Anisotropy of the Sommerfeld Coefficient in Magnesium Diboride Single Crystals

Abstract

The anisotropic field dependence of the Sommerfeld coefficient gamma has been measured down to B-->0 by combining specific heat and Hall probe magnetization measurements in MgB2 single crystals. We find that gamma(B,theta) is the sum of two contributions arising from the sigma and pi band, respectively. We show that gammasigma(B,theta)=B/Bc2(theta) where Bc2(theta)=Bc2ab/sqrt[sin2theta+Gamma2cos2theta] with Gamma approximately 5.4 (theta being the angle between the applied field and the c axis) and gammapi(B,theta)=gammapi(B)=B/Bpi(B). The "critical field" of the pi band Bpi is fully isotropic but field dependent increasing from approximately 0.25 T for B< or =0.1 T up to 3 T approximately Bc2c for B-->3 T. Because of the coupling of the two bands, superconductivity survives in the pi band up to 3 T but is totally destroyed above for any orientation of the field.

Multiband Superconductivity in KFe2As2: Evidence for One Isotropic and Several Lilliputian Energy Gaps

“…1(b)). We prove the existence of several additional extremely small gaps (∆ 0 < 1.0 k B T c ) and show that they have a profound impact on the low-temperature and low-field behavior, as previously shown experimentally [17][18][19][20][21] and theoretically [22][23][24][25][26] for MgB 2 , CeCoIn 5 and PrOs 4 Sb 12 . The zero-field heat capacity is analyzed in a realistic self-consistent 4-band BCS model which qualitatively reproduces the recent laser ARPES results of Okazaki et al 14 We also find that extremely low-temperature measurements, i.e.…”

“…17,19,45,46 Figure 4(a) shows the field dependence of the electron heat capacity γ(H) at 0.12 K (i.e. T/T c = 0.035) for H parallel to c. Similar to MgB 2 , we find that γ(H) is very non-linear with applied magnetic field.…”

Specific heat measurements of a superconductingNbS2single crystal in an external magnetic field: Energy gap structure

“…The observed γ(H) behavior resembles the two-gap case of MgB 2 (see Fig. 3c, left curve) [21], where the initial rapid increase of γ(H) due to dominant role of the π band with the small gap changes at fields where the σ band with the large gap comes into play.…”

First-Order Transition in the Magnetic Vortex Matter in SuperconductingMgB2Tuned by Disorder

“…(1) may occur in MgB 2 for very large irradiation doses (i.e., small H OD values) due to the influence of H a on the two gaps [24]. However, in our case, H OD lies well above the field for which the small gap is suppressed [28] and MgB 2 behaves here as a single () band system. Finally, note that j In conclusion, the discontinuity in the equilibrium flux density accompanying the critical current peak effect in electron-irradiated single-crystalline MgB 2 , and the marked history dependence of the irreversible magnetization, strongly support the presence of a first-order phase transition.…”