Session X53: Superconducting, Electronic and Chemical Properties of 2D Materials

Andreev Reflection in Pseudospin-1 System

An incoming electron can be reflected as a hole at a normal-metal (N)/superconductor (S) interface, known as Andreev reflection. And a perfect Andreev reflection is predicted at two-dimensional metal/s-wave superconductor interface. Compared with previous models in which the N sides are of two-dimensional electron gas and graphene, we find a perfect Andreev reflect with a larger incident angle allowed at pseudospin-1 metal/s-wave superconductor interface. Additionally, an all-angle perfect Andreev reflection occurs in the limits. To support our discoveries, we study the differential conductance which can be observed directly in experiments.   

Tunneling spectroscopy as a probe of fractionalized excitations in RuCl3/graphene stacks

Recent studies of alpha-RuCl3 have spurred a significant deal of interest. This material is predicted to well approximate a Kitaev quantum spin liquid, a topologically-ordered state that supports fractionalized spin excitations. Several experimental studies have focused on large bulk samples, while few of them have dealt with lateral transport in proximity to thin multilayers finding unexpected results. In this work, we model the electron tunnelling through thin alpha-RuCl3 samples encapsulated with graphene and study signatures of fractionalised excitations in spin-unpolarized tunnelling currents. Since such devices can be produced by several labs in the world, our aim is to validate tunnelling spectroscopy as a tool to address the intriguing physics at play in thin alpha-RuCl3 multilayers.   

On-Chip Strain Enhanced Superconductivity in 1T'-MoTe2

Two-dimensional transition metal ditellurides have been found to have unique electronic/structural phase transitions with respect to strain. Bulk Weyl semimetal 1T'-MoTe₂ has been shown to exhibit enhanced superconductivity from the nominal value of 100 mK up to 7 K under applied hydrostatic pressure. In this work, we explore the use of on-chip thin film stress capping layers to achieve the same effect from in-plane strain on exfoliated 1T'-MoTe₂. These stress capping layers are analogous to the SiN_x capping layers used in industrial strained silicon processes. We examine 1T'-MoTe₂ channels under various thin film stressors and observe superconducting onsets at temperatures as high as 4 K. Devices are sensitive to both geometry and stressor layer composition. Control devices eliminate the potential of this effect arising from defect formation or unintended doping. Challenges related to low-temperature thin film strain transfer, and thermal matching of the stressor layers will be explored. Upon further optimization and standardization, this technique can be used to explore the superconducting phase diagram of MoTe₂ with respect to strain, temperature, and thickness.   

Unconventional Superconductivity in Semiconductor Artificial Graphene

Unconventional superconductivity featuring large pairing energies has attracted immense interest, yet tractable microscopic theories have proven elusive. A major breakthrough has been the advent of twisted bilayer graphene (TBG), which serves as a simple model system to `look under the hood' of unconventional superconductivity. We propose a new model, within current experimental reach, to investigate the microscopics of strong--binding superconductivity. Our proposed device is semiconductor artificial graphene, a two dimensional electron gas overlaid with a periodic potential. We demonstrate that this system realises a new mechanism for pairing, whereby the topology of the Dirac points gives rise to an attraction in the $p+ip$ channel. The pairing interaction, which originates from charge dynamics on the interlocking sublattices, dominates due to antiscreening, and -- in contrast to graphene -- can be strongly enhanced through device engineering. The unconventional superconducting gap provides a realisation of a Fulde-Ferrell-Larkin-Ovchinnikov state. The pairing strength is similar to TBG, and within the accuracy of our calculations we find $T_c$ up to 20 K for InAs heterostructures.   

Structural, Topological, and Superconducting Properties of 2D Tellurium Allotropes from ab initio Predictions

2D Tellurium, will extended the realm of two-dimensional (2D) materials to group-VI elements beyond the discovery of graphene, phosphorene, and borophene, and it will provide new candidates for next-generation electronic and optoelectronic device applications. Through the particle-swarm optimization searches combined with ab initio calculations, we further predict 31 novel stable 2D tellurium allotropes with various geometric structures which sorted by different thickness of atomic layers (termed as M-α, B-α, T-α, T+α, and so on). Their thermodynamic stabilities have been carefully verified by the phonon modes calculations. We show that four (the B-α, B-β, B-γ, and B-η phases) exhibit topological insulators, and three (B-β, B-ε and T-κ phases) are intrinsic superconductors with Tc ~8 K. Interestingly, B-β phase possesses both topological and superconducting properties, which is the 2D monolayer material that have such intrinsic properties and may provide an appealing platform for exploring topological superconductivity in 2D systems.   

Proximity induced spin-orbit splitting in graphene nanoribbons on transition metal dichalcogenides

We study the effect of transition metal dichalcogenides (TMD) on the electronic structure of graphene nanoribbons (GNRs) from first-principles. We consider both semiconducting TMDs and metallic TMDs and different stacking configurations. We find that when the TMD is semiconducting the effects on the band structure of the GNRs are small. In particular the spin-splitting induced by proximity on the GNR's bands is only of the order of few meV irrespective of the stacking configuration. When the TMD is metallic, such as NbSe2, we find that the spin-splitting induced in the GNRs can be very large and strongly dependent on the stacking configuration. For optimal stacking configurations the proximity-induced spin-splitting is of the order of 20 meV for armchair graphene nanoribbons, and as high as 40 meV for zigzag graphene nanoribbons. This results are encouraging for the prospect of using GNR-TMD heterostructures to realize quasi one-dimensional topological superconducting states.   

Electron-electron interactions, valleys, and band nesting in gated MoS2 quantum dots.

We report on the effect of valley, spin and band nesting on the many-electron properties of gated MoS₂ quantum dots. We start with single electron atomistic calculation for a computational box with periodic boundary conditions containing up to 10⁶ atoms, using a tight binding model developed from ab-initio methods for MoS₂ [1,2]. The effect of the metallic gates is modelled as a parabolic lateral confining potential. We find a twofold degenerate energy spectrum of confined electrons originating from the two non-equivalent valleys K and -K as well as a sixfold degenerate ladder of states associated with six secondary conduction band minima Q [3]. These electronic states up to 5 K-derived shells are then populated with up to 6 electrons and electron-electron interactions are turned on. We demonstrate that the large intra-valley exchange interaction determines the many-electron ground state of these QDs. As a consequence, with varying QD size and confining potential, a valley-polarized and spin-polarised broken symmetry ground states emerge.
[1] T. Scrace, et al. Nature Nanotech. 10, 7, (2015), 603–607.
[2] M. Bieniek, M. Korkusinski, L. Szulakowska, P. Potasz, I. Ozfidan, and P. Hawrylak, PRB 97, 085153 (2018).
[3] M. Bieniek, L. Szulakowska, and P. Hawrylak, PRB, in review, (2019).   

Ultra-thin van der Waals crystals as semiconductor quantum wells

Control over the electronic spectrum at low energy is at the heart of the functioning of modern advanced electronics which highly relies on meticulous engineering of the size quantization of electrons in quantum wells. This avenue hasn’t yet been explored in 2D materials. Here we transfer this concept onto the van der Waals heterostructures which utilize few-layers films of InSe as quantum wells. Precise control over the energy of the subbands and their uniformity guarantees extremely high quality of the electronic transport in such systems. Using novel tunnelling and light emitting devices, we reveal the full subbands structure by studying resonance features in the tunnelling current, photoabsorption and light emission. [1]

[1] J. Zultak et al, arXiv:1910.04215   

Observation of tunable plasmons in large area type-II Weyl semimetal films

The observation of the highly tunable and confined plasmons in Graphene has stimulated the exploration of plasmons in other 2D materials. Here, for the first time, plasmonic resonance modes are observed in CVD-grown large area films of WTe₂, which is known as a type-II Weyl semimetal, by far-infrared absorption spectroscopy. Plasmon-phonon hybridization is revealed by mapping the plasmon dispersion on Si/SiO2 substrates. The plasmon frequency is tunable by changing the temperature and film thickness. Moreover, the plasmon modes are still observable with film thickness down to 8 nm, indicating the potential for tuning by gate. Our results represent a first look at the plasmon in large area CVD-grown Weyl semimetal films and suggest a practical platform to experimentally study the topological plasmons.   

The Origin of Hydrophilicity on 1T′-MoS2

Single-layer MoS₂ has been shown to excel in many applications like as a catalyst, supercapacitor, transistor, and biosensor. It has been shown that there is a correlation between the catalytic activity and the metallic character of the surface. Consequently, the metallic phase of MoS₂ has been shown to surpass the performance of the semiconducting phase. However, the metallic phase is less energetically favorable than the semiconducting 2H phase, and it is unstable in air. Recently, it has been shown that a metallic phase of MoS₂ is stable in water. This metallic phase is also found to be hydrophilic which provides additional advantages in applications as a supercapacitor or biosensor. We attribute this metallic phase to 1T′-MoS₂ and investigate the origin of the hydrophilic behavior. Through first-principles calculations, we found that, with a sulfur vacancy on the surface, dissociative adsorption of a water molecule is favorable only on the 1T′ surface. The water dissociation on the surface not only increases the hydrophilicity of the surface, but it also explains the enhanced stability of 1T′-MoS₂ in water by reducing the energy difference between the 1T′ and 2H phases.   

Metal-based nanomats as catalysts for self-oscillating Belousov-Zhabotinsky reactions

Chemical reactions exhibiting self-sustained oscillations have been used to replicate “life-like” characteristics in various synthetic systems. Belousov-Zhabotinsky (BZ) reactions are such reactions that exhibit a periodic color change of the solution, facilitated by oxidation and reduction of the metal-ion catalyst. Recently, we have shown that by using 0D-2D heterostructures as catalytic mats, specifically graphene-based nanosheets decorated with Ce or Ru nanoparticles, the frequency of oscillations for the BZ reactions are significantly enhanced. Here, however, we establish that bare metal nanosheets can also be used to catalyze BZ reactions. In particular, we show that when bare Ceria and Ru nanosheets are used as catalysts for BZ reactions, the oscillations change depending upon the presence of oxy-functional groups. Also, we designed hybrid 0D-2D metal catalysts, decorating Ce and Ru NPs on respective Ru and Ce nanosheets. Our experiments reveal that the frequency of oscillations is dependent on the conductivity of the bare metal nanosheets and the NP loading. Although our work focusses on BZ reactions, our findings open up new avenues to tune characteristics of dynamical systems through the use of catalytic nanomaterials.   

Stable out-of-plane piezoelectric group-IV monochalcogenide monolayers with a buckled honeycomb structure

We used DFT to study 12 group-IV monochalcogenide monolayers MX (M=Si, Ge, Sn, Pb and X =S, Se, Te) with a 2-fold degenerate honeycomb structure. The exchange of M and X atoms yields a two-fold degenerate structure and the bucking of atoms in the unit cell provides a thickness Δz and an out-of-plane electric polarization. A 2D structural phase transformation is discussed from the perspective of elastic energy barriers J [1,2,3,4]; J is the smallest energy required to change the buckled unit cell (Δz>0) onto a planar unit cell (Δz=0). These materials will not undergo a 2D phase transformation. For comparison, silicene has a similar structure and a lower J and undergoes the 2D structural transformation [5]. This study increases the understanding of relationships among the structure, ferroelectric behavior, and structural phase transformations in 2D materials with structural degeneracies.

References:
1. M. Mehobodi et al., Nano Lett. 16, 1704 (2016)
2. T. Bishop et al., PRL 122, 015703 (2019)
3. S. Barraza-Lopez et al., PRB 97, 024110 (2018)
4. S. P. Poudel et al., under review
5. G. G. Naumis et al., Rep. Prog. Phys. 80, 096501 (2017)   

A spectrum of exfoliable 1D van der Waals materials and their electronic properties

Data-mining efforts suggest there are hundreds of bulk materials with the potential to be exfoliated into one-dimensional (1D) structures. We combine data-mining with DFT calculations to elucidate the properties of 1D wires. We identify several materials that have the potential to be more readily exfoliable than the commonly studied 1D-like materials tellurium and selenium and examine the bulk and single-wire electronic characteristics. While layered materials have received significant attention, their 1D van der Waals counterparts have received relatively small amounts, yet are likely to have many of the same qualities that make layered materials of interest for a number of applications.