Decoherence without dissipation?

In a recent article, Ford, Lewis and O'Connell (PRA 64, 032101 (2001)) discuss a thought experiment in which a Brownian particle is subjected to a double-slit measurement. Analyzing the decay of the emerging interference pattern, they derive a decoherence rate that is much faster than previous results and even persists in the limit of vanishing dissipation. This result is based on the definition of a certain attenuation factor, which they analyze for short times. In this note, we point out that this attenuation factor captures the physics of decoherence only for times larger than a certain time t_mix, which is the time it takes until the two emerging wave packets begin to overlap. Therefore, the strategy of Ford et al of extracting the decoherence time from the regime t < t_mix is in our opinion not meaningful. If one analyzes the attenuation factor for t > t_mix, one recovers familiar behaviour for the decoherence time; in particular, no decoherence is seen in the absence of dissipation. The latter conclusion is confirmed with a simple calculation of the off-diagonal elements of the reduced density matrix.Comment: 8 pages, 4 figure

Optical conductivity near finite-wavelength quantum criticality

We study the optical conductivity sigma(Omega) of an electron system near a quantum-critical point with finite-wavelength ordering. sigma(Omega) vanishes in clean Galilean-invariant systems, unless electrons are coupled to dynamical collective modes, which dissipate the current. This coupling introduces a nonuniversal energy scale. Depending on the parameters of each specific system, a variety of responses arise near criticality: scaling peaks at a temperature- and doping-dependent frequency, peaks at a fixed frequency, or no peaks to be associated with criticality. Therefore the lack of scaling in the far-infrared conductivity in cuprates does not necessarily call for new concepts of quantum criticality.Comment: 4 pages, 4 figures; version as publishe

The Role of Vortices in the Mutual Coupling of Superconducting and Normal-Metal Films

I propose a possible explanation to a recently observed ``cross-talk'' effect in metal-insulator-metal trilayers, indicating a sharp peak near a superconducting transition in one of the metal films. Coulomb interactions are excluded as a dominant coupling mechanism, and an alternative is suggested, based on the local fluctuating electric field induced by mobile vortices in the superconducting layer. This scenario is compatible with the magnitude of the peak signal and its shape; most importantly, it addresses the {\it non-reciprocity} of the effect in exchanging the roles of the films.Comment: 13 pages, RevTe

Collective modes and electromagnetic response of a chiral superconductor

Motivated by the recent controversy surrounding the Kerr effect measurements in strontium ruthenate \cite{xia:167002}, we examine the electromagnetic response of a clean chiral p-wave superconductor. When the contributions of the collective modes are accounted for, the Hall response in a clean chiral superconductor is smaller by several orders of magnitude than previous theoretical predictions and is too small to explain the experiment. We also uncover some unusual features of the collective modes of a chiral superconductor, namely, that they are not purely longitudinal and couple to external transverse fields.Comment: 8 page

Quantum Master Equation of Particle in Gas Environment

The evolution of the reduced density operator $\rho$ of Brownian particle is discussed in single collision approach valid typically in low density gas environments. This is the first succesful derivation of quantum friction caused by {\it local} environmental interactions. We derive a Lindblad master equation for $\rho$, whose generators are calculated from differential cross section of a single collision between Brownian and gas particles, respectively. The existence of thermal equilibrium for $\rho$ is proved. Master equations proposed earlier are shown to be particular cases of our one.Comment: 6 pages PlainTeX, 23-March-199

Theory of Coherent cc-Axis Josephson Tunneling between Layered Superconductors

We calculate exactly the Josephson current for $c$-axis coherent tunneling between two layered superconductors, each with internal coherent tight-binding intra- and interlayer quasiparticle dispersions. Our results also apply when one or both of the superconductors is a bulk material, and include the usually neglected effects of surface states. For weak tunneling, our results reduce to our previous results derived using the tunneling Hamiltonian. Our results are also correct for strong tunneling. However, the $c$-axis tunneling results of Tanaka and Kashiwaya are shown to be incorrect in any limit. In addition, we consider the $c$-axis coherent critical current between two identical layered superconductors twisted an angle $\phi_0$ about the $c$-axis with respect to each other. Regardless of the order parameter symmetry, our coherent tunneling results using a tight-binding intralayer quasiparticle dispersion are inconsistent with the recent $c$-axis twist bicrystal Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ twist junction experiments of Li {\it et al.}Comment: 11 pages, 13 figures, submitted to Physical Review

Thin Films of 3He -- Implications on the Identification of 3 He -A

Recently the identification of 3He-A with the axial state has been questioned. It is suggested that the A-phase can actually be in the axiplanar state. We point out in the present paper that experiments in a film geometry may be useful to distinguish the above two possibilities. In particular a second order phase transition between an axial and an axiplanar state would occur as a function of thickness or temperature.Comment: 3 pages, no figures latex- revtex aps accepted by J. of Low Temperature Physic

Resonance Enhanced Tunneling

Time evolution of tunneling in thermal medium is examined using the real-time semiclassical formalism previously developed. Effect of anharmonic terms in the potential well is shown to give a new mechanism of resonance enhanced tunneling. If the friction from environment is small enough, this mechanism may give a very large enhancement for the tunneling rate. The case of the asymmetric wine bottle potential is worked out in detail.Comment: 12 pages, LATEX file with 5 PS figure

Josephson scanning tunneling microscopy

We propose a set of scanning tunneling microscopy experiments in which the surface of superconductor is scanned by a superconducting tip. Potential capabilities of such experimental setup are discussed. Most important anticipated results of such an experiment include the position-resolved measurement of the superconducting order parameter and the possibility to determine the nature of the secondary component of the order parameter at the surface. The theoretical description based on the tunneling Hamiltonian formalism is presented.Comment: 6 pages, 7 figures, submitted to Phys. Rev.

Langevin Simulations of Two Dimensional Vortex Fluctuations: Anomalous Dynamics and a New IVIV-exponent

The dynamics of two dimensional (2D) vortex fluctuations are investigated through simulations of the 2D Coulomb gas model in which vortices are represented by soft disks with logarithmic interactions. The simulations trongly support a recent suggestion that 2D vortex fluctuations obey an intrinsic anomalous dynamics manifested in a long range 1/t-tail in the vortex correlations. A new non-linear IV-exponent a, which is different from the commonly used AHNS exponent, a_AHNS and is given by a = 2a_AHNS - 3, is confirmed by the simulations. The results are discussed in the context of earlier simulations, experiments and a phenomenological description.Comment: Submitted to PRB, RevTeX format, 28 pages and 13 figures, figures in postscript format are available at http://www.tp.umu.se/~holmlund/papers.htm