We report an ab initio study of the effect of pressure on vibrational and electronic properties of ${\mathrm{K}}_3\mathrm{Bi}$ and ${\mathrm{Rb}}_3\mathrm{Bi}$ in the cubic $Fm\overline{3}m$ structure. It is shown that the high-temperature cubic phase of ${\mathrm{K}}_3\mathrm{Bi}$ and ${\mathrm{Rb}}_3\mathrm{Bi}$ is dynamically unstable at $T=0$ but can be stabilized by pressure. The electronic spectra of alkali bismuthides are found to possess the bulk band touching at the Brillouin zone center and an inverted spin-orbit bulk band structure. Upon hydrostatic compression the compounds transform from the topologically nontrivial semimetal (${\mathrm{K}}_3\mathrm{Bi}$)/metal (${\mathrm{Rb}}_3\mathrm{Bi}$) into a trivial semiconductor (metal) with a conical Dirac-type dispersion of electronic bands at the point of the topological transition. In ${\mathrm{K}}_3\mathrm{Bi}$ the dynamical stabilization occurs before the system undergoes the topological phase transition.

We describe a mechanism for insulator-to-metal transition triggered by spin canting following femtosecond laser excitation of insulating antiferromagnetic (AFM) states of colossal magnetoresistive (CMR) manganites. We show that photoexcitation of composite fermion quasiparticles dressed by spin fluctuations results in the population of a broad metallic conduction band due to canting of the AFM background spins via strong electron-spin local correlation. By inducing spin canting, photoexcitation can increase the quasiparticle energy dispersion and quench the charge excitation energy gap. This increases the critical Jahn-Teller (JT) lattice displacement required to maintain an insulating state. We present femtosecond-resolved pump-probe measurements showing biexponential relaxation of the differential reflectivity below the AFM transition temperature. We observe a nonlinear dependence of the ratio of the femtosecond and picosecond relaxation component amplitudes at the same pump fluence threshold where we observe femtosecond magnetization photoexcitation. We attribute this correlation between nonlinear femtosecond spin and charge dynamics to spin/charge/lattice coupling and population inversion between the polaronic majority carriers and metallic quasielectron minority carriers as the lattice displacement becomes smaller than the critical value required to maintain an insulating state following laser-induced spin canting.

Motivated by the unusual temperature dependence of the specific heat in MnSi, comprising a combination of a sharp first-order feature accompanied by a broad hump, we study the extended Heisenberg model with competing exchange $J$ and anisotropic Dzyaloshinskii-Moriya $D$ interactions in a broad range of ratio $D/J$. Utilizing classical Monte Carlo simulations we find an evolution of the temperature dependence of the specific heat and magnetic susceptibility with variation of $D/J$. Combined with an analysis of the Bragg intensity patterns, we clearly demonstrate that the observed puzzling hump in the specific heat of MnSi originates from smearing out of the virtual ferromagnetic second-order phase transition by helical fluctuations which manifest themselves in the transient multiple spiral state. These fluctuations finally condense into the helical ordered phase via a first-order phase transition, as is indicated by the specific heat peak. Thus the model demonstrates a crossover from a second-order to a first-order transition with increasing $D/J$. Upon further increasing $D/J$ another crossover from a first-order to a second-order transition takes place in the system. Moreover, the results of the calculations clearly indicate that these competing interactions are the primary factors responsible for the appearance of first-order phase transitions in helical magnets with the Dzyaloshinskii-Moriya interaction.

X-ray absorption spectroscopy was used to determine the valence state in $\mathrm{L}{\mathrm{a}}_2\mathrm{C}{\mathrm{o}}_{1-x}\mathrm{M}{\mathrm{n}}_{1+x}{\mathrm{O}}_6$ ($x\approx 0.23$) thin films. We found that in spite of the nonstoichiometry, Co is in a divalent state while Mn ions show a mixed valence state. The relation of this finding with the magnetic properties of the films is discussed. X-ray magnetic circular dichroism measurements prove that magnetic anisotropy originates from Co spin-orbit coupling and it is strain dependent: a strong increase of the angular contribution to the magnetic moment is found when in-plane (out-of-plane) cell parameters get expanded (compressed). This behavior is reproduced by first order perturbation theory calculations.

Based on numerical simulations, we demonstrate thermally induced magnetic switching in synthetic ferrimagnets composed of multilayers of rare-earth and transition metals. Our findings show that deterministic magnetization reversal occurs above a certain threshold temperature if the ratio of transition-metal atoms to rare-earth atoms is sufficiently large. Surprisingly, the total thickness of the multilayer system has little effect on the occurrence of switching. We further provide a simple argument to explain the temperature dependence of the reversal process.

Intensive studies for more than three decades have elucidated multiple superconducting phases and odd-parity Cooper pairs in a heavy fermion superconductor ${\mathrm{UPt}}_3$. We identify a time-reversal invariant superconducting phase of ${\mathrm{UPt}}_3$ as a recently proposed topological nonsymmorphic superconductivity. Combining the band structure of ${\mathrm{UPt}}_3$, order parameter of $E_{2\mathrm{u}}$ representation allowed by $P{6}_3/mmc$ space group symmetry, and topological classification by $K$ theory, we demonstrate the nontrivial ${\mathbb{Z}}_2$-invariant of three-dimensional DIII class enriched by glide symmetry. Correspondingly, double Majorana cone surface states appear at the surface Brillouin zone boundary. Furthermore, we show a variety of surface states and clarify the topological protection by crystal symmetry. Majorana arcs corresponding to tunable Weyl points appear in the time-reversal symmetry broken B phase. Majorana cone protected by mirror Chern number and Majorana flat band by glide-winding number are also revealed.

The perovskite iridates ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ and ${\mathrm{Sr}}_3{\mathrm{Ir}}_2{\mathrm{O}}_7$ represent novel systems for exploring the electronic structure that is characteristic of Mott insulators upon carrier doping. Using angle-resolved photoemission spectroscopy (ARPES), we reveal a previously unobserved suppression of spectral weight near the Fermi level in the conduction band of very lightly electron-doped ${\left({\mathrm{Sr}}_{1-x}{\mathrm{La}}_x\right)}_3{\mathrm{Ir}}_2{\mathrm{O}}_7$ followed by a loss of coherence at high temperature. The doping and temperature dependence of this suppression suggests a correspondence with the antiferromagnetic Mott state. These results connect ${\left({\mathrm{Sr}}_{1-x}{\mathrm{La}}_x\right)}_3{\mathrm{Ir}}_2{\mathrm{O}}_7$ to other doped Mott insulators and add to the growing evidence of universal physics in these systems.

We study fermionic ladders with identical disorder along the leg direction. Following recent experiments, we focus, in particular, on how an initial occupation imbalance evolves in time. By considering different initial states and different ladder geometries, we find strong evidence that in generic cases interchain coupling leads to a destruction of the imbalance over time, both for Anderson and for many-body localized systems.

We present a quasianalytical solution of a spin-orbital model of ${\mathrm{KCuF}}_3$, using the variational method for Green's functions. By analyzing the spectra for different partial bosonic compositions as well as the full solution, we show that hole propagation needs both orbiton and magnon excitations to develop, but the orbitons dominate the picture. We further elucidate the role of the different bosons by analyzing the self-energies for simplified models, establishing that because of the nature of the spin-orbital ground state, magnons alone do not produce a full quasiparticle band, in contrast to orbitons. Finally, using the electron-hole transformation between the $e_g$ states of ${\mathrm{KCuF}}_3$ and ${\mathrm{LaMnO}}_3$ we suggest the qualitative scenario for photoemission experiments in ${\mathrm{LaMnO}}_3$.

We study the effect of nonmagnetic ${\mathrm{Zn}}^{2+}$ (spin-0) and magnetic ${\mathrm{Ni}}^{2+}$ (spin-1) impurities on the ground state and low-lying excitations of the quasi-one-dimensional spin-$1/2$ Heisenberg antiferromagnet ${\mathrm{Sr}}_2{\mathrm{CuO}}_3$ using inelastic neutron scattering, specific heat, and bulk magnetization measurements. We show that 1% ${\mathrm{Ni}}^{2+}$ doping in ${\mathrm{Sr}}_2{\mathrm{CuO}}_3$ results in a sizable spin gap in the spinon excitations, analogous to the case of Ni doped ${\mathrm{SrCuO}}_2$ previously reported [Simutis et al., Phys. Rev. Lett. 111, 067204 (2013)]. However, a similar level of ${\mathrm{Zn}}^{2+}$ doping in ${\mathrm{SrCuO}}_2$, investigated here for comparison, did not reveal any signs of a spin gap. Magnetic ordering temperature was found to be suppressed in the presence of both ${\mathrm{Zn}}^{2+}$ and ${\mathrm{Ni}}^{2+}$ impurities; however, the rate of suppression due to ${\mathrm{Ni}}^{2+}$ was found to be much more pronounced than for ${\mathrm{Zn}}^{2+}$. Effect of magnetic field on the ordering temperature is investigated. We found that with increasing magnetic field, not only the magnetic ordering temperature gradually increases but the size of specific heat anomaly associated with the magnetic ordering also progressively enhances, which can be qualitatively understood as due to the field induced suppression of quantum fluctuations.

We present the direct measurements of magnetoexciton transport. Excitons give the opportunity to realize the high magnetic-field regime for composite bosons with magnetic fields of a few tesla. Long lifetimes of indirect excitons allow the study of kinetics of magnetoexciton transport with time-resolved optical imaging of exciton photoluminescence. We performed spatially, spectrally, and time-resolved optical imaging of transport of indirect excitons in high magnetic fields. We observed that an increasing magnetic field slows down magnetoexciton transport. The time-resolved measurements of the magnetoexciton transport distance allowed for an experimental estimation of the magnetoexciton diffusion coefficient. An enhancement of the exciton photoluminescence energy at the laser excitation spot was found to anticorrelate with the exciton transport distance. A theoretical model of indirect magnetoexciton transport is presented and is in agreement with the experimental data.

We perform full time-resolved tomographic measurements of the polarization state of pairs of photons emitted during the radiative cascade of the confined biexciton in a semiconductor quantum dot. The biexciton was deterministically initiated using a $\pi$-area pulse into the biexciton two-photon absorption resonance. Our measurements demonstrate that the polarization states of the emitted photon pair are maximally entangled. We show that the measured degree of entanglement depends solely on the temporal resolution by which the time difference between the emissions of the photon pair is determined. A route for fabricating an on-demand source of maximally polarization entangled photon pairs is thereby provided.

We report an observation of a topologically protected transport of surface carriers in a quasiballistic $\mathrm{C}{\mathrm{d}}_3\mathrm{A}{\mathrm{s}}_2$ nanowire. The nanowire is thin enough for the spin-textured surface carriers to form 1D subbands, demonstrating conductance oscillations with gate voltage even without magnetic field. The $\pi$-phase shift of Aharonov-Bohm oscillations can periodically appear/disappear by tuning gate voltage continuously. Such a $\pi$-phase shift stemming from the Berry phase demonstrates the topological nature of surface states. The topologically protected transport of the surface states is further revealed by four-terminal nonlocal measurements.

It is proposed that strain-induced pseudomagnetic fields in Dirac and Weyl materials could be used as valley and chirality sensitive lenses for beams of Weyl quasiparticles. The study of the (pseudo)magnetic lenses is performed by using the eikonal approximation for describing the Weyl quasiparticles propagation in magnetic and strain-induced pseudomagnetic fields. Analytical expressions for the locations of the principal foci and the focal lengths are obtained in the paraxial approximation in the models with uniform as well as nonuniform effective magnetic fields inside the lens. The results show that the left- and right-handed quasiparticles can be focused at different spatial locations when both magnetic and pseudomagnetic fields are applied. It is suggested that the use of magnetic and pseudomagnetic lenses could open new ways of producing and manipulating beams of chiral Weyl quasiparticles.

Understanding the nature of two-level tunneling defects is important for minimizing their disruptive effects in various nanodevices. By exploiting the resonant coupling of these defects to a superconducting qubit, one can probe and coherently manipulate them individually. In this work, we utilize a phase qubit to induce Rabi oscillations of single tunneling defects and measure their dephasing rates as a function of the defect's asymmetry energy, which is tuned by an applied strain. The dephasing rates scale quadratically with the external strain and are inversely proportional to the Rabi frequency. These results are analyzed and explained within a model of interacting defects, in which pure dephasing of coherent high-frequency (gigahertz) defects is caused by interaction with incoherent low-frequency thermally excited defects. Our analysis sets an upper bound for the relaxation rates of thermally excited defects interacting strongly with strain fields.

Charge density wave (CDW) phases are symmetry-reduced states of matter in which a periodic modulation of the electronic charge frequently leads to drastic changes of the electronic spectrum, including the emergence of energy gaps. We analyze the CDW state in a $1T–{\mathrm{TiSe}}_2$ monolayer within a density functional theory framework and show that, similarly to its bulk counterpart, the monolayer is unstable towards a commensurate $2\times 2$ periodic lattice distortion (PLD) and CDW at low temperatures. Analysis of the electron and phonon spectrum establishes the PLD as the stable $T=0$ K configuration with a narrow band gap, whereas the undistorted and semimetallic state is stable only above a threshold temperature. The lattice distortions as well as the unfolded and reconstructed band structure in the CDW phase agree well with experimental results. We also address evidence in our results for the role of electron-electron interactions in the CDW instability of $1T–{\mathrm{TiSe}}_2$ monolayers.

We have performed two-dimensional Fourier transform spectroscopy on intrinsic and modulation doped quantum wells in external magnetic fields up to 10 T. In the undoped sample, the strong Coulomb interactions and the increasing separations of the electron and hole charge distributions with increasing magnetic fields lead to a nontrivial in-plane dispersion of the magneto-excitons. Thus, the discrete and degenerate Landau levels are coupled to a continuum. The signature of this continuum is the emergence of elongated spectral line shapes at the Landau level energies, which are exposed by the multidimensional nature of our technique. Surprisingly, the elongation of the peaks is completely absent in the lowest Landau level spectra obtained from the modulation doped quantum well at high fields.

Simultaneous time-and-space resolved reflectivity and interferometric measurements over a temporal span of 300 ps have been performed in fused silica and sapphire samples excited with 800 nm, 120 fs laser pulses at energies slightly and well above the ablation threshold. The experimental results have been simulated in the frame of a multiple-rate equation model including light propagation. The comparison of the temporal evolution of the reflectivity and the interferometric measurements at 400 nm clearly shows that the two techniques interrogate different material volumes during the course of the process. While the former is sensitive to the evolution of the plasma density in a very thin ablating layer at the surface, the second yields an averaged plasma density over a larger volume. It is shown that self-trapped excitons do not appreciably contribute to carrier relaxation in fused silica at fluences above the ablation threshold, most likely due to Coulomb screening effects at large excited carrier densities. For both materials, at fluences well above the ablation threshold, the maximum measured plasma reflectivity shows a saturation behavior consistent with a scattering rate proportional to the plasma density in this fluence regime. Moreover, for both materials and for pulse energies above the ablation threshold and delays in the few tens of picoseconds range, a simultaneous “low reflectivity” and “low transmission” behavior is observed. Although this behavior has been identified in the past as a signature of femtosecond laser-induced ablation, its origin is alternatively discussed in terms of the optical properties of a material undergoing strong isochoric heating, before having time to substantially expand or exchange energy with the surrounding media.

We used Brillouin light scattering spectroscopy to independently determine the in-plane magnetocrystalline anisotropy and the Dzyaloshinskii-Moriya interaction (DMI) in out-of-plane magnetized Au/Co/W(110). We found that the DMI strength is 2–3 times larger along the bcc$\left[\overline{1}10\right]$ than along the bcc[001] direction. We use analytical considerations to illustrate the relationship between the crystal symmetry of the stack and the anisotropy of microscopic DMI. Such an anisotropic DMI is the first step to realize isolated elliptical skyrmions or antiskyrmions in thin-film systems with $C_{2v}$ symmetry.

We investigate the temperature-dependent microwave absorption spectrum of an yttrium iron garnet sphere as a function of temperature (5 to $300\mathrm{K}$) and frequency (3 to $43.5\mathrm{GHz}$). At temperatures above $100\mathrm{K}$, the magnetic resonance linewidth increases linearly with temperature and shows a Gilbert-like linear frequency dependence. At lower temperatures, the temperature dependence of the resonance linewidth at constant external magnetic fields exhibits a characteristic peak which coincides with a non-Gilbert-like frequency dependence. The complete temperature and frequency evolution of the linewidth can be modeled by the phenomenology of slowly relaxing rare-earth impurities and either the Kasuya-LeCraw mechanism or the scattering with optical magnons. Furthermore, we extract the temperature dependence of the saturation magnetization, the magnetic anisotropy, and the $g$ factor.