arXiv subjects

D. C. Johnston

Publications and source records attributed to D. C. Johnston.

117 records · Page 7Linked to original sources

arXiv2000

Thermodynamics of Spin S = 1/2 Antiferromagnetic Uniform and Alternating-Exchange Heisenberg Chains

The magnetic susceptibility chi and specific heat C versus temperature T of the spin-1/2 antiferromagnetic alternating-exchange (J1 and J2) Heisenberg chain are studied for the entire range 0 \leq alpha \leq 1 of the alternation parameter alpha = J2/J1. For the uniform chain (alpha = 1), detailed comparisons of the high-accuracy chi(T) and C(T) Bethe ansatz data of Kluemper and Johnston are made with the asymptotically exact low-T field theory predictions of Lukyanov. QMC simulations and TMRG calculations of chi(alpha,T) are presented. From the low-T TMRG data, the spin gap Delta(alpha)/J1 is extracted for 0.8 \leq alpha \leq 0.995. High accuracy fits to all of the above numerical data are obtained. We examine in detail the theoretical predictions of Bulaevskii for chi(alpha,T) and compare them with our results. Our experimental chi(T) and C(T) data for NaV2O5 single crystals are modeled in detail. The chi(T) data above the spin dimerization temperature Tc = 34 K are not in agreement with the prediction for the uniform Heisenberg chain, but can be explained if there is a moderate ferromagnetic interchain coupling and/or if J changes with T. By fitting the chi(T) data, we obtain Delta(T = 0) = 103(2) K, alternation parameter delta(0) = (1 - alpha)/(1 + alpha) = 0.034(6) and average exchange constant J(0) = 640(80) K. The delta(T) and Delta(T) are derived from the data. A spin pseudogap with a large magnitude \approx 0.4 Delta(0) is consistently found just above Tc, which decreases with increasing T. Analysis of our C(T) data indicates that at Tc, at least 77% of the entropy change due to the transition at Tc and associated order parameter fluctuations arise from the lattice and/or charge degrees of freedom and less than 23% from the spin degrees of freedom.

cond-mat.str-el↗

arXiv2000

Modeling of the Magnetic Susceptibilities of the Ambient- and High-Pressure Phases of (VO)_{2}P_{2}O_{7}

The magnetic susceptibilities chi versus temperature T of powders and single crystals of the ambient-pressure (AP) and high-pressure (HP) phases of (VO)_{2}P_{2}O_{7} are analyzed using an accurate theoretical prediction of chi(T, J1, J2) for the spin-1/2 antiferromagnetic alternating-exchange (J1, J2) Heisenberg chain. The results are consistent with recent models with two distinct types of alternating-exchange chains in the AP phase and a single type in the HP phase. The spin gap for each type of chain is derived from the respective set of two fitted alternating exchange constants and the one-magnon dispersion relation for each of the two types of chains in the AP phase is predicted. The influences of interchain coupling on the derived intrachain exchange constants, spin gaps, and dispersion relations are estimated using a mean-field approximation for the interchain coupling. The accuracies of the spin gaps obtained using fits to the low-T chi(T) data by theoretical low-T approximations are determined. The results of these studies are compared with previously reported estimates of the exchange couplings and spin gaps in the AP and HP phases and with the magnon dispersion relations in the AP phase measured previously using inelastic neutron scattering.

cond-mat.str-el↗

arXiv2000

Thermodynamics of the Spin-1/2 Antiferromagnetic Uniform Heisenberg Chain

We present a new application of the traditional thermodynamic Bethe ansatz to the spin-1/2 antiferromagnetic uniform Heisenberg chain and derive exact nonlinear integral equations for just {\em two} functions describing the elementary excitations. Using this approach the magnetic susceptibility $χ$ and specific heat C versus temperature T are calculated to high accuracy for $5\times10^{-25}\leq T/J\leq 5$. The $χ(T)$ data agree very well at low T with the asymptotically exact theoretical low-T prediction of S. Lukyanov, Nucl. Phys. B {\bf 522}, 533 (1998). The unknown coefficients of the second and third lowest-order logarithmic correction terms in Lukyanov's theory for C(T) are estimated from the C(T) data.

cond-mat↗

arXiv2000

Magnetic Susceptibilities of Spin-1/2 Antiferromagnetic Heisenberg Ladders and Applications to Ladder Oxide Compounds

A comprehensive theoretical and experimental study is presented of the magnetic susceptibility versus temperature χ(T) of spin S = 1/2 two- and three-leg Heisenberg ladders and ladder oxide compounds. Extensive quantum Monte Carlo simulations of χ(T) were carried out for both isolated and coupled two-leg ladders with spatially anisotropic intraladder exchange. Accurate fits to these and related literature QMC data were obtained. We have also calculated the one- and two-magnon dispersion relations and the dynamical spin structure factor for anisotropic isolated 2 x 12 ladders. The exchange constants in the two-leg ladder compound SrCu2O3 are estimated from LDA+U calculations. We report the detailed crystal structure of SrCu2O3 and of the three-leg ladder compound Sr2Cu3O5. New experimental χ(T) data are reported for the two-leg ladder cuprates SrCu2O3 and LaCuO_{2.5}, and for the (nominally) two-leg ladder vanadates CaV2O5 and MgV2O5. The new and literature χ(T) data for these compounds and for Sr2Cu3O5 are modeled using our QMC χ(T) simulation fits, and the exchange coupling constants between the spins-1/2 are thereby estimated for each material. The surpisingly strong spatial anisotropy of the bilinear intraladder exchange constants in the cuprate compounds is discussed together with the results of other experiments sensitive to this anisotropy. Recent theoretical predictions are discussed including those which indicate that a four-spin cyclic exchange interaction within a Cu4 plaquette is important to determining the magnetic properties and which can significantly influence the exchange interactions estimated from χ(T) data assuming the presence of only bilinear exchange.

cond-mat.str-el↗

arXiv1999

Heavy Fermion Behaviors in LiV_{2}O_{4}

Experiments of various types on the metallic transition metal oxide compound LiV_{2}O_{4} with the fcc normal-spinel structure are reviewed. The low-temperature T < 10 K data consistently indicate heavy fermion (HF) behaviors characteristic of those of the heaviest-mass f-electron HF compounds. A crossover is observed above about 50 K in the magnetic susceptibility, ^{7}Li spin-lattice relaxation rate and neutron magnetic scattering function to a magnetic state variously described as an antiferromagnetically-coupled local-moment metal or, at the opposite extreme, as an itinerant nearly-ferromagnetic metal. Recent theoretical investigations to understand these properties of LiV_{2}O_{4} are discussed.

cond-mat.str-el↗

arXiv1999

Spin dynamics in hole-doped two-dimensional S=1/2 Heisenberg antiferromagnets: ^{63}Cu NQR relaxation in La_{2-x}Sr_xCuO_4 for $x\leq 0.04$

The effects on the correlated Cu^{2+} S = 1/2 spin dynamics in the paramagnetic phase of La_{2-x}Sr_xCuO_4 (for $x \lesssim 0.04$) due to the injection of holes are studied by means of ^{63}Cu NQR spin-lattice relaxation time T_1 measurements. The results are discussed in the framework of the connection between T_1 and the in-plane magnetic correlation length $ξ_{2D}(x,T)$. It is found that at high temperatures the system remains in the renormalized classical regime, with a spin stiffness constant $ρ_s(x)$ reduced by small doping to an extent larger than the one due to Zn doping. For $x\gtrsim 0.02$ the effect of doping on $ρ_s(x)$ appears to level off. The values for $ρ_s(x)$ derived from T_1 for $T\gtrsim 500$ K are much larger than the ones estimated from the temperature behavior of sublattice magnetization in the ordered phase ($T\leq T_N$). It is argued that these features are consistent with the hypothesis of formation of stripes of microsegregated holes.

cond-mat.str-el↗

arXiv1998

Synthesis, Characterization and Magnetic Susceptibility of the Heavy Fermion Transition Metal Oxide LiV_{2}O_{4}

The preparative method, characterization and magnetic susceptibility χmeasurements versus temperature T of the heavy fermion transition metal oxide LiV_{2}O_{4} are reported in detail. The intrinsic χ(T) shows a nearly T-independent behavior below ~ 30 K with a shallow broad maximum at about 16 K, whereas Curie-Weiss-like behavior is observed above 50-100 K. Field-cooled and zero-field-cooled magnetization M measurements in applied magnetic fields H = 10 to 100 G from 1.8 to 50 K showed no evidence for spin-glass ordering. Crystalline electric field theory for an assumed cubic V point group symmetry is found insufficient to describe the observed temperature variation of the effective magnetic moment. The Kondo and Coqblin-Schrieffer models do not describe the magnitude and T dependence of χwith realistic parameters. In the high T range, fits of χ(T) by the predictions of high temperature series expansion calculations provide estimates of the V-V antiferromagnetic exchange coupling constant J/k_{B} ~ 20 K, g-factor g ~ 2 and the T-independent susceptibility. Other possible models to describe the χ(T) are discussed. The paramagnetic impurities in the samples were characterized using isothermal M(H) measurements with 0 < H <= 5.5 Tesla at 2 to 6 K. These impurities are inferred to have spin S_{imp} ~ 3/2 to 4, g_{imp} ~ 2 and molar concentrations of 0.01 to 0.8 %, depending on the sample.

cond-mat.str-el↗

arXiv1998

Specific Heat (1.2-108 K) and Thermal Expansion (4.4-297 K) Measurements of the 3d Heavy Fermion Compound LiV_{2}O_{4}

Specific heat C_{p}(T) measurements of the heavy fermion normal-spinel structure compound LiV_{2}O_{4} were carried out using a heat-pulse calorimeter over the temperature T range from 1.2 to 108 K. The electronic specific heat C_{e}(T) of LiV_{2}O_{4} is extracted from the C_{p}(T) data using the lattice contribution obtained for LiTi_{2}O_{4}, a superconductor with T_{c} = 11.8 K. The electronic specific heat coefficient γ(T) = C_{e}(T)/T of LiV_{2}O_{4} is found to be 0.42 and 0.43 J/mol K^2 at T = 1 K for two different high magnetic purity samples, respectively. γ(T) decreases rapidly with increasing temperature from 4 to 30 K and then decreases much more slowly from 0.13 J/mol K^2 at 30 K to 0.08 J/mol K^2 at 108 K. The C_{e}(T) is compared with theoretical predictions for the spin S = 1/2 Kondo model, a generic Fermi liquid model, and an antiferromagnetically coupled quantum-disordered metal. Each of these theories can adequately describe C_{e}(T) in the Fermi liquid regime at low (~ 1 to 10 K) temperatures, consistently yielding a large extrapolated γ(0) = 428(3) mJ/mol K^2. However, none of these theories describes C_{e}(T) from ~ 10 K to 108 K. Our C_{e}(T) data are also in severe disagreement with the magnetic specific heat of the spin S = 1/2 Heisenberg model. Thermal expansion measurements of LiV_{2}O_{4} were carried out from 4.4 to 297 K using a differential capacitance dilatometer. Strong increases in the thermal expansion coefficient and Grüneisen parameter Γare found below \~ 20 K, confirming the results of Chmaissem et al. [Phys. Rev. Lett. 79, 4866 (1997)] obtained using neutron diffraction. We estimate that Γ(0) is about 11.4.

cond-mat.str-el↗

arXiv1995

Intercalation and Staging Behavior in Super-Oxygenated $La_2CuO_{4 + δ}$

A high temperature electrochemical oxidation process has been used to produce large single crystals of $La_2CuO_{4 + δ}$ suitable for neutron scattering experiments. Below room temperature the oxygen-rich phases have structural superlattice scattering peaks which indicate new periodicities ranging from 2 to 6.6 layers perpendicular to the copper oxide planes. A model structure originally proposed for $La_2NiO_{4 + δ}$ can account for the superlattice peaks as a result of anti-phase domain boundaries between different tilt directions of the CuO$_6$ octahedra. Within this model, the changes in CuO$_6$ tilt directions are induced by segregated layers of interstitial oxygen which order in a manner similar to intercalants in graphite. This structural model thus clarifies previous work and establishes $La_2CuO_{4 + δ}$ as a unique lamellar superconducting system with annealed disorder.

cond-mat↗