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J. Oitmaa

Publications and source records attributed to J. Oitmaa.

At least 37 records · Page 2Linked to original sources
arXiv2006

Critical Behaviour of One-particle Spectral Weights in the Transverse Ising Model

We investigate the critical behaviour of the spectral weight of a single quasiparticle, one of the key observables in experiment, for the particular case of the transverse Ising model.Series expansions are calculated for the linear chain and the square and simple cubic lattices. For the chain model, a conjectured exact result is discovered. For the square and simple cubic lattices, series analyses are used to estimate the critical exponents. The results agree with the general predictions of Sachdev.

cond-mat.str-el↗
arXiv2006

The J1-J2 model: First order phase transition versus deconfinement of spinons

We revisit the phase transition from the Néel ordered to a valence bond solid (VBS) state in the two-dimensional $J_1-J_2$ antiferromagnetic Heisenberg model. In the first part we address the question whether or not this transition could be an example of a second order phase transition due to a deconfinement of spinons. We give arguments based on series expansion and spin-wave theory that this is not the case and the transition is most likely first order. The method proposed here to detect first order phase transitions seems to be very sensitive and might be useful in other models as well. In the second part we analyze possible VBS patterns in the magnetically disordered phase based on numerical data for different susceptibilities, obtained in the ordered phase, which test the breaking of lattice symmetries. We conclude that a columnar dimerization pattern is the most likely candidate.

cond-mat.str-el↗
arXiv2005

Hard Core Bosons on the Triangular Lattice at Zero Temperature: A Series Expansion Study

We use high order linked cluster series to investigate the hard core boson model on the triangular lattice, at zero temperature. Our expansions, in powers of the hopping parameter $t$, probe the spatially ordered `solid' phase and the transition to a uniform superfluid phase. At the commensurate fillings $n=1/3, 2/3$ we locate a quantum phase transition point at $(t/V)_c\simeq 0.208(1)$, in good agreement with recent Monte Carlo studies. At half-filling ($n=1/2$) we find evidence for a solid phase, which persists to $t/V\simeq 0.06$.

cond-mat.other↗
arXiv2005

Further Series Studies of the Spin-1/2 Heisenberg Antiferromagnet at T=0: Magnon Dispersion and Structure Factors

We have extended our previous series studies of quantum antiferromagnets at zero temperature by computing the one-magnon dispersion curves and various structure factors for the linear chain, square and simple cubic lattices. Many of these results are new; others are a substantial extension of previous work. These results are directly comparable with neutron scattering experiments and we make such comparisons where possible.

cond-mat.str-el↗
arXiv2005

Magnon and Hole Excitations in the Two-Dimensional Half-filled Hubbard Model

Spin and hole excitation spectra and spectral weights are calculated for the half-filled Hubbard model, as a function of $t/U$. We find that the high energy spin spectra are sensitive to charge fluctuations. The energy difference $Δ(π,0)- Δ(π/2,π/2)$, which is negative for the Heisenberg model, changes sign at a fairly small $t/U\approx 0.053(5)$. The hole bandwidth is proportional to $J$, and considerably larger than in the t-J models. It has a minimum at ($π/2,π/2$) and a very weak dispersion along the antiferromagnetic zone boundary. A good fit to the measured spin spectra in La$_2$CuO$_4$ at $T=10K$ is obtained with the parameter values $U=3.1{\rm eV}$, $t=0.35{\rm eV}$.

cond-mat.str-el↗
arXiv2004

Curie and Neel Temperatures of Quantum Magnets

We estimate, using high-temperature series expansions, the transition temperatures of the spin 1/2, 1 and 3/2 Heisenberg ferromagnet and antiferromagnet in 3-dimensions. The manner in which the difference between Curie and Neel temperatures vanishes with increasing spin quantum number is investigated.

cond-mat.supr-con↗
arXiv2004

Ground state and finite temperature signatures of quantum phase transitions in the half-filled Hubbard model on a honeycomb lattice

We investigate ground state and finite temperature properties of the half-filled Hubbard model on a honeycomb lattice using quantum monte carlo and series expansion techniques. Unlike the square lattice, for which magnetic order exists at T=0 for any non-zero $U$, the honeycomb lattice is known to have a semi-metal phase at small $U$ and an antiferromagnetic one at large $U$. We investigate the phase transition at T=0 by studying the magnetic structure$and compressibility using quantum monte carlo simulations and by calculating the sublattice magnetization, uniform susceptibility, spin-wave and single hole %single-particle dispersion using series expansions around the ordered phase. Our results are consistent with a single continuous transition at $U_c/t$ in the range 4-5. Finite temperature signatures of this phase transition are seen in the behavior of the specific heat, $C(T)$, which changes from a two-peaked structure for $U>U_c$ to a one-peaked structure for $U < U_c$. Furthermore, the $U$ dependence of the low temperature coefficient of $C(T)$ exhibits an anomaly at $U \approx U_c$.

cond-mat.str-el↗
arXiv2004

Order and disorder in the triangular-lattice t-J-V model at 2/3 electron density

Motivated by the recent discovery of superconductivity in Na$_x$CoO$_2\cdot y$H$_2$O, we use series expansion methods and cluster mean-field theory to study spontaneous charge order, Neel order, ferromagnetic order, dimer order and phase-separation in the triangular-lattice t-J-V model at 2/3 electron density. We find that for t<0, the charge ordered state, with electrons preferentially occupying a honeycomb lattice, is very robust. Quite surprisingly, hopping to the third sublattice can even enhance Neel order. At large negative t and small V, the Nagaoka ferromagnetic state is obtained. For large positive t, charge and Neel order vanish below a critical V, giving rise to an itinerant antiferromagnetically correlated state.

cond-mat.str-el↗
arXiv2003

Ferrimagnetism and compensation points in a decorated 3D Ising models

We give a precise numerical solution for decorated Ising models on the simple cubic lattice which show ferromagnetism, compensation points, and reentrant behaviour. The models, consisting of $S={1\over 2}$ spins on a simple cubic lattice, and decorating S=1 or $S=3/2$ spins on the bonds, can be mapped exactly onto the normal spin-1/2 Ising model, whose properties are well known.

cond-mat↗
arXiv2003

A closer look at symmetry breaking in the collinear phase of the $J_1-J_2$ Heisenberg Model

The large $J_2$ limit of the square-lattice $J_1-J_2$ Heisenberg antiferromagnet is a classic example of order by disorder where quantum fluctuations select a collinear ground state. Here, we use series expansion methods and a meanfield spin-wave theory to study the excitation spectra in this phase and look for a finite temperature Ising-like transition, corresponding to a broken symmetry of the square-lattice, as first proposed by Chandra et al. (Phys. Rev. Lett. 64, 88 (1990)). We find that the spectra reveal the symmetries of the ordered phase. However, we do not find any evidence for a finite temperature phase transition. Based on an effective field theory we argue that the Ising-like transition occurs only at zero temperature.

cond-mat.str-el↗
arXiv2003

The Mixed Spin S=(1/2,1) XXZ Ferrimagnet at Zero Temperature

Linked cluster series expansions about the Ising limit are used to study ground state preperties, viz. ground state energy, magnetization and excitation spectra, for mixed spin S=(1/2,1) quantum ferrimagnets on simple bipartite lattices in 1, 2, and 3-dimensions. Results are compared to second-order spin wave theory and, in general, excellent agreement is obtained.

cond-mat.str-el↗
arXiv2002

Zero Temperature Series Expansions for the Kondo Lattice Model

We present new results for the Kondo lattice model of strongly correlated electrons, in 1-, 2-, and 3-dimensions, obtained from high-order linked-cluster series expansions. Results are given for varies ground state properties at half-filling, and for spin and charge excitations. The existence and nature of the predicted quantum phase transition are explored.

cond-mat.str-el↗
arXiv2001

Series Expansions for Variable Electron Density

We have developed a series expansion method for calculating the zero-temperature properties of lattice electron models for variable electron density, i.e. for finite doping away from the half-filled case. This is done by introducing particle fluctuation terms in both the unperturbed Hamiltonian and perturbation. The method is demonstrated by application to the 2-chain $t-J$ ladder, where we provide comparison with previous work as well as obtaining a number of new results.

cond-mat.str-el↗
arXiv2001

Critical dynamics of singlet and triplet excitations in strongly frustrated spin systems

New data for the two-dimensional J1-J2 model shows that the critical points for singlet and triplet excitations near J2/J1=0.38 are very close together, or coincident. We propose that this is a more general result: in strongly frustrated spin systems the critical dynamics of singlet and triplet excitations are closely related. An effective field theory for such a critical point, or points, is developed, and its implications discussed. The analysis suggests the existance of gapless or almost gapless singlet excitations in the magnetically disordered phase.

cond-mat.str-el↗
arXiv2001

Realization of a large J_2 quasi-2D spin-half Heisenberg system: Li2VOSiO4

Exchange couplings are calculated for Li2VOSiO4 using LDA. While the sum of in-plane couplings J_1 + J_2 = 9.5 \pm 1.5 K and the inter-plane coupling J_{perp} \sim 0.2 - 0.3 K agree with recent experimental data, the ratio J_2/J_1 \sim 12 exceeds the reported value by an order of magnitude. Using geometrical considerations, high temperature expansions and perturbative mean field theory, we show that the LDA derived exchange constants lead to a remarkably accurate description of the properties of these materials including specific heat, susceptibility, Neel temperature and NMR spectra.

cond-mat.str-el↗
arXiv2001

The Phase Diagram of the Shastry-Sutherland Antiferromagnet

The Shastry-Sutherland model, which provides a representation of the magnetic properties of SrCu$_2$(BO$_3$)$_2$, has been studied by series expansion methods at T=0. Our results support the existence of an intermediate phase between the Néel long-range ordered phase and the short-range dimer phase. They provide strong evidence against the existence of helical order in the intermediate phase, and and somewhat less strong evidence against a plaquette-singlet phase. The nature of the intermediate phase thus remains elusive. It appears to be gapless, or nearly so.

cond-mat.str-el↗
arXiv2001

Two-dimensional randomly-frustrated spin 1/2 Heisenberg model

We investigate the properties of S=1/2 Heisenberg clusters with random frustration using exact diagonalizations. This is a model for a quantum spin glass. We show that the average ground state spin is $S \propto \sqrt{N}$, where N is the number of sites. We also calculate the magnetic susceptibility and the spin stiffness and low-energy excitations, and discuss these in terms of a semiclassical picture.

cond-mat.str-el↗