M. A. Kastner

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.

Defect chemistry of lone-pair semiconductors

Abstract A chemical-bond approach reveals the possibility of several unusual bonding configurations that have relatively low energy in materials with lone-pair valence electrons. Valence-alternation pairs (V.A.P.) are the lowest energy charged defects. The interconversion between positive and negative centres is described in detail. The nature of V.A.P.s in pnictides (group V elements) is discussed. The possibility that valence-alternation centres can form intimate or bound pairs is explored. It is pointed out that certain unusual bonding configurations (configuration distortions) arise even in an ideal lone-pair material. It is stressed that the unusual bonding configurations discussed are a result of the special bonding in lone-pair materials and are, therefore, expected to arise in crystals as well as amorphous materials.

Transient photoconductivity and photo-induced optical absorption in amorphous semiconductors

Abstract Amorphous semiconductors and insulators display the phenomenon of dispersive transport: the average mobility of the carriers decreases with time after pulsed excitation. A simple model explains how multiple trapping (MT) in a continuous distribution of localized states gives rise to dispersion. This model is then used to show what can be learned about the material from a complete study of its dispersive transport when MT is known to be the origin of the dispersion. Transient photo-induced optical absorption (PA) provides a direct test of the presence of the MT mechanism. Transient photocurrent (PC) then provides the spectrum of the localized states when examined in the time regime before recombination begins. Using this density of states, many of the parameters that characterize transport and recombination, both monomolecular (MR) and bimolecular (BR), can be determined. The assumption that thermal excitation from localized states to higher-energy transport states limits both thermalization and r...

Transport properties of annealed CdSe colloidal nanocrystal solids

Transport properties of artificial solids composed of colloidal CdSe nanocrystals (NCs) are studied from 6 to 250 K, before and after annealing. After the solids are annealed, three changes are observed. First, transmission electron micrographs show that the separation between NCs decreases with annealing. Second, the optical absorption spectrum changes: the excitonic peaks of the NC solids shift to lower energies and broaden with annealing. These redshifts can result from the change of the dielectric environment around the NCs. Last, annealing results in greatly enhanced dark current and photocurrent. This increased current can be attributed to the enhancement of interdot tunneling caused by the decreased separation between NCs and by chemical changes in their organic cap. In addition, the dark current is an exponential function of the applied electric field and it is only weakly temperature dependent. Our measurements also suggest that Coulomb interactions between charges on neighboring NCs play an impo...

Thermalization and recombination in amorphous semiconductors

Abstract The fate of carriers excited in an amorphous semiconductor by a pulse of light is described. Two processes are discussed: thermalization of carriers within a distribution of localized states, and carrier recombination.

Neutron-scattering study of spin-density wave order in the superconducting state of excess-oxygen-doped La{sub 2}CuO{sub 4+y}

We report neutron-scattering measurements of spin-density wave order within the superconducting state of a single crystal of predominately stage-4 La{sub 2}CuO{sub 4+y} with a T{sub c} (onset) of 42 K. The low-temperature elastic magnetic scattering is incommensurate with the lattice and is characterized by long-range order in the copper-oxide plane with the spin direction identical to that in the insulator. Between neighboring planes, the spins exhibit short-range correlations with a stacking arrangement reminiscent of that in the undoped antiferromagnetic insulator. The elastic magnetic peak intensity appears at the same temperature within the errors as the superconductivity, suggesting that the two phenomena are strongly correlated. These observations directly reveal the persistent influence of the antiferromagnetic order as the doping level increases from the insulator to the superconductor. In addition, our results confirm that spin-density wave order for incommensurabilities near 1/8 is a robust feature of the La{sub 2}CuO{sub 4}-based superconductors. {copyright} {ital 1999} {ital The American Physical Society}

Electronic transport in films of colloidal CdSe nanocrystals

We present results for electronic transport measurements on large three-dimensional arrays of CdSe nanocrystals. In response to a step in the applied voltage, we observe a power-law decay of the current over five orders of magnitude in time. Furthermore, we observe no steady-state dark current for fields up to

Electrical Control of Spin Relaxation in a Quantum Dot

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