True Critical Current Density Research Articles

Enhancement of critical current density and vortex activation energy in proton-irradiated Co-dopedBaFe2As2

The effect of proton irradiation in Ba(Fe0.93Co0.07)2As2 single crystals is reported. We analyze temperature dependence of current density and normalized flux relaxation rate in the framework of collective creep model. Glassy exponent and barrier height for flux creep are directly determined by Maley's method. Our model functions for barrier height and critical current density in the absence of flux creep are explained by the superposition of \deltaTc- and \deltal-pinning. We also approach true critical current density by means of generalized inversion scheme, and the obtained result is in reasonable agreement with our model function. Proton irradiation effect on temperature dependence of current density and normalized relaxation rate can be summarized as doubling of barrier height at the beginning of flux creep.

Magnetic-field-dependent pinning potential in LiFeAs superconductor from its Campbell penetration depth

A true critical current density, $j_{c}$, as opposite to commonly measured relaxed persistent (Bean) current, $j_{B}$, was extracted from the Campbell penetration depth, $\lambda_{C}(T,H)$ measured in single crystals of LiFeAs. The effective pinning potential is non-parabolic, which follows from the magnetic field - dependent Labusch parameter $\alpha$. At the equilibrium (upon field - cooling), $\alpha(H)$ is non-monotonic, but it is monotonic at a finite gradient of the vortex density. This behavior leads to a faster magnetic relaxation at the lower fields and provides a natural \emph{dynamic} explanation for the fishtail (second peak) effect. We also find the evidence for strong pinning at the lower fields. The inferred field dependence of the pinning potential is consistent with the evolution from strong pinning, through collective pinning and, eventually, to a disordered vortex lattice. The values of $j_{c}(2\text{K}) \simeq 2\times10^{6}$ A/cm$^{2}$ provide an upper estimate of the current carrying capability of LiFeAs. Overall, vortex behavior of almost isotropic, fully-gapped LiFeAs is very similar to highly anisotropic d-wave cuprate superconductors, the similarity that requires further studies in order to understand unconventional superconductivity in cuprates and pnictides.

Vortex pinning by natural linear defects in thin films ofYBa2Cu3O7−δ

The behavior of the superconducting current density ${j}_{s}(B,T)$ and the dynamical relaxation rate $Q(B,T)$ of ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ thin films exhibits a number of features typical for strong pinning of vortices by growth induced linear defects. At low magnetic fields ${j}_{s}(B)$ and $Q(B)$ are constant up to a characteristic field ${B}^{*},$ that is directly proportional to the linear defect density ${n}_{\mathrm{disl}}.$ The pinning energy ${U}_{c}(B=0)\ensuremath{\approx}600\mathrm{K}$ can be explained by half-loop excitations determining the thermal activation of vortices at low magnetic fields. Extending the Bose glass theory [D. R. Nelson and V. M. Vinokur, Phys. Rev. B 48, 13 060 (1993)], we derive a different expression for the vortex pinning potential ${\ensuremath{\varepsilon}}_{r}(R),$ which is valid for all defect sizes and describes its renormalization due to thermal fluctuations. With this expression we explain the temperature dependence of the true critical current density ${j}_{c}(0,T)$ and of the pinning energy ${U}_{c}(0,T)$ at low magnetic fields. At high magnetic fields ${\ensuremath{\mu}}_{0}H\ensuremath{\gg}{B}^{*}$ the current density experiences a power law behavior ${j}_{s}(B)\ensuremath{\sim}{B}^{\ensuremath{\alpha}},$ with $\ensuremath{\alpha}\ensuremath{\approx}\ensuremath{-}0.58$ for films with low ${n}_{\mathrm{disl}}$ and $\ensuremath{\alpha}\ensuremath{\approx}\ensuremath{-}0.8$ to -1.1 for films with high ${n}_{\mathrm{disl}}.$ The pinning energy in this regime, ${U}_{c}(\mathrm{high}B)\ensuremath{\approx}60--200\mathrm{K}$ is independent of magnetic field, but depends on the dislocation density. This implies that vortex pinning is still largely determined by the linear defects, even when the vortex density is much larger than the linear defect density. Our results show that natural linear defects in thin films form an analogous system to columnar tracks in irradiated samples. There are, however, three essential differences: (i) typical matching fields are at least one order of magnitude smaller, (ii) linear defects are smaller than columnar tracks, and (iii) the distribution of natural linear defects is nonrandom, whereas columnar tracks are randomly distributed. Nevertheless the Bose glass theory, that has successfully described many properties of pinning by columnar tracks, can be applied also to thin films. A better understanding of pinning in thin films is thus useful to put the properties of irradiated samples in a broader perspective.

Voltage Response of AC Susceptibility and Pinning Effects in Hg0.69Pb0.31Ba2Ca2Cu3O8+δ

We have studied the voltage response of AC susceptibility,and from the imaginary part of the susceptibility experimental results a new method is presented in terms of the material equation to investigate the pinning properties and the dynamical response in our Hg 0.69 Pb 0.31 Ba 2 Ca 2 Cu 3 O 8+δ sample. We also discuss the differences between the voltage criterion and our AC response measurement method in order to determine the value of the true critical current density. The result shows that the later method reflects the true critical state of high-temperature superconductivity better than the general voltage criterion.

Evidence for the lattice-mismatch-stress-field induced flux pinning in (Gd 1 − xY x)BA 2Cu 3O 7 − σ thin films

By partially substituting Gd with Y in GdBa 2Cu 3O 7 − σ thin films, the measured critical current density ( j s) was increased, while the relaxation rate ( S) was clearly lowered, which was attributed to the presence of extra pinning centers. The temperature dependence of the true critical current density j c( T) and the pinning potential U c( T) were determined from the experimental data both for the pure and the substituted thin films by means of a generalized inversion scheme (GIS). It is found that the experimental results for the pure film are close to the predictions of the σl pinning model, while those for the substituted thin film follow the theoretical expressions derived for the single-vortex collectively pinned by the small size stress-field pinning centers, which may suggest that the enhanced pinning in the substituted sample is probably due to the presence of a local stress field induced by the lattice mismatch.

Dynamic contribution to the fishtail effect in a twin-free DyBa 2Cu 3O 7−δ single crystal

Superconducting current densities j s and dynamic relaxation rates Q  d ln j s/d In(d B e/d t), where d B e/d t is the sweep rate of the external magnetic field B e, were measured as a function of temperature (5 K < T < 65 K) in magnetic fields up to 7 T on a twin-free DyBa 2Cu 3O 7−δ single crystal by means of a high-sensitivity capacitance torque magnetometer. Above 15 K, we observe a “fishtail” effect, i.e. a pronounced minimum in the j s( B e) curve at fields around B e = 1 T. The relaxation rate Q shows an anomalous increase at low fields which is correlated to the minimum in the j s( B e) curve. Both the j s versus B e and Q versus B e data are used as input parameters into the generalized inversion scheme developed by Schnack et al. [Phys. Rev. B 48 (1993) 13178] to calculate the true critical current density j c which is by definition independent of relaxation effects. Interestingly, the j c( B e, T) curves derived in this way do not show a minimum. This points clearly to a dynamic contribution to the fishtail effect. The true critical current density j c( B e, T) decreases weakly with increasing B e over the entire measured temperature and field range, as expected for single-vortex pinning. This indicates that the observed fishtail effect is not caused by a crossover from single-vortex pinning to pinning of flux bundles. The temperature dependence of j c is in good agreement with the predictions of a model based on single-vortex pinning caused by spatial fluctuations in the charge-carrier mean free path.

A study of the power handling ability of gallium arsenide and silicon, single and double drift IMPATT diodes

The concept of a critical current density effect on the operation of silicon and gallium arsenide IMPATT diodes is examined using large signal analysis. This critical current density effect does not appear to exist in the form that is generally thought of to-date. However, other physical processes develop at high current densities which gradually degrade diode efficiencies. These processes are worse in silicon diodes than in gallium arsenide diodes because at a given frequency of operation silicon diodes need a lower doping density than gallium arsenide diodes due to the lower saturated drift velocities of carriers in gallium arsenide. Reasons are suggested which explain why these other processes develop before a true critical current density limit is seen. New scaling data for limits on power handling ability vs frequency of gallium arsenide IMPATT diodes are presented. In addition the advantages of double drift structures over single drift structures are re-examined in the light of the suggestion that silicon and gallium arsenide IMPATT diodes are thermally and impedance limited rather than current density limited at high frequencies. The problem of tunnelling is also examined and is shown to be unimportant at all frequencies up to 120 GHz.