245 research outputs found
Hall resistivity of granular metals
We calculate the Hall conductivity \sig_{xy} and resistivity of
a granular system at large tunneling conductance . We show that in
the absence of Coulomb interaction the Hall resistivity depends neither on the
tunneling conductance nor on the intragrain disorder and is given by the
classical formula , where differs from the carrier
density inside the grains by a numerical coefficient determined by the
shape of the grains. The Coulomb interaction gives rise to logarithmic in
temperature correction to in the range \Ga \lesssim T \lesssim
\min(g_T E_c,\ETh), where \Ga is the tunneling escape rate, is the
charging energy and \ETh is the Thouless energy of the grain.Comment: 4 pages, 1 figur
Electron screening and excitonic condensation in double-layer graphene systems
We theoretically investigate the possibility of excitonic condensation in a
system of two graphene monolayers separated by an insulator, in which electrons
and holes in the layers are induced by external gates. In contrast to the
recent studies of this system, we take into account the screening of the
interlayer Coulomb interaction by the carriers in the layers, and this
drastically changes the result. Due to a large number of electron species in
the system (two projections of spin, two valleys, and two layers) and to the
suppression of backscattering in graphene, the maximum possible strength of the
screened Coulomb interaction appears to be quite small making the weak-coupling
treatment applicable. We calculate the mean-field transition temperature for a
clean system and demonstrate that its highest possible value
is extremely small
( is the Fermi energy). In addition, any sufficiently short-range
disorder with the scattering time would
suppress the condensate completely. Our findings renders experimental
observation of excitonic condensation in the above setup improbable even at
very low temperatures.Comment: 4+ pages, 3 figure
Enhancement of Superconductivity in Disordered Films by Parallel Magnetic Field
We show that the superconducting transition temperature T_c(H) of a very thin
highly disordered film with strong spin-orbital scattering can be increased by
parallel magnetic field H. This effect is due to polarization of magnetic
impurity spins which reduces the full exchange scattering rate of electrons;
the largest effect is predicted for spin-1/2 impurities. Moreover, for some
range of magnetic impurity concentrations the phenomenon of {\it
superconductivity induced by magnetic field} is predicted: superconducting
transition temperature T_c(H) is found to be nonzero in the range of magnetic
fields .Comment: 4 pages, 2 figure
Hall Transport in Granular Metals and Effects of Coulomb Interactions
We present a theory of Hall effect in granular systems at large tunneling
conductance . Hall transport is essentially determined by the
intragrain electron dynamics, which, as we find using the Kubo formula and
diagrammatic technique, can be described by nonzero diffusion modes inside the
grains. We show that in the absence of Coulomb interaction the Hall resistivity
depends neither on the tunneling conductance nor on the intragrain
disorder and is given by the classical formula , where
differs from the carrier density inside the grains by a numerical
coefficient determined by the shape of the grains and type of granular lattice.
Further, we study the effects of Coulomb interactions by calculating
first-order in corrections and find that (i) in a wide range of
temperatures T \gtrsim \Ga exceeding the tunneling escape rate \Ga, the
Hall resistivity and conductivity \sig_{xy} acquire logarithmic
in corrections, which are of local origin and absent in homogeneously
disordered metals; (ii) large-scale ``Altshuler-Aronov'' correction to
\sig_{xy}, relevant at T\ll\Ga, vanishes in agreement with the theory of
homogeneously disordered metals.Comment: 29 pages, 16 figure
Bose-Einstein condensation of quasiparticles in graphene
The collective properties of different quasiparticles in various graphene
based structures in high magnetic field have been studied. We predict
Bose-Einstein condensation (BEC) and superfluidity of 2D spatially indirect
magnetoexcitons in two-layer graphene. The superfluid density and the
temperature of the Kosterlitz-Thouless phase transition are shown to be
increasing functions of the excitonic density but decreasing functions of
magnetic field and the interlayer separation. The instability of the ground
state of the interacting 2D indirect magnetoexcitons in a slab of superlattice
with alternating electron and hole graphene layers (GLs) is established. The
stable system of indirect 2D magnetobiexcitons, consisting of pair of indirect
excitons with opposite dipole moments, is considered in graphene superlattice.
The superfluid density and the temperature of the Kosterlitz-Thouless phase
transition for magnetobiexcitons in graphene superlattice are obtained.
Besides, the BEC of excitonic polaritons in GL embedded in a semiconductor
microcavity in high magnetic field is predicted. While superfluid phase in this
magnetoexciton polariton system is absent due to vanishing of
magnetoexciton-magnetoexciton interaction in a single layer in the limit of
high magnetic field, the critical temperature of BEC formation is calculated.
The essential property of magnetoexcitonic systems based on graphene (in
contrast, e.g., to a quantum well) is stronger influence of magnetic field and
weaker influence of disorder. Observation of the BEC and superfluidity of 2D
quasiparticles in graphene in high magnetic field would be interesting
confirmation of the phenomena we have described.Comment: 13 pages, 5 figure
Neutron lifetime measurements using gravitationally trapped ultracold neutrons
Our experiment using gravitationally trapped ultracold neutrons (UCN) to
measure the neutron lifetime is reviewed. Ultracold neutrons were trapped in a
material bottle covered with perfluoropolyether. The neutron lifetime was
deduced from comparison of UCN losses in the traps with different
surface-to-volume ratios. The precise value of the neutron lifetime is of
fundamental importance to particle physics and cosmology. In this experiment,
the UCN storage time is brought closer to the neutron lifetime than in any
experiments before:the probability of UCN losses from the trap was only 1% of
that for neutron beta decay. The neutron lifetime
obtained,878.5+/-0.7stat+/-0.3sys s, is the most accurate experimental
measurement to date.Comment: 38 pages, 19 figures,changed conten
Excitonic condensation in a double-layer graphene system
The possibility of excitonic condensation in a recently proposed electrically
biased double-layer graphene system is studied theoretically. The main emphasis
is put on obtaining a reliable analytical estimate for the transition
temperature into the excitonic state. As in a double-layer graphene system the
total number of fermionic "flavors" is equal to N=8 due to two projections of
spin, two valleys, and two layers, the large- approximation appears to be
especially suitable for theoretical investigation of the system. On the other
hand, the large number of flavors makes screening of the bare Coulomb
interactions very efficient, which, together with the suppression of
backscattering in graphene, leads to an extremely low energy of the excitonic
condensation. It is shown that the effect of screening on the excitonic pairing
is just as strong in the excitonic state as it is in the normal state. As a
result, the value of the excitonic gap \De is found to be in full agreement
with the previously obtained estimate for the mean-field transition temperature
, the maximum possible value ( is the Fermi energy) of both being in
range for a perfectly clean system. This proves that the energy scale really sets the upper bound for the transition temperature
and invalidates the recently expressed conjecture about the high-temperature
first-order transition into the excitonic state. These findings suggest that,
unfortunately, the excitonic condensation in graphene double-layers can hardly
be realized experimentally.Comment: 21 pages, 5 figures, invited paper to Graphene special issue in
Semiconductor Science and Technolog
Excitonic condensation in a double-layer graphene system
The possibility of excitonic condensation in a recently proposed electrically
biased double-layer graphene system is studied theoretically. The main emphasis
is put on obtaining a reliable analytical estimate for the transition
temperature into the excitonic state. As in a double-layer graphene system the
total number of fermionic "flavors" is equal to N=8 due to two projections of
spin, two valleys, and two layers, the large- approximation appears to be
especially suitable for theoretical investigation of the system. On the other
hand, the large number of flavors makes screening of the bare Coulomb
interactions very efficient, which, together with the suppression of
backscattering in graphene, leads to an extremely low energy of the excitonic
condensation. It is shown that the effect of screening on the excitonic pairing
is just as strong in the excitonic state as it is in the normal state. As a
result, the value of the excitonic gap \De is found to be in full agreement
with the previously obtained estimate for the mean-field transition temperature
, the maximum possible value ( is the Fermi energy) of both being in
range for a perfectly clean system. This proves that the energy scale really sets the upper bound for the transition temperature
and invalidates the recently expressed conjecture about the high-temperature
first-order transition into the excitonic state. These findings suggest that,
unfortunately, the excitonic condensation in graphene double-layers can hardly
be realized experimentally.Comment: 21 pages, 5 figures, invited paper to Graphene special issue in
Semiconductor Science and Technolog
Excitonic condensation in a double-layer graphene system
The possibility of excitonic condensation in a recently proposed electrically
biased double-layer graphene system is studied theoretically. The main emphasis
is put on obtaining a reliable analytical estimate for the transition
temperature into the excitonic state. As in a double-layer graphene system the
total number of fermionic "flavors" is equal to N=8 due to two projections of
spin, two valleys, and two layers, the large- approximation appears to be
especially suitable for theoretical investigation of the system. On the other
hand, the large number of flavors makes screening of the bare Coulomb
interactions very efficient, which, together with the suppression of
backscattering in graphene, leads to an extremely low energy of the excitonic
condensation. It is shown that the effect of screening on the excitonic pairing
is just as strong in the excitonic state as it is in the normal state. As a
result, the value of the excitonic gap \De is found to be in full agreement
with the previously obtained estimate for the mean-field transition temperature
, the maximum possible value ( is the Fermi energy) of both being in
range for a perfectly clean system. This proves that the energy scale really sets the upper bound for the transition temperature
and invalidates the recently expressed conjecture about the high-temperature
first-order transition into the excitonic state. These findings suggest that,
unfortunately, the excitonic condensation in graphene double-layers can hardly
be realized experimentally.Comment: 21 pages, 5 figures, invited paper to Graphene special issue in
Semiconductor Science and Technolog
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