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Domenico Giuliano

Publications and source records attributed to Domenico Giuliano.

At least 55 records · Page 3Linked to original sources
arXiv2016

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.

cond-mat.str-el↗
arXiv2016

Transfer matrix approach to the persistent current in hybrid normal-superconducting rings

Using the properties of the transfer matrix of one-dimensional quantum mechanical systems, we derive a technique to exactly compute the persistent current across a hybrid normal-superconducting-mesoscopic ring pierced by a magnetic flux Phi as a single integral of a known function of the system's parameters. Our approach provides exact results at zero temperature, which can be readily extended to finite temperatures T much below the superconducting gap. We apply our technique to derive the persistent current through p-wave and s-wave superconducting-normal hybrid rings, recovering at once a number of effects such as the crossover in the current periodicity on increasing the size of the ring and the signature of the topological phase transition in the p-wave case. In the limit of a large ring size, resorting to a systematic expansion in inverse powers of the ring length, we derive exact analytic closed-form formulas, applicable to a number of cases of physical interest.

cond-mat.str-el↗
arXiv2016

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 extreme regimes, 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.

cond-mat.str-el↗
arXiv2015

Dual fermionic variables and renormalization group approach to junctions of strongly interacting quantum wires

Making a combined use of bosonization and fermionization techniques, we build nonlocal transformations between dual fermion operators, describing junctions of strongly interacting spinful one-dimensional quantum wires. Our approach allows for trading strongly interacting (in the original coordinates) fermionic Hamiltonians for weakly interacting (in the dual coordinates) ones. It enables us to generalize to the strongly interacting regime the fermionic renormalization group approach to weakly interacting junctions. As a result, on one hand, we are able to pertinently complement the information about the phase diagram of the junction obtained within bosonization approach; on the other hand, we map out the full crossover of the conductance tensors between any two fixed points in the phase diagram connected by a renormalization group trajectory.

cond-mat.str-el↗
arXiv2014

The dc Josephson current in a long multi-channel quantum wire

The dc Josephson current across a multi-channel SNS junction is computed by summing contributions from sub-gap Andreev bound states, as well as from continuum states propagating within the superconducting leads. We show that, in a long multi-channel SNS-junction, at low temperatures, all these contributions add up, so that the current can be entirely expressed in terms of single-particle normal- and Andreev reflection amplitudes at the Fermi level at both SN interfaces. Our derivation applies to a generic number of channels in the normal region and/or in the superconducting leads, without assumptions about scattering processes at the SN interfaces: if the channels within the central region have the same dispersion relation, it leads to simple analytical formulas for the current at low temperatures; if the channels within the central region have different dispersion relations, it allows for expressing the current in terms of a simple integral involving only scattering amplitudes at the Fermi level. Our result motivates using a low energy effective boundary Hamiltonian formalism for computing the current, which is crucial for treating Luttinger liquid interaction effects.

cond-mat.str-el↗
arXiv2014

Screening Clouds and Majorana Fermions

Ken Wilson developed the Numerical Renormalization Group technique which greatly enhanced our understanding of the Kondo effect and other quantum impurity problems. Wilson's NRG also inspired Philippe Nozieres to propose the idea of a large "Kondo screening cloud". While much theoretical evidence has accumulated for this idea it has remained somewhat controversial and has not yet been confirmed experimentally. Recently a new possibility for observing an analogous crossover length scale has emerged, involving a Majorana fermion localized at the interface between a topological superconductor quantum wire and a normal wire. We give an overview of this topic both with and without interactions included in the normal wire.

cond-mat.str-el↗
arXiv2013

Realization of a two-channel Kondo model with Josephson junction networks

We show that- in the quantum regime- a Josephson junction rhombi chain (i.e. a Josephson junction chain made by rhombi formed by joining 4 Josephson junctions) may be effectively mapped onto a quantum Hamiltonian describing Ising spins in a transverse magnetic field with open boundary conditions. Then, we elucidate how a Y-shaped network fabricated with 3 Josephson Junction Rhombi chains may be used as a quantum device realizing the two channel Kondo model recently proposed by Tsvelik. We point out that the emergence of a 2 channel Kondo effect in this superconducting network may be probed through the measurement of a pertinent Josephson current.

cond-mat.str-el↗
arXiv2013

Josephson current through a long quantum wire

The dc Josephson current through a long SNS junction receives contributions from both Andreev bound states localized in the normal region as well as from scattering states incoming from the superconducting leads. We show that in the limit of a long junction, this current, at low temperatures, can be expressed entirely in terms of properties of the Andreev bound states at the Fermi energy: the normal and Andreev reflection amplitudes at the left-hand and at the right-hand S-N interface. This has important implications for treating interactions in such systems.

cond-mat.supr-con↗
arXiv2013

XXZ spin-1/2 representation of a finite-U Bose-Hubbard chain at half-integer filling

Using a similarity Hamiltonian renormalization procedure, we determine an effective spin-1/2 representation of the Bose-Hubbard model at half-integer filling and at a finite on-site interaction energy U. By means of bosonization, we are able to recast the effective Hamiltonian as that of a spin-1/2 XXZ magnetic chain with pertinently renormalized coupling and anisotropy parameters. We use this mapping to provide analytical estimates of the correlation functions of the Bose-Hubbard model. We then compare such results with those based on DMRG numerical simulations of the Bose-Hubbard model for various values of U and for a number L of lattice sites as low as L ~ 30. We find an excellent agreement up to 10% between the output of analytical and numerical computations, even for relatively small values of U. Our analysis implies that, also at finite U, the 1D Bose-Hubbard model with suitably chosen parameters may be seen as a quantum simulator of the XXZ chain.

cond-mat.str-el↗
arXiv2011

Enhanced Coherence of a Quantum Doublet Coupled to Tomonaga-Luttinger Liquid Leads

We use boundary field theory to describe the phases accessible to a tetrahedral qubit coupled to Josephson junction chains acting as Tomonaga-Luttinger liquid leads. We prove that, in a pertinent range of the fabrication and control parameters, an attractive finite coupling fixed point emerges due to the geometry of the composite Josephson junction network. We show that this new stable phase is characterized by the emergence of a quantum doublet which is robust not only against the noise in the external control parameters (magnetic flux, gate voltage) but also against the decoherence induced by the coupling of the tetrahedral qubit with the superconducting leads. We provide protocols allowing to read and to manipulate the state of the emerging quantum doublet and argue that a tetrahedral Josephson junction network operating near the new finite coupling fixed point may be fabricated with today's technologies.

cond-mat.supr-con↗
arXiv2010

Competing Boundary Interactions in a Josephson Junction Network with an Impurity

We analyze a perturbation of the boundary Sine-Gordon model where two boundary terms of different periodicities and scaling dimensions are coupled to a Kondo-like spin degree of freedom. We show that, by pertinently engineering the coupling with the spin degree of freedom, a competition between the two boundary interactions may be induced, and that this gives rise to nonpertubative phenomena, such as the emergence of novel quantum phases: indeed, we demonstrate that the strongly coupled fixed point may become unstable as a result of the "deconfinement" of a new set of phase-slip operators -the short instantons- associated with the less relevant boundary operator. We point out that a Josephson junction network with a pertinent impurity located at its center provides a physical realization of this boundary double Sine-Gordon model. For this Josephson junction network, we prove that the competition between the two boundary interactions stabilizes a robust finite coupling fixed point and, at a pertinent scale, allows for the onset of $4e$ superconductivity.

cond-mat.supr-con↗
arXiv2009

Y-junction of superconducting Josephson chains

We show that, for pertinent values of the fabrication and control parameters, an attractive finite coupling fixed point emerges in the phase diagram of a Y-junction of superconducting Josephson chains. The new fixed point arises only when the dimensionless flux $f$ piercing the central loop of the network equals $π$ and, thus, does not break time-reversal invariance; for $f \neq π$, only the strongly coupled fixed point survives as a stable attractive fixed point. Phase slips (instantons) have a crucial role in establishing this transition: we show indeed that, at $f = π$, a new set of instantons -the W-instantons- comes into play to destabilize the strongly coupled fixed point. Finally, we provide a detailed account of the Josephson current-phase relationship along the arms of the network, near each one of the allowed fixed points. Our results evidence remarkable similarities between the phase diagram accessible to a Y-junction of superconducting Josephson chains and the one found in the analysis of quantum Brownian motion on frustrated planar lattices.

cond-mat.str-el↗
arXiv2006

Boundary Field Theory Approach to the Renormalization of SQUID Devices

We show that the quantum properties of some Josephson SQUID devices are described by a boundary sine Gordon model. Our approach naturally describes multi-junction SQUID devices and, when applied to a single junction SQUID (the rf-SQUID), it reproduces the known results of Glazman and Hekking. We provide a detailed analysis of the regimes accessible to an rf-SQUID and to a two-Josephson junction SQUID device (the dc-SQUID). We then compute the normal component of the current-response of a SQUID device to an externally applied voltage and show that the equation describing the current-voltage characteristic function reduces to well-known results when the infrared cutoff is suitably chosen. Our approach helps in establishing new and interesting connections between superconducting devices, quantum brownian motion, fermionic quantum wires and, more generally, quantum impurity problems.

cond-mat.mes-hall↗
arXiv2005

QED_3 on a space-time lattice: a comparison between compact and noncompact formulation

Quantum electrodynamics in a (2+1)-dimensional space-time has been object of studies both as effective theory for the pseudogap phase of high-T_c superconductors and for the theoretical investigation of mechanisms of confinement in presence of matter fields. We discretize the theory using both compact and noncompact formulations, analyze the behavior of the chiral condensate and of the monopole density and compare them. Finally we draw some conclusions about the possible equivalence of the two lattice formulations.

hep-lat↗
arXiv2005

Effective Boundary Field Theory for a Josephson Junction Chain with a Weak Link

We show that a finite Josephson Junction (JJ) chain, ending with two bulk superconductors, and with a weak link at its center, may be regarded as a condensed matter realization of a two-boundary Sine-Gordon model. Computing the partition function yields a remarkable analytic expression for the DC Josephson current as a function of the phase difference across the chain. We show that, in a suitable range of the chain parameters, there is a crossover of the DC Josephson current from a sinusoidal to a sawtooth behavior, which signals a transition from a regime where the boundary term is an irrelevant operator to a regime where it becomes relevant.

cond-mat.supr-con↗
arXiv2004

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 Dot

cond-mat.mes-hall↗
arXiv2004

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 $π-$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.

cond-mat.str-el↗