Absorption of twisted photons for quadrupole ${}^{4}f\text{−}{}^{4}f$ transitions in $\mathrm{R}{\mathrm{e}}^{3+}$ impurity ions in micro- and macrocrystals is considered. Two types of twisted photon beams—the Laguerre-Gaussian and the Bessel-Gaussian beams—are considered and compared. Twisted photons have a nonzero transverse field gradient, which leads to selection rules for quadrupole transitions other than those in the case of the usual quadrupole transitions caused by a longitudinal field gradient. This allows one to observe new spectral lines which are absent in the usual absorption spectra. For microcrystals placed at the beam axis of twisted photons the otherwise dominating forced dipole ${}^{4}f\text{−}{}^{4}f$ transitions are absent, which simplifies the observation of new spectral lines.

In this paper, we demonstrate the expressibility of artificial neural networks (ANNs) in quantum many-body physics by showing that a feed-forward neural network with a small number of hidden layers can be trained to approximate with high precision the ground states of some notable quantum many-body systems. We consider the one-dimensional free bosons and fermions, spinless fermions on a square lattice away from half-filling, as well as frustrated quantum magnetism with a rapidly oscillating ground-state characteristic function. In the latter case, an ANN with a standard architecture fails, while that with a slightly modified one successfully learns the frustration-induced complex sign rule in the ground state and approximates the ground states with high precisions. As an example of practical use of our method, we also perform the variational method to explore the ground state of an antiferromagnetic $J_1\text{−}{J}_2$ Heisenberg model.

We report on the radiative interaction of two single quantum dots (QDs) each in a separate InP/GaInP-based microdisk cavity via resonant whispering gallery modes. The investigations are based on as-fabricated coupled disk modes. We apply optical spectroscopy involving a $4f$ setup, as well as mode-selective real-space imaging and photoluminescence mapping to discern single QDs coupled to a resonant microdisk mode. Excitation of one disk of the double cavity structure and detecting photoluminescence from the other yields proof of single-photon emission of a QD excited by incoherent energy transfer from one disk to the other via a mode in the weak-coupling regime. Finally, we present evidence of photons emitted by a QD in one disk that are transferred to the other disk by a resonant mode and are subsequently resonantly scattered by another QD.

We addressed the carrier dynamics in so-called G-centers in silicon (consisting of substitutional-interstitial carbon pairs interacting with interstitial silicons) obtained via ion implantation into a silicon-on-insulator wafer. We performed detailed photoluminescence experiments as a function of excitation energy, incident power, irradiation fluence, and temperature in order to study the impact of radiative and nonradiative recombination channels on the spectrum, yield, and lifetime of G-centers. In the framework of the Huang-Rhys theory with nonperturbative calculations of the acoustic phonon sidebands, we reach an estimation of $1.6\pm 0.1\AA$ for the spatial extension of the electronic wave function in the G-center. The radiative recombination time measured at low temperature lies in the 6 ns range. The estimation of both radiative and nonradiative recombination rates as a function of temperature further demonstrates a constant radiative lifetime.

We study via first-principles modeling and simulation two-dimensional spintronic junctions made of metal-organic frameworks consisting of two Mn-phthalocyanine ferromagnetic metal leads and semiconducting Ni-phthalocyanine channels of various lengths. These systems exhibit a large tunneling magnetoresistance ratio; the transmission functions of such junctions can be tuned using gate voltage by three orders of magnitude. We find that the origin of this drastic change lies in the orbital alignment and hybridization between the leads and the center electronic states. With physical insight into the observed on-off phenomenon, we predict a gate-controlled spin current switch based on two-dimensional crystallines and offer general guidelines for designing spin junctions using 2D materials.

Weyl semimetals are novel topological conductors that host Weyl fermions as emergent quasiparticles. In this Rapid Communication, we propose a new type of Weyl semimetal state that breaks both time-reversal symmetry and inversion symmetry in the $R\mathrm{AlGe}$ ($R=\mathrm{rare}-\mathrm{earth}$) family. Compared to previous predictions of magnetic Weyl semimetal candidates, the prediction of Weyl nodes in $R\mathrm{AlGe}$ is more robust and less dependent on the details of the magnetism because the Weyl nodes are generated already by the inversion breaking and the ferromagnetism acts as a simple Zeeman coupling that shifts the Weyl nodes in $k$ space. Moreover, $R\mathrm{AlGe}$ offers remarkable tunability, which covers all varieties of Weyl semimetals including type I, type II, inversion breaking, and time-reversal breaking, depending on a suitable choice of the rare-earth elements. Furthermore, the unique noncentrosymmetric and ferromagnetic Weyl semimetal state in $R\mathrm{AlGe}$ enables the generation of spin currents.

Proposing a theoretical model of a layered ferromagnetic (FM) -nonmagnetic (NM) heterostructure under few-cycle optical driving, we render the light-wave-induced charge transport spin asymmetric and achieve petahertz frequency control of spins. Transition-metal trichalcogenides (TMTCs) are suggested for the FM layer. With the strong FM TMTCs (i.e., high-spin ${\mathrm{TM}}^{3+}$ therein), major and minor spins undergo cross transfer to the NM and FM layers, respectively, whereas with the weak FM TMTCs (i.e., low-spin ${\mathrm{TM}}^{3+}$), only major spins are transferred to the NM layer due to the Coulomb blockade for minor spins. These findings present prototypes of petahertz spin devices, broadening the horizon of spintronics up to the subfemtosecond time span.

The spectroscopic fingerprints of the point defects of titanium dioxide remain highly controversial. Seemingly indisputable experiments lead to conflicting conclusions in which oxygen vacancies and titanium interstitials are alternately referred to as the primary origin of the Ti $3d$ band-gap states. We report on experiments performed by electron energy loss spectroscopy whose key is the direct annealing of only the very surface of rutile ${\mathrm{TiO}}_2\left(110\right)$ crystals and the simultaneous measurement of its temperature via the Bose-Einstein loss/gain ratio. By surface preparations involving reactions with oxygen and water vapor, in particular, under electron irradiation, vacancy- and interstitial-related band-gap states are singled out. Off-specular measurements reveal that both types of defects contribute to a unique charge distribution that peaks in subsurface layers with a common dispersive behavior.

We use first-principles simulations to investigate the magnetic and the electronic transport properties of functionalized graphene layers upon the presence of extended linear defects (ELDs). We have considered electron-donor molecules, tetrathiafulvalene (TTF), lying on the graphene sites neighboring the ELD (TTF/ELD). Those molecules bond to the graphene sheet mediated by van der Waals interactions, giving rise to a net (molecule$\to$graphene) charge transfer. The $n$-type doping of graphene, as well as the molecule-graphene interaction, are strengthened by the presence of the ELD. There is a charge density accumulation on the C atoms along the defect sites, promoting the formation of magnetic states in TTF/ELD. We show that such a net magnetization can be modified through an external electric field ($E_{\text{field}}$). Further electronic transport calculations reveal that the transmission coefficients exhibit a spin anisotropy which can be controlled by the $E_{\text{field}}$, and thus showing that TTF/ELD is a quite interesting system to realize tunable spin-polarized electronic currents on graphene.

We consider the dimer Hubbard model within dynamical mean-field theory to study the interplay and competition between Mott and Peierls physics. We describe the various metal-insulator transition lines of the phase diagram and the breakdown of the different solutions that occur along them. We focus on the specific issue of the debated Mott-Peierls insulator crossover and describe the systematic evolution of the electronic structure across the phase diagram. We found that at low intradimer hopping, the emerging local magnetic moments can unbind above a characteristic singlet temperature $T^{*}$. Upon increasing the interdimer hopping, subtle changes occur in the electronic structure. Notably, we find Hubbard bands of a mix character with coherent and incoherent excitations. We argue that this state might be relevant for materials such as ${\mathrm{VO}}_2$ and its signatures may be observed in spectroscopic studies, and possibly through pump-probe experiments.

We investigate the reflectionlessness and invisibility properties of the transverse electric–mode solution of a linear homogeneous optical system which comprises $\mathcal{PT}$-symmetric structures covered by graphene sheets. We derive analytic expressions, indicate the roles of each parameter governing the optical system with graphene, and prove that optimal conditions of these parameters give rise to broadband and wide-angle invisibility. The presence of graphene turns out to shift the invisible wavelength range and to reduce the required gain value considerably, based on its chemical potential and temperature. We substantiate that our results yield broadband reflectionless and invisible configurations for realistic materials of small refractive indices, usually around $\eta =1$, and of small thicknesses with graphene sheets of rather low temperatures and chemical potentials. Finally, we demonstrate that pure $\mathcal{PT}$-symmetric graphene yields invisibility at low temperatures and chemical potentials.

Topological quantum computation using non-Abelian Majorana zero modes localized in proximitized semiconductor nanowires requires careful electrostatic control of wire junctions so as to manipulate and braid the zero modes enabling anyonic fault-tolerant gate operations. We theoretically investigate the topological superconducting properties of such elementary wire junctions, the so-called T junctions, finding that the existence of the junction may nonperturbatively affect the Majorana behavior by introducing spurious nontopological subgap states mimicking zero modes. We propose a possible solution to this potentially serious problem by showing that junctions made lithographically from two-dimensional (2D) electron gas systems may manifest robust subgap topological properties without any spurious zero modes. We propose a 2D structure that enables multiprobe tunneling experiments providing position-dependent spectroscopy, which can decisively settle outstanding open questions related to the origin of the zero-bias conductance peaks observed experimentally. We also find that junctions with trivial superconductors may result in local perturbations that induce extrinsic low-energy states similar to those associated with wire junctions.

We study room-temperature spin transport in graphene devices encapsulated between a layer-by-layer-stacked two-layer-thick chemical vapor deposition (CVD) grown hexagonal boron nitride (hBN) tunnel barrier, and a few-layer-thick exfoliated-hBN substrate. We find mobilities and spin-relaxation times comparable to that of ${\mathrm{SiO}}_2$ substrate-based graphene devices, and we obtain a similar order of magnitude of spin relaxation rates for both the Elliott-Yafet and D'Yakonov-Perel' mechanisms. The behavior of ferromagnet/two-layer-CVD-hBN/graphene/hBN contacts ranges from transparent to tunneling due to inhomogeneities in the CVD-hBN barriers. Surprisingly, we find both positive and negative spin polarizations for high-resistance two-layer-CVD-hBN barrier contacts with respect to the low-resistance contacts. Furthermore, we find that the differential spin-injection polarization of the high-resistance contacts can be modulated by dc bias from $-0.3$ to $+0.3$ V with no change in its sign, while its magnitude increases at higher negative bias. These features point to the distinctive spin-injection nature of the two-layer-CVD-hBN compared to the bilayer-exfoliated-hBN tunnel barriers.

Internal friction is analyzed in an atomic-scale model of amorphous silica. The potential energy landscape of more than 100 glasses is explored to identify a sample of about 700 two-level systems (TLSs). We discuss the properties of TLSs, particularly their energy asymmetry and barrier as well as their deformation potential, computed as longitudinal and transverse averages of the full deformation potential tensors. The discrete sampling is used to predict dissipation in the classical regime. Comparison with experimental data shows a better agreement with poorly relaxed thin films than well relaxed vitreous silica, as expected from the large quench rates used to produce numerical glasses. The TLSs are categorized in three types that are shown to affect dissipation in different temperature ranges. The sampling is also used to discuss critically the usual approximations employed in the literature to represent the statistical properties of TLSs.

Despite their crystalline nature, thermoelectric clathrates exhibit a strongly reduced lattice thermal conductivity. While the reason for this unexpected behavior is known to lie in the peculiarities of the complex crystal structure and the interplay of the underlying guest-host framework, their respective roles are still not fully disentangled and understood. Our ab initio study of the most simple type-I clathrate phase, the binary compound ${\mathrm{Ba}}_8{\mathrm{Si}}_{46}$ and its derivatives ${\mathrm{Ba}}_{8-x}{\mathrm{Si}}_{46}$ seeks to identify these mechanisms and provides insight into their origin. Indeed, the strongly decreased lattice thermal conductivity in thermoelectric clathrates is a consequence of a reduction of the acoustic phonon bandwidth, a lowering of the acoustic phonon group velocities, and the amplification of three-phonon-scattering processes. While the complexity of the crystal structure is demonstrated not to be the leading factor, the reasons are manifold. A modified Si-Si interaction causes a first decrease of the sound velocity, whereas the presence of flat Ba modes results in an additional lowering. These modes correspond to confined Bloch states that are localized on the Ba atoms and significantly increase the scattering phase space and, together with an increased anharmonicity of the interatomic interactions, strongly affect the phonon lifetimes.

A general theory of the tilting of dispersion bands in phononic crystals whose properties are being slowly and periodically modulated in space and time is established. The ratio of tilt to modulation speed is calculated, for the first time, in terms of Berry's phase and curvature and is proven to be a robust integer-valued Chern number. Derivations are based on a version of the adiabatic theorem for elastic waves demonstrated thanks to WKB asymptotics. Findings are exemplified in the case of a 3-periodic discrete spring-mass lattice. Tilted dispersion diagrams plotted using fully numerical simulations and semianalytical calculations based on a numerically gauge invariant expression of Berry's phase show perfect agreement. One-way blocking of waves due to the tilt, and ultimately to the breaking of reciprocity, is illustrated numerically and shown to be highly significant across a limited number of unit cells, suggesting the feasibility of experimental demonstrations. Finally, a version of the bulk-edge correspondence principle relating the tilt of bulk bands to the number of one-way gapless edge states is demonstrated.

We study superconducting pairing in the doped Kitaev-Heisenberg model by taking into account the recently proposed symmetric off-diagonal exchange $\mathrm{\Gamma }$. By performing a mean-field analysis, we classify all possible superconducting phases in terms of symmetry, explicitly taking into account effects of spin-orbit coupling. Solving the resulting gap equations self-consistently, we map out a phase diagram that involves several topologically nontrivial states. For $\mathrm{\Gamma }<0$, we find a competition between a time-reversal symmetry-breaking chiral phase with Chern number $\pm 1$ and a time-reversal symmetric nematic phase that breaks the rotational symmetry of the lattice. On the other hand, for $\mathrm{\Gamma }\ge 0$ we find a time-reversal symmetric phase that preserves all the lattice symmetries, thus yielding clearly distinguishable experimental signatures for all superconducting phases. Both of the time-reversal symmetric phases display a transition to a ${\mathbb{Z}}_2$ nontrivial phase at high doping levels. Finally, we also include a symmetry-allowed spin-orbit coupling kinetic energy and show that it destroys a tentative symmetry-protected topological order at lower doping levels. However, it can be used to tune the time-reversal symmetric phases into a ${\mathbb{Z}}_2$ nontrivial phase even at lower doping.

We have revealed $\mathrm{S}{\mathrm{b}}_2{\mathrm{S}}_3$ to have two phase transitions and to undergo metallization using a diamond anvil cell at around 5.0, 15.0, and 34.0 GPa, respectively. These results were obtained on the basis of high-pressure Raman spectroscopy, temperature-dependent conductivity measurements, atomic force microscopy, high-resolution transmission electron microscopy, and first-principles calculations. The first phase transition at $\sim 5.0\mathrm{GPa}$ is an isostructural phase transition, which is manifested in noticeable changes in five Raman-active modes and the slope of the conductivity because of a change in the electronic structure. The second pressure-induced phase transition was characterized by a discontinuous change in the slope of conductivity and a new low-intensity Raman mode at $\sim 15.0\mathrm{GPa}$. Furthermore, a semiconductor-to-metal transition was found at $\sim 34.0\mathrm{GPa}$, which was accompanied by irreversible metallization, and it could be attributed to the permanently plastic deformation of the interlayer spacing. This high-pressure behavior of $\mathrm{S}{\mathrm{b}}_2{\mathrm{S}}_3$ will help us to understand the universal crystal structure evolution and electrical characteristics for $A_2{B}_3$-type compounds, and to facilitate their application in electronic devices.

We have investigated the effect of spin-orbit coupling (SOC) and strong covalency on the magnetism of double-perovskite iridates (${\mathrm{Sr}}_2{M\mathrm{IrO}}_6$, $M=\mathrm{Ca}$, Mg) using first principles as well as model Hamiltonian calculations. Our calculation shows that strong Ir-$d$ and O-$p$ covalency in these double-perovskite iridates results in the deviation from the expected $6+$ charge state of Ir, leading to an unquenched orbital moment in the presence of SOC. We have studied in detail the nature of magnetism and our calculations of isotropic exchange interactions for these double-perovskite iridates suggest the systems to be frustrated. We argue that the SOC driven easy plane anisotropy possibly promotes long range magnetic order suggested for these systems indicating the importance of SOC in both these systems. Further, our model calculation considering ideal $d^3$ electronic configuration for Ir emphasizes the role of SOC, noncubic crystal field and Ir-Ir hopping in the reduction of the spin moment.