High purity crystalline materials comprise the backbone of modern electronics, and Thrust 1 seeks to develop a new suite of electronic materials suitable for quantum information science. Many of the most promising quantum entangled states in materials are those whose coherence is protected by the symmetries of the material itself. Such states are theorized to be robust against environmental decoherence and have the potential to realize qubits with a substantially diminished need for error correction. Key examples of this are quasiparticles manifesting non-Abelian statistics, such as Majorana zero modes that arise in topological superconductors and anyonic excitations of emergent gauge fields in certain quantum spin liquids. Both host protected edge states and are highly sought-after platforms for quantum information science.
Thrust 1 will develop materials that inherently manifest these states through tightly coupled feedback between theory and experiment. The motivation here is that while known examples are exceedingly rare and often contentious, new theoretical tools and experimental techniques for synthesizing and verifying natively entangled materials are now coming to fruition. This, combined with new data-driven approaches to materials discovery, makes a foundry-based effort for accelerating their development particularly timely. In this thrust, promising new classes of intrinsic topological superconductors and quantum spin liquid states will be explored theoretically, complemented by ab initio modeling of material candidates as well as data-driven approaches to identifying the most promising space for exploration. Ultrahigh purity bulk crystal and thin film synthesis of targeted materials will then couple into a suite of new experimental tools developed to detect and characterize entangled states.
- Year 1 Summary
In Thrust 1 the first year focused on the identification and synthesis of new candidate materials that can host natively entangled states and the development and assembly of unique synthesis and characterization instrumentation. The accelerated discovery of electronic materials suitable for quantum information science is based on a strong connection between data-driven approaches for the identification of promising material spaces, ab initio modeling of material candidates, ultrahigh purity bulk crystal and thin film synthesis of compounds, and a newly developed experimental suite to detect and characterize entangled states.
An approach to natively entangled materials is to search for superconducting compounds with a topologically nontrivial band structure. One promising class of materials are those which possess a so-called Kagome lattice layer crystal structure, as they can exhibit topologically nontrivial electronic band structures. If such a Kagome metal can be tuned to be superconducting in the bulk while exhibiting topologically protected surface states, a system is obtained that can support nonabelian Majorana modes under application of a magnetic field. Researchers in Thrust 1 have identified a new class of materials AV3Sb5 (A = Na, Rb, Cs) that exhibits a layered Kagome structure of vanadium and antimony ions with large alkali metal ions separating the layers. The electronic band structure of these materials shows topologically non-trivial surface states close to the Fermi energy and a continuous direct bandgap throughout the Brillouin zone in band structure calculations. This allows for the classification of these compounds as Z2 topological metals. Researchers of the Foundry have synthesized single crystals of CsV3Sb5 and confirmed the electronic structure calculations in angle-resolved photoemission spectroscopy experiments. The compound exhibits a transition into a superconducting ground state with a critical temperature of TC = 2.7 K. Subsequent experiments in Thrust 1 will address the topological character of the surface superconductivity in this material. Furthermore, a theory specific to the topological superconductivity for these AV3Sb5 Kagome metals using realistic model Hamiltonians is being developed.
In addition to bulk crystal growth, thin film growth via molecular beam epitaxy (MBE) is another powerful method for exploring the emergent properties of candidate topological and superconducting materials. Many of these materials are still in very early stages of discovery, and thus the optimal growth conditions necessary to grow high-quality films must be found before measurements of their electronic properties can be reliably performed. Early attempts by Thrust 1 researchers to find growth conditions for several such materials have been promising and are ongoing. Additionally, advances in understanding of relatively unexplored MBE source materials, such as SnO2, have provided researchers with insights into how films grown using these sources could be improved.
The low critical temperatures of many topological superconductor candidate materials require specific instrumentation for the detection and manipulation of non-abelian quasiparticles in these materials. A general-purpose helium-3 closed-cycle optical cryostat acquired by the Foundry will reach a base temperature of 300 mK and allow for in-dept characterization of entangled states in materials developed in Thrust 1. This cryostat will enable ultrafast optical experiments and magneto-optical microscopy to image and manipulate superconducting vortices that can be used as pinning points for Majorana zero modes. This manipulation technique can be employed to foster braiding of Majorana quasiparticles and has potential use in a topological quantum computing architecture. Furthermore, researchers in Thrust 1 developed a nanoscale superconducting quantum interference device (SQUID) fabricated on the tip of a quartz tube that will enable researchers to probe magnetic and thermal properties of entangled materials at the nanoscale. The system design will be able to achieve a spatial resolution of 30 nm and will include a portable vacuum cell. Using this addition, extremely air sensitive samples can be easily loaded from a glovebox into the instrument and studied without the need to break vacuum.