Spin-orbit controlled quantum capacitance of a polar heterostructure

Oxide heterostructures with polar films display special electronic properties, such as the electronic reconstruction at their internal interfaces with the formation of two-dimensional metallic states. Moreover, the electrical field from the polar layers is inversion-symmetry breaking and generates a Rashba spin-orbit coupling (RSOC) in the interfacial electronic system. We investigate the quantum capacitance of a heterostructure in which a sizeable RSOC at a metallic interface is controlled by the electric field of a surface electrode. Such a structure is, for example, given by a LaAlO_3 film on a SrTiO_3 substrate which is gated by a top electrode. Such heterostructures can exhibit a strong enhancement of their capacitance [Li et al., Science 332, 825 (2011)]. The capacitance is related to the electronic compressibility of the heterostructure, but the two quantities are not equivalent. In fact, the transfer of charge to the interface controls the relation between capacitance and compressibility. We find that due to a strong RSOC, the quantum capacitance can be larger than the classical geometric value. However, in contrast to the results of recent investigations [Caprara et al., Phys. Rev. Lett. 109, 196401 (2012); Bucheli et al., Phys. Rev. B 89, 195448 (2014); Seibold et al., Europhys. Lett. 109, 17006 (2015)] the compressibility does not become negative for realistic parameter values for LaAlO_3/SrTiO_3 and, therefore, we find that no phase-separated state is induced by the strong RSOC at these interfaces

Superconductivity with Finite-Momentum Pairing in Zero Magnetic Field

In the BCS theory of superconductivity, one assumes that all Cooper pairs have the same center of mass momentum. This is indeed enforced by self consistency, if the pairing interaction is momentum independent. Here, we show that for an attractive nearest neighbor interaction, this is different. In this case, stable solutions with pairs with momenta q and -q coexist and, for a sufficiently strong interaction, one of these states becomes the groundstate of the superconductor. This finite-momentum pairing state is accompanied by a charge order with wave vector 2q. For a weak pairing interaction, the groundstate is a d-wave superconductor

Fractional Flux Quantization in Loops of Unconventional Superconductors

The magnetic flux threading a conventional superconducting ring is typically quantized in units of $\Phi_0=hc/2e$. The factor 2 in the denominator of $\Phi_0$ originates from the existence of two different types of pairing states with minima of the free energy at even and odd multiples of $\Phi_0$. Here we show that spatially modulated pairing states exist with energy minima at fractional flux values, in particular at multiples of $\Phi_0/2$. In such states condensates with different center-of-mass momenta of the Cooper pairs coexist. The proposed mechanism for fractional flux quantization is discussed in the context of cuprate superconductors, where $hc/4e$ flux periodicities as well as uniaxially modulated superconducting states were observed.Comment: 5 pages, 3 figure

Flux-Periodicity Crossover from hc/e in Normal Metallic to hc/2e in Superconducting Loops

The periodic response of a metallic or a superconducting ring to an external magnetic flux is one of the most evident manifestations of quantum mechanics. It is generally understood that the oscillation period hc/2e in the superconducting state is half the period hc/e in the metallic state, because the supercurrent is carried by Cooper pairs with a charge 2e. On the basis of the Bardeen-Cooper-Schrieffer theory we discuss, in which cases this simple interpretation is valid and when a more careful analysis is needed. In fact, the knowledge of the oscillation period of the current in the ring provides information on the electron interactions. In particular, we analyze the crossover from the hc/e periodic normal current to the hc/2e periodic supercurrent upon turning on a pairing interaction in a metal ring. Further, we elaborate on the periodicity crossover when cooling a metallic loop through the superconducting transition temperature Tc.Comment: To be bublished in "Superconductors", InTech (Rijeka), 2012 (ISBN 979-953-307-798-6

The Demographics of Non-motor Vehicle Associated Railway Injuries Seen at Trauma Centers in the United States 2007 - 2014

Introduction The majority of railway injury studies are limited by small sample size, restricted to a small geographical distribution, or located outside the United States (US). The aim of our study was to assess the demographic patterns associated with non-motor vehicle railway injuries in the US using a national trauma center database. Materials and Methods Data from the National Trauma Data Bank data from 2007 - 2014 were used; 3,506 patients were identified. For all statistical analyses, a p-value < 0.05 was considered significant. Results The patients were 81% male with an average age of 38.6 + 17.1 years and an Injury Severity Score (ISS) of 16.8 + 13.8. Males compared to females were younger (37.7 vs 42.5 years, p = 0.000002), had greater length of stays (12.7 vs 9.8 days, p = 0.000006), and higher ISS scores (17.1 vs 15.4, p = 0.0007). The geographic distribution within the US was most common in the South (32.0%) and least in the Northeast (18.9%). The racial composition was 67.5% White, 19.1% Black, 11.5% Hispanic/Latino, and 1.9% others. The most common mechanisms of injury were hitting/colliding with rolling stock (38.6%), followed by a fall in or from a train (19.5%), and collision with an object (13.5%). The majority of patients were pedestrians or passengers (68.5%); employees accounted for 12.5%. Although the majority were pedestrian/passengers for all regions, the Midwest had a greater proportion of employees (22.0%) compared to the other regions (7.8% to 12.2%) (p < 10-6), and thus injuries were more commonly work-related (24.6% vs 6.7% - 13.7%, p < 10-6). Work-related injuries were less severe (ISS 11.2 vs 17.3 - p < 10-6) and more commonly occurred due to a fall (32.8% vs 17.9%, p < 10-6). Alcohol and/or drug involvement was present in 40.7% and was less in those with work-related injuries (2.2%). Overall mortality was 6.4% and was less in those having a work-related injury (2.0 vs 6.6% p = 0.000004). Conclusion For non-motor vehicle USA railway injuries, the average age was 38.5 years; 80.6% were male. The injuries were least common in the Northeast and most common in the South. Racial distribution mirrored that of the US population. Alcohol involvement was present in 29%, lower than in previous studies. Mortality was 6.4%, also lower than previously reported

Echoing the Past: A Proposal for a Counter-Monument

To hear an echo is to witness a past event; it is a past event in the here and now. The phenomenon of the echo is not an event cut off from its conception, that first outburst of noise or speech. But rather it is that birthing event, delayed and distorted but nonetheless that past moment in this present; the echo is its own past made present. The echo is presented - made present - as an ephemeral event, departed yet connected from its own materialisation in the past; a distinct spatio-temporal activity and phenomenon where the past is dematerialised to erupt upon the present. The echo appears to deliver a unique paradigm for memorialisation and remembrance, the opportunity to access and (de)materialise a past event within a concrete present. But the echo performs a distinct spatio-temporal praxis is in antithesis to traditional monumentalism. The performance is not a practice of material making but immaterial un-making; a practice of absence and ephemerality. An art practice of echoing would appear to conform to what James E. Young describes as the “counter-monument.” These are practices of memorialisation that are against the ideological, painfully self-conscious of their very premise and deploy “a strategy of evoking rather than invoking”. This paper will present the phenomenon of the echo within a psycho-mythological framework, and will resolve the echo as a paradigm for a type of counter-monumental art practice. Through discussing the public art project “Daughter of a Voice” (2011), an artwork that confronts both the echo as a mythological device via the legend of Le Timbaler del Bruc (The Drummer Boy on Bruc Hill) and the echo as a material intervention into urban space, a practice will be proposed that eschews a representation of a past but performs an act of presence. This is a performance that places the burden of remembrance on active spectatorship rather than passive memory or reflection

Momentum-Space Spin Texture in a Topological Superconductor

A conventional superconductor with spin-orbit coupling turns into a topological superconductor beyond a critical strength of the Zeeman energy. The spin-expectation values $\mathbf{S}(\mathbf{k})$ in momentum space trace this transition via a characteristic change in the topological character of the spin texture within the Brillouin zone. At the transition the skyrmion counting number switches from 0 to 1/2 identifying the topological superconductor via its meron-like spin texture. The change in the skyrmion counting number is crucially controlled by singular points of the map $\mathbf{S}(\mathbf{k})/|\mathbf{S}(\mathbf{k})|$ from the Brillouin zone, i.e. a torus, to the unit sphere. The complexity of this spin-map is discussed at zero temperature as well as for the extension to finite temperatures.Comment: 16 pages, 9 figure

Occupying Infrastructure

Infrastructure shapes and conditions the manner in which we live in the industrialised world, putting into circulation the matter essential for our living, a cycle of energy, materials, data, and even excrement. Infrastructures are not neutral but embedded within systems of power and inequality. As globalising networks, infrastructures trouble conventional temporalities, from high-speed communication at the speed of light to the ‘deep time’ of nuclear waste. The essential condition of infrastructure means that it is paradoxically only revealed when its mechanisms cease to function normally. A multi-layered complexity performing with a global resolution, infrastructure exemplifies a planetary ontological condition for the human. Nonetheless, infrastructures are essentially nonhuman environments, designed and configured to direct and manage the material. These are spaces designed not for occupation by the human, yet embody the contemporary planetary status of our species, and the problematics that such a status proposes. This article explores proposes strategies of occupation for these more-than-human environments. These speculative proposals with be sited within an expanded discourse of the planetary, framing the various modes of occupation to expound a critical discourse that contributes to contemporary discussions of climate change, the Anthropocene and the impact of the human on the planet. The research project will draw attention to the hidden spaces of infrastructure that exist in the everyday around us, and asks what new activities or ways of being might emerge from these hybrid spaces and planetary networks

Optimization of an Electromagnetic Energy Harvesting Device

This paper presents the modeling and optimization of an electromagnetic-based generator for generating power from ambient vibrations. Basic equations describing such generators are presented and the conditions for maximum power generation are described. Two-centimeter scale prototype generators, which consist of magnets suspended on a beam vibrating relative to a coil, have been built and tested. The measured power and modeled results are compared. It is shown that the experimental results confirm the optimization theory