Maintaining a population inversion in electron distributions is the first step towards lasing. There is a strong interest in realizing the inversion in a Dirac conical band structure, because broad-band lasing may then be realized owing to the zero-gap nature of the Dirac cone. Here we show that the population inversion can be elongated to $>7$ ps at 8 K and $>10$ ps at 300 K on the surface of a $p$-type topological insulator ${\left({\mathrm{Sb}}_{0.73}{\mathrm{Bi}}_{0.27}\right)}_2{\mathrm{Te}}_3$. Time- and angle-resolved photoemission spectroscopy gives us the direct evidence for the elongated duration of the inversion in the topological surface states. We hereby provide a guideline to prolong the population inversion at finite temperatures. Our study strengthens the route toward the Dirac materials to be a lasing medium.

The weak spin-orbit interaction in graphene was predicted to be increased, e.g., by hydrogenation. This should result in a sizable spin Hall effect (SHE). We employ two different methods to examine the spin Hall effect in weakly hydrogenated graphene. For hydrogenation we expose graphene to a hydrogen plasma and use Raman spectroscopy to characterize this method. We then investigate the SHE of hydrogenated graphene in the H-bar method and by direct measurements of the inverse SHE. Although a large nonlocal resistance can be observed in the H-bar structure, comparison with the results of the other method indicate that this nonlocal resistance has a non-spin-related origin.

What determines the glass property remains one of the major unsolved problems in both condensed matter physics and materials science. Despite extensive research attempting to identify possible structural features as property signatures in glasses obeying the conventional philosophy of “structure determines property” in matter, the hidden rule about why some proposed structures predict properties effectively but others do not is still poorly understood. Here we revisit some earlier proposed successful “structural descriptors” in glasses, e.g., vibrational mean-squared displacement, flexibility volume, participation fraction of low-frequency vibrational modes, and two-body excess entropy, correlating them with the long-time dynamic property of model glass probed via the activation energy for local structural excitation. We find that all four structural descriptors correlate strongly with the activation energy, presenting large Pearson's correlation coefficients. By examining the spatial nature of the activation energy and the structural descriptors, a common rule for the robustness of structure–property relationships in glasses is established, according to which there exists a critical characteristic correlation length. We further demonstrate the concept that complex structures determine the glass property by manipulating the cutoff distance used to define the two-body excess entropy. Only if this structural descriptor is defined involving atoms beyond the first nearest neighbor does it reproduce the feature of common spatial correlation range in glass. The presence of a common spatial correlation length strongly indicates that it is necessary to include the spatial correlation of a complex structure accounting for the dynamic property of metallic glasses.

The field of double perovskites is now advancing to three magnetic elements on the A, B and ${\mathrm{B}}^{\prime }$ sites. A series of iridium-based double perovskite compounds, $L{n}_2{\mathrm{CoIrO}}_6$ $(Ln=\mathrm{Eu}, \mathrm{Tb}, \mathrm{Ho})$, with three magnetic elements was synthesized as polycrystalline samples. The compounds crystalize in monoclinic structures with the space group $P{2}_1/n$. Magnetic properties of these hetero-tri-spin $3d\text{−}5d\text{−}4f$ systems were studied by magnetic susceptibility and field dependent magnetization in both DC and pulsed magnetic fields. All these compounds show ferrimagnetic transitions at temperatures $T_C$ above 100 K, which are attributable to antiferromagnetic coupling between ${\mathrm{Co}}^{2+}$ and ${\mathrm{Ir}}^{4+}$ spins. For ${\mathrm{Eu}}_2{\mathrm{CoIrO}}_6$, the magnetic properties are similar to those of ${\mathrm{La}}_2{\mathrm{CoIrO}}_6$. The ${\mathrm{Eu}}^{3+}$ spins show Van Vleck paramagnetism, and do not significantly interact with transition-metal cations. In contrast, ${\mathrm{Tb}}_2{\mathrm{CoIrO}}_6$ and ${\mathrm{Ho}}_2{\mathrm{CoIrO}}_6$ reveal a second transition to antiferromagnetic order below a lower temperature $T_N$. The temperature-induced ferrimagnetic-to-antiferromagnetic phase transition might be explained by a spin-reorientation transition. Moreover, a magnetic-field-induced spin-flop-like transition with a small hysteresis was observed below $T_N$ in these two compounds. The magnetic moments of all three compounds do not saturate up to 60 T at low temperatures. A moderate magnetocaloric effect was also observed in all three compounds. Our results should motivate further investigation of the spin configuration on single crystals of these iridium-based double perovskites.

The transport properties of a disordered two-dimensional (2D) honeycomb lattice are examined numerically using the spectral approach to the 2D percolation problem, characterized by an Anderson-type Hamiltonian. In our model, disorder is represented by two parameters: a distribution of random on-site energies ${\epsilon }_i$ (positional disorder) and a concentration of doping energies $p$ (substitutional disorder). The results indicate the existence of extended energy states for nonzero disorder and the emergence of a transition towards localized behavior for critical doping concentration $n_D>0.3\%$, in agreement with the experimentally observed metal-to-insulator transition in a graphene sheet doped with hydrogen.

A Dirac-cone-like dispersion at the center of the Brillouin zone where the wave number $\vec{k}=0$ [L. Brillouin, Wave Propagation in Periodic Structures (Dover, New York, 1953)], is rare and only happens due to accidental degeneracy. At certain frequencies, the Dirac cone breaks the time-reversal symmetry of acoustic waves, which has not yet been fully explored. In this paper, even the simplest geometric microarchitecture of phononic crystals (PnCs) in a periodic structure can be modulated to obtain the accidental triple degeneracies that make a Dirac-like cone at the Γ point ($\vec{k}=0$). While doing so, it was observed that the frequency of a nondispersive "deaf" band obtained from any arbitrary periodic structure made of similar PnCs remains unaltered. Then, a deaf band based predictive modulation of the PnCs is realized, and multiple occurrences of the Dirac-like points are demonstrated. The claims are validated using a numerical and experimental study of a baseline periodic structure having a square array of cylindrical polyvinylchloride inclusions in an air matrix. Phenomena such as orthogonal wave transport, negative refraction, and wave vortex are verified to exist at the deaf band based engineered Dirac cone.

We study the behavior of electrons interacting with a classical magnetic background via a strong Hund coupling. The magnetic background results from a Hamiltonian which favors at low temperature the emergence of a phase with nonzero scalar chirality. The strong Hund's coupling combined with the total chirality of the classical spins induces in the electrons an effective flux which results in the realization of a band structure with nonzero Chern number. First, we consider as a magnetic background a classical spin system with spontaneous net chirality. We study the density of states and Hall conductance in order to analyze the topological transitions as a function of the Fermi energy and the temperature of the classical spins. We also study a similar model in which the Chern number of the filled bands can be “tuned” with the external magnetic field, resulting in a topological insulator in which the direction of the chiral edge mode can be reverted by reversing the orientation of the magnetic field applied to the classical magnetic system.

Recently, there has been a surge of interest in using Rényi entropies as quantifiers of correlations in many-body quantum systems. However, it is well known that in general these entropies do not satisfy the strong subadditivity inequality, which is a central property ensuring the positivity of correlation measures. In fact, in many cases they do not even satisfy the weaker condition of subadditivity. In the present paper we shed light on this subject by providing a detailed survey of Rényi entropies for bosonic and fermionic Gaussian states. We show that for bosons the Rényi entropies always satisfy subadditivity, but not necessarily strong subadditivity. Conversely, for fermions both do not hold in general. We provide the precise intervals of the Rényi index $\alpha$ for which subadditivity and strong subadditivity are valid in each case.

Two-dimensional (2D) magnetic materials are essential to developing high-performance spintronic devices. Recent experimental discoveries of several atomic thin 2D ferromagnetic materials have stimulated great interest in further exploring this fascinating class of materials. Here, combining an advanced crystal structure search method and extensive first-principles energetic and dynamic calculations, we have identified a planar ${\mathrm{CoB}}_6$ monolayer as a stable 2D ferromagnet. We show that the ferromagnetic ground state of the ${\mathrm{CoB}}_6$ monolayer remains robust in the ambient environment, and the magnetic stability and moment can be remarkably enhanced and tuned by external strain. Moreover, we propose feasible synthesis routes for the the newly predicted ${\mathrm{CoB}}_6$ monolayer, either by Co atom adsorption on the recently proposed ${\delta }_4$ boron sheet or by direct chemical growth. The present results establish a fundamental material and physics basis for synthesis and characterization of the ${\mathrm{CoB}}_6$ monolayer among the emerging 2D ferromagnetic materials.

Electric dipole spin resonance is studied theoretically for a shallow donor formed in a nanowire with spin-orbit coupling in a magnetic field. Such a system may represent a donor-based qubit. The single discrete energy level of the donor is accompanied by a set of continuum states, which provide a nontrivial interplay for the picture of electric dipole spin resonance driven by an external monochromatic field. Strongly nonlinear dependencies of spin-flip time as well as of the coordinate mean values on the electric field amplitude are observed, demonstrating the significance of coupling to the continuum for spin-based qubit manipulation in nanostructures.

Noncentrosymmetric superconductors with $\alpha$-manganese structure has attracted much attention recently, after the discovery of time-reversal symmetry breaking in all the members of the ${\mathrm{Re}}_{6}X$ ($X=\text{Ti}$, Hf, Zr) family. Similar to ${\mathrm{Re}}_{6}X$, ${\mathrm{NbOs}}_2$ also adopts $\alpha \text{−}\text{Mn}$ structure and is found to be superconducting with critical temperature $T_c\simeq 2.7$ K. The results of the resistivity, magnetization, specific heat, and muon spin relaxation/rotation measurements show that ${\mathrm{NbOs}}_2$ is a weakly coupled type-II superconductor. Interestingly, the zero-field muon experiments indicate that the time-reversal symmetry is preserved in the superconducting state. The low-temperature transverse-field muon measurements and the specific-heat data evidence a conventional isotropic fully gapped superconductivity. However, the calculated electronic properties in this material show that the ${\mathrm{NbOs}}_2$ is positioned close to the band of unconventionality of the Uemura plot, indicating that ${\mathrm{NbOs}}_2$ potentially borders an unconventional superconducting ground state.

In this work we numerically investigate the fate of the Bardeen-Cooper-Schrieffer (BCS) pairing in the presence of quenched phase under Peierls substitution using time-dependent real space and momentum space Bogoliubov-de Gennes equation methods and Anderson pseudospin representation method. This kind of phase imprint can be realized by modulating electric field in ultracold atoms and illumining of THz optical pump pulse in solids with conventional and unconventional superconductors. In the case of weak phase imprint, the BCS pairing is stable, while in the strong phase imprint, instability towards finite-momentum pairing is allowed, in which the real space and momentum space methods yield different results. In the pulsed gauge potential, we find that this instability will not happen even with much stronger vector potential. We also show that the uniform and staggered gauge potentials yield different behaviors. While the staggered potential induces transition from the BCS pairing to the overdamped phase, the uniform gauge may enhance the pairing and will not induce to the overdamped phase. These results may shade light on the realization of finite momentum pairing, such as Fulde-Ferrell-Larkin-Ovchinnikov phase with dynamical modulation.

Path-integral Monte Carlo calculations have been carried out to investigate physical properties of a ${}^4\mathrm{He}$ monolayer adsorbed on a single 6,6,12-graphyne sheet, which is one of the graphyne families possessing a rectangular symmetry. To characterize elusive quantum phases of an adsorbed ${}^4\mathrm{He}$ monolayer on 6,6,12-graphyne, we model the ${}^4\mathrm{He}$-graphyne interaction by the pairwise sum of empirical ${}^4\mathrm{He}\text{−}\mathrm{C}$ interatomic potentials. At partially filled ${}^4\mathrm{He}$ coverages, we identify three commensurate solids of the $C_{3/4},C_{4/4}$, and $C_{6/4}$ structures from the two-dimensional density distribution. These solids show the rectangular symmetry inherited from the symmetry of 6,6,12-graphyne, which were confirmed with the analysis of their static structure factors. At high helium coverages near its completion, the ${}^4\mathrm{He}$ monolayer is predicted to exhibit a transition from a rectangular commensurate structure to a triangular incommensurate structure, after going through inhomogeneous structures mixed with domains of triangular and rectangular orderings. This symmetry-changing transition has not been observed in ${}^4\mathrm{He}$ monolayers adsorbed on other carbon substrates.

Room temperature compression of graphitic materials leads to interesting superhard $s{p}^3$ rich phases which are sometimes transparent. In the case of graphite itself, the $s{p}^3$ rich phase is proposed to be monoclinic $M$-carbon; however, for disordered materials such as glassy carbon the nature of the transformation is unknown. We compress glassy carbon at room temperature in a diamond anvil cell, examine the structure in situ using x-ray diffraction, and interpret the findings with molecular dynamics modeling. Experiment and modeling both predict a two-stage transformation. First, the isotropic glassy carbon undergoes a reversible transformation to an oriented compressed graphitic structure. This is followed by a phase transformation at $\sim 35$ GPa to an unstable, disordered $s{p}^3$ rich structure that reverts on decompression to an oriented graphitic structure. Analysis of the simulated $s{p}^3$ rich material formed at high pressure reveals a noncrystalline structure with two different $s{p}^3$ bond lengths.

The structural, phonon, magnetic, dielectric, and magnetodielectric responses of the pure bulk brownmillerite compound $\mathrm{C}{\mathrm{a}}_2\mathrm{FeCo}{\mathrm{O}}_5$ are reported. This compound showed giant magnetodielectric response (10–24%) induced by strong spin-lattice coupling across its spin reorientation transition (150–250 K). The role of two Debye temperatures pertaining to differently coordinated sites in the dielectric relaxations is established. The positive giant magnetodielectricity is shown to be a direct consequence of the modulations in the lattice degrees of freedom through applied external field across the spin reorientation transition. Our study illustrates control of magnetodielectricity by the spin reorientation transition in a material that possesses strong spin-lattice coupling.

Current state-of-the-art devices for detecting and manipulating Majorana fermions commonly consist of networks of Majorana wires and tunnel junctions. We study a key ingredient of these networks—a topological Josephson junction with charging energy—and we pinpoint crucial features for device implementation. The phase-dependent tunneling term contains both the usual $2\pi$-periodic Josephson term and a $4\pi$-periodic Majorana tunneling term representing the coupling between Majoranas on both sides of the junction. In nontopological junctions when the charging energy is small compared to the Josephson tunneling scale, the low-energy physics is described by $2\pi$ phase slips. By contrast, in a topological junction, due to the $4\pi$ periodicity of the tunneling term, it is usually expected that only $4\pi$ phase slips are possible while $2\pi$ phase slips are suppressed. However, we find that if the ratio between the strengths of the Majorana assisted tunneling and the Josephson tunneling is small, as is likely to be the case for many setups, $2\pi$ phase slips occur and may even dominate the low-energy physics. In this limit, one can view the $4\pi$ phase slips as a pair of $2\pi$ phase slips with arbitrarily large separation. We provide an effective description of the system in terms of $2\pi$ and $4\pi$ phase slips valid for all values of the tunneling ratio. Comparing the spectrum of the effective models with numerical simulations, we determine the crossover between the $4\pi$ phase slip regime to the $2\pi$ phase slip dominated regime. We also discuss the role of the charging energy as well as the implications of our results on the dissipative phase transitions expected in such a system.

We present a framework of semiclassical superconductivity (SC) dynamics that properly includes effects of spatial fluctuations for the attractive Hubbard model. We consider both coherent and adiabatic limits. To model the coherent SC dynamics, we develop a real-space von Neumann equation based on the time-dependent Hartree-Fock-Bogoliubov theory. Applying our method to interaction quenches in the negative-$U$ Hubbard model, we show that the relaxation of SC order at weak coupling is dominated by Landau-damping. At strong coupling, we find a two-stage relaxation of the pairing field: a collapse of the synchronized oscillation of Cooper pairs due to spatial inhomogeneity, followed by a slow relaxation to a quasistationary state. SC dynamics in the adiabatic limit is described by a quantum Landau-Lifshitz equation with Ginzburg-Landau relaxation. Numerical simulations of the pump-probe process show that long-time recovery of the pairing field is dominated by defect dynamics. Our results demonstrate the important role of spatial fluctuations in both limits.

The parameter retrieval is a procedure in which effective material properties are assigned to a given metamaterial. A widely used technique bases on the inversion of reflection and transmission from a metamaterial slab. Thus far, local constitutive relations have been frequently considered in this retrieval procedure to describe the metamaterial at the effective level. This, however, is insufficient. The retrieved local material properties frequently fail to predict reliably the optical response from the slab in situations that deviate from those that have been considered in the retrieval, e.g., when illuminating the slab at a different incidence angle. To significantly improve the situation, we describe here a parameter retrieval, also based on the inversion of reflection and transmission from a slab, that describes the metamaterial at the effective level with nonlocal constitutive relations. We retrieve the effective material parameters at the example of a basic metamaterial, namely, dielectric spheres on a cubic lattice but also on a more advanced, anisotropic metamaterial of current interest, i.e., the fishnet metamaterial. We demonstrate that the nonlocal constitutive relation can describe the optical response much better than local constitutive relation would do. Our approach is widely applicable to a large class of metamaterials.