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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. Measurement of the dynamic charge susceptibility near the charge density wave transition in ErTe ₃

   https://doi.org/10.1073/pnas.2424430122  

   Chaudhuri, Dipanjan; Jiang, Qianni; Guo, Xuefei; Chen, Jin; Kengle, Caitlin S; Hoveyda-Marashi, Farzaneh; Bernal-Choban, Camille; de_Vries, Niels; Chiang, Tai-Chang; Fradkin, Eduardo; et al (June 2025, Proceedings of the National Academy of Sciences)

   A charge density wave (CDW) is a phase of matter characterized by a periodic modulation of valence electron density coupled with lattice distortion. Its formation is closely tied to the dynamical charge susceptibility, $\chi (q,\omega )$, which reflects the collective electron dynamics of the material. Despite decades of study, $\chi (q,\omega )$near a CDW transition has never been measured at nonzero momentum, $q$, with meV energy resolution. Here, we investigate the canonical CDW transition in ErTe ${}_3$using momentum-resolved electron energy loss spectroscopy, a technique uniquely sensitive to valence band charge excitations. Unlike phonons, which soften via the Kohn anomaly, we find the electronic excitations exhibit purely relaxational dynamics well described by a diffusive model, with the diffusivity peaking just below the critical temperature, $T_{C1}$. Additionally, we report for the first time a divergence in the real part of $\chi (q,\omega )$in the static limit ( $\omega \to 0$), a long-predicted hallmark of CDWs. Unexpectedly, this divergence occurs as $T\to 0$, with only a weak thermodynamic signature at $T=T_{C1}$. Our study necessitates a reexamination of the traditional description of CDW formation in quantum materials. 

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3. 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 $T_{\text{g}}\left(z\right)$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 $\tilde{G}\left(z\right)$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 $T_{\text{g}}\left(z\right)$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 $\tilde{G}\left(z\right)$spanning $\approx$180 nm between the glassy and rubbery domains of PS and PB. We suggest these broad profiles in $T_{\text{g}}\left(z\right)$and $\tilde{G}\left(z\right)$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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4. Anyon delocalization transitions out of a disordered fractional quantum anomalous Hall insulator

   https://doi.org/10.1073/pnas.2520608122  

   Shi, Zhengyan Darius; Senthil, T (December 2025, Proceedings of the National Academy of Sciences)

   Motivated by the experimental discovery of the fractional quantum anomalous Hall effect, we develop a theory of doping-induced transitions out of the $\nu =2/3$lattice Jain state in the presence of quenched disorder. We show that disorder strongly affects the evolution into the conducting phases described in our previous work. The delocalization of charge $2/3$anyons leads to a chiral superconductor through a direct second-order transition for a smooth random potential with long-wavelength modulations. The longitudinal resistance has a universal peak at the associated quantum critical point. Close to the transition, we show that the superconducting ground state is an “Anomalous Vortex Glass” stabilized in the absence of an external magnetic field. For short-wavelength disorder, this transition generically splits into three distinct ones with intermediate insulating topological phases. If instead, the charge $1/3$anyon delocalizes, then at low doping the resulting phase is a Reentrant Integer Quantum Hall state with ${\rho }_{\mathit{xy}}=h/e^2$. At higher doping this undergoes a second transition to a Fermi liquid metal. We show that this framework provides a plausible explanation for the complex phase diagram recently observed in twisted MoTe₂near $\nu =2/3$and discuss future experiments that can test our theory in more detail. 

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5. Fluctuation theorems for autonomous work

   https://doi.org/10.1073/pnas.2524775122  

   Jarzynski, Christopher; Deffner, Sebastian; Rahav, Saar (December 2025, Proceedings of the National Academy of Sciences)

   Classical fluctuation theorems for work have been obtained theoretically, and verified experimentally, within a nonautonomous framework in which work is performed on a system of interest, $\mathcal{S}$, by the external manipulation of a work parameter, such as a piston’s position. Here, we obtain fluctuation theorems within an autonomous framework in which $\mathcal{S}$exchanges energy with a reversible work source, $\mathcal{R}$. The two subsystems, $\mathcal{R}$and $\mathcal{S}$, interact with one another as they evolve under Hamiltonian or stochastic dynamics, without external intervention. In this setting, we must account for the backaction of $\mathcal{S}$on $\mathcal{R}$, which is absent in the nonautonomous setting. We obtain autonomous versions of standard fluctuation theorems for work and entropy production. In each case, we argue, the autonomous fluctuation theorem reduces to its nonautonomous counterpart when $\mathcal{R}$’s inertia becomes infinitely large. 

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