D.R. Taylor

Interpretation of electric susceptibility and order-parameter measurements near the first- and second-order Jahn-Teller phase transitions in DyAsO4 and DyVO4 using correlated-effective-field theory

Precise measurements of the order parameter and the c axis electric susceptibility have been made near the Jahn-Teller phase transitions in DyAsO4 and DyVO4. Both compounds show large departures from simple mean-field (MF) theory. The phase transition in DyAsO4 is of first order while that of DyVO4 is continuous. The data are analysed using a correlated-effective-field (CEF) model in which a simple approximation, consistent with classical critical behaviour, is made for the dependence on q of the effective exchange constant J(q). The CEF model gives an excellent description of the data over a wide range of temperatures and magnetic fields, both below and above TD. This agreement establishes that the competing effects of short- and long-range interactions are responsible for the deviations from MF behaviour; the model accounts for the first- and second-order nature of the phase transitions in these compounds. The large c axis dielectric anomalies imply the existence of appreciable electric dipole moments and are interpreted in terms of coupling to odd-symmetry q=0 modes. From the measurements they find that the odd-symmetry (electric dipolar) couplings are in order of magnitude larger in DyVO4 than in DyAsO4. These interactions of relatively long range appear to be strong enough in DyVO4 to ensure a continuous phase transition, but are too weak to do so in DyAsO4.

Dielectric studies and interpretation of the first-order cooperative Jahn-Teller phase transition in DyAsO4

The first-order Jahn-Teller phase transition in DyAsO4 has been studied using precise dielectric susceptibility measurements. Discontinuities in the order parameter and the c-axis susceptibility are observed at the transition temperature. They persist in low magnetic fields but are progressively suppressed by higher fields. The data are interpreted using a molecular field model in which the ion-ion interactions are allowed to depend on the order parameter.

Interpretation of Jahn-Teller phase transitions in mixed crystals using the random-field ising model

Abstract Random-field effects on Ising phase transitions can be studied in mixed rare-earth Jahn-Teller systems. Phase transitions in (Tb x Tm 1- x )AsO 4 are well described by mean-field RF theory. Order parameter and susceptibility data for Dy(As x V 1- x )O 4 show non-classical critical behaviour, as well as evidence of time- and history-dependent ordering.

Observation of continuous and discontinuous ordering in a random-field Ising model system

Abstract The effects of random fields on the Ising model structural phase transition in Tb(As x V 1− x )O 4 have been examined by optical diffraction and birefringence. The mixed compounds show discontinuous ordering features on cooling which are attributed to the formation of a domain state followed by a transition to long-range order. On warming, the transition is nearly continuous, with an apparent order-parameter critical exponent unchanged from pure TbVO 4 .

Optical measurements of critical exponents in the random-field Ising system Dy(AsxV1−x)O4

Abstract Optical measurements of β and γ, the order-parameter and susceptibility critical exponents, are reported for the structural random-field Ising system Dy(AsxV1−x)O4 with x=0.154. The results were compared with exponents measured for pure DyVO4 (x=0). γ was found to increase substantially in random fields, from 1.15±0.10 to 1.57±0.15, but β remained unchanged at 0.31±0.05. The latter result conflicts with scaling theory as well as estimates from previous experiments and simulations.

Crossover from classical to random-field critical exponents in As-doped TbVO 4

Using birefringence techniques we have measured the critical exponents beta, gamma, and delta in As-doped TbVO4, a structural realization of the random-field Ising model where random strain fields are introduced by V-As size mismatch. For pure TbVO4 we observe the expected classical critical exponents, while for a mixed sample with 15% As concentration our results are beta=0.31 +/-0.03, gamma=1.22 +/-0.07, and delta=4.2 +/-0.7. These values are consistent with the critical exponents for the short range pure Ising model in three dimensions in agreement with a prediction by Toh. The susceptibility data showed a crossover with temperature from classical to random field critical behaviour.

Homogeneity and composition of mixed flux-grown Dy(AsxV1 − x)O4 crystals

Abstract Large flux-grown single crystals of Dy(As x V 1 − x )O 4 have been studied and characterized by X-ray powder diffraction, PIXE and EDX analyses, and optical birefringence. The crystal structure remains zircon throughout the composition range, and Vegards law for the dependence of unit cell parameters on composition is obeyed well. The crystal As concentrations were much higher than those of the ingredient powders. The relative As/V incorporation in this lattice was found to be 3.23. Absolute indices of refraction were determined by observing white-light fringes in a Michelson interferometer. Birefringence measurements showed that As concentration gradients were small, typically less than 10 −4 mm −1 in regions of good optical quality. Flux techniques are thus able to yield large mixed crystals of excellent uniformity.

Critical x-ray scattering studies of Jahn-Teller phase transitions in TbV 1 − x As x O 4

The critical behavior associated with the cooperative Jahn-Teller phase transitions in

Ultrasonic Velocity and Modified Critical Behaviour in the Random-Field Ising System DyAsxV1-xO4

{\text{TbV}}_{1\ensuremath{-}x}{\text{As}}_{x}{\text{O}}_{4}

Random-field effects on Ising Jahn-Teller phase transitions

(where