Session J37: New Probes of Correlated Oxides

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Recently we have developed a new experimental method that can make a direct image of the ground state d-orbitals in transition metal compounds [1,2]. The method, s-core-level x-ray Raman or non-resonant inelastic x-ray scattering (s-NIXS), relies on dipole-forbidden transitions which become available when the experiment is carried out with high momentum transfers. New opportunities are opened up for the investigation of the ground state, especially for quantum materials that are too complex to be handled by ab-initio theories. Here we will go one step further and explore the spectroscopy aspect of s-NIXS in order to study the excited states which are most often dominated by many-body atomic multiplet interactions. We will show that we are able to obtain images by which we can identify the orbital character of those excited states. This in turn facilitates the extraction of important energy parameters in correlated materials.

[1] Yavas et al., Nature Physics 15, 559 (2019)
[2] Leedahl et al., Nature Communications 10, 5447 (2019)   

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Spin-driven multiferroics is a research field which explores nontrivial phenomena emerged by spin-orbit coupling (SOC). The effect of SOC in quasi-particle spectra has been investigated in high-energy range; electromagnon, hybridization of magnon and phonon, and renormalization of optical magnon. The effect of SOC to acoustic magnon in low-energy range which plays important roles in bulk properties, however, has not been studied. Here we demonstrate that the acoustic magnon in multiferroics Ba₂MnGe₂O₇ [1-3] is hybridized with orbitals by measuring the temperature variations of magnetization curves and neutron scattering spectra. The temperature dependence of the anisotropy energy is scaled not by the spin moment but by the electric polarization. The observation evidences renormalization of orbital effect into a spin-nematic interaction and the hybridized magnon in an emergent field. Our finding exhibits that the spin-flop transition field is a sensitive probe for SOC, which could be a key for search of new magnetoelectric materials.
[1] H. Murakawa et al., Phys. Rev. B 85, 174106 (2012).
[2] T. Masuda et al., Phys. Rev. B 81 100402(R) (2010).
[3] Y. Iguchi et al., Phys. Rev. B 98, 064416 (2018).   

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Over the past decade, Ruddlesden-Popper iridates emerged as a versatile playground for investigations of exotic phenomena, such as a doped Mott insulator state, Fermi arcs, density wave instabilities and spin-related orders. We use spin-polarized scanning tunneling microscopy (SP-STM) to visualize spin-resolved modulations arising from the antiferromagnetic (AF) order in doped Mott insulators Sr₂IrO₄ and Sr₃Ir₂O₇. We find that near insulator-to-metal transition, the long-range antiferromagnetic order melts into a fragmented state with short-range correlations. We investigate the impact of chemical disorder on the distribution of these AF domains to pinpoint a defect most strongly correlated with the domain arrangement. Thermal erasure and re-entry into the AF state leads to a partial spatial reorganization of AF modulations, indicating multiple stable AF configurations at low temperature. Interestingly, regardless of this rearrangement, the AF puddles maintain scale-invariant fractal geometry. Our experiments shed light onto the sensitivity of the AF order to different types of atomic-scale disorder, and reveal its surprising fluidity with thermal cycling.   

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CaCo_1.86As₂ is a unique example of a metallic square-lattice system with nearly perfect frustration despite the compound having A-type antiferromagnetic (AF) order below a Néel temperature of T_N = 52 K. Our recent neutron diffraction data on Ca(Co_1-xFe_x)_2-yAs₂ show that the A-type AF order is suppressed past x = 0.12, and here we give results from INS experiments made at T = 6 K on single-crystal samples of x = 0.15. We find diffuse spin fluctuations giving rise to walls of magnetic scattering with similar anisotropy to that found for x = 0, however, the fluctuating moment is extremely weak. A simultaneous analysis of magnetic susceptibility and neutron scattering data shows that the total magnetic moment is gradually reduced with increasing x. This suggests that the development of magnetic order has a Stoner-like mechanism where Fe-doping acts to tune (and decrease) the Stoner parameter UN(E_F). This highlights that cobalt arsenides are uniquely frustrated and weakly itinerant magnets.   

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Despite the observation of strong stripe antiferromagnetic fluctuations, the paradigmatic cobalt pnictide metallic magnet SrCo2As2 exhibits an intriguing lack of magnetic order. Minute amounts of substitutions of Co by Ni, corresponding to electron doping, however, induce long-range spiral magnetic order in the metallic magnet. To understand these puzzling observations, we investigate a realistic interacting multi-orbital model for SrCo2As2 that takes both Co d-orbitals and As p-orbitals into account. We find that the static paramagnetic RPA spin susceptibility exhibits a crossover from dominantly ferromagnetic to dominantly stripe antiferromagnetic fluctuations as a function of both doping and modifications of Hubbard to Hund interactions. In agreement with experimental observations, our model shows a clear tendency towards the development of magnetic order under electron doping, while hole doping moves the system further into the paramagnetic regime. By analyzing the orbital character of the leading magnetic fluctuations, our study elucidates the proposed mechanism of itinerant magnetic frustration as the origin of the observed lack of magnetic order.   

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The quantum coherence of spins is usually studied in highly diluted magnetic ions in a diamagnetic medium. We propose a new paradigm: a non-magnetic defect, like a break in the translational symmetry of Heisenberg’s spin chain, polarized many spins around it.
We show that this correlated spin system that is made of hundreds of coupled spins bear an overall spin S = 1/2 and can be manipulated as a single spin. By means of pulsed electron paramagnetic resonance, we show the quantum coherence of this object by measuring the Rabi oscillations. We show that the spin bath decoherence is strongly reduced by the strong isotropic exchange interaction.

1. J. Zeisner, et al Phys. Rev. B. 100, 224414 (2019). arXiv:1909.10301
2. L. Soriano, et al. Applied Magnetic Resonance (2020), arXiv:2008.10897.   

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We utilize intense ultrafast mid-infrared (mid-IR) circularly polarized pump pulses to preferentially excite spin degrees of freedom through coherent two-magnon excitation as measured via the magneto-optic Kerr effect using 800 nm pulses. Our previous results [1] on the spin-orbit coupled Mott-Hubbard insulator (MHI) Sr₂IrO₄ indicate that magnon generation using mid-IR pulses is nearly an order of magnitude more efficient in comparison to above gap doublon generation. To clarify the potential of mid-IR pulses for coherent magnon generation, we are investigating the canonical MHI nickel oxide which serves as a well characterized material for comparison with theoretical predictions [2]. This presentation will provide an update on these experiments with a view towards mode-selective spin excitation in quantum materials.
[1] Gu-Feng Zhang, Xiang Chen, Urban F.P. Seifert, Jingdi Zhang, Kevin Cremin, Leon Balents, Stephen D. Wilson and Richard D. Averitt. “Ultrafast magnetic control in a spin-orbit coupled iridate.” Manuscript in Preparation.
[2] Seifert, Urban FP, and Leon Balents. "Optical excitation of magnons in an easy-plane antiferromagnet: Application to Sr 2 IrO 4." Physical Review B 100, no. 12 (2019): 125161.   

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The antiferromagnetic insulator Bi2CuO4 has been of research interest due to its unusual crystal structure of staggered CuO_4 plaquettes. While the first synthesis of this compound was reported in 1976, the exact spin structure of the magnetic order that develops below 43.5 K remains a mystery. In addition, recent studies of this compound report the presence of a magneto-electric effect in this material. We present the results of time-domain THz studies of single crystals of Bi2CuO4 grown via the traveling solvent technique in a laser floating zone furnace .   

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AC calorimetry is a steady-state measurement technique capable of detecting small heat-capacity signals and variations. Recently, Na2Co2TeO6 is proposed as a candidate material for realizing the Kitaev-Heisenberg model on a honeycomb lattice, and its low-temperature thermal phase behaviors are currently under exploration. Equipped with a home-made AC calorimetry setup, we have measured the heat capacity of Na2Co2TeO6 single crystals with a resolution that is significantly better than commercial equipment. We find an unexpected small heat-capacity peak at low temperature and in zero magnetic field.   

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We performed magnetoelastic measurements of the piezomagnetic antiferromagnet UO₂ via the Fiber Bragg Grating method in pulsed magnetic fields up to 150 T generated by a single-turn coil setup. While still insufficient to suppress the robust low temperature magnetically ordered state, we show that the uniquely short timescales of a few micro seconds of the magnetic-field pulse excite mechanical elastic resonances in the sample due to the piezomagnetic coupling. This results in standing-wave magnetoelastic oscillations superimposed on the magnetostriction signal. We compare the resonances with natural resonance frequencies obtained by a resonant ultrasound technique. The piezomagnetic switching behavior of the antiferromagnetic ordering vector below T_N=30.5K, which was revealed in a previous study [M. Jaime et al., Nature Communications 8, 99 (2017).] results in an apparent phase shift of π in the lattice oscillations. We further present a model to explain our data.   

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The rare earth compound CeSb₂ belongs to a strongly correlated system, and it shows a rich magnetic phase diagram depending on the temperature. And the magnetic transport measurements show the highly anisotropic nature of the magnetization of CeSb₂ in the ab-plane. In order to determine the electronic properties of CeSb₂, previous study by synchrotron radiation photomission spectroscopy but without angular resolution shows no indication of Kondo-like behavior in the bulk properties. Reexamination of the resonance behavior of Ce 4f by tracking the evolution of Fermi surface and electronic structure is needed.
Here, we performed the angular-resolved photomission spectroscopy (ARPES) of CeSb₂ and obtained the 3D electronic structure of CeSb₂ for the first time by applying the changeable photon energy. We acquired the intrinsic bulk electronic structure including both in plane and out plane. Furthermore our ARPES measurements reveal the orbital components of the electronic state for the first time and the hybridization information of CeSb₂ conduction band and Ce 4f states were obtained. Importantly, we also conducted the temperature-dependent measurements on CeSb₂ to study the phase transition, which will be helpful to understand the magnetic transport properties.