STM Studies and First-Principles Calculations on Van der Waals Structures


Student thesis: Doctoral Thesis

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Award date11 Dec 2020


In this thesis, I focus on some van der Waals (VdW) structures and materials. VdW structures have nonbonding properties. Hence, interactions between the different parts are usually weak compared with structures that interact via chemical bonds. Though VdW interactions are weak, they may have considerable influence on VdW structures and materials both in real space and k-space (i.e., electronic structures). Furthermore, these weak couplings make manipulating, engineering, and tailoring VdW materials and materials feasible. Interesting physics such as superconductivity, strong correlation effects, and spin-valley locking (SVL) and applications in spintronics and valleytronics can be realized based on the properties of VdW structures.

Part I of this thesis presents STM studies on an unusual self-assembled layer structure of chloroaluminium phthalocyanine (ClAlPc) molecules on highly ordered pyrolytic graphite (HOPG), in which a close-packed well-ordered monolayer is separated from the substrate by a relatively disordered buffer layer. The observed close-packed monolayer has a nearly rectangular lattice, which is distinctly different from the square lattice of the more commonly observed well-ordered bilayer structure. This may be due to the dominance of the intermolecular interactions within the monolayer when it is shielded from the influence of the substrate by the buffer layer. Density-functional theory calculations and reduced density gradient analysis indicate that the dominant intermolecular interaction within the unusual layer is likely the London dispersion force. The idea that introducing an inert buffer layer as a shield from the effect of the substrate can be extended to other fields where weak interactions occur must be taken into consideration.

Part II is on the general theory of C-paired SVL and its related properties, and specifically, the first-principles calculations on V2Se2O, which is a two-dimensional (2D) VdW material. Valleys in a crystal refer to the degenerate energy extrema well separated in momentum space. By coupling this “pseudospin” freedom to real spin freedom, one can realize SVL. This coupled mechanism can significantly increase the decoherence time of both spin and valley polarization because flipping one of them requires flipping the other simultaneously, which is physically difficult. The general theories on a new type of SVL: the C-paired SVL and its induced properties, giant pizeomagnetism, and angle-dependent spin current are discussed. In contrast to the conventional SVL originates in monolayer transition metal dichalcogenides, C-paired SVL can occur without spin-orbit coupling (SOC), and the valleys that constitute an emergent quantum degree of freedom are related in a crystalline symmetry instead of the time-reversal symmetry. Thus, a strong valley polarization can be generated by simply breaking the corresponding crystalline symmetry, which can give rise to a wealth of interesting phenomena highly desirable for versatile device applications. Typically, one can use a strain field to induce large net valley polarization or magnetization and use a charge current to generate a large noncollinear spin current. Predictions of the first candidate of C-paired SVL in monolayer V2Se2O are made using first-principles calculations. I have performed the calculations on a family of materials that share a common chemical formula M2X2O where M is the 3d transition metal and X is Se or Te. This family of materials has the same VdW layered structure, which is easily exfoliated to a few layers for experimental detection. The monolayer V2Se2O calculated exhibits giant piezomagnetism and can generate a large transverse spin current with an equivalent spin Hall angle of approximately 0.7, which is about several times larger than the conventional spin Hall effect induced by SOC. These findings provide unprecedented opportunities for integrating versatile dynamic controls of valley and spin with nonvolatile information storage into a single 2D material. The magnetic properties of both the bulk and monolayer are also thoroughly studied via a combination of first-principles calculations, crystal field theory analysis, Heisenberg model study and superexchange interactions analysis. The mechanical properties of V2Se2O, including cleavage energy, in-plane stiffness, and dynamical stability, are systematically studied.

    Research areas

  • Van der Waals structures, STM, molecular self-assembly, first-principles calculations, spin-valley locking, giant piezomagnetism, angle-dependent spin current