Specific Heat of the Triangular Lattice t-J Model

We perform exact diagonalization (ED) to calculate the specific heat \(C\) of the triangular lattice t-J model, defined by the Hamiltonian:

\[ \mathcal{H} = -t \sum_{\langle i,j \rangle, \sigma} P\left(c^\dagger_{i\sigma}c_{j\sigma} + \text{h.c.}\right) + J\sum_{\langle i,j \rangle}\left(\boldsymbol{S}_i\cdot \boldsymbol{S}_j - \frac{1}{4}n_i n_j\right), \]

where \(P\) is the projection operator onto the space of non-doubly occupied sites, \(c^\dagger_{i\sigma}\) creates an electron at site \(i\) with spin \(\sigma\), and the spin operators are defined as \(\boldsymbol{S}_i=\frac{1}{2} c^\dagger_{i\alpha}\boldsymbol{\sigma}_{\alpha\beta}c_{i\beta}\). The sums \(\langle i,j \rangle\) run over nearest-neighbor bonds on a triangular lattice. This model has been studied extensively due to its relevance for correlated electron systems and frustrated magnetism, particularly in the context of triangular lattice compounds and high-temperature superconductors [1].

Lattice Geometry

In this example, we consider an 11-site triangular lattice (arranged in a 4-4-3 geometry) with one hole in the system. The number of electrons is thus \(N_{\text{sites}} - 1\) (with equal numbers of spin-up and spin-down electrons).

Simulation Overview

• Model Parameters:
• Hopping parameter: \(t = 1.0\)
• Spin interaction parameter: \(J = 0.4\)
• System Parameters:
• Total lattice sites: \(N_{\text{sites}} = 11\)
• Electrons: 10 (5 spin‑up, 5 spin‑down)
• Specific Heat Calculation:
  For a given temperature \(T\), we compute the Boltzmann weights $$ w_i = \exp\left(-\frac{E_i}{T}\right), $$ where the eigenvalues \(\{E_i\}\) are obtained from exact diagonalization of the t‑J Hamiltonian. The partition function is $$ Z = \sum_i w_i, $$ and the average energy is given by $$ \langle E \rangle = \frac{1}{Z}\sum_i E_i\,w_i. $$ The specific heat is then calculated using the relation $$ C(T) = \frac{1}{T^2}\left(\frac{1}{Z}\sum_i E_i^2\,w_i - \langle E \rangle^2\right). $$

The figure below shows the specific heat as a function of \(T/J\):

Code

Julia

using XDiag                # Package for exact diagonalization
using LinearAlgebra        # Standard library for linear algebra operations
using GLMakie              # Package for interactive plotting
using LaTeXStrings         # Package for LaTeX formatted strings in plots

function main()
    # Model parameters
    Nsites = 11                         # Total number of lattice sites
    electrons = Nsites - 1              # Number of electrons (one hole in the system)
    nup = electrons ÷ 2                 # Number of spin-up electrons
    ndn = electrons - nup               # Number of spin-down electrons
    println("Triangular t-J Model: Nsites=$Nsites, nup=$nup, ndn=$ndn (holes=$(Nsites-electrons))")

    # Define nearest-neighbor bonds for the 11-site triangular lattice (3-4-4 geometry)
    bonds = [
        (1, 2), (2, 3),
        (1, 4), (1, 5), (2, 5), (2, 6), (3, 6), (3, 7),
        (4, 5), (5, 6), (6, 7),
        (4, 8), (5, 8), (5, 9), (6, 9), (6, 10), (7, 10), (7, 11),
        (8, 9), (9, 10), (10, 11)
    ]

    # Build the Hamiltonian using the t-J model parameters
    t = 1.0   # Hopping parameter
    J = 0.4   # Interaction parameter
    H = OpSum()
    for (i, j) in bonds
        H += J * Op("SdotS", [i, j]) + t * Op("Hop", [i, j])
    end

    # Define the block structure for the Hamiltonian with+out any symmetry
    block = tJ(Nsites, nup, ndn)
    Hmat = matrix(H, block)

    # Perform ED
    evals = eigvals(Hermitian(Hmat))

    # Define temperature range 
    Temp = LinRange(0.01, 1.0, 64)
    C = zeros(Float64, length(Temp)) 

    # Calculate specific heat at each temperature
    for (idx, T) in enumerate(Temp)
        w = exp.(-evals ./ T)         # Boltzmann factors
        Z = sum(w)                    # Partition function
        E = sum(evals .* w) / Z       # Average energy
        C[idx] = (sum(evals.^2 .* w) / Z - E^2) / T^2  # Specific heat
    end

    # Create the figure
    fig = Figure(size = (800, 600))

    ax = Axis(fig[1, 1],
        xlabel = L"T/J",
        ylabel = L"\text{Specific Heat}",
        title = L"\text{Triangular t-J Model (11-site)}",
        xticklabelsize = 22,
        yticklabelsize = 22,
        xlabelsize = 22,
        ylabelsize = 22,
        titlegap = 14,
        titlesize = 22
    )

    # Plot 
    scatterlines!(ax, Temp ./ J, C, markersize = 10, color = :blue)

    # Save the figure
    save("specific_heat_tJ_triangular.png", fig)

end

main()  

References

[1]
J. O. Haerter, M. R. Peterson, and B. S. Shastry, Finite temperature properties of the triangular lattice t-J model, PRB 74, 245118 (2006).