高压下过渡金属二硫属化物的超导电性

过渡金属二硫属化物是一类典型的二维类石墨烯层状结构的材料,相比于石墨烯的全碳元素组成以及无带隙的电子结构特点,具有更丰富的元素组成、多样的微观结构和奇异的物理性质。过渡金属二硫属化物强烈的各向异性以及在催化、光伏器件和储能材料等领域的优异表现,引起了科学家们浓厚的研究兴趣。它们的层间范德瓦耳斯间隙、层间范德瓦耳斯相互作用、层间堆垛次序对压力非常敏感,易于通过压力调控其晶体结构和电子能带结构,进而发生电子基态的变化。过渡金属二硫属化物的电子基态可以是莫特绝缘体、激子绝缘体、电荷密度波、半导体、(拓扑)半金属、金属,甚至是超导体。在常压条件下,部分过渡金属二硫属化物具有超导电性。实验表明,压力可以诱导过渡金属二硫属化物非超导母体发生超导转变,或者提高超导母体的超导转变温度。文章以典型的过渡金属二硫属化物为例,概述了其在高压调控下超导电性的响应,并简要讨论产生超导电性的物理机制。     

The metallic nature of two-dimensional transition-metal dichalcogenides and MXenes

Metallic two-dimensional (2D) materials such as transition-metal dichalcogenides (TMDCs) and MXenes exhibit intriguing properties, including superconductivity, magnetism and electrocatalysis. Studies on the correlation between their nanoscale structures and properties can facilitate the development of photodetectors, supercapacitors, nanocatalysts, etc., but this topic has not been reviewed systematically. Here, we provide a comprehensive overview on the key factors that dictate the structures and properties of these 2D metals. We examine their phase transitions induced by structural or electronic modifications based on microscopic imaging, spectral characterization, and electrical measurements. From the perspective of surface and interface engineering, we elucidate the influences of lattice defects, dopants, and intercalated species between adjacent layers. Moreover, heterostructures involving highly conductive 2D component(s) are discussed, which may enable the observation of fascinating phenomena and/or synergistic effects due to the interlayer interactions. Finally, we provide insights into opportunities for new applications, e.g., radio-frequency antennas and electromagnetic interference shields. Feasible routes are also proposed to overcome the current challenges.     

Strong-coupling superconductivity induced by calcium intercalation in bilayer transition-metal dichalcogenides

We theoretically investigate the possibility of achieving a superconducting state in transition-metal dichalcogenide bilayers through intercalation, a process previously and widely used to achieve metallization and superconducting states in novel superconductors. For the Ca-intercalated bilayers MoS₂ and WS₂, we find that the superconducting state is characterized by an electron–phonon coupling constant larger than 1.0 and a superconducting critical temperature of 13.3 and 9.3 K, respectively. These results are superior to other predicted or experimentally observed two-dimensional conventional superconductors and suggest that the investigated materials may be good candidates for nanoscale superconductors. More interestingly, we proved that the obtained thermodynamic properties go beyond the predictions of the mean-field Bardeen–Cooper–Schrieffer approximation and that the calculations conducted within the framework of the strong-coupling Eliashberg theory should be treated as those that yield quantitative results.     

Hyperquadratic plasmon dispersion in indium

Experimental evidence suggested that the plasmon dispersion in aluminum has two branches with a discontinuous slope change at low momentum. In a recent experiment with high momentum resolution [J. Sprösser-Prou, A. vom Felde and J. Fink, Phys. Rev. B, 40 (1989) 5799] this discontinuity could not be found. For indium, the dielectric behavior of which closely resembles that of a free electron gas, there are only two experiments, both reporting a discontinuous slope change [T. Aiyama and K. Yada, J. Phys. Soc. Jpn., 38 (1975) 1357. T. Bornemann, J. Eickmans and A. Otto, Solid State Commun., 65 (1988) 381]. We could not find such a discontinuity for indium; on the contrary, the dispersion is hyperquadratic, similar to the findings of Sprösser-Prou et al. for aluminum.     

Effect of dimension on charge order domain in Ruddlesden-Popper manganites

The charge order (CO) domains of dimensionally controlled manganites Pr_1−xCa_xMnO₃ and Pr_1−xCa_1+xMnO₄ (x=0.5, 0.6 and 0.67), which have three-dimensional (3D) and two-dimensional (2D) Mn-O networks, respectively, have been studied by transmission electron microscopy (TEM). Although the electron diffraction data show similar dependences of the modulation wave vector on hole doping x, there are distinctive differences between the 3D and 2D systems in terms of the CO domain sizes. In the 2D system, the TEM images show that the domain size is almost constant irrespective of hole doping x. On the other hand, in the 3D system, the domain size of the incommensurate CO for x=0.6 is much smaller than those of the commensurate CO for x=0.5 and 0.67. Namely, in the 3D system, the CO states are strongly influenced by the incommensurability for the parent lattice. This difference indicates that the dimension of the Mn-O network plays a crucial role in the CO domain and suggests that the electron-lattice coupling of the 3D system is stronger than that of the 2D system.     

A metal-semiconductor transition triggered by atomically flat zigzag edge in monolayer transition-metal dichalcogenides

Due to the structure of three stacked layers, monolayer transition-metal dichalcogenides (TMDs) is different from graphene. Creating atomically flat graphene-like edges in them has long been expected, which is crucial to the modulation of electronic structures in two-dimensional systems. Recently, by thermal annealing, Chen et al. [21] successfully synthesized atomically flat Mo-terminated edge in monolayer MoS₂. Inspired by this, through first-principles calculations, we studied the electronic and transport properties of typical TMD monolayers with transition atom-terminated flat zigzag edges, i.e., ScS₂, VS₂, CrS₂, FeS₂, NiS₂, MoS₂ and WS₂. It is found that the nanoribbons with and without flat edges are both metallic. Interestingly, the vacancy in the flat edge could open a transmission gap at the Fermi level in the ScS₂ ribbon, and trigger a metal-semiconductor transition. Further analysis shows that, the opening of bandgap around the Fermi level induced by the specific pattern of vacancies is the mechanism behind, which could be used as an modulating method for electronic structures. We believe our results are quite beneficial for the development of many other monolayer transition-metal dichalcogenides configurations, showing great application potential.     

Anisotropy of plasmon dispersion in Al

The plasmon dispersion of Al is evaluated for the [100], [110], and [111] directions. The energy eigenvalues and wave functions used have been obtained from energy-band calculations based on the empirical pseudopotential method with 15 plane waves. The anisotropy of the dispersion is found to be in fair agreement with the experimental results.     

Band structure effects on the plasmon dispersion in simple metals

The real part of the effective dielectric function is derived analytically in the pseudopotential approximation. For anyk-direction, the theory allows the evaluation of the plasmon dispersion, _p (k), which is found to be anisotropic in general due to band structure effects. Application of the theory to Al yields an isotropic shift atk=0 of _p (0) _v – 0.3 eV and explains the recently observed anisotropic behaviour of the dispersion very well fork < 0.6k _F .     

Effect of a charge layer on the plasmon-polariton dispersion curve in doped semiconductor superlattices

We consider the effects of a charge layer (accumulation and depletion layers) on the spectrum of collective plasmon-polariton modes of a semiconductor superlattice consisting of alternating mediaA andB, where mediumA is an-doped semiconductor and mediumB an insulator. The effect of the charge layer is taken into account assuming a linear position dependence for the free-carrier concentration and for the dielectric function, within mediumA. Applications are made considering mediumA asn-type semiconductor and mediumB being vacuum. We compare our results with those obtained in the abscence of the charge layer and we discuss their differences.     

Exchange effects in the plasmon dispersion of one-dimensional systems

The influence of the exchange interaction on the plasmon dispersion in one-dimensional model systems has been studied by applying the dynamical exchange decoupling method. In a tight-binding model, the exchange effects result in a decrease of the slope of the plasmon branch for nearly half filled bands, and can even yield a negative dispersion.     

APW dispersion relation for plasmon in metals

Following Ichikawa's technique we derived a dispersion relation for plasmons in metals using APW (Augmented Plane Wave) wave functions. The formula contained herein may be used to acquire information concerning the filling of the energy bands and so data needed in explaining electrical and magnetic properties of metals.     

The effect of grain size on the dispersion of the volume plasmon in Al-Mg-alloys

Dispersion measurements on the volume plasmon in fine dispersed Al-Mg-alloys of the two phase α + β structure show one or two plasmon peaks, depending on wavector k. The relative intensities of the two peaks are k-dependent.     

Thickness dependence of the surface plasmon dispersion in ultrathin aluminum films on silicon

The collective excitation in Al films deposited on Si(1 1 1)-7 × 7 surface was investigated by high-resolution electron-energy-loss spectroscopy (HREELS), X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM). At the Al film thickness d < 10 ML, the surface plasmon of Al film has only a small contribution to the observed energy-loss peaks in the long wavelength limit (q_∥≈0), while its contribution becomes significant for q_∥>d^-1. More interestingly, for thin Al films, the initial slope of the surface plasmon dispersion curve is positive at q_∥∼0, in a sharp contrast to bulk Al surface where the energy dispersion is negative. These observations may be explained based on the screening interaction of the space charge region at the Al-Si interface.