Fenghua Chen

Oriented domains in a thin film of La0.8Sr0.2MnO3 prepared by laser molecular-beam epitaxy

Abstract A thin film of La0 8Sr0.2MnO3, prepared by computer-controlled laser molecular-beam epitaxy, on a single-crystal SrTiO3 substrate, has been characterized by transmission electron microscopy. Electron microdiffraction and high-resolution imaging reveal that the as-received thin film with a thickness of 200 nm is epitaxially grown on the SrTiO3(001) substrate. The microstructures in the whole film are clarified in terms of the oriented microdomains. The crystallographic relationships of these domains are discussed on the basis of an orthorhombic unit cell. Theoretical calculations based on a geometrical model that was recently proposed and applied to a number of epitaxial systems have been carried out to rationalize the present observations.

Magnetic evolution of spinel Mn1−xZnxCr2O4 single crystals

Mn1−xZnxCr2O4 (0 ≤ x ≤ 1) single crystals have been grown using the chemical vapor transport (CVT) method. The crystallographic, magnetic, and thermal transport properties of the single crystals were investigated by room-temperature X-ray diffraction, magnetization M(T) and specific heat CP(T) measurements. Mn1−xZnxCr2O4 crystals show a cubic structure, the lattice constant a decreases with the increasing content x of the doped Zn2+ ions and follows the Vegard law. Based on the magnetization and heat capacity measurements, the magnetic evolution of Mn1−xZnxCr2O4 crystals has been discussed. For 0 ≤ x ≤ 0.3, the magnetic ground state is the coexistence of the long-range ferrimagnetic order (LFIM) and the spiral ferrimagnetic one (SFIM), which is similar to that of the parent MnCr2O4. When x changes from 0.3 to 0.8, the SFIM is progressively suppressed and spin glass-like behavior is observed. When x is above 0.8, an antiferromagnetic (AFM) order presents. At the same time, the magnetic specific heat (Cmag.) was also investigated and the results are coincident with the magnetic measurements. The possible reasons based on the disorder effect and the reduced molecular field effect induced by the substitution of Mn2+ ions by nonmagnetic Zn2+ ones in Mn1−xZnxCr2O4 crystals have been discussed.

Resistivity plateau and large magnetoresistance in the charge density wave system TaTe4

Due to the time reversal symmetry, the metallic surface of the topological insulator (TI) survives and results in the resistivity plateau at low temperature, which is the transport signature of the metallic surface state. Such a universal character has been observed in many materials, such as Bi2Te2Se, SmB6, and so on. Recently, a similar behavior has also been found in the possible topological semimetal LaSb/Bi and the metallic compounds Nb/TaAs2. Herein, we have mainly explored the resistivity plateau and the magnetoresistance (MR) of TaTe4 single crystal with the charge density wave (CDW) transition temperature TCDWu2009=u2009475u2009K. There are some interesting observations: (i) The large MR (MR is about 1200% in a magnetic field 16u2009T at Tu2009=u20092u2009K) is observed at low temperature; (ii) A field-induced universal TI resistivity with a plateau at roughly Tu2009=u200910u2009K and high quantum mobility of carriers in the plateau region are present; (iii) Quantum oscillations with the angle dependence of a two-dimensional Fermi surf...

Polymer precursor synthesis of TaC–SiC ultrahigh temperature ceramic nanocomposites

TaC–SiC ultrahigh temperature ceramic nanocomposites were prepared by cross-linking and subsequent pyrolysis of a novel soluble blend precursor in argon atmosphere. The precursor was synthesized by blending polycarbosilane (PCS) with polytantaloxane (PT). The geometry of metal–ligand interaction of polymer PT was determined by the X-ray absorption fine structure (XAFS). The prepared ceramic nanomaterials were investigated with respect to their chemical and phase composition, by means of FT-IR, TG, X-ray diffraction (XRD), Micro-Raman spectroscopy, scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HR-TEM). Annealing experiments of the TaC–SiC samples at temperatures in the range of 1000–1800 °C showed conversion into nanostructured ultrahigh temperature ceramic composites with a trace amount of free carbon. The average grain sizes of the precursor-derived TaC and SiC ceramics were both less than 50 nm. Ta, Si and C elements were homogeneously distributed in the sample at the submicron scale.

Mobility spectrum analytical approach for the type-II Weyl semimetal Td-MoTe2

The extreme magnetoresistance (XMR) in orthorhombic W/MoTe2 arises from the combination of the perfect electron-hole (e-h) compensation effect and the unique orbital texture topology, which have comprised an intriguing research field in materials physics. Herein, we apply a special analytical approach as a function of mobility (μ-spectrum) without any hypothesis. Based on the interpretations of longitudinal and transverse electric transport of Td-MoTe2, the types and the numbers of carriers can be obtained. There are three observations: the large residual resistivity ratio can be observed in the MoTe2 single crystal sample, which indicates that the studied crystal is of high quality; we observed three electron-pockets and three hole-ones from the μ-spectrum and that the ratio of h/e is much less than 1, which shows that MoTe2 is more e-like; different from the separated peaks obtained from the hole-like μ-spectrum, those of the electron-like one are continuous, which may indicate the topological feature of electron-pockets in Td-MoTe2. The present results may provide an important clue to understanding the mechanism of the XMR effect in Td-MoTe2.The extreme magnetoresistance (XMR) in orthorhombic W/MoTe2 arises from the combination of the perfect electron-hole (e-h) compensation effect and the unique orbital texture topology, which have comprised an intriguing research field in materials physics. Herein, we apply a special analytical approach as a function of mobility (μ-spectrum) without any hypothesis. Based on the interpretations of longitudinal and transverse electric transport of Td-MoTe2, the types and the numbers of carriers can be obtained. There are three observations: the large residual resistivity ratio can be observed in the MoTe2 single crystal sample, which indicates that the studied crystal is of high quality; we observed three electron-pockets and three hole-ones from the μ-spectrum and that the ratio of h/e is much less than 1, which shows that MoTe2 is more e-like; different from the separated peaks obtained from the hole-like μ-spectrum, those of the electron-like one are continuous, which may indicate the topological feature o...

Critical behavior of two-dimensional intrinsically ferromagnetic semiconductor CrI3

CrI3, which belongs to a rare category of two-dimensional (2D) ferromagnetic semiconductors, is of great interest for spintronic device applications. Unlike CrCl3 whose magnetism presents a 2D-Heisenberg behavior, CrI3 exhibits a larger van der Waals gap, smaller cleavage energy, and stronger magnetic anisotropy which could lead to a 3D magnetic characteristic. Hence, we investigate the critical behavior of CrI3 in the vicinity of magnetic transition. We use the modified Arrott plot and Kouvel-Fisher method, and conduct critical isotherm analysis to estimate the critical exponents near the ferromagnetic phase transition. This shows that the magnetism of CrI3 follows the crossover behavior of a 3D-Ising model with mean field type interactions where the critical exponents b{eta}, {gamma}, and {delta} are 0.323, 0.835, and 3.585, respectively, at the Curie temperature of 64 K. We propose the crossover behavior can be attributed to the strong uniaxial anisotropy and inevitable interlayer coupling. Our experiment demonstrates the applicability of crossover behavior to a 2D ferromagnetic semiconductor.

Evolution of the formation of a covalent triazine-based framework catalyzed by p-toluenesulfonic acid monohydrate

A porous covalent triazine-based framework (CTF) was prepared by a two-step strategy, where p-toluenesulfonic acid monohydrate (TsOH·H2O) was used as a catalyst. The porosity and gas storage capacity of the as-obtained framework was evaluated. In addition, the evolution of the chemical structure during the two stages was studied. Crystallite size was calculated to study the formation of the framework; the evolution of internal porosity has been discussed based on the results obtained. In the first step, a triazine-based polymer was formed at relatively low temperatures, and limited crystallites were developed. However, in the second step, a treatment at higher temperatures performed in an open system resulted in the purification of the chemical structure; furthermore, CTF lamellae with an improved ordered stacking developed, resulting in a dramatic increase in internal porosity.

Anomalous Hall effect in two-dimensional non-collinear antiferromagnetic semiconductor Cr0.68Se

Cr0.68Se single crystals with two-dimensional (2D) character have been grown, and the detailed magnetization M(T), electrical transport properties (including longitudinal resistivity and Hall resistivity and thermal transport ones (including heat capacity Cp(T) and thermoelectric power (TEP) S(T)) have been measured. There are some interesting phenomena: (i) Cr0.68Se presents a non-collinear antiferromagnetic (AFM) semiconducting behavior with the Neel temperature TN = 42 K and the activated energy Eg=3.9 meV; (ii) It exhibits the anomalous Hall effect (AHE) below TN and large negative magnetoresistance (MR) about 83.7% (2 K, 8.5 T). The AHE coefficient RS is 0.385 cm-3/C at T=2 K and the AHE conductivity {sigma}H is about 1 ohm-1cm-1 at T=40 K, respectively; (iii) The scaling behavior between the anomalous Hall resistivity and the longitudinal resistivity is linear and further analysis implies that the origin of the AHE in Cr0.68Se is dominated by the skew-scattering mechanism. Our results may be helpful for exploring the potential application of these kind of 2D AFM semiconductors.

Multilayered structures of SrNb0.1Ti0.9O3/La0.8Sr0.2MnO3/SrTiO3 prepared by laser molecular beam epitaxy

Multilayer oxides of SrNb0.1Ti0.9O3/La0.8Sr0.2MnO3/SrTiO3 have been grown by computer-controlled laser molecular beam epitaxy and characterized by transmission electron microscopy. Electron microdiffractions and high-resolution imaging reveal that the as-prepared thin film of La0.8Sr0.2MnO3 with thickness of 200 nm is epitaxially grown on the SrTiO3(001) substrate and the SrNb0.1Ti0.9O3 With thickness of 250 nm epitaxially on the as-received La0.8Sr0.2MnO3 film. The microstructures in the La0.8Sr0.2MnO3 film are clarified in terms of the oriented rnicrodomains. In contrast, microstructures in SNTO are featured by the fort-nation of superstructures due to charge ordering. Crystallographic relationships of these domains are discussed on the basis of an orthorhombic cell and rationalized by theoretical calculations based on a geometrical model.