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Register a new userFirst-Principles Calculation of the Bulk Photovoltaic Effect in the Polar Compounds LiAsS$_text{2}$, LiAsSe$_text{2}$, and NaAsSe$_text{2}$
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We calculate the shift current response, which has been identified as the dominant mechanism for the bulk photovoltaic effect, for the polar compounds LiAsS$_text{2}$, LiAsSe$_text{2}$, and NaAsSe$_text{2}$. We find that the magnitudes of the photovoltaic responses in the visible range for these compounds exceed the maximum response obtained for BiFeO$_text{3}$ by 10 - 20 times. We correlate the high shift current response with the existence of $p$ states at both the valence and conduction band edges, as well as the dispersion of these bands, while also showing that high polarization is not a requirement. With low experimental band gaps of less than 2 eV and high shift current response, these materials have potential for use as bulk photovoltaics.
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The iron-based superconductor Ba$_{1-x}$K$_x$Fe$_text{2}$As$_text{2}$ is emerging as a key material for high magnetic field applications owing to the recent developments in superconducting wires and bulk permanent magnets. Epitaxial thin films play important roles in investigating and artificially tuning physical properties; nevertheless, the synthesis of Ba$_{1-x}$K$_x$Fe$_2$As$_2$ epitaxial thin films remained challenging because of the high volatility of K. Herein, we report the successful growth of epitaxial Ba$_{1-x}$K$_x$Fe$_text{2}$As$_text{2}$ thin films by molecular-beam epitaxy with employing a combination of fluoride substrates (CaF$_text{2}$, SrF$_text{2}$, and BaF$_text{2}$) and a low growth temperature (350$-$420$^circ$C). Our epitaxial thin film grown on CaF$_text{2}$ showed sharp superconducting transition at an onset critical temperature of 36 K, slightly lower than bulk crystals by ~2 K due presumably to the strain effect arising from the lattice and thermal expansion mismatch. Critical current density ($J$$_text{c}$) determined by the magnetization hysteresis loop is as high as 2.2 MA/cm$^text{2}$ at 4 K under self-field. In-field $J$$_text{c}$ characteristics of the film are superior to the bulk crystals. The realization of epitaxial thin films opens opportunities for tuning superconducting properties by epitaxial strain and revealing intrinsic grain boundary transport of Ba$_{1-x}$K$_x$Fe$_text{2}$As$_text{2}$.
Scanning nano-focused X-ray diffraction (nXRD) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) are used to investigate the crystal structure of ramp-edge junctions between superconducting electron-doped Nd$_text{1.85}$Ce$_text{0.15}$CuO$_text{4}$ and superconducting hole-doped La$_text{1.85}$Sr$_text{0.15}$CuO$_text{4}$ thin films, the latter being the top layer. On the ramp, a new growth mode of La$_text{1.85}$Sr$_text{0.15}$CuO$_text{4}$ with a 3.3 degree tilt of the c-axis is found. We explain the tilt by developing a strain accommodation model that relies on facet matching, dictated by the ramp angle, indicating that a coherent domain boundary is formed at the interface. The possible implications of this growth mode for the creation of artificial domains in morphotropic materials are discussed.
Electric current has been experimentally demonstrated to be able to drive the insulator-to-metal transition (IMT) in VO$_2$. The main mechanisms involved are believed to be the Joule heating effect and the strong electron-correlation effect. These effects are often entangled with each other in experiments, which complicates the understanding of the essential nature of the observations. We formulate a phase-field model to investigate theoretically in mesoscale the pure correlation effect brought by the current on the IMT in VO$_2$, i.e., the isothermal process under the current. We find that a current with a large density ($sim 10^1$ nA/nm$^2$) induces a few-nanosecond ultrafast switch in VO$_2$, in agreement with the experiment. The temperature-current phase diagram is further calculated, which reveals that the current may induce the M2 phase at low temperatures. The current is also shown capable of driving domain walls to move. Our work may assist related experiments and provide guidance to the engineering of VO$_2$-based electric switching devices.
Metal-ion doping can effectively regulate the metal-insulator transition temperature in $mathrm{VO}_2$. Experiments found that the pentavalent and hexavalent ion doping dramatically reduces the transition temperature while the trivalent ion doping increases the transition temperature and induces intermediate phases. Based on the phase-field model of the metal-insulator transition in $mathrm{VO}_2$ we developed previously, we formulate a Landau potential of the metal-ion-doped $mathrm{VO}_2$ taking account of the effects of doping on the electron correlation and lattice structure. The effect of metal-ion doping on the lattice structure is accounted for in a phenomenological way. Using the Landau potential, we calculate the temperature-dopant-concentration phase diagrams of $mathrm{VO}_2$ doped with various metal ions consistent with the experiments and provide explanation to the different behaviors of different metal-ion doping. The phenomenological theory can provide estimations of phase diagrams of $mathrm{VO}_2$ doped with other metal ions.
Epitaxial perovskite oxide interfaces with different symmetry of the epitaxial layers have attracted considerable attention due to the emergence of novel behaviors and phenomena. In this paper, we show by aberration corrected transmission electron microscopy that orthorhombic $text{LaInO}_text{3}$ films grow in form of three different types of domains on the cubic $text{BaSnO}_text{3}$ pseudosubstrate. Quantitative evaluation of our TEM data shows that $c_{pc}$-oriented and $a_{pc}/b_{pc}$-oriented domains are present with similar probability. While continuum elasticity theory suggests that $c_{pc}$-oriented domains should exhibit a significantly higher strain energy density than $a_{pc}/b_{pc}$-oriented domains, density functional calculations confirm that $c_{pc}$- and $a_{pc}$-oriented domains on $text{BaSnO}_text{3}$ have similar energies.
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