Quantum materials

The idea of realizing and harnessing coherent quantum bits in scalable solid-state environments has attracted widespread attention in the past decade. One of the milestones in the field has been the coherent manipulation of the single nitrogen-vacancy (NV) defect spin in diamond. However, inherent difficulties in growing and controlling the lattice of C diamond pose severe limitations to the use of the NV center for scalable quantum technologies. In close collaboration with experiments, we are searching for analogs to this defect in carbide and nitride materials. We are also studying the fundamental properties of several other quantum materials within the quantum sciences research areas at the University of Chicago (http://quantum.uchicago.edu/). Part of our work is supported by the Chicago MRSEC (https://mrsec.uchicago.edu/).

Quantum coherence of defect spins in solids

Long coherence times are key to the performance of quantum bits (qubits). In quantum computing, long spin coherence times are necessary for executing quantum algorithms with many gates. Qubits with robust coherence are also ideal systems for developing applications such as collective quantum memories and ultra-sensitive quantum sensors. We theoretically and experimentally showed that the Hahn-echo coherence time (T2) of electron spins associated with divacancy defects in 4H-SiC reaches 1.3 ms, one of the longest T2 times of an electron spin in a naturally isotopic crystal. Using a first-principles microscopic quantum-bath model, we identified key factors determining the unusually robust coherence. Our results point to polyatomic crystals as promising hosts for coherent qubits in the solid state. In addition, we investigated large metal ion-vacancy pairs in both SiC and AlN and found them to be promising qubit candidates.

Surface defects for atomic scale electronics

Surface defects created and probed with scanning tunneling microscopes are a promising platform for atomic-scale electronics and quantum information technology applications. Using first-principles calculations we demonstrate how to engineer dangling bond (DB) defects on hydrogenated Si(100) surfaces, which give rise to isolated impurity states that can be used in atomic-scale devices. In particular, we show that sample thickness and biaxial strain can serve as control parameters to design the electronic properties of DB defects. While in thick Si samples the neutral DB state is resonant with bulk valence bands, ultrathin samples (1-2 nm) lead to an isolated impurity state in the gap; similar behavior is seen for DB pairs and DB wires. Strain further isolates the DB from the valence band, with the response to strain heavily dependent on sample thickness. These findings suggest new methods for tuning the properties of defects on surfaces for electronic and quantum information applications. Finally, we present a consistent and unifying interpretation of many results presented in the literature for DB defects on hydrogenated silicon surfaces, rationalizing apparent discrepancies between different experiments and simulations.

Defect spins in functional ionic crystals

To date, defect qubits have only been realized in materials with strong covalent bonds. We recently introduce a strain-driven scheme to rationally design defect qubits in functional ionic crystals. Using a combination of state-of-the-art ab-initio calculations based on hybrid density functional and many-body perturbation theory, we predicted that the negatively charged nitrogen vacancy center in piezoelectric aluminum nitride exhibits spin-triplet ground states under realistic uni- and bi-axial strain conditions; such states may be harnessed for the realization of qubits.

Boron: a frustrated element

All elements, except for helium, appear to solidify into crystalline forms at zero temperature, and it is generally assumed that the introduction of lattice defects results in an increase in internal energy. By using lattice Monte Carlo techniques combined with ab initio calculations, we find that β-Boron is stabilized by a macroscopic amount of intrinsic defects that are responsible not only for entropic effects but also for a reduction in internal energy. These defects enable the conversion of two-center to three-center bonds and are accompanied by the presence of localized, nonconductive electronic states in the optical gap. The ab initio Ising model describing the partial occupancy of β-boron has macroscopic residual entropy, suggesting that boron is a frustrated system analogous to ice and spin ice.

BaHfN2: a superconductor?

We have examined the electronic and vibrational structure of the ternary nitride BaHfN2 within density-functional theory. We find that BaHfN2 has chemical and electronic similarities with high Tc metallochloronitrides MNCl’s M=Ti,Hf,Zr, so its candidacy as another high Tc superconducting nitride is plausible. The basic electronic and vibrational properties of the undoped insulating phase provide a basis for an understanding of the behavior of BaHfN2 upon doping.