1,055 research outputs found
Effect of external magnetic field on electron spin dephasing induced by hyperfine interaction in quantum dots
We investigate the influence of an external magnetic field on spin phase
relaxation of single electrons in semiconductor quantum dots induced by the
hyperfine interaction. The basic decay mechanism is attributed to the
dispersion of local effective nuclear fields over the ensemble of quantum dots.
The characteristics of electron spin dephasing is analyzed by taking an average
over the nuclear spin distribution. We find that the dephasing rate can be
estimated as a spin precession frequency caused primarily by the mean value of
the local nuclear magnetic field. Furthermore, it is shown that the hyperfine
interaction does not fully depolarize electron spin. The loss of initial spin
polarization during the dephasing process depends strongly on the external
magnetic field, leading to the possibility of effective suppression of this
mechanism.Comment: 10 pages, 2 figure
Radiation-Induced Magnetoresistance Oscillations in a 2D Electron Gas
Recent measurements of a 2D electron gas subjected to microwave radiation
reveal a magnetoresistance with an oscillatory dependence on the ratio of
radiation frequency to cyclotron frequency. We perform a diagrammatic
calculation and find radiation-induced resistivity oscillations with the
correct period and phase. Results are explained via a simple picture of current
induced by photo-excited disorder-scattered electrons. The oscillations
increase with radiation intensity, easily exceeding the dark resistivity and
resulting in negative-resistivity minima. At high intensity, we identify
additional features, likely due to multi-photon processes, which have yet to be
observed experimentally.Comment: 5 pages, 3 figures; final version as published in Phys Rev Let
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
Observation of Apparently Zero-Conductance States in Corbino Samples
Using Corbino samples we have observed oscillatory conductance in a
high-mobility two-dimensional electron system subjected to crossed microwave
and magnetic fields. On the strongest of the oscillation minima the conductance
is found to be vanishingly small, possibly indicating an insulating state
associated with these minima.Comment: 4 pages, 3 figures, RevTex
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
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
Spectral and Transport Properties of d-Wave Superconductors With Strong Impurities
One of the remarkable features of disordered d-wave superconductors is strong
sensitivity of long range properties to the microscopic realization of the
disorder potential. Particularly rich phenomenology is observed for the --
experimentally relevant -- case of dilute distributions of isolated impurity
centers. Building on earlier diagrammatic analyses, the present paper derives
and analyses a low energy effective field theory of this system. Specifically,
the results of previous diagrammatic T-matrix approaches are extended into the
perturbatively inaccessible low energy regimes, and the long range (thermal)
transport behaviour of the system is discussed. It turns out that in the
extreme case of a half-filled tight binding band and infinitely strong
impurities (impurities at the unitary limit), the system is in a delocalized
phase.Comment: 14 pages, two figures include
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
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