5,857 research outputs found
Spin transport in a one-dimensional anisotropic Heisenberg model
We analytically and numerically study spin transport in a one-dimensional
Heisenberg model in linear-response regime at infinite temperature. It is shown
that as the anisotropy parameter Delta is varied spin transport changes from
ballistic for Delta<1 to anomalous at the isotropic point Delta=1, to diffusive
for finite Delta>1, ending up as a perfect isolator in the Ising limit of
infinite Delta. Using perturbation theory for large Delta a quantitative
prediction is made for the dependence of diffusion constant on Delta.Comment: 5 pages, 4 figures; v2.: few comments added and typos corrected;
published versio
Spin diffusion and the anisotropic spin-1/2 Heisenberg chain
Measurements of the spin-lattice relaxation rate 1/T_1 by nuclear magnetic
resonance for the one-dimensional Heisenberg antiferromagnet Sr_2CuO_3 have
provided evidence for a diffusion-like contribution at finite temperature and
small wave-vector. By analyzing real-time data for the auto- and
nearest-neighbor spin-spin correlation functions obtained by the density-matrix
renormalization group I show that such a contribution indeed exists for
temperatures T>J, where J is the coupling constant, but that it becomes
exponentially suppressed for T << J. I present evidence that the
frequency-dependence of 1/T_1 in the Heisenberg case is smoothly connected to
that in the free fermion case where the exponential suppression of the
diffusion-like contribution is easily understood.Comment: 9 pages, 7 figure
Transport properties of a spin-1/2 Heisenberg chain with an embedded spin-S impurity
The finite temperature transport properties of a spin-1/2 anisotropic
Heisenberg chain with an embedded spin-S impurity are studied. Using primarily
numerical diagonalization techniques, we study the dependence of the dynamical
spin and thermal conductivities on the lattice size, the magnitude of the
impurity spin, the host-impurity coupling, the easy axis anisotropy, as well as
the dependence on temperature. Particularly for the temperature dependence, we
discuss the screening of the impurity by the chain eventually leading to the
cutting or healing of the host chain. Numerical results are supported by
analytical arguments obtained in the strong host-impurity coupling regime.Comment: 7 pages, 10 figure
Determination of the relative resistance to ignition of selected turbopump materials in high-pressure, high-temperature, oxygen environments, volume 1
Advances in the design of the liquid oxygen, liquid hydrogen engines for the Space Transportation System call for the use of warm, high-pressure oxygen as the driving gas in the liquid oxygen turbopump. The NASA Lewis Research Center requested the NASA White Sands Test Facility (WSTF) to design a test program to determine the relative resistance to ignition of nine selected turbopump materials: Hastelloy X, Inconel 600, Invar 36, Monel K-500, nickel 200, silicon carbide, stainless steel 316, and zirconium copper. The materials were subjected to particle impact and to frictional heating in high-pressure oxygen
Increased temperature in urban ground as source of sustainable energy
This paper is part of the Proceedings of the 10th International Conference on Urban Regeneration and Sustainability (Sustainable City 2015). http://www.witconferences.comDensely urbanized areas are characterized by special microclimatic conditions with typically elevated temperatures in comparison with the rural surrounding. This phenomenon is known as the urban heat island (UHI) effect, but not restricted exclusively to the atmosphere. We also find significant warming of the urban subsurface and shallow groundwater bodies. Here, main sources of heat are elevated ground surface temperatures, direct thermal exploitation of aquifers and heat losses from buildings and other infrastructure. By measuring the shallow groundwater temperature in several European cities, we identify that heat sources and associated transport processes interact at multiple spatial and temporal scales. The intensity of a subsurface UHI can reach the values of above 4 K in city centres with hotspots featuring temperatures up to +20°C. In comparison with atmospheric UHIs, subsurface UHIs represent long-term accumulations of heat in a relatively sluggish environment. This potentially impairs urban groundwater quality and permanently influences subsurface ecosystems. From another point of view, however, these thermal anomalies can also be seen as hidden large-scale batteries that constitute a source of shallow geothermal energy. Based on our measurements, data surveys and estimated physical ground properties, it is possible to estimate the theoretical geothermal potential of the urban groundwater bodies beneath the studied cities. For instance, by decreasing the elevated temperature of the shallow aquifer in Cologne, Germany, by only 2 K, the obtained energy could supply the space-heating demand of the entire city for at least 2.5 years. In the city of Karlsruhe, it is estimated that about 30% of annual heating demand could be sustainably supplied by tapping the anthropogenic heat loss in the urban aquifer. These results reveal the attractive potential of heated urban ground as energy reservoir and storage, which is in place at many places worldwide but so far not integrated in any city energy plans.This work was supported by the Swiss National Science Foundation (SNSF) under grant number 200021L 144288, and the German Research Foundation (DFG), under grant number BL 1015/4-1
Critical core mass for enriched envelopes: the role of H2O condensation
Context. Within the core accretion scenario of planetary formation, most
simulations performed so far always assume the accreting envelope to have a
solar composition. From the study of meteorite showers on Earth and numerical
simulations, we know that planetesimals must undergo thermal ablation and
disruption when crossing a protoplanetary envelope. Once the protoplanet has
acquired an atmosphere, the primordial envelope gets enriched in volatiles and
silicates from the planetesimals. This change of envelope composition during
the formation can have a significant effect in the final atmospheric
composition and on the formation timescale of giant planets.
Aims. To investigate the physical implications of considering the envelope
enrichment of protoplanets due to the disruption of icy planetesimals during
their way to the core. Particular focus is placed on the effect on the critical
core mass for envelopes where condensation of water can occur.
Methods. Internal structure models are numerically solved with the
implementation of updated opacities for all ranges of metallicities and the
software CEA to compute the equation of state. CEA computes the chemical
equilibrium for an arbitrary mixture of gases and allows the condensation of
some species, including water. This means that the latent heat of phase
transitions is consistently incorporated in the total energy budget.
Results. The critical core mass is found to decrease significantly when an
enriched envelope composition is considered in the internal structure
equations. A particular strong reduction of the critical core mass is obtained
for planets whose envelope metallicity is larger than Z=0.45 when the outer
boundary conditions are suitable for condensation of water to occur in the top
layers of the atmosphere. We show that this effect is qualitatively preserved
when the atmosphere is out of chemical equilibrium.Comment: Accepted for publication in A&
Theoretical and material studies on thin-film electroluminescent devices
The effect of surface nucleation processes on the quality of ZnS layers grown on (001) GaAs substrates by molecular beam epitaxy is reported. Reflection high energy electron diffraction indicated that nucleation at high temperatures produced more planar surfaces than nucleation at low temperatures, but the crystalline quality as accessed by x ray double crystal diffractometry is relatively independent of nucleation temperature. A critical factor in layer quality was the initial roughness of the GaAs surfaces
Device and method for frictionally testing materials for ignitability
Test apparatus for determining ignition characteristics of various metal in oxidizer environments simulating operating conditions for materials is invented. The test apparatus has a chamber through which the oxidizing agent flows, and means for mounting a stationary test sample therein, a powered, rotating shaft in the chamber rigidly mounts a second test sample. The shaft is axially movable to bring the samples into frictional engagement and heated to the ignition point. Instrumentation connected to the apparatus provides for observation of temperatures, pressures, loads on and speeds of the rotating shaft, and torques whereby components of stressed oxygen systems can be selected which will avoid accidental fires under working conditions
Open XXZ spin chain: Nonequilibrium steady state and strict bound on ballistic transport
Explicit matrix product ansatz is presented, in first two orders in the
(weak) coupling parameter, for the non-equilibrium steady state of the
homogeneous, nearest neighbor Heisenberg XXZ spin-1/2 chain driven by Lindblad
operators which act only at the edges of the chain. The first order of the
density operator becomes in thermodynamic limit an exact pseudo-local
conservation law and yields -- via Mazur inequality -- a rigorous lower bound
on the high temperature spin Drude weight. Such Mazur bound is a non-vanishing
fractal function of the anisotropy parameter Delta for |Delta|<1.Comment: Slightly longer but essentially equivalent to a published versio
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