We study the Anderson model as a description of the quantum $RC$ circuit for spin-$1/2$ electrons and a single level connected to a single lead. Our analysis relies on the Fermi liquid nature of the ground state, which fixes the form of the low-energy effective model. The constants of this effective model are extracted from a numerical solution of the Bethe ansatz equations for the Anderson model. They allow us to compute the charge relaxation resistance $R_q$ in different parameter regimes. In the Kondo region, the peak in $R_q$ as a function of the magnetic field is recovered and proven to be in quantitative agreement with previous numerical renormalization group results. In the valence-fluctuation region, the peak in $R_q$ is shown to persist, with a maximum value of $h/2e^2$, and an analytical expression is obtained using perturbation theory. We extend our analysis to the SU(4) Anderson model where we also derive the existence of a giant peak in the charge relaxation resistance.

We study the effect of the Dresselhaus spin-orbit interaction on the electronic states and spin relaxation rates of cylindrical quantum dots defined on quantum wires having wurtzite lattice structure. The linear and cubic contributions of the bulk Dresselhaus spin-orbit coupling (SOC) are taken into account, along with the influence of a weak external magnetic field. The previously found analytic solution for the electronic states of cylindrical quantum dots with zinc blende lattice structures with Rashba interaction is extended to the case of quantum dots with wurtzite lattices. For the electronic states in InAs dots, we determine the spin texture and the effective $g$ factor, which shows a scaling collapse when plotted as a function of an effective renormalized dot-size-dependent spin-orbit coupling strength. The acoustic-phonon-induced spin relaxation rate is calculated and the transverse piezoelectric potential is shown to be the dominant one.

We propose an analytical two-band model for surface plasmon polaritons on corrugated metal surfaces. The generalized eigenvalue equation is derived from Maxwell's equation and matching conditions for electromagnetic fields at the metal surface. Our basic equation is Hermitian for the bands outside of the light cone and perturbation theory is therefore justified. We solve the equation and show the dispersion of the one-dimensional plasmonic crystal, and compare the behaviors of eigenfrequencies with previous works for analytic plasmonic crystals and the band theory for electrons in solids.

We introduce the photonic analog of electronic van Hove singularities (VHS) in artificial media (metamaterials) with hyperbolic dispersion. Unlike photonic and electronic crystals, the VHS in metamaterials are unrelated to the underlying periodicity and occur due to slow-light modes in the structure. We show that the VHS characteristics are manifested in the near-field local density of optical states in spite of the losses, dispersion, and finite unit-cell size of the hyperbolic metamaterial. Finally, we show that this work should lead to quantum, thermal, nanolasing, and biosensing applications of van Hove singularities in hyperbolic metamaterials achievable by current fabrication technology.

A weak topological insulator (WTI) bears, generally, an even number of Dirac cones on its surface; they are susceptible to doubling, while on the surface of a certain orientation it shows no Dirac cone. On this “dark” surface of a WTI, we predict that a single pair of isolated one-dimensional perfectly conducting channels emerges and forms either a closed loop or a segment of a line. The former is associated typically with a single-atomic-layer-thick island formed on the dark surface, while the latter is shown to be the consequence of a pair of crystal (screw) dislocations terminating on the dark surface.

Using resonant inelastic x-ray scattering (RIXS) at the Cu $L$-absorption edge, we have observed intense, dispersive spin excitations in highly overdoped Tl${}_2$Ba${}_2$CuO${}_{6+\delta }$ (superconducting $T_c=6$ K), a model compound whose normal-state charge transport and thermodynamic properties have been shown to exhibit canonical Fermi-liquid behavior. Complementary RIXS experiments on slightly overdoped Tl${}_2$Ba${}_2$CuO${}_{6+\delta }$ ($T_c=89$ K) and on Y${}_{1-x}$Ca${}_x$Ba${}_2$Cu${}_3$O${}_{6+\delta }$ compounds spanning a wide range of doping levels indicate that these excitations exhibit energies and energy-integrated spectral weights closely similar to those of antiferromagnetic magnons in undoped cuprates, indicating the persistence of substantial antiferromagnetic spin correlations over a wide doping range. The surprising coexistence of such correlations with Fermi-liquid-like charge excitations in highly overdoped cuprates poses a challenge to current theoretical models of correlated-electron metals.

We report detailed inelastic neutron-scattering measurements on single crystals of the frustrated two-leg ladder ${\text{BiCu}}_2{\text{PO}}_6$, whose ground state is described as a spin liquid phase with no long-range order down to 6 K. Two branches of steeply dispersing long-lived spin excitations are observed with excitation gaps of ${\Delta }_1=1.90\left(9\right)$ meV and ${\Delta }_2=3.95\left(8\right)$ meV. Significant frustrating next-nearest-neighbor interactions along the ladder leg drive the minimum of each excitation branch to incommensurate wave vectors ${\zeta }_1=0.574\pi$ and ${\zeta }_2=0.553\pi$ for the lower and upper energy branches, respectively. The temperature dependence of the excitation spectrum near the gap energy is consistent with thermal activation into singly and doubly degenerate excited states. The observed magnetic excitation spectrum as well as earlier thermodynamic data could be consistently explained by the presence of strong anisotropic interactions in the ground-state Hamiltonian.

The complete piezoelectric tensor of ferroelectric SrTiO${}_3$ at low temperature is computed by ab initio theoretical simulations. Both direct and converse—coupled with elastic compliance—piezoelectricity are computed and interpreted in terms of electronic and nuclear contributions. The role of the ferroelectric soft phonon mode on this property is found to be dramatic thus leading to a possible giant piezoelectric response at very low temperature. Two possible space groups are considered for the ferroelectric phase of SrTiO${}_3$, both compatible with the available experimental data: a tetragonal $I4cm$ and an orthorhombic $Ima2$ one. The piezoelectric response of the two symmetries is predicted to be rather different and could be experimentally detected to clarify the (still unknown) structure of the ferroelectric phase of SrTiO${}_3$.

The low-temperature (∼5 K) phonon-assisted relaxation of the 2$p_0$ state of phosphorus donors in isotopically pure, monocrystalline ${}^{28}$Si has been studied in the time domain using a pump-probe technique. The lifetime of the 2$p_0$ state in ${}^{28}$Si:P is found to be 235 ps, which is 16$\%$ larger than the lifetime of a reference Si:P sample with a natural isotope composition. The interaction of the 2$p_0$ state with intervalley $g$-type longitudinal acoustic and $f$-type transverse acoustic phonons determines its lifetime. This interaction, which depends on the homogeneity of the crystal, becomes weaker in ${}^{28}$Si because of its more perfect crystal lattice compared to natural Si, and this leads to a longer lifetime. The difference between the linewidths of the 1$s$($A_1$) → 2$p_0$ transition in ${}^{28}$Si:P and natural Si:P is more than a factor of two. It follows that linewidth broadening due to isotopic composition is an inhomogeneous process.

The large effect of a small magnetic field on the current, magnetoconductance (MC), in organic semiconductors—so-called organic magnetoresistance—has puzzled the field of organic spintronics during the last decade. Although the microscopic mechanisms regarding spin mixing are well understood by now, it is still unknown which pairs of spin carrying particles are influencing the current in such a drastic manner. Here, a model for the MC is presented based on the spin selective formation of metastable trions from triplet exciton-polaron pairs. Additionally, the magnetic-field and voltage dependence of the MC are experimentally investigated in materials showing large effects. Using a combination of analytical and numerical calculations, it is shown that the MC is perfectly described by a process in which trions are created at polaron trap sites.

By combining high-resolution transmission electron microscopy experiments and first-principles calculations, we study production, diffusion, and agglomeration of sulfur vacancies in monolayer MoS${}_2$ under electron irradiation. Single vacancies are found to be mobile under the electron beam and tend to agglomerate into lines. Different kinds of such extended defects are identified in the experiments, and their atomic structures and electronic properties are determined with the help of calculations. The orientation of line defects is found to be sensitive to mechanical strain. Our calculations also indicate that the electronic properties of the extended defects can be tuned by filling vacancy lines with other atomic species, thereby suggesting a way for strain and electron-beam-assisted engineering of MoS${}_2$-based nanostructures.

We show that magnetoresistance of Bernal-stacked graphite (with the magnetic field $\mathbf{B}$ parallel to the $c$ axis and the current in the $ab$ plane ) scales linearly with the magnetic field over an interval of classically weak fields. The linearity is related to the presence of extremely light, Dirac-like carriers near the $H$($H^{\prime }$) points of the Brillouin zone. The Hall resistivity in this interval also shows a nonanalytic $B\ln \left|B\right|$ behavior, and is dominated by holes.

Electronic properties of bilayer graphene are distinct from both the conventional two dimensional electron gas and monolayer graphene due to its particular chiral properties and excitation charge carrier dispersions. We study the effect of strain on the electronic structure, the edge states and charge transport of bilayer graphene nanoribbon at zero temperature. We demonstrate a valley polarized quantum Hall effect in biased bilayer graphene when the system is subjected to a perpendicular magnetic field. In this system a topological phase transition from a quantum valley Hall to a valley polarized quantum Hall phase can occur by tuning the interplanar strain. Furthermore, we study the layer-resolved transport properties by calculating the layer polarized quantity by using the recursive Green's function technique and show that the resulting layer polarized value confirms the obtained phases. These predictions can be verified by experiments, and our results demonstrate the possibility for exploiting strained bilayer graphene in the presence of external fields for electronics and valleytronics devices.

We report the results of second-harmonic optical spectroscopy of LaAlO${}_3$ films grown on SrTiO${}_3$ (001) substrates having either TiO${}_2$ or SrO atomic terminations. The atomic termination is found to have a large effect on the observed spectra, with a strong dependence on the $\omega$-$2\omega$ light polarizations. These results are analyzed through a model based on symmetry-controlled selection rules and atomic orbital overlaps. The analysis shows that, as suspected from transport measurements, the SrO termination prevents the formation of a mobile interfacial electron gas. Yet, by explicitly probing and identifying the electronic states at the O${}^{2-}$–Ti${}^{3+}$ band gap we now obtain unique and direct evidence that this is caused by the absence of any detectable charge injection in the interfacial region rather than by carrier localization.

A systematic theory of the conductance measurements of noninvasive (weak probe) scanning gate microscopy is presented that provides an interpretation of what precisely is being measured. A scattering approach is used to derive explicit expressions for the first- and second-order conductance changes due to the perturbation by the tip potential in terms of the scattering states of the unperturbed structure. In the case of a quantum point contact, the first-order correction dominates at the conductance steps and vanishes on the plateaux where the second-order term dominates. Both corrections are nonlocal for a generic structure. Only in special cases, such as that of a centrally symmetric quantum point contact in the conductance quantization regime, can the second-order correction be unambiguously related with the local current density. In the case of an abrupt quantum point contact, we are able to obtain analytic expressions for the scattering eigenfunctions and thus evaluate the resulting conductance corrections.

We report the synthesis and characterization of a bulk diluted magnetic semiconductor (La${}_{1-x}$Ba${}_x$)(Zn${}_{1-x}$Mn${}_x$)AsO (0 $⩽$ $x$ $⩽$ 0.2) with a layered crystal structure identical to that of the 1111-type FeAs superconductors. No ferromagnetic order occurs with (Zn,Mn) substitution in the parent compound LaZnAsO without charge doping. Together with carrier doping via (La,Ba) substitution, a small amount of Mn substituting for Zn results in ferromagnetic order with $T_C$ up to $\sim$40 K, although the system remains semiconducting. Muon spin relaxation measurements confirm the development of ferromagnetic order in the entire volume, with the relationship between the internal field and $T_C$ consistent with the trend found in (Ga,Mn)As and the 111-type Li(Zn,Mn)As and the 122-type (Ba,K)(Zn,Mn)${}_2$As${}_2$ systems.

We present strong numerical evidence for the existence of a localization-delocalization transition in the eigenstates of the 1D Anderson model with long-range hierarchical hopping. Hierarchical models are important because of the well-known mapping between their phases and those of models with short-range hopping in higher dimensions, and also because the renormalization group can be applied exactly without the approximations that generally are required in other models. In the hierarchical Anderson model, we find a finite critical disorder strength $W_c$ where the average inverse participation ratio goes to zero; at small disorder, $W<W_c$, the model lies in a delocalized phase. This result is based on numerical calculation of the inverse participation ratio in the infinite volume limit using an exact renormalization group approach facilitated by the model's hierarchical structure. Our results are consistent with the presence of an Anderson transition in short-range models with $D>2$ dimensions, which was predicted using renormalization group arguments. Our finding should stimulate interest in the hierarchical Anderson model as a simplified and tractable model of the Anderson localization transition, which occurs in finite-dimensional systems with short-range hopping.

We use time- and angle-resolved photoemission spectroscopy in the extreme ultraviolet to measure the time- and momentum-dependent electronic structures of photoexcited K${}_{0.3}$MoO${}_3$. Prompt depletion of the charge-density wave condensate launches coherent oscillations of the amplitude mode, observed as a 1.7-THz-frequency modulation of the bonding band position. In contrast, the antibonding band oscillates at about half this frequency. We attribute these oscillations to coherent excitation of phasons via parametric amplification of phase fluctuations.

The longitudinal in-plane magnetoresistance (LMR) has been measured in different Ba(Fe${}_{1-x}$Co${}_x$)${}_2$As${}_2$ single crystals and in LiFeAs. For all these compounds, we find a negative LMR in the paramagnetic phase whose magnitude increases as $H^2$. We show that this negative LMR can be readily explained in terms of suppression of the spin fluctuations by the magnetic field. In the Co-doped samples, the absolute value of the LMR coefficient is found to decrease with doping content in the paramagnetic phase. The analysis of its $T$ dependence in an itinerant nearly antiferromagnetic Fermi-liquid model evidences that the LMR displays a qualitative change of $T$ variation with increasing Co content. The latter occurs at optimal doping for which the antiferromagnetic ground state is suppressed. The same type of analysis for the negative LMR measured in LiFeAs suggests that this compound is on the verge of magnetism.

We analyze the physics of cold atoms in honeycomb optical lattices with on-site repulsion and spin-dependent hopping that breaks time-reversal symmetry. Such systems, at half filling and large on-site repulsion, have been proposed as a possible realization of the Kitaev model. The spin-dependent hopping breaks the spin degeneracy and, if strong enough, leads to four nonoverlapping bands in the noninteracting limit. These bands carry nonzero Chern number and therefore the noninteracting system has nonzero angular momentum and chiral edge states at 1/4 and 3/4 filling. We have investigated the effect of interactions on a quarter-filled system using the variational cluster perturbation theory and found that the critical spin-dependent hopping that separates the metal from the quantum Hall state is affected by interactions.