116 research outputs found
Dielectric Susceptibility and Heat Capacity of Ultra-Cold Glasses in Magnetic Field
Recent experiments demonstrated unexpected, even intriguing properties of
certain glassy materials in magnetic field at low temperatures. We have studied
the magnetic field dependence of the static dielectric susceptibility and the
heat capacity of glasses at low temperatures. We present a theory in which we
consider the coupling of the tunnelling motion to nuclear quadrupoles in order
to evaluate the static dielectric susceptibility. In the limit of weak magnetic
field we find the resonant part of the susceptibility increasing like
while for the large magnetic field it behaves as 1/B. In the same manner we
consider the coupling of the tunnelling motion to nuclear quadrupoles and
angular momentum of tunnelling particles in order to find the heat capacity.
Our results show the Schotky peak for the angular momentum part, and
dependence for nuclear quadrupoles part of heat capacity, respectively. We
discuss whether or not this approach can provide a suitable explanation for
such magnetic properties.Comment: 10 pages, 1 figur
Elastic response of [111]-tunneling impurities
We study the dynamic response of a [111] quantum impurity, such as lithium or
cyanide in alkali halides, with respect to an external field coupling to the
elastic quadrupole moment. Because of the particular level structure of a
eight-state system on a cubic site, the elastic response function shows a
biexponential relaxation feature and a van Vleck type contribution with a
resonance frequency that is twice the tunnel frequency . This
basically differs from the dielectric response that does not show relaxation.
Moreover, we show that the elastic response of a [111] impurity cannot be
reduced to that of a two-level system. In the experimental part, we report on
recent sound velocity and internal friction measurements on KCl doped with
cyanide at various concentrations. At low doping (45 ppm) we find the dynamics
of a single [111] impurity, whereas at higher concentrations (4700 ppm) the
elastic response rather indicates strongly correlated defects. Our theoretical
model provides a good description of the temperature dependence of
and at low doping, in particular the relaxation peaks, the absolute
values of the amplitude, and the resonant contributions. From our fits we
obtain the value of the elastic deformation potential eV.Comment: 19 pages, 5 figure
On the theory of resonant susceptibility of dielectric glasses in magnetic field
The anomalous magnetic field dependence of dielectric properties of
insulating glasses in the temperature interval is considered. In
this temperature range, the dielectric permittivity is defined by the resonant
contribution of tunneling systems. The external magnetic field regulates
nuclear spins of tunneling atoms. This regulation suppresses a nuclear
quadrupole interaction of these spins with lattice and, thus, affects the
dielectric response of tunneling systems. It is demonstrated that in the
absence of an external magnetic field the nuclear quadrupole interaction
results in the correction to the permittivity in the
temperature range of interest. An application of a magnetic field results in a
sharp increase of this correction approximately by a factor of two when the
Zeeman splitting approaches the order of . Further increase of the
magnetic field results in a relatively smooth decrease in the correction until
the Zeeman splitting approaches the temperature. This smooth dependence results
from tunneling accompanied by a change of the nuclear spin projection. As the
magnetic field surpasses the temperature, the correction vanishes. The results
obtained in this paper are compared with experiment. A new mechanism of the low
temperature nuclear spin-lattice relaxation in glasses is considered.Comment: 9 Pages, 5 Figures, To be submitted to the Physical Review B, please
send comment
Four-well tunneling states and elastic response of clathrates
We present resonant ultrasound elastic constant measurements of the clathrate
compounds Eu8Ga16Ge30 and Sr8Ga16Ge30. The elastic response of the Eu clathrate
provides clear evidence for the existence of a new type of four-well tunneling
states, described by two nearly degenerate four level systems (FLS). The FLS's
are closely linked with the fourfold split positions of Eu known from neutron
diffraction density profiles. Using a realistic potential we estimate the
tunneling frequencies and show that the energy gap between the two FLS's is of
the same order as the Einstein oscillator frequency. This explains why the
observed harmonic oscillator type specific heat is not modified by tunneling
states. In addition the quadrupolar interaction of FLS's with elastic strains
explains the pronounced depression observed in elastic constant measurements.
In the case of the Sr clathrate, we show that the shallow dip in the elastic
constant c44 is explained using the same type of quadrupolar interaction with a
soft Einstein mode instead of a FLS.Comment: 4 pages, 4 figures; accepted for publication in Physical Review
Letter
Interacting quantum rotors in oxygen-doped germanium
We investigate the interaction effect between oxygen impurities in
crystalline germanium on the basis of a quantum rotor model. The dipolar
interaction of nearby oxygen impurities engenders non-trivial low-lying
excitations, giving rise to anomalous behaviors for oxygen-doped germanium
(Ge:O) below a few degrees Kelvin. In particular, it is theoretically predicted
that Ge:O samples with oxygen-concentration of 10cm show (i)
power-law specific heats below 0.1 K, and (ii) a peculiar hump in dielectric
susceptibilities around 1 K. We present an interpretation for the power-law
specific heats, which is based on the picture of local double-well potentials
randomly distributed in Ge:O samples.Comment: 13 pages, 11 figures; to be published in Phys. Rev.
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
Cubic Defects: Comparing the Eight-State-System with its Two-Level-Approximation
Substitutional defects in a cubic symmetry (such as a lithium defect in a KCl
host crystal) can be modeled appropriately by an eight-state-system. Usually
this tunneling degree of freedom is approximated by a two-level-system. We
investigate the observable differences between the two models in three
contexts. First we show that the two models predict different relations between
the temperature dependence of specific heat and static susceptibility. Second
we demonstrate that in the presence of external forces (pressure and electric
field) the eight-state-system shows features that cannot be understood within
the framework of the two-level-approximation. In this context we propose an
experiment for measuring the parameter for tunneling along the face diagonal.
Finally we discuss the differences between the models appearing for strongly
coupled pairs. Geometric selection rules and particular forms of asymmetry lead
to clear differences between the two models.Comment: 19 pages, Latex, submitted to J. of Phys., some small supplement
Evidence for Magnetic Field Induced Changes of the Phase of Tunneling States: Spontaneous Echoes in (KBr)(KCN) in Magnetic Fields
Recently, it has been discovered that in contrast to expectations the
low-temperature dielectric properties of some multi-component glasses depend
strongly on magnetic fields. In particular, the low-frequency dielectric
susceptibility and the amplitude of coherent polarization echoes show striking
non-monotonic magnetic field dependencies. The low-temperature dielectric
response of these materials is governed by atomic tunneling systems. We now
have investigated the coherent properties of tunneling states in a crystalline
host in magnetic fields up to 230mT. Two-pulse echo experiments have been
performed on a KBr crystal containing about 7.5% CN. Like in glasses, but
perhaps even more surprising in the case of a crystalline system, we observe a
very strong magnetic field dependence of the echo amplitude. Moreover, for the
first time we have direct evidence that magnetic fields change the phase of
coherent tunneling systems in a well-defined way. We present the data and
discuss the possible origin of this intriguing effect.Comment: 4 pages, 3 figures, submitted to PR
Free energy of colloidal particles at the surface of sessile drops
The influence of finite system size on the free energy of a spherical
particle floating at the surface of a sessile droplet is studied both
analytically and numerically. In the special case that the contact angle at the
substrate equals a capillary analogue of the method of images is
applied in order to calculate small deformations of the droplet shape if an
external force is applied to the particle. The type of boundary conditions for
the droplet shape at the substrate determines the sign of the capillary
monopole associated with the image particle. Therefore, the free energy of the
particle, which is proportional to the interaction energy of the original
particle with its image, can be of either sign, too. The analytic solutions,
given by the Green's function of the capillary equation, are constructed such
that the condition of the forces acting on the droplet being balanced and of
the volume constraint are fulfilled. Besides the known phenomena of attraction
of a particle to a free contact line and repulsion from a pinned one, we
observe a local free energy minimum for the particle being located at the drop
apex or at an intermediate angle, respectively. This peculiarity can be traced
back to a non-monotonic behavior of the Green's function, which reflects the
interplay between the deformations of the droplet shape and the volume
constraint.Comment: 24 pages, 19 figure
Effective interactions of colloids on nematic films
The elastic and capillary interactions between a pair of colloidal particles
trapped on top of a nematic film are studied theoretically for large
separations . The elastic interaction is repulsive and of quadrupolar type,
varying as . For macroscopically thick films, the capillary interaction
is likewise repulsive and proportional to as a consequence of
mechanical isolation of the system comprised of the colloids and the interface.
A finite film thickness introduces a nonvanishing force on the system (exerted
by the substrate supporting the film) leading to logarithmically varying
capillary attractions. However, their strength turns out to be too small to be
of importance for the recently observed pattern formation of colloidal droplets
on nematic films.Comment: 13 pages, accepted by EPJ
- …