Researchers are making progress to tuning and creating strained graphene where they want. This could lead to valleytronics and ultratiny free electron lasers and other applications. Strained graphene can create pseudo magnetic fields of 25,000 tesla. This is about 1000 times more than the magnetic fields from superconducting magnets.
Electronics and spintronics have their strengths and weaknesses when it comes to establishing the on-off states that are so critical for digital logic. Valleytronics offer another degree of freedom in electrons that avoids weaknesses of electronics and spintronics and maximizes the strengths. Valleytronics uses the energy level in relation to their momentum.
Valleytronics could be faster and more efficient computer logic systems and data storage chips in next-generation devices.
There are a number of theoretical proposals based on strain engineering of graphene and other two-dimensional materials, however purely mechanical control of strain fields in these systems has remained a major challenge. The two approaches mostly used so far either couple the electrical and mechanical properties of the system simultaneously or introduce some unwanted disturbances due to the substrate. Here, we report on silicon micromachined comb-drive actuators to controllably and reproducibly induce strain in a suspended graphene sheet in an entirely mechanical way. We use spatially resolved confocal Raman spectroscopy to quantify the induced strain, and we show that different strain fields can be obtained by engineering the clamping geometry, including tunable strain gradients of up to 1.4%/μm. Our approach also allows for multiple axis straining and is equally applicable to other two-dimensional materials, opening the door to investigating their mechanical and electromechanical properties. Our measurements also clearly identify defects at the edges of a graphene sheet as being weak spots responsible for its mechanical failure.
Other researchers have created 25000 tesla pseudo magnetic fields with strained graphene
Super-high pseudo magnetic fields from strained graphene can make table top free electron lasers or free electron lasers on computer chips for communication and other applications.
Reseachers report the use of nearly strain-free PECVD-grown graphene to induce controllable strain and pseudo-magnetic fields (BS) by placing graphene on synthesized tetrahedron nanocrystals (55nm laterally and 45nm in height). The nanocrystals were spin-coated on a Si substrates and then covered by a monolayer of h-BN followed by a monolayer of graphene. Scanning tunneling spectroscopic studies revealed giant BS values up to ~25000 Tesla in isolated tetrahedrons. In contrast, the maximum BS value became reduced to ~600 Tesla along the ridge of two correlated tetrahedrons. The effect of pseudo-magnetic field-induced local time-reversal symmetry breaking on valley polarization was confirmed by the alternating presence and absence of zeroth Landau level at two inequivalent sublattice sites. These empirical results were compared with Molecular Dynamics (MD) simulations for the magnitude and spatial distribution of strain-induced pseudo-magnetic fields, and the consistency ensured that properly designed arrays of nanostructures could induce the desirable BS values and spatial distributions to yield realistic valleytronic devices.
Other progress to valleytronics with graphene
We report a theoretical study on the valley-filter and valley-valve effects in the monolayer graphene system by using electrostatic potentials, which are assumed to be electrically controllable. Based on a lattice model, we find that a single extremely strong electrostatic-potential barrier, with its strength exceeding the hopping energy of electrons, will significantly block one valley but allow the opposite valley flowing in the system, and this is dependent on the sign of the potential barrier as well as the flowing direction of electrons. In a valley-valve device composed of two independent potential barriers, the valley-valve efficiency can even amount to 100% that the electronic current is entirely prohibited or allowed by reversing the sign of one of potential barriers. The physics origin is attributed to the valley mixing effect in the strong potential barrier region. Our findings provide a simple electric way of controlling the valley transport in the monolayer graphene system.
Graphene subject to high levels of shear strain leads to strong pseudo-magnetic fields resulting in the emergence of pseudo-Landau levels. Here we show that, with modest levels of strain, graphene can also sustain a classical valley Hall effect (VHE) that can be detected in nonlocal transport measurements. We provide a theory of the strain-induced VHE starting from the quantum Boltzmann equation. This allows us to show that, averaging over short-range impurity configurations destroys quantum coherence between valleys, leaving the elastic scattering time and inter-valley scattering rate as the only parameters characterizing the transport theory. Using the theory, we compute the nonlocal resistance of a Hall bar device in the diffusive regime. Our theory is also relevant for the study of moderate strain effects in the (nonlocal) transport properties of other two-dimensional materials and van der Walls heterostructures.