Dipolar Order Research Articles

Dipolar Order Parameters in Large Systems With Fast Spinning.

Order parameters are a useful tool for quantifying amplitudes of molecular motions. Here we measure dipolar order parameters by recoupling heteronuclear dipole-dipole couplings under fast spinning. We apply symmetry based recoupling methods to samples spinning under magic angle at 60 kHz by employing a variable flip angle compound inversion pulse. We validate the methods by measuring site-specific 15N-1H order parameters of a microcrystalline protein over a small temperature range and the same protein in a large, precipitated complex with antibody. The measurements of the order parameters in the complex are consistent with the observed protein undergoing overall motion within the assembly.

Sensitivity Enhancement by Progressive Saturation of the Proton Reservoir: A Solid-State NMR Analogue of Chemical Exchange Saturation Transfer.

Chemical exchange saturation transfer (CEST) enhances solution-state NMR signals of labile and otherwise invisible chemical sites, by indirectly detecting their signatures as a highly magnified saturation of an abundant resonance—for instance, the 1H resonance of water. Stimulated by this sensitivity magnification, this study presents PROgressive Saturation of the Proton Reservoir (PROSPR), a method for enhancing the NMR sensitivity of dilute heteronuclei in static solids. PROSPR aims at using these heteronuclei to progressively deplete the abundant 1H polarization found in most organic and several inorganic solids, and implements this 1H signal depletion in a manner that reflects the spectral intensities of the heteronuclei as a function of their chemical shifts or quadrupolar offsets. To achieve this, PROSPR uses a looped cross-polarization scheme that repeatedly depletes 1H–1H local dipolar order and then relays this saturation throughout the full 1H reservoir via spin-diffusion processes that act as analogues of chemical exchanges in the CEST experiment. Repeating this cross-polarization/spin-diffusion procedure multiple times results in an effective magnification of each heteronucleus’s response that, when repeated in a frequency-stepped fashion, indirectly maps their NMR spectrum as sizable attenuations of the abundant 1H NMR signal. Experimental PROSPR examples demonstrate that, in this fashion, faithful wideline NMR spectra can be obtained. These 1H-detected heteronuclear NMR spectra can have their sensitivity enhanced by orders of magnitude in comparison to optimized direct-detect experiments targeting unreceptive nuclei at low natural abundance, using modest hardware requirements and conventional NMR equipment at room temperature.

Influence of processing parameters on the ferroelectric-relaxor crossover in BNT-based piezoelectric ceramics

The effects of sintering time on the ferroelectric to relaxor crossover were systematically investigated for Sr(Hf0.5Zr0.5)O3-modified Bi0.5(Na0.8K0.2)TiO3 ceramics, prepared using the conventional solid-state mixed-oxide route. Scanning electron microscopy indicated a modest increase in grain size from 1.0 ± 0.2 to 2.0 ± 0.5 μm when the sintering time increased from 2 to 24 h. Furthermore, it was observed that the sintering time does not affect the long-range average crystal structure, as x-ray diffraction data suggest the presence of a single pseudocubic phase for all the samples, irrespective of the sintering time. Interestingly, ferroelectric and piezoelectric characterization showed evidence of a ferroelectric to relaxor transition when the sintering time increased from 2 to 6 h. This transition was marked by a sudden decrease in remanent polarization, a loss in negative strain along with a drastic increase in the maximum electromechanical strain. This was further exemplified in the unipolar strain data, which showed a transition from linear to non-linear dependence with electric field when the sintering time increased from 2 to 6 h. The piezoelectric properties were enhanced with further increase in sintering time up to 12 h, with the corresponding normalized strain value (Smax/Emax) d33∗=647pm/V. However, the d33∗ decreased with further increase in sintering time to 24 h. As the sintering time increased, temperature-dependent dielectric data show a decrease in the maximum permittivity along with the slight shift of the Tmax (temperature of maximum permittivity) to a higher temperature. In addition, results from impedance spectroscopy indicate that the DC resistivity increased by approximately two orders of magnitude when the sintering time increased from 2 to 12 h. These results suggest that while sintering time has a minimal impact on either the microstructure or the long-range average structure, it has a strong influence on the ferroelectric to relaxor crossover, which is often associated with enhanced electromechanical properties. This work presents further evidence that the crossover phenomenon is closely tied to the local structure, where disruption of the long-range dipole order results in stabilization of the relaxor state.

Relaxation of nuclear dipolar energy

Under typical conditions for dynamic nuclear polarization (DNP)—temperature about 1 K or below and magnetic field about 3 T or higher—the polarization agent causes nuclear dipolar order to relax up to four orders of magnitude faster than nuclear polarization. However, as far as we know, this ultra-fast dipolar relaxation has thus far not been explained in a satisfactory way.We report similar ultra-fast dipolar relaxation of proton spins in naphthalene due to the photo-excited triplet spin of pentacene and propose a three-step mechanism that explains such ultra-fast dipolar relaxation by ground state electron spins as well as by photo-excited triplet spins: nuclear spin diffusion transfers nuclear dipolar order—that is nuclear dipolar energy—spatially to near the electron spins. Flip-flop transitions between nuclear spins near the electron spins convert this dipolar energy into electron-nuclear interaction energy. Finally electron spin–lattice relaxation or decay of the triplet spin transfers the latter type of energy to the lattice. We will show that this mechanism quantitatively explains the observed dipolar relaxation rate.The proposed mechanism is expected to contribute to dipolar relaxation in any spin system containing more than one spin species. It tends to create a stationary state, in which all dipolar interactions are combined in a single energy reservoir described by a single spin temperature. As an example we suggest that the addition of a relaxation agent in samples used for DNP may significantly accelerate the relaxation of the dipolar energy of the polarization agent, and as a result could possibly reduce the contribution of thermal mixing (TM) to DNP.

Protonation tuned dipolar order mediated 1H→13C cross-polarization for dissolution-dynamic nuclear polarization experiments

A strategy of dipolar order mediated nuclear spin polarization transfer has recently been combined with dissolution-dynamic nuclear polarization (dDNP) and improved by employing optimized shaped radiofrequency pulses and suitable molecular modifications. In the context of dDNP experiments, this offers a promising means of transferring polarization from high-gamma 1H spins to insensitive 13C spins with lower peak power and lower energy compared with state-of-the-art cross-polarization schemes. The role of local molecular groups and the glassing matrix protonation level are both postulated to play a key role in the polarization transfer pathway via an intermediary reservoir of dipolar spin order. To gain appreciation of the mechanisms involved in the dipolar order mediated polarization transfer under dDNP conditions, we investigate herein the influence of the pivotal characteristics of the sample makeup: (i) revising the protonation level for the constituents of the DNP glass; and (ii) utilizing deuterated molecular derivatives. Experimental demonstrations are presented for the case of [1–13C]sodium acetate. We find that the proton sample molarity has a large impact on both the optimal parameters and the performance of the dipolar order mediated cross-polarization sequence, with the 13C signal build-up time drastically shortened in the case of high solvent protonation levels. In the case of a deuterated molecular derivative, we observe that the nearby 2H substituted methyl group is deleterious to the 1H→13C transfer phenomenon (particularly at low levels of sample protonation). Overall, increased solvent protonation makes the dipolar order governed polarization transfer significantly faster and more efficient. This study sheds light on the influential sample formulation traits which govern the dipolar order-controlled transfer of polarization and indicates that the polarization transfer efficiencies of deuterated molecules can be boosted and reach high performances simply by adequate solvent protonation.

Probing Polymer Chain Folding in Solution Using Second Harmonic Light Scattering.

Periodically grafted amphiphilic copolymers (PGACs) were earlier shown by us to adopt a zigzag folded conformation in the solid state, which enabled the backbone and pendant segments to segregate and occupy alternate layers in a lamellar structure. The conformational transition from a random coil to a zigzag folded chain in solution is an interesting problem, which is largely unexplored. To examine this, an orthogonally clickable parent polyester was sequentially clicked with two types of poly(ethylene glycol) (PEG) segments: one is a simple PEG and the other is a PEG that carries a dipolar chromophore. These two hydrophilic PEG segments, installed in a periodic and alternating fashion along the hydrocarbon-rich (HC) polyester backbone, ensure that the Janus folded chains are formed upon folding and carry chromophoric dipoles oriented along the same direction, thereby generating a large net dipole. The folding-induced alignment of chromophores in solution was followed using second harmonic light scattering (SHLS), wherein the intensity of the frequency-doubled scattered light (I2ω) is measured. Folding was induced by adding a polar solvent, like methanol, to a chloroform solution of the polymer; methanol desolvates the HC backbone but solubilizes the pendant PEG segments, thus inducing folding. The second harmonic intensity (I2ω) increased initially with methanol concentration and then saturated; in contrast, I2ω remained invariant with the solvent composition in the case of an analogous model chromophore. Furthermore, in a model PGAC carrying chromophore-bearing PEG segments on every repeat unit, I2ω decreased with increasing methanol composition, revealing the formation of a centrosymmetric folded chain, wherein the chromophoric dipoles on either side cancel each other. Thus, this study clearly reveals that the zigzag chain folding of PGACs can be induced by a segment-selective solvent, resulting in the rather elusive directional ordering of chromophoric dipoles in solution.

Magnonics vs. Ferronics

Magnons are the elementary excitations of the magnetic order that carry spin, momentum, and energy. Here we compare the magnon with the ferron, i.e. the elementary excitation of the electric dipolar order, that transports polarization, momentum, and energy in ferroelectrics.

Influence of Cation Size on the Local Atomic Structure and Electronic Properties of Ta Perovskite Oxynitrides.

Partial anion substitution in transition metal oxides provides rich opportunities to control and tune physical and chemical properties, for example, combining the merits of oxides and nitrides. In addition, the possibility of resulting anion sublattice order provides a means to target polar and chiral structures based on a wide array of accessible structural archetypes by design. Here, we investigate the local structures of a family of perovskite tantalum oxynitrides-ATaO2N (A = Ba, Sr, and Ca)-using a combination of experimental and theoretical approaches including neutron total scattering, density functional theory (DFT), and ab initio molecular dynamics (AIMD) simulations. We present the first experimental study of chemical short-range order (CSRO) in CaTaO2N, confirming local cis N ordering of the anion sub-lattice. Our systematic exploration of a local structure across the A cation size series (from the larger Ba to the smaller Ca) reveals a perovskite motif increasingly distorted with respect to long-range average structures. DFT and AIMD simulations support the observed trends. Overall, structures with cis ordering of the nitrogen anions in each TaO4N2 octahedron are favored over those with trans ordering. With diminishing A cation size, local cis ordering and Ta off-centering play decreasing roles in overall lattice stability, overshadowed by the stabilizing effects of octahedral tilting. The influence of these factors on local dipole formation and frustrated dipole ordering are discussed.

Boosting dissolution-dynamic nuclear polarization by multiple-step dipolar order mediated 1H→13C cross-polarization

Dissolution-dynamic nuclear polarization can be boosted by employing multiple-contact cross-polarization techniques to transfer polarization from 1H to 13C spins. The method is efficient and significantly reduces polarization build-up times, however, it involves high-power radiofrequency pulses in a superfluid helium environment which limit its implementation and applicability and prevent a significant scaling-up of the sample size. We propose to overcome this limitation by a stepwise transfer of polarization using a low-energy and low-peak power radiofrequency pulse sequence where the 1H→13C polarization transfer is mediated by a dipolar spin order reservoir. An experimental demonstration is presented for [1-13C]sodium acetate. A solid-state 13C polarization of ∼43.5% was achieved using this method with a build-up time constant of ∼5.1 minutes, leading to a ∼27.5% 13C polarization in the liquid-state after sample dissolution. The low-power multiple-step polarization transfer efficiency achieved with respect to the most advanced and highest-power multiple-contact cross-polarization approach was found to be ∼0.69.

Electromagnetic-Induced Transparency and Slow Light in Plasmonic Metasurfaces

We report numerically electromagnetic-induced transparency (EIT) and Fano resonances in simple plasmonic metasurfaces consisting of gold nanobars arranged in pi, H, and four-shaped fashion. The bright and dark elements in the metasurfaces are responsible for the emergence of EIT and Fano effects in the transmission spectrum. The concept of symmetry breaking is also introduced by incorporating multiple cavities in the metasurface, which relaxes the dipole coupling selection rules resulting in a mixture of dipole and higher order modes that interact and engenders EIT and Fano modes simultaneously in a nanostructure. Furthermore, the EIT and Fano resonances experience a significant red shift by increasing the refractive index of the background medium due to which high sensitivity of around 574 nmRIU−1, figure of merit of 32, and contrast ratio of 41% are realized. Moreover, the effective group index of the proposed metasurface is retrieved and is observed to be very high around the steep asymmetric Fano line shape and within the EIT window, signifying its potential use in slow light applications.

Extreme Ultraviolet Second Harmonic Generation Spectroscopy in a Polar Metal.

The coexistence of ferroelectricity and metallicity seems paradoxical, since the itinerant electrons in metals should screen the long-range dipole interactions necessary for dipole ordering. The recent discovery of the polar metal LiOsO3 was therefore surprising [as discussed earlier in Y. Shi et al., Nat. Mater. 2013, 12, 1024]. It is thought that the coordination preferences of the Li play a key role in stabilizing the LiOsO3 polar metal phase, but an investigation from the combined viewpoints of core-state specificity and symmetry has yet to be done. Here, we apply the novel technique of extreme ultraviolet second harmonic generation (XUV-SHG) and find a sensitivity to the broken inversion symmetry in the polar metal phase of LiOsO3 with an enhanced feature above the Li K-edge that reflects the degree of Li atom displacement as corroborated by density functional theory calculations. These results pave the way for time-resolved probing of symmetry-breaking structural phase transitions on femtosecond time scales with element specificity.

Mechanical and Electrical Interaction of Biological Membranes with Nanoparticles and Nanostructured Surfaces.

In this review paper, we theoretically explain the origin of electrostatic interactions between lipid bilayers and charged solid surfaces using a statistical mechanics approach, where the orientational degree of freedom of lipid head groups and the orientational ordering of the water dipoles are considered. Within the modified Langevin Poisson–Boltzmann model of an electric double layer, we derived an analytical expression for the osmotic pressure between the planar zwitterionic lipid bilayer and charged solid planar surface. We also show that the electrostatic interaction between the zwitterionic lipid head groups of the proximal leaflet and the negatively charged solid surface is accompanied with a more perpendicular average orientation of the lipid head-groups. We further highlight the important role of the surfaces’ nanostructured topography in their interactions with biological material. As an example of nanostructured surfaces, we describe the synthesis of TiO2 nanotubular and octahedral surfaces by using the electrochemical anodization method and hydrothermal method, respectively. The physical and chemical properties of these nanostructured surfaces are described in order to elucidate the influence of the surface topography and other physical properties on the behavior of human cells adhered to TiO2 nanostructured surfaces. In the last part of the paper, we theoretically explain the interplay of elastic and adhesive contributions to the adsorption of lipid vesicles on the solid surfaces. We show the numerically predicted shapes of adhered lipid vesicles corresponding to the minimum of the membrane free energy to describe the influence of the vesicle size, bending modulus, and adhesion strength on the adhesion of lipid vesicles on solid charged surfaces.

Magnetic order and 5d1 multipoles in a rhenate double perovskite Ba2MgReO6

Structural and magnetic transitions in a double perovskite hosting 5d1 Re ions are discussed on the basis of recently published high-resolution x-ray diffraction patterns [D. Hirai, et al., Phys. Rev. Res. 2, 022063(R) (2020)]. A reported structural transition below room temperature, from cubic to tetragonal symmetry, appears not to be driven by T2g-type quadrupoles, as suggested. A magnetic motif at lower temperature is shown to be composed of two order parameters, associated with propagation vectors k = (0, 0, 1) and k = (0, 0, 0). Findings from our studies, for structural and magnetic properties of Ba2MgReO6, surface in predicted amplitudes for x-ray diffraction at rhenium L2 and L3 absorption edges, and magnetic neutron Bragg diffraction. Specifically, entanglement of anapole and spatial degrees of freedom creates a quadrupole in the neutron scattering amplitude. It would be excluded in an unexpected scenario whereby the rhenium atomic state is a manifold. Also, a chiral signature visible in resonant x-ray diffraction will be one consequence of predicted electronic quadrupole and magnetic dipole orders. A model Re wave function consistent with all current knowledge is a guide to electronic and magnetic multipoles engaged in x-ray and neutron diffraction investigations.

Dipole ordering of water molecules in cordierite: Monte Carlo simulations

Electric dipoles of water molecules, enclosed singly in regularly spaced nanopores of a cordierite crystal, become ordered at low temperature due to their mutual interaction and show the frequency dependence of their dielectric susceptibility, typical for relaxor ferroelectrics, according to recent experimental data. The corresponding phase transition is accompanied by anomalies in thermodynamic quantities, such as heat capacity and dielectric susceptibility, which are calculated here using the Monte Carlo method, and their agreement with the experimental data is discussed. Despite the increase in the correlation length, the partially filled dipole lattice at low temperatures, according to the calculations, does not have long-range order and corresponds to a dipole glass. This simulation gives a microscopical insight into the formation of polar nanoregions in relaxor ferroelectrics and the temperature dependence of their size.

A pectin methyltransferase modulates polysaccharide dynamics and interactions in Arabidopsis primary cell walls: Evidence from solid-state NMR

Plant cell walls contain cellulose embedded in matrix polysaccharides. Understanding carbohydrate structures and interactions is critical to the production of biofuel and biomaterials using these natural resources. Here we present a solid-state NMR study of cellulose and pectin in 13C-labeled cell walls of Arabidopsis wild-type and mutant plants. Using 1D 13C and 2D 13C–13C correlation experiments, we detected a highly branched arabinan structure in qua2 and tsd2 samples, two allelic mutants for a pectin methyltransferase. Both mutants show close physical association between cellulose and the backbones of pectic homogalacturonan and rhamnogalacturonan-I. Relaxation and dipolar order parameters revealed enhanced microsecond dynamics due to polymer disorder in the mutants, but restricted motional amplitudes due to tighter pectin-cellulose associations. These molecular data shed light on polymer structure and packing in these two pectin mutants, helping to elucidate how pectin could influence cell wall architecture at the nanoscale, cell wall mechanics, and plant growth.

How Photoswitchable Lipids Affect the Order and Dynamics of Lipid Bilayers and Embedded Proteins.

Altering the properties of phospholipid membranes by light is an attractive option for the noninvasive manipulation of membrane proteins and cellular functions. Lipids with an azobenzene group within their acyl chains such as AzoPC are suitable tools for manipulating lipid order and dynamics through a light-induced trans-to-cis isomerization. However, the action of these photoswitchable lipids at the atomic level is still poorly understood. Here, liposomes containing AzoPC, POPE, and POPG have been characterized by solid-state NMR through chemical shift and dipolar CH order parameter measurements. Upon UV-light illumination, an efficient trans-to-cis conversion can be achieved resulting in a localized reduction of the CH order parameter within the bulk lipid acyl chains. This effect is even more pronounced in liposomes containing the integral membrane protein E.coli diacylglycerol kinase. The protein responds to the light-induced trans-to-cis isomerization by a site-specific increase in the molecular dynamics as observed by altered cross peak intensities in NCA spectra. This study represents a proof-of-concept demonstration for the use of photoswitchable lipids to modulate membrane properties by light for inducing dynamic changes within an embedded membrane protein.

First Observation of Superlattice Reflections in the Hidden Order at 105 K of Spin–Orbit Coupled Iridium Oxide Ca5Ir3O12

We report the inelastic X-ray scattering (IXS) experimental results of iridium oxide Ca5Ir3O12 with a strong spin–orbit interaction, showing the hidden order at 105 K where no superlattice reflections were observed so far. We measured the IXS spectra of Ca5Ir3O12 along Γ–A, Γ–M, Γ–K–M, M–L, and K–H directions in the Brillouin zone of a hexagonal lattice down to 20 K. The obtained phonon spectra show almost no change on cooling; there are no soft phonon modes. However, the superlattice reflections specified by wavevector \(\textbf{q} = (1/3,1/3,1/3)\) are observed below 105 K. For the order parameter in the hidden order, the characteristic on intensity for observed superlattice reflections can lead to the irreducible representation A2 order parameter in the point group 31m. Furthermore, the theoretical study indicates that the hidden order at 105 K comes from an electric toroidal dipole or higher-order multipole ordering.

Quantum phases of a weakly disordered Josephson ladder

The interplay of interactions and disorder in low-dimensional superconductors supports the formation of multiple quantum phases, as possible instabilities of the Superconductor-Insulator Transition (SIT) at a singular quantum critical point. We explore a one-dimensional model which exhibits such variety of phases in the strongly quantum fluctuations regime. Specifically, we study the effect of weak disorder on a two-leg Josephson ladder with comparable Josephson and charging energies ($E_J$~$E_C$). An additional key feature of our model is the requirement of perfect $\mathbb{Z}_2$-symmetry, respected by all parameters including the disorder. Using a perturbative renormalization-group (RG) analysis, we derive the phase diagram and identify at least one intermediate phase between a full-fledged superconductor and a disorder-dominated insulator. Most prominently, for repulsive interactions on the rungs we identify two distinct mixed phases: in both of them the longitudinal charge mode is a gapless superconductor, however one phase exhibits a dipolar charge density order on the rungs, while the other is disordered. This latter phase is characterized by coexisting superconducting (phase-locked) and charge-ordered rungs, and encompasses the potential of evolving into a Griffith's phase characteristic of the random-field Ising model in the strong disorder limit.

Thermoelastic wave and thermal shock based on dipolar gradient elasticity and fractional-order generalized thermoelasticity

In the current work, the thermoelastic wave propagation and the thermal shock problems are studied based on the dipolar gradient elasticity and fractional order generalized thermoelasticity in order to take into account the microstructure effect and the thermal effect at a small scale. First, the fractional-order governing equations for coupled thermoelastic wave propagation are derived and the effects of microstructure parameters and fractional order on the dispersion and the attenuation are discussed; next, the transient problems by the thermal shock and the mechanical shock on the surface of half-space are studied. The Laplace transformation method and the state transfer equation method are used to solve this problem. The numerical results for the temperature, the displacement, the stress and the high order stress distributions are provided and shown graphically. The effects of the fractional order and the microstructure parameter on the thermoelastic behavior are investigated. Some interesting phenomenons and conclusions are presented at last.

Molecular dipoles in designer honeycomb lattices

Recent advances in ultracold atoms in optical lattices and developments in surface science have allowed for the creation of artificial lattices as well as the control of many-body interactions. Such systems provide new settings to investigate interaction-driven instabilities and non-trivial topology. In this paper, we explore the interplay between molecular electric dipoles on a two-dimensional triangular lattice with fermions hopping on the dual decorated honeycomb lattice which hosts Dirac and flat band states. We show that short-range dipole-dipole interaction can lead to ordering into various stripe and vortex crystal ground states. We study these ordered states and their thermal transitions as a function of the interaction range using simulated annealing and Monte Carlo methods. For the special case of zero wave vector ferrodipolar order, incorporating dipole-electron interactions and integrating out the electrons leads to a six-state clock model for the dipole ordering. Finally, we discuss the impact of the various dipole orders on the electronic band structure and the local tunneling density of states. Our work may be relevant to studies of "molecular graphene" -- CO molecules arranged on the Cu(111) surface -- which have been explored using scanning tunneling spectroscopy, as well as ultracold molecule-fermion mixtures in optical lattices.