7,182 research outputs found
Quantum entanglement and fixed-point bifurcations
How does the classical phase space structure for a composite system relate to
the entanglement characteristics of the corresponding quantum system? We
demonstrate how the entanglement in nonlinear bipartite systems can be
associated with a fixed point bifurcation in the classical dynamics. Using the
example of coupled giant spins we show that when a fixed point undergoes a
supercritical pitchfork bifurcation, the corresponding quantum state - the
ground state - achieves its maximum amount of entanglement near the critical
point. We conjecture that this will be a generic feature of systems whose
classical limit exhibits such a bifurcation.Comment: v2: Structure of the paper changed for clarity, reduced length, now 9
pages with 6 figure
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.
Phase diagram of the one-dimensional Holstein model of spinless fermions
The one-dimensional Holstein model of spinless fermions interacting with
dispersionless phonons is studied using a new variant of the density matrix
renormalisation group. By examining various low-energy excitations of finite
chains, the metal-insulator phase boundary is determined precisely and agrees
with the predictions of strong coupling theory in the anti-adiabatic regime and
is consistent with renormalisation group arguments in the adiabatic regime. The
Luttinger liquid parameters, determined by finite-size scaling, are consistent
with a Kosterlitz-Thouless transition.Comment: Minor changes. 4 pages, 4 figures. To appear in Physical Review
Letters 80 (1998) 560
Universal subgap optical conductivity in quasi-one-dimensional Peierls systems
Quasi-one-dimensional Peierls systems with quantum and thermal lattice
fluctuations can be modeled by a Dirac-type equation with a Gaussian-correlated
off-diagonal disorder. A powerful new method gives the exact disorder-averaged
Green function used to compute the optical conductivity. The strong subgap tail
of the conductivity has a universal scaling form. The frequency and temperature
dependence of the calculated spectrum agrees with experiments on KCP(Br) and
trans-polyacetylene.Comment: 11 pages (+ 3 figures), LATEX (REVTEX 3.0
Temperature dependence of the interlayer magnetoresistance of quasi-one-dimensional Fermi liquids at the magic angles
The interlayer magnetoresistance of a quasi-one-dimensional Fermi liquid is
considered for the case of a magnetic field that is rotated within the plane
perpendicular to the most-conducting direction. Within semi-classical transport
theory dips in the magnetoresistance occur at integer amgic angles only when
the electronic dispersion parallel to the chains is nonlinear. If the field
direction is fixed at one of the magic angles and the temperature is varied the
resulting variation of the scattering rate can lead to a non-monotonic
variation of the interlayer magnetoresistance with temperature. Although the
model considered here gives a good description of some of the properties of the
Bechgaard salts, (TMTSF)2PF6 for pressures less than 8kbar and (TMTSF)2ClO4 it
gives a poor description of their properties when the field is parallel to the
layers and of the intralayer transport.Comment: 10pages, RevTeX + epsf, 3 figure
Effect of quantum nuclear motion on hydrogen bonding
This work considers how the properties of hydrogen bonded complexes,
D-H....A, are modified by the quantum motion of the shared proton. Using a
simple two-diabatic state model Hamiltonian, the analysis of the symmetric
case, where the donor (D) and acceptor (A) have the same proton affinity, is
carried out. For quantitative comparisons, a parametrization specific to the
O-H....O complexes is used. The vibrational energy levels of the
one-dimensional ground state adiabatic potential of the model are used to make
quantitative comparisons with a vast body of condensed phase data, spanning a
donor-acceptor separation (R) range of about 2.4-3.0 A, i.e., from strong to
weak bonds. The position of the proton and its longitudinal vibrational
frequency, along with the isotope effects in both are discussed. An analysis of
the secondary geometric isotope effects, using a simple extension of the
two-state model, yields an improved agreement of the predicted variation with R
of frequency isotope effects. The role of the bending modes in also considered:
their quantum effects compete with those of the stretching mode for certain
ranges of H-bond strengths. In spite of the economy in the parametrization of
the model used, it offers key insights into the defining features of H-bonds,
and semi-quantitatively captures several experimental trends.Comment: 12 pages, 8 figures. Notation clarified. Revised figure including the
effect of bending vibrations on secondary geometric isotope effect. Final
version, accepted for publication in Journal of Chemical Physic
Temperature Dependence of the Magnetic Susceptibility for Triangular-Lattice Antiferromagnets with spatially anisotropic exchange constants
We present the temperature dependence of the uniform susceptibility of
spin-half quantum antiferromagnets on spatially anisotropic
triangular-lattices, using high temperature series expansions. We consider a
model with two exchange constants, and on a lattice that
interpolates between the limits of a square-lattice (), a
triangular-lattice (), and decoupled linear chains (). In all
cases, the susceptibility which has a Curie-Weiss behavior at high
temperatures, rolls over and begins to decrease below a peak temperature,
. Scaling the exchange constants to get the same peak temperature, shows
that the susceptibilities for the square-lattice and linear chain limits have
similar magnitudes near the peak. Maximum deviation arises near the
triangular-lattice limit, where frustration leads to much smaller
susceptibility and with a flatter temperature dependence. We compare our
results to the inorganic materials CsCuCl and CsCuBr and to a
number of organic molecular crystals. We find that the former (CsCuCl
and CsCuBr) are weakly frustrated and their exchange parameters
determined through the temperature dependence of the susceptibility are in
agreement with neutron-scattering measurements. In contrast, the organic
materials are strongly frustrated with exchange parameters near the isotropic
triangular-lattice limit.Comment: 10 pages, 9 figures and 1 table, revised versio
Apparent Violation of the Wiedemann-Franz law near a magnetic field tuned metal-antiferromagnetic quantum critical point
The temperature dependence of the interlayer electrical and thermal
resistivity in a layered metal are calculated for Fermi liquid quasiparticles
which are scattered inelastically by two-dimensional antiferromagnetic spin
fluctuations. Both resistivities have a linear temperature dependence over a
broad temperature range. Extrapolations to zero temperature made from this
linear- range give values that appear to violate the Wiedemann-Franz law.
However, below a low-temperature scale, which becomes small close to the
critical point, a recovery of this law occurs. Our results describe recent
measurements on CeCoIn near a magnetic field-induced quantum phase
transition. Hence, the experiments do not necessarily imply a non-Fermi liquid
ground state.Comment: 4 pages, 2 figures; accepted to Phys. Rev. Let
Mott Transition, Compressibility Divergence and P-T Phase Diagram of Layered Organic Superconductors: An Ultrasonic Investigation
The phase diagram of the organic superconductor
-(BEDT-TTF)Cu[N(CN)Cl has been investigated by ultrasonic
velocity measurements under helium gas pressure. Different phase transitions
were identified trough several elastic anomalies characterized from isobaric
and isothermal sweeps. Our data reveal two crossover lines that end on the
critical point terminating the first-order Mott transition line. When the
critical point is approached along these lines, we observe a dramatic softening
of the velocity which is consistent with a diverging compressibility of the
electronic degrees of freedom.Comment: 4 pages, 5 figure
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