Phase transitions with nonvanishing order parameter and Landau theory

AbstractSpontaneous C4-symmetry breaking phases are ubiquitous in layered quantum materials, and often compete with other phases such as superconductivity. Preferential suppression of the symmetry broken phases by light has been used to explain non-equilibrium light induced superconductivity, metallicity, and the creation of metastable states. Key to understanding how these phases emerge is understanding how C4 symmetry is restored. A leading approach is based on time-dependent Ginzburg-Landau theory, which explains the coherence response seen in many systems. However, we show that, for the case of the single layered manganite La0.5Sr1.5MnO4, the theory fails. Instead, we find an ultrafast inhomogeneous disordering transition in which the mean-field order parameter no longer reflects the atomic-scale state of the system. Our results suggest that disorder may be common to light-induced phase transitions, and methods beyond the mean-field are necessary for understanding and manipulating photoinduced phases.

Download Full-text

Phase Diagrams for Incommensurate Systems

MRS Proceedings ◽

10.1557/proc-21-75 ◽

1982 ◽

Vol 21 ◽

Author(s):

P. Toledano

Keyword(s):

Free Energy ◽

Phase Transitions ◽

Energy Density ◽

Phase Diagrams ◽

Order Parameter ◽

Landau Theory ◽

Free Energy Density ◽

Lock In

ABSTRACTPhase diagrams, for systems undergoing one or several incommensurate and lock-in transitions, are discussed in the framework of the Landau theory of phase transitions. It is shown that their essential features can be deduced from the explicit forms of the free-energy density ø and the k-dispersion of the coefficient α(→k)of the quadratic contribution of the order-parameter components in ø. Two families of phase diagrams are distinguished depending on symmetry considerations.

Download Full-text

Unique atom hyper-kagome order in Na4Ir3O8and in low-symmetry spinel modifications

Acta Crystallographica Section A Foundations and Advances ◽

10.1107/s2053273315003848 ◽

2015 ◽

Vol 71 (3) ◽

pp. 301-318 ◽

Cited By ~ 12

Author(s):

V. M. Talanov ◽

V. B. Shirokov ◽

M. V. Talanov

Keyword(s):

Experimental Data ◽

Phase Transitions ◽

Order Parameter ◽

Landau Theory ◽

Atomic Order ◽

Structure Phase ◽

Spinel Structures ◽

Low Symmetry ◽

Critical Order

Group-theoretical and thermodynamic methods of the Landau theory of phase transitions are used to investigate the hyper-kagome atomic order in structures of ordered spinels and a spinel-like Na4Ir3O8crystal. The formation of an atom hyper-kagome sublattice in Na4Ir3O8is described theoretically on the basis of the archetype (hypothetical parent structure/phase) concept. The archetype structure of Na4Ir3O8has a spinel-like structure (space group Fd\bar 3m) and composition [Na1/2Ir3/2]16d[Na3/2]16cO32e4. The critical order parameter which induces hypothetical phase transition has been stated. It is shown that the derived structure of Na4Ir3O8is formed as a result of the displacements of Na, Ir and O atoms, and ordering of Na, Ir and O atoms, orderingdxy,dxz,dyzorbitals as well. Ordering of all atoms takes place according to the type 1:3. Ir and Na atoms form an intriguing atom order: a network of corner-shared Ir triangles called a hyper-kagome lattice. The Ir atoms form nanoclusters which are named decagons. The existence of hyper-kagome lattices in six types of ordered spinel structures is predicted theoretically. The structure mechanisms of the formation of the predicted hyper-kagome atom order in some ordered spinel phases are established. For a number of cases typical diagrams of possible crystal phase states are built in the framework of the Landau theory of phase transitions. Thermodynamical conditions of hyper-kagome order formation are discussed by means of these diagrams. The proposed theory is in accordance with experimental data.

Download Full-text