Lev Gasparov

Infrared and Raman studies of the Verwey transition in magnetite

We present infrared and Raman measurements of magnetite (Fe3O4). This material is known to undergo a metal-insulator and a structural transition (Verwey transition) at T-V = 120 K. The structural aspect of the Verwey transition is disclosed by the appearance of additional infrared-active and Raman-active phonons. The frequencies of the infrared-active phonons show no significant singularities at the transition whereas their linewidths increase. The frequency and linewidth of the Raman-active phonon at 670 cm(-1) change abruptly at the transition. For T<T-V, we observe fine structure in the infrared and Raman spectra which may indicate strong anharmonicity of the system below the transition. The effective mass of the mobile carriers is estimated to be m*approximate to 100m, where m is the electronic mass.

Polaronic Conductivity in the Photoinduced Phase of 1T−TaS2

The transient optical conductivity of photoexcited 1T-TaS2 is determined over a three-order-of-magnitude frequency range. Prompt collapse and recovery of the Mott gap is observed. However, we find important differences between this transient metallic state and that seen across the thermally driven insulator-metal transition. Suppressed low-frequency conductivity, Fano phonon line shapes, and a midinfrared absorption band point to polaronic transport. This is explained by noting that the photoinduced metallic state of 1T-TaS2 is one in which the Mott gap is melted but the lattice retains its low-temperature symmetry, a regime only accessible by photodoping.

Phonon anomaly at the charge ordering transition in 1T-TaS2

The infrared reflectance of the transition-metal chalcogenide 1T-TaS2 has been measured at temperatures from 30 K to 360 K over 30-45 000 cm(-1) (4 meV-5.5 eV). The optical conductivity was obtained by Kramers-Kronig analysis. At 360 K only modest traces of the phonon lines are noticeable. The phonon modes are followed by a pseudo-gap-like increase of the optical conductivity, with direct optical transitions observed at frequencies above 1 eV. As the temperature decreases, the low-frequency conductivity also decreases, phonon modes become more pronounced, and a pseudogap develops into a gap at 800 cm(-1) (100 meV). We observe an anomalous frequency dependence of the 208 cm(-1) infrared-active phonon mode. This mode demonstrates softening as the temperature decreases below the 180-K transition. The same mode demonstrates strong hysteresis of the frequency and linewidth changes, similar in its temperature behavior to the hysteresis in the dc resistivity. We discuss a possible relation of the observed softening of the mode to the structural changes and changes in electronic properties associated with the 180-K transition.

Magnetite: Raman study of the high-pressure and low-temperature effects

We report the results of a low-temperature (300 K-15 K) high-pressure (up to 22 GPa) Raman study of the Verwey transition in magnetite (Fe3O4). We use additional Raman modes observed below the Verwey transition to determine how the transition temperature changes with the quasihydrostatic pressure. Increase of the pressure results in the linear decrease of the Verwey transition temperature, with no discontinuity. The corresponding pressure coefficient dT(v)/dP is found to be -5.16 +/- 1.19 K/GPa. Such a decrease is substantially larger than the one predicted by the mean-field Coulomb interaction model of the transition. (c) 2005 American Institute of Physics.

ELECTRONIC RAMAN SCATTERING IN TL2BA2CUO6+DELTA : SYMMETRY OF THE ORDER PARAMETER, OXYGEN DOPING EFFECTS, AND NORMAL-STATE SCATTERING

Single crystals of the optimally doped, moderately and strongly overdoped high temperature superconductor Tl2Ba2CuO6+x (Tl-2201) with Tc=80, 56 and 30K, respectively, have been investigated by polarized Raman scattering. By taking the peak position of the B_1g component of electronic Raman scattering as 2Delta_0 we found that the reduced gap value (2Delta_0/k_BT_c) strongly decreases with increasing doping. The behavior of the low frequency scattering for the B_1g and B_2g scattering components is similar for optimally doped and overdoped crystals and can be described by a w^3 - and w -law, respectively, which is consistent with a d-wave symmetry of the order parameter. In contrast to the optimally doped Tl-2201 in both, moderately and strongly overdoped Tl-2201, the relative (compared to the B_1g) intensity of the A_1g scattering component is suppressed. We suggest that the van Hove singularity is responsible for the observed changes of Raman intensity and reduced gap value with doping. Electronic Raman scattering in the normal state is discussed in the context of the scattering from impurities and compared to the existing infrared data. The scattering rate evaluated from the Raman measurements is smaller for the overdoped samples, compared to the moderately overdoped samples.

Raman study of Tl-based superconducting single crystals: Phonons assignment and temperature dependence

Abstract We report Raman spectra for three members of the high- T c superconductor family Tl m Ba 2 Ca n −1 Cu n O 2( n +1)+ m . A factor group analysis of the zone center phonons is also given. All phonons observed have A 1g symmetry and their frequencies increase as temperature decreases. The absence of B 1g phonons in Raman spectra of Tl 2 Ba 2 CaCu 2 O 7− x and TlBa 2 CaCuO 6− x compounds is discussed. Asymmetric lineshape of the number of phonons was revealed.

Preparation and magnetic properties of (Bi,Pb)-1212

Abstract Polycrystalline samples of (Bi,Pb)Sr 2 (Y,Ca)Cu 2 O 7− z [(Bi,Pb)-1212] were prepared to study the influence of one missing BiO-layer in the BSCCO system [Bi-212, (Bi,Pb)-2223] on its physical properties. In a structural analysis, the high percentage of the (Bi,Pb)-1212 phase (≈80%) was proven by XRD and EDX, and the structure of the 1212 phase as well as the incorporation of the 2212 phase in the 1212 matrix were evidenced by a HRTEM analysis. The best samples exhibit T c R =0 = 92 K and T c,onset = 110 K. The pinning properties were investigated by means of SQUID-magnetometry. The critical current density J c shows a strong decrease with temperature, similar to that of Bi-2212 and Bi-2223. Furthermore, regimes of pinning of individual flux lines and pinning of small and large flux bundles were identified by measurements of the magnetic relaxation rate. The transition temperatures separating these regimes lie in the same range as for Bi-2212, indicating a comparable anisotropy of the two compounds.

Investigation of the Spin-Peierls transition in CuGeO3 by Raman scattering

Abstract Raman experiments on the Spin-Peierls compound CuGeO3 and the substituted (Cu2−x,Znx)GeO3 and Cu(Ge1−x,GaxO3 compounds were performed in order to investigate the response of specific magnetic excitations of the one-dimensional spin-chain to spin anisotropies and substitution-induced disorder. In pure CuGeO3, in addition to normal phonon scattering which is not affected at all by the spin-peierls transition, four types of magnetic scattering features were observed. Below TSP = 14 K a singlet-triplet excitation at 30 cm−1, two-magnon scattering from 30 to 227 cm−1 and folded phonon modes at 369 and 819 cm−1 were identified. They were assigned by their temperature dependence and lineshape. For temperatures between the spin-peierls transition TSP and approsimately 100 K a broad intensity maximum centered at 300 cm−1 is observed. The temperature dependence of this intensity is similar to the behavior of lattice fluctuations recently observed in an electron diffraction study. This scattering is attributed to dynamical spin-peierls fluctuations of the weakly or non-static dimerized spin chain.

Manifestation of the orthorhombic symmetry in Raman spectra of untwinned single crystals of YBa2Cu3O7−x

Abstract The Raman scattering spectra of the untwinned region of orthorhombic single crystals have been measured in all polarizations. The observed vibrations are classified.

COEXISTENCE OF FERROMAGNETISM AND HIGH-TEMPERATURE SUPERCONDUCTIVITY IN Dy-DOPED BiPbSrCaCuO

Ferromagnetism was found to coexist with superconductivity in Dy-doped BiPbSrCaCuO-2212 single crystals up to the superconducting critical temperature Tc ~ 80 K. Several experimental tests indicated that the phenomenon is intrinsic to the entire specimen rather than due to separate phases or to isolated impurities.