arXiv subjects

S. Lupi

Publications and source records attributed to S. Lupi.

At least 37 records · Page 2Linked to original sources

arXiv2013

Phase diagram and optical conductivity of La1.8-xEu0.2SrxCuO4

La1.8-xEu0.2SrxCuO4 (LESCO) is the member of the 214 family which exhibits the largest intervals among the structural, charge ordering (CO), magnetic, and superconducting transition temperatures. By using new dc transport measurements and data in the literature we construct the phase diagram of LESCO between x = 0.8 and 0.20. This phase diagram has been further probed in ac, by measuring the optical conductivity σ1(ω) of three single crystals with x = 0.11, 0.125, and 0.16 between 10 and 300 K in order to associate the extra-Drude peaks often observed in the 214 family with a given phase. The far-infrared peak we detect in underdoped LESCO is the hardest among them, survives up to room temperature and is associated with charge localization rather than with ordering. At the CO transition for the commensurate doping x = 0.125 instead the extra-Drude peak hardens and a pseudogap opens in σ1(ω), approximately as wide as the maximum superconducting gap of LSCO.

cond-mat.str-el↗

arXiv2013

A combined experimental and computational study of the pressure dependence of the vibrational spectrum of solid picene C_22H_14

We present high-quality optical data and density functional perturbation theory calculations for the vibrational spectrum of solid picene (C$_{22}$H$_{14}$) under pressure up to 8 GPa. First-principles calculations reproduce with a remarkable accuracy the pressure effects on both frequency and intensities of the phonon peaks experimentally observed . Through a detailed analysis of the phonon eigenvectors, We use the projection on molecular eigenmodes to unambiguously fit the experimental spectra, resolving complicated spectral structures, in a system with hundreds of phonon modes. With these projections, we can also quantify the loss of molecular character under pressure. Our results indicate that picene, despite a \sim 20 % compression of the unit cell, remains substantially a molecular solid up to 8 GPa, with phonon modes displaying a smooth and uniform hardening with pressure. The Grueneisen parameter of the 1380 cm^{-1} a_1 Raman peak ($γ_p=0.1$) is much lower than the effective value ($γ_d=0.8$) due to K doping. This is an indication that the phonon softening in K doped samples is mainly due to charge transfer and electron-phonon coupling.

cond-mat.supr-con↗

arXiv2013

Observation of Dirac plasmons in a topological insulator

Plasmons are the quantized collective oscillations of electrons in metals and doped semiconductors. The plasmons of ordinary, massive electrons are since a long time basic ingredients of research in plasmonics and in optical metamaterials. Plasmons of massless Dirac electrons were instead recently observed in a purely two-dimensional electron system (2DEG)like graphene, and their properties are promising for new tunable plasmonic metamaterials in the terahertz and the mid-infrared frequency range. Dirac quasi-particles are known to exist also in the two-dimensional electron gas which forms at the surface of topological insulators due to a strong spin-orbit interaction. Therefore,one may look for their collective excitations by using infrared spectroscopy. Here we first report evidence of plasmonic excitations in a topological insulator (Bi2Se3), that was engineered in thin micro-ribbon arrays of different width W and period 2W to select suitable values of the plasmon wavevector k. Their lineshape was found to be extremely robust vs. temperature between 6 and 300 K, as one may expect for the excitations of topological carriers. Moreover, by changing W and measuring in the terahertz range the plasmonic frequency vP vs. k we could show, without using any fitting parameter, that the dispersion curve is in quantitative agreement with that predicted for Dirac plasmons.

cond-mat.mes-hall↗

arXiv2013

Electrodynamics of BaFe2As2 from infrared measurements under pressure

We report on an infrared study on the undoped compound BaFe2As2 as a function of both 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. The whole optical results indicate that lattice modifications can not be recognized as the only parameter determining the low-energy electrodynamics in these compounds.

cond-mat.str-el↗

arXiv2013

Mott-Hubbard transition in V2O3 revisited

The isostructural metal-insulator transition in Cr-doped V2O3 is the textbook example of a Mott-Hubbard transition between a paramagnetic metal and a paramagnetic insulator. We review recent theoretical calculations as well as experimental findings which shed new light on this famous transition. In particular, the old paradigm of a doping-pressure equivalence does not hold, and there is a microscale phase separation for Cr-doped V2O3.

cond-mat.str-el↗

arXiv2012

Deconstructing the Hubbard Hamiltonian by Ultrafast Quantum Modulation Spectroscopy in Solid-state Mott Insulators

Most available theories for correlated electron transport are based on the Hubbard Hamiltonian. In this effective theory, renormalized hopping and interaction parameters only implicitly incorporate the coupling of correlated charge carriers to microscopic degrees of freedom. Unfortunately, no spectroscopy can individually probe such renormalizations, limiting the applicability of Hubbard models. We show here that the role of each individual degree of freedom can be made explicit by using a new experimental technique, which we term 'quantum modulation spectroscopy' and we demonstrate here in the one-dimensional Mott insulator ET-F2TCNQ. We explore the role on the charge hopping of two localized molecular modes, which we drive with a mid infrared optical pulse. Sidebands appear in the modulated optical spectrum, and their linshape is fitted with a model based on the dynamic Hubbard Hamiltonian. A striking asymmetry between the renormalization of doublons and holons is revealed. The concept of quantum modulation spectroscopy can be used to systematically deconstruct Hubbard Hamiltonians in many materials, exposing the role of any mode, electronic or magnetic, that can be driven to large amplitude with a light field.

cond-mat.str-el↗

arXiv2012

Optical conductivity of Bismuth-based topological insulator

The optical conductivity and the spectral weight of four topological insulators with increasing chemical compensation (Bi2Se3, Bi2-xCaxSe3, Bi2Se2Te, Bi2Te2Se) have been measured from 5 to 300 K and from sub-THz to visible frequencies. The effect of compensation is clearly observed in the infrared spectra, through the suppression of an extrinsic Drude term and the appearance of strong absorption peaks, that we assign to electronic transitions among localized states. From the far-infrared spectral weight of the most compensated sample (Bi2Te2Se) one can estimate a density of charge-carriers in the order of 10^17/cm^3 in good agreement with transport data. Those results demonstrate that the low-energy electrodynamics in single crystals of topological insulators, even at the highest degree of compensation presently achieved, is still affected by extrinsic charge excitations.

cond-mat.mtrl-sci↗

arXiv2012

Universal conductivity and the electrodynamics of graphite at high pressures

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.

cond-mat.mtrl-sci↗

arXiv2012

Vibrational spectrum of solid picene (C_22H_14)

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.

cond-mat.mtrl-sci↗

arXiv2011

An extended infrared study of the (p,T) phase diagram of the p-doped Cu-O plane

The ab-plane optical conductivity of eleven single crystals, belonging to the families Sr2-xCuO2Cl2, Y1-xCaxBa2Cu3O6, Bi2Sr2-xLaxCuO6, and Bi2Sr2CaCu2O8 has been measured with hole concentrations p between 0 and 0.18, and for 6 K < T < 500 K to obtain an infrared picture of the p,T phase diagram of the Cu-O plane. At extreme dilution (p = 0.005), a narrow peak is observed at 1570 cm-1 (195 meV), that we assign to a single-hole bound state. For increasing doping, that peak broadens into a far-infrared (FIR) band whose low-energy edge sets the insulating gap. The insulator-to-metal transition (IMT) occurs when the softening of the FIR band closes the gap thus evolving into a Drude term. In the metallic phase, a multi-band analysis identifies a mid-infrared band which weakly depends on temperature and softens for increasing p, while the extended-Drude analysis leads to an optical scattering rate larger than the frequency, as found in other cuprates. The infrared spectral weight W(T) is consistent with a Fermi liquid renormalized by strong correlations, provided that the T^4 term of the Sommerfeld expansion is included above 300 K. In the superconducting phase, the optical response of single-layer Bi2Sr2-xLaxCuO6 at optimum doping is similar to that of the corresponding optimally-doped bilayer Bi2Sr2CaCu2O8.

cond-mat.supr-con↗

arXiv2010

Optical Properties of (SrMnO3)n/(LaMnO3)2n superlattices: an insulator-to-metal transition observed in the absence of disorder

We measure the optical conductivity of (SrMnO3)n/(LaMnO3)2n superlattices (SL) for n=1,3,5, and 8 and 10 < T < 400 K. Data show a T-dependent insulator to metal transition (IMT) for n \leq 3, driven by the softening of a polaronic mid-infrared band. At n = 5 that softening is incomplete, while at the largest-period n=8 compound the MIR band is independent of T and the SL remains insulating. One can thus first observe the IMT in a manganite system in the absence of the disorder due to chemical doping. Unsuccessful reconstruction of the SL optical properties from those of the original bulk materials suggests that (SrMnO3)n/(LaMnO3)2n heterostructures give rise to a novel electronic state.

cond-mat.mtrl-sci↗

arXiv2010

A Microscopic View on the Mott transition in Chromium-doped V2O3

V2O3 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 (PI). 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 V2O3 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 which drives a genuine Mott transition to an eventually homogeneous metallic state.

cond-mat.str-el↗

arXiv2010

Far infrared properties of the rare-earth scandate DyScO3

We present reflectance measurements in the infrared region on a single crystal the rare earth scandate DyScO3. Measurements performed between room temperature and 10 K allow to determine the frequency of the infrared-active phonons, never investigated experimentally, and to get information on their temperature dependence. A comparison with the phonon peak frequency resulting from ab-initio computations is also provided. We finally report detailed data on the frequency dependence of the complex refractive index of DyScO3 in the terahertz region, which is important in the analysis of terahertz measurements on thin films deposited on DyScO3.

cond-mat.mtrl-sci↗

arXiv2010

Multi-gap superconductivity in a BaFe1.84Co0.16As2 film from optical measurements at terahertz frequencies

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 $Δ_A$ 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 $Δ_A$ gap results to be well determined ($Δ_A$=15.5$\pm$0.5 cm$^{-1}$), while the value of the high-energy gap $Δ_B$ is more uncertain ($Δ_B$=55$\pm$7 cm$^{-1}$). Our results provide evidence of a nodeless isotropic double-gap scenario, with the presence of two optical gaps corresponding to 2$Δ/kT_c$ values close to 2 and 7.

cond-mat.supr-con↗

arXiv2010

High-temperature optical spectral weight and Fermi liquid renormalization in Bi-based cuprates

The optical conductivity and the spectral weight W(T) of two superconducting cuprates at optimum doping, Bi2Sr2-xLaxCuO6 and Bi2Sr2CaCu2O8, have been first measured up to 500 K. Above 300 K, W(T) deviates from the usual T2 behavior in both compounds, even though the zero-frequency extrapolation of the optical conductivity remains larger than the Ioffe-Regel limit. The deviation is surprisingly well described by the T4 term of the Sommerfeld expansion, but its coefficients are enhanced by strong correlation. This renormalization is due to strong correlation, as shown by the good agreement with dynamical mean field calculations.

cond-mat.supr-con↗

arXiv2009

Characterization of dipolar current-based metamaterials showing a single resonance in the terahertz spectrum

We will report on the electromagnetic response due to induced dipolar currents in metamaterials of 2-dimensional array of metallic elements. Used as frequency selectors, the metamaterial transmittance presents a single resonance in the region from 1 to 8 THz that can be easily selected and scaled maintaining unaltered the quality factor by choosing the size and shape of the planar metallic element and exploiting the scalability properties of the Maxwell equations. Basing on these studies, we have designed and tested a series of simple and inexpensive frequency selective metamaterials fabricated by using lithographic processes.

physics.optics↗

arXiv2009

Substrateless metamaterials at mid-infrared frequencies

We report on the fabrication and mid-infrared transmission properties of free-standing thin metal films, periodically patterned with holes at periods down to 2 microns and area of 3x3 mm2. Square grids were fabricated by electron beam lithography and deep-etching techniques and display substrateless holes, with the metal being supported by a patterned dielectric silicon nitride membrane. The mid-infrared transmission spectra of the substrateless grid display extraordinary transmission peaks and resonant absorption lines with a Q-factor up to 22. These spectral features are due to the interaction of the radiation with surface plasmon modes. The high transmittivity and the negative value of the dielectric constant at selected frequencies make our substrateless structures ideal candidates for the fabrication of mid-infrared metamaterials.

physics.optics↗

arXiv2009

Pressure and alloying effects on the metal to insulator transition in NiS{2-x}Se{x} studied by infrared spectroscopy

The metal to insulator transition in the charge transfer NiS{2-x}Se{x} 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.

cond-mat.str-el↗