We present a comprehensive study of the structure and dynamics of ${\mathrm{Bi}}_2{\mathrm{O}}_3$ with different Y-doping concentrations $\mathrm{[}{\left({\mathrm{Bi}}_{1–x}{\mathrm{Y}}_x\right)}_2{\mathrm{O}}_3(x=0$, 0.2, and 0.4)] using inelastic neutron scattering (INS) and Raman scattering experiments supplemented by ab initio molecular dynamics (AIMD) and lattice dynamics simulations. Our INS and Raman experiments show a significant change in the vibrational spectrum between normal $\alpha \text{−}{\mathrm{Bi}}_2{\mathrm{O}}_3$ and superionic $\delta \text{−}{\mathrm{Bi}}_2{\mathrm{O}}_3$, and a pronounced change with temperature in Y-doped compounds, which reveal the presence of soft anharmonic phonons and the role of host dynamics in fast oxygen diffusion. The results corroborate very well the AIMD simulations. Further, the AIMD simulations show a faster oxygen diffusion with 20% Y-doping than with 40%. Our analysis reveals that the ${\mathrm{Bi}}_4\mathrm{O}$ tetrahedral volume distribution plays a crucial role in O diffusion; a larger mean volume of ${\mathrm{Bi}}_4\mathrm{O}$ leads to faster oxygen diffusion.

This study presents the experimental realization of a spin-1/2 zigzag-square lattice in a verdazyl-based complex, namely ($m\text{−}\mathrm{Py}\text{−}\mathrm{V}\text{−}2,6\text{−}{\mathrm{F}}_2)\left[\mathrm{Cu}{\left(\mathrm{hfac}\right)}_2\right]$. Molecular orbital calculations suggest the presence of five types of frustrated exchange couplings. Our observations reveal an incremental increase in the magnetization curve beyond a critical field, signifying a phase transition from the antiferromagnetic ordered state to a quantum state characterized by a 1/2 plateau. This intriguing behavior arises from the effective stabilization of a zigzag chain by the external fields. These results provide evidence for field-induced dimensional reduction in a zigzag-square lattice attributed to the effects of frustration.

Liquid crystal (LC), an anisotropic and tunable material, is widely used in phase shifters, polarization converters, and chiral devices. However, it is difficult for simple LC devices to achieve strong chiral response in the terahertz regime, even for cholesteric LCs. In this paper, we observe a strong optical rotation effect in a twisted nematic (TN) LC cell with submillimeter thickness; moreover, the magnetic field disturbance breaks the mirror symmetry of the whole structure and a discontinuous TN LC molecular alignment is formed, thus realizing the strong circular dichroism effect. The above two effects can also be actively tuned by electrically controlling the arrangement of LC molecules. First, without applying an external magnetic field, the dynamic modulation of the optical rotation angle in the above TN LC device can be as high as about ${75}^{\circ }$. Then, when a magnetic field in the $x$-axis direction is applied, the dynamic manipulation range of circular dichroism in the TN LC device can be up to 35 dB. Meanwhile, the dynamic polarization conversion function (from LP to CP or LP to LP) can also be achieved. The optical rotation and circular dichroism effect of this TN LC with submillimeter thickness offer a clear physical picture for the development of efficient and active terahertz polarization conversion and chiral devices.

Magnetic garnets such as yttrium iron garnet (${\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$, YIG) are widely used in spintronic and magnonic devices. Their magnetic and magneto-optical properties can be modified over a wide range by tailoring their chemical composition. Here, we report the successful growth of Al-substituted yttrium iron garnet (YAlIG) thin films via radio frequency sputtering in combination with an ex situ annealing step. Upon selecting appropriate process parameters, we obtain highly crystalline YAlIG films with different ${\mathrm{Al}}^{3+}$ substitution levels on both single crystalline ${\mathrm{Y}}_3{\mathrm{Al}}_5{\mathrm{O}}_{12}$ (YAG) and ${\mathrm{Gd}}_3{\mathrm{Ga}}_5{\mathrm{O}}_{12}$ (GGG) substrates. With increasing ${\mathrm{Al}}^{3+}$ substitution levels, we observe a reduction of the saturation magnetization as well as a systematic decrease of the magnetic ordering temperature to values well below room temperature. YAlIG thin films thus provide an interesting material platform for spintronic and magnonic experiments in different magnetic phases.

Heterostructures based on ultrawide-bandgap (UWBG) semiconductors $(\mathrm{bandgap}>4.0\mathrm{eV})$, such as BN and diamond, hold significant importance for the development of high-power electronics in the next generation. However, achieving in situ heteroepitaxy of BN/diamond or vice versa remains exceptionally challenging due to the complex growth kinetics involved. In this work, we grew BN thin film on (100) single-crystal diamonds using pulsed laser deposition and investigated its structural, magnetic, optical, and thermal properties. The structural analyses confirmed the growth of BN films, which exhibited diamagnetic behavior at room temperature. Notably, the film demonstrated anisotropic refractive index characteristics within the visible-to-near-infrared wavelength range. The room-temperature cross-plane thermal conductivity of BN is 1.53 ± 0.77 W/mK, while the thermal conductance of the BN/diamond interface is $20\pm 2\mathrm{MW}/{\mathrm{m}}^2\mathrm{K}$. These findings have significant implications for a range of device applications based on UWBG BN/diamond heterostructures.

We revisit the problem of polytypism in layered ${MX}_2$ materials, with a view to reinterpreting the phase space accessible to this family. Our starting point is to develop a simple, constructive, and compact label for the most commonly observed stacking arrangements, similar to the Glazer notation used to label tilt systems in perovskites. The key advantage of this label in the context of ${MX}_2$ systems is that it contains sufficient information to generate the corresponding stacking sequences uniquely. Using a related approach, we generate a Cartesian representation of the phase space containing all possible ${MX}_2$ polytypes, with the most common structures appearing as limiting cases. We argue that variation in, e.g., composition, temperature, or pressure may allow navigation of this phase space along continuous paths. This interpretation is shown to be consistent with the structural evolution of stacking-faulted ${MX}_2$ systems as a function in temperature and pressure. In this way, our study highlights the potential for controlling composition/structure/property relationships among layered ${MX}_2$ materials in ways that might not previously have been obvious.

In density functional theory, traditional explicit density functionals such as the local density approximation and generalized gradient approximations cannot accurately predict the band gap of solids for a fundamental reason: They lack the exchange-correlation derivative discontinuity. By comparing Kohn-Sham and generalized Kohn-Sham calculations, we here show that the nonempirical meta-generalized-gradient-approximation (meta-GGA) TASK from Aschebrock and Kümmel [Phys. Rev. Res. 1, 033082 (2019)] predicts the right gaps for the right reason, i.e., as a combination of a proper Kohn-Sham gap and a substantial derivative discontinuity contribution. For many materials from small-gap semiconductors to large-gap insulators, the proper band gap is thus obtained. We further study a group of metal-halide perovskites for which the band gap is notoriously hard to predict. For these materials, TASK yields band gaps very similar to the nonlocal screened hybrid Heyd-Scuseria-Ernzerhof functional, yet at a fraction of the hybrid functional's computational cost. We discuss the influence of correlation functionals, and open questions in the comparison of calculated band gaps with experimental ones.

Based on available datasets prepared by numerical simulations and machine learning, maps of properties for materials that have not yet been synthesized can be developed. These maps can be used to select promising materials for synthetic experiments. With a single objective function, the ranking of the optimal solutions can be simply obtained based on the values of the target property. However, applications with multiple target properties require the calculation of Pareto optimal solutions to visualize trade-offs. These solutions are generally ranked manually, selecting the weight of the multiple objectives based on prior knowledge. In this study, to provide an automated ranking of Pareto solutions, we introduced the most-isolated Pareto solution (MIPS) score, which is defined by a projection free energy. Using the MIPS ranking, it is possible to appropriately select the most isolated materials predicted in the property space. To verify the effectiveness of the proposed method, we used a database of semiconductors created by density-functional theory. Our method was able to correctly select and rank the most isolated solutions in both convex and concave two-dimensional Pareto frontiers, outperforming the most relevant outlier detection methods. We also demonstrated that our approach can be easily extended to three-dimensional property spaces.

We report a quasi-one-dimensional $S=1$ spin chain compound $\mathrm{BaNi}{\mathrm{Te}}_2{\mathrm{O}}_7$. This magnetic system has been investigated by magnetic susceptibility, specific heat, and neutron powder diffraction. These results indicate that $\mathrm{BaNi}{\mathrm{Te}}_2{\mathrm{O}}_7$ develops a short-range magnetic correlation around $T\sim 22\mathrm{K}$. With further cooling, an antiferromagnetic phase transition is observed at $T_{\mathrm{N}}\sim 5.4\mathrm{K}$. Neutron powder diffraction revealed antiferromagnetic noncollinear order with a commensurate propagation vector $\mathbf{k}=\left(1/2,1,0\right)$. The refined magnetic moment size of ${\mathrm{Ni}}^{2+}$ at 1.5 K is $1.84{\mu }_{\mathrm{B}}$, and its noncollinear spin texture is confirmed by first-principles calculations. Inelastic neutron-scattering results and density functional theory calculations confirmed the quasi-one-dimensional nature of the spin systems.

Using a combined approach of first-principles and Boltzmann transport theory, we conducted a systematic investigation of the thermal and electrical transport properties of the unexplored ternary quasi-two-dimensional KMgSb system of the $\mathrm{KMg}X$ ($X=\mathrm{P},\mathrm{As},\mathrm{Sb}, \text{and} \mathrm{Bi}$) family. In this paper, we present the transport properties of KMgSb under the influence of hydrostatic pressure and alloy engineering. At a carrier concentration of $\sim 8\times {10}^{19}{\mathrm{cm}}^{-3}$, we observed a close match in the figure of merit (zT; $\sim 0.75$, at 900 K) for both $n$-type and $p$-type KMgSb, making it an attractive choice for engineering thermoelectric devices with uniform materials in both legs. This characteristic is particularly advantageous for high-performance thermoelectric applications. Additionally, as pressure decreases, the zT value exhibits an increasing trend, further enhancing its potential for use in thermoelectric devices. Substitutional doping (replacing 50% of Sb atoms with Bi atoms) resulted in a significant $\sim 49\%$ (in-plane) increase in the peak thermoelectric figure of merit. Notably, after alloy engineering, the maximum figure-of-merit value obtained reached $\sim 1.45$ at 900 K temperature. Hydrostatic pressure emerges as an effective tool for tuning the lattice thermal conductivity ${\kappa }_L$. Our observations indicate that negative-pressure-like effects can be achieved through the chemical doping of larger atoms, especially when investigating ${\kappa }_L$ properties. Through our computational investigation, we elucidate that hydrostatic pressure and alloy engineering hold the potential to dramatically enhance thermoelectric performance in this compound.

The prediction of phase diagrams in the search for new phases is a complex and computationally intensive task. Density functional theory provides, in many situations, the desired accuracy, but its throughput becomes prohibitively limited as the number of species involved grows, even when used with local and semilocal functionals. Here, we explore the possibility of integrating machine-learning models in the workflow for the construction of ternary convex hull diagrams. In particular, we train a set of spectral neighbor-analysis potentials (SNAPs) over readily available binary phases, and we establish whether this is good enough to predict the energies of novel ternaries. Such a strategy does not require any new calculations specific for the construction of the model, but just avails of data stored in binary-phase-diagram repositories. We find that a so-constructed SNAP is capable of accurate total-energy estimates for ternary phases close to the equilibrium geometry but, in general, is not able to perform atomic relaxation. This is because during a typical relaxation path, a given phase traverses regions in the parameter space poorly represented by the training set. Different metrics are then investigated to assess how well an unknown structure is described by a given SNAP model, and we find that the standard deviation of an ensemble of SNAPs provides a fast and non-specie-specific metric. Overall, we show that it is possible to train machine-learning interatomic potentials on readily available binary-compound data to effectively screen ternary compounds in a high-throughput search.

Geometrical frustration, as in pyrochlore lattices made of corner-sharing tetrahedra, precludes the onset of conventional magnetic ordering, enabling the stabilization of fluctuating spin states at low temperature. Disorder is a subtle ingredient that can modify the nature of these exotic nonordered phases. Here, we study the interplay between disorder and magnetic frustration in the new pyrochlore ${\mathrm{Tb}}_2{\mathrm{ScNbO}}_7$ where the nonmagnetic site presents a charge disorder ${\mathrm{Nb}}^{5+}/{\mathrm{Sc}}^{3+}$. Its quantification with sophisticated diffraction techniques (electrons, x-rays, neutrons) allows us to estimate the distribution of the splitting of the magnetic ${\mathrm{Tb}}^{3+}$ non-Kramers ground-state doublets and to compare it with excitations measured in inelastic neutron scattering. Combining macroscopic and neutron scattering measurements, we show that a spin glass transition occurs at 1 K while retaining strong spin liquid correlations. Our results suggest that ${\mathrm{Tb}}_2{\mathrm{ScNbO}}_7$ stabilizes one of the novel disorder-induced quantum spin liquid or topological glassy phases recently proposed theoretically.

Single crystals of the Zintl compound ${\mathrm{Eu}}_7{\mathrm{Ga}}_6{\mathrm{Sb}}_8$ were synthesized using a Ga-Sb flux. We report the temperature ($T$) and magnetic field ($H$) dependence of the magnetic susceptibility ($\chi$), magnetization ($M$), resistivity ($\rho$), specific heat ($C$), and thermal expansion ($\alpha$). We also report high-resolution powder x-ray diffraction data that support a structural phase transition with accompanying signatures seen in $C\left(T\right), \rho \left(T\right)$, and $\alpha \left(T\right)$. We find ${\mathrm{Eu}}_7{\mathrm{Ga}}_6{\mathrm{Sb}}_8$ exhibits antiferromagnetic ordering at $T_{N1}=9.0$ K from anomalies seen in $\chi \left(T\right), C\left(T\right)$, and $\alpha \left(T\right)$, as well as a potential reorientation of ${\text{Eu}}^{2+}$ spins at $T_{N2}=7.5$ K and $T_{N3}=7.2$ K. Density functional theory calculations predict ${\mathrm{Eu}}_7{\mathrm{Ga}}_6{\mathrm{Sb}}_8$ to be semiconducting; however, electrical resistivity measurements show bad-metal behavior that indicates the presence of disorder.

Atomic-level defects dictate the mechanical properties of carbon fibers and strong correlations have been established between the crystallite sizes and mechanical properties. We recently demonstrated similar correlations with hydrogen content, but reliably quantifying the hydrogen content is not possible using only inelastic neutron scattering experiments. Here, we present prompt-gamma activation analysis (PGAA) experiments collected on 20 commercially available carbon fibers to quantify the hydrogen content of carbon fibers and find correlations between fiber modulus and hydrogen content. We then evaluate the role of hydrogen defect type and connect the PGAA results to both newly acquired and recently reported inelastic neutron scattering experiments. We find that intercalated hydrogen defects are preferentially removed at carbonization temperatures required for high-modulus fibers, potentially giving rise to voids within the carbon fibers that undermine their tensile strength.

Disordering processes in oxide materials are complicated by the presence of interfaces, which can serve as either point defect sinks or accumulation sites; the response depends on factors such as interfacial structure, chemistry, and termination. We have characterized the disordering of epitaxial ${\mathrm{Fe}}_3{\mathrm{O}}_4\left(111\right)/{\mathrm{Cr}}_2{\mathrm{O}}_3\left(0001\right)$ thin film heterostructures after 400 keV ${\mathrm{Ar}}^{2+}$ radiation at room temperature. The density of misfit dislocations in both the ${\mathrm{Fe}}_3{\mathrm{O}}_4$ overlayer and ${\mathrm{Cr}}_2{\mathrm{O}}_3$ buffer layer is varied by changing the thickness of ${\mathrm{Cr}}_2{\mathrm{O}}_3$ to be pseudomorphically strained to the ${\mathrm{Al}}_2{\mathrm{O}}_3$ (0001) substrate (5 nm thick) or partially relaxed (20 nm thick), as confirmed by Bragg filtering analysis of scanning transmission electron microscopy images. In both cases, irradiation leads to damage accumulation on the ${\mathrm{Fe}}_3{\mathrm{O}}_4$ side of the heterointerface, as shown by Rutherford backscattering spectrometry measurements in the channeling geometry. However, the interface with more misfit dislocations exhibits disordering at a faster rate than the less-defective interface. Likewise, layer-resolved electron energy loss spectroscopy reveals interfacial reduction of Fe after irradiation at the more defective interface. Intermixing of Cr across the interface is observed by atom probe tomography, which is likely facilitated by the generation of Cr interstitials in ${\mathrm{Cr}}_2{\mathrm{O}}_3$ under irradiation.

Two-dimensional topological insulators (2DTIs) or quantum spin Hall insulators are attracting increasing attention due to their potential applications in next-generation spintronic devices. Despite their promising prospects, realizable 2DTIs are still limited. Recently, ${\mathrm{Ta}}_2{\mathrm{Pd}}_3{\mathrm{Te}}_5$, a semiconducting van der Waals material, has shown spectroscopic evidence of quantum spin Hall states. However, achieving controlled preparation of few to monolayer samples, a crucial step in realizing quantum spin Hall devices, has not yet been achieved. In this work, we fabricated few to monolayer ${\mathrm{Ta}}_2{\mathrm{Pd}}_3{\mathrm{Te}}_5$ and performed systematic thickness- and temperature-dependent Raman spectroscopy measurements. Our results demonstrate that Raman spectra can provide valuable information to determine the thickness of ${\mathrm{Ta}}_2{\mathrm{Pd}}_3{\mathrm{Te}}_5$ thin flakes. Moreover, our angle-resolved polarized Raman (ARPR) spectroscopy measurements show that the intensities of the Raman peaks are strongly anisotropic due to the quasi-one-dimensional atomic structure, providing a straightforward method to determine its crystalline orientation. Our findings may stimulate further efforts to realize quantum devices based on few or monolayer ${\mathrm{Ta}}_2{\mathrm{Pd}}_3{\mathrm{Te}}_5$.

The exceptional electronic and spintronic properties of magnetic Heusler alloys, which include half-metals and Weyl semimetals, are strongly sensitive to deviations from the ideal atomic structure. To ensure that these materials have been produced with the desired properties, it is necessary to determine both the structural ordering and the electronic structure, which can be challenging. Here, we present the results of a far-infrared-to-visible optical spectroscopy study of films of room-temperature ferromagnetic Weyl semimetal $\text{C}{\text{o}}_2\text{MnGa}$. Combined with a determination of the level of ordering from x-ray diffraction, we have investigated near Fermi energy valence and conduction band intra- and interband transitions and their dependence on the atomic order. Motivated by band structure calculations, we have modeled our optical spectra with two Drude terms and two Lorentz oscillators, where the latter are assigned to interband transitions. The scattering rate of the itinerant carriers, determined from the width of the Drude term, increases threefold with increasing disorder, while the carrier density to effective mass ratio is unchanged. Based on our band structure and the joint density of states calculations, we have assigned the oscillator that dominates the interband spectral region near 1 eV to transitions across the minority spin gap along the Γ-$X$ direction. It is found that the energy of this transition is strongly sensitive to the degree of order and decreases rapidly with increasing disorder as states fill a decreasing minority spin gap. Our results demonstrate optical spectroscopy is a sensitive way to fingerprint structural order in the technologically relevant near Fermi level electronic states in Heusler alloys.

Quantum spin Hall insulators are a class of topological materials that has been extensively studied during the past decade. One of their distinctive features is the presence of a finite band gap in the bulk and gapless, topologically protected edge states that are spin-momentum locked. These materials are characterized by a ${\mathbb{Z}}_2$ topological order where, in the two-dimensional case, a single topological invariant can be even or odd for a trivial or a topological material, respectively. Thanks to their interesting properties, such as the realization of dissipationless spin currents, spin pumping, and spin filtering, they are of great interest in the field of electronics, spintronics, and quantum computing. In this work we perform a high-throughput screening of quantum spin Hall insulators starting from a set of 783 two-dimensional exfoliable materials, recently identified from a systematic screening of the Inorganic Crystal Structure Database, Crystallography Open Database, and Materials Platform for Data Science databases. We find four ${\mathbb{Z}}_2$ topological insulators and seven direct gap metals that have the potential of becoming quantum spin Hall insulators under a reasonably weak external perturbation.

Using density functional theory (DFT), we study charge transfer between hexagonal boron nitride (h-BN) point defects and graphene in h-BN/graphene heterostructures using illustrative examples of intrinsic defects: nitrogen vacancy, boron vacancy, nitrogen antisite, and boron antisite. We show that traditional methods that calculate charge transfer by spatial discrimination of charge to different atoms suffer from the misallocation of charge and introduce an alternative method that relies on the integration of the density of states (DOS). We also show that DFT calculations of charge transfer have cell size dependencies due to a change in the DOS in the vicinity of the defect levels. Our results indicate that the nitrogen and boron anitsites do not participate in charge transfer, whereas the nitrogen and boron vacancies experience the transfer of a whole electron. Additionally, we show that a change in the geometry of a defect corresponds to a change in the charge state of the defect. The results of this paper will be important for a wide variety of device applications that involve charge transfer between h-BN defects and graphene in h-BN/graphene heterostructures, while our methodology can be feasibly extended to a wide range of point defects and heterostructures.

Modifying the normal state charge carriers and the related Fermi surface can significantly affect a material's superconducting state. The recently discovered superconducting dome as a function of chemical concentration in transition metal nitrides provides a promising platform for achieving such control. However, this effort was hindered by synthesis techniques that cannot stabilize the material's structure in the presence of a significant number of nitrogen vacancies. In this study, we employed oxygen-assisting nitrogen gas flow with radio frequency magnetron sputtering to stabilize the crystal structure of nitrogen-deficient zirconium nitride thin films and explore the impact of normal state charge carrier types on the superconducting state. Our electrical and thermoelectrical transport measurements indicate a fine-tuning of the superconducting transition temperature, $T_{\mathrm{c}}$, through a shift from hole-type to electron-type charge carriers. Additionally, a concurrent strain release, reflected in the change of the film's crystal orientation, is observed in the process.