77Se Nuclear Magnetic Resonance Research Articles

Spin-Orbital-Intertwined Nematic State in FeSe

The importance of the spin-orbit coupling (SOC) effect in Fe-based superconductors (FeSCs) has recently been under hot debate. Considering the Hund's coupling-induced electronic correlation, the understanding of the role of SOC in FeSCs is not trivial and is still elusive. Here, through a comprehensive study of 77Se and 57Fe nuclear magnetic resonance, a nontrivial SOC effect is revealed in the nematic state of FeSe. First, the orbital-dependent spin susceptibility, determined by the anisotropy of the 57Fe Knight shift, indicates a predominant role from the 3dxy orbital, which suggests the coexistence of local and itinerant spin degrees of freedom (d.o.f.) in the FeSe. Then, we reconfirm that the orbital reconstruction below the nematic transition temperature (Tnem ~ 90 K) happens not only on the 3dxz and 3dyz orbitals but also on the 3dxy orbital, which is beyond a trivial ferro-orbital order picture. Moreover, our results also indicate the development of a coherent coupling between the local and itinerant spin d.o.f. below Tnem, which is ascribed to a Hund's coupling-induced electronic crossover on the 3dxy orbital. Finally, due to a nontrivial SOC effect, sizable in-plane anisotropy of the spin susceptibility emerges in the nematic state, suggesting a spin-orbital-intertwined nematicity rather than simply spin- or orbital-driven nematicity}. The present work not only reveals a nontrivial SOC effect in the nematic state but also sheds light on the mechanism of nematic transition in FeSe.

Separate tuning of nematicity and spin fluctuations to unravel the origin of superconductivity in FeSe

The interplay of orbital and spin degrees of freedom is the fundamental characteristic in numerous condensed matter phenomena, including high-temperature superconductivity, quantum spin liquids, and topological semimetals. In iron-based superconductors (FeSCs), this causes superconductivity to emerge in the vicinity of two other instabilities: nematic and magnetic. Unveiling the mutual relationship among nematic order, spin fluctuations, and superconductivity has been a major challenge for research in FeSCs, but it is still controversial. Here, by carrying out 77Se nuclear magnetic resonance (NMR) measurements on FeSe single crystals, doped by cobalt and sulfur that serve as control parameters, we demonstrate that the superconducting transition temperature Tc increases in proportion to the strength of spin fluctuations, while it is independent of the nematic transition temperature Tnem. Our observation therefore directly implies that superconductivity in FeSe is essentially driven by spin fluctuations in the intermediate coupling regime, while nematic fluctuations have a marginal impact on Tc.

77Se NMR Probes the Protein Environment of Selenomethionine.

Sulfur is critical for the correct structure and proper function of proteins. Yet, lacking a sensitive enough isotope, nuclear magnetic resonance (NMR) experiments are unable to deliver for sulfur in proteins the usual wealth of chemical, dynamic, and structural information. This limitation can be circumvented by substituting sulfur with selenium, which has similar physicochemical properties and minimal impact on protein structures but possesses an NMR compatible isotope (77Se). Here we exploit the sensitivity of 77Se NMR to the nucleus' chemical milieu and use selenomethionine as a probe for its proteinaceous environment. However, such selenium NMR spectra of proteins currently resist a reliable interpretation because systematic connections between variations of system variables and changes in 77Se NMR parameters are still lacking. To start narrowing this knowledge gap, we report here on a biological 77Se magnetic resonance data bank based on a systematically designed library of GB1 variants in which a single selenomethionine was introduced at different locations within the protein. We recorded the resulting isotropic 77Se chemical shifts and relaxation times for six GB1 variants by solution-state 77Se NMR. For four of the GB1 variants we were also able to determine the chemical shift anisotropy tensor of SeM by solid-state 77Se NMR. To enable interpretation of the NMR data, the structures of five of the GB1 variants were solved by X-ray crystallography to a resolution of 1.2 Å, allowing us to unambiguously determine the conformation of the selenomethionine. Finally, we combine our solution- and solid-state NMR data with the structural information to arrive at general insights regarding the execution and interpretation of 77Se NMR experiments that exploit selenomethionine to probe proteins.

Observation of orbital ordering and origin of the nematic order in FeSe

In iron-based superconductors the interactions driving the nematic order that breaks the lattice four-fold rotational symmetry in the iron plane may also facilitate the Cooper pairing, but experimental determination of these interactions is challenging because the temperatures of the nematic order and the order of other electronic phases appear to match each other or to be close to each other. Here we performed field-dependent 77Se-nuclear magnetic resonance (NMR) measurements on single crystals of iron-based superconductor FeSe, with magnetic field B0 up to 16 T. The 77Se-NMR spectra and Knight shift split when the direction of B0 is away from the direction perpendicular to the iron planes (i.e. B0 ∥ c) upon cooling in temperature, with a significant change in the distribution and magnitude of the internal magnetic field at the 77Se nucleus, but these do not happen when B0 is perpendicular to the iron planes, thus demonstrating that there is an orbital ordering. Moreover, stripe-type antiferromagnetism is absent, while giant antiferromagnetic spin fluctuations measured by the NMR spin-lattice relaxation gradually developed starting at ∼40 K, which is far below the nematic order temperature Tnem = 89 K. These results provide direct evidence of orbital-driven nematic order in FeSe.

(Invited) Molecular-Level Understanding of Ion Intercalation Mechanisms in Aluminum and Zinc Battery Electrodes Revealed By Solid-State NMR Spectroscopy

Solid-state nuclear magnetic resonance (NMR) spectroscopy is a powerful and quantitative characterization tool that can elucidate the local chemical, structural, and electronic changes that battery materials undergo upon electrochemical cycling. However, to date no solid-state NMR experiments have been reported that directly probe multivalent aluminum or zinc cations within intercalation electrodes, in part due to their challenging quadrupolar nature and in part due to the small number of structures that reversibly intercalate multivalent ions. Here, multi-nuclear solid-state magic-angle-spinning (MAS) experiments will be presented on rechargeable aluminum and zinc intercalation electrodes for the first time, revealing insights into their ion intercalation and charge transfer mechanisms. For aluminum or zinc batteries using the thio-Chevrel Mo6S8 and seleno-Chevrel Mo6Se8 as cathode materials, solid-state 27Al and 67Zn NMR experiments establish quantitatively the relative populations of intercalated aluminum or zinc cations in different local environments as a function of state-of-charge. For the seleno-chevrel electrodes, solid-state 77Se NMR experiments reveal the effects of aluminum- and zinc-ion intercalation on the local electronic structures of the Mo6Se8 frameworks. For comparison, aluminum-graphite batteries will also be analyzed, where solid-state 27Al NMR measurements yield insights into the local environments of monovalent chloroaluminate anions after intercalation into natural graphite. Opportunities and challenges will be discussed regarding the application of NMR spectroscopy to aluminum and zinc battery materials. Overall, the solid-state NMR and electrochemical results pave the way towards a better understanding of material design principles aimed at realizing multivalent intercalation electrodes with enhanced electrochemical properties.

77Se Nuclear Magnetic Resonance Study of the Surface Effect in Topological Insulator Bi2Se3 Nanoparticles

We have carried out 77Se nuclear magnetic resonance (NMR) measurements in order to probe the surface state of topological insulator Bi2Se3 nanoparticles physically mixed with insulating Al2O3 nanoparticles. Two distinct peaks in the NMR spectra were separately assigned to the surface and the bulk regions in the samples with different mixing ratios, manifesting an increase in the surface effect.

The Selenium-Nitrogen Bond as Basis for Reactive Selenium Species with Pronounced Antimicrobial Activity

Aim and Objective: Selenium (Se) compounds are often associated with good reactivity and selectivity due to specific modifications of thiol groups in peptides, proteins and enzymes. Among them, selenazolinium salts are of particular interest, as they react readily with their thiol targets. This study was undertaken to verify whether this reactivity translates into biological activity against a few selected organisms. Materials and Methods: To screen the activity of selenazolinium salts, we performed nematicidal activity assay using Steinernema feltiae. To determine their impact on microbial proliferation, viability of Escherichia coli and Saccharomyces cerevisiae cells was monitored. For a chemical genetic phenotyping focused on a redox link, 32 redox-related S. cerevisiae mutants were used. DNA double-strand breakage caused by selenazolinium salts was investigated using pulsed-field gel electrophoresis and their physico-chemical properties were assessed using nuclear magnetic resonance (NMR). Results: Some of selenazolinium salts are toxic against S. feltiae at a concentration of 100-500 µM. In E. coli, selenazolium salts display no toxicity at a concentration of 100 µM; however, at a concentration of 500 µM some of them show a statistically relevant toxicity. Similar findings were obtained in wild-type S. cerevisiae cells. Only a few redox-related mutants show higher sensitivity to selenazolinium salts compared to wild-type cells. Selenazolinium salts induce DNA double-strand breaks at moderate doses (10-100 µM). 77Se-NMR shifts reflect some of the trends observed in the biological assays. Conclusion: Our results confirmed that several of selenazolinium salts show a significant biological activity that is executed via an electrophilic attack. Keywords: Antimicrobial activity, charge density, chemogenetic screening, DNA damage, reactive selenium species, redox modulation, 77Se NMR, selenazolinium salts.

Novel P,P-diphenylphosphinic amide-TEMPO radicals family: Synthesis, crystal structures, spectroscopic characterization, magnetic properties and DFT calculations

A new family of three P,P-diphenylphosphinic amide-TEMPO radicals with general formula (C6H5)2P(X)-NH-TEMPO, X = O (1), S (2) and Se (3) were obtained by reaction of chlorodiphenylphosphine with 4-amino-2,2,6,6-tetramethylpiperidine-1-oxyl radical (NH2-TEMPO) and subsequent oxidation with H2O2 and elemental sulfur or selenium. These radicals were characterized by UV–Vis, FT-IR and 1H, 31P and 77Se nuclear magnetic resonance spectroscopy. The crystal structures of all radicals were determined by single-crystal X-ray diffraction. The magnetic study of all diphenylphosphinic amide-TEMPO radicals reveals very weak intermolecular ferromagnetic interactions. Density functional theory (DFT) calculations have been also performed to support the interpretation of magnetic properties and electronic structure.

Pseudogap Behavior of the Nuclear Spin–Lattice Relaxation Rate in FeSe Probed by 77Se-NMR

We conducted $^{77}$Se-nuclear magnetic resonance studies of the iron-based superconductor FeSe in magnetic fields of 0.6 to 19 T to investigate the superconducting and normal-state properties. The nuclear spin-lattice relaxation rate divided by the temperature $(T_1T)^{-1}$ increases below the structural transition temperature $T_\mathrm{s}$ but starts to be suppressed below $T^*$, well above the superconducting transition temperature $T_\mathrm{c}(H)$, resulting in a broad maximum of $(T_1T)^{-1}$ at $T_\mathrm{p}(H)$. This is similar to the pseudogap behavior in optimally doped cuprate superconductors. Because $T^*$ and $T_\mathrm{p}(H)$ decrease in the same manner as $T_\mathrm{c}(H)$ with increasing $H$, the pseudogap behavior in FeSe is ascribed to superconducting fluctuations, which presumably originate from the theoretically predicted preformed pair above $T_\mathrm{c}(H)$.

Structural mechanism of iodine incorporation in As-Se-I glasses and its effects on physical properties

The structural evolution of ternary AsxSe100−x−yIy glasses (x=20, 30 and y=10, 20) upon progressive substitution of Se with I is studied using a combination of high-resolution, two-dimensional 77Se nuclear magnetic resonance (NMR) and Raman spectroscopy. The results suggest that the As-Se bonds in the As-Se-Se linkages in these glasses are severed and terminated by As-I bonds with a concomitant increase in the relative fraction of Se-Se-Se linkages and, in the case of As30Se70−yIy glasses, also in the relative fraction of As-Se-As linkages. This structural modification decreases the average connectivity of the network and its packing efficiency, which are manifested in an increase in molar volume and fragility and a decrease in glass transition temperature Tg and isothermal viscosity. Additionally, Tg and fragility are found to be only a function of average coordination number regardless of chemical composition. When taken together, these results indicate a relatively homogeneous distribution of the I atoms in the glass structure in “mixed-anion” AsSe(3−x)/2Ix pyramidal units.

Structure of Amorphous Selenium by 2D 77Se NMR Spectroscopy: An End to the Dilemma of Chain versus Ring

AbstractAmorphous selenium, owing to its tremendous technological importance and perhaps to its chemical simplicity, has been studied for nearly a century and yet an unequivocal structural description of this material remains lacking to date. The primary controversy regarding the structure of amorphous Se relates to the relative fraction of Se atoms residing in chains versus in Se8 rings. Herein we present the results of a two‐dimensional solid‐state 77Se nuclear magnetic resonance (NMR) spectroscopic study of the chain and ring crystalline allotropes of Se as well as of amorphous Se to unequivocally demonstrate that 1) the Se8 rings and the chains are characterized by their unique 77Se NMR signatures and 2) the structure of amorphous Se consists exclusively of chains.

Structure of Amorphous Selenium by 2D 77 Se NMR Spectroscopy: An End to the Dilemma of Chain versus Ring.

Amorphous selenium, owing to its tremendous technological importance and perhaps to its chemical simplicity, has been studied for nearly a century and yet an unequivocal structural description of this material remains lacking to date. The primary controversy regarding the structure of amorphous Se relates to the relative fraction of Se atoms residing in ∞1Se chains versus in Se8 rings. Herein we present the results of a two-dimensional solid-state 77 Se nuclear magnetic resonance (NMR) spectroscopic study of the chain and ring crystalline allotropes of Se as well as of amorphous Se to unequivocally demonstrate that 1) the Se8 rings and the ∞1Se chains are characterized by their unique 77 Se NMR signatures and 2) the structure of amorphous Se consists exclusively of ∞1Se chains.

Structural and Topological Evolution in SixSe1-x Glasses: Results from 1D and 2D 29Si and 77Se NMR Spectroscopy.

The coordination environments of Si and Se atoms and their connectivity in binary SixSe1-x glasses with 0.05 ≤ x ≤ 0.33 are investigated using a combination of one- and two-dimensional 29Si and 77Se nuclear magnetic resonance (NMR) and Raman spectroscopy. The high-resolution correlated isotropic and anisotropic 29Si and 77Se NMR spectra allow for the identification and quantitation of a variety of Si and Se environments. The results suggest that the structure of these glasses are characterized by a network with essentially perfect short-range chemical order, but with strong clustering at the intermediate range. Initial addition of Si to Se results in cross-linking of Se chain segments with nanoclusters of corner- and edge-shared SiSe4/2 tetrahedra. These clusters percolate via coalescence near x ≥ 0.2 to finally form a low-dimensional network with high molar volume, at the stoichiometric composition (x = 0.33) that is composed of chains of edge-sharing tetrahedra cross-linked by corner-shared tetrahedra. This structural evolution is shown to be consistent with the compositional variation of the glass transition temperature and the molar volume of these glasses.

Recent Advances in Multinuclear NMR Spectroscopy for Chiral Recognition of Organic Compounds.

Nuclear magnetic resonance (NMR) is a powerful tool for the elucidation of chemical structure and chiral recognition. In the last decade, the number of probes, media, and experiments to analyze chiral environments has rapidly increased. The evaluation of chiral molecules and systems has become a routine task in almost all NMR laboratories, allowing for the determination of molecular connectivities and the construction of spatial relationships. Among the features that improve the chiral recognition abilities by NMR is the application of different nuclei. The simplicity of the multinuclear NMR spectra relative to 1H, the minimal influence of the experimental conditions, and the larger shift dispersion make these nuclei especially suitable for NMR analysis. Herein, the recent advances in multinuclear (19F, 31P, 13C, and 77Se) NMR spectroscopy for chiral recognition of organic compounds are presented. The review describes new chiral derivatizing agents and chiral solvating agents used for stereodiscrimination and the assignment of the absolute configuration of small organic compounds.

Dissolution behaviour and activation of selenium in phosphonium based ionic liquids.

The dissolution behaviour of grey selenium in phosphonium based ionic liquids (ILs) has been investigated for the first time by 31P and 77Se nuclear magnetic resonance (NMR) experiments. The investigations evidence the formation of trialkylphosphane selenides which can serve as a selenium reservoir in the subsequent formation of metal selenides.

Synthesis and structural characterization of stoichiometric Li-Ga-Ge Sulfo-selenide glasses

Homogeneous glasses in the mixed-chalcogen pseudo-ternary system Li2S-Ga2Se3-GeSe2 are synthesized and their structure is characterized using Raman and one– and two-dimensional 6Li, 77Se, and 71Ga nuclear magnetic resonance (NMR) spectroscopy. The structure of these glasses can be described as a charge-compensated network predominantly consisting of corner sharing (Ga/Ge) (Se,S)4/2 tetrahedra. The compositional evolution of the atomic structure is heavily influenced by the Li2S:Ga2Se3 ratio R where charge compensation is accommodated by the formation of different structural units and preferential chemical ordering for S atoms. Glasses with R<1 are deficient in chalcogens required to satisfy the tetrahedral coordination of Ga and consequently form ethane-like X3Ge-GeX3 (X=S, Se) units and S atoms preferentially participate in these structural units. On the other hand, the structure of chalcogen-excess glasses with R>1 are characterized by the formation of non-bridging Se (NBSe) and S (NBS) sites. The Se atoms show a preference over S for these non-bridging sites and form Ge-NBSe linkages, while the S atoms preferentially bond to Ga, resulting in the formation of GaS4/2 tetrahedra. This structural scenario is shown to be consistent with the corresponding changes in the glass transition temperature.

Stress induced alignment of Se chains in supercooled Ge5Se95 liquid

The structural modifications in Ge5Se95 supercooled liquid under the application of a uniaxial compressive load are investigated using one- and two-dimensional 77Se nuclear magnetic resonance spectroscopy. The spectroscopic results clearly demonstrate a stress-induced alignment of selenium chains in the structure of the liquid in planes perpendicular to the loading axis. The topological modifications due to this selenium chain alignment result in inefficient atomic packing in the liquid structure that is manifested in a decrease in the glass density and may have important implications in understanding the pronounced non-Newtonian shear thinning behavior observed in Se-rich chalcogenide glasses/liquids.

Spin-rotation symmetry breaking in the superconducting state of CuxBi2Se3

Spontaneous symmetry breaking is an important concept for understanding physics ranging from the elementary particles to states of matter. For example, the superconducting state breaks global gauge symmetry, and unconventional superconductors can break additional symmetries. In particular, spin rotational symmetry is expected to be broken in spin-triplet superconductors. However, experimental evidence for such symmetry breaking has not been conclusively obtained so far in any candidate compounds. Here, by 77Se nuclear magnetic resonance measurements, we show that spin rotation symmetry is spontaneously broken in the hexagonal plane of the electron-doped topological insulator Cu0.3Bi2Se3 below the superconducting transition temperature Tc=3.4 K. Our results not only establish spin-triplet superconductivity in this compound, but may also serve to lay a foundation for the research of topological superconductivity.

Selenium Chain Length Distribution in GexSe100-x Glasses: Insights from (77)Se NMR Spectroscopy and Quantum Chemical Calculations.

The statistics of selenium chain length distribution in GexSe100-x glasses with 5 ≤ x ≤ 20 are investigated using a combination of high-resolution, two-dimensional (77)Se nuclear magnetic resonance (NMR) spectroscopy and quantum chemical calculations. This combined approach allows for the distinction of various selenium chain environments on the basis of subtle but systematic effects of next-nearest neighbors of Se atoms in -Se-Se-Se- linkages on the (77)Se chemical shift tensor parameters. Simulation of the experimental (77)Se NMR spectral line shapes indicates that Se chain speciation in these chalcogenide glasses follows the Flory-Schulz distribution, originally developed for organic chain polymers.

Nematicity and in-plane anisotropy of superconductivity inβ−FeSedetected bySe77nuclear magnetic resonance

The recent study of $^{77}$Se nuclear magnetic resonance (NMR) in a $\beta$-FeSe single crystal proposed that ferro-orbital order breaks the $90^\circ$ $C_4$ rotational symmetry, driving nematic ordering. Here, we report an NMR study of the impact of small strains generated by gluing on nematic state and spin fluctuations. We observe that the local strains strongly affect the nematic transition, considerably enhancing its onset temperature. On the contrary, no effect on low-energy spin fluctuations was found. Furthermore we investigate the interplay of the nematic phase and superconductivity. Our study demonstrates that the twinned nematic domains respond unequivalently to superconductivity, evidencing the twofold $C_2$ symmetry of superconductivity in this material. The obtained results are well understood in terms of the proposed ferro-orbital order.