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Quantum spin Hall (QSH) materials promise revolutionary device applications based on dissipationless propagation of spin currents. They are two-dimensional (2D) representatives of the family of topological insulators, which exhibit conduction channels at their edges inherently protected against scattering. Initially predicted for graphene, and eventually realized in HgTe quantum wells, in the QSH systems realized so far, the decisive bottleneck preventing applications is the small bulk energy gap of less than 30 meV, requiring cryogenic operation temperatures in order to suppress detrimental bulk contributions to the edge conductance. Room-temperature functionalities, however, require much larger gaps. Here we show how this can be achieved by making use of a new QSH paradigm based on substrate-supported atomic monolayers of a high-Z element. Experimentally, the material is synthesized as honeycomb lattice of bismuth atoms, forming "bismuthene", on top of the wide-gap substrate SiC(0001). Consistent with the theoretical expectations, the spectroscopic signatures in experiment display a huge gap of ~0.8 eV in bismuthene, as well as conductive edge states. The analysis of the layer-substrate orbitals arrives at a QSH phase, whose topological gap - as a hallmark mechanism - is driven directly by the atomic spin-orbit coupling (SOC). Our results demonstrate how strained artificial lattices of heavy atoms, in contact with an insulating substrate, can be utilized to evoke a novel topological wide-gap scenario, where the chemical potential is located well within the global system gap, ensuring pure edge state conductance. We anticipate future experiments on topological signatures, such as transport measurements that probe the QSH effect via quantized universal conductance, notably at room temperature., Comment: 4 figures

We report on the epitaxial fabrication and electronic properties of a topological phase in strained \alpha-Sn on InSb. The topological surface state forms in the presence of an unusual band order not based on direct spin-orbit coupling, as shown in density functional and GW slab-layer calculations. Angle-resolved photoemission including spin detection probes experimentally how the topological spin-polarized state emerges from the second bulk valence band. Moreover, we demonstrate the precise control of the Fermi level by dopants., Comment: version 2 with supplementary information

In conventional and high transition temperature copper oxide and iron pnictide superconductors, the Cooper pairs all have even parity. As a rare exception, Sr$_2$RuO$_4$ is the first prime candidate for topological chiral p-wave superconductivity, which has time-reversal breaking odd-parity Cooper pairs known to exist before only in the neutral superfluid $^3$He. However, there are several key unresolved issues hampering the microscopic description of the unconventional superconductivity. Spin fluctuations at both large and small wavevectors are present in experiments, but how they arise and drive superconductivity is not yet clear. Spontaneous edge current is expected but not observed conclusively. Specific experiments point to highly band- and/or momentum-dependent energy gaps for quasiparticle excitations in the superconducting state. Here, by comprehensive functional renormalization group calculations with all relevant bands, we disentangle the various competing possibilities. In particular we show the small wavevector spin fluctuations, driven by a single two-dimensional band, trigger p-wave superconductivity with quasi-nodal energy gaps., Comment: 6 pages, 4 figures

One central challenge in high-$T_c$ superconductivity (SC) is to derive a detailed understanding for the specific role of the $Cu$-$d_{x^2-y^2}$ and $O$-$p_{x,y}$ orbital degrees of freedom. In most theoretical studies an effective one-band Hubbard (1BH) or t-J model has been used. Here, the physics is that of doping into a Mott-insulator, whereas the actual high-$T_c$ cuprates are doped charge-transfer insulators. To shed light on the related question, where the material-dependent physics enters, we compare the competing magnetic and superconducting phases in the ground state, the single- and two-particle excitations and, in particular, the pairing interaction and its dynamics in the three-band Hubbard (3BH) and 1BH-models. Using a cluster embedding scheme, i.e. the variational cluster approach (VCA), we find which frequencies are relevant for pairing in the two models as a function of interaction strength and doping: in the 3BH-models the interaction in the low- to optimal-doping regime is dominated by retarded pairing due to low-energy spin fluctuations with surprisingly little influence of inter-band (p-d charge) fluctuations. On the other hand, in the 1BH-model, in addition a part comes from "high-energy" excited states (Hubbard band), which may be identified with a non-retarded contribution. We find these differences between a charge-transfer and a Mott insulator to be renormalized away for the ground-state phase diagram of the 3BH- and 1BH-models, which are in close overall agreement, i.e. are "universal". On the other hand, we expect the differences - and thus, the material dependence to show up in the "non-universal" finite-T phase diagram ($T_c$-values)., Comment: 17 pages, 9 figures

We carry out a detailed numerical study of the three-band Hubbard model in the underdoped region both in the hole- as well as in the electron-doped case by means of the variational cluster approach. Both the phase diagram and the low-energy single-particle spectrum are very similar to recent results for the single-band Hubbard model with next-nearest-neighbor hoppings. In particular, we obtain a mixed antiferromagnetic+superconducting phase at low doping with a first-order transition to a pure superconducting phase accompanied by phase separation. In the single-particle spectrum a clear Zhang-Rice singlet band with an incoherent and a coherent part can be seen, in which holes enter upon doping around $(\pi/2,\pi/2)$. The latter is very similar to the coherent quasi-particle band crossing the Fermi surface in the single-band model. Doped electrons go instead into the upper Hubbard band, first filling the regions of the Brillouin zone around $(\pi,0)$. This fact can be related to the enhanced robustness of the antiferromagnetic phase as a function of electron doping compared to hole doping., Comment: 14 pages, 15 eps figures

Theories based on the coupling between spin fluctuations and fermionic quasiparticles are among the leading contenders to explain the origin of high-temperature superconductivity, but estimates of the strength of this interaction differ widely. Here we analyze the charge- and spin-excitation spectra determined by angle-resolved photoemission and inelastic neutron scattering, respectively, on the same crystals of the high-temperature superconductor YBa2Cu3O6.6. We show that a self-consistent description of both spectra can be obtained by adjusting a single parameter, the spin-fermion coupling constant. In particular, we find a quantitative link between two spectral features that have been established as universal for the cuprates, namely high-energy spin excitations and "kinks" in the fermionic band dispersions along the nodal direction. The superconducting transition temperature computed with this coupling constant exceeds 150 K, demonstrating that spin fluctuations have sufficient strength to mediate high-temperature superconductivity., Comment: 25 pages, 7 figures, including supplementary information, accepted for publication in Nature Physics

We report the first electrical manipulation and detection of the mesoscopic intrinsic spin-Hall effect (ISHE) in semiconductors through non-local electrical measurement in nano-scale H-shaped structures built on high mobility HgTe/HgCdTe quantum wells. By controlling the strength of the spin-orbit splittings and the n-type to p-type transition by a top-gate, we observe a large non-local resistance signal due to the ISHE in the p-regime, of the order of kOhms, which is several orders of magnitude larger than in metals. In the n-regime, as predicted by theory, the signal is at least an order of magnitude smaller. We verify our experimental observation by quantum transport calculations which show quantitative agreement with the experiments., Comment: 12 pages, incl. 5 figures

Two-particle (2-p) excitations such as spin and charge excitations play a key role in high-Tc cuprate superconductors (HTSC). On the basis of a parameter-free theory, which extends the Variational Cluster Approach (a recently developed embedded cluster method) to 2-p excitations, the magnetic excitations of HTSC are shown to be reproduced for a Hubbard model within the relevant strong-coupling regime. In particular, the resonance mode in the underdoped regime, its intensity and "hour-glass" dispersion are in good overall agreement with experiments., Comment: 5 pages, 3 figures, version as published

We investigate the charge- and spin dynamical structure factors for the 2D one-band Hubbard model in the strong coupling regime within an extension of the Dynamical Cluster Approximation (DCA) to two-particle response functions. The full irreducible two-particle vertex with three momenta and frequencies is approximated by an effective vertex dependent on the momentum and frequency of the spin/charge excitation. In the spirit of the DCA, the effective vertex is calculated with quantum Monte Carlo methods on a finite cluster. On the basis of a comparison with high temperature auxiliary field quantum Monte Carlo data we show that near and beyond optimal doping, our results provide a consistent overall picture of the interplay between charge, spin and single-particle excitations., Comment: 8 pages, 11 figures

Cooperation and competition between the antiferromagnetic, d-wave superconducting and Mott-insulating states are explored for the two-dimensional Hubbard model including nearest and next-nearest-neighbor hoppings at zero temperature. Using the variational cluster approach with clusters of different shapes and sizes up to 10 sites, it is found that the doping-driven transition from a phase with microscopic coexistence of antiferromagnetism and superconductivity to a purely superconducting phase is discontinuous for strong interaction and accompanied by phase separation. At half-filling the system is in an antiferromagnetic Mott-insulating state with vanishing charge compressibility. Upon decreasing the interaction strength U below a certain critical value of roughly U=4 (in units of the nearest-neighbor hopping), however, the filling-dependent magnetic transition changes its character and becomes continuous. Phase separation or, more carefully, the tendency towards the formation of inhomogeneous states disappears. This critical value is in contrast to previous studies, where a much larger value was obtained. Moreover, we find that the system at half-filling undergoes the Mott transition from an insulator to a state with a finite charge compressibility at essentially the same value. The weakly correlated state at half-filling exhibits superconductivity microscopically admixed to the antiferromagnetic order. This scenario suggests a close relation between phase separation and the Mott-insulator physics., Comment: 7 pages, 8 figures, revised version to be published in Phys. Rev. B

Using the local spin-density approximation (LSDA) and the (non self-consistent) GW approach, the (quasi-particle) band structure is calculated for MnTe in zinc-blende geometry. Different parameters characterizing the electronic structure are computed for an antiferromagnetic and the ferromagnetic phase and compared with the experiment. The strong Hubbard-type repulsion on the Mn-3d orbitals and the p-d hybridization are seen to be responsible for substantial defects found in the LSDA picture. It is discussed to which extent these can be improved upon by means of the GW approach., Comment: 9 pages, accepted for publication in pssc (special issue on II-VI semiconductors)

Recent excperiments (ARPES, Raman) suggest the presence of two distinct energy gaps in high-Tc superconductors (HTSC), exhibiting different doping dependences. Results of a variational cluster approach to the superconducting state of the two-dimensional Hubbard model are presented which show that this model qualitatively describes this gap dichotomy: One gap (antinodal) increases with less doping, a behavior long considered as reflecting the general gap behavior of the HTSC. On the other hand, the near-nodal gap does even slightly decrease with underdoping. An explanation of this unexpected behavior is given which emphasizes the crucial role of spin fluctuations in the pairing mechanism., Comment: 4 pages, 5 figures, revised version to be published in Phys. Rev. Lett

Using the variational cluster approach (VCA), we study the transition from the antiferromagnetic to the superconducting phase of the two-dimensional Hubbard model at zero temperature. Our calculations are based on a new method to evaluate the VCA grand potential which employs a modified Lanczos algorithm and avoids integrations over the real or imaginary frequency axis. Thereby, very accurate results are possible for cluster sizes not accessible to full diagonalization. This is important for an improved treatment of short-range correlations, including correlations between Cooper pairs in particular. We investigate the cluster-size dependence of the phase-separation tendency that has been proposed recently on the basis of calculations for smaller clusters. It is shown that the energy barrier driving the phase separation decreases with increasing cluster size. This supports the conjecture that the ground state exhibits microscopic inhomogeneities rather than macroscopic phase separation. The evolution of the single-particle spectum as a function of doping is studied in addtion and the relevance of our results for experimental findings is pointed out., Comment: 7 pages, 6 figures, published version

We study the renormalization of the electron-spin-fluctuation (el-sp) vertex in a two-dimensional Hubbard model with nearest-neighbor (t) and next-nearest-neighbor (t') hopping by a Quantum-Monte-Carlo calculation. Our results show that for t'=0, the renormalized el-sp vertex decreases quite generally with decreasing temperature at all spin-fluctuation momentum transfers. The suppression of the el-sp vertex results in a substantial reduction of the effective pairing interaction mediated by antiferromagnetic spin fluctuations in both the intermediate- and strong-correlation regimes. The inclusion of a finite t'/t<0, increases the Landau damping rate of spin fluctuations, especially in the overdoped region. The increased damping rate leads to smaller vertex corrections, in agreement with earlier diagrammatic calculations. Still, the vertex correction reduces the spin-fermion vertex, as at t'=0., Comment: 7 pages, 7 figures

The competition between d-wave superconductivity (SC) and antiferromagnetism (AF) in the high-Tc cuprates is investigated by studying the hole- and electron-doped two-dimensional Hubbard model with a recently proposed variational quantum-cluster theory. The approach is shown to provide a thermodynamically consistent determination of the particle number, provided that an overall shift of the on-site energies is treated as a variational parameter. The consequences for the single-particle excitation spectra and for the phase diagram are explored. By comparing the single-particle spectra with quantum Monte-Carlo (QMC) and experimental data, we verify that the low-energy excitations in a strongly-correlated electronic system are described appropriately. The cluster calculations also reproduce the overall ground-state phase diagram of the high-temperature superconductors. In particular, they include salient features such as the enhanced robustness of the antiferromagnetic state as a function of electron doping and the tendency towards phase separation into a mixed antiferromagnetic-superconducting phase at low-doping and a pure superconducting phase at high (both hole and electron) doping., Comment: 13 pages, 10 figures, revised version accepted by PRB

We present a numerical study of the doping dependence of the spectral function of the n-type cuprates. Using a variational cluster-perturbation theory approach based upon the self-energy-functional theory, the spectral function of the electron-doped two-dimensional Hubbard model is calculated. The model includes the next-nearest neighbor electronic hopping amplitude $t'$ and a fixed on-site interaction $U=8t$ at half filling and doping levels ranging from $x=0.077$ to $x=0.20$. Our results support the fact that a comprehensive description of the single-particle spectrum of electron-doped cuprates requires a proper treatment of strong electronic correlations. In contrast to previous weak-coupling approaches, we obtain a consistent description of the ARPES experiments without the need to introduce a doping-dependent on-site interaction $U$., Comment: 7 pages 4 eps figures

We study numerically and partly diagrammatically the renormalization of the electron-spin interaction or vertex in a two-dimensional one-band Hubbard model with spin-fluctuation momentum transfer ${\vv q}=(\pi,\pi)$. We find that the renormalized electron-spin vertex decreases quite generally with decreasing temperature at all doping densities. As a combination of two concurring effects, i.e. the decrease of the vertex and the increase of the spin susceptibility, the effective pairing interaction increases with lowering temperature in the intermediate-correlation regime, but {\it flattens off} in the strong-correlation regime. Our findings indicate that in the high-T$_c$ cuprates the pairing mediated by antiferromagnetic spin fluctuations is substantially reduced due to vertex corrections., Comment: Final version, accepted for publication in Europhysics Letters

One of the most remarkable properties of the high-temperature superconductors is a pseudogap regime appearing in the underdoped cuprates above the superconducting transition temperature T_c. The pseudogap continously develops out of the superconducting gap. In this paper, we demonstrate by means of a detailed comparison between theory and experiment that the characteristic change of quasiparticle dispersion in crossing T_c in the underdoped cuprates can be understood as being due to phase fluctuations of the superconducting order parameter. In particular, we show that within a phase fluctuation model the characteristic back-turning BCS bands disappear above T_c whereas the gap remains open. Furthermore, the pseudogap rather has a U-shape instead of the characteristic V-shape of a d_{x^2-y^2}-wave pairing symmetry and starts closing from the nodal k=(pi/2,pi/2) directions, whereas it rather fills in at the anti-nodal k=(pi,0) regions, yielding further support to the phase fluctuation scenario., Comment: 6 pages, 4 eps-figures

Based on the self-energy-functional approach proposed recently [M. Potthoff, Eur. Phys. J. B 32, 429 (2003)], we present an extension of the cluster-perturbation theory to systems with spontaneously broken symmetry. Our method applies to models with local interactions and accounts for both short-range correlations and long-range order. Short-range correlations are accurately taken into account via exact diagonalization of finite clusters. Long-range order is described by variational optimization of a ficticious symmetry-breaking field. In comparison with related cluster methods, our approach is more flexible and, for a given cluster size, less demanding numerically, especially at zero temperature. An application of the method to the antiferromagnetic phase of the Hubbard model at half-filling shows good agreement with results from quantum Monte-Carlo calculations. We demonstrate that the variational extension of the cluster-perturbation theory is crucial to reproduce salient features of the single-particle spectrum., Comment: 13 pages, accepted by PRB, v2 with minor changes

Using the Quantum Monte Carlo (QMC) technique within frozen-phonon, we studied the effects of the half-breathing O$(\pi,0)$ phonon mode on the ground-state properties of the three-band Peierls-Hubbard model. Our simulations are performed for both ionic and covalent electron-phonon couplings. The effects of lattice displacements on the ground-state energies and charge fluctuations are similar in magnitude for both hole- and electron-doped cases. However, the effects of lattice displacements on the magnetic properties are rather different. In the hole-doped case, the normalized next-nearest-neighbor Cu-Cu spin correlations are dramatically modified by both ionic and covalent electron-phonon couplings. On the other hand, in the electron-doped case, much smaller effects are observed. The distinct spin-phonon couplings, in conjunction with the spin-bag picture of the quasiparticle, could explain a strong mass renormalization effect in the p-type cuprates and a weaker effect in the n-type cuprates., Comment: 5 pages, 5 figures

Using quantum Monte Carlo techniques, we study the effects of electronic correlations on the effective electron-phonon (el-ph) coupling in a two-dimensional one-band Hubbard model. We consider a momentum-independent bare ionic el-ph coupling. In the weak- and intermediate-correlation regimes, we find that the on-site Coulomb interaction $U$ acts to effectively suppress the ionic el-ph coupling at all electron- and phonon- momenta. In this regime, our numerical simulations are in good agreement with the results of perturbation theory to order $U^2$. However, entering the strong-correlation regime, we find that the forward scattering process stops decreasing and begins to substantially increase as a function of $U$, leading to an effective el-ph coupling which is peaked in the forward direction. Whereas at weak and intermediate Coulomb interactions, screening is the dominant correlation effect suppressing the el-ph coupling, at larger $U$ values irreducible vertex corrections become more important and give rise to this increase. These vertex corrections depend crucially on the renormalized electronic structure of the strongly correlated system., Comment: 5 pages, 4 eps-figures, minor changes

We study the scaling properties of the quantum ``projected'' SO(5) model in three dimensions by means of a highly accurate Quantum-Monte-Carlo analysis. Within the parameter regime studied (temperature and system size), we show that the scaling behavior is consistent with a SO(5)-symmetric critical behavior in a relative extendent transient regime. This holds both when the symmetry breaking is caused by quantum fluctuations only as well as when also the static (mean-field) symmetry is moderately broken. We argue that possible departure away from the SO(5)-symmetric scaling occurs only in an extremely narrow parameter regime, which is inaccessible both experimentally and numerically., Comment: Completely revised and expanded version. Includes scaling analisis for more general systems away from the "pSO(5)" condition 7 pages revtex4, 9 .eps figures

Recent reflectivity measurements provide evidence for a "violation" of the in-plane optical integral in the underdoped high-T_c compound Bi_2Sr_2CaCu_2O_{8+\delta} up to frequencies much higher than expected by standard BCS theory. The sum rule violation may be related to a loss of in-plane kinetic energy at the superconducting transition. Here, we show that a model based on phase fluctuations of the superconducting order parameter can account for this change of in-plane kinetic energy at T_c. The change is due to a transition from a phase-incoherent Cooper-pair motion in the pseudogap regime above T_c to a phase-coherent motion at T_c., Comment: 5 pages, 3 eps-figures

Angular-resolved photoemission data on half-filled doped cuprate materials are compared with an exact-diagonalization analysis of the three-band Hubbard model, which is extended to the infinite lattice by means of a perturbation in the intercluster hopping (cluster perturbation theory). A study of the band dispersion and spectral weight of the insulating cuprate Sr$_2$CuO$_2$Cl$_2$ allows us to fix a consistent parameter set, which turns out to be appropriate at finite dopings as well. In the overdoped regime, our results for the spectral weight and for the Fermi surface give a good description of the experimental data on Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$. In particular, the Fermi surface is hole-like and centered around $k=(\pi,\pi)$. Finally, we introduce a hopping between two layers and address the issue of bilayer splitting., Comment: Accepted for publication in JLTP as a contribution to the "Festschrift" dedicated to Prof. P. Woelfle's sixtieth birthday

The single-particle density of states and the tunneling conductance are studied for a two-dimensional BCS-like Hamiltonian with a d_{x^2-y^2}-gap and phase fluctuations. The latter are treated by a classical Monte Carlo simulation of an XY model. Comparison of our results with recent scanning tunneling spectra of Bi-based high-T_c cuprates supports the idea that the pseudogap behavior observed in these experiments can be understood as arising from phase fluctuations of a d_{x^2-y^2} pairing gap whose amplitude forms on an energy scale set by T_c^{MF} well above the actual superconducting transition., Comment: 5 pages, 6 eps-figures

We present numerical studies of a quantum ``projected'' SO(5) model which aims at a unifying description of antiferromagnetism and superconductivity in the high-T$_c$ cuprates, while properly taking into account the Mott insulating gap. Our numerical results, obtained by the Quantum Monte Carlo technique of Stochastic Series Expansion, show that this model can give a realistic description of the global phase diagram of the high-$T_c$ superconductors and accounts for many of their physical properties. Moreover, we address the question of dynamic restoring of the SO(5) symmetry at the critical point., Comment: Final version. Accepted for publication in Phys. Rev. Lett

We propose a phenomenological model, comprising a microscopic \sof model plus the on-site Hubbard interaction $U$ (``projected \sof model'') to understand the interrelation between the d-wave-gap modulation observed by recent angle-resolved photoemission experiments in the insulating antiferromagnet Ca$_2$CuO$_2$Cl$_2$ and the d-wave gap of high-Tc superconducting materials. The on-site interaction $U$ is important in order to produce a Mott gap of the correct order of magnitude, which would be absent in an exact \sof theory. The projected \sof-model explains the gap characteristics, namely both the symmetry and the different order of magnitude of the gap modulations between the \af and the \sc phases. Furthermore, it is shown that the projected \sof theory can provide an explanation for a recent observation [E. Pavarini et al., Phys. Rev. Lett. 87, 47003 (2001)], i. e. that the maximum Tc observed in a large variety of high-Tc cuprates scales with the next-nearest-neighbor hopping matrix element $t'$., Comment: Substantially revised version. Included discussion about relation between Tc_max and t'

By combining fermion Quantum Monte Carlo (QMC) simulations with diagrammatic theory, we have calculated the dielectric screening and the screened potential, in a quasi 2D Hubbard model for cuprate superconductors with and without 1/r Coulomb potential. At half filling, we find that the Hubbard electrons contribute only a minor fraction, Delta(epsilon) ~0.9 of the observed in-plane dielectric constant of the cuprates, epsilon ~4.7. With increasing doping x, the 1/r interaction is rapidly suppressed by metallic screening. Surprisingly, near x ~5%, the low-frequency part of the screened potential V_S becomes attractive, at distances r \ge 1. At r=1, it reaches maximum attraction strength for dopings x ~13-15% and becomes repulsive again for for x ~23-25%. Similar results are found for the pure 2D Hubbard model. The 1/r interaction enhances the on-site and 1st neighbor overscreening attraction already present in the pure model at finite doping. Our results are potentially relevant for the d-wave pairing mechanism in the cuprates, suggesting that the screened 1/r potential could actually increase the d-wave attraction, in the 5-25% doping regime. They may also have implications for the isotope effect and its doping dependence. At larger dopings, x>15%, the screened potential becomes attractive even on-site, suggesting that it could support or enhance s-wave pairing. We also give a rigorous analytical proof that the screened on-site interaction must become attractive near half-filling in the repulsive large-U-limit, with and without 1/r interaction. We present a simple physical interpretation of this result in terms of retardation effects. We also point out that on-site overscreening implies singularities in the imaginary frequency dependence of the irreducible polarization insertion and its 3-point vertex function., Comment: 14 pages, revtex

We study the competition between long- and short-range interactions among charge carriers in strongly-correlated electronic systems employing a new method which combines the density-matrix renormalization-group technique with a self-consistent treatment of the long-range interactions. We apply the method to an extended t-J model which exhibits ``stripe'' order. The Coulomb interactions, while not destroying stripes, induce large transverse stripe fluctuations with associated charge delocalization. This leads to a substantial Coulomb-repulsion-induced {\it enhancement} of long-range superconducting pair-field correlations., Comment: Appendix included

We consider the projected SO(5) bosonic model introduced in order to connect the SO(5) theory of high-T$_c$ superconductivity with the physics of the Mott-insulating gap, and derive the corresponding effective functional describing low-energy degrees of freedom. At the antiferromagnetic-superconducting transition, SO(5) symmetry-breaking effects due to the gap are purely quantum mechanical and become irrelevant in the neighborhood of a possible finite-temperature multicritical point separating the normal from the antiferromagnetic and the superconducting phases. A difference in the magnon and hole-pair mobility always takes the system away from the SO(5)-symmetric fixed point towards a region of instability, and the phase transition between the normal and the two ordered phases becomes first order before merging into the antiferromagnetic-superconducting line. Quantum fluctuations at intermediate temperatures, while introducing symmetry-breaking terms in the case of equal mobilities, tend to cancel the symmetry-breaking effects in the case of different mobilities., Comment: 11 pages, no figures. Dedicated to Prof. Franz Wegner on the occasion of his 60th birthday

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

Using quantum Monte Carlo (QMC) simulations combined with Maximum Entropy analytic continuation as well as analytical methods, we examine the one- and two-particle dynamical properties of the Hubbard model on two coupled chains at small doping. The behavior of the single-particle spectral weight $A({\bf k},\omega)$ as a function of hopping anisotropy $t_\perp/t$ at intermediate interaction strength is dominated by the transition from one-band behavior at large $t_\perp/t$ to two-band behavior at small $t_\perp/t$, although interaction effects such as band-narrowing, a shift of spectral weight to higher energies in the unoccupied antibonding band and reflected structures due to short-range antiferromagnetic correlations are also present. A single-particle gap is resolved in the intermediate $t_\perp/t$ Luther-Emery phase using Density Matrix Renormalization Group calculations. The dynamical spin and charge susceptibilities show features of the expected bonding-band Luttinger liquid behavior, as well as higher-energy features due to local excitations between the chains at large $t_\perp/t$, and evolve towards the behavior of two uncoupled chains as $t_\perp/t$ is reduced. For the one hole, large $t_\perp/t$ case, we make a detailed comparison between the QMC data and an approximation based on local rung states. At isotropic coupling and somewhat larger doping, we find that the dispersion of the single-particle bands is essentially unrenormalized from that of the noninteracting system, and that the spin and charge response functions have features also seen in random-phase-approximation calculations., Comment: 12 pages, 17 figure captions; a version with embedded figures (1.6 MByte zipped) is available for download at our institute server (http://theorie.physik.uni-wuerzburg.de/publications.shtml, ftp://ftp.physik.uni-wuerzburg.de/pub/preprint/1998/WUE-ITP-98-027.ps.gz); hardcopies of figures or the original files can be obtained upon email request to M.G. Zacher (mailto:gaston@physik.uni-wuerzburg.de)

We investigate, within the fluctuation-exchange approximation, a correlated-electron model for Y2Ba4Cu7O15 represented by two inequivalent Hubbard layers coupled by an interlayer hopping $t_\perp$. An energy offset $\delta$ is introduced in order to produce a different charge carrier concentration in the two layers. We compare several single-particle and magnetic excitations, namely, the single particle scattering rate, the spectral function and the spin lattice as well as spin-spin relaxation times in the two layers as a function of $\delta$. We show that the induced interlayer magnetic coupling produces a tendency to ``equalization'' of the magnetic properties in the two layers whereby antiferromagnetic fluctuations are suppressed in the less doped layer and enhanced in the heavily doped one. The strong antiferromagnetic bilayer coupling causes the charge carriers in the plane with larger doping concentration to behave similar to those of the underdoped layer, they are coupled to. This effect grows for decreasing temperature. For high temperatures or if both layers are optimally or overdoped, i.e. when the antiferromagnetic correlation length becomes of the order or smaller than one lattice site the charge carrier and magnetic dynamics of the two layers is disconnected and the equalization effect disappears. These results are in good agreement with NMR experiments on Y2Ba4Cu7O15 by Stern et al. [Phys. Rev B 51, 15478 (1995)]. We also compare the results with calculations on bilayer systems with equivalent layers as models for the constituent compounds YBa2Cu3O7 and YBa2Cu4O8., Comment: 11 Pages, 16 ps figures. This is an extended version of cond-mat/9806155

Numerical and analytical results are reviewed, which support SO(5) symmetry as a concept unifying superconductivity and antiferromagnetism in the high-temperature superconductors. Exact cluster diagonalizations verify that the low-energy states of the two-dimensional t-J and Hubbard models, widely used microscopic models for the high-Tc cuprates, form SO(5) symmetry multiplets. Apart from a small standard deviation ~J/10, these multiplets become degenerate at a critical chemical potential (transition into doped system). As a consequence, the d-wave superconducting states away from half-filling are obtained from the higher spin states at half-filling through SO(5) rotations. Between one and two dimensions, using weak-coupling renormalization, a rather general ladder Hamiltonian including next-nearest-neighbor hopping can be shown to flow to an SO(5) symmetric point. Experimental tests and consequences such as the existence of a pi-Goldstone mode both in the insulator and superconductor and, in particular, the relationship between the photoemission spectra of the insulator and superconductor, are emphasized., Comment: LaTeX, 12 pages, 9 postscript figures. To appear in: Festkoerperprobleme/Advances in Solid State Physics

We investigate the interplane magnetic coupling of the multilattice compound Y_2Ba_4Cu_7O_{15} by means of a bilayer Hubbard model with inequivalent planes. We evaluate the spin response, effective interaction and the intra- and interplane spin-spin relaxation times within the fluctuation exchange approximation. We show that strong in-plane antiferromagnetic fluctuations are responsible for a magnetic coupling between the planes, which in turns leads to a tendency of the fluctuation in the two planes to equalize. This equalization effect grows whit increasing in-plane antiferromagnetic fluctuations, i. e., with decreasing temperature and decreasing doping, while it is completely absent when the in-layer correlation length becomes of the order of one lattice spacing. Our results provide a good qualitative description of NMR and NQR experiments in Y_2Ba_4Cu_7O_{15}., Comment: Final version, to appear. in Phys. Rev. B (Rapid Communications), sched. Jan. 99

We show that doping-induced charge fluctuations in strongly correlated Hubbard electron systems near the 1/2-filled, insulating limit cause overscreening of the electron-electron Coulomb repulsion. The resulting attractive screened interaction potential supports d_{x^2-y^2}-pairing with a strongly peaked, doping dependent pairing strength at lower doping, followed by s-wave pairing at larger doping levels., Comment: 4 pages, 3 figures, Revtex

We discuss properties of an exactly SO(5) symmetric ladder model. In the strong coupling limit we demonstrate how the SO(3)-symmetric description of spin ladders in terms of bond Bosons can be upgraded to an SO(5)-symmetric bond-Boson model, which provides a particularly simple example for the concept of SO(5) symmetry. Based on this representation we show that antiferro- magnetism on one hand and superconductivity on the other hand can be understood as condensation of either magnetic or charged Bosons into an RVB vacuum. We identify exact eigenstates of a finite cluster with general multiplets of the SO(5) group, and present numerical results for the single particle spectra and spin/charge correlation functions of the SO(5)-symmetric model and identify `fingerprints' of SO(5) symmetry in these. In particluar we show that SO(5) symmetry implies a `generalized rigid band behavior' of the photoemission spectrum, i.e. spectra for the doped case are rigorously identical to spectra for spin-polarized states at half-filling. We discuss the problem of adiabatic continuity between the SO(5) symmetric ladder and the actual t-J ladder and demonstrate the feasibility of a `Landau mapping' between the two models., Comment: Revtex-file, 16 pages with 15 eps-figures. Hardcopies of Figures (or the entire manuscript) obtainable by e-mail request to eder@physik.uni-wuerzburg.de

We present results for the zero and finite temperature Drude weight D(T) and for the Meissner fraction of the attractive and the repulsive Hubbard model, as well as for the model with next nearest neighbor repulsion. They are based on Quantum Monte Carlo studies and on the Bethe ansatz. We show that the Drude weight is well defined as an extrapolation on the imaginary frequency axis, even for finite temperature. The temperature, filling, and system size dependence of D is obtained. We find counterexamples to a conjectured connection of dissipationless transport and integrability of lattice models., Comment: 10 pages, 14 figures. Published version

We analyze single-particle electronic and two-particle magnetic properties of the Hubbard model in the underdoped and optimally-doped regime of \YBCO by means of a modified version of the fluctuation-exchange approximation, which only includes particle-hole fluctuations. Comparison of our results with Quantum-Monte Carlo (QMC) calculations at relatively high temperatures ($T\sim 1000 K$) suggests to introduce a temperature renormalization in order to improve the agreement between the two methods at intermediate and large values of the interaction $U$. We evaluate the temperature dependence of the spin-lattice relaxation time $T_1$ and of the spin-echo decay time $T_{2G}$ and compare it with the results of NMR measurements on an underdoped and an optimally doped \YBCO sample. For $U/t=4.5$ it is possible to consistently adjust the parameters of the Hubbard model in order to have a good {\it semi-quantitative} description of this temperature dependence for temperatures larger than the spin gap as obtained from NMR measurements. We also discuss the case $U/t\sim 8$, which is more appropriate to describe magnetic and single-particle properties close to half-filling. However, for this larger value of $U/t$ the agreement with QMC as well as with experiments at finite doping is less satisfactory., Comment: Final version, to appear in Phys. Rev. B (sched. Feb. 99)

Using angle-resolved synchrotron-radiation photoemission spectroscopy we have determined the dispersion of the valence bands of BeTe(100) along $\Gamma X$, i.e. the [100] direction. The measurements are analyzed with the aid of a first-principles calculation of the BeTe bulk band structure as well as of the photoemission peaks as given by the momentum conserving bulk transitions. Taking the calculated unoccupied bands as final states of the photoemission process, we obtain an excellent agreement between experimental and calculated spectra and a clear interpretation of almost all measured bands. In contrast, the free electron approximation for the final states fails to describe the BeTe bulk band structure along $\Gamma X$ properly., Comment: 21 pages plus 4 figures

We study the occurrence of SO(5) symmetry in the low-energy sector of two-chain Hubbard-like systems by analyzing the flow of the running couplings ($g$-ology) under renormalization group in the weak-interaction limit. It is shown that SO(5) is asymptotically restored for low energies for rather general parameters of the bare Hamiltonian. This holds also with inclusion of a next-nearest-neighbor hopping which explicitly breaks particle-hole symmetry provided one accounts for a different single-particle weight for the quasiparticles of the two bands of the system. The physical significance of this renormalized SO(5) symmetry is discussed., Comment: Final version: to appear in Phys. Rev. Lett., sched. Mar. 99

We construct an SO(5) symmetric electron model on a two-chain ladder with purely local interactions on a rung. The ground state phase diagram of this model is determined in the strong-coupling limit. The relationship between the spin-gap magnon mode of the spin-gap insulator and the $\pi$ resonance mode of the d-wave pairing phase is discussed. We also present the exact ground state for an SO(5) superspin model., Comment: 28 pages including figures

The problem of spin-charge separation is analyzed numerically in the metallic phase of the one-band Hubbard model in one dimension by studying the behavior of the single-particle Green's function and of the spin and charge susceptibilities. We first analyze the Quantum-Monte Carlo data for the imaginary-time Green's function within the Maximum Entropy method in order to obtain the spectral function at real frequencies. For some values of the momentum sufficiently away from the Fermi surface two separate peaks are found, which can be identified as charge and spin excitations. In order to improve our accuracy and to be able to extend our study to a larger portion of the Brillouin zone, we also fit our data with the imaginary-time Green's function obtained from the Luttinger-model solution with two different velocities as fitting parameters. The excitation energies associated with these velocities turn out to agree, in a broad range of momenta, with the ones calculated from the charge and spin susceptibilities. This allows us to identify these single-particle excitations as due to a separation of spin and charge. Remarkably, the range of momenta where spin-charge separation is seen extends well beyond the region of linear dispersion about the Fermi surface. We finally discuss a possible extension of our method to detect spin-charge separation numerically in two dimensions., Comment: 7 pages, 10 figures. Final version to appear in Phys. Rev. B. Minor misprints corrected

We study the low-temperature transport of a doped two-chain ladder system of interacting fermions in the presence of a barrier or of a low concentration of impurities. Above a certain value of the interaction, the conductance is suppressed, like for a single chain, despite the presence of dominant superconducting correlations. There is, however, a region of repulsive interaction where perfect transmission across the barrier occurs unlike the single-chain case. We provide a possible explanation for the temperature maximum of the resistivity in the normal state of \srca., Comment: 4 pages, 2 figures, to be published in Phys. Rev. Lett

A quantitative analysis of a microscopic model for the intrinsic Josephson effect in high-temperature superconductors based on interlayer tunneling is presented. The pairing correlations in the CuO_2-planes are modelled by a 2-D Hubbard-model with attractive interaction, a model which accounts well for some of the observed features such as the short planar coherence length. The stack of Hubbard planes is arranged on a torus, which is threaded by a magnetic flux. The current perpendicular to the planes is calculated as a function of applied flux (i. e. the phase), and -- after careful elimination of finite-size effects due to single-particle tunneling -- found to display a sinusoidal field dependence in accordance with interlayer Josephson tunneling. Studies of the temperature dependence of the supercurrent reveal at best a mild elevation of the Josephson transition temperature compared to the planar Kosterlitz-Thouless temperature. These and other results on the dependence of the model parameters are compared with a standard BCS evaluation., Comment: 11 pages, Rev-Tex, 8 figures

In this work, we present numerical results which support SO(5) symmetry as a concept unifying superconductivity and antiferromagnetism in the high-temperature superconductors. Using exact cluster diagonalization, we verify the recently proposed SO(5) multiplet structures for a widely used microscopic model, the t-J model. Our results show that the d-wave superconducting ground states away from half-filling are obtained from the higher spin states at half-filling through SO(5) rotations, that the dominant low energy resonance in the dynamical spin correlation function corresponds to a `pi-resonance' in the entire low doping region, and that the SO(5) symmetry is robust against inclusion of longer ranged Coulomb repulsion and next-nearest neighbor hopping., Comment: Revtex file, 7 pages and 7 eps-figures

We show that important anomalous features of the normal-state thermoelectric power S of high-Tc materials can be understood as being caused by doping dependent short-range antiferromagnetic correlations. The theory is based on the fluctuation-exchange approximation applied to Hubbard model in the framework of the Kubo formalism. Firstly, the characteristic maximum of S as function of temperature can be explained by the anomalous momentum dependence of the single-particle scattering rate. Secondly, we discuss the role of the actual Fermi surface shape for the occurrence of a sign change of S as a function of temperature and doping., Comment: 4 pages, with eps figures