We theoretically investigate the effects of interaction between an optical dipole (semiconductor quantum dot or molecule) and metal nanoparticles. The calculated absorption spectra of hybrid structures demonstrate strong effects of interference coming from the exciton-plasmon coupling. In particular, the absorption spectra acquire characteristic asymmetric line shapes and strong antiresonances. We present here an exact solution of the problem beyond the dipole approximation and find that the multipole treatment of the interaction is crucial for the understanding of strongly interacting exciton-plasmon nanosystems. Interestingly, the visibility of the exciton resonance becomes greatly enhanced for small interparticle distances due to the interference phenomenon, multipole effects, and electromagnetic enhancement. We find that the destructive interference is particularly strong. Using our exact theory, we show that the interference effects can be experimentally observed in the exciting systems even at room temperature.

We compute the tunneling current in a double point contact geometry of a quantum Hall system at filling fraction $\nu =\frac{5}{2}$, as a function of voltage and temperature, in the weak tunneling regime. We quantitatively compare two possible candidates for the state at $\nu =\frac{5}{2}$: the Moore-Read Pfaffian state, and its particle-hole conjugate, the anti-Pfaffian. We find that both possibilities exhibit the same qualitative behavior, and both have an even-odd effect that reflects their non-Abelian nature, but differ quantitatively in their voltage and temperature dependence.

In two dimensions, the laws of physics permit the existence of anyons, particles with fractional statistics which are neither Fermi nor Bose. That is, upon exchange of two such particles, the quantum state of a system acquires a phase which is neither 0 nor $\pi$, but can be any value. The elementary excitations (Laughlin quasiparticles) of a fractional quantum Hall fluid have a fractional electric charge and are expected to obey fractional statistics. In this paper we report experimental realization of a Laughlin quasiparticle interferometer, where quasiparticles of the $1∕3$ fluid execute a closed path around an island of the $2∕5$ fluid and thus acquire statistical phase. Interference fringes are observed as conductance oscillations as a function of magnetic flux, similar to the Aharonov-Bohm effect. We observe the interference shift by one fringe upon introduction of five magnetic flux quanta $(5h∕e)$ into the island. The corresponding $2e$ charge period is confirmed directly in calibrated gate experiments. These results constitute direct observation of fractional statistics of Laughlin quasiparticles.

The phase diagram and critical properties of the $N$-component London superconductor are studied both analytically and through large-scale Monte Carlo simulations in $d=2+1$ dimensions (components here refer to different replicas of the complex scalar field). Examples are given of physical systems to which this model is applicable. The model with different bare phase stiffnesses for each component is a model of superconductivity, which should arise out of metallic phases of light atoms under extreme pressure. A projected mixture of electronic and protonic condensates in liquid metallic hydrogen under extreme pressure is the simplest example, corresponding to $N=2$. These are such that Josephson coupling between different matter field components is precisely zero on symmetry grounds. The $N$-component London model is dualized to a theory involving $N$ vortex fields with highly nontrivial interactions. We compute critical exponents $\alpha$ and $\nu$ for $N=2$ and $N=3$. Direct and dual gauge field correlators for general $N$ are given and the $N=2$ case is studied in detail. The model with $N=2$ shows two anomalies in the specific heat when the bare phase stiffnesses of each matter field species are different. One anomaly corresponds to an inverted $3\mathrm{D}xy$ fixed point, while the other corresponds to a $3\mathrm{D}xy$ fixed point. Correspondingly, for $N=3$, we demonstrate the existence of two neutral $3\mathrm{D}xy$ fixed points and one inverted charged $3\mathrm{D}xy$ fixed point. For the general case, there are $N$ fixed points, namely one charged inverted $3\mathrm{D}xy$ fixed point, and $N-1$ neutral $3\mathrm{D}xy$ fixed points. We explicitly identify one charged vortex mode and $N-1$ neutral vortex modes. The model for $N=2$ and equal bare phase stiffnesses corresponds to a field theoretical description of an easy-plane quantum antiferromagnet. In this case, the critical exponents are computed and found to be non-$3\mathrm{D}xy$ values. The $N$-component London superconductor model in an external magnetic field, with no interspecies Josephson coupling, will be shown to have a different feature, namely $N-1$ superfluid phases arising out of $N$ charged condensates. In particular, for $N=2$ we point out the possibility of two different types of field-induced phase transitions in ordered quantum fluids: (i) A phase transition from a superconductor to a superfluid or vice versa, driven by tuning an external magnetic field. This sets the superconducting phase of liquid metallic hydrogen apart from other known quantum fluids. (ii) A phase transition corresponding to a quantum fluid analogue of sublattice melting, where a composite field-induced Abrikosov vortex lattice is decomposed and disorders the phases of the constituent condensate with lowest bare phase stiffness. Both transitions belong to the $3\mathrm{D}xy$ universality class. For $N⩾3$, there is a feature not present in the cases $N=1$ and $N=2$, namely a partial decomposition of composite field-induced vortices driven by thermal fluctuations. A “color electric charge” concept, useful for establishing the character of these phase transitions, is introduced.

We present transport critical current measurements on microfibers consisting of a $80\text{−}\mathrm{nm}$ thick layer of polycrystalline $\mathrm{Mg}\mathrm{C}{\mathrm{Ni}}_3$ synthesized directly onto $7\text{−}\mu \mathrm{m}$ diameter carbon fibers. Near the transition temperature $T_c$, the critical current density $J_c$ is well described by the power law form ${[1-{(T∕T_c)}^2]}^{\alpha }$, where $\alpha =2$ with no crossover to the Ginzburg-Landau exponent $\alpha =1.5$. We extrapolate $J_c\left(0\right)\approx 4\times {10}^7\mathrm{A}∕{\mathrm{cm}}^2$, which is an order of magnitude greater than estimates obtained from magnetization measurements of polycrystalline powders. The field dependence is purely exponential $J_c(T,H)=J_c\left(T\right)\exp (-H∕H_0)$ over the entire field range of $0\text{to}9\mathrm{T}$. The unconventional scaling behavior of the critical current may be rooted in an anomalous temperature dependence of the London penetration depth.

We have performed magnetization measurements on ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4.$ A hysteresis in the temperature dependence of low field magnetization is observed well above the superconducting transition temperature. The behavior of the hysteresis resembles fundamental properties of the superconducting state and is discussed in terms of superconducting correlations.

Surface modes are generally believed to be an undesirable feature of finite photonic crystals (PC), unlike point or line defect modes. However, it is possible to make the surface mode radiate by appropriate corrugation of the PC interface. In this paper we show theoretically that the coherent action of these surface indentations can be engineered to collimate within a few degrees the light exiting a PC waveguide, or to funnel light coming from free space into the waveguide.

We present an efficient scheme for calculating the Kohn-Sham ground state of metallic systems using pseudopotentials and a plane-wave basis set. In the first part the application of Pulay’s DIIS method (direct inversion in the iterative subspace) to the iterative diagonalization of large matrices will be discussed. Our approach is stable, reliable, and minimizes the number of order ${\mathit{N}}_{\mathrm{atoms}}^3$ operations. In the second part, we will discuss an efficient mixing scheme also based on Pulay’s scheme. A special ‘‘metric’’ and a special ‘‘preconditioning’’ optimized for a plane-wave basis set will be introduced. Scaling of the method will be discussed in detail for non-self-consistent and self-consistent calculations. It will be shown that the number of iterations required to obtain a specific precision is almost independent of the system size. Altogether an order ${\mathit{N}}_{\mathrm{atoms}}^2$ scaling is found for systems containing up to 1000 electrons. If we take into account that the number of k points can be decreased linearly with the system size, the overall scaling can approach ${\mathit{N}}_{\mathrm{atoms}}$. We have implemented these algorithms within a powerful package called VASP (Vienna ab initio simulation package). The program and the techniques have been used successfully for a large number of different systems (liquid and amorphous semiconductors, liquid simple and transition metals, metallic and semiconducting surfaces, phonons in simple metals, transition metals, and semiconductors) and turned out to be very reliable. © 1996 The American Physical Society.

The electrical resistivity of Fe-Cr-Fe layers with antiferromagnetic interlayer exchange increases when the magnetizations of the Fe layers are aligned antiparallel. The effect is much stronger than the usual anisotropic magnetoresistance and further increases in structures with more than two Fe layers. It can be explained in terms of spin-flip scattering of conduction electrons caused by the antiparallel alignment of the magnetization.

We present a theory for tunneling between a real surface and a model probe tip, applicable to the recently developed ‘‘scanning tunneling microscope.’’ The tunneling current is found to be proportional to the local density of states of the surface, at the position of the tip. The effective lateral resolution is related to the tip radius R and the vacuum gap distance d approximately as [(2 Å)(R+d${\left)\right]}^{1/2}$. The theory is applied to the 2×1 and 3×1 reconstructions of Au(110); results for the respective corrugation amplitudes and for the gap distance are all in excellent agreement with experimental results of Binnig et al. if a 9-Å tip radius is assumed. In addition, a convenient approximate calculational method based on atom superposition is tested; it gives reasonable agreement with the self-consistent calculation and with experiment for Au(110). This method is used to test the structure sensitivity of the microscope. We conclude that for the Au(110) measurements the experimental ‘‘image’’ is relatively insensitive to the positions of atoms beyond the first atomic layer. Finally, tunneling to semiconductor surfaces is considered. Calculations for GaAs(110) illustrate interesting qualitative differences from tunneling to metal surfaces.

It is shown that the quantization of the Hall conductivity of two-dimensional metals which has been observed recently by Klitzing, Dorda, and Pepper and by Tsui and Gossard is a consequence of gauge invariance and the existence of a mobility gap. Edge effects are shown to have no influence on the accuracy of quantization. An estimate of the error based on thermal activation of carriers to the mobility edge is suggested.