18,302 research outputs found
Large thermoelectric power and figure of merit in a ferromagnetic-quantum dot-superconducting device
We investigate the thermoelectric properties of a quantum dot coupled to
ferromagnetic and superconducting electrodes. The combination of spin polarized
tunneling at the ferromagnetic-quantum dot interface and the application of an
external magnetic field that Zeeman splits the dot energy level leads to large
values of the thermopower (Seebeck coefficient). Importantly, the thermopower
can be tuned with an external gate voltage connected to the dot. We compute the
figure of merit that measures the efficiency of thermoelectric conversion and
find that it attains high values. We discuss the different contributions from
Andreev reflection processes and quasiparticle tunneling into and out of the
superconducting contact. Furthermore, we obtain dramatic variations of both the
magnetothermopower and the spin Seebeck effect, which suggest that in our
device spin currents can be controlled with temperature gradients only.Comment: 9 pages, 6 figure
Thermoelectric effects in quantum Hall systems beyond linear response
We consider a quantum Hall system with an antidot acting as an energy
dependent scatterer. In the purely charge case, we find deviations from the
Wiedemann-Franz law that take place in the nonlinear regime of transport. We
also discuss Peltier effects beyond linear response and describe both effects
using magnetic-field asymmetric transport coefficients. For the spin case such
as that arising along the helical edge states of a two-dimensional topological
insulator, we investigate the generation of spin currents as a result of
applied voltage and temperature differences in samples attached to
ferromagnetic leads. We find that in the parallel configuration the spin
current can be tuned with the leads' polarization even in the linear regime of
transport. In contrast, for antiparallel magnetizations the spin currents has a
strict nonlinear dependence on the applied fields.Comment: 17 pages, 8 figure
Cross thermoelectric coupling in normal-superconductor quantum dots
We discuss the nonlinear current of an interacting quantum dot coupled to
normal and superconducting reservoirs with applied voltage and temperature
differences. Due to the particle-hole symmetry introduced by the
superconducting lead, the pure (subgap) thermoelectric response vanishes.
However, we show that the Andreev bound states shift as the thermal gradient
increases. As a consequence, the -- characteristic can be tuned with a
temperature bias if the system is simultaneously voltage biased. This is a
cross effect that occurs beyond linear response only. Furthermore, we emphasize
the role of quasiparticle tunneling processes in the generation of high
thermopower sensitivities.Comment: 6 pages, 5 figure
Models of galaxy collisions in Stephan\u27s Quintet and other interacting systems
This dissertation describes numerical studies of three interacting galaxy systems. First, hydrodynamical models of the collisions in the famous compact galaxy group, Stephan\u27s Quintet, were constructed to investigate the dynamical interaction history and evolution of the intergalactic gas. It has been found that with a sequence of two-at-a-time collisions, most of the major morphological and kinematical features of the group were well reproduced in the models. The models suggest the two long tails extending from NGC 7319 toward NGC 7320c may be formed simultaneously from a strong collisional encounter between the two galaxies, resulting in a thinner and denser inner tail than the outer one. The tails then also run parallel to each other as observed. The model results support the idea that the group-wide shock detected in multi-wavelength observations between NGC 7319 and 7318b and the starburst region north of NGC 7318b are triggered by the current high-speed collision between NGC 7318b and the intergalactic gas. It is expected that other compact groups containing rich extended features like Stephan\u27s Quintet can be modeled in similar ways, and that sequences of two-at-a-time collisions will be the general rule.
The second set of hydrodynamical simulations were performed to model the peculiar galaxy pair, Arp 285. This system possesses a series of star-forming complexes in an unusual tail-like feature extending out perpendicular to the disk of the northern galaxy. Several conceptual ideas for the origin of the tail-like feature were examined. The models suggest that the bridge material falling into the gravitational potential of the northern disk overshoots the disk; as more bridge material streams into the region, compression drives star formation. This work on star-formation in the pile-up region can be extended to the studies of the formation of tidal dwarf galaxies or globular clusters.
Thirdly, the development of spiral waves was studied with numerical models of fast galaxy collisions involving a disk with a high value of the Toomre Q parameter. The models found that spirals slowly developed in the disk do not dissipate in a few outer disk orbital time. The waves in the models persist for a long time, more than 10 outer disk rotation periods, while winding ever tighter. Since fast collisions are common in many galaxy groups and cluster environments, the phenomena and effect presented in the work may be one of the several processes that contribute to galaxy harassment, and a contributor to the Butcher-Oemler effect
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