137 research outputs found
Technique for Magnetic Susceptibility Determination in the High Doped Semiconductors by Electron Spin Resonance
Method for determining the magnetic susceptibility in the high doped
semiconductors is considered. A procedure that is based on double integration
of the positive part of the derivative of the absorption line having a Dyson
shape and takes into account the depth of the skin layer is described. Analysis
is made for the example of arsenic doped germanium samples at a rather high
concentration corresponding to the insulator metal phase transition.Comment: Pages 13, figures 9, references 1
Electrical properties of isotopically enriched neutron-transmutation-doped ^{70} Ge:Ga near the metal-insulator transition
We report the low temperature carrier transport properties of a series of
nominally uncompensated neutron-transmutation doped (NTD) ^{70} Ge:Ga samples
very close to the critical concentration N_c for the metal-insulator
transition. The concentration of the sample closest to N_c is 1.0004N_c and it
is unambiguously shown that the critical conductivity exponent is 0.5.
Properties of insulating samples are discussed in the context of Efros and
Shklovskii's variable range hopping conduction.Comment: 8 pages using REVTeX, 8 figures, published versio
Electronic structure and light-induced conductivity in a transparent refractory oxide
Combined first-principles and experimental investigations reveal the
underlying mechanism responsible for a drastic change of the conductivity (by
10 orders of magnitude) following hydrogen annealing and UV-irradiation in a
transparent oxide, 12CaO.7Al2O3, found by Hayashi et al. The charge transport
associated with photo-excitation of an electron from H, occurs by electron
hopping. We identify the atoms participating in the hops, determine the exact
paths for the carrier migration, estimate the temperature behavior of the
hopping transport and predict a way to enhance the conductivity by specific
doping.Comment: 4 pages including 4 figure
Anomalous Hopping Exponents of Ultrathin Films of Metals
The temperature dependence of the resistance R(T) of ultrathin
quench-condensed films of Ag, Bi, Pb and Pd has been investigated. In the most
resistive films, R(T)=Roexp(To/T)^x, where x=0.75. Surprisingly, the exponent x
was found to be constant for a wide range of Ro and To in all four materials,
possibly implying a consistent underlying conduction mechanism. The results are
discussed in terms of several different models of hopping conduction.Comment: 6 pages, 5 figure
Anderson Transitions
The physics of Anderson transitions between localized and metallic phases in
disordered systems is reviewed. The term ``Anderson transition'' is understood
in a broad sense, including both metal-insulator transitions and
quantum-Hall-type transitions between phases with localized states. The
emphasis is put on recent developments, which include: multifractality of
critical wave functions, criticality in the power-law random banded matrix
model, symmetry classification of disordered electronic systems, mechanisms of
criticality in quasi-one-dimensional and two-dimensional systems and survey of
corresponding critical theories, network models, and random Dirac Hamiltonians.
Analytical approaches are complemented by advanced numerical simulations.Comment: 63 pages, 39 figures, submitted to Rev. Mod. Phy
Correlated conformation and charge transport in multiwall carbon nanotube - conducting polymer nanocomposites
The strikingly different charge transport behaviors in nanocomposites of
multiwall carbon nanotubes (MWNTs) and conducting polymer polyethylene
dioxythiophene - polystyrene sulfonic acid (PEDOT-PSS) at low temperatures are
explained by probing their conformational properties using small angle X-ray
scattering (SAXS). The SAXS studies indicate assembly of elongated PEDOT-PSS
globules on the walls of nanotubes, coating them partially thereby limiting the
interaction between the nanotubes in the polymer matrix. This results in a
charge transport governed mainly by small polarons in the conducting polymer
despite the presence of metallic MWNTs. At T > 4 K, hopping of the charge
carriers following 1D-VRH is evident which also gives rise to a positive
magnetoresistance (MR) with an enhanced localization length (~ 5 nm) due to the
presence of MWNTs. However, at T < 4 K, the observation of an unconventional
positive temperature coefficient of resistivity (TCR) is attributed to small
polaron tunnelling. The exceptionally large negative MR observed in this
temperature regime is conjectured to be due to the presence of quasi-1D MWNTs
that can aid in lowering the tunnelling barrier across the nanotube - polymer
boundary resulting in large delocalization.Comment: Accepted J. Phys.: Condens. Matte
Universal Crossover between Efros-Shklovskii and Mott Variable-Range-Hopping Regimes
A universal scaling function, describing the crossover between the Mott and
the Efros-Shklovskii hopping regimes, is derived, using the percolation picture
of transport in strongly localized systems. This function is agrees very well
with experimental data. Quantitative comparison with experiment allows for the
possible determination of the role played by polarons in the transport.Comment: 7 pages + 1 figure, Revte
Wave function multifractality and dephasing at metal-insulator and quantum Hall transitions
We analyze the critical behavior of the dephasing rate induced by short-range
electron-electron interaction near an Anderson transition of metal-insulator or
quantum Hall type. The corresponding exponent characterizes the scaling of the
transition width with temperature. Assuming no spin degeneracy, the critical
behavior can be studied by performing the scaling analysis in the vicinity of
the non-interacting fixed point, since the latter is stable with respect to the
interaction. We combine an analytical treatment (that includes the
identification of operators responsible for dephasing in the formalism of the
non-linear sigma-model and the corresponding renormalization-group analysis in
dimensions) with numerical simulations on the Chalker-Coddington
network model of the quantum Hall transition. Finally, we discuss the current
understanding of the Coulomb interaction case and the available experimental
data.Comment: 33 pages, 7 figures, elsart styl
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