Column density and temperature effects on narrow resonance structures in atomic photoionization and photoabsorption

Abstract

We consider a formation of the Larkin-Ovchinnikov-Fulde-Ferrell (LOFF) phase in a quasi-onedimensional (Q1D) conductor in a magnetic field, parallel to its conducting chains, where we take into account both the paramagnetic spin-splitting and orbital destructive effects against superconductivity. We show that, due to a relative weakness of the orbital effects in a Q1D case, the LOFF phase appears in (TMTSF)2ClO4 superconductor for real values of its Q1D band parameters. We compare our theoretical calculations with the recent experimental data by Y. Maeno's group [S. Yonezawa et al., Phys. Rev. Lett. 100, 117002 (2008)] and show that there is a good qualitative and quantitative agreement between the theory and experimental data.PACS numbers: 74.70. Kn, 74.20.Rp, 74.25.Op Since a discovery of superconductivity in organic (TMTSF) 2 X conductors (X=PF 6 and ClO 4 ) [1], their physical properties have been intensively studied both experimentally and theoretically [2,3]. From the beginning, it was clear that their superconducting properties were unconventional. Indeed, it was found [4] that superconductivity was destroyed by non-magnetic impurities, which was recently unequivocally confirmed [5]. In addition, it was shown [6] that the conventional for s-wave superconductivity Hebel-Slichter peak was absent in the NMR experiments. Note that the experimental results [4][5][6] provide strong arguments that superconducting order parameter changes its sign on a quasi-one-dimensional (Q1D) Fermi surface (FS) of (TMTSF) 2 X superconductors. On the other hand, they do not contain information about a spin-part of a superconducting order parameter and, thus, do not distinguish between singlet and triplet pairings. At the moment, the problem about a spin part of a superconducting order parameter in (TMTSF) 2 X conductors is still controversial. Indeed, early measurements of the Knight shift in (TMTSF) 2 PF 6 conductor [7] in a magnetic field H = 1.43 T showed that spin susceptibility was unchanged through the superconducting transition. These data were interpreted as an evidence for triplet superconductivity. On the other hand, more recent Knight shift data in (TMTSF) 2 ClO 4 conductor [8], obtained in a magnetic field H = 0.957 T , were interpreted in favor of a singlet superconducting pairing.Another unconventional feature of superconductivity in (TMTSF) 2 X conductors is very large upper critical fields for a magnetic field parallel to their conducting planes and perpendicular to their conducting chains, H b ′ [9-14]. These fields exceed both the quasi-classical orbital upper critical field [15,16] and so-called Clogston paramagnetic limit [17]. Note that in (TMTSF) 2 PF 6 superconductor H b ′ c2 is very large [10,11] due to a formation of domain walls in the vicinity of antiferromagnetic phase. In contrast, in (TMTSF) 2 ClO 4 conductor large upper critical field H b ′ c2 [12-14] is prescribed to 3D → 2D dimensional crossover in a magnetic field, predicted in Ref. [18] and elaborated in Refs. [19][20][21][22][23]. In addi...