Search Results (144 total)

We report on a photoemssion study of A-site ordered perovskite oxides ACu₃Co₄O₁₂ (A=Ca and Y) which are metallic for A=Ca and insulating for A=Y. If the Cu valence is +2 as stabilized by the square planar coordination, the Co valence for A=Ca is formally +4 and the system could be a Mott insulator while that for A=Y should be +3.75 and the system is a doped Mott insulator. The photoemission study reveals that the Cu valence for A=Ca and Y is formally +3 with the d⁹L configuration (L: an O 2p hole) which corresponds to the Zhang-Rice singlet state. In addition, it is found that the Co valence for A=Y is formally +3 with the low-spin d⁶ configuration consistent with its insulating behavior.

Employing electron energy-loss spectroscopy we show that small values of doping in the system Ca_2−xNa_xCuO₂Cl₂ strongly influence the formation of charge-transfer excitons in the Cu-O plane. We find a remarkable redistribution of spectral weight between the two modes—a bound exciton and a continuum state—seen in the insulator yielding a single sharp feature for nonzero doping accompanied by a strong suppression of the dispersion. Our data may provide evidence for a prominent role of the magnetic background on the dynamics of charge-transfer excitations in underdoped cuprates.

Single protein molecules are regarded as contact networks of amino-acid residues. Relationships between the shortest path lengths and the numbers of residues within single molecules in the native structures are examined for various sized proteins. A universal scaling among proteins is obtained, which shows that the residue networks are fractal networks. This universal fractal network is characterized with three kinds of dimensions: the network topological dimension D_c≈1.9, the fractal dimension D_f≈2.5, and the spectral dimension D_s≈1.3. These values are in surprisingly good coincidence with those of the three-dimensional critical percolation cluster. Hence the residue contact networks in the protein native structures belong to the universality class of the three-dimensional percolation cluster. The criticality is relevant to the ambivalence in the protein native structures, the coexistence of stability and instability, both of which are necessary for protein functions.

We report on a first-principles study of SrFeO₂, an infinite-layer oxide with Fe atoms in a perfect square-planar coordination down to essentially 0 K. Our results reveal that this striking behavior relies on the double occupation of the 3d_z² orbitals of high-spin Fe²⁺. Such an electronic state results from electrostatic and hybridization effects, the latter enabling a large reduction in the intra-atomic exchange splitting of the z² electrons.

Perovskite-type BiNiO₃ is an insulating antiferromagnet in which a charge disproportionation occurs at the Bi site. La substitution for Bi suppresses the charge disproportionation and makes the system metallic, and for 0.05≤x≤0.1 a broad metal-insulator transition (MIT) occurs as a function of temperature. We have measured the temperature dependence of the photoemission and x-ray absorption (XAS) spectra of Bi_1−xLa_xNiO₃ to investigate how the electronic structure changes across the MIT. From the Ni  2p XAS spectra of x=0.05, we found almost no change in the valence of Ni across the MIT. In the valence-band photoemission spectra, the Fermi cutoff disappeared for x=0.05 at a low temperature, whereas for x=0.1 and 0.2, it remained at all temperatures but the intensity at the Fermi level decreased gradually with decreasing temperature. Our experimental results suggest that the MIT is caused by the localization of holes in the O  2p band and that the “insulating” phase below the MIT is indeed a mixture of insulating and metallic regions.

Ca_1−xSr_xRuO₃, which is ferromagnetic for Sr concentration x>0.3, has been studied by x-ray magnetic circular dichroism (XMCD) in Ru  3p and O  1s core-level x-ray absorption. XMCD signals appear at x ∼ 0.3 and monotonically increase with x in the ferromagnetic phase. While the monotonic increase of the XMCD signals with x is of a typical Stoner-type, the absence of appreciable change in the spectral line shapes of both the Ru  3p and O  1s XMCD spectra indicate that the itinerant-electron ferromagnetism in Ca_1−xSr_xRuO₃ is influenced by strong electron correlation.

In nonequilibrium steady states (NESS) far from equilibrium, it is known that the Einstein relation is violated. Then, the ratio of the diffusion coefficient to the mobility is called an effective temperature, and the physical relevance of this effective temperature has been studied in several works. Although the physical relevance is not yet completely clear, it has been found that the role of an effective temperature in NESS is indeed analogous to that of the temperature in equilibrium systems in a number of respects. In this paper, we find further evidence establishing this analogy. We employ a nonequilibrium Langevin system as a thermostat for a Hamiltonian system and find that the kinetic temperature of this Hamiltonian system is equal to the effective temperature of the thermostat.

A-site ordered perovskites CaCu₃Ge₄O₁₂ and CaCu₃Sn₄O₁₂, which are isostructural to antiferromagnetic CaCu₃Ti₄O₁₂, were found to be ferromagnets. The special alignment of the CuO₄ planes in the perovskite structure plays an important role in the magnetic properties of Cu²⁺ spins. Direct exchange interaction gives rise to the ferromagnetic behavior in CaCu₃Ge₄O₁₂ and CaCu₃Sn₄O₁₂, whereas involvement of Ti-3d orbitals produces the antiferromagnetic superexchange interaction in CaCu₃Ti₄O₁₂. This finding demonstrates that either ferromagnetic or antiferromagnetic behavior can appear within the same structural framework.

A cubic ordered perovskite BiCu₃Mn₄O₁₂ has been synthesized under 6  GPa and 1000  °C. BiCu₃Mn₄O₁₂ is a ferrimagnet with T_C=350  K and its saturated magnetic moment is 10.5  μ_B∕f.u. The material showed low-resistive metallic behavior and magnetoresistance (MR) below T_C. Its MR was observed over a wide temperature range, and the low-field MR reached −28% at 5  K. An electronic structure calculation revealed that it had a half-metallic nature and that the large MR observed under a low magnetic field was attributed to a spin-polarized tunneling or spin-dependent scattering effect at grain boundaries.

A two-staged, uniaxial magnetoresistive effect has been discovered in SrCo₆O₁₁ having a layered hexagonal structure. Conduction electrons and localized Ising spins are in different sublattices but their interpenetration makes the conduction electrons sensitively pick up the stepwise field dependence of magnetization. The stepwise field dependence suggests two competitive interlayer interactions between ferromagnetic Ising-spin layers, i.e., a ferromagnetic nearest-layer interaction and an antiferromagnetic next-nearest-layer interaction. This oxide offers a unique opportunity to study nontrivial interplay between conduction electrons and Ising spins, the coupling of which can be finely controlled by a magnetic field of a few Tesla.

To elucidate the nature of the single-particle excitations in the undoped parent cuprates, we have performed a detailed study of Ca₂CuO₂Cl₂ using photoemission spectroscopy. The photoemission line shapes of the lower Hubbard band are found to be well-described by a polaron model. By comparing the line shape and temperature dependence of the lower Hubbard band with additional O 2p and Ca 3p states, we conclude that the dominant broadening mechanism arises from the interaction between the photohole and the lattice. The strength of this interaction was observed to be strongly anisotropic and may have important implications for the momentum dependence of the first doped hole states.

Recent scanning-tunneling spectroscopy (STS) studies of the cupric oxychloride Na_xCa_2−xCuO₂Cl₂ (NCCOC) have uncovered a periodic 4a×4a density of states (DOS) modulation, termed a “checkerboard” [T. Hanaguri , Nature (London) 430, 1001 (2004)]. The periodicity of this phase is the same as that of the “stripe” charge order observed with neutron scattering in the very similar systems La_1.48Nd_0.4Sr_0.12CuO₄ (LNSCO) [J. M. Tranquada , Nature (London) 375, 561 (1995)] and La_1.875Ba_0.125CuO₄ (LBCO) [M. Fujita , Phys. Rev. Lett. 88, 167008 (2002)]. This raises the question of whether the stripes are, in fact, actually checkerboards. Unfortunately, NCCOC samples are very small and LBCO and LNSCO samples do not cleave, so neutron and STS measurements cannot be carried out on the same system. To determine the relationship between stripes and checkers, we used resonant soft-x-ray scattering, previously applied to LBCO [P. Abbamonte , Nat. Phys. 1, 155 (2005)], to study single crystals of NCCOC. No evidence was seen for a 4a×4a DOS modulation, indicating that the checkerboard effect is not directly related to the stripe modulation in LBCO. We place an upper bound on the product of the charge amplitude and the square of the in-plane correlation length of 2.3×10³ hole Å². Our measurements suggest that the checkers in NCCOC are either glassy or are nucleated by the surface, as suggested by Brown [Phys. Rev. B 71, 224512 (2005)].

We have deduced the chemical potential shift Δμ in the high-T_c superconductor Ca_2−xNa_xCuO₂Cl₂ (Na-CCOC) using core-level x-ray photoemission spectroscopy. The derived Δμ is rigid-band-like, and almost linear in hole concentration x, quantitatively consistent with the shift estimated from a recent angle resolved photoemission spectroscopy study. Also, Δμ in Na-CCOC is much larger than that in La_2−xSr_xCuO₄ (LSCO) and as large as that in Bi₂Sr₂CaCu₂O_8+y. Qualitatively different behavior of Δμ between Na-CCOC and LSCO is discussed in relation to the different behaviors of charge ordering.

Phosphate PbCu₂(PO₄)₂ was found in the Pb-Cu-P-O system. Its structure parameters were refined by the Rietveld method using synchrotron x-ray diffraction data [space group Pccn (No. 56); Z=8; a=8.03810(10) Å, b=15.46574(14) Å, and c=10.36820(10) Å]. PbCu₂(PO₄)₂ is isotypic with SrCu₂(PO₄)₂. Magnetic properties of PbCu₂(PO₄)₂ were investigated by magnetic susceptibility, magnetization up to 65 T, electron-spin resonance, and specific-heat measurements. Magnetic susceptibilities were fit by a linear four-spin cluster model, Cu1-Cu2-Cu2-Cu1 [Hamiltonian: −2J₁(S₁S₂+S₃S₄)−2J₂S₂S₃], with −2J₁∕k_B=97.0 K for Cu1-Cu2 and −2J₂∕k_B=86.5 K for Cu2-Cu2. The spin gap (Δ∕k_B) estimated from the high-field magnetization was 43 K. The g values of PbCu₂(PO₄)₂, g_∕∕=2.167 and g_⊥=2.222, suggest the 3z²-r² type of the orbital ground state for Cu²⁺ ions. Specific heat showed that PbCu₂(PO₄)₂ does not undergo a long-range magnetic order transition down to 0.45 K in agreement with its spin-singlet ground state.

An oxychloride superconductor Ca_2−xCuO₂Cl₂ with a single CuO₂ plane in the unit cell was prepared without cation substitution using high-pressure synthesis. The highest T_C was 38 K, 10 K higher than that of Ca_2−xNa_xCuO₂Cl₂. Structure analysis based on synchrotron x-ray and neutron powder diffractions revealed that the Ca deficiency was the origin of the hole carrier. This compound has two structural features as compared to Ca_2−xNa_xCuO₂Cl₂; fewer defects and a shorter Cu-Cl bond length. Postannealing at 773 K led to a further increase of the T_C to 43 K. This superconductor with a flat CuO₂ plane might form the basis for future discussions about the factors that determine the T_C of single-layer cuprates.

Perovskite-type BiNiO₃ is an insulating antiferromagnet in which a charge disproportionation occurs at the Bi site. La substitution for Bi suppresses the charge disproportionation and makes the system metallic. We have measured the photoemission and x-ray-absorption (XAS) spectra of Bi_1−xLa_xNiO₃ to investigate how the electronic structure changes with La doping. From Ni 2p XAS, we observed an increase of the valence of Ni from 2+ toward 3+. Combined with the core-level photoemission study, it was found that the average valence of Bi remains ∼4+ and that the Ni valence behaves as ∼(2+x)+, that is, La substitution results in a hole doping at the Ni sites. In the valence-band photoemission spectra, we observed a Fermi cutoff for x>0, consistent with the metallic behavior of the La-doped compounds. The Ni 2p XAS, Ni 2p core-level photoemission, and valence-band photoemission spectra were analyzed by configuration-interaction cluster-model calculation, and the spectral line shapes were found to be consistent with the gradual Ni²⁺→Ni³⁺ valence change.

We have measured magnetic susceptibility and resistivity of Sr_1−xY_xCoO_3−δ (x=0.1, 0.15, 0.2, 0.215, 0.225, 0.25, 0.3, and 0.4), and found that Sr_1−xY_xCoO_3−δ is a room temperature ferromagnet with a Curie temperature of 335 K in a narrow compositional range of 0.2⩽x⩽0.25. This is the highest transition temperature among perovskite Co oxides. The saturation magnetization for x=0.225 is 0.25 μ_B∕Co at 10 K, which implies that the observed ferromagnetism is a bulk effect. We attribute this ferromagnetism to a peculiar Sr∕Y ordering.

Metal-insulator transitions strongly coupled with lattice were found in Bi_1−xLa_xNiO₃. Synchrotron x-ray powder diffraction revealed that pressure (P∼3 GPa, T=300 K), temperature (T∼340 K, x=0.05), and La substitution (x∼0.075, T=300 K) caused the similar structural change from a triclinic (insulating) to an orthorhombic (metallic) symmetry, suggesting melting of the A-site charge disproportionation. Comparing crystal structure and physical properties with the other ANiO₃ series, an electronic state of the metallic phase can be described as [A³⁺Ḻ^δ, Ni²⁺Ḻ^1−δ], where a ligand-hole Ḻ contributes to a conductivity. We depicted a schematic P-T phase diagram of BiNiO₃ including a critical point (3 GPa, 300 K) and an inhomogeneous region, which implies universality of ligand-hole dynamics in ANiO₃ (A=Bi, Pr, Nd… ).

La₄Cu₃MoO₁₂ is a cluster antiferromagnet where copper spin-1∕2 trimers form a network of strongly coupled spin trimers. The magnetic properties of this material have been examined using magnetic neutron scattering. At low temperatures, excitations from the ground state are observed at 7.5(3) meV and 132.5(5) meV. An additional peak in the neutron scattering spectrum, which appears at 125.0(5) meV on heating, is ascribed to a transition between excited states. The wave vector and temperature dependence of the inelastic magnetic scattering cross section are consistent with intratrimer transitions. Magnetic neutron diffraction reveals antiferromagnetic order below T_N=2.6 K with a wave vector (1/200). The ordered magnetic structure is described as intertrimer order where spin correlations within trimers are controlled by the strong intratrimer interactions. Combining the information derived from elastic and inelastic magnetic neutron scattering with group theoretical analysis, a consistent set of intratrimer interactions and ordered magnetic structures is derived. The experiment provides a simple worked example of magnetism associated with interatomic composite degrees of freedom in the extreme quantum limit.

There is a controversy about the major determinants of the folding rate of small single-domain proteins. To shed light on this issue, we examined a possibility that the major determinants may change depending on temperature by conducting molecular dynamics simulations for 17 small single-domain proteins using an off-lattice Go-like model over a wide range of temperature. It was shown that the rank order of the folding rates is temperature dependent, which indicates that the major determinants are dependent on temperature. It was also found that as temperature is decreased, the correlation of the folding rate with the chain length becomes weakened, whereas that with the native topology becomes enhanced. Our simulation results, therefore, may provide a clue to reconcile the apparent controversy between the study by Plaxco based on experimental data and the previous theoretical and subsequent simulation studies: the former showed that the folding rate of two-state folders does not correlate with the chain length but correlates well with the native topology, whereas the latter showed that the folding rate does correlate with the chain length. We propose a possible scenario reconciling the controversy, explaining the reason why the correlation of the folding rate with the chain length became weakened and that with the native topology became enhanced with decreasing temperature.

Temperature-dependent micro-Raman studies of orthorhombic CaFeO₃ and rhombohedral La_0.33Sr_0.67FeO₃ were carried out with an aim to study the role of a lattice in the formation of the charge-disproportioned state (Fe⁴⁺→Fe⁵⁺+Fe³⁺) below the transition temperature (T_co) of 290 and 200 K, respectively. Shell-model lattice-dynamical calculations were performed for CaFeO₃ to assign the Raman modes and determine their vibrational pattern. The temperature dependence of the peak positions and the peak widths of various modes for both systems show distinct changes across their respective transition temperatures. In CaFeO₃, the symmetric-stretching mode at 707 cm⁻¹ splits into two modes, 707 cm⁻¹ and 684 cm⁻¹, corresponding to the breathing-type distortion of the FeO₆ octahedra. In comparison, the spectral feature at 704 cm⁻¹ in La_0.33Sr_0.67FeO₃, which has been assigned to the Raman-forbidden symmetric-stretching mode, disappears below T_co. These observations indicate the presence of finite Jahn-Teller distortions of the FeO₆ octahedra in CaFeO₃ in the entire temperature range, whereas these distortions are present only above T_co in La_0.33Sr_0.67FeO₃. Two modes at 307 cm⁻¹ and 380 cm⁻¹ in La_0.33Sr_0.67FeO₃ approach each other at T_co, indicating a reduction of rhombohedral distortions below T_co.

Inelastic neutron-scattering experiments were performed on A₃Cu₃(PO₄)₄ (A=Ca, Sr, and Pb), which consists of one-dimensional array of linear copper spin trimers. It was found that the magnetic excitations are well described by the linear Heisenberg antiferromagnetic spin trimer model, in which the main interaction is the nearest-neighbor intratrimer interaction and the weak next-nearest-neighbor intratrimer and intertrimer interactions are expected. The intratrimer coupling constants are determined to be 9.45(3), 10.04(3), and 9.13(2) meV for A₃Cu₃(PO₄)₄ with A=Ca, Sr, and Pb, respectively. The other interactions were found to be very small.

The antiferromagnetic-to-ferromagnetic transition in SrFe_1−xCo_xO₃ (x_c≃0.15) induced by Co doping has been studied by magnetic circular x-ray dichroism. The orbital and spin magnetic moments of the Fe and Co 3d states under the magenetic field of 2 T are found to show different x dependences: The spin polarization of Fe 3d gradually increases with Co concentration; on the other hand, a large spin polarization of Co 3d is induced already in the antiferromagnetic phase, indicating that the Co moment is nearly fully alligned already in the antiferromagnetic phase. This suggests that the alignment of the Fe magnetic moment in SrFe_1−xCo_xO₃ is induced by interaction with doped Co magnetic moment. Possible formation of ferromagnetic Co rich region is discussed.

We examine the momentum and energy dependence of the scattering rate of the high-temperature cuprate superconductors using angle-resolved photoemission spectroscopy. The scattering rate is of the form a+bω around the Fermi surface for under-and optimal doping. The inelastic coefficient b is found to be isotropic. The elastic term a, however, is found to be highly anisotropic for under-and optimally doped samples, with an anisotropy which correlates with that of the pseudogap. This is contrasted with heavily overdoped samples, which show an isotropic scattering rate and an absence of the pseudogap above T_c. We find this to be a generic property for both single-and double-layer compounds.

The evolution of Ca_2-xNa_xCuO₂Cl₂ from Mott insulator to superconductor was studied using angle-resolved photoemission spectroscopy. By measuring both the excitations near the Fermi energy as well as nonbonding states, we tracked the doping dependence of the electronic structure and the chemical potential with unprecedented precision. Our work reveals failures in the standard weakly interacting quasiparticle scenario, including the broad line shapes of the insulator and the apparently paradoxical shift of the chemical potential within the Mott gap. To resolve this, we develop a model where the quasiparticle is vanishingly small at half filling and grows upon doping, allowing us to unify properties such as the dispersion and Fermi wave vector with the chemical potential.