Boundary conductance in macroscopic bismuth crystals

The interface between a solid and vacuum can become electronically distinct from the bulk. This feature, encountered in the case of quantum Hall effect, has a manifestation in insulators with topologically protected metallic surface states. Non-trivial Berry curvature of the Bloch waves or periodically driven perturbation are known to generate it. Here, by studying the angle-dependent magnetoresistance in prismatic bismuth crystals of different shapes, we detect a robust surface contribution to electric conductivity when the magnetic field is aligned parallel to a two-dimensional boundary between the three-dimensional crystal and vacuum. The effect is absent in antimony, which has an identical crystal symmetry, a similar Fermi surface structure and equally ballistic carriers, but an inverted band symmetry and a topological invariant of opposite sign. Our observation confirms that the boundary interrupting the cyclotron orbits remains metallic in bismuth, which is in agreement with what was predicted by Azbel decades ago. However, the absence of the effect in antimony indicates an intimate link between band symmetry and this boundary conductance.

Boundary conductance protected by topology in macroscopic bismuth crystals

The interface between a solid and vacuum can become electronically distinct from the bulk. This feature, encountered in the case of quantum Hall effect, has a manifestation in insulators with topologically protected metallic surface states. Non-trivial Berry curvature of the Bloch waves or periodically driven perturbation are known to generate it. Here, by studying the angle-dependent magnetoresistance in prismatic bismuth crystals of different shapes, we detect a robust surface contribution to electric conductivity when the magnetic field is aligned parallel to a two-dimensional boundary between the three-dimensional crystal and vacuum. The effect is absent in antimony, which has an identical crystal symmetry, a similar Fermi surface structure and equally ballistic carriers, but an inverted band symmetry and a topological invariant of opposite sign. Our observation points to the relevance of band symmetries to survival of metallicity at the boundary interrupting the cyclotron orbits.

Conformational aspects of polymorphs and phases of 2-propyl-1H-benzimidazole

This paper reports on the polymorphism of 2-propyl-1H-benzimidazole (2PrBzIm) induced by temperature change. Upon heating, an irreversible reconstructive-type phase transition at T = 384 K from the ordered form I (P212121) to a new polymorph, form II HT (Pcam), was observed. The structural transformation between forms I and II involves significant changes in the crystal packing, as well as a key conformational variation around the propyl chain of the molecule. After the first irreversible phase transition, the II HT form undergoes two further (reversible) phase transitions upon cooling at 361 K (II RT) and 181 K (II LT). All three phases (forms II HT, II RT and II LT) have almost identical crystal packing and, given the reversibility of the conversions as a function of temperature, they are referred to as form II temperature phases. They differ, however, with respect to conformational variations around the propyl chain of 2PrBzIm. Energy calculations of the gas-phase conformational energy landscape of this compound about its flexible bonds allowed us to classify the observed conformational variations of all forms into changes and adjustments of conformers. This reveals that forms I and II are related by conformational change, and that two of the form II phases (HT and RT) are related by conformational adjustment, whilst the other two (RT and LT) are related by conformational change. We introduce the term `conformational phases' for different crystal phases with almost identical packing but showing changes in conformation.

CrystalCMP: an easy-to-use tool for fast comparison of molecular packing

A new approach is introduced for the comparison of molecular packing and the identification of identical crystal structure motifs. It has been tested on data sets for the solid forms of benzamide, cabergoline and trospium. In this approach, the packing similarity is calculated using a simple formula involving the distances between molecular centres and the relative orientations of molecular entities inside a finite molecular cluster. The approach is independent of the atomic labelling, the unit-cell parameters, the space group setting and the number of molecules in the asymmetric part of the unit cell. Owing to its low sensitivity to volume changes, this approach allows the comparison of various solid forms (such as polymorphs, hydrates, solvates, co-crystals or salts) of identical or similar molecular compounds. The method is also suitable for identifying similar results from direct space methods, which are often used in powder diffraction.

Scalable solution-phase epitaxial growth of symmetry-mismatched heterostructures on two-dimensional crystal soft template

Epitaxial heterostructures with precisely controlled composition and electronic modulation are of central importance for electronics, optoelectronics, thermoelectrics, and catalysis. In general, epitaxial material growth requires identical or nearly identical crystal structures with small misfit in lattice symmetry and parameters and is typically achieved by vapor-phase depositions in vacuum. We report a scalable solution-phase growth of symmetry-mismatched PbSe/Bi2Se3 epitaxial heterostructures by using two-dimensional (2D) Bi2Se3 nanoplates as soft templates. The dangling bond–free surface of 2D Bi2Se3 nanoplates guides the growth of PbSe crystal without requiring a one-to-one match in the atomic structure, which exerts minimal restriction on the epitaxial layer. With a layered structure and weak van der Waals interlayer interaction, the interface layer in the 2D Bi2Se3 nanoplates can deform to accommodate incoming layer, thus functioning as a soft template for symmetry-mismatched epitaxial growth of cubic PbSe crystal on rhombohedral Bi2Se3 nanoplates. We show that a solution chemistry approach can be readily used for the synthesis of gram-scale PbSe/Bi2Se3 epitaxial heterostructures, in which the square PbSe (001) layer forms on the trigonal/hexagonal (0001) plane of Bi2Se3 nanoplates. We further show that the resulted PbSe/Bi2Se3 heterostructures can be readily processed into bulk pellet with considerably suppressed thermal conductivity (0.30 W/m·K at room temperature) while retaining respectable electrical conductivity, together delivering a thermoelectric figure of merit ZT three times higher than that of the pristine Bi2Se3 nanoplates at 575 K. Our study demonstrates a unique epitaxy mode enabled by the 2D nanocrystal soft template via an affordable and scalable solution chemistry approach. It opens up new opportunities for the creation of diverse epitaxial heterostructures with highly disparate structures and functions.

Phase composition of Bi2O3 specimens doped with Ti, Zr and Hf

Powder mixtures of ?-Bi2O3 containing 2, 5 and 10 mole % of TiO2, ZrO2 or HfO2 were homogenized, heated at 820?C for 24 h and quenched in air. X-ray powder diffraction technique was used to characterize the prepared samples. In all cases metastable Bi2O3 polymorphs, ?-Bi2O3 or ?-Bi2O3, are found as single or major phases. Addition of Ti4+ ions stabilizes ?-Bi2O3 polymorph, while either Zr4+ or Hf4+ ions stabilize ?-Bi2O3 polymorph. In the samples with 2 and 5 mole % of TiO2 the presence of even two ?-Bi2O3 phases (Bi12TiO20 compound and a very low Ti-doped ?-Bi2O3) was established. Similarly, in the sample with 2 mole % of HfO2 two ?-Bi2O3 phases were found. Phase composition of prepared samples, values of unit cell parameters and the appearance of two polymorphs with identical crystal structure but different unit cell parameters are discussed and compared with known data.

Hydrolyses of α- and β-cellobiosyl fluorides by Cel6A (cellobiohydrolase II) of Trichoderma reesei and Humicola insolens

We have measured the hydrolyses of α- and β-cellobiosyl fluorides by the Cel6A [cellobiohydrolase II (CBHII)] enzymes of Humicola insolens and Trichoderma reesei, which have essentially identical crystal structures [Varrot, Hastrup, Schülein and Davies (1999) Biochem. J. 337, 297-304]. The β-fluoride is hydrolysed according to Michaelis-Menten kinetics by both enzymes. When the ~ 2.0% of β-fluoride which is an inevitable contaminant in all preparations of the α-fluoride is hydrolysed by Cel7A (CBHI) of T. reesei before initial-rate measurements are made, both Cel6A enzymes show a sigmoidal dependence of rate on substrate concentration, as well as activation by cellobiose. These kinetics are consistent with the classic Hehre resynthesis-hydrolysis mechanism for glycosidase-catalysed hydrolysis of the ‘wrong’ glycosyl fluoride for both enzymes. The Michaelis-Menten kinetics of α-cellobiosyl fluoride hydrolysis by the T. reesei enzyme, and its inhibition by cellobiose, previously reported [Konstantinidis, Marsden and Sinnott (1993) Biochem. J. 291, 883-888] are withdrawn. 1H NMR monitoring of the hydrolysis of α-cellobiosyl fluoride by both enzymes reveals that in neither case is α-cellobiosyl fluoride released into solution in detectable quantities, but instead it appears to be hydrolysed in the enzyme active site as soon as it is formed.

Electron-Doped High Tc Superconductors

Since the discovery of high temperature superconductivity in layered copper-oxide compounds in the latter part of 1986, an enormous amount of research has been carried out on these remarkable materials. Prior to 1989, the prevailing view was that the charge carriers responsible for superconductivity in these materials were holes that move through conducting CuO2 planes. The CuO2 planes are the basic building blocks of the crystal structures of all the presently known oxides with superconducting critical temperatures Tc greater than ~30 K. Recently, new superconducting materials have been discovered in Japan and the United States in which the charge carriers involved in the superconductivity appear to be electrons, rather than holes, that reside within the conducting CuO2 planes. These findings could have important implications regarding viable theories of high temperature superconductivity as well as strategies for finding new high temperature superconductors.The new electron-doped materials have the chemical formula Ln2-xMxCuO4-y and exhibit superconductivity with superconducting critical temperatures Tc as high as ~25 K for x ≍ 0.15 and y ≍ 0.02. Superconductivity has been discovered for M = Ce and Ln = Pr, Nd, Sm, and Eu, and for M = Th and Ln = Pr, Nd, and Sm. A related compound with the identical crystal structure, Nd2CuO4-x-y Fx, has also been found to display superconductivity withTc ≍ 25 K. Recently, it has been observed that superconductivity with Tc ≍ 25 K can even be induced in nonsuperconducting Nd2-xCexCuO4-y compounds by substituting Ga or In for Cu. Thus, it appears that the CuO2 planes can be doped with electrons, rendering the Ln2CuO4-y parent compounds metallic and superconducting, by substituting electron donor elements at sites within, as well as outside, the CuO2 planes; i.e., by substituting (1) Ce4+ or Th4+ ions for Ln3+ ions; (2) F1- ions for O2- ions; and (3) Ga3+ or In3+ ions for Cu2+ ions.

Superconductivity and Stoichiometry in Tl2Ca2Ba2Cu3Ox Single Crystals

Magnetization and resistivity data are presented which show a substantial difference in superconducting transitions for nominal Tl2Ca2Ba2Cu3Ox plates with identical crystal structures grown from two different “melt” compositions. A Tl-O and CuO-rich flux yielded approximately stoichiometric plates which have sharp transitions beginning near 111 K, while a second Tl-O-rich flux produced plates containing more Tl and less Ba with very broad transitions starting near 96 K. These data demonstrate the extreme sensitivity of superconductivity to cation site disorder in the Tl-Ca-Ba-Cu-O system.