Whereas magnetic frustration is typically associated with local-moment magnets in special geometric arrangements, here we show that ${\mathrm{SrCo}}_2{\mathrm{As}}_2$ is a candidate for frustrated itinerant magnetism. Using inelastic neutron scattering (INS), we find that antiferromagnetic (AF) spin fluctuations develop in the square Co layers of ${\mathrm{SrCo}}_2{\mathrm{As}}_2$ below $T\approx 100$ K centered at the stripe-type AF propagation vector of $(\frac{1}{2},\frac{1}{2})$, and that their development is concomitant with a suppression of the uniform magnetic susceptibility determined via magnetization measurements. We interpret this switch in spectral weight as signaling a temperature-induced crossover from an instability toward ferromagnetism ordering to an instability toward stripe-type AF ordering on cooling, and show results from Monte-Carlo simulations for a $J_1-J_2$ Heisenberg model that illustrates how the crossover develops as a function of the frustration ratio $-J_1/\left(2J_2\right)$. By putting our INS data on an absolute scale, we quantitatively compare them and our magnetization data to exact-diagonalization calculations for the $J_1-J_2$ model [N. Shannon et al., Eur. Phys. J. B 38, 599 (2004).], and show that the calculations predict a lower level of magnetic frustration than indicated by experiment. We trace this discrepancy to the large energy scale of the fluctuations ($J_{\text{avg}}\gtrsim 75$ meV), which, in addition to the steep dispersion, is more characteristic of itinerant magnetism.

By combining high spinning speed (60 kHz) and low-field (4.7 T) ${}^{31}\mathrm{P}$ solid-state NMR with magnetic susceptibility measurements, we experimentally characterized a series of solid solutions belonging to the $\mathrm{L}{\mathrm{a}}_x\mathrm{E}{\mathrm{u}}_{1-x}\mathrm{P}{\mathrm{O}}_4$ ($0\le x\le 1$) series. Analyses of the magnetic susceptibility data were carried out using the free ion model and crystal field theory calculations allowing to extract the electronic structure. The paramagnetic shifts of the P sites having one $\mathrm{E}{\mathrm{u}}^{3+}$ cation in their surrounding were predicted by combining the determined crystal field and energy level values with density functional theory (DFT) calculations. For the $\mathrm{L}{\mathrm{a}}_{0.9}\mathrm{E}{\mathrm{u}}_{0.1}\mathrm{P}{\mathrm{O}}_4$ sample, these theoretical shifts gave a very good overall trend allowing the unambiguous attribution of each P site. This study paves the way for the future analysis of both magnetic susceptibility and NMR data for a broad range of materials containing paramagnetic rare-earth cations.

The magnetization-density distribution in the metallic ferromagnet ${\mathrm{SrRuO}}_3$ was studied by means of polarized neutron diffraction. The analyzes by multipole refinements and by the maximum entropy method consistently reveal a strong polarization of all oxygen sites carrying 30% of the total magnetization. The spin-density distribution on the Ru site exhibits a nearly cubic shape in agreement with an almost equal occupation of $t_{2g}$ orbitals and $pd$ hybridization. The experimental analysis is well reproduced by density functional calculations. There is no qualitative change in the magnetization distribution between 2 and 200 K.

Materials either have a high hardness or excellent ductility, but rarely both at the same time. ${\mathrm{Mo}}_2\mathrm{BC}$ is one of the only crystalline materials that simultaneously has a high Vickers hardness and is also moderately ductile. The origin of this unique balance is revealed here using stress-strain calculations. The results show an anisotropic nonlinear elastic response including an intermediate tensile strain-stiffening behavior and a two-step sequential failure under shear strain that resembles the behavior of soft materials such as biological systems or polymer networks rather than hard, refractory metals. The mechanism of the unusual nonlinear elasticity is established by analyzing changes in the electronic structure and the chemical bonding environments under mechanical perturbation. The optimized structure under extreme strain shows the formation of a pseudogap in DOS and dimerization of the structure's boron-boron zigzag chain. These mechanisms delay the ultimate failure, establishing a pathway for developing the next generation of structural materials with high hardness and ductility.

The anomalous Hall effect reflects abundant spin physics encompassing the Berry phase, spin texture, impurity scatterings, etc. Here, we study the anomalous Hall effect in epitaxial thin films of ${\mathrm{SrCoO}}_3$, a perovskite oxide hosting the unique intermediate spin state. We observe a sign reversal in the anomalous Hall resistivity when either the carrier density or the film thickness decreases. It reflects the dominance of the Berry-phase-induced anomalous Hall effect, when the extrinsic anomalous Hall component caused by skew scattering gets suppressed. The experimental finding is further supported by density functional theory calculations.

Understanding the mechanisms of thermal conduction in graphene is a longstanding research topic due to its high thermal conductivity. Studies based on the Peierls-Boltzmann transport equation (PBTE) have revealed many unique phonon transport properties in graphene, but most previous works considered only three-phonon scatterings and relied on interatomic force constants (IFCs) extracted at 0 K. Recently, the importance of four-phonon scattering in graphene was revealed by Feng and Ruan [Phys. Rev. B 97, 045202 (2018)]. In this paper, we explore the temperature-dependent IFCs with regard to phonon transport in graphene through our PBTE calculations. We demonstrate that the strength of four-phonon scatterings was severely overestimated by previous work that used the IFCs extracted at 0 K. By using IFCs at finite temperatures, we find that four-phonon scattering is weakened but still significantly reduced the thermal conductivity of graphene by around 50%, even at room temperature. Furthermore, in order to reproduce the prediction from molecular dynamics simulations, phonon frequency broadening has to be taken into account when determining the phonon scattering rates. Our study elucidates the phonon transport properties of graphene at finite temperatures and could be extended to other crystalline materials.

By self-consistently resolving the disentanglement of the spin Hall angle (SHA) and the spin diffusion length (SDL) through the ratio of Hanle and spin Hall magnetoresistances, the SHA in heavy metals has been for the first time rigorously extracted. For Pt on ferromagnetic insulator ${\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$, the genuine SHA scales as an exponential function of the Pt layer thickness. The spin Hall effect is proved to be dominated by the Berry-phase-induced intrinsic mechanism. In particular, the giant interfacial SHA of 0.33 is observed and is attributed to the broken-symmetry-induced nonspecular electronic scattering at the ${\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$/Pt interface. At last, the spin relaxation process is found to be dominated by the Elliott-Yafet and D'yakonov-Perel' mechanisms for thin and thick Pt layers, respectively.

We study, theoretically, the single-photon response of a strongly disordered thin superconducting strip in the flux flow state. We find that this resistive state, at a current $I$ larger than the critical current $I_c$, jumps to the normal state by the absorption of a single optical photon. The absorbed photon creates a beltlike region with suppressed superconductivity and fast moving Josephson-like vortices across the strip. The formed Josephson-like link is not stable in such a superconductor and evolves into a normal domain which expands along the length of the superconducting strip, leading to the transition to the normal state.

A second-order topological insulator is designed on a platform of a two-dimensional (2D) square lattice with all coupling coefficients having the same sign. Simulated results show the existence of two types of nontrivial corner states in this system, with one type being identified as bound states in the continuum (BIC). The non-BIC corner states are also found by surrounding a nontrivial sample by a trivial one, and interestingly, these perfectly confined corner states can be gradually delocalized and merge into edge states by tuning the intersystem coupling coefficient. Both BIC and non-BIC corner states originate from bulk dipole moments rather than quantized quadrupole moments, with the corresponding topological invariant being the 2D Zak phase. Full wave simulations based on realistic acoustic waveguide structures are demonstrated. Our proposal provides an experimentally feasible platform for the study of the interplay between BIC and a high-order topological insulator, and the evolution from corner states to edge states.

A determinant formalism in combination with ab initio calculations was proposed recently and has paved a way for simulating and interpreting x-ray excitation spectra in condensed-phase systems. The method systematically takes into account many-electron effects in the Mahan-Noziéres-De Dominicis (MND) theory, including core-level excitonic effects, the Fermi-edge singularity, shakeup excitations, and wave function overlap effects such as the orthogonality catastrophe, all within a universal framework using many-electron configurations. A heuristic search algorithm was introduced to search for the configurations that are important for defining the x-ray spectral line shape, instead of enumerating them in a brute-force way. The algorithm has proven to be efficient for calculating $\mathrm{O}$ $K$ edges of transition metal oxides, which converge at the second excitation order (denoted as $f^{\left(n\right)}$ with $n=2$), i.e., the final-state configurations with two e-h pairs (with one hole being the core hole). However, it remains unknown how the determinant x-ray spectra converge for general cases and at which excitation order $n$ should one stop the determinant calculation. Even with the heuristic algorithm, the number of many-electron configurations still grows exponentially with the excitation order $n$. In this work we prove two theorems that can indicate the order of magnitude of the contribution of the $f^{\left(n\right)}$ configurations, so that one can estimate their contribution very quickly without actually calculating their amplitudes. The two theorems are based on singular-value decomposition (SVD) analysis, a method that is widely used to quantify entanglement between two quantum many-body systems. We examine the $K$ edges of several metallic systems with the determinant formalism up to $f^{\left(5\right)}$ to illustrate the usefulness of the theorems.

We study the effects of disorder and Coulomb interactions on the physics of three-dimensional type-I Weyl fermions with tilted and anisotropic dispersions in a renormalization-group approach. To lowest nontrivial loop order, we show that the tendency of the Coulomb interactions to restore the symmetry of the dispersion in the semimetallic region of the phase diagram dominates the stabilization of the tilt and anisotropy favored by weak disorder. We argue that the topology of the renormalization flow of the disorder and Coulomb couplings is essentially determined by gauge invariance so that these findings continue to hold qualitatively at any order in perturbation theory.

A long-lasting puzzle of the conducting $\mathrm{LaAl}{\mathrm{O}}_3/\mathrm{SrTi}{\mathrm{O}}_3$ (LAO/STO) interface is that the excess electron density counted in the transport measurements is much less than 0.5 electrons per unit cell as predicted by the polar catastrophe model. In this study, via first-principles calculations, we show that the excess electrons at the LAO/STO interface favor the formation of small polarons. These electrons interact strongly with the interfacial Ti ions, forming localized midgap states, which make an insignificant contribution to the conductivity. We also find that the interaction between the neighboring spin-polarized small polarons is weak and does not lead to long-range magnetic ordering. Compared with bulk STO, the formation of the polarons is more favorable at the LAO/STO interface, which is ascribed to the reduced symmetry of the crystal field and the increased lattice distortion. Our results suggest that a large number of the excess electrons at the LAO/STO interface are localized in the form of small polarons, which can partially explain the unexpected fewer electrons in the transport measurements, and also shed light on understanding various properties of other complex perovskite oxide interfaces.

We investigate subgap quasiparticles of a quantum dot coupled to the superconducting and normal leads, whose energy level is periodically driven by an external potential. Using the Floquet formalism we determine the quasienergies and analyze redistribution of their spectral weights between individual harmonics upon varying the frequency and amplitude of the driving potential. We propose feasible spectroscopic methods for probing the in-gap quasiparticles observable in the differential conductance of the charge current averaged over a period of oscillations.

We apply the recently formulated torque equilibrium spin wave theory (TESWT) to compute the $1/S$-order interacting $K$-edge bimagnon resonant inelastic x-ray scattering (RIXS) spectra of an anisotropic triangular lattice antiferromagnet with Dzyaloshinskii-Moriya (DM) interaction. We extend the interacting torque equilibrium formalism, incorporating the effects of DM interaction, to appropriately account for the zero-point quantum fluctuation that manifests as the emergence of spin Casimir effect in a noncollinear spin spiral state. Using inelastic neutron scattering data from ${\mathrm{Cs}}_2{\mathrm{CuCl}}_4$ we fit the $1/S$-corrected TESWT dispersion to extract exchange and DM interaction parameters. We use these new fit coefficients alongside other relevant model parameters to investigate, compare, and contrast the effects of spatial anisotropy and DM interaction on the RIXS spectra at various points across the Brillouin zone. We highlight the key features of the bi- and trimagnon RIXS spectrum at the two inequivalent rotonlike points, $M(0,2\pi /\sqrt{3})$ and $M^{\prime }(\pi ,\pi /\sqrt{3})$, whose behavior is quite different from an isotropic triangular lattice system. While the roton RIXS spectrum at the $M$ point undergoes a spectral downshift with increasing anisotropy, the peak at the $M^{\prime }$ location loses its spectral strength without any shift. With the inclusion of DM interaction the spiral phase is more stable and the peak at both $M$ and $M^{\prime }$ point exhibits a spectral upshift. Our calculation offers a practical example of how to calculate interacting RIXS spectra in a noncollinear quantum magnet using TESWT. Our findings provide an opportunity to experimentally test the predictions of interacting TESWT formalism using RIXS, a spectroscopic method currently in vogue.

Donor qubits in bulk doped silicon have many competitive advantages for quantum computation in the solid state: not only do they offer a fast way to scalability, but they also show some of the longest coherence times found in any quantum computation proposal. We determine the densities of entangling gates in randomly doped silicon comprising two different dopant species. First, we define conditions and plot maps of the relative locations of the dopants necessary for them to form exchange interaction-mediated entangling gates. Second, using nearest neighbor Poisson point process theory, we calculate the doping densities necessary for maximal densities of single and dual-species gates. Third, using the moving-average cluster expansion technique, we make predictions for a proof of principle experiment demonstrating the control of the far-from-equilibrium magnetization dynamics of one species by the orbital excitation of another. We find agreement of our results with a Monte Carlo simulation that handles multiple donor structures and scales optimally with the number of dopants. The simulator can also extract donor structures not captured by our Poisson point process theory. The combined approaches to density optimization in random distributions presented here may be useful for other condensed matter systems as well as applications outside physics.

We use an out-of-time-order commutator (OTOC) to diagnose the propagation of chaos in one-dimensional long-range power law interaction system. We map the evolution of OTOC to a classical stochastic dynamics problem and use a Brownian quantum circuit to exactly derive the master equation. We vary two parameters: The number of qubits $N$ on each site (the on-site Hilbert space dimension) and the power law exponent $\alpha$. Three light cone structures of OTOC appear at $N=1$: (1) logarithmic when $0.5<\alpha \lesssim 0.8$, (2) sublinear power law when $0.8\lesssim \alpha \lesssim 1.5$, and (3) linear when $\alpha \gtrsim 1.5$. The OTOC scales as $exp\left(\lambda t\right)/x^{2\alpha }$ and $t^{2\alpha /\zeta }/x^{2\alpha }$, respectively, beyond the light cones in the first two cases. When $\alpha \ge 2$, the OTOC has essentially the same diffusive broadening as systems with short-range interactions, suggesting a complete recovery of locality. In the large $N$ limit, it is always a logarithmic light cone asymptotically, although a linear light cone can appear before the transition time for $\alpha \gtrsim 1.5$. This implies the locality is never fully recovered for finite $\alpha$. Our result provides a unified physical picture for the chaos dynamics in a long-range power law interaction system.