Department of Physics, Department of Chemistry, and JILA, University of Colorado, Boulder, Colorado, 80309, USA.
Department of Physics, University of Washington, Seattle, Washington 98195, USA.
Nat Commun. 2015 Apr 21;6:6849. doi: 10.1038/ncomms7849.
The insulator-metal transition (IMT) of vanadium dioxide (VO2) has remained a long-standing challenge in correlated electron physics since its discovery five decades ago. Most interpretations of experimental observations have implicitly assumed a homogeneous material response. Here we reveal inhomogeneous behaviour of even individual VO2 microcrystals using pump-probe microscopy and nanoimaging. The timescales of the ultrafast IMT vary from 40±8 fs, that is, shorter than a suggested phonon bottleneck, to 200±20 fs, uncorrelated with crystal size, transition temperature and initial insulating structural phase, with average value similar to results from polycrystalline thin-film studies. In combination with the observed sensitive variations in the thermal nanodomain IMT behaviour, this suggests that the IMT is highly susceptible to local changes in, for example, doping, defects and strain. Our results suggest an electronic mechanism dominating the photoinduced IMT, but also highlight the difficulty to deduce microscopic mechanisms when the true intrinsic material response is yet unclear.
五十年前发现的二氧化钒（VO2）的绝缘-金属转变（IMT）一直是关联电子物理学中的一个长期挑战。大多数对实验观察的解释都隐含地假设了材料的均匀响应。在这里，我们使用泵浦-探测显微镜和纳米成像技术揭示了即使是单个 VO2 微晶体的非均匀行为。超快 IMT 的时间尺度从 40±8fs 变化，即短于建议的声子瓶颈，到 200±20fs，与晶体尺寸、转变温度和初始绝缘结构相无关，平均值与多晶薄膜研究的结果相似。结合观察到的热纳米域 IMT 行为的敏感变化，这表明 IMT 极易受到掺杂、缺陷和应变等局部变化的影响。我们的结果表明，电子机制主导了光致 IMT，但也突出了在真正的内在材料响应尚不清楚的情况下，推断微观机制的困难。
