Engineered Designer Lattices for Quantum Systems

Summary

Engineering programmable quantum materials at the nanoscale is a main challenge in quantum nanoelectronics. With recent advances in nanoscale patterning methods, superlattices at a resolution of above 10 nm can be formed on complex oxide interfaces and ferroelectric materials, making it possible to imprint patterns onto van der Waals materials such as graphene and transition metal dichalcogenides (TMDs). Exposing low-dimensional materials to a superlattice potential not only modified their band structures – enabling phases absent in their intrinsic crystal lattices – but also provides an important playground to study the Hubbard model, a key step toward understanding interaction-driven phases in solid-state systems. Combined with the world’s frontier scanning tunneling microscopy (STM) capability, the local property of each single site can be precisely probed, understood, and even controlled, and the electron density of state can be imaged. This opens a new pathway to understand the strong correlation in the system, also potentially serving as a solid-state platform for analog quantum simulation. 

Description

Exposing the low-dimensional materials to a superlattice periodic potential is an effective way to engineer their electronic and other solid-state properties. One example is the moiré superlattice, created by stacking two-dimensional layers at a twist angle, which leads to novel band structures and emergent phases such as Mott-like insulator state and superconductivity. Beyond moiré systems, certain lattice geometries – such as Kagome and Lieb lattices - are recognized for hosting flat bands, now understood to be crucial for introducing strong correlation to the system. 

Recent developments and improvements in nanoscale patterning methods allow for superlattice formation at complex oxide heterointerface electron gas and ferroelectric thin films with down to 10 nm resolution, which can then be imprinted into van der Waals materials placed on top. Specifically, ultra-low-voltage electron beam direct irradiation leverages precise control of beam energy and dose to achieve high-resolution, resist-free patterning. It extends the possibilities beyond moiré systems, where strain relaxation, single-site control, and limited lattice geometries have posed challenges. This approach provides more degrees of freedom for generating quantum materials in a programmable fashion.  

Superlattices not only permit us to modify and explore electronic properties, they also offer a promising platform for analog quantum simulation. In particular, superlattices enable testing of theoretical models, including the Hubbard model, which describes interactions in many-body systems. While systems like ultra-cold atom lattices have provided a successful route to simulating such Hamiltonians, there are still challenges, such as lowering the temperature or having more tuning capabilities. There is a strong motivation to develop a solid-state platform for analog quantum simulation. 

Quantum materials which we are patterning include: 

Complex Oxides  
Two-dimensional electron gases at SrTiO₃ (STO) or KTaO₃ (KTO)-based interfaces are of particular interest.  For STO - the first found superconducting semiconductor - the exact pairing mechanism remains under debate. There is a superconducting 2-dimensional electron gas (2DEG) formed at the interface when a thin layer of another insulator, LaAlO3 (LAO), is grown on top. It has been reported that this LAO/STO 2DEG has superconductivity, magnetism, tunable spin-orbit coupling, and a tunable metal-to-insulator transition. Using a biased AFM tip, nanodevices such as a single-electron transistor and a ballistic transport waveguide have been demonstrated. These devices show emergent behavior, such as electron pairing without superconductivity and one-dimensional superconductivity, demonstrating strong correlations in the system that are not well understood. KTO-based 2DEG, such as LAO/KTO or AlO_x/KTO, features an anisotropic superconductivity tied to its crystal orientations. With electron beam patterning, superconducting nanoscale devices as well as superlattices can be generated at their interfaces. Together with the STM, which can measure the electron density of state locally, one of our goals is to understand the pairing mechanism of these superconductors. 

Fig. 1. Schematic of ultra-low voltage ebeam (ULVEB) writing of polarization pattern to induce a pn junction potential in the top layer graphene. 

Credit: NIST

Ferroelectric Materials  
By applying direct e-beam irradiation, the ferroelectric polarization switches and enables the creation of well-controlled ferroelectric domains. The polarization patterns can then be used to induce a potential landscape in 2D materials such as graphene, as shown in Fig. 1.

Van der Waals Materials
Patterned potential on a programmable substrate can be transferred to vdW layers by electrostatic gating, allowing for tuning of electronic properties in these atomically thin materials. Both ferroelectric patterns, as shown in Fig. 1, or patterns at complex oxide interfaces can be transferred electrostatically into vdW materials.