Observation of an amplitude collapse and revival of chirped coherent phonons in bismuth

We have studied the A(1g) coherent phonons in bismuth generated by high fluence ultrashort laser pulses. We observed that the nonlinear regime, where the phonons' oscillation parameters depend on fluence, consists of subregimes with distinct dynamics. Just after entering the nonlinear regime, the phonons become chirped. Increasing the fluence further leads to the emergence of a collapse and revival, which next turns into multiple collapses and revivals. This is explained by the dynamics of a wave packet in an anharmonic potential, where the packet periodically breaks up and reconstitutes in its original form, giving convincing evidence that the phonons are in a quantum state, with no classical analog.     

Coherent phonons in NdBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7−<Emphasis Type="Italic">x</Emphasis></Subscript> single crystals: Optical-response anisotropy and hysteretic behavior

Femtosecond pump-probe measurements of reflection from crystallographic planes are performed to investigate lattice relaxation dynamics in the NdBa₂Cu₃O_{7? x} high-temperature superconductor. Ultrafast phonon response is examined over a wide temperature range for various orientations of the pump and probe polarization vectors with respect to particular crystallographic axes. The initial phases of coherent phonons are measured, and hysteretic behavior is revealed in the transition between two temperature regions above T_c for the ac plane.     

Reactions of photogenerated fluorine atoms with molecules trapped in solid argon

Isolated radicals^.NH₂ and radical-molecule complexes^.NH₂−HF, which are products of the reactions of mobile fluorine atoms with NH₃ molecules in solid argon, were identified by EPR spectroscopy. The isotropic HFC constants of the complex (a _N=1.20,a _H=2.40, anda _F=0.70 mT) were determined experimentally. The constant of isotropic HFC with the nucleus of hydrogen atom of the HF molecule is less than 0.1 mT. This assignment was confirmed in the experiments on isotope substitution of atoms (H→D),¹⁴N→¹⁵N) in the NH₃ molecule. According to quantum-chemical calculations, the free complex^.NH₂−HF has a planar structure withC ₂, summetry and a binding energy of 12 kcal mol⁻¹. Optimization of the arrangement of the complex in the crystal showed that its structure is only slightly distorted in the Ar lattice so that the equilibrium configuration is close to that obtained from gas-phase calculations. Different ratios of relative intensities of the proton triplet lines in the EPR spectra of isolated^.NH₂ radicals and^.NH₂−HF complexes were qualitatively explained by different heights of the barriers to rotation of the NH₂ fragment in the Ar lattice.     

Generation of coherent off-diagonal raman-active phonons by femtosecond laser pulses in high-temperature superconductor YBa2Cu3O7−x

The time-resolved optical response from various crystallographic planes of a YBa₂Cu₃O_7?x monocrystal is investigated. In such measurements of temporal behavior, the phonon system is driven by an ultrashort (subpicosecond) pulse into a coherent state and is probed by a second ultrashort pulse with a given temporal delay. A comparison of the Fourier transforms of the temporal responses with the spontaneous Raman scattering spectra shows that the contribution to a temporal response comes not only from fully symmetric phonons, but also from off-diagonal modes. The mechanism of generation of coherent phonons in high-temperature superconductors is discussed.     

Matrix isolation ESR spectroscopy and quantum chemical calculations on 5-methylhexa-1,2,4-triene-1,3-diyl, a highly delocalized triplet "hybrid" carbene

The ESR spectrum of 5-methylhexa-1,2,4-triene-1,3-diyl (1) was recorded in an argon matrix at 15 K. The derived zero-field splitting (ZFS) parameters (D = 0.5054 ± 0.0006 cm(-1) and E = 0.0045 ± 0.0002 cm(-1)) fall between those determined previously for propargylene (2) and vinylcarbene (3). DFT and ab initio (CAS and MRCI) quantum-chemical calculations of the ZFS parameters of 1, 2, and 3 were performed. These calculations indicate that multireference methods are needed to successfully predict ZFS parameters of delocalized carbenes/biradicals such as 1-3. The calculated singly occupied MOs and spin density distributions show that the spin is more delocalized in 1 than in 2 and 3, indicating that 1 is a "hybrid" of the constituent ethynyl- and vinylcarbenes, 2 and 3, respectively. The dominant contribution to the D-value in 1 and 2 is found to result from spin-spin interactions on the C atoms of the propylidene moiety, which is strongly affected by spin polarization. Accurate values for the D-parameter are also predicted for other types of delocalized triplet carbenes such as HC(5)H and HCCN.     

Effect of high pressure on raman spectra of T1-based superconducting crystals

Abstract

Raman spectra for two members of the high-Tc superconductor family T1_m Ba₂ Ca₂ cuo_6+m were measured at pressure up to 3 0 GPa. It is shown that pressure induces nonuniform deformation of the unit cell. Analysis of Raman shifts indicates the applicability of a Gruneisen model for these high-Tc superconductors. As evidenced by the absence of changes of the Raman spectra, no phase transitions occur in the pressure range studied.     

Michelson interferometer with multichannel interferogram recording

A schematic diagram of a Michelson interferometer with a diffraction grating for which the length of one arm is a function of the transverse coordinate of the light beam is proposed. The possibility of recording the interferograms without mechanical displacement of optical elements of the interferometer is demonstrated.     

On the nature of “coherent artifact”

The coherent interaction of femtosecond laser pulses in the pump-probe regime has been experimentally studied in the time domain by monitoring light reflection from a tellurium single crystal. The optical response of the probed medium exhibits periodic variations at a frequency equal to that of the exciting laser radiation. Experimental dependences of the observed “coherent artifact” on the pump/probe intensity ratio, the number of accumulated pulses, and the mutual orientation of the polarization vectors of electromagnetic fields and the crystallographic axes are well described by the proposed phenomenological model.     

Anisotropy of the Raman scattering measured in the xy plane of a nontwinned YBa2Cu3O7−x single crystal

The temperature dependence of anisotropy of the phonon and electron Raman scattering components was studied by measurements in the xy plane of a nontwinned YBa₂Cu₃O_7?x single crystal. It is shown that the sign of the orthorhombicity parameter γ=(I _yy ?I _xx )/(I _yy +I _xx ) for the full-symmetry phonons (150, 340, and 435 cm^?1 modes) generated by the displacements of ions in the CuO₂ plane is opposite to the sign of this parameter for the phonons generated by the out-of-plane barium and bridging oxygen displacements (120 and 500 cm^?1 modes). In the superconducting state, the γ value decreases in the region of low frequencies, but the frequency renormalization of the 340 cm^?1 mode measured in the xx and yy spectra is the same to within the experimental error.     

Superconducting gap observed in Raman spectra of Bi2Sr2CaCu2O8+x

Investigation of the temperature behavior of electronic Raman scattering in Bi₂Sr₂CaCu₂O_8+x superconducting crystals indicates the frequency of the mode whose spectral position is customarily used to derive the width of the superconducting gap to be only weakly temperature dependent. Measurements carried out under different resonance conditions can be interpreted as due to spatial dispersion of the superconducting gap. Fiz. Tverd. Tela (St. Petersburg) 40, 998–1001 (June 1998)