Search

Materials with strong electronic correlations host remarkable -- and technologically relevant -- phenomena such as magnetism, superconductivity and metal-insulator transitions. Harnessing and controlling these effects is a major challenge, on which key advances are being made through lattice and strain engineering in thin films and heterostructures, leveraging the complex interplay between electronic and structural degrees of freedom. Here we show that the electronic structure of LaNiO3 can be tuned by means of lattice engineering. We use different substrates to induce compressive and tensile biaxial epitaxial strain in LaNiO3 thin films. Our measurements reveal systematic changes of the optical spectrum as a function of strain and, notably, an increase of the low-frequency free carrier weight as tensile strain is applied. Using density functional theory (DFT) calculations, we show that this apparently counter-intuitive effect is due to a change of orientation of the oxygen octahedra.The calculations also reveal drastic changes of the electronic structure under strain, associated with a Fermi surface Lifshitz transition. We provide an online applet to explore these effects. The experimental value of integrated spectral weight below 2 eV is significantly (up to a factor of 3) smaller than the DFT results, indicating a transfer of spectral weight from the infrared to energies above 2 eV. The suppression of the free carrier weight and the transfer of spectral weight to high energies together indicate a correlation-induced band narrowing and free carrier mass enhancement due to electronic correlations. Our findings provide a promising avenue for the tuning and control of quantum materials employing lattice engineering., Comment: 12 pages, 11 figures

We study the temperature dependence of the optical conductivity of rare-earth nickelate films of varying composition and strain close to the antiferromagnetic ordering temperature, TN. Two prominent peaks at 0.6 and 1.3 eV, which are characteristic of the insulating phase, display a small but significant increase in intensity when the material passes from para- to antiferromagnetic. This observation indicates the presence of a positive feedback between antiferromagnetic (AF) and bond disproportionation (BD) order. By analyzing the temperature dependence near TN, and using a Landau-type free-energy expression for BD and AF order, we infer that BD order is a necessary condition for the AF phase to appear, and that the antiferromagnetism contributes to stabilization of the bond disproportionation. This model also explains why hysteresis is particularly strong when the transition into the insulating state occurs simultaneously with antiferromagnetic order., Comment: 5 pages, 5 figures

Selective optical excitation of a substrate lattice can drive phase changes across hetero-interfaces. This phenomenon is a non-equilibrium analogue of static strain control in heterostructures and may lead to new applications in optically controlled phase change devices. Here, we make use of time-resolved non-resonant and resonant x-ray diffraction to clarify the underlying physics, and to separate different microscopic degrees of freedom in space and time. We measure the dynamics of the lattice and that of the charge disproportionation in NdNiO3, when an insulator-metal transition is driven by coherent lattice distortions in the LaAlO3 substrate. We find that charge redistribution propagates at supersonic speeds from the interface into the NdNiO3 film, followed by a sonic lattice wave. When combined with measurements of magnetic disordering and of the metal-insulator transition, these results establish a hierarchy of events for ultrafast control at complex oxide hetero-interfaces., Comment: 20 pages, 4 figures

Taking advantage of the large electron escape depth of soft x-ray angle resolved photoemission spectroscopy we report electronic structure measurements of (111)-oriented [LaNiO3/LaMnO3] superlattices and LaNiO3 epitaxial films. For thin films we observe a 3D Fermi surface with an electron pocket at the Brillouin zone center and hole pockets at the zone vertices. Superlattices with thick nickelate layers present a similar electronic structure. However, as the thickness of the LaNiO3 is reduced the superlattices become insulating. These heterostructures do not show a marked redistribution of spectral weight in momentum space but exhibit a pseudogap of 50 meV.

We investigate the non-equilibrium insulator-metal transition driven in a SmNiO3 thin film by coherent optical excitation of the LaAlO3 substrate lattice. By probing the transient optical properties over a broad frequency range (100 - 800 cm^-1), we analyze both the time dependent metallic plasma and the infrared optical phonon line shapes. We show that the light-induced metallic phase in SmNiO3 has the same carrier density as the equilibrium metallic phase. We also report that the LaAlO3 substrate acts as a transducer only at the earlier time delays, as the vibrations are driven coherently. No long-lived structural rearrangement takes place in the substrate. Finally, we show that the transient insulator-metal transition occurs both below and above the N\'eel temperature. We conclude that the supersonic melting of magnetic order measured with ultrafast x-rays is not the driving force of the formation of the metallic phase. We posit that the insulator metal transition may origin from the rearrangement of ordered charges at the interface propagating into the film., Comment: 12 pages, 3 figures

Using a combination of spectroscopic ellipsometry and DC transport measurements, we determine the temperature dependence of the optical conductivity of NdNiO$_3$ and SmNiO$_{3}$ films. The optical spectra show the appearance of a characteristic two-peak structure in the near-infrared when the material passes from the metal to the insulator phase. Dynamical mean-field theory calculations confirm this two-peak structure, and allow to identify these spectral changes and the associated changes in the electronic structure. We demonstrate that the insulating phase in these compounds and the associated characteristic two-peak structure are due to the combined effect of bond-disproportionation and Mott physics associated with half of the disproportionated sites. We also provide insights into the structure of excited states above the gap., Comment: 12 pages, 13 figures

Bulk NdNiO_3 and thin films grown along the pseudocubic (001)_pc axis display a 1st order metal to insulator transition (MIT) together with a N\'eel transition at T=200K. Here, we show that for NdNiO3 films deposited on (111)_pc NdGaO_3 the MIT occurs at T=335K and the N\'eel transition at T=230 K. By comparing transport and magnetic properties of layers grown on substrates with different symmetries and lattice parameters, we demonstrate a particularly large tuning when the epitaxy is realized on (111)_pc surfaces. We attribute this effect to the specific lattice matching conditions imposed along this direction when using orthorhombic substrates.

Nickelates are known for their metal to insulator transition (MIT) and an unusual magnetic ordering, occurring at T=T_N\'eel. Here, we investigate thin films of SmNiO_3 subjected to different levels of epitaxial strain. We find that the original bulk behavior (T_N\'eel

We present and analyze high-resolution optical spectra of the Algol binaries S Equ and KO Aql. New accurate radial velocities for the hotter primary components are obtained. Thanks to the cross-correlation procedure, we were able to measure, for the first time to our knowledge, radial velocities also for the cool secondary components of S Equ and KO Aql. By combining the parameters obtained from the solution of the radial velocity curves with those obtained from the light curve analysis, reliable absolute parameters of the systems have been derived. The rotational velocity of the hotter components of S Equ and KO Aql has been measured and it is found that the gainers of both systems rotate about 30 % faster than synchronously. This is likely due to mass transfer across the Lagrangian L1 point from the cooler to the hotter component. The lower luminosity of the mass-gaining components of these systems compared to normal main-sequence stars of the same mass can be also an effect of the mass transfer. The H-alpha profiles reveal clear evidence of mass transfer and accretion structures. In both systems we clearly observed extra-absorption lines. From the integrated absorption and the radial velocity variations of these features, we found that the mass accretion is very dense around the impact region of the hotter components. A double-peaked emission in the spectra of S Equ was seen outside the eclipses. One of these peaks is likely originated in a region between the center of mass and the cooler component, which is occupied by the flowing matter. Furthermore, the H-alpha difference spectra of S Equ and KO Aql display also emission features, which should be arising from the magnetic activity of the cooler components., Comment: 19 pages

We test our capability of deriving stellar physical parameters of giant stars by analysing a sample of field stars and the well studied open cluster IC 4651 with different spectroscopic methods. The use of a technique based on line-depth ratios (LDRs) allows us to determine with high precision the effective temperature of the stars and to compare the results with those obtained with a classical LTE abundance analysis. (i) For the field stars we find that the temperatures derived by means of the LDR method are in excellent agreement with those found by the spectral synthesis. This result is extremely encouraging because it shows that spectra can be used to firmly derive population characteristics (e.g., mass and age) of the observed stars. (ii) For the IC 4651 stars we use the determined effective temperature to derive the following results. a) The reddening E(B-V) of the cluster is 0.12+/-0.02, largely independent of the color-temperature calibration used. b) The age of the cluster is 1.2+/-0.2 Gyr. c) The typical mass of the analysed giant stars is 2.0+/-0.2M_sun. Moreover, we find a systematic difference of about 0.2 dex in log g between spectroscopic and evolutionary values. We conclude that, in spite of known limitations, a classical spectroscopic analysis of giant stars may indeed result in very reliable stellar parameters. We caution that the quality of the agreement, on the other hand, depends on the details of the adopted spectroscopic analysis., Comment: 11 pages, 9 figures, version accepted for publication in A&A. Major changes: revised paper, new discussion of results, references added, one additional figure

We present a high-resolution spectroscopic analysis of two samples of evolved stars selected in the field and in the intermediate-age open cluster IC 4651, for which detailed measurements of chemical composition were made in the last few years. Applying the Gray's method based on ratios of line depths, we determine the effective temperature and compare our results with previous ones obtained by means of the curves of growth of iron lines. The knowledge of the temperature enables us to estimate other fundamental stellar parameters, such as color excess, age, and mass., Comment: 4 pages, 1 figure, to appear in the Proceedings of the ESO Workshop "Precision Spectroscopy in Astrophysics", eds. L. Pasquini, M. Romaniello, N.C. Santos, A. Correia

The dependence on the temperature of photospheric line-depth ratios (LDRs) in the spectral range 6190-6280 A is investigated by using a sample of 174 ELODIE Archive stellar spectra of luminosity class from V to III. The rotational broadening effect on LDRs is also studied. We provide useful calibrations of effective temperature versus LDRs for giant and main sequence stars with 3800

The ratio of the depths of spectral lines is a powerful indicator of the effective temperature. The method based on this analysis is capable of discerning small temperature variations of individual stars. We apply this spectroscopic data analysis to three type of stars, namely an RS CVn type binary system, a young solar-type star and a Cepheid variable. We show that individual LDRs converted into temperature through calibration relations lead to rotational and pulsational modulation of the average surface temperature with amplitudes of 127 K, 48 K and 1466 K in the three types of stars, with average estimated errors of some tens Kelvin degrees., Comment: 10 pages, 8 figures, Published in the Proceedings of "Astronomical Data Analysis III"

As part of our program to study stellar photospheric and chromospheric activity, we have examined several young solar type stars with activity levels intermediate between the Sun and the very active RS CVn binaries. We have analysed contemporaneous spectroscopic data obtained at Catania Observatory (Serra La Nave station, Mt. Etna) and photometric data acquired in the Stromgren bands with an automatic photometric telescope (APT) at Fairborn Observatory (Arizona, USA). Surface inhomogeneities have been detected from the rotational modulation of stellar brightness as well as from the modulation of several photospheric line-depth ratios (LDRs). The presence of chromospheric plage-like regions has been inferred from the rotational modulation of the Halpha line equivalent width (EW_Halpha) evaluated with the spectral synthesis method. The most relevant results are the strong correlation between the brightness and temperature curves derived respectively from photometry and the LDRs as well as the striking anti-correlation between brightness and Halpha emission. This suggests a close spatial association of spots and plages, as frequently observed for the largest sunspot groups (e.g., Catalano et al. (1998)) and for some very active RS CVn systems (Catalano et al. 2002). Moreover, a simple spot/plage model applied to the observed flux curves allows a rough reconstruction of photospheric and chromospheric features of young main sequence stars., Comment: 4 pages, 5 figures, Proceeding for the "13th Cool Star Workshop"

We report the detection of short period oscillations in the hot subdwarf B (sdB) star PG 1613+426 from time-series photometry carried out with the 91-cm Cassegrain telescope of the Catania Astrophysical Observatory. This star, which is brighter than the average of the presently known sdB pulsators, with B = 14.14 mag, has $T_{\rm eff}=34 400 {\rm K}$ and $\log g = 5.97$, its position is near the hot end of the sdB instability strip, and it is a pulsator with a well observed peak in the power spectrum at $144.18\pm 0.06 \rm s$. This star seems to be well suited for high precision measurements, which could detect a possible multi-mode pulsation behaviour, Comment: 3 pages, 4 figures. to appear on A&A

Time-resolved observations of a 6' x 6' field, containing the position error boxes of the X-ray source 1WGA J1958.2+3232, were performed in June 1999 with the 91 cm Catania telescope, equipped with a Photon Counting Intensified CCD. The star recently proposed as the optical counterpart of the 1WGA J1958.2+3232, exhibited a strong optical modulation with a period compatible with that seen in X-ray (~12 min). The optical modulation was detected again in September and October 1999. These results confirm the identification of the optical counterpart and support the classification of 1WGA J1958.2+3232 as a cataclysmic variable, possibly an Intermediate Polar. Modulation at period twice as large was also found in one observation run, suggesting that the true spin period of the White Dwarf could be 24 min rather than 12 min, thus 1WGA J1958.2+3232 appears to be, among the IPs, the slowest rotator which exhibit double peaked spin profile., Comment: 7 pages, 6 figures, accepted by A&A