S. L. Qiu

Time dependence of the soliton density in K2ZnCl4: Dielectric evidence for pair annihilation in the intrinsic chaotic state

Abstract The excess dielectric constant or dielectric tail below T c has been investigated in K 2 ZnCl 4 . With decreasing impurity content the tail becomes longer, and its relaxation at constant temperature becomes slower, indicating that the metastable chaotic soliton state is intrinsic rather than impurity induced. Observation of the decay at different temperatures in a well-annealed crystal suggests that once the soliton density decays below a critical value n s * , it then follows a square-law curve characteristic of pair-annihilation mediated decay.

The formation of metal–oxygen species at low temperatures

The interaction of solid molecular oxygen with Li, Cs, K, La, Ag, Cu, and Ba has been studied at 35 K or below using photoemission. A feature near 535 eV in the O 1s core‐level spectra was observed when Li, Cs, K, and La were deposited on solid oxygen. This feature was identified with one electron being donated to an oxygen molecule, i.e., the superoxide species, which as far as we know has not been previously reported for La and Li. A feature at about 531.5–533 eV was identified as a peroxide species where two electrons were donated to an oxygen molecule. Finally, features at about 528–530.5 eV were identified as oxide phases where the molecular oxygen was dissociated into atomic O with formal oxidation state of −2. These identifications are crucial in the determinations of the exotic features in the x‐ray photoelectron spectroscopy (XPS) O 1s spectra of the high Tc superconductors.

Structural anomalies in hcp metals under pressure: Zn and Cd

First-principles total-energy calculations with WIEN2k using a procedure that finds the equilibrium states of hcp structures under pressure from minima of the Gibbs free energy have found a structural anomaly in hcp Zn and in hcp Cd under pressure. The calculated small but definite anomaly in the pressure dependences of the structural parameters of hcp Zn allows a reinterpretation of the data to show the anomaly. We find a similar but stronger anomaly in Cd than in Zn. The calculated anomaly in Zn contradicts a recent theoretical conclusion that found that the anomaly disappears at a large number of k-points in the Brillouin zone. The calculation also shows that the uncertainty in locating equilibrium found in another recent paper is absent here. Reasons are given for computational differences from previous work. Evaluation of the pressure dependence of various elastic quantities which are much more sensitive to the anomaly shows the anomalies in hcp Zn and hcp Cd exist over a considerable range of pressure; several abrupt changes in the electron distribution are thereby indicated in that pressure range.

The mechanism of antiferromagnetism in chromium

It is shown that two special properties of Cr are needed to explain its antiferromagnetism. One special property is the well known sensitivity to antiferromagnetic spin-density waves due to nesting of the Fermi surface. A second new special property comes from first-principles total-energy calculations on bcc Cr, which show that, although the lowest energy state is nonmagnetic, a small expansion of the lattice brings a second-order transition into a type-I antiferromagnetic phase with rapidly rising local moments. The combined properties provide a mechanism for stabilization of the unusual antiferromagnetic ground state, since a spin-density wave which modulates the moments of the antiferromagnetic phase can be used to compensate the strain energy of the lattice expansion. This combined mechanism also explains various properties of Cr, such as the great sensitivity of the antiferromagnetism to pressure, that are otherwise puzzling.

Magnetic structure in FeCr and FeCo

The occurrence of magnetic phases in FeCr and FeCo in the CsCl structure has been studied by first-principles total-energy calculations with a sensitive and accurate method, using a four-atom unit cell. When both atoms in such binary compounds can be strongly magnetic, unusual structures are found. These materials have ferromagnetic (FM) and antiferromagnetic (AF) phases, but the equilibrium state for both is FM. However at 3% expansion of the lattice constant the ground state of FeCr becomes AF, whereas in FeCo the AF phase is never the ground state. The AF phase in both FeCr and FeCo has an unusual structure in that both the Fe and the Cr or Co sublattices are separately AF. In both the FM and AF phases at the equilibrium volume, the Fe moment is reduced in FeCr, but enhanced in FeCo from that in pure body-centered-cubic (bcc) Fe; also the Cr moment is increased from that in pure bcc Cr, while the Co moment is similar to the moment in hexagonal-dose-packed Co. In the FM phase of FeCr, but not in FeCo, t...

Phases of Ca from first principles

Structures and properties of many of the phases of Ca under pressure are calculated from first principles by a systematic procedure that minimizes total energy E with respect to structure under the constraint of constant volume V. The minima of E are followed on successive sweeps of lattice parameters for 11 of 14 Bravais symmetries for one-atom-per-cell structures. The structures include the four orthorhombic phases. Also included are the hexagonal close-packed and cubic diamond phases with two atoms per primitive cell. No uniquely orthorhombic phases are found; all one-atom orthorhombic phases over a mega-bar pressure range are identical to higher-symmetry phases. The simple cubic phase is shown to be stable where it is the ground state. The number of distinct one-atom phases reduces to five plus the two two-atom phases. For each of these phases the Gibbs free energy at pressure p, G(p), is calculated for a non-vibrating lattice; the functions G(p) give the ground state at each p, the relative stabilities of all phases and the thermodynamic phase transition pressures for all phase transitions over a several-megabar range.

On the importance of the free energy for elasticity under pressure

It is shown that the theory of elasticity under hydrostatic pressure p at zero temperature is unified and simplified by the use of the Gibbs free energy G, rather than the energy E. The minima of G, but not of E, give the equilibrium structure; the second derivatives of G, but not of E, with respect to strains at the equilibrium structure give the zero-temperature elastic constants; the stability of a phase at p is then determined by the same Born stability conditions used at p = 0 when applied to the elastic constants from G. Examples are noted of mistakes due to use of E rather than G.

Tetragonal equilibrium states of Mn and Fe

Tetragonal equilibrium states of Mn and Fe have been found by total-energy calculations at constant volume as a function of c/a with the full-potential linearized-augmented-plane-wave method using two different potentials: (1) the local-spin-density approximation without relativistic corrections and (2) the Perdew–Burke–Ernzerhof exchange-correlation potential in a generalized-gradient approximation with relativistic corrections. Comparison of potential (1) with potential (2) shows that the energy curves relative to the lowest minimum of each are very similar and have minima at the same c/a values. However, potential (2) makes the magnetic phases more magnetic. Both Mn and Fe are shown to have stable and metastable tetragonal equilibrium states in each of several magnetic phases. The antiferromagnetic (AF) energy versus c/a curve of Mn shows a stable tetragonal state at c/a=0.96, close to the experimental value for γ-Mn at c/a=0.95, and a metastable body-centered-tetragonal state at c/a=0.60. However the ...

The magnetic phases of face-centred-cubic iron

The energy bands and magnetic moments of the magnetic phases of face-centred-cubic iron are calculated from first principles with the augmented spherical wave method and the fixed-spin-moment procedure. An antiferromagnetic phase that requires a four-atom cell to exist is added. This phase is the current ground state of the bulk layers of Fe film epitaxial on Cu(001). A ferrimagnetic phase and a very-low-spin phase are also found and compared with previous work. In a narrow range of volume that includes the bulk layers of the Fe films on Cu(001) at least eight magnetic phases exist.

K promoted oxidation of Al and Ta

Abstract Core level and valence band photoemission have been used to study the response to oxygen exposure of potassium-precovered, polycrystalline aluminum and tantalum substrates. Deposition of potassium overlayers in the coverage regime from zero to one monolayer, results in an increase of the rate of oxygen uptake for both substrates. The increased oxygen uptake is, however, relatively stronger on aluminum. Correlated measurements of work function and oxygen uptake show that there is a correspondence between these two parameters for low K coverages, i.e., lower work function results in higher oxygen uptake. These results may be understood in terms of a model in which the alkali atoms are increasing the rate of dissociation of oxygen molecules at the surface, as well as lowering the barrier for oxygen penetration into the surface, through a reduction in the system work function.