Critical behavior of the surface of FeBO3 single crystals

The behavior of the surface and near-surface layers of macroscopic FeBO₃ single crystals is studied over the temperature range from 291 K to Neél temperature (T _N ) using depth-selective conversion-electron Mössbauer spectroscopy. Three different phases or states, namely, an antiferromagnetically ordered phase (similar to the crystal bulk state), a surface phase, and a transition layer between them coexist near the Neél point in a surface layer ～500 nm thick. The critical parameters found for the bulk phase agree well with the theoretical critical index ν_th?0.63 predicted by the 3D Ising model. As the crystal surface is approached, the critical parameter β increases to 0.51(2) but remains smaller than the value of β=0.8 for the surface of a semi-infinite Heisenberg model. Therefore, the effective dimensionality of the system, being equal to 3 in the bulk, decreases at the crystal surface.     

Nonclassical rotational inertia in helium crystals

It has been proposed that the observed nonclassical rotational inertia (NCRI) in solid helium results from the superflow of thin liquid films along interconnected grain boundaries within the sample. We have observed NCRI in large (4)He crystals grown at constant temperature and pressure, demonstrating that the superfluid grain boundary model cannot explain the phenomenon.     

Thermal effects in the anomalous growth of helium crystals

Variations in pressure and thermal response have been synchronously measured beginning from the moment of nucleation of a helium crystal, at the onset of anomalously fast growth and during this growth, and in the course of relaxation to the normal kinetics. The measurements were performed at temperatures within 0.4–0.7 K in a range of pressures from the boundary of the anomalous growth region up to an overpressure of ΔP ≈ 25 mbar. A superconductor bolometer with an rms noise of about 10 μK was used as the fast-response thermal sensor. Temperature variations related to changes in the pressure and the heat dissipation during crystal growth were observed. The upper boundary of possible variations in the internal energy of the crystal is estimated as 1–20% of the excess energy of the system prior to the onset of anomalous growth.     

Equilibrium shape of rotating helium crystals

We have studied theoretically the effects of rotation on the equilibrium shape of the interface between superfluid and solid helium. Surface structures in the shape of hillocks and ridges appear in the presence of a vortex lattice in the superfluid. These structures are very sensitive to the orientation of the interface boundary with respect to the crystal planes when surface stiffness is very anisotropic, as occurs well below the roughening transition. We predict the appearance of ring shaped facets for fast rotation speeds. These effects should be observable by using optical techniques.     

Propagation of heat in anthracene crystals at helium temperatures

Measurement of temperature pulses with time resolution less than 5 nsec based, on the high-speed registration of the Debye-Waller factor in the fluourescence spectrum of an impurity layer deposited on a sample, has been achieved. In anthracene single crystals, the velocity of heat propagation was found to be equal to 0.7?1.0.10⁵ cm/sec, approaching that sound; the mean free path of phonons was found to exceed 30 μm.     

Kinematic multiplication of elementary steps on the surface of helium crystals

The nonlinear dynamics of elementary steps on an atomically smooth crystal-liquid interface is considered within the framework of a weak coupling approximation. In fact, the proposed theory describes faceting of the interface between solid and superfluid helium at low temperatures, when dissipative processes weakly influence the dynamics of growing steps. The main results are obtained by means of numerical calculations. The law of dispersion of a step is determined and the dependence of its shape on the propagation velocity is analyzed. It is established that a kinematic multiplication of steps is possible at a sufficiently high velocity, whereby a single step becomes unstable with respect to the formation of a pair of steps of opposite signs that leads to the appearance of a new atomic layer. Under certain conditions, the collision of steps with opposite signs may lead (in addition to the usual annihilation of steps) either to the step “transfer” to the adjacent row with the formation of a new atomic layer or to the reflection of steps from each other. Thus, a qualitatively new mechanism is proposed for the growth of crystals with atomically smooth faces in the absence of renewable sources such as screw dislocations. The effect of dissipation and external supersaturation on the dynamics of steps is considered. The possibility of experimental observation of the proposed mechanisms of step multiplication, as well as their probable relation to the unusual regimes of helium crystal growth at low temperatures, are discussed.     

Critical casimir effect and wetting by helium mixtures

We have measured the contact angle of the interface of phase-separated 3He-4He mixtures against a sapphire window. We have found that this angle is finite and does not tend to zero when the temperature approaches T(t), the temperature of the tricritical point. On the contrary, it increases with temperature. This behavior is a remarkable exception to what is generally observed near critical points, i.e., "critical point wetting." We propose that it is a consequence of the "critical Casimir effect" which leads to an effective attraction of the 3He-4He interface by the sapphire near T(t).     

Critical behaviour of the exciton relaxation constants in magnetic crystals

The critical indices defining the divergence of the relaxation constants in ferro-and antiferro-dielectrics are determined.     

金属钛中氦团簇融合的分子动力学模拟

运用分子动力学方法研究了金属钛中氦的扩散聚集行为.在300—800K的温度范围内,模拟了钛基底中氦团簇之间的融合过程.研究发现,温度的升高会加快氦团簇的融合.在300—800K,融合后的氦团簇在所模拟的时间尺度内三维结构保持不变.模拟结果还表明,常温下氦团簇之间的吸引力是导致氦团簇融合的重要因素. 关键词： 氦团簇 团簇融合 分子动力学模拟     

First principle study of the behavior of helium in plutonium dioxide

The incorporation and solution of helium in plutonium dioxide have been investigated based on density functional theory. The GGA and GGA + U approximations were used with the projector-augmented-wave method. Several defects that are likely to accommodate the incorporation of helium in PuO₂, such as oxygen vacancy, plutonium vacancy, divacancy and Schotty defects were considered in this work. With GGA approach, the lowest incorporation energy corresponds to neutral trivacancy, followed by divacancy and plutonium vacancy, while the GGA + U scheme gave us that oxygen vacancy is the most favorable incorporation site for He. Both SP-GGA and SP-GGA + U methods obtained a same conclusion that the most favorable solution site for He is oxygen vacancy, interstitial site and plutonium vacancy for under-, perfect and over-stoichiometry, respectively. Additionally, the concentrations of the point defects and the solution energy of He for the different incorporation sites as a function of the stoichiometry were also obtained based on the point-defect model.     

Critical behavior of the Chayes-Machta-Swendsen-Wang dynamics

We study the dynamic critical behavior of the Chayes-Machta dynamics for the Fortuin-Kasteleyn random-cluster model, which generalizes the Swendsen-Wang dynamics for the q-state Potts model to noninteger q, in two and three spatial dimensions, by Monte Carlo simulation. We show that the Li-Sokal bound z >or= alpha/nu is close to but probably not sharp in d = 2 and is far from sharp in d = 3, for all q. The conjecture z >or= beta/nu is false (for some values of q) in both d = 2 and d = 3.     

Critical power for self-focussing of a femtosecond laser pulse in helium

The critical power for self-focussing of a femtosecond laser pulse in helium has been measured using the moving focus method. The experimental value is (1 atm) ∼268 GW. Using this value, the nonlinear refractive index is inferred to be  ∼ 3.6 × 10^-21 cm²/W. In addition, the plots of the electron densities versus energy and pressure have also been used to determine the critical power of helium, based on the intensity clamping of the filamentation process. The value agrees well with the one by the moving focus method.     

Critical behavior in type-II superconductors

The high-field critical behavior of type-II superconductors with weak disorder is dominated by the Landau levels of Cooper pairs. The macroscopic degeneracy of Landau manifolds suppresses phase coherence and eliminates the Abrikosov transition in dimensions two and three. A novel phase transition, unrelated to the conventional Abrikosov transition, is predicted to take its place. At this transition the normal state is unstable to the charge-density wave of Cooper pairs. The nature of this new state is discussed. This phase should be most readily observable in layered materials at fields > 1–10T.