Keldysh and Doi-Peliti Techniques for out-of-Equilibrium Systems

Lecture notes presented at Windsor NATO school on "Field Theory of Strongly Correlated Fermions and Bosons in Low-Dimensional Disordered Systems" (August 2001). The purpose of these lectures is to give a brief modern introduction to Keldysh non-equilibrium field theory and its classical analog - Doi-Peliti technique. The special emphasis is put on stressing the analogy between the two approaches.Comment: 19 page

Near-field heat transfer between disordered conductors

We study heat transfer mediated by near-field fluctuations of the electromagnetic field. In case of metals the latter are dominated by Coulomb interactions between thermal fluctuations of electronic density. We show that an elastic scattering of electrons, leading to diffusive propagation of density fluctuations, results in a qualitative change of the radiation law. While the heat flux between clean metals follows the Stefan-Boltzmann-like $T^4$ dependence, the heat exchange between disordered conductors is significantly enhanced and scales as $T^3$ at low temperatures.Comment: 4 pages, 2 figure

Many-body theory of non-equilibrium systems

Lectures notes for 2004 Les Houches Summer School on "Nanoscopic Quantum Transport". These lectures contain an introduction to Keldysh formalism for interacting bosonic and fermionic systems, presented in the functional integral framework. Covered topics include: kinetic theory, relation to classical techniques (such as Martin--Siggia--Rose and Fokker--Planck), non--linear sigma model for disordered fermions, etc.Comment: 70 pages, 7 figure

Optimization of spin-torque switching using AC and DC pulses

We explore spin-torque induced magnetic reversal in magnetic tunnel junctions using combined AC and DC spin-current pulses. We calculate the optimal pulse times and current strengths for both AC and DC pulses as well as the optimal AC signal frequency, needed to minimize the Joule heat lost during the switching process. The results of this optimization are compared against numeric simulations. Finally we show how this optimization leads to different dynamic regimes, where switching is optimized by either a purely AC or DC spin-current, or a combination AC/DC spin-current, depending on the anisotropy energies and the spin-current polarization.Comment: 10 pages, 9 figure

From the Zero--bias Anomaly to the Coulomb Blockade: an Exactly Solvable Model.

A microscopic theory of zero wavelength (q=0) interaction in finite--size systems is proposed. Its exact solution interpolates between the Coulomb blockade and the perturbative Altshuler--Aronov theory, in the strong and weak interaction limits respectively. The tunneling density of states and the quasiparticle life--time are calculated. The physical nature of the q=0 component of the interaction is discussed.Comment: 10 pages, 2 figures, REVTEX 3.0. Figures are tarred compressed, no changes in the text

Forming doublons by a quantum quench

Repulsive interactions between particles on a lattice may lead to bound states, so called doublons. Such states may be created by dynamically tuning the interaction strength, e.g. using a Feshbach resonance, from attraction to repulsion. We study the doublon production efficiency as a function of the tuning rate at which the on-site interaction is varied. An expectation based on the Landau- Zener law suggests that exponentially few doublons are created in the adiabatic limit. Contrary to such an expectation, we found that the number of produced doublons scales as a power law of the tuning rate with the exponent dependent on the dimensionality of the lattice. The physical reason for this anomaly is the effective decoupling of doublons from the two-particle continuum for center of mass momenta close to the corners of the Brillouin zone. The study of doublon production may be a sensitive tool to extract detailed information about the band structure.Comment: 5 pages, 3 figure

Symmetry Protected Topological Metals

We show that sharply defined topological quantum phase transitions are not limited to states of matter with gapped electronic spectra. Such transitions may also occur between two gapless metallic states both with extended Fermi surfaces. The transition is characterized by a discontinuous, but not quantized, jump in an off-diagonal transport coefficient. Its sharpness is protected by a symmetry, such as e.g. particle-hole, which remains unbroken across the transition. We present a simple model of this phenomenon, based on 2D $p+ip$ superconductor with an applied supercurrent, and discuss its geometrical interpretation.Comment: 5 pages, 5 figure

Phonon-mediated Casimir interaction between mobile impurities in one-dimensional quantum liquids

Virtual phonons of a quantum liquid scatter off impurities and mediate a long-range interaction, analogous to the Casimir effect. In one dimension the effect is universal and the induced interaction decays as $1/r^3$, much slower than the van der Waals interaction $\sim1/r^6$, where $r$ is the impurity separation. The sign of the effect is characterized by the product of impurity-phonon scattering amplitudes, which take a universal form and have been seen to vanish for several integrable impurity models. Thus, if the impurity parameters can be independently tuned to lie on opposite sides of such integrable points, one can observe an attractive interaction turned into a repulsive one.Comment: 4+4 pages, 1+1 figures. Published version [Editors' Suggestion

Coulomb blockade with neutral modes

We study transport through a quantum dot in the fractional quantum Hall regime with filling factors \nu=2/3 and \nu=5/2, weakly coupled to the leads. We account for both injection of electrons to/from the leads, and quasiparticle rearrangement processes between the edge and the bulk of the quantum dot. The presence of neutral modes introduces topological constraints that modify qualitatively the features of the Coulomb blockade (CB). The periodicity of CB peak spacings doubles and the ratio of spacing between adjacent peaks approaches (in the low temperature and large dot limit) a universal value: 2:1 for \nu=2/3 and 3:1 for \nu=5/2. The corresponding CB diamonds alternate their width in the direction of the bias voltage and allow for the determination of the neutral mode velocity, and of the topological numbers associated with it.Comment: 5 pages, 4 figure

Charge Fluctuations in a Quantum Dot with a Dissipative Environment

We consider a multiple tunneling process into a quantum dot capacitively coupled to a dissipative environment. The problem is mapped onto an anisotropic Kondo model in its Coulomb gas representation. The tunneling barrier resistance and the dissipative resistance of the environment correspond to the transverse and the longitudinal Kondo couplings respectively. We thus identify a line in the parameter space of the problem which corresponds to a zero-temperature Berezinskii-Kosterlitz-Thouless like phase transition. The physics of coupling to the environment is elucidated and experimental consequences of the predicted transition are discussed.Comment: 13 pages, RevTe