We report inelastic x-ray scattering measurements of the temperature dependence of phonon dispersion in the prototypical charge-density-wave (CDW) compound 2H-NbSe2. Surprisingly, acoustic phonons soften to zero frequency and become overdamped over an extended region around the CDW wave vector. This extended phonon collapse is dramatically different from the sharp cusp in the phonon dispersion expected from Fermi surface nesting. Instead, our experiments, combined with ab initio calculations, show that it is the wave vector dependence of the electron-phonon coupling that drives the CDW formation in 2H-NbSe2 and determines its periodicity. This mechanism explains the so far enigmatic behavior of CDW in 2H-NbSe2 and may provide a new approach to other strongly correlated systems where electron-phonon coupling is important.

We report high-resolution inelastic x-ray measurements of the soft phonon mode in the chargedensity-wave compound TiSe2. We observe a complete softening of a transverse optic phonon at the L point, i.e. q = (0.5, 0, 0.5), at T ≈ TCDW . Renormalized phonon energies are observed over a large wavevector range (0.3, 0, 0.5) ≤ q ≤ (0.5, 0, 0.5). Detailed ab-initio calculations for the electronic and lattice dynamical properties of TiSe2 are in quantitative agreement with experimental frequencies for the phonon branch involving the soft mode. The observed broad range of renormalized phonon frequencies is directly related to a broad peak in the electronic susceptibility stabilizing the chargedensity-wave ordered state. Our analysis demonstrates that a conventional electron-phonon coupling mechanism can explain a structural instability and the charge-density-wave order in TiSe2 although other mechanisms might further boost the transition temperature.PACS numbers: 71.45. Lr, 63.20.kd, 63.20.dd, 63.20.dk The origin of charge-density-wave (CDW ) order, i.e., a periodic modulation of the electronic density, is a long-standing problem relevant to a number of important issues in condensed matter physics, such as the role of stripes in cuprates [1] and charge fluctuations in the colossal magnetoresistive manganites [2]. Chan and Heine derived the criterion for a stable CDW phase with a modulation wavevector q as [3]where η q is the electron-phonon coupling (EPC) matrix element associated with a mode at an unrenormalized energy of ω bare , χ q is the dielectric response of the conduction electrons, and U q and V q are their Coulomb and exchange interactions. Static CDW order typically is taken as a result of a divergent electronic susceptibility χ q due to nesting, i.e. parallel sheets of the Fermi surface (FS) separated by twice the Fermi wavevector 2k f . Electron-phonon coupling (EPC) is required to stabilize the structural distortion and, hence, an acoustic phonon mode at the CDW wavevector q CDW = 2k f softens to zero energy at the transition temperature T CDW [3,4]. However, when electronic probes reported only small and not well nested Fermi surfaces this scenario has been discarded for the prototypical CDW compound TiSe 2 [5][6][7].Alternative scenarios such as indirect or band-type Jahn-Teller effects [6][7][8] and, most prominently, exciton formation [5,9,10] are discussed in the theoretical as well as experimental literature. More recently, van Wezel et al. have invoked a model including exciton formation as well as EPC [11] and have shown that it can explain data from angle-resolved photoemission spectroscopy (ARPES) [12], formerly taken as evidence of an excitonic insulating phase in TiSe 2 [9]. Determining the origin of CDW formation in TiSe 2 is all the more important with respect to the nature of superconductivity, which emerges both as function of pressure [13] and Cu intercalation [14]. In particular, pressure induced superconductivity is expected to be closely linked to the nature of the parent CDW state.I...

We present an investigation of the lattice dynamics of the charge-density-wave compound 2H-NbSe 2 . We analyze the precise nature of the wave vector dependent electron-phonon coupling (EPC) and derive the bare dispersion of the charge-density-wave (CDW) soft phonon mode using inelastic x-ray scattering combined with ab-initio calculations. Experimentally, phonon modes along the Γ -M line, i.e. q = (h, 0, 0) with 0 ≤ h ≤ 0.5, with the same longitudinal symmetry (Σ 1 ) as the CDW soft mode were investigated up to 32 meV. In agreement with our calculations we observe significant EPC in the optic modes at h ≤ 0.2. We analyze the EPC in the optic as well as acoustic modes and show that the q dependences stem from scattering processes between two bands at the Fermi surface both having Nb 4d character. Finally, we demonstrate that the soft mode dispersion at T = 33 K (= T CDW ) can be well described on the basis of a strongly q dependent EPC matrix element and an acoustic-like bare phonon dispersion in agreement with observations near room temperature.

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The increasing production of silver nanoparticles (AgNPs) and titanium dioxide nanoparticles (TiONPs) has resulted in their elevated concentrations in the environment. This study was, therefore, aimed at determining the distribution, redox parameters, and genotoxic effects in male Wistar rats that were treated with either AgNP or TiONP individually, as well as under a co-exposure scenario. Animals were exposed via oral gavage to either sodium citrate buffer (vehicle), 0.5 mg/kg/day TiONP, 0.5 mg/kg/day AgNP or a mixture of TiONPs and AgNPs. Exposure lasted 45 days after which rats were sacrificed, and tissue biodistribution of Ag and Ti measured. The blood concentration of glutathione (GSH) and activities of glutathione peroxidase (GPx) and catalase (CAT) were determined while the genotoxicity was analyzed using the comet assay in peripheral blood and liver cells. The tissue concentrations of Ag followed the order; blood > liver > kidneys while for Ti the order was kidneys > liver > blood. There was no significant change in the measured redox parameters in animals that were exposed to TiONPs. However, there was a significant increase in GSH levels accompanied by a reduction in the GPx activity in AgNP-treated and co-exposed groups. The individual or co-exposure to TiONP and AgNP did not markedly induce genotoxicity in blood or liver cells. Data showed that TiONP did not produce significant oxidative stress or genotoxicity in rats at the dose used in this study while the same dose level of AgNPs resulted in oxidative stress, but no noticeable adverse genotoxic effects.

We investigate the three-dimensional electronic structure of the seminal charge-density-wave (CDW) material 2H-NbSe2 by soft x-ray angle-resolved photoelectron spectroscopy and density-functional theory. Our results reveal the pronounced 3D character of the electronic structure formed in the quasitwo-dimensional layered crystal structure. In particular, we find a strong dispersion along excluding a nesting-driven CDW formation based on experimental data. The 3D-like band structure of 2H-NbSe2 has strong implications for the intriguing phase competition of CDW order with superconductivity.

The competition between superconductivity and other ground states of solids is one of the challenging topics in condensed matter physics. Apart from high-temperature superconductors

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