Recently we proposed a multichannel Dyson equation, in which we coupled the one- and the three-body Green's functions, to model photoemission spectra. In this work we extend this concept to higher-order Green's functions and to other spectroscopies. We show the general structure of the equations and how one can systematically approximate the corresponding multichannel self-energy. As a particular case, we focus on the coupling of the two-body and the four-body Green's functions in the electron-hole channel to describe neutral excitations. This formulation allows for the description of important many-body effects, such as biexcitons, in a natural way. We illustrate our approach by applying it to a two-level model system, which, in a one-particle picture, exhibits single and double excitations. Our method can correctly describe both kinds of excitation, unlike standard approaches, and in good agreement with the exact results.

The continuous scaling of metal-oxide-semiconductor field-effect transistors (MOSFETs) has led to device geometries where charged carriers are increasingly confined to ever smaller channel cross sections. This development is associated with reduced screening of long-range Coulomb interactions. To accurately predict the behavior of such ultra-scaled devices, electron-electron (e-e) interactions must be explicitly incorporated in their quantum transport simulation. In this paper, we present an ab initio atomistic simulation framework based on density functional theory, the nonequilibrium Green's function formalism, and the self-consistent GW approximation to perform this task. The implementation of the method and its approximations are first tested with a carbon nanotube structure before being applied to calculate the transfer characteristics of a silicon nanowire MOSFET in a gate-all-around configuration. As a consequence of e-e scattering, the energy and spatial distribution of the carrier and the spectral current density both significantly change, while the on-current of the transistor decreases owing to the Coulomb repulsion between the electrons. Furthermore, we demonstrate how the resulting bandgap modulation of the nanowire channel as a function of the gate-to-source voltage could potentially improve device performance. This study reports large-scale atomistic quantum transport simulations of nanodevices under nonequilibrium conditions and in the presence of e-e interactions within the GW approximation.

In a density-imbalanced bilayer Wigner crystal, where the ratio of electron densities in separate layers deviates slightly from unity, defects spontaneously form in one or both layers in the ground state of the system. Due to quantum tunneling, these defects become mobile and the system becomes a defect liquid. Motivated by this idea, we numerically study the semiclassical energetics of individual and paired point defects in the bilayer Wigner crystal system. We use these results in combination with a simple defect model to map the phase diagram of the defect liquid as a function of electron density and interlayer distance. Our results should be relevant for present experimental bilayer Wigner crystal systems.

We analyze the resonance energy transfer (RET) rate between quantum emitters (QEs) near a phosphorene sheet under the effects of uniaxial strain. Using a low-energy tight-binding model, we describe the electronic structure of strained phosphorene in an experimentally feasible situation. Due to the anisotropic electronic structure of phosphorene, we demonstrate that the RET rate drastically depends on the direction in which the QEs are separated relative to the phosphorene lattice. More specifically, we obtain a large variation in the RET rate when the QEs are separated along the zigzag direction, in contrast to a rather small variation when separated along the armchair direction of phosphorene's crystalline structure. Furthermore, our results reveal that the RET rate can be highly modulated by uniaxial strain in phosphorene when considering emitters placed along the zigzag direction. Finally, by means of a simple toy model, we also show that this anisotropy in the RET rate is a general characteristic produced by anisotropic two-dimensional materials.

The boson peak was recently detected experimentally for the first time in a 2D material (bilayer silica). The experiment was performed using helium atom scattering. Here we apply helium atom scattering in further experimental investigations of the presence of the boson peak in 2D materials. We investigate two crystalline 2D materials (graphene and AB-bilayer graphene) as well as one amorphous 2D material: a 2D silica sample containing nanoscale holes, thus displaying a different mass density from the 2D silica investigated in the previously published experiment. The crystalline materials graphene and bilayer graphene show no indication of the presence of a boson peak. The boson peak in the bilayer silica containing a random distribution of holes is identified at $7\pm 1$ meV, which is within the error bars of the value for the first bilayer silica sample previously investigated.

The magnetic insulator ${\mathrm{Nd}}_3{\mathrm{BWO}}_9$ has been previously proposed to realize the highly frustrated breathing kagome lattice model. We report a combination of single-crystal neutron scattering studies and numerical simulations that debunk this interpretation. We show that it is the interplane couplings that determine the physics. To explain the exotic magnetism, we derive a simple one-dimensional Ising model composed of twisted triangular spin tubes, i.e., triple braids of Ising spin chains with almost-orthogonal anisotropy frames and competing ferro-antiferromagnetic interactions. This model can account for the ground state, excitations, the numerous field-induced fractional magnetization plateau phases, and incommensurate magnetic correlations at elevated temperatures. ${\mathrm{Nd}}_3{\mathrm{BWO}}_9$ constitutes a rare example where rich magnetic phenomena can be understood and simulated quantitatively in terms of a simple classical Hamiltonian.

The electromotive force transverse to a bias field under irradiation of circularly polarized light, namely the photovoltaic Hall response or light-induced anomalous Hall effect, has attracted considerable attention to investigate the topologically nontrivial states in Floquet engineering and the inverse spin Hall effect of spin-polarized carriers in spintronics. However, taking into account inversion symmetry breaking by the bias field, the circularly polarized light can excite photocarriers with asymmetric momentum distribution, which generates injection current transverse to the bias field. Therefore, the field-induced circular photogalvanic effect (FI-CPGE) should also emerge in the very same experimental configuration for light-induced anomalous Hall effect but has been overlooked in the literature. In this work, using terahertz pulses as a bias field for a semiconductor GaAs, we conduct two-dimensional Fourier analysis and demonstrate that the FI-CPGE can play a major role in the photovoltaic Hall response. Counterintuitively, the FI-CPGE is significantly enhanced when the photocarriers are excited near the band gap with small density of states and low group velocity, which can be explained by a three-level resonant nonlinear interaction near the band degeneracy point. We also clarified that the FI-CPGE would be further largely detected in the contact-type measurement using electrodes because of the absence of a filtering effect inherent to terahertz pulses. This work provides a comprehensive, generalized view of the photovoltaic Hall response in biased materials, paving a different avenue for detecting topological monopoles in momentum space hidden in equilibrium using third-order nonlinear responses.

Understanding the phase behavior of ammonia-water hydrates at high pressures is crucial for modeling the interiors of icy planets. In this study, the behavior of ammonia quarterhydrate [$4\mathrm{N}{\mathrm{H}}_3·{\mathrm{H}}_2\mathrm{O}$ (AQH)] has been measured using Raman spectroscopy and x-ray diffraction up to 33.8 GPa at room temperature. Our results reveal that AQH experienced four phase transitions. At 2.8 GPa, liquid AQH transforms into AQH-II, where ammonia and water molecules randomly occupy the lattice sites of the $P{2}_1/c$ space group. Within the 2.8–26.4-GPa range, the two twists in the axial ratio indicate that AQH undergoes two types of isostructural transitions. The first, at 14.6 GPa, AQH-II′ is caused by the rearrangement of hydrogen atoms on ammonia molecules. In the second, at 20.4 GPa, an ionic-molecular phase (AQH-II′ ionic) results from the ionization of ammonia molecules. Above 26.4 GPa, the AQH-II′ ionic phase transforms into the cubic disordered ionic-molecular alloy phase.

Despite being a pillar of quantum mechanics, little attention has been paid to the onset of the Fermi golden rule as a discrete microscopic bath of modes approaches the macroscopic thermodynamic limit and forms a continuum. Motivated by recent experiments in circuit quantum electrodynamics, we tackle this question through the lens of single-photon decay in a finite transmission line coupled to a qubit (“quantum impurity”). We consider a single-photon state, coupled via the nonlinear impurity to several baths formed by multiphoton states with different number of photons, which are inherently discrete due to the finite length of the line. We focus on the late-time dynamics of the single-photon, and uncover the conditions under which the photon decoherence rate approaches the decay rate predicted by the Fermi golden rule. We show that it is necessary to keep a small but finite escape rate (unrelated to the impurity) for each single-photon mode to obtain a finite long-time inelastic decay rate. We analyze the contribution of the baths formed by many-body states with different number of photons, and illustrate how the decay rate induced by some bath of $n$ photon states is enhanced by the presence of other baths of $m\ne n$ photon states, highlighting the contribution of cascade photon decay processes. Our formalism could be used to analyze recent experiments in superconducting circuits.

We report the propagation of fast and stable wave packets in classical non-Hermitian lattices, where the group velocity is controlled by the non-Hermiticity parameters, and can exceed that of the Hermitian counterpart. Specifically, we design a $\mathcal{PT}$-symmetric system that supports a constant group velocity increase, and which evolves into a nonconstant increase, i.e., an acceleration, when we induce spatial inhomogeneity. A dedicated relation between the control parameters keeps the lattice system dynamically stable for the achieved velocities. As a result, wave packets are propagating along the lattice with a nongrowing amplitude. We demonstrate the system experimentally in a topoelectrical metamaterial, where the non-Hermiticity is generated by embedded operational amplifiers in a feedback setup. Our design paves the way to new forms of fast and stable signal transmission, which can be directly controlled on demand.

Altermagnetism has attracted considerable attention for its remarkable combination of spin-polarized band structures and zero net magnetization, making it a promising candidate for spintronics applications. We demonstrate that this magnetic phase represents a case of unconventional magnetism, first proposed nearly two decades ago by one of the present authors as part of a broader framework for understanding Landau-Pomeranchuk instabilities in the spin channel, driven by many-body interactions. By systematically analyzing the altermagnetism in ${\mathrm{RuO}}_2$ with first-principles calculations, we reconcile conflicting experimental and theoretical reports by attributing it to ${\mathrm{RuO}}_2$'s proximity to a quantum phase transition. We emphasize the critical role of tuning parameters, such as the Hubbard $U$, hole doping, and epitaxial strain, in modulating quasiparticle interactions near the Fermi surface. This work provides fresh insights into the origin and tunability of altermagnetism in ${\mathrm{RuO}}_2$, highlighting its potential as a platform for investigating quantum phase transitions and the broader realm of unconventional magnetism.

Bismuth ferrite ($\mathrm{BiFe}{\mathrm{O}}_3$) is one of the rare materials that exhibits multiferroic properties already at room temperature. Therefore, it offers tremendous potential for future technological applications, such as in memory and logic devices. However, a weak magnetoelectric coupling together with the presence of a noncollinear cycloidal spin order restricts various practical applications of $\mathrm{BiFe}{\mathrm{O}}_3$. Therefore, there is a large interest in the search for suitable methods for the modulation of the spin cycloid in $\mathrm{BiFe}{\mathrm{O}}_3$. By performing neutron diffraction experiments using a triple-axis instrument we have determined that the spin cycloid can be systematically suppressed by applying a high magnetic field of 10 T in a $\mathrm{BiFe}{\mathrm{O}}_3$ thin film of about 100 nm grown on a (110)-oriented $\mathrm{SrTi}{\mathrm{O}}_3$ substrate. As predicted by theoretical calculations, we observed that the required critical magnetic field to suppress the spin cycloid in a $\mathrm{BiFe}{\mathrm{O}}_3$ thin film was lower as compared to the previously reported critical magnetic field for bulk $\mathrm{BiFe}{\mathrm{O}}_3$. Our experiment reveals that the spin cycloid continuously expands with increasing magnetic field before the complete transformation into a G-type antiferromagnetic spin order. Such tuning of the length of the spin cycloid up to a complete suppression offers further functionalities for future technological applications as in spintronics or magnonics.

Ternary metal boron carbides represent a fascinating class of materials that have attracted significant interest in materials science due to their unique combination of excellent electrically conductive and mechanical properties. The strong boron-carbon covalent framework within these compounds is a critical factor underlying their desirable characteristics. In this study, we have successfully predicted a series of low-enthalpy superconducting phases in Li-B-C systems using a particle swarm optimization algorithm in combination with first-principles calculations, including Imm2-$\mathrm{Li}{\mathrm{B}}_5{\mathrm{C}}_5$, Pba2-$\mathrm{Li}{\mathrm{B}}_6{\mathrm{C}}_4, \mathit{Cc}-\mathrm{Li}{\mathrm{B}}_7{\mathrm{C}}_3$ and $P2\text{/}m\text{−}\mathrm{Li}{\mathrm{B}}_8{\mathrm{C}}_2$. Strikingly, Imm2-$\mathrm{Li}{\mathrm{B}}_5{\mathrm{C}}_5$ demonstrates remarkable superconductivity that can be sustained at atmospheric pressure, with a relatively high critical temperature ($T_c$) of 33.8 K. This phenomenon arises from the strong $s{p}^3$ hybridization in the B-C tunnel sublattice, as well as the nested Fermi surface and phonon softening associated with the vibrations of lithium atoms within the channels. Furthermore, Imm2-$\mathrm{Li}{\mathrm{B}}_5{\mathrm{C}}_5$ exhibits excellent mechanical properties, with a high hardness value of 49.3 GPa attributed to the presence of the covalent B-C framework, indicating its potential applications as a superhard and superconducting material under specific conditions. This work opens avenues for the discovery of superconductors at ambient pressure within covalent B-C based materials.

Theoretically, the relative change of the Heisenberg-type nearest-neighbor coupling $J_1$ and next-nearest-neighbor coupling $J_2$ in a face-centered-cubic lattice can give rise to three main antiferromagnetic orderings: type I, type II, and type III. However, it is difficult to tune the $J_2/J_1$ ratio in real materials. Here, we report studies of the influence of ${\mathrm{Te}}^{6+}$ and ${\mathrm{W}}^{6+}$ ion replacement on the magnetic interactions and the magnetic ground states in the double-perovskite compounds ${\mathrm{Ba}}_{2}M{\mathrm{Te}}_{1-x}{\mathrm{W}}_x{\mathrm{O}}_6$ ($M$ = Mn, Co). For ${\mathrm{Ba}}_2{\mathrm{MnTe}}_{1-x}{\mathrm{W}}_x{\mathrm{O}}_6, {\mathrm{W}}^{6+}$ doping on the ${\mathrm{Te}}^{6+}$ site is successful in the range $0.02\le x\le 0.9$, with short-range orders of type I ($0.02\le x\le 0.08$) and type II ($0.1\le x\le 0.9$). In ${\mathrm{Ba}}_2{\mathrm{CoTe}}_{1-x}\mathrm{W}{}_x{\mathrm{O}}_6$, x-ray diffraction measurements reveal two crystal structures, including the trigonal phase ($0\le x\le 0.1$) and the cubic phase ($0.5\le x\le 1$), between which is a miscibility gap. Two magnetic transitions are identified in the trigonal phase due to two magnetic subsystems, and the type II magnetic order is observed in the cubic phase. Magnetic phase diagrams of ${\mathrm{Ba}}_{2}M{\mathrm{Te}}_{1-x}{\mathrm{W}}_x{\mathrm{O}}_6$ ($M$ = Mn, Co) are established. Our work shows that the magnetic interactions and ground states of ${\mathrm{Ba}}_{2}M{\mathrm{Te}}_{1-x}{\mathrm{W}}_x{\mathrm{O}}_6$ can be tuned effectively by the replacement of ${\mathrm{Te}}^{6+}$ with ${\mathrm{W}}^{6+}$ ions.

Recent theory-guided discoveries of hydrides exhibiting high superconducting temperatures ($T_{\mathrm{c}}$) exceeding 200 K above 150 GPa have sparked the enthusiasm for the pursuit of room-temperature ambient-pressure superconductors. While efforts to increase the $T_{\mathrm{c}}$'s of these hydrides are underway, optimizing the demanding high-pressure synthesis conditions remains a challenge. Several studies suggested that a multielemental alloying strategy could be a feasible approach to tackle this formidable task. Here, we present an example using $\mathrm{LaBe}{\mathrm{H}}_8$ to establish several alloy hydrides via the substitution of La atoms with similar metal atoms, such as Y, Ce, and Th. Quasiquaternary alloy hydride $\left(\mathrm{La},\mathrm{Y},\mathrm{Ce},\mathrm{Th}\right)\mathrm{Be}{\mathrm{H}}_8$ is computed to become thermodynamically stable at 70 GPa and a typically realistic temperature of ∼2000 K for synthesizing high-$T_{\mathrm{c}}$ hydrides, compared with the calculated results showing thermodynamic instability above 300 GPa without considering entropy. Our in-depth analysis revealed that not only does entropy play a crucial role in stabilizing alloy hydrides at moderate pressures and high temperatures, but also the energy contribution from vibrational entropy is much more significant than configurational entropy, where configurational entropy has long been believed to play a key role in the stability of multielemental alloy. Furthermore, extensive electron-phonon simulations indicate that the estimated $T_{\mathrm{c}}$ of $\left(\mathrm{La},\mathrm{Y},\mathrm{Ce},\mathrm{Th}\right)\mathrm{Be}{\mathrm{H}}_8$ could reach near ∼100 K at 70 GPa. The present work offers a fresh perspective on the design and realization of high-$T_{\mathrm{c}}$ multielemental alloy hydrides towards near-ambient pressure.

The recent surge of interest in moiré superlattices of twisted van der Waals compounds has spotlighted the emergence of unconventional superconductivity and novel electronic phases. However, the range of moiré phenomena can be dramatically expanded by incorporating complex oxide materials into twisted heterostructures. In this study, motivated by the recent breakthroughs in the synthesis of freestanding oxide membranes, we explore the emergent structural and electronic properties of twisted oxide bilayers. We focus on the classic perovskite oxide, $\mathrm{SrTi}{\mathrm{O}}_3$, and design $\mathrm{SrTi}{\mathrm{O}}_3$ bilayers with a relative twist between the individual layers. Using density functional theory calculations, we predict the appearance of vortex-antivortex polarization patterns at the interface of the $\mathrm{SrTi}{\mathrm{O}}_3$ bilayers driven by twist. We also predict charge modulation of the interfacial Ti ions induced by varying local coordination, which follow the moiré pattern. Furthermore, we forecast the emergence of flat bands at large twist angles and the associated localized electronic states with moiré-periodic charge density, originating from the interlayer bonding effects resulting in the formation of dangling bonds. Finally, we predict that hole doping induces unconventional $d^0$ magnetism in otherwise nonmagnetic $\mathrm{SrTi}{\mathrm{O}}_3$, driven by the exchange splitting of the high-density O-$p$ bands and producing the spin density with moiré periodicity. These results demonstrate a broad landscape of emergent phenomena that may occur in moiré-engineered oxide heterostructures showing far-reaching perspectives of these material systems for further fundamental studies and potential applications.