Hui Xing

Mechanical twinning and omega transition by ⟨111⟩ {112} shear in a metastable β titanium alloy

{112} ⟨111⟩ mechanical twinning and stress-induced omega transition were observed by high-resolution transmission electron microscope in a metastable β titanium alloy with chemical composition of Ti-23Nb-0.7Ta-2Zr-1.2O at.u2009% after deformation. The orientation relationships between the ω phase and β parent matrix are (1¯010)ω‖(211)β, [12¯10]ω‖[01¯1]β and [0001]ω‖[1¯11]β, and the habit plane of (1¯010)ω‖(211)β for the stress-induced ω transition is different from that of (0001)ω‖(111)β often observed for the thermal ω transition. Both mechanical twinning and ω transition arise from the shear along ⟨111⟩ {112}. A dislocation mechanism for mechanical twinning and stress-induced ω transition was discussed additionally.

Structural stability and elastic property of the L12 ordered Co3(Al,W) precipitate

The method of augmented plane wave plus local orbitals within the generalized gradient approximation has been employed to investigate the structural stability and elastic properties of the ternary Co3(Al,W) precipitate. The results showed that the stable Co3(Al,W) compound has L12 ordered structure. The calculated results of elastic properties show that the L12 Co3(Al,W) precipitate has a strengthening effect in the disordered fcc cobalt matrix, owing to the larger modulus differences between Co3(Al,W) and cobalt matrix. The theoretical calculations also show that the Co3(Al,W) compound is brittle in nature.

Elastic properties of β, α′′ and ω metastable phases in Ti–Nb alloy from first-principles

The method of pseudopotentials within the generalized gradient approximation has been employed to calculate the phase stability, electronic structure and elastic constants of β, α and ω metastable phases in the Ti–25 at.% Nb alloy. The bulk, shear and Youngs moduli of those metastable phases are estimated from the theoretical elastic constants by the Voigt–Reuss–Hill averaging method. The results show that the phase stability of those different metastable phases follows the order of α>ω>β from the viewpoint of energetic and electronic structure. The bulk moduli of β, α and ω phases are close, but the shear and Youngs moduli increase with β, α and ω phase in that order.

Origin of substantial plastic deformation in Gum Metals

The elastic property and structure of dislocation in Gum Metal are investigated by anisotropic elastic theory and high-resolution transmission electron microscopy (HRTEM). The results show that the elastic energy coefficients for the 1∕2⟨111⟩ perfect dislocations nearly equal to zero as the shear modulus along ⟨111⟩{110}, {112}, {123} when the valence electron number e∕a reaches 4.2, which implies a low intrinsic critical resolved shear stress for dislocation glide. HRTEM observations further revealed 70.53° dislocations in Gum Metal after severe cold working. The substantial plastic deformation is considered to originate from the conventional dislocation mechanism, rather than from the dislocation-free mechanism in Gum Metals.

Lithium ion intercalation in thin crystals of hexagonal TaSe2 gated by a polymer electrolyte

Ionic liquid gating has been used to modify the properties of layered transition metal dichalcogenides (TMDCs), including two-dimensional (2D) crystals of TMDCs used extensively recently in the device work, which has led to observations of properties not seen in the bulk. The main effect comes from the electrostatic gating due to the strong electric field at the interface. In addition, ionic liquid gating also leads to ion intercalation when the ion size of the gate electrolyte is small compared to the interlayer spacing of TMDCs. However, the microscopic processes of ion intercalation have rarely been explored in layered TMDCs. Here, we employed a technique combining photolithography device fabrication and electrical transport measurements on the thin crystals of hexagonal TaSe2 using multiple channel devices gated by a polymer electrolyte LiClO4/Polyethylene oxide (PEO). The gate voltage and time dependent source-drain resistances of these thin crystals were used to obtain information on the intercalati...

Existence of electron and hole pockets and partial gap opening in the correlated semimetal Ca 3 Ru 2 O 7

The electronic band structure of correlated Ca3Ru2O7 featuring an antiferromagnetic as well as a structural transition has been determined theoretically at high temperatures, which has led to the understanding of the remarkable properties of Ca3Ru2O7 such as the bulk spin valve effects. However, its band structure and Fermi surface (FS) below the structural transition have not been resolved even though a FS consisting of electron pockets was found experimentally. Here we report magneto electrical transport and thermoelectric measurements with the electric current and temper- ature gradient directed along a and b axes of an untwined single crystal of Ca3Ru2O7 respectively. The thermopower obtained along the two crystal axes were found to show opposite signs at low temperatures, demonstrating the presence of both electron and hole pockets on the FS. In addition, how the FS evolves across T* = 30 K at which a distinct transition from coherent to incoherent behavior occurs was also inferred - the Hall and Nernst coefficient results suggest a temperature and momentum dependent partial gap opening in Ca3Ru2O7 below the structural transition, with a pos- sible Lifshitz transition occurring at T*. The experimental demonstration of a correlated semimetal ground state in Ca3Ru2O7 calls for further theoretical studies of this remarkable material.

Dimensional reduction and ionic gating induced enhancement of superconductivity in atomically thin crystals of 2H-TaSe 2

The effects of dimensional reduction and ion intercalation on superconductivity (SC) in the presence of charge density waves (CDWs) in two-dimensional crystals of 2H-TaSe2 were characterized. We prepared atomically thin crystals by mechanical exfoliation and performed electrical transport measurements on devices made by photolithography. The superconducting transition temperature (T c SC ) was found to increase monotonically as the thickness decreased, changing from 0.14 K in the bulk to higher than 1.4 K for a 3-nm-thick crystal. The temperature dependence of upper critical field was found to be anomalous. The CDW transition temperature (T c CDW ) was found to decrease, but to a less extent than T c SC , from 120 K in the bulk to around 113 K for the 3-nm-thick crystal. In addition, ion intercalation was found to increase T c SC and suppress T c CDW in an atomically thin crystal of 2H-TaSe2. The implications of these findings are discussed. We suggest that dimensional reduction and ion intercalation are potentially effective ways to engineer material properties for layered transition metal chalcogenides.

Band dependence of charge density wave in quasi-one-dimensional Ta2NiSe7 probed by orbital magnetoresistance

Ta2NiSe7 is a quasi-one-dimensional (quasi-1D) transition-metal chalcogenide with Ta and Ni chain structure. An incommensurate charge-density wave (CDW) in this quasi-1D structure was well studied previously using tunnelling spectrum, X-ray and electron diffraction, whereas its transport property and the relation to the underlying electronic states remain to be explored. Here we report our results of magnetoresistance (MR) on Ta2NiSe7. A breakdown of the Kohlers rule is found upon entering the CDW state. Concomitantly, a clear change of curvature in the field dependence of MR is observed. We show that the curvature change is well described by two-band orbital MR, with the hole density being strongly suppressed in the CDW state, indicating that the

Probing the intrinsic charge transport in indacenodithiophene-co-benzothiadiazole thin films

p

Evolution of Microstructure and Texture during Recrystallization of the Cold-Swaged Ti-Nb-Ta-Zr-O Alloy

orbitals from Se atoms dominate the change in transport through the CDW transition.