arXiv subjects

E. Wach

Publications and source records attributed to E. Wach.

4 recordsLinked to original sources

arXiv2017

Pauli blockade microscopy of quantum dots

We propose a spin-sensitive scanning probe microscopy experiment on double quantum dots in Pauli blockade conditions. Electric spin resonance is induced by an AC voltage applied to the scanning gate which induces lifting of the Pauli blockade of the current. The stationary Hamiltonian eigenstates are used as a basis for description of the spin dynamics with the AC potential of the probe. For the two-electron system we evaluate the transitions rates from triplet $\mathrm{T}_+$ state to singlet $\mathrm{S}$ or triplet $\mathrm{T}_0$ states, i.e. to conditions in which the Pauli blockade of the current is lifted. The rates of the spin-flip transitions are consistent with the transition matrix elements and strongly dependent on the tip position. Probing the spin densities and identification of the final transition state are discussed.

cond-mat.mes-hall↗

arXiv2014

Imaging transition to fractional quantum Hall regime by Coulomb blockade microscopy

We consider electron systems in quantum dots and imaging of the confined charge density by the Coulomb blockade microscopy (CBM) with the scanning probe technique. We apply an exact diagonalization method to study the reaction of the electron system to the potential induced by the model potential of the probe and calculate the energy maps as functions of the position of the probe. The charge densities derived from the energy maps are confronted to the exact charge densities. We focus on the transition of the electron system to the fractional quantum Hall conditions in external magnetic field. For magnetic fields corresponding to the integer fillings of the lowest Landau level the electron system exhibits a liquid-like reaction to the potential of the probe and the confined charge density can be quite accurately mapped by the CBM. For fractional fillings of the lowest Landau level the single-electron charge density islands nucleate in presence of the tip. In circular quantum dots the single-electron islands evade imaging by CBM. We demonstrate that mapping the molecular charge densities is possible for confinement potentials of symmetry that is lower and consistent with the geometry of the lowest-energy charge distribution of the single-electron islands.

cond-mat.mes-hall↗

arXiv2013

Confined states in quantum dots defined within finite flakes of bilayer graphene: Coupling to the edge, ionization threshold, and valley degeneracy

We study quantum dots defined by external potentials within finite flakes of bilayer graphene using the tight-binding approach. We find that in the limit of large flakes containing zigzag edges the dot-localized energy levels appear within the energy continuum formed by extended states. As a consequence no ionization threshold for the carriers contained within the dot exists. For smaller flakes with zigzag boundaries the dot-localized energy levels appear interlaced with the energy levels outside the flake, so in a charging experiment the electrons will be added alternately to the dot area and to its neighborhood. We demonstrate that for flakes with armchair boundaries only, an energy window accessible uniquely to the dot-localized states is opened. Then a number of electrons can be added to the dot before the external states start to be occupied. We also discuss coupling of the dot-localized states to the edge states in the context of the valley degeneracy lifting. Moreover, we extract smooth envelope wave functions from the tight-binding solution and discuss their spatial symmetries. The coupling of the dot localized energy levels with reconstructed zigzag edges and atomic vacancies present within the layers is also considered.

cond-mat.mes-hall↗

arXiv2013

Charge density mapping of strongly-correlated few-electron two-dimensional quantum dots by scanning probe technique

We perform a numerical simulation of mapping of charge confined in quantum dots by the scanning probe technique. We solve the few-electron Schrödinger equation with the exact diagonalization approach and evaluate the energy maps in function of the probe position. Next, from the energy maps we try to reproduce the charge density distribution using an integral equation given by the perturbation theory. The reproduced density maps are confronted with the original ones. The present study covers two-dimensional quantum dots of various geometries and profiles with the one-dimensional (1D) quantum dot as a limit case. We concentrate on large quantum dots for which strong electron-electron correlations appear. For circular dots the correlations lead to formation of Wigner molecules that in the presence of the tip appear in the laboratory frame. The unperturbed rotationally-symmetric charge density is surprisingly well reproduced by the mapping. We find in general that the size of the confined droplet as well as the spatial extent of the charge density maxima is underestimated for repulsive tip potential and overestimated for the attractive tip. In lower-symmetry quantum dots the Wigner molecules with single-electron islands nucleate for some electron numbers even in the absence of the tip. These charge densities are well resolved by the mapping. The single-electron islands appear in the laboratory frame provided that classical point charge density distribution is unique, in the 1D limit of confinement in particular. We demonstrate that for electron systems which possess a few equivalent classical configurations the repulsive probe switches between the configurations. In consequence the charge density evades mapping by the repulsive probe.

cond-mat.mes-hall↗