1,396 research outputs found
The feasibility of a fluidic respiratory flow meter
A study was undertaken to determine the feasibility of adapting a fluidic airspeed sensor for use as a respiratory flowmeter. A Pulmonary Function Testing Flowmeter was developed which should prove useful for mass screening applications. The fluidic sensor threshold level was not reduced sufficiently to permit its adaptation to measuring the low respiratory flow rates encountered in many respiratory disorders
Observation of a Spin-Peierls Transition in a Heisenberg Antiferromagnetic Linear-Chain System
Magnetic-susceptibility and EPR measurements are reported which provide the first unambiguous evidence for a spin-Peierls transition in a system of linear one-dimensional antiferromagnetic Heisenberg chains. The material studied is TTFCuS4C4(CF3)4 (TFF stands for tetrathiafulvalinium). At 12 K, the spin-lattice system undergoes a second-order phase transition to a singlet ground state
Ising spin glass under continuous-distribution random magnetic fields: Tricritical points and instability lines
The effects of random magnetic fields are considered in an Ising spin-glass
model defined in the limit of infinite-range interactions. The probability
distribution for the random magnetic fields is a double Gaussian, which
consists of two Gaussian distributions centered respectively, at and
, presenting the same width . It is argued that such a
distribution is more appropriate for a theoretical description of real systems
than its simpler particular two well-known limits, namely the single Gaussian
distribution (), and the bimodal one (). The
model is investigated by means of the replica method, and phase diagrams are
obtained within the replica-symmetric solution. Critical frontiers exhibiting
tricritical points occur for different values of , with the possibility
of two tricritical points along the same critical frontier. To our knowledge,
it is the first time that such a behavior is verified for a spin-glass model in
the presence of a continuous-distribution random field, which represents a
typical situation of a real system. The stability of the replica-symmetric
solution is analyzed, and the usual Almeida-Thouless instability is verified
for low temperatures. It is verified that, the higher-temperature tricritical
point always appears in the region of stability of the replica-symmetric
solution; a condition involving the parameters and , for the
occurrence of this tricritical point only, is obtained analytically. Some of
our results are discussed in view of experimental measurements available in the
literature.Comment: 23 pages, 8 figures, accept for publication in Phys. Rev.
Spin-Peierls transitions in magnetic donor-acceptor compounds of tetrathiafulvalene (TTF) with bisdithiolene metal complexes
The spin-Peierls transition is considered as a progressive spin-lattice dimerization occurring below a transition temperature in a system of one-dimensional antiferromagnetic Heisenberg chains. In the simplest theories, the transition is second order and the ground state is a singlet with a magnetic gap. The historical origins and theoretical development of the concept are examined. Magnetic susceptibility and EPR measurements on the π-donor-acceptor compounds TTF·MS4C4(CF3)4 (M=Cu, Au; TTF is tetrathiafulvalene) are reported. These compounds exhibit clearly the characteristics of the spin-Peierls transition in reasonably good agreement with a mean-field theory. The susceptibility of each compound has a broad maximum near 50 K, while the transitions occur at 12 and 2.1 K for M=Cu and Au, respectively. EPR linewidth observations over a broad temperature range are examined. Areas for further experimental and theoretical work are indicated, and a critical comparison is made of related observations on other materials
Against politicization of science: Comment on S. Keller: Scientization: putting global climate change on the scientific agenda since 1970 and the role of the IPCC
Universal aspects of vacancy-mediated disordering dynamics: the effect of external fields
We investigate the disordering of an initially phase-segregated binary alloy,
due to a highly mobile defect which couples to an electric or gravitational
field. Using both mean-field and Monte Carlo methods, we show that the late
stages of this process exhibit dynamic scaling, characterized by a set of
exponents and scaling functions. A new scaling variable emerges, associated
with the field. While the scaling functions carry information about the field
and the boundary conditions, the exponents are universal. They can be computed
analytically, in excellent agreement with simulation results.Comment: 15 pages, 6 figure
Hysteresis and hierarchies: dynamics of disorder-driven first-order phase transformations
We use the zero-temperature random-field Ising model to study hysteretic
behavior at first-order phase transitions. Sweeping the external field through
zero, the model exhibits hysteresis, the return-point memory effect, and
avalanche fluctuations. There is a critical value of disorder at which a jump
in the magnetization (corresponding to an infinite avalanche) first occurs. We
study the universal behavior at this critical point using mean-field theory,
and also present preliminary results of numerical simulations in three
dimensions.Comment: 12 pages plus 2 appended figures, plain TeX, CU-MSC-747
Speckle from phase ordering systems
The statistical properties of coherent radiation scattered from
phase-ordering materials are studied in detail using large-scale computer
simulations and analytic arguments. Specifically, we consider a two-dimensional
model with a nonconserved, scalar order parameter (Model A), quenched through
an order-disorder transition into the two-phase regime. For such systems it is
well established that the standard scaling hypothesis applies, consequently the
average scattering intensity at wavevector _k and time t' is proportional to a
scaling function which depends only on a rescaled time, t ~ |_k|^2 t'. We find
that the simulated intensities are exponentially distributed, with the
time-dependent average well approximated using a scaling function due to Ohta,
Jasnow, and Kawasaki. Considering fluctuations around the average behavior, we
find that the covariance of the scattering intensity for a single wavevector at
two different times is proportional to a scaling function with natural
variables mt = |t_1 - t_2| and pt = (t_1 + t_2)/2. In the asymptotic large-pt
limit this scaling function depends only on z = mt / pt^(1/2). For small values
of z, the scaling function is quadratic, corresponding to highly persistent
behavior of the intensity fluctuations. We empirically establish a connection
between the intensity covariance and the two-time, two-point correlation
function of the order parameter. This connection allows sensitive testing,
either experimental or numerical, of existing theories for two-time
correlations in systems undergoing order-disorder phase transitions. Comparison
between theory and our numerical results requires no adjustable parameters.Comment: 18 pgs RevTeX, to appear in PR
Tricritical Points in the Sherrington-Kirkpatrick Model in the Presence of Discrete Random Fields
The infinite-range-interaction Ising spin glass is considered in the presence
of an external random magnetic field following a trimodal (three-peak)
distribution. The model is studied through the replica method and phase
diagrams are obtained within the replica-symmetry approximation. It is shown
that the border of the ferromagnetic phase may present first-order phase
transitions, as well as tricritical points at finite temperatures. Analogous to
what happens for the Ising ferromagnet under a trimodal random field, it is
verified that the first-order phase transitions are directly related to the
dilution in the fields (represented by ). The ferromagnetic boundary at
zero temperature also exhibits an interesting behavior: for , a single tricritical point occurs, whereas if
the critical frontier is completely continuous; however, for
, a fourth-order critical point appears. The stability
analysis of the replica-symmetric solution is performed and the regions of
validity of such a solution are identified; in particular, the Almeida-Thouless
line in the plane field versus temperature is shown to depend on the weight
.Comment: 23pages, 7 ps figure
Breakdown of Scale Invariance in the Phase Ordering of Fractal Clusters
Our numerical simulations with the Cahn-Hilliard equation show that
coarsening of fractal clusters (FCs) is not a scale-invariant process. On the
other hand, a typical coarsening length scale and interfacial area of the FC
exhibit power laws in time, while the mass fractal dimension remains invariant.
The initial value of the lower cutoff is a relevant length scale. A
sharp-interface model is formulated that can follow the whole dynamics of a
diffusion controlled growth, coarsening, fragmentation and approach to
equilibrium in a system with conserved order parameter.Comment: 4 pages, 4 figures, RevTex, submitted to PR
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