9,587 research outputs found
On the low energy properies of fermions with singular interactions
We calculate the fermion Green function and particle-hole susceptibilities
for a degenerate two-dimensional fermion system with a singular gauge
interaction. We show that this is a strong coupling problem, with no small
parameter other than the fermion spin degeneracy, N. We consider two
interactions, one arising in the context of the model and the other in
the theory of half-filled Landau level. For the fermion self energy we show in
contrast to previous claims that the qualitative behavior found in the leading
order of perturbation theory is preserved to all orders in the interaction. The
susceptibility at a general wavevector retains
the fermi-liquid form. However the susceptibility either
diverges as or remains finite but with nonanalytic wavevector,
frequency and temperature dependence. We express our results in the language of
recently discussed scaling theories, give the fixed-point action, and show that
at this fixed point the fermion-gauge-field interaction is marginal in ,
but irrelevant at low energies in .Comment: 21 pages, uuencoded LATEX file with included Postscript figures, R
Quasi-particle behavior of composite fermions in the half-filled Landau level
We calculate the effect of infrared fluctuations of the Chern-Simons gauge
field on the single-particle Green's function of composite fermions in the
half-filled Landau level via higher-dimensional bosonization on a curved Fermi
surface. We find that composite fermions remain well-defined quasi-particles,
with an effective mass given by the mean-field value, but with anomalously
large damping and a spectral function that contains considerable weight away
from the quasi-particle peak.Comment: reference added; accepted for publication in Phys. Rev. Let
Impurity correlations in dilute Kondo alloys
The single impurity Kondo model is often used to describe metals with dilute
concentrations (n_i) of magnetic impurities. Here we examine how dilute the
impurities must be for this to be valid by developing a virial expansion in
impurity density. The O(n_i^2) term is determined from results on the
2-impurity Kondo problem by averaging over the RKKY coupling. The non-trivial
fixed point of the 2-impurity problem could produce novel singularities in the
heat capacity of dilute alloys at O(n_i^2).Comment: 6 pages, no figure
Nonequilibrium relaxation of the two-dimensional Ising model: Series-expansion and Monte Carlo studies
We study the critical relaxation of the two-dimensional Ising model from a
fully ordered configuration by series expansion in time t and by Monte Carlo
simulation. Both the magnetization (m) and energy series are obtained up to
12-th order. An accurate estimate from series analysis for the dynamical
critical exponent z is difficult but compatible with 2.2. We also use Monte
Carlo simulation to determine an effective exponent, z_eff(t) = - {1/8} d ln t
/d ln m, directly from a ratio of three-spin correlation to m. Extrapolation to
t = infinity leads to an estimate z = 2.169 +/- 0.003.Comment: 9 pages including 2 figure
Specific heat and validity of quasiparticle approximation in the half-filled Landau level
We calculate the specific heat of composite fermion system in the half-filled
Landau level. Two different methods are used to examine validity of the
quasiparticle approximation when the two-body interaction is given by (). The singular part of the specific heat
is calculated from the free energy of the gauge field, which is compared with
the specific heat calculated from the quasiparticle approximation via the
singular self-energy correction due to the gauge field fluctuations. It turns
out that two results are in general different and they coincide only for the
case of the Coulomb interaction (). This result supports the fact
that the quasiparticle approximation is valid only for the case of the Coulomb
interaction. It is emphasized that this result is obtained by looking at a
gauge-invariant quantity -- the specific heat.Comment: 8 pages, Revte
Influence of gauge-field fluctuations on composite fermions near the half-filled state
Taking into account the transverse gauge field fluctuations, which interact
with composite fermions, we examine the finite temperature compressibility of
the fermions as a function of an effective magnetic field ( is the density of electrons) near the half-filled state. It is
shown that, after including the lowest order gauge field correction, the
compressibility goes as for , where . Here we assume that the interaction between
the fermions is given by , where is a dependent constant. This result can be
interpreted as a divergent correction to the activation energy gap and is
consistent with the divergent renormalization of the effective mass of the
composite fermions.Comment: Plain Tex, 24 pages, 5 figures available upon reques
Effective Mass of Composite Fermions and Fermionic Chern-Simons Theory in Temporal Gauge
The definitions of the effective mass of the composite fermion are discussed
for the half-filled Landau level problem. In a recent work, Shankar and Murthy
show a finite effective mass of the composite fermion by a canonical
transformation while the perturbative calculation gives the logarithmic
divergence of the effective mass at the Fermi surface. We will emphasize that
the different definition of the effective mass has the different physical
processes. The finite one could be defined for any momentum of the composite
fermion while the divergence only appears at the Fermi surface. We work with
the standard Halperin-Lee-Read model but in the temporal gauge. The advantage
of this gauge to be employed is that the finite effective mass could be
calculated in the Hartree-Fock approximation. Furthermore, it is precisely
equal to the result that Shankar and Murthy obtained which is well-fit with the
numerical calculation from the ground state energy analysis and a
semi-classical estimation. However, if we consider the random phase
approximation, one sees that the divergence of the effective mass of the
quasiparticle at the Fermi surface emerges again no matter that we work on the
temporal or Coulomb gauges. We develop an effective theory where the finite
effective mass serves as a `bare' effective mass and show that the same
divergence of the RPA effective mass. On the other hand, the correct behavior
of the response functions in the small band mass limit could be seen clearly in
the temporal gauge since there is a self-interaction among the magnetoplasmons.Comment: 27 pages,6 eps figure
Quantum Oscillations of Electrons and of Composite Fermions in Two Dimensions: Beyond the Luttinger Expansion
Quantum oscillation phenomena, in conventional 2-dimensional electron systems
and in the fractional quantum Hall effect, are usually treated in the
Lifshitz-Kosevich formalism. This is justified in three dimensions by
Luttinger's expansion, in the parameter . We show that in two
dimensions this expansion breaks down, and derive a new expression, exact in
the limit where rainbow graphs dominate the self-energy. Application of our
results to the fractional quantum Hall effect near half-filling shows very
strong deviations from Lifshitz-Kosevich behaviour. We expect that such
deviations will be important in any strongly-interacting 2-dimensional
electronic system.Comment: 4 pages, 3 figures, LaTe
Instantons and the spectral function of electrons in the half-filled Landau level
We calculate the instanton-anti-instanton action in
the gauge theory of the half-filled Landau level. It is found that for a class of interactions between electrons. This means that the instanton-anti-instanton
pairs are confining so that a well defined `charged' composite fermion can
exist. It is also shown that can be used to calculate
the spectral function of electrons from the microscopic theory within a
semiclassical approximation. The resulting spectral function varies as at low
energies.Comment: 13 pages, Plain Tex, MIT-CMT-APR-9
A sol-gel method for growing superconducting MgB2 films
In this paper we report a new sol-gel method for the fabrication of MgB2
films. Polycrystalline MgB2 films were prepared by spin-coating a precursor
solution of Mg(BH_4)_2 diethyl ether on (001)Al2O3 substrates followed with
annealing in Mg vapor. In comparison with the MgB2 films grown by other
techniques, our films show medium qualities including a superconducting
transition temperature of Tc ~ 37 K, a critical current density of Jc(5 K, 0 T)
~ 5 {\times} 10^6 A cm^{-2}, and a critical field of H_{c2}(0) ~ 19 T. Such a
sol-gel technique shows potential in the commercial fabrication of practically
used MgB2 films as well as MgB2 wires and tapes.Comment: 8 pages, 5 figure
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