Polar Metal Research Articles

A Rare Case of Magnesium Alkyl Mediated CO-to-Alkoxide Conversion.

Alcohols can be produced by nucleophilic addition at the C=O functional group of ketones or aldehydes. Such C-C bond formations generally proceed smoothly and selective. In contrast, nucleophilic addition at carbon monoxide is considerably more complex and unselective due to the instability of polar metal acyl intermediates. Herein, we report a magnesium alkyl complex, stabilized with a dipyrromethenide ligand, and its reactivity with CO2 and CO. Whilst the reaction with CO2 gave the expected carboxylate, reaction with CO took a surprising course, resulting in an alkoxide by formation of three new C-C bonds in a single reaction. An intertwined experimental and computational approach sheds light on the unusual reactivity of a carbene intermediate.

Two-Dimensional Topological Ferroelectric Metal with Giant Shift Current.

The pursuit for "ferroelectric metal," which combines seemingly incompatible spontaneous electric polarization and metallicity, has been assiduously ongoing but remains elusive. Unlike traditional ferroelectrics with a wide band gap, ferroelectric (FE) metals can naturally incorporate nontrivial band topology near the Fermi level, endowing them with additional exotic properties. Here, we show first-principles evidence that the metallic PtBi_{2} monolayer is an intrinsic two-dimensional (2D) topological FE metal, characterized by out-of-plane polarization and a moderate switching barrier. Moreover, it exhibits a topologically nontrivial electronic structure with Z_{2} invariant equal to 1, leading to a significant FE bulk photovoltaic effect. A slight strain can further enhance this effect to a remarkable level, which far surpasses that of previously reported 2D and 3D FE materials. Our Letter provides an important step toward realizing intrinsic monolayer topological FE metals and paves a promising way for future nonlinear optical devices.

Fully spin-polarized two dimensional polar semi-metallic phase in Eu substituted GdI2 monolayer

Abstract The coexistence of seemingly mutually exclusive properties such as ferromagnetism, ferroelectricity and metallicity in atomically thin materials is the requirement of the hour in electronics as the Moore’s law faces an impending end. Only a few 2D multiferroic materials have been predicted/realized so far. The polar metals with simultaneous presence of polarity and conductivity are also equally rare. Here, we predict, based on first-principles calculations that an Eu-substituted rare-earth halide GdI2 monolayer showcases ferromagnetism, ferroelasticity while being polar and a fully spin-polarized semi-metal at the same time. The ferroelasticity and polarity are shown to be coupled making it possible to switch the polar direction using external mechanical stress. Further, it is observed that an application of biaxial tensile strain of 5% causes the spin easy-axis to shift from out-of-plane to in-plane direction. Thus, spin easy axis gets coupled with the direction of polarization in the strained monolayer making the switching of magnetization also possible using external strain. Simultaneous coexistence and coupling of the ferroic orders in a metallic 2D material makes the Eu substituted GdI2 monolayer an incredibly rare material for nano-electronics and spintronics applications.

Production of rGO based hybrid composite buckypaper cathodes by using copper oxide polysulfide adsorbent for Li-S batteries

Lithium-sulfur batteries are particularly noteworthy for applications requiring large capacity and long battery life. Transition metal oxides have emerged as polar host materials for lithium polysulfides to further modulate the bonding energy with polysulfides and increase the coupling density of the electrodes. Polar metal oxides naturally adsorb polysulfides on their hydrophilic surfaces. In this study, the polysulfide adsorbing ability of CuO against the shuttle effect is utilized. For this purpose, copper(II) oxide (CuO) is used as polysulfide adsorbent and CuO - reduced Graphene Oxide / Sulphur (CuO-rGO/S) hybrid composite buckypaper electrodes are produced as cathodes in Li-S batteries without using binders. In this work, optical, morphological and structural analyzes of composite films were carried out by Fourier transformed infrared spectroscopy (FT-IR), Raman spectroscopy, field emission gun scanning electron microscopy (FEG-SEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Brunauer-Emmett-Teller (BET) analysis, respectively. After the CR2032 button cell was assembled using the produced cathode materials, cyclic voltammetry (CV), charge-discharge performance tests, rate capability and electrochemical impedance spectroscopy (EIS) analyzes were performed to investigate the Li-S battery capacities. With the CuO-rGO/S hybrid composite cathode containing 2 % CuO, a discharge capacity of 1205 mAh g−1 was obtained in the first cycle. At the end of the 1000 cycles, the discharge capacity of the cathode was achieved as high as 696 mAh g−1. As a result, the battery capacity of rGO/S-based composite electrodes has been improved by taking advantage of CuO added to the cathode structure to adsorb polysulfides.

The utility of MOF-based materials in direct air capture (DAC) application to ppm-level CO2

Metal-organic frameworks (MOFs) are well-suited materials for CO2 removal and have robust capture capacity and selectivity. Although the adsorption of CO2 in MOFs has been studied, the implementation of ppm-level CO2 uptake in MOFs and the effects of the pore size and charge have not been fully explored. We performed grand canonical Monte Carlo (GCMC) simulations combined with the Density Functional Theory plus U (DFT + U) charge method to investigate MOF screening for ppm-level CO2 uptake and its application in a direct air capture (DAC) system. Three types of MOFs containing eight members were studied: i.e., ZIF-68, 69, 70; UiO-66, 67, 68; CAU-10; and MIL-125. The pore landscape characterization, electrostatic field-induced enhancement, and preferential binding sites of these MOFs were examined for CO2 capture. MOFs with pore limited diameters (PLD) 1.5 times the size of CO2 molecules and with large cavity diameters (LCD) smaller than 10 Å exhibit robust confinement capacity. Polar functional groups and metal ions dominate the electrostatic contributions and subsequently enhance the surface adhesion of CO2 molecules. For a given framework, favorable CO2 binding occurs in the following order: small pores/cages > polar functional group/metal ions > larger pores/cages. ZIF-69 which comprises smaller pores (7.5 Å) and robust polar functional groups (–Cl) collectively enhances CO2 capture; thus, ZIF-69 outperforms other MOFs; the performance of ZIF-69 is followed by that of CAU-10 which has an optimal pore size of 6 Å. These findings are of fundamental and practical importance for the application of MOFs in DAC technologies for CO2 removal.

From Fractional Quantum Anomalous Hall Smectics to Polar Smectic Metals: Nontrivial Interplay Between Electronic Liquid Crystal Order and Topological Order in Correlated Topological Flat Bands.

Symmetry-breaking orders can not only compete with each other, but also be interwined, and the interwined topological and symmetry-breaking orders make the situation more intriguing. This work examines the archetypal correlated flat band model on a checkerboard lattice at filling ν = 2/3 and we find the unique interplay between smectic charge order and topological order gives rise to two novel quantum states. As the interaction strength increases, the system first transitions from a Fermi liquid into FQAH smectic (FQAHS) state, where FQAH topological order coexists cooperatively with smectic charge order with enlarged ground-state degeneracy and interestingly, the Hall conductivity is σxy = ν = 2/3, different from the band-folding or doping scenarios. Further increasing the interaction strength, the system undergoes another quantum phase transition and evolves into a polar smectic metal (PSM) state. This emergent PSM is an anisotropic non-Fermi liquid, whose interstripe tunneling is irrelevant while it is metallic inside each stripe. Different from the FQAHS and conventional smectic orders, this PSM spontaneously breaks the two-fold rotational symmetry, resulting in a nonzero electric dipole moment and ferroelectric order. In addition to the exotic ground states, large-scale numerical simulations are also used to study low-energy excitations and thermodynamic characteristics. We find the onset temperature of the incompressible FQAHS state, which also coincides with the onset of non-polar smectic order, is dictated by the magneto-roton modes. Above this onset temperature, the PSM state exists at intermediate-temperature regime. Although the T = 0 quantum phase transition between PSM and FQAHS is first order, the thermal FQAHS-PSM transition could be continuous. We expect the features of the exotic states and thermal phase transitions could be accessed in future experiments.

Realization of Two-Dimensional Intrinsic Polar Metal in a Buckled Honeycomb Binary Lattice.

Structural topology and symmetry of a two-dimensional (2D) network play pivotal roles in defining its electrical properties and functionalities. Here, a binary buckled honeycomb lattice with C3v symmetry, which naturally hosts topological Dirac fermions and out-of-plane polarity, is proposed. It is successfully achieved in a group IV-V compound, namely monolayer SiP epitaxially grown on Ag(111) surface. Combining first-principles calculations with angle-resolved photoemission spectroscopy, the degeneration of the Dirac nodal lines to points due to the broken horizonal mirror symmetry is elucidated. More interesting, the SiP monolayer manifests metallic nature, which is mutually exclusive with polarity in conventional materials. It is further found that the out-of-plane polarity is strongly suppressed by the metallic substrate. This study not only represents a breakthrough of realizing intrinsic polarity in 2D metallic material via ingenious design but also provides a comprehensive understanding of the intricate interplay of many exotic low-dimensional quantum phenomena.

Polar Metallicity Controlled by Epitaxial Strain Engineering.

The discovery of polar metal opens the door to incorporating electric polarization into electronics with the potential to invigorate next-generation multifunctional electronic devices. Especially, electric polarization can be induced by geometric design in non-polar perovskite oxides. Here, the epitaxial strain exerted on the deposited single-crystalline NdNiO3 thin films is systematically varied in both sign and amplitude by choosing substrates with different lattice mismatch. The pseudocubic NdNiO3(111) film, which is non-polar in its bulk state, is induced to be polar under both compressive and tensile strain. The fine-tuning of epitaxial strain is realized by continuously varying the film thickness using the "thickness-wedge" growth technique, and from the elucidated thickness dependence, the electric polarization and metallicity can be further optimized. Moreover, transitioning from isotropic to anisotropic epitaxial strain gives rise to an ideal polar metal state in the pseudocubic NdNiO3(102) film on an orthorhombic substrate, achieving a remarkably low resistivity of 173 µΩcm at room temperature. The metal-insulator transition in NdNiO3 is completely suppressed and the polar metal state becomes the ground state at all temperatures. These results demonstrate alluring possibilities of induction and manipulation of both electric polarization and electric transport properties in functional perovskite oxides by epitaxial strain engineering.

Revealing a distortive polar order buried in the Fermi sea.

Polar metals are challenging to identify spectroscopically because the fingerprints of electric polarization are often obscured by the presence of screening charges. Here, we unravel unambiguous signatures of a distortive polar order buried in the Fermi sea by probing the nonlinear optical response of materials driven by tailored terahertz fields. We apply this strategy to investigate the topological crystalline insulator Pb1-xSnxTe, tracking its soft phonon mode in the time domain and observing the occurrence of inversion symmetry breaking as a function of temperature. By combining measurements across the material's phase diagram with ab initio calculations, we demonstrate the generality of our approach. These results highlight the potential of terahertz driving fields to reveal polar orders coexisting with itinerant electrons, thus opening additional avenues for material discovery.

Achieving High Operating-Temperature Self-powered X-Ray Detection in Multilayered Hybrid Perovskites through Arylamine Intercalation.

Polar metal halide hybrid perovskites (PHPs) that exhibit outstanding bulk photovoltaic effect (BPVE), excellent semiconductor features, and strong radiation absorption ability, have shown prominent advantages in highly sensitive direct X-ray detection. However, it is still a challenge to explore PHPs with high BPVE temperature ranges, answering the demand of developing thermally stable passive X-ray detection. Herein, by intercalating arylamine into lead tribromide and inducing order-disorder phase transition, a 2D multilayered PHPs (BZA)2(MA)Pb2Br7 (BZPB, BZA = benzylamine, MA = methylamine) is synthesized. BZPB crystallizes in a polar space group Aea2 at a low-temperature phase and demonstrates a significant open-circuit of 0.3V deriving from BPVE under X-ray irradiation. Meanwhile, the strong X-ray absorption coefficient and outstanding carrier transport capability of the bilayered lead halide framework associated with the polar BPVE give BZPB excellent X-ray detection abilities. At 0V bias, the impressive sensitivity of BZPB is 98 µC Gy-1 cm-2. Importantly, the introduction of the rigid BZA ring increases the energy barrier of phase transition and thus dramatically enhances the X-ray detection operating temperature of BZPB up to 409 K without significant performance degradation. This work strongly reveals the great potential of rational design of metal halide hybrid perovskites for X-ray detection applications.

MPX3 van der Waals magnets under pressure (M = Mn, Ni, V, Fe, Co, Cd; X = S, Se)

van der Waals antiferromagnets with chemical formula MPX3 (M = V, Mn, Fe, Co, Ni, Cd; X = S, Se) are superb platforms for exploring the fundamental properties of complex chalcogenides, revealing their structure-property relations and unraveling the physics of confinement. Pressure is extremely effective as an external stimulus, able to tune properties and drive new states of matter. In this review, we summarize experimental and theoretical progress to date with special emphasis on the structural, magnetic, and optical properties of the MPX3 family of materials. Under compression, these compounds host inter-layer sliding and insulator-to-metal transitions accompanied by dramatic volume reduction and spin state collapse, piezochromism, possible polar metal and orbital Mott phases, as well as superconductivity. Some responses are already providing the basis for spintronic, magneto-optic, and thermoelectric devices. We propose that strain may drive similar functionality in these materials.

Above-room-temperature chiral skyrmion lattice and Dzyaloshinskii–Moriya interaction in a van der Waals ferromagnet Fe3−xGaTe2

Skyrmions in existing 2D van der Waals (vdW) materials have primarily been limited to cryogenic temperatures, and the underlying physical mechanism of the Dzyaloshinskii–Moriya interaction (DMI), a crucial ingredient for stabilizing chiral skyrmions, remains inadequately explored. Here, we report the observation of Néel-type skyrmions in a vdW ferromagnet Fe3−xGaTe2 above room temperature. Contrary to previous assumptions of centrosymmetry in Fe3−xGaTe2, the atomic-resolution scanning transmission electron microscopy reveals that the off-centered FeΙΙ atoms break the spatial inversion symmetry, rendering it a polar metal. First-principles calculations further elucidate that the DMI primarily stems from the Te sublayers through the Fert–Lévy mechanism. Remarkably, the chiral skyrmion lattice in Fe3−xGaTe2 can persist up to 330 K at zero magnetic field, demonstrating superior thermal stability compared to other known skyrmion vdW magnets. This work provides valuable insights into skyrmionics and presents promising prospects for 2D material-based skyrmion devices operating beyond room temperature.

Elemental topological ferroelectrics and polar metals of few-layer materials

Ferroelectricity can exist in elemental phases as a result of charge transfers between atoms occupying inequivalent Wyckoff positions. We investigate the emergence of ferroelectricity in two-dimensional elemental materials with buckled honeycomb lattices. Various multibilayer structures hosting ferroelectricity are designed by stacking engineering. Ferroelectric materials candidates formed by group IV and V elements are predicted theoretically. Ultrathin Bi films show layer-stacking-dependent physical properties of ferroelectricity, topology, and metallicity. The two-bilayer Bi film with a polar stacking sequence is found to be an elemental topological ferroelectric material. Three and four bilayers Bi films with polar structures are ferroelectriclike elemental polar metals with topological nontrivial edge states. For Ge and Sn, trivial elemental polar metals are predicted. Our work reveals the possibility of designing two-dimensional elemental topological ferroelectrics and polar metals by stacking engineering. Published by the American Physical Society 2024

Two-Dimensional Os2Se3 Nanosheet: A Ferroelectric Metal with Room-Temperature Ferromagnetism.

Two-dimensional (2D) ferroelectric metals (FEMs) possess intriguing characteristics, such as unconventional superconductivity and the nonlinear anomalous Hall effect. However, their occurrence is exceedingly rare due to mutual repulsion between ferroelectricity and metallicity. In addition, further incorporating other features like ferromagnetism into FEMs to enhance their functionalities poses a significantly greater challenge. Here, via first-principles calculations, we demonstrate a case of an FEM that features a coexistence of room-temperature ferromagnetism, ferroelectricity, and metallicity in a thermodynamically stable 2D Os2Se3. It presents a vertical electric polarization of 3.00 pC/m that exceeds those of most FEMs and a moderate polarization switching barrier of 0.22 eV per formula unit. Moreover, 2D Os2Se3 exhibits robust ferromagnetism (Curie temperature TC ≈ 527 K) and a sizable magnetic anisotropy energy (-30.87 meV per formula unit). Furthermore, highly magnetization-dependent electrical conductivity is revealed, indicative of strong magnetoelectric coupling. Berry curvature calculation suggests that the FEM might exhibit nontrivial band topology.

A correlated ferromagnetic polar metal by design.

Polar metals have recently garnered increasing interest because of their promising functionalities. Here we report the experimental realization of an intrinsic coexisting ferromagnetism, polar distortion and metallicity in quasi-two-dimensional Ca3Co3O8. This material crystallizes with alternating stacking of oxygen tetrahedral CoO4 monolayers and octahedral CoO6 bilayers. The ferromagnetic metallic state is confined within the quasi-two-dimensional CoO6 layers, and the broken inversion symmetry arises simultaneously from the Co displacements. The breaking of both spatial-inversion and time-reversal symmetries, along with their strong coupling, gives rise to an intrinsic magnetochiral anisotropy with exotic magnetic field-free non-reciprocal electrical resistivity. An extraordinarily robust topological Hall effect persists over a broad temperature-magnetic field phase space, arising from dipole-induced Rashba spin-orbit coupling. Our work not only provides a rich platform to explore the coupling between polarity and magnetism in a metallic system, with extensive potential applications, but also defines a novel design strategy to access exotic correlated electronic states.

Promoting exciton dissociation by metal ion modification in polymeric carbon nitride for photocatalysis

Carbon nitride, a typical low-dimensional conjugated polymer photocatalyst, features a high exciton binding energy due to the weak dielectric screening and the strong Coulombic attraction of photogenerated electrons and holes. The reduction of the exciton binding energy of carbon nitride to promote the conversion from excitons into free carriers is the first priority for the improvement of charge-transfer-dependent photocatalytic reaction activity. In this paper, by introducing a variety of polar metal cations to carbon nitride, it is demonstrated that the charge distribution of the heptazine ring can be improved by ion polarization, which effectively promotes the dissociation of excitons into electrons and holes. The sodium ion shows the best modification effect, which enhances the rate of both photocatalytic hydrogen and hydrogen peroxide production by about 50 %. Characterization shows that the introduction of strongly polar metal cations contributes to the reduction of the exciton dissociation energy of carbon nitride. This study provides a new perspective and a convenient method for the exciton modulation engineering of low-dimensional photocatalysts.

Universal responses in nonmagnetic polar metals

We demonstrate that two phenomena, the kinetic magnetoelectric effect and the nonlinear Hall effect, are universal to polar metals as a consequence of their coexisting and contraindicated polarization and metallicity. We show that measurement of the effects provides a complete characterization of the nature of the polar metal, in that the nonzero response components indicate the direction of the polar axis, and the coefficients change signs on polarization reversal and become zero in the nonpolar phase. We illustrate our findings for the case of electron-doped PbTiO3 using a combination of density functional theory and model Hamiltonian-based calculations. Our model Hamiltonian analysis provides crucial insight into the microscopic origin of the effects, showing that they originate from inversion-symmetry-breaking-induced interorbital hoppings that correlate to an asymmetric charge density quantified by odd-parity charge multipoles. Our paper both heightens the relevance of the kinetic magnetoelectric and nonlinear Hall effects, and broadens the platform for investigating and detecting odd-parity charge multipoles in polar metals. Published by the American Physical Society 2024

Polar metals with coexisting ferroelectricity and high-density conduction electrons

Polar metals, a family of materials with exclusive but coexisting electric polarization and metallicity, have attracted plenty of studies recently. Experimentally, how these two exclusive states interact with each other is still an open question thus far. Here, we report on the existence of ferroelectric metal states with switchable electric polarization and unexpected high carrier density in Ba0.5La0.5TiO3 films. A combination of atomic resolution scanning transmission electron microscopy, high-resolution x-ray diffraction, piezoresponse force microscopy, optical second harmonic generation, and electrical transport was utilized to investigate the crystal and electronic structures of Ba0.5La0.5TiO3 films. Unexpectedly, with the modulation of ferroelectricity, the density of conduction electrons can be tuned from 1019 to 1021 cm−3. Our results provide a way to design polar metals with coexisting switchable electric polarization and high-density conduction electrons.

Realization of a strong ferroelectric metal by Nb-doping in strained EuTiO3

Ferroelectric (FE) metals have been attracting attention as they possess both metallicity and ferroelectricity, the two seemingly incompatible physical properties. An important problem for both fundamental research and potential applications is how to realize a strong FE metal. One strategy is to dope a strong FE insulator and turn it into an FE metal, provided that the induced free electrons do not destroy FE distortion. Strained EuTiO3 is a strong FE ferromagnet and a promising candidate for such a strategy. Here, we calculate the structural, electronic, and polarization properties of Nb-doped strained EuTiO3 using the hybrid density-functional theory. The results show that the strained EuTi0.875Nb0.125O3 is a strong FE metal as the Nb doping induces metallicity without weakening the FE distortion. The underlying atomistic mechanism of the coexistence of metallicity and strong ferroelectricity is discussed. The findings show that combining doping and strain engineering is a promising way to realize new EuTiO3-based strong FE metals and may be used in other materials as well.

Panoply of Ni-Doping-Induced Reconstructions, Electronic Phases, and Ferroelectricity in 1T-MoS2.

The distorted phases of monolayer 1T-MoS2 have distinct electronic properties, with potential applications in optoelectronics, catalysis, and batteries. We theoretically investigate the use of Ni-doping to generate distorted 1T phases and find not only the ones usually reported but also two further phases (3 × 3 and 4 × 4), depending on the concentration and the substitutional or adatom doping site. Corresponding pristine phases are stable after removal of dopants, which might offer a potential route to experimental synthesis. We find large ferroelectric polarizations, most notably in 3 × 3 which─compared to the recently measured 1T″─has 100 times greater ferroelectric polarization, a lower energy, and a larger band gap. Doped phases include exotic multiferroic semimetals, ferromagnetic polar metals, and improper ferroelectrics with only in-plane polarization switchable. The pristine phases have unusual multiple gaps in the conduction bands with possible applications for intermediate band solar cells, transparent conductors, and nonlinear optics.