Siyu Yan

Abstract:

2D lead halide perovskites (2DPs) offer chemical compatibility with 3D perovskites and enhanced stability, which are attractive for applications in photovoltaic and light‐emitting devices. However, such lowered structural dimensionality causes increased excitonic effects and highly anisotropic charge‐carrier transport. Determining the diffusivity of excitations, in particular for out‐of‐plane or inter‐layer transport, is therefore crucial, yet challenging to achieve. Here, an effective method is demonstrated for monitoring inter‐layer diffusion of photoexcitations in (PEA)2PbI4 thin films by tracking time‐dependent changes in photoluminescence spectra induced by photon reabsorption effects. Selective photoexcitation from either substrate‐ or air‐side of the films reveals differences in diffusion dynamics encountered through the film profile. Time‐dependent diffusion coefficients are extracted from spectral dynamics through a 1D diffusion model coupled with an interference correction for refractive index variations arising from the strong excitonic resonance of 2DPs. Such analysis, together with structural probes, shows that minute misalignment of 2DPs planes occurs at distances far from the substrate, where efficient in‐plane transport consequently overshadows the less efficient out‐of‐plane transport in the direction perpendicular to the substrate. Through detailed analysis, a low out‐of‐plane excitation diffusion coefficient of (0.26 ± 0.03) ×10−4 cm2 s−1 is determined, consistent with a diffusion anisotropy of ≈4 orders of magnitude.

Abstract:

Deformation of metal halide perovskite can induce many interesting properties. This study focuses on a manganese-based organic-inorganic perovskite with a unique structure in which tetrahedral and octahedral coordination coexist in single crystal unit cell. This perovskite emits at 519 and 615 nm at room temperature. In contrast to conventional perovskites, this perovskite regulates the Dexter energy transfer between the two coordination modes through asymmetric deformation without phase transition, producing a reversible and tunable photoluminescence. Notably, under atmospheric pressure, as temperature increases from liquid nitrogen temperature to 135 °C, the luminescence color shifts progressively from red with a CIE coordinate of (0.59, 0.27) to yellow green with a CIE coordinate of (0.33, 0.56), with excellent reversibility. Additionally, at room temperature, the luminescence color shifts progressively from orange with a CIE coordinate of (0.54, 0.42) to red with a CIE coordinate of (0.61, 0.27) as pressure increases from 1 atm to 7.5 GPa. This novel tetrahedral and octahedral coexisting perovskite has a temperature-pressure equivalence effect in modulating luminescent color changes. It tunes emission by forming asymmetric deformations through the contraction (or expansion) of tetrahedra and expansion (or contraction) of octahedra upon stimulation, providing a new pathway to tune the emission of perovskites.

Abstract:

A search is presented for the pair production of higgsinos 𝜒˜ in gauge-mediated supersymmetry models, where the lightest neutralinos 𝜒˜ 0 1 decay into a light gravitino 𝐺˜ either via a Higgs ℎ or 𝑍 boson. The search is performed with the ATLAS detector at the Large Hadron Collider using 139 fb−1 of proton–proton collisions at a centre-of-mass energy of √ 𝑠 = 13 TeV. It targets final states in which a Higgs boson decays into a photon pair, while the other Higgs or 𝑍 boson decays into a 𝑏𝑏¯ pair, with missing transverse momentum associated with the two gravitinos. Search regions dependent on the amount of missing transverse momentum are defined by the requirements that the diphoton mass should be consistent with the mass of the Higgs boson, and the 𝑏𝑏¯ mass with the mass of the Higgs or 𝑍 boson. The main backgrounds are estimated with data-driven methods using the sidebands of the diphoton mass distribution. No excesses beyond Standard Model expectations are observed and higgsinos with masses up to 320 GeV are excluded, assuming a branching fraction of 100% for 𝜒˜ 0 1 → ℎ𝐺˜. This analysis excludes higgsinos with masses of 130 GeV for branching fractions to ℎ𝐺˜ as low as 36%, thus providing complementarity to previous ATLAS searches in final states with multiple leptons or multiple 𝑏-jets, targeting different decays of the electroweak bosons.

Abstract:

Lattice dynamics are critical to photovoltaic material performance, governing dynamic disorder, hot-carrier cooling, charge-carrier recombination, and transport. Soft metal-halide perovskites exhibit particularly intriguing dynamics, with Raman spectra exhibiting an unusually broad low-frequency response whose origin is still much debated. Here, we utilize ultra-low frequency Raman and infrared terahertz time-domain spectroscopies to provide a systematic examination of the vibrational response for a wide range of metal-halide semiconductors: FAPbI3, MAPbI x Br3–x , CsPbBr3, PbI2, Cs2AgBiBr6, Cu2AgBiI6, and AgI. We rule out extrinsic defects, octahedral tilting, cation lone pairs, and “liquid-like” Boson peaks as causes of the debated central Raman peak. Instead, we propose that the central Raman response results from an interplay of the significant broadening of Raman-active, low-energy phonon modes that are strongly amplified by a population component from Bose–Einstein statistics toward low frequency. These findings elucidate the complexities of light interactions with low-energy lattice vibrations in soft metal-halide semiconductors emerging for photovoltaic applications.