A model for the electronic structure of PrBa2Cu307 is proposed with three independent components: insulating CuC>2 planes with oxidation state Cu 11 , mixed valent Pr ions with almost equal numbers of Pr ,n and Pr IV , and CUO3 chains described by a t-J model with filling ^ 7. The difference from the high-r c superconductors comes from an enhanced stability of the Pr IV state due to the hybridization with O neighbors, and involves a transfer of holes from primarily planar O 2p a to 2p n states. The insulating nature of PrBa2Cu307 is ascribed to the extreme sensitivity of the dc conductivity of the mixed valent Pr ions and the CUO3 chains to vacancies and impurities.
Abstract. We contrast the effects of non-magnetic impurities on the properties of superconductors having a d x 2 −y 2 order parameter, and a highly anisotropic s-wave (ASW) gap with the same nodal structure. The non-vanishing, impurity induced, offdiagonal self-energy in the ASW state is shown to gap out the low energy excitations present in the clean system, leading to a qualitatively different impurity response of the single particle density of states compared to the d x 2 −y 2 state. We discuss how this behaviour can be employed to distinguish one state from the other by an analysis of high-resolution angle-resolved photoemission spectra.
A simple physical picture for the ac absorption in high-T, superconductors in the mixed state is proposed. We argue that the transition in the real part and the peak in the imaginary part of the ac susceptibility are due to the skin size eA'ect and occur when the skin penetration depth is of the order of the size of the sample. The frequency of the peak is proportional to the resistivity p(H, T), excluding other field-or temperature-dependent factors. Experiments to determine the true irreversibility line associated with the transition between the vortex-glass and the vortexliquid state are suggested.The unusual behavior of the high-T, superconductors in the mixed state attracts considerable attention. One of the most intriguing phenomena is the existence of the socalled irreversibility line in the 0-T plane. This line separates the regions of reversible (high temperature T and high field H) from irreversible (low T and H) behavior of the system. The irreversibility line was observed by means of magnetization measurements:' above the irreversibility line the magnetization showed reversible behavior, while at
The critical current I, of a long one-dimensional (1D) Josephson junction in the presence of different types of structural disorder is investigated both analytically and numerically. It is shown that most properties of I, can be understood from the behavior of the elementary pinning force (PF) of a single defect, which we calculate exactly as a function of the external magnetic field H"pinning-center size, and strength. The following types of disorder are discussed: (i) For a given field, and pinning centers with equal strength, a unique arrangement of pins that maximizes I, is found. (ii) In the case of a periodic pinning-center lattice, we reproduce the commensurability peaks in the field dependence of the critical current, I,(H, ), previously reported by Oboznov and Ustinov [Phys. Lett. A 139, 481 (1989)]. In addition, we predict that a peak can be damped or disappear, if its position coincides with a field value at which the elementary PF vanishes. (iii) The most interesting effects appear in the presence of random disorder. Using the exact expression for the elementary PF, we extend the collective pinning analysis of Koshelev and Vinokur (Zh. Eksp. Teor. Fiz. 97, 976 (1990) [Sov. Phys. JETP 70, 547 (1990]) to arbitrary fields and properties of the disorder, and compare the obtained predictions with the results of numerical simulations. The agreement between the two approaches is extremely good. In particular, we find that the appearance of a plateau in I, (H, ) for large fields depends strongly on the ratio rp/AJ between the average pinning center size ro and the average Josephson penetration depth A, If ro/A, , =1, there is no plateau at all, and in the case ro/k, ((1,a plateau is found up to fields for which the vortex spacing becomes of the order of ro. Furthermore, we predict the possibility of a dimensional crossover from a 1D behavior at low fields to OD behavior at large fields. Finally, we present a possible explanation of the experimentally observed plateau in the j, (H, ) dependence of granular high-T, materials.
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