595 research outputs found
Theory of the ferroelectric phase in organic conductors: optics and physics of solitons
Recently the ferroelectric anomaly (Nad, Monceau, et al) followed by the
charge disproportionation (Brown, et al) have been discovered in (TMTTF)2X
compounds. The corresponding theory of the combined Mott-Hubbard state
describes both effects by interference of the build-in nonequivalence of bonds
and the spontaneous one of sites. The state gives rise to three types of
solitons: \pi solitons (holons) are observed via the activation energy \Delta
in the conductivity ; noninteger \alpha solitons (the FE domain walls)
provide the frequency dispersion of the ferroelectric response; combined
spin-charge solitons determine G(T) below subsequent structural transitions of
the tetramerisation. The photoconductivity gap 2\Delta is determined by
creations of soliton - antisoliton pairs. The optical edge lies well below,
given by the collective ferroelectric mode which coexists with the combined
electron-phonon resonance and the phonon antiresonance. The charge
disproportionation and the ferroelectricity can exist hiddenly even in the Se
subfamily giving rise to the unexplained yet low frequency optical peak, the
enhanced pseudogap and traces of phonons activation.Comment: small fixe
Competing Interactions, the Renormalization Group and the Isotropic-Nematic Phase Transition
We discuss 2D systems with Ising symmetry and competing interactions at
different scales. In the framework of the Renormalization Group, we study the
effect of relevant quartic interactions. In addition to the usual constant
interaction term, we analyze the effect of quadrupole interactions in the self
consistent Hartree approximation. We show that in the case of repulsive
quadrupole interaction, there is a first order phase transition to a stripe
phase in agreement with the well known Brazovskii result. However, in the case
of attractive quadrupole interactions there is an isotropic-nematic second
order transition with higher critical temperature.Comment: 4 pages, no figures, version to be published in Physical Review
Letters. Some scaling dimensions corrected, conclusions are the sam
Topological Confinement of Spins and Charges: Spinons as pi-junctions
Topologically nontrivial states, the solitons, emerge as elementary
excitations in 1D electronic systems. In a quasi 1D material the topological
requirements originate the spin- or charge- roton like excitations with charge-
or spin- kinks localized in the core. They result from the spin-charge
recombination due to confinement and the combined symmetry. The rotons possess
semi-integer winding numbers which may be relevant to configurations discussed
in connection to quantum computing schemes. Practically important is the case
of the spinon functioning as the single electronic pi- junction in a quasi 1D
superconducting material.Comment: 3 figure
New Routes to Solitons in Quasi One-Dimensional Conductors
We collect evidences on existence of microscopic solitons, and their
determining role in electronic processes of quasi-1D conductors. The
ferroelectric charge ordering gives access to several types of solitons in
conductivity and permittivity, and to solitons' bound pairs in optics - both in
insulating and conducting cases of TMTTF and TMTSF subfamilies. The excursion
to physics of conjugated polymers allows to suggest further experiments.
Internal tunnelling in Charge Density Waves goes through the channel of
"amplitude solitons", which correspond to the long sought quasi-particle - the
spinon. The same experiment gives an access to the reversible reconstruction of
the junction via spontaneous creation of a lattice of 2Pi solitons - a grid of
dislocations. The individual 2Pi solitons have been visually captured in recent
STM experiments. Junctions of organic and oxide conductors are anticipated to
show similar effects of reconstruction.Comment: Proceedings of ISCOM 2007, to be published in Solid State Sciences
(2008
Statistics of charged solitons and formation of stripes
The 2-fold degeneracy of the ground state of a quasi-one-dimensional system
allows it to support topological excitations such as solitons. We study the
combined effects of Coulomb interactions and confinement due to interchain
coupling on the statistics of such defects. We concentrate on a 2D case which
may correspond to monolayers of polyacetylene or other charge density waves.
The theory is developped by a mapping to the 2D Ising model with long-range
4-spin interactions. The phase diagram exhibits deconfined phases for liquids
and Wigner crystals of kinks and confined ones for bikinks. Also we find
aggregated phases with either infinite domain walls of kinks or finite rods of
bikinks. Roughening effects due to both temperature and Coulomb repulsion are
observed. Applications may concern the melting of stripes in doped correlated
materials.Comment: 16 pages, 7 figure
Solitons: from Charge Density Waves to FFLO in superconductors
This short review aims to summarize on "What the Charge Density Waves can
tell to other inhomogeneous states in strongly correlated systems, particularly
to spin-polarized superconductors". We shall update on expanding observations
of solitons in quasi 1D CDW conductors and link them to the growing information
and demands related to inhomogeneous spin-polarized states in superconductors.
The related theory, existent or awaited for, stretches from solitons in 1D
models to vortex-like elementary excitations in 2D,3D ordered incommensurate
CDWs and superconductors.Comment: After presentations given at the conferences STRIPES 2008 and ECRYS
200
Soliton Wall Superlattice in Quasi-One-Dimensional Conductor (Per)2Pt(mnt)2
We suggest a model to explain the appearance of a high resistance high
magnetic field charge-density-wave (CDW) phase, discovered by D. Graf et al.
[Phys. Rev. Lett. v. 93, 076406 (2004)] in (Per)2Pt(mnt)2. In particular, we
show that the Pauli spin-splitting effects improve the nesting properties of a
realistic quasi-one-dimensional electron spectrum and, therefore, a high
resistance Peierls CDW phase is stabilized in high magnetic fields. In low and
very high magnetic fields, a periodic soliton wall superlattice (SWS) phase is
found to be a ground state. We suggest experimental studies of the predicted
phase transitions between the Peierls and SWS CDW phases in (Per)2Pt(mnt)2 to
discover a unique SWS phase.Comment: 10 pages, 3 figures. Submitted to Physical Review Letters (February
19, 2007
Soliton phase near antiferromagnetic quantum critical point in Q1D conductors
In the frameworks of a nesting model for Q1D organic conductor at the
antiferromagnetic (SDW) quantum critical point the first-order transition
separates metallic state from the soliton phase having the periodic domain
structure. The low temperature phase diagram also displays the 2nd-order
transition line between the soliton and the uniformly gapped SDW phases. The
results agree with the phase diagram of (TMTSF)PF near critical
pressure [T. Vuletic et al., Eur. Phys. J. B 25, 319 (2002)]. Detection of the
2nd-order transition line is discussed. We comment on superconductivity at
lowest temperature.Comment: 4 pages, 1 figur
Phase transitions in ensembles of solitons induced by an optical pumping or a strong electric field
The latest trend in studies of modern electronically and/or optically active
materials is to provoke phase transformations induced by high electric fields
or by short (femtosecond) powerful optical pulses. The systems of choice are
cooperative electronic states whose broken symmetries give rise to topological
defects. For typical quasi-one-dimensional architectures, those are the
microscopic solitons taking from electrons the major roles as carriers of
charge or spin. Because of the long-range ordering, the solitons experience
unusual super-long-range forces leading to a sequence of phase transitions in
their ensembles: the higher-temperature transition of the confinement and the
lower one of aggregation into macroscopic walls. Here we present results of an
extensive numerical modeling for ensembles of both neutral and charged solitons
in both two- and three-dimensional systems. We suggest a specific Monte Carlo
algorithm preserving the number of solitons, which substantially facilitates
the calculations, allows to extend them to the three-dimensional case and to
include the important long-range Coulomb interactions. The results confirm the
first confinement transition, except for a very strong Coulomb repulsion, and
demonstrate a pattern formation at the second transition of aggregation.Comment: 16 pages, 16 figure
A Field Theory for Fermionic Ladder with Generic Intrachain Interactions
An effective low energy field theory is developed for a system of two chains.
The main novelty of the approach is that it allows to treat generic intrachain
repulsive interactions of arbitrary strength. The chains are coupled by a
direct tunneling and four-fermion interactions. At low energies the individual
chains are described as Luttinger liquids with an arbitrary ratio of spin
and charge velocities. A judicious choice of the basis for the decoupled
chains greatly simplifies the description and allows one to separate high and
low energy degrees of freedom. In a direct analogy to the bulk cuprates the
resulting effective field theory distinguishes between three qualitatively
different regimes: (i) small doping (), (ii) optimal doping () and (iii) large doping (). I discuss the excitation
spectrum and derive expressions for the electron spectral function which turns
out to be highly incoherent. The degree of incoherence increases when one
considers an array of ladders (stripe phase).Comment: 32 pages, 4 figures. A section explaining adiabatic approximation is
modified. Typos correcte
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