Dynamical Dzyaloshinsky-Moriya interaction in KCuF3: Raman evidence for an antiferrodistortive lattice instability

In the orbitally ordered, quasi-one dimensional Heisenberg antiferromagnet KCuF3 the low-energy Eg and B1g phonon modes show an anomalous softening (25% and 13%) between room temperature and the characteristic temperature T_S = 50 K. In this temperature range a freezing-in of F ion dynamic displacements is proposed to occur. In addition, the Eg mode at about 260 cm-1 clearly splits below T_S. The width of the phonon lines above T_S follows an activated behavior with an activation energy of about 50 K. Our observations clearly evidence a reduction of the structural symmetry below T_S and indicate a strong coupling of lattice and spin fluctuations for T>T_S.Comment: 7 pages, 9 figure

Phase-amplitude retrieval:SHG FROG vs. SPIDER

Frequency resolved optical gating (FROG) and spectral phase interferometry for direct electric-field reconstruction (SPIDER) are nowadays leading techniques that provide access to phase-amplitude pulse retrieval. Each of these techniques has a number of outstanding features that establish the method applicability under certain experimental conditions. For instance, FROG is perfectly suited for pulse characterization precisely at the position of the sample in most spectroscopic applications because it utilizes similar excited-probe geometry. On the other hand, SPIDER has the advantage of real-time pulse measurement at high repetition rates. While in an ideal case both methods allow the precise amplitude-phase reconstruction of an ultrashort pulse, in practice specific experimental conditions such as phase-matching, detector noise etc. affect the reconstruction quality. For instance, experimental comparison of the techniques in the case characterization of a single pulse has shown some discrepancies in the retrieved parameters. We present a comparative study of SPIDER and second-harmonic generation (SHG) FROG techniques. Two main sources of errors are analyzed: the limited phase matching bandwidth of the nonlinear medium and the detector noise. We show that under similar experimental conditions SPIDER performs somewhat better than SHG FROG

Low temperature mixed spin state of Co3+ in LaCoO3 evidenced from Jahn-Teller lattice distortions

One- and multi-phonon excitations of the single crystalline LaCoO3 were studied using Raman spectroscopy in the temperature region of 5 K - 300 K. First-order Raman spectra show a larger number of phonon modes than allowed for the rhombohedral structure. Additional phonon modes are interpreted in terms of activated modes due to lattice distortions, arising from the Jahn-Teller (JT) activity of the intermediate-spin (IS) state of Co3+ ions. In particular, the 608-cm-1 stretching-type mode shows anomalous behavior in peak energy and scattering intensity as a function of temperature. The anomalous temperature dependence of the second-order phonon excitations spectra is in accordance with the Franck-Condon mechanism that is characteristic for a JT orbital order.Comment: 11 pages, 9 figures, to be published in J. Low. Temp. Physic