1,592 research outputs found
Universal dielectric loss in amorphous solids from simultaneous bias and microwave field
We derive the ac dielectric loss in glasses due to resonant processes created
by two-level systems and a swept electric field bias. It is shown that at
sufficiently large ac fields and bias sweep rates the nonequilibrium loss
tangent created by the two fields approaches a universal maximum determined by
the bare linear dielectric permittivity. In addition this nonequilibrium loss
tangent is derived for a range of bias sweep rates and ac amplitudes and show
that the loss tangent creates a predicted loss function that can be understood
in a Landau-Zener theory and which can be used to extract the TLS density,
dipole moment, and relaxation rate.Comment: To appear in Physical Review Letters, 4 pages, 3 figure
The effect of Aharanov-Bohm phase on the magnetic-field dependence of two-pulse echos in glasses at low temperatures
The anomalous response of glasses in the echo amplitude experiment is
explained in the presence of a magnetic field. We have considered the low
energy excitations in terms of an effective two level system. The effective
model is constructed on the flip-flop configuration of two interacting two
level systems. The magnetic field affects the tunneling amplitude through the
Aharanov-Bohm effect. The effective model has a lower scale of energy in
addition to the new distribution of tunneling parameters which depend on the
interaction. We are able to explain some features of echo amplitude versus a
magnetic field, namely, the dephasing effect at low magnetic fields, dependence
on the strength of the electric field, pulse separation effect and the
influence of temperature. However this model fails to explain the isotope
effects which essentially can be explained by the nuclear quadrupole moment. We
will finally discuss the features of our results.Comment: 8 pages, 7 figure
Memory effects in transport through a hopping insulator: Understanding two-dip experiments
We discuss memory effects in the conductance of hopping insulators due to
slow rearrangements of many-electron clusters leading to formation of polarons
close to the electron hopping sites. An abrupt change in the gate voltage and
corresponding shift of the chemical potential change populations of the hopping
sites, which then slowly relax due to rearrangements of the clusters. As a
result, the density of hopping states becomes time dependent on a scale
relevant to rearrangement of the structural defects leading to the excess time
dependent conductivity
Low temperature breakdown of coherent tunneling in amorphous solids induced by the nuclear quadrupole interaction
We consider the effect of the internal nuclear quadrupole interaction on
quantum tunneling in complex multi-atomic two-level systems. Two distinct
regimes of strong and weak interactions are found. The regimes depend on the
relationship between a characteristic energy of the nuclear quadrupole
interaction and a bare tunneling coupling strength
. When , the internal interaction is
negligible and tunneling remains coherent determined by . When
, coherent tunneling breaks down and an effective
tunneling amplitude decreases by an exponentially small overlap factor
between internal ground states of left and right wells of a
tunneling system. This affects thermal and kinetic properties of tunneling
systems at low temperatures . The theory is applied for
interpreting the anomalous behavior of the resonant dielectric susceptibility
in amorphous solids at low temperatures mK where the nuclear
quadrupole interaction breaks down coherent tunneling. We suggest the
experiments with external magnetic fields to test our predictions and to
clarify the internal structure of tunneling systems in amorphous solids.Comment: To appear in the Physical Review
Effect of nuclear quadrupole interactions on the dynamics of two-level systems in glasses
The standard tunneling model describes quite satisfactorily the thermal
properties of amorphous solids at temperatures in terms of an ensemble
of two-level systems possessing logarithmically uniform distribution over their
tunneling amplitudes and uniform distribution over their asymmetry energies. In
particular, this distribution explains the observable logarithmic temperature
dependence of the dielectric constant. Yet, experiments have shown that at
ultralow temperatures such a temperature behavior breaks down and the
dielectric constant becomes temperature independent (plateau effect). In this
letter we suggest an explanation of this behavior exploiting the effect of the
nuclear quadrupole interaction on tunneling. We show that below a temperature
corresponding to the characteristic energy of the nuclear quadrupole
interaction the effective tunneling amplitude is reduced by a small overlap
factor of the nuclear quadrupole ground states in the left and right potential
wells of the tunneling system. It is just this reduction that explains the
plateau effect . We predict that the application of a sufficiently large
magnetic field should restore the logarithmic dependence because of the
suppression of the nuclear quadrupole interaction.Comment: To appear in the Physical Review Letter
Influence of radiative interatomic collisions on an atom laser
We discuss the role of light absorption by pairs of atoms (radiative
collisions) in the context of a model for an atom laser. The model is applied
to the case of VSCPT cooling of metastable triplet helium. We show that,
because of radiative collisions, for positive detuning of the driving light
fields from an atomic resonance the operating conditions for the atom laser can
only be marginally met. It is shown that the system only behaves as an atom
laser if a very efficient sub-Doppler precooling mechanism is operative. In the
case of negative frequency detuning the requirements on this sub-Doppler
mechanism are less restricting, provided one avoids molecular resonances.Comment: 19 pages, 2 Postscript figure
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