深澤 英人

Zero-field and longitudinal-field muon spin relaxation (μSR) measurements have been performed in optimally doped and overdoped iron-based oxypnictide superconductors LaFeAsO1-xFx in order to investigate the spin dynamics over a wide temperature range. We have observed no sign of the slowing-down behavior of the spin fluctuations in all samples below and above the superconducting transition temperature. The results of the present study suggest that spins, which would play an important role in the appearance of superconductivity with the characteristics of the Fermi surface, fluctuate rapidly from the viewpoint of μSR and their fluctuation frequency is beyond the upper limit of the characteristic time window of μSR (∼10-11 s) in optimally doped and overdoped LaFeAsO 1-xFx. © 2011 The Physical Society of Japan.

We have measured electrical resistivity under pressure P and 75As NMR on single crystal and poly crystalline BaFe 2As2 and (Ba1-xKx)Fe 2As2 to elucidate mechanism of antiferromagnetic/ superconducting phase transition in BaFe2As2. This system is an antiferromagnet, AF, with a transition temperature of 141 K, which shows superconductivity by applying P or by a carrier doping of K for Ba. From high P resistivity measurements, we understood that the transition was highly sensitive to the P homogeneity. Small uniaxial stress strongly suppresses AF order and stabilizes superconductivity at much lower pressure. 75As NMR study in (Ba1-xKx)Fe2As2 shows existence of large AF spin fluctuations in tetragonal phase having a superconducting ground state. The AF spin fluctuations decrease gradually with increasing K concentration. © Published under licence by IOP Publishing Ltd.

We report the results of the angular-dependent magnetoresistance oscillations (AMROs), which can determine the shape of bulk Fermi surfaces (FSs) in quasi-two-dimensional (Q2D) systems, in a highly hole-doped Fe-based superconductor KFe2As2 with Tc3.7K. From the AMROs, we determined the two Q2D FSs with rounded-square cross sections, correspond to 12% and 17% of the first Brillouin zone. The rounded-squared shape of the FS cross section is also confirmed by the analyses of the interlayer transport under in-plane fields. From the obtained FS shape, we infer the character of the 3d orbitals that contribute to the FSs. © 2010 The American Physical Society.

We have succeeded in observing de Haas-van Alphen oscillations in KFe 2As2. Cylindrical Fermi surfaces are found. Effective masses of electrons range from about 6-9 times the free electron mass for B∥c. © 2009 Elsevier B.V. All rights reserved.

We report the 75As nuclear magnetic resonance (NMR) measurement of hole-doped superconductor Ba1-xKxFe2As 2 with superconducting transition temperature Tc ≃ 38 K for x = 0.4 and Tc ≃ 35 K for x = 0.55. 75As-NMR spectra revealed that the magnetically ordered and paramagnetic phases are microscopically separated. The spin-lattice relaxation rate 1/T1 in the normal state reflects the existence of a large two-dimensional antiferromagnetic (AF) spin fluctuation. The 1/T1 in the superconducting state down to the lowest measurement temperature varies close to T3. In addition, it exhibits no coherence peak just below T c. From the difference of the 1/T1 of Ba 0.6K0.4Fe2As2 in the AF state at 1.5 K and that in the SC state, it is suggested that the superconductivity does not occur in the phase-separated AF state. © 2009 Elsevier B.V. All rights reserved.

Centimeter-sized platelet single crystals of KFe2As2 were grown using a self-flux method. An encapsulation technique using a commercial stainless steel container allowed the stable crystal growth to last for more than 2 weeks. Ternary K-Fe-As systems with various starting compositions were examined to determine the optimal growth conditions. The employment of KAs flux led to the growth of large single crystals with a typical size of as large as 15 × 10 × 0.4 mm3. The grown crystals exhibit a sharp superconducting transition at 3.4K with the transition width 0.2 K, as well as the very large residual resistivity ratio exceeding 450, evidencing the good sample quality. © 2010 The Physical Society of Japan.

Among the iron-based pnictide superconductors the material KFe2 As2 is unusual in that its Fermi surface does not consist of quasinested electron and hole pockets. Here we report measurements of the temperature dependent London penetration depth of very clean crystals of this compound with residual resistivity ratio >1200. We show that the superfluid density at low temperatures exhibits a strong linear-in-temperature dependence which implies that there are line nodes in the energy gap on the large zone-centered hole sheets. The results indicate that KFe2 As2 is an unconventional superconductor with strong electron correlations. © 2010 The American Physical Society.

A Comment on the Letter by J.K. Dong et al., Phys. Rev. Lett. 104, 087005 (2010)PRLTAO0031-900710.1103/PhysRevLett.104.087005. The authors of the Letter offer a Reply. © 2010 The American Physical Society.

We have performed electrical resistivity measurements on single-crystal BaFe2 As2 under high pressure P up to 16 GPa with a cubic anvil apparatus, and up to 3 GPa with a modified Bridgman anvil cell. The samples were obtained from the same batch, which was grown with a self-flux method. A cubic anvil apparatus provides highly hydrostatic pressure, and a modified Bridgman anvil cell, which contains liquid-pressure transmitting medium, provides quasihydrostatic pressure. For highly hydrostatic pressure, the crystal phase and magnetic-transition temperature decreases robustly with P and disappears at around 10 GPa. The superconducting phase appears adjacent to magnetic phase in narrow pressure region between 11 and 14 GPa. The tiny difference in hydrostaticity between the cubic anvil apparatus and modified Bridgman anvil cell induces a drastic effect on the phase diagram of BaFe 2 As2. This result indicates that small uniaxial stress along c axis strongly suppresses the structural/antiferromagnetic ordering and stabilizes superconductivity at much lower pressure. © 2010 The American Physical Society.

We report on a band structure calculation and de Haas-van Alphen measure ements of KFe2As2. Three cylindrical Fermi surfaces are found. Effective masses of electrons range from 6 to 18me, m e being the free electron mass. Remarkable discrepancies between the calculated and observed Fermi surface areas and the large mass enhancement (≳3) highlight the importance of electronic correlations in determining the electronic structures of iron pnicitide superconductors. © 2010 The Physical Society of Japan.

We performed resistivity and zero-external-field (ZF) 75As nuclear magnetic resonance (NMR) measurements of Ba1-xK xFe2As2 (x 0 and 0.4) under high pressure. ZF 75As NMR measurements revealed that the magnetic ordered state is robust against pressure. On the other hand, superconducting transition temperature Tc has a tendency to decrease monotonically under pressure. The spin-lattice relaxation rate 1/T1 in antiferromagnetic and superconducting states suggests that the coexistence of the both states is not microscopic but is phase separated. © 2010 IOP Publishing Ltd.

The high pressure nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) are conventionally performed up to 3 GPa using piston cylinder cell. However, the NMR/NQR measurements beyond this pressure range are scarcely performed owing to the technical difficulty. Recently, we developed new high pressure NMR/NQR technique using cubic anvil apparatus in which highly hydrostatic pressure was obtained. Using the new method, the 63Cu-NQR signal of Cu2O was observed up to 7.2GPa with high sensitivity. The use of MgO gasket in mini-cubic anvil apparatus was examined for enlarging pressure range. © 2010 IOP Publishing Ltd.

We report the 75As nuclear quadrupole resonance (NQR) and specific heat measurements of the heavily hole-doped superconductor KFe 2As 2 (superconducting transition temperature T c 3.5 K). The spin-lattice relaxation rate 1/T 1 in the superconducting state exhibits a gradual temperature dependence with no coherence peak below T c. The quasiparticle specific heat C qp/T shows a small jump, which is about 30% of the electronic specific heat coefficient just below T c. The C qp/T suggests the existence of low-energy quasiparticle excitation at the lowest measurement temperature T - 0.4 K T c/10. The T dependences of 1/T 1 and C qp/T can be explained by a multiple nodal superconducting gap scenario rather than by a multiple fully gapped s ±-wave scenario determined using simple gap analysis. © 2009 The Physical Society of Japan.

We report the 75As nuclear magnetic resonance (NMR) measurement of the hole-doped superconductor Ba 1-xK xFe 2As 2 with different lattice parameters and different superconducting volume fractions (T c ≃ 38 K). 75As-NMR spectra revealed that the magnetically ordered and superconducting phases are microscopically separated. The spin-lattice relaxation rate 1/T 1 in the normal state reflects the existence of a large two-dimensional antiferromagnetic spin fluctuation. The 1/T 1 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 T c. This shows a T dependence similar to those of other iron pnictides. © 2009 The Physical Society of Japan.

Filled skutterudite TbRu4P12 exhibits antiferromagnetic ordering below 20 K. Additional magnetic ordering below 10 K exists under zero field and the origin of this successive magnetic ordering might be related to multipole ordering. In order to reveal the detailed properties of the magnetic transition in TbRu4P12, we performed zero-field 31P nuclear magnetic resonance(NMR), 101Ru nuclear quadrupole resonance(NQR), muon spin rotation(μSR). The zero-field 31P-NMR spectra indicate the magnetic transition only at 20 K, while the time spectra of μSR of TbRu4P12 indicate the additional increase of the internal field below 10 K. Moreover, we observed an anomalous temperature dependence of the spin-lattice relaxation rate T1-1 just below 20 K and small anomaly at around 10 K in 101Ru-NQR. © 2009 IOP Publishing Ltd.

The high pressure nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) are conventionally performed up to 3 GPa using piston cylinder cell. However, the NMR/NQR measurements beyond this pressure range are scarcely performed owing to the technical difficulty. Recently, we developed new high pressure NMR/NQR technique using cubic anvil apparatus with which highly hydrostatic pressure was obtained. Using the new method, the 63Cu-NQR signal of Cu2O was observed up to 7.2GPa in nearly hydrostatic condition with high sensitivity. It was noted that the appropriate choice of pressure-transmitting medium is important for accurate experiments. © 2009 IOP Publishing Ltd.

In this article, we review recent NMR studies of Fe-based superconductors. We also report our 75As-NMR and dc resistivity measurement of BaFe 2As 2 under high pressure P. The T-P phase diagram of BaFe 2As 2 determined from resistivity anomalies and the 75As-NMR clearly revealed that the antiferromagnetic anomaly is quite robust against P. The NMR measurement under P beyond 3 GPa is needed in order to determine the ground state of BaFe 2As 2 under P. In addition, we report recent development of NMR technique under high P.

We report the 75As nuclear magnetic resonance (NMR) measurement of the ternary iron arsenide BaFe2As2. 75As-NMR spectra clearly revealed that magnetic transition occurs at around 131K in our samples, which corresponds to the emergence of spin density wave. The temperature dependence of the internal magnetic field suggests that the transition is likely of the first order. The critical slowing-down phenomenon in the spin-lattice relaxation rate 1=T1 is not pronounced in this material. ©2008 The Physical Society of Japan.

We report the detailed analysis of zero-external-field (ZF) 75As nuclear-magnetic-resonance (NMR) measurement of the ternary iron arsenide BaFe2As2. The ZF 75As-NMR spectrum clearly revealed that the magnetically ordered state is commensurate. Its internal magnetic field at the As site is 1.39(2) T.

We briefly report pressure dependence of electrical resistivity in fluorine-free, oxygen-deficient, NdFeAsO1-y (1-y = 0.85, 0.8 and 0.6, nominal compositions) under hydrostatic high pressure up to 18 GPa. Superconducting temperatures, TC decrease in every composition by applying pressure. In NdFeAsO0.85, which shows no superconductivity at ambient pressure, steep drop of resistivity appears at around 20 K above 5 GPa as resistivity anomaly at around 150 K is suppressed by applying pressure. ©2008 The Physical Society of Japan.

Ce 2MIn 8 (M = Co, Rh) are heavy-fermion compounds: at ambient pressure, Ce 2CoIn 8 is a superconductor with TSC = 0.4K and Ce 2RhIn 8 is an antiferromagnet with T N = 2.8K and Ce 2RhIn 8 becomes superconducting below 1.4K at 2.0 GPa. Their crystal structure can be viewed as successive layers of the heavy-fermion compounds CeMIn 5 and CeIn 3. In order to determine their physical properties, we perform a nuclear quadrupole resonance (NQR) measurement of 115In nuclei of Ce 2MIn 8. The spin-lattice relaxation rate 1/T 1 of Ce 2CoIn 8 decreases with decreasing temperature T and is proportional to T through T SC. This is in contrast to the T 1/2 dependence of the 1/T 1 of pressurized Ce 2RhIn 8 above T SC. ©2007 The Physical Society of Japan.

The Sm-based filled skutterudite phosphides SmT4 P12 (T = Fe, Ru, Os) have been studied by 31P-NMR (nuclear magnetic resonance) and μSR (muon spin relaxation). The ferromagnetic order and the heavy fermion behavior in SmFe4 P12 and the antiferromagnetic order in SmOs4 P12 were confirmed by 31P-NMR and μSR. In SmRu4 P12, a successive anomaly around the metal-insulator transition temperature and around the Néel temperature was observed by 31P-NMR in high applied magnetic fields. The μSR results of SmRu4 P12 indicate the magnetic order below the metal-insulator transition temperature, which supports the existence of the octupolar order in SmRu4 P12. © 2007.

We report 75As nuclear quadrupole resonance (NQR) measurements of the filled skutterudite superconductors LaRu 4As 12 and PrRu 4As 12. The temperature dependence of the 75As-NQR line and the nuclear spin-lattice relaxation rate 1/T 1 show that Pr 3+ has a crystal electric field (CEF) singlet ground state with a separate first excited state. In the superconducting states of PrRu 4As 12 and LaRu 4As 12, 1/T 1 has a coherence peak just below the transition temperature and decreases exponentially at low temperatures. This behavior shows that the s-wave superconductivity occurs in both compounds. ©2007 The Physical Society of Japan.

63Cu Nuclear Quadrupole Resonance (NQR) measurements in Cu 2O have been performed under high pressures (P) up to 10 GPa at 1.6K. The pressure cell, based on a technique using modified Bridgman anvils with a liquid pressure medium in Teflon capsule, generates nearly hydrostatic pressure with a volume of 1×10-3 ml, which is sufficiently large for the detection of NMR/NQR signals. The narrow 63Cu NQR was observed at 26.82 MHz (P = 0), and the resonance frequency varies linearly with P up to 9 GPa. This result shows that Cu NQR line in Cu2O becomes a convenient and precise P manometer. © 2007 The Physical Society of Japan.

NMR and nuclear quadrupole resonance (NQR) are powerful methods for the microscopic study of the magnetism and superconductivity. The NMR/NQR measurements were performed under pressure P up to 3 GPa using a conventional piston and cylinder pressure cell. The application of P induces phase transitions in many strongly correlated f- electron systems. In CeRhIn 5 and Ce2RhIns, the antiferromagnetic AF order is suppressed, and the superconductivity appears with increasing P. Our measurements show appearance of non s- wave superconductivity. In URu 2Si2,the 29Si NMR spectra show that the transition occurs from some curious ordered state at low P to the antiferromagnetic state at high P with increasing P. © 2007 The Physical Society of Japan.

We report C 63 u nuclear quadrupole resonance (NQR) measurement of Cu2 O under pressure up to about 10 GPa at low temperatures. Because the lattice parameter of Cu2 O changes with increasing pressure, the electric field gradient at the Cu site also changes correspondingly with pressure. This enables us to use the Cu2 O as an in situ manometer for high pressure nuclear magnetic resonance/NQR up to about 9 GPa. © 2007 American Institute of Physics.

The 31P-NMR (nuclear magnetic resonance) and <TEX>$\mu$</TEX>SR (muon spin relaxation) measurements on the filled skutterudite system SmFe4P12 have been carried out. The temperature <TEX>$T$</TEX>dependence of the 31P-NMR spectra indicates the existence of the crystalline electric field effect splitting of the Sm3+ (<TEX>$J=5/2$</TEX>) multiplet into a ground state and an excited state of about 70 K. The spin–lattice relaxation rate <TEX>$1/T_{1}$</TEX>shows the typical behavior of the Kondo system, i.e., <TEX>$1/T_{1}$</TEX>is nearly <TEX>$T$</TEX>independent above 30 K, and varies in proportion to <TEX>$T$</TEX>(the Korringa behavior, <TEX>$1/T_{1}\propto T$</TEX>) between 7.5 and 30 K. The <TEX>$T$</TEX>dependence deviated from the Korringa behavior below 7.5 K, which is independent of <TEX>$T$</TEX>in the applied magnetic field of 1 kOe, and suppressed strongly in higher fields. The behavior is explained as <TEX>$1/T_{1}$</TEX>is determined by ferromagnetic fluctuations of the uncovered Sm3+ magnetic moments by conduction electrons. The <TEX>$\mu$</TEX>SR measurements in zero field show the appearance of a static internal field associated with the ferromagnetic order below 1.6 K.

The 31P-NMR (nuclear magnetic resonance) and μSR (muon spin relaxation) measurements on the filled skutterudite system SmFe 4P12 have been carried out. The temperature T dependence of the 31P-NMR spectra indicates the existence of the crystalline electric field effect splitting of the Sm3+ (J = 5/2) multiplet into a ground state and an excited state of about 70 K. The spin-lattice relaxation rate 1/T1 shows the typical behavior of the Kondo system, i.e., 1/T1 is nearly T independent above 30 K, and varies in proportion to T (the Korringa behavior, 1/T1 ∝ T) between 7.5 and 30 K. The T dependence deviated from the Korringa behavior below 7.5 K, which is independent of T in the applied magnetic field of 1 kOe, and suppressed strongly in higher fields. The behavior is explained as 1/T1 is determined by ferromagnetic fluctuations of the uncovered Sm3+ magnetic moments by conduction electrons. The μSR measurements in zero field show the appearance of a static internal field associated with the ferromagnetic order below 1.6 K. ©2006 The Physical Society of Japan.

The 31P-NMR (nuclear magnetic resonance) measurements of the filled skutterudite SmRu4P12 have been carried out in several applied magnetic fields. The line width of the 31P-NMR spectrum rapidly increases below the metal-insulator transition temperature TMI, which indicates the appearance of an internal field below the temperature. Though no distinct anomaly was observed below TMI in low fields, a complicated structure was observed between the Néel temperature TN and TMI (TN < TMI) above 70 kOe. The spin-lattice relaxation rate 1/T1 is almost independent of temperature above TMI, and rapidly decreases below TMI. The 1/T1 in low fields decreases monotonously, whereas the strong suppression of 1/T1 occurs between TN and TMI above 70 kOe. The structure of the spectra and the suppression of 1/T1 become apparent with increasing field. ©2006 The Physical Society of Japan.

Ce2 CoIn8 is a superconductor with TSC = 0.4 K and Ce2 RhIn8 is an antiferromagnet with TN = 2.8 K. Ce2 RhIn8 becomes superconducting below 1.4 K at 2.0 GPa. Their crystal structure can be viewed as the successive layers of the heavy fermion compounds Ce T In5 and CeIn3. In this study, we have performed nuclear quadrupole resonance (NQR) measurement on 115In nuclei of Ce2 T In8. Spin-lattice relaxation rate 1 / T1 of Ce2 CoIn8 decreases with decreasing temperature T and is proportional to T just above TSC. This is contrast with the T1 / 2 dependence of the 1 / T1 of pressurized Ce2 RhIn8 at low temperatures. © 2006 Elsevier B.V. All rights reserved.

The 31P nuclear magnetic resonance (31P-NMR) and muon spin relaxation ( μ SR ) of the filled skutterudite compounds SmRu4P12 and GdRu4P12 have been carried out. The 31P-NMR spectra of SmRu4P12 indicate a complicated magnetic structure in high magnetic fields. A successive anomaly of the spin-lattice relaxation rate in SmRu4P12 was also observed in high fields. The μ SR of both SmRu4P12 and GdRu4P12 shows the appearances of static internal magnetic fields below the metal-insulator transition temperature of 16.5 K and below the antiferromagnetic transition temperature of 22 K, respectively. © 2006 Elsevier B.V. All rights reserved.

The 31P nuclear magnetic resonance (31P-NMR) and muon spin relaxation ( μSR) of the filled skutterudite compounds Sm T4 P12 (T: Fe, Os) have been carried out. From 31P-NMR, heavy fermion behavior and ferromagnetic spin fluctuations in SmFe4 P12 and simple antiferromagnetic spin fluctuations in SmOs4 P12 were observed. From μSR, the appearances of static internal magnetic fields below each transition temperature were clarified, which indicates that each ground state is a magnetically ordered state. © 2006 Elsevier B.V. All rights reserved.

The 5f-band system UCoAl, which crystallizes in the hexagonal ZrNiAl-type structure, has the paramagnetic ground state. The magnetic fields H as low as 0.6 T oriented along the c-axis induce the metamagnetic transition below 17 K. In order to study the magnetic property of UCoAl, we performed 27Al and 59Co NMR/NQR measurements in UCoAl, UCo0.98Fe0.02Al and UCo0.95Ni0.05Al single crystals. The substitution of Fe stabilizes the ferromagnetic state, and that of Ni stabilizes the paramagnetic state. The nuclear spin-lattice relaxation rate 1 / T1 obtained with the crystal c-axis perpendicular to H is nearly six times larger than the 1 / T1 with the c-axis parallel to H, which reflects the anisotropy of the spin fluctuations of the system. © 2006 Elsevier B.V. All rights reserved.

We have carried out115In nuclear quadrupole resonance (NQR) measurements in CeRhIn5 and CeCoIn5 under presure P. The nuclear spin-lattice relaxation rates, 1/T1, of115In indicated that the anisotropic superconductivity with line nodes occurred near an antiferromagnetic state. The application of P suppressed the antiferromagnetic, AF, spin fluctuations and moves the system away from the AF state to the non-magnetic Fermi liquid state. The P dependence of 1/T1 in the superconducting state of CeCoIn5 shows that the magnitude of the superconducting energy gap scales with its TC up to 3.0 GPa. © 2005 Elsevier B.V. All rights reserved.

The31P nuclear magnetic resonance (31P-NMR) and muon spin relaxation (μS R) of the filled skutterudite compound SmRu 4P12 have been carried out. The line width of the 31P- NMR spectrum rapidly increases below the metal-insulator transition temperature Tmi but not below the Néel temperature Tn. The temperature dependence of the spin-lattice relaxation rate shows a successive anomaly at Tmi and Tn. TheμSR measurements indicate that a static internal field appears below Tmi, which means that the ordering below Tmi is a magnetic ordering with the time reversal symmetry breakdown. These results support a scenario that a magnetic octupolar ordering occurs below Tmi in SmRu 4P12. © 2006 The Physical Society of Japan.

The 5f-band system UCoAl, which crystallizes in the hexagonal ZrNiAl-type structure, has the enhanced paramagnetic ground state. The magnetic fields as low as 0.67 T oriented along the c-axis induce the metamagnetic transition below 17 K. Substitution effect for Co was studied by using Ni and Fe; the substitution of Ni stabilizes the paramagnetic state, and that of Fe stabilizes the ferromagnetic state. In order to study the origin of the magnetic-property change of UCo1-xTxAl (T = Ni, Fe), we performed 27A1 and57Co NMR/NQR measurements in UCo 0.95Ni0.05Al and UCo0.98Fe0.02Al single crystals. The Knight shift and the nuclear spinlattice relaxation rate T-11 show the existence of the anisotropic spin fluctuations which are related with the potential ferromagnetic fluctuations among 5f electrons in these materials. © 2006 The Physical Society of Japan.

Muon spin relaxation measurements on the filled skutterudite compound Sm Ru4 P12 have been carried out in zero and longitudinal fields. The temperature dependence of both the initial asymmetry and the muon spin depolarization rate shows the appearance of a magnetically ordered state below the metal-insulator transition temperature TMI. The present study indicates that the ordering below TMI is not a nonmagnetic antiferroquadrupolar ordering but a magnetic one, which supports a scenario that magnetic octupolar ordering occurs below TMI in Sm Ru4 P12. © 2006 The American Physical Society.

Resonant x-ray diffraction performed at the LII and LIII absorption edges of Ru has been used to investigate the magnetic and orbital ordering in Ca2RuO4 single crystals. A large resonant enhancement due to electric dipole 2p→4d transitions is observed at the wave-vector characteristic of antiferromagnetic ordering. Besides the previously known antiferromagnetic phase transition at TN=110K, an additional phase transition, between two paramagnetic phases, is observed around 260 K. Based on the polarization and azimuthal angle dependence of the diffraction signal, this transition can be attributed to orbital ordering of the Ru t2g electrons. The propagation vector of the orbital order is inconsistent with some theoretical predictions for the orbital state of Ca2RuO4. © 2005 The American Physical Society.

We investigated the electronic structure of a layered Ca2-xSrxRuO4 in a metallic regime (0.15 &lt;= x &lt;= 2.0) using a polarization dependent O 1s x-ray absorption spectroscopy. The spectrum shows strong variations with the polarization especially in Ru 4d region, which enables us to identify the electronic states. The spectral line shape gradually changes with increase of Sr concentration, and agrees well with an unrestricted Hartree-Fock analysis, which suggests evolution of the orbital states and provides clues for how the lattice distortion affects the orbital occupations.

We investigated the electronic structure of a layered Ca2-x Srx Ru O4 in a metallic regime (0.15≤x≤2.0) using a polarization dependent O 1s x-ray absorption spectroscopy. The spectrum shows strong variations with the polarization especially in Ru 4d region, which enables us to identify the electronic states. The spectral line shape gradually changes with increase of Sr concentration, and agrees well with an unrestricted Hartree-Fock analysis, which suggests evolution of the orbital states and provides clues for how the lattice distortion affects the orbital occupations. © 2005 The American Physical Society.

We have succeeded in detecting ferro-type orbital states in Ca2-xSrxRuO4, which is the first outcome in a 4d Mott transition system by the resonant x-ray scattering interference technique. For x=0 (Mott insulator), the resonant signal for dxy orbital ordering is observed even at room temperature, in which the Jahn-Teller distortion is negligible. The signal disappears near the metal-insulator transition. On the other hand, in a metallic phase for x=0.5, orbital polarization with dyz/zx character dominates. With lowering temperature, the magnitude of the resonant signal slightly decreases owing to the additional influence of the γ band with dxy character. © 2005 The American Physical Society.

We present a Raman scattering study of Ca2RuO4, in which we investigate the temperature dependence of the lattice dynamics and the electron-phonon interaction below the metal-insulator transition temperature (TMI). Raman spectra obtained in a backscattering geometry with light polarized in the ab plane reveal nine B1g phonon modes (140, 215, 265, 269, 292, 388, 459, 534, and 683cm-1) and nine Ag phonon modes (126, 192, 204, 251, 304, 322, 356, 395, and 607cm-1) for the orthorhombic crystal structure (Pbca-D2h15). With increasing temperature toward TMI, the observed phonon modes shift to lower energies and exhibit reduced spectral weights, reflecting structural changes associated with the elongation of the RuO6 octahedra. Interestingly, the phonons exhibit significant increases in linewidths and asymmetries as temperature is raised through the Néel temperature (TN). These results indicate that there is an increase in the effective number of electrons and the electron-phonon interaction strengths as the temperature is raised through TN, suggesting the presence of orbital fluctuations in the temperature regime TN<T<TMI. © 2005 The American Physical Society.

We have carried out 115In nuclear quadrupole resonance (NQR) measurements in CeCoIn5 under pressure P. The obtained nuclear spin-lattice relaxation rate, 1/T1, of 115In at ambient pressure varies in proportion to T1/4 in the normal state, and T3 below the superconducting transition temperature TC. The results show that the system locates just at the antiferromagnetic AF instability and the anisotropic superconductivity with line nodes appears. The value of 1/T1 is strongly suppressed with increasing P, indicating that P moves the system away from the AF instability. The Fermi-liquid behavior was observed at the high P region. The P dependence of 1/T1 in the superconducting state shows that the magnitude of the superconducting energy gap scales with its TC up to 3 GPa. © 2005 Elsevier B.V. All rights reserved.

We have performed nuclear quadrupole resonance (NQR) measurements on 115In nuclei of Ce2CoIn8. Spin-lattice relaxation rate 1/T1 remains nearly constant at high temperatures. With decreasing temperature T, 1/T1 decreases and is proportional to T above TSC. 1/T1 of Ce2CoIn8 is quite similar to that of pressurized CeIn3 at about 2.7 GPa. The sign of superconductivity was observed at around 0.4 K. © 2005 Elsevier B.V. All rights reserved.

We present Raman scattering studies of the magnetic, electronic, and structural properties of Ca2-xSrxRuO4 as functions of temperature and Sr substitution. Raman spectra from Ca2RuO4 reveal a two-magnon scattering response for T<TN and an increase of the electron-phonon interaction strength through TN. The two-magnon energy at low temperature is relatively insensitive to Sr substitution. In contrast, Sr substitution causes a much more dramatic renormalization of the two-magnon energy and linewidth with increasing temperature toward TN. Moreover, substitution of Sr for Ca results in a significant increase in the electron-phonon interaction strength. © 2005 Published by Elsevier B.V.

We have studied the electronic structure of Sr2RuO4 and Ca2RuO4 using x-ray photoemission spectroscopy (XPS) and subsequent model calculations. While the t(2g) band of Sr2RuO4 has substantial spectral weight at the Fermi level, that of Ca2RuO4 has no spectral weight at E-F and shows a peak at -1.8 eV. In the valence-band XPS spectrum of Sr2RuO4, a satellite structure of the t(2g) 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 Ca2RuO4. 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.

A mechanism characterizing the hopping transport in the Mott insulators was analyzed. The study was carried by using X ray powdered diffraction. It was found that the hopping exponent α shows a systematic closer to the transitions. It was also observed that the moderate randomness intrinsic to the Sr substitution leads to the formation of strongly localized states around the closing of Mott-Hubbard gap.