Optical Resonances in Reflectivity near Crystal Modes with Spatial Dispersion

We study the effect of spatial dispersion of crystal modes on optical properties such as the reflectivity $R$. As an example for isotropic media, we investigate the simplest model for phonons in ionic crystals and compare with previous results for highly anisotropic plasmons, which are now understood from a more general point of view. As a consequence of the wave vector dependence of the dielectric function small changes in the lineshape are predicted. Beyond that, if the frequency of minimal $R$ is near a pole of the dispersionless dielectric function, the relative amplitude of dips in $R$ with normal and anomalous dispersion differ significantly, if dissipation and disorder are low.Comment: 4 pages, 7 eps figures, minor change

Occupational burden of disease in the Netherlands

Ongunstige arbeidsomstandigheden veroorzaken 2 tot 4% van de totale ziektelast in Nederland. Het begrip ziektelast is een maatstaf om de gevolgen van ziekte uit te drukken. Het combineert gezondheidsverlies door verminderde kwaliteit van leven en door vroegtijdig overlijden. Hoge werkdruk, blootstelling aan schadelijke stoffen (inclusief passief roken) en beeldschermwerk zijn de ongunstigste arbeidsomstandigheden. Zij zorgen voor veel ziektelast door burn-out, de chronische luchtwegaandoening COPD, longkanker en klachten van arm, nek en schouder (KANS). In het rapport is de positieve invloed van arbeid op de gezondheid niet meegenomen. Het RIVM laat voor het eerst zien welke arbeidsgerelateerde aandoeningen veel ziektelast in Nederland veroorzaken. Deze gegevens bieden aanknopingspunten voor verder onderzoek en voor maatregelen om de ziektelast door deze aandoeningen te verminderen. Dat is niet alleen goed voor werknemers en werkgevers, maar ook voor de volksgezondheid en de samenleving in haar geheel: health is wealth. In eerdere ziektelastberekeningen van de wereldgezondheidsorganisatie (WHO) is de arbeidsgerelateerde ziektelast in Nederland sterk onderschat. In Nederland veroorzaken niet zozeer de 'klassieke' risico's en aandoeningen, zoals arbeidsongevallen en slechthorendheid door lawaai, de meeste ziektelast. Juist 'nieuwe' aandoeningen, zoals burn-out en KANS, leiden hiertoe, en die zijn niet meegenomen in de WHO-schattingen. Behalve negatieve effecten op de gezondheid hebben ongunstige arbeidsomstandigheden een nadelig effect op de arbeidsproductiviteit, het ziekteverzuim en arbeidsongeschiktheid. In theorie blijkt het mogelijk om deze effecten te berekenen. Het is aan te bevelen om uit te zoeken of dit in de praktijk haalbaar is.Occupational health risks cause 2 to 4% of the total burden of disease in the Netherlands. The concept of burden of disease is a criterion to express the consequences of disease. It combines the time lived with disability and the time lost due to premature mortality. A high workload, exposure to harmful chemicals (including environmental tobacco smoke) and working with a computer are the most unfavourable working conditions. They contribute most to the occupational burden of disease caused by: burn-out, chronic obstructive pulmonary disease, lung cancer and complaints of arm, neck and shoulder (CANS). The health benefits of work were not included in this report. For the first time, the RIVM shows in this study which occupational diseases contribute most to the burden of disease in the Netherlands. The data offer starting points for further research and measures to reduce the burden of disease caused by these complaints. This is good not only for employers and employees but also for public health and society as a whole: health is wealth. The World Health Organization (WHO) underestimated the occupational burden of disease in the Netherlands in its earlier burden of disease assessments. In the Netherlands, most of the burden of disease is not caused by the 'classical' risks and diseases like accidents and hearing impairment caused by noise. Rather 'new' diseases like burn-out and CANS cause considerable loss of health and were not taken into account in the assessments of the WHO. Unfavourable working conditions do not only have negative effects on health, but also have a disadvantageous effect on productivity, absenteeism and incapacity for work. In theory it is possible to estimate these adverse effects. We therefore advise to study whether estimating them is feasible in practice.SZ

Relation between Vortex core charge and Vortex Bound States

Spatially inhomogeneous electron distribution around a single vortex is discussed on the basis of the Bogoliubov-de Gennes theory. The spatial structure and temperature dependence of the electron density around the vortex are presented. A relation between the vortex core charge and the vortex bound states (or the Caroli-de Gennes-Matricon states) is pointed out. Using the scanning tunneling microscope, information on the vortex core charge can be extracted through this relation.Comment: 5 pages, 3 figures; minor changes; Version to appear in JPSJ 67, No.10, 199

Collective pinning of a frozen vortex liquid in ultrathin superconducting YBa_2Cu_3O_7 films

The linear dynamic response of the two-dimensional (2D) vortex medium in ultrathin YBa_2Cu_3O_7 films was studied by measuring their ac sheet impedance Z over a broad range of frequencies \omega. With decreasing temperature the dissipative component of Z exhibits, at a temperature T*(\omega) well above the melting temperature of a 2D vortex crystal, a crossover from a thermally activated regime involving single vortices to a regime where the response has features consistent with a description in terms of a collectively pinned vortex manifold. This suggests the idea of a vortex liquid which, below T*(\omega), appears to be frozen at the time scales 1/\omega of the experiments.Comment: 4 pages, 3 figures, submitted to Phys. Rev. Let

Topologically protected quantum bits from Josephson junction arrays

All physical implementations of quantum bits (qubits), carrying the information and computation in a putative quantum computer, have to meet the conflicting requirements of environmental decoupling while remaining manipulable through designed external signals. Proposals based on quantum optics naturally emphasize the aspect of optimal isolation, while those following the solid state route exploit the variability and scalability of modern nanoscale fabrication techniques. Recently, various designs using superconducting structures have been successfully tested for quantum coherent operation, however, the ultimate goal of reaching coherent evolution over thousands of elementary operations remains a formidable task. Protecting qubits from decoherence by exploiting topological stability, a qualitatively new proposal due to Kitaev, holds the promise for long decoherence times, but its practical physical implementation has remained unclear so far. Here, we show how strongly correlated systems developing an isolated two-fold degenerate quantum dimer liquid groundstate can be used in the construction of topologically stable qubits and discuss their implementation using Josephson junction arrays.Comment: 6 pages, 4 figure

Critical depinning force and vortex lattice order in disordered superconductors

We simulate the ordering of vortices and its effects on the critical current in superconductors with varied vortex-vortex interaction strength and varied pinning strengths for a two-dimensional system. For strong pinning the vortex lattice is always disordered and the critical depinning force only weakly increases with decreasing vortex-vortex interactions. For weak pinning the vortex lattice is defect free until the vortex-vortex interactions have been reduced to a low value, when defects begin to appear with a simultaneous rapid increase in the critical depinning force. In each case the depinning force shows a maximum for non-interacting vortices. The relative height of the peak increases and the peak width decreases for decreasing pinning strength in excellent agreement with experimental trends associated with the peak effect. We show that scaling relations exist between the distance between defects in the vortex lattice and the critical depinning force.Comment: 5 pages, 6 figure

Temperature dependence of Vortex Charges in High Temperature Superconductors

Using a model Hamiltonian with d-wave superconductivity and competing antiferromagnetic (AF) interactions, the temperature (T) dependence of the vortex charge in high T_c superconductors is investigated by numerically solving the Bogoliubov-de Gennes equations. The strength of the induced AF order inside the vortex core is T dependent. The vortex charge could be negative when the AF order with sufficient strength is present at low temperatures. At higher temperatures, the AF order may be completely suppressed and the vortex charge becomes positive. A first order like transition in the T dependent vortex charge is seen near the critical temperature T_{AF}. For underdoped sample, the spatial profiles of the induced spin-density wave and charge-density wave orders could have stripe like structures at T < T_s, and change to two-dimensional isotropic ones at T > T_s. As a result, a vortex charge discontinuity occurs at T_s.Comment: 5 pages, 5 figure

Magnetic quantization of electronic states in d-wave superconductors

We derive a general quasiclassical approach for long-range magnetic-field quantization effects in superconductors. The method is applied to superclean d-wave superconductors in the mixed state. We study the delocalized states with energies $\epsilon \gg \Delta_{0}\sqrt{H/H_{c2}}$. We find that the energy spectrum consists of narrow energy bands whose centers are located at the Landau levels calculated in absence of the vortex potential. We show that transitions between the states belonging to the different Landau levels give rise to resonances in the a.c. quasiparticle conductivity and in the a.c. vortex friction.Comment: 11 pages, no figure

Fluctuating diamagnetism in underdoped high temperature superconductors

The fluctuation induced diamagnetism of underdoped high temperature superconductors is studied in the framework of the Lawrence-Doniach model. By taking into account the fluctuations of the phase of the order parameter only, the latter reduces to a layered XY-model describing a liquid of vortices which can be either thermally excited or induced by the external magnetic field. The diamagnetic response is given by a current-current correlation function which is evaluated using the Coulomb gas analogy. Our results are then applied to recent measurements of fluctuation diamagnetism in underdoped YBCO. They allow to understand both the observed anomalous temperature dependence of the zero-field susceptibility and the two distinct regimes appearing in the magnetic field dependence of the magnetization.Comment: 12 pages, 4 figures included, accepted for publication in PR

Strong Pinning in High Temperature Superconductors

Detailed measurements of the critical current density jc of YBa2Cu3O7 films grown by pulsed laser deposition reveal the increase of jc as function of the filmthickness. Both this thickness dependence and the field dependence of the critical current are consistently described using a generalization of the theory of strong pinning of Ovchinnikov and Ivlev [Phys. Rev. B 43, 8024 (1991)]. From the model, we deduce values of the defect density (10^21 m^-3) and the elementary pinning force, which are in good agreement with the generally accepted values for Y2O3-inclusions. In the absence of clear evidence that the critical current is determined by linear defects or modulations of the film thickness, our model provides an alternative explanation for the rather universal field dependence of the critical current density found in YBa2Cu3O7 films deposited by different methods.Comment: 11 pages; 8 Figures; Published Phys. Rev. B 66, 024523 (2002