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The pyrochlore magnet ${\mathrm{Tb}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$ shows a lack of magnetic order to low temperatures and is considered to be a quantum spin liquid candidate. We perform time-domain THz spectroscopy on high-quality ${\mathrm{Tb}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$ crystals and study the low-energy excitations as a function of [111] magnetic field with high energy resolution. The low-energy crystal-field excitations change their energies anomalously under magnetic field. Despite several sharp field-dependent changes, we show that the material's spectrum can be described not by phase transitions but by field-dependent hybridization between the low-energy crystal-field levels. We highlight the strong coupling between spin and lattice degrees of freedom in ${\mathrm{Tb}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$ as evidenced by the magnetic-field tunable crystal-field environment. Calculations based on single ion physics with field-induced symmetry reduction of the crystal-field environment can reproduce our data.

Ferromagnetically interacting Ising spins on the pyrochlore lattice of corner-sharing tetrahedra form a highly degenerate manifold of low-energy states. A spin flip relative to this “spin-ice” manifold can fractionalize into two oppositely charged magnetic monopoles with effective Coulomb interactions. To understand this process, we have probed the low-temperature magnetic response of spin ice to time-varying magnetic fields through stroboscopic neutron scattering and SQUID magnetometry on a new class of ultrapure Ho_2Ti_2O_7 crystals. Covering almost 10 decades of time scales with atomic-scale spatial resolution, the experiments resolve apparent discrepancies between prior measurements on more disordered crystals and reveal a thermal crossover between distinct relaxation processes. Magnetic relaxation at low temperatures is associated with monopole motion through the spin-ice vacuum, while at elevated temperatures, relaxation occurs through reorientation of increasingly spin-like monopolar bound states. Spin fractionalization is thus directly manifest in the relaxation dynamics of spin ice., 論文

Experiments investigated how vernier acuity for dot targets is affected by dot spatial separation and retinal eccentricity. Beck and Schwanz [ Vision Res . 19 , 313–319. 1979] found that the vernier thresholds increased linearly with interdot separation from 7.5 to 30 min arc. Experiment 1 showed that the vernier thresholds increased linearly with interdot separation from 0.5 to 8 deg arc. Experiments 2 and 3 showed that moving the dots into the periphery while keeping the interdot separation constant increased the threshold little if at all. The results are interpreted as supporting the hypothesis that there is (1) an encoding of dot positions in retinal coordinates, and (2) that the relative positions of the dots are made explicit in terms of the slope of the virtual line joining them. The increase in threshold with dot separation indicates that the visual system is unable to directly access and compare the retinal positions of the dots. The constancy of the threshold with eccentricity indicates that the visual system encodes the retinal positions of two well-separated dots as accurately in the periphery as in the fovea.