Search

We report the synthesis of a new layered iridium oxyfluoride, $\mathrm{S}{\mathrm{r}}_{2}\mathrm{Ir}{(\mathrm{O},\mathrm{F})}_{6\ensuremath{-}\ensuremath{\delta}}$, using a topochemical fluorination method. The $c$ axis is elongated compared to that in the mother compound $\mathrm{S}{\mathrm{r}}_{2}\mathrm{Ir}{\mathrm{O}}_{4}$, because the fluorine layer is inserted into the rock-salt layer while tetragonal symmetry is preserved. Resistivity measurements for the compound show nonmetallic behavior similar to that of $\mathrm{S}{\mathrm{r}}_{2}\mathrm{Ir}{\mathrm{O}}_{4}$. In magnetization measurements, suppression of antiferromagnetic order and paramagnetic behavior was observed after topochemical fluorination. Meanwhile, muon spin relaxation (\ensuremath{\mu}SR) experiments suggest the development of antiferromagnetic spin correlation between Ir spins. Furthermore, fluorine content was estimated at 1.5 the chemical composition of the fluorinated product being $\mathrm{S}{\mathrm{r}}_{2}\mathrm{Ir}{\mathrm{O}}_{x}{\mathrm{F}}_{1.5}$ with $x=3.25$, assuming $\mathrm{I}{\mathrm{r}}^{4+}$. Suppression of magnetic order in $\mathrm{S}{\mathrm{r}}_{2}\mathrm{Ir}{(\mathrm{O},\mathrm{F})}_{6\ensuremath{-}\ensuremath{\delta}}$ is attributed to antiferromagnetic instability between $\mathrm{Ir}{\mathrm{O}}_{2}$ layers by elongation of the $c$ axis due to fluorine intercalation.

We performed X-ray diffraction (XRD) and DC magnetic susceptibility measurements to elucidate the crystal structure and superconducting properties of KSr2Nb3O10. From the diffraction pattern indexing, it was found that KSr2Nb3O10 crystallizes with monoclinic symmetry, space group P21/m(11). We succeeded in preparing high temperature (HT) and low temperature (LT) phases of KSr2Nb3O10 powder samples synthesized by a conventional solid state reaction and an ion-exchange reaction, respectively. Superconductivity was observed at 4 K by Li intercalation and it was found that the superconducting volume fraction of the LT phase (~ 1.4%) is clearly larger than that of the HT phase (0.07%).

Herein, we investigated the pressure dependence of electric transport in a new type of superconducting metal iridate compound, SrIr

Carbon-based superconductors can be roughly categorized into two groups. The first includes relatively new superconductors, such as boron-doped diamond, graphite-intercalated compounds (GIC), carbon nanotube (CNT) superconductors and fullerene superconductors.

The extended X-ray-absorption fine structure of a spinel CuIr2(S0.95Se0.05)4 powder sample, which has the charge-ordered temperature T CO = 190 K, was determined at the Ir L 3 edge for a wide temperature range. All the local structure parameters on the Ir–Ir bonds signified the second structural change at T * ~ 70 K. Remarkably, the number of Ir dimers, N D ( T ) , decreased by ~ 10% below T * [ N D ( 0 ) / N D ( T * ) ~ 0.9 ], which indicates partial dedimerization by X-ray irradiation. We also reanalyzed the resistance data of CuIr2S4 [Furubayashi et al., Solid State Comm. 126, 617 (2003)] with the variable-range hopping model by focusing on the density of states at E F , g. The X-ray-induced decrease in resistance was quantitatively explained as a result of the increase in g, which was attributed to the release of holes from the 10%-dedimerized Ir ions. We suppose that the released holes hop over the ligand chalcogen site and discuss the origin of X-ray-induced structural modulations [Kiryukhin et al., Phys. Rev. Lett. 97, 225503 (2006)] in terms of segregation of broken dimers.

We report here the superconducting properties of a Laves phase superconductor SrIr2, which has a cubic MgCu2 structure. SrIr2 is a type-II superconductor, with a T c of 5.9 K. The estimated superconducting parameters of lower critical field µ 0 H c1 and upper critical field µ 0 H c2, coherence length ξ(0), penetration depth λ(0) and Ginzburg-Landau (GL) parameter κ(0) are approximately µ 0 H c1 = 101 Oe, µ 0 H c2(0) = 5.9 T, ξ(0) = 7.47 nm, λ(0) = 237 nm, and κ(0) = 31.7, respectively. The specific-heat data indicate that SrIr2 is a strong-coupling superconductor because the value of ΔC/γT c is approximately 1.71, which is larger than the value of 1.43 that is expected from the BCS theory. The physical properties obtained in this study are explained well by theoretical calculations including spin-orbit coupling (SOC). This result indicates that the physical properties of SrIr2 are strongly affected by the presence of SOC.

The local structure of correlated spin-orbit insulator Sr2-xMxIrO4 (M = K, La) has been investigated by Ir L3-edge extended X-ray absorption fine structure measurements. The measurements were performed as a function of temperature for different dopings induced by substitution of Sr with La or K. It is found that Ir-O bonds have strong covalency and they hardly show any change across the Néel temperature. In the studied doping range, neither Ir-O bonds nor their dynamics, measured by their mean square relative displacements, show any appreciable change upon carrier doping, indicating the possibility of nanoscale phase separation in the doped system. On the other hand, there is a large increase of the static disorder in Ir-Sr correlation, larger for K doping than La doping. Similarities and differences with respect to the local lattice displacements in cuprates are briefly discussed.

We report the crystal structure and physical properties of Sr2-xLaxIrO4 synthesized by mechanical alloying. The magnetic transition temperature T-N and electrical resistivity decreased with increasing La doping, consistent with previous studies involving single-crystalline samples. We also identified the relationship between T-N and tetragonal distortion (c/a) in this system. This result suggests that the magnetism of the Sr214 system is strongly correlated with its crystal structure. Zero-field muon spin rotation/relaxation studies revealed that short-range antiferromagnetic ordering is realized in Sr1.9La0.1IrO4; also, the spin-glass state is stabilized in the low-temperature region. The Ir moment estimated from the longitudinal field mu SR results is 0.045 mu(B), ten times smaller than that of Sr2IrO4 (similar to 0.4 mu(B)), indicating that electrons are introduced into the Ir atoms.

The local structure of correlated spin-orbit insulator Sr$_{2-x}$M$_x$IrO$_4$ (M = K, La) has been investigated by Ir L$_3$-edge extended x-ray absorption fine structure measurements. The measurements were performed as a function of temperature for different dopings induced by substitution of Sr with La or K. It is found that Ir-O bonds have strong covalency and they hardly show any change across the N\'eel temperature. In the studied doping range, neither Ir-O bonds nor their dynamics, measured by their mean square relative displacements, show any appreciable change upon carrier doping, indicating possibility of a nanoscale phase separation in the doped system. On the other hand, there is a large increase of the static disorder in Ir-Sr correlation, larger for K doping than La doping. Similarities and differences with respect to the local lattice displacements in cuprates are briefly discussed., Comment: Main text: 6 pages, 4 figures, Supplemental information: 2 pages, 2 figures

The electronic structure of the lightly electron-doped correlated spin-orbit insulator Sr2IrO4 has been studied by angle-resolved photoelectron spectroscopy. We have observed the coexistence of a lower Hubbard band and an in-gap band; the momentum dependence of the latter traces that of the band calculations without on-site Coulomb repulsion. The in-gap state remained anisotropically gapped in all observed momentum areas, forming a remnant Fermi-surface state, evolving towards the Fermi energy by carrier doping. These experimental results show a striking similarity with those observed in deeply underdoped cuprates, suggesting the common nature of the nodal liquid states observed in both compounds.

Herein, we investigated the pressure dependence of electric transport in a new type of superconducting metal iridate compound, SrIr2, that exhibits a superconducting transition temperature, T c, as high as 6.6 K at ambient pressure, in order to complete the T c-pressure (p ) phase diagram. Very recently, this sample's superconductivity was discovered by our group, but the superconducting behavior has not yet been clarified under pressure. In this study, we fully investigated this sample's superconductivity in a wide pressure range. The T c value decreased with an increase in pressure, but the onset superconducting transition temperature, [Formula: see text], increased above a pressure of 8 GPa, indicating an unconventional superconductivity different from a BCS-type superconductor. The magnetic field dependence of electric resistance (R) against temperature (R - T plot) recorded at 7.94 and 11.3 GPa suggested an unconventional superconductivity, followed by a p -wave polar model, supporting the deviation from a simple s-wave pairing. Moreover, we fully investigated the pressure dependence of crystal structure in SrIr2 and discussed the correlation between superconductivity and crystal structure. This is the first systematic study on superconducting behavior of a new type of metal iridate compound, MIr2 (M: alkali-earth metal atom), under pressure.

We report on the crystal structure and superconducting properties of a novel iridium-silicide, namely Li2IrSi2. It has a Ag2NiO2-type structure (space group R-3m) with the lattice parameters a = 4.028 30(6) Å and c = 13.161 80(15) Å. The crystal structure comprises IrSi2 and double Li layers stacked alternately along the c-axis. The IrSi2 layer includes a two-dimensional Ir equilateral-triangular lattice. Electrical resistivity and static magnetic measurements revealed that Li2IrSi2 is a type-II superconductor with critical temperature (T c) of 3.3 K. We estimated the following superconducting parameters: lower critical field H c1(0) ∼ 42 Oe, upper critical field H c2(0) ∼ 1.7 kOe, penetration depth λ 0 ∼ 265 nm, coherence length ξ 0 ∼ 44 nm, and Ginzburg–Landau parameter κ GL ∼ 6.02. The specific-heat data suggested that superconductivity in Li2IrSi2 could be attributed to weak-coupling Cooper pairs.

Understanding the overall features of magnetic excitation is essential for clarifying the mechanism of Cooper pair formation in iron-based superconductors. In particular, clarifying the relationship between magnetism and superconductivity is a central challenge because magnetism may play a key role in their exotic superconductivity. BaFe2As2 is one of ideal systems for such investigation because its superconductivity can be induced in several ways, allowing a comparative examination. Here we report a study on the spin fluctuations of the hole-overdoped iron-based superconductors Ba1-xKxFe2As2 (x = 0.5 and 1.0; Tc = 36 K and 3.4 K, respectively) over the entire Brillouin zone using inelastic neutron scattering. We find that their spin spectra consist of spin wave and chimney-like dispersions. The chimney-like dispersion can be attributed to the itinerant character of magnetism. The band width of the spin wave-like dispersion is almost constant from the non-doped to optimum-doped region, which is followed by a large reduction in the overdoped region. This suggests that the superconductivity is suppressed by the reduction of magnetic exchange couplings, indicating a strong relationship between magnetism and superconductivity in iron-based superconductors., Comment: 16 pages, 4 figures

The mechanism of Cooper pair formation in iron-based superconductors remains a controversial topic. The main question is whether spin or orbital fluctuations are responsible for the pairing mechanism. To solve this problem, a crucial clue can be obtained by examining the remarkable enhancement of magnetic neutron scattering signals appearing in a superconducting phase. The enhancement is called spin resonance for a spin fluctuation model, in which their energy is restricted below twice the superconducting gap value (2Ds), whereas larger energies are possible in other models such as an orbital fluctuation model. Here we report the doping dependence of low-energy magnetic excitation spectra in Ba1-xKxFe2As2 for 0.5, Comment: 20 pages, 8 figures

Using Co L 2,3 resonant soft X-ray scattering measurement, we reveal that the magnetic orderings with q = (1/2, 0, 1/2) and (1/2, 0, 1) originate from different magnetic domains. Temperature dependence of the magnetic ordering peaks clearly shows that there is a finite difference between the transition temperatures of these magnetic orderings.

${\mathrm{SrFeO}}_{2}$ is an insulating antiferromagnet with a remarkably high transition temperature in spite of its quasi-two-dimensional crystal structure. The magnetic exchange coupling is, however, very sensitive to a local mode involving transverse displacements of O and Fe, resulting in zigzag patterns of distortion. The buckling driven by rising temperatures is enhanced just as the Fe magnetic moment is reduced, implying a strong spin-lattice coupling. It is suggested that the undulations lead to orbital disorder by distorting the three possible paths to exchange interactions.

Neutron scattering experiments were performed on the single crystals of layered-perovskite cobaltate La2−xSrxCoO4. For a commensurate value of x=0.5, magnetic reflections due to high-spin (HS) state of Co2+ are observed in (H, 0, L) plane. Further, in (H, H, L) plane, we found super-lattice reflections with wave vector qm=(1/4, 1/4, 1). The latter magnetic peaks cannot be understood by the Co2+ spin configurations, indicating that these peaks are originated from Co3+ spin ordering. From the hole-doping dependence, short ranged charge and spin orderings were observed below x=0.5, while those of peaks were suppressed at x=0.6. Our results suggest that double exchange interaction between the HS state of Co2+ and the intermediate spin (IS) state of Co3+ mediate the IS state in this system.

We examined checkerboard charge and spin order in La 1.5 Ca 0.5 CoO 4 by elastic and inelastic neutron scattering. We discovered the sharp magnetic peaks with l =half-integer and broad magnetic peaks with l =integer in (0.5, 0, l). The origin of magnetic scattering can be explained by the minority and majority stacking arrangement of Co 2+ /Co 3+ spins along the c -axis since the same magnetic reflections arise from the two stacking vectors in La 0.5 Sr 1.5 MnO 4 . Spin-wave dispersion at majority stacking is seen up to 16 meV, in contrast to 50 meV in La 1.5 Sr 0.5 NiO 4 . This indicates a much weaker exchange interaction in the cobaltates than in nickelates. The dispersion at majority stacking has almost a same one with that of minority stacking.

Magnetic properties were investigated for single crystals of La2−xCaxCoO4 ( 0.5 ⩽ x ⩽ 0.8 ) and for polycrystalline samples of La2−xBaxCoO4 ( 0.5 ⩽ x ⩽ 0.9 ) . In the Ca system, the effective moment μeff decreased with increasing x from 4.0 μB ( x = 0.5 ) to 3.0 μB ( x = 0.7 – 0.8 ) , accompanied by a steep reduction of Weiss temperature Θ. These drastic changes of the magnetic properties suggest a spin-state transition of the Co3+ ions from the high-spin state to the intermediate-spin state. On the other hand, the effective moment of La2−xBaxCoO4 was almost 4.0 μB and there is no change of Weiss temperature Θ. These facts suggest that the intermediate-spin state is related to the Co–O bond length, i.e. crystal fields.

Magnetic and electrical properties were characterized for newly synthesized La1−xSrxOCuSe (0

SrFeO2 is an insulating antiferromagnet with a remarkably high transition temperature in spite of its quasi-two-dimensional crystal structure. The magnetic exchange coupling is, however, very sensitive to a local mode involving transverse displacements of O and Fe, resulting in zigzag patterns of distortion. The buckling driven by rising temperatures is enhanced just as the Fe magnetic moment is reduced, implying a strong spin-lattice coupling. It is suggested that the undulations lead to orbital disorder by distorting the three possible paths to exchange interactions.

Single crystals of Ba1−xKxFe2As2 with 0.15 ≤ x ≤ 1 have been successfully synthesized by a KAs self-flux method. The potassium (K) concentration x of the grown crystals was systematically changed by changing the mixing ratio of Ba to Fe in the starting materials. The crystals have flat surfaces corresponding to the crystallographic (001) planes, whose planar dimensions increase with decreasing thickness when x increases. The superconducting transition temperature systematically changes with x, ranging from 38 K at x = 0.4 to 3.4 K at x = 1.0, in good agreement with the results for polycrystalline samples.

In this study, we performed high-pressure electrical resistivity measurements of polycrystalline FeSe in the pressure range of 1-16.0 GPa at temperatures of 4-300 K. A precise evaluation of Tc from zero-resistivity temperatures revealed that Tc shows a slightly distorted dome-shaped curve, with maximum Tc (30 K) at 6 GPa, which is lower than a previously reported Tc value (~37 K). With the application of pressure, the temperature dependence of resistivity above Tc changes dramatically to a linear dependence; a non-Fermi-liquid-like "high-Tc" phase appears above 3 GPa. We found a striking correlation between Tc and the Se height: the lower the Se height, the more enhanced is Tc. Moreover, this relation is broadly applicable to other iron pnictides, strongly indicating that high-temperature superconductivity can appear only around the optimum anion height (~1.38A). On the basis of these results, we suggest that the anion height should be considered as a key determining factor of Tc of iron-based superconductors containing various anions., Comment: 6 pages, 6 figures

The isovalent substitution of Te for Se in the superconducting $\ensuremath{\alpha}\text{-FeSe}$ raises ${T}_{C}$ where the average chalcogen--Fe bond angle decreases and the chalcogen-Fe distance increases. Locally, however, the Se and Te ions do not share the same site and have two distinct $z$ coordinates, in contrast to what is presumed in the $P4/nmm$ symmetry. The local bond angle between the chalcogens and Fe increases with the substitution, consistent with the rise in ${T}_{C}$, the Fe--Te bonds become shorter than in the binary FeTe, while the Fe--Se bonds stay the same as in the binary. Ab initio calculations based on spin density functional theory yielded an optimized structure with distinct $z$ coordinates for Se and Te, in addition to a stronger hybridization of Te with Fe.

Using elastic neutron scattering on single crystals of ${\text{La}}_{1\ensuremath{-}x}{A}_{x}{\text{CoO}}_{3}$ ($A={\text{Ca}}^{2+}$, ${\text{Sr}}^{2+}$, and ${\text{Ba}}^{2+}$), we found the development of magnetic superstructures below the global magnetic transition to be strongly dependent on the size of the $A$-site dopant, $⟨{r}_{A}⟩$, in an unusual way. Upon reducing the $⟨{r}_{A}⟩$ (i.e., as with Ca doping), only a commensurate ferromagnetic cluster phase is evident. On expanding the $⟨{r}_{A}⟩$, the tendency toward coexistence of competing ferromagnetic and antiferromagnetic orders increases giving rise to an inhomogeneous ground state. The antiferromagnetic ordered state, initially incommensurate, continuously strengthens and becomes commensurate with long-range order and a characteristic cubic wave vector of ${\stackrel{P\vec}{Q}}_{c}=(0.25,0.25,0.25)$ with $x$. The two competing order parameters become comparable in magnitude indicative of the phase-separated nature of the cobalt perovskite system.

The lattice dynamics of Ba1-xKxFe2As2 (x = 0.00, 0.27) have been studied by inelastic X-ray scattering measurement at room temperature. K doping induces the softening and broadening of phonon modes in the energy range E = 10-15 meV. Analysis with a Born-von Karman force-constant model indicates that the softening results from reduced interatomic force constants around (Ba,K) sites following the displacement of divalent Ba by monovalent K. The phonon broadening may be explained by the local distortions induced by the K substitution. Extra phonon modes are observed around the wave vector q = (0.5,0,0) at E = 16.5 meV for the x = 0.27 sample. These modes may arise either from the local disorder induced by K doping or from electron-phonon coupling., Comment: J. Phys. Soc. Jpn. (in press)

We have measured the longitudinal and transverse sound velocities of ${\text{La}}_{2\ensuremath{-}x}{\text{Ca}}_{x}{\text{CoO}}_{4}$ $(0.4\ensuremath{\le}x\ensuremath{\le}0.7)$ propagating along [100], [110], and [001] from 200 K down to 3 K with a frequency of 10 MHz. It was found that ${c}_{66}$ shows a remarkable softening below ${\text{Co}}^{2+}$ spin-ordering temperature ${T}_{N}$ in the whole doping range, while $\frac{{c}_{11}\ensuremath{-}{c}_{12}}{2}$ exhibits a clear hardening below ${\text{Co}}^{3+}$ spin-ordering temperature ${T}_{N2}$ only when $x=0.5$. To explain the observed anomaly, the Landau free energy for the ${\text{La}}_{2\ensuremath{-}x}{\text{Ca}}_{x}{\text{CoO}}_{4}$ system has been constructed, including the coupling energy between elastic strains and a two-dimensional order parameter $({Q}_{1},{Q}_{2})$, having a spatial modulation along [110] or $[\overline{1}10]$. We argued that this represents orbital order, and it is this order that leads to the softening of ${c}_{66}$.

Neutron scattering experiments were performed on single crystals of layered cobalt-oxides La2-xCaxCoO4 (LCCO) to characterize the charge and spin orders in a wide hole-doping range of 0.3, Comment: 11pages, 9 figures. Submitted to J. Phys. Soc. Jpn

The antiferromagnetic insulator La1.5Ca0.5CoO4 has been investigated by Co L2,3-edge and O K-edge X-ray absorption spectroscopy (XAS) measurements and Co L2,3-edge resonant soft X-ray magnetic scattering (RXMS) measurement to determine the Co electronic structures associated with magnetic ordering. Co L2,3-edge linear-dichroic XAS shows that Co2+ takes a high-spin (HS) state and Co3+ takes a low-spin (LS) state. Using Co L2,3-edge RXMS, we directly determined that an antiferromagnetic order is formed with a HS state of Co2+ ions. Moreover, the spin and orbital angular momenta of the Co2+ HS state are quantitatively estimated to be 1.1 ± 0.1 and 1.0 ± 0.1, respectively, and to align parallel in the ab plane by utilizing the cluster model calculation. The large orbital angular momentum of the Co2+ HS state originates from the small D4h-symmetry crystal field splitting of t2g levels, which is comparable with the spin–orbit coupling constant of the Co 3d orbital.

In resonant soft X-ray scattering measurement, we need the detector which can tune its solid angle continuously in vacuum. We have constructed a 4-blade-slit detector system for high vacuum and set it in the quasi-3-axis diffractometer. With this system, we have studied two magnetic structures in antiferromagnetic insulator La1.5Ca0.5CoO4 by Co L2,3-edge resonant soft X-ray scattering. We revealed that the underlying Co electronic structures for these magnetic structures are the same, since the observed energy spectra take the same lineshape.

We report the crystal structure parameters of clathrate-type copper-oxide Cu6O8YCl, which belongs to the Cu6O8MX (M = cation, X = anion) family, investigated by the powder neutron diffraction technique and the Rietveld method. The crystal structure of Cu6O8YCl was identified to be a cubic structure with the space group Fmm. The Cl atom is located at the 4a site, which is the center position of the Cu6O8 cage, and the Y atom occupies the 4b site in the cuboid space. The Y and Cl atoms have anomalously large atomic displacement parameters (Uiso) at room temperature, which suggests that Y and Cl atoms respectively present in a cuboid and in a Cu6O8 cage exhibit intensive motion such as rattling, as recently observed for clathrate compounds.

Lattice vibrations from 20 K to room temperature in lithium alanates LiAlH4 and Li3AlH6, which decompose to release hydrogen over 423 K, were investigated by neutron spectroscopy. For both alanates, the overall spectra already start to broaden at 100–150 K with increasing temperature. The spectral lines assigned to the librational (rotational) modes of the polyhedral Al hydrido complexes, [AlH4]− and [AlH6]3−, exhibit not only broadening but also softening at around 250–300 K. These results suggest that the decomposition stems from low-temperature bulk lattice instability/anharmonicity, in particular, due to the weakening of binding between the complexes and their surroundings.

Neutron scattering experiments were performed on single crystals of magnetic impurity doped cobalt oxides La1.5Ca0.5CoO4 to characterize the charge and spin orders. We newly found contrasting impurity effects. Two types of magnetic peaks are observed at q = (0.5,0,L) with L = half-integer and integer in La1.5Ca0.5CoO4, while magnetic peak at L = half-integer (integer) was only observed in Mn (Fe)-substituted sample. Although Mn and Fe impurities degrade charge and magnetic order, Cr impurity stabilizes the ordering at x = 0.5. Based on the crystal structural analysis of Cr doped sample, we found that the excess oxygen and change of octahedron around Co3+ were realized in Cr doped sample.

We performed magnetic susceptibility and inelastic neutron scattering measurements on the frustrated spinel Mg1−xCr2O4−x (0.02 ≲ x ≲ 0.11). Susceptibility shows that Curie–Weiss temperature is almost independent of x, meaning that exchange interactions sustain. Inelastic neutron scattering reveals that the spin-hexamer excitations also persist with almost the same energy around 4 meV in a lowest temperature phase, whereas the resonance width increases with increasing x. These results suggest that nearest-neighboring Cr···Cr direct exchange predominantly forms the hexamers regardless of lattice defect and superexchange via Mg and O sites.

A renewed Ge-crystal monochromator was installed in a neutron spectrometer at JRR-3. The beam size at the sample position is vertically about 1.5 times as large as before, while the neutron luminosity is unchanged. Well-collimated, monochromatic beams with quite different incident energy ( E i ) are available through a slight rotation in the monochromator-crystal angle by making use of discrete Bragg reflections of Ge. The ability of inelastic-neutron-scattering measurements is examined through observation of magnetic Brillouin scattering with extremely high resolution and with a supplementary use of two E i s.

A transition to a superconducting state was recently observed in the binary alloy of FeSe1-xTex system where TC rises with increasing x. The substitution of the larger Te for Se ion results in no additional charges but increases the internal chemical pressure. Earlier studies suggested that the crystal structure maintains the tetragonal P4/nmm symmetry with the substitution of Te where the average bond angle, α, decreases considerably from ~ 104° in FeSe to 100.5° in the mixed phase of FeSe0.5Te0.5. With the use of pulsed neutron power diffraction and the Rietveld analysis, the crystal structure refinement for FeSe0.5Te0.5 yielded very large thermal factors in the superconducting phase indicative of the presence of structural distortions that may be significant in understanding the electronic and magnetic properties of this system.

We studied magnetic phase diagram and x -dependence of spiral spin correlation in magnetically ordered phase of (Co 1- x Zn x )Cr 2 O 4 by magnetization and neutron scattering experiments. The spiral correlation survives in a wide x range from 0 to 0.9. As x increases, the correlation length decreases from ξ∼23 A at x =0, and saturates at ξ∼2.5 A at around x =0.75, which is close to the nearest neighboring Cr 3+ -Cr 3+ distance. The pitch is almost constant from x =0 to 0.75, then abruptly shortens above around x =0.75. The correlation is isotropic at x =0.25 but anisotropic at x =0.75. We discuss that the spiral correlation is of short-range owing to unquenched geometric frustration, and begins to collapse with the minimization of correlation length above around x =0.75, possibly as the precursor of dynamic spin hexamer caused by high geometric frustration.

We determined the crystal structural parameters of FeSe 1- x Te x ( x =0.5, 0.625, 0.75, 0.875) and FeSe by using the neutron powder diffraction technique. It was found that the superconducting transition temperature T c of FeSe 1- x Te x increased with the (Se,Te)–Fe–(Se,Te) bond angle α in the Te-doped samples. However, T c of FeSe did not exhibit a similar relation. In order to explain this contradiction, we studied the temperature dependence of the z position of chalcogens, which is strongly coupled to the spin fluctuation by theoretical calculations. The z position in FeSe showed a remarkable shift, indicating that strong spin fluctuation can affect the superconductivity in FeSe.

High-pressure resistivity experiments were performed on FeSe 0.5 Te 0.5 . The onset temperature of T c ( T c onset ) increases rapidly from 13.5 to 26.2 K upon applying pressure up to 2 GPa. Above 2 GPa, T c decreases linearly and the metallic phase was observed at P =14 GPa. The same relationship between normalized T c and pressure in both FeSe 0.5 Te 0.5 and FeSe suggests a universal pressure dependence in this system. This report presents the first observation of a phase transition from the superconducting to the metallic phase on the P – T c phase diagram.

Using neutron and resonant X-ray scattering, we studied crystal structure and charge-ordering pattern in La 1.5 Ca 0.5 CoO 4 . Taking A 2 m m with a – b type twin structure into account, Co 2+ /Co 3+ checkerboard charge ordering pattern was suggested in La 1.5 Ca 0.5 CoO 4 by neutron scattering structure analysis. In order to clarify the charge ordering pattern and the valences of the two Co sites, we performed a resonant X-ray scattering (RXS) measurement near the Co K -edge. We found a resonant signal at (3, 0, 0) reflection which corresponds to the charge ordering with a Co 2+ /Co 3+ checkerboard pattern, and the Co valences of charge ordering were fully charge disproportionated. From the X-ray fluorescence spectra, we found the pre-edge peak structures at E =7.710 and 7.713 keV, corresponding to 1 s → t 2 g and 1 s → e g transitions, respectively. The polarization dependence of the latter one suggests that the anisotropy e g orbital is realized in La 1.5 Ca 0.5 CoO 4 .