26 research outputs found
Hydrodynamic charge and heat transport on inhomogeneous curved spaces
We develop the theory of hydrodynamic charge and heat transport in strongly
interacting quasi-relativistic systems on manifolds with inhomogeneous spatial
curvature. In solid-state physics, this is analogous to strain disorder in the
underlying lattice. In the hydrodynamic limit, we find that the thermal and
electrical conductivities are dominated by viscous effects, and that the
thermal conductivity is most sensitive to this disorder. We compare the effects
of inhomogeneity in the spatial metric to inhomogeneity in the chemical
potential, and discuss the extent to which our hydrodynamic theory is relevant
for experimentally realizable condensed matter systems, including suspended
graphene at the Dirac point.Comment: 15+8 pages, 4+1 figures; v2: added references, published versio
Spin connection as Lorentz gauge field: propagating torsion
We propose a modified gravitational action containing besides the
Einstein-Cartan term some quadratic contributions resembling the Yang-Mills
lagrangian for the Lorentz spin connections. We outline how a propagating
torsion arises and we solve explicitly the linearised equations of motion on a
Minkowski background. We identify among torsion components six degrees of
freedom: one is carried by a pseudo-scalar particle, five by a tachyon field.
By adding spinor fields and neglecting backreaction on the geometry, we point
out how only the pseudo-scalar particle couples directly with fermions, but the
resulting coupling constant is suppressed by the ratio between fermion and
Planck masses. Including backreaction, we demonstrate how the tachyon field
provides causality violation in the matter sector, via an interaction mediated
by gravitational waves.Comment: 7 pages, no figures, new section adde
Regularization dependence of the OTOC. Which Lyapunov spectrum is the physical one?
We study the contour dependence of the out-of-time-ordered correlation
function (OTOC) both in weakly coupled field theory and in the
Sachdev-Ye-Kitaev (SYK) model. We show that its value, including its Lyapunov
spectrum, depends sensitively on the shape of the complex time contour in
generic weakly coupled field theories. For gapless theories with no thermal
mass, such as SYK, the Lyapunov spectrum turns out to be an exception; their
Lyapunov spectra do not exhibit contour dependence, though the full OTOCs do.
Our result puts into question which of the Lyapunov exponents computed from the
exponential growth of the OTOC reflects the actual physical dynamics of the
system. We argue that, in a weakly coupled theory, a kinetic theory
argument indicates that the symmetric configuration of the time contour, namely
the one for which the bound on chaos has been proven, has a proper
interpretation in terms of dynamical chaos. Finally, we point out that a
relation between these OTOCs and a quantity which may be measured
experimentally --- the Loschmidt echo --- also suggests a symmetric contour
configuration, with the subtlety that the inverse periodicity in Euclidean time
is half the physical temperature. In this interpretation the chaos bound reads
.Comment: Comment on regularization dependence in 2d-CFTs added. Published
versio
Kinetic theory for classical and quantum many-body chaos
For perturbative scalar field theories, the late-time-limit of the
out-of-time-ordered correlation function that measures (quantum) chaos is shown
to be equal to a Boltzmann-type kinetic equation that measures the total gross
(instead of net) particle exchange between phase space cells, weighted by a
function of energy. This derivation gives a concrete form to numerous attempts
to derive chaotic many-body dynamics from ad hoc kinetic equations. A period of
exponential growth in the total gross exchange determines the Lyapunov exponent
of the chaotic system. Physically, the exponential growth is a front
propagating into an unstable state in phase space. As in conventional Boltzmann
transport, which follows from the dynamics of the net particle number density
exchange, the kernel of this kinetic integral equation is also set by the
2-to-2 scattering rate. This provides a mathematically precise statement of the
known fact that in dilute weakly coupled gases transport and scrambling (or
ergodicity) are controlled by the same physics.Comment: V2: 10 pages, 1 figure. References and clarifications added.
Published versio
Reducing the environmental impact of surgery on a global scale: systematic review and co-prioritization with healthcare workers in 132 countries
Abstract
Background
Healthcare cannot achieve net-zero carbon without addressing operating theatres. The aim of this study was to prioritize feasible interventions to reduce the environmental impact of operating theatres.
Methods
This study adopted a four-phase Delphi consensus co-prioritization methodology. In phase 1, a systematic review of published interventions and global consultation of perioperative healthcare professionals were used to longlist interventions. In phase 2, iterative thematic analysis consolidated comparable interventions into a shortlist. In phase 3, the shortlist was co-prioritized based on patient and clinician views on acceptability, feasibility, and safety. In phase 4, ranked lists of interventions were presented by their relevance to high-income countries and low–middle-income countries.
Results
In phase 1, 43 interventions were identified, which had low uptake in practice according to 3042 professionals globally. In phase 2, a shortlist of 15 intervention domains was generated. In phase 3, interventions were deemed acceptable for more than 90 per cent of patients except for reducing general anaesthesia (84 per cent) and re-sterilization of ‘single-use’ consumables (86 per cent). In phase 4, the top three shortlisted interventions for high-income countries were: introducing recycling; reducing use of anaesthetic gases; and appropriate clinical waste processing. In phase 4, the top three shortlisted interventions for low–middle-income countries were: introducing reusable surgical devices; reducing use of consumables; and reducing the use of general anaesthesia.
Conclusion
This is a step toward environmentally sustainable operating environments with actionable interventions applicable to both high– and low–middle–income countries
Improved performance in flexible organic solar cells by using copolymeric phase-separation modulators
One of the main problems related to the low performance of the organic solar cells (OSCs), concerns the low mobility of the materials constituting the heterojunction. Indeed, the poor charge transport in the active layer is the principal cause of a competition between separation and recombination of the photogenerated carriers. In this regard, a major obstacle to enhance OSCs efficiency is developing strategies to optimize the exciton dissociation and, consequently, the charge collection at the electrodes. Donor and acceptor systems must be well mixed on the length scale of 5 – 20 nm (exciton diffusion length) to meet the criteria for efficient exciton dissociation. In addition, the network structure should involve continuous donor-acceptor pathways for efficient carrier transport. The most common practice to achieve this goal is by thermal or solvent annealing of active layer.[1] However, this approach often leads to an unwanted phase segregation with formation of large domains where only a small fraction of excitons could diffuse to the donor-acceptor interface.[2] In this work, we show how this challenge is achievable by incorporating phase-separation modulators into bulk heterojunction (BHJ). In particular, three copolymers based on polythiophene and C60 units have been designed, easily synthesized, characterized, and employed as additive in P3HT:PCBM devices. The effect of the thienyl spacer length between C60 monomers on optoelectronic properties, morphology, and structure of heterojunction has been examined using several techniques (NMR, FTIR, XPS, XRD and AFM). We observed that small quantities of these systems can play a critical role in tuning the device morphology by improving the phase separation in thin film heterojunction.[3] In particular, these copolymers act as phase separation modulators by controlling the growth of donor/acceptor domains in the heterojunction, during the thermal annealing process. Indeed, by employing copolymers containing oligothiophenic chains with size of about 8 nm, a large number of domains with a size comparable to the length scale of exciton diffusion are generated, resulting in the highest power conversion efficiency (PCE) (4.46 %) and short current density (JSC) (16.15 mA cm-2) values reported so far for P3HT:PCBM solar cells on plastic substrates. Moreover, the results obtained in preliminary investigations on the other devices containing different fullerene acceptors seem to show the effectiveness and the generality of our approach. Finally, bending tests showed that OSCs with copolymers maintain higher level of performance than reference devices, thus giving new perspectives to applications of flexible photovoltaics
Donor-Acceptor Interfaces by Engineered Nanoparticles Assemblies for Enhanced Efficiency in Plastic Planar Heterojunction Solar Cells
Precisely positioning functionalized gold nanoparticles assemblies at planar donor-acceptor interfaces results in 14-fold enhancement of power conversion efficiency in P3HT/PCBM organic solar cells on plastic (ITO/PET) substrates. This result has been achieved by employing naphthalenethiol-capped gold nanoparticles (NT-Au-NPs) produced by laser ablation in liquid and size varied in the 10-30 nm range. Upon surface functionalization with the aromatic thiol, these particles self-assemble in submicrometer aggregates, which give increased light scattering. When these aggregates are deposited in the planar heterojunction between the donor and the acceptor systems, the localized scattering leads to a larger exciton formation just in the region of interest for charge transfer
Low angle bending detection semi-transparent piezoresistive sensor
We designed, fabricated, and validated a piezoresistive bending sensor, a fundamental component of wearable electronic devices for monitoring human motion. The most diffused opaque carbon-based resistance flex sensors suffer from low detection for small bending angles. The sensor we here present is based on a semi-transparent active material (fulleropyrrolidine bisadducts polymer) and has the remarkable advantage of good electrical properties for low bending angles. The fabrication steps are effective since a pre-patterned ITO/PET surface is functionalized by chronoamperometric deposition, and the silver electrical contacts are inkjet printed. We propose a fitting function of the measured thin film resistance curve vs. the bending angle, showing promising properties as a complimentary bending sensor to the most diffused flex sensors. The results pave the way to new applications and more performant wearables
Aurora-a is essential for the tumorigenic capacity and chemoresistance of colorectal cancer stem cells.
Colorectal cancer stem cells (CR-CSC) are responsible for the generation and maintenance of intestinal tumors and are highly resistant to conventional chemotherapeutic agents. Aurora-A, a serine-threonine kinase involved in mitosis regulation, plays multiple key functions in tumor initiation and progression. We found that Aurora-A is overexpressed in primary colorectal tumor cells, in the CR-CSC fraction, and in stem cell-derived differentiated cells, compared with normal colon tissue. Aurora-A expression was functionally linked to centrosome amplification in CR-CSC, as indicated by the decrease in cells with multiple centrosomes that followed Aurora-A silencing. Knockdown of Aurora-A resulted in growth inhibition of CR-CSC, alteration of cell cycle kinetics, and downregulation of the expression levels of antiapoptotic Bcl-2 family members, strongly sensitizing to chemotherapy-induced cell death. Moreover, Aurora-A silencing compromised the ability to form tumor xenografts in immunocompromised mice and reduced the migratory capacity of CR-CSC. Altogether, these results indicate that Aurora-A is essential for CR-CSC regeneration and resistance to cytotoxic stimuli and suggest that therapies directed against Aurora-A may effectively target the stem cell population in colorectal cancer