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The evolution of the content of heavy elements in galaxies, the relative chemical abundances, their spatial distribution, and how these scale with various galactic properties, provide unique information on the galactic evolutionary processes across the cosmic epochs. In recent years major progress has been made in constraining the chemical evolution of galaxies and inferring key information relevant to our understanding of the main mechanisms involved in galaxy evolution. In this review we provide an overview of these various areas. After an overview of the methods used to constrain the chemical enrichment in galaxies and their environment, we discuss the observed scaling relations between metallicity and galaxy properties, the observed relative chemical abundances, how the chemical elements are distributed within galaxies, and how these properties evolve across the cosmic epochs. We discuss how the various observational findings compare with the predictions from theoretical models and numerical cosmological simulations. Finally, we briefly discuss the open problems and the prospects for major progress in this field in the nearby future.

Engineering the electronic band structure of material systems enables the unprecedented exploration of new physical properties that are absent in natural or as-synthetic materials. Half metallicity, an intriguing physical property arising from the metallic nature of electrons with singular spin polarization and insulating for oppositely polarized electrons, holds a great potential for a 100% spin-polarized current for high-efficiency spintronics. Conventionally synthesized thin films hardly sustain half metallicity inherited from their 3D counterparts. A fundamental challenge, in systems of reduced dimensions, is the almost inevitable spin-mixed edge or surface states in proximity to the Fermi level. Here, we predict electric field-induced half metallicity in bilayer A-type antiferromagnetic van der Waals crystals (i.e., intralayer ferromagnetism and interlayer antiferromagnetism), by employing density functional theory calculations on vanadium diselenide. Electric fields lift energy levels of the constituent layers in opposite directions, leading to the gradual closure of the gap of singular spin-polarized states and the opening of the gap of the others. We show that a vertical electrical field is a generic and effective way to achieve half metallicity in A-type antiferromagnetic bilayers and realize the spin field effect transistor. The electric field-induced half metallicity represents an appealing route to realize 2D half metals and opens opportunities for nanoscale highly efficient antiferromagnetic spintronics for information processing and storage.

While Hermiticity lies at the heart of quantum mechanics, recent experimental advances in controlling dissipation have brought about unprecedented flexibility in engineering non-Hermitian Hamiltonians in open classical and quantum systems. Examples include parity-time-symmetric optical systems with gain and loss, dissipative Bose-Einstein condensates, exciton-polariton systems, and biological networks. A particular interest centers on the topological properties of non-Hermitian systems, which exhibit unique phases with no Hermitian counterparts. However, no systematic understanding in analogy with the periodic table of topological insulators and superconductors has been achieved. In this paper, we develop a coherent framework of topological phases of non-Hermitian systems. After elucidating the physical meaning and the mathematical definition of non-Hermitian topological phases, we start with one-dimensional lattices, which exhibit topological phases with no Hermitian counterparts and are found to be characterized by an integer topological winding number even with no symmetry constraint, reminiscent of the quantum-Hall insulator in Hermitian systems. A system with a nonzero winding number, which is experimentally measurable from the wave-packet dynamics, is shown to be robust against disorder, a phenomenon observed in the Hatano-Nelson model with asymmetric hopping amplitudes. We also unveil a novel bulk-edge correspondence that features an infinite number of (quasi)edge modes. We then apply theKtheory to systematically classify all the non-Hermitian topological phases in the Altland-Zirnbauer (AZ) classes in all dimensions. The obtained periodic table unifies time-reversal and particle-hole symmetries, leading to highly nontrivial predictions such as the absence of non-Hermitian topological phases in two dimensions. We provide concrete examples for all the nontrivial non-Hermitian AZ classes in zero and one dimensions. In particular, we identify aZ2topological index for arbitrary quantum channels (completely positive trace-preserving maps). Our work lays the cornerstone for a unified understanding of the role of topology in non-Hermitian systems.

Recent experimental observations have showed some signatures of superconductivity close to 80 K in La 3 Ni 2 O 7 under pressure and have raised the hope of achieving high-temperature superconductivity in bulk nickelates. However, a zero-resistance state—a key characteristic of a superconductor—was not observed. Here we show that the zero-resistance state does exist in single crystals of La 3 Ni 2 O 7− δ using a liquid pressure medium at up to 30 GPa. We also find that the system remains metallic under applied pressures, suggesting the absence of a metal–insulator transition proximate to the superconductivity. Moreover, analysis of the normal state T -linear resistance reveals a link between this strange-metal behaviour and superconductivity. The association between strange-metal behaviour and high-temperature superconductivity is very much in line with other classes of unconventional superconductors, including the cuprates and Fe-based superconductors. Further investigations exploring the interplay of strange-metal behaviour and superconductivity, as well as possible competing electronic or structural phases, are essential to understand the mechanism of superconductivity in this system. Some features resembling superconductivity at high temperature have been seen under pressure in La 3 Ni 2 O 7 , but a transition to a zero-resistance state has not been observed. Now transport studies demonstrate this transition, along with strange metallicity.

The formation of our Milky Way can be split up qualitatively into different phases that resulted in its structurally different stellar populations: the halo and the disk components 1 – 3 . Revealing a quantitative overall picture of our Galaxy’s assembly requires a large sample of stars with very precise ages. Here we report an analysis of such a sample using subgiant stars. We find that the stellar age–metallicity distribution p ( τ , [Fe/H]) splits into two almost disjoint parts, separated at age τ  ≃ 8 Gyr. The younger part reflects a late phase of dynamically quiescent Galactic disk formation with manifest evidence for stellar radial orbit migration 4 – 6 ; the other part reflects the earlier phase, when the stellar halo 7 and the old α -process-enhanced (thick) disk 8 , 9 formed. Our results indicate that the formation of the Galaxy’s old (thick) disk started approximately 13 Gyr ago, only 0.8 Gyr after the Big Bang, and 2 Gyr earlier than the final assembly of the inner Galactic halo. Most of these stars formed around 11 Gyr ago, when the Gaia-Sausage-Enceladus satellite merged with our Galaxy 10 , 11 . Over the next 5–6 Gyr, the Galaxy experienced continuous chemical element enrichment, ultimately by a factor of 10, while the star-forming gas managed to stay well mixed. A sample of approximately 250,000 subgiant stars enables an alternative view of the Milky Way’s assembly history, especially the early formation history of the old disk and halo.

Transmission spectroscopy 1 – 3 of exoplanets has revealed signatures of water vapour, aerosols and alkali metals in a few dozen exoplanet atmospheres 4 , 5 . However, these previous inferences with the Hubble and Spitzer Space Telescopes were hindered by the observations’ relatively narrow wavelength range and spectral resolving power, which precluded the unambiguous identification of other chemical species—in particular the primary carbon-bearing molecules 6 , 7 . Here we report a broad-wavelength 0.5–5.5 µm atmospheric transmission spectrum of WASP-39b 8 , a 1,200 K, roughly Saturn-mass, Jupiter-radius exoplanet, measured with the JWST NIRSpec’s PRISM mode 9 as part of the JWST Transiting Exoplanet Community Early Release Science Team Program 10 – 12 . We robustly detect several chemical species at high significance, including Na (19 σ ), H 2 O (33 σ ), CO 2 (28 σ ) and CO (7 σ ). The non-detection of CH 4 , combined with a strong CO 2 feature, favours atmospheric models with a super-solar atmospheric metallicity. An unanticipated absorption feature at 4 µm is best explained by SO 2 (2.7 σ ), which could be a tracer of atmospheric photochemistry. These observations demonstrate JWST’s sensitivity to a rich diversity of exoplanet compositions and chemical processes. A broad-wavelength 0.5–5.5 µm atmospheric transmission spectrum of WASP-39b, a 1,200 K, roughly Saturn-mass, Jupiter-radius exoplanet, demonstrates JWST’s sensitivity to a rich diversity of exoplanet compositions and chemical processes.

Measuring the abundances of carbon and oxygen in exoplanet atmospheres is considered a crucial avenue for unlocking the formation and evolution of exoplanetary systems 1 , 2 . Access to the chemical inventory of an exoplanet requires high-precision observations, often inferred from individual molecular detections with low-resolution space-based 3 – 5 and high-resolution ground-based 6 – 8 facilities. Here we report the medium-resolution ( R  ≈ 600) transmission spectrum of an exoplanet atmosphere between 3 and 5 μm covering several absorption features for the Saturn-mass exoplanet WASP-39b (ref.  9 ), obtained with the Near Infrared Spectrograph (NIRSpec) G395H grating of JWST. Our observations achieve 1.46 times photon precision, providing an average transit depth uncertainty of 221 ppm per spectroscopic bin, and present minimal impacts from systematic effects. We detect significant absorption from CO 2 (28.5 σ ) and H 2 O (21.5 σ ), and identify SO 2 as the source of absorption at 4.1 μm (4.8 σ ). Best-fit atmospheric models range between 3 and 10 times solar metallicity, with sub-solar to solar C/O ratios. These results, including the detection of SO 2 , underscore the importance of characterizing the chemistry in exoplanet atmospheres and showcase NIRSpec G395H as an excellent mode for time-series observations over this critical wavelength range 10 . The medium-resolution transmission spectrum of the exoplanet WASP-39b, described using observations from the Near Infrared Spectrograph G395H grating aboard JWST, shows significant absorption from CO 2 and H 2 O and detection of SO 2 .

Measurements of the atmospheric carbon (C) and oxygen (O) relative to hydrogen (H) in hot Jupiters (relative to their host stars) provide insight into their formation location and subsequent orbital migration 1 , 2 . Hot Jupiters that form beyond the major volatile (H 2 O/CO/CO 2 ) ice lines and subsequently migrate post disk-dissipation are predicted have atmospheric carbon-to-oxygen ratios (C/O) near 1 and subsolar metallicities 2 , whereas planets that migrate through the disk before dissipation are predicted to be heavily polluted by infalling O-rich icy planetesimals, resulting in C/O < 0.5 and super-solar metallicities 1 , 2 . Previous observations of hot Jupiters have been able to provide bounded constraints on either H 2 O (refs. 3 – 5 ) or CO (refs. 6 , 7 ), but not both for the same planet, leaving uncertain 4 the true elemental C and O inventory and subsequent C/O and metallicity determinations. Here we report spectroscopic observations of a typical transiting hot Jupiter, WASP-77Ab. From these, we determine the atmospheric gas volume mixing ratio constraints on both H 2 O and CO (9.5 × 10 −5 –1.5 × 10 −4 and 1.2 × 10 −4 –2.6 × 10 −4 , respectively). From these bounded constraints, we are able to derive the atmospheric C/H ( 0.35 − 0.10 + 0.17  × solar) and O/H ( 0.32 − 0.08 + 0.12  × solar) abundances and the corresponding atmospheric carbon-to-oxygen ratio (C/O = 0.59 ± 0.08; the solar value is 0.55). The sub-solar (C+O)/H ( 0.33 − 0.09 + 0.13  × solar) is suggestive of a metal-depleted atmosphere relative to what is expected for Jovian-like planets 1 while the near solar value of C/O rules out the disk-free migration/C-rich 2 atmosphere scenario. The C/O ratio of the transiting hot Jupiter WASP-77Ab is measured here and found to be approximately solar, though the (C+O)/H ratio is subsolar.

Photochemistry is a fundamental process of planetary atmospheres that regulates the atmospheric composition and stability 1 . However, no unambiguous photochemical products have been detected in exoplanet atmospheres so far. Recent observations from the JWST Transiting Exoplanet Community Early Release Science Program 2 , 3 found a spectral absorption feature at 4.05 μm arising from sulfur dioxide (SO 2 ) in the atmosphere of WASP-39b. WASP-39b is a 1.27-Jupiter-radii, Saturn-mass (0.28  M J ) gas giant exoplanet orbiting a Sun-like star with an equilibrium temperature of around 1,100 K (ref.  4 ). The most plausible way of generating SO 2 in such an atmosphere is through photochemical processes 5 , 6 . Here we show that the SO 2 distribution computed by a suite of photochemical models robustly explains the 4.05-μm spectral feature identified by JWST transmission observations 7 with NIRSpec PRISM (2.7 σ ) 8 and G395H (4.5 σ ) 9 . SO 2 is produced by successive oxidation of sulfur radicals freed when hydrogen sulfide (H 2 S) is destroyed. The sensitivity of the SO 2 feature to the enrichment of the atmosphere by heavy elements (metallicity) suggests that it can be used as a tracer of atmospheric properties, with WASP-39b exhibiting an inferred metallicity of about 10× solar. We further point out that SO 2 also shows observable features at ultraviolet and thermal infrared wavelengths not available from the existing observations. Observations from the JWST show the presence of a spectral absorption feature at 4.05 μm arising from SO 2 in the atmosphere of the gas giant exoplanet WASP-39b, which is produced by photochemical processes and verified by numerical models.