Session H9: Focus Session: Complex Bulk Oxides: Ruthenates

Nonlinear conduction phenomena in the Mott insulator Ca$_{2}$RuO$_{4}$

The 4$d$-electron Mott insulator Ca$_2$RuO$_4$ has attracted considerable attention because of its rich electronic states dramatically varied by temperature change, pressure, and isovalent Sr substitution. Recently, Nakamura {\it et al}. reported an intriguing result of current-voltage characteristics in Ca$_2$RuO$_4$ and found an $E$-induced insulator-to-metal transition caused by relatively low electric field $E$ $\sim$ 50 V/cm [1]. In this study we investigate the nonlinear conduction phenomena in the Mott insulating phase of Ca$_2$RuO$_4$ with a proper evaluation for self-heating effects. By utilizing a non-contact infrared thermometer, sample temperature was accurately determined even in the presence of large Joule heating. The resistivity shows a typical insulating behavior featured by thermal activation with an energy gap, but clearly depends on the applied currents. The result is highlighted by a strong suppression of the energy gap by the electrical currents. A striking similarity to the current dependence of charge-order gap in organic insulators is discussed in terms of the nonequilibrium phase transition. \\[4pt] [1] F. Nakamura {\it et al.}, (submitted).   

Emergent electronic and magnetic state in Ca$_{3}$Ru$_{2}$O$_{7}$ induced by Ti doping

We report an emergent electronic and magnetic state in the bilayer ruthenate Ca$_{3}$Ru$_{2}$O$_{7}$ upon doping with a small concentration of Ti on the Ru sites. In contrast to a quasi-two-dimensional metallic state in Ca$_{3}$Ru$_{2}$O$_{7}$, which has an antiferromagnetic state formed by ferromagnetic bilayers stacked antiferromagnetically along the c-axis [1,2], we find an insulating state with a ``G''-type nearest neighbor antiferromagnetic order in Ca$_{3}$(Ru$_{1-x}$Ti$_{x})_{2}$O$_{7}$ for $x$ $>$= 0.03 [3]. The close proximity of these two distinct electronic and magnetic states demonstrates unique competing magnetic interactions in Ca$_{3}$Ru$_{2}$O$_{7}$, which provides a rare opportunity to investigate the interplay between correlated metal physics and Mott physics. Work supported by U.S. DOE. \\[4pt] [1] W. Bao \textit{et al}., Phys. Rev. Lett. \textbf{100}, 247203 (2008).\\[0pt] [2] X. Ke \textit{et al}., Phys. Rev. B \textbf{84}, 014422 (2011).\\[0pt] [3] X. Ke \textit{et al}., Phys. Rev. B \textbf{84}, 201102 (R) (2011).   

From quasi-2D metal with ferromagnetic bilayers to Mott insulator with G-type antiferromagnetic order in Ca$_{3}$(Ru$_{1-x}$Ti$_{x}$)$_{2}$O$_{7}$

Ca$_{3}$Ru$_{2}$O$_{7}$ exhibits unique electronic and magnetic properties, such as giant magnetoresistance, a quasi-2D metallic ground state, and antiferromagnetic (AFM) order comprising of ferromagnetic bilayers coupled antiferromagnetically along the c-axis [1-3]. In this talk, we will show that only a few percent of Ti-doping at Ru sites can tune the ground state to a Mott-insulating state with ``G''-type, nearest neighbor AFM order [4]. We have established the electronic and magnetic phase diagram of this doped system to address the underlying physics of such a Mott-transition. We find that a strong scattering effect due to the Ti ions'' empty 3d orbital significantly reduces electrons'' itinerancy, playing a pivotal role in the suppression of the bilayer ferromagnetism and inducing the Mott transition. These findings imply that Ca$_{3}$Ru$_{2}$O$_{7}$ involves competition between the antiferromagnetism due to the Mott transition and the itinerant ferromagnetism due to a Stoner instability. [1] X.N. Lin et al., Phys. Rev. Lett. 95, 017203(2005). [2] W. Bao et al., Phys. Rev. Lett. 100, 247203 (2008). [3] Y. Yoshida et al., Phys. Rev. B 69, 220411 (R) (2004). [4] X. Ke et al., Phys. Rev. B 84, 201102 (R) (2011).   

Orbital selective phase transition

Orbital selective phase transition (OSPT), proposed to be responsible for the coexistence of localized and itinerant electrons, has attracted extensive interest from both experimentalists and theoreticians, since the observation of an anomalous behavior with a Curie-Weiss-like local spin in the metallic phase of Ca$_{2-x}$Sr$_x$RuO$_4$ at $0.2 \leq x \leq 0.5$. Recently, even more attentions have been paid to OSPT since the coexistence of localized and itinerant electrons may reconcile the strong debates on how to understand the origin of magnetism in various iron-based superconductors. Here, various mechanisms for OSPT are reviewed and a new mechanism will be proposed. The distinct band dispersion of different orbitals, which should be generally satisfied in various materials, is identified to be the crucial point for OSPT with magnetic order. Such an OSPT are not sensitive to the strength of Hund's rule coupling. Heavy doping favors collinear antiferromagnetic state over the OSPT. Discussions are made related to the pnictides.   

Magnetic order arising from chemical chaos in A2MnRuO6 (A = Sr, Ca) double perovskites

Experimentally Sr$_{2}$MnRuO$_{6}$ is observed to be a c-type antiferromagnetic insulator with a tetragonal structure, while Ca$_{2}$MnRuO$_{6}$ is found to be a metallic ferrimagnet with an orthorhombic structure. Both compounds display magnetic ordering even in the absence of any recognizable chemical ordering of Mn and Ru ions. In this work, we present first principles calculations to show that the change in properties of the two compounds is only a consequence of pressure and hence can be tuned either by epitaxial growth or by alloying at A-site with Sr$^{2+}$ or Ca$^{2+}$ ions. We find that the Mn $e_{g}$-states are highly sensitive to pressure. Compression raises their energy in Sr$_{2}$MnRuO$_{6}$, accompanied by a transfer of electrons from the eg states to the Ru-Mn hybridized $t_{2g}$-states. This results in a transition from c-type antiferromagnetism to ferrimagnetism. Ferrimagnetic ordering in-turn allows for greater delocalization of the up-spin Ru $t_{2g}$-electrons, hence increasing the conductivity. We also show that in the presence of disorder, the Mn$_{Ru}$ antisites couple ferromagnetically with their neighboring Mn$_{Mn}$ sites. This allows for the interesting possibility of preserving highly spin-polarized conduction even in the absence of chemical ordering.   

Surface dynamics and electronic properties of parent and Mn doped Sr$_{3}$Ru$_{2}$O$_{7}$

High resolution Electron Energy Loss Spectroscopy has been utilized to measure the temperature dependence of the low energy excitations at the surface of cleaved Sr$_{3}$(Ru$_{1-x}$Mn$_{x})_{2}$O$_{7}$ single crystals (x = 0, 0.16). Four loss peaks are observed and assigned as the A$_{1g}$(2), A$_{2u}$(3), A$_{2u}$(2), and A$_{1g}$(1) vibration modes. The continuous electronic excitation spectra, Drude tail, shows that the parent compound is metallic at all temperatures investigated (80K to RT). But the A$_{1g}$(1) mode splits into two peaks between 145K and 210K indicating a structural transition at the surface. For the 16{\%} Mn doped samples the bulk is insulating and antiferromagnetic below 160K. In contrast, the surface is always metallic. Upon cooling from RT the A$_{1g}$(1) mode hardens with its width broadening from RT to 160K, and then softens and narrows quickly until 80K. The Drude tail exhibits similar behavior. Evidently the presence of the surface suppresses AF ordering and kills the insulating phase.   

The Physics of Hunds metals and its relevance for ruthenades, iron pnictides and chalchogenides

I will discuss the physics of Hund's metals. In these systems the Coulomb interaction among the electrons is not strong enough to fully localize them, but it significantly slows them down, such that low-energy emerging quasiparticles have a substantially enhanced mass. This enhanced mass emerges not because of the Hubbard interaction U, but because of the Hund's rule interactions J that tend to align electrons with the same spin but different orbital quantum numbers when they find themselves on the same atom. I will show a few examples of such Hund's metals, including Sr2RuO4, iron pnictides and iron chalchogenides materials. The electronic structure, computed by the Dynamical Mean Field Theory in combination with Density Functional Theory, successfully reproduces several experimental results and explains the key properties of these material: such as the mass renormalizations and anisotropy of quasiparticles, the crossover into an incoherent regime above a low temperature scale, the magnetic moments in iron compunds, etc. While at very low temperature our simulations predict these materials to be Fermi liquids, at finite temperature they strongly deviate from Fermi liquid prediction and can be characterized by self-energy which follows a powerlaw, with non-integer exponents. The origin of this non-Fermi liquid fixed point will be discussed.   

Anomalous $E-$type Antiferromagnetism in the ground state of Mn-substituted Sr$_{3}$Ru$_{2}$O$_{7}$

The bi-layer perovskite, Sr$_{3}$Ru$_{2}$O$_{7}$, has sparked a lot of interest because of the quantum critical behavior---related to a metamagnetic (magnetic field-tuned) phase transition. One of the key issues is related to the magnetism in the system. Here we report an investigation of the effects on magnetism resulting from chemical substitution in this compound. Our neutron scattering investigation reveals an unusual $E-$type antiferromagnetic (AFM) structure induced by Mn-substitution in the ground state of Sr$_{3}$(Ru$_{1-x}$Mn$_{x})_{2}$O$_{7}$ ($x$ = 0.16). The AFM structure exhibits a long-range order in \textit{ab}-plane but almost only a single bilayer-thickness correlation along the $c$-direction, thus characterizing the system as a quasi-two-dimensional antiferromagnet while the AFM order parameter shows almost three-dimensional-like scaling character as $T$ approaches $T_{N}$ ($\sim $ 82 K). The magnetic moments are aligned along the c-axis with an upper limit of $\sim $ 0.70 \textit{$\mu $}$_{B}$/Ru site. The induced AFM order most likely results from the enhancement of super-exchange interactions rather than from structural distortions or from freezing of electronic instabilities due to the nesting character of Fermi surface in the parent compound.   

Anisotropy of magnetoresistivities in Sr$_{3}$Ru$_{2}$O$_{7}$: Evidence for orbital-dependent metamagnetism

Sr$_{3}$Ru$_{2}$O$_{7}$ has been studied extensively due to its rich electronic and magnetic ground state properties, such as its quantum criticality and electronic nematic phase [1,2]. In this talk we will present the results of in-plane angle-resolved directional magnetotransport anisotropy measurements, a technique we used previously to elucidate the orbital-selective nature of the itinerant metamagnetism in Sr$_{4}$Ru$_{3}$O$_{10}$ [3]. We find that the \textit{c}-axis magnetoresistivity anisotropy undergoes a drastic change in symmetry from fourfold to twofold through the metamagnetic transition, consistent with the behavior expected for the strong spin polarization. In contrast, the in-plane magnetoresistivity anisotropy remains fourfold through the transition accompanied by only a gradual shift in phase, and only trends towards twofold symmetry at fields well above the transition. These findings suggest the $d_{xz,yz}$ bands, should play a pivotal role in the metamagnetic transition.\\[4pt] [1] S. Grigera \textit{et al}., Science \textbf{294}, 329 (2001)\\[0pt] [2] R. Borzi \textit{et al}., Science \textbf{315}, 214 (2007)\\[0pt] [3] D. Fobes \textit{et al}., Phys. Rev. B \textbf{81}, 172402 (2010)   

Study of an electronic nematic with precise control of the applied magnetic field vector

The layered perovskite metal Sr$_{3}$Ru$_{2}$O$_{7}$ has gained considerable interest since the discovery of its field tuned quantum criticality [1] and the subsequent discovery of a new electronic phase with a high magnetoresistive anisotropy, consistent with the existence of an electronic nematic fluid [2]. This anisotropy may be oriented by applying a moderate field ($H_{ab}$) in the plane of the RuO layers. The study of the behaviour of the electronic nematic state requires precise control over both the magnitude and direction of $H_{ab}$. For this purpose, we operate a 3-axis 9/1/1 tesla vector magnet, which offers full control of the magnetic field vector with a high degree of precision. Here, we present recent magnetotransport data for Sr$_{3}$Ru$_{2}$O$_{7}$ measured in the vector magnet. We confirm the two-fold to four-fold rotational symmetry breaking and show that it occurs even in the limit of small values of $H_{ab}$. Additionally, we address pinning of the anisotropy underlying crystal lattice. Finally, we show the dependence of the anisotropy on magnetic field and temperature, which may help explain its origin at the microscopic scale. \\[4pt] [1] S. A. Grigera \textit{et al}., \textit{Science }\textbf{294}, 329 (2001) [2] R. A. Borzi \textit{et al}., \textit{Science} \textbf{315}, 214 (2007)   

Optical Polarization Microscopy of the Electron Nematic Phase in Sr$_3$Ru$_2$O$_7$

We report the implementation of a fiber-based optical microscope, capable of operating at temperatures below 100 mK and in magnetic fields in excess of 9 Tesla, with sub-micron spatial resolution. This microscope is integrated into the bore of a dilution refrigerator with an optical fiber coupling light to an external optical table. Bench-top optical elements allow for polarization analysis of the reflected light from a surface and thus the detection of magnetic or other polarization-sensitive properties of mater at low temperature and high fields. As a first application of the instrument, we are studying the proposed electron nematic phase of the n=2 Ruddlesden-Popper material Sr$_3$Ru$_2$O$_7$, which exhibits a low-temperature phase transition in the form of an in-plane conduction anisotropy. We plan to detect this phase optically by analyzing the polarization rotation of the reflected light through the phase boundary, with the aim of imaging domain structure in the nematic phase.   

Suppression of An Antiferromagnetic Insulating Phase in Sr$_{3}$(Ru$_{1-x}$Mn$_{x})_{2}$O$_{7}$ by Magnetic Field

Double-layered Sr$_{3}$Ru$_{2}$O$_{7 }$is a paramagnetic metal. The partial substitution of Mn for Ru results in metal-insulator transition at T$_{MIT}$ and antiferromagnetic ordering at T$_{M}$ in Sr$_{3}$(Ru$_{1-x}$Mn$_{x})_{2}$O$_{7. }$Interestingly, both T$_{MIT}$ and T$_{M}$ can be easily suppressed by the application of magnetic field, especially for low-doping compounds (x $<$ 0.1). This behavior can be explained as Mn-doping-induced antiferromagnetic-insulating domains below T$_{MIT}$. The application of magnetic field suppresses the antiferromagnetic coupling, thus converting the insulating domains back to metallic.   

The Orbital-Selected Mott Phase of the Nondegenerate Two-Orbital Hubbard Model

We use the dynamical mean-field theory to study the optical conductivity and orbital susceptibility of the nondegenerate two-orbital Hubbard model in the orbital-selective Mott phase. The optical conductivity of the wide band presents expectedly a nonzero Drude peak, while the localization character is observed for the optical conductivity of the narrow band. Particularly, a rapidly reverse shape in the orbital susceptibility emerges right at Fermi surface, implying the coexistence of the orbital ordering with the orbital-selected Mott phase. We also find that the orbital-selected Mott transition can be suppressed by the negative crystal field splitting. Applying the present findings to compound Ca$_{2-x}$Sr$_{x}$RuO$_{4}$, we demonstrate that the orbital-selected Mott phase can not survive in wide doping region from $x$=0.2 to 2.0.