We report the feasibility of using magnetoentropic mapping for the rapid identification of magnetic cycloid and skyrmion phases in uniaxial systems, based on the ${\mathrm{GaV}}_4{\mathrm{S}}_8$ and ${\mathrm{GaV}}_4{\mathrm{Se}}_8$ model skyrmion hosts with easy-axis and easy-plane anisotropies, respectively. We show that these measurements can be interpreted with the help of a simple numerical model for the spin Hamiltonian to yield unambiguous assignments for both single-phase regions and phase boundaries. In the two lacunar spinel chemistries, we obtain excellent agreement between the measured magnetoentropic features and a minimal spin Hamiltonian built on Heisenberg exchange, single-ion anisotropy, and anisotropic Dzyaloshinskii-Moriya interactions. In particular, we identify characteristic high-entropy behavior in the cycloid phase that serves as a precursor to the formation of skyrmions at elevated temperatures and is a readily measurable signature of this phase transition. Our results demonstrate that rapid magnetoentropic mapping guided by numerical modeling is an effective means of understanding the complex magnetic phase diagrams innate to skyrmion hosts. One notable exception is the observation of an anomalous, low-temperature high-entropy state in the easy-plane system ${\mathrm{GaV}}_4{\mathrm{Se}}_8$, which is not captured in the numerical model. Possible origins of this state are discussed.

Realization of semimetals with nontrivial topologies such as Dirac and Weyl semimetals has provided a boost to the study of these quantum materials. The presence of electron correlation makes the system even more exotic due to enhanced scattering of charge carriers, Kondo screening, etc. Here, we studied the electronic properties of high-quality single-crystalline SmBi employing magnetic and resistivity measurements. The temperature dependence of the magnetization reveals two magnetic transitions: an antiferromagnetic order with a Néel temperature of $\sim 9$ K and a second transition at a lower temperature (= 7 K). The electrical resistivity shows an upturn typical of a Kondo system, and the estimated Kondo temperature is found to be close to the Néel temperature. The magnetization as a function of the applied magnetic field exhibits pronounced quantum oscillations at low temperatures. Using a Landau level fan diagram analysis, a nontrivial Berry phase is identified for a Fermi pocket suggesting topological properties of this material. These results provide an example of the fermiology in the antiferromagnetic state of a Kondo system and open up a new paradigm to explore the Dirac fermion physics in correlated topological materials via interplay of Kondo interaction, magnetism, and topological order.

Layered two-dimensional (2D) transition metal phosphorous chalcogenides (TMPCs) are now in intense research focus due to their interesting ferroelectric and magnetic properties and compatibility with 2D electronic devices. Here, we have employed first-principles density functional theory calculations to investigate the electric and magnetic properties of ${\mathrm{ABP}}_2{\mathrm{S}}_6$ (A = Cu, Ni; B = Cr, Mn) TMPCs. We have systematically investigated four TMPCs compounds, namely, ${\mathrm{CuCrP}}_2{\mathrm{S}}_6, {\mathrm{CuMnP}}_2{\mathrm{S}}_6, {\mathrm{NiCrP}}_2{\mathrm{S}}_6$, and ${\mathrm{NiMnP}}_2{\mathrm{S}}_6$, and reported unusual antiferroelectric/ferroelectric (AFE/FE) and electronic properties. We have found a more stable ferroelectric state in van der Waals (vdW) gap with higher polarization compared to the usual ferroelectric phase in the insulating state. In case of ${\mathrm{CrMnP}}_2{\mathrm{S}}_6$ and ${\mathrm{NiMnP}}_2{\mathrm{S}}_6$, we have proposed them as polar half-metals as our analysis have revealed that ferroelectric distortions can persist in these systems in the metallic phase. Moreover, our analysis have shown that ${\mathrm{NiMnP}}_2{\mathrm{S}}_6$ can undergo metal-to-insulator transition driven by the polar distortion. We have identified two modes namely, ${\mathrm{\Gamma }}_2^{-}$ & ${\mathrm{\Gamma }}_1^{+}$, those are responsible for driving ferroelectricity and antiferroelectricity, respectively, into these systems. We could observe a rare and interesting phenomena, where the antipolar mode ${\mathrm{\Gamma }}_1^{+}$ that is responsible for antiferroelectric distortion leads to a ferroelectric distortion when applied excessively on the system, which is unusual. Finally, we have performed molecular dynamics simulations at various finite temperatures. In case of ${\mathrm{NiCrP}}_2{\mathrm{S}}_6$ and ${\mathrm{NiMnP}}_2{\mathrm{S}}_6$ at 300 K, we have observed that the ferroelectric state within the vdW gap is stable. Interestingly, we have discovered a hybrid inter-intra layer antiferroelectric configuration within the vdW gap for ${\mathrm{CuCrP}}_2{\mathrm{S}}_6$. The layers start to move opposite to each other due to the temperature effect, which leads to the switching of the AFE state in the first layer. Further, increase of temperature results the switching in both the layers. The reported in-gap FE/AFE states in vdW gap can be tuned by uniaxial strain along the perpendicular direction of the 2D layer and thus the materials studied here can be considered as potential piezoelectric materials.

Polar metal is a material that hosts both polar distortion and metallicity. Such a material is expected to show exotic magneto-electric phenomena if it superconducts. Here, we theoretically explore ferroelectric and superconducting properties in a series of perovskite-type oxyhydrides $A\mathrm{Ti}{\mathrm{O}}_2\mathrm{H}$ ($A=\mathrm{K}$, Rb, Cs) under hole-doping conditions using first-principles calculations based on density functional theory. Our simulation shows that these compounds host spontaneous polarization and superconductivity at optimal doping concentration. The unusual coexistence of superconductivity and large polarization ($\sim 100\mu \mathrm{C}/\mathrm{c}{\mathrm{m}}^2$) originates from the weak coupling of the polar distortion and superconducting states, the reason for which is the separation of the displaced atoms and spatially confined metallic carriers. Additionally, superconductivity is enhanced by the unique electronic properties near the valence band maximum: quartic band dispersion with a sizable contribution of hydrogen $1s$ states. This paper thus features the oxyhydrides as possible model polar superconducting systems, which may be utilizable for future magneto-electric devices.

We investigate precipitation dynamics in the presence of a local solute gradient using phase-field simulations. During the homogenization heat treatment of the solidified Inconel 718 alloy, high Nb concentration within the Laves phases or at the core of the secondary arms results in Nb diffusion into the $\gamma$ matrix. The volume fraction and spatial distribution of precipitation during subsequent annealing can be controlled by tailoring the Nb concentration gradient in the matrix during homogenization. We use a surrogate Ni-Fe-Nb alloy for Inconel 718 to explore the growth dynamics of $\delta$ precipitates related to the local Nb concentration levels. The simulations indicate that in the presence of a Nb concentration gradient the growth rate of $\delta$ precipitates is higher than in a matrix of uniform average Nb concentration. The higher growth rate is a result of the higher local thermodynamic driving force at the interface between the solute-rich matrix and the $\delta$ interface. We propose a phenomenological model to describe the diffusion-controlled growth kinetics of the $\delta$ phase under a solute concentration gradient.

We report on the real-time structure formation and growth of two thiophene-based organic semiconductors, $5,5^{\prime }$-bis(naphth-2-yl)-$2,2^{\prime }$-bi- and $5,5^{\prime \prime }$-bis(naphth-2-yl)-$2,2^{\prime }$:$5^{\prime },2^{\prime \prime }$-terthiophene (NaT2 and NaT3), studied in situ during vacuum deposition by grazing-incidence x-ray diffraction and supported by atomic force microscopy and photoabsorption spectroscopy measurements on corresponding ex situ samples. On device-relevant silicon dioxide substrates, for both molecules the growth is observed to transition from two-dimensional (2D) layer-by-layer growth to three-dimensional (3D) growth after the formation of a few-molecule-thick wetting layer. The crystal structure of the NaT2 film is considerably more ordered than the NaT3 counterpart, and there is a significant collective change in the unit cell during the initial stage of growth, indicating strain relief from substrate induced strain as the growth transitions from two to three dimensions. In addition, the orientation of the film molecules are controlled by employing substrates of horizontally and vertically oriented few-layer molybdenum disulfide. Both molecules form needle-like crystals on horizontally oriented ${\mathrm{MoS}}_2$ layers, while the NaT3 molecules form tall, isolated islands on vertically oriented ${\mathrm{MoS}}_2$ layers. The molecules are standing on silicon dioxide and on vertically oriented ${\mathrm{MoS}}_2$, but lying flat on horizontally oriented ${\mathrm{MoS}}_2$. These results demonstrate the importance of film-substrate interactions on the thin-film growth and microstructure formation in naphthyl-terminated oligothiophenes.

Over the last few years, $\mathrm{Re}{\mathrm{S}}_2$ has generated a myriad of unattended queries regarding its structure, the concomitant thickness-dependent electronic properties, and its apparently contrasting experimental optical response. In this paper, with elaborate first-principles investigations, using density functional theory (DFT) and time-dependent DFT, we identify the structure of $\mathrm{Re}{\mathrm{S}}_2$, which is capable of reproducing and analyzing the layer-dependent optical response. The theoretical results are further validated by an in-depth structural, chemical, optical, and optoelectronic analysis of the large-area $\mathrm{Re}{\mathrm{S}}_2$ thin films, grown by the chemical vapor deposition (CVD) process. Micro-Raman, x-ray photoelectron spectroscopy, cross-sectional transmission electron microscopy, and energy-dispersive x-ray analysis have enabled the optimization of the uniform growth of the CVD films. The correlation between the optical and electronic properties was established by static photoluminescence and excited state transient absorption measurements. Sulfur vacancy-induced localized mid-gap states render a significantly long lifetime of the excitons in these films. The ionic gel top-gated photodetectors, fabricated from the as-prepared CVD films, exhibit a large photo-response of $\sim 5$ A/W and a remarkable detectivity of $\sim {10}^{11}$ Jones. The outcome of this paper will be useful in promoting the application of vertically grown large-area films in the field of optics and opto-electronics.

Modern high-performance permanent magnets are made from alloys of rare earth and transition metal elements, and large magnetization is achieved in the alloys with a high concentration of transition metals. We applied an evolutionary search scheme based on first-principles calculations to the Y-Co-B system and predicted 37 cobalt-rich compounds with a high probability of being stable. Focusing on remarkably cobalt-rich compounds, ${\mathrm{YCo}}_{16}$ and ${\mathrm{YCo}}_{20}$, we found that, although they are metastable phases, the phase stability is increased with an increase of temperature due to the contribution of vibrational entropy. The magnetization and Curie temperature are higher by 0.22 T and 204 K in ${\mathrm{YCo}}_{16}$ and by 0.29 T and 204 K in ${\mathrm{YCo}}_{20}$ than those of ${\mathrm{Y}}_2{\mathrm{Co}}_{17}$, which has been well studied as a strong magnetic compound.

In the last decade rare-earth hexaborides have been investigated for their fundamental importance in condensed matter, and for their applications in advanced technological fields. Among these compounds, $\mathrm{La}{\mathrm{B}}_6$ has a special place, being a traditional $d$-band metal without additional $f$ bands. In order to understand the bulk electronic structure of the more complex rare-earth hexaborides, in this paper we investigate the bulk electronic structure of $\mathrm{La}{\mathrm{B}}_6$ using tender/hard x-ray photoemission spectroscopy, measuring both core-level and angle-resolved valence-band spectra. Furthermore, we compare the La $3d$ core level spectrum to cluster model calculations in order to understand the bulklike core-hole screening effects. The results show that the La $3d$ well-screened peak is at a lower binding energy compared to the main poorly screened peak; the relative intensity between these peaks depends on how strong the hybridization is between La and B atoms. We show that the recoil effect, negligible in the soft x-ray regime, becomes prominent at higher kinetic energies for lighter elements, such as boron, but is still negligible for heavy elements, such as lanthanum. In addition, we report the bulklike band structure of $\mathrm{La}{\mathrm{B}}_6$ determined by tender/hard x-ray angle-resolved photoemission spectroscopy (HARPES). We compare HARPES experimental results to the free-electron final-state calculations and to the more precise one-step photoemission theory including matrix element and phonon excitation effects. The agreement between the features present in the experimental ARPES data and the theoretical calculations is very good. In addition, we consider the nature and the magnitude of phonon excitations in order to interpret HARPES experimental data measured at different temperatures and excitation energies. We demonstrate that the one-step theory of photoemission and HARPES experiments provides, at present, the only approach capable of probing, both experimentally and theoretically, true “bulklike” electronic band structure of rare-earth hexaborides and strongly correlated materials.

Tunneling two-level systems (TLSs) dominate the physics of glasses at low temperatures. Yet TLSs are extremely rare, and it is thus difficult to directly observe them in silico. Developing simple structural predictors, which can provide markers for determining if a TLS is present in a given glass region, is crucial for a more efficient search. It has been speculated that vibrational quasilocalized modes (QLMs) are closely related to TLSs, and that one can extract information about TLSs from QLMs. In this work we address this possibility. In particular, we investigate the degree to which a linear or nonlinear vibrational mode analysis can predict the location of TLSs independently found by energy landscape exploration. We find that even though there is a notable spatial correlation between QLMs and TLSs, in general TLSs are strongly nonlinear, and their global properties cannot be predicted by a simple vibrational mode analysis.

The complex crystal chemistry of elemental boron has led to numerous proposed structures with distinctive motifs as well as contradictory findings. Herein, evolutionary structure searches performed at 100 GPa have uncovered a series of metastable phases of boron, and bonding analyses were carried out to elucidate their electronic structure. These polymorphs, dynamically stable at 100 GPa, were grouped into two families. The first was derived from the thermodynamic minimum at these conditions, $\alpha$-Ga, whereas channels comprised the second. Two additional intergrowth structures were uncovered, and it was shown they could be constructed by stacking layers of $\alpha$-Ga-like and channel-like allotropes on top of each other. A detailed bonding analysis revealed networks of four-center $\sigma$-bonding functions linked by two-center B-B bonds in the $\alpha$-Ga based structures, and networks that were largely composed of three-center $\sigma$-bonding functions in the channel-based structures. Seven of these high-pressure phases were found to be metastable at atmospheric conditions, and their Vickers hardnesses were estimated to be $\approx 36$ GPa.

Out-of-plane spin tunneling through the two-dimensional (2D) van der Waals ${\mathrm{CrI}}_3$ multilayer has recently been deeply explored, and giant magnetoresistance has been achieved in various ${\mathrm{CrI}}_3$ magnetic tunneling junctions (MTJs) via the control of interlayer antiferromagnetic coupling by both magnetic and electric fields. In contrast, knowledge of the in-plane spin-transport properties of 2D ${\mathrm{CrI}}_3$ is currently very limited. Here, based on quantum transport simulations, we study the in-plane transport properties of the photocurrent in a ${\mathrm{CrI}}_3$ MTJ with a bilayer/monolayer/bilayer configuration. The photogalvanic effect (PGE) is induced under vertical illumination of elliptically polarized light, giving rise to a robust photocurrent in a broad visible range at zero bias. A perfect spin-valve effect can be achieved with a magnetoresistance of 100% for some photon energies with an appropriate light helicity. Moreover, the PGE photocurrent for the antiparallel configuration is enhanced, as compared to the parallel configuration due to the increased device asymmetry. Our results show that a PGE-driven ${\mathrm{CrI}}_3$ photodetector is a promising candidate for low-power 2D spintronic devices.

Two-dimensional (2D) group-IV monochalcogenides have raised the attention of the material science community owing to their rich physical properties and promising applications in 2D functional devices. Based on ab initio calculations, we explore three square/octagon Haeckelite structures as additional stable allotropes of 2D GeS. All the predicted allotropes are semiconducting with wide [2.5–2.7 eV at the Heyd-Scuseria-Ernzerhof exchange-correlation functional (HSE06) level] and tunable band gaps. Meanwhile, $h_1$- and $h_2$-GeS exhibit unusually high carrier mobilities up to ${10}^4{\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$. Exotic electric polarization orders, including antiferroelectricity and noncollinear polarization with atomic-scale dipole vortices, are found in the predicted allotropes. All three Haeckelite allotropes can be connected to the corresponding hexagonal GeS allotropes laterally with an extremely low energy cost, which indicates the possibility for the coexistence of different phases and the formation of heterostructures in the same GeS layer.

We report complex magnetic, magnetoresistance (MR), and magnetocaloric properties of ${\mathrm{Gd}}_4\mathrm{RhAl}$ and ${\mathrm{Tb}}_4\mathrm{RhAl}$, forming in the ${\mathrm{Gd}}_4\mathrm{RhIn}$-type cubic structure. Though the synthesis of the compounds was reported long ago, to our knowledge, no attempt was made to investigate the properties of these compounds. The present results of ac and dc magnetization, electrical resistivity, and heat-capacity measurements down to 1.8 K establish that these compounds undergo antiferromagnetic order initially, followed by complex spin-glass features with decreasing temperature. These characteristic temperatures are as follows: for the Gd case, $T_N=\sim 46\mathrm{K}$ and $T_G=\sim 21\mathrm{K}$, and for Tb, ∼32 and ∼28 K, respectively. Additionally, there are field-induced magnetic effects, interestingly leading to nonmonotonic variations in MR. There is a significant MR over a wide temperature range above $T_N$, similar to the behavior of the magnetocaloric effect (MCE) as measured by isothermal entropy change (ΔS). An intriguing finding we made is that ΔS at the onset of magnetic order is significantly larger for the Tb compound than that observed for the Gd analog near its $T_N$. On the basis of this observation in a cubic material, we raise the question of whether the aspherical nature of the $4f$ orbital can play a role to enhance MCE under favorable circumstances—a clue that could be useful to find materials for magnetocaloric applications.

We report on a self-organized process initiated on a potassium-tungsten-tellurite glass surface under repetitive femtosecond laser irradiation in a regime where cumulative effects lead to a localized melting. Specifically, we show that self-organized periodic patterns consisting of parallel nanoplanes perpendicular to the laser polarization are forming and extending beyond the zone under direct laser exposure. Examination of the modified regions revealed a phase change from a glassy tellurium-oxide to a crystalline tellurium. In addition, we observe that this self-organization process, associated with elemental redistribution and deoxygenation, is triggered by the optical-field strength. We suggest that early self-organized nanostructures formed by a local-field enhancement is subsequently reinforced by a metallization event in an open-air atmosphere.

We studied structural relaxation in the bulk metallic glass forming alloy ${\mathrm{Zr}}_{46.8}{\mathrm{Ti}}_{8.2}{\mathrm{Cu}}_{7.5}{\mathrm{Ni}}_{10}{\mathrm{Be}}_{27.5}$ on different timescales and length scales, with emphasis on the supercooled liquid state. Using x-ray photon correlation spectroscopy, we determined the microscopic structural relaxation time covering timescales of more than two decades in the supercooled liquid region, down to the subsecond regime. Upon heating across the glass transition, the intermediate scattering function changes from a compressed to a stretched decay, with a smooth transition in the stretching exponent and characteristic relaxation time. In the supercooled liquid state, the macroscopic and microscopic relaxation time and the melt viscosity all exhibit the same temperature dependence. This points to a relaxation mechanism via intrinsic structural relaxation of the majority component Zr, with its microscopic timescale controlling both the stress relaxation and viscous flow of the melt.

${\mathrm{PdCoO}}_2$, belonging to a family of triangular oxides called delafossite, is one of the most conducting oxides. Its in-plane conductivity is comparable to those of the best metals and exhibits hydrodynamic electronic transport with an extremely long mean free path at cryogenic temperatures. Nonetheless, it is nonmagnetic despite the presence of the cobalt ion. Here, we show that a mild hydrogenation process reduces ${\mathrm{PdCoO}}_2$ thin films to an atomically mixed alloy of PdCo with strong out-of-plane ferromagnetism and sign-tunable anomalous Hall effect. Considering that many other compounds remain little affected under a similar hydrogenation condition, this discovery may provide a route to creating novel spintronic heterostructures combining strong ferromagnetism, involving oxides and other functional materials.

Intercalation of foreign atoms in two-dimensional (2D) hosts has been considered a quite promising route to engineer the electronic and magnetic properties in 2D platforms. In the present study, we performed a first-principles theoretical investigation of the energetic stability and the magnetic/electronic properties of 2D ${\mathrm{GeP}}_3$ doped by Cr atoms. Our total energy results reveal the formation of thermodynamically stable Cr doped ${\mathrm{GeP}}_3$ bilayer and quadrilayer, characterized by interstitial Cr atoms lying in the van der Waals gap between the stacked ${\mathrm{GeP}}_3$ layers. The Cr-doped systems become magnetic, and the magnetic ordering can be tuned through the application of compressive mechanical strain. Moreover, the systems are metallic, characterized by the emergence of strain-induced spin-polarized channels at the Fermi level. These findings reveal that the atomic intercalation offers a set of degree of freedom not only to design but also to control the magnetic/electronic properties by mechanical strain in 2D systems.

We derive a gap equation for bilayer excitonic systems based on density functional theory and benchmark our results against quantum Monte Carlo simulations and recent experiments on double bilayer graphene. The gap equation has a mean-field form but includes a consistent treatment of dynamical screening. We show that the gap survives at much higher densities than previously thought from mean-field estimates which gives strong indications that the double-bilayer graphene systems at zero magnetic field can be used as model systems to investigate the BCS-BEC crossover. Furthermore, we show that Josephson-like transfer of pairs can be substantial for small band gaps and densities.