R. Fittipaldi - Academia.edu (original) (raw)

Papers by R. Fittipaldi

npj Quantum Materials

The single-layered ruthenate Sr2RuO4 is one of the most enigmatic unconventional superconductors.... more The single-layered ruthenate Sr2RuO4 is one of the most enigmatic unconventional superconductors. While for many years it was thought to be the best candidate for a chiral p-wave superconducting ground state, desirable for topological quantum computations, recent experiments suggest a singlet state, ruling out the original p-wave scenario. The superconductivity as well as the properties of the multi-layered compounds of the ruthenate perovskites are strongly influenced by a van Hove singularity in proximity of the Fermi energy. Tiny structural distortions move the van Hove singularity across the Fermi energy with dramatic consequences for the physical properties. Here, we determine the electronic structure of the van Hove singularity in the surface layer of Sr2RuO4 by quasi-particle interference imaging. We trace its dispersion and demonstrate from a model calculation accounting for the full vacuum overlap of the wave functions that its detection is facilitated through the octahedra...

Physical Review D

We present directed searches for continuous gravitational waves from the neutron stars in the Cas... more We present directed searches for continuous gravitational waves from the neutron stars in the Cassiopeia A (Cas A) and Vela Jr. supernova remnants. We carry out the searches in the LIGO detector data from the first six months of the third Advanced LIGO and Virgo observing run using the Weave semi-coherent method, which sums matched-filter detection-statistic values over many time segments spanning the observation period. No gravitational wave signal is detected in the search band of 20-976 Hz for assumed source ages greater than 300 years for Cas A and greater than 700 years for Vela Jr. Estimates from simulated continuous wave signals indicate we achieve the most sensitive results to date across the explored parameter space volume, probing to strain magnitudes as low as ∼6.3 × 10 −26 for Cas A and ∼5.6 × 10 −26 for Vela Jr. at frequencies near 166 Hz at 95% efficiency.

Physical Review Research, 2021

arXiv: Strongly Correlated Electrons, 2020

We present a combined oxygen KKK-egde x-ray absorption spectroscopy (XAS) and resonant inelastic ... more We present a combined oxygen KKK-egde x-ray absorption spectroscopy (XAS) and resonant inelastic x-ray scattering (RIXS) study of the bilayer ruthenate Ca$_3$Ru$_2$O$_7$. Our RIXS experiments on Ca$_3$Ru$_2$O$_7$ were carried out on the overlapping in-plane and inner apical oxygen resonances, which are distinguishable from the outer apical one. Comparison to equivalent oxygen KKK-edge spectra recorded on band-Mott insulating Ca$_2$RuO$_4$ is made. In contrast to Ca$_2$RuO$_4$ spectra, which contain excitations linked to Mott physics, Ca$_3$Ru$_2$O$_7$ spectra feature only intra-$t_{2g}$ ones that do not directly involve the Coulomb energy scale. As found in Ca$_2$RuO$_4$, we resolve two intra-$t_{2g}$ excitations in Ca$_3$Ru$_2$O$_7$. Moreover, the lowest lying excitation in Ca$_3$Ru$_2$O$_7$ shows a significant dispersion, revealing a collective character differently from what is observed in Ca$_2$RuO$_4$. Theoretical modelling supports the interpretation of this lowest energy exci...

A. Kreisel, C. A. Marques, L. C. Rhodes, X. Kong, T. Berlijn, R. Fittipaldi, V. Granata, A. Vecch... more A. Kreisel, C. A. Marques, L. C. Rhodes, X. Kong, T. Berlijn, R. Fittipaldi, V. Granata, A. Vecchione, P. Wahl, and P. J. Hirschfeld Institut für Theoretische Physik, Universität Leipzig, D-04103 Leipzig, Germany SUPA, School of Physics and Astronomy, University of St Andrews, North Haugh, St Andrews, Fife KY16 9SS, United Kingdom Center For Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA CNR-SPIN, UOS Salerno, Via Giovanni Paolo II 132, Fisciano, I-84084, Italy Dipartimento di Fisica “E. R. Caianiello”, Università di Salerno, I-84084 Fisciano, Salerno, Italy Department of Physics, University of Florida, Gainesville, Florida 32611, USA (Dated: March 10, 2021)

npj Quantum Materials, 2021

Electronic band structures in solids stem from a periodic potential reflecting the structure of e... more Electronic band structures in solids stem from a periodic potential reflecting the structure of either the crystal lattice or electronic order. In the stoichiometric ruthenate Ca3Ru2O7, numerous Fermi surface-sensitive probes indicate a low-temperature electronic reconstruction. Yet, the causality and the reconstructed band structure remain unsolved. Here, we show by angle-resolved photoemission spectroscopy, how in Ca3Ru2O7 a C2-symmetric massive Dirac semimetal is realized through a Brillouin-zone preserving electronic reconstruction. This Dirac semimetal emerges in a two-stage transition upon cooling. The Dirac point and band velocities are consistent with constraints set by quantum oscillation, thermodynamic, and transport experiments, suggesting that the complete Fermi surface is resolved. The reconstructed structure—incompatible with translational-symmetry-breaking density waves—serves as an important test for band structure calculations of correlated electron systems.

Physical Review B, 2020

We present a combined soft x-ray and high-resolution vacuum-ultraviolet angle-resolved photoemiss... more We present a combined soft x-ray and high-resolution vacuum-ultraviolet angle-resolved photoemission spectroscopy study of the electron-overdoped cuprate Pr 1.3−x La 0.7 Ce x CuO 4 (PLCCO). Demonstration of its highly two-dimensional band structure enabled precise determination of the in-plane self-energy dominated by electron-electron scattering. Through analysis of this self-energy and the Fermi liquid cutoff energy scale, we find-in contrast to hole-doped cuprates-a momentum isotropic and comparatively weak electron correlation in PLCCO. Yet, the self-energies extracted from multiple oxide systems combine to demonstrate a logarithmic divergent relation between the quasiparticle scattering rate and mass. This constitutes a spectroscopic version of the Kadowaki-Woods relation with an important merit-the demonstration of Fermi liquid quasiparticle lifetime and mass being set by a single energy scale.

Physical Review B, 2018

We report, for the first time, measurements of the third order, 3 and fifth order, 5, susceptibil... more We report, for the first time, measurements of the third order, 3 and fifth order, 5, susceptibilities in an itinerant oxide metamagnet, Sr3Ru2O7 for magnetic fields both parallel and perpendicular to the c-axis. These susceptibilities exhibit maxima in their temperature dependence such that T1 ⇡ 2T3 ⇡ 4T5 where the Ti are the position in temperature where a peak in the i-th order susceptibility occurs. These features taken together with the scaling of the critical field with the temperature T1 observed in a diverse variety of itinerant metamagnets find a natural explanation in a single band model with one Van Hove singularity (VHS) and onsite repulsion U. The separation of the VHS from the Fermi energy , sets a single energy scale, which is the primary driver for the observed features of itinerant metamagnetism at low temperatures.

Physical Review B, 2019

A comprehensive study of the behavior of the Mott insulator Ca2RuO4 under electrical current driv... more A comprehensive study of the behavior of the Mott insulator Ca2RuO4 under electrical current drive is performed by combining two experimental probes: the macroscopic electrical transport and the microscopic X-Ray diffraction. The resistivity, ρ, vs electric current density, J, and temperature, T , ρ(J,T), resistivity map is drawn. In particular, the meta-stable state, induced between the insulating and the metallic thermodynamic states by current biasing Ca2RuO4 single crystals, is investigated. Such an analysis, combined with the study of the resulting RuO6 octahedra energy levels, reveals that a metallic crystal phase emerges in the meta-stable regime. The peculiar properties of such a phase, coexisting with the well-established orthorhombic insulating and tetragonal metallic phases, allow to explain some of the unconventional and puzzling behaviors observed in the experiments, as a negative differential resistivity.

Physical Review Letters, 2019

Physica B: Condensed Matter, 2018

We present magnetization measurements on Sr 4 Ru 3 O 10 as a function of temperature and magnetic... more We present magnetization measurements on Sr 4 Ru 3 O 10 as a function of temperature and magnetic field applied perpendicular to the magnetic easy c-axis inside the ferromagnetic phase. Peculiar metamagnetism evolves in Sr 4 Ru 3 O 10 below the ferromagnetic transition T C as a double step in the magnetization at two critical fields H c1 and H c2. We map the H − T phase diagram with special focus on the temperature range 50 K ≤ T ≤ T C. We find that the critical field H c1 (T) connects the field and temperature axes of the phase diagram, whereas the H c2 boundary starts at 2.8 T for the lowest temperatures and ends in a critical endpoint at (1 T; 80 K). We conclude from the temperature dependence of the ratio Hc1 Hc2 (T) that the double metamagnetic transition is an intrinisc effect of the material and it is not caused by sample stacking faults such as twinning or partial in-plane rotation between layers.

Femtosecond laser surface structuring of silicon with Gaussian and optical vortex beams

Applied Surface Science, 2017

Abstract We report an experimental analysis of femtosecond laser induced surface structuring of s... more Abstract We report an experimental analysis of femtosecond laser induced surface structuring of silicon by exploiting both Gaussian and Optical Vortex beams. In particular, we show how different surface patterns, consisting of quasi-periodic ripples and grooves, can be obtained by using different states of polarization offered by optical vortex beams. Both for Gaussian and optical vortex beams, an increase of the number of laser pulses, N, or beam energy, E0, leads to a progressive predominance of the grooves coverage, with ripples confined in specific regions of the irradiated area at lower fluence. The average period of ripples and grooves shows a different dependence as a function of both E0 and N, underlying important differences in mechanisms leading to the formation of ripples and grooves. In particular, our experimental characterization allows identifying a preliminary stage of grooves generation with rudimental surface structures, preferentially directed parallel to the laser polarization. This supports the idea that one possible mechanism of grooves formation lies in the progressive aggregation of clusters of nanoparticles densely decorating the ripples. Our experimental findings provide important indications on the basic understanding of the processes involved in laser surface structuring with ultrashort pulses that can guide the design of the surface patterns.

Journal of Applied Physics, 2014

Laser surface structuring of copper is induced by laser ablation with a femtosecond optical vorte... more Laser surface structuring of copper is induced by laser ablation with a femtosecond optical vortex beam generated via spin-to-orbital conversion of the angular momentum of light by using a q-plate. The variation of the produced surface structures is studied as a function of the number of pulses, N, and laser fluence, F. After the first laser pulse (N ¼ 1), the irradiated surface presents an annular region characterized by a corrugated morphology made by a rather complex network of nanometer-scale ridges, wrinkles, pores, and cavities. Increasing the number of pulses (2 < N < 100), the surface texture progressively evolves towards larger structures, while the central, non-ablated area is gradually decorated by nanoparticles produced during laser ablation. At large number of pulses (200 < N < 1000), a micro-tip with a nanostructured surface forms in the center of the irradiated area, which eventually disappears at still larger number of pulses (N > 1000) and a deep crater is formed. The nanostructure variation with the laser fluence, F, also evidences an interesting dependence, with a coarsening of the structure morphology as F increases. Our experimental findings demonstrate that direct femtosecond laser ablation with optical vortex beams produces interesting patterns not achievable by the more standard beams with a Gaussian intensity profile. They also suggest that appropriate tuning of the experimental conditions (F, N) can allow generating micro-and/or nano-structured surface for any specific application. V

Nature Communications, 2017

A paradigmatic case of multi-band Mott physics including spin-orbit and Hund’s coupling is realiz... more A paradigmatic case of multi-band Mott physics including spin-orbit and Hund’s coupling is realized in Ca2RuO4. Progress in understanding the nature of this Mott insulating phase has been impeded by the lack of knowledge about the low-energy electronic structure. Here we provide—using angle-resolved photoemission electron spectroscopy—the band structure of the paramagnetic insulating phase of Ca2RuO4 and show how it features several distinct energy scales. Comparison to a simple analysis of atomic multiplets provides a quantitative estimate of the Hund’s coupling J=0.4 eV. Furthermore, the experimental spectra are in good agreement with electronic structure calculations performed with Dynamical Mean-Field Theory. The crystal field stabilization of the dxy orbital due to c-axis contraction is shown to be essential to explain the insulating phase. These results underscore the importance of multi-band physics, Coulomb interaction and Hund’s coupling that together generate the Mott insu...

Physical Review B, 2011

The identification of electronic states and the analysis of their evolution with n is key to unde... more The identification of electronic states and the analysis of their evolution with n is key to understanding n-layered ruthenates. To this end, we combine polarization-dependent O 1s x-ray absorption spectroscopy, high-purity Sr n+1 Ru n O 3n+1 (n = 1,2,3) single crystals, and ab initio and many-body calculations. We find that the energy splitting between the empty x 2 − y 2 and 3z 2 − 1 state is considerably smaller than previously suggested and that, remarkably, Sr bands are essential to understanding the spectra. At low energy, we identify the main difference among the materials with a substantial rearrangement of t 2g orbital occupations with increasing n. This rearrangement is controlled by the interplay of Coulomb repulsion, dimensionality, and changes in the t 2g crystal field.