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首页> 外文期刊>Physical review >I-V curve signatures of nonequilibrium-driven band gap collapse in magnetically ordered zigzag graphene nanoribbon two-terminal devices
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I-V curve signatures of nonequilibrium-driven band gap collapse in magnetically ordered zigzag graphene nanoribbon two-terminal devices

机译：磁序曲折石墨烯纳米带两末端器件中非平衡驱动带隙塌陷的I-V曲线特征

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摘要

Motivated by the very recent fabrication of sub-10-nm-wide semiconducting graphene nanoribbons [X. Li et al., Science 319, 1229 (2008)], where some of their band gaps extracted from transport measurements were closely 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 (ZGNRs) 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's 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 direct current rnthrough it. The threshold voltage for the onset of band gap collapse depends on the ZGNR length and the rncontact transparency. If the contact resistance is adjusted to experimentally measured value of ≌ 60 kΩ, the rnthreshold voltage for sub-10-nm-wide ZGNR with intercontact distance of ≌ 7 nm is ≈ 0.4 V. For some device setups, including 60 kΩ contacts, the room-temperature I-V curves demonstrate steplike current increase by one order of magnitude at the threshold voltage and can exhibit a hysteresis as well. On the other hand, poorly transmitting contacts can almost completely eliminate abrupt jump in the I-V characteristics. The threshold voltage increases with the ZGNR length (e.g., reaching ≈0.8 V for ≌13-nm-long ZGNR) which provides possible explanation of why the recent experiments [Wang et al., Phys. Rev. Lett. 100, 206803 (2008)] on ～100-nm-long GNR field-effect transistors with bias voltage <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 metallic 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.
机译：受到最近制造的亚10纳米宽半导体石墨烯纳米带的驱动[X. Li et al。，Science 319，1229（2008）]，其中从输运测量中提取的某些带隙与密度函数理论预测紧密匹配，这些预测是沿之字形边缘的磁序导致绝缘基态，我们计算连接到金属触点的有限长度之字形石墨烯纳米带（ZGNR）的电流-电压（IV）特性。使用非平衡格林函数形式主义和适用于重现Hb的局部自旋密度近似描述的Hubbard模型的均值场形式，可以获得在线性响应范围以外的源漏偏置电压下此类器件的传输特性。磁性排序。我们的结果表明，通过使足够大的直流电流通过ZGNR，可以完全消除ZGNR中的磁序和相应的带隙。带隙塌陷开始的阈值电压取决于ZGNR长度和接触透明性。如果将接触电阻调整为实验测量值≌60kΩ，则接触距离为≌7 nm的亚10纳米以下ZGNR的阈值电压为≈0.4V。对于某些器件设置（包括60kΩ触点），室温IV曲线表明，在阈值电压下，阶梯状电流增加了一个数量级，并且也表现出滞后现象。另一方面，传输不良的触点几乎可以完全消除I-V特性的突变。阈值电压随ZGNR长度的增加而增加（例如，对于≌13nm长的ZGNR达到≈0.8V），这为最近进行实验的原因提供了可能的解释[Wang等，Phys。牧师100，206803（2008）]在偏置电压<1 V的约100 nm长的GNR场效应晶体管上未检测到带隙塌陷的I-V曲线特征。因此，观察到具有短ZGNR通道和透明金属触点的两端子器件的IV曲线中突然突变，可以通过全电测量以及电流驱动的磁绝缘子来确认其Z字形边缘的磁排序。非磁性金属非平衡相变。

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