Conductance calculations for quantum wires and interfaces: mode matching and Green functions

Landauer's formula relates the conductance of a quantum wire or interface to transmission probabilities. Total transmission probabilities are frequently calculated using Green function techniques and an expression first derived by Caroli. Alternatively, partial transmission probabilities can be calculated from the scattering wave functions that are obtained by matching the wave functions in the scattering region to the Bloch modes of ideal bulk leads. An elegant technique for doing this, formulated originally by Ando, is here generalized to any Hamiltonian that can be represented in tight-binding form. A more compact expression for the transmission matrix elements is derived and it is shown how all the Green function results can be derived from the mode matching technique. We illustrate this for a simple model which can be studied analytically, and for an Fe|vacuum|Fe tunnel junction which we study using first-principles calculations.Comment: 14 pages, 5 figure

Detection of the spin character of Fe(001) surface states by scanning tunneling microscopy: A theoretical proposal

We consider the magnetic structure on the Fe(001) surface and theoretically study the scanning tunneling spectroscopy using a spin-polarized tip (SP-STM). We show that minority-spin surface states induce a strong bias dependence of the tunneling differential conductance which largely depends on the orientation of the magnetization in the SP-STM tip relative to the easy magnetization axis in the Fe(001) surface. We propose to use this effect in order to determine the spin character of the Fe(001) surface states. This technique can be applied also to other magnetic surfaces in which surface states are observed.Comment: 5 pages, 4 figure

Reversal of spin polarization in Fe/GaAs (001) driven by resonant surface states: First-principles calculations

A minority-spin resonant state at the Fe/GaAs(001) interface is predicted to reverse the spin polarization with voltage bias of electrons transmitted across this interface. Using a Green's function approach within the local spin density approximation we calculate spin-dependent current in a Fe/GaAs/Cu tunnel junction as a function of applied bias voltage. We find a change in sign of the spin polarization of tunneling electrons with bias voltage due to the interface minority-spin resonance. This result explains recent experimental data on spin injection in Fe/GaAs contacts and on tunneling magnetoresistance in Fe/GaAs/Fe magnetic tunnel junctions

First-principles calculations of magnetization relaxation in pure Fe, Co, and Ni with frozen thermal lattice disorder

The effect of the electron-phonon interaction on magnetization relaxation is studied within the framework of first-principles scattering theory for Fe, Co, and Ni by displacing atoms in the scattering region randomly with a thermal distribution. This "frozen thermal lattice disorder" approach reproduces the non-monotonic damping behaviour observed in ferromagnetic resonance measurements and yields reasonable quantitative agreement between calculated and experimental values. It can be readily applied to alloys and easily extended by determining the atomic displacements from ab initio phonon spectra

Writing and Reading antiferromagnetic Mn2_2Au: N\'eel spin-orbit torques and large anisotropic magnetoresistance

Antiferromagnets are magnetically ordered materials which exhibit no net moment and thus are insensitive to magnetic fields. Antiferromagnetic spintronics aims to take advantage of this insensitivity for enhanced stability, while at the same time active manipulation up to the natural THz dynamic speeds of antiferromagnets is possible, thus combining exceptional storage density and ultra-fast switching. However, the active manipulation and read-out of the N\'eel vector (staggered moment) orientation is challenging. Recent predictions have opened up a path based on a new spin-orbit torque, which couples directly to the N\'eel order parameter. This N\'eel spin-orbit torque was first experimentally demonstrated in a pioneering work using semimetallic CuMnAs. Here we demonstrate for Mn$_2$Au, a good conductor with a high ordering temperature suitable for applications, reliable and reproducible switching using current pulses and readout by magnetoresistance measurements. The symmetry of the torques agrees with theoretical predictions and a large read-out magnetoresistance effect of more than $\simeq 6$~$\%$ is reproduced by ab initio transport calculations.Comment: 5 pages, 4 figure

Enabling Factor Analysis on Thousand-Subject Neuroimaging Datasets

The scale of functional magnetic resonance image data is rapidly increasing as large multi-subject datasets are becoming widely available and high-resolution scanners are adopted. The inherent low-dimensionality of the information in this data has led neuroscientists to consider factor analysis methods to extract and analyze the underlying brain activity. In this work, we consider two recent multi-subject factor analysis methods: the Shared Response Model and Hierarchical Topographic Factor Analysis. We perform analytical, algorithmic, and code optimization to enable multi-node parallel implementations to scale. Single-node improvements result in 99x and 1812x speedups on these two methods, and enables the processing of larger datasets. Our distributed implementations show strong scaling of 3.3x and 5.5x respectively with 20 nodes on real datasets. We also demonstrate weak scaling on a synthetic dataset with 1024 subjects, on up to 1024 nodes and 32,768 cores

Cotunneling thermopower of single electron transistors

We study the thermopower of a quantum dot weakly coupled to two reservoirs by tunnel junctions. At low temperatures the transport through the dot is suppressed by charging effects (Coulomb blockade). As a result the thermopower shows an oscillatory dependence on the gate voltage. We study this dependence in the limit of low temperatures where the transport through the dot is dominated by the processes of inelastic cotunneling. We also obtain a crossover formula for intermediate temperatures which connects our cotunneling results to the known sawtooth behavior in the sequential tunneling regime. As the temperature is lowered, the amplitude of thermopower oscillations increases, and their shape changes qualitatively.Comment: 9 pages, including 4 figure

Compositional disorder and its influence on the structural, electronic and magnetic properties of MgC(Ni_{1-x}Co_{x})_{3} alloys using first-principles

First-principles, density-functional based electronic structure calculations are carried out for MgC(Ni_{1-x}Co_{x})_{3} alloys over the concentration range 0\leq x\leq1, using Korringa-Kohn-Rostoker coherent-potential approximation (KKR CPA) method in the atomic sphere approximation (ASA). The self-consistent calculations are used to study the changes as a function of x in the equation of state parameters, total and partial densities of states, magnetic moment and the on-site exchange interaction parameter. To study the magnetic properties as well as its volume dependence, fixed-spin moment calculations in conjunction with the phenomenological Landau theory are employed. The salient features that emerge from these calculations are (i) a concentration independent variation in the lattice parameter and bulk modulus at x~0.75 with an anomaly in the variation of the pressure derivative of bulk modulus, (ii) the fixed-spin moment based corrections to the overestimated magnetic ground state for 0.0\leq x\leq0.3 alloys, making the results consistent with the experiments, and (iii) the possibility of multiple magnetic states at x~0.75, which, however, requires further improvements in the calculations