We report measurements of mesoscopic fluctuations of Coulomb blockade peaks in a shapedeformable GaAs quantum dot. Distributions of peak heights agree with predicted universal functions for both zero and nonzero magnetic fields. Parametric fluctuations of peak height and position, measured using a two-dimensional sweep over gate voltage and magnetic field, yield autocorrelations of height fluctuations consistent with a predicted Lorentzian-squared form for the unitary ensemble. We discuss the dependence of the correlation field on temperature and coupling to the leads as the dot is opened.
The thermovoltage of a chaotic quantum dot is measured using a current heating technique. The fluctuations in the thermopower as a function of magnetic field and dot shape display a nonGaussian distribution, in agreement with simulations using Random Matrix Theory. We observe no contributions from weak localization or short trajectories in the thermopower. 72.20.Pa, 73.20.Dx, 05.45+b The electrical conductance of small -characteristic size much smaller than the electron mean free path -confined electron systems (usually denoted as quantum dots) shows distinct fluctuations. These fluctuations display correlations as a function of an external parameter such as shape or magnetic field, which can be described in a statistical manner. The electrons can, in fact, be viewed as billiard balls moving in a classically chaotic system where many random reflections at the system walls occur. Because of the wave-like nature of the electrons, quantum mechanics is needed to describe these systems fully. Chaos in quantum dots has been investigated [1][2][3] in conductance measurements but the analysis turns out to be difficult. So-called short trajectories [4] and weak localization effects [1,5] add up to the signature of chaotic motion. Moreover, current heating of the electrons in the dot appears to be unavoidable in conductance measurements. Electron heating effects in the dot smear out the underlying chaotic statistics and therefore the observed fluctuations exhibit mostly a Gaussian distribution, although theory predicts non-Gaussian distributions when a small number of electron modes is admitted to the dot [6]. Only when dephasing (modelled as extra modes coupling the dot to the environment) is included, Random Matrix Theory (RMT) [1,7] gives a Gaussian distribution. Very recently, Huibers et al. [8] observed small deviations from a Gaussian distribution in conductance measurements. However, other transport properties calculated from these data exhibit again Gaussian distributions in contrast to theoretical predictions.An alternative for the conductance measurements pursued so far (which inherently are accompanied by electron heating inside the dot) is to investigate the thermoelectric properties of a system. Thermopower measurements have already been used to study semiconductor nanostructures like quantum point-contacts [9] and quantum dots in the Coulomb blockade regime [10,11]. The thermopower S measures directly the parametric derivative of the conductance, S ∝ G −1 ∂G/∂X with X = E (energy), and thus yields both similar and additional information on the electron transport processes as can be obtained from conductance measurements. The distribution of parametric derivatives (X = E, B, shape, . . .) of the conductance of a quantum dot is the subject of recent 13]. The probability distribution for the thermopower is again expected to be non-Gaussian for chaotic conductors, exhibiting cusps at zero amplitude and non-exponential tails [13,14].In this paper, we present magneto-thermopower measurements of a statistical en...
From accurate measurements of the energy states in a double quantum dot, we deduce the change in magnetization due to single electron tunneling. We observe crossings and anticrossings in the energy spectrum as a function of magnetic field. The change in magnetization exhibits wiggles as a function of magnetic field with maximum values of a few effective Bohr magnetons in GaAs. These wiggles are a measure of the chaotic motion of the discrete energy states versus magnetic field. Our results show good agreement with a numeric calculation but deviate significantly from semiclassical estimates.[ S0031-9007(98)06132-8] PACS numbers: 07.55.Jg, 85.30.Vw Orbital magnetization of small electron systems has become an important issue in the field of mesoscopics, for instance, in relation to the issue of persistent currents in rings [1]. Altshuler et al. [2] have pointed out that a nonzero orbital magnetization can be present in any mesoscopic electron system, regardless of the precise geometry. The point of interest is that the magnetization measures the cumulative motion of the occupied quantum states as a function of magnetic field. Generally, this motion is chaotic, except for very specific conditions of separable geometries [3]. The statistical properties of the chaotic motion are supposed to be universal in the sense that they do not depend on the details of the microscopic structure. Direct measurements of the magnetization of a mesoscopic object is a challenging task, since it requires the detection of tiny magnetic moments [4].In quantum dots, the chaotic nature can be measured in electron transport [5]. For instance, fluctuations in the Coulomb peak heights have been measured and successfully explained by random matrix theory (RMT) [6,7]. Because of the noninteracting character of RMT, it has not been possible to describe the results of several studies on peak spacings (addition energies) [8]. Here, we report on an experimental study of the magnetization of a quantum dot. We show that semiclassical estimates cannot explain our results, implying that a system with ϳ50 electrons is too small to be described by RMT.We measure the energy evolution versus B of energy states near the Fermi energy E F . The resolution is high enough that, for the first time, avoided crossings in the spectrum of a quantum dot can be resolved. We then obtain the magnetization by taking the derivative of energy with respect to B. Although this magnetization includes only contributions from states near E F , this part largely determines the total magnetization [1]. Measurements of single-particle states versus B have previously been reported on single quantum dot devices [4,9,10], but have not been analyzed in terms of their magnetization. In this paper, we address a double quantum dot system which allows for a much better energy resolution compared to single dots. From the energy dependence on B we calculate the magnetization. The advantage of our method is that the background magnetization of the whole heterostructure [11] is not me...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.