spin-Peierls Material Research Articles

Zero field- and longitudinal field- [formula omitted] studies of quasi-one-dimensional organic conductor, [formula omitted

Zero and longitudinal field muon-spin-relaxation ( μ SR ) experiments were carried out in organic spin-Peierls material, TMTTF 2 PF 6 . An enhancement of muon spin depolarization rate is observed at the temperature somewhat lower than spin-Peierls transition T sP . Moreover, we observed an unusual magnetic state in a weak field of 0.1 mT below about 1 K, which is enough lower than the spin-Peierls transition temperature.

Crystal structure and chemistry of conichalcite, CaCu(AsO4)(OH)

The crystal structures of conichalcite [Ca(Cu,Mg)(AsO4)(OH)] samples obtained from Gozaisho mine, Fukushima, Japan, and Higgins mine, Arizona, USA, were refined by single-crystal X-ray diffraction (XRD) using an imaging plate detector. The results revealed that conichalcite is orthorhombic and belongs to the P212121 space group. The positions of the hydrogen atom and the donor and accepter atoms in the structure were determined by difference Fourier and bond-valence sum methods. The crystal comprised three highly distorted coordination polyhedra: an AsO4 tetrahedron, a CuO4(OH)2 octahedron, and a CaO7(OH) square antiprism. The CuO4(OH)2 octahedron was distorted by the Jahn-Teller effect. The observed As-O distances were shorter than those predicted by ionic bonding. The electronic structure of the atoms affected the distortion of the polyhedra that did not have a symmetry center. The CuO4(OH)2 octahedron shared its edges to form linear chains, which were further linked by the vertices of the AsO4 tetrahedron and the CaO7(OH) square antiprism to form a three-dimensional network. The arrangement of these linear CuO4(OH)2 chains was very similar to that of CuO6 chains in CuGeO3, a spin-Peierls material. Comparisons with several isomorphous minerals revealed that the Jahn-Teller distortion effect caused by Cu atoms differed significantly between the minerals in the Ca series and those in the Pb series. Because of hydrogen bonding, the Cu-O(5) and Cu-O(5)* distances tended to become shorter than the other Cu-O distances in all isomorphous minerals.

Synchrotron X-ray Diffraction Studies of the Incommensurate Phase of a Spin–Peierls System CuGeO3in Strong Magnetic Fields

Synchrotron X-ray diffraction measurements on a spin–Peierls material CuGeO 3 in applied magnetic fields, H , up to 15 T are made. We find that the temperature, T , dependence of the incommensurate Bragg peak at a lower H is quite different from that at a higher H . At sufficiently high fields, we find that the lattice incommensurability, δ l , is almost independent of T , while at H slightly above the critical field = 12.25 T for the commensurate to incommensurate transition, δ l decreases with increasing T . We interpret that this finding is due to a stabilization of the incommensurate state by a strong magnetic field which suppresses thermal fluctuations.

Chasing ghosts in reciprocal space—a novel inelastic neutron multiple scattering process

We have discovered that a recently reported weak excitation branch in the spin-Peierls material CuGeO 3 is in fact a ghost image of the primary magnetic excitation shifted in reciprocal space by a novel multiple scattering process. A model is developed that predicts the occurrence of such multiple scattering and accounts for the observations in CuGeO 3. New ‘ghostons’ can occur when the magnetic unit cell is smaller than the structural, while mixing of intensities from different reciprocal space zones jeopardize accurate polarisation analysis and the study of weak modes in general.

Pressure effect of spin-Peierls material CuGeO 3

We studied the pressure effect of Cu 1− x Mg x GeO 3 by the magnetic susceptibility measurements. We observed the revival of spin-gap, which is supposed to be spin-Peierls gap, in the sample without the spin-Peierls phase at ambient pressure and the sudden decrease of antiferromagnetic (AF)-phase transition temperature at the same pressure. The observation suggests the existence of the phase transition between AF phase with spin-gap excitation and conventional AF phase, which is similar to that between dimerized-AF and uniform-AF phases in the temperature vs. concentration phase diagram previously studied (T. Masuda et al., Phys. Rev. Lett. 80 (1998) 4566).

Mixing of magnetic and phononic excitations in incommensurate spin-Peierls systems

We analyze the excitation spectra of a spin-phonon coupled chain in the presence of a soliton. This is taken as a microscopic model of a Spin-Peierls material placed in a high magnetic field. We show, by using a semiclassical approximation in the bosonized representation of the spins that a trapped magnetic state obtained in the adiabatic approximation is destroyed by dynamical phonons. Low energy states are phonons trapped by the soliton. When the magnetic gap is smaller than the phonon frequencies the only low energy state is a mixed magneto-phonon state with the energy of the gap. We emphasize that our results are relevant for the Raman spectra of the inorganic Spin-Peierls material CuGeO$_3$.

Reentrant spin-Peierls transition in Mg-DopedCuGeO3

We report a synchrotron x-ray scattering study of the diluted spin-Peierls (SP) material Cu_{1-x}Mg_xGeO_3. In a recent paper we have shown that the SP dimerization attains long-range order only for x < x_c = 0.022(0.001). Here we report that the SP transition is reentrant in the vicinity of the critical concentration x_c. This is manifested by broadening of the SP dimerization superlattice peaks below the reentrance temperature, T_r, which may mean either the complete loss of the long-range SP order or the development of a short-range ordered component within the long-range ordered SP state. Marked hysteresis and very large relaxation times are found in the samples with Mg concentrations in the vicinity of x_c. The reentrant transition is likely related to the competing Neel transition which occurs at a temperature similar to T_r. We argue that impurity-induced competing interchain interactions play an essential role in these phenomena.

Giant magnetostrictive effects in magnetic oxides

Experimental research on giant magnetostrictive effects in magnetic oxides is reviewed. Particular attention is paid to manganates, cobaltates, high-temperature superconductors and spin-Peierls material CuGeO 3. The magnetostrictive effects are analysed in terms of recently developed theoretical models. It is shown that strong electron–phonon coupling is responsible for intrinsic giant magnetostriction in most of the oxides presented. In the case of high-temperature superconductors the giant forced magnetostriction is due to extrinsic effects related to flux-pinning and consequently to lattice defects.

Static spin structure factor of CuGeO 3 above spin-Peierls transition temperature

We study, by means of the quantum transfer matrix method, the finite-temperature behavior of the S=1/2 antiferromagnetic chain with nearest neighbor and competing next nearest neighbor interactions, which is a model for the spin-Peierls material, CuGeO 3. In particular, the static spin structure factor is found to display an interesting phenomenon as a consequence of the competition: a characteristic wave number for the maximum value of the structure factor takes a commensurate or an incommensurate value, depending on temperature. The boundary, called a Lifshitz temperature, can be found in this material. We discuss such behavior in connection with the experimental result of the neutron scattering in CuGeO 3 above the spin-Peierls transition temperature.

Structural Critical Scattering Study of Mg-DopedCuGeO3

We report a synchrotron x-ray scattering study of the diluted spin-Peierls (SP) material Cu_(1-x)Mg_xGeO_3. We find that for x>0 the temperature T_m at which the spin gap is established is significantly higher than the temperature T_s at which the SP dimerization attains long-range order. The latter is observed only for x<x_c =0.021, while for x>x_c the SP correlation length quickly decreases with increasing x. We argue that impurity-induced competing interactions play a central role in these phenomena.

Domain excitations in spin-Peierls systems

We study a model of a Spin-Peierls material consisting of a set of antiferromagnetic Heisenberg chains coupled with phonons and interacting among them via an inter-chain elastic coupling. The excitation spectrum is analyzed by bosonization techniques and the self-harmonic approximation. The elementary excitation is the creation of a localized domain structure where the dimerized order is the opposite to the one of the surroundings. It is a triplet excitation whose formation energy is smaller than the magnon gap. Magnetic internal excitations of the domain are possible and give the further excitations of the system. We discuss these results in the context of recent experimental measurements on the inorganic Spin-Peierls compound CuGeO$_3$

Surface flatness and structure of thebc-cleaved plane of spin-Peierls materialCuGeO3

The surface structure of the first inorganic spin-Peierls material ${\mathrm{CuGeO}}_{3}$ has been investigated by atomic force microscopy. The $\mathrm{bc}$ surface of ${\mathrm{CuGeO}}_{3}$ single crystals grown by the floating-zone method was easily prepared by cleavage, where we found extraordinarily flat terraces compared to other oxide materials. One of the large-area images demonstrates that two atomically flat surfaces spread over micrometers, which are separated by a monolayer step of 4 \AA{} height. A narrow-area image also reveals a unique structure of ${\mathrm{CuGeO}}_{3},$ where we found that the one-dimensional atomic row image of Cu-O is observed everywhere on the terraces. The model structure consistent with the observed image is proposed and the surface stability is discussed in detail.

Phase diagram of a new spin-Peierls compound: p-CyDOV

Susceptibility and magnetization measurements were performed to investigate the magnetic behavior of the verdazyl crystal p-CyDOV. The magnetic phase diagram was examined to check whether p-CyDOV is a real spin-Peierls material or not. Observed field dependence of the transition temperature shows good agreement with the theory for a spin-Peierls system. Magnetic phase diagram obtained agrees well with those of other spin-Peierls materials. As a result, it has been concluded that the p-CyDOV is a spin-Peierls compound.

Spin-Peierls transition in CuGeO 3

The first inorganic spin-Peierls material CuGeO 3 has attracted much attention. The advantage of CuGeO 3 in comparison with organic spin-Peierls compounds is that we can investigate effects of substitution on the spin-Peierls system. Studies of effects of substitution have provided a novel interesting topic, i.e., coexistence of a lattice dimerization and antiferromagnetic long-range order. In this paper, we summarize recent studies of CuGeO 3 doped with impurities.

The Observation ofg-Shifts in Spin-Peierls Material CuGeO3by Submillimeter Wave ESR

The g -shifts in spin-Peierls (SP) and magnetic (M) phases of the inorganic spin-Peierls material CuGeO 3 have been observed by submillimeter wave ESR. The temperature dependence of the g -shifts from U phase to SP or M phase showed a continuous change. Since such g -shifts were also observed in the organic spin-Peierls material, for example TTF-AuBDT, such g -shifts can be considered the universal phenomena due to the lattice distortion in the spin-Peierls materials. But, in the case of CuGeO 3 , the g -shifts are so large that we cannot explain them in the framework of the point charge model with considering the continuous lattice distortion.

Neutron-Scattering Study of Magnetism in Single-CrystalCu1-xZnxGeO3

A recent study of the magnetic susceptibility in single crystals of the spin-Peierls material CuGeO 3 doped with Zn indicates antiferromagnetic order at low temperatures. In the present publication, we report a neutron scattering investigation of the static and dynamic magnetic properties of single-crystal Cu 0.966 Zn 0.034 GeO 3 . Below the transition temperature of 4.2 ± 0.3 K, sharp magnetic Bragg peaks are observed at the magnetic superlattice positions (0 k l /2) with both k and l odd. However, the intensities of these peaks are not saturated even at 1.4 K, and the long range ordered magnetic moment is only ∼0.2 µ B per Cu site. Moreover, no well-defined spin-wave-like excitations are seen. Instead, overdamped magnetic excitations appear around the magnetic Bragg peaks at low temperatures. These results suggest an unusual state of magnetic order incorporating considerable magnetic and possibly structural disorder. We speculate that this unusual order is caused by a spatial distribution of spin states...

Antiferromagnetic phase in Zn- and Ni-doped CuGeO3

The magnetic susceptibility of single crystals Cu1−xMxGeO3 (M=Zn, Ni) were investigated to study the effects of impurities on the inorganic spin-Peierls material CuGeO3. We succeeded in the growth of Zn-doped single crystals with 0.007≤x≤0.09 and Ni-doped ones withx=0.033. We determined temperature vsx phase diagram of the Zn-doped single crystals, which is almost identical to that of the polycrystaline samples. In both Cu1−x ZnxGeO3 and Cu1−xNixGeO3, the antiferromagnetic transition occurs below 5 K. It is proved that the easy axis of Ni-doped CuGeO3 is nearly along thea axis, while that of Zn-doped CuGeO3 is along thec axis as previously reported. This difference of spin orderings between Zn-doped and Ni-doped CuGeO3 is caused by that Ni2+ ions carry spinS=1 while Zn2+ ions are nonmagnetic.

Observation of magnetization saturation of CuGeO3 in ultrahigh magnetic fields up to 500 T.

The magnetization in a spin-Peierls material ${\mathrm{CuGeO}}_{3}$ was measured by means of the Faraday rotation in ultrahigh magnetic fields up to 500 T. The experimental magnetization curve shows a nonlinear increase against the magnetic field which is a typical behavior of a one-dimensional quantum spin system. A distinct saturation of the magnetization was observed at 253 T, from which the exchange parameter is evaluated as 183 K. A large hysteresis of the magnetization was observed in the magnetic phase, whereas it was indiscernible in the uniform phase.

The lattice evolution of the spin-Peierls material CuGeO 3 studied by neutron scattering

CuGeO 3 is an inorganic compound which possesses a spin-Peierls transition. We have studied the crystal structure and the phonon dynamics of CuGeO 3 by neutron scattering. We found that a lattice anomaly starts to evolve significantly above the transition temperature, T sp, and there is a drastic change below T sp. The phonon mode over a wide energy range changes drastically at high temperature. Therefore we conclude that the inherent structural instability in the crystal structure provides favorable conditions for the lattice-spin interaction and induces the spin-Peierls transition, i.e. lattice dimerization.

Discovery of a spin-singlet ground state with an energy gap in CaCuGe2O6.

The magnetic properties of ${\mathrm{CaCuGe}}_{2}$${\mathrm{O}}_{6}$, a cuprate related to the spin-Peierls material ${\mathrm{CuGeO}}_{3}$, were studied. The magnetic susceptibility and the magnetization curve were measured. They revealed that this compound has a spin-singlet ground state and a finite energy gap between the ground and excited states. We discuss the origin of such a property of ${\mathrm{CaCuGe}}_{2}$${\mathrm{O}}_{6}$.