Raman and combined trasmission and reflectivity mid infrared measurements have been carried out on monoclinic VO2 at room temperature over the 0-19 GPa and 0-14 GPa pressure ranges, respectively. The pressure dependence obtained for both lattice dynamics and optical gap shows a remarkable stability of the system up to P*∼10 GPa. Evidence of subtle modifications of V ion arrangements within the monoclinic lattice together with the onset of a metallization process via band gap filling are observed for P>P*. Differently from ambient pressure, where the VO2 metal phase is found only in conjunction with the rutile structure above 340 K, a new room temperature metallic phase coupled to a monoclinic structure appears accessible in the high pressure regime, thus opening to new important queries on the physics of VO2. PACS numbers:Since the first observation of the metal to insulator transition (MIT) in several vanadium oxides, these materials attracted considerable interest because of the huge and abrupt change of the electrical properties at the MIT. As usual in transition metal oxides, electronic correlation strongly affects the conduction regime of vanadium oxides, although, in some compounds, lattice degrees of freedom seem to play an important role. This is the case of VO 2 , which undergoes a first order transition from a high temperature metallic rutile (R) phase to a low temperature insulating monoclinic (M1) one. At the MIT temperature, T c =340 K, the opening of an optical gap in the mid-infrared (MIR) conductivity and a jump of several order of magnitude in the resistivity are observed [1]. The interest on this compound is thus mainly focused on understanding the role and the relative importance of the electron-electron and the electron-lattice interaction in driving the MIT. Despite the great experimental and theoretical efforts [2], the understanding of this transition is still far from being complete [3,4,5,6,7]. In the R phase the V atoms, each surrounded by an oxygen octahedron, are equally spaced along linear chains in the c-axis direction and form a body-centered tetragonal lattice. On entering the M1 insulating phase the dimerization of the vanadium atoms and the tilting of the pairs with respect to the c axis lead to a doubling of the unit cell, with space group changing from C 5 2h (R) to D 14 4h (M1) [8,9]. As first proposed by Goodenough [10], the V-V pairing and the off-axis zig-zag displacement of the dimers lead to a band splitting with the formation of a Peierls-like gap at the Fermi level. First principle electronic structure calculations based on local density approximation (LDA) showed the band splitting on entering the monoclinic phase, but failed to yield the opening of the band gap [11,12]. In fact, as early pointed out [13], the electron-electron correlation has to be taken into account to obtain the insulating phase. A recent theoretical paper where the electronic Coulomb repulsion U is properly accounted for, shows that calculations carried out joining dynamical mean field theory with...

The experimental determination of the quantum critical point (QCP) that triggers the self-organization of charged striped domains in cuprate perovskites is reported. The phase diagram of doped cuprate superconductors is determined by a first variable, the hole doping δ, and a second variable, the micro-strain ε of the Cu-O bond length, obtained from the Cu K-edge extended x-ray absorption fine structure. For a fixed optimum doping, δ c = 0.16, we show the presence of the QCP for the onset of local lattice distortions and stripe formation at the critical micro-strain ε c. The critical temperature T c (ε, δ) reaches its maximum at the quantum critical point (ε c , δ c) for the formation of bubbles of superconducting stripes. The critical charge, orbital and spin fluctuations near this strain QCP provide the interaction for the pairing.

The temperature dependence of the London penetration depth lambda was measured for an untwinned single crystal of YBa_{2}Cu_{3}O_{7-delta} along the three principal crystallographic directions (a, b, and c). Both in-plane components (lambda_{a};{-2} and lambda_{b};{-2}) show an inflection point in their temperature dependence which is absent in the component along the c direction (lambda_{c};{-2}). The data provide convincing evidence that the in-plane superconducting order parameter is a mixture of (s+d)-wave symmetry whereas it is mainly s wave along the c direction. In conjunction with previous results it is concluded that coupled s+d-order parameters are universal and intrinsic to cuprate superconductors.

Raman and infrared transmission and reflectivity measurements were carried out at room temperature and high pressure ͑0-15 GPa͒ on V 1−x Cr x O 2 compounds. Raman spectra were collected at ambient conditions on the x = 0.007 and 0.025 materials, which are characterized by different insulating monoclinic phases ͑M3 and M2, respectively͒, while infrared spectra were collected on the x = 0.025 sample only. The present data were compared with companion results on undoped VO 2 ͓E. Arcangeletti et al., Phys. Rev. Lett. 98, 196406 ͑2007͔͒, which is found at ambient conditions in a different, third insulating monoclinic phase, named M1. This comparison allowed us to investigate the effects of different extents of structural distortions ͑Peierls distortion͒ on the lattice dynamics and the electronic properties of this family of compounds. The pressure dependence of the Raman spectrum of VO 2 and Cr-doped samples shows that all the systems retain the monoclinic structure up to the highest explored pressure, regardless the specific monoclinic structure ͑M1, M2, and M3͒ at ambient condition. Moreover, the Raman spectra of the two Cr-doped samples, which exhibit an anomalous behavior over the low-pressure range ͑P Ͻ 8 GPa͒, merge into that of VO 2 in the high-pressure regime and are all found into a common monoclinic phase ͑a possible fourth kind phase͒. Combining Raman and infrared results on both the VO 2 and the present data, we found that a common metallic monoclinic phase appears accessible in the high-pressure regime at room temperature for both undoped and Cr-doped samples independently of the different extents of Peierls distortion at ambient conditions. This finding differs from the behavior observed at ambient pressure, where the metallic phase is found only in conjunction with the rutile structure. The whole of these results suggests a major role of the electron correlations, rather than of the Peierls distortion, in driving the metal-insulator transition in vanadium dioxide systems, thus opening to new experimental and theoretical queries.

In high T c superconductors the magnetic and electronic properties are determined by the probability that valence electrons virtually jump from site to site in the CuO 2 planes, a mechanism opposed by on-site Coulomb repulsion and favored by hopping integrals. The spatial extent of the latter is related to transport properties, including superconductivity, and to the dispersion relation of spin excitations (magnons). Here, for three antiferromagnetic parent compounds (single-layer Bi 2 Sr 0.99 La 1.1 CuO 6+ , double-layer Nd 1.2 Ba 1.8 Cu 3 O 6 and infinite-layer CaCuO 2 ) differing by the number of apical atoms, we compare the magnetic spectra measured by resonant inelastic x-ray scattering over a significant portion of the reciprocal space and with unprecedented accuracy. We observe that the absence of apical oxygens increases the in-plane hopping range and, in CaCuO 2 , it leads to a genuine 3D exchange-bond network. These results establish a corresponding relation between the exchange interactions and the crystal structure, and provide fresh insight into the materials dependence of the superconducting transition temperature.