Charge separation and many-body interactions at the interface of the light-absorbing semiconductor and contact layer are of crucial importance to the photophysical properties and optoelectronic device performance. Here, we report the exciton many-body interactions and charge transfer dynamics at the interface of metal halide perovskite nanocrystals and graphene derivatives [graphene oxide (GO) and reduced GO (RGO)] using ultrafast transient absorption (TA) and time-resolved photoluminescence (PL) measurements. At the early timescales, the TA spectra of ${\mathrm{CsPbBr}}_3/\mathrm{GO}$ and ${\mathrm{CsPbBr}}_3/\mathrm{RGO}$ show an asymmetric derivative feature originating from the exciton many-body interactions. The band gap renormalization and binding energies of exciton and biexciton of ${\mathrm{CsPbBr}}_3$ nanocrystals are significantly reduced in ${\mathrm{CsPbBr}}_3/\mathrm{GO}$(RGO) due to the charge transfer and change in the dielectric environment, respectively. More specifically, the exciton (biexciton) binding energy of ${\mathrm{CsPbBr}}_3$ nanocrystals, originally $38\pm 2$ ($34\pm 1$) meV, decreases to $27\pm 1$ ($22\pm 1$) meV in ${\mathrm{CsPbBr}}_3/\mathrm{RGO}$ and $17\pm 1$ ($15\pm 1$) meV in ${\mathrm{CsPbBr}}_3/\mathrm{GO}$. Furthermore, we observe a reduction in the Auger recombination rate and exciton PL quenching in ${\mathrm{CsPbBr}}_3/\mathrm{GO}$ and ${\mathrm{CsPbBr}}_3/\mathrm{RGO}$, corroborating the charge transfer mechanism. Our systematic studies successfully describe photoexcited charge transfer from ${\mathrm{CsPbBr}}_3$ nanocrystals to GO (RGO) in $7.0\pm 0.4$ ($4.2\pm 0.1$) ps, which is one order of magnitude faster than the charge transfer for other acceptor materials such as metal oxide, fullerene, anthraquinone, 1-aminopyrene, and phenothiazine. Our results offer insights and guidance for perovskite-based high-performance optoelectronic devices.

Using ab initio dynamical mean-field theory we explore the electronic and magnetic states of layered ${\mathrm{Li}}_x{\mathrm{MnO}}_2$ as a function of $x$, the state-of-charge. Constructing real-space Wannier projections of Kohn-Sham orbitals based on the low-energy subspace of Mn $3d$ states and solving a multi-impurity problem, our approach focuses on local correlations at Mn sites. The antiferromagnetic insulating state in ${\mathrm{LiMnO}}_2$ has a moderate Néel temperature of $T_N=296\text{K}$ in agreement with experimental studies. Upon delithiation the system proceeds through a number of states: ferrimagnetic correlated metals at $x=0.92$, 0.83; multiple charge disproportionated ferromagnetic correlated metals with large quasiparticle peaks at $x=0.67$, 0.50, 0.33; ferromagnetic metals with small quasiparticle peaks at $x=0.17$, 0.08 and an antiferromagnetic insulator for the fully delithiated state, $x=0.0$. At moderate states of charge, $x=0.67-0.33$, a mix of $+3/+4$ formal oxidation states of Mn is observed, while the overall nominal oxidation of Mn state changes from $+3$ in ${\mathrm{LiMnO}}_2$ to $+4$ in ${\mathrm{MnO}}_2$. In all these cases the high-spin state emerges as the most likely state in our calculations considering the full $d$ manifold of Mn based on the proximity of $e_g$ levels in energy to $t_{2g}$. We observe a crossover from coherent to incoherent behavior on delithiation as function of state-of-charge.

We study the effects of anisotropy on the magnetoresistance of Weyl semimetals in the ultraquantum regime. We utilize the fact that many Weyl semimetals are approximately axially anisotropic. We find that anisotropy manifests itself in the strong dependence of the magnetoresistance on the polar and azimuthal angles determining the orientation of the anisotropy axis with respect to the applied magnetic field and electric current. We also predict that the ratio of magnetoresistances in the geometries, where the magnetic field and anisotropy axes are aligned and where they are orthogonal, scales as ${(v_{\perp }/v_{\parallel })}^2$ where $v_{\perp }$ and $v_{\parallel }$ are the corresponding Fermi velocities.

We performed angle-resolved photoemission spectroscopy (ARPES) experiments using soft x rays to investigate the electronic structure of the heavy-fermion superconductor ${\mathrm{UPt}}_3$. The overall band structure revealed by the present ARPES measurements is compared with density-functional calculations for ${\mathrm{UPt}}_3$ and the non-$5f$ reference compound ${\mathrm{ThPt}}_3$. We showed that the calculation for ${\mathrm{ThPt}}_3$ gives a better description of the experimental band structure, except for the $5f$-derived heavy quasiparticle bands. This situation is reminiscent of Ce-based heavy-fermion systems, whose band structures are often very similar to those of non-$4f$ La-based compounds. The narrow heavy quasiparticle bands are located just below the Fermi level ($E_{\mathrm{F}}$), and their band structure could not be resolved by the present energy resolution. We also showed that most of the U $5f$ spectral weight exists not as the coherent heavy-fermion bands but as incoherent components that distribute over a wide energy range, spanning from near $E_{\mathrm{F}}$ to approximately 2 eV. This result indicates that the heavy quasiparticle bands are enormously renormalized by the electron correlation effect. Additionally, we showed that the spectral intensity of the heavy-fermion bands is almost temperature independent, at least up to 100 K, where the magnetic susceptibility follows a Curie-Weiss behavior.

Weyl points in crystalline materials can be treated as magnetic monopoles in momentum space, with topological charges inscribed by Chern numbers. The main method of finding Weyl states with topological charges greater than one, i.e., unconventional Weyl states, is to take advantage of crystal symmetries, but the maximum topological charge that can be achieved is four. Here we provide a unified scheme for realizing a pair of Weyl points using long-range spin-orbit coupling. This scheme requires no additional symmetry other than the translational symmetry of the crystal and allows the realization of the Weyl point with arbitrary topological charges. We give a concrete method for its realization in a circuit system and verify it experimentally. This discovery reveals a scheme for realizing ideal unconventional Weyl states and using long-range spin-orbit coupling for exotic states.

Lorentz reciprocity places important constraints on the response of systems to wave excitations, and surpassing these constraints is of both fundamental and applied interest. In the context of scattering, reciprocity requires that the total extinction of an object is identical when excited from opposite directions, regardless of the asymmetry of the scatterer. Here, we demonstrate that, by combining multiple scatterers that break time-reversal symmetry, this extinction symmetry can be largely violated. We consider spinning cylinders, demonstrating that nonreciprocity must be paired with broken parity symmetry to enable large extinction contrast. As a dramatic example, we show a system that strongly scatters when excited from one side, but it is cloaked for the opposite excitation. Our results pave the way for approaches to asymmetric audibility and scattering manipulation, and they may readily translate to other wave domains.

The semiconductor BAs has drawn significant interest due to experimental reports of simultaneous high thermal conductivity and ambipolar charge mobility. The ab initio prediction of high electron and hole mobility assumed the dominance of charge carrier scattering by one phonon. Recently, higher-order electron-phonon scattering processes in polar and nonpolar semiconductors have been reported to have a non-negligible impact on charge transport properties, suggesting they may play a role in BAs as well. Here, we report an ab initio study of two-phonon electron and hole scattering processes in BAs. We find that inclusion of these higher-order processes reduces the computed room-temperature electron and hole mobility in BAs by around 40% from the one-phonon value, resulting in an underestimate of experimental values by a similar percentage. We suggest an experimental approach to test these predictions using luminescence spectroscopy that is applicable to the defective samples which are presently available.

We show that a synergy of the covalency effect associated with the $A$-site and $B$-site cations in ${\mathrm{K}}_{0.5}{\mathrm{Bi}}_{0.5}\mathrm{Ti}{\mathrm{O}}_3$ (KBT) makes it unique among the Pb-free ferroelectrics by imparting a rare property of negative and zero thermal expansion (ZTE). We tuned this synergy in two different KBT alloys, ${\mathrm{K}}_{0.5}{\mathrm{Bi}}_{0.5}\mathrm{Ti}{\mathrm{O}}_3-{\mathrm{Na}}_{0.5}{\mathrm{Bi}}_{0.5}\mathrm{Ti}{\mathrm{O}}_3$ and ${\mathrm{K}}_{0.5}{\mathrm{Bi}}_{0.5}\mathrm{Ti}{\mathrm{O}}_3-\mathrm{Bi}\left({\mathrm{Mg}}_{0.5}{\mathrm{Ti}}_{0.5}\right){\mathrm{O}}_3$, and identified a common critical tetragonality below which the ZTE behavior disappears completely in both the systems.

In this Letter, we combine atomistic simulations and a continuum model, based on the Gurtin-Murdoch theory of surface elasticity, to explain the warp previously observed in $\langle 111\rangle$ gold nanowires. This phenomenon, which is characterized by the spontaneous formation of nonplanar cross sections, is primarily due to the crystallographic orientation of the surfaces that can form around a $\langle 111\rangle$ wire. As a result, the surface stress tensor expressed in the wire cylindrical coordinates basis presents a nondiagonal shear component denoted $S_{\mathrm{\Theta }Z}^S\left(\mathrm{\Theta }\right)$ whose anisotropy (its variation with azimuth $\mathrm{\Theta }$) induces bulk stress and warp in the wire. This result is demonstrated by computing $S_{\mathrm{\Theta }Z}^S$ as a function of $\mathrm{\Theta }$ and by integrating these atomistic inputs into the continuum model. By adopting this approach, the warp found in the atomistic simulations is well reproduced analytically for both Au and Cu nanowires having a circular cross section. This behavior is a fine example of the coupling between surface and bulk stress tensors involving nondiagonal components. An extension of this modeling to the case of nanowires with a hexagonal cross section is discussed.

For using machine learning to predict material properties, the feature representation of the materials given to the model plays a fundamental role. A model describes material properties as a function of any given material system expressed as a fixed-length numeric vector, often called a descriptor. However, in most cases, the variables of interest are nontrivial for encoding their compositional or structural features, such as molecules, crystal systems, chemical compositions, and composite materials, into a fixed-length vector. Conventionally, to translate such a multicomponent system into a fixed-length vector, the distribution of predefined component features is summarized into a few summary statistics. The disadvantage of this reduction operation is that some distributional information, such as multimodality, is lost in the vectorization process. Here, we present a general class of material descriptors motivated by the machine-learning theory of kernel mean embedding. Unlike conventional descriptors, kernel mean embedding can retain all information regarding the distribution of component features in the vectorization process. Furthermore, the kernel mean descriptor uniquely determines the inverse map to the original material space. We demonstrate the expressive power and versatility of the kernel mean descriptor in various applications, including the prediction of the formation energy of inorganic compounds, prediction of the chemical composition to form quasicrystalline materials, and the use of force-field parameters to characterize polymeric materials.

Analytical gradients of the total energy are worked out for local density, generalized-gradient, and hybrid approximations to generalized Kohn-Sham spin-current density functional theory (SCDFT) of materials. It is shown that gradients can be determined analytically, in a two-component framework, including spin-orbit coupling (SOC), with high accuracy. We introduce an implementation in the Crystal program of analytical SOC gradients (with respect to both atomic coordinates and lattice cell parameters, i.e. the stress tensor) within a DFT and SCDFT framework. We show through selected applications that renormalization of the electron-electron potential by SOC-induced spin-currents can account for considerable modification of crystal structures. In the case of Iodine-based molecular crystals, the effect amounts to more than half of the total modification of the structure by SOC. Such effects necessitate an SCDFT, rather than DFT, formulation, in which exchange-correlation functionals are endowed with an explicit dependence on spin-current densities.

The spin structure of a Mn triple layer grown pseudomorphically on a W(001) surface is studied using spin-polarized scanning tunneling microscopy (SP-STM) and density functional theory (DFT). In SP-STM images a $\mathrm{c}(4\times 2)$ superstructure is found. The magnetic origin of this contrast is verified by contrast reversal and using the $\mathrm{c}(2\times 2)$ antiferromagnetic state of the Mn double layer as a reference. SP-STM simulations show that this contrast can be explained by a spin spiral propagating along the [110] direction with an angle close to ${90}^{\circ }$ between magnetic moments of adjacent Mn rows. To understand the origin of this spin structure, DFT calculations have been performed for a large number of competing collinear and noncollinear magnetic states including the effect of spin-orbit coupling (SOC). Surprisingly, a collinear state in which the magnetic moments of the top Mn layer and the central Mn layer are aligned antiparallel and those of the bottom Mn layer are aligned parallel to those of the central layer is the energetically lowest state. We show that in this so-called “up-down-down” ($\uparrow \downarrow \downarrow$) state the magnetic moments in the Mn bottom layer are only induced by those of the central Mn layer. Flat spin spirals propagating in either one, two, or all Mn layers are shown to be energetically unfavorable to the collinear $\uparrow \downarrow \downarrow$ state even upon including the Dzyaloshinskii-Moriya interaction (DMI). However, conical spin spirals with a small opening angle of about ${10}^{\circ }$ are only slightly energetically unfavorable within DFT and could explain the experimental observations. Surprisingly, the DFT energy dispersion of conical spin spirals including SOC cannot be explained if only the DMI is taken into account. Therefore higher-order interactions such as chiral biquadratic terms need to be considered, which could explain the stabilization of a conical spin spiral state.

We have successfully synthesized single crystals, solved the crystal structure, and studied the magnetic properties of a new family of copper halides $\left({\mathrm{C}}_4{\mathrm{H}}_{14}{\mathrm{N}}_2\right){\mathrm{Cu}}_2{X}_6$ $(X=\mathrm{Cl},\mathrm{Br})$. These compounds crystallize in an orthorhombic crystal structure with space group $Pnma$. The crystal structure features ${\mathrm{Cu}}^{2+}$ dimers arranged parallel to each other that makes a zigzag two-leg ladderlike structure. Further, there exists a diagonal interaction between two adjacent dimers which generates interdimer frustration. Both the compounds manifest a singlet ground state with a large gap in the excitation spectrum. Magnetic susceptibility is analyzed in terms of both interacting spin-$\frac{1}{2}$ dimer and two-leg ladder models followed by exact diagonalization calculations. Our theoretical calculations in conjunction with the experimental magnetic susceptibility establish that the spin lattice can be described well by a frustrated two-leg ladder model with strong rung coupling [$J_0/k_{\mathrm{B}}=116.0\left(2\right)$ and 300.0(2) K], weak leg coupling [$J^{\prime \prime }/k_{\mathrm{B}}=18.6\left(2\right)$ and 105.0(2) K], and equally weak diagonal coupling [$J^{\prime }/k_{\mathrm{B}}=23.2\left(2\right)$ and 90.0(2) K] for Cl and Br compounds, respectively. These exchange couplings set the critical fields very high, making them experimentally inaccessible. The correlation function decays exponentially as expected for a gapped spin system. The structural aspects of both the compounds are correlated with their magnetic properties. The calculation of entanglement witness divulges strong entanglement in both the compounds which persists up to high temperatures, even beyond 370 K for the Br compound.

Understanding the complex multiphase properties of Ce and its alloys remains a long-standing scientific challenge. Here, we report experimental observations of the shock responses of Ce-5wt. %La alloy dynamically compressed up to 20 GPa using plate-impact shock experiments. The Ce-5wt. %La alloy is observed to undergo a volume collapse phase transition (PT) from the $\gamma$ to the $\alpha$ phase with a volume change of $\sim 20\%$ and the shock-compression PT region is found to be between 0.997 and 4.835 GPa. Thermodynamic analysis shows that the few La additives markedly elevate the PT threshold pressure of Ce. The obtained experimental $P-V$ and $u_s-u_p$ results confirm the validity of the law of additive volumes for ideal mixtures for Ce-5wt. %La in an experimental pressure range from the $\alpha$ phase to the liquid phase. These data and observations can provide a better understanding of the multiphase properties of dynamically compressed Ce and its alloys, and have potential industrial applications in the synthesis of new materials at high pressures.

This study investigates the predictive capabilities of common density functional theory (DFT) methods (GGA, $\mathrm{GGA}+U$, and $\mathrm{GGA}+U+V$) for determining the transition temperature of antiferromagnetic insulators. We utilize a data set of 29 compounds and derive Heisenberg exchanges based on DFT total energies of different magnetic configurations. To obtain exchange parameters within a supercell, we have devised an innovative method that utilizes null-space analysis to identify and address the limitations imposed by the supercell on these exchange parameters. With obtained exchanges, we construct Heisenberg Hamiltonian to compute transition temperatures using classical Monte Carlo simulations. To refine the calculations, we apply linear response theory to compute onsite ($U$) and intersite ($V$) corrections through a self-consistent process. Our findings reveal that GGA significantly overestimates the transition temperature (by 113%), while $\mathrm{GGA}+U$ underestimates it (by 53%). To improve $\mathrm{GGA}+U$ results, we propose adjusting the DFT results with the $(S+1)/S$ coefficient to compensate for quantum effects in Monte Carlo simulation, resulting in a reduced error of 44%. Additionally, we discover a high Pearson correlation coefficient of approximately 0.92 between the transition temperatures calculated using the $\mathrm{GGA}+U$ method and the experimentally determined transition temperatures. Furthermore, we explore the impact of geometry optimization on a subset of samples. Using consistent structures with $\mathrm{GGA}+U$ and $\mathrm{GGA}+U+V$ theories reduced the error.

In this paper, we study the ground-state quantum Fisher information (QFI) in one-dimensional spin-1 models, as witness to multipartite entanglement. The models addressed are the bilinear-biquadratic model, the most general isotropic $\text{SU}\left(2\right)$-invariant spin-1 chain, and the $XXZ$ spin-1 chain, both with nearest-neighbor interactions and open boundary conditions. We show that the scaling of the QFI of strictly nonlocal observables can be used for characterizing the phase diagrams and, in particular, for studying topological phases, where it scales maximally. Analyzing its behavior at the critical phases, we are also able to recover the scaling dimensions of the order parameters, both for local and string observables. The numerical results have been obtained by exploiting the density-matrix renormalization-group algorithm and tensor network techniques.