Dipolar Order Research Articles

Sensitivity Enhancement of Ultra-Wideline NMR by Progressive Saturation of the Proton Reservoir Under Magic-Angle Spinning.

Solid-state NMR of low-γ nuclides is often characterized by low sensitivity and by significant spectral broadenings induced by the quadrupolar and the chemical-shift anisotropy interactions. Herein, we introduce an indirect acquisition method, termed PROgressive Saturation of the Proton Reservoir Under Spinning (PROSPRUS), which could facilitate the acquisition of ultra-wideline NMR spectra under magic-angle spinning (MAS), in systems with a sufficiently long dipolar relaxation time, T1D. PROSPRUS NMR relies on the generation of so-called second-order dipolar order among abundant protons undergoing MAS, and on the subsequent depletion of this dipolar order by a series of looped cross-polarization events, transferring the proton order into polarization of the low-γ I-nuclei as a function of the latter's offsets. While the spin dynamics of the ensuing experiment is complex, particularly when dealing with narrow I spectral lines, it is shown that PROSPRUS can lead to faithful lineshapes for ultra-wideline spin-1/2 and spin-1 species, providing high sensitivity with extremely low RF power requirements. It is also shown that the ensuing 1H-detected PROSPRUS experiments can efficiently characterize I-spin lineshapes in excess of 1 MHz without having to retune electronics, while providing improvements in sensitivity per unit time over current broadband direct-detection methods by up to a factor of four.

Incommensurate charge super-modulation and hidden dipole order in layered kitaev material α-RuCl3

The magnetism of Kitaev materials has been widely studied, but their charge properties and the coupling to other degrees of freedom are less known. Here we investigate the charge states of α-RuCl3, a promising Kitaev quantum spin liquid candidate, in proximity to graphite. We discover that few-layered α-RuCl3 experiences a clear modulation of charge states, where a Mott-insulator to weak charge-transfer-insulator transition in the 2D limit occurs by means of heterointerfacial polarization. More notably, distinct signals of incommensurate charge and lattice super-modulations, regarded as an unconventional charge order, accompanied in the insulator. Our theoretical calculations have reproduced the incommensurate charge order by taking into account the antiferroelectricity of α-RuCl3 that is driven by dipole order in the internal electric fields. The findings imply that there is strong coupling between the charge, spin, and lattice degrees of freedom in layered α-RuCl3 in the heterostructure, which offers an opportunity to electrically access and tune its magnetic interactions inside the Kitaev compounds.

Observation of Antiferroelectric Domain Walls in a Uniaxial Hyperferroelectric.

Ferroelectric domain walls are a rich source of emergent electronic properties and unusual polar order. Recent studies show that the configuration of ferroelectric walls can go well beyond the conventional Ising-type structure. Néel-, Bloch-, and vortex-like polar patterns have been observed, displaying strong similarities with the spin textures at magnetic domain walls. Here, the discovery of antiferroelectric domain walls in the uniaxial ferroelectric Pb5Ge3O11 is reported. Highly mobile domain walls with an alternating displacement of Pb atoms are resolved, resulting in a cyclic 180° flip of dipole direction within the wall. Density functional theory calculations show that Pb5Ge3O11 is hyperferroelectric, allowing the system to overcome the depolarization fields that usually suppress the antiparallel ordering of dipoles along the longitudinal direction. Interestingly, the antiferroelectric walls observed under the electron beam are energetically more costly than basic head-to-head or tail-to-tail walls. The results suggest a new type of excited domain-wall state, expanding previous studies on ferroelectric domain walls into the realm of antiferroic phenomena.

Giant chiral magnetoelectric oscillations in a van der Waals multiferroic

Helical spin structures are expressions of magnetically induced chirality, entangling the dipolar and magnetic orders in materials1–4. The recent discovery of helical van der Waals multiferroics down to the ultrathin limit raises prospects of large chiral magnetoelectric correlations in two dimensions5,6. However, the exact nature and magnitude of these couplings have remained unknown so far. Here we perform a precision measurement of the dynamical magnetoelectric coupling for an enantiopure domain in an exfoliated van der Waals multiferroic. We evaluate this interaction in resonance with a collective electromagnon mode, capturing the impact of its oscillations on the dipolar and magnetic orders of the material with a suite of ultrafast optical probes. Our data show a giant natural optical activity at terahertz frequencies, characterized by quadrature modulations between the electric polarization and magnetization components. First-principles calculations further show that these chiral couplings originate from the synergy between the non-collinear spin texture and relativistic spin–orbit interactions, resulting in substantial enhancements over lattice-mediated effects. Our findings highlight the potential for intertwined orders to enable unique functionalities in the two-dimensional limit and pave the way for the development of van der Waals magnetoelectric devices operating at terahertz speeds.

Anisotropic relaxation of nuclear spins dipolar energy of water molecules in two-dimensional nanopores - A single crystal NMR study

Energy transfer from Zeeman to dipolar order discovered by Jeener et al. is usually observed in solids with a strong dipole-dipole interaction of nuclear spins. It is not observed in liquids, where fast molecular motion completely averages this interaction. The intermediate case, when the dipole-dipole interaction of nuclear spins is only partially averaged, has been poorly studied. We report on the first measurement of an angular-dependent proton spin relaxation of a dipolar reservoir in mobile water molecules confined in the interlayer pores of a vermiculite single crystal. In this layered crystal, the intramolecular dipole-dipole interactions of nuclear spins are only partially averaged due to the restricted anisotropic molecular motion in nanopores. We show that this allows the formation of dipolar echo. We measured the spin-lattice relaxation times of the dipolar order T1D at different angles between the normal to the crystal surface and the applied magnetic field and obtained a distinct angular dependence of T1D. The minimum relaxation rate R1D was found around the magic angle of 54.74°.

Dipolar Order Controls Dielectric Response of Glass-Forming Liquids.

The dielectric response of liquids reflects both reorientation of single molecular dipoles and collective modes, i.e., dipolar cross-correlations. A recent theory predicts the latter to produce an additional slow peak in the dielectric loss spectrum. Following this idea we argue that in supercooled liquids the high-frequency power law exponent of the dielectric loss β should be correlated with the degree of dipolar order, i.e., the Kirkwood correlation factor g_{K}. This notion is confirmed for 25 supercooled liquids. While our findings support recent theoretical work the results are shown to violate the earlier Kivelson-Madden theory.

Dipolar Order Induced Electron Spin Hyperpolarization.

The structure of coupled electron spin systems is of fundamental interest to many applications, including dynamic nuclear polarization (DNP), enhanced nuclear magnetic resonance (NMR), the generation of electron spin qubits for quantum information science (QIS), and quantitative studies of paramagnetic systems by electron paramagnetic resonance (EPR). However, the characterization of electron spin coupling networks is nontrivial, especially at high magnetic fields. This study focuses on a system containing high concentrations of trityl radicals that give rise to a DNP enhancement profile of 1H NMR characteristic of the presence of electron spin clusters. When this system is subject to selective microwave saturation through pump-probe ELectron DOuble Resonance (ELDOR) experiments, electron spin hyperpolarization is observed. We show that the generation of an out-of-equilibrium longitudinal dipolar order is responsible for the transient hyperpolarization of electron spins. Notably, the coupled electron spin system needs to form an AX-like system (where the difference in the Zeeman interactions of two spins is larger than their coupling interaction) such that selective microwave irradiation can generate signatures of electron spin hyperpolarization. We show that the extent of dipolar order, as manifested in the extent of electron spin hyperpolarization generated, can be altered by tuning the pump or probe pulse length, or the interpulse delay in ELDOR experiments that change the efficiency to generate or readout longitudinal dipolar order. Pump-probe ELDOR with selective saturation is an effective means for characterizing coupled electron spins forming AX-type spin systems that are foundational for DNP and quantum sensing.

Fast collective motions of backbone in transmembrane α helices are critical to water transfer of aquaporin.

Fast collective motions are widely present in biomolecules, but their functional relevance remains unclear. Herein, we reveal that fast collective motions of backbone are critical to the water transfer of aquaporin Z (AqpZ) by using solid-state nuclear magnetic resonance (ssNMR) spectroscopy and molecular dynamics (MD) simulations. A total of 212 residue site-specific dipolar order parameters and 158 15N spin relaxation rates of the backbone are measured by combining the 13C- and 1H-detected multidimensional ssNMR spectra. Analysis of these experimental data by theoretic models suggests that the small-amplitude (~10°) collective motions of the transmembrane α helices on the nanosecond-to-microsecond timescales are dominant for the dynamics of AqpZ. The MD simulations demonstrate that these collective motions are critical to the water transfer efficiency of AqpZ by facilitating the opening of the channel and accelerating the water-residue hydrogen bonds renewing in the selectivity filter region.

Relaxor ferroelectricity and elevated thermal expansion behaviour of (1−x)Bi0.5Na0.5TiO3-xBaSnO3 lead-free piezoelectric ceramics

The study on piezoelectric and ferroelectric properties of sodium bismuth titanate-based materials is essential for piezoelectric electronic components. In this study, a series of lead-free relaxor ferroelectric (1−x)Bi0.5Na0.5TiO3-xBaSnO3 (BNT-xBS) ceramics were prepared by a conventional solid-state reaction method. The phase performance, fracture morphologies, permittivity, ferroelectricity, piezoelectricity, and thermal expansion for various BS contents (x) were systematically studied. With an increase in x, the phase transition from R3c to P4bm, and the grain size marginally decreased from 1.79 to 1.51 μm. Meanwhile, the degree of dielectric relaxor and diffused phase transition improved, and the ferroelectricity and piezoelectricity initially increased and then decreased. The optimal electrical properties of Pr = 41.6 μC/cm2, d33 = 167.8 pC/N, and kp = 0.318 were observed at x = 0.06. The paper discusses how site disorder, long-range electric dipole ordering, and grain size influence various electrical properties. The mechanism of the thermal expansion behaviour was investigated based on Born-Lande theory. This study could serve as a landmark for studying multifunctional BNT-based piezoelectric materials for potential energy-harvesting devices.

Field-Induced Antiferroelectric-Ferroelectric Transformation in Organometallic Perovskite Displaying Giant Negative Electrocaloric Effect.

Antiferroelectric materials with an electrocaloric effect (ECE) have been developed as promising candidates for solid-state refrigeration. Despite the great advances in positive ECE, reports on negative ECE remain quite scarce because of its elusive physical mechanism. Here, a giant negative ECE (maximum ΔS ∼ -33.3 J kg-1 K-1 with ΔT ∼ -11.7 K) is demonstrated near room temperature in organometallic perovskite, iBA2EA2Pb3I10 (1, where iBA = isobutylammonium and EA = ethylammonium), which is comparable to the greatest ECE effects reported so far. Moreover, the ECE efficiency ΔS/ΔE (∼1.85 J cm kg-1 K-1 kV-1) and ΔT/ΔE (∼0.65 K cm kV-1) are almost 2 orders of magnitude higher than those of classical inorganic ceramic ferroelectrics and organic polymers, such as BaTiO3, SrBi2Ta2O9, Hf1/2Zr1/2O2, and P(VDF-TrFE). As far as we know, this is the first report on negative ECE in organometallic hybrid perovskite ferroelectric. Our experimental measurement combined with the first-principles calculations reveals that electric field-induced antipolar to polar structural transformation results in a large change in dipolar ordering (from 6.5 to 45 μC/cm2 under the ΔE of 18 kV/cm) that is closely related to the entropy change, which plays a key role in generating such giant negative ECE. This discovery of field-induced negative ECE is unprecedented in organometallic perovskite, which sheds light on the exploration of next-generation refrigeration devices with high cooling efficiency.

Exploring low-temperature dynamics in triple perovskite ruthenates using nonlinear dielectric susceptibility measurements

We report the nonlinear dielectric properties of three triple-perovskite ruthenates: Ba3CoRu2O9, Ba3BiRu2O9, and Sr3CaRu2O9. These compounds exhibit notable correlations among their spin, charge, lattice, and polar degrees of freedom. Ba3CoRu2O9 displays a pronounced frequency-dependent relaxation in χ2,3 just above the magnetoelastic transition, occurring around 100 K, followed by an abrupt loss of polarization within the ordered phase. In Ba3BiRu2O9, we encounter the possibility of multiple coexisting relaxation behaviors, indicating a complex phase strongly influenced by the spin-gap opening at 175 K. Lastly, Sr3CaRu2O9 displays anomalies with strong dispersion effects close to its magnetic transitions, indicating a robust coupling between magnetic and dipolar orders in the system. These measurements highlight the significance of higher-order (hyper-)susceptibilities in providing profound insight into the dynamics of a system, offering information otherwise inaccessible through the linear polarization response alone.

Numerical extraction of three-dimensional acoustic polarizabilities for acoustically small scatterers

Polarizability is a convenient descriptor for scattering from low-ka inhomogeneities in both electromagnetism, where it originated, and in acoustics, where it is increasingly used in the design of metamaterial elements. In two or three dimensions, the polarizability is represented by a block matrix that couples the local pressure and particle velocity fields to the lowest-order components of the multipole expansion, usually truncated to dipole order. A fully dense acoustic polarizability matrix has components that couple spatially uniform pressure and velocity oscillations to dipole and monopole scattering, respectively. This unique coupling leads to acoustic bianisotropy, also known as Willis coupling, for a collection of scatterers. The design of Willis materials necessitates computationally efficient methods to extract all components of the polarizability matrix and, at present, only two-dimensional analytical extraction methods are found in the literature. In this work, we present an algorithm to extract all components of the three-dimensional polarizability matrix from a scatterer with arbitrary geometry and composition. The method presented here is numerically efficient and yields results that are in agreement with an analytically obtained benchmark polarizability, providing a useful tool for acoustic metamaterial design.

Emergence of Improper Electronic Ferroelectricity and Flat Band in Twisted Bilayer Tl2S.

Two-dimensional (2D) ferroelectrics possessing out-of-plane (OP) polarization are highly desirable for applications and fundamental physics. Here, by first-principles calculations, we reveal that large-angle interlayer twisting can efficiently stabilize an unexpected ordering of sizable electric dipoles, producing OP polarization out of the centrosymmetric ground-state structure of Tl2S, in great contrast to the recently proposed interlayer-sliding ferroelectricity. The ferroelectricity originates from a nonlinear coupling between a polar order dominantly contributed by electrons and an unstable phonon mode associated with a commensurate k point (1/3, 1/3, 0) in the two constituent monolayers, therefore indicating an improper and electronic ferroelectric nature. More interestingly, a flat band and a van Hove singularity occur in its electronic structures just below the Fermi level in the large-angle twisted bilayer Tl2S. The unusual coexistence of improper electronic ferroelectricity, a flat band, and a van Hove singularity in one 2D material offers exceptional opportunities for exploring novel physics and applications.

Ordering/displacive ferrielectricity in 2D CuInP2S6

Using DFT-based molecular dynamics simulation of Cu+ cations flipping and In3+ cations displacive dynamics, we clarify the dipole ordering of CuInP2S6 ferrielectrics through the second order Jahn-Teller effect which determines the double-well potential for copper cations as well as three-well potential for indium cations inside the structural layers. The temperature dependence of the spatial distribution of Cu+ and In3+ cations is also described within the quantum anharmonic oscillator model. In3+ cations play a decisive role in the character of the polar ordering: at ambient or for positive pressures, the first order transition occurs; with the rise of negative compression, the ferrielectric transition evolves to the second order. The peculiarities of spontaneous polarization switching are related to the contributions of copper and indium cationic sublattices.

A Quantum Chemical Study on the Enormously Large Nonlinear Optical Response of Doped Endohedral Superalkali Na2F and Superhalogens MF4 (M = B, Al) with CNT (C56H16) Cage

AbstractIn the present communication, the interaction of single‐wall carbon nanotubes (SWCNTs) CNT (C56H16) with endohedral doped superhalogens (MF4:M = B, Al) placed outside of CNT cage and superalkali (Na2F) placed inside the cage has been studied by using a combination of DFT/B3LYP method and 6–311G(d, p) basis set. The geometry and stability of MF4@CNT‐Li2F have been studied using optimization parameters, highest occupied molecular orbit (HOMO), lowest unoccupied molecular orbital (LUMO), vibration frequencies, and thermodynamic parameters of absorption reactions. The quantum theory in atoms in molecules (QTAIM) analysis is used to analyze the nature of interactions between MF4 (M = B, Al) and Na2F@CNT. Several electronic parameters are computed by using HOMO–LUMO energy. The dipole moment, polarizability, hyperpolarizability, order parameters, anisotropic polarizability, and molar refractivity of MF4@CNT‐Li2F (M = Al, B) are used to calculate the nonlinear optical parameters (MR). The NBO analysis is used to calculate the transfer of charge to stabilized system. The calculated hyperpolarizability of Na2F@CNT‐BF4/Na2F@CNT‐AlF4 is nearly 81 times that of reference material urea (40 a.u.). The intramolecular charge transfer (ICT) moment of π electron cloud is responsible for the nonlinear optical behavior of the system.

Superacidic Properties of Protonated PtFn (n = 1–6) and Their Ability to Form Supersalts: DFT Study

AbstractIn the present communication, this study has modeled protonated PtFn (n = 1–6), a new series of superacids. The calculated vertical de‐attachment energy (VDE) of PtFn (n = 1–6) reestablishes the superhalogenic behavior. The vibrational frequency, dissociation energy through the HF channel, and optimized geometries are utilized to discuss the stability of HPtFn (n = 1–6). The deprotonation energy of HPtFn is used to calculate the acidic strength of HPtFn (n = 1–6). The acidity of protonated PtFn (n = 1–6) is directly related to its counterpart superhalogen anions. The computed correlation factor (R2 = 0.9754) shows that VDE is directly associated with acidity. The mechanism of acidity of HPtFn (n = 1–6) is described by using natural bond orbital analysis charges on atoms. The electronic properties of HPtFn (n = 1–6) are calculated using the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy. The nature of chemical reactivity is also determined by the HOMO–LUMO plot of HPtFn (n = 1–6). It has formed the supersalts using superacids HPtFn (n = 1–6) and superbase (OLi3OH), and the nature of supersalts is compared with traditional salt LiF. The most stable conformers of supersalts PtFnOLi3 (n = 2–6) are analyzed for optoelectronic properties. It has computed the dissociation of supersalts through neutral and anions through superhalogns and superalkalis. The computed results show that PtF4OLi3 favors dissociation through anions as in traditional salts. The nonlinear optical parameters are calculated using dipole moment, mean polarizability, anisotropic polarizability, molar reflectivity (MR), order parameters, and hyperpolarizability of PtFnOLi3 (n = 2–4). The estimated nonlinear optical properties parameters are compared with corresponding parameters of traditional salt LiF by the same level theory.

Unsolved problem of long-range interactions: dipolar spin-ice study

Long-range interactions derive various strange phenomena. As illustrated by cutoff simulations of water, increasing cutoff length does not improve the simulation result necessarily; on the contrary, it makes the result worse. In the extreme situation, the structure of water transforms into a layer structure. In this study, to explore the underlying mechanism of this phenomenon, we performed Monte Carlo simulations on dipolar spins arranged on a pyrochlore spin-ice lattice. Like the water case, the present dipolar spin system also showed cutoff-induced dipole ordering and layer formation. The width of the layers depended on the cutoff length; and longer cutoff length led to a broader layer. These features are certainly consistent with the previous water case. This indicates that layer formation is the general behavior of dipolar systems whose interactions are truncated within a finite distance. The result is important for future exploration of the relationship between long-range interactions and resulting structures. In addition, it emphasizes the necessity of rigorous treatment of long-range interactions because increasing the cutoff length prevents convergence and provides an entirely different result from the rigorous Ewald calculation.

Quadruple-well ferroelectricity and topological domain structures in strained Bi2O2Se

We demonstrate that biaxially strained Bi2O2Se holds unusual quadruple-well dipole orders, a two-step 90° ferroelectric switching and various types of spontaneous topological domain structures.

Cross polarization from dipolar-order under magic angle spinning: The ADRF-CPMAS NMR experiment

Techniques for enhancing the signals arising from low-γ, insensitive (I) nuclei are central to solid-state nuclear magnetic resonance. One of the leading and best-established methods to sensitize these unreceptive species is Hartmann–Hahn cross polarization (HH-CP), a polarization transfer mechanism often executed under MAS. Herein, we explore the possibility of utilizing the 1H dipolar order created via adiabatic demagnetization in the rotating frame (ADRF), to enhance the unreceptive spins under MAS. It is found that an efficient polarization transfer via ADRF-CPMAS is not only possible but can exceed, at least in some instances involving plastic crystals, the efficiency of an optimized HH-CPMAS transfer. The experiment requires low radiofrequency nutation fields on both the 1H- and the I-spin channels, and displays unusual matching conditions that are reminiscent of the zero- and double-quantum matching conditions arising under CPMAS, albeit centered at zero frequency and demanding the simultaneous involvement of several spins. The origin of these multi-spin transfer processes is analytically derived and numerically simulated in predictions that compare well with experimental 13C and 15N results collected on model compounds at different spinning speeds. These derivations start from descriptions that depart from traditional thermodynamic arguments, and treat instead the ADRF processes in static and spinning solids on the basis of coherent evolutions. The predictions of these analytical derivations are corroborated by numerical simulations. The effects of additional factors, including chemical shift anisotropies, J-couplings, and radiofrequency inhomogeneities, are also theoretically and experimentally explored.

Effects of the substituent position on the structural order, work function change, and thermopower of dichloro-substituted benzenethiolate self-assembled monolayers on Au(1 1 1)

The surface structures, electrochemical behaviors, work function changes, and thermopower of dichloro-substituted benzenethiolate self-assembled monolayers (SAMs) on gold surfaces prepared by vapor deposition were investigated using scanning tunneling microscopy (STM), cyclic voltammetry, Kelvin probe force microscopy, and thermoelectric junction measurements to understand the effects of their substituent positions. STM observations revealed that the adsorption of 2,4-dichlorobenzenethiol (2,4-DCBT) on Au(111) at 363 K for 1 h led to the formation of short-range ordered domains with a (5 × 6√7)R20° structure, while the SAMs formed at 363 K for 4 h consisted of long-range well-ordered domains with a (√3 × 5) structure. 3,4-Dichlorobenzenethiol (3,4-DCBT) SAMs showed coexistence of partial ordered domains and a disordered phase, while 3,4-DCBT SAMs showed fully disordered phases regardless of deposition time. Moreover, the Seebeck coefficient (S) increased in the order of 3,4-DCBT (4.6 ± 0.1 μV/K) < 2,4-DCBT (5.0 ± 0.4 μV/K) < 3,5-DCBT (5.2 ± 0.1 μV/K). Interestingly, the trend of S values was qualitatively consistent with that of the work function change (ΔΦ) of the SAMs. The reductive desorption behavior, work function shifts, and thermopower of aromatic thiolate SAMs were significantly modified by the structural order and direction of molecular dipoles of dichloro-substituted benzenethiols.