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- ItemSwitchable magnetic bulk photovoltaic effect in the two-dimensional magnet CrI3(London : Nature Publishing Group, 2019) Zhang, Y.; Holder, T.; Ishizuka, H.; de Juan, F.; Nagaosa, N.; Felser, C.; Yan, B.The bulk photovoltaic effect (BPVE) rectifies light into the dc current in a single-phase material and attracts the interest to design high-efficiency solar cells beyond the pn junction paradigm. Because it is a hot electron effect, the BPVE surpasses the thermodynamic Shockley–Queisser limit to generate above-band-gap photovoltage. While the guiding principle for BPVE materials is to break the crystal centrosymmetry, here we propose a magnetic photogalvanic effect (MPGE) that introduces the magnetism as a key ingredient and induces a giant BPVE. The MPGE emerges from the magnetism-induced asymmetry of the carrier velocity in the band structure. We demonstrate the MPGE in a layered magnetic insulator CrI3, with much larger photoconductivity than any previously reported results. The photocurrent can be reversed and switched by controllable magnetic transitions. Our work paves a pathway to search for magnetic photovoltaic materials and to design switchable devices combining magnetic, electronic, and optical functionalities.
- ItemA diuranium carbide cluster stabilized inside a C80 fullerene cage(London : Nature Publishing Group, 2018) Zhang, X.; Li, W.; Feng, L.; Chen, X.; Hansen, A.; Grimme, S.; Fortier, S.; Sergentu, D.-C.; Duignan, T.J.; Autschbach, J.; Wang, S.; Wang, Y.; Velkos, G.; Popov, A.A.; Aghdassi, N.; Duhm, S.; Li, X.; Li, J.; Echegoyen, L.; Schwarz, W.H.E.; Chen, N.Unsupported non-bridged uranium-carbon double bonds have long been sought after in actinide chemistry as fundamental synthetic targets in the study of actinide-ligand multiple bonding. Here we report that, utilizing I h(7)-C80 fullerenes as nanocontainers, a diuranium carbide cluster, U=C=U, has been encapsulated and stabilized in the form of UCU@I h(7)-C80. This endohedral fullerene was prepared utilizing the Krätschmer-Huffman arc discharge method, and was then co-crystallized with nickel(II) octaethylporphyrin (NiII-OEP) to produce UCU@I h(7)-C80·[NiII-OEP] as single crystals. X-ray diffraction analysis reveals a cage-stabilized, carbide-bridged, bent UCU cluster with unexpectedly short uranium-carbon distances (2.03 Å) indicative of covalent U=C double-bond character. The quantum-chemical results suggest that both U atoms in the UCU unit have formal oxidation state of +5. The structural features of UCU@I h(7)-C80 and the covalent nature of the U(f1)=C double bonds were further affirmed through various spectroscopic and theoretical analyses.
- ItemAmorphous martensite in β-Ti alloys(London : Nature Publishing Group, 2018) Zhang, L.; Zhang, H.; Ren, X.; Eckert, J.; Wang, Y.; Zhu, Z.; Gemming, T.; Pauly, S.Martensitic transformations originate from a rigidity instability, which causes a crystal to change its lattice in a displacive manner. Here, we report that the martensitic transformation on cooling in Ti-Zr-Cu-Fe alloys yields an amorphous phase instead. Metastable β-Ti partially transforms into an intragranular amorphous phase due to local lattice shear and distortion. The lenticular amorphous plates, which very much resemble α′/α″ martensite in conventional Ti alloys, have a well-defined orientation relationship with the surrounding β-Ti crystal. The present solid-state amorphization process is reversible, largely cooling rate independent and constitutes a rare case of congruent inverse melting. The observed combination of elastic softening and local lattice shear, thus, is the unifying mechanism underlying both martensitic transformations and catastrophic (inverse) melting. Not only do we reveal an alternative mechanism for solid-state amorphization but also establish an explicit experimental link between martensitic transformations and catastrophic melting.
- ItemHighly indistinguishable and strongly entangled photons from symmetric GaAs quantum dots(London : Nature Publishing Group, 2017) Huber, D.; Reindl, M.; Huo, Y.; Huang, H.; Wildmann, J.S.; Schmidt, O.G.; Rastelli, A.; Trotta, R.The development of scalable sources of non-classical light is fundamental to unlocking the technological potential of quantum photonics. Semiconductor quantum dots are emerging as near-optimal sources of indistinguishable single photons. However, their performance as sources of entangled-photon pairs are still modest compared to parametric down converters. Photons emitted from conventional Stranski-Krastanov InGaAs quantum dots have shown non-optimal levels of entanglement and indistinguishability. For quantum networks, both criteria must be met simultaneously. Here, we show that this is possible with a system that has received limited attention so far: GaAs quantum dots. They can emit triggered polarization-entangled photons with high purity (g (2) (0) = 0.002±0.002), high indistinguishability (0.93±0.07 for 2 ns pulse separation) and high entanglement fidelity (0.94±0.01). Our results show that GaAs might be the material of choice for quantum-dot entanglement sources in future quantum technologies.
- ItemSpectral dynamics of shift current in ferroelectric semiconductor SbSI(Washington : National Academy of Sciences, 2019) Sotome, M.; Nakamura, M.; Fujioka, J.; Ogino, M.; Kaneko, Y.; Morimoto, T.; Zhang, Y.; Kawasaki, M.; Nagaosa, N.; Tokura, Y.; Ogawa, N.Photoexcitation in solids brings about transitions of electrons/ holes between different electronic bands. If the solid lacks an inversion symmetry, these electronic transitions support spontaneous photocurrent due to the geometric phase of the constituting electronic bands: the Berry connection. This photocurrent, termed shift current, is expected to emerge on the timescale of primary photoexcitation process. We observe ultrafast evolution of the shift current in a prototypical ferroelectric semiconductor antimony sulfur iodide (SbSI) by detecting emitted terahertz electromagnetic waves. By sweeping the excitation photon energy across the bandgap, ultrafast electron dynamics as a source of terahertz emission abruptly changes its nature, reflecting a contribution of Berry connection on interband optical transition. The shift excitation carries a net charge flow and is followed by a swing over of the electron cloud on a subpicosecond timescale. Understanding these substantive characters of the shift current with the help of first-principles calculation will pave the way for its application to ultrafast sensors and solar cells.