arXiv subjects

Siegfried Guertler

Publications and source records attributed to Siegfried Guertler.

7 recordsLinked to original sources

arXiv2014

Kagome Lattice Hubbard model at half filling

We investigate the Kagome lattice Hubbard model at half-filling by variational Monte-Carlo with testing the U(1)-Dirac spin liquid, uniform and valence bond crystal states. Even for the large-$U$ case the U(1) Dirac state, being the optimal state in the Heisenberg model, cannot be recovered. While the finite $U$ Hubbard model allows the introduction of vacancies in a different manner compared to the $t-J$ model, the physics appears to have many similarities. In particular a valence bond crystal is formed in the intermediate-$U$ regime. We observe an impact of the formation of this valence bond crystal on a possible Mott-transition in this model and discuss the properties of the wave-function.

cond-mat.str-el↗

arXiv2013

Unveiling the physics of the doped phase of the Kagome $t-J$ model

We investigate the ground state properties of the Kagome lattice $t-J$ model at low doping. We propose a new state of matter. Our results suggest an interesting phase with a balance of spin-exchange and kinetic exchange through the building blocks of stars and their internal hexagons, being linked by triangles. The particles favor to hop in the hexagons, while the spin-exchange is taking place mainly on the stars. The combined effect leads to the formation of the "Star of David" valence bond crystal. We discuss our result in connection with static impurities, and show the likely relevance to the diluted Kagome lattice Heisenberg model, describing actual compounds.

cond-mat.str-el↗

arXiv2012

The classical $J_1$-$J_2$ Heisenberg model on the Kagome lattice

Motivated by earlier simulated annealing studies and materials with large spin on the Kagome lattice, we performed large scale parallel tempering simulations on the Kagome lattice for the extended classical Heisenberg model including next nearest neighbor interactions. We find that even a small inclusion of a $J_2$ term induces anti-ferromagnetic order which prevails in the thermodynamic limit. The magnitude of this effect is surprising. While at $J_2=0$ the finite-size behaviour does not suggest a phase-transition, at other points the numerical result is consistent with one. Close to $J_2=0$ and for a positive sign of $J_2$ two subsequent phase-transitions/crossovers are found, one of them connecting to the crossover for the $J_2=0$ case, shedding light to the pure case. The universality classes of the transitions were explored.

cond-mat.stat-mech↗

arXiv2011

Doping on the kagome lattice: A variational Monte-Carlo study of the $t-J$ model

We study doping on the Kagome lattice by exploring the $t-J$-model with variational Monte-Carlo. We use a number of Gutzwiller projected spin-liquid and valence bond-crystal states and compare their energies at several system-sizes. We find that introducing mobile holes drives the system away from the Spin-Liquid state proposed by Ran et al for the undoped system, towards a uniform state with zero-flux. On top of the uniform-state a VBC of the Hastings-type is formed for low doping. The results are compared to exact diagonalization on small clusters. This agrees well.

cond-mat.str-el↗

arXiv2008

Variational Monte-Carlo studies of Gossamer Superconductivity

We use a partially Gutzwiller projected BCS d-wave wavefunction with an antiferromagentic weighting factor to study the ground state phase diagram of a half filled Hubbard-Heisenberg model in a square lattice with nearest neighbor hopping $t$ and a diagonal hopping $t'$. The calculations are carried out by using variational Monte Carlo method which treats the Gutzwiller projection explicitly. At large on-site Coulomb interaction $U$, the ground state is antiferromagnetic. As $U$ decreases, the ground state becomes superconducting and eventually metallic. The phase diagram is obtained by extensive calculations. As compared to the strong effect of $U/t$, the phase boundaries turn out to be less sensitive to $t'/t$. The result is consistent with the phase diagram in layered organic conductors, and is compared to the earlier mean field result based on the Gutzwiller approximation.

cond-mat.supr-con↗

arXiv2007

Quantum Monte-Carlo study of a two-species boson Hubbard model

We consider a two-species hard-core boson Hubbard model for a supersolid, where the two types of bosons represent vacancies and interstitials doped into a commensurate crystal. The on-site inter-species interaction may create bound states of vacancies and interstitials facilitating vacancy condensation at lower energies than in a single-species model, as suggested in an earlier mean field study. Here we carry out quantum Monte Carlo simulation to study possible supersolid phases of the model, corresponding to superfluid phases of the vacancies or interstitials. At low temperatures, we find three distinct superfluid phases. The extent of the phases and the nature of the phase transitions are discussed in comparison to mean-field theory.

cond-mat.other↗

arXiv2004

Directional Ordering of Fluctuations in a Two-dimensional Compass Model

In the Mott insulating phase of the transition metal oxides, the effective orbital-orbital interaction is directional both in the orbital space and in the real space. We discuss a classical realization of directional coupling in two dimensions. Despite extensive degeneracy of the ground state, the model exhibits partial orbital ordering in the form of directional ordering of fluctuations at low temperatures stabilized by an entropy gap. Transition to the disordered phase is shown to be in the Ising universality class through exact mapping and multicanonical Monte Carlo simulations.

cond-mat.str-el↗