1,608 research outputs found
Comment on "A Tale of Two Theories: Quantum Griffiths Effects in Metallic Systems" by A. H. Castro-Neto and B. A. Jones
In a recent paper Castro-Neto and Jones argue that because the observability
of quantum Griffiths-McCoy effects in metals is controlled by non-universal
quantities, the quantum Griffiths-McCoy scenario may be a viable explanation
for the non-fermi-liquid behavior observed in heavy fermion compounds. In this
Comment we point out that the important non-universal quantity is the damping
of the spin dynamics by the metallic electrons; quantum Griffiths-McCoy effects
occur only if this is parametrically weak relative to other scales in the
problem, i.e. if the spins are decoupled from the carriers. This suggests that
in heavy fermion materials, where the Kondo effect leads to a strong
carrier-spin coupling, quantum Griffiths-McCoy effects are unlikely to occur.Comment: 2 page
Ion optics for high power 50-cm-diam ion thrusters
The process used at the NASA-Lewis to fabricate 30 and 50-cm-diameter ion optics is described. The ion extraction capabilities of the 30 and 50-cm diameter ion optics were evaluated on divergent field and ring-cusp discharge chambers and compared. Perveance was found to be sensitive to the effects of the type and power of the discharge chamber and to the accelerator electrode hole diameter. Levels of up to 0.64 N and 20 kW for thrust and input power, respectively, were demonstrated with the divergent-field discharge chamber. Thruster efficiencies and specific impulse values up to 79 percent and 5000 sec., respectively, were achieved with the ring-cusp discharge chamber
Unconventional properties of superconducting cuprates
We present an explanation of the unusual peak/dip/hump features observed in
photoemission experiments on Bi2212 at . We argue that these
features arise from the interaction of the fermionic quasi-particles with
overdamped spin fluctuations. We show that the strong spin-fermion interaction
combined with the feedback effect on the spin damping due to superconductivity
yields a Fermi-liquid form of the fermionic spectral function for where is the maximum value of the superconducting gap, and a
non-Fermi-liquid form for . In the Fermi-liquid regime,
the spectral function displays a quasiparticle peak at
; in the non-Fermi-liquid regime it possesses a broad
maximum (hump) at . In between the two regimes, the
spectral function has a dip at . We argue that our
theory also explains the tunneling data for the superconducting density of
states.Comment: 4 pages, RevTeX, 4 eps figures embedded in the tex
The Gutzwiller wave function as a disentanglement prescription
The Gutzwiller variational wave function is shown to correspond to a
particular disentanglement of the thermal evolution operator, and to be
physically consistent only in the temperature range U<<kT<<E_F, the Fermi
energy of the non-interacting system. The correspondence is established without
using the Gutzwiller approximation. It provides a systematic procedure for
extending the ansatz to the strong-coupling regime. This is carried out to
infinite order in a dominant class of commutators. The calculation shows that
the classical idea of suppressing double occupation is replaced at low
temperatures by a quantum RVB-like condition, which involves phases at
neighboring sites. Low-energy phenomenologies are discussed in the light of
this result.Comment: Final version as accepted in EPJ B, 10 pages, no figure
Various series expansions for the bilayer S=1/2 Heisenberg antiferromagnet
Various series expansions have been developed for the two-layer, S=1/2,
square lattice Heisenberg antiferromagnet. High temperature expansions are used
to calculate the temperature dependence of the susceptibility and specific
heat. At T=0, Ising expansions are used to study the properties of the
N\'{e}el-ordered phase, while dimer expansions are used to calculate the
ground-state properties and excitation spectra of the magnetically disordered
phase. The antiferromagnetic order-disorder transition point is determined to
be . Quantities computed include the staggered
magnetization, the susceptibility, the triplet spin-wave excitation spectra,
the spin-wave velocity, and the spin-wave stiffness. We also estimates that the
ratio of the intra- and inter-layer exchange constants to be for cuprate superconductor .Comment: RevTeX, 9 figure
Cooperative Jahn-Teller Effect and Electron-Phonon Coupling in
A classical model for the lattice distortions of \lax is derived and, in a
mean field approximation, solved. The model is based on previous work by
Kanamori and involves localized Mn d-electrons (which induce tetragonal
distortions of the oxygen octahedra surrounding the Mn) and localized holes
(which induce breathing distortions). Parameters are determined by fitting to
the room temperature structure of . The energy gained by formation of
a local lattice distortion is found to be large, most likely eV
per site, implying a strong electorn-phonon coupling and supporting polaronic
models of transport in the doped materials. The structural transition is shown
to be of the order-disorder type; the rapid x-dependence of the transition
temperature is argued to occur because added holes produce a "random" field
which misaligns the nearby sites.Comment: 24 pages. No figures. One Table. Late
Fictive Impurity Approach to Dynamical Mean Field Theory: a Strong-Coupling Investigation
Quantum Monte Carlo and semiclassical methods are used to solve two and four
site cluster dynamical mean field approximations to the square lattice Hubbard
model at half filling and strong coupling. The energy, spin correlation
function, phase boundary and electron spectral function are computed and
compared to available exact results. The comparision permits a quantitative
assessment of the ability of the different methods to capture the effects of
intersite spin correlations. Two real space methods and one momentum space
representation are investigated. One of the two real space methods is found to
be significantly worse: in it, convergence to the correct results is found to
be slow and, for the spectral function, nonuniform in frequency, with
unphysical midgap states appearing. Analytical arguments are presented showing
that the discrepancy arises because the method does not respect the pole
structure of the self energy of the insulator. Of the other two methods, the
momentum space representation is found to provide the better approximation to
the intersite terms in the energy but neither approximation is particularly
acccurate and the convergence of the momentum space method is not uniform. A
few remarks on numerical methods are made.Comment: Errors in previous versions corrected; CDMFT results adde
Optical spectral weights and the ferromagnetic transition temperature of CMR manganites: relevance of double-exchange to real materials
We present a thorough and quantitative comparison of double-exchange models
to experimental data on the colossal magnetoresistance manganese perovskites.
Our results settle a controversy by showing that physics beyond double-exchange
is important even in LaSrMnO, which has been regarded as a
conventional double-exchange system. We show that the crucial quantity for
comparisons of different calculations to each other and to data is the
conduction band kinetic energy , which is insensitive to the details of the
band structure and can be experimentally determined from optical conductivity
measurements. The seemingly complicated dependence of on the Hund's
coupling and carrier concentration is shown to reflect the variation of
with , and temperature. We present results for the optical
conductivity which allow interpretation of experiments and show that a feature
previously interpreted in terms of the Hund's coupling was misidentified. We
also correct minor errors in the phase diagram presented in previous work.Comment: 13 pages, 7 eps figure
Monte Carlo Simulations for the Magnetic Phase Diagram of the Double Exchange Hamiltonian
We have used Monte Carlo simulation techniques to obtain the magnetic phase
diagram of the double exchange Hamiltonian. We have found that the Berry's
phase of the hopping amplitude has a negligible effect in the value of the
magnetic critical temperature. To avoid finite size problems in our simulations
we have also developed an approximated expression for the double exchange
energy. This allows us to obtain the critical temperature for the ferromagnetic
to paramagnetic transition more accurately. In our calculations we do not
observe any strange behavior in the kinetic energy, chemical potential or
electron density of states near the magnetic critical temperature. Therefore,
we conclude that other effects, not included in the double exchange
Hamiltonian, are needed to understand the metal-insulator transition which
occurs in the manganites.Comment: 6 pages Revtex, 8 PS figure
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