Phase diagram and isotope effect in cobaltites with spin-state transitions

The oxygen isotope effect was investigated in cobalt oxides (Pr_{1 − y} Eu _y )_0.7Ca_0.3CoO₃ (0.12 < y < 0.26). The measurements of magnetization, electric resistivity, differential magnetic susceptibility, thermal expansion, and specific heat show that as the Eu concentration increases, a transition from “ferromagnetic metal” to “weakly magnetic insulator” is observed at y ≈ 0.18. In the insulating ground state, the transition occurs with a Co spin-state change that is suppressed in the ferromagnetic (FM) metallic phase. The spin-state transition at y > 0.18 is accompanied by a substantial oxygen isotope effect that is virtually absent in the FM phase (y < 0.18).     

Dome-shaped magnetic phase diagram of thermoelectric layered cobaltites

Using muon spin spectroscopy we have found that, for both NaxCoO(2) (0.6相似文献   

Charge-transfer transitions and optical properties of cobaltites

Specific features of the charge-transfer states and transitions of the O 2p → Co 3d type in octahedral complexes (CoO₆)^9? and (CoO₆)^10? are considered in the cluster approach. The reduced matrix elements of the electric dipole transition operator are calculated on the many-electron wave functions of the complexes corresponding to the initial (high-or low-spin) state and the final state at a charge-transfer transition. The energies of the many-electron charge-transfer transitions and their intensities are calculated within the Tanabe-Sugano theory taking into account the mixing of different configurations of the same symmetry. Simulation of the optical spectrum of cobaltites showed the presence of a wide band consisting of many lines due to the charge-transfer transitions. The results of the simulation are in agreement with experiment and demonstrate the limited validity of the generally accepted concepts of a simple structure of the spectrum of charge-transfer transitions.     

The phase diagram and the pathway of phase transitions for traffic flow in a circular one-lane roadway

This paper demonstrates that patient driving habits lead to homogenous congested flow while impatient driving habits lead to wide-moving jam flow in the high density region based on the numerical simulation of the intelligent driver model proposed by M.Treiber [M. Treiber, A.Hennecke, D. Helbing, Phys. Rev. E 62 (2) (2000), 1805-1824]. In a circular one lane traffic system which includes homogeneous drivers, we obtain the stable condition of homogenous flow and the phase diagram of traffic flow based on the linearization analysis. The phase diagram shows three possible pathways of phase transition along with the increase of global density: from the homogenous free flow to the homogenous congested flow directly, from the homogenous free flow to the synchronized flow then to the homogenous congested flow, or from the homogenous free flow to synchronized flow then to the wide-moving jam flow. The paper also analyzes the traffic flow including heterogenous drivers, and the results indicate that homogenous congested flow will lose its stability when the proportion of impatient drivers reaches a critical value and some new kinds of traffic flow emerge: wide-moving jam flow or a mixture of synchronized flow and wide-moving jam flow.     

First order phase transitions in unbounded spin systemsI: Construction of the phase diagram

The phase diagram and the corresponding infinite volume Gibbs states are constructed for a large class of continuous, unbounded spin models. Our construction relies on a partition of unity mapping our system onto an interacting contour system, a generalisation of Zahradnik's approach to Piragov Sinai theory to interacting contour systems, and a suitable mean field expansion around the minimas of the Hamiltonian.     

On some features of the phase diagram of a ferroic with the cubic paraphase

The phase diagram of a ferroic with the paraphase of symmetry O _h has been studied for a three-dimensional order parameter. In the simplest model, which allows for the existence of a phase of the general position, i.e., the triclinic phase, the region of its existence has been investigated analytically. The invariance of the shape of this region is shown. The conditions providing the formation of a “swallowtail”-type feature in the region of existence of the monoclinic phase are considered. A convenient parametrization is proposed for the representation of the phase diagram, including all possible phases.     

Phase diagram and phase transitions of Krypton on graphite in the extended monolayer regime

We report high resolution x-ray diffraction studies of the structures and phase transitions of monolayer krypton, adsorbed on both powder and single crystal graphite substrates. A comprehensive series of powder diffraction profiles is used to construct the two dimensional phase diagram. The melting of the commensurate solid is shown to be strongly first order throughout the region where tricritical behavior was previously thought to occur; fluid solid coexistence extends up to the termination of the commensurate phase at 130 K. A disordered weakly incommensurate phase is shown to be a reentrant fluid, a system which may be described as a disordered network of domain walls and which evolves continuously into a more conventional 2D fluid. This evolution is marked by the disappearance of satellite peaks which are caused by the modulation of the overlayer by the substrate. The freezing of the reentrant fluid into the commensurate phase is shown to be consistent with a chiral Potts transition, its freezing into the incommensurate solid consistent with a dislocation binding transition.Single crystal experiments reveal the orientation of the weakly incommensurate phase. The reentrant fluid is found to have no visible orientational fluctuations, manifesting isotropic diffraction peaks. This is attributed to the strong epitaxy of domain walls. The incommensurate solid is shown to undergo an aligned-rotated transition which is well described by zerotemperature calculations.     

First order phase transitions in unbounded spin systems. II. Completeness of the phase diagram

We continue our analysis of unbounded spin systems with nearest neighbor interactionW and a single spin potentialV which hasN deep and widely separated minima. In this second part we show that all translation invariant phases obeying a certain regularity condition are convex combinations of the stable phases determined in the first part of this paper. For periodic boundary conditions each stable phase contributes with the same weight in the infinite volume limit.     

Specific features of temperature-and strain-induced order-disorder phase transitions

The regularities of temperature-and strain-induced order-disorder phase transitions are determined from analyzing X-ray diffraction data. The results obtained are used to reveal common features in order-disorder transformations induced under energy actions of different types. It is established that, in both cases, the transition from the long-range order state to the short-range order state is accompanied by an increase in the number of defects in the structure of the alloy and the most rapid decrease in the long-range order parameter is associated with the generation of antiphase boundaries and their smearing.     

Generic features of vacuum phase transitions in the early universe

A simple Higgs model is utilized to show the occurrence of a four-phase pattern of vacuum symmetry. As temperature changes, an interplay of spontaneous symmetry breaking and spontaneous symmetry restoration ensues, and resonant field interchange occurs. The generality of models which may contain a sequence of vacuum phase transitions is emphasized. The laboratory for these multi-phase transitions is the early Universe.     

Specific features of magnetic phase diagram of an MnSi helimagnet

Magnetization curves of MnSi single crystals have been measured in a range of temperatures T = 5.5–35 K and magnetic field strengths H ≤ 11 kOe for H ∥ [1 1 1], [0 0 1], and [1 1 0]. Special attention has been paid to the temperature interval near T _N = 28.8 K, where MnSi exhibits a transition to the state with a long-period helical magnetic structure. Some new features in the magnetic behavior of MnSi have been found. In particular, in an intermediate temperature region above the transition (28.8 K = T _N ≤ T < 31.5 K), the dM(H)/dH curves exhibit anomalies that are not characteristic of the typical paramagnetic state. It is established that the line of the characteristic field H*(T) of this anomaly is a natural extrapolation of the temperature dependence of the field of the transition from a conical phase to an induced ferromagnetic phase observed at T < T _N. It is concluded that the properties of MnSi in the indicated intermediate region are related to and governed by those of the conical phase (rather than of the A phase). Based on these data, magnetic phase diagrams of MnSi for H ∥ [1 1 1], [0 0 1], and [1 1 0] are plotted and compared to diagrams obtained earlier by other methods.     

Metal-ligand interplay in strongly correlated oxides: a parametrized phase diagram for pressure-induced spin transitions

We investigate the magnetic properties of archetypal transition-metal oxides MnO, FeO, CoO, and NiO under very high pressure by x-ray emission spectroscopy at the Kbeta line. We observe a strong modification of the magnetism in the megabar range in all the samples except NiO. The results are analyzed within a multiplet approach including charge-transfer effects. The spectral changes are well accounted for by changes of the ligand field acting on the d electrons and allows us to extract the d-hybridization strength, O-2p bandwidth and ionic crystal field across the magnetic transition. This approach allows first-hand insight into the mechanism of the pressure-induced spin transition.     

Magnetic phase diagram of MnAs: Effect of magnetic field on structural and magnetic transitions

The boundary, in the T?H₀ plane, separating the MnP-type and NiAs-type crystallographic phases of MnAs was determined from magnetization and volume measurements. Transitions on one part of the boundary are of first order, but are of second order on the remaining part. The two parts are separated by a tricitical point.     

Specific features of phase transitions in molecular crystals of diols

A comparative analysis of the phase transitions in molecular crystals of diols with different chain lengths [1,15-pentadecanediol (CH₂)₁₄(OH)₂ and 1,16-hexadecanediol (CH₂)₁₅(OH)₂] has been performed using differential scanning calorimetry. A quantitative analysis of the temperature dependence of the heat capacity within the theory of diffuse (Λ-shaped) first-order phase transitions and the use of the temperature hysteresis have revealed a number of new effects associated with the specific features of the phase transitions in molecular crystals of diols.     

Plasticity-induced characteristic changes of pattern dynamics and the related phase transitions in small-world neuronal networks

Phase transitions widely exist in nature and occur when some control parameters are changed. In neural systems, their macroscopic states are represented by the activity states of neuron populations, and phase transitions between different activity states are closely related to corresponding functions in the brain. In particular, phase transitions to some rhythmic synchronous firing states play significant roles on diverse brain functions and disfunctions, such as encoding rhythmical external stimuli, epileptic seizure, etc. However, in previous studies, phase transitions in neuronal networks are almost driven by network parameters （e.g., external stimuli）, and there has been no investigation about the transitions between typical activity states of neuronal networks in a self-organized way by applying plastic connection weights. In this paper, we discuss phase transitions in electrically coupled and lattice-based small-world neuronal networks （LBSW networks） under spike-timing-dependent plasticity （STDP）. By applying STDP on all electrical synapses, various known and novel phase transitions could emerge in LBSW networks, particularly, the phenomenon of self-organized phase transitions （SOPTs）： repeated transitions between synchronous and asynchronous firing states. We further explore the mechanics generating SOPTs on the basis of synaptic weight dynamics.     

Hybridization effect on phase transitions in the Falicov-Kimball model

The Falicov-Kimball model extended by the hybridization between the localized electron states and the conduction band states is studied in Hartree-Fock approximation. Taking into account the periodicity of the system, the occupation number of the localized levels always varies continuously with the level energy for non-vanishing hybridization. This result is in contradiction with earlier mean-field calculations which are not self- consistent for the periodic model. Since first order phase transitions in intermediate valence compounds such as SmS have been observed experimentally, it follows that their explanation needs a more general theoretical frame than the model considered here.