We discuss the effects of fluctuations of the local density
of charged dopants near a first order phase transition in
electronic systems, that is driven by change of charge
carrier density controlled by doping level. Using a
generalization of the Imry-Ma argument, we find that the
first order transition is rounded by disorder at or below
the lower critical dimension d_c=3, when at least one of the
two phases has no screening ability. The increase of d_c
from 2 (as in the random field Ising model) to 3 is due to
the long-range nature of the Coulomb interaction. This
result suggests that large clusters of both phases will
appear near such transitions due to disorder, in both two
and three dimensions. Possible implications of our results
on manganites and underdoped cuprates (where meso-scale
cluster formation has been found) will be discussed.
[H8.002] Transport equation of interacting electrons with disorder
Jun Sun, Qimiao Si (Department of Physics and Astronomy, Rice University), Chandra Varma (Bell Labs, Lucent Technologies)
We develop a transport formalism for interacting electrons
in the presence of quenched disorder, which generalizes the
work of Betbeder-Matibet and Nozieres. Quantum effects on
transport, due both to quantum interference and interaction
effects, are incorporated through non-analytic terms in the
irreducible interactions and appropriate contributions to
the electron self-energy. We use our formalism to recover
the standard results on perturbative quantum corrections to
the Drude conductivity, as well as the competition between
singlet and triplet interaction corrections previously
treated in the scaling theory. We then go on to use our
formalim to discuss the nature of the strong coupling fixed
point in two dimensions; here, the Coulomb gap in the
density of states plays a key role.
[H8.003] On the Ordering Instability of Weakly-Interacting Electrons in a Dirty Metal
Xiao Yang, Chetan Nayak (Department of Physics, University of California, Los Angeles)
In a dirty metal, electron-electron interactions in the
spin-triplet channel lead to singular corrections to a
variety of physical quantities. We show that these
singularities herald the emergence of ferromagnetism by
calculating the effective action for the magnetic moment of
weakly-interacting electrons in a dirty metal. A state with
finite ferromagnetic moment minimizes this effective action.
We discuss the physics of the ferromagnetic state with
particular regard to thermal fluctuations and localization
effects.
[H8.004] Glassy Dynamics in a 2D Electron System in Si MOSFETs
Dragana Popovi\'c (National High Magnetic Field Laboratory, Florida State University, Tallahassee, Florida 32310)
Studies of low-frequency resistance noise demonstrate that
glassy freezing occurs in a two-dimensional electron system
in Si MOSFETs in the vicinity of the metal-insulator
transition. The size of the metallic glass phase, which
separates the 2D metal and the (glassy) insulator, depends
strongly on disorder, becoming extremely small in
high-mobility (low-disorder) samples, in agreement with
theoretical predictions. The glass transition is manifested
by a sudden and dramatic slowing down of the electron
dynamics, and by a very abrupt change to the sort of
statistics characteristic of complicated multistate systems.
In particular, the behavior of the second spectrum, an
important fourth-order noise statistic, indicates the
presence of long-range correlations between fluctuators in
the glassy phase, consistent with the hierarchical picture
of glassy dynamics. This work has been supported by NSF.
[H8.005] Logarithmic temperature dependence of thick granular films near metal-insulator transition
T. J. Li, J. J. Lin (National Chiao Tung University, Taiwan), H. Liu, X. X. Zhang (Hong Kong University of Science & Technology, Hong Kong)
We have measured the resistances as a function of
temperature in a series of thick (1 \mum) nano-granular
Cu_x(SiO_2)_1-x films near the metal-insulator
transition. Although there have been many works concerned
with the variations of resistance on either the metallic or
insulating side, there have been very few studies focusing
on the metallic regime but close to the metal-insulator
transition. In this regime, a T^1/3 law has been
proposed to hold for a narrow temperature range at
liquid-helium temperatures. Instead, our thick nano-granular
Cu_x(SiO_2)_1-xfilms, with x around 0.45, reveal
a logarithmic temperature dependence of resistance from 1 K
up to above 40 K.
[H8.006] Landau theory of the Fermi-liquid to electron glass transition
Denis Dalidovich, Vladimir Dobrosavljevic (Department of Physics and National High Magnetic Field Laboratory, Florida State University, Tallahassee, Florida 32306)
A lattice model of spinless interacting electrons is used to
formulate the Landau theory of the Fermi liquid to electron
glass quantum phase transition. We demonstrate that the
presence of additional random site energies does not affect
the character of the transition, once the replica symmetry
breaking is considered self-consistently at the mean-field
level. Inside the glass phase, the low temperature
conductivity assumes a non-Fermi liquid \sim T^3/2 form, in
agreement with recent experiments.
[H8.007] ‘Aging’ effects in conductivity – electron glass ?
Ady Vaknin (The Rowland Institute of Science; and Department of Molecular and Cellular Biology, Harvard University.)
Slow dynamics, memory, and ‘aging’ are common
characteristics of glassy systems. Similar features are
observed in the conductivity of Anderson insulators. Such
systems are disordered conductors where the conduction band
consists of localized electronic states. Glassy behavior in
these systems might result from the interplay between the
disorder and the electron-electron interactions.
[H8.008] Electrodynamics of Coulomb Glasses
N. Peter Armitage, Erik Helgren, George Gruner (Dept. of Physics and Astronomy, UCLA)
Strong long range Coulomb interactions are known to play a
central role in the electronic properties of disordered
solids, of which doped semiconductors in the insulating
‘glass’ phase are a canonical example. It is surprising
that, even in a material as thoroughly researched as doped
bulk silicon, no consensus on the nature of the ground state
and its low lying excitations have been reached. We have
performed measurements of the complex AC conductivity in the
quantum limit, w > T, of insulating uncompensated n-type
silicon over a broad doping range. At low energies, the
observed frequency dependence shows characteristics
consistent with many theories of an interacting
‘Coulomb-glass.’ At higher excitation energies there is
evidence for a crossover from this Coulomb glass-like
behavior to a non-interacting ‘Fermi-glass’ regime. This is
interesting behavior, because unlike the case of ‘typical’
interacting systems the non-interacting regime is not
recovered in the low energy limit. Although it is expected
from theory that a crossover should be observed when the
photon energy exceeds the interaction energy of a typical
excitation pair, this crossover is sharper than predicted
and the form for the conductivity cannot be fit by extant
theories. Despite this, the measured crossover energy
compares favorably to the predicted Coulomb interaction
energy and reasonable estimates of the localization length
and scaling exponents can be obtained. We put our results in
the context of previous measurements of the insulating
amorphous semiconductor NbSi, where in addition to a Coulomb
glass phase we were able to observe a crossover to a quantum
critical regime as the metal insulator transition was
approached.
[H8.009] Hopping Transport in Insulating Quench-Condensed Ultrathin Beryllium Films
Edward Bielejec, Wenhao Wu (University of Rochester)
We have measured the temperature dependence of the sheet
resistance, R(T), in highly disordered, quench-condensed
ultrathin Be films, as the sheet resistance at 20 K was
progressively decreased from 600 to 20 kØmega/\Box. For
temperatures above 5 K, the sheet resistances for all the
films follow the Efros-Shklovskii hopping law, R(T) =
R_0exp(T_0/T)^1/2, with all the curves focusing onto
a single pre-factor R_0 \sim (1/2)R_Q \approx 13
kØmega/\Box. We will discuss these results in connection
with the recently measured tunneling density of states near
the Fermi energy, as well as the dielectric constant,
\kappa, and the localization length, \xi, for these
highly disordered films.
[H8.010] Disorder Screening in Strongly Correlated Systems
Darko Tanaskovic, Vladimir Dobrosavljevic (Department of Physics and National High Magnetic Field Laboratory, Florida State University, Tallahassee, Florida 32306), Elihu Abrahams, Gabriel Kotliar (Serin Physics Laboratory, Rutgers University, P.O. Box 849, Piscataway, New Jersey 08855)
Electron-electron interactions generally reduce the residual
resistivity at low temperatures due to the screening of the
impurity potential by the electron gas. In the weak coupling
limit, the magnitude of this screening effect is determined
by the thermodynamic compressibility. We show that when
strong correlations are present, although the
compressibility is reduced, the screening effect is
nevertheless strongly enhanced. This phenomenon is traced to
the same non-perturbative Kondo-like processes that lead to
strong mass enhancements, but which are absent in weak
coupling approaches.
[H8.011] Current-Driven Insulator-to-Superconductor Transition in Quench-Condensed Ultrathin Beryllium Films
Wenhao Wu (Department of Physics and Astronomy, University of Rochester), Edward Bielejec (Sandia National Laboratory)
We have studied at various temperatures and magnetic fields
the nonlinear I-V characteristics for quench-condensed Be
films just on the insulating side of the thickness-tuned
superconductor-insulator transition. These films are about 1
nm in thickness and 3mm by 3mm in size. The superconducting
phase coherence length measured in such films decreases with
decreasing temperature as strong disorder leads to a
localization of the Cooper pairs at low temperatures. The
I-V curves are linear with a positive slope in the
zero-current limit. However, the slope of the I-V curves
becomes negative as the current increases above 100 nA.
Further increasing the current to above 300 nA but below a
robust critical current of \sim 15 \muA, the voltage
becomes zero and the dynamical resistance is effectively
zero. Such I-V curves indicate a current-driven
insulator-to-superconductor transition. We have also
measured the critical fields of the induced superconducting
state at various temperatures and bias currents. We will
discuss how the applied current suppresses the localization
effects and restores the phase coherence of the Cooper
pairs.
[H8.012] Electronic Delocalization in Finite One-Dimensional Correlated--Disordered Binary Solids
Plamen Ch. Ivanov (Boston University and Harvard Medical School, Boston, USA), Pedro Carpena, Pedro Bernaola--Galvan (E.T.S.I. de Telecomunicacion, Universidad de Malaga, Spain)
We report a strong electronic delocalization when long-range power-law correlations are introduced in 1-D disordered binary solids. We perform numerical simulations using a 1-D tight-binding model, and we find that the long-range correlations in the system lead to a considerable extension of the localization length for a large fraction of the electrons in the system within a broad energy band. We discuss the possible relevance of this result for DNA, which displays long-range correlations and was recently reported to be a 1-D disordered conductor.