Muons and magnets

Abstract:

We present a first-principles investigation of the spin-ice state in Dy2Ti2O7 using a magnetic source-free exchange and correlation functional, implemented in the CASTEP electronic-structure code. By comparing results from the conventional local spin-density approximation, we show that a spin-ice state in Dy2Ti2O7 can be reliably obtained by removing the magnetic sources from the exchange and correlation contributions to the potential, and we contrast this against the computed ground states of other frustrated pyrochlore magnets.

Abstract:

Y-kapellasite [ ${\mathrm{Y}}_3{\mathrm{Cu}}_9{(\mathrm{OH})}_{19}{\mathrm{Cl}}_8$ ], which hosts an original anisotropic kagome sublattice, is a promising candidate for studying elusive and complex correlated physics. It exhibits a theoretically predicted in-plane $(1/3,1/3)$ magnetic order [Hering , ], but its magnetic interaction values place it close to a phase boundary to a spin liquid state [Chatterjee , ]. Our $\mathrm{\mu }\mathrm{SR}$ measurements under hydrostatic pressure demonstrate the complete suppression of static magnetism in favor of a fully dynamical ground state at 2.3 GPa. Complementary high-pressure x-ray and optical phonon measurements reveal a gradual reduction of the kagome anisotropy, enhancing magnetic frustration without structural transitions. Our results establish Y-kapellasite as a rare clean kagome model in which long-range order is suppressed by pressure-tuned frustration, the first fingerprint for the realization of a quantum spin liquid without strong disorder.

Abstract:

Hydrogels containing C=O groups and calcium cations show an unexpected paramagnetic effect that may have biomedical applications.

Abstract:

We report muon-spin relaxation (μ+SR) measurements of copper acetate [Cu(CH₃CO₂)₂·H₂O], a model spin-1/2 Heisenberg antiferromagnetic dimer chain with an alternation parameter α = 0.001. Zero-field μ⁺SR data collected from 2 to 200 K revealed an oscillatory asymmetry that was analyzed using a model based on the muon-stopping site determined by density-functional theory +μ calculations. Below 50 K, the fitted parameters capture spin dynamics characteristic of the singlet ground state, while at higher temperatures, an additional relaxation was observed due to the thermally populated triplet state, affecting the local magnetic field around the muon-stopping site. The temperature dependence of the fitting parameters was found to exhibit characteristics similar to those of a bipartite entanglement measure, “distance between the states,” obtained from the magnetic susceptibility data. Longitudinal-field μ⁺SR measurements reveal field-dependent relaxation at field values much lower than the field values required to close the spin singlet-triplet gap, emphasizing the importance of quantum fluctuations in the spin dynamics of the dimerized copper acetate.