Magnetic topological materials, in which strong correlations between magnetic and electronic properties of matter, give rise to various exotic phenomena such as the anomalous Hall effect (AHE), topological Hall effect (THE), and skyrmion lattice. Here, we report on the electronic, magnetic, and topological properties of ${\mathrm{Mn}}_{3-\mathit{x}}{\mathrm{Fe}}_{\mathit{x}}\mathrm{Sn}$ single crystals ($\mathit{x}=0$, 0.25, and 0.35). Low-temperature magnetic properties have been significantly changed with Fe doping. Most importantly, we observe that large uniaxial magnetocrystalline anisotropy that is induced by the Fe doping in combination with competing magnetic interactions at low temperature produce nontrivial spin texture, leading to a large topological Hall effect in the doped systems at low temperatures. Our studies further show that the topological properties of ${\mathrm{Mn}}_{3-\mathit{x}}{\mathrm{Fe}}_{\mathit{x}}\mathrm{Sn}$ are very sensitive to the Fe doping.

Bi-isotropic media constitute a proper scenario for scrutinizing optical effects stemming from magnetoelectric parameters. Chiral magnetic current is a macroscopic effect arising from the chiral magnetic effect that enriches the phenomenology of a classical dielectric medium. This paper examines the optical aspects of bi-isotropic media in the presence of Ohmic and magnetic conductivity. The full isotropic scenario manifests circular birefringence described by a dispersive rotatory power that changes sign at a given frequency. For a bi-isotropic medium with antisymmetric magnetic conductivity, an intricate dispersive rotatory power is attained, supplied with sign reversal as well. This scenario occurs in the absence or presence of Ohmic conductivity. It also indicates a handedness reversion of the medium, an unusual property in dielectrics, which may work as a signature of bi-isotropic media supporting chiral magnetic current.

We study theoretically electron dynamics of a graphene nanoring placed in the field of an ultrashort optical pulse. We describe the graphene nanoring within an effective model with infinite mass boundary conditions. For an optical pulse with a duration of just a few femtoseconds, the electron dynamics is coherent and is described by a time-dependent Schrödinger equation. If the optical pulse is circularly polarized, then two valleys of graphene are populated differently, resulting in a finite valley polarization of the system after the pulse. Such a valley polarization is a unique property of graphene nanoscale systems, while for a graphene monolayer, a circularly polarized pulse does not produce any valley polarization. The valley polarization of the graphene nanoring depends on parameters of the system, such as inner and outer radii. With the system's size increasing, the valley polarization monotonically decreases, converging to its zero value for the infinite graphene monolayer.

We report the density functional calculations with the hybrid approximation to the exchange-correlation energy that clarify the atomic and electronic structures of the N vacancy (${\mathrm{V}}_{\mathrm{N}}$) in ${\mathrm{Si}}_3{\mathrm{N}}_4$. We find that the gap states induced by the Si dangling bonds around ${\mathrm{V}}_{\mathrm{N}}$ show interesting interplay between the spin splitting and the Jahn-Teller splitting depending on their charges. More surprisingly, we find that a peculiar electron state distributed in the internal space of the host SiN is hidden in the conduction bands and converted to a localized floating state in the fundamental energy gap due to the presence of ${\mathrm{V}}_{\mathrm{N}}$. This theoretical finding indicates ${\mathrm{V}}_{\mathrm{N}}$ being multiply charged from $+1$ to $-5$: The charge states from $+1$ to $-3$ are stable at respective Fermi-level positions in the gap, whereas the states $-4$ and $-5$ are metastable for the Fermi level near the conduction band bottom. A possibility of multilevel memory function of ${\mathrm{V}}_{\mathrm{N}}$ in SiN is suggested.

The optical force density as a function of position and time provides fundamental information to model local and, through integration, macroscopic kinetic motion of condensed matter. Here, the boundary condition associated with the optical force density is developed and investigated using an expression stemming from the work of Einstein and Laub, and in conjunction with Maxwell's equations to describe the electromagnetic fields. Consequently, a constraint is formed that allows a unique relationship between the total force and the force density, one that is achieved by virtue of the conservation principles for physical materials and described by locally homogenized constitutive parameters. Further insight can be garnered from new experimental studies, as summarized. The mathematical steps presented form a basis for modeling various optomechanical phenomena, including optical forces in and on solid-state systems such as membranes, beams, cantilevers, and waveguides, and can be interpreted in terms of a suite of related theoretical work. This specification of the force density boundary condition is relevant for basic scientific fields including those involved with various quantum cooling issues, molecular optomechanics, photochemistry, and biophysics (including mechanotransduction). The technologies impacted encompass integrated optomechanics (silicon photonics, where new optical device concepts can be enabled), communication systems (in which optical forces could supplant electronic switching), remote control and actuation, propulsion, sensing, and navigation.

We investigate quantum charge pumping through the surface states of a topological insulator (TI), i.e., ${\mathrm{Bi}}_2{\mathrm{Te}}_3$. We consider a device with two oscillating potentials for the generation of a dc current in the adiabatic pumping regime. Applying the exchange magnetic field, we show that the device can work as a memory read-out. Also, we show that by applying a static gate voltage the pumping device can work as a field-effect transistor. In addition, the pumped current obtained in our proposed device is significantly (more than three orders of magnitude) greater than the pumped current obtained previously for TI-based devices. These properties show that ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ can be considered a good candidate for the fabrication of nanoelectronic devices based on TI as well as low-power and low-energy consumption devices in quantum computing.

We calculate the nonreciprocal charge current in a bulk semiconductor with Rashba spin-orbit coupling subjected to crossed electric and magnetic fields. By using a second-order distribution function derived in a perturbative approach that considers the change in the local electron energy induced by the electric field, we find that, in contrast with previous theoretical estimates, a charge current proportional to the applied magnetic field exists for all values of the chemical potential, above or below the band crossing point (BCP), the energy where the two chiral conduction bands intersect. The persistence of the quadratic electric current across the BCP is a consequence of the chiral dependence of the relaxation times, an effect neglected before.

Time-resolved optically detected magnetic resonance (ODMR) is a valuable technique to study the local deformation of the crystal lattice around magnetic ion as well as the ion spin-relaxation time. Here we utilize selective Mn doping to additionally enhance the inherent locality of the ODMR technique. We present the time-resolved ODMR studies of single (Cd,Mg)Te/(Cd,Mn)Te quantum wells with manganese ions located at different positions along the growth axis—in the center or on the sides of the quantum well. We observe that spin-lattice relaxation of ${\mathrm{Mn}}^{2+}$ significantly depends on the ion-carrier wave-function overlap at low-magnetic fields. Interestingly, the effect is clearly observed despite very low carrier density, which suggests the potential for control of the ${\mathrm{Mn}}^{2+}$ ion relaxation rate by means of the electric field in future experiments.

Here, we comprehensively investigate the atomic structures and electronic properties of different antiphase boundaries in III-V semiconductors with different orientations and stoichiometries, including {110}, {100}, {111}, {112}, and {113}, based on first-principle calculations. Especially, we demonstrate how the ladder or zigzag chemical bond configuration can lead, for the different cases, to a gapped semiconducting band structure, to a gapped metallic band structure, or to a nongapped metallic band structure. In addition, we evidence that the ladder antiphase boundary (APB) configurations more significantly impact the Fermi energy levels than the zigzag APB configurations. We finally discuss how these different band structures can have some consequences on the operation of monolithic III-V/Si devices for photonics or energy harvesting.

Quenched disorder may prevent the formation of the widely sought quantum spin-liquid states (QSLs) or mask their signatures by inducing a spin-glass state, which is why considerable experimental efforts are directed at purifying materials that may host QSLs. However, in geometrically frustrated (GF) magnets, the largest class of materials in which QSLs are sought, the glass-transition temperature $T_g$ grows with decreasing the density of vacancy defects, accompanied by a simultaneous growth of the magnetic susceptibility. In this Letter, we develop a phenomenological theory of glass transitions and magnetic susceptibility in three-dimensional geometrically frustrated magnetic materials. We consider a model of a GF magnet in which the glass transition occurs in the absence of vacancies, e.g., due to other types of quenched disorder. We show that disorder that creates weak local perturbations, e.g., weak random strain, leads to the growth of the transition temperature $T_g$. By contrast, vacancies reduce $T_g$ for small vacancy concentrations. Another consequence of the presence of vacancies is the creation of quasispins, effective magnetic moments localized near the vacancies, that contribute to the magnetic susceptibility of the system together with the bulk spins. We show that increasing the vacancy density leads to an increase of the total magnetic susceptibility.

Hund metals are multiorbital systems with $3d$ or $4d$ electrons exhibiting both an itinerant character and local moments, and they feature Kondo-like screenings of local orbital and spin moments, with suppressed coherence temperatures driven by Hund's coupling $J_H$. They often exhibit magnetic order at low temperature, but how the interaction between the Kondo-like screening and long-range magnetic order is manifested in the quasiparticle spectrum remains an open question. Here, we present the spectroscopic signature of such an interaction in a Hund metal candidate MnSi exhibiting weak ferromagnetism. Our photoemission measurements reveal renormalized quasiparticle bands near the Fermi level with strong momentum dependence: The ferromagnetism manifests through possibly exchange-split bands (Q1) below $T_C$, while the spin/orbital screenings lead to the gradual development of quasiparticles (Q2) upon cooling. Our results demonstrate how the characteristic spin/orbital coherence in a Hund metal could coexist and compete with the magnetic order to form a weak itinerant ferromagnet, via quasiparticle bands that are well separated in momentum space and exhibit distinct temperature dependence. Our results imply that the competition between the spin/orbital screening and the magnetic order in a Hund metal bears interesting similarities to the Kondo lattice systems.

Here we report the observation of the quadruple Weyl point of charge 4 in phononic crystals, the highest topological charge a twofold degenerate node can carry. Besides the quadruple Weyl point, the phononic semimetal also hosts the conventional, quadratic, and spin-1 Weyl points, which stands as a system with yet the richest types of Weyl points. The quadruple-helicoid surface states, specific to the quadruple Weyl point, are demonstrated. The finding of the high-charge Weyl point enriches the knowledge of Weyl semimetals and may stimulate related researches in other systems, such as photonic, mechanical, and cold atom systems.

Aperiodic crystals possess long-range order without translational symmetry. They constitute a state of matter that has forced a profound paradigm shift in solid-state physics. A common feature of aperiodic crystals is that they recover periodicity in higher dimensional spaces, the so-called crystallographic superspaces. Much work has been dedicated to the structural order within these superspaces and also to their specific dynamics. This paper focuses on the phase transitions within crystallographic superspaces. Aperiodic crystals are divided into three families: incommensurately modulated crystals, composite crystals, and quasicrystals. Incommensurately modulated crystals have been studied heavily and appear now to be relatively well understood, since they possess a periodic high-symmetry phase. The other two members of the family are aperiodic by construction. In this paper, we present a comprehensive and systematic study of a prototype composite host-guest family of $n$-alkane/urea inclusion compounds $[\mathit{n}-{\text{C}}_n{\text{H}}_{2n+2}/\text{CO}{\left({\text{NH}}_2\right)}_2]$. For these materials, which exhibit a rich sequence of phases, the phase transitions are described in terms of group/subgroup symmetry breaking within crystallographic superspaces. Such phase transitions may decrease, increase, or maintain the dimension of the crystallographic superspace. These results highlight the multiplicity of specific structural solutions that aperiodicity offers.

We present a theoretical study on the current and frequency windows of self-oscillations induced by the dampinglike spin-orbit torque in a biaxial antiferromagnet. By the linear stability analysis and averaging technique, we analytically formulate the lower and upper thresholds and the frequency of self-oscillations. We find that the self-oscillation range is highly sensitive to the damping and magnetic anisotropies, which have a variety of options in abundant antiferromagnets. Beyond a critical damping, the self-oscillation can arise after the instability of an antiferromagnetic state, with its range widening for a heavier damping. Below it, a spin-flip transition occurs when increasing the current, similar to the spin flip under an increasing magnetic field. Meanwhile, we examine the role of anisotropies. With the spin polarization along the easy axis, the weak easy- and hard-axis anisotropies allow self-oscillations, the range of which is broadened for weaker anisotropies. At the same time, the spin-flip transition is permitted for strong anisotropies. If the spin polarization coincides with the hard axis, the transition types are mainly determined by the easy-axis anisotropy. For a weak easy-axis anisotropy, the self-oscillation can develop with its range expanding for a stronger hard-axis anisotropy. Finally, we envision that (tunneling) anisotropic magnetoresistance may act as an effective means to probe the oscillations.

A quantum spin liquid (QSL) is an exotic state in which electron spins are highly entangled, yet keep fluctuating even at zero temperature. Experimental realization of model QSLs has been challenging due to imperfections, such as antisite disorder, strain, and extra or a lack of interactions in real materials compared to the model Hamiltonian. Here we report the magnetic susceptibility, thermodynamic, inelastic neutron scattering (INS), and muon-spin relaxation studies on a polycrystalline sample of ${\mathrm{PrZnAl}}_{11}{\mathrm{O}}_{19}$, where the ${\mathrm{Pr}}^{3+}$ ions form an ideal two-dimensional triangular lattice. Our results demonstrate that this system does not order nor freeze, but keeps fluctuating down to 50 mK despite large antiferromagnetic couplings ($\sim -10$ K). Furthermore, the INS and specific-heat data suggest that ${\mathrm{PrZnAl}}_{11}{\mathrm{O}}_{19}$ is best described as a gapless QSL.

Recent experiments demonstrated robust ferroelectricity in Mg-doped ZnO (ZMO) of the wurtzite structure, hinting at a promising strategy to substantially expand the list of ferroelectrics by doping conventional piezoelectrics. We investigate the origin of ferroelectricity in ZMO with first-principles density functional theory (DFT). The general argument that the Mg alloying could soften the ionic potential energy surface of ZMO for polarization reversal is overly simplified. Our DFT calculations reveal that even at a high Mg concentration, the energy difference ($\mathrm{\Delta }U$) between the polar and nonpolar phases remains prohibitively large for ZMO systems when the strain is fixed to the polar phase. Interestingly, the magnitude of $\mathrm{\Delta }U$ becomes substantially smaller when the strain relaxation is allowed, approaching the value of typical perovskite ferroelectrics such as ${\mathrm{PbTiO}}_3$ with increasing Mg doping concentrations. The enabled switchability of ZMO systems is attributed to a hexagonal phase of MgO that is much lower in energy than its wurtzite counterpart. Detailed orbital and bonding analysis supports that the intra-atomic $3d_{z^2}\text{−}4p_z$ orbital self-mixing of Zn plays an important role in stabilizing the polar wurtzite phase, the lack of which is responsible for the low-energy nonpolar hexagonal phase of MgO.