Temperature Dependence Of Hc1 Research Articles

Effect of vortex pinning by point defects on the lower critical field in layered superconductors

The lower critical field Hc1 in layered superconductors is calculated under the assumption that vortex pinning by point defects is strong in these materials. We consider the case of a purely electromagnetic coupling of vortex pancakes and the case of both the electromagnetic and Josephson couplings of the pancakes in a vortex line. In the latter case, singularities in the temperature dependence of Hc1 are predicted at certain characteristic temperatures.

Two energy gaps in superconducting Lu2Fe3Si5 single crystal derived from the temperature dependence of lower critical field Hc1(T)

We report a detailed study of in-plane lower critical field Hc1(T) and penetration depth λab(T) of the Lu2Fe3Si5 superconductor. Two pronounced changes of the Hc1 curvature are observed on the Hc1(T) curve around 0.4Tc and 0.65Tc, respectively, which are attributed to the two-band nature of superconductivity in Lu2Fe3Si5. The penetration depth of λab(T) follows a power law (∼Tn) below 0.4Tc, with n=2.5 close to the values reported for the FeAs-pnictides, suggesting possible s±-wave pairing in Lu2Fe3Si5. Temperature dependence of Hc1 can be well fitted by the two-gap BCS model in the full temperature region using two superconducting gaps Δ1=0.19±0.1meV and Δ2=1.02±0.1meV.

Enhancement of Hc1 and its influence on the irreversibility lines in multilayer YBa2Cu3O7/PrBa2Cu3O7 thin films

The irreversibility line Hirr, cannot cross Hc1, and the temperature dependence of Hirr(T) differs from Hc1(T). Therefore there exists an intersection between them, and it is expected that the simple power-law behaviour of the irreversibility line Hirr(T)~(1-T/Tc)alpha will break down over a wide range of temperature. We have observed this interesting phenomenon in our three YBa2Cu3O7/PrBa2Cu3O7 (YBCO/PrBCO) multilayer thin films with different YBCO superconducting layers for the first time due to the enhancement of Hc1 in a parallel (|| CuO plane) magnetic field. The temperature dependence of Hc1(T) in these three films is in good agreement with calculations of the enhanced Hc1 derived from the London approximation for the case of a slab of thickness tau which is thinner than the London penetration depth lambda , when the applied field is parallel to the sample surface, and they mimic the results observed by Civale and co-workers in thinner pure YBCO ultra-thin films (~25 nm).

Low field ultrasonic studies on melt-textured YBa2Cu3O7−δ

Ultrasonic attenuation and velocity measurements have proved to be a useful probe for the study of conventional superconductors, where the electron–phonon coupling is the dominant pairing mechanism. Since the discovery of high Tc superconductors, ultrasonic measurements have been performed on these systems with moderate success. In the absence of large single crystals, measurements have been performed on isotropic sintered pellets and partially oriented sintered pellets of these high Tc compounds. Low field ultrasonic measurements in the melt textured YBa2Cu3O7 have proved to be interesting. With shear waves one can see a coupling of the sound waves to the flux lattice. As the field is increased, there is an abrupt jump in both the sound velocity (ΔV) and the attenuation (Δα) at a field Hc1’, which is close to Hc1. The temperature dependence of Hc1’ is the same as that of Hc1. ΔV is found to be proportional to the square root of Hcl’, that is inversely to the average distance between the flux lines. Δα goes to zero at Tc and at T=0, going through a maximum at ∼55 K. Measurements have been done for various orientations of the magnetic field. As no anisotropy is seen in the value of Hc1’, it is surmised that Hc1’ is the lower critical field for the intergranular matrix. [Work supported by ONR.]

Temperature dependence of Hc1 and Jc of æ-(BEDT-TTF)2Cu(NCS)2: Critical state model

Temperature dependence of Hc1 and Jc of æ-(BEDT-TTF)2Cu(NCS)2: Critical state model

Low-field magnetization measurements on small TaS2(aniline)3/4 crystals

Sharp magnetic transitions were obtained in carefully selected superconducting TaS2(aniline)3/4 crystals which are believed to represent the intrinsic material. The temperature dependence of Hc1 was determined and compared with other data and found to be much stronger than expected.

Anisotropy of the Ginzburg-Landau parameter κ in NbSe2

On small single crystals of 2H NbSe 2 we have measured the temperature and angular dependence ofH c2 as well as the temperature dependence ofH c1 close toT c. Using an effective mass approach to the Ginzburg-Landau equations we have analyzed our data in terms of an anisotropic κ and obtained values for the coherence distances and penetration depths parallel and perpendicular to the layers. The connection between these parameters and normal-state properties is discussed and shown to be more complex than in the isotropic case.