Fast Quantum Computing with Buckyballs

We have found that encapsulated atoms in fullerene molecules, which carry a spin, can be used for fast quantum computing. We describe the scheme for performing quantum computations, going through the preparation of the qubit state and the realization of a two-qubit quantum gate. When we apply a static magnetic field to each encased spin, we find out the ideal design for the preparation of the quantum state. Therefore, adding to our system a time dependent magnetic field, we can perform a phase-gate. The operational time related to a $\pi-$phase gate is of the order of $ns$. This finding shows that, during the decoherence time, which is proportional to $\mu s$, we can perform many thousands of gate operations. In addition, the two-qubit state which arises after a $\pi-$gate is characterized by a high degree of entanglement. This opens a new avenue for the implementation of fast quantum computation.Comment: 9 pages, 4 figure

Switching and symmetry breaking behaviour of discrete breathers in Josephson ladders

We investigate the roto-breathers recently observed in experiments on Josephson ladders subjected to a uniform transverse bias current. We describe the switching mechanism in which the number of rotating junctions increases. In the region close to switching we find that frequency locking, period doubling, quasi-periodic behaviour and symmetry breaking all occur. This suggests that a chaotic dynamic occurs in the switching process. Close to switching the induced flux increases sharply and clearly plays an important role in the switching mechanism. We also find three critical frequencies which are independent of the dissipation constant $\alpha$, provided that $\alpha$ is not too large

Direct current generation due to harmonic mixing: From bulk semiconductors to semiconductor superlattices

We discuss an effect of dc current and dc voltage (stopping bias) generation in a semiconductor superlattice subjected by an ac electric field and its phase-shifted n-th harmonic. In the low field limit, we find a simple dependence of dc voltage on a strength, frequency, and relative phase of mixing harmonics for an arbitrary even value of n. We show that the generated dc voltage has a maximum when a frequency of ac field is of the order of a scattering constant of electrons in a superlattice. This means that for typical semiconductor superlattices at room temperature operating in the THz frequency domain the effect is really observable. We also made a comparison of a recent paper describing an effect of a directed current generation in a semiconductor superlattice subjected by ac field and its second harmonic (n=2) [K.Seeger, Appl.Phys.Lett. 76(2000)82] with our earlier findings describing the same effect [K.Alekseev et al., Europhys. Lett. 47(1999)595; cond-mat/9903092 ]. For the mixing of an ac field and its n-th harmonic with n>=4, we found that additionally to the phase-shift controlling of the dc current, there is a frequency control. This frequency controlling of the dc current direction is absent in the case of n=2. The found effect is that, both the dc current suppression and the dc current reversals exist for some particular values of ac field frequency. For typical semiconductor superlattices such an interesting behavior of the dc current should be observable also in the THz domain. Finally, we briefly review the history of the problem of the dc current generation at mixing of harmonics in semiconductors and semiconductor microstructures.Comment: 9 pages, 1 figure, RevTEX, EPS

Exotic structures on magnetic multilayers

To characterize the possible magnetic structures created on magnetic multilayers a model has been formulated and studied. The interlayer inhomogeneous structures found indicate either (i) a regular periodic, (ii) a quasiperiodic change in the magnetization or (iii) spatially chaotic glass states. The magnetic structures created depend mainly on the ratio of the magnetic anisotropy constant to the exchange constant. With the increase of this ratio the periodic structures first transform into the quasiperiodic and then into the chaotic glass states. The same tendency arises with the depolarization of the magnetic moments of the first layer deposited on the substrate.Comment: 4 pages, 1 figur

Gravitational vortices and clump formation In Saturn's F ring during an encounter with Prometheus

Saturn rings are most beautiful and dynamic places in the solar system, consisting of ice particles in a constant battle between the gravitational forces of Saturn and its many moons. Fan, spiral, propellers, moonlets and streamer-channels observed by CASSINI in the F-ring have been attributed to encounters by Prometheus on the F ring, with investigations of optical thickness revealing large populations of transient moonlets. Taking into account gravitational interaction between particles and a multi-stranded F-ring structure we show that Prometheus' encounters create rotational flows, like atmospheric vortices and the self-gravity enhances the accelerated growth and size of moonlets. Vortex patches form caustics, which is a primary cause of the transient particle density clumps of 20 km width and 100 km length, and they are elongated to cover an area of 1600 km by 150 km, which may eventually combine into a vortex sheet

Chaos and rectification of electromagnetic wave in a lateral semiconductor superlattice

We find the conditions for a rectification of electromagnetic wave in a lateral semiconductor superlattice with a high mobility of electrons. The rectification is assisted by a transition to a dissipative chaos at a very high mobility. We show that mechanism responsible for the rectification is a creation of warm electrons in the superlattice miniband caused by an interplay of the effects of nonlinearity and finite band width.Comment: 4 pages, 4 figures (2 color figs). Sufficient revision in comparison with version1: More explanations on physics of the effect are added. Removed from version1 material will be published elsewher

Two-Dimensional Ising Model with Competing Interactions as a Model for Interacting pi-Rings

We investigate the Ising model on a square lattice with antiferromagnetic exchange between nearest and next-nearest neighbors and show that at low temperatures stripe-like and droplet-like superstructures appear. We show that the competing interactions introduce a strong frustration that could plausibly describe the systems with dipole{dipole type interactions and pay particular attention to arrays of interacting Josephson ¼-rings