The electron excitation spectra of the Li-intercalated graphene (${\mathrm{LiC}}_2$) are obtained using a high-accuracy ab initio density function theory calculation within the random phase approximation. The low-energy parts of the spectra are analyzed to predict the existence of two quasi-two-dimensional collective modes: a Dirac plasmon similar to the one in the doped graphene but significantly stronger, and an acoustic plasmon.

We investigate the evolution of the magnetic properties in FeS under pressure and show that these cannot be explained solely in terms of the spin-state transition from a high to low spin state due to an increase of the crystal field. Using a combination of density functional theory and dynamical mean-field theory (DFT+DMFT), our calculations show that at normal conditions the ${\mathrm{Fe}}^{2+}$ ions are in the $3d^6$ high-spin ($S=2$) state, with some admixture of a $3d^7\underline{L}$ ($S=3/2$) configuration, where $\underline{L}$ stands for the ligand hole. Suppressing the magnetic moment by uniform compression is related to a substantial increase in electron delocalization and occupation of several lower spin configurations. The electronic configuration of Fe ions cannot be characterized by a single ionic state, but only by a mixture of the $3d^7\underline{L},3d^8{\underline{L}}^2$, and $3d^9{\underline{L}}^3$ configurations at pressures $\sim 7.5\mathrm{GPa}$. The local spin-spin correlation function shows well-defined local magnetic moment, corresponding to a large lifetime in the high spin state at normal conditions. Under pressure FeS demonstrates a transition to a mixed state with small lifetimes in each of the spin configurations.

We report first-principles density-functional theory results on the electronic and magnetic properties of the recently discovered superconducting FeS, which reveals important differences with the other members of the iron-chalcogenides (FeSe and FeTe). The band structure of FeS is characterized by two hole bands at the Fermi energy with a fully occupied $d_{xy}$ band at $\mathrm{\Gamma }$. A stripe-antiferromagnetic phase with a small magnetic moment is the most stable magnetic solution, but different magnetic phases have comparable energies indicating a tight competition. Including local interactions treated within dynamical mean-field theory, we find significant correlation effects with orbital-dependent strength and character, even if all the fingerprints of correlations are slightly weaker than in FeSe. The study of the effect of pressure reveals significant changes in the electronic structure of the material and of the correlation effects. These results point toward further studies on the possible superconducting phase stabilized by pressure effects or two dimensionality, in analogy with pressurized and monolayer FeSe.

We theoretically investigate the $d^4$ (${\mathrm{Cr}}^{2+}$) compound ${\mathrm{BaCr}}_2{\mathrm{As}}_2$ and show that, despite non-negligible differences in the electronic structure, its many-body physics mirrors that of ${\mathrm{BaFe}}_2{\mathrm{As}}_2$, which has instead a $d^6$ (${\mathrm{Fe}}^{2+}$) configuration. This reflects a symmetry of the electron correlation effects around the half-filled $d^5$ Mott insulating state. Dynamical mean-field calculations correctly reproduce the experimental antiferromagnetic phase and, for realistic values of the interactions, they show a moderate mass enhancement of order $\sim 2$. This value can be insensitive to, or even be lowered by, an increase of the interaction parameters, as a result of a larger magnetic moment. In the paramagnetic phase however, correlation effects are much stronger, and the influence of the half-filled Mott insulator shows up as a crossover from a weakly correlated to an orbitally differentiated “Hund's metal” phase. This mirrors an analogous phenomenon in $d^6$ iron compounds including a strong enhancement of the compressibility in a zone just inside the frontier between the normal and the Hund's metal. The experimental evidence and our theoretical description place ${\mathrm{BaCr}}_2{\mathrm{As}}_2$ at interaction strength slightly below the crossover which implies that negative pressures and/or electron doping (e.g., Cr $\to$ Mn, Fe or Ba $\to$ Sc, Y, La) might strongly enhance the compressibility, thereby possibly inducing high-$T_c$ superconductivity in this nonsuperconducting compound.

We present a detailed study of the lifetime of optical spectral holes due to population storage in Zeeman sublevels of ${\mathrm{Nd}}^{3+}:{\mathrm{Y}}_2{\mathrm{SiO}}_5$. The lifetime is measured as a function of magnetic field strength and orientation, temperature, and ${\mathrm{Nd}}^{3+}$ doping concentration. At the lowest temperature of 3 K we find a general trend where the lifetime is short at low field strengths, then increases to a maximum lifetime at a few hundred mT, and then finally decays rapidly for high field strengths. This behavior can be modeled with a relaxation rate dominated by ${\mathrm{Nd}}^{3+}-{\mathrm{Nd}}^{3+}$ cross relaxation at low fields and spin lattice relaxation at high magnetic fields. The maximum lifetime depends strongly on both the field strength and orientation, due to the competition between these processes and their different angular dependencies. The cross relaxation limits the maximum lifetime for concentrations as low as 30 ppm of ${\mathrm{Nd}}^{3+}$ ions. By decreasing the concentration to less than 1 ppm we could completely eliminate the cross relaxation, reaching a lifetime of 3.8 s at 3 K. At higher temperatures the spectral hole lifetime is limited by the magnetic-field-independent Raman and Orbach processes. In addition we show that the cross relaxation rate can be strongly reduced by creating spectrally large holes of the order of the optical inhomogeneous broadening. Our results are important for the development and design of new rare-earth-ion doped crystals for quantum information processing and narrow-band spectral filtering for biological tissue imaging.

We determine experimentally the spinor interaction constants of lower polaritons ${\alpha }_1$ and ${\alpha }_2$ using a resonant pump-probe spectroscopy with a spectrally narrow pump pulse. Our experimental findings are analyzed with the Bogoliubov-type theory and a mean-field two-channel model based on the lower polariton and biexciton basis. We find an enhancement of the attractive interaction and a dissipative nonlinearity of lower polaritons with antiparallel spins in the vicinity of the biexciton resonance when the energy of two lower polaritons approaches energetically that of the biexciton. These observations are consistent with the existence of a scattering resonance between lower polaritons and biexcitons (polaritonic Feshbach resonance).

The effects of external electric and magnetic fields on the energy spectrum of quantum rings made out of a bidimensional semiconductor material with anisotropic band structures are investigated within the effective-mass model. The interplay between the effective-mass anisotropy and the radial confinement leads to wave functions that are strongly localized at two diametrically opposite regions where the kinetic energy is lowest due to the highest effective mass. We show that this quantum phenomenon has clear consequences on the behavior of the energy states in the presence of applied in-plane electric fields and out-of-plane magnetic fields. In the former, the quantum confined Stark effect is observed with either linear or quadratic shifts, depending on the direction of the applied field. As for the latter, the usual Aharonov-Bohm oscillations are not observed for a circularly symmetric confining potential, however they can be reinstated if an elliptic ring with an appropriate aspect ratio is chosen.

We develop a multiscale approach to quantify the increase in the recombined fraction of point defects under irradiation resulting from dilute solute additions to a solid solution. This methodology provides design principles for radiation-resistant materials. Using an existing database of solute diffusivities, we identify Sb as one of the most efficient solutes for this purpose in a Cu matrix. We perform density-functional-theory calculations to obtain binding and migration energies of Sb atoms, vacancies, and self-interstitial atoms in various configurations. The computed data informs the self-consistent mean-field formalism to calculate transport coefficients, allowing us to make quantitative predictions of the recombined fraction of point defects as a function of temperature and irradiation rate using homogeneous rate equations. We identify two different mechanisms according to which solutes lead to an increase in the recombined fraction of point defects; at low temperature, solutes slow down vacancies (kinetic effect), while at high temperature, solutes stabilize vacancies in the solid solution (thermodynamic effect). Extension to other metallic matrices and solutes are discussed.

Surface-plasmon polaritons are naturally generated upon excitation of metals with high-frequency electromagnetic waves. However, the concept of spoof plasmons has made it possible to generate plasmoniclike effects in microwave electrodynamics, magnetics, and even acoustics. Similarly, in this paper, the concept of localized surface plate modes (SPMs) is introduced. It is demonstrated that SPMs can be generated on a two-dimensional (clamped or stress-free) cylindrical surface with subwavelength corrugations, which resides on a thin elastic plate, under excitation by an incident flexural plane wave. Numerical characterization of this corrugated rigid structure shows that it is elastically equivalent to a cylindrical scatterer with dispersive but uniformly negative flexural rigidity. This, indeed, suggests that plasmoniclike elastic materials can be engineered with potential applications in various areas including earthquake sensing and elastic imaging and cloaking.

The strength of spin-orbit torque in ferromagnetic structures is characterized by fieldlike and dampinglike effective fields. Conventionally, two distinct measurement approaches are employed to quantify the magnitude of the respective effective fields in structures with in-plane magnetic anisotropy. Here, we propose and demonstrate a self-validating method, which enables simultaneous quantification of both the fieldlike and dampinglike terms in structures with in-plane magnetic anisotropy. An analytical expression is derived and validated by harmonic Hall resistance measurement. Both the fieldlike and dampinglike effective fields are extracted from a single measurement using the derived fitting functions for the second harmonic Hall resistance. The first harmonic Hall resistance acts as a reference to confirm that the experimental parameters are consistent with the derived equations.

We study a one-dimensional topological superconductor, the Kitaev chain, under the influence of a non-Hermitian but $\mathcal{PT}$-symmetric potential. This potential introduces gain and loss in the system in equal parts. We show that the stability of the topological phase is influenced by the gain/loss strength and explicitly derive the bulk topological invariant in a bipartite lattice as well as compute the corresponding phase diagram using analytical and numerical methods. Furthermore, we find that the edge state is exponentially localized near the ends of the wire despite the presence of gain and loss of probability amplitude in that region.

In the temperature-magnetic field phase diagram, the binary metallic compound MnSi exhibits three magnetic phases below $T_{\mathrm{c}}\approx 29$ K. An unconventional helicoidal phase is observed in zero field. At moderate field intensity a conical phase sets in. Near $T_{\mathrm{c}}$, in an intermediate field range, a skyrmion lattice phase appears. Here we show the magnetic structure in the conical phase to strongly depend on the field direction and to deviate substantially from a conventional conical structure.

We have investigated electric-field induced magnetic phase transition between the skyrmion lattice and the helix in the multiferroic ${\mathrm{Cu}}_2{\mathrm{OSeO}}_3$ by means of small-angle soft x-ray scattering at the Cu $L_3$ absorption edge. By application of electric fields, the skyrmion lattice transforms into helices with distinct modulation vectors depending on the sign of the electric field. In particular, the helix realized under the positive electric field never appears unless the electric field is applied.

We demonstrate that in diffusive superconductor/ferromagnet/superconductor (S/F/S) junctions a finite, anomalous Josephson current can flow even at zero phase difference between the S electrodes. The conditions for the observation of this effect are noncoplanar magnetization distribution and a broken magnetization inversion symmetry of the superconducting current. The latter symmetry is intrinsic for the widely used quasiclassical approximation and prevented previous works based on this approximation from obtaining the Josephson anomalous current. We show that this symmetry can be removed by introducing spin-dependent boundary conditions for the quasiclassical equations at the superconducting/ferromagnet interfaces in diffusive systems. Using this recipe, we consider generic multilayer magnetic systems and determine the ideal experimental conditions in order to maximize the anomalous current.

The band gaps of a few selected semiconductors/insulators are obtained from the self-consistent solution of the Hedin's equations. Two different schemes to include the vertex corrections are studied: (i) the vertex function of the first order (in the screened interaction $W$) is applied in both polarizability $P$ and self-energy $\mathrm{\Sigma }$, and (ii) the vertex function obtained from the Bethe-Salpeter equation is used in $P$ whereas the vertex of the first order is used in $\mathrm{\Sigma }$. Both schemes show considerable improvement in the accuracy of the calculated band gaps as compared to the self-consistent $GW$ approach ($\mathrm{sc}GW$) and to the self-consistent quasiparticle $GW$ approach ($\mathrm{QS}GW$). To further distinguish between the performances of two vertex-corrected schemes one has to properly take into account the effect of the electron-phonon interaction on the calculated band gaps which appears to be of the same magnitude as the difference between schemes (i) and (ii).

The authors report ${}^{29}\mathrm{Si}$ nuclear spin-echo oscillations and decay measurements on a single crystal of ${\mathrm{YbRh}}_2{\mathrm{Si}}_2$. These quantities are found to be $T$ independent from 300 K down to $\sim 20$ K, showing, however, a strong $T$ dependence below 20 K. These results indicate that electronic states near the Fermi level are modified at low temperatures. The observed spin-echo oscillations can be interpreted with the Ruderman-Kittel and pseudodipolar interactions between nearest-neighbor Si nuclei driven by conduction electron scattering at the Fermi surface. Possible modifications to the Fermi surface at low temperatures are discussed.

In the Fock representation, we propose a framework to construct the generalized matrix product states (MPS) for topological phases with ${\mathbb{Z}}_p$ parafermions. Unlike the ${\mathbb{Z}}_2$ Majorana fermions, the ${\mathbb{Z}}_p$ parafermions form intrinsically interacting systems. Here we explicitly construct two topologically distinct classes of irreducible ${\mathbb{Z}}_3$ parafermionic MPS wave functions, characterized by one or two parafermionic zero modes at each end of an open chain. Their corresponding parent Hamiltonians are found as the fixed point models of the single ${\mathbb{Z}}_3$ parafermion chain and two-coupled parafermion chains with ${\mathbb{Z}}_3\times {\mathbb{Z}}_3$ symmetry. Our results thus pave the road to investigate all possible topological phases with ${\mathbb{Z}}_p$ parafermions within the matrix product representation in one dimension.

We study the Fermi-Hubbard model in the strongly correlated Mott phase under the influence of a harmonically oscillating hopping rate $J\left(t\right)=J_0+\mathrm{\Delta }Jcos\left(\omega t\right)$. Apart from the well-known fundamental resonance, where the frequency $\omega$ of this oscillation equals (or a little exceeds) the Mott gap, we also find higher-order resonances where multiple particle-hole pairs are created when $\omega$ is near an integer multiple of the gap. These findings should be relevant for experimental realizations such as ultracold fermionic atoms in optical lattices or pump-probe experiments using laser pulses incident on correlated electrons in solid-state materials.

Motivated by recent experimental research on the temperature-induced unfold of ${\mathrm{C}}_{60}$ molecules, carbon nanostructures are proposed based on the complex assemblies of molecules resulting from the hierarchical unzipping of fullerene cages. Such assemblies can result in either 1D ribbons or 2D porous membrane systems whose electronic structure is studied here using ab initio calculations. These ribbons and membranes show versatile electronic behaviors such as direct or indirect band gaps that can be fine tuned as a function of the details of the atomic structure, such as the type of assembly and ribbon width.