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Register a new userHybrid $mathbf{kcdot p}$-tight-binding model for intersubband optics in atomically thin InSe films
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We propose atomic films of n-doped $gamma$-InSe as a platform for intersubband optics in the infrared (IR) and far infrared (FIR) range, coupled to out-of-plane polarized light. Depending on the film thickness (number of layers) of the InSe film these transitions span from $sim 0.7$ eV for bilayer to $sim 0.05$ eV for 15-layer InSe. We use a hybrid $mathbf{k} cdot mathbf{p}$ theory and tight-binding model, fully parametrized using density functional theory, to predict their oscillator strengths and thermal linewidths at room temperature.
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We present a density functional theory parametrized hybrid k$cdot$p tight binding model for electronic properties of atomically thin films of transition-metal dichalcogenides, 2H-$MX_2$ ($M$=Mo, W; $X$=S, Se). We use this model to analyze intersubband transitions in $p$- and $n$-doped $2{rm H}-MX_2$ films and predict the line shapes of the intersubband excitations, determined by the subband-dependent two-dimensional electron and hole masses, as well as excitation lifetimes due to emission and absorption of optical phonons. We find that the intersubband spectra of atomically thin films of the 2H-${MX_2}$ family with thicknesses of $N=2$ to $7$ layers densely cover the infrared spectral range of wavelengths between $2$ and $30 {rm mu m}$. The detailed analysis presented in this paper shows that for thin $n$-doped films, the electronic dispersion and spin-valley degeneracy of the lowest-energy subbands oscillate between odd and even number of layers, which may also offer interesting opportunities for quantum Hall effect studies in these systems.
Artificial graphene consisting of honeycomb lattices other than the atomic layer of carbon has been shown to exhibit electronic properties similar to real graphene. Here, we reverse the argument to show that transport properties of real graphene can be captured by simulations using theoretical artificial graphene. To prove this, we first derive a simple condition, along with its restrictions, to achieve band structure invariance for a scalable graphene lattice. We then present transport measurements for an ultraclean suspended single-layer graphene pn junction device, where ballistic transport features from complex Fabry-Perot interference (at zero magnetic field) to the quantum Hall effect (at unusually low field) are observed and are well reproduced by transport simulations based on properly scaled single-particle tight-binding models. Our findings indicate that transport simulations for graphene can be efficiently performed with a strongly reduced number of atomic sites, allowing for reliable predictions for electric properties of complex graphene devices. We demonstrate the capability of the model by applying it to predict so-far unexplored gate-defined conductance quantization in single-layer graphene.
We present a tight-binding (TB) model and $mathbf{kcdot p}$ theory for electrons in monolayer and few-layer InSe. The model is constructed from a basis of all $s$ and $p$ valence orbitals on both indium and selenium atoms, with tight-binding parameters obtained from fitting to independently computed density functional theory (DFT) band structures for mono- and bilayer InSe. For the valence and conduction band edges of few-layer InSe, which appear to be in the vicinity of the $Gamma$ point, we calculate the absorption coefficient for the principal optical transitions as a function of the number of layers, $N$. We find a strong dependence on $N$ of the principal optical transition energies, selection rules, and optical oscillation strengths, in agreement with recent observations cite{Bandurin2016}. Also, we find that the conduction band electrons are relatively light ($m propto 0.14-0.18 m_e$), in contrast to an almost flat, and slightly inverted, dispersion of valence band holes near the $Gamma$-point, which is found for up to $N propto 6$.
Light-matter interaction at the atomic scale rules fundamental phenomena such as photoemission and lasing, while enabling basic everyday technologies, including photovoltaics and optical communications. In this context, plasmons --the collective electron oscillations in conducting materials-- are important because they allow manipulating optical fields at the nanoscale. The advent of graphene and other two-dimensional crystals has pushed plasmons down to genuinely atomic dimensions, displaying appealing properties such as a large electrical tunability. However, plasmons in these materials are either too broad or lying at low frequencies, well below the technologically relevant near-infrared regime. Here we demonstrate sharp near-infrared plasmons in lithographically-patterned wafer-scale atomically-thin silver crystalline films. Our measured optical spectra reveal narrow plasmons (quality factor $sim4$), further supported by a low sheet resistance comparable to bulk metal in few-atomic-layer silver films down to seven Ag(111) monolayers. Good crystal quality and plasmon narrowness are obtained despite the addition of a thin passivating dielectric, which renders our samples resilient to ambient conditions. The observation of spectrally sharp and strongly confined plasmons in atomically thin silver holds great potential for electro-optical modulation and optical sensing applications.
A realistic tight-binding model is developed and employed to elucidate the resistivity size effect due to steps on Ru thin films. The resistivity of two different film orientations, $(0001)$ and $(1 bar{1}00)$, is computed for transport along a $[1 1 bar{2} 0]$ direction both for smooth surfaces and for surfaces with monolayer-high steps. In the case of smooth films, the systems are also studied using solutions to the Boltzmann transport equation (BTE). Interestingly, the resistivity of $(1 bar{1}00)$ surfaces exhibits a significant size effect even in the absence of surface steps. When monolayer-high steps are spaced $sim 10$ nm apart, the resistivity is shown to increase due to scattering from the steps. However, only a small increase was found which cannot explain the large effect seen in recent experiments with Ru thin films. This highlights the need for further elucidation of the resistivity size effect. Theoretical analysis suggest that films made from materials with a relatively large ballistic conductance per area like Ru should exhibit a reduced resistivity size effect. This result points to Ru as a promising interconnect material. Finally, because a very efficient algorithm for computing resistivity based on the kernel polynomial method (KPM) is used, the approach fulfills a need for realistic models that can span length scales directly relevant to experimental results. The calculations described here include films approaching $5$ nm in thickness, with in-plane distances up to $sim 160$ nm and $3.8times10^{5}$ atomic sites.
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