Recently, topological semimetals featured with symmetry-protected nodal line degeneracies in three-dimensional momentum space have attracted great attention. In this work, we have constructed a three-dimensional acoustic metacrystal that hosts topological nodal lines pinning to the edges of a Brillouin zone. Markedly different from the nodal rings or chains observed previously, here the nodal lines are geometrically straight and fully stabilized by the crystal symmetry, which are highly unique and favored for detection. In addition to the symmetry-enforced nodal lines identified by transmission measurements, exotic waterslidelike surface states have been unveiled through scanning surface fields. Excellent agreements are found between our experiments and simulations. Our study may provoke new possibilities for controlling sound, such as realizing unusual sound radiation and scattering.

We calculate the bipartite von Neumann and second Rényi entanglement entropies of the ground states of spin-1/2 dimerized Heisenberg antiferromagnets on a square lattice. Two distinct dimerization patterns are considered: columnar and staggered. In both cases, we concentrate on the valence bond solid (VBS) phase and describe such a phase with the bond-operator representation. Within this formalism, the original spin Hamiltonian is mapped into an effective interacting boson model for the triplet excitations. We study the effective Hamiltonian at the harmonic approximation and determine the spectrum of the elementary triplet excitations. We then follow an analytical procedure, which is based on a modified spin-wave theory for finite systems and was originally employed to calculate the entanglement entropies of magnetic ordered phases, and calculate the entanglement entropies of the VBS ground states. In particular, we consider one-dimensional (line) subsystems within the square lattice, a choice that allows us to consider line subsystems with sizes up to $L^{\prime }=1000$. We combine such a procedure with the results of the bond-operator formalism at the harmonic level and show that, for both dimerized Heisenberg models, the entanglement entropies of the corresponding VBS ground states obey an area law as expected for gapped phases. For both columnar-dimer and staggered-dimer models, we also show that the entanglement entropies increase but they seem to not diverge as the dimerization decreases and the system approaches the Néel-VBS quantum phase transition. Finally, the entanglement spectra associated with the VBS ground states are presented.

The effects of structural distortion and $A$-site composition on the band gaps and band-edge orbitals of perovskite niobate and tantalate photocatalysts are explored using density-functional theory and crystal-field theory. When filled $d$ orbitals are introduced to the $A$ site by replacing alkali metal cations with silver, the crystal-field splitting energies of the $A$-site $d$ orbitals play a dominant role in determining the effect of silver in narrowing the band gap. It is demonstrated that an intuitive, geometry-based metric can usually predict within 0.1 eV the extent to which the presence of silver narrows the band gap. The ability to make such predictions is both valuable and potentially generalizable toward the design of photocatalysts.

Exciton transport is particularly intriguing in molecules with intramolecular charge-transfer states that show thermally-activated delayed fluorescence (such as 4CzBN), as in these cases it is defined by multiple cycles through singlet and triplet spin configurations that have significantly different transport properties. Thus, the overall exciton transport has contributions from both the singlet and triplet exciton diffusion. Herein, we investigate with Monte Carlo simulations how the singlet and triplet diffusion lengths can be experimentally established for this unique class of molecules. We then consider how the efficiency of diffusive transfer to a fluorescence acceptor (hyperfluorescence) is affected by modifications of the triplet and singlet transfer rates. Importantly, we theoretically predict that high-efficiency hyperfluorescence can be achieved in an active layer by ensuring that triplet transport between thermally-activated delayed fluorescence molecules is poor.

Here we present a combined experimental and theoretical study of graphene nanoribbons (GNRs), where detailed multiwavelength Raman measurements are integrated by accurate ab initio simulations. Our study covers several ultranarrow GNRs, obtained by means of solution-based bottom-up synthetic approach, allowing to rationalize the effect of edge morphology, position and type of functional groups, as well as the length on the GNR Raman spectrum. We show that the low-energy region, especially in the presence of bulky functional groups, is populated by several modes, and a single radial breathinglike mode cannot be identified. In the Raman optical region, we find that, except for the fully brominated case, all GNRs functionalized at the edges with different side groups show a characteristic dispersion of the D peak (8–22 ${\mathrm{cm}}^{-1}$/eV). This has been attributed to the internal degrees of freedom of these functional groups, which act as dispersion-activating defects. The G peak shows small to negligible dispersion in most of the cases, with larger values only in the presence of poor control of the edge functionalization, exceeding the values reported for highly defective graphene. In conclusion, we have shown that the characteristic dispersion of the G and D peaks offers further insight into the GNR structure and functionalization, by making Raman spectroscopy an important tool for the characterization of GNRs.

Braiding is a key aspect of topological quantum computations. Such a proposal has been made based on Majorana fermions. In this paper, it is shown that the braiding behavior is simulated in electric circuits by constructing Majorana-like corner states. First, we simulate the Kitaev model by an $LC$ electric circuit and the $p_x+i{p}_y$ model by an $LC$ circuit together with operational amplifiers. Zero-energy edge states emerge in the topological phase, which are detectable by measuring impedance. Next, we simulate the Bernevig-Hughes-Zhang model by including an effective magnetic field without breaking the particle-hole symmetry, where zero-energy corner states emerge in the topological phase. It is demonstrated that they are Ising anyons subject to braiding. Namely, we derive ${\sigma }^2=-1$ for them, where $\sigma$ denotes the single-exchange operation. We also study non-Hermitian generalizations of these models by requiring particle-hole symmetry. It is shown that braiding holds also in certain reciprocal non-Hermitian generalizations.

At high pressure, bismuth acquires a complex incommensurate host-guest structure, only recently discovered. Characterizing the structure and properties of this incommensurate phase from first principles is challenging owing to its nonperiodic nature. In this study we use large-scale density functional theory calculations to model commensurate approximants to the Bi-III phase, and in particular to describe the atomic modulations with respect to their ideal positions, shown here to strongly affect the electronic structure of the lattice and its stability. The equation of state and range of stability of Bi-III are reproduced in excellent agreement with experiment using a fully relativistic model. We demonstrate the importance of employing large unit cells for the accurate description of the geometric and electronic configuration of Bi-III. In contrast, accurate description of the equation of state of bismuth is found to be primarily sensitive to the choice of pseudopotential and exchange-correlation function, while almost completely insensitive to the commensurate approximation.

Here, we reinvestigate the underlying charge and magnetic correlations in ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CoO}}_4$ within a combined study of muon spin relaxation ($\mu \mathrm{SR}$), x-ray photon correlation spectroscopy, synchrotron radiation single-crystal x-ray diffraction, and neutron-scattering measurements. For ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CoO}}_4$ around 1/3 hole-doping, the significant charge correlations that are responsible for the emergence of hour-glass magnetic spectra are found to be slowly fluctuating checkerboard charge ordering (CBCO) correlations with an onset temperature being independent of hole-doping. This reveals a local origin of the CBCO correlations in the rather ionic cobaltates, thus being distinct from the more covalent nickelates. Moreover, we report the observation of a very similar temperature dependence of the intensities of in-plane high- and low-energy excitations within the ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CoO}}_4$ hour-glass spectrum. This observation shows that nano phase separation is distinct from conventional phase separation and that there is a coupling between nanometer-sized undoped and hole-doped islands. Based on this, it is possible to propose a microscopic scenario for the suppression of the outwards-dispersing branches in hour-glass magnetic spectra.

Using Langevin molecular dynamics simulations we study relaxation processes of interacting skyrmion systems with and without quenched disorder. Using the typical diffusion length as the time-dependent length characterizing the relaxation process, we find that clean systems always display dynamical scaling, and this even in cases where the typical length is not a simple power law of time. In the presence of the Magnus force, two different regimes are identified as a function of the noise strength. The Magnus force has also a major impact when attractive pinning sites are present, as this velocity-dependent force helps skyrmions to bend around defects and avoid caging effects. With the exception of the limit of large noise, for which dynamical scaling persists even in the presence of quenched disorder, attractive pinning sites capture a substantial fraction of skyrmions which results in a complex behavior of the two-time autocorrelation function that is not reproduced by a simple aging scaling ansatz.

We present a theory of standing spin waves (SSW) in a monoaxial chiral helimagnet. Motivated by experimental findings on the magnetic-field dependence of the resonance frequency in thin films of $\mathrm{Cr}{\mathrm{Nb}}_3{\mathrm{S}}_6$ [Goncalves et al., Phys. Rev. B 95, 104415 (2017)], we examine the SSW over a chiral soliton lattice (CSL) excited by an ac magnetic field applied parallel and perpendicular to the chiral axis. For this purpose, we generalize Kittel-Pincus theories of the SSW in ferromagnetic thin films to the case of a noncollinear helimagnet with the surface end spins, which are softly pinned by an anisotropy field. Consequently, we found that there appear two types of modes. One is a Pincus mode that is composed of a long-period Bloch wave and a short-period ripple originated from the periodic structure of the CSL. Another is a short-period Kittel ripple excited by space-periodic perturbation, which exists only in the case in which the ac field is applied perpendicular the chiral axis. We demonstrate that the existence of the Pincus mode and the Kittel ripple is consistent with the experimentally found double resonance profile.

Recently, a honeycomb borophene was reported to grow successfully on Al(111) surface. Since the metallic $\sigma$-bonding bands of honeycomb boron sheet play a crucial role in the 39 K superconductivity of ${\mathrm{MgB}}_2$, it is physically interesting to examine whether a similar property exists in this material. We have calculated the electronic structures and the electron-phonon coupling for honeycomb borophene by explicitly considering the substrate effect using first-principles density functional theory in conjunction with the Wannier interpolation technique. We find that the $s{p}^2$-hybridized $\sigma$-bonding bands of honeycomb borophene are metallized due to moderate charge transfer from the Al substrate, similar to ${\mathrm{MgB}}_2$. However, the electron-phonon coupling in honeycomb borophene is much weaker than in ${\mathrm{MgB}}_2$ due to the hardening of the bond-stretching boron phonon modes and the reduction of phonon density of states. Nevertheless, the interlayer coupling between Al-associated phonons and electrons in borophene is strong. Based on this observation, we predict that a superconducting transition temperature of 6.5 K can be observed in a free-standing borophene decorated by a single Al layer, namely monolayer ${\mathrm{AlB}}_2$. Accordingly, a similar superconducting transition temperature could be expected in honeycomb borophene on Al(111).

Two conflicting ground states, i.e., an antiferromagnetic ground state at 3.8 K and a ferromagnetic ground state at 5.1 K, have been reported for $\mathrm{CePtG}{\mathrm{e}}_2$, which has two nonequivalent crystallographic Ce sites. In this study, we have prepared polycrystalline and single-crystalline $\mathrm{CePtG}{\mathrm{e}}_2$ samples and measured their magnetization, specific heat, resistivity, and Seebeck coefficient. The observed bend in resistivity, peak in magnetic susceptibility, and λ-type anomaly in specific heat at $T_N=3.8\mathrm{K}$ and the metamagnetic transition in this $\mathrm{CePtG}{\mathrm{e}}_2$ suggest antiferromagnetism at low temperatures. No evidence of ferromagnetic order was found in the studied temperature range, indicating possible crystal polymorphism with ferromagnetic character. The large susceptibility around $T_N$ and the Schottky-type specific heat indicate a well-localized $f$-electron state. However, the large residual γ and χ(0) indicate an itinerant heavy $f$-electron state. These findings suggest the coexistence of localized and heavy itinerant states corresponding to $f$ electrons at the two Ce sites in this antiferromagnetic $\mathrm{CePtG}{\mathrm{e}}_2$. The field dependence of magnetization indicates a sudden increase in magnetization at weak magnetic fields $(\sim 0.2\mathrm{T})$, followed by a plateau at fields above 0.2 T, resulting in a possible different polarized metamagnetic state via interaction between localized and itinerant $f$ electrons.

Implementing high-fidelity two-qubit gates in single-electron spin qubits in silicon double quantum dots is still a major challenge. In this work we employ analytical methods to design control pulses that generate high-fidelity entangling gates for quantum computers based on this platform. Using realistic parameters and initially assuming a noise-free environment, we present simple control pulses that generate cnot, cphase, and cz gates with average fidelities greater than 99.99% and gate times as short as 45 ns. Moreover, using the local invariants of the system's evolution operator, we show that a simple square pulse generates a cnot gate in less than 27 ns and with a fidelity greater than 99.99%. Last, we use the same analytical methods to generate two-qubit gates locally equivalent to $\sqrt{\mathrm{CNOT}}$ and $\sqrt{\mathrm{CZ}}$ that are used to implement simple two-piece pulse sequences that produce high-fidelity cnot and cz gates in the presence of low-frequency noise.

Electrical quantum conductors coupled to microwave resonators have in the last decade emerged as a versatile test bed for controllable light-matter interaction on the nanometer scale. Recent experimental progress with high-impedance resonators has resulted in conductor-resonator systems with a large, dimensionless coupling parameter $\lambda \gtrsim 0.1$, well beyond the small-coupling regime, $\lambda \ll 1$. Motivated by this progress, we here analyze theoretically the joint statistics of transported electrons and emitted photons in a single-level quantum dot coupled to a microwave resonator, for arbitrarily large $\lambda$. Describing the electron-photon dynamics via a number-resolved master equation, we evaluate the joint long-time probability distribution as well as joint short-time, $g^{\left(2\right)}\left(t\right)$, correlation functions. Considering the high-bias regime, with sequential electron tunneling and working in the damping basis, allows us to obtain analytical results for both transport cumulants and $g^{\left(2\right)}\left(t\right)$ functions. It is found that the photons emitted out of the resonator are bunched and display a super-Poissonian statistics, for all system parameters. However, the electron transport properties are found to be unaffected by the coupling to the resonator, antibunched and with sub-Poissonian statistics. From the joint distribution we identify regimes of electron-tunneling-induced photon cascades and very large $g^{\left(2\right)}\left(t\right)$ functions. All $g^{\left(2\right)}\left(t\right)$ functions are found to be independent of $\lambda$. We also identify conditions for and transport signatures of a thermal resonator photon state.

Nanoporous materials are of broad interest for various applications, in particular, advanced thermoelectric materials. The introduction of nanoscale porosity, even at modest levels, has been known to drastically reduce a material's thermal conductivity, in some cases even below its amorphous limit, thereby significantly increasing its thermoelectric figure of merit ZT. The details of the important attributes that drive these large reductions, however, are not yet clear. In this work, we employ large-scale equilibrium molecular dynamics to perform an exhaustive atomistic-scale investigation of the effect of porosity on thermal transport in nanoporous bulk silicon. Thermal transport is computed for over 50 different geometries, spanning a large number of geometrical degrees of freedom, such as cylindrical pores and voids, different porosities, diameters, neck sizes, pore/void numbers, and surface-to-volume ratios, placed in ordered fashion, or fully disordered. We thus quantify and compare the most important parameters that determine the thermal conductivity reductions in nanoporous materials. Ultimately, we find that, even at the nanoscale, the effect of merely reducing the line-of-sight of phonons, i.e., the clear pathways that phonons can utilize during transport, plays the most crucial role in reducing the thermal conductivity in nanoporous materials, beyond other metrics such as porosity and surface/boundary scattering.

The recent development of all-electrical electron spin resonance (ESR) in a scanning tunneling microscope (STM) setup has opened the door to vast applications. Despite the fast-growing number of experimental works on STM-ESR, the fundamental principles remain unclear. By using a cotunneling picture, we show that the spin resonance signal can be explained as a time-dependent variation of the tunnel barrier induced by the alternating electric driving field. We demonstrate how this variation translates into the resonant frequency response of the direct current. Our cotunneling theory explains the main experimental findings. Namely, the linear dependence of the Rabi flop rate with the alternating bias amplitude, the absence of resonant response for spin-unpolarized currents, and the weak dependence on the actual atomic species.

The quasi-one-dimensional linear chain compound ${\mathrm{HfTe}}_3$ is experimentally and theoretically explored in the few- to single-chain limit. Confining the material within the hollow core of carbon nanotubes allows isolation of the chains and prevents the rapid oxidation which plagues even bulk ${\mathrm{HfTe}}_3$. High-resolution transmission electron microscopy combined with density functional theory calculations reveals that, once the triple-chain limit is reached, the normally parallel chains spiral about each other, and simultaneously a short-wavelength trigonal antiprismatic rocking distortion occurs that opens a significant energy gap. This results in a size-driven metal-insulator transition.

We report the anomalous Hall effect (AHE) in antiperovskite ${\mathrm{Mn}}_3\mathrm{NiN}$ with substantial doping of Cu on the Ni site (i.e., ${\mathrm{Mn}}_3{\mathrm{Ni}}_{1-x}{\mathrm{Cu}}_x\mathrm{N}$), which stabilizes a noncollinear antiferromagnetic (AFM) order compatible with the AHE. Observed on both sintered polycrystalline pieces and single crystalline films, the AHE does not scale with the net magnetization, contrary to the conventional ferromagnetic case. The existence of the AHE is explained through symmetry analysis based on the ${\mathrm{\Gamma }}_{4\mathrm{g}}$ AFM order in Cu doped ${\mathrm{Mn}}_3\mathrm{NiN}$. Density functional theory calculations of the intrinsic contribution to the AHE reveal the nonvanishing Berry curvature in momentum space due to the noncollinear magnetic order. Combined with other attractive properties, the antiperovskite ${\mathrm{Mn}}_3\mathrm{AN}$ system offers great potential in AFM spintronics.