W. Hanke - Academia.edu (original) (raw)

Papers by W. Hanke

Journal of Physics and Chemistry of Solids, 2002

Numerical studies of a recently constructed projected SO(5) model [1] are presented, which aim at... more Numerical studies of a recently constructed projected SO(5) model [1] are presented, which aim at unifying antiferromagnetic and superconducting phases of the cuprates, while at the same time projecting out the high energy Coulomb scale of the Mott gap. We use a new implementation of the Quantum Monte Carlo technique of Stochastic Series Expansion [2±4] with extremely favorable scaling behavior towards large system sizes and low temperatures. Our numerical results show that the projected SO(5) 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.

The European Physical Journal B, 1999

We investigate, within the fluctuation-exchange approximation, a correlated-electron model for Y2... more 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 ⊥. An energy offset δ 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 δ. 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.

As their most prominent universal feature, high-temperature superconductors always display antife... more As their most prominent universal feature, high-temperature superconductors always display antiferromagnetism and d−wave superconductivity in close proximity, in their phase diagram. A unifying theory has been proposed, according to which these two at first sight radically different phases are "two faces of one and the same coin". They are unified by a common symmetry principle, the SO(5) symmetry. Recently, it was proposed that this theory had to be supplemented with a so-called "Gutzwiller projection" in order to resolve several inconsistencies with experimental results on high-Tc superconductors. Here, we present a numerical study of an effective bosonic model which describes the low-energy physics of this "projected" SO(5) theory. Our numerical results, obtained by the Quantum Monte Carlo technique of Stochastic Series Expansion, show that this model provides a realistic description of the global phase diagram of the high-Tc superconductors and accounts for many of their physical properties. Moreover, we address the question of asymptotic restoring of the SO(5) symmetry at the critical point.

Physical Review Letters, 1974

A method of calculating local-field effects in the time-dependent Hartree-Fock approximation util... more A method of calculating local-field effects in the time-dependent Hartree-Fock approximation utilizing the Wannier representation is presented and applied to the optical spectrum of diamond. The results establish the importance of including both the local-field effects and the exchange effects beyond the random-phase approximation.

Scientific reports, Jan 27, 2015

Recent theoretical studies employing density-functional theory have predicted BaBiO3 (when doped ... more Recent theoretical studies employing density-functional theory have predicted BaBiO3 (when doped with electrons) and YBiO3 to become a topological insulator (TI) with a large topological gap (~0.7 eV). This, together with the natural stability against surface oxidation, makes the Bismuth-Oxide family of special interest for possible applications in quantum information and spintronics. The central question, we study here, is whether the hole-doped Bismuth Oxides, i.e. Ba1-xKxBiO3 and BaPb1-xBixO3, which are "high-Tc" bulk superconducting near 30 K, additionally display in the further vicinity of their Fermi energy EF a topological gap with a Dirac-type of topological surface state. Our electronic structure calculations predict the K-doped family to emerge as a TI, with a topological gap above EF. Thus, these compounds can become superconductors with hole-doping and potential TIs with additional electron doping. Furthermore, we predict the Bismuth-Oxide family to contain an ...

Physical Review B, 2002

The single-particle density of states and the tunneling conductance are studied for a twodimensio... more The single-particle density of states and the tunneling conductance are studied for a twodimensional 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-Tc 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 M F c well above the actual superconducting transition.

Using the Quantum Monte Carlo (QMC) technique within frozen-phonon, we studied the effects of the... more Using the Quantum Monte Carlo (QMC) technique within frozen-phonon, we studied the effects of the half-breathing O(π, 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.

Nature Physics, 2009

Theories based on the coupling between spin fluctuations and fermionic quasiparticles are among t... more 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 1. Here, we analyse 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 YBa 2 Cu 3 O 6.6. 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 2-7 and 'kinks' in the fermionic band dispersions along the nodal direction 8-12. The superconducting transition temperature computed with this coupling constant exceeds 150 K, demonstrating that spin fluctuations have sufficient strength to mediate high-temperature superconductivity. Looking back at conventional superconductors, the most convincing demonstration of the electron-phonon interaction as the source of electron pairing was based on the quantitative correspondence between features in the electronic tunnelling conductance and the phonon spectrum measured by inelastic neutron scattering (INS; for reviews, see the articles by Scalapino, McMillan and Rowell in ref. 13). The rigorous comparison of fermionic and bosonic spectra was made possible by the Eliashberg theory, which enabled the tunnelling conductance to be derived from the experimentally determined phonon spectrum. Various difficulties have impeded a similar approach to the origin of high-temperature superconductivity. First, the d-wave pairing state found in these materials implies a strongly momentum-dependent pairing interaction. A more elaborate analysis based on data from momentum-resolved experimental techniques such as INS and angle-resolved photoemission spectroscopy (ARPES) is thus required. These methods, in turn, impose conflicting constraints on the materials. To avoid surface-related problems, most ARPES experiments have been carried out on the electrically neutral BiO cleavage plane in Bi 2 Sr 2 Ca n−1 Cu n O 2(n+2)+δ (ref. 8). However, as a consequence of electronic inhomogeneity, this family of materials exhibits broad INS spectra that greatly complicate a quantitative comparison with ARPES data 7. Conversely, compounds with sharp spin excitations, including YBa 2 Cu 3 O 6+x , have generated problematic ARPES spectra due to polar surfaces with charge distributions different from the bulk 8. Finally, an analytically rigorous treatment of the spin-fluctuation-mediated pairing

Physical Review Letters, 2007

Recent excperiments (ARPES, Raman) suggest the presence of two distinct energy gaps in high-Tc su... more 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.

Physical Review Letters, 1997

On the basis of Quantum Monte Carlo simulations of the two-dimensional Hubbard model which cover ... more On the basis of Quantum Monte Carlo simulations of the two-dimensional Hubbard model which cover the doping range from the under-to the over-doped regime, we find that the single-particle spectral weight A(k, ω) qualitatively reproduces both the momentum (d x 2 −y 2-symmetry) and doping dependence of the pseudogap as found in photoemission experiments. The drastic doping dependence of the spin response χs(q, ω) which is sharp in both q (≈ (π, π)) and ω in the under-doped regime but broad and structureless otherwise, identifies remnants of the antiferromagnetic order as the driving mechanism behind the pseudogap and its evolution with doping.

We show that doping-induced charge fluctuations in strongly correlated Hubbard electron systems n... more 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.

$Research paper thumbnail of Renormalization of the electron-spin-fluctuation interaction in the tt^{′}-U Hubbard model$

The competition between d-wave superconductivity (SC) and antiferromagnetism (AF) in the high-Tc ... more 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.

$Research paper thumbnail of Phase Diagram and Dynamics of the Projected SO (5) Symmetric Model of High-T_ {c} Superconductivity$

We present numerical studies of a quantum "projected" SO(5) model which aims at a unifying descri... more We present numerical studies of a quantum "projected" SO(5) model which aims at a unifying description of antiferromagnetism and superconductivity in the high-Tc 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-Tc 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.

Physical Review Letters, 1997

On the basis of Quantum Monte Carlo simulations of the two-dimensional Hubbard model which cover ... more On the basis of Quantum Monte Carlo simulations of the two-dimensional Hubbard model which cover the doping range from the under-to the over-doped regime, we find that the single-particle spectral weight A( k, ω) qualitatively reproduces both the momentum (d x 2 −y 2 -symmetry) and doping dependence of the pseudogap as found in photoemission experiments. The drastic doping dependence of the spin response χs( q, ω) which is sharp in both q (≈ (π, π)) and ω in the under-doped regime but broad and structureless otherwise, identifies remnants of the antiferromagnetic order as the driving mechanism behind the pseudogap and its evolution with doping.

Journal of Physics and Chemistry of Solids, 2002

The European Physical Journal B, 1999

Physical Review Letters, 1974

Scientific reports, Jan 27, 2015

Physical Review B, 2002

Nature Physics, 2009

Physical Review Letters, 2007

Physical Review Letters, 1997

$Research paper thumbnail of Renormalization of the electron-spin-fluctuation interaction in the tt^{′}-U Hubbard model$

$Research paper thumbnail of Phase Diagram and Dynamics of the Projected SO (5) Symmetric Model of High-T_ {c} Superconductivity$

Physical Review Letters, 1997

On the basis of Quantum Monte Carlo simulations of the two-dimensional Hubbard model which cover ... more On the basis of Quantum Monte Carlo simulations of the two-dimensional Hubbard model which cover the doping range from the under-to the over-doped regime, we find that the single-particle spectral weight A( k, ω) qualitatively reproduces both the momentum (d x 2 −y 2 -symmetry) and doping dependence of the pseudogap as found in photoemission experiments. The drastic doping dependence of the spin response χs( q, ω) which is sharp in both q (≈ (π, π)) and ω in the under-doped regime but broad and structureless otherwise, identifies remnants of the antiferromagnetic order as the driving mechanism behind the pseudogap and its evolution with doping.