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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., Comment: 15 pages, 12 figures

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, Comment: 7 pages, 7 figures

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., Comment: 13 pages, 9 figures

The two-dimensional electron system in (110)Al$_2$O$_{3-\delta}$/SrTiO$_3$ heterostructures displays anisotropic electronic transport. Largest and lowest conductivity and electron mobility $\mu$ are observed along the $[001]$ and $[1\bar{1}0]$ direction, respectively. The anisotropy of the sheet resistance and $\mu$ 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., Comment: 11 pages, 11 Figures

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

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., Comment: 12 pages, 11 Postscript figures

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., Comment: 18 pages, 16 Postscript figures

The known analytical properties of the Green's function and self-energy rule out an ambiguity of the self-energy. The noninteracting Green's functions obtained by Kozik et al. in arXiv:1407.5687 likely have pathological properties., Comment: 1 page, no figures

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.

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 N\'eel 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., Comment: 12 pages, 13 figures

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)., Comment: 11 pages, 9 figures

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., Comment: Revtex file, 17 pages, 14 eps-figures

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., Comment: 17 pages, 18 figures

Mott transition and ferrimagnetism are studied in the Hubbard model on the anisotropic kagom\'e 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., Comment: 5 pages, 6 figures

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., Comment: 11 pages, 9 Postscript figures

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., Comment: Submitted to Europhysics Letters

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., Comment: 9 pages, 3 figures

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., Comment: Revtex file with 10 eps figures

We report on the determination of the electronic heat capacity of a slightly overdoped (x = 0.075) Ba(Fe1-xCox)2As2 single crystal with a Tc of 21.4 K. Our analysis of the temperature dependence of the superconducting-state specific heat provides strong evidence for a two-band s-wave order parameter with gap amplitudes 2D1(0)/kBTc=1.9 and 2D2(0)/kBTc=4.4. Our result is consistent with the recently predicted s+- order parameter [I. I. Mazin et al., Phys. Rev. Lett. 101, 057003 (2008)]., Comment: 4 pages, 3 figures

Epitaxially strained LaCoO3 (LCO) thin films were grown with different film thickness, t, on (001) oriented (LaAlO3)0.3(SrAl0.5Ta0.5O3)0.7 (LSAT) substrates. After initial pseudomorphic growth the films start to relieve their strain partly by the formation of periodic nano-twins with twin planes predominantly along the <100> direction. Nano-twinning occurs already at the initial stage of growth, albeit in a more moderate way. Pseudomorphic grains, on the other hand, still grow up to a thickness of at least several tenths of nanometers. The twinning is attributed to the symmetry lowering of the epitaxially strained pseudo-tetragonal structure towards the relaxed rhombohedral structure of bulk LCO. However, the unit-cell volume of the pseudo-tetragonal structure is found to be nearly constant over a very large range of t. Only films with t > 130 nm show a significant relaxation of the lattice parameters towards values comparable to those of bulk LCO., Comment: 31 pages, 10 figures

The variational cluster approximation proposed by Potthoff is applied to the calculation of the single-particle spectral function of the transition metal oxides MnO, CoO and NiO. Trial self-energies and the numerical value of the Luttinger-Ward functional are obtained by exact diagonalization of a TMO6-cluster. The single-particle parameters of this cluster serve as variational parameters to construct a stationary point of the grand potential of the lattice system. The stationary point is found by a crossover procedure which allows to go continuously from an array of disconnected clusters to the lattice system. The self-energy is found to contain irrelevant degrees of freedom which have marginal impact on the grand potential and which need to be excluded to obtain meaningful results. The obtained spectral functions are in good agreement with experimental data., Comment: 14 pages, 17 figures

We present a variational approach based on the string picture to analyze the internal structure and dispersion of spin polarons with different symmetries in an antiferromagnet. We then use this to discuss the properties of underdoped cuprate superconductor within the `doped insulator' picture. The theory explains the remnant Fermi surface for the undoped compunds, as well as hole pockets, Fermi arcs, high energy pseudogap and the the mid-infrared band in doped materials. Destructive interference between the phases of a photohole near $\Gamma$ and the internal phases of the Zhang Rice singlet combined with our theory moreover explains the `waterfall' phenomenon., Comment: 19 pages, 16 figures

The variational cluster approximation is applied to the calculation of the single particle spectral function of NiO. Trial self energies and the numerical value of the Luttinger-Ward functional are obtained by exact diagonalization of NiO6-clusters and the single particle parameters of the clusters serve as variational parameters to obtain a stationary point of the grand potential of the lattice system. Good agreement with experiment is obtained., Comment: Revtex, 4 pages, 2 eps-figures

We propose a many-body method for band-structure calculations in strongly correlated electron systems and apply it to NiO. The method may be viewed as a translationally invariant version of the cluster method of Fujimori and Minami. Thereby the Coulomb interaction within the d-shells is treated by exact diagonalization and the d-shells then are coupled to a solid by an extension of the cluster perturbation theory due to Senechal et al. The method is computationally no more demanding than a conventional band structure calculation and for NiO we find good agreement between the calculated single particle spectral function and the experimentally measured band structure., Comment: Revtex-file, 15 pages with 12 eps figures

The electronic structure and the single-particle spectral density of a stripe array formed by ladder-like domain walls (DWs) and by antiferromagnetic (AF) domains of width 2 lattice spacings are computed and compared with ARPES spectra from some doped cuprates belonging to the 214 family of compounds. We assume that bond order is formed on legs in DWs and that the phase of the sublattice magnetization changes by Pi across each DW. The intensity map plotted in the coordination frame momentum-energy reproduces quite well the ARPES spectra obtained at the doping level of 15%. We consider this agreement as an argument for a scenario of coexisting bond-ordered regions and AF regions in the stripe phase of moderately doped cuprates., Comment: 16 pages, 8 figures

Motivated by recent inelastic neutron scattering experiments on cuprates, we discuss the formation of bond order in the stripe phase. We suggest that the spin Peierls order emerges in hole-rich domain walls (DWs) formed between hole-poor regions in which long-range antiferromagnetic (AF) correlations exist. On the example of a single stripe we analyze the stability of such structures. The motion of a hole inside the DW which takes the form of a bond ordered ladder is in principle unrestricted. The hole hopping in domains is to some extent obscured by the fact that a moving hole spoils AF correlations. The propagation of a hole along the stripe which takes the form of the ladder-like domain wall that separates antiphase AF domains is a combination of these two types of motion occurring in two different environments. By analyzing the energy dispersion of a quasiparticle propagating along the bond-centered, bond-ordered stripe and of a quasiparticle propagating along the site-centered stripe we deduce that bond ordered stripes are stable at the total doping level 1/8 and the linear stripe-filling level 1/2. This conclusion seems to be relevant to the nature of the stripe phase in La_1.875 Ba_0.125 Cu O_4., Comment: 18 pages, 9 figures

We investigate the effects of the next-nearest-neighbor ($t'$) and the third-nearest-neighbor (t") hopping terms on superconductivity (SC) correlation in the 2D hole-doped extended t-J model based on the variational Monte-Carlo (VMC), mean-field (MF) calculation, and exact diagonalization (ED) method. Despite of the diversity of the methods employed, the results all point to a consistent conclusion: While the d-wave SC correlation is slightly suppressed by t' and t" in underdoped regions, it is greatly enhanced in the optimal and overdoped regions. The optimal T_c is a result upon balance of these two opposite trends., Comment: 5 figures, submitted to Phys. Rev. Lett

The 2D t-J model with and without t' and t'' hopping-terms is studied by exact diagonalization on a 5 x 4 cluster, which realizes a hole stripe in y-direction in a dimerized singlet background. Next nearest hopping terms with a sign appropriate for hole-doped cuprates enhance the stripe formation. The dispersion of the quasiparticle-peaks in the single-particle spectrum is in good agreement with a simple bond operator theory for hole motion in the spin-Peierls phase, particularly so for realistic values of t' and t''. The resulting spectral weight distribution and Fermi surface agree well with experimental ARPES spectra on La_1.28 Nd_0.6 Sr_0.12 Cu O_4., Comment: 4 pages with 3 eps-figures

The hole-doped 2D t-J model is studied by exact diagonalization on a 5 x 4 cluster which, unlike the standard tilted square clusters, can in principle accomodate an antiphase domain wall. For hole concentration 10% and J/t >= 0.5 the ground state energy/site is lower than the conventional tilted square 20-site-cluster. In the ground state two holes form a loosely bound pair pinned to an antiphase domain wall. The dynamical density correlation function shows sharp quasiparticle-like peaks, reminiscent of the `holons' in 1D chains, which suggest the existence of soliton-like, propagating domain walls. The dynamical correlation function of the bond-singlet operator has a low-energy peak structure characteristic of columnar Spin-Peierls order, the dynamical spin correlation function shows an intense and isolated `resonance peak' near (pi,pi)., Comment: 4 pages with 5 eps-figures

We derive an effective Hamiltonian for spin polarons forming in weakly doped antiferromagnets and demonstrate that the system becomes superconducting at finite doping. We argue that the driving mechanism which gives rise to superconductivity is lowering of the kinetic energy by formation of mobile antiferromagnetic spin bipolarons. That source of attraction between holes is by definition effective if the antiferromagnetic correlation length is longer than the radius of forming polarons. Notwithstanding that the attraction is strongest in the undoped system with long range order, the superconducting order parameter vanishes when the doping parameter decreases which should be attributed to emptying the spin polaron band and approaching the Mott insulator phase. Since the hypothetical normal phase of low density gas of fermions is unstable against formation of bound hole pairs the intensity of low energy excitations is suppressed and the pseudogap forms in the underdoped region., Comment: 14 pages, 4 figures

Motivated by recent Raman and resonant inelastic X-ray scattering experiments performed for Mott insulators, which suggest formation of excitons in these systems, we present a theory of exciton formation in the upper Hubbard band. The analysis based on the spin polaron approach is performed in the framework of an effective t-J model for the subspace of states with one doubly occupied site. Our results confirm the existence of excitons and bear qualitative resemblance to experimental data despite some simplifications in our approach. They prove that the basic underlying mechanismof exciton formation is the same as that which gives rise to binding of holes in weakly doped antiferromagnets., Comment: 4 pages, 1 figure

We present a theory for the photon energy and polarization dependence of ARPES intensities from the CuO$_2$ plane in the framework of strong correlation models. We show that for electric field vector in the CuO$_2$ plane the `radiation characteristics' of the $O$ $2p_\sigma$ and $Cu$ $3d_{x^2-y^2}$ orbitals are strongly peaked along the CuO$_2$ plane, i.e. most photoelectrons are emitted at grazing angles. This suggests that surface states play an important role in the observed ARPES spectra, consistent with recent data from Sr$_2$CuCl$_2$O$_2$. We show that a combination of surface state dispersion and Fano resonance between surface state and the continuum of LEED-states may produce a precipitous drop in the observed photoelectron current as a function of in-plane momentum, which may well mimic a Fermi-surface crossing. This effect may explain the simultaneous `observation' of a hole-like and an electron-like Fermi surfaces in Bi2212 at different photon energies. We show that by suitable choice of photon polarization one can on one hand `focus' the radiation characteristics of the in-plane orbitals towards the detector and on the other hand make the interference between partial waves from different orbitals `more constructive'., Comment: 19 pages, 18 figures

We demonstrate that patterns formed by the current-current correlation function are landmarks which indicate that spin bipolarons form in doped antiferromagnets. Holes which constitute a spin bipolaron reside at opposite ends of a line (string) formed by the defects in the antiferromagnetic spin background. The string is relatively highly mobile, because the motion of a hole at its end does not raise extensively the number of defects, provided that the hole at the other end of the line follows along the same track. Appropriate coherent combinations of string states realize some irreducible representations of the point group C_4v. Creep of strings favors d- and p-wave states. Some more subtle processes decide the symmetry of pairing. The pattern of the current correlation function, that defines the structure of flux, emerges from motion of holes at string ends and coherence factors with which string states appear in the wave function of the bound state. Condensation of bipolarons and phase coherence between them puts to infinity the correlation length of the current correlation function and establishes the flux in the system., Comment: 5 pages, 6 figures

Using variational trial wave function based on the string picture we study the motion of a single mobile hole in the stripe phase of the doped antiferromagnet. The holes within the stripes are taken to be static, the undoped antiferromagnetic domains in between the hole stripes are assumed to have alternating staggered magnetization, as is suggested by neutron scattering experiments. The system is described by the t-t'-t''-J model with realistic parameters and we compute the single particle spectral density., Comment: RevTex-file, 9 PRB pages with 15 .eps and .gif files. To appear in PRB. Hardcopies of figures (or the entire manuscript) can be obtained by e-mail request to: eder@physik.uni-wuerzburg.de

We present results of a systematic Quantum-Monte-Carlo study for the single-band Hubbard model. Thereby we evaluated single-particle spectra (PES & IPES), two-particle spectra (spin & density correlation functions), and the dynamical correlation function of suitably defined diagnostic operators, all as a function of temperature and hole doping. The results allow to identify different physical regimes. Near half-filling we find an anomalous `Hubbard-I phase', where the band structure is, up to some minor modifications, consistent with the Hubbard-I predictions. At lower temperatures, where the spin response becomes sharp, additional dispersionless `bands' emerge due to the dressing of electrons/holes with spin excitatons. We present a simple phenomenological fit which reproduces the band structure of the insulator quantitatively. The Fermi surface volume in the low doping phase, as derived from the single-particle spectral function, is not consistent with the Luttinger theorem, but qualitatively in agreement with the predictions of the Hubbard-I approximation. The anomalous phase extends up to a hole concentration of 15%, i.e. the underdoped region in the phase diagram of high-T_c superconductors. We also investigate the nature of the magnetic ordering transition in the single particle spectra. We show that the transition to an SDW-like band structure is not accomplished by the formation of any resolvable `precursor bands', but rather by a (spectroscopically invisible) band of spin 3/2 quasiparticles. We discuss implications for the `remnant Fermi surface' in insulating cuprate compounds and the shadow bands in the doped materials., Comment: RevTex-file, 20 PRB pages, 16 figures included partially as gif. A full ps-version including ps-figures can be found at http://theorie.physik.uni-wuerzburg.de/~eder/condmat.ps.gz Hardcopies of figures (or the entire manuscript) can also be obtained by e-mail request to: eder@physik.uni-wuerzburg.de

We reanalyze the Hubbard-I approximation by showing that it is equivalent to an effective Hamiltonian describing Fermionic charge fluctuations, which can be solved by Bogoliubov transformation. As the most important correction in the limit of large U and weak spin correlations we augment this Hamiltonian by further effective particles, which describe composite objects of a Fermionic charge fluctuation and a spin-, density- or eta- excitation. The scheme is valid for positive and negative U. We present results for the single particle Green's function for the two-dimensional Hubbard model with and without t' and t'' terms, and compare to Quantum Monte-Carlo (QMC) results for the paramagnetic phase. The overall agreement is significantly improved over the conventional Hubbard-I or two-pole approximation., Comment: RevTex-file, 10 PRB pages with 6 eps figures. Hardcopies of figures (or the entire manuscript) can be obtained by e-mail request to: eder@physik.uni-wuerzburg.de

We present a Quantum Monte Carlo study of the temperature dependent dynamics of the two-dimensional Kondo insulator. Working at the so-called symmetrical point allows to perform minus-sign free QMC simulations. Study of the temperature dependence of the single-particle Green's function and the dynamical spin correlation function provides evidence for two characteristic temperatures, which we associate with the Kondo and coherence temperature, whereby the system shows a metal-insulator transition at the coherence temperature. The data shows evidence for two distinct types of spin excitations and we show that despite strong antiferromagnetic ordering at low temperature the system cannot be described by spin-density-wave theory., Comment: RevTex-file, 9 PRB pages with 7 eps figures. Hardcopies of figures (or the entire manuscript) can be obtained by e-mail request to: eder@physik.uni-wuerzburg.de

We study the electronic structure of the doped paramagnetic insulator by finite temperature Quantum Monte-Carlo simulations for the 2D Hubbard model. Throughout we use the moderately high temperature T=0.33t, where the spin correlation length has dropped to < 1.5 lattice spacings, and study the evolution of the band structure with hole doping. The effect of doping can be best described as a rigid shift of the chemical potential into the lower Hubbard band, accompanied by some transfer of spectral weight. For hole dopings <20% the Luttinger theorem is violated, and the Fermi surface volume, inferred from the Fermi level crossings of the `quasiparticle band', shows a similar topology and doping dependence as predicted by the Hubbard I and related approximations., Comment: RevTex-file, 4 PRB pages with 5 eps figures. Hardcopies of figures (or the entire manuscript) can be obtained by e-mail request to: eder@physik.uni-wuerzburg.de

We present a Quantum Monte Carlo (QMC) study of the 1D Kondo lattice at non-integer filling, i.e. a one-dimensional version of a heavy electron metal. For this special system the minus-sign problem turns out to be strongly reduced, which allows QMC simulations for temperatures as low as 1% of the conduction electron bandwidth. The single particle Green's function shows and intricate network of low-energy bands at low temperature, with a Fermi surface volume that comprises both c and f-electrons. As temperature increases the system evolves through two distinct crossover temperatures into a very simple band structure with one free c-electron band and a disconnected upper and lower Hubbard band for f-electrons. The f-electrons thus `drop out' of the Fermi surface volume as temperature increases., Comment: RevTex-file, 4 PRB pages with 4 eps figures. Hardcopies of figures (or the entire manuscript) can be obtained by e-mail request to: eder@physik.uni-wuerzburg.de