Critical Field Curve Research Articles

Initial slope of the critical magnetic field curve of yttrium thorium sesquicarbide

The initial slope of the critical magnetic field curve of yttrium thorium sesquicarbide was observed to be 2.4±0.2 × 10 4 Oe/K. Assuming no paramagnetic limitation, as was the case for lanthanum sesquicarbide, the zero temperature critical-magnetic field is estimated to be 280 ± 24 kOe.

Electronic and superconducting properties of theTi3P-type compoundsNb3As andNb3Si

Superconductivity has been observed below temperatures of about 0.3 K in the tetragonal ${\mathrm{Ti}}_{3}$P-type compounds ${\mathrm{Nb}}_{3}$Si and ${\mathrm{Nb}}_{3}$As. Measurements of the electrical resistivity, heat capacity, superconducting transition temperature, and the critical magnetic field curve are reported. From these measurements, values of the electronic specific-heart coefficient $\ensuremath{\gamma}$, the Debye temperature $\ensuremath{\Theta}$, the electron-phonon coupling constant $\ensuremath{\lambda}$, and the Ginzburg-Landau parameter $\ensuremath{\kappa}$, are deduced. Both compounds have low $\ensuremath{\gamma}$ values (${\ensuremath{\gamma}}_{{\mathrm{Nb}}_{3}\mathrm{Si}}\ensuremath{\approx}2.15$ mJ/${\mathrm{K}}^{2}$g-atom and ${\ensuremath{\gamma}}_{{\mathrm{Nb}}_{3}\mathrm{As}}\ensuremath{\approx}0.9$ mJ/${\mathrm{K}}^{2}$g-atom) which probably accounts for the low transition temperatures. A comparison of their electronic and superconducting properties with those of ${A}_{3}B$ compounds of the $A15$ type suggests that the electron-phonon interaction is quite large in these ${\mathrm{Ti}}_{3}$P-type materials.

Transport properties of superconducting and non superconducting sputtered thin films of Nb-N

We have prepared and studied various kinds of sputered Nb-N samples. By changing the preparation conditions we can indeed obtain superconducting samples or on the opposite very resistive samples. In the first case we have studied the behaviour of the slope of the critical field versus temperature curves near T <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</inf> and we have shown that <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">(\frac{dH_{c2}}{dT})_{T_{c}}</tex> is a regular increasing function of the Ginzburg-Landau parameter κ. On the other hand have measured the perature dependent resistivity <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\rho(T)</tex> , of the nonsuperconducting samples. The results can be interpreted in terms of hopping conduction in disordered materials, the resistivity indeed fit well Mott's relation, <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\rho=\rho_{0}\exp(\frac{T}{T_{o}})^{-1/4}</tex> .

Superconducting properties of Pb xSn 1− xTe

Superconducting transition temperatures and critical magnetic field curves on low carrier concentration Pb x Sn 1− x Te samples where 0.25 ⩽ x⩽0.55 are reported. These data are interpreted in terms of a changing band structure from that of SnTe to one which is similar to that of GeTe. The importance of carriers (holes) in secondary maxima of the valence band is emphasized.

The pressure dependence of superconducting energy gaps from high pressure critical field measurements

It is shown that the Toxen relationship Δ/ kT c = T c / H c (0) (d H c /d T) T c can be used to obtain Δ/ kT c as a function of pressure from critical field curves taken at high pressures. Data for Sn, In, Zn, Al, Cd, Ga, and Th are analyzed and Δ( p)/ kT c are tabulated. For Sn and In a comparison was made of energy gaps so determined with those measured directly by tunnelling.

Effect of Nonmagnetic Localized States on the Superconducting Critical-Field Curves ofThU Alloys

A detailed study of the superconducting critical field curves for ThU alloys is reported. They are found to follow the BCS theory over a wide range of concentrations. Thus the very rapidd depression of the superconducting transition temperature of thorium with the addition of uranium is caused by an interaction which is time reversal invariant. It is found that nonmagnetic resonant impurity states can be extremely detrimental to superconductivity even though they have no local moment and show no sign of lifetime broadening of the Cooper pairs. 25 references.

Specific heat of osmium between 0.18 and 4.2 K

The specific heat of a polycrystalline osmium sample (resistivity ratio 500) has been measured between 0.18 and 4.2 K in zero magnetic field. The transition from superconducting to normal state occurs at T 0 = 0.638±0.002 K. In the normal state, the coefficient of the electronic contribution was found to be 2.050±0.003 mJ/mole deg 2, and the Debye temperature at absolute zero, θ 0 = 467±6 K. In the superconducting state, the electronic specific heat was found to agree with Ces = ayT 0e -bT 0 /T where a = 9.8 and b = 1.51. The critical-field parameter H 0 was found to be 77.5 G, and the deviation of the critical-field curve from the parabolic law was 4.3%. The value of the superconducting energy gap derived from γ, T 0, and H 0 is 3.55 kT 0.

Superconductivity in the presence of short-range magnetic order

Superconducting critical field curves have been measured for La98Lu2−xTbx alloys in a regime where the Tb ions show short-range magnetic order. The concentration dependence of the superconducting transition temperature and the critical field curves both show large deviations from the simple multiple pair-breaking theories and indicate the presence of substantial exchange fields. The onset of short-range order is very detrimental to superconductivity in this system.

Die elektrischen Eigenschaften des Kondo-Supraleiters (La, Ce)Al2

In the ternary alloy (La, Ce)Al2 the Kondo anomalies due to the Cerium impurities are studied in the normal and superconducting state. A vanishing of superconductivity in certain (La, Ce)Al2 alloys below a second transition temperature has been observed for the narrow range of 0.6–0.7 at% Ce-additions. The transition back into the normal state has been observed by means of dc-conductivity and low frequency mutual inductance measurements down to 30 mK. Curves of the upper critical field versus temperature show a maximum at finite temperatures even for lower Ce-concentrations. The temperature dependence of the pair-breaking parameter has been deduced empirically from the upper critical field curves, assuming additive pair breaking theory. In contradiction to the theory no decrease of the pair-breaking parameter forT → 0 can be observed. Therefore the reentrance of superconductivity at ultra low temperatures cannot be expected.

Anomalous critical field curves of the Kondo superconductor (La 1−xCe x)Al 2

We have investigated the critical field curves of (La 1−xCe x)Al 2 alloys and have observed a maximum in H c 2 ( T) for finite temperatures between 0.4 and 0.6°K for Ce concentrations x > 0.003. Our results are in rough agreement with the theory of additive pair-breaking as applied to a Kondo superconductor.

Superconductivity and structural features of the TeGeAs system

Alloys in the TeGeAs system are superconducting below 1 K. X-ray studies show a correlation between a distorted GeTe lattice and elevated transition temperature. Scanning electron micrographs show an inverse trend between typical microstructure dimensions and the slope of the critical field curve. These observations suggest a model of interconnected superconducting GeTe regions in a background of normal Te.

Critical-Field Curve of Superconducting Lead

The critical field of pure monocrystalline lead has been measured throughout its range. A vibrating phase-boundary technique was used, permitting continuous observation of the boundary between the normal and superconducting phases within the sample. Because of the co-existence of both phases at all times, supercooling and superheating effects are avoided. Departures from thermodynamic equilibrium are small, as is indicated by the very weak hysteresis (as low as several parts in 10 000). The deviation curve is strongly positive, as expected for strong-coupling superconductors. The measured values of the usual superconductive parameters are ${T}_{c}=(7.195\ifmmode\pm\else\textpm\fi{}0.006)$ \ifmmode^\circ\else\textdegree\fi{}K, ${H}_{0}=803.4\ifmmode\pm\else\textpm\fi{}0.3$ Oe, ${\frac{d{H}_{c}}{\mathrm{dT}}|}_{{T}_{c}}=237.3\ifmmode\pm\else\textpm\fi{}1.1$ Oe/\ifmmode^\circ\else\textdegree\fi{}K.

Low-temperature annealing effects upon the superconducting properties of V 3Au

The superconducting transition temperature, T 0, and the initial slope of the upper critical magnetic field curve of V 3Au have been measured as functions of low (400°C–800°C) temperature annealing. T 0 ranges from no superconductivity down to 0.015 K to a T 0 as high as 3.22 K. X-ray diffractometer data indicated that all the samples employed in this study had the A15 structure. Metallographic studies indicated the presence of a few percent of a second Au rich phase in these equilibrated samples. The X-ray data also indicated that the low-temperature anneals did change the degree of crystallographic long range order present in the samples and our data clearly indicate that T 0 increases quite markedly as the Bragg-Williams long-range order parameter approaches the value of unity. The magnitude of the initial slope of the critical magnetic field curve also increases with increase in T 0( from -22 × 10 3 Oe/ deg to -37 × 10 3 Oe/ deg for T 0 = 0.89 K and 3.1 K respectively) with one exception, and that is the sample with the highest T 0 for which the slope has its smallest value, namely -16 × 10 3 Oe/ deg.

Critical magnetic fields of aluminium, gallium and thorium under pressure and the isotope effect of cadmium

Critical field curves of the superconductors Al, Ga and Th under pressures up to 25 kbar and down to temperatures of 0.3 K are given. Improved measurements on the isotope effect in cadmium are also reported. The electronic Grüneisen constant is deduced for the investigated metals.

Superconductivity and Magnetic Order in La–Ce Alloys

Superconductivity and magnetic order have been studied both above and below the Kondo temperature for the La–Ce system. Electrical resistivity measurements on La 0.2, 1.0, 2.0, 3.2, and 4.0 wt% Ce have been made from 0.060 to 20.0 K. All samples showed a Kondo-like minimum Tmin in the resistivity with a subsequent maximum Tmax. Tmin was found to be proportional to n1/7, where n is the concentration, and Tmax(H=0) was proportional to n2. The resistivity due to the magnetic impurities went as αln (T/T0) on both sides of Tmax. Above Tmax, α was almost independent of n, and below Tmax it was proportional to n2. Applied magnetic fields had little effect on α even through Tmax increased linearly with H. The magnitude of the resistivity at the peak was essentially independent of n, but decreased for increased H. The superconducting transition temperature was depressed as n increased, but more slowly than predicted by Abrikosov and Gor'kov, and the critical field curves were considerably depressed below the multiple pair breaking theory of Fulde and Maki.

Lead-Alloying Effects on the Superconducting Critical Field Curve of Indium

Lead-Alloying Effects on the Superconducting Critical Field Curve of Indium

Coexistence of Superconductivity and Short-Range Magnetic Order

Thermal conductivity measurements on the La-Lu-Tb alloy system show that short-range magnetic order in the magnetic-impurity spin system does not change the superconducting properties in a substantial way. There appears to be no anomaly in the thermal conductivity at the temperature where short-range order occurs. The critical-field curves are depressed far below the prediction of the multiple-pair-breaking theories, and this seems to indicate the presence of a strong exchange-field enhancement.

Specific Heat of Rhenium between 0.15 and 4.0 K

The specific heat of rhenium has been measured in the normal state between 0.15 and 4.0 K and in the superconducting state between 0.3 and 1.7 K. The specific heat in the normal state between 0.15 and 4.0 K was found to be ${C}_{n}=\ensuremath{\gamma}T+\ensuremath{\alpha}{T}^{3}+\frac{A}{{T}^{2}}$, where $\ensuremath{\gamma}=2290\ifmmode\pm\else\textpm\fi{}20$ \ensuremath{\mu}J/mole ${\mathrm{deg}}^{2}$, $\ensuremath{\alpha}=27\ifmmode\pm\else\textpm\fi{}2$ \ensuremath{\mu}J/mole ${\mathrm{deg}}^{4}$, and $A=49\ifmmode\pm\else\textpm\fi{}2$ \ensuremath{\mu}J deg/mole. Below ${T}_{0}=1.700$ K, the electronic contribution to the specific heat in the superconducting state was found to be ${C}_{s}=\ensuremath{\gamma}{T}_{0}a{e}^{\ensuremath{-}\frac{b{T}_{0}}{T}}$, where $a=8.14$ and $b=1.413$. The above parameters were consistent with the vanishing of the entropy difference ${S}_{n}\ensuremath{-}{S}_{s}$ at ${T}_{0}$. The value of the Debye temperature at absolute zero, ${\ensuremath{\Theta}}_{0}=416$ K, agrees with the value derived from measurement of elastic constants. The density of states at the Fermi level, $N(\ensuremath{\zeta})$, derived from the measured value of $\ensuremath{\gamma}$, is 0.484 states of one spin per eV atom. The value for the superconducting energy gap derived from the value of $b$ deduced in this work is $3.43k{T}_{0}$, compared to a value of $3.30k{T}_{0}$ deduced from thermal-conductivity measurements. The critical-field parameter ${H}_{0}$ was found to be 211 Oe, and the deviation of the critical-field curve from parabolic dependence was less than 3.7%. The resonant frequencies corresponding to the interaction between the nuclear quadrupole moment and the crystalline field are estimated to be 41 and 82 MHz.

Critical magnetic field curve of superconducting tungsten

The critical magnetic field of superconducting tungsten has been measured as a function of temperature down to 3 mK. In common with other weak-coupling superconductors the agreement with the theoretical BCS curve is very good. The critical temperatureT cof our specimen is found to be 15.4±0.2 mK. Analysis of the low temperature data gives the values 1.15±0.01 G forH 0and 0.90±0.06 mJ/mole K2 for the electronic specific heat parameter γ. Very large supercooling and superheating effects are observed for an electropolished specimen. The Ginzburg-Landau parameter estimated from the supercooling field is κ =(2.6 ± 1.6) × 10−3. The problem of providing thermal contact at very low temperatures to the sample is discussed with emphasis on our use of liquid He3.

The electronic specific heat of the high pressure phase of gallium

We have measured the critical magnetic field curve of the high pressure phase of superconducting Ga. For a pressure of about 20 katm, the critical field at T = 0, HO, is found to be 620 Oe and the critical temperature Tc is 6.24K.