Giant Magnetoresistance Effect in the Metal-Insulator Transition of Pyrochlore Oxide Nd2Ir2O7

We investigated the magnetoresistance (MR) effect of the pyrochlore oxide Nd2Ir2O7, which shows a metal-insulator transition at T_MI =33 K. A small positive MR effect was observed in the metallic state above T_MI, while a large negative MR effect was observed in the insulating state below T_MI . MR effects exceeding 3000% were found at 1 K at a field of 9 T. As a result, we confirmed the crossover from the insulating state to a state with a small or partial band gap in a field up to 56 T. Furthermore, from the MR effect in Eu2Ir2O7 (T_MI = 120 K) and Gd$_2$Ir$_2$O$_7$ (T_MI = 127 K), we revealed that the large negative MR effect of the pyrochlore iridate Ln2Ir2O7 depends on the magnetism of the lanthanide Ln^{3+} ion. The d-f interaction plays a significant role in the large negative MR effect in the insulating state.Comment: 10 pages, 4 figure

Transport properties in normal metal Bi2Pd3S2 and superconducting Bi2Pd3Se2

The transport properties of the parkerite-related compounds Bi2Pd3X2 (X=S,Se) were studied. The electrical resistivities of both compounds show typical metallic behavior up to 400 K. Resistivity and specific heat measurements at low temperatures reveal that Bi2Pd3Se2 is superconducting below 1 K. On the other hand, Bi2Pd3S2 does not show a bulk superconducting transition down to 0.35 K. In the normal state, the electronic specific heat coefficient γ and the Debye temperature θD are found to be 5.9 mJ/mol K2 and 170 K, respectively for Bi2Pd3S2, and 8.3 mJ/mol K2 and 150 K, respectively for Bi2Pd3Se2. In the superconducting state for Bi2Pd3Se2, the upper critical field at zero temperature for Bi2Pd3Se2 is 290 mT. From the electronic specific heat in the superconducting temperature range, it was found that Bi2Pd3Se2 belongs to an s-wave weakcoupling superconductor

Metal–Insulator Transitions in Pyrochlore Oxides Ln2Ir2O7

We report the physical properties of Ln2Ir2O7 (Ln = Nd, Sm, Eu, Gd, Tb, Dy, and Ho), which exhibit metal-insulator transitions (MITs) at different temperatures. The transition temperature TMI increases with a reduction in the ionic radius of Ln. The ionic radius　boundary for MITs in Ln2Ir2O7 lies between Ln = Pr and Nd. MITs in Ln2Ir2O7 have some common features. They are second-order transitions. Under the field cool condition, a weak ferromagnetic component (»10−3 μB/f.u.) caused by Ir 5d electrons is observed below TMI.The entropy associated with MITs for Ln = Nd, Sm, and Eu is estimated to be 0.47, 2.0, and 1.4 J/K mole, respectively. The change in entropy is much smaller than 2R ln 2 [11.5 J /K mole] expected in a magnetic transition due to localized moments of S = 1/2. The feature of continuous MITs in Ln2Ir2O7 is discussed

Neutron scattering study of dipolar spin ice Ho2Sn2O7: Frustrated pyrochlore magnet

By means of neutron scattering techniques we have investigated the frustrated pyrochlore magnetHo2Sn2O7, which was found to show ferromagnetic spin-ice behavior below T.1.4 K by susceptibilitymeasurements. High-resolution powder neutron diffraction shows no detectable disorder of the lattice, whichimplies the appearance of a random magnetic state solely by frustrated geometry, i.e., the corner sharingtetrahedra. Magnetic inelastic scattering spectra show that Ho magnetic moments behave as an Ising spinsystem at low temperatures and that the spin fluctuation has static character. The system remains in a shortrange-ordered state down to at least T50.4 K. By analyzing the wave-vector dependence of the magneticscattering using a mean-field theory, it is shown that the Ising spins interact via the dipolar interaction.Therefore we conclude that Ho2Sn2O7 belongs to the dipolar-spin-ice family. Slow spin dynamics is exhibitedas thermal hysteresis and time dependence of the magnetic scattering

Charge-density-wave superconductor Bi2Rh3Se2

We discovered a superconducting transition with the charge-density-wave state in a ternary compound Bi2Rh3Se2. This compound crystallizes in the parkerite-type structure composed of sheets containing one-dimensional Rh-Rh chains. The electrical resistivity, magnetic susceptibility, thermoelectric power, sample length change, and x-ray diffraction measurements reveal that this compound is in the CDW state below 240 K. Furthermore, the specific heat and electrical resistivity measurements show a superconducting transition at ~0.7 K. The various superconducting parameters were determined, and the GL parameter (0) shows the considerably large value of 151 indicating an extreme type-II superconductor

Magnetic order in pyrochlore iridate Nd2_2Ir2_2O7_7 probed by muon spin relaxation

Muon-spin relaxation results on the pyrochlore iridate Nd$_2$Ir$_2$O$_7$ are reported. Spontaneous coherent muon-spin precession below the metal-insulator transition (MIT) temperature of about 33 K is observed, indicating the appearance of a long-ranged magnetic ordering of Ir$^{4+}$ moments. With further decrease in temperature, the internal field at the muon site increases again below about 9 K. The second increase of internal field suggests the ordering of Nd$^{3+}$ moments, which is consistent with a previous neutron experiment. Our results suggest that the MIT and magnetic ordering of Ir$^{4+}$ moments have a close relationship and that the large spin-orbit coupling of Ir 5\textit{d} electrons plays a key role for both MIT and the mechanism of the magnetic ordering in pyrochlore iridates in the insulting ground state.Comment: 5 pages, 3 figures. Accepted by Physical Review B (rapid communications

Metal–Insulator Transitions in Pyrochlore Oxides Ln2Ir2O7

We report the physical properties of Ln2Ir2O7 (Ln = Nd, Sm, Eu, Gd, Tb, Dy, and Ho), which exhibit metal-insulator transitions (MITs) at different temperatures. The transition temperature TMI increases with a reduction in the ionic radius of Ln. The ionic radius　boundary for MITs in Ln2Ir2O7 lies between Ln = Pr and Nd. MITs in Ln2Ir2O7 have some common features. They are second-order transitions. Under the field cool condition, a weak ferromagnetic component (»10−3 μB/f.u.) caused by Ir 5d electrons is observed below TMI. The entropy associated with MITs for Ln = Nd, Sm, and Eu is estimated to be 0.47, 2.0, and 1.4 J/K mole, respectively. The change in entropy is much smaller than 2R ln 2 [11.5 J /K mole] expected in a magnetic transition due to localized moments of S = 1/2. The feature of continuous MITs in Ln2Ir2O7 is discussed

Carrier doping effect of magnetic and transport properties on the geometrically frustrated iridate Ca5Ir3O12

We show the carrier doping effect on magnetic and transport properties of Ca5Ir3O12, indicating semiconductivity and an antiferromagnetic ordering below Néel temperature TN = 7.8 K. The average valence of Ir ions in Ca5Ir3O12 is +4.67. Carrier doping for 5d electrons of Ir ions is achieved by substitution of Ca2+ by Na+, La3+, and Bi3+; Na substitution leads to hole doping, and La or Bi substitution leads to electron doping. The electrical resistivity decreases in both case of the hole doping or the electron doping. For La10% doped samples, TN does not change but Curie–Weiss temperature ΘCW becomes 3.5 times larger than 9.9 K for Ca5Ir3O12. For Bi10% doped samples, TN decreases to 5 K but ΘCW becomes 3 times larger than that of undoped compound. On the other hand, for Na10% doped samples, TN decreases to 4 K and ΘCW decreases to 8.3 K. The frustration index f which is defined by ΘCW /TN is enhanced by electron doping. The magnetic frustration becomes stronger with increase of filling of Ir 5d electrons on the triangular lattice

Systematic Study of Lattice Specific Heat of Filled Skutterudites

The lattice specific heat C lat of La-based filled skutterudites La T 4 X 12 ( T = Fe, Ru and Os; X = P, As, and Sb) has been systematically studied, and both the Debye temperature Θ D and the Einstein temperature Θ E of La T 4 X 12 were carefully estimated. We confirmed that a correlation exists between Θ D and the reciprocal of the square root of average atomic mass for La T 4 P 12 , La T 4 As 12 , and La T 4 Sb 12 . The Θ D of filled skutterudites was found to depend mainly on the nature of the species X forming the cage. The temperature dependence of C lat / T 3 for La T 4 X 12 exhibited a large broad maximum at low temperatures (10–30 K), which suggests a nearly dispersionless low-energy optical mode characterized by Einstein specific heat. Since no such broad maximum exists for the unfilled skutterudite RhP 3 , the low-energy optical modes are associated with vibration involving La ions in the X 12 cage (the so-called “guest ion modes”). The Θ E of filled skutterudites was found to roughly correspond to the energy of low-energy guest ion optical modes. Furthermore, a good correlation was shown to exist between Θ E and r R–X - r R3+ , where r R–X is the R – X distance and r R3+ is the effective ionic radius of R 3+ . As r R–X - r R3+ increased, Θ E was found to decrease

Magnetic Moments and Ordered States in Pyrochlore Iridates Nd2Ir2O7 and Sm2Ir2O7 Studied by Muon-Spin Relaxation

Magnetic-ordered states of the pyrochlore iridates Nd2Ir2O7 (Nd227) and Sm2Ir2O7 (Sm227), showing metal–insulator transitions at 33 and 117 K, respectively, were studied by both the muon-spin-relaxation (μSR) method and density functional theory (DFT) calculations. A long-range magnetic ordering of Ir moments appeared in conjunction with the metal insulator transition, and additional long-range-ordered states of Nd/Sm moments were confirmed at temperatures below about 10 K. We found that the all-in all-out spin structure most convincingly explained the present μSR results of both Nd227 and Sm227. Observed internal fields were compared with values derived from DFT calculations. The lower limits of the sizes of magnetic moments were estimated to be 0.12 μB and 0.2 μB for Ir and Nd moments in Nd227, and 0.3 μB and 0.1 μB for Ir and Sm moments in Sm227, respectively. Further analysis indicated that the spin coupling between Ir and Nd/Sm moments was ferromagnetic for Nd227 and antiferromagnetic for Sm227