Engineering topics

Sacuto, A.

Publications and source records attributed to Sacuto, A..

4 records

DOE OSTI2022

Spin singlet and quasiparticle excitations in cuprate superconductors

We followed step by step the transition from an antiferromagnetic (AFM) Mott insulator to a superconducting (SC) metal in the Bi 2 Sr 2 CaCu 2 O 8 + δ (Bi-2212) cuprate using electronic Raman scattering spectroscopy. This was achieved by tracking the doping dependence of the spin singlet excitation (SSE) originating from the AFM Mott insulator, the normal-state quasiparticle excitation related to the mobile charge carriers, and the Bogoliubov quasiparticles related to the SC gap. We show that the signature of the pseudogap phase which develops during this transition can be interpreted as the blocking of charge carriers by the enhancement of the AFM correlations as the temperature drops. We find that the energy scale of the pseudogap, Δ pg (p), closely follows that of the SSE, Δ sse (p) with doping p. The quasiparticle lifetime considerably increases with doping when the pseudogap collapses. We reveal that the maximum amplitude of the SC gap Δ$^{ma}_{sc}$ and the SC transition temperature T c are linked in an extended range of doping, such as Δ$^{ma}_{sc}$(p)∝Δ sse (p)T c (p). This relation suggests that the AFM correlations play a key role in the mechanism of superconductivity.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2021

Lattice-shifted nematic quantum critical point in FeSe 1-x S x

We report the evolution of nematic fluctuations in FeSe 1-x S x single crystals as a function of Sulfur content x across the nematic quantum critical point (QCP) x c ~ 0.17 via Raman scattering. The Raman spectra in the B1g nematic channel consist of two components, but only the low energy one displays clear fingerprints of critical behavior and is attributed to itinerant carriers. Curie–Weiss analysis of the associated nematic susceptibility indicates a substantial effect of nemato-elastic coupling, which shifts the location of the nematic QCP. We argue that this lattice-induced shift likely explains the absence of any enhancement of the superconducting transition temperature at the QCP. The presence of two components in the nematic fluctuations spectrum is attributed to the dual aspect of electronic degrees of freedom in Hund’s metals, with both itinerant carriers and local moments contributing to the nematic susceptibility.

36 MATERIALS SCIENCE↗

DOE OSTI2019

Nematic fluctuations in the cuprate superconductor Bi 2 Sr 2 CaCu 2 O 8+δ

Establishing the presence and the nature of a quantum critical point in their phase diagram is a central enigma of the high-temperature superconducting cuprates. It could explain their pseudogap and strange metal phases, and ultimately their high superconducting temperatures. Yet, while solid evidences exist in several unconventional superconductors of ubiquitous critical fluctuations associated to a quantum critical point, in the cuprates they remain undetected until now. Here using symmetry-resolved electronic Raman scattering in the cuprate Bi 2 Sr 2 CaCu 2 O 8+δ , we report the observation of enhanced electronic nematic fluctuations near the endpoint of the pseudogap phase. While our data hint at the possible presence of an incipient nematic quantum critical point, the doping dependence of the nematic fluctuations deviates significantly from a canonical quantum critical scenario. The observed nematic instability rather appears to be tied to the presence of a van Hove singularity in the band structure.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2019

A magnetic phase diagram for nanoscale epitaxial BiFeO 3 films

BiFeO 3 thin films have attracted considerable attention by virtue of their potential application in low-energy spintronic and magnonic devices. BiFeO 3 possesses an intricate magnetic structure, characterized by a spin cycloid with period ~62 nm that governs the functional magnonic response, and which can be modulated or even destroyed by strain, magnetic and electric fields, or chemical doping. The literature on (110)-oriented BiFeO 3 films is not explicit in defining the conditions under which this cycloid persists, as its presence depends on synthesis method and thin-film boundary conditions, especially in the sub-100 nm thickness regime. This report aims to end “trial and error” approaches in determining the conditions under which this cycloid and its associated functional magnonic response exist. We show that in specific crystallographic orientations of epitaxial BiFeO 3 , an unexplored strain parameter—the distortion in the ab plane of the monoclinic unit cell—significantly influences the spin structure. Combining Mössbauer spectroscopy and low-energy Raman spectroscopy with first-principles-based effective Hamiltonian calculations, we show that both average strain and this distortion destroy the cycloid. For films grown on (110)-oriented SrTiO 3 substrates, if the BiFeO 3 lattice parameters a and b differ by more than about 1.2%, the cycloid is destabilized, resulting in a pseudocollinear magnetic order ground state. We are thereby able to construct a phase diagram of the spin structure for nanoscale epitaxial BiFeO 3 films, which aims to resolve long-standing literature inconsistencies and provide powerful guidelines for the design of future magnonic and spintronic devices.

36 MATERIALS SCIENCE↗