201 research outputs found
Magnetic Field Dependent Tunneling in Glasses
We report on experiments giving evidence for quantum effects of
electromagnetic flux in barium alumosilicate glass. In contrast to expectation,
below 100 mK the dielectric response becomes sensitive to magnetic fields. The
experimental findings include both, the complete lifting of the dielectric
saturation by weak magnetic fields and oscillations of the dielectric response
in the low temperature resonant regime. As origin of these effects we suggest
that the magnetic induction field violates the time reversal invariance leading
to a flux periodicity in the energy levels of tunneling systems. At low
temperatures, this effect is strongly enhanced by the interaction between
tunneling systems and thus becomes measurable.Comment: 4 pages, 4 figure
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
Field-induced structural aging in glasses at ultra low temperatures
In non-equilibrium experiments on the glasses Mylar and BK7, we measured the
excess dielectric response after the temporary application of a strong electric
bias field at mK--temperatures. A model recently developed describes the
observed long time decays qualitatively for Mylar [PRL 90, 105501, S. Ludwig,
P. Nalbach, D. Rosenberg, D. Osheroff], but fails for BK7. In contrast, our
results on both samples can be described by including an additional mechanism
to the mentioned model with temperature independent decay times of the excess
dielectric response. As the origin of this novel process beyond the "tunneling
model" we suggest bias field induced structural rearrangements of "tunneling
states" that decay by quantum mechanical tunneling.Comment: 4 pages, 4 figures, accepted at PRL, corrected typos in version
On the identification of quasiprimary scaling operators in local scale-invariance
The relationship between physical observables defined in lattice models and
the associated (quasi-)primary scaling operators of the underlying field-theory
is revisited. In the context of local scale-invariance, we argue that this
relationship is only defined up to a time-dependent amplitude and derive the
corresponding generalizations of predictions for two-time response and
correlation functions. Applications to non-equilibrium critical dynamics of
several systems, with a fully disordered initial state and vanishing initial
magnetization, including the Glauber-Ising model, the Frederikson-Andersen
model and the Ising spin glass are discussed. The critical contact process and
the parity-conserving non-equilibrium kinetic Ising model are also considered.Comment: 12 pages, Latex2e with IOP macros, 2 figures included; final for
Fermionic renormalization group methods for transport through inhomogeneous Luttinger liquids
We compare two fermionic renormalization group methods which have been used
to investigate the electronic transport properties of one-dimensional metals
with two-particle interaction (Luttinger liquids) and local inhomogeneities.
The first one is a poor man's method setup to resum ``leading-log'' divergences
of the effective transmission at the Fermi momentum. Generically the resulting
equations can be solved analytically. The second approach is based on the
functional renormalization group method and leads to a set of differential
equations which can only for certain setups and in limiting cases be solved
analytically, while in general it must be integrated numerically. Both methods
are claimed to be applicable for inhomogeneities of arbitrary strength and to
capture effects of the two-particle interaction, such as interaction dependent
exponents, up to leading order. We critically review this for the simplest case
of a single impurity. While on first glance the poor man's approach seems to
describe the crossover from the ``perfect'' to the ``open chain fixed point''
we collect evidence that difficulties may arise close to the ``perfect chain
fixed point''. Due to a subtle relation between the scaling dimensions of the
two fixed points this becomes apparent only in a detailed analysis. In the
functional renormalization group method the coupling of the different
scattering channels is kept which leads to a better description of the
underlying physics.Comment: 25 pages, accepted for publication in NJP, remarks added on the poor
man's RG treatment of the Y-junction and the Breit-Wigner line shape
Evidence for a Second Order Phase Transition in Glasses at Very Low Temperatures -- A Macroscopic Quantum State of Tunneling Systems
Dielectric measurements at very low temperature indicate that in a glass with
the eutectic composition BaO-AlO-SiO a phase transition occurs at
5.84 mK. Below that temperature small magnetic fields of the order of 10 T
cause noticeable changes of the dielectric constant although the glass is
insensitive to fields up to 20 T above 10 mK. The experimental findings may be
interpreted as the signature of the formation of a new phase in which many
tunneling systems perform a coherent motion resulting in a macroscopic wave
function.Comment: 4 pages, 4 figures, submitted to Phys. Rev. Let
Prospects for measuring the 229Th isomer energy using a metallic magnetic microcalorimeter
The Thorium-229 isotope features a nuclear isomer state with an extremely low
energy. The currently most accepted energy value, 7.8 +- 0.5 eV, was obtained
from an indirect measurement using a NASA x-ray microcalorimeter with an
instrumental resolution 26 eV. We study, how state-of-the-art magnetic metallic
microcalorimeters with an energy resolution down to a few eV can be used to
measure the isomer energy. In particular, resolving the 29.18 keV doublet in
the \gamma-spectrum following the \alpha-decay of Uranium-233, corresponding to
the decay into the ground and isomer state, allows to measure the isomer
transition energy without additional theoretical input parameters, and increase
the energy accuracy. We study the possibility of resolving the 29.18 keV line
as a doublet and the dependence of the attainable precision of the energy
measurement on the signal and background count rates and the instrumental
resolution.Comment: 32 pages, 8 figures, eq. (3) correcte
Ageing in the contact process: Scaling behavior and universal features
We investigate some aspects of the ageing behavior observed in the contact
process after a quench from its active phase to the critical point. In
particular we discuss the scaling properties of the two-time response function
and we calculate it and its universal ratio to the two-time correlation
function up to first order in the field-theoretical epsilon-expansion. The
scaling form of the response function does not fit the prediction of the theory
of local scale invariance. Our findings are in good qualitative agreement with
recent numerical results.Comment: 20 pages, 3 figure
Functional renormalization group for Luttinger liquids with impurities
We improve the recently developed functional renormalization group (fRG) for
impurities and boundaries in Luttinger liquids by including renormalization of
the two-particle interaction, in addition to renormalization of the impurity
potential. Explicit flow-equations are derived for spinless lattice fermions
with nearest neighbor interaction at zero temperature, and a fast algorithm for
solving these equations for very large systems is presented. We compute
spectral properties of single-particle excitations, and the oscillations in the
density profile induced by impurities or boundaries for chains with up to
1000000 lattice sites. The expected asymptotic power-laws at low energy or long
distance are fully captured by the fRG. Results on the relevant energy scales
and crossover phenomena at intermediate scales are also obtained. A comparison
with numerical density matrix renormalization results for systems with up to
1000 sites shows that the fRG with the inclusion of vertex renormalization is
remarkably accurate even for intermediate interaction strengths.Comment: 35 pages, 16 figures, revised version as publishe
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