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تسجيل مستخدم جديدVisualizing Strain-induced Pseudo magnetic Fields in Graphene through an hBN Magnifying Glass
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The remarkable properties of graphene are inherent to its 2D honeycomb lattice structure. Its low dimensionality, which makes it possible to rearrange the atoms by applying an external force, offers the intriguing prospect of mechanically controlling the electronic properties. In the presence of strain, graphene develops a pseudo-magnetic field (PMF) which reconstructs the band structure into pseudo Landau levels (PLLs). However, a feasible route to realizing, characterizing and controlling PMFs is still lacking. Here we report on a method to generate and characterize PMFs in a graphene membrane supported on nano-pillars. A direct measure of the local strain is achieved by using the magnifying effect of the Moire pattern formed against a hexagonal Boron Nitride (hBN) substrate under scanning tunneling microscopy (STM). We quantify the strain induced PMF through the PLLs spectra observed in scanning tunneling spectroscopy (STS). This work provides a pathway to strain induced engineering and electro-mechanical graphene based devices.
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Strain-inducing deformations in graphene alter charge distributions and provide a new method to design specific features in the band structure and transport properties. Novel approaches implement engineered substrates to induce specifically targeted
strain profiles. Motivated by this technique, we study the evolution of charge distributions with an increasing number of out-of-plane deformations as an example of a finite size periodic substrate. We first analyze a system of two overlapping deformations and determine the quantitative relation between geometrical parameters and features in the local density of states. We extend the study to sets of 3 and 4 deformations in linear and two-dimensional arrays and observe the emergence of moire patterns that are more pronounced for a hexagonal cell composed of 7 deformations. A comparison between the induced strain profile and spatial maps of the local density of states at different energies provides evidence for the existence of states confined by the pseudo-magnetic field in bounded regions, reminiscent of quantum dots structures. Due to the presence of these states, the energy level scaling to be observed by local probes should exhibit a linear dependence with the pseudo-field, in contrast to the expected scaling of pseudo-Landau levels.
Using a simple setup to bend a flexible substrate, we demonstrate deterministic and reproducible in-situ strain tuning of graphene electronic devices. Central to this method is the full hBN encapsulation of graphene, which preserves the exceptional q
uality of pristine graphene for transport experiments. In addition, the on-substrate approach allows one to exploit strain effects in the full range of possible sample geometries and at the same time guarantees that changes in the gate capacitance remain negligible during the deformation process. We use Raman spectroscopy to spatially map the strain magnitude in devices with two different geometries and demonstrate the possibility to engineer a strain gradient, which is relevant for accessing the valley degree of freedom with pseudo-magnetic fields. Comparing the transport characteristics of a suspended device with those of an on-substrate device, we demonstrate that our new approach does not suffer from the ambiguities encountered in suspended devices.
The observation of novel physical phenomena such as Hofstadters butterfly, topological currents and unconventional superconductivity in graphene have been enabled by the replacement of SiO$_2$ with hexagonal Boron Nitride (hBN) as a substrate and by
the ability to form superlattices in graphene/hBN heterostructures. These devices are commonly made by etching the graphene into a Hall-bar shape with metal contacts. The deposition of metal electrodes, the design and specific configuration of contacts can have profound effects on the electronic properties of the devices possibly even affecting the alignment of graphene/hBN superlattices. In this work we probe the strain configuration of graphene on hBN contacted with two types of metal contacts, two-dimensional (2D) top-contacts and one-dimensional (1D) edge-contacts. We show that top-contacts induce strain in the graphene layer along two opposing leads, leading to a complex strain pattern across the device channel. Edge-contacts, on the contrary, do not show such strain pattern. A finite-elements modelling simulation is used to confirm that the observed strain pattern is generated by the mechanical action of the metal contacts clamped to the graphene. Thermal annealing is shown to reduce the overall doping whilst increasing the overall strain, indicating and increased interaction between graphene and hBN. Surprisingly, we find that the two contacts configurations lead to different twist-angles in graphene/hBN superlattices, which converge to the same value after thermal annealing. This observation confirms the self-locking mechanism of graphene/hBN superlattices also in the presence of strain gradients. Our experiments may have profound implications in the development of future electronic devices based on heterostructures and provide a new mechanism to induce complex strain patterns in 2D materials.
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