spin-Peierls Compound CuGeO3 Research Articles

Observation of an antiferromagnetic resonance in the spin-Peierls compound CuGeO3 doped with Zn.

The results of electron-spin-resonance (ESR) measurements on a single crystal of the spin-Peierls compound CuGe${\mathrm{O}}_{3}$ doped with 4% Zn are reported. We have observed ESR signals in the antiferromagnetic phase below the N\'eel temperature (${T}_{N}=4.2$ K). The frequency vs magnetic-field relation of these ESR signals is well explained by the classical theory of antiferromagnetic resonance with orthorhombic anisotropy. The excitation energies at zero field are 26.4 GHz (0.11 meV) and 40.6 GHz (0.17 meV).

Hysteresis in the incommensurate phase of the spin-Peierls compound CuGeO3: Electron spin resonance in high field.

The far-infrared electron spin resonance (ESR) technique is used to probe the incommensurate $I$ phase of the spin-Peierls compound CuGe${\mathrm{O}}_{3}$. At sufficiently low temperature, the ESR signal is seen to display hysteresis effects. Depending on the sweeping field direction, drastic changes are observed on both the linewidth and the resonance field ${H}_{r}$. From accurate determinations of ${H}_{r}$, the observed hysteresis is defined quantitatively. Observed on a large field range, this hysteretic behavior appears to be a new and basic property of the incommensurate $I$ phase.

Investigation of the Spin-Peierls transition in CuGeO 3 by Raman scattering

Raman experiments on the Spin-Peierls compound CuGeO 3 and the substituted (Cu 2− x ,Zn x )GeO 3 and Cu(Ge 1− x ,Ga x O 3 compounds were performed in order to investigate the response of specific magnetic excitations of the one-dimensional spin-chain to spin anisotropies and substitution-induced disorder. In pure CuGeO 3, in addition to normal phonon scattering which is not affected at all by the spin-peierls transition, four types of magnetic scattering features were observed. Below T SP = 14 K a singlet-triplet excitation at 30 cm −1, two-magnon scattering from 30 to 227 cm −1 and folded phonon modes at 369 and 819 cm −1 were identified. They were assigned by their temperature dependence and lineshape. For temperatures between the spin-peierls transition T SP and approsimately 100 K a broad intensity maximum centered at 300 cm −1 is observed. The temperature dependence of this intensity is similar to the behavior of lattice fluctuations recently observed in an electron diffraction study. This scattering is attributed to dynamical spin-peierls fluctuations of the weakly or non-static dimerized spin chain.

Microscopic magnetic property and electronic state of the spin-Peierls compound CuGeO 3: 63,65Cu and 17O NMR studies

63,65Cu and 17O NMR measurements have been performed to study microscopic magnetic properties and the electronic state of CuGeO 3 which undergoes the spin-Peierls transition at T sp ∼ 14 K. The Cu 2+ electronic state is discussed within an ionic model in a rhombic crystal field. The spin density at the oxygen sites is estimated.

Investigation of the spin-Peierls phase diagram of CuGeO3: Far-infrared electron spin resonance in high field

Electron spin resonance on single crystals of the spin-Peierls compound CuGeO3 has been performed in magnetic fields up to 17 T in the temperature range 1.8–20 K. Using far-infrared laser frequencies of 245 and 525 GHz we could observe resonance conditions in the three different magnetic phases, i.e., the uniform, the dimerized, and the incommensurate phases for a magnetic field applied parallel to the a axis of the crystal. So for the first time data on electron spin dynamics in the incommensurate phase are reported for fields far from the transition to the dimerised phase. Striking effects concerning the g factor, linewidth, and hysteresis are observed.

Quantum effect on magnetic excitation

Inelastic neutron scattering have been performed on the spin-Peierls compound CuGeO3 in order to investigate the quantum effect on the magnetic excitation. In one-dimensional Heisenberg antiferromagnet system with S=1/2, the quantum effect is so strong that there is no long range order, and the existence of so-called ‘Spin Wave Continuum’ above the des Cloizeaux-Pearson mode can be expected. In this work, the whole spectrum of the magnetic excitation in the Brillouin zone has been firstly elucidated and it has made clear the quantum effect on the ground-state of the spin-Peierls system.

Spin dynamics of the spin-Peierls compound CuGeO3:63Cu NMR relaxation

The63Cu nuclear spin-lattice relaxation rate 1/T 1 and the nuclear spin-spin relaxation rate due to the indirect nuclear coupling 1/T 2G have been measured to study the spin dynamics in the spin-Peierls compound CuGeO3.

Heat capacity of the spin-Peierls compound CuGeO3 under magnetic field up to 22 T

We report on heat capacity (C p ) uneasurements of the spin-Peierls compound CuGeO3 under magnetic fields between 0 and 22 teslas. We will focus on the transition lines between uniform (U) and incommensurate (I) phases, and betwene dimerized (D) and I phases, always forH>12 teslas. We point out specific properties of the I phase, such as the step-discontinuities ofC p observed by varying the field at constantT, in accordance with the solition picture, and the persistence of a gap in the magnetic excitation spectrum at the largest fields.

Magnetostriction of the spin-Peierls compound CuGeO3

The magnetostriction λ of spin-Peierls compound CuGeO3 was measured in magnetic field up to 18 T both above, and below the transition temperature Tsp=14.3 K. In uniform phase at T>Tsp λ in b-c plane is negative and isotropic, while in dimerized phase at T λc). At low temperatures λ increases sharply at transition from dimerized to magnetic phase.

Heat capacity of the spin-Peierls compound CuGeO 3

We report on heat capacity measurements of the spin-Peierls compound CuGeO 3 under magnetic fields between 0 and 22 T. We have determined the magnetic phase diagram between the three different phases (U, M, D) of this system, unknown up to now for H > 14 T. We have studied the second-order phase transition for H = const. at the crossing of D-U and M-U phases, and we have defined the different contributions (lattice, magnetic excitations) to the specific heat and studied their variation under H.

High field heat capacities of some singlet ground state systems at very low temperatures

Several aspects of high field heat capacity observed in different types of singlet ground state system are discussed. The ground state energy gap and its magnetic field dependence are also studied in the Haldane gap compounds NENP, NINO and TMNIN. The origin of the observed minimum gap in applied magnetic field is discussed. In the spin-Peierls compound CuGeO 3, the energy gap of the Cu 2+ spin system in the dimerized phase and the gapless state in the high-field magnetic phase are confirmed.

Far-infrared spectroscopy in high magnetic fields

A far-infrared spectrometer. which can be used in the energy range between 3 and 40 cm −1, has been constructed to measure magneto-optical spectra with the Tsukuba Magnet Laboratories hybrid magnet system. The spectrometer was examined by measuring ESR of the inorganic spin-Peierls (SP) compound CuGeO 3 with a superconducting magnet instead of the hybrid magnet. ESR lines were observed at steady fields for both the SP phase and the field-induced incommensurate M phase.

Predictions for neutron scattering and photoemission experiments on CuGeO3.

Applying numerical techniques to a model recently proposed for the one dimensional spin-Peierls compound CuGeO_3, we calculate dynamical properties that can be directly compared with inelastic neutron scattering data and angle-resolved photoemission experiments. The momentum and energy dependence of the dynamical structure factor S(q,omega) are discussed, as well as the static structure factor S(q). The spectral function A(q,omega) calculated from the one particle Green's function at half-filling is shown at several values of the hole hopping amplitude t. The results have some unique features characteristic of one dimensional systems including small weight near the Fermi energy. The presence of ``shadow bands'' induced by short distance antiferromagnetic correlations is predicted to appear in ARPES experiments for CuGeO_3 and also for Sr_2CuO_3.

Lattice-dynamics and spin-excitations in the spin-Peierls compound CuGeO 3

The acoustic phonon modes of the spin-Peierls compound CuGeO 3 have been measured along the principal directions at various temperatures. The longitudinal mode along b *, which is perpendicular to the Cu 2+ chain direction, lies below the transverse mode in the same direction and is quite broad in energy due to damping. Measurements of the spin-excitation spectrum reveal that there remains considerable intensity spreading over a wide energy range above T SP = 14.2 K, and persisting up to about 50 K.

Magnetic excitations in CuGeO 3

In the inorganic spin-Peierls compound CuGeO 3, we found a pronounced anisotropic magnetic energy dispersion, i.e. intra-chain exchange J c ≈ 10.4 meV, inter-chain exchanges J b ≈ 0.1 J c and J a ≈ − 0.01 J c and gap energy 2.1 meV extrapolated to T = 0 K. We also observed critical scattering at (0, 1, ½) above T sp. The ratio of the correlation lengths ξ c / ξ b is in good agreement with that obtained from X-ray scattering at (½, 1, ½) by Pouget et al. This fact suggests that the Cu 2+ spins fluctuate to form singlet-pair short-range order, but not to form ordinary antiferromagnetic order above T sp .

Specific heat of the spin-Peierls compound CuGeO3

The specific heat of the quasi-onedimensional magnetic compound CuGeO3 shows a sharp anomaly at the spin-Peierls transition temperatureT sp . The experimental decrease of the magnetic specific heat belowT sp indicates the presence of a spin gap as observed previously with inelastic neutron scattering. A magnetic field of 6T suppressesT sp only slightly but reduces the spin gap by a factor of two.

Direct observation of the q = 0 spin-Peierls gap in CuGeO3 by high-field ESR

We present high-field Electron Spin Resonance (ESR) measurements on the spin-Peierls compound CuGeO3. We distinguish between two types of ESR transitions in the dimerized phase. They are associated with two energy gaps located at different points in the Brillouin zone. The renormalization of a q = 0 gap at E′g = 64 K as a function of temperature is measured.

Heat capacity of the spin-peierls compound CuGeO3

We report on heat capacity measurements in the temperature range between 40 mK and 35 K of a single crystal of the spin-Peierls compound CuGeO3. It exhibits a sharp anomaly at Tc = 14.2K corresponding to a second order phase transition with strong critical fluctuations. Below 6K analysis of data is compatible with an exponential decay of the magnetic contribution: the corresponding energy gap agrees with the B.C.S. theory.

Cu NMR study of a single crystal of the spin-peierls compound CuGeO 3

We have measured the nuclear quadrupole resonance (NQR) and the nuclear magnetic resonance (NMR) of copper nuclei in a single crystal of CuGeO 3, at 4K. The NQR frequency is practically independent of temperature whereas the spin-lattice relaxation time T 1 increases drastically as the temperature decreases. This behaviour of T 1 can be qualitatively understood by means of the fluctuation-dissipation theorem, which relates the imaginary part of the electronic susceptibility to the nuclear relaxation. The absence of a static Cu 2+ hyperfine field indicates that there is no long-range order. NMR was used to investigate the local symmetry of the Cu sites. The local symmetry axes, x, y, z were then determined. The asymmetry parameter of the electric field gradient was found to be 0.16. The principal values of the Cu 2+ g tensor were deduced as g x = 2.070, g y = 2.004 and g z = 2.407.

AC susceptibility in the spin-Peierls compound CuGeO 3 under high magnetic fields

The AC susceptibility in the cuprate compound CuGeO 3, in which the spin-Peierls (SP) transition occurs around 14K in the absence of magnetic field, was measured in the magnetic field region up to 20 T to establish the magnetic phase diagram of CuGeO 3. It was found that the isofield AC susceptibility shows a “knee” or peak at the phase boundary between the SP and uniform (U) phases and that between another magnetic (M) and U phases. The SP-M phase transition was clearly observed in the isothermal AC susceptibility. The determined phase diagram is consistent with the theoretical one after Cross and those of three organic SP materials.