Phase transitions in 1-D magnetic materials

Ferromagnetic ↔ antiferromagnetic phase transitions for one-dimensional systems are investigated using the Monte Carlo technique. Ground state diagrams are constructed within the framework of the Ising model. Using the Metropolis algorithm, the effects of external magnetic field and temperature variations on phase transitions are examined. Kinetic features of these transitions are included into consideration. It is shown that the correlational length coefficient v for ferromagnetic materials decreases with increase in the external magnetic field strength, while for antiferromagnetic this dependence is reverse. Behavior of the critical dynamic coefficient z under field variations for small one-dimensional magnetic is similar to that of coefficient v. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp.54–58, March, 2006.     

Phase transitions in SbSI

In ferroelectric SbSI the phase transitions are studied by means of precise birefringence measurement. It is observed that the transition from D^162h phase to C⁹_2v phase occurs at 15.70°C. At ? 36.61°C, the second order transition from C⁹_2v phase to C¹_s or C²_s phase is observed. From the behavior of birefringence near the second order transition point, the mechanism of the phase transition in SbSI is considered.     

Phase transitions in triamantane

The triple-cage triamantane, C₁₈H₂₄, in addition to the previously-reported transition to a plastic pre-melting phase at 150°C, is now found to undergo a gradual transition from an ordered phase at 0°C to a disordered phase at 30°C involving an increasing proportion of molecules re-orienting about one axis. The mechanism for this transition is established and a disorder parameter defined and estimated at three temperatures from X-ray measurements.     

Phase transitions in geometrothermodynamics

Using the formalism of geometrothermodynamics, we investigate the geometric properties of the equilibrium manifold for diverse thermodynamic systems. Starting from Legendre invariant metrics of the phase manifold, we derive thermodynamic metrics for the equilibrium manifold whose curvature becomes singular at those points where phase transitions of first and second order occur. We conclude that the thermodynamic curvature of the equilibrium manifold, as defined in geometrothermodynamics, can be used as a measure of thermodynamic interaction in diverse systems with two and three thermodynamic degrees of freedom.     

Phase transitions in the Ashkin-Teller model

For some values of the coupling constants, a proof of the existence of two phase transitions in the Ashkin-Teller model is given. Only correlation inequalities are used.     

Phase transitions in the frustration network

The distribution of the frustrated plaquettes is calculated as a function of disorder for two and three dimensional random Ising- andXY-models. It is shown that in all cases the frustrated plaquettes are highly correlated and that at a critical value of disorder a phase transition occurs in the frustration network which can be viewed as a dissociation of plaquettes. The critical disorder is calculated using duality transformations. We show that in the two and three dimensional random Ising systems two types of nonmagnetic phases exist which differ qualitatively by their distribution of frustrated plaquettes and consequently by their domain wall structure.     

Phase transitions in doped antiferromagnets

The spin flop phase transition is compared in K₂FeF₅ and K₂Fe_0.99Ga_0.01F₅ in terms of spin flop field, hysteresis and coexistence of phases.     

Phase transitions in quantum plasmas

We present a theory for a critical point and phase transitions in quantum plasmas (QPs). We use a newly obtained Lennard-Jones-type interaction potential or an oscillating exponential-screened Coulomb potential around ions that are screened by degenerate electrons in an unmagnetized QP. Expressions for the free energy and an equation of state for non-degenerate ions are obtained. It is found that the existence of a critical point and phase separations in our QPs depends on the Wigner–Seitz electron radius and the equilibrium ion temperature. Our results are relevant for understanding superdense astrophysical objects such as the cores of white dwarfs.     

Phase transitions in angle variables

Phase transitions in angle variables are studied. An example of angular phase transition, an axially to triaxially deformed "shape" transition in nuclei, is discussed. Spectroscopic signatures for the occurrence of these transitions are suggested. Preliminary experimental evidence is presented.     

Phase transitions in hadronic matter

We formulate an equation of state for strongly interacting matter, which leads to a phase transition from massive resonance excitation to ideal gas behaviour. The structural similarity to the Van der Waals equation is discussed, as are extensions to describe hadron to quark matter transitions.     

Phase transitions in manganese-containing perovskites

Solid solutions with a perovskite structure of the general formula A′_1?xA″_xMnO₃ (including the previously unknown compounds with A′=La or Pr and A″=Cd or Bi) are studied. It is found that the structure of the orthorhombic phases formed from rhombohedral phases with a decrease in the temperature is characterized by strong anisotropy of the Mn-O bond lengths. This anisotropy drastically decreases at lower temperatures.     

Phase transitions in social networks

We study a model of network with clustering and desired node degree. The original purpose of the model was to describe optimal structures of scientific collaboration in the European Union. The model belongs to the family of exponential random graphs. We show by numerical simulations and analytical considerations how a very simple Hamiltonian can lead to surprisingly complicated and eventful phase diagram.     

Phase transitions in isolated molecules

It is shown that an isolated polyatomic molecule can undergo a structural phase transition when optically pumped.