Piezoelectric mechanism of orientation of stripe structures in two-dimensional electron systems

A piezoelectric mechanism of orientation of stripes in two-dimensional quantum Hall systems in GaAs heterostructures is considered. The anisotropy of the elastic moduli and the boundary of the sample are taken into account. It is found that in the average the stripes line up with the [110] axis. In double layer systems the wave vector of the stripe structure rotates from the [110] to [100] axis if the period of density modulation becomes large than the interlayer distance. From the experimental point of view it means that in double layer systems anisotropic part of resistivity changes its sign under variation of the external magnetic field.Comment: 8 page

Tunneling into Multiwalled Carbon Nanotubes: Coulomb Blockade and Fano Resonance

Tunneling spectroscopy measurements of single tunnel junctions formed between multiwalled carbon nanotubes (MWNTs) and a normal metal are reported. Intrinsic Coulomb interactions in the MWNTs give rise to a strong zero-bias suppression of a tunneling density of states (TDOS) that can be fitted numerically to the environmental quantum-fluctuation (EQF) theory. An asymmetric conductance anomaly near zero bias is found at low temperatures and interpreted as Fano resonance in the strong tunneling regime.Comment: 4 pages, 4 figure

The induced representations of Brauer algebra and the Clebsch-Gordan coefficients of SO(n)

Induced representations of Brauer algebra $D_{f}(n)$ from $S_{f_{1}}\times S_{f_{2}}$ with $f_{1}+f_{2}=f$ are discussed. The induction coefficients (IDCs) or the outer-product reduction coefficients (ORCs) of $S_{f_{1}}\times S_{f_{2}}\uparrow D_{f}(n)$ with $f\leq 4$ up to a normalization factor are derived by using the linear equation method. Weyl tableaus for the corresponding Gel'fand basis of SO(n) are defined. The assimilation method for obtaining CG coefficients of SO(n) in the Gel'fand basis for no modification rule involved couplings from IDCs of Brauer algebra are proposed. Some isoscalar factors of $SO(n)\supset SO(n-1)$ for the resulting irrep $[\lambda_{1},~\lambda_{2},~ \lambda_{3},~\lambda_{4},\dot{0}]$ with $\sum\limits_{i=1}^{4}\lambda_{i}\leq .Comment: 48 pages latex, submitted to Journal of Phys.

Nanoladder cantilevers made from diamond and silicon

We present a "nanoladder" geometry that minimizes the mechanical dissipation of ultrasensitive cantilevers. A nanoladder cantilever consists of a lithographically patterned scaffold of rails and rungs with feature size $\sim$ 100 nm. Compared to a rectangular beam of the same dimensions, the mass and spring constant of a nanoladder are each reduced by roughly two orders of magnitude. We demonstrate a low force noise of $158 (+62)(-42)\,$zN and $190 (+42)(-33)\,$zN in a one-Hz bandwidth for devices made from silicon and diamond, respectively, measured at temperatures between 100--150 mK. As opposed to bottom-up mechanical resonators like nanowires or nanotubes, nanoladder cantilevers can be batch-fabricated using standard lithography, which is a critical factor for applications in scanning force microscopy

Magnetoresistivity in a Tilted Magnetic Field in p-Si/SiGe/Si Heterostructures with an Anisotropic g-Factor: Part II

The magnetoresistance components $\rho_{xx}$ and $\rho_{xy}$ were measured in two p-Si/SiGe/Si quantum wells that have an anisotropic g-factor in a tilted magnetic field as a function of temperature, field and tilt angle. Activation energy measurements demonstrate the existence of a ferromagnetic-paramagnetic (F-P) transition for a sample with a hole density of $p$=2$\times10^{11}$\,cm$^{-2}$. This transition is due to crossing of the 0$\uparrow$ and 1$\downarrow$ Landau levels. However, in another sample, with $p$=7.2$\times10^{10}$\,cm$^{-2}$, the 0$\uparrow$ and 1$\downarrow$ Landau levels coincide for angles $\Theta$=0-70$^{\text{o}}$. Only for $\Theta$ > 70$^{\text{o}}$ do the levels start to diverge which, in turn, results in the energy gap opening.Comment: 5 pages, 6 figure

A q-Deformed Schr\"odinger Equation

We found hermitian realizations of the position vector $\vec{r}$, the angular momentum $\vec{\Lambda}$ and the linear momentum $\vec{p}$, all behaving like vectors under the $su_q(2)$ algebra, generated by $L_0$ and $L_\pm$. They are used to introduce a $q$-deformed Schr\" odinger equation. Its solutions for the particular cases of the Coulomb and the harmonic oscillator potentials are given and briefly discussed.Comment: 14 pages, latex, no figure

Evaluation of additively manufactured microchannel heat sinks

Microchannel heat sinks allow removal of dense heat loads from high-power electronic devices at modest chip temperature rises. Such heat sinks are produced primarily using conventional subtractive machining techniques or anisotropic chemical etching, which restricts the geometric features that can be produced. Owing to their layer-by-layer and direct-write approaches, additive manufacturing (AM) technologies enable more design-driven construction flexibility and offer improved geometric freedom. Various AM processes and materials are available, but their capability to produce features desirable for microchannel heat sinks has received limited assessment. Following a survey of commercially mature AM techniques, direct metal laser sintering (DMLS) was used in this work to produce both straight and manifold microchannel designs with hydraulic diameters of 500 μm in an aluminum alloy (AlSi10Mg). Thermal and hydraulic performance were characterized over a range of mass fluxes from 500 kg/m2s to 2000 kg/m2s using water as the working fluid. The straight microchannel design allows these experimental results to be directly compared against widely accepted correlations from the literature. The manifold design demonstrates a more complex geometry that offers a reduced pressure drop. A comparison of the measured and predicted performance confirms that the nominal geometry is reproduced accurately enough to predict pressure drop based on conventional hydrodynamic theory, albeit with roughness-induced early transition to turbulence; however, the material properties are not known with sufficient accuracy to allow for a priori thermal design. New design guidelines are needed to exploit the benefits of additive manufacturing while avoiding undesired or unanticipated performance impacts

A Permeable-Membrane Microchannel Heat Sink Made by Additive Manufacturing

Microchannel heat sinks are capable of removing dense heat loads from high-power electronic devices with low thermal resistance, but suffer from high pressure drops due to the small channel dimensions. Features that reduce the pressure drop, such as manifolds, increase fabrication complexity and are constrained by traditional subtractive manufacturing approaches. Additive manufacturing technologies offer improved design freedom and reduced geometric restrictions, expanding the types of features that can be produced and integrated into a heat sink. In this work, a novel permeable membrane microchannel (PMM) heat sink geometry is proposed and fabricated using direct metal laser sintering (DMLS) of an aluminum alloy (AlSi10Mg). In this PMM design, the cooling fluid is forced through thin, porous walls that act as both conducting fins and membranes that allow flow through their fine internal flow features for efficient heat exchange. The design leverages the ability of this fabrication process to incorporate complex, arbitrarily curved structures having internal porosity to enhance heat transfer and reduce pressure drop across the heat sink. The PMM heat sink geometry is benchmarked against a low-pressure-drop manifold microchannel (MMC) heat sink. A reduced-order model is used to explore the relative performance trends between the designs. Both heat sinks are experimentally characterized at flow rates of 50–500 mL/min using deionized water as the working fluid. At a constant pumping power of 0.018 W, the permeable membrane microchannel design offers both lower thermal resistance (17% reduction) and lower pressure drop (28% reduction) compared to the manifold microchannel heat sink