Lattice Polarization Research Articles

Born effective charges and electric polarization in bulk ε-Fe2O3: An ab-initio approach

Limitation in the synthesis of single-crystal has restricted the complete understanding of the magnetoelectric properties of bulk ε-Fe2O3. In order to understand the electric polarization of bulk ε-Fe2O3, we have employed a first-principles Berry-phase method and the density functional perturbation theory to calculate the spontaneous electric polarization and Born effective charges (BECs) of ε-Fe2O3. The polarization quanta of the ferroelectric bulk ε-Fe2O3 and its polarization lattice are also reported. The polarization switching through a paraelectric transition phase of a low barrier is found to be favorable. The polarization and BECs calculated by both methods are in good agreement.

Heterotic strings on (K3 \xd7 T2)/\u21243 and their dual Calabi-Yau threefolds

In this paper we study compactifications of the mathcal{N} = 2 heterotic E8× E8 string on (K3 × T2)/ℤ3 with various gauge backgrounds and calculate the topological couplings in the effective supergravity action that arise from one-loop amplitudes. We then identify candidates for dual type IIA compactifications on Calabi-Yau threefolds and compare the heterotic results with the corresponding topological string amplitudes. We find that the dual Calabi-Yau geometries are K3 fibrations that are also genus one fibered with three- sections. Moreover, we show that the intersection form on the polarization lattice of the K3 fibration has to be three times the intersection form on the Narain lattice Γ1,1.

Graphene‐Assisted Epitaxy of Nitrogen Lattice Polarity GaN Films on Non‐Polar Sapphire Substrates for Green Light Emitting Diodes

AbstractLattice polarity is a key point for hexagonal semiconductors such as GaN. Unfortunately, only Ga‐polarity GaN have been achieved on graphene till now. Here, the epitaxy of high quality nitrogen‐polarity GaN films on transferred graphene on non‐polar sapphire substrates by molecular beam epitaxy is reported. This success is achieved through atomic nitrogen irradiation, where CN bonds are formed in graphene and provide nucleation sites for GaN and leading to N‐polarity GaN epitaxy. The N‐polarity characteristics are confirmed by chemical etching and transmission electron microscopy measurement. Due to the higher growth temperature of InGaN at N‐polarity than that at Ga‐polarity, green light emitting diodes are fabricated on the graphene‐assisted substrate, where a large redshift of emission wavelength is observed. These results open a new avenue for the polarity modulation of III‐nitride films based on 2D materials, and also pave the way for potential application in longer wavelength light emitting devices.

Epitaxial growth of InAs/GaAs quantum dots on {113}-faceted Ge/Si (001) hollow substrate

The direct epitaxial growth of GaAs on Si suffers from their nature of lattice mismatch, thermal mismatch and polarity difference induced anti-phase domains (APDs). Here, we report the high quality and thin GaAs film grown on {113}-faceted Ge/Si (001) hollow substrate by in-situ hybrid molecular beam epitaxy. By directly growth of Ge on U-shape patterned Si (001), a strain-relaxed high-quality Ge sawtooth hollow structure with {113} facets was obtained. With an additional 400 nm GaAs deposition, an APD-free surface with a root-mean-square roughness of merely 0.67 nm is obtained on such Ge {113} /Si (001) substrate. The lattice mismatch dislocation between Ge and Si is found to terminate mostly at the sidewalls of the hollow structures. The {113}-faceted Ge surface is acting as an equivalent to the miscut substrate, which annihilates the APDs at the GaAs/Ge interface. High-resolution X-ray diffraction characterization reveals that the hollow structures can effectively reduce the thermal strain, leading to a crack-free GaAs film up to 7 µm. Five-layer InAs/GaAs quantum dots (QDs) on such virtual GaAs/Ge {113} /Si (001) substrate without any dislocation filter layers exhibits almost the same photoluminescence (PL) intensity as that on the GaAs substrate, providing a promising method for integrating III-V QD lasers with silicon photonic platform.

Searching high spin polarization ferromagnet in Heusler alloy via machine learning

In order to search for stable ferromagnets with high spin polarization in Heusler alloys for spintronic applications, we develop an efficient machine learning workflow based on a deep neural network, whose training data were collected from the open quantum materials database and high throughput calculation by first-principle calculations. The lattice constants, formation energy and spin polarization of 10 577 candidate materials were predicted, and 192 materials with high spin polarization were selected according to a spin polarization greater than 0.87 and formation energy less than 80 meV/atom. 57 of these alloys have been reported as Half-metal (100% spin polarization) according to previous researches, and 18 have been reported as semiconductors. Especially, 6 Heusler alloys were identified as promising half-metallic ferromagnets, and some of them have high Curie temperature above room temperature. Our study suggests this approach is an efficient method for the discovery of superior spintronic materials, which should be also suitable for exploring other functional materials.

Multiferroic behavior from synergetic response of multiple ordering parameters in BiFeO3 single crystal under high magnetic field up to 50 Tesla

BiFeO3, a typical multiferroic compound, owns complicated interactions among its charge, lattice, and spin degrees, while largely remaining a mystery. In this work, the BiFeO3 single crystal has been investigated in pulsed high magnetic fields up to 50 T. The lattice, spin, and charge orderings are found to couple with each other strongly. High magnetic field could drive the spin cycloid phase to a canted antiferromagnetic phase while accompanied by a clear magnetostriction response and weakening of ferroelectric polarization. The critical magnetic field for the magnetic phase transition was shifted to the lower magnetic field region by an external electric field over a broad temperature range, which indicates the complex interaction among magnetization, electrical polarization, and crystal lattice. This work has provided a clear picture to show the synergetic response of multiple ordering parameters during the magnetic phase transition in BiFeO3, which could benefit the design of BiFeO3 based spintronic devices and novel multiferroic materials with strong magnetoelectric coupling.

Defective crystal plane-oriented induced lattice polarization for the photocatalytic enhancement of ZnO

The mechanism of the photocatalytic reaction of defective ZnO systems was determined.

Interface-induced magnetic polar metal phase in complex oxides

Polar metals are commonly defined as metals with polar structural distortions. Strict symmetry restrictions make them an extremely rare breed as the structural constraints favor insulating over metallic phase. Moreover, no polar metals are known to be magnetic. Here we report on the realization of a magnetic polar metal phase in a BaTiO3/SrRuO3/BaTiO3 heterostructure. Electron microscopy reveals polar lattice distortions in three-unit-cells thick SrRuO3 between BaTiO3 layers. Electrical transport and magnetization measurements reveal that this heterostructure possesses a metallic phase with high conductivity and ferromagnetic ordering with high saturation moment. The high conductivity in the SrRuO3 layer can be attributed to the effect of electrostatic carrier accumulation induced by the BaTiO3 layers. Density-functional-theory calculations provide insights into the origin of the observed properties of the thin SrRuO3 film. The present results pave a way to design materials with desired functionalities at oxide interfaces.

Crystal Engineering of Bi2WO6 to Polar Aurivillius-Phase Oxyhalides

The Aurivillius phases of complex bismuth oxides have attracted considerable attention because of their lattice polarization (ferroelectricity) and photocatalytic activity. We report a first-princi...

Modified transverse Ising model for the dielectric properties of SrTiO3 films and interfaces

The transverse Ising model (TIM), with pseudospins representing the lattice polarization, is often used as a simple description of ferroelectric materials. However, we demonstrate that the TIM, as it is usually formulated, provides an incorrect description of SrTiO3 films and interfaces because of its inadequate treatment of spatial inhomogeneity. We correct this deficiency by adding a pseudospin anisotropy to the model. We demonstrate the physical need for this term by comparison of the TIM to a typical Landau–Ginzburg–Devonshire model. We then demonstrate the physical consequences of the modification for two model systems: a ferroelectric thin film, and a metallic LaAlO3/SrTiO3 interface. We show that, in both cases, the modified TIM has a substantially different polarization profile than the conventional TIM. In particular, at low temperatures the formation of quantized states at LaAlO3/SrTiO3 interfaces only occurs in the modified TIM.

Superconductivity near a Ferroelectric Quantum Critical Point in Ultralow-Density Dirac Materials

The experimental observation of superconductivity in doped semimetals and semiconductors, where the Fermi energy is comparable to or smaller than the characteristic phonon frequencies, is not captured by the conventional theory. In this paper, we propose a mechanism for superconductivity in ultralow-density three-dimensional Dirac materials based on the proximity to a ferroelectric quantum critical point. We derive a low-energy theory that takes into account both the strong Coulomb interaction and the direct coupling between the electrons and the soft phonon modes. We show that the Coulomb repulsion is strongly screened by the lattice polarization near the critical point even in the case of vanishing carrier density. Using a renormalization group analysis, we demonstrate that the effective electron-electron interaction is dominantly mediated by the transverse phonon mode. We find that the system generically flows towards strong electron-phonon coupling. Hence, we propose a new mechanism to simultaneously produce an attractive interaction and suppress strong Coulomb repulsion, which does not require retardation. For comparison, we perform same analysis for covalent crystals, where lattice polarization is negligible. We obtain qualitatively similar results, though the screening of the Coulomb repulsion is much weaker. We then apply our results to study superconductivity in the low-density limit. We find strong enhancement of the transition temperature upon approaching the quantum critical point. Finally, we also discuss scenarios to realize a topological $p$-wave superconducting state in covalent crystals close to the critical point.

Ni2+ guided phase/structure evolution and ultra-wide bandwidth microwave absorption of CoxNi1-x alloy hollow microspheres

A series of bimetallic hexagonal close-packed (HCP) and face-centered cubic (FCC) CoxNi1−x (x = 1, 0.858, 0.813, 0.714, 0.662, 0.137) alloy hollow microspheres (AHMs) with continuously tunable composition and wall thickness were successfully synthesized via one-pot liquid phase reduction. In addition to prolonged reaction time, decreased metal ion concentration, and elevated reaction temperature, Ni2+ addition favors an inside-out Ostwald ripening and Kirkendall diffusion, leading to the formation of CoxNi1−x AHMs with thin wall thickness. Moreover, Ni2+ addition induced the phase transformation from HCP Co to FCC Co and modulated crystal size, internal strain, lattice constant, composition, and wall thickness. Accordingly, Ms decreased linearly as Ni content increased. Alloying of Ni with Co induced significantly enhanced μ″, negative ε′ and ultra-wide bandwidth microwave absorption due to the Plasmon resonance, defect polarization and lattice polarization. The wax-based composites containing 35 wt% Co0.81Ni0.19, 45 wt% Co0.86Ni0.14, or 50 wt% Co0.66Ni0.34 AHMs possessed significantly enhanced absorption capability with maximum RL values of −35.3, −47.3, and −54.6 dB and ultra-wide bandwidths (RL ≤ −10 dB) of 8.16, 9.2, and 10.08 GHz, corresponding to layer thicknesses of 1.9, 1.8, and 2.6 mm, respectively. CoxNi1−x AHMs exhibited stronger absorption, broader bandwidth, and lighter weight than those of other absorbers. Superior microwave absorption performance (MAP) were mainly creditable to the synergy of enhanced permittivity and permeability, high attenuation, and good impedance matching caused by hollow structure, Ni incorporation, and Plasmon resonance. CoxNi1−x AHMs with tunable Ms and excellent MAP might solve electromagnetic pollution and interference problems for further applications in microwave absorption and shielding fields.

Enhancing electrical properties of high-Curie temperature piezoelectric ceramics BNT-PZT and their mechanism

Due to the urgent demands for high-Curie temperature (TC/Tm) piezoelectric materials in geothermal exploration, aerospace and related fields, high-TC/Tm ferroelectrics have attracted booming research attention. The high-Tm 0.25Bi(Ni1/2Ti1/2)O3-0.75 Pb(Zr1/2Ti1/2)O3 (0.25BNT-0.75PZT) ceramics were prepared by solid-state sintering method and via partial oxalate route, where the 0.25BNT-0.75PZT ceramics prepared via the partial oxalate route exhibit better electrical properties (Tm = 232 °C (10 kHz), εm = 10963, Pr = 26.14 μC/cm2, Ec = 15.61 kV/cm, d33 = 521 pC/N, d33* = 553.3 pm/V, Kp = 47.1%, and Qm = 21.6). The nano-scale domain configuration of the ceramics was revealed by piezoelectric force microscopy (PFM), and the relationship between the micro-structure and macro-electrical properties was analyzed. The ferroelectric phase transition mechanism was studied by temperature dependent Raman spectroscopy. The reduction of energy barrier of lattice distortion and polarization deflection is caused by nanometer-sized domain structure, low symmetric polar nano-regions and/or coexistence of multi-ferroelectric phases, contributing to the excellent electrical properties of the 0.25BNT-0.75PZT ceramics prepared via the partial oxalate route.

A Solid with Liquid-like Diffusion: A Unique Superionic Conductor

Understanding fast ionic diffusion in solids is key to engineering solid electrolytes for energy storage devices such as all-solid-state batteries. In this issue of Chem, Hautier and co-workers conduct atomistic modeling and nuclear magnetic resonance experiments to illustrate how the unique structure of LiTi2(PS4)3 enables unprecedented Li-ion diffusivity.

Surface and Volume Phonon Polaritons in Boron Nitride Nanotubes.

Phonon polaritons (PhPs) are quasiparticles created by coupling of photons to polar lattice vibrations. Previously, PhPs have been observed as both surface and volume confined waves. The dispersion of the polariton depends strongly on the nature of the material. Volume polaritons show asymptotic behavior near the longitudinal optical phonon frequency of the material, whereas surface polaritons instead approach the surface phonon frequency. Boron nitride nanotubes (BNNTs) were found to exhibit the dispersion of volume modes below the surface phonon frequency. However, around and above the surface phonon frequency, the behavior becomes that of a surface wave with an amplified near-field response. These findings improve our understanding of photonics within BNNTs and suggest potential applications that take advantage of the high fields and density of states in that spectral region.

History of the reciprocal lattice

History of the development of the reciprocal lattice is reviewed. The reciprocal lattice as an essential tool for the study of diffraction experiments by ordered structures and characterization of their structural properties is widely taught in any text of solid state or chemistry, but usually without discussion of its history. This article aims to give a coherent historical perspective on the reciprocal lattice. First, a basic introduction to the reciprocal lattice concept, its mathematical foundation and physical origin, and its relationship with the direct lattice is provided. Then a detailed chronicle of ideas leading to the concept of the reciprocal lattice is presented, including a review of the contributions of Gibbs, Ewald, and others. The polar lattice concept, the great ancestor of the reciprocal lattice, is presented.

Tailoring the dielectric properties of KDP crystals by shock waves for microelectronic and optoelectronic applications

In the present research article, authors report on tailoring the dielectric properties of potassium dihydrogen phosphate (KDP) crystals using supersonic shock waves that is proposed and demonstrated. The present experimentation involves an investigation of loading single pulse of shock waves with different Mach numbers such as 1.7, 1.9, 2.2, 2.4 and 2.5 on KDP crystals using shock tube. The crystalline nature and dielectric behavior of pre and post shock loaded KDP crystals are evaluated by PXRD and impedance analyzer techniques, respectively. Interestingly, shock wave loaded test materials show low dielectric constant compared to pre shock test crystal. Moreover, Mach 1.9 shock wave loaded test crystal shows the lowest dielectric constant compared to other Mach numbers and it is due to the reduction of lattice polarization and domain orientation changes. The experimental results clearly show that the test crystal can be a potential material for microelectronic and optoelectronic applications because it acquires low dielectric constant on shock exposure.

Calabi\u2013Yau threefolds fibred by high rank lattice polarized K3 surfaces

We study threefolds fibred by K3 surfaces admitting a lattice polarization by a certain class of rank 19 lattices. We begin by showing that any family of such K3 surfaces is completely determined by a map from the base of the family to the appropriate K3 moduli space, which we call the generalized functional invariant. Then we show that if the threefold total space is a smooth Calabi–Yau, there are only finitely many possibilities for the polarizing lattice and the form of the generalized functional invariant. Finally, we construct explicit examples of Calabi–Yau threefolds realizing each case and compute their Hodge numbers.

Non-overlapping small polaron tunneling conduction coupled dielectric relaxation in weak ferromagnetic NiAl2O4

The inverse spinel nickel aluminate micro-particles was successfully synthesized via solid-state synthesis techniques. The single phase cubic structure with space group Fd3m of the as-prepared sample was entrenched from Rietveld refinement of x-ray diffraction pattern. In addition, the photoluminescence (PL) and FTIR spectra were also performed to give strong evidence of its pure phase formation. The narrow hysteresis m–h loop and UV-DRS spectra at room temperature demonstrate its weak ferromagnetic and semiconducting nature with saturation magnetization 64.96 × 10−3 emu gm−1 and direct optical band gap 2.03 eV respectively. The high-resolution FESEM micrographs and EDS elemental analysis exhibit its grain growth in µm range (217 µm) and presence of elemental compound Ni, Al and O respectively. The electrical transport properties were accomplished by complex impedance spectroscopy as a function of frequency (100 Hz–1 MHz) with the evolution of high temperature (300 °C–500 °C). The Nyquist plots (Z″ versus Z′) were well fitted with an equivalent circuit model (QR) (QR) consisting of a series combination of intra and inter-granular contribution. Furthermore, the imaginary modulus spectra were also fitted with Kohlrausch–Williams–Watts (KWW) function, which represents two thermally activated peaks of grain and grain boundary effects. The low-frequency dispersive ac conductivity was elucidated using the following equation: . The increasing nature of temperature dependent frequency exponent (n) suggests the quantum mechanical tunneling: non-overlapping small polaron tunneling (NSPT) concept for conduction mechanism. In low-frequency region, the lattice and charge carrier polarization simultaneously contribute in dielectric permittivity.

Influence of screening dynamics on excitons in Ga2O3 polymorphs

Exciton binding energies EB measured for α- and β-Ga2O3 crystals seem to be not explainable in the Wannier-Mott picture. However, we demonstrate theoretically that including screening dynamics and effective mass anisotropy, reasonable values EB = 184 meV (α) or 230 meV (β) are derived. Since the binding energies are larger than the phonon frequencies, the exciton formation is so fast that only about 5% of the lattice polarizability contributes to the screening of the electron-hole attraction. Effective dielectric constants possess values between the complete static ones and the electronic high-frequency dielectric constants. They indeed depend on the polymorph.