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Register a new userAntibonding ground states in semiconductor artificial molecules
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The spin-orbit interaction is a crucial element of many semiconductor spintronic technologies. Here we report the first experimental observation, by magneto-optical spectroscopy, of a remarkable consequence of the spin-orbit interaction for holes confined in the molecular states of coupled quantum dots. As the thickness of the barrier separating two coupled quantum dots is increased, the molecular ground state changes character from a bonding orbital to an antibonding orbital. This result is counterintuitive, and antibonding molecular ground states are never observed in natural diatomic molecules. We explain the origin of the reversal using a four band k.p model that has been validated by numerical calculations that account for strain. The discovery of antibonding molecular ground states provides new opportunities for the design of artificially structured materials with complex molecular properties that cannot be achieved in natural systems.
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The ground state of the diatomic molecules in nature is inevitably bonding, and its first excited state is antibonding. We demonstrate theoretically that, for a pair of distant adatoms placed buried in three-dimensional-Dirac semimetals, this natural order of the states can be reversed and an antibonding ground state occurs at the lowest energy of the so-called bound states in the continuum. We propose an experimental protocol with the use of a scanning tunneling microscope tip to visualize the topographic map of the local density of states on the surface of the system to reveal the emerging physics.
We fabricate artificial molecules composed of heavy atom lead on a van der Waals crystal. Pb atoms templated on a honeycomb charge-order superstructure of IrTe2 form clusters ranging from dimers to heptamers including benzene-shaped ring hexamers. Tunneling spectroscopy and electronic structure calculations reveal the formation of unusual relativistic molecular orbitals within the clusters. The spin-orbit coupling is essential both in forming such Dirac electronic states and stabilizing the artificial molecules by reducing the adatom-substrate interaction. Lead atoms are found to be ideally suited for a maximized relativistic effect. This work initiates the use of novel two dimensional orderings to guide the fabrication of artificial molecules of unprecedented properties.
We study the excitation spectroscopy of few-electron, parallel coupled double quantum dots (QDs). By applying a finite source drain voltage to a double QD (DQD), the first excited states observed in nonequilibrium charging diagrams can be classified into two kinds in terms of the total effective electron number in the DQD, assuming a core filling. When there are an odd (even) number of electrons, one (two)-electron antibonding (triplet) state is observed as the first excited state. On the other hand, at a larger source drain voltage we observe higher excited states, where additional single-particle excited levels are involved. Eventually, we identify the excited states with a calculation using the Hubbard model and, in particular, we elucidate the quadruplet state, which is normally forbidden by the spin blockade caused by the selection rule.
The many-body state of carriers confined in a quantum dot is controlled by the balance between their kinetic energy and their Coulomb correlation. In coupled quantum dots, both can be tuned by varying the inter-dot tunneling and interactions. Using a theoretical approach based on the diagonalization of the exact Hamiltonian, we show that transitions between different quantum phases can be induced through inter-dot coupling both for a system of few electrons (or holes) and for aggregates of electrons and holes. We discuss their manifestations in addition energy spectra (accessible through capacitance or transport experiments) and optical spectra.
Recent advancement in fabrication technologies enable the construction of nano-objects with rather rich internal structures such as double or triple quantum dots, which can then be regarded as artificial molecules. The main new ingredient in the study of the Kondo effect in such artificial (and also in natural) molecules is the internal symmetry of the nano-object, which proves to play a crucial role in the construction of the effective exchange Hamiltonian. This internal symmetry combines continuous spin symmetry SU(2) and discrete point symmetry (such as mirror reflections for double dots or discrete $C_{3v}$ rotation for equilateral triangular dots. When these artificial molecules are attached to metallic leads, the set of dot operators appearing in the effective exchange Hamiltonian generate a group which is refereed to as the dynamical symmetry group of the system dot-leads [mostly SO(n) or SU(n)], and the pertinent group parameters (such as the value of $n$) can be controlled by experiment. In this short review we clarify and expand these concepts and discuss some specific examples. In particular we concentrate on the difference between the chain geometry and the ring geometry. When a perpendicular magnetic field is applied in the ring geometry, its gauge symmetry U(1) is involved in the interplay with the spin and orbital dynamics of the dot.
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