278 research outputs found
Tracking primary thermalization events in graphene with photoemission at extreme timescales
Direct and inverse Auger scattering are amongst the primary processes that
mediate the thermalization of hot carriers in semiconductors. These two
processes involve the annihilation or generation of an electron-hole pair by
exchanging energy with a third carrier, which is either accelerated or
decelerated. Inverse Auger scattering is generally suppressed, as the
decelerated carriers must have excess energies higher than the band gap itself.
In graphene, which is gapless, inverse Auger scattering is instead predicted to
be dominant at the earliest time delays. Here, femtosecond
extreme-ultraviolet pulses are used to detect this imbalance, tracking both the
number of excited electrons and their kinetic energy with time- and
angle-resolved photoemission spectroscopy. Over a time window of approximately
25 fs after absorption of the pump pulse, we observe an increase in conduction
band carrier density and a simultaneous decrease of the average carrier kinetic
energy, revealing that relaxation is in fact dominated by inverse Auger
scattering. Measurements of carrier scattering at extreme timescales by
photoemission will serve as a guide to ultrafast control of electronic
properties in solids for PetaHertz electronics.Comment: 16 pages, 8 figure
Enhanced electron-phonon coupling in graphene with periodically distorted lattice
Electron-phonon coupling directly determines the stability of cooperative
order in solids, including superconductivity, charge and spin density waves.
Therefore, the ability to enhance or reduce electron-phonon coupling by optical
driving may open up new possibilities to steer materials' functionalities,
potentially at high speeds. Here we explore the response of bilayer graphene to
dynamical modulation of the lattice, achieved by driving optically-active
in-plane bond stretching vibrations with femtosecond mid-infrared pulses. The
driven state is studied by two different ultrafast spectroscopic techniques.
Firstly, TeraHertz time-domain spectroscopy reveals that the Drude scattering
rate decreases upon driving. Secondly, the relaxation rate of hot
quasi-particles, as measured by time- and angle-resolved photoemission
spectroscopy, increases. These two independent observations are quantitatively
consistent with one another and can be explained by a transient three-fold
enhancement of the electron-phonon coupling constant. The findings reported
here provide useful perspective for related experiments, which reported the
enhancement of superconductivity in alkali-doped fullerites when a similar
phonon mode was driven.Comment: 12 pages, 4 figure
rEHR: An R package for manipulating and analysing Electronic Health Record data
Research with structured Electronic Health Records (EHRs) is expanding as data becomes more accessible; analytic methods advance; and the scientific validity of such studies is increasingly accepted. However, data science methodology to enable the rapid searching/extraction, cleaning and analysis of these large, often complex, datasets is less well developed. In addition, commonly used software is inadequate, resulting in bottlenecks in research workflows and in obstacles to increased transparency and reproducibility of the research. Preparing a research-ready dataset from EHRs is a complex and time consuming task requiring substantial data science skills, even for simple designs. In addition, certain aspects of the workflow are computationally intensive, for example extraction of longitudinal data and matching controls to a large cohort, which may take days or even weeks to run using standard software. The rEHR package simplifies and accelerates the process of extracting ready-for-analysis datasets from EHR databases. It has a simple import function to a database backend that greatly accelerates data access times. A set of generic query functions allow users to extract data efficiently without needing detailed knowledge of SQL queries. Longitudinal data extractions can also be made in a single command, making use of parallel processing. The package also contains functions for cutting data by time-varying covariates, matching controls to cases, unit conversion and construction of clinical code lists. There are also functions to synthesise dummy EHR. The package has been tested with one for the largest primary care EHRs, the Clinical Practice Research Datalink (CPRD), but allows for a common interface to other EHRs. This simplified and accelerated work flow for EHR data extraction results in simpler, cleaner scripts that are more easily debugged, shared and reproduced
Momentum resolved spin dynamics of bulk and surface excited states in the topological insulator
The prospective of optically inducing a spin polarized current for spintronic
devices has generated a vast interest in the out-of-equilibrium electronic and
spin structure of topological insulators (TIs). In this Letter we prove that
only by measuring the spin intensity signal over several order of magnitude in
spin, time and angle resolved photoemission spectroscopy (STAR-PES) experiments
is it possible to comprehensively describe the optically excited electronic
states in TIs materials. The experiments performed on
reveal the existence of a Surface-Resonance-State in the 2nd bulk band gap
interpreted on the basis of fully relativistic ab-initio spin resolved
photoemission calculations. Remarkably, the spin dependent relaxation of the
hot carriers is well reproduced by a spin dynamics model considering two
non-interacting electronic systems, derived from the excited surface and bulk
states, with different electronic temperatures.Comment: 5 pages and 4 figure
Probing the structure and dynamics of molecular clusters using rotational wavepackets
The chemical and physical properties of molecular clusters can heavily depend
on their size, which makes them very attractive for the design of new materials
with tailored properties. Deriving the structure and dynamics of clusters is
therefore of major interest in science. Weakly bound clusters can be studied
using conventional spectroscopic techniques, but the number of lines observed
is often too small for a comprehensive structural analysis. Impulsive alignment
generates rotational wavepackets, which provides simultaneous information on
structure and dynamics, as has been demonstrated successfully for isolated
molecules. Here, we apply this technique for the firsttime to clusters
comprising of a molecule and a single helium atom. By forcing the population of
high rotational levels in intense laser fields we demonstrate the generation of
rich rotational line spectra for this system, establishing the highly
delocalised structure and the coherence of rotational wavepacket propagation.
Our findings enable studies of clusters of different sizes and complexity as
well as incipient superfluidity effects using wavepacket methods.Comment: 5 pages, 6 figure
Possible observation of parametrically amplified coherent phasons in K0.3MoO3 using time-resolved extreme-ultraviolet ARPES
We use time- and angle-resolved photoemission spectroscopy (tr-ARPES) in the
Extreme Ultraviolet (EUV) to measure the time- and momentum-dependent
electronic structure of photo-excited K0.3MoO3. Prompt depletion of the Charge
Density Wave (CDW) condensate launches coherent oscillations of the amplitude
mode, observed as a 1.7-THz-frequency modulation of the bonding band position.
In contrast, the anti-bonding band oscillates at about half this frequency. We
attribute these oscillations to coherent excitation of phasons via parametric
amplification of phase fluctuations.Comment: 4 figure
Modelling Conditions and Health Care Processes in Electronic Health Records : An Application to Severe Mental Illness with the Clinical Practice Research Datalink
BACKGROUND: The use of Electronic Health Records databases for medical research has become mainstream. In the UK, increasing use of Primary Care Databases is largely driven by almost complete computerisation and uniform standards within the National Health Service. Electronic Health Records research often begins with the development of a list of clinical codes with which to identify cases with a specific condition. We present a methodology and accompanying Stata and R commands (pcdsearch/Rpcdsearch) to help researchers in this task. We present severe mental illness as an example. METHODS: We used the Clinical Practice Research Datalink, a UK Primary Care Database in which clinical information is largely organised using Read codes, a hierarchical clinical coding system. Pcdsearch is used to identify potentially relevant clinical codes and/or product codes from word-stubs and code-stubs suggested by clinicians. The returned code-lists are reviewed and codes relevant to the condition of interest are selected. The final code-list is then used to identify patients. RESULTS: We identified 270 Read codes linked to SMI and used them to identify cases in the database. We observed that our approach identified cases that would have been missed with a simpler approach using SMI registers defined within the UK Quality and Outcomes Framework. CONCLUSION: We described a framework for researchers of Electronic Health Records databases, for identifying patients with a particular condition or matching certain clinical criteria. The method is invariant to coding system or database and can be used with SNOMED CT, ICD or other medical classification code-lists
A Simulation Model for Assessing Alternate Strategies for Beef Production with Land, Energy and Economic Constraints
A computer model has been developed to analyze alternate management strategies and energy and economic constraints. Daily production of beef animals and growing crops is simulated in response to prevailing conditions and system interactions using the GASP IV simulation language. Complete inventories of plant dry matter, animal status, production resources and economic net worth are maintained over the simulation period
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