771 research outputs found
Hearing Characteristics and Doppler Shift Compensation in South Indian CF-FM Bats
1. Echolocation pulses, Doppler shift compensation behaviour under laboratory conditions and frequency response characteristics of hearing were recorded inRhinolophus rouxi, Hipposideros speoris andHipposideros bicolor.
2. The frequencies of the constant frequency portions of the CF-FM pulses lie at about 82.8 kHz forR. rouxi from Mahabaleshwar, at 85.2 kHz forR. rouxi from Mysore. Hipposiderid bats have considerably higher frequencies at 135 kHz inH. speoris and 154.5 kHz inH. bicolor. The mean sound durations were 50 ms, 6.4 ms and 4.7 ms, respectively.
3. R. rouxi compensates for Doppler shifts in a range up to typically 4 kHz of positive Doppler shifts (Fig. 2). The Doppler shift compensation behaviour is almost identical to that ofR. ferrumequinum.
4. H. speoris andH. bicolor do not compensate for Doppler shifts under laboratory conditions. Doppler shifts in the echoes induce emission frequency changes which are not correlated to the presented Doppler shifts (Fig. 3).
5. The frequency response characteristics of hearing ofR. rouxi show characteristic sensitivity changes near the bat's reference frequency as also found inR. ferrumequinum. The threshold differences between the low threshold at the reference frequency and a few hundred Hz below are 40 to 50 dB in awake bats (Fig. 5).
6. Frequency sensitivity changes near the emitted CF-frequency of the bats are less pronounced inH. speoris or almost absent inH. bicolor
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
Audition in vampire bats, Desmodus rotundus
1. Within the tonotopic organization of the inferior colliculus two frequency ranges are well represented: a frequency range within that of the echolocation signals from 50 to 100 kHz, and a frequency band below that of the echolocation sounds, from 10 to 35 kHz. The frequency range between these two bands, from about 40 to 50 kHz is distinctly underrepresented (Fig. 3B).
2. Units with BFs in the lower frequency range (10–25 kHz) were most sensitive with thresholds of -5 to -11 dB SPL, and units with BFs within the frequency range of the echolocation signals had minimal thresholds around 0 dB SPL (Fig. 1).
3. In the medial part of the rostral inferior colliculus units were encountered which preferentially or exclusively responded to noise stimuli. — Seven neurons were found which were only excited by human breathing noises and not by pure tones, frequency modulated signals or various noise bands. These neurons were considered as a subspeciality of the larger sample of noise-sensitive neurons. — The maximal auditory sensitivity in the frequency range below that of echolocation, and the conspicuous existence of noise and breathing-noise sensitive units in the inferior colliculus are discussed in context with the foraging behavior of vampire bats
Mechanical and dielectric relaxation spectra in seven highly viscous glass formers
Published dielectric and shear data of six molecular glass formers and one
polymer are evaluated in terms of a spectrum of thermally activated processes,
with the same barrier density for the retardation spectrum of shear and
dielectrics. The viscosity, an independent parameter of the fit, seems to be
related to the high-barrier cutoff time of the dielectric signal, in accordance
with the idea of a renewal of the relaxing entities after this critical time.
In the five cases where one can fit accurately, the temperature dependence of
the high-barrier cutoff follows the shoving model. The Johari-Goldstein peaks,
seen in four of our seven cases, are describable in terms of gaussians in the
barrier density, superimposed on the high-frequency tail of the
-process. Dielectric and shear measurements of the same substance find
the same peak positions and widths of these gaussians, but in general a
different weight.Comment: Contribution to the Ngai Fest issue of J. Non-Cryst. Solids; 8 pages,
8 figures, 30 reference
Semiclassical time evolution of the density matrix and tunneling
The time dependent density matrix of a system with potential barrier is
studied using path integrals. The characterization of the initial state, which
is assumed to be restricted to one side of the barrier, and the time evolution
of the density matrix lead to a three-fold path integral which is evaluated in
the semiclassical limit. The semiclassical trajectories are found to move in
the complex coordinate plane and barrier penetration only arises due to
fluctuations. Both the form of the semiclassical paths and the relevant
fluctuations change significantly as a function of temperature. The
semiclassical analysis leads to a detailed picture of barrier penetration in
the real time domain and the changeover from thermal activation to quantum
tunneling. Deep tunneling is associated with quasi-zero modes in the
fluctuation spectrum about the semiclassical orbits in the long time limit. The
connection between this real time description of tunneling and the standard
imaginary time instanton approach is established. Specific results are given
for a double well potential and an Eckart barrier.Comment: 27 pages, 8 figures, to be published in Phys. Rev.
Dissipative Quantum Systems with Potential Barrier. General Theory and Parabolic Barrier
We study the real time dynamics of a quantum system with potential barrier
coupled to a heat-bath environment. Employing the path integral approach an
evolution equation for the time dependent density matrix is derived. The time
evolution is evaluated explicitly near the barrier top in the temperature
region where quantum effects become important. It is shown that there exists a
quasi-stationary state with a constant flux across the potential barrier. This
state generalizes the Kramers flux solution of the classical Fokker-Planck
equation to the quantum regime. In the temperature range explored the quantum
flux state depends only on the parabolic approximation of the anharmonic
barrier potential near the top. The parameter range within which the solution
is valid is investigated in detail. In particular, by matching the flux state
onto the equilibrium state on one side of the barrier we gain a condition on
the minimal damping strength. For very high temperatures this condition reduces
to a known result from classical rate theory. Within the specified parameter
range the decay rate out of a metastable state is calculated from the flux
solution. The rate is shown to coincide with the result of purely thermodynamic
methods. The real time approach presented can be extended to lower temperatures
and smaller damping.Comment: 29 pages + 1 figure as compressed ps-file (uufiles) to appear in
Phys. Rev.
Особенности и закономерности изменения восстановлености углей башкирского яруса Западного Донбасса
В статье приведена детальная петрографическая характеристика углей башкирского яруса Западного Донбасса. Проведена классификация по восстановлености в соответствии с петрографическими типами. Установлены стратиграфические и площадные закономерности изменения степени восстановлености.У статті наведена детальна петрографічна характеристика вугілля башкирського ярусу Західного Донбасу. Проведена класифікація відновленості, згідно з петрографічними типами. Встановлені стратиграфічні та просторові закономірності зміни ступеню відновленості.The article gives detailed petrographic characteristics of coal of Bashkirian formation of Western Donbas. The classifications for recovery in accordance with petrographic types are given. The stratigraphic and areal patterns of change in the degree of recovery are established
Radiative corrections to the excitonic molecule state in GaAs microcavities
The optical properties of excitonic molecules (XXs) in GaAs-based quantum
well microcavities (MCs) are studied, both theoretically and experimentally. We
show that the radiative corrections to the XX state, the Lamb shift
and radiative width , are
large, about of the molecule binding energy , and
definitely cannot be neglected. The optics of excitonic molecules is dominated
by the in-plane resonant dissociation of the molecules into outgoing
1-mode and 0-mode cavity polaritons. The later decay channel,
``excitonic molecule 0-mode polariton + 0-mode
polariton'', deals with the short-wavelength MC polaritons invisible in
standard optical experiments, i.e., refers to ``hidden'' optics of
microcavities. By using transient four-wave mixing and pump-probe
spectroscopies, we infer that the radiative width, associated with excitonic
molecules of the binding energy meV, is
meV in the microcavities and
meV in a reference GaAs single quantum
well (QW). We show that for our high-quality quasi-two-dimensional
nanostructures the limit, relevant to the XX states, holds at
temperatures below 10 K, and that the bipolariton model of excitonic molecules
explains quantitatively and self-consistently the measured XX radiative widths.
We also find and characterize two critical points in the dependence of the
radiative corrections against the microcavity detuning, and propose to use the
critical points for high-precision measurements of the molecule bindingenergy
and microcavity Rabi splitting.Comment: 16 pages, 11 figures, accepted for publication in Phys. Rev.
Energy landscape - a key concept for the dynamics of glasses and liquids
There is a growing belief that the mode coupling theory is the proper
microscopic theory for the dynamics of the undercooled liquid above a critical
temperature T_c. In addition, there is some evidence that the system leaves the
saddlepoints of the energy landscape to settle in the valleys at this critical
temperature. Finally, there is a microscopic theory for the entropy at the
calorimetric glass transition T_g by Mezard and Parisi, which allows to
calculate the Kauzmann temperature from the atomic pair potentials.
The dynamics of the frozen glass phase is at present limited to
phenomenological models. In the spirit of the energy landscape concept, one
considers an ensemble of independent asymmetric double-well potentials with a
wide distribution of barrier heights and asymmetries (ADWP or Gilroy-Phillips
model). The model gives an excellent description of the relaxation of glasses
up to about T_g/4. Above this temperature, the interaction between different
relaxation centers begins to play a role. One can show that the interaction
reduces the number of relaxation centers needed to bring the shear modulus down
to zero by a factor of three.Comment: Contribution to the III Workshop on Nonequilibrium Phenomena in
Supercooled Fluids, Glasses and Amorphous Materials, 22-27 September 2002,
Pisa; 14 pages, 3 figures; Version 3 takes criticque at Pisa into account;
final version 4 will be published in J.Phys.: Condens.Matte
Influence of parallel magnetic fields on a single-layer two-dimensional electron system with a hopping mechanism of conductivity
Large positive (P) magnetoresistance (MR) has been observed in parallel
magnetic fields in a single 2D layer in a delta-doped GaAs/AlGaAs
heterostructure with a variable-range-hopping (VRH) mechanism of conductivity.
Effect of large PMR is accompanied in strong magnetic fields by a substantial
change in the character of the temperature dependence of the conductivity. This
implies that spins play an important role in 2D VRH conductivity because the
processes of orbital origin are not relevant to the observed effect. A possible
explanation involves hopping via double occupied states in the upper Hubbard
band, where the intra-state correlation of spins is important.Comment: 10 pages, 4 jpeg figure
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