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Physical review. B, Condensed matter

Thermodynamics of the two-dimensionalt-Jmodel

Heinrich Röder, Holger Fehske, V. Waas, H. Büttner

Published Jun 1, 1992• 4 citations
Publisher

Abstract

We present results for the thermodynamics of the two-dimensional t-J model on finite square lattices in the grand canonical ensemble. The specific heat has a double-peak structure independent of J. The susceptibility shows antiferromagnetic behavior above a critical value ${\mathit{J}}_{\mathit{c}}$ where for J${\mathit{J}}_{\mathit{c}}$ the ground state of the one-hole sector has maximal spin. We give evidence that phase separation observed in the ground state of the t-J model at large J may be suppressed at any finite, nonzero T.

DOI
10.1103/physrevb.45.13092

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Physical review. B, Condensed matter•Mar 1, 1993•81 citations
We present results for the thermodynamics of the two-dimensional t-J model on finite square lattices in the grand canonical ensemble. The specific heat has a double-peak structure independent of J. The susceptibility shows antiferromagnetic behavior above a critical value Jc{\mathit{J}}_{\mathit{c}}Jc​ where for JJc{\mathit{J}}_{\mathit{c}}Jc​ the ground state of the one-hole sector has maximal spin. We give evidence that phase separation observed in the ground state of the t-J model at large J may be suppressed at any finite, nonzero T.
The two-dimensional spin-1/2 XY model is investigated via an extensive quantum Monte Carlo simulation on square lattices as large as 128\ifmmode\times\else\texttimes\fi{}128. The transverse susceptibility and correlation length show precise thermodynamic scaling behaviors of a phase transition of the Kosterlitz-Thouless type at kTc{\mathit{kT}}_{\mathit{c}}kTc​/J=0.353(3). The correlation functions near the transition point exhibit a universal scaling behavior. Various critical exponents are determined and our results demonstrate that the universality class of the two-dimensional quantum system with continuous symmetry is the same as its classical analog. The specific heat exhibits a finite peak around kT/J=0.45, decreases very rapidly near Tc{\mathit{T}}_{\mathit{c}}Tc​, and falls off as T2{\mathit{T}}^{2}T2 near T=0, validating the spin-wave treatment. The ground-state energy is -0.5543(1)J per spin.
The occurrance of ferromagnetism for the t-J model on a square lattice is investigated using high-temperature series for the Helmholtz free energy F and the uniform magnetic susceptibility ${\mathrm{\ensuremath{\chi}}}_{0}$. For J=0 fourteenth-order series have been calculated for F and ${\mathrm{\ensuremath{\chi}}}_{0}$ and we estimate F(T=0) to find the ground-state energy. A region of fully polarized ferromagnetism is found for J0. A ferrimagnetic region is observed for larger J/t where ${\mathrm{\ensuremath{\chi}}}_{0}$ is divergent, but F(T=0) is below that of spinless fermions. Our results do not support Nagaoka's theorem at a thermodynamic density of holes.
Abstract We study the temperature dependence of the thermodynamic properties of spin-1/2 antiferromagnets on two-dimensional lattices. Our analysis employs the sine-square deformation (SSD), in which a real-space envelope function is applied to the Hamiltonian so that the local energy scale is smoothly reduced to zero at the system boundaries. The quantum eigenstates of the SSD Hamiltonian exhibit bulk-like behavior near the system center, effectively mimicking the thermodynamic limit even in small finite-size calculations. Using these fictitious bulk states, we compute the energy density, specific heat, and magnetic susceptibility as functions of temperature. We find that both the triangular- and kagome-lattice antiferromagnets show either a shoulder or a pronounced double-peak structure in the low-temperature specific heat, whereas the kagome case particularly shows a strong enhancement of magnetic susceptibility down to the lowest temperature range. These direct comparisons, together with the square-lattice and one-dimensional cases, reveal that although both frustrated systems retain a substantial amount of entropy, the low-energy excitations below ∼ 0.5 J ∼0.5J\sim 0.5J∼0.5J of the kagome lattice are predominantly governed by the magnetic excited states, whereas not much for the triangular lattice.
The ground state of the two-dimensional (2D) t-J model is studied by exact diagonalization for seven different unit cells with N=10, 16, 18, 20, 26, 32, and 36 sites on a square lattice. In the limit of low electron densities (up to six electrons) we calculate the lowest eigenvalues and classify the ground state according to wave vector K, total spin S, and point-group symmetry both in the limit J=0 (U=\ensuremath{\infty} Hubbard model) and for finite J>0. The transition to a completely phase-separated state for J>JPS{\mathit{J}}_{\mathrm{PS}}JPS​(N) is examined by calculating the free energy as a function of the electron concentration. We find that JPS{\mathit{J}}_{\mathrm{PS}}JPS​(N) increases with the lattice size N. An extrapolation of our finite-size results to the thermodynamic limit gives JPS{\mathit{J}}_{\mathrm{PS}}JPS​(\ensuremath{\infty})/t=4.1 (\ifmmode\pm\else\textpm\fi{}0.1).
We investigate the thermodynamics of the one-dimensional $t\ensuremath{-}J$ model using transfer matrix renormalization group algorithms and present results for quantities like particle number, specific heat, spin susceptibility, and compressibility. Based on these results we confirm a phase diagram consisting of a Tomonaga-Luttinger liquid (TLL) phase for small J/tJ/tJ/t and a phase-separated state for J/tJ/tJ/t large. Close to phase separation we find a spin-gap (Luther-Emery) phase at low densities consistent with predictions by other studies. At the supersymmetric point we compare our results with exact results from the Bethe ansatz and find excellent agreement. In particular we focus on the calculation of correlation lengths and static correlation functions and study the crossover from the nonuniversal high-TTT lattice into the quantum critical regime. At the supersymmetric point we compare in detail with predictions by conformal field theory and TLL theory and show the importance of logarithmic corrections.
Hole-density correlation functions in the t-J model on lattices with square geometry are studied by the method of exact diagonalization for systems of various sizes with up to 26 sites. For the largest sizes we employ only a restricted basis, whereby for the extrapolation we introduce and test a scaling procedure. For two holes in an antiferromagnet our results confirm their binding for J>Jc{\mathit{J}}_{\mathit{c}}Jc​\ensuremath{\sim}0.2t. For four holes the two-point and four-point density correlations, calculated for N=18,20 sites, show clear evidence for hole-clustering instabilities for J>Js{\mathit{J}}_{\mathit{s}}Js​\ensuremath{\sim}1.5t; a first instability is likely to be towards a striped phase, with holes forming domain walls along the (0,1) or (1,0) direction. Signs for the phase separation into a hole-rich and a hole-free phase appear only at J>Js∗{\mathit{J}}_{\mathit{s}}^{\mathrm{*}}Js∗​\ensuremath{\sim}2.5t. Results in the intermediate regime 0.4J/t1.2 are consistent with the pairing picture, although there are also indications for a large weight of configurations representing domain walls along the (1,1) direction.