Quantum escape of the phase in a strongly driven Josephson junction

A quantum mechanical analysis of the Josephson phase escape in the presence of both dc and ac bias currents is presented. We find that the potential barrier for the escape of the phase is effectively suppressed as the resonant condition occurs, i.e. when the frequency $\omega$ of the ac bias matches the Josephson junction energy level separation. This effect manifests itself by a pronounced drop in the dependence of the switching current $I_s$ on the power $W$ of the applied microwave radiation and by a peculiar double-peak structure in the switching current distribution $P(I_s)$. The developed theory is in a good accord with an experiment which we also report in this paper. The obtained features can be used to characterize certain aspects of the quantum-mechanical behavior of the Josephson phase, such as the energy level quantization, the Rabi frequency of coherent oscillations and the effect of damping.Comment: 4 pages, 3 figures, to be published in Physical Review B (Rapid Communication

Secondary atomization of coal-water fuels for gas turbine applications

The main research objective is to determine the effect of coal-water fuel (CWF) treatment on atomization quality when applied to an ultrafine coal water fuel (solids loading - 50%) and at elevated pressures. The fuel treatment techniques are expected to produce secondary atomization, i.e., disruptive shattering of CWF droplets subsequent to their leaving the atomizing nozzle. Upon combustion, the finer fuel droplets would then yield better burnout and finer fly ash size distribution, which in turn could reduce problems of turbine blade erosion. The parallel objective was to present quantitative information on the spray characteristics of CWF (average droplet size and spray shape and angle) with and without fuel treatment for purposes of application to the design of CWF-burning gas turbine combustors. The experiments include laser diffraction droplet size measurements and high speed photographic studies of CWF sprays in the MIT Spray Test Facility to determine mean droplet size (mass median diameter), droplet size distribution, and spray shape and angle. For the spray tests at elevated pressures, pressure vessels were constructed and installed in the spray test rig. For support of data analyses, a capillary tube viscometer was used to measure the CWF viscosity at the high shear rate that occurs in an atomizer (> 104 sec' ). A semi-empirical relationship was developed giving the CWF spray droplet size as a function of the characteristic dimensionless parameters of twin-fluid atomization, including the Weber number, the Reynolds number, and the air-to-fuel mass flow ratio. The correlation was tested experimentally and good agreement was found between calculated and measured drop sizes when the high shear viscosity of the CWF was used in the semi-empirical equation. Water and CWF spray tests at elevated pressure were made. Average droplet sizes measured as a function of atomizing air-to-fuel ratios (AFRs) at various chamber pressures show that the droplet mass median diameter (MMD) decreases with increasing AFR at a given chamber pressure and increases with increasing chamber pressure at a given AFR. In particular, the results show that droplet sizes of CWF sprays decrease with increasing chamber pressure if the atomizing air velocity is held constant. Of the fuel treatment techniques investigated, the heating of CWF (flash-atomization) was found to be very effective in reducing droplet size, not only at atmospheric pressure but also at elevated pressure. Secondary atomization by C02 absorption (used in a previous study) had given favorable results on CWF combustion, but in this present case this fuel treatment did not seem to have any observable effect on the drop size distribution of the CWF spray at room temperature. The spray angle was observed to reduce with increasing chamber pressure for given atomizing conditions (AFR, fuel flow rate, fuel temperature). The decreasing entrainment rate per unit length of spray with increasing chamber pressure was mainly responsible for the reduction of the spray angle. The heating of the CWF increased the spray angle, both at atmospheric and elevated pressures. A model was developed to predict spray angle change for the effects of the flash-atomization as a function of AFR, fuel flow rate, and the superheat of the water

Multifractal characterisation of length sequences of coding and noncoding segments in a complete genome

The coding and noncoding length sequences constructed from a complete genome are characterised by multifractal analysis. The dimension spectrum $D_{q}$ and its derivative, the 'analogous' specific heat $C_{q}$, are calculated for the coding and noncoding length sequences of bacteria, where $q$ is the moment order of the partition sum of the sequences. From the shape of the $$ and $C_{q}$ curves, it is seen that there exists a clear difference between the coding/noncoding length sequences of all organisms considered and a completely random sequence. The complexity of noncoding length sequences is higher than that of coding length sequences for bacteria. Almost all $D_{q}$ curves for coding length sequences are flat, so their multifractality is small whereas almost all $D_{q}$ curves for noncoding length sequences are multifractal-like. We propose to characterise the bacteria according to the types of the $C_{q}$ curves of their noncoding length sequences.Comment: 15 pages with 5 figures, Latex, Accepted for publication in Physica

Decoherence in a Josephson junction qubit

The zero-voltage state of a Josephson junction biased with constant current consists of a set of metastable quantum energy levels. We probe the spacings of these levels by using microwave spectroscopy to enhance the escape rate to the voltage state. The widths of the resonances give a measurement of the coherence time of the two states involved in the transitions. We observe a decoherence time shorter than that expected from dissipation alone in resonantly isolated 20 um x 5 um Al/AlOx/Al junctions at 60 mK. The data is well fit by a model including dephasing effects of both low-frequency current noise and the escape rate to the continuum voltage states. We discuss implications for quantum computation using current-biased Josephson junction qubits, including the minimum number of levels needed in the well to obtain an acceptable error limit per gate.Comment: 4 pages, 6 figure

Running anti-de Sitter radius from QCD-like strings

We consider renormalization effects for a bosonic QCD-like string, whose partons have $1/p^{2}$ propagators instead of Gaussian. Classically this model resembles (the bosonic part of) the projective light-cone (zero-radius) limit of a string on an AdS${}_5$ background, where Schwinger parameters give rise to the fifth dimension. Quantum effects generate dynamics for this dimension, producing an AdS${}_5$ background with a running radius. The projective light-cone is the high-energy limit: Holography is enforced dynamically.Comment: 12 page

Magnetoinductance of Josephson junction array with frozen vortex diffusion

The dependence of sheet impedance of a Josephson junction array on the applied magnetic field is investigated in the regime when vortex diffusion between array plaquettes is effectively frozen due to low enough temperature. The field dependent contribution to sheet inductance is found to be proportional to f*ln(1/f), where f<<1 is the magnitude of the field expressed in terms of flux quanta per plaquette.Comment: 5 pages, no figure

Mixing-induced CP violating sources for electroweak baryogenesis from a semiclassical approach

The effects of flavor mixing in electroweak baryogenesis is investigated in a generalized semiclassical WKB approach. Through calculating the nonadiabatic corrections to the particle currents it is shown that extra CP violation sources arise from the off-diagonal part of the equation of motion of particles moving inside the bubble wall. This type of mixing-induced source is of the first order in derivative expansion of the Higgs condensate, but is oscillation suppressed. The numerical importance of the mixing-induced source is discussed in the Minimal Supersymmetric Standard Model and compared with the source term induced by semiclassical force. It is found that in a large parameter space where oscillation suppression is not strong enough, the mixing-induced source can dominate over that from the semiclassical force.Comment: 19 pp, 2 figs, 1 table, some comments added, to appear in Eur.Phys.J.

Electric-field dependent spin diffusion and spin injection into semiconductors

We derive a drift-diffusion equation for spin polarization in semiconductors by consistently taking into account electric-field effects and nondegenerate electron statistics. We identify a high-field diffusive regime which has no analogue in metals. In this regime there are two distinct spin diffusion lengths. Furthermore, spin injection from a ferromagnetic metal into a semiconductor is enhanced by several orders of magnitude and spins can be transported over distances much greater than the low-field spin diffusion length.Comment: 5 pages, 3 eps figure

Quasi-local Energy for Spherically Symmetric Spacetimes

We present two complementary approaches for determining the reference for the covariant Hamiltonian boundary term quasi-local energy and test them on spherically symmetric spacetimes. On the one hand, we isometrically match the 2-surface and extremize the energy. This can be done in two ways, which we call programs I (without constraint) and II (with additional constraints). On the other hand, we match the orthonormal 4-frames of the dynamic and the reference spacetimes. Then, if we further specify the observer by requiring the reference displacement to be the timelike Killing vector of the reference, the result is the same as program I, and the energy can be positive, zero, or even negative. If, instead, we require that the Lie derivatives of the two-area along the displacement vector in both the dynamic and reference spacetimes to be the same, the result is the same as program II, and it satisfies the usual criteria: the energies are non-negative and vanish only for Minkowski (or anti-de Sitter) spacetime.Comment: 16 pages, no figure

Large Aperiodic Semigroups

The syntactic complexity of a regular language is the size of its syntactic semigroup. This semigroup is isomorphic to the transition semigroup of the minimal deterministic finite automaton accepting the language, that is, to the semigroup generated by transformations induced by non-empty words on the set of states of the automaton. In this paper we search for the largest syntactic semigroup of a star-free language having $n$ left quotients; equivalently, we look for the largest transition semigroup of an aperiodic finite automaton with $n$ states. We introduce two new aperiodic transition semigroups. The first is generated by transformations that change only one state; we call such transformations and resulting semigroups unitary. In particular, we study complete unitary semigroups which have a special structure, and we show that each maximal unitary semigroup is complete. For $n \ge 4$ there exists a complete unitary semigroup that is larger than any aperiodic semigroup known to date. We then present even larger aperiodic semigroups, generated by transformations that map a non-empty subset of states to a single state; we call such transformations and semigroups semiconstant. In particular, we examine semiconstant tree semigroups which have a structure based on full binary trees. The semiconstant tree semigroups are at present the best candidates for largest aperiodic semigroups. We also prove that $2^n-1$ is an upper bound on the state complexity of reversal of star-free languages, and resolve an open problem about a special case of state complexity of concatenation of star-free languages.Comment: 22 pages, 1 figure, 2 table