arXiv subjects

H. Schoeller

Publications and source records attributed to H. Schoeller.

At least 19 recordsLinked to original sources

arXiv2021

Quantitative analysis of interaction effects in generalized Aubry-Andŕe-Harper models

We present a quantitative analysis of two-particle interaction effects in generalized, one-dimensional Aubry-André-Harper models with the Fermi energy placed in one of the band gaps. We investigate systems with periodic as well as open boundary conditions; for the latter focusing on the number of edge states and the boundary charge. Both these observables are important for the classification of noninteracting topological systems. In our first class of models the unit cell structure stems from periodically modulated single-particle parameters. In the second it results from the spatial modulation of the two-particle interaction. For both types of models, we find that the single-particle band gaps are renormalized by the interaction in accordance with expectations employing general field theoretical arguments. While interaction induced effective edge states can be found in the local single-particle spectral function close to a boundary, the characteristics of the boundary charge are not modified by the interaction. This indicates that our results for the Rice-Mele and Su-Schriefer-Heeger model [Phys. Rev. B 102, 085122 (2020)] are generic and can be found in lattice models with more complex unit cells as well.

cond-mat.str-el↗

arXiv2020

The interacting Rice-Mele model: bulk and boundaries

We investigate the interacting, one-dimensional Rice-Mele model, a prototypical fermionic model of topological properties. To set the stage, we firstly compute the single-particle spectral function, the local density, and the boundary charge in the absence of interactions. The boundary charge is fully determined by bulk properties indicating a bulk-boundary correspondence. In a large parameter regime it agrees with the one obtained from an effective low-energy theory (arXiv:2004.00463). Secondly, we investigate the robustness of our results towards two-particle interactions. To resum the series of leading logarithms for small gaps, which dismantle plain perturbation theory in the interaction, we use an essentially analytical renormalization group approach. It is controlled for small interactions and can directly be applied to the microscopic lattice model. We benchmark the results against numerical density matrix renormalization group data. The main interaction effect in the bulk is a power-law renormalization of the gap with an interaction dependent exponent. The important characteristics of the boundary charge are unaltered and can be understood from the renormalized bulk properties, elevating the bulk-boundary correspondence to the interacting regime. This requires a consistent treatment not only of the low-energy gap renormalization but also of the high-energy band width one. In contrast to low-energy field theories our renormalization group approach also provides the latter. We show that the interaction spoils the relation between the bulk properties and the number of edge states, consistent with the observation that the Rice-Mele model with finite potential modulation does not reveal any zero-energy edge states.

cond-mat.str-el↗

arXiv2013

Scaling of the Kondo zero bias peak in a hole quantum dot at finite temperatures

We have measured the zero bias peak in differential conductance in a hole quantum dot. We have scaled the experimental data with applied bias and compared to real time renormalization group calculations of the differential conductance as a function of source-drain bias in the limit of zero temperature and at finite temperatures. The experimental data show deviations from the T=0 calculations at low bias, but are in very good agreement with the finite T calculations. The Kondo temperature T_K extracted from the data using T=0 calculations, and from the peak width at 2/3 maximum, is significantly higher than that obtained from finite T calculations.

cond-mat.str-el↗

arXiv2013

Oscillatory dynamics and non-markovian memory in dissipative quantum systems

The nonequilibrium dynamics of a small quantum system coupled to a dissipative environment is studied. We show that (1) the oscillatory dynamics close to a coherent-to-incoherent transition is surprisingly different from the one of the classical damped harmonic oscillator and that (2) non-markovian memory plays a prominent role in the time evolution after a quantum quench.

cond-mat.str-el↗

arXiv2012

A renormalization-group analysis of the interacting resonant level model at finite bias: Generic analytic study of static properties and quench dynamics

Using a real-time renormalization group method we study the minimal model of a quantum dot dominated by charge fluctuations, the two-lead interacting resonant level model, at finite bias voltage. We develop a set of RG equations to treat the case of weak and strong charge fluctuations, together with the determination of power-law exponents up to second order in the Coulomb interaction. We derive analytic expressions for the charge susceptibility, the steady-state current and the conductance in the situation of arbitrary system parameters, in particular away from the particle-hole symmetric point and for asymmetric Coulomb interactions. In the generic asymmetric situation we find that power laws can be observed for the current only as function of the level position (gate voltage) but not as function of the voltage. Furthermore, we study the quench dynamics after a sudden switch-on of the level-lead couplings. The time evolution of the dot occupation and current is governed by exponential relaxation accompanied by voltage-dependent oscillations and characteristic algebraic decay.

cond-mat.str-el↗

arXiv2010

Non-equilibrium current and relaxation dynamics of a charge-fluctuating quantum dot

We study the steady-state current in a minimal model for a quantum dot dominated by charge fluctuations and analytically describe the time evolution into this state. The current is driven by a finite bias voltage V across the dot, and two different renormalization group methods are used to treat small to intermediate local Coulomb interactions. The corresponding flow equations can be solved analytically which allows to identify all microscopic cutoff scales. Exploring the entire parameter space we find rich non-equilibrium physics which cannot be understood by simply considering the bias voltage as an infrared cutoff. For the experimentally relevant case of left-right asymmetric couplings, the current generically shows a power-law suppression for large V. The relaxation dynamics towards the steady state features characteristic oscillations as well as an interplay of exponential and power-law decay.

cond-mat.mes-hall↗

arXiv2010

Charge transport through single molecules, quantum dots, and quantum wires

We review recent progresses in the theoretical description of correlation and quantum fluctuation phenomena in charge transport through single molecules, quantum dots, and quantum wires. A variety of physical phenomena is addressed, relating to co-tunneling, pair-tunneling, adiabatic quantum pumping, charge and spin fluctuations, and inhomogeneous Luttinger liquids. We review theoretical many-body methods to treat correlation effects, quantum fluctuations, nonequilibrium physics, and the time evolution into the stationary state of complex nanoelectronic systems.

cond-mat.mes-hall↗

arXiv2009

Transport signature of pseudo-Jahn-Teller dynamics in a single-molecule transistor

We calculate the electronic transport through a molecular dimer, in which an excess electron is delocalized over equivalent monomers, which can be locally distorted. In this system the Born-Oppenheimer approximation breaks down resulting in quantum entanglement of the mechanical and electronic motion. We show that pseudo Jahn-Teller (pJT) dynamics of the molecule gives rise to conductance peaks that indicate this violation. Their magnitude, sign and position sharply depend on the electro-mechanical properties of the molecule, which can be varied in recently developed three-terminal junctions with mechanical control. The predicted effect depends crucially on the degree of intramolecular delocalization of the excess electron, a parameter which is also of fundamental importance in physical chemistry.

cond-mat.mes-hall↗

arXiv2008

Quasiparticles in the Kondo lattice model at partial fillings of the conduction band

We study the spectral properties of the one-dimensional Kondo lattice model as function of the exchange coupling, the band filling, and the quasimomentum in the ferromagnetic and paramagnetic phase. Using the density-matrix renormalization group method, we compute the dispersion relation of the quasiparticles, their lifetimes, and the Z-factor. As a main result, we provide evidence for the existence of the spinpolaron at partial band fillings. We find that the quasiparticle lifetime differs by orders of magnitude between the ferromagnetic and paramagnetic phase and depends strongly on the quasimomentum.

cond-mat.str-el↗

arXiv2008

Fermionic renormalization group methods for transport through inhomogeneous Luttinger liquids

We compare two fermionic renormalization group methods which have been used to investigate the electronic transport properties of one-dimensional metals with two-particle interaction (Luttinger liquids) and local inhomogeneities. The first one is a poor man's method setup to resum ``leading-log'' divergences of the effective transmission at the Fermi momentum. Generically the resulting equations can be solved analytically. The second approach is based on the functional renormalization group method and leads to a set of differential equations which can only for certain setups and in limiting cases be solved analytically, while in general it must be integrated numerically. Both methods are claimed to be applicable for inhomogeneities of arbitrary strength and to capture effects of the two-particle interaction, such as interaction dependent exponents, up to leading order. We critically review this for the simplest case of a single impurity. While on first glance the poor man's approach seems to describe the crossover from the ``perfect'' to the ``open chain fixed point'' we collect evidence that difficulties may arise close to the ``perfect chain fixed point''. Due to a subtle relation between the scaling dimensions of the two fixed points this becomes apparent only in a detailed analysis. In the functional renormalization group method the coupling of the different scattering channels is kept which leads to a better description of the underlying physics.

cond-mat.str-el↗

arXiv2006

Kondo-transport spectroscopy of single molecule magnets

We demonstrate that in a single molecule magnet (SMM) strongly coupled to electrodes the Kondo effect involves all magnetic excitations. This Kondo effect is induced by the quantum tunneling of the magnetic moment (QTM). Importantly, the Kondo temperature $T_K$ can be much larger than the magnetic splittings. We find a strong modulation of the Kondo effect as function of the transverse anisotropy parameter or a longitudinal magnetic field. For both integer and half-integer spin this can be used for an accurate transport spectroscopy of the magnetic states in low magnetic fields on the order of the easy-axis anisotropy parameter. We set up a relationship between the Kondo effects for successive integer and half-integer spins.

cond-mat.str-el↗

arXiv2006

Quantum tunneling induced Kondo effect in single molecular magnets

We consider transport through a single-molecule magnet strongly coupled to metallic electrodes. We demonstrate that for half-integer spin of the molecule electron- and spin-tunneling \emph{cooperate} to produce both quantum tunneling of the magnetic moment and a Kondo effect in the linear conductance. The Kondo temperature depends sensitively on the ratio of the transverse and easy-axis anisotropies in a non-monotonic way. The magnetic symmetry of the transverse anisotropy imposes a selection rule on the total spin for the occurrence of the Kondo effect which deviates from the usual even-odd alternation.

cond-mat.str-el↗

arXiv2006

Spin quantum tunneling in single molecular magnets: fingerprints in transport spectroscopy of current and noise

We demonstrate that transport spectroscopy of single molecular magnets shows signatures of quantum tunneling at low temperatures. We find current and noise oscillations as function of bias voltage due to a weak violation of spin selection rules by quantum tunneling processes. The interplay with Boltzmann suppression factors leads to fake resonances with temperature-dependent position which do not correspond to any charge excitation energy. Furthermore, we find that quantum tunneling can completely suppress transport if the easy-plane anisotropy has a high symmetry.

cond-mat.str-el↗

arXiv2005

Spin blockade in a charge-switchable molecular magnet

We consider the effect of adding electrons to a single molecule on its magnetic properties and the resulting transport fingerprints. We analyze a generic model for a metal-organic complex consisting of orbitals with different Coulomb repulsions. We find that by modulating the charge of the molecule by a single electron the total spin can be switched from zero to the maximal value supported by the added electrons, $S=3/2$. The Nagaoka mechanism is responsible for this charge-sensitivity of the molecular spin. It is shown that fingerprints of these maximal spin states, either as groundstates or low-lying excitations, can be experimentally observed in current-spectroscopy. as either spin blockade at low bias voltage or negative differential conductance and complete current suppression at finite bias.

cond-mat.str-el↗

arXiv2005

Charge-induced maximal spin states of a polynuclear transition-metal complex

We theoretically investigate the ground state spin of a polynuclear transition-metal complex as a function of the number of added electrons taking into account strong electron correlations. Our phenomenological model of the so-called [$2\times2$]-grid molecule incorporates the relevant electronic degrees of freedom on the four transition-metal centers (either Fe$^{2+}$ or Co$^{2+}$) and the four organic bridging ligands. Extra electrons preferably occupy redox orbitals on the ligands. Magnetic interactions between these ligands are mediated by transition-metal ions {\em and vice versa}. Using both perturbation theory and exact diagonalization we find that for certain charge states the maximally attainable total spin (either $S_{\mathsf{tot}}=3/2$ or $S_{\mathsf{tot}}=7/2$) may actually be achieved. Due to the Nagaoka mechanism, all unpaired electron spins couple to a total maximal spin, including unpaired electron spins on the metal-ions in the case of Co$^{2+}$. The parameters are chosen to be consistent with cyclovoltammetry experiments in which up to twelve redox states have been observed. The above effect may also be realized in other complexes with an appropriate connectivity between the redox sites. The maximal spin states of such a charge-switchable molecular magnet may be experimentally observed as spin-blockade effects on the electron tunneling in a three-terminal transport setup.

cond-mat.str-el↗

arXiv2002

Current collapse in tunneling transport through benzene

We investigate the electrical transport through a system of benzene coupled to metal electrodes by electron tunneling. Using electronic structure calculations, a semi--quantitative model for the pi-electrons of the benzene is derived that includes general two-body interactions. After exact diagonalization of the benzene model the transport is computed using perturbation theory for weak electrode-benzene coupling (golden rule approximation). We include the effect of an applied electric field on the molecular states, as well as radiative relaxation. We predict a current collapse and strong negative differential conductance due to a ``blocking'' state when the electrode is coupled to the para-position of benzene. In contrast, for coupling to the meta-position, a series of steps in the I-V curve is found.

cond-mat.mes-hall↗

arXiv2002

Interference in interacting quantum dots with spin

We study spectral and transport properties of interacting quantum dots with spin. Two particular model systems are investigated: Lateral multilevel and two parallel quantum dots. In both cases different paths through the system can give rise to interference. We demonstrate that this strengthens the multilevel Kondo effect for which a simple two-stage mechanism is proposed. In parallel dots we show under which conditions the peak of an interference-induced orbital Kondo effect can be split.

cond-mat.mes-hall↗

arXiv1997

Photon-Assisted Transport Through Ultrasmall Quantum Dots: Influence of Intradot Transitions

We study transport through one or two ultrasmall quantum dots with discrete energy levels to which a time-dependent field is applied (e.g., microwaves). The AC field causes photon-assisted tunneling and also transitions between discrete energy levels of the dot. We treat the problem by introducing a generalization of the rotating-wave approximation to arbitrarily many levels. We calculate the dc-current through one dot and find satisfactory agreement with recent experiments by Oosterkamp et al. . In addition, we propose a novel electron pump consisting of two serially coupled single-level quantum dots with a time-dependent interdot barrier.

cond-mat.mes-hall↗