Theoretical Physics

Abstract: The ferrimagnet
Mn
3
Si
2
Te
6
attracts attention because of a recently discovered colossal magnetoresistance (CMR) with unique magnetic field properties. An improved magnetic structure for the material has emerged from a neutron diffraction study linked to understanding the CMR. A deeper theoretical investigation of the magnetic structure has now revealed anapole, chiral, and orbital states of manganese ions not previously mentioned. Moreover, it is shown that existence of these states in the low temperature form of
Mn
3
Si
2
Te
6
, with a magnetic field applied, can be tested by neutron and resonant xray diffraction.

Jun 2023


Theoretical Physics

Open Access
Abstract: Scrutiny of an established monoclinic magnetic space group for
NdFeO
3
reveals hitherto unknown properties of the orthoferrite. Future experiments using neutron and xray diffraction techniques can verify them. Neodymium ions possess Dirac multipoles, both time odd (magnetic) and parity odd (polar), that come with unique diffraction conditions. Nonmagnetic polar Nd multipoles are permitted even though the monoclinic space group is centrosymmetric. Dirac multipoles are forbidden by symmetry at sites occupied by ferric ions. Available diffraction patterns have not been analyzed for Dirac multipoles, nor all permitted components of the axial dipoles and quadrupoles. In the case of neutron diffraction, magnetic quadrupoles are correlations between anapole and orbital degrees of freedom. We give conditions for the observation of TempletonTempleton scattering of x rays, created by angular anisotropy in the electronic charge distribution. Axial multipoles are the sole providers of dichroic signals.

Jun 2023


Theoretical Physics

Open Access
Abstract: The Landau free energy of a compound that benefits from a linear coupling of an electric field and a magnetic field includes a product of the two fields, one polar and time even and one axial and time odd. Evidently, the coefficient of the product of fields is unchanged by a simultaneous change in the directions of space and time, a symmetry operation labeled antiinversion. Invariance with respect to antiinversion is the defining symmetry of the linear magnetoelectric (ME) effect included in 58 of 122 magnetic crystal classes, 19 of which prohibit higherorder (nonlinear) contributions to the free energy. In ME compounds, expectation values of some atomic magnetic tensors are invariant with respect to antiinversion. An invariance shared by the Dirac monopole (an element of charge allowed in Maxwell's equations that has not been observed) and a Zel'dovich anapole, is also known as a Dirac dipole. From the science of materials perspective, it has been established that Dirac multipoles contribute to the diffraction of x rays and neutrons. We identify Dirac monopoles in bulk magnetic properties of iron tellurate (
Fe
2
Te
O
6
) and a spin ladder (
Sr
Fe
2
S
2
O
). They are visible in the diffraction of light using an iron electric dipole–magnetic dipole absorption event. Both cited compounds present a simple antiferromagnetic configuration of axial dipoles, and their different magnetic crystal classes allow a linear ME effect. However, the Kerr effect is symmetry allowed in the spin ladder and forbidden in iron tellurate. Anapoles are forbidden in iron tellurate and allowed in the spin ladder compound, a difference evident in diffraction patterns fully informed by symmetry. More generally, we identify a raft of Dirac multipoles, and axial multipoles beyond dipoles, visible in future experiments using standard techniques with beams of neutrons or x rays tuned in energy to an iron atomic resonance. ME invariance imposes a phase relationship between nuclear (charge) and magnetic contributions to neutron (xray) diffraction amplitudes. In consequence, intensities of Bragg spots in an xray pattern do not change when helicity in the primary beam is reversed. A like effect occurs in the magnetic diffraction of polarized neutrons.

Apr 2023


Theoretical Physics

Abstract: Circular dichroism in resonant elastic xray scattering (CDREXS) has recently been observed in chiral
structures composed of multiferroic materials as well as magnetic moments or electric polarization vectors. In order to comprehensively understand the experimental results of these previous studies, we present here in detail the analytical formulation of CDREXS for onedimensional helices composed of magnetic moments and electric polarization vectors, respectively. In particular, by comparing CDREXS for a properscrewshaped Blochtype helix and cycloidshaped Néeltype helix, we found that CDREXS for both magnetic and polar helices can discriminate between both types of helices.We also found that the xray polarization factor depending on the scattering geometry is a significant factor in determining the characteristics of CDREXS for chiral structures. The results obtained from the detailed formulas can be intuitively understood using the concept of mirror reflection. In particular, in this way it can be understood that Bloch and Néeltype helices correspond to truly chiral and achiral structures, respectively, and why CDREXS is able to distinguish between these two types of helices.

Jul 2022


Theoretical Physics

Open Access
Abstract: The nature and origin of electronic nematicity remains a significant challenge in our understanding of the ironbased superconductors. This is particularly evident in the iron chalcogenide, FeSe, where it is currently unclear how the experimentally determined Fermi surface near the M point evolves from having two electron pockets in the tetragonal state, to exhibiting just a single electron pocket in the nematic state. This has posed a major theoretical challenge, which has become known as the missing electron pocket problem of FeSe, and is of central importance if we wish to uncover the secrets behind nematicity and superconductivity in the wider ironbased superconductors. Here, we review the recent experimental work uncovering this nematic Fermi surface of FeSe from both ARPES and STM measurements, as well as current theoretical attempts to explain this missing electron pocket of FeSe, with a particular focus on the emerging importance of incorporating the dxy orbital into theoretical descriptions of the nematic state. Furthermore, we will discuss the consequence this missing electron pocket has on the theoretical understanding of superconductivity in this system and present several remaining open questions and avenues for future research.

May 2022


Theoretical Physics

Abstract: Rutiletype RuO2 likely supports a simple antiferromagnetic structure which can be verified by xray Bragg
diffraction. Three magnetic motifs that do not break translation symmetry are explored in calculations of amplitudes suitable for diffraction enhanced by tuning the primary xray energy to a ruthenium atomic resonance. Coupling to xray helicity through a chargemagnetic interference is common to all motifs, together with magnetic and charge intensities in quadrature in the rotated channel of polarization. Necessary conditions for these diffraction phenomena are a centrosymmetric crystal structure, null magnetic propagation vector, and absence of a linear magnetoelectric effect. Published xray diffraction data for RuO2 were analyzed by the authors against a
magnetic motif that does not satisfy the conditions. A polarized neutron study of antiferromagnetic domains can be achieved with a sample that meets the stated crystal and magnetic symmetries.

Jan 2022


Theoretical Physics

Open Access
Abstract: A threefold symmetric kagome lattice that has negative spin chirality can give a nonzero xray magnetic circular dichroism (XMCD) signal, despite the total spin moment amounting to zero. In order to explain this, I present here a rule for the rotational symmetry invariance of the XMCD signal. A necessary condition is the existence of an anisotropic XMCD signal for the local magnetic atom, which can arise from a spin anisotropy either in the ground state or the final state. The angular dependence of the XMCD as a function of the beam direction has an unusual behavior. The maximum dichroism is not aligned along the spin direction, but depends on the relative orientation of the spin with respect to the atomic orientation. Therefore, different geometries can result in the same angular dependence, and the spin direction can only be determined if the atomic orientation is known. The consequences for the xray magnetooptical sum rules are given. The integrated XMCD signals are proportional to the anisotropy in the orbital moment and the magnetic dipole term, where the isotropic spin moment drops out.

Sep 2021


Theoretical Physics

Frank M. F.
De Groot
,
Hebatalla
Elnaggar
,
Federica
Frati
,
RuPan
Wang
,
Mario U.
DelgadoJaime
,
Michel
Van Veenendaal
,
Javier
FernandezRodriguez
,
Maurits W.
Haverkort
,
Robert J.
Green
,
Gerrit
Van Der Laan
,
Yaroslav
Kvashnin
,
Atsushi
Hariki
,
Hidekazu
Ikeno
,
Harry
Ramanantoanina
,
Claude
Daul
,
Bernard
Delley
,
Michael
Odelius
,
Marcus
Lundberg
,
Oliver
Kuhn
,
Sergey I.
Bokarev
,
Eric
Shirley
,
John
Vinson
,
Keith
Gilmore
,
Mauro
Stener
,
Giovanna
Fronzoni
,
Piero
Decleva
,
Peter
Kruger
,
Marius
Retegan
,
Yves
Joly
,
Christian
Vorwerk
,
Claudia
Draxl
,
John
Rehr
,
Arata
Tanaka
Open Access
Abstract: This review provides an overview of the different methods and computer codes that are used to interpret 2p xray absorption spectra of 3d transition metal ions. We first introduce the basic parameters and give an overview of the methods used. We start with the semiempirical multiplet codes and compare the different codes that are available. A special chapter is devoted to the user friendly interfaces that have been written on the basis of these codes. Next we discuss the first principle codes based on band structure, including a chapter on Density Functional theory based approaches. We also give an overview of the firstprinciple multiplet codes that start from a cluster calculation and we discuss the wavefunction based methods, including multireference methods. We end the review with a discussion of the link between theory and experiment and discuss the open issues in the spectral analysis.

Apr 2021


Theoretical Physics

Abstract: Nonlinear optics, and particularly second harmonic generation (SHG), is increasingly used in many modern
disciplines from material characterization in physical sciences to bioimaging in medicine and optical signal processing in information technology. We present a theoretical analysis yielding a strong estimate of the energy integrated SHG response. Compact spherical multipoles are provided for the corresponding natural and magnetic circular dichroic signals. Like symmetry requirements in time and space are traced in the amplitude for magnetic neutron scattering, which includes all axial and polar (Dirac) contributions. Our method of working in terms of,now standard, electronic multipoles and Racah algebra, with full implementation of discrete symmetries, could be of use in a variety of other probes of matter.

Mar 2021


Theoretical Physics

Abstract: A recent polarized neutron diffraction experiment on the 5d2 rhenium double perovskite Ba2YReO6 held at a low temperature uncovered weak magnetic diffraction peaks. Data analysis inferred a significantly reduced Re dipole moment, and longrange order compatible with an antiferromagnetic, noncollinear motif. To interpret the experimental findings, we present a model wave function for Re ions derived from the crystal field potential, Coulomb interaction, and spinorbit coupling that fully respects the symmetry of the low temperature ordered state. It is used to calculate in analytic form all multipole moments visible in neutron and resonance enhanced xray diffraction. A minimal model consistent with available neutron diffraction data predicts significant multipolar moments up to the hexadecapole and, in particular, a dominant charge like quadrupole moment. Calculated diffraction patterns embrace single crystal xray diffraction at the Re L edge, and renewed neutron diffraction, to probe the presumed underlying multipolar order.

Mar 2021

