H.C. Li

Evidence for the lattice-mismatch-stress-field induced flux pinning in (Gd1 − xYx)BA2Cu3O7 − σ thin films

Abstract By partially substituting Gd with Y in GdBa 2 Cu 3 O 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.

Heteroepitaxial multilayer of YBa2Cu3O7 and PrBa2Cu3O7 on SrTiO3 and LaAlO3 substrates by sputtering

We have successfully fabricated epitaxially grown YBa2Cu3O7/PrBa2Cu3O7 (YBCO/PBCO) multilayer thin films on SrTiO3 and LaAlO3 substrates by dc/rf magnetron sputtering. The thicknesses of YBCO and PBCO varied from 1 to 8 unit cells. Satellite peaks in x‐ray diffraction patterns clearly indicate the formation of periodic modulation structures of different wavelengths. At a certain thickness of the YBCO layer, the zero resistance transition temperature Tc0 decreased with the increase of the PBCO layer thickness. In contrast, Tc0 increased with the increase of the YBCO layer thickness at a constant PBCO layer thickness.

NUMB negatively regulates the epithelial-mesenchymal transition of triple-negative breast cancer by antagonizing Notch signaling

Triple-negative breast cancer (TNBC), an aggressive subtype of breast cancer with higher rates of early relapse and metastasis, is frequently associated with aberrant activation of epithelial-mesenchymal transition (EMT). Nonetheless, how EMT is initiated and regulated during TNBC progression is not well understood. Here, we report that NUMB is a negative regulator of EMT in both human mammary epithelial cells and breast cancer cells. Reduced NUMB expression was significantly associated with elevated EMT in TNBC. Conversely, overexpression of NUMB strongly attenuated the EMT program and metastasis of TNBC cell lines. Interestingly, we showed that NUMB employs different molecular mechanisms to regulate EMT. In normal mammary epithelial cells and breast cancer cells expressing wild-type p53, NUMB suppressed EMT by stabilizing p53. However, in TNBC cells, loss of NUMB facilitated the EMT program by activating Notch signaling. Consistent with these findings, low NUMB expression and high Notch activity were significantly correlated with the TNBC subtype in patients. Collectively, these findings reveal novel molecular mechanisms of NUMB in the regulation of breast tumor EMT, especially in TNBC.

The nature of the relaxation of resistivity in high-Tc superconductors

Abstract The relaxation of resistivity (Rff(t) in high-Tc is discussed both experimentally and theoretically. The two-kind-of-flux-creep model is proposed. The flux related to the irreversible magnetization part is defined as relaxation flux, which may creep under the Lorentz force of an average transport current density Jt and cause a time-dependent resistivity. The creep of conventional flux under the Lorentz force of Jt causes a time-independent resistivity. A general formula of Rff(t) is obtained. A specific Rff(t) formula is derived from the general one, using the logarithmic U-J relation, which describes quantitatively the experimental data well. Experiments support the proposed model.

Critical current density and flux pinning in Gd1−xYxBa2Cu3O7−y epitaxial thin films

Low-electric-field E=j(s) curves, normalized dynamical relaxation rate Q(T)=d ln(j(s))/d In(E), and experimental critical current density j(s)(E = E(c)) were measured from 10 to 85 K with the magnetic sweeping method. By taking a general assumption for the activation energy U(j(s), T) = U-c( T)f [j(s)(T)/j(c)(T)], with t = T/T-c, we obtained the U(j, T=O K) relation by scaling all the E-j(s) curves of different temperatures to one smooth curve with U-c(T) and j(c)(T) determined by the so-called generalized inversion scheme. It is shown that the important parameter c = In(upsilon(o)B), which was actually taken as a fitting parameter in, the scaling process in previous work, can be predetermined. It is also found that the resulting U(j, T=O K) relation based on the E=j(s) curves for T less than or equal to 79 K can be well interpreted with the collective-pinning model. However, the data for T>79 K start to deviate from this interpretation, which may be attributed to the reverse hopping of vortices.

THE VORTEX-GLASS TRANSITION IN TL2BA2CACU2O8 SUPERCONDUCTING THIN-FILMS

Abstract We have measured the I–V curves of Tl2Ba2CaCu2O8 thin films at different temperatures in fixed magnetic field. The curves collapsed nicely onto two branches by plotting V/I/|T − Tg|v(z−1) vs. I/T|Tg − T|2v, with the temperature Tg, critical exponents v and z determined from experiments. When T

Resistance relaxation resulting from current diffusion in superconducting thin films

The current diffusion process in superconducting thin films was investigated numerically. The attention was concentrated on the influence of current diffusion on experimental results of transport measurements. It was found that the voltage and the resistance of the film sample in transport measurements were time dependent. The calculated relaxation curves of resistance differed obviously from those observed in magnetic relaxation measurements. Transport measurements on a GdBawCu3O7 thin film were carried out to examine our numerical investigation. Good agreement between the experimental results and our calculation was obtained

A new method for study the mixed state of high-Tc superconductors by using the relaxation of resistivity

Abstract A new method to study the mixed state of high- T c superconductors by the relaxation of resistivity R ff ( t ) experiments is theoretically and experimentally discussed. The theoretical method about how to calculate the decayed magnetization current J i ( R ff ( t )) and the effective pinning energy of flux lines U ( R ff ( t ), J i ( t )) from R ff ( t ) is discussed. Two equations about J i ( R ff ( t ) and U ( R ff ( t ), J i ( t )), are obtained by the two-kind-of-flux-creeop model. R ff ( t ) experiments are performed at various temperatures and magnetic fields. U - J data are calculated. Reasonable physical parameters are derived by fitting the U - J data with two U - J relation models. Experiments and the U - J relation models support the new method and the two-kind-of-flux-creep model.

Paramagnetism and scaling behavior of volume flux pinning force density in a GdBa2Cu3O6+x thin film

Magnetic measurements have been performed on a GdBa2Cu3O6+x superconducting thin film. The paramagnetism carried by rare earth Gd3+ ions in the film tilts the magnetic hysteresis loop and broadens the width of the magnetic hysteresis Delta M, then the magnetization critical current density and the volume flux pinning force density based on the Bean critical state model deviate from intrinsic values. Therefore, in order to get useful information on the pinning mechanism, correction for the paramagnetism is essential. And after correction for the paramagnetism, a scaling law of the volume flux pinning force density is obtained as f(b)=(b/4)(0.5)(1-b/4)(1.5), based on which the possible pinning mechanism in the film is discussed. In the end, deviation from the scaling law at high fields is interpreted by the collective pinning theory

The pinning process of GdBa2Cu3O7−x films

Abstract We measured the pinning force density F p in high quality GdBa 2Cu 3 O 7−x films as a function of temperature ranging 63–78 K and magnetic field up to 7 T. The results indicate that the process of pinning of a -axis oriented platelets in GdBa 2 Cu 3 O 7−x films can be successfully explained by a transition from individual pinning. The anisotropic pinning possibly results from a two-dimensional pinning center, and flux-lattice melting for H ∥ C below and for H ⊥ C at H c 2 and an anisotropic vortex lattice.