Superconductivity and Hc2-anisotropy in vanadium

Superconductivity and Hc2-anisotropy in vanadium

Ph),sica I07B (1981) 295-296 North-Holland Publishing Company FA 3 SUPERCONDUCTIVITY AND Hc2-ANISOTROPY IN VANADIUM H.W.Weber, E.Moser and E.Seidl A...

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Ph),sica I07B (1981) 295-296 North-Holland Publishing Company

FA 3

SUPERCONDUCTIVITY AND Hc2-ANISOTROPY IN VANADIUM H.W.Weber, E.Moser and E.Seidl Atominstitut der ~sterreichischen Universit~ten, A-I020 Wien, Austria F.A.Schmidt Ames Laboratory, Iowa State University, Ames, IA 50011, U.S.A. We report on a systematic investigation of the superconductivity in vanadium and i t s impurity dependence. Starting from a high-purity s~ngle crystal (? = 1200) we find that no changes of the Fermi surface seem to occur for r e s i s t i v i t y ratios'~450. On the contrary, the second anisotropy coefficient a6 is s t i l l sensitive to variations of the impurity parameter in this range indicating contributions of an anisotropic electron-phonon coupling. The overall Hc2-anisotropy is larger than in Nb and s t i l l s i g n i f i c a n t at an impurity parameter ~ = 2.81. The attractive f l u x line interaction is found to be comparable with the results reported for Nb. INTRODUCTION In continuation of our previous work on the cubic superconductors Nb and Ta [ I - 3 ] and based on the a v a i l a b i l i t y of a h i g h - p u r i t y vanadium single c r y s t a l , systematic experiments on the superconductive properties and, in p a r t i c u l a r , the anisotropy of the upper c r i t i c a l f i e l d H~o have been made. In order to achieve a v a r i a t i o n of the electron mean free path, i n t e r s t i t i a l l y dissolved nitrogen was used again. The data to be presented in the f o l l o w i n g sections are well suited f o r an analysis in terms of microscopic t h e o r i e s , which should help to c l a r i f y the role of various possible mechanisms c o n t r i b u t i n g to the observed anisotropy e f f e c t . SUPERCONDUCTIVE PROPERTIES OF THE SYSTEM V - N As the starting material a [110]-oriented single crystal, which had been purified by electrotransport [4], was used. Detailed measurements of the r e s i s t i v i t y rati6 and the magnetization in the temperature range 2.0 ~ T ~ Tc were made and evaluated in terms of the i n t r i n s i c superconductive parameters of vanadium. A summary of this data is presented in Table I. The values of the BCS-coherence length, the London penetration depth and the Ginzburg-Landau parameter in the clean l i m i t , ~ , are rated as highly reliable because of t h e i r consistency with the following evaluation of the impurity dependence and the obviously reasonable trend with regard to previous work on less pure single crystals [5,6]. I t is interesting to note, that neither the magnitude of the Fermi velocity nor the average value of Hc2.are changed upon increasing the residual r e s l s t i v i t y ratio beyond ~ 450. On the contrary, the analysis of Hc2-anisotropy shows a s t i l l growing contribution of the second anisotropy coefficient a6 (cf. below). These observations may be explained - although without further theoretical analysis only q u a l i t a t i v e ly - by assuming unchanged contributions of the Fermi surface, but s t i l l growing contributions of.the electron-phonon coupling to the observed anlsotropy. 0378-4363/81/0000-0000/$02.50 © North-HollandPublishingCompany

Furthermore, the results of magnetization measurements clearly demonstrate the existence of an attractive f l u x line interaction through an induction jump B0 at the lower c r i t i c a l f i e l d Hcl. The magnitude of BO/POHc is comparable with tile results reported for Nb [ 7 ] . Unfortunately, because of the cylindrical sample geometry used in our experiments, which makes an evaluation of B0 much more d i f f i c u l t than with a spherical geometry [7], the data are not s u f f i c i e n t l y accurate, in order to establish a phase boundary for the transition from type-II/1 to typeI I / 2 superconductivity as a function of Ginzburg Landau parameters. As mentioned in the introduction the starting material was loaded with nitrogen, in order to reduce the electron mean free path. The results on the superconductive properties, which w i l l be presented in more detail elsewhere [8], are in close agreement with the prediction of the Gor'kov-Goodman relation up to the highest impurity parameter (a = 2.81). Table I: Superconductive parameters of vanadium Ref.

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[5} 140 0.18 5 . 3 7 9 45.0 39.8 0.848 [6] 450 0.043 5 . 4 3 46.4 3 9 . 6 0.82 This work 1200 0.026 5 . 4 5 4 6 . 6 3 7 . 9 0.78 Hc2-ANISOTROPY The anisotropy of H.~ was measured with the equlpment descrlbed In [I] in the temperature range 1.6 ~ T ~ 4.2. The angular dependence of Hc? within the (110)-plane ( F i g . l ) is qualitat i v e l y equivalent to a l l the results reported for cubic superconductors so f a r ; for a~O and a given reduced temperature the relative variation of Hc~ is even larger than in Nb. A detailed-analysis of the data was made in terms of cubic harmonic functions H£, the normalized expansion coefficients a~ are shown in Fig.2 for different impurity parameters. The results may be summarized as follows: 295

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Phys.Rev. B5 (1972) 904 [7] Kerchner, H.P., Christen, D.K. and Sekula, S.T., Equilibrium properties of the fluxoid l a t t i c e in s i n g l e - c r y s t a l niobium, Phys.Rev. B21 (1980) 86 [8] Moser, E., Seidl, E., Weber, H.W., Schmidt, F.A., to be published

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I ) The f i r s t c o e f f i c i e n t a4 decreases slowly with increasing impurity content and is s t i l l s i g n i f i c a n t at the highest value of ~ (2.81) at a l l temperatures. 2) The second c o e f f i c i e n t a6 shows a drastic response to the introduction of small impurity concentrations and disappears at ~ 2 . 3) A f i t of the data including the ~ : 8 component y i e l d s a reasonable temperature dependence of a8 in samples with ~ < 0.9. However, the values of a8 are close to the error l i m i t s , which makes a d e f i n i t i v e establishment of the existence and magnitude of a8 difficult. 4) The values of a4 and a6 are larger than in Nb (by about 14 and 3~, respectively, for t = 0.5 and ~ = 0.1). 5) The temperature dependence of a4 shows a sign i f i c a n t kink in the purest samples, which had been observed in s i m i l a r form on high-purity Nb [1].

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{2] Weber, H.W., Sporna, J.F. and Seidl, E., Transition from t y p e - l l to type-I supercond u c i t i v i t y with magnetic f i e l d d i r e c t i o n , Phys.Rev.Lett. 41 (1978) 1502 [3] Sporna, J.F., Seidl, E. and Weber, H.W., Anisotropy of the superconductive to normal t r a n s i t i o n in tantalum-nitrogen single crys t a l s , J.Low Temp.Phys. 37 (1979) 639 [4] Carlson, O.N., Schmidt, F.A. and Alexander, D.G., Electrotransport p u r i f i c a t i o n and some characterization studies of vanadium metal, Met.Trans. 3 (1972) 1249 [5] Radebaugh, R. and Keesom, P.H., Low-temperature thermodynamic properties of vanadium, Phys.Rev. 149 (1966) 209 [6] Sekula, S.T. and Kernohan, R.H., Magnetic properties of superconducting vanadium,

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REFERENCES [ I ] Seidl, E., Weber, H.W. and Teichler, H., Hc2-anisotropy in the system niobium-nitrogen, J.Low Temp.Phys. 30 (1978) 795

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A detailed analysis of these observations and, in particular, an attempt of a theoretical explanation are under way [8].

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