We show that confinement together with intrinsic polarization induce hole doping of the CuO${}_2$ plane in the proposed SrTiO${}_3$/[SrO-La${}_2$CuO${}_4$-SrO]/LaAlO${}_3$ heterostructure without resorting to chemical substitution, using the generalized gradient approximation and its combination with Coulomb correlation $U$. Hole concentration can be rationally manipulated via tuning the LaAlO${}_3$ thickness and in-plane strain. The stable antiferromagnetic insulating bulk state up to 8$\%$ doping in this disorder-free structure is remarkable; moreover, this antiferromagnetic state coexists with two-dimensional electron gas on the top surface of LaAlO${}_3$, which originates from out-of-plane charge transfer mainly between the La-$d_{3z^2-1}$ state at the surface and Cu-$e_g$ states in the CuO${}_2$ plane. In addition, a possible SrTiO${}_3$ capping layer is introduced in which, instead of La-$d_{3z^2-1}$, Ti-$t_{2g}$ orbitals exchange holes with electrons of Cu-$e_g$ orbitals.

We present a large-scale nonadiabatic molecular dynamics simulation to study carrier transport in an organic monolayer. This simulation calculates a 4802-atom system for 825 fs in about 3 h using 51 744 computer cores, while deploying a plane-wave pseudopotential density-functional theory Hamiltonian. A new approach is developed that makes such large-scale calculation possible. Our simulation on the pentathiophene butyric acid monolayer reveals the mechanism for the carrier transport in the system: the hole wave functions are localized by thermal fluctuation-induced disorder, while the hole transport is via charge transfer during state energy crossing. The simulation also shows that the system is never in a thermodynamic equilibrium in terms of adiabatic-state populations according to Boltzmann distribution.

The real-time dynamics of local occupation numbers in a Hubbard model on a $6\times 6$ square lattice is studied by means of the nonequilibrium generalization of the cluster-perturbation theory. The cluster approach is adapted to studies of two-dimensional lattice systems by using concepts of multiple-scattering theory and a component decomposition of the nonequilibrium Green's function on the Keldysh-Matsubara contour. We consider “classical” initial states formed as tensor products of states on $2\times 2$ plaquettes and trace the effects of the interplaquette hopping in the final-state dynamics. Two different initially excited states are considered on an individual plaquette, a fully polarized staggered spin state (Néel) and a fully polarized charge-density wave (CDW). The final-state dynamics is constrained by a dynamical symmetry; i.e., the time-evolution operator and certain observables are invariant under an antiunitary transformation composed of time reversal, an asymmetric particle-hole, and a staggered sign transformation. We find an interesting interrelation between this dynamical symmetry and the separation of energy and time scales: In the case of a global excitation with all plaquettes excited, the initial Néel and the initial CDW states are linked by the transformation. This prevents an efficient relaxation of the CDW state on the short time scale governing the dynamics of charge degrees of freedom. Contrarily, the CDW state is found to relax much faster than the Néel state in the case of a local excitation on a single plaquette where the symmetry relation between the two states is broken by the coupling to the environment.

A superexchange calculation is performed for multiorbital band models with broken inversion symmetry. Orbital-changing hopping terms allowed by the symmetry-breaking electric field lead to a new kind of orbital exchange interaction closely resembling the Dzyaloshinskii-Moriya spin exchange. Inversion symmetry breaking as present in surfaces and interfaces and a strong on-site repulsion, but not the spin-orbit interaction, are the requirements to observe the proposed effect. The mean-field phase diagram exhibits a rich structure including anti-ferro-orbital, ferro-orbital, and both single and multiple spiral-orbital phases, in close analogy with the Skyrmion spin crystal phase recently discovered in thin-film chiral magnets.

We report on direct measurements of the Yb valence in the heavy Fermion compound YbCuAl as a function of temperature and pressure using resonant x-ray emission spectroscopy. The increase of the Yb${}^{2+}$ component at $T<100$ K and ambient pressure, well described by the single impurity Anderson model, is found to compensate for the thermal contraction of the unit cell volume. Under pressure, the Yb valence increases continuously up to 25 GPa, albeit a marked leveling off close to the critical pressure, at $P\ge 13$ GPa. This finding is reminiscent of a recent report on YbCu${}_2$Si${}_2$ and further confirms the interplay between electronic and magnetic fluctuations near the magnetic instability point of rare-earth intermediate-valence materials.

We report the observation of optical near fields in a photonic waveguide of conductive indium tin oxide (ITO) using multiphoton photoemission electron microscopy (PEEM). Nonlinear two-photon photoelectron emission is enhanced at field maxima created by interference between incident 410-nm and coherently excited guided photonic waves, providing strong phase contrast. Guided modes are observed under both transverse magnetic field (TM) and transverse electric field (TE) polarized illuminations and are consistent with classical electromagnetic theory. Implications on the role of multiphoton PEEM in optical near-field imaging are discussed.

It is shown that for the hopping regime, the thermopowers in both finite two- and three-terminal systems are governed by the edges of the samples. This is due to the fact that the energy transfer between a transport electron and a conducting terminal is determined by the site most strongly coupled to that terminal. One-dimensional systems with both nearest-neighbor and variable-range transport as well as certain types of two-dimensional systems are considered. For a given sample, the changes in the thermopowers due to modifying the bulk are quite limited, compared with those of the conductance. When the small thermopower changes exist, their average over a large ensemble of mesoscopic samples will vanish. We also obtain the distribution of the thermopower in such an ensemble and show that its width approaches a finite limit with increasing sample length. This contrasts with the distribution of conductances in such systems, whose width vanishes in the long sample limit. Finally, we find that the thermal conductances in the three-terminal case have a boundary-dominated contribution, due to nonpercolating conduction paths. This contribution can become dominant when the usual conductance is small enough. All our theoretical statements are backed by numerical computations.

The investigation of the switching times of the magnetization reversal of two interacting CoFeB nanomagnets, with dimensions small enough to maintain a single-domain structure, has been carried out. A quasistatic approximation is shown to give valid results and to compare well to the damped dynamical solutions of the Landau-Lifshitz-Gilbert equations. The characteristics of the switching are shown in the associated hysteresis loops and we build a complete phase diagram of the various parallel, antiparallel, and scissoring states of the magnetization in terms of the coupling energy between the nanomagnets, magnetic anisotropy, and the interaction with an applied magnetic field. The phase diagram summarizes the different kinds of hysteresis associated with the magnetization reversal phenomena. The switching fields and times are estimated and the vulnerabilities of the magnetic phases to thermally induced magnetic field variations are examined. The stability of the phases is a fine balance between intrinsic and extrinsic magnetism and we examine its precarious nature. Our work identifies the structures that have the most robust magnetization states and hence a design ethic for creating nanomagnetic heterostructures with outstanding magnetoresistance properties based upon the two magnetic elements.

${}^{139}$La NMR and relaxation measurements have been performed on La${}_{1.8-x}$Eu${}_{0.2}$Sr${}_x$CuO${}_4$ ($x=0.13$ and 0.2) single crystals. The temperature dependence of the ${}^{139}$La NMR spectra in all the structural phases [high-temperature tetragonal (HTT) $\to$ low-temperature orthorhombic (LTO) $\to$ low-temperature tetragonal (LTT)] reveals the nonvanishing tilt angle of the CuO${}_6$ octahedra in the HTT phase, opposed to the case of La${}_{2-x}$Sr${}_x$CuO${}_4$ where the tilt angle disappears immediately above the transition. Since ${}^{139}$La relaxation data provide evidence of the thermodynamic critical fluctuations associated with the structural phase transitions, HTT $\to$ LTO and LTO $\to$ LTT, we conclude that the structural transitions in Eu-doped La${}_{2-x}$Sr${}_x$CuO${}_4$ should be of the order-disorder type rather than of the displacive type observed in La${}_{2-x}$Sr${}_x$CuO${}_4$. The change of the nature of the structural transitions caused by doping with Eu appears to be consistent with the LTO $\to$ LTT transition that is absent in La${}_{2-x}$Sr${}_x$CuO${}_4$.

We consider two weakly coupled Richardson models to study the formation of a relative phase and the Josephson dynamics between two mesoscopic attractively interacting fermionic systems. Our results apply to superconducting properties of coupled ultrasmall metallic grains and to Cooper-pairing superfluidity in neutral systems with a finite number of fermions. We discuss how a definite relative phase between the two systems emerges and how it can be conveniently extracted from the many-body wave function, finding that a definite relative phase difference emerges even for very small numbers of pairs ($\sim$10). The Josephson dynamics and the current-phase characteristics are then investigated, showing that the critical current has a maximum at the BCS-BEC crossover. For the considered initial conditions a two-state model gives a good description of the dynamics and of the current-phase characteristics.

The NMR results in iron pnictides BaFe${}_2$(As${}_{1-x}$P${}_x$)${}_2$ and Ba(Fe${}_{1-x}$Co${}_x$)${}_2$As${}_2$ are analyzed based on the self-consistent renormalization (SCR) spin-fluctuation theory. The temperature dependence of the NMR relaxation rate $T_1^{-1}$ as well as the electrical resistivity is well reproduced by a SCR model where two-dimensional antiferromagnetic (AF) spin fluctuations are dominant. The successful description of the crossover feature from non-Fermi-liquid to Fermi-liquid behavior strongly suggests that low-lying spin fluctuations in BaFe${}_2$(As${}_{1-x}$P${}_x$)${}_2$ and Ba(Fe${}_{1-x}$Co${}_x$)${}_2$As${}_2$ possess an itinerant AF nature, and that chemical substitution in the two compounds tunes the distance of these systems to an AF quantum critical point. The close relationship between spin fluctuations and superconductivity is discussed compared with the other unconventional superconductors, cuprate and heavy fermion superconductors. In addition, it is suggested that magnetism and lattice instability in these pnictides are strongly linked via orbital degrees of freedom.

We examine the influence of the main approximations employed in density functional theory descriptions of the solid phase of molecular hydrogen near dissociation. We consider the importance of nuclear quantum effects on equilibrium properties and find that they strongly influence intramolecular properties, such as bond fluctuations and stability. We demonstrate that the combination of both thermal and quantum effects make a drastic change to the predicted optical properties of the molecular solid, suggesting a limited value to dynamical, e.g., finite-temperature predictions based on classical ions and static crystals. We also consider the influence of the chosen exchange-correlation density functional on the predicted properties of hydrogen, in particular, the pressure dependence of the band gap and the zero-point energy. Finally, we use our simulations to make an assessment of the accuracy of typically employed approximations to the calculation of the Gibbs free energy of the solid, namely the quasi-harmonic approximation for solids. We find that, while the approximation is capable of producing free energies with an accuracy of $\approx 10$ meV, this is not enough to make reliable predictions of the phase diagram of hydrogen from first principles due to the small free energy differences seen between several potential structures for the solid; direct free energy calculations for quantum protons are required in order to make definite predictions.

The electronic structures of $R$ $4f$ and Fe $3d$ states of $R$Fe${}_2$O${}_4$ ($R=\text{Er}$, Tm, Lu) have been investigated by employing soft x-ray absorption spectroscopy (XAS) and magnetic circular dichroism at the Fe $2p$ and $R$ $3d$ absorption edges. It is found that the valence states of Fe and $R$ ions are nearly Fe${}^{2.5+}$ and $R^{3+}$, and that the net magnetic moments of Fe${}^{2+}$ and Fe${}^{3+}$ ions are antiparallel to each other. Both $R$ $3d$ and O $1s$ XAS spectra show that the localized $R$ $4f$ states do not contribute to the multiferroicity of $R$Fe${}_2$O${}_4$. On the other hand, the magnetization data for $R$Fe${}_2$O${}_4$ at low temperature ($T$) reveal the cluster glass behavior for $R=\text{Tm}$ and Lu, but not for $R=\text{Er}$. This work suggests that the $T$-dependent structural differences among $R$Fe${}_2$O${}_4$, caused by different $R$ ions, are closely related to the magnetoelectricity at low temperature.

Nanostripes with varying widths are lithographed on Co thin films with strong magnetic anisotropy resulting from the epitaxial growth onto vicinal Si(111) substrate. The competition between magnetocrystalline and shape anisotropies is used to tune the magnetic behavior of Co nanostripes. Single domain configuration is observed for nanostructures where magnetocrystalline and shape anisotropies go along the same direction. However, more complicated configurations such as open stripe domains can be developed when both anisotropies compete. The nanostructures have been experimentally characterized by longitudinal magneto-optical Kerr effect and magnetic force microscopy (MFM). Micromagnetic simulations performed by finite-element and finite difference codes are in good agreement with the experimental results. The use of MFM based techniques such as the variable field magnetic force microscopy and the so-called three-dimensional modes has allowed us to follow the evolution of the domains and domain walls under externally applied magnetic fields, i.e., to deeply understand the magnetization reversal process in the multidomain nanostripes. In particular, the nanostripes with competing anisotropies and a high aspect ratio present vortex configuration along the domain walls which have a key role in the magnetization reversal process.

The spin-polarized electronic structure of iron octaethylporphyrin (FeOEP) molecules adsorbed on a pristine and on a $c$($2\times 2$) oxygen-reconstructed Co(100) surface has been analyzed by means of spin-polarized photoemission spectroscopy (SPPES) and first-principles density functional theory with the on-site Coulomb repulsion $U$ term ($\mathrm{DFT}+U$) calculations with and without Van der Waals corrections. The aim is to examine the magnetic exchange mechanism between the FeOEP molecules and the Co(100) substrate in the presence or absence of the oxygen mediator. The results demonstrate that the magnetic coupling from the ferromagnetic substrate to the adsorbed FeOEP molecules is ferromagnetic, whereas, the coupling is antiferromagnetic for the FeOEP on the $c$($2\times 2$)O/Co(100) system. Spin-resolved partial densities of states extracted from ab initio $\mathrm{DFT}+U$ modeling are in fairly good comparison with the electronic spectral densities seen in angle-integrated SPPES energy dispersion curves for submonolayer coverages of FeOEP. Through combined analysis of these spectra and theoretical results, we determine that hybridization of 2$p$ orbitals of N and O with Co 3$d$ orbitals facilitates indirect magnetic exchange interactions between Fe and Co, whereas, a direct Fe-Co interaction involving the Fe $d_{z^2}$ orbital is also found for FeOEP on Co. It is observed through SPPES that the spin polarization of the photoemission-visible molecular overlayers decreases to zero as coverage is increased beyond the submonolayer regime, indicating that only interfacial magnetic coupling is at work. Microspot low-energy electron diffraction and low-energy electron microscopy were performed to characterize the physical order of the molecular coverage, revealing that FeOEP structural domains are orders of magnitude greater in size on $c$($2\times 2$)O/Co(100) than on clean Co(100), which coincides with reduced scattering from the disorder and sharper features seen in SPPES.

Motivated by exploring spin-orbit-coupled magnetism in $5d$-based transition metal oxides (TMOs) beyond the iridates, we present a powder inelastic neutron scattering study of magnetic excitations in Ba${}_2$FeReO${}_6$—a member of the double-perovskite family of materials which exhibit half-metallic behavior and high Curie temperatures $T_c$. We find clear evidence of two well-defined dispersing magnetic modes in its low-temperature ferrimagnetic state. We develop a local moment model, which incorporates the interaction of Fe spins with spin-orbital locked magnetic moments on Re and show that it captures our experimental observations. This allows us to extract moment sizes and exchange couplings, explain the magnitude of $T_c$, and infer that magnetostructural locking terms are weak. Our study further opens up Re-based compounds as model systems to explore the interplay of strong correlations and spin-orbit coupling in $5d$ TMOs.

Conflicting predictions have been made for the ground state of the SU(3) Heisenberg model on the honeycomb lattice: Tensor network simulations found a plaquette order [Zhao et al., Phys. Rev. B 85, 134416 (2012)], where singlets are formed on hexagons, while linear flavor-wave theory suggested a dimerized, color-ordered state [Lee and Yang, Phys. Rev. B 85, 100402 (2012)]. In this work we show that the former state is the true ground state by a systematic study with infinite projected-entangled pair states (iPEPS), for which the accuracy can be systematically controlled by the so-called bond dimension $D$. Both competing states can be reproduced with iPEPS by using different unit cell sizes. For small $D$ the dimer state has a lower variational energy than the plaquette state; however, for large $D$ it is the latter which becomes energetically favorable. The plaquette formation is also confirmed by exact diagonalizations and variational Monte Carlo studies, according to which both the dimerized and plaquette states are nonchiral flux states.

We study particle current and occupation distribution in a recently proposed model for coherent quantum transport. In this model a system connected to mesoscopic Fermi reservoirs (mesoreservoir) is driven out of equilibrium by the action of superreservoirs with prescribed temperatures and chemical potentials described by a simple dissipative mechanism with the Lindblad equation. We compare exact (numerical) results for the nonequilibrium steady-state particle current with theoretical expectations based on the Landauer formula and show that the model reproduces the behavior of coherent quantum systems in the expected parameter region. We also obtain the mesoreservoir occupation distribution in the nonequilibrium steady-state and compare them with the occupation distribution in the leads in usual description of coherent quantum transport.

Silicene—the silicon counterpart of graphene—has a two-dimensional structure that leads to a host of interesting physical and chemical properties of significant utility. We report here an investigation with nonequilibrium molecular dynamics simulations of thermal transport in a single-layer silicene sheet under uniaxial stretching. We discovered that, contrary to its counterpart of graphene and despite the similarity of their honeycomb lattice structure, silicene exhibits an anomalous thermal response to tensile strain: The thermal conductivity of silicene and silicene nanoribbons first increases significantly with applied tensile strain rather than decreasing and then fluctuates at an elevated plateau. By quantifying the relative contribution from different phonon polarizations, we show first that the phonon transport in silicene is dominated by the out-of-plane flexural modes, similar to graphene. We attribute subsequently the unexpected and markedly different behavior of silicene to the interplay between two competing mechanisms governing heat conduction in a stretched silicene sheet, namely, (1) uniaxial stretching modulation in the longitudinal direction significantly depressing the phonon group velocities of longitudinal and transverse modes (phonon softening) and hindering heat conduction, and (2) phonon stiffening in the flexural modes counteracting the phonon softening effect and facilitating thermal transport. The abnormal behavior of the silicene sheet is further correlated to the unique deformation characteristics of its hexagonal lattice. Our study offers perspectives of modulating the thermal properties of low-dimensional structures for applications such as thermoelectric, photovoltaic, and optoelectronic devices.