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arXiv:cond-mat/0503465 · Physical Review B

Quantum Monte Carlo study of the superconducting Kosterlitz-Thouless transition of the attractive Hubbard model on a triangular lattice

Tsuguhito Nakano, Kazuhiko Kuroki

Published Nov 2, 2006• 6 citations
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Abstract

We study the superconducting Kosterlitz-Thouless (KT) transition of the attractive Hubbard model on a two-dimensional triangular lattice using auxiliary field quantum Monte Carlo method for system sizes up to $14\ifmmode\times\else\texttimes\fi{}14$ sites. Combining three methods to analyze the numerical data, we find, for the attractive interaction of $U=\ensuremath{-}4t$, that the transition temperature is broadly maximized around the band filling of $1.2<n<1.6$, while it is found to be rather low in the range of $n<1$ and $n>1.8$. We discuss that this band filling dependence may be explained in terms of the band filling dependence of the superfluid density and the effective mass. Our analysis also teaches us that the estimation of the KT transition temperature requires some caution when the density of states near the Fermi level is small and the calculation is restricted to small system sizes.

DOI
10.1103/physrevb.74.174502

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We study the superconducting Kosterlitz-Thouless (KT) transition of the attractive Hubbard model on a two-dimensional triangular lattice using auxiliary field quantum Monte Carlo method for system sizes up to 14\ifmmode×\else\texttimes\fi1414\ifmmode\times\else\texttimes\fi{}1414\ifmmode×\else\texttimes\fi14 sites. Combining three methods to analyze the numerical data, we find, for the attractive interaction of U=\ensuremath−4tU=\ensuremath{-}4tU=\ensuremath−4t, that the transition temperature is broadly maximized around the band filling of 1.2<n<1.61.2<n<1.61.2<n<1.6, while it is found to be rather low in the range of n<1n<1n<1 and n>1.8n>1.8n>1.8. We discuss that this band filling dependence may be explained in terms of the band filling dependence of the superfluid density and the effective mass. Our analysis also teaches us that the estimation of the KT transition temperature requires some caution when the density of states near the Fermi level is small and the calculation is restricted to small system sizes.
Susceptibilities in the static limit ω→0 are evaluated to show that the superconducting Kosterlitz–Thouless transition exists in the two-dimensional Hubbard model for optimal band parameters. By using quantum Monte Carlo methods, we have examined the size dependence of the spin susceptibility χ( Q ) at half filling and the pair susceptibilities with d - and s -wave symmetries for the repulsive and attractive interactions, respectively. The numerical results for systems of up to 14×14 sites show that the s -wave pair susceptibility scales as χ pair ∼ L 2 for the attractive case, in agreement with previous quantum Monte Carlo studies. The scaling χ pair ∼ L 2 also holds for the d -wave pair susceptibility for the repulsive Hubbard model if we adjust the band parameter t '. Therefore, it is likely that the superconducting Kosterlitz–Thouless transition exists in the two-dimensional repulsive Hubbard model.
Based on extensions of the grand-canonical quantum Monte Carlo algorithm to incorporate magnetic fields, we provide numerical data confirming the existence of a Kosterlitz-Thouless transition in the attractive Hubbard model. Here, we calculate the temperature derivative of the superfluid density, [\ensuremath{\partial}\ensuremath{\beta}Ds{\mathit{D}}_{\mathit{s}}Ds​(\ensuremath{\beta})]/\ensuremath{\partial}\ensuremath{\beta}, to pin down the transition. The latter quantity is obtained from the difference in internal energy of systems which differ by a phase twist \ensuremath{\pi}/2 in the boundary condition along one lattice direction. Away from half-band filling, [\ensuremath{\partial}\ensuremath{\beta}Ds{\mathit{D}}_{\mathit{s}}Ds​(\ensuremath{\beta})]/\ensuremath{\partial}\ensuremath{\beta} shows a response which increases with lattice size at the transition temperature. In contrast, such a signal is not observed at half-band filling.
The values of the insulator gap Δ in one-dimensional systems of interacting bosons described by the Hubbard Hamiltonian are calculated at low temperatures by the quantum world-line Monte Carlo algorithm. The dependence of Δ on the size of the system, the temperature, and the parameters of the model is investigated. It is shown that a chain with N a =50 sites is already sufficient to estimate the thermodynamic value of the critical quantity ( t/U ) c for which a transition from the insulator into the superfluid state occurs in a commensurate system. To within the computational error, this value, ( t/U ) c =0.300±0.005, agrees with the value ( t/U ) c =0.304±0.002 obtained previously by the combined “exact diagonalization + renormalization-group analysis” method. The characteristic Kosterlitz-Thouless behavior of the insulator gap is demonstrated near the critical region: Δ∼exp[− b (1− t/t c ) −1/2 ].
We present results of Monte Carlo simulations of the three-dimensional half-filled Hubbard model on simple-cubic 43{4}^{3}43 lattices and some 63{6}^{3}63 lattices with periodic boundary conditions, as well as of small clusters embedded in an effective medium. Magnetic properties were studied for values of the Hubbard repulsion U ranging from 0 to the bandwidth W=12t (t is the hopping matrix element). Our results show that antiferromagnetism is enhanced up to U\ensuremath{\sim}(5/6)W and then suppressed. The q=0 magnetic susceptibility is enhanced by U, although much less than predicted by the random-phase approximation. The transition temperature to an antiferromagnetic state is found to have a broad maximum around U\ensuremath{\sim}(5/6)W, with a maximum value of approximately W/18. The implications of our results for superconductivity in the attractive Hubbard model are also discussed.
The critical temperature for superconductivity for the two-dimensional attractive (negative- U ) Hubbard model on a square lattice is calculated as a function of the strength of the interaction and of the density of electrons. The critical temperature is obtained by calculating the helicity modulus associated with a wavelike distortion of the gap parameter in the BCS approximation as a function of temperature and subsequent comparison with the Kosterlitz-Thouless relation between critical temperature and helicity modulus. The adequacy of the approximations is discussed and a comparison with recent Monte Carlo calculations is made.
A two-dimensional triangular Ising lattice with the nearest-neighbor antiferromagnetic interaction J 1 and the next-nearest-neighbor ferromagnetic interaction - J 2 is studied by the Monte Carlo simulation. It is shown that the system with a finite J 2 undergoes the successive phase transition, paramagnetic→Kosterlitz-Thouless (KT)-like→ferrimagnetic phases, as temperature decreases. The sub-lattice switching phenomenon observed in our previous work is shown to be an intrinsic property inherent to the present system which has an XY character. The marginal exponent of the order-parameter correlation function at the KT-like transition point is seen to increase from 1/4 as the ratio J 2 / J 1 decreases, namely, it reveals a deviation from the universal behavior of a genuine XY model.