Calculating electron momentum densities and Compton profiles using the linear tetrahedron method

Anti-symmetric Compton scattering in LiNiPO 4: Towards a direct probe of the magneto-electric multipole moment

BACKGROUND

Magnetoelectric multipoles, which break both space-inversion and time-reversal symmetries, play an important role in the magnetoelectric response of a material. Motivated by uncovering the underlying fundamental physics of the magnetoelectric multipoles and the possible technological applications of magnetoelectric materials, understanding as well as detecting such magnetoelectric multipoles has become an active area of research in condensed matter physics. Here we employ the well-established Compton scattering effect as a possible probe for the magnetoelectric toroidal moments in LiNiPO 4.

METHODS

We employ combined theoretical and experimental techniques to compute as well as detect the antisymmetric Compton profile in LiNiPO 4. For the theoretical investigation we use density functional theory to compute the anti-symmetric part of the Compton profile for the magnetic and structural ground state of LiNiPO 4. For the experimental verification, we measure the Compton signals for a single magnetoelectric domain sample of LiNiPO 4, and then again for the same sample with its magnetoelectric domain reversed. We then take the difference between these two measured signals to extract the antisymmetric Compton profile in LiNiPO 4.

RESULTS

Our theoretical calculations indicate an antisymmetric Compton profile in the direction of the t y toroidal moment in momentum space, with the computed antisymmetric profile around four orders of magnitude smaller than the total profile. The difference signal that we measure is consistent with the computed profile, but of the same order of magnitude as the statistical errors and systematic uncertainties of the experiment.

CONCLUSIONS

While the weak difference signal in the measurements prevents an unambiguous determination of the antisymmetric Compton profile in LiNiPO 4, our results motivate  further theoretical work to understand the factors that influence the size of the antisymmetric Compton profile, and to identify materials exhibiting larger effects.

Hidden k-Space Magnetoelectric Multipoles in Nonmagnetic Ferroelectrics

In condensed matter systems, the electronic degrees of freedom are often entangled to form complex composites, known as hidden orders, which give rise to unusual properties, while escaping detection in conventional experiments. Here we demonstrate the existence of hidden k-space magnetoelectric multipoles in nonmagnetic systems with broken space-inversion symmetry. These k-space magnetoelectric multipoles are reciprocal to the real-space charge dipoles associated with the broken inversion symmetry. Using the prototypical ferroelectric PbTiO_{3} as an example, we show that their origin is a spin asymmetry in momentum space resulting from the broken space inversion symmetry associated with the ferroelectric polarization. In PbTiO_{3}, the k-space spin asymmetry corresponds to a pure k-space magnetoelectric toroidal moment, which can be detected using magnetic Compton scattering, an established tool for probing magnetism in ferromagnets or ferrimagnets with a net spin polarization, which has not been exploited to date for nonmagnetic systems. In particular, the k-space magnetoelectric toroidal moment combined with the spin-orbit interaction manifest in an antisymmetric magnetic Compton profile that can be reversed using an electric field. Our work suggests an experimental route to directly measuring and tuning hidden k-space magnetoelectric multipoles via specially designed magnetic Compton scattering measurements.

Extreme Fermi Surface Smearing in a Maximally Disordered Concentrated Solid Solution

We show that the Fermi surface can survive the presence of extreme compositional disorder in the equiatomic alloy Ni_{0.25}Fe_{0.25}Co_{0.25}Cr_{0.25}. Our high-resolution Compton scattering experiments reveal a Fermi surface which is smeared across a significant fraction of the Brillouin zone (up to 40% of 2π/a). The extent of this smearing and its variation on and between different sheets of the Fermi surface have been determined, and estimates of the electron mean free path and residual resistivity have been made by connecting this smearing with the coherence length of the quasiparticle states.

Electron Localization Function and Compton Profiles of Cu2O

The nature of bonding in the cubic cuprous oxide is studied by means of the theoretical tools, namely, the electron localization function and Compton profiles. The isotropic Compton profiles together with the anisotropies in the directional Compton profiles are presented. Taking free-atom Compton profiles, the charge-transfer model is also applied. The first-principles calculations based on the GGA are performed, and the self-interaction correction is incorporated, adopting the GGA+U approach. Both types of calculations are performed deploying the linearized augmented plane-wave (LAPW) method. The effect of self-interaction correction on the electron localization function, Compton profiles, and anisotropies is discussed. The electron localization function reveals ionic behavior in the (110) plane and covalent nature in the Cu-O bond intersecting plane. The GGA+U exhibits more covalent nature. The two LAPW calculations of the Compton profiles show better agreement with the available experimental data than the free-atom profiles. Among all of the calculations undertaken, the GGA+U shows the best agreement with the experiment. The GGA+U calculation shows more anisotropic behavior in directional Compton profiles.

Vacancies, disorder-induced smearing of the electronic structure, and its implications for the superconductivity of anti-perovskite MgC0.93Ni2.85

The anti-perovskite superconductor MgC_0.93Ni_2.85 was studied using high-resolution x-ray Compton scattering combined with electronic structure calculations. Compton scattering measurements were used to determine experimentally a Fermi surface that showed good agreement with that of our supercell calculations, establishing the presence of the predicted hole and electron Fermi surface sheets. Our calculations indicate that the Fermi surface is smeared by the disorder due to the presence of vacancies on the C and Ni sites, but does not drastically change shape. The 20% reduction in the Fermi level density-of-states would lead to a significant (~70%) suppression of the superconducting T_c for pair-forming electron-phonon coupling. However, we ascribe the observed much smaller T_c reduction at our composition (compared to the stoichiometric compound) to the suppression of pair-breaking spin fluctuations.

Spin-Resolved Fermi Surface of the Localized Ferromagnetic Heusler Compound Cu₂MnAl Measured with Spin-Polarized Positron Annihilation

We determined the bulk electronic structure of the prototypical Heusler compound Cu(2)MnAl by measuring the angular correlation of annihilation radiation using spin-polarized positrons. To this end, a new algorithm for reconstructing 3D densities from projections is introduced that allows us to corroborate the excellent agreement between our electronic structure calculations and the experimental data. The contribution of each individual Fermi surface sheet to the magnetization was identified, and summed to a total spin magnetic moment of 3.6±0.5 μ(B)/f.u..

Magnetic frustration, short-range correlations and the role of the paramagnetic Fermi surface of PdCrO2

Frustrated interactions exist throughout nature, with examples ranging from protein folding through to frustrated magnetic interactions. Whilst magnetic frustration is observed in numerous electrically insulating systems, in metals it is a rare phenomenon. The interplay of itinerant conduction electrons mediating interactions between localised magnetic moments with strong spin-orbit coupling is likely fundamental to these systems. Therefore, knowledge of the precise shape and topology of the Fermi surface is important in any explanation of the magnetic behaviour. PdCrO2, a frustrated metallic magnet, offers the opportunity to examine the relationship between magnetic frustration, short-range magnetic order and Fermi surface topology. By mapping the short-range order in reciprocal space and experimentally determining the electronic structure, we have identified the dual role played by the Cr electrons in which the itinerant ones on the nested paramagnetic Fermi surface mediate the frustrated magnetic interactions between local moments.

Proposed parameter-free model for interpreting the measured positron annihilation spectra of materials using a generalized gradient approximation

Positron annihilation spectroscopy is often used to analyze the local electronic structure of materials of technological interest. Reliable theoretical tools are crucial to interpret the measured spectra. Here, we propose a parameter-free gradient correction scheme for a local-density approximation obtained from high-quality quantum Monte Carlo data. The results of our calculations compare favorably with positron affinity and lifetime measurements, opening new avenues for highly precise and advanced positron characterization of materials.