Correlations between Ground and Excited State Spectra of a Quantum Dot

We have studied the ground and excited state spectra of a semiconductor quantum dot for successive numbers of electron occupancy using linear and nonlinear magnetoconductance measurements. We present the first observation of direct correlation between the mth excited state of the N electron system and the ground state of the N+m electron system for m up to 4. Results are consistent with a non-spin-degenerate single particle picture of the filling of levels. Electron-electron interaction effects are also observed as a perturbation to this model. Magnetoconductance fluctuations of ground states are shown as anticrossings where wavefunction characteristics are exchanged between adjacent levels.Comment: 8 pages pdf; gzipped ps available at http://www-leland.stanford.edu/group/MarcusLab/grouppubs.htm

Mesoscopic Coulomb Blockade in One-channel Quantum Dots

Signatures of "mesoscopic Coulomb blockade" are reported for quantum dots with one fully transmitting point-contact lead, T1 = 1, T2 << 1. Unlike Coulomb blockade (CB) in weak-tunneling devices (T1, T2 << 1), one-channel CB is a mesoscopic effect requiring quantum coherence. Several distinctive features of mesoscopic CB are observed, including a reduction in CB upon breaking time-reversal symmetry with a magnetic field, relatively large fluctuations of peak position as a function of magnetic field, and strong temperature dependence on the scale of the quantum level spacing.Comment: 12 pages, including 4 figure

Distributions of the Conductance and its Parametric Derivatives in Quantum Dots

Full distributions of conductance through quantum dots with single-mode leads are reported for both broken and unbroken time-reversal symmetry. Distributions are nongaussian and agree well with random matrix theory calculations that account for a finite dephasing time, $\tau_\phi$, once broadening due to finite temperature $T$ is also included. Full distributions of the derivatives of conductance with respect to gate voltage $P(dg/dV_g)$ are also investigated.Comment: 4 pages (REVTeX), 4 eps figure

Spin Degeneracy and Conductance Fluctuations in Open Quantum Dots

The dependence of mesoscopic conductance fluctuations on parallel magnetic field is used as a probe of spin degeneracy in open GaAs quantum dots. The variance of fluctuations at high parallel field is reduced from the low-field variance (with broken time-reversal symmetry) by factors ranging from roughly two in a 1 square-micron dot at low temperature, to four or greater in 8 square-micron dots. The factor of two is expected for simple Zeeman splitting of spin degenerate channels. A possible explanation for the unexpected larger factors in terms of field-dependent spin orbit scattering is proposed.Comment: Includes new reference to related theoretical work, cond-mat/0010064. Other minor changes. Related papers at http://marcuslab.harvard.ed

Coulomb Blockade Fluctuations in Strongly Coupled Quantum Dots

Quantum fluctuations of Coulomb blockade are investigated as a function of the coupling to reservoirs in semiconductor quantum dots. We use fluctuations in the distance between peaks $\Delta N$ apart to characterize both the amplitude and correlation of peak motion. For strong coupling, peak motion is greatly enhanced at low temperature, but does not show an increase in peak-to-peak correlation. These effects can lead to anomalous temperature dependence in the Coulomb valleys, similar to behavior ascribed to Kondo physics.Comment: figures made smaller so download works. Revised, including new data. Related papers at http://www.stanford.edu/group/MarcusLab/grouppubs.htm

Statistics of Coulomb Blockade Peak Spacings

Distributions of Coulomb blockade peak spacing are reported for large ensembles of both unbroken (magnetic field B = 0) and broken (B 0) time reversal symmetry in GaAs quantum dots. Both distributions are symmetric and roughly gaussian with a width ~ 2-6% of the average spacing, with broad, non-gaussian tails. The distribution is systematically wider at B = 0 by a factor of ~ 1.2 +- 0.1. No even-odd spacing correlations or bimodal structure in the spacing distribution is found, suggesting an absence of spin-degeneracy. There is no observed correlation between peak spacing and peak height.Comment: To appear in PRL; 13 pages, one table, 3 figures; pdf available at http://www-leland.stanford.edu/group/MarcusLab/papers/Patel_peakspacing.pd