Graph partitioning induced phase transitions

We study the percolation properties of graph partitioning on random regular graphs with N vertices of degree k. Optimal graph partitioning is directly related to optimal attack and immunization of complex networks. We find that for any partitioning process (even if nonoptimal) that partitions the graph into essentially equal sized connected components (clusters), the system undergoes a percolation phase transition at f = fc = 1-2/k where f is the fraction of edges removed to partition the graph. For optimal partitioning, at the percolation threshold, we find S approximately N 0.4 where S is the size of the clusters and l approximately N 0.25 where l is their diameter. Also, we find that S undergoes multiple nonpercolation transitions for f相似文献   

Surface magnetic phase transitions in films

Using a simple Landau model, we discuss the different possibilities of generating magnetic effects at a second-order transition for films. Varying the sample size d$d$ and/or surface coupling γ$\gamma$ one can decrease or increase substantially the surface critical temperature _Ts$T_{\mathrm{s}}$ and the saturation magnetization _Ms$M_{\mathrm{s}}$. In the case of γ>0$\gamma >0$, _Ms$M_{\mathrm{s}}$ and _Ts$T_{\mathrm{s}}$ decrease from the bulk values as the film thickness is reduced. These theoretical results are in nice agreement with the experimental data on superconducting _MgB2${\mathrm{MgB}}_2$ thin films. By contrast, for γ<0$\gamma <0$, an enhancement of both quantities is expected. This extraordinary transition has rarely been observed experimentally and, usually, the situation is far from being clear. We analyze a new experiment on _NiFe2O4${\mathrm{NiFe}}_2{\mathrm{O}}_4$ ultra-thin films, where a very strong enhancement of the saturation magnetization is observed.     

Adatom-adatom interaction mediated by an underlying surface phase transition

Low temperature scanning tunneling microscopy measurements on the adsorption of single Pb adatoms on Si(111)-(square root 3 x square root 3)-Pb surfaces reveal the vertical displacement patterns induced on the substrate by these Pb adatoms as well as a novel adatom-adatom interaction. The origin of both can be traced back to the (square root 3 x square root 3)<-->(3 x 3) phase transition taking place at lower temperatures. A Landau-like approach explains the displacement patterns as due to the corresponding order parameter and shows that the vicinity of a surface phase transition gives rise to a nonmonotonic adatom-adatom interaction.     

Surface phase transitions of multiple-site associating fluids

Surface phase transitions of Lennard–Jones (LJ) based two- and four-site associating fluids have been studied for various associating strengths using grand-canonical transition matrix Monte Carlo simulations. Our results suggest that, in the case of a smooth surface, represented by a LJ 9-3-type potential, multiple-site associating fluids display a prewetting transition within a certain temperature range. However, the range of the prewetting transition decreases with increasing associating strength and increasing number of sites on the fluid molecules. With the addition of associating sites on the surface, a quasi-2D vapor–liquid transition may appear, which is observed at a higher surface site density for weaker associating fluids. The prewetting transition at lower associating strength is found to shift towards the quasi-2D vapor–liquid transition with increasing surface site density. However, for highly associating fluids, the prewetting transition is still intact, but shifts slightly towards the lower temperature range. Adsorption isotherms, chemical potentials and density profiles are used to characterize surface phase transitions.     

Surface induced disordering at first-order bulk transitions

Semi-infinite systems may undergo surface induced disordering transitions. These transitions exhibit both critical surface behaviour and interface delocalization phenomena. As a consequence, various surface exponents can be defined although there are no bulk exponents. It is shown that the corresponding power laws can be derived from a scaling form for the surface free energy where two independent surface exponentsΔ ₁ and α _s enter. In addition, global phase diagrams with finite symmetry breaking fields are also briefly discussed.     

Ultrafast vibronic phase transitions induced in semiconductors by femtosecond laser pulses

The intermode anharmonic interaction in the theory of ultrafast (t∼10⁻¹³ s) vibronic phase transitions induced on semiconductor surfaces (Si, GaAs) by femtosecond laser pulses is calculated. The conditions for plasma-induced transitions either to a state of chaotic disorder in the positions of the atoms (“cold liquid”) or into a state with crystal symmetry different from the initial symmetry (a new crystalline phase) are determined. It is shown that a NaCl-type structure is realized in GaAs for a transition of the second type, the transition being due to the instability of the longitudinal optical phonon branch. The corresponding numerical estimates are made for Si and GaAs. Fiz. Tverd. Tela (St. Petersburg) 41, 1462–1466 (August 1999)     

Nonequilibrium phase transitions induced by multiplicative noise: effects of self-correlation

A recently introduced lattice model, describing an extended system which exhibits a reentrant (symmetry-breaking, second-order) noise-induced nonequilibrium phase transition, is studied under the assumption that the multiplicative noise leading to the transition is colored. Within an effective Markovian approximation and a mean-field scheme it is found that when the self-correlation time tau of the noise is different from zero, the transition is also reentrant with respect to the spatial coupling D. In other words, at variance with what one expects for equilibrium phase transitions, a large enough value of D favors disorder. Moreover, except for a small region in the parameter subspace determined by the noise intensity sigma and D, an increase in tau usually prevents the formation of an ordered state. These effects are supported by numerical simulations.     

Theory of photoinduced phase transitions in itinerant electron systems

Theoretical progress in the research of photoinduced phase transitions is reviewed with closely related experiments. After a brief introduction of stochastic evolution in statistical systems and domino effects in localized electron systems, we treat photoinduced dynamics in itinerant-electron systems. Relevant interactions are required in the models to describe the fast and ultrafast charge-lattice-coupled dynamics after photoexcitations. First, we discuss neutral-ionic transitions in the mixed-stack charge-transfer complex, TTF-CA. When induced by intrachain charge-transfer photoexcitations, the dynamics of the ionic-to-neutral transition are characterized by a threshold behavior, while those of the neutral-to-ionic transition by an almost linear behavior. The difference originates from the different electron correlations in the neutral and ionic phases. Second, we deal with halogen-bridged metal complexes, which show metal, Mott insulator, charge-density-wave, and charge–polarization phases. The latter two phases have different broken symmetries. The charge-density-wave to charge–polarization transition is much more easily achieved than the reverse transition. This is clarified by considering microscopic charge-transfer processes. The transition from the charge-density-wave to Mott insulator phases and that from the Mott insulator to metal phases proceed much faster than those between the low-symmetry phases. Next, we discuss ultrafast, inverse spin-Peierls transitions in an organic radical crystal and alkali-TCNQ from the viewpoint of intradimer and interdimer charge-transfer excitations. Then, we study photogenerated electrons in the quantum paraelectric perovskite, SrTiO₃, which are assumed to couple differently with soft-anharmonic phonons and breathing-type high-energy phonons. The different electron–phonon couplings result in two types of polarons, a “super-paraelectric large polaron” with a quasi-global parity violation, and an “off-center-type self-trapped polaron” with only a local parity violation. The former is equivalent to a charged and conductive ferroelectric domain, which greatly enhances both the quasi-static electric susceptibility and the electric conductivity. Finally, we outline the development of time-resolved X-ray diffraction experiments, which directly accesses the dynamics of electronic, atomic and molecular motions in photoexcited materials. They are extremely useful when a three-dimensional structural long-range order is established and changes the symmetry.     

Bubble-nucleation rates for radiatively induced first-order phase transitions

We present a consistent calculation of bubble-nucleation rates in theories of two scalar fields. Our approach is based on the notion of a coarse-grained free energy that incorporates the effects of fluctuations with momenta above a given scale k. We establish the reliability of the method for a variety of two-scalar models and confirm the conclusions of previous studies in one-field theories: Langer's theory of homogeneous nucleation is applicable as long as the expansion around the semi-classical saddle point associated with tunnelling is convergent. This expansion breaks down when the exponential suppression of the rate by the saddle-point action becomes comparable to the pre-exponential factor associated with fluctuations around the saddle point. We reconfirm that Langer's theory is not applicable to the case of weakly first-order phase transitions. We also find that the same is true in general for radiatively induced first-order phase transitions. We discuss the relevance of our results for the electroweak phase transition and the metastability bound on the Higgs-boson mass.     

Jahn-Teller effect induced phase transitions in CsCuCl3

Occurence of the three Jahn-Teller effect induced phase transitions of CsCuCl₃ at 423, 510 and 535 K has been established and the nature of the transitions examined by X-ray crystallography, far infrared spectroscopy and other techniques.     

Simulation of phase transitions induced in gallium arsenide by combined laser radiation

We performed a numerical simulation of phase transitions in gallium arsenide that are induced by the combined action of nanosecond laser pulses initiating melting and an additional neodymium-glass laser irradiation enabling the control of the interface velocity. In the case of counterpropagating laser beams, a strong temperature dependence of the absorption factor at 1.06 μm occurs. It causes a thermal wave, which separates from the melting front and, propagating towards the neodymium-glass laser beam, screens the melt. For copropagating laser beams, regimes with a nonmonotonic time dependence of the melt depth may exist.     

Ferroelastic phase transitions induced by the orbital and rotation ordering in perovskites

The phases formed in cubic crystals upon ferrodistortion and antiferrodistortion orientational ordering of anion polyhedra distorted as a result of the Jahn-Teller effect are established in terms of the statistical model in the mean-field approximation. For the four-sublattice three-minimum model, there are four types of ordered phases, and one of them is the intrinsic ferroelastic (ferrodistortion) phase. The conditions providing the appearance of ordering of a particular type are determined. The effect of orbital ordering on the rotational structural distortions is discussed. The conditions of the formation of ordered phases (D _4h ⁵ , D _4h ¹⁸ ) that are most frequently encountered in perovskites are found within the eight-sublattice three-minimum model. It turns out that, under certain conditions, these phases can coexist in the same crystal and transform into each other. The variants of their mutual transformations are considered.     

Surface spin dynamics of magnets near spin-orientational phase transitions

The sufficient conditions for the formation of a dipole-exchange generalized surface spin wave which softens near a volume spin-orientational phase transition are found for a definite form of the high-frequency magnetic susceptibility tensor. Fiz. Tverd. Tela (St. Petersburg) 40, 1532–1536 (August 1998)     

Surface critical behavior in systems with nonequilibrium phase transitions

We study the surface critical behavior of branching-annihilating random walks with an even number of offspring (BARW) and directed percolation (DP) using a variety of theoretical techniques. Above the upper critical dimensions d(c), with d(c)=4 (DP) and d(c)=2 (BARW), we use mean field-theory to analyze the surface phase diagrams using the standard classification into ordinary, special, surface, and extraordinary transitions. For the case of BARW, at or below the upper critical dimension d相似文献   

Pressure and temperature induced phase transitions in LiInSe2

Abstract

At 4.1 GPa LiInSe₂ transforms from the β-NaFeO₂ - type structure to the NaCl-type structure LiInSe₂-hpI (cubic; Fm3m; a=546.4(3)pm, Z=2, D _x =5.75g/cm³; 4.1GPa) which remains metastable at normal conditions. Heating to 210°C at 1.8 GPa causes ordering of the cations and a phase transition from LiInSe₂-hpI to the α-NaFeO₂ - type structure LiInSe₂-hpII (rhombohedral; R3m; a=393.4pm, c=1919.7pm, Z=3, D _x =5.53g/cm³; 1.8GPa). Heating to 210°C at 0.27 GPa results in a phase transformation from LiInSe₂-hpII to the chalcopyrite-type phase LiInSe₂-hpIII (tetragonal; 142d; a=580.7(8)pm, c=1181.0(31)pm, Z=4, D _x =4.66g/cm³; 0.27 GPa).     

Insulator-superconductor-metal transitions induced by spin fluctuations in the paramagnetic phase of doped insulators

AbstractThe band structure of cuprates as a doped 2D insulator is modeled assuming that the excess charge carriers are associated with the corresponding substitution atoms, and the phase diagram of the paramagnetic states as a function of the degree x of doping at zero temperature is studied. The Hamiltonian contains electronic correlations on impurity orbitals and hybridization between them and the initial band states of the insulator. It is shown that the change in the electronic structure of a doped compound includes the formation of impurity bands of distributed and localized electronic states in the initial insulator gap. It is established that in the case of one excess electron per substitution atom the spin fluctuations (1) give rise to an insulator state of the doped compound for x < x _thr, 1, (2) lead to a superconducting state for x _thr, 1 < x < x _thr, 2, and (3) decay as x > x _thr, 2 increases further, and the doped compound transforms into a paramagnetic state of a “poor” metal with a high density of localized electronic states at the Fermi level.