The dependence of the optical and electrical properties of two-dimensional transition metal dichalcogenides on the number of layers has garnered significant interest. In particular, the indirect-to-direct band gap transition and the resulting changes, such as improved quantum yield, have been widely studied. However, an experimental investigation of the dependence of the optical transition for a wide range of photon energies is still lacking. Here, we report the broadband optical response of large-area $\mathrm{MoS}{\mathrm{e}}_2$ grown from monolayer to pentalayer thicknesses by molecular beam epitaxy, for photon energies in the 0.9–5.5 eV range. We observed a dramatic evolution of the absorption spectrum that depends on the number of layers. Using the density functional theory, we show that this feature is related to a change in the energy and geometric shape of the band structure at the Г point in the Brillouin zone. The dependence of these optical properties on the number of layers yields insights into the underlying physics and is promising for photonic and optoelectronic applications.

The dependence of the excitonic photoluminescence (PL) spectrum of monolayer transition metal dichalcogenides (TMDs) on the tilt angle of an applied magnetic field is studied. Starting from a four-band Hamiltonian we construct a theory which quantitatively reproduces the available experimental PL spectra for perpendicular and in-plane magnetic fields. In the presence of a tilted magnetic field, we demonstrate that the dark exciton PL peaks brighten due to the in-plane component of the magnetic field and split for light with different circular polarizations as a consequence of the perpendicular component of the magnetic field. This splitting is more than twice as large as the splitting of the bright exciton peaks in tungsten-based TMDs. We propose an experimental setup that will allow for accessing the predicted splitting of the dark exciton peaks in the PL spectrum.

Parameter-free atomistic simulations of entangled solid-state paramagnetic defects may aid in the rational design of devices for quantum information science. This work applies time-dependent density functional theory (TDDFT) embedded-cluster simulations to a prototype entangled-defect system, namely two adjacent singlet-coupled $F$ color centers in lithium fluoride. TDDFT calculations accurately reproduce the experimental visible absorption of both isolated and coupled $F$ centers. The most accurate results are obtained by combining spin symmetry breaking to simulate strong correlation, a large fraction of exact (Hartree-Fock-like) exchange to minimize the defect electrons' self-interaction error, and a standard semilocal approximation for dynamical correlations between the defect electrons and the surrounding ionic lattice. These results motivate application of two-reference correlated ab initio approximations to the M-center, and application of TDDFT in parameter-free simulations of more complex entangled paramagnetic defect architectures.

We propose a general method for constructing system-dependent basis functions for correlated quantum calculations. Our construction combines features from several traditional approaches: plane waves, localized basis functions, and wavelets. In a one-dimensional mimic of Coulomb systems, it requires only 2–3 basis functions per electron to achieve high accuracy, and reproduces the natural orbitals. We illustrate its effectiveness for molecular energy curves and chains of many one-dimensional atoms. We discuss the promise and challenges for realistic quantum chemical calculations.

Substrate strain offers an extra dimension in enriching the functionality of perovskite thin films. The recent observation of the coexistence of ferromagnetism (FM) and antiferromagnetism (AFM) in the ${\mathrm{LuMnO}}_3$ thin films on (110)-oriented ${\mathrm{YAlO}}_3$ substrate illustrated another interesting example [Phys. Rev. Lett. 111, 037201 (2013); Phys. Rev. B 94, 054423 (2016)]. The FM signal arises from a thin atomic layer near the ${\mathrm{YAlO}}_3$ substrate. We propose in this paper that vertical strain relaxation may stabilize the FM structure in the most compressively strained region near the substrate, while the AFM structure is stabilized in less compressively strained outer surfaces of thin films. This physical picture was supported by the magnetic phase diagram vs compressive strain derived from comprehensive first-principles calculations. The critical $\left[1\overline{1}0\right]$ strain separating E-AFM and FM structures is $\epsilon \approx -0.0425$, which is close to the value $\epsilon \approx -0.047$ dictated by ${\mathrm{YAlO}}_3$ substrate. From the half-metal feature of the FM structure and ferroelectric property of E-AFM structure, the greatly reduced ferroelectric polarization and averaged magnetic moment of $0.5{\mu }_B/\mathrm{Mn}$ can be understood.

By using the local-density approximation + dynamical mean-field theory approach, we study the low-energy electronic properties and the optical conductivity of the layered ruthenates ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ and ${\mathrm{Sr}}_3{\mathrm{Ru}}_2{\mathrm{O}}_7$. We study the interplay of spin-orbit, crystal-field, and Coulomb interactions, including the tetragonal terms of the Coulomb tensor. We show that the spin-orbit interaction is multifaced; depending on the parameter regime, filling, and temperature, it can either enhance or reduce the effective strength of correlations. We compare the results based on the two common approximations for the screened Coulomb parameters, the constrained random-phase approximation (cRPA) and the constrained local-density approximation. We show that the experimental Drude peak is better reproduced by the cRPA parameters, hinting to relatively small mass renormalizations. We find that including the spin-orbit interaction is, however, important, for a realistic description. We show that Coulomb terms with tetragonal $D_{4h}$ symmetry have a strong effect on the mass-enhancement anisotropy, but they do not affect sizably the total spectral function or the in-plane conductivity.

The Kronig-Penney model, an exactly solvable one-dimensional model of crystal in solid physics, shows how the allowed and forbidden bands are formed in solids. In this paper, we study this model in the presence of both strong spin-orbit coupling and the Zeeman field. We analytically obtain four transcendental equations that represent an implicit relation between the energy and the Bloch wave vector. Solving these four transcendental equations, we obtain the spin-orbital bands exactly. In addition to the usual band gap opened at the boundary of the Brillouin zone, a much larger spin-orbital band gap is also opened at some special sites inside the Brillouin zone. The $x$ component of the spin-polarization vector is an even function of the Bloch wave vector, while the $z$ component of the spin-polarization vector is an odd function of the Bloch wave vector. At the band edges, the optical transition rates between adjacent bands are nonzero.

Using first-principles calculations, we systematically study the elastic stiffness constants, mechanical properties, elastic wave velocities, Debye temperature, melting temperature, and specific heat of several thermodynamically stable crystal structures of ${\mathrm{Bi}}_x{\mathrm{Sb}}_{1-x}$ ($0<x<1$) binaries, which are of great interest due to their numerous inherent rich properties, such as thermoelectricity, thermomagnetic cooling, strong spin-orbit coupling (SOC) effects, and topological features in the electronic band structure. We analyze the bulk modulus ($B$), Young's modulus ($E$), shear modulus ($G$), $B/G$ ratio, and Poisson's ratio ($\nu$) as a function of the Bi concentration in ${\mathrm{Bi}}_x{\mathrm{Sb}}_{1-x}$. The effect of SOC on the above-mentioned properties is further investigated. In general, we observe that the SOC effects cause elastic softening in most of the studied structures. Three monoclinic structures of Bi-Sb binaries are found to exhibit significantly large auxetic behavior due to the hingelike geometric structure of bonds. The Debye temperature and the magnitude of the elastic wave velocities monotonically increase with increasing Sb concentration. However, anomalies were observed at very low Sb concentration. We also discuss the specific-heat capacity versus temperature data for all studied binaries. Our theoretical results are in excellent agreement with the existing experimental and theoretical data. The comprehensive understanding of the material properties such as hardness, mechanical strength, melting temperature, propagation of the elastic waves, auxeticity, and heat capacity is vital for practical applications of the studied binaries.

Tin titanate ($\mathrm{SnTi}{\mathrm{O}}_3$) has been notoriously impossible to prepare as a thin-film ferroelectric, probably because high-temperature annealing converts much of the $\mathrm{S}{\mathrm{n}}^{2+}$ to $\mathrm{S}{\mathrm{n}}^{4+}$. In the present paper, we show two things: first, perovskite phase $\mathrm{SnTi}{\mathrm{O}}_3$ can be prepared by atomic-layer deposition directly onto $p$-type Si substrates; and second, these films exhibit ferroelectric switching at room temperature, with $p$-type Si acting as electrodes. X-ray diffraction measurements reveal that the film is single-phase, preferred-orientation ferroelectric perovskite $\mathrm{SnTi}{\mathrm{O}}_3$. Our films showed well-saturated, square, and repeatable hysteresis loops of around $3\mu \mathrm{C}/\mathrm{c}{\mathrm{m}}^2$ remnant polarization at room temperature, as detected by out-of-plane polarization versus electric field and field cycling measurements. Furthermore, photovoltaic and photoferroelectricity were found in $\mathrm{Pt}/\mathrm{SnTi}{\mathrm{O}}_3/\mathrm{Si}/\mathrm{SnTi}{\mathrm{O}}_3/\mathrm{Pt}$ heterostructures, the properties of which can be tuned through band-gap engineering by strain according to first-principles calculations. This is a lead-free room-temperature ferroelectric oxide of potential device application.

The magnetovolume effects (MVEs) of $\mathrm{M}{\mathrm{n}}_3\mathrm{G}{\mathrm{a}}_{1-x}\mathrm{S}{\mathrm{n}}_x\mathrm{N}$ antiperovskite compounds have been investigated by means of neutron powder diffraction. Increasing the Sn-doping content at the Ga site leads to the broadening of the magnetic phase transition temperature range and the thermal expansion behavior changes from negative to positive. We establish the relationship between the square of the ordered magnetic moment $m^2$ and the volume variation $\mathrm{\Delta }{\omega }_m$ for the antiferromagnetic phase (${\mathrm{\Gamma }}^{5g}$ magnetic structure with rhombohedral symmetry $R\overline{3}m$). The temperature variations of $\mathrm{\Delta }{\omega }_m\left(T\right), m^2\left(T\right)$ and the magnetoelastic coupling constant $C\left(T\right)$ are also quantitatively analyzed according to the itinerant-electron theory. Moreover, the increase of the phonon contribution to the thermal expansion induced by Sn doping and the corresponding decrease of dm/dT are revealed to be the key parameters for tuning the MVEs. Our results allow elucidating and quantifying the mechanism of the spin-lattice coupling and can be used to design magnetic functional materials with controlled thermal expansion behaviors for specific applications.

We study the electron and phonon thermalization in simple metals excited by a laser pulse. The thermalization is investigated numerically by solving the Boltzmann equation taking into account the relevant scattering mechanism: electron-electron, electron-phonon (e-ph), phonon-electron (ph-e), and phonon-phonon (ph-ph) scattering. In the initial stage of the relaxation, most of the excitation energy is transferred from the electrons to phonons through the e-ph scattering. This creates hot high-frequency phonons due to the ph-e scattering, followed by an energy redistribution between phonon subsystems through the ph-ph scattering. This yields an overshoot of the total longitudinal-acoustic phonon energy at a time, across which a crossover occurs from a nonequilibrium state, where the e-ph and ph-e scattering frequently occur, to a state, where the ph-ph scattering occurs to reach a thermal equilibrium. This picture is quite different from the scenario of the well-known two-temperature model (2TM). The behavior of the relaxation dynamics is compared with those calculated by several models, including the 2TM, the four-temperature model, and nonequilibrium electron or phonon models. The relationship between the relaxation time and the initial distribution function is also discussed.

We consider the Abelian-Higgs model in $2+1$ dimensions with instanton-monopole defects. This model is closely related to the phases of quantum antiferromagnets. In the presence of ${\mathbb{Z}}_2$ preserving monopole operators, there are two confining ground states in the monopole phase, corresponding to the valence bond solid (VBS) phase of quantum magnets. We show that the domain wall carries a 't Hooft anomaly in this case. The anomaly can be saturated by, e.g., charge-conjugation breaking on the wall or by the domain wall theory becoming gapless (a gapless model that saturates the anomaly is $SU{\left(2\right)}_1$ WZW). Either way the fundamental scalar particles (i.e., spinons) which are confined in the bulk are deconfined on the domain wall. This ${\mathbb{Z}}_2$ phase can be realized either with spin-1/2 on a rectangular lattice or spin-1 on a square lattice. In both cases the domain wall contains spin-1/2 particles (which are absent in the bulk). We discuss the possible relation to recent lattice simulations of domain walls in VBS. We further generalize the discussion to Abrikosov-Nielsen-Olsen (ANO) vortices in a dual superconductor of the Abelian-Higgs model in $3+1$ dimensions and to the easy-plane limit of antiferromagnets. In the latter case the wall can undergo a variant of the BKT transition (consistent with the anomalies) while the bulk is still gapped. The same is true for the easy-axis limit of antiferromagnets. We also touch upon some analogies to Yang-Mills theory.

Domain wall displacement in Co/Pt thin films induced by not only femtosecond but also picosecond laser pulses is demonstrated using time-resolved magneto-optical Faraday imaging. We evidence multipulse helicity-dependent laser-induced domain wall motion in all-optical switchable Co/Pt multilayers with a laser energy below the switching threshold. Domain wall displacement of $\sim 2$ nm per 2-ps pulse is achieved. By investigating separately the effect of linear and circular polarization, we reveal that laser-induced domain wall motion results from a complex interplay between pinning, temperature gradient, and helicity effect. Then, we explore the microscopic origin of the helicity effect acting on the domain wall. These experimental results enhance the understanding of the mechanism of all-optical switching in ultrathin ferromagnetic films.

We report first-principles investigations of magnetocrystalline anisotropy energy (MCAE) oscillations as a function of capping layer thickness in Heusler alloy ${\mathrm{Co}}_2\mathrm{FeAl}/\mathrm{Ta}$ heterostructures. A substantial oscillation is observed in the FeAl-interface structure. According to $k$-space and band-decomposed charge-density analyses, this oscillation is mainly attributed to the Fermi-energy-vicinal quantum well states (QWSs) which are confined between the ${\mathrm{Co}}_2\mathrm{FeAl}/\mathrm{Ta}$ interface and Ta/vacuum surface. The smaller oscillation magnitude in the Co-interface structure can be explained by the smooth potential transition at the interface. These findings clarify that MCAE in ${\mathrm{Co}}_2\mathrm{FeAl}/\mathrm{Ta}$ is not a local property of the interface and that the quantum well effect plays a dominant role in MCAE oscillations of the heterostructures. This work presents the possibility of tuning MCAE by QWSs in capping layers and paves the way for artificially controlling magnetic anisotropy energy in magnetic tunnel junctions.

We present evidence of coexistence of electron paramagnetic resonance signal and antiferromagnetic resonance signals above the antiferromagnetic (AFM) transition $\left(T_N\right)$ in ${\mathrm{Pr}}_{0.5}{\mathrm{Ca}}_{0.5}{\mathrm{MnO}}_3$. We identify the latter with AFM rare regions within the “Griffiths-like” phase scenario with the associated temperature scale $T^{*}$ extending above room temperature.

We develop a theoretical approach for modeling a wide range of semiconductor-superconductor structures with arbitrary potential barriers and a spatially dependent superconducting order parameter. We demonstrate asymmetry in the conductance spectrum as a result of a Schottky barrier shape. We further show that the Andreev reflection process can be significantly enhanced through resonant tunneling with appropriate barrier configuration, which can incorporate the Schottky barrier as a contributing component of the device. Moreover, we show that resonant tunneling can be achieved in superlattice structures as well. These theoretically demonstrated effects along with our modeling approach enable much more efficient Cooper pair injection into semiconductor-superconductor structures, including superconducting optoelectronic devices.

In many superconducting devices, including qubits, quasiparticle excitations are detrimental. A normal metal $\left(N\right)$ in contact with a superconductor $\left(S\right)$ can trap these excitations; therefore, such a trap can potentially improve the device's performances. The two materials influence each other, a phenomenon known as proximity effect which has drawn attention since the 1960's. Here, we study whether this mutual influence places a limitation on the possible performance improvement in superconducting qubits. We first revisit the proximity effect in uniform $NS$ bilayers; we show that the density of states is of the Dynes type above the minigap. We then extend our results to describe a nonuniform system in the vicinity of a trap edge. Using these results together with a phenomenological model for the suppression of the quasiparticle density due to the trap, we find in a transmon qubit an optimum trap-junction distance at which the qubit relaxation rate is minimized. This optimum distance, of the order of 4 to 20 coherence lengths, originates from the competition between proximity effect and quasiparticle density suppression. We conclude that the harmful influence of the proximity effect can be avoided so long as the trap is farther away from the junction than this optimum.

We study magnetic pair breaking due to Mn impurities in the optimally electron-doped superconductor $\mathrm{Sr}{\left({\mathrm{Fe}}_{0.88}{\mathrm{Co}}_{0.12}\right)}_2{\mathrm{As}}_2$. We found that the as-grown $\mathrm{Sr}{\left({\mathrm{Fe}}_{0.88-y}{\mathrm{Co}}_{0.12}{\mathrm{Mn}}_y\right)}_2{\mathrm{As}}_2$ single crystals exhibit a $T_c$ suppression rate of $\sim 30$ mK/$\mu \mathrm{\Omega }\text{cm}$. This rate is slow but in good agreement with the previous reports on various magnetic/nonmagnetic impurities doped in other structurally analogous iron-based superconductors. The slow $T_c$ suppression rate for magnetic impurities is often cited as an evidence for the nonvalidity of the $s_{++}$-wave symmetry, which should have suppressed $T_c$ in accordance with the Abrikosov-Gor'kov theory. Here, we show that the crystallographic defects are the main source of pair breaking in the as-grown crystals. Once these defects are healed by a low-temperature annealing, the true $T_c$ suppression rate due to Mn impurities is revealed. We thus estimate the actual $T_c$ suppression rate due to Mn alone to be $\ge 325 \mathrm{mK}/\mu \mathrm{\Omega }\text{cm}$, and that due to the nonmagnetic crystallographic defects to be nearly 35 mK/$\mu \mathrm{\Omega }\text{cm}$. These findings can be reconciled with the fully gapped $s_{+-}$-wave symmetry provided the interband scattering is rather weak. On the other hand, the $s_{++}$-wave symmetry, which is resilient to the nonmagnetic defects and fragile against the magnetic impurities, can be a possible pairing symmetry in the optimally electron-doped ${\mathrm{SrFe}}_2{\mathrm{As}}_2$. The crucial information that we provide here is that the magnetic pair breaking in these superconductors is not as weak as is generally believed.

The physics of quantum impurities coupled to a many-body environment is among the most important paradigms of condensed-matter physics. In particular, the formation of polarons, quasiparticles dressed by the polarization cloud, is key to the understanding of transport, optical response, and induced interactions in a variety of materials. Despite recent remarkable developments in ultracold atoms and solid-state materials, the direct measurement of their ultimate building block, the polaron cloud, has remained a fundamental challenge. We propose and analyze a platform to probe time-resolved dynamics of polaron-cloud formation with an interferometric protocol. We consider an impurity atom immersed in a two-component Bose-Einstein condensate where the impurity generates spin-wave excitations that can be directly measured by the Ramsey interference of surrounding atoms. The dressing by spin waves leads to the formation of magnetic polarons and reveals a unique interplay between few- and many-body physics that is signified by single- and multi-frequency oscillatory dynamics corresponding to the formation of many-body bound states. Finally, we discuss concrete experimental implementations in ultracold atoms.