Muons and magnets

Abstract:

The crystal structure, phonon density-of-states, and magnetic properties of Mn(dca)2(o-phen) (dca = dicyanamide, N(CN)2-; o-phen = 1,10-phenanthroline) have been studied using several methods including magnetization, muon-spin relaxation, and neutron and X-ray scattering. From X-ray crystallography the structure was shown to consist of octahedral MnN 6 sites that are connected via four μ1,5-bridging dca ligands while the two remaining cis-positions are occupied by N-donors of the o-phen ligand. Two Mn2+ ions are bridged together by two dca anions to afford [Mn2(dca)2(o-phen)2]2+ "dimers." These dimers are linked in 2D via μ1,5-dca ligands to afford a polymeric structure whereas the o-phen ligands act as spacers. Bulk magnetic susceptibility data show a characteristic broad maximum at 2.65 K while neutron scattering and muon-spin relaxation data establish TN = 1.85 K. For T ≪ TN, the magnetization, as obtained in pulsed-fields, saturates at 5.75 T and reaches a moment typical of an isotropic S = 5/2 ion. The magnetic structure was determined and found to consist of antiferromagnetically ordered Mn2+ moments oriented in the ac-plane with no spin-canting being evident. The magnetic space group symmetry for the Mn2+ moments is P21′/c with each Mn 2+ ion carrying a magnetic moment of 4.7(1) μB, in good agreement with the value of 5.0 μB expected for an isotropic, high-spin S = 5/2 ion. The fact that LRO is observed in Mn(dca) 2(o-phen) is unusual among low-dimensional dicyanamide complexes especially since only μ1,5-dca modes are involved.

Abstract:

The low-field phase of the organic superconductor (TMTSF)(2)ClO(4) is studied by muon-spin rotation. The zero temperature limit of the magnetic penetration depth within the TMTSF layers is obtained to be λ(ab)(0) = 0.86(2) μm. Temperature dependence of the muon-spin relaxation shows no indication of gap nodes on the Fermi surface nor of any spontaneous fields due to time-reversal-symmetry breaking. The weight of evidence suggests that the symmetry of this low-field phase is odd-frequency p-wave singlet, a novel example of odd-frequency pairing in a bulk superconductor.