No abstract

Doping bismuth selenide (Bi2Se3) with elements such as copper and strontium (Sr) can induce superconductivity, making the doped materials interesting candidates to explore potential topological superconducting behaviors. It was thought that the superconductivity of doped Bi2Se3 was induced by dopant atoms intercalated in van der Waals gaps. However, several experiments have shown that the intercalation of dopant atoms may not necessarily make doped Bi2Se3 superconducting. Thus, the structural origin of superconductivity in doped Bi2Se3 remains an open question.Herein, we combined material synthesis and characterization, high-resolution transmission electron microscopy, and first-principles calculations to study the doping structure of Sr-doped Bi2Se3. We found that the emergence of superconductivity is strongly related with n-type dopant atoms. Atomic-level energy-dispersive X-ray mapping revealed various n-type Sr dopants that occupy intercalated and interstitial positions. First-principles calculations showed that the formation energy of a specific interstitial Sr doping position depends strongly on Sr doping level. This site changes from a metastable position at low Sr doping level to a stable position at high Sr doping level. The calculation results explain why quenching is necessary to obtain superconducting samples when the Sr doping level is low and also why slow furnace cooling can yield superconducting samples when the Sr doping level is high. Our findings suggest that Sr atoms doped at interstitial locations, instead of those intercalated in van der Waals gaps, are most likely to be responsible for the emergence of superconductivity in Sr-doped Bi2Se3.3

Single monolayer FeSe film grown on a Nb-doped SrTiO_{3}(001) substrate shows the highest superconducting transition temperature (T_{C}∼100  K) among the iron-based superconductors (iron pnictides), while the T_{C} value of bulk FeSe is only ∼8  K. Although bulk FeSe does not show antiferromagnetic order, calculations suggest that the parent FeSe/SrTiO_{3} films are antiferromagnetic. Experimentally, because of a lack of a direct probe, the magnetic state of FeSe/SrTiO_{3} films remains mysterious. Here, we report direct evidence of antiferromagnetic order in the parent FeSe/SrTiO_{3} films by the magnetic exchange bias effect measurements. The magnetic blocking temperature is ∼140  K for a single monolayer film. The antiferromagnetic order disappears after electron doping.

Properly cutting graphene into certain high-quality micro-/nanoscale structures in a cost-effective way has a critical role. Here, we report a novel approach to pattern graphene films by H 2 O-based magnetic-assisted ultraviolet (UV) photolysis under irradiation at 184.9 nm. By virtue of the paramagnetic characteristic, the photo-dissociated hydroxyl [OH(X 2 Π)] radicals are magnetized and have their oxidation capability highly enhanced through converting into an accelerated directional motion. Meanwhile, the precursor of H 2 O(X ̃1A 1 ) molecules distributes uniformly thanks to its weak diamagnetic characteristic, and there exists no instable diamagnetic intermediate to cause lateral oxidation. Possessing these unique traits, the H 2 O-based magneticassisted UV photolysis has the capability of making graphene microscale patterns with the linewidth down to 8.5 μm under a copper grid shadow mask. Furthermore, it is feasible to pattern graphene films into 40 nm-wide ribbons under ZnO nanowires and realize hybrid graphene/ZnO nanoribbon field-effect transistors with a hole mobility up to 7200 cm 2 •V −1 •s −1 . The X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry analyses reveal that OH(X 2 Π) radicals act as a strong oxidant and that another product of H(1 2 S) adsorbs weakly on graphene.

The low-energy electronic structure of a ferromagnetic insulator Cr2Ge2Te6 was studied by angle-resolved photoemission spectroscopy (ARPES). A broadening of the ARPES spectra, which we ascribe to a splitting of the valence band, has been observed at low temperature. Through temperature dependent measurements, we found that the band splitting presented a transition at ∼60–70 K, which is consistent with the ferromagnetic transition temperature of Cr2Ge2Te6. Therefore, we suggested that the splitting of the valence band is due to the spin polarization effect of Te 5p states by Cr 3d states, which is compatible with the superexchange coupling through Cr-Te-Cr to form long-range ferromagnetic order in Cr2Ge2Te6.