Mixed-state properties of superconductingMgB2single crystals

Abstract

We report on measurements of the magnetic moment in superconducting MgB2 single crystals. We find µ0H c c2 (0) = 3.2 T, µ0H ab c2 (0) = 14.5 T, γ = 4.6, µ0Hc(0) = 0.28 T, and κ(Tc) = 4.7. The standard Ginzburg-Landau and London model relations lead to a consistent data set and indicate that MgB2 is a clean limit superconductor of intermediate coupling strength with very pronounced anisotropy effects.PACS numbers: 74.25. Ha, 74.60.Ec, 74.70.Ad The recent discovery of superconductivity in MgB 2 [1] has attracted a lot of attention. Especially the rather high transition temperature of nearly 40 K in such a simple compound is of interest for applications, but also for an analysis of the physical mechanism leading to superconductivity. Several experiments indicate a phonon mediated s -wave BCS mechanism [2,3]. Different models are proposed to explain the particular properties of MgB 2 [4,5]. Their correctness has to be checked by experiments, but only a few results are available on single crystals [6,7,8,9,10,11,12].We report in this Letter on magnetization measurements on single crystalline MgB 2 in magnetic fields applied parallel and perpendicular to the uniaxial crystallographic (≡ c) axis. A detailed evaluation allows us to obtain the temperature dependence of the most important reversible mixed state parameters, such as the critical magnetic fields, the characteristic lengths, the Ginzburg-Landau (GL) parameter and the anisotropy. We will show that MgB 2 is a clean limit superconductor of intermediate coupling strength with very pronounced anisotropy effects.Several single crystals of MgB 2 were grown using high pressure cubic anvils. Details of the process will be published elsewhere [13]. Two crystals (sample A:3 ) were investigated by magnetic methods. The transition temperature (T c ) of each sample was obtained from the ac -susceptibility measured in a 1 T quantum interference device (SQUID) magnetometer. Sample A shows an onset of T c at 38 K and a rather broad transition of about 1 K. A linear fit of H c c2 vs. T near T c indicates a "bulk transition temperature" of 37.5 K (see inset of fig. 1a). In sample B we find T c = 38.3 K, ∆ T c = 0.3 K and a "bulk T c " of 38.2 K. A simple analysis [14] of the slope of the magnetic moment after reversing the applied field demonstrates that the size of the domain, in which the supercurrents flow without impedance, is identical to the sample size. Furthermore, a comparison of the calculated and the measured magnetization in the Meissner regime indicates a superconducting volume fraction of about 100 %. The further evaluation of the mixed state parameters did not show significant differences between these two crystals.The measurements of the magnetic moment were carried out in the 1 T and in an 8 T (SHE) SQUID magnetometer (for details, cf. [15]). Fig. 1a shows the upper critical field of MgB 2 for applied fields H a c (H c c2 ) and H a ab (H ab c2 ). H c2 (T ) is determined either from the onset of the superconducting signal in the m(T ) curve ("T c (H a )") or ...

Enhancement of the upper critical field by nonmagnetic impurities in dirty two-gap superconductors

“…(32) and (33), the ratio H || c2 /H ⊥ c2 is minimum at T c and increases as T decreases. This behavior has indeed been observed on many, mostly clean, M gB 2 samples 11,12,13,14,15,16,17,18,19,20,21 . By contrast, if impurities (like Al, Mg vacancies) cause the strongest intraband scattering in the "weak" π-band, (D 2 ≪ D 1 ), then the anisotropy parameter H || c2 /H ⊥ c2 would decrease as T decreases, The temperature dependence of H || c2 /H ⊥ c2 for these limiting cases is shown in Fig.…”

“…(12) for the current density obtained from the Uzadel equations coincides with the general expression J = δF/δA, where F is defined by Eqs. (19) and (20). Static GL equations (13)- (20) and their time-dependent generalization 45 have been suggested phenomenologically to describe various distributions of the order parameter and vortex properties in two-gap superconductors 44,45 .…”

“…So far all attempts to increase the critical temperature T c of M gB 2 by doping have been unsuccessful 9 . On the other hand, the significant potential of M gB 2 for applications 10 is limited by comparatively low upper critical fields H ⊥ c2 (0) ≃ 3 − 5T and H || c2 (0) ≃ 15 − 19T of hexagonal M gB 2 single crystals 11,12,13,14,15,16,17,18,19,20,21 , where the indices ⊥ and || correspond to the magnetic field H perpendicular and parallel to the ab plane, respectively. As far as H c2 is concerned, it can be increased by adding nonmagnetic impurities 22,23 , which are especially effective in the dirty limit, 2πk B T c < /τ , where τ is the elastic scattering time, is the Planck constant, and k B is the Boltzmann constant.…”

Measurement of the magnetic penetration depth of a superconducting MgB2thin film with a large intraband diffusivity

“…This value is smaller than those previously obtained in polycrystalline MgB 2 by specific heat measurements 36 and the values reported in clean single crystals. 37,38 As discussed earlier, the superconducting properties in multiband superconductors are affected by the interactions among the bands. 37 We obtain ξ ab (0) = 5.7 nm using H c2 (0) = Φ 0 /2πξ 2 (0) and our experimental value H c c2 (0) = 10 T. We can then use the Ginzburg-Landau theory to estimate the thermodynamic critical field in the film, H c = Φ 0 /2 √ 2πλ(0)ξ(0) = 2100 ± 300 Oe.…”

Plasma modes in layered superconductors

“…This material also has a layered structure with the boron atoms forming layers of two-dimensional honeycomb lattices (single layer). Recent studies with the growth of single crystals [34] show anisotropy of physical properties in MgB 2 . Our results can be applied also to MgB 2 in the limit, when d/a tends to zero.…”