The structural and electronic properties of the skutterudite-related Ce${}_3$$M$${}_4$Sn${}_{13}$ and La${}_3$$M$${}_4$Sn${}_{13}$ intermetallic compounds with $M=\text{Co}$, Ru, and Rh have been investigated. A subtle structural transition from a simple cubic phase (Yb${}_3$Rh${}_4$Sn${}_{13}$-type structure) to the superlattice variant at $T_D\sim 160$ K has been evidenced by x-ray diffraction, resistivity, and specific-heat data. Recently, a high charge density between metal $M$ and Sn atoms was calculated for Ce${}_3$Rh${}_4$Sn${}_{13}$ and Ce${}_3$Co${}_4$Sn${}_{13}$. The strong charge accumulation implies a strong $M$-Sn2 covalent bonding interaction. In consequence, a local distortion of the trigonal Sn prisms around metal $M$ can modify the electronic structure of the system. The main goal of this report is to study the electronic structure in the family of RE${}_3$$M$${}_4$Sn${}_{13}$, where $\text{RE}=\text{Ce}$ or La above and below $T_D$. We show that the Sn $4d$ x-ray photoelectron spectroscopy spectra have the complex structure, which can be interpreted as a result of different charge distribution around Sn atoms. The shape of the $4d$ Sn lines is also temperature dependent, which we attribute to the local distortion of the Sn2 cages in RE${}_3$$M$${}_4$Sn${}_{13}$.

A ${\left(LaV{O}_3\right)}_6$/${\left(SrV{O}_3\right)}_3$ superlattice is studied with a combination of sub-Å resolved scanning transmission electron microscopy and monochromated electron energy-loss spectroscopy. The V oxidation state is mapped with atomic spatial resolution enabling us to investigate electronic reconstruction at the $LaV{O}_3$/$SrV{O}_3$ interfaces. Surprisingly, asymmetric charge distribution is found at adjacent chemically symmetric interfaces. The local structure is proposed and simulated with a double channeling calculation which agrees qualitatively with our experiment. We demonstrate that local strain asymmetry is the likely cause of the electronic asymmetry of the interfaces. The electronic reconstruction at the interfaces extends much further than the chemical composition, varying from 0.5 to 1.2 nm. This distance corresponds to the length of charge transfer previously found in the ${\left(LaV{O}_3\right)}_m$/${\left(SrV{O}_3\right)}_n$ metal/insulating and the ${\left(LaAl{O}_3\right)}_m$/${\left(SrTi{O}_3\right)}_n$ insulating/insulating interfaces.

We have performed a low-temperature scanning tunneling microscopy and spectroscopy study of the iron chalcogenide superconductor ${\mathrm{FeSe}}_{0.4}{\mathrm{Te}}_{0.6}$ with $T_{\mathrm{C}}\approx 14$ K. Spatially resolved measurements of the superconducting gap reveal substantial inhomogeneity on a nanometer length scale. Analysis of the structure of the gap seen in tunneling spectra by comparison with calculated spectra for different superconducting order parameters ($s$-wave, $d$-wave, and anisotropic $s$-wave) yields the best agreement for an order parameter with anisotropic $s$-wave symmetry with an anisotropy of $\sim$40%. The temperature dependence of the superconducting gap observed in places with large and small gap size indicates that it is indeed the superconducting transition temperature which is inhomogeneous. The temperature dependence of the gap size has been found to differ from the one predicted by BCS theory. An analysis of the local gap size in relation to the local chemical composition shows almost no correlation with the local concentration of Se/Te atoms at the surface.

By means of space-group symmetry arguments, we argue that the electronic pairing in iron-based high-temperature superconductors shows a structure which is a linear combination of planar $s$-wave and $d$-wave symmetry channels, both preserving the three-dimensional $A_{1g}$ irreducible representation of the corresponding crystal point group. We demonstrate that the $s$- and $d$-wave channels are determined by the parity under reflection of the electronic orbitals through the iron planes and by improper rotations around the iron sites. We provide evidence of these general properties by performing accurate quantum Monte Carlo (QMC) ab initio calculations of the pairing function, for a FeSe lattice with tetragonal experimental geometry at ambient pressure. We find that this picture survives even in the FeSe under pressure and at low temperatures, when the tetragonal point-group symmetry is slightly broken. In order to achieve a higher resolution in momentum space we introduce a BCS model that faithfully describes our QMC variational pairing function on the simulated 4x4 FeSe unit cell. This allows us to provide a k-resolved image of the pairing function and show that nonisotropic contributions in the BCS gap function are related to the improper $s$-wave symmetry. Our theory can rationalize and explain a series of contradictory experimental findings, such as the observation of twofold symmetry in the FeSe superconducting phase, the anomalous drop of $T_c$ with Co impurity in LaFeAsO${}_{(1-x)}$F${}_x$, the $s$- to $d$-wave gap transition in BaFe${}_2$As${}_2$ under K doping, and the nodes appearing in the LiFeAs superconducting gap upon P isovalent substitution.

Recent experiments [Hutchison, O’Carroll, Schwartz, Genet, and Ebbesen, Angew. Chem. Int. Ed. 50, 2085 (2011)] have demonstrated that optical transmission through an array of subwavelength holes in a metal film can be enhanced by the intentional presence of dyes in the system. As the transmission maximum occurs spectrally close to the absorption resonances of the dyes, this phenomenon was christened “absorption induced transparency”. Here, a theoretical study on absorption induced transparency is presented. The results show that the appearance of transmission maxima requires that the absorbent fills the holes and that it occurs also for single holes. Furthermore, it is shown that the transmission process is nonresonant, being composed by a sequential passage of the electromagnetic field through the hole. Finally, the physical origin of the phenomenon is demonstrated to be nonplasmonic, which implies that absorption induced transparency should also occur at the infrared or terahertz frequency regimes.

The unusual electronic properties of the quantum spin Hall or Chern insulator become manifest in the form of robust edge states when samples with boundaries are studied. In this work, we ask if and how the topologically nontrivial electronic structure of these two-dimensional systems can be passed on to their zero-dimensional relatives, namely, fullerenes or other closed-cage molecules. To address this question, we study Haldane's honeycomb lattice model on polyhedral nanosurfaces. We find that for sufficiently large surfaces, characteristic corner states appear for parameters for which the planar model displays a quantized Hall effect. In the electronic structure, these corner states show up as in-gap modes which are well separated from the quasicontinuum of states. We discuss the role of finite-size effects and how the coupling between the corner states lifts the degeneracy in a characteristic way determined by the combined Berry phases which leads to an effective magnetic monopole of charge 2 at the center of the nanosurface. Experimental implications for fullerenes in the large spin-orbit regime are also pointed out.

We perform and analyze a series of time-resolved Faraday rotation measurements of coherent spin dynamics in a room-temperature ensemble of CdSe nanocrystal quantum dots (NCQDs) to study the decoherence and dephasing mechanisms that limit the transverse spin lifetime. Coherent spin lifetimes on the order of nanoseconds have been previously observed in CdSe NCQDs, but the presence of multiple components with distinct dynamics and strong inhomogeneous dephasing have made it difficult to study the relevant spin decay mechanisms quantitatively. Here, we obtain reliable fitting results by ensuring that cross-correlations between model parameters are minimized for the parameters of interest. Furthermore, we characterize the morphological inhomogeneity of the NCQD ensemble using transmission electron microscopy to constrain the model parameters that specify inhomogeneous dephasing. We find that $g$-factor inhomogeneity-induced dephasing (gID) is not sufficient to explain the magnetic-field-dependent decay of the spin signal. We propose an additional decoherence mechanism arising from rapid transitions between the fine structure states of the exciton referred to as fine-structure decoherence (FSD). By including both gID and FSD in the model, excellent fits are obtained to the data, including a prominent short-time-scale feature, which has typically been excluded from the fits in previous work.

First-principles calculations are performed to investigate the topological properties of TiTe${}_2$ under hydrostatic pressure, uniaxial strain, and biaxial strain. It is found that the system is unusually accessible to strain effects and the first compound that under hydrostatic pressure (up to experimentally reasonable 30 GPa) is subject to a series of four topological phase transitions, which are related to band inversions at different points of the Brillouin zone. Therefore, TiTe${}_2$ enables experimental access to all these transitions in a single compound.

An update of the effective screening medium (ESM) method [Otani and Sugino, Phys. Rev. B 73, 115407 (2006)] is presented, extending the ability to simulate electrified interfaces in an efficient and flexible way. The need for an artificial vacuum buffer between the molecular system and the screening medium is removed by defining a smooth transition of the ESM dielectric permittivity (smooth ESM), precluding numerical instabilities when molecules come in contact with the ESM. Moreover, at short distances, the smooth ESM acts as a repulsive wall, and thus the simulation cell can serve as a natural container for molecules in molecular-dynamics simulations. Consequently, the smooth ESM method is a substantial advancement in modeling solid-liquid interfaces under electric bias.

The possibility to observe and manipulate Majorana fermions as end states of one-dimensional topological superconductors has been actively discussed recently. In a quantum wire with strong spin-orbit coupling placed in proximity to a bulk superconductor, a topological superconductor has been expected to be realized when the band energy is split by the application of a magnetic field. When a periodic lattice modulation is applied, multiple topological superconductor phases appear in the phase diagram. Some of them occur for higher filling factors compared to the case without the modulation. We study the effects of phase jumps and argue that the topologically nontrivial state of the whole system is retained even if they are present. We also study the effect of the spatial modulation in the hopping parameter.

We present a tight-binding theory of triangular graphene quantum dots (TGQD) with zigzag edge and broken sublattice symmetry in an external magnetic field. The lateral size quantization opens an energy gap, and broken sublattice symmetry results in a shell of degenerate states at the Fermi level. We derive a semianalytical form for zero-energy states in a magnetic field and show that the shell remains degenerate in a magnetic field, in analogy to the zeroth Landau level of bulk graphene. The magnetic field closes the energy gap and leads to the crossing of valence and conduction states with the zero-energy states, modulating the degeneracy of the shell. The closing of the gap with increasing magnetic field is present in all graphene quantum dot structures investigated irrespective of shape and edge termination.

The change of the quasiparticle density of states in a superconductor due to the Zeeman splitting can lead to highly responsive spintronic devices. This is demonstrated here in the magnetotunneling studies of a supercondcutor/insulator/ferromagnet tunnel junction. A tunnel magnetoresistance (TMR) as large as 36% is obtained, resulting from the conductance variation due to spin-split quasiparticle tunneling, and only occurs in the superconducting state. Our results show that in addition to the naturally existent spin imbalance in ferromagnets (upon which conventional TMR is obtained), we can manipulate tunnel conductance by tailoring spin dependent density of states with interfacial exchange fields, and even with a tunnel junction with both superconducting electrodes.

We investigate spin and charge transport in both single and bilayer graphene nonlocal spin-valve devices. An inverse dependence of the spin lifetime ${\tau }_s$ on the carrier mobility $\mu$ is observed in devices with large contact-resistance-area products ($R_{c}A>1\mathrm{k}\Omega \mu {\mathrm{m}}^2$). Furthermore, we observe an increase of ${\tau }_s$ with increasing $R_{c}A$ values, demonstrating that spin transport is limited by spin dephasing underneath the electrodes. In charge transport, we measure a second contact-induced Dirac peak at negative gate voltages in devices with larger $R_{c}A$ values, demonstrating different transport properties in contact-covered and bare graphene parts. We argue that the existence of the second Dirac peak complicates the analysis of the carrier mobilities and the spin scattering mechanisms.

We present a straightforward, noniterative projection scheme that can represent the electronic ground state of a periodic system on a finite atomic-orbital-like basis, up to a predictable number of electronic states and with controllable accuracy. By cofiltering the projections of plane-wave Bloch states with high-kinetic-energy components, the richness of the finite space and thus the number of exactly-reproduced bands can be selectively increased at a negligible computational cost, an essential requirement for the design of efficient algorithms for electronic structure simulations of realistic material systems and massive high-throughput investigations.

The metallic character of the GeBi${}_2$Te${}_4$ single crystals is probed using a combination of structural and physical properties measurements, together with density functional theory (DFT) calculations. The structural study shows distorted Ge coordination polyhedra, mainly of the Ge octahedra. This has a major impact on the band structure, resulting in bulk metallic behavior of GeBi${}_2$Te${}_4$, as indicated by DFT calculations. Such calculations place GeBi${}_2$Te${}_4$ in a class of a few known nontrivial topological metals, and explains why an observed Dirac point lies below the Fermi energy at about −0.12 eV. A topological picture of GeBi${}_2$Te${}_4$ is confirmed by the observation of surface state modulations by scanning tunneling microscopy.

Using circular dichroism-angle resolved photoemission spectroscopy, we report a study of the effect of angular momentum transfer between polarized photons and topological surface states on the surface of the insulating topological insulator Bi${}_2$Te${}_2$Se. The photoelectron dichroism is found to be strongly modulated by the frequency of the helical photons including a dramatic sign flip. Our results suggest that the observed dichroism and its sign flip are consequences of strong coupling between the photon field and the spin-orbit nature of the Dirac modes on the surface. Our studies reveal the intrinsic dichroic behavior of topological surface states and point toward the potential utility of bulk insulating topological insulators in opto-spintronics device applications.

We study in this paper quantum spin liquid states (QSLs) at the vicinity of the metal-insulator transition. Assuming that the low-energy excitations in the QSLs are labeled by “spinon” occupation numbers with the same Fermi surface structure as in the corresponding metal (Fermi-liquid) side, we propose a phenomenological Landau-like low-energy theory for the QSLs and show that the usual $U\left(1\right)$ QSLs is a representative member of this class of spin liquids. Based on our effective low-energy theory, an alternative picture to the Brinkman-Rice picture of the Mott metal-insulator transition is proposed. The charge, spin, and thermal responses of QSLs are discussed under such a phenomenology.

We study the dynamics resulting from an abrupt change of the two-particle interaction in two models of closed one-dimensional Fermi systems: (a) the field theoretical Tomonaga-Luttinger model and (b) a microscopic lattice model. Using a nonperturbative approach which is controlled for small two-particle interactions, we are able to reach large times allowing us to access the properties of the steady state of the lattice model. Comparing those to the exact solution of the full dynamics in the Tomonaga-Luttinger model, we provide evidence for universal Luttinger liquid behavior.

The phase transition driven by electron correlations in a Chern insulator is investigated within the dynamical mean-field theory. The Chern insulator is described by the Haldane model and the electron correlations are incorporated by introducing the short-range interaction between the itinerant electrons and localized fermions. In the preservation of the inversion symmetry, the electron correlations drive the system from the Chern insulator to a renormalized pseudogap metal, and then to the topologically trivial Mott insulator. When the inversion symmetry is broken, a charge ordering and a nontrivial Chern topological invariant coexist.

We present a comprehensive experimental and theoretical study of the power dependence of coherently controlled currents in bulk GaAs. Currents are optically induced by phase-stable femtosecond $\omega /2\omega$ pulse pairs. For moderate irradiances, these currents are linked to the third-order optical nonlinearity ${\chi }^{\left(3\right)}(0;\omega ,\omega ,-2\omega )$. Here we focus on elevated irradiances where absorption saturation and ultimately the onset of Rabi oscillations contribute to the optical response. Current diagnostics is achieved electrically by recording the photoresponse of contacted specimens of low temperature grown GaAs as a function of the relative phase of the $\omega$ and $2\omega$ pulses. For stronger $\omega$ irradiance we find the magnitude of the coherently controlled current to be markedly reduced when compared to the ${\chi }^{\left(3\right)}$ expectation $dJ/dt\propto E_{\omega }^2{E}_{2\omega }$. Additional pump-probe type experiments corroborate that this current saturation is indeed predominantly linked to macroscopic band filling. Theoretical simulations for the coherently controlled current based on a 14 band $\mathbf{k}·\mathbf{p}$ model agree well with the experimental trends.