U. Meirav

Kondo effect in a single-electron transistor

How localized electrons interact with delocalized electrons is a central question to many problems in sold-state physics. The simplest manifestation of this situation is the Kondo effect, which occurs when an impurity atom with an unpaired electron is placed in a metal. At low temperatures a spin singlet state is formed between the unpaired localized electron and delocalized electrons at the Fermi energy. Theories predict that a Kondo singlet should form in a single-electron transistor (SET), which contains a confined ‘droplet’ of electrons coupled by quantum-mechanical tunnelling to the delocalized electrons in the transistors leads. If this is so, a SET could provide a means of investigating aspects of the Kondo effect under controlled circumstances that are not accessible in conventional systems: the number of electrons can be changed from odd to even, the difference in energy between the localized state and the Fermi level can be tuned, the coupling to the leads can be adjusted, voltage differences can be applied to reveal non-equilibrium Kondo phenomena, and a single localized state can be studied rather than a statistical distribution. But for SETs fabricated previously, the binding energy of the spin singlet has been too small to observe Kondo phenomena. Ralph and Buhrman have observed the Kondo singlet at a single accidental impurity in a metal point contact, but with only two electrodes and without control over the structure they were not able to observe all of the features predicted. Here we report measurements on SETs smaller than those made previously, which exhibit all of the predicted aspects of the Kondo effect in such a system.

FROM THE KONDO REGIME TO THE MIXED-VALENCE REGIME IN A SINGLE-ELECTRON TRANSISTOR

We demonstrate that the conductance through a single-electron transistor at low temperature is in quantitative agreement with predictions of the equilibrium Anderson model. When an unpaired electron is localized within the transistor, the Kondo effect is observed. Tuning the unpaired electrons energy toward the Fermi level in nearby leads produces a cross-over between the Kondo and mixed-valence regimes of the Anderson model.

Fano resonances in electronic transport through a single-electron transistor

We have observed asymmetric Fano resonances in the conductance of a single electron transistor resulting from interference between a resonant and a nonresonant path through the system. The resonant component shows all the features typical of quantum dots, but the origin of the non-resonant path is unclear. A unique feature of this experimental system, compared to others that show Fano line shapes, is that changing the voltages on various gates allows one to alter the interference between the two paths.

The Metalliclike Conductivity of a Two-Dimensional Hole System

We report on a zero magnetic field transport study of a two-dimensional, variable-density, hole system in GaAs. As the density is varied we observe, for the first time in GaAs-based materials, a crossover from an insulating behavior at low-density, to a metallic-like behavior at high-density, where the metallic behavior is characterized by a large drop in the resistivity as the temperature is lowered. These results are in agreement with recent experiments on Si-based two-dimensional systems by Kravchenko et al. and others. We show that, in the metallic region, the resistivity is dominated by an exponential temperature-dependence with a characteristic temperature which is proportional to the hole density, and appear to reach a constant value at lower temperatures.

Temperature dependence of Fano line shapes in a weakly coupled single-electron transistor

We report the temperature dependence of the zero-bias conductance of a single-electron transistor in the regime of weak coupling between the quantum dot and the leads. The Fano line shape, convoluted with thermal broadening, provides a good fit to the observed asymmetric Coulomb charging peaks. However, the width of the peaks increases more rapidly than expected from the thermal broadening of the Fermi distribution in a temperature range for which Fano interference is unaffected. The intrinsic width of the resonance extracted from the fits increases approximately quadratically with temperature. Above about 600 mK the asymmetry of the peaks decreases, suggesting that phase coherence necessary for Fano interference is reduced.

ABSENCE OF SCALING IN THE INTEGER QUANTUM HALL EFFECT

We have studied the conductivity peak in the transition region between the two lowest integer Quantum Hall states using transmission measurements of edge magnetoplasmons. The width of the transition region is found to increase linearly with frequency but remains finite when extrapolated to zero frequency and temperature. Contrary to prevalent theoretical pictures, our data does not show the scaling characteristics of critical phenomena.These results suggest that a different mechanism governs the transition in our experiment.

The Kondo effect in a single-electron transistor

Abstract How localized electrons interact with delocalized electrons is a question central to many of the problems at the forefront of solid state physics. The simplest example, the Kondo effect, occurs when an impurity atom with an unpaired electron is placed in a metal, and the energy of the unpaired electron is far below the Fermi energy. At low temperatures a spin singlet state is formed between the unpaired localized electron and delocalized electrons at the Fermi energy. The consequences of this singlet formation were first observed over 60 years ago in metals with magnetic impurities, but full theoretical understanding was slow to come. Today, the situation is reversed: scaling theories and recent renormalization group calculations (T.A. Costi, A.C. Hewson (1994) J. Phys.: Cond. Mat. 6, 2519) can predict quantitatively the bonding strength of the singlet state, and the singlets effect on the conduction electrons at all temperatures. The detailed dependence of these properties on parameters such as the energy of the localized electron cannot be tested experimentally in the classic Kondo systems, since the relevant parameters cannot easily be tuned for impurities in a metal. Recently it has become possible to test these predictions with a new experimental approach — creating an artificial Kondo system by nanofabrication (D. Goldhaber-Gordon et al. (1998), Nature 391, 156). The confined droplet of electrons interacting with the leads of a single electron transistor (SET) is closely analogous to an impurity atom interacting with the delocalized electrons in a metal, as described in the Anderson model (Y. Meir, N.S. Wingreen, P.A. Lee, Phys. Rev. Lett. (1993) 70 2601–2604). We review here measurements on a new generation of SETs that display all the aspects of the Kondo effect: the spin singlet forms and causes an enhancement of the zero-bias conductance when the number of electrons on the artificial atom is odd but not when it is even. The singlet is altered by applying a voltage or magnetic field or by increasing the temperature, all in ways that agree with predictions (N.S. Wingreen, Y. Meir (1994), Phys. Rev. B 49, 11040; T.A. Costi, A.C. Hewson (1994), J. Phys.: Cond. Mat. 6, 2519; W. Izumida, O. Sakai, Y. Shimizu (1998), J. Phys. Soc. Jpn. 67; D. Goldhaber-Gordon et al. (1998), Nature 391, 156; D. Goldhaber-Gordon, J. Gores, M.A. Kastner, H. Shtrikman, D. Mahalu, U. Meirav (1998), Phys. Rev. Lett. 81, 5225).

Short-period surface superlattices formed by plasma etching

We report the development of a process for fabricating etched surface superlattices (SSL). We utilize low-voltage electron cyclotron resonance plasma etching in conjunction with electron beam lithography to form a short-pitch grating relief on GaAs/AlGaAs heterostructures hosting a high-mobility two-dimensional electron gas (2DEG). The process minimizes damage to the 2DEG and results in highly uniform etched gratings. A Schottky gate covering the etched surface appears to improve the electrical properties of the SSLs. Magnetotransport measurements show the effectiveness of this technique in realizing high-quality SSLs with periods down to 100 nm.

Refractory metal-based low-resistance ohmic contacts for submicron GaAs heterostructure devices

Abstract A simple and reliable process for ohmic contacts to Al x Ga 1−x /AsGaAs heterostructures is presented. The key feature is the use of niobium refractory metal as a barrier between the eutectic AuGe alloy and the capping Au layer, leading to excellent contact morphology and good electrical properties. Such ohmic contacts fabricated for a two-dimensional electron gas have achieved a resistance of 0.2 Ω mm. The smooth morphology allows subsequent electron beam lithography alignment to the ohmic level, and is crucial for short-gate transistors.

Very low density two-dimensional hole gas in an inverted GaAs/AlAs interface

We utilize an inverted heterostructure grown on (311)A GaAs to realize a two-dimensional hole gas (2DHG) with a built-in back gate. The density of the 2DHG is easily and reproducibly varied between 5×109 and 5×1011 cm−2. The mobility of the 2DHG is highly anisotropic in the (311)A plane.