arXiv subjects

M. Stopa

Publications and source records attributed to M. Stopa.

14 recordsLinked to original sources

arXiv2026

Geometric blockade in a quantum dot coupled to two-dimensional and three dimensional electron gases

We fabricated a quantum dot coupled laterally to a two-dimensional electron gas and vertically to a three-dimensional electron gas in order to investigate the eigenstate dependence of tunneling rate to these gases. We observed a bias-dependent ``geometric" current blockade. By tunneling via the asymmetric couplings, population inversion is induced and a dark metastable triplet state is revealed. The metastable state stops the current transport process, suppresses the current and asymmetrically widens the Coulomb diamond. By analyzing the current as a function of source-drain and gate voltage and the magnetic field, we concluded that this effect is due to the geometric shape of the electronic states in the dot and the current is limited by the tunneling rate due to the eigenstates, that is, artificial $\sigma$-coupling and $\pi$-coupling.

cond-mat.mes-hall↗

arXiv2010

Fast sensing of double-dot charge arrangement and spin state with an rf sensor quantum dot

Single-shot measurement of the charge arrangement and spin state of a double quantum dot are reported, with measurement times down to ~ 100 ns. Sensing uses radio-frequency reflectometry of a proximal quantum dot in the Coulomb blockade regime. The sensor quantum dot is up to 30 times more sensitive than a comparable quantum point contact sensor, and yields three times greater signal to noise in rf single-shot measurements. Numerical modeling is qualitatively consistent with experiment and shows that the improved sensitivity of the sensor quantum dot results from reduced screening and lifetime broadening.

cond-mat.mes-hall↗

arXiv2010

Dynamic Nuclear Polarization in Double Quantum Dots

We theoretically investigate the controlled dynamic polarization of lattice nuclear spins in GaAs double quantum dots containing two electrons. Three regimes of long-term dynamics are identified, including the build up of a large difference in the Overhauser fields across the dots, the saturation of the nuclear polarization process associated with formation of so-called "dark states," and the elimination of the difference field. We show that in the case of unequal dots, build up of difference fields generally accompanies the nuclear polarization process, whereas for nearly identical dots, build up of difference fields competes with polarization saturation in dark states. The elimination of the difference field does not, in general, correspond to a stable steady state of the polarization process.

cond-mat.mes-hall↗

arXiv2009

Inhomogeneous Nuclear Spin Flips

We discuss a feedback mechanism between electronic states in a double quantum dot and the underlying nuclear spin bath. We analyze two pumping cycles for which this feedback provides a force for the Overhauser fields of the two dots to either equilibrate or diverge. Which of these effects is favored depends on the g-factor and Overhauser coupling constant A of the material. The strength of the effect increases with A/V_x, where V_x is the exchange matrix element, and also increases as the external magnetic field B_{ext} decreases.

cond-mat.mes-hall↗

arXiv2006

Magnetic Field Control of Exchange and Noise Immunity in Double Quantum Dots

We employ density functional calculated eigenstates as a basis for exact diagonalization studies of semiconductor double quantum dots, with two electrons, through the transition from the symmetric bias regime to the regime where both electrons occupy the same dot. We calculate the singlet-triplet splitting $J(ε)$ as a function of bias detuning $ε$ and explain its functional shape with a simple, double anti-crossing model. A voltage noise suppression "sweet spot," where $dJ(ε)/dε=0$ with nonzero $J(ε)$, is predicted and shown to be tunable with a magnetic field $B$.

cond-mat.mes-hall↗

arXiv2005

Electronic Structure of Multiple Dots

We calculate, via spin density functional theory (SDFT) and exact diagonalization, the eigenstates for electrons in a variety of external potentials, including double and triple dots. The SDFT calculations employ realistic wafer profiles and gate geometries and also serve as the basis for the exact diagonalization calculations. The exchange interaction J between electrons is the difference between singlet and triplet ground state energies and reflects competition between tunneling and the exchange matrix element, both of which result from overlap in the barrier. For double dots, a characteristic transition from singlet ground state to triplet ground state (positive to negative J) is calculated. For the triple dot geometry with 2 electrons we also find the electronic structure with exact diagonalization. For larger electron number (18 and 20) we use only SDFT. In contrast to the double dot case, the triple dot case shows a quasi-periodic fluctuation of J with magnetic field which we attribute to periodic variations of the basis states in response to changing flux quanta threading the triple dot structure.

cond-mat.mes-hall↗

arXiv2004

Single-dot spectroscopy via elastic single-electron tunneling through a pair of coupled quantum dots

We study the electronic structure of a single self-assembled InAs quantum dot by probing elastic single-electron tunneling through a single pair of weakly coupled dots. In the region below pinch-off voltage, the non-linear threshold voltage behavior provides electronic addition energies exactly as the linear, Coulomb blockade oscillation does. By analyzing it, we identify the s and p shell addition spectrum for up to six electrons in the single InAs dot, i.e. one of the coupled dots. The evolution of shell addition spectrum with magnetic field provides Fock-Darwin spectra of s and p shell.

cond-mat.mes-hall↗

arXiv2004

Molecular states observed in a single pair of strongly coupled self-assembled InAs quantum dots

Molecular states in a SINGLE PAIR of strongly coupled self-assembled InAs quantum dots are investigated using a sub-micron sized single electron transistor containing just a few pairs of coupled InAs dots embedded in a GaAs matrix. We observe a series of well-formed Coulomb diamonds with charging energy of less than 5 meV, which are much smaller than those reported previously. This is because electrons are occupied in molecular states, which are spread over both dots and occupy a large volume. In the measurement of ground and excited state single electron transport spectra with magnetic field, we find that the electrons are sequentially trapped in symmetric and anti-symmetric states. This result is well-explained by numerical calculation using an exact diagonalization method.

cond-mat.mes-hall↗

arXiv2003

Magnetically induced chessboard pattern in the conductance of a Kondo quantum dot

We quantitatively describe the main features of the magnetically induced conductance modulation of a Kondo quantum dot -- or chessboard pattern -- in terms of a constant-interaction double quantum dot model. We show that the analogy with a double dot holds down to remarkably low magnetic fields. The analysis is extended by full 3D spin density functional calculations. Introducing an effective Kondo coupling parameter, the chessboard pattern is self-consistently computed as a function of magnetic field and electron number, which enables us to quantitatively explain our experimental data.

cond-mat.mes-hall↗

arXiv1999

Bandgap renormalization of modulation doped quantum wires

We measure the photoluminescence spectra for an array of modulation doped, T-shaped quantum wires as a function of the 1d density n_e which is modulated with a surface gate. We present self-consistent electronic structure calculations for this device which show a bandgap renormalization which, when corrected for excitonic energy and its screening, are largely insensitive to n_e and which are in quantitatively excellent agreement with the data. The calculation (cf. cond-mat/9908349) shows the importance of including orthogonality between the screening electrons and the electron(s) bound to the hole. The calculations show that electron and hole remain bound up to 3 x 10^6 cm^-1 and that therefore the stability of the exciton far exceeds the conservative Mott criterion.

cond-mat.mes-hall↗

arXiv1999

Bandgap renormalization and excitonic binding in T-shaped quantum wires

We calculate the electronic structure for a modulation doped and gated T-shaped quantum wire using density functional theory. We calculate the bandgap renormalization as a function of the density of conduction band electrons, induced by the donor layer and/or the gate, for the translationally invariant wire, incorporating all growth and geometric properties of the structure completely. We show that most of the bandgap renormalization arises from exchange-correlation effects, but that a small shift also results from the difference of wave function evolution between electrons and holes. We calculate the binding energy of excitons in the wire, which breaks translational invariance, using a simpler, cylindrical model of the wire. For a single hole and a one dimensional electron gas of density n_e, screening of the exciton binding energy is shown to approximately compensate for bandgap renormalization, suggesting that the recombination energy remains approximately constant with n_e, in agreement with experiment. We find that the nature of screening, as treated within our non-linear model, is significantly different from that of the various linear screening treatments, and the orthogonality of free carrier states with the bound electron states has a profound effect on the screening charge. We find that the electron and hole remain bound for all densities up to about 3 x 10^6 cm^{-1} and that, as n_e increases from zero, trion and even ``quadron'' formation becomes allowed.

cond-mat.mes-hall↗

arXiv1997

Fluctuations in quantum dot charging energy

We show that quasi-periodic fluctuations in the charging energy E_C of small, chaotic quantum dots result from strongly scarred states which are the remnant of periodic orbits in the classical confining potential. We perform self-consistent, density functional and spin density functional calculations for the dots used in the recent experimental studies of Sivan et al. [PRL 77, 1123 (1996)]. We directly compute the direct Coulomb matrix elements W_{pq}, screened by the gates, between self-consistent states. We show that diagonal elements (denoted U_p) are uniformly larger than off diagonal elements, resulting in spin polarization of the dot. We show that strongly scarred states, which are quasi-one dimensional, have particularly large values of U_p which causes them to remain, partially occupied, at the Fermi surface as gate voltage V_g is swept and more homogeneous, ``chaotic'' states pass through. We show that ultimate double-filling of these scars results in large upward fluctuations of E_C. In addition, we show that V_g dependence of capacitances and level energies substantially enhances fluctuations in the gate voltage spacing dV_g of Coulomb oscillations, as compared to and distinct from fluctuations in the ``inverse compressibility'' (i.e. E_C). Moreover, these dependences modify the distribution of dV_g fluctuations so that purely upward fluctuations of E_C produce symmetric fluctuations in dV_g. Consequently, gate voltage fluctuations (normalized by the appropriate capacitance ratio) are substantially greater than the single particle level spacing, Delta, but, as we show by direct calculation, their temperature T scaling is consistent with Delta/T and not E_C/T, in complete agreement with experiment.

cond-mat.mes-hall↗

arXiv1996

Comment on ``New Class of Resonances at the Edge of the Two-Dimensional Electron Gas:'' self-consistent electronic structure

Self-consistent electronic structure calculations, for devices recently fabricated and studied by Zhitenev et al. for capacitance spectroscopy in the quantum Hall regime, demonstrate that reproducible resonances in the coupling between adiabatically free edge states and isolated ``puddles'' of electrons of higher filling factor under the gate proceed from an interference phenomenon as the edge states bend upon entering and leaving the gated region. We note that results of experiment and theory which we published earlier this year relate to a similar, if not identical, phenomenon.

cond-mat.mes-hall↗

arXiv1996

Quantum dot self consistent electronic structure and the Coulomb blockade

We employ density functional theory to calculate the self consistent electronic structure, free energy and linear source-drain conductance of a lateral semiconductor quantum dot patterned via surface gates on the 2DEG formed at the interface of a $GaAs-AlGaAs$ heterostructure. The Schrödinger equation is reduced from 3D to multi-component 2D and solved via an eigenfunction expansion in the dot. This permits the solution of the electronic structure for dot electron number $N \sim 100$. We present details of our derivation of the total dot-lead-gates interacting free energy in terms of the electronic structure results, which is free of capacitance parameters. Statistical properties of the dot level spacings and connection coefficients to the leads are computed in the presence of varying degrees of order in the donor layer. Based on the self-consistently computed free energy as a function of gate voltages, $V_i$, and N, we modify the semi-classical expression for the tunneling conductance as a function of gate voltage through the dot in the linear source-drain, Coulomb blockade regime. Among the many results presented, we demonstrate the existence of a shell structure in the dot levels which (a) results in envelope modulation of Coulomb oscillation peak heights, (b) which influences the dot capacitances and should be observable in terms of variations in the activation energy for conductance in a Coulomb oscillation minimum, and (c) which possibly contributes to departure of recent experimental results from the predictions of random matrix theory.

cond-mat↗