System CuGe Research Articles

Absence of Edge Localized Moments in the Doped Spin-Peierls SystemCuGe1−xSixO3

We report the observation of nuclear quadrupole resonance (NQR) of Cu from the sites near the doping center in the spin-Peierls system CuGe 1-xSixO3. The signal appears as the satellites in the Cu NQR spectrum, and has a suppressed nuclear spin-lattice relaxation rate indicative of a singlet correlation rather than an enhanced magnetic correlation near the doping center. Signal loss of Cu nuclei with no neighboring Si is also observed. We conclude from these observations that the doping-induced moments are not in the vicinity of the doping center but rather away from it.

Cu NMR and NQR Studies of the Doped Spin-Peierls System CuGe1-xSixO3

Nuclear magnetic and nuclear quadrupole resonance (NMR and NQR) of Cu have been measured in the doped spin-Peierls system CuGe 1 - x Si 2 O 3 . We find evidence that the staggered polarization of Cu 2 + spins due to doping is not induced near the Si doping center. It occurs in-between Si atoms, wiping out a part of Cu nuclei far from Si while leaving those near Si observable as NQR/NMR satellites. Local susceptibility measurements via the NMR shifts suggest preformation of singlet dimers near Si above the spin-Peierls transition temperature. These observations may be understood by effective interruption of a linear chain of Cu by Si, and the formation of strong bonds on the edges of a segment which is favored by an inherent spin-phonon coupling.

Application of Berezinskii's Diagram Method to Bond Disordered System with the Dimerization and the Staggered Field

Based on the Berezinskii's diagram method, the density of states (DOS) of a bond disordered system has been calculated in the presence of the dimerization and the staggered field. It is found that the DOS near the center of the band behaves anomalously and that it is indicated that this feature is the cause of the coexistence phase of antiferromagnetism (AF) and dimerization in the disordered quasi-one-dimensional spin-Peierls (SP) system CuGe 1- y Si y O 3 .

Quantum Spin Excitations in the Spin-Peierls System CuGeO3.

The complete spectrum of the double spinon excitation in the spin-Peierls system CuGe${\mathrm{O}}_{3}$ is mapped as a function of temperature for the first time. The spin dynamics of the lower boundary and the excitation continuum evolve quite differently. Moreover, the dimerization of the lattice produces a sharp excitation in the lower energy boundary at the edge of the Brillouin zone, as well as the well known spin-gap opening at the zone center.

Inelastic-neutron-scattering investigation of the spin-Peierls system CuGeO3.

We have investigated the spin dynamics of the spin-Peierls system CuGe${\mathrm{O}}_{3}$ by inelastic neutron scattering. The measurements have been performed as a function of wave vector, temperature, and magnetic field, for fields perpendicular to the chain axis (up to 10 T). Our neutron results confirm the occurrence of a crystalline distortion below ${T}_{\mathrm{SP}}\ensuremath{\approx}14.2$ K, corroborated by the appearance of superlattice peaks indexed with a commensurate wave vector ${\mathbf{k}}_{\mathrm{SP}}$=(\textonehalf{},0,\textonehalf{}). At low temperature, the spin-excitation spectrum exhibits a well defined energy gap $\ensuremath{\Delta}\ensuremath{\approx}2 \mathrm{meV}\ensuremath{\approx}1.66 k{T}_{\mathrm{SP}}$ at the antiferromagnetic point ${\mathbf{k}}_{\mathrm{AF}}$=(0,1,\textonehalf{}), distinct from ${\mathbf{k}}_{\mathrm{SP}}$. The experimental results for $T\ensuremath{\ll}{T}_{\mathrm{SP}}$ are quantitatively understood from an alternating-exchange Hamiltonian with an exchange parameter $(2{J}_{1})\ensuremath{\approx}10.6$ meV and an alternation parameter $\ensuremath{\alpha}=\frac{{J}_{2}}{{J}_{1}}\ensuremath{\approx}0.92$, despite the existence of sizable interchain couplings both along the $a$ and $b$ axes ($\frac{{J}_{a}^{\ensuremath{'}}}{{J}_{1}}\ensuremath{\approx}0.011$ and $\frac{{J}_{b}^{\ensuremath{'}}}{{J}_{1}}\ensuremath{\approx}0.11$, respectively). We present the temperature dependence of the spin dynamics on both sides of ${T}_{\mathrm{SP}}$. Our results confirm the persistence of a pseudogap in the excitation spectrum for q=${\mathbf{k}}_{\mathrm{AF}}$, but only in the immediate vicinity of the spin-Peierls transition temperature. The pseudogap vanishes rapidly with $T$, following decreasing size of dimerized segments. The stronger inelasticity observed above ${T}_{\mathrm{SP}}$ at q=${\mathbf{k}}_{\mathrm{SP}}$ has been attributed to an effect of the dispersion of magnetic excitations due to interchain couplings ${J}_{a}^{\ensuremath{'}}$ and ${J}_{b}^{\ensuremath{'}}$. Under a magnetic field applied perpendicular to the chain axis, the excited triplet is split into three components, with gap values depending linearly on the field. The spin-Peierls transition temperature ${T}_{\mathrm{SP}}$ decreases with increasing field, following a quadratic dependence on $H$, in agreement with theoretical as well as other experimental results. We have determined the field dependence of the dimerization peak intensities up to 10 T, which show very small changes. In particular, no trace of pretransitional incommensurability could be detected, at least up to $0.77{H}_{c}$.

Crystal growth and investigation of the solid solutions of the system CuGe 2P 3-I 2-IV-VI 3

The compounds Cu 2GeS 3, Cu 2SiS 3 and Cu 2SiSe 3 were dissolved in the ternary compound CuGe 2P 3. A modified Bridgman technique was used in the preparation, and good quality single crystals were grown for single phase samples. A complete solid solution for the system CuGe 2P 3-Cu 2GeS 3 was found, with lattice parameters changing from 5.3678Å to 5.2895Å for 90% CuGe 2P 3 and obeying Vegards law. In the CuGe 2P 3-Cu 2GeSe 3 system the existence of the solid solution appears in the region of 0.25≧ x≧0.9 when x is CuGe 2P 3. However, the compound CuGe 2P 3 does not form any solid solution with the compounds Cu 2SiS 3 and Cu 2SiSe 3.

Electronic band structure of h. c. p. electron phases based on the noble metals

Measurements of the electronic specific heat coefficient and of the limiting Debye temperature are reported for several close-packed hexagonal intermediate phases in the systems Cu-Ge, Ag-Sn and Ag-Cd. The obtained results, together with previous similar work, conform to a general pattern which suggests that the electronic structure of all h. c. p. phases based on the noble metals can be described in terms of a common nearly rigid band for the conduction electrons. The details of the trend in the density of states are similar to the calculated density of states trends for the h. c. p. metals Zn and Be. The main feature is a large peak in the range of electron concentration between 1.4 and 1.8 which is most likely the result of Fermi surface contacts with the {10.1} Brillouin zone planes, and overlaps across the {10.0} zone planes. A secondary feature is the onset of overlaps across the {00.2} zone planes. A model of an inscribed Fermi sphere into the Brillouin zone is sufficient to indicate the expected behaviour of the trend in the electronic heat capacities in individual phases. Electron-phonon enhancement in the h. c. p. phases appears to be a nearly constant factor, amounting to about 30% of the measured electronic heat capacity values.

A study of mechanical twinning in?-solid solutions of the system Cu-Ge

Electron and optical microscopy have been used to study the structure and distribution of mechanical twins in polycrystalline Cu-Ge alloys containing up to 9.5 at.% Ge which had been deformed at room temperature. The probability of twinning occurring increases continuously with rise in concentration of the alloy up to the solubility limit. A reduction in the grain size of a polycrystalline aggregate impedes the development of twinning. It is shown that the twins have an imperfect structure even in the initial stage of growth.

On the Variation of Wire Texture with Stacking Fault Energy in Alpha-Brasses

Young's modulus was used as an indication of the changes of texture and stacking fault energy. For alpha-brasses a maximum was found around 1% zinc (γ≈101 ergs/cm 2 ). The change of the stacking fault energy for alpha-brasses is comparable with the characteristic of the concentration-dependence of the stacking fault energy in the system Cu-Ge and Cu-Ga. The results found by English and Chin (both high and low stacking fault energy results in sharp -textures) as a contrary to the theory of Brown was discussed.

The lattice spacings and the stability of close-packed hexagonal Cu-Ga, Cu-Ge and Ag-Al alloys

The lattice spacings of close-packed hexagonal ζ-phases in the systems Cu-Ge, Cu-Ga and Ag-Al have been determined as a function of composition and electron: atom ratio, using alloys quenched from 550°C. The results how that overlap of occupied states across the {101̄0} faces of the Brillouin zone occurs at the electron: atom ratio of 1.415 in Cu-Ge alloys, 1.43 in Cu-Ga alloys and at 1.47 in Ag-Al alloys. In all three systems the relative distortion of the α lattice spacings due to overlap increases linearly at higher values of overlap at a rate of 3.5 × 10 −3 Å per 0.1 increase in the electron : atom ratio. In the system Ag-Al the change of slope in the c lattice spacings near the electron : atom value of 1.62 may be interpreted in terms of interaction of the Fermi surface with the {101̄0} faces of the Brillouin zone before contact.

Liquid⇌solid equilibrium in copper and silver alloyr; with an appendix on the equilibrium diagram of the system copper-arsenic

Abstract The liquidus and solidus curves for the α-solid solutions, of Cd, In, Sn, and Sb in Ag, and of Zn, Ga, Ge, and As, in Cu are discussed. In the Ag-series a given depression of the solidus is produced by atomic percentages of Cd, In, Sn, and Sb proportional to 1/22: 1/32: 1/43: 1/62, and in the Cu-series by amounts of Zn and Ga proportional to 1/32: 1/42. The increases in mean lattice spacing produced by a given atomic percentage of Cd, In, Sn, and Sb dissolved in Ag are as 2: 3: 4: 6, and by a given atomic percentage of Zn and Ga dissolved in Cu are as 3: 4. The solidus curves and lattice distortions thus involve the same whole number sequences, but this correspondence breaks down in the system Cu-As where the electrochemical factor is high, and the same effect to a lesser degree is apparent in the system Cu-Ge. In the systems Ag-Cd, Ag-In, Ag-Sn, Cu-Zn, Cu-Ga, and Cu-Ge the liquidue and solidus curves in dilute solutions are connected by a relation of the type x 1 = k √ x 5 where x 1 and x 3 are ...