Dipolar Order Research Articles

Modeling magnetic multipolar phases in density functional theory

Multipolar magnetic phases in correlated insulators represent a great challenge for density functional theory (DFT) due to the coexistence of intermingled interactions, typically spin-orbit coupling, crystal field and complex noncollinear and high-rank intersite exchange, creating a complected configurational space with multiple minima. Although the $+\mathrm{U}$ correction to DFT allows, in principle, the modeling of such magnetic ground states, its results strongly depend on the initially symmetry breaking, constraining the nature of order parameter in the converged $\mathrm{DFT}+\mathrm{U}$ solution. As a rule, $\mathrm{DFT}+\mathrm{U}$ calculations starting from a set of initial on-site magnetic moments result in a conventional dipolar order. A more sophisticated approach is clearly needed in the case of magnetic multipolar ordering, which is revealed by a null integral of the magnetization density over spheres centered on magnetic atoms, but with nonzero local contributions. Here we show how such phases can be efficiently captured using an educated constrained initialization of the on-site density matrix, which is derived from the multipolar-ordered ground state of an ab initio effective Hamiltonian. Various properties of such exotic ground states, like their one-electron spectra, become therefore accessible by all-electron $\mathrm{DFT}+\mathrm{U}$ methods. We assess the reliability of this procedure on the ferro-octupolar ground state recently predicted in ${\mathrm{Ba}}_{2}M{\mathrm{OsO}}_{6}\phantom{\rule{0.28em}{0ex}}(\mathrm{M}=\mathrm{Ca},\mathrm{Mg},\mathrm{Zn})$ [Phys. Rev. Lett. 127, 237201 (2021)].

Antisymmetric thermopolarization by electric toroidicity

We investigate electric polarizations that emerge perpendicular to an applied thermal gradient in insulating systems. The thermally induced electric polarization, known as thermopolarization, has been studied conventionally in the case where an electric polarization appears along the thermal gradient. Here, we focus on the antisymmetric component of the thermopolarization tensor, and we reveal that it becomes nonzero due to the ferrotype order for electric-toroidal dipole moments. To describe local electric polarizations originating from the disproportionation of localized electronic clouds, we introduce a two-dimensional three-orbital model with localized $s$ and two $p$ orbitals, where the electric polarization at each site interacts with the neighboring one as dipole-dipole interactions. We find that a vortex-type configuration of local electric polarizations appears as a mean-field ground state, corresponding to a ferrotype electric-toroidal dipole order. By taking account of collective modes from this ordered state, we calculate the coefficient of the thermopolarization based on the linear-response theory. The antisymmetric component is nonzero in the presence of the electric-toroidal dipole order. We clarify that fluctuations in the $p$ orbitals are crucial in enhancing the antisymmetric thermopolarization. We discuss the appearance conditions based on the symmetry argument and the relevance to real materials.

Improper multiferroiclike transition in a metal

Magnetic frustration in insulators often induces cooperative atomic displacements to form a polar structure with long-range ordered electric dipoles, commonly known as improper multiferroics. In metals, such a multiferroic phenomenon is not expected to occur because free carriers tend to screen internal electrostatic forces, eliminating long-range ordered electric dipoles. To date, intrinsic multiferroiclike metals have rarely been reported. Here we present a spin-frustration-induced long-range antiparallel ordering of electric dipoles in metallic CrN with an unusual antiferromagnetic ground state. Two equivalent magnetic structures are also iden- tified and linked to two distinct antiparallel polar orders. Such a scenario is analogous to that of conventional improper multiferroics with a magnetic origin of polar transition. The order parameter of structural distortion is linearly coupled to the magnetic order due to a strong magnetoelectric effect. The multiferroiclike metal is in striking contrast to insulating multiferroics, offering an extraordinary archetype for probing new underlying physics and exploring unusual properties for practical applications.

NMR evidence against a spin-nematic nature of the presaturation phase in the frustrated magnet SrZnVO(PO4)2

Using $^{31}$P nuclear magnetic resonance (NMR) we investigate the recently discovered presaturation phase in the highly frustrated two-dimensional spin system SrZnVO(PO$_4$)$_2$ [F. Landolt et al., Phys. Rev. B 104, 224435 (2021)]. Our data provide two pieces of evidence against the presumed spin-nematic character of this phase: i) NMR spectra reveal that it hosts a dipolar spin order and ii) the 1/$T_1$ relaxation rate data recorded above the saturation field can be fitted by the sum of a single-magnon term, exponential in the gap, and a critical second-order term, exponential in the triple gap, leaving no space for a nematic spin dynamics, characterized by a double-gap exponential. We explain the unexpectedly broad validity of the simple fit and the related critical spin dynamics.

Solid‐state heteronuclear multiple‐quantum spectroscopy under a magic‐angle spinning frequency of 150kHz

AbstractWe hereby demonstrate that1H detected15N‐1H heteronuclear multiple‐quantum spectroscopy can be carried out at a magic angle spinning frequency of 150 kHz. While the15N‐1H multiple‐quantum coherences can be directly excited from the dipolar order created by the method of adiabatic demagnetization in the rotating frame, it is technically more advantageous to acquire the chemical shift evolution of the heteronuclear multiple‐quantum coherence by two separate chemical shift evolution periods for1H and15N. We also show that the heteronuclear multiple‐quantum correlation spectrum can be obtained by shearing the corresponding heteronuclear single‐quantum correlation spectrum.

Mechanical Nanoscale Polarization Control in Ferroelectric PVDF‐TrFE Films

AbstractFerroelectric polymer films offer strong advantages like mechanical flexibility, biocompatibility, optical transparency, and low‐cost processing. However, their dielectric or piezoelectric performance is often inferior to that of oxide ferroelectric materials. Key to the dielectric or piezoelectric performance of semicrystalline polymers is the enhancement of electric dipolar order that is naturally lower than in crystalline ferroelectrics. Here, reorientation and alignment of the electric polarization in thin films by the mechanical effect of a scanning unbiased force microscopy tip is demonstrated as a versatile tool for nanoscale domain writing. Thin films (50–150 nm) of PVDF‐TrFE (78:22) on graphite are prepared with dense (110)‐oriented β‐phase lamellae randomly oriented in the film plane. The in‐plane polarization can be poled “mechanically” along any deliberately chosen direction in the film plane after vertical electric poling. Domain patterns with resolution down to ≈50 nm are written with four (out of six possible) local polarization orientations. Written domains show excellent long‐time stability. The surface roughening from the mechanical treatment is moderate (rms roughness of 2–3 nm). A ferroelastic origin of the mechanical polarization switching is discussed. Finally, suggestions are made how to utilize the domain patterns in thin film devices.

Hybrid improper dipolar density wave in NaLaCoWO6

Hybrid improper ferroelectricity (HIF) allows the generation of an electrical polarization in the AA${}^{\ensuremath{'}}$BB${}^{\ensuremath{'}}{\mathrm{O}}_{6}$ double perovskite materials thanks to the combination of two nonpolar octahedral distortions. Nevertheless, for selected combination of the A/A${}^{\ensuremath{'}}$ cations a nonpolar incommensurate phase is observed with average symmetry $C2/m$. Thanks to a detailed crystallographic description of the incommensurate phase, based on electron, neutron, and x-ray diffraction data, we show that the incommensurate modulation is related to an abrupt change of the out-of-phase tilting along the $a$ and $c$ axis, whereas the tilting along the $b$ axis remains constant across the structure. By using group theory and symmetry analysis we show that we observe an incommensurate analog of HIF, which induces a hybrid improper dipolar density wave in ${\mathrm{NaLaCoWO}}_{6}$. The dipolar ordering is due also in this case to a trilinear invariant involving the commensurate and incommensurate octahedra tilting.

Orientation dependence of inhomogeneous magnetization transfer and dipolar order relaxation rate in phospholipid bilayers

Inhomogeneous magnetization transfer (ihMT) is a novel MRI technique used to measure white matter myelination in the brain and spinal cord. In the brain, ihMT has a strong orientation dependence which is likely to arise from the anisotropy of dipolar couplings between protons on oriented lipids in the myelin bilayers. We measured the orientation dependence of the second moment (M2) of the lineshape, dipolar order relaxation rate (R1D), and ihMT ratio (ihMTR) in an oriented phospholipid bilayer at 9.4 T. We found a strong orientation dependence in all three parameters. ihMTR and R1D were maximized when the bilayers were aligned perpendicular to B0 and minimized near the magic angle (∼54.7°). M2 followed an orientation dependence given by the second Legendre polynomial squared as predicted by the form of the secular dipolar Hamiltonian. These results were used to calculate the orientation dependence of R1D and ihMTR in a diffusionless myelin sheath model, which showed ihMTR was maximised for fibers perpendicular to B0 and minimised at 45°, similar to ex-vivo spinal cord with a larger prepulse frequency offset, but in contrast to in vivo brain findings. Adding fiber dispersion to this model smoothed the orientation dependence curve as expected. Our results suggest the importance of the effects of lipid diffusion and prepulse offset frequency on ihMTR.

Comprehensive characterization of glycerol/ZnO green nanofluids for advances in multifunctional soft material technologies

Nanofluids (NFs) are increasingly used in most of the areas of soft condensed matter technologies, especially for heat transfer, energy harvesting, electrical insulation, and in the development of a variety of optoelectronic devices and controllable systems. Nowadays, green nanofluids are of considerable interest to researchers from an environmental safety point of view. Therefore, in this work, green nanofluids having biodegradable glycerol (GLY) as base fluid with homogeneous dispersion of eco-friendly zinc oxide (ZnO) nanoparticles (concentration range 0.01–1.00 wt%) were prepared and comprehensively characterized by employing various advanced techniques for exploring their promising multifunctional properties. The X-ray diffraction study of these GLY/ZnO green nanofluids, which is the first of its kind on nanofluids, confirmed the existence of monodispersed ZnO nanoparticles in the arranged 3D supramolecular structural network of the glycerol fluid. The ZnO nanoparticles having higher suspension stability in the GLY fluid were able to exhibit all the characteristic crystalline diffraction peaks for the concentration ≥ 0.30 wt%, at which the visible range photons absorption became almost saturated. The detailed analysis of the UV–Vis absorbance spectra of these nanofluids with increasing ZnO concentration explained substantial alterations in the electronic transition energy for the lone electron pairs of the hydroxyl oxygen atoms of glycerol molecules and also the valence band electrons of the ZnO nanomaterial. Dielectric and electrical spectra of these nanofluid materials are carried out in the harmonic field of 20 Hz to 1 MHz range, at 25 °C. A small change in the static dielectric permittivity of these GLY/ZnO nanofluids with the increase of ZnO concentration reveals some modifications in the parallel arranged dipolar ordering of the glycerol molecules. The conductivity relaxation process ruled the electrical conductivity of the GLY/ZnO nanofluids, but the low conductivity of these materials confirmed them as potential candidates for electric insulating nanodielectrics. The decreased viscosity and the increased thermal conductivity with some constructive changes in the acoustic parameters of these nanofluids were noted with the increase of ZnO concentration. The experimental results on the structural, optical, dielectric, electrical, and thermophysical properties of the GLY/ZnO nanofluids are technologically appealing and recognize these as innovative multifunctional soft materials.

Field-induced heterophase state in PbZrO3 thin films

Applications of epitaxial antiferroelectrics face scientific challenges, such as limited understanding of degraded (in comparison to bulk) switching behavior and its relation to nanoscale structural organization. We report on an unusual structural response of ${\mathrm{PbZrO}}_{3}/{\mathrm{SrRuO}}_{3}/{\mathrm{SrTiO}}_{3}$ (001) heterostructures to precritical (lower than required for switching) electric fields. In situ x-ray diffraction shows a ferrielectric-like structure, which forms gradually upon increasing the applied field and makes up a heterophase state with the host antiferroelectric phase. The field-induced structure is similar to the antiferroelectric parent phase in the octahedral-tilt pattern, but differs from it in the lead-ion displacement pattern. The latter can be encoded as $\ensuremath{\uparrow}\ensuremath{\uparrow}\ensuremath{\uparrow}\ensuremath{\uparrow}\phantom{\rule{4pt}{0ex}}\ensuremath{\downarrow}\ensuremath{\uparrow}\ensuremath{\uparrow}\ensuremath{\downarrow}$ provided that the antiferroelectric structure is encoded as $\ensuremath{\downarrow}\ensuremath{\downarrow}\ensuremath{\uparrow}\ensuremath{\uparrow}$. We propose that the unusual commensurateness (as opposed to the more ubiquitous incommensuration in similar materials) between the modulation periods of host and guest phases can be explained by accounting for the energy of heterophase boundaries, which is important in dense nanostructures due to the high surface-to-bulk ratio of nanodomains. An analysis using the ab-initio-correlated, but empirical (parametrized) energy model suggests that the observed field-induced structure is likely to be selected in ${\mathrm{PbZrO}}_{3}$ instead of the others in the case when the above commensuration effect is at play. The results point to the mechanism leading to the smearing of polarization-field loops in such heterostructures and suggest a perspective for the controlled creation of delicate dipolar orderings for ferroic-based memory.

Antiferroelectric negative capacitance from a structural phase transition in zirconia

Crystalline materials with broken inversion symmetry can exhibit a spontaneous electric polarization, which originates from a microscopic electric dipole moment. Long-range polar or anti-polar order of such permanent dipoles gives rise to ferroelectricity or antiferroelectricity, respectively. However, the recently discovered antiferroelectrics of fluorite structure (HfO2 and ZrO2) are different: A non-polar phase transforms into a polar phase by spontaneous inversion symmetry breaking upon the application of an electric field. Here, we show that this structural transition in antiferroelectric ZrO2 gives rise to a negative capacitance, which is promising for overcoming the fundamental limits of energy efficiency in electronics. Our findings provide insight into the thermodynamically forbidden region of the antiferroelectric transition in ZrO2 and extend the concept of negative capacitance beyond ferroelectricity. This shows that negative capacitance is a more general phenomenon than previously thought and can be expected in a much broader range of materials exhibiting structural phase transitions.

Double frustration and magnetoelectroelastic excitations in collinear multiferroic materials

We discuss a model scenario for multiferroic systems of type II (collinear spins) where the electric dipolar order competes with a frustrated magnetic order in determining the elastic distortions of the lattice ion positions. High magnetic frustration due to second-neighbors exchange and small spin easy-axis anisotropy lead to the appearance of the so-called quantum magnetic plateau states. Increasing the magnetic field above the plateau border produces composite excitations, where fractionalized spin tertions arise together with spontaneous dipolar flips (in the form of domain walls) and enhanced localized elastic distortions. This peculiar magnetoelectric effect may be described by magneto-electric-elastic quasiparticles that could be detected by x-ray and neutron diffraction techniques. Our results are supported by extensive DMRG computations on the spin sector and self-consistent equations for the lattice distortions.

Evidence for a single-layer van der Waals multiferroic.

Multiferroic materials have attracted wide interest because of their exceptional static1-3 and dynamical4-6 magnetoelectric properties. In particular, type-II multiferroics exhibit an inversion-symmetry-breaking magnetic order that directly induces ferroelectric polarization through various mechanisms, such as the spin-current or the inverse Dzyaloshinskii-Moriya effect3,7. This intrinsic coupling between the magnetic and dipolar order parameters results in high-strength magnetoelectric effects3,8. Two-dimensional materials possessing such intrinsic multiferroic properties have been long sought for to enable the harnessing of magnetoelectric coupling in nanoelectronic devices1,9,10. Here we report the discovery of type-II multiferroic order in a single atomic layer of the transition-metal-based van der Waals material NiI2. The multiferroic state of NiI2 is characterized by a proper-screw spin helix with given handedness, which couples to the charge degrees of freedom to produce a chirality-controlled electrical polarization. We use circular dichroic Raman measurements to directly probe the magneto-chiral ground state and its electromagnon modes originating from dynamic magnetoelectric coupling. Combining birefringence and second-harmonic-generation measurementswith theoretical modelling and simulations, we detect a highly anisotropic electronic state that simultaneously breaks three-fold rotational and inversion symmetry, and supports polar order. The evolution of the optical signatures as a function of temperature and layer number surprisingly reveals an ordered magnetic polar state that persists down to the ultrathin limit of monolayer NiI2. These observations establish NiI2 and transition metal dihalides as a new platform for studying emergent multiferroic phenomena, chiral magnetic textures and ferroelectricity in the two-dimensional limit.

Kinetic Yvon-Born-Green theory of the linear dielectric constant and complex permittivity of isotropic polar fluids

The theory of the linear static dielectric constant and linear complex permittivity of isotropic polar fluids is formulated starting from the coupled Langevin equationsdescribing the rototranslational dynamics of long-range interacting molecules with thermal agitation and subjected to external forces and torques. To this aim, adequate reduced densities are introduced and equationsgoverning their dynamics derived. In the equilibrium zero frequency limit, integral expressions for the Kirkwood correlation factor g_{K} are given, transparently showing that the popular method consisting in comparing g_{K} with 1 in order to deduce pair dipolar ordering has no serious theoretical grounding. In the dynamical situation, the complex permittivity spectrum of a simple liquid is shown to exhibit an infinite discrete set of relaxation times, some of which may have thermally activated behavior. The theory is also shown to contain all previous results derived in the area provided molecular inertial effects are ignored, so restricting the range of validity of the theory to frequencies much below the far-infrared region. Finally, the theory can be adapted without much effort to relaxation of interacting magnetic nanoparticles for which macroscopic magnetic anisotropy arising from the assembly of nanoparticles is neglected.

Voltage sensitive ion channels: hardware or soft matter? Can we participate in a dialogue?

Standard models of voltage sensing in voltage-sensitive ion channels assume rigidity in gates and other everyday devices, but if rigidity requires the cooperation of numerous strong interatomic bonds, can it exist in a macromolecule at physiological temperature? Can an ion channel be pierced by a water-filled pore without suffering electrical breakdown? The Channel Activation by Electrostatic Repulsion (CAbER) hypothesis proposes that threshold reduction of the electric field magnitude across the excitable channel allows thermal disorder to overcome a partial order of electric dipoles induced by the high resting field.

Interplay of lattice, spin, and dipolar properties in CoTeMoO6 : Emergence of Griffiths-like phase, metamagnetic transition, and magnetodielectric effect

The physical properties of the noncentrosymmetric and quasi-two-dimensional canted antiferromagnetic material ${\mathrm{CoTeMoO}}_{6}$ (CTMO) are studied. We unravel a spectrum of intriguing phenomena, including a correlated short-range-ordered Griffiths-like phase near ${T}_{G}\ensuremath{\sim}64\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, a sharp step-like metamagnetic behavior of isothermal magnetization induced by a spin-flop transition at a critical magnetic field ${H}_{\mathrm{c}}\ensuremath{\sim}1.2\phantom{\rule{0.16em}{0ex}}\mathrm{T}$, a strong dielectric anomaly, a pronounced magnetodielectric effect, and the associated weak magnetoelectric coupling at its magnetic transition temperature, ${T}_{N}\ensuremath{\sim}23.4\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. The crucial roles played by spin canting in CTMO are revealed in the aforementioned strongly entangled behavior of the spin and dipolar order parameters. The observed magnetostriction effect coupled with spin-phonon coupling seems to be a key factor in influencing the observed magnetodielectric effect. In addition, our theoretical first-principles calculations predict an insulating state with a canted antiferromagnetic spin configuration that matches well with the experimental results.

Remotely controllable supramolecular rotor mounted inside a porphyrinic cage

Remotely controllable supramolecular rotor mounted inside a porphyrinic cage

Effect of Iodine Octahedral Rotations on Dipole Ordering in Organic–Inorganic Hybrid Perovskite CH3NH3PbI3

Organic–inorganic hybrid perovskite MAPbI3 (MA = CH3NH3) has been rapidly growing as one of the most promising materials for future solar cells. One of the mechanisms that contribute to its high efficiency is related to the ferroelectric ordering of the MA molecules inside the inorganic framework. Here, we use first-principles calculations to explore structural distortions that can lead to the dipolar ordering of the organic molecules in the tetragonal phase, which is relevant to room-temperature applications. We find that the existence of antiphase rotation of the PbI6 octahedra has a significant impact, whereas tetragonal change of the lattice parameters is relatively unimportant for either ferroelectric or antiferroelectric configurations.

Slow and fast relaxation times of quantum lattice model with local multi-well potentials: phenomenological dynamics for Sn2P2S6 ferroelectric crystals

As a continuation of the previously published work [Velychko O. V., Stasyuk I. V., Phase Transitions, 2019, 92, 420], a phenomenological framework for the relaxation dynamics of quantum lattice model with multi-well potentials is given in the case of deformed Sn2P2S6 ferroelectric lattice. The framework is based on the combination of statistical equilibrium theory and irreversible thermodynamics. In order to study these dynamics in a connected way we assume that the dipole ordering or polarization (η) and volume deformation (u) can be treated as fluxes and forces in the sense of Onsager theory. From the linear relations between the forces and fluxes, the rate equations are derived and characterized by two relaxation times (τS , τF) which describe the irreversible process near the equilibrium states. The behaviors of τS and τF in the vicinity of ferroelectric phase transitions are studied.

An intriguing canting dipole configuration and its evolution under an electric field in La-doped Pb(Zr,Sn,Ti)O3 perovskites

Despite the fact that electric dipole ordering plays a key role in the unique physical properties of dielectric materials, electric dipole configurations mostly appear simply as either parallel or antiparallel. Here, we report a canting electric dipole configuration in La-doped Pb(Zr,Sn,Ti)O3 perovskites based on advanced neutron, synchrotron X-ray and three-dimensional electron diffraction techniques. It is revealed that, arising from the coupling between the atomic displacement and oxygen octahedral tilting, this unique electric dipole configuration displays a canting arrangement aligned in the (110)p plane that possesses an antiparallel component along the [110]P direction and a parallel component along the [001]P direction. Remarkably, under an in-situ electric field, the electric dipoles continuously rotate with a gradually reduced canting angle, as confirmed by phase-field simulations, and ultimately evolve into a ferroelectric ordering. Such an evolution gives rises to a small hysteresis and an equivalent lattice strain to the macroscopic strain. These findings enrich the current understanding of the types of electric dipole configurations in dielectric materials and are expected to aid the design of new dielectric materials with emergent properties.