We revisit the low-energy physics of one-dimensional spinless fermion liquids, showing that with sufficiently strong interactions the conventional Luttinger liquid can give way to a strong-pairing phase. While the density fluctuations in both phases are described by a gapless Luttinger liquid, single fermion excitations are gapped only in the strong-pairing phase. Smooth spatial Interfaces between the two phases lead to topological degeneracies in the ground-state and low-energy spectrum of the density fluctuations. Using a concrete microscopic model, with both single-particle and pair hopping, we show that the strong-pairing state is established through emergence of a new low-energy fermionic mode. We characterize the two phases with numerical calculations using the density matrix renormalization group. In particular we find enhancement of the central charge from $c=1$ in the two Luttinger liquid phases to $c=3/2$ at the critical point, which gives direct evidence for an emergent critical Majorana mode. Finally, we confirm the existence of topological degeneracies in the low-energy phonon spectrum, associated with spatial interfaces between the two phases.

Magnetism in solids generally originates from the localized $d$ or $f$ orbitals that are hosted by heavy transition-metal elements. Here, we demonstrate a mechanism for designing a half-metallic $f$-orbital Dirac fermion from superlight $sp$ elements. Combining first-principles and model calculations, we show that bare and flat-band-sandwiched (FBS) Dirac bands can be created when ${\mathrm{C}}_{20}$ molecules are deposited into a two-dimensional hexagonal lattice, which are composed of $f$-molecular orbitals (MOs) derived from $sp$-atomic orbitals (AOs). Furthermore, charge doping of the FBS Dirac bands induces spontaneous spin polarization, converting the system into a half-metallic Dirac state. Based on this discovery, a model of a spin field effect transistor is proposed to generate and transport 100% spin-polarized carriers. Our finding illustrates a concept to realize exotic quantum states by manipulating MOs, instead of AOs, in orbital-designed molecular crystal lattices.

We derive a density matrix (DM) theory for quantum cascade lasers (QCLs) that describes the influence of scattering on coherences through a generalized scattering superoperator. The theory enables quantitative modeling of QCLs, including localization and tunneling effects, using the well-defined energy eigenstates rather than the ad hoc localized basis states required by most previous DM models. Our microscopic approach to scattering also eliminates the need for phenomenological transition or dephasing rates. We discuss the physical interpretation and numerical implementation of the theory, presenting sets of both energy-resolved and thermally averaged equations, which can be used for detailed or compact device modeling. We illustrate the theory's applications by simulating a high performance resonant-phonon terahertz (THz) QCL design, which cannot be easily or accurately modeled using conventional DM methods. We show that the theory's inclusion of coherences is crucial for describing localization and tunneling effects consistent with experiment.

We present a theoretical investigation of anisotropic $g$-factor tensors of single holes confined in droplet epitaxial GaAs/AlGaAs quantum dots under electrical and mechanical controls using the gauge-invariant discretization method within the framework of four-band Luttinger-Kohn $\vec{k}·\vec{p}$ theory. We reveal an intrinsic obstacle to realize the electrical sign reversal of the hole $g$-factors, being a key condition required for a full spin control in the scheme of $g$-tensor modulation, for the quantum dots solely with electrical bias control. Constructively, our studies show that, besides electrical gating, slightly stressing an inherently unstrained droplet epitaxial GaAs/AlGaAs quantum dot can offset the transverse hole $g$-factor to be nearly zero and make the electrical sign reversal of the hole $g$-factors feasible.

The interlayer interaction in graphene/boron-nitride heterostructures is studied using density functional theory calculations with the correction for van der Waals interactions. It is shown that the use of the experimental interlayer distance allows one to describe the potential energy surface at the level of more accurate but expensive computational methods. On the other hand, it is also demonstrated that the dependence of the interlayer interaction energy on the relative in-plane position of the layers can be fitted with high accuracy by a simple expression determined by the system symmetry. The use of only two independent parameters in such an approximation suggests that various physical properties of flat graphene/boron-nitride systems are interrelated and can be expressed through these two parameters. Here we estimate some of the corresponding physical properties that can be accessed experimentally, including the correction to the period of the moiré superstructure for the highly incommensurate ground state of the graphene/boron-nitride bilayer coming from the interlayer interaction, the width of stacking dislocations in slightly incommensurate systems of boron nitride on stretched graphene, and shear mode frequencies for commensurate graphene/boron-nitride systems, such as a flake on a layer. We propose that the commensurate-incommensurate phase transition can be observed in boron nitride on stretched graphene and that experimental measurements of the corresponding critical strain can be also used to gain insight into graphene/boron-nitride interactions.

Ferroelectric relaxors are complex materials with distinct properties. The understanding of their dielectric susceptibility, which strongly depends on both temperature and probing frequency, has been a challenge for researchers for many years. Here we report a macroscopic and phenomenological approach based on statistical modeling to investigate how the dielectric response of a relaxor depends on temperature. Employing the Maxwell-Boltzmann distribution and considering temperature-dependent dipolar orientational polarizability, we propose a minimum statistical model and specific equations to understand and fit numerical and experimental dielectric responses versus temperature. We show that the proposed formula can successfully fit the dielectric response of typical relaxors, including $\mathrm{Ba}(\mathrm{Zr},\mathrm{Ti}){\mathrm{O}}_3,0.87\mathrm{Pb}\left({\mathrm{Zn}}_{1/3}{\mathrm{Nb}}_{2/3}\right){\mathrm{O}}_3\text{−}0.13{\mathrm{PbTiO}}_3,0.95\mathrm{Pb}\left({\mathrm{Mg}}_{1/3}{\mathrm{Nb}}_{2/3}\right){\mathrm{O}}_3\text{−}0.05\mathrm{Pb}\left({\mathrm{Zr}}_{0.53}{\mathrm{Ti}}_{0.47}\right){\mathrm{O}}_3$, and Bi-based compounds, which demonstrates the general applicability of this approach.

We propose and numerically simulate a spintronic device layout consisting of a nanocontact on top of an extended Co/Pt bilayer. The interfacial Dzyaloshinskii-Moriya interaction in such bilayer systems can lead to the possible existence of metastable skyrmions. A small dc current injected through the nanocontact enables the manipulation of the size as well as the annihilation of an initially present skyrmion, while ps-long current pulses allow for the controlled nucleation of single skyrmions underneath the nanocontact. The results are obtained from micromagnetic simulations and can be potentially used for future magnetic storage implementations.

We derive the reciprocal cluster mean-field method to study the strongly interacting bosonic Harper-Hofstadter-Mott model. The system exhibits a rich phase diagram featuring band insulating, striped superfluid, and supersolid phases. Furthermore, for finite hopping anisotropy, we observe gapless uncondensed liquid phases at integer fillings, which are analyzed by exact diagonalization. The liquid phases at fillings $\nu =1,3$ exhibit the same band fillings as the fermionic integer quantum Hall effect, while the phase at $\nu =2$ is $\mathcal{C}\mathcal{T}$-symmetric with zero charge response. We discuss how these phases become gapped on a quasi-one-dimensional cylinder, leading to a quantized Hall response, which we characterize by introducing a suitable measure for nontrivial many-body topological properties. Incompressible metastable states at fractional filling are also observed, indicating competing fractional quantum Hall phases. The combination of reciprocal cluster mean-field and exact diagonalization yields a promising method to analyze the properties of bosonic lattice systems with nontrivial unit cells in the thermodynamic limit.

The use of proton beams in several modern technologies to probe or modify the properties of materials, such as proton beam lithography or ion beam cancer therapy, requires us to accurately know the extent to which the energy lost by the swift projectiles in the medium is redistributed radially around their tracks, since this determines several endpoints, such as the resolution of imaging or manufacturing techniques, or even the biological outcomes of radiotherapy. In this paper, the radial distribution of the energy deposited around swift-proton tracks in polymethylmethacrylate (PMMA) by the transport of secondary electrons is obtained by means of a detailed Monte Carlo simulation. The initial energy and angular distributions of the secondary electrons generated by proton impact, as well as the electronic cross sections for the ejection of these electrons, are reliably calculated in the framework of the dielectric formalism, where a realistic electronic excitation spectrum of PMMA is accounted for. The cascade of all secondary electrons generated in PMMA is simulated taking into account the main interactions that occur between these electrons and the condensed phase target. After analyzing the influence that several angular distributions of the electrons generated by the proton beam have on the resulting radial profiles of deposited energy, we conclude that the widely used Rudd and Kim formula should be replaced by the simpler isotropic angular distribution, which leads to radial energy distributions comparable to the ones obtained from more realistic angular distributions. By studying the dependence of the radial dose on the proton energy we recommend lower proton energies than previously published for reducing proximity effects around a proton track. The obtained results are of relevance for assessing the resolution limits of proton beam based imaging and manufacturing techniques.

We study non-Hermitian photonic lattices that exhibit competition between conservative and non-Hermitian (gain/loss) couplings. A bipartite sublattice symmetry enforces the existence of non-Hermitian flat bands, which are typically embedded in an auxiliary dispersive band and give rise to nondiffracting “compact localized states”. Band crossings take the form of non-Hermitian degeneracies known as exceptional points. Excitations of the lattice can produce either diffracting or amplifying behaviors. If the non-Hermitian coupling is fine-tuned to generate an effective $\pi$ flux, the lattice spectrum becomes completely flat, a non-Hermitian analog of Aharonov-Bohm caging in which the magnetic field is replaced by balanced gain and loss. When the effective flux is zero, the non-Hermitian band crossing points give rise to asymmetric diffraction and anomalous linear amplification.

We analyze a model of a double quantum dot Cooper pair splitter coupled to two ferromagnetic detectors and demonstrate the possibility of determination of spin correlation by current measurements. We use perturbation theory, taking account of the exchange interaction with the detectors, which leads to complex spin dynamics in the dots. This affects the measured spin and restricts the use of ferromagnetic detectors to the nonlinear current-voltage characteristic regime at the current plateau, where the relevant spin projection is conserved, in contrast to the linear current-voltage characteristic regime, in which the spin information is distorted. Moreover, we show that for separable states the spin correlation can only be determined in a limited parameter regime, much more restricted than in the case of entangled states. We propose an entanglement test based on the Bell inequality.

We systematically investigate the normal and superconducting properties of noncentrosymmetric ${\mathrm{Re}}_6\mathrm{Zr}$ using magnetization, heat capacity, and electrical resistivity measurements. Resistivity measurements indicate ${\mathrm{Re}}_6\mathrm{Zr}$ has poor metallic behavior and is dominated by disorder. ${\mathrm{Re}}_6\mathrm{Zr}$ undergoes a superconducting transition at $T_{\mathrm{c}}=\left(6.75\pm 0.05\right)$ K. Magnetization measurements give a lower critical field, ${\mu }_0{H}_{\mathrm{c}1}=\left(10.3\pm 0.1\right)$ mT. The Werthamer-Helfand-Hohenberg model is used to approximate the upper critical field ${\mu }_0{H}_{\mathrm{c}2}=\left(11.2\pm 0.2\right)\mathrm{T}$, which is close to the Pauli limiting field of 12.35 T and which could indicate singlet-triplet mixing. However, low-temperature specific-heat data suggest that ${\mathrm{Re}}_6\mathrm{Zr}$ is an isotropic, fully gapped $s$-wave superconductor with enhanced electron-phonon coupling. Unusual flux pinning resulting in a peak effect is observed in the magnetization data, indicating an unconventional vortex state.

We formulate an analytical recursive method to generate the wave function of doped short-range resonating valence bond (RVB) states as a tool to efficiently estimate multisite entanglement as well as other physical quantities in doped quantum spin ladders. We prove that doped RVB ladder states are always genuine multipartite entangled. Importantly, our results show that within specific doping concentration and model parameter regimes, the doped RVB state essentially characterizes the trends of genuine multiparty entanglement in the exact ground states of the Hubbard model with large on-site interactions, in the limit that yields the $t\text{−}J$ Hamiltonian.

We study the problem of injecting single electrons into interacting one-dimensional quantum systems, a fundamental building block for electron quantum optics. It is well known that such injection leads to charge and energy fractionalization. We elucidate this concept by calculating the nonequilibrium electron distribution function in the momentum and energy domains after the injection of an energy-resolved electron. Our results shed light on how fractionalization occurs via the creation of particle-hole pairs by the injected electron. In particular, we focus on systems with a pair of counterpropagating channels, and we fully analyze the properties of each chiral fractional excitation which is created by the injection. We suggest possible routes to access their energy and momentum distribution functions in topological quantum Hall or quantum spin-Hall edge states.

Excitonic effects play a particularly important role in the optoelectronic behavior of two-dimensional semiconductors. To facilitate the interpretation of experimental photoabsorption and photoluminescence spectra we provide (i) statistically exact diffusion quantum Monte Carlo binding-energy data for a Mott-Wannier model of (donor/acceptor-bound) excitons, trions, and biexcitons in two-dimensional semiconductors in which charges interact via the Keldysh potential, (ii) contact pair-distribution functions to allow a perturbative description of contact interactions between charge carriers, and (iii) an analysis and classification of the different types of bright trions and biexcitons that can be seen in single-layer molybdenum and tungsten dichalcogenides. We investigate the stability of biexcitons in which two charge carriers are indistinguishable, finding that they are only bound when the indistinguishable particles are several times heavier than the distinguishable ones. Donor/acceptor-bound biexcitons have similar binding energies to the experimentally measured biexciton binding energies. We predict the relative positions of all stable free and bound excitonic complexes of distinguishable charge carriers in the photoluminescence spectra of ${\mathrm{WSe}}_2$ and ${\mathrm{MoSe}}_2$.

Gradient metasurfaces have been extensively used in the past few years for advanced wave manipulation over a thin surface. These metasurfaces have been mostly designed based on the generalized laws of reflection and refraction. However, it was recently revealed that metasurfaces based on this approach tend to suffer from inefficiencies and complex design requirements. We have recently proposed a different approach to the problem of efficient beam steering using a surface, based on bianisotropic particles in a periodic array. Here, we show highly efficient reflective metasurfaces formed by pairs of isotropic dielectric rods, which can offer asymmetrical scattering of normally incident beams with unitary efficiency. Our theory shows that moderately broadband anomalous reflection can be achieved with suitably designed periodic arrays of isotropic nanoparticles. We also demonstrate practical designs using ${\mathrm{TiO}}_2$ cylindrical nanorods to deflect normally incident light toward a desired direction. The proposed structures may pave the way to a broader range of light management opportunities, with applications in energy harvesting, signaling, and communications.

We report on nonlinear transport measurements in a two-dimensional electron gas hosted in GaAs/AlGaAs heterostructures. Upon application of direct current, the low-temperature differential resistivity acquires a positive correction, which exhibits a pronounced maximum followed by a plateau. With increasing temperature, the nonlinearity diminishes and disappears. These observations can be understood in terms of a crossover from the Bloch-Grüneisen regime to the quasielastic scattering regime as the electrons are heated by direct current. Calculations considering the interaction of electrons with acoustic phonons provide a reasonable description of our experimental findings.

Electronic structures of electron- and hole-doped ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ and nondoped ${\mathrm{SrFe}}_2{\mathrm{As}}_2$ have been studied systematically by x-ray emission spectroscopy at Fe and As $K$-absorption edges. The electron and hole doping causes slight increase of the integrated absolute difference (IAD) values of the Fe $K\beta$ x-ray emission spectra which correlate to the local magnetic moment. Pressure decreases the IAD values and local magnetic moment, and induces the lower-spin states in these compounds. The pre-edge peak intensity of the XAS spectra at the Fe $K$-absorption edge increases with pressure in both compounds. This indicates an increase of the Fe $3d$–As $4p$ hybridization. It was found that pressure induced a discontinuous increase of the prepeak intensity of the PFY-XAS spectra at the As $K$-absorption edge at low pressures in the ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ systems. Our results may suggest that the Fe $3d$–As $4p$ hybridization plays a key role in suppressing the AFM order by the doping or pressure and fluctuation of the local magnetic moment and the electron-electron correlation may also play a role on the physical properties of the iron superconductors.

For a generic lattice Hamiltonian of the electron states in Weyl semimetals, we calculate the electric charge and current densities in the first order in background electromagnetic and strain-induced pseudoelectromagnetic fields. We show that the resulting expressions for the densities contain contributions of two types. The contributions of the first type coincide with those in the chiral kinetic theory. The contributions of the second type contain the information about the whole Brillouin zone and cannot be reproduced in the chiral kinetic theory. Remarkably, the latter coincide exactly with the Bardeen-Zumino terms that are usually introduced in relativistic quantum field theory in order to define the consistent anomaly. We demonstrate the topological origin of the Bardeen-Zumino (or, equivalently, Chern-Simons) corrections by expressing them in terms of the winding number in the lattice Hamiltonian model.