Chirality of matter can produce unique responses in optics, electricity and magnetism. In particular, magnetic crystals transmit their handedness to the magnetism via antisymmetric exchange interaction of relativistic origin, producing helical spin orders as well as their fluctuations. Here we report for a chiral magnet MnSi that chiral spin fluctuations manifest themselves in the electrical magnetochiral effect, i.e. the nonreciprocal and nonlinear response characterized by the electrical resistance depending on inner product of current and magnetic field. Prominent electrical magnetochiral signals emerge at specific temperature-magnetic field-pressure regions: in the paramagnetic phase just above the helical ordering temperature and in the partially-ordered topological spin state at low temperatures and high pressures, where thermal and quantum spin fluctuations are conspicuous in proximity of classical and quantum phase transitions, respectively. The finding of the asymmetric electron scattering by chiral spin fluctuations may explore new electromagnetic functionality in chiral magnets.

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We investigate skyrmion formation in both a single crystalline bulk and epitaxial thin films of MnSi by measurements of planar Hall effect. A prominent stepwise field profile of planar Hall effect is observed in the well-established skyrmion phase region in the bulk sample, which is assigned to anisotropic magnetoresistance effect with respect to the magnetic modulation direction. We also detect the characteristic planar Hall anomalies in the thin films under the inplane magnetic field at low temperatures, which indicates the formation of skyrmion strings lying in the film plane. Uniaxial magnetic anisotropy plays an important role in stabilizing the in-plane skyrmions in the MnSi thin film.

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Orthoexcitonic gas in cuprous oxide is generated by one and two photon resonant excitations at different excitation intensities and at different temperatures between 1.8 and 4.2 K. The experimental results are analyzed by simulation with a Boltzmann equation. When the exciton density is low, the observed luminescence is found to originate from a nonequilibrium excitonic gas where the exciton-LA phonon scattering dominates. When the exciton density is very high, not only the exciton-LA phonon scattering but also the exciton-exciton scattering is important. The observed luminescence consists of two systems: one is from an exciton system that is distributed according to the usual Bose-Einstein statistics with chemical potential ϭ0, while the other is from excitons with zero kinetic energy. The two systems were found to be in thermal equilibrium. The latter system might be a form of Bose-Einstein condensation. ͓S0163-1829͑97͒05644-0͔

Nonlinear Hall effect related to current-driven skyrmion string motion is observed experimentally and analyzed theoretically.