Large Noncollinearity and Spin Reorientation in the Novel Mn2RhSn Heusler Magnet

Noncollinear magnets provide essential ingredients for the next generation memory technology. It is a new prospect for the Heusler materials, already well known due to the diverse range of other fundamental characteristics. Here, we present a combined experimental and theoretical study of novel noncollinear tetragonal Mn2RhSn Heusler material exhibiting unusually strong canting of its magnetic sublattices. It undergoes a spin-reorientation transition, induced by a temperature change and suppressed by an external magnetic field. Because of the presence of Dzyaloshinskii-Moriya exchange and magnetic anisotropy, Mn2RhSn is suggested to be a promising candidate for realizing the Skyrmion state in the Heusler family

Finite-temperature magnetism of Fex_xPd1−x_{1-x} and Cox_xPt1−x_{1-x} alloys

The finite-temperature magnetic properties of Fe$_x$Pd$_{1-x}$ and Co$_x$Pt$_{1-x}$ alloys have been investigated. It is shown that the temperature-dependent magnetic behaviour of alloys, composed of originally magnetic and non-magnetic elements, cannot be described properly unless the coupling between magnetic moments at magnetic atoms (Fe,Co) mediated through the interactions with induced magnetic moments of non-magnetic atoms (Pd,Pt) is included. A scheme for the calculation of the Curie temperature ($T_C$) for this type of systems is presented which is based on the extended Heisenberg Hamiltonian with the appropriate exchange parameters $J_{ij}$ obtained from {\em ab-initio} electronic structure calculations. Within the present study the KKR Green's function method has been used to calculate the $J_{ij}$ parameters. A comparison of the obtained Curie temperatures for Fe$_x$Pd$_{1-x}$ and Co$_x$Pt$_{1-x}$ alloys with experimental data shows rather good agreement.Comment: 10 pages, 12 figure

Decoherence produces coherent states: an explicit proof for harmonic chains

We study the behavior of infinite systems of coupled harmonic oscillators as t->infinity, and generalize the Central Limit Theorem (CLT) to show that their reduced Wigner distributions become Gaussian under quite general conditions. This shows that generalized coherent states tend to be produced naturally. A sufficient condition for this to happen is shown to be that the spectral function is analytic and nonlinear. For a rectangular lattice of coupled oscillators, the nonlinearity requirement means that waves must be dispersive, so that localized wave-packets become suppressed. Virtually all harmonic heat-bath models in the literature satisfy this constraint, and we have good reason to believe that coherent states and their generalizations are not merely a useful analytical tool, but that nature is indeed full of them. Standard proofs of the CLT rely heavily on the fact that probability densities are non-negative. Although the CLT generally fails if the probability densities are allowed to take negative values, we show that a CLT does indeed hold for a special class of such functions. We find that, intriguingly, nature has arranged things so that all Wigner functions belong to this class.Comment: Final published version. 17 pages, Plain TeX, no figures. Online at http://astro.berkeley.edu/~max/gaussians.html (faster from the US), from http://www.mpa-garching.mpg.de/~max/gaussians.html (faster from Europe) or from [email protected]

Decoherence of a Pointer by a Gas Reservoir

We study the effect of the environment on the process of the measurement of a state of a microscopic spin half system. The measuring apparatus is a heavy particle, whose center of mass coordinates can be considered at the end of the measurement as approximately classical, and thus can be used as a pointer. The state of the pointer, which is the result of its interaction with the spin, is transformed into a mixed state by the coupling of the pointer to the environment. The environment is considered to be a gas reservoir, whose particles interact with the pointer. This results in a Fokker-Planck equation for the reduced density matrix of the pointer. The solution of the equation shows that the quantum coherences, which are characteristic to the entangled state between the probabilities to find the pointer in one of two positions, decays exponentially fast in time. We calculate the exponential decay function of this decoherence effect, and express it in terms of the parameters of the model.Comment: 41 pages, 1 figur

Magnetization damping in a local-density approximation

The linear response of itinerant transition metal ferromagnets to transverse magnetic fields is studied in a self-consistent adiabatic local-density approximation. The susceptibility is calculated from a microscopic Hamiltonian, including spin-conserving impurities, impurity induced spin-orbit interaction and magnetic impurities using the Keldysh formalism. The Gilbert damping constant in the Landau-Lifshitz-Gilbert equation is identified, parametrized by an effective transverse spin dephasing rate, and is found to be inversely proportional to the exchange splitting. Our result justify the phenomenological treatment of transverse spin dephasing in the study of current-induced magnetization dynamics in weak, itinerant ferromagnets by Tserkovnyak \textit{et al.}. We show that neglect of gradient corrections in the quasiclassical transport equations leads to incorrect results when the exchange potential becomes of the order of the Fermi energy.Comment: 11 pages, 41 references, no figure

Importance of correlation effects in hcp iron revealed by a pressure-induced electronic topological transition

We discover that hcp phases of Fe and Fe0.9Ni0.1 undergo an electronic topological transition at pressures of about 40 GPa. This topological change of the Fermi surface manifests itself through anomalous behavior of the Debye sound velocity, c/a lattice parameter ratio and M\"ossbauer center shift observed in our experiments. First-principles simulations within the dynamic mean field approach demonstrate that the transition is induced by many-electron effects. It is absent in one-electron calculations and represents a clear signature of correlation effects in hcp Fe

Following a "Collapsing" Wavefunction

I study the quantum mechanics of a spin interacting with an ``apparatus''. Although the evolution of the whole system is unitary, the spin evolution is not. The system is chosen so that the spin exhibits loss of quantum coherence, or ``wavefunction collapse'', of the sort usually associated with a quantum measurement. The system is analyzed from the point of view of the spin density matrix (or ``Schmidt paths''), and also using the consistent histories approach. These two points of view are contrasted with each other. Connections between the results and the form of the Hamiltonian are discussed in detail.Comment: 30 pages, plain LaTex, 3 figures in a separate uuencoded fil

Decoherence: Concepts and Examples

We give a pedagogical introduction to the process of decoherence - the irreversible emergence of classical properties through interaction with the environment. After discussing the general concepts, we present the following examples: Localisation of objects, quantum Zeno effect, classicality of fields and charges in QED, and decoherence in gravity theory. We finally emphasise the important interpretational features of decoherence.Comment: 24 pages, LATEX, 9 figures, needs macro lamuphys.sty, to appear in the Proceedings of the 10th Born Symposiu

Magnons in real materials from density-functional theory

We present an implementation of the adiabatic spin-wave dynamics of Niu and Kleinman. This technique allows to decouple the spin and charge excitations of a many-electron system using a generalization of the adiabatic approximation. The only input for the spin-wave equations of motion are the energies and Berry curvatures of many-electron states describing frozen spin spirals. The latter are computed using a newly developed technique based on constrained density-functional theory, within the local spin density approximation and the pseudo-potential plane-wave method. Calculations for iron show an excellent agreement with experiments.Comment: 1 LaTeX file and 1 postscript figur

First-principles scattering matrices for spin-transport

Details are presented of an efficient formalism for calculating transmission and reflection matrices from first principles in layered materials. Within the framework of spin density functional theory and using tight-binding muffin-tin orbitals, scattering matrices are determined by matching the wave-functions at the boundaries between leads which support well-defined scattering states and the scattering region. The calculation scales linearly with the number of principal layers N in the scattering region and as the cube of the number of atoms H in the lateral supercell. For metallic systems for which the required Brillouin zone sampling decreases as H increases, the final scaling goes as H^2*N. In practice, the efficient basis set allows scattering regions for which H^{2}*N ~ 10^6 to be handled. The method is illustrated for Co/Cu multilayers and single interfaces using large lateral supercells (up to 20x20) to model interface disorder. Because the scattering states are explicitly found, ``channel decomposition'' of the interface scattering for clean and disordered interfaces can be performed.Comment: 22 pages, 13 figure