Magnetic field dependence of the coherence length and penetration depth of MgB2 single crystals

We report on specific heat and Hall probe magnetization measurements in magnesium diboride single crystals. A magnetic field dependence of the coherence length (xi) has been deduced from the former assuming that the electronic excitations are localized in field dependent vortex cores in which case xi is related to the Sommerfeld coefficient gamma=Delta C-p/T parallel to(T -> 0) throughout, gamma proportional to[xi(H)/a(0)](2) (a(0) being the vortex spacing). The reversible part of the magnetization has been analyzed with a phenomenological Ginzburg-Landau model introducing field dependent parameters (i.e., penetration depth lambda and xi) which account for the decreasing contribution of the pi-band with increasing field. This approach perfectly reproduces the experimental data by combining the field dependence of xi deduced from C-p (1/xi(2)similar to root B) with an almost linear increase of lambda from similar to 450 A at low field to similar to 700 A 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.