Luca Capriotti

Long-Range Néel Order in the Triangular Heisenberg Model

We have studied the Heisenberg model on the triangular lattice using several Quantum Monte Carlo (QMC) techniques (up to 144 sites), and exact diagonalization (ED) (up to 36 sites). By studying the spin gap as a function of the system size we have obtained a robust evidence for a gapless spectrum, confirming the existence of long range Neel order. Our best estimate is that in the thermodynamic limit the order parameter m= 0.41 +/- 0.02 is reduced by about 59% from its classical value and the ground state energy per site is e0=-0.5458 +/- 0.0001 in unit of the exchange coupling. We have identified the important ground state correlations at short distance.

Superconductivity in the two-dimensional t-J model

Using computational techniques, it is shown that pairing is a robust property of hole-doped antiferromagnetic insulators. In one dimension and for two-leg ladder systems, a BCS-like variational wave function with long-bond spin singlets and a Jastrow factor provides an accurate representation of the ground state of the t-J model, even though strong quantum fluctuations destroy the off-diagonal superconducting long-range order in this case. However, in two dimensions it is argued-and numerically confirmed using several techniques, especially quantum Monte Carlo-that quantum fluctuations are not strong enough to suppress superconductivity.

Wave-vector power spectrum of the local tunneling density of states: Ripples in a d -wave sea

A weak scattering potential imposed on a

Algorithmic differentiation and the calculation of forces by quantum Monte Carlo.

{\mathrm{CuO}}_{2}

Ising transition driven by frustration in a 2D classical model with continuous symmetry

layer of a cuprate superconductor modulates the local density of states

Mott metal-insulator transition in the half-filled Hubbard model on the triangular lattice

N(x,\ensuremath{\omega}).

Thermodynamics of the quantum easy-plane antiferromagnet on the triangular lattice

In recently reported experimental studies, scanning-tunneling maps of

Low-Temperature Broken-Symmetry Phases of Spiral Antiferromagnets

N(x,\ensuremath{\omega})

Ising transition in the two-dimensional quantum J1 − J2 Heisenberg model

have been Fourier transformed to obtain a wave-vector power spectrum. Here, for the case of a weak scattering potential, we discuss the structure of this power spectrum and its relationship to the quasiparticle spectrum and the structure factor of the scattering potential. Examples of quasiparticle interferences in normal metals and s- and d-wave superconductors are discussed.

Finite-size spin-wave theory of the triangular Heisenberg model

We describe an efficient algorithm to compute forces in quantum Monte Carlo using adjoint algorithmic differentiation. This allows us to apply the space warp coordinate transformation in differential form, and compute all the 3M force components of a system with M atoms with a computational effort comparable with the one to obtain the total energy. Few examples illustrating the method for an electronic system containing several water molecules are presented. With the present technique, the calculation of finite-temperature thermodynamic properties of materials with quantum Monte Carlo will be feasible in the near future.