Electronic properties of TaAs2topological semimetal investigated by transport and ARPES.

We have performed electron transport and angle-resolved photo-emission spectroscopy (ARPES) measurements on single crystals of transition metal dipnictide TaAs2cleaved along the (2¯01) surface which has the lowest cleavage energy. A Fourier transform of the Shubnikov-de Haas oscillations shows four different peaks whose angular dependence was studied with respect to the angle between magnetic field and the [2¯01] direction. The results indicate elliptical shape of the Fermi surface cross-sections. Additionally, a mobility spectrum analysis was carried out, which also reveals at least four types of carriers contributing to the conductance (two kinds of electrons and two kinds of holes). ARPES spectra were taken on freshly cleaved (2¯01) surface and it was found that bulk states pockets at constant energy surface are elliptical, which confirms the magnetotransport angle dependent studies. First-principles calculations support the interpretation of the experimental results. The theoretical calculations better reproduce the ARPES data if the theoretical Fermi level (FL) is increased, which is due to a small n-doping of the samples. This shifts the FL closer to the Dirac point, allowing investigating the physics of the Dirac and Weyl points, making this compound a platform for the investigation of the Dirac and Weyl points in three-dimensional materials.

A multiferroic molecular magnetic qubit.

The chiral Fe3O(NC5H5)3(O2CC6H5)6 molecular cation, with C3 symmetry, is composed of three six-fold coordinated spin-carrying Fe3+ cations that form a perfect equilateral triangle. Experimental reports demonstrating the spin-electric effect in this system also identify the presence of a magnetic uniaxis and suggest that this molecule may be a good candidate for an externally controllable molecular qubit. Here, we demonstrate, using standard density-functional methods, that the spin-electric behavior of this molecule could be even more interesting as there are energetically competitive reference states associated with both high and low local spins (S = 5/2 vs S = 1/2) on the Fe3+ ions. Each of these structures allow for spin-electric ground states. We find that qualitative differences in the broadening of the Fe(2s) and O(1s) core levels, shifts in the core-level energies, and the magnetic signatures of the single-spin anisotropy Hamiltonian may be used to confirm whether a transition between a high-spin manifold and a low spin manifold occurs.

Wurtzite (Ga,Mn)As nanowire shells with ferromagnetic properties.

(Ga,Mn)As having a wurtzite crystal structure was coherently grown by molecular beam epitaxy on the {1100} side facets of wurtzite (Ga,In)As nanowires and further encapsulated by (Ga,Al)As and low temperature GaAs. For the first time, a truly long-range ferromagnetic magnetic order is observed in non-planar (Ga,Mn)As, which is attributed to a more effective hole confinement in the shell containing Mn by the proper selection/choice of both the core and outer shell materials.

Trend of the magnetic anisotropy for individual Mn dopants near the (1 1 0) GaAs surface.

Using a microscopic finite-cluster tight-binding model, we investigate the trend of the magnetic anisotropy energy as a function of the cluster size for an individual Mn impurity positioned in the vicinity of the (1 1 0) GaAs surface. We present results of calculations for large cluster sizes containing approximately 10(4) atoms, which have not been investigated so far. Our calculations demonstrate that the anisotropy energy of a Mn dopant in bulk GaAs, found to be non-zero in previous tight-binding calculations, is purely a finite size effect that vanishes with inverse cluster size. In contrast to this, we find that the splitting of the three in-gap Mn acceptor energy levels converges to a finite value in the limit of the infinite cluster size. For a Mn in bulk GaAs this feature is related to the nature of the mean-field treatment of the coupling between the impurity and its nearest neighbor atoms. We also calculate the trend of the anisotropy energy in the sublayers as the Mn dopant is moved away from the surface towards the center of the cluster. Here the use of large cluster sizes allows us to position the impurity in deeper sublayers below the surface, compared to previous calculations. In particular, we show that the anisotropy energy increases up to the fifth sublayer and then decreases as the impurity is moved further away from the surface, approaching its bulk value. The present study provides important insights for experimental control and manipulation of the electronic and magnetic properties of individual Mn dopants at the semiconductor surface by means of advanced scanning tunneling microscopy techniques.

Chern number spins of Mn acceptor magnets in GaAs.

We determine the effective total spin J of local moments formed from acceptor states bound to Mn ions in GaAs by evaluating their magnetic Chern numbers. When individual Mn atoms are close to the sample surface, the total spin changes from J=1 to J=2, due to quenching of the acceptor orbital moment. For Mn pairs in bulk, the total J depends on pair orientation in the GaAs lattice and on the separation between the Mn atoms. We point out that Berry curvature variation as a function of local moment orientation can profoundly influence the quantum-spin dynamics of these magnetic entities.

Theory of tunneling spectroscopy in a Mn12 single-electron transistor by density-functional theory methods.

We consider tunneling transport through a Mn12 molecular magnet using spin density functional theory. A tractable methodology for constructing many-body wave functions from Kohn-Sham orbitals allows for the determination of spin-dependent matrix elements for use in transport calculations. The tunneling conductance at finite bias is characterized by peaks representing transitions between spin multiplets, separated by an energy on the order of the magnetic anisotropy. The energy splitting of the spin multiplets and the spatial part of their many-body wave functions, describing the orbital degrees of freedom of the excess charge, strongly affect the electronic transport, and can lead to negative differential conductance.

Tunneling anisotropic magnetoresistance in Co/AlOx/Au tunnel junctions.

We observe spin-valve-like effects in nanoscaled thermally evaporated Co/AlOx/Au tunnel junctions. The tunneling magnetoresistance is anisotropic and depends on the relative orientation of the magnetization direction of the Co electrode with respect to the current direction. We attribute this effect to a two-step magnetization reversal and an anisotropic density of states resulting from spin-orbit interaction. The results of this study points to future applications of novel spintronics devices involving only one ferromagnetic layer.

Probing spin accumulation in Ni/Au/Ni single-electron transistors with efficient spin injection and detection electrodes.

We have investigated spin accumulation in Ni/Au/Ni single-electron transistors assembled by atomic force microscopy. The fabrication technique is unique in that unconventional hybrid devices can be realized with unprecedented control, including real-time tunable tunnel resistances. A grid of Au disks, 30 nm in diameter and 30 nm thick, is prepared on a SiO2 surface by conventional e-beam writing. Subsequently, 30 nm thick ferromagnetic Ni source, drain, and side-gate electrodes are formed in similar process steps. The width and length of the source and drain electrodes were different to exhibit different coercive switching fields. Tunnel barriers of NiO are realized by sequential Ar and O2 plasma treatment. By use of an atomic force microscope with specially designed software, a single nonmagnetic Au nanodisk is positioned into the 25 nm gap between the source and drain electrodes. The resistance of the device is monitored in real time while the Au disk is manipulated step-by-step with angstrom-level precision. Transport measurements in magnetic field at 1.7 K reveal no clear spin accumulation in the device, which can be attributed to fast spin relaxation in the Au disk. From numerical simulations using the rate-equation approach of orthodox Coulomb blockade theory, we can put an upper bound of a few nanoseconds on the spin-relaxation time for electrons in the Au disk. To confirm the magnetic switching characteristics and spin injection efficiency of the Ni electrodes, we fabricated a test structure consisting of a Ni/NiO/Ni magnetic tunnel junction with asymmetric dimensions of the electrodes similar to those of the single-electron transistors. Magnetoresistance measurements on the test device exhibited clear signs of magnetic reversal and a maximum tunneling magnetoresistance of 10%, from which we deduced a spin polarization of about 22% in the Ni electrodes.

Electron-magnon coupling and nonlinear tunneling transport in magnetic nanoparticles.

We present a theory of single-electron tunneling transport through a ferromagnetic nanoparticle in which particle-hole excitations are coupled to spin collective modes. The model employed to describe the interaction between quasiparticles and collective excitations captures the salient features of a recent microscopic study. Our analysis of nonlinear quantum transport in the regime of weak coupling to the external electrodes is based on a rate-equation formalism for the nonequilibrium occupation probability of the nanoparticle many-body states. For strong electron-boson coupling, we find that the tunneling conductance as a function of bias voltage is characterized by a large and dense set of resonances. Their magnetic field dependence in the large-field regime is linear, with slopes of the same sign. Both features are in agreement with recent tunneling experiments.

Chern numbers for spin models of transition metal nanomagnets.

We argue that ferromagnetic transition metal nanoparticles with fewer than approximately 100 atoms can be described by an effective Hamiltonian with a single giant spin degree of freedom. The total spin S of the effective Hamiltonian is specified by a Berry curvature Chern number that characterizes the topologically nontrivial dependence of a nanoparticle's many-electron wave function on magnetization orientation. The Berry curvatures and associated Chern numbers have a complex dependence on spin-orbit coupling in the nanoparticle and influence the semiclassical Landau-Liftshitz equations that describe magnetization-orientation dynamics.

Theory of tunneling spectroscopy in ferromagnetic nanoparticles.

We present a theory of the low-energy excitations of a ferromagnetic metal nanoparticle. In addition to the particle-hole excitations, which occur in a paramagnetic metal nanoparticle, we predict a branch of excitations involving the magnetization-orientation collective coordinate. Tunneling matrix elements are in general sizable for several different collective states associated with the same band configuration. We point out that the average change in ground state spin per added electron differs from noninteracting quasiparticle expectations, and that the change in the spin polarization, due to Zeeman coupling, is strongly influenced by Coulomb blockade physics.