1,479 research outputs found
Damping and decoherence of a nanomechanical resonator due to a few two level systems
We consider a quantum model of a nanomechanical flexing beam resonator
interacting with a bath comprising a few damped tunneling two level systems
(TLS's). In contrast with a resonator interacting bilinearly with an ohmic free
oscillator bath (modeling clamping loss, for example), the mechanical resonator
damping is amplitude dependent, while the decoherence of quantum superpositions
of mechanical position states depends only weakly on their spatial separation
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 Interaction on the Relaxation in Amorphous Solids
Recently it has been experimentally demonstrated that certain glasses display
an unexpected magnetic field dependence of the dielectric constant. In
particular, the echo technique experiments have shown that the echo amplitude
depends on the magnetic field. The analysis of these experiments results in the
conclusion that the effect seems to be related to the nuclear degrees of
freedom of tunneling systems. The interactions of a nuclear quadrupole
electrical moment with the crystal field and of a nuclear magnetic moment with
magnetic field transform the two-level tunneling systems inherent in amorphous
dielectrics into many-level tunneling systems. The fact that these features
show up at temperatures , where the properties of amorphous materials
are governed by the long-range interaction between tunneling systems,
suggests that this interaction is responsible for the magnetic field dependent
relaxation. We have developed a theory of many-body relaxation in an ensemble
of interacting many-level tunneling systems and show that the relaxation rate
is controlled by the magnetic field. The results obtained correlate with the
available experimental data. Our approach strongly supports the idea that the
nuclear quadrupole interaction is just the key for understanding the unusual
behavior of glasses in a magnetic field.Comment: 18 pages, 9 figure
Low temperature dipolar echo in amorphous dielectrics: Significance of relaxation and decoherence free two level systems
The nature of dielectric echoes in amorphous solids at low temperatures is
investigated. It is shown that at long delay times the echo amplitude is
determined by a small subset of two level systems (TLS) having negligible
relaxation and decoherence because of their weak coupling to phonons. The echo
decay can then be described approximately by power law time dependencies with
different powers at times longer and shorter than the typical TLS relaxation
time. The theory is applied to recent measurements of two and three pulse
dipolar echo in borosilicate glass BK7 and provides a perfect data fit in the
broad time and temperature ranges under the assumption that there exist two TLS
relaxation mechanisms due to TLS-phonons and TLS-TLS interaction. This
interpretation is consistent with the previous experimental and theoretical
investigations. Further experiments verifying the theory predictions are
suggested.Comment: 10 pages, 8 figure
Low temperature acoustic properties of amorphous silica and the Tunneling Model
Internal friction and speed of sound of a-SiO(2) was measured above 6 mK
using a torsional oscillator at 90 kHz, controlling for thermal decoupling,
non-linear effects, and clamping losses. Strain amplitudes e(A) = 10^{-8} mark
the transition between the linear and non-linear regime. In the linear regime,
excellent agreement with the Tunneling Model was observed for both the internal
friction and speed of sound, with a cut-off energy of E(min) = 6.6 mK. In the
non-linear regime, two different behaviors were observed. Above 10 mK the
behavior was typical for non-linear harmonic oscillators, while below 10 mK a
different behavior was found. Its origin is not understood.Comment: 1 tex file, 6 figure
Turbulence and jet-driven zonal flows: Secondary circulation in rotating fluids due to asymmetric forcing
We report on experiments and modeling on a rotating confined liquid that is forced by circumferential jets coaxial with the rotation axis, wherein system-scale secondary flows are observed to emerge. The jets are evenly divided in number between inlets and outlets and have zero net mass transport. For low forcing strengths the sign of this flow depends on the sign of a sloped end cap, which simulates a planetary β plane. For increased forcing strengths the secondary flow direction is insensitive to the slope sign, and instead appears to be dominated by an asymmetry in the forcing mechanism, namely, the difference in radial divergence between the inlet and outlet jet profiles. This asymmetry yields a net radial velocity that is affected by the Coriolis force, inducing secondary zonal flow
Optical absorption spectra and monomer interaction in polymers. Investigation of exciton coupling in DNA hairpins
We investigate the effect of exciton coupling on the optical absorption
spectrum of polymer molecules under conditions of strong inhomogeneous
broadening. We demonstrate that the dependence of the maximum in the rescaled
absorption spectrum on the number of monomers is determined by the average
monomer excitation energies and their resonant coupling and insensitive to the
inhomogeneous broadening. Thus the absorption spectrum can be used to determine
optical interactions between monomers. The results are applied to the
absorption spectra of poly-A poly-T DNA hairpins and used to interpret the
dependence of the absorption spectrum on the number of monomers. We also
discuss exciton localization in these hairpins.Comment: Submitted to Journal of Chemical Physic
Random Resonators and Prelocalized Modes in Disordered Dielectric Films
Areal density of disorder-induced resonators with a high quality factor,
, in a film with fluctuating refraction index is calculated
theoretically. We demonstrate that for a given , where is the light
wave vector, and is the transport mean free path, when {\em on average} the
light propagation is diffusive, the likelihood for finding a random resonator
increases dramatically with increasing the correlation radius of the disorder.
Parameters of {\em most probable} resonators as functions of and are
found.Comment: 6 pages including 2 figure
Quantum Statistical Physics of Glasses at Low Temperatures
We present a quantum statistical analysis of a microscopic mean-field model
of structural glasses at low temperatures. The model can be thought of as
arising from a random Born von Karman expansion of the full interaction
potential. The problem is reduced to a single-site theory formulated in terms
of an imaginary-time path integral using replicas to deal with the disorder. We
study the physical properties of the system in thermodynamic equilibrium and
develop both perturbative and non-perturbative methods to solve the model. The
perturbation theory is formulated as a loop expansion in terms of two-particle
irreducible diagrams, and is carried to three-loop order in the effective
action. The non-perturbative description is investigated in two ways, (i) using
a static approximation, and (ii) via Quantum Monte Carlo simulations. Results
for the Matsubara correlations at two-loop order perturbation theory are in
good agreement with those of the Quantum Monte Carlo simulations.
Characteristic low-temperature anomalies of the specific heat are reproduced,
both in the non-perturbative static approximation, and from a three-loop
perturbative evaluation of the free energy. In the latter case the result so
far relies on using Matsubara correlations at two-loop order in the three-loop
expressions for the free energy, as self-consistent Matsubara correlations at
three-loop order are still unavailable. We propose to justify this by the good
agreement of two-loop Matsubara correlations with those obtained
non-perturbatively via Quantum Monte Carlo simulations.Comment: 13 pages, 6 figure
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