Experimental absence of the non-perovskite ground state phases of MaPbI₃ explained by a Funnel Hopping Monte Carlo study based on a neural network potential

Methylammonium lead iodide is a material known for its exceptional opto-electronic properties that make it a promising candidate for many high performance applications, such as light emitting diodes or solar cells. A recent computational structure search revealed two previously unknown non-perovskite polymorphs, that are lower in energy than the experimentally observed perovskite phases. To investigate the elusiveness of the non-perovskite phases in experimental studies, we extended our Funnel Hopping Monte Carlo (FHMC) method to periodic systems and performed extensive MC simulations driven by a machine learned potential. FHMC simulations that also include these newly discovered non-perovskite phases show that above temperatures of 200 K the perovskite phases are thermodynamically preferred. A comparison with the quasi-harmonic approximation highlights the importance of anharmonic effects captured by FHMC

Spin generation away from boundaries by nonlinear transport

In several situations of interest, spin polarization may be generated far from the boundaries of a sample by nonlinear effects of an electric current, even when such a generation is forbidden by symmetry in the linear regime. We present an analytically solvable model where spin accumulation results from a combination of current gradients, nonlinearity, and cubic anisotropy. Further, we show that even with isotropic conductivity, nonlinear effects in a low symmetry geometry can generate spin polarization far away from boundaries. Finally, we find that drift from the boundaries results in spin polarization patterns that dominate in recent experiments on GaAs by Sih et al. [Phys. Rev. Lett. 97, 096605 (2006)]

Construction of a quasiconserved quantity in the Henon-Heiles problem using a single set of variables

The problem of finding the coefficients of a simple series expansion for a quasiconserved quantity K for the Henon-Heiles Hamiltonian H using a single set of variables is solved. In the past, this type of approach has been problematic because the solution to the equations determining the coefficients in the expansion is not unique. As a result, the existence of a consistent expression for K to all orders had not previously been established. We show how to deal with this arbitrariness in the expansion coefficients for K in a consistent way. Due to this arbitrariness, we find a class of expansions for K, in contrast to the single unique expansion for K generated by the normal-form approach of Gustavson [Astron. J. 71, 670 (1966)]. It may be possible to devise a criterion for deciding which one of our expansions is optimally convergent, although we do not deal with this question here. We proceed by introducing a single set of dynamic variables that have simple symmetry properties and that also diagonalize the problem of finding the coefficients of K. No canonical transformations are required. A straightforward constructive procedure is given for generating the power series to any order for quantities having the symmetry of the Hamiltonian that -are formally conserved. This leads to a very practical method for calculating a quasiconserved quantity in the Henon-Heiles problem. A comparison is made through several orders of the terms generated by this approach and those generated in the original Gustavson expansion in normal form

Domain Patterns in the Microwave-Induced Zero-Resistance State

It has been proposed that the microwave-induced ``zero-resistance'' phenomenon, observed in a GaAs two-dimensional electron system at low temperatures in moderate magnetic fields, results from a state with multiple domains, in which a large local electric field \bE(\br) is oriented in different directions. We explore here the questions of what may determine the domain arrangement in a given sample, what do the domains look like in representative cases, and what may be the consequences of domain-wall localization on the macroscopic dc conductance. We consider both effects of sample boundaries and effects of disorder, in a simple model, which has a constant Hall conductivity, and is characterized by a Lyapunov functional.Comment: 19 pages, 5 figures; submitted to a special issue of Journal of Statistical Physics, in honor of P. C. Hohenberg and J. S. Lange

Being homeless in a small city : the case of the Open Arms Day Centre : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Psychology at Massey University, Albany, New Zealand

Homelessness in Aotearoa/New Zealand has been a significant issue for many years, with Māori being consistently overrepresented in local and national statistics. Whether living on the streets in an urban centre, small city, or rural area, homeless people frequently experience negative health consequences, stigma, and displacement. Due to the recent Covid-19 outbreak, street homeless people have been confronted with additional challenges, such as increased prejudices and social isolation. However, non-urban street life, such as in Whangārei, has been predominantly overlooked, and responses have remained scant and narrowly focused. Nevertheless, recent estimates demonstrate a significant increase in Whangārei’s street homeless population. In order to effectively address this development, comprehensive understandings about the lifeworlds of those experiencing street homelessness in non-metropolitan areas must be sought. The present research addresses this knowledge gap in the context of the Open Arms Day Centre [OADC], a space of refuge, routine, and inclusion for Whangārei’s street dwellers. Adopting a relational Māori-centred ethnographic case study approach, the experiences and standpoints of three OADC staff members, two volunteers, and three previously homeless service users [whānau] were explored. Insights into homelessness in Whangārei and the significance of the OADC were gained through the utilisation of a reflective research journal, photographic exercises, and in-depth semi- structured interviews. Bricolage was used as a method of analysis, permitting a wide range of considerations. These included diverse personal, systemic, and Māori cultural dynamics and understandings, as well as inputs from various perspectives, disciplines and experiences. Culturally orientated services, based on relational, inclusive, and empathetic conceptualisations of whānau and their needs, such as the OADC, were identified as pivotal for people’s resilience and pathways out of homelessness. Future research on non-metropolitan homelessness is necessary to invoke transformative change

Ternary Phase Diagram of Nitrogen Doped Lutetium Hydrides

This paper presents the results of an extensive structural search of ternary solids containing lutetium, nitrogen and hydrogen. Based on thousands of thermodynamically stable structures, available online, the convex hull of the formation enthalpies is constructed. To obtain the correct energetic ordering, the highly accurate RSCAN DFT functional is used in high quality all-electron calculations. In this way possible pseudopotential errors are eliminated. A novel lutetium hydride structure (HLu$_2$) that is on the convex hull is found in our search. An electron phonon analysis however shows that it is not a candidate structure for near ambient superconductivity. Besides this structure, which appears to have been missed in previous searches, possibly due to different DFT methodologies, our results agree closely with the results of previously published structure search efforts. This shows, that the field of crystal structure prediction has matured to a state where independent methodologies produce consistent and reproducible results, underlining the trustworthiness of modern crystal structure predictions. Hence it is quite unlikely that a structure, that would give rise within standard BCS theory to the superconducting properties, claimed to have been observed by Dasenbrock-Gammon et al. 10.1038/s41586-023-05742-0 , exists. This solidifies the evidence that no structure with conventional superconducting properties exists that could explain the experimental observation made by Dasenbrock-Gammon et al. 10.1038/s41586-023-05742-

A Fourth-Generation High-Dimensional Neural Network Potential with Accurate Electrostatics Including Non-local Charge Transfer

Machine learning potentials have become an important tool for atomistic simulations in many fields, from chemistry via molecular biology to materials science. Most of the established methods, however, rely on local properties and are thus unable to take global changes in the electronic structure into account, which result from long-range charge transfer or different charge states. In this work we overcome this limitation by introducing a fourth-generation high-dimensional neural network potential that combines a charge equilibration scheme employing environment-dependent atomic electronegativities with accurate atomic energies. The method, which is able to correctly describe global charge distributions in arbitrary systems, yields much improved energies and substantially extends the applicability of modern machine learning potentials. This is demonstrated for a series of systems representing typical scenarios in chemistry and materials science that are incorrectly described by current methods, while the fourth-generation neural network potential is in excellent agreement with electronic structure calculations.Comment: 13 pages, 11 figure