The Ising triangular-lattice antiferromagnet neodymium heptatantalate as a quantum spin liquid candidate.

Disordered magnetic states known as spin liquids are of paramount importance in both fundamental and applied science. A classical state of this kind was predicted for the Ising antiferromagnetic triangular model, while additional non-commuting exchange terms were proposed to induce its quantum version-a quantum spin liquid. However, these predictions have not yet been confirmed experimentally. Here, we report evidence for such a state in the triangular-lattice antiferromagnet NdTa7O19. We determine its magnetic ground state, which is characterized by effective spin-1/2 degrees of freedom with Ising-like nearest-neighbour correlations and gives rise to spin excitations persisting down to the lowest accessible temperature of 40 mK. Our study demonstrates the key role of strong spin-orbit coupling in stabilizing spin liquids that result from magnetic anisotropy and highlights the large family of rare-earth (RE) heptatantalates RETa7O19 as a framework for realization of these states, which represent a promising platform for quantum applications.

Origin of Magnetic Ordering in a Structurally Perfect Quantum Kagome Antiferromagnet.

The ground state of the simple Heisenberg nearest-neighbor quantum kagome antiferromagnetic model is a magnetically disordered spin liquid, yet various perturbations may lead to fundamentally different states. Here we disclose the origin of magnetic ordering in the structurally perfect kagome material YCu_{3}(OH)_{6}Cl_{3}, which is free of the widespread impurity problem. Ab initio calculations and modeling of its magnetic susceptibility reveal that, similar to the archetypal case of herbertsmithite, the nearest-neighbor exchange is by far the dominant isotropic interaction. Dzyaloshinskii-Moriya (DM) anisotropy deduced from electron spin resonance, susceptibility, and specific-heat data is, however, significantly larger than in herbertsmithite. By enhancing spin correlations within kagome planes, this anisotropy is essential for magnetic ordering. Our study isolates the effect of DM anisotropy from other perturbations and unambiguously confirms the predicted phase diagram.

Synthesis and characterization of a family of M(2+) complexes supported by a trianionic ONO(3-) pincer-type ligand: towards the stabilization of high-spin square-planar complexes.

High-spin square-planar molecular compounds are rare. In an effort to access this unique combination of geometry and spin state, we report the synthesis of a series of M(II) compounds stabilized by a trianionic pincer-type ligand, highlighting the formation of a high-spin square-planar Co(II) complex. Low-temperature, variable-frequency EPR measurements reveal that the ground electronic state of the Co(II) analogue is a highly anisotropic Kramers doublet (effective g values 7.35, 2.51, 1.48). This doublet can be identified with the lowest doublet of a quartet, S = 3/2 spin state that exhibits a very large ZFS, D ≥ 50 cm(-1). The observation of an effective g value considerably greater than the largest spin-only value 6, demonstrates that the orbital angular moment is essentially unquenched along one spatial direction. Density Functional Theory (DFT) and time-dependent DFT calculations reveal the electronic configurations of the ground and excited orbital states. A qualitative crystal field description of the geff tensor shows that it originates from the spin-orbit coupling acting on states obtained through the transfer of a ß electron from the doubly occupied xy to the singly-occupied {xz/yz} orbitals.

A high-spin square-planar Fe(ii) complex stabilized by a trianionic pincer-type ligand and conclusive evidence for retention of geometry and spin state in solution.

Square-planar high-spin Fe(ii) molecular compounds are rare and the only three non-macrocyclic or sterically-driven examples reported share a common FeO4 core. Using an easily modifiable pincer-type ligand, the successful synthesis of the first compound of this type that breaks the FeO4 motif was achieved. In addition, we present the first evidence that geometry and spin state persist in solution. Extensive characterization includes the first high-field EPR and variable field/temperature Mössbauer spectra for this class of compounds. Analysis of the spectroscopic data indicates this complex exhibits a large and positive zero-field splitting tensor. Furthermore, the unusually small ΔEQ value determined for this compound is rationalized on the basis of DFT calculations.

Intense turquoise and green colors in brownmillerite-type oxides based on Mn5+ in Ba2In(2-x)Mn(x)O(5+x).

Brownmillerite-type oxides Ba(2)In(2-x)Mn(x)O(5+x) (x = 0.1-0.7) have been prepared and characterized. Magnetic measurements indicate that manganese in as-prepared samples is substituting predominantly as Mn(5+) for all values of x with observed paramagnetic spin-only moments close to values expected for two unpaired electrons. Electron paramagnetic resonance measurements indicate that this Mn(5+) is present in a highly distorted tetrahedral environment. Neutron diffraction structure refinements show that Mn(5+) occupies tetrahedral sites for orthorhombic (x = 0.1) and tetragonal (x = 0.2) phases. For Mn ≥ 0.3 samples, neutron refinements show that the phases are cubic with disordered cations and oxygen vacancies. The colors of the phases change from light yellow (x = 0) to intense turquoise (x = 0.1) to green (x = 0.2, 0.3) or to dark green (x ≥ 0.4). Under reducing conditions, Mn(5+) is reduced to Mn(3+), and Ba(2)In(2-x)Mn(x)O(5+x) phases become black Ba(2)In(2-x)Mn(x)O(5) phases still with the brownmillerite structure.

Spin dynamics of the S = 5/2 2D triangular antiferromagnet Ba3NbFe3Si2O14.

We report pulse-field magnetization, ac susceptibility, and 100 GHz electron spin resonance (ESR) measurements on the S = 5/2 two-dimensional triangular compound Ba3NbFe3Si2O14 with the Néel temperature T(N) = 26 K. The magnetization curve shows an almost linear increase up to 60 T with no indication of a one-third magnetization plateau. An unusually large frequency dependence of the ac susceptibility in the temperature range of T = 20-100 K reveals a spin-glass behavior or superparamagnetism, signaling the presence of frustration-related slow magnetic fluctuations. The temperature dependence of the ESR linewidth exhibits two distinct critical regimes; (i) ΔH(pp)(T) is proportional to (T-T(N))(-p) with the exponent p = 0.2(1)-0.2(3) for temperatures above 27 K, and (ii) ΔH(pp)(T) is proportional to (T-T*)(-p) with T* = 12 K and p = 0.8(1)-0.8(4) for temperatures between 12 and 27 K. This is interpreted as indicating a dimensional crossover of magnetic interactions and the persistence of short-range correlations with a helically ordered state.

Role of antisymmetric exchange in selecting magnetic chirality in Ba3NbFe3Si2O14.

We present an electron spin resonance (ESR) investigation of the acentric Ba(3)NbFe(3)Si(2)O(14), featuring a unique single-domain double-chiral magnetic ground state. Combining simulations of the ESR linewidth anisotropy and the antiferromagnetic-resonance modes allows us to single out the Dzyaloshinsky-Moriya (DM) interaction as the leading magnetic anisotropy term. We demonstrate that the rather minute out-of-plane DM component d(c)=45 mK is responsible for selecting a unique ground state, which endures thermal fluctuations up to astonishingly high temperatures.

Characterization of (Fe'Zr,Ti - VO..). defect dipoles in (La,Fe)-codoped PZT 52.5/47.5 piezoelectric ceramics by multifrequency electron paramagnetic resonance spectroscopy.

Ferroelectric 1 mol.% La(3)+ and 0.5 mol.% Fe(3)+ codoped Pb[Zr0(0.54)Ti0(0.46)]O(3) ceramics were studied by means of multifrequency electron paramagnetic resonance (EPR) spectroscopy. The obtained results prove that iron is incorporated at the [Zr,Ti]-site, acting as an acceptor and building a charged Fe'(Zr,Ti) - V(O)..)(.) defect dipole with a directly coordinated oxygen vacancy for partial charge compensation. This feature of the defect associates has hitherto been identified only in hard, exclusively Fe(3)+-doped PZT compounds. The present results show, however, that a similar defect association of the Fe3+ functional center with a V(O)..) also exists in soft, donor-acceptor (La(3)+,Fe(3)+)-codoped PZT. According to models developed by Arlt et al. electric dipoles from defect associates, such as the Fe'(Zr,Ti) - V(O)..)(.) defect associate, which may give rise to an internal bias field that is discussed being responsible for ferroelectric aging.

Characterization of the tryptophan residues of Escherechia coli alkaline phosphatase by phosphorescence and optically detected magnetic resonance spectroscopy.

The phosphorescence and zero field optically detected magnetic resonance (ODMR) of the tryptophan (Trp) residues of alkaline phosphatase from Escherechia coli are examined. Each Trp is resolved optically and identified with the aid of the W220Y mutant and the terbium complex of the apoenzyme. Trp(109), known from earlier work to be the source of room-temperature phosphorescence (RTP), emits a highly resolved low-temperature phosphorescence (LTP) spectrum and has the narrowest ODMR bands observed thus far from any protein site, revealing a uniquely homogeneous local environment. The decay kinetics of Trp(109) at 1.2 K reveals that the major triplet population (70%) undergoes inefficient crystallike spin-lattice relaxation by direct interaction with lattice phonons, the remainder being relaxed efficiently by local disorder modes. The latter population is smaller than is typical for protein sites, suggesting an unusual degree of local rigidity and order consistent with the long-lived RTP. Trp(220) emits a broader LTP spectrum originating to the blue of Trp(109). It has typically broad ODMR bands consistent with local heterogeneity. The LTP of Trp(268) has an ill-defined origin blue shifted relative to Trp(220) and ODMR frequencies consistent with a greater degree of solvent exposure. Trp(268) has noticeable dispersion of its decay kinetics, consistent with quenching at the triplet level by a nearby disulfide residue.

Determination of relative triplet sublevel populating rates during optical pumping using ODMR.

A method is introduced, based on optical detection of triplet state magnetic resonance (ODMR), to determine the relative populating rates of photoexcited triplet state sublevels during optical pumping. Phosphorescence transients induced by microwave rapid passage during optical pumping are analyzed globally utilizing kinetic parameters obtained from separate microwave-induced delayed phosphorescence measurements to obtain relative sublevel populating rates. Results are unaffected by phosphorescence from triplet populations that do not yield an ODMR response. The method is applied to the triplet state of the indole chromophore in various environments to reveal the effects of local interactions on the pattern of intersystem crossing. Enhanced spin--orbit coupling effects are attributed to interactions that reduce the planar symmetry of the indole chromophore.

Phosphorescence and optically detected magnetic resonance of HIV-1 nucleocapsid protein complexes with stem-loop sequences of the genomic Psi-recognition element.

The binding of NCp7, the nucleocapsid protein of human immunodeficiency virus type 1, to oligonucleotide stem--loop (SL) sequences of the genomic Psi-recognition element has been studied using fluorescence, phosphorescence, and optically detected magnetic resonance (ODMR). RNA SL2, SL3, and SL4 constructs bind with higher affinity than the corresponding DNAs. G to I substitutions in the SL3 DNA loop sequence lead to reduced binding affinity and significant changes in the triplet state properties of Trp37 of NCp7, implicating these bases in contacts with aromatic amino acid residues of the zinc finger domains of NCp7, in agreement with the NMR structure of the 1:1 complex of NCp7 and SL3 RNA [DeGuzman, R. N., Wu, Z. R., Stalling, C. C., Pappaladro, L., Borer, P. N., and Summers, M. F. (1998) Science 279, 384-388]. The NCp7 to SL binding stoichiometry is 2:1 for intact SL sequences but is reduced to 1:1 for SL variants with an abasic or hydrocarbon loop. It is proposed that Delta D/Delta E(0,0), where Delta D is the change in the zero-field splitting D parameter and Delta E(0,0) is the shift of the tryptophan phosphorescence origin, provides a measure of aromatic stacking interactions with nucleic acid bases. Values on the order of 10(-5) indicate significant stacking interactions, while values closer to 10(-6) result from interactions not involving aromatic stacking. Binding of NCp7 to oligonucleotide substrates produces shortened Trp37 triplet state lifetimes by enhancement of k(x) and an increase of the relative value of P(x), the intersystem crossing rate to the T(x) sublevel. These effects are attributed to a reduction in the degree of electronic symmetry of Trp37 in the complexes. Guanine and adenine triplet states produced by optical pumping of SL3 DNA are characterized. We find, as with tryptophan, that D < 3E.

Effect of nucleic acid binding on the triplet state properties of tetrapeptides containing tryptophan and 6-methyltryptophan: a study by phosphorescence and ODMR spectroscopy.

Complexes of four peptides [KWGK, KGWK, K(6MeW)GK, KG(6MeW)K] with the nucleic acids [poly(A), poly(C), poly(U), poly(I), and rG(8)] have been investigated by phosphorescence and optically detected magnetic resonance (ODMR) spectroscopy. The intrinsic spectroscopic probes used in these studies are tryptophan (W) and 6-methyltryptophan (6MeW). Binding to the nucleic acids results in a red-shift of the phosphorescence 0,0-band (delta E(0,0)) of the aromatic residue as well as a reduction of its zero-field splitting parameter (delta D). Results are compared with earlier studies of the HIV-1 nucleocapsid protein, NCp7, that contains a single tryptophan residue (Trp37) within a retroviral zinc finger sequence. Binding of poly(A) or poly(U) to the tetrapeptides induces larger delta E(0,0) and delta D than when bound to NCp7, indicating stronger stacking interactions. Poly(I), on the other hand, produces larger shifts in Trp37 of NCp7. Binding of rG(8) produces sequence-dependent effects in the peptides. When bound to NCp7, but in contrast with tetrapeptide binding, nucleic acids produce large changes in triplet state kinetics consistent with enhanced spin-orbit coupling. These results are discussed in terms of three limiting types of tryptophan-base interaction: intercalation, aromatic stacking, and edge-on interaction. These should have differing effects on the properties of the triplet state.

Ligand-induced conformational changes in lactose repressor: a phosphorescence and ODMR study of single-tryptophan mutants.

Phosphorescence and optically detected magnetic resonance (ODMR) measurements are reported on four single-tryptophan mutants of lac repressor protein from Escherichia coli: H74W/Wless, W201Y, Y273W/Wless, and F293W/Wless, where Wless represents a protein background containing the double mutation W201Y/W220Y. The single-tryptophan residues are located in the protein core region, either in the monomer-monomer interface of the tetrameric protein or in the region of the inducer binding cleft. Inducer binding elicits large changes in the energy (0,0-band wavelength shifts) and zero-field splitting energies (ZFS) of the triplet states for each of the mutant proteins except W201Y which exhibits more modest effects. F293W/Wless exists in two distinguishable conformations, only one of which appears to be sensitive to the presence of inducer. These effects of inducer binding can be attributed to a conformational change that alters specific polar interactions that occur at each affected tryptophan site. Changes in the tryptophan triplet state indicator depend on the existence of specific polar interactions that are altered by local atomic relocations.

Phosphorescence and optically detected magnetic resonance characterization of the environments of tryptophan analogues in staphylococcal nuclease, its V66W mutant, and Delta 137-149 fragment.

Phosphorescence and optically detected magnetic resonance (ODMR) measurements are reported on the triplet states of the tryptophan analogues, 7-azatryptophan (7AW), 5-hydroxytryptophan (5HW), and 4-, 5-, and 6-fluorotryptophan (4FW, 5FW, 6FW), when incorporated at position 140 of wild-type Staphylococcal nuclease (7AW-nuclease, etc. ), positions 66 and 140 of its V66W mutant (7AW-V66W, etc.), and the deletion fragment of the latter, Delta 137-149 (7AW-V66W', etc.). These measurements point to the retention of protein structure at position 140 in each of the wild-type nuclease analogues. Substitution of the analogue at both tryptophan sites of V66W leads to structured sites with differentiated triplet-state properties for all analogues except 7AW-V66W, whose structure is destabilized. 5HW-V66W' is the only fragment that apparently lacks structure at position 66. All other V66W' analogues exhibit a structured environment at position 66 (4FW-V66W' was not studied), but in each case this site can be differentiated readily from the corresponding site in intact V66W. 7AW-V66W' is resolved by ODMR into two discrete structures with slightly differing zero field splittings (ZFS). Interaction of the protein with 5HW at position 66 of 5HW-V66W induces a 2-fold increase in the ZFS E parameter, which is reduced to its normal value upon formation of the fragment, 5HW-V66W'. Analogous effects occur for 5FW, but on a smaller scale.

Study of complexes of a tryptophan-free mutant of E. coli trp aporepressor with tryptophan analogues using optically detected magnetic resonance (ODMR).

Phosphorescence and optically detected magnetic resonance (ODMR) spectra of tryptophan (W) and several of its analogues (4-, 5-, 6-methyltryptophan (MeW); 4-, 5-, 6-fluorotryptophan (FW); 5-bromotryptophan) are compared with those of complexes formed with the W-free trp aporepressor from Escherichia coli (W19,99F). W19,99F binds W and each analogue except 4-FW with an estimated KD < or = 30 microM; triplet state spectroscopic and kinetic effects that accompany binding at the corepressor site are reported. ODMR data for the MeW isomers are presented for the first time. No binding of 7-azaW is observed, in agreement with the low affinity found by previous workers.

Binding of the nucleocapsid protein of type 1 human immunodeficiency virus to nucleic acids studied using phosphorescence and optically detected magnetic resonance.

The binding of p7 nucleocapsid protein of type 1 human immunodeficiency virus (HIV-1) to various oligonucleotides and polynucleotides has been investigated by phosphorescence and optically detected magnetic resonance (ODMR) spectroscopy. The intrinsic spectroscopic probe used in these studies is the photoexcited triplet state of Trp37, which is associated with the C-terminal zinc finger of p7 and is its only tryptophan residue. Complex formation produces a red-shift of the phosphorescence 0, 0-band (DeltaE0,0) of Trp37 as well as a reduction of the zero field splitting (zfs) D parameter. Increases of -DeltaE0,0 (A < C < U < G

Fluorescence, phosphorescence, and optically detected magnetic resonance studies of the nucleic acid association of the nucleocapsid protein of the murine leukemia virus.

Fluorescence, phosphorescence, and optical detection of triplet state magnetic resonance (ODMR) are employed to investigate the interaction of p10, the nucleocapsid protein of the Moloney murine leukemia virus, with nucleic acids. p10 is a 55-amino acid protein containing a single zinc finger motif, C26C29H34C39, that includes Y at position 28 and W at position 35. In addition, the interactions of a zinc finger peptide, p10-ZF, comprising residues 24-41 of p10, and a doubly mutated 24-41 peptide, p10-ZF' in which the positions of Y and W are interchanged, also are reported. The measurements focus on the direct involvement of the sole W residue in the nucleic acid interaction. Fluorescence quenching and salt-back titrations indicate complex formation of p10 with several octanucleotides--(dT)8, (dI)8, (dU)7dT, and (5-BrdU)7dT--and with the polynucleotides poly(dT) and poly(dI). Poly(dI) binds with the highest affinity. Apparent binding constants and salt-back midpoints are reported. Neither p10-ZF nor p10-ZF' exhibits significant fluorescence quenching by these DNA substrates. Binding of p10-ZF to fluorescent poly(ethenoadenylic acid) was detected with greatly reduced affinity relative to p10, but binding of p10-ZF' was undetectable. These results are in general agreement with phosphorescence and ODMR measurements monitoring W. Addition of poly(I) to p10 leads to a phosphorescence red shift, reduction in the zero-field splitting (ZFS) parameters D and E, and a significantly reduced phosphorescence lifetime, each consistent with aromatic stacking interactions between W and the nucleobases. These effects are smaller with p10-ZF and undetectable with p10-ZF'. Poly(U) produces no significant changes in the triplet state parameters of W; no stacking interactions are observed even for p10. (5-BrdU)7dT yields large phosphorescence red shifts in p10 and p10-ZF, and reductions of D, but no significant heavy atom effects. These effects probably are due to enhanced local polarizability caused by Br, but any stacking interactions in these complexes would exclude van der Waals contacts between W and the Br atoms.