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Valleytronics in Atomically Thin Semiconductors

Name: Mandar Sohoni

Supervisor: Prof. Anshuman Kumar

Department: Physics

Topic of research: Valleytronics in Atomically Thin Semiconductors

Description of research work:

Valleytronics in Atomically Thin Semiconductors

In recent years, valleytronics – the technology to manipulate the electronic valley degree of freedom’ in two dimensional gapped Dirac systems, which possess pairs of degenerate band extrema or valleys, has received enormous attention for quantum information processing applications. In contrast to conventional information storage techniques that utilize charges or spins, gapped Dirac systems offer valleys in their electronic band structures as quantum information storage bits, i.e., qubits. In hexagonal two-dimensional materials, these two disparate valleys occur at high symmetry points in the band structure. Transition metal dichalcogenides (TMDCs) are one class of hexagonal two dimensional semiconductors that have shown great promise for such technologies. From a quantum technology perspective, one of the most advantageous aspects of valley qubits in atomically thin TMDCs is the ease of creating an initial state and measuring the final state. It turns out that the transition dipole moments of valley excitons in these semiconductors couple to orthogonally polarized light. Consequently, only an optical field is required for quantum control of this pseudo-spin. The promise of this field motivated me to choose this problem for my Dual degree thesis project. I was also influenced by the research carried out in the same group (LOQM, Physics Department, IIT Bombay) by one of my batchmates, Muralidhar Nalabothula (EP B. Tech. ‘19 batch) whose work in a similar field led to a published research paper [1].  

 

Our work focuses on creating and controlling coherence in the valley pseudo-spin of interlayer excitons in atomically thin TMDC heterostructures. The novelty of this work is opening up the study of spontaneously generated coherence using an anisotropic electromagnetic vacuum in interlayer excitons. Previous works have studied such phenomena in monolayer TMDC excitons, however, there is one serious limitation with them. In monolayer excitons, the valley-pseudo spin state is restricted to only one plane in the Bloch sphere due to the way the anisotropic vacuum is generated (see Figure 1 (a)). By bringing interlayer excitons into the picture, this work has shown that almost the entire Bloch sphere can be accessed.

   a)                                      b)                                        c)

                                             Figure 1 (borrowed from[2])

 

A nano-antenna based metasurface was used to generate the anisotropic vacuum. The idea here was to suppress the radiative rate for a x polarized dipole, while leaving a y dipole unaffected (radiative rates given by  and  respectively), thus creating an anisotropic vacuum. As we have shown, by changing the radiative rates, the interlayer valley exciton can almost access the entire Bloch sphere (see Figures 1 (b) and (c)). This work, with me as the lead author, is currently undergoing peer review and a preprint is available on arXiv where further details can be found [2]. 

 

Our work has opened up the potential for many applications of using Moire lattices (bilayers with a twist angle between the single layers, see Figure 2) as a platform for quantum information science, as they provide an array of such interlayer valley excitons. These Moire lattices are a very hot field right now in several fields of physics, in particular superconductivity and topological insulators. A moire pattern is formed when two non identical patterns are superimposed on each other -- we have all played with toys that create this pattern in our childhood. Our current study, in conjunction with a recent publication of generating anisotropic vacuums using anisotropic plasmon modes (surface waves) in phosphorene [1], also opens up the possibility of observing spontaneous coherent oscillations in the valley pseudo-spin, which would be a first in such systems.

Figure 2

 

[1] “Engineering valley quantum interference in anisotropic van der Waals heterostructures” by Muralidhar Nalabothula, Pankaj K. Jha, Tony Low, and Anshuman Kumar, Physical Review B (accepted, 2020), preprint available here - https://arxiv.org/abs/1910.03952

[2] “Interlayer Exciton Valleytronics in Bilayer Heterostructures Interfaced with a Metasurface” by Mandar Sohoni, Pankaj K. Jha, Muralidhar Nalabothula, Anshuman Kumar, under peer review, preprint available here - https://arxiv.org/abs/1912.12080