6,777 research outputs found
Competition between Charge Ordering and Superconductivity in Layered Organic Conductors -(BEDT-TTF)Hg(SCN) (M = K, NH)
While the optical properties of the superconducting salt
-(BEDT-TTF)NHHg(SCN) remain metallic down to 2 K, in the
non-superconducting K-analog a pseudogap develops at frequencies of about 200
cm for temperatures T < 200 K. Based on exact diagonalisation
calculations on an extended Hubbard model at quarter-filling we argue that
fluctuations associated with short range charge ordering are responsible for
the observed low-frequency feature. The different ground states, including
superconductivity, are a consequence of the proximity of these compounds to a
quantum phase charge-ordering transition driven by the intermolecular Coulomb
repulsion.Comment: 4 pages, 3 figure
13C NMR study of superconductivity near charge instability realized in beta"-(BEDT-TTF)4[(H3O)Ga(C2O4)3]C6H5NO2
To investigate the superconducting (SC) state near a charge instability, we
performed ^{13}C NMR experiments on the molecular superconductor
beta"-(BEDT-TTF)_{4}[(H_{3}O)Ga(C_{2}O_{4})_{3}]C_{6}H_{5}NO_{2}, which
exhibits a charge anomaly at 100 K. The Knight shift which we measured in the
SC state down to 1.5 K demonstrates that Cooper pairs are in spin-singlet
state. Measurements of the nuclear spin-lattice relaxation time reveal strong
electron-electron correlations in the normal state. The resistivity increase
observed below 10 K indicates that the enhanced fluctuation has an electric
origin. We discuss the possibility of charge-fluctuation-induced
superconductivity.Comment: 5 pages, 4 figure
Spin Liquid State in an Organic Mott Insulator with Triangular Lattice
H NMR and static susceptibility measurements have been performed in an
organic Mott insulator with nearly isotropic triangular lattice,
-(BEDT-TTF)Cu(CN), which is a model system of
frustrated quantum spins. The static susceptibility is described by the spin
= 1/2 antiferromagnetic triangular-lattice Heisenberg model with the
exchange constant 250 K. Regardless of the large magnetic
interactions, the H NMR spectra show no indication of long-range magnetic
ordering down to 32 mK, which is four-orders of magnitude smaller than .
These results suggest that a quantum spin liquid state is realized in the close
proximity of the superconducting state appearing under pressure.Comment: 4 pages, 4 figure
Superconductivity in Na_xCoO_2yH_2O by charge fluctuation
A new mechanism for superconductivity in the newly discovered Co-based oxide
is proposed by using charge fluctuation. A single-band extended Hubbard model
on the triangular lattice is studied within random phase approximation.
-wave triplet superconductivity is stabilized in the vicinity of
charge-density-wave instability, which is in sharp contrast with the
square-lattice case. The physical origin of the realization of the -wave
triplet state as well as the relevance to experiments are discussed
Feynman scaling violation on baryon spectra in pp collisions at LHC and cosmic ray energies
A significant asymmetry in baryon/antibaryon yields in the central region of
high energy collisions is observed when the initial state has non-zero baryon
charge. This asymmetry is connected with the possibility of baryon charge
diffusion in rapidity space. Such a diffusion should decrease the baryon charge
in the fragmentation region and translate into the corresponding decrease of
the multiplicity of leading baryons. As a result, a new mechanism for Feynman
scaling violation in the fragmentation region is obtained. Another numerically
more significant reason for the Feynman scaling violation comes from the fact
that the average number of cutted Pomerons increases with initial energy. We
present the quantitative predictions of the Quark-Gluon String Model (QGSM) for
the Feynman scaling violation at LHC energies and at even higher energies that
can be important for cosmic ray physics.Comment: 21 pages, 11 figures, and 1 table. arXiv admin note: substantial text
overlap with arXiv:1107.1615, arXiv:1007.320
Ferromagnetism, paramagnetism and a Curie-Weiss metal in an electron doped Hubbard model on a triangular lattice
Motivated by the unconventional properties and rich phase diagram of NaxCoO2
we consider the electronic and magnetic properties of a two-dimensional Hubbard
model on an isotropic triangular lattice doped with electrons away from
half-filling. Dynamical mean-field theory (DMFT) calculations predict that for
negative inter-site hopping amplitudes (t<0) and an on-site Coulomb repulsion,
U, comparable to the bandwidth, the system displays properties typical of a
weakly correlated metal. In contrast, for t>0 a large enhancement of the
effective mass, ferromagnetism and a Curie-Weiss magnetic susceptibility are
found in a broad electron doping range. Our observation of Nagaoka
ferromagnetism is consistent with the A-type antiferromagnetism (i.e.
ferromagnetic layers stacked antiferromagnetically) observed in neutron
scattering experiments on NaxCoO2. We propose that `Curie-Weiss metal' phase
observed in NaxCoO2 is a consequence of the crossover from ``bad metal'' with
incoherent quasiparticles at temperatures T>T* and Fermi liquid behavior with
enhanced parameters below T*, where T* is a low energy coherence scale induced
by strong local Coulomb electron correlations. We propose a model which
contains the charge ordering phenomena observed in the system which, we
propose, drives the system close to the Mott insulating phase even at large
dopings.Comment: 24 pages, 15 figures; accepted for publication in Phys. Rev.
Correlation gap in the optical spectra of the two-dimensional organic metal (BEDT-TTF)_4[Ni(dto)_2]
Optical reflection measurements within the highly conducting (a,b)-plane of
the organic metal (BEDT-TTF)_4[Ni(dto)_2] reveal the gradual development of a
sharp feature at around 200 cm as the temperature is reduced below 150 K. Below
this frequency a narrow Drude-like response is observed which accounts for the
metallic behavior. Since de Haas-von Alphen oscillations at low temperatures
confirm band structure calculations of bands crossing the Fermi energy, we
assign the observed behavior to a two-dimensional metallic state in the
proximity of a correlation induced metal-insulator transition.Comment: 4 pages, 2 figure
Phase Diagram of Spinless Fermions on an Anisotropic Triangular Lattice at Half-filling
The strong coupling phase diagram of the spinless fermions on the anisotropic
triangular lattice at half-filling is presented. The geometry of inter-site
Coulomb interactions rules the phase diagram. Unconventional charge ordered
phases are detected which are the recently reported pinball liquid and the
striped chains. Both are induced by the quantum dynamics out of classical
disordered states and afford extremely correlated metallic states and the
particular domain wall-type of excitations, respectively. The disorder once
killed by the quantum effect revives at the finite temperature, which is
discussed in the terms of the organic -ET.Comment: 4pages 6figure
Cyclotron effective masses in layered metals
Many layered metals such as quasi-two-dimensional organic molecular crystals
show properties consistent with a Fermi liquid description at low temperatures.
The effective masses extracted from the temperature dependence of the magnetic
oscillations observed in these materials are in the range, m^*_c/m_e \sim 1-7,
suggesting that these systems are strongly correlated. However, the ratio
m^*_c/m_e contains both the renormalization due to the electron-electron
interaction and the periodic potential of the lattice. We show that for any
quasi-two-dimensional band structure, the cyclotron mass is proportional to the
density of states at the Fermi energy. Due to Luttinger's theorem, this result
is also valid in the presence of interactions. We then evaluate m_c for several
model band structures for the \beta, \kappa, and \theta families of
(BEDT-TTF)_2X, where BEDT-TTF is bis-(ethylenedithia-tetrathiafulvalene) and X
is an anion. We find that for \kappa-(BEDT-TTF)_2X, the cyclotron mass of the
\beta-orbit, m^{*\beta}_c, is close to 2 m^{*\alpha}_c, where m^{*\alpha}_c is
the effective mass of the \alpha- orbit. This result is fairly insensitive to
the band structure details. For a wide range of materials we compare values of
the cyclotron mass deduced from band structure calculations to values deduced
from measurements of magnetic oscillations and the specific heat coefficient.Comment: 12 pages, 3 eps figure
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