Publications (23)29.91 Total impact
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Article: Optical Conductivity Measurements of GaTa_{4}Se_{8} Under High Pressure: Evidence of a Bandwidth-Controlled Insulator-to-Metal Mott Transition.
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ABSTRACT: The optical properties of a GaTa_{4}Se_{8} single crystal are investigated under high pressure. At ambient pressure, the optical conductivity exhibits a charge gap of ≈0.12 eV and a broad midinfrared band at ≈0.55 eV. As pressure is increased, the low energy spectral weight is strongly enhanced and the optical gap is rapidly filled, pointing to an insulator to metal transition around 6 GPa. The overall evolution of the optical conductivity demonstrates that GaTa_{4}Se_{8} is a Mott insulator which undergoes a bandwidth-controlled Mott metal-insulator transition under pressure, in remarkably good agreement with theory. With the use of our optical data and ab initio band structure calculations, our results were successfully compared to the (U/D, T/D) phase diagram predicted by dynamical mean field theory for strongly correlated systems.Physical Review Letters 01/2013; 110(3):037401. · 7.37 Impact Factor -
Article: High-pressure phases of crystalline tellurium: A combined Raman and ab initio study
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ABSTRACT: A room temperature study of the high-pressure phases of crystalline Te was carried out by combining Raman spectroscopy and density functional theory (DFT)-based calculations. The pressure dependence of the experimental phonon spectrum reveals the occurrence of phase transitions, confirming the high-pressure scenario recently proposed by Hejny and McMahon. The effects of the incommensurate lattice modulation on the vibrational properties of Te are discussed. DFT-based calculations are consistent with the present and previous experimental data and show that the metallization process at 4 GPa is due to charge-bridges between atoms belonging to adjacent chains. A first-principles study of the stability of the 4 GPa phase is here reported and discussed against the insurgence of lattice modulation.Phys. Rev. B. 08/2012; 86(6). -
Article: Vibrational spectrum of solid picene (C_22H_14)
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ABSTRACT: Recently, Mitsuhashi et al., have observed superconductivity with transition temperature up to 18 K in potassium doped picene (C22H14), a polycyclic aromatic hydrocarbon compound [Nature 464 (2010) 76]. Theoretical analysis indicate the importance of electron-phonon coupling in the superconducting mechanisms of these systems, with different emphasis on inter- and intra-molecular vibrations, depending on the approximations used. Here we present a combined experimental and ab-initio study of the Raman and infrared spectrum of undoped solid picene, which allows us to unanbiguously assign the vibrational modes. This combined study enables the identification of the modes which couple strongly to electrons and hence can play an important role in the superconducting properties of the doped samples.05/2012; -
Article: Vibrational spectrum of solid picene (C22H14).
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ABSTRACT: Recently, Mitsuhashi et al observed superconductivity with a transition temperature up to 18 K in potassium doped picene (C(22)H(14)), a polycyclic aromatic hydrocarbon compound (Mitsuhashi et al 2010 Nature 464 76). Theoretical analysis indicates the importance of electron-phonon coupling in the superconducting mechanisms of these systems, with different emphasis on inter- and intra-molecular vibrations, depending on the approximations used. Here we present a combined experimental and ab initio study of the Raman and infrared spectrum of undoped solid picene, which allows us to unambiguously assign the vibrational modes. This combined study enables the identification of the modes which couple strongly to electrons and hence can play an important role in the superconducting properties of the doped samples.Journal of Physics Condensed Matter 05/2012; 24(25):252203. · 2.55 Impact Factor -
Article: Universal conductivity and the electrodynamics of graphite at high pressures
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ABSTRACT: We address the in-plane pressure-dependent electrodynamics of graphite through synchrotron based infrared spectroscopy and ab initio Density Functional Theory calculations. The Drude term remarkably increases upon pressure application, as a consequence of an enhancement of both electron and hole charge densities. This is due to the growth of the band dispersion along the k_z direction between the K and H points of the Brillouin zone. On the other hand, the mid-infrared optical conductivity between 800 and 5000 cm-1 is almost flat, and very weakly pressure dependent, at least up to 7 GPa. This demonstrates a surprising robustness of the graphene-like universal quantum conductance of graphite, even when the interlayer distance is significantly reduced.11/2011; -
Article: Metal–insulator transition in NiS2−x Se x : chemical vs external pressure effects
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ABSTRACT: The Se alloying (x)- and the pressure (P)-induced metal–insulator transitions on the strongly correlated NiS2−x Se x system have been investigated through Raman and infrared (IR) spectroscopies. Raman and IR responses of NiS2 to lattice compression are correlated to a metallization transition, occurring at 4 GPa. This result suggests a strong interaction between lattice and electronic degrees of freedom. In particular, IR measurements carried out by applying P on NiS2 (i.e. lattice contraction) and on Se alloying (i.e. lattice expansion) reveal that in both cases a metallic state is obtained. Our optical spectroscopy results deviate from the idea of a simple scaling factor between P and x previously claimed by transport measurements, but, on the contrary, point out the substantially different microscopic origin of the two transitions.High Pressure Research 03/2011; 31(1):18-22. · 0.78 Impact Factor -
Article: Combined Raman and infrared investigation of the insulator-to-metal transition in NiS2−xSex compounds
Physical Review B 01/2011; 84:235134. · 3.69 Impact Factor -
Article: Multiband conductivity and a multigap superconducting phase in V_ {3} Si films from optical measurements at terahertz frequencies
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ABSTRACT: The possibility of multiband conductivity and multigap superconductivity is explored in oriented V3Si thin films by means of reflectance and transmittance measurements at terahertz frequencies. The temperature dependence of the transmittance spectra in the normal state gives evidence of two bands contributing to the film conductivity. This outcome is consistent with electronic structure calculations performed within density functional theory. On this basis, we performed a detailed data analysis and found that all optical data can be consistently accounted for within a two-band framework, with the presence of two optical gaps in the superconducting state corresponding to 2Δ/kTc values close to 1.8 and 3.8.Physical Review B 03/2010; 81(9):092509. · 3.69 Impact Factor -
Article: 1. “Evidence for a monoclinic metallic phase in high-pressure VO2”,
High Pressure Research 03/2010; · 0.78 Impact Factor -
Article: Multi-gap superconductivity in a BaFe1.84Co0.16As2 film from optical measurements at terahertz frequencies
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ABSTRACT: We measured the THz reflectance properties of a high quality epitaxial thin film of the Fe-based superconductor BaFe$_{1.84}$Co$_{0.16}$As$_2$ with T$_c$=22.5 K. The film was grown by pulsed laser deposition on a DyScO$_3$ substrate with an epitaxial SrTiO$_3$ intermediate layer. The measured $R_S/R_N$ spectrum, i.e. the reflectivity ratio between the superconducting and normal state reflectance, provides clear evidence of a superconducting gap $\Delta_A$ close to 15 cm$^{-1}$. A detailed data analysis shows that a two-band, two-gap model is absolutely necessary to obtain a good description of the measured $R_S/R_N$ spectrum. The low-energy $\Delta_A$ gap results to be well determined ($\Delta_A$=15.5$\pm$0.5 cm$^{-1}$), while the value of the high-energy gap $\Delta_B$ is more uncertain ($\Delta_B$=55$\pm$7 cm$^{-1}$). Our results provide evidence of a nodeless isotropic double-gap scenario, with the presence of two optical gaps corresponding to 2$\Delta/kT_c$ values close to 2 and 7. Comment: Published Version03/2010; -
Article: Pressure and alloying effects on the metal to insulator transition in NiS_ {2− x} Se_ {x} studied by infrared spectroscopy
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ABSTRACT: The metal to insulator transition in the charge-transfer NiS2−xSex compound has been investigated through infrared reflectivity. Measurements performed by applying pressure to pure NiS2 (lattice contraction) and by Se alloying (lattice expansion) reveal that in both cases an anomalous metallic state is obtained. We find that optical results are not compatible with the linear Se-alloying vs pressure-scaling relation previously established through transport, thus pointing out the substantially different microscopic origin of the two transitions.Phys. Rev. B. 08/2009; 80(7). -
Article: Multi-band conductivity and multi-gap superconductivity in V3Si fromoptical measurements on films at terahertz frequencies
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ABSTRACT: The possibility of multi-band conductivity and multi-gap superconductivity is explored in oriented V3Si thin films by means of reflectance and transmittance measurements at terahertz frequencies. The temperature dependence of the transmittance spectra in the normal state gives evidence of two bands contributing to the film conductivity. This outcome is consistent with electronic structure calculations performed within density functional theory. On this basis, we performed a detailed data analysis and found that all optical data can be consistently accounted for within a two-band framework, with the presence of two optical gaps in the superconducting state corresponding to 2D=kTc values close to 1.8 and 3.8.Physical Review B 07/2009; 81:092509. · 3.69 Impact Factor -
Article: Pressure dependence of the single particle excitation in the charge-density-wave CeTe$_3$ system
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ABSTRACT: We present new data on the pressure dependence at 300 K of the optical reflectivity of CeTe$_3$, which undergoes a charge-density-wave (CDW) phase transition well above room temperature. The collected data cover an unprecedented broad spectral range from the infrared up to the ultraviolet, which allows a robust determination of the gap as well as of the fraction of the Fermi surface affected by the formation of the CDW condensate. Upon compressing the lattice there is a progressive closing of the gap inducing a transfer of spectral weight from the gap feature into the Drude component. At frequencies above the CDW gap we also identify a power-law behavior, consistent with findings along the $R$Te$_3$ series (i.e., chemical pressure) and suggestive of a Tomonaga-Luttinger liquid scenario at high energy scales. This newest set of data is placed in the context of our previous investigations of this class of materials and allows us to revisit important concepts for the physics of CDW state in layered-like two-dimensional systems.11/2008; -
Article: The optical phonon spectrum of SmFeAsO
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ABSTRACT: We measured the Raman and the Infrared phonon spectrum of SmFeAsO polycrystalline samples. We also performed Density Functional Theory calculations within the pseudopotential approximation to obtain the structural and dynamical lattice properties of both the SmFeAsO and the prototype LaFeAsO compounds. The measured Raman and Infrared phonon frequencies are well predicted by the optical phonon frequencies computed at the Gamma point, showing the capability of the employed ab-initio methods to describe the dynamical properties of these materials. A comparison among the phonon frequencies of different oxypnictides suggests a possible role of the high frequency phonons in the pairing mechanism leading to superconductivity in these materials. Comment: text file (.tex) and 2 figures (.eps)10/2008; -
Article: Optical properties of V_ {1− x} Cr_ {x} O_ {2} compounds under high pressure
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ABSTRACT: Raman and infrared transmission and reflectivity measurements were carried out at room temperature and high pressure (0–15 GPa) on V1−xCrxO2 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, respectively), while infrared spectra were collected on the x=0.025 sample only. The present data were compared with companion results on undoped VO2 [ E. Arcangeletti et al. Phys. Rev. Lett. 98 196406 (2007)], 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 distortion) on the lattice dynamics and the electronic properties of this family of compounds. The pressure dependence of the Raman spectrum of VO2 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 M3) at ambient condition. Moreover, the Raman spectra of the two Cr-doped samples, which exhibit an anomalous behavior over the low-pressure range (P<8 GPa), merge into that of VO2 in the high-pressure regime and are all found into a common monoclinic phase (a possible fourth kind phase). Combining Raman and infrared results on both the VO2 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.Phys. Rev. B. 06/2008; 77(23). -
Article: Evidence of a pressure-induced metallization process in monoclinic VO2.
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ABSTRACT: Raman and combined infrared transmission and reflectivity measurements were carried out at room temperature (RT) on monoclinic VO2 over the 0-19 GPa and 0-14 GPa pressure ranges. Both lattice dynamics and optical gap show a remarkable stability up to P* approximately 10 GPa whereas 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 P=0, where the VO2 metallic phase is found only in conjunction with the rutile structure above 340 K, a new RT metallic phase within a monoclinic structure appears accessible in the high pressure regime.Physical Review Letters 05/2007; 98(19):196406. · 7.37 Impact Factor -
Article: Evidence of a pressure-induced metallization process in monoclinic VO$_2$
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ABSTRACT: Raman and combined trasmission and reflectivity mid infrared measurements have been carried out on monoclinic VO$_2$ 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*$\sim$10 GPa. Evidence of subtle modifications of V ion arrangements within the monoclinic lattice together with the onset of a metallization process via band gap filling are observed for P$>$P*. Differently from ambient pressure, where the VO$_2$ 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 VO$_2$. Comment: 5 pages, 3 figures11/2006; -
Article: Electrodynamics of BaFe_ {2} As_ {2} from infrared measurements under pressure
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ABSTRACT: We report on an infrared study on the undoped compound BaFe2As2 as a function of pressure (up to about 10 GPa) at three temperatures (300, 160, and 110 K). The evolution with pressure and temperature of the optical conductivity shows that by increasing pressure, the mid-infrared absorptions associated with magnetic order are lowered while the Drude term increases, indicating the evolution towards a conventional metallic state. We evaluate the spectral weight dependence on pressure comparing it to that previously found upon doping. All the optical results indicate that lattice modifications cannot be recognized as the only parameter determining the low-energy electrodynamics in these compounds.Phys. Rev. B. 85(17). -
Article: Multi-gap superconductivity in a BaFe 1.84 Co 0.16 As 2 film from optical measurements at terahertz frequencies
The European Physical Journal B - Condensed Matter and Complex Systems. 77(1):25-30. -
Article: Multi-gap superconductivity in a BaFe
http://dx.doi.org/10.1140/epjb/e2010-00242-y.
Top co-authors
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Institutions
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2007–2012
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Sapienza University of Rome
- Department of Physics
Roma, Latium, Italy
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2011
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European Synchrotron Radiation Facility
Grenoble, Rhone-Alpes, France
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