Nonequilibrium Transport through a Kondo Dot: Decoherence Effects

We investigate the effects of voltage induced spin-relaxation in a quantum dot in the Kondo regime. Using nonequilibrium perturbation theory, we determine the joint effect of self-energy and vertex corrections to the conduction electron T-matrix in the limit of transport voltage much larger than temperature. The logarithmic divergences, developing near the different chemical potentials of the leads, are found to be cut off by spin-relaxation rates, implying that the nonequilibrium Kondo-problem remains at weak coupling as long as voltage is much larger than the Kondo temperature.Comment: 16 pages, 4 figure

Non-Equilibrium Transport through a Kondo-Dot in a Magnetic Field: Perturbation Theory and Poor Man's Scaling

We consider electron transport through a quantum dot described by the Kondo model in the regime of large transport voltage V in the presence of a magnetic field B with max(V,B) >> T_K. The electric current I and the local magnetization M are found to be universal functions of V/T_K and B/T_K, where T_K is the equilibrium Kondo temperature. We present a generalization of the perturbative renormalization group to frequency dependent coupling functions, as necessitated by the structure of bare perturbation theory. We calculate I and M within a poor man's scaling approach and find excellent agreement with experiment.Comment: version accepted in PRL, notations changed, parts rewritten, figures modified, references and some corrections adde

Giant mass and anomalous mobility of particles in fermionic systems

We calculate the mobility of a heavy particle coupled to a Fermi sea within a non-perturbative approach valid at all temperatures. The interplay of particle recoil and of strong coupling effects, leading to the orthogonality catastrophe for an infinitely heavy particle, is carefully taken into account. We find two novel types of strong coupling effects: a new low energy scale $T^{\star}$ and a giant mass renormalization in the case of either near-resonant scattering or a large transport cross section $\sigma$. The mobility is shown to obey two different power laws below and above $T^{\star}$. For $\sigma\gg\lambda_f^2$, where $\lambda_f$ is the Fermi wave length, an exponentially large effective mass suppresses the mobility.Comment: 4 pages, 4 figure

Zero temperature optical conductivity of ultra-clean Fermi liquids and superconductors

We calculate the low-frequency optical conductivity sigma(w) of clean metals and superconductors at zero temperature neglecting the effects of impurities and phonons. In general, the frequency and temperature dependences of sigma have very little in common. For small Fermi surfaces in three dimensions (but not in 2D) we find for example that Re sigma(w>0)=const. for low w which corresponds to a scattering rate Gamma proportional to w^2 even in the absence of Umklapp scattering when there is no T^2 contribution to Gamma. In the main part of the paper we discuss in detail the optical conductivity of d-wave superconductors in 2D where Re sigma(w>0) \propto w^4 for the smallest frequencies and the Umklapp processes typically set in smoothly above a finite threshold w_0 smaller than twice the maximal gap Delta. In cases where the nodes are located at (pi/2, pi/2), such that direct Umklapp scattering among them is possible, one obtains Re sigma(w) \propto w^2.Comment: 7 pages, 3 figure

Heat transport of clean spin-ladders coupled to phonons: Umklapp scattering and drag

We study the low-temperature heat transport in clean two-leg spin ladder compounds coupled to three-dimensional phonons. We argue that the very large heat conductivities observed in such systems can be traced back to the existence of approximate symmetries and corresponding weakly violated conservation laws of the effective (gapful) low--energy model, namely pseudo-momenta. Depending on the ratios of spin gaps and Debye energy and on the temperature, the magnetic contribution to the heat conductivity can be positive or negative, and exhibit an activated or anti-activated behavior. In most regimes, the magnetic heat conductivity is dominated by the spin-phonon drag: the excitations of the two subsystems have almost the same drift velocity, and this allows for an estimate of the ratio of the magnetic and phononic contributions to the heat conductivity.Comment: revised version, 8 pages, 3 figures, added appendi

Interplay of disorder and spin fluctuations in the resistivity near a quantum critical point

The resistivity in metals near an antiferromagnetic quantum critical point (QCP) is strongly affected by small amounts of disorder. In a quasi-classical treatment, we show that an interplay of strongly anisotropic scattering due to spin fluctuations and isotropic impurity scattering leads to a large regime where the resistivity varies as T^alpha, with an anomalous exponent, alpha, 1 <= alpha <= 1.5, depending on the amount of disorder. I argue that this mechanism explains in some detail the anomalous temperature dependence of the resistivity observed in CePd_2Si_2, CeNi_2Ge_2 and CeIn_3 near the QCP.Comment: 4 pages, 4 eps figures, published version, only small change

Nonequilibrium Singlet-Triplet Kondo Effect in Carbon Nanotubes

The Kondo-effect is a many-body phenomenon arising due to conduction electrons scattering off a localized spin. Coherent spin-flip scattering off such a quantum impurity correlates the conduction electrons and at low temperature this leads to a zero-bias conductance anomaly. This has become a common signature in bias-spectroscopy of single-electron transistors, observed in GaAs quantum dots as well as in various single-molecule transistors. While the zero-bias Kondo effect is well established it remains uncertain to what extent Kondo correlations persist in non-equilibrium situations where inelastic processes induce decoherence. Here we report on a pronounced conductance peak observed at finite bias-voltage in a carbon nanotube quantum dot in the spin singlet ground state. We explain this finite-bias conductance anomaly by a nonequilibrium Kondo-effect involving excitations into a spin triplet state. Excellent agreement between calculated and measured nonlinear conductance is obtained, thus strongly supporting the correlated nature of this nonequilibrium resonance.Comment: 21 pages, 5 figure