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Register a new userCritical role of the sample preparation in experiments using piezoelectric actuators inducing uniaxial or biaxial strains
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We report on a systematic study of the stress transferred from an electromechanical piezo-stack into GaAs wafers under a wide variety of experimental conditions. We show that the strains in the semiconductor lattice, which were monitored in situ by means of X-ray diffraction, are strongly dependent on both the wafer thickness and on the selection of the glue which is used to bond the wafer to the piezoelectric actuator. We have identified an optimal set of parameters that reproducibly transfers the largest distortions at room temperature. We have studied strains produced not only by the frequently used uniaxial piezostressors but also by the biaxial ones which replicate the routinely performed experiments using substrate-induced strains but with the advantage of a continuously tunable lattice distortion. The time evolution of the strain response and the sample tilting and/or bending are also analyzed and discussed.
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In this paper strain transfer efficiencies from single crystalline piezoelectric lead magnesium niobate-lead titanate (PMN-PT) substrate to a GaAs semiconductor membrane bonded on top are investigated using state-of-the-art x-ray diffraction (XRD) techniques and finite-element-method (FEM) simulations. Two different bonding techniques are studied, namely gold-thermo-compression and polymer-based SU8 bonding. Our results show a much higher strain-transfer for the soft SU8 bonding in comparison to the hard bonding via gold-thermo-compression. A comparison between the XRD results and FEM simulations allows to explain this unexpected result with the presence of complex interface structures between the different layers.
Graphite-like carbon nitride (g-$mathrm{C_3N_4}$) is considered as a promising candidate for energy materials. In this work, the biaxial strain (-4%-4%) effects on piezoelectric properties of g-$mathrm{C_3N_4}$ monolayer are studied by density functional theory (DFT). It is found that the increasing strain can reduce the elastic coefficient $C_{11}$-$C_{12}$, and increases piezoelectric stress coefficient $e_{11}$, which lead to the enhanced piezoelectric strain coefficient $d_{11}$. Compared to unstrained one, strain of 4% can raise the $d_{11}$ by about 330%. From -4% to 4%, strain can induce the improved ionic contribution to $e_{11}$ of g-$mathrm{C_3N_4}$, and almost unchanged electronic contribution, which is different from $mathrm{MoS_2}$ monolayer (the enhanced electronic contribution and reduced ionic contribution). To prohibit current leakage, a piezoelectric material should be a semiconductor, and g-$mathrm{C_3N_4}$ monolayer is always a semiconductor in considered strain range. Calculated results show that the gap increases from compressive strain to tensile one. At 4% strain, the first and second valence bands cross, which has important effect on transition dipole moment (TDM). Our works provide a strategy to achieve enhanced piezoelectric effect of g-$mathrm{C_3N_4}$ monolayer, which gives a useful guidence for developing efficient energy conversion devices.
We report on domain formation and magnetization reversal in epitaxial Fe films on ferroelectric BaTiO3 substrates with ferroelastic a-c stripe domains. The Fe films exhibit biaxial magnetic anisotropy on top of c domains with out-of-plane polarization, whereas the in-plane lattice elongation of a domains induces uniaxial magnetoelastic anisotropy via inverse magnetostriction. The strong modulation of magnetic anisotropy symmetry results in full imprinting of the a-c domain pattern in the Fe films. Exchange and magnetostatic interactions between neighboring magnetic stripes further influence magnetization reversal and pattern formation within the a and c domains.
Ever since high Tc superconductivity was discovered in La-based mixed oxide system by Bednorz and Muller, enormous efforts have been put in by several researchers around the world in understanding the origin and mechanism of superconductivity in these, as well as in systems derived from them. It is a proven fact that the superconductivity in RE-123 superconductors is governed by the oxygen content, which in turn is responsible for the carrier concentration in the system. Due to their dependence on oxygen content, RE-123 superconductors undergo structural transformation from orthorhombic to tetragonal as a function of oxygen content making them very difficult compounds to work with, in terms of technological applications, such as device fabrication. Hence, it would be interesting to obtain a stable compound whose superconducting properties, are not only insensitive directly to oxygen content but, having dependence of its carrier concentration and Tc on the nature and amount of the substituted cation. In the present work, we focus our investigations for such a compound, which has been derived from a tetragonal RE-123 superconducting system. In this chapter, we present a brief review of the studies carried out on La-2125 compounds to elucidate the role of dopants in modifying the superconducting properties and establish a structure-property correlation in them.
We demonstrate the continuous and reversible tuning of the optical band gap of suspended monolayer MoS2 membranes by as much as 500 meV by applying very large biaxial strains. By using chemical vapor deposition (CVD) to grow crystals that are highly impermeable to gas, we are able to apply a pressure difference across suspended membranes to induce biaxial strains. We observe the effect of strain on the energy and intensity of the peaks in the photoluminescence (PL) spectrum, and find a linear tuning rate of the optical band gap of 99 meV/%. This method is then used to study the PL spectra of bilayer and trilayer devices under strain, and to find the shift rates and Gruneisen parameters of two Raman modes in monolayer MoS2. Finally, we use this result to show that we can apply biaxial strains as large as 5.6% across micron sized areas, and report evidence for the strain tuning of higher level optical transitions.
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