We investigated electronic structure of 5d transition-metal oxide Sr 2 IrO 4 using angle-resolved photoemission, optical conductivity, and x-ray absorption measurements and first-principles band calculations. The system was found to be well described by novel effective total angular momentum J eff states, in which relativistic spin-orbit (SO) coupling is fully taken into account under a large crystal field. Despite of delocalized Ir 5d states, the J eff -states form so narrow bands that even a small correlation energy leads to the J eff = 1/2 Mott ground state with unique electronic and magnetic behaviors, suggesting a new class of the J eff quantum spin driven correlated-electron phenomena.

We report transport and thermodynamic properties of single-crystal SrIrO 3 as a function of temperature T and applied magnetic field H. We find that SrIrO 3 is a nonFermi-liquid metal near a ferromagnetic instability, as characterized by the following properties: (1) small ordered moment but no evidence for long-range order down to 1.7 K; (2) strongly enhanced magnetic susceptibility that diverges as T -γ at low temperatures with 1/2 < γ < 1, depending on the applied field; (3) heat capacity C(T,H) ~ -Tlog T that is readily amplified by low applied fields; (4) a strikingly large Wilson ratio at T< 4K;and (5) a T 3/2 -dependence of electrical resistivity over the range 1.7 < T < 120 K. A phase diagram based on the data implies SrIrO 3 is a rare example of a stoichiometric oxide compound that exhibits non-Fermi-liquid behavior near a quantum critical point (T = 0 and μ ο H = 0.23 T). model system for studies of quantum criticality both experimentally and theoretically.It is now qualitatively understood that the extended 5d orbital significantly enhances crystalline electric field splittings causing a partial breakdown of Hund's rule, leading to a low spin state, S = 1/2 for Ir 4+ (5d 5 ) ions, and a d-p hybridization between the transition metal and the oxygen octahedron enclosing it. These interactions generate strong electron-lattice coupling that can alter and distort the metal-oxygen bond lengths and angles, lifting the degeneracies of 5d-orbitals and precipitating orbital ordering.Single crystals were grown using flux techniques described elsewhere [7]. The crystal structure was determined from a small fragment (0.05×0.05×0.05 mm 3 ) using MoKα radiation and a Nonius Kappa CCD single-crystal diffractometer. Heat capacity measurements were performed with a Quantum Design PPMS that utilizes a thermal- χ ab at T <5 K.) Indeed, the isothermal magnetization M at T = 1.7 K is already saturated at μ ο H ~3 T, as seen in the inset to Fig. 1a. On the other hand, the corresponding ordered moment μ or is less than 3% of that expected (1 μ B /Ir) for an S = 1/2 system and decreases with increasing T, which is indicative of a nearby Stoner instability. 4The reciprocal susceptibilities χ c -1 and χ ab -1 display linear T-dependences, consistent with a Curie-Weiss behavior for T > 120 K, as shown in Fig.1b follow non-standard power laws that range from T 1/2 for B < 0.3 T to linear-T for B > 0.8 T, as shown in Fig. 1c. This high sensitivity of the temperature exponent to low applied magnetic fields again suggests the rapid approach to a ferromagnetic instability.The low temperature specific heat C(T,H) data acquired over 1.8 < T < 24 K and μ ο H < 8 T offer important insights into the low energy excitations of SrIrO 3 . For T > 12 K, the specific heat is well described by C(T) = γT + βT 3 with γ = 1.50 mJ/mole K 2 and β = 0.28 mJ/mole K 4 , suggesting that only electronic and phonon contributions are significant in this temperature range (data not shown). The small γ-value implies that there is essentially no mass enhancem...

We report results of a magnetic and transport study of SrRu 1-x Mn x O 3 (0≤ x<0.60), i.e., Mn doped SrRuO 3 . The Mn doping drives the system from the itinerant ferromagnetic state (T C =165 K for x=0) through a quantum critical point at x c =0.39 to an insulating antiferromagnetic state. The onset of antiferromagnetism is abrupt with a Néel temperature increasing from 205 K for x=0.44 to 250 K for x=0.59. Accompanying this quantum phase transition is a drastic change in resistivity by as much as 8 orders of magnitude as a function of x at low temperatures. The critical composition x c =0.39 sharply separates the two distinct ground states, namely the ferromagnetic metal from the antiferromagnetic insulator.

We report a structural, thermodynamic, and transport study of the newly synthesized Sr 5 Rh 4 O 12 , Ca 5 Ir 3 O 12 , and Ca 4 IrO 6 single crystals. These quasi-one-dimensional insulators consist of a triangular lattice of spin chains running along the c axis, and are commonly characterized by a partial antiferromagnetic ͑AFM͒ order, a small entropy removal associated with the phase transitions, and a sizable low-temperature specific heat linearly proportional to temperature. Sr 5 Rh 4 O 12 is antiferromagnetically ordered below 23 K with strong evidence for an Ising character. Isothermal magnetization of Sr 5 Rh 4 O 12 exhibits two steplike transitions leading to a ferrimagnetic state at 2.4 T and a ferromagnetic state at 4.8 T, respectively. Ca 5 Ir 3 O 12 and Ca 4 IrO 6 are also antiferromagnetically ordered below 7.8 and 12 K, respectively, and show an unusually large ratio of the Curie-Weiss temperature to the Néel temperature. In particular, Ca 5 Ir 3 O 12 , which includes both Ir 4+ and Ir 5+ ions, reveals that only S =1/2 spins of the Ir 4+ ions are involved in the magnetic ordering, whereas S = 1 spins of the Ir 5+ ions remain disordered. All results suggest the presence of a geometrical frustration that causes incomplete long-range AFM order in these quasi-one-dimensional compounds.

The 5d-electron based BaIrO 3 is a nonmetallic weak ferromagnet with a Curie temperature at Tc=175 K. Its largely extended orbitals generate strong electron-lattice coupling, and magnetism and electronic structure are thus critically linked to the lattice degree of freedom. Here we report results of our transport and magnetic study on slightly Sr doped BaIrO 3 . It is found that dilute Sr-doping drastically suppresses Tc, and instantaneously leads to a nonmetal-metal transition at high temperatures. All results highlight the instability of the ground state and the subtle relation between magnetic ordering and electron mobility. It is clear that BaIrO 3 along with very few other systems represents a class of materials where the magnetic and transport properties can effectively be tuned by slight alterations in lattice parameters.The layered iridates BaIrO 3 [1][2][3][4][5] and Sr n+1 Ir n O 3n+1 with n=1 and 2 [6-11] possess two unique features not found in other materials, namely, high-temperature weak ferromagnetism and an insulating ground state with phase proximity of a metallic state.These features defy common wisdom that 4d and 5d transition metal oxides should be much more conducting than their 3d counterparts because of the more extended d-orbitals that significantly reduce the Coulomb interaction. In fact, this extended characteristic significantly enhances crystalline field splittings and the d-p hybridization between the transition metal and the oxygen octahedron surrounding it. This, in turn, leads to strong electron-lattice coupling, which very often alters and distorts the metal-oxygen bonding lengths and angles, lifting degeneracies of t 2g orbitals and thus precipitating possible orbital ordering. It is this feature that defines a high sensitivity of the ground state to the atomic stacking sequence and structural distortions. This characteristic is illustrated in recent studies on 4d-electron based ruthenates where magnetism and electric transport change drastically and systematically with structural dimensionality. Although the layered iridates distinguish themselves from the ruthenates by showing conspicuously low magnetic moments and yet high Curie temperatures, magnetic and transport behavior of these 5d-electron systems is also critically linked to the lattice degree of freedom, requiring only dilute impurity doping to tip the balance across the metal-insulator borderline accompanying drastic changes in magnetic ordering.The Ir 4+ (5d 5 ) ions in these layered iridates are presumed to be in a low spin state with S=1/2 since the large extension of the 5d orbitals leads to large crystal field splittings and a reduced Coulomb repulsion [1][2][3][4][5][6][7][8][9][10][11]. BaIrO 3 is a nonmetallic weak ferromagnet with an exotic ground state. Recent experimental studies reveal evidence for 2 a simultaneous onset of weak ferromagnetism and charge density wave (CDW) formation at Tc=175 K [4], an unexpected occurrence that is also supported by results of tightbinding band structure calculations, whi...