E.W. Fenton

Ordering in Si1−xGex crystals

Abstract Evidence in transmission electron microscopy and Raman scattering supports the claim by Ourmazd and Bean that the unit cell in Si 1− x Ge x grown by molecular beam epitaxy is doubled in a 〈111〉 direction by weak long-range order of Si and Ge atoms.

Characterization of MBE grown Si/GexSi1−x strained layer superlattices

Abstract The physical properties of both single GexSi1−x epilayers and GexSi1−x/Si strained layer superlattices (SLSs) grown by MBE on Si(100) substrates have been investigated. Raman scattering studies of Si/GexSi1−x superlattices of period d = dSi + d GexSi1−x, with 17 ⩽ d ⩽ 65 nm, 10 ⩽ dSi ⩽ 45 nm, 4 ⩽ dGexSi1−x ⩽ 20 nm and 0.2 ⩽ × ⩽ 0.5, revealed a series of low-frequency Raman lines associated with zone-folded acoustic phonons. The frequency shifts of optic phonons in SLSs compared with bulk alloys were used to reveal the strain distribution and composition of GexSi1−x epilayers. Specific phonon features are observed in the alloy spectra that are attributed to Si-Ge ordering. Crystalline perfection and interface abruptness were assessed using cross-sectional transmission electron microscopy (TEM), secondary ion mass spectroscopy (SIMS) and Rutherford backscattering spectrometry (RBS). In addition, the compositional uniformity, the strain tensor and superlattice periodicity were evaluated using X-ray diffraction.

Proximity and Josephson effects for heavy-fermion superconductors

Abstract At an interface or junction, strong spin-orbit coupling combined with pairing inhomogeneity and small pair size on the heavy-electron side mean that proximity or Josephson effects occur for singlet to triplet as well as for singlet to singlet pairing states. Large electron mass contrast at a metal-metal interface means that transmission of electron probability current is at most of order one percent. Singlet-triplet mixing of pair states by inhomogeneous magnetism is much smaller than mixing due to strong spin-orbit coupling combined with small pair size.

Infrared conductivity from spin and charge density waves: (TMTSF) 2X

Abstract Infrared conductivity from an incommensurate spin density wave occurs due to even-order charge density wave harmonics which interact with the host lattice. Phonon states within the density-wave-induced energy gap for single-particle excitations lead to conductivity much different from that of an incommensurate charge density wave including counter-ion ordering. The conductivity expected for relaxed and quenched states of (TMTSF)2ClO4 is discussed.

Copper pairing in heavy-Fermion superconductors

Abstract It is often stated that for heavy-Fermion systems, the coulomb pseudopotential within the usual framework of superconductivity theory should be much larger than usual, because the bandwidth for heavy electrons is smaller than the Debye frequency rather than much larger. This conclusion holds only for heavy band-mass electrons and is completely misleading for quasi-particles where the heavy mass arises mainly from a many-body effect. Quasi-particles, interactions, and Cooper pairing are discussed.

Lowest energy Cooper pairing in the presence of antiferromagnetism

Abstract Cooper pairing of electron eigenstates of an antiferromagnet involves considerable complexity in spin space and in phases of the order parameters. With a BCS interaction the pairing scheme with lowest free energy has anisotropic spin-matrix order parameter ( k ); however + ( k ) ( k )=|△| 2 . Magnitude of the anisotropic order parameter satisfies the simple BCS gap equation but with electron energies of the antiferromagnet eigenstates appearing instead of Bloch state energies of the nonmagnetic crystal.

Absence of proximity effect between s-wave and p-wave superconductors

Abstract In the absence of magnetic impurities, the coherent coupling between s-wave and p-wave condensates is exactly zero in the non-relativistic limit, because of conservation of S pair. S-wave to p-wave coupling due to relativistic effects is of negligible amplitude λ f ξ 0 E f mc 2 V pairing for an inhomogeneous system, and is zero for a homogeneous system.

Correlations and spin susceptibility of lowst-energy Cooper pairing in an antiferromagnet

Abstract Lowest-energy Cooper pairing of electron eigenfunctions in an antiferromagnetic metal is transformed to k -wave or nonmagnetic-Bloch-function space, where the pairing is resolved into wave-vector and spin components. The spin susceptibility is calculated using an eight-component field in Bloch function space.

Cooper pairing in the presence of antiferromagnetism

Abstract With a BCS interaction, the free energy for usual BCS pairing of electrons but in the presence of antiferromagnetism is shown to be lower than for a different pairing scheme where pairs are formed from electron eigenstates of the antiferromagnet. In both pairing schemes, super-conductivity in the presence of antiferromagnetism is always a mixture of spin-singlet-even-parity-orbital and spin-triplet-odd-parity-orbital electron pairs.

On possible coexistence of superconductivity and spin density waves in organic superconductors

Abstract If superconductivity coexists with a spin density wave for the same electrons, then both spin-singlet and spin-triplet Cooper pairs must occur. When the electron-electron interaction varies strongly over the Fermi surface, spin-singlet superconductivity occuring alone at high pressures must have T c sensitive to nonmagnetic impurities as observed experimentally. The theory predicts that T c becomes less sensitive to impurities and the a-axis critical magnetic field increases for the coexistence state.