Energy-gap driven low-temperature magnetic and transport properties in Cr1/3MS2(M = Nb, Ta)
Anomalous magnetic exchange in a dimerized quantum magnet composed of unlike spin species
Abstract:
We present here a study of the magnetic properties of the antiferromagnetic dimer material CuVOF4(H2O)6 · H2O, in which the dimer unit is composed of two different S = 1/2 species, Cu(II) and V(IV). An applied magnetic field of μ0Hc1 = 13.1(1) T is found to close the singlet-triplet energy gap, the magnitude of which is governed by the antiferromagnetic intradimer J0 ≈ 21 K, and interdimer J' ≈ 1 K, exchange energies, determined from magnetometry and electron-spin resonance measurements. The results of density functional theory (DFT) calculations are consistent with the experimental results. The DFT calculations predict antiferromagnetic coupling along all nearest-neighbor bonds, with the magnetic ground state comprising spins of different species aligning antiparallel to one another, while spins of the same species are aligned parallel. The magnetism in this system cannot be accurately described by the overlap between localized V orbitals and magnetic Cu orbitals lying in the Jahn-Teller (JT) plane, with a tight-binding model based on such a set of orbitals incorrectly predicting that interdimer exchange should be dominant. DFT calculations indicate significant spin density on the bridging oxide, suggesting instead an unusual mechanism in which intradimer exchange is mediated through the O atom on the Cu(II) JT axis.Intrinsic nature of spontaneous magnetic fields in superconductors with time-reversal symmetry breaking
Abstract:
We present a systematic investigation of muon-stopping states in superconductors that reportedly exhibit spontaneous magnetic fields below their transition temperatures due to time-reversal symmetry breaking. These materials include elemental rhenium, several intermetallic systems, and Sr2RuO4. We demonstrate that the presence of the muon leads to only a limited and relatively localized perturbation to the local crystal structure, while any small changes to the electronic structure occur several electron volts below the Fermi energy, leading to only minimal changes in the charge density on ions close to the muon. Our results imply that the muon-induced perturbation alone is unlikely to lead to the observed spontaneous fields in these materials, whose origin is more likely intrinsic to the time-reversal symmetry-broken superconducting state.