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La_0.5Ca_0.5MnO₃ (LCMO) thin films grown by pulsed laser deposition (PLD) and annealed at different temperatures were investigated by high angle X-ray diffraction, atomic force microscope (AFM), scanning electron microscope (SEM), and energy dispersive spectroscopy (SEM-EDS). The lattice parameters, surface morphology as well as the metal compositions of the films were obtained. It was found that the surface morphology of the films strongly depends on the annealing temperatures. The difference of the thermal expansion coefficients between the film and the substrate plays an important role in determining the morphology of the film surface. It induces an in-plane compressive stress in the LCMO films. The strains in the film can be relaxed by nanoscale grains and cracks.

Morphotropic phase boundary (MPB) compositions of (1-x)PMN-(x)PT system, where x=0.30, 0.32 and 0.34, were prepared in single perovskite phase by the B-site Columbite precursor method. Temperature dependence of dielectric behavior, measured at different frequencies, revealed the transformation of the system from relaxor to normal ferroelectric behavior and decrease in dielectric dispersion near transition temperature (T_c) with the increase in PT content. Appearance of an abnormal hump in the dielectric behavior of PMNT 68/32 composition confirms the MPB nature of the system. Remnant polarization, P_r was found to be ~ 23, 22 and 27 μC/cm², whereas coercive field, E_c was found to be ~ 3.3, 3.9 and 3.6 kV/cm for PMNT 70/30, 68/32 and 66/34 compositions respectively. Strain versus electric field (S-E) behavior revealed the piezoelectric nature of these compositions with a maximum strain ~ 0.75% at 31 kV/cm applied electric field in PMNT 68/32 composition. Temperature variation of the pyroelectric coefficient, P_i, confirms T_c, as obtained by dielectric measurements.

Theoretical calculation based on density function theory (DFT) and generalized gradient approximation (GGA) has been carried out in studying the magnetic properties of nitrogen-doped ZnO. The results show that ferromagnetism (FM) coupling between N atoms is more stable for the majority of 11 geometrically distinct configurations, and N atoms in ZnO have a clear clustering tendency. In addition, the formation and ionization energy of native defects in ZnO is analyzed and discussed. The effect of native defects on FM properties of nitrogen-doped ZnO has also been investigated. It is found that FM state is more favored than the AFM state in the presence of zinc vacancy or oxygen interstitial. In the paper, we also analyze strain effect on FM of nitrogen-doped ZnO.

Using the first principle method based on density function theory (DFT), we study the electronic and ferromagnetic stability in Cu-doped ZnO. The calculated results based on the local density approximation (LDA) showed that the ferromagnetism (FM) coupling between Cu atoms is more energetically favorable for eight geometrically distinct configurations. In this paper, we also analyze the ferromagnetic properties of Cu-doped ZnO within LDA + U scheme. The dominant ferromagnetic interaction is due to the hybridization between O 2p and Cu 3d. We investigate the effects of oxygen vacancies and nitrogen impurities on FM properties of Cu-doped ZnO. It is obvious that oxygen vacancies and nitrogen impurities are unfavorable in stabilizing the FM of Cu-doped ZnO. In addition, the origin of the FM state of Cu-doped ZnO has also been discussed by analyzing the coupling of Cu 3d levels. Also in this paper, we analyze the strain effect on the FM properties of Cu-doped ZnO.

The nanoscale deformation field of twin boundary dislocations in nanocrystalline aluminum was experimentally investigated using a combination of high-resolution transmission electron microscopy and geometric phase analysis. The entire strain field of the twin boundary dislocations was mapped and then compared with those of the Peierls–Nabarro and elastic theory dislocation models. The comparison results demonstrated that the Peierls–Nabarro dislocation model best describes the strain field of the twin boundary dislocations in nanocrystalline aluminum.

The effect of strain on the electronic properties of BC₃ sheet was studied by using first-principles density functional theory. It is found that the band gap of BC₃ sheet increases gradually when the applied tensile strain ranges from 0% to 12.5%. While the band gap decreases as the compressive strain is applied, especially resulting in the semiconductor-metal transition at some strain. Further analysis shows that the change of band gap mainly results from the variation of the energy of valence band maximum (VBM), which is related to the strength of the bonding state. The proposed mechanical control of the electronic properties will widen the application of BC₃ sheet in future nanotechnology.

In this paper, we investigate the structural stability of silicane and germanane under biaxial strain by employing the lattice dynamics calculations within the frame of density functional theory. Our results show that silicane and germanane become unstable even under 1% compressive strain, while maintaining stable under tensile strain. Further calculations about the thermodynamical properties of silicane and germanane show that the phonon contribution to Helmholtz free energy, entropy and specific capacity are insensitive to the tensile strain.

We investigate the band structure of narrow armchair-edge graphene nanoribbons (AGNRs) under tensile strain by means of an extension of the Extended Hückel method. The strain-induced band gap modulation presents asymmetric behavior. The asymmetric modulation of band gap is derived from the different changes of conduction and valence bands near Fermi level under tensile strain. Further analysis suggests that the asymmetric variation of band structure near Fermi level only appear in narrow armchair-edge graphene nanoribbons.

The transport properties in ultrasmall single-wall carbon nanotubes (SWCNTs) under tensile strain have been theoretically investigated. The regular negative differential resistance (NDR) induced by the strain undergoes a process from enhancement to weakening in the zigzag (3,0) SWCNT. The NDR achieves maximum with applying 4% tensile strain. Compared to the case of (3,0) SWCNT, that NDR cannot be manipulated by applying strain clearly in (4,0) and (5,0) ultrasmall SWCNTs with tensile strain lower than 10%. It proposes this strain-induced NDR effect to demonstrate the possibility of finding potential applications in SWCNT-based NDR nanodevices such as in memory devices, oscillators and fast switching devices.

We present first-principles calculations to study the stability and electronic properties of stanene on WS₂ hybrid structure. It can be seen that the stanene is bound to WS₂ substrate with an interlayer distance of about 3.0 Å with a binding energy of −51.8 meV per Sn atom, suggesting a weak interaction between stanene and WS₂. The nearly linear band dispersion character of stanene can be preserved with a sizeable band gap in stanene on WS₂ hybrid structure due to the difference of onsite energy induced by WS₂ substrate, which is more helpful to the on–off current ratio in the logical devices made of stanene/WS₂. Moreover, the band gaps, the position of Dirac point with respect to Fermi level, and electron effective mass (EEM) of stanene on WS₂ hybrid structure can be tuned by the interlayer distance, external electric field and strains. These results indicate that stanene on WS₂ hybrid structure is a promising candidate for stanene-based field-effect transistor (FET) with a finite band gap and high carrier mobility.

This paper presents a theoretical description of the effects of strain induced by out-of-plane deformations on charge distributions and transport on graphene. A review of a continuum model for electrons using the Dirac formalism is complemented with elasticity theory to represent strain fields. The resulting model is cast in terms of scalar and pseudo-magnetic fields that control electron dynamics. Two distinct geometries, a bubble and a fold, are chosen to represent the most commonly observed deformations in experimental settings. It is shown that local charge accumulation regions appear in deformed areas, with a peculiar charge distribution that favors occupation of one sublattice only. This unique phenomenon that allows to distinguish each carbon atom in the unit cell, is the manifestation of a sublattice symmetry broken phase. For specific parameters, resonant states appear in localized charged regions, as shown by the emergence of discrete levels in band structure calculations. These findings are presented in terms of intuitive pictures that exploit analogies with confinement produced by square barriers. In addition, electron currents through strained regions are spatially separated into their valley components, making possible the manipulation of electrons with different valley indices. The degree of valley filtering (or polarization) for a specific system can be controlled by properly designing the strained area. The comparison between efficiencies of filters built with this type of geometries identifies extended deformations as better valley filters. A proposal for their experimental implementations as component of devices, and a discussion for potential observation of novel physics in strained structures are presented at the end of the paper.

A three-dimensional (3D) digital image correlation (DIC) method is presented for measuring the deformations of vinyl chloride-coated metal (VCM) multilayer sheets and their composites. The calculations and the principle of strain and deformation measurements using the DIC method are described. A VCM multilayer sheet consists of a substrate [steel plate cold commercial (SPCC) and steel plate cold elongation (SPCE)] and a clad (a VCM film). The corresponding deformations of VCM deep-drawing multilayer sheets (SPCE as a substrate and a VCM film as a clad), VCM nondeep-drawing multilayer sheets (SPCC as a substrate and a VCM film as a clad), nondeep-drawing substrates (SPCC), deep-drawing substrates (SPCE) and clads (VCM films) were captured along the x- and y-directions in uniaxial tension experiments and using the DIC method. The maximal measured strains along the x-direction for the VCM deep-drawing multilayer sheets, VCM nondeep-drawing multilayer sheets, nondeep-drawing substrates, deep-drawing substrates and clads were, respectively, 637.835%, 132.210%, 31.688.632%, 107.102%, and 118.937%. The maximal measured strains along the $y$-direction were 739.028%, −11.174%, −9.678%, −13.273% and 12.120%, respectively. These data show that the mechanical properties of VCM multilayer sheets are better than those of their substrates and clads. The effectiveness and accuracy of the presented DIC method for VCM multilayer sheet measurements were confirmed in a series of experiments.

First-principles calculation has been performed to investigate the effect of strain on the magnetic moment of Fe-doped MoTe₂ monolayer. Our results show that the Fe-doped MoTe₂ monolayer is semiconductor with the magnetic moment of 2.037 ${\mu }_{\mathrm{\text{B}}}$. By analyzing the density of states, we find that the magnetic moment is mainly contributed by the Fe atom. When the biaxial strain is applied along the layer, the results show that the magnetic moment is almost unchanged when the compressive strain is under 5% and tensile strain is under 7%. However, as the strain increases, the magnetic moment decreases to almost zero with compressive strain larger than 7%, and the magnetic moment begins to increase with the tensile strain larger than 8%, which indicates the different effects of compressive strain and tensile strain on the magnetism of Fe-doped MoTe₂.

The study of complex oxides and oxide heterostructures has dominated the field of experimental and theoretical condensed matter research for the better part of the last few decades. Powerful experimental techniques such as molecular beam epitaxy and pulsed laser deposition have made fabrication of oxide heterostructures with atomically sharp interfaces possible, whereas more and more sophisticated handling of exchange and correlations within first principles methods including density functional theory (DFT) supplemented with Hubbard U corrections and hybrid functionals, and beyond DFT techniques such as dynamical mean field theory (DMFT) have made understanding of such correlated oxides and oxide interfaces easier. The emergence of the high-mobility two-dimensional electron gas with fascinating properties such as giant photoconductance, large negative magnetoresistance, superconductivity, ferromagnetism, and the mysterious coexistence of the latter two have indeed caught the attention of condensed matter community at large. Similarly, strain tuning of oxides have generated considerable interest particularly after the recent discovery of piezoelectric methods of strain generation. Theoretical understanding and prediction of the possible exotic phases emerging in such complex oxides both under strain and in heterostructures will eventually lead to better design of device applications in this new emerging field of oxide electronics, along with possible discovery of exotic physics in condensed matter systems, which may be of wider significance! In this review, we briefly look at theoretical studies of novel phenomena in oxides under strain and oxide heterostructure, and try to understand the role of exchange and particularly correlation in giving rise to such exotic electronic states. This review though primarily focuses on the theoretical aspects on understanding the different mechanism of the phenomena of emergence of exotic phases, does provide a unique overview of the experimental literature as well, accompanied by the theoretical understanding such that relevant device applications can be envisaged.

The two-dimensional transition-metal dichalcogenides (2D TMDs) WX₂ (S, Se, Te) have received extensive attention and research since they have excellent physical properties and have been widely used in the fields of photoelectronics. Monolayer (ML) WX₂ has excellent physical properties and can be modified by simple strain. Using the first principles based on density functional theory (DFT), this paper mainly studies the electronic properties of ML WS₂, WSe₂ and Wte₂. We also study the stabilities of three ML structures, the changes of Raman spectra and the movement of Raman peaks under biaxial tensile and compressive strains. Under the control of strain not only does the bandgap changes, but also the band properties shift between the direct bandgap and the indirect bandgap. With the increase of strain, bond length and bond angle change in the opposite trend. At the same time, we also studied the phonon dispersion relations of WX₂ under different strains. We found that three structures showed good thermodynamic stabilities under the tensile strain (1–10%). When the compressive strain is 2%, one of the acoustic modes of WS₂ or Wse₂ becomes imaginary at $\mathrm{\Gamma }$ point, which indicates the structural instability. When tensile strain Raman summit blueshifts and when compressive strains, the redshift occurs.

In this paper, the electrical and optical properties of single-layer MoS₂ and single-layer WS₂ in three strain states: biaxial tension, biaxial compression, biaxial tension and compression are systematically studied. All calculations are based on the first-principle of density functional theory. The results show that after biaxial tension strain, biaxial compression strain, and biaxial tension-compression strain are applied, the atomic structure, energy band structure, and optical absorption coefficient will show disparate changing trends. When the biaxial tension and compression strain intensity is less than 15%, the bond length, bond angle, and light absorption peak will have little fluctuation with the increase of strain intensity. However, compared with the other two strain states, these two crystal structures are the most volatile at this time. In addition, when 15% biaxial tensile strain is applied, the two crystals can still maintain their kinetic stability.

As a building block, the icosahedral gold 13-atom cluster has attracted much attention for many years. In this paper, the tensile and compressive deformation of the icosahedral gold 13-atom cluster are investigated and some interesting results different from bulks and nanowires are obtained. It is found that the elastic strain limits of the cluster are much larger than those of the gold bulks and the nanowires. Within the elastic strain limit, the loading force–strain relationship is not linear. And the stiffness coefficient decreases with increasing strain under the tensile loading, and increases with increasing strain under the compressive loading. Under the influence of temperature, the loading force and the stiffness coefficient decrease with the increasing temperature at the same strain. The elastic strain limit and the break-up strain are also reduced as the temperature rises. Although the bulks and nanowires cannot return to their original configurations when they are in a plastic state, however, the calculation shows that the cluster can return spontaneously to its original icosahedral structure even if the cluster has been at plastic deformation when the loading is released above a certain temperature. A monatomic chain is formed when the cluster is close to rupture. The interatomic distance and the tensile force for the monatomic chain are consistent with the experimental data.

In this paper, the exciton binding energy and radiation lifetime in type I and II structures of CdTe/Cd${}_{1-x}$Zn_xS strained core/shell spherical quantum dots under the hydrostatic pressure have been studied by using the variational method within the continuous dielectric model and effective mass approximation. The results show that for these two structures, the exciton binding energies with and without strain decrease with the increasing core radius but increase with the increasing pressure. The exciton binding energy with strain is smaller than that without strain. For type I structure, the effect of strain is small, and the radiation lifetime decreases monotonically with the increasing pressure. By contrast, for type II structure, the effects of shell radius and strain on the exciton binding energy are obvious, and the radiation lifetime increases first and then decreases with the increasing pressure.

Based on the first principles, the crystal structure, photoelectric properties, and structural stability of two transition metal dichalcogenides (TMDCs) under different strain treatments are systematically calculated. The properties of NbX₂ under tensile and compressive strains are discussed for the first time. Recently, a single-layer 1T structure with X atoms surrounding the transition metal atoms was synthesized in experiments. The 1T multi-forms were octahedral coordination structures, and some of the systems showed excellent semiconductor properties and stability. The results show that NbS₂ has an indirect band gap under different strains and tends to transition to semiconductors which can effectively improve the material activity. NbSe₂ has more excellent properties in the optical field and can be used to manufacture infrared lenses, anti-reflection coatings, and UV reflectors. This study provides a new understanding of the unconventional structure of NbX₂ and provides theoretical guidance for the work in the field of TMDCs.

In this paper, the effects of transition metal doping on the electronic structure of monolayer NbS₂ were studied through the first principles. The electronic structure changes caused by doping transition metal (TM) atoms were recorded, including the energy band, the density of states, binding energy, and optical properties. Studies show that all doping systems are metal. Still, under strain regulation, some doping systems offer an indirect bandgap; NbS₂ transforms into a narrow bandgap diluted semiconductor and can improve the activity. Doping atoms lead to $n(p)$ type doping in NbS₂.

Regarding optical properties, IVB group metals are selected as the typical dopant of three periods. Composite NbS₂ has excellent reflector characteristics and can be applied to infrared and ultraviolet regions. The spectral response and electromagnetic wave absorption range of low-energy areas are also improved. This study effectively solves the problem of impurity states introduced by doping and provides a solution for the doping modification of monolayer NbS₂, which will lay a foundation for nanoelectronics.