Quasiparticle interference-induced spatial standing waves in local density of states, i.e., Friedel oscillations, are first visualized in rhombohedral trilayer graphene (rTG) by using scanning tunneling microscopy and spectroscopy. We show that the long-range standing-wave patterns of rTG can be created not only by the scattering off usual potential barriers including defects and step edges, but also by ABC-ABA stacking domain walls. For step edges as scatterers, both the surface step edge and the underlying step edge can effectively generate quasiparticle standing waves on the surface layer. For all observed types of scatterers in rTG, the Friedel oscillations always exhibit a $1/r$ spatial decay. This decay rate of Friedel oscillations is consistent with that in bilayer graphene, while slower than that in monolayer graphene, directly confirming previous theoretical predictions. Our results provide fundamental knowledge of the nature of scatterers in rTG, which would help us to better understand their microscopic scattering mechanisms.

We present a theoretical study of circular dichroism in the second-harmonic signal of single dielectric nanostructures with different symmetries and noncentrosymmetric materials. We show that this effect is defined not only by macroscopic (particle) and lattice symmetries but also by their relative orientation. Using symmetry and modal analysis, we provide a general and very simple formula to determine whether the dichroism exists and a chart with the most significant cases.

Recording field ion microscope images under field-evaporating conditions and subsequently reconstructing the underlying atomic configuration, called three-dimensional field ion microscopy (3D-FIM), is one of the few techniques capable of resolving crystalline defects at an atomic scale. However, the quantification of the observed vacancies and their origins are still a matter of debate. It was suggested that high electrostatic fields (1–5 V/Å) used in 3D-FIM could introduce artifact vacancies. To investigate such effects, we used density functional theory simulations. Stepped nickel and platinum surfaces with kinks were modeled in the repeated-slab approach with a (971) surface orientation. An electrostatic field of up to 4 V/Å was introduced on one side of the slab using the generalized dipole correction. Contrary to what was proposed, we show that the formation of vacancies on the electrified metal surface is more difficult compared to a field-free case. We also find that the electrostatic field can introduce kinetic barriers to a potential “vacancy annihilation” mechanism. We rationalize these findings by comparing to insights from field evaporation models.

The thermal equation of states for fcc iridium (Ir) is obtained from first-principles molecular dynamics up to 3000 K and 540 GPa. The equation of state (EoS) is globally fitted to a simplified free-energy model and various parameters are derived. The theoretical principal Hugoniot is compared with shock wave experiments, where discrepancy suggests formation of new Ir phases. A few representative EoS parameters, such as bulk modulus $K_T$, thermal expansivity $\alpha$, Grüneisen parameter $\gamma$, constant pressure capacity $C_P$, and Debye temperature ${\mathrm{\Theta }}_{\mathrm{D}}$, are computed to compare with experimental data.

We present a comprehensive experimental and ab initio study of the $S=1/2{\mathrm{Mo}}^{5+}$ system, ${\mathrm{KMoOP}}_2{\mathrm{O}}_7$, and show that it realizes the $S=\frac{1}{2}$ Heisenberg chain antiferromagnet model. Powder neutron diffraction reveals that ${\mathrm{KMoOP}}_2{\mathrm{O}}_7$ forms a magnetic network comprised of pairs of ${\mathrm{Mo}}^{5+}$ chains within its monoclinic $P{2}_1/n$ structure. Antiferromagnetic interactions within the ${\mathrm{Mo}}^{5+}$ chains are identified through magnetometry measurements and confirmed by analysis of the magnetic specific heat. The latter reveals a broad feature centered on $T_{\mathrm{N}}=0.54$ K, which we ascribe to the onset of long-range antiferromagnetic order. No magnetic Bragg scattering is observed in powder neutron-diffraction data collected at 0.05 K, however, which is consistent with a strongly suppressed ordered moment with an upper limit ${\mu }_{\mathrm{ord}}<0.15{\mu }_{\mathrm{B}}$. The one-dimensional character of the magnetic correlations in ${\mathrm{KMoOP}}_2{\mathrm{O}}_7$ is verified through analysis of inelastic neutron-scattering data, resulting in a model with $J_2\approx 34$ K and $J_1\approx -2$ K for the intrachain and interchain exchange interactions, respectively. The origin of these experimental findings are addressed through density-functional theory calculations.

We employ micromagnetic simulations to model the effect of pore-type microstructural defects on the magnetic small-angle neutron scattering cross section and the related pair-distance distribution function of spherical magnetic nanoparticles. Our expression for the magnetic energy takes into account the isotropic exchange interaction, the magnetocrystalline anisotropy, the dipolar interaction, and an externally applied magnetic field. The signatures of the defects and the role of the dipolar energy are highlighted and the effect of a particle-size distribution is studied. The results serve as a guideline to the experimentalist.

In model Hamiltonians, like Fröhlich's, the electron-phonon interaction is assumed to be screened from the beginning. The same occurs when this interaction is obtained by using the state-of-the-art density functional perturbation theory as starting point. In this work I formally demonstrate that these approaches are affected by a severe overscreening error. By using an out-of-equilibrium many-body technique I discuss how to merge the many-body approach with density functional perturbation theory in order to correct the overscreening error. A symmetric statically screened phonon self-energy is obtained by downfolding the exact Baym-Kadanoff equations. The statically screened approximation proposed here is shown to have the same long-range spatial limit of the exact self-energy and to respect the fluctuation-dissipation theorem. The doubly screened approximation, commonly used in the literature, is shown, instead, to be overscreened, to violate several many-body properties and to have a wrong spatial long-range decay. The accuracy of the proposed approximation is tested against the exact solution of an extended model Fröhlich Hamiltonian and it is applied to a paradigmatic material: ${\mathrm{MgB}}_2$. I find that the present treatment enhances the linewidths by $57\%$ with respect to what has been previously reported for the anomalous $E_{2g}$ mode. I further discover that the $A_{2u}$ mode is also anomalous (its strong coupling being completely quenched by the overscreened expression). The present results deeply question methods based on state-of-the-art approaches and impact a wide range of fields such as thermal conductivity, phononic instabilities, and nonequilibrium lattice dynamics.

We show that an interlayer Dzyaloshinskii-Moriya interaction in combination with nonlocal damping gives rise to unidirectional magnetic coupling. That is, the coupling between two magnetic layers—say, the left and right layer—is such that the dynamics of the left layer leads to the dynamics of the right layer, but not vice versa. We discuss the implications of this result for the magnetic susceptibility of a magnetic bilayer, electrically actuated spin-current transmission, and unidirectional spin-wave packet generation and propagation. Our results may enable a route towards spin-current and spin-wave diodes and further pave the way to design spintronic devices via reservoir engineering.

A theoretical investigation is conducted of high-order harmonic generation (HHG) in silicon thin films to elucidate the effect of light propagation in reflected and transmitted waves. The first-principles simulations are performed of the process in which an intense pulsed light irradiates silicon thin films up to $3\mu \mathrm{m}$ thickness. Our simulations are carried within the time-dependent density functional theory (TDDFT) with the account of coupled dynamics of the electromagnetic fields and the electronic motion. It was found that the intensity of transmission HHG gradually decreases with the thickness, while the reflection HHG becomes constant from a certain thickness. Detailed analyses show that transmission HHG has two origins: the HHG generated near the front edge and propagates to the back surface and the HHG generated near the back edge and emitted directly. The dominating mechanism of the transmission HHG is found to depend on the thickness of the thin film and the frequency of the HHG. At the film thickness of $1\mu \mathrm{m}$, the transmission HHG with the frequency below 20 eV is generated near the back edge, while that with the frequency above 20 eV is generated near the front edge and propagates from there to the back surface.

We propose a practically feasible time-periodic sinusoidal drive protocol in an on-site mass term to generate the two-dimensional (2D) Floquet second-order topological superconductor, hosting both the regular 0- and anomalous $\pi$-Majorana corner modes (MCMs) while starting from a static 2D topological insulator/$d$-wave superconductor heterostructure setup. We theoretically study the local density spectra and the time dynamics of MCMs in the presence of such drive. The dynamical MCMs are topologically characterized by employing the average quadrupolar motion. Furthermore, we employ the Floquet perturbation theory (FPT) in the strong driving amplitude limit to provide analytical insight into the problem. We compare our exact (numerical) and the FPT results in terms of the eigenvalue spectra and the time dynamics of the MCMs. We emphasize that the agreement between the exact numerical and the FPT results is more prominent in the higher-frequency regime for close to the zero-quasienergy mode.

The breaking of the out of plane mirror symmetry in the Janus transition-metal dichalcogenides (TMDs) has made them appealing candidates for optoelectronic, spintronic, valleytronic, and photocatalytic applications. In this study, we compare the structural and electronic properties of the Janus structure with the more stable TMD alloys of the form $M\mathrm{SSe} (M=\mathrm{Mo},\mathrm{W})$. The charge distribution analysis and two-dimensional electron localization function analysis indicate that the geometry of the system plays a vital role in determining the magnitude of the metal to ligand charge transfer, which in turn dictates the stability of the alloys. The arrangement of the chalcogen atoms in the more stable alloy allows for a better charge distribution, resulting in superior stability. Band structure calculations reveal that both structures are semiconducting with identical direct band gaps and exhibit Rashba splitting near the Γ point in the presence of the spin-orbit coupling (SOC) effects. While in the Janus structures, the broken mirror symmetry gives rise to an out of plane uniform electric field, the reduced symmetry of the more stable alloy structure produces an anisotropic arrangement of the perpendicular dipole moments. The nonuniformity of the electric dipole moments is reflected in the Rashba effect, which has an anisotropic nature, unlike the isotropic Rashba splitting observed in the Janus structures.

We developed a complete semiclassical Wentzel-Kramers-Brillouin (WKB) theory for $\alpha \text{−}{\mathcal{T}}_3$ model which describes a wide class of existing pseudospin-1 Dirac cone materials. By expanding the sought wave functions in a series of powers of Planck's constant $h$, we obtained the leading-order expansion term which is the key quantity required for calculating the electronic and tunneling properties of a semiclassical electron in $\alpha \text{−}{\mathcal{T}}_3$ materials. Based on this, we further derived the WKB transport equations relating two consecutive orders of the wave-function expansion and solved them analytically to obtain the first-order WKB wave function. Meanwhile, we discussed the applicability of the employed approximation as well as how these results could be applied to study various tunneling and transport properties of $\alpha \text{−}{\mathcal{T}}_3$ materials with nontrivial potential profiles. Finally, our results could also be useful for designing next-generation electronic transistors and devices with help from innovative flat-band Dirac materials.

Enhanced circular dichroism (CD) signal is crucial for ultrasensitive detection of chiral species. This purpose strongly relies on the construction of superchiral field in achiral resonant nanostructures. Despite the progress, the research on superchiral fields and their related physics is fundamentally restricted. Here, we demonstrate an approach to construct superchiral field in achiral metasurfaces and explore its impact on chiral mode interaction. While the chiral field at individual bound states in the continuum (BIC) is small, all criteria for giant superchiral fields can be satisfied when two BICs with orthogonal polarizations are merged. Consequently, the typical photonic crystal slabs can produce superchiral fields with enhancement factors ($C\text{/}{C}_{\mathrm{CPL}}$) (where CPL is circular polarized light) orders of magnitude higher than state of the art in nanostructures. More interestingly, a type of chiral light-matter interaction has been revealed by introducing chiral active materials. The nearly orthogonal BICs couple one another at the merging point and result in chiral mode splitting.

In recent years, Chebyshev polynomial expansions of tight-binding Green's functions have been successfully applied to the study of a wide range of spectral and transport properties of materials. However, the application of the Chebyshev approach to the study of quantum transport properties of noninteracting mesoscopic systems with leads has been hampered by the lack of a suitable Chebyshev expansion of Landaeur's formula or one of its equivalent formulations in terms of Green's functions in Keldysh's perturbation theory. Here, we tackle this issue by means of a hybrid approach that combines the efficiency of Chebyshev expansions with the convenience of complex absorbing potentials to calculate the conductance of two-terminal devices in a computationally expedient and accurate fashion. The versatility of the approach is demonstrated for mesoscopic twisted bilayer graphene (TBG) devices with up to $2.3\times {10}^6$ atomic sites. Our results highlight the importance of moiré effects, interlayer scattering events, and twist-angle disorder in determining the conductance curves in devices with a small twist angle near the TBG magic angle ${\theta }_m\approx 1.1^{\circ }$.

Elastic modulus (EM) is usually considered to be a relatively stable physical quantity; even phase transformation only results in a little change in EM, while this work demonstrates that it is not so straightforward. Internal friction tests reveal that the EM of a 60% cold-worked CoCrMoNi (MP35N) alloy increases by over 17% after exposed at $100\text{–}700{}^{\circ }\mathrm{C}$, with no change in phase or grains. Analysis of nanoscale structure of the alloy by high-resolution transmission electron microscope shows that high-density nanosized unstable stacking faults and incoherent nanotwins are formed in the cold-worked MP35N alloy, which severely weaken the bonding between the atom planes and decrease the EM. During the exposure process, the unstable fault interfaces and incoherent nanotwins transform into coherent nanotwins or annihilate to decrease the system energy, which repairs the crystal structure and increases the interatomic bonding force and the EM. The segregation of solute atoms at the twin boundaries and stacking faults also contribute a little to the increase in EM. These mechanisms should also suit for the faced centered cubic (FCC)-structured alloys with low-level stacking-fault energy.

The exchange bias effect, namely the horizontal shift in the magnetic hysteretic loop, is known as a fundamentally and technologically important property of magnetic systems. Though the exchange bias effect has been widely observed in normal magnetic heterostructure, it is desirable to raise such pinning coupling in topology-based multilayer structure. Furthermore, the exchange bias effect was theoretically proposed to be able to further open the surface magnetization gap in the recently discovered intrinsic magnetic topological insulator $\mathrm{Mn}{\mathrm{Bi}}_2{\mathrm{Te}}_4$. Such an exchange interaction can be ensured and programmed in the heterojunction, or applied to spintronics. Here we report the electrically tunable exchange bias in the van der Waals $\mathrm{Mn}{\mathrm{Bi}}_2{\mathrm{Te}}_4/{\mathrm{Cr}}_2{\mathrm{Ge}}_2{\mathrm{Te}}_6$ heterostructure. The exchange bias emerges over a critical magnetic field and reaches the maximum value near the band gap. Moreover, the exchange bias is experienced by net ferromagnetic (FM) odd-layers $\mathrm{Mn}{\mathrm{Bi}}_2{\mathrm{Te}}_4$ rather than the pure FM insulator ${\mathrm{Cr}}_2{\mathrm{Ge}}_2{\mathrm{Te}}_6$. Accompanied by nonlocal signal, an unfamiliar antisymmetric peak endows a domain-related structure within interface of the heterostructure. Such van der Waals heterostructure provides a promising platform to study the novel exchange bias effect and explore the possible application of spintronics or $\mathrm{high}-{\mathrm{T}}_{\mathrm{c}}$ quantum anomalous Hall effect.

We study entanglement fluctuations and quantum error correction in the weakly monitored volume-law phase of quantum automaton circuits subject to repeated local measurements. We numerically observe that the entanglement entropy exhibits strong fluctuation with the exponent close to the “growth exponent” of the Kardar-Parisi-Zhang (KPZ) universality class, the same as other local random circuits studied previously. We also investigate the dynamically generated quantum error correction code in the purification process and show that this model has different contiguous code distances for two types of errors that exhibit similar sublinear power-law scaling. We give an interpretation of these results by mapping them to various quantities in a classical particle model. We demonstrate that the subleading correction term of the entanglement entropy and the sublinear power-law scaling of the contiguous code distance in the volume-law phase are both the emergent phenomena of the hybrid random dynamics. Finally, we show that this classical particle dynamics itself has a type of error correction ability and can dynamically generate a classical linear code.

Here we propose a type of ferromagnetic semiconductors: ferromagnetic negative charge-transfer energy insulator (FNCTEI). In FNCTEI, the negative charge-transfer states strongly enhance the ferromagnetic (FM) exchange interactions and the orbital hybridization gap permits the magnetic molecular orbitals as the underlying magnetic units rather than local atomic orbitals. Thus, the FM exchange interactions are rather strong and decay slowly due to the large spreading of magnetic molecular orbitals. This is distinct from the superexchange mechanism where FM exchange interactions are quite weak as summarized in the well-known Goodenough-Kanamori-Anderson semiempirical rules. Through first-principle calculations with the hybrid functional, PbO-type CrAs monolayer is mapped out to be a FNCTEI, which possesses a band gap $\sim 0.35$ eV, FM nearest-/next-nearest-neighbor exchange coupling strength $\sim 57/40$ meV, and a high $T_c\sim 1500$ K respectively. It is believed that the theoretical prediction of FNCTEI supports the hypothesis by Khomskii and Sawatzky in 1997 [Solid State Commun. 102, 87 (1997)].