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Register a new userBipolaronic blockade effect in quantum dots with negative charging energy
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We investigate single-electron transport through quantum dots with negative charging energy induced by a polaronic energy shift. For weak dot-lead tunnel couplings, we demonstrate a bipolaronic blockade effect at low biases which suppresses the oscillating linear conductance, while the conductance resonances under large biases are enhanced. Novel conductance plateau develops when the coupling asymmetry is introduced, with its height and width tuned by the coupling strength and external magnetic field. It is further shown that the amplitude ratio of magnetic-split conductance peaks changes from 3 to 1for increasing coupling asymmetry. Though we demonstrate all these transport phenomena in the low-order single-electron tunneling regime, they are already strikingly different from the usual Coulomb blockade physics and are easy to observe experimentally.
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Numerical analysis of the simplest odd-numbered system of coupled quantum dots reveals an interplay between magnetic ordering, charge fluctuations and the tendency of itinerant electrons in the leads to screen magnetic moments. The transition from local-moment to molecular-orbital behavior is visible in the evolution of correlation functions as the inter-dot coupling is increased. Resulting novel Kondo phases are presented in a phase diagram which can be sampled by measuring the zero-bias conductance. We discuss the origin of the even-odd effects by comparing with the double quantum dot.
A dilute concentration of magnetic impurities can dramatically affect the transport properties of an otherwise pure metal. This phenomenon, known as the Kondo effect, originates from the interactions of individual magnetic impurities with the conduction electrons. Nearly a decade ago, the Kondo effect was observed in a new system, in which the magnetic moment stems from a single unpaired spin in a lithographically defined quantum dot, or artificial atom. The discovery of the Kondo effect in artificial atoms spurred a revival in the study of Kondo physics, due in part to the unprecedented control of relevant parameters in these systems. In this review we discuss the physics, origins, and phenomenology of the Kondo effect in the context of recent quantum dot experiments.
We present a theoretical study of the charging effects in single and double layer black phosphorus quantum dots (BPQDs) with lateral sizes of 2 nm and 3 nm. We demonstrate that the charging of BPQDs are able to store up to an $N_{max}$ electron (that depends on the lateral size and number of layers in the QD), after which structural instabilities arises. For example, 3 nm wide hydrogen-passivated single layer BPQDs can hold a maximum of 16 electrons, and an additional electron causes the expelling of hydrogen atoms from the QD borders. We also calculated the additional energy ($E_A$) spectrum. For single layer QDs with 2 and 3 nm of lateral sizes, the average $E_A$ is around 0.4 eV and 0.3 eV, respectively. For double layer QDs with the same sizes, the average $E_A$ is around 0.25 eV and 0.2 eV, respectively.
We investigate with the aid of numerical renormalization group techniques the thermoelectric properties of a molecular quantum dot described by the negative-U Anderson model. We show that the charge Kondo effect provides a mechanism for enhanced thermoelectric power via a correlation induced asymmetry in the spectral function close to the Fermi level. We show that this effect results in a dramatic enhancement of the Kondo induced peak in the thermopower of negative-U systems with Seebeck coefficients exceeding 50$mu V/K$ over a wide range of gate voltages.
We study an interacting quantum dot in contact with a small superconducting island described by the interacting pairing model with charging (Coulomb) energy $E_c$. This charge-conserving Hamiltonian admits a compact matrix-product-operator representation and can be accurately solved using the density-matrix renormalization group. We investigate the effects of the $E_c$ term which controls the number of electrons on the superconducting island. Most prominently, the energies of the subgap excited states induced by the impurity are no longer symmetric with respect to the chemical potential and may undergo discontinuous changes as a function of gate voltages. Phase diagrams of spin-singlet and spin-doublet ground states reveal a cross-over from the regime governed by the Yu-Shiba-Rusinov physics to the charge quantization (Coulomb blockade) regime characterized by even-odd electron-number effects. In this regime we find subgap states for both even and odd superconductor occupancy, but with distinctive subgap excitation spectra.
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