Double-island Coulomb blockade in (Ga,Mn)As nanoconstrictions (original) (raw)
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Physical Review B, 2009
We report on magnetotransport measurements of nanoconstricted (Ga,Mn)As devices showing very large resistance changes that can be controlled by both an electric and a magnetic field. Based on the bias voltage and temperature dependent measurements down to the millikelvin range we compare the models currently used to describe transport through (Ga,Mn)As nanoconstrictions. We provide an explanation for the observed spin-valve like behavior during a magnetic field sweep by means of the magnetization configurations in the device. Furthermore, we prove that Coulombblockade plays a decisive role for the transport mechanism and show that modeling the constriction as a granular metal describes the temperature and bias dependence of the conductance correctly and allows to estimate the number of participating islands located in the constriction.
Coulomb blockade phenomena observed in supported metallic nanoislands
Frontiers in Physics, 2013
The electron transport properties of single crystalline metallic nanostructures in the Coulomb blockade (CB) regime have been investigated by low-temperature scanning tunneling spectroscopy. To this end, nanoscale flat-top Pb islands with well-defined geometries are grown on NaCl-covered Ag(111) substrates. The tunneling spectra acquired at 4.6 K on the Pb nanoislands reflect the presence of single electron tunneling processes across the double-barrier tunnel junction (DBTJ). By a controlled change of the tip-island tunnel distance, the spectra display the characteristic evolution from CB to Coulomb staircase (CS) regime. Simulations within the semi-classical orthodox theory allow us to extract quantitatively the parameters characterizing the DBTJ, i.e., the resistances, capacitances, and the residual charge Q 0 . Manipulation of Q 0 is achieved by controlled application of voltage pulses on the Pb islands. Moreover, under specific tunneling conditions, the influence of the tip-island junction on Q 0 is revealed in topographic images of the Pb islands.
Electron transport through nanocontacts: Ohmic behavior, zero-bias anomaly and Coulomb blockade
2010
Employing low-temperature scanning tunneling spectroscopy we show that the electrical conductance of individual Pb nanocontacts with metallic, semimetallic, semiconducting and partially insulating substrates, varies strikingly upon reduction of the contact area. Charging effects across the junction formed by the nanocontact lead to zero-bias anomalies (Si) or to Coulomb blockade (HOPG, BN/Ni, NaCl/Ag), while for Pb on Cu(111) charging effects remain
Large spin effects in Coulomb blockade of Fe/MgO/Fe tunnel junctions
PHYSICAL REVIEW B, 2005
The Coulomb blockade across Fe and Pd nanoislands grown on single-crystalline MgO insulator films on Fe͑001͒ was investigated with scanning tunneling microscopy. In I͑U͒ spectra of individual islands, the characteristic steps of the Coulomb staircase were observed as a function of island size. A detailed analysis of the Coulomb staircase energetics in terms of the island capacitance reveals the expected linear behavior for electrostatic interactions. For ferromagnetic Fe islands, however, a significant offset of 1 eV was found, which is missing in the case of nonmagnetic Pd islands. This effect is explained by the spin dependence of the electronic transport across the MgO barrier in combination with a quasihalf-metallic density of states of the Fe islands, as corroborated by first-principles electronic-structure calculations.
Coulomb blockade anisotropic magnetoresistance effect in a (Ga, Mn) As single-electron transistor
Physical review letters, 2006
We observe low-field hysteretic magnetoresistance in a Ga; MnAs single-electron transistor which can exceed 3 orders of magnitude. The sign and size of the magnetoresistance signal are controlled by the gate voltage. Experimental data are interpreted in terms of electrochemical shifts associated with magnetization rotations. This Coulomb blockade anisotropic magnetoresistance is distinct from previously observed anisotropic magnetoresistance effects as it occurs when the anisotropy in a band structure derived parameter is comparable to an independent scale, the single-electron charging energy. Effective kineticexchange model calculations in Ga; MnAs show chemical potential anisotropies consistent with experiment and ab initio calculations in transition metal systems suggest that this generic effect persists to high temperatures in metal ferromagnets with strong spin-orbit coupling.
Quantum limitation on Coulomb blockade observed in a 2D electron system
Physical Review Letters, 1993
The evolution of the Coulomb blockade of conduction through a small 2D electron island of large density of states, realized in a GaAs/Ga(Al)As heterojunction, is studied from the weak tunnel coupling to the Ohmic regime. Cotunneling arising from quantum charge Auctuations is identified, at low voltage, by its dependence on the product of junction conductances, and the current at threshold voltage is well described by a nonperturbative model. Small Coulomb conductance oscillations are found to persist even when both junctions exceed the conductance quantum.
Investigations of the Coulomb Blockade effect in double tunnel junctions
Metrology and Measurement Systems, 2006
The Coulomb Blockade effect (CBE) is observed in double tunnel junctions where a small metallic cluster is precisely located between two outer electrodes. This electrically isolated nanocluster exhibits capacitances of attofarads. Such extremely small capacitance may cause that the charging energy EC = e/2C which an individual electron needs to be placed at the cluster may be bigger than the electron’s thermal energy kBT. The Coulomb potential of the electron localized at the cluster keeps the other electrons from flowing through the tunnel junction. Tunnel current increase is possible only after a discrete increase of bias voltage, what is observed as a so called Coulomb staircase in I-V curves. In our investigations, we have created an ultra-thin oxide (barrier) layer on the Si(111) 7x7 surface via controlled oxidation process inside the UHV chamber. One of the tunnel junctions was made by deposition of Ag nanoclusters onto an oxidized Si(111) surface. The other tunnel junction wa...
Single electron tunneling near the Coulomb blockade threshold
In the vicinity of the Coulomb blockade threshold virtual electron tunneling leads to effective screening of the charge on small metallicjunctions. We present a nonperturbative calculation of the junction ground state energy and level splitting close to the edges of the Brillouin zone. We show that the electron tunneling rate can be substantially suppressed due to quantum fluctuations of the charge and the junction current-voltage characteristic becomes nonlinear near the Coulomb gap. We also investigate linear transport properties of a single tunnel junction in the presence of Coulomb effects.
Coulomb Blockade in Graphene Nanoribbons
Physical Review Letters, 2007
We propose that recent transport experiments revealing the existence of an energy gap in graphene nanoribbons may be understood in terms of Coulomb blockade. Electron interactions play a decisive role at the quantum dots which form due to the presence of necks arising from the roughness of the graphene edge. With the average transmission as the only fitting parameter, our theory shows good agreement with the experimental data. 73.23.Hk, 81.05.Uw Graphene, a two-dimensional allotrope of carbon on a honeycomb lattice, was isolated a few years ago [1] creating a great excitement in the physics community due to its close connections to high-energy particle physics [2, 3] and its tantalizing possible technological applications . It is now experimentally established that a great deal of the properties of graphene [6] can be described in terms of non-interacting (or weakly interacting) linearly dispersing Dirac quasi-particles . The only accepted exception maybe when graphene is subject to strong magnetic fields in the quantum Hall regime [9], when the electronic kinetic energy is quenched by the appearance of Landau levels, and the fourfold degeneracy of the Landau levels is split by electron-electron interactions . Nevertheless, in the absence of an applied magnetic field, because of the vanishing of the density of states for Dirac fermions in two dimensions , the electrons in graphene interact through strong, essentially unscreened, long-range Coulomb interactions . The fact that Coulomb interactions do not show up in bulk experiments remains a puzzle in the physics of graphene.