Novel magnetism of Ir5+(5d4) Ions in the double perovskite Sr2YIrO6.

We synthesize and study single crystals of a new double-perovskite Sr2YIrO6. Despite two strongly unfavorable conditions for magnetic order, namely, pentavalent Ir5+(5d4) ions which are anticipated to have Jeff=0 singlet ground states in the strong spin-orbit coupling (SOC) limit and geometric frustration in a face-centered cubic structure formed by the Ir5+ ions, we observe this iridate to undergo a novel magnetic transition at temperatures below 1.3 K. We provide compelling experimental and theoretical evidence that the origin of magnetism is in an unusual interplay between strong noncubic crystal fields, local exchange interactions, and "intermediate-strength" SOC. Sr2YIrO6 provides a rare example of the failed dominance of SOC in the iridates.

Ferromagnetic exchange anisotropy from antiferromagnetic superexchange in the mixed 3d-5d transition-metal compound Sr3CuIrO6.

We report a combined experimental and theoretical study of the unusual ferromagnetism in the one-dimensional copper-iridium oxide Sr(3)CuIrO(6). Utilizing Ir L(3) edge resonant inelastic x-ray scattering, we reveal a large gap magnetic excitation spectrum. We find that it is caused by an unusual exchange anisotropy generating mechanism, namely, strong ferromagnetic anisotropy arising from antiferromagnetic superexchange, driven by the alternating strong and weak spin-orbit coupling on the 5d Ir and 3d Cu magnetic ions, respectively. From symmetry consideration, this novel mechanism is generally present in systems with edge-sharing Cu(2+)O(4) plaquettes and Ir(4+)O(6) octahedra. Our results point to unusual magnetic behavior to be expected in mixed 3d-5d transition-metal compounds via exchange pathways that are absent in pure 3d or 5d compounds.

Combined estimation of B1and T1for dynamic contrast-enhanced MRI by accounting for incomplete spoiling of transverse magnetization.

Objective.The variable flip angle (VFA) method for longitudinal relaxation time (T1) measurement is inherently sensitive to inaccuracies in the radiofRequency transmit field (B1) and incomplete spoiling of transverse magnetization. The objective of this study is to devise a computational method that addresses the problems of incomplete spoiling andB1inhomogeneity in the estimation ofT1using VFA method.Approach. Using an analytical expression of the gradient echo signal with account of incomplete spoiling, we first showed that ill-posedness in the simultaneous estimation ofB1andT1can be lifted with the use of flip angles larger than the Ernst angle. We then devised a nonlinear optimization method based on this signal model of incomplete spoiling for simultaneous estimation ofB1andT1.Main results. We evaluated the proposed method on a graded-concentration phantom to show that the derivedT1estimates offers an improvement over the regular VFA method and compares well with reference values measured by inversion recovery. Reduction of the number of flip angles from 17 to 5 yielded consistent results indicating that the proposed method is numerically stable.T1estimates derived from in-vivo brain imaging were consistent with literature values for gray and white matter tissues.Significance. Contrary to the common notion thatB1correction in the VFA method forT1mapping should be performed separately, we show that combined estimation ofB1andT1is feasible by the proposed method simply with the acquisition of 5 flip angles, as demonstrated on both phantom and in-vivo imaging data.

Testing the validity of the strong spin-orbit-coupling limit for octahedrally coordinated iridate compounds in a model system Sr3CuIrO6.

The electronic structure of Sr3CuIrO6, a model system for the 5d Ir ion in an octahedral environment, is studied through a combination of resonant inelastic x-ray scattering and theoretical calculations. Resonant inelastic x-ray scattering spectra at the Ir L3 edge reveal an Ir t(2g) manifold that is split into three levels, in contrast to the expectations of the strong spin-orbit-coupling limit. Effective Hamiltonian and ab inito quantum chemistry calculations find a strikingly large noncubic crystal field splitting comparable to the spin-orbit coupling, which results in a strong mixing of the j(eff)=1/2 and j(eff)=3/2 states and modifies the isotropic wave functions on which many theoretical models are based.

Negative volume thermal expansion via orbital and magnetic orders in Ca2Ru1-(x)Cr(x)O4(0 < x < 0.13).

Ca2RuO4 undergoes a metal-insulator transition at T(MI)=357 K, followed by a well-separated transition to antiferromagnetic order at T(N)=110 K. Dilute Cr doping for Ru reduces the temperature of the orthorhombic distortion at T(MI) and induces ferromagnetic behavior at T(C). The lattice volume V of Ca2Ru1-(x)Cr(x)O4 (0 < x < 0.13) abruptly expands with cooling at both T(MI) and T(C), giving rise to a total volume expansion ΔV/V ≈ 1%, which sharply contrasts the smooth temperature dependence of the few known examples of negative volume thermal expansion driven by anharmonic phonon modes. In addition, the near absence of volume thermal expansion between T(C) and T(MI) represents an Invar effect. The two phase transitions, which surprisingly mimic the classic freezing transition of water, suggest an exotic ground state driven by an extraordinary coupling between spin, orbit, and lattice degrees of freedom.

[Exploration on the new situation of HIV/ADIS epidemic and the necessity of interdisciplinary concern in Tanzania].

Through the effort in nearly 40 years of Tanzania, the total number of reported cases, the prevalence of HIV/AIDS and the number of death have declined, while the number of patients receiving antiretroviral therapy has increased significantly in Tanzania. At the same time, however, there are new challenges that require more attention, such as the HIV spread to rural areas, middle and lower social classes, and female teenagers. Although the overall performance of the HIV/AIDS prevention and control response is great, there is still a need to survey the new trends of this disease at the micro-level in Tanzania. In addition, there is a necessity of interdisciplinary concern due to the prevalence of heterosexual anal sex in Tanzania.

A simple B 1 correction method for dynamic contrast-enhanced MRI.

The double-angle method (DAM) is commonly used as a reference standard in radiofrequency field (B 1) mapping studies. This study explored two aspects of DAM: (i) use of small flip angle pairs to reduce the repetition time (TR) needed for adequate longitudinal relaxation (T 1); and (ii) the effect of using different flip angle ratios for B 1 mapping. Results of phantom studies show that B 1 correction using small flip angle pairs ≤ 60° with TR = 5000 ms can allow for accurate estimation of T 1 up to about 1500 ms; and that increasing the ratio of the two flip angles used for B 1 correction resulted in more accurate estimation of T 1. These modifications allow 3-dimensional (3D) B 1 mapping to be consistently performed with the same 3D spoiled gradient echo sequence used for T 1 mapping in dynamic contrast-enhanced MRI.

X-ray Absorption Spectroscopy Study of the Effect of Rh doping in Sr2IrO4.

We investigate the effect of Rh doping in Sr2IrO4 using X-ray absorption spectroscopy (XAS). We observed appearance of new electron-addition states with increasing Rh concentration (x in Sr2Ir1-xRhxO4) in accordance with the concept of hole doping. The intensity of the hole-induced state is however weak, suggesting weakness of charge transfer (CT) effect and Mott insulating ground states. Also, Ir Jeff = 1/2 upper Hubbard band shifts to lower energy as x increases up to x = 0.23. Combined with optical spectroscopy, these results suggest a hybridisation-related mechanism, in which Rh doping can weaken the (Ir Jeff = 1/2)-(O 2p) orbital hybridisation in the in-planar Rh-O-Ir bond networks.

Experimental electronic structure of the metallic pyrochlore iridate Bi2Ir2O7.

Angle-resolved photoemission measurements have been performed on Bi2Ir2O7 single crystals, a metallic end-member of the family of pyrochlore iridates. The density of states, the Fermi surface, and the near-Fermi-level band dispersion in the plane perpendicular to the (1, 1, 1) direction were all measured and found to be in rough overall agreement with our LDA + SOC density functional calculations. Assuming that this same calculation approach will extend to other members of the pyrochlore iridates, the overall agreement we found increases the possibility that some of the novel predicted phases such as quantum spin-ice or Weyl Fermion states will exist in this family of compounds.

Pressure induced second-order structural transition in Sr3Ir2O7.

We conducted in situ angle dispersive high pressure x-ray diffraction experiments on Sr3Ir2O7 up to 23.1 GPa at 25 K with neon as the pressure transmitting medium. Pressure induces a highly anisotropic compressional behavior seen where the tetragonal plane is compressed much faster than the perpendicular direction. By analyzing different aspects of the diffraction data, a second-order structural transition is observed at approximately 14 GPa, which is accompanied by the insulating state to nearly metallic state at 13.2 GPa observed previously (Li et al 2013 Phys. Rev. B 87 235127). Our results highlight the coupling between electronic state and lattice structure in Sr3Ir2O7 under pressure.

Magnetic symmetries in neutron and resonant x-ray Bragg diffraction patterns of four iridium oxides.

The magnetic properties of Sr(2)IrO(4), Na(2)IrO(3), Sr(3)Ir(2)O(7) and CaIrO(3) are discussed, principally in the light of experimental data in recent literature for Bragg intensities measured in x-ray diffraction with enhancement at iridium L-absorption edges. The electronic structure factors we report, which incorporate parity-even and acentric entities, serve the immediate purpose of making full use of crystal and magnetic symmetry to refine our knowledge of the magnetic properties of the four iridates from resonant x-ray diffraction data. They also offer a platform on which to interpret future investigations, using dichroic signals, resonant x-ray diffraction and neutron diffraction, for example, as well as ab initio calculations of electronic structure. Unit-cell structure factors, suitable for x-ray Bragg diffraction enhanced by an electric dipole-electric dipole (E1-E1) event, reveal exactly which iridium multipoles are visible, e.g., a magnetic dipole parallel to the crystal c-axis (z-axis) and an electric quadrupole with yz-like symmetry in the specific case of CaIrO(3). Magnetic space-groups are assigned to Sr(2)IrO(4), Sr(3)Ir(2)O(7) and CaIrO(3), namely, P(I)cca, P(A)ban and Cm'cm', respectively, in the Belov-Neronova-Smirnova notation. The assignment for Sr(2)IrO(4) is possible because of our new high-resolution neutron diffraction data, gathered on a powder sample. In addition, the new data are used to show that the ordered magnetic moment of an Ir(4+) ion in Sr(2)IrO(4) does not exceed 0.29(4) µ(B). Na(2)IrO(3) has two candidate magnetic space-groups that are not resolved with currently available resonant x-ray data.

Strong magnetic instability in correlated metallic Bi2Ir2O7.

We report an experimental/theoretical study of single-crystal Bi(2)Ir(2)O(7) that possesses a metallic state with strongly exchange-enhanced paramagnetism. The ground state of Bi(2)Ir(2)O(7) is characterized by the following features: (1) a divergent low-temperature magnetic susceptibility that indicates no long-range order down to 50 mK; (2) strongly field-dependent coefficients of the low-temperature T and T(3) terms of the specific heat; (3) a conspicuously large Wilson ratio R(W) ≈ 53.5; and (4) unusual temperature and field dependences of the Hall resistivity that abruptly change below 80 K, without any clear correlation with the magnetic behavior. All these unconventional properties suggest the existence of an exotic ground state in Bi(2)Ir(2)O(7).

Correlated giant dielectric peaks and antiferromagnetic transitions near room temperature in pure and alkali-doped BaMnO(3-δ).

We report structural, magnetic, dielectric and thermal properties of single-crystal BaMnO(2.99) and its derivatives BaMn(0.97)Li(0.03)O(3) and Ba(0.97)K(0.03)MnO(3). The hexagonal 15R-BaMnO(2.99) perovskite phase is a known antiferromagnetic insulator that orders at a Néel temperature T(N) = 220 K. We find dilute Li and K doping change the ratio of cubic to hexagonal layers and cause drastic changes in the dielectric and magnetic properties. Unusually large high-temperature magnetoelectric shifts (up to 85%) are observed near temperatures at which pronounced peaks in the dielectric constant are observed for applied electric fields along either the c or a axis, respectively. The temperatures of the dielectric peaks are strongly correlated with anomalies in the c- or a-axis magnetic susceptibility and the specific heat for all compositions studied. All our data suggest that the strongly anisotropic magnetic and dielectric anomalies (which occur near, or above room temperature) originate from the same Mn ion sites, which implies these materials form an exceptional class of magnetoelectrics.