Low-cost, stable, environment-friendly solar cells are the key aspects of modern-day photovoltaics, and oxide absorbers have become the focus of work towards stable devices. Here we explore the viability of semiconducting transition-metal spinel oxide $\mathrm{ZnF}{\mathrm{e}}_2{\mathrm{O}}_4$ (ZFO) as an absorber layer in solution-processed all-oxide planar solar cells. A 300 nm thick spin-coated ZFO shows a typical cubic spinel structure with a calculated band gap of 1.68 eV, as estimated from density functional theory. Absorption and emission spectra show a direct band gap of 2.19 eV with a maximum absorption coefficient of ${10}^5/\mathrm{cm}$, and the film shows a steady increase in photocurrent on illumination by white light. A conventional solution-grown $\mathrm{Ti}{\mathrm{O}}_2/\mathrm{ZFO}/\mathrm{NiO}$ all-oxide thin film heterojunction solar cell prepared on the fluorine-doped tin oxide coated glass substrate shows an open circuit voltage ($V_{\mathrm{OC}}$) of 519 mV under air mass 1.5 solar illumination. Furthermore, upon doping the $\mathrm{Ti}{\mathrm{O}}_2$ layer with lithium, enhancement in recombination resistance and reduction of charge transfer resistance results in an increased $V_{\mathrm{OC}}$ of 640 mV. Our result introduces spinel oxide as a stable alternative for a photoabsorber in all-oxide photovoltaics, enhancing the multifunctionality of spinel ferrites.

The role of Cr-doping on the structural and magnetic ground states of ${\mathrm{Mn}}_5{\mathrm{Si}}_3$ alloy has been investigated through temperature-dependent neutron powder diffraction (NPD) and x-ray absorption fine structure (XAFS) techniques. All the Cr-doped alloys of nominal composition ${\mathrm{Mn}}_{5-x}{\mathrm{Cr}}_x{\mathrm{Si}}_3$ (for $x$ = 0.05, 0.1, and 0.2) undergo two first-order magnetostructural phase transitions, i.e., hexagonal (space group $P{6}_3/mcm$) paramagnetic $\to$ orthorhombic (space group $Ccmm$) collinear antiferromagnetic $\to$ orthorhombic (space group $Cc2m$) noncollinear antiferromagnetic phase, on cooling from room temperature. NPD studies at different constant temperatures indicate that both antiferromagnetic phases are commensurate in nature and can be represented by the $q=(0,1,0)$ magnetic propagation vector for all the Cr-doped alloys. Such doping at the Mn site results in a significant modification of the noncollinear antiferromagnetic structure (both moment size and orientation) and hence affects the unusual magnetic properties, such as inverted hysteresis loop, thermomagnetic irreversibility, etc. The XAFS measurements were performed to interpret the local environment of doped Cr atoms in detail, which is critical for a microscopic understanding of the unusual properties of this class of Cr-doped ${\mathrm{Mn}}_5{\mathrm{Si}}_3$ alloys. The analysis confirms the elemental state of Cr in the doped alloys and indicates a high degree of preservation of local crystallographic structure with varying Cr concentration and sample temperature. Doping induces intriguing changes in XAFS patterns, elucidated through different types of scattering mechanisms associated with the central absorbing Cr atom.

Beyond quantum confinement, impurity doping of semiconductor nanocrystals (NCs) offers a degree of control over their optical and electronic properties, which are of interest for potential applications. Manganese (Mn)-doped ${\mathrm{CsPbCl}}_3$ NCs, for example, show synergistic effects in the visible, with large enhancements in both blue (exciton) and red (dopant) emission over a narrow range of dopant concentration. Although Mn concentration is the primary parameter, NC size offers an additional degree of control over the number of dopants per NC. Size-resolved ${\mathrm{CsPbCl}}_3$ NCs prepared in the limit of strong Mn doping highlight the emergence of defective NCs with deviations from anticipated confinement trends. The transition correlates with a redshift and increase in the amplitude of dopant emission, and we discuss these observations in the context of atomistic simulations of Ruddlesden-Popper defects in ${\mathrm{CsPbCl}}_3$.

Using high-energy polarization-dependent reflectivity, we discover high-energy resonant excitons at room temperature in single-layer graphene. The resonant excitons occur at $\sim 10.04$ and $\sim 12.29$ eV arising from transitions between parallel $\sigma$ and $\pi *$ states along $\mathrm{\Gamma }\text{−}K$ as well as $\sigma$ and $\sigma *$ states at the $M$ point, respectively. The observation of spectral weight transfer signifies electronic corelation and that the two-dimensional nature and low-energy properties are strongly coupled with high-energy bands. Our result shows the importance of electron-electron and electron-hole interactions in determining electronic and optical properties of graphene and two-dimensional systems.

Chiral materials, characterized by their nonsuperimposable mirror-image symmetry, introduce a multitude of fascinating phenomena and applications in present-day science. Here we predict the presence of the spin Hall effect (SHE) and current-induced spin polarization (CISP) within chiral selenium crystals, employing symmetry analysis and first-principles computational techniques. Notably, band structure calculations unveiled the presence of Rashba splitting, a manifestation of spin-orbit coupling, in these noninversion-symmetric materials. Moreover, our findings reveal that bulk selenium holds promise as a thermoelectric material by adjusting the carrier concentration accordingly. The ZT value we have obtained for t-Se is similar to that of the layered thermoelectric material SnSe and higher than its t-Te counterpart. The unique properties of chiral Se crystals, such as the Rashba splitting and the emergence of spin-related phenomena, together with their thermoelectric features, hold promise for potential applications of this chiral crystal in emerging technologies.

Based on ab initio calculations, we demonstrate that a Mott insulator ${\mathrm{LaTiO}}_3$ (LTO), not inspected previously as an altermagnetic material, shows the characteristic features of altermagnets, i.e., (i) fully compensated antiferromagnetism and (ii) $k$-dependent spin-split electron bands in the absence of spin-orbit coupling. The altermagnetic ground state of LTO is protected by the crystal symmetry and specifically ordered $d$ orbitals of Ti ions with the orbital momentum $l=2.$ The altermagnetism occurs when sites of Ti pairs in the unit cell are occupied by single electrons with $m=-1,s_z=+1/2$ and $m=+1,s_z=-1/2$ per site, with $m$ and $s_z-$ being the $z\text{−}$component of the orbital momentum and spin, respectively. By further simulating orbital disorder within the Green's function method, we disclose its damaging character on the spin splitting and the resulting altermagnetism. When the single-electron spin-polarized state at each Ti site is contributed almost equally by two or three $t_{2g}$ orbitals, LTO becomes antiferromagnetic. The effect of the spin-orbit coupling, which can cause orbital disorder and suppress altermagnetism, is discussed.

The two stable surfaces of ${\mathrm{GdRu}}_2{\mathrm{Si}}_2$ are studied using spin-polarized scanning tunneling microscopy (SP-STM). Depending on the applied magnetic field different magnetic phases have been found and the presented measurements are in agreement with the respective previously proposed multi-$\mathbf{Q}$ spin textures. In particular, the multi-$\mathbf{Q}$ nature of the zero magnetic field state, for which previous experiments could not rule out the coexistence of single-$\mathbf{Q}$ states, can be confirmed by our spin-resolved measurements on the Si-terminated surface. The surfaces of ${\mathrm{GdRu}}_2{\mathrm{Si}}_2$ exhibit strong magnetism-induced modulations of the spin-averaged density of states. We find that while the magnetic contribution to the tunnel signal can be clearly identified for the Si-terminated surface this proves to be much more difficult for the Gd-terminated surface. However, the magnetic field dependent spatial modulations on the Gd-terminated surface demonstrate that additional magnetic phase transitions occur for the surface layer compared to those identified for bulk ${\mathrm{GdRu}}_2{\mathrm{Si}}_2$ and possible spin textures are presented.

A great deal of interest is directed nowadays toward the development of innovative technologies in the field of quantum information and quantum computing, with emphasis on obtaining reliable qubits as building blocks. The realization of highly stable, controllable, and accessible hole spin qubits is strongly dependent on the quality of the materials hosting them. Ultraclean germanium/silicon-germanium heterostructures have been predicted and proven to be promising candidates and, due to their large scalability potential, they are opening the door toward the development of realistic and reliable solid state, all-electric, silicon-based quantum computers. In order to obtain ultraclean germanium/silicon-germanium heterostructures in a reverse grading approach, the understanding and control over the growth of Ge virtual substrates and thin films is key. Here we present a detailed study on the growth kinetics, morphology, and crystal quality of Ge thin films grown via chemical vapor deposition by investigating the effects of growth temperature, partial pressure of the precursor gas, and the use of Ar or ${\mathrm{H}}_2$ atmosphere. The presence of carrier gases catalyzes the deposition rate and induces a smoothening on the surfaces of films grown at low temperatures. We investigated the surface roughness and threading dislocation density as a function of deposition temperature, partial pressure, and gas mixture. Ge thin films deposited by diluting $\mathrm{Ge}{\mathrm{H}}_4$ in Ar or ${\mathrm{H}}_2$ were employed as virtual substrates for the growth of full Ge/SiGe quantum well heterostructures. Their defect density was analyzed and their electric transport properties were characterized via Hall measurements. Similar results were obtained for both carrier gases used.

Superconductivity is one of the most intriguing properties of matter, described by an attractive interaction that binds electrons into Cooper pairs. To date, the highest critical temperature at ambient conditions is achieved in copper oxides. While layered nickel oxides were long proposed to be analogous to cuprates, superconductivity was only demonstrated in 2019 albeit without clarifying the pairing mechanism. Here we use density functional theory to show that superconductivity in nickelates is driven by an electron-phonon coupling originating from a charge ordering. Due to an intrinsic electronic instability in half-doped compounds, $\mathrm{N}{\mathrm{i}}^{1.5+}$ cations dismutate into more stable $\mathrm{N}{\mathrm{i}}^{+}$ and $\mathrm{N}{\mathrm{i}}^{2+}$ cations, which is accompanied by a bond disproportionation of $\mathrm{Ni}{\mathrm{O}}_4$ complexes producing an insulating charge-ordered state. Once doping suppresses the instability, the bond disproportionation vibration is sufficient to reproduce the key characteristic of nickelates observed experimentally, notably the dome of $T_c$ as a function of doping content. These phenomena are identified if relevant degrees of freedom as well as an exchange-correlation functional that sufficiently amends self-interaction errors are involved in the simulations. Finally, despite the presence of correlation effects inherent to $3d$ elements that favor the formation of local Ni spins, the mechanism behind the formation of Cooper pairs in nickelate superconductors appears similar to that of nonmagnetic bismuth oxide superconductors.

Deciphering atomic packing around “defects” in glasses remains a grand challenge in the glass community. Amorphous silicon (a-Si), as a prototypical network-forming glass, often exhibits opposite structure-property relations compared to metallic glasses (MGs), and the classic concept of free volume widely used for MGs is also invalid to a-Si. In this work, we surprisingly discovered that a-Si and MGs share the same structural origin responsible for their orientation-dependent local mechanical response. In addition, the favorite motif in a-Si, i.e., tetrahedra, centered on the atoms with the lowest plastic susceptibility have two dominant preferred orientations. This is in contrast to the case in MGs. These results will enhance our understanding of structure-property relationships in glasses and also provide some optimism for establishing a universal picture regarding “defects” for the two classes of glasses.

Supercooling of liquids is a crucial phenomenon to understand and control crystallization and vitrification in various research fields. In particular, deep supercooling beyond a certain limit, called hypercooling, is practically important for manipulating glass formation as well as crystal nucleation and growth. However, it is still very ambiguous how hypercooling occurs and impacts glass formation. In this work, we find that the hypercooling behavior of liquids is determined by the combination of undercoolability and hypercoolability, unlike the common belief that deep supercooling is the prerequisite to observe hypercooling. This provides an answer to a long-standing question of why certain materials exhibit hypercooling behavior, even though their liquids only have small degree of supercooling. Moreover, we find a clear connection between the hypercooling limit and glass forming ability, which explicitly reveals the hidden role of the hypercooling limit in glass formation from both thermodynamic and kinetic viewpoints. The present results provide not only a key parameter for materials design but also an insight into understanding glass formation.

Femtosecond time-resolved second-harmonic generation (SHG) measurements were performed at room temperature on a single crystal of ${\mathrm{LuFe}}_2{\mathrm{O}}_4$, which has recently attracted attention as an electronic ferroelectric material in which polarization occurs due to the real-space arrangement of $\mathrm{F}{\mathrm{e}}^{2+}$ and $\mathrm{F}{\mathrm{e}}^{3+}$. We irradiated the crystal with 800-nm laser pulses, traced the subsequent SHG changes, and observed anisotropic polarization dynamics: immediately after photoexcitation, the in-plane ($ab$-plane) component of the SHG greatly reduced, while the interplane ($c$ axis) SHG increased. This means that a hidden photoexcited state, in which the in-plane charge ordering is broken but the polarization along the $c$-axis direction enhanced, has been created by photoexcitation, indicating that the electronic ferroelectricity can be controlled on a femtosecond scale.

Motivated by previous investigations that showed very promising high ionic conductivities within relatively stable framework structures, we report a systematic first-principles study of the extended family of lithium (thio)boracites, consisting of eight chemical compositions. Three of the compositions—${\mathrm{Li}}_4{\mathrm{B}}_7{\mathrm{O}}_{12}\mathrm{Cl}$, ${\mathrm{Li}}_4{\mathrm{Al}}_3{\mathrm{B}}_4{\mathrm{O}}_{12}\mathrm{Cl}$, and ${\mathrm{Li}}_6{\mathrm{B}}_7{\mathrm{S}}_{13}\mathrm{Cl}$—are comparable to synthesized and analyzed materials reported in the experimental literature. The five additional compositions—${\mathrm{Li}}_4{\mathrm{B}}_7{\mathrm{S}}_{12}\mathrm{Cl}$, ${\mathrm{Li}}_4{\mathrm{Al}}_3{\mathrm{B}}_4{\mathrm{S}}_{12}\mathrm{Cl}$, ${\mathrm{Li}}_6{\mathrm{B}}_7{\mathrm{O}}_{13}\mathrm{Cl}$, ${\mathrm{Li}}_6{\mathrm{Al}}_3{\mathrm{B}}_4{\mathrm{O}}_{13}\mathrm{Cl}$, and ${\mathrm{Li}}_6{\mathrm{Al}}_3{\mathrm{B}}_4{\mathrm{S}}_{13}\mathrm{Cl}$—are predicted from the computational modeling and analysis presented in this paper. For each material, idealized ordered rhombohedral, cubic, or monoclinic ground-state structures are determined. Through various methodologies including thermodynamic, voltage window, and harmonic phonon analyses, stability is assessed for all eight Li (thio)boracite-derived compositions. Based on the dominant energetics of density functional theory, an analysis of the thermodynamically accessible phases predicts stability for ${\mathrm{Li}}_4{\mathrm{B}}_7{\mathrm{O}}_{12}\mathrm{Cl}$ only. The analysis of the voltage windows of these materials suggests that the sulfur materials are much more reactive in contact with lithium metal than their oxygen counterparts. This reactivity problem has been identified in other highly conducting solid electrolytes and various mitigation methods discussed in the literature look promising. Within the harmonic approximation, phonon analysis predicts that all eight materials are dynamically stable in their ground-state structures. Future investigations will focus on the performance of the family of lithium (thio)boracites, including ionic conductivity predictions and ion migration mechanisms.

We investigate the properties of oxygen-functionalized carbides of lanthanide elements with the composition $M_2{\mathrm{CO}}_2$ $(M=\mathrm{Gd}, \mathrm{Tb}, \mathrm{Dy})$ that form two-dimensional structures. Our ab initio calculations reveal that oxygen termination turns $M_2\mathrm{C}$ monolayers into semiconductors with two dynamically stable phases. Of these, the energetically favored $\alpha$ phase becomes ferroelectric, whereas the $\beta$ phase turns antiferroelectric. Applying in-plane biaxial strain may transform one phase into the other, changes the ferroelectric polarization of the $\alpha$ phase in a linear fashion, and modifies the size and nature of the fundamental band gap from direct to indirect. The structure with a direct band gap exhibits in-plane isotropic electronic and optical properties. This previously unexplored class of systems also exhibits excellent photon absorption in the ultraviolet range.

The physical properties of manganites depend sensitively on their lattice degree of freedom. While lattice parameter can be controlled by growing epitaxial thin films on substrates with different lattice constants or application of high pressure, the range of lattice variation is limited to 2–3% before dislocations start to emerge. In contrast, fabrication of freestanding thin films allows one to change the uniaxial lattice parameter up to 8.2%, which opens up a new platform to investigate emergent phenomena under large lattice-constant change. In this work, we fabricate freestanding ${\left({\mathrm{La}}_{2/3}{\mathrm{Pr}}_{1/3}\right)}_{5/8}{\mathrm{Ca}}_{3/8}{\mathrm{MnO}}_3$ (LPCMO) thin films and investigate how the physical properties are affected by the absence of substrate. We find large length-scale electronic phase separation is greatly suppressed in the freestanding film after being released from the tensile strain of the ${\mathrm{SrTiO}}_3$ substrate. Consequently, the ferromagnetism is boosted in the freestanding film. The physical properties of the freestanding LPCMO thin film are observed to be comparable to those of LPCMO thin film epitaxially grown on nearly strain-free substrate, revealing that freestanding thin film retains the intrinsic physical properties of LPCMO system, which provides a reliable platform to study effects of large strain and strain gradient (by curvature) in manganite systems.

Oxygen vacancies play a key role in many energy-related applications, and the investigation of the thermodynamic forces driving their formation across different materials can be tackled through the use of ab initio methods. In this context, however, computational studies targeting finite-temperature properties such as entropy remain scarce. Wider-ranging studies are of interest to deepen the understanding of the role of the entropy of oxygen vacancy formation and its different components in processes like the ones involved in solar thermochemical hydrogen (STCH) production, and to investigate the existence of common trends and differences between materials. In this work, we use density functional theory and harmonic phonon calculations to compute the vibrational entropy of oxygen vacancy formation ($\mathrm{\Delta }{S}_{\text{vib}}$) for 10 different metal oxide compounds. The computation is carried out by taking the difference between the vibrational entropy of a vacancy-containing and a pristine structure, while the entropic contribution from the final gaseous state of the oxygen lost from the material is accounted for separately with a gas entropy term. We first examine the temperature dependence of $\mathrm{\Delta }{S}_{\text{vib}}$ and highlight the presence of an initial peak around room temperature, followed by a steady decrease resulting from the presence of an additional O atom (the one becoming vacant) in the defect-free structure. We then inspect the atomic contributions to $\mathrm{\Delta }{S}_{\text{vib}}$, and highlight similarities and differences between compounds. Finally, we consider other significant sources of entropy, and find $\mathrm{\Delta }{S}_{\text{vib}}$ to provide a smaller, yet non-negligible, contribution to the total entropy of vacancy formation. We also compare the temperature dependence of $\mathrm{\Delta }{S}_{\text{vib}}$ to that of the gas entropy, and show that the two largely counterbalance each other at high temperatures.

Mechanical properties such as plasticity are fundamental and important properties in processing and applications of materials. Protein crystals are one of the macroscopic molecular crystals composed of protein molecules with nanometer size. The application of protein crystals has been explored not only for the structure analysis of proteins, but also for their application as solids. Generally, it is known that native protein crystals are quite brittle. However, it is empirically known that the mechanical property of protein crystals is enhanced by a cross-linking technique. One of the qualitative reasons is that chemical cross-linking enhances the mechanical strength, but the detailed mechanical properties are not clear. Herein, we investigate the macroscopic elastic and plastic behaviors of cross-linked tetragonal hen-egg-white lysozyme crystals under a compression test. The cross-linked crystals exhibit plastic deformation whereas the native crystals show brittle fracture. The plastic behavior of the cross-linked crystals clearly shows yield phenomena with upper and lower yield points. Stress-induced dislocations are also noted. The Burgers vectors of the moving dislocations of the crystals are characterized by synchrotron x-ray topography, and the results indicate that the slip systems are controlled by cross-linking. Thus, cross-linking leads to the macroscopic plastic deformation of protein crystals. The ductile properties of the cross-linked protein crystals indicate their great potential for material applications such as biocatalysis and biosensing.

Transition-metal dichalcogenides (TMDs) have received much attention in the past decade not only due to the new fundamental physics, but also due to the emergent applications in these materials. Currently chalcogenide deficiencies in TMDs are commonly believed either during the high-temperature growth procedure or in the nanofabrication process resulting in significant changes of their reported physical properties in the literature. Here, we perform a systematic study involving pristine stochiometric ${\mathrm{HfSe}}_2$, Se-deficient ${\mathrm{HfSe}}_{1.9}$, and ${\mathrm{HfSe}}_{1.8}$. Stochiometric ${\mathrm{HfSe}}_2$ transport results show semiconducting behavior with a gap of 1.1eV. Annealing ${\mathrm{HfSe}}_2$ under high vacuum at room temperature causes the Se loss resulting in ${\mathrm{HfSe}}_{1.9}$, which shows unconventionally large magnetoresistivity following the extended Kohler rule at low temperatures below 50 K. Moreover, a clear electrical resistivity crossover, mimicking the metal-insulator transition, is observed in the ${\mathrm{HfSe}}_{1.9}$ single crystal. Further increasing the degree of deficiency in ${\mathrm{HfSe}}_{1.8}$ results in complete metallic electrical transport at all temperatures down to 2 K. Such a drastic difference in the transport behaviors of stoichiometric and Se-deficient ${\mathrm{HfSe}}_2$ further emphasizes the defect control and engineering could be an effective method that could be used to tailor the electronic structure of 2D materials, potentially unlock new states of matter, or even discover new materials.