Abstract
Sr2RuO4 and Fe-pnictide superconductors belong to the same point group symmetry D4h. Many experimental data confirm odd pairs in Sr2RuO4 and even pairs in Fe-pnictides, but opposite conclusions also exist. Recent NMR results of Pustogow et al., which revealed even Cooper pairs in Sr2RuO4, require reconsideration of symmetry treatment of its SOP (superconducting order parameter). In the present work making use of the Mackey–Bradley theorem on symmetrized squares, a group theoretical investigation of possible pairing states in D4h symmetry is performed. It is obtained for I4/mmm , i.e., space group of Sr2RuO4, that triplet pairs with even spatial parts are possible in kz direction and in points M and Y. For the two latter cases pairing of equivalent electrons with nonzero total momentum is proposed. In P4/nmm space group of Fe- pnictides in point M, even and odd pairs are possible for singlet and triplet cases. It it shown that even and odd chiral states with angular momentum projection m=±1 have nodes in vertical planes, but Eg is nodal , whereas Eu is nodeless in the basal plane. It is also shown that the widely accepted assertion that the parity of angular momentum value is directly connected with the spatial parity of a pair is not valid in a space-group approach to the wavefunction of a Cooper pair.
Highlights
Pnictide superconductors [1] and Sr2 RuO4 [2] have been characterized in a great number of experiments [3,4] whose results are in some cases mutually excluding, and no consensus is currently achievable
A slight increase in the spin susceptibility originating from the Ru-4d electrons in superconducting state of Sr2 RuO4 was connected with a triplet pairing [9]
Group theoretical basis functions with topological phase winding of the present work are similar to commonly used chiral functions (1), (4) and (5), and it is important to establish the reasons for the appearance of significant differences, namely vertical line nodes and nonunitary structure
Summary
Pnictide superconductors [1] and Sr2 RuO4 [2] have been characterized in a great number of experiments [3,4] whose results are in some cases mutually excluding, and no consensus is currently achievable. NMR spectrum on passing through Tc confirms oddness of the SOP (superconducting order parameter) of Sr2 RuO4 [5]. Phase-sensitive measurements indicate that the phase of the SOP in Sr2 RuO4 changes by π under inversion and confirmed the odd nature of pairing [6]. Odd superconductivity in Sr2 RuO4 is in agreement with the results of penetration measurements [7,8]. A slight increase in the spin susceptibility originating from the Ru-4d electrons in superconducting state of Sr2 RuO4 was connected with a triplet pairing [9]. A dramatic increase in Tc from 1.5 K to 3.5 K and increase in Hc2 ( by a factor of 20 ) in strained samples of Sr2 RuO4 [10] was connected with singlet phase
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