Vortex sublattice melting in a two-component superconductor

We consider the vortices in a superconductor with two individually conserved condensates in a finite magnetic field. The ground state is a lattice of cocentered vortices in both order parameters. We find two phase transitions: (i) a "vortex sublattice melting" transition where vortices in the field with lowest phase stiffness ("light vortices") lose cocentricity with the vortices with large phase stiffness ("heavy vortices"), entering a liquid state (the structure factor of the light vortices vanishes continuously; this transition is in the 3Dxy universality class); (ii) a first-order melting transition of the lattice of heavy vortices, in a liquid of light vortices.     

面心立方铁纳米粒子的相变与并合行为的分子动力学研究

本文采用分子动力学模拟结合Finnis-Sinclair多体势研究了面心立方铁纳米粒子在加温过程中的相变与并合行为. 模拟结果表明: 纳米粒子在熔化之前均发生了由面心立方至体心立方的马氏体相变; 大小相等的两纳米粒子在并合之前发生了相对转动; 而大小不等的两纳米粒子在并合过程中并未出现转动, 小纳米粒子倾向于吸附在大纳米粒子上, 并随着温度的升高而熔化, 最终形成更大的纳米粒子. 关键词： 纳米粒子 相变 并合 分子动力学     

Coulomb clusters: melting and potential barriers

The melting of two-dimensional and three-dimensional Coulomb micro- and macroclusters is studied. Temperature dependences of radial and angular square deviations of particles are investigated. The melting of microclusters has two stages: at lower temperature there is a transition from a frozen phase to a state with a rotatory reorientation of “crystalline” shells relative to each other, different pairs of shells melting at different temperatures. In the case of large N and high triangular symmetry inside the cluster, orientational melting takes place only for external pairs of shells. In this case external shells lose their order. At higher temperature a transition with a loss of radial shell order occurs. The origin of two-stage melting is in the smallness of the barrier energy relative to the rotation of shells in comparison with the barrier corresponding to the radial disordering of shells. It is shown also that the temperatures of orientational and total melting are at 5–15 times lower than the temperatures of disappearance of corresponding potential barriers. The influence of confinement anisotropy on the character of cluster melting is considered. It is found that at some degree of anisotropy the melting becomes one stage. The last is connected with an increase of the ratios of barriers of intershell rotation to barriers of jumps of a particle between the shells.     

The S-FFT calculation of Collins formula and its application in digital holography

We study analytically and numerically the properties of one-dimensional chain of cold ions placed in a periodic potential of optical lattice and global harmonic potential of a trap. In close similarity with the Frenkel-Kontorova model, a transition from sliding to pinned phase takes place with the increase of the optical lattice potential for the density of ions incommensurate with the lattice period. We show that at zero temperature the quantum fluctuations lead to a quantum phase transition and melting of pinned instanton glass phase at large values of dimensional Planck constant. After melting the ion chain can slide in an optical lattice. The obtained results are also relevant for a Wigner crystal placed in a periodic potential.     

Two-dimensional melting of dislocation vector systems

Dislocation vector systems with various dislocation core energies are simulated, and the nature and the mechanism of the melting phase transition there is determined by means of the energy, specific heat, dislocation density, renormalized coupling constant, shear modulus and orientational stiffness constant as well as microscopic configurations of dislocation vectors. For a system with a large core energy the melting transition is found to be continuous, caused by the dislocation unbinding mechanism predicted by Kosterlitz-Thouless and Halperin-Nelson-Young. For a system with a small core energy, grain boundary loops are nucleated in the process of melting and the phase transition turns out to be first order. The latter agrees with most of the computer experiments on atomistic systems.     

Melting from one to two to three dimensions

The transformation of a crystalline solid into a liquid, seeming to have no precursor and no intermediate states, has challenged scientists for over a century. The search for the fundamental mechanism stimulated the development of quantum mechanics, concepts of the roles of dimensionality and topological order in condensed matter, and experimental techniques to test the theories. We now understand that the transition begins at lower temperatures than the melting point of the bulk. It starts at the edges of crystal planes, progresses across the surface, evolves into the successive melting of atomic layers, and ends in bulk phase coexistence. The memory of the process remains within a few molecular distances at the crystal-melt interface.     

Differential thermal analysis at high pressures— VIII: Phase behaviour of solid cyclopentanone,cyclopentanol, and cyclohexanone up to 3 KBAR

The phase behaviour of solid cyclopentanone, cyclopentanol, and cyclohexanone was investigated from 80 K to the melting temperature and up to 3 Kbar, using a low-temperature high-pressure differential thermal analysis apparatus. The melting temperature of cyclopentanone rises from 221.7 K at atmospheric pressure to 278 K at 2900 bar. No solid solid transition was observed. The melting temperature of cyclohexanone rises from 242.4 K at atmospheric pressure to 312 K at 2840 bar. Its well-known solid solid transition at atmospheric pressure (220.3 K) splits into two different solid solid transitions at elevated pressures. The melting temperature of cyclopentanol rises from 256 K at atmospheric pressure to 328 K at 2600 bar. Cyclopentanol exhibits two well-known solid solid transitions (236.4 K and 202.5 K at 1 atm), but an additional metastable form has been observed in the present work. the transition temperature being 195 K at 1 atm. Volume changes accompanying the phase transitions have been calculated using the Clausius Clapeyron equation.     

Statistical mechanics of melting mediated by two types of defects

We propose a defect-mediated melting theory based on the statistics of two types of lattice defects, the point defects and dislocation pairs. The model predicts a first-order phase transition. Based on the model, phase transition temperature, latent heat and other thermodynamic functions are derived. Melting occurs due to discontinuous growth of point defects into dislocation pairs. The calculated phase transition temperature for five alkali metallic crystals are in fair agreement with measured melting temperatures, and the Richards' rule is derived by the model also.     

Solid-to-molecular-orientational-hexatic melting induced by local environment determined defect proliferations

Two-dimensional (2D) melting is a fundamental research topic in condensed matter physics, which can also provide guidance on fabricating new functional materials. Nevertheless, our understanding of 2D melting is still far from being complete due to existence of possible complicate transition mechanisms and absence of effective analysis methods. Here, using Monte Carlo simulations, we investigate 2D melting of 60° rhombs which melt from two different surface-fully-coverable crystals, a complex hexagonal crystal (cHX) whose primitive cell contains three rhombs, and a simple rhombic crystal (RB) whose primitive cell contains one rhomb. The melting of both crystals shows a sequence of solid, hexatic in molecular orientation (H^mo), and isotropic phases which obey the Berezinskii-Kosterlitz-Thouless-Halperin-Nelson-Young (BKTHNY) theory. However, local polymorphic configuration (LPC) based analysis reveals different melting mechanisms: the cHX-H^mo transition is driven by the proliferation of point-like defects during which defect-associated LPCs are generated sequentially, whereas the RB-H^mo transition is driven by line defects where defect-associated LPCs are generated simultaneously. These differences result in the observed different solid-H^mo transition points which are φ_A=0.812 for the cHX-H^mo and φ_A=0.828 for the RB-H^mo. Our work will shed light on the initial-crystal-dependence of 2D melting behavior.     

X-ray study of molecular oxygen adsorbed on graphite

We have performed a detailed X-ray diffraction study of O₂ adsorbed on UCAR-ZYX and Le Carbon Lorraine vermicular exfoliated graphite between 15 and 50 K. At least four phases of physisorbed oxygen are found. The monolayer δ phase consists of a centered parallelogram lattice, with the molecular axes parallel to the graphite surface. The data are consistent with a triple point at 26 K. The melting transition at a coverage of one monolayer appears to be first order. At higher coverages the molecules undergo a lying-down to standing-up transition; the higher coverage ζ phase froms an approximately triangular lattice with the molecular axes perpendicular to the graphite surface. Satellite peaks around the (1, 0) Bragg peak indicate, however, that this cannot be a simple triangular lattice; possible explanations include successively incommensurate layers or a sinusoidal density modulation. For coverages in the two-layer region the ζ phase modulation peaks disappear at 37 K, and at 40 K the adsorbed oxygen appears to undergo a first order melting transition into a fluid phase. With increasing coverage, the 2D X-ray diffraction profiles and phase boundaries do not connect smoothly onto those of the 3D α and β phases. At low temperatures (T < 30 K) the ζ phase always coexists with bulk crystallites; for temperatures near the 2D melting transition the 3D peaks are not observable. These data, taken together with the heat capacity results, suggest a wetting transition with only the bilayer lamellar phase or bulk O₂ being stable at low temperatures.     

Anomalous Scenario for Termination of Metastable Melting in Simplified Coulomb Models in the Zero‐Temperature Limit

Two non‐standard scenarios of melting termination in deep metastable states are studied in the zero‐temperature limit on the base of two variants of modified one‐component Coulomb models. These additional scenarios supplement the previously studied standard case of “spinodal decomposition” (Iosilevskiy and Chigvintsev, arXiv:0609059) when liquid binodal of metastable freezing (liquidus) is terminated in intersection with gas‐liquid spinodal. In the first new scenario hypothetical unique crystal‐fluid global phase coexistence is realized as smooth superposition of melting and sublimation transitions (without gas‐liquid transition and corresponding critical point). The second new type of “spinodal decomposition” scenario is related to the situation when solid binodal of metastable melting (solidus) intersects spinodal of metastable isostructural crystal‐crystal phase transition. Modified one‐component Coulomb models allow one to investigate in details all features of such “spinodal decomposition” scenarios (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Theory of the vortex matter transformations in high-Tc superconductor YBCO

Flux line lattice in type II superconductors undergoes a transition into a "disordered" phase such as vortex liquid or vortex glass, due to thermal fluctuations and random quenched disorder. We quantitatively describe the competition between the thermal fluctuations and the disorder using the Ginzburg-Landau approach. The following T-H phase diagram of YBCO emerges. There are just two distinct thermodynamical phases, the homogeneous and the crystalline one, separated by a single first order transition line. The line, however, makes a wiggle near the experimentally claimed critical point at 12 T. The "critical point" is reinterpreted as a (noncritical) Kauzmann point in which the latent heat vanishes and the line is parallel to the T axis. The magnetization, the entropy, and the specific heat discontinuities at melting compare well with experiments.     

Thermodynamics and phase transitions in two-dimensional Yukawa systems

The results of numerical simulations of strongly-coupled two-dimensional dissipative Yukawa systems are presented. The thermodynamic characteristics of these systems were studied, namely the internal energy, the specific heat and the entropy. For the first time, it is discovered that the considered characteristics have two singular points on the melting line; one of these points corresponds to the first-order phase transition from crystal to the hexatic phase, and another point corresponds to the second-order phase transition from the hexatic phase to the isotropic liquid. The obtained results are compared to the existing numerical and analytical data.     

Spin model for inverse melting and inverse glass transition

A spin model that displays inverse melting and inverse glass transition is presented and analyzed. Strong degeneracy of the interacting states of an individual spin leads to entropic preference of the "ferromagnetic" phase, while lower energy associated with the noninteracting states yields a "paramagnetic" phase as temperature decreases. An infinite range model is solved analytically for constant paramagnetic exchange interaction, while for its random exchange analogous results based on the replica symmetric solution are presented. The qualitative features of this model are shown to resemble a large class of inverse melting phenomena. First and second order transition regimes are identified.     

Phase Transitions of Colloidal Suspensions Under External Potential

We review phase transitions in a system of charge-stabilized colloids subject to laser beams with wavevectors tuned to the ordering wavevector of the liquid. Density-functional theory shows that the first-order freezing transition becomes continuous for large strengths of the modulation potential, if the wavevectors satisfy certain symmetry requirements. Computer simulations indicate that, apart from laser induced freezing (LIF), there is laser induced melting (LIM) as well. Recent experiments have shown the existence of both transitions. Theoretical arguments, based on the dislocation-mediated melting scenario in two dimensions; confirm the LIM phenomenon.     

Flux line lattice melting and the formation of a coherent quasiparticle Bloch state in the ultraclean URu2Si2 superconductor

We find that in the ultraclean heavy-fermion superconductor URu(2)Si(2) (T_{c0}=1.45 K) a distinct flux line lattice melting transition with outstanding characters occurs well below the mean-field upper critical fields. We show that a very small number of carriers with heavy mass in this system results in exceptionally large thermal fluctuations even at sub-Kelvin temperatures, which are witnessed by a sizable region of the flux line liquid phase. The uniqueness is further highlighted by an enhancement of the quasiparticle mean free path below the melting transition, implying a possible formation of a quasiparticle Bloch state in the periodic flux line lattice.     

Crystal melting processes of propylene carbonate and 1,3-propanediol investigated by the reed-vibration mechanical spectroscopy for liquids

The melting of crystals is one of the most common and general phase transition phenomena.However, the mechanism of crystal melting is not well understood, and more experimental measurements and explorations are still needed.The mechanical spectra of propylene carbonate and 1,3-propanediol during the crystal melting processes are measured by the reed vibration mechanical spectroscopy for liquids(RMS-L) for the first time.The experimental results show that as the temperature increases, the real part of the complex Young modulus first decreases slowly, and then quickly drops to zero;meanwhile, its imaginary part increases slowly at first, then goes up and drops quickly to zero, showing a peak of internal friction.Preliminary analyses indicate that both the real and imaginary parts can present some characteristics of the melting process, such as the transition from the disconnected liquid regions to the connected liquid regions, that from the connected crystal regions to the disconnected crystal regions, and so on.In addition, the results show that the melting rate per unit volume of crystalline phase versus temperature satisfies the Arrhenius relation at the initial stage of melting, and deviates from this relation as the temperature increases to a certain value.Therefore, the RMS-L will provide an effective supplement for the further study of melting.     

晶体相场法研究预变形对熔点附近六角相/正方相相变的影响

应用双模晶体相场模型计算二维相图,并模拟了在熔点附近预变形和保温温度对六角相晶界演化以及六角相/正方相相变的影响.研究发现:在相变初期,当预变形为零、保温温度离熔点很近时在晶界发生缺陷诱发预熔;增大预变形,变形与缺陷的交互作用在熔点附近诱发预熔;随着预变形的进一步增大,变形在畸变处同时诱发液相和正方相,且预变形越大、保温温度越接近熔点,液相生长越明显,反之正方相生长明显.持续保温使得畸变能释放,晶粒最终完全转变为平衡正方相.模拟结果表明:预变形六角相在熔点附近保温时,由于晶界固有缺陷和预变形双重作用使得原子无序度增加,从而在晶界或其他缺陷处产生液相,待能量释放后晶粒再转变成平衡正方相,进而延缓了六角相/正方相相变时间.     

Fate of the peak effect in a type-II superconductor: multicriticality in the Bragg-Glass transition

We have used small-angle neutron scattering (SANS) and ac magnetic susceptibility to investigate the global magnetic field H vs temperature T phase diagram of a Nb single crystal in which a first-order transition of Bragg-glass melting (disordering), a peak effect, and surface superconductivity are all observable. It was found that the disappearance of the peak effect is directly related to a multicritical behavior in the Bragg-glass transition. Four characteristic phase boundary lines have been identified on the H-T plane: a first-order line at high fields, a mean-field-like continuous transition line at low fields, and two continuous transition lines associated with the onset of surface and bulk superconductivity. All four lines are found to meet at a multicritical point.