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We present a theory of magnon-polaritons in quantum Ising materials, and develop a formalism describing the coupling between light and matter as an Ising system is tuned through its quantum critical point. The theory is applied to Ising materials having multilevel single-site Hamiltonians, in which multiple magnon modes are present, such as the insulating Ising magnet LiHoF$_4$ . We find that the magnon-photon coupling strengths may be tuned by the applied transverse field, with the coupling between the soft mode present in the quantum Ising material and a photonic resonator mode diverging at the quantum critical point of the material. A fixed system of spins will not exhibit the diamagnetic response expected when light is coupled to mobile spins or atoms. Without the diamagnetic response, one expects a divergent magnon-photon coupling strength to lead to a superradiant quantum phase transition. However, this neglects the effects of damping and decoherence present in any real system. We show that damping and decoherence may block the superradiant quantum phase transition, and lead to weak coupling between the soft magnon mode and the resonator mode. The results of the theory are applied to experimental data on the model system LiHoF$_4$ in a microwave resonator., 22 pages, 6 figures

The transition metal monopnictide family of Weyl semimetals recently has been shown to exhibit anomalously strong second-order optical nonlinearity, which is theoretically attributed to a highly asymmetric polarization distribution induced by their polar structure. We experimentally test this hypothesis by measuring optical second harmonic generation (SHG) from TaAs across a pressure-tuned polar-to-nonpolar structural phase transition. Despite the high-pressure structure remaining noncentrosymmetric, the SHG yield is reduced by more than 60 % by 20 GPa as compared to the ambient pressure value. By examining the pressure dependence of distinct groups of SHG susceptibility tensor elements, we find that the yield is primarily controlled by a single element that governs the response along the polar axis. Our results confirm a connection between the polar axis and the giant optical nonlinearity of Weyl semimetals and demonstrate pressure as a means to tune this effect $in$ $situ$., Comment: 5 pages main text, 4 figures, 2 pages supplementary information

The 2-dimensional layered oxide material SrCu

The hallmark of materials science is the ability to tailor the microstructure of a given material to provide a desired response. Carbon mixed with iron provides the steel of buildings and bridges; impurities sprinkled in silicon single crystals form the raw materials of the electronics revolution; pinning centres in superconductors let them become powerful magnets. Here, we show that either adding a few parts per million of the proper chemical impurities to indium antimonide, a well-known semiconductor, or redesigning the material's structure on the micrometre scale, can transform its response to an applied magnetic field. The former approach is purely quantum mechanical; the latter a classical outgrowth of disorder, turned to advantage. In both cases, the magnetoresistive response--at the heart of magnetic sensor technology--can be converted to a simple, large and linear function of field that does not saturate. Harnessing the effects of disorder has the further advantage of extending the useful applications range of such a magnetic sensor to very high temperatures by circumventing the usual limitations imposed by phonon scattering.

When performed in the proper low-field, low-frequency limits, measurements of the dynamics and the nonlinear susceptibility in the model Ising magnet in a transverse field LiHo(x)Y(1-x)F(4) prove the existence of a spin-glass transition for x=0.167 and 0.198. The classical behavior tracks for the two concentrations, but the behavior in the quantum regime at large transverse fields differs because of the competing effects of quantum entanglement and random fields.

Effective-medium theory and inelastic neutron scattering are used to study the magnetic excitations in the Ising ferromagnet LiHoF4 near its magnetic-field-induced quantum phase transition (QPT). As expected, the dominant mode softens at the QPT, but reaches a finite, rather than vanishing, limit due to the hyperfine interaction. The experimental phase diagram and excitation spectra are well described by an effective-medium theory to first order in the 1/z expansion. There are some differences between theory and experiments, which may be due to domain walls, their dynamics and shape effects, and to magnetoelastic couplings.

Magnetic fields change the way that electrons move through solids. The nature of these changes reveals information about the electronic structure of a material and, in auspicious circumstances, can be harnessed for applications. The silver chalcogenides, Ag2Se and Ag2Te, are non-magnetic materials, but their electrical resistance can be made very sensitive to magnetic field by adding small amounts--just 1 part in 10,000--of excess silver. Here we show that the resistance of Ag2Se displays a large, nearly linear increase with applied magnetic field without saturation to the highest fields available, 600,000 gauss, more than a million times the Earth's magnetic field. These characteristics of large (thousands of per cent) and near-linear response over a large field range make the silver chalcogenides attractive as magnetic-field sensors, especially in physically tiny megagauss (10(6) G) pulsed magnets where large fields have been produced but accurate calibration has proved elusive. High-field studies at low temperatures reveal both oscillations in the magnetoresistance and a universal scaling form that point to a quantum origin for this material's unprecedented behaviour.