Orbital glass in HTSC: a new state of condensed matter

We show that in granular high-temperature superconductors (HTSC) at small magnetic fields the Meissner effect may disappear. A possible explanation for this effect may be related to the existence of special loops of Josephson junctions with positive and negative Josephson couplings. These loops have been observed in a large number of experiments in granular HTSC. On the Josephson loop with an odd number of negative couplings a spontaneous orbital moment is created. The new state of the HTSC with orbital moments, which indeed is a new state of Condensed Matter, is characterized by the coexistence of the orbital paramagnetic state with the superconducting state. If the Coulomb blockade is taken into account, the critical field H_c0 of the transition into the new orbital glass state increases. The new state exists only for magnetic fields H≤H_c0. We have estimated the value of orbital moments and also the influence of the charging energy on orbital magnetism. We show that the critical field H_c0 of the orbital paramagnetic glass transition is inversely proportional to the surface area S of an average loop of Josephson junctions in granular HTSC or the cross-sectional area of an average grain S: H_c0∼ΦH₀/S, where Φ₀ is the elementary flux quantum. Recent experiments demonstrate that at small magnetic fields (0.1-2 Oe) the Meissner effect in granular HTSC (2212-Bi compound) and in other compounds disappears. If we insert in our formula an appropriate value for the cross-section of an average grain, we obtain H_c0 of the order of 2 Oe. This confirms the prediction of our theory on the existence of a new critical field H_c0, which we call the orbital glass critical field.