arXiv subjects

Harald O. Jeschke

Publications and source records attributed to Harald O. Jeschke.

139 records · Page 8Linked to original sources

arXiv2009

Dynamical cluster approximation study of the anisotropic two-orbital Hubbard model

We investigate the properties of a two-orbital Hubbard model with unequal bandwidths on the square lattice in the framework of the dynamical cluster approximation (DCA) combined with a continuous-time quantum Monte Carlo (CT QMC) algorithm. We explore the effect of short-range spatial fluctuations on the nature of the metal-insulator transition and the possible occurrence of an orbital-selective Mott transition (OSMT), as a function of cluster size $N_c$. We observe that for $N_c=2$ no OSMT is present, instead a band insulator state for both orbitals is stabilized at low temperatures due to the appearance of an artificial local ordered state. For $N_c=4$ the DCA calculations suggest the presence of five different phases which originate out of the cooperation and competition between spatial fluctuations and orbitals of different bandwidths and a OSMT phase is stabilized. Based on our results, we discuss the nature of the gap opening.

cond-mat.str-el↗

arXiv2009

Microscopic origin of pressure-induced phase transitions in iron-pnictide $AFe_2As_2$ superconductors: an {ab initio} molecular-dynamics study

Using {\it ab initio} molecular dynamics we investigate the electronic and lattice structure of $A$Fe$_2$As$_2$ ($A$=Ca, Sr, Ba) under pressure. We find that the structural phase transition (orthorhombic to tetragonal symmetry) is always accompanied by a magnetic phase transition in all the compounds, while the nature of the transitions is different for the three systems. Our calculations explain the origin of the existence of a collapsed tetragonal phase in CaFe$_2$As$_2$ and its absence in BaFe$_2$As$_2$. We argue that changes of the Fermi surface nesting features dominate the phase transition under pressure rather than spin frustration or a Kondo scenario. The consequences for superconductivity are discussed.

cond-mat.supr-con↗

arXiv2009

Pressure-driven phase transitions in correlated systems

Motivated by recent experimental measurements on pressure-driven phase transitions in Mott-insulators as well as the new iron pnictide superconductors, we show that first principles Car-Parrinello molecular dynamics calculations are a powerful method to describe the microscopic origin of such transitions. We present results for (i) the pressure-induced insulator to metal phase transition in the prototypical Mott insulator TiOCl as well as (ii) the pressure-induced structural and magnetic phase transitions in the family of correlated metals $A$Fe$_2$As$_2$ ($A$=Ca,Sr,Ba). Comparison of our predictions with existing experimental results yields very good agreement.

cond-mat.str-el↗

arXiv2009

First principles determination of the model parameters in $κ$-(ET)$_2$Cu$_2$(CN)$_3$

We present a detailed study of the derivation of the Hubbard model parameters for $κ$-(ET)$_2$Cu$_2$(CN)$_3$ in the framework of {\it ab initio} Density Functional Theory. We show that calculations with different (i) wavefunction basis, (ii) exchange correlation functionals and (iii) tight-binding models provide a reliable benchmark for the parameter values. We compare our results with available extended Hückel molecular orbital calculations and discuss its implications for the description of the properties of $κ$-(ET)$_2$Cu$_2$(CN)$_3$. The electronic properties of $κ$-(ET)$_2$Cu(SCN)$_2$ are also briefly discussed.

cond-mat.str-el↗

arXiv2009

Revision of model parameters for kappa-type charge transfer salts: an ab initio study

Intense experimental and theoretical studies have demonstrated that the anisotropic triangular lattice as realized in the kappa-(BEDT-TTF)2X family of organic charge transfer (CT) salts yields a complex phase diagram with magnetic, superconducting, Mott insulating and even spin liquid phases. With extensive density functional theory (DFT) calculations we refresh the link between manybody theory and experiment by determining hopping parameters of the underlying Hubbard model. This leads us to revise the widely used semiempirical parameters in the direction of less frustrated, more anisotropic triangular lattices. The implications of these results on the systems' description are discussed.

cond-mat.str-el↗

arXiv2009

A fast impurity solver based on equations of motion and decoupling

In this paper a fast impurity solver is proposed for dynamical mean field theory (DMFT) based on a decoupling of the equations of motion for the impurity Greens function. The resulting integral equations are solved efficiently with a method based on genetic algorithms. The Hubbard and periodic Anderson models are studied with this impurity solver. The method describes the Mott metal insulator transition and works for a large range of parameters at finite temperature on the real frequency axis. This makes it useful for the exploration of real materials in the framework of LDA+DMFT.

cond-mat.str-el↗

arXiv2008

Microscopic model for transitions from Mott to spin-Peierls insulator in TiOCl

On the basis of ab initio density functional theory (DFT) calculations, we derive the underlying microscopic model Hamiltonian for TiOCl, a unique system that shows two consecutive phase transitions from a Mott insulator to a spin-Peierls insulator through a structurally incommensurate phase. We show with our model that the presence of magnetic frustration in TiOCl leads to a competition with the spin-Peierls distortion, which results in the novel incommensurate phase. In addition, our calculations indicate that the spin-Peierls state is triggered by adiabatic phonons, which is essential for understanding the nature of the phase transition.

cond-mat.str-el↗

arXiv2008

Two pressure-induced transitions in TiOCl: Mott insulator to anisotropic metal

Using Car-Parrinello molecular dynamics calculations we investigate the behavior of the low-dimensional multiorbital Mott insulator TiOCl under pressure. We show that the system undergoes {\it two} consecutive phase transitions, first at $P_\text{c}$ from a Mott-insulator to a metallic phase in the $ab$ plane with a strong Ti-Ti dimerization along $b$. At a pressure $P'_\text{c} > P_\text{c}$ the dimerization disappears and the system behaves as a uniform metal. This second transition has not yet been reported experimentally. We show that the insulator-to-metal transition at $P_\text{c}$ is driven by the widening of the bandwidth rather than structural changes or reduction of crystal field splittings and it shows a redistribution of the electronic occupation within the $t_{2g}$ bands. Our computed pressure-dependent lattice parameters are consistent with experimental observations and the existing controversy on the change of crystal symmetry at high pressures is discussed.

cond-mat.str-el↗

arXiv2008

Microscopic modeling of a spin crossover transition

In spin crossover materials, an abrupt phase transition between a low spin state and a high spin state can be driven by temperature, pressure or by light irradiation. Of a special relevance are Fe(II) based coordination polymers where, in contrast to molecular systems, the phase transition between a spin S=0 and a spin S=2 state shows a pronounced hysteresis which is desirable for technical applications. A satisfactory microscopic explanation of this large cooperative phenomenon has been sought for a long time. The lack of X-ray data has been one of the reasons for the absence of microscopic studies. In this work, we present an efficient route to prepare reliable model structures and within an ab initio density functional theory analysis and effective model considerations we show that in polymeric spin crossover compounds magnetic exchange between high spin Fe(II) centers is as important as elastic couplings for understanding the phase transition. We discuss the relevance of these interactions for the cooperative behavior in these materials.

cond-mat.mtrl-sci↗

arXiv2007

Classical and ab initio preparation of reliable structures for polymeric coordination compounds

The detailed investigation of electronic and magnetic properties of polymeric coordination materials with accurate ab initio quantum mechanical methods is often computationally extremely demanding because of the large number of atoms in the unit cell. Moreover, usually the available structural data are insufficient or poorly determined, especially when the structure contains hydrogen atoms. In order to be able to perform controlled ab initio calculations on reliable structures, we use a two-step approach to systematically prepare model structures for polymeric coordination compound systems and to relax them to their equilibrium configuration. First, a structure is constructed on the basis of a crystallographic database and optimized by force field methods; in the second step, the structure is relaxed by ab initio quantum mechanical molecular dynamics. With this structure, we perform accurate electronic structure calculations. We will apply this procedure to a Fe(II) triazole compound and to a coordination polymer of Cu(II) ions with 2,5-bis(pyrazol-1-yl)-1,4-dihydroxybenzene.

cond-mat.mtrl-sci↗

arXiv2007

Cu-based metalorganic systems: an ab initio study of the electronic structure

Within a first principles framework, we study the electronic structure of the recently synthesized polymeric coordination compound Cu(II)-2,5-bis(pyrazol-1-yl)-1,4-dihydroxybenzene (CuCCP), which has been suggested to be a good realization of a Heisenberg spin-1/2 chain with antiferromagnetic coupling. By using a combination of classical with ab initio quantum mechanical methods, we design on the computer reliable modified structures of CuCCP aimed at studying effects of Cu-Cu coupling strength variations on this spin-1/2 system. For this purpose, we performed two types of modifications on CuCCP. In one case, we replaced H in the linker by i) an electron donating group (NH2) and ii) an electron withdrawing group (CN), while the other modification consisted in adding H2O and NH3 molecules in the structure which change the local coordination of the Cu(II) ions. With the NMTO-downfolding method we provide a quantitative analysis of the modified electronic structure and the nature of the Cu-Cu interaction paths in these new structures and discuss its implications for the underlying microscopic model.

cond-mat.mtrl-sci↗

arXiv2006

Microscopic model for the novel frustrated Cu(II)-spin tetrahedron-based Cu4Te5O12X4 (X=Cl, Br) systems

We present a microscopic study of the electronic and magnetic properties of the newly synthesized Cu(II)-spin tetrahedron-based Cu4Te5O12Cl4 compound based on Density Functional calculations and on ab initio-derived effective models. In view of these results, we discuss the origin of the observed differences in behavior between this system and the structurally similar and much studied Cu2Te2O5Cl2. Since the Br analog of the title compound has not been synthesized yet, we derive the crystal structure of Cu4Te5O12Br4 by geometry optimization in an ab initio molecular dynamics calculation and investigate the effect of substituting Cl by Br as well as the possible magnetic behavior of this system and compare with the recently studied sister compound, Cu2Te2O5Br2.

cond-mat.str-el↗

arXiv2004

Decoupling method for dynamical mean field theory calculations

In this paper we explore the use of an equation of motion decoupling method as an impurity solver to be used in conjunction with the dynamical mean field self-consistency condition for the solution of lattice models. We benchmark the impurity solver against exact diagonalization, and apply the method to study the infinite $U$ Hubbard model, the periodic Anderson model and the $pd$ model. This simple and numerically efficient approach yields the spectra expected for strongly correlated materials, with a quasiparticle peak and a Hubbard band. It works in a large range of parameters, and therefore can be used for the exploration of real materials using LDA+DMFT.

cond-mat.str-el↗