Showing papers in "Phase Transitions in 2002"

X-ray Photoelectron Spectroscopy of Sb2S3 Crystals

Abstract: The paper presents the X-ray photoelectron spectra (XPS) of the valence band and core levels of semiconductor ferroelectric Sb2S3 single crystals, which show weak phase transitions and anomalies of various physical properties. The XPS were measured with monochromatized Al K α radiation in the energy range 0-1450 eV and the temperature range 160-450 K. The valence band is located 0.8-7.5 eV below the Fermi level. Experimental results of the valence band and core levels are compared with the results of theoretical ab initio calculations of the molecular model of Sb2S3 crystal. The chemical shifts in Sb2S3 crystal for the Sb and S states are obtained. Results revealed that the small structural rearrangements at the phase transition T c1 = 300 K shift the Fermi level and all electronic spectrum. Also, temperature dependence of a spontaneous polarisation shifts the electronic spectra of the valence band and core levels. Specific temperature-dependent excitations in Sb 3d core levels are also revealed.

First-Order Transition of Fe 2 P and Anti-Metamagnetic Transition

Abstract: A fixed-spin-moment (FSM) calculation is carried out for Fe 2 P using the linear muffin-tin orbital method. It is found that the Fe atom at the 3f site shows a metamagnetic behavior, while the Fe atom at the 3g site is ferromagnetic. In order to study the magnetic properties of this compound at finite temperatures, a simple model based on the Landau-Ginzburg theory is proposed, by taking into account the effect of spin fluctuations. The Landau parameters included in this model are estimated from the results obtained by fixed-spin moment band-calculations. It is shown that the first-order transition at T C observed for Fe 2 P is attributed to the metamagnetic Fe atoms at the 3f site. The present mechanism of the first-order transition is connected with the anti-metamagnetic transition observed in some rare-earth and Co compounds.

Capillary Pressure and Phonons in Ag, Au, Cu and Ni Nanoparticles

Abstract: Spherical nanoparticles of fcc metals with diameters between 2 and 10 nm have been studied by molecular-dynamics simulations. The results show that despite of the small size of the clusters the Kelvin-equation accounts for the capillary pressure building up in the particles. Furthermore, the simulations show that the capillary pressure leads to a shift of the local vibrational density of states in the core of the particles to higher energies. The cluster's total vibrational density of states is found to be broadened by the contributions from surface atoms. The connection between this broadening and the density of states in nanocrystalline materials is discussed.

LOW-TEMPERATURE PHASE TRANSITIONS IN THE TRIGONAL MODIFICATION OF Cs3Bi2Br9 AND Cs3Sb2I9

Abstract: The trigonal (P-3 m1) modification of Cs 3 Bi 2 Br 9 and Cs 3 Sb 2 I 9 have been studied using NQR, X-ray single crystal and powder pattern methods. Moreover, the heat capacity was measured in a wide temperature interval: 4-300 K. In Cs 3 Bi 2 Br 9 a second-order phase transition was found at T C = 96 K. The low-temperature phase is monoclinic (C12/c1), with the unit cell doubled along the [001] direction. Cs 3 Sb 2 I 9 has a sequence of phase transitions at T C = 85 K, T i = 78 K and T L = 72.1 K. The monoclinic structure below 85 K is isomorphic with the low-temperature structure of Cs 3 Bi 2 Br 9 . According to calorimetric data the lock-in transition at 72.1 K is discontinuous.

Organic-Inorganic Hybrid Spin-Valve: A Novel Approach to Spintronics

Abstract: The combined use of highly correlated solids opens the possibility to explore new phenomena and to generate new hybrid devices for Spintronics. We have recently demonstrated the spin injection of carriers into an organic semiconductor, sexithienyl (T 6 ). A spin-valve device has been realized by using two colossal magnetoresistance (CMR) half metallic electrodes separated by the organic semiconductor. The total magnetoresistance as large as 30% was detected for channel lengths lower than 140 nm.

Slow Relaxation in Ice and Ice Clathrates and its Connection to the Low-Temperature Phase Transition Induced by Dopants

Abstract: We have used dielectric spectroscopy (frequency range: 10 6 Hz-10 m 3 Hz) and differential scanning calorimetry down to a temperature of 77 K, to study the effect of various dopants on the molecular relaxation in hexagonal ice ( I h ) and some clathrate hydrates ( I c ). The nonelectrolytic dopants did not affect a large change in the dynamics. However, when I h and I c are doped with the alkali hydroxides: KOH, NaOH, LiOH and Ca(OH) 2 , a drastic fall in the relaxation times of the order of 10 7 -10 12 has been noticed. It appears that the alkali metal- and hydroxyl-ion pair exert a large influence on the orientational mobility of water molecules by way of polarization over a domain. The amount of phase that gets transformed to the low temperature (low- T ) ordered phase depends on the solid solubility of the dopant. The nature of the relaxation and variation of the static dielectric constant have been examined critically to get an insight into the actual mechanism responsible for the above phenomena. Ou...

A Ginzburg-Landau Theory For Ni-Mn-Ga

Abstract: We present an effective Ginzburg-Landau theory for ferromagnetic cubic Ni-Mn-Ga in which structural and magnetic phase transitions take place. The general problem of a cubic ferromagnet is taken into account by fitting the parameters of the Ginzburg-Landau functional to experimental data. This enables us to obtain a realistic phase diagram for Ni-Mn-Ga.

High pressure and high temperature studies on manganese oxides

Abstract: High pressure and high temperature quench experiments on f -MnO 2 , Mn 2 O 3 and sol gel derived manganese oxides have been carried out to identify any new phases to which the materials may transform under high pressure and high temperature conditions Results of ESR, DTA and TGA investigations on sol gel derived manganese oxide have shown it to be hausmannite Mn 3 O 4 , instead of n -Mn 2 O 3 as reported earlier in the literature The sol gel derived manganese oxide transforms to n -Mn 2 O 3 when heated above 700°C Sol gel derived Mn 3 O 4 , when quenched from 5 GPa and temperature range 800-1200°C, gives a mixture of Mn 3 O 4 (hausmannite) and a phase having CaMn 2 O 4 (marokite)-type structure f -MnO 2 undergoes partial amorphization when pressure-quenched from 8 GPa at room temperature The high pressure and high temperature quench experiments up to 5 GPa and 700°C showed that the decomposition temperature of f -MnO 2 increases with pressure The new phase reported by Liu (1976) from diamond-anvil c

Photo-Induced Phase Transition to a Striped Polaron Crystal in Cuprates

Abstract: Here we report the evidence of a disorder-to-order photo-induced phase transition (PIPT) under X-ray (h x = 12.4 keV) illumination in oxygen doped La 2 CuO 4 + i . The commensurate striped polaron crystalline phase is induced by the irradiation of X-rays in a cuprate perovskite at hole doping i ¨ 1/8 and high Cu-O micro-strain, k >7%. The nucleation of long range crystalline phases of holes on the oxygen 2 p x,y orbitals in the CuO 2 plane associated with pseudo Jahn Teller local lattice distortions (LLD) is favored by mobile counter-ions (interstitial oxygen ions in the La 2 O 2 layer) in the temperature range 200-350 K. The PIPT appears beyond a threshold dose of 10 17 photons/cm 2 at 220 K and a power law of the polaron crystal growth is observed as a function of the X-ray illumination dose.

Morphological Phase Transitions in Spontaneous Dewetting of Thin Films on Homogeneous and Heterogeneous Surfaces

Abstract: An overview of recent work on the stability, morphology and intermolecular forces in thin liquid films is presented. Various stages of evolution of the surface instability and pattern formation are investigated for unstable thin (∼ 100 nm) films subjected to a long-range van der Waals attraction and a shorter-range repulsion. Numerical solutions of the nonlinear thin film equation provide 3-d morphologies, which are compared with experiments on thin (< 300 nm) films of polydimethylsiloxane (PDMS) sandwiched between water and bimodal brushes of PDMS chemically grafted on silicon wafers. Initially, random disturbances are quickly reorganized into a small-amplitude undulating structure consisting of long "hills" and "valleys". Two distinct pathways of morphological evolution and dewetting are found thereafter - depending on the initial film thickness vis-a-vis location of the minimum in the spinodal parameter (intermolecular force per unit volume curve). While dewetting of relatively thick films occurs by th...

Atomistic Modeling of Diffusion in Aluminum

Abstract: We present molecular-dynamics simulations of self-diffusion in Al. In order to facilitate the description of elastic and vibrational properties as well as vacancy migration, an embedded-atom method potential was used in the simulations. This potential was specifically designed to reproduce the T = 0 K equation of state of Al obtained by ab initio total-energy calculations. We show that the temperature dependent self-diffusion coefficient obeys an Arrhenius law and that the resulting dynamical migration energy is slightly larger than the static migration energy obtained by using classical rate theory.

Photo-reversible valence tautomerism in a Co compound

Abstract: Photo-illumination effects have been measured for the compound [Co III-LS (tmpda)(3,6-DBSQ)(3,6-DBCat)]. When the aforementioned compound was illuminated at 5 K, the magnetization value increased from w eff = 1.87 to 2.03 w B . The absorption spectra after illumination show that the absorption band at around 730 nm, which is characteristic of the [Co II-HS (tmpda)(3,6-DBSQ) 2 ] state, increased in intensity and that the charge transfer band from 3,6-DBCat to 3,6-DBSQ at 2500 nm was reduced in intensity. IR spectra show that the C-O stretching peak at 1279 cm m 1 is decreased in intensity. These results confirm that a photo-induced intra-molecular electron transfer, [Co III-LS (tmpda)(3,6-DBSQ)(3,6-DBCat)] to; [Co II-HS (tmpda)(3,6-DBSQ) 2 ], was induced. The lifetime of the metastable state at 5 K was 15 min. Furthermore, it was found that reverse valence tautomerism can be induced by exciting the Co II-HS to 3,6-DBSQ charge transfer band in metastable [Co II-HS (tmpda)(3,6-DBSQ) 2 ].

Atomistic Investigations of the Thermodynamical Stability and Martensitic Nucleation of Fe 80 Ni 20 Nanoparticles

Abstract: The study of the thermodynamical stability of Fe 80 Ni 20 nanoparticles in the framework of embedded-atom method and molecular-dynamics simulations shows evidence of large surface contributions to the free energy for particles with diameters up to 15 nm or more. The change of the free energy compared to its bulk value leads to a reduction of the martensitic equilibrium temperature, M 0 , and can suppress martensitic transformations in small particles. In addition the formation of martensite can be suppressed due to the lack of pre-existing nucleation sites necessary to overcome the energy barrier along the transformation path from austenite to martensite.

SnO 2 : Bulk and Surface Simulations by an Ab Initio Numerical Local Orbitals Method

Abstract: The results of first-principles crystal structure optimization for the bulk rutile-type tin dioxide and its (110) and (001) surfaces as obtained by applying the siesta code, that incorporates norm-conserving pseudopotentials and strictly localized basis of pseudoatomic orbitals, are summarized. The relaxation near the (110) surface has been studied with the use of small supercells Sn 6 O 10 and Sn 6 O 12 , representing reduced and stoichiometric compositions, respectively,and compared with previously known results of other simulations. Further on, the effect of relaxation has been studied for a larger (stoichiometric) supercell Sn 10 O 20 . While qualitatively the same as obtained for small supercells, the relaxation pattern shows however numerical differences. The relaxation study at the (001) surface shows, as the major effect, the inward relaxation of the upper tin layer and the outward displacement of upper oxygen atoms (by , 0.3 A) with respect to it. When going deep into the crystal, the values of S...

A First-Principles Study of 11-Cis-Retinal: Modelling the Chromophore-Protein Interaction In Rhodopsin

Abstract: The 11- cis -retinal protonated Schiff base is the chromophore of rhodopsin, the photoreceptor in the vertebrate eye. The photochemical isomerization from 11- cis to the all- trans form triggers a ...

X-ray diffraction investigation of oleic acid under high pressure

Abstract: Pressure has a specific influence on the light scattering in those liquids in which all molecules have an unsaturated double bond C=C. Strong changes of intensity of transmitted and scattered light in oleic acid were observed during its transition to the high-pressure phase. In this paper, local order in liquid of oleic acid has been studied using the X-ray diffraction photographic Laue method. Experiments were run at room temperature under atmospheric pressure as well as under pressures up to 0.25 GPa. At pressures above 0.2 GPa a strong increase of ordering of molecules, similar to a smectic liquid crystal phase, has been observed. This confirms results obtained previously by studies of light scattering under high pressure.

Inherent Structure Approach to the Study of Glass-Forming Liquids

Abstract: The inherent structure approach, wherein thermodynamic and structural changes in glass-forming liquids are analyzed in terms of local potential energy minima that the liquid samples, has recently been applied extensively to the study of thermodynamic aspects of glass-forming liquids. The evaluation of the configurational entropy, which arises from the multiplicity of local energy minima, plays a central role in such analyses. Results are presented here concerning the calculation of configurational entropy based on computer simulations of a model liquid; and the application of the inherent structure formalism to the study of the glass transition locus, and the fragility of glass-forming liquids.

Phase diagram of a two-species lattice model with a linear instability

Abstract: We discuss the properties of a one-dimensional lattice model of a driven system with two species of particles in which the mobility of one species depends on the density of the other. This model was introduced by Lahiri and Ramaswamy (Phys. Rev. Lett., 79, 1150 (1997)) in the context of sedimenting colloidal crystals, and its continuum version was shown to exhibit an instability arising from linear gradient couplings. In this paper we review recent progress in understanding the full phase diagram of the model. There are three phases. In the first, the steady state can be determined exactly along a representative locus using the condition of detailed balance. The system shows phase separation of an exceptionally robust sort, termed strong phase separation, which survives at all temperatures. The second phase arises in the threshold case where the first species evolves independently of the second, but the fluctuations of the first influence the evolution of the second, as in the passive scalar problem. The second species then shows phase separation of a delicate sort, in which long-range order coexists with fluctuations which do not damp down in the large-size limit. This fluctuation-dominated phase ordering is associated with power law decays in cluster size distributions and a breakdown of the Porod law. The third phase is one with a uniform overall density, and along a representative locus the steady state is shown to have product measure form. Density fluctuations are transported by two kinematic waves, each involving both species and coupled at the nonlinear level. Their dissipation properties are governed by the symmetries of these couplings, which depend on the overall densities. In the most interesting case,, the dissipation of the two modes is characterized by different critical exponents, despite the nonlinear coupling.

Non-linear relaxation of the photo-induced high spin state in spin-crossover solids: effect of correlations

Abstract: We investigate a microscopic model based on the pair approximation, for the dynamical properties of the photo-induced high-spin state in spin-crossover solids at low temperature. The model uses the Ising-like hamiltonian and combines long- and short-range interactions. The stochastic treatment of the latter provides a set of two coupled differential equations for the macroscopic short- and long-range order parameters, which reproduce successfully the main features of the experimental relaxation curves.

Superspin Glass Behavior of Interacting Ferromagnetic Nanoparticles in Discontinuous Magnetic Multilayers

Abstract: Discontinuous magnetic multilayers [Co 80 Fe 20 ( t )/Al 2 O 3 (3 nm)] 10 with t = 0.9 and 1.0 nm are studied by Superconducting Quantum Interference Device (SQUID) magnetometry and ac susceptibility. Owing to dipolar interaction the superparamagnetic cluster systems undergo collective glass-like freezing upon cooling. While both samples exhibit very similar glass temperatures T g , 45 K and critical exponents z x , 10 and n , 1.4 as obtained from the temperature dependencies of the relaxation time, ‰ , and the nonlinear susceptibility, h 3 , dynamical scaling reveals different critical exponents, g (0.9 nm) , 1.0 and g (1.0 nm) , 0.6, respectively.

Layer-Resolved Magneto-Optical Kerr Effect In Semi-Infinite Inhomogeneous Layered Systems

Abstract: The contour integration technique applied to calculate the optical conductivity tensor at finite temperatures in the case of inhomogeneous surface layered systems within the framework of the spin-polarized relativistic screened Korringa-Kohn-Rostoker band structure method is extended to arbitrary polarizations of the electric field. It is shown that besides the inter-band contribution, the contour integration technique also accounts for the intra-band contribution. Introducing a layer-resolved complex Kerr angle, the importance of the first, non-magnetic buffer layer below the ferromagnetic surface on the magneto-optical Kerr effect in the Co | Pt m multilayer system is shown. Increasing the thickness of the buffer Pt, the layer-resolved complex Kerr angles follow a linear dependence with respect to m only after nine Pt mono-layers.

Ab Initio Study of Iron and Cr/Fe(001)

Abstract: In the present paper interfacial mixing of a thin chromium overlayer on bcc iron is studied. The calculations are performed in the framework of a pseudo-potential technique using the generalized gr...

A Comparison of Different Orthogonal Tight-Binding Molecular Dynamics Simulation Methods for Silicon Clusters

Abstract: In the present work different orthogonal tight-binding molecular dynamics methods have been employed for describing small silicon clusters. The cohesive energies calculated using these methods have been compared with those found from the first principles Car-Parrinello method. The comparison shows that the orthogonal tight binding matrix elements and repulsive potentials need to include the radial cutoff up to fourth neighbor distance in diamond structure in order to reproduce ab initio results. The environmental correction is not needed in this orthogonal tight-binding method.

First Observation of a Composition-Controlled Low-Moment/High-Moment Transition in the Fcc Fe-Ni System: Implications Regarding Invar and Anti-Invar Behaviors

Abstract: We report the first direct observation of a high-moment (HM)/low-moment (LM) transition occurring in face centered cubic (FCC) Fe-Ni alloys. 57 Fe Mossbauer isomer shifts (ISs) give local electronic densities that exhibit a large discontinuity of , 0.4 el./ $ a_{0}^3 $ at the transition that spans the concentration range ∼ 65-75 apc (atomic percent) Fe, in agreement with ab initio predictions. In the most Fe-rich alloys that have LM ground states (including n -Fe), we show that thermal stabilization of the HM state occurs at high temperatures, thereby providing an experimental proof that anti-Invar behavior is due to such HM stabilization. In Invar (Fe 65 Ni 35 ) and at near-Invar compositions, we observe temperature-induced changes in electronic density that follow the spontaneous magnetization curves and find that Invar is predominantly a HM phase at all temperatures where an Invar effect occurs. We show that LM phase thermal excitation cannot cause the Invar effect and that such excitation would cause ...

INVAR BEHAVIOR IN Fe-Ni ALLOYS IS PREDOMINANTLY A LOCAL MOMENT EFFECT ARISING FROM THE MAGNETIC EXCHANGE INTERACTIONS BETWEEN HIGH MOMENTS

Abstract: I argue that the main models that have been advanced to explain Invar behavior in Fe-Ni alloys (the original, classical, Invar system) can all be shown to be critically deficient, except one: The local moment frustration model of Rancourt and Dang ( Phys. Rev. B , 54 , 12225, 1996). The latter model explains all the measured structural, magnetic, and magnetovolume features of the Fe-Ni alloys with 0-65 apc (atomic percent) Fe, based on the assumptions that these systems are predominantly high-moment in character at the temperatures of interest and that the Fe-Fe pairs have large inter-atomic separation dependencies of their magnetic exchange parameters. The large magnetovolume Fe-Fe couplings are understood (based on ab initio electronic structure calculations) as a precursor effect of the low-moment/high-moment (LM/HM) transition that has recently been observed to occur at larger Fe concentrations, as a continuous transition occurring in the range , 65-75 apc Fe (Lagarec, Ph.D. thesis, 2001).

Instabilities and Self-Organization Phenomena in High Density Exciton Systems

Abstract: The influence of final value of exciton lifetime on exciton condensed phase formation is investigated in deterministic and stochastic approach. It is shown that in some region of pumping, the periodical distribution of exciton density takes place. The numerical solution for periodical distribution of exciton density is obtained for different values of pumping. There are critical values of exciton lifetime for appearance of exciton condensed phase. The distribution density function becomes narrower with increasing pumping.

Smectic A-C-A Liquid Crystal Reentrance: A Photon Transmission Study

Abstract: Ultraviolet/visible photon transmission measurements were applied to study liquid crystal phase transitions in the binary BOPDOB-BOPOOB mixture. A sequence exhibiting a new reentrant, namely isotropic-nematic-smectic A-C-A as temperature is lowered, is identified. For the smectic A-C transition, the critical exponent g is seen to cross over from the mean-field 0.501 - 0.008 to the helium-like 0.336 - 0.012 as the transition is approached. The reentrant C-A transition is first-order, signalling a tricritical point in the phase diagram.

Inhomogeneous relaxation in vibrated granular media: consolidation waves

Abstract: We investigate the spatial dependence of the density of vibrated granular beds, using simulations based on a hybrid Monte Carlo algorithm. We find that the initial consolidation is typically inhomogeneous, both in the presence of a constant shaking intensity and when the granular bed is subjected to "annealed shaking". We also present a theoretical model which explains such inhomogeneous relaxation in terms of a "consolidation wave", in good qualitative agreement with our simulations. Our results are also in qualitative agreement with recent experiments.

Layer-by-layer growth for pulsed laser deposition

Abstract: Pulsed laser deposition (PLD) is a popular growth method, which has been successfully used for fabricating thin films. Compared to continuous deposition (like molecular beam epitaxy) the pulse intensity can be used as an additional parameter for tuning the growth behavior, so that under certain circumstances PLD improves layer-by-layer growth. We present kinetic Monte-Carlo simulations for PLD in the submonolayer regime and give a description of the island distance versus intensity. Furthermore we discuss a theory for second layer nucleation and the impact of Ehrlich-Schwoebel barriers on the growth behavior. We find an exact analytical expression for the probability of second layer nucleation during one pulse for high Ehrlich-Schwoebel barriers.

Thermochromism and Magnetic Phase Transition in an Organic Radical 1,3,5-Trithia-2,4,6-Triazapentalenyl

Abstract: Thermochromic and magnetic properties of an organic radical, 1,3,5-trithia-2,4,6-triazapentalenyl (TTTA) crystal were investigated through polarized reflection and EPR measurements. TTTA shows a first-order magnetic phase transition with a wide thermal hysteresis loop near room temperature. The high temperature (HT) phase is paramagnetic due to a uniform one-dimensional stacking of the radical molecules, while the low temperature (LT) phase is diamagnetic due to strong dimerization along the stacking direction. The colors of the HT and LT phases are purple and green, respectively, and both phases show a strong dichroism. We also found that part of the LT phase changes to the HT phase by laser light irradiation. The experimental results suggest the possibility of optical control of magnetic bistabitity in TTTA crystal.