When performed in the proper low-field, low-frequency limits, measurements of the dynamics and the nonlinear susceptibility in the model Ising magnet in a transverse field LiHo(x)Y(1-x)F(4) prove the existence of a spin-glass transition for x=0.167 and 0.198. The classical behavior tracks for the two concentrations, but the behavior in the quantum regime at large transverse fields differs because of the competing effects of quantum entanglement and random fields.

The hydrogen-bond network formed from a crystalline solution of ferroelectric RbH 2 PO 4 and antiferroelectric NH 4 H 2 PO 4 demonstrates glassy behavior, with proton tunneling the dominant mechanism for relaxation at low temperature. We characterize the dielectric response over seven decades of frequency and quantitatively fit the long-time relaxation by directly measuring the local potential energy landscape via neutron Compton scattering. The collective motion of protons rearranges the hydrogen bonds in the network. By analogy with vortex tunneling in superconductors, we relate the logarithmic decay of the polarization to the quantum-mechanical action. DOI: 10.1103/PhysRevLett.97.145501 PACS numbers: 61.43.Fs, 66.35.+a, 77.22.ÿd, 77.84.Fa Glasses become trapped in a restricted set of local configurations and evolve extraordinarily slowly over time. Delineating the relationship between the structure and the dynamics is a critical element in establishing the essential nature of the glassy state, but this correspondence remains difficult to access in most systems. The proton glass [1], a structural analogue to magnetic spin glasses [2] with competing ferroelectric (FE) and antiferroelectric (AFE) order, is different. By combining dielectric spectroscopy over seven decades in frequency with a direct mapping of the real-space energy potential via neutron Compton scattering [3], we are able to describe quantitatively the highly choreographed proton dynamics within a hydrogen-bond network. Proton tunneling controls correlations between neighboring hydrogen bonds and dominates the long-time relaxation.The hydrogen bond plays an important role in physics, chemistry, and biology [4], but it is probably most familiar from ice, where the freedom of different static configurations to accommodate Pauling's rule generates a macroscopic ground state and extra entropy contributing to the latent heat [5]. The hydrogen-bond network formed by Pauling's rule exists as well in other crystalline materials, for example, the piezoelectric KH 2 PO 4 (KDP) family [1,6]. Protons in four O-H-O bonds attached to each PO 4 3ÿ group are constrained to maintain singly charged H 2 PO 4 ÿ groups. Six distinct electrical dipole moments along three orthogonal axes can be formed with different Slater configurations [6]. While pure phases of RbH 2 PO 4 (RDP) and NH 4 H 2 PO 4 (ADP) are FE and AFE, respectively, at low temperature, a solid state solution creates a dipolar glass, Rb 1ÿx NH 4 x H 2 PO 4 (RADP:100x), for concentrations 0:22 < x < 0:74 [1]. The disorder, combined with the ''frustration'' arising from the competition between FE ordering along the c axis in RDP and AFE ordering within the ab plane for ADP, leads to the characteristic broad and frequency-dependent peak in the dielectric susceptibility vs temperature T [1,2] and no more than short-range structural correlations [7]. The proton's pivotal role in relaxation is confirmed by the strong isotope effect on low T thermal properties [8].The dielectric spectroscopy is performed f...

No abstract

No abstract

Polycrystalline samples of (La1−xAx)0.67Ba0.33Mn1.05O3−δ, with A being a mixture of lanthanides containing 66% La, 22% Nd, 8% Pr, and 4% Ce, were prepared by the glycine-nitrate method, with target compositions of x=0, 0.33, 0.67, and 1. The effect of the mixture of lanthanides on the Curie temperature, TC, and the magnetocaloric properties was investigated. The prepared samples are single phase, with space group R3¯c. The lattice parameters and average A-site ionic radius, rA decrease linearly with x while the size disorder, as characterized by the variance, σ2, increases from 0.014 to 0.017 with x. As x is increased, the increase in σ2 drives a linear decrease in TC. The magnetic entropy change, ΔSM, for a field change of 1 T is obtained by magnetization measurements, giving a maximum value of 1.7 J/kg K, and a relative cooling power of 43 J/kg, independent of x. These results suggest that replacing lanthanum with a mixture of lanthanides is a realistic way to tune the working temperature of magnetocaloric manganites while maintaining a constant magnetocaloric effect.