Semiconductor InSb nanowires are expected to provide an excellent material platform for the study of Majorana fermions in solid state systems. Here, we report on the realization of a Nb-InSb nanowire-Nb hybrid quantum device and the observation of a zero-bias conductance peak structure in the device. An InSb nanowire quantum dot is formed in the device between the two Nb contacts. Due to the proximity effect, the InSb nanowire segments covered by the superconductor Nb contacts turn to superconductors with a superconducting energy gap Δ(InSb) ∼ 0.25 meV. A tunable critical supercurrent is observed in the device in high back gate voltage regions in which the Fermi level in the InSb nanowire is located above the tunneling barriers of the quantum dot and the device is open to conduction. When a perpendicular magnetic field is applied to the devices, the critical supercurrent is seen to decrease as the magnetic field increases. However, at sufficiently low back gate voltages, the device shows the quasi-particle Coulomb blockade characteristics and the supercurrent is strongly suppressed even at zero magnetic field. This transport characteristic changes when a perpendicular magnetic field stronger than a critical value, at which the Zeeman energy in the InSb nanowire is E(z) ∼ Δ(InSb), is applied to the device. In this case, the transport measurements show a conductance peak at the zero bias voltage and the entire InSb nanowire in the device behaves as in a topological superconductor phase. We also show that this zero-bias conductance peak structure can persist over a large range of applied magnetic fields and could be interpreted as a transport signature of Majorana fermions in the InSb nanowire.

Abstract. When laser light illuminates a diffuse object, it produces a random interference effect known as a speckle pattern. If there is movement in the object, the speckles fluctuate in intensity. These fluctuations can provide information about the movement. A simple way of accessing this information is to image the speckle pattern with an exposure time longer than the shortest speckle fluctuation time scale-the fluctuations cause a blurring of the speckle, leading to a reduction in the local speckle contrast. Thus, velocity distributions are coded as speckle contrast variations. The same information can be obtained by using the Doppler effect, but producing a two-dimensional Doppler map requires either scanning of the laser beam or imaging with a high-speed camera: laser speckle contrast imaging (LSCI) avoids the need to scan and can be performed with a normal CCD-or CMOS-camera. LSCI is used primarily to map flow systems, especially blood flow. The development of LSCI is reviewed and its limitations and problems are investigated.

A programmable ferroelectric single electron transistor Appl. Phys. Lett. 102, 053505 (2013); 10.1063/1.4791601Experimental study on quantum confinement effects in silicon nanowire metal-oxide-semiconductor field-effect transistors and single-electron transistors

We report on magnetotransport measurements on InSb nanowire quantum dots. The measurements show that the quantum levels of the InSb quantum dots have giant g factors, with absolute values up to approximately 70, the largest value ever reported for semiconductor quantum dots. We also observe that the values of these g factors are quantum level dependent and can differ strongly between different quantum levels. The presence of giant g factors indicates that considerable contributions from the orbital motion of electrons are preserved in the measured InSb nanowire quantum dots, while the level-to-level fluctuations arise from spin-orbit interaction. We have deduced a value of Delta(SO) = 280 mueV for the strength of spin-orbit interaction from an avoided level crossing between the ground state and first excited state of an InSb nanowire quantum dot with a fixed number of electrons.

We report tunneling spectroscopy measurements of the Zeeman spin splitting in InAs few-electron quantum dots. The dots are formed between two InP barriers in InAs nanowires with a wurtzite crystal structure which are grown using chemical beam epitaxy. The values of the electron g-factors of the first few electrons entering the dot are found to strongly depend on dot size. They range from close to the InAs bulk value in large dots |g * | = 13 down to |g * | = 2.3 for the smallest dots.PACS numbers: 73.23. Hk, 73.63.Kv, 71.70.Ej The spin of an electron in a quantum dot (QD) is one of the candidates for a scaleable quantum bit, the fundamental unit in quantum computation and quantum communication schemes [1]. Experimental realizations are on the one hand pursued using top-down approaches. This usually involves lateral gate electrodes electrostatically confining few or a single electron in a two dimensional electron gas close to the surface of a Ga(Al)As based heterostructure [2]. Such systems offer good tunability and controlled coupling of multiple spins has been demonstrated [3]. On the other hand, bottom up systems such as self assembled QDs [4] and carbon nanotubes [5] are expected to scale more easily. Semiconductor nanowires have emerged as a promising bottom-up fabricated system for electronic and optical device applications [6]. We have recently demonstrated the creation of few-electron QDs using InAs nanowire heterostructures [7] with two InP barriers. In the following we set out to investigate the spin properties of the first few orbital levels of these QDs.We utilize transport spectroscopy to measure the Zeeman splitting of the energy levels as a function of magnetic field and thereby determine the effective electron g-factor (g * ). The g-factor of bulk InAs, which crystalizes in the zinc-blende (ZB) structure, has been found to be g * = −14.7 [8]. However, InAs nanowires can exhibit both zinc-blende and wurtzite (WZ) type crystal structure[9] depending on diameter and growth conditions and so far very little is known about band parameters in WZ InAs. In low-dimensional semiconductor heterostructures the g factor depends critically on system size and dimensionality [10]. We show that varying the size of our nanowire dots allows us to tune g * from a value close to the InAs bulk value down to |g * | = 2.3±0.3. The possibility to have multiple dots along a nanowire, each with a different g-factor, makes such systems interesting candidates for realizations of individually addressable spin qubits.Using chemical beam epitaxy InAs nanowires containing QDs were grown catalytically from Au nanoparticles deposited on a <111>B InAs substrate [11,12]. The * Electronic address: andreas.fuhrer@ftf.lth.se nanowires typically grow perpendicular to the substrate and high resolution scanning transmission electron microscope (STEM) images indicate that most of them

Abstract:A new method for estimating the measurement depth and volume in laser Doppler flowmetry (LDF) is presented. The method is based on Monte Carlo simulations of light propagation in tissue. The contribution from each individual Doppler shift is calculated and thereby multiple Doppler shifts are handled correctly. Different LDF setups for both probe based (0.0, 0.25, 0.5, and 1.2 mm source-detector separation) and imaging systems (0.5 and 2.0 mm beam diameter) are considered, at the wavelengths 453 nm, 633 nm, and 780 nm. Non-linear speckle pattern effects are accounted for in the imaging system setups. The effects of tissue optical properties, blood concentration, and blood oxygen saturation are evaluated using both homogeneous tissue models and a layered skin model. The results show that the effect on the measurement depth of changing tissue properties is comparable to the effect of changing the system setup, e.g. sourcedetector separation and wavelength. Skin pigmentation was found to have a negligible effect on the measurement depth. Examples of measurement depths are (values are given for a probe based system with 0.25 mm source-detector separation and an imaging system with a 0.5 mm beam diameter, respectively, both operating at 780 nm): muscle -0.55/0.79 mm; liver -0.40/0.53 mm; gray matter -0.48/0.68 mm; white matter -0.20/0.20 mm; index finger pulp -0.41/0.53 mm; forearm skin -0.53/0.56 mm; heat provoked forearm skin -0.66/0.67 mm.

Abstract. The tissue fraction of red blood cells (RBCs) and their oxygenation and speed-resolved perfusion are estimated in absolute units by combining diffuse reflectance spectroscopy (DRS) and laser Doppler flowmetry (LDF). The DRS spectra (450 to 850 nm) are assessed at two source-detector separations (0.4 and 1.2 mm), allowing for a relative calibration routine, whereas LDF spectra are assessed at 1.2 mm in the same fiber-optic probe. Data are analyzed using nonlinear optimization in an inverse Monte Carlo technique by applying an adaptive multilayered tissue model based on geometrical, scattering, and absorbing properties, as well as RBC flow-speed information. Simulations of 250 tissue-like models including up to 2000 individual blood vessels were used to evaluate the method. The absolute root mean square (RMS) deviation between estimated and true oxygenation was 4.1 percentage units, whereas the relative RMS deviations for the RBC tissue fraction and perfusion were 19% and 23%, respectively. Examples of in vivo measurements on forearm and foot during common provocations are presented. The method offers several advantages such as simultaneous quantification of RBC tissue fraction and oxygenation and perfusion from the same, predictable, sampling volume. The perfusion estimate is speed resolved, absolute (% RBC × mm∕s), and more accurate due to the combination with DRS. © The Authors. Published by SPIE under a Creative Commons Attribution 3.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI.

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