${\mathrm{Bi}}_2{\mathrm{Se}}_3$ has been the focus of intense interest over the past decade due to its topological properties. Bi surfaces are known to form on ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ upon exposure to atmosphere, but their electronic structure has not been investigated. We report band structure measurements of such Bi surfaces using angle-resolved photoemission spectroscopy. Measured spectra can be well explained by the band structure of a single bilayer of Bi on ${\mathrm{Bi}}_2{\mathrm{Se}}_3$, and show that Bi surfaces consistently dominate the photoemission signal for air exposure times of at least one hour. These results demonstrate that atmospheric effects should be taken into consideration when identifying two-dimensional transport channels, and when designing surface-sensitive measurements of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$, ideally limiting air exposure to no more than a few minutes.

${\mathrm{LiNb}}_{1-x}{\mathrm{Ta}}_x{\mathrm{O}}_3$ solid solutions are investigated from first principles and by optical spectroscopy. The ground- and excited-state properties of the solid solutions are modeled within density functional theory as a function of the Ta concentration using special quasirandom structures spanning the entire composition range between ${\mathrm{LiNbO}}_3$ and ${\mathrm{LiTaO}}_3$. Deviations from a Vegard behavior are predicted for the lattice parameters, the heat capacity, the electronic band gap, and consequently the absorption edge. The latter is measured for crystals of different composition by low-temperature optical spectroscopy, qualitatively confirming the theoretical predictions. The ${\mathrm{LiNb}}_{0.11}{\mathrm{Ta}}_{0.89}{\mathrm{O}}_3$ composition is found to be a highly unusual crystal with a permanent macroscopic electric polarization and nonetheless zero birefringence.

We report the tunable magnetic structures in the helimagnet $\mathrm{YBa}{\left(\mathrm{C}{\mathrm{u}}_{1-x}\mathrm{F}{\mathrm{e}}_x\right)}_2{\mathrm{O}}_5$ by changing various Cu/Fe ratios. The magnetic properties and structures of the samples were studied by using neutron- and x-ray powder diffraction and susceptibility measurements. The parent material $\mathrm{YBaCuFe}{\mathrm{O}}_5$ (YBCFO) exhibits two antiferromagnetic (AFM) transitions at $T_{N1}\sim 450$ K and $T_{N2}\sim 175$ K. At $T_{N1}$, YBCFO shows a commensurate (CM) spin ordering with a q-wave vector ${\mathit{k}}_{\mathit{c}\mathbf{1}}=(1/21/21/2)$, which transfers to a spiral magnetic ordering at $T_{N2}$ with an incommensurate $q$-wave vector (1/2 1/2 1/2 ±δ), δ  being the incommensurability. Using neutron powder diffraction for samples with $0.490\le x\le 0.505, T_{N2}$ was observed to systematically increase from 125 to 236 K as a function of the Cu/Fe ratio. For $x=0.510$ and 0.515, additional magnetic reflections with a propagation vector ${\mathit{k}}_{\mathit{c}\mathbf{2}}=(1/21/20)$ appear below $T_{N3}\sim 245$ and 269 K, respectively. This CM phase shows a collinear magnetic ordering. These transition temperatures are consistent with the AFM transitions of the magnetization data. Experiments were also conducted to synthesize the samples with different thermal treatments, demonstrating the $T_{N2}$ and $T_{N3}$ to be sensitive to the annealing conditions. These results can be understood within the framework of the random distributions of the dopants (impurities) at the $B$ site of a double-perovskite lattice and demonstrate that the magnetic ordering in the helimagnet YBCFO can be systematically tuned by changing the Cu/Fe ratio.

Barium tantalate with a tetragonal tungsten bronze structure, ${\mathrm{Ba}}_3{\mathrm{Ta}}_5{\mathrm{O}}_{15}$, and compounds where Ba is partly substituted by various rare earths ($R$), ${\mathrm{Ba}}_{3-x}{R}_x{\mathrm{Ta}}_5{\mathrm{O}}_{15}$, were synthesized as single crystals, and their transport, magnetic, and magnetotransport properties were investigated. It was found that the compounds with Eu substitution show a relatively large negative magnetoresistance arising from the interaction between the conduction electrons in the Ta $5d$ orbital and the localized spins in the Eu $4f$ orbital.

The ferroelectric to paraelectric phase transition in ${\mathrm{LiTaO}}_3$ and in pure as well as Mg-doped ${\mathrm{LiNbO}}_3$ is investigated theoretically by atomistic calculations in the framework of the density functional theory, as well as experimentally by calorimetry and electrical conductivity measurements. First-principles models within the stochastic self-consistent harmonic approximation (SSCHA) allow to consider anharmonic effects and thus to obtain a realistic estimate of the Curie temperature $T_C$ of both ferroelectrics. Ab initio molecular dynamics (AIMD) calculations performed on large supercells confirm the Curie temperatures estimated with the SSCHA approach. Moreover, they also suggest that the structural phase transition is a continuous process beginning at temperatures well below $T_C$. According to AIMD, significant ionic displacements occur already at temperatures of about 100 K and 300 K below $T_C$ in ${\mathrm{LiTaO}}_3$ and ${\mathrm{LiNbO}}_3$, respectively. To asses whether and how far the ionic displacements affect the materials properties, the AIMD results are compared with measurements of the electrical conductivity and of the heat capacity across the phase transition. Our first-principles calculations moreover show that Mg ions, a frequently employed dopant, raise the Curie temperature in ${\mathrm{LiNbO}}_3$.

The series of $\mathrm{Li}M{\mathrm{O}}_2$ ($M$: transition metal) materials are highly relevant as cathode materials of Li-ion batteries. The stability of such systems remains an important factor for their usability in batteries, and depends strongly on the electronic configuration of the transition-metal ions. In particular, the promising class of multi-transition-metal systems exhibits complicated valence states due to intermetallic charge transfer and charge disproportionation. Here we perform a systematic study on the valence of the transition-metal ions using x-ray absorption spectroscopy on the $M-L_{2,3}$ edges and O-$K$ edges. In $\mathrm{Li}{\left({\mathrm{Ni}}_{0.5}{\mathrm{Mn}}_{0.5}\right)}_{1-x}{\mathrm{Co}}_x{\mathrm{O}}_2$ we established that the valence is ${\mathrm{Co}}^{3+}$ and ${\mathrm{Ni}}_{0.5}^{2+}{\mathrm{Mn}}_{0.5}^{4+}$ throughout the whole series. Meanwhile, in ${\mathrm{LiNi}}_{1-x}{\mathrm{Co}}_x{\mathrm{O}}_2$ we found that the Ni displays a behavior consistent with a charge disproportionated negative charge transfer system, and that with increased concentration of ${\mathrm{Co}}^{3+}$, the disproportionation signal decreases. Since the number of O $2p$ holes also gets reduced, we infer that the material will also become more unstable.

Freestanding tubular crystals offer a general description of crystalline order on deformable surfaces with cylindrical topology, such as single-walled carbon nanotubes, microtubules, and recently reported colloidal assemblies. These systems exhibit a rich interplay between the crystal's helicity (the chiral angle) on its periodic surface, the deformable geometry of that surface, and the motions of topological defects within the crystal. Previously, in simulations of tubular crystals as elastic networks, we found that dislocations in nontrivial patterns can costabilize with kinks in the tube shape, producing mechanical multistability. Here, we extend that work with detailed Langevin dynamics simulations in order to explore defect dynamics efficiently and without the constraints imposed by elastic network models. Along with the predicted multistability of dislocation glide, we find a variety of irreversible defect transformations, including vacancy formation, particle extrusions, and “reactions” that reorient dislocation pairs. Moreover, we report spontaneous sequences of several such defect transformations, which are unique to tubular crystals. We demonstrate a simple method for controlling these sequences through a time-varying external force.

Atomically thin van der Waals magnetic materials offer exceptional opportunities to mechanically and electrically manipulate magnetic states and spin textures. The possibility of efficient spin transport in these materials makes them promising for the development of novel nanospintronics technology. Using atomistic spin dynamics simulations, we investigate magnetic ground state, magnon dispersion, critical temperature, and magnon spin transport in ${\mathrm{CrCl}}_3$ bilayers in the absence and presence of compressive and tensile strains. We show that in the presence of mechanical strain, the magnon band gap at the $\mathrm{\Gamma }$ point and the critical temperature of the bilayer are increased. Furthermore, our simulations show that the magnon diffusion length is reduced in the presence of strain. Moreover, by exciting magnons through the spin Seebeck effect and spin Hall-induced torque, we illustrate distinctions between magnon spin transport in the antiferromagnetic state, under compressive strains, and ferromagnetic states, under tensile strains or in the unstrained case.

The high domain wall mobility in ferroelectric multidomain state superlattices contributes to an enhancement of the dielectric permittivity and can locally over-respond to an applied charge giving rise to a negative capacitance behavior in the ferroelectric layers. Here the results of dielectric investigations using impedance spectroscopy are reported on $\mathrm{Pb}\left({\mathrm{Zr}}_{0.2}{\mathrm{Ti}}_{0.8}\right){\mathrm{O}}_3/{\mathrm{SrTiO}}_3$ superlattices made of 5 unit cells of $\mathrm{Pb}\left({\mathrm{Zr}}_{0.2}{\mathrm{Ti}}_{0.8}\right){\mathrm{O}}_3$, but with varying the thickness of the ${\mathrm{SrTiO}}_3$ in the superlattice period. X-ray diffraction has evidenced the presence of ferroelectric nanodomains consistent with dielectric enhancement. The temperature dependence of the dielectric constant ε′, the dissipation factor tanδ, the ac conductivity σ′, and the electric modulus M″ were investigated. We evidence thermally activated long-range conduction, which is not associated to an interfacial polarization. The temperature behavior of the ac conductivity depends on the STO layer thickness in the period of the superlattice. Regardless of the STO thickness, a Maxwell-Wagner effect occurs for all samples around 600 K. A negative capacitance regime in the superlattices was evidenced in a temperature range where no Maxwell-Wagner relaxation takes place, showing that $\mathrm{Pb}\left({\mathrm{Zr}}_{0.2}{\mathrm{Ti}}_{0.8}\right){\mathrm{O}}_3/{\mathrm{SrTiO}}_3$ superlattices provide an alternative system to ${\mathrm{PbTiO}}_3/{\mathrm{SrTiO}}_3$ superlattices to investigate the relationship between ferroelectric topological structures and related dielectric properties.

Basic vacancy defects in two-dimensional silicon carbide (2D-SiC) are examined by means of density functional theory calculations to explore their magneto-optical properties as well as their potential in quantum technologies. In particular, the characteristic hyperfine tensors and optical excited states of carbon-vacancy, silicon-vacancy, and carbon antisite-vacancy pair defects in 2D-SiC are determined that are the key fingerprints of these defects that may be observed in electron paramagnetic resonance and photoluminescence experiments, respectively. Besides the fundamental characterization of the most basic native defects, we show that the negatively charged carbon antisite-vacancy defect is a promising candidate for realizing a near-infrared single-photon quantum emitter with a spin doublet ground state, where the negative charge state may be provided by nitrogen doping of 2D-SiC. We find that the neutral carbon vacancy with a spin-triplet ground state might be used for quantum sensing with a broad emission in the visible.

Samples synthesized as Nb substituted onto Bi sites from the nominal composition of ${\mathrm{Nb}}_x{\mathrm{Bi}}_{2-x}{\mathrm{Se}}_3$ resulted in superconducting critical temperatures ( $T_c$ 's) in transport measurements reaching 5.6 K and 5.4 K, for $x$ = 0.2 and 0.25, respectively. All, significantly higher than reported in earlier studies of compositions aiming at Nb intercalation ${\mathrm{Nb}}_x{\mathrm{Bi}}_2{\mathrm{Se}}_3$. Bulk susceptibility showed lower responses for powders with a large variety, including $T_c$ 's ranging from 1.8 K to 4.5 K. Magnetotransport experiments were conducted with angular dependence at the High Field Magnet Laboratory (HFML). Clear Shubnikov–de Haas oscillations were observed in all substituted samples, and a thorough study of the angular dependence of the Shubnikov–de Haas frequencies is presented together with estimates of the Fermi energy ($E_F$). All samples contain the phases of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ and the misfit compound $({\mathrm{BiSe})}_{1.1}{\mathrm{NbSe}}_2$. However, only some areas of the crystal boules contain the third phase of BiSe, and the calculated niobium contents from the nominal composition and the results from the phase analysis show large variations. The results obtained on these substituted samples are compared to the more studied Nb-doped ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ stoichiometry, thereby giving further insight into this system currently under high interest.

The pyrochlore oxide $\mathrm{P}{\mathrm{b}}_2\mathrm{R}{\mathrm{e}}_2{\mathrm{O}}_{7-\delta }$ (PRO) is a candidate spin-orbit coupled metal (SOCM) that exhibits a structural phase transition with inversion symmetry breaking. In this paper, we report on the results of detailed x-ray diffraction (XRD) measurements on single crystals of PRO to clarify the crystal structure below the phase transition temperature at $T_{\mathrm{s}}=300$ K. In the XRD patterns, a clear peak splitting is observed below $T_{\mathrm{s}}$, indicating a cubic-to-tetragonal transition. Based on the group-subgroup relationship and the observed reflection conditions, the space group of the low-temperature phase is proposed to be $I{4}_{1}22$, which agrees with optical second harmonic generation measurements. This space group is the same as that of the lowest-temperature structure of the analogous SOCM $\mathrm{C}{\mathrm{d}}_2\mathrm{R}{\mathrm{e}}_2{\mathrm{O}}_7$ (CRO), which is realized by the emergence of odd-parity multipole order. The comparison between PRO and CRO allows for advancing our understanding of the symmetry-lowering complex order exhibited by SOCMs.

We report on the epitaxial growth of pyrochlore NRO thin films utilizing a reactive off-axis sputtering technique. The growth process employed Ru and Nd metal targets with a repetitive substrate temperature sequence. X-ray diffraction, magnetic susceptibility, and spectroscopic ellipsometry measurements confirm that the structural, magnetic, and electronic properties of near-stoichiometric NRO films match those of the bulk material. Our spin-polarized density functional calculations based on ${\text{Nd}}_2{\text{Ru}}_2{\text{O}}_7$ structural parameters accurately describe the optical spectra and assign Hubbard bands to the interband transitions observed above the optical band gap of 0.2 eV. By utilizing the technique's capability to adjust the degree of ruthenium deficiency, we investigated ${\text{Nd}}_2{\text{Ru}}_2{\text{O}}_7$ films across a wide range of stoichiometric variations. Decreasing the Ru/Nd ratio results in lattice expansion, an increase in the optical band gap, and the suppression of Ru $4d$ intersite optical transitions. Additionally, this adjustment facilitates the elimination of minor inclusions of a ferromagnetic ${\mathrm{NdO}}_x$ impurity phase, influencing the magnetic properties of stoichiometric films at low temperatures. The successful growth of ${\text{Nd}}_2{\text{Ru}}_2{\text{O}}_7$ films opens up promising opportunities for designing and exploring strain- and light-induced states in pyrochlore ruthenates.

Millimeter-scale growth of ${\mathrm{YCrB}}_4$ crystals was achieved by postannealing, followed by conventional arc melting. The bulk crystal was semiconducting, whereas the surface was metallic, as observed by photoemission spectroscopy. First-principles calculations consistently reproduced the metallicity in a model of the boron-terminated surface of the ${\mathrm{YCrB}}_4$ crystal. These results support further investigations of different states on the surface of metal boride bulk materials.

While the connection between colossal magnetoresistance and phase separation in praseodymium-doped rare-earth manganites has been well established, the underlying mechanisms enabling this connection still need to be fully understood. This paper thoroughly examines the phase separation in $\mathrm{L}{\mathrm{a}}_{5/8-x}\mathrm{P}{\mathrm{r}}_x\mathrm{C}{\mathrm{a}}_{3/8}\mathrm{Mn}{\mathrm{O}}_3$ ($x=0.40$) using an integrated approach including magnetotransport, electron spin resonance, and magnetic susceptibility. We observed that the initially fluidlike magnetic phase separation at intermediate temperatures transforms into the solidlike phase separation via a glass transition at low temperatures. Magnetization dynamics measurements revealed that this transition is concurrent with the reentrance of the paramagnetic phase. Our findings offer a perspective on the freezing of phase separation fluidlike behavior resulting in the advent of the low-temperature solidlike state, contributing to a deeper understanding of the temperature-dependent behaviors of praseodymium-doped rare-earth manganites.

Retention of hydrogen isotopes (protium, deuterium, and tritium) in tungsten is one of the most severe issues in the design of fusion power plants, since significant trapping of tritium may cause exceeding radioactivity safety limits in future reactors. Hydrogen isotopes in tungsten can be detected using the nuclear reaction analysis method in the channeling mode (NRA/C). However, the information hidden within the experimental spectra is subject to interpretation. In this work, we propose the methodology to interpret the response of the experimental NRA/C spectra to the specific lattice locations of deuterium by simulations of the NRA/C spectra from atomic structures as obtained from the first-principles calculations. We show that trapping conditions, i.e., states of local crystal structures retaining deuterium, affect the lattice locations of deuterium and the change of lattice locations can be detected by the ion channeling method. By analyzing the experimental data, we are able to determine specific information on the deuterium trapping conditions, including the number of deuterium atoms trapped by one vacancy as well as the presence of impurity atoms along with deuterium in vacancies.

Superionic materials possess mobile atoms with liquidlike behavior in the rigid frameworks of other atoms. Theoretically, the diffusion behavior of the mobile atoms is usually probed by ab initio molecular dynamics simulations where enormous computing resources are requested for a complete thorough study. Thus, only limited cases are investigated without providing the most critical quantity, such as the diffusion barrier. To address this shortcoming, we perform molecular dynamics simulations based on machine-learning interatomic potentials, fitted from ab initio molecular dynamics simulations, to have complete studies for ${\mathrm{Ag}}_2\mathrm{S}, {\mathrm{Ag}}_8{\mathrm{SiTe}}_6, {\mathrm{Cu}}_2\mathrm{S}$, and ${\mathrm{Zn}}_{3.6+x}{\mathrm{Sb}}_3$ systems. Our results indicate that the Arrhenius equation can describe very well the diffusion behaviors of the studied superionic systems where the activation barriers range from 0.09–0.22 eV. The small diffusion barrier provides the fundamental origin for the liquid behaviors of superionic materials.

GeSn alloys hold promise for silicon-compatible integrated applications in electronics, photonics, and topological quantum devices. However, understanding their intricate structures using density functional theory (DFT) calculations is hindered by spatiotemporal constraints. To overcome this limitation, we develop highly accurate and efficient machine-learning interatomic potentials based on a neuroevolution potential approach with farthest point sampling on a comprehensive DFT data set. The application of the developed machine-learning potential in large-scale atomistic simulations bridges the spatiotemporal gap between modeling and advanced characterizations, and facilitates the discovery of structural intricacies in GeSn alloys. Through extensive statistical sampling, we identify a type of short-range order (SRO) that is distinguished by both its structural signature and electronic band gap from the SRO structure previously predicted. Modeling based on a large simulation cell reveals the coexistence of nano SRO domains with various degrees of ordering, demonstrating a complex spatial heterogeneity of SRO structure. Our study not only reinforces the significance of fine-level structural information in alloys, but it also constitutes an effective framework for exploring SRO in a broad range of complex alloys based on highly accurate and effective machine-learning potentials.

At ambient pressure, ${\mathrm{HfTe}}_5$ is a material at the boundary between a weak and a strong topological phase, which can be tuned by changes in its crystalline structure or by the application of high magnetic fields. It exhibits a Lifshitz transition upon cooling, and three-dimensional (3D) quantum Hall effect (QHE) plateaus can be observed at low temperatures. Here, we have investigated the electrical transport properties of ${\mathrm{HfTe}}_5$ under hydrostatic pressure up to 3 GPa. We find a pressure-induced crossover from a semimetallic phase at low pressures to an insulating phase at about 1.5 GPa. Our data suggest the presence of a pressure-induced Lifshitz transition at low temperatures within the insulating phase around 2 GPa. The quasi-3D QHE is confined to the low-pressure region in the semimetallic phase. This reveals the importance of the semimetallic ground state for the emergence of the QHE in ${\mathrm{HfTe}}_5$ and thus favors a scenario based on a low carrier density metal in the quantum limit for the observed signatures of the quasiquantized 3D QHE.

The recent discovery of the spin supersolid candidate ${\mathrm{Na}}_2\mathrm{BaCo}({\mathrm{PO}}_4{)}_2$ has stimulated a great deal of research on triangular-lattice transition-metal phosphates. Here we report a comprehensive study on the structural, magnetic, and magnetocaloric properties of polycrystalline ${\mathrm{Na}}_{2}AT({\mathrm{PO}}_4{)}_2$ ($A$ = Ba,Sr; $T$ = Co,Ni,Mn). X-ray and neutron diffraction measurements confirm that ${\mathrm{Na}}_2\mathrm{Ba}T({\mathrm{PO}}_4{)}_2$ ($\mathrm{NB}T\mathrm{P}$) crystallizes in a trigonal structure, while ${\mathrm{Na}}_2\mathrm{Sr}T({\mathrm{PO}}_4{)}_2$ ($\mathrm{NS}T\mathrm{P}$) forms a monoclinic structure with a slight distortion of the triangular network of $T^{2+}$ ions. The dc magnetization data show that all six compounds order antiferromagnetically below 2 K, and the Néel temperatures of $\mathrm{NS}T\mathrm{P}$ are consistently higher than those of $\mathrm{NB}T\mathrm{P}$ for $T$ = Co, Ni, and Mn, due to the release of geometrical frustration by monoclinical distortions. Furthermore, magnetocaloric measurements show that trigonal $\mathrm{NB}T\mathrm{P}$ can reach a lower temperature in the quasiadiabatic demagnetization process and thus it demonstrates a better performance in the magnetic refrigeration, compared with monoclinic $\mathrm{NS}T\mathrm{P}$. Our findings highlight the outstanding magnetocaloric performances of the trigonal transition-metal phosphates and disclose two necessary ingredients for a superior magnetic coolant that can reach an ultralow temperature, including a perfect geometrically frustrated lattice and a small effective spin number associated with the magnetic ions.