834 research outputs found
Low-frequency method for magnetothermopower and Nernst effect measurements on single crystal samples at low temperatures and high magnetic fields
We describe an AC method for the measurement of the longitudinal (Sxx) and
transverse (Sxy, i.e. Nernst) thermopower of mm-size single crystal samples at
low temperatures (T30 T). A low-frequency (33
mHz) heating method is used to increase the resolution, and to determine the
temperature gradient reliably in high magnetic fields. Samples are mounted
between two thermal blocks which are heated by a sinusoidal frequency f0 with a
p/2 phase difference. The phase difference between two heater currents gives a
temperature gradient at 2f0. The corresponding thermopower and Nernst effect
signals are extracted by using a digital signal processing method due. An
important component of the method involves a superconducting link, YBa2Cu3O7+d
(YBCO), which is mounted in parallel with sample to remove the background
magnetothermopower of the lead wires. The method is demonstrated for the quasi
two-dimensional organic conductor a-(BEDT-TTF)2KHg(SCN)4, which exhibits a
complex, magnetic field dependent ground state above 22.5 T at low
temperatures.Comment: 11 pages, 6 figures, 15 reference
Violation of Kohler's rule by the magnetoresistance of a quasi-two-dimensional organic metal
The interlayer magnetoresistance of the quasi-two-dimensional metal
-(BEDT-TTF)KHg(SCN) is considered. In the temperature range
from 0.5 to 10 K and for fields up to 10 tesla the magnetoresistance has a
stronger temperature dependence than the zero-field resistance. Consequently
Kohler's rule is not obeyed for any range of temperatures or fields. This means
that the magnetoresistance cannot be described in terms of semiclassical
transport on a single Fermi surface with a single scattering time. Possible
explanations for the violations of Kohler's rule are considered, both within
the framework of semi-classical transport theory and involving incoherent
interlayer transport. The issues considered are similar to those raised by the
magnetotransport of the cuprate superconductors.Comment: 5 pages, RevTeX + epsf, 2 figures. Slightly revised version to appear
in Physical Review B, May 15, 199
Ontological Problem-Solving Framework for Assigning Sensor Systems and Algorithms to High-Level Missions
The lack of knowledge models to represent sensor systems, algorithms, and missions makes opportunistically discovering a synthesis of systems and algorithms that can satisfy high-level mission specifications impractical. A novel ontological problem-solving framework has been designed that leverages knowledge models describing sensors, algorithms, and high-level missions to facilitate automated inference of assigning systems to subtasks that may satisfy a given mission specification. To demonstrate the efficacy of the ontological problem-solving architecture, a family of persistence surveillance sensor systems and algorithms has been instantiated in a prototype environment to demonstrate the assignment of systems to subtasks of high-level missions
What is the Priestley–Taylor wet-surface evaporation parameter? Testing four hypotheses
This study compares four different hypotheses regarding the nature of the Priestley–Taylor parameter α. They are as follows: α is a universal constant. The Bowen ratio (H/LE, where H is the sensible heat flux, and LE is the latent heat flux) for equilibrium (i.e., saturated air column near the surface) evaporation is a constant times the Bowen ratio at minimal advection (Andreas et al., 2013). Minimal advection over a wet surface corresponds to a particular relative humidity value. α is a constant fraction of the difference from the minimum value of 1 to the maximum value of α proposed by Priestley and Taylor (1972).
Formulas for α are developed for the last three hypotheses. Weather, radiation, and surface energy flux data from 171 FLUXNET eddy covariance stations were used. The condition LEref=LEp \u3e0.90 was taken as the criterion for nearly saturated conditions (where LEref is the reference, and LEp is the apparent potential evaporation rate from the equation by Penman, 1948). Daily and monthly average data from the sites were obtained. All formulations for α include one model parameter which is optimized such that the root mean square error of the target variable was minimized. For each model, separate optimizations were done for predictions of the target variables α, wet-surface evaporation (α multiplied by equilibrium evaporation rate) and actual evaporation (the latter using a highly successful version of the complementary relationship of evaporation). Overall, the second and fourth hypotheses received the best support from the data
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