K. Kakurai | Tohoku University (original) (raw)
Papers by K. Kakurai
Journal of Physics: Condensed Matter, 2017
CuAl2O4 is a normal spinel oxide having quantum spin, S=1/2 for Cu 2+. It is a rather unique feat... more CuAl2O4 is a normal spinel oxide having quantum spin, S=1/2 for Cu 2+. It is a rather unique feature that the Cu 2+ ions of CuAl2O4 sit at a tetrahedral position, not like the usual octahedral position for many oxides. At low temperatures, it exhibits all the thermodynamic evidence of a quantum spin glass. For example, the polycrystalline CuAl2O4 shows a cusp centered at ~2 K in the low-field dc magnetization data and a clear frequency dependence in the ac magnetic susceptibility while it displays logarithmic relaxation behavior in a time dependence of the magnetization. At the same time, there is 2 a peak at ~2.3 K in the heat capacity, which shifts towards higher temperature with magnetic fields. On the other hand, there is no evidence of new superlattice peaks in the high-resolution neutron powder diffraction data when cooled from 40 to 0.4 K. This implies that there is no long-ranged magnetic order down to 0.4 K, thus confirming a spin glasslike ground state for CuAl2O4. Interestingly, there is no sign of structural distortion either although Cu 2+ is a Jahn-Teller active ion. Thus, we claim that an orbital liquid state is the most likely ground state in CuAl2O4. Of further interest, it also exhibits a large frustration parameter, f = CW/Tm ~67, one of the largest values reported for spinel oxides. Our observations suggest that CuAl2O4 should be a rare example of a frustrated quantum spin glass with a good candidate for an orbital liquid state.
Physical Review B, 1990
The three-dimensional (3D) magnetism present in Nd&Cu04 and Pr2Cu04 has been examined in neutron-... more The three-dimensional (3D) magnetism present in Nd&Cu04 and Pr2Cu04 has been examined in neutron-scattering experiments. Antiferromagnetic long-range order of the Cu'+ moments develops below T& =255 K in both compounds. In Nd2Cu04 a series of magnetic phase transitions in which the Cu'+ spins reorient has also been observed below T&, while in Pr2Cu04 the 3D Cu + spin structure is stable. No changes in the crystalline structure of Nd&Cu04 associated with the spinreorientation transitions were found. Induced ordering of the rare-earth magnetic moments has been observed in both compounds. The Nd + (Pr +) magnetic moments align parallel (antiparallel) to Cu + moments which are nearest neighbors along the c axis, and the ordered moments of the rare-earth ions were determined to be-1. 3p& for Nd'+ at 0.4 K and-0. 08p& for Pr'+ at 8 K. The temperature-dependent Pr'+ moment is shown to be proportional to the Cu'+ moment times the bulk susceptibility. In this paper we also discuss the role of the XY anisotropy in the 3D ordering.
Physical Review Letters, 2000
Inelastic neutron scattering is used to study transverse-polarized magnetic excitations in the qu... more Inelastic neutron scattering is used to study transverse-polarized magnetic excitations in the quasione-dimensional S = 1/2 antiferromagnet BaCu2Si2O7, where the saturation value for the Néel order parameter is m0 = 0.12 µB per spin. At low energies the spectrum is totally dominated by resolution-limited spin wave-like excitations. An excitation continuum sets in above a well-defined threshold frequency. Experimental results are discussed in the context of current theories for weaklyinteracting quantum half-integer spin chains.
Physica C: Superconductivity, 1989
A new method of fitting the magnetic penetation depth from transverse field muo spi rotation (TF ... more A new method of fitting the magnetic penetation depth from transverse field muo spi rotation (TF muSR) data is demonstrated with oriented and unoriented sintered powders and single crystals of high Tc superconducting oxides.
Physica B: Condensed Matter, 2003
The phonon profiles of CuGeO 3 have been measured at Q ¼ ð0; 3; 0Þ and EB1 meV at several tempera... more The phonon profiles of CuGeO 3 have been measured at Q ¼ ð0; 3; 0Þ and EB1 meV at several temperatures by inelastic neutron scattering experiments. The temperature dependence of the central energy and the full-width at halfmaximum changes little, but the integral intensity decreases exponentially below 150 K. This anomalous phenomenon is related to the spin-Peierls transition regarded not as displacive type but as order-disorder type of oxygen atoms.
Physica B: Condensed Matter, 2003
Neutron measurements and transport studies have been carried out on single crystals of the pyroch... more Neutron measurements and transport studies have been carried out on single crystals of the pyrochlore ferromagnet Nd 2 Mo 2 O 7 ; which has quite unusual Hall resistivity r H : The magnetic structure has been determined at 1:6 K as a function of H; and based on this magnetic structure, the H-dependence of r H is deduced by using the recently proposed chiral order mechanism. The results have turned out not to describe the
Physical Review B, 1989
Using large single crystals (-2 cm), we have extended previous elastic and inelastic neutron scat... more Using large single crystals (-2 cm), we have extended previous elastic and inelastic neutron scattering studies of the spin dynamics in the two-dimensional (2D) quantum antiferromagnet La2Cu04. We observe well-defined magnetic peaks for constant energy scans across the 2D magnetic rod for a wide range of temperatures and energies. The spectra are compared quantitatively with theoretical models. Simple spin-wave theory satisfactorily describes the experimental results well below T&. The thermal excitations of the low-energy (Ace & 3 meV) spectra, however, appear to deviate from the theory as T& is approached. The overall temperature and energy dependence of the spectra above Tz (245-520 K) can be explained by the formula of Chakravarty, Halperin, Nelson, and Tyc for the dynamical structure factor S(q, cu) with no adjustable parameters. We do, however, observe a 2D elastic component which grows rapidly as T& is approached; it is hypothesized that this arises primarily from magnetic defects and is analogous to the "central peak" observed for many structural phase transitions.
Physica B: Condensed Matter, 1995
Polarized-neutron transmission experiments on a triangular-lattice antiferromagnet, LiNiO2, have ... more Polarized-neutron transmission experiments on a triangular-lattice antiferromagnet, LiNiO2, have been performed. The transmitted beam through the sample was depolarized below the lower transition temperature TN2. The transmitted beam also includes small-angle neutron-scattering, and magnetic small-angle neutron-scattering separated from the transmission was observed below the upper transition temperature TNr
Physica C: Superconductivity and its Applications, 2010
ABSTRACT
Physica B: Condensed Matter, 2002
The magnetic form factor in the pressure-induced ferromagnetic superconductor UGe 2 has been meas... more The magnetic form factor in the pressure-induced ferromagnetic superconductor UGe 2 has been measured with polarized neutrons. The experiments were performed on single crystal samples at 2 K by applying a magnetic field of 2 T along the easy magnetization axis a under pressures of 0 and 1:4 GPa: In spite of the itinerant nature of the 5f electrons, the shapes of the observed magnetic form factors at both 0 and 1:4 GPa are very close to the theoretical 5f form factor of the free U 3þ or U 4þ ions calculated by the dipole approximation. The effect of pressure on the 5f form factor seems to be small in spite of the reduction of the magnetic moment.
Physica C: Superconductivity, 1996
ABSTRACT
Journal of Physics and Chemistry of Solids, 1999
The magnetic ground state in the hole-doped two-leg ladder system Sr 2.5 Ca 11.5 Cu 24 O 41 both ... more The magnetic ground state in the hole-doped two-leg ladder system Sr 2.5 Ca 11.5 Cu 24 O 41 both at ambient pressure and under high pressure up to 3 GPa was investigated by a neutron scattering experiment. The spin gap value for this system at ambient pressure is found to be 32.1^0.3 meV, being almost the same as that of the parent material Sr 14 Cu 24 O 41. With increasing pressure, the spin gap energy is almost unchanged, however, the inelastic neutron scattering intensity at the zone center of the ladder is decreased progressively. We also observed antiferromagnetic long range ordering below T N (ϳ2.1 K), which was increased with increasing pressure. A possible magnetic structure of this system is also discussed. These results show the coexistence of the spin liquid state and antiferromagnetic ordering in the hole-doped two-leg ladder system.
Physica B: Condensed Matter, 2009
By applying spherical neutron polarimetry technique to Cr 1Àx Mo x B 2 (x ¼ 0, 0.15), we successf... more By applying spherical neutron polarimetry technique to Cr 1Àx Mo x B 2 (x ¼ 0, 0.15), we successfully determined its precise magnetic structures. We emphasize here that the ratio of magnetic component was determined by measuring the two satellite reflections and rotating the incident neutron spin directions.
Physical Review Letters, 1990
Neutron-scattering measurements have been performed on (Nd, Pr)~-, Ce, Cu04 (0.0 & x & 0. 125) si... more Neutron-scattering measurements have been performed on (Nd, Pr)~-, Ce, Cu04 (0.0 & x & 0. 125) single crystals. All samples exhibit three-dimensional (3D) long-range magnetic order; the temperature dependence of the 2D magnetic correlation lengths in all is similar in character to that observed in pure and lightly doped La~CuO& albeit with renormalized spin stiffness constants p,. Thus, in the nonsuperconducting doping regime the holeand electron-doped systems exhibit analogous magnetic behavior except for the drastically different dopant-concentration scales for the destruction of magnetic order,
Physical Review B, 1999
The antiferromagnetic ͑AF͒ compound BaCu 2 Si 2 O 7 is studied by magnetic susceptibility and neu... more The antiferromagnetic ͑AF͒ compound BaCu 2 Si 2 O 7 is studied by magnetic susceptibility and neutronscattering techniques. The observed behavior is dominated by the presence of loosely coupled Sϭ1/2 chains with the intrachain AF exchange constant J ʈ Ϸ24.1 meV. Long-range Néel ordering is observed below T N ϭ9.2 K. The results are discussed within the framework of the mean-field RPA model for weakly interacting quantum spin chains. ͓S0163-1829͑99͒03433-5͔
Physical Review B, 1995
We report inelastic neutron-scattering measurements on the spin-Peierls inorganic compound CuGeO3... more We report inelastic neutron-scattering measurements on the spin-Peierls inorganic compound CuGeO3 under high pressure up to 1.8 GPa. The pressure dependences of lattice constants under hydrostatic pressure are qualitatively similar to their temperature dependences when the temperature is decreased, but are enhanced by a factor of 4 along the b axis. The magnetic spin-Peierls gap energy increases with increasing pressure
Solid State Communications, 1990
Abstract The results of an inelastic neutron scattering study on the quasi one dimensional ferrom... more Abstract The results of an inelastic neutron scattering study on the quasi one dimensional ferromagnet (CH3)4N NiBr3 (TMNB) are reported. From the spin wave dispersion in the chain direction the intrachain exchange of J/kB=(4.5±0.2)K was deduced in accordance with the previous susceptibility measurement. An anisotropy in the easy plane, causing a gap of E0/kB=(1.8±1.0)K, had to be introduced to obtain a single site planar anisotropy of A/kB=3.4K consistent with the previous susceptibility measurement. The phase transition from hexagonal to monoclinic structure is found by the temperature dependence of the elastic incoherent scattering to be at Tc=133K.
Journal of Physics: Condensed Matter, 2017
CuAl2O4 is a normal spinel oxide having quantum spin, S=1/2 for Cu 2+. It is a rather unique feat... more CuAl2O4 is a normal spinel oxide having quantum spin, S=1/2 for Cu 2+. It is a rather unique feature that the Cu 2+ ions of CuAl2O4 sit at a tetrahedral position, not like the usual octahedral position for many oxides. At low temperatures, it exhibits all the thermodynamic evidence of a quantum spin glass. For example, the polycrystalline CuAl2O4 shows a cusp centered at ~2 K in the low-field dc magnetization data and a clear frequency dependence in the ac magnetic susceptibility while it displays logarithmic relaxation behavior in a time dependence of the magnetization. At the same time, there is 2 a peak at ~2.3 K in the heat capacity, which shifts towards higher temperature with magnetic fields. On the other hand, there is no evidence of new superlattice peaks in the high-resolution neutron powder diffraction data when cooled from 40 to 0.4 K. This implies that there is no long-ranged magnetic order down to 0.4 K, thus confirming a spin glasslike ground state for CuAl2O4. Interestingly, there is no sign of structural distortion either although Cu 2+ is a Jahn-Teller active ion. Thus, we claim that an orbital liquid state is the most likely ground state in CuAl2O4. Of further interest, it also exhibits a large frustration parameter, f = CW/Tm ~67, one of the largest values reported for spinel oxides. Our observations suggest that CuAl2O4 should be a rare example of a frustrated quantum spin glass with a good candidate for an orbital liquid state.
Physical Review B, 1990
The three-dimensional (3D) magnetism present in Nd&Cu04 and Pr2Cu04 has been examined in neutron-... more The three-dimensional (3D) magnetism present in Nd&Cu04 and Pr2Cu04 has been examined in neutron-scattering experiments. Antiferromagnetic long-range order of the Cu'+ moments develops below T& =255 K in both compounds. In Nd2Cu04 a series of magnetic phase transitions in which the Cu'+ spins reorient has also been observed below T&, while in Pr2Cu04 the 3D Cu + spin structure is stable. No changes in the crystalline structure of Nd&Cu04 associated with the spinreorientation transitions were found. Induced ordering of the rare-earth magnetic moments has been observed in both compounds. The Nd + (Pr +) magnetic moments align parallel (antiparallel) to Cu + moments which are nearest neighbors along the c axis, and the ordered moments of the rare-earth ions were determined to be-1. 3p& for Nd'+ at 0.4 K and-0. 08p& for Pr'+ at 8 K. The temperature-dependent Pr'+ moment is shown to be proportional to the Cu'+ moment times the bulk susceptibility. In this paper we also discuss the role of the XY anisotropy in the 3D ordering.
Physical Review Letters, 2000
Inelastic neutron scattering is used to study transverse-polarized magnetic excitations in the qu... more Inelastic neutron scattering is used to study transverse-polarized magnetic excitations in the quasione-dimensional S = 1/2 antiferromagnet BaCu2Si2O7, where the saturation value for the Néel order parameter is m0 = 0.12 µB per spin. At low energies the spectrum is totally dominated by resolution-limited spin wave-like excitations. An excitation continuum sets in above a well-defined threshold frequency. Experimental results are discussed in the context of current theories for weaklyinteracting quantum half-integer spin chains.
Physica C: Superconductivity, 1989
A new method of fitting the magnetic penetation depth from transverse field muo spi rotation (TF ... more A new method of fitting the magnetic penetation depth from transverse field muo spi rotation (TF muSR) data is demonstrated with oriented and unoriented sintered powders and single crystals of high Tc superconducting oxides.
Physica B: Condensed Matter, 2003
The phonon profiles of CuGeO 3 have been measured at Q ¼ ð0; 3; 0Þ and EB1 meV at several tempera... more The phonon profiles of CuGeO 3 have been measured at Q ¼ ð0; 3; 0Þ and EB1 meV at several temperatures by inelastic neutron scattering experiments. The temperature dependence of the central energy and the full-width at halfmaximum changes little, but the integral intensity decreases exponentially below 150 K. This anomalous phenomenon is related to the spin-Peierls transition regarded not as displacive type but as order-disorder type of oxygen atoms.
Physica B: Condensed Matter, 2003
Neutron measurements and transport studies have been carried out on single crystals of the pyroch... more Neutron measurements and transport studies have been carried out on single crystals of the pyrochlore ferromagnet Nd 2 Mo 2 O 7 ; which has quite unusual Hall resistivity r H : The magnetic structure has been determined at 1:6 K as a function of H; and based on this magnetic structure, the H-dependence of r H is deduced by using the recently proposed chiral order mechanism. The results have turned out not to describe the
Physical Review B, 1989
Using large single crystals (-2 cm), we have extended previous elastic and inelastic neutron scat... more Using large single crystals (-2 cm), we have extended previous elastic and inelastic neutron scattering studies of the spin dynamics in the two-dimensional (2D) quantum antiferromagnet La2Cu04. We observe well-defined magnetic peaks for constant energy scans across the 2D magnetic rod for a wide range of temperatures and energies. The spectra are compared quantitatively with theoretical models. Simple spin-wave theory satisfactorily describes the experimental results well below T&. The thermal excitations of the low-energy (Ace & 3 meV) spectra, however, appear to deviate from the theory as T& is approached. The overall temperature and energy dependence of the spectra above Tz (245-520 K) can be explained by the formula of Chakravarty, Halperin, Nelson, and Tyc for the dynamical structure factor S(q, cu) with no adjustable parameters. We do, however, observe a 2D elastic component which grows rapidly as T& is approached; it is hypothesized that this arises primarily from magnetic defects and is analogous to the "central peak" observed for many structural phase transitions.
Physica B: Condensed Matter, 1995
Polarized-neutron transmission experiments on a triangular-lattice antiferromagnet, LiNiO2, have ... more Polarized-neutron transmission experiments on a triangular-lattice antiferromagnet, LiNiO2, have been performed. The transmitted beam through the sample was depolarized below the lower transition temperature TN2. The transmitted beam also includes small-angle neutron-scattering, and magnetic small-angle neutron-scattering separated from the transmission was observed below the upper transition temperature TNr
Physica C: Superconductivity and its Applications, 2010
ABSTRACT
Physica B: Condensed Matter, 2002
The magnetic form factor in the pressure-induced ferromagnetic superconductor UGe 2 has been meas... more The magnetic form factor in the pressure-induced ferromagnetic superconductor UGe 2 has been measured with polarized neutrons. The experiments were performed on single crystal samples at 2 K by applying a magnetic field of 2 T along the easy magnetization axis a under pressures of 0 and 1:4 GPa: In spite of the itinerant nature of the 5f electrons, the shapes of the observed magnetic form factors at both 0 and 1:4 GPa are very close to the theoretical 5f form factor of the free U 3þ or U 4þ ions calculated by the dipole approximation. The effect of pressure on the 5f form factor seems to be small in spite of the reduction of the magnetic moment.
Physica C: Superconductivity, 1996
ABSTRACT
Journal of Physics and Chemistry of Solids, 1999
The magnetic ground state in the hole-doped two-leg ladder system Sr 2.5 Ca 11.5 Cu 24 O 41 both ... more The magnetic ground state in the hole-doped two-leg ladder system Sr 2.5 Ca 11.5 Cu 24 O 41 both at ambient pressure and under high pressure up to 3 GPa was investigated by a neutron scattering experiment. The spin gap value for this system at ambient pressure is found to be 32.1^0.3 meV, being almost the same as that of the parent material Sr 14 Cu 24 O 41. With increasing pressure, the spin gap energy is almost unchanged, however, the inelastic neutron scattering intensity at the zone center of the ladder is decreased progressively. We also observed antiferromagnetic long range ordering below T N (ϳ2.1 K), which was increased with increasing pressure. A possible magnetic structure of this system is also discussed. These results show the coexistence of the spin liquid state and antiferromagnetic ordering in the hole-doped two-leg ladder system.
Physica B: Condensed Matter, 2009
By applying spherical neutron polarimetry technique to Cr 1Àx Mo x B 2 (x ¼ 0, 0.15), we successf... more By applying spherical neutron polarimetry technique to Cr 1Àx Mo x B 2 (x ¼ 0, 0.15), we successfully determined its precise magnetic structures. We emphasize here that the ratio of magnetic component was determined by measuring the two satellite reflections and rotating the incident neutron spin directions.
Physical Review Letters, 1990
Neutron-scattering measurements have been performed on (Nd, Pr)~-, Ce, Cu04 (0.0 & x & 0. 125) si... more Neutron-scattering measurements have been performed on (Nd, Pr)~-, Ce, Cu04 (0.0 & x & 0. 125) single crystals. All samples exhibit three-dimensional (3D) long-range magnetic order; the temperature dependence of the 2D magnetic correlation lengths in all is similar in character to that observed in pure and lightly doped La~CuO& albeit with renormalized spin stiffness constants p,. Thus, in the nonsuperconducting doping regime the holeand electron-doped systems exhibit analogous magnetic behavior except for the drastically different dopant-concentration scales for the destruction of magnetic order,
Physical Review B, 1999
The antiferromagnetic ͑AF͒ compound BaCu 2 Si 2 O 7 is studied by magnetic susceptibility and neu... more The antiferromagnetic ͑AF͒ compound BaCu 2 Si 2 O 7 is studied by magnetic susceptibility and neutronscattering techniques. The observed behavior is dominated by the presence of loosely coupled Sϭ1/2 chains with the intrachain AF exchange constant J ʈ Ϸ24.1 meV. Long-range Néel ordering is observed below T N ϭ9.2 K. The results are discussed within the framework of the mean-field RPA model for weakly interacting quantum spin chains. ͓S0163-1829͑99͒03433-5͔
Physical Review B, 1995
We report inelastic neutron-scattering measurements on the spin-Peierls inorganic compound CuGeO3... more We report inelastic neutron-scattering measurements on the spin-Peierls inorganic compound CuGeO3 under high pressure up to 1.8 GPa. The pressure dependences of lattice constants under hydrostatic pressure are qualitatively similar to their temperature dependences when the temperature is decreased, but are enhanced by a factor of 4 along the b axis. The magnetic spin-Peierls gap energy increases with increasing pressure
Solid State Communications, 1990
Abstract The results of an inelastic neutron scattering study on the quasi one dimensional ferrom... more Abstract The results of an inelastic neutron scattering study on the quasi one dimensional ferromagnet (CH3)4N NiBr3 (TMNB) are reported. From the spin wave dispersion in the chain direction the intrachain exchange of J/kB=(4.5±0.2)K was deduced in accordance with the previous susceptibility measurement. An anisotropy in the easy plane, causing a gap of E0/kB=(1.8±1.0)K, had to be introduced to obtain a single site planar anisotropy of A/kB=3.4K consistent with the previous susceptibility measurement. The phase transition from hexagonal to monoclinic structure is found by the temperature dependence of the elastic incoherent scattering to be at Tc=133K.