Session G21: Emergent Properties: Superconductivity and Spin-Orbit Coupling

Superconductivity in infinite layer nickelate films using aluminium and in-situ synthesis

The addition of the nickelates to the list of non-conventional superconductors represents an opportunity to understand the physics of these materials and develop new superconductors. An additional twist has been added through the recent discovery of Europium as a hole dopant to the Ni t2g bands in Nd_1-xEu_xNiO₂. This material is fabricated by first growing a perovskite parent material, Nd_1-xEu_xNiO₃ using oxide molecular beam epitaxy and subsequently reducing it by depositing Al metal in ultra high vacuum. This process results in superconducting films with a Tc as high as 21 K. Measurements of x-ray absorption spectroscopy as a function of doping concentration x, point to a novel aliovalent hole doping mechanism, where the Eu 4f bands lie near the Fermi level and leading to the coexistence of Eu³⁺ and Eu²⁺ oxidation states. We present x-ray spectroscopic measurements to understand the microscopic mechanism of this doping. The role of magnetism in this material is explored using a combination of high-field (35 T) magneto transport measurements, where the upper critical field is found to exceed the Pauli limit. Through this combination of synthesis and characterization we highlight the unique properties of this new Eu-based superconductor.   

In-plane Broken Symmetry with Strong Perpendicular Magnetization in Ru-substituted Manganite Films

Understanding the origin of magnetic anisotropy provides insights into the interplay of various factors, including crystal symmetry, spin-orbit coupling, and exchange interactions, thereby playing a crucial role in designing the required magnetic materials for various applications. Recent studies on the Ru substituted La_0.70Sr_0.30MnO₃ (Ru-LSMO) films have illustrated non-trivial magnetic topologies, and enhanced anomalous Hall effect, where tilted magnetic anisotropy (TMA) plays a crucial role. While perpendicular magnetization has been demonstrated with Ru-LSMO, anisotropic in-plane magnetization was not addressed. Here we describe anisotropic in-plane magnetization coexisting with strong perpendicular magnetization in 10% Ru substituted LSMO films with Curie temperature near room temperature. In our recent work, we unveil a monoclinic, fully-strained phase, and 1D periodic in-plane structural domains. We determine that the in-plane symmetry breaking by the monoclinic crystal structure induces weekly anisotropic in-plane magnetization, which is further enhanced by the 1D periodic structural modulation. We further illustrate how the interplay of in-plane crystal symmetry breaking, octahedral rotation, spin-orbit coupling, and the magnetic interaction between Ru and Mn ions induce such a complex TMA. Such TMA is attractive for cutting edge spintronic memories such as spin-orbit-torque based deterministic perpendicular magnetization switching.   

Orbital-driven spin-singlet dimerization in La2Ru4O10

La₄Ru₂O₁₀ exhibits an interplay between spin, orbital, and lattice degrees of freedom. This quasi-two-dimensional compound consists of LaRuO₄ layers, formed by the corner-sharing RuO₆ octahedra, separated by buckled LaO layers. At T_S = 160 K, La₄Ru₂O₁₀ undergoes a strong first-order structural transition, accompanied by an orbital ordering of the Ru 4d electrons, resulting in a non-magnetic ground state due to the dimerization of Ru⁴⁺ S=1 spins.

In this talk, we present results from inelastic neutron scattering (INS) and X-ray diffraction studies on single-crystal La₄Ru₂O₁₀. In the INS data, we observed well-separated bands of oscillatory excitations centered at around 45 meV and 60 meV, which are indicative of the presence of strongly coupled Ru-Ru dimers, with weak inter-dimer couplings. We apply the pair distribution function (PDF) analyses on the X-ray diffraction data at various temperatures, to explore the correlations between the spin-singlet dimerization and the lattice distortion through the monoclinic to triclinic structural phase transition.   

Vibronic effect on resonant inelastic x-ray scattering in cubic iridium hexahalides

In resonant inelastic x-ray scattering (RIXS) spectra of K₂IrCl₆, the peak for the j = 3/2 multiplet states shows a splitting that resembles a noncubic crystal-field effect, although the compound is cubic down to 0.3 K. Here we theoretically describe the RIXS spectra concomitantly treating the spin-orbit and vibronic interactions. We found that the dynamic Jahn-Teller effect in the j = 3/2 multiplet states gives rise to the splitting of the RIXS spectra and the broadening of the spectra in raising the temperature. The validity of the interaction parameters for the simulations is supported by our ab initio calculations. Our results suggest that in cubic iridium compounds, the dynamic Jahn-Teller effect induces the splitting of RIXS spectra without lowering the symmetry.   

Sketched Nanoscale KTaO3-Based Superconducting Quantum Interference Device

The discovery of two-dimensional superconductivity in LAO/KTO (111) and (110) interfaces has raised significant interest in this system. Here we report creation of nanoscale direct current superconducting quantum interference device (DC-SQUID), created by conductive atomic force microscope (c-AFM) lithographic control of conductivity at the LAO/KTO(110) interface. The field modulation of its critical current, , is dominated by the large kinetic inductance of superconducting 2DEG in KTO. The critical current of the SQUID is tunable by electrical gating from the back, due to the large dielectric constant of KTO. The demonstration of SQUID effect in KTO opens up the possibilities for probing the underlying physics of KTO superconductivity, such as the role of spin-orbit-coupling, pairing symmetry, and inhomogeneity. It also further implies that KTO could serve as a foundation for future quantum devices.   

Realization of large-volume crystals of Sr2IrO4 grown via high-pressure laser floating zone

The study of 5d transition metal oxides provides a rich bed for exploring the interplay of spin-orbit coupling with electron-electron interactions. Of these, the spin-orbit-coupled Mott insulator Sr₂IrO₄ has received particular attention, particularly for its similarity to La₂CuO₄, a parent material to high-T_c superconductivity upon doping. For studies utilizing weakly interacting probes such as neutron scattering, obtaining large-volume, high-quality crystals is of the utmost importance. Historically, studies on bulk crystals of Sr₂IrO₄ have been limited by the small (<5 mm²), plate-like crystals resulting from conventional flux growth techniques. The state-of-the-art growth method for large crystals of transition metal oxides is the floating zone growth technique, though iridates have historically suffered from extreme material losses and decomposition due to the volatility of IrO₂ upon melting. Here we present an advancement in iridate synthesis using a custom high-pressure laser-based floating zone furnace to realize the first floating zone samples of high-quality Sr₂IrO₄ grown from a self-flux under a high pressure 80:20 Ar:O₂ gas mixture. We report characterization of the microstructure, magnetic, and electronic properties and compare these to flux-grown crystals.   

Peierls-like distortion drives anion ordering in rutile TiOF

Using first principles density functional theory calculations, we examine the effect of multiple anions on the Peierls distortion in rutile compounds with the oxyfluoride TiOF. We use a structure enumeration approach and obtain the ground state for TiOF and identify the driving force behind the experimentally observed two-dimensional chemical ordering along rutile (110) planes. We find that the anion bridge connecting adjacent edge-shared octahedra comprises like atoms with an –O-O/F-F/O-O/F-F– pattern along the rutile [001] direction and a Peierls-like distortion leading to the formation of a singlet state. This anion ordering arising from the competition between electrostatic interactions from the Ti-F cation-anion pairs and the tendency of the d¹ Ti³⁺ cation to form a metallic Ti-Ti dimer. We find that the addition of strong on-site Coulombic interactions suppresses the formation of the singlet state. By increasing the correlation strength, we uncover two first-order phase transitions: first, from a nonmagnetic insulator to a ferromagnetic half-metal, and then second to a ferromagnetic Mott insulator. Our work shows that anion ordering in oxyfluorides can be driven by cation-cation interactions, enabling design of ordered heteroanionic materials exhibiting collective phenomena through cation sublattice control.    

Ferromagnetism in fluorine doped LaMnAsO

The electrical and magnetic properties of F-doped LaMnAsO have been investigated using x-ray diffraction, magnetization, electrical resisitivity and magnetoresistance measurements. LaMnAsO is wellestablished as an insulator and exhibits antiferromagnetic behavior below its Neel temperature T_N=360 K. With F-doping, the electrical and magnetic characteristics undergo a change from an insulating antiferromagnetic state to a metallic ferromagnetic state. With F-doping levels in the range x ∼ 0.15 to 0.25, the resistivity shows a strong reduction by five orders of magnitude, reaching values of approximately ∼0.05 Ω·cm. Furthermore, the temperature-dependent resistivity profile demonstrates a decidedly metallic behavior in this range of composition. The transition temperatures (Tc) exhibit a spread of approximately 20K around 300K. Notably, the transverse magnetoresistance is approximately 12% at 300 K.   

Hidden magnetism and the emergence of flat bands in insulator-metal transition in VO2

Flat bands near the Fermi level offer a platform to tune the interaction between electrons for realizing exotic physical effects such as insulator-metal transition and superconductivity. Most materials with flat bands fall into the category of flat topological band systems such as Kagome crystals that lack a knob to switch the flat bands. Here, we focus on a class of split-off flat bands induced by symmetry breaking, where the latter can be used as knobs to switch on and off the flat bands and demonstrate such flat bands in the quintessential insulator-metal transition system VO₂. Such flat bands in VO₂ are split down into the band gap from the wide principal conduction band by cation-cation dimerization, forming the occupied, rather narrow V-d valence band. We predict rather low energy barrier for the insulator-metal transition and find that strong dimerization will suppress magnetism, leading to the nonmagnetic insulating state, whereas magnetism appears when dimerization is reduced, suggesting hidden magnetism during the insulator-metal transition and a potential magnetic metallic state. This study opens the way to design novel functional quantum materials with symmetry breaking-induced flat bands.   

Controlling errors in fixed node diffusion Monte Carlo calculations for spin transitions in Co3+ clusters

Previously, our investigations revealed that diffusion Monte Carlo (DMC) computations suggested an antiferromagnetic arrangement for LaCoO₃ [1], in contrast to experimental observations that characterize the ground state of LaCoO₃ as non-magnetic. This discrepancy is intriguing, considering that DMC calculations employing similar configurations have proven successful in predicting the stable polymorphs and formation energies of transition metal oxides [ref]. In this context, we propose a systematic examination of the LaCoO₃ issue by specifically addressing the fixed node and locality approximations within the DMC framework. We will study the [CoHe₆]³⁺ cluster as a representative model to understand DMC’s accuracy in calculating the spin states of Co-octahedra in LaCoO₃.We address the locality approximation error through our recently developed L2 pseudopotentials.  Our model calculation results will be evaluated with respect to ab-initio DMC and configuration interaction (CI) calculations hence providing an explanation for the contradiction between the theory and experiment.