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N. Andrei

Publications and source records attributed to N. Andrei.

At least 19 recordsLinked to original sources
arXiv2020

Roadmap on Atomtronics: State of the art and perspective

Atomtronics deals with matter-wave circuits of ultra-cold atoms manipulated through magnetic or laser-generated guides with different shapes and intensities. In this way, new types of quantum networks can be constructed, in which coherent fluids are controlled with the know-how developed in the atomic and molecular physics community. In particular, quantum devices with enhanced precision, control and flexibility of their operating conditions can be accessed. Concomitantly, new quantum simulators and emulators harnessing on the coherent current flows can also be developed. Here, we survey the landscape of atomtronics-enabled quantum technology and draw a roadmap for the field in the near future. We review some of the latest progresses achieved in matter-wave circuits design and atom-chips. Atomtronic networks are deployed as promising platforms for probing many-body physics with a new angle and a new twist. The latter can be done both at the level of equilibrium and non-equilibrium situations. Numerous relevant problems in mesoscopic physics, like persistent currents and quantum transport in circuits of fermionic or bosonic atoms, are studied through a new lens. We summarize some of the atomtronics quantum devices and sensors. Finally, we discuss alkali-earth and Rydberg atoms as potential platforms for the realization of atomtronic circuits with special features.

cond-mat.quant-gas↗
arXiv2019

Universal Non-equilibrium I-V Curve Near Two-channel Kondo-Luttinger Quantum Critical Point

The Fermi liquid paradigm for metals has contributed enormously to our understanding of condensed matter systems. However a growing number of quantum critical systems have been shown to exhibit non Fermi liquid behavior. A full understanding of such systems is still lacking and in particular analytical results away from equilibrium are rare. In this work, we provide a distinct example of such kind in a two channel Kondo Luttinger model where a Kondo impurity couples to two voltage biased interacting electron leads, experimentally realizable in a dissipative quantum dot. An exotic quantum phase transition has been known to exist for two decades from the one channel to two channel Kondo ground states by enhancing electron interactions in the leads, but a controlled theoretical approach to this quantum critical point has not yet been established. We present a controlled method to this problem and obtain an analytical form for the universal nonequilibrium differential conductance near the transition. The relevance of our results for recent experiments is discussed.

cond-mat.str-el↗
arXiv2014

Quench between a Mott insulator and a Lieb-Liniger liquid

In this work we study a quench between a Mott insulator and a repulsive Lieb-Liniger liquid. We find explicitly the stationary state when a long time has passed after the quench. It is given by a GGE density matrix which we completely characterize, calculating the quasiparticle density describing the system after the quench. In the long time limit we find an explicit form for the local three body density density density correlation function and the asymptotic long distance limit of the density density correlation function. The later is shown to have a Gaussian decay at large distances.

cond-mat.quant-gas↗
arXiv2011

Electrically Controlled Pumping of Spin Currents in Topological Insulators

Pure spin currents are shown to be generated by an electrically controlled quantum pump applied at the edges of a topological insulator. The electric rather than the more conventional magnetic control offers several advantages and avoids, in particular, the necessity of delicate control of magnetization dynamics over tiny regions. The pump is implemented by pinching the sample at two quantum point contacts and phase modulating two external gate voltages between them. The spin current is generated for the full range of parameters. On the other hand, pumping via amplitude modulation of the inter-boundary couplings generates both charge and spin currents, with a pure charge current appearing only for special values of the parameters for which the Bohm-Aharonov flux takes integer values. Our setup can therefore serve to fingerprint the helical nature of the edges states with the zeros of the pumped spin and charge currents occurring at distinct universal locations where the Fabry-Perot or the Aharonov-Bohm phases take integer values.

cond-mat.mes-hall↗
arXiv2007

Resistivity of Inhomogeneous Superconducting Wires

We study the contribution of quantum phase fluctuations in the superconducting order parameter to the low--temperature resistivity $ρ(T)$ of a dirty and inhomogeneous superconducting wire. In particular, we account for random spatial fluctuations of arbitrary size in the wire thickness. For a typical wire thickness above the critical value for superconductor--insulator transition, phase--slips processes can be treated perturbatively. We use a memory formalism approach, which underlines the role played by weak violation of conservation laws in the mechanism for generating finite resistivity. Our calculations yield an expression for $ρ(T)$ which exhibits a smooth crossover from a homogeneous to a ``granular'' limit upon increase of $T$, controlled by a ``granularity parameter'' $D$ characterizing the size of thickness fluctuations. For extremely small $D$, we recover the power--law dependence $ρ(T)\sim T^α$ obtained by unbinding of quantum phase--slips. However in the strongly inhomogeneous limit, the exponent $α$ is modified and the prefactor is {\em exponentially enhanced}. We examine the dependence of the exponent $α$ on an external magnetic field applied parallel to the wire. Finally, we show that the power--law dependence at low $T$ is consistent with a series of experimental data obtained in a variety of long and narrow samples. The values of $α$ extracted from the data, and the corresponding field dependence, are consistent with known parameters of the corresponding samples.

cond-mat.supr-con↗
arXiv2007

Heat transport of clean spin-ladders coupled to phonons: Umklapp scattering and drag

We study the low-temperature heat transport in clean two-leg spin ladder compounds coupled to three-dimensional phonons. We argue that the very large heat conductivities observed in such systems can be traced back to the existence of approximate symmetries and corresponding weakly violated conservation laws of the effective (gapful) low--energy model, namely pseudo-momenta. Depending on the ratios of spin gaps and Debye energy and on the temperature, the magnetic contribution to the heat conductivity can be positive or negative, and exhibit an activated or anti-activated behavior. In most regimes, the magnetic heat conductivity is dominated by the spin-phonon drag: the excitations of the two subsystems have almost the same drift velocity, and this allows for an estimate of the ratio of the magnetic and phononic contributions to the heat conductivity.

cond-mat.str-el↗
arXiv2007

Theory of inelastic scattering from quantum impurities

We use the framework set up recently to compute non-perturbatively inelastic scattering from quantum impurities [G. Zaránd {\it et al.}, Phys. Rev. Lett. {\bf 93}, 107204 (2004)] to study the the energy dependence of the single particle $S$-matrix and the inelastic scattering cross section for a number of quantum impurity models. We study the case of the spin $S=1/2$ two-channel Kondo model, the Anderson model, and the usual $S=1/2$ single-channel Kondo model. We discuss the difference between non-Fermi liquid and Fermi liquid models and study how a cross-over between the non-Fermi liquid and Fermi liquid regimes appears in case of channel anisotropy for the $S=1/2$ two-channel Kondo model. We show that for the most elementary non-Fermi liquid system, the two-channel Kondo model, half of the scattering remains inelastic even at the Fermi energy. Details of the derivation of the reduction formulas and a simple path integral approach to connect the $T$-matrix to local correlation functions are also presented.

cond-mat.str-el↗
arXiv2006

Scattering theories for the 1D Hubbard model

In one-dimensional (1D) non-perturbative many-electron problems such as the 1D Hubbard model the electronic charge and spin degrees of freedom separate into exotic quantum objects. However, there are two different representations for such objects and associated scattering quantities whose relation is not well understood. Here we solve the problem by finding important information about the relation between the corresponding alternative choices for one-particle scattering states. Our study reveals why one of these representations, the {\it pseudofermion} representation, is the most suitable for the description of the unusual finite-energy spectral and dynamical properties of the model. This is a problem of physical importance, since the exotic independent charge and spin finite-energy spectral features observed by angle-resolved photoelectron spectroscopy in quasi-1D metals was found recently to correspond to the charge and spin quantum objects of the pseudofermion representation.

cond-mat.str-el↗
arXiv2005

Two-channel Anderson Impurity Model: Single-electron Green's function, self-energies, and resistivity

We compute exactly the low-energy single-electron Green's function, the impurity and electron self-energies, and the resistivity for the two-channel Anderson impurity model. These results are obtained by exploiting the boundary conformal field theory identified from the Bethe Ansatz solution of the model. Using that solution we can make contact with the parameters of the original Hamiltonian and provide the detailed crossover between the two integer valence limits. Our results generalize those obtained previously in the context of the two-channel Kondo model.

cond-mat.str-el↗
arXiv2005

Solution of the Multi-Channel Anderson Impurity Model: Ground state and thermodynamics

We present the solution of the SU(N) x SU(M) Anderson impurity model using the Bethe-Ansatz. We first explain what extensions to the formalism were required for the solution. Subsequently we determine the ground state and derive the thermodynamics over the full range of temperature and fields. We identify the different regimes of valence fluctuation at high temperatures, followed by moment formation or intrinsic mixed valence at intermediate temperatures and a low temperature non-Fermi liquid phase. Among other things we obtain the impurity entropy, charge valence and specific heat over the full range of temperature. We show that the low-energy physics is governed by a line of fixed points. This describes non-Fermi-liquid behavior in the integral valence regime, associated with moment formation, as well as in the mixed valence regime where no moment forms.

cond-mat.str-el↗
arXiv2005

Conformal Field Theory Approach to the Two-channel Anderson Model

The two-channel Anderson impurity model serves as a prototype for describing heavy-fermion materials with a possible mixed-valent regime with both quadrupolar and magnetic character. We report on the low-energy physics of the model, using a conformal field theory approach with exact Bethe Ansatz results as input.

cond-mat.str-el↗
arXiv2004

On the Multi-channel Anderson Model of Uranium Compounds

In this talk we will present the solution of the two-channel Anderson impurity model, proposed in the context of the heavy fermion compound UBe13, and discuss briefly the more general multi-channel case. We will show results for the thermodynamics in the full range of temperature and fields and make the connection with the current experimental situation.

cond-mat.str-el↗
arXiv2004

Critical Theory of the Two-Channel Anderson Impurity Model

We construct the boundary conformal field theory that describes the low-temperature behavior of the two-channel Anderson impurity model. The presence of an exactly marginal operator is shown to generate a line of stable fixed points parameterized by the charge valence of the impurity. We calculate the exact zero-temperature entropy and impurity thermodynamics along the fixed line. We also derive the critical exponents of the characteristic Fermi edge singularities caused by time-dependent hybridization between conduction electrons and impurity. Our results suggest that in the mixed-valent regime the electrons participate in two competing processes, leading to frustrated screening of spin and channel degrees of freedom. By combining the boundary conformal field theory with the Bethe Ansatz solution we obtain a complete description of the low-energy dynamics of the model.

cond-mat.str-el↗
arXiv2003

Pumping in an interacting quantum wire

We study charge and spin pumping in an interacting one-dimensional wire. We show that a spatially periodic potential modulated in space and time acts as a quantum pump inducing a dc-current component at zero bias. The current generated by the pump is strongly affected by the interactions. It has a power law dependence on the frequency or temperature with the exponent determined by the interaction in the wire, while the coupling to the pump affects the amplitudes only. We also show that pure spin-pumping can be achieved, without the presence of a magnetic field.

cond-mat.mes-hall↗
arXiv2002

Solution of the Two-Channel Anderson Impurity Model - Implications for the Heavy Fermion UBe$_{13}$ -

We solve the two-channel Anderson impurity model using the Bethe-Ansatz. We determine the ground state and derive the thermodynamics, obtaining the impurity entropy and specific heat over the full range of temperature. We show that the low temperature physics is given by a line of fixed points decribing a two-channel non Fermi liquid behavior in the integral valence regime associated with moment formation as well as in the mixed valence regime where no moment forms. We discuss relevance for the theory of UBe$_{13}$.

cond-mat.str-el↗
arXiv2002

Optical Conductivity and Pseudo-Momentum Conservation in Anisotropic Fermi Liquids

Umklapp scattering determines the conductivity of clean metals. In typical quasi one-dimensional Fermi liquids with an open Fermi surface, certain pseudo-momenta do not decay by 2-particle collisions even in situations where Umklapp scattering relaxes the momentum of the quasi particles efficiently. Due to this approximate conservation of pseudo-momentum, a certain fraction of the electrical current decays very slowly and a well-pronounced low-frequency peak emerges in the optical conductivity. We develop simple criteria to determine under what conditions approximate pseudo-momentum conservation is relevant and calculate within in Fermi liquid theory the weights of the corresponding low-frequency peaks and the temperature dependence of the various relevant decay rates. Based on these considerations, we obtain a qualitative picture of the frequency and temperature dependence of the optical conductivity of an anisotropic Fermi liquid.

cond-mat.str-el↗
arXiv2001

Thermodynamics of the anisotropic two-channel Kondo problem

We construct and solve numerically the thermodynamic Bethe Ansatz equations for the spin-anisotropic two-channel Kondo model in arbitrary external field $h$. At high temperatures the specific heat and the susceptibility show power law dependence. For $h \to 0$ and at temperatures below the Kondo temperature $T_K$ a two-channel Kondo effect develops characterized by a Wilson ratio 8/3, and a logarithmic divergence of the susceptibility and the linear specific heat coefficient. Finite magnetic field, $h>0$ drives the system to a Fermi liquid fixed point with an unusual Wilson ratio which depends sensitively on $h$.

cond-mat.str-el↗