Strong spin-momentum coupling in topological insulators give rise to topological surface states, protected against disorder scattering by time reversal symmetry. The study of these exotic quantum states not only provides an opportunity to explore fundamental phenomenon in condensed matter physics such as the spin hall effect, but also lays the foundation for applications in quantum computing to spintronics. Conventional electrical measurements suffer from substantial bulk interference, making it difficult to clearly identify topological surface state from the bulk. We use terahertz time-domain spectroscopy to study the temperature-dependent optical behavior of a 23-quintuple-thick film of bismuth selenide (Bi2Se3) allowing the deconvolution of the surface state response from the bulk. The signatures of the topological surface state at low temperatures (< 30 K) with the optical conductance of Bi2Se3 exhibiting a metallic behavior are observed. Measurement of carrier dynamics, obtain an optical mobility, exceeding 2000 cm2/V•s at 4 K, indicative of a surface-dominated response. A scattering lifetime of ~0.18 ps and a carrier density of 6×1012 cm-2 at 4 K were obtained from the terahertz time-domain spectroscopy measurement. The terahertz conductance spectra reveal characteristic features at ~1.9 THz, attributed to the optical phonon mode, which becomes less prominent with temperature. The electrical transport measurements reveal weak antilocalization behavior in our Bi2Se3 sample. We obtain the number of surface state modes, α as 0.5, and the coherence length, Lφ, as 380 nm at low temperatures < 10 K, further confirming the presence of a single topological surface state mode.
Owing to the unique chemical and electronic properties arising from 3d-electrons, substitution with transition metal ions is one of the key routes for engineering new functionalities into materials. While this approach has been used extensively in complex metal oxide perovskites, metal halide perovskites have largely resisted facile isovalent substitution. In this work, it is demonstrated that the substitution of Co2+ into the lattice of methylammonium lead triiodide imparts magnetic behavior to the material while maintaining photovoltaic performance at low concentrations. In addition to comprehensively characterizing its magnetic properties, the Co2+ ions themselves are utilized as probes to sense the local electronic environment of Pb in the perovskite, thereby revealing the nature of their incorporation into the material. A comprehensive understanding of the effect of transition metal incorporation is provided, thereby opening the substitution gateway for developing novel functional perovskite materials and devices for future technologies.
We report the structural, electronic, and magnetic study of Cr-doped Sb2Te3 thin films grown by a two-step deposition process using molecular-beam epitaxy (MBE). The samples were investigated using a variety of complementary techniques, namely, x-ray diffraction (XRD), atomic force microscopy, SQUID magnetometry, magneto-transport, and polarized neutron reflectometry (PNR). It is found that the samples retain good crystalline order up to a doping level of (in CrxSb2−xTe3), above which degradation of the crystal structure is observed by XRD. Fits to the recorded XRD spectra indicate a general reduction in the c-axis lattice parameter as a function of doping, consistent with substitutional doping with an ion of smaller ionic radius. The samples show soft ferromagnetic behavior with the easy axis of magnetization being out-of-plane. The saturation magnetization is dependent on the doping level, and reaches from to almost per Cr ion. The transition temperature depends strongly on the Cr concentration and is found to increase with doping concentration. For the highest achievable doping level for phase-pure films of , a of 125 K was determined. Electric transport measurements find surface-dominated transport below ∼10 K. The magnetic properties extracted from anomalous Hall effect data are in excellent agreement with the magnetometry data. PNR studies indicate a uniform magnetization profile throughout the film, with no indication of enhanced magnetic order towards the sample surface.
Strong many-body interactions in solids yield a host of fascinating and potentially useful physical properties. Here, from angle-resolved photoemission experiments and ab initio many-body calculations, we demonstrate how a strong coupling of conduction electrons with collective plasmon excitations of their own Fermi sea leads to the formation of plasmonic polarons in the doped ferromagnetic semiconductor EuO. We observe how these exhibit a significant tunability with charge carrier doping, leading to a polaronic liquid that is qualitatively distinct from its more conventional lattice-dominated analogue. Our study thus suggests powerful opportunities for tailoring quantum many-body interactions in solids via dilute charge carrier doping.
Magnetic doping with transition metal ions is the most widely used approach to break time-reversal symmetry in a topological insulator (TI)---a prerequisite for unlocking the TI's exotic potential. Recently, we reported the doping of ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ thin films with rare-earth ions, which, owing to their large magnetic moments, promise commensurately large magnetic gap openings in the topological surface states. However, only when doping with Dy has a sizable gap been observed in angle-resolved photoemission spectroscopy, which persists up to room temperature. Although disorder alone could be ruled out as a cause of the topological phase transition, a fundamental understanding of the magnetic and electronic properties of Dy-doped ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ remained elusive. Here, we present an x-ray magnetic circular dichroism, polarized neutron reflectometry, muon-spin rotation, and resonant photoemission study of the microscopic magnetic and electronic properties. We find that the films are not simply paramagnetic but that instead the observed behavior can be well explained by the assumption of slowly fluctuating, inhomogeneous, magnetic patches with increasing volume fraction as the temperature decreases. At liquid helium temperatures, a large effective magnetization can be easily introduced by the application of moderate magnetic fields, implying that this material is very suitable for proximity coupling to an underlying ferromagnetic insulator or in a heterostructure with transition-metal-doped layers. However, the introduction of some charge carriers by the Dy dopants cannot be excluded at least in these highly doped samples. Nevertheless, we find that the magnetic order is not mediated via the conduction channel in these samples and therefore magnetic order and carrier concentration are expected to be independently controllable. This is not generally the case for transition-metal-doped topological insulators, and Dy doping should thus allow for improved TI quantum devices.
The character of the organic matter of Chattanooga shale kerogen was determined by studying the products obtained by oxidation of the shale and kerogen with concentrated nitric acid. A violent reaction, which was partly caused by pyrite, occurred on treating the raw shale with concentrated nitric acid, but considerable oxidation occurred as evidenced by the 35.7 and 5.8% of the total carbon appearing as carbon dioxide and acetic acid, respectively. A methcd was developed in which large samples of shale were extracted with hydrofluoric and nitric acids to give a sample of kerogen containing 1 to 3% ash in a relatively short period of time. To avoid foaming of reaction mixture, a two-stage oxidation procedure was developed which consisted of first treating with concentrated nitric acid at 75 deg C for one hour followed by retreatment of the products insoluble in dilute nitric acid with concentrated nitric acid at reflux for one hour. The acids insoluble in dilute nitric acid from both the first and second stages were shown to be nitrated and oxidized tc a considerable extent. Equivalent weights of 211 and 159 were found for the acids from the first and second stages respectively compared with an equivalentmore » weight of 776 for the kerogen. Infrared spectral data, atomic hydrogen/carbon ratios, and empirical formulas, calculated from the chemical analyses, showed that the acids were a complex mixture containing both aliphatic and aromatic components. The 75 deg C soluble acids, isolated in the first stage, and the 120 deg C soluble acids, isolated from the second state, appeared to be composed of principally allphatic structures. Free sulfuric acid which was found in both of the acids is assumed to have originated from organic structures since no sulfate was observed in either the kerogen or the insoluble acids. An equivalent weight of 277 was observed for the 75 deg C soluble acids and 144 for the 120 deg C soluble acids. Hydrogen/carbon ratios of 1.28 and 1.17 were observed for the 75 and 120 deg C soluble acids, respectively, while the oxygen/carbon ratio of the 120 deg C acids, 0.45, was higher than that of the 75 deg C soluble acids, 0.28. Infrared spectra and empirical formulas of both acids emphasized the predominantly aliphatic character. They are thought to be composed of predominantly alicyclic, which may contain unsaturated linkages, and/or unsaturated open-chain organic structures. It is estimated that from 60 to 70% of the carbon in the organic matter of Chattanooga shale is of an aliphatic nature. (auth)« less
Using soft x-ray absorption spectroscopy we determined the chemical and magnetic properties of the magnetic topological insulator (MTI) Cr:${\mathrm{Sb}}_{2}{\mathrm{Te}}_{3}$. X-ray magnetic circular dichroism (XMCD) at the Cr ${L}_{2,3}$, Te ${M}_{4,5}$, and Sb ${M}_{4,5}$ edges shows that the Te $5p$ moment is aligned antiparallel to both the Cr $3d$ and Sb $5p$ moments, which is characteristic for carrier-mediated ferromagnetic coupling. Comparison of the Cr ${L}_{2,3}$ spectra with multiplet calculations indicates a hybridized Cr state, consistent with the carrier-mediated coupling scenario. We studied the enhancement of the Curie temperature, ${T}_{\mathrm{C}}$, of the MTI thin film through the magnetic proximity effect. Arrott plots, measured using the Cr ${L}_{3}$ XMCD, show a ${T}_{\mathrm{C}}\phantom{\rule{4pt}{0ex}}\ensuremath{\approx}87$ K for the as-cleaved film. After deposition of a thin layer of ferromagnetic Co onto the surface, the ${T}_{\mathrm{C}}$ increases to $\ensuremath{\sim}93$ K, while the Co and Cr moments are parallel. This increase in ${T}_{\mathrm{C}}$ is unexpectedly small compared to similar systems reported earlier. The XMCD spectra demonstrate that the Co/MTI interface remains intact, i.e., no reaction between Co and the MTI takes place. Our results are a useful starting point for refining the physical models of Cr-doped ${\mathrm{Sb}}_{2}{\mathrm{Te}}_{3}$, which is required for making use of them in device applications.
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