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Unusual Suppression of the Superconducting Energy Gap and Critical Temperature in Atomically Thin NbSe2
Nano Letters ( IF 9.6 ) Pub Date : 2018-03-12 00:00:00 , DOI: 10.1021/acs.nanolett.8b00443 E. Khestanova 1 , J. Birkbeck 1 , M. Zhu 1 , Y. Cao 1, 2 , G. L. Yu 1 , D. Ghazaryan 1 , J. Yin 1 , H. Berger 3 , L. Forró 3 , T. Taniguchi 4 , K. Watanabe 4 , R. V. Gorbachev 1, 2 , A. Mishchenko 1 , A. K. Geim 1, 2 , I. V. Grigorieva 1
Nano Letters ( IF 9.6 ) Pub Date : 2018-03-12 00:00:00 , DOI: 10.1021/acs.nanolett.8b00443 E. Khestanova 1 , J. Birkbeck 1 , M. Zhu 1 , Y. Cao 1, 2 , G. L. Yu 1 , D. Ghazaryan 1 , J. Yin 1 , H. Berger 3 , L. Forró 3 , T. Taniguchi 4 , K. Watanabe 4 , R. V. Gorbachev 1, 2 , A. Mishchenko 1 , A. K. Geim 1, 2 , I. V. Grigorieva 1
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It is well-known that superconductivity in thin films is generally suppressed with decreasing thickness. This suppression is normally governed by either disorder-induced localization of Cooper pairs, weakening of Coulomb screening, or generation and unbinding of vortex–antivortex pairs as described by the Berezinskii–Kosterlitz–Thouless (BKT) theory. Defying general expectations, few-layer NbSe2, an archetypal example of ultrathin superconductors, has been found to remain superconducting down to monolayer thickness. Here, we report measurements of both the superconducting energy gap Δ and critical temperature TC in high-quality monocrystals of few-layer NbSe2, using planar-junction tunneling spectroscopy and lateral transport. We observe a fully developed gap that rapidly reduces for devices with the number of layers N ≤ 5, as does their TC. We show that the observed reduction cannot be explained by disorder, and the BKT mechanism is also excluded by measuring its transition temperature that for all N remains very close to TC. We attribute the observed behavior to changes in the electronic band structure predicted for mono- and bi- layer NbSe2 combined with inevitable suppression of the Cooper pair density at the superconductor-vacuum interface. Our experimental results for N > 2 are in good agreement with the dependences of Δ and TC expected in the latter case while the effect of band-structure reconstruction is evidenced by a stronger suppression of Δ and the disappearance of its anisotropy for N = 2. The spatial scale involved in the surface suppression of the density of states is only a few angstroms but cannot be ignored for atomically thin superconductors.
中文翻译：
原子薄NbSe 2中超导能隙和临界温度的异常抑制
众所周知，通常随着厚度的减小而抑制薄膜中的超导性。这种抑制作用通常是由Beroperskii-Kosterlitz-Thouless（BKT）理论描述的，由无序诱导的库珀对定位，减弱的库仑筛选或涡旋-反涡旋对的产生和解离控制。与一般预期相反，已经发现超薄超导体的典型例子：很少层的NbSe 2仍然可以超导至单层厚度。在这里，我们报告了几层NbSe 2的高质量单晶中超导能隙Δ和临界温度T C的测量结果，使用平面结隧穿光谱和横向传输。我们观察到一个充分发展的间隙即迅速减少与层的数目的装置Ñ ≤5一样，其Ť Ç。我们表明观察到的减少不能用无序来解释，并且通过测量其所有N保持非常接近T C的转变温度也可以排除BKT机制。我们将观察到的行为归因于为单层和双层NbSe 2预测的电子能带结构的变化，并不可避免地抑制了超导体-真空界面处的库珀对密度。我们对N的实验结果> 2与在后一种情况下预期的Δ和T C的依赖性很好地吻合，而通过N的更强的Δ抑制和各向异性的消失证明了带结构重建的效果。所涉及的空间尺度表面密度的抑制仅几埃，但是对于原子上薄的超导体是不容忽视的。
更新日期：2018-03-12
中文翻译：
原子薄NbSe 2中超导能隙和临界温度的异常抑制
众所周知，通常随着厚度的减小而抑制薄膜中的超导性。这种抑制作用通常是由Beroperskii-Kosterlitz-Thouless（BKT）理论描述的，由无序诱导的库珀对定位，减弱的库仑筛选或涡旋-反涡旋对的产生和解离控制。与一般预期相反，已经发现超薄超导体的典型例子：很少层的NbSe 2仍然可以超导至单层厚度。在这里，我们报告了几层NbSe 2的高质量单晶中超导能隙Δ和临界温度T C的测量结果，使用平面结隧穿光谱和横向传输。我们观察到一个充分发展的间隙即迅速减少与层的数目的装置Ñ ≤5一样，其Ť Ç。我们表明观察到的减少不能用无序来解释，并且通过测量其所有N保持非常接近T C的转变温度也可以排除BKT机制。我们将观察到的行为归因于为单层和双层NbSe 2预测的电子能带结构的变化，并不可避免地抑制了超导体-真空界面处的库珀对密度。我们对N的实验结果> 2与在后一种情况下预期的Δ和T C的依赖性很好地吻合，而通过N的更强的Δ抑制和各向异性的消失证明了带结构重建的效果。所涉及的空间尺度表面密度的抑制仅几埃，但是对于原子上薄的超导体是不容忽视的。
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