We fabricated topological insulating Sb2Te3/Bi2Te3 p–n heterostructures by means of molecular beam epitaxy and characterized the topography of the films by scanning tunneling microscopy. Due to the van der Waals growth mode of the layered Te compounds, X-ray diffraction measurements show that the heterostructure is fully relaxed on the Si(111) substrate. Furthermore, scanning transmission electron microscopy measurements unveil the crystalline structure of the p–n interface. Energy dispersive X-ray spectroscopy and atom probe tomography enable the mapping of the chemical element distribution. We conclude that a diffusion of Sb and Bi during growth causes the formation of ternary compounds. In addition a Sb and Te accumulation at the substrate interface could be detected. Transport measurements prove the tunability of the carrier concentration via thickness variation of the p–n heterostructure.

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Quantitative imaging of the human brain is of great interest in clinical research as it enables the identification of a range of MR biomarkers useful in diagnosis, treatment and prognosis of a wide spectrum of diseases. Here, a 3D two-point method for water content and relaxation time mapping is presented and compared to established gold standard methods. The method determines free water content, H2O, and the longitudinal relaxation time, T1, quantitatively from a two-point fit to the signal equation including corrections of the transmit and receive fields. In addition, the effective transverse relaxation time, T2*, is obtained from an exponential fit to the multi-echo signal train and its influence on H2O values is corrected. The phantom results obtained with the proposed method show good agreement for H2O and T1 values with known and spectroscopically measured values, respectively. The method is compared in vivo to already established gold standard quantitative methods. For H2O and T2* mapping, the 3D two-point results were compared to a measurement conducted with a multiple-echo GRE with long TR and T1 is compared to results from a Look-Locker method, TAPIR. In vivo results show good overall agreement between the methods, but some systematic deviations are present. Besides an expected dependence of T2* on voxel size, T1 values are systematically larger in the 3D approach than those obtained with the gold standard method. This behaviour might be due to imperfect spoiling, influencing each method differently. Results for H2O differ due to differences in the saturation of cerebrospinal fluid and partial volume effects. In addition, ground truth values of in vivo studies are unknown, even when comparing to in vivo gold standard methods. A detailed region-of-interest analysis for H2O and T1 matches well published literature values.

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Background Body mass index (BMI) is increasing in a large number of elderly persons. This increase in BMI is known to put one at risk for many "diseases of aging," although less is known about how a change in BMI may affect the brains of the elderly. Purpose To investigate the relationship between BMI and quantitative water content, T1, T2*, and the semi‐quantitative magnetization transfer ratio (MTR) of various structures in elderly brains. Study Type Cross‐sectional. Subjects Forty‐two adults (BMI range: 19.1–33.5 kg/m2, age range: 58–80 years). Field Strength 3T MRI (two multi‐echo gradient echoes, actual flip angle imaging, magnetization prepared rapid gradient echo, fluid attenuated inversion recovery). Assessment The 3D two‐point method was used to derive (semi‐)quantitative parameters in global white (WM) and gray matter (GM) and their regions as defined by the Johns Hopkins University and the Montreal Neurological Institute atlases. Statistical Tests Multivariate linear regression with BMI as principal regressor, corrected for the additional regressors age, gender, and glycated hemoglobin. Spearman correlation between quantitative parameters of the regions showing significant changes and the lipid spectra / C‐reactive protein (CRP). Voxel‐based morphometry and analysis of covariance (ANCOVA) to explore changes in the GM volume. Results T1 increased significantly (P < 0.05) in the frontal, temporal, and parietal cortices, while the bilateral corona radiata, right superior longitudinal fasciculus, as well as the corpus callosum showed significant changes in the WM regions. T2* increased significantly in the global WM and left corona radiata. Changes in MTR and the free water content did not reach significance. No significant correlation between any quantitative parameter and the lipid spectra or CRP could be identified. Data Conclusion These results suggest that an elevated BMI predominantly affects T1 in WM as well as GM structures in the elderly human brain. Level of Evidence: 3 Technical Efficacy: Stage 3 J. Magn. Reson. Imaging 2020;51:514–523.

In a topological insulator (TI) the bulk electronic band structure behaves like an ordinary band insulator. However, at the surface topologically protected surface states occur, which give rise to a spin-locked, dissipation-less electronic transport. Hence, topological insulator materials, such as Sb 2 Te 3 or Bi 2 Te 3 , are of great interest for spintronic devices or quantum computing. In order to achieve the dissipation-less transport, the Fermi level has to be exactly tuned within the Dirac cone like band structure at the surface. To date intrinsic doping by vacancies or antisite defects renders Sb 2 Te 3 and Bi 2 Te 3 p-and n-type, respectively, which results in hole or electron transport in the bulk. Recently we solved this problem by growing p-n junctions made of Sb 2 Te 3 and Bi 2 Te 3 [1]. In this approach the carrier concentration at the surface is reduced by formation of the space charge layer at the buried heterointerface. Structure-wise the X 2 Te 3 (X=Bi, Sb) rhombohedral unit cell consists of three Te-X-Te-X-Te quintuple layers, which are linked by van der Waals forces. In order to achieve layers of high structural perfection on Si(111) substrates careful control of the growth parameters of the molecular beam epitaxy is required. In particular, we could demonstrate that the suppression of twin domains, which are the most prominent structural defects, is possible by van der Waals epitaxy [2]. Here we report on advanced scanning transmission electron microscopy and energy dispersive X-ray studies on MBE grown Sb 2 Te 3 /Bi 2 Te 3 heterostructures. Figure 1 displays STEM bright-field and dark-field images, where the quintuple layers within the heterostructures and a highly perfect interface to the Si substrate are atomically resolved. The EDX measurement in Figure 2 reveals, that at the heterointerface a Sb and Bi gradient extends over about 4 nm, effectively introducing a ternary compound as interlayer. Corresponding electrical transport measurements demonstrate the tunability of the intrinsic carrier concentration by variation of the thickness of the individual films [3].