Properties of parallel upper critical field within continuous Ginzburg–Landau model

In this paper, we employ a continuous Ginzburg–Landau model to study the behaviors of the parallel upper critical field of an intrinsically layered superconductor. Near T_c where the order parameter is nearly homogeneous, the parallel upper critical field is found to vary as (1−T/T_c)^1/2. With a well-localized order parameter, the same field temperature dependence holds over the whole temperature range. The profile of the order parameter at the parallel upper critical field is of a Gaussian type, which is consistent with the usual Ginzburg–Landau theory. In addition, the influences of the unit cell dimension and the average effective masses on the parallel upper critical field and the associated order parameter are also addressed.     

Photoinduced semiconductor-metal phase transition in the surface layer of vanadium dioxide

The photoinduced semiconductor-metal phase transition occurring for a time Δt < 1 ps in the surface layer of vanadium dioxide is studied theoretically. A nonthermal mechanism of instability development is considered. An equation for the order parameter ξ of the photoinduced semiconductor-metal phase transition is derived. It is shown that the transition of the surface layer of VO₂ to the metallic state requires irradiation by a laser pulse whose energy density W exceeds a critical value W _c. The phase transition is initiated at the surface, after which the interface propagates deep into the sample. The critical energy density W _c, the velocity of propagation of the metal-semiconductor interface, the thickness z ₀, and the characteristic time Δt of formation of the metal layer are calculated. The theoretical results obtained are in agreement with the experimental data on irradiation of vanadium dioxide single crystals by high-intensity laser pulses.     

Surface anchoring and temperature variations of the pitch in thin cholesteric layers

The temperature variations of the cholesteric pitch in thin planar layers of cholesterics and their dependence on the surface anchoring force are investigated theoretically. It is shown that the temperature variations of the pitch in a layer are of a universal character. This is manifested in the fact that they depend not separately on the parameters of the sample but only on one dimensionless parameter S _d =K ²²/dW, where K ₂₂ is the torsional modulus in the Frank elastic energy, W is the height of the surface-anchoring potential, and d is the thickness of the layer. The investigation is performed the parameter S _d in a range where the change per unit number of cholesteric half-turns within the thickness of the layer accompanying a change in the temperature is due to the slipping of the director on the surface of the layer through the potential barrier for surface anchoring. The critical values of the parameter S _d (which are most easily attained experimentally by varying the thickness of the layer), determining the region of applicability of the approach employed, are presented. The temperature variations of the free energy of the layer and the pitch of the cholesteric helix in the layer as well as the temperature hysteresis in the variations of the pitch with increasing and decreasing temperature are investigated for the corresponding values of S _d . Numerical calculations of the quantities mentioned above are performed using the Rapini anchoring potential.     

Temperature dependence of the order parameter and of diffuse scattering in the Hg<Subscript>2</Subscript>Cl<Subscript>2</Subscript> model ferroelastics

The fundamental and diffuse x-ray reflections from Brillouin zone-edge X points of the paraelastic phase of Hg₂Cl₂ crystals (whose integrated intensity is related to the order parameter and its fluctuations) were studied. Information was obtained on the temperature dependence of the order parameter and of diffuse scattering, and the critical exponents were determined. The conclusion is drawn that the ferroelastic phase transition in these crystals is close to the tricritical point.     

Irradiation-induced suppression of the critical temperature in high-<Emphasis Type="Italic">T</Emphasis><Subscript><Emphasis Type="Italic">c</Emphasis></Subscript> superconductors: Pair breaking versus phase fluctuations

Experiments on the irradiation-induced suppression of the critical temperature in high-T _c superconductors are analyzed within the mean-field Abrikosov-Gor’kov-like approach. It is shown that the experimental data for YBa₂Cu₃O_7-δ single crystals can be quantitatively explained by the pair-breaking effects under the assumption of the combined effect of potential and spin-flip scattering on the critical temperature and with an accounting for a nonpure d-wave superconducting order parameter.     

New method in bardeen-cooper-schrieffer theory: Tiplet pairing in superfluid dense neutron matter

On the basis of the method outlined in the first part of this review, the properties of superfluid dense neutron matter are analyzed in the density region where the spin of a Cooper pair and its total angular momentum are S=1 and J=2, respectively. An analytic solution to the problem of ³ P ₂ pairing in neutron matter is presented. Basic features of the structure and of the energy spectrum of superfluid phases are discussed. Degeneracy that is absolutely dissimilar to that which is associated with the phase transformation of the order parameter in the S-pairing problem is a distinct feature of the structure of the aforementioned phases. It appears that one or even a few numbers characterizing the weight of components associated with different values of the projection M of the total angular momentum J=2 of a Cooper pair can be chosen arbitrarily, while the others adjust to them in accordance with universal laws. As a result, the structure of any phase depends neither on the density, nor on the temperature, nor on any other input parameter. The phases found here form two groups degenerate in energy. One of these groups comprises phases for which the sign of the order parameter remains unchanged over the entire Fermi surface, while the other consists of phases whose order parameter has a zero. The energy splitting between the phases from the different groups is calculated analytically as a function of temperature. The relative magnitude of this splitting changes from approximately 3% at T=0 to zero in the vicinity of the critical point T _c. The role of tensor forces in dense neutron matter is analyzed. It is shown that the mixing of the orbital angular momenta L=1 and L=3 of Cooper pairs that is induced by tensor forces completely removes degeneracy peculiar to the ³ P ₂-pairing problem—the number of phases and their structure at a given temperature are tightly fixed, while the energy spectrum of the phases splits completely.     

X-ray structural investigations of the phase transition in benzyl crystals

The Bragg (fundamental) and diffuse reflections from the M-points of the Brillouin zone boundary of the paraphase of the benzyl crystals, whose integrated intensity is related to the order parameter and its fluctuations, respectively, have been investigated. The original information on the temperature behavior of the order parameter and the diffuse scattering is obtained and the values of the corresponding critical exponents are determined. The temperature behavior of the correlation radius and its anisotropy are analyzed. The conclusion is drawn that the phase transition in these crystals is close to the tricritical point. The mechanism of the phase transition in the benzyl crystals alternative to the “trigger” mechanism is suggested.     

Scaling analysis of high temperature superconductors under pressure

The scaling relation of single parameter scaling hypothesis is applied to the study of the scaling behavior of high temperature superconductors under pressure. The data of resistance and specific heat coefficient under various pressures are scaled onto a universal curve according to this scaling relation. The scaling parameters are pressure dependent while temperature independent. It is found that the controlling parameter Bi equals to the relative critical temperature tcP, which indicates that the superconducting energy gap at the zero temperature 2Δs0 is the controlling parameter in this scaling.     

Phase transition to anticlinic texture in free-standing smectic c films

Experimental data on the optical reflectance of free-standing smectic C films were analyzed within the framework of a phenomenological Landau approach. At a certain temperature T _0N (determined from experimental data), which exceeds the known temperature T _c of the volume phase transition from smectic A to smectic C state, a surface phase transition takes place whereby molecules in the surface layer become sloped relative to the normal of the smectic layers. The transition temperatures T _0N ^s,a for N-layer films possessing synclinic (symmetric) and anticlinic (antisymmetric) textures of the order parameter (tilt angle θ) were determined. A comparison of the theoretical and experimental data allowed all parameters of the model to be determined (including critical indices of the correlation length and the surface order parameter). Three possible models of the transition from the state with transverse polarization (perpendicular to the molecular tilt plane) to the state with longitudinal polarization (parallel to this plane) are analyzed. The transition takes place at low (°–°) values of the order parameter θ in the middle layer of the film.     

Phase transitions and interface fluctuations in double wedges and bi-pyramids with competing surface fields

The interplay between surface and interface effects on binary AB mixtures that are confined in unconventional geometries is investigated by Monte Carlo simulations and phenomenological considerations. Both double-wedge and bi-pyramid confinements are considered and competing surface fields are applied at the two opposing halves of the system. Below the bulk critical temperature, domains of opposite order parameter are stabilized at the corresponding corners and an interface runs across the middle of the bi-partite geometry. Upon decreasing the temperature further one encounters a phase transition at which the AB symmetry is broken. The interface is localized in one of the two wedges or pyramids, respectively, and the order parameter is finite. In both cases, the transition becomes discontinuous in the thermodynamic limit but it is not a first-order phase transition. In an antisymmetric double wedge geometry the transition is closely related to the wedge-filling transition. Choosing the ratio of the cross-section L × L of the wedge and its length L _y according to L _y /L ³ = const., simulations and phenomenological consideration show that the new type of phase transition is characterized by critical exponents α = 3/4, β = 0, and γ = 5/4 for the specific heat, order parameter, and susceptibility, respectively. In an antisymmetric bi-pyramid the transition occurs at the cone-filling transition of a single pyramid. The important critical fluctuations are associated with the uniform translation of the interface and they can be described by a Landau-type free energy. Monte Carlo results provide evidence that the coefficients of this Landau-type free energy exhibit a system-size dependence, which gives rise to critical amplitudes that diverge with system size and result in a transition that becomes discontinuous in the thermodynamic limit.     

Critical behavior of dilute electrolyte solutions

A theory of the critical behavior of a dilute ionic solution is constructed. An expression for the susceptibility in a wide temperature range is obtained. It is shown that ionic solutions belong to the universality class of the Ising model. The Ginzburg parameter of the ionic solutions decreases with the increase of the solvent concentration. In the general case, the critical exponent of susceptibility nonmonotonically depends on the temperature in the crossover region from the Ising-like to the mean-field behavior. In the vicinity of the transition point, the Debye-Hückel screening radius is proportional to the correlation length. As T→T _c, the screening radius tends to infinity and the screening disappears. The voltage between the two phases of the ionic solution is proportional to the order parameter and changes as |T/T _c?1|^β in the vicinity of the phase transition point.     

Monte Carlo study of three-dimensional QCD at finite temperature

Three-dimensional QCD at finite temperature is analyzed using Monte Carlo calculations. It is shown that confinement at low temperature is lost through a second-order phase transition. At high temperature, static quarks are liberated and screened and it is argued that timelike (A₀) gluons are liberated. Just above the critical temperature electric correlation functions seem to be dominated by a resonance of two timelike gluons, whose existence is suggested by perturbation theory. The influence of an external magnetic field close to the critical point is considered.     

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.     

Electronic properties of iron arsenic high temperature superconductors revealed by angle resolved photoemission spectroscopy (ARPES)

We present an overview of the electronic properties of iron arsenic high temperature superconductors with emphasis on low energy band dispersion, Fermi surface and superconducting gap. ARPES data is compared with full-potential linearized plane wave (FLAPW) calculations. We focus on single layer NdFeAsO_0.9F_0.1 (R1111) and two layer Ba_1?xK_xFe₂As₂ (B122) compounds. We find general similarities between experimental data and calculations in terms of character of Fermi surface pockets, and overall band dispersion. We also find a number of differences in details of the shape and size of the Fermi surfaces as well as the exact energy location of the bands, which indicate that magnetic interaction and ordering significantly affects the electronic properties of these materials. The Fermi surface consists of several hole pockets centered at Γ and electron pockets located in zone corners. The size and shape of the Fermi surface changes significantly with doping. Emergence of a coherent peak below the critical temperature T_c and diminished spectral weight at the chemical potential above T_c closely resembles the spectral characteristics of the cuprates, however the nodeless superconducting gap clearly excludes the possibility of d-wave order parameter. Instead it points to s-wave or extended s-wave symmetry of the order parameter.     

Landau theory of phase transitions in thick films

A simple expression is used for the free energy density with a one-component order parameter, and the boundary conditions at the surfaces of a film of thickness L are given by means of an extrapolation length δ. Exact expressions are given for the critical temperature and the order parameter profile in terms of elliptic functions, and the nature of the phase transition is discussed.     

An exactly soluble model with tricritical points for structural-phase transitions

We present and study a lattice-dynamical model whose static and dynamic properties can be described exactly for all dimensionsd≧3 (d an integer) and which, in addition, exhibits tricritical points. For certain model parameters, the tricritical behaviour is found to be identical to that of the spherical model. By changing the model parameters continuously however, the transition suddenly becomes of first order at a tricritical point (TCP). The order parameter and the susceptibility are given explicitly ford≧3. The tricritical exponents are Gaussian. The critical dynamics is also discussed.     

Estimation of critical exponents for the ferroelectric transition in Mn2+-doped tris-sarcosine calcium chloride (TSCC) by means of EPR

The static rotation angle of the EPR fine-structure tensor of Mn²⁺ centers of ferroelectric TSCC correlates with the order parameter. Its temperature dependence shows two distinct regions. In the vicinity of the phase transition temperature the critical exponent is β = 0.322 ± 0.012, below this critical region its temperature dependence points at a phase transition of weak first order.     

Quantum critical behavior of clean itinerant ferromagnets

We consider the quantum ferromagnetic transition at zero temperature in clean itinerant electron systems. We find that the Landau-Ginzburg-Wilson order parameter field theory breaks down since the electron-electron interaction leads to singular coupling constants in the Landau- Ginzburg-Wilson functional. These couplings generate an effective long-range interaction between the spin or order parameter fluctuations of the form 1 <r ² d?1, with d the spatial dimension. This leads to unusual scaling behavior at the quantum critical point in 1 < d ≤ 3, which we determine exactly. We also discuss the quantum-to-classical crossover at small but finite temperatures, which is characterized by the appearance of multiple temperature scales. A comparison with recent results on disordered itinerant ferromagnets is given.