmetry of the atomic orbitals involved in
the tight-binding states. In insulators there are only inter-band charge
oscillations. If atomic dipolar transitions are allowed, the corresponding
collective modes reduce in the asymptotic limit of vanishing bandwidths to
Frenkel excitons for an atomic insulator with weak on-site interactions. The
finite bandwidths renormalize the dispersion of these modes and introduce a
continuum of incoherent inter-band electron-hole excitations. The possible
Landau damping of collective modes due to the presence of this continuum is
discussed in detail.Comment: 25 pages, LaTeX, to appear in Z.Phys.
Pauli and orbital effects of magnetic field on charge density waves
Taking into account both Pauli and orbital effects of external magnetic field
we compute the mean field phase diagram for charge density waves in
quasi-one-dimensional electronic systems. The magnetic field can cause
transitions to CDW states with two types of the shifts of wave vector from its
zero-field value. It can also stabilize the field-induced charge density wave.
Furthermore, the critical temperature shows peaks at a new kind of magic
angles.Comment: 3 pages, 1 figure include
Photo-emission properties of quasi-one-dimensional conductors
We calculate the self-energy of one-dimensional electron band with the
three-dimensional long range Coulomb interaction within the random phase
approximation, paying particular attention to the contribution coming from the
electron scatterings on the collective plasmon mode. It is shown that the
spectral density has a form of wide feature at thr frequency scale of the
plasmon frequency, without the presence of quasi-particle delta-peaks. The
relevance of this result with respect to experimental findings and to the
theory of Luttinger liquids is discussed.Comment: 4 pages, 2 figure
Acoustic collective excitations and static dielectric response in incommensurate crystals with real order parameter
Starting from the basic Landau model for the incommensurate-commensurate
materials of the class II, we derive the spectrum of collective modes for all
(meta)stable states from the corresponding phase diagram. It is shown that all
incommensurate states posses Goldstone modes with acoustic dispersions. The
representation in terms of collective modes is also used in the calculation and
discussion of static dielectric response for systems with the commensurate wave
number in the center of the Brillouin zone.Comment: 7 pages, 4 figures, REVTe
Magnetic oscillations and field induced spin density waves in (TMTSF)_2ClO_4
We report an analysis of the effects of magnetic field on a
quasi-one-dimensional band of interacting electrons with a transverse
dimerizing potential. One-particle problem in bond-antibond representation is
solved exactly. The resulting propagator is used to calculate the
spin-density-wave (SDW) response of the interacting system within the matrix
RPA for the SDW susceptibility. We predict the magnetic field induced
transition of the first order between interband SDW_0 and intraband SDW_(+-)
phases. We reproduce the rapid oscillations with a period of 260 Tesla and the
overal profile of the TMTSF_2ClO_4 phase diagram.Comment: 6 pages, 3 figure
Dielectric properties of multiband electron systems: I - Tight-binding formulation
The screened electron-electron interaction in a multi-band electron system is
calculated within the random phase approximation and in the tight-binding
representation. The obtained dielectric matrix contains, beside the usual
site-site correlations, also the site-bond and bond-bond correlations, and thus
includes all physically relevant polarization processes. The arguments are
given that the bond contributions are negligible in the long wavelength limit.
We analyse the system with two non-overlapping bands in this limit, and show
that the corresponding dielectric matrix reduces to a form. The
intra-band and inter-band contributions are represented by diagonal matrix
elements, while the off-diagonal elements contain the mixing between them. The
latter is absent in insulators but may be finite in conductors. Performing the
multipole expansion of the bare long-range interaction, we show that this
mixing is directly related to the symmetry of the atomic orbitals participating
in the tight-binding electronic states. In systems with forbidden atomic
dipolar transitions, the intra-band and inter-band polarizations are separated.
However, when the dipolar transitions are allowed, the off-diagonal elements of
the dielectric matrix are of the same order as diagonal ones, due to a finite
monopole-dipole interaction between the intra-band and inter-band charge
fluctuations.Comment: 32 pages, LaTeX, to appear in Z.Phys.
Phase Diagram for Charge Density Waves in a Magnetic Field
The influence of an external magnetic field on a quasi one-dimensional system
with a charge density wave (CDW) instability is treated within the random phase
approximation which includes both CDW and spin density wave correlations. We
show that the CDW is sensitive to both orbital and Pauli effects of the field.
In the case of perfect nesting, the critical temperature decreases monotonously
with the field, and the wave vector of the instability starts to shift above
some critical value of magnetic field. Depending on the ratio between the spin
and charge coupling constants and on the direction of the applied magnetic
field, the wave vector shift is either parallel ( order) or
perpendicular ( order) to the most conducting direction. The
order is a field dependent linear combination of the charge and spin density
waves and is sensible only to the Pauli effect. The wave vector shift in
depends on the interchain coupling, but the critical temperature does
not. This order is affected by the confinement of the electronic orbits. By
increasing the relative strength of the orbital effect with respect to the
Pauli effect, one can destroy the , establishing either a , or a
(corresponding to perfect nesting wave vector). We also show that by
increasing the imperfect nesting parameter, one passes from the regime where
the critical temperature decreases with the field to the regime where it is
initially enhanced by the orbital effect and eventually suppressed by the Pauli
effect. For a bad nesting, the quantized phases of the field-induced CDW
appear.Comment: 30 pages (LaTeX) + 15 figure
Collective modes in uniaxial incommensurate-commensurate systems with the real order parameter
The basic Landau model for uniaxial systems of the II class is nonintegrable,
and allows for various stable and metastable periodic configurations, beside
that representing the uniform (or dimerized) ordering. In the present paper we
complete the analysis of this model by performing the second order variational
procedure, and formulating the combined Floquet-Bloch approach to the ensuing
nonstandard linear eigenvalue problem. This approach enables an analytic
derivation of some general conclusions on the stability of particular states,
and on the nature of accompanied collective excitations. Furthermore, we
calculate numerically the spectra of collective modes for all states
participating in the phase diagram, and analyze critical properties of
Goldstone modes at all second order and first order transitions between
disordered, uniform and periodic states. In particular it is shown that the
Goldstone mode softens as the underlying soliton lattice becomes more and more
dilute.Comment: 19 pages, 16 figures, REVTeX, to be published in Journal of Physics
A: Mathematical and Genera