Showing papers in "Reports on Progress in Physics in 2023"

Teleparallel gravity: from theory to cosmology

Abstract: Abstract Teleparallel gravity (TG) has significantly increased in popularity in recent decades, bringing attention to Einstein’s other theory of gravity. In this Review, we give a comprehensive introduction to how teleparallel geometry is developed as a gauge theory of translations together with all the other properties of gauge field theory. This relates the geometry to the broader metric-affine approach to forming gravitational theories where we describe a systematic way of constructing consistent teleparallel theories that respect certain physical conditions such as local Lorentz invariance. We first use TG to formulate a teleparallel equivalent of general relativity (GR) which is dynamically equivalent to GR but which may have different behaviors for other scenarios, such as quantum gravity. After setting this foundation, we describe the plethora of modified teleparallel theories of gravity that have been proposed in the literature. We attempt to connect them together into general classes of covariant gravitational theories. Of particular interest, we highlight the recent proposal of a teleparallel analogue of Horndeski gravity which offers the possibility of reviving all of the regular Horndeski contributions. In the second part of the Review, we first survey works in teleparallel astrophysics literature where we focus on the open questions in this regime of physics. We then discuss the cosmological consequences for the various formulations of TG. We do this at background level by exploring works using various approaches ranging from dynamical systems to Noether symmetries, and more. Naturally, we then discuss perturbation theory, firstly by giving a concise approach in which this can be applied in TG theories and then apply it to a number of important theories in the literature. Finally, we examine works in observational and precision cosmology across the plethora of proposal theories. This is done using some of the latest observations and is used to tackle cosmological tensions which may be alleviated in teleparallel cosmology. We also introduce a number of recent works in the application of machine learning to gravity, we do this through deep learning and Gaussian processes, together with discussions about other approaches in the literature.

Measuring glacier mass changes from space—a review

Abstract: Glaciers distinct from the Greenland and Antarctic ice sheets are currently losing mass rapidly with direct and severe impacts on the habitability of some regions on Earth as glacier meltwater contributes to sea-level rise and alters regional water resources in arid regions. In this review, we present the different techniques developed during the last two decades to measure glacier mass change from space: digital elevation model (DEM) differencing from stereo-imagery and synthetic aperture radar interferometry, laser and radar altimetry and space gravimetry. We illustrate their respective strengths and weaknesses to survey the mass change of a large Arctic ice body, the Vatnajökull Ice Cap (Iceland) and for the steep glaciers of the Everest area (Himalaya). For entire regions, mass change estimates sometimes disagree when a similar technique is applied by different research groups. At global scale, these discrepancies result in mass change estimates varying by 20%–30%. Our review confirms the need for more thorough inter-comparison studies to understand the origin of these differences and to better constrain regional to global glacier mass changes and, ultimately, past and future glacier contribution to sea-level rise.

The Rigid Unit Mode model: review of ideas and applications.

Abstract: We review a set of ideas concerning the flexibility of network materials, broadly defined as structures in which atoms form small polyhedral units that are connected at corners. One clear example is represented by the family of silica polymorphs, with structures composed of corner-lined SiO4tetrahedra. TheRigid Unit Mode(RUM) is defined as any normal mode in which the structural polyhedra can translate and/or rotate without distortion, and since forces associated with changing the size and shape of the polyhedra are much stronger than those associated with rotations of two polyhedra around a shared vertex, the RUMs might be expected to have low frequencies compared to all other phonon modes. In this paper we discuss the flexibility of network structures, and how RUMs can arise in such structures, both in principle and in a number of specific examples of real systems. We also discuss applications of the RUM model, particularly for our understanding of phenomena such as displacive phase transitions and negative thermal expansion in network materials. .

Hydrogen in actinides: electronic and lattice properties

Abstract: Hydrides of actinides, their magnetic, electronic, transport, and thermodynamic properties are discussed within a general framework of H impact on bonding, characterized by volume expansion, affecting mainly the 5f states, and a charge transfer towards H, which influences mostly the 6d and 7s states. These general mechanisms have diverse impact on individual actinides, depending on the degree of localization of their 5f states. Hydrogenation of uranium yields UH2 and UH3, binary hydrides that are strongly magnetic due to the 5f band narrowing and reduction of the 5f-6d hybridization. Pu hydrides become magnetic as well, mainly as a result of the stabilization of the magnetic 5f 5 state and elimination of the admixture of the non-magnetic 5f 6 component. Ab-initio computational analyses, which for example suggest that the ferromagnetism of β-UH3 is rather intricate involving two non-collinear sublattices, are corroborated by spectroscopic studies of sputter-deposited thin films, yielding a clean surface and offering a variability of compositions. It is found that valence-band photoelectron spectra cannot be compared directly with the 5f n ground-state density of states. Being affected by electron correlations in the excited final states, they rather reflect the atomic 5f n −1 multiplets. Similar tendencies can be identified also in hydrides of binary and ternary intermetallic compounds. H absorption can be used as a tool for fine tuning of electronic structure around a quantum critical point. A new direction is represented by actinide polyhydrides with a potential for high-temperature superconductivity.

Statistical Genetics and Direct Coupling Analysis in and out of Quasi-Linkage Equilibrium.

Abstract: This review is about statistical genetics, an interdisciplinary topic between statistical physics and population biology. The focus is on the phase of \emph{quasi-linkage equilibrium} (QLE). Our goals here are to clarify under which conditions the QLE phase can be expected to hold in population biology and how the stability of the QLE phase is lost. The QLE state, which has many similarities to a thermal equilibrium state in statistical mechanics, was discovered by M Kimura for a two-locus two-allele model, and was extended and generalized to the global genome scale by Neher and Shraiman (2011). What we will refer to as the Kimura-Neher-Shraiman (KNS) theory describes a population evolving due to the mutations, recombination, natural selection and possibly genetic drift. A QLE phase exists at sufficiently high recombination rate (r) and/or mutation rates (μ) with respect to selection strength. We show how in QLE it is possible to infer the epistatic parameters of the fitness function from the knowledge of the (dynamical) distribution of genotypes in a population. We further consider the breakdown of the QLE regime for high enough selection strength. We review recent results for the selection-mutation and selection-recombination dynamics. Finally, we identify and characterize a new phase which we call the non-random coexistence (NRC) where variability persists in the population without either fixating or disappearing.

Wetting ridges on slippery liquid-infused porous surfaces

Abstract: Slippery liquid-infused porous surfaces (SLIPS) show remarkable liquid repellency, making them useful for many coating applications. The outstanding repellency of SLIPS comes from a lubricant layer stabilized within and at the surface of a porous template. The stability of this lubricant layer is key for SLIPS to exhibit their unique functionality. The lubricant layer, however, is depleted over time, causing degradation of liquid repellency. The formation of wetting ridges surrounding liquid droplets on the surface of SLIPS is one of the primary sources of lubricant depletion. Here, we present the fundamental understanding and characteristics of wetting ridges and highlight the latest developments that enable the detailed investigation and suppression of wetting ridge formation on SLIPS. In addition, we offer our perspectives on new and exciting directions for SLIPS.

Thermally activated delayed fluorescence materials for organic light emitting diodes.

Abstract: Recently, remarkable advances in thermally activated delayed fluorescence (TADF) materials have attracted dramatic attention due to 100% exciton utilization efficiency in organic light emitting diode (OLED). Although the commercialization of TADF materials is at an early stage, they exhibit enormous potential for next generation OLED for the comparable electroluminescence (EL) performance to metal phosphorescent complexes counterparts but without the presence of precious metal elements. This review summarizes the different types of TADF small molecules with various photophysical properties and the state-of-the-art molecular design strategies. Furthermore, the device engineering and emerging optoelectronic applications, such as organic light-emitting electrochemical cells, organic lasing, and organic scintillators are introduced. It is anticipated that the review can clarify the design of efficient TADF emitters and point out the direction of future development.

MICROSCOPE’s view at gravitation

Abstract: The weak equivalence principle (WEP) is the cornerstone of general relativity (GR). Testing it is thus a natural way to confront GR to experiments, which has been pursued for four centuries with increasing precision. MICROSCOPE is a space mission designed to test the WEP with a precision of 1 in 1015 parts, two orders of magnitude better than previous experimental constraints. After completing its two-year mission, from 2016 to 2018, MICROSCOPE delivered unprecedented precise constraints η(Ti,Pt)=[−1.5±2.3 (stat)±1.5 (syst)]×10−15 (at 1σ in statistical errors) on the Eötvös parameter between one proof mass made of titanium and another made of platinum. This bound allowed for improved constraints on alternative theories of gravitation. This review discusses the science beyond MICROSCOPE—GR and its alternatives, with an emphasis on scalar–tensor theories—before presenting the experimental concept and apparatus. The mission’s science returns are then discussed before future tests of the WEP are introduced.

Mechanical metamaterials.

Abstract: Mechanical metamaterials, also known as architected materials, are rationally designed composites, aiming at elastic behaviors and effective mechanical properties beyond ("meta") those of their individual ingredients - qualitatively and/or quantitatively. Due to advances in computational science and manufacturing, this field has progressed considerably throughout the last decade. Here, we review its mathematical basis in the spirit of a tutorial, and summarize the conceptual as well as experimental state-of-the-art. This summary comprises disordered, periodic, quasi-periodic, and graded anisotropic functional architectures, in one, two, and three dimensions, covering length scales ranging from below one micrometer to tens of meters. Examples include extreme ordinary linear elastic behavior from artificial crystals, e.g., auxetics and pentamodes, "negative" effective properties, behavior beyond classical linear elasticity, e.g., arising from local resonances, chirality, beyond-nearest-neighbor interactions, quasi-crystalline mechanical metamaterials, topological band gaps, cloaking based on coordinate transformations and on scattering cancellation, seismic protection, nonlinear and programmable metamaterials, as well as space-time-periodic architectures.

Analogies of phonon anomalies and electronic gap features in the infrared response of Sr 14−x Ca x Cu24O41 and underdoped YBa2Cu3O 6+x

Abstract: We present an experimental and theoretical study which compares the phonon anomalies and the electronic gap features in the infrared response of the weakly coupled two-leg-ladders in Sr 14−x Ca x Cu24O41 (SCCO) with those of the underdoped high-T c superconductor YBa2Cu3O 6+x (YBCO) and thereby reveals some surprising analogies. Specifically, we present a phenomenological model that describes the anomalous doping- and temperature-dependence of some of the phonon features in the a-axis response (field along the rungs of the ladders) of SCCO. It assumes that the phonons are coupled to charge oscillations within the ladders. Their changes with decreasing temperature reveal the formation of a crystal (density wave) of hole pairs that are oriented along the rungs. We also discuss the analogy to a similar model that was previously used to explain the phonon anomalies and an electronic plasma mode in the c-axis response (field perpendicular to the CuO2 planes) of YBCO. We further confirm that an insulator-like pseudogap develops in the a-axis conductivity of SCCO which closely resembles that in the c-axis conductivity of YBCO. Most surprisingly, we find that the c-axis conductivity (field along the legs of the ladders) of SCCO is strikingly similar to the in-plane one (field parallel to the CuO2 planes) of YBCO. Notably, in both cases a dip feature develops in the normal state spectra that is connected with a spectral weight shift toward low frequencies and can thus be associated with precursor superconducting pairing correlations that are lacking macroscopic phase coherence. This SCCO–YBCO analogy indicates that collective degrees of freedom contribute to the low-energy response of underdoped high T c cuprates and it even suggests that the charges in the CuO2 planes tend to segregate forming quasi-one-dimensional structures similar to the two-leg ladders, as predicted for the stripe-scenario or certain intertwinned states.

Machine learning for observational cosmology

Abstract: An array of large observational programs using ground-based and space-borne telescopes is planned in the next decade. The forthcoming wide-field sky surveys are expected to deliver a sheer volume of data exceeding an exabyte. Processing the large amount of multiplex astronomical data is technically challenging, and fully automated technologies based on machine learning (ML) and artificial intelligence are urgently needed. Maximizing scientific returns from the big data requires community-wide efforts. We summarize recent progress in ML applications in observational cosmology. We also address crucial issues in high-performance computing that are needed for the data processing and statistical analysis.

Stochastic equations and cities

Abstract: Stochastic equations constitute a major ingredient in many branches of science, from physics to biology and engineering. Not surprisingly, they appear in many quantitative studies of complex systems. In particular, this type of equation is useful for understanding the dynamics of urban population. Empirically, the population of cities follows a seemingly universal law—called Zipf’s law—which was discovered about a century ago and states that when sorted in decreasing order, the population of a city varies as the inverse of its rank. Recent data however showed that this law is only approximate and in some cases not even verified. In addition, the ranks of cities follow a turbulent dynamics: some cities rise while other fall and disappear. Both these aspects—Zipf’s law (and deviations around it), and the turbulent dynamics of ranks—need to be explained by the same theoretical framework and it is natural to look for the equation that governs the evolution of urban populations. We will review here the main theoretical attempts based on stochastic equations to describe these empirical facts. We start with the simple Gibrat model that introduces random growth rates, and we will then discuss the Gabaix model that adds friction for allowing the existence of a stationary distribution. Concerning the dynamics of ranks, we will discuss a phenomenological stochastic equation that describes rank variations in many systems—including cities—and displays a noise-induced transition. We then illustrate the importance of exchanges between the constituents of the system with the diffusion with noise equation. We will explicit this in the case of cities where a stochastic equation for populations can be derived from first principles and confirms the crucial importance of inter-urban migrations shocks for explaining the statistics and the dynamics of the population of cities.