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1. Low mechanical loss and high refractive index in amorphous ${\mathrm{Ta}}_2{\mathrm{O}}_5$ films grown by magnetron sputtering

   https://doi.org/10.1103/PhysRevMaterials.8.035603  

   Molina-Ruiz, M; Shukla, K; Ananyeva, A; Vajente, G; Abernathy, M R; Metcalf, T H; Liu, X; Markosyan, A; Bassiri, R; Fejer, M M; et al (March 2024, Physical Review Materials)

   The ability to observe astronomical events through the detection of gravitational waves relies on the quality of multilayer coatings used on the optical mirrors of interferometers. Amorphous Ta2O5 (including TiO2:Ta2O5) currently limits detector sensitivity due to high mechanical loss. In this paper, mechanical loss measured at both cryogenic and room temperatures of amorphous Ta2O5 films grown by magnetron sputtering and annealed in air at 500 ◦C is shown to decrease for elevated growth temperature. Films grown at 310 ◦C and annealed yield a mechanical loss of 3.1×10−4 at room temperature, the lowest value reported for pure amorphous Ta2O5 grown by magnetron sputtering to date, and comparable to the lowest values obtained for films grown by ion beam sputtering. Additionally, the refractive index n increases 6% for elevated growth temperature, which could lead to improved sensitivity of gravitational-wave detectors by allowing a thickness reduction in the mirrors’ coatings. Structural characterization suggests that the observed mechanical loss reduction in amorphous Ta2O5 films with increasing growth temperature correlates with a reduction in the coordination number between oxygen and tantalum atoms, consistent with TaOx polyhedra with increased corner-sharing and reduced edge- and facesharing structures. 

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2. Unveiling the cation ratio mediated structural modifications in TiO ₂ :GeO ₂ mixtures for gravitational-wave detectors

   https://doi.org/10.1088/1361-6382/ad3c08  

   Bhowmick, Sangita; Osovsky, Ruth; Davenport, Aaron; Schiettekatte, François; Chicoine, Martin; Menoni, Carmen S (April 2024, Classical and Quantum Gravity)

   Abstract Amorphous thin films of Ti doped GeO₂are of interest for coatings of the mirrors in gravitational wave detectors (GWDs) due to their low internal friction (Vajenteet al2021Phys. Rev. Lett.127071101). The addition of Ti to amorphous GeO₂(a-GeO₂) enables tailoring of the optical and structural properties of the mixtures. However, the specific modifications that occur in the amorphous network with the addition of Ti are not known. In this work, x-ray photoelectron spectroscopy is used to identify modifications to the bonding of Ge and Ti atoms in mixtures of Ti dopeda-GeO₂with different Ti cation content. The formation of (Ti–O–Ge) bonds is evidenced from: (1) the presence of a peak which intensity increases with Ti content and causes a shift to lower binding energy (BE) of the core level O 1speak; (2) the shift to higher BE of the Ti 2p_3/2peak and a decrease in the energy split; and (3) the shift to lower BE of the Ge 3d_5/2peak and increase in the energy split. These changes reflect modifications to the bonding when Ge replaces Ti in Ti–O–Ti bonds and Ti replaces Ge in Ge–O–Ge bonds due to their difference in electronegativity. A decrease in the O–O nearest-neighbour distance due to the incorporation of Ti atom is also observed from the broadening of the valence band spectra. The results show the 0.44 Ti dopeda-GeO₂mixture has a balance between the (Ti–O–Ge) and the (Ge–O–Ge) networks, not observed in Ti poor and Ti rich mixtures. This finding could have important consequences in the optimisation of amorphous Ti dopeda-GeO₂mixtures for low internal friction coatings of GWDs. 

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3. Valence transition theory of the pressure-induced dimensionality crossover in superconducting ${\mathrm{Sr}}_{14-x}{\mathrm{Ca}}_x{\mathrm{Cu}}_{24}{\mathrm{O}}_{41}$

   https://doi.org/10.1103/PhysRevB.108.134510  

   Song, Jeong-Pil; Clay, R. Torsten; Mazumdar, Sumit (October 2023, Physical Review B)

   Stephen E. Nagler (Ed.)

   One of the strongest justifications for the continued search for superconductivity within the single-band Hubbard Hamiltonian originates from the apparent success of single-band ladder-based theories in predicting the occurrence of superconductivity in the cuprate coupled-ladder compound Sr{14−x}Ca{x}Cu{24}O{41}. Recent theoretical works have, however, shown the complete absence of quasi-long-range superconducting correlations within the hole-doped multiband ladder Hamiltonian including realistic Coulomb repulsion between holes on oxygen sites and oxygen-oxygen hole hopping. Experimentally, superconductivity in Sr{14−x}Ca{x}Cu{24}O{41} occurs only under pressure and is preceded by dramatic transition from one to two dimensions that remains not understood. We show that understanding the dimensional crossover requires adopting a valence transition model within which there occurs transition in Cu-ion ionicity from +2 to +1 , with transfer of holes from Cu to O ions [S. Mazumdar, Phys. Rev. B 98, 205153 (2018)]. The driving force behind the valence transition is the closed-shell electron configuration of Cu^{1+} , a feature shared by cations of all oxides with a negative charge-transfer gap. We make a falsifiable experimental prediction for Sr{14−x}Ca{x}Cu{24}O{41} and discuss the implications of our results for layered two-dimensional cuprates. 

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4. Surface acoustic waves and lattice vibrations in two-dimensional ${\mathrm{Ti}}_3{\mathrm{C}}_2{T}_x$ $(T=\mathrm{O}, \mathrm{F})$ MXene films

   https://doi.org/10.1103/PhysRevB.111.214103  

   Hamilton, Parker; Kalia, Rajiv; Wixom, Ryan; Dingreville, Rémi (June 2025, Physical Review B)

   We investigated surface acoustic wave (SAW) propagation and lattice vibrations in two-dimensional (2D) titanium carbide $\left({\mathrm{Ti}}_3{\mathrm{C}}_2{T}_{\mathrm{x}}\right)$MXene films as a function of surface termination and layer stacking, using atomistic simulations. We found that SAW propagation velocity is highly sensitive to both single-layer properties and interlayer bonding. Surface terminations significantly modulate wave behavior, with oxygen and fluorine terminations producing distinct effects on wave propagation, with oxygen-terminated monolayers exhibiting 20% higher wave speeds than fluorine counterparts due to strengthened intralayer bonds. Key observations include the transition from one to two layers causing wave speed variations, and the development of interlayer modes that generate more dispersed lattice vibrations. As the film layer thickness increases, SAW propagation becomes predominantly confined to the upper surface, with coherence of vibrational modes diminishing in multilayer structures. These findings suggest MXene terminations and layer stacking are crucial parameters for controlling SAW behavior, offering promising avenues for novel acoustic wave device applications. Published by the American Physical Society2025 

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5. First-principles electron-phonon interactions and polarons in the parent cuprate ${\mathrm{La}}_2{\mathrm{CuO}}_4$

   https://doi.org/10.1103/PhysRevResearch.7.L012073  

   Chang, Benjamin K; Timrov, Iurii; Park, Jinsoo; Zhou, Jin-Jian; Marzari, Nicola; Bernardi, Marco (March 2025, Physical Review Research)

   Understanding electronic interactions in high-temperature superconductors is an outstanding challenge. In the widely studied cuprate materials, experimental evidence points to strong electron-phonon ( $e$-ph) coupling and broad photoemission spectra. Yet, the microscopic origin of this behavior is not fully understood. Here, we study $e$-ph interactions and polarons in a prototypical parent (undoped) cuprate, ${\mathrm{La}}_2{\mathrm{CuO}}_4$(LCO), by means of first-principles calculations. Leveraging parameter-free Hubbard-corrected density functional theory, we obtain a ground state with the band gap and Cu magnetic moment in nearly exact agreement with experiments. This enables a quantitative characterization of $e$-ph interactions. Our calculations reveal two classes of longitudinal optical (LO) phonons with strong $e$-ph coupling to hole states. These modes consist of bond stretching and bond bending in the Cu-O plane as well as vibrations of apical O atoms. The hole spectral functions, obtained with a cumulant method that can capture strong $e$-ph coupling, exhibit broad quasiparticle peaks with a small spectral weight ( $Z\approx 0.25$) and pronounced LO-phonon sidebands characteristic of polaron effects. Our calculations predict features observed in photoemission spectra, including a 40-meV peak in the $e$-ph coupling distribution function not explained by existing models. These results show that the universal strong $e$-ph coupling found experimentally in doped lanthanum cuprates is also present in the parent compound, and elucidate its microscopic origin. 

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