Phys. Rev. B 89, 045143 (2014) We consider an interacting nanowire/superconductor heterostructure attached
to metallic leads. The device is described by an unusual low-energy model
involving spin-1 ...conduction electrons coupled to a nonlocal spin-1/2 Kondo
impurity built from Majorana fermions. The topological origin of the resulting
Kondo effect is manifest in distinctive non-Fermi-liquid (NFL) behavior, and
the existence of Majorana fermions in the device is demonstrated unambiguously
by distinctive conductance lineshapes. We study the physics of the model in
detail, using the numerical renormalization group, perturbative scaling and
abelian bosonization. In particular, we calculate the full scaling curves for
the differential conductance in AC and DC fields, onto which experimental data
should collapse. Scattering t-matrices and thermodynamic quantities are also
calculated, recovering asymptotes from conformal field theory. We show that the
NFL physics is robust to asymmetric Majorana-lead couplings, and here we
uncover a duality between strong and weak coupling. The NFL behavior is
understood physically in terms of competing Kondo effects. The resulting
frustration is relieved by inter-Majorana coupling which generates a second
crossover to a regular Fermi liquid.
We study theoretically a triangular cluster of three magnetic impurities, hybridizing locally with conduction electrons of a metallic host. Such a cluster is the simplest to exhibit frustration - an ...important generic feature of many complex molecular systems in which different interactions compete. Here, low-energy doublet states of the trimer are favored by effective exchange interactions produced by strong electronic repulsion in localized impurity orbitals. Parity symmetry protects a level-crossing of such states on tuning microscopic parameters, while an avoided crossing arises in the general distorted case. On coupling to a metallic host, the behavior is shown to be immensely rich, since collective quantum many-body effects now also compete. In particular, impurity degrees of freedom are totally screened at low temperatures in a Kondo-screened Fermi liquid phase, while degenerate ground states persist in a local moment phase. Local frustration drives the quantum phase transition between the two, which may be first order or of Kosterlitz-Thouless type, depending on symmetries. Unusual mechanisms for local moment formation and Kondo screening are found, due to the orbital structure of the impurity trimer. Our results are of relevance for triple quantum dot devices. The problem is studied by a combination of analytical arguments and the numerical renormalization group.
The Numerical Renormalization Group is used to solve quantum impurity problems, which describe magnetic impurities in metals, nanodevices, and correlated materials within DMFT. Here we present a ...simple generalization of the Wilson Chain, which improves the scaling of computational cost with the number of channels/bands, bringing new problems within reach. The method is applied to calculate the t-matrix of the three-channel Kondo model at T=0, which shows universal crossovers near non-Fermi liquid critical points. A non-integrable three-impurity problem with three bands is also studied, revealing a rich phase diagram and novel screening/overscreening mechanisms.
We investigate theoretically the possibility of observing two-channel Kondo (2CK) physics in tunnel-coupled double quantum dots (TCDQDs), at both zero and finite magnetic fields; taking the ...two-impurity Anderson model (2AIM) as the basic TCDQD model, together with effective low-energy models arising from it by Schrieffer-Wolff transformations to second and third order in the tunnel couplings. The models are studied primarily using Wilson's numerical renormalization group. At zero-field our basic conclusion is that while 2CK physics arises in principle provided the system is sufficiently strongly-correlated, the temperature window over which it could be observed is much lower than is experimentally feasible. This finding disagrees with recent work on the problem, and we explain why. At finite field, we show that the quantum phase transition known to arise at zero-field in the two-impurity Kondo model (2IKM), with an essentially 2CK quantum critical point, persists at finite fields. This raises the prospect of access to 2CK physics by tuning a magnetic field, although preliminary investigation suggests this to be even less feasible than at zero field.
We examine several effects of an applied magnetic field on Anderson-type models for both single- and two-level quantum dots, and make direct comparison between numerical renormalization group (NRG) ...calculations and recent conductance measurements. On the theoretical side the focus is on magnetization, single-particle dynamics and zero-bias conductance, with emphasis on the universality arising in strongly correlated regimes; including a method to obtain the scaling behavior of field-induced Kondo resonance shifts over a very wide field range. NRG is also used to interpret recent experiments on spin-1/2 and spin-1 quantum dots in a magnetic field, which we argue do not wholly probe universal regimes of behavior; and the calculations are shown to yield good qualitative agreement with essentially all features seen in experiment. The results capture in particular the observed field-dependence of the Kondo conductance peak in a spin-1/2 dot, with quantitative deviations from experiment occurring at fields in excess of \(\sim\) 5 T, indicating the eventual inadequacy of using the equilibrium single-particle spectrum to calculate the conductance at finite bias.
We consider an Anderson impurity model in which the locally correlated orbital is coupled to a host with a gapped density of states. Single-particle dynamics are studied, within a perturbative ...framework that includes both explicit second-order perturbation theory and self-consistent perturbation theory to all orders in the interaction. Away from particle-hole symmetry the system is shown to be a generalized Fermi liquid (GFL) in the sense of being perturbatively connectable to the non-interacting limit; and the exact Friedel sum rule for the GFL phase is obtained. We show by contrast that the particle-hole symmetric point of the model is not perturbatively connected to the non-interacting limit, and as such is a non-Fermi liquid for all non-zero gaps. Our conclusions are in agreement with NRG studies of the problem.
To compare the effect of treatment decisions guided by phenotypic resistance testing (PRT) or standard of care (SOC) on short-term virological response.
A prospective, randomized, controlled clinical ...trial conducted in 25 university and private practice centers in the United States.
A total of 272 subjects who failed to achieve or maintain virological suppression (HIV-1-RNA plasma level > 2000 copies/ml) with previous exposure to two or more nucleoside reverse transcriptase inhibitors and one protease inhibitor.
Randomization was to antiretroviral therapy guided by PRT or SOC.
The percentage of subjects with HIV-1-RNA plasma levels less than 400 copies/ml at week 16 (primary); change from baseline in HIV-1-RNA plasma levels and number of "active" (less than fourfold resistance) antiretroviral agents used (secondary).
At week 16, using intent-to-treat (ITT) analysis, a greater proportion of subjects had HIV-1-RNA levels less than 400 copies/ml in the PRT than in the SOC arm (P = 0.036, ITT observed; P = 0.079, ITT missing equals failure). An ITT observed analysis showed that subjects in the PRT arm had a significantly greater median reduction in HIV-1-RNA levels from baseline than the SOC arm (P = 0.005 for 400 copies/ml; P = 0.049 for 50 copies/ml assay detection limit). Significantly more subjects in the PRT arm were treated with two or more "active" antiretroviral agents than in the SOC arm (P = 0.003).
Antiretroviral treatment guided prospectively by PRT led to the increased use of "active" antiretroviral agents and was associated with a significantly better virological response.
The local moment approach is extended to the orbitally-degenerate SU(2N) Anderson impurity model (AIM). Single-particle dynamics are obtained over the full range of energy scales, focussing here on ...particle-hole symmetry in the strongly correlated regime where the onsite Coulomb interaction leads to many-body Kondo physics with entangled spin and orbital degrees of freedom. The approach captures many-body broadening of the Hubbard satellites, recovers the correct exponential vanishing of the Kondo scale for all N, and its universal scaling spectra are found to be in very good agreement with numerical renormalization group (NRG) results. In particular the high-frequency logarithmic decays of the scaling spectra, obtained here in closed form for arbitrary N, coincide essentially perfectly with available numerics from the NRG. A particular case of an anisotropic Coulomb interaction, in which the model represents a system of N `capacitively-coupled' SU(2) AIMs, is also discussed. Here the model is generally characterised by two low-energy scales, the crossover between which is seen directly in its dynamics.
We study correlated two-level quantum dots, coupled in effective 1-channel fashion to metallic leads; with electron interactions including on-level and inter-level Coulomb repulsions, as well as the ...inter-orbital Hund's rule exchange favoring the spin-1 state in the relevant sector of the free dot. For arbitrary dot occupancy, the underlying phases, quantum phase transitions (QPTs), thermodynamics, single-particle dynamics and electronic transport properties are considered; and direct comparison is made to conductance experiments on lateral quantum dots. Two distinct phases arise generically, one characterised by a normal Fermi liquid fixed point (FP), the other by an underscreened (USC) spin-1 FP. Associated QPTs, which occur in general in a mixed valent regime of non-integral dot charge, are found to consist of continuous lines of Kosterlitz-Thouless transitions, separated by first order level-crossing transitions at high symmetry points. A `Friedel-Luttinger sum rule' is derived and, together with a deduced generalization of Luttinger's theorem to the USC phase (a singular Fermi liquid), is used to obtain a general result for the T=0 zero-bias conductance, expressed solely in terms of the dot occupancy and applicable to both phases. Relatedly, dynamical signatures of the QPT show two broad classes of behavior, corresponding to the collapse of either a Kondo resonance, or antiresonance, as the transition is approached from the Fermi liquid phase; the latter behavior being apparent in experimental differential conductance maps. The problem is studied using the numerical renormalization group method, combined with analytical arguments.
J. Phys.: Condens. Matter 17, 6959 (2005) Single-particle dynamics of the Anderson impurity model are studied using
both the numerical renormalization group (NRG) method and the local moment
approach ...(LMA). It is shown that a 'two-self-energy' description of dynamics
inherent to the LMA, as well as a conventional 'single-self-energy'
description, arise within NRG; each yielding correctly the same local
single-particle spectrum. Explicit NRG results are obtained for the broken
symmetry spectral constituents arising in a two-self-energy description, and
the total spectrum. These are also compared to analytical results obtained from
the LMA as implemented in practice. Very good agreement between the two is
found, essentially on all relevant energy scales from the high-energy Hubbard
satellites to the low-energy Kondo resonance.
