We report a photovoltaic effect in ferroelectric ${\text{BiFeO}}_3$ thin films. The all-oxide heterostructures with ${\text{SrRuO}}_3$ bottom and tin doped indium oxide top electrodes are characterized by open-circuit voltages $\sim 0.8–0.9\text{V}$ and external quantum efficiencies up to $\sim 10\%$ when illuminated with the appropriate light. Efficiencies are at least an order of magnitude larger than the maximum efficiency under sunlight (AM 1.5) thus far reported for ferroelectric-based devices. The dependence of the measured open-circuit voltage on film thickness suggests contributions to the large open-circuit voltage from both the ferroelectric polarization and band offsets at the ${\text{BiFeO}}_3/\text{tin}$ doped indium oxide interface.

The optical properties of epitaxial $\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}_3$ thin films have been characterized in the visible range. Variable temperature spectra show an absorption onset near $2.17\mathrm{eV}$ , a direct gap ($2.667\pm 0.005\mathrm{eV}$ at $300\mathrm{K}$ ), and charge transfer excitations at higher energy. Additionally, we report photoconductivity in $\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}_3$ films under illumination from a $100\mathrm{mW}∕{\mathrm{cm}}^2$ white light source. A direct correlation is observed between the magnitude of the photoconductivity and postgrowth cooling pressure. Dark conductivities increased by an order of magnitude when comparing films cooled in 760 and $0.1\text{Torr}$ . Large increases in photoconductivity are observed in light.

$\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}_3$ thin films have been deposited on (001) $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$ substrates by adsorption-controlled reactive molecular-beam epitaxy. For a given bismuth overpressure and oxygen activity, single-phase $\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}_3$ films can be grown over a range of deposition temperatures in accordance with thermodynamic calculations. Four-circle x-ray diffraction reveals phase-pure, epitaxial films with $\omega$ rocking curve full width at half maximum values as narrow as $29\mathrm{arc}\sec$ (0.008°). Multiple-angle spectroscopic ellipsometry reveals a direct optical band gap at $2.74\mathrm{eV}$ for stoichiometric as well as 5% bismuth-deficient single-phase $\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}_3$ films.

CrSiTe₃ has attracted recent interest as a candidate single-layer ferromagnetic semiconductor, but relatively little is known about the bulk properties of this material. Here, we report single-crystal X-ray diffraction, magnetic properties, thermal conductivity, vibrational, and optical spectroscopies and compare our findings with complementary electronic structure and lattice dynamics principles calculations. The high temperature paramagnetic phase is characterized by strong spin-lattice interactions that give rise to glassy behavior, negative thermal expansion, and an optical response that reveals that CrSiTe₃ is an indirect gap semiconductor with indirect and direct band gaps at 0.4 and 1.2 eV, respectively. Measurements of the phonons across the 33 K ferromagnetic transition provide additional evidence for strong coupling between the magnetic and lattice degrees of freedom. The Si-Te stretching and Te displacement modes are sensitive to the magnetic ordering transition, a finding that we discuss in terms of the superexchange mechanism. Spin-lattice coupling constants are also extracted.

Using spectroscopic ellipsometry, the room temperature refractive index and absorption versus wavelength of the ferroelectric antiferromagnet bismuth ferrite, $\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}_3$ , are reported. The material has a direct band gap at $442\mathrm{nm}$ wavelength $\left(2.81\mathrm{eV}\right)$ . Using optical second harmonic generation, the nonlinear optical coefficients were determined to be $d_{15}∕d_{22}=0.20\pm 0.01$ , $d_{31}∕d_{22}=0.35\pm 0.02$ , $d_{33}∕d_{22}=-11.4\pm 0.20$ , and $\mid d_{22}\mid =298.4\pm 6.1\mathrm{pm}∕\mathrm{V}$ at a fundamental wavelength of $800\mathrm{nm}$ .

We measured the optical properties of epitaxial CoFe₂O₄ thin films and compared our findings with complementary electronic structure calculations and similar studies on the Ni analog. Our work reveals CoFe₂O₄ to be an indirect band gap material (1.2 eV, X → Γ in the spin-down channel) with a direct gap at 2.7 eV. The latter is robust up to 800 K. Compared to NiFe₂O₄, the indirect gap is ≈0.5 eV lower, a difference we discuss in terms of size and covalency effects in spinel ferrites.