The chiral magnetic effect is the generation of electric current induced by chirality imbalance in the presence of magnetic field. It is a macroscopic manifestation of the quantum anomaly 1,2 in rel-ativistic field theory of chiral fermions (massless spin 1/2 particles with a definite projection of spin on momentum)-a dramatic phenomenon arising from a collective motion of particles and antiparti-cles in the Dirac sea. The recent discovery 3-5 of Dirac semimetals with chiral quasi-particles opens a fascinating possibility to study this phenomenon in condensed matter experiments. Here we report on the first observation of chiral magnetic effect through the measurement of magneto-transport in zirconium pentatelluride, ZrTe 5. Our angle-resolved photoemission spectroscopy experiments show that this material's electronic structure is consistent with a 3D Dirac semimetal. We observe a large negative magnetoresistance when magnetic field is parallel with the current. The measured quadratic field dependence of the magnetoconductance is a clear indication of the chiral magnetic effect. The observed phenomenon stems from the effective transmutation of Dirac semimetal into a Weyl semimetal induced by the parallel electric and magnetic fields that represent a topologically nontrivial gauge field background.

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High-pressure optical experiments have been performed on midinfrared excitations unique to the layered cuprates, yet whose physical origin has been unresolved. Measurements of high-pressure midinfrared spectra of Sr 2 CuCl 2 o 2 unambiguously identify the ϳ2800-cm Ϫ1 absorption as an unusual quasibound two-magnon ϩ phonon excitation. We further show that the broad peak centered near ϳ4000 cm Ϫ1 is a four-magnon ϩ phonon excitation, with spectral weight ϳ10 2 times larger than predicted by theory. These excitations reveal unusual latent interactions coupling charge, spin, and lattice degrees of freedom in the CuO 2 planes. PRB 62 V. V. STRUZHKIN et al.

The reflectivity and optical conductivity of single crystal La 2 Cu 1Ϫx Li x O 4 are presented over a temperature range 15-365 K and comparisons are made to its superconducting counterpart La 2Ϫx Sr x CuO 4 . Much like all superconducting cuprates La 2 Cu 1Ϫx Li x O 4 transfers spectral weight from the charge-transfer band to the midinfrared ͑MIR͒ with increasing doping concentrations. However, unlike La 2Ϫx Sr x CuO 4 , La 2 Cu 1Ϫx Li x O 4 is nonmetallic up to Li concentrations as high as xϭ0.50. Furthermore, the charge-transfer ͑CT͒ band of La 2 Cu 1Ϫx Li x O 4 is robust compared to the CT band of the prototypical superconducting cuprates while the MIR band grows at a comparable rate. This and the absence of a Drude tail in La 2 Cu 1Ϫx Li x O 4 suggest that the spectral weight of the MIR band takes in part from excitations above the charge-transfer gap. Despite intensive theoretical and experimental effort to understand the origin of the MIR band there is no universal agreement to date on its mechanism. A few models are mentioned in the summary and will be addressed more thoroughly elsewhere.

Selected findings from the initial 5 years of a longitudinal study of political learning in young children are examined with regard to their theoretical implications. To a limited extent, traditional social learning theory accounted for children’s recognition of political symbols, awareness of public policy issues, and knowledge of the electoral-political party process. Also a limited success was the effort to construct a three-level six-stage framework of political understanding consistent with cognitive developmental theory in the Piagetian or Kohlbergian tradition. A more satisfactory, comprehensive theory of political learning must be able to deal with both incremental and structural change in knowledge. Contemporary information processing theories may prove useful in this regard.