We propose helicoidal excitonic phase in a Coulomb-coupled two-dimensional electron-hole double-layer (EHDL) system with relativistic spin-orbit interaction. Previously, it was demonstrated that layered InAs/AlSb/GaInSb heterostructure is an ideal experimental platform for searching excitonic condensate phases, while its electron layer has non-negligible Rashba interaction. We clarify that due to the Rashba term, the spin-triplet (spin-1) exciton field in the EHDL system forms a helicoidal structure and the helicoid plane can be controlled by an in-plane Zeeman field. We show that due to a small but finite Dirac term in the heavy hole layer the helicoidal structure of the excitonic field under the in-plane field results in a helicoidal magnetic order in the electron layer. Based on linearization analyses, we further calculate momentum-energy dispersions of low-energy Goldstone modes in the helicoidal excitonic phase. We discuss possible experimental probes of the excitonic phase in the EHDL system.

The presence of defects in the narrow gap semiconductors GaSb and InSb affects their dopability and hence applicability for a range of optoelectronic applications. Here, we report hybrid density functional theory (DFT)-based calculations of the properties of intrinsic point defects in the two systems, including spin-orbit coupling effects, which influence strongly their band structures. With the hybrid DFT approach adopted, we obtain excellent agreement between our calculated band dispersions and structural, elastic, and vibrational properties and available measurements. We compute point defect formation energies in both systems, finding that antisite disorder tends to dominate, apart from in GaSb under certain conditions, where cation vacancies can form in significant concentrations. Calculated self-consistent Fermi energies and equilibrium carrier and defect concentrations confirm the intrinsic $n$- and $p$-type behavior of both materials under anion-rich and anion-poor conditions. Moreover, by computing the compensating defect concentrations due to the presence of ionized donors and acceptors, we explain the observed dopability of GaSb and InSb.

We present a method for calculating the frequencies and intensities of the vibrational modes of adsorbates on surfaces. Our method is based on density functional perturbation theory (DFPT) and provides accurate estimates of the vibrational intensities even when the vibrations are strongly anharmonic. Furthermore, it does so at a negligible additional computation cost compared to conventional DFPT calculations. We illustrate our method by calculating the vibrational spectra of three example systems—ethylidyne on Rh(111), benzene on Rh(111) coadsorbed with CO, and terephthalic acid on Cu(100)—and comparing them to experimental measurements performed using high-resolution electron energy loss spectroscopy. We find excellent agreement between our predictions and the experimentally measured frequencies and intensities in all three cases.

The high-order wave mixing by two-color multiphoton excitation in nonlinear regime along with the harmonics generation in two-dimensional (2D) nanostructures such as graphene, silicene, germanene, and stanene is investigated. We consider the coherent part of the spectra corresponding to high harmonic generation by two-color intense laser pulses. The developed theory of interaction of free carriers with the driving effective field of two-color waves covers the full Brillouin zone of 2D hexagonal nanostructures. We also consider the case when one of the driving waves is in one-photon resonance with the van Hove singularity at the $M$ point of the Brillouin zone. Obtained results show that novel 2D nanostructures can serve as an effective medium for high-order wave mixing and harmonic generation at the two-color multiphoton excitation with strong laser pulses.

The spin-polarized surface states of topological insulators have attracted interest both from a fundamental and applied point of view. A recent proposal describes a method of probing these surface states with ferromagnetic contacts, which was subsequently applied to a variety of materials. In this paper, we use this method on the potential topological insulator ${\mathrm{SmB}}_6$ with a design based on the Corbino geometry. Though the signal behaves as predicted for all orientations of current and magnetic field, its magnitude is much larger than expected. Possible parasitic effects such as stray field-induced Hall voltages are excluded, leaving the origin of the observations uncertain. This corroborates the need for careful analysis when interpreting results of similar experiments.

We investigate the real-time dynamics of the half-filled one-dimensional extended Hubbard model in the strong-coupling regime, when driven by a transient laser pulse. Starting from a wide regime displaying a charge-density wave in equilibrium, a robust photoinduced in-gap state appears in the optical conductivity, depending on the parameters of the pulse. Here, by tuning its conditions, we maximize the overlap of the time-evolving wave function with excited states displaying the elusive bond-ordered wave of this model. Finally, we make a clear connection between the emergence of this order and the formation of the aforementioned in-gap state, suggesting the potential observation of purely electronic (i.e., not associated with a Peierls instability) bond-ordered waves in experiments involving molecular crystals.

An unusual behavior of the exchange energy scale of a quantum Hall ferromagnet with $\nu =1$ was found in strongly correlated two-dimensional electron systems based on MgZnO/ZnO heterostructures. The exchange contribution, entering the energy of a collective excitation, was probed by means of inelastic light scattering. It was established that, in a wide range of electron densities corresponding to the Wigner-Seitz parameter $7<r_s<11$, this contribution is on the order of the cyclotron energy, which is notably different from the typical scale of $e^2/\varepsilon {\ell }_B$ that is typical for weakly interacting systems. The same trend was confirmed via numerical calculations.

The ground-state equilibrium properties of copper-gold alloys have been explored with the state-of-the-art random phase approximation (RPA). Our estimated lattice constants agree with the experiment within a mean absolute percentage error of 1.4 percent. Semilocal functionals, such as the generalized gradient approximation (GGA) of Perdew, Burke, and Ernzerhof (PBE) and the strongly constrained and appropriately normed (SCAN) functional, fail to provide accurate bulk moduli, which indicates their inability to describe the system in a stretched or compressed state with respect to the equilibrium geometry. The PBE, PBE revised for solids, and revised Tao-Perdew-Staroverov-Scuseria functionals by Perdew et al. predict too-low formation energies, while the SCAN slightly overestimates it. The inclusion of thermal correction or the long-range dispersion provides a negligible contribution to the formation energies estimated with semilocal density functional theory. The spin-orbit coupling improves the formation energies of PBE only by 7–8 meV, while it intensifies the overestimation of SCAN. We found that the nonlocality present in RPA is able to describe the transition between two delocalized electron densities (bulk elemental constituents to crystallized alloys), as required to provide accurate formation energies without any further corrections. Based on our results, we conclude that it is difficult to find a universal density functional which can give accurate results for a wide range of properties of intermetallic alloys. However, RPA can capture different bonding situations, often better than other density functionals. It gives accurate results for a wide range of ground-state properties for the alloys, generated from metals with completely filled $d$ shells.

We develop recursion relations, in particle number, for all (unprojected) Jain composite fermion (CF) wave functions. These recursions generalize a similar recursion originally written down by Read for Laughlin states, in mixed first-second quantized notation. In contrast, our approach is purely second-quantized, giving rise to an algebraic, “pure guiding center” definition of CF states that de-emphasizes first quantized many-body wave functions. Key to the construction is a second-quantized representation of the flux attachment operator that maps any given fermion state to its CF counterpart. An algebra of generators of edge excitations is identified. In particular, in those cases where a well-studied parent Hamiltonian exists, its properties can be entirely understood in the present framework, and the identification of edge state generators can be understood as an instance of “microscopic bosonization.” The intimate connection of Read's original recursion with “nonlocal order parameters” generalizes to the present situation, and we are able to give explicit second-quantized formulas for nonlocal order parameters associated with CF states.

We show that a domain wall separating single-layer graphene and AA-stacked bilayer graphene (AA-BLG) can be used to generate highly collimated electron beams which can be steered by a magnetic field. Two distinct configurations are studied, namely, locally delaminated AA-BLG and terminated AA-BLG whose terminal edge types are assumed to be either zigzag or armchair. We investigate the electron scattering using semiclassical dynamics and verify the results independently with wave-packet dynamics simulations. We find that the proposed system supports two distinct types of collimated beams that correspond to the lower and upper cones in AA-BLG. Our computational results also reveal that collimation is robust against the number of layers connected to AA-BLG and terminal edges.

Majorana bound states have been proposed to induce current-current correlations (CCCs) that are completely different from those induced by low-energy fermionic Andreev bound states. Such characteristics can be used as a signature to detect Majorana bound states and their nonlocality. Herein, we studied the Majorana and fermionic Andreev bound states in a two-dimensional topological insulator system. We found that the coupling of each pair of fermionic Andreev bound states coexists with that of the pair of Majorana bound states, and that the coupling strengths of all pairs of bound states depend on system parameters in the same pattern. Majorana and fermionic Andreev bound states show the same differential CCCs characteristics, thereby indicating a universal behavior for both types of bound states in a two-dimensional topological insulator system. The maximal cross-differential CCCs are robust to the structural asymmetry of the system.

MgO is a major constituent of the ${\text{MgO-FeO-SiO}}_2$ system that comprises the Earth's mantle and that of super-Earth exoplanets. Knowledge of its high-pressure behavior is important for modeling the more complex compounds. This paper presents measurements of the principal Hugoniot, sound velocity, and temperature of MgO, shocked to pressures of 710 to 2300 GPa using laser-driven compression. The Hugoniot and temperature measurements compare favorably to previous results constraining the shock response of MgO at extreme conditions. The Grüneisen parameter was calculated from the Hugoniot and sound velocity data and was found to be underpredicted by tabular models. The sound velocity of liquid MgO is overpredicted by models implying that the quantity of partial melt required to match decreased wave speeds in ultralow velocity zones in the lower mantle may be less than previously assumed and experiments at lower-mantle pressures are needed.

In the emerging field of magnonics, spin waves are considered for information processing and transmission at high frequencies. Accordingly, the manipulation of propagating spin waves in nanostructured waveguides for novel functionality has attracted increased attention. Excitations with uniform magnetic fields in such waveguides favor symmetric spin-wave modes with odd quantization numbers. Interference between multiple odd spin-wave modes leads to a periodic self-focusing effect of the propagating spin waves. In this work we demonstrate how antisymmetric spin-wave modes with even quantization numbers can be induced by local magnetic fields in a well-controlled fashion. The resulting interference patterns are discussed within an analytical model and experimentally demonstrated using microfocused Brillouin light scattering.

The interaction between lattice and spins is at the heart of an extremely intriguing ultrafast dynamics in magnetic materials. In this paper, we formulate a general nonequilibrium theory that disentangles the complex interplay between phonons and magnons in a THz laser-excited antiferromagnetic insulator. The theory provides a quantitative description of the transient energy flow between the spin and lattice subsystems, subject to magnon-phonon and phonon-phonon scatterings, giving rise to finite lifetimes of the quasiparticles and to the equilibration time of the system. We predict a kind of scattering process where two magnons of opposite polarizations decay into a phonon, previously omitted in the literature. The theory is combined with first-principles calculations and then applied to simulate realistic dynamics of NiO. The main relaxation channels and hot spots in the reciprocal space, giving the strongest contribution to the energy transfer between phonons and magnons are identified. The diverse interaction strengths lead to distinct coupled dynamics of the lattice and spin systems and subsequently to different equilibration timescales.

Dynamics of magnetic skyrmions in hybrid ferromagnetic films harbors interesting physical phenomena and holds promise for technological applications. In this work, we discuss the behavior of magnetic skyrmions when coupled to superconducting vortices in a ferromagnet-superconductor heterostructure. We use numerical simulations and analytic arguments within London and Thiele formalisms to reveal broader possibilities for manipulating the skyrmion-vortex dynamic correlations in the hybrid system, that are not possible in its separated constituents. We explore the thresholds of particular dynamic phases, and quantify the phase diagram as a function of the relevant material parameters, applied current, and induced magnetic torques. Finally, we demonstrate the broad and precise tunability of the skyrmion Hall angle in the presence of vortices, with respect to currents applied to either or both the superconductor and the ferromagnet within the heterostructure.

Motivated by the observation of biskyrmions in centrosymmetric magnetic films [X. Z. Yu et al., Nat. Commun. 5, 3198 (2014), W. Wang et al., Adv. Mater. 28, 6887 (2016)], we investigate analytically and numerically the stability of biskyrmions in films of finite thickness, taking into account the nearest-neighbor exchange interaction, perpendicular magnetic anisotropy (PMA), dipole-dipole interaction (DDI), and the discreteness of the atomic lattice. The biskyrmion is characterized by the topological charge $Q=2$, the spatial scale $\lambda$, and another independent length $d$ that can be interpreted as a separation of two $Q=1$ skyrmions inside a $Q=2$ topological defect in the background of uniform magnetization. We find that biskyrmions with $d$ of order $\lambda$ can be stabilized by the magnetic field within a certain range of the ratio of PMA to DDI in a film having a sufficient number of atomic layers $N_z$. The shape of biskyrmions has been obtained by the numerical minimization of the energy of interacting spins in a $1000\times 1000\times N_z$ atomic lattice. It is close to the exact solution of the Belavin-Polyakov model when $d$ is below the width of the ferromagnetic domain wall. We compute the magnetic moment of a biskyrmion and discuss ways of creating biskyrmions in experiment.

The spontaneous formation of a lattice structure of quantized vortices is a characteristic feature of superfluidity in closed systems under thermal equilibrium. In an exciton-polariton Bose-Einstein condensate, which is a typical example of a macroscopic quantum state in open systems, spontaneous vortex lattices have also been proposed but not yet observed. Here, we take into account the finite decay rate of the exciton reservoir and theoretically investigate the vortex structures in a circularly pumped polariton Bose-Einstein condensate. Our results show that a decreasing reservoir decay rate can reduce the number of vortices and destabilize the lattice structure, and hence is unfavorable to the formation and observation of vortex lattices. These detrimental effects can be prevented by applying an external angular momentum.

Slow-light waveguides play an important role in optical pulse delay lines, but are practically limited by disorder-induced attenuation. Topological edge states, unidirectional and robust against disorder, have been proposed as a way to address this issue. Here, we study the transmission phase and pulse propagation dynamics through unidirectional systems in the presence of strong discontinuities. We first investigate a magnetically biased slow-light channel, demonstrating broadband pulse delays in small footprints. We uncover a nonreciprocal etalonlike resonance, sustained by propagating forward and evanescent backward modes, affecting pulse propagation and delay. We then show the existence of these features in topologically protected edge states, highlighting their universality. These exotic resonances provide a new degree of freedom in unidirectional waveguides to engineer their response, and they may therefore prove highly useful in various nanophotonic applications.

We report the experimental realization of a space-time crystal with tunable periodicity in time and space in a magnon Bose-Einstein condensate (BEC), formed in a room-temperature yttrium iron garnet (YIG) film by a microwave space-homogeneous magnetic field. The magnon BEC is prepared to have a well-defined frequency and nonzero wave vector. We demonstrate how the crystalline “density” as well as the time and space textures of the resulting crystal may be tuned by varying the experimental parameters: External static magnetic field, temperature, thickness of the YIG film, and power of the microwave field. The proposed space-time crystals provide an additional dimension for exploring dynamical phases of matter and can serve as a model nonlinear Floquet system, that brings in touch the rich fields of classical nonlinear waves, magnonics, and periodically driven systems.