We show, through first-principles calculations, that by applying an experimentally accessible transverse electric field, certain boron and nitrogen periodically codoped graphene nanoribbons (GNRs) have tunable topological phases. The tunability arises from a field-induced band inversion due to an opposite response of the conduction- and valence-band states to the electric field. 

With a spatially varying applied field, segments of GNRs of distinct topological phases are created, resulting in a field-programmable array of topological junction states, each may be occupied with charge or spin.

Our study has provided a new way of controlling topological junction or end states in 1D systems which may be used in building quantum dot spin qubits with tunable couplings, and this is a promising new approach for designing future GNR-based quantum electronic devices. This work has been published in Physical Review Letters. 

The ferromagnetically ordered edge states in zigzag graphene nanoribbons have sparked renewed interest in carbon-based spintronics. Despite recent advances in the bottom-up synthesis of GNRs, the magnetic edge states of zigzag GNRs (ZGNRs) has long been obscured from direct observation by a strong hybridization of the edges states with the surface states of the underlying support. Our experimental collaborator, the Felix Fischer group, is able to synthesize a periodically nitrogen doped ZGNR and decouple the N-doped ZGNR from the metallic substrate by the tip of scanning tunneling microscope (STM). 

Our first-principles GW calculations predicted two low-lying nitrogen lone pair flat bands with a giant spin splitting, and the splitting is due to the magnetic edge state in the ZGNR. Our experimental collaborator measured this spin splitting which matches quantitatively well with our first-principles GW calculations. Our findings is the first experimental evidence to the magnetic order in ZGNRs and provide a robust platform for their exploration and functional integration into nanoscale sensing and logic devices. 

Our results have been published in Nature. 

Trap-assisted nonradiative recombination is a key mechanism limiting the efficiency of optoelectronic devices such as light-emitting diodes. It has been challenging, however, to identify defect-related sources of loss in wider-band-gap materials emitting in the blue or ultraviolet. The rate of trap-assisted recombination via multiphonon emission (MPE) become negligibly low in materials with band gaps larger than about 2.5 eV. Experimentally, however, trap-assisted recombination is observed to persist at larger band gaps [4]. 

We have found that trap-assisted Auger-Meitner (TAAM) recombination can resolve this puzzle. Similar to band-to-band Auger-Meitner recombination, a TAAM process enables capture by exciting a carrier to a higher-energy state, and can be an alternative mechanism to complete the nonradiative recombination cycle when MPE capture has a negligibly low rate.  

I have developed a practical first-principles methodology to calculate the TAAM rate for defects and impurities in semiconductors and insulators. In or test case of Ca substitutional impurity in GaN, we found that for band gaps larger than 2.5 eV the combination of hole capture by MPE and electron capture by TAAM results in recombination rates that are more than a billion times larger than the recombination rate governed by MPE alone. As a result, the inclusion of the TAAM-assisted processes can account for the observed nonradiative recombination rate in wider-band-gap materials, where rates due to MPE alone become negligibly low. Our result is published in Physical Review Letters.  

The research achievement is reported as top news in the college of engineering in UC Santa Barbara.