Bulk Transition Research Articles

Re-orientation transition in molecular thin films: Potts model with dipolar interaction

We study the low-temperature behavior and the phase transition of a thin film by Monte Carlo simulation. The thin film has a simple cubic lattice structure where each site is occupied by a Potts parameter which indicates the molecular orientation of the site. We take only three molecular orientations in this paper, which correspond to the three-state Potts model. The Hamiltonian of the system includes (i) the exchange interaction Jij between nearest-neighbor sites i and j, (ii) the long-range dipolar interaction of amplitude D truncated at a cutoff distance rc, and (iii) a single-ion perpendicular anisotropy of amplitude A. We allow Jij = Js between surface spins, and Jij = J otherwise. We show that the ground state depends on the ratio D/A and rc. For a single layer, for a given A, there is a critical value Dc below (above) which the ground-state (GS) configuration of molecular axes is perpendicular (parallel) to the film surface. When the temperature T is increased, a re-orientation transition occurs near Dc: the low-T in-plane ordering undergoes a transition to the perpendicular ordering at a finite T, below the transition to the paramagnetic phase. The same phenomenon is observed in the case of a film with a thickness. Comparison with the Fe/Gd experiment is given. We show that the surface phase transition can occur below or above the bulk transition depending on the ratio Js/J. Surface and bulk order parameters as well as other physical quantities are shown and discussed.

Unusual application of porous matrix made from quartzite schist

Results of dielectric spectroscopy of quartzite slate are presented. New type of a porous matrix based on quartzite schist was described. The matrix was filled by a ferroelectric TGS-crystal. The phase transition of the TGS bulk was shown to occur and compared with the phase transition of the component quartzite schist filled with TGS (QSM-TGS). The resonance phenomena in the QSM were shown and described from the point of view filled material.

Crossover From Under to Overdamped Dynamics in Free-Standing Smectic Films Close to the Smectic-A-Nematic Second Order Phase Transition

The hydrodynamic properties of free-standing smectic films close to a second order smectic-A (SmA) — nematic (N) phase transition are investigated. By using an extended McMillan model, the profiles of the smectic elastic constants are computed at different temperatures. We show that the amplitude of smectic fluctuations has a strong dependence on the film temperature. By considering the critical behavior of viscoelastic coefficients of thin films, we show that the typical underdamped relaxation of the smectic order is replaced by an overdamped behavior close to the bulk transition temperature.

Polarized Neutron on URu2Si2

Abstract Polarized neutron scattering has shed a new light on URu 2 Si 2 . This compound studied for almost thirty years presents two tran- sitions: a superconducting state below 1.2K (T sc ) and a well-defined bulk transition at 17.8K (T 0 ). In spite of intensive research, no local probe has defined the order parameter associated with this transition (the famous Hidden Order (HO)). Neutron Larmor diffraction measurements combined with uniaxial stress experiments have shown that the relevant parameter that governs the magnetic properties is the a -lattice parameter. Induced magnetization distribution measured by elastic polarized neutron scattering shows a subtle change when entering the HO state that suggests a freezing of rank 5 multipoles (i.e. dotriacontapole). Polarized inelastic neutron measurements have shown the existence of two well defined excitations on the top of a broad continuum. How- ever only the AF excitation vanishes at T 0 and as well as when URu 2 Si 2 switches between HO to AF state under pressure. These excitations seem to correspond also to transitions between singlet levels.

Towards an all-solid-state battery: Preparation of conversion anodes by electrodeposition–oxidation processes

A thin film electrode consisting of a mixture of iron and nickel oxides is prepared by a single route. Electrochemical deposition from Fe2+ and Ni2+ solutions on a titanium substrate and further thermal treatment at low temperature allowed obtaining a nanocomposite thin film of NiO and Fe2O3. The electrochemical studies performed on lithium cells assembled with this electrode material revealed the smallest irreversibility from first to second discharge ever reported during the conversion reactions of metal oxides at ∼0.8 V. Unlike bulk transition metal oxides, the poorly-crystalline oxidized material exhibited an unusually low discharge polarization. The 57Fe Mössbauer spectra of partially discharged and charged electrodes evidenced the reversibility of the conversion reaction with lithium and the efficiency of the recovery of the local environment of iron after the first cycle.

Predicting the temperature for the solid–solid phase transition in II–VI semiconductor alloys

At atmospheric pressure, many of the II–VI semiconducting alloys are known to undergo a zinc-blende to wurtzite solid–solid transition. Experimental values for these transitions temperatures have only been reported for two alloys. We show here that chemical potentials for one of the components in a solid solution with the other can be used to estimate the transition temperature. The non-ideal behavior of the solvent component is addressed via an activity coefficient which is determined using the quasi-chemical model. The chemical potentials for each case, zinc-blende and wurtzite are then taken to be equal at the transition temperature. Predicted transition temperatures are reported here for ZnS, CdS, ZnSe, ZnTe, and CdSe. In the case of ZnS and CdS these values agree with the experimental. For ZnSe, ZnTe, and CdSe the values are above their respective melting points. This result suggests that there is no complete bulk transition below the melting point for these three alloys but calculations predict that the two phases are nearly isoenergetic.

Application of screened hybrid functionals to the bulk transition metals Rh, Pd, and Pt

We present the results of calculations on bulk transition metals Rh, Pd, and Pt using the screened hybrid functional YS-PBE0 [F. Tran and P. Blaha, Phys. Rev. B 83, 235118 (2011)]. The results for the equilibrium geometry are compared with those obtained from (semi)local functionals, namely, the local density approximation and the generalized gradient approximation PBE of Perdew et al. [J. P. Perdew, K. Burke, and M. Ernzerhof, Phys. Rev. Lett. 77, 3865 (1996)]. It is shown that the screened hybrid functional yields more accurate equilibrium geometry than PBE, but, overall, it is not more accurate than the local density approximation. However, in contradiction with experiment, we find that the screened hybrid functional favors a ferromagnetic state as the ground state for all three transition metals. Therefore, the use of hybrid functionals for, e.g., the study of catalytically active systems with correlated oxides on a metal support is questionable.

Mechanical properties of bulk and nanostructured La0.61Sr0.39MnO3 perovskite manganite materials

Ultrasonic non-destructive measurement is a versatile and sensitive tool for evaluating the mechanical properties of solids. La0.61Sr0.39MnO3 perovskite magnetic material in bulk and at nanoscale was prepared by solid-state reaction and sonochemical reactor methods, respectively. Evaluation of the mechanical properties of the prepared bulk and nano samples was made through on-line ultrasonic velocity measurements over a wide range of temperature from 300 to 400K. The observed anomalous behaviour in elastic moduli was used to explore the phase transition temperature (Curie temperature TC), that is, from paramagnetic (PM) to ferromagnetic (FM) phase, both in bulk and nanostructured perovskite samples. Further, the behaviour of phase transition both in bulk and nano perovskites was explored through the observed anomalous behaviour.

Quantum interface unbinding transitions

We consider interfacial phenomena accompanying bulk quantum phase transitions in presence of surface fields. On general grounds we argue that the surface contribution to the system free energy involves a line of singularities characteristic of an interfacial phase transition, occurring below the bulk transition temperature T_c down to T=0. This implies the occurrence of an interfacial quantum critical regime extending into finite temperatures and located within the portion of the phase diagram where the bulk is ordered. Even in situations, where the bulk order sets in discontinuously at T=0, the system's behavior at the boundary may be controlled by a divergent length scale if the tricritical temperature is sufficiently low. Relying on an effective interfacial model we compute the surface phase diagram in bulk spatial dimensionality $d\geq 2$ and extract the values of the exponents describing the interfacial singularities in $d\geq 3$.

Phase transitions in single crystal tubes formed from C60 molecules under high pressure

Pure single crystal tubes formed from C60 molecules, with a face-centered cubic (fcc) structure were fabricated by a liquid–liquid interfacial precipitation method using C60 powder. A bulk transition from fcc to a simple cubic structure and a surface transition from (1×1) to (2×2) have been observed around 246K (bulk transition temperature TB) and 214K (surface transition temperature TS), respectively, during the measurement of the temperature dependence of electrical resistance. The initiation of the two transitions under pressure was investigated using a piston cylinder high pressure apparatus and it was found that both TB and TS increase with increasing pressure. And the C60 molecules at the surface of the tube exhibit the same behavior of that in the bulk at a pressure of about 2.1GPa.

Critical Point Coupling and Proximity Effects inHe4at the Superfluid Transition

We report measurements of the superfluid fraction ρ(s)/ρ and specific heat c(p) near the superfluid transition of 4He when confined in an array of (2 μm)3 boxes at a separation of S=2 μm and coupled through a 32.5 nm film. We find that c(p) is strongly enhanced when compared with data where coupling is not present. An analysis of this excess signal shows that it is proportional to the finite-size correlation length in the boxes ξ(t,L), and it is measurable as far as S/ξ∼30-50. We obtain ξ(0,L) and the scaling function (within a constant) for ξ(t,L) in an L3 box geometry. Furthermore, we find that ρ(s)/ρ of the film persists a full decade closer to the bulk transition temperature T(λ) than a film uninfluenced by proximity effects. This excess in ρ(s)/ρ is measurable even when S/ξ>100, which cannot be understood on the basis of mean field theory.

Finite-Size Scaling Approach for Critical Wetting: Rationalization in Terms of a Bulk Transition with an Order Parameter Exponent Equal to Zero

Clarification of critical wetting with short-range forces by simulations has been hampered by the lack of accurate methods to locate where the transition occurs. We solve this problem by developing an anisotropic finite-size scaling approach and show that then the wetting transition is a "bulk" critical phenomenon with order parameter exponent equal to zero. For the Ising model in two dimensions, known exact results are straightforwardly reproduced. In three dimensions, it is shown that previous estimates for the location of the transition need revision, but the conclusions about a slow crossover away from mean-field behavior remain unaltered.

Front Cover: Surface studies of BiFeO3 with interdigital electrodes (Phys. Status Solidi A 7/2012)

In 1974, nearly forty years ago, Maria Polomska and her colleagues discovered an anomaly in bismuth ferrite (BiFeO3) at 458 K. Despite a large number of publications on this material (more than a thousand since 2005), the origin of this anomaly has proved enigmatic. In the study on pp. 1207–1212, by use of in-plane interdigital electrodes, Ashok Kumar et al. have shown that it is a surface phase transition. Two other surface phase transitions are confirmed at 201 K and 548 K, bringing the total of bulk and surface transitions in BiFeO3 to nine. Bismuth ferrite is very important from the standpoint of basic physics and devices, because it is the only room-temperature multiferroic magnetoelectric known with device-worthy switched polarization in the range of μC/cm2 (other recent discoveries at or near room temperature switch nC/cm2 - a 1000× less charge). The new work is of special importance for devices that involve surfaces in any way, such as THz emitters.

Textured superconductivity in the presence of a coexisting order: Ce115s and other heavy-fermion compounds

Superconductivity in strongly correlated electron systems frequently emerges in proximity to another broken symmetry. In heavy-electron superconductors, the nearby ordered state most commonly is magnetism, and the so-called Ce115 heavy-electron compounds have been particularly instructive for revealing new relationships between magnetism and superconductivity. From measurements of the resistive and bulk transitions to superconductivity in these materials, we find that the resistive transition appears at a temperature considerably higher than the bulk transition when superconductivity and magnetic order coexist, but this temperature difference disappears in the absence of long-range magnetic order. Further, in the pressure–temperature region of coexistence in CeRhIn5, a new anisotropy in the resistive transition develops even though the tetragonal crystal structure apparently remains unchanged, implying a form of textured superconductivity. We suggest that this texture may be a generic response to coexisting order in these and other heavy-fermion superconductors.

Order Parameter Distribution and Phase Transition Temperature for a Thin Film With Asymmetric Boundaries

The phenomenological theory was developed for the ferroic phase transition in a thin film with asymmetric boundary conditions which simulate the real situation of a film on the substrate or a layer between two different materials. The solutions of the boundary value problem for the order parameter were obtained numerically for some particular sets of phenomenological parameters in the Landau expansion and extrapolation lengths. The main attention was focused on the case of opposite signs of the extrapolation lengths at film surfaces. It was shown that the mean order parameter is close to that in bulk and phase transition temperature remains weakly dependent on film thickness when the extrapolation lengths have similar absolute values and opposite signs.

Influence of quenched disorder created by nanosilica network on phase transitions in tetracosane

Calorimetric measurements on composites of a long chain n-alkane, tetracosane, doped with nanosilica aerosil particles decorated with a corona of hydrophobic/hydrophilic nature are described. The weakly perturbing random field created by the addition of the aerosil particles has the general effect of weakening all the transitions to, and between, the three different rotator phases that the pure alkane exhibits. One of highlights of the studies is that the strong first order transitions of the pure alkane, viz., from the isotropic liquid to hexagonal rotator phase (Iso–R2), and tilted monoclinic rotator phase to the crystalline phase (R5–Cr), are accompanied by a much weaker supplementary peak at lower temperatures. In analogy with observations made in aerosil composites of liquid crystalline systems, and additionally with information from preliminary Xray diffraction measurements on the currently studied materials, it is reasoned that the subsidiary peaks are nothing but the transformations between the same phases as the corresponding main peaks due to the bulk of the material, but occurring in the vicinity of the surface of the aerosil particles. The nature of the corona of the aerosil particles also seems to play an important role. For the bulk transitions, the reduction in the transition enthalpy is higher for the composites with the hydrophilic particles, than those with hydrophobic particles; the opposite is true for the surface transitions. The data also suggest that the lowering of the transition temperature of the surface transition with respect to its bulk counterpart is more for the hydrophobic composites than the hydrophilic ones. We provide an explanation for these features based on the surface interactions between the particles and tetracosane molecules, and also the strength of the gel network.

Deoxygenation mechanisms on Ni-promoted MoS2 bulk catalysts: A combined experimental and theoretical study

Through a combined experimental and density functional study, we investigate the deoxygenation of ethyl heptanoate catalyzed by three relevant unsupported (bulk) transition metal sulfide catalysts: MoS2, Ni3S2 and Ni-promoted MoS2 (with various Ni/Mo ratio). Two pathways compete for this reaction: hydrodeoxygenation (HDO) and decarboxylation/decarbonylation (DCO). It is shown experimentally that the presence of Ni either in the NiMoS mixed phase or in the Ni monosulfide phase may change the selectivity between the HDO and DCO pathway. To understand the origin of this selectivity, we study the deoxygenation pathways of two relevant intermediates: carboxylic acid and aldehyde. In particular, DFT calculations highlight that the aldehyde decarbonylation occurs via its dehydrogenation into the alkanoyl and/or ketene intermediates on the Ni3S2 (111) surface. These pathways are found to be more favorable on Ni3S2 than on MoS2-based catalysts. This trend is explained by the presence of Ni3 triangular facets on Ni3S2 (111) enhancing the formation of the unsaturated alkanoyl and/or ketene intermediates as well as the CO product. We finally propose a detailed analysis of the promoting effect of Ni on MoS2, when present in the NiMoS structure.

QUARK MATTER NUCLEATION AT NEUTRON STAR CORES: RELEVANCE OF ENERGY-DENSITY FLUCTUATIONS

We study the deconfinement of hadronic matter into quark matter in a protoneutron star focusing on the effects of the finite size on the formation of just-deconfined color superconducting quark droplets embedded in the hadronic environment. We show that energy-density fluctuations are much more relevant for deconfinement than temperature and neutrino density fluctuations. We calculate the critical size spectrum of energy-density fluctuations that allows deconfinement as well as the nucleation rate of each critical bubble. We find that drops with any radii smaller than 800 fm can be formed at a huge rate when matter achieves the bulk transition limit of 5–6 times the nuclear saturation density.

Diode-less bilayer oxide (WOx–NbOx) device for cross-point resistive memory applications

The combination of a threshold switching device and a resistive switching (RS)device was proposed to suppress the undesired sneak current for the integrationof bipolar RS cells in a cross-point array type memory. A simulation for thishybrid-type device shows that the matching of key parameters between switch elementand memory element is an important issue. Based on the threshold switchingoxides, a conceptual structure with a simple metal–oxide 1–oxide 2–metal stackwas provided to accommodate the evolution trend. We show that electroformedW–NbOx–Pt devices can simultaneously exhibit both threshold switching and memory switching.A qualitative model was suggested to elucidate the unique properties in aW–NbOx–Pt stack, where threshold switching is associated with a localized metal–insulator transition in theNbOx bulk, and the bipolar RS derives from a redox at the tip of the localized filament at theWOx–NbOx interface. Such a simple metal–oxide–metal structure, with functionally separated bulk andinterface effects, provides a fabrication advantage for future high-density cross-pointmemory devices.

Surface melting of electronic order in La0.5Sr1.5MnO4

We report temperature-dependent surface x-ray scattering studies of the orbital ordered surface in La$_{0.5}$Sr$_{1.5}$MnO$_4$. We find that the interfacial width of the electronic order grows as the bulk ordering temperature is approached from below, though the bulk correlation length remains unchanged. Close to the transition, the surface is so rough that there is no well-defined electronic surface, despite the presence of bulk electronic order, that is the electronic surface has melted. Above the bulk transition, finite-sized isotropic fluctuations of orbital order are observed, with a correlation length equal to that of the electronic surfaces' in-plane correlation length at the transition temperature.