Quantum magnetism and quantum phase transitions

Abstract:

We have investigated the optimum conditions for the growth of Na xCoO2 single crystals by the optical floating-zone technique. A significant reduction in Na loss, was achieved by using a high-pressure growth atmosphere. A high-pressure oxygen-rich atmosphere during crystal growth was found to reduce the presence of CO3O4 impurity phase in the grown crystal. The magnetic properties of powder and single crystal samples were measured by superconducting quantum interference device (SQUID) magnetometry and found to vary according to the preparation conditions. © 2004 Elsevier B.V. All rights reserved.

Abstract:

We present the temperature dependence of the uniform susceptibility of spin-half quantum antiferromagnets on spatially anisotropic triangular-lattices, using high temperature series expansions. We consider a model with two exchange constants, $J_1$ and $J_2$ on a lattice that interpolates between the limits of a square-lattice ($J_1=0$), a triangular-lattice ($J_2=J_1$), and decoupled linear chains ($J_2=0$). In all cases, the susceptibility which has a Curie-Weiss behavior at high temperatures, rolls over and begins to decrease below a peak temperature, $T_p$. Scaling the exchange constants to get the same peak temperature, shows that the susceptibilities for the square-lattice and linear chain limits have similar magnitudes near the peak. Maximum deviation arises near the triangular-lattice limit, where frustration leads to much smaller susceptibility and with a flatter temperature dependence. We compare our results to the inorganic materials Cs$_2$CuCl$_4$ and Cs$_2$CuBr$_4$ and to a number of organic molecular crystals. We find that the former (Cs$_2$CuCl$_4$ and Cs$_2$CuBr$_4$) are weakly frustrated and their exchange parameters determined through the temperature dependence of the susceptibility are in agreement with neutron-scattering measurements. In contrast, the organic materials are strongly frustrated with exchange parameters near the isotropic triangular-lattice limit.

Abstract:

We have investigated the spin fluctuations at energy transfers up to ~110 meV, well above the resonance energy (33 meV) in the YBa2Cu3O6.5 ortho-II superconductor using neutron time-of-flight and triple-axis techniques. The spectrum at high energies differs from the low-energy incommensurate modulations previously reported where the incommensurate wave vector is largely independent of energy. Well above the resonance the peak of the spin response lies at wave vectors that increase with energy. Within error the excitations at all energies above the resonance are best described by a ring around the (pi, pi) position. The isotropic wave-vector pattern differs from a recently reported square pattern in different but related systems. The spin spectral weight at high-energies is similar to that in the insulator but the characteristic velocity is ~40% lower. We introduce a method of extracting the acoustic and optic weights at all energies from time-of-flight data. We find that the optic spectral weight extends to surprisingly low-energies of ~25 meV, and infer that the bilayer spin correlations weaken with increase in hole doping. When the low-energy optic excitations are taken into account we measure the total integrated weight around (pi, pi), for energies below 120 meV, to agree with that expected from the insulator. As a qualitative guide, we compare spin-wave calculations for an ordered and a disordered stripe model and describe the inadequacy of this and other stripe models for the high-energy fluctuations.

Abstract:

The spin excitation spectrum in the 2D square lattice Heisenberg antiferromagnet CFTD has been investigated using time-of-flight neutron scattering. The magnon dispersion and intensities have been determined as a function of wavevector Q in the full Brillouin zone. Both quantities are found to deviate significantly from the predictions of linear spin wave theory for wavevectors in the vicinity of Q=(π,0), whereas good agreement is found near Q=(π/2,π/2). © 2004 Published by Elsevier B.V.

Abstract:

We explore the magnetic excitations as a function of applied field in the quasi 1D s=12 XY-like magnet Cs2CoCl4. A transition from spin-flop antiferromagnetic order to a quantum paramagnet occurs at the critical field BC=2.15 T and fields higher than 2.5 T fully align the spins along the field. In the spin-flop phase gapped scattering continua dominate, identified with the creation of pairs of quantum domain walls. In the saturated phase sharp peaks are observed, as expected for magnons in a ferromagnetically ordered ground state. © 2003 Elsevier B.V. All rights reserved.