We study parafermion chains with ${\mathbb{Z}}_k$ symmetry subject to a periodic binary drive, focusing on the case $k=3$. We find that the chains support different Floquet edge modes at nontrivial quasienergies, distinct from those for the static system. We map out the corresponding phase diagram by a combination of analytics and numerics, and provide the location of $2\pi /3$ modes in parameter space. We also show that the modes are robust to weak disorder. While the previously studied ${\mathbb{Z}}_2$-invariant Majorana systems possess a transparent weakly interacting limit where the existence of a $\pi$-Majorana mode is manifest, our intrinsically strongly interacting generalization demonstrates that the existence of such a limit is not necessary.

We show how to compute the electro-optical functions (absorption, reflection, and transmission) when Rydberg exciton-polaritons appear, including the effect of the coherence between the electron-hole pair and the electromagnetic field. With the use of the real density matrix approach, numerical calculations applied for the ${\mathrm{Cu}}_2\mathrm{O}$ crystal are performed. We also examine in detail and explain the dependence of the resonance displacement on the state number and applied electric field strength. We report a fairly good agreement with recently published experimental data.

We study the dark exciton's behavior as a coherent physical two-level spin system (qubit) using an external magnetic field in the Faraday configuration. Our studies are based on polarization-sensitive intensity autocorrelation measurements of the optical transition resulting from the recombination of a spin-blockaded biexciton state, which heralds the dark exciton and its spin state. We demonstrate control over the dark exciton eigenstates without degrading its decoherence time. Our observations agree well with computational predictions based on a master equation model.

Interference effects in quantum dots between different transport channels can lead to a strong suppression of conductance, which cuts like a canyon through the common conductance plot [Phys. Rev. Lett. 104, 186804 (2010)]. In the present work we consider the thermoelectric transport properties of the canyon of conductance suppression using the second-order von Neumann approach. We observe a characteristic signal for the zeros of the thermopower. This demonstrates that thermoelectric measurements are an interesting complimentary tool to study complex phenomena for transport through confined systems.

Core-shell coating structures and concentration gradient materials may enhance Li-ion battery performance by integrating advantages of core and shell components without introducing unfavorable problems associated with general coatings. The fundamental thermodynamic properties of concentration gradient composite materials are complex due to the multicomponent nature of the problem. We systematically study the thermodynamics of ordering and phase separation in the quaternary spinel ${\mathrm{Li}}_y{\square }_{1-y}{\mathrm{Mn}}_x{\mathrm{Ni}}_{2-x}{\mathrm{O}}_4$ ($\square$ means vacancy) system by density functional theory calculations, together with the coupled cluster expansion method with interactions within and between (Li/$\square$) and (Mn/Ni) sublattices. On the basis of coupled cluster expansion interactions and Monte Carlo simulations, we calculate quaternary phase diagrams as a function of temperature as well as voltage profiles of single ordered phases and multiphase composite structures. The phase diagram and voltage results are in good agreement with available experimental observations. We also predict a stable high-voltage ordered compound ${\mathrm{LiMnNiO}}_4$, with a very high delithiation voltage of 4.76 V. For the composite (Mn-$\text{rich}+\text{Ni}$-rich) cathode materials, the voltage profiles show combinations of plateaus from each component compound. The computational strategy of combining quaternary phase diagrams with voltage calculations provides a pathway to understand and design concentration gradient materials.

The metallic compound ${\mathrm{EuCo}}_2{\mathrm{P}}_2$ with the body-centered tetragonal ${\mathrm{ThCr}}_2{\mathrm{Si}}_2$ structure containing Eu spins-7/2 was previously shown from single-crystal neutron diffraction measurements to exhibit a helical antiferromagnetic (AFM) structure below $T_{\mathrm{N}}=66.5$ K with the helix axis along the $c$ axis and with the ordered moments aligned within the $ab$ plane. Here we report crystallography, electrical resistivity, heat capacity, magnetization, and magnetic susceptibility measurements on single crystals of this compound. We demonstrate that ${\mathrm{EuCo}}_2{\mathrm{P}}_2$ is a model molecular-field helical Heisenberg antiferromagnet from comparisons of the anisotropic magnetic susceptibility $\chi$, high-field magnetization, and magnetic heat capacity of ${\mathrm{EuCo}}_2{\mathrm{P}}_2$ single crystals at temperature $T\le T_{\mathrm{N}}$ with the predictions of our recent formulation of molecular-field theory. Values of the Heisenberg exchange interactions between the Eu spins are derived from the data. The low-$T$ magnetic heat capacity $\sim T^3$ arising from spin-wave excitations with no anisotropy gap is calculated and found to be comparable to the lattice heat capacity. The density of states at the Fermi energy of ${\mathrm{EuCo}}_2{\mathrm{P}}_2$ and the related compound ${\mathrm{BaCo}}_2{\mathrm{P}}_2$ are found from the heat capacity data to be large, 10 and 16 states/eV per formula unit for ${\mathrm{EuCo}}_2{\mathrm{P}}_2$ and ${\mathrm{BaCo}}_2{\mathrm{P}}_2$, respectively. These values are enhanced by a factor of $\sim 2.5$ above those found from DFT electronic structure calculations for the two compounds. The calculations also find ferromagnetic Eu–Eu exchange interactions within the $ab$ plane and AFM interactions between Eu spins in nearest- and next-nearest planes, in agreement with the MFT analysis of ${\chi }_{ab}(T\le T_{\mathrm{N}})$.

The ultrafast demagnetization of small iron clusters initiated by an intense optical excitation is studied from the time-dependent spin density functional theory (TDSDFT). In particular we investigate the effect of the spin-orbit interaction on the onset of the demagnetization process. It is found that demagnetization occurs locally, in the vicinity of the atomic sites, and the initial rate of spin loss, coherent with the laser field, is proportional to the square of the ionic spin-orbit coupling strength $\lambda$. A simplified quantum spin model comprising spin-orbit interaction and a time-dependent magnetic field is found to be the minimal model able to reproduce our ab initio results. The model predicts the ${\lambda }^2$ dependence of the onset rate of demagnetization when it is solved either analytically for the small $t$ regime, or numerically integrated in the time domain. Our findings are supported by additional TDSDFT simulations of clusters made of Co and Ni.

We report a neutron spin-echo study of the critical dynamics in the $S=\frac{5}{2}$ antiferromagnets ${\mathrm{MnF}}_2$ and ${\mathrm{Rb}}_2{\mathrm{MnF}}_4$ with three-dimensional (3D) and two-dimensional (2D) spin systems, respectively, in zero external field. Both compounds are Heisenberg antiferromagnets with a small uniaxial anisotropy resulting from dipolar spin-spin interactions, which leads to a crossover in the critical dynamics close to the Néel temperature, $T_N$. By taking advantage of the $\mu \text{eV}$ energy resolution of the spin-echo spectrometer, we have determined the dynamical critical exponents $z$ for both longitudinal and transverse fluctuations. In ${\mathrm{MnF}}_2$, both the characteristic temperature for crossover from 3D Heisenberg to 3D Ising behavior and the exponents $z$ in both regimes are consistent with predictions from the dynamical scaling theory. The amplitude ratio of longitudinal and transverse fluctuations also agrees with predictions. In ${\mathrm{Rb}}_2{\mathrm{MnF}}_4$, the critical dynamics crosses over from the expected 2D Heisenberg behavior for $T\gg T_N$ to a scaling regime with exponent $z=1.387\left(4\right)$, which has not been predicted by theory and may indicate the influence of long-range dipolar interactions.

Lattice defects are inevitably present in two-dimensional materials, with direct implications on their physical and chemical properties. We show that the formation energy of a lattice defect in buckled two-dimensional crystals is not uniquely defined as it takes different values for different boundary conditions even in the thermodynamic limit, as opposed to their perfectly planar counterparts. Also, the approach to the thermodynamic limit follows a different scaling: inversely proportional to the logarithm of the system size for buckled materials, rather than the usual power-law approach. In graphene samples of $\sim 1000$ atoms, different boundary conditions can cause differences exceeding 10 eV. Besides presenting numerical evidence in simulations, we show that the universal features in this behavior can be understood with simple bead-spring models. Fundamentally, our findings imply that it is necessary to specify the boundary conditions for the energy of the lattice defects in the buckled two-dimensional crystals to be uniquely defined, and this may explain the lack of agreement in the reported values of formation energies in graphene. We argue that boundary conditions may also have an impact on other physical observables such as the melting temperature.

All-optical magnetic switching (AOS) is a recently observed rich and puzzling phenomenon that offers promising technological applications. However, a fundamental understanding of the underlying mechanisms remains elusive. Here we present a model for multishot helicity-dependent AOS in ferromagnetic materials based on a purely heat-driven mechanism in the presence of magnetic circular dichroism (MCD). We predict that AOS should be possible with as little as 0.5% of MCD, after a minimum number of laser shots heat the sample close to the Curie temperature. Finally, we qualitatively reproduce the all-optically switched domain patterns observed experimentally by numerically simulating the result of multiple laser shots on an FePtC granular ferromagnetic film.

The melting curve of sodium for a pressure range up to 100 GPa has been evaluated by the orbital free ab initio molecular dynamics method. This method uses the electronic density as the basic variable combined with an approximate electronic kinetic energy functional and a local ionic pseudopotential and makes it possible to perform simulations with a large number of particles and for long simulation times. The calculated melting curve shows a maximum melting temperature at a pressure around 30 GPa followed by a steep decrease up to 100 GPa. For various pressures and temperatures we have evaluated several static properties, including average and local structure, electronic properties, like the electron localization function (ELF), and dynamic properties, both single-particle and collective ones, from which some transport coefficients are deduced. Despite the accurate reproduction of the available experimental data, we do not observe any indication of an early transition from a bcc-like to an fcc-like liquid, as suggested previously by other authors, but rather pressure-induced change in the variation of icosahedral-like order and bcc-like order, with no sign of fcc-like structures in the whole liquid range studied. We also consider the evolution of the ELF within this type of local arrangement upon pressurization. In the dynamic realm, we find an enlarged wave-vector region where atomic collisions play an important role in the dynamic properties of the system as pressure is increased and temperature decreased along the melting line, leading to a peculiar behavior of the dynamic properties.

To guide improved properties coincident with reduction of critical materials in permanent magnets, we investigate via density functional theory (DFT) the intrinsic magnetic properties of a promising system, $R{\left({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x\right)}_{11}\mathrm{Ti}Z$ with $R=\text{Y}$, Ce and interstitial doping $(Z=\text{H},\text{C},\text{N})$. The magnetization $M$, Curie temperature $T_{\text{C}}$, and magnetocrystalline anisotropy energy $K$ calculated in local density approximation to DFT agree well with measurements. Site-resolved contributions to $K$ reveal that all three Fe sublattices promote uniaxial anisotropy in ${\mathrm{YFe}}_{11}\mathrm{Ti}$, while competing anisotropy contributions exist in ${\mathrm{YCo}}_{11}\mathrm{Ti}$. As observed in experiments on $R{\left({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x\right)}_{11}\mathrm{Ti}$, we find a complex nonmonotonic dependence of $K$ on Co content and show that anisotropy variations are a collective effect of MAE contributions from all sites and cannot be solely explained by preferential site occupancy. With interstitial doping, calculated $T_{\text{C}}$ enhancements are in the sequence of $\mathrm{N}>\mathrm{C}>\mathrm{H}$, with volume and chemical effects contributing to the enhancement. The uniaxial anisotropy of $R{\left({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x\right)}_{11}\mathrm{Ti}Z$ generally decreases with C and N; although, for $R=\text{Ce}$, C doping is found to greatly enhance it for a small range of $0.7<x<0.9$.

The symmetry of the superconducting order parameter in quasi-two-dimensional bis-ethylenedithio-tetrathiafulvalene (BEDT-TTF) organic superconductors is a subject of ongoing debate. We report ab initio density-functional-theory calculations for a number of organic superconductors containing $\kappa$-type layers. Using projective Wannier functions, we derive the parameters of a common low-energy Hamiltonian based on individual BEDT-TTF molecular orbitals. In a random-phase approximation spin-fluctuation approach, we investigate the evolution of the superconducting pairing symmetry within this model, and we point out a phase transition between extended $s+d_{x^2-y^2}$ and $d_{xy}$ symmetry. We discuss the origin of the mixed order parameter and the relation between the realistic molecule description and the widely used dimer approximation. Based on our ab initio calculations, we position the investigated materials in the obtained molecule model phase diagram, and we simulate scanning tunneling spectroscopy experiments for selected cases. Our calculations show that many $\kappa$-type materials lie close to the phase-transition line between the two pairing symmetry types found in our calculation, possibly explaining the multitude of contradictory experiments in this field.

We obtain the exchange parameter of hybrid functionals by imposing the fundamental condition of a piecewise linear total energy with respect to electron number. For the Perdew-Burke-Ernzerhof (PBE) hybrid family of exchange-correlation functionals (i.e., for an approximate generalized Kohn-Sham theory) this implies that (i) the highest occupied molecular orbital corresponds to the ionization potential $\left(I\right)$, (ii) the energy of the lowest unoccupied molecular orbital corresponds to the electron affinity $\left(A\right)$, and (iii) the energies of the frontier orbitals are constant as a function of their occupation. In agreement with a previous study [N. Sai et al., Phys. Rev. Lett. 106, 226403 (2011)], we find that these conditions are met for high values of the exact exchange admixture $\alpha$ and illustrate their importance for the tetrathiafulvalene-tetracyanoquinodimethane complex for which standard density functional theory functionals predict artificial electron transfer. We further assess the performance for atomization energies and weak interaction energies. We find that atomization energies are significantly underestimated compared to PBE or PBE0, whereas the description of weak interaction energies improves significantly if a $1/R^6$ van der Waals correction scheme is employed.

We study the impurity scattering effect in black phosphorene (BP) in this work. For a single impurity, we calculate the impurity-induced local density of states (LDOS) in momentum space numerically based on a tight-binding Hamiltonian. In real space, we calculate the LDOS and Friedel oscillation analytically. The LDOS shows strong anisotropy in BP. Many impurities in BP are investigated using the $T$-matrix approximation when the density is low. Midgap states appear in the band gap with peaks in the DOS. The peaks of midgap states are dependent on the impurity potential. For finite positive potential, the impurity tends to bind negative charge carriers and vice versa. The infinite-impurity-potential problem is related to chiral symmetry in BP.

The study of electron transport and scattering processes limiting electron mobility in high-quality semiconductor structures is central to solid-state electronics. Here, we uncover an unavoidable source of electron scattering which is caused by fluctuations of nuclear spins. We calculate the momentum relaxation time of electrons in quantum wells governed by the hyperfine interaction between electrons and nuclei and show that this time depends greatly on the spatial correlation of nuclear spins. Moreover, the scattering processes accompanied by a spin flip are a source of the backscattering of Dirac fermions at conducting surfaces of topological insulators.

We study the low temperature conductivity of the electron accumulation layer induced by the very strong electric field at the surface of an ${\text{SrTiO}}_3$ sample. Due to the strongly nonlinear lattice dielectric response, the three-dimensional density of electrons $n\left(x\right)$ in such a layer decays with the distance from the surface $x$ very slowly as $n\left(x\right)\propto 1/x^{12/7}$. We show that when the mobility is limited by the surface scattering the contribution of such a tail to the conductivity diverges at large $x$ because of growing time electrons need to reach the surface. We explore truncation of this divergence by the finite sample width, by the bulk scattering rate, by the back gate voltage, or by the crossover to the bulk linear dielectric response with the dielectric constant $\kappa$. As a result we arrive at the anomalously large mobility, which depends not only on the rate of the surface scattering, but also on the physics of truncation. Similar anomalous behavior is found for the Hall factor, the magnetoresistance, and the thermopower.

Electron transport properties of nanoelectronics can be significantly influenced by the inevitable and randomly distributed impurities/defects. For theoretical simulation of disordered nanoscale electronics, one is interested in both the configurationally averaged transport property and its statistical fluctuation that tells device-to-device variability induced by disorder. However, due to the lack of an effective method to do disorder averaging under the nonequilibrium condition, the important effects of disorders on electron transport remain largely unexplored or poorly understood. In this work, we report a general formalism of Green's function based nonequilibrium effective medium theory to calculate the disordered nanoelectronics. In this method, based on a generalized coherent potential approximation for the Keldysh nonequilibrium Green's function, we developed a generalized nonequilibrium vertex correction method to calculate the average of a two-Keldysh-Green's-function correlator. We obtain nine nonequilibrium vertex correction terms, as a complete family, to express the average of any two-Green's-function correlator and find they can be solved by a set of linear equations. As an important result, the averaged nonequilibrium density matrix, averaged current, disorder-induced current fluctuation, and averaged shot noise, which involve different two-Green's-function correlators, can all be derived and computed in an effective and unified way. To test the general applicability of this method, we applied it to compute the transmission coefficient and its fluctuation with a square-lattice tight-binding model and compared with the exact results and other previously proposed approximations. Our results show very good agreement with the exact results for a wide range of disorder concentrations and energies. In addition, to incorporate with density functional theory to realize first-principles quantum transport simulation, we have also derived a general form of conditionally averaged nonequilibrium Green's function for multicomponent disorders.

We calculate the resistance and shot noise in the edge states of a two-dimensional topological insulator that result from the exchange of electrons between these states and conducting puddles in the bulk of the insulator. The two limiting cases where the energy relaxation is either absent or very strong are considered. A finite time of spin relaxation in the puddles is introduced phenomenologically. Depending on this time and on the strength of coupling between the edge states and the puddles, the Fano factor $\mathcal{F}=S_I/2eI$ ranges from 0 to 1/3, which is in an agreement with the available experimental data.

The nickel self-diffusion coefficient was measured in a ${\mathrm{Ni}}_{75}{\mathrm{Si}}_{25}$ alloy in the liquid and undercooled state by quasielastic neutron scattering. The undercooled state was achieved by applying aerodynamic levitation. That containerless technique allowed an undercooling of $\mathrm{\Delta }T\approx 90\text{K}$ over 4 h of measurement time. The temperature dependence of the derived diffusion coefficients follows an Arrhenius-type behavior. The activation energy for the diffusion process is about 10% larger than in pure nickel and is probably the reason for the slower self-diffusion coefficient compared to pure Ni. With increasing Si content more covalent bonding is formed, which might be the origin for the reduced mobility. Molecular dynamics simulations predicted a change in dynamics from an Arrhenius-type behavior to a power-law for temperatures as high as twice the glass transition temperature. Our data are compatible with a power-law behavior for the Ni self-diffusion.