Transport in magnetically ordered Pt nanocontacts

Pt nanocontacts, like those formed in mechanically controlled break junctions, are shown to develop spontaneous local magnetic order. Our density functional calculations predict that a robust local magnetic order exists in the atoms presenting low coordination, i.e., those forming the atom-sized neck. We thus find that the electronic transport can be spin polarized, although the net value of the conductance still agrees with available experimental information. Experimental implications of the formation of this new type of nanomagnet are discussed.

Spin depolarization in the transport of holes across GaxMn1−xAs∕GayAl1−yAs∕p-GaAs

We study the spin polarization of tunneling holes injected from ferromagnetic GaMnAs into a p-doped semiconductor through a tunneling barrier. We find that spin-orbit interaction in the barrier and in the drain limits severely spin injection. Spin depolarization is stronger when the magnetization is parallel to the current than when it is perpendicular to it.

Emergence of half-metallicity in suspended NiO chains: Ab initio electronic structure and quantum transport calculations

Contrary to the antiferromagnetic and insulating character of bulk NiO, one-dimensional chains of this material can become half metallic due to the lower coordination of their atoms. Here we present ab initio electronic structure and quantum transport calculations of ideal infinitely long NiO chains and of more realistic short ones suspended between Ni electrodes. While infinite chains are insulating, short suspended chains are half-metallic minority-spin conductors that displays very large magnetoresistance and a spin-valve behavior controlled by a single atom.

Coherent transport in graphene nanoconstrictions

We study the effect of a structural nanoconstriction on the coherent transport properties of otherwise ideal zigzag-edged infinitely long graphene ribbons. The electronic structure is calculated with the standard one-orbital tight-binding model and the linear conductance is obtained using the Landauer formula. We find that, since the zero-bias current is carried in the bulk of the ribbon, this is very robust with respect to a variety of constriction geometries and edge defects. In contrast, the curve of zero-bias conductance versus gate voltage departs from the (2n+1)e2∕h staircase of the ideal case as soon as a single atom is removed from the sample. We also find that wedge-shaped constrictions can present nonconducting states fully localized in the constriction close to the Fermi energy. The interest of these localized states in regards to the formation of quantum dots in graphene is discussed.

Single-electron transport in electrically tunable nanomagnets

We study a single-electron transistor (SET) based upon a II–VI semiconductor quantum dot doped with a single-Mn ion. We present evidence that this system behaves like a quantum nanomagnet whose total spin and magnetic anisotropy depend dramatically both on the number of carriers and their orbital nature. Thereby, the magnetic properties of the nanomagnet can be controlled electrically. Conversely, the electrical properties of this SET depend on the quantum state of the Mn spin, giving rise to spin-dependent charging energies and hysteresis in the Coulomb blockade oscillations of the linear conductance.

Topologically protected quantum transport in locally exfoliated bismuth at room temperature

We report electrical conductance measurements of Bi nanocontacts created by repeated tip-surface indentation using a scanning tunneling microscope at temperatures of 4 and 300 K. As a function of the elongation of the nanocontact, we measure robust, tens of nanometers long plateaus of conductance G0=2e2/h at room temperature. This observation can be accounted for by the mechanical exfoliation of a Bi(111) bilayer, a predicted quantum spin Hall (QSH) insulator, in the retracing process following a tip-surface contact. The formation of the bilayer is further supported by the additional observation of conductance steps below G0 before breakup at both temperatures. Our finding provides the first experimental evidence of the possibility of mechanical exfoliation of Bi bilayers, the existence of the QSH phase in a two-dimensional crystal, and, most importantly, the observation of the QSH phase at room temperature.

Singular Temperatures Connected to Charge Transport Mechanism Transitions in Perylene Bisimides from Steady-State Photocurrent Measurements

Perylene bisimides (PBIs) are n-type semiconducting and photogenerating materials widely used in a variety of optoelectronic devices. Particularly interesting are PBIs that are simultaneously water-soluble and liquid-crystalline (PBI-W+LC) and, thus, attractive for the development of high-performing easily processable applications in biology and “green” organic electronics. In this work, singular temperatures connected to charge transport mechanism transitions in a PBI-W+LC derivative are determined with high accuracy by means of temperature-dependent photocurrent studies. These singular temperatures include not only the ones observed at 60 and 110 °C, corresponding to phase transition temperatures from crystalline to liquid-crystalline (LC) and from LC to the isotropic phase, respectively, as confirmed by differential scanning calorimetry (DSC), but also a transition at 45 °C, not observed by DSC. By analyzing the photocurrent dependence simultaneously on temperature and on light intensity, this transition is interpreted as a change from monomolecular to bimolecular recombination. These results might be useful for other semiconducting photogenerating materials, not necessarily PBIs or even organic semiconductors, which also show transport behavior changes at singular temperatures not connected with structural or phase transitions.

Transport across two interacting quantum dots: Bulk Kondo, Kondo box, and molecular regimes

We analyze the transport properties of a double quantum dot device with both dots coupled to perfect conducting leads and to a finite chain of N noninteracting sites connecting both of them. The interdot chain strongly influences the transport across the system and the local density of states of the dots. We study the case of a small number of sites, so that Kondo box effects are present, varying the coupling between the dots and the chain. For odd N and small coupling between the interdot chain and the dots, a state with two coexisting Kondo regimes develops: the bulk Kondo due to the quantum dots connected to leads and the one produced by the screening of the quantum dot spins by the spin in the finite chain at the Fermi level. As the coupling to the interdot chain increases, there is a crossover to a molecular Kondo effect, due to the screening of the molecule (formed by the finite chain and the quantum dots) spin by the leads. For even N the two Kondo temperatures regime does not develop and the physics is dominated by the usual competition between Kondo and antiferromagnetism between the quantum dots. We finally study how the transport properties are affected as N is increased. For the study we used exact multiconfigurational Lanczos calculations and finite-U slave-boson mean-field theory at T=0. The results obtained with both methods describe qualitatively and also quantitatively the same physics.