I-V curve signatures of nonequilibrium-driven band gap collapse in magnetically ordered zigzag graphene nanoribbon two-terminal devices

TL;DR

This paper predicts that passing a large enough current through zigzag graphene nanoribbons can eliminate their magnetic ordering and band gap, leading to a measurable abrupt change in the I-V characteristics.

Contribution

It introduces a theoretical model showing current-induced collapse of magnetic order and band gap in ZGNRs, linking it to observable electrical signatures.

Findings

Magnetic ordering in ZGNR can be destroyed by high bias voltage.

Threshold voltage for gap collapse increases with ZGNR length.

Short ZGNR devices may exhibit abrupt I-V jumps confirming magnetic phase transition.

Abstract

Motivated by the very recent fabrication of sub-10-nm-wide semiconducting graphene nanoribbons [X. Li et al., Science 319, 1229 (2008)], whose band gaps extracted from transport measurements were fitted to density functional theory predictions for magnetic ordering along zigzag edges that is responsible for the insulating ground state, we compute current-voltage (I-V) characteristics of finite-length zigzag graphene nanoribbons (ZGNR) attached to metallic contacts. The transport properties of such devices, at source-drain bias voltages beyond the linear response regime, are obtained using the nonequilibrium Green function formalism combined with the mean-field version of the Hubbard model fitted to reproduce the local spin density approximation description of magnetic ordering. Our results indicate that magnetic ordering and the corresponding band gap in ZGNR can be completely eliminated by passing large enough DC current through it. The threshold voltage increases with the ZGNR length (e.g., reaching $\approx 0.8$ V for $\simeq 13$ nm long ZGNR) which provides possible explanation of why the recent experiments [Wang et al., Phys. Rev. Lett. 100, 206803 (2008)] on $\sim 100$ nm long GNR field-effect transistors with bias voltage less than 1 V did not detect the I-V curve signatures of the band gap collapse. Thus, observation of predicted abrupt jump in the I-V curve of two-terminal devices with short ZGNR channel and transparent contacts will confirm its zigzag edge magnetic ordering via all-electrical measurements, as well as a current-flow-driven magnetic-insulator--nonmagnetic-metal nonequilibrium phase transition.