Lattice Polarization Research Articles

Dual modulation of lattice strain and charge polarization induced by Co(OH)2/Ni(OH)2 interfaces for efficient oxygen evolution catalysis

Co(OH)2/Ni(OH)2 heterostructure nanomaterials were constructed by an in situ etching hydrothermal method and exhibited excellent oxygen evolution reaction activity due to the dual modulation of lattice strain and charge polarization induced by Co(OH)2/Ni(OH)2 interfaces.

Improved Performance of GaN-Based Ultraviolet LEDs With Electron Blocking Layers Composed of Double-Peak p-Type Al x Ga1−x N/GaN Superlattice Layers

Past studies have demonstrated the positive impact of step-graded $p$ -type Al x Ga1− x N/GaN superlattice (SL) electron blocking layer (EBL) structures on the efficiency performance of ultraviolet (UV) GaN-based light-emitting diodes (LEDs). However, the optimal Al-grading structure of these SL EBLs remains unclear owing to a lack of systematic investigation. The present work addresses this issue by applying EBL composed of alternating half-peak, single-peak, and double-peak $p$ -type Al x Ga1− x N/GaN SL structures with varying values of $x$ ranging from 0.05 to 0.15 in 0.05 step increments. Simulation analysis is employed to obtain the internal quantum efficiency (IQE), energy band diagrams, polarization compensation factor, hole concentration, and electron concentration of GaN-based UV LEDs with three different SL-EBL structures. The results obtained at an injection current of 200 mA demonstrate that UV LEDs with double-peak SL-EBL structures provide the maximum IQE, which is ~38% greater than that of devices employing the conventional EBL in simulation experiment. This SL-EBL is demonstrated to improve the hole injection and electron overflow performance of GaN-based UV LEDs owing to the polarization charge and lattice mismatch at the p- Al x Ga1− x N/GaN interfaces. The reduced Al composition on the p- GaN side reduces the potential barrier of hole injection. Moreover, the predicted increase in the IQE of GaN-based UV LEDs with the optimal SL-EBL is verified experimentally.

Hidden spontaneous polarisation in the chalcohalide photovoltaic absorber Sn2SbS2I3.

Perovskite-inspired materials aim to replicate the optoelectronic performance of lead-halide perovskites, while eliminating issues with stability and toxicity. Chalcohalides of group IV/V elements have attracted attention due to enhanced stability provided by stronger metal-chalcogen bonds, alongside compositional flexibility and ns2 lone pair cations – a performance-defining feature of halide perovskites. Following the experimental report of solution-grown tin-antimony sulfoiodide (Sn2SbS2I3) solar cells, with power conversion efficiencies above 4%, we assess the structural and electronic properties of this emerging photovoltaic material. We find that the reported centrosymmetric Cmcm crystal structure represents an average over multiple polar Cmc21 configurations. The instability is confirmed through a combination of lattice dynamics and molecular dynamics simulations. We predict a large spontaneous polarisation of 37 μC cm−2 that could be active for electron–hole separation in operating solar cells. We further assess the radiative efficiency limit of this material, calculating ηmax > 30% for film thicknesses t > 0.5 μm.

The Study on Structural and Photoelectric Properties of Zincblende InGaN via First Principles Calculation

In this paper, the structure and photoelectric characteristics of zincblende InxGa1−xN alloys are systematically calculated and analyzed based on the density functional theory, including the lattice constant, band structure, distribution of electronic states, dielectric function, and absorption coefficient. The calculation results show that with the increase in x, the lattice constants and the supercell volume increase, whereas the bandgap tends to decrease, and InxGa1−xN alloys are direct band gap semiconductor materials. In addition, the imaginary part of the dielectric function and the absorption coefficient are found to redshift with the increase in indium composition, expanding the absorption range of visible light. By analyzing the lattice constants, polarization characteristics, and photoelectric properties of the InxGa1−xN systems, it is observed that zincblende InxGa1−xN can be used as an alternative material to replace the channel layer of wurtzite InxGa1−xN heterojunction high electron mobility transistor (HEMT) devices to achieve the manufacture of HEMT devices with higher power and higher frequency. In addition, it also provides a theoretical reference for the practical application of InxGa1−xN systems in optoelectronic devices.

Ultrafast strain engineering and coherent structural dynamics from resonantly driven optical phonons in LaAlO3

Strain engineering has been extended recently to the picosecond timescales, driving ultrafast metal–insulator phase transitions and the propagation of ultrasonic demagnetization fronts. However, the nonlinear lattice dynamics underpinning interfacial optoelectronic phase switching have not yet been addressed. Here we perform time-resolved all-optical pump-probe experiments to study ultrafast lattice dynamics initiated by impulsive light excitation tuned in resonance with a polar lattice vibration in LaAlO3 single crystals, one of the most widely utilized substrates for oxide electronics. We show that ionic Raman scattering drives coherent rotations of the oxygen octahedra around a high-symmetry crystal axis. By means of DFT calculations we identify the underlying nonlinear phonon–phonon coupling channel. Resonant lattice excitation is also shown to generate longitudinal and transverse acoustic wave packets, enabled by anisotropic optically induced strain. Importantly, shear strain wave packets are found to be generated with high efficiency at the phonon resonance, opening exciting perspectives for ultrafast material control.

Spontaneous cycloidal order mediating a spin-reorientation transition in a polar metal

We show how complex modulated order can spontaneously emerge when magnetic interactions compete in a metal with polar lattice distortions. Combining neutron and resonant x-ray scattering with symmetry analysis, we reveal that the spin reorientation in Ca$_3$Ru$_2$O$_7$ is mediated by a magnetic cycloid whose eccentricity evolves smoothly but rapidly with temperature. We find the cycloid to be highly sensitive to magnetic fields, which appear to continuously generate higher harmonic modulations. Our results provide a unified picture of the rich magnetic phases of this correlated, multi-band polar metal.

Dynamic polaronic screening for anomalous exciton spin relaxation in two-dimensional lead halide perovskites.

Two-dimensional lead halide perovskites with confined excitons have shown exciting potentials in optoelectronic applications. It is intriguing but unclear how the soft and polar lattice redefines excitons in layered perovskites. Here, we reveal the intrinsic exciton properties by investigating exciton spin dynamics, which provides a sensitive probe to exciton coulomb interactions. Compared to transition metal dichalcogenides with comparable exciton binding energy, we observe orders of magnitude smaller exciton-exciton interaction and, counterintuitively, longer exciton spin lifetime at higher temperature. The anomalous spin dynamics implies that excitons exist as exciton polarons with substantially weakened inter- and intra-excitonic interactions by dynamic polaronic screening. The combination of strong light matter interaction from reduced dielectric screening and weakened inter-/intra-exciton interaction from dynamic polaronic screening explains their exceptional performance and provides new rules for quantum-confined optoelectronic and spintronic systems.

Screening of hot electrons in the ferroelectric semiconductor In2Se3

In a conventional semiconductor, electrons $(e)$ scatter efficiently with longitudinal-optical phonons $(\mathrm{p}{\mathrm{h}}_{\mathrm{LO}})$ via the Fr\ohlich interaction, e.g., in polaron formation or hot-electron cooling. Recent studies on lead halide perovskites have suggested that the Fr\ohlich interaction may be sufficiently screened by the polar lattice. Here we show that hot-electron cooling in a layered ferroelectric semiconductor $\mathrm{I}{\mathrm{n}}_{2}\mathrm{S}{\mathrm{e}}_{3}$ slows down by 50% as temperature is increased from 115 to 387 K; this decrease is about four times more than that predicted by the Fr\ohlich model. We suggest that screening of Fr\ohlich interaction by polar domains or fluctuations in the ferroelectric semiconductor may be responsible for the reduced $\mathrm{e}\text{\ensuremath{-}}\mathrm{p}{\mathrm{h}}_{\mathrm{LO}}$ scattering. Such enhanced screening in polar materials featuring anharmonic lattice instability may also reduce carrier scattering with other charge carriers or with charged defects, leading to improved optoelectronic properties.

Pressure-Regulated Dynamic Stereochemical Role of Lone-Pair Electrons in Layered Bi2O2S.

Lone-pair electrons (LPEs) ns2 in subvalent 14 and 15 groups lead to highly anharmonic lattice and strong distortion polarization, which are responsible for the groups' outstanding thermoelectric and optoelectronic properties. However, their dynamic stereochemical role in structural and physical properties is still unclear. Here, by introducing pressure to tune the behavior of LPEs, we systematically investigate the lone-pair stereochemical role in a Bi2O2S. The gradually suppressed LPEs during compression show a nonlinear repulsive electrostatic force, resulting in two anisotropic structural transitions. An orthorhombic-to-tetragonal phase transition happens at 6.4 GPa, caused by the dynamic cation centering. This structural transformation effectively modulates the optoelectronic properties. Further compression beyond 13.2 GPa induces a 2D-to-3D structural transition due to the disappearance of the Bi-6s2 LPEs. Therefore, the pressure-induced LPE reconfiguration dominates these anomalous variations of lattice, electronic, and optical properties. Our findings provide new insights into the materials optimization by regulating the characters of LPEs.

Comparative study of oxidizing ambient infused with varying nitrogen flow rates for fabrication of ternary nitrided AlZrO based MOS capacitor

High dielectric constant nitrided aluminum zirconium oxide films were successfully fabricated on silicon substrate via an oxidation process infused with different nitrogen gas flow rates at 800°C. The presence of nitrogen piling up at the interface could control the thickness of interfacial layer formation. An increase in the nitrogen gas flow rate might be effective in controlling the overall thickness of the films, yet the nitrogen piling up at the interface impeded the diffusion of oxygen to the interface. Consequently, the interface quality was affected, leading to degradation in the interface-state density. In addition, the generation of oxygen vacancies in the films, as well as the presence of nitrogen at the interstitial sites, would create a lattice polarization, which decreased the k values. Corresponding effects were also discussed in corroboration with energy band gap, effective oxide charges, and slow trap density, as well as interface-state density calculated from capacitance and conductance methods.

Spin/charge density waves at the boundaries of transition metal dichalcogenides

One-dimensional grain boundaries of two-dimensional semiconducting {\MX} (M= Mo,W; X=S,Se) transition metal di-chalcogenides are typically metallic at room temperature. The metallicity has its origin in the lattice polarization, which for these lattices with $D_{3h}$ symmetry is a topological invariant, and leads to one-dimenional boundary states inside the band gap. For boundaries perpendicular to the polarization direction, these states are necessarily 1/3 occupied by electrons or holes, making them susceptible to a metal-insulator transition that triples the translation period. Using density-functional-theory calculations we demonstrate the emergence of combined one-dimensional spin density/charge density waves of that period at the boundary, opening up a small band gap of $\sim 0.1$ eV. This unique electronic structure allows for soliton excitations at the boundary that carry a fractional charge of $\pm 1/3\ e$.

Connecting the Multiscale Structure with Macroscopic Response of Relaxor Ferroelectrics

AbstractLead‐based relaxor ferroelectrics are characterized by outstanding piezoelectric and dielectric properties, making them useful in a wide range of applications. Despite the numerous models proposed to describe the relation between their nanoscale polar structure and the large properties, the multiple contributions to these properties are not yet revealed. Here, by combining atomistic and mesoscopic‐scale structural analyses with macroscopic piezoelectric and dielectric measurements across the (100–x)Pb(Mg1/3Nb2/3)O3–xPbTiO3 (PMN–xPT) phase diagram, a direct link is established between the multiscale structure and the large nonlinear macroscopic response observed in the monoclinic PMN‐xPT compositions. The approach reveals a previously unrecognized softening effect, which is common to Pb‐based relaxor ferroelectrics and arises from the displacements of low‐angle nanodomain walls, facilitated by the nanoscale polar character and lattice strain disorder. This comprehensive comparative study points to the multiple, distinct mechanisms that are responsible for the large piezoelectric response in relaxor ferroelectrics.

Hot Carrier Relaxation in CsPbBr3-Based Perovskites: A Polaron Perspective.

Long-standing interpretations for the exceptional photovoltaic and optoelectronic properties showcased by the perovskite family pertain to the underlying complicated interplay of polaron formation and hot carrier cooling. This Perspective primarily focuses on reassessing the existing status of polaron studies conducted on CsPbBr3-based systems in particular, in the framework of transient absorption investigations. The role of the key aspect that is ultimately accountable for deciding the fate of polaron formation, i.e., the carrier-longitudinal optical phonon coupling, has been comprehensively evaluated in terms of diverse factors which affect this Fröhlich interaction-mediated coupling. The study provides a detailed discussion regarding the alterations in lattice polarity, surrounding dielectric medium, lattice temperature, and system dimensionality which can influence the charge screening extent and thereby the polaron formation. Such studies concerning strategies for achieving easily attainable modulations in polaron formation in CsPbBr3-based systems are highly relevant for technological advancement.

Structural, electronic and magnetic properties of full-Heusler alloy Co2CrAl

The first-principle calculations combined with the full-potential linearized augmented plane wave method with the help of density functional theory (DFT) implemented in WIEN2k within the Perdew-Burke-Ernzerhof of generalized gradient approximations (GGA-PBE), LSDA, GGA+U (local spin density approximation (LSDA)+U) are used to investigate the structural, electronic and magnetic properties of full-Heusler alloy Co2CrAl. We add the parameter, “U” to GGA and LSDA approximations to improve certain physical parameters. We found that the structure Fm-3m is stable and we also determined the parameters of the equilibrium lattice using the optimizing method. The lattice constant, bulk modulo, energy gap, spin polarization and density of have been calculated. The LSDA, LSDA+U, GGA and GGA+U approximations are used to calculate the density of state and band structure analysis together with spin magnetic moments. It is found that Co2CrAl has a half metallic character for a wide range of lattice parameters. The exchange energy calculated confirms that the ground state ferromagnetic (FM) is more stable than the non-magnetic (NM) states of L21 phase. The magnetic moment of Co and Cr is obtained in Co2CrAl using LSDA+U and GGA+U approaches. The thermal magnetization, magnetic entropy change, adiabatic temperature change and relative cooling power are obtained via Monte Carlo simulation using the value of exchange couplings found by Ab initio calculations. The curie temperature has been established. The half metallic is one of the key properties of magnetic materials for the application of spintronics devices and magnetic refrigeration.

Mirror symmetry for K3 surfaces

For certain K3 surfaces, there are two constructions of mirror symmetry that are very different. The first, known as BHK mirror symmetry, comes from the Landau-Ginzburg model for the K3 surface; the other, known as LPK3 mirror symmetry, is based on a lattice polarization of the K3 surface in the sense of Dolgachev's definition. There is a large class of K3 surfaces for which both versions of mirror symmetry apply. In this class we consider the K3 surfaces admitting a certain purely nonsymplectic automorphism of order 4, 8, or 12, and we complete the proof that these two formulations of mirror symmetry agree for this class of K3 surfaces.

Polarization-resolved Er emission in Er doped GaN bulk crystals

Erbium-doped GaN (Er:GaN) quasi-bulk crystals are emerging as a promising novel gain medium for high energy lasers emitting at the retina-safe wavelength window of 1.5 μm. We report the polarization-resolved photoluminescence (PL) emission spectroscopy studies, which revealed that the pumping efficiency with the excitation polarization parallel to the c-axis of GaN (E⇀||c⇀) is significantly higher than that with the excitation polarization perpendicular to the c-axis of GaN (E⇀⊥c⇀). This phenomenon is a direct consequence of the inherent polar wurtzite GaN lattice, giving rise to a net local field, surrounding each Er ion, along the c-axis of GaN. The temperature dependent behaviors of the PL emission spectra were explained in terms of the Boltzmann population distributions among sublevels within the 4I15/2 ground state and the 4I13/2 first excited state of Er3+ in GaN, thereby providing an improved understanding regarding the origin of the dominant emission lines observed near 1.5 μm. The results suggested that the polarization field in GaN can be exploited to enhance the effective Er excitation cross section by manipulating the polarization of the excitation light source.

Phase segregation induced third order nonlinear saturable absorption behavior in Erbium doped ZnO nanoparticles synthesized by facile hydrothermal method

Abstract Here in this work, we present the dominance of rare earth dopant (Er) in association with the surfactant (Oleylamine) on the structural, surface morphology, optical, and nonlinear optical properties of hydrothermally grown ZnO nanoparticles. The polycrystalline nature of the prepared nanostructures is confirmed by the XRD diffractograms. The formation of erbium related compound (erbium oxide) in the highest Er doped ZnO nanoparticles is visible from the XRD diffractogram. The incorporation of Er at different doping concentrations (1 at. wt.%, 3 at. wt.%, and 5 at. wt.%) brings noticeable morphological changes in the FESEM images. The Raman spectra reveal the retention of wurtzite structure in undoped and Er doped nanoparticles. The disappearance of A1 (TO) mode in ZnEr3 and ZnEr5 nanoparticles shows the deteriorated polar lattice bond strength. The phase segregation observed from XRD spectrum of ZnEr5 nanoparticles is further confirmed by the detection of local vibrational mode (LVM). A severe decrement in the bandgap of the Er doped nanoparticles is noticed from UV–Vis spectroscopy. A continuous fall in the intensity in the NBE emission with increasing Er doping concentration is attributed to the formation of surface defects below the conduction band edge. The nonlinear optical parameters of the nanoparticles are quantified with the assistance of open aperture and closed aperture Z-scan curves. A remarkable switching from reverse saturable absorption (RSA) to saturable absorption (SA) and self-defocusing to self-focusing mechanisms in ZnEr5 shed light on the impact of phase segregation on the nonlinear response of the Er doped nanoparticles. The quantified values of nonlinear parameters look promising in the fabrication of photonic devices.

Polar meron lattice in strained oxide ferroelectrics.

A topological meron features a non-coplanar structure, whose order parameters in the core region are perpendicular to those near the perimeter. A meron is half of a skyrmion, and both have potential applications for information carrying and storage. Although merons and skyrmions in ferromagnetic materials can be readily obtained via inter-spin interactions, their behaviour and even existence in ferroelectric materials are still elusive. Here we observe using electron microscopy not only the atomic morphology of merons with a topological charge of 1/2, but also a periodic meron lattice in ultrathin PbTiO3 films under tensile epitaxial strain on a SmScO3 substrate. Phase-field simulations rationalize the formation of merons for which an epitaxial strain, as a single alterable parameter, plays a critical role in the coupling of lattice and charge. This study suggests that by engineering strain at the nanoscale it should be possible to fabricate topological polar textures, which in turn could facilitate the development of nanoscale ferroelectric devices.

Terahertz-infrared electrodynamics of single-crystalline Ba0.2Pb0.8Al1.2Fe10.8O19 M-type hexaferrite

Spectral response of single-crystalline Ba0.2Pb0.8Al1.2Fe10.8O19 synthesized by a modified Czochralski method is investigated using terahertz-infrared spectroscopy. Reflectivity, transmissivity, and complex dielectric permittivity spectra of the compound are studied in the temperature range from 6 to 300 K and in the frequency interval 8-8000 cm−1 for two principle polarizations of the radiation electric field relative to the crystallographic c-axis, namely E||c and E⊥c. The resonance absorption lines observed above 80 cm−1 are assigned to polar lattice vibrations basing on a factor group analysis and a comparison with a dielectric response of isostructural compounds. A set of absorption bands is observed in a range of 8–80 cm−1. To clarify their nature, a model is developed that considers electronic transitions within the fine-structured ground state of four-fold coordinated Fe2+. It is shown that the trigonal distortions of the crystal field lead to lowering of the symmetry of 4f1 and 4e tetrahedral site-positions of Fe2+ and, as a result, to further splitting of the ground state spin-orbital sub-levels. Electro-dipole transitions between the corresponding sub-levels are associated with the absorption lines observed in a low-energy response (at 8-80 cm−1) of the Ba0.2Pb0.8Al1.2Fe10.8O19 compound. The study paves the way for the development of low cost materials with high dielectric permittivity (about 30) at terahertz frequencies that are promising for the manufacture of electronic devices with enhanced characteristics.

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.