Unveiling the nature of the bosonic excitations that mediate the formation of Cooper pairs is a key issue for understanding unconventional superconductivity. A fundamental step toward this goal would be to identify the relative weight of the electronic and phononic contributions to the overall frequency (Ω) dependent bosonic function, Π(Ω). We perform optical spectroscopy on Bi2Sr2Ca0.92Y0.08Cu2O 8+δ crystals with simultaneous time-and frequency-resolution; this technique allows us to disentangle the electronic and phononic contributions by their different temporal evolution. The strength of the interaction (λ∼1.1) with the electronic excitations and their spectral distribution fully account for the high critical temperature of the superconducting phase transition. [7]. Inelastic neutron and X-ray scattering experiments found evidence for both QP-phonon anomalies [8] and bosonic excitations attributed to spin fluctuations [7,9] and loop currents [10]. Dip features in tunnelling experiments have been used to alternatively support the scenarios of dominant electron-phonon interactions [11] or antiferromagnetic spin fluctuations [12]. The frequency-dependent dissipation of the Drude optical conductivity, σ (ω), measured by equilibrium optical spectroscopies, has been interpreted [13][14][15] as the coupling of electrons to bosonic excitations, in which the separation of the phononic and electronic contributions is impeded by their partial coexistence on the same energy scale (<90 meV).We disentangle the electronic and phononic contributions to Π(Ω) through a non-equilibrium optical spectroscopy, in which the femtosecond time-resolution is combined with an energy-resolution smaller than 10 meV, over a wide photon energy range (0.5-2 eV). Our approach is based on the widely-used assumption [16,17] that, after the interaction between a superconductor and a short laser pulse (1.55 eV photon energy), the effective electronic temperature (T e ) relaxes toward its equilibrium value through energy exchange with the different degrees of freedom that linearly contribute to Π(Ω). In a more formal description, the total bosonic function is given by Π(Ω)=Π be (Ω)+Π SCP (Ω)+Π lat (Ω) where Π be refers to the bosonic excitations of electronic origin at the effective temperature T be , Π SCP to the small fraction of strongly-coupled phonons (SCPs) at T SCP [16] and Π lat to all other * Electronic address: s.f.p.dalconte@tue.nl † Electronic address: claudio.giannetti@unicatt.it
One of the pivotal questions in the physics of high-temperature superconductors is whether the low-energy dynamics of the charge carriers is mediated by bosons with a characteristic timescale. This issue has remained elusive as electronic correlations are expected to greatly accelerate the electron-boson scattering processes, confining them to the very femtosecond timescale that is hard to access even with state-of-the-art ultrafast techniques. Here we simultaneously push the time resolution and frequency range of transient reflectivity measurements up to an unprecedented level, enabling us to directly observe the ∼16 fs build-up of the e ective electron-boson interaction in hole-doped copper oxides. This extremely fast timescale is in agreement with numerical calculations based on the t-J model and the repulsive Hubbard model, in which the relaxation of the photo-excited charges is achieved via inelastic scattering with short-range antiferromagnetic excitations.A fter almost 30 years of intensive experimental and theoretical efforts to understand the origin of high-temperature superconductivity in copper oxides, a consensus about the microscopic process responsible for the superconducting pairing is still lacking. The large Coulomb repulsion U 1 eV between two electrons occupying the same lattice site is believed to have fundamental consequences for the normal state of these systems 1 , and it is not clear whether a BCS-like bosonic glue that mediates the electron interactions and eventually leads to pairing can still be defined [2][3][4] . The fundamental issue can be reduced to the question whether the electronic interactions are essentially unmediated and instantaneous, or whether the low-energy physics, including superconductivity, can be effectively described in terms of interactions among the fermionic charge carriers mediated by the exchange of bosons. The problem can be rationalized by considering the Hubbard model, in which the instantaneous virtual hopping of holes into already occupied sites (with an energy cost of U ) inherently favours an antiferromagnetic (AF) coupling J = 4t 2 h /U between neighbouring sites, where t h is the nearestneighbour hopping energy. As a consequence, antiferromagnetic fluctuations with a high-energy cutoff of 2J U naturally emerge as a candidate 5 for mediating the low-energy electronic interactions, on a characteristic retarded timescale of the order ofh/2J .In principle, time-resolved optical spectroscopy 6 may be used to prove the existence of an effective retarded boson-mediated interaction, provided that the temporal resolution is of the order of the inverse bosonic-fluctuation scale (for example,h/2J for AF fluctuations) and the optical properties are probed over a sufficiently broad frequency range, to extract the dynamics of the electron-boson coupling. Recent advances in ultrafast optical spectroscopy have succeeded in separately fulfilling these requirements. For example, high-temporal-resolution (<15 fs) experiments 7,8 have been carried out to investigate the...
High-frequency surface acoustic waves can be generated by ultrafast laser excitation of nanoscale patterned surfaces. Here we study this phenomenon in the hypersonic frequency limit. By modeling the thermomechanics from first-principles, we calculate the system’s initial heat-driven impulsive response and follow its time evolution. A scheme is introduced to quantitatively access frequencies and lifetimes of the composite system’s excited eigenmodes. A spectral decomposition of the calculated response on the eigemodes of the system reveals asymmetric resonances that result from the coupling between surface and bulk acoustic modes. This finding allows evaluation of impulsively excited pseudosurface acoustic wave frequencies and lifetimes and expands our understanding of the scattering of surface waves in mesoscale metamaterials. The model is successfully benchmarked against time-resolved optical diffraction measurements performed on one-dimensional and two-dimensional surface phononic crystals, probed using light at extreme ultraviolet and near-infrared wavelengths.
The mechanical vibrations of individual gold nanodisks nanopatterned on a sapphire substrate are investigated using ultrafast time-resolved optical spectroscopy. The number and characteristics of the detected acoustic modes are found to vary with nanodisk geometry. In particular, their quality factors strongly depend on nanodisk aspect ratio (i.e., diameter over height ratio), reaching a maximal value of ≈70, higher than those previously measured for substrate-supported nano-objects. The peculiarities of the detected acoustic vibrations are confirmed by finite-element simulations, and interpreted as the result of substrate-induced hybridization between the vibrational modes of a nanodisk. The present findings demonstrate novel possibilities for engineering the vibrational modes of nano-objects.
The ultrafast thermal and mechanical dynamics of a two-dimensional lattice of metallic nanodisks has been studied by near infrared pump-probe diffraction measurements, over a temporal range spanning from 100 fs to several nanoseconds. The experiments demonstrate that, in these systems, a two-dimensional surface acoustic wave (2DSAW), with a wavevector given by the reciprocal periodicity of the array, can be excited by ∼120 fs Ti:sapphire laser pulses. In order to clarify the interaction between the nanodisks and the substrate, numerical calculations of the elastic eigenmodes and simulations of the thermodynamics of the system are developed through finite-element analysis. At this light, we unambiguously show that the observed 2DSAW velocity shift originates from the mechanical interaction between the 2DSAWs and the nano-disks, while the correlated 2DSAW damping is due to the energy radiation into the substrate.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.