$(\mathrm{Mg},\mathrm{Fe})\mathrm{Si}{\mathrm{O}}_3$ perovskite is the most abundant mineral of the Earth's lower mantle, and compounds with the perovskite structure are perhaps the most widely employed ceramics. Hence, they attract both geophysicists and material scientists. Several investigations attempted to predict their structural evolution at high pressure, and recent advancements highlighted that perovskites having ions with the same formal valence at both polyhedral sites (i.e., $3+:3+)$ define different compressional patterns when transition metal ions (TMI) are involved. In this study, in situ high-pressure synchrotron XRD measurements coupled with ab initio simulations of the electronic population of $\mathrm{NdCr}{\mathrm{O}}_3$ perovskite are compared with the compressional feature of $\mathrm{NdGa}{\mathrm{O}}_3$. Almost identical from a steric point of view $(\mathrm{C}{\mathrm{r}}^{3+}$ and $\mathrm{G}{\mathrm{a}}^{3+}$ have almost the same ionic radius), the different electronic configuration of octahedrally coordinated ions – which leads to a redistribution of electrons at the $3d$ orbitals for $\mathrm{C}{\mathrm{r}}^{3+}$ – allows the crystal field stabilization energy (CFSE) to act as a vehicle of octahedral softening in $\mathrm{NdCr}{\mathrm{O}}_3$ or it turns octahedra into rigid units when CFSE is null as in $\mathrm{NdGa}{\mathrm{O}}_3$. Besides to highlight that different electronic configurations can act as a primary effect during compression of perovskite compounds, our findings have a deep repercussion on the way the compressibility of perovskites have to be modeled.

We present a general framework to study quantum disordered systems in the context of the Kikuchi's cluster variational method (CVM). The method relies in the solution of message passing-like equations for single instances or in the iterative solution of complex population dynamic algorithms for an average case scenario. We first show how a standard application of the Kikuchi's CVM can be easily translated to message passing equations for specific instances of the disordered system. We then present an “ad hoc” extension of these equations to a population dynamic algorithm representing an average case scenario. At the Bethe level, these equations are equivalent to the dynamic population equations that can be derived from a proper cavity ansatz. However, at the plaquette approximation, the interpretation is more subtle and we discuss it taking also into account previous results in classical disordered models. Moreover, we develop a formalism to properly deal with the average case scenario using a replica-symmetric ansatz within this CVM for quantum disordered systems. Finally, we present and discuss numerical solutions of the different approximations for the quantum transverse Ising model and the quantum random field Ising model in two-dimensional lattices.

In a recent publication [W. Dou et al., Phys. Rev. Lett. 119, 046001 (2017)], using the quantum-classical Liouville equation (QCLE), we derived a very general form for the electronic friction felt by a molecule moving near one or many metal surfaces. Moreover, we have already proved the equivalence of the QCLE electronic friction with the Head-Gordon–Tully model as well as a generalized version of von Oppen's nonequilibrium Green's function (NEGF) method at equilibrium [W. Dou and J. E. Subotnik, Phys. Rev. B 96, 104305 (2017)]. In the present paper, we now further prove the equivalence between the QCLE friction and the NEGF friction for the case of multiple metal surfaces and an out-of-equilibrium electronic current without electron-electron interactions. The present results reinforce our recent claim that there is only one universal electronic friction tensor arising from the Born-Oppenheimer approximation.

In chiral $p$-wave superconductors, magnetic flux patterns may appear spontaneously when translational symmetry is broken such as at surfaces, domain walls, or impurities. However, in the candidate material ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ no direct signs of such magnetic fields have been detected experimentally. In this paper, the flux pattern at the edge of a disk-shaped sample is examined using the phenomenological Ginzburg-Landau approach. The detailed shape of the flux pattern, including self-screening, is computed numerically for different surface types by systematically scanning a range of boundary conditions. Moreover, specific features of the electronic structure are included qualitatively through the coefficients in the Ginzburg-Landau functional. Both the shape and the magnitude of the flux pattern are found to be highly sensitive to all considered parameters. In conclusion, such spontaneous magnetic flux patterns are not a universal feature of chiral $p$-wave superconductors.

${}^{139}\mathrm{La}$ NMR is suited for investigations into magnetic properties of ${\mathrm{La}}_2{\mathrm{CuO}}_4$-based cuprates in the vicinity of their magnetic instabilities, owing to the modest hyperfine interactions between ${}^{139}\mathrm{La}$ nuclear spins and Cu electron spins. We report comprehensive ${}^{139}\mathrm{La}$ NMR measurements on a single-crystal sample of high-$T_c$ superconductor ${\mathrm{La}}_{1.885}{\mathrm{Sr}}_{0.115}{\mathrm{CuO}}_4$ in a broad temperature range across the charge and spin order transitions $(T_{\mathrm{charge}}\simeq 80$ K, $T_{\mathrm{spin}}^{\mathrm{neutron}}\simeq T_c=30$ K). From the high-precision measurements of the linewidth for the nuclear spin $I_z=+1/2$ to $-1/2$ central transition, we show that paramagnetic line broadening sets in precisely at $T_{\mathrm{charge}}$ due to enhanced spin correlations within the ${\mathrm{CuO}}_2$ planes. Additional paramagnetic line broadening ensues below $\sim 35$ K, signaling that Cu spins in some segments of ${\mathrm{CuO}}_2$ planes are on the verge of three-dimensional magnetic order. A static hyperfine magnetic field arising from ordered Cu moments along the $ab$ plane, however, begins to develop only below $T_{\mathrm{spin}}^{\mu SR}=15–20$ K, where earlier muon spin rotation measurements detected Larmor precession for a small volume fraction $\left(\sim 20\%\right)$ of the sample. Based on the measurement of ${}^{139}\mathrm{La}$ nuclear-spin-lattice relaxation rate $1/T_1$, we also show that charge order triggers enhancement of low-frequency Cu spin fluctuations inhomogeneously; a growing fraction of ${}^{139}\mathrm{La}$ sites is affected by enhanced low-frequency spin fluctuations toward the eventual magnetic order, whereas a diminishing fraction continues to exhibit a behavior analogous to the optimally superconducting phase even below $T_{\mathrm{charge}}$. These ${}^{139}\mathrm{La}$ NMR results corroborate our recent ${}^{63}\mathrm{Cu}$ NMR observation that a very broad, anomalous winglike signal gradually emerges below $T_{\mathrm{charge}}$, whereas the normally behaving, narrower main peak is gradually wiped out [T. Imai et al., Phys. Rev. B 96, 224508 (2017)]. Furthermore, we show that the enhancement of low-energy spin excitations in the low-temperature regime below $T_{\mathrm{spin}}^{\mathrm{neutron}}\left(\simeq T_c\right)$ depends strongly on the magnitude and orientation of the applied magnetic field.

${}^{73}\mathrm{Ge}$ and ${}^{59}\mathrm{Co}$ nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements have been performed on a ${}^{73}\mathrm{Ge}$-enriched single-crystalline sample of the ferromagnetic superconductor UCoGe in the paramagnetic state. The ${}^{73}\mathrm{Ge}$ NQR parameters deduced from NQR and NMR are close to those of another isostructural ferromagnetic superconductor URhGe. The Knight shifts of the Ge and Co sites are well scaled to each other when the magnetic field is parallel to the $b$ or $c$ axis. The hyperfine coupling constants of Ge are estimated to be close to those of Co. The large difference of spin susceptibilities between the $a$ and $b$ axes could lead to the different response of the superconductivity and ferromagnetism with the field parallel to these directions. The temperature dependence of the nuclear spin-lattice relaxation rates $1/T_1$ at the two sites is similar to each other above 5 K. These results indicate that the itinerant U-$5f$ electrons are responsible for the ferromagnetism in this compound, consistent with previous studies. The similarities and differences in the three ferromagnetic superconductors are discussed.

Simple hexagonal graphite, also known as AA graphite, is a metastable configuration of graphite. Using tight-binding approximation, it is easy to show that AA graphite is a metal with well-defined Fermi surface. The Fermi surface consists of two sheets, each shaped like a rugby ball. One sheet corresponds to electron states, another corresponds to hole states. The Fermi surface demonstrates good nesting: a suitable translation in the reciprocal space superposes one sheet onto another. In the presence of the electron-electron repulsion, a nested Fermi surface is unstable with respect to spin-density-wave ordering. This instability is studied using the mean-field theory at zero temperature, and the spin-density-wave order parameter is evaluated.

Renormalization of Landau level energies in graphene in strong magnetic field due to Coulomb interaction is studied theoretically, and calculations are compared with two experiments on carrier-density dependent scanning tunneling spectroscopy. An approximate preservation of the square-root dependence of the energies of Landau levels on their numbers and magnetic field in the presence of the interaction is examined. Many-body calculations of the renormalized Fermi velocity with the statically screened interaction taken in the random-phase approximation show good agreement with both experiments. The crucial role of the screening in achieving quantitative agreement is found. The main contribution to the observed rapid logarithmic growth of the renormalized Fermi velocity on approach to the charge neutrality point turned out to be caused not by mere exchange interaction effects, but by weakening of the screening at decreasing carrier density. The importance of a self-consistent treatment of the screening is also demonstrated.

We investigate the spin transport properties of a topological magnon insulator, a magnetic insulator characterized by topologically nontrivial bulk magnon bands and protected magnon edge modes located in the bulk band gaps. Employing the Landau-Lifshitz-Gilbert phenomenology, we calculate the spin current driven through a normal $\mathrm{metal}|\mathrm{topological}$ magnon $\mathrm{insulator}|\mathrm{normal}$ metal heterostructure by a spin accumulation imbalance between the metals, with and without random lattice defects. We show that bulk and edge transport are characterized by different length scales. This results in a characteristic system size where the magnon transport crosses over from being bulk dominated for small systems to edge dominated for larger systems. These findings are generic and relevant for topological transport in systems of nonconserved bosons.

We report on the nonequilibrium spin noise of a single InGaAs quantum dot charged by a single hole under strong driving by a linearly polarized probe light field. The spectral dependency of the spin noise power evidences a homogeneous broadening and negligible charge fluctuations in the environment of the unbiased quantum dot. Full analysis of the spin noise spectra beyond the fluctuation-dissipation theorem yields the heavy-hole spin dynamics as well as the trion spin dynamics. Moreover, the experiment reveals an additional much weaker noise contribution in the Kerr rotation noise spectra. This additional noise contribution has a maximum at the quantum dot resonance and shows a significantly longer correlation time. Magnetic-field-dependent measurements in combination with theoretical modeling prove that this additional noise contribution unveils a charge reoccupation noise which is intrinsic in naturally charged quantum dots.

The spinel-structured lithium manganese oxide (${\mathrm{LiMn}}_2{\mathrm{O}}_4$) is a material currently used as cathode for secondary lithium-ion batteries, but whose properties are not yet fully understood. Here, we report a computational investigation of the inversion thermodynamics and electronic behavior of ${\mathrm{LiMn}}_2{\mathrm{O}}_4$ derived from spin-polarized density functional theory calculations with a Hubbard Hamiltonian and long-range dispersion corrections (DFT+U–D3). Based on the analysis of the configurational free energy, we have elucidated a partially inverse equilibrium cation distribution for the ${\mathrm{LiMn}}_2{\mathrm{O}}_4$ spinel. This equilibrium degree of inversion is rationalized in terms of the crystal field stabilization effects and the difference between the size of the cations. We compare the atomic charges with the oxidation numbers for each degree of inversion. We found segregation of the Mn charge once these ions occupy the tetrahedral and octahedral sites of the spinel. We have obtained the atomic projections of the electronic band structure and density of states, showing that the normal ${\mathrm{LiMn}}_2{\mathrm{O}}_4$ has half-metallic properties, while the fully inverse spinel is an insulator. This material is in the ferrimagnetic state for the inverse and partially inverse cation arrangement. The optimized lattice and oxygen parameters, as well as the equilibrium degree of inversion, are in agreement with the available experimental data. The partial equilibrium degree of inversion is important in the interpretation of the lithium ion migration and surface properties of the ${\mathrm{LiMn}}_2{\mathrm{O}}_4$ spinel.

We classify the sources of second harmonic generation (SHG) of the Weyl semimetal TaAs by collisionless quantum kinetic equation into three kinds: i.e., injection current from the canonical band dispersion, shift current from a gauge invariant shift vector, and anomalous current from Berry curvature associated with the Fermi surface. Importantly, by using the realistic band model for TaAs, we predict that the SHG in TaAs is predominately contributed by the shift current, while the anomalous current has a minute contribution when the Weyl point is exactly located on the Fermi surface. Moreover, we highlight that the SHG contributed by the anomalous current decays fast with the increasing frequency of incident photons, and could be enhanced by proper electron or hole doping of TaAs.

Optical effects arising from spin-induced gain anisotropy such as threshold reduction and emission intensity enhancement, hitherto unobserved in electrically injected polariton lasers, are predicted theoretically for a bulk GaN-based exciton-polariton diode laser operated with electrical injection of spin-polarized electrons. These phenomena are deduced from a simplified spin-dependent rate equation model. We also demonstrate an electrical excitation scheme, which can amplify the degree of a deterministic circular polarization of the output emission by an order of magnitude, compared to the injected electron spin polarization, above threshold.

We demonstrate that plasmons in graphene can be manipulated using a dc current. A source-drain current lifts the forward/backward degeneracy of the plasmons, creating two modes with different propagation properties parallel and antiparallel to the current. We show that the propagation length of the plasmon propagating parallel to the drift current is enhanced, while the propagation length for the antiparallel plasmon is suppressed. We also investigate the scattering of light off graphene due to the plasmons in a periodic dielectric environment and we find that the plasmon resonance separates in two peaks corresponding to the forward and backward plasmon modes. The narrower linewidth of the forward propagating plasmon may be of interest for refractive index sensing and the dc current control could be used for the modulation of mid-infrared electromagnetic radiation.

We report an experimental and numerical study of femtosecond multipulse laser-induced densification in vitreous silica ($v-{\mathrm{SiO}}_2$) and its signature in Raman spectra. We compare the experimental findings to the recently developed molecular dynamics (MD) approach accounting for bond breaking due to laser irradiation, together with a dynamical matrix approach and bond polarizability model based on first-principles calculations for the estimation of Raman spectra. We observe two stages of the laser-induced densification and Raman spectrum evolution: growth during several hundreds of pulses followed by further saturation. At the medium range, the network connectivity change in $v-{\mathrm{SiO}}_2$ is expressed in reduction of the major ring fractions leading to more compacted structure. With the help of the Sen and Thorpe model, we also study the short-range order transformation and derive the interbonding Si–O–Si angle change from the Raman measurements. Experimental findings are in excellent agreement with our MD simulations and hence support a bond-breaking mechanism of laser-induced densification. Thus, our modeling explains well the laser-induced changes both in the short-range order caused by the appearance of Si coordination defects and medium-range order connected to evolution of the ring distribution. Finally, our findings disclose similarities between sheared, permanently densified, and laser-induced glass and suggest interesting future experiments in order to clarify the impact of the thermomechanical history on glasses under shear, cold and hot compression, and laser-induced densification.

Inspired by the quantum spin Hall effect shown by topological insulators, we propose a plate structure that can be used to demonstrate the pseudospin Hall effect for flexural waves. The system consists of a thin plate with periodically arranged resonators mounted on its top surface. We extend a technique based on the plane-wave expansion method to identify a double Dirac cone emerging due to the zone-folding in frequency band structures. This particular design allows us to move the double Dirac cone to a lower frequency than the resonating frequency of local resonators. We then manipulate the pattern of local resonators to open subwavelength Bragg band gaps that are topologically distinct. Building on this method, we verify numerically that a waveguide at an interface between two topologically distinct resonating plate structures can be used for guiding low-frequency, spin-dependent one-way flexural waves along a desired path with bends.

We study the spin dynamics in antiferromagnetic nanowire under an applied temperature gradient using micromagnetic simulations on a classical spin model with a uniaxial anisotropy. The entropic torque driven domain-wall motion and the Brownian motion are discussed in detail, and their competition determines the antiferromagnetic wall motion towards the hotter or colder region. Furthermore, the spin dynamics in an antiferromagnet can be well tuned by the anisotropy and the temperature gradient. Thus, this paper not only strengthens the main conclusions obtained in earlier works [Kim et al., Phys. Rev. B 92, 020402(R) (2015); Selzer et al., Phys. Rev. Lett. 117, 107201 (2016)], but more importantly gives the concrete conditions under which these conclusions apply, respectively. Our results may provide useful information on the antiferromagnetic spintronics for future experiments and storage device design.

We present systematic temperature-dependent resistance noise measurements on a series of ferromagnetic ${\mathrm{Ga}}_{1-x}{\mathrm{Mn}}_x\mathrm{As}$ epitaxial thin films covering a large parameter space in terms of the Mn content $x$ and other variations regarding sample fabrication. We infer that the electronic noise is dominated by switching processes related to impurities in the entire temperature range. While metallic compounds with $x>2\%$ do not exhibit any significant change in the low-frequency resistance noise around the Curie temperature $T_{\mathrm{C}}$, we find indications for an electronic phase separation in films with $x<2\%$ in the vicinity of $T_{\mathrm{C}}$, manifesting itself in a maximum in the noise power spectral density. These results are compared with noise measurements on an insulating ${\mathrm{Ga}}_{1-x}{\mathrm{Mn}}_x\mathrm{P}$ reference sample, for which the evidence for an electronic phase separation is even stronger and a possible percolation of bound magnetic polarons is discussed. Another aspect addressed in this work is the effect of ion-irradiation-induced disorder on the electronic properties of ${\mathrm{Ga}}_{1-x}{\mathrm{Mn}}_x\mathrm{As}$ films and, in particular, whether any electronic inhomogeneities can be observed in this case. Finally, we put our findings into the context of the ongoing debate on the electronic structure and the development of spontaneous magnetization in these materials.