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1. Low-energy electrodynamics and a hidden Fermi liquid in the heavy-fermion compound ${\mathrm{CeCoIn}}_5$

   https://doi.org/10.1103/PhysRevB.111.165113  

   Shi, L Y; Tagay, Zhenisbek; Liang, Jiahao; Duong, Khoan; Wu, Yi; Ronning, F; Schlom, Darrell G; Shen, K M; Armitage, N P (April 2025, Physical Review B)

   We present time-domain THz spectroscopy of thin films of the heavy-fermion superconductor CeCoIn5. Below the ≈40 K Kondo coherence temperature, a narrow Drude-like peak forms, as a result of the 𝑓-orbital–conduction-electron hybridization and the formation of the heavy-fermion state. The complex optical conductivity is analyzed through a Drude model and extended Drude model analysis. Via the extended Drude model analysis, we measure the frequency-dependent scattering rate (1/𝜏) and effective mass (𝑚*/𝑚𝑏). This scattering rate shows a linear dependence on temperature, which matches the dependence of the resistivity as expected. Nevertheless, the width of the low-frequency Drude peak itself that is set by the renormalized quasiparticle scattering rate (1/𝜏*=𝑚𝑏/𝑚*⁢𝜏) shows a 𝑇^2 dependence. This is the scattering rate that characterizes the relaxation time of the renormalized quasiparticles. This gives evidence for a Fermi liquid state, which in conventional transport experiments is hidden by the strong temperature dependent mass. 

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2. Controllable suppression of the unconventional superconductivity in bulk and thin-film ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ via high-energy electron irradiation

   https://doi.org/10.1103/PhysRevResearch.6.033178  

   Ruf, Jacob P; Noad, Hilary_M L; Grasset, Romain; Miao, Ludi; Zhakina, Elina; McGuinness, Philippa H; Nair, Hari P; Schreiber, Nathaniel J; Kikugawa, Naoki; Sokolov, Dmitry; et al (August 2024, Physical Review Research)

   In bulk ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$, the strong sensitivity of the superconducting transition temperature $T_{\text{c}}$to nonmagnetic impurities provides robust evidence for a superconducting order parameter that changes sign around the Fermi surface. In superconducting epitaxial thin-film ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$, the relationship between $T_{\text{c}}$and the residual resistivity ${\rho }_0$, which in bulk samples is taken to be a proxy for the low-temperature elastic scattering rate, is far less clear. Using high-energy electron irradiation to controllably introduce point disorder into bulk single-crystal and thin-film ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$, we show that $T_{\text{c}}$is suppressed in both systems at nearly identical rates. This suggests that part of ${\rho }_0$in films comes from defects that do not contribute to superconducting pairbreaking and establishes a quantitative link between the superconductivity of bulk and thin-film samples. Published by the American Physical Society2024 

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3. Low leakage current in heteroepitaxial ${\mathrm{Al}}_{0.7}{\mathrm{Sc}}_{0.3}\mathrm{N}$ ferroelectric films on $\mathrm{Ga}\mathrm{N}$

   https://doi.org/10.1103/PhysRevApplied.23.014036  

   Yazawa, Keisuke; Evans, Charles; Dickey, Elizabeth C; Tellekamp, M Brooks; Brennecka, Geoff L; Zakutayev, Andriy (January 2025, Physical Review Applied)

   Wurtzite $(\mathrm{Al},\mathrm{Sc})\mathrm{N}$ferroelectrics are attractive for microelectronics applications due to their chemical and structural compatibility with wurtzite semiconductors, such as $\mathrm{Ga}\mathrm{N}$and $(\mathrm{Al},\mathrm{Ga})\mathrm{N}$. However, the leakage current in epitaxial stacks reported to date should be reduced for reliable device operation. Here, we demonstrate low leakage current in epitaxial ${\mathrm{Al}}_{0.7}{\mathrm{Sc}}_{0.3}\mathrm{N}$films on $\mathrm{Ga}\mathrm{N}$with well-saturated ferroelectric hysteresis loops that are orders of magnitude lower (i.e., 0.07 A ${\mathrm{cm}}^{-2}$) than previously reported films (1–19 A ${\mathrm{cm}}^{-2}$) having similar or better structural characteristics. We also show that, for these high-quality epitaxial $(\mathrm{Al},\mathrm{Sc})\mathrm{N}$films, structural quality (edge and screw dislocations), as measured by diffraction techniques, is not the dominant contributor to leakage. Instead, the small leakage in our films is limited by thermionic emission across the interfaces, which is distinct from the large leakage due to trap-mediated bulk transport in the previously reported $(\mathrm{Al},\mathrm{Sc})\mathrm{N}$films. To support this conclusion, we show that ${\mathrm{Al}}_{0.7}{\mathrm{Sc}}_{0.3}\mathrm{N}$on lattice-matched ${\mathrm{In}}_{0.18}{\mathrm{Ga}}_{0.82}\mathrm{N}$buffers with improved structural characteristics but higher interface roughness exhibit increased leakage characteristics. This demonstration of low leakage current in heteroepitaxial $(\mathrm{Al},\mathrm{Sc})\mathrm{N}$films and understanding of the importance of interface barrier and surface roughness can guide further efforts toward improving the reliability of wurtzite ferroelectric devices. Published by the American Physical Society2025 

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4. Observation of an antiferromagnetic quantum critical point in high-purity LaNiO3

   https://doi.org/10.1038/s41467-020-15143-w  

   Liu, Changjiang; Humbert, Vincent F.; Bretz-Sullivan, Terence M.; Wang, Gensheng; Hong, Deshun; Wrobel, Friederike; Zhang, Jianjie; Hoffman, Jason D.; Pearson, John E.; Jiang, J. Samuel; et al (December 2020, Nature Communications)

   null (Ed.)

   Abstract Amongst the rare-earth perovskite nickelates, LaNiO 3 (LNO) is an exception. While the former have insulating and antiferromagnetic ground states, LNO remains metallic and non-magnetic down to the lowest temperatures. It is believed that LNO is a strange metal, on the verge of an antiferromagnetic instability. Our work suggests that LNO is a quantum critical metal, close to an antiferromagnetic quantum critical point (QCP). The QCP behavior in LNO is manifested in epitaxial thin films with unprecedented high purities. We find that the temperature and magnetic field dependences of the resistivity of LNO at low temperatures are consistent with scatterings of charge carriers from weak disorder and quantum fluctuations of an antiferromagnetic nature. Furthermore, we find that the introduction of a small concentration of magnetic impurities qualitatively changes the magnetotransport properties of LNO, resembling that found in some heavy-fermion Kondo lattice systems in the vicinity of an antiferromagnetic QCP. 

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5. Symmetry-driven large tunneling magnetoresistance in ${\mathrm{SrRuO}}_3$ magnetic tunnel junctions with perpendicular magnetic anisotropy

   https://doi.org/10.1103/PhysRevMaterials.8.L091401  

   Goossens, Anouk S; Samanta, Kartik; Jaman, Azminul; Boubaker, Wissem; van_Rijn, Job_J L; Tsymbal, Evgeny Y; Banerjee, Tamalika (September 2024, Physical Review Materials)

   Magnetic tunnel junctions (MTJs) that are comprised of epitaxially-grown complex oxides offer a versatile platform to control the symmetry of tunneling states and tailor magnetic anisotropy useful for practical applications. This work employs thin films of SrTiO3 as an insulating barrier deposited between two ferromagnetic SrRuO3 electrodes to form fully epitaxial MTJs and demonstrate these functionalities. Transport measurements demonstrate large tunneling magnetoresistance (TMR), significantly exceeding previously found values of TMR in MTJs based on SrRuO3 electrodes. These results are explained by perpendicular magnetic anisotropy of SrRuO3 and matching (mismatching) between symmetry and spin across the SrTiO3/SrRuO3 (001) interface for the parallel (antiparallel) MTJ magnetization state, supported by density functional (DFT) calculations. The angular varia- tion of TMR indicates that the SrRuO3 electrodes contain multiple magnetic domains, allowing the devices to exhibit at least three stable resistance states. 

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