We report Raman scattering experiments on the strongly correlated electron system NaxCoO2 with x= 0.71 and ordered Na vacancies. In this doping regime, NaxCoO2 exhibits a large and unusual thermopower and becomes superconducting upon hydration. Our R
aman scattering data reveal pronounced low energy fluctuations that diverge in intensity at low temperatures. Related to these fluctuations is a drastic decrease of an electronic scattering rate from 50 to 3 cm-1. This observation is quite different from the behavior of Na disordered samples that have a larger and temperature independent scattering rate. Simultaneously with the evolution of the scattering rate, phonon anomalies point to an increasing out-of-plane coherence of the lattice with decreasing temperature. These observations may indicate the condensation of spin polarons into an unusual, highly dynamic ground state.
In the giant Rashba semiconductor BiTeI electronic surface scattering with Lorentzian linewidth is observed that shows a strong dependence on surface termination and surface potential shifts. A comparison with the topological insulator Bi2Se3 evidenc
es that surface confined quantum well states are the origin of these processes. We notice an enhanced quasiparticle dynamics of these states with scattering rates that are comparable to polaronic systems in the collision dominated regime. The Eg symmetry of the Lorentzian scattering contribution is different from the chiral (RL) symmetry of the corresponding signal in the topological insulator although both systems have spin-split surface states.
In the spin ladder compound BiCu$_2$PO$_6$ there exists a decisive dynamics of spin excitations that we classify and characterize using inelastic light scattering. We observe low-energy singlets and a broad triplon continuum extending from 36 cm$^{-1
}$ to 700 cm$^{-1}$ in ($aa$), ($bb$), and ($cc$) light scattering polarizations. Though isolated spin ladder physics can roughly account for the observed excitations at high energies, frustration and interladder interactions need to be considered to fully describe the spectral distribution and scattering selection rules at low and intermediate energies. More significantly, an interladder singlet bound mode at 24 cm$^{-1}$, lying below the continuum, shows its largest scattering intensity in interladder ($ab$) polarization. In contrast, two intraladder bound states at 62 cm$^{-1}$ and 108 cm$^{-1}$ with energies comparable to the continuum are observed with light polarization along the leg ($bb$) and the rung ($cc$). We attribute the rich spectrum of singlet bound modes to a melting of a dimer crystal. Our study provides evidence for a Z$_2$ quantum phase transition from a dimer to a resonating valence bond state driven by singlet fluctuations.
The low-dimensional s=1/2 compound (NO)[Cu(NO3)3] has recently been suggested to follow the Nersesyan-Tsvelik model of coupled spin chains. Such a system shows unbound spinon excitations and a resonating valence bond ground state due spin frustration
. Our Raman scattering study demonstrates phonon anomalies as well as the suppression of a broad magnetic scattering continuum for temperatures below a characteristic temperature, T<T*=100K. We interpret these effects as evidence for a dynamical interplay of spin and lattice degrees of freedom that might lead to a further transition into a dimerized or structurally distorted phase at lower temperatures.
We present evidence for a concomitant structural and ferroelectric transformation around $T_Ssim 360$ K in multiferroic BiFeO$_3$/LaAlO$_3$ thin films close to the tetragonal phase. Phonon excitations are investigated by using Raman scattering as a f
unction of temperature. The low-energy phonon modes at 180-260 cm$^{-1}$ related to the FeO$_6$ octahedron tilting show anomalous behaviors upon cooling through $T_S$; (i) a large hardening amounting to 15 cm$^{-1}$, (ii) an increase of intensity by one order of magnitude, and (iii) an appearance of a dozen new modes. In contrast, the high-frequency modes exhibit only weak anomalies. This suggests an intimate coupling of octahedron tilting to ferroelectricity leading to a simultaneous change of structural and ferroelectric properties.
We report on Raman scattering experiments of the undoped SrFe2As2 and superconducting Sr0.85K0.15Fe2As2 (Tc=28K) and Ba0.72K0.28Fe2As2 (Tc=32K) single crystals. The frequency and linewidth of the B1g mode at 210 cm-1 exhibits an appreciable temperatu
re dependence induced by the superconducting and spin density wave transitions. We give estimates of the electron-phonon coupling related to this renormalization. In addition, we observe a pronounced quasi-elastic Raman response for the undoped compound, suggesting persisting magnetic fluctuations to low temperatures. In the superconducting state the renormalization of an electronic continuum is observed with a threshold energy of 61cm-1.
Raman scattering in the spin-crossover system [Fe(pmd)(H2O){Au(CN)2}2]*H2O reveals a complex three-phase spin-state transition in contrast to earlier observations in magnetization measurements. We observe different spin state phases as function of te
mperature and electromagnetic radiation in the visible spectral range. There exists a fluctuating spin state phase with an unexpected wipeout of the low frequency phonon scattering intensity. Furthermore we observe one phase with reduced symmetry that is attributed to a cooperative Jahn-Teller effect. Pronounced electron-phonon interaction manifests itself as a strong Fano-resonance of phonons related to {FeN6} and {FeN4O2} coordination octahedra. Density functional theory supports this interpretation.
We report inelastic light scattering experiments on CaFe_2As_2 in the temperature range of 4 to 290 K. In in-plane polarizations two Raman-active phonon modes are observed at 189 and 211 cm-1, displaying A_1g and B_1g symmetries, respectively. Upon h
eating through the tetragonal-to-orthorhombic transition at about T_S=173 K, the B_1g phonon undergoes a discontinuous drop of the frequency by 4 cm-1 whereas the A_1g phonon shows a suppression of the integrated intensity. Their linewidth increases strongly with increasing temperature and saturates above T_S. This suggests (i) a first-order structural phase transition and (ii) a drastic change of charge distribution within the FeAs plane through T_S.
We report magnetic susceptibility, specific heat, and Raman scattering investigations of alpha-TeVO4 containing V-O edge-sharing chains. These chains promote a system of ferromagnetic/antiferromagnetic spin-1/2 Heisenberg alternating exchange chains
with pronounced spin frustration. The magnetic susceptibility and Raman scattering evidence a crossover at T* = 85 K with different slopes of the reciprocal susceptibility and a magnetic phase transition into a long-range-ordered state at Tc = 16 K. From Raman scattering data a strong mutual coupling between lattice and magnetic degrees of freedom is deduced. A comparison to model calculations and prior Raman scattering on other chain systems yield a plausible interpretation of the microscopic mechanism for the crossover behavior.
