We demonstrate that the onset of complex spin orders in ACr2O4 spinels with magnetic and Jahn-Teller active A=Fe and Cu ions lowers the lattice symmetry. This is clearly indicated by the emergence of anisotropic lattice dynamics-i.e., by the pronounced phonon splittings-even when experiments probing static distortions fail. The crystal symmetry in the magnetic phase is reduced from tetragonal to orthorhombic for both compounds. The conical spin ordering in FeCr2O4 is also manifested in the hardening of the phonon frequencies. In contrast, the multiferroic CoCr2O4 with no orbital degrees of freedom shows tiny deviations from cubic structure even in its ground state.
Dynamical properties of the lattice structure were studied by optical spectroscopy in ACr 2 O 4 chromium spinel oxide magnetic semiconductors over a broad temperature region of T = 10-335 K. The systematic change of the A-site ions (A = Mn, Fe, Co, Ni and Cu) showed that the occupancy of 3d orbitals on the A site has strong impact on the lattice dynamics. For compounds with orbital degeneracy (FeCr 2 O 4 , NiCr 2 O 4 , and CuCr 2 O 4 ), clear splitting of infrared-active phonon modes and/or activation of silent vibrational modes have been observed upon the Jahn-Teller transition and at the onset of the subsequent long-range magnetic order. Although MnCr 2 O 4 and CoCr 2 O 4 show multiferroic and magnetoelectric character, no considerable magnetoelasticity was found in spinel compounds without orbital degeneracy as they closely preserve the high-temperature cubic spinel structure even in their magnetic ground state. Aside from lattice vibrations, intra-atomic 3d-3d transitions of the A 2+ ions were also investigated to determine the crystal field and Racah parameters and the strength of the spin-orbit coupling.
We studied the effect of pressure on the electronic properties of bundled single-walled carbon nanotubes by infrared transmission measurements. Different pressure transmitting media were employed to verify their influence on the observed pressure dependence. A redshift of the optical absorption bands is observed under pressure. Irrespective of the pressure transmitting medium, the pressure-induced frequency shifts of the optical transitions show an anomaly at a critical pressure p c ¼ 2-3 GPa, which can be attributed to the predicted circularto-elliptical structural phase transition. We find a second anomaly in the frequency shifts of the optical transitions at 5-6 GPa, which we interpret in terms of the change of the nanotubes' cross-section from elliptical to race-track or peanut type, in agreement with earlier reports.
We present the pressure-dependent infrared absorbance spectra of unoriented single-walled carbon nanotube films for pressures up to 8 GPa. Various pressure transmitting media (helium, argon, CsI, and alcohol mixture) were employed to verify the influence of the pressure medium on the observed pressure effects. For all pressure transmitting media, a pressure-induced redshift of the absorption bands is observed, with an anomaly at the critical pressure P
c = 2−3 GPa. This anomaly can be attributed to the circular-to-oval structural phase transition. The pressure transmitting medium only affects the results quantitatively, namely, the value of P
c and the broadening of the bands.
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