Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userCritical Field of MGB2 : Crossover from Clean to Dirty Regimes
82
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
We have studied the upper critical field, Bc2, in poly-crystalline MgB2 samples in which disorder was varied in a controlled way to carry selectively p and s bands from clean to dirty limit. We have found that the clean regime survives when p bands are dirty and s bands are midway between clean and dirty. In this framework we can explain the anomalous behaviour of Al doped samples, in which Bc2 decreases as doping increases.
rate research
Read More
We study the quasiparticle energy relaxation processes in superconducting Nb films of different thicknesses corresponding to different electron mean free paths in a state far from equilibrium, that is the highly dissipative flux-flow state driven up to the instability point. From the measured current-voltage curves we derive the vortex critical velocity $v^{*}$ for several temperatures. From the $v^{*}(T)$ values, the quasiparticle energy relaxation time $tau_{epsilon}$ is evaluated within the Larkin-Ovchinnikov model and numerical calculations of the quasiparticle energy relaxation rates are carried out to support the experimental findings. Besides the expected constant behavior of $tau_{epsilon}(T)$ for the dirty samples, we observe a strong temperature dependence of the quasiparticle energy relaxation time in the clean samples. This feature is associated with the increasing contribution from the electron-phonon scattering process as the dirty limit is approached from the clean regime.
Low resistivity (clean) MgB2 bulk samples annealed in Mg vapor show an increase in upper critical field Hc2(T) and irreversibility field Hirr(T) by a factor of 2 in both transport and magnetic measurements. The best sample displayed Hirr above 14 T at 4.2 K and 6 T at 20 K. These changes were accompanied by an increase of the 40 K resistivity from 1.0 to 18 microohm-cm and a lowering of the resistivity ratio from 15 to 3, while the critical temperature Tc decreased by only 1-2 K. These results point the way to make prepare MgB2 attractive for magnet applications.
We report microwave surface impedances of FeSe$_{0.4}$Te$_{0.6}$ single crystals measured at 12, 19, and 44 GHz. The penetration depth exhibits a power law behavior, $delta lambda_L=lambda_L (T)-lambda_L (0) propto CT^n$ with an exponent $nsimeq 2$, which is considered to result from impurity scattering. This behavior is consistent with $spm$-wave pairing symmetry. The temperature dependence of the superfluid density largely deviates from the behavior expected in the BCS theory. We believe that this deviation is caused by the crossover from the dirty regime near $T_c$ to the clean regime at low temperatures, which is supported by the rapid increase of the quasiparticle scattering time obtained from the microwave conductivity. We also believe that the previously published data of the superfluid density can be interpreted in this scenario.
Magnetic field of up to 12 T was applied during the sintering process of pure MgB2 and carbon nanotube (CNT) doped MgB2 wires. We have demonstrated that magnetic field processing results in grain refinement, homogeneity and significant enhancement in Jc(H) and Hirr. The Jc of pure MgB2 wire increased by up to a factor of 3 to 4 and CNT doped MgB2 by up to an order of magnitude in high field region respectively, compared to that of the non-field processed samples. Hirr for CNT doped sample reached 7.7 T at 20 K. Magnetic field processing reduces the resistivity in CNT doped MgB2, straightens the entangled CNT and improves the adherence between CNTs and MgB2 matrix. No crystalline alignment of MgB2 was observed. This method can be easily scalable for a continuous production and represents a new milestone in the development of MgB2 superconductors and related systems.
There is a hot debate on the anomalous behavior of superfluid density $rho_s$ in overdoped La$_{2-x}$Sr$_x$CuO$_4$ films in recent years. Its linear temperature dependence $rho_s(0)-rho_s(T)propto T$ infers the superconductors are clean, but the zero temperature value $rho_s(0)propto T_c$ is a hallmark of the dirty limit in the Bardeen-Cooper-Schrieffer (BCS) framework (Bozovic et al., 2016). In this work, we show that the apical oxygen vacancies can lead to an anisotropic scattering rate $Gamma_dcos^2(2theta)$, which can explain the above two linear scalings simultaneously, and thus provides a plausible solution to this clean-dirty paradox. Furthermore, by analyzing the optical conductivity, it may also explain the ``missing Drude weight upon doping as reported in the THz experiment (Mahmood et al., 2019). Therefore, we conclude that the superconducting states of the overdoped cuprates are consistent with the disordered BCS theory.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
