435 research outputs found
Quantum metamaterials: Electromagnetic waves in a Josephson qubit line
We consider the propagation of a classical electromagnetic wave through a
transmission line, formed by identical superconducting charge qubits inside a
superconducting resonator. Since the qubits can be in a coherent superposition
of quantum states, we show that such a system demonstrates interesting new
effects, such as a ``breathing'' photonic crystal with an oscillating bandgap,
and a ``quantum Archimedean screw'' that transports, at an arbitrary controlled
velocity, Josephson plasma waves through the transmission line. The key
ingredient of these effects is that the optical properties of the Josephson
transmission line are controlled by the quantum coherent state of the qubits.Comment: References adde
Transport of free surface liquid films and drops by external ratchets and self-ratcheting mechanisms
We discuss the usage of ratchet mechanisms to transport a continuous phase in
several micro-fluidic settings. In particular, we study the transport of a
dielectric liquid in a heterogeneous ratchet capacitor that is periodically
switched on and off. The second system consists of drops on a solid substrate
that are transported by different types of harmonic substrate vibrations. We
argue that the latter can be seen as a self-ratcheting process and discuss
analogies between the employed class of thin film equations and Fokker-Planck
equations for transport of discrete objects in a 'particle ratchet'.Comment: 10 pages, 13 figure
Discontinuous conductance of bichromatically ac-gated quantum wires
We study the electron transport through a quantum wire under the influence of
external time-dependent gate voltages. The wire is modelled by a tight-binding
Hamiltonian for which we obtain the current from the corresponding
transmission. The numerical evaluation of the dc current reveals that for
bichromatic driving, the conductance depends sensitively on the
commensurability of the driving frequencies. The current even possesses a
discontinuous frequency dependence. Moreover, we find that the conductance as a
function of the wire length oscillates with a period that depends on the ratio
between the driving frequencies.Comment: 7 pages, 4 figure
Controlling the motion of interacting particles: Homogeneous systems and binary mixtures
We elaborate on recent results on the transport of interacting particles for both single-species and binary mixtures subject to an external driving on a ratchetlike asymmetric substrate. Moreover, we also briefly review motion control without any spatial asymmetric potential (i.e., no ratchet). Our results are obtained using an analytical approach based on a nonlinear Fokker–Planck equation as well as via numerical simulations. By increasing the particle density, the net dc ratchet current in our alternating (ac)-driven systems can either increase or decrease depending on the temperature, the drive amplitude, and the nature of the inter-particle interactions. This provides an effective control of particle motion by just changing the particle density. At low temperatures, attracting particles can condense at some potential minima, thus breaking the discrete translational symmetry of the substrate. Depending on the drive amplitude, an agglomeration or condensation results either in a drop to zero or in a saturation of the net particle velocity at densities above the condensation density—the latter case producing a very efficient rectification mechanism. For binary mixtures we find three ways of controlling the particle motion of one (passive) BB species by means of another (active) AA species: (i) Dragging the target particles BB by driving the auxiliary particles AA, (ii) rectifying the motion of the BB particles on the asymmetric potential created by the A–BA–B interactions, and (iii) dynamically modifying (pulsating) this potential by controlling the motion of the AA particles. This allows to easily control the magnitude and direction of the velocity of the target particles by changing either the frequency, phase and/or amplitude of the applied ac drive(s).Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87889/2/026112_1.pd
Coherent emission from disordered arrays of driven Josephson vortices
We propose a mechanism of coherent emission from driven vortices in stacked
intrinsic Josephson junctions. In contrast to super-radiance, which occurs only
for highly ordered vortex lattices, we predict resonant radiation emission from
weakly correlated vortex arrays. Our analytical results for the THz wave
intensity, resonance frequencies, and the dependence of THz emission power on
dissipation are in good agreement with the ones obtained by recent simulations.Comment: 2 figure
Generation of tunable Terahertz out-of-plane radiation using Josephson vortices in modulated layered superconductors
We show that a moving Josephson vortex in spatially modulated layered
superconductors generates out-of-plane THz radiation. Remarkably, the magnetic
and in-plane electric fields radiated are of the same order, which is very
unusual for any good-conducting medium. Therefore, the out-of-plane radiation
can be emitted to the vacuum without the standard impedance mismatch problem.
Thus, the proposed design can be more efficient for tunable THz emitters than
previous proposals, for radiation only propagating along the ab-plane.Comment: 7 pages, 1 figure. Phys. Rev. B (2005), in pres
Molecular dynamics simulations of oxide memristors: crystal field effects
We present molecular-dynamic simulations of memory resistors (memristors)
including the crystal field effects on mobile ionic species such as oxygen
vacancies appearing during operation of the device. Vacancy distributions show
different patterns depending on the ratio of a spatial period of the crystal
field to a characteristic radius of the vacancy-vacancy interaction. There are
signatures of the orientational order and of spatial voids in the vacancy
distributions for some crystal field potentials. The crystal field stabilizes
the patterns after they are formed, resulting in a non-volatile switching of
the simulated devices.Comment: 9 pages, 3 figure
"Unusual" critical states in type-II superconductors
We give a theoretical description of the general critical states in which the
critical currents in type-II superconductors are not perpendicular to the local
magnetic induction. Such states frequently occur in real situations, e.g., when
the sample shape is not sufficiently symmetric or the direction of the external
magnetic field changes in some complex way. Our study is restricted to the
states in which flux-line cutting does not occur. The properties of such
general critical states can essentially differ from the well-known properties
of the usual Bean critical states. To illustrate our approach, we analyze
several examples. In particular, we consider the critical states in a slab
placed in a uniform perpendicular magnetic field and to which two components of
the in-plane magnetic field are then applied successively. We also analyze the
critical states in a long thin strip placed in a perpendicular magnetic field
which then is tilted towards the axis of the strip.Comment: 15 pages including 11 figure
Ring-shaped luminescence patterns in a locally photoexcited electron-hole bilayer
We report the results of molecular dynamics simulation of a spatiotemporal
evolution of the locally photoexcited electrons and holes localized in two
separate layers. It is shown that the ring-shaped spatial pattern of
luminescence forms due to the strong in-layer Coulomb interaction at high
photoexcitation power. In addition, the results predict (i) stationary spatial
oscillations of the electron density in quasi one-dimensional case and (ii)
dynamical phase transition in the expansion of two-dimensional electron cloud
when threshold electron concentration is reached. A possible reason of the
oscillations and a theoretical interpretation of the transition are suggested.Comment: 6 pages, 5 figures. Final version as published + Erratum has been
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