M

Vortex dynamics and pinning properties analysis of MgB2bulk samples by ac susceptibility measurements

“…The comparison between the experiments and the simulations shows that the behaviour encountered in the VG approach is the one which better agrees with the frequency dependence of the measured χ' 3 (T) curves, as it is visible in Fig.4 for the particular values B ac =4 G and B dc =200 G. As a support of this agreement, it can be noted that the features reported in Fig.2, namely the decrease of |χ 3 |(T) for increasing frequencies, are also predicted by the numerical simulations in the VG approach [23], as well as it is the reduction of the non-linearity, evidenced from the χ" 1 (T) curves in Fig.1. Therefore, our results make possible to state that the experimental ac magnetic response of our MgB 2 samples is governed by a vortex glass dynamic regime, likely due to the presence of quenched random point-size disorder [19] which promotes the highly disordered glass phase in the vortex matter, as already reported in literature for this kind of material [24].…”

Macroscopic anisotropy in superconductors with anisotropic gaps

“…To a large extent, motivation for this work was to understand why experiments on different samples of MgB 2 done with different techniques yield widely varying values for γ. [10][11][12][13][14][15][16] The anisotropic Ginzburg-Landau (GL) equations, derived for clean superconductors with an arbitrary gap anisotropy in the seminal work by L.Gor'kov and T.Melik-Barkhudarov, 17 led to the commonly used concept of a single parameter γ defined as ξ a /ξ c ≡ λ c /λ a (ξ is the coherence length, λ is the penetration depth, and a, c are principal crystal directions). Formally, this came out because the same mass tensor enters both the first GL equation which determines the anisotropy of ξ (and of the upper critical fields H c2 ) and the equation for the current which defines the anisotropy of λ.…”

“…The ratio of λ's can be obtained, e.g., from the angular dependence of the reversible torque on single crystals in intermediate magnetic fields tilted relative to the principal crystal directions. 29,30 Some torque data for MgB 2 were reported by M. Andst et al, 14 but the T dependence was not examined in detail. The ratio H c2,ab /H c2,c was shown to drop with increasing T from about 6 at 15 K to 2.8 at 35 K (see also Refs.…”

Competition between anisotropy and superconductivity in organic and cuprate superconductors

“…Blue points -underdoped (UD), green -optimally doped (OpD) and red -overdoped (OD). Transition metal dichalcogenides [22][23][24][25][26][27]; (ET)2 X compounds [28][29][30][31][32][33][34][35][36]; Sr2RuO4 [37,38]; MgB2 [39][40][41]; Nb [42,43]; Pb [43]; Nb Josephson junctions [44]; α−Mo1−x Gex [45]. The dotted line is Eq.…”