Chul Koo Kim

Composite acoustic medium with simultaneously negative density and modulus.

We report observation of reverse Doppler effect in a double negative acoustic metamaterial. The metamaterial exhibited negative phase velocity and positive group velocity. The dispersion relation is such that the wavelength corresponding to higher frequency is longer. We observed that the frequency was down-shifted for the approaching source, and up-shifted when the source receded. ∗ To whom correspondence should be addressed. E-mail: ckkim@yonsei.ac.kr 1 ar X iv :0 90 1. 27 72 v2 [ co nd -m at .o th er ] 2 2 Ja n 20 09 Metamaterials with negative constitutive parameters [1, 2, 3, 4, 5] brought many phenomena previously regarded impossible into reality [6, 7, 8, 9]. Electromagnetic waves have been demonstrated to propagate with negative phase velocity in the double negative (DNG) matematerials which have negative electric permittivity and negative magnetic permeability simultaneously. For acoustic waves, negative phase velocity is expected if density and modulus can be made simultaneously negative [10]. We combined negative density material [11, 12] and negative modulus material [13, 14] to fabricate an acoustic metamaterial that exhibits the acoustic DNG property. The negative phase velocity introduces a dispersion relation in which the wavelengths corresponding to higher frequencies become longer. By measuring the frequency shifts in the DNG metamaterial, we observed the reverse Doppler effect of sound, where the frequency was down-shifted for the approaching source and up-shifted when the source receded. In comparison with the numerous theoretical and experimental research results on electromagnetic metamaterials, research efforts on the acoustic metamaterials are relatively few and mainly concentrated on theoretical aspects [10, 15, 16, 17, 18, 19, 20]. Only, recently, Fang et al. reported realization of negative modulus in a metamaterial consisting of Helmholtz resonators [11]. Also negative mass has been realized using a membrane by Yang et al. [13]. The authors also have investigated single negative acoustic metamaterials [12, 14]. Here, we present fabrication of a new acoustic DNG metamaterial by combining the two single negative metamaterials. Fig. 1a shows schematically the structure of a negative density material [12], which has a regular array of tensioned thin membranes placed inside a tube. The metamaterial shown in Fig. 1b exhibits negative modulus due to the motion of air column in the side holes [14]. Fig. 1c shows the acoustic DNG metamaterial with the membranes and side holes alternately placed along the tube. Acoustic double negativity of this structure is due to the actions of the membranes and side holes. The membrane forms a paraboloid when pushed by the fluid, and when the spacing between the membranes is much smaller than the wavelength (the long-wavelength limit), the restoring force from the tension gives the static pressure gradient 5p = −κ~ ξ where p, κ, and ~ ξ are the pressure, the elastic modulus, and the displacement of the fluid, respectively. In the dynamic case, due to the force from the membranes, Newton’s equation becomes −5p = ρ′∂~u/∂t+κ~ ξ, where ~u = ∂~ ξ/∂t is the longitudinal velocity of the fluid. The volume-averaged density ρ′ of the fluid (air in the present case) and the membranes, giving

Band structure of ternary compound semiconductors beyond the virtual crystal approximation

A simple pseudopotential scheme, which incorporates compositional disorder as an effective potential, is proposed for calculation of the band structure of ternary compound semiconductors. It is shown that the present theory, which is free from any additional parameter, satisfactorily produces the band-gap bowings of ternary compounds when the lattice mismatch is small.

DNA damage in the kidneys of diabetic rats exhibiting microalbuminuria

8-Hydroxydeoxyguanosine (8-OHdG), an oxygen radical induced modification of purine residue in DNA, was measured in the liver, pancreas, and kidney of streptozotocin-induced diabetic rats (STZR) exhibiting microalbuminuria. At 4 weeks after the injection of streptozotocin (50 mg/kg, i.v.), the rate of urinary albumin excretion was 0.5 +/- 0.1 and 2.0 +/- 0.2 mg/24 h in age-matched control rats (CR) and STZR, respectively. Compared to CR, STZR also showed a significantly increased level of 8-OHdG in the kidney but not the liver and pancreas. Amounts of 8-OHdG/10(5) dG for CR and STZR were 3.4 +/- 0.3 and 5.1 +/- 0.2 for renal cortices, and 4.1 +/- 0.2 and 20.0 +/- 3.7 for renal papillae. Daily injection of insulin (2 U, SC) starting on the third day after streptozotocin treatment significantly reduced both urinary albumin excretion and papillary 8-OHdG formation, which suggests that these are associated with the diabetic state induced by streptozotocin rather than a direct nephrotoxic effect of the drug. This study suggests that formation of 8-OHdG and, therefore, oxidative damage are closely related in the process of diabetic nephropathy.

Dependence of the indirect energy gap of silicon on hydrostatic pressure

A diamond anvil optical cell is employed to measure the pressure dependence of the fundamental indirect Γ-X transition of crystalline Si. The result is E (eV) = (1.110 ± 0.002) − (1.41 ± 0.06) × 10−3P where the pressure P is in kbar. Between 115 and 126 a transformation takes place to a phase opaque to electromagnetic radiation of wavelength between 0.3 and 2.0 μm. We believe this is the same phase transition reported by Minomura and Drickamer. A pseudopotential calculation assuming a relatively soft core is carried out and its results are in rather good agreement with experiment.

PRESSURE DEPENDENCE OF THE LOWEST DIRECT ABSORPTION EDGE OF ZnSe

Abstract The variation of the lowest direct absorption edge (E0) of ZnSe with hydrostatic pressure has been measured at room temperature with a diamond anvil cell for pressures up to the phase transition (13.5±0.2 GPa). The gap varies sublinearly with pressure. However, when the gap is plotted as a function of the relative change of the lattice constant (-Δa/a0) it shows a slight supralinearly in contrast to most III–V tetrahedral semiconductors which retain a slight sublinearly even as a function of (-Δa/a0). The experimental results are compared to theoretical calculations based (a) on the local density approximation combined with the self-consistent band calculations using the LMTO method (LD-LMTO) and (b) on a local empirical pseudopotential. Both calculations predict a sublinear dependence of E0 as a function of (-Δa/a0). The semiempirical Murnaghans equation of state that was calculated with the LD-LMTO method, has also been tested by determining the lattice constant under pressure with an energy dispersive x-ray diffractometer.

Quasiparticle scattering and the protected nature of the topological states in a parent topological insulator Bi 2 Se 3

We report on angle resolved photoemission spectroscopic studies on a parent topological insulator (TI), Bi

Temperature dependence of the fundamental energy gap in GaAs

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Acoustic wave propagation in one-dimensional phononic crystals containing Helmholtz resonators

Se

Fano resonance in electron transport through parallel double quantum dots in the Kondo regime

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Chiral Orbital-Angular Momentum in the Surface States of Bi2Se3

. The line width of the spectral function (inverse of the quasi-particle lifetime) of the topological metallic (TM) states shows an anomalous behavior. This behavior can be reasonably accounted for by assuming decay of the quasi-particles predominantly into bulk electronic states through electron-electron interaction and defect scattering. Studies on aged surfaces reveal that topological metallic states are very much unaffected by the potentials created by adsorbed atoms or molecules on the surface, indicating that topological states could be indeed protected against weak perturbations.