Nanostructured surfaces.
Temperature dependence of coherence lengths for optimally-doped YBa2Cu3O7-δ thin films
Petrenko E.V.1, Omelchenko L.V.1, Solovjov A.L.1,2, A. V. Terekhov1,         Yu. A. Kolesnichenko1, Sergeyev D.M.3 
1 Department of Point-Contact Spectroscopy, B. Verkin Institute for Low Temperature Physics and Engineering of NAS of Ukraine, Nauky Ave., 47, Kharkiv-61103, Ukraine.               E-mail: petrenko@ilt.kharkov.ua                                          
2Institute for Low Temperatures and Structural Research of PAS, ul. Okolna 2, P.O. Box 1410, 50-950 Wroclaw, Poland.                                                  3Department of Physics, K. Zhubanov Aktobe Regional State University,  A. Moldagulova Prospect, 34, Aktobe-030000, Kazakhstan.

A high value of superconducting transition temperature Tc in cuprate high-temperature superconductors (HTSCs) is likely due to a small size of Cooper pairs, which is determined by the short in-plane coherence length ξab(0) [1]. It is well established that, due to anisotropy, ξab is approximately an order of magnitude larger than the coherence length along the c-axis ξc. Interestingly, in cuprates, the exact temperature dependence of both ξab and ξc above and, more surprisingly, below Tc has not yet been fully elucidated. To determine ξab(T) and ξc(T), it is necessary to obtain the temperature dependences of the upper critical magnetic field µ0Hc2(T), preferably in a wide temperature range and in a magnetic field applied parallel to either the ab- plane or the c-axis, since the coherence lengths and the upper critical field are mutually correlated.
For the first time, having calculated the upper critical fields µ0Hc2(0) of the optimally doped YBCO film both within the Ginzburg-Landau (GL) [2] and Wertamer-Helfand-Hochenberg (WHH) [3] theories, we report results concerning the temperature dependences of the coherence lengths in the ab-plane ξab(T) and along the c-axis ξc(T). The effect of a small coherence length on the behavior of Cooper pairs will be discussed during the conference in detail. 

1.	Robinson, N. J., Johnson, P. D., Rice, T. M. & Tsvelik, A. M. Anomalies in the pseudogap phase of the cuprates: Competing ground states and the role of umklapp scattering // Rep. Prog. Phys. - 2019. -  82. - P. 126501.
2.	Ginzburg V.L. and Landau L.D. On the theory of superconductivity // Zh. Eksp. Teor. Fiz. - 1950. - 20, P. 1064-1082.
3.	Werthamer N . R., Helfand K., and Hohenberg P. C. Temperature and Purity Dependence of the Superconducting Critical Field, Hc2. III. Electron Spin and Spin-Orbit Effects // Phys. Rev. -1066. – 147. P 295-302.