Bulk Critical Temperature Tc Research Articles

Tunneling spectroscopy of slightly overdoped pnictides BaFe1.88Ni0.12As2 in the superconducting and normal state

Using tunneling spectroscopy, we measured current-voltage characteristics (CVC) and dynamic conductance spectra 𝑑𝐼(𝑉 )/𝑑𝑉 of tunneling contacts in single crystals of slightly overdoped superconducting ferro-pnictide of nominal composition BaFe1.88Ni0.12As2 with bulk critical temperature 𝑇𝑐 = 16−18 K. Besides the Andreev structure, a strong nonlinearity is observed in the obtained curves both in the superconducting and normal state. For the temperature range 4.3 − 26.4 K we determined the positions and temperature dependences of the observed features irrelated to the superconducting state. Additionally, we discuss the possible origins for the appearance of the observed features.

Strain Engineering at Heterointerfaces: Application to an Iron Pnictide Superconductor, Cobalt-Doped BaFe2As2.

We propose a unique strategy to apply stronger strain at heterointerfaces than conventional epitaxial strain methods to extract hidden attractive physical/chemical properties in materials. This strategy involves precisely accounting for the epitaxial strain induced by lattice mismatch as well as the differences in the thermal expansion coefficients and compressibilities of epitaxial films and substrates. We selected optimally cobalt-doped BaFe2As2(Ba122:Co), an iron-based superconductor with a bulk critical temperature (Tc) of 22 K, as a model material and four types of single-crystal substrates. Ba122:Co was selected because its Tc is robust to hydrostatic pressure but sensitive to epitaxial strain (i.e., one of the anisotropic strains), and the selected substrates entirely cover the positive/negative lattice mismatches, thermal expansion coefficients, and compressibilities with respect to Ba122:Co. With strong anisotropic strain successfully induced by film growth, external hydrostatic pressurizing, and cooling processes, we observed unique carrier transport properties in Ba122:Co epitaxial films on CaF2 and BaF2 substrates including (i) upturn behavior in the temperature dependence of the longitudinal resistivity, (ii) negative magnetoresistance, (iii) large enhancement of anomalous Hall effects in the epitaxial films on CaF2, and (iv) enhancement of Tc to 27 K in the epitaxial films on BaF2. These results demonstrate the effectiveness of our strategy, and this approach can be further extended to other inorganic materials in thin-film form.

Exact results for the Casimir force of a three-dimensional model of relativistic Bose gas in a film geometry

It has recently recently been suggested that a relativistic Bose gas of some type may play a role in issues such as dark matter, dark energy, and in some other cosmological problems. In this article, we investigate one known exactly solvable model of a three-dimensional statistical-mechanical model of relativistic Bose gas which takes into account the existence of both particles and antiparticles. We derive exact expressions for the behavior of the Casimir force for a system subjected to film geometry under periodic boundary conditions. We show that the Casimir force between the plates is attractive, monotonic as a function of the temperature scaling variable, and has a scaling function that, at low temperatures, approaches a universal negative constant equal to the corresponding one for two-component three-dimensional Gaussian system. The force decays with the distance in a power-law near and below the bulk critical temperature Tc of the Bose condensate, and exponentially above Tc. We obtain a closed-form exact expression for the Casimir amplitude . We establish the precise correspondence of the scaling function of the free energy of the model with the scaling functions of two other well-known models of statistical mechanics: the spherical model, and the imperfect Bose gas model.

Mössbauer Study of a New Superconductor GdOFeAs

The 57Fe-Mossbauer spectroscopy measurements on the polycrystalline superconductor GdO0.88F0.12FeAs with the bulk critical temperature TC=53 K were carried out at temperatures between 5 and 300 K. Two nonequivalent Fe2+ sites were revealed in the Fe-As superconducting planes. Iron ions show clear nonmagnetic behavior in all temperature ranges. There were no observed significant changes in the hyperfine Mossbauer parameters below and above TC. The magnetic component appearing below 75 K indicates the presence of a small fraction of impurity phase FeAs.

Fluctuation Conductivity of La0.5RE0.5BaCaCu3O7−d (RE=Y, Sm, Dy) Superconductor

We report production, conductivity fluctuation and magnetoconductivity fluctuation measurements of the system La0.5RE0.5BaCaCu3O7−δ RE=(Y,Sm,Dy) high-temperature superconductors. Samples were synthesized by the standard solid state reaction recipe. By Rietveld-like refinement of X-ray diffraction patterns we determined that the material crystallizes in a tetragonal structure. DC resistivity measurements show a bulk critical temperature Tc (71, 72 and 72 K for RE=Y, Sm, Dy, respectively) for the analyzed samples. In order to examine the effect of substitution of rare earth into the lanthanum structural sites on the pairing mechanism close to Tc, we performed conductivity fluctuation analysis by using the method of logarithmic temperature derivative of the conductivity excess. We found the occurrence of Gaussian and genuinely critical fluctuations. Our results are in agreement with reports on non-doped systems, but we observe minor values of the coherence length of the fluctuation regimes for the doped case. Correlations of critical exponents with the dimensionality of the fluctuation system for Gaussian regimes were made by using the Aslamazov–Larkin theory. The genuinely critical exponent was interpreted by the 3D-XY.

Tunneling break-junction measurements of the superconducting gap in Y2C3

Tunneling measurements of the electron spectrum in the superconducting state have been carried out in Y2C3 samples with the bulk (resistive and diamagnetic) critical temperature Tc=15–16K. A break-junction technique was crucial to find the intrinsic gap features. The representative gap structure at 4K reveals distinct BCS-like peaks with the gap value of 2Δ(4K)=3–4meV. Assuming local Tc values of 10–13K, the characteristic gap to Tc ratio 2Δ/kBTc (kB is the Boltzmann constant) was found to possess the BCS weak-coupling value 3.5±0.2, appropriate to isotropic s-wave superconductors.

Enhancement of the critical temperature in iron pnictide superconductors by finite-size effects

Recent experiments have shown that, in agreement with previous theoretical predictions, superconductivity in metallic nanostructures can be enhanced with respect to the bulk limit. Motivated by these results we study finite size effects (FSE) in an iron-pnictide superconductor. For realistic values of the bulk critical temperature Tc ~ 20-50K, we find that, in the nanoscale region L ~ 10 nm, Tc(L) has a complicated oscillating pattern as a function of the system size L. A substantial enhancement of Tc with respect to the bulk limit is observed for different boundary conditions, geometries and two microscopic models of superconductivity. Thermal fluctuations, which break long range order, are still small in this region. Finally we show that the differential conductance, an experimental observable, is also very sensitive to FSE.

Coherence transition in granular YBa2Cu3O7-δ, YBa2Cu2.95Zn0.05O7-δ, and YBa1.75Sr0.25Cu3O7-δ superconductors

We have studied experimentally the electrical magneto-conductivity near the superconducting transition of YBa2Cu3O7-δ, YBa2 (Cu2.95Zn0.05)O7-δ and Y(Ba1.75Sr0.25)Cu3O7-δ polycrystalline samples. The measurements were performed in magnetic fields ranging from 0 to 400 Oe applied parallel to the current orientation. The results show that the resistive transition of these systems proceeds in two stages. The pairing transition occurs at the bulk critical temperature Tc, where superconductivity is stabilized within small and homogeneous regions of the sample generically called grains. The regime of approach to the zero resistance state reveals the occurrence of a coherence transition at a lower temperature Tc0. This transition is related to the connective nature of the granular samples and is controlled by fluctuations of the order-parameter phase of individual grains. Our experiments show that the Zn-doping, besides depressing the pairing critical temperature, strongly enlarges the temperature range dominated by effects related to the coherence transition. The substitution of Ba by Sr causes only a small reduction of Tc, but also enhances significantly the effects related to the grain coupling phenomenology. In general, our results indicate that these impurity substitutions in YBa2Cu3O7-δ produce or magnify the granularity at a microscopic level, enhancing the effects of phase fluctuations in the conductivity near the transition.

Water in nanopores: II. The liquid–vapour phase transition near hydrophobicsurfaces

The liquid–vapour phase transition near a weakly attractive surface is studied bysimulations of the coexistence curves of water in hydrophobic pores. There is apronounced gradual density depletion of the liquid phase near the surface without anytrend to the formation of a vapour layer below the bulk critical temperatureTC. The temperature dependence of the order parameter in the surface layer follows the powerlaw with a value of the exponent β1 close to the critical exponent β1 = 0.82 of the ordinary transition in the Ising model. The order parameter profiles in the subcriticalregion are consistent with the behaviour of an ordinary transition and their temperatureevolution is governed by the bulk correlation length. Density profiles of waterat supercritical temperatures are consistent with the behaviour of the normaltransition caused by the preferential adsorption of voids. The relation betweennormal and ordinary transitions in the Ising model and in fluids is discussed.

On the finite-size behaviour of systems with asymptotically large critical shift

Exact results of the finite-size behaviour of the susceptibility in three-dimensional mean spherical model films under Dirichlet–Dirichlet, Dirichlet–Neumann and Neumann–Neumann boundary conditions are presented. The corresponding scaling functions are explicitly derived and their asymptotics close to, above and below the bulk critical temperature Tc are obtained. The results can be incorporated in the framework of the finite-size scaling theory where the exponent λ characterizing the shift of the finite-size critical temperature with respect to Tc is smaller than 1/ν, with ν being the critical exponent of the bulk correlation length.

Adsorption hysteresis in ink-bottle pore

To examine the mechanism of the adsorption hysteresis in ink-bottle pores, we measured the temperature dependence of the adsorption–desorption isotherms of argon, oxygen, and carbon dioxide onto SBA-16 ordered mesoporous material with cagelike pores. The hysteresis loop always shrank with increasing temperature and eventually disappeared at a hysteresis temperature (Th), well below the bulk critical temperature (Tc). When the relative pressures p/p0 of the capillary condensation and evaporation are plotted as a function of reduced temperature T/Tc, all the data including the transition pressures for nitrogen reported previously are represented by a common curve. We also calculated the temperature dependence of the capillary condensation and evaporation pressures of nitrogen under the assumption that adsorption and desorption in an ink-bottle pore may be regarded as the process of the disappearance and formation of a gas bubble in a liquid droplet confined to the pore. A fit between the observed and calculated transition pressures in a wide temperature range was reasonable in light of several assumptions and approximations used. This clearly indicates that the energy barrier for the formation and disappearance of vapor bubbles in the liquid confined to the pores is responsible for the appearance of the adsorption hysteresis and the hysteresis temperature is not concerned with the so-called capillary criticality. At temperatures higher than Th, the reversible capillary condensation takes place, because the energy barrier between a full liquid pore and the vapor coexisting with the liquid film becomes surmountable.

Effect of bulk magnetic field on critical Ising films

Two-dimensional Ising films L x infinity in a nonvanishing bulk magnetic field H are studied at the bulk critical temperature Tc for two choices of surface fields (a) H1 = HL = 0 (ordinary transition), and (b) H1 = HL = infinity (normal transition) by the density-matrix renormalization-group method. Universal scaling functions for magnetization profiles, the excess magnetization gamma, the longitudinal correlation length xi parallel, and for the analog of the solvation force fsolv are found and discussed. When H1 = 0 the scaling function for fsolv has two symmetric minima at y = sgn(H)L magnitude of H nu/delta approximately +/- 1 with an amplitude at the minimum about 3.8 times the value at H = 0, the Casimir amplitude. For the normal transition the scaling function for fsolv has a single minimum near the continuation of the pseudocoexistence (capillary condensation) line, with an amplitude about 100 times the Casimir amplitude.

Finite size dependence of the helicity modulus within the mean spherical model

The validity of the finite-size scaling prediction about the existence of logarithmic corrections in the helicity modulusγ of three-dimensional O(n)-symmetric order parameter systems in confined geometries is studied for the three-dimensional mean spherical model of geometryL3/s-d′×∞d′, 0⩽d′<3. For a fully finite geometry the general case ofdp⩾0 periodic,da⩾0 antiperiodic,d0⩾0 free, andd1⩾0 fixed (dp+da+d0+d1=d, d=3) boundary conditions is considered, whereas for film (d′=2) and cylinder (d′=1) geometries only the case of antiperiodic and/or periodic boundary conditions is investigated. The corresponding expressions for the finite-size scaling function of the helicity modulus and its asymptotics in the vicinity, below, and above the bulk critical temperatureTc and the shifted critical temperatureTc,L are derived. The obtained results are not in agreement with the hypothesis of the existence of a log(L) correction term to the finite-size behavior of the helicity modulus in the finite-size critical region if d=3. In the case of film and cylinder geometries there are no logarithmic corrections. In the case of a fully finite geometry a universal logarithmic correction term −[(d0 −d1)/4π−2da−1/π2] lnL/L is obtained only for (Tc-T) L≫lnL.

Finite-size dependence of the helicity modulus within the mean spherical model

The validity of the finite-size scaling prediction about the existence of logarithmic corrections in the helicity modulusγ of three-dimensional O(n)-symmetric order parameter systems in confined geometries is studied for the three-dimensional mean spherical model of geometryL 3/s-d′×∞d′, 0⩽d′<3. For a fully finite geometry the general case ofd p⩾0 periodic,d a⩾0 antiperiodic,d 0⩾0 free, andd 1⩾0 fixed (d p+da+d0+d1=d, d=3) boundary conditions is considered, whereas for film (d′=2) and cylinder (d′=1) geometries only the case of antiperiodic and/or periodic boundary conditions is investigated. The corresponding expressions for the finite-size scaling function of the helicity modulus and its asymptotics in the vicinity, below, and above the bulk critical temperatureT c and the shifted critical temperatureT c,L are derived. The obtained results are not in agreement with the hypothesis of the existence of a log(L) correction term to the finite-size behavior of the helicity modulus in the finite-size critical region if d=3. In the case of film and cylinder geometries there are no logarithmic corrections. In the case of a fully finite geometry a universal logarithmic correction term −[(d 0 −d 1)/4π−2da−1/π2] lnL/L is obtained only for (T c-T) L≫lnL.

Finite size effects in critical dynamics and the renormalization group

Systems representable as a time-dependent Ginzburg-Landau model with nonconserved order parameter are considered in a block (V=L d) geometry with periodic boundary conditions, both for space dimensionalitiesd≧4 andd=4−e. A systematic approach for studying finite size effects on dynamic critical behavior is developed. The method consists in constructing an effective reduced dynamics for the lowest-energy (q=0) mode by integrating out the remaining degrees of freedom, and generalizes recent analytic approaches for studying static finite size effects to dynamics. Above four dimensions, the coupling to the other (q≠0) modes is irrelevant and the probability densityP(Φ,t) for the normalized order parameterΦ=∫dd xϕ(x,t)/V satisfies a Fokker-Planck equation. The dynamics is equivalently described by the Langevin equation for a particle moving in a |Φ|4 potential or by a supersymmetric quantum mechanical Hamiltonian. Dynamic finite size scaling is found to be broken, e.g. the order parameter relaxation rate varies at the bulk critical temperatureT c,∞ as ωυ(T c,∞ L)∼L −d/2 asL→∞. By contrast, ford<4, the coupling to the other (q≠0) modes cannot be ignored and dynamic finite size scaling is valid. The asymptotic behavior of correlation and response functions can be studied within the framework of an expansion in powers of ɛ1/2. The scaling function associated with ωυ is computed to one-loop order. Finally, the many component (n→∞) limit is briefly considered.

Magnetization at corners in two-dimensional Ising models

We investigate the corner spin magnetization of two-dimensional ferromagnetic Ising models in various wedge geometries. Results are obtained for triangular and square lattices by numerical studies on finite wedges using the star-triangle transformation, as well as analytic calculations using corner transfer matrices and a fermionic representation of the row-to-row transfer matrix. The corner magnetizations vanish at the bulk critical temperature Tc with an exponentβc differing from the bulk exponent. For isotropic systems with free edges we find thatβ c is given simply byβ c =π/2θ, whereθ is the angle at the corner. For apex magnetizations of conical lattices we obtain the strikingly similar resultαa=π/4θ. These formulas apply equally well to anisotropic lattices if the angleθ is interpreted as an effective angle,θeff, determined by the relative strengths of the interactions.

Molecular dynamics calculations for systems with a localized “soft-mode”

Abstract Molecular dynamics simultations of Id and 2d systems with a single defect are presented. The frequency dependent scattering function shows a peak at γ = 0 for temperatures above the bulk critical temperature TC bulk. This indicates a locally polarized state. If T approaches TC bulK, the life time and the spatial extension of the localized state become large.

Thermodynamics of an anisotropic boundary of a two-dimensional Ising model

We consider a semi-infinite two-dimensional Ising model with its spins on the boundary row having a different interaction energy E′1 from the ferromagnetic bulk. We find that the boundary specific heat has two divergent terms: one of which diverges linearly at the bulk critical temperature Tc, and the other, logaritmically. The linearly divergent term is independent of E′1, and the coefficient of the logaritmically divergent term is a decreasing function of E′1. There is a boundary latent heat at Tc, which is identical to McCoy and Wu's result. The boundary spins, which can be either ferromagnetic or antiferromagnetic, are aligned for temperature lower than Tc. The boundary spontaneous magnetization approaches zero in the form of A(E′1)(1−T/Tc)1/2, and the boundary zero field magnetic susceptibility diverges at Tc in the form −B (E′1)ln|1−T/Tc|, where A(E′1) and B(E′) are increasing functions of E′1.

127. Sorption properties of Ceto-alloys with titanium and zirconium.: K Nesteruk

This paper discusses recent experimental results in the liquid-vapour critical region of mercury. These results show that the existence of the metal-non-metal transition noticeably influences electronic, thermodynamic, structural and interfacial features. Optical reflectivity experiments were conducted for mercury in contact with an optically transparent sapphire substrate. The experiments were carried out with the mercury close to the vapour-liquid coexistence curve. The temperature and density dependence of the reflectivity reveal clearly the existence of a prewetting transition of mercury on the sapphire substrate. At high temperature, the transition line terminates at a surface critical temperature Tsc lying below the bulk critical temperature Tc. At low temperature, it terminates at the wetting transition temperature Tw. Furthermore, the transition lies close to the bulk vapour-liquid coexistence curve and occurs when the bulk vapour pressure is within a few percent of its saturation value.