arXiv subjects

C. J. Hamer

Publications and source records attributed to C. J. Hamer.

At least 19 recordsLinked to original sources

arXiv2011

Proceedings of the 35th Annual Australian/New Zealand Condensed Matter and Materials Meeting

The 35th Australian/New Zealand Annual Condensed Matter and Materials Meeting was held at the Charles Sturt University campus in Wagga Wagga, NSW, Australia from the 1st to the 4th of February 2011. The conference was attended by 92 delegates from a range of universities across Australia, New Zealand and further afield. There were a total of 9 invited and 21 contributed talks during the three days of scientific sessions, as well as 2 poster sessions with a total of 49 poster presentations. All presenters were invited to submit a manuscript for publication in the conference proceedings. The length limits where six pages for invited papers and four pages for contributed papers. Each manuscript was reviewed by two anonymous referees and 18 papers were accepted for publication. The accepted manuscripts are also available at the online publication section of the Australian Institute of Physics national web site (http://www.aip.org.au/).

cond-mat.mtrl-sci↗

arXiv2011

A Frustrated 3-Dimensional Antiferromagnet: Stacked $J_{1}-J_{2}$ Layers

We study a frustrated 3D antiferromagnet of stacked $J_1 - J_2$ layers. The intermediate 'quantum spin liquid' phase, present in the 2D case, narrows with increasing interlayer coupling and vanishes at a triple point. Beyond this there is a direct first-order transition from N{\' e}el to columnar order. Possible applications to real materials are discussed.

cond-mat.str-el↗

arXiv2010

The Quantum Compass Model on the Square and Simple Cubic Lattices

We use high-temperature series expansions to obtain thermodynamic properties of the quantum compass model, and to investigate the phase transition on the square and simple cubic lattices. On the square lattice we obtain evidence for a phase transition, consistent with recent Monte Carlo results. On the simple cubic lattice the same procedure provides no sign of a transition, and we conjecture that there is no finite temperature transition in this case.

cond-mat.str-el↗

arXiv2009

The Fate of the Two-Magnon Bound State in the Heisenberg Antiferromagnet

The energy spectrum of the two-magnon bound states in the Heisenberg-Ising antiferromagnet on the square lattice are calculated using series expansion methods. The results confirm an earlier spin-wave prediction of Oguchi and Ishikawa, that the bound states vanish into the continuum before the isotropic Heisenberg limit is reached.

cond-mat.stat-mech↗

arXiv2008

Critical Behaviour of Structure Factors at a Quantum Phase Transition

We review the theoretical behaviour of the total and one-particle structure factors at a quantum phase transition for temperature T=0. The predictions are compared with exact or numerical results for the transverse Ising model, the alternating Heisenberg chain, and the bilayer Heisenberg model. At the critical wavevector, the results are generally in accord with theoretical expectations. Away from the critical wavevector, however, different models display quite different behaviours for the one-particle residues and structure factors.

cond-mat.stat-mech↗

arXiv2008

Two-particle bound states and one-particle structure factor in a Heisenberg bilayer system

The S=1/2 Heisenberg bilayer spin model at zero temperature is studied in the dimerized phase using analytic triplet-wave expansions and dimer series expansions. The occurrence of two-triplon bound states in the S=0 and S=1 channels, and antibound states in the S=2 channel, is predicted by the triplet-wave theory, and confirmed by series expansions. All bound states are found to vanish at or before the critical coupling separating the dimerized phase from the Neel phase. The critical behaviour of the total and single-particle static transverse structure factors is also studied by series, and found to conform with theoretical expectations. The single-particle state dominates the structure factor at all couplings.

cond-mat.str-el↗

arXiv2008

The Spin-1 Heisenberg Antiferromagnet: New Results from Series Expansions

We calculate ground state properties (energy, magnetization, susceptibility) and one-particle spectra for the $S = 1$ Heisenberg antiferromagnet with easy-axis or easy-plane single site anisotropy, on the square lattice. Series expansions are used, in each of three phases of the system, to obtain systematic and accurate results. The location of the quantum phase transition in the easy-plane sector is determined. The results are compared with spin-wave theory.

cond-mat.other↗

arXiv2006

A modified triplet-wave expansion method applied to the alternating Heisenberg chain

An alternative triplet-wave expansion formalism for dimerized spin systems is presented, a modification of the 'bond operator' formalism of Sachdev and Bhatt. Projection operators are used to confine the system to the physical subspace, rather than constraint equations. The method is illustrated for the case of the alternating Heisenberg chain, and comparisons are made with the results of dimer series expansions and exact diagonalization. Some discussion is included of the phenomenon of 'quasiparticle breakdown', as it applies to the two-triplon bound states in this model.

cond-mat.str-el↗

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↗

arXiv2005

The t-Jz ladder

The phase diagram of the two-leg t-Jz ladder is explored, using the density matrix renormalization group method. Results are obtained for energy gaps, electron density profiles and correlation functions for the half-filled and quarter-filled cases. The effective Lagrangian velocity parameter is shown to vanish at half-filling. The behaviour of the one-hole gap in the Nagaoka limit is investigated, and found to disagree with theoretical predictions. A tentative phase diagram is presented, which is quite similar to the full t-J ladder, but scaled up by a factor of about two in coupling. Near half-filling a Luther-Emery phase is found, which may be expected to show superconducting correlations, while near quarter-filling the system appears to be in a Tomonaga-Luttinger phase.

cond-mat.stat-mech↗

arXiv2005

The Union Jack model: a quantum spin model with frustration on the square lattice

A new quantum spin model with frustration, the `Union Jack' model on the square lattice, is analyzed using spin-wave theory. For small values of the frustrating coupling $α$, the system is N{\' e}el ordered as usual, while for large $α$ the frustration is found to induce a canted phase. The possibility of an intermediate spin-liquid phase is discussed.

cond-mat.str-el↗

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

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

The Hamiltonian limit of (3+1)D SU(3) lattice gauge theory on anisotropic lattices

The extreme anisotropic limit of Euclidean SU(3) lattice gauge theory is examined to extract the Hamiltonian limit, using standard path integral Monte Carlo (PIMC) methods. We examine the mean plaquette and string tension and compare them to results obtained within the Hamiltonian framework of Kogut and Susskind. The results are a significant improvement upon previous Hamiltonian estimates, despite the extrapolation procedure necessary to extract observables. We conclude that the PIMC method is a reliable method of obtaining results for the Hamiltonian version of the theory. Our results also clearly demonstrate the universality between the Hamiltonian and Euclidean formulations of lattice gauge theory. It is particularly important to take into account the renormalization of both the anisotropy, and the Euclidean coupling $ β_E $, in obtaining these results.

hep-lat↗

arXiv2003

An Application of Feynman-Kleinert Approximants to the Massive Schwinger Model on a Lattice

A trial application of the method of Feynman-Kleinert approximants is made to perturbation series arising in connection with the lattice Schwinger model. In extrapolating the lattice strong-coupling series to the weak-coupling continuum limit, the approximants do not converge well. In interpolating between the continuum perturbation series at large fermion mass and small fermion mass, however, the approximants do give good results. In the course of the calculations, we picked up and rectified an error in an earlier derivation of the continuum series coefficients.

hep-lat↗

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↗

arXiv2002

Spectral weight contributions of many-particle bound states and continuum

Cluster expansion methods are developed for calculating the spectral weight contributions of multiparticle excitations - continuum and bound states - to high orders. A complete 11th order calculation is carried out for the alternating Heisenberg chain. For $λ=0.27$, relevant to the material $Cu(NO_3)_2.2.5D_2O$, we present detailed spectral weights for the two-triplet continuum and all bound states. We also examine variation of the relative weights of one and two-particle states with bond alternation from the dimerized to the uniform chain limit.

cond-mat.str-el↗