arXiv subjects

Gerd Czycholl

Publications and source records attributed to Gerd Czycholl.

18 recordsLinked to original sources

arXiv2019

Nonlocal Exchange Interactions in Strongly Correlated Electron Systems

We study the influence of ferromagnetic nonlocal exchange on correlated electrons in terms of a $SU(2)$-Hubbard-Heisenberg model and address the interplay of on-site interaction induced local moment formation and the competition of ferromagnetic direct and antiferromagnetic kinetic exchange interactions. In order to simulate thermodynamic properties of the system in a way that largely accounts for the on-site interaction driven correlations in the system, we advance the correlated variational scheme introduced in [M. Schüler et al., Phys. Rev. Lett. 111, 036601 (2013)] to account for explicitily symmetry broken electronic phases by introducing an auxiliary magnetic field. After benchmarking the method against exact solutions of a finite system, we study the $SU(2)$-Hubbard-Heisenberg model on a square lattice. We obtain the $U$-$J$ finite temperature phase diagram of a $SU(2)$-Hubbard-Heisenberg model within the correlated variational approach and compare to static mean field theory. While the generalized variational principle and static mean field theory yield transitions from dominant ferromagnetic to antiferromagnetic correlations in similar regions of the phase diagram, we find that the nature of the associated phase tranistions differs between the two approaches. The fluctuations accounted for in the generalized variational approach render the transitions continuous, while static mean field theory predicts discontinuous transitions between ferro- and antiferromagnetically ordered states.

cond-mat.str-el↗

arXiv2017

Optically and electrically controllable adatom spin-orbital dynamics in transition metal dichalcogenides

We analyze the interplay of spin-valley coupling, orbital physics and magnetic anisotropy taking place at single magnetic atoms adsorbed on semiconducting transition-metal dichalcogenides, MX$_2$ (M = Mo, W; X = S, Se). Orbital selection rules turn out to govern the kinetic exchange coupling between the adatom and charge carriers in the MX$_2$ and lead to highly orbitally dependent spin-flip scattering rates, as we illustrate for the example of transition metal adatoms with $d^9$ configuration. Our ab initio calculations suggest that $d^9$ configurations are realizable by single Co, Rh, or Ir adatoms on MoS$_2$, which additionally exhibit a sizable magnetic anisotropy. We find that the interaction of the adatom with carriers in the MX$_2$ allows to tune its behavior from a quantum regime with full Kondo screening to a regime of "Ising spintronics" where its spin-orbital moment acts as classical bit, which can be erased and written electronically and optically.

cond-mat.mes-hall↗

arXiv2017

Non-invasive control of excitons in two-dimensional materials

We investigate how external screening shapes excitons in two-dimensional (2d) semiconductors embedded in laterally structured dielectric environments. An atomic scale view of these elementary excitations is developed using models which apply to a variety of materials including transition metal dichalcogenides (TMDCs). We find that structured dielectrics imprint a peculiar potential energy landscape on excitons in these systems: While the ground-state exciton is least influenced, higher excitations are attracted towards regions with high dielectric constant of the environment. This landscape is "inverted" in the sense that low energy excitons are less strongly affected than their higher energy counterparts. Corresponding energy variations emerge on length scales of the order of a few unit cells. This opens the prospect of trapping and guiding of higher excitons by means of tailor-made dielectric substrates on ultimately small spatial scales.

cond-mat.mtrl-sci↗

arXiv2014

Determination of the Fermi Level Position in Dilute Magnetic Ga$_{1-x}$Mn$_{x}$N Films

We report on a combined theoretical and experimental determination of the Fermi level position in wurtzite Ga$_{1-x}$Mn$_{x}$N films with $x=4\%$ and $x=10\%$ as grown by molecular beam epitaxy. By means of ellipsometric measurements the real part of the frequency-dependent conductivity is determined. An electronic model in the framework of the effective bond-orbital model is parameterized in order to theoretically reproduce the measured transport properties. Predictions for the long-wavelength behaviour as a function of the Fermi level are made. The corresponding density of states obtained in this model is in qualitative agreement with first-principle calculations. The absence of a significant experimental peak in the AC conductivity for small frequencies indicates that the Fermi level lies in a gap between two Mn-related impurity bands in the host band gap.

cond-mat.mtrl-sci↗

arXiv2013

Quantum-Confined Stark Effect in polar and nonpolar Wurtzite InN/GaN Heterostructures: Influence on Electronic Structure and Compensation by Coulomb Attraction

In this paper we systematically analyze the electronic structures of polar and nonpolar wurtzite-InN/GaN quantum dots and their modification due to the quantum-confined Stark effect caused by intrinsic fields. This is achieved by combining continuum elasticity theory with an empirical tight binding model to describe the elastic and single-particle electronic properties in these nitride systems. Based on these results, a many-body treatment is used to determine optical absorption spectra. The efficiency of optical transitions depends on the interplay between the Coulomb interaction and the quantum-confined Stark effect. We introduce an effective confinement potential which represents the electronic structure under the influence of the intrinsic polarization fields and calculate the needed strength of Coulomb interaction to diminish the separation of electrons and holes.

cond-mat.mtrl-sci↗

arXiv2012

Effective Heisenberg exchange integrals of diluted magnetic semiconductors determined within realistic multi-band tight-binding models

Diluted magnetic semiconductors (DMS) like Ga$_{1-x}$Mn$_{x}$As are described by a realistic tight-binding model (TBM) for the (valence) bands of GaAs, by a Zener (J-)term modeling the coupling of the localized Mn-spins to the spins of the valence band electrons, and by an additional potential scattering (V-) term due to the Mn-impurities. We calculate the effective (Heisenberg) exchange interaction between two Mn-moments mediated by the valence electrons. The influence of the number of bands taken into account (6-band or 8-band TBM) and of the potential (impurity) scattering V-term is investigated. We find that for realistic values of the parameters Ð the indirect exchange integrals show a long-range, oscillating (RKKY-like) behavior, if the V-term is neglected, probably leading to spin-glass behavior rather than magnetic order. But by including a V-term of a realistic magnitude the exchange couplings become short ranged and mainly positive allowing for the possibility of ferromagnetic order. Our results are in good agreement with available results of ab-initio treatments.

cond-mat.str-el↗

arXiv2012

Theory of band gap bowing of disordered substitutional II-VI and III-V semiconductor alloys

For a wide class of technologically relevant compound III-V and II-VI semiconductor materials AC and BC mixed crystals (alloys) of the type A(x)B(1-x)C can be realized. As the electronic properties like the bulk band gap vary continuously with x, any band gap in between that of the pure AC and BC systems can be obtained by choosing the appropriate concentration x, granted that the respective ratio is miscible and thermodynamically stable. In most cases the band gap does not vary linearly with x, but a pronounced bowing behavior as a function of the concentration is observed. In this paper we show that the electronic properties of such A(x)B(1-x)C semiconductors and, in particular, the band gap bowing can well be described and understood starting from empirical tight binding models for the pure AC and BC systems. The electronic properties of the A(x)B(1-x)C system can be described by choosing the tight-binding parameters of the AC or BC system with probabilities x and 1-x, respectively. We demonstrate this by exact diagonalization of finite but large supercells and by means of calculations within the established coherent potential approximation (CPA). We apply this treatment to the II-VI system Cd(x)Zn(1-x)Se, to the III-V system In(x)Ga(1-x)As and to the III-nitride system Ga(x)Al(1-x)N.

cond-mat.mtrl-sci↗

arXiv2011

Origins of shortcomings in recent realistic multiband Monte-Carlo studies for GaMnAs

Magnetic properties of Mn-doped GaAs are re-investigated within a realistic multiband description of the host valence bands. We explicitely demonstrate that the recent Monte Carlo (MC) simulations performed on a large scale supercomputer suffer from severe shortcomings. Indeed, it is shown, using identical parameters that (i) the calculated Zeeman-splitting largely underestimates that obtained from first principle studies, (ii) the couplings exhibit strong RKKY oscillations, (iii) the stability region for ferromagnetism is much narrower than obtained previously and (iv) the calculated Curie temperatures appear to be at least one order of magnitudes smaller. We show that the proposed choice of physical parameters cannot describe the physics in (Ga,Mn)As.

cond-mat.str-el↗

arXiv2010

Multiband tight-binding theory of disordered ABC semiconductor quantum dots: Application to the optical properties of alloyed CdZnSe nanocrystals

Zero-dimensional nanocrystals, as obtained by chemical synthesis, offer a broad range of applications, as their spectrum and thus their excitation gap can be tailored by variation of their size. Additionally, nanocrystals of the type ABC can be realized by alloying of two pure compound semiconductor materials AC and BC, which allows for a continuous tuning of their absorption and emission spectrum with the concentration x. We use the single-particle energies and wave functions calculated from a multiband sp^3 empirical tight-binding model in combination with the configuration interaction scheme to calculate the optical properties of CdZnSe nanocrystals with a spherical shape. In contrast to common mean-field approaches like the virtual crystal approximation (VCA), we treat the disorder on a microscopic level by taking into account a finite number of realizations for each size and concentration. We then compare the results for the optical properties with recent experimental data and calculate the optical bowing coefficient for further sizes.

cond-mat.mes-hall↗

arXiv2010

Band gap bowing of binary alloys: Experimental results compared to theoretical tight-binding supercell calculations for CdZnSe

Compound semiconductor alloys of the type ABC find widespread applications as their electronic bulk band gap varies continuously with x, and therefore a tayloring of the energy gap is possible by variation of the concentration. We model the electronic properties of such semiconductor alloys by a multiband tight-binding model on a finite ensemble of supercells and determine the band gap of the alloy. This treatment allows for an intrinsic reproduction of band bowing effects as a function of the concentration x and is exact in the alloy-induced disorder. In the present paper, we concentrate on bulk CdZnSe as a well-defined model system and give a careful analysis on the proper choice of the basis set and supercell size, as well as on the necessary number of realizations. The results are compared to experimental results obtained from ellipsometric measurements of CdZnSe layers prepared by molecular beam epitaxy (MBE) and photoluminescence (PL) measurements on catalytically grown CdZnSe nanowires reported in the literature.

cond-mat.mtrl-sci↗

arXiv2010

Multiband effective bond-orbital model for nitride semiconductors with wurtzite structure

A multiband empirical tight-binding model for group-III-nitride semiconductors with a wurtzite structure has been developed and applied to both bulk systems and embedded quantum dots. As a minimal basis set we assume one s-orbital and three p-orbitals, localized in the unit cell of the hexagonal Bravais lattice, from which one conduction band and three valence bands are formed. Non-vanishing matrix elements up to second nearest neighbors are taken into account. These matrix elements are determined so that the resulting tight-binding band structure reproduces the known Gamma-point parameters, which are also used in recent kp-treatments. Furthermore, the tight-binding band structure can also be fitted to the band energies at other special symmetry points of the Brillouin zone boundary, known from experiment or from first-principle calculations. In this paper, we describe details of the parametrization and present the resulting tight-binding band structures of bulk GaN, AlN, and InN with a wurtzite structure. As a first application to nanostructures, we present results for the single-particle electronic properties of lens-shaped InN quantum dots embedded in a GaN matrix.

cond-mat.mes-hall↗

arXiv2008

A comparison of atomistic and continuum theoretical approaches to determine electronic properties of GaN/AlN quantum dots

In this work we present a comparison of multiband k.p-models, the effective bond-orbital approach, and an empirical tight-binding model to calculate the electronic structure for the example of a truncated pyramidal GaN/AlN self-assembled quantum dot with a zincblende structure. For the system under consideration, we find a very good agreement between the results of the microscopic models and the 8-band k.p-formalism, in contrast to a 6+2-band k.p-model, where conduction band and valence band are assumed to be decoupled. This indicates a surprisingly strong coupling between conduction and valence band states for the wide band gap materials GaN and AlN. Special attention is paid to the possible influence of the weak spin-orbit coupling on the localized single-particle wave functions of the investigated structure.

cond-mat.mtrl-sci↗

arXiv2008

Influence of disorder on the transport properties of heavy-fermion systems

The influence of substitutional disorder on the transport properties of heavy-fermion systems is investigated. We extend the dynamical mean-field theory treatment of the periodic Anderson model (PAM) to a coherent-potential approximation for disordered strongly correlated electron systems. Considering two distinct local environments of a binary alloy $A_c B_{1-c}$ with arbitrary concentration $c$, we explore two types of disorder: on the f site and on the ligand sites. We calculate the spectral functions and self-energies for the disordered PAM as well as the temperature dependence of the resistivity and the thermoelectric power. The characteristic concentration dependence as well as the order of magnitude of transport properties are reproduced for metallic heavy-fermion systems and Kondo insulators. In particular, sign changes of the Seebeck coefficient as function of temperature and concentration are observed.

cond-mat.str-el↗

arXiv2008

Weak-coupling Treatment of Electronic (Anti-)Ferroelectricity in the Extended Falicov-Kimball Model

We study the (spinless) Falicov-Kimball model extended by a finite band width (hopping $t_f$) of the localized (f-) electrons in infinite dimensions in the weak-coupling limit of a small local interband Coulomb correlation $U$ for half filling. In the case of overlapping conduction- and f-bands different kinds of ordered solutions are possible, namely charge-density wave (CDW) order, electronic ferroelectricity (EFE) and electronic antiferroelectricity (EAFE). The order parameters are calculated as a function of the model parameters and of the temperature. There is a first-order phase transition from the CDW-phase to the EFE- or EAFE-phase. The total energy is calculated to determine the thermodynamically stable solution. The quantum phase diagrams are calculated.

cond-mat.str-el↗

arXiv2006

Transport properties of heavy-fermion systems

We calculate the temperature dependence of the transport properties of heavy-fermion systems such as resistivity, optical conductivity, thermoelectric power, the electronic part of the thermal conductivity, and the "figure of merit." The one-particle properties of the periodic Anderson model are obtained within dynamical mean-field theory for the paramagnetic phase using Wilson's numerical renormalization group and the modified perturbation theory as impurity solvers. We discuss the dependence of the transport properties on the band filling, valence, and Coulomb correlation $U$. The typical experimental findings can be reproduced and understood, in particular the temperature dependence of the resistance and the thermoelectric power and their absolute magnitude for both metallic heavy-fermion systems and Kondo insulators. For large values of $U$, we find a negative Seebeck coefficient $S(T)$ for an intermediate-temperature regime as observed in $S(T)$ of CeCu$_2$Si$_2$. We analyze different estimates for possible characteristic low-temperature scales of the lattice. Our results indicate a one-parameter scaling of thermodynamic and some transport properties with a strongly occupancy-dependent scaling function. This is consistent with a strong-coupling local Fermi-liquid fixed point of the effective site governing all low-lying excitations for $T\to 0$ in the paramagnetic phase.

cond-mat.str-el↗

arXiv2005

Tight-Binding model for semiconductor nanostructures

An empirical $s_cp^3_a$ tight-binding (TB) model is applied to the investigation of electronic states in semiconductor quantum dots. A basis set of three $p$-orbitals at the anions and one $s$-orbital at the cations is chosen. Matrix elements up to the second nearest neighbors and the spin-orbit coupling are included in our TB-model. The parametrization is chosen so that the effective masses, the spin-orbit-splitting and the gap energy of the bulk CdSe and ZnSe are reproduced. Within this reduced $s_cp_a^3$ TB-basis the valence (p-) bands are excellently reproduced and the conduction (s-) band is well reproduced close to the $Γ$-point, i.e. near to the band gap. In terms of this model much larger systems can be described than within a (more realistic) $sp^3s^*$-basis. The quantum dot is modelled by using the (bulk) TB-parameters for the particular material at those sites occupied by atoms of this material. Within this TB-model we study pyramidal-shaped CdSe quantum dots embedded in a ZnSe matrix and free spherical CdSe quantum dots (nanocrystals). Strain-effects are included by using an appropriate model strain field. Within the TB-model, the strain-effects can be artifically switched off to investigate the infuence of strain on the bound electronic states and, in particular, their spatial orientation. The theoretical results for spherical nanocrystals are compared with data from tunneling spectroscopy and optical experiments. Furthermore the influence of the spin-orbit coupling is investigated.

cond-mat.other↗

arXiv1997

Accurate mapping of quantum Heisenberg magnetic models of spin $s$ on strong-coupling magnon systems

An infinite-$U$ term is introduced into the Holstein-Primakoff-transformed magnon hamiltonian of quantum Heisenberg magnetic models of spin $s$. This term removes the unphysical spin wave states on every site and truncates automatically the expansion in powers of the magnon occupation operator. The resultant strong-coupling magnon hamiltonians are accurately equivalent to the original spin hamiltonians. The on-site $U$ levels and their implications are studied. Within a simple decoupling approximation for our strong-coupling magnon models we can easily reproduce the results for the (sublattice) magnetizations obtained previously for the original spin model. But our bosonic hamiltonians without any unphysical states allow for substantially improved values for the spectral weight in the ground state and for lower ground-state energies than those obtained within previous approximations.

cond-mat↗

arXiv1997

Infinite coupling magnon theory of quantum Heisenberg magnetic models of spin s

An infinity magnon coupling term is introduced into the Holstein-Primakoff transformed forms of the Heisenberg ferromagnetic and antiferromagnetic models of any spin $s$ to rigorously remove the unphysical magnon states. This term makes the series expansion of the square root of the magnon operators become a finite series of magnon operator products. Under a simple Hubbard-like approximation our infinite coupling theory yields much better result than the existing spin wave theories, especially near transition temperatures.

cond-mat↗