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Oxide molecular beam epitaxy is a powerful synthesis technique capable of creating complex layered structures with elements in high oxidation states. The authros start with the Sr${}_{n+1}$Cr${}_n$O${}_{3n+1}$ Ruddlesden-Popper series. This system contains the magnetic Cr${}^{4+}$ cation which gives rise to electronic correlations that vary as a function of structural dimensionality: Sr${}_2$CrO${}_4$ and Sr${}_3$Cr${}_2$O${}_7$ possess enhanced spin and orbital ordering temperatures compared to the SrCrO${}_3$ end member. In this work, they synthesize films for $n=1$ to $n=5$, uncovering a metal to insulator transition. They seek the physical origins of the concomitant spin and orbital orderings – both experimentally with x-ray absorption spectroscopy measurements, and theoretically with density functional theory calculations. Their results unveil the basis of these exotic ground states, including additional structural distortions that play a key role in the system and enable the metal-insulator transitions.

When two different polymers are blended, they form phase-separated emulsions with weak interfaces. These recycled materials are inferior to the original plastics, posing a fundamental challenge for recycling mixed plastic waste streams. An emerging solution to this problem is the addition of a multiblock copolymer compatibilizer to “stitch together” the interface. Using coarse-grained molecular dynamics simulations, Collanton and Dorfman examined how interfacial toughness is impacted by the number of blocks in the compatibilizer and the copolymer loading, connecting the microstructural features of the interface to its failure mechanism.

In this study, the authors take advantage of the emergence time-resolved mechanical spectroscopy to investigate the formation and aging of enzymatic milk gels, made of soft natural colloids. By coupling rheometric measurements with structural characterizations, they reveal two sequential steps in the aging process. First, the open particulate network rapidly matures into a compact network, increasing gel elasticity and evolving the viscoelastic spectrum. Second, the microstructure “freezes” at a critical time, after which aging proceeds through contact-driven mechanisms. This two-step aging process is crucial for industrial cheese processing and contrasts with the aging of hard particle colloidal gels, where the overall network structure remains constant throughout aging.

Hydrogen embrittlement severely impacts structural materials such as iron and its alloys. This study aims to understand hydrogen-enhanced decohesion in ferritic steel grain boundaries (GBs). Density functional theory calculations are carried out to investigate the influence of H on the decohesion of the $\mathrm{\Sigma }$5(310)[001] and $\mathrm{\Sigma }$3(112)[1-10] symmetrical tilt GBs in body-centered cubic (bcc) Fe and Fe+$X$ systems, where $X$ = C, Cr, V, or Mn. The findings indicate that higher local concentrations of hydrogen significantly reduce GB cohesive strength, especially at the $\mathrm{\Sigma }$5 GB. However, at finite stress, the $\mathrm{\Sigma }$3 GB becomes more favorable for hydrogen segregation, suggesting hydrogen redistribution due to stresses in the microstructure can enhance hydrogen resistance.

Low or quasi reduced-dimensional crystal structures are associated with enhanced electronic correlations and emergent quantum behavior. Despite wide interest, identification of new material examples remains restricted by the lack of chemical rules for predicting the structure of extended solids. The authors present the antagonistic pairs approach to discover intermetallic compounds with reduced dimensional structural motifs. By using a pair of strongly immiscible atoms (an antagonistic pair) with a mutually compatible third element, they show that ternary compounds can be formed in which the compatible third element separates the immiscible elements into distinct crystallographic substructures. Quasi-low dimensional structural units, such as sheets, chains, or clusters are the consequence of the immiscible atoms trying to avoid close contact in the solid state. As an example, the authors outline the discovery of La${}_4$Co${}_4$$X$ ($X$ = Pb, Bi, Sb), a family of intermetallic compounds in which the La separates the highly immiscible Co-Pb or Co-Bi atoms into a quasi-layered structure. They anticipate that their approach is a generalizable design principle for discovering new materials and new structure types containing low-dimensional substructures.

A thin film of a topological magnet displays a large thermoelectric effect that doesn’t require an applied magnetic field—a behavior that could lead to new energy-harvesting devices.

Synopsis on:
Shun'ichiro Kurosawa et al.
Phys. Rev. Materials 8, 054206 (2024)

Inspired by the nonmagnetic topological materials database, the authors investigated the 3D fermiology and band topology of the Topological Crystalline Insulator (TCI) candidate SrAg${}_4$Sb${}_2$. The fermiology, revealed by angular-dependent quantum oscillations, shows excellent agreement with first-principles calculations. Symmetry and topology analysis result in two potential sets of topological invariants, suggesting the emergence of crystal-symmetry-protected gapless Dirac surface states either on the as-grown ab planes or on both the ab planes and as-grown mirror planes. Their findings provide evidence that SrAg${}_4$Sb${}_2$ is a promising TCI for exploring topological surface states protected by crystal symmetry.

A reliable interlayer band structure of the kagome superconductor CsV${}_3$Sb${}_5$ is critical for understanding emergent phenomena like the charge density wave ordering and for classifying the topology. Here, the authors present a survey of computational techniques aimed at comparing the electronic interactions between kagome layers in CsV${}_3$Sb${}_5$. This study highlights the computational parameters and plotting methods that lead to differing band behaviors. Within conventional DFT, the parameters employed during structural relaxation are critical in determining the electronic structure between kagome layers. However, higher levels of computational theory contrast these results and point to the increased role of interlayer interactions.

The study focuses on the systematic growth and characterization of material properties, as well as the low-temperature transport properties, of ultrashallow heavily strained quantum wells. A new characterization method, called Density of Stress Accumulation Points, has been introduced for assessing quantum well strain. An ultrashallow heavily constrained quantum well with a remarkable mobility of 3.382×105 cm${}^2$/Vs was successfully achieved. This achievement serves as the foundation for the development of fully electrically controlled and microwave cavity-coupled quantum dot materials.

APS has selected 156 Outstanding Referees for 2024 who have demonstrated exceptional work in the assessment of manuscripts published in the Physical Review journals. A full list of the Outstanding Referees is available online.

Block copolymers provide both a model system for understanding symmetry breaking in soft matter and a unique platform for the design of nanostructured materials.

A discussion of the focus on materials related research in the Physical Review journals.