Surface Recombination in Ultra-Fast Carrier Dynamics of Perovskite Oxide La0.7Sr0.3MnO3 Thin Films

Electrostatic Shaping of Magnetic Transition Regions in La_{0.7}Sr_{0.3}MnO_{3}

We report a magnetic transition region in La_{0.7}Sr_{0.3}MnO_{3} with gradually changing magnitude of magnetization, but no rotation, stable at all temperatures below T_{C}. Spatially resolved magnetization, composition and Mn valence data reveal that the magnetic transition region is induced by a subtle Mn composition change, leading to charge transfer at the interface due to carrier diffusion and drift. The electrostatic shaping of the magnetic transition region is mediated by the Mn valence, which affects both magnetization by Mn^{3+}-Mn^{4+} double exchange interaction and free carrier concentration.

6-Aza-2-thio-thymine-gold nanoclusters: an excellent candidate in the photoelectrochemical field

The high performance of the photoelectrochemical (PEC) properties of AuNCs can be achieved with 6-aza-2-thio-thymine-AuNCs (ATT-AuNCs) as a photoactive material. The ATT-AuNCs yielded a cathodic photocurrent density as high as 88 μA cm^-2 with O₂ as electron acceptor, which is three orders of magnitude higher than those of other AuNCs in aqueous solutions. Moreover, ATT-AuNCs also show a higher carrier density, shorter Debye length, and smaller depletion layer width than those of reported AuNCs. This work not only reveals the PEC performance and mechanism of ATT-AuNCs, but also establishes a framework for in-depth design and studying the PEC performance of AuNCs.

Contribution of the Sub-Surface to Electrocatalytic Activity in Atomically Precise La0.7 Sr0.3 MnO3 Heterostructures

Electrocatalytic reactions are known to take place at the catalyst/electrolyte interface. Whereas recent studies of size-dependent activity in nanoparticles and thickness-dependent activity of thin films imply that the sub-surface layers of a catalyst can contribute to the catalytic activity as well, most of these studies consider actual modification of the surfaces. In this study, the role of catalytically active sub-surface layers was investigated by employing atomic-scale thickness control of the La_0.7 Sr_0.3 MnO₃ (LSMO) films and heterostructures, without altering the catalyst/electrolyte interface. The activity toward the oxygen evolution reaction (OER) shows a non-monotonic thickness dependence in the LSMO films and a continuous screening effect in LSMO/SrRuO₃ heterostructures. The observation leads to the definition of an "electrochemically-relevant depth" on the order of 10 unit cells. This study on the electrocatalytic activity of epitaxial heterostructures provides new insight in designing efficient electrocatalytic nanomaterials and core-shell architectures.

Magnetism manipulated by ferroelectric polarization and epitaxial strain in a La0.75Sr0.25MnO3/BaTiO3 system

Exploring the manipulation of magnetism in perovskite oxides is scientifically interesting and of great technical importance in next-generation magnetic memory. Dual control of magnetism in superlattices through epitaxial strain and ferroelectric polarization may induce rich physical properties. In this work, we demonstrated a strong magnetoelectric coupling that appears in an La0.75Sr0.25MnO3/BaTiO3 superlattice. Reversible transitions in ferromagnetism, ferrimagnetism and anti-ferromagnetism, with strong magnetoelectric coupling, are achieved by precisely controlling the magnitude and spin-direction of the magnetic moments of Mn. Half-metallicity is demonstrated in the MnO2 layers, accompanied by the spin polarization of the superlattice varying from 100% to 0%. We realize the coexistence of ferroelectric polarization and metallicity, i.e., "ferroelectric metal". The variation in strain and re-orientation of polarization lead to a change in interfacial Ti-O and Mn-O bond lengths, and hence a hybridization state, determining the magnetism of our system. The purpose-designed LSMO/BTO superlattice with intrinsic magnetoelectric coupling is a particularly interesting model system that can provide guidance for the development of spintronic devices.

Application of wavelet analysis on transient reflectivity in ultra-thin films

Applications of wavelet analysis in ultra-thin film transient reflectivity (TR) measurements have been investigated. Advantages of utilizing different localized wavelet bases, in position and time, have been addressed on the residual TR signals. Morse wavelets have been used to obtain information from the abrupt oscillatory modes in the signal, which are not distinguishable with conventional methods such as Fourier transforms. These abrupt oscillatory modes are caused by the surface, interface, or any short-lived oscillatory modes which are suppressed in the TR signal in ultra-thin films. It is demonstrated that by choosing different Morse wavelets, information regarding different oscillatory modes in the TR signal of a heterostructure thin film is achievable. Moreover, by performing wavelet analysis on multiferroic heterostructures, oscillatory modes with very close energy ranges are easily distinguishable. For illustration, residuals of the TR signals have been obtained by a pump-probe setup in reflectivity mode on La_0.7Sr_0.3MnO₃/SrTiO₃ and BaTiO₃/La_0.7Sr_0.3MnO₃/SrTiO₃ samples, where sufficient signal to noise ratios have been achieved by taking multiple scans. The residual signals have been analyzed with Morse wavelets, and multiple oscillatory modes with close energy ranges have been observed and distinguished. This approach can isolate the location of various oscillatory modes at the surface, interface and in the bulk of the heterostructure sample.