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Daghofer M.,Institute For Theoretische Festkorperphysik | Hohenadler M.,University of Wurzburg
Physical Review B - Condensed Matter and Materials Physics | Year: 2014

We use exact diagonalization and cluster perturbation theory to address the role of strong interactions and quantum fluctuations for spinless fermions on the honeycomb lattice. We find quantum fluctuations to be very pronounced both at weak and strong interactions. A weak second-neighbor Coulomb repulsion V2 induces a tendency toward an interaction-generated quantum anomalous Hall phase, as borne out in mean-field theory. However, quantum fluctuations prevent the formation of a stable quantum Hall phase before the onset of the charge-modulated phase predicted at large V2 by mean-field theory. Consequently, the system undergoes a direct transition from the semimetal to the charge-modulated phase. For the latter, charge fluctuations also play a key role. While the phase, which is related to pinball liquids, is stabilized by the repulsion V2, the energy of its low-lying charge excitations scales with the electronic hopping t, as in a band insulator. © 2014 American Physical Society.


Fuji Y.,Chiba University | Nishimoto S.,Institute For Theoretische Festkorperphysik | Ohta Y.,Chiba University
Journal of Physics: Conference Series | Year: 2012

Using the density-matrix renormalization group and exact diagonalization methods the ground-state properties of the S = 3/2 three-leg Heisenberg tube with leg (J∥) and rung (J⊥) exchange couplings are studied. We find that the spin-excitation gap associated with a spontaneous dimerization opens in the entire region of the coupling strength, as seen in the S = 1/2 three-leg tube. However, in contrast to the case of the S = 1/2 tube, the gap develops very slowly with increasing the rung coupling strength in the weak-coupling regime and its size remains only about 5 % of the leg coupling strength even at J⊥/J∥ = 5. We also calculate the quantized Berry phase and suggest that there exist three types of the valence-bond-solid states depending on the ratio of leg and rung coupling strengths. Moreover, an effective model is derived by the perturbative expansion to check the numerical results in the strong rung-coupling limit.


Brzezicki W.,Jagiellonian University | Brzezicki W.,University of Salerno | Daghofer M.,Institute For Theoretische Festkorperphysik | Oles A.M.,Jagiellonian University | Oles A.M.,Max Planck Institute for Solid State Research
Acta Physica Polonica A | Year: 2015

We explore the propagation of a single hole in the generalized quantum compass model which interpolates between fully isotropic antiferromagnetic phase in the Ising model and nematic order of decoupled antiferromagnetic chains for frustrated compass interactions. We observe coherent hole motion due to either interorbital hopping or due to the three-site effective hopping, while quantum spin fluctuations in the ordered background do not play any role.


Hohenadler M.,University of Wurzburg | Wessel S.,RWTH Aachen | Daghofer M.,Institute For Theoretische Festkorperphysik | Assaad F.F.,University of Wurzburg
Physical Review B - Condensed Matter and Materials Physics | Year: 2012

Experiments on quasi-one-dimensional systems such as quantum wires and metallic chains on surfaces suggest the existence of electron-electron interactions of substantial range and hence physics beyond the Hubbard model. We therefore investigate one-dimensional, quarter-filled chains with a Coulomb potential with variable screening length by quantum Monte Carlo methods and exact diagonalization. The Luttinger liquid interaction parameter K ρ decreases with increasing interaction strength and range. Experimentally observed values close to 1/4 require strong interactions and/or large screening lengths. As predicted by bosonization, we find a metal-insulator transition at K ρ=1/4. Upon increasing the screening length, the charge and spin correlation functions reveal the crossover from dominant 2k F spin correlations to dominant 4k F charge correlations, and a strong enhancement of the charge velocity. In the metallic phase, the signatures of spin-charge separation in the single-particle spectrum, spinon and holon bands, remain robust even for rather long-ranged interactions. The charge-density-wave state exhibits backfolded shadow bands. © 2012 American Physical Society.


Nishimoto S.,Institute For Theoretische Festkorperphysik | Fuji Y.,Chiba University | Ohta Y.,Chiba University
Physical Review B - Condensed Matter and Materials Physics | Year: 2011

The ground-state properties of the three-leg S=32 straight Heisenberg tube are studied using the density-matrix renormalization group method. We find that the spin-excitation gap associated with a spontaneous dimerization opens for the whole coupling regime, as seen in the three-leg S=12 straight Heisenberg tube. However, in contrast to the case of the S=12 straight tube, the gap increases very slowly with increasing the rung coupling, and its size is only a few percent or less of the leg exchange interaction in the weak- and intermediate-coupling regimes. We thus argue that, unless the rung coupling is substantially larger than the leg coupling, the gap may be quite hard to be observed experimentally. We also calculate the quantized Berry phase to show that there exist three kinds of valence-bond-solid states depending on the ratio of leg and rung couplings. © 2011 American Physical Society.

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