直 竹下 - Academia.edu (original) (raw)
Papers by 直 竹下
日本物理学会講演概要集, Aug 25, 2004
日本物理学会講演概要集, Aug 26, 2013
日本物理学会講演概要集, Sep 10, 2000
日本物理学会講演概要集 秋の分科会, Aug 16, 1994
日本物理学会講演概要集, Mar 10, 2000
We report the 75 As nuclear magnetic resonance (NMR) measurement of the hole-doped superconductor... more We report the 75 As nuclear magnetic resonance (NMR) measurement of the hole-doped superconductor Ba1−xKxFe2As2 with different lattice parameters and different superconducting volume fractions (Tc ≃ 38 K). 75 As-NMR spectra revealed that the magnetically ordered and superconducting phases are microscopically separated. The spin-lattice relaxation rate 1/T1 in the normal state reflects the existence of a large two-dimensional antiferromagnetic spin fluctuation. The 1/T1 in the superconducting state down to the lowest measurement temperature T varies close to T 3 . In addition, it exhibits no coherence peak just below Tc. This shows a T dependence similar to those of other iron pnictides.
Physical review letters, Jan 17, 2002
We have investigated the pressure effect on T(c) and the Hall coefficient in the static stripe-or... more We have investigated the pressure effect on T(c) and the Hall coefficient in the static stripe-ordered phase of La(1.48)Nd(0.4)Sr(0.12)CuO(4) crystal under hydrostatic pressure. We found a dramatic change of the Hall coefficient and an abrupt increase of T(c) at low pressure of about 0.1 GPa. The results are indicative of a transition from one- to two-dimensional charge transport, associated with the suppression of low-temperature-tetragonal (LTT) phase. From the uniaxial pressure measurements it turns out that the observed critical change is induced primarily due to the in-plane compression of the CuO(2) planes which would make the pinning potential of the LTT lattice distortions weaker.
Synthetic Metals, 1999
ABSTRACT
Review of Scientific Instruments, 2007
We report (63)Cu nuclear quadrupole resonance (NQR) measurement of Cu(2)O under pressure up to ab... more We report (63)Cu nuclear quadrupole resonance (NQR) measurement of Cu(2)O under pressure up to about 10 GPa at low temperatures. Because the lattice parameter of Cu(2)O changes with increasing pressure, the electric field gradient at the Cu site also changes correspondingly with pressure. This enables us to use the Cu(2)O as an in situ manometer for high pressure nuclear magnetic resonance/NQR up to about 9 GPa.
Review of Scientific Instruments, 2002
Physical Review Letters, 2007
We measured the temperature dependent resistivity, varrho(T), of the intercalated graphite superc... more We measured the temperature dependent resistivity, varrho(T), of the intercalated graphite superconductor CaC6 as a function of pressure up to 16 GPa. We found a large linear increase of critical temperature, Tc, from the ambient pressure value 11.5 K up to 15.1 K, the largest value for intercalated graphite, at 7.5 GPa. At approximately 8 GPa, a jump of varrho and a sudden drop of Tc down to approximately 5 K indicates the occurrence of a phase transition. Our data analysis suggests that a pressure-induced phonon softening related to an in-plane Ca phonon mode is responsible for the Tc increase and that higher pressures greater, similar8 GPa lead to a structural transition into a new phase with a low Tc less, similar3 K.
Physical Review Letters, 2005
High-pressure effects on the superconducting transitions of beta-pyrochlore oxide superconductors... more High-pressure effects on the superconducting transitions of beta-pyrochlore oxide superconductors AOs(2)O(6) (A = Cs,Rb,K) are studied by measuring resistivity under high pressures up to 10 GPa. The superconducting transition temperature T(c) first increases with increasing pressure in every compound and then exhibits a broad maximum at 7.6 K (6 GPa), 8.2 K (2 GPa), and 10 K (0.6 GPa) for A = Cs, Rb, and K, respectively. Finally, the superconductivity is suppressed completely at a critical pressure near 7 GPa and 6 GPa for A = Rb and K and probably above 10 GPa for A = Cs. Characteristic changes in the coefficient A of the T(2) term in resistivity and residual resistivity are observed, both of which are synchronized with the corresponding change in T(c).
Physical Review Letters, 2006
The pyrochlore molybdate Gd2MO2O7 locates near the phase boundary between the ferromagnetic-metal... more The pyrochlore molybdate Gd2MO2O7 locates near the phase boundary between the ferromagnetic-metallic and the spin-glass insulating state. This metal-insulator transition is governed on a large energy scale by the electron-correlation effect, while the geometrical frustration causes the random potential. The magnetic field can tune the randomness of the potential and control, under a suitable pressure, the continuous Mott-Anderson transition precisely. The critical exponent (mu = 1.04 +/- 0.1) of the Mott-Anderson transition has been determined for this ferromagnetic orbital-degenerate electron system.
Physical Review Letters, 2009
The electron-correlation driven metal-insulator (Mott) transition in pyrocholore-type R2Mo2O7(R b... more The electron-correlation driven metal-insulator (Mott) transition in pyrocholore-type R2Mo2O7(R being rare-earth-metal ions) is accompanied by the change of the magnetic state from ferromagnetic to spin glass due to the competing double-exchange and superexchange interactions on the frustrated lattice. By application of high pressures on the compounds with Mott criticality, however, a new unique paramagnetic metal phase is observed to show up with nearly temperature-independent high resistivity close to the Ioffe-Regel limit. A possible non-Fermi-liquid character of this anomalously diffuse metallic state is argued in terms of the extended double-exchange model with the magnetically frustrated local S=1/2 spins.
Physical Review Letters, 2009
We investigate LiVS2 and LiVSe2 with a triangular lattice as itinerant analogues of LiVO2, known ... more We investigate LiVS2 and LiVSe2 with a triangular lattice as itinerant analogues of LiVO2, known for the formation of valence bond solid (VBS) state out of S = 1 frustrated magnet. LiVS2, which is located at the border between a metal and a correlated insulator, shows a first ordered transition from a paramagnetic metal to a VBS insulator at Tc ∼ 305 K upon cooling. The presence of VBS state in the close vicinity of insulator-metal transition may suggest the importance of itinerancy in the formation of VBS state. We argue that the high temperature metallic phase of LiVS2 has a pseudo-gap, likely originating from the VBS fluctuation. LiVSe2 was found to be a paramagnetic metal down to 2 K. PACS numbers: 71.30.+h, 61.14.-x, 61.50.Ks, 61.66.Fn Transition metal compounds with a geometrically frustrated lattice, such as a triangular and a pyrochlore lattice, often form a valence bond solid (VBS) state at low temperatures. When the t 2g orbitals are partially occupied, utilizing orbital degrees of freedom, complex "molecular" clusters in a spin singlet state are often formed: trimer in LiVO 2 [1, 2, 3, 4, 5], heptamer in AlV 2 O 4 [6], helical dimer in MgTi 2 O 4 [7] and octamer in CuIr 2 S 4 [8]. A VBS state was found also in organic systems with geometrical frustration [9]. Very recently, Shimizu et al. demonstrated that the VBS state can be melted by applying hydrostatic pressure P on the organic compound EtMe 3 P[Pd(dmit) 2 ] 2 with a triangular lattice. Remarkably, superconductivity appears as soon as the VBS state is suppressed by P [10].
Physical Review B, 2008
We have studied the uniaxial-stress effects on the half-doped charge-orbital ordered (CO-OO) mang... more We have studied the uniaxial-stress effects on the half-doped charge-orbital ordered (CO-OO) manganites Eu0.5Ca1.5MnO4 and Pr(Sr1-yCay)2Mn2O7(y0.9) . Eu0.5Ca1.5MnO4 with single Mn-O layer structure is ferroelastic and the application of weak uniaxial stress (<8.5MPa) along the orbital stripe direction drives a rotation of the orbital stripes by 90° through the lattice orthorhombicity channel. For Pr(Sr1-yCay)2Mn2O7(y0.9) with double Mn-O layer structure, it is revealed that the phase stability of two CO-OO states with different orbital stripe direction can be controlled by the external uniaxial pressure. In addition, the application of higher uniaxial pressure also causes a rotation of the lattice orthorhombicity itself and hence rotates the orbital stripes as well as the electric polarization axis in the low-temperature CO-OO state.
日本物理学会講演概要集, Aug 25, 2004
日本物理学会講演概要集, Aug 26, 2013
日本物理学会講演概要集, Sep 10, 2000
日本物理学会講演概要集 秋の分科会, Aug 16, 1994
日本物理学会講演概要集, Mar 10, 2000
We report the 75 As nuclear magnetic resonance (NMR) measurement of the hole-doped superconductor... more We report the 75 As nuclear magnetic resonance (NMR) measurement of the hole-doped superconductor Ba1−xKxFe2As2 with different lattice parameters and different superconducting volume fractions (Tc ≃ 38 K). 75 As-NMR spectra revealed that the magnetically ordered and superconducting phases are microscopically separated. The spin-lattice relaxation rate 1/T1 in the normal state reflects the existence of a large two-dimensional antiferromagnetic spin fluctuation. The 1/T1 in the superconducting state down to the lowest measurement temperature T varies close to T 3 . In addition, it exhibits no coherence peak just below Tc. This shows a T dependence similar to those of other iron pnictides.
Physical review letters, Jan 17, 2002
We have investigated the pressure effect on T(c) and the Hall coefficient in the static stripe-or... more We have investigated the pressure effect on T(c) and the Hall coefficient in the static stripe-ordered phase of La(1.48)Nd(0.4)Sr(0.12)CuO(4) crystal under hydrostatic pressure. We found a dramatic change of the Hall coefficient and an abrupt increase of T(c) at low pressure of about 0.1 GPa. The results are indicative of a transition from one- to two-dimensional charge transport, associated with the suppression of low-temperature-tetragonal (LTT) phase. From the uniaxial pressure measurements it turns out that the observed critical change is induced primarily due to the in-plane compression of the CuO(2) planes which would make the pinning potential of the LTT lattice distortions weaker.
Synthetic Metals, 1999
ABSTRACT
Review of Scientific Instruments, 2007
We report (63)Cu nuclear quadrupole resonance (NQR) measurement of Cu(2)O under pressure up to ab... more We report (63)Cu nuclear quadrupole resonance (NQR) measurement of Cu(2)O under pressure up to about 10 GPa at low temperatures. Because the lattice parameter of Cu(2)O changes with increasing pressure, the electric field gradient at the Cu site also changes correspondingly with pressure. This enables us to use the Cu(2)O as an in situ manometer for high pressure nuclear magnetic resonance/NQR up to about 9 GPa.
Review of Scientific Instruments, 2002
Physical Review Letters, 2007
We measured the temperature dependent resistivity, varrho(T), of the intercalated graphite superc... more We measured the temperature dependent resistivity, varrho(T), of the intercalated graphite superconductor CaC6 as a function of pressure up to 16 GPa. We found a large linear increase of critical temperature, Tc, from the ambient pressure value 11.5 K up to 15.1 K, the largest value for intercalated graphite, at 7.5 GPa. At approximately 8 GPa, a jump of varrho and a sudden drop of Tc down to approximately 5 K indicates the occurrence of a phase transition. Our data analysis suggests that a pressure-induced phonon softening related to an in-plane Ca phonon mode is responsible for the Tc increase and that higher pressures greater, similar8 GPa lead to a structural transition into a new phase with a low Tc less, similar3 K.
Physical Review Letters, 2005
High-pressure effects on the superconducting transitions of beta-pyrochlore oxide superconductors... more High-pressure effects on the superconducting transitions of beta-pyrochlore oxide superconductors AOs(2)O(6) (A = Cs,Rb,K) are studied by measuring resistivity under high pressures up to 10 GPa. The superconducting transition temperature T(c) first increases with increasing pressure in every compound and then exhibits a broad maximum at 7.6 K (6 GPa), 8.2 K (2 GPa), and 10 K (0.6 GPa) for A = Cs, Rb, and K, respectively. Finally, the superconductivity is suppressed completely at a critical pressure near 7 GPa and 6 GPa for A = Rb and K and probably above 10 GPa for A = Cs. Characteristic changes in the coefficient A of the T(2) term in resistivity and residual resistivity are observed, both of which are synchronized with the corresponding change in T(c).
Physical Review Letters, 2006
The pyrochlore molybdate Gd2MO2O7 locates near the phase boundary between the ferromagnetic-metal... more The pyrochlore molybdate Gd2MO2O7 locates near the phase boundary between the ferromagnetic-metallic and the spin-glass insulating state. This metal-insulator transition is governed on a large energy scale by the electron-correlation effect, while the geometrical frustration causes the random potential. The magnetic field can tune the randomness of the potential and control, under a suitable pressure, the continuous Mott-Anderson transition precisely. The critical exponent (mu = 1.04 +/- 0.1) of the Mott-Anderson transition has been determined for this ferromagnetic orbital-degenerate electron system.
Physical Review Letters, 2009
The electron-correlation driven metal-insulator (Mott) transition in pyrocholore-type R2Mo2O7(R b... more The electron-correlation driven metal-insulator (Mott) transition in pyrocholore-type R2Mo2O7(R being rare-earth-metal ions) is accompanied by the change of the magnetic state from ferromagnetic to spin glass due to the competing double-exchange and superexchange interactions on the frustrated lattice. By application of high pressures on the compounds with Mott criticality, however, a new unique paramagnetic metal phase is observed to show up with nearly temperature-independent high resistivity close to the Ioffe-Regel limit. A possible non-Fermi-liquid character of this anomalously diffuse metallic state is argued in terms of the extended double-exchange model with the magnetically frustrated local S=1/2 spins.
Physical Review Letters, 2009
We investigate LiVS2 and LiVSe2 with a triangular lattice as itinerant analogues of LiVO2, known ... more We investigate LiVS2 and LiVSe2 with a triangular lattice as itinerant analogues of LiVO2, known for the formation of valence bond solid (VBS) state out of S = 1 frustrated magnet. LiVS2, which is located at the border between a metal and a correlated insulator, shows a first ordered transition from a paramagnetic metal to a VBS insulator at Tc ∼ 305 K upon cooling. The presence of VBS state in the close vicinity of insulator-metal transition may suggest the importance of itinerancy in the formation of VBS state. We argue that the high temperature metallic phase of LiVS2 has a pseudo-gap, likely originating from the VBS fluctuation. LiVSe2 was found to be a paramagnetic metal down to 2 K. PACS numbers: 71.30.+h, 61.14.-x, 61.50.Ks, 61.66.Fn Transition metal compounds with a geometrically frustrated lattice, such as a triangular and a pyrochlore lattice, often form a valence bond solid (VBS) state at low temperatures. When the t 2g orbitals are partially occupied, utilizing orbital degrees of freedom, complex "molecular" clusters in a spin singlet state are often formed: trimer in LiVO 2 [1, 2, 3, 4, 5], heptamer in AlV 2 O 4 [6], helical dimer in MgTi 2 O 4 [7] and octamer in CuIr 2 S 4 [8]. A VBS state was found also in organic systems with geometrical frustration [9]. Very recently, Shimizu et al. demonstrated that the VBS state can be melted by applying hydrostatic pressure P on the organic compound EtMe 3 P[Pd(dmit) 2 ] 2 with a triangular lattice. Remarkably, superconductivity appears as soon as the VBS state is suppressed by P [10].
Physical Review B, 2008
We have studied the uniaxial-stress effects on the half-doped charge-orbital ordered (CO-OO) mang... more We have studied the uniaxial-stress effects on the half-doped charge-orbital ordered (CO-OO) manganites Eu0.5Ca1.5MnO4 and Pr(Sr1-yCay)2Mn2O7(y0.9) . Eu0.5Ca1.5MnO4 with single Mn-O layer structure is ferroelastic and the application of weak uniaxial stress (<8.5MPa) along the orbital stripe direction drives a rotation of the orbital stripes by 90° through the lattice orthorhombicity channel. For Pr(Sr1-yCay)2Mn2O7(y0.9) with double Mn-O layer structure, it is revealed that the phase stability of two CO-OO states with different orbital stripe direction can be controlled by the external uniaxial pressure. In addition, the application of higher uniaxial pressure also causes a rotation of the lattice orthorhombicity itself and hence rotates the orbital stripes as well as the electric polarization axis in the low-temperature CO-OO state.