Maximum Critical Temperature Research Articles

Estimativa do conforto de matrizes de frango de corte baseada em análise do comportamento de preferência térmica

Considerando o comportamento social, é sugestivo que a freqüência e a intensidade de interações agressivas, o total de coesão social e compreensão da extensão de vícios sociais possam ser utilizados para avaliação de bem-estar. Este trabalho propõe que a freqüência de utilização de determinados locais dentro do galpão possa ser usada como variável para monitorar estados de bem-estar térmico e/ou estresse das aves alojadas. A partir de dados registrados no verão de 2000-2001, esta pesquisa identificou as temperaturas críticas máximas (tc máx) de matrizes pesadas, individualmente, por meio de análise estatística da freqüência de uso de locais previamente estabelecidos, utilizando a tecnologia de identificação eletrônica. Os valores das tc máx individuais variaram de 30 °C ± 0,2 ºC a 32,3 ºC ± 0,2 ºC e a temperatura crítica máxima média para o grupo foi 30,9 ºC ± 0,8 ºC.

Superconducting MgB2 Microstrips

The MgB2 superconducting thin films were prepared on NbN buffered Si(100) substrate by e-beam sequential evaporation. An in-situ fabrication process allows to prepare polycrystalline (with grain size ≈10 nm) MgB2 thin films with a maximum zero resistance critical temperature as high as 36 K. The normal state and superconducting transport properties of these films as well as patterned microstrips (width 10 - 30 μm, length 10 - 30 μm) have been studied. Results following from temperature dependences of critical current density and current-voltage characteristics show that a flux-creep is responsible for critical current limitation in these microsptrips.

Very high Tc superconductors in hydroxyl and fluoride-containing Bi 2Sr 2Ca 2Cu 3O y ceramics systems

Experimental data of ceramic superconductors for maximum T c, up to 135 K, are reported with regard to fluoride- and hydroxyl-containing Bi 2Sr 2Ca 2Cu 3O y systems. Both magnetic and resistivity measurements confirmed a maximum critical superconductive temperature at 135 K, distinctly higher than the previous record value at 125 K observed in Tl 2Ba 2Ca 2Cu 3O 10 ceramics. This increase in T c is presumably associated with superconductive structural phase transition or phase equilibrium for [CuO 6] octahedra, occupied by hydroxyl or fluorine ions in oxygen vacancies.

Symmetry of the neutron and proton superfluidity effects in cooling neutron stars

We investigate the combined effect of neutron and proton superfluidities on the cooling of neutron stars whose cores consist of nucleons and electrons. We consider the singlet state paring of protons and the triplet pairing of neutrons in the cores of neutron stars. The critical superfluid temperatures T c are assumed to depend on the matter density. We study two types of neutron pairing with different components of the total angular momentum of a Cooper pair along the quantization axis (|m J |=0 or 2). Our calculations are compared with the observations of thermal emission from isolated neutron stars. We show that the observations can be interpreted by using two classes of superfluidity models: (1) strong proton superfluidity with a maximum critical temperature in the stellar core T maxc ≳4×109 K and weak neutron superfluidity of any type (T maxc ≲2×108 K); (2) strong neutron superfluidity (pairing with m J =0) and weak proton superfluidity. The two types of models reflect an approximate symmetry with respect to an interchange of the critical neutron and proton pairing temperatures.

Electrical and structural properties of MgB2 films prepared by sequential deposition of B and Mg on the NbN-buffered Si(100) substrate

We introduce a simple method of a MgB2 film preparation using a sequential electron-beam evaporation of B-Mg bilayer (followed by in-situ annealing) on the NbN-buffered Si(100) substrate. The transmission electron microscopy analyses confirm a growth of homogeneous nanogranular MgB2 films without the presence of crystalline MgO. A sensitive measurement of temperature dependence of microwave losses shows a presence of intergranular weak links close to the superconducting transition only. The MgB2 films obtained, about 200-nm thick, exhibit a maximum zero resistance critical temperature of 36K and a critical current density of 3×107A∕cm2 at 13.2K.

A new equation for the temperature dependence of the excess Gibbs energy of solution phases

During CALPHAD optimization procedures, the excess Gibbs energy of solution phases is usually described by the Redlich–Kister (RK) polynomial. The RK polynomial includes interaction parameters ( L i ), usually described as linear functions of temperature ( L i = a i − b i T ), with semi-empirical parameters a i and b i usually having the same signs. Consequently, the excess Gibbs energy will change its sign at a certain high temperature and will have an infinite value at infinitely high temperature. This is in conflict with thermodynamic constraints and can also lead to artificial phase stabilization in some of the calculated phase diagrams, such as the appearance of an artificial miscibility gap having no maximum critical temperature. Such artifacts can usually be avoided, if the following new semi-empirical equation is used for the interaction energy of solution phases: L i = h 0 i ⋅ exp ( − T τ 0 i ) . Parameter h 0 i (J/mol) is the enthalpy part of L i at T = 0 K, while parameter τ 0 i (K) is the temperature at which L i would change its sign if it were extrapolated linearly from T = 0 K. Parameter h 0 i can have any sign, but parameter τ 0 i must be always positive. The new equation can be extended to describe more complex behavior in certain systems as: L i = h 0 i ⋅ exp ( − T τ 0 i ) ⋅ [ 1 + ∑ j = 1 m a i j ⋅ ( T τ 0 i ) j ] , with a i j being adjustable semi-empirical parameters for the given L i .

Spin triplet pairing and superconducting states in square lattices

In this work we study the pairing symmetry in a square lattice within a generalized Hubbard model, in which a second-neighbor correlated hopping is included in addition to the on-site and nearest-neighbor repulsions. When an infinitesimal distortion of the right angles in the square lattice is considered, we observe a spin triplet p-wave ground state for the case of electrons, but for holes a finite distortion is required, contrary to the d-wave case. In general, this spatial distortion induces a splitting of the doubly degenerate p-wave pairing states and one of them could become the ground state. On the other hand, this study has been extended to a finite density of electrons within the BCS formalism and the results show a maximum critical temperature of p-wave superconductivity around a 2/3 band-filling, close to the Fermi energy in the γ-band of Sr 2RuO 4 estimated by band-structure calculations.

Nucleation of superconductivity in a superconducting disk with magnetic dot

We have analysed the onset of superconductivity in an Al mesoscopic disk, with a perpendicularly magnetized magnetic dot on the top. An insulating Si spacer layer between the Al disk and magnetic dot ensures that the interaction between them occurs only through the stray field of the magnetic dot. The superconducting T c( B) phase boundary is asymmetric with respect to the polarity of the applied magnetic field. The maximum critical temperature is achieved for a finite value of the perpendicular applied magnetic field, which is parallel to the magnetization of the magnetic dot.

Little-Parks effect in a superconducting loop with a magnetic dot

We have studied the nucleation of superconductivity in a mesoscopic Al loop, enclosing magnetic dot with perpendicular magnetization. The superconducting phase boundary Tc(B), determined from transport measurements, is asymmetric with respect to the polarity of an applied magnetic field. The maximum critical temperature has been found for a finite applied magnetic field, which is antiparallel to the magnetization of the dot. Theoretical phase boundary shows a good agreement with the experimental data.

In situ sputtering growth and characterization of MgB/sub 2/ films for microwave applications

High-quality c-axis oriented MgB/sub 2/ superconducting thin films were grown on MgO and Al/sub 2/O/sub 3/ single crystal substrates by a d.c. planar magnetron sputtering technique. The films were obtained starting by Mg rich Mg-B precursor films, subsequently annealed in situ at 800 /spl deg/C for 10 min in a In sealed Nb box in the presence of saturated Mg vapor. The process is highly reproducible and can be easily scaled to produce large area films. Resistive transition of the 1 /spl mu/m thick resulting films showed a maximum critical temperature T/sub c/ of 35 K and a transition width lower than 0.5 K. The residual resistivity ratio was 1.6 for the best samples. The films were characterized by a variety of structural and electronic techniques including profilometry, XRD, EDS, and STM-AFM analyses, critical current, upper critical field and penetration depth measurements. In view of possible device applications, the microwave response has been studied in detail. The measured dependence of the surface impedance from temperature and r.f. field amplitude at 20 GHz via a dielectric resonator technique is reported and discussed.

Hg-cuprate thin films prepared using Re and fluorides based precursor thin films

We present the preparation, structural and electrical properties of Hg-based superconducting thin films prepared by two step process from different precursor films. Ba-Ca-Cu-(O,F) precursor films were prepared by sequential evaporation method and Re-Ba-Ca-Cu-O precursor films by high-pressure sputtering on CeO 2 buffered R-plane sapphire substrates. Then, Hg-Re-Ba-Ca-Cu-O pellets as a source of Hg-vapour were applied in heat-treatment. The Hg cuprate thin films with maximum zero resistance critical temperature Too ( 119.8 K were continuous with a sufficient adhesion for patterning. In the case of the films prepared by Re-doped precursor film we observed uniform surface morphology and superconducting properties.

Hg-based cuprate superconducting thin films prepared on CeO2buffered R-plane sapphire substrate

Hg-Ba-Ca-Cu-O films have been grown on CeO2/R-plane sapphiresubstrates using a two-step process. First, a 200 nm thick Ba-Ca-Cu-(O, F)precursor film was prepared by sequential evaporation, thenHg-Re-Ba-Ca-Cu-O pellets as a source of Hg-vapour were applied using heattreatment. The prepared films were continuous with sufficient adhesion to thesubstrate and a maximum zero-resistance critical temperature ofTC0 = 118 K. The x-ray diffraction measurements of all the prepared filmsconfirmed the Hg-1212 phase as well as the minor Hg-1223 phase in some cases.We found that the best results were obtained at the mercuration temperature of775 °C and an optimal annealing time within the range 0.5-5 h. Thesuperconducting properties of the films prepared under unsuitable conditionswere mostly improved after annealing in oxygen atmosphere. Darker parts of thefilm exhibited a different surface morphology and the presence of the Hg-1223phase in plate-like grains.

Anomalous exchange interactions in III-V dilute magnetic semiconductors

Based on local-density functional calculations, we study the exchange interactions between magnetic dopants Cr, Mn, and Fe in the III-V compounds GaAs, GaN, and AlN. We show the magnetic exchange interactions deviate strongly in behavior expected from simple models, and may explain the observed maximum in critical temperature with impurity concentration. Additionally the magnetism is responsible for a strong, short-range attraction between the magnetic dopants, thus creating an anomalous effective alloy hamiltonian. This suggests that the impurities may aggregate into small nanoclusters of a few magnetic atoms.

On the relation between (maximum) critical temperature and c-axis layered structure in cuprates. : (II). Quantitative analysis on the basis of indirect exchange pairing

In this paper, we continue, from Physica A 289 (2001) 165, the analysis of the relation between maximum critical temperatures of the oxygen-doped high-temperature superconducting cuprates Hg(n), Tl 2(n) and Bi 2( n) and the number n of CuO 2 layers per molecular unit, on the basis of a model of indirect-exchange pairing between conduction electrons (quasiparticles) via oxygen anions as the origin of Cooper-pair formation, previously used in numerous applications. A separability property of the coupling strength essentially simplifies the analysis. Numerical results are presented for the Hg( n) series with n=1–7. Since the equation determining these maximum T C 's, as a function of n, cannot be solved directly, we take a detour via an associated series obtained formally from the oxygen-doped systems by subsequent selective cation doping. Relations between the two series then enable conclusions to be drawn concerning the dependence on n of maximum critical temperatures for the oxygen-doped systems. It is found conclusively, in agreement with experiment, that T C ( n) reaches a maximum at n=3, deviating qualitatively from an asymptotic n-dependence as proposed in many earlier analyses. We also elucidate the cause of the deviating behavior of the cuprate series Tl 1( n), i.e., nominally Tl 1Ba 2Ca n−1 Cu n O 2 n+3 , where the maximum occurs at n=4, in the framework of indirect-exchange pairing.

Metal-insulator transition in donor-acceptor type superconductor, (BETS) 2(X 2TCNQ)

We found that (BETS) 2 (Cl 2 TCNQ) becomes superconducting under pressure, showing a maximum critical temperature of 1.3K at 3.5kbar. The superconductivity disappears at higher pressures and is replaced by an insulating phase. Its isostructural complex with larger cell parameters, (BETS) 2 (Br 2 TCNQ), displays a metal-insulator transition at 13K under ambient pressure.

The other high-temperature superconductors

Since the discovery of the high-temperature superconducting cuprates in 1986, there have been four classes of superconductors discovered that have members with critical temperature higher than the highest known in 1986. These classes are typified by (in chronological order) the representatives: Ba 1− x K x BiO 3, Cs 3C 60, YPd 2B 2C, and Na x HfNCl, with maximum critical temperatures, respectively, of 35, 40, 23, and 25 K. These new superconductors are mostly different from the older superconductors, and certainly different from the layered cuprates. For several characteristics that are possibly important for superconductivity, two or three of the classes share similarities. There seems, however, to be almost no feature that is shared by all four classes. However, the three for which there is isotope shift data clearly qualify as phonon-coupled superconductors.

On the relation between (maximum) critical temperature and c-axis layered structure in cuprates. (I). Evaluation of existing analyses

The present paper is the first part of an analysis aimed at an explanation of the observed relation between maximum critical temperature T C ( n) and number n of CuO 2 layers, per molecular unit, in the three cuprate series Tl 2( n), Bi 2( n) and Hg( n). We scrutinize relevant models proposed in the literature based on (A) Anderson et al.'s interlayer tunneling (ILT) approach, (B) the Lawrence–Doniach (LD) version of Ginzburg–Landau (GL) phenomenology of phase transitions, (C) a recent scenario for high-temperature superconductivity proposed by Leggett, and (D) the effect of different doping of non-equivalent CuO 2 layers ( n⩾3). It is established that models A–C inevitably lead to an asymptotically increasing T C ( n) with the number of layers, in contrast with the observed pronounced maximum at n=3. Regarding the effect of differently doped non-equivalent CuO 2 layers, the available analyses lead to results which are either non-committal, inapplicable, or highly speculative.

LOCAL LATTICE FLUCTUATIONS AS A FUNCTION OF CHEMICAL PRESSURE

Here we report local lattice fluctuations as a function of chemical pressure in the high T c superconductors at the optimum doping (δ = 0.16). The chemical pressure is determined by the mismatch between the rock-salt M-O (M=Ba, Sr, La) layers and the CuO 2 layers. High k-resolution Cu K-edge EXAFS has been used to measure the strain of the Cu-O bond lengths and the amplitude of local lattice distortions probed by the Debye-Waller factor of the Cu-O bonds in different materials under increasing chemical pressure. We find that in the normal metallic state, showing non-Fermi liquid transport behavior, the Debye-Waller factor reaches a maximum at a critical value of the strain η c of the CuO 2 plane. The results provide a direct evidence that at the maximum critical temperature T c the system has diverging local lattice fluctuations.

Superconductivity of In-doped Sn0.62Pb0.33Ge0.05Te alloys

A study of the Hall and Seebeck coefficients and of resistivity has been carried out on an Sn0.62Pb0.33Ge0.05Te alloy doped by 5 and 10 at. % In. A superconducting transition with the maximum critical temperature TC∼4 K has been discovered in samples with hole concentrations p≥1×1021 cm−3. The dependence of TC on hole concentration has been established to be of a threshold nature. The onset of superconductivity is accompanied practically simultaneously by a growth of the resistivity and a sharp drop of the Seebeck coefficient. These features in the experimental data indicate the existence of a band of In resonance states within the allowed valence-band spectrum and strong resonance hole scattering to impurity states. The threshold character of the TC(p) dependence is connected with the holes filling the resonance states. A positive correlation between the resonance scattering intensity and the critical temperature is observed.

Effects of the correlated hopping on the d-wave superconductivity

A \({\text{d}}_{x^2 - y^2 } \) superconducting ground state has been obtained within a two-dimensional (2D) generalized Hubbard model, in which a next-nearest-neighbor correlated-hopping interaction is included. In spite of its smaller strength in comparison with other terms of the model, we found its key participation in the d-channel hole pairing. The hole singlet ground-state phase diagram shows a large d-symmetry pairing region that is enhanced by the on-site repulsive Coulomb interaction. For finite density of holes (nh), the mean-field BCS theory is applied to the model. The results show a maximum d-wave critical temperature (Tc) around nh = 0.35, and this Tc is favored by the presence of an antiferromagnetic background.