Session Z36: Focus Session: Bulk Properties of Complex Oxides -- 4d and 5d Systems

Giant Magneto-electric Effect in the J$_{eff}$ = $\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} $ Mott Insulator Sr$_{2}$IrO$_{4}$

Our magnetic, electrical, and thermal measurements on single-crystals of the J$_{eff}$ = 1/2 Mott insulator, Sr$_{2}$IrO$_{4}$, reveal a novel giant magneto-electric effect (GME) arising from a frustrated magnetic/ferroelectric state [1] whose signatures are: (1) a strongly enhanced electric permittivity that peaks near a newly observed magnetic anomaly at 100 K, and (2) a large ($\sim $100{\%}) magneto-dielectric shift that occurs near a metamagnetic transition. The GME hinges on a spin-orbit gapping of 5d-bands, rather than the magnitude and spatial dependence of magnetization, as traditionally accepted. \\[4pt] [1] S. Chikara, et al, Phys. Rev. B 80, 140407 (R) (2009)   

Evolution of Physical Properties with Mn Content in Sr$_{3}$(Ru$_{1-x}$Mn$_{x})_{2}$O$_{7}$ Single Crystals

We have systematically studied the doping dependence of physical properties of Sr$_{3}$(Ru$_{1-x}$Mn$_{x})_{2}$O$_{7 }$with 0.0$\le $x$\le $1.0. Although the undoped Sr$_{3}$Ru$_{2}$O$_{7}$ is a paramagnetic metal, partial substitution of Ru by Mn results not only in metal-insulating transition (T$_{MIT})$ but also in complex magnetic ordering at T$_{M}$. Interestingly, the difference between T$_{M}$ and T$_{MIT}$ (T$_{M}<$T$_{MIT})$ increases with increasing doping level x. The correlation between these two transitions will be reported.   

Continuous metal-insulator transition at 410 K of the antiferromagnetic perovskite NaOsO$_3$

Newly synthesized perovskite NaOsO$_3$ shows a Curie-Weiss metallic nature at high temperature and suddenly goes into an antiferromagnetically insulating state at 410 K on cooling. Electronic specific heat at the low temperature limit is absent, indicating that the band gap fully opens. $In situ$ observation in electron microscopy undetected any lattice anomalies in the vicinity of the transition temperature. It is most likely that the antiferromagnetic correlation plays an essential role in the gap opening. Use of the Advanced Photon Source was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. This research was supported in part by the WPI Initiative on Materials Nanoarchitectonics from MEXT, Japan, and the Grants-in-Aid for Scientific Research (20360012) from JSPS.   

Pressure-Induced Insulating State in Ba$_{1-x}$RE$_{x}$IrO$_{3}$ (RE = Gd, Eu) Single Crystals

BaIrO$_{3}$ is a novel insulator with coexistent weak ferromagnetism, charge and spin density wave. Dilute RE doping for Ba induces a metallic state, whereas application of modest pressure ($\le $ 12.1 kbar) readily restores an insulating state characterized by a three-order-of-magnitude increase of resistivity. A pressure-induced insulating state is not common, and has never been observed in 5d-electron materials. The profoundly dissimilar responses of the ground state to light doping and low hydrostatic pressures signal an unusual, delicate interplay between structural and electronic degrees of freedom in BaIrO$_{3}$.   

Unusual interlayer magnetic coupling in quasi 2-D heavy-mass nearly ferromagnetic state of (Sr$_{1-x}$Ca$_{x})_{3}$Ru$_{2}$O$_{7}$

Perovskite ruthenates exhibit a wide range of complex magnetic ground states. In this talk we focus on an unusual heavy-mass, nearly ferromagnetic state with an extremely large Wilson ratio (Z. Qu \textit{et al.}, Phys. Rev. B \textbf{78} R180407 (2008)). Despite considerable FM correlations, this state never develops long-range FM order, instead freezing into a cluster-spin-glass (CSG) state. We have further investigated this magnetic state through in-plane angular dependence of magnetoresistivity and magnetization on (Sr$_{0.62}$Ca$_{0.38})_{3}$Ru$_{2}$O$_{7}$. The in-plane magnetoresistivity $\rho_{ab}(\phi)$ at high magnetic fields reveals a change in anisotropy symmetry from 2-fold to 4-fold at the frozen temperature $T_{f}$ of the CSG phase, whereas inter-planar magnetoresistivity $\rho_{c}(\phi)$ at high fields only exhibits 4-fold symmetry. For low magnetic fields, both $\rho_{ab}$ and $\rho_{c}$ only exhibit anisotropy below $T_{f}$, also with 4-fold symmetry. Angle-dependent magnetization data reveal that at high field the anisotropy exhibits 8-fold symmetry for $T > T_{f}$. However, for $T < T_{f}$, an additional asymmetric 2-fold anisotropy develops. These results may indicate non-traditional interlayer magnetic coupling, one possible scenario involving perpendicular spin stacking between alternate layers.   

Ferromagnetism in perovskites Sr$_{1-x}$Pb$_{x}$RuO$_{3}$ (0 $\le $ x $\le $ 1)

Orthorhombic SrRuO$_{3}$ is a well-known metallic ferromagnet with T$_{c}$ $\approx $ 165 K.$^{1}$ We have demonstrated that a complete solid solution of the orthorhombic perovskites Sr$_{1-x}$Pb$_{x}$RuO$_{3}$ can be made with high-pressure synthesis. Whereas the ground state for the whole series remains metallic, the Pb substitution reduces the Curie temperature and eventually ferromagnetic phase is totally suppressed at x $\approx $ 0.5. Moreover, an unusual low-temperature phase with the Imma space group is developed through a first-order transition below T$_{t} \quad \approx $ 90 K in the orthorhombic PbRuO$_{3}$. Based on physical properties measurements on a small grain of the high-pressure product PbRuO$_{3}$, we have shown that the transition at T$_{t}$ is a metal-metal transition. T$_{t}$ is suppressed below 10 K under high pressure 3GPa as verified by the structural study with a diamond anvil cell with synchrotron radiation. Suppression of ferromagnetism in this system can be attributed to the hybridization of the Pb$^{2+}$ 6s electrons with the Ru 4d electrons that broadens the Ru 4d band. $^{1}$ G. Cao, et al., Phys. Rev. B \textbf{56}, 321 (1997). $^{2 }$S. A. J. Kimber, et al., Phys. Rev. Lett. \textbf{102}, 046409 (2009). $^{3 }$J.-G. Cheng, et al., Phys. Rev. B, in press.   

Intermediate-Valence Behavior in the Transition-Metal Oxide CaCu$_3$Ru$_4$O$_{12}$

A detailed study of the electronic properties of strongly correlated CCRO will be presented. Along with transport, $\chi(T)$ and specific heat, we focus on NMR/NQR. CCRO is a metallic system with perovskite structure showing non-Fermi-liquid behavior below 2${\,}$K which is inferred from $C_p/{\,}T\propto -ln(T)$ and a deviation of $1/{\,}T_1 (T)$ from the Korringa law at the copper site [PRB \textbf{78}, 165126 (2008)]. On closer inspection, a volume change could be detected by neutron diffraction which comes along with a corresponding anomaly in the specific heat around 150${\,}$K. Furthermore, the $^{99}$Ru Knight shift shows a cross-over between paramagnetic behavior of localized moments at high $T$ and itinerant band states at low $T$, respectively. Complementary density-functional calculations (LDA+DFT) relate these phenomena to the ruthenium \emph{d}-electron number. We conclude that dynamic charge fluctuations originating from the strong electronic correlations are present in CCRO and give rise to the intermediate valence of the ruthenium ions [PRB \textbf{80}, 121101(R) (2009)].   

Surface state van Hove singularity effect to bulk electron-bosonic mode coupling in Sr$_{2}$RuO$_{4}$

Discovery of spin triplet superconductivity in Sr$_{2}$RuO$_{4}$ brought attention to the electronic structure studies on the system, especially by using angle resolved photoemission (ARPES). There are three bands that cross the Fermi level. The system is particularly interesting because these bands have different orbitals with different characters such as dimensionality. Along the way, it was found that there are surface states due to RuO$_{6}$ octahedral rotation on the surface layer. RuO$_{6}$ octahedral rotation results in a dramatic change in the surface electronic structure such as van Hove singularity. By controlling temperatures, we can traverse or kill the van Hove singularity in the surface electronic states. Through comparison of measured bulk electronic structures at different temperatures, we elucidate the effect of surface state van Hove singularity to bulk electron-bosonic mode coupling. We will discuss the importance of the surface states in interpretation of bulk properties measured by ARPES.   

Electronic density of states of Ca3Ru2O7 measured by tunneling spectroscopy

The ruthenates are perhaps one of the most diverse of group of materials known up to date. These compounds exhibit a wide array of behaviors ranging from the exotic p-wave superconductivity in Sr$_{2}$RuO$_{4}$, to the itinerant ferromagnetism in SrRuO$_{3}$, and the Mott-insulating behavior in Ca$_{2}$RuO$_{4}$. One of the most intriguing compounds belonging to this group is Ca$_{3}$Ru$_{2}$O$_{7}$ which is known to undergo an antiferromagnetic ordering at 56K and an insulating transition at 48K. We have prepared Ca$_{3}$Ru$_{2}$O$_{7}$/Al$_{2}$O$_{3}$/Ag planar junctions and used tunneling spectroscopy to study the density of states of Ca$_{3}$Ru$_{2}$O$_{7}$ from room temperature to 4.2K. Distinguish gap structure can be observed in all spectra at temperatures below 48K. The tunneling spectra behave very differently and the gap has dissimilar values along the three crystal axes directions. We have also studied the effect of magnetic field on these spectra by applying an external field up to 7T along the a-axis direction. In this presentation we will summarize our results and discuss their implications in relation to the competition between different interactions in the ground state of this material. This research was supported by NSF grants DMR-0800367, DMR-0856234 and EPS-0814194.   

Experimental studies of structure-property relationship in nine- and four-layer BaRuO$_{3}$

BaRuO$_3$ is a fascinating material as it possesses four different crystalline forms that possess rather different physical properties. We report results of our low-temperature magnetotransport measurements on single crystals of nine-layer rhombohedra (9R) and four-layer hexagonal (4H) BaRuO$_3$, two of the four different crystalline structures adopted by this material. Structurally a very short Ru-Ru distance was found in both 4H and 9R BaRuO$_3$, leading to metal-metal bonding, while 9R BaRuO$_3$ features three but 4H BaRuO$_3$ two face-sharing RuO$_6$ octahedra. For both 9R and 4H BaRuO$_3$, the magnetoresistance was found to become significant only below 30 K. More importantly, the magnetoresistance of 9R BaRuO$_3$ was found to be negative, while that of 4H BaRuO$_3$ positive. We suggest that the difference in the sign of magnetoresistance is associated with the difference in the two crystalline forms and electronic states of BaRuO$_3$, with only 9R BaRuO$_3$ featuring the previously proposed energy gap on certain part of the Fermi surface. Specific heat measurements are also being pursued to seek for additional insight into the physics of BaRuO$_3$. The work is supported by DOD ARO and NSF.   

Unusual Fermi surface reconstruction driven by moderate magnetic fields in Sr$_{4}$Ru$_{3}$O$_{10}$

Trilayered ruthenate Sr$_{4}$Ru$_{3}$O$_{10}$ exhibits puzzling magnetic properties. For field applied along the c-axis it exhibits typical itinerant ferromagnetic (FM) behavior, while moderate field (2-3T) applied within the \textit{ab}-plane can induce a metamagnetic (MM) transition [1]. Such coexistence of ferromagnetism and metamagnetism has been shown to be associated with a multiple band effect; FM bands derived from the $d_{xy}$ orbital coexists with MM bands from the $d_{xz,yz}$ orbitals [2,3]. In this talk, we report on Hall effect studies of this compound. Our results reveal that Fermi surface of Sr$_{4}$Ru$_{3}$O$_{10}$ changes dramatically through the MM transition. This Fermi surface change leads the Hall resistivity \textit{$\rho $}$_{xy}$ to strongly deviate from the scaling relation with magnetization, i.e. \textit{$\rho $}$_{xy}$ = $R_{0}H$ + 4$\pi R_{s}M$, which holds at fields well below and above the MM transition field. Such a significant change of the FS is unexpected for typical itinerant MM transitions.\\[4pt] [1] Cao \textit{et al}., Phys. Rev. B 68, 174409 (2003)\\[0pt] [2] J. Jo \textit{et al}., Phys. Rev. B 75, 094413 (2007)\\[0pt] [3] Fobes \textit{et al}., unpublished.   

Influence of spin-orbit coupling on the metamagnetic transition in Sr$_3$Ru$_2$O$_7$

Clean single crystals of Sr$_3$Ru$_2$O$_7$ undergo a metamagnetic transition at low temperatures. This transition shows a strong anisotropy in the applied field direction with a critical field $H_c$ ranging from $5.1$T for the case of $H\perp c$ to almost $8$T for $H\parallel c$. In addition, studies on ultra-pure samples revealed a bifurcation of the metamagnetic line for fields in $c$-direction and it is argued that a nematic phase emerges between the magnetization jumps. The aim of this study is to explain the field anisotropy of these phenomena. Based on a microscopic tight-binding model, we introduce the metamagnetic transition by means of a van Hove singularity scenario. We show that the rotation of the O-octahedra around the $c$-axis expected for this material introduces a staggered Rashba-like spin-orbit coupling within the planes and naturally leads to an anisotropy of the magnetic response. We describe the low-temperature phase as a nematic state favored by forward scattering processes. The spin-orbit coupling shows an influence on both, the critical field $H_c$ and the occurence of the nematic phase.   

XPS and ARPES study of the metal-insulator transition in Mn-substituted Sr$_3$Ru$_2$O$_7$

We have studied the metal-insulator transition in Mn-substituted Sr$_3$Ru$_2$O$_7$ by core-level x-ray photoemission (XPS) and angle-resolved photoemission spectroscopy (ARPES). In XPS, both the surface- and bulk-sensitive spectra show a two-peak structure, corresponding to the well screened and the unscreened excitations. The intensity of the well-screened peak is suppressed upon increasing the concentration of Mn, reflecting a metal-to-insulator transition induced by Mn impurities. In ARPES, changes in Fermi surface topology and band dispersions are observed as the system crosses over from a metal to a - possibly Mott - insulator. We observed a variation and enhancement of the Fermi-surface nesting upon Mn substitution, which might be connected to the emergence of the magnetic superstructure revealed by our resonant elastic soft x-ray scattering results [1].\\[4pt] [1] M.A. Hossain {\it et al.}, arXiv:0906.0035 (2009).   

Heavy d-Electron Quasiparticle Interference and Real-space Electronic Structure of $Sr_{3}Ru_{2}O_{7}$

The intriguing idea that strongly interacting electrons can generate electronic liquid crystalline phases is already a decade old, but these systems still represent an unexplored frontier of condensed matter physics. One reason is that visualization of the many-body quantum states generated by the strong interactions, and of the resulting nematic electronic phases, has not been achieved. $Sr_{3}Ru_{2}O_{7}$ possesses (i) a very strongly renormalized ``heavy'' d-electron Fermi liquid , and (ii) exhibits a field-induced transition to an electronic nematic phase. We present the first observation of scattering interference of heavy d-electron quasiparticles at individual Ti dopant atoms substituted in $Sr_{3}(Ru_{1-x}Ti_{x})_{2}O_{7}$, and the associated discovery that it derives from a band formed from the Ru $d_{xz}, d_{yz}$ orbitals recently postulated to be behind the formation of the nematic state. Simultaneously, we achieve direct sub-atomic imaging of r-space electronic structure which can be associated with these same d-orbitals within the many-body state.   

Magnetic Superstructure and Metal-Insulator Transition in Mn-Substituted Sr$_3$Ru$_2$O$_7$

We present a temperature-dependent resonant elastic soft x-ray scattering (REXS) study of the metal-insulator transition in Sr$_3$(Ru$_{1-x}$Mn$_x$)$_2$O$_7$, performed at both Ru and Mn $L$-edges. Resonant magnetic superstructure reflections, which indicate an incipient instability of the parent compound, are detected below the transition. Based on modelling of the REXS intensity from randomly distributed Mn impurities, we establish the inhomogeneous nature of the metal-insulator transition, with an effective percolation threshold corresponding to an anomalously low $x\sim 0.05$ Mn substitution. In collaboration with A.G. Cruz Gonzalez, J.D. Denlinger (Berkeley Lab), I. Zegkinoglou, M.W. Haverkort (MPI, Stuttgart), I.S. Elfimov, D.G. Hawthorn (UBC), R. Mathieu, S. Satow, H. Takagi (Tokyo), H.-H. Wu and C. Sch\"{u}\ss ler-Langeheine (Cologne).