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Recently, superconductivity was discovered with a superconducting transition temperature (Tc) of 2 K in strained (110)-oriented RuO2 films grown on TiO2(110) single-crystal substrates. In this work, we predict and realize superconductivity in strained (100)-oriented RuO2 thin films grown on TiO2(100) single-crystal substrates. We show that while density functional theory predicts the Tc of strained RuO2(100) films to be even higher than the RuO2(110) films, our transport and angle-resolved photoemission spectroscopy measurements find the Tc to be about 1 K in strained RuO2(100) films grown on TiO2(100) substrates. Nonetheless, the thickness dependence of the Tc follows a similar trend in both cases. Our scanning SQUID measurements reveal a local superfluid response, indicating a 100 mK inhomogeneity in Tc over a 100 μm scale. 

Improper ferroelectrics, in which electric polarization emerges as a secondary effect of primary order parameters such as nonpolar structural distortions, are theoretically predicted to exhibit no intrinsic thickness limit. Yet, this prediction is inconsistent with experiment where ferroelectricity is observed to disappear in improper ferroelectric films under six formula units thick. We transcend this limitation by moving beyond the conventional epitaxial design paradigm that focuses largely on in-plane lattice mismatch and chemical bonding. By considering structural compatibility in the out-of-plane direction, we realize undiminished improper ferroelectricity at the monolayer limit. Our findings confirm the theoretically predicted absence of a critical thickness for improper ferroelectrics and establish a broadly applicable strategy for designing additional ultrathin ferroelectric materials. 

The discovery of superconductivity in La3Ni2O7 under pressure has motivated the investigation of a parent spin density wave (SDW) state, which could provide the underlying pairing interaction. Here, we employ resonant soft x-ray scattering and polarimetry on thin films of bilayer La3Ni2O7 to determine that the magnetic structure of the SDW forms unidirectional diagonal spin stripes with moments lying within the NiO2 plane and perpendicular to QSDW, but without evidence of the strong charge disproportionation typically associated with other nickelates. These stripes form anisotropic domains with shorter correlation lengths perpendicular versus parallel to QSDW, revealing nanoscale rotational and translational symmetry breaking analogous to the cuprate and Fe-based superconductors, with possible Bloch-like antiferromagnetic domain walls separating orthogonal domains. 

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. 

We demonstrate the epitaxial growth of single-phase (100) BaTiO3 films on (100) β-Ga2O3 substrates at substrate temperatures ranging from 600 to 700 °C using molecular-beam epitaxy. Characterization of a 47 nm thick BaTiO3 film by atomic force microscopy reveals a step-and-terrace morphology with unit-cell-high BaTiO3 steps and an rms surface roughness of 0.26 nm. Scanning transmission electron microscopy (STEM) images show that in some regions the β-Ga2O3 substrate terminates with a (100)A plane as it transitions to BaTiO3 and in other regions with a (100)B plane. The (100) BaTiO3 films are fully relaxed and consist of a mixture of two types of a-axis domains: a1 and a2. The orientation relationship determined by X-ray diffraction and confirmed by STEM is (100) BaTiO3 || (100) β-Ga2O3 and [011] BaTiO3||[010] β-Ga2O3. Despite the average linear lattice mismatch of 3.8%, BaTiO3 films with rocking curve full width at half maximum widths as narrow as 28 arc sec are achieved. From capacitance–voltage measurements on a metal–oxide–semiconductor capacitor structure with a-axis BaTiO3 as the oxide layer and Si-doped β-Ga2O3 as the semiconducting layer, we extract a dielectric constant of K11 = 670 for the BaTiO3 epitaxially integrated with (100) β-Ga2O3. We anticipate that this high-K epitaxial dielectric will be useful for electric-field management in β-Ga2O3-based device structures.