9 research outputs found
Magnetism and Quantum Melting in Moir\'e-Material Wigner Crystals
Recent experiments have established that semiconductor-based moir\'e
materials can host incompressible states at a series of fractional
moir\'e-miniband fillings. These states have been identified as generalized
Wigner crystals in which electrons localize on a subset of the available
triangular-lattice moir\'e superlattice sites. In this article, we use
momentum-space exact diagonalization to investigate the many-body ground state
evolution at rational fillings from the weak-hopping classical lattice gas
limit, in which only spin degrees-of-freedom are active at low energies, to the
strong-hopping metallic regime where the Wigner crystals melt. We specifically
address the nature of the magnetic ground states of the generalized Wigner
crystals at fillings = 1/3 and = 2/3.Comment: 12 pages, 8 figure
Itinerant ferromagnetism in transition metal dichalcogenides moir\'e superlattices
Moir\'e materials are artificial crystals formed at van der Waals
heterojunctions that have emerged as a highly tunable platform to realize much
of the rich quantum physics of electrons in atomic scale solids, also providing
opportunities to discover new quantum phases of matter. Here we use finite-size
exact diagonalization methods to explore the physics of single-band itinerant
electron ferromagnetism in semiconductor moir\'e materials. We predict where
ferromagnetism is likely to occur in triangular-lattice moir\'e systems, and
where it is likely to yield the highest Curie temperatures.Comment: 15 pages, 14 figure
Prevalence of oxygen defects in an in-plane anisotropic transition metal dichalcogenide
Atomic scale defects in semiconductors enable their technological
applications and realization of novel quantum states. Using scanning tunneling
microscopy and spectroscopy complemented by ab-initio calculations we determine
the nature of defects in the anisotropic van der Waals layered semiconductor
ReS. We demonstrate the in-plane anisotropy of the lattice by directly
visualizing chains of rhenium atoms forming diamond-shaped clusters. Using
scanning tunneling spectroscopy we measure the semiconducting gap in the
density of states. We reveal the presence of lattice defects and by comparison
of their topographic and spectroscopic signatures with ab initio calculations
we determine their origin as oxygen atoms absorbed at lattice point defect
sites. These results provide an atomic-scale view into the semiconducting
transition metal dichalcogenides, paving the way toward understanding and
engineering their properties.Comment: 9 pages, 4 figures; Supp 5 pages, 4 figure
Sublattice engineering and voltage control of magnetism in triangular single and bi-layer graphene quantum dots
When a Dirac electron is confined to a triangular graphene quantum dot with zigzag edges, its low-energy spectrum collapses to a shell of degenerate states at the Fermi level leading to a magnetized edge. The shell degeneracy and the total magnetization are proportional to the edge size and can be made macroscopic. In this review, we start with a general discussion of magnetic properties of graphene structures and its relation to broken sublattice symmetry. Then, we discuss single electronic properties of single and bilayer triangular graphene quantum dots, focusing on the nature of edge states. Finally, we investigate the role of electronic correlations in determining the nature of ground state and excitation spectra of triangular graphene quantum dots as a function of dot size and filling fraction of the shell of zero-energy states. The interactions are treated by a combination of tight-binding, Hartree-Fock and configuration interaction methods. We show that the spin polarization of the triangular graphene quantum dots can be controlled through gating, i.e., by adding or removing electrons. In bilayer graphene dots, the relative filling of edge states in each layer and the magnetization can be tuned down to single localized spin using an external vertical electrical field.The Science Academy, Turkey, under the BAGEP program; TUBITAK, under the BIDEP program; NSERC; University of Ottawa; NRC Canada (IP2012 007372
Graphene-based integrated electronic, photonic and spintronic circuit
To create carbon-based nanoscale integrated electronic, photonic, and spintronic circuit one must demonstrate the three functionalities in a single material, graphene quantum dots (GQDs), by engineering lateral size, shape, edges, number of layers and carrier density. We show theoretically that spatial confinement in GQDs opens an energy gap tunable from UV to THz, making GQDs equivalent to semiconductor nanoparticles. When connected to leads, GQDs act as single-electron transistors. The energy gap and absorption spectrum can be tuned from UV to THz by size and edge engineering and by external electric and magnetic fields. The sublattice engineering in, e.g., triangular graphene quantum dots (TGQDs) with zigzag edges generates a finite magnetic moment. The magnetic moment can be controlled by charging, electrical field, and photons. Addition of a single electron to the charge-neutral system destroys the ferromagnetic order, which can be restored by absorption of a photon. This allows for an efficient spin-photon conversion. These results show that graphene quantum dots have potential to fulfill the three functionalities: electronic, photonic, and spintronic, realized with different materials in current integrated circuits, as well as offer new functionalities unique to graphene
Electronic and optical properties of semiconductor and graphene quantum dots
Our recent work on the electronic and optical properties of semiconductor and graphene quantum dots is reviewed. For strained self-assembled InAs quantum dots on GaAs or InP substrate atomic positions and strain distribution are described using valence-force field approach and continuous elasticity theory. The strain is coupled with the effective mass, k \ub7 p, effective bond-orbital and atomistic tight-binding models for the description of the conduction and valence band states. The single-particle states are used as input to the calculation of optical properties, with electron-electron interactions included via configuration interaction (CI) method. This methodology is used to describe multiexciton complexes in quantum dot lasers, and in particular the hidden symmetry as the underlying principle of multiexciton energy levels, manipulating emission from biexcitons for entangled photon pairs, and optical control and detection of electron spins using gates. The self-assembled quantum dots are compared with graphene quantum dots, one carbon atom-thick nanostructures. It is shown that the control of size, shape and character of the edge of graphene dots allows to manipulate simultaneously the electronic, optical, and magnetic properties in a single material system.Peer reviewed: YesNRC publication: Ye