Antiferromagnetic-spin-fluctuation-mediated pairing as a likely mechanism for unconventional superconductivity in LaAg1−cMnc alloys

Abstract

Electrical resistivity, ac magnetic susceptibility, specific heat, dc magnetization, and dc magnetic susceptibility of superconducting LaAg1−cMnc alloys with c=0.0, 0.025, 0.05. 0.1, 0.2, and 0.3 have been measured in the temperature range of 0.35 K≤T≤300 K at external magnetic fields ranging from 0 to 90 kOe with a view to unravel the exact nature of the superconducting ground state. In these alloys, each Mn atom carries a magnetic moment of ≅4μB. A comparison of the results of these investigations with the predictions of the existing theoretical models permits us to make a number of interesting observations that include the following. The intermetallic compound LaAg is an archetypal Bardeen–Cooper–Schrieffer (BCS) spin-singlet isotropic even-parity s-wave superconductor with a superconducting transition temperature of Tc=0.97 K. At low solute concentrations of c≈0.03, Mn substitutes for La at the La sublattice sites in the LaAg parent compound and Tc suddenly drop from 0.97 to temperatures below 0.35 K, reflecting thereby the destruction of conventional phonon-mediated s-wave superconductivity of the LaAg host by pair-breaking magnetic (Mn) impurities. At a threshold concentration of Mn, c≅0.05 (which corresponds to the antiferromagnetic instability/critical phase boundary in the magnetic phase diagram), superconducting gap opens up, Tc abruptly shoots up to 5 K, and unconventional superconductivity sets in at ambient pressure for T≤Tc. Beyond this threshold concentration, Mn has exclusive site preference for Ag at the Ag sublattice sites in LaAg and Tc increases from 5 to 6 K. The unconventional nature of superconductivity at these solute concentrations is signaled by strong departures from the BCS predictions. We present ample experimental evidence that favors antiferromagnetic-spin-fluctuation-mediated pairing as the most likely mechanism for the unconventional (d-wave) superconductivity observed in LaAg1−cMnc alloys with c≥0.05.