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Register a new userElectron transport across a metal/MoS$_2$ interface: dependence on contact area and binding distance
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We investigate the nature of electron transport through monolayer molybdenum dichalcogenides (MoX$_2$, X=S, Se) suspended between Au and Ti metallic contacts. The monolayer is placed ontop of the close-packed surfaces of the metal electrodes and we focus on the role of the metal-MoX$_2$ binding distance and the contact area. Based on emph{ab initio} transport calculations we identify two different scattering mechanisms which depend differently on the metal-MoX$_2$ binding distance: (i) An interface resistance between the metal and the supported part of MoX$_2$ which decreases with decreasing binding distance and increasing contact area. (ii) An edge resistance across the 1D interface between metal-supported and free-standing MoX$_2$ which increases with decreasing binding distance and is independent on contact area. The origin of the edge resistance is a metal-induced potential shift within the MoX$_2$ layer. The optimal metal thus depends on the junction geometry. In the case of MoS$_2$, we find that for short contacts, L$<$6 nm, Ti electrodes (with short binding distance) gives the lowest resistance, while for longer contacts, Au (large binding distance) is a better electrode metal.
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The low-energy band structure of few-layer MoS$_2$ is relevant for a large variety of experiments ranging from optics to electronic transport. Its characterization remains challenging due to complex multi band behavior. We investigate the conduction band of dual-gated three-layer MoS$_2$ by means of magnetotransport experiments. The total carrier density is tuned by voltages applied between MoS$_2$ and both top and bottom gate electrodes. For asymmetrically biased top and bottom gates, electrons accumulate in the layer closest to the positively biased electrode. In this way, the three-layer MoS$_2$ can be tuned to behave electronically like a monolayer. In contrast, applying a positive voltage on both gates leads to the occupation of all three layers. Our analysis of the Shubnikov--de Haas oscillations originating from different bands lets us attribute the corresponding carrier densities in the top and bottom layers. We find a twofold Landau level degeneracy for each band, suggesting that the minima of the conduction band lie at the $pm K$ points of the first Brillouin zone. This is in contrast to band structure calculations for zero layer asymmetry, which report minima at the $Q$ points. Even though the interlayer tunnel coupling seems to leave the low-energy conduction band unaffected, we observe scattering of electrons between the outermost layers for zero layer asymmetry. The middle layer remains decoupled due to the spin-valley symmetry, which is inverted for neighboring layers. When the bands of the outermost layers are energetically in resonance, interlayer scattering takes place, leading to an enhanced resistance and to magneto-interband oscillations.
Ideal monolayers of common semiconducting transition metal dichalcogenides (TMDCs) such as MoS$_2$, WS$_2$, MoSe$_2$, and WSe$_2$ possess many similar electronic properties. As it is the case for all semiconductors, however, the physical response of these systems is strongly determined by defects in a way specific to each individual compound. Here we investigate the ability of exfoliated monolayers of these TMDCs to support high-quality, well-balanced ambipolar conduction, which has been demonstrated for WS$_2$, MoSe$_2$, and WSe$_2$, but not for MoS$_2$. Using ionic-liquid gated transistors we show that, contrary to WS$_2$, MoSe$_2$, and WSe$_2$, hole transport in exfoliated MoS$_2$ monolayers is systematically anomalous, exhibiting a maximum in conductivity at negative gate voltage (V$_G$) followed by a suppression of up to 100 times upon further increasing V$_G$. To understand the origin of this difference we have performed a series of experiments including the comparison of hole transport in MoS$_2$ monolayers and thicker multilayers, in exfoliated and CVD-grown monolayers, as well as gate-dependent optical measurements (Raman and photoluminescence) and scanning tunneling imaging and spectroscopy. In agreement with existing {it ab-initio} calculations, the results of all these experiments are consistently explained in terms of defects associated to chalcogen vacancies that only in MoS$_2$ monolayers -- but not in thicker MoS$_2$ multilayers nor in monolayers of the other common semiconducting TMDCs -- create in-gap states near the top of the valence band that act as strong hole traps. Our results demonstrate the importance of studying systematically how defects determine the properties of 2D semiconducting materials and of developing methods to control them.
In a number of current experiments in the field of spin-caloritronics a temperature gradient across a nanostructured interface is applied and spin-dependent transport phenomena are observed. However, a lack in the interpretation and knowledge let it unclear how the temperature drop across a magnetic nanostructured interface looks like where both phonons and electrons may contribute to thermal transport. We answer this question for the case of a magnetic tunnel junction (MTJ) where the tunneling magneto Seebeck effect occurs. Nevertheless, our results can be extended to other nanostructured interfaces as well. Using an textit{ab initio} method we explicitly calculate phonon and electron thermal conductance across the Fe/MgO/Fe-MTJs by using Greens function method. Further, by estimating the electron-phonon interaction in the Fe leads we are able to calculate the electron and phonon temperature profile across the Fe/MgO/Fe-MTJ. Our results show that there is an electron-phonon temperature imbalance at the Fe-MgO interfaces. In consequence, a revision of the interpretation of current experimental measurements might be necessary.
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.
We investigate the electron transport properties of a model magnetic molecule formed by two magnetic centers whose exchange coupling can be altered with a longitudinal electric field. In general we find a negative differential conductance at low temperatures originating from the different scattering amplitudes of the singlet and triplet states. More interestingly, when the molecule is strongly coupled to the leads and the potential drop at the magnetic centers is only weakly dependent on the magnetic configuration, we find that there is a critical voltage V_C at which the current becomes independent of the temperature. This corresponds to a peak in the low temperature current noise. In such limit we demonstrate that the quadratic current fluctuations are proportional to the product between the conductance fluctuations and the temperature.
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