深澤 英人

We have synthesized new compounds R2Nb2O7 (R = Dy, Yb) based on the pyrochlore structure. Both are insulating at room temperature. Susceptibility measurements reveal that Dy2Nb 2O7 exhibits a spin freezing around 1 K without any long range ordering down to 0.3 K, while the Yb-based compound does not show any ordering down to 0.3 K. Specific heat measurements show that both compounds have a well-isolated doublet ground state. Magnetization and magnetic contribution to the specific heat for both systems solely come from rare earth ions, which suggests that the Nb site network has nonmagnetic state as reported for Y 2Nb2O7. © 2004 The Physical Society of Japan.

We have studied the electronic structure of Sr2RuO4 and Ca2RuO4 using x-ray photoemission spectroscopy (XPS) and subsequent model calculations. While the t2g band of Sr 2RuO4 has substantial spectral weight at the Fermi level, that of Ca2RuO4 has no spectral weight at EF and shows a peak at -1.8 eV. In the valence-band XPS spectrum of Sr 2RuO4, a satellite structure of the t2g band is observed. In order to explain the spectral features, we have carried out band-structure calculations using the unrestricted Hartree-Fock (HF) approximation and found good agreement with the experimental result for Ca 2RuO4. In order to explain the satellite structure of Sr2RuO4, we have performed second-order perturbation calculations of the self-energy corrections around the unrestricted HF solution of Sr2RuO4.

We report the specific heat of single crystals of the spin ice compound Dy2Ti2O7 at temperatures down to 100 mK in the so-called Kagome ice state. In our previous paper, we showed the anisotropic release of residual entropy in different magnetic field directions and reported new residual entropy associated with spin frustration in the Kagome slab for field in the [111] direction. In this paper, we confirm the first-order phase transition line in the field-temperature phase diagram and the presence of a critical point at (0.98 T, 400 mK), previously reported from the magnetization and specific-heat data. We newly found another peak in the specific heat at 1.25 T below 0.3 K. One possible explanation for the state between 1 T and 1.25 T is the coexistence of states with different spin configurations including the 2-in 2-out state (Kagome ice state), the 1-in 3-out state (ordered state) and paramagnetic state (free-spin state). © 2004 The Physical Society of Japan.

We reported the optical conductivity spectra of the Ruddlesden-Popper series ruthenates, i.e., Srn+1RunO3n+1 and Can+1RunO3n+1, where n=1, 2, and ∞. Among various optical transitions, we investigated two Ru-O related modes, i.e., the charge-transfer excitation and the transverse stretching phonon. We found that their frequency shifts are not much affected by a structural dimensionality, but are closely related to the Ru-O bond length. Through the quantitative analysis of the charge-transfer excitation energy, we could demonstrate that the p-d hybridization should play an important role in determining their electronic structure. In addition, we discussed how the electronic excitation could contribute the lattice dynamics in the metallic ruthenates.

We report here results of DC and AC magnetization, and magnetic specific heat measurements of a geometrically frustrated pyrochlore Tb2Nb 2O7. The DC susceptibility χ is consistent with collective paramagnetism down to a temperature scale set by effective nearest-neighbor interactions, Jeff≈-1.5K. However, AC susceptibility χac peak exhibits a frequency-dependence at T f∼0.8K, signaling a spin-glass-like freezing of Tb3+ spins in Tb2Nb2O7. © 2003 Elsevier B.V. All rights reserved.

The nature of metal-insulator (MI) transition in a t(2g)-electron system Ca2-xSrxRuO4 has been investigated using O 1s x-ray absorption spectroscopy (XAS). The O 1s XAS data strongly depend on temperature and demonstrate that the hysteresis of the MI transition is accompanied by the hysteresis of the orbital change. An unrestricted Hartree-Fock analysis indicates that the amount of yz/zx hole is discontinuously reduced at the MI transition. In the insulating phase, the amount of 4d yz/zx hole gradually decreases on heating, suggesting that partial orbital fluctuation increases with temperature.

The nature of metal-insulator (MI) transition in a t2g-electron system Ca2-xSrxRuO4 has been investigated using O 1s x-ray absorption spectroscopy (XAS). The O 1s XAS data strongly depend on temperature and demonstrate that the hysteresis of the MI transition is accompanied by the hysteresis of the orbital change. An unrestricted Hartree-Fock analysis indicates that the amount of yz/zx hole is discontinuously reduced at the MI transition. In the insulating phase, the amount of 4d yz/zx hole gradually decreases on heating, suggesting that partial orbital fluctuation increases with temperature.

We report on the specific heat and the entropy of single crystals of the spin ice compound Dy2Ti2O7 down to 100 mK under magnetic fields along the [111] direction in order to study detailed low temperature properties of the Kagome ice state. Previously, we reported anisotropic release of the residual entropy reflecting the Ising anisotropy. We pointed out that the anisotropic behaviour can be viewed as the difference in frustration dimensionality. In an intermediate field, the entropy for the [111] field direction is different from those of the other principal directions. This is because the frustration structure changes from that of a three-dimensional (3D) pyrochlore to that of a two-dimensional (2D) Kagome lattice with constraint from the ice rule, accompanied by a different zero-point entropy value. We call this state 'the Kagome ice state'. In the Kagome ice state, the value of the residual entropy, ΔSKagome, is estimated as 0.598 J K-1/(mol Dy) by Pauling's method. In the present experiment, we observed that the specific heat at 1 T exhibits a sharp peak at 400 mK and an anomalous upturn at lower temperature. This peak indicates the entropy release from a 1-in 3-out state with no residual entropy to a 1-in 3-free state.

DC (χ) and ac (χac) magnetic susceptibilities, field dependence of isothermal magnetization and specific heat CP measurements have been performed on a pyrochlore geometrically frustrated niobate Tb2Nb2O7. The χ characteristics is consistent with collective paramagnetism down to at least T≈2 K with effective nearest-neighbor antiferromagnetic interactions, Jeff=−1.5 K, among magnetic ions. Crystal-field calculation shows that Tb3+ ground state is an isolated Ising-like S=1/2 doublet with anisotropic g tensor and Tb3+ spins pointing along local 〈111〉 directions of tetrahedral unit of pyrochlore structure. The ground-state doublet is further modified by the short-range spin-spin correlations, giving rise to a rounded peak at 1.9 K in zero-field magnetic specific heat Cmag. In addition, the frequency-dependent peak in χac at about 0.8 K indicates spin-glass-like freezing of Tb3+ spins in Tb2Nb2O7. Tb2Nb2O7 is compared with other geometrically frustrated Tb-based pyrochlores, e.g., a spin-glass Tb2Mo2O7 and a collective paramagnet-spin liquid Tb2Ti2O7. © 2003 The American Physical Society.

We use Raman scattering to study spin, charge, and lattice dynamics in various phases of Ca2-xSrxRuO4. With increasing substitution of Ca by Sr in the range 0≤x<0.2, we observe (1) evidence for an increase of the electron-phonon interaction strength, (2) an increased temperature dependence of the two-magnon energy and linewidth in the antiferromagnetic insulating phase, and (3) evidence for charge gap development significantly below the metal-insulator transition (TM1), and hysteresis associated with the structural phase change, both of which are indicative of a first-order metal-insulator transition and a coexistence of metallic and insulating components for T<TM1.

We show that application of very modest pressures to pure Ca 2RuO4 transforms it form an antiferromagnetic Mott insulator to a quasi-2D metal with a ferromagnetic ground state. The smallness of the moment compared with the saturated moment of the localized Ru 4+ ion reflects the itinerant character of the magnetism. © 2003 Elsevier Science B.V. All rights reserved.

Dy2Ti2O7 is regarded as a good example of geometrically frustrated spin systems exhibiting spin ice behavior. We report specific heat Cp of single crystalline Dy2Ti 2O7 in the characteristic magnetic-field direction [1 0 0] down to 0.35 K and up to 5 T. We observed a single peak in Cp with the peak temperature dependent on the field strength. For this direction, the component of the magnetic moment parallel to the field is equivalent for every spin in each tetrahedron. Thus in high fields, this peak is expressed as a Schottky peak characterized by the Zeemann splitting; in lower fields, however, the peak becomes representative of the energy of spin-spin interactions. © 2003 Elsevier Science B.V. All rights reserved.

We have investigated a pyrochlore Mott insulator Y2Ir2O7 with an expectation that 5d(5) electrons of Ir ions in the t(2g) orbitals give rise to an S = 1/2 Heisenberg system and form a quantum spin-liquid state. However, it exhibits spin-glass ordering below about 170 K. Such spin-glass ordering is also observed in its isomorphs Y2Mo2O7 and Y2Ru2O7 with S = 1.

We report dc magnetization and specific heat of a series of insulating pyrochlore oxides Y2Nb2-xTixO7. Although the end member Y2Nb2O7 contains magnetic tetravalent Nb ions (4d1, S= 1/2), it does not exhibit any sign of magnetic ordering at least between 0.35 and 800 K. In addition, it exhibits Curie-Weiss behavior originating from spins S = 1/2 on less than 0.3% of the Nb sites; the remaining spins of Nb ions apparently disappear. With the substitution of nonmagnetic Ti ions, this magnetic component increases only up to 4.7% of the B sites, with Nb and Ti combined. These results are ascribable to a strongly correlated spinless state, which is a specific character derived from a band structure of d-electron pyrochlore oxides.

We use Raman scattering to study spin, charge, and lattice dynamics in various phases of Ca2−xSrxRuO4. With increasing substitution of Ca by Sr in the range 0 < x < 0.2, we observe (1) evidence for an increase of the electron-phonon interaction strength, (2) an increased temperature dependence of the two-magnon energy and linewidth in the antiferromagnetic insulating phase, and (3) evidence for charge gap development significantly below the metal-insulator transition (TMI), and hysteresis associated with the structural phase change, both of which are indicative of a first-order metal-insulator transition and a coexistence of metallic and insulating components for T < TMI. © 2003 The American Physical Society.

We report the specific heat and entropy of single crystals of the spin ice compound Dy2Ti2O7 at temperatures down to 0.35 K. We apply magnetic fields along the four characteristic directions [100], [110], [111], and [112]. Because of Ising anisotropy, we observe anisotropic release of the residual zero-point entropy, attributable to the difference in frustration dimensionality. In the high magnetic field along these four directions, the residual entropy is almost fully released and the activation entropy reaches Rln2. However, in the intermediate-field region, the entropy in the fields along the [111] direction is different from those for the other three field directions. For the [111] direction, the frustration structure changes from that of the three-dimensional pyrochlore to that of a two-dimensional Kagome-like lattice with constraint due to the ice rule, leading to different values of the zero-point entropy. © 2003 The American Physical Society.

The doping- and temperature-dependent optical conductivity spectra of Ca2-xSrxRuO4 were investigated. The 3-orbital Hubbard model was used to explain the temperature-dependent evolution of two correlation-induced excitations in terms of orbital arrangements.

The pressure-induced collapse of the Mott-like phases of Ca3Ru2O7 and Ca2RuO4 were discussed using the Raman spectroscopic studies. The floating zone technique for Ca2RuO4 and the flux method for Ca3Ru2O7 were used to obtain the single crystal. The pressure-tuned critical point in Ca3Ru2O7 was larger than that in Ca2RuO4 indicating a more robust Mott insulating phase in the former.

We show that, the pressure-temperature phase diagram of the Mott insulator Ca2RuO4 features a metal-insulator transition at 0.5 GPa: at 300 K from paramagnetic insulator to paramagnetic quasi-two-dimensional metal, and at T≤12 K from antiferromagnetic insulator to ferromagnetic, highly anisotropic, three-dimensional metal. We compare the metallic state to that of the structurally related p-wave superconductor Sr2RuO4, and discuss the importance of structural distortions, which are expected to couple strongly to pressure.

We report magnetization and ac susceptibility of single crystals of the spin-ice compound Dy2Ti2O7. Saturated moments at 1.8 K along the characteristic axes [100] and [110] agree with the expected values for an effective ferromagnetic nearest-neighbor Ising pyrochlore with local (111) anisotropy, where each magnetic moment is constrained to obey the "ice rule." At high enough magnetic fields along the [111] axis, the saturated moment exhibits a breaking of the ice rule; it agrees with the value expected for a three-in, one-out spin configuration. Assuming the realistic magnetic interaction between Dy ions given by the dipolar spin ice model, we completely reproduce the results at 2 K by Monte Carlo calculations. However, down to at least 60 mK, we have not found any experimental evidence of the long-range magnetic ordering predicted by this model to occur at around 180 mK. Instead, we confirm the spin freezing of the system below 0.5 K.

We report a Raman scattering study of the pressure-induced collapse of the Mott-like phases of [Formula presented] ([Formula presented]) and [Formula presented] ([Formula presented]). The pressure dependence of the phonon and two-magnon excitations in these materials indicate (i) a [Formula presented] pressure-induced collapse of the antiferromagnetic (AF) insulating phase above [Formula presented] in [Formula presented] and [Formula presented] in [Formula presented], (ii) a remarkable insensitivity of the exchange interaction to pressure in both systems, and (iii) evidence for persistent AF correlations above the critical pressure of [Formula presented], suggestive of phase separation involving AF insulator and ferromagnetic metal phases. © 2002 The American Physical Society.

We report the specific heat of single crystals of the spin ice compound Dy2Ti2O7 under magnetic fields down to 1.8 K. Our polycrystalline results down to 0.4 K are fully consistent with the previous report by Ramirez et al. For the magnetic field along the [1 0 0] direction, Cspin/T is interpreted as one Schottky peak characterized by the Zeemann splitting of the Ising levels of all four equivalent spins for each tetrahedron. However, for the magnetic field along the [1 1 1] direction, Cspin/T is interpreted as two-component Schottky peak reflecting the fact that there are two kinds of spin components. Along the [1 1 0] direction, it is also interpreted as two-component Schottky peak because of experimental misalignment. We found that the features of the polycrystalline data in magnetic fields cannot be reconstructed by simply taking appropriate averages of the present anisotropic single-crystalline data. © 2002 Elsevier Science Ltd. All rights reserved.

Magnetic properties in the normal state of spin-triplet superconductor Sr2RuO4 are discussed on the basis of Ru- and O-NMR and neutron-diffraction (ND) measurements reported so far. Besides strong incommensurate antiferromagnetic (IC-AFM) fluctuations probed by the ND measurements, it is shown that q-independent fluctuations are present over a wide temperature range up to 500 K from the comparison between NMR and ND results. From an anisotropy in the nuclear spin lattice relaxation rate 1/T1 at both the Ru and O sites, it was found that the IC-AFM fluctuations are anisotropic so that the out-of-plane component of low-frequency dynamical susceptibility, X″IC,out(q,ω) is by a factor of ∼3 larger than the in-plane one, X″IC,in(q,ω). These unique characters in magnetic fluctuations are implied to have some relevance for the occurrence of spin-triplet superconductivity.

O 1s x-ray absorption study of the Mott insulator Ca2RuO4 shows that the orbital population of the 4d t2g band dramatically changes with temperature. In addition, spin-resolved circularly polarized photoemission study of Ca2RuO4 shows that a substantial orbital angular momentum is induced in the Ru 4d t2g band. Based on the experimental results and model Hartree-Fock calculations, we argue that the cooperation between the strong spin-orbit coupling in the Ru 4d t2g band and the small distortion of the RuO6 octahedra causes the interesting changeover of the spin and orbital anisotropy as a function of temperature. © 2001 The American Physical Society.

We have grown single crystals of electron doping system Ca2-xLaxRuO4 (0.00 ≤ x ≤ 0.20) by a floating zone method. The first order metal/non-metal transition and canted antiferromagnetic ordering occur for 0.00 < x < 0.15, similar to those in the bandwidth controlled system Ca2-xSrxRuO4 (CSRO). However, comparing with CSRO, we found a rather different metallic ground state adjacent to the non-metallic ground state with canted antiferromagnetic order. Instead of short-range antiferromagnetic correlation found in CSRO (0.20 ≤ x < 0.50), the metallic ground state of the present system is characterized by strong ferromagnetic correlation.

Magnetic properties in the normal state of spin-triplet superconductor (formula presented) are discussed on the basis of Ru- and O-NMR and neutron-diffraction (ND) measurements reported so far. Besides strong incommensurate antiferromagnetic (IC-AFM) fluctuations probed by the ND measurements, it is shown that q-independent fluctuations are present over a wide temperature range up to 500 K from the comparison between NMR and ND results. From an anisotropy in the nuclear spin lattice relaxation rate (formula presented) at both the Ru and O sites, it was found that the IC-AFM fluctuations are anisotropic so that the out-of-plane component of low-frequency dynamical susceptibility, (formula presented) is by a factor of (formula presented) larger than the in-plane one, (formula presented) These unique characters in magnetic fluctuations are implied to have some relevance for the occurrence of spin-triplet superconductivity. © 2001 The American Physical Society.

We report 99Ru Knight-shift (99K) measurement on the high-quality Sr2RuO4 with a superconducting transition temperature Tc∼ 1.5 K. Its spin component, 99Ks due to the core polarization effect caused by Ru-4d spins, is nearly one order of magnitude larger than that of the 17O Knight shift (17Ks). The 4d-spin susceptibility that has been more precisely measured from 99Ks than 17Ks shows no change across Tc at all. The present result reinforces that Sr2RuO4 is the spin-triplet p-wave superconductor in which electrons are bound together in parallel-spin pairs parallel to the RuO2 plane.

We describe the details of crystal growth of Sr2RuO4 by a floating-zone method. The superconducting transition temperature Tc can be controlled by adjustment of crystal growth conditions. By optimizing the growth conditions, crystals with Tc up to 1.50 K have been obtained. In addition, we observed three kinds of two-phase composite structures grown under different conditions. They include (1) a Sr2RuO4-Ru eutectic system, in which lamellae of Ru are embedded in the primary phase of Sr2RuO4, resulting in a remarkable enhancement in Tc by a factor of about 2; (2) epitaxial-like intergrowth of ferromagnetic SrRuO3 on Sr2RuO4; and (3) eutectic growth of Sr2RuO4 and a new insulating phase with the possible composition of Sr3RuO5+δ.

We will describe the growth of single crystals of the layered perovskite Ca2 RuO4 by a floating-zone method. We have confirmed that stoichiometric Ca2RuO4 is a Mott insulator. Canted antiferromagnetic (CAP) ordering occurs below 113 K. We also discuss the development of a short-range CAP order below 150 K. © 2000 Elsevier Science B.V. All rights reserved.