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We study non-hermitian many-body physics in the interacting Hatano-Nelson model with open boundary condition. The violation of reciprocity, resulting from an imaginary vector potential, induces the non-hermitian skin-effect and causes exponential localization for all single particle eigenfunctions in the non-interacting limit. Nevertheless, the density profile of the interacting system becomes only slightly tilted relative to the average filling. The Friedel oscillations exhibit a beating pattern due to the modification of the Fermi wavenumber. The probability distribution of particles over any finite interval is the normal distribution, whose mean scales with the imaginary vector potential and the variance is symmetric to the center of the chain. This is confirmed by several numerically exact methods even for relatively small systems. These features are expected to be generic not only for fermions, which naturally repel each other due to Pauli's exclusion principle, but for interacting bosons as well. Our findings indicate that many-body effects can significantly alter and conceal the single particle properties and the skin effect in non-hermitian systems., Comment: 5 pages, 3 figures

We propose a novel many-body framework combining the density matrix renormalization group (DMRG) with the valence-space (VS) formulation of the in-medium similarity renormalization group. This hybrid scheme admits for favorable computational scaling in large-space calculations compared to direct diagonalization. The capacity of the VS-DMRG approach is highlighted in ab initio calculations of neutron-rich nickel isotopes based on chiral two- and three-nucleon interactions, and allows us to perform converged ab initio computations of ground and excited state energies. We also study orbital entanglement in the VS-DMRG, and investigate nuclear correlation effects in oxygen, neon, and magnesium isotopes. The explored entanglement measures reveal nuclear shell closures as well as pairing correlations., Comment: 5 pages, 4 Figures, version published at Phys. Lett. B

We study the dissipative dynamics of one-dimensional fermions, described in terms of the sine-Gordon model in its free massive boson or semi-classical limit, while keeping track of forward scattering processes. The system is prepared in the gapped ground state, and then coupled to environment through local currents within the Lindblad formalism. The heating dynamics of the system is followed using bosonization. The single particle density matrix exhibits correlations between the left and right moving particles. While the density matrix of right movers and left movers is translationally invariant, the left-right sector is not, corresponding to a translational symmetry breaking charge density wave state. Asymptotically, the single particle density matrix decays exponentially with exponent proportional to $-\gamma t|x|\Delta^2$ where $\gamma$ and $\Delta$ are the dissipative coupling and the gap, respectively. The charge density wave order parameter decays exponentially in time with an interaction independent decay rate. The second R\'enyi entropy grows linearly with time and is essentially insensitive to the presence of the gap., Comment: 9 pages, 3 figures

The time-dependent transport through single-molecule magnets coupled to magnetic or non-magnetic electrodes is studied in the framework of the generalized master equation method. We investigate the transient regime induced by the periodic switching of the source and drain contacts. If the electrodes have opposite magnetizations the quantum turnstile operation allows the stepwise writing of intermediate excited states. In turn, the transient currents provide a way to read these states. Within our approach we take into account both the uniaxial and transverse anisotropy. The latter may induce additional quantum tunneling processes which affect the efficiency of the proposed read-and-write scheme. An equally weighted mixture of molecular spin states can be prepared if one of the electrodes is ferromagnetic., Comment: 19 pages, 6 figures

Quantum critical systems derive their finite temperature properties from the influence of a zero temperature quantum phase transition. The paradigm is essential for understanding unconventional high-Tc superconductors and the non-Fermi liquid properties of heavy fermion compounds. However, the microscopic origins of quantum phase transitions in complex materials are often debated. Here we demonstrate experimentally, with support from numerical renormalization group calculations, a universal crossover from quantum critical non-Fermi liquid behavior to distinct Fermi liquid ground states in a highly controllable quantum dot device. Our device realizes the non-Fermi liquid two-channel Kondo state, based on a spin-1/2 impurity exchange-coupled equally to two independent electronic reservoirs. Arbitrarily small detuning of the exchange couplings results in conventional screening of the spin by the more strongly coupled channel for energies below a Fermi liquid scale T*. We extract a quadratic dependence of T* on gate voltage close to criticality and validate an asymptotically exact description of the universal crossover between strongly correlated non-Fermi liquid and Fermi liquid states., Comment: Text and supplementary information. Text: 14 pages, 3 figures. Supplementary information: 11 pages, 6 figures

We analyze the phase diagram associated with a pair of magnetic impurities trapped in a superconducting host. The natural interplay between Kondo screening, superconductivity and exchange interactions leads to a rich array of competing phases, whose transitions are characterized by discontinuous changes of the total spin. Our analysis is based on a combination of numerical renormalization group techniques as well as semi-classical analytics. In addition to the expected screened and unscreened phases, we observe a new molecular doublet phase where the impurity spins are only partially screened by a single extended quasiparticle. Direct signatures of the various Shiba molecule states can be observed via RF spectroscopy., Comment: 13 pages, 7 figures

We study resonant all-electric adiabatic spin pumping through a quantum dot with two nearby levels by using a Fermi liquid approach in the strongly interacting regime, combined with a projective numerical renormalization group (NRG) theory. Due to spin-orbit coupling, a strong spin pumping resonance emerges at every charging transition, which allows for the transfer of a spin $~ \hbar/2$ through the device in a single pumping cycle. Depending on the precise geometry of the device, controlled pure spin pumping is also possible., Comment: 5 pages, 5 figures

Central to condensed matter physics are quantum impurity models, which describe how a local degree of freedom interacts with a continuum. Surprisingly, these models are often universal in that they can quantitatively describe many outwardly unrelated physical systems. Here we develop a double quantum dot-based experimental realization of the SU(4) Kondo model, which describes the maximally symmetric screening of a local four-fold degeneracy. As demonstrated through transport measurements and detailed numerical renormalization group calculations, our device affords exquisite control over orbital and spin physics. Because the two quantum dots are coupled only capacitively, we can achieve orbital state- or "pseudospin"-resolved bias spectroscopy, providing intimate access to the interplay of spin and orbital Kondo effects. This cannot be achieved in the few other systems realizing the SU(4) Kondo state., Comment: Text and supplementary information. Text: 17 pages, 4 figures. Supplementary information: 24 pages, 9 figures

We analyze the equilibrium frequency-dependent spin current noise and spin conductance through a quantum dot in the local moment regime. Spin current correlations behave markedly differently from charge correlations. Equilibrium spin correlations are characterized by two universal scaling functions in the absence of an external field: one of them is related to charge correlations, while the other one describes cross-spin correlations. We characterize these functions using a combination of perturbative and non-perturbative methods. We find that at low temperatures spin cross-correlations are suppressed at frequencies below the Kondo scale, $T_K$, and a dynamical spin accumulation resonance is found at the Kondo energy, $\omega \sim T_K$. At higher temperatures, $T>T_K$, surprising low-frequency anomalies related to overall spin conservation appear in the spin noise and spin conductance, and the Korringa rate is shown to play a distinguished role. The transient spin current response also displays universal and singular properties., Comment: 17 pages, 15 figures

A theory of thermal transport in a two-channel Kondo system, such as the one formed by a small quantum dot coupled to two leads and to a larger dot, is formulated. The interplay of the two screening constants allows an exploration of the Fermi liquid and non-Fermi liquid regimes. By using analytical, as well as numerical renormalization group methods, we study the temperature dependence of the thermal conductance and the Lorentz number. We find that in the low temperature limit, the Lorentz number attains its universal value, irrespective of the nature of the ground state., Comment: 4 pages, 4 eps figures

We construct a real time current-conserving functional renormalization group (RG) scheme on the Keldysh contour to study frequency-dependent transport and noise through a quantum dot in the local moment regime. We find that the current vertex develops a non-trivial non-local structure in time, governed by a new set of RG equations. Solving these RG equations, we compute the complete frequency and temperature-dependence of the noise spectrum. For voltages large compared to the Kondo temperature, $eV \gg k_BT_K$, two sharp anti-resonances are found in the noise spectrum at frequencies $\hbar \omega = \pm e V$, and correspondingly, two peaks in the ac conductance through the dot., Comment: 4 pages, 4 figures

We analyze the equilibrium and non-equilibrium frequency-dependent spin current noise and spin conductance through a quantum dot in the local moment regime. Spin current correlations are shown to behave markedly differently from charge correlations: Equilibrium spin cross-correlations are suppressed at frequencies below the Kondo scale, and are characterized by a universal function that we determine numerically for zero temperature. For asymmetrical quantum dots dynamical spin accumulation resonance is found for frequencies of the order of the Kondo energy. At higher temperatures surprising low-frequency anomalies related to overall spin conservation appear.

We construct a semi-microscopic theory, to describe the optical conductivity of GaMnAs in the dilute limit, x = 1%. We construct an effective Hamiltonian that captures inside-impurity band optical transitions as well as transitions between the valence band and the impurity band. All parameters of the Hamiltonian are computed from microscopic variational calculations. We find a metal-insulator transition within the impurity band in the concentration range, x = 0.2 -0.3 for uncompensated and x = 1-3% for compensated samples, in good agreement with the experiments. We find an optical mass m_opt = m_e, which is almost independent of the impurity concentration excepting in the vicinity of the metal-insulator transition, where it reaches values as large as m_opt = 10 m_e. We also reproduce a mid-infrared peak at \hbar \omega = 200 meV, which redshifts upon doping, in quantitative agreement with the experiments., Comment: 17 pages, 16 figures, submitted to Physical Review B

Quantum impurity models describe interactions between some local degrees of freedom and a continuum of non-interacting fermionic or bosonic states. The investigation of quantum impurity models is a starting point towards the understanding of more complex strongly correlated systems, but quantum impurity models also provide the description of various correlated mesoscopic structures, biological and chemical processes, atomic physics and describe phenomena such as dissipation or dephasing. Prototypes of these models are the Anderson impurity model, or the single- and multi-channel Kondo models. The numerical renormalization group method (NRG) proposed by Wilson in mid 70's has been used in its original form for a longtime as one of the most accurate and powerful methods to deal with quatum impurity problems. Recently, a number of new developments took place: First, a spectral sum-conserving density matrix NRG approach (DM-NRG) has been developed, which has also been generalized for non-Abelian symmetries. In this manual we introduce some of the basic concepts of the NRG method and present recently developed Flexible DM-NRG code. This code uses user-defined non-Abelian symmetries dynamically, computes spectral functions, expectation values of local operators for user-defined impurity models. The code can also use a uniform density of states as well as a user-defined density of states. The current version of the code assumes fermionic bath's and it uses any number of U(1), SU(2) charge SU(2) or Z(2) symmetries. The Flexible DM-NRG code can be downloaded from http://www.phy.bme.hu/~dmnrg, Comment: The Flexible DM-NRG code can be downloaded from http://www.phy.bme.hu/~dmnrg

Changes dopant ion concentrations in the sides of a symmetric quantum well are known to create a random Rashba-type spin-orbit coupling. Here we demonstrate that, as a consequence, a finite size spin-Hall effect is also present. Our numerical algorithm estimates the result of the Kubo formula for the spin-Hall conductivity, by using a tight-binding approximation of the Hamiltonian in the framework of a time-dependent Green's function formalism, well suited for very large systems., Comment: 4 pages, 4 figures in eps format

We generalize the spectral sum rule preserving density matrix numerical renormalization group (DM-NRG) method in such a way that it can make use of an arbitrary number of not necessarily Abelian, local symmetries present in the quantum impurity system. We illustrate the benefits of using non-Abelian symmetries by the example of calculations for the T-matrix of the two-channel Kondo model in the presence of magnetic field, for which conventional NRG methods produce large errors and/or take a long run-time., Comment: 12 pages, 6 figures, PRB format

The Kubo formula is used to calculate the spin-Hall conductivity in a spin-polarized two-dimensional electron system with Rashba-type spin-orbit interaction. As in the case of the unpolarized electron system, spin Hall conductivity is entirely determined by states at the Fermi level, a property that persists in the presence of magnetic impurities. In the clean limit, the spin-Hall conductivity decreases monotonically with the Zeeman splitting, a result of the ordering effect on the electron spins produced by the magnetic field. In the presence of magnetic impurities, the spin-dependent scattering determines a finite renormalization of the static part of the fully dressed vertex correction of the velocity operator that leads to an enhancement of the \sigma_{sH}, an opposite behaviour to that registered in the presence of spin-independent disorder. The variation of \sigma_{sH} with the strength of the Rashba coupling and the Zeeman splitting is studied., Comment: 13 pages, 5 figures

We compare experimental resistivity data on Ga_{1-x}Mn_xAs films with theoretical calculations using a scaling theory for strongly disordered ferromagnets. All characteristic features of the temperature dependence of the resistivity can be quantitatively understood through this approach as originating from the close vicinity of the metal-insulator transition. In particular, we find that the magnetic field induced changes in resistance cannot be explained within a mean-field treatment of the magnetic state, and that accounting for thermal fluctuations is crucial for a quantitative analysis. Similarly, while the non-interacting scaling theory is in reasonable agreement with the data, we find clear evidence in favor of interaction effects at low temperatures., Comment: 4 pages, 3 figures

Within the Kubo formalism, we estimate the spin-Hall conductivity in a two-dimensional electron gas with Rashba spin-orbit interaction and study its variation as a function of disorder strength and system size. The numerical algorithm employed in the calculation is based on the direct numerical integration of the time-dependent Schrodinger equation in a spin-dependent variant of the particle source method. We find that the spin-precession length, L_s controlled by the strength of the Rashba coupling, establishes the critical lengthscale that marks the significant reduction of the spin-Hall conductivity in bulk systems. In contrast, the electron mean free path, inversely proportional to the strength of disorder, appears to have only a minor effect., Comment: 5 pages, 3 figures

We derive an effective Hamiltonian for ${\rm Ga}_{1-x}{\rm Mn}_x {\rm As}$ in the dilute limit, where ${\rm Ga}_{1-x}{\rm Mn}_x {\rm As}$ can be described in terms of spin $F=3/2$ polarons hopping between the {\rm Mn} sites and coupled to the local {\rm Mn} spins. We determine the parameters of our model from microscopic calculations using both a variational method and an exact diagonalization within the so-called spherical approximation. Our approach treats the extremely large Coulomb interaction in a non-perturbative way, and captures the effects of strong spin-orbit coupling and Mn positional disorder. We study the effective Hamiltonian in a mean field and variational calculation, including the effects of interactions between the holes at both zero and finite temperature. We study the resulting magnetic properties, such as the magnetization and spin disorder manifest in the generically non-collinear magnetic state. We find a well formed impurity band fairly well separated from the valence band up to $x_{\rm active} \lesssim 0.015$ for which finite size scaling studies of the participation ratios indicate a localization transition, even in the presence of strong on-site interactions, where $x_{\rm active}

We study the magnetic properties of metallic GaMnAs. We calculate the effective RKKY interaction between Mn spins using several realistic models for the valence band structure of GaAs. We also study the effect of positional disorder of the Mn on the magnetic properties. We find that the interaction between two Mn spins is anisotropic due to spin-orbit coupling within both the so-called spherical approximation and in the more realistic six band model. The spherical approximation strongly overestimates this anistropy, especially for short distances between Mn ions. Using the obtained effective Hamiltonian we carry out Monte Carlo simulations of finite and zero temperature magnetization and find that, due to orientational frustration of the spins, non-collinear states appear in both valence band approximations for disordered, uncorrelated Mn impurities in the small concentration regime. Introducing correlations among the substitutional Mn positions or increasing the Mn concentration leads to an increase in the remnant magnetization at zero temperature and an almost fully polarized ferromagnetic state., Comment: 17 Pages, 13 Figures

Experimental data obtained from thermodynamic measurements in underdoped high temperature superconductors show unusual anomalies in the temperature dependence of the electronic specific heat both in the normal state and at the critical point associated to the superconducting phase transition. The observed deviations from the standard behavior are probably linked with the opening of a pseudogap in the energy spectrum of the single-particle excitations associated with the normal state. Based on a phenomenological description of the pseudogap phase we perform analytical and numerical calculations for the temperature dependence of the specific heat for both the superconducting and normal state. The reduced specific heat jump at the transition point can be explained by a modified electronic single particle contribution to the specific heat in the presence of the normal state pseudogap. The hump observed in the normal state specific heat can be explained by the electronic pair contribution associated with strong fluctuations of the order parameter in the critical region. The obtained theoretical results are discussed in connection with experimental data for cuprates., Comment: 7 pages, 3 EPS figures. to appear in EPJB

Within a two-band, strong coupling model we analyze SIS-tunneling conductance, Raman scattering and optical conductivity measured on $MgB_2$ samples. We find that features observed in tunneling and Raman spectroscopy at intermediate energies $\omega=\Delta_{\sigma}+\Delta_{\pi}\simeq 8\div 10meV$ can be consistently explained when incoherent scattering between $\sigma$ and $\pi$ band is considered. We calculate the optical conductivity and find that strong coupling effects are important for explaining the presently available data, and predict the location and magnitude of interband scattering features for this spectroscopic probe as well., Comment: 5 pages, 3 figures

When pairing correlations in a quasi two dimensional electron system induce a pseudogap in the single particle density of states, the specific heat must also contain a sizeable pair contribution. The theoretically calculated specific heat for such a system is compared to the experimental results of Loram and his collaborators for underdoped YBa_2Cu_3O_{6+x} and La_{2-x}Sr_{x}CuO_4 samples. The size and doping dependence of the extracted pseudogap energy scale for both materials is comparable to the values obtained from a variety of other experiments., Comment: 4 pages, 5 eps figures

We performed a self-consistent calculation of the transport properties of a d-wave superconductor. We used for calculations the T-matrix approximation. The coresponding equations were evaluated numerically directly on the real frequecy axis. We studied the ab-plane charge dynamics in the coherent limit. For the c-axis charge dynamics, we considered both, the coherent and the incoherent limit. We also have calculated the penetration depth in this model., Comment: 7 pages, 6 figures, to appear in Physica C

We performed a self-consistent calculation using T-Matrix approximation for a quasi-bidimensional system. We calculated the one particle spectrum function A(k,\omega) in the presence of strong d-wave attractive interaction. The c-axis charge dynamics was studied by considering incoherent interlayer hopping and ab-plane charge dynamics was studied in the coherent limit. It is shown that the c-axis charge dynamics is mainly governed by the scattering from the in plane fluctuations. We also present results for c-axis and ab-plane resistivity and for thermopower coefficient., Comment: 10 pages, 13 figures, accepted for publication in Physica C

We study the Ginsburg-Landau expansion for the non-Fermi model proposed by Anderson. We analyze the deviations of the main properties of a non-Fermi superconductor from the isotropic s-wave bidimensiona superconductor., Comment: 7 pages, 3 figures, accepted for publication in Physica C

Using the field-theoretical methods we studied the evolution from BCS description of a non-Fermi superconductor to that of Bose-Einstein condensation (BEC) in one loop approximation. We showed that the repulsive interaction between composite bosons is determined by the exponent $\alpha$ of the Anderson propagator in a two dimensional model. For $\a\neq 0$ the crossover is also continous and for $\a=0$ we obtain the case of the Fermi liquid.

We showed that an electronic system with weak disorder considered in the finite-charge infinite U Hubbard model can present a non-Fermi behavior. The imaginary pert of the self-energy has been calculated and a linear temperature dependence was obtained. This result is in agreement with the non-Fermi behavior observed at the insulator-metal crossover in La_{2-x}Sr_xCuO_4.

Using the renormalization approach proposed by Millis for the itinerant electron systems we calculated the specific heat coefficient $\gamma(T)$ for the magnetic fluctuations with susceptibility $\chi^{-1}\sim |\delta+\omega|^\alpha+f(q)$ near the Lifshitz point. The constant value obtained for $\alpha=4/5$ and the logarithmic temperature dependence, specific for the non-Fermi behavior, have been obtained in agreement with the experimental dat., Comment: 6 pages, Revtex

We studied the influence of the amplitude fluctuations of a non-Fermi superconductor on the energy spectrum of the 2D Anderson non-Fermi system. The classical fluctuations give a temperature dependence in the pseudogap induced in the fermionic excitations., Comment: revtex file

Using the flow-equations method we showed analytically the occurence of dispersion for the local bosons in a model of hybridized of local bosons and fermions., Comment: REVTeX file

The flow equation method has been used to calculate the energy of single impurity in a superconductor for the Anderson model with $U \neq 0$. We showed that the energy of the impurity depends only of the $\Delta^{2}_{R}$ (renormalized order parameter) which depends on the renormalized Hubbard repulsion$U^R$. For a strong Hubbard repulsion $U^R = U$ and $\Delta^R = \Delta ^I$ the effect of the $s-d$ interactions are nonrelevant, a result which is expected for this model., Comment: Latex file