Session B01: Design and Synthesis of New Quantum Materials II

Invited: Synthesis of Electronic-Grade Quantum Heterostructures by Hybrid PLD

Modern quantum materials are inherently sensitive to point defects, and require a new synthesis route to produce epitaxial oxide thin films and interfaces clean enough to probe fundamental quantum phenomena.  The recent discovery of robust superconductivity at KTaO₃ (111) and KTaO₃ (110) heterointerfaces on KaTaO₃ bulk single crystals offers new insights into the role of incipient ferroelectricity and strong spin-orbit coupling.  Electronic grade epitaxial thin film platforms will facilitate investigation and control of the interfacial superconductivity and understanding the fundamental mechanisms of the superconductivity in KTaO₃. The major challenge of research on KTaO₃ system is that it is difficult to grow high-quality KTaO₃ epitaxial thin films due to potassium volatility. Recently, we have developed the hybrid PLD method for electronic grade KTaO₃ thin film growth, which successfully achieves this by taking advantage of the unique capabilities of PLD to instantly evaporate Ta₂O₅ in a controlled manner and evaporation of K₂O to maintain sufficient overpressure of volatile species. We successfully synthesized heteroepitaxial KTaO₃ thin films on 111-oriented KTaO₃ bulk single crystal substrates with a SmScO₃ template by hybrid PLD, followed by a LaAlO₃ overlayer. Electrical transport data show a superconducting transition temperature of ~ 1.35K. We anticipate that the ability to synthesize high-quality epitaxial complex oxides such as KTaO₃ that contain volatile elements will provide a new platform for exploring new physics and technological applications arising from unique characteristics such as large spin-orbit coupling. 

This works has been done in collaboration with Jieun Kim, Jungwoo Lee, Muqing Yu, Neil Campbell, Shun-Li Shang, Jinsol Seo, Zhipeng Wang, Sangho Oh, Zi-Kui Liu, Mark S. Rzchowski, Jeremy Levy.   

Freestanding nickelate membrane

Perovskite nickelates have been heavily studied as a textbook example of how the interplay of structural, orbital, and charge/spin degrees of freedom drive strongly correlated phenomena in 3d transition metal oxides. The recent discovery of superconductivity in the infinite-layer nickelate thin films has provided the field another d⁹ electron configuration similar to cuprates. Releasing nickelate thin films from the substrate constraint would provide a novel experimental avenue to explore the strain tunability of the metal-insulator transition in the perovskite nickelates and superconductivity in the infinite-layer nickelates. In this talk, we will discuss our progress on synthesizing freestanding perovskite nickelate membranes and bridging them to the infinite-layer nickelate phase.   

Oxidation and reduction processes of CaCoO2 – CaCoO2.5 thin films investigated by operando optical spectroscopic ellipsometry

Recently, new calcium cobaltate thin films, i.e., CaCoO_x, are found to exhibit strong lattice distortions indicative of intriguing orbital and spin structures [1], which might result in a drastic change in their electronic and magnetic properties. It has attracted attention since it offers a way of tuning materials’ functional properties by oxidation and reduction processes. However, the oxidation and reduction processes of this system are not fully understood. 

In this presentation, we will discuss our experimental studies on the oxidation and reduction processes of CaCoO_x (x = 2.0 or 2.5) using operando optical spectroscopic ellipsometry. We have successfully observed drastic changes in the real-time optical spectra while the thin film samples undergo oxidation and reduction. Spectral analyses have provided key for understanding the intermediate electronic structures of the samples. A clear difference was observed between the dynamics of oxidation and reduction processes. Moreover, the oxidation process was affected by SrTiO₃ capping-layer thicknesses, indicating that the oxidation process is dominated by oxygen diffusion through the capping layer. Our finding provides valuable insight for understanding this system.

 

 

[1] W. J. Kim et al., Nature, 615, 237 (2023)   

Topological Hall Effect in Free-standing NiCo2O4 Membranes

We report the observation of topological Hall effect (THE) in 30 nm suspended NiCo₂O₄ (NCO) membranes. Free-standing NCO (001) membranes are achieved via epitaxial growth on Sr₃Al₂O­₆ (SAO) buffered (001) LaAlO₃ (LAO) substrates via off-axis RF magnetron sputtering followed by water etching. X-ray diffraction measurements confirm the single crystalline phase of the membranes. The NCO membranes are then transferred on SiO₂/Si substrate and patterned into Hall bar devices. We have studied the magnetoresistance (MR) and Hall effect in both NCO/SAO/LAO films and NCO membranes. The samples exhibit insulating temperature dependence, and the Curie temperature T_C are above 300 K. Below T_C, we observe anomalous Hall effect (AHE), which shows a nonmonotonic temperature dependence and reverses sign at about 240 K. For NCO membranes, topological Hall effect (THE) signal emerges below 10 K upon magnetic field cooling, a phenomenon that is absent in epitaxial NCO thin films. The THE signal changes sign when the magnetic field is reversed. The MR, AHE, and THE in suspended NCO membranes point to the intricate roles of strain and disorder in determining the magnetic properties of NCO.   

Investigating magnetic anisotropy and the stability of Jeff =1/2 state in Sr2IrO4 thin films under orthorhombic distortion

We have investigated the complex interplay between orthorhombic distortion and magnetic anisotropy in Sr₂IrO₄ thin films. By inducing a pronounced orthorhombic distortion on Sr₂IrO₄ thin films due to Ca₃Ru₂O₇ substrates, we effectively control the uniaxial magnetic anisotropy. From x-ray magnetic circular dichroism (XMCD), we have observed divergent responses along the system’s magnetic easy and hard axes, thereby indicating the existence of magnetic anisotropy. The magnetic anisotropy energy estimated from the spin flop transition, observed when the magnetic field is parallel to the hard axis, well matches the value calculated from the single magnon peak energy measured using Raman spectroscopy. Notably, the observed anisotropy energy remains lower than the theoretical estimates, emphasizing the persistence of the J_eff =1/2 state. This conclusion is supported by negligible XMCD intensity ratios and the expectation value of the spin-orbit coupling determined from the branching ratio using x-ray absorption spectroscopy. Our findings highlight the proximity of Sr₂IrO₄ to the J_eff =1/2 state despite substantial biaxial distortion, enhancing our understanding of the interplay between magnetic anisotropy and strain in materials with emergent quantum phenomena.   

Synthesis and characterization of epitaxial Sr2RhO4 thin films

A 4d layered oxide, Sr2RhO4, has unquenched spin-orbit coupling resulting in intriguing collective modes such as spin-orbit excitons [1] despite its moderate spin-orbit coupling compared to 5d transition metal oxides. Recent studies on single crystals have shown that the metallic ground state of Sr2RhO4 is a confluence of the spin-orbit coupling, electron correlation, and structural distortion. In this presentation, we will discuss the synthesis and characterization of epitaxial Sr2RhO4 thin films. We have successfully deposited high-quality thin films on various oxide substrates using pulsed laser deposition. We have observed that Sr2RhO4 thin films are coherently strained and their optical spectra are overall consistent with that of single crystals. However, its resistivity has increased when they are under relatively large strain (> 1%), presumably due to weak localization. These epitaxial Sr2RhO4 thin films provide a tunable model system to explore the physics of moderate spin-orbit interaction and its competition with electron correlation in 4d transition metal oxides.   

Atomically flat epitaxial Cr2O3 films on atomically flat Pd electrodes for magnetoelectric and topological applications.

Cr₂O₃ is one of the best magnetoelectric antiferromagnetic materials and is also an excellent substrate for many hexagonal topological materials. To fully utilize these functionalities, it is essential to have epitaxial pin-hole-free Cr₂O₃ films on conducting electrodes. So far, V₂O₃ has been identified as the best conducting bottom electrode for epitaxial Cr₂O₃ due to their matching crystal structures. However, V₂O₃ is a poor conductor at room temperature and becomes insulating at low temperatures, so its utility is very limited.

Here, we demonstrate atomically flat epitaxial Cr₂O₃ films with excellent electronic properties on highly conducting Pd films grown on Al₂O₃ (0001) substrates. It turns out that achieving epitaxial and atomically flat Pd film is critical to grow the high quality Cr₂O₃ films. Along the way, we have found that growth of epitaxial Pd film on sapphire substrate is highly challenging despite their excellent lattice match because Pd film has a strong tendency to ball up on sapphire. However, with a new growth recipe, we were able to convert the morphology of epitaxial Pd film from discontinuous, microscopic islands to contiguous, atomically flat regime.  The availability of epitaxial Cr₂O₃ films on metallic electrodes will open many new opportunities for both magnetoelectric and topological applications.   

Transversality-Enforced Tight-Binding Model for 3D Photonic Crystals aided by Topological Quantum Chemistry

Tight-binding (TB) models can accurately predict crystalline systems' band structure and topology. They have been heavily used in solid-state physics due to their versatility and low computational cost.

It is quite straightforward to build an accurate TB model of any crystalline system using the crystal's maximally localized Wannier functions (WF) as a basis.

Unfortunately, in 3D photonic crystals (PhCs), the transversality condition of Maxwell's equations precludes constructing a basis of maximally localized WF via usual techniques.

In this work, we show how to overcome this problem by using topological quantum chemistry, allowing us to express the band structure of the PhC as a difference of elementary band representations (EBRs).

This can be achieved by introducing a set of auxiliary modes recently proposed by Soljačić et al., which regularize the Γ-point obstruction arising from the transversality constraint of Maxwell's equations.

The decomposition into EBRs allows us to isolate a set of orbitals that permit us to construct an accurate transversality-enforced TB model that matches the dispersion, symmetry content, and topology of the 3D PhC under study. 

Moreover, we show how to introduce the effects of a gyrotropic bias in the framework, modeled via non-minimal coupling to a static magnetic field.

Our work provides the first systematic method to analytically model the photonic bands of the lowest transverse modes over the entire Brillouin zone via a transversality-enforced TB model.