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

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

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

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

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

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

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

Harmonic voltage response to AC current in the nonlinear conductivity of iridium oxide Ca5Ir3O12

We have carried out harmonic voltage response experiments by application of AC current along the c-axis of Ca5Ir3O12, which has a nonlinear electrical conductivity in a non-ordered state. This AC current method can allow us to investigate a detail of the nonlinear conductivity by application of small current. We observed the harmonics up to 7th order below 200 K. The results reveal that the nonlinear conductivity exists even in application of current close to zero. In addition, the temperature dependence of the resistance R0 estimated at the zero current limit is expressed by ln R0 ∝T-2/3, which is explained by an adaptation of Efros-Shklovskii variable range hopping or Fogler-Teber-Shklovskii variable range hopping. As a field assisted motion of charge career occurs in hopping conduction, from this analysis result, the nonlinear conductivity comes form the field-assisted hopping conduction

Observation of a Liquid-Gas-Type Transition in the Pyrochlore Spin Ice Compound Dy2Ti2O7 in a Magnetic Field

Low temperature magnetization measurements on the pyrochlore spin ice compound Dy2Ti2O7 reveal that the ice-rule breaking spin flip, appearing at H∼0.9  T applied parallel to the [111] direction, turns into a novel first-order transition for T<0.36   K which is most probably of a liquid-gas type. T-linear variation of the critical field observed down to 0.03 K suggests the unusual situation that the entropy release across the transition remains finite [∼0.5   (J/K)⋅mol−Dy] as T→0, in accordance with a breaking of the macroscopic degeneracy in the intermediate “kagomé ice” state