Atomic-Scale Observation of Spontaneous Hole Doping and Concomitant Lattice Instabilities in Strained Nickelate Films

Thin oxide films are of vast opportunities for modern electronics and can facilitate emergent phenomena by factors absent in the bulk counterparts, such as the ubiquitous epitaxial strain and interfacial charge doping. Here, we demonstrate the twisting of intended bulk-metallic phases in 10-unit-cell LaNiO3, PrNiO3, and NdNiO3 films on (001)-oriented SrTiO3 into distinct charge-lattice entangled states by epitaxial strains. Using atomically-resolved electron microscopy and spectroscopy, the interfacial electron doping into SrTiO3 in the conventional context of band alignments are discounted. Instead, spontaneously doped holes that are localized and at the order of 1013 cm-2 are atomically unraveled across all three heterointerfaces and associated with strain mitigations by the accompanied atomic intermixing with various ionic radii. The epitaxial strains also lead to condensations of monoclinic-C2/c lattice instabilities, which are hidden to the bulk phase diagram. The group-theoretical analysis of characteristic transition pathways unveils the strain resurrection of the hidden C2/c symmetry. While this strain-induced monoclinic phase in LaNiO3 remains metallic at room temperature, those in PrNiO3 and NdNiO3 turn out to be insulating. Such strain-induced monoclinic lattice instabilities and parasitic localized holes go beyond the classical elastic deformations of films upon epitaxial strains and hint on plausible hidden orders in versatile oxide heterostructures with unexpected properties, of which the exploration is only at the infancy and full of potentials.

Hidden order and multipolar exchange striction in a correlated f-electron system

The nature of order in low-temperature phases of some materials is not directly seen by experiment. Such “hidden orders” (HOs) may inspire decades of research to identify the mechanism underlying those exotic states of matter. In insulators, HO phases originate in degenerate many-electron states on localized f or d shells that may harbor high-rank multipole moments. Coupled by intersite exchange, those moments form a vast space of competing order parameters. Here, we show how the ground-state order and magnetic excitations of a prototypical HO system, neptunium dioxide NpO2, can be fully described by a low-energy Hamiltonian derived by a many-body ab initio force theorem method. Superexchange interactions between the lowest crystal-field quadruplet of Np4+ ions induce a primary noncollinear order of time-odd rank 5 (triakontadipolar) moments with a secondary quadrupole order preserving the cubic symmetry of NpO2. Our study also reveals an unconventional multipolar exchange striction mechanism behind the anomalous volume contraction of the NpO2 HO phase.

Applications of Machine Learning to Estimating the Sizes and Market Impact of Hidden Orders in the BRICS Financial Markets

The research aims to investigate the role of hidden orders on the structure of the average market impact curves in the five BRICS financial markets. The concept of market impact is central to the implementation of cost-effective trading strategies during financial order executions. The literature is replicated using the data of visible orders from the five BRICS financial markets. We repeat the implementation of the literature to investigate the effect of hidden orders. We subsequently study the dynamics of hidden orders. The research applies machine learning to estimate the sizes of hidden orders. We revisit the methodology of the literature to compare the average market impact curves in which true hidden orders are added to visible orders to the average market impact curves in which hidden orders sizes are estimated via machine learning. The study discovers that: (1) hidden orders sizes could be uncovered via machine learning techniques such as Generalized Linear Models (GLM), Artificial Neural Networks (ANN), Support Vector Machines (SVM), and Random Forests (RF); and (2) there exist no set of market features that are consistently predictive of the sizes of hidden orders across different stocks. Artificial Neural Networks produce large R2 and small Mean Squared Error on the prediction of hidden orders of individual stocks across the five studied markets. Random Forests produce the most appropriate average price impact curves of visible and estimated hidden orders that are closest to the average market impact curves of visible and true hidden orders. In some markets, hidden orders produce a convex power-law far-right tail in contrast to visible orders which produce a concave power-law far-right tail. Hidden orders may affect the average price impact curves for orders of size less than the average order size; meanwhile, hidden orders may not affect the structure of the average price impact curves in other markets. The research implies ANN and RF as the recommended tools to uncover hidden orders.

‘opened out […] like an oyster shell': the Roach bed Portland stone cladding of the Smithsons’ Economist Building

Alison & Peter Smithson's Economist Building in London (1960–1964) has been much written about, but one of its most distinctive features – the Roach bed Portland stone cladding – has been relatively little discussed. Although there have been various retrospective explanations for the choice of this previously unused stone, whose surface is marked by cavities of now vanished shells, no direct evidence from the time fully accounts for the decision. Reliance upon inference, and upon retrospective statements, mean that no definitive explanation for the choice is now possible. Some new evidence from archives and interviews, and a re-examination of the contemporary concerns of the Smithsons, and of their artist friends Eduardo Paolozzi and Nigel Henderson, fellow members of the Independent Group during the 1950s, however, suggest a plausible, though speculative, interpretation. When considered in light of the Smithsons' and their friends' interests in pattern, in hidden orders, and in the problems of ‘human association’, the choice of Roach bed Portland stone seems less surprising. More than just ‘pretty’, as Peter Smithson was later to describe the material, it can be seen to connect with a fascination with underlying, non-human, systems of order, while at the same time providing a particularly expressive form of protection for human identity in the modern city.

Hidden and Fast Liquidity - Hidden Orders and High-Frequency Trading

This work focuses on two of the more frequent practices in financial (especially capital) markets -the use of hidden orders and High-Frequency Trading (HFT). Although the use of each of them may reach 40% of the market turnover - even 60% for HFT, the actual knowledge on how they affect liquidity, prices, and market structure is still limited - especially if they are combined. The presence of both of these practices may look controversial, as it seems to be going in the opposite direction to what some of the goals that market regulators try to reach - transparency and increase of market liquidity. Additionally, their use suggests first, to give a clear advantage to some traders while not knowing the exact consequences to others. The aim of this paper is, by performing a literature study, to structure the current knowledge on a very specific topic in the area of market microstructure - the use of hidden orders and High-Frequency Trading. This paper tries to show the motivations, strategies, and eventual price effects behind hidden orders and High-Frequency Trading. It is also important to mention that this paper is based on scarce empirical research available (mainly for the US market) and as such, it is intended to encourage further analysis and research on this important topic.

Unveiling hidden multipolar orders with magnetostriction

Broken symmetries in solids involving higher order multipolar degrees of freedom are historically referred to as “hidden orders” due to the formidable task of detecting them with conventional probes. In this work, we theoretically propose that magnetostriction provides a powerful and novel tool to directly detect higher-order multipolar symmetry breaking—such as the elusive octupolar order—by examining scaling behaviour of length change with respect to an applied magnetic field h. Employing a symmetry-based Landau theory, we focus on the family of Pr-based cage compounds with strongly correlated f-electrons, Pr(Ti,V,Ir)2(Al,Zn)20, whose low energy degrees of freedom are purely higher-order multipoles: quadrupoles $${\cal{O}}_{20,22}$$ O 20 , 22 and octupole $${\cal{T}}_{xyz}$$ T x y z . We demonstrate that a magnetic field along the [111] direction induces a distinct linear-in-h length change below the octupolar ordering temperature. The resulting “magnetostriction coefficient” is directly proportional to the octupolar order parameter, thus providing clear access to such subtle order parameters.