65 research outputs found
Hamiltonian theory of the strongly-coupled limit of the Kondo problem in the overscreened case
By properly generalizing Nozie`res' Fermi liquid theory, we construct an
Hamiltonian approach to the scattering of conduction electrons off a spin-1/2
impurity in the ovescreneed Kondo regime, as T -> 0. We derive the S-matrix at
the interacting fixed point, and the corresponding phase shifts, together with
leading energy corrections to the unitary limit. We apply our results to obtain
the low-temperature dependence of the 2-channel Kondo conductance, and we
relate it to possible transport experiments in a Quantum DotComment: 22 pages, 1 figur
Junction of three off-critical quantum Ising chains and two-channel Kondo effect in a superconductor
We show that a junction of three off-critical quantum Ising chains can be
regarded as a quantum spin chain realization of the two-channel spin-1/2
overscreened Kondo effect with two superconducting leads. We prove that, as
long as the Kondo temperature is larger than the superconducting gap, the
equivalent Kondo model flows towards the 2 channel Kondo fixed point. We argue
that our system provides the first controlled realization of 2 channel Kondo
effect with superconducting leads. This, besides its the theoretical interest,
is of importance for potential applications to a number of context, including
the analysis of the quantum entanglement properties of a Kondo system.Comment: 14 pages, 4 .eps figure
Fano versus Kondo Resonances in a Multilevel "Semi-Open" Quantum Dot
Linear conductance across a large quantum dot via a single level e_0 with
large hybridization to the contacts is strongly sensitive to quasi-bound states
localized in the dot and weakly coupled to e_0. It oscillates with the gate
voltage due to interference of the Fano type. At low temperature and Coulomb
blockade, Kondo correlations damp the oscillations on an extended range of gate
voltage values, by freezing the occupancy of the e_0 level itself. As a
consequence, antiresonances of Fano origin are washed out. The results are in
good correspondence with experimental data for a large quantum dot in the
semi-open regime.Comment: 4 eps figures, RevTex format, revised version, to appear in Phys.
Rev. Letter
High critical temperature nodal superconductors as building block for time-reversal invariant topological superconductivity
We study possible applications of high critical temperature nodal
superconductors for the search for Majorana bound states in the DIII class. We
propose a microscopic analysis of the proximity effect induced by d-wave
superconductors on a semiconductor wire with strong spin-orbit coupling. We
characterize the induced superconductivity on the wire employing a numerical
self-consistent tight-binding Bogoliubov-de Gennes approach, and analytical
considerations on the Green's function. The order parameter induced on the
wire, the pair correlation function, and the renormalization of the Fermi
points are analyzed in detail, as well as the topological phase diagram in the
case of weak coupling. We highlight optimal Hamiltonian parameters to access
the nontrivial topological phase which could display time-reversal invariant
Majorana doublets at the boundaries of the wire
Suppression of Kondo-assisted co-tunneling in a spin-1 quantum dot with Spin-Orbit interaction
Kondo-type zero-bias anomalies have been frequently observed in quantum dots
occupied by two electrons and attributed to a spin-triplet configuration that
may become stable under particular circumstances. Conversely, zero-bias
anomalies have been so far quite elusive when quantum dots are occupied by an
even number of electrons greater than two, even though a spin-triplet
configuration is more likely to be stabilized there than for two electrons. We
propose as an origin of this phenomenon the spin-orbit interaction, and we show
how it profoundly alters the conventional Kondo screening scenario in the
simple case of a laterally confined quantum dot with four electrons.Comment: 5 pages, 3 figures, submitted 05May201
Charge dynamics effects in conductance through a large semi-open quantum dot
Fano lineshapes in resonant transmission in a quantum dot imply interference
between localized and extended states. The influence of the charge accumulated
at the localized levels, which screens the external gate voltage acting on the
conduction channel is investigated. The modified Fano q parameter and the
resonant conduction is derived starting from a microscopic Hamiltonian. The
latest experiments on "charge sensing" and ``Coulomb modified Fano sensing ``
compare well with the results of the present model.Comment: 5 pages, 4 figures, RevTex styl
Josephson versus Kondo coupling in a quantum dot connected to two superconductors
We apply a Gutzwiller-like variational technique to study Josephson
conduction across a quantum dot with an odd number of electrons connected to
two superconducting leads. Our method projects out all states on the dot but
the Kondo singlet and is valid when Kondo correlations are dominant and no
Andreev bound states localized at the dot are available for Kondo screening. In
these conditions superconducting pairing is a competing effect and the junction
is like, to optimize antiferromagnetic correlations on the dot. As the
superconducting gap increases, the Josephson current also increases, but its
phase dependence becomes strongly non sinusoidal
Linear Kondo conductance in a quantum dot
In a tunneling experiment across a quantum dot it is possible to change the
coupling between the dot and the contacts at will, by properly tuning the
trasparency of the barriers and the temperature. Gate voltages allow for
changes of the relative position of the dot addition energies and the Fermi
level of the leads. Here we discuss the two limiting cases: weak and strong
coupling in the tunneling Hamiltonian. In the latter case Kondo resonant
conductance can emerge at low temperature in a Coulomb blockade valley. We give
a pedagogical approach to the single-channel Kondo physics at equilibrium and
review the Nozieres scattering picture of the correlated fixed point. We
emphasize the effect of an applied magnetic field and show how an orbital Kondo
effect can take place in vertical quantum dots tuned both to an even and to an
odd number of electrons at a level crossing. We extend the approach to the
two-channel overscreened Kondo case and discuss recent proposals for detecting
the non-Fermi liquid fixed point which could be reached at strong coupling.Comment: 31 pages, invited review articl
From four- to two-channel Kondo effect in junctions of XY spin chains
We consider the Kondo effect in Y-junctions of anisotropic XY models in an applied magnetic field along the critical lines characterized by a gapless excitation spectrum. We find that, while the boundary interaction Hamiltonian describing the junction can be recasted in the form of a four-channel, spin-1/2 antiferromagnetic Kondo Hamiltonian, the number of channels effectively participating in the Kondo effect depends on the chain parameters, as well as on the boundary couplings at the junction. The system evolves from an effective four-channel topological Kondo effect for a junction of XX-chains with symmetric boundary couplings into a two-channel one at a junction of three quantum critical Ising chains. The effective number of Kondo channels depends on the properties of the boundary and of the bulk. The XX-line is a "critical" line, where a four-channel topological Kondo effect can be recovered by fine-tuning the boundary parameter, while along the line in parameter space connecting the XX-line and the critical Ising point the junction is effectively equivalent to a two-channel topological Kondo Hamiltonian. Using a renormalization group approach, we determine the flow of the boundary couplings, which allows us to define and estimate the critical couplings and Kondo temperatures of the different Kondo (pair) channels. Finally, we study the local transverse magnetization in the center of the Y-junction, eventually arguing that it provides an effective tool to monitor the onset of the two-channel Kondo effect. \ua9 2016 The Author(s)
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