Model of multiphoton transitions in a current-biased Josephson junction

We present a simple model for multiphoton transitions between the quasi-bound states of a current-driven Josephson junction. The transitions are induced by applying an ac voltage with controllable frequency and amplitude across the junction. The voltage induces transitions when the ac frequency equals n times the splitting between the ground and first excited quasi-bound state of the junction. We calculate the transition matrix elements as functions of the dc bias current I, and the frequency and amplitude of the ac voltage, for representative junction parameters. We also calculate the frequency-dependent absorption coefficient by solving the relevant Bloch equations when the ac amplitude is sufficiently small. In this regime, the absorption coefficient is a sum of Lorentzian lines centered at the n-photon absorption frequency, of strength proportional to the squared matrix elements. For fixed ac voltage amplitude, the n-photon transition rate usually decreases with increasing n. We also find a characteristic even-odd effect: The absorption coefficient typically increases with I for n even but decreases for n odd. Our results agree qualitatively with recent experiments.Comment: 15 pages, 13 figures, accepted for publication in Physical Review

Model of the Longitudinal Spin Seebeck Coefficient of InSb in a Magnetic Field

We develop a simple theory for the longitudinal spin Seebeck effect in n-doped InSb in an external magnetic field. We consider spin-$1/2$ electrons in the conduction band of InSb with a temperature gradient parallel to the applied magnetic field. In the absence of spin-orbit interactions, a Boltzmann equation approach leads to a spin current parallel to the field and proportional to the temperature gradient. The calculated longitudinal spin Seebeck coefficients oscillates as a function of magnetic field B; the peak positions are approximately periodic in 1/B. The oscillations arise when the Fermi energy crosses the bottom of a Landau band.Comment: 7 pages, 6 figure

Theory of plasmonic waves on a chain of metallic nanoparticles in a liquid crystalline host

A chain of metallic particles, of sufficiently small diameter and spacing, allows linearly polarized plasmonic waves to propagate along the chain. In this paper, we describes how these waves are altered when the liquid crystal host is a nematic or a cholesteric liquid crystal (NLC or CLC) with or without an applied magnetic field. We find that, in general, the liquid crystal host, either NLC or CLC, alters the dispersion relations of the transverse ($T$) and longitudinal ($L$) waves significantly from the dispersion relations for an isotropic host. We show that by altering the director axis of the liquid crystal relative to the long axis of the metallic chain, that the $T$ branch can be split into two non-degenerate linearly polarized branches (NLC host) or two non-degenerate elliptically polarized branches (CLC host). When an external magnetic field is applied parallel to both the long axis of the metallic particles and the director of the CLC host, we find that the dispersion relations are odd in an exchange in sign for $\omega$ for the non-degenerate elliptically polarized $T$ branches. That is, the application of an external magnetic field leads to the realization of a one-way waveguide.Comment: 9 Pages, 3 Figures. arXiv admin note: text overlap with arXiv:1210.150

Graphene with adatoms: tuning the magnetic moment with an applied voltage

We show that, in graphene with a small concentration of adatoms, the total magnetic moment $\mu_T$ can be switched on and off by varying the Fermi energy $E_F$, either by applying a gate voltage or by suitable chemical doping. Our calculation is carried out using a simple tight-binding model described previously, combined with a mean-field treatment of the electron-electron interaction on the adatom. The values of $E_F$ at which the moment is turned on or off are controlled by the strength of the hopping between the graphene sheet and the adatom, the on-site energy of the adatom, and the strength of the electron-electron correlation energy U. Our result is in qualitatively consistent with recent experiments by Nair {\it et al.} [Nat.\ Commun.\ {\bf 4}, 2010 (2013)].Comment: 4 Pages, 1 Figur

Numerical Study of Energy Loss by a Nanomechanical Oscillator Coupled to a Cooper Pair Box

We calculate the dynamics of a nanomechanical oscillator (NMO) coupled capacitively to a Cooper pair box (CPB), by solving a stochastic Schrodinger equation with two Lindblad operators. Both the NMO and the CPB are assumed dissipative, and the coupling is treated within the rotating wave approximation. We show numerically that, if the CPB decay time is smaller than the NMO decay time, the coupled NMO will lose energy faster, and the coupled CPB more slowly, than do the uncoupled NMO and CPB. The results show that the efficiency of energy loss by an NMO can be substantially increased if the NMO is coupled to a CPB.Comment: 10 pages, 3 figure