黒江 晴彦

Cu2Fe2Ge4O13 is a quantum magnet consisting of Cu spin dimers and Fe spin chains. It shows an antiferromagnetic phase transition at low temperatures due to the magnetic moment of Fe. Since the two parts are weakly coupled, magnetic excitations are well separated into high-energy Cu dimer part and low-energy Fe chain part. We study the magnetic excitations in the ordered phase of Cu2Fe 2Ge4O13 based on a theory used for coupled spin dimer systems such as TlCuCl3. Analyzing the dynamical spin correlation functions, we find several high-energy modes with longitudinal spin fluctuations peculiar to the coupled spin dimer and spin chain system. This is consistent with the recent experimental result of inelastic neutron scattering. A possibility to detect such longitudinal modes is discussed by means of magnetic Raman scattering. © 2010 IOP Publishing Ltd.

We report the magnetization curve under magnetic fields H up to 30 T at low temperatures in a tetrahedral spin-chain system Cu3Mo2O9 single crystal. This material shows successive phase transitions to an antiferromagnetic phase at T-N = 7.9 K and a weak ferromagnetic (WF) phase at T-c similar or equal to 2.5 K in low magnetic fields. As the magnetic field increases below T-N, two rapid increases in magnetization occur at 0 - 0.1 and 19.3 T for H // a and 0.3 - 0.8 and 17.5 T for H // c. However, these increases do not occur for H // b. These facts refute the conventional spin flop transition. The rapid increases disappear above T-N. The magnetic moments are canted toward the a c plane by the Dzyaloshinskii-Moriya interaction in the WF phase. Consequently, these rapid increases in magnetization at high magnetic fields are interpreted by sudden rotations of the WF moments around the b axis.

Raman spectra in TlCuCl3 under the magnetic field up to 12 T are reported. Above the critical magnetic field of the quantum phase transition to the magnon Bose-Einstein condensation phase, the one-magnon Raman peak is observed. Above 10 T, the one-magnon energy approaches the energy of the A g phonon (3.1 meV) and the anticrossing effect is observed. By using the Green-function theory with three adjustable parameters, we estimate the amplitude of the attractive magnon-phonon interaction as 0.043 meV. The consistency between the present results and those of previous works is discussed. © 2009 IOP Publishing Ltd.

By means of Raman scattering we have studied the spin-Peierls (SP) transition in Ni-doped CuGeO3 crystals. The folded-phonon peak extremely broadens and the two-magnetic-excitation mode disappears when the Ni concentration is above about 2.0%, indicating that the SP phase collapses. The spin-gap mode is activated in lightly Ni-doped samples. This peak neither splits nor shifts under magnetic fields in the SP phase, but its frequency increases in the incommensurate phase. The activation of the spin-gap mode is interpreted in terms of a mixing between the spin-singlet ground state and the Sz=0 triplet excited one by staggered fields around the doped impurities. Meanwhile, the two-magnetic-excitation mode gradually diminishes in intensity with increasing magnetic field in the incommensurate phase.

We studied magnetism of two cuprates using inelastic neutron scattering and magnetization measurements on powder samples. A spin gap of 6.2 meV (72 K) was observed in beta-AgCuPO(4). The spin system was determined as a spin-1/2 twofold-period (alternating) antiferromagnetic (AF) chain with two exchange interaction values of 80 and 14 K. Contrary to common knowledge, the strongest exchange interaction exists in a Cu-Cu bond with the distance of 5.20 A in spite of the existence of short Cu-Cu bonds. Cu(3)(P(2)O(6)OD)(2) is known to have a spin-1/2 threefold-period chain with J(1)-J(2)-J(2) interactions, where J(1) and J(2) denote two AF exchange interaction parameters, showing a 1/3 magnetization plateau. A spin gap of 9.8 meV (114 K) was found, which generated the magnetization plateau. The gap corresponds to a singlet-triplet-like excitation of an AF dimer formed by the dominant J(1) interaction. We determined that J(1)=111 K and J(2)=30 K. (c) 2008 American Institute of Physics.

Investigation of the magnetism of Cu3Mo2O9 single crystal, which has antiferromagnetic (AF) linear chains interacting with AF dimers, reveals an AF second-order phase transition at T-N=7.9 K. Although weak-ferromagnetic-like behavior appears at lower temperatures in low magnetic fields, complete remanent magnetization cannot be detected down to 0.5 K. However, a jump is observed in the magnetization below weak-ferromagnetic (WF) phase transition at T-c similar or equal to 2.5 K when a tiny magnetic field along the a axis is reversed, suggesting that the coercive force is very weak. A component of magnetic moment parallel to the chain forms AF long-range order (LRO) below T-N, while a perpendicular component is disordered above T-c at zero magnetic field and forms WF-LRO below T-c. Moreover, the WF-LRO is also realized by applying magnetic fields even between T-c and T-N. These results are explainable by both magnetic frustration among symmetric exchange interactions and competition between symmetric and asymmetric Dzyaloshinskii-Moriya exchange interactions.

We report the observation of the A(g)-symmetric one-magnon Raman peak in the magnon Bose-Einstein condensation phase of TlCuCl(3). Its Raman shift traces the one-magnon energy at the magnetic Gamma point, and its intensity is proportional to the squared transverse magnetization. The appearance of the one-magnon Raman scattering originates from the exchange magnon Raman process and reflects the change of the magnetic-state symmetry. By using the bond-operator representation, we theoretically clarify the Raman selection rules, which are consistent with the experimental results.

One-magnon Raman scattering for interacting spin dimer systems is studied theoretically based on exchange light scattering process. We focus on a pressure-induced ordered phase, where a longitudinal excitation mode exists. We find that the Raman scattering probes the longitudinal mode selectively. The Raman intensity is finite only in the ordered phase and develops with pressure. We apply the theory to spin gapped systems such as TlCuCl3 and NiCl2-4SC(NH2)(2) to demonstrate that the Raman scattering is a suitable probe for the longitudinal mode.

We studied the effects of Zn doped into Cu sites on the magnetism of Cu3Mo2O9. The spin system consists of spin-1/2 antiferromagnetic (AF) uniform chains and AF dimers. Each chain is coupled to neighboring dimers by other interactions, giving rise to magnetic frustration. A component parallel to the chain of magnetic moment in the chain forms AF long-range order (LRO) below the transition temperature T-N, whereas a perpendicular component is disordered in zero magnetic field and above 2.0K. The perpendicular component forms weak ferromagnetic LRO (WF-LRO) only in finite magnetic fields H's. We have found that WF-LRO in H disappears with a very small amount of Zn doping, although the decrease of TN is negligible at corresponding Zn concentrations. These results are explained by the enhancement of magnetic frustration among symmetric exchange interactions caused by Zn doping.

We studied magnetism of beta-AgCuPO(4) using inelastic neutron scattering and magnetic susceptibility measurements on powder samples. A well-defined asymmetric broad inelastic peak was observed at 6.2 meV in constant-Q (scattering vector) scan spectra, indicating spin-gap excitation. From results of these measurements, we have obtained that the strongest exchange interaction exists in a Cu-O-P-O-Cu path in spite of the existence of Cu-O-Cu paths and that the antiferromagnetic alternating chain model with two exchange interaction values of 80 and 14 K can explain the experimental results quantitatively. Our results are consistent with the prediction of Yahia [Inorg. Chem. 45, 5501 (2006)].

We report magnetic Raman scattering in a distorted-triangular-lattice Ising-like antiferromagnet TlCoCl₃. Below the Néel temperature, we observe several new Raman peaks from the quantized domain-wall pair excitations (Zeeman-ladder excitations). From their sequencies, we obtain the magnetic parameters of this system. We find that the orthorhombic lattice distortion causes a reduction of exchange interactions between the magnetic chains shifting to the opposite sides. The origin is discussed within a framework of superexchange interaction.

We have studied magnetic excitations in the coupled spin dimer system Tl(Cu1-xMgx)Cl-3 by means of Raman scattering. A broad two-magnon Raman band extending from 2 Delta to about 150cm(-1) is observed at low temperatures, where Delta is the spin-gap energy. With increasing nonmagnetic impurity Mg, the intensity of the broad Raman band weakens and its onset energy, 2 Delta, shifts to higher energy. The quasi-elastic scattering appears and the onset of the two-magnon Raman band gets smeared when x = 0.05. The Raman spectrum hardly changes when a magnetic field is applied, which indicates a creation of a pair of triplets whose total S is zero in the second-order Raman process. (c) 2006 Elsevier B.V. All rights reserved.

We report magnetic susceptibility (X)(T) and magnetization M(H) in A(2)Cu(2)Mo(3)O(12) (A = Rb, Cs) under hydrostatic pressures. (X)(T) shows a broad peak at low temperatures. When the pressure is applied to the Rb compound, the peak in X(T) grows and shifts to a lower temperature, the M(H) curve becomes steeper, and the saturation field decreases. Comparing with the calculation of X(T) by DMRG method, accompanied with a correction term (alpha 1/T), these are qualitatively explained by a decrease of vertical bar alpha vertical bar, where alpha is the ratio between the nearest-neighbor and the next-nearest-neighbor interactions. vertical bar alpha vertical bar is reduced approximately by 0.01 per 1 GPa. The Cs compound has a different pressure dependence from the Rb compound. (c) 2006 Elsevier B.V. All rights reserved.

We study the spin-gap mode in the "devil's staircase"-type phase of NaV2O5 by means of Raman scattering under high pressures at low temperatures. With increasing pressure the spin-gap mode decreases in frequency and Raman intensity. It is not observed between 0.9 and 1 GPa, but it appears again between 1 and about 2.7 GPa, indicating that it is observed only in C-0 and Q(1/4) phases of the phase diagram. It is found that the normal phase is stable down to low temperature above about 2.7 GPa. In C-0 phase, i.e., above 1 GPa, the transition temperatures obtained by Raman scattering and X-ray diffraction show good agreement. However, the obtained transition temperature is about 10 degrees lower than the X-ray diffraction result in C-1/4 phase. (c) 2006 Elsevier B.V. All rights reserved.

We report the temperature and magnetic-field dependences of Raman spectra in a distorted-triangular-lattice Ising-like antiferromagnet TlCoCl3. A broad Raman band from the continuous domain-wall pair excitations has been observed at 30 K. It is quantized in the Neel state and has been observed as a series of Raman peaks at 6 K. We also report the change of the phonon modes at the successive structural phase transitions. (c) 2006 Elsevier B.V. All rights reserved.

We have studied phonon mode behavior in GaN nanocolumns and GaN/AlN multiple quantum disk nanocolumns, which are self-organized on Al 2O3 and Si by RF-MBE, by means of Raman scattering. In GaN nanocolumns, we observe a Fröhlich mode localized at the surface of nanocolumns. The frequency and line width strongly depend on the column density and its inhomogeneity. It is demonstrated that Raman scattering is a useful method to characterize the crystal properties of the nanocolumns. In GaN/AlN multiple quantum disk nanocolumns, we observe several confined, quasi-confined and interface phonon modes, whose wave vectors along the axial direction may be quantized by the columnar nanostructures. 10.1380/ejssnt.2006.227 © 2006 The Surface Science Society of Japan.

We report the magnetic susceptibility for Rb2Cu 2Mo3O12 under hydrostatic pressure up to 1.4 GPa. Applied pressure increases the peak value of the magnetic susceptibility, whereas the associated temperature decreases. This pressure dependence of magnetic susceptibility is explained qualitatively by an increase of α, where α is the ratio between the next-nearest-neighbor antiferromagnetic and the nearest-neighbor ferromagnetic exchange interactions. Moreover, a small increase of magnetic susceptibility with decreasing temperature is observed, suggesting a change of ground state. © 2006 American Institute of Physics.

Magnetic susceptibility and magnetization showed that A(2)Cu(2)Mo(3)O(12) (A=Rb or Cs) includes a one-dimensional spin-1/2 Heisenberg system with ferromagnetic first-nearest-neighbor (INN) and antiferromagnetic second-nearest-neighbor (2NN) competing exchange interactions (competing system). Values of the INN and 2NN interactions are estimated as J(1)=-138 K and J(2)=51 K for A = Rb or J(1) = -93 K and J(2) = 33 K for A = Cs. Rb2Cu2Mo3O12 is a suitable compound to study the competing system with ferromagnetic J(1), because other interactions are small. The value Of J(2)/J(1) indicates that Rb2Cu2Mo3O12 has a spin-singlet incommensurate ground state. (c) 2005 American Institute of Physics.

We have investigated magnetic properties of Rb2Cu2Mo3O12 powder. Temperature dependence of magnetic susceptibility and magnetic-field dependence of magnetization have shown that this cuprate can be considered as a compound of a one-dimensional spin-1/2 Heisenberg system with ferromagnetic first-nearest-neighbor (1NN) and antiferromagnetic second-nearest-neighbor (2NN) competing interactions (competing system). Values of the 1NN and 2NN exchange interactions are estimated as J(1)=-138 K and J(2)=51 K (alphaequivalent toJ(2)/J(1)=-0.37). This value of alpha suggests that the ground state is a spin-singlet incommensurate state. In spite of relatively large J(1) and J(2), no magnetic phase transition appears down to 2 K, while an antiferromagnetic transition occurs in other model compounds of the competing system with ferromagnetic 1NN interaction. For that reason, Rb2Cu2Mo3O12 is a suitable compound to study properties of the incommensurate ground state that are unconfirmed experimentally.

We have studied the effects of the magnetic field up to 10 T on Raman spectra at 40 K in (Nd0.5Sr0.50)MnO3. The folded-phonon modes due to the superlattice in the charge-ordered phase disappear at 10 T, indicating the phase transition to the ferromagnetic metallic phase. We have observed a clear magnetic-field hysteresis. (C) 2003 Elsevier B.V. All rights reserved.

By means of Raman scattering we have observed the appearance of the folded phonon modes and the two-magnetic-excitation bound state under high pressures in 3.5% Mg-doped CuGeO3 crystals. The spin-Peierls (SP) gap mode is also observed. These facts indicate clear evidence that the SP phase transition extinguished by Mg doping is revived by the application of high pressure. The revived SP state has a large spin gap and a very weak lattice dimerization when compared with pure sample, which is interpreted in terms of the strongly frustrated spin chain with a spontaneous spin gap. Moreover, a new phonon peak is observed above 2.8 GPa in the SP phase and even far above the SP transition temperature, indicating a pressure-induced structural phase transition. We propose a P-T phase diagram for this sample. © 2003 The American Physical Society.

Raman-scattering study has been carried out in 3.5% Mg-doped CuGeO3 crystals under high pressures at low temperatures. This sample does not undertake the spin-Peierls (SP) transition at ambient pressure. The folded phonon modes and the two-magnetic-excitation bound state appear under high pressures. The SP gap mode is also observed. These facts give a clear evidence that the SP phase transition is revived by the application Of high pressure. A new phonon peak is observed above 2.8 GPa, indicating a pressure-induced structural phase transition. It remains observable far above the SP transition temperature. We propose a P-T phase diagram for this sample.

We report the Sr-concentration dependence of Raman spectra in (Nd1-xSrx)MnO3 (x similar to 1/2). In the charge-ordered phase of the x = 0.50 and 0.49 samples, several new Raman peaks appear. In the x = 0.48 sample, no new Raman peak is observed, indicating that the charge-ordered phase transition does not occur. We propose that the symmetry of the lattice in the charge-ordered phase is Pnmm. (C) 2003 Elsevier Science B.V. All rights reserved.

The effect of pressure is studied on highly Mg-doped CuGeO3, which at ambient pressure shows an antiferromagnetic transition but no spin-Peierls transition. The magnetic susceptibility under pressure was measured; the broad maximum around 60 K shifts to lower temperatures and the spin-Peierls (SP) transition reappears. The temperature-concentration phase diagram at various pressures was constructed and it was found that the SP phase region enlarges with pressure. These results mean that the enhancement of frustration due to nearest-neighbor and next-nearest-neighbor exchange interactions overcomes the effect of interchain interaction, and that the spin-Peierls phase is more stabilized than the paramagnetic or antiferromagnetic phases. In the temperature-pressure-concentration phase diagram, the compositional phase transition between the dimerized-antiferromagnetic phase and the uniform-antiferromagnetic phase is second order at P>0.2 GPa, but first order at P<0.2 GPa. We propose that the change in order of this transition is also due to enhancement of the frustration. We show an example of frustration effects in a one-dimensional quantum spin system.

We have measured the temperature dependence of Raman spectra in (Nd0.5Sr0.5)MnO3. New phonon modes are observed in the charge-ordered phase. A change of the lattice symmetry at the charge-order phase transition and extraordinary behavior of the low-frequency Raman spectra are reported. (C) 2002 Elsevier Science B.V. All rights reserved.

We have studied Zn- and Si-doping effects on the 366-cm(-1) folded phonon in the spin-Peierls (SP) systems Cu1-xZnxGeO3 and CuGe1-ySixO3 by means of Raman scattering. The folded phonon is induced by the formation of the superlattice. The Raman intensity at 6 K decreases almost linearly with increasing dopant concentration. The temperature dependence of Raman intensity and the broad line shape in the x = 0.021 Zn-doped crystal indicate that the SP long-range order is not formed in this sample. In the low-temperature antiferromagnetic (AF) phases in lightly doped crystals, we observe the sharp folded phonon mode, indicating that the lattice dimerization persists in this phase. No sharp Raman line from the folded phonon is observed in the AF phase of the x = 0.021 Zn- and y = 0.04 Si-doped crystals, indicating the existence of the uniform AF phase in these samples.

The dopant-concentration dependence of the folded phonon at 368 cm(-1) in the spin-Peierls (SP) system CuGeO3 has been studied by Raman scattering. With increasing dopant concentration, the Raman spectrum from the folded phonon weakens in intensity and broadens. In the weakly doped samples, the sharp folded phonon is observed at low temperatures, giving evidence for the coexistence of the SP and antiferromagnetic (AF) orders. In the 4% Si-doped sample, a broad Raman band probably reflecting the one-phonon density of states is observed in the AF phase, indicating the existence of the uniform AF phase. In the 2.1% Zn-doped sample, the 368 cm(-1) broad peak is superimposed on the broad band. This concentration is near the critical concentration between the dimerized AF and uniform AF phases. The intensity of the folded phonon in the ME-doped sample is stronger than those in Zn- and Si-doped samples. The luminescence emitted by the phonon cascade process is observed in the 8.1% Mg-doped sample. (C) 1999 Elsevier Science B.V. All rights reserved.

We study a spin-Peierls gap and a bound and a resonant states of two magnetic excitations in Cu1-xZnxGeO3 and CuGe1-ySiyO3. Breakdown of the selection rule in the doped samples enables us to observe the spin-Peierls gap excitation in the first-order Raman process. It is found that this excitation shifts to lower frequency and merges into the tail of the elastic scattering together with its line broadening with increasing dopant concentration. The bound state of a particle-hole pair is observed just below the bottom of the second-order Raman band in pure CuGeO3. It changes into a resonant state and disappears rapidly as the concentration of the dopant is increased, owing to the weakening of attractive interaction and the extreme increase of the dampings of the magnetic excitations. These features are demonstrated theoretically by using Green's function.

A spin-Peierls gap excitation is observed in the first-order Raman process in Zn-and Si-doped CuGeO3, because the selection rule is broken. It shifts to lower frequency together with its line broadening as the dopant concentration is increased. The two-magnetic-excitation bound state changes into a resonant state and disappears with increasing dopant concentration, mainly due to the shortening of the lifetime of magnetic excitation. These features are demonstrated theoretically by using a Green's function. (C) 1998 Elsevier Science B.V. All rights reserved.

We have measured the temperature dependence of low-frequency Raman spectra in CuGeO3, and have observed quasielastic scattering in the (c,c) polarization above the spin-Peierls transition temperature. We attribute this to the fluctuations of the energy density in the spin system. The magnetic specific heat and an inverse of the magnetic correlation length can be derived from the quasielastic scattering. The inverse of the magnetic correlation length is proportional to (T-T-SP)(1/2) at high temperatures. We compare the specific heat with a competing-J model. This model cannot explain quantitatively both the magnetic specific heat and the magnetic susceptibility with the same parameters. The origin of this discrepancy is discussed.

We measured Raman scattering below 400 cm(-1) in Cu1-xZnxGeO3 (x = 0, 0.013, 0.017, and 0.0212) to study the Zn-doping effect in the spin-Peierls phase. When Zn atoms (S = 0) are doped, a two-magnetic-exciton bound state disappears in the spin-Peierls phase. In the sample of x = 0.0212, the folded phonon mode at 370 cm(-1) and the two-magnetic-exciton Raman spectra below 250 cm(-1) were observed even in the uniform phase. In pure CuGeO3, the quasi-elastic scattering was observed at high temperatures. The origin of this scattering is considered as a spin fluctuation interacting with lattice.

Successive structural phase transitions in C-70 crystal were studied by heat capacity and Raman scattering. Heat capacity was measured between 4.2 and 350 K by means of the ac calorimetry. We observed two anomalies at 62 K and around 255 K. The latter is assigned as an orientational ordering transition in the molecular rotation about the long axis. The 62-K anomaly probably comes from the freezing of the jumping motion. Raman spectra of C-70 crystal were measured in the temperature region 14-340 K. A weak Raman peak at 1089 cm(-1) appear below about 152 K, but no anomaly was found at this temperature in the heat capacity.