We report on noise measurements in a quantum dot in the presence of Kondo correlations. Close to the unitary limit, with the conductance reaching 1.8e2 /h, we observed an average backscattered charge of e*~5e/3, while weakly biasing the quantum dot. This result held to bias voltages up to half the Kondo temperature.Away from the unitary limit, the charge was measured to be e as expected. These results confirm and extend the prediction by E. Sela et al. [1], that suggested that twoelectron backscattering processes dominate over single-electron backscattering processes near the unitary limit, with an average backscattered charge e*~5e/3.
We present new results of the "0.7"(2e 2 /h) structure or quasi plateau in some of the most strongly confined point contacts so far reported. This strong confinement is obtained by a combination of shallow etching and metal gate deposition on modulation doped GaAs/GaAlAs heterostructures. The resulting subband separations are up to 20 meV, and as a consequence the quantized conductance can be followed at temperatures up to 30 K, an order of magnitude higher than in conventional split gate devices. We observe pronounced quasi plateaus at several of the lowest conductance steps all the way from their formation around 1 K to 30 K, where the entire conductance quantization is smeared out thermally. We study the deviation of the conductance from ideal integer quantization as a function of temperature, and we find an activated behavior, exp(−T a /T ), with a density dependent activation temperature T a of the order of 2 K. We analyze our results in terms of a simple theoretical model involving scattering against plasmons in the constriction. I. IntroductionThe quantized conduction through a narrow point contact is one of the key effects in mesoscopic physics believed to demonstrate e.g. the validity of the single particle Fermi liquid picture in terms of the LandauerBüttiker formalism, a central formalism in the field. However, the electron system in the narrow constriction forms a quasi one-dimensional electron liquid, and such systems have long ago been predicted to exhibit significant deviations from the ordinary Fermi liquid behavior. Thus the quantum point contact (QPC) remains an important testing ground for the description of mesoscopic phenomena. Indeed recently, significant deviations from the Landauer-Büttiker theory have in fact been observed in quantum point contacts in the temperature dependence of the conductance quantization [1, 2] and as a so called "0.7" structure or quasi plateau, appearing around 0.7 times the conductance quantum 2e 2 /h [3]. Invoking a Luttinger liquid approach [4] the deviations have been discussed in terms of interaction effects [5,6,7] and spin polarization of the one-dimensional electron liquid [8]. However, firm conclusions have been difficult to obtain partly due to the narrow temperature range (0.1 K -4 K) in which the effect can be studied in conventional split gate quantum point contacts, where relatively close lying one-dimensional subbands are formed. One major point in this work is the fabrication of QPCs with large subband spacings which allow a more detailed study of the temperature dependence of the deviations from the standard single particle picture.
We report on the phase measurements on a quantum dot containing a single electron in the Kondo regime. Transport takes place through a single orbital state. Although the conductance is far from the unitary limit, we measure directly, for the first time, a transmission phase as theoretically predicted of pi/2. As the dot's coupling to the leads is decreased, with the dot entering the Coulomb blockade regime, the phase reaches a value of pi. Temperature shows little effect on the phase behavior in the range 30-600 mK, even though both the two-terminal conductance and amplitude of the Aharonov-Bohm oscillations are strongly affected. These results also suggest that previous phase measurements involved transport through more than a single level.
The electrical characteristics of shallow etched GaAs/GaAlAs quantum point contacts (QPCs) of various shapes have been studied as a function of temperature above 0.3 K. Quantized conductance was observed up to 36 K, and from the temperature dependence of the conductance staircase we find energy separations between the lowest one-dimensional subbands up to 20 meV. This value exceeds the highest values so far reported for laterel QPC constrictions in GaAs/GaAlAs heterostructures. In addition, very well behaved quantized conductance plateaus were observed at the lowest temperatures.
A new consistent approach to forming the 0.7 structure by using a quantum dot rather than a quantum point contact is demonstrated. With this new scheme, it was possible to tune on and off the 0.7 structure. The new 0.7 structure continuously evolved into a normal integer conductance plateau by varying the tuning condition. Unlike the conventional 0.7 plateau, the new 0.7 structure was observed even at low electron temperatures down to 100 mK, with unprecedented flatness.From our results, it is concluded that electron interference as well as the electron interaction effect should be taken into consideration to explain the 0.7 structure.
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.