Venkatesh Narayanamurti

Bio: Venkatesh Narayanamurti is an academic researcher from Harvard University. The author has contributed to research in topics: Ballistic electron emission microscopy & Phonon. The author has an hindex of 49, co-authored 258 publications receiving 9399 citations. Previous affiliations of Venkatesh Narayanamurti include Cornell University & Sandia National Laboratories.

Direct measurement of quasiparticle-lifetime broadening in a strong-coupled superconductor

Abstract: We have measured the quasiparticle recombination time in the strong-coupled superconductor ${\mathrm{Pb}}_{0.9}$${\mathrm{Bi}}_{0.1}$ directly by measuring the lifetime-broadened energy gap edge. This is done by measuring the $I\ensuremath{-}V$ characteristics of a superconducting tunnel junction of the type ${\mathrm{Pb}}_{0.9}$${\mathrm{Bi}}_{0.1}$-insulator-${\mathrm{Pb}}_{0.9}$${\mathrm{Bi}}_{0.1}$. Agreement with the calculated value is excellent.

Size-dependent surface luminescence in ZnO nanowires

Abstract: Nanometer sized whiskers (nanowires) offer a vehicle for the study of size-dependent phenomena. While quantum-size effects are commonly expected and easily predicted, size reduction also causes more atoms to be closer to the surface. Here we show that intensity relations of below-band-gap and band-edge luminescence in ZnO nanowires depend on the wire radius. Assuming a surface layer wherein the surface-recombination probability is 1 (surface-recombination approximation), we explain this size effect in terms of bulk-related to surface-related material-volume ratio that varies almost linearly with the radius. This relation supports a surface-recombination origin for the deep-level luminescence we observe. The weight of this surface-luminescence increases as the wire radius decreases at the expense of the band-edge emission. Using this model, we obtain a radius of 30 nm, below which in our wires surface-recombination prevails. More generally, our results suggest that in quantum-size nanowires, surface-recombination may entirely quench band-to-band recombination, presenting an efficient sink for charge carriers that unless deactivated may be detrimental for electronic devices.

Zero-resistance states induced by electromagnetic-wave excitation in GaAs/AlGaAs heterostructures

Abstract: The observation of vanishing electrical resistance in condensed matter has led to the discovery of new phenomena such as, for example, superconductivity, where a zero-resistance state can be detected in a metal below a transition temperature Tc (ref. 1). More recently, quantum Hall effects were discovered from investigations of zero-resistance states at low temperatures and high magnetic fields in two-dimensional electron systems (2DESs)2,3,4. In quantum Hall systems and superconductors, zero-resistance states often coincide with the appearance of a gap in the energy spectrum1,2,4. Here we report the observation of zero-resistance states and energy gaps in a surprising setting5: ultrahigh-mobility GaAs/AlGaAs heterostructures that contain a 2DES exhibit vanishing diagonal resistance without Hall resistance quantization at low temperatures and low magnetic fields when the specimen is subjected to electromagnetic wave excitation. Zero-resistance-states occur about magnetic fields B = 4/5 Bf and B = 4/9 Bf, where Bf = 2πfm*/e,m* is the electron mass, e is the electron charge, and f is the electromagnetic-wave frequency. Activated transport measurements on the resistance minima also indicate an energy gap at the Fermi level6. The results suggest an unexpected radiation-induced, electronic-state-transition in the GaAs/AlGaAs 2DES.

Continuous and Discontinuous Semiconductor-Metal Transition in Samarium Monochalcogenides Under Pressure

Abstract: Resistivity and lattice-constant measurements under high pressure on SmS show that a $4f\ensuremath{\rightarrow}5d$ electronic transition in SmS occurs discontinuously at 6.5 kbar at room temperature, whereas such a transition takes place continuously over a broad pressure range in SmTe and SmSe. The pressure-induced semiconductor-to-metal transition in the Sm chalcogenides and their pressure-volume relationship are consistent with the conversion of ${\mathrm{Sm}}^{2+}$ to ${\mathrm{Sm}}^{3+}$. Optical-absorption measurements in these materials correlate well with the resistivity data under pressure. The semiconductor-to-metal transition in Sm chalcogenides appears to fit the model recently proposed by Falicov and Kimball for a system with a localized state and a conduction band.

Tunneling States of Defects in Solids

Abstract: The various optical, caloric, dielectric, elastic, and microwave investigations which have been used to study the tunneling states connected with the motion of atomic and molecular impurities in alkali halides are reviewed.

疟原虫var基因转换速率变化导致抗原变异[英]／Paul H, Robert P, Christodoulou Z, et al//Proc Natl Acad Sci U S A

Abstract: 抗原变异可使得多种致病微生物易于逃避宿主免疫应答。表达在感染红细胞表面的恶性疟原虫红细胞表面蛋白1（PfPMP1）与感染红细胞、内皮细胞、树突状细胞以及胎盘的单个或多个受体作用，在黏附及免疫逃避中起关键的作用。每个单倍体基因组var基因家族编码约60种成员，通过启动转录不同的var基因变异体为抗原变异提供了分子基础。

Spintronics: Fundamentals and applications

Abstract: Spintronics, or spin electronics, involves the study of active control and manipulation of spin degrees of freedom in solid-state systems. This article reviews the current status of this subject, including both recent advances and well-established results. The primary focus is on the basic physical principles underlying the generation of carrier spin polarization, spin dynamics, and spin-polarized transport in semiconductors and metals. Spin transport differs from charge transport in that spin is a nonconserved quantity in solids due to spin-orbit and hyperfine coupling. The authors discuss in detail spin decoherence mechanisms in metals and semiconductors. Various theories of spin injection and spin-polarized transport are applied to hybrid structures relevant to spin-based devices and fundamental studies of materials properties. Experimental work is reviewed with the emphasis on projected applications, in which external electric and magnetic fields and illumination by light will be used to control spin and charge dynamics to create new functionalities not feasible or ineffective with conventional electronics.

Band parameters for III–V compound semiconductors and their alloys

Abstract: We present a comprehensive, up-to-date compilation of band parameters for the technologically important III–V zinc blende and wurtzite compound semiconductors: GaAs, GaSb, GaP, GaN, AlAs, AlSb, AlP, AlN, InAs, InSb, InP, and InN, along with their ternary and quaternary alloys. Based on a review of the existing literature, complete and consistent parameter sets are given for all materials. Emphasizing the quantities required for band structure calculations, we tabulate the direct and indirect energy gaps, spin-orbit, and crystal-field splittings, alloy bowing parameters, effective masses for electrons, heavy, light, and split-off holes, Luttinger parameters, interband momentum matrix elements, and deformation potentials, including temperature and alloy-composition dependences where available. Heterostructure band offsets are also given, on an absolute scale that allows any material to be aligned relative to any other.

Thin-film thermoelectric devices with high room-temperature figures of merit

Abstract: Thermoelectric materials are of interest for applications as heat pumps and power generators. The performance of thermoelectric devices is quantified by a figure of merit, ZT, where Z is a measure of a material's thermoelectric properties and T is the absolute temperature. A material with a figure of merit of around unity was first reported over four decades ago, but since then-despite investigation of various approaches-there has been only modest progress in finding materials with enhanced ZT values at room temperature. Here we report thin-film thermoelectric materials that demonstrate a significant enhancement in ZT at 300 K, compared to state-of-the-art bulk Bi2Te3 alloys. This amounts to a maximum observed factor of approximately 2.4 for our p-type Bi2Te3/Sb2Te3 superlattice devices. The enhancement is achieved by controlling the transport of phonons and electrons in the superlattices. Preliminary devices exhibit significant cooling (32 K at around room temperature) and the potential to pump a heat flux of up to 700 W cm-2; the localized cooling and heating occurs some 23,000 times faster than in bulk devices. We anticipate that the combination of performance, power density and speed achieved in these materials will lead to diverse technological applications: for example, in thermochemistry-on-a-chip, DNA microarrays, fibre-optic switches and microelectrothermal systems.

Thinking fast and slow.

Abstract: In 1974 an article appeared in Science magazine with the dry-sounding title “Judgment Under Uncertainty: Heuristics and Biases” by a pair of psychologists who were not well known outside their discipline of decision theory. In it Amos Tversky and Daniel Kahneman introduced the world to Prospect Theory, which mapped out how humans actually behave when faced with decisions about gains and losses, in contrast to how economists assumed that people behave. Prospect Theory turned Economics on its head by demonstrating through a series of ingenious experiments that people are much more concerned with losses than they are with gains, and that framing a choice from one perspective or the other will result in decisions that are exactly the opposite of each other, even if the outcomes are monetarily the same. Prospect Theory led cognitive psychology in a new direction that began to uncover other human biases in thinking that are probably not learned but are part of our brain’s wiring.