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...

Silicon, arguably the most important technological semiconductor, is predicted to exhibit a range of new and interesting properties when grown in the hexagonal crystal structure. To obtain pure hexagonal silicon is a great challenge because it naturally crystallizes in the cubic structure. Here, we demonstrate the fabrication of pure and stable hexagonal silicon evidenced by structural characterization. In our approach, we transfer the hexagonal crystal structure from a template hexagonal gallium phosphide nanowire to an epitaxially grown silicon shell, such that hexagonal silicon is formed. The typical ABABAB... stacking of the hexagonal structure is shown by aberration-corrected imaging in transmission electron microscopy. In addition, X-ray diffraction measurements show the high crystalline purity of the material. We show that this material is stable up to 9 GPa pressure. With this development, we open the way for exploring its optical, electrical, superconducting, and mechanical properties.

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V 2 O 3 is the prototype system for the Mott transition, one of the most fundamental phenomena of electronic correlation. Temperature, doping or pressure induce a metal-to-insulator transition (MIT) between a paramagnetic metal (PM) and a paramagnetic insulator. This or related MITs have a high technological potential, among others, for intelligent windows and field effect transistors. However the spatial scale on which such transitions develop is not known in spite of their importance for research and applications. Here we unveil for the first time the MIT in Cr-doped V 2 O 3 with submicron lateral resolution: with decreasing temperature, microscopic domains become metallic and coexist with an insulating background. This explains why the associated PM phase is actually a poor metal. The phase separation can be associated with a thermodynamic instability near the transition. This instability is reduced by pressure, that promotes a genuine Mott transition to an eventually homogeneous metallic state.

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 this paper we provide an accurate high-pressure structural and optical study of MAPbI3 hybrid perovskite. Structural data show the presence of a phase transition towards an orthorhombic structure around 0.3 GPa followed by full amorphization of the system above 3 GPa. After releasing pressure the systems keeps the high-pressure orthorhombic phase. The occurrence of these structural transitions is further confirmed by pressure induced variations of the photoluminescence signal at high pressure. These variations clearly indicate that the bandgap value and the electronic structure of MAPI change across the phase transition.3

We investigate the pressure dependence of the optical properties of CeTe3, which exhibits an incommensurate charge-density-wave (CDW) state already at 300 K. Our data are collected in the mid-infrared spectral range at room temperature and at pressures between 0 and 9 GPa. The energy for the single particle excitation across the CDW gap decreases upon increasing the applied pressure, similarly to the chemical pressure by rare-earth substitution. The broadening of the bands upon lattice compression removes the perfect nesting condition of the Fermi surface and therefore diminishes the impact of the CDW transition on the electronic properties of RTe3. The physical properties of low-dimensional systems have fascinated researchers for a great part of the last century, and have recently become one of the primary centers of interest in condensed matter research. Lowdimensional systems not only experience strong quantum and thermal fluctuations, but also admit ordering tendencies which are difficult to realize in three-dimensional materials. Prominent examples are spin-and chargedensity waves in quasi-one-dimensional compounds [1]. Moreover, the competition among several possible order parameters leads to rich phase diagrams, which can be tuned by external variables as temperature, magnetic field, and both chemical and applied pressure [1,2]. Tunable external parameters also affect the effective dimensionality of the interacting electron gas, which plays an essential role in defining the intrinsic electronic properties of the investigated systems.The rare-earth tri-tellurides RTe 3 (R= La-Tm, excepting Eu [3]) are the latest paramount examples of low dimensional systems exhibiting an incommensurate chargedensity-wave (CDW) state, stable across the available rare-earth series [4,5]. The lattice constant decreases on going from R = La to R = Tm [6,7], i.e. by chemically compressing the lattice, as consequence of the reduced ionic radius of the rare-earth atom. The CDW state in RTe 3 can be then investigated as a function of the inplane lattice constant a, which is directly related to the Te-Te distance in the Te-layers.Recently, we have reported on the first optical measurements of RTe 3 [8]. Our data, collected over an extremely broad spectral range, allowed us to observe both the Drude component and the single-particle peak, ascribed to the contributions due to the free charge carriers and to the excitation across the charge-density-wave gap, respectively. We established a diminishing impact of the charge-density-wave condensate on the electronic properties of RTe 3 with decreasing a across the rare-earth series [8]. On decreasing a, a reduction of the CDW gap together with an enhancement of the metallic (Drude) contribution were observed in the absorption spectrum. This is the consequence of a quenching of the nesting condition, driven by the modification of the Fermi surface (FS) because of the lattice compression [8].We present in this letter infrared optical investigations of the pressure dependence of the optical ...

Raman spectra of La(0.75)Ca(0.25)MnO(3) have been collected for the first time over a wide pressure range (0-14 GPa) using a diamond anvil cell. The frequency range explored (200-1100 cm(-1)) and the very good quality of the data allowed us to carefully analyze the pressure evolution of the phonon modes of the MnO(6) octahedra. The results show an abrupt transition at approximately 7.5 GPa with an evident deviation from the linear trend of the frequency of the Jahn-Teller phonon versus the applied pressure, accompanied by a strong phonon broadening. This behavior disagrees with the predicted insulator to metal transition and, on the contrary, indicates the occurrence of a new unpredicted phase in the very high pressure regime.