Wanyan Wang

Superconducting Phases in Potassium-Intercalated Iron Selenides

The ubiquitous coexistence of majority insulating 245 phases and minority superconducting (SC) phases in A(x)Fe(2-y)Se(2) (A = K, Cs, Rb, Tl/Rb, Tl/K) formed by high-temperature routes makes pure SC phases highly desirable for studying the intrinsic properties of this SC family. Here we report that there are at least two pure SC phases, K(x)Fe(2)Se(2)(NH(3))(y) (x ≈ 0.3 and 0.6), determined mainly by potassium concentration in the K-intercalated iron selenides formed via the liquid ammonia route. K(0.3)Fe(2)Se(2)(NH(3))(0.47) corresponds to the 44 K phase with lattice constant c = 15.56(1) Å and K(0.6)Fe(2)Se(2)(NH(3))(0.37) to the 30 K phase with c = 14.84(1) Å. With higher potassium doping, the 44 K phase can be converted into the 30 K phase. NH(3) has little, if any, effect on superconductivity. Thus, the conclusions should apply to both K(0.3)Fe(2)Se(2) and K(0.6)Fe(2)Se(2) SC phases. K(0.3)Fe(2)Se(2)(NH(3))(0.47) and K(0.6)Fe(2)Se(2)(NH(3))(0.37) stand out among known superconductors as their structures are stable only at particular potassium doping levels, and hence the variation of T(c) with doping is not dome-like.

Stable Oxoborate with Edge‐Sharing BO4 Tetrahedra Synthesized under Ambient Pressure

Analysis of an atom s coordination and the linkage of polyhedra is of vital importance for understanding crystal structures, especially considering our general inability to forecast structure types for new systems of elements. As a general rule, the presence of shared edges or faces of polyhedra in a coordinated structure is common for large cations, but scarcely seen for high-valence low coordinated small cations. This conclusion is the main thrust of the Pauling s third and fourth rules and is especially strict for compounds such as borates, 6] silicates, and phosphates. Until now, edge-sharing of those cation–oxygen (cation = B, Si, P) polyhedra was considered impossible except under extreme conditions. Unlike silicon and carbon, boron has the ability to bind to either three or four oxygen atoms to form a BO3 triangle or a BO4 tetrahedron. Polymerization of those B O blocks can give omnifarious types of anion groups (the BO3 and BO4 groups can occur isolated or linked in the form of rings, chains, layers, or networks) and endow over 1000 borate compounds with amazing structural diversity from triclinic symmetry to cubic symmetry. 11] On the basis of the borate structures discovered, Ross and Edwards in 1967 postulated that B O groups can only link to each other through common corners, not by edge-sharing or face-sharing. This hypothesis reduced the number of possible fundamental building blocks (FBB, the repeat B O block of the structure) greatly, making it possible for subsequent researchers to develop concise theories and clearer nomenclature for the unique borate structural chemistry. The hypothesis is valid except under extreme conditions. In 2002, Huppertz and van der Eltz claimed first violation of this hypothesis as they synthesized Dy4B6O15 under high pressure (HP) (8 GPa, 1000 K). Since then, several more edge-sharing HP borates have been synthesized under high-pressure/high-temperature conditions; thus, the appearance of edge-sharing BO4 is a significant phenomenon for distinguished HP borates. Herein, we present a novel borate KZnB3O6 synthesized under ambient pressure which is built from edge-sharing BO4 tetrahedra and is stable up to its melting point. Our work demonstrates that high pressure is not an indispensable prerequisite for the formation of edge-sharing BO4 polyhedra, and that the original hypothesis should be reexamined. KZnB3O6 was synthesized through solid-state reaction in air with K2CO3, H3BO3, and ZnO powders as the starting materials. The compound thus obtained is airand waterstable. The crystal structure was solved and refined on the basis of single-crystal data, which confirms the title compound to be the first ambient pressure borate with the edgesharing BO4 tetrahedra. Figure 1 shows the structure, in which the metal–borate framework is built up from corner-sharing B6O12 and Zn2O6 blocks, and weakly bonded K ions are

Observation of glassy ferromagnetism in Al-doped 4H-SiC.

Glassy ferromagnetism is observed in diluted magnetic semiconductor Al-doped 4H-SiC. We propose a possible explanation for the origin of ferromagnetism order that is the coeffect of sp(2)/sp(3) configuration along with the structural defects. This result unambiguously demonstrates the existence of intrinsic ferromagnetism order in nonmagnetic sp systems.

The perpendicular anisotropy of Co40Fe40B20 sandwiched between Ta and MgO layers and its application in CoFeB/MgO/CoFeB tunnel junction

Magnetic anisotropy of Co40Fe40B20 thin films sandwiched between Ta and MgO layers was investigated. Magnetic properties of CoFeB layers deposited on top and bottom of MgO layer are different. The thickness of the CoFeB layer and annealing temperature are the critical parameters to achieve and keep the perpendicular magnetic anisotropy. The phase diagram of perpendicular anisotropic strength of CoFeB layers on annealing temperatures and thicknesses of CoFeB layers is observed. According to phase diagrams, perpendicular CoFeB/MgO/CoFeB tunnel junctions were fabricated, and tunneling magnetoresistance (TMR) ratio was higher than 30% at low temperatures.

Magnetic properties of Mn-doped 6H-SiC

We report the synthesis and characterizations of low Mn-doped (<10−3u2002molar fraction) 6H-SiC. Raman scattering studies show an unusual shift in Raman peak with altering Mn contents. The magnetic properties measurement shows the typical ferromagnetic order was established at as low Mn-doped concentration as 10−4u2002molar fraction at around 250 K. It is speculated that the defects-related effects other than the Mn content play a more important role to determine the magnetic ordering.

Chemical diffusion: Another factor affecting the magnetoresistance ratio in Ta/CoFeB/MgO/CoFeB/Ta magnetic tunnel junction

This letter investigates the microstructure and mean inner potential (MIP) profile of Ta/CoFeB/MgO/CoFeB/Ta magnetic tunnel junctions (MTJs) by high resolution transmission electron microscopy (HRTEM) and electron holography, respectively. The inconspicuous crystallization of MgO barrier is confirmed by HRTEM in the post-annealed sample at 250u2009°C. An obvious MIP difference is displayed in the Ta layers between the top and bottom of the MTJ, and elemental content difference of them is confirmed by energy dispersive spectroscopy. These results imply that the chemical diffusion can also give rise to a lower tunnel magnetoresistance ratio besides the inconspicuous crystallization of MgO barrier.

Improved thermal stability of wet-oxidized AlAs

Lateral thermal wet oxidization of the AlAs layer in a GaAs/AlAs/GaAs sandwiched structure is studied by Raman spectroscopy and Nomarski microscopy. A significant improvement in thermal stability of the oxidized AlAs layer has been achieved by optimizing the oxidation conditions, which can be used to fabricate reliable devices. We show that the thermal stability is strongly related to the removal of volatile products, such as As and As2O3, as evidenced by the Raman spectroscopy measurement.

Exploring FeSe-based superconductors by liquid ammonia method

Our recent progress on the preparation of a series of new FeSe-based superconductors and the clarification of SC phases in potassium-intercalated iron selenides are reviewed here. By the liquid ammonia method, metals Li, Na, Ca, Sr, Ba, Eu, and Yb are intercalated in between FeSe layers and form superconductors with transition temperatures of 30 K~46 K, which cannot be obtained by high-temperature routes. In the potassium-intercalated iron selenides, we demonstrate that at least two SC phases exist, KxFe2Se2(NH3)y (x ≈ 0.3 and 0.6), determined mainly by the concentration of potassium. NH3 has little, if any, effect on superconductivity, but plays an important role in stabilizing the structures. All these results provide a new starting point for studying the intrinsic properties of this family of superconductors, especially for their particular electronic structures.

Epitaxial graphene on 4H-SiC by pulsed electron irradiation

Controlled sublimation of silicon on a SiC surface based on pulsed electron irradiation (PEI) is presented as an effective route to quality graphene. The PEI allows us to obtain graphene in millimetre-scale within three monolayers, and is a potential candidate for preparing high quality large graphene with controlled layers.

Experimental observation of ferromagnetism evolution in nanostructured semiconductor InN

Recently, undoped semiconductors have been pushed to the frontiers of diluted magnetic semiconductors (DMSs) since no spurious ferromagnetism (FM) signal from metal segregation occurs. However, some aspects still remain untouched in undoped semiconductors nanostructures, including: 1) the evolution of spin moments during the fabrication process; 2) possible intrinsic relationship between the magnetic properties and morphological features, the latter being also dependent on formation process. In this article, we synthesized undoped InN nanostructures and observed a gradual magnetic transition from diamagnetic to FM, and an obvious morphology evolution from tube-shape to wire-shape products with the increase of nitridation time. It is speculated that increasing surface defects such as N vacancies in the morphology transition process are the intrinsic causes for the enhanced FM order with increase of nitridation time. The results provide some useful clues in understanding the true magnetic origin in nanostructured DMSs and reported properties inconsistencies in nanostructured materials.