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Spectral and magnetic properties of Na<sub>2</sub>RuO<sub>3</sub>
Gapontsev, Vladimir V,Kurmaev, Ernst Z,Sathish, Clastin I,Yun, Seokhwan,Park, Je-Geun,Streltsov, Sergey V IOP 2017 Journal of physics, an Institute of Physics journa Vol.29 No.40
<P>We present measurements of resistivity, x-ray absorption (XAS) and emission (XES) spectroscopy together with <I>ab initio</I> band structure calculations for quasi two dimensional ruthenate Na<SUB>2</SUB>RuO<SUB>3</SUB>. Density function calculations (DFT) and XAS and XES spectra both show that Na<SUB>2</SUB>RuO<SUB>3</SUB> is a semiconductor with an activation energy of ∼80 meV. Our DFT calculations reveal large magneto-elastic coupling in Na<SUB>2</SUB>RuO<SUB>3</SUB> and predict that the ground state of Na<SUB>2</SUB>RuO<SUB>3</SUB> should be antiferromagnetic zig–zag.</P>
Nirmala, R,Jang, Kwang-Hyun,Sim, Hasung,Cho, Hwanbeom,Lee, Junghwan,Yang, Nam-Geun,Lee, Seongsu,Ibberson, R M,Kakurai, K,Matsuda, M,Cheong, S-W,Gapontsev, V V,Streltsov, S V,Park, Je-Geun Institute of Physics 2017 Journal of Physics, Condensed Matter Vol.29 No.13
<P>CuAl<SUB>2</SUB>O<SUB>4</SUB> is a normal spinel oxide having quantum spin, <I>S</I> = 1/2 for Cu<SUP>2+</SUP>. It is a rather unique feature that the Cu<SUP>2+</SUP> ions of CuAl<SUB>2</SUB>O<SUB>4</SUB> sit at a tetrahedral position, not like the usual octahedral position for many oxides. At low temperatures, it exhibits all the thermodynamic evidence of a quantum spin glass. For example, the polycrystalline CuAl<SUB>2</SUB>O<SUB>4</SUB> shows a cusp centered at ~2 K in the low-field dc magnetization data and a clear frequency dependence in the ac magnetic susceptibility while it displays logarithmic relaxation behavior in a time dependence of the magnetization. At the same time, there is a peak at ~2.3 K in the heat capacity, which shifts towards a higher temperature with magnetic fields. On the other hand, there is no evidence of new superlattice peaks in the high-resolution neutron powder diffraction data when cooled from 40 to 0.4 K. This implies that there is no long-ranged magnetic order down to 0.4 K, thus confirming a spin glass-like ground state for CuAl<SUB>2</SUB>O<SUB>4</SUB>. Interestingly, there is no sign of structural distortion either although Cu<SUP>2+</SUP> is a Jahn–Teller active ion. Thus, we claim that an orbital liquid state is the most likely ground state in CuAl<SUB>2</SUB>O<SUB>4</SUB>. Of further interest, it also exhibits a large frustration parameter, <I>f</I> = |<I>θ</I> <SUB>CW</SUB>/<I>T</I> <SUB>m</SUB>| ~ 67, one of the largest values reported for spinel oxides. Our observations suggest that CuAl<SUB>2</SUB>O<SUB>4</SUB> should be a rare example of a frustrated quantum spin glass with a good candidate for an orbital liquid state.</P>