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R. Eder

Publications and source records attributed to R. Eder.

At least 19 recordsLinked to original sources
arXiv2019

Bond particle theory for the pseudogap phase of underdoped cuprates

We present a theory for the lightly doped t-J model which is of possible relevance for the normal state of underdoped cuprates. Starting from an arbitrary dimer covering of the plane an exact representation of the t-J Hamiltonian in terms of bond bosons and fermions can be derived. Since all dimer coverings must give identical results for observable quantities we construct an approximate but translationally invariant Hamiltonian by averaging the bond particle Hamiltonian over all possible dimer coverings. Treating the resulting Hamiltonian in mean-field approximation we find fermi pockets centered near (pi/2,pi/2) with a total area of x/2 (with x the hole concentration) and a gapped spin-wave-like band of triplet excitations.

cond-mat.str-el↗
arXiv2019

Antiferromagnetic phase of the Kondo-insulator

We discuss the quasiparticle band structure of the antiferromagnetic phase of the planar Kondo lattice model with a half-filled conduction band, the so-called Kondo insulator. The band structure is obtained by bond fermion technique and good agreement is obtained with the single particle spectral function obtained by Dynamical Cluster Approximation. In particular, various changes of the band structure with J/t observed in the numerical spectra are reproduced qualitatively by the calculation. In the absence of Fermi surface nesting we find a semimetallic phase for sufficiently small exchange constant and possible experimental consequences are discussed

cond-mat.str-el↗
arXiv2019

Quasiparticle Band Structure and Spin Excitation Spectrum of the Kondo Lattice

A formulation of the Kondo lattice Hamiltonian in terms of bond particles is derived and solved in two different approximations. The bond particles correspond to the eigenstates of a single unit cell and are bosons for states with even electron number and fermions for states with an odd electron number. A s a check various physical quantities are calculated for the 1D Kondo insulator a nd good agreement with numerical results is obtained for J/t>1.

cond-mat.str-el↗
arXiv2018

Anisotropic electronic transport and Rashba effect of the two-dimensional electron system in (110) SrTiO$_3$-based heterostructures

The two-dimensional electron system in (110)Al$_2$O$_{3-δ}$/SrTiO$_3$ heterostructures displays anisotropic electronic transport. Largest and lowest conductivity and electron mobility $μ$ are observed along the $[001]$ and $[1\bar{1}0]$ direction, respectively. The anisotropy of the sheet resistance and $μ$ likewise leads to a distinct anisotropic normal magnetotransport (MR) for T < 30K. However, at temperatures T<5K and magnetic field B<2T MR is dominated by weak antilocalization.Despite the rather strong anisotropy of the Fermi surfaces, the in-plane anisotropic magnetoresistance (AMR) displays two-fold non-crystalline anisotropy. However, the AMR-amplitude is found to be anisotropic with respect to the current direction, leading to a 60% larger AMR amplitude for current I along the $[001]$ direction compared to I parallel to $[1\bar{1}0]$. Tight binding calulations evidence an anisotropic Rashba-induced band splitting with dominant linear k-dependence. In combination with semiclassical Boltzmann theory the non-crystalline AMR is well described, despite the anisotropic Fermi surface.

cond-mat.str-el↗
arXiv2016

Strong Correlations, Strong Coupling and s-wave Superconductivity in Hole-doped BaFe2As2 Single Crystals

We present a comprehensive study of the low-temperature heat capacity and thermal expansion of single crystals of the hole-doped Ba1-xKxFe2As2 series (0<x<1) and the end-members RbFe2As2 and CsFe2As2. A large increase of the Sommerfeld coefficient is observed with both decreasing band filling and isovalent substitution (K, Rb, Cs) revealing a strong enhancement of electron correlations and the possible proximity of these materials to a Mott insulator. This trend is well reproduced theoretically by our Density-Functional Theory + Slave-Spin (DFT+SS) calculations, confirming that 122-iron pnictides are effectively Hund metals, in which sizable Hund's coupling and orbital selectivity are the key ingredients for tuning correlations. We also find direct evidence for the existence of a coherence-incoherence crossover between a low-temperature heavy Fermi liquid and a highly incoherent high-temperature regime similar to heavy fermion systems. In the superconducting state, clear signatures of multiband superconductivity are observed with no evidence for nodes in the energy gaps, ruling out the existence of a doping-induced change of symmetry (from s to d-wave). We argue that the disappearance of the electron band in the range 0.4<x<1.0 is accompanied by a strong-to-weak coupling crossover and that this shallow band remains involved in the superconducting pairing, although its contribution to the normal state fades away. Differences between hole- and electron-doped BaFe2As2 series are emphasized and discussed in terms of strong pair breaking by potential scatterers beyond the Born limit.

cond-mat.supr-con↗
arXiv2016

Antiferromagnetic Phases of the Kondo Lattice

We discuss the paramagnetic and Neel-ordered phases of the Kondo lattice Hamiltonian on the 2D square lattice by means of bond Fermions. In the doped case we find two antiferromagnetic solutions, the first one with small ordered moment, heavy bands, and an antiferromagetically folded large Fermi surface - i.e. including the localized spins - the second one with large ordered moment, light bands and an antiferromagnetically folded conduction electron-only Fermi surface. The zero temperature phase diagram as a function of Kondo coupling and conduction electron density shows first and second order transition lines between the three different phases and agrees qualitatively with previous numerical studies. We compare to experiments on CeRh(1-x)Co(x)In(5) and find qualitative agreement.

cond-mat.str-el↗
arXiv2016

Of Substitution and Doping: Spatial and Electronic Structure in Fe Pnictides

A highly intriguing aspect in iron-pnictide superconductors is the composition-dependent electronic structure, in particular the question if and how charge carriers are introduced to the system upon substitution of Ba by alkali metals or of Fe by other transition metals, TM. We report on a systematic study of spatial structure and electronic states by x-ray diffraction and x-ray absorption on a large number of compositions in the (Ba,K)(Fe,TM)2As2 family. The coherent combination of detailed structural information with an in-depth analysis of the electronic structure allows us to sensitively disentangle (charge-carrier) "doping" effects from "substitutional" effects. Results include a doping character that is site-decoupled, as well as TM 3d energy-level schemes that exhibit non-standard level sequences and even t2-e level crossings. Our study indicates that doping per se seems to play a lesser role than expected for pnictide superconductivity and magnetism.

cond-mat.supr-con↗
arXiv2015

Electronic Structure of NiO: Antiferromagnetic Transition and Photoelectron Spectra in the Ordered Phase

The thermodynamics of the antiferromagnetic ordering transition in NiO and the photoelectron spectra in the antiferromagnetic phase are studied by the Variational Cluster Approximation. Using realistic Racah parameters to describe the Coulomb interaction in the Ni 3d shell and a Slater-Koster parameter pds which is slightly (10%) increased over the band structure estimate the calculated Neel temperature is 481 Kelvin (experimental value: 523 Kelvin). The magnetic susceptibility above TN has Curie-Weiss form. A significant contribution to the stabilization of the antiferromagnetic phase comes from electron hopping between oxygen which would be missed in theories that consider superexchange along a single bond only. The single particle spectral function in the ordered phase is in good agreement with experiment, in particular a number of dispersionless bands which are not reproduced by most calculations are obtained correctly. These flat bands are shown to be direct experimental evidence for a dispersionless electronic self-energy with several poles in the energy range of the valence band which originate from the multiplets of the Ni3+ ion. Small but possibly experimentally detectable changes of the photoelectron spectra with temperature are discussed, in p articular a widening of the insulating gap in the paramagnetic phase by approximately 10% is predicted.

cond-mat.str-el↗
arXiv2013

Magnetic properties and Mott transition in the square-lattice Hubbard model with frustration

The magnetic properties and Mott transition of the Hubbard model on the square lattice with frustration are studied at half-filling and zero temperature by the variational cluster approximation. When the on-site repulsion $U$ is large, magnetically disordered state is realized in highly frustrated region between the Néel and collinear phases, and no imcommensurate magnetic states are found there. As for the Mott transition, in addition to the Mott gap and double occupancy, which clarify the nature of the transition, the structure of the self-energy in the spectral representation is studied in detail below and above the Mott transition point. The spectral structure of the self-energy is almost featureless in the metallic phase, but clear single dispersion, leading to the Mott gap, appears in the Mott insulator phase.

cond-mat.str-el↗
arXiv2013

Mott transition and ferrimagnetism in the Hubbard model on the anisotropic kagomé lattice

Mott transition and ferrimagnetism are studied in the Hubbard model on the anisotropic kagomé lattice using the variational cluster approximation and the phase diagram at zero temperature and half-filling is analyzed. The ferrimagnetic phase rapidly grows as the geometric frustration is relaxed, and the Mott insulator phase disappears in moderately frustrated region, showing that the ferrimagnetic fluctuations stemming from the relaxation of the geometric frustration is enhanced by the electron correlations. In metallic phase, heavy fermion behavior is observed and mass enhancement factor is computed. Enhancement of effective spatial anisotropy by the electron correlations is also confirmed in moderately frustrated region, and its effect on heavy fermion behavior is examined.

cond-mat.str-el↗
arXiv2011

A BCS-BEC crossover in the extended Falicov-Kimball model: Variational cluster approach

We study the spontaneous symmetry breaking of the excitonic insulator state induced by the Coulomb interaction $U$ in the two-dimensional extended Falicov-Kimball model. Using the variational cluster approximation (VCA) and Hartree-Fock approximation (HFA), we evaluate the order parameter, single-particle excitation gap, momentum distribution functions, coherence length of excitons, and single-particle and anomalous excitation spectra, as a function of $U$ at zero temperature. We find that in the weak-to-intermediate coupling regime, the Fermi surface plays an essential role and calculated results can be understood in close correspondence with the BCS theory, whereas in the strong-coupling regime, the Fermi surface plays no role and results are consistent with the picture of BEC. Moreover, we find that HFA works well both in the weak- and strong-coupling regime, and that the difference between the results of VCA and HFA mostly appears in the intermediate-coupling regime. The reason for this is discussed from a viewpoint of the self-energy. We thereby clarify the excitonic insulator state that typifies either a BCS condensate of electron-hole pairs (weak-coupling regime) or a Bose-Einstein condensate of preformed excitons (strong-coupling regime).

cond-mat.str-el↗
arXiv2011

Doping driven Small-to-Large Fermi surface transition and d-wave superconductivity in a two-dimenional Kondo lattice

We study the two-dimensional Kondo lattice model with an additional Heisenberg exchange between localized spins. In a first step we use mean-field theory with two order parameters. The first order parameter is a complex pairing amplitude between conduction electrons and localized spins which describes condensation of Kondo (or Zhang-Rice) singlets. A nonvanishing value implies that the localized spins contribute to the Fermi surface volume. The second order parameter describes singlet-pairing between the localized spins and competes with the Kondo-pairing order parameter. Reduction of the carrier density in the conduction band reduces the energy gain due to the formation of the large Fermi surface and induces a phase transition to a state with strong singlet correlations between the localized spins and a Fermi surface which comprises only the conduction electrons. The model thus shows a doping-driven change of its Fermi surface volume. At intermediate doping and low temperature there is a phase where both order parameters coexist, which has a gapped large Fermi surface and d-wave superconductivity. The theory thus qualitatively reproduces the phase diagram of cuprate superconductors. In the second part of the paper we show how the two phases with different Fermi surface volume emerge in a strong coupling theory applicable in limit of large Kondo exchange. The large-Fermi-surface phase corresponds to a `vacuum' of localized Kondo singlets with uniform phase and the quasiparticles are spin-1/2 charge fluctuations around this fully paired state. In the small-Fermi-surface phase the quasiparticles correspond to propagating Kondo-singlets or triplets whereby the phase of a given Kondo-singlet corresponds to its momentum. In this picture a phase transition occurs for low filling of the conduction band as well.

cond-mat.str-el↗
arXiv2011

Self-energy and Fermi surface of the 2-dimensional Hubbard model

We present an exact diagonalization study of the self-energy of the two-dimensional Hubbard model. To increase the range of available cluster sizes we use a corrected t-J model to compute approximate Greens functions for the Hubbard model. This allows to obtain spectra for clusters with 18 and 20 sites. The self-energy has several `bands' of poles with strong dispersion and extended incoherent continua with k-dependent intensity. We fit the self-energy by a minimal model and use this to extrapolate the cluster results to the infinite lattice. The resulting Fermi surface shows a transition from hole pockets in the underdoped regime to a large Fermi surface in the overdoped regime. We demonstrate that hole pockets can be completely consistent with the Luttinger theorem. Introduction of next-nearest neighbor hopping changes the self-energy stronlgy and the spectral function with nonvanishing next-nearest-neighbor hopping in the underdoped region is in good agreement with angle resolved photoelectron spectroscopy.

cond-mat.str-el↗
arXiv2010

Theory of the waterfall phenomenon in cuprate superconductors

Based on exact diagonalization and variational cluster approximation calculations we study the relationship between charge transfer models and the corresponding single band Hubbard models. We present an explanation for the waterfall phenomenon observed in angle resolved photoemission spectroscopy (ARPES) on cuprate superconductors. The phenomenon is due to the destructive interference between the phases of the O2p orbitals belonging to a given Zhang-Rice singlet and the Bloch phases of the photohole which occurs in certain regions of k-space. It therefore may be viewed as a direct experimental visualisation of the Zhang-Rice construction of an effective single band model for the CuO2 plane.

cond-mat.str-el↗
arXiv2010

Doping evolution of superconducting gaps and electronic densities of states in Ba(Fe1-xCox)2As2 iron pnictides

An extensive calorimetric study of the normal- and superconducting-state properties of Ba(Fe1-xCox)2As2 is presented for 0 < x < 0.2. The normal-state Sommerfeld coefficient increases (decreases) with Co doping for x < 0.06 (x > 0.06), which illustrates the strong competition between magnetism and superconductivity to monopolize the Fermi surface in the underdoped region and the filling of the hole bands for overdoped Ba(Fe1-xCox)2As2. All superconducting samples exhibit a residual electronic density of states of unknown origin in the zero-temperature limit, which is minimal at optimal doping but increases to the normal-state value in the strongly under- and over-doped regions. The remaining specific heat in the superconducting state is well described using a two-band model with isotropic s-wave superconducting gaps.

cond-mat.supr-con↗
arXiv2010

Theory of the Lightly Doped Mott Insulator

A theory for the Hubbard model appropriate in the limit of large U/t, small doping away from half-filling and short-ranged antiferromagnetic spin correlations is presented. Despite the absence of any broken symmetry the Fermi surface takes the form of elliptical hole pockets centered near (pi/2,pi/2) with a volume proportional to the hole concentration. Short range antiferromagnetic correlations render the nearest neighbor hopping almost ineffective so that only second or third nearest neighbor hopping contributes appreciably to the dispersion relation.

cond-mat.str-el↗
arXiv2009

Spin state transition in LaCoO3 by variational cluster approximation

The variational cluster approximation is applied to the calculation of thermodynamical quantities and single-particle spectra of LaCoO3. Trial self-energies and the numerical value of the Luttinger-Ward functional are obtained by exact diagonalization of a CoO6 cluster. The VCA correctly predicts LaCoO3 as a paramagnetic insulator and a gradual and relatively smooth increase of the occupation of high-spin Co3+ ions causes the temperature dependence of entropy and magnetic susceptibility. The single particle spectral function agrees well with experiment, the experimentally observed temperature dependence of photoelectron spectra is reproduced satisfactorily. Remaining discrepancies with experiment highlight the importance of spin orbit coupling and local lattice relaxation.

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