Finite bias Cooper pair splitting

In a device with a superconductor coupled to two parallel quantum dots (QDs) the electrical tunability of the QD levels can be used to exploit non-classical current correlations due to the splitting of Cooper pairs. We experimentally investigate the effect of a finite potential difference across one quantum dot on the conductance through the other completely grounded QD in a Cooper pair splitter fabricated on an InAs nanowire. We demonstrate that the electrical transport through the device can be tuned by electrical means to be dominated either by Cooper pair splitting (CPS), or by elastic co-tunneling (EC). The basic experimental findings can be understood by considering the energy dependent density of states in a QD. The reported experiments add bias-dependent spectroscopy to the investigative tools necessary to develop CPS-based sources of entangled electrons in solid-state devices.Comment: 4 pages, 4 figure

Wet etch methods for InAs nanowire patterning and self-aligned electrical contacts

Advanced synthesis of semiconductor nanowires (NWs) enables their application in diverse fields, notably in chemical and electrical sensing, photovoltaics, or quantum electronic devices. In particular, Indium Arsenide (InAs) NWs are an ideal platform for quantum devices, e.g. they may host topological Majorana states. While the synthesis has been continously perfected, only few techniques were developed to tailor individual NWs after growth. Here we present three wet chemical etch methods for the post-growth morphological engineering of InAs NWs on the sub-100 nm scale. The first two methods allow the formation of self-aligned electrical contacts to etched NWs, while the third method results in conical shaped NW profiles ideal for creating smooth electrical potential gradients and shallow barriers. Low temperature experiments show that NWs with etched segments have stable transport characteristics and can serve as building blocks of quantum electronic devices. As an example we report the formation of a single electrically stable quantum dot between two etched NW segments.Comment: 9 pages, 5 figure

Stable isotopic disequilibrium in high-T metamorphic systems

A principal use of stable isotopes in metamorphic rocks is as thermometers, or as tests for isotopic equilibrium where metamorphic temperatures are known. Applications are often complicated when apparent isotopic temperatures are discordant and disagree with petrologic temperatures, indicating a failure of isotopic systems to record and/or preserve equilibrium, peak-T fractionations. In low-T, fluid-hosted environments such features often clearly reflect open system exchange. However, in high-T metamorphic environments a slow cooling history can be sufficient to produce such features by retrograde, closed system exchange between coexisting minerals

Decidability of the Monadic Shallow Linear First-Order Fragment with Straight Dismatching Constraints

The monadic shallow linear Horn fragment is well-known to be decidable and has many application, e.g., in security protocol analysis, tree automata, or abstraction refinement. It was a long standing open problem how to extend the fragment to the non-Horn case, preserving decidability, that would, e.g., enable to express non-determinism in protocols. We prove decidability of the non-Horn monadic shallow linear fragment via ordered resolution further extended with dismatching constraints and discuss some applications of the new decidable fragment.Comment: 29 pages, long version of CADE-26 pape

Petrological Constraints on the Recycling of Mafic Crystal Mushes and Intrusion of Braided Sills in the Torres del Paine Mafic Complex (Patagonia)

Cumulate and crystal mush disruption and reactivation are difficult to recognize in coarse-grained, shallow plutonic rocks. Mafic minerals included in hornblende and zoned plagioclase provide snapshots of early crystallization and cumulate formation, but are difficult to interpret in terms of the dynamics of magma ascent and possible links between silicic and mafic rock emplacement. This study presents the field relations, the microtextures and the mineral chemistry of the Miocene mafic sill complex of the Torres del Paine intrusive complex (Patagonia, Chile) and its subvertical feeder zone. We summarize a number of observations that occur in structurally different, shallow, plutonic rocks, as follows. (1) The mafic sill complex was built up by a succession of braided sills of shoshonitic and high-K calc-alkaline porphyritic hornblende-gabbro and fine-grained monzodiorite sills. Local diapiric structures and felsic magma accumulation between sills indicate limited separation of intercumulus liquid from the mafic sills. Anhedral hornblende cores, with olivine + clinopyroxene ± plagioclase ± apatite inclusions, crystallized at temperatures >900°C and pressures of ∼300 to ∼400 MPa. The corresponding rims and monzodiorite matrix crystallized at 950°C) than estimated from the composition of the granite minimum. We show that hornblende-plagioclase thermobarometry is a useful monitor for the determination of the segregation conditions of granitic magmas from gabbroic crystal mushes, and for monitoring the evolution of shallow crustal magmatic crystallization, decompression and coolin

Contrasting motivation and learning strategies of ex-mathematics and ex-mathematical literacy students

Abstract: This inquiry contrasts motivation and learning strategies of ex-Mathematics (Maths) and ex-Mathematical Literacy (ML) students. ML ideally delivers candidates who can make sense of and actively participate in a world of numbers and numerical arguments, but ex-ML students are excluded from many undergraduate studies at most South African higher education institutions (HEIs). Institutions employ various strategies in enhancing student transition to higher education (HE), however, such options are rare for ex-ML students. A year-long foundation programme offered by a private HEI is one exception. This inquiry employed the Motivated Strategies for Learning Questionnaire and t-test, detecting significant differences in motivation and learning strategies between 111 ex-Maths and 81 ex-ML students. The intrinsic goal orientation, task value, self-efficacy, effort regulation and test anxiety-handling abilities of ex-Maths students were significantly superior. An integrated solution process addressing academic content and social-psychological attributes to improve the motivation of ex-ML students in support of their academic development is proffered

Petrological Constraints on the Recycling of Mafic Crystal Mushes and Intrusion of Braided Sills in the Torres del Paine Mafic Complex (Patagonia)

Cumulate and crystal mush disruption and reactivation are difficult to recognize in coarse-grained, shallow plutonic rocks. Mafic minerals included in hornblende and zoned plagioclase provide snapshots of early crystallization and cumulate formation, but are difficult to interpret in terms of the dynamics of magma ascent and possible links between silicic and mafic rock emplacement. This study presents the field relations, the microtextures and the mineral chemistry of the Miocene mafic sill complex of the Torres del Paine intrusive complex (Patagonia, Chile) and its subvertical feeder zone. We summarize a number of observations that occur in structurally different, shallow, plutonic rocks, as follows. (1) The mafic sill complex was built up by a succession of braided sills of shoshonitic and high-K calc-alkaline porphyritic hornblende-gabbro and fine-grained monzodiorite sills. Local diapiric structures and felsic magma accumulation between sills indicate limited separation of intercumulus liquid from the mafic sills. Anhedral hornblende cores, with olivine + clinopyroxene ± plagioclase ± apatite inclusions, crystallized at temperatures >900°C and pressures of ∼300 to ∼400 MPa. The corresponding rims and monzodiorite matrix crystallized at <830°C, ∼70 MPa. This abrupt compositional variation suggests stability and instability of hornblende during recycling of the mafic roots of the complex and subsequent decompression. (2) The near lack of intercumulus crystals in the subvertical feeder zone layered gabbronorite and pyroxene–hornblende gabbronorite stocks testifies that melt is more efficiently extracted than in sills, resulting in a cumulate signature in the feeding system. Granitic liquids were extracted at a higher temperature (T >950°C) than estimated from the composition of the granite minimum. We show that hornblende–plagioclase thermobarometry is a useful monitor for the determination of the segregation conditions of granitic magmas from gabbroic crystal mushes, and for monitoring the evolution of shallow crustal magmatic crystallization, decompression and cooling

X-ray computed micro tomography as complementary method for the characterization of activated porous ceramic preforms

X-ray computed micro tomography (CT) is an alternative technique to the classical methods such as mercury intrusion (MIP) and gas pycnometry (HP) to obtain the porosity, pore-size distribution, and density of porous materials. Besides the advantage of being a nondestructive method, it gives not only bulk properties, but also spatially resolved information. In the present work, uniaxially pressed porous alumina performs activated by titanium were analyzed with both the classical techniques and CT. The benefits and disadvantages of the applied measurement techniques were pointed out and discussed. With the generated data, development was proposed for an infiltration model under ideal conditions for the production of metal matrix composites (MMC) by pressureless melt infiltration of porous ceramic preforms. Therefore, the reliability of the results, received from different investigation techniques, was proved statistically and stereologicall

Time and duration of chondrule formation: Constraints from 26Al-26Mg ages of individual chondrules

Chondrules from unequilibrated ordinary and carbonaceous chondrites belong to the oldest and most primitive materials from the early solar system and record chemical and isotopic signatures relating to their formation and evolution. These signatures allow tracing protoplanetary disk processes that eventually led to the formation of planetary building blocks and rocky planets. 26Al-26Mg ages based on mineral-mesostasis isochrons of 31 porphyritic ferromagnesian chondrules, that belong mainly to type-II, constrain the time of chondrule melting prior to incorporation into the respective chondrite parent bodies. For this study chondrules from the unequilibrated L, L(LL) and LL ordinary chondrites (UOCs) NWA 5206, NWA 8276, MET 96503, MET 00452, MET 00526, NWA 7936 and QUE 97008 were selected, which are of petrologic types 3.00-3.15 and were thus least metamorphosed after formation. Magnesium and Al isotopes were measured in-situ by Secondary Ion Mass Spectrometry (SIMS) using a CAMECA 1280 ims. 26Mg excess from in-situ decay of 26Al correlating with 27Al/24Mg has been detected in the mesostasis of all but one chondrule. The initial Al isotopic compositions (26Al/27Al)0 and 26Mg/24Mg ratios (d26Mg*0) deduced from internal mineral isochron regressions range from (9.5 ± 2.8) × 10-6 to (3.1 ± 1.2) × 10-6 and -0.020 ± 0.028‰ to 0.011 ± 0.039‰, respectively. The corresponding chondrule ages (∆tCAI), calculated relative to calcium-aluminum-rich inclusions (CAIs) using the canonical 26Al/27Al = (5.23 ± 0.13) × 10-5, are between 1.76_(-0.27)^(+0.36) and 2.92_(-0.34)^(+0.51) Ma and date the melt formation and thus primary chondrule formation from dust-like precursors or reprocessing of older chondrules. The age range agrees with those acquired with different short-lived chronometers and with published 26Al-26Mg ages, the majority of which were obtained for chondrules from the Bishunpur and Semarkona meteorites, although no chondrule with (26Al/27Al)0 > 10-5 was found. Chondrules in single chondrite samples or between different chondrite groups show no distinct age distributions. The initial 26Al/27Al of the oldest chondrules in the L(LL)/LL and L chondrite samples are identical within their 1σ uncertainties and yield a mean age of 1.99_(-0.08)^(+0.08) Ma and 1.81_(-0.10)^(+0.11) Ma, respectively. The oldest chondrules from six of the seven studied samples record a mean age of 1.94_(-0.06)^(+0.07) Ma. Since heating events in the protoplanetary disk could have partially reset the Al-Mg systematics in pre-existing chondrules and this would have shifted recorded 26Al-26Mg ages toward younger dates, the oldest mean age of 1.81_(-0.10)^(+0.11) Ma recorded in L chondrite chondrules is interpreted to date the rapid and punctuated onset of chondrule formation. The density distribution of chondrule ages from this study, which comprises the largest single dataset of OC chondrule ages, combined with published ages for chondrules from ordinary and carbonaceous chondrites reveals major age peaks for OC chondrules at 2.0 and 2.3 Ma. Chondrules in ordinary and carbonaceous chondrites formed almost contemporaneously (with a possible distinction between CC groups) in two chemically distinct reservoirs, probably in density-enriched regions at the edges of Jupiter’s orbit. The young formation ages of chondrules suggest that they do not represent precursors but rather by-products of planetesimal accretion

Real-space imaging of quantum Hall effect edge strips

We use dynamic scanning capacitance microscopy (DSCM) to image compressible and incompressible strips at the edge of a Hall bar in a two-dimensional electron gas (2DEG) in the quantum Hall effect (QHE) regime. This method gives access to the complex local conductance, Gts, between a sharp metallic tip scanned across the sample surface and ground, comprising the complex sample conductance. Near integer filling factors we observe a bright stripe along the sample edge in the imaginary part of Gts. The simultaneously recorded real part exhibits a sharp peak at the boundary between the sample interior and the stripe observed in the imaginary part. The features are periodic in the inverse magnetic field and consistent with compressible and incompressible strips forming at the sample edge. For currents larger than the critical current of the QHE break-down the stripes vanish sharply and a homogeneous signal is recovered, similar to zero magnetic field. Our experiments directly illustrate the formation and a variety of properties of the conceptually important QHE edge states at the physical edge of a 2DEG.Comment: 7 page