Molecular beam epitaxy of CuMnAs

We present a detailed study of the growth of the tetragonal polymorph of antiferromagnetic CuMnAs by the molecular beam epitaxy technique. We explore the parameter space of growth conditions and their effect on the microstructural and transport properties of the material. We identify its typical structural defects and compare the properties of epitaxial CuMnAs layers grown on GaP, GaAs and Si substrates. Finally, we investigate the correlation between the crystalline quality of CuMnAs and its performance in terms of electrically induced resistance switching.Comment: 10 pages, 8 figures and supplementary materia

Ferromagnetic MnSb2Te4: A p-type topological insulator with magnetic gap closing at high Curie temperatures of 45-50K

Resumen del trabajo presentado al APS March Meeting, celebrado de forma virtual del 13 al 19 de marzo de 2021Mn enables the formation of intrinsic magnetic topological insulatorsfor the quantum anomalous Hall effect with A1B2C4 stoichiometry, e. g., antiferromagnetic MnBi2Te4 with 25 K Néel temperature. Here, we showthat p-type MnSb2Te4, previously considered topologically trivial, is a ferromagnetic topological insulator with high Curie temperature of 45 to 50 K.It displays out-of-plane magnetic anisotropy, the nontrivial topology is robust in band structure calculations towards magnetic disorder, provides aDirac point of the topological surface state close to the Fermi level with out-of-plane spin polarization in spin-ARPES, and exhibits a magneticallyinduced band gap of 17 meV that closes at the Curie temperature as demonstrated by scanning tunneling spectroscopy. Moreover, it displays acritical exponent of magnetization β≈1, indicating the vicinity of a quantum critical point. We identify the influences of structural and magneticdisorder that render MnSb2Te4 the ideal system for tuning electric and magnetic properties of quantum anomalous Hall systems.Peer reviewe

Long-range Interactions for the A3\text{}^{3}1u\text{}_{u} and B3\text{}^{3}0u+\text{}_{u}^{+} States of Cd2\text{}_{2} from Line Shape Measurements

We report on very precise measurements of the absorption profile of the 326.1 nm resonance line of cadmium broadened by interactions with the ground state Cd atoms under thermal equilibrium. The experimental red wing profiles, corrected for the Boltzmann factor at different temperatures, are analysed in terms of the long-range potential parameters including dispersion and resonant interactions. The resonant interaction energy constant C$\text{}_{3}$ is calculated to be 2.31×10$\text{}^{-2}$ eV Å$\text{}^{3}$ and the van der Waals constant C$\text{}_{6}$ for difference potentials between the ground state X$\text{}^{1}$0$\text{}_{ }^{+}$g and the excited states B$\text{}^{3}$0$\text{}_{u}^{+}$ and A$\text{}^{3}$1$\text{}_{u}$ of Cd$\text{}_{2}$ derived experimentally are found to be (182.6± 8) eV Å$\text{}^{6}$ and (289±4) eV Å$\text{}^{6}$, respectively. The former is determined for the first time and the latter is considerably corrected in comparison to earlier determination. The van der Waals constant C$\text{}_{6}$ for the ground state of Cd$\text{}_{2}$ is discussed and estimated to be C$\text{}_{6}^{g}$=(247± 40) eV Å$\text{}^{6}$

Ochrana hematitových elektrod pomocí krycích TiO2 vrstev vytvořených ALD

Iron (III) oxide, in the form of hematite (alpha-Fe2O3), is a n-type semiconductor which is photoactive in the visible spectral region. Therefore, use in photoelectrocatalysis and photoassisted water electrolysis may be suggested. For such implementations, stability of contacts with liquid phases is mandatory. Hematite is stable in alkaline media but less stable in acidic media. For the first time the coverage of porous photoactive Sn doped hematite by thin capping layers of TiO2, deposited by Atomic Layer Deposition (ALD) and its impact on photocurrent and chemical stability of hematite is shown. The nominal thicknesses of the TiO2 ALD coatings were 0.5, 2 and 7.5 nm. The presence of the TiO2 coatings was evidenced by X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy (HR-TEM) and scanning TEM coupled with energy dispersive X-ray (EDX) spectroscopy. HR-TEM analyses revealed that the TiO2 capping layers were amorphous and conformal. Exposure of uncovered hematite layers to 1 M sulfuric acid led to a nominal dissolution rate of 0.23 nm/h which was halved when a TiO2 ALD coating (7.5 nm thin) was applied. Due to mismatch of the valence band positions of the two semiconductors, photocurrents were strongly diminished as the capping layer thickness was increased. Post calcination of as deposited ALD films on hematite resulted in an increase of photocurrent, which only exceeded photocurrents of pristine hematite when the ALD thickness was not more than 0.5 nm.Oxid železitý ve formě hematitu (α-Fe2O3) je polovodič n-typu a je fotoaktivní ve viditelné spektrální oblasti. Proto lze využít ve fotoelektrokatalýze a fotoasistovaném rozkladu vody. Pro tyto využití je důležitá jeho stabilita v kontaktu s kapalnou fází. Hematit je stabilní v alkalickém prostředí, ale méně stabilní v kyselém prostředí. V této práci poprvé ukazueme pokrytí porézního Sn-dopovaného hematitu tenkými krycími vrstvami TiO2, deponovanými depozicí atomárních vrstev (ALD) a dopad těchto vrstev na chemickou stabilitu a stabilitu fotoproudů. Nominální tlouštky ALD TiO2 vrstev byly 0.5, 2 a 7.5 nm. Přítomnost TiO2 vrstev byla detekována pomocí rentgenové fotoelektronové spektroskopie, transmisní elektronové mikroskopie s vysokým rozlišením (HR-TEM) a skenovací TEM s energiově disperzním detektorem (EDX). HR-TEM analýzy odhalili, že TiO2 vrstavy byly rovnoměrné a zárověn amorfní. Jejich expozice nepokrytého hematitu v 1M kyselině sírové způsobila rozpouštění o rychlosti 0.23 nm/h, přičemž tato hodnota poklesla o polovinu při pokrytí TiO2 vrstvou (7.5 nm). Díky energetické nevyrovnanosti valenčních pásů obou polovodičů docházelo ke graduálnímu poklesu vygenerovaných fotoproudů s rostoucí tloušťkou krycí vrstvy. Následné žíhání deponovaných hematitů zvýšilo fotoproudy, které byly v případě nejtenčí krycí vrstvy (0.5 nm) dokonce vyšší než fotoproudy samotného hematitu

Mn rich MnSb2Te4 A topological insulator with magnetic gap closing at high Curie temperatures of 45 50 K

Ferromagnetic topological insulators exhibit the quantum anomalous Hall effect, which is potentially useful for high precision metrology, edge channel spintronics, and topological qubits. The stable 2 state of Mn enables intrinsic magnetic topological insulators. MnBi2Te4 is, however, antiferromagnetic with 25 K N el temperature and is strongly n doped. In this work, p type MnSb2Te4, previously considered topologically trivial, is shown to be a ferromagnetic topological insulator for a few percent Mn excess. i Ferromagnetic hysteresis with record Curie temperature of 45 50 K, ii out of plane magnetic anisotropy, iii a 2D Dirac cone with the Dirac point close to the Fermi level, iv out of plane spin polarization as revealed by photoelectron spectroscopy, and v a magnetically induced bandgap closing at the Curie temperature, demonstrated by scanning tunneling spectroscopy STS , are shown. Moreover, a critical exponent of the magnetization amp; 946; amp; 8776; 1 is found, indicating the vicinity of a quantum critical point. Ab initio calculations reveal that Mn Sb site exchange provides the ferromagnetic interlayer coupling and the slight excess of Mn nearly doubles the Curie temperature. Remaining deviations from the ferromagnetic order open the inverted bulk bandgap and render MnSb2Te4 a robust topological insulator and new benchmark for magnetic topological insulator