The identification of a microscopic configuration of point defects acting as quantum bits is a key step in the advance of quantum information processing and sensing. Among the numerous candidates, silicon-vacancy related centers in silicon carbide (SiC) have shown remarkable properties owing to their particular spin-3/2 ground and excited states. Although, these centers were observed decades ago, two competing models, the isolated negatively charged silicon vacancy and the complex of negatively charged silicon vacancy and neutral carbon vacancy [Phys. Rev. Lett. 115, 247602 (2015)], are still argued as an origin. By means of high-precision first-principles calculations and high-resolution electron spin resonance measurements, we here unambiguously identify the Si-vacancy related qubits in hexagonal SiC as isolated negatively charged silicon vacancies. Moreover, we identify the Si-vacancy qubit configurations that provide room-temperature optical readout.

For decades, the three-phonon scattering process has been considered to govern thermal transport in solids, while the role of higher-order four-phonon scattering has been persistently unclear and so ignored. However, recent quantitative calculations of three-phonon scattering have often shown a significant overestimation of thermal conductivity as compared to experimental values. In this Rapid Communication we show that four-phonon scattering is generally important in solids and can remedy such discrepancies. For silicon and diamond, the predicted thermal conductivity is reduced by 30% at 1000 K after including four-phonon scattering, bringing predictions in excellent agreement with measurements. For the projected ultrahigh-thermal conductivity material, zinc-blende BAs, a competitor of diamond as a heat sink material, four-phonon scattering is found to be strikingly strong as three-phonon processes have an extremely limited phase space for scattering. The four-phonon scattering reduces the predicted thermal conductivity from 2200 to 1400 W/m K at room temperature. The reduction at 1000 K is 60%. We also find that optical phonon scattering rates are largely affected, being important in applications such as phonon bottlenecks in equilibrating electronic excitations. Recognizing that four-phonon scattering is expensive to calculate, in the end we provide some guidelines on how to quickly assess the significance of four-phonon scattering, based on energy surface anharmonicity and the scattering phase space. Our work clears the decades-long fundamental question of the significance of higher-order scattering, and points out ways to improve thermoelectrics, thermal barrier coatings, nuclear materials, and radiative heat transfer.

The spinel/perovskite heterointerface $\gamma -{\mathrm{Al}}_2{\mathrm{O}}_3/{\mathrm{SrTiO}}_3$ hosts a two-dimensional electron system (2DES) with electron mobilities exceeding those in its all-perovskite counterpart ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ by more than an order of magnitude, despite the abundance of oxygen vacancies which act as electron donors as well as scattering sites. By means of resonant soft x-ray photoemission spectroscopy and ab initio calculations, we reveal the presence of a sharply localized type of oxygen vacancies at the very interface due to the local breaking of the perovskite symmetry. We explain the extraordinarily high mobilities by reduced scattering resulting from the preferential formation of interfacial oxygen vacancies and spatial separation of the resulting 2DES in deeper ${\mathrm{SrTiO}}_3$ layers. Our findings comply with transport studies and pave the way towards defect engineering at interfaces of oxides with different crystal structures.

Extremely large magnetoresistance (XMR), observed in transition-metal dichalcogenides, ${\mathrm{WTe}}_2$, has attracted recently a great deal of research interest as it shows no sign of saturation up to a magnetic field as high as 60 T, in addition to the presence of type-II Weyl fermions. Currently, there is a great deal of discussion on the role of band structure changes in the temperature-dependent XMR in this compound. In this contribution, we study the band structure of ${\mathrm{WTe}}_2$ using angle-resolved photoemission spectroscopy and first-principles calculations to demonstrate that the temperature-dependent band structure has no substantial effect on the temperature-dependent XMR, as our measurements do not show band structure changes upon increasing the sample temperature between 20 and 130 K. We further observe an electronlike surface state, dispersing in such a way that it connects the top of bulk holelike band to the bottom of bulk electronlike band. Interestingly, similarly to bulk states, the surface state is also mostly intact with the sample temperature. Our results provide valuable information in shaping the mechanism of temperature-dependent XMR in ${\mathrm{WTe}}_2$.

As lattice analogs of fractional quantum Hall systems, fractional Chern insulators (FCIs) exhibit enigmatic physical properties resulting from the intricate interplay between single-body and many-body physics. In particular, the design of ideal Chern band structures as hosts for FCIs necessitates the joint consideration of energy, topology, and quantum geometry of the Chern band. We devise an analytical optimization scheme that generates prototypical FCI models satisfying the criteria of band flatness, homogeneous Berry curvature, and isotropic quantum geometry. This is accomplished by adopting a holomorphic coordinate representation of the Bloch states spanning the basis of the Chern band. The resultant FCI models not only exhibit extensive tunability despite having only few adjustable parameters but are also amenable to analytically controlled truncation schemes to accommodate any desired constraint on the maximum hopping range or density-density interaction terms. Together, our approach provides a starting point for engineering ideal FCI models that are robust in the face of specifications imposed by analytical, numerical, or experimental implementation.

Nanoribbons of molybdenum disulfide (${\mathrm{MoS}}_2$) are interesting one-dimensional (1D) nanostructures with intriguing electronic properties, consisting of a semiconducting bulk bounded by edges with metallic character. Edges of similar character can also be expected in other transition-metal dichalcogenide (TMDC) nanostructures. We report first-principles electronic structure calculations for the total energy and the band structure of four representative TMDCs, ${\mathrm{MoS}}_2, {\mathrm{MoSe}}_2, {\mathrm{WS}}_2, {\mathrm{WSe}}_2$, in various 1D nanoribbon configurations. We compare the thermodynamic stability and the electronic structure of the 2D bulk and 35 different quasi-1D nanoribbons for each of the four materials. In each case, we consider the reconstructions of the zigzag metal-terminated edge by adding different amounts of chalcogen adatoms. The 1D structures we investigated have positive edge energies when the chalcogen chemical potential is close to the energy of the bulk chalcogen phase, and negative edge energies for higher chemical potential values. We find that the reconstruction with two chalcogen adatoms per edge metal atom is the most stable under usual experimental conditions and that all 1D nanoribbon structures exhibit metallic character.

We predict a different type of ultrafast third-order nonlinearity of surface plasmon polaritons (SPPs) in planar magnetoplasmonic structures caused by the inverse Faraday effect (IFE). Planar SPPs with a significant longitudinal component of the electric field act via the IFE as an effective transverse magnetic field. Its response to the plasmon propagation leads to strong ultrafast self-action which manifests itself through a third-order nonlinearity. We derive a general formula and analytical expressions for the IFE-related nonlinear susceptibility for two specific planar magnetoplasmonic structures from the Lorentz reciprocity theorem. Our estimations predict a very large nonlinear third-order nonlinear susceptibility exceeding those of typical metals such as gold.

The local application of mechanical stress in piezoelectric materials gives rise to boundaries across which the electric polarization changes. Polarization charges appear along such polar discontinuities and the ensuing electric fields drive a charge reconstruction with the accumulation of free carriers at the boundaries. This is particularly relevant for two-dimensional materials that can sustain very large strains and display record piezoelectric responses. Here we show by first-principles simulations the emergence of one-dimensional wires of free electrons and holes along strain interfaces, taking SnSe as a paradigmatic material. We complement this by developing a Schrödinger-Poisson approach specifically designed for two-dimensional materials that is able to reproduce the ab initio results and also to extend them to regimes of parameters and system sizes that would be unaffordable in first-principles calculations. This model allows us to assess the degree of tunability for the free charge in the wires coming from strain values and profiles, and to obtain the critical size at which the interfaces start to be metallic.

The reaction of tantalum with molecular hydrogen was studied by x-ray diffraction in a diamond-anvil cell at room temperature and pressures from 1 to 41 GPa. At pressures up to 5.5 GPa, a substoichiometric tantalum monohydride with a distorted bcc structure was shown to be stable. Its hydrogen content gradually increased with the pressure increase, reaching $\mathrm{H}/\mathrm{Ta}=0.92\left(5\right)$ at 5 GPa. At higher pressures, a new dihydride phase of tantalum was formed. This phase had an hcp metal lattice, and its hydrogen content was virtually independent of pressure. When the pressure was decreased, the tantalum dihydride thus obtained transformed back to the monohydride at $P=2.2\mathrm{GPa}$. Single-phase samples of tantalum dihydride also were synthesized at a hydrogen pressure of 9 GPa in a toroid-type high-pressure apparatus, quenched to the liquid-${\mathrm{N}}_2$ temperature, and studied at ambient pressure. X-ray diffraction showed them to have an hcp metal lattice with $a=3.224\left(3\right)$ and $c=5.140\left(5\right)\AA$ at $T=85\mathrm{K}$. The hydrogen content determined by thermal desorption was $\mathrm{H}/\mathrm{Ta}=2.2\left(1\right)$.

We establish, both theoretically and experimentally, that metamaterials for water waves reach a much higher degree of anisotropy than the one predicted using the analogy between water waves and their electromagnetic or acoustic counterparts. This is due to the fact that this analogy, based on the two-dimensional shallow water approximation, is unable to account for the three-dimensional near field effects in the water depth. To properly capture these effects, we homogenize the fully three-dimensional problem and show that a subwavelength layered structuration of the bathymetry produces significant anisotropic parameters in the shallow water regime. Furthermore, we extend the validity of the homogenized prediction by proposing an empirical anisotropic version of the dispersion relation.

Density functional theory (DFT) simulations, at the generalized gradient approximation (GGA) level, are being routinely used for material discovery based on high-throughput descriptor-based searches. The success of descriptor-based material design relies on eliminating bad candidates and keeping good candidates for further investigation. While DFT has been widely successfully for the former, oftentimes good candidates are lost due to the uncertainty associated with the DFT-predicted material properties. Uncertainty associated with DFT predictions has gained prominence and has led to the development of exchange correlation functionals that have built-in error estimation capability. In this work, we demonstrate the use of built-in error estimation capabilities within the BEEF-vdW exchange correlation functional for quantifying the uncertainty associated with the magnetic ground state of solids. We demonstrate this approach by calculating the uncertainty estimate for the energy difference between the different magnetic states of solids and compare them against a range of GGA exchange correlation functionals as is done in many first-principles calculations of materials. We show that this estimate reasonably bounds the range of values obtained with the different GGA functionals. The estimate is determined as a postprocessing step and thus provides a computationally robust and systematic approach to estimating uncertainty associated with predictions of magnetic ground states. We define a confidence value (c-value) that incorporates all calculated magnetic states in order to quantify the concurrence of the prediction at the GGA level and argue that predictions of magnetic ground states from GGA level DFT is incomplete without an accompanying c-value. We demonstrate the utility of this method using a case study of Li-ion and Na-ion cathode materials and the c-value metric correctly identifies that GGA-level DFT will have low predictability for ${\mathrm{NaFePO}}_4\mathrm{F}$. Further, there needs to be a systematic test of a collection of plausible magnetic states, especially in identifying antiferromagnetic (AFM) ground states. We believe that our approach of estimating uncertainty can be readily incorporated into all high-throughput computational material discovery efforts and this will lead to a dramatic increase in the likelihood of finding good candidate materials.

Using an infinite matrix product state (iMPS) technique based on the time-dependent variational principle (TDVP), we study two major types of dynamical phase transitions (DPT) in the one-dimensional transverse-field Ising model (TFIM) with long-range power-law ($\propto 1/r^{\alpha }$ with $r$ interspin distance) interactions out of equilibrium in the thermodynamic limit—DPT-I: based on an order parameter in a (quasi-)steady state, and DPT-II: based on nonanalyticities (cusps) in the Loschmidt-echo return rate. We construct the corresponding rich dynamical phase diagram, while considering different quench initial conditions. We find a nontrivial connection between both types of DPT based on their critical lines. Moreover, and very interestingly, we detect a new DPT-II dynamical phase in a certain range of interaction exponent $\alpha$, characterized by what we call anomalous cusps that are distinct from the regular cusps usually associated with DPT-II. Our results provide the characterization of experimentally accessible signatures of the dynamical phases studied in this work.

Magnetic frustrated materials are of great interest for their novel spin-dependent transport properties. We report an anisotropic anomalous Hall effect in the triangular itinerant ferromagnet ${\mathrm{Fe}}_3{\mathrm{GeTe}}_2$. When the current flows along the $ab$ plane, ${\mathrm{Fe}}_3{\mathrm{GeTe}}_2$ exhibits the conventional anomalous Hall effect below the Curie temperature $T_c$, which can be depicted by Karplus–Luttinger theory. On the other hand, the topological Hall effect shows up below $T_c$ with current along the $c$ axis. The enhancement of Hall resistivity can be attributed to the chiral effect during the spin-flop process.

Correlation functions of dimer operators, the product operators of spins on two adjacent sites, are studied in the spin-$\frac{1}{2}$ XXZ chain in the critical regime. The amplitudes of the leading oscillating terms in the dimer correlation functions are determined with high accuracy as functions of the exchange anisotropy parameter and the external magnetic field through the combined use of bosonization and density-matrix renormalization-group methods. In particular, for the antiferromagnetic Heisenberg model with SU(2) symmetry, logarithmic corrections to the dimer correlations due to the marginally irrelevant operator are studied, and the asymptotic form of the dimer correlation function is obtained. The asymptotic form of the spin-Peierls excitation gap including logarithmic corrections is also derived.

Using density-functional calculations we investigate the possibility and underlying mechanism of generating ferromagnetism (FM) in ferroelectric ${\mathrm{BaTiO}}_3$ by native vacancies. For the same vacancy species but different charge states (e.g., ${\mathrm{V}}_{\mathrm{O}}^0$ vs ${\mathrm{V}}_{\mathrm{O}}^{2+}$), our paper reveals a marked difference in magnetic behaviors. For instance, while ${\mathrm{V}}_{\mathrm{O}}^0$ is ferromagnetic, ${\mathrm{V}}_{\mathrm{O}}^{2+}$ is not. This sensitive dependence, which has often been overlooked, highlights the critical importance of taking into account different charge states. Furthermore, while oxygen vacancies have been often used in experiments to explain the vacancy-induced FM, our calculation demonstrates that Ti vacancies, in particular ${\mathrm{V}}_{\mathrm{Ti}}^{3-}$ and ${\mathrm{V}}_{\mathrm{Ti}}^{2-}$ with low formation energies, generate even stronger ferromagnetism in ${\mathrm{BaTiO}}_3$, with a magnetic moment which is 400% larger than that of ${\mathrm{V}}_{\mathrm{O}}^0$. Interestingly, this strong FM of ${\mathrm{V}}_{\mathrm{Ti}}^{}$ can be further enhanced by hole doping. Although both cation vacancies (${\mathrm{V}}_{\mathrm{Ti}}^{\mathrm{q}}$) and anion vacancies (${\mathrm{V}}_{\mathrm{O}}^0$) induce FM, their mechanisms differ drastically. FM of anion vacancies originates from the spin-polarized electrons at Ti sites, but FM of cation vacancies stems from the spin-polarized holes at O sites. This paper also sheds light on vacancy-induced FM by discovering that the spin densities of all three considered vacancy species are highly extended in real space, distributed far away from the vacancy. Moreover, we predict that the ferromagnetism caused by ${\mathrm{V}}_{\mathrm{Ti}}^{3-}$ is able to survive at high temperatures, which is promising for room-temperature spintronic or multiferroic applications.

This work aims at determining whether artificial intelligence can recognize a phase transition without prior human knowledge. If this were successful, it could be applied to, for instance, analyzing data from the quantum simulation of unsolved physical models. Toward this goal, we first need to apply the machine learning algorithm to well-understood models and see whether the outputs are consistent with our prior knowledge, which serves as the benchmark for this approach. In this work, we feed the computer data generated by the classical Monte Carlo simulation for the $XY$ model in frustrated triangular and union jack lattices, which has two order parameters and exhibits two phase transitions. We show that the outputs of the principal component analysis agree very well with our understanding of different orders in different phases, and the temperature dependences of the major components detect the nature and the locations of the phase transitions. Our work offers promise for using machine learning techniques to study sophisticated statistical models, and our results can be further improved by using principal component analysis with kernel tricks and the neural network method.

${\mathrm{Sr}}_2\mathrm{Ir}{\mathrm{O}}_4$ is an extensively studied spin-orbit coupling induced insulator with antiferromagnetic ground state. The delicate balance between competing energy scales plays a crucial role for its low-temperature phase and the route of chemical substitution has often been used to tune these different energy scales. Here, we report an evolution of structural, magnetic, and electronic properties in doped ${\mathrm{Sr}}_2{\mathrm{Ir}}_{1-x}{\mathrm{Cu}}_x{\mathrm{O}}_4$ ($x\le 0.2$). The substitution of ${\mathrm{Cu}}^{2+}$ ($3d^9$) for ${\mathrm{Ir}}^{4+}$ ($5d^5$) acts for electron doping, though it tunes the related parameters such as spin-orbit coupling, electron correlation, and Ir charge state. Moreover, both ${\mathrm{Ir}}^{4+}$ and ${\mathrm{Cu}}^{2+}$ has single unpaired spin, though it occupies different $d$ orbitals. With Cu substitution, the system retains its original structural symmetry but the structural parameters show systematic changes. X-ray photoemission spectroscopy measurements show that ${\mathrm{Ir}}^{4+}$ equivalently converts to ${\mathrm{Ir}}^{5+}$ and a significant enhancement in the density of states has been observed at the Fermi level due to the contribution from the Cu $3d$ orbitals, which supports the observed decrease in the resistivity with Cu substitution. While the long-range magnetic ordering is much weakened and the highest-doped sample shows almost paramagneticlike behavior, the overall system remains insulator. Analysis of the resistivity data shows a mode of charge conduction in the whole series follows a two-dimensional variable-range-hopping model, but the range of validity varies with temperature. The whole series of samples exhibits negative magnetoresistance at low temperature, which is considered to be a signature of a weak-localization effect in a spin-orbit coupled system, and its evolution with Cu appears to follow the variation of resistivity with $x$.

“Spin pumping” is the injection of spin angular momentum by a time-dependent magnetization into an adjacent normal metal proportional to the spin mixing conductance. We study the role of electrostatic interactions in the form of crystal fields on the pumped spin currents generated by insulators with exchange-coupled local moments at the interface to a metal. The crystal field is shown to render the spin currents anisotropic, which implies that the spin mixing conductance of insulator $|$ normal metal bilayers depends on crystal cut and orientation. We interpret the interface “effective field” (imaginary part of the spin mixing conductance) in terms of the coherent motion of the equilibrium spin density induced by proximity in the normal metal.

The electrodynamics of free carriers in the semimetallic Dirac material $\mathrm{SrMnB}{\mathrm{i}}_2$ was investigated using optical spectroscopy and first-principles calculations. Using a two-carrier-model analysis, the total free-carrier response was successfully decomposed into individual contributions from Dirac fermions and non-Dirac free carriers. Possible roles of chiral pseudospin, spin-orbit interaction (SOI), antiferromagnetism, and electron-phonon $\left(e\text{−}ph\right)$ coupling in the Dirac fermion transport were also addressed. The Dirac fermions possess a low scattering rate of $\sim 10\mathrm{meV}$ at low temperature and thereby experience coherent transport. However, at high temperatures, we observed that the Dirac fermion transport becomes significantly incoherent, possibly due to strong $e\text{−}ph$ interactions. The SOI-induced gap and antiferromagnetism play minor roles in the electrodynamics of the free carriers in $\mathrm{SrMnB}{\mathrm{i}}_2$. We also observed a seemingly optical-gap-like feature near 120 meV, which emerges at low temperatures but becomes filled in with increasing temperature. This gap-filling phenomenon is ascribed to phonon-assisted indirect transitions promoted at high temperatures.