We report results of Raman scattering experiments on twin-free ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ with the main focus placed on understanding the influence of electronic and spin degrees of freedom on the lattice dynamics. In particular, we scrutinize the $E_g$ modes and the As $A_{1g}$ mode. Each of the two $E_g$ phonons in the tetragonal phase is observed to split into a $B_{2g}$ and a $B_{3g}$ mode upon entering the orthorhombic stripe-magnetic phase. The splitting amounts to approximately 10 ${\mathrm{cm}}^{-1}$ and less than 5 ${\mathrm{cm}}^{-1}$ for the low- and the high-energy $E_g$ mode, respectively. The detailed study of the fully symmetric As mode using parallel incident and outgoing photon polarizations along either the antiferromagnetic or the ferromagnetic Fe-Fe direction reveals an anisotropic variation of the spectral weight with the energy of the exciting laser indicating a polarization-dependent resonance effect. Along with the experiments we present results from density functional theory calculations of the phonon eigenvectors, the dielectric function, and the Raman tensor elements. The comparison of theory and experiment indicates that (i) orbital-selective electronic correlations are crucial to understand the lattice dynamics and (ii) all phonon anomalies originate predominantly from the magnetic ordering and the corresponding reconstruction of the electronic bands at all energies.

Nitrogen-vacancy (NV) defect centers in diamond are strong candidates to generate entangled states in solid-state environments even at room temperature. Quantum correlations in spatially separated NV systems, for distances between NVs ranging from a few nanometers to a few kilometers, have been recently reported. In the present paper, we consider the entanglement transfer from two-mode microwave-squeezed (entangled) photons, which are in resonance with the two lowest NV electron spin states, to initially unentangled NV centers. We first demonstrate that the entanglement transfer process from quantum microwaves to isolated NV electron spins is feasible. We then proceed to extend the previous results to more realistic scenarios where ${}^{13}\mathrm{C}$ nuclear spin baths surrounding each NV are included, quantifying the entanglement transfer efficiency and robustness under the effects of dephasing/dissipation noisy nuclear baths. Finally, we address the issue of assessing the possibility of entanglement transfer from the squeezed microwave light to two remote nuclear spins closely linked to different NV centers.

We consider a two-orbital tight-binding model defined on a layered three-dimensional hexagonal lattice to investigate the properties of topological nodal lines and their associated drumhead surface states. We examine these surface states in centrosymmetric systems, where the bulk nodal lines are of Dirac type (i.e., fourfold degenerate), as well as in noncentrosymmetric systems with strong Rashba and/or Dresselhaus spin-orbit coupling, where the bulk nodal lines are of Weyl type (i.e., twofold degenerate). We find that in noncentrosymmetric systems the nodal lines and their corresponding drumhead surface states are fully spin polarized due to spin-orbit coupling. We show that unique signatures of the topologically nontrivial drumhead surface states can be measured by means of quasiparticle scattering interference, which we compute for both Dirac and Weyl nodal line semimetals.

We reveal the nature of propagation and reflection of light in spin-orbit coupled Dirac metals under external magnetic fields. Such applied magnetic fields split the fourfold degeneracy of a spin-orbit coupled Dirac metal state into a pair of a twofold degeneracy along the direction of the applied magnetic field, resulting in a Weyl band structure. These Weyl metals turn out to play the role of a chiral prism, whose electromagnetic properties are described by axion electrodynamics: An incident monochromatic wave can split into three differently polarized modes (eigenvectors), propagating with different wave numbers (eigenvalues). In particular, the axion electrodynamics allows the longitudinal component naturally inside the Weyl metal state. We evaluate both transmission/reflection coefficients and Faraday/Kerr rotation angles as a function of both an external magnetic field and frequency for various configurations of light propagation. The helicity of the propagating/reflected light is determined by $\mathbf{\nabla }\theta \times {\mathit{E}}_{\mathrm{light}}={\mathit{B}}_{\mathrm{ext}}\times {\mathit{E}}_{\mathrm{light}}$, where $\mathbf{\nabla }\theta ={\mathit{B}}_{\mathrm{ext}}$ is the gradient of the $\theta$ field in the axion term given by the applied magnetic field and ${\mathit{E}}_{\mathrm{light}}$ is the electric field of the incident light. This implies that the direction of the external magnetic field controls the Faraday/Kerr rotation. We find several interesting optical properties of the Weyl metal phase. First, longitudinal oscillating charge density fluctuations along the light propagating direction arise when the pair of Weyl nodes are aligned along the direction of the oscillating magnetic field, which gives rise to the longitudinal component of the electromagnetic wave. Second, the Weyl metal phase becomes more reflective when the external magnetic field is enhanced to be along with $\mathbf{E}//{\mathit{B}}_{\mathrm{ext}}$ due to the longitudinal negative magnetoresistivity, which is a fingerprint of the Weyl metal phase. Third, eigenmodes can have various structures, depending on a parameter $\eta$, which corresponds to a ratio between the conventional Hall effect from normal electrons and the anomalous Hall effect from Weyl electron. We propose these strong magnetic field dependencies of the optical response as the fingerprints of the axion electrodynamics.

Self-similar bulk plasmon-polariton modes in quasiperiodic photonic superlattices were used here for a giant enhancement of second harmonic generation. Results for Fibonacci and Thue-Morse-like superlattices indicate that not only the disorder but also the symmetry of the unit cell plays a crucial role for enhancing the efficiency. In particular, a giant enhancement is shown for Thue-Morse-like systems when compared with the enhancement for Fibonacci-like superlattices.

We discuss herein the emergence of a large entropy change in metallic ${\mathrm{Li}}_{0.33}{\mathrm{VS}}_2$ derived from orbitally assisted loose $\sigma$ bond formation. Comprehensive structural studies based on synchrotron x-ray and neutron diffraction analyses clarify the fabrication of ribbon chains at 375 K, consisting of multiple three-centered two-electron $\sigma$ bonds based on the viewpoint of local chemical bonding. Although the metallic conductivity persists down to the lowest temperature measured, an exceptionally large entropy change as a metal, as much as $\mathrm{\Delta }S=6.6{\mathrm{J}\mathrm{mol}}^{-1}{\mathrm{K}}^{-1}$, appears at the transition. The emergence of a large entropy change in a metallic state may possibly lead to different functional materials, such as a heat-storage material with a rapid thermal response.

Kondo insulators are emerging as a simplified setting to study both magnetic and insulator-to-metal quantum phase transitions. Here, we study the half-filled Anderson lattice model defined on a magnetically frustrated Shastry-Sutherland geometry. We determine a “global” phase diagram that applies to both the local-moment and intermediate-valence regimes. This provides the theoretical basis for understanding how tuning a Kondo insulator by external parameters can close its hybridization gap, liberate the local-moment spins from the conduction electrons, and lead to a magnetically correlated metal. We also calculate the momentum distribution of the single-particle excitations in the Kondo insulating state, and show how Fermi-surface-like features emerge as a precursor to the actual Fermi surfaces of the Kondo-destroyed metals. The implications for an incipient Fermi surface and quantum phase transitions of Kondo insulators including ${\mathrm{SmB}}_6$ are discussed.

Searching for simple models that possess nontrivial controlling properties is one of the central tasks in the field of quantum technologies. In this work, we construct a quantum spin-$1/2$ chain of finite size, termed as controllable spin wire (CSW), in which we have ${\widehat{S}}^z{\widehat{S}}^z$ (Ising) interactions with a transverse field in the bulk and ${\widehat{S}}^x{\widehat{S}}^z$ and ${\widehat{S}}^z{\widehat{S}}^z$ couplings with a canted field on the boundaries. The Hamiltonians on the boundaries, dubbed as tuning Hamiltonians (TH's), bear the same form as the effective Hamiltonians emerging in the so-called “quantum entanglement simulator” that is originally proposed for mimicking infinite models. We show that tuning the TH's (parametrized by $\alpha$) can trigger nontrivial controlling of the bulk properties, including the degeneracy of energy/entanglement spectra, and the response to the magnetic field $h_{bulk}$ in the bulk. A universal point dubbed as ${\alpha }^s$ emerges. For $\alpha >{\alpha }^s$, the ground-state diagram versus $h_{bulk}$ consists of three “phases,” which are NeéL and polarized phases, and an emergent pseudomagnet phase, distinguished by entanglement and magnetization. For $\alpha <{\alpha }^s$, the phase diagram changes completely, with no steplike behaviors to distinguish phases. Due to its controlling properties and simplicity, the CSW could potentially serve in future the experiments for developing quantum devices.

We discuss electronic properties and their evolution for the linear chain of $H_2$ molecules in the presence of a uniform external force $f$ acting along the chain. The system is described by an extended Hubbard model within a fully microscopic approach. Explicitly, the microscopic parameters describing the intra- and intersite Coulomb interactions are determined together with the hopping integrals, by optimizing simultaneously the system ground state energy and the single-particle wave functions in the correlated state. The many-body wave function is taken in the Jastrow form and the variational Monte Carlo (VMC) method is used in combination with an ab initio approach to determine the energy. Both the effective Bohr radii of the renormalized single-particle wave functions and the many-body wave function parameters are determined for each $f$, which is the only external parameter in the whole analysis. Hence the evolution of the system can be analyzed in detail as a function of the equilibrium intermolecular distance, which in turn is determined for each $f$ value. The transition to the atomic state, including the Peierls distortion stability, can thus be studied in a systematic manner, particularly near the threshold of the dissociation of the molecular into an atomic chain. We also show that interelectronic correlations enhance the Peierls distortion. The computational reliability of the VMC approach is also estimated.

The Andreev transport through a large-spin magnetic molecule, such as a single molecular magnet, attached to superconducting and ferromagnetic leads is studied theoretically by means of the real-time diagrammatic technique. It is shown that due to the proximity effect, molecular Andreev bound states form in the system, with energies depending on the intrinsic parameters of the molecule. We study the spin-resolved Andreev current, conductance, and tunnel magnetoresistance in both the linear and nonlinear response regimes and find regions of negative differential conductance, as well as either enhanced or negative tunnel magnetoresistance. The mechanisms leading to those effects are thoroughly discussed. It is also shown that the tunnel magnetoresistance can provide information about particular spin multiplets responsible for the Andreev reflection processes.

We present a new study of the nonlinear transport of a two-dimensional electron crystal on the surface of liquid helium confined in a $10\text{−}\mu \mathrm{m}$-wide channel in which the effective length of the crystal can be varied from 10 to $215\mu \mathrm{m}$. At low driving voltages, the moving electron crystal is strongly coupled to deformation of the liquid surface arising from resonant excitation of surface capillary waves, ripplons, while at higher driving voltages the crystal decouples from the deformation. We find strong dependence of the decoupling threshold of the driving electric field acting on the electrons, on the size of the crystal. In particular, the threshold electric field significantly decreases when the length of the crystal becomes shorter than $25\mu \mathrm{m}$. We explain this effect as arising from weakening of surface deformations due to radiative loss of resonantly excited ripplons from an electron crystal of finite size, and we account for the observed effect using an instructive analytical model.

It has been long accepted that the second-order Raman bands in carbon nanotubes are enhanced through the double-resonance mechanism. Although separate aspects of this mechanism have been studied for a few second-order Raman bands, including the most intense defect-induced D band and the two-phonon 2D band, a complete computational approach to the second-order bands is still lacking. Here, we propose such an approach, entirely based on a symmetry-adapted nonorthogonal tight-binding model with ab initio derived parameters. As a case study, we consider a nanotube $(6,5)$, for which we calculate the two-phonon spectrum. We investigate in detail the 2D band and identify three contributions to it: a nondispersive one and two dispersive ones, which are found to depend on the electron and phonon dispersion of the nanotube, and on the laser excitation. We also predict two-phonon bands, which are not allowed in the parent structure graphene. The obtained two-phonon bands are in very good agreement with the available experimental data. The symmetry-adapted formalism makes feasible the calculation of the two-phonon Raman bands of any observable nanotube.

We develop a theory for strain control of Majorana zero energy modes and the Josephson effect in black phosphorus (BP) devices proximity coupled to a superconductor. Employing realistic values for the band parameters subject to strain, we show that the strain closes the intrinsic band gap of BP; however, the proximity effect from the superconductor reopens it and creates Dirac and Weyl nodes. Our results illustrate that Majorana zero energy flat bands connect the nodes within the band-inverted regime in which their associated density of states is localized at the edges of the device. In a ferromagnetically mediated Josephson configuration, the exchange field induces superharmonics in the supercurrent phase relation in addition to a ${\phi }_0$ phase shift, corresponding to a spontaneous supercurrent, and strain offers an efficient tool to control these phenomena. We analyze the experimental implications of our findings and show that they can pave the way for creating a rich platform for studying two-dimensional Dirac and Weyl superconductivity.

This work brings together thermodynamics and nonequilibrium quantum theory, by showing that a real-time diagrammatic technique on the Keldysh contour is an equivalent of stochastic thermodynamics for non-Markovian quantum machines (heat engines, refrigerators, etc.). Symmetries are found between quantum trajectories and their time reverses on the Keldysh contour, for any interacting quantum system coupled to ideal reservoirs of electrons, phonons, or photons. These lead to quantum fluctuation theorems the same as the well-known classical ones (Jarzynski and Crooks equalities, integral fluctuation theorem, etc.), whether the system's dynamics are Markovian or not. Some of these are also shown to hold for nonfactorizable initial states. The sequential tunneling approximation and the cotunneling approximation are both shown to respect the symmetries that ensure the fluctuation theorems. For all initial states, energy conservation ensures that the first law of thermodynamics holds on average, while the above symmetries ensure that the second law of thermodynamics holds on average, even if fluctuations violate it.

Two-dimensional conducting interfaces in ${\mathrm{SrTiO}}_3$-based heterostructures display a variety of coexisting and competing physical phenomena, which can be tuned by the application of a gate voltage. (111) oriented heterostructures have recently gained attention due to the possibility of finding exotic physics in these systems due to their hexagonal surface crystal symmetry. In this work, we use magnetoresistance to study the evolution of spin-orbit interaction and magnetism in (111) oriented (${\mathrm{La}}_{0.3}{\mathrm{Sr}}_{0.7})\left({\mathrm{Al}}_{0.65}{\mathrm{Ta}}_{0.35}\right){\mathrm{O}}_3/{\mathrm{SrTiO}}_3$. At more positive values of the gate voltage, which correspond to high carrier densities, we find that transport is multiband, and dominated by high-mobility carriers with a tendency toward weak localization. At more negative gate voltages, the carrier density is reduced, the high-mobility bands are depopulated, and weak antilocalization effects begin to dominate, indicating that spin-orbit interaction becomes stronger. At millikelvin temperatures, at gate voltages corresponding to the strong spin-orbit regime, we observe hysteresis in magnetoresistance, indicative of ferromagnetism in the system. Our results suggest that in the (111) (${\mathrm{La}}_{0.3}{\mathrm{Sr}}_{0.7})\left({\mathrm{Al}}_{0.65}{\mathrm{Ta}}_{0.35}\right){\mathrm{O}}_3/{\mathrm{SrTiO}}_3$ system, low-mobility carriers that experience strong spin-orbit interactions participate in creating magnetic order in the system.

Recently reported synthesis of ${\mathrm{FeO}}_2$ at high pressure has stimulated great interest in exploring this new iron oxide and elucidating its properties. Here, we present a systematic computational study of crystal structure, chemical bonding, and sound velocity of ${\mathrm{FeO}}_2$ in a wide range of pressure. Our results establish thermodynamic stability of the experimentally observed pyrite phase (P-phase) of ${\mathrm{FeO}}_2$ at pressures above 74 GPa and unveil two metastable ${\mathrm{FeO}}_2$ phases in $Pbcn$ and $P{4}_2/mnm$ symmetry at lower pressures. Simulated x-ray diffraction (XRD) spectra of $Pbcn$ and $P{4}_2/mnm{\mathrm{FeO}}_2$ match well with measured XRD data of the decompression products of P-phase ${\mathrm{FeO}}_2$, providing compelling evidence for the presence of these metastable phases. Energetic calculations reveal unusually soft O-O bonds in P-phase ${\mathrm{FeO}}_2$ stemming from a low-frequency libration mode of ${\mathrm{FeO}}_6$ octahedra, rendering the O-O bond length highly sensitive to computational and physical environments. Calculated sound-velocity profiles of P-phase ${\mathrm{FeO}}_2$ are markedly different from those of the $Pbcn$ and $P{4}_2/mnm$ phases, underscoring their distinct seismic signatures. Our findings offer insights for understanding the rich structural, bonding, and elastic behaviors of this newly discovered iron oxide.

Formation of isolated chiral skyrmions by homogeneous and heterogeneous nucleation has been studied in thin layers of cubic helimagnets via elongation of torons and chiral bobbers, respectively. Both torons and bobbers are localized in three dimensions, contain singularities, and, according to the theoretical analysis within the standard phenomenological models, can exist as metastable states in saturated and modulated phases of noncentrosymmetric ferromagnets. Their elongation into the defect-free skyrmion filament is facilitated by small anisotropic contributions making skyrmion cores negative with respect to the surrounding parental state. We show that isolated magnetic torons pose the same problem of compatibility with a surrounding phase as the torons in confinement-frustrated chiral nematics [I. I. Smalyukh et al., Nat. Mater. 9, 139 (2010)]. We underline the distinct features of magnetic and liquid-crystal torons and calculate phase diagrams indicating their stability regions.

Reversed structures of artificial spin-ice systems, where elongated holes with elliptical shape (antidots) are arranged into a square array with two orthogonal sublattices, are referred to as antisquared spin ice. Using Brillouin light-scattering spectroscopy and plane-wave-method calculations, we investigate the spin-wave propagation perpendicular to the applied field direction for two 20-nm-thick Permalloy nanostructures which differ by the presence of single- and double-elliptical antidots. For the spin-wave propagation along the principal antidot lattice axis, the spectrum consists of flat bands separated by several frequency gaps which are the effect of spin-wave amplitude confinement in the regions between antidots. Contrarily, for propagation direction at 45° with respect to the antidot symmetry axis, straight and narrow channels of propagation are formed, leading to broadening of bands and closing of the magnonics gaps. Interestingly, in this case, extra magnonic band gaps occur due to the additional periodicity along this direction. The width and the position of these gaps depend on the presence of single or double antidots. In this context, we discuss possibilities for the tuning of spin-wave spectra in antisquared spin-ice structures.

Motivated by the recent experimentally observed manifestations of the quantum critical point fluctuations in the thermodynamic properties of multiband superconductors, we derive a general expression for the Josephson current of various junctions between two superconductors in the phase of superconductivity coexistence with the spin-density wave. We demonstrate that the critical current peaks at the quantum critical point that separates pure and mixed superconducting phases. We argue that our results are generic and, in particular, can be adopted to explain the recent observations of a nonmonotonic dependence of the supercurrent on the external pressure in the heavy fermion superconductor ${\mathrm{CeRhIn}}_5$ and on the chemical doping in iron-based superconductors such as $\mathrm{Ba}{\left({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x\right)}_2{\mathrm{As}}_2$.