130 research outputs found
Dielectric response of charge induced correlated state in the quasi-one-dimensional conductor (TMTTF)2PF6
Conductivity and permittivity of the quasi-one-dimensionsional organic
transfer salt (TMTTF)2PF6 have been measured at low frequencies (10^3-10^7 Hz)
between room temperature down to below the temperature of transition into the
spin-Peierls state. We interpret the huge real part of the dielectric
permittivity (up to 10^6) in the localized state as the realization in this
compound of a charge ordered state of Wigner crystal type due to long range
Coulomb interaction.Comment: 11 pages, 3 .eps figure
Electron-lattice coupling and the broken symmetries of the molecular salt (TMTTF)SbF
(TMTTF)SbF is known to undergo a charge ordering (CO) phase
transition at and another transition to an
antiferromagnetic (AF) state at . Applied pressure causes a
decrease in both and . When , the CO is largely
supressed, and there is no remaining signature of AF order. Instead, the ground
state is a singlet. In addition to establishing an expanded, general phase
diagram for the physics of TMTTF salts, we establish the role of
electron-lattice coupling in determining how the system evolves with pressure.Comment: 4 pages, 5 figure
Charge Ordering in the One-Dimensional Extended Hubbard Model: Implication to the TMTTF Family of Organic Conductors
We study the charge ordering (CO) in the one-dimensional (1D) extended
Hubbard model at quarter filling where the nearest-neighbor Coulomb repulsion
and dimerization in the hopping parameters are included. Using the cluster
mean-field approximation to take into account the effect of quantum
fluctuations, we determine the CO phase boundary of the model in the parameter
space at T=0 K. We thus find that the dimerization suppresses the stability of
the CO phase strongly, and in consequence, the realistic parameter values for
quasi-1D organic materials such as (TMTTF)PF are outside the region of
CO. We suggest that the long-range Coulomb interaction between the chains
should persist to stabilize the CO phase.Comment: 5 pages, 4 eps figures, to appear in 15 Nov. 2001 issue of PR
Effect of nearest neighbor repulsion on the low frequency phase diagram of a quarter-filled Hubbard-Holstein chain
We have studied the influence of nearest-neighbor (NN) repulsion on the low
frequency phase diagram of a quarter-filled Hubbard-Holstein chain. The NN
repulsion term induces the apparition of two new long range ordered phases (one
CDW for positive and one CDW for
negative ) that did not exist in the V=0 phase diagram. These results
are put into perspective with the newly observed charge ordered phases in
organic conductors and an interpretation of their origin in terms of
electron-molecular vibration coupling is suggested.Comment: 10 pages, 10 figure
Competition of Dimerization and Charge Ordering in the Spin-Peierls State of Organic Conductors
The effect of the charge ordering on the spin-Peierls (SP) state has been
examined by using a Peierls-Hubbard model at quarter-filling with dimerization,
on-site and nearest-neighbor repulsive interactions. By taking account of the
presence of dimerization, a bond distortion is calculated variationally with
the renormalization group method based on bosonization. When the charge
ordering appears at V=V_c with increasing the nearest-neighbor interaction (V),
the distortion exhibits a maximum due to competition between the dimerization
and the charge ordering. It is shown that the second-order phase transition
occurs from the SP state with the bond alternation to a mixed state with an
additional component of the site alternationat V = V_c.Comment: 11 pages, 13 figures, to be published in J. Phys. Soc. Jpn. 72 No.6
(2003
Charge and spin order in one-dimensional electron systems with long-range Coulomb interactions
We study a system of electrons interacting through long--range Coulomb forces
on a one--dimensional lattice, by means of a variational ansatz which is the
strong--coupling counterpart of the Gutzwiller wave function. Our aim is to
describe the quantum analogue of Hubbard's classical ``generalized Wigner
crystal''. We first analyse charge ordering in a system of spinless fermions,
with particular attention to the effects of lattice commensurability. We argue
that for a general (rational) number of electrons per site there are three
regimes, depending on the relative strength of the long--range Coulomb
interaction (as compared to the hopping amplitude ). For very large the
quantum ground state differs little from Hubbard's classical solution, for
intermediate to large values of we recover essentially the Wigner crystal
of the continuum model, and for small the charge modulation amounts to a
small--amplitude charge--density wave. We then include the spin degrees of
freedom and show that in the Wigner crystal regimes (i.e. for large ) they
are coupled by an antiferromagnetic kinetic exchange , which turns out to be
smaller than the energy scale governing the charge degrees of freedom. Our
results shed new light on the insulating phases of organic quasi--1D compounds
where the long--range part of the interaction is unscreened, and magnetic and
charge orderings coexist at low temperatures.Comment: 11 pages, 7 figures, accepted for publication on Phys. Rev.
Crossover from Quarter-Filling to Half-Filling in a One-Dimensional Electron System with a Dimerized and Quarter-Filled Band
The interplay between quarter-filled and half-filled umklapp scattering has
been examined by applying the renormalization group method to a one-dimensional
quarter-filled electron system with dimerization, on-site (U) and
nearest-neighbor (V) repulsive interactions. The phase diagram on the U-V plane
is obtained at absolute zero temperature where the Mott insulator (the charge
ordered insulator) is found for smaller (larger) V. By choosing the moderate
parameter in the region of Mott insulator, it is shown that the resistivity
exhibits a crossover from behavior of quarter-filling to that of half-filling
with decreasing temperature.Comment: 4 pages, 4 figures, submitted to J. Phys. Soc. Jp
Competition and coexistence of bond and charge orders in (TMTTF)2AsF6
(TMTTF)2AsF6 undergoes two phase transitions upon cooling from 300 K. At
Tco=103 K a charge-ordering (CO) occurs, and at Tsp(B=9 T)=11 K the material
undergoes a spin-Peierls (SP) transition. Within the intermediate, CO phase,
the charge disproportionation ratio is found to be at least 3:1 from carbon-13
NMR 1/T1 measurements on spin-labeled samples. Above Tsp, up to about 3Tsp,
1/T1 is independent of temperature, indicative of low-dimensional magnetic
correlations. With the application of about 0.15 GPa pressure, Tsp increases
substantially, while Tco is rapidly suppressed, demonstrating that the two
orders are competing. The experiments are compared to results obtained from
calculations on the 1D extended Peierls-Hubbard model.Comment: 4 pages, 5 figure
Re-integerization of fractional charges in the correlated quarter-filled band
Previous work has demonstrated the existence of soliton defect states with
charges +/- e/2 in the limits of zero and of infinite on-site Coulomb
interactions in the one-dimensional (1D) quarter-filled band. For large but
finite on-site Coulomb interaction, the low temperature 2k_F bond distortion
that occurs within the 4k_F bond-distorted phase is accompanied by
charge-ordering on the sites. We show that a ``re-integerization'' of the
defect charge occurs in this bond-charge density wave (BCDW) state due to a
``binding'' of the fractional charges. We indicate briefly possible
implications of this result for mechanisms of organic superconductivity.Comment: 4 eps figure
Multiferroicity in an organic charge-transfer salt: Electric-dipole-driven magnetism
Multiferroics, showing simultaneous ordering of electrical and magnetic
degrees of freedom, are remarkable materials as seen from both the academic and
technological points of view. A prominent mechanism of multiferroicity is the
spin-driven ferroelectricity, often found in frustrated antiferromagnets with
helical spin order. There, similar to conventional ferroelectrics, the
electrical dipoles arise from an off-centre displacement of ions. However,
recently a different mechanism, namely purely electronic ferroelectricity,
where charge order breaks inversion symmetry, has attracted considerable
interest. Here we provide evidence for this exotic type of ferroelectricity,
accompanied by antiferromagnetic spin order, in a two-dimensional organic
charge-transfer salt, thus representing a new class of multiferroics. Quite
unexpectedly for electronic ferroelectrics, dipolar and spin order arise nearly
simultaneously. This can be ascribed to the loss of spin frustration induced by
the ferroelectric ordering. Hence, here the spin order is driven by the
ferroelectricity, in marked contrast to the spin-driven ferroelectricity in
helical magnets.Comment: 8 pages, 9 figures (including 4 pages and 6 figures in supplementary
information). Version 2 with minor errors corrected (legend of Fig. 3c and
definition of vectors e and Q
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