Charge density wave formation accompanying ferromagnetic ordering in quasi-one-dimensional BaIrO3

Abstract

The magnetic, transport, optical, and structural properties of quasi-one-dimensional BaIrO 3 show evidence for the simultaneous onset of electronic density wave formation and ferromagnetism at T c3 175 K: Two additional features in the chain direction dc conductivity show a sudden change to metallic behavior below T c2 80 K and then a Mott-like transition at T c1 26 K: Highly non-linear dc conductivity, optical gap formation at Ϸ9k B T c3 , additional phonon modes, and emergent X-ray satellite structure support density wave formation. Even at very high (30 T) fields the saturation Ir moment is very small, Ϸ0.04m B /Ir. ᭧ 2000 Elsevier Science Ltd. All rights reserved. Transition metal oxides (TMO) with low crystalline symmetry are known to exhibit electronic density wave formation [1][2][3]. However, to our knowledge, density wave formation has not yet been observed accompanying the onset of ferromagnetic order. However, the ferromagnetism at T c3 175 K in BaIrO 3 [4] appears to be accompanied by and possibly driven by a collective electronic excitation or at least partial gapping of the Fermi surface. This demonstrates once again the strong coupling between spin and charge in the heavy (4d-and 5d-based) TMOs [5][6][7]. BaIrO 3 has a highly anisotropic quasi-one-dimensional structure [8][9][10] and this gives rise, in our single crystal samples, to large anisotropy of r(T), the electrical resistivity, with the quasi-one-dimensional axis, the c-axis, having much lower resistivity. This kind of low-dimensional structure is necessary for the formation of an insulating charge density wave (CDW) ground state, which is a collective electron mode normally incommensurate with the underlying lattice for partially filled bands [3].Evidence for density wave formation comes from: (1) A discontinuous increase in the slope of r (T) vs. T at T c3 T C ; the Curie temperature-an abrupt transition to a more insulating phase. (Two additional features of r (T) along the c-axis, at T c2 80 K and T c1 26 K; mark a sudden return to "metallic" behavior (possibly a crossover from partial toward full gapping of the Fermi surface) and a well-defined Mott-like metal-insulator transition, respectively). (2) An abrupt feature in the non-linear conductivity showing negative differential resistivity. (3) Gap formation at about 1200 cm Ϫ1 in the electron excitation spectrum and a splitting of a phonon mode at 350 cm Ϫ1 , which appear for T Ͻ T c3 (This was determined by optical reflectivity studies in the far and near infrared.). (4) Additional satellite formation for T Ͻ T C3 in the X-ray diffraction spectrum.The structure of BaIrO 3 is monoclinic and consists of Ir 3 O 12 trimers of face-sharing IrO 6 octahedra which are vertex-linked to other trimeric clusters forming columns roughly parallel to the c-axis. These clusters form channels accommodating Ba ions. The space group is C2/m and the

Synthesis, Crystal Structure, and Physical Properties of the Perovskite Iridates

“…It is the first known ferromagnetic insulator with T c ≈ 180 K among the 5d TMOs [17]. Detailed studies on single crystals revealed a charge-density-wave (CDW) formation below the ferromagnetic order [18,51]. Recent experimental [52] and theoretical [53] investigations further revealed it as an exotic spin-orbit Mott insulator that is of great current research interest.…”

“…The sample should be cooled down slowly for the last sintering in order to ensure an oxygen stoichiometry [54]. Single crystals have been reported to grow out of the BaCl 2 flux at a relatively low temperature of 1,000 K [18]. HPHT synthesis is needed for all the other polytypes [19,21,55,56].…”

“…Cao et al [18] performed the first detailed experimental study on the single-crystal samples and uncovered a CDW formation accompanying the ferromagnetic order at T c ≈ 180…”

“…The experimental evidences in support of the CDW formation included [18]: (1) a sudden increase of resistivity at T c , (2) the presence of non-linear conductivity with negative differential resistivity below T c , (3) an optical gap formation at ~ 1,200 cm -1 ≈ 9κ B T c in the electron excitation spectrum and a splitting of a phonon mode at 350 cm -1 for T < T c , and (4) the emergent X-ray satellite structure below T c . Besides the transitions at T c ≈ 180 K, two additional anomalies have also been observed [18] upon cooling on the c-axis resistivity ρ c , which first changes to a metallic behaviour below T 1 = 80 K and then suddenly enters a Mott-like insulating state below T 2 = 26 K, Figure 10. These two additional transitions are absent for the resistivity within the ab plane.…”

Tuning theJeff=12insulating state via electron doping and pressure in the double-layered iridate Sr3IrLi1, Kong2, Qi3 et al. 2013Phys. Rev. BSelf Cite

“…This conventional wisdom conflicts with two trends observed among layered iridates such as the Ruddlesden-Popper phases, Sr n+1 Ir n O 3n+1 (n = 1 and 2; n defines the number of Ir-O layers in a unit cell) [1-5] and hexagonal BaIrO 3 [6]. First, complex magnetic states occur with high critical temperatures but unusually low ordered moments.…”