Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session D56: Strontium and Calcium Ruthenates |
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Sponsoring Units: DCMP Chair: Sung-Kwan Mo, Lawrence Berkeley National Laboratory Room: Mile High Ballroom 2C |
Monday, March 2, 2020 2:30PM - 2:42PM |
D56.00001: Van Hove singularity and stress-induced Fermi surface tuning in Sr2RuO4 Aaron Chronister, Andrej Pustogow, Yongkang Luo, Yue-Shun Su, Andrew Mackenzie, Clifford W Hicks, Eric Bauer, Naoki Kikugawa, Stuart Brown Application of in-plane uniaxial stress to the quasi-2D correlated material Sr2RuO4 results in pronounced changes to the physical properties; most familiar is a factor 2.5 increase in superconducting transition temperature. The normal state of Sr2RuO4 is also impacted. Specifically, the crossover temperature to standard Fermi Liquid properties can be tuned from T=30 K to almost 0 K with -0.44% strain. These anomalous properties are associated with the proximity of the Fermi energy (EF) to a sharp singularity in the density of states, which can be tuned to EF and thus varies strongly on the scale of the Zeeman interaction and thermal energies. Reported here are 17O NMR hyperfine shifts over a wide range of stress, field, and temperature. Simple modeling of the results indicates that the difference between EF and the energy of the van Hove singularity plays a dominant role in the normal state properties for temperatures of order 300 K and below. |
Monday, March 2, 2020 2:42PM - 2:54PM |
D56.00002: Density wave at the reconstructed surface of Sr2RuO4 and implications for quantum criticality Carolina De Almeida Marques, Luke Rhodes, Veronica Granata, Rosalba Fittipaldi, Antonio Vecchione, Andreas Rost, Peter Wahl Many of the exciting properties of strongly correlated electron materials, including high-temperature superconductivity, are intricately linked to proximity to a quantum phase transition, a transition through which a material can be tuned as a function of an external control parameter other than temperature. Here, we propose that the reconstructed surface layer of Sr2RuO4 constitutes an ideal material to study the physics of quantum criticality. |
Monday, March 2, 2020 2:54PM - 3:06PM |
D56.00003: QSGW+DMFT+BSE description of superconductivity in Sr2RuO4 Swagata Acharya, Dimitar Pashov, Mark Schilfgaarde The nature of pairing in Sr2RuO4 continues to be widely debated, in particular, the possibility of a triplet origin of Cooper pairs. Its complexity, with multiple low-energy scales involving subtle interplay among spin, charge and orbital degrees of freedom, calls for advanced theoretical approaches which treat on equal footing all electronic effects. We present a novel approach, a detailed \emph{ab initio} theory, coupling quasiparticle self-consistent GW approximation with dynamical mean field theory (DMFT). We report that the superconducting instability has multiple triplet and singlet components. In the unstrained case the triplet eigenvalues are larger than the singlets. Under uniaxial strain, the triplet eigenvalues drop rapidly and the singlet components increase. This is concomitant with our observation of spin and charge fluctuations shifting closer to wave-vectors favoring singlet pairing in the Brillouin zone. We identify a complex mechanism where charge fluctuations and spin fluctuations co-operate in the even-parity channel under strain leading to increment in Tc, thus proposing a novel mechanism for pushing the frontier of Tc in unconventional "triplet" superconductors. |
Monday, March 2, 2020 3:06PM - 3:18PM |
D56.00004: Non-monotonic temperature dependence of the Hall coefficient in Hund’s metals Manuel Zingl, Jernej Mravlje, Markus Aichhorn, Olivier Parcollet, Antoine Georges A non-monotonic temperature dependence of the Hall coefficient has been reported for several strongly correlated materials, e.g. Sr2RuO4, Sr3Ru2O7, LiFeAs and FeSe [1]. One common signature of these Hund’s metals is a strong orbital selectivity of electronic correlations. On the example of Sr2RuO4, where the Hall coefficient exhibits two sign reversals, we show that the behavior of the Hall coefficient is directly linked to the temperature dependence of the ratio of inelastic scattering rates between the different correlated orbitals [2]. In our picture the sign reversals reflect two important crossovers in the physics of this material: (I) from a high-T incoherent to a coherent regime at low-T associated with a remarkably large ratio of scattering rates, and (II) from inelastic to impurity-dominated scattering. This qualitative picture is supported by quantitative calculations using Boltzmann transport theory in combination with dynamical mean-field theory, taking into account the effect of spin–orbit coupling. |
Monday, March 2, 2020 3:18PM - 3:30PM |
D56.00005: Fluctuation stablised spin-density-wave order and the phase diagram of Sr3Ru2O7 Stephen Hayden, Richard Waite, Chris Lester, Robin Perry, Silvia Ramos, Dmitry Khalyavin, Fabio Orlandi, Pascal Manuel Sr2Ru3O7 is a unique metal in which a magnetic field of about 8 Tesla can induce spin density wave (SDW) order [1]. There are two SDW ordered phases (A and B) with different incommensurate wavevectors. Here we use magnetic neutron diffraction to characterise the magnetic order as a function of magnetic field strength and direction. The order is observed for all field directions investigated. Our results suggest that the SDW is closely linked to Fermi surface nesting and is probably stabilised by magnetic fluctuations. Thus the SDW order explains many (magnetic, transport and thermal) anomalies in this material. |
Monday, March 2, 2020 3:30PM - 3:42PM |
D56.00006: Interplay between Spatially Modulated Nematic and Spin-Density Wave Order in Sr3Ru2O7 Jonathan Clepkens, Hae-Young Kee The anisotropic resistivity observed in Sr3Ru2O7 [1] has led to a variety of theoretical studies proposing microscopic routes to a nematic phase in the material. However, the discovery of spin-density wave (SDW) order in the same region of phase space [2] has cast some doubt on this picture, even though there are open issues that cannot be understood within the SDW scenario. Here we study the interplay between spatially modulated nematic and SDW order from a microscopic viewpoint. Mean-field results illustrating the formation of the finite momentum nematic and SDW order will be presented. Implications towards the importance of bilayer coupling, spin-orbit coupling and the presence of a van-Hove singularity in Sr3Ru2O7 will also be discussed. |
Monday, March 2, 2020 3:42PM - 3:54PM |
D56.00007: Metal-Insulator and Magnetic Phase Diagram of Ca2RuO4 from Auxiliary Field Quantum Monte Carlo and Dynamical Mean Field Theory Hao Shi, Hongxia Hao, Antoine Georges, Andrew Millis, Brenda M Rubenstein, Qiang Han Layered perovskite ruthenium oxides exhibit a striking series of metal-insulator and magnetic-nonmagnetic phase transitions easily tuned by temperature, pressure, epitaxy, and nonlinear drive. In this work, we combine results from two complementary states of the art many-body methods, Auxiliary Field Quantum Monte Carlo and Dynamical Mean Field Theory, to determine the low-temperature phase diagram of Ca2RuO4. Both methods predict a low-temperature pressure-driven metal-insulator transition coincident with a structural transition and accompanied by a ferromagnetic-antiferromagnetic transition. The properties of the ferromagnetic state are dominated by the ruthenium xy orbital while properties of the antiferromagnetic state are dominated by the xz and yz orbitals. Differences of detail in the predictions of the two methods are analyzed. The work is theoretically important as an application of the auxiliary field quantum Monte Carlo method to an orbitally degenerate system with both Mott and Hunds physics and provides an important comparison of the dynamical mean-field and auxiliary field quantum Monte Carlo methods. |
Monday, March 2, 2020 3:54PM - 4:06PM |
D56.00008: Spin-orbit interpretation of the Higgs mode in Ca2RuO4 Paul Sarte, Chris Stock, Brenden Ortiz, Stephen Wilson The role played by spin-orbit coupling in the determination of a system's magnetism has recently been the subject of intense study, with particular interest being placed on magnets based on Ru4+ and Ir4+, revealing a wealth of novel emergent phenomena. Stemming from the complex interplay of comparable values for J and λ, the parameterization for such phenomena has proven challenging. One particular example corresponds to the modeling of the low energy magnetic fluctuations, where conventional approaches often require complicated phenomenological Hamiltonians. Using the 4d layered antiferromagnet Ca2RuO4 as a case study, we demonstrate that a mean-field multilevel spin-orbit exciton model employing a minimalist Hamiltonian captures the main features of the low energy magnetic excitation spectrum. Corresponding to a tetragonally-distorted A-centered antiferromagnetic unit cell, the model accounts for the softening and longitudinal polarization of the Higgs mode. The success of such a minimalist Hamiltonian in capturing such a rich spectrum for a 4d magnet, suggests that the spin-orbit exciton model may be a suitable alternative to traditional approaches. |
Monday, March 2, 2020 4:06PM - 4:18PM |
D56.00009: Strain-mediated Mott transition in Ca2RuO4 induced by a dc current Alfred Zong, Anshul Kogar, Hengdi Zhao, Qian Li, Yifan Su, Samuel D Marks, Gang Cao, Haidan Wen, Nuh Gedik In a Mott insulator, the delicate balance among several competing energy scales gives rise to a plethora of ground states, which may be tuned into one another by pressure, electric field, photoexcitation among other perturbations. Owing to many degrees of freedom present in the system, understanding Mott transitions remains difficult. Recently, Ca2RuO4 has emerged as a new platform for studying the insulator-to-metal Mott transition. A dramatic change in its structural and optical properties across the phase boundary allows a multimodal approach to investigate the transition in space and time. Here, we report in situ transport, optical, and X-ray microscopy measurements on current-induced Mott transition in Ca2RuO4. We find that lattice strain plays a central role in this spatially-inhomogeneous transition, whose temporal evolution is characterized by local fluctuations in temperature. The present work clarifies important aspects of the current-induced Mott transition and provides insights into future applications that harness such electromagnetic control. |
Monday, March 2, 2020 4:18PM - 4:30PM |
D56.00010: Tailoring the ferromagnetic easy axis of Ca2RuO4 via epitaxial strain Ludi Miao, Hari Nair, Nathaniel Schreiber, Jacob Ruf, Matthew Fu, Yonghun Lee, Celesta Chang, Jacob Ruff, David Muller, Darrell Schlom, Kyle M Shen Magnetism in strongly correlated ruthenates has been a central topic in condensed matter physics, where the spin degree of freedom is has a strong interplay with lattice, charge and orbital degrees of freedom. For instance, the Mott insulator Ca2RuO4 (CRO) can be turned into a ferromagnetic metal under epitaxial strain or hydrostatic pressure. Here, we have grown coherently strained CRO thin films on various substrates including SrLaAlO4 and YAlO3 via molecular beam epitaxy and investigated the strain effect on their magnetic properties. We have used angle-resolved magnetoresistance measurements to characterize the ferromagnetic easy axes of these films, and found that they are also drastically different from CRO bulk, and show how the magnetic easy axes can be tuned as a function of epitaxial strain. Our results manifest the complex interplay between the spin, lattice and orbital degree of freedom in CRO, as well as demonstrate a practical approach to tailor the magnetic properties in strongly correlated oxide materials via epitaxial strain, in general. |
Monday, March 2, 2020 4:30PM - 4:42PM |
D56.00011: Imaging the current-driven metal–insulator transition in Ca2RuO4 Giordano Mattoni, Shingo Yonezawa, Fumihiko Nakamura, Yoshiteru Maeno The flow of electric current is a powerful tool to alter the ground state of strongly correlated ruthenates, where novel material properties emerge in non-equilibrium steady states [1,2,3]. Despite the efforts to investigate these phenomena, the microscopic mechanism and the physical quantities responsible for the current-induced phase changes have not been clarified yet. |
Monday, March 2, 2020 4:42PM - 4:54PM |
D56.00012: Pump-probe Nano-spectroscopy of Mott Insulating Ca2RuO4 Rocco Vitalone, Aaron Sternbach, Ben Foutty, Alexander McLeod, Chanchal Sow, Fumihiko Nakamura, Yoshiteru Maeno, Abhay Pasupathy, Dmitri Basov Ca2RuO4 is a Mott Insulator that exhibits a temperature-induced insulator to metal transition occurring slightly above room temperature. Further, Ca2RuO4 exhibits a current-driven IMT at room temperature and current-induced diamagnetism below 50K. These discoveries, particularly the latter, imply that the application of current can expose a previously hidden state in Ca2RuO4. Motivated by the current induced transition, we conducted nanoscale pump-probe measurements to explore the phase diagram further. In our experiments, we study both the static and photo-induced response with nano-FTIR measurements to interrogate the dynamics of the spectral response throughout the mid-IR. The static spectra reveal a large phonon resonance of the in-plane stretching mode of the Ru-O bonds in the insulating state, while the metallic state demonstrates a flat, Drude like response. The photo-induced spectra reveal an initial suppression of the phonon mode and a noticeable increase in the Drude response at later times. We interpret the response as an initial injection of charge carriers that screen the phonon mode while their thermalization at later times leads to the Drude-like response. |
Monday, March 2, 2020 4:54PM - 5:06PM |
D56.00013: Magnetic phase tuning in Ca$_2$Ru$_{1-x}$Fe$_x$O$_4$: A structural perspective Songxue Chi, Feng Ye, Gang Cao, Huibo Cao, Jaime Fernandez-Baca The crystalline and magnetic structures of Ca$_2$Ru$_{1-x}$Fe$_x$O$_4$ (x=0.02, 0.05, 0.08 and 0.12) have been studied using neutron scattering and X-ray diffraction. The Fe-doping reduces the Ru-O bond length in both apical and planar directions. The smaller Ru(Fe)O$_6$ octahedra leads to its reduced distortion. The $Pbca$ space group is maintained and so is the apical flattening. Warming has similar effect as Fe-doping in releasing the distorted octahedra except an abrupt change of flattening across the Neel temperature in x=0.08. Two types of antiferromagnetic orders, $A$- and $B$-centered phases with the same inplane spin arrangement but different stack sequences, have been found. The Fe induced structure with relaxed octahedra seems to prefer the $B$-phase to the $A$-centered one, which is systematically overwhelmed and replaced as the Fe-doping increases. The nature of the magnetic phase competition and its connection to its host structure is discussed. |
Monday, March 2, 2020 5:06PM - 5:18PM |
D56.00014: Emergence of Competing Stripe Phase near the Mott Transition in Ti-doped Bilayer Calcium Ruthenates Ashish Gangshettiwar, Yanglin Zhu, Jin Peng, Yu Wang, Zhanzhi Jiang, Zhiqiang Mao, Keji Lai The physics of nanoscale phase separation is at the heart of strongly correlated materials, where multiple degrees of freedom such as charge, spin, lattice, and orbital are simultaneously active. Using microwave impedance microscopy, we spatially resolved the coexisting phases on a Ca3(Ru0.9Ti0.1)2O7 bulk crystal during the metal-insulator transition. Different from a typical first- order phase transition where coexistence of the two terminal phases takes place, a new stripe phase oriented along the in-plane crystalline axes emerges inside both the G-type antiferromagnetic insulating state and paramagnetic metallic state. The effect of this electronic state can be observed in macroscopic measurements, allowing us to construct a phase diagram that takes into account the energetically competing phases. Our work provides a model approach to correlate macroscopic properties and mesoscopic phase separation in complex oxides. |
Monday, March 2, 2020 5:18PM - 5:30PM |
D56.00015: Nano-resolved insulator-metal domain textures in a polar bilayer ruthenate Alexander McLeod, Ran Jing, Jedrzej Wieteska, Leixin Miao, Ben Foutty, Rui Zu, Giuliano Chiriaco, Qiang Han, Danilo Puggioni, James Rondinelli, Andrew Millis, Venkatraman Gopalan, Zhiqiang Mao, Nasim Alem, Abhay Pasupathy, Dimitri Basov The 4d transition metal oxides of the Can+1RunO3n+1 perovskite family have recently garnered interest for their correlated electron physics and strong sensitivity to external stimuli like strain, temperature, and even electric current. The bilayer ruthenate Ca3Ru2O7 exhibits a structural distortion producing a polar metal and, under Ti substitution for Ru, a correlated antiferromagnetic insulator. Through low-temperature nano-infrared imaging, we reveal a spontaneous striped texture of coexisting insulating and metallic domains in single crystals across their insulator-metal phase transition at T=70-100K. Under in situ uniaxial strain, we image anisotropic nucleation and growth of these domains, rationalized through on-demand control of a spontaneous Jahn-Teller distortion. Through high resolution transmission electron microscopy, we also reveal the detailed interplay between this textured phase coexistence and the displacive orientations, volumetric structures, and domain boundaries among polar twin domains in these crystals. These novel imaging methods afford new insights into strain- and structure-mediated manipulation of the insulator-metal transition in 4d metal oxides and the ubiquity of phase coexistence even in pristine single crystals. |
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