arXiv subjects

R. Bulla

Publications and source records attributed to R. Bulla.

At least 19 recordsLinked to original sources

arXiv2008

Dissipative Two-Electron Transfer

We investigate non-equilibrium two-electron transfer in a model redox system represented by a two-site extended Hubbard model and embedded in a dissipative environment. The influence of the electron-electron interactions and the coupling to a dissipative bosonic bath on the electron transfer is studied in different temperature regimes. At high temperatures Marcus transfer rates are evaluated and at low temperatures, we calculate equilibrium and non-equilibrium population probabilities of the donor and acceptor with the non-perturbative Numerical Renormalization Group approach. We obtain the non-equilibrium dynamics of the system prepared in an initial state of two electrons at the donor site and identify conditions under which the electron transfer involves one concerted two-electron step or two sequential single-electron steps. The rates of the sequential transfer depend non-monotonically on the difference between the inter-site and on-site Coulomb interaction which become renormalized in the presence of the bosonic bath. If this difference is much larger than the hopping matrix element, the temperature as well as the reorganization energy, simultaneous transfer of both electrons between donor and acceptor can be observed.

cond-mat.str-el↗

arXiv2007

Kondo quantum dot coupled to ferromagnetic leads: Numerical renormalization group study

We 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 states, to identify the effects of (i) a finite spin polarization in the leads (at the Fermi-surface), (ii) a Stoner splitting in the bands (governed by the band edges) and (iii) an arbitrary shape of the leads density of states. For a generic lead density of states the quantum dot favors being occupied by a particular spin-species due to exchange interaction with ferromagnetic leads leading to a suppression and splitting of the Kondo resonance. The application of a magnetic field can compensate this asymmetry restoring the Kondo effect. We study both the gate-voltage dependence (for a fixed band structure in the leads) and the spin polarization dependence (for fixed gate voltage) of this compensation field for various types of bands. Interestingly, we find that the full recovery of the Kondo resonance of a quantum dot in presence of leads with an energy dependent density of states is not only possible by an appropriately tuned external magnetic field but also via an appropriately tuned gate voltage. For flat bands simple formulas for the splitting of the local level as a function of the spin polarization and gate voltage are given.

cond-mat.mes-hall↗

arXiv2006

Modeling molecular conduction in DNA wires: Charge transfer theories and dissipative quantum transport

Measurements of electron transfer rates as well as of charge transport characteristics in DNA produced a number of seemingly contradictory results, ranging from insulating behaviour to the suggestion that DNA is an efficient medium for charge transport. Among other factors, environmental effects appear to play a crucial role in determining the effectivity of charge propagation along the double helix. This chapter gives an overview over charge transfer theories and their implication for addressing the interaction of a molecular conductor with a dissipative environment. Further, we focus on possible applications of these approaches for charge transport through DNA-based molecular wires.

cond-mat.soft↗

arXiv2004

Dynamical Mean-Field Theory and Its Applications to Real Materials

Dynamical mean-field theory (DMFT) is a non-perturbative technique for the investigation of correlated electron systems. Its combination with the local density approximation (LDA) has recently led to a material-specific computational scheme for the ab initio investigation of correlated electron materials. The set-up of this approach and its application to materials such as (Sr,Ca)VO_3, V_2O_3, and Cerium is discussed. The calculated spectra are compared with the spectroscopically measured electronic excitation spectra. The surprising similarity between the spectra of the single-impurity Anderson model and of correlated bulk materials is also addressed.

cond-mat.str-el↗

arXiv2004

Quantum Critical Points in Quantum Impurity Systems

The numerical renormalization group method is used to investigate zero temperature phase transitions in quantum impurity systems, in particular in the soft-gap Anderson model, where an impurity couples to a non-trivial fermionic bath. In this case, zero temperature phase transitions occur between two different phases whose fixed points can be built up of non-interacting single-particle states. However, the quantum critical point cannot be described by non-interacting fermionic or bosonic excitations.

cond-mat.str-el↗

arXiv2004

Gate-controlled spin-splitting in quantum dots with ferromagnetic leads in the Kondo regime

The effect of a gate voltage ($V_g$) on the spin-splitting of an electronic level in a quantum dot (QD) attached to ferromagnetic leads is studied in the Kondo regime using a generalized numerical renormalization group technique. We find that the $V_g$-dependence of the QD level spin-splitting strongly depends on the shape of the density of states (DOS). For one class of DOS shapes there is nearly no $V_g$-dependence, for another, $V_g$ can be used to control the magnitude and sign of the spin-splitting, which can be interpreted as a local exchange magnetic field. We find that the spin-splitting acquires a new type of logarithmic divergence. We give an analytical explanation for our numerical results and explain how they arise due to spin-dependent charge fluctuations.

cond-mat.mtrl-sci↗

arXiv2003

Phase diagram of the frustrated Hubbard model

The Mott-Hubbard metal-insulator transition in the paramagnetic phase of the one-band Hubbard model has long been used to describe similar features in real materials like V$_2$O$_3$. Here we show that this transition is hidden inside a rather robust antiferromagnetic insulator even in the presence of comparatively strong magnetic frustration. This result raises the question of the relevance of the Mott-Hubbard metal-insulator transition for the generic phase diagram of the one-band Hubbard model.

cond-mat.str-el↗

arXiv2003

Mott transitions in correlated electron systems with orbital degrees of freedom

Mott metal-insulator transitions in an M-fold orbitally degenerate Hubbard model are studied by means of a generalization of the linearized dynamical mean-field theory. The method allows for an efficient and reliable determination of the critical interaction U_c for any integer filling n and different M at zero temperature. For half-filling a linear dependence of U_c on M is found. Inclusion of the (full) Hund's rule exchange J results in a strong reduction of U_c. The transition turns out to change qualitatively from continuous for J=0 to discontinuous for any finite J.

cond-mat.str-el↗

arXiv2002

Nonresonant inelastic light scattering in the Hubbard model

Inelastic light scattering from electrons is a symmetry-selective probe of the charge dynamics within correlated materials. Many measurements have been made on correlated insulators, and recent exact solutions in large dimensions explain a number of anomalous features found in experiments. Here we focus on the correlated metal, as described by the Hubbard model away from half filling. We can determine the B1g Raman response and the inelastic X-ray scattering along the Brillouin zone diagonal exactly in the large dimensional limit. We find a number of interesting features in the light scattering response which should be able to be seen in correlated metals such as the heavy fermions.

cond-mat.str-el↗

arXiv2002

Quantum Phase Transitions in Models of Magnetic Impurities

Zero temperature phase transitions not only occur in the bulk of quantum systems, but also at boundaries or impurities. We review recent work on quantum phase transitions in impurity models that are generalizations of the standard Kondo model describing the interaction of a localized magnetic moment with a metallic fermionic host. Whereas in the standard case the moment is screened for any antiferromagnetic Kondo coupling as T to 0, the common feature of all systems considered here is that Kondo screening is suppressed due to the competition with other processes. This competition can generate unstable fixed points associated with phase transitions, where the impurity properties undergo qualitative changes. In particular, we discuss the coupling to both non-trivial fermionic and bosonic baths as well as two-impurity models, and make connections to recent experiments.

cond-mat.str-el↗

arXiv2002

Gap formation and soft phonon mode in the Holstein model

We investigate electron-phonon coupling in many-electron systems using dynamical mean-field theory in combination with the numerical renormalization group. This non-perturbative method reveals significant precursor effects to the gap formation at intermediate coupling strengths. The emergence of a soft phonon mode and very strong lattice fluctuations can be understood in terms of Kondo-like physics due to the development of a double-well structure in the effective potential for the ions.

cond-mat.str-el↗

arXiv2002

Photoemission study of polycrystalline Gd_{1-x}Sr_xTiO_{3+delta}: correlation and surface effects

We report photoemission studies of polycrystalline samples of Sr-doped GdTiO_{3+delta}, which undergoes a Mott-Hubbard-like metal-insulator transition at a Ti-concentration of about 20%. The Ti 3d-derived valence band near the Fermi level displays a two-peak structure consistent with a Hubbard-model-type interpretation as quasiparticle and lower Hubbard band. Contrary to the theoretical expectation the lower Hubbard band does not change its energy with doping. However, the analysis of the core level spectra and elemental mapping with a scanning electron microscope indicate a strongly inhomogeneous doping concentration in the probed surface regions. This questions the intrinsic character of the valence band spectra and demonstrates a general difficulty when using polycrystalline samples for such studies.

cond-mat.str-el↗

arXiv2002

Magnetism and Phase Separation in the Ground State of the Hubbard Model

We discuss the ground state magnetic phase diagram of the Hubbard model off half filling within the dynamical mean-field theory. The effective single-impurity Anderson model is solved by Wilson's numerical renormalization group calculations, adapted to symmetry broken phases. We find a phase separated, antiferromagnetic state up to a critical doping for small and intermediate values of U, but could not stabilise a Neel state for large U and finite doping. At very large U, the phase diagram exhibits an island with a ferromagnetic ground state. Spectral properties in the ordered phases are discussed.

cond-mat.str-el↗

arXiv2001

Nonresonant B1g Raman scattering in the Hubbard model

The numerically exact solution for nonresonant B1g Raman scattering is presented for the half-filled Hubbard model in infinite dimensions. This solution illustrates the modifications of the Raman response (in a system tuned through the quantum-critical point of a metal-insulator transition) due to Fermi-liquid properties in the metallic phase. In the insulating phase, we recover the predicted universal behavior, while we find the Raman response is quite anomalous on the metallic side of the transition. Our calculated results are similar to those measured in FeSi, SmB6, and underdoped cuprates.

cond-mat.str-el↗

arXiv2001

Critical behaviour near the metal-insulator transition of a doped Mott insulator

We have studied the critical behaviour of a doped Mott insulator near the metal-insulator transition for the infinite-dimensional Hubbard model using a linearized form of dynamical mean-field theory. The discontinuity in the chemical potential in the change from hole to electron doping, for $U$ larger than a critical value $U_c$, has been calculated analytically and is found to be in good agreement with the results of numerical methods. We have also derived analytic expressions for the compressibility, the quasiparticle weight, the double occupancy and the local spin susceptibility near half-filling as functions of the on-site Coulomb interaction and the doping.

cond-mat.str-el↗

arXiv2001

Phase diagram of the Mott Transition in a two-band Hubbard model in infinite dimensions

The Mott metal-insulator transition in the two-band Hubbard model in infinite dimensions is studied by using the linearized dynamical mean-field theory recently developed by Bulla and Potthoff. The phase boundary of the metal-insulator transition is obtained analytically as a function of the on-site Coulomb interaction at the $d$-orbital, the charge-transfer energy between the $d$- and $p$-orbitals and the hopping integrals between $p-d$, $d-d$ and $p-p$ orbitals. The result is in good agreement with the numerical results obtained from the exact diagonalization method.

cond-mat.str-el↗

arXiv2000

Finite temperature numerical renormalization group study of the Mott-transition

Wilson's numerical renormalization group (NRG) method for the calculation of dynamic properties of impurity models is generalized to investigate the effective impurity model of the dynamical mean field theory at finite temperatures. We calculate the spectral function and self-energy for the Hubbard model on a Bethe lattice with infinite coordination number directly on the real frequency axis and investigate the phase diagram for the Mott-Hubbard metal-insulator transition. While for T T_c there is a smooth crossover from metallic-like to insulating-like solutions.

cond-mat.str-el↗

arXiv2000

Anderson impurity in a correlated conduction band

We investigate the physics of a magnetic impurity with spin 1/2 in a correlated metallic host. Describing the band by a Hubbard Hamiltonian, the problem is analyzed using dynamical mean-field-theory in combination with Wilson's nonperturbative numerical renormalization group. We present results for the single-particle density of states and the dynamical spin susceptibility at zero temperature. New spectral features (side peaks) are found which should be observable experimentally. In addition, we find a general enhancement of the Kondo scale due to correlations. Nevertheless, in the metallic phase, the Kondo scale always vanishes exponentially in the limit of small hybridization.

cond-mat.str-el↗