Session A01: Design and Synthesis of New Quantum Materials I

A Heterolayered Fe-based Silicide Superconductor

Superconductivity is among the most intensively sought states of matter. However, the discovery of new superconductors often occurs by chance from a wide range of materials largely due to the lack of understanding of the true nature of superconductivity. Significantly, the discovery of cuprate and Fe-based superconductors, many of which contain heterolayered building blocks of [CuO₂]^2- and [FeAs]^- layers respectively, suggests a rational design approach for unconventional superconductors by combining functional layers with non-magnetic spacer layers, indicating the potential for a systematic approach to discovering new superconductors

Here we demonstrate an encouraging example of this strategy with a newly synthesized Fe-based Heterolayered Silicide, Y₄FeSi₈, in single-crystal form. The crystal structure of Y₄FeSi₈ consists of two distinct functional layers (of edge-sharing Fe-Si square pyramidal) separated by non-magnetic spacer layers ( of double vertical honeycomb Y-Si layers). The synthesis method of this new heterolayered Fe-based silicide will be discussed along with physical properties confirming the superconductivity transition.   

Heteroanionic Stabilization of Ni1+ with Nonplanar Coordination in Layered Nickelates

We present electronic structure calculations on layered nickelate oxyfluorides derived from the Ruddlesden-Popper arisotype structure in search of unidentified materials that may host nickelate superconductivity. By performing anion exchange of oxygen with fluorine, we design two heteroanionic polymorphs with Ni¹⁺ in 4-coordinate and 5-coordinate square planar and square pyramidal geometries, respectively. We suggest chemical reactions to guide their synthesis. These oxyfluorides are weakly correlated antiferromagnetic insulators and their nonmagnetic phases exhibit quasi-2D Fermi surfaces dominated by Ni d_x2−y2 states, which strikingly resemble undoped cuprate superconductors. We use our understanding to propose doping strategies and layered nickelate oxyfluorides with tunable electronic and magnetic structures for experimentation.   

High throughput search for delafossite-hosted honeycomb and kagome phases

Delafossites (ABO₂), are layered materials that can exhibit a wide range of electronic and optical properties. Modifying host delafossites via doping provides a possible route to create ordered kagome or honeycomb phases. In this study, we explore many possible candidate kagome and honeycomb phases via high-throughput density functional theory calculations. Potential dopants were selected from the parent compounds of known ternary delafossite oxides from the Inorganic Crystal Structure Database. We show that for A-site doping, only limited range of elemental species are thermodynamically stable, and those that are display only low propensity for mixing or ordering. In contrast, we show that the B-site oxide sub-units more readily take up guest species and show higher rates of ordering. We identify four candidate B-site kagome and fifteen candidate B-site honeycombs with formation energes 50 meV/f.u. below other competing phases. A number of these constitute novel correlated metals, which may be of interest for subsequent efforts in synthesis as well as theory efforts to identify potentially non-trival topological states.   

Hidden Magnetic Toroidicity in Collinear Spins

A magnetic toroidal moment is the order parameter for ferro-toroidicity, playing a prominent role in condensed matter physics by its close ties to off-diagonal conductivity and non-reciprocal transport. This concept shows promise for applications in quantum computing devices and quantum communication protocols. Typically, a magnetic toroidal moment refers to the axial vector of a spin vortex, suggesting that collinear spins cannot host a toroidal moment parallel to the spins. However, in this talk, I will present the experimental observation of emergent out-of-plane magnetic toroidal moment in a triangular Co²⁺-based collinear antiferromagnet. The magnetic structure determined by single crystal and powder neutron diffraction exhibits an A-type collinear antiferromagnetic order with k = (0, 0, 0) on an R-3 crystallographic lattice. A significant magnetic toroidal moment is evidenced by a pronounced off-diagonal linear magnetoelectricity. Symmetry analysis reveals a scenario that a combination of a diagonal linear magnetoelectric sublattice plus a ferro-rotation type structural distortion gives rise to an effective magnetic toroidal moment. These results demonstrate a rare-earth-free magnetoelectric material with excellent performance, and that the symmetry analysis is a powerful tool in predicting emergent phenomena and designing functional quantum materials.   

Fe-based Ternary Magnetic Nitrides are Negative Charge Transfer Compounds

Nitrides display unique chemical and structural characteristics rendering them attractive for a wide variety of applications that complement those of oxides. However, limited by the inherent stability of dinitrogen and competition with oxide formation, only a small number of transition metal ternary nitrides are known. To this end, we employ first-principles calculations to study the magnetic and electronic properties of a ternary iron nitrides with Fe³⁺ d⁵ electronic configurations. We examine the electronic structure in detail and explain how σ- and π-bonding interactions contribute to the stability of various antiferromagnetic insulating and potentially metallic phases. Our density functional theory (DFT) results suggest the ternary iron nitrides are proximate to the negative charge transfer insulator-metal boundary owing to the strong local π-donor character of the N^3- to Fe³⁺. This material family may offer a promising new materials platform for exploring Mott and Slater metal-insulator transitions.   

Operando electrical transport during topotactic reduction of nickelate superconductors

The use of topotactic reduction reactions has enabled the exploration of new classes of quantum materials. However, the reduction process remains largely unexplored due to the limited accessibility of characterization techniques during the reaction. To address this, we have developed an apparatus allowing operando measurement of (magneto-)electrical transport properties throughout the topotactic reduction process, spanning temperatures from 620 K to 2 K. Using this tool, we successfully synthesized superconducting infinite-layer nickelates via topotactic reduction from perovskite nickelate thin films. Guided by electrical transport, we have identified distinct structural phases during the reduction reactions, tracked the evolution of superconductivity with changing oxygen stoichiometry, and determined optimal reduction conditions. Our method offers a versatile approach applicable to a growing family of metastable materials accessible via topochemistry.   

Integration of van der Waals Heterostructures with Programmable Ferroelectric Thin Films

We present the ability to modulate the electronic properties of van der Waals heterostructures with high resolution ferroelectric programming as a novel pathway to the realization of a solid-state 2D quantum simulator. Using a precision ultra-low voltage electron beam lithography technique to switch the polarization of ferroelectric Al1-xBxN ferroelectricity at the nanoscale, sub-micron ferroelectric domains are patterned on Al1-xBxN thin films. We demonstrate this programmability by the induced ferroelectric field effect on a graphene/AlBN device, where the manipulation of ferroelectric polarization is used to create a p-n junction in the graphene layer. We discuss some of the complications in fabrication of high-quality devices that integrate 2D heterostructures with ferroelectric thin film substrates.   

Engineering topologically nontrivial structures by stacking topologically trivial materials

Nonmagnetic topological insulators (TIs) are crystalline materials with conserved time-reversal symmetry that feature robust metallic surface states coexisting with an insulating bulk phase and are characterized by non-zero topological invariants. Certain non-centrosymmetric crystals exhibit remarkably strong Rashba spin-orbit coupling, leading to a significant splitting of the uppermost valence band from the other bands, with the former surpassing the bottom-most conduction band. This often results in a band inversion near the Fermi level, transforming a topologically trivial material into a nontrivial one. In 2D TIs, this kind of transformation gives rise to the emergence of 1D metallic edge states, which form the basis of the quantum spin Hall effect. In this work, we theoretically propose a novel approach to create topologically nontrivial 2D structures by strategically layering multiple topologically trivial constituent layers of Rashba semiconductors. We present our findings by considering the BiSb and GeTe monolayer structures as prototypes. In their free-standing form, these monolayers are topologically trivial. However, by strategically stacking these seemingly trivial layers, we unveil the potential for generating intriguing topologically nontrivial properties.