Session U61: Metal-Insulator transition in 4d/5d oxides

Emergent transport properties on the verge of metal-insulator transitions in pyrochlore 5d/4d oxides

Magnetic Weyl semimetals have been proved to generate emergent transport properties such as giant anomalous Hall effect due to the band-crossing points acting as magnetic monopoles of Berry curvature.
A variety of 5d/4d oxides, in which the magnitude of the relativistic spin-orbit coupling is comparable to that of the electron correlation, are suggested to host such a topologically-nontrivial property. For instance, pyrochlore iridates (R₂Ir₂O₇) are firstly proposed as a candidate for the magnetic Weyl semimetal. Recent studies uncover that Nd₂Ir₂O₇ shows salient magnetotransport properties on the verge of metal-insulator transitions, attributable to the realization of multiple topological semimetals with different magnetic ordering patterns. Here, we report another examples of such states in Pr₂Ir₂O₇, which is known as a paramagnetic Luttinger semimetal. The resistivity is largely enhanced in the 3-in 1-out magnetic configuration of Pr-4f moments, whereas it is relatively small in the magnetically metastable state (so-called Kagome-ice state) accompanied with the plateau structure of Hall resistivity. It indicates that the quadratic-band-touching electronic structure is significantly sensitive to the localized magnetic configurations even in the weak-coupling system.
We also demonstrate that the pyrochlore 4d oxides provide a fertile playground. Particularly, Ruthenates (A²⁺₂Ru⁵⁺₂O₇: 4d³or R³⁺₂Ru⁴⁺₂O₇: 4d⁴) exhibit wide range of magnetic/electronic phases from an antiferromagnetic insulator to a ferromagnetic-like metal as the valence of A site changes. Furthermore, the sizable anomalous Hall conductivity is observed in the intermediate A-site composition, which can be understood in the context of the topological nature of the band structures.   

Multiorbital effects in Sr2IrO4 under an external field

Many parallels have been drawn between the spin-orbit coupled Mott insulator Sr₂IrO₄ and the high-T_c superconducting cuprates. In the undoped compound, a one band J_eff=1/2 model has been shown to capture the observed magnetic order. In other regimes, the applicability of this model is less clear, and the correlated phases are less understood due to potentially complex multiorbital physics. We use a multiple order parameter, self-consistent mean-field approach to explore the relevance of multiorbital effects to possible exotic phases in Sr₂IrO₄ in an external magnetic field. We study the contributions from both spin and orbital degrees of freedom to the magnetic moment. We thus unravel what role spin orbit coupling plays for emerging orders. Implications for superconductivity and thermal Hall measurements are discussed.   

Electrical resistivity relaxation under uniaxial strain in BaIrO3 at room temperature

We investigate the 5d transition metal oxide BaIrO₃, in which the ground state is strongly dependent on the corner sharing Ir-O-Ir buckling angle between the face sharing octahedral trimers. We compare nanoindentation and resistivity under uniaxial strain, finding a strong hysteretic behavior in the resistivity that is driven by a strain dependent relaxation. This relaxation exhibits logarithmic time dependence, and also emerges in the lattice relaxation as creep. These observations highlight the strong mechanical and electrical coupling in BaIrO₃, and point to the relaxation mechanisms sharing the same origin. Our results promote the value of utilizing uniaxial strain testing and nanoindentation as a novel technique to probe the structural sensitivity of quantum materials.   

In-situ anisotropic strain controlled metamagnetism of Sr2IrO4 by pseudo-Jahn-Teller distortion

In recent years, iridates have been recognized as an ideal material platform where the emergent many-body phenomena mesh with Antiferromagnetic (AF) functionalities. As the prototype, Sr₂IrO₄ is a quasi-two-dimensional AF Mott insulator of pseudospin-half electrons that exhibits remarkable phenomenological analogy to the high-T_c cuprates. Meanwhile, significant magnetic responses of the AF order in both bulk and thin film samples have been demonstrated, including metamagnetism, giant magnetoresistance, and anisotropic magnetoresistance. The high controllability of the AF order is largely related to the unique electronic structure that the strong spin-orbit coupling of Ir stabilizes the pseudospin-half Kramer doublet while makes no contribution to the magnetic anisotropy. In this study, we drove the metamagnetic transition of Sr₂IrO₄ bulk crystal into distinct regimes by applying in-situ anisotropic strain. The system was studied with resonant magnetic x-ray scattering and transport measurement. We found the application of anisotropic strain is seen to modulate the pseudospin-lattice coupling, leading to highly efficient magnetoelastic control of the J_eff=1/2 metamagnetism as well as the associated elasto- and magnetoresistance.   

Thickness Dependent Magnetic Reversal in Sr4Ru3O10 Nanosheets

While ruthenium oxide Sr₄Ru₃O₁₀ has been studied extensively in the last two decades, the origin of the unusual second magnetic anomaly near T_M~50 K remains elusive. Here, we investigated the thickness-dependent anisotropic magnetoresistance (AMR) of pure and 2% Fe-doped Sr₄Ru₃O₁₀ nanosheets. It was clearly found that the AMR properties in pure nanosheets present thickness-dependent behavior and an unusual reversal near T_M, but Fe-doping nansheet completely suppresses the second anomaly down to low temperatures. Analysis of the data demonstrates that this AMR reversal is caused by the magnetization reversal from the ab plane ferromagnetic order at T>T_M to the c direction at T<T_M due to the competition between the shape anisotropy and the inherent magnetocrystalline anisotropy. Our result suggests that the second magnetic transition in Sr₄Ru₃O₁₀ nanosheets is originated from spin reorientation.   

Transport phenomena in ultraclean 111 oriented SrVO3 thin films

SrVO₃ has proven to be an ideal testbed for studying the effects of strong electron correlation due to its simple cubic structure and single d-orbital electron. Ultraclean SrVO₃ films with residual ratios exceeding 200 have recently been demonstrated using self-regulated growth by hybrid MBE, allowing accurate interpretation of the intrinsic features of electron correlation [1]. In addition to allowing high quality growth, we demonstrate that SrVO₃ can also be grown epitaxially on (111) oriented LSAT substrates. We compare the temperature dependent electronic transport properties of (111) and (001) oriented SrVO₃ thin films. In addition, we discuss the formation of a low temperature Fermi liquid like phase in each case.

[1] M. Brahlek, et al Appl. Phys. Lett. 107, 143108 (2015)
  

Growth and Characterization of Heterostructures of Ferromagnetic SrRuO3 and Superconducting Sr2RuO4 by Molecular-Beam Epitaxy

Sr₂RuO₄ single crystals have been shown to exhibit unconventional superconductivity with a T_c of 1.5 K. Our group recently reported a thermodynamic growth window for Sr_n+1Ru_nO_3n+1 thin films, including the demonstration of the reproducible growth of high-RRR Sr₂RuO₄ and SrRuO₃ thin films. The growth of Sr₂RuO₄ thin films and the recent ¹⁷O NMR results on single crystals offer an opportunity for the growth of Sr₂RuO₄-based heterostructures, in order to perform measurements to determine the superconducting order parameter. In this talk, we describe the growth of SrRuO₃/Sr₂RuO₄ heterostructures using oxide molecular-beam epitaxy. We characterize the crystallinity and structure of these heterostructures using X-ray diffraction, X-ray reflectivity, and scanning transmission electron microscopy, and finally we present magnetometry and electrical resistivity measurements on these heterostructures.   

Intrinsic tunable anomalous Hall effect induced by momentum-space Berry curvature in SrRuO3

Broken time-reversal symmetry and spin-orbit coupling (SOC) in strongly correlated ferromagnetic materials are expected to induce novel phenomena. One of these ferromagnetic materials, SrRuO₃ (SRO), is well-known for showing non-monotonous anomalous Hall effect (AHE), which is produced due to magnetic monopoles, or large Berry curvature in momentum space. Recently, we observed an abnormal behavior of AHE in SRO ultrathin films. To explain this, we measured angle-resolved photoemission spectroscopy (ARPES) of SRO ultrathin film. We found that non-monotonous AHE is induced by band-structure nodal lines, which open a gap due to SOC and magnetization. By changing temperature and thickness of SRO, the sign of AHE can be tunable with a variation of large Berry curvature.   

Spectroscopic study on band evolution and formation of relativistic Mott insulating phase in Ir doped Sr2RuO4

Recent research showed 4d, 5d transition metal oxide provides rich novel physical properties. A discovery of superconductivity in Sr₂RuO₄ is still controversial to verify the mechanism, and relativisitc Mott phase of Sr₂IrO₄ expanded wide research field of spin-orbit coupling-driven physics. As one step to explore the field, doping study also become vivid that it can alter the system's key parameters and artificially create a perturbed physical system. Rh doping in Sr₂IrO₄ revealled variation of spin-orbit coupling, and doping on electrons on Sr₂IrO₄ showed charge carrier dependent collapse of Mott phase. Now we tried to study doped compound Sr₂(Ru,Ir)O₄ to show the electronic propagation of bands and evolution of relativistic Mott phase with angle-resolved photoemission spectrscopy technique. With this presentation we would clarify the role of spin-orbit coupling on the crystal-field-dominating band structure of Sr₂RuO₄, and provide a step by step evolution of relativistic Mott transition from varying spin-orbit coupling parameter.   

Spectroscopic evidence for electron-boson coupling in electron-doped Sr2IrO4

The pseudogap, d-wave superconductivity and electron-boson coupling are three intertwined key ingredients in the phase diagram of the cuprates. Sr₂IrO₄ is a 5d-electron counterpart of the cuprates in which both the pseudogap and a d-wave instability have been observed. Here, we report spectroscopic evidence for the presence of the third key player in electron-doped Sr₂IrO₄: electron-boson coupling. A kink in nodal dispersion is observed. The strength of the kink changes with doping, but the energy scale remains the same. These results provide the first non-cuprate platform for exploring the relationship between the pseudogap, d-wave instability and electron-boson coupling in doped Mott insulators.   

Relaxation Dynamics at High-symmetry Points in Sr2IrO4 through Time- and Angle-resolved Photoemission Spectroscopy

The unresolved mechanism that underlies high-T_c superconductivity in cuprates has stimulated the search for new material platforms that could shed light on this long-standing puzzle. A promising candidate is electron-doped Sr₂IrO₄ – an antiferromagnetic Mott insulator with strong spin-orbit coupling. Pioneering studies have shown that chemical doping modifies the electronic band structure in electron-doped Sr₂IrO₄. However, instead of chemical doping, which provides a static modification of the electronic band structure, transient photo-doping with an ultrafast laser pulse can be considered. Through time- and angle-resolved photoemission spectroscopy, we performed momentum-resolved pump-probe experiments and compared the electron relaxation behavior at high-symmetry points (Γ, M, and X) of the Brillouin zone. This time- and momentum-resolved study provides new insights into the transient charge dynamics in this Mott insulator.   

Magnetic-field-dependent second-harmonic generation study of Sr2IrO4

The quasi-two-dimensional layered oxide Sr₂IrO₄ has earned recognition for its novel spin-orbit Mott state and various analogies to cuprate physics. Neutron diffraction and x-ray scattering measurements show that it possesses a centrosymmetric magnetic ground state, consisting of a canted intralayer antiferromagnetic order with nonzero net magnetic moments in each layer stacked antiferromagnetically [1-4]. However, the onset of a second-harmonic generation (SHG) signal below the Néel temperature has suggested the presence of a distinct hidden magnetic order [5]. Using small in-plane magnetic fields, it is possible to manipulate the Néel order and help distinguish it from hidden order. In this talk, we will present magnetic-field-dependent SHG rotational anisotropy and microscopy studies that further elucidate the nature of this hidden state.

[1] Kim, B. J. et al. Science 323, 1329–1332 (2009).
[2] Ye, F. et al. Phys. Rev. B 87, 140406 (2013).
[3] Dhital, C. et al. Phys. Rev. B 87, 144405 (2013).
[4] Boseggia, S. et al. J. Phys. Condens. Matter 25, 422202 (2013).
[5] Zhao, L. et al. Nat. Phys. 12, 32–36 (2015).   

Raman measurements on Sr2IrO4 grown in magnetic field

Recent experiments found a massive reduction in the low-temperature resistivity in Sr₂IrO₄ when the crystals were grown in the presence of a magnetic field. Here I will present a comparison of Raman scattering data from normal and field-grown Sr₂IrO₄. The main differences between the two appeared in scattering from magnetic excitations. Two-magnon scattering was significantly broader in the field-grown sample and exhibited unusual temperature dependence. Scattering from single magnons, clearly visible in the normal sample, was completely absent in the field-grown sample down to an energy of at most 5 cm^-1. Latest results including on doped samples will be presented.