We report results of a muon spin rotation ($\mu \mathrm{SR}$) study of the cuprate-analog nickelate ${\mathrm{La}}_4{\mathrm{Ni}}_3{\mathrm{O}}_8$, which undergoes a transition at 105 K to a low-temperature phase with charge-stripe and antiferromagnetic (AFM) order on square planar ${\mathrm{NiO}}_2$ layers. Zero-field $\mu \mathrm{SR}$ shows that the AFM transition is abrupt, commensurate, and has a quasi-two-dimensional character below $\sim 25$ K. Comparison of observed muon precession frequencies with Ni dipolar field calculations yields Ni moments $\lesssim 0.5{\mu }_B$. Dynamic muon spin relaxation above 105 K suggests critical slowing of Ni spin fluctuations, but is inconsistent with corresponding ${}^{139}\mathrm{La}$ NMR results. Critical slowing and an abrupt transition are also observed in the planar cuprate AFM ${\mathrm{La}}_2{\mathrm{CuO}}_{4+\delta }$, where they are taken as evidence for weakly interplanar-coupled two-dimensional AFM spin fluctuations, but our $\mu \mathrm{SR}$ data do not agree quantitatively with theoretical predictions for this scenario when applied to the nickelate.

We report the prediction of a two-dimensional (2D) allotrope common to group-IV and group-V elements and III-V compounds, which consist of two nonplanar atomic layers connected by vertical bonds and form deformed octagon, hexagon, and squares (dohs) with threefold and fourfold coordinated atoms. Specifically for silicon, it is a semiconductor with cohesion stronger than silicene and can be chemically doped to have localized donor and acceptor states in the band gap. This allotrope can be functionalized to construct quasi-2D clathrates with transition metal atoms and attain spin polarized metallic, half-metallic, or semiconducting states. It is demonstrated that these properties can be maintained, when it is grown on a specific substrate. Stringent tests show that the atomic structure is dynamically stable and can sustain thermal excitation at high temperatures. Additionally, stable bilayer, as well as 3D layeredlike structures, can be constructed by the vertical stacking of single-layer dohs. Surprisingly, C, Ge, AlP, and GaAs can form also similar 2D semiconducting structures. In contrast to semiconducting black and blue phosphorene, P-dohs is a semimetal with band inversion. While the premise of using well-developed silicon technology in 2D electronics has been hampered by the semimetallic silicene, the realization of this 2D, semiconducting allotrope of silicon and compounds can constitute a productive direction in 2D nanoelectronics/spintronics.

The ordered adsorbate layer Sn/Si(111) ($\sqrt{3}\times \sqrt{3}$) with coverage of one third of a monolayer is considered as a realization of strong electronic correlation in surface physics. Our theoretical analysis shows that electron-hole pair excitations in this system can be long lived, up to several hundred nanoseconds, since the decay into surface phonons is found to be a highly nonlinear process. We combine first-principles calculations with help of a hybrid functional (HSE06) with modeling by a Mott-Hubbard Hamiltonian coupled to phononic degrees of freedom. The calculations show that the Sn/Si(111) ($\sqrt{3}\times \sqrt{3}$) surface is insulating and the two Sn-derived bands inside the substrate band gap can be described as the lower and upper Hubbard band in a Mott-Hubbard model with $U=0.75$ eV. Furthermore, phonon spectra are calculated with particular emphasis on the Sn-related surface phonon modes. The calculations demonstrate that the adequate treatment of electronic correlations leads to a stiffening of the wagging mode of neighboring Sn atoms; thus, we predict that the onset of electronic correlations at low temperature should be observable in the phonon spectrum, too. The deformation potential for electron-phonon coupling is calculated for selected vibrational modes and the decay rate of an electron-hole excitation into multiple phonons is estimated, substantiating the very long lifetime of these excitations.

Graphene in the presence of a strong external magnetic field is a unique attraction for investigations of the fractional quantum Hall (fQH) states with odd and even denominators of the fraction. Most of the attempts to understand graphene in the strong-field regime were made through exploiting the universal low-energy effective description of Dirac fermions emerging from the nearest-neighbor hopping model of electrons on a honeycomb lattice. We highlight that accounting for the next-nearest-neighbor hopping terms in doped graphene can lead to a unique redistribution of magnetic fluxes within the unit cell of the lattice. While this affects all the fQH states, it has a striking effect at a half-filled Landau-level state: It leads to a gapless composite fermion state that is equivalent to the doped topological Chern insulator on a honeycomb lattice coupled to a Chern Simons gauge field. At energies comparable to the Fermi energy, this state possesses a Haldane gap in the bulk proportional to the next-nearest-neighbor hopping and density of dopants. We argue that this microscopically derived energy gap survives the projection to the lowest band. We also conjecture that the gap should be present in a microscopic theory giving the recently proposed particle-hole symmetric Dirac composite fermion scenario of the half-filled Landau level. The proposed gap is lower than the chemical potential, and is predicted to be parametrically separated from the Dirac point in the latter description. Finally we conclude by proposing experiments to detect this gap; the associated boundary mode; and encourage cold-atom setups to test other predictions of the theory.

A metal-dielectric composite metasurface of the broadband epsilon-near-zero property is designed with the nanoscale airtube superlattice microstructure embedded in the metallic host. The design strategy rigorously starts from the theoretical analysis on the spectral representation of effective permittivity in quasistatic conditions for the Hashin-Shtrikman coated-cylinder microstructure, then improves in full-wave conditions for the nanoscale airtube superlattice microstructure. Through the computational simulations, the correctness and robustness of the strategy are verified and the broadband epsilon-near-zero property of the designed metasurface is demonstrated. Furthermore, the physical mechanism behind the broadband epsilon-near-zero property, including the potential application in broadband deep subwavelength electromagnetic wave tunneling, is also straightforwardly revealed.

We propose and analyze a “flopping-mode” mechanism for electric dipole spin resonance based on the delocalization of a single electron across a double quantum dot confinement potential. Delocalization of the charge maximizes the electronic dipole moment compared to the conventional single-dot spin resonance configuration. We present a theoretical investigation of the flopping-mode spin qubit properties through the crossover from the double- to the single-dot configuration by calculating effective spin Rabi frequencies and single-qubit gate fidelities. The flopping-mode regime optimizes the artificial spin-orbit effect generated by an external micromagnet and draws on the existence of an externally controllable sweet spot, where the coupling of the qubit to charge noise is highly suppressed. We further analyze the sweet spot behavior in the presence of a longitudinal magnetic field gradient, which gives rise to a second-order sweet spot with reduced sensitivity to charge fluctuations.

We compute the Holstein polaron spectral function on a one-dimensional ring using the finite-temperature $T$ Lanczos method. With increasing $T$ additional features in the spectral function emerge even at temperatures below the phonon frequency. We observe a substantial spread of the spectral weight towards lower frequencies and the broadening of the quasiparticle (QP) peak. In the weak-coupling regime the QP peak merges with the continuum in the high-$T$ limit. In the strong-coupling regime the main features of the low-$T$ spectral function remain detectable up to the highest $T$ used in our calculations. The effective polaron mass shows a nonmonotonic behavior as a function of $T$ at small phonon frequency but increases with $T$ at larger frequencies. The self-energy remains $k$ independent even at elevated $T$ in the frequency range corresponding to the polaron band, while at higher frequencies it develops a distinguishable $k$ dependence. Analytical expressions for the first few frequency moments are derived, and they agree well with those extracted from numerical calculations in a wide-$T$ regime.

The nonzero spectral gap of the original two-dimensional Affleck-Kennedy-Lieb-Tasaki (AKLT) models has remained unproven for more than three decades. Recently, Abdul-Rahman et al. (arXiv:1901.09297) provided an elegant approach and proved analytically the existence of a nonzero spectral gap for the AKLT models on the decorated honeycomb lattice (for the number $n$ of spin-1 decorated sites on each original edge no less than 3). We perform calculations for the decorated square lattice and show that the corresponding AKLT models are gapped if $n\ge 4$. Combining both results, we also show that a family of decorated hybrid AKLT models, whose underlying lattice is of mixed vertex degrees 3 and 4, are also gapped for $n\ge 4$. We develop a numerical approach that extends beyond what was accessible previously. Our numerical results further improve the nonzero gap to $n\ge 2$, including the establishment of the gap for $n=2$ in the decorated triangular and cubic lattices. The latter case is interesting, as this shows that the AKLT states on the decorated cubic lattices are not Néel ordered, in contrast to the state on the undecorated cubic lattice.

We present a simple and efficient tensor network method to accurately locate phase boundaries of two-dimensional classical lattice models. The method utilizes only the information-theoretic (von Neumann) entropy of quantities that automatically arise along tensor renormalization group [Phys. Rev. Lett. 99, 120601 (2007)] flows of partition functions. We benchmark the method against theoretically known results for the square-lattice $q$-state Potts models, which includes first-order, weakly first-order, and continuous phase transitions, and find good agreement in all cases. We also compare against previous Monte Carlo results for the frustrated square lattice $J_1-J_2$ Ising model and find good agreement.

We report the local (NMR) and bulk (magnetization and heat capacity) properties of the vanadium-based $S=\frac{1}{2}$ uniform spin chain compound ${\mathrm{Bi}}_6{\mathrm{V}}_3{\mathrm{O}}_{16}\left({\mathrm{Bi}}_4{\mathrm{V}}_2{\mathrm{O}}_{10.66}\right)$. In the low-temperature $\alpha$ phase, the magnetic ions $\left({\mathrm{V}}^{4+}\right)$ are arranged in one-dimensional chains. The magnetic susceptibility shows a broad maximum around 50 K signifying a short-range magnetic order. Heat capacity measurements also reveal low-dimensional magnetism. The ${}^{51}\mathrm{V}$ magic angle spinning nuclear magnetic resonance measurements clearly show that the magnetic ${\mathrm{V}}^{4+}$ and nonmagnetic ${\mathrm{V}}^{5+}$ species are located on different crystallographic sites with no mixed occupation. The spin susceptibility calculated from the shift of the ${}^{51}\mathrm{V}$ NMR spectra reproduces the behavior observed in magnetic susceptibility and agrees well with the $S=\frac{1}{2}$ uniform spin chain model with $J=113\left(5\right)$ K.

We report the discovery of superconductivity in ${\mathrm{W}}_3{\mathrm{Al}}_2\mathrm{C}$ ($T_c=7.6\mathrm{K}$) synthesized by high-pressure method. ${\mathrm{W}}_3{\mathrm{Al}}_2\mathrm{C}$ is isostructural to ${\mathrm{Mo}}_3{\mathrm{Al}}_2\mathrm{C}$ (space group $P{4}_{1}32$) but with stronger spin-orbit coupling (SOC). Different from the ${\mathrm{Mo}}_3{\mathrm{Al}}_2\mathrm{C}$ with metallic nature, the resistivity of the normal state of ${\mathrm{W}}_3{\mathrm{Al}}_2\mathrm{C}$ shows a nonmetallic behavior. A specific-heat jump of $\mathrm{\Delta }{C}_{es}/\gamma T_c=2.7$ and gap energy of $2\mathrm{\Delta }\left(0\right)/\gamma T_c=5.43$ are observed, which are much larger than that of ${\mathrm{Mo}}_3{\mathrm{Al}}_2\mathrm{C}$ (2.1 and 4.03) and the expectation of Bardeen-Cooper-Schrieffer theory (1.43 and 3.52). However, the Sommerfeld coefficient of ${\mathrm{W}}_3{\mathrm{Al}}_2\mathrm{C}$ is less than half of that of its Mo counterpart, and the specific heat above 2 K shows a power-law divergence following $C_{es}/\gamma T_c\sim {(T/T_c)}^{3.3}$ rather than an exponential relation. Theoretical calculations show that the Fermi surface of ${\mathrm{W}}_3{\mathrm{Al}}_2\mathrm{C}$ is dominated by W-$5d$ electrons and the inclusion of SOC significantly changes its band structure, density of states, and Fermi surface topology. The realization of superconductivity by replacing $4d$ Mo toward $5d\mathrm{W}$ provides a candidate for the search of potential triplet superconductors with enhanced SOC.

Oxide interfaces are a source of spin-orbit coupling which can lead to novel spin-to-charge conversion effects. In this Rapid Communication, the contribution of the $\mathrm{B}{\mathrm{i}}_2{\mathrm{O}}_3$ interface to the anomalous Hall effect (AHE) of Co is experimentally studied in $\mathrm{Co}/\mathrm{B}{\mathrm{i}}_2{\mathrm{O}}_3$ bilayers. We evidence a variation of ∼40% in the AHE of Co when a $\mathrm{B}{\mathrm{i}}_2{\mathrm{O}}_3$ capping layer is added to the ferromagnet. This strong variation is attributed to an additional source of asymmetric transport in $\mathrm{Co}/\mathrm{B}{\mathrm{i}}_2{\mathrm{O}}_3$ bilayers that originates from the $\mathrm{Co}/\mathrm{B}{\mathrm{i}}_2{\mathrm{O}}_3$ interface and contributes to the skew scattering.

We investigate the stability and dynamics of the skyrmion lattice in antiferromagnetic thin films subjected to fieldlike torques such as, e.g., those induced by an electric current in CuMnAs and ${\mathrm{Mn}}_2\mathrm{Au}$ via the inverse spin-galvanic effect. The skyrmion crystal phase represents the ground state of the antiferromagnet in a substantial area of the phase diagram, the latter being parametrized by the effective staggered field and uniaxial anisotropy constant. Experimental signatures of the skyrmion crystal phase and readout schemes based on topological transport (e.g., the spin Hall effect) are discussed. We also estimate qualitatively the effect of thermal and current fluctuations, including shot noise, on the stability of the skyrmion lattice.

The frustrated rare-earth pyrochlore ${\mathrm{Yb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ is remarkable among magnetic materials: despite a ferromagnetically ordered ground state it exhibits a broad, nearly gapless, continuum of excitations. This broad continuum connects smoothly to the sharp one-magnon excitations expected, and indeed observed, at high magnetic fields, raising the question: how does this picture of sharp magnons break down as the field is lowered? In this paper, we consider the effects of magnon interactions in ${\mathrm{Yb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$, showing that their inclusion greatly extends the reach of spin-wave theory. First, we show that magnon interactions shift the phase boundary between the (splayed) ferromagnet (SFM) and the antiferromagnetic ${\mathrm{\Gamma }}_5$ phase so that ${\mathrm{Yb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ lies very close to it. Next, we show how the high-field limit connects to lower fields; this includes corrections to the critical fields for the [111] and $\left[1\overline{1}0\right]$ directions, bringing them closer to the observed experimental values, as well as accounting for the departures from linear spin-wave theory that appear in [001] applied fields below $3\mathrm{T}$ [Thompson et al., Phys. Rev. Lett. 119, 057203 (2017)]. Turning to low fields, though the extent of the experimentally observed broadening is not quite reproduced, we find a rough correspondence between nonlinear spin-wave theory and inelastic neutron scattering data on both a single-crystal sample as well as on a powder sample [Peçanha-Antonio et al., Phys. Rev. B 96, 214415 (2017)]. We conclude with an outlook on implications for future experimental and theoretical work on ${\mathrm{Yb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ and related materials, highlighting the importance of proximity to the splayed ferromagnet-${\mathrm{\Gamma }}_5$ phase boundary and its potential role in intrinsic or extrinsic explanations of the low-field physics of ${\mathrm{Yb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$.

Three-magnon scattering can be used to evaluate the lowest frequency of the magnon band in a ferromagnet. Both the ferromagnetic-resonant (FMR) frequency $f_{\mathrm{FMR}}$ and the lowest frequency of the magnon band $f_{\min }$ in narrow-shaped NiFe strips were electrically measured using the anisotropic-magnetoresistance effect. The comparison with a micromagnetic simulation shows that $f_{\min }$ of the magnon band can be controlled independent of $f_{\mathrm{FMR}}$ by varying the width $w$ and thickness $t$ of the NiFe strip while maintaining a constant $t/w$ ratio. In addition, we found that the frequency difference, $f_{\mathrm{FMR}}-f_{\min }$, can be greatly increased in thicker NiFe strips. Our results show that narrow-shaped ferromagnets allow us to tune the magnon-band structures by varying their $w$ and $t$. This ability is important for designing magnon circuits in integrated magnonic devices and for improving the quantitative study on the Bose-Einstein condensation of magnons.

We study the electromagnetic wave propagation in a graphenelike triangular metallic photonic structure and show that, associated to trigonally warped Dirac cones, the transport properties resulting from an incident beam at the armchair edge display drastically different behaviors as compared to the results obtained for other analogous systems. Namely, at $K$ point, only the center beam associated to the lower Dirac cone and the side beams to the upper cone and, at $K^{\prime }$ point, only the center beam associated to the lower cone display significant intensities. The other expected beams are all strongly inhibited. This can be related to the metallic nature of the structure, where the low frequency electromagnetic bands are formed by local resonance modes confined inside the structural units. A propagating wave should follow the structure local patterns that govern its field distribution, whose symmetry properties determine its excitation by an incident wave.

High-$T_{\mathrm{C}}$ superconductors show anomalous transport properties in their normal states, such as the bad-metal and pseudogap behaviors. To discuss their origins, it is important to speculate whether these behaviors are material-dependent or universal phenomena in the proximity of the Mott transition by investigating similar but different material systems. An organic Mott transistor is suitable for this purpose owing to the adjacency between the two-dimensional Mott insulating and superconducting states, simple electronic properties, and high doping/bandwidth tunability in the same sample. Here we report the temperature dependence of the transport properties under electron and hole doping in an organic Mott electric-double-layer transistor. At high temperatures, the bad-metal behavior widely appears except at half filling, regardless of the doping polarity. At lower temperatures, the pseudogap behavior is observed only under hole doping, while the Fermi-liquid-like behavior is observed under electron doping. The bad-metal behavior seems a universal high-energy-scale phenomenon, while the pseudogap behavior is based on lower-energy-scale physics that can be influenced by details of the band structure.

The collective and quantum behavior of many-body systems may be harnessed to achieve fast charging of energy storage devices, which have been recently dubbed quantum batteries. In this paper, we present an extensive numerical analysis of energy flow in a quantum battery described by a disordered quantum Ising chain Hamiltonian, whose equilibrium phase diagram presents many-body localized (MBL), Anderson localized (AL), and ergodic phases. We demonstrate that (i) the low amount of entanglement of the MBL phase guarantees much better work extraction capabilities, measured by the ergotropy, than the ergodic phase and (ii) interactions suppress temporal energy fluctuations in comparison with those of the noninteracting AL phase. Finally, we show that the statistical distribution of values of the optimal charging time is a clear-cut diagnostic tool of the MBL phase.

Motivated by the puzzling report of the observation of a ferromagnetic insulating state in ${\mathrm{LaMnO}}_3/{\mathrm{SrTiO}}_3$ heterostructures, we calculate the electronic and magnetic state of ${\mathrm{LaMnO}}_3$, coherently matched to a ${\mathrm{SrTiO}}_3$ square substrate within a “strained-bulk” geometry. We employ three different density functional theory based computational approaches: (a) density functional theory (DFT) supplemented with Hubbard $U\left(\mathrm{DFT}+U\right)$, (b) DFT $+$ dynamical mean-field theory (DMFT), and (c) a hybrid functional treatment of the exchange-correlation functional. While the first two approaches include local correlations and exchange at Mn sites, treated in a static and dynamic manner, respectively, the last one takes into account the effect of nonlocal exchange at all sites. We find in all three approaches that the compressive strain induced by the square substrate of ${\mathrm{SrTiO}}_3$ turns ${\mathrm{LaMnO}}_3$, from an antiferromagnet with sizable orbital polarization, to a ferromagnet with suppressed Jahn-Teller distortion, in agreement with experiment. However, while both $\mathrm{DFT}+U$ and $\mathrm{DFT}+\mathrm{DMFT}$ provide a metallic solution, only the hybrid calculations result in an insulating solution, as observed in experiment. This insulating behavior is found to originate from an electronic charge disproportionation. Our conclusions remain valid when we investigate ${\mathrm{LaMnO}}_3/{\mathrm{SrTiO}}_3$ within the experimental setup of a superlattice geometry using $\mathrm{DFT}+U$ and hybrid calculations.

Phosphorene, a single layer of black phosphorus, is a direct band gap two-dimensional semiconductor with promising charge and spin transport properties. The electronic band structure of phosphorene is strongly affected by the structural anisotropy of the underlying crystal lattice. We describe the relevant conduction and valence bands close to the $\mathrm{\Gamma }$-point by four- and six-band (with spin) $k·p$ models, including the previously overlooked interband spin-orbit coupling which is essential for studying anisotropic crystals. All the $k·p$ parameters are obtained by a robust fit to ab initio data, by taking into account the nominal band structure and the $k$-dependence of the effective mass close to the $\mathrm{\Gamma }$-point. The inclusion of interband spin-orbit coupling allows us to determine dipole transitions along both armchair and zigzag directions. The interband coupling is also key to determine the effective $g$-factors and Zeeman splittings of the Landau levels. We predict the electron and hole $g$-factor correction of $\approx 0.03$ due to the intrinsic contributions in phosphorene, which lies within the existing range of experimental data. Furthermore, we investigate excitonic effects using the $k·p$ models and find exciton binding energy (0.81 eV) and exciton diameters consistent with experiments and ab initio based calculations. The proposed $k·p$ Hamiltonians should be useful for investigating magnetic, spin, transport, and optical properties and many-body effects in phosphorene.