Abstract
Knowledge of magnetic symmetry is vital for exploiting nontrivial surface states of magnetic topological materials. EuIn
2
As
2
is an excellent example, as it is predicted to have collinear ...antiferromagnetic order where the magnetic moment direction determines either a topological-crystalline-insulator phase supporting axion electrodynamics or a higher-order-topological-insulator phase with chiral hinge states. Here, we use neutron diffraction, symmetry analysis, and density functional theory results to demonstrate that EuIn
2
As
2
actually exhibits low-symmetry helical antiferromagnetic order which makes it a stoichiometric magnetic topological-crystalline axion insulator protected by the combination of a 180
∘
rotation and time-reversal symmetries:
$${C}_{2}\times {\mathcal{T}}={2}^{\prime}$$
C
2
×
T
=
2
′
. Surfaces protected by
$${2}^{\prime}$$
2
′
are expected to have an exotic gapless Dirac cone which is unpinned to specific crystal momenta. All other surfaces have gapped Dirac cones and exhibit half-integer quantum anomalous Hall conductivity. We predict that the direction of a modest applied magnetic field of
μ
0
H
≈ 1 to 2 T can tune between gapless and gapped surface states.
Iron pnictides and related materials have been a topic of intense research for understanding the complex interplay between magnetism and superconductivity. Here we report on the magnetic structure of ...SrMn2As2 that crystallizes in a trigonal structure (P3¯m1) and undergoes an antiferromagnetic (AFM) transition at TN=118(2) K. The magnetic susceptibility remains nearly constant at temperatures T TN with H∥c whereas it decreases significantly with H∥ab. This shows that the ordered Mn moments lie in the ab plane instead of aligning along the c-axis as in tetragonal BaMn2As2. Single-crystal neutron diffraction measurements on SrMn2As2 demonstrate that the Mn moments are ordered in a collinear Néel AFM phase with 180∘ AFM alignment between a moment and all nearest neighbor moments in the basal plane and also perpendicular to it. Moreover, quasi-two-dimensional AFM order is manifested in SrMn2As2 as evident from the temperature dependence of the order parameter.
Features of low-dimensional magnetism resulting from a square-net arrangement of Co atoms in trirutile CoTa2O6 are studied in the present work by means of density functional theory and are compared ...with the experimental results of specific heat and neutron diffraction. The small total energy differences between the ferromagnetic (FM) and antiferromagnetic (AFM) configuration of CoTa2O6 shows that competing magnetic ground states exist, with the possibility of transition from FM to AFM phase at low temperature. Our calculation further suggests the semiconducting behavior for CoTa2O6 with a band gap of ∼0.41 eV. The calculated magnetic anisotropy energy is ∼2.5 meV with its easy axis along the 100 (in-plane) direction. Studying the evolution of magnetism in Co1−xMgxTa2O6 (x=0, 0.1, 0.3, 0.5, 0.7, and 1), it is found that the sharp AFM transition exhibited by CoTa2O6 at TN=6.2 K in its heat capacity vanishes with Mg dilution, indicating the obvious effect of weakening the superexchange pathways of Co. The current specific heat study reveals the robust nature of TN for CoTa2O6 in applied magnetic fields. Clear indication of short-range magnetism is obtained from the magnetic entropy, however, diffuse components are absent in neutron diffraction data. At TN, CoTa2O6 enters a long-range ordered magnetic state which can be described using a propagation vector, (14140). Upon Mg dilution at x≥0.1, the long-range ordered magnetism is destroyed. The present results should motivate an investigation of magnetic excitations in this low-dimensional anisotropic magnet.
The perovskite TbFe0.5Cr0.5O3 shows two anomalies in its magnetic susceptibility at TN=257 K and TSR=190 K which are, respectively, the antiferromagnetic and spin-reorientation transition that occur ...in the Fe/Cr sublattice. Magnetic susceptibility of this compound reveals canonical signatures of a Griffiths-like phase: a negative deviation from the ideal Curie-Weiss law and in less-than-unity power-law susceptibility exponents. Neutron-diffraction data analysis confirms two spin-reorientation transitions in this compound. The first one from Γ2 (Cx, Gy, Fz) to Γ4 (Ax, Fy, Gz) occurs at TN=257 K and a second one from Γ4 (Ax, Fy, Gz) to Γ2 (Cx, Gy, Fz) at TSR=190 K in the Pnma space-group setting. The Γ2 (Cx, Gy, Fz) structure is stable down to 7.7 K, leading to an ordered moment of 3.34(1) μB/Fe3+(Cr3+). In addition to the long-range magnetic order, experimental indication of diffuse magnetism is observed in neutron-diffraction data at 7.7 K. Tb develops a ferromagnetic component along the z axis at 20 K. Thermal conductivity and spin-phonon coupling of TbFe0.5Cr0.5O3 studied through Raman spectroscopy are also presented in the paper. The magnetic anomalies at TN and TSR do not appear in the thermal conductivity of TbFe0.5Cr0.5O3, which appears to be robust up to 9 T. On the other hand, they are revealed in the temperature dependence of full-width-at-half-maximum curves derived from Raman intensities. An antiferromagnetic structure with ↑↓↑↓ arrangement of Fe/Cr spins is found as the ground state through first-principles energy calculations, supporting the experimentally determined magnetic structure at 7.7 K. The spin-resolved total and partial density of states show that TbFe0.5Cr0.5O3 is insulating with a band gap of ∼0.12 (2.4) eV within GGA (GGA+U) functionals.
We report results from neutron scattering experiments on single crystals of YbBiPt that demonstrate antiferromagnetic order characterized by a propagation vector, tau sub(AFM) = (1/21/21/2), and ...ordered moments that align along the 1 1 1 direction of the cubic unit cell. We describe the scattering in terms of a two-Gaussian peak fit, which consists of a narrower component that appears below T sub(N) approximately 0.4 K and corresponds to a magnetic correlation length of xi sub(n) approximately 80 A, and a broad component that persists up to T* approximately 0.7 K and corresponds to antiferromagnetic correlations extending over xi sub(b)20 A. Our results illustrate the fragile magnetic order present in YbBiPt and provide a path forward for microscopic investigations of the ground states and fluctuations associated with the purported quantum critical point in this heavy-fermion compound.
The spin fluctuation spectra from nonsuperconducting Cu-substituted, and superconducting Co-substituted, BaFe(2)As(2) are compared quantitatively by inelastic neutron scattering measurements and are ...found to be indistinguishable. Whereas diffraction studies show the appearance of incommensurate spin-density wave order in Co and Ni substituted samples, the magnetic phase diagram for Cu substitution does not display incommensurate order, demonstrating that simple electron counting based on rigid-band concepts is invalid. These results, supported by theoretical calculations, suggest that substitutional impurity effects in the Fe plane play a significant role in controlling magnetism and the appearance of superconductivity, with Cu distinguished by enhanced impurity scattering and split-band behavior.
Marcasite class of compounds provide a facile platform to explore novel phenomena of fundamental and technological importance, such as unconventional superconductivity or high-performance ...electrocatalyst. We report the synthesis and experimental investigation of a marcasite CoSeAs in this paper. Experimental investigation of this material using neutron scattering measurements reveals weak magnetic correlation of cobalt ions below T=36.2 K. The modest isotropic exchange interaction between cobalt moments, inferred from random phase approximation analysis, hints of a magnetically unstable environment. It is a desirable characteristic to induce unconventional superconductivity via chemical pressure application.
We study the rectification of vortex motion in an asymmetric ring channel in a Corbino setup. With an applied ac current, the motion of vortices in the channel is rectified by the asymmetric ...potential and induces a dc net flow. The net flow in such a system strongly depends on vortex density, and we distinguish "single-vortex" rectification regime (for low density, when each vortex is rectified individually) determined by the potential-energy landscape inside each cell of the channel and "multi-vortex", or "collective", rectification (high density case) when the interaction between vortices becomes important.
The influence of lignin, lignin model compounds, and black liquor from the kraft pulping process on the hydrolysis of xylan by xylanase was investigated. Addition of vanillic acid, acetovanillone, ...and protocatechuic acid increased the rate of hydrolysis of xylan by as much as 18–50% at low concentrations, but reached maxima at about 0.05% concentration. Addition of vanillin caused a 15% improvement in xylan hydrolysis, while addition of guaiacol more than doubled the hydrolysis rate. Increasing concentrations of either lignin or black liquor also increased the hydrolysis rate of xylan. Circular dichroism spectroscopy indicated a change in the structure of xylanase in the presence of black liquor.
PDF
