Thomas C. Schulthess

Electronic structure, exchange interactions, and Curie temperature of FeCo

Fe–Co alloys in the α phase are soft magnetic materials which have high saturation inductions over a wide range of compositions. However, above about 1250 K, an α to γ phase transition occurs. The fcc-based, γ, high-temperature phase is paramagnetic at this temperature. In this work the low-temperature ordered B2, or α′, phase, as well as the disordered bcc phase of FeCo alloys, have been studied with first-principles electronic-structure calculations using the layer Korringa–Kohn–Rostoker method. The variation of moment with composition (Slater–Pauling curve) is discussed. For equiatomic FeCo, interatomic exchange couplings are derived from first principles. These exchange interactions are compared to those obtained for pure Fe and Co, and are used within a mean-field theory to estimate the hypothetical Curie temperature of the α phase.

COUPLING MECHANISMS IN EXCHANGE BIASED FILMS (INVITED)

We use an atomistic Heisenberg model in conjunction with the classical Landau Lifshitz equation for the spin motion to study coupling mechanisms between ferromagnetic (FM) and antiferromagnetic (AFM) films. Calculations for CoO/FM illustrate that there are two coupling mechanisms at work, the spin–flop coupling and an AFM–FM coupling through uncompensated defects. While the latter accounts for exchange bias and related phenomena, the former gives rise to a large coercivity and perpendicular alignment between FM spins and AFM easy axis. A combination of the two mechanisms explains apparent discrepancies between reversible and irreversible measurements of the AFM–FM coupling.

Wang-landau algorithm for continuous models and joint density of states

We present a modified Wang-Landau algorithm for models with continuous degrees of freedom. We demonstrate this algorithm with the calculation of the joint density of states of ferromagnet Heisenberg models and a model polymer chain. The joint density of states contains more information than the density of states of a single variable-energy, but is also much more time consuming to calculate. We present strategies to significantly speed up this calculation for large systems over a large range of energy and order parameter.

Systematic Study of d-Wave Superconductivity in the 2D Repulsive Hubbard Model

The cluster size dependence of superconductivity in the conventional two-dimensional Hubbard model, commonly believed to describe high-temperature superconductors, is systematically studied using the dynamical cluster approximation and quantum Monte Carlo simulations as a cluster solver. Because of the nonlocality of the d-wave superconducting order parameter, the results on small clusters show large size and geometry effects. In large enough clusters, the results are independent of the cluster size and display a finite temperature instability to d-wave superconductivity.

The effect of Ta on the magnetic thickness of permalloy (Ni81Fe19) films

The effect of Ta and Ta/Cu seed layers, and Ta and Cu cap layers on the effective magnetic thickness of ultrathin permalloy (Ni81Fe19) was investigated for MRAM applications. The films were deposited by Ion Beam Deposition. The magnetic moment of each as-deposited permalloy film was measured using a B-H looper and a SQUID magnetometer. The films were further annealed at either 525 K for 1/2 h or 600 K for 1 h to study the effect of thermally driven interdiffusion on the magnetic moment of the permalloy film. Our theoretical calculations showed that the presence of 12% intermixing at the interface reduces the Ni moments to zero. Experimentally, it was shown that the tantalum rather than the copper interfaces are primarily responsible for the magnetically dead layers. The Ta seed layer interface produces a loss of moment equivalent to a magnetically dead layer of thickness 0.6±0.2 nm. The Ta metal in the cap layer results in a loss of moment equivalent to a dead layer of thickness 1.0±0.2 nm. Upon annealing...

Toward an atomistic model for predicting transcription-factor binding sites.

Identifying the specific DNA‐binding sites of transcription‐factor proteins is essential to understanding the regulation of gene expression in the cell. Bioinformatics approaches are fast compared to experiments, but require prior knowledge of multiple binding sites for each protein. Here, we present an atomistic force‐field method to predict binding sites based only on the X‐ray structure of a related bound complex. Specific flexible contacts between the protein and DNA are modeled by a library of amino acid side‐chain rotamers. Using the example of the mouse transcription factor, Zif268, a well‐studied zinc‐finger protein, we show that the protein sequence alone, without the detailed experimental structure, gives a strong bias toward the consensus binding site. Proteins 2004.

Mott transition of MnO under pressure : A comparison of correlated band theories

The electronic structure, magnetic moment, and volume collapse of MnO under pressure are obtained from four different correlated band theory methods; local density approximation+ Hubbard U LDA+U, pseudopotential self-interaction correction pseudo-SIC, the hybrid functional combined local exchange plus Hartree-Fock exchange, and the local spin density SIC SIC-LSD method. Each method treats correlation among the five Mn 3d orbitals per spin, including their hybridization with three O 2p orbitals in the valence bands and their changes with pressure. The focus is on comparison of the methods for rocksalt MnO neglecting the observed transition to the NiAs structure in the 90– 100 GPa range. Each method predicts a first-order volume collapse, but with variation in the predicted volume and critical pressure. Accompanying the volume collapse is a moment collapse, which for all methods is from high-spin to low-spin 5 → 1 , not to nonmagnetic as the simplest scenario would have. The specific manner in which the transition occurs varies considerably among the methods: pseudo-SIC and SIC-LSD give insulator-to-metal, while LDA+U gives insulator-toinsulator and the hybrid method gives an insulator-to-semimetal transition. Projected densities of states above and below the transition are presented for each of the methods and used to analyze the character of each transition. In some cases the rhombohedral symmetry of the antiferromagnetically ordered phase clearly influences the character of the transition.

Electronic structure and magnetic interactions in Mn doped semiconductors

The electronic structures of several magnetic semiconductors were calculated using first-principles based electronic structure techniques. Calculations were performed both for periodic supercells and for disordered systems which were treated within the coherent potential approximation. Our results differ from the conventional atomic model for the Mn impurity. We find strong hybridization between the majority Mn d states and the majority valence band. We find that substitutional Mn impurities add states and electrons to the majority valence band but not to the minority. We find relatively strong carrier induced magnetic interactions between impurities. The sign of these interactions may however, change sign as a function of the distance between the impurities.

Accuracy and performance of graphics processors: A Quantum Monte Carlo application case study

The tradeoffs of accuracy and performance are as yet an unsolved problem when dealing with Graphics Processing Units (GPUs) as a general-purpose computation device. Their high performance and low cost makes them a desirable target for scientific computation, and new language efforts help address the programming challenges of data parallel algorithms and memory management. But the original task of GPUs - real-time rendering - has traditionally kept accuracy as a secondary goal, and sacrifices have sometimes been made as a result. In fact, the widely deployed hardware is generally capable of only single precision arithmetic, and even this accuracy is not necessarily equivalent to that of a commodity CPU. In this paper, we investigate the accuracy and performance characteristics of GPUs, including results from a preproduction double precision-capable GPU. We then accelerate the full Quantum Monte Carlo simulation code DCA++, similarly investigating its tolerance to the precision of arithmetic delivered by GPUs. The results show that while DCA++ has some sensitivity to the arithmetic precision, the single-precision GPU results were comparable to single-precision CPU results. Acceleration of the code on a fully GPU-enabled cluster showed that any remaining inaccuracy in GPU precision was negligible; sufficient accuracy was retained for scientifically meaningful results while still showing significant speedups.

NONCOLLINEAR MAGNETISM IN SUBSTITUTIONALLY DISORDERED FACE-CENTERED-CUBIC FEMN

We use first principles electronic structure techniques to study the magnetic structure of γ-FeMn using the Korringa–Kohn–Rostocker multiple-scattering approach in conjunction with an extension of the single site coherent potential approximation to noncollinear magnetic structures. Our results show that the noncollinear 3Q and 2Q structures are both stable solutions with the former being slightly lower in energy. The collinear solutions could only be converged in a traditional spin-polarized calculation and are unstable in a noncollinear treatment.