18 research outputs found
Theory of Josephson Phenomena in Superfluid 3He
Quite detailed theoretical description of superfluid 3He is possible on
length scales that are much larger than the atomic scale. We discuss weak links
between two bulk states of 3He-B. The current through the weak link is
determined by the bound states at the link. The bound state energies are spin
split depending on the order parameters in the bulk. As a result, unusual
current-phase relations with pi state appear. For not too weak links, the order
parameter in the bulk is modified because of the Josephson coupling. This leads
to a stronger pi state and to an additional current at constant pressure bias.
The theoretical results are compared with experiments.Comment: 6 pages, 9 figures, talk on LT-24, Orlando, Florid
Comment on "Ballistic SNS sandwich as a Josephson junction"
The old problem of supercurrent in a long ballistic SNS Josephson junction is
interpreted using a simple picture of the fermion energy levels in the normal
and superconducting regions. We argue that a recent paper on the topic by Sonin
contains erroneous results.Comment: 5 pages, 1 figure, in version 2 an appendix containing mathematical
details is adde
Stability of A-B phase boundary in a constriction
Abstract We report on a Ginzburg-Landau calculation of the stability of a boundary between A and B phases of superfluid 3 He in a two-dimensional constriction. In the macroscopic limit the stability follows a well-known relation, which depends on the surface tension s AB of the A-B boundary. In the narrow-constriction limit the surface tension is not well defined, but the interface is always stable, and a weak link between the A and B phases is obtained. Pinning of A-B interface Properties of the A-B interface in superfluid It is well known that in the macroscopic limit the pinning stability of the interface is determined only by its finite surface tension s AB where Df AB ¼ f A À f B is the difference in bulk condensation energy densities of the two phases. However, the macroscopic concept of surface tension is not well defined in the limit of small constrictions, and Eq. Numerical results Our computational method employs a standard minimization routine for the GL free energy functional
Bloch gain in dc-ac-driven semiconductor superlattices in the absence of electric domains
We theoretically study the feasibility of amplification and generation of terahertz radiation in dc-ac-driven semiconductor superlattices in the absence of electric domains. We find that if in addition to a dc bias a strong terahertz pump field is applied, a Bloch gain profile for a small terahertz signal can be achieved under the conditions of a positive static differential conductivity. Here, the positive differential conductivity arises, similarly to the case of a large-signal amplification scheme [H. Kroemer, arXiv:cond-mat/0009311 (unpublished)], due to modifications in dc current density caused by the application of a high-frequency ac field [ K. Unterrainer et al. Phys. Rev. Lett. 76 2973 (1996)]. Whereas the sign of absorption at low and zero frequencies is sensitive to the ac fields, the gain profile in the vicinity of the gain maximum is robust. We suggest to use this ac-induced effect in a starter for a terahertz Bloch oscillator. Our analysis demonstrates that the application of a short terahertz pulse to a superlattice allows the suppression of the undesirable formation of electric domains and the achievement of a sustained large-amplitude operation of the dc-biased Bloch oscillator
Terahertz Bloch oscillator with suppressed electric domains: Effect of elastic scattering
We theoretically consider the amplification of THz radiation in a
superlattice Bloch oscillator. The main dilemma in the realization of THz Bloch
oscillator is finding operational conditions which allow simultaneously to
achieve gain at THz frequencies and to avoid destructive space-charge
instabilities. A possible solution to this dilemma is the extended Limited
Space-Charge Accumulation scheme of Kroemer (H. Kroemer, cond-mat/0009311).
Within the semiclassical miniband transport approach we extend its range of
applicability by considering a difference in the relaxation times for electron
velocity and electron energy. The kinetics of electrons and fields establishing
a stationary signal in the oscillator is also discussed.Comment: Submitted to proceedings of the summer school-conference of AQDJJ
programme of ESF, Kiten, Bulgaria, 9-24 June 200