We study random dense packings of Heisenberg dipoles by numerical simulation. The dipoles are at the centers of identical spheres that occupy fixed random positions in space and fill a fraction $\mathrm{\Phi }$ of the spatial volume. The parameter $\mathrm{\Phi }$ ranges from rather low values, typical of amorphous ensembles, to the maximum $\mathrm{\Phi }=0.64$ that occurs in the random-close-packed limit. We assume that the dipoles can freely rotate and have no local anisotropies. As well as the usual thermodynamical variables, the physics of such systems depends on $\mathrm{\Phi }$. Concretely, we explore the magnetic ordering of these systems in order to depict the phase diagram in the temperature-$\mathrm{\Phi }$ plane. For $\mathrm{\Phi }\gtrsim 0.49$ we find quasi-long-range ferromagnetic order coexisting with strong long-range spin-glass order. For $\mathrm{\Phi }\lesssim 0.49$ the ferromagnetic order disappears giving way to a spin-glass phase similar to the ones found for Ising dipolar systems with strong frozen disorder.

Spin-wave-based circuits and logic devices have been considered as an alternative to current electronic devices as they approach the physical limit of miniaturization. Asymmetrical propagation of spin waves, also known as nonreciprocity, provides an additional degree of freedom to these spin-wave-based devices, increasing their flexibility. In thin films, nonreciprocity can be induced by the Dzyaloshinskii-Moriya interaction (DMI) at heavy-metal/ferromagnet bilayers, and by the dipolar coupling in multilayers. Here, we show that in an antiferromagnetically coupled multilayer with interfacial DMI, the frequency nonreciprocity induced by the DMI is enhanced when both heavy metals are the same as long as the multilayer remains in an antiparallel state. Furthermore, we show that the interplay between the dipolar and Dzyaloshinskii-Moriya interaction enhances the nonreciprocity of one oscillation mode and reduces the nonreciprocity of the other. Which mode is enhanced depends on the sign of the induced Dzyaloshinskii-Moriya interaction at the interfaces and the magnetic moments of the layers. Finally, we show that it is possible to change the frequency nonreciprocity of Pt/Co/Cu/Co/Pt and Pt/Co/Cu/Py/Pt multilayers by $\sim 7$ GHz when applying an in-plane magnetic field of 130 mT. This includes a change in the sign of the nonreciprocity, which could be used to control the direction of the flux of information in spin-wave devices.

Future spintronic devices based on skyrmions will require precise control of the skyrmion motion. We show that this goal can be achieved through the use of magnetic antidot arrays. We perform micromagnetic simulations and semianalytical calculations based on the Thiele equation, where the skyrmion motion is driven by applied electric current via spin transfer torque (STT) or spin orbit torque (SOT) mechanism. For both torque mechanisms we demonstrate that an antidot array can guide the skyrmions in different directions depending on the parameters of the applied current pulse. Despite the fixed direction of the net driving current, due to the nontrivial interplay between the repulsive potential introduced by the antidots, the skyrmion Hall effect, and the nonuniform current distribution, full control of skyrmion motion in a 2D lattice can be achieved. Moreover, we demonstrate that the direction of skyrmion motion can be controlled by tuning only a single parameter of the current pulse, i.e., current magnitude.

We have investigated the electronic and magnetic properties of the pyrochlore iridates ${\mathrm{Y}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$, ${\mathrm{Pr}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$, and ${\mathrm{Eu}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$ within the density-functional theory (DFT) using the generalized gradient approximation with taking into account strong Coulomb correlations ($\mathrm{GGA}+U$) in the framework of the fully relativistic spin-polarized Dirac linear muffin-tin orbital band-structure method. The x-ray absorption spectra (XAS) and x-ray magnetic circular dichroism (XMCD) at the Ir $L_{2,3}$, Pr $L_{2,3}$, Pr $M_{4,5}$, and O $K$ edges in the pyrochlore iridate ${\mathrm{Pr}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$ have been investigated theoretically from first principles. The calculated results are in good agreement with experimental data. We have also investigated theoretically the resonant inelastic x-ray scattering (RIXS) spectra at the Ir $L_3$ edge in the pyrochlore iridates ${\mathrm{R}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$ (R = Y, Pr, and Eu). We have found that these materials exhibit qualitatively similar excitation spectra. The experimentally measured RIXS spectra of all three compounds in addition to the elastic scattering peak at 0 eV possess sharp features below 2 eV corresponding to transitions within the Ir $t_{2g}$ levels, and a strong intense peak stretching from 2.5 to 5 eV that according to the calculations corresponds to $d\text{−}d$ transitions between the Ir $t_{2g}$ and $e_g$ manifolds. We have found that the double peak at low energy can be assigned to local excitations between the filled $J_{\text{eff}}=1/2$ and empty $J_{\text{eff}}=1/2$ manifolds; the latter states are split into two subbands.

We study the quantum phase transition between the superfluid and valence bond solid in easy-plane J-Q models on the square lattice. The Hamiltonian we study is a linear combination of two model Hamiltonians: (1) an SU(2) symmetric model, which is the well known J-Q model that does not show any direct signs of a discontinuous transition on the largest lattices and is presumed continuous and (2) an easy-plane version of the J-Q model, which shows clear evidence for a first-order transition even on rather small lattices of size $L\approx 16$. A parameter $0\le \lambda \le 1$ [$\lambda =0$ being the easy-plane model and $\lambda =1$ being the SU(2) symmetric J-Q model] allows us to smoothly interpolate between these two limiting models. We use stochastic series expansion (SSE) quantum Monte Carlo (QMC) to investigate the nature of this transition as $\lambda$ is varied—here we present studies for $\lambda =0,0.5,0.75,0.85,0.95$, and 1. While we find that the first-order transition weakens as $\lambda$ is increased from 0 to 1, we find no evidence that the transition becomes continuous until the SU(2) symmetric point, $\lambda =1$. We thus conclude that the square-lattice superfluid-VBS transition in the two-component easy-plane model is generically first order.

We report on the precise determination, in ${\mathrm{Ag}}_2\mathrm{Zn}\mathrm{Sn}{\mathrm{Se}}_4$ epitaxial layer, of both the band gap $E_{\text{g}}$ and the characteristic Urbach energy $U$ that describes the density of localized, defect states in the gap. Various origins for these defect band tail states have been considered, together with the corresponding modeling for their density of states, in order to fit the whole of the optical spectral data. The interest of the methodology developed here is to account quantitatively not only for the absorption and steady-state photoluminescence data but also for the time-resolved photoluminescence spectra. We compare the different origins of localized band tail states to select the standard textbook, Urbach tail model that corresponds to short-range band gap fluctuations. Such an approach is different from the one most often used to evaluate the energy extent of the localized states, which is the Stokes shift between the energies of the photoluminescence emission and the absorption threshold. The advantage of the present method is that no arbitrary choice of the low power excitation has to be done to select the photoluminescence emission spectrum and its peak energy. Thanks to this systematic study of both photoluminescence excitation and time-resolved photoluminescence spectra at low temperature (6 K), the values $E_{\text{g}}=1226\pm 5$ meV and $U=20\pm 3$ meV are found for this promising absorber for thin films photovoltaics.

By means of muon spin relaxation measurements we unraveled the temperature spin dynamics in monodisperse maghemite spherical nanoparticles with different surface to volume ratio, in two samples with a full core (diameter $D\sim 4$ and $D\sim 5\mathrm{nm}$) and one with a hollow core (external diameter $D\sim 7.4\mathrm{nm}$). The behavior of the muon longitudinal relaxation rates as a function of temperature allowed us to identify two distinct spin dynamics. The first is well witnessed by the presence of a characteristic peak for all the samples around the so-called muon blocking temperature $T_B^{\mu +}$. A Bloembergen-Purcell-Pound (BPP)-like model reproduces the experimental data around the peak and at higher temperatures $\left(20<T<100\mathrm{K}\right)$ by assuming the Néel reversal time of the magnetization as the dominating correlation time. An additional dynamic emerges in the samples with higher surface to volume ratio, namely, full 4 nm and hollow samples. This is witnessed by a shoulder of the main peak for $T<20\mathrm{K}$ at low longitudinal field (${\mu }_{0}H\approx 15\mathrm{mT})$, followed by an abrupt increase of the relaxation rate at $T<10\mathrm{K}$, which is more evident for the hollow sample. These unusual anomalies of the longitudinal relaxation rate for $T<T_B^{\mu +}$ are suggested to be due to the surface spins’ dynamical behavior. Furthermore, for weak applied longitudinal magnetic field (${\mu }_{0}H\approx 15\mathrm{mT}$) and $T<T_B^{\mu +}$ we observed damped coherent oscillations of the muon asymmetry, which are a signature of a quasistatic local field at the muon site as probed by muons implanted in the inner magnetic core of the nanoparticles. The muon spin relaxation technique turns out to be very successful to study the magnetic behavior of maghemite nanoparticles and to detect their unusual local spin dynamics in low magnetic field conditions.

Strongly coupled light-matter systems can carry information over long distances and realize low-threshold polariton lasing, condensation, and superfluidity. These systems are highly nonequilibrium in nature, so constant nonzero fluxes manifest themselves even at the steady state and are set by a complicated interplay between nonlinearity, dispersion, pumping, dissipation, and interactions between the various constituents of the system. Based on the mean-field governing equations of lasers or polariton condensates, we develop a method for engineering and controlling the velocity profiles by manipulating the spatial pumping and dissipation in the system. We present analytically exact pumping and dissipation profiles that lead to a large variety of spatially periodic density and velocity profiles. Besides these, any physically relevant velocity profiles can be engineered by finding the stationary state of the conservative nonlinear Schrödinger equation in an external potential related to the velocity. Our approach opens the way to the controllable implementation of laser or polariton flows for ultrafast information processing, integrated circuits, and analog simulators.

The success of all-graphene electronics is severely hindered by the challenging realization and subsequent integration of semiconducting channels and metallic contacts. Here, we comprehensively investigate the electronic transport across width-modulated heterojunctions consisting of a graphene quantum dot of varying lengths and widths embedded in a pair of armchair-edged metallic nanoribbons, of the kind recently fabricated via on-surface synthesis. We show that the presence of the quantum dot enables the opening of a width-dependent transport gap, thereby yielding built-in one-dimensional metal-semiconductor-metal junctions. Furthermore, we find that, in the vicinity of the band edges, the conductance is subject to a smooth transition from an antiresonant to a resonant transport regime upon increasing the channel length. These results are rationalized in terms of a competition between quantum-confinement effects and quantum dot-to-lead coupling. Overall, our work establishes graphene quantum dot nanoarchitectures as appealing platforms to seamlessly integrate gap-tunable semiconducting channels and metallic contacts into an individual nanoribbon, hence realizing self-contained carbon-based electronic devices.

We analyze several methods of obtaining the accurate relativistic total energy (TE) variations using traditional perturbation theories (PTs) and proposed coupling constant integration (CCI) methods. For this purpose, we perform benchmark calculations within the density functional theory taking the spin-orbit coupling (SOC) and its derivative as a perturbation. The TE change due to SOC addition obtained from both PTs and CCI is shown to reach the accuracy of fully self-consistent TE calculations. Similar accuracy is also obtained even for the magnetocrystalline anisotropy energy (MAE). The real advantage of the proposed methods is to use PTs and CCI methods in those electronic structure methods where accurate total energies currently cannot be obtained with required accuracy. Correspondingly, we demonstrate the applicability of suggested methods for calculations of MAE in different magnetic materials using a dynamic mean-field method. All suggested PTs and CCI methods also provide convenient site, orbital, and spin decompositions of the TE variation, creating a powerful way to analyze microscopic physics in strongly correlated materials.

The genesis of the Curie-Weiss magnetic response observed in most transition metals that are Fermi liquids at low temperatures has been an enigma for decades and has not yet been fully explained from microscopic principles. We show on the single-impurity Anderson model how the quantum dynamics of strong electron correlations leads to the Curie-Weiss magnetic susceptibility sufficiently above the Kondo temperature. Such behavior has not yet been demonstrated and can be observed only when the bare interaction is substantially screened (renormalized) and a balance between quantum and thermal fluctuations is kept. We set quantitative criteria for the existence of the Curie-Weiss law.

We performed density-functional calculations to estimate the formation energies of intermetallic alloys. We used two semilocal approximations, the generalized gradient approximation (GGA) by Perdew-Burke-Ernzerhof (PBE), and the strongly constrained and appropriately normed (SCAN) meta-GGA. In addition, we utilized two nonlocal DFT functionals, the hybrid HSE06, and the state-of-the-art random phase approximation (RPA). The nonlocal functionals such as HSE06 and RPA yield accurate formation energies of binary alloys with completely filled $d$-band metals, where semilocal functionals underperform. The accuracy of the nonlocal functionals is greatly reduced when a partially filled $d$-band metal is present in an alloy, while PBE-GGA outperforms in these cases. We show that the accurate prediction of formation energies by any DFT method depends on its ability to predict the accurate electronic properties, e.g., valence $d$-band contribution to the density of states (DOS). The SCAN meta-GGA often corrects the PBE-DOS, however, it does not provide accurate formation energies compared to PBE. This is assumed to be due to the lack of proper error cancellation that should be expected due to the similar bulk nature of both alloys and their constituents, which may improve with the modification of meta-GGA ingredients. RPA yields too-negative formation energies of alloys with partially filled $d$-band metals. RPA results can be corrected by restoring the exchange-correlation kernel, thereby improving the short-range electron-electron correlation in metallic densities.

Two-dimensional ($2D$) layered semiconductors with both ultrawide band gap and strong second-harmonic generation (SHG) are essential for expanding the nonlinear optical (NLO) applications to deep-ultraviolet (DUV) region in nanoscale. Unfortunately, these materials are rare in nature and have not been discovered until now. Here, we predict the ${\mathrm{B}}_2{\mathrm{S}}_2{\mathrm{O}}_9$ (BSO), an existing layered oxide, can exhibit both DUV band gap and strong SHG effects, among the highest values in all known DUV NLO bulks. The strong SHG intensities in BSO, originated from the ordered arrangement of polar $\mathrm{S}{\mathrm{O}}_4$ and $\mathrm{B}{\mathrm{O}}_4$ tetrahedra forming planar structure, are linearly tunable by the layer thickness. Surprisingly, the spontaneous rotations of rigid tetrahedra under strains can induce the near-zero Poisson ratios in BSO, which simultaneously result in the robust SHG effects against large strains, fundamentally differing from other known $2D$ NLO semiconductors. The discovery of BSO may provide an unprecedented opportunity to explore DUV NLO materials and applications in the $2D$ limit.

We show that it is possible to successfully, rapidly, and robustly transfer a topological vibrational edge mode across a time-varying mechanical chain. The stiffness values of the springs of the chain are arranged in an alternating staggered way such that we obtain a mechanical analog of the quantum Su-Schrieffer-Heeger model, which exhibits a nontrivial topological phase. Using optimal control methods, we are able to design control schemes for driving the stiffness parameters such that the transfer is done with high fidelity, speed, and robustness against disorder as well as energy amplification of the target edge mode.

The stationary interatomic force constant (IFC) matrix method proposed by Calandra et al. [Phys. Rev. B 82, 165111 (2010)] is extended to the ultrasoft pseudopotential method. A key ingredient of the method is the stationary property of the IFC matrix with respect to the first-order charge density. We derive the stationary IFC expression with ultrasoft pseudopotentials, which permits accurate and efficient Brillouin-zone sampling in the phonon calculation with the computational effort considerably smaller than the linear-response calculation, particularly for the systems containing an atom with localized orbitals such as first-row elements and transition metals. The method is applied for vanadium to validate the formulation.

Optically induced phase transitions of the manganite ${\mathrm{Pr}}_{1/3}{\mathrm{Ca}}_{2/3}\mathrm{Mn}{\mathrm{O}}_3$ have been simulated by using a model Hamiltonian that captures the dynamics of strongly correlated charge, orbital, lattice, and spin degrees of freedom. Its parameters have been extracted from first-principles calculations. Beyond a critical intensity of a femtosecond light pulse, the material undergoes an ultrafast and nonthermal magnetic phase transition from a noncollinear to collinear antiferromagnetic phase. The light-pulse excites selectively either a spin-nematic or a ferroelectric phase, depending on the light polarization. The behavior can be traced to an optically induced ferromagnetic coupling between Mn trimers, i.e., polarons which are delocalized over three Mn sites. The polarization guides the polymerization of the polaronic crystal into distinct patterns of ferromagnetic chains determining the target phase.

Nonlinear interactions between collective modes play a definitive role in far out of equilibrium dynamics of strongly correlated electron systems. Understanding and utilizing these interactions is crucial to photocontrol of quantum many-body states. One of the most surprising examples of strong mode coupling is the interaction between apical oxygen phonons and Josephson plasmons in bilayer ${{\mathrm{YBa}}_2{\mathrm{Cu}}_3\mathrm{O}}_{6+\mathrm{x}}$ superconductors. Experiments by Hu et al. [Nat. Mater. 13, 705 (2014)] and Kaiser et al. [Phys. Rev. B 89, 184516 (2014)] showed that below $T_c$, photoexcitation of phonons leads to enhancement and frequency shifts of Josephson plasmon edges, while above $T_c$, photoexcited phonons induce plasmon edges even when there are no discernible features in the equilibrium reflectivity spectrum. Recent experiments by von Hoegen et al. (arXiv:1911.08284) also observed parametric generation of Josephson plasmons from photoexcited phonons both below $T_c$ and in the pseudogap phase. In this paper, we present a theoretical model of three-wave phonon-plasmon interaction arising from changes of the in-plane superfluid stiffness caused by the apical oxygen motion. Analysis of the parametric instability of plasmons based on this model gives frequencies of the most unstable plasmons that are in agreement with experimental observations. We also discuss how strong parametric excitation of Josephson plasmons can explain pump-induced changes in the terahertz reflectivity of ${{\mathrm{YBa}}_2{\mathrm{Cu}}_3\mathrm{O}}_{6+\mathrm{x}}$ in the superconducting state, including frequency shifts and sharpening of Josephson plasmon edges, as well as appearance of a new peak around 2 THz. An interesting feature of this model is that overdamped Josephson plasmons do not give any discernible features in reflectivity in equilibrium, but can develop plasmon edges when parametrically excited. We suggest that this mechanism explains photoinduced plasmon edges in the pseudogap phase of ${{\mathrm{YBa}}_2{\mathrm{Cu}}_3\mathrm{O}}_{6+\mathrm{x}}$.