1,573 research outputs found
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
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
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
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
Tunneling dynamics of side chains and defects in proteins, polymer glasses, and OH-doped network glasses
Simulations on a Lennard-Jones computer glass are performed to study effects
arising from defects in glasses at low temperatures. The numerical analysis
reveals that already a low concentration of defects may dramatically change the
low temperature properties by giving rise to extrinsic double-well potentials
(DWP's). The main characteristics of these extrinsic DWP's are (i) high barrier
heights, (ii) high probability that a defect is indeed connected with an
extrinsic DWP, (iii) highly localized dynamics around this defect, and (iv)
smaller deformation potential coupling to phonons. Designing an extension of
the Standard Tunneling Model (STM) which parametrizes this picture and
comparing with ultrasound experiments on the wet network glass -BO
shows that effects of OH-impurities are accurately accounted for. This model is
then applied to organic polymer glasses and proteins. It is suggested that side
groups may act similarly like doped impurities inasmuch as extrinsic DWP's are
induced, which possess a distribution of barriers peaked around a high barrier
height. This compares with the structurlessly distributed barrier heights of
the intrinsic DWP's, which are associated with the backbone dynamics. It is
shown that this picture is consistent with elastic measurements on polymers,
and can explain anomalous nonlogarithmic line broadening recently observed in
hole burning experiments in PMMA.Comment: 34 pages, Revtex, 9 eps-figures, accepted for publication in J. Chem.
Phy
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
Vibrational instability, two-level systems and Boson peak in glasses
We show that the same physical mechanism is fundamental for two seemingly
different phenomena such as the formation of two-level systems in glasses and
the Boson peak in the reduced density of low-frequency vibrational states
g(w)/w^2. This mechanism is the vibrational instability of weakly interacting
harmonic modes. Below some frequency w_c << w_0 (where w_0 is of the order of
Debye frequency) the instability, controlled by the anharmonicity, creates a
new stable universal spectrum of harmonic vibrations with a Boson peak feature
as well as double-well potentials with a wide distribution of barrier heights.
Both are determined by the strength of the interaction I ~ w_c between the
oscillators. Our theory predicts in a natural way a small value for the
important dimensionless parameter C ~ 10^{-4} for two-level systems in glasses.
We show that C ~ I^{-3} and decreases with increasing of the interaction
strength I. We show that the number of active two-level systems is very small,
less than one per ten million of oscillators, in a good agreement with
experiment. Within the unified approach developed in the present paper the
density of the tunneling states and the density of vibrational states at the
Boson peak frequency are interrelated.Comment: 28 pages, 3 figure
Universality in Glassy Low-Temperature Physics
We propose a microscopic translationally invariant glass model which exhibits
two level tunneling systems with a broad range of asymmetries and barrier
heights in its glassy phase. Their distribution is qualitatively different from
what is commonly assumed in phenomenological models, in that symmetric
tunneling systems are systematically suppressed. Still, the model exhibits the
usual glassy low-temperature anomalies. Universality is due to the collective
origin of the glassy potential energy landscape. We obtain a simple explanation
also for the mysterious {\em quantitative} universality expressed in the
unusually narrow universal glassy range of values for the internal friction
plateau.Comment: 4 pages, 5 figures, uses RevTeX
Stability of Bose Einstein condensates of hot magnons in YIG
We investigate the stability of the recently discovered room temperature
Bose-Einstein condensate (BEC) of magnons in Ytrrium Iron Garnet (YIG) films.
We show that magnon-magnon interactions depend strongly on the external field
orientation, and that the BEC in current experiments is actually metastable -
it only survives because of finite size effects, and because the BEC density is
very low. On the other hand a strong field applied perpendicular to the sample
plane leads to a repulsive magnon-magnon interaction; we predict that a
high-density magnon BEC can then be formed in this perpendicular field
geometry.Comment: Submitted to Physical Review Letter
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