Enhancement of the Two-channel Kondo Effect in Single-Electron boxes

The charging of a quantum box, coupled to a lead by tunneling through a single resonant level, is studied near the degeneracy points of the Coulomb blockade. Combining Wilson's numerical renormalization-group method with perturbative scaling approaches, the corresponding low-energy Hamiltonian is solved for arbitrary temperatures, gate voltages, tunneling rates, and energies of the impurity level. Similar to the case of a weak tunnel barrier, the shape of the charge step is governed at low temperatures by the non-Fermi-liquid fixed point of the two-channel Kondo effect. However, the associated Kondo temperature TK is strongly modified. Most notably, TK is proportional to the width of the level if the transmission through the impurity is close to unity at the Fermi energy, and is no longer exponentially small in one over the tunneling matrix element. Focusing on a particle-hole symmetric level, the two-channel Kondo effect is found to be robust against the inclusion of an on-site repulsion on the level. For a large on-site repulsion and a large asymmetry in the tunneling rates to box and to the lead, there is a sequence of Kondo effects: first the local magnetic moment that forms on the level undergoes single-channel screening, followed by two-channel overscreening of the charge fluctuations inside the box.Comment: 21 pages, 19 figure

Interplay of disorder and magnetic field in the superconducting vortex state

We calculate the density of states of an inhomogeneous superconductor in a magnetic field where the positions of vortices are distributed completely at random. We consider both the cases of s-wave and d-wave pairing. For both pairing symmetries either the presence of disorder or increasing the density of vortices enhances the low energy density of states. In the s-wave case the gap is filled and the density of states is a power law at low energies. In the d-wave case the density of states is finite at zero energy and it rises linearly at very low energies in the Dirac isotropic case (\alpha_D=t/\Delta_0=1, where t is the hopping integral and \Delta_0 is the amplitude of the order parameter). For slightly higher energies the density of states crosses over to a quadratic behavior. As the Dirac anisotropy increases (as \Delta_0 decreases with respect to the hopping term) the linear region decreases in width. Neglecting this small region the density of states interpolates between quadratic and back to linear as \alpha_D increases. The low energy states are strongly peaked near the vortex cores.Comment: 12 REVTeX pages, 15 figure

Decoupling of the S=1/2 antiferromagnetic zig-zag ladder with anisotropy

The spin-1/2 antiferromagnetic zig-zag ladder is studied by exact diagonalization of small systems in the regime of weak inter-chain coupling. A gapless phase with quasi long-range spiral correlations has been predicted to occur in this regime if easy-plane (XY) anisotropy is present. We find in general that the finite zig-zag ladder shows three phases: a gapless collinear phase, a dimer phase and a spiral phase. We study the level crossings of the spectrum,the dimer correlation function, the structure factor and the spin stiffness within these phases, as well as at the transition points. As the inter-chain coupling decreases we observe a transition in the anisotropic XY case from a phase with a gap to a gapless phase that is best described by two decoupled antiferromagnetic chains. The isotropic and the anisotropic XY cases are found to be qualitatively the same, however, in the regime of weak inter-chain coupling for the small systems studied here. We attribute this to a finite-size effect in the isotropic zig-zag case that results from exponentially diverging antiferromagnetic correlations in the weak-coupling limit.Comment: to appear in Physical Review

Network Economics and the Digital Divide in Rural South Asia

The concept of a 'global digital divide' is now common, and many cross-country studies of determinants of differences in computer and Internet penetration have been performed. The main conclusions and policy implications from these studies are relatively blunt: get richer, have more telephones, and regulate telecommunications better. In this paper, we examine an alternative approach to bridging the digital divide, through organizational innovations that provide low cost Internet access in developing countries, within the existing conditions of income levels, telecommunications infrastructure and regulatory environment. We use survey data from 500 individuals in three South Asian countries, Bangladesh, Nepal and Sri Lanka, to examine factors influencing patterns of computer and Internet use. These individuals were in situations where computer and Internet access has been provided by a developmental agency (government or non-government). We estimate logit and multinomial logit models, using explanatory variables such as income, household size, education, and occupation, as well as infrastructure factors such as quality of electricity supply, and availability of telephones and televisions. Thus we are able to go beyond simple analyses of penetration at the country level, to understand the microeconomics of computer and Internet use in rural South Asia

Network Economics and the Digital Divide in Rural India

The idea of a 'global digital divide' is well accepted, and cross-country studies of determinants of differences in computer and Internet penetration have identified income, telecommunications infrastructure, and regulatory quality as key influencing factors. The policy implications from these studies are relatively blunt: get richer, have more telephones, and regulate telecommunications better. In this paper, we examine an alternative policy approach to bridging the digital divide, through organizational innovations that provide low cost Internet access in developing countries, within the existing levels of income, telecommunications infrastructure and regulatory environment. We use survey data from 500 individuals in four states of India: Haryana, Madhya Pradesh, Punjab and Rajasthan, to examine factors influencing patterns of computer and Internet use. The situations in which data was collected were ones where computer and Internet access was being provided by a developmental agency (government or non-government). We estimate logit and multinomial logit models, using explanatory variables such as income, household size, education, and occupation, as well as infrastructure factors such as quality of electricity supply, and availability of telephones and televisions. Thus we are able to go beyond simple analyses of penetration at the country level, to understand the microeconomics of computer and Internet use in rural India. In particular, by examining patterns of use, we are able to comment on the importance of network externalities for diffusion of computers and the Internet in these local rural contexts

Superconductivity in the SU(N) Anderson Lattice at U=\infty

We present a mean-field study of superconductivity in a generalized N-channel cubic Anderson lattice at U=\infty taking into account the effect of a nearest-neighbor attraction J. The condition U=\infty is implemented within the slave-boson formalism considering the slave bosons to be condensed. We consider the $f$-level occupancy ranging from the mixed valence regime to the Kondo limit and study the dependence of the critical temperature on the various model parameters for each of three possible Cooper pairing symmetries (extended s, d-wave and p-wave pairing) and find interesting crossovers. It is found that the d- and p- wave order parameters have, in general, very similar critical temperatures. The extended s-wave pairing seems to be relatively more stable for electronic densities per channel close to one and for large values of the superconducting interaction J.Comment: Seven Figures; one appendix. Accepted for publication in Phys. Rev.

Quantum phase transition in a two-channel-Kondo quantum dot device

We develop a theory of electron transport in a double quantum dot device recently proposed for the observation of the two-channel Kondo effect. Our theory provides a strategy for tuning the device to the non-Fermi-liquid fixed point, which is a quantum critical point in the space of device parameters. We explore the corresponding quantum phase transition, and make explicit predictions for behavior of the differential conductance in the vicinity of the quantum critical point

Orbital Kondo behavior from dynamical structural defects

The interaction between an atom moving in a model double-well potential and the conduction electrons is treated using renormalization group methods in next-to-leading logarithmic order. A large number of excited states is taken into account and the Kondo temperature $T_K$ is computed as a function of barrier parameters. We find that for special parameters $T_K$ can be close to $1 {\rm K}$ and it can be of the same order of magnitude as the renormalized splitting $\Delta$. However, in the perturbative regime we always find that T_K \alt \Delta with a T_K \alt 1 {\rm K} [Aleiner {\em et al.}, Phys. Rev. Lett. {\bf 86}, 2629 (2001)]. We also find that $\Delta$ remains unrenormalized at energies above the Debye frequency, $\omega_{\rm Debye}$.Comment: 9 pages, 9 figures, RevTe

Multi-Channel Kondo Necklace

A multi--channel generalization of Doniach's Kondo necklace model is formulated, and its phase diagram studied in the mean--field approximation. Our intention is to introduce the possible simplest model which displays some of the features expected from the overscreened Kondo lattice. The $N$ conduction electron channels are represented by $N$ sets of pseudospins \vt_{j}, $j=1, ... , N$, which are all antiferromagnetically coupled to a periodic array of |\vs|=1/2 spins. Exploiting permutation symmetry in the channel index $j$ allows us to write down the self--consistency equation for general $N$. For $N>2$, we find that the critical temperature is rising with increasing Kondo interaction; we interpret this effect by pointing out that the Kondo coupling creates the composite pseudospin objects which undergo an ordering transition. The relevance of our findings to the underlying fermionic multi--channel problem is discussed.Comment: 29 pages (2 figures upon request from [email protected]), LATEX, submitted for publicatio

Low-Temperature Specific Heat of an Extreme-Type-II Superconductor at High Magnetic Fields

We present a detailed study of the quasiparticle contribution to the low-temperature specific heat of an extreme type-II superconductor at high magnetic fields. Within a T-matrix approximation for the self-energies in the mixed state of a homogeneous superconductor, the electronic specific heat is a linear function of temperature with a linear-$T$ coefficient $\gamma_s(H)$ being a nonlinear function of magnetic field $H$. In the range of magnetic fields H\agt (0.15-0.2)H_{c2} where our theory is applicable, the calculated $\gamma_s(H)$ closely resembles the experimental data for the borocarbide superconductor YNi$_2$B$_2$C.Comment: 7 pages, 2 figures, to appear in Physical Review