Session W56: Twisted Heterostructures and Straintronics

A novel platform for studying twisted 2D heterostructures, Part I:

The relative twist angle between the layers in 2D heterostructures is an excellent knob to tune their electronic properties and has led to the discovery of a plethora of new phenomena. We have developed a novel platform for controlling the twist angle between 2D crystals dynamically and accurately, allowing us to probe their electronic behavior and structure. We show the spectroscopic capabilities of our new platform by presenting measurements of momentum resolved tunneling between these twisted 2D heterostructures. In this first talk of our joint presentation, I will describe our setup and basic transport measurements.      

A novel platform for studying twisted 2D heterostructures, Part II:

The relative twist angle between the layers in 2D heterostructures is an excellent knob to tune their electronic properties and has led to the discovery of a plethora of new phenomena. We have developed a novel platform for controlling the twist angle between 2D crystals dynamically and accurately, allowing us to probe their electronic behavior and structure. We show the spectroscopic capabilities of our new platform by presenting measurements of momentum resolved tunneling between these twisted 2D heterostructures. In this second talk of our joint presentation, I will present our measurements on momentum resolved tunneling.       

Infrared magneto-spectroscopy in the Hofstadter butterfly regime of graphene-boron nitride moire superlattices

Graphene stacked on hexagonal boron nitride forms a moire superlattice system. Rotational angles close to zero degrees provide ideal moire length scales, of order ten nanometers, enabling experimental access to the fractal Hofstadter spectrum at lab scale magnetic fields. Using infrared magneto-spectroscopy, we report observations of cyclotron resonance transitions in a graphene-hexagonal boron nitride sample, having a relative alignment between the layers of ~0.9 degrees. The device shows strong satellite Dirac peaks in electronic transport that indicate it is in the Hofstadter regime. We observe splittings of the cyclotron resonance lines beyond what is seen in intrinsic monolayer graphene, likely to arise from the fractal energy levels.   

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Monolayer epitaxial graphene (EG) is a suitable candidate for a variety of electronic applications. One advantage of EG growth on the Si face of SiC is that it develops as a single crystal, as does the layer below, referred to as the interfacial buffer layer (IBL), whose properties include an electronic bandgap. Though much research has been conducted to learn about the electrical properties of the IBL, not nearly as much work has been reported on the optical properties of the IBL. Previously, combining the experimental measurements and density functional theory (DFT), we have studied the electronic property and dielectric function of the heterostructure bilayers. We have found that the dielectric function of the IBL, the permittivity of the material, is within the energy range of 1 eV to 8.5 eV. DFT results revealed the wave nature of the IBL graphene, which matches the experimental results. According to our new DFT calculations, the length of the wave is 6.19 Å which is a 3x3x1 supercell for the IBL graphene. Additionally, we have found that the formation of the wave entirely depends on the lattice mismatch between graphene, SiC, and mechanical strain.   

Controlling twist angle and strain in van der Waals heterostructures

In the low twist angle limit for bilayer graphene, reconstruction of the moiré superlattice leads to strain and twist angle inhomogeneities [1]. While strain is ubiquitous, its influence on the resulting bandstructure remains poorly understood and prevents reliable fabrication of such heterostructures as well as the observation of correlated states for twist angles lower than 1.1° as the bandstructures varies widely with smal perturbations of the moiré unit cell [2].

Here, we demonstrate a novel fabrication method that addresses these questions by using the tip of an atomic force microscope to mechanically bend graphene ribbons on various substrates, thus continuously and homogeneously varying both twist angle and strain. We confirm through piezo force microscopy and Raman spectroscopy that our technique allows for a continuous and independent variation of both twist angle and strain within a single device. Finally, we show this technique can be applied to any vdW materials, limited only by material stiffness.

[1] S. Dai et al. Nano Lett.16, 5923-5927 (2016)

[2] Z. Bi et al. Phys. Rev. B 100, 035448 (2019)   

Heterostrain Engineering Twisted and Non-Twisted 2D Bilayers with Process-Induced Strain

Two-dimensional (2D) bilayers have exotic (opto)electronic properties that may be controlled with strain and twist angle between the two layers. Industrially, process induced strain engineering has long been used to strain Si to enhance electron or hole mobility by capping transistors with stressed thin films. We have applied the same concept in 2D systems by evaporating optically transparent thin films with high stress onto graphene and MoS₂¹. We observed that 2D systems have nonhomogeneous strain transfer in the c-axis, where the interlayer coupling of the given system dictates how many layers strain can propagate from the top strained layer². Graphene and MoS₂ have weak interlayer coupling, therefore heterostrain is present in a bilayer structure as no strain can be transferred to the bottom layer (this layer is fixed to the substrate). Here, we discuss using Raman spectroscopic mapping to probe heterostrain in graphene and MoS₂ homobilayers. We then investigate twisted bilayer graphene structures, where we probe heterostrain by observing shifts in twist induced Raman modes (R-band) with varying thin film force application. Since these techniques are robust at low temperature, this can potentially be used to probe correlated electron physics with respect to heterostrain.

[1] T. Peña et al., 2D Mater. 8, 045001 (2021).

[2] S. A. Chowdhury, et. al.  J. Eng. Mater. Technol. 144, 011006 (2021).   

Zero-field superconducting diode effect in twisted trilayer graphene with spin-orbital coupling

In a 2D superconductor, simultaneously breaking time reversal symmetry (TRS) and inversion symmetry could give rise to the superconducting diode effect, where the critical DC current is direction dependent. We report the observation of zero-field superconducting diode effect in a heterostructure consisting of twisted trilayer graphene (tTLG) and tungsten diselenide (WSe), the latter of which induces strong Rashba spin-orbital coupling and breaks the inversion symmetry.

The evidence for the diode effect comes from the observed nonreciprocity in the current-voltage characteristic of the superconducting phase at B=0. Notably, the sign of nonreciprocity can be trained using an applied magnetic field and reversed by simply varying field-effect doping.

Such experimental control on the zero-field nonreciprocity points towards spontaneously TRS breaking resulting from isospin ferromagnetic order which coexists with the superconducting phase. The observed nonreciprocal superconducting transport behavior also provides strong evidence for finite momentum superconductivity.   

Critical current photoconductivity of Fermi-velocity limited carriers in graphene superlattices

In the absence of a magnetic field, the most profound photoconductive phenomena typically manifest in gapped systems and has led to the development of state-of-the-art opto-electronic devices. In contrast, metallic systems tend to exhibit much weaker photoconductivity because the effects of photo-excited carriers are masked by the large density of equilibrium carriers in the band.  Here, we show experimentally that the metallic states of graphene-based moiré superlattices exhibit a striking photoconductive effect with a magnitude comparable to those typically observed in strongly gapped systems. The effect manifests when the moiré superlattices are biased to an out-of-equilibrium critical state¹ that marks a transition from a Fermi-velocity limited current regime to a highly dissipative electron-hole liquid. Analogous to superconducting bolometers, the Fermi-velocity limited metallic state is particularly sensitive to photoexcitation so that incident radiation causes a shift in the critical current and a giant change in the differential resistivity. From the perspective of photodetection, our mid-infrared photocurrent measurements show that the critical metallic state competes with state-of-the-art gapped MCT detectors at low temperatures, exhibiting responsivities as high as 10⁵ V/W for wavelengths 5-11 um. The results demonstrate a unique type of opto-electronic response based on non-equilibrium phenomena in moiré superlattices.   

Topological piezoelectric response in moiré graphene systems

We theoretically study the piezoelectric effects in moiré graphene systems. Since the strain couples to the electrons in the system as an effective vector field, which has opposite signs for the K and K′ valleys of graphene, its effects on the two valleys with opposite Chern numbers do not cancel out, but adds up. As a result, some components of the piezoelectric tensor in these systems, which typically have non-trivial topology in their flat bands, are nearly quantized in terms of the valley Chern numbers. Such a conclusion is verified by numerical calculations of the in-plane piezoelectric response of hBN-aligned twisted bilayer graphene, twisted bilayer-monolayer graphene, and twisted double bilayer graphene systems using both continuum model and atomistic tight-binding model. We find that by tuning the vertical displacement field and/or twist angle, which may induce gap closures between the flat bands and remote bands in these systems, plateau shapes of the piezoelectric response curves are obtained, with abrupt jumps across the topological phase transitions. We propose that such nearly quantized piezoelectric response may serve as a direct experimental probe for the valley Chern numbers of the flat bands in moiré graphene systems.   

Realizing a Topological Charge Pump in a Sliding Moiré Heterostructure

In a Thouless charge pump, a sliding lattice potential pumps a quantized amount of charge across an insulator.  We describe an experiment to realize a Thouless charge pump by sliding a moiré superlattice formed at the interface of a graphene-hexagonal boron nitride (hBN) heterostructure. Here, the sliding process is enabled by the low interfacial friction between the van der Waals (vdW) material interfaces. Furthermore, unique to a moiré system, translating one layer by one atomic lattice constant leads to an amplified translation of the moiré by one superlattice constant. In order to achieve the sliding, we have developed a cryogenic nanomanipulation apparatus that can mechanically modify the vdW heterostructure while performing electrical measurements. I will discuss our progress on cryogenic nanomanipulation (sliding) as well as measuring the pumped charge in a graphene-hBN moiré system. I will also discuss other experiments enabled by our technique such as twist dependent studies and scanning gate experiments.    

Synthetic correlated electron system with twisted bilayergraphene quantum dots

We study a synthetic strongly correlated system realised in bilayer graphene quantum dots with a relative twist between the layers. Doing so, we combine two concepts - triangular graphene quantum dots with zig-zag edges [1] (TGQD), and twisted bilayer graphene finite systems [2] (TBG), both of which were shown to allow for the design of the degenerate electronic shell. The two degenerate electronic shells of each TGQD combine to a single degenerate shell, which properties can be manipulated with a vertical electric field and a twist. We study the effect of the moiré potential on the internal structure and wave function properties of the zero-energy shell. We also investigate the possibilities of manipulating moiré/field-confined QD-like states properties, e.g. state symmetry, using the twist angle as a new mean of control.   

Tuning the optoelectronic properties of a WS2-monolayer in different dielectric environments

Monolayers of transition metal dichalcogenides (TMDs) are direct-gap semiconductors with their optoelectronic properties strongly affected by the dielectric environment. We investigate the optical properties of WS₂ monolayers mechanically transferred on a pre-patterned Si/SiO₂ substrate with cylindrical wells of 3 μm in diameter. A protocol has been developed in order to easily obtain areas fully conformed to the well (strained) or suspended due to air being trapped in some wells. As a result, exactly the same monolayer is investigated under different dielectric environments. Detailed Raman mapping was used to quantify the strain and a fully T-dependent spectroscopic characterization using PL and Reflectivity was performed. Comparison of suspended to strained areas reveals a 10-fold enhanced PL efficiency with strong neutral excitonic emission at 78K. A T-dependent polarization PL spectroscopy was also performed for the strained and suspended parts of the flake in order to study the spin-valley depolarization behavior. Our work offers a useful approach to better understand the fundamental intrinsic properties of TMDs and a simple experimental procedure towards device fabrication with spatially controlled valley optoelectronic properties.

    

Composite super-moiré lattices in double aligned graphene heterostructures

Van der Waals heterostructures, as vertical stacks assembled by different 2D crystals, have been widely used to produce combinations with predetermined functionalities. Apart from the selection and the sequence of 2D crystals, controlling the twist angle between stacking layers opened the use of another degree of freedom, especially for two crystals with similar lattice mismatch. Unlike the singly aligned heterostructures with one moire pattern, we reported a doubly aligned structure in which fully encapsulated graphene simultaneously aligned to the top and bottom hBNs. In this case, two periodic potentials due to the moire pattern are applied on graphene simultaneously. Their differential will create another set of supermoires, among which the one with the largest period can be independent of the difference in the lattice constants between two crystals and thus break through the restrictions of this lattice mismatch to achieve the period much larger than 14nm. This would open up the prospect for the design of graphene band reconstruction at arbitrary low Fermi energies. Based on this platform, some interesting phenomena on transport will be discussed in this presentation.   

Studying interplay between non-linear electron transport and correlated physics in graphene-based flat band systems

Recent observations of correlated physics such as superconductivity, orbital ferromagnetism, and many-body insulating states in twisted graphene devices drove enormous enthusiasm in the study of flat band systems. It is commonly believed that such physics can be observed only under extremely small excitation currents, thus so far flat band systems are studied mostly in a linear response regime. Here we make the first step towards the understanding of non-linear properties of those systems and studying electron transport under high bias in small-angle twisted monolayer-bilayer graphene (tMBG) devices [1,2].

 First, we observe a non-linear current propagation around the single body gaps (around fillings ν= -4, 0, 4 per Moiré unit cell). We show that this non-linear regime is governed by the out-of-equilibrium criticalities general to graphene superlattices [3]. Second, when we tune the Fermi level close to the many-body correlated insulating states (ν=1,2, and 3 electrons per Moiré unit cell) we observe similar out-of-equilibrium criticalities with sharp peaks in differential resistance and superconducting-like I-V curves. These criticalities originate from the strong shift of the Fermi surface at the many-body band edge.

Overall, we show that many-body gaps in tMBG are robust against the significant shift of the Fermi surface happening under high current. Our results provide the first insights into the interplay between high bias electron transport and correlated physics in flat band systems. 

[1] Nat. Phys. 17, 374-380 (2021)

[2] Nat. Phys. 17, 619-626 (2021) 

[3] arXiv:2106.12609