arXiv subjects

Ulrich Aschauer

Publications and source records attributed to Ulrich Aschauer.

42 records · Page 3Linked to original sources

arXiv2016

Interplay between strain, defect charge state and functionality in complex oxides

We use first-principles calculations to investigate the interplay between strain and the charge state of point defect impurities in complex oxides. Our work is motivated by recent interest in using defects as active elements to provide novel functionality in coherent epitaxial films. Using oxygen vacancies as model point defects, and CaMnO$_3$ and MnO as model materials, we calculate the changes in internal strain caused by changing the charge state of the vacancies, and conversely the effect of strain on charge-state stability. Our results show that the charge state is a degree of freedom that can be used to control the interaction of defects with strain and hence the concentration and location of defects in epitaxial films. We propose the use of field-effect gating to reversibly change the charge state of defects and hence the internal strain and corresponding strain-induced functionalities.

cond-mat.mtrl-sci↗

arXiv2016

Strain-induced structural instability in FeRh

We perform density functional calculations to investigate the structure of the inter-metallic alloy FeRh under epitaxial strain. Bulk FeRh exhibits a metamagnetic transition from a low-temperature antiferromagnetic (AFM) phase to a ferromagnetic (FM) phase at 350K, and its strain dependence is of interest for tuning the transition temperature to the room-temperature operating conditions of typical memory devices. We find an unusually strong dependence of the structural energetics on the choice of exchange-correlation functional, with the usual local density approximation (LDA) yielding the wrong ground-state structure, and generalized gradient (GGA) extensions being in better agreement with the bulk experimental structure. Using the GGA we show the existence of a metastable face-centered-cubic (fcc)-like AFM structure that is reached from the ground state body-centered-cubic (bcc) AFM structure by following the epitaxial Bain path. We predict that this metastable fcc-like structure has a significantly higher conductivity than the bcc AFM phase. We show that the behavior is well described using non-linear elasticity theory, which captures the softening and eventual sign change of the orthorhombic shear modulus under compressive strain, consistent with this structural instability. Finally, we predict the existence of an additional unit-cell-doubling lattice instability, which should be observable at low temperature.

cond-mat.mtrl-sci↗

arXiv2015

Quantum critical origin of the superconducting dome in SrTiO$_3$

We investigate the origin of superconductivity in doped SrTiO$_3$ (STO) using a combination of density functional and strong coupling theories within the framework of quantum criticality. Our density functional calculations of the ferroelectric soft mode frequency as a function of doping reveal a crossover from quantum paraelectric to ferroelectric behavior at a doping level coincident with the experimentally observed top of the superconducting dome. Based on this finding, we explore a model in which the superconductivity in STO is enabled by its proximity to the ferroelectric quantum critical point and the soft mode fluctuations provide the pairing interaction on introduction of carriers. Within our model, the low doping limit of the superconducting dome is explained by the emergence of the Fermi surface, and the high doping limit by departure from the quantum critical regime. We predict that the highest critical temperature will increase and shift to lower carrier doping with increasing $^{18}$O isotope substitution, a scenario that is experimentally verifiable.

cond-mat.supr-con↗

arXiv2015

Effect of epitaxial strain on cation and anion vacancy formation in MnO

Biaxial strain in coherent epitaxial thin films can have a pronounced effect on the point defect profile in the film material. Detailed fundamental knowledge of the interaction of strain with point defects is crucial in understanding the stoichiometry and resulting properties of strained thin films. Here we investigate the effect of biaxial strain on the formation energy of cation and anion vacancies using MnO as a model system. Our density functional theory calculations show that, as expected from local volume arguments, compressive strain favours the formation of cation vacancies. Interestingly, we find that small compressive and tensile strains lead to ordering of the resulting holes along the in-plane and normal direction respectively, which should manifest in different anisotropic properties in the two strain states.

cond-mat.mtrl-sci↗

arXiv2013

Competition and cooperation between antiferrodistortive and ferroelectric instabilities in SrTiO$_3$

We use density functional theory to explore the interplay between octahedral rotations and ferroelectricity in the model compound SrTiO$_3$. We find that over the experimentally relevant range octahedral rotations suppress ferroelectricity as is generally assumed in the literature. Somewhat surprisingly we observe that at larger angles the previously weakened ferroelectric instability strengthens significantly. By analysing geometry changes, energetics, force constants and charges, we explain the mechanisms behind this transition from competition to cooperation with increasing octahedral rotation angle.

cond-mat.mtrl-sci↗

arXiv2013

Strain controlled oxygen vacancy formation and ordering in CaMnO$_3$

We use first-principles calculations to investigate the stability of bi-axially strained \textit{Pnma} perovskite CaMnO$_3$ towards the formation of oxygen vacancies. Our motivation is provided by promising indications that novel material properties can be engineered by application of strain through coherent heteroepitaxy in thin films. While it is usually assumed that such epitaxial strain is accommodated primarily by changes in intrinsic lattice constants, point defect formation is also a likely strain relaxation mechanism. This is particularly true at the large strain magnitudes ($>$4%) which first-principles calculations often suggest are required to induce new functionalities. We find a strong dependence of oxygen vacancy defect formation energy on strain, with tensile strain lowering the formation energy consistent with the increasing molar volume with increasing oxygen deficiency. In addition, we find that strain differentiates the formation energy for different lattice sites, suggesting its use as a route to engineering vacancy ordering in epitaxial thin films.

cond-mat.mtrl-sci↗