608 research outputs found
On the structure of the energy distribution function in the hopping regime
The impact of the dispersion of the transport coefficients on the structure
of the energy distribution function for charge carriers far from equilibrium
has been investigated in effective-medium approximation for model densities of
states. The investigations show that two regimes can be observed in energy
relaxation processes. Below a characteristic temperature the structure of the
energy distribution function is determined by the dispersion of the transport
coefficients. Thermal energy diffusion is irrelevant in this regime. Above the
characteristic temperature the structure of the energy distribution function is
determined by energy diffusion. The characteristic temperature depends on the
degree of disorder and increases with increasing disorder. Explicit expressions
for the energy distribution function in both regimes are derived for a constant
and an exponential density of states.Comment: 16 page
Taxonomic diversity and identification problems of oncaeid microcopepods in the Mediterranean Sea
The species diversity of the pelagic microcopepod
family Oncaeidae collected with nets of 0.1-mm mesh
size was studied at 6 stations along a west-to-east transect
in the Mediterranean Sea down to a maximum depth of
1,000 m. A total of 27 species and two form variants have
been identified, including three new records for the
Mediterranean. In addition, about 20, as yet undescribed,
new morphospecies were found (mainly from the genera
Epicalymma and Triconia) which need to be examined
further. The total number of identified oncaeid species was
similar in the Western and Eastern Basins, but for some cooccurring
sibling species, the estimated numerical dominance
changed. The deep-sea fauna of Oncaeidae, studied
at selected depth layers between 400 m and the near-bottom
layer at >4,200 m depth in the eastern Mediterranean
(Levantine Sea), showed rather constant species numbers
down to ∼3,000 m depth. In the near-bottom layers, the
diversity of oncaeids declined and species of Epicalymma
strongly increased in numerical importance. The taxonomic
status of all oncaeid species recorded earlier in the
Mediterranean Sea is evaluated: 19 out of the 46 known
valid oncaeid species are insufficiently described, and most
of the taxonomically unresolved species (13 species) have
originally been described from this area (type locality). The
deficiencies in the species identification of oncaeids cast
into doubt the allegedly cosmopolitan distribution of some
species, in particular those of Mediterranean origin. The
existing identification problems even of well-described
oncaeid species are exemplified for the Oncaea mediacomplex,
including O. media Giesbrecht, O. scottodicarloi
Heron & Bradford-Grieve, and O. waldemari Bersano &
Boxshall, which are often erroneously identified as a single
species (O. media). The inadequacy in the species identification
of Oncaeidae, in particular those from the Atlantic
and Mediterranean, is mainly due to the lack of reliable
identification keys for Oncaeidae in warm-temperate and/or
tropical seas. Future efforts should be directed to the
construction of identification keys that can be updated
according to the latest taxonomic findings, which can be
used by the non-expert as well as by the specialist. The
adequate consideration of the numerous, as yet undescribed,
microcopepod species in the world oceans, in
particular the Oncaeidae, is a challenge for the study of the
structure and function of plankton communities as well as
for global biodiversity estimates
Optical absorption and activated transport in polaronic systems
We present exact results for the optical response in the one-dimensional
Holstein model. In particular, by means of a refined kernel polynomial method,
we calculate the ac and dc electrical conductivities at finite temperatures for
a wide parameter range of electron phonon interaction. We analyze the
deviations from the results of standard small polaron theory in the
intermediate coupling regime and discuss non-adiabaticity effects in detail.Comment: 7 pages, 8 figure
Polarons and slow quantum phonons
We describe the formation and properties of Holstein polarons in the entire
parameter regime. Our presentation focuses on the polaron mass and radius,
which we obtain with an improved numerical technique. It is based on the
combination of variational exact diagonalization with an improved construction
of phonon states, providing results even for the strong coupling adiabatic
regime. In particular we can describe the formation of large and heavy
adiabatic polarons. A comparison of the polaron mass for the one and three
dimensional situation explains how the different properties in the static
oscillator limit determine the behavior in the adiabatic regime. The transport
properties of large and small polarons are characterized by the f-sum rule and
the optical conductivity. Our calculations are approximation-free and have
negligible numerical error. This allows us to give a conclusive and impartial
description of polaron formation. We finally discuss the implications of our
results for situations beyond the Holstein model.Comment: Final version, 10 pages, 10 figure
Scaling of THz-conductivity at metal-insulator transition in doped manganites
Magnetic field and temperature dependence of the Terahertz conductivity and
permittivity of the colossal magnetoresistance manganite
Pr_{0.65}Ca_{0.28}Sr_{0.07}MnO_3 (PCSMO) is investigated approaching the
metal-to-insulator transition (MIT) from the insulating side. In the
charge-ordered state of PCSMO both conductivity and dielectric permittivity
increase as function of magnetic field and temperature. Universal scaling
relationships between the changes in permittivity and conductivity are observed
in a broad range of temperatures and magnetic fields. Similar scaling is also
seen in La_{1-x}Sr_xMnO_3 for different doping levels. The observed
proportionality points towards the importance of pure ac-conductivity and
phononic energy scale at MIT in manganites.Comment: 5 pages, 4 figure
Quantum creep and variable range hopping of one-dimensional interacting electrons
The variable range hopping results for noninteracting electrons of Mott and
Shklovskii are generalized to 1D disordered charge density waves and Luttinger
liquids using an instanton approach. Following a recent paper by Nattermann,
Giamarchi and Le Doussal [Phys. Rev. Lett. {\bf 91}, 56603 (2003)] we calculate
the quantum creep of charges at zero temperature and the linear conductivity at
finite temperatures for these systems. The hopping conductivity for the short
range interacting electrons acquires the same form as for noninteracting
particles if the one-particle density of states is replaced by the
compressibility. In the present paper we extend the calculation to dissipative
systems and give a discussion of the physics after the particles materialize
behind the tunneling barrier. It turns out that dissipation is crucial for
tunneling to happen. Contrary to pure systems the new metastable state does not
propagate through the system but is restricted to a region of the size of the
tunneling region. This corresponds to the hopping of an integer number of
charges over a finite distance. A global current results only if tunneling
events fill the whole sample. We argue that rare events of extra low tunneling
probability are not relevant for realistic systems of finite length. Finally we
show that an additional Coulomb interaction only leads to small logarithmic
corrections.Comment: 15 pages, 3 figures; references adde
Hopping Conductivity of a Nearly-1d Fractal: a Model for Conducting Polymers
We suggest treating a conducting network of oriented polymer chains as an
anisotropic fractal whose dimensionality D=1+\epsilon is close to one.
Percolation on such a fractal is studied within the real space renormalization
group of Migdal and Kadanoff. We find that the threshold value and all the
critical exponents are strongly nonanalytic functions of \epsilon as \epsilon
tends to zero, e.g., the critical exponent of conductivity is \epsilon^{-2}\exp
(-1-1/\epsilon). The distribution function for conductivity of finite samples
at the percolation threshold is established. It is shown that the central body
of the distribution is given by a universal scaling function and only the
low-conductivity tail of distribution remains -dependent. Variable
range hopping conductivity in the polymer network is studied: both DC
conductivity and AC conductivity in the multiple hopping regime are found to
obey a quasi-1d Mott law. The present results are consistent with electrical
properties of poorly conducting polymers.Comment: 27 pages, RevTeX, epsf, 5 .eps figures, to be published in Phys. Rev.
Effect of electron-phonon interaction range on lattice polaron dynamics: a continuous-time quantum Monte Carlo study
We present the numerically exact ground state energy, effective mass, and
isotope exponents of a one-dimensional lattice polaron, valid for any range of
electron-phonon interaction, applying a new continuous-time Quantum Monte Carlo
(QMC) technique in a wide range of coupling strength and adiabatic ratio. The
QMC method is free from any systematic finite-size and finite-time-step errors.
We compare our numerically exact results with analytical weak-coupling theory
and with the strong-coupling expansion. We show that the exact
results agree well with the canonical Fr\"ohlich and Holstein-Lang-Firsov
theories in the weak and strong coupling limits, respectively, for any range of
interaction. We find a strong dependence of the polaron dynamics on the range
of interaction. An increased range of interaction has a similar effect to an
increased (less adiabatic) phonon frequency: specifically, a reduction in the
effective mass.Comment: 27 pages, 16 figures, to appear Phys Rev B. Introduction rewritten,
comparison with other authors extended, description of method shortened,
improved treatment of weak coupling theor
The Essential Interactions in Oxides and Spectral Weight Transfer in Doped Manganites
We calculate the value of the Fr\"ohlich electron-phonon interaction in
manganites, cuprates, and some other charge-transfer insulators and show that
this interaction is much stronger than any relevant magnetic interaction. A
polaron shift due to the Fr\"ohlich interaction, which is about 1 eV, suggests
that carriers in those systems are small (bi)polarons at all temperatures and
doping levels, in agreement with the oxygen isotope effect and other data. An
opposite conclusion, recently suggested in the literature, is shown to be
incorrect. The frequency and temperature dependence of the optical conductivity
of ferromagnetic manganites is explained within the framework of the bipolaron
theory.Comment: 6 pages, REVTeX 3.1 with 3 eps-figures. Journal versio
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