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Review: Superconductivity in ultrasmall metallic grainsJan 03 2001We review recent experimental and theoretical work on superconductivity in ultrasmall metallic grains, i.e. grains sufficiently small that the conduction electron energy spectrum becomes discrete. The discrete excitation spectrum of an individual grain ... More

Functional Renormalization Group Approach for Inhomogeneous Interacting Fermi-SystemsNov 13 2013Jan 23 2014The functional renormalization group (fRG) approach has the property that, in general, the flow equation for the two-particle vertex generates $\mathcal{O}(N^4)$ independent variables, where $N$ is the number of interacting states (e.g. sites of a real-space ... More

Identifying Symmetry-Protected Topological Order by Entanglement EntropyJun 24 2013Jan 08 2014According to the classification using projective representations of the SO(3) group, there exist two topologically distinct gapped phases in spin-1 chains. The symmetry-protected topological (SPT) phase possesses half-integer projective representations ... More

The quantum transverse-field Ising chain in circuit QED: effects of disorder on the nonequilibrium dynamicsJan 16 2013Apr 19 2013We study several dynamical properties of a recently proposed implementation of the quantum transverse-field Ising chain in the framework of circuit QED. Particular emphasis is placed on the effects of disorder on the nonequilibrium behavior of the system. ... More

Multiloop functional renormalization group for general modelsJul 14 2017Feb 14 2018We present multiloop flow equations in the functional renormalization group (fRG) framework for the four-point vertex and self-energy, formulated for a general fermionic many-body problem. This generalizes the previously introduced vertex flow [F. B. ... More

Comment on ``Point-Contact Study of Fast and Slow Two-Level Fluctuators in Metallic Glasses'' by Keijsers, Shklyarevskii and van KempenJan 23 1998We point out that a recent experiment by Keijsers, Shklyarevskii and van Kempen [Phys. Rev. Lett. 77, 3411 (1996)] on metallic glass point contacts containing two-level systems (TLS) effectively measures for the first time the conductance contributions ... More

Observing the Nonequilibrium Dynamics of the Quantum Transverse-Field Ising Chain in Circuit QEDAug 07 2012Jan 10 2013We show how a quantum Ising spin chain in a time-dependent transverse magnetic field can be simulated and experimentally probed in the framework of circuit QED with current technology. The proposed setup provides a new platform for observing the nonequilibrium ... More

Fermi-edge singularity and the functional renormalization groupJun 21 2017May 13 2018We study the Fermi-edge singularity, describing the response of a degenerate electron system to optical excitation, in the framework of the functional renormalization group (fRG). Results for the (interband) particle-hole susceptibility from various implementations ... More

Multiloop functional renormalization group that sums up all parquet diagramsMar 19 2017Feb 14 2018We present a multiloop flow equation for the four-point vertex in the functional renormalization group (fRG) framework. The multiloop flow consists of successive one-loop calculations and sums up all parquet diagrams to arbitrary order. This provides ... More

Derivation of exact flow equations from the self-consistent parquet relationsJul 08 2018Dec 27 2018We exploit the parquet formalism to derive exact flow equations for the two-particle-reducible four-point vertices, the self-energy, and typical response functions, circumventing the reliance on higher-point vertices. This includes a concise, algebraic ... More

Reply to Comment on "Superradiant Phase Transitions and the Standard Description of Circuit QED"Feb 14 2012Jun 25 2012A Reply to the Comment by C. Ciuti and P. Nataf [arXiv:1112.0986v1] on our Letter "Superradiant Phase Transitions and the Standard Description of Circuit QED" [Phys. Rev. Lett. 107, 113602 (2011)].

Poor man's derivation of the Bethe-Ansatz equations for the Dicke modelAug 11 2010We present an elementary derivation of the exact solution (Bethe-Ansatz equations) of the Dicke model, using only commutation relations and an informed Ansatz for the structure of its eigenstates.

Finite-Size Bosonization of 2-Channel Kondo Model: a Bridge between Numerical Renormalization Group and Conformal Field TheoryJul 06 1998We generalize Emery and Kivelson's (EK) bosonization-refermionization treatment of the 2-channel Kondo model to finite system size and on the EK-line analytically construct its exact eigenstates and finite-size spectrum. The latter crosses over to conformal ... More

Efficient simulation of infinite tree tensor network states on the Bethe latticeSep 11 2012Nov 27 2012We show that the simple update approach proposed by Jiang et. al. [H.C. Jiang, Z.Y. Weng, and T. Xiang, Phys. Rev. Lett. 101, 090603 (2008)] is an efficient and accurate method for determining the infinite tree tensor network states on the Bethe lattice. ... More

Transport and Dephasing in a Quantum Dot: Multiply Connected Graph ModelOct 10 2011Using the theory of diffusion in graphs, we propose a model to study mesoscopic transport through a diffusive quantum dot. The graph consists of three quasi-1D regions: a central region describing the dot, and two identical left- and right- wires connected ... More

Keldysh Derivation of Oguri's Linear Conductance Formula for Interacting FermionsApr 19 2017We present a Keldysh-based derivation of a formula, previously obtained by Oguri using the Matsubara formalisum, for the linear conductance through a central, interacting region coupled to non-interacting fermionic leads. Our starting point is the well-known ... More

Hundness versus Mottness in a three-band Hubbard-Hund model: On the origin of strong correlations in Hund metalsAug 29 2018Dec 21 2018Hund metals are multi-orbital systems with moderate Coulomb interaction, $U$, and sizeable Hund's rule coupling, $J<U$, that aligns the spins in different orbitals. They show strong correlation effects, like very low Fermi-liquid coherence scales and ... More

The dephasing rate formula in the many body contextSep 08 2009Nov 11 2009We suggest a straightforward approach to the calculation of the dephasing rate in a fermionic system, which correctly keeps track of the crucial physics of Pauli blocking. Starting from Fermi's golden rule, the dephasing rate can be written as an integral ... More

Equilibrium Fermi-liquid coefficients for the fully screened N-channel Kondo modelMar 03 2014May 25 2014We analytically and numerically compute three equilibrium Fermi-liquid coefficients of the fully screened $N$-channel Kondo model, namely $c_B$, $c_T$ and $c_\varepsilon$, characterizing the magnetic field and temperature dependence of the resisitivity, ... More

Nonequilibrium Steady-State Transport in Quantum Impurity Models: a Thermofield and Quantum Quench Approach using Matrix Product StatesAug 21 2017Jan 03 2019The numerical renormalization group (NRG) is tailored to describe interacting impurity models in equilibrium, but faces limitations for steady-state nonequilibrium, arising, e.g., due to an applied bias voltage. We show that these limitations can be overcome ... More

Spin fluctuations in the 0.7-anomaly in quantum point contactsMar 08 2017It has been argued that the 0.7 anomaly in quantum point contacts (QPCs) is due to an enhanced density of states at the top of the QPC-barrier (van Hove ridge), which strongly enhances the effects of interactions. Here, we analyze their effect on dynamical ... More

Fermi-edge exciton-polaritons in doped semiconductor microcavities with finite hole massJul 26 2017Oct 27 2017The coupling between a 2D semiconductor quantum well and an optical cavity gives rise to combined light-matter excitations, the exciton-polaritons. These were usually measured when the conduction band is empty, making the single polariton physics a simple ... More

Hexagon-singlet solid ansatz for the spin-1 kagome antiferromagnetDec 22 2014Jun 25 2015We perform a systematic investigation on the hexagon-singlet solid (HSS) states, which are a class of spin liquid candidates for the spin-1 kagome antiferromagnet. With the Schwinger boson representation, we show that all HSS states have exponentially ... More

Critical and strong-coupling phases in one- and two-bath spin-boson modelsOct 28 2011Mar 06 2012For phase transitions in dissipative quantum impurity models, the existence of a quantum-to-classical correspondence has been discussed extensively. We introduce a variational matrix product state approach involving an optimized boson basis, rendering ... More

Functional Renormalization Group treatment of the 0.7-analog in quantum point contactsMay 16 2018We use a recently developed fRG method (extendend Coupled-Ladder Approximation) to study the 0.7-analog in quantum point contacts, arising at the crossing of the 1st and 2nd band at suf- ficiently high magnetic fields. We reproduce the main features of ... More

Lindblad-Driven Discretized Leads for Non-Equilibrium Steady-State Transport in Quantum Impurity Models: Recovering the Continuum LimitApr 07 2016The description of interacting quantum impurity models in steady-state non-equilibrium is an open challenge for computational many-particle methods: the numerical requirement of using a finite number of lead levels and the physical requirement of describing ... More

Ohmic and step noise from a single trapping center hybridized with a Fermi seaApr 06 2005Oct 27 2005We show that single electron tunneling devices such as the Cooper-pair box or double quantum dot can be sensitive to the zero-point fluctuation of a single trapping center hybridized with a Fermi sea. If the trap energy level is close to the Fermi sea ... More

Microscopic model of critical current noise in Josephson-junction qubits: Subgap resonances and Andreev bound statesJun 28 2009Sep 19 2009We propose a microscopic model of critical current noise in Josephson-junctions based on individual trapping-centers in the tunnel barrier hybridized with electrons in the superconducting leads. We calculate the noise exactly in the limit of no on-site ... More

Lindblad-Driven Discretized Leads for Non-Equilibrium Steady-State Transport in Quantum Impurity Models: Recovering the Continuum LimitApr 07 2016Nov 07 2016The description of interacting quantum impurity models in steady-state nonequilibrium is an open challenge for computational many-particle methods: the numerical requirement of using a finite number of lead levels and the physical requirement of describing ... More

Orbital differentiation in Hund metalsApr 24 2019Orbital differentiation is a common theme in multi-orbital systems, yet a complete understanding of it is still missing. Here, we consider a minimal model for orbital differentiation in Hund metals with a highly accurate method: We use the numerical renormalization ... More

Anderson Orthogonality in the Dynamics After a Local Quantum QuenchAug 29 2011Sep 03 2011We present a systematic study of the role of Anderson orthogonality for the dynamics after a quantum quench in quantum impurity models, using the numerical renormalization group. As shown by Anderson in 1967, the scattering phase shifts of the single-particle ... More

Nonequilibrium Kondo effect in a magnetic field: Auxiliary master equation approachAug 18 2017Mar 01 2018We study the single-impurity Anderson model out of equilibrium under the influence of a bias voltage $\phi$ and a magnetic field $B$. We investigate the interplay between the shift ($\omega_B$) of the Kondo peak in the spin-resolved density of states ... More

Simplex valence-bond crystal in the spin-1 kagome Heisenberg antiferromagnetJun 23 2014Mar 19 2015We investigate the ground state properties of a spin-1 kagome antiferromagnetic Heisenberg model using tensor-network (TN) methods. We obtain the energy per site {$e_0=-1.41090(2)$ with $D^*=8$ multiplets retained (i.e., a bond dimension of $D=24$), and ... More

Emergent spin-1 trimerized valence bond crystal in the spin-1/2 Heisenberg model on the star latticeAug 14 2015Feb 26 2018We explore the frustrated spin-$1/2$ Heisenberg model on the star lattice with antiferromagnetic (AF) couplings inside each triangle and ferromagnetic (FM) inter-triangle couplings ($J_e<0$), and calculate its magnetic and thermodynamic properties. We ... More

Two-color Fermi liquid theory for transport through a multilevel Kondo impurityFeb 01 2018Mar 27 2018We consider a quantum dot with ${\cal K}{\geq} 2$ orbital levels occupied by two electrons connected to two electric terminals. The generic model is given by a multi-level Anderson Hamiltonian. The weak-coupling theory at the particle-hole symmetric point ... More

Two-bath spin-boson model: Phase diagram and critical propertiesOct 14 2014The spin-boson model, describing a two-level system coupled to a bath of harmonic oscillators, is a generic model for quantum dissipation, with manifold applications. It has also been studied as a simple example for an impurity quantum phase transition. ... More

Thermal Tensor Renormalization Group Simulations of Square-Lattice Quantum Spin ModelsApr 12 2019In this work, we benchmark the well-controlled and numerically accurate exponential thermal tensor renormalization group (XTRG) in the simulation of interacting spin models in two dimensions. Finite temperature introduces a thermal correlation length, ... More

On the relation between the 0.7-anomaly and the Kondo effect: Geometric Crossover between a Quantum Point Contact and a Kondo Quantum DotSep 11 2014Quantum point contacts (QPCs) and quantum dots (QDs), two elementary building blocks of semiconducting nanodevices, both exhibit famously anomalous conductance features: the 0.7-anomaly in the former case, the Kondo effect in the latter. For both the ... More

Two-temperature scales in the triangular-lattice Heisenberg antiferromagnetNov 04 2018Apr 11 2019The anomalous thermodynamic properties of the paradigmatic frustrated spin-1/2 triangular lattice Heisenberg antiferromagnet (TLH) has remained an open topic of research over decades, both experimentally and theoretically. Here we further the theoretical ... More

Two Temperature Scales in the Triangular Lattice Heisenberg AntiferromagnetNov 04 2018The anomalous thermodynamic properties of the paradigmatic frustrated spin-1/2 triangular lattice Heisenberg antiferromagnet (TLH) has remained an open topic of research over decades, both experimentally and theoretically. Here we further the theoretical ... More

Open Wilson chains for quantum impurity models: Keeping track of all bath modesNov 16 2016Apr 03 2017When constructing a Wilson chain to represent a quantum impurity model, the effects of truncated bath modes are neglected. We show that their influence can be kept track of systematically by constructing an "open Wilson chain" in which each site is coupled ... More

Influence Functional for Decoherence of Interacting Electrons in Disordered ConductorsOct 21 2005Apr 10 2008We have rederived the controversial influence functional approach of Golubev and Zaikin (GZ) for interacting electrons in disordered metals in a way that allows us to show its equivalence, before disorder averaging, to diagrammatic Keldysh perturbation ... More

Decoherence of interacting electrons in disordered conductors: on the relation between influence functional and diagrammatic approachesOct 29 2002We establish a connection between the influence functional approach of Golubev and Zaikin (GZ) and Keldysh diagrammatic perturbation theory for calculating the decoherence time of interacting electrons in disordered metals; we show how the standard diagrams ... More

Simple Bosonization Solution of the 2-channel Kondo Model: I. Analytical Calculation of Finite-Size Crossover SpectrumDec 10 1998We present in detail a simple, exact solution of the anisotropic 2-channel Kondo (2CK) model at its Toulouse point. We reduce the model to a quadratic resonant-level model by generalizing the bosonization-refermionization approach of Emery and Kivelson ... More

Sum-rule Conserving Spectral Functions from the Numerical Renormalization GroupJul 19 2006Aug 20 2007We show how spectral functions for quantum impurity models can be calculated very accurately using a complete set of ``discarded'' numerical renormalization group eigenstates, recently introduced by Anders and Schiller. The only approximation is to judiciously ... More

Algebraic Bethe Ansatz for a discrete-state BCS pairing modelJun 20 2001We show in detail how Richardson's exact solution of a discrete-state BCS (DBCS) model can be recovered as a special case of an algebraic Bethe Ansatz solution of the inhomogeneous XXX vertex model with twisted boundary conditions: by implementing the ... More

Superconductivity in Ultrasmall Grains: Introduction to Richardson's Exact SolutionNov 04 1999Studies of pairing correlations in ultrasmall metallic grains have commonly been based on a simple reduced BCS-model describing the scattering of pairs of electrons between discrete energy levels that come in time-reversed pairs. This model has an exact ... More

Fixed-N Superconductivity: The Exact Crossover from the Bulk to the Few-Electron LimitJul 26 1999We use two truly canonical approaches to describe superconductivity in ultrasmall metallic grains: (a) a variational fixed-N projected BCS-like theory and (b) an exact solution of the model Hamiltonian developed by Richardson in context with Nuclear Physics. ... More

Nonequilibrium excitations in Ferromagnetic NanoparticlesOct 17 2001In recent measurements of tunneling transport through individual ferromagnetic Co nanograins, Deshmukh, Gu\'eron, Ralph et al. \cite{mandar,gueron} (DGR) observed a tunneling spectrum with discrete resonances, whose spacing was much smaller than what ... More

Bosonization for Beginners --- Refermionization for ExpertsMay 21 1998Oct 30 1998This tutorial gives an elementary and self-contained review of abelian bosonization in 1 dimension in a system of finite size $L$, following and simplifying Haldane's constructive approach. As a non-trivial application, we rigorously resolve (following ... More

Fixed-N Superconductivity: The Crossover from the Bulk to the Few-Electron LimitOct 13 1998Jan 18 1999We present a truly canonical theory of superconductivity in ultrasmall metallic grains by variationally optimizing fixed-N projected BCS wave-functions, which yields the first full description of the entire crossover from the bulk BCS regime (mean level ... More

Superconductivity in Ultrasmall Metallic GrainsJan 16 1998We develop a theory of superconductivity in ultrasmall (nm-scale) metallic grains having a discrete electronic eigenspectrum with a mean level spacing of order of the bulk gap. The theory is based on calculating the eigenspectrum using a generalized BCS ... More

Josephson effect between superconducting nanograins with discrete energy levelsMay 15 2003We investigate the Josephson effect between two coupled superconductors, coupled by the tunneling of pairs of electrons, in the regime that their energy level spacing is comparable to the bulk superconducting gap, but neglecting any charging effects. ... More

Matrix product state approach for a two-lead, multi-level Anderson impurity modelApr 03 2008Mar 16 2010We exploit the common mathematical structure of the numerical renormalization group and the density matrix renormalization group, namely, matrix product states, to implement an efficient numerical treatment of a two-lead, multi-level Anderson impurity ... More

Decoherence without dissipation?Jun 03 2003In a recent article, Ford, Lewis and O'Connell (PRA 64, 032101 (2001)) discuss a thought experiment in which a Brownian particle is subjected to a double-slit measurement. Analyzing the decay of the emerging interference pattern, they derive a decoherence ... More

Spin tunneling in the Kagomé antiferromagnetNov 03 1992The collective tunneling of a small cluster of spins between two degenerate ground state configurations of the Kagom\'{e}-lattice quantum Heisenberg antiferromagnet is \mbox{studied}. The cluster consists of the six spins on a hexagon of the lattice. ... More

Measuring the size of a Schroedinger cat stateSep 01 2006We propose a measure for the "size" of a Schroedinger cat state, i.e. a quantum superposition of two many-body states with (supposedly) macroscopically distinct properties, by counting how many single-particle operations are needed to map one state onto ... More

Functional Renormalization Group Approach for Inhomogeneous One-Dimensional Fermi Systems with Finite-Ranged InteractionsSep 23 2016Jan 16 2017We introduce an equilibrium formulation of the functional renormalization group (fRG) for inhomogeneous systems capable of dealing with spatially finite-ranged interactions. In the general third order truncated form of fRG, the dependence of the two-particle ... More

Matrix product state comparison of the numerical renormalization group and the variational formulation of the density matrix renormalization groupApr 01 2008Wilson's numerical renormalization group (NRG) method for solving quantum impurity models yields a set of energy eigenstates that have the form of matrix product states (MPS). White's density matrix renormalization group (DMRG) for treating quantum lattice ... More

Spectroscopy of discrete energy levels in ultrasmall metallic grainsJan 02 2001Jan 03 2001We review recent experimental and theoretical work on ultrasmall metallic grains, i.e. grains sufficiently small that the conduction electron energy spectrum becomes discrete. The discrete excitation spectrum of an individual grain can be measured by ... More

Functional Renormalization Group Approach for Inhomogeneous One-Dimensional Fermi Systems with Finite-Ranged InteractionsSep 23 2016We introduce an equilibrium formulation of the functional renormalization group (fRG) for inhomogeneous systems capable of dealing with spatially finite-ranged interactions. In the general third order truncated form of fRG, the dependence of the two-particle ... More

Destructive quantum interference in spin tunneling problemsAug 15 1992In some spin tunneling problems, there are several different but symmetry-related tunneling paths that connect the same initial and final configurations. The topological phase factors of the corresponding tunneling amplitudes can lead to destructive interference ... More

Symmetric Minimally Entangled Typical Thermal StatesJun 10 2015Sep 05 2015We extend White's minimally entangled typically thermal states approach (METTS) to allow Abelian and non-Ablian symmetries to be exploited when computing finite-temperature response functions in one-dimensional (1D) quantum systems. Our approach, called ... More

Anderson Orthogonality and the Numerical Renormalization GroupApr 15 2011Anderson Orthogonality (AO) refers to the fact that the ground states of two Fermi seas that experience different local scattering potentials, say |G_I> and |G_F>, become orthogonal in the thermodynamic limit of large particle number N, in that |<G_I|G_F>| ... More

Superradiant Phase Transitions and the Standard Description of Circuit QEDMar 23 2011We investigate the equilibrium behaviour of a superconducting circuit QED system containing a large number of artificial atoms. It is shown that the currently accepted standard description of circuit QED via an effective model fails in an important aspect: ... More

Flow equation renormalization of a spin-boson model with a structured bathFeb 18 2003We discuss the dynamics of a spin coupled to a damped harmonic oscillator. This system can be mapped to a spin-boson model with a structured bath, i.e. the spectral function of the bath has a resonance peak. We diagonalize the model by means of infinitesimal ... More

Kondo quantum dot coupled to ferromagnetic leads: Numerical renormalization group studyJun 27 2007We systematically study the influence of ferromagnetic leads on the Kondo resonance in a quantum dot tuned to the local moment regime. We employ Wilson's numerical renormalization group method, extended to handle leads with a spin asymmetric density of ... More

Effect of spin-orbit interactions on the 0.7 anomaly in quantum point contactsAug 04 2014Jan 05 2015We study how the conductance of a quantum point contact is affected by spin-orbit interactions, for systems at zero temperature both with and without electron-electron interactions. In the presence of spin-orbit coupling, tuning the strength and direction ... More

Paramagnetic Breakdown of Superconductivity in Ultrasmall Metallic GrainsApr 22 1997Jul 31 1997We study the magnetic-field-induced breakdown of superconductivity in nm-scale metal grains having a mean electron level spacing $d \simeq \tilde\Delta$ (bulk gap). Using a generalized variational BCS approach that yields good qualitative agreement with ... More

The Kondo Box: A Magnetic Impurity in an Ultrasmall Metallic GrainSep 30 1998We study the Kondo effect generated by a single magnetic impurity embedded in an ultrasmall metallic grain, to be called a ``Kondo box''. We find that the Kondo resonance is strongly affected when the mean level spacing in the grain becomes larger than ... More

A numerical algorithm for the explicit calculation of SU(N) and SL(N,C) Clebsch-Gordan coefficientsSep 02 2010Mar 28 2011We present an algorithm for the explicit numerical calculation of SU(N) and SL(N,C) Clebsch-Gordan coefficients, based on the Gelfand-Tsetlin pattern calculus. Our algorithm is well-suited for numerical implementation; we include a computer code in an ... More

The 2-Channel Kondo Model II: CFT Calculation of Non-Equilibrium Conductance through a Nanoconstriction containing 2-Channel Kondo ImpuritiesFeb 05 1997Sep 20 1999Recent experiments by Ralph and Buhrman on zero-bias anomalies in quenched Cu nanoconstrictions (reviewed in the preceding paper, I), are in accord with the assumption that the interaction between electrons and nearly degenerate two-level systems in the ... More

Theory of inelastic scattering from magnetic impuritiesMar 29 2004Mar 31 2004We use the numerical renormalization group method tocalculate the single particle matrix elements $\cal T$ of the many body $T$-matrix of the conduction electrons scattered by a magnetic impurity at T=0 temperature. Since $\cal T$ determines both the ... More

Dephasing in Metals by Two-Level Systems in the 2-Channel-Kondo RegimeFeb 11 1999Sep 28 1999We point out a novel, non-universal contribution to the dephasing rate 1/\tau_\phi \equiv \gamma_\phi of conduction electrons in metallic systems: scattering off non-magnetic two-level systems (TLSs) having almost degenerate Kondo ground states. In the ... More

Decoherence in weak localization II: Bethe-Salpeter calculation of CooperonOct 21 2005This is the second in a series of two papers (I and II) on the problem of decoherence in weak localization. In paper I, we discussed how the Pauli principle could be incorporated into an influence functional approach for calculating the Cooperon propagator ... More

Decoherence in weak localization I: Pauli principle in influence functionalOct 21 2005This is the first in a series of two papers (I and II), in which we revisit the problem of decoherence in weak localization. The basic challenge addressed in our work is to calculate the decoherence of electrons interacting with a quantum-mechanical environment, ... More

At which magnetic field, exactly, does the Kondo resonance begin to split?Sep 20 2016We extend a recently-developed Fermi-liquid (FL) theory for the asymmetric single-impurity Anderson model [C. Mora et al., Phys. Rev. B, 92, 075120 (2015)] to the case of an arbitrary local magnetic field. To describe the system's low-lying quasiparticle ... More

Dynamic structure factor of the spin-1/2 XXZ chain in a transverse fieldJun 10 2016Aug 22 2016The spin-$\frac{1}{2}$ XXZ chain with easy-plane anisotropy in a transverse field describes well the thermodynamic properties of the material ${\rm Cs_2CoCl_4}$ in a wide range of temperatures and fields including the region close to the spin-flop Ising ... More

Well-defined quasiparticles in interacting metallic grainsApr 26 2004We analyze spectral functions of mesoscopic systems with large dimensionless conductance, which can be described by a universal Hamiltonian. We show that an important class of spectral functions are dominated by one single state only, which implies the ... More

Fermi-liquid theory for the single-impurity Anderson modelSep 11 2014Aug 25 2015We generalize Nozi\`eres' Fermi-liquid theory for the low-energy behavior of the Kondo model to that of the single-impurity Anderson model. In addition to the electrons' phase shift at the Fermi energy, the low-energy Fermi-liquid theory is characterized ... More

SU(3) Anderson impurity model: A numerical renormalization group approach exploiting non-Abelian symmetriesAug 03 2012We show how the density-matrix numerical renormalization group (DM-NRG) method can be used in combination with non-Abelian symmetries such as SU(N), where the decomposition of the direct product of two irreducible representations requires the use of a ... More

Interplay of mesoscopic and Kondo effects for transmission amplitude of few-level quantum dotsMay 20 2008The magnitude and phase of the transmission amplitude of a multi-level quantum dot is calculated for the mesoscopic regime of level spacing large compared to level width. The interplay between Kondo correlations and the influence by neighboring levels ... More

Numerical renormalization group calculation of near-gap peaks in spectral functions of the Anderson model with superconducting leadsMar 08 2008Jun 12 2008We use the numerical renormalization group method (NRG) to investigate a single-impurity Anderson model with a coupling of the impurity to a superconducting host. Analysis of the energy flow shows, in contrast to previous belief, that NRG iterations can ... More

Charge oscillations in Quantum Dots: Renormalization group and Hartree method calculationsAug 04 2004Nov 08 2005We analyze the local level occupation of a spinless, interacting two-level quantum dot coupled to two leads by means of Wilson's numerical renormalization group method. A gate voltage sweep, causing a rearrangement of the charge such that the system's ... More

Transmission Phase Shift of a Quantum Dot with Kondo CorrelationsSep 28 1999Apr 26 2000We study the effects of Kondo correlations on the transmission phase shift of a quantum dot in an Aharonov-Bohm ring. We predict in detail how the development of a Kondo resonance should affect the dependence of the phase shift on transport voltage, gate ... More

Data-adaptive estimation of time-varying spectral densitiesDec 02 2015Jun 12 2018This paper introduces a data-adaptive non-parametric approach for the estimation of time-varying spectral densities from nonstationary time series. Time-varying spectral densities are commonly estimated by local kernel smoothing. The performance of these ... More

Flavor fluctuations in 3-level quantum dots: Generic SU(3)-Kondo fixed point in equilibrium and non-Kondo fixed points in nonequilibriumFeb 27 2018We study a $3$-level quantum dot in the singly occupied cotunneling regime coupled via a generic tunneling matrix to several multi-channel leads in equilibrium or nonequilibrium. We derive an effective model where also each reservoir has three channels ... More

Emergent spin-1 trimerized valence bond crystal in the spin-1/2 Heisenberg model on the star latticeAug 14 2015We explore the frustrated spin-$1/2$ Heisenberg model on the star lattice with antiferromagnetic (AF) couplings inside each triangle and ferromagnetic (FM) inter-triangle couplings ($J_e<0$), and calculate its magnetic and thermodynamic properties. We ... More

At which magnetic field, exactly, does the Kondo resonance begin to split? A Fermi liquid description of the low-energy properties of the Anderson modelSep 20 2016Jul 17 2018This paper is a corrected version of Phys. Rev. B 95, 165404 (2017), which we have retracted because it contained a trivial but fatal sign error that lead to incorrect conclusions. --- We extend a recently-eveloped Fermi-liquid (FL) theory for the asymmetric ... More

Two-dimensional cavity grid for scalable quantum computation with superconducting circuitsJun 25 2007Mar 24 2009Superconducting circuits are among the leading contenders for quantum information processing. This promising avenue has been strengthened with the advent of circuit quantum electrodynamics, underlined by recent experiments coupling on-chip microwave resonators ... More

Two-color Fermi liquid theory for transport through a multilevel Kondo impurityFeb 01 2018Feb 08 2018We consider a quantum dot with ${\cal K}{\geq} 2$ orbital levels occupied by two electrons connected to two electric terminals. The generic model is given by a multi-level Anderson Hamiltonian. The weak-coupling theory at the particle-hole symmetric point ... More

Dependency-Based Information Flow Analysis with Declassification in a Program LogicSep 14 2015We present a deductive approach for the analysis of secure information flows with support for fine-grained policies that include declassifications in the form of delimited information release. By explicitly tracking the dependencies of program locations ... More

Chebyshev matrix product state approach for spectral functionsJan 31 2011We show that recursively generated Chebyshev expansions offer numerically efficient representations for calculating zero-temperature spectral functions of one-dimensional lattice models using matrix product state (MPS) methods. The main features of this ... More

Stroboscopic observation of quantum many-body dynamicsFeb 08 2011Jan 12 2012Recent experiments have demonstrated single-site resolved observation of cold atoms in optical lattices. Thus, in the future it may be possible to take repeated snapshots of an interacting quantum many-body system during the course of its evolution. Here ... More

Parity-Affected Superconductivity in Ultrasmall Metallic GrainsApr 11 1996We investigate the breakdown of BCS superconductivity in {\em ultra}\/small metallic grains as a function of particle size (characterized by the mean spacing $d$ between discrete electronic eigenstates), and the parity ($P$ = even/odd) of the number of ... More

Constrained optimization of sequentially generated entangled multiqubit statesOct 06 2008Sep 20 2009We demonstrate how the matrix-product state formalism provides a flexible structure to solve the constrained optimization problem associated with the sequential generation of entangled multiqubit states under experimental restrictions. We consider a realistic ... More

Density matrix renormalization group study of a quantum impurity model with Landau-Zener time-dependent HamiltonianOct 16 2008Apr 01 2009We use the adaptive time-dependent density matrix renormalization group method (t-DMRG) to study the nonequilibrium dynamics of a benchmark quantum impurity system which has a time-dependent Hamiltonian. This model is a resonant-level model, obtained ... More

Exact Study of the Effect of Level Statistics in Ultrasmall Superconducting GrainsSep 01 1999The reduced BCS model that is commonly used for ultrasmall superconducting grains has an exact solution worked out long ago by Richardson in the context of nuclear physics. We use it to check the quality of previous treatments of this model, and to investigate ... More

Signatures of Mottness and Hundness in archetypal correlated metalsAug 18 2017Jan 15 2019Physical properties of multi-orbital materials depend not only on the strength of the effective interactions among the valence electrons but also on their type. Strong correlations are caused by either Mott physics that captures the Coulomb repulsion ... More

A Model for Ferromagnetic Nanograins with Discrete Electronic StatesMar 30 2001We propose a simple phenomenological model for an ultrasmall ferromagnetic grain, formulated in terms of the grain's discrete energy levels. We compare the model's predictions with recent measurements of the discrete tunneling spectrum through such a ... More