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1. Structural phase purification of bulk HfO 2 :Y through pressure cycling

   https://doi.org/10.1073/pnas.2312571121  

   Musfeldt, J. L. ; Singh, Sobhit ; Fan, Shiyu ; Gu, Yanhong ; Xu, Xianghan ; Cheong, S.-W. ; Liu, Z. ; Vanderbilt, David ; Rabe, Karin M. ( January 2024 , Proceedings of the National Academy of Sciences)

   We combine synchrotron-based infrared absorption and Raman scattering spectroscopies with diamond anvil cell techniques and first-principles calculations to explore the properties of hafnia under compression. We find that pressure drives HfO${}_2$:7%Y from the mixed monoclinic ($P{2}_1/c$)$+$antipolar orthorhombic ($\mathit{Pbca}$) phase to pure antipolar orthorhombic ($\mathit{Pbca}$) phase at approximately 6.3 GPa. This transformation is irreversible, meaning that upon release, the material is kinetically trapped in the$\mathit{Pbca}$metastable state at 300 K. Compression also drives polar orthorhombic ($Pca{2}_1$) hafnia into the tetragonal ($P{4}_2/nmc$) phase, although the latter is not metastable upon release. These results are unified by an analysis of the energy landscape. The fact that pressure allows us to stabilize targeted metastable structures with less Y stabilizer is important to preserving the flat phonon band physics of pure HfO${}_2$.

    

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2. Development of broad modulus profile upon polymer–polymer interface formation between immiscible glassy–rubbery domains

   https://doi.org/10.1073/pnas.2312533120  

   Gagnon, Yannic J ; Burton, Justin C ; Roth, Connie B ( January 2024 , Proceedings of the National Academy of Sciences)

   Interfaces of glassy materials such as thin films, blends, and composites create strong unidirectional gradients to the local heterogeneous dynamics that can be used to elucidate the length scales and mechanisms associated with the dynamic heterogeneity of glasses. We focus on bilayer films of two different polymers with very different glass transition temperatures ($T_{\text{g}}$) where previous work has demonstrated a long-range (∼200 nm) profile in local$T_{\text{g}}(z)$is established between immiscible glassy and rubbery polymer domains when the polymer–polymer interface is formed to equilibrium. Here, we demonstrate that an equally long-ranged gradient in local modulus$\tilde{G}(z)$is established when the polymer–polymer interface ($\approx$5 nm) is formed between domains of glassy polystyrene (PS) and rubbery poly(butadiene) (PB), consistent with previous reports of a broad$T_{\text{g}}(z)$profile in this system. A continuum physics model for the shear wave propagation caused by a quartz crystal microbalance across a PB/PS bilayer film is used to measure the viscoelastic properties of the bilayer during the evolution of the PB/PS interface showing the development of a broad gradient in local modulus$\tilde{G}(z)$spanning$\approx$180 nm between the glassy and rubbery domains of PS and PB. We suggest these broad profiles in$T_{\text{g}}(z)$and$\tilde{G}(z)$arise from a coupling of the spectrum of vibrational modes across the polymer–polymer interface as a result of acoustic impedance matching of sound waves with$\lambda \sim 5$nm during interface broadening that can then trigger density fluctuations in the neighboring domain.

    

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3. Quantum interference in superposed lattices

   https://doi.org/10.1073/pnas.2315787121  

   Feng, Yejun ; Wang, Yishu ; Rosenbaum, T. F. ; Littlewood, P. B. ; Chen, Hua ( February 2024 , Proceedings of the National Academy of Sciences)

   Charge transport in solids at low temperature reveals a material’s mesoscopic properties and structure. Under a magnetic field, Shubnikov–de Haas (SdH) oscillations inform complex quantum transport phenomena that are not limited by the ground state characteristics and have facilitated extensive explorations of quantum and topological interest in two- and three-dimensional materials. Here, in elemental metal Cr with two incommensurately superposed lattices of ions and a spin-density-wave ground state, we reveal that the phases of several low-frequency SdH oscillations in${\sigma }_{\mathit{xx}} \left({\rho }_{\mathit{xx}}\right)$and${\sigma }_{\mathit{yy}} \left({\rho }_{\mathit{yy}}\right)$are no longer identical but opposite. These relationships contrast with the SdH oscillations from normal cyclotron orbits that maintain identical phases between${\sigma }_{\mathit{xx}} \left({\rho }_{\mathit{xx}}\right)$and${\sigma }_{\mathit{yy}} \left({\rho }_{\mathit{yy}}\right)$ . We trace the origin of the low-frequency SdH oscillations to quantum interference effects arising from the incommensurate orbits of Cr’s superposed reciprocal lattices and explain the observed$\pi$-phase shift by the reconnection of anisotropic joint open and closed orbits.

    

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4. Two-dimensional turbulence above topography: Vortices and potential vorticity homogenization

   https://doi.org/10.1073/pnas.2308018120  

   Siegelman, Lia ; Young, William R. ( October 2023 , Proceedings of the National Academy of Sciences)

   The evolution of unforced and weakly damped two-dimensional turbulence over random rough topography presents two extreme states. If the initial kinetic energy$E$is sufficiently high, then the topography is a weak perturbation, and evolution is determined by the spontaneous formation and mutual interaction of coherent axisymmetric vortices. High-energy vortices roam throughout the domain and mix the background potential vorticity (PV) to homogeneity, i.e., in the region between vortices, which is most of the domain, the relative vorticity largely cancels the topographic PV. If$E$is low, then vortices still form but they soon become locked to topographic features: Anticyclones sit above topographic depressions and cyclones above elevated regions. In the low-energy case, with topographically locked vortices, the background PV retains some spatial variation. We develop a unified framework of topographic turbulence spanning these two extreme states of low and high energy. A main organizing concept is that PV homogenization demands a particular kinetic energy level$E_{♯}$.$E_{♯}$is the separator between high-energy evolution and low-energy evolution.

    

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5. Quantifying the ultrafast and steady-state molecular reduction potential of a plasmonic photocatalyst

   https://doi.org/10.1073/pnas.2305932120  

   Warkentin, Christopher L. ; Frontiera, Renee R. ( October 2023 , Proceedings of the National Academy of Sciences)

   Plasmonic materials are promising photocatalysts as they are well suited to convert light into hot carriers and heat. Hot electron transfer is suggested as the driving force in many plasmon-driven reactions. However, to date, there are no direct molecular measures of the rate and yield of plasmon-to-molecule electron transfer or energy of these electrons on the timescale of plasmon decay. Here, we use ultrafast and spectroelectrochemical surface-enhanced Raman spectroscopy to quantify electron transfer from a plasmonic substrate to adsorbed methyl viologen molecules. We observe a reduction yield of 2.4 to 3.5% on the picosecond timescale, with plasmon-induced potentials ranging from$-$3.1 to$-$4.5 mV. Excitingly, some of these reduced species are stabilized and persist for tens of minutes. This work provides concrete metrics toward optimizing material–molecule interactions for efficient plasmon-driven photocatalysis.

    

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