We report on specific heat, high magnetic field transport and ac−susceptibility measurments on magnesium diboride single crystals. The upper critical field Hc2 for magnetic fields perpendicular and parallel to the Mg and B planes is presented for the first time in the entire temperature range. A very different temperature dependence has been observed in the two directions which yields to a temperature dependent anisotropy with Γ ∼ 5 at low temperatures and about 2 near Tc. A peak effect is observed in the susceptibility measurments for µ0H ∼2 T parallel to the c−axis and the critical current density presents a sharp maximum for H parallel to the ab−plane.
The temperature dependence of the upper (H(c2)) and lower (H(c1)) critical fields has been deduced from Hall probe magnetization measurements of high quality MgB2 single crystals along the two main crystallographic directions. We show that Gamma(H(c2))=H(c2 axially ab)/H(c2 axially c) and Gamma(H(c1))=H(c1 axially c)/H(c1 axially ab) differ significantly at low temperature (being approximately 5 and approximately 1, respectively) and have opposite temperature dependencies. We suggest that MgB2 can be described by a single field dependent anisotropy parameter gamma(H) (=lambda(c)/lambda(ab)=xi(ab)/xi(c)) that increases from Gamma(H(c1)) at low field to Gamma(H(c2)) at high field.
We report on specific heat and Hall probe magnetization measurements in magnesium diboride single crystals. A magnetic field dependence of the coherence length ͑͒ has been deduced from the former assuming that the electronic excitations are localized in field dependent vortex cores in which case is related to the Sommerfeld coefficient ␥ = ⌬C p / T͉ T→0 throughout, ␥ ϰ ͓͑H͒ / a 0 ͔ 2 ͑a 0 being the vortex spacing͒. The reversible part of the magnetization has been analyzed with a phenomenological Ginzburg-Landau model introducing field dependent parameters ͑i.e., penetration depth and ͒ which account for the decreasing contribution of the-band with increasing field. This approach perfectly reproduces the experimental data by combining the field dependence of deduced from C p ͑1/ 2 ϳ ͱ B͒ with an almost linear increase of from ϳ450 Å at low field to ϳ700 Å close to H c2. These field dependences can then be used to consistently describe the field dependence of the critical current density, small angle neutron scattering form factor, and muon spin relaxation rate.
The lower (Hc1) and upper (Hc2) critical fields of Mg1−xAlxB2 single crystals (for x = 0, 0.1 and 0.2) have been deduced from specific heat and local magnetization measurements, respectively. We show that Hc1 and Hc2 are both decreasing with increasing doping content. The corresponding anisotropy parameter ΓH c2 (0) = H Very similar values have also been obtained by point contact spectroscopy measurements. The evolution of those gaps with Al concentration suggests that both band filling and interband scattering effects are present.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.