Magnetic excitations and magnetic structure of EuRbFe4As4 were investigated by inelastic neutron scattering (INS), neutron diffraction, and random phase approximation (RPA) calculations. Below the ...superconducting transition temperature Tc=36.5 K, the INS spectra exhibit the neutron spin resonances at Qres=1.27(2) and 1.79(3)Å−1. They correspond to the Q=(0.5,0.5,1) and (0.5,0.5,3) nesting wave vectors, showing three-dimensional nature of the band structure. The characteristic energy of the neutron spin resonance is Eres=17.7(3) meV corresponding to 5.7(1)kBTc. Observation of the neutron spin resonance mode and our RPA calculations in conjunction with the recent optical conductivity measurements are indicative of the s± superconducting pairing symmetry in EuRbFe4As4. In addition to the neutron spin resonance mode, upon decreasing temperature below the magnetic transition temperature TN=15 K, the spin wave excitation originating in the long-range magnetic order of the Eu sublattice was observed in the low-energy inelastic channel. Single-crystal neutron diffraction measurements demonstrate that the magnetic propagation vector of the Eu sublattice is k=(0,0,0.25), representing the three-dimensional antiferromagnetic order. Linear spin wave calculations assuming the obtained magnetic structure with the intra- and interplane nearest neighbor exchange couplings of J1/kB=−1.31 K and Jc/kB=0.08 K can reproduce quantitatively the observed spin wave excitation. Our results show that superconductivity and long-range magnetic order of Eu coexist in EuRbFe4As4, whereas the coupling between them is rather weak.
We study the correlated quantum magnet YbCl3 with neutron scattering, magnetic susceptibility, and heat capacity measurements. The crystal field Hamiltonian is determined through simultaneous ...refinements of the inelastic neutron scattering and magnetization data. The ground-state doublet is well isolated from the other crystal field levels and results in an effective spin-1/2 system with local easy plane anisotropy at low temperature. Cold neutron spectroscopy shows low-energy excitations peaked at 0.5 meV that are consistent with nearest-neighbor antiferromagnetic correlations.
Entanglement of spin and orbital degrees of freedom drives the formation of novel quantum and topological physical states. Here we report resonant inelastic x-ray scattering measurements of the ...transition metal oxides Ca_{3}LiOsO_{6} and Ba_{2}YOsO_{6}, which reveals a dramatic spitting of the t_{2g} manifold. We invoke an intermediate coupling approach that incorporates both spin-orbit coupling and electron-electron interactions on an even footing and reveal that the ground state of 5d^{3}-based compounds, which has remained elusive in previously applied models, is a novel spin-orbit entangled J=3/2 electronic ground state. This work reveals the hidden diversity of spin-orbit controlled ground states in 5d systems and introduces a new arena in the search for spin-orbit controlled phases of matter.
Abstract
The influence of spin-orbit coupling (SOC) on the physical properties of the 5
d
2
system Sr
2
MgOsO
6
is probed via a combination of magnetometry, specific heat measurements, elastic and ...inelastic neutron scattering and density functional theory calculations. Although a significant degree of frustration is expected, we find that Sr
2
MgOsO
6
orders in a type I antiferromagnetic structure at the remarkably high temperature of 108 K. The measurements presented allow for the first accurate quantification of the size of the magnetic moment in a 5
d
2
system of 0.60(2) μ
B
–a significantly reduced moment from the expected value for such a system. Furthermore, significant anisotropy is identified via a spin excitation gap and we confirm by first principles calculations that SOC not only provides the magnetocrystalline anisotropy, but also plays a crucial role in determining both the ground state magnetic order and the size of the local moment in this compound. Through comparison to Sr
2
ScOsO
6
, it is demonstrated that SOC-induced anisotropy has the ability to relieve frustration in 5
d
2
systems relative to their 5
d
3
counterparts, providing an explanation of the high
T
N
found in Sr
2
MgOsO
6
.
Proteins and RNA are unique among known polymers in their ability to adopt compact and well-defined folding patterns. These two biopolymers can perform complex chemical operations such as catalysis ...and highly selective recognition, and these functions are linked to folding in that the creation of an active site requires proper juxtaposition of reactive groups. So the development of new types of polymeric backbones with well-defined and predictable folding propensities ('foldamers') might lead to molecules with useful functions. The first step in foldamer development is to identify synthetic oligomers with specific secondary structural preferences. Whereas alpha-amino acids can adopt the well-known alpha-helical motif of proteins, it was shown recently that beta-peptides constructed from carefully chosen beta-amino acids can adopt a different, stable helical conformation defined by interwoven 14-membered-ring hydrogen bonds (a 14-helix; Fig. 1a). Here we report that beta-amino acids can also be used to design beta-peptides with a very different secondary structure, a 12-helix (Fig. 1a). This demonstrates that by altering the nature of beta-peptide residues, one can exert rational control over the secondary structure.
The x-ray crystal structure of dimeric (+)-bornyl diphosphate synthase, a metal-requiring monoterpene cyclase from Salvia officinalis, is reported at 2.0-Å resolution. Each monomer contains two ...α-helical domains: the C-terminal domain catalyzes the cyclization of geranyl diphosphate, orienting and stabilizing multiple reactive carbocation intermediates; the N-terminal domain has no clearly defined function, although its N terminus caps the active site in the C-terminal domain during catalysis. Structures of complexes with aza analogues of substrate and carbocation intermediates, as well as complexes with pyrophosphate and bornyl diphosphate, provide "snapshots" of the terpene cyclization cascade.
In this paper we report low-temperature magnetic properties of the rare-earth perovskite material YbAlO3. Results of elastic and inelastic neutron scattering experiment, magnetization measurements ...along with the crystalline electrical field (CEF) calculations, suggest that the ground state of Yb moments is a strongly anisotropic Kramers doublet, and the moments are confined in the ab plane, pointing at an angle of φ=±23.5∘ to the a axis. With temperature decreasing below TN=0.88 K, Yb moments order into the coplanar but noncollinear antiferromagnetic (AFM) structure AxGy, where the moments are pointed along their easy axes. In addition, we highlight the importance of the dipole-dipole interaction, which selects the type of magnetic ordering and may be crucial for understanding magnetic properties of other rare-earth orthorhombic perovskites. Further analysis of the broad diffuse neutron scattering shows that one-dimensional interaction along the c axis is dominant and suggests YbAlO3 as a new member of one-dimensional quantum magnets.
Overexpression of the zinc enzyme carbonic anhydrase (CA; EC 4.2.1.1) XII is observed in certain human cancers. This bitopic membrane protein contains an N-terminal extracellular catalytic domain, a ...membrane-spanning α-helix, and a small intracellular C-terminal domain. We have determined the three-dimensional structure of the extracellular catalytic domain of human CA XII by x-ray crystallographic methods at 1.55-Å resolution. The structure reveals a prototypical CA fold; however, two CA XII domains associate to form an isologous dimer, an observation that is confirmed by studies of the enzyme in solution. The identification of signature GXXXG and GXXXS motifs in the transmembrane sequence that facilitate helix-helix association is additionally consistent with dimeric architecture. The dimer interface is situated so that the active site clefts of each monomer are clearly exposed on one face of the dimer, and the C termini are located together on the opposite face of the dimer to facilitate membrane interaction. The amino acid composition of the active-site cleft closely resembles that of the other CA isozymes in the immediate vicinity of the catalytic zinc ion, but differs in the region of the nearby α-helical "130's segment." The structure of the CA XII-acetazolamide complex is also reported at 1.50-Å resolution, and prospects for the design of CA XII-specific inhibitors of possible chemotherapeutic value are discussed.
Understanding spin textures in frustrated magnets Paddison, J. A. M.; Zhang, H.; Rai, B. ...
Acta crystallographica. Section A, Foundations and advances,
08/2023, Letnik:
79, Številka:
a2
Journal Article
We report the results of inelastic neutron scattering measurements on particular phonons of a superconducting (SC) Ca10Pt4As8(Fe1−xPtxAs)10 with the onset transition temperature Tc ~ 33 K to ...investigate mainly what roles orbital fluctuation plays in Cooper pairing, where we observed a slight softening of the in-plane transverse acoustic mode corresponding to the elastic constant C66. This softening starts at temperature T well above the SC Tc, as T decreases. An anomalously strong change of the scattering intensity of in-plane optical modes was observed at the M point of the pseudo tetragonal reciprocal space in the range of 35 < ω < 40 meV with decreasing T from far above Tc. Because this ω region mainly corresponds to the motion of Fe and As atoms in the FeAs planes, the finding presents information on the coupling between the orbital fluctuation of Fe 3d electrons and the lattice system, useful for studying the possible roles of orbital fluctuation in the pairing mechanism and/or the appearance of the so-called nematic phase.
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