112 research outputs found
Transient magnetoconductivity of photoexcited electrons
Transient magnetotransport of two-dimensional electrons with
partially-inverted distribution excited by an ultrashort optical pulse is
studied theoretically. The time-dependent photoconductivity is calculated for
GaAs-based quantum wells by taking into account the relaxation of electron
distribution caused by non-elastic electron-phonon interaction and the
retardation of the response due to momentum relaxation and due to a finite
capacitance of the sample. We predict large-amplitude transient oscillations of
the current density and Hall field (Hall oscillations) with frequencies
corresponding to magnetoplasmon range, which are initiated by the instability
owing to the absolute negative conductivity effect.Comment: 21 pages, 6 fig
Re-entrance and entanglement in the one-dimensional Bose-Hubbard model
Re-entrance is a novel feature where the phase boundaries of a system exhibit
a succession of transitions between two phases A and B, like A-B-A-B, when just
one parameter is varied monotonically. This type of re-entrance is displayed by
the 1D Bose Hubbard model between its Mott insulator (MI) and superfluid phase
as the hopping amplitude is increased from zero. Here we analyse this
counter-intuitive phenomenon directly in the thermodynamic limit by utilizing
the infinite time-evolving block decimation algorithm to variationally minimize
an infinite matrix product state (MPS) parameterized by a matrix size chi.
Exploiting the direct restriction on the half-chain entanglement imposed by
fixing chi, we determined that re-entrance in the MI lobes only emerges in this
approximate when chi >= 8. This entanglement threshold is found to be
coincident with the ability an infinite MPS to be simultaneously
particle-number symmetric and capture the kinetic energy carried by
particle-hole excitations above the MI. Focussing on the tip of the MI lobe we
then applied, for the first time, a general finite-entanglement scaling
analysis of the infinite order Kosterlitz-Thouless critical point located
there. By analysing chi's up to a very moderate chi = 70 we obtained an
estimate of the KT transition as t_KT = 0.30 +/- 0.01, demonstrating the how a
finite-entanglement approach can provide not only qualitative insight but also
quantitatively accurate predictions.Comment: 12 pages, 8 figure
Effect of radiation on transport in graphene
We study transport properties of graphene-based p-n junctions irradiated by
an electromagnetic field (EF). The resonant interaction of propagating
quasiparticles with an external monochromatic radiation opens dynamical gaps in
their spectrum, resulting in a strong modification of current-voltage
characteristics of the junctions. The values of the gaps are proportional to
the amplitude of EF. We find that the transmission of the quasiparticles in the
junctions is determined by the tunneling through the gaps, and can be fully
suppressed when applying a sufficiently large radiation power. However, EF can
not only suppress the current but also generate it. We demonstrate that if the
height of the potential barrier exceeds a half of the photon energy, the
directed current (photocurrent) flows through the junction without any dc bias
voltage applied. Such a photocurrent arises as a result of inelastic
quasiparticle tunneling assisted by one- or two-photon absorption. We calculate
current-voltage characteristics of diverse graphene based junctions and
estimate their parameters necessary for the experimental observation of the
photocurrent and transmission suppression.Comment: 21 pages, 15 figure
Formation of a "Cluster Molecule" (C20)2 and its thermal stability
The possible formation of a "cluster molecule" (C20)2 from two single C20
fullerenes is studied by the tight-binding method. Several (C20)2 isomers in
which C20 fullerenes are bound by strong covalent forces and retain their
identity are found; actually, these C20 fullerenes play the role of "atoms" in
the "cluster molecule". The so-called open-[2+2] isomer has a minimum energy.
Its formation path and thermal stability at T = 2000 - 4000 K are analyzed in
detail. This isomer loses its molecular structure due to either the decay of
one of C20 fullerenes or the coalescence of two C20 fullerenes into a C40
cluster. The energy barriers for the metastable open-[2+2] configuration are
calculated to be U = 2 - 5 eV.Comment: 21 pages, 8 figure
Decay and fusion as two different mechanisms of stability loss for the (C_20)_2 cluster dimer
The thermal stability of the (C_20)_2 cluster dimer consisting of two C_20
fullerenes is examined using a tight-binding approach. Molecular dynamics
simulations of the (C_20)_2 dimer at temperatures T = 2000 - 3500 K show that
the finite lifetime \tau of this metastable system is determined by two
fundamentally different processes, the decay of one of the C_20 fullerenes and
the fusion of two C_20 fullerenes into the C_40 cluster. The activation
energies for these processes Ea = 3.4 and 2.7 eV, respectively, as well as
their frequency factors, have been determined by analyzing the dependence of
\tau on T.Comment: Slightly modified version of the paper to appear in JETP Let
Metastable quasi-one-dimensional ensembles of nitrogen clusters N_8
By means of ab initio and tight-binding calculations it is shown that
metastable nitrogen clusters N_8 (boats) can form quasi-one-dimensional
ensembles in which the nearest clusters N_8 are bound to each other by covalent
bonds. Those ensembles are characterized by rather high energy barriers (~ 0.3
eV) that prevent the fission of the ensembles into isolated N_8 clusters and/or
N_2 molecules.Comment: 3 pages, 4 ps figure
From underdoped to overdoped cuprates: two quantum phase transitions
Several experimental and theoretical studies indicate the existence of a
critical point separating the underdoped and overdoped regions of the high-T_c
cuprates' phase diagram. There are at least two distinct proposals on the
critical concentration and its physical origin. First one is associated with
the pseudogap formation for p<p*, with p~0.2. Another one relies on the Hall
effect measurements and suggests that the critical point and the quantum phase
transition (QPT) take place at optimal doping, p_{opt}~0.16. Here we have
performed a precise density of states calculation and found that there are two
QPTs and the corresponding critical concentrations associated with the change
of the Fermi surface topology upon doping
Exact Quantum Monte Carlo Process for the Statistics of Discrete Systems
We introduce an exact Monte Carlo approach to the statistics of discrete
quantum systems which does not rely on the standard fragmentation of the
imaginary time, or any small parameter. The method deals with discrete objects,
kinks, representing virtual transitions at different moments of time. The
global statistics of kinks is reproduced by explicit local procedures, the key
one being based on the exact solution for the biased two-level system.Comment: 4 pages, latex, no figures, English translation of the paper
Prismane C_8: A New Form of Carbon?
Our numerical calculations on small carbon clusters point to the existence of
a metastable three-dimensional eight-atom cluster C which has a shape of a
six-atom triangular prism with two excess atoms above and below its bases. We
gave this cluster the name "prismane". The binding energy of the prismane
equals to 5.1 eV/atom, i.e., is 0.45 eV/atom lower than the binding energy of
the stable one-dimensional eight-atom cluster and 2.3 eV/atom lower than the
binding energy of the bulk graphite or diamond. Molecular dynamics simulations
give evidence for a rather high stability of the prismane, the activation
energy for a prismane decay being about 0.8 eV. The prismane lifetime increases
rapidly as the temperature decreases indicating a possibility of experimental
observation of this cluster.Comment: 5 pages (revtex), 3 figures (eps
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