Polar Distortion Research Articles

Directed synthesis of a hybrid improper magnetoelectric multiferroic material

Preparing materials which simultaneously exhibit spontaneous magnetic and electrical polarisations is challenging as the electronic features which are typically used to stabilise each of these two polarisations in materials are contradictory. Here we show that by performing low-temperature cation-exchange reactions on a hybrid improper ferroelectric material, Li2SrTa2O7, which adopts a polar structure due to a cooperative tilting of its constituent TaO6 octahedra rather than an electronically driven atom displacement, a paramagnetic polar phase, MnSrTa2O7, can be prepared. On cooling below 43 K the Mn2+ centres in MnSrTa2O7 adopt a canted antiferromagnetic state, with a small spontaneous magnetic moment. On further cooling to 38 K there is a further transition in which the size of the ferromagnetic moment increases coincident with a decrease in magnitude of the polar distortion, consistent with a coupling between the two polarisations.

Interplay between Polar Distortions and Superconductivity in SrTiO3

Interplay between Polar Distortions and Superconductivity in SrTiO₃

Thermo-optical properties of terbium sesquioxide (Tb2O3) ceramics at room temperature.

Thermo-optical properties of several (Tb1-xYx)2O3 ceramic samples were investigated in this Letter. The linear absorption and thermal conductivity coefficients, as well as the power dependence of thermally induced phase and polarization distortions of laser radiation, were measured. In addition, the effective thermo-optical constants Peff and Qeff were estimated. Thermo-optical properties of the studied ceramics were compared with those of the widely used terbium gallium garnet. It was shown that the material under consideration is highly promising for Faraday isolators operating at high average power laser radiation.

Designing multifunctional two-dimensional layered transition metal phosphorous chalcogenides

Layered two-dimensional (2D) transition metal phosphorous chalcogenides (TMPCs) are now in intense research focus due to their interesting ferroelectric and magnetic properties and compatibility with 2D electronic devices. Here, we have employed first-principles density functional theory calculations to investigate the electric and magnetic properties of ${\mathrm{ABP}}_{2}{\mathrm{S}}_{6}$ (A = Cu, Ni; B = Cr, Mn) TMPCs. We have systematically investigated four TMPCs compounds, namely, ${\mathrm{CuCrP}}_{2}{\mathrm{S}}_{6}, {\mathrm{CuMnP}}_{2}{\mathrm{S}}_{6}, {\mathrm{NiCrP}}_{2}{\mathrm{S}}_{6}$, and ${\mathrm{NiMnP}}_{2}{\mathrm{S}}_{6}$, and reported unusual antiferroelectric/ferroelectric (AFE/FE) and electronic properties. We have found a more stable ferroelectric state in van der Waals (vdW) gap with higher polarization compared to the usual ferroelectric phase in the insulating state. In case of ${\mathrm{CrMnP}}_{2}{\mathrm{S}}_{6}$ and ${\mathrm{NiMnP}}_{2}{\mathrm{S}}_{6}$, we have proposed them as polar half-metals as our analysis have revealed that ferroelectric distortions can persist in these systems in the metallic phase. Moreover, our analysis have shown that ${\mathrm{NiMnP}}_{2}{\mathrm{S}}_{6}$ can undergo metal-to-insulator transition driven by the polar distortion. We have identified two modes namely, ${\mathrm{\ensuremath{\Gamma}}}_{2}^{\ensuremath{-}}$ ${\mathrm{\ensuremath{\Gamma}}}_{1}^{+}$, those are responsible for driving ferroelectricity and antiferroelectricity, respectively, into these systems. We could observe a rare and interesting phenomena, where the antipolar mode ${\mathrm{\ensuremath{\Gamma}}}_{1}^{+}$ that is responsible for antiferroelectric distortion leads to a ferroelectric distortion when applied excessively on the system, which is unusual. Finally, we have performed molecular dynamics simulations at various finite temperatures. In case of ${\mathrm{NiCrP}}_{2}{\mathrm{S}}_{6}$ and ${\mathrm{NiMnP}}_{2}{\mathrm{S}}_{6}$ at 300 K, we have observed that the ferroelectric state within the vdW gap is stable. Interestingly, we have discovered a hybrid inter-intra layer antiferroelectric configuration within the vdW gap for ${\mathrm{CuCrP}}_{2}{\mathrm{S}}_{6}$. The layers start to move opposite to each other due to the temperature effect, which leads to the switching of the AFE state in the first layer. Further, increase of temperature results the switching in both the layers. The reported in-gap FE/AFE states in vdW gap can be tuned by uniaxial strain along the perpendicular direction of the 2D layer and thus the materials studied here can be considered as potential piezoelectric materials.

Coexistence of spontaneous polarization and superconductivity in hole-doped oxyhydrides ATiO2H ( A=K,Rb,Cs ): First-principles study

The polar metal is a material that hosts both polar distortion and metallicity. Such a material is expected to show exotic magneto-electric phenomena if superconducts. Here, we theoretically explore ferroelectric and superconducting properties in a series of perovskite-type oxyhydrides ATiO$_2$H (A=K, Rb, Cs) under hole-doping conditions using the first-principles calculations based on the density functional theory. Our simulation shows that these compounds host spontaneous polarization and superconductivity at optimal doping concentration. The unusual coexistence of superconductivity and large polarization (~100 ${\mu}$C/cm$^2$) originates from weak coupling of the polar distortion and superconducting states, the reason of which is separation of the displaced atoms and spacially confined metallic carriers. Besides, the superconductivity is enhanced by the unique electronic properties near the valence band maximum: quartic band dispersion with a sizable contribution of hydrogen 1s states. Our study thus feature the oxyhydrides as possible model polar superconducting systems, which may be utilizable for future magneto-electric devices.

Functional Transition Metal Perovskite Oxides with 6s2 Lone Pair Activity Stabilized by High-Pressure Synthesis

Perovskite ABO3 oxides that have Bi and Pb at the A site and transition metals at the B site, when stabilized by high-pressure synthesis at several gigapascals, provide a rich parameter space of fascinating properties. Stereochemical 6 s2 lone pairs of Bi3+ and Pb2+ induce polar or antipolar distortions. 6 s2 and 6 s0 (Bi5+ and Pb4+) charge degree of freedom enable intermetallic charge transfer transitions. The structural distortion and the charge degree of freedom are coupled with magnetism of transition metals, resulting in various functionalities. In particular, we highlight magnetization reversal by electric field and polarization rotation in BiFe1− xCo xO3, negative thermal expansion in modified BiNiO3 and PbVO3, and systematic charge distribution changes in Pb MO3 ( M = 3 d transition metal).

Efficient Correction Method for Polarimetric Distortion Error of Polarimetric Scatterometer

The full-polarization scatterometer is a new type of microwave scatterometer that can simultaneously measure the co-polarization and cross-polarization backscatter of the target. It can solve the wind direction ambiguity problem of the traditional scatterometer in the sea surface wind retrieval and improve the wind measurement accuracy. Moreover, the cross-polarization backscatter can effectively increase the sea surface wind speed measurement range and can be used for typhoon monitoring. High-precision sea surface wind retrieval using full-polarization scatterometer data first needs to correct the polarization distortion error of the full-polarization scatterometer system. To solve this problem, the polarimetric scattering coefficient model influenced by polarimetric distortion error is built based on the signal model of polarimetric scatterometer, and then the influence of polarimetric distortion error in quantitatively analyzed. Based on the polarimetric scattering coefficient model, some assumptions of ocean scattering characteristic and polarimetric calibration are introduced to establish the correction model of polarimetric scattering coefficients. Simulation data based on RADARSAT-2 polarimetric scattering coefficients of different sea region is corrected and results show that the proposed method can effectively correct of polarimetric distortion error and greatly improve the measurement accuracy of polarimetric scattering coefficients.

Ferroelectricity and multiferroicity in anti–Ruddlesden–Popper structures

Combining ferroelectricity with other properties such as visible light absorption or long-range magnetic order requires the discovery of new families of ferroelectric materials. Here, through the analysis of a high-throughput database of phonon band structures, we identify a structural family of anti-Ruddlesden-Popper phases [Formula: see text]O (A=Ca, Sr, Ba, Eu, X=Sb, P, As, Bi) showing ferroelectric and antiferroelectric behaviors. The discovered ferroelectrics belong to the new class of hyperferroelectrics that polarize even under open-circuit boundary conditions. The polar distortion involves the movement of O anions against apical A cations and is driven by geometric effects resulting from internal chemical strains. Within this structural family, we show that [Formula: see text]O combines coupled ferromagnetic and ferroelectric order at the same atomic site, a very rare occurrence in materials physics.

Cooperative evolution of polar distortion and nonpolar rotation of oxygen octahedra in oxide heterostructures.

Polarity discontinuity across LaAlO3/SrTiO3 (LAO/STO) heterostructures induces electronic reconstruction involving the formation of two-dimensional electron gas (2DEG) and structural distortions characterized by antiferrodistortive (AFD) rotation and ferroelectric (FE) distortion. We show that AFD and FE modes are cooperatively coupled in LAO/STO (111) heterostructures; they coexist below the critical thickness (t c) and disappear simultaneously above t c with the formation of 2DEG. Electron energy-loss spectroscopy and density functional theory (DFT) calculations provide direct evidence of oxygen vacancy (V O) formation at the LAO (111) surface, which acts as the source of 2DEG. Tracing the AFD rotation and FE distortion of LAO reveals that their evolution is strongly correlated with V O distribution. The present study demonstrates that AFD and FE modes in oxide heterostructures emerge as a consequence of interplay between misfit strain and polar field, and further that their combination can be tuned to competitive or cooperative coupling by changing the interface orientation.

SOA-Based Multilevel Polarization Shift On–Off Keying Transmission for Free-Space Optical Communication

This paper proposes a semiconductor optical amplifier (SOA)-based multilevel polarization shift on–off keying (MPS-OOK) transmission for free-space optical (FSO) communication. At the transmitter end, the MPS-OOK signal is modulated with a constant linear state of polarization (SOP) at the high-intensity level and various SOPs at the low-intensity level in order to improve the spectral efficiency (SE) with the transmitted power efficiency. At the receiver end, first, a polarization-independent SOA was introduced to optically suppress the turbulence-induced scintillation effect and equalize the intensities of the various SOPs in the deep gain saturation state without polarization distortion. Then, a linear polarizer (LP) with a high extinction ratio (ER) was deployed to convert the equalized SOPs into a known intensity. Finally, the converted MPS-OOK signal was detected using a single photodiode (PD) and distinguished using a multilevel fixed-threshold decision (M-FTD). The proposed technique was evaluated using experiments. A Mach–Zehnder modulator (MZM)-based fading simulator was introduced to emulate the turbulence-induced scintillation effect. The experimental results demonstrated that the scintillation effect was effectively mitigated and the SE was improved by up to 2 bit/s/Hz using the proposed four-level polarization shift on–off keying (4PS-OOK) transmission.

Carrier-mediated control over the soft mode and ferroelectricity in BaTiO3

We calculate the effects of conduction band electrons (CBEs), introduced by doping or photoexcitation, on the ferroelectricity and phonon dynamics of ${\mathrm{BaTiO}}_{3}$ (BTO). We show that CBEs destabilize ferroelectricity, which would lower the Curie temperature and coercive field, and might help to improve the speed or efficiency with which polarization domains can be switched in ferroelectric devices. We show that CBEs lower the frequencies of the ${A}_{1}$ soft and ferroelectric modes in BTO's ferroelectric phases, and raise the soft-mode frequency in its paraelectric phase. We also show that femtosecond laser pulses could be used to selectively excite a coherent ${A}_{1}$ ferroelectric-mode phonon. This would allow this much-studied excitation to be monitored by pump-probe spectroscopy as it decays into other modes. We show that many of the properties of doped and undoped BTO have simple and intuitive explanations, within an ionic picture of BTO's bonding, if it is assumed that ferroelectricity is not driven by long-range interactions, but by the attraction between Ti and O neighbors. Most of the effects of CBEs are consequences of them reducing ions' charges and increasing their polarizabilities, thereby weakening the Ti-O attraction. As the CBE density increases, so does the density of delocalized interstitial electrons, which would increase conductivity. We argue that a polar metallic phase exists if the threshold for metallic conductivity can be reached before the CBEs make polar distortions energetically unfavorable by critically weakening the Ti-O attraction.

Study of the characteristics of the synchronization algorithm for a quantum key distribution system based on comparing the number of samples from an adjacent pair of time windows with a threshold level

The analysis of time and probabilistic characteristics is carried out for a two-stage synchronization algorithm in a quantum key distribution (QKD) system with automatic compensation of polarization distortions. Formulas are obtained for calculating the missing probabilities a signal pair of windows, false alarm and successful synchronization. The dependences of the missing probability a signal pair on the average number of photons and noise pulses are investigated during the analysis of a pair of windows for different threshold levels. The numerical material obtained in the course of the study indicates the possibility of using for express calculations the probability of detecting a signal pair of windows formulas oriented to the normal distribution of the number of recorded pulses at the output of a single-photon avalanche photodiode.

Effects of Motion Compensation Residual Error and Polarization Distortion on UAV-Borne PolInSAR

Polarimetric Interferometric Synthetic Aperture Radar (PolInSAR) can improve interferometric coherence and phase quality, which has good application potential. With the development of the Mini-SAR system, Unmanned Aerial Vehicle borne (UAV-borne) PolInSAR systems became a reality. However, UAV-borne PolInSAR is easily affected by air currents and other factors, which may cause large motion errors and polarization distortion inevitably exists. However, there are few pieces of research which are about motion compensation residual error (MCRE) and polarization distortion effects on PolInSAR. Though the effects of MCRE on Interferometric SAR (InSAR) and polarization distortion on PolInSAR were studied, respectively, these two parts are independently modeled and analyzed. In this paper, a model that simultaneously considers the effects of these two kinds of errors is proposed, and simulation results are given to validate the model. Then, a quantitative analysis based on a real Quadcopter UAV PolInSAR system is performed according to the model, which is valuable for system design and practical application of the UAV-borne PolInSAR system.

Coexistence of polar distortion and conduction in doped 2D group-IV ferroelectrics: SiGe, SiSn, and GeSn

Since the concept of ferroelectric metal predicted in the 1960s has been experimentally realized in the bulk Weyl semimetal WTe2 [Sharma et al 2019 Sci. Adv. 5, eaax5080], it is significant to find the ultrathin polar metal or ferroelectric metal due to the demand of miniature of electronic nanodevices. Here, 2D buckled monolayers composed of group-IV elements such as SiGe, SiSn, and GeSn are selected as prototype. Then, the stability of 2D ferroelectricity in the above monolayers are confirmed based on the results of first-principles calculations. Most interesting, a robustly metallic polar state has been found in the above 2D ferrolectrics under both the electron doping and hole doping, and the polar distortion becomes even more remarkable when the electrons are doped as compared with the undoped system. Thus, the coexistence of polar state and conduction is theoretically verified in the doped group-IV monolayers. We hope the 2D ferroelectric materials can be used as a starting point to look for the polar metals with atomic thickness, and further broaden their applications in 2D electronics or spintronics in the future.

Microscopic mechanisms behind hyperferroelectricity

Hyperferroelectrics are receiving a growing interest thanks to their unique property to retain a spontaneous polarization even in presence of a depolarizing field. Nevertheless, general microscopic mechanisms driving hyperferroelectricity, which is ascribed to the softening of a polar $LO$ mode, are still missing. Here, by means of phonons calculations and force constants analysis in two class of hyperferroelectrics, the ABO$_3$-LiNbO3-type systems and the prototypical hexagonal-ABC systems, we unveiled common features in the dynamical properties of a hyperferroelectric behind such $LO$ instability: negative or vanishing on-site force constant associated to the cation driving the $LO$ polar distortion, and destabilizing cation-anion interactions, both induced by short-range forces. We also predict possible enhancement of the hyperferroelectric properties by applying an external positive pressure; pressure strengthens the destabilizing short-range interactions. Particularly, the increase in the mode effective charges associated to the unstable $LO$ mode under pressure suggests an eventual enhancement of the $D$=0 polarization under compressive strain.

Tuning the electronic structure of d0 perovskite oxides by combining distortive modes

Transition metal oxides in the ${d}^{0}$ perovskite class, defined broadly to include $\mathrm{W}{\mathrm{O}}_{3}$, have shown promise as light absorbers for photocatalytic applications. These compounds are highly tunable in their composition and atomic structure, and consequently, in their electronic structure and properties. Through the lens of atomic orbital interactions and density functional theory calculations, the present work explores how and why certain structural distortions tune the band gaps and band edges of $\mathrm{W}{\mathrm{O}}_{3}$ and ${d}^{0}$ perovskite oxides. It is determined that, because an individual distortive mode (e.g., polar distortion along or octahedral rotation around a cubic perovskite axis) changes the character of only some band-edge orbitals, combinations of distortive modes may tune band gaps much more significantly than the sum of individual modes. The implications of this tunability are discussed for $\mathrm{W}{\mathrm{O}}_{3}$ as a photocatalyst, and for ${d}^{0}$ perovskite oxides more generally.

The influence of the 6s2 configuration of Bi3+ on the structures of A'BiNb2O7 (A' = Rb, Na, Li) layered perovskite oxides.

Solid state compounds which exhibit non-centrosymmetric crystal structures are of great interest due to the physical properties they can exhibit. The 'hybrid improper' mechanism - in which two non-polar distortion modes couple to, and stabilize, a further polar distortion mode, yielding an acentric crystal structure - offers opportunities to prepare a range of novel non-centrosymmetric solids, but examples of compounds exhibiting acentric crystal structures stabilized by this mechanism are still relatively rare. Here we describe a series of bismuth-containing layered perovskite oxide phases, RbBiNb2O7, LiBiNb2O7 and NaBiNb2O7, which have structural frameworks compatible with hybrid-improper ferroelectricity, but also contain Bi3+ cations which are often observed to stabilize acentric crystal structures due to their 6s2 electronic configurations. Neutron powder diffraction analysis reveals that RbBiNb2O7 and LiBiNb2O7 adopt polar crystal structures (space groups I2cm and B2cm respectively), compatible with stabilization by a trilinear coupling of non-polar and polar modes. The Bi3+ cations present are observed to enhance the magnitude of the polar distortions of these phases, but are not the primary driver for the acentric structure, as evidenced by the observation that replacing the Bi3+ cations with Nd3+ cations does not change the structural symmetry of the compounds. In contrast the non-centrosymmetric, but non-polar structure of NaBiNb2O7 (space group P212121) differs significantly from the centrosymmetric structure of NaNdNb2O7, which is attributed to a second-order Jahn-Teller distortion associated with the presence of the Bi3+ cations.

Designing new ferromagnetic double perovskites: the coexistence of polar distortion and half-metallicity.

Advancing technology and growing interdisciplinary fields raise the need for new materials that simultaneously possess several significant physics quantities to meet human demands. In this research, using density functional theory, we aim to design A2MnVO6 (A = Ca, Ba) as new double perovskites and investigate their structural, electronic, and magnetic properties. Structural calculations based on the total energies show the optimized monoclinic and orthorhombic crystal structures for the Ca2MnVO6 (CMVO) and Ba2MVO6 (BMVO) compounds, respectively. Through performing calculations, we reveal that the Jahn-Teller effect plays an important role in polar distortions of VO6 and elongation of MnO6 octahedra, resulting from the V5+(3d0) and Mn3+(3d4:t32ge1g) electron configurations. The spin-polarized calculations predict the half-metallic ferromagnetic ground state for CMVO and BMVO with a total magnetic moment of 4.00 μB f.u.-1 Our findings introduce CMVO and BMVO double perovskites as promising candidates for designing ferromagnetic polar half-metals and spintronic applications.

Effect of the polar distortion on the thermoelectric properties of GeTe

First principle calculations are performed to investigate the effect of polar order strength on the thermoelectric properties of GeTe compound in its rhombohedral structure. Different magnitudes of polarization exhibit a noticeable effect on the thermoelectric properties of GeTe. In particular, polar structures with higher polarization tend to show higher thermoelectric efficiencies. Thus, it is shown that polarization engineering is an important factor in designing efficient thermoelectric devices. In particular, we proposed that high thermoelectric performances could be achieved by growing epitaxial GeTe films that are bi-axially compressed in the directions perpendicular to the polar axis in order to achieve larger polarization states.

Preliminary estimate of instrumental polarization effects in the LST-3 Large Solar Telescope

This article presents a polarization calculation for the LST-3 large solar telescope. It describes the interaction of the telescope mirrors and includes a calculation of the transmittance of the telescope’s optical system for incident radiation with various polarization states. It estimates the instrumental polarization, cross-talk, and depolarization at Gregorian focus F2 and after tilted mirror M4 of the telescope. The estimate is made using modeling of an aluminum coating on the mirrors for axial and peripheral image points at reference wavelengths in the 400–2000 nm region. The mean Mueller matrices are found at Gregorian focus F2 and after tilted mirror M4. The polarization distortions at the Gregorian focus are insignificant, but they are substantial after tilted flat mirror M4.