We discuss plasmonic excitations in a thin slab of topological insulators. In the limit of no hybridization of the surface states and same electronic density of the two layers, the electrostatic coupling between the top and bottom layers leads to optical and acoustic plasmons which are purely charge and spin collective oscillations. We then argue that a recent experiment on the plasmonic excitations of Bi${}_2$Se${}_3$ [Di Pietro et al., Nat. Nanotechnol. 8, 556 (2013)] must be explained by including the charge response of the two-dimensional electron gas of the depletion layer underneath the two surfaces. We also present an analytic formula to fit their data.

We calculate the dynamical polarization function and solve a self-consistent gap equation in the random phase approximation in undoped ABC-stacked $n$-layer graphene. We find that the gap is maximal in trilayer graphene and decreases monotonously for $n\ge 4$ because the effects of screening in the gap equation win over those connected with the flattening of electron bands as $n$ increases.

We study the renormalization of the Fermi velocity by the long-range Coulomb interactions between the charge carriers in the Dirac-cone approximation for the effective low-energy description of the electronic excitations in graphene at half-filling. Solving the coupled system of Dyson-Schwinger equations for the dressing functions in the corresponding fermion propagator with various approximations for the particle-hole polarization, we observe that Fermi velocity renormalization effects generally lead to a considerable increase of the critical coupling for dynamical gap generation and charge-density-wave formation at the semimetal-insulator transition.

Understanding of stationary charge transport in disordered organic semiconductors has matured during recent years. However, charge-carrier relaxation in nonstationary situations is still poorly understood. Such relaxation can be studied in dark injection experiments, in which the bias applied over an unilluminated organic semiconductor device is abruptly increased. The resulting transient current reveals both charge-carrier transport and relaxation characteristics. We performed such experiments on hole-only devices of a polyfluorene-based organic semiconductor. Modeling the dark injection by solving a one-dimensional master equation using the equilibrium carrier mobility leads to a too-slow current transient, since this approach does not account for carrier relaxation. Modeling by solving a three-dimensional time-dependent master equation does take into account all carrier transport and relaxation effects. With this modeling, the time scale of the current transient is found to be in agreement with experiment. With a disorder strength somewhat smaller than extracted from the temperature-dependent stationary current-voltage characteristics, also the shape of the experimental transients is well described.

Electronic and magnetic properties of molybdenum pyrochlores $A_2$Mo${}_2$O${}_7$ are studied by the fully relativistic density-functional theory plus on-site repulsion ($U$) method, with a focus on the spin-glass insulating material Y${}_2$Mo${}_2$O${}_7$. We find that the system exhibits peculiar competition in energy between different magnetic states in the large-$U$ insulating region. The magnetic competition cannot be explained by the conventional picture based on the geometrical frustration of isotropic Heisenberg antiferromagnetic exchange interactions. Through an analysis by using a generalized spin model, we find that the effective spin interactions are distinct from the simple Heisenberg form and strongly anisotropic in spin space. We also reveal that they give rise to keen competition between antiferromagnetic and ferromagnetic states. The complex form of the magnetic interactions indicates a crucial role of the orbital degree of freedom. Analyzing a three-orbital Hubbard model, we clarify that the magnetic competition is tightly connected with orbital frustration in the $4d^2$ electronic configuration through the spin-orbital interplay. The results challenge the conventional picture of the spin-glass behavior that attributes the origin to the geometrical frustration of purely antiferromagnetic exchange interactions.

We have investigated the electronic and magnetic properties of Co-doped Pr${}_{0.8}$Ca${}_{0.2}$MnO${}_3$ thin films using various spectroscopic techniques. X-ray absorption and hard x-ray photoemission spectroscopy revealed that the substituted Co ions are in the divalent state, resulting in hole doping on the Mn atoms. Studies of element-selective magnetic properties by x-ray magnetic circular dichroism found a large orbital magnetic moment for the Co ions. These spectroscopic studies reveal that the substituted Co ions play several roles of hole doping for Mn, ferromagnetic superexchange coupling between the Co${}^{2+}$ and Mn${}^{4+}$ ions, and orbital magnetism of the Co${}^{2+}$ ions. Competition among these complex interactions produces the unique electronic and magnetic behaviors including enhanced coercivity of the Co-doped Pr${}_{0.8}$Ca${}_{0.2}$MnO${}_3$.

A long-range magnetic order was discovered in the quasi-one-dimensional spin-ladder compound SrCa${}_{13}$Cu${}_{24}$O${}_{41}$ by susceptibility, specific heat, and neutron diffraction experiments. The temperature dependence of the magnetic Bragg peak intensity could be well fitted to the power law with a transition temperature $T_N$ $=$ 4.23 K and a critical exponent β $=$ 0.28, indicating a three-dimensional phase transition for a low-dimensional magnet. A computer program was coded and found two possible magnetic structure models fitting best with all the observed magnetic peaks. These models suggest the spin-ladder sublattice is magnetically ordered with Cu moments aligning along the $a$ axis. The spin interactions are primarily antiferromagnetic along rungs and legs, while there are ferromagnetic clusters along legs. Surprisingly, the singlet-triplet spin-gap excitation is observable above and below $T_N$, indicating a coexistence of the spin-singlet ground state and long-range magnetic ordering state in this compound.

A long-standing challenge in spintronics is the development of a stable, processable and tunable organic magnetic semiconductor. We reveal, through first-principles calculations, that a $p$-electron organic molecular magnet, lithium phthalocyanine (LiPc), can display surprisingly strong antiferromagnetic coupling. The strong coupling, far exceeding that observed in the widely studied transition-metal phthalocyanines, is found to be due to the delocalized spin orbital of the ligand which facilitates intermolecular interactions. The enhanced hopping between the $\pi$-conjugated orbitals is also responsible for the wide bandwidth required for high spin mobility. The interactions are a strong function of the intermolecular arrangement and increase when approaching a face-on geometry resulting in a crossover to an itinerant spin density wave ground state, which we propose as an explanation for the unusual spin susceptibility in the related $x$-LiPc phase [M. Brinkmann et al., J. Mater. Chem. 8, 675 (1998)]. This strong coupling, in conjunction with the structural flexibility of the metal phthalocyanine organic semiconductors, suggests a promising route for the fabrication of transition-metal-free, room-temperature, chain magnets for spintronic applications.

We study the finite-size corrections to the free-energy density in disordered spin systems on sparse random graphs, using both replica theory and the cavity method. We derive analytical expressions for the $O(1/N)$ corrections in the replica symmetric phase as a linear combination of the free energies of open and closed chains. We perform a numerical check of the formulas on the random-field Ising model at zero temperature by computing finite-size corrections to the ground-state energy density.

We report powder and single-crystal neutron diffraction measurements, combined with x-ray powder diffraction data, to unravel the complex magnetic phase diagram and exchange coupling in Co${}_3$TeO${}_6$. The magnetic structures of the various phases differ markedly from those proposed by Ivanov et al. [Mater. Res. Bull. 47, 63 (2012)] on the basis of only powder diffraction data. The dominant exchange interactions are identified by considering the geometrical arrangement of severely distorted CoO${}_6$ octahedra and CoO${}_4$ tetrahedra, which naturally divide into two different types of layers, one of which consists of zigzag chains. These zigzag chains are the first to develop magnetic order at $T_{\mathrm{M}1}$ $=$ 26 K, which is incommensurate in nature. The other separate layer of Co spins develops antiferromagnetic order of ${\Gamma }_4$ symmetry at zero wave vector at $T_{\mathrm{M}2}$ $=$ 19.5 K. Our results are consistent with the previous findings of a spontaneous polarization below $T_{\mathrm{M}3}$ $=$ 18 K. Our neutron powder diffraction data indicate that the increase in the single-crystal (600) Bragg peak is due to a relief of extinction rather than to magnetic effects associated with the observed anomalous variation in the incommensurate wave vector at $T_{\mathrm{M}4}$ $=$ 16 K. The commensurate order parameter is shown to have a small dependence on the applied electric field, whereas no such effect is found for the incommensurate ordering. Below $T_{\mathrm{M}3}$, the thermal expansion is negative, and it also exhibits anomalies at $T_{\mathrm{M}2}$ and $T_{\mathrm{M}4}$. A symmetry analysis and comprehensive phase diagram are given.

We have studied the field-induced magnetic structures of Tb${}_2$Ti${}_2$O${}_7$ pyrochlore by single-crystal neutron diffraction under a field applied along the [111] axis, up to $H=12$ T and down to $T=40$ mK. We refined the magnetic structures with $\mathit{k}=\mathbf{0}$ propagation vector by performing a symmetry analysis in the space group $R\overline{3}m$, reducing the number of free parameters to three only. The Tb moments gradually reorient towards the field direction, keeping close to a “3-in, 1-out / 1-in, 3-out” spin structure (magnetic space group $R\overline{3}{m}^{\prime }$) in the whole measured field range 0.05–12 T. Our results rule out the “all-in/all-out” structure previously proposed and do not support the existence of a magnetization plateau. We perform a quantitative comparison with mean-field calculations and we propose the presence of a low-temperature dynamic symmetry breaking of the local trigonal symmetry, akin to a dynamic Jahn-Teller effect, i.e., preserving the overall cubic symmetry. We discuss the possible origin of this off-diagonal mixing term in the crystal field Hamiltonian in terms of quadrupole-quadrupole interaction or magnetoelastic effects.

We have measured the elastic constant ($C_{11}$-$C_{12}$)/2 in URu${}_2$Si${}_2$ by means of high-frequency ultrasonic measurements in pulsed magnetic fields $H\parallel$ [001] up to 61.8 T in a wide temperature range from 1.5 to 116 K. We found a reduction of ($C_{11}$-$C_{12}$)/2 that appears only in the temperature and magnetic field region in which URu${}_2$Si${}_2$ exhibits a heavy-electron state and hidden order. This change in ($C_{11}$-$C_{12}$)/2 appears to be a response of the 5$f$ electrons to an orthorhombic and volume conservative strain field ${\epsilon }_{xx}$-${\epsilon }_{yy}$ with ${\Gamma }_3$ symmetry. This lattice instability is likely related to a symmetry-breaking band instability that arises due to the hybridization of the localized $f$ electrons with the conduction electrons and is probably linked to the hidden-order parameter of this compound.

We investigate surface plasmons at a planar interface between a normal dielectric and a topological insulator, where the Fermi energy lies inside the bulk gap of the topological insulator and gives rise to a two-dimensional charge distribution of free Dirac electrons. We develop the methodology for the calculation of plasmon dispersions using the framework of classical electrodynamics, with modified constituent equations due to Hall currents in the topological insulator, together with a Lindhard-type description for the two-dimensional charge distribution of free Dirac electrons. For a system representative for Bi${}_2$X${}_3$ binary compounds, we find in agreement with recent related work that the modified constituent equations have practically no impact on the surface plasmon dispersion but lead to a rotation of the magnetic polarization of surface plasmons out of the interface plane.

The strained {105} facet, fundamental in the heteroepitaxial growth of Ge/Si(100), is investigated through a combination of scanning tunneling microscopy, reflectance anisotropy spectroscopy, and density functional theory simulations. Besides providing a strong independent confirmation of the proposed structural model, optical measurements give insight into Si/Ge intermixing, reveal hidden signatures of the buried interface, and give access to a complementary viewpoint of the epitaxial growth with respect to standard top-layer probing. Strained subsurface atoms are found to strongly determine the electronic and optical properties of the whole reconstruction. Moreover, we demonstrate how their unique spectral fingerprint is a sensitive probe of the local chemical bonding environment and allows the stoichiometry of atomic bonds to be monitored within and beneath the surface layer.

We use the $N$-terminal scheme for studying the edge-state transport in two-dimensional topological insulators. We find the universal nonlocal response in the ballistic transport approach. This macroscopic exhibition of the topological order offers different areas for applications.

We study the effect of the coupling between the electronic ground state of high-spin alkaline-earth fermionic atoms and their metastable optically excited state, when the system is confined in a one-dimensional chain, and show that the system provides a possible realization of a finite momentum pairing (Fulde-Ferrell-Larkin-Ovchinnikov-like) state without spin- or bare mass imbalance. We determine the $\beta$-functions of the renormalization group trajectories for general spin and analyze the structure of the possible gapped and gapless states in the hydrodynamic limit. Due to the SU($N$) symmetry in the spin space, complete mode separation can not be observed even in the fully gapless 2$N$-component Luttinger liquid state. Contrary, four velocities characterize the system. We solve the renormalization group equations for spin-9/2 strontium-87 isotope and analyze in detail its phase diagram. The fully gapless Luttinger liquid state does not stabilize in the two-orbital system of the ${}^{87}$Sr atoms, instead, different gapped non-Gaussian fixed points are identified either with dominant density or superconducting fluctuations. The superconducting states are stable in a nontrivial shaped region in the parameter space as a consequence of the coupling between the two electronic states.

We report on the structural and electronic properties of the Bi/Au(110)–$1\times 4$ surface, by combining scanning tunneling microscopy, angle-resolved photoemission spectroscopy, and first-principles calculations. The analysis of the precursor $1\times 8$ moiré structure shows that the $1\times 4$ reconstruction forms at an optimum coverage of one monolayer. A hard-sphere model is proposed for the $1\times 4$ structure and further confirmed by calculations. In this model, topmost Bi atoms form rows supported by a Bi overlayer, with no significant alloying with the substrate. This has important consequences regarding the electronic properties and the spin texture. The photoemission measurements evidence typical $p$ Bi-induced states, that can have either quasi-one- or two-dimensional character depending on their binding energy. These states show no Rashba spin splitting, in agreement with the results of first-principles calculations. This finding is discussed by considering the role of hybridization with the substrate in the emergence of the Rashba effect.

The exact reduced density-matrix functional is derived from the Luttinger-Ward functional of the single-particle Green's function. Thereby, a formal link is provided between diagrammatic many-body approaches using Green's functions on the one hand and theories based on many-body wave functions on the other. This link can be used to explicitly construct approximations for the density-matrix functional that are equivalent to standard diagrammatic resummation techniques and to nonperturbative dynamical mean field theory in particular. Contrary to functionals of the Green's function, the exact density-matrix functional is convex and thus provides a true minimum principle which facilitates the calculation of the grand potential and derived equilibrium properties. The benefits of the proposed Green's-function-based density-matrix functional theory for geometrical structure optimization of strongly correlated materials are discussed.

Heterostructures made of transition metal oxides are new tailor-made materials which are attracting much attention. We have constructed a six-band k$·$p Hamiltonian and used it within the envelope function method to calculate the subband structure of a variety of LaAlO${}_3$/SrTiO${}_3$ heterostructures. By use of density functional calculations, we determine the k$·$p parameters describing the conduction band edge of SrTiO${}_3$: the three effective mass parameters $L=0.6104\text{eV}{\text{Å}}^2$, $M=9.73\text{eV}{\text{Å}}^2$, $N=-1.616\text{eV}{\text{Å}}^2$, the spin orbit splitting ${\Delta }_{\text{SO}}=28.5$ meV, and the low temperature tetragonal distortion energy splitting ${\Delta }_T=2.1$ meV. For confined systems we find strongly anisotropic nonparabolic subbands. As an application we calculate bands, density of states, and magnetic energy levels and compare the results to Shubnikov–de Haas quantum oscillations observed in high magnetic fields. For typical heterostructures we find that electric field strength at the interface of $F=0.1$ meV/Å for a carrier density of $7.2\times {10}^{12}$ cm${}^{-2}$ results in a subband structure that is similar to experimental results.

The effects of electronic correlations and orbital degeneracy on thermoelectric properties are studied within the context of multiorbital Hubbard models on different lattices. We use dynamical mean field theory with a modified version of iterative perturbation theory as a solver to calculate the self-energy of the models in a wide range of interaction strengths. The Seebeck coefficient, which measures the voltage drop in response to a temperature gradient across the system, shows a nonmonotonic behavior with temperature in the presence of strong correlations. This anomalous behavior is associated with a crossover from a Fermi liquid metal at low temperatures to a bad metal with incoherent excitations at high temperatures, and is qualitatively captured by the Kelvin formula but not quantitatively. We find that for interactions comparable to the bandwidth, the Seebeck coefficient acquires large values at low temperatures. Moreover, for strongly correlated cases, where the interaction is larger than the bandwidth, the figure of merit is enhanced over a wide range of temperatures because of decreasing electronic contributions to the thermal conductivity. We also find that multiorbital systems will typically yield larger thermopower and figure of merit compared to single-orbital models over a temperature range possibly relevant to applications.