6,804 research outputs found
Why Study Noise due to Two Level Systems: A Suggestion for Experimentalists
Noise is often considered to be a nuisance. Here we argue that it can be a
useful probe of fluctuating two level systems in glasses. It can be used to:
(1) shed light on whether the fluctuations are correlated or independent
events; (2) determine if there is a low temperature glass or phase transition
among interacting two level systems, and if the hierarchical or droplet model
can be used to describe the glassy phase; and (3) find the lower bound of the
two level system relaxation rate without going to ultralow temperatures.
Finally we point out that understanding noise due to two level systems is
important for technological applications such as quantum qubits that use
Josephson junctions.Comment: 15 pages, 4 figures, Latex, to be published in J. Low Temp. Phys.
issue in honor of S. Hunklinge
Noise spectra of stochastic pulse sequences: application to large scale magnetization flips in the finite size 2D Ising model
We provide a general scheme to predict and derive the contribution to the
noise spectrum of a stochastic sequence of pulses from the distribution of
pulse parameters. An example is the magnetization noise spectra of a 2D Ising
system near its phase transition. At , the low frequency spectra is
dominated by magnetization flips of nearly the entire system. We find that both
the predicted and the analytically derived spectra fit those produced from
simulations. Subtracting this contribution leaves the high frequency spectra
which follow a power law set by the critical exponents.Comment: 4 pages, 5 figures. We improved text and included a predicted noise
curve in Figure 4. 2 examples from Figure 3 are remove
Hydrogen as a Source of Flux Noise in SQUIDs
Superconducting qubits are hampered by flux noise produced by surface spins
from a variety of microscopic sources. Recent experiments indicated that
hydrogen (H) atoms may be one of those sources. Using density functional theory
calculations, we report that H atoms either embedded in, or adsorbed on, an
a-Al2O3(0001) surface have sizeable spin moments ranging from 0.81 to 0.87 uB
with energy barriers for spin reorientation as low as ~10 mK. Furthermore, H
adatoms on the surface attract gas molecules such as O2, producing new spin
sources. We propose coating the surface with graphene to eliminate H-induced
surface spins and to protect the surface from other adsorbates.Comment: 12 pages, 4 figure
Hierarchical Lattice Models of Hydrogen Bond Networks in Water
We develop a graph-based model of the hydrogen bond network in water, with a
view towards quantitatively modeling the molecular-level correlational
structure of the network. The networks are formed are studied by the
constructing the model on two infinite-dimensional lattices. Our models are
built \emph{bottom up}, based on microscopic information coming from atomistic
simulations, and we show that the predictions of the model are consistent with
known results from ab-initio simulations of liquid water. We show that simple
entropic models can predict the correlations and clustering of
local-coordination defects around tetrahedral waters observed in the atomistic
simulations. We also find that orientational correlations between bonds are
longer ranged than density correlations, and determine the directional
correlations within closed loops and show that the patterns of water wires
within these structures are also consistent with previous atomistic
simulations. Our models show the existence of density and compressibility
anomalies, as seen in the real liquid, and the phase diagram of these models is
consistent with the singularity-free scenario previously proposed by Sastry and
co-workers (Sastry et al, PRE 53, 6144 (1996)).Comment: 17 pages, published versio
Why phonon scattering in glasses is universally small at low temperatures
We present a novel view of the standard model of tunneling two level systems
(TLS) to explain the puzzling universal value of a quantity, , that characterizes phonon scattering in glasses below 1 K as
reflected in thermal conductivity, ultrasonic attenuation, internal friction,
and the change in sound velocity. Physical considerations lead to a broad
distribution of phonon-TLS couplings that (1) exponentially renormalize
tunneling matrix elements, and (2) reduce the TLS density of states through
TLS-TLS interactions. We find good agreement between theory and experiment for
a variety of individual glasses.Comment: Resubmission: several typos were corrected and the notation clarified
in some places. No major change
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