599 research outputs found
Microwave dielectric study of spin-Peierls and charge ordering transitions in (TMTTF)PF salts
We report a study of the 16.5 GHz dielectric function of hydrogenated and
deuterated organic salts (TMTTF)PF. The temperature behavior of the
dielectric function is consistent with short-range polar order whose relaxation
time decreases rapidly below the charge ordering temperature. If this
transition has more a relaxor character in the hydrogenated salt, charge
ordering is strengthened in the deuterated one where the transition temperature
has increased by more than thirty percent. Anomalies in the dielectric function
are also observed in the spin-Peierls ground state revealing some intricate
lattice effects in a temperature range where both phases coexist. The variation
of the spin-Peierls ordering temperature under magnetic field appears to follow
a mean-field prediction despite the presence of spin-Peierls fluctuations over
a very wide temperature range in the charge ordered state of these salts.Comment: 7 pages, 6 figure
Superconducting pairing and density-wave instabilities in quasi-one-dimensional conductors
Using a renormalization group approach, we determine the phase diagram of an
extended quasi-one-dimensional electron gas model that includes interchain
hopping, nesting deviations and both intrachain and interchain repulsive
interactions. d-wave superconductivity, which dominates over the
spin-density-wave (SDW) phase at large nesting deviations, becomes unstable to
the benefit of a triplet -wave phase for a weak repulsive interchain
backscattering term , despite the persistence of dominant SDW
correlations in the normal state. Antiferromagnetism becomes unstable against
the formation of a charge-density-wave state when exceeds some
critical value. While these features persist when both Umklapp processes and
interchain forward scattering () are taken into account, the effect
of alone is found to frustrate nearest-neighbor interchain - and
-wave pairing and instead favor next-nearest-neighbor interchain singlet or
triplet pairing. We argue that the close proximity of SDW and
charge-density-wave phases, singlet d-wave and triplet -wave superconducting
phases in the theoretical phase diagram provides a possible explanation for
recent puzzling experimental findings in the Bechgaard salts, including the
coexistence of SDW and charge-density-wave phases and the possibility of a
triplet pairing in the superconducting phase.Comment: 19 pages, 13 figure
Superconductivity and Antiferromagnetism in Quasi-one-dimensional Organic Conductors
We review the current understanding of superconductivity in the
quasi-one-dimensional organic conductors of the Bechgaard and Fabre salt
families. We discuss the interplay between superconductivity,
antiferromagnetism, and charge-density-wave fluctuations. The connection to
recent experimental observations supporting unconventional pairing and the
possibility of a triplet-spin order parameter for the superconducting phase is
also presented.Comment: (v1) 30 pages, 13 figures; Review article for the 20th anniversary of
high-Tc superconductivity, to appear in J. Low Temp. Phys. (v2) 1 Ref. adde
CDW Ordering in Stripe Phase of Underdoped Cuprates
The in-plane resistivity and out-of-plane resistivity of non-superconducting
RBCO (R = Y, Tm) and Fe-doped Bi2212 single crystals are discussed. The
comparison of electrical transport properties of the cuprates and quasi-one
dimensional (1D) (TMTSF)2PF6 organic conductor suggests that RBCO and Bi2212
exhibit 1D transport properties, and the step rise at low temperatures in the
resistivities of the cuprates and quasi-1D organic conductor is due to
charge-density-wave ordering. We discuss also phonon-electron interactions in
cuprates at low temperatures.Comment: 10 pages including 4 figure
Pressure-induced Spin-Peierls to Incommensurate Charge-Density-Wave Transition in the Ground State of TiOCl
The ground state of the spin-Peierls system TiOCl was probed using
synchrotron x-ray diffraction on a single-crystal sample at T = 6 K. We tracked
the evolution of the structural superlattice peaks associated with the
dimerized ground state as a function of pressure. The dimerization along the b
axis is rapidly suppressed in the vicinity of a first-order structural phase
transition at Pc = 13.1(1) GPa. The high-pressure phase is characterized by an
incommensurate charge density wave perpendicular to the original spin chain
direction. These results show that the electronic ground state undergoes a
fundamental change in symmetry, indicating a significant change in the
principal interactions.Comment: 5 pages, 4 figure
Renormalization Group calculations with k|| dependent couplings in a ladder
We calculate the phase diagram of a ladder system, with a Hubbard interaction
and an interchain coupling . We use a Renormalization Group method, in
a one loop expansion, introducing an original method to include
dependence of couplings. We also classify the order parameters corresponding to
ladder instabilities. We obtain different results, depending on whether we
include dependence or not. When we do so, we observe a region with
large antiferromagnetic fluctuations, in the vicinity of small ,
followed by a superconducting region with a simultaneous divergence of the Spin
Density Waves channel. We also investigate the effect of a non local backward
interchain scattering : we observe, on one hand, the suppression of singlet
superconductivity and of Spin Density Waves, and, on the other hand, the
increase of Charge Density Waves and, for some values of , of triplet
superconductivity. Our results eventually show that is an influential
variable in the Renormalization Group flow, for this kind of systems.Comment: 20 pages, 19 figures, accepted in Phys. Rev. B 71 v. 2
Triplet superconducting pairing and density-wave instabilities in organic conductors
Using a renormalization group approach, we determine the phase diagram of an
extended quasi-one-dimensional electron gas model that includes interchain
hopping, nesting deviations and both intrachain and interchain repulsive
interactions. We find a close proximity of spin-density- and
charge-density-wave phases, singlet d-wave and triplet f-wave superconducting
phases. There is a striking correspondence between our results and recent
puzzling experimental findings in the Bechgaard salts, including the
coexistence of spin-density-wave and charge-density-wave phases and the
possibility of a triplet pairing in the superconducting phase.Comment: 4 pages, 5 eps figure
Diamagnetism of doped two-leg ladders and probing the nature of their commensurate phases
We study the magnetic orbital effect of a doped two-leg ladder in the
presence of a magnetic field component perpendicular to the ladder plane.
Combining both low-energy approach (bosonization) and numerical simulations
(density-matrix renormalization group) on the strong coupling limit (t-J
model), a rich phase diagram is established as a function of hole doping and
magnetic flux. Above a critical flux, the spin gap is destroyed and a Luttinger
liquid phase is stabilized. Above a second critical flux, a reentrance of the
spin gap at high magnetic flux is found. Interestingly, the phase transitions
are associated with a change of sign of the orbital susceptibility. Focusing on
the small magnetic field regime, the spin-gapped superconducting phase is
robust but immediately acquires algebraic transverse (i.e. along rungs) current
correlations which are commensurate with the 4k_F density correlations. In
addition, we have computed the zero-field orbital susceptibility for a large
range of doping and interactions ratio J/t : we found strong anomalies at low
J/t only in the vicinity of the commensurate fillings corresponding to delta =
1/4 and 1/2. Furthermore, the behavior of the orbital susceptibility reveals
that the nature of these insulating phases is different: while for delta = 1/4
a 4k_F charge density wave is confirmed, the delta = 1/2 phase is shown to be a
bond order wave.Comment: 15 pages, 17 figure
Tomonaga-Luttinger parameters for doped Mott insulators
The Tomonaga--Luttinger parameter determines the critical behavior
in quasi one-dimensional correlated electron systems, e.g., the exponent
for the density of states near the Fermi energy. We use the numerical
density-matrix renormalization group method to calculate from the
slope of the density-density correlation function in momentum space at zero
wave vector. We check the accuracy of our new approach against exact results
for the Hubbard and XXZ Heisenberg models. We determine in the phase
diagram of the extended Hubbard model at quarter filling, , and
confirm the bosonization results on the critical
line and at infinitesimal doping of the
charge-density-wave (CDW) insulator for all interaction strengths. The doped
CDW insulator exhibits exponents only for small doping and strong
correlations.Comment: 7 pages, 4 figure
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