Session H07: Ruthenates and Nickelates

Current-controlled switching of diamagnetism in Ca3(Ru1-xTix)2O7

It was recently shown that a strong Landau diamagnetism may emerge in strongly correlated electron systems (SCES) under nonequilibrium steady state (NESS) conditions. However, due to high resistivity of the insulating state, in situ sign-switching of magnetism was not possible in the previous study. Here, we demonstrate that DC current can trigger in situ switching between the impurity-induced Mott insulating state and diamagnetic semimetallic state of the bilayered ruthenate Ca₃Ru₂O₇. By performing simultaneous magnetic and resistive measurements, we map out the temperature vs current-density phase diagram in the NESS of this material. The present results open up the possibility of creating novel electronic states in a variety of SCES by NESS conditions under DC current.   

How does optical response of SrRuO3 and CaRuO3 thin films deviate from Fermi liquid predictions?

Orthorhombic pervoskites SrRuO₃ and CaRuO₃ are strongly correlated metals with unusual transport properties. Previous optics studies in the infrared and terahertz range shows non-Drude dynamics and fractional scattering rates at low temperatures which seems to contradict a Fermi-liquid picture with long-lived quasiparticles. Here we present time domain THz measurements of clean thin films of SrRuO₃ and CaRuO₃. Our results demonstrate for both materials the low temperature conductivity shows a narrow Drude-like peak and can be fitted with two Drude terms. The conductivity crosses over to become less coherent at higher temperatures. Comparison with Fermi liquid scaling theories shows that CaRuO₃ is more anomalous than SrRuO₃ probably owing to spin fluctuations in the paramagnetic phase.   

17O NMR Studies Applied to the Uniaxial Stress-Tuned Fermi-Liquid Crossover in Sr2RuO4

Application of in-plane uniaxial stress to the quasi two-dimensional strongly correlated system Sr₂RuO₄ results in substantial changes of the physical properties, including a more than doubled superconducting transition temperature as well as normal state transport properties deviating from conventional Fermi-liquid behavior. Both observations are attributed to a nearby Lifshitz transition, associated with moving the Fermi energy through a van Hove singularity (vHs). ¹⁷O NMR measurements reveal that the Lifshitz point is accompanied by strong field and temperature dependence of the spin susceptibility, persisting to temperatures less than 100 mK. Our NMR measurements indicate that the non-Fermi-liquid behavior is strongly influenced by the proximity of the Lifshitz transition and a large Stoner enhancement, in a system where three energy scales are nearly balanced: width of vHs, as well as Zeeman and thermal energies.   

Collective modes in Sr2RuO4 measured with momentum-resolved EELS (M-EELS)

Measurements of the charge susceptibility χ(q,ω) of the high-T_c cuprate Bi-2212 using Momentum-resolved inelastic electron scattering (M-EELS) have shown that bad-metallic behavior can be characterized by a continuum of non-propagating density fluctuations, in stark contrast to the plasmons of simple metals [1]. Here, we report on M-EELS measurements of Sr₂RuO₄, a bad metal which crosses over to a Fermi Liquid at T=40K. Like Bi-2212, we find that the bad metal phase of Sr₂RuO₄ exhibits a continuum of density fluctuations. However, below 40K, spectral weight is redistributed to form a plasmon-like excitation around 1.4 eV. Surprisingly, we also discover a coherent low-energy (sub-80meV) 2D acoustic plasmon in the bad metal phase, whose velocity is strongly renormalized from 0.7 to 0.4eV Å below the Fermi liquid crossover, indicating a coupled 2D Fermi Liquid and 3D bad metal in a single solid. We will contrast this coupled system to a closely related ruthenate, the Mott-insulator Ca₂RuO₄.

[1] M. Mitrano, A. A. Husain, et al. PNAS 115 (21) 5392-5396 (2018)   

Magnetisation density and electronic structure of Sr3Ru2O7.

Sr3Ru2O7 is itinerant metamagnet, exhibiting a sharp increase in magnetisation in a field of 5.5 – 7.7 T (depending on the applied field direction) at temperatures around 1 K. This metamagnetic transition is thought to be related to the behaviour of the density of states near the Fermi level. We have performed a study of the magnetisation density in the metamagnetic state combining a magnetic Compton scattering experiment with electronic structure calculations. Our initial density functional calculations were fixed to reproduce the value of the experimentally determined spin moment, but do not agree well with the experimentally measured spin density. In the talk, we will show how it is possible to gain a good agreement with the experimental spin densities from the calculations and the consequences of this in relation to the electronic structure and underlying physics.   

DMFT+NRG study of orbital differentiation in three-orbital Hubbard models

The orbital-selective Mott phase (OSMP) in multi-orbital Hubbard models has been of high interest for a long time. While it has early been understood [1] that different bandwidths of the orbitals can lead to different critical interaction energies, the OSMP solely driven by crystal-field splitting (CFS) is more intricate. Here, we present a real-frequency dynamical mean-field theory and numerical renormalization group (DMFT+NRG) analysis of the OSMP driven by CFS in a three-orbital model and illustrate the singular Fermi-liquid properties of this phase [2]. On the example of Sr₂RuO₄, we show how our method can be used to study low-energy physics in a real-materials framework.

[1] V.I. Anisimov et al., EPJB 25, 191-201 (2002)
[2] M. Greger et al., arXiv:1312.0100 (2013)   

Multi-Wavelength Near-Field Imaging of the Temperature & Current Induced Metal to Insulator Transitions in Ca2RuO4

Ca₂RuO_4, a strongly correlated metal oxide and Mott Insulator, exhibits an insulator to metal transition and a large structural distortion along the c-axis concurrently at 365K with a hysteresis of about 30K. In this study, we examine the temperature driven insulator-metal transition (IMT) in single crystals of Ca₂RuO₄ at the nanoscale with wavelengths ranging from 10 µm to 16 µm, where a strong phonon resonance characterizes the insulating phase. Through this study, we clearly resolve the phonon mode at 16 µm among insulating domains coexisting with the metallic phase, in accord with the far-field optical response of this material. We also resolve a novel pattern of striped phase coexistence emerging on the sub-micron scale that we attribute to the large strain induced by the structural difference between the metallic and insulating phases. We discuss how the development of this stripe pattern spontaneously minimizes strain energy within the crystal. Our interrogation of the metallic and phonon responses of these coexisting phases provides nano-imaging evidence that the temperature and current driven IMTs proceed through fundamentally different microscopic mechanisms.   

Effect of uniaxial strain on the metal-insulator transition in Ti-substituted Ca3Ru2O7

We present results of electrical transport studies on the metal-insulator transition (MIT) in 10% Ti-substituted Ca3Ru2O7 under uniaxial strain. We observe a large (~10K) modulation of the Mott transition temperature under application of high (<1%) ab-plane uniaxial strain. For low strains we find an anomaly across T_{MIT} in the linear response elastoresistive susceptibility. Complementary scanning near-field optical microscopy measurements have been performed on the same samples.   

Novel uniaxial strain device for transport and scanning probe experiments

In recent years uniaxial strain has emerged as an important probe of condensed matter systems, coupling to phenomena such as nematicity [1], superconductivity [2], magnetism [3], and metal insulator transitions [4]. Existing methods for in-situ application of high strains generally have been based off of extension piezoelectric actuators [5]. We present a method of strain application that relies on stacks of shear piezoelectric actuators to apply up to 1% uniaxial extension and compression at cryogenic temperatures homogeneously to up to 100 um square regions of materials ranging from transition metal dichalcogenides to ruthenates. Our method has been successfully applied to electrical transport, scanning tunneling microscopy, and scanning near-field optical microscopy measurements. Some early results using this device include measurement of a large strain effect on superconductivity in 2H-NbSe₂, the formation of solitons in 2H-MoSe₂, and the insulator-metal transition in 10% Ti-doped Ca₃Ru₂O₇.
[1] Jiun-Haw Chu et al, Science 337, 710 (2012)
[2] Clifford W. Hicks et al, Science 344, 283 (2014)
[3] Joonbum Park et al, Phys. Rev. B 97, 024411 (2018)
[4] Haruka Taniguchi et al, Phys. Rev. B 88, 205111 (2013)
[5] Clifford W. Hicks et al, Rev. Sci. Instruments 85, 065003 (2014)   

Nano-resolved strain control and polar modulation of the Mott transition in a bilayer ruthenate

The 4d transition metal oxides of the Ca_n+1Ru_nO_3n+1perovskite family have recently garnered interest for their correlated electron physics and strong sensitivity to external stimuli like strain, temperature, and even electric current. These place the n=1,2 members in a rich phase diagram of competing and tunable metallic, magnetic, and insulating phases. The bilayer ruthenate Ca₃Ru₂O₇ exhibits a structural distortion producing a polar metal and, under Ti substitution for Ru, a polar Mott insulator. We report cryogenic (T<100K) infrared nano-imaging (nano-IR) of 5% and 10% Ti-doped crystals revealing spontaneous patterns of striped phase coexistence through the thermal first-order Mott transition, and demonstrate nucleation and manipulation of metallic stripes through in situ uniaxial strain. Energy-resolved nano-IR imaging combined with surface work function mapping reveal suppression or enhancement of the Mott insulating state at polar domain boundaries in the crystal. Verified by second-harmonic polarimetry and transmission electron microscopy, we find the polar orientation and state of charge at these polar domain walls can selectively modulate the Mott transition, thus opening new routes towards manipulation of this canonical insulator-metal transition at sub-micron scales.   

Ionic gating of correlated perovskite nickelates

In this work, we demonstrate that an ionic liquid gate can drive the fast motion of Li⁺ into and out of correlated perovskite nickelates such as SmNiO₃ and NdNiO₃. This modulation effect can successfully engineer the phase of the material and tune the effective gap by nearly 3 eV. X-ray absorption spectra characterization showed that the Ni-site orbital occupancy control upon the gate modulation is the primary reason for the phase transition. Electrical transport measurements illustrated that such lithiation behavior induced the drastic change in electronic transport by several orders of magnitude and is independent of temperature. The lattice dilatation can be monitored as a function of Li⁺ doping concentration via synchrotron spectroscopy and provides structural insights during the phase transition. We will discuss hydrogen versus lithium doping of the nickelates and discuss the strain-orbital filling relationships.

  

First Reconstruction of the Antiferromagnetic Order Parameter in NdNiO3 from Resonant Soft X-ray Bragg Ptychography.

Cubic perovskite nickelates with a rare earth ion (ANiO3) exhibit both a metal to insulator transition and a transition to antiferromagnetic order, which coincide in the case of NdNiO3. The many-body nature of these oxides and the interplay between antiferromagnetic order and charge disproportionation leads to a complex landscape involving significant inhomogeneity at the nanoscale. This is exhibited in the form of phase segregation near both the antiferromagnetic and metal to insulator transitions [1]. We report on a first step towards fully characterizing that nanoscale texture using resonant soft x-ray coherent Bragg ptychography at the CSX beamline of NSLS-II. In addition, our work demonstrates for the first time the feasibility of Bragg ptychography in the soft x-ray regime using the weak resonant scattering signals associated with electronic ordering wavevectors.   

Similarities and Differences Between the Nickelate LaNiO2 and Isoelectronic Cuprate Superconductors

It is an open question whether layered nickelate compounds and derived heterostructures could mimic a set of distinct properties which are suspected to invoke high-temperature superconductivity in cuprates, that is, a 2D layered crystal structure, spin 1/2 and strong antiferromagnetic correlations in the parent compound, absence of orbital degeneracy, and hybridization with oxygen ligands. To address this issue, we have investigated LaNiO₂ and isostructural SrCuO₂ thin films by x-ray absorption spectroscopy (XAS) and resonant inelastic x-ray scattering (RIXS) at the O K -and Ni L-edges. Despite several formal similarities between the nickelate and the cuprate, our study unveils severe differences in orbital configuration, antiferromagnetic order, and ligand-hybridization.   

On the mysterious magnetic susceptibility of LaNiO3

Ever since Mott's time, understanding of the enhanced Pauli susceptibility observed in many metallic compounds has remained a great challenge[1]. This, in particular, holds in regard to the temperature dependence of the magnetic susceptibility of LaNiO₃, which, up until recently, was thought to be greatly enhanced yet almost temperature independent. However, recent advances in growth of highly stoichiometric LaNiO₃ single crystals have revealed that the magnetic susceptibility of LaNiO₃ has in fact a much more complex temperature dependence[2,3], which has invoked a renewed theoretical interest in the basic electronic structure of this material. In this talk, we propose an explanation of the recent data assuming temperature dependent disproportionation of localized magnetic moments.

1. N.F. Mott, Metal-insulator transitions 2nd ed., Taylor & Francis, 1990, ISBN:0850667836, p.102-103, 132-143
2. J. Zhang et.al., Cryst. Growth Des. 2017, 17, 2730-2735
3. H. Guo et.al., Nature communications, (2018) 9:43