861 research outputs found
Structural and Financial Characteristics of U.S. Farms: 2001 Family Farm Report
Family farms vary widely in size and other characteristics, ranging from very small retirement and residential farms to establishments with sales in the millions of dollars. The farm typology developed by the Economic Research Service (ERS) categorizes farms into groups based primarily on occupation of the operator and sales class of the farm. The typology groups reflect operators' expectations from farming, position in the life cycle, and dependence on agriculture. The groups differ in their importance to the farm sector, product specialization, program participation, and dependence on farm income. These (and other) differences are discussed in this report.Agricultural Resource Management Study (ARMS), family farms, farm businesses, farm financial situation, farm operator household income, farm operators, farm structure, farm typology, female farm operators, government payments, spouses of farm operators, taxes, Agricultural Finance, Farm Management,
Microwave conductivity of a d-wave superconductor disordered by extended impurities: a real-space renormalization group approach
Using a real-space renormalization group (RSRG) technique, we compute the
microwave conductivity of a d-wave superconductor disordered by extended
impurities. To do this, we invoke a semiclassical approximation which naturally
accesses the Andreev bound states localized near each impurity. Tunneling
corrections (which are captured using the RSRG) lead to a delocalization of
these quasiparticles and an associated contribution to the microwave
conductivity.Comment: 8 pages, 4 figures. 2 figures added to previous versio
Microwave Conductivity due to Scattering from Extended Linear Defects in d-Wave Superconductors
Recent microwave conductivity measurements of detwinned, high-purity,
slightly overdoped YBaCuO crystals reveal a linear
temperature dependence and a near-Drude lineshape for temperatures between 1
and 20 K and frequencies ranging from 1 to 75 GHz. Prior theoretical work has
shown that simple models of scattering by point defects (impurities) in d-wave
superconductors are inconsistent with these results. It has therefore been
suggested that scattering by extended defects such as twin boundary remnants,
left over from the detwinning process, may also be important. We calculate the
self-energy and microwave conductivity in the self-consistent Born
approximation (including vertex corrections) for a d-wave superconductor in the
presence of scattering from extended linear defects. We find that in the
experimentally relevant limit (), the
resulting microwave conductivity has a linear temperature dependence and a
near-Drude frequency dependence that agrees well with experiment.Comment: 13 pages, 7 figure
A conceptual basis for surveying fouling communities at exposed and protected sites at sea: Feasible designs with exchangeable test bodies for in-situ biofouling collection
The enhanced inertia load caused by biofouling on device components, such as the foundations of wind turbines or other structures at sea, modifies the hydrodynamic properties, and increases the stress to structures, predominantly in upper water layers with high impact from wave dynamics. This compromises the stability, functioning, operation as well as the durability of these devices especially in exposed environments. A main challenge is the quantification of the impact of hydrodynamic forces on irregular bodies being overgrown by soft- and hard-bodied biofouling organisms. Therefore, test bodies from the upper 1–5 m water depth and thus exposed to the strongest wave actions close to the surface shall be overgrown by biofouling and used in measurement trials in a wave and current flume. These measurements shall shed light on the varying roughness and its influence on the load bearing capacity of foundation piles. Consequently, the main aims of the present work were the development of two independent test stations as holding devices for artificial test bodies for the collection of biofouling organisms during field studies: a carrying unit floating at the surface in an exposed area (System A) and a sampling device with access from a land-based facility (System B). Both systems are relatively easy to access, exhibit straightforward handling, and are reasonable cost-effective. A Test Body Support Unit (TBSU, System A) was designed and mounted on a spare buoy to carry the test bodies (cylinders), which serve as substrate for the fouling. The system was sufficiently robust to withstand several periods of rough sea conditions over the first two years. This system can only be accessed by vessels. System B (MareLift) provided the robustness and functionality needed for areas exhibiting harsh conditions but can be operated from land. The here used test bodies (steel panels) exhibited a sound basis for the monitoring of succession processes in the biofouling development. System B offered the possibility to analyse two habitats (intertidal and subtidal) and revealed clear differences in the composition and development of their fouling communities. Overall, both systems provide advantages in obtaining standardized biofouling samples compared to previous approaches. Such test stations play an important role in the risk management of marine sectors as they could help characterising biofouling communities over different geographical areas. System A and B provide a sound basis for biofouling research but potentially also for other potential research approaches in exposed areas as they provide space for future developments
Fermi-Liquid Interactions in d-Wave Superconductor
This article develops a quantitative quasiparticle model of the
low-temperature properties of d-wave superconductors which incorporates both
Fermi-liquid effects and band-structure effects. The Fermi-liquid interaction
effects are found to be classifiable into strong and negligible renormalizaton
effects, for symmetric and antisymmetric combinations of the energies of
and quasiparticles, respectively. A particularly
important conclusion is that the leading clean-limit temperature-dependent
correction to the superfluid density is not renormalized by Fermi-liquid
interactions, but is subject to a Fermi velocity (or mass) renormalization
effect. This leads to difficulties in accounting for the penetration depth
measurements with physically acceptable parameters, and hence reopens the
question of the quantitative validity of the quasiparticle picture.Comment: 4 page
Optical and Thermal-Transport Properties of an Inhomogeneous d-Wave Superconductor
We calculate transport properties of disordered 2D d-wave superconductors
from solutions of the Bogoliubov-de Gennes equations, and show that weak
localization effects give rise to a finite frequency peak in the optical
conductivity similar to that observed in experiments on disordered cuprates. At
low energies, order parameter inhomogeneities induce linear and quadratic
temperature dependencies in microwave and thermal conductivities respectively,
and appear to drive the system towards a quasiparticle insulating phase.Comment: 5 pages,3 figure
Effects of an in-plane magnetic field on c-axis sum rule and superfluid density in high- cuprates
In layered cuprates, the application of an in-plane magnetic field changes the c-axis optical sum rule and superfluid density . For
pure incoherent c-axis coupling, has no effect on either quantities
but it does if an additional coherent component is present. For the coherent
contribution, different characteristic variations on and on
temperature result from the constant part of the hopping matrix
element and from the part which has zero on the diagonal of the
Brillouin zone. Only the constant part leads to a dependence on
the direction of as well as on its magnitude.Comment: 3 figure
Microwave Photoconductivity in Two-Dimensional Electron Systems due to Photon-Assisted Interaction of Electrons with Leaky Interface Phonons
We calculate the contribution of the photon-assisted interaction of electrons
with leaky interface phonons to the dissipative dc photoconductivity of a
two-dimensional electron system in a magnetic field. The calculated
photoconductivity as a function of the frequency of microwave radiation and the
magnetic field exhibits pronounced oscillations. The obtained oscillation
structure is different from that in the case of photon-assisted interaction
with impurities. We demonstrate that at a sufficiently strong microwave
radiation in the certain ranges of its frequency (or in certain ranges of the
magnetic field) this mechanism can result in the absolute negative
conductivity.Comment: 3 pages, 1 figur
Absolute Negative Conductivity in Two-Dimensional Electron Systems Associated with Acoustic Scattering Stimulated by Microwave Radiation
We discuss the feasibility of absolute negative conductivity (ANC) in
two-dimensional electron systems (2DES) stimulated by microwave radiation in
transverse magnetic field. The mechanism of ANC under consideration is
associated with the electron scattering on acoustic piezoelectric phonons
accompanied by the absorption of microwave photons. It is demonstrated that the
dissipative components of the 2DES dc conductivity can be negative
() when the microwave frequency is
somewhat higher than the electron cyclotron frequency or its
harmonics. The concept of ANC associated with such a scattering mechanism can
be invoked to explain the nature of the occurrence of zero-resistance
``dissipationless'' states observed in recent experiments.Comment: 7 pager, 2 figure
Radiation induced oscillations of the Hall resistivity in two-dimensional electron systems
We consider the effect of microwave radiation on the Hall resistivity in
two-dimension electron systems. It is shown that the photon-assisted impurity
scattering of electrons can result in oscillatory dependences of both
dissipative and Hall components of the conductivity and resistivity tensors on
the ratio of radiation frequency to cyclotron frequency. The Hall resistivity
can include a component induced by microwave radiation which is an even
function of the magnetic field. The phase of the dissipative resistivity
oscillations and the polarization dependence of their amplitude are compared
with those of the Hall resistivity oscillations. The developed model can
clarify the results of recent experimental observations of the radiation
induced Hall effect.Comment: 4 pages, 1 figur
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