Cascade Of Phase Transitions Research Articles

Cascade of phase transitions in GdFe3(BO3)4

Cascade of phase transitions in GdFe3(BO3)4 at 156, 37, and 9 K has been detected by specific heat measurements and further studied by Raman scattering and Nd3+ spectroscopic probe method. A weakly first-order structural phase transition at 156 K is followed by a second-order antiferromagnetic ordering phase transition at 37 K and a first-order spin-reorientational phase transition at 9 K.

π-phase magnetism in ferromagnet-superconductor superlattices

New 0π and ππ Larkin-Ovchinnikov-Fulde-Ferrell (LOFF) states with antiferromagnetic orientation of magnetizations in the neighboring layers of a ferromagnetic metal (FM) are predicted for FM/superconductor (FM/S) superlattices. Under certain conditions, the critical temperature T c of these states is higher than for the known 00 and π0 LOFF states with ferromagnetic ordering of the FM layers. It is shown that the nonmonotonic behavior of T c in the FM/S superlattices with S-layer thickness d s less than the threshold value d s π is due to the phase transition cascade 0π-ππ-0π At d s >d s π , the T c oscillations are caused by the 00-π0-00 transitions. New logic elements based on the FM/S structures and combining the advantages of the superconducting and magnetic data-record channels in a single sample are proposed.

Cascade of phase transitions in Tm1−xSmxS due to Tm and Sm valence changes

Studies of the physical properties of Tm1−xSmxS solid solutions have been carried out. We have measured the lattice constant a at 300 K, the resistivity ρ (4.2–300 K), the Hall constant RH (at 4.2, 77 and 300 K) and the X-ray line shifts (300 K). We have found two phase transitions at x = 0.84 and 0.75 which are related to changes in the Tm and Sm valence states. We argue that for 1 > x ⩾ 0.84 the Tm3+ ions form donor centres.

Domain patterns in incommensurate systems with the uniaxial real order parameter.

The basic Landau model for the incommensurate-commensurate transition to the uniform or dimerized uniaxial ordering is critically reexamined. The previous analyses identified only sinusoidal and homogeneous solutions as thermodynamically stable and proposed a simple phase diagram with the first order phase transition between these configurations. By performing the numerical analysis of the free energy and the Euler-Lagrange equation we show that the phase diagram is more complex. It also contains a set of metastable solutions present in the range of coexistence of homogeneous and sinusoidal solutions. These new configurations are periodic patterns of homogeneous domains connected by sinusoidal segments. They are Lyapunov unstable, very probably due to the nonintegrability of the free energy functional. We also discuss some other mathematical aspects of the model, and compare it with the essentially simpler sine-Gordon model for the transitions to the states with higher commensurabilities. The present results might be a basis for the explanation of phenomena like thermal hystereses, cascades of phase transitions, and memory effects, observed in materials exhibiting the transitions to uniform or dimerized state. Some particular examples are discussed in detail.

Spectra and gap amplification for systems with two widely different incommensurate periodicities.

We derive analytically the spectrum for the Schroedinger equation for quasiperiodic systems with two length scales: one large ''macroscopic'' scale (e.g., a-italic cos(2..pi..x-italic/lambda)) and one small ''microscopic'' scale (e.g., v-italic cos(2..pi..x-italic)). The phase diagram includes regimes with exponentially narrow gaps due to the slowly varying potential, regimes where the rapidly varying potential amplifies these narrow gaps, and regimes with exponentially narrow ''Landau bands.'' The full ''devil's-staircase'' spectrum with gaps at wave vectors q-italic = m-italic..pi..+n-italic..pi../lambda develops in a hierarchical manner as a-italic increases. The results apply to systems with superlattices, to celestial orbits with two periodic perturbations, to systems with slowly varying lattice distortions, and, in particular, to quasi-one-dimensional magnets such as bis(tetramethyltetraselenafulvalene) perchlorate ((TMTSF)/sub 2/ClO/sub 4/) in magnetic fields, where our findings may provide insight into the experimentally observed cascade of phase transitions.

Low-temperature analysis of the ANNNI model in an external magnetic field: Cascades of phase transitions, complete devil's staircases

The behavior of the axial next-nearest-neighbor Ising (ANNNI) model in an external magnetic field is investigated using a low-temperature expansion of the free energy. Unusual cascades of phase transitions and “complete devil's staircases,” unexpected for the ANNNI model, are found.