Showing papers on "Transition temperature published in 1976"

Saturation of the high-temperature normal-state electrical resistivity of superconductors

Abstract: The high temperature, normal state resistivity of strong coupled superconducting transition metal compounds and other materials with high resistivities saturates at a value corresponding to an electron mean free path of order the interatomic spacing. This accounts for the nonlinear temperature dependence of the resistivity observed in these compounds at high temperature.

Ionic influences on the phase transition of dipalmitoylphosphatidylserine

Abstract: The ionization and phase behavior of 1,2-dipalmitoyl-sn-glycero-3-phosphoserine have been investigated under a variety of condtions by several different methods. As measured by turbidity changes, the temperature of the crystal-liquid crystal phase transition of this lipid is influenced by pH and mono- and divalent cation concentrations. The pH-transition temperature curve is congruent with the curve relating temperature to the degree of ionization of the carboxyl group of the crystalline form. The transition temperature falls from an upper plateau of 72 degrees C at low pH values, where the carboxyl group is fully protonated, to a lower plateau of 55 degrees C at high pH values, where this group is fully ionized. The apparent pK (pH at 50% ionization) of the crystalline form shifts from 6.0 to 4.6 to 3.7 with an increase of NaCl concentration from 10(-3) to 0.1 to l.0 M, respectively. These observations are in accord with a simple theoretical analysis that utilizes diffuse double layer theory and the influence of surface potential on surface concentration of protons. In qualitative terms, an increase in electrolyte concentration reduces the surface potential, the result of which is a diminution of the surface-bulk pH difference and a lowering of the apparent pK. Assuming an area of 50 A2/molecule, the intrinsic pKa (apparent pK corrected for surface pH) of the carboxyl group is 2.7. A 1000-fold change of NaCl concentration produces a very large change in surface potential without influencing the transition temperature of the ionized form of the lipid.

Mössbauer effect study on low-spin 1A1 ⇌ high-spin 5T2 transition in tris(2-picolylamine) iron chloride I. Dilution effect in [FexZn1-x(2-pic)3]Ch2·C2H5OH

Abstract: The 57 Fe Mossbauer effect of the solid solutions [Fe x Zn 1- x (2-pic) 3 ]Ch 2 ·EtOH ( x = 1.0, 0.8, 0.6 and 0.2) has been studied in detail between 10 and 300 K (2-pic = 2 picolylamine). All specimens exhibited a cooperative phase transition due to low-spin ( 1 A 1 ) XXX high-spin ( 5 T 2 ) transition. The coexistence of two quadrupole doublets arising from the high-spin and low-spin states, respectively, near the transition temperature T C indicates a first-order nature for the spin transition. The quadrupole splitting Δ E Q and the isomer shift b are independent of x at all temperatures, whereas T C varies linearly with the iron concentration in the range 0.2 ⩽ x ⩽ 1.0: T C ( x /K = 40x + 82. This concentration dependence of T C is different from those observed in usual magnetic random systems showing a phase transition due to spin ordering. An explanation for this as well as the possible mechanism of the spin transition are discussed.

Systematic study of the transitions in tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ) and its selenium analogs

Abstract: We have carried out a comparative study of the dc conductivity in tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ) and its isostructural selenium analogs TSeF-TCNQ (tetraselenafulvalene) and cis/trans-DSeDTF-TCNQ (diselenadithiafulvalene) below their metal-insulator transitions. Our study reveals the presence of a second transition in TTF-TCNQ at \ensuremath{\sim}38 K and the absence of a similar transition in TSeF-TCNQ. We have characterized the 38-K transition and shown that it involves primarily the TTF chains. From the sharpness of the two transitions in TTF-TCNQ and the single transition at \ensuremath{\sim}29 K in TSeF-TCNQ it is argued that they are related to a two- or three-dimensional ordering. The potentially disordered system DSeDTF-TCNQ does not possess any sharp transition. It is pointed out that the transition temperature defined as the sharp peak in the logarithmic derivative of the conductivity and not the conductivity itself relates more closely to the underlying thermodynamic transition. It is observed that the ratio of the zero-temperature gap to the transition temperature is the same in these systems and in many, if not all, other quasi-one-dimensional systems which undergo a metal-insulator transition. This ratio has a value of 8-10 and is considerably larger than 3.5 which is predicted by the mean-field theory. A model which is consistent with the observation of the transitions in these systems is suggested in which the coupling betwen TCNQ chains drives a two- or three-dimensional ordering of the fluctuation distortion giving rise to a metal-insulator transition.

NMR experiments on the superfluid phases of 3 He in restricted geometries

Abstract: We report transverse cw NMR measurements on the superfluid phases of3He at temperatures between 3 and 0.7 mK. Nuclear demagnetization of copper was used for refrigeration. For thermometry we employed pulsed NMR on platinum powder immersed in the liquid. The measurements on3He were carried out in two NMR coil assemblies in which the liquid was confined between parallel Mylar foils with separations of 0.37 mm and 4 µm. The transition temperatureT c was measured at pressures between 32 bars and the saturated vapor pressure; a pressure-independent increase of ∼11% was observed inT c with respect to earlier data obtained with the same apparatus. We found that our temperature scale is not proportional to that used in La Jolla. In the 4-µm stack we observe a reduction in the B → A transition temperature. In our measurements on the orientational anisotropy of the B phase we found qualitative agreement with the theory of Brinkman et al. We also measured the longitudinal resonance frequencies of the A and B phases between 32 bars and the polycritical point. In the 4-µm stack we found a negative NMR shift in the A phase when the field was oriented perpendicular to the Mylar plates, in agreement with the prediction of Takagi. The static susceptibility XB of the B phase was measured as a function of temperature at 18.7 and 29 bars; its low-temperature limiting value was observed to be (0.33±0.02)XN , independent of pressure. We use our data to estimate the strong coupling corrections to the size of the energy gap. The initial slope of the reduced gap in the A phase, ΔA/T c , was found to increase by ∼25% when the pressure increased from 21.1 bars to the melting curve, whereas in the low-temperature limit ΔB(0)/T c was found to be independent of pressure and close to its weak coupling value.

Superconductivity and electron-phonon interaction in impure simple metals

Abstract: Nonmagnetic impurities change the electron-phonon interaction in a metal and therefore they influence the superconducting properties. This is demonstrated for an impure simple metal by deriving the Eliashberg equation. It turns out that (1) this equation is of the same formal structure as in the clean case, (2) there exists an additional coupling of low-frequency transverse phonons to the electrons, whereas (3) the interaction between longitudinal phonons and electrons is reduced. Since the former effect is much larger than the latter, the effective phonon density of states α2F(ω) is considerably enlarged in the low-frequency regime, which generally leads to an increased transition temperature and energy gap. Calculated values of the change in the transition temperature are compared for various impurity concentrations with experimental results and satisfactory agreement is obtained.

Superconducting critical fields of alkali and alkaline-earth intercalates of MoS2

Abstract: Results are reported for measurements of the critical-field anisotropy and temperature dependence of group-VIB semiconductor MoS2 intercalated with the alkali and alkaline-earth metals Na, K, Rb, Cs, and Sr. The temperature dependences are compared with present theories on the relation between critical field and transition temperature in the clean and dirty limits over the reduced-temperature range from 1 to 0.1. The critical-field anisotropy data are compared with predictions based on coupled-layers and thin-film ('independent-layers') models. It is found that the critical-field boundaries are steep in all cases, that the fields are greater than theoretical predictions at low temperatures, and that an unusual positive curvature in the temperature dependence appears which may be related to the high anisotropy of the layer structure. The results show that materials with the largest ionic intercalate atom diameters and hexagonal structures (K, Rb, and Cs compounds) have the highest critical temperatures, critical fields, and critical-boundary slopes; the critical fields of these materials are observed to exceed the paramagnetic limiting fields.

Time-temperature indicator

Abstract: A time-temperature indicator having a time-temperature sensitive component including a layer of material which at temperatures above a predetermined transition temperature changes from an impermeable, psuedo-crystalline state to a permeable, liquid-crystalline state. A reactant material having a predetermined rate of diffusion through the layer at temperatures above its transition temperature is maintained out of contact with an indicator by the layer at temperatures below the transition temperature. A container for the time-temperature component includes a transparent face for viewing the time-temperature component. The container is adapted for attachment to an object, such as to the package of a time and temperature sensitive product to visually indicate the product has been exposed above a particular temperature far in excess of a certain period of time.

Proposed model of a high-temperature excitonic superconductor

Abstract: We present a detailed calculation of the transition temperature of a model filamentary excitonic superconductor. The proposed structure consists of a linear chain of transition-metal atoms to which is complexed a ligand system of highly polarizable dyelike molecules. Calculations of the electronic properties and experimental data on related materials are used to estimate the strength of the excitonic interaction, Coulomb repulsion, and band structure. From this the superconducting transition temperature was calculated by integration of the gap equation. For the particular structure proposed, transition temperatures of several hundred degrees are calculated. However, we find superconductivity only in those systems where the excitonic medium is within a covalent bond length of, and completely surrounds, the conductive spine. This imposes severe constraints on the structure of any excitonic superconductor. We show that for the structure proposed the momentum dependence of the exciton interaction results in the superconducting state being favored over the Peierls state and in vertex corrections to the electron-exciton interaction which are small.

Muon-Spin Depolarization in Spin-Glasses

Abstract: Polarized ${\ensuremath{\mu}}^{+}$ particles were stopped in spin-glass alloys CuMn and AuFe. A sharp onset of the relaxation of the muon's polarization was observed in the vicinity of the transition temperature and is interpreted as arising from the dipolar fields of frozen impurity moments. Increased temperature width of the transition in an applied magnetic field is observed as a broadening of the internal-field distribution, extending through the transition.

Low-temperature dielectric properties and phase transition in BaMnF/sub 4/

Abstract: The temperature dependences of the static dielectric properties of BaMnF/sub 4/ were investigated from 4 to 300 K at 1 kHz to 9.8 GHz with emphasis on the behavior near the structural phase transition at 247 K. Anomalies in the dielectric constants epsilon'/sub i/ and losses tandelta/sub i/ at the transition were observed along the major crystallographic directions, the main feature being a lambda-shaped anomaly in epsilon' along the polar a axis. This anomaly persists to very high frequencies (9.8 GHz). Anomalies were also observed in epsilon'/sub a/ at approx. 30 K and in epsilon'/sub b/ at approx. 70 K. These are believed to be related to the onset of spin ordering at these two temperatures. The effects of pressure on the dielectric properties and transition temperature T/sub c/ were also investigated. T/sub c/ increases nonlinearly with pressure at low pressures, reaches a maximum and then decreases at higher pressures. This is discussed briefly in terms of the lattice-dynamical origin of the transition. The crystal is highly anisotropic and this is reflected in the epsilon'/sub i/. This anisotropy is due to the lattice contribution to epsilon'/sub i/ and specifically the relevant lattice resonance frequencies. The over-all results and their implicationsmore » are discussed. (AIP)« less

Critical effects of the light scattering intensity at an order–disorder phase transition: KCN

Abstract: The temperature dependence of the Raman spectrum of potassium cyanide between 140 and 220 cm−1 was measured in the two low temperature phases. With decreasing temperature the band at 190 cm−1 grows out of a broad feature of the spectrum and becomes distinct below the transition temperature at 83 °K. We suppose that the Raman spectrum of KCN in the measured energy range is disorder induced above 83 °K, and thus of low intensity. Below the transition temperature only the intensity of the Raman allowed modes increases. We analyze the data by assuming that fluctuating clusters of the ordered low temperature phase enhance the intensity of these modes. From the spectra we determine the critical exponents ν=0.68 and β=0.33 and estimate a lower limit for the correlation length.

Effect of trace elements, molybdenum, and intercritical heat treatment on temper embrittlement of 2-1/4Cr-1 Mo steel

Abstract: To minimize the temper-embrittlement susceptibility of 2 l/4Cr-lMo steel, minor changes in trace-element content and/or heat treatment will provide an improvement. Specimens containing two trace-element and six molybdenum levels were quenched and tempered to 100 ksi (690 N/mm2) tensile strength and embrittled by either isothermal aging at 900‡F (480°C) for 1000 h or step cooling. When 33 ppm of P, Sn, As, and Sb are present, fracture appearance transition temperature (FATT) remains at about -130°F (-90°C) between 0.5 and 1.4 pct Mo. When 405 ppm of the trace elements are present, embrittled FATT reaches a minimum -50‡F (-45°C). Thus, temper embrittlement can be suppressed by maintaining low trace-element concentrations. Intercritical heat treatment at 1575°F (855°C) also reduces temper embrittlement in air-cooled l/2 in. (13 mm) thick plates which simulate water-quenched heavy plates.

Influence of Disordering by Low-Temperature Neutron Irradiation on the Superconducting Transition Temperature of Nb_sub.3_Sn

Abstract: The superconducting transition temperatureT c of a Nb3Sn diffusion wire has been studied as a function of low-temperature neutron irradiation (T=10 K).T c is observed to be essentially constant until the fast neutron dose φt (for neutron energiesE>0.1 MeV) exceeds a value of 1018 n/cm2; after this valueT c decreases linearly with φt up to the maximum applied dose of 1.05×1019 n/cm2. Comparison with theory and other experimental data indicates that radiation-induced disorder is the primary mechanism for the observedT c changes.

Theory of Liquid—Liquid Interface of Polymer Systems

Abstract: Top of pageAbstract A theory for the interfacial tension and interface thickness of demixed polymer mixtures and polymer solutions is presented on the basis of the theory of Cahn and Hilliard, taking into account the change in the dimension of polymer coil at the interface. The following results are obtained from the theory: (1) Near critical temperature Tc in a polymer mixture with the same chain length m (a symmetrical system), interfaical tension σ increases with m in the 1/2 power of m, whereas in a polymer solution, σ decreases with m in the—1/4 power, compared at the same reduced temperature T/Tc. On the other hand, the thickness of the interface near Tc increases with increasing m, proportional to the unperturbed chain dimension, i.e., to m1/2, in a symmetrical polymer system, and to the 1/4 power in a polymer solution. (2) In high polymer systems, a first order transition from a diffuse interface to a sharp interface is predicted. The transition temperature reduced by Tc increase with increasing m. The thickness of the interface at a low temperature does not greatly depend on m and is of the order of a segment length. Keywords: Interfacial Tension; Interface Thickness; Transition; Polymer; Non-Uniform System; Mixture; Solution; Critical Phenomena

The central mode in the critical scattering of X-rays by SrTiO3

Abstract: The dynamics of the phase transition in SrTiO3 (Tc approximately 105K) is complicated by the presence of a central mode in the scattering at the R point of the cubic Brillouin zone in addition to the expected soft-mode scattering. Experiments using Mossbauer gamma rays are reported in which 'elastic' scattering with an energy change of less than 1.5*10-8 eV is detected at the R-point for temperatures as high as 170K. Furthermore, by using X-rays of different incident energy, it is shown that above Tc the scattering power of the crystal at the R point is depth-dependent. The intensity of the scattering above Tc can be altered by annealing the crystal. The results are interpreted in terms of a depth-dependent transition temperature; the crystal begins to transform to the lower-temperature phase first at the surface, and as Tc is approached from above the transformed surface region extends further into the bulk.

Effect of thermodynamic fluctuations on the shape and width of the resistive transition in three-dimensional amorphous superconducting alloys

Abstract: Measurements of the fluctuation conductivity above T/subc/ have been made for several bulk amorphous superconducting alloys. The results are compared to existing theories for fluctuation conductivity in three-dimensional superconductors. Agreement with theory is obtained very near T/subc/, but significant deviations from the predictions of the theory are found for higher temperatures. This is attributed to a breakdown of the Ginzburg-Landau approach. The observed temperature dependence of the fluctuation conductivity was similar in all samples studied and suggests a universal dependence on temperature for bulk amorphous superconductors. The effect of statistical variations in alloy composition (occurring on the scale of the superconducting coherence length xi) on the width of the superconducting transition widths was observed. (AIP)

Phase transition studies of pure and flux-grown barium titanate crystals

Abstract: The temperature dependence of the axial lattice spacings of a pure BaTiO3 crystal, grown from a `top-seeded' TiO2-rich melt, is presented for the four phases. This behaviour is compared with that of a crystal grown using a potassium fluoride flux. The lower ferroelectric paraelectric transition temperature and the smaller lattice parameter discontinuities exhibited by the latter crystal are attributed to the incorporation of impurities from the flux.

Dynamics of the order parameter of superconducting aluminum films

Abstract: The imaginary part of the pair-field susceptibility of dirty-limit, superconducting, aluminum films has been obtained both above and below the transition temperature by analyzing the I–V characteristic of asymmetric superconducting tunneling junctions. Results obtained above the transition temperature have been found to be consistent with a diffusive time-dependent Ginzburg-Landau equation. Below the transition temperature two modes have been found, one propagating and the other diffusing. The resonant frequency and damping constant of the propagating mode have been compared with the predictions of a phenomenological two-fluid hydrodynamic theory given by Bray and Schmidt. A somewhat more detailed comparison has been made with a microscopic theory given by Schon and Schmid in which the propagating and diffusing modes, respectively, are associated with transverse and longitudinal fluctuations of the order parameter. In both the microscopic and macroscopic theories the propagating mode is a high-frequency superconducting analog of second sound in that it consists of a counter motion of the normal and superfluid.

Synthesis and stability of Nb3Ge made by electron‐beam codeposition

Abstract: Niobium‐germanium (Nb‐Ge) films which remain superconducting up to about 22.5 °K have been prepared using high‐rate (33–133 A/sec) electron‐beam codeposition. The presence of oxygen in the deposition chamber is necessary for stabilizing the high‐Tc A15 phase. Analysis of Nb‐Ge films shows that oxygen extends, with some temperature dependence, the Ge‐rich range of the A15 phase with an accompanying decrease in the lattice constant. An amorphous component in the films is first detected below a deposition temperature of 800 °C. It increases rapidly at lower temperatures and becomes dominant at about 600 °C. The A15 phase diagram was determined for the first time for temperatures below 875 °C.

Thermal transitions between collisionally and tight‐complex limited kinetics in the gas phase. Reactions involving carbonium ions

Abstract: H− transfer reactions of C2H5+ and s‐C3H7+ with C4–C8 tertiary hydrocarbons were found to exhibit transition from slow kinetics at high temperatures to fast kinetics at low temperatures. Above the transition temperature, characteristic to each reaction, the reactions proceed with less than unit efficiency and the rate constants exhibit significant negative temperature dependencies with the functional form k=AT−1.5 to k=AT−2.5. Below its transition temperature, each reaction becomes a typical fast ion–molecule reaction. In these lower temperature regions the rates are comparable to the calculated ion–molecule collision rates and are independent of further lowering of the temperature. The transition between temperature dependent and temperature independent behavior is interpreted on the basis of a mechanism proceeding through two stages each of which may be rate limiting. The first step is an ion–molecule collision whose rate is independent of temperature. The second step involves the formation of a tight c...

Critical Heat Capacities of Two-Dimensional Ising-Like Antiferromagnets K2CoF4 and Rb2CoF4

Abstract: Precise measurement of the specific heat of two-dimensional Ising-like antiferromagnets K 2 CoF 4 and Rb 2 CoF 4 near the transition temperature are reported. A symmetric logarithmic singularity is observed at the transition temperature in both materials. The result is in excellent agreement with Onsager's solution for the two-dimensional Ising model. Further, to clarify the short-range order effect in the two-dimensional Ising-like system, temperature variation of magnetic specific heat of K 2 CoF 4 is also examined over a wide temperature range.