Deviations from Matthiessen’s rule and resistivity saturation effects in Gd and Fe from first principles

According to earlier first-principles calculations, the spin-disorder contribution to the resistivity of rare-earth metals in the paramagnetic state is strongly underestimated if Matthiessen’s rule is assumed to hold. To understand this discrepancy, the resistivity of paramagnetic Fe and Gd is evaluated by taking into account both spin and phonon disorder. Calculations are performed using the supercell approach within the linear muffin-tin orbital method. Phonon disorder is modeled by introducing random displacements of the atomic nuclei, and the results are compared with the case of fictitious Anderson disorder. In both cases, the resistivity shows a nonlinear dependence on the square of the disorder potential, which is interpreted as a resistivity saturation effect. This effect is much stronger in Gd than in Fe. The nonlinearity makes the phonon and spin-disorder contributions to the resistivity nonadditive, and the standard procedure of extracting the spin-disorder resistivity by extrapolation from high temperatures becomes ambiguous. An “apparent” spin-disorder resistivity obtained through such extrapolation is in much better agreement with experiment compared to the results obtained by considering only spin disorder. By analyzing the spectral function of the paramagnetic Gd in the presence of Anderson disorder, the resistivity saturation is explained by the collapse of a large area of the Fermi surface due to the disorder-induced mixing between the electron and hole sheets

Topology of the three-qubit space of entanglement types

The three-qubit space of entanglement types is the orbit space of the local unitary action on the space of three-qubit pure states, and hence describes the types of entanglement that a system of three qubits can achieve. We show that this orbit space is homeomorphic to a certain subspace of R^6, which we describe completely. We give a topologically based classification of three-qubit entanglement types, and we argue that the nontrivial topology of the three-qubit space of entanglement types forbids the existence of standard states with the convenient properties of two-qubit standard states.Comment: 9 pages, 3 figures, v2 adds a referenc

Ab-initio and model studies of spin fluctuation effects in transport and thermodynamics of magnetic metals

Magnetic materials are vital to many devices and the manipulation of spins is central to the operation of novel devices such as spin transistors. It is important to understand the effect of spin fluctuations on such systems. In this dissertation, first-principles calculations and models further the understanding of spin fluctuation effects in the transport and thermodynamics of magnetic metals. A simple classical spin-fluctuation Hamiltonian with a single itinerancy parameter is studied using the mean-field approximation, Monte Carlo simulations, and a generalized Onsager cavity field method. The results of these different methods are in agreement. It is found that the thermodynamics are sensitive to the choice of phase space measure and that short-range order is weak for all degrees of itinerancy. Spin injection from a half-metallic electrode in the presence of thermal spin disorder is analyzed using a combination of random matrix theory, spin-diffusion theory, and explicit simulations for the tight-binding s-d model. It is shown that spin-flip scattering from the interface destroys spin coherence. Spin injection is possible and is constrained by the mean-free path and spin diffusion length in the semiconductor. The spin-disorder resistivity (SDR) is calculated for the Gd-Tm series in the paramagnetic state using two complimentary first-principles approaches. The SDR in the series follows an almost universal dependence on the exchange splitting and is underestimated when compared with experiment. Frozen atomic displacements (phonons) are then introduced along with spin disorder and the total resistivity is calculated as a function of the mean-square displacement for Fe and Gd. The resistivity increases non-linearly for small displacements and transitions to a linear dependence at larger displacements that, when fitted, enhances the SDR. The enhancement observed in Gd is substantial. The enhancements are electronic in origin, and the rapid increase observed in Gd is traced to a strong, disorder-induced interaction between the electron and hole Fermi surfaces, while the linear trend at large displacements is a saturation effect brought on by strong disorder. Adviser: Kirill D. Belashchenk

First-principles study of spin-disorder resistivity of heavy rare-earth metals: Gd–Tm series

Electrical resistivity of heavy rare-earth metals has a dominant contribution from thermal spin-disorder scattering. Here this spin-disorder resistivity is calculated for the Gd-Tm series of metals in the paramagnetic state. Calculations are performed within the tight-binding linear muffin-tin orbital method using two complementary methods: (1) averaging of the Landauer-Büttiker conductance of a supercell over random noncollinear spin-disorder configurations, and (2) linear response calculations with the spin-disordered state described in the coherent potential approximation. The agreement between these two methods is found to be excellent. The spin-disorder resistivity in the series follows an almost universal dependence on the exchange splitting. While the crystallographic anisotropy of the spin-disorder resistivity agrees well with experiment, its magnitude is significantly underestimated. These results suggest that the classical picture of slowly rotating self-consistent local moments is inadequate for rare-earth metals. A simple quantum correction improves agreement with experiment but does not fully account for the discrepancy, suggesting that more complicated scattering mechanisms may be important

The Predictive Power of Different Projector-Augmented Wave Potentials for Nuclear Quadrupole Resonance

The projector-augmented wave (PAW) method is used to calculate electric field gradients (EFG) for various PAW potentials. A variety of crystals containing reactive nonmetal, simple metal, and transition elements, are evaluated in order to determine the predictive ability of the PAW method for the determination of nuclear quadrupole resonance frequencies in previously unstudied materials and their polymorphs. All results were compared to experimental results and, where possible, to previous density functional theory (DFT) calculations. The EFG at the 14N site of NaNO2 is calculated by DFT for the first time. The reactive nonmetal elements were not very sensitive to the variation in PAW potentials, and calculations were quite close to experimental values. For the other elements, the various PAW potentials led to a clear spread in EFG values, with no one universal potential emerging. Within the spread, there was agreement with other ab initio models

Spin-disorder resistivity of ferromagnetic metals from first principles: The disordered-local-moment approach

The paramagnetic spin-disorder resistivity (SDR) of transition-metal ferromagnets Fe, Co, Ni, ordered transition metal alloys Ni3Mn and Fe3Si as well as Ni2MnX (X = In,Sn,Sb) Heusler alloys is determined from first principles. SDR is evaluated similar to the residual resistivity by using the disordered local moment (DLM) model combined with the Kubo-Greenwood linear response calculation. The electronic structure is determined within the tight-binding linear muffin-tin orbital method and the coherent potential approximation (CPA) applied to the DLM state. We also estimate the temperature dependence of the resistivity below the Curie temperature using a simple model. The results agree well with the supercell Landauer-Buttiker calculations and, generally, with experimental data. For the Ni2MnSb Heusler alloy it is necessary to include substitutional disorder of B2-type to explain the experimental data