The exact mechanism responsible for the significant enhancement of the superconducting transition temperature (Tc) of monolayer iron selenide (FeSe) films on SrTiO3 (STO) over that of bulk FeSe is an open issue. We present the results of a coordinated study of electrical transport, low temperature electron energy-loss spectroscopy (EELS), and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) measurements on FeSe/STO films of different thicknesses. HAADF-STEM imaging together with EELS mapping across the FeSe/STO interface shows direct evidence of electrons transferred from STO to the FeSe layer. The transferred electrons were found to accumulate within the first two atomic layers of the FeSe films near the STO substrate. An additional Se layer is also resolved to reside between the FeSe film and the TiO x -terminated STO substrate. Our transport results found that a positive backgate applied from STO is particularly effective in enhancing Tc of the films while minimally changing the carrier density. This increase in Tc is due to the positive backgate that "pulls" the transferred electrons in FeSe films closer to the interface and thus enhances their coupling to interfacial phonons and also the electron-electron interaction within FeSe films.
Background Contrast-enhanced ultrasound (CEUS) shows potential for the differential diagnosis of breast lesions in general, but its effectiveness remains unclear for the differential diagnosis of lesions highly suspicious for breast cancers. Objective This study aimed to evaluate the diagnostic value of CEUS in differentiating pathological subtypes of suspicious breast lesions defined as category 4 of US-BI-RADS. Methods The dataset of 150 breast lesions was prospectively collected from 150 patients who underwent routine ultrasound and CEUS examination and were highly suspected of having breast cancers. All lesions were pathologically confirmed by US-guided needle biopsy and surgery. The qualitative features and the quantitative parameters of CEUS of these breast lesions were analyzed. The CEUS and biopsy examinations were performed after informed consent. Results In the qualitative features, crab clam-like enhancement, the presence of more than two enhanced vessels within lesions, and surrounding enriched vessels inserting into lesions were able to differentiate atypical fibroadenomas (FIB) and mass-like non-puerperal mastitis (NPM) from invasive ductal carcinomas (IDC) and ductal carcinomas in situ (DCIS) ( p < 0.05). The enlarged scope, irregular shape, and perfusion deficiency were valuable to the differential diagnosis of FIB from the others ( p < 0.05). In the four quantitative parameters of CEUS, only the peak intensity (IMAX) contributed to the differential diagnosis between malignant and benign tumors ( p < 0.05, ROCAUC: 0.61, sensitivity: 60.4% and specificity: 65.9%, accuracy: 62.1%). However, IMAX did not show any difference in the paired comparison of IDC, DCIS, FIB, and NPM ( p > 0.05). The logistic regression analysis results showed that heterogeneous perfusion, crab clam-like enhancement, and partial_ IMAX were independent risk factors for benign and malignant breast lesions (p < 0.05) . The area under a receiver operating characteristic of the integrated model was 0.89. In the diagnosis of benign and malignant pathological subtypes of breast lesions, independent risk factors and integrated models had no statistical significance in the diagnosis of IDC and DCISs, FIB, and NPM ( p > 0.05). Conclusion Some qualitative risk features of CEUS can distinguish malignant breast lesions from NPM and atypical FIB with a high score of US-BI-RADS, aiding physicians to reduce the misdiagnosis of suspicious breast lesions in clinical practice.
A quantum anomalous Hall (QAH) insulator coupled to an s-wave superconductor is predicted to harbor a topological superconducting phase, the elementary excitations of which (i.e. Majorana fermions) can form topological qubits upon non-Abelian braiding operations. A recent transport experiment interprets the half-quantized two-terminal conductance plateau as the presence of chiral Majorana fermions in a millimeter-size QAH-Nb hybrid structure. However, there are concerns about this interpretation because non-Majorana mechanisms can also generate similar signatures, especially in a disordered QAH system. Here, we fabricated QAH-Nb hybrid structures and studied the QAH-Nb contact transparency and its effect on the corresponding two-terminal conductance. When the QAH film is tuned to the metallic regime by electric gating, we observed a sharp zero-bias enhancement in the differential conductance, up to 80% at zero magnetic field. This large enhancement suggests high probability of Andreev reflection and transparent interface between the magnetic topological insulator (TI) and Nb layers. When the magnetic TI film is in the QAH state with well-aligned magnetization, we found that the two-terminal conductance is always half-quantized. Our experiment provides a comprehensive understanding of the superconducting proximity effect observed in QAH-superconductor hybrid structures and shows that the half-quantized conductance plateau is unlikely to be induced by chiral Majorana fermions.
The 122$^{*}$ series of iron-chalcogenide superconductors, for example K$_x$Fe$_{2-y}$Se$_{2}$, only possesses electron Fermi pockets. Their distinctive electronic structure challenges the picture built upon iron pnictide superconductors, where both electron and hole Fermi pockets coexist. However, partly due to the intrinsic phase separation in this family of compounds, many aspects of their behavior remain elusive. In particular, the evolution of the 122$^{*}$ series of iron-chalcogenides with chemical substitution still lacks a microscopic and unified interpretation. Using angle-resolved photoemission spectroscopy, we studied a major fraction of 122$^{*}$ iron-chalcogenides, including the isovalently `doped' K$_x$Fe$_{2-y}$Se$_{2-z}$S$_z$, Rb$_x$Fe$_{2-y}$Se$_{2-z}$Te$_z$ and (Tl,K)$_x$Fe$_{2-y}$Se$_{2-z}$S$_z$. We found that the bandwidths of the low energy Fe \textit{3d} bands in these materials depend on doping; and more crucially, as the bandwidth decreases, the ground state evolves from a metal to a superconductor, and eventually to an insulator, yet the Fermi surface in the metallic phases is unaffected by the isovalent dopants. Moreover, the correlation-driven insulator found here with small band filling may be a novel insulating phase. Our study shows that almost all the known 122$^{*}$-series iron chalcogenides can be understood {\it via} one unifying phase diagram which implies that moderate correlation strength is beneficial for the superconductivity.
Topological crystalline insulator is a recently discovered topological phase of matter. It possesses multiple Dirac surface states, which are protected by the crystal symmetry. This is in contrast to the time-reversal symmetry that is operative in the well-known topological insulators. In the presence of a Zeeman field and/or strain, the multiple Dirac surface states are gapped. The high-Chern-number quantum anomalous Hall (QAH) state is predicted to emerge if the chemical potential resides in all the Zeeman gaps. Here, we use molecular-beam epitaxy to grow 12 double-layer (DL) pure and Cr-doped SnTe (111) thin film on heat-treated $\mathrm{SrTi}{\mathrm{O}}_{3}$ (111) substrate using a quintuple layer of insulating ${(\mathrm{B}{\mathrm{i}}_{0.2}\mathrm{S}{\mathrm{b}}_{0.8})}_{2}\mathrm{T}{\mathrm{e}}_{3}$ topological insulator as a buffer film. The Hall traces of Cr-doped SnTe film at low temperatures display square hysteresis loops indicating long-range ferromagnetic order with perpendicular anisotropy. The Curie temperature of the $12\mathrm{DL}\phantom{\rule{0.16em}{0ex}}\mathrm{S}{\mathrm{n}}_{0.9}\mathrm{C}{\mathrm{r}}_{0.1}\mathrm{Te}$ film is \ensuremath{\sim}110 K. Due to the chemical potential crossing the bulk valence bands, the anomalous Hall resistance of $12\mathrm{DL}\phantom{\rule{0.16em}{0ex}}\mathrm{S}{\mathrm{n}}_{0.9}\mathrm{C}{\mathrm{r}}_{0.1}\mathrm{Te}$ film is substantially lower than the predicted quantized value $(\ensuremath{\sim}1/4\phantom{\rule{0.28em}{0ex}}h/{e}^{2})$. It is possible that with systematic tuning the chemical potential via chemical doping and electrical gating, the high-Chern-number QAH state can be realized in the Cr-doped SnTe (111) thin film.
When a ferromagnet is placed in contact with a superconductor, owing to incompatible spin order, the Cooper pairs from the superconductor cannot survive more than one or two nanometers inside the ferromagnet. This is confirmed in the measurements of ferromagnetic nickel (Ni) nanowires contacted by superconducting niobium (Nb) leads. However, when a 3 nm thick copper oxide (CuO) buffer layer made by exposing an evaporated or a sputtered 3 nm Cu film to air, is inserted between the Nb electrodes and the Ni wire, the spatial extent of the superconducting proximity range is dramatically increased from 2 to a few tens of nanometers. Scanning transmission electron microscope study confirms the formation of a 3 nm thick CuO layer when an evaporated Cu film is exposed to air. Magnetization measurements of such a 3 nm CuO film on a SiO2/Si substrate and on Nb/SiO2/Si show clear evidence of ferromagnetism. One way to understand the long-range proximity effect in the Ni nanowire is that the CuO buffer layer with ferromagnetism facilitates the conversion of singlet superconductivity in Nb into triplet supercurrent along the Ni nanowires.
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