CC BY
3
ХИМИЯ И МАТЕРИАЛОВЕДЕНИЕ
УДК 537.3
Лысенок Ю.И. студент магистратуры 1 курса факультет «Отдел магистратуры» Волкова Т.В., к.филос.н.
доцент
кафедра "Научно-технический перевод и профессиональная коммуникация " Донской государственный технический университет
Россия, г. Ростов-на-Дону AGAIN TO THE QUESTION OF D- OR S-SYMMETRY
Annotation. At present, it can be considered practically established that the complex order parameter in high-temperature superconductors has d-wave symmetry. In traditional superconductors, the phase of the order parameter does not depend on the coordinates, and its amplitude can vary with direction. A similar calculation for lead in the framework of the theory of a strong electron-phonon coupling (to which we will return below) was carried out almost 35 years ago by Bennett [1]. He found that the main source of anisotropy of superconducting properties is the angular dependence of the phonon spectrum, although a certain contribution is made by the anisotropy of the Fermi surface. As a result, it turned out that in the equatorial plane the maximum spread of the gap values for lead is about 10%.
Keywords: superconductors, anisotropy, tunneling, additive, injector, component, Josephson curre.
A different matter is high-temperature superconductors. In them, the dependence of the order parameter on the angle in the ab plane, which determines the superconductivity of HTSC materials, has the form of four lobes shifted by 90 ° with respect to each other. In each of the petals, the phase of the order parameter is constant, but in two neighboring lobes it differs by?.
However, there are indications that everything is not so simple, and a small fraction of the s-wave is mixed with the d-wave. Perhaps the most direct in this regard is the Josephson tunneling experiment from the ordinary s-superconductor in HTSCs, oriented so that the normal to its surface coincides with the c axis. Since the Josephson current through the contact is proportional to the sine of the phase difference of its shores, the direction of the current for the neighboring lobes, which differ in phase by?, Will be opposite. The currents compensate each other, and the final result must be zero. It turned out that this is not so.
The corresponding experiment revealed the existence of a Josephson current in the c-axis direction for YBCO [2]. However, there are no special problems here. The symmetry of the 123-connection allows simultaneous existence of both d- and s-waves, i.e. the presence of the d + ks state, where the coefficient k characterizes
"Теория и практика современной науки" №11(29) 2017
402
the fraction of the s-additive. This share is apparently small - a few percent, but it is enough to explain the Josephson experiments on YBCO. However, as it turned out, the Josephson current also exists when tunneling along the c-axis of the BSCCO material [3].
Although the effect here is very small (almost 100 times less than for YBCO), but the fact of its detection is of fundamental importance, since the BSCCO symmetry does not allow the simultaneous presence of d and s waves. The critical field from the magnetic field had a usual Fraunhofer dependence [3], which indicates the uniformity of the current along the transition. To save the situation, it was necessary to put forward the hypothesis that the symmetry with respect to time reversal is violated on the surface of HTSC, and then the complex order parameter has the form d + iks.
There is some experimental evidence in support of this assertion. However, an alternative explanation can be put forward. He was suggested by A.I.M.Rae of the University of Birmingham in a recent publication [4]. The Josephson experiment was performed on BSCCO with a lead injector. Lead, as noted at the beginning of this note, is an anisotropic superconductor, and therefore tunneling into two adjacent lobes of the d-wave may turn out to be nonequivalent. The corresponding currents do not compensate each other, and during tunneling along the c axis a small Josephson component will remain. Where, then, will the necessary nonequivalence take place when rotating by 90 if lead has cubic symmetry? When this argument is advanced, it is assumed that we are dealing with tunneling from the plane of the base of the cube (that is, the [001] direction in the Pb crystal is normal to the xy-plane of the transition). In a real experiment, the cube can be rotated. Let us imagine that we placed it on the plane of the tunnel junction, and then, leaving one edge lying on this plane, the second was rotated by 45 normal to the transition made the direction [011] in lead. Now the x and y axes are already nonequivalent, which was what the author needed [4]. Numerical calculations [4] showed that if a lot of Pb crystallites are present in a real experiment (and in [3], there are quite a few Pb crystallites so oriented, the Josephson experiment for bismuth HTSC is fully explained in the framework of Bennett calculations [1 ] and without the assumption of symmetry breaking with respect to time reversal. How to find out the truth? A.I.M.Rae suggests repeating a Josephson experiment with a c-oriented BSCCO, but with a more isotropic (in comparison with lead) superconductor as one of the plates or simply with a "dirty" material in which all anisotropy is washed away. By the way, he quotes the authors of [3] who write that the best results, i.e. the presence of a weak Josephson current was observed with pure lead. So, the word for the new experiment.
Literature:
1. A.J.Bennett, Phys. Rev., 1966, 140, p. A1902
2. A.G.Sun et al., Phys. Rev. Lett., 1994, 72, p. 2267
3. M.Mosle, R.Kleiner, Phys. Rev. B, 1999, 59, p. 4486
4. A.I.M.Rae, Phys. Rev. Lett., 2000, 84, p. 2235
"Теория и практика современной науки" №11(29) 2017
403
