Zone Face Research Articles

Evidence for Quasiparticle Decay in Photoemission from Underdoped Cuprates

I argue that the ``gap'' recently observed at the Brillouin zone face of cuprate superconductors in photoemission by Marshall et al [Phys. Rev. Lett. 76, 4841 (1996)] and Ding et al [Nature 382, 54 (1996)] is evidence for the decay of the injected hole into a spinon-holon pair.

Odd parity and line nodes in heavy-fermion superconductors.

Group theory arguments have demonstrated that a general odd parity order parameter cannot have line nodes in the presence of spin-orbit coupling. In this paper, it is shown that these arguments do not hold on the $k_z = \pi/c$ zone face of a hexagonal close packed lattice. In particular, three of the six odd parity representations vanish identically on this face. This has potential relevance to the heavy fermion superconductor $UPt_3$.

Single-hole properties in thet-Jand strong-coupling models

We report numerical results for the single-hole properties in the $t$-$J$ model and the strong-coupling approximation to the Hubbard model in two dimensions. Using the hopping basis with over $10^6$ states we discuss (for an infinite system) the bandwidth, the leading Fourier coefficients in the dispersion, the band masses, and the spin-spin correlations near the hole. We compare our results with those obtained by other methods. The band minimum is found to be at ($\pi/2,\pi/2$) for the $t$-$J$ model for $0.1 \leq t/J \leq 10$, and for the strong-coupling model for $1 \leq t/J \leq 10$. The bandwidth in both models is approximately $2J$ at large $t/J$, in rough agreement with loop-expansion results but in disagreement with other results. The strong-coupling bandwidth for $t/J\agt6$ can be obtained from the $t$-$J$ model by treating the three-site terms in first-order perturbation theory. The dispersion along the magnetic zone face is flat, giving a large parallel/perpendicular band mass ratio.

Hole motion in quantum antiferromagnets: String and hopping results.

We have investigated the {ital t}-{ital J} model and its variants using the string basis (of Shraiman, Siggia, Emery, and Lin) and the hopping basis (of Trugman); both bases are generated by the hopping term in the Hamiltonian applied repeatedly to the Neel state with a single hole. For the string basis, states to order {ital t}{sup 10} are treated exactly, so that the Green function is obtained to order {ital t}{sup 20}; the tree approximation (of Brinkman and Rice) is used for the remainder. The hopping basis, which relaxes constraints imposed in the string basis, contains all states generated by up to eight hops. Properties studied include the wave function, the ground-state energy, the effective mass, the bandwidth, the spectral function, the self-energy, and the density of states. For the {ital U}={infinity} model, there are no quasiparticles; the ferromagnetic polaron is missed. For the {ital t}-{ital J}{sub {ital z}} model, both the string and hopping bases provide excellent results for {ital J}{sub {ital z}}{gt}0.1{ital t}. The Ising limit of the {ital t}-{ital J} model is treated well by the string basis, but the Heisenberg limit {ital J}{sub {perpendicular}}={ital J}{sub {ital z}} requires the hopping basis, which gives apparently goodmore » results for all {ital J}{gt}0.1{ital t}. The mass is highly anisotropic; for example, at {ital J}=0.4{ital t} the masses parallel and perpendicular to the magnetic zone face are {ital m}{sub N}=20.4{ital m} and {ital m}{sub {perpendicular}}=2.1{ital m}.« less

Small-cluster study of a single-hole t-J model.

We have studied hole states on square lattices with 8, 10, 16, 18, and 20 sites by exactly diagonalizing the single-hole t-J model with twisted boundary conditions in the whole Brillouin zone. The bottom of the hole band is found to be extremely flat and anisotropic. The hole effective mass is highly anisotropic and enhanced in a quantum antiferromagnet. The momentum of a hole in the ground state is not located at high-symmetry points such as (\ifmmode\pm\else\textpm\fi{}\ensuremath{\pi}/2,\ifmmode\pm\else\textpm\fi{}\ensuremath{\pi}/2), (\ifmmode\pm\else\textpm\fi{}\ensuremath{\pi},0), or (0,\ifmmode\pm\else\textpm\fi{}\ensuremath{\pi}). They are strongly lattice-size dependent and weakly parameter J/t (t is the hopping constant and J is the exchange constant) dependent. As the lattice size increases, the momentum of the ground state approaches the zone face center (\ifmmode\pm\else\textpm\fi{}\ensuremath{\pi}/2,\ifmmode\pm\else\textpm\fi{}\ensuremath{\pi}/2).

Positron annihilation in InPb alloy single crystals

Electron momenta in Pb and PbIn alloys were measured by the positron annihilation technique. The distributions of momentum observed in single crystals of the specimens showed appreciable anisotropy. A model of the known Fermi surface of Pb near the (111) zone face was adjusted to fit the data. A similar model for the alloys was used to determine kF as a function of In concentration in the range 0 < In < 48 at.%. The results are remarkably like free electron theory.

A possible resolution of the valence-band offset controversy in HgTe/CdTe superlattices

The valence-band offset controversy in HgTe/CdTe superlattices can be simply resolved by showing, as we do here, that a large offset value (Λ>300 meV) is indeed consistent with the magneto-optical data of Berroir et al. [Phys. Rev. B 34, 891 (1986)]. The superlattice (100 Å HgTe/36 Å CdTe) becomes semimetallic (SM) when Λ is increased from small values, but reverts to semiconducting (SC) behavior for Λ>300 meV. The band gap in this new regime occurs at the superlattice Brillouin zone face. The sensitivity to layer thicknesses of such SC→SM→SC transitions is discussed and experiments suggested.

The effect of proximity of the Fermi level to singularities in the electron state density on elastic and thermodynamic properties of metals and alloys

The authors investigate the effect of the proximity of the Fermi surface (FS) to a singularity in the electron state density on atomic properties of a nearly free-electron metal. Expressions for thermodynamic potentials in the improved version of the pseudopotential perturbation theory are given which generalise the conventional formulae to the case of small distances eta between the FS and the Brillouin zone (BZ) face and also take into account the Fermi liquid interaction effects. The case of the FS proximity to the BZ edge is also considered. The character of the eta dependence of atomic properties is illustrated by model calculations for disordered Li1-xMgx alloys in the BCC, FCC and HCP structures. They calculate the concentration dependencies of the elastic constants cik and their volume derivatives delta cik/ delta u, the low-temperature phonon and electron contributions to the specific heat Cv and to the thermal expansion coefficient beta T, and the equilibrium lattice parameters. All the quantities have peculiar anomalies at small eta which can be rather large, especially in delta cik/ delta u and beta T. The results are compared with the available experimental data.

On the size and shape of the process zone

We discuss a procedure for determining the size and shape of the process zone in an elastoplastic material. The basic features of Rice-Johnson elastoplastic analysis are utilized together with a linear distribution of the process zone forces and the smooth closure condition for the process zone faces. Three different methods are adopted for the calculation: one based on the elimination of the plastic singularity at the crack tip, another on the balance of energy at the onset of crack motion, and the third on an expression for the crack opening angle. Numerical results pertaining to these calculations are listed.

Magnetic breakdown above the “compressibility collapse” transition in ReO 3

We report a new cross-sectional area of the ReO 3 Fermi surface above the compressibility collapse transition. Magnetic breakdown at low fields destroys the gaps at the new Brillouin zone faces of the high-pressure phase so that the observed Fermi surface is essentially identical in both phases at the fields where it can be observed.

Experimental uniaxial stress and strain derivatives of the Fermi surface of rhenium

The derivatives of the areas of extremal orbits on all the small sheets of the Fermi surface of rhenium, with respect to stress and strain along the hexad axis, have been determined from simultaneous measurements of Landau quantum oscillations in magnetostriction and torque, and also in sound velocity and torque. Strong anisotropy is observed in the stress derivatives of orbits in zones five and six as the direction of the magnetic field defining the normal to the orbit is varied; the anisotropy is most pronounced for orbits which come close to the line of degeneracy AL on the hexagonal Brillouin zone face. The derivatives of the small void in zone eight are found to be very large; this is consistent with the results of band structure calculations which show that this feature of the Fermi surface is very sensitive to small changes in the Fermi energy. Cyclotron effective masses for a number of orbits on the void have also been measured.

Structural properties of Gd(Co xIr 1−x) 2

The pseudobinary series Gd( Co x Ir 1− x ) 2 has been found to retain the C15 cubic Laves phase structure of its terminal compounds. It shows correlated sharp changes in structure and magnetic properties around x = 0.4. The lattice contracts while a Co moment begins to form. These effects may be interpreted as arising from an interaction between the Fermi surface and Brillouin zone faces.

The Fermi surface of the HCP Cu-Ge alloys containing 12-15 at.% Ge studied by positron annihilation

The Fermi surface of the Cu-Ge alloy in the HCP zeta phase field is studied by measurements of momentum distributions of annihilation photons from two single crystals at e/a=1.38 and 1.45. The Fermi surface of the alloy at e/a=1.38 lies inside the Jones zone, and has a high degree of contact with the zone faces. A normal to (10.0) and a small degree of contact with the zone faces B normal to (00.1). The overlap of the Fermi surface across the A faces is observed for the alloy at e/a=1.45. Remarkable bulges of the Fermi surface are detected along the directions towards the faces C normal to (10.1) in both the compositions. The present results reveal the correlation between the overlapping of the Fermi surface across the zone faces and the change of lattice constant. The relative stability of the HCP zeta phase is discussed in the light of the experimental results.

Composition dependence of the thermodynamic activity and lattice parameter of ζ nickel-indium

The vapor pressure of indium over six alloys in the ζ phase of the nickel-indium system was measured by the method of atomic absorption. Values of thermodynamic activity were calculated from the vapor pressure, and partial heat and entropy of indium were calculated from the temperature coefficients. The lattice parameters of the hexagonal B8 2 unit cell of all alloys were calculated from X-ray diffraction powder patterns. It was found that the a lattice parameter passed through a minimum at the same composition that the excess chemical potential showed a sharp change of slope, when graphed as a function of composition. These effects were similar to those observed previously which have been attributed to overlap by the Fermi surface of a Brillouin zone face. In the present case they were attributed to overlap of the Fermi surface across faces tentatively identified as the {110} faces of the Brillouin zone of the B8 2 structure. The influence of substitutional disorder was also considered as a cause of the thermodynamic effects, but this was rejected because it does not explain the minimum in lattice parameter.

Angular Correlation of Photons from Positron Annihilation in Copper-Nickel Alloys

Angular correlation measurements of radiation from positrons annihilating in copper-nickel alloys have been carried out with crossedslit geometry using single crystals containing less than 73 at % Ni. The neck radius of the Fermi surface decreases linearly with increasing nickel content, but the neck does not detach from the hexagonal zone face even at the highest nickel content. No marked change in the composition dependence of neck radius is observed at the magnetic phase boundary near 70% Ni. The neck radius of paramagnetic nickel is estimated to be about 0.13 in unit of that for copper from the extrapolation of the observed data.

An Examination of the Angular Correlation of Annihilation Radiation in Silicon Single Crystal

The angular correlation of annihilation radiation along the [110]-axis in silicon has been measured at 120∼130 K with a special care in orienting the crystal. As predicted by a recent many-body theory of the annihilation of free positrons in a simplest nearly-free-electron system (K. Fujiwara, T. Hyodo and J. Ohyama: J. Phys. Soc. Japan 33 (1972) 1047), the cut-off of the narrow component at the angle corresponding to the filled zone face has been found to be sharper than that given by the independent-particle computation of Stroud and Ehrenreich.

Observation of de Haas-van Alphen oscillations in a lithium dispersion

De Haas-van Alphen oscillations of magnetic susceptibility have been observed in a lithium dispersion in paraffin wax. Analysis of the results provides an upper bound for the distortion of the Fermi surface, which is inconsistent with contact with the Brillouin zone faces. An estimate has been obtained for the mean value of the cyclotron mass.

Enhancement of Annihilation Rate of Electron-Positron pairs in Periodic Fields

The enhancement of the partial annihilation rate of electron-positron pairs in crystals has been examined by assuming a two-band structure of electrons and using the lowest-order ladder graphs with static interaction. The intraband transitions of electrons due to the attractive force of positrons cause the enhancement of the rate over all the components of the momentum, while the interband transitions from the lower to the upper give a marked dehancement of HMC's (higher-momentum components) despite that the total annihilation rate remains still enhanced. Numerical computation has been made for several one-dimensional cases, and an asymmetry with respect to the zone face has been found even in the enhancement due to intraband transitions. An angular-correlation measurement has been made for silicon, and an evidence for the dehancement of HMC's has been found by comparing the date with the independent-particle theory of Stroud and Ehrenreich (Phys. Rev. 171 (1968) 399).

Model calculation of the electron phonon coupling parameter λ

A simple model is described, in which the influence of two parallel Brillouin zone faces on the electron phonon coupling parameter λ can be studied. In this model it is observed that λ, considered as a function of the Fermi energyEitF, has a peculiar structure when the Fermi body just passes through the two Bragg planes. Both an enhancement as well as a reduction of the more familiar density of states effect on λ and therefore on the critical temperatureTitc of superconductors appears to be possible. In this respect approaching a Brillouin zone face turns out to be very favourable.

The Hall Coefficients of Li-Mg Alloys

The temperature and concentration dependences of the Hall coefficients in β-phase of lithium-magnesium alloys (0∼70 at.%Mg) have been measured from 77°K to the room temperature. In the temperature range investigated, Hall coefficient is almost independent of the temperature except pure lithium. The dependence of Hall coefficient ( R H ) on electron-atom ratio (e/a) is approximately represented by a third-degree polynomial; R H ×10 11 (m 3 /A.s)=85.58(e/a) 3 -333.31(e/a) 2 +443.00(e/a)-210.80. Although there is fluctuation to some extent in the experimental data, a distinct agreement is found between the experimental results and the prediction by four-OPW calculation. For this reason, the four-OPW model is thought to be a good approximation for this alloy phase. The experimental result suggests the contact of the Fermi surface to the {110} zone boundary at e/a=1.3. The result also suggests that overlap through {110} zone face does not occur in the measured range.