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Linear dispersion near the Dirac points in the band structure of graphenes can give rise to novel physical properties. We calculate the electronic contribution to the Raman spectra in graphenes, which also shows novel features. In the clean limit, the Raman spectrum in the undoped graphene is linear (with a universal slope against impurity scattering) at low energy due to the linear dispersion near the Dirae points, and it peaks at a position corresponding to the van Hove singularity in the band structure. In a doped graphene, the electronic Raman absorption is forbidden up to a vertical inter-band particle-hole gap. Beyond the gap the spectrum follows the undoped case. In the presence of impurities, absorption within the gap (in the otherwise clean case) is induced, which is identified as the intra-band contribution. The Drude-like intra-band contribution is seen to be comparable to the higher energy inter-band Raman peak. The results are discussed in connection to experiments. 相似文献
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Theoretical Prediction of Superconductivity in Boron Kagome Monolayer:MB3(M=Be,Ca,Sr) and the Hydrogenated CaB3
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Using first-principles calculations, we predict a new type of two-dimensional(2D) boride MB3(M = Be,Ca, Sr), constituted by boron kagome monolayer and the metal atoms adsorbed above the center of the boron hexagons. The band structures show that the three MB3 compounds are metallic, thus the possible phononmediated superconductivity is explored. Based on the Eliashberg equation, for BeB3, CaB3, and SrB3, the calculated electron–phonon coupli... 相似文献
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本文利用基于密度泛函理论(DFT)的第一性原理计算研究了它们的电子结构和光学性质.光学性质的计算结果和实验相一致.结果表明,Fe或Ag掺杂后,K2Ti6O13的带隙中出现了杂质带且其带隙值变小,因而使掺杂后的K2Ti6O13的吸收边发生红移并实现了其对可见光吸收.其中杂质带主要由Fe 3d态或Ag 4d态与Ti 3d态和O 2p态杂化而成.对于Fe掺杂的K2Ti6O13,杂质带位于带隙中间,因此可以作为电子从价带跃迁到导带的桥梁.对于Ag掺杂的K2Ti6O13,杂质带位于价带顶附近为受主能级,可以降低光生载流子的复合概率.实验和计算研究表明,通过Fe或Ag的掺杂可以实现了K2Ti6O13对可见光的吸收,这对进一步研究K2Ti6O13的光学性质具有重要意义. 相似文献
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Mechanism of Pseudogap Detected by Electronic Raman Scattering: Phase Fluctuation or Hidden Order?
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We study the electronic Raman scattering in the cuprates to distinguish the two possible scenarios of the pseudogap normal state. In one scenario, the pseudogap is assumed to be caused by phase fluctuations of the preformed Cooper pairs. We find that pair-breaking peaks appear in both the B1g and B2g Raman channels, and they axe smeared and tend to shift to the same energy with the increasing strength of phase fluctuations. Thus both channels reflect the same pairing energy scale, irrespectively of the doping level. In another scenario, the pseudogap is assumed to be caused by a hidden order that competes with the superconducting order. As an example, we assume that the hidden order is the d-density-wave (DDW) order. We find analytically and numerically that in the DDW normal state there is no Raman peak in the B2g channel in a tight-binding model up to the second nearest-neighbor hopping, while the Raman peak in the Big channel reflects the energy gap caused by the DDW order. This behavior is in agreement with experiments in the pseudogap normal state. To gain further insights, we also calculate the Raman spectra in the DDW+SC state. We study the doping and temperature dependence of the peak energy in both channels and find a two-gap behavior, which is in agreement with recent Raman experiments. Therefore, our results shed light on the hidden order scenario for the pseudogap. 相似文献
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The isovalent iron chalcogenides,FeSe_(0.5)Te_(0.5) and FeS,share similar lattice structures but behave very differently in superconducting properties.We study the underlying mechanism theoretically.By first principle calculations and tightbinding fitting,we find the spectral weight of the d_(X~2-Y~2) orbital changes remarkably in these compounds.While there are both electron and hole pockets in FeSe_(0.5)Te_(0.5) and Fe S,a small hole pocket with a mainly d_(X~2-Y~2) character is absent in FeS.We find the spectral weights of d_(X~2-Y~2) orbital change remarkably,which contribute to electron and hole pockets in FeSe_(0.5)Te_(0.5) but only to electron pockets in FeS.We then perform random-phase-approximation and unbiased singular-mode functional renormalization group calculations to investigate possible superconducting instabilities that may be triggered by electron-electron interactions on top of such bare band structures.For FeSe_(0.5)Te_(0.5) ,we find a fully gapped s~±-wave pairing that can be associated with spin fluctuations connecting electron and hole pockets.For Fe S,however,a nodal dxy(or d_(x~2-y~2) in an unfolded Broullin zone)is favorable and can be related to spin fluctuations connecting the electron pockets around the corner of the Brillouin zone.Apart from the difference in chacogenide elements,we propose the main source of the difference is from the d_(X~2-Y~2) orbital,which tunes the Fermi surface nesting vector and then influences the dominant pairing symmetry. 相似文献
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电子拉曼实验表明在空穴型掺杂的铜氧化物超导体中存在两能隙行为,即在欠掺杂区,随着掺杂浓度的降低,一个能隙逐渐增大而且在超导转变温度以上仍然存在,而另一个能隙逐渐减小且在DDW态依然存在.解释两能隙行为非常重要因为它与赝能隙的机理密切相关.本文计算了超导序和d-density-wave(DDW)序竞争机理下相图上不同区域的电子拉曼谱,发现欠掺杂区能隙表现出两能隙行为,与实验一致.特别地,本文发现B1g峰对应能量由超导和DDW序共同决定,且随着掺杂浓度的降低而增大,在D
关键词:
两能隙
电子拉曼散射
竞争序 相似文献
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