arXiv subjects

S. Lupi

Publications and source records attributed to S. Lupi.

At least 55 records · Page 3Linked to original sources

arXiv2009

Multi-band conductivity and multi-gap superconductivity in V3Si from optical measurements on films at terahertz frequencies

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.

cond-mat.supr-con↗

arXiv2009

Far-infrared absorption and the metal-to-insulator transition in hole-doped cuprates

By studying the optical conductivity of BSLCO and YCBCO, we show that the metal-to-insulator transition (MIT) in these hole-doped cuprates is driven by the opening of a small gap at low T in the far infrared. Its width is consistent with the observations of Angle-Resolved Photoemission Spectroscopy in other cuprates, along the nodal line of the k-space. The gap forms as the Drude term turns into a far-infrared absorption, whose peak frequency can be approximately predicted on the basis of a Mott-like transition. Another band in the mid infrared softens with doping but is less sensitive to the MIT.

cond-mat.supr-con↗

arXiv2008

Pressure dependence of the single particle excitation in the charge-density-wave CeTe$_3$ system

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.

cond-mat.str-el↗

arXiv2008

The optical phonon spectrum of SmFeAsO

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.

cond-mat.supr-con↗

arXiv2008

Transport and infrared properties of SmFeAs(O1-xFx): from SDW to superconducting ordering

We report measurements of resistivity, magnetoresistivity, Hall effect, Seebeck coefficient, infrared reflectivity of undoped SmFeAsO and lightly doped SmFeAs(O0.93F0.07) oxypnictides. All the properties measured on SmFeAsO are characterized by clear signatures of the magnetic instability. A self-consistent picture emerges in which below the magnetic transition carrier condensation occurs due to the opening of spin density wave (SDW) gap. This is accompanied by the mobility increase of not gapped carriers due to the suppression of electron-electron scattering. SmFeAs(O0.93F0.07) exhibits an increase of the metallic character on cooling consistent with electron doping, even though at room temperature values of all the properties nearly overlaps with those of SmFeAsO. However, with temperature decrease all anomalies related to the SDW instability are missed and the superconducting transition occurs. This suggests that doping breaks abruptly the symmetries of the Fermi surface inhibiting the SDW formation in favor of the superconducting transition, with no substantial changes in the density of states or in the effective mass.

cond-mat.supr-con↗

arXiv2008

Quasiparticle evolution and pseudogap formation in V2O3: An infrared spectroscopy study

The infrared conductivity of V2O3 is measured in the whole phase diagram. Quasiparticles appear above the Neel temperature TN and eventually disappear further enhancing the temperature, leading to a pseudogap in the optical spectrum above 425 K. Our calculations demonstrate that this loss of coherence can be explained only if the temperature dependence of lattice parameters is considered. V2O3 is therefore effectively driven from the metallic to the insulating side of the Mott transition as the temperature is increased.

cond-mat.str-el↗

arXiv2008

Two-band parallel conductivity at terahertz frequencies in the superconducting state of MgB$_2$.

The optical response of the two-band superconductor MgB$_2$ has been studied in the 0.7-4 THz range on films with very low impurity level. The effect of the high-energy $σ$-gap is observed in the ratio $R_S/R_N$ between the normal and superconducting state reflectance, while in a neutron irradiated film with a slightly higher impurity level mainly the effect of the $π$-gap is evident as reported in previous experiments. At terahertz frequencies, the electrodynamic of MgB$_2$ can be well described by the two-band parallel conductivity model and is dominated by the $π$-bands when the impurity level is only slightly higher than that of an ultra-clean sample.

cond-mat.supr-con↗

arXiv2007

Pressure dependence of the optical properties of the charge-density-wave compound LaTe$_2$

We report the pressure dependence of the optical response of LaTe$_2$, which is deep in the charge-density-wave (CDW) ground state even at 300 K. The reflectivity spectrum is collected in the mid-infrared spectral range at room temperature and at pressures between 0 and 7 GPa. We extract the energy scale due to the single particle excitation across the CDW gap and the Drude weight. We establish that the gap decreases upon compressing the lattice, while the Drude weight increases. This signals a reduction in the quality of nesting upon applying pressure, therefore inducing a lesser impact of the CDW condensate on the electronic properties of LaTe$_2$. The consequent suppression of the CDW gap leads to a release of additional charge carriers, manifested by the shift of weight from the gap feature into the metallic component of the optical response. On the contrary, the power-law behavior, seen in the optical conductivity at energies above the gap excitation and indicating a weakly interacting limit within the Tomonaga-Luttinger liquid scenario, seems to be only moderately dependent on pressure.

cond-mat.str-el↗

arXiv2007

Sub-THz electrodynamics of the graphene-like superconductor CaAlSi

We report the first optical study of CaAlSi, a superconductor which displays both the crystal structure of MgB2 and the electronic band structure of intercalated graphites. The reflectivity of a CaAlSi single crystal was measured down to sub-THz frequencies and to 3.3 K, with the use of Coherent Synchrotron Radiation. A superconducting gap in the hexagonal planes, two gaps along the c axis were found and measured, as expected from the structure of the CaAlSi Fermi surface. The anisotropic optical parameters of the normal state were also determined.

cond-mat.supr-con↗

arXiv2007

Electrodynamics near the Metal-to-Insulator Transition in V3O5

The electrodynamics near the metal-to-insulator transitions (MIT) induced, in V3O5 single crystals, by both temperature (T) and pressure (P) has been studied by infrared spectroscopy. The T- and P-dependence of the optical conductivity may be explained within a polaronic scenario. The insulating phase at ambient T and P corresponds to strongly localized small polarons. Meanwhile the T-induced metallic phase at ambient pressure is related to a liquid of polarons showing incoherent dc transport, in the P-induced metallic phase at room T strongly localized polarons coexist with partially delocalized ones. The electronic spectral weight is almost recovered, in both the T and P induced metallization processes, on an energy scale of 1 eV, thus supporting the key-role of electron-lattice interaction in the V3O5 metal-to-insulator transition.

cond-mat.str-el↗

arXiv2006

Evidence of a pressure-induced metallization process in monoclinic VO$_2$

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$_2$.

cond-mat.mtrl-sci↗

arXiv2006

Pressure dependence of the charge-density-wave gap in rare-earth tri-tellurides

We investigate the pressure dependence of the optical properties of CeTe$_3$, 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 $R$Te$_3$.

cond-mat.str-el↗

arXiv2006

Effects of Superconductivity and Charge Order on the sub-Terahertz reflectivity of La$_{1.875}$Ba$_{0.125-y}$Sr$_{y}$CuO$_4$

The reflectivity $R (ω)$ of both the $ab$ plane and the c axis of two single crystals of La$_{1.875}$Ba$_{0.125-y}$Sr$_{y}$CuO$_4$ has been measured down to 5 cm$^{-1}$, using coherent synchrotron radiation below 30 cm$^{-1}$. For $y$ = 0.085, a Josephson Plasma Resonance is detected at $T \ll T_c$ = 31 K in $R_{c} (ω)$, and a far-infrared peak (FIP) appears in the optical conductivity below 50 K, where non-static charge ordering (CO) is reported by X-ray scattering. For $y$ = 0.05 ($T_c$ = 10 K), a FIP is observed in the low-temperature tetragonal phase below the ordering temperature $T_{CO}$. At 1/8 doping the peak frequency scales linearly with $T_{CO}$, confirming that the FIP is an infrared signature of CO, either static or fluctuating.

cond-mat.supr-con↗

arXiv2006

Low-energy electrodynamics of superconducting diamond

Heavily-boron-doped diamond films become superconducting with critical temperatures $T_c$ well above 4 K. Here we first measure the reflectivity of such a film down to 5 cm$^{-1}$, by also using Coherent Synchrotron Radiation. We thus determine the optical gap, the field penetration depth, the range of action of the Ferrell-Glover-Tinkham sum rule, and the electron-phonon spectral function. We conclude that diamond behaves as a dirty BCS superconductor.

cond-mat.supr-con↗

arXiv2006

Infrared observation of the Hund's mechanism in an electron-doped manganite

In the mid-infrared absorption of Sr{1-x}Ce{x}MnO{3} at low electron doping (x = 0.05), a band at 0.3 eV is fully replaced by another one at 0.9 eV as the system becomes antiferromagnetic (AF) of type $G$. A weaker effect occurs at x = 0.10 for an AF phase of type $C$. One thus directly measures the electron hopping energies for spin parallel and anti-parallel to that of the host ion. The Hund's, crystal-field, and Jahn-Teller splittings for the Mn{3+} ions in a Mn{4+} matrix, can also be derived.

cond-mat.str-el↗

arXiv2005

Temperature dependence of the optical spectral weight in the cuprates: Role of electron correlations

We compare calculations based on the Dynamical Mean-Field Theory of the Hubbard model with the infrared spectral weight $W(Ω,T)$ of La$_{2-x}$Sr$_x$CuO$_4$ and other cuprates. Without using fitting parameters we show that most of the anomalies found in $W(Ω,T)$ with respect to normal metals, including the existence of two different energy scales for the doping- and the $T$-dependence of $W(Ω,T)$, can be ascribed to strong correlation effects.

cond-mat.str-el↗

arXiv2005

Infrared Conductivity and Charge Ordering in NaCoO2

The infrared conductivity of NaxCoO2 is studied as a function of doping and temperature for x between 0.5 and 1. Charge localization in CoO2 layers shows up through a far-infrared peak (FIP) in the infrared conductivity which coexists with a small Drude contribution. Long-range ordering at x = 0.5 is confirmed to create a far-infrared gap, in addition to the FIP. At high x, the formation of a Spin-Density Wave reported below 22 K dramatically shifts the FIP to higher energy when x is incommensurate with the lattice, indicating an abrupt deepening of the localizing potential. The in-plane E1u phonon lifetime is shown to be sensitive to both "freezing" and ordering of the mobile Na+ ions. A comparison with the behavior of the FIP shows that such "freezing" is not the only origin of charge localization in the CoO layers.

cond-mat.str-el↗

arXiv2004

Frequency-dependent Thermal Response of the Charge System and Restricted Sum Rules in La(2-x)Sr(x)CuO(4)

By using new and previous measurements of the $ab$-plane conductivity $σ_1^{ab} (ω,T)$ of La$_{2-x}$Sr$_x$CuO$_{4}$ (LSCO) it is shown that the spectral weight $W = \int_0^Ω{σ_1^{ab} (ω,T) dω}$ obeys the same law $W = W_0 - B(Ω) T^2$ which holds for a conventional metal like gold, for $Ω$'s below the plasma frequency. However $B(Ω)$, which measures the "thermal response" of the charge system, in LSCO exhibits a peculiar behavior which points towards correlation effects. In terms of hopping models, $B(Ω)$ is directly related to an energy scale $t_T$, smaller by one order of magnitude than the full bandwidth $t_0 \sim W_0$.

cond-mat.supr-con↗