Ac current-driven motions of spiral spin textures can give rise to emergent electric fields acting on conduction electrons. This in turn leads to the emergent electromagnetic induction effect which may realize quantum inductor elements of micrometer size. ${\mathrm{YMn}}_6{\mathrm{Sn}}_6$ is a helimagnet with a short helical period (2–3 nm) that shows this type of emergent inductance beyond room temperature. To identify the optimized materials conditions for ${\mathrm{YMn}}_6{\mathrm{Sn}}_6$-type room-temperature emergent inductors, we have investigated emergent electromagnetic inductance (EEMI) as the magnetism is modified through systematic partial substitution of Y by Tb. By small angle neutron scattering and inductance measurements, we have revealed that the pinning effect on the spin-helix translational mode by Tb doping selectively and largely suppresses the negative component of EEMI, while sustaining the positive inductance arising from the spin tilting mode. We also find that, in addition to the spin helix, even the spin-collinear antiferromagnetic structure can host the positive EEMI due to thermally enhanced spin fluctuations. The present study highlights the facile control of both the magnitude and sign of EEMI beyond room temperature and thus suggests a route to expand the range of emergent inductor candidate materials.

The half-filled Kondo lattice model is studied at low temperatures on a simple cubic lattice using the self-consistent theory developed by Ram and Kumar [Phys. Rev. B 96, 075115 (2017)]. It is found to have three distinct insulating phases in the temperature-hopping plane, namely, the strong-coupling Kondo singlet (KS) phase, the inverted Kondo singlet (iKS) phase distinguished from the KS by inversion, and the antiferromagnetic (AFM) phase. The quasiparticle density of states across the inversion transition is noted to exhibit a dimensional reduction, which can differentiate between the KS and iKS insulators in experiments. Magnetic quantum oscillations obtained in the iKS and AFM phases are found to show a Lifshitz-Kosevich-like behavior with temperature as well as quantum fluctuations induced by the Kondo interaction.

Twisted graphene multilayers have been recently demonstrated to share several correlation-driven behaviors with twisted bilayer graphene. In general, the van Hove singularities (VHSs) can be used as a proxy of the tendency for correlated behaviors. In this paper, we adopt an atomistic method by combining the tight-binding method with the semiclassical molecular dynamics to investigate the electronic structures of twisted trilayer graphene (TTG) with two independent twist angles. The two independent twist angles can lead to the interference of the moiré patterns forming a variety of commensurate/incommensurate complex supermoiré patterns. In particular, the lattice relaxation, twist angle and angle disorder effects on the VHS are discussed. We find that the lattice relaxation significantly influences the position and magnitude of the VHSs. In the supermoiré TTG, the moiré interference provides constructive or destructive effects depending on the relative twist angle. By modulating the two independent twist angles, novel superstructures, for instance, the Kagome-like lattice, could be constructed via the moiré pattern. Moreover, we demonstrate that a slight change in twist angles (angle disorder) provides a significant suppression of the peak of the VHSs. Apart from the moiré length, the evolution of the VHSs and the LDOS mapping in real space could be used to identify the twist angles in the complicated TTG. In practice, our work could provide a guide for exploring the flat band behaviors in the supermoiré TTG experimentally.

The ${\mathrm{La}}_{1-x}{\mathrm{Ca}}_x{\mathrm{MnO}}_3$ series of compounds with antiferromagnetic ground states ($x\ge 1/2$) have been extensively studied due to the novel spin, orbital, and charge-ordering states observed when the calcium concentration is a simple fraction ($x=1/2,2/3$, and 3/4). The ground states of these compositions have been explained by the Goodenough charge, orbital, and spin ordering model. An important issue remaining is the elucidation of how the ground state changes when $x$ is not a simple number. Here we study the magnetic structure of ${\mathrm{La}}_{1-x}{\mathrm{Ca}}_x{\mathrm{MnO}}_3$ for $0.51\le x\le 0.69$ using powder neutron diffraction measurements supported by magnetization data. For compositions with $0.51\le x\le 0.56$, the magnetic structure, which we term as an incommensurate charge exchange (CE) structure can be described by two propagation vectors ${\mathbf{k}}_{\mathrm{C}}=[1/2,0,1/2]$ and ${\mathbf{k}}_{\mathrm{E}}=[{\varepsilon }_{\mathrm{E}},0,1/2]$. In the second one, the component parallel to the ${\mathbf{a}}^{*}$ axis of the reciprocal lattice changes with the ${\mathrm{Mn}}^{4+}$ concentration $x$ as ${\varepsilon }_{\mathrm{E}}\approx x-1/2$ providing, thus, an unambiguous signature of the adoption of an incommensurate magnetic structure. As $x$ gradually increases, the diffraction data reveal that two magnetic phases—one adopting the incommensurate CE, and one adopting the commensurate “2/3” magnetic structure–co-exist in the concentration regime of $0.57\le x\le 0.61$. Around the simple fraction $x=2/3$, the magnetic structure can be also described by three propagation vectors, the commensurate ${\mathbf{k}}_{\mathrm{E}}=[0,0,1/2]$, ${\mathbf{k}}_{\mathrm{C}}=[1/2,0,1/2]$, and an incommensurate ${\mathbf{k}}_{2/3}=[1/3+{\varepsilon }_{2/3},0,1/2]$ propagation vector with ${\varepsilon }_{2/3}$ taking negative/zero/positive values for $x$ smaller than/equal to/larger than 2/3, respectively. Our experimental results for $0.51\le x\le 0.56$ are neither in favor of a stripe structure consisting of a fine mixture of $x=1/2$ and $x=2/3$ phases (phase separation) nor of a defect structure in which an appropriate amount of ${\mathrm{Mn}}^{3+}$-O sheets have been replaced by ${\mathrm{Mn}}^{4+}$-O sheets (defect structure). A sinusoidal modulated structure has been used as a possible candidate in explaining the experimental neutron diffraction magnetic Bragg peaks. This result may be linked to the presence of a mixed orbital state of the manganese ions.

By introducing electron screening in the extended Koopmans' theorem we correctly describe the band-gap opening in weakly as well as strongly correlated systems. We show this by applying our method to bulk LiH, Si, and paramagnetic as well as antiferromagnetic NiO. Although incorrect features remain in the full photoe- mission spectra, this is a remarkable result for an ab-initio electronic structure method and it opens the way to a unified description of photoemission spectra at weak and strong correlation.

Using electron emission spectroscopy, measurement and analysis were conducted on the energy distribution of vacuum emitted electrons from electrically driven InGaN/GaN green (peak wavelengths $\lambda \approx 515\mathrm{nm}$) light-emitting diodes (LEDs) with and without a prewell superlattice (SL). We report on the detection of a high-energy upper valley at $\sim 1.7\mathrm{eV}$ above the $\mathrm{\Gamma }$ valley from samples with no prewell SL. We propose that these upper valley electrons originate predominantly from trap-assisted Auger recombination (TAAR) in green LEDs, as the intensity of these peaks is found to have quadratic dependence on the carrier density $n$ [see Espenlaub et al., J. Appl. Phys. 126, 184502 (2019)]. The high-energy upper valley peak was not observed in the sample with a prewell SL which is attributed to gettering by the prewell SL of still unidentified impurities that act as TAAR centers.

Interfaces of dye molecules and two-dimensional transition metal dichalcogenides (TMDCs) combine strong molecular dipole excitations with high carrier mobilities in semiconductors. Förster type energy transfer is one key mechanism for the coupling between both constituents. We report microscopic calculations of a spectrally resolved Förster induced transition rate from dye molecules to a TMDC layer. Our approach is based on microscopic Bloch equations which are solved self-consistently together with Maxwell's equations. This approach allows to incorporate the dielectric environment of a TMDC semiconductor, sandwiched between donor molecules and a substrate. Our analysis reveals transfer rates in the meV range for typical dye molecules in closely stacked structures, with a nontrivial dependence of the Förster rate on the molecular transition energy resulting from unique signatures of dark, momentum forbidden TMDC excitons.

As disorder strength increases in quantum many-body systems a new phase of matter, the so-called many-body localization, emerges across the whole spectrum. This transition is energy dependent, a phenomenon known as mobility edge, such that the mid-spectrum eigenstates tend to localize at larger values of disorder in comparison to eigenstates near the edges of the spectrum. Many-body localization becomes more sophisticated in long-range interacting systems. Here, by focusing on several quantities, we draw the phase diagram as a function of disorder strength and energy spectrum, for a various range of interactions. Regardless of the underlying transition type, either second-order or Kosterlitz-Thouless, our analysis consistently determines the mobility edge, i.e., the phase boundary across the spectrum. We show that, long-range interaction enhances the localization effect and shifts the phase boundary towards smaller values of disorder. In addition, we establish a hierarchy among the studied quantities concerning their corresponding transition boundary and critical exponents. Interestingly, we show that deliberately discarding some information of the system can mitigate finite-size effects and provide results in line with the analytical predictions at the thermodynamic limit.

Materials normally exhibit positive thermal expansion, and materials with negative thermal expansion coefficient are of great interest for both fundamental study and applications such as high-precision microscale actuators, thermal expansion compensators, etc. Here, the coefficients of linear thermal expansion (${\alpha }_L$) along the $c$ axis of the layered ferroelectric $\mathrm{CuIn}{\mathrm{P}}_2{\mathrm{S}}_6$ have been measured. In the single-crystalline bulk specimen, ${\alpha }_L$ exhibits large negative thermal expansion up to $-20.05\times {10}^{-6}{\mathrm{K}}^{-1}$ in the temperature range below 150 K. In contrast to the bulk, exfoliated ultrathin $\mathrm{CuIn}{\mathrm{P}}_2{\mathrm{S}}_6$ flakes exhibit an overall positive ${\alpha }_L\sim 26.8\times {10}^{-6}{\mathrm{K}}^{-1}$. The thickness-dependent thermal expansion of $\mathrm{CuIn}{\mathrm{P}}_2{\mathrm{S}}_6$ crystal may facilitate the design of van der Waals heterostructures and ferroelectric devices with tunable thermal responses.

In this paper, we carry out a theoretical study of the electronic ballistic transport in gated phosphorene superlattices. Our work is focused on the effects by the random introduction of structural disorder in the electrostatic potential barriers. Specifically, we consider various degrees of disorder in the width and height of the barriers. To explain the transport of the charge carriers (electrons and holes) in the armchair direction of phosphorene, we employ a low-energy two-band Hamiltonian derived from the tight-binding model. The transmission and conductance are calculated by the transfer matrix technique and the Landauer-Büttiker formalism, respectively. We have found that both types of disorder affect the transmission and conductance in different ways, and we also found distinct effects on the charge carriers. In general, the disorder significantly reduces the transmission as well as the peak zones associated with the tunneling effect. Particularly, for disorder in the width, the transmission miniband structure is greatly degraded, while for disorder in the height, the miniband structure is preserved. The variation in the width of the barriers interferes directly with the Fabry-Pérot resonances of the propagating waves, while the variation in the height mainly reduces the magnitude of the transmission due to the dominance of evanescent waves. Consequently, the conductance loses its oscillatory characteristic and experiences flattening under the influence of disorder of the width, in contrast with the disorder of the height in which the oscillation prevails. Finally, we found that both types of disorder induce a redistribution of electron states. Specifically, the allowed energy bands are increased, and the occupation number as well as the accumulation of states are greatly reduced, which is in accordance with the effects of disorder observed in the transport properties.

A classic route for destroying long-lived electronic quasiparticles in a weakly interacting Fermi liquid is to couple them to other low-energy degrees of freedom that effectively act as a bath. We consider here the problem of electrons scattering off the spin fluctuations of a geometrically frustrated antiferromagnet, whose nonlinear Landau-Lifshitz dynamics, which remains nontrivial at all temperatures, we model in detail. At intermediate temperatures and in the absence of any magnetic ordering, the fluctuating local moments lead to a nontrivial angular anisotropy of the scattering rate along the Fermi surface, which disappears with increasing temperature, elucidating the role of “hot spots.” Over a remarkably broad window of intermediate and high temperatures, the electronic properties can be described by employing a local approximation for the dynamical spin response. This we contrast with the more familiar setup of electrons scattering off classical phonons, whose high-temperature limit differs fundamentally on account of their unbounded Hilbert space. We place our results in the context of layered magnetic delafossite compounds.

The problem of the normal-superconducting phase boundary for films in a parallel magnetic field, discussed in the classical paper by Ginzburg and Landau for temperatures close to the critical, is revisited with the help of the microscopic BCS theory for arbitrary temperatures taking pair-breaking and transport scattering into account. Although confirming experimental findings of the $T_c$ enhancement by the magnetic field, we find that the transport scattering pushes the phase transition curve to higher fields and higher temperatures for nearly all practical scattering rates. Still, the $T_c$ enhancement disappears in the dirty limit. We also consider intriguing changes, such as reentrant superconductivity, caused to the phase boundary by pair-breaking magnetic ions spread on one of the film faces. These features await experimental verification.

Ferroelectric perovskites have been ubiquitously applied in piezoelectric devices for decades, among which ecofriendly lead-free $\left(\mathrm{K},\mathrm{Na}\right)\mathrm{Nb}{\mathrm{O}}_3\text{−}\mathrm{based}$ materials have been recently demonstrated to be an excellent candidate for sustainable development. Molecular dynamics is a versatile theoretical calculation approach for the investigation of the dynamical properties of ferroelectric perovskites. However, molecular dynamics simulation of ferroelectric perovskites has been limited to simple systems, since the conventional construction of interatomic potential is rather difficult and inefficient. In the present study, we construct a machine-learning interatomic potential of $\mathrm{KNb}{\mathrm{O}}_3$ [as a representative system of $\left(\mathrm{K},\mathrm{Na}\right)\mathrm{Nb}{\mathrm{O}}_3$] by using a deep neural network model. Including first-principles calculation data into the training data set ensures the quantum-mechanics accuracy of the interatomic potential. The molecular dynamics based on machine-learning interatomic potential shows good agreement with the first-principles calculations, which can accurately predict multiple fundamental properties, e.g., atomic force, energy, elastic properties, and phonon dispersion. In addition, the interatomic potential exhibits satisfactory performance in the simulation of domain wall and temperature-dependent phase transition. The construction of interatomic potential based on machine learning could potentially be transferred to other ferroelectric perovskites and consequently benefit the theoretical study of ferroelectrics.

We have used nanoelectromechanical resonators to probe superfluid ${}^4\mathrm{He}$ at different temperature regimes, spanning over four orders of magnitude in damping. These regimes are characterized by the mechanisms which provide the dominant contributions to damping and the shift of the resonance frequency: tunneling two-level systems at the lowest temperatures, ballistic phonons and rotons at few hundred $\mathrm{m}\mathrm{K}$, and laminar drag in the two-fluid regime below the superfluid transition temperature as well as in the normal fluid. Immersing the nanoelectromechanical resonators in fluid increases their effective mass substantially, decreasing their resonance frequency. Dissipationless superflow gives rise to a unique possibility to dramatically change the mechanical resonance frequency in situ, allowing rigorous tests on different damping models in mechanical resonators. We apply this method to characterize tunneling two-level system losses and magnetomotive damping in the devices.

The discovery of superconductivity in infinite-layer nickelates is presently an important topic in condensed-matter physics, and potential similarities to and differences from cuprates are under intense debate. We determine general features of the charge excitation spectrum in nickelates from two opposite viewpoints: (i) Nickelates are regarded as strongly correlated electron systems like cuprate superconductors and thus can be described by the $t\text{−}J$ model, and (ii) electron correlation effects are not as strong as in cuprates, and thus, random-phase approximation (RPA) calculations may capture the essential physics. We find that in both cases, plasmon excitations are realized around the momentum transfer $\mathbf{q}=(0,0,q_z)$, although they tend to be damped more strongly in the RPA. In particular, this damping is enhanced by the relatively large interlayer hopping expected in nickelates. Besides reproducing the optical plasmon at $\mathbf{q}=(0,0,0)$ observed in ${\mathrm{Nd}}_{0.8}{\mathrm{Sr}}_{0.2}{\mathrm{NiO}}_2$, we obtain low-energy plasmons with gaps of $\sim 360$ and $\sim 560$ meV at $\mathbf{q}=(0,0,q_z)$ for finite $q_z$ in cases (i) and (ii), respectively. The present work offers a possible theoretical hint to answer whether nickelates are cupratelike or not and contributes to the general understanding of the charge dynamics in nickelates.

Spherical nanoclusters and nanoparticles are rising materials whose functional design provides many useful applications ranging from catalysis, molecular sensing, and gas storage to drug targeting and delivery. Here, we develop phenomenological crystallization theory of such spherical structures with octahedral symmetries $O$ and $O_h$. Within the developed theory, we propose a method based on constructing irreducible octahedral density functions and allows us to predict the positions of structural units in the spherical nanoobjects. The proposed theory explains the structures of the simplest known metal nanoclusters, some metal-organic polyhedra and membrane protein polyhedral nanoparticles, and also predicts more complex chiral spherical structures and achiral assemblies characterized by the geometry of semiregular polyhedra. A relationship between the constructed irreducible octahedral functions and spherical lattices, obtained by mapping a plane hexagonal order onto a spherical surface through an octahedron net, is discussed as well.

The melting of the corner of a crystal is a classical, real-world, nonequilibrium statistical mechanics problem which has shown several connections with other branches of physics and mathematics. For a perfect, classical crystal in two and three dimensions the solution is known: The crystal melts reaching a certain asymptotic shape, which keeps expanding ballistically. In this paper, we move onto the quantum realm and show that the presence of quenched disorder slows down severely the melting process. Nevertheless, we show that there is no many-body localization transition, which could impede the crystal to be completely eroded. We prove such claim both by a perturbative argument, using the forward approximation, and via numerical simulations. At the same time we show how, despite the lack of localization, the erosion dynamics is slowed from ballistic to logarithmic, therefore pushing the complete melting of the crystal to extremely long timescales.