Sn(II) and Pb(II) compounds, especially chalcogenides, are important functional materials. The complementary properties of these two families of compounds, with differences related to the differences in stability of the divalent state and relativistic effects on the band structures, provide tunability of properties and also can guide searches for new phases. In particular, it suggests the existence of previously unknown ternary Sn(II) and Pb(II) chalcogenide phases. Here, we predict the existence of stable compounds via first-principles global optimization structure searches on the alkaline-earth metal Sn(II) and Pb(II) chalcogenide systems. This leads to five new stoichiometries. These unreported phases, ${\mathrm{SrSnS}}_2$, ${\mathrm{SrSnSe}}_2$, ${\mathrm{SrPbSe}}_2$, ${\mathrm{BaSnSe}}_2$, and ${\mathrm{BaPbS}}_2$, are thermodynamically preferred by Sn-rich (or Pb-rich) conditions, which favor the divalent state. They are semiconducting with band gaps ranging from 0.80 to 1.85 eV, strong visible light absorption and relatively light hole and electron effective masses ($<1$ ${\mathrm{m}}_0$). The valence band maxima of these compounds have antibonding character involving metal $s$ and chalcogen $p$ character. This is a feature that is often associated with defect tolerant behavior. In addition the compounds show complex band structures of a type that is favorable for thermoelectric performance.

The mechanical properties of many supramolecular materials are often determined by noncovalent interactions that arise from an interplay between chemical composition and molecular microstructural organization. The reversible nature of noncovalent interactions gives supramolecular materials responsive properties that are otherwise difficult to obtain, such as becoming rigid as a response to mechanical stress. How exactly noncovalent interactions emerge from microstructure, and how they might change in response to applied force or deformation, is not understood. Here we combine nuclear magnetic resonance (NMR) and rheology to directly probe the role of chain proximity in polymer complexes. We observe an increase in chain proximity in response to imposed flow, which we hypothesize to originate from enhanced hydrogen bonding. The chain proximity is directly correlated to rod climbing and shear banding. Flow persists only when applied stresses are low, suggesting a stress-induced thickening mechanism. We verify that hydrogen bond disruptors can turn off both the nontrivial flow behavior and the spectroscopic evidence of chain proximity. The combined rheo-NMR approach shows that it is possible to directly observe the molecular origins behind supramolecular mechanics, paving the way for further study into mechanochemical properties of supramolecular materials.

While most semiconductor materials are susceptible to radiation damage, we report here an observation of enhancements in the conductivity of undoped composite amorphous/nanocrystalline silicon thin films after irradiation with high-energy protons. When a series of films for which the nanocrystal concentration is varied were irradiated with 16-MeV protons with fluences from $2\times {10}^{13}$ to ${10}^{15}\mathrm{protons}/\mathrm{c}{\mathrm{m}}^2$, the dark conductivity following irradiation is increased by up to a factor of 10. Unlike the persistent photoconductivity effect observed in amorphous semiconductors, this enhancement is permanent and is not removed by annealing. Various mechanisms are tested to explain this effect, but none are able to fully account for our observations.

Here we clarify the central role of the miscut during group III-V/group IV crystal growth. We show that the miscut impacts the initial antiphase domain distribution, with two distinct nucleation-driven (miscut typically >1°) and terraces-driven (miscut typically <0.1°) regimes. It is then inferred how the antiphase domain distribution mean phase and mean lateral length are affected by the miscut. An experimental confirmation is given through the comparison of antiphase domain distributions in GaP and GaSb/AlSb samples grown on nominal and vicinal Si substrates. The antiphase domain burying step of GaP/Si samples is then observed at the atomic scale by scanning tunneling microscopy. The steps arising from the miscut allow growth rate imbalance between the two phases of the crystal and the growth conditions can deeply modify the imbalance coefficient, as illustrated with GaAs/Si. We finally explain how a monodomain III-V semiconductor configuration can be achieved even on low miscut substrates.

Modeling thermal transport at the nanoscale is a difficult task, especially when external time-varying heating sources and the complexity of the studied systems make computational schemes that rely on accurate particlelike simulations nonaffordable. Alternative strategies based on corrections of the Fourier law could satisfy the trade-off between accuracy and computational efficiency, since they can be implemented in partial differential equation solvers. This continuum approach could also allow for the coupling between thermal transport and other evolving fields related to the generalized temperature field. Here we demonstrate that corrections due to the finite phonon mean free paths can be suitably included in annealing process simulations of three-dimensional nanosystems. Quantitative predictions can be obtained and readily compared with the experimental characterization of the processed samples.

The performance of $\mathrm{C}{\mathrm{u}}_2\mathrm{ZnSnS}{\mathrm{e}}_4$ solar cells is presently limited by low values of open-circuit voltage which are a consequence of strong band tailing and high level of nonradiative recombination. Recently, the partial substitution of Cu, Zn, and Sn by other elements has shown the potential to overcome this limitation. We explored the structural changes and the effect on the optoelectronic properties of the partial substitution of Cu with Ag in $\mathrm{C}{\mathrm{u}}_2\mathrm{ZnSnS}{\mathrm{e}}_4$. This paper clarifies the crystal structure of ${\left(\mathrm{A}{\mathrm{g}}_{1-x}\mathrm{C}{\mathrm{u}}_x\right)}_2\mathrm{ZnSnS}{\mathrm{e}}_4$ solid solution series, deducing possible cationic point defects and paying special attention to the presence of Cu/Zn disorder with a combination of neutron and x-ray diffraction. The optoelectronic properties of the solid solution series are assessed using reflection and quantitative photoluminescence spectroscopy, which allows us to estimate the fraction of nonradiative recombination, which would contribute to the open-circuit voltage loss in devices. The results strongly suggest Ag incorporation as a promising route to eliminate Cu/Zn disorder and to reduce nonradiative recombination losses in $\mathrm{C}{\mathrm{u}}_2\mathrm{ZnSnS}{\mathrm{e}}_4$.

In this paper, we report on the synthesis and magnetic properties of a series of double perovskites $\mathit{Ln}{}_2{\mathrm{ZnIrO}}_6$ with $\mathit{Ln}$ = Nd, Sm, Eu, Gd. These compounds present examples of the rare case of double perovskites (general formula $A{}_{2}BB{}^{\prime }{\mathrm{O}}_6$) with a magnetic $4f$ ion on the $A$ site in combination with the strongly spin-orbit coupled 5d transition-metal ion ${\mathrm{Ir}}^{4+}$ on the $B$ sublattice. We discuss the impact of different rare earths on the macroscopic magnetic properties. ${\mathrm{Gd}}_2{\mathrm{ZnIrO}}_6$ and ${\mathrm{Eu}}_2{\mathrm{ZnIrO}}_6$ show ferrimagnetic or canted antiferromagnetic order below $T_N$ = 23 and 12 K, respectively. ${\mathrm{Sm}}_2{\mathrm{ZnIrO}}_6$ orders antiferromagnetically at $T_N$ = 13 K. ${\mathrm{Nd}}_2{\mathrm{ZnIrO}}_6$ exhibits more complex magnetic properties with a strong field dependence ranging from a two-step spin reorientation at ${\mu }_{0}H$ = 0.01 T to an antiferromagnetic ground state at intermediate external fields to a spin-flop phase for ${}_{0}H \ge$ 4 T. This unique behavior suggests an interesting interplay between ${\mathrm{Nd}}^{3+}$ and ${\mathrm{Ir}}^{4+}$. To further shed light on the relevant magnetic interaction, the zero-field magnetic ground state of ${\mathrm{Nd}}_2{\mathrm{ZnIrO}}_6$ is examined via neutron powder diffraction. In general, the magnetic properties of $\mathit{Ln}{}_2{\mathrm{ZnIrO}}_6$ hint towards a substantial contribution of the rare-earth ions and a significant correlation between $5d$ and $4f$ magnetism.

The Hall effect in ${\mathrm{SrRuO}}_3$ thin films near the thickness limit for ferromagnetism shows an extra peak in addition to the ordinary and anomalous Hall effects. This extra peak has been attributed to a topological Hall effect due to two-dimensional skyrmions in the film around the coercive field; however, the sign of the anomalous Hall effect in ${\mathrm{SrRuO}}_3$ can change as a function of saturation magnetization. Here we report Hall peaks in ${\mathrm{SrRuO}}_3$ in which volumetric magnetometry measurements and magnetic force microscopy indicate that the peaks result from the superposition of two anomalous Hall channels with opposite sign. These channels likely form due to thickness variations in ${\mathrm{SrRuO}}_3$, creating two spatially separated magnetic regions with different saturation magnetizations and coercive fields. The results are central to the development of strongly correlated materials for spintronics.

Controlling changes in magnetic anisotropy across epitaxial film interfaces is an important prerequisite for many spintronic devices. For the canonical dilute magnetic semiconductor GaMnAs, magnetic anisotropy is highly tunable through strain and doping, making it a fascinating model system for exploration of anisotropy control in a carrier-mediated ferromagnet. Here, we have used transmission electron microscopy and polarized neutron reflectometry to characterize the interface between GaMnAs-based layers designed to have anisotropy vectors oriented at right angles from one another. For a bilayer of ${\mathrm{Ga}}_{1-x}{\mathrm{Mn}}_x{\mathrm{As}}_{1-y}{\mathrm{P}}_y$ and ${\mathrm{Ga}}_{1-x}{\mathrm{Mn}}_x\mathrm{As}$, we find that the entirety of the ${\mathrm{Ga}}_{1-x}{\mathrm{Mn}}_x\mathrm{As}$ layer exhibits in-plane magnetic anisotropy and that the $majority$ of the ${\mathrm{Ga}}_{1-x}{\mathrm{Mn}}_x{\mathrm{As}}_{1-y}{\mathrm{P}}_y$ exhibits perpendicular anisotropy. However, near the ${\mathrm{Ga}}_{1-x}{\mathrm{Mn}}_x\mathrm{As}$ interface, we observe a thin Mn-rich region of the nominally perpendicular ${\mathrm{Ga}}_{1-x}{\mathrm{Mn}}_x{\mathrm{As}}_{1-y}{\mathrm{P}}_y$ that instead exhibits $in-plane$ anisotropy. Using first-principles energy considerations, we explain this sublayer as a natural consequence of interfacial carrier migration.

We report on the structural and magnetic properties of the elusive Jahn-Teller active compound ${\mathrm{NaCrF}}_3$, for first time synthesized in large quantities allowing detailed characterization. The crystal structure of ${\mathrm{NaCrF}}_3$ is initially described from a DFT model, which helped serve as the basis for indexing and structure determination confirmed by high-resolution synchrotron x-ray diffraction experiments. ${\mathrm{NaCrF}}_3$ adopts the triclinic space group $P\overline{1}$ (isostructural with ${\mathrm{NaCuF}}_3$). Magnetometry studies at low temperature show that ${\mathrm{NaCrF}}_3$ is a weak antiferromagnet, Weiss temperature $\theta =-4$ K. The Néel temperature is $T_N=21.3$ K and the paramagnetic moment $\mu =4.47{\mu }_{{}_B}$ is in accordance with the theoretical $S=2$. Field-dependent measurements between 2 and 12 K unveil the onset of metamagnetic behavior. Our experiments revealed a weakly canted $A$-type magnetic structure observed by neutron powder diffraction, with a magnetic propagation vector $(1/2,1/2,0)$ and a magnetic moment of $3.51{\mu }_B$ at 1.5 K. Our results shed further light on the Jahn-Teller effects and strong correlations as a function of $A$-ion size in the family $A{\mathrm{CrF}}_3$.

We studied the surface and electronic structures of barium stannate (${\mathrm{BaSnO}}_3$) thin films by low-energy electron diffraction (LEED) and angle-resolved photoemission spectroscopy (ARPES) techniques. ${\mathrm{BaSnO}}_3/{\mathrm{Ba}}_{0.96}{\mathrm{La}}_{0.04}{\mathrm{SnO}}_3/{\mathrm{SrTiO}}_3$ (10 nm/100 nm/0.5 mm) samples were grown using the pulsed-laser deposition (PLD) method and were ex situ transferred from the PLD chamber to ultrahigh vacuum (UHV) chambers for annealing, LEED, and ARPES studies. UHV annealing starting from $300{}^{\circ }\mathrm{C}$ up to $550{}^{\circ }\mathrm{C}$, followed by LEED and ARPES measurements, show $1\times 1$ surfaces with nondispersive energy-momentum bands. The $1\times 1$ surface reconstructs into a $\sqrt{2}\times \sqrt{2}R{45}^{\circ }$ one at an annealing temperature of $700{}^{\circ }\mathrm{C}$ where the ARPES data show clear dispersive bands with a valence band maximum located around 3.3 eV below the Fermi level. While the $\sqrt{2}\times \sqrt{2}R{45}^{\circ }$ surface reconstruction is stable under further UHV annealing, it is reversed to a $1\times 1$ surface by annealing the sample in 400 mTorr oxygen at $600{}^{\circ }\mathrm{C}$. Another UHV annealing at $600{}^{\circ }\mathrm{C}$, followed by LEED and ARPES measurements, suggests that LEED $\sqrt{2}\times \sqrt{2}R{45}^{\circ }$ surface reconstruction and ARPES dispersive bands are reproduced. Our results provide a better picture of the electronic structure of ${\mathrm{BaSnO}}_3$ surfaces and are suggestive of the role of oxygen vacancies in reversible $\sqrt{2}\times \sqrt{2}R{45}^{\circ }$ surface reconstruction.

Quantum dots (QDs) formed in semiconductor nanowires (NWs) form a basis for studying interesting quantum phenomena and provide an exciting platform for various device applications, where in-depth understanding of the formation and electronic properties of such QDs is a requirement. In this work we present detailed investigations of structural, electronic, and optical properties of nitrogen-induced self-assembled QDs in novel Ga(N,As,P) NWs. We show that the three-dimensional quantum confinement of the excitons caused by short-range fluctuations in the N content leads to single-photon emission. We demonstrate that valence band character varies between the QD emitters and attribute it to a varying degree of hole localization within the QDs induced by fluctuations in the As/P ratio. These fluctuations in the chemical compositions are also believed to be at least partly responsible for the observed large variation in exciton lifetimes of the QDs, ranging between 0.5 and 10 ns. The rather long lifetimes demonstrate the resilience of the QDs to nonradiative recombination—a merit for efficient single-photon emitters. Furthermore, we show that the principal axes of the formed QDs are oriented radially, normal to the NW side facets, in contrast to the N-induced QDs in GaNAs NWs which are primarily aligned along the NW axis. Our results, therefore, underline the impacts of the host alloy on the QD formation, which could provide an additional degree of freedom for band structure engineering in nanowires.

The trion, a quasiparticle comprising one exciton and an additional charge carrier, offers unique opportunities for generating spin-photon interfaces that can be used in developing quantum networks. Trions are also actively sought after for integrated optoelectronic devices including photovoltaics, photodetectors, and spintronics. However, formation of trions in strongly confined low-dimensional materials is often deemed detrimental. This is because trion emission in such materials is typically prohibited due to the predominant nonradiative Auger recombination processes. Semiconductor nanoplatelets with their strong confinement in the thickness direction and extended lateral geometries exhibit large exciton coherence sizes and reduced carrier-carrier interactions that may enable unprecedented trion properties. Here, we perform optical spectroscopic studies of individual CdSe nanoplatelets at cryogenic temperatures and observe bright trion emission with intensities comparable to that of neutral exciton emission. We perform carrier dynamics studies of the nanoplatelets and find that due to their extended lateral geometry, the fast radiative decay rate of the nanoplatelets at cryogenic temperatures is comparable to the inhibited Auger recombination rate, leading to the bright trion emission. Our tight-binding theory further reveals distinct size-tunable trion emission in the nanoplatelets that is advantageous for efficient trion emission. These properties make semiconductor nanoplatelets potential candidates as photon sources for optoelectronic and quantum logic devices.

Bulk ${\mathrm{SrTiO}}_3$ is a quantum paraelectric in which an antiferrodistortive distortion below $\approx 105$ K and quantum fluctuations at low temperature preclude the stabilization of a long-range ferroelectric state. However, biaxial mechanical stress, impurity doping, and Sr nonstoichiometry, among other mechanisms, are able to stabilize a ferroelectric or relaxor ferroelectric state at room temperature, which develops into a longer-range ferroelectric state below 250 K. In this paper, we show that epitaxial ${\mathrm{SrTiO}}_3$ thin films grown under tensile strain on ${\mathrm{DyScO}}_3$ exhibit a large reduction of thermal conductivity, of $\approx 60\%$ at room temperature, with respect to identical strain-free or compressed films. The thermal conductivity shows a further reduction below 250 K, a temperature concurrent with the peak in the dielectric constant [J. H. Haeni et al., Nature (London) 430, 758 (2004)]. These results suggest that strain gradients in the relaxor and ferroelectric phase of ${\mathrm{SrTiO}}_3$ are very effective phonon scatterers, limiting the thermal transport in this material.

Two-dimensional (2D) transition-metal nitrides and carbides (MXenes), containing a few atomic layers only, are novel materials which have become a hub of research in many applied technological fields, ranging from catalysis, to environmental scrubber materials, up to batteries. MXenes are obtained by removing the A element from precursor MAX phases, and it is for this reason that it is often assumed that the resulting 2D material displays the MAX atomic layer stacking—an ABC sequence with trigonal $\left(D_{3d}\right)$ symmetry. By means of density functional theory calculations, including dispersion, this work thoroughly explores the stability of alternative ABA stacking, with $D_{3h}$ hexagonal symmetry, for a total of 54 MXene materials with ${\mathrm{M}}_2\mathrm{X}, {\mathrm{M}}_3{\mathrm{X}}_2$, and ${\mathrm{M}}_4{\mathrm{X}}_3$ stoichiometries ($\mathrm{M}=\mathrm{Ti}$, Zr, Hf, V, Nb, Ta, Cr, Mo, or W; and $\mathrm{X}=\mathrm{C}$ or N), revealing that for clean MXenes, the ABA stacking is fostered (i) by the number of $d$ electrons in M, (ii) when $\mathrm{X}=\mathrm{N}$ rather than $\mathrm{X}=\mathrm{C}$, and (iii) when the surface is terminated by oxygen adatoms. The results suggest that stacking phase transitions are likely to take place under working operando conditions, surmounting affordable layer sliding energy barriers, in accordance with the experimentally observed layer distortions in $\mathrm{M}{\mathrm{o}}_2\mathrm{N}$. Finally, we tackled the adsorptive and catalytic capabilities implications of such layer phase transition by considering ${\mathrm{N}}_2$ adsorption, dissociation, and hydrogenation on selected ABC and ABA stacked MXenes. Results highlight changes in adsorption energies of up to ∼1 eV, and in ${\mathrm{N}}_2$ dissociation energy barriers of up to ∼0.3 eV, which can critically change the reaction step rate constant by three to four orders of magnitude for working temperatures in the 400–700 K range. Consequently, it is mandatory to carefully determine the atomic structure of MXenes and to use models with the most stable stacking when inspecting their chemical or physical properties.

${\mathrm{MnBi}}_4{\mathrm{Te}}_7$ and ${\mathrm{MnBi}}_6{\mathrm{Te}}_{10}$ are two members with $n=2$ and 3 in the family of ${\mathrm{MnBi}}_{2n}{\mathrm{Te}}_{3n+1}$ where the $n=1$ member, ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$, has been intensively investigated as the first intrinsic antiferromagnetic topological insulator. Here we report the A-type antiferromagnetic order in these two compounds by measuring magnetic properties, electrical and thermal transport, specific heat, and single-crystal neutron diffraction. Both compounds order into an A-type antiferromagnetic structure as does ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ with ferromagnetic planes coupled antiferromagnetically along the crystallographic $c$ axis. While no evidence for any in-plane ordered moment is found for ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ or ${\mathrm{MnBi}}_6{\mathrm{Te}}_{10}$, weak reflections at half-$L$ positions along the [0 0 $L]$ direction are observed for ${\mathrm{MnBi}}_4{\mathrm{Te}}_7$ suggesting an in-plane ordered moment around $0.15{\mu }_B/\mathrm{Mn}$. The ordering temperature, $T_N$, is 13 K for ${\mathrm{MnBi}}_4{\mathrm{Te}}_7$ and 11 K for ${\mathrm{MnBi}}_6{\mathrm{Te}}_{10}$. The magnetic order is also manifested in the anisotropic magnetic properties. For both compounds, the interlayer coupling is weak and a spin-flip transition occurs when a magnetic field of around 1.6 kOe is applied along the $c$ axis at 2 K. As observed in ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$, when cooling across $T_N$, no anomaly was observed in the temperature dependence of thermopower. On the other hand, critical scattering effects are observed in thermal conductivity although the effect is less pronounced than that in ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$.