Since the discovery of topological phases in optical systems, most of the efforts have been focused on the periodic and quasiperiodic optical structures which have both long-range and short-range orders. In this paper, we construct a kind of two-dimensional aperiodic optical systems, named the amorphous magnetic photonic lattices (AMPLs), which only have short-range orders, and investigate the topological states in these AMPLs. By using the supercell method, we build supercell magnetic photonic crystals as approximate equivalents of the AMPLs. Based on the band structure calculation and full wave simulation, we demonstrate the existence of single-mode and multimode topological edge states in these AMPLs. Through calculating the gapless edge states in reciprocal space, our research gives an intuitive understanding of the topology underlying aperiodic optical systems. Our system provides a platform to explore the relationship between short-range orders and topology and also paves an easy way to fabricate topological optical devices in application.

We report on the modulation of the fine-structure splitting of quantum-confined excitons in localized quantum emitters hosted by a monolayer transition metal dichalcogenide (TMDC). The monolayer TMDC, tungsten diselenide (${\mathrm{WSe}}_2$), is encapsulated in a van der Waals heterostructure which enables the application of an external electric field on the quantum-dot-like emitters hosted by the monolayer flake. The emitters exhibit quantum-confined Stark effect and a modulation in the fine-structure splitting (FSS) as a function of electric field. A maximum modulation of $1500\mu \mathrm{eV}$ is observed in the FSS from the studied emitters. Finally, we measure the polarization response of the localized exciton emission as a function of electric field exhibiting strong circular polarization with decreasing fine-structure splitting, further confirming the suppression of the anisotropic electron-hole exchange interaction thats causes the FSS.

Asymmetry in a one-dimensional optical Fabry-Perot cavity is shown to produce a large net pressure, the total on the two mirrors. Consequently, asymmetric cavity structures that are formed in this manner can experience a net force that is greater than that resulting from the excitation light illuminating a perfect mirror. The conditions for this to occur are a modest quality factor regime, where some influence of the cavity is needed, but when the quality factor becomes very large the enhancement diminishes. This result is used to illustrate how structuring a metal surface, thereby forming a plasmonic cavity, can substantially increase the optical pressure over that possible with a planar interface. It is shown that the force on one mirror in an asymmetric arrangement can be increased relative to the other. Importantly, the sum of the pressures on both mirrors increases through asymmetry and with quality factor, while adhering to conservation of energy. Using cavity quality factor as a measure, the one-dimensional Fabry-Perot cavity pressure results are related to pressure enhancement with a structured metal surface where a different type of mode in an asymmetric cavity is excited, the lowest order metal-insulator-metal surface plasmon mode. In principle, an optical cavity or cavity array formed with any material should display this enhanced pressure phenomenon. The length scale of the resonant structures for visible light can be as small as a few tens of nanometers, in the case of metals. With this understanding guiding the design of structured metallic and dielectric materials, a many-fold increase in pressure over that on a perfect mirror is possible. Consequently, the relatively weak optical force can become more effective in a variety of scientific and technological applications.

Metal alloys have a wide range of technological applications, from structural materials to catalysts. In many situations, the transport of hydrogen, whether intentionally for hydrogen storage and fuel cell applications or unintentionally in the case of tritium uptake in nuclear materials, is an important concern. Fe-Cr binary alloys, in particular, may be viewed as a simple model system to represent ferritic steels used in nuclear energy systems and, more generally, as a model binary alloy for examining the role of alloying elements on transport. In this work, we used density functional theory, cluster expansion, and kinetic Monte Carlo to study hydrogen kinetics in Fe-Cr alloys. In random homogeneous alloys, we observed that hydrogen diffusivity first decreased with increasing Cr content and then increased to its levels in pure Cr. Furthermore, the effects of heterogeneity, as might be induced by irradiation, were explored and it was concluded that local structural heterogeneities for the same overall Cr concentration may significantly affect the hydrogen diffusivity. The effect was attributed to the relative binding energy of hydrogen in different metals and this understanding was then utilized to predict hydrogen transport behavior in different element-segregated grain/grain boundary combinations and thus identify solute/solvent alloys where hydrogen transport might be either hindered or enhanced. Finally, we comment on the potential impact of radiation-induced segregation on hydrogen behavior in nuclear energy systems.

We apply a general first-principles approach to derive the phase diagram of metallic lithium at ambient pressure between 0 and 350 K, including identification of candidate phases. We use ab initio random structure searching to identify competing phases and supplement the results with calculations of vibrational properties and relevant derived neutron diffraction patterns. Strong quantum nuclear effects are present, prompting a careful treatment of vibrations. We directly map the Born-Oppenheimer surface of Li, allowing the extension of the normal quasi-harmonic treatment of vibrations to a “quasi-anharmonic” approach, where the effects of anharmonicity are included. The Gibbs free energies of the fcc, bcc, hcp, and 9R phases are derived using a variety of equations of state. We find that the anharmonic contribution to the Gibbs free energies of Li phases is of the same order as the differences between phases. Anharmonicity also makes a noticeable difference to the 0-K phonon dispersion of the bcc phase, with the largest difference at the $N$-point phonon. The ordering of phase transitions that we find agrees with the calculations of Ackland et al. [Science 356, 1254 (2017)], even when anharmonic effects are included, suggesting that a quasi-harmonic treatment is sufficient for correct phase behavior. We show explicitly that the martensitic phase transition from close-packed to bcc lithium upon heating is driven by entropic contributions to the phonon free energy.

Magnetic skyrmion motion under parametric pumping is numerically investigated. Parametric pumping with a perpendicularly oscillating electric field can excite the breathing modes of a skyrmion because an electric field can modify the magnetic anisotropy. Interestingly, using an in-plane static magnetic field to break the rotational symmetry, a perpendicularly oscillating electric field can drive a skyrmion undergoing a wiggling motion along a well-defined trajectory. The skyrmion velocity can reach several meters per second when the frequency of the electric field is close to the frequency of the skyrmion breathing motion. The physics is revealed in a generalized Thiele equation where a net spin current excited by the parametric pumping can drive the skyrmion propagation through angular momentum transfer. Our results open new possibilities for manipulating skyrmions in both metals and insulators with low-power consumption. Moreover, the oscillating skyrmion can act as a microwave generator for future spintronic applications such as a nanotool on a diamond nitrogen-vacancy center.

Thermoelectric materials enables the harvest of waste heat and direct conversion into electricity. In search of high efficient thermoelectric materials, low thermal conductivity of a material is essential and critical. Here, we have theoretically investigated the lattice thermal conductivity and thermoelectric properties of layered intermetallic ${\mathrm{Na}}_2\mathrm{MgSn}$ and ${\mathrm{Na}}_2\mathrm{MgPb}$ based on the density functional theory and linearized Boltzmann equation with the single-mode relaxation-time approximation. It is found that both materials exhibit very low and anisotropic intrinsic lattice thermal conductivity. Despite the very low mass density and simple crystal structure of ${\mathrm{Na}}_2\mathrm{MgSn}$, its lattice thermal conductivities along $a$ and $c$ axes are only 1.77 and $0.81\mathrm{W}/\mathrm{m}\mathrm{K}$ respectively at room temperatures. When Sn is replaced by the heavier element Pb, its lattice thermal conductivities decrease remarkably to 0.56 and $0.31\mathrm{W}/\mathrm{m}\mathrm{K}$ respectively along $a$ and $c$ axes at room temperature. We show that the low lattice thermal conductivities of both materials are mainly due to their short phonon lifetimes and phonon mean free path. Combined with previous experimental measurements, the metallic ${\mathrm{Na}}_2\mathrm{MgPb}$ cannot be a good thermoelectric material. However, we predict that the semiconducting ${\mathrm{Na}}_2\mathrm{MgSn}$ is a potential room-temperature thermoelectric material with a considerable $ZT$ of 0.36 at 300 K. Our calculations not only imply that the intermetallic ${\mathrm{Na}}_2\mathrm{MgSn}$ is a potential thermoelectric material, but also can motivate more theoretical and experimental works on the thermoelectric researches in simple layered intermetallic compounds.

We theoretically and numerically investigate magnetic domain wall dynamics in a nanowire of an easy-cone magnet. The easy-cone domain wall exhibits several distinguishing dynamic features in comparison to the easy-axis domain wall. They are related to the generation of additional chiral spin textures due to the domain wall precession, which occurs commonly for various driving sources such as magnetic fields and spin-transfer torque. We envision that the unique easy-cone domain wall dynamics can enrich the study of magnetic domain walls both for fundamental interest and for device applications.

Interface effect in complex oxide thin-film heterostructures lies at the vanguard of current research to design technologically relevant functionality and explore emergent physical phenomena. While most of the previous works focus on the perovskite/perovskite heterostructures, the study of perovskite/brownmillerite interfaces remains in its infancy. Here, we investigate spontaneously stabilized perovskite-ferromagnet ($\mathrm{SrCo}{\mathrm{O}}_{3-\delta }$)/brownmillerite-antiferromagnet $\left(\mathrm{SrCo}{\mathrm{O}}_{2.5}\right)$ bilayer with $T_{\mathrm{N}}>T_{\mathrm{C}}$ and discover an unconventional interfacial magnetic exchange bias effect. From magnetometry investigations, it is rationalized that the observed effect stems from the interfacial ferromagnet/antiferromagnet coupling. The possibility for coupled ferromagnet/spin-glass interface engendering such effect is ruled out. Strikingly, a finite coercive field persists in the paramagnetic state of $\mathrm{SrCo}{\mathrm{O}}_{3-\delta }$, whereas the exchange bias field vanishes at $T_{\mathrm{C}}$. We conjecture the observed effect to be due to the effective external quenched staggered field provided by the antiferromagnetic layer for the ferromagnetic spins at the interface. Our results not only unveil a paradigm to tailor the interfacial magnetic properties in oxide heterostructures without altering the cations at the interface, but also provide a purview to delve into the fundamental aspects of exchange bias in such unusual systems, paving a big step forward in thin-film magnetism.

By comparison of the electrical conductivity of ceria doped with penta- and hexavalent ions, we separate the total electron localization energy into the two contributions originating from the small polaron effects and the Coulomb interaction with the donor ions. The upper bound of the itinerant small polaron hopping energy is estimated at $66\pm 20$ meV. The binding energy of the ${\mathrm{Ce}}^{3+}–M^{5+/6+}$ defect complex increases from 121 meV for $M={\mathrm{Nb}}^{5+}/{\mathrm{Ta}}^{5+}$ to 243 meV for $M={\mathrm{W}}^{6+}/{\mathrm{U}}^{6+}$. The first-principles simulations are in qualitative agreement with the experimental findings. At low temperatures the $f$ electrons bound to the donor defects show dielectric relaxation with the lowest activation energy of 2.7 and 17 meV for Nb(Ta)- and W-doped ceria, respectively. Remarkably, these energies are significantly smaller than the hopping energy of the itinerant small polarons. While both the electron-lattice and the electron-defect interactions cause the $f$ electron localization in real-case ceria, the latter effects seem to be the dominant.

In search for a new generation of spintronics hardware, material candidates for room temperature quantum spin Hall effect (QSHE) have become a contemporary focus of investigation. Inspired by the original proposal for QSHE in graphene, several heterostructures have been synthesized, aiming at a hexagonal monolayer of heavier group IV elements promoting the QSHE bulk gap via increased spin-orbit coupling. So far, the monolayer/substrate coupling, which can manifest itself in strain, deformation, and hybridization, has proven to be detrimental to the aspired QSHE conditions for the monolayer. For stanene, the Sn analog of graphene, we investigate how an interposing buffer layer mediates between monolayer and substrate in order to optimize the QSHE setting. From a detailed density functional theory study, we highlight the principal mechanisms induced by such a buffer layer to accomplish quasifreestanding stanene in its QSHE phase. We complement our theoretical predictions by presenting attempts to grow a buffer layer on SiC(0001) on which stanene can be deposited.

We extend the standard Su-Schrieffer-Heeger (SSH) model to include long-range hopping amplitudes and disorder, and analyze how the electronic and topological properties are affected. We show that long-range hopping can change the symmetry class and the topological invariant, while diagonal and off-diagonal disorder lead to Anderson localization. Interestingly, we find that the Lyapunov exponent $\gamma \left(E\right)$ can be linked in two ways to the topological properties in the presence of disorder—either due to the different response of midgap states to increasing disorder or due to an extra contribution to $\gamma$ due to the presence of edge modes. Finally, we discuss its implications in realistic transport measurements.

The basic features of vanadium (V)-doped $\mathrm{LiNb}{\mathrm{O}}_3$, such as V doping sites, local lattice distortions, and electronic structures are investigated via hybrid density functional theory. The interaction between V and the intrinsic point defects is also studied in this work. V is found to prefer to substitute Li (${\mathrm{V}}_{\mathrm{Li}}$) at its highest charge state of +4 in most $\mathrm{LiNb}{\mathrm{O}}_3$ samples, and begins to substitute Nb to form a neutral ${\mathrm{V}}_{\mathrm{Nb}}$ defect as the Fermi level is increased. Furthermore, ${\mathrm{V}}_{\mathrm{Li}}$ exhibits different polaronic behaviors in stoichiometric and congruent $\mathrm{LiNb}{\mathrm{O}}_3$. The most stable ${\mathrm{V}}_{\mathrm{Li}}^{4+}$ tend to form a ${\mathrm{V}}_{\mathrm{Li}}^{2+}$ small bound polaron by simultaneously capturing two electrons in stoichiometric $\mathrm{LiNb}{\mathrm{O}}_3$, and form a bound bipolaron along the nonpolarization axis in the congruent samples. Moreover, both bound bipolarons along the polarization and nonpolarization axes are found in congruent $\mathrm{LiNb}{\mathrm{O}}_3$ by capturing two more electrons.

van der Waals heterostructures employing graphene and hexagonal boron nitride (hBN) crystals have emerged as a promising platform for plasmonics thanks to the tunability of their collective modes with carrier density and record values for plasmonics figures of merit. In this paper we investigate theoretically the role of moiré-pattern superlattices in nearly aligned graphene on hBN by using a continuum-model Hamiltonian derived from ab initio calculations. We calculate the system's energy-loss function for a range of chemical potential values that are accessible in gated devices. Our calculations reveal that the electron-hole asymmetry of the moiré bands leads to a remarkable asymmetry of the plasmon dispersion between positive and negative chemical potentials, showcasing the intricate band structure and rich absorption spectrum across the secondary Dirac point gap for the hole bands.

We report specific heat, magnetization, and resistivity measurements on single crystals of ${\mathrm{Ce}}_x{\mathrm{La}}_{1-x}{\mathrm{TiGe}}_3$ systems. When the Ce concentration $x$ is increased, the system changes from a single-ion Kondo system for $x=0.05$ into a ferromagnetic Kondo lattice for $x=1$, where the magnetic part of electrical resistivity reveals a single-ion scaling with $x$. The isoelectric substitution of Ce by La atoms causes a change of the relative strength of competing energy scales of Kondo and RKKY interaction and crystalline electric field (CEF). The substitutions induce the continuous evolution of the Kondo temperature $T_K$ and the linear variation of ferromagnetic ordering temperature $T_c$, which are accompanied by a change of the CEF level scheme of the Ce ions. The composition-temperature ($x\text{−}T$) phase diagram for ${\mathrm{Ce}}_x{\mathrm{La}}_{1-x}{\mathrm{TiGe}}_3$ is constructed by a combination of magnetization, specific heat, and resistivity measurements. The ferromagnetic ordering temperature is linearly suppressed as $x$ decreases and vanishes near the critical concentration $x_c=0.1$, but conventional quantum criticality is absent near $x_c$. The specific heat measurement for $x=0.05$ reveals the power law increase of the electronic specific heat coefficient $C_m/T\propto 1/T$ with a large value of $\sim 3.5$ J/mol ${\mathrm{K}}^2$ at $T=0.4\mathrm{K}$. The magnetic susceptibility for $x=0.05$ also shows a power law dependence $\chi \left(T\right)\propto 1/T$ below 10 K.

We perform an exact-diagonalization study of quasihole excitations for the two-component Halperin (221) state in the lowest Landau level and for several $\nu =1/3$ bosonic fractional Chern insulators in topological flat bands with Chern number $\left|C\right|$=2. Properties including the quasihole size, charge, and braiding statistics are evaluated. For the Halperin (221) model state, we observe isotropic quasiholes with a clear internal structure, and obtain the quasihole charge and statistics matching the theoretical values. Interestingly, we also extract the same quasihole size, charge, and braiding statistics for the continuum model states of $\left|C\right|$=2 fractional Chern insulators, although the latter possess a “color-entangled” nature that does not exist in ordinary two-component Halperin states. We also consider two real lattice models with a band having $\left|C\right|$=2. There we find that a quasihole can exhibit much stronger oscillations of the density profile, while having the same charge and statistics as those in the continuum models.

We present a hydrodynamic theory for electron-hole magnetotransport in graphene incorporating carrier-population imbalance, energy, and momentum relaxation processes. We focus on the electric response and find that the carrier and energy imbalance relaxation processes strongly modify the shear viscosity, so that an effective viscosity can be negative in the vicinity of charge neutrality. We predict an emergent eddy flow pattern of swirling currents and explore its manifestation in nonlocal resistivity oscillations in a strip of graphene driven by a source current.

Magnetization-induced circular dichroism (CD) in optical second harmonic generation (SHG) is observed in planar symmetrical arrays of U-shaped permalloy nanostructures. We demonstrate that extrinsic chirality of an array leads to the CD effect that strongly depends on the azimuthal orientation of the sample. In the absence of magnetization, the value of the SHG CD effect averaged over $2\pi$ azimuthal angle is zero, while up to 6% averaged SHG CD appears as the dc magnetic field is applied along the symmetry axis of the structure. Importantly the averaged SHG CD effect changes its sign under the reversal of the magnetic field, which corresponds to the appearance of magnetization driven true chirality in an array of symmetric magnetic nanostructures.

We investigate the spin conductivity of topological insulators taking into account both the surface and quasi-two-dimensional bulk states. We apply a low-energy expansion of the Hamiltonian up to the third order in momentum and take into account the vertex corrections arising due to the short-range disorder. Hexagonal warping gives rise to the additional anisotropic components in the spin conductivity tensor. Typically, the isotropic part of the spin conductivity is larger than the anisotropic one. The helical regime for the bulk states, in which the electrons in the Fermi level have the same projection of the spin on the direction of momentum, have been studied in more detail. In this regime, a substantial increase of the spin conductivity contribution from the bulk states at the Fermi level is observed. We find that the bulk spin conductivity is insensitive to disorder if Rashba spin-orbit coupling is larger than disorder strength, otherwise, it is strongly suppressed. The contribution to the spin conductivity from the surface states is almost independent of the chemical potential, robust to disorder and its value is comparable to the spin conductivity contribution from the bulk states per layer. The obtained results are in agreement with experimental data.

Interstitial defects are inevitably present in doped semiconductors that enable modern-day electronic, optoelectronic, or thermoelectric technologies. Understanding the stability of interstitials and their bonding mechanisms in the silicon lattice was accomplished only recently with the advent of first-principles modeling techniques, supported by powerful experimental methods. However, much less attention has been paid to the effect of different naturally occurring interstitials on the thermal and electrical properties of silicon. In this work, we present a systematic study of the variability of heat and charge transport properties of bulk silicon, in the presence of randomly distributed interstitial defects (Si, Ge, C, and Li). We find through atomistic lattice dynamics and molecular dynamics studies that interstitial defects scatter heat-carrying phonons to suppress thermal transport—1.56% of randomly distributed Ge and Li interstitials reduce the thermal conductivity of silicon by $\sim 30$ and 34 times, respectively. Using first-principles density functional theory and semiclassical Boltzmann transport theory, we compute electronic transport coefficients of bulk Si with 1.56% neutral Ge, C, Si, and Li interstitials, in energetically favorable hexagonal, tetrahedral, split-interstitial, and bond-centered sites. We demonstrate that hexagonal-Si and hexagonal-Ge interstitials minimally impact charge transport. As an illustration of the relevance of this work for practical applications, we predict the thermoelectric property of an experimentally realizable bulk Si sample that contains Ge interstitials in different symmetry sites. Our research establishes a direct relationship between the variability of structures dictated by fabrication processes and heat and charge transport properties of silicon. The relationship provides guidance to accurately estimate performance of Si-based materials for various technological applications.