59 research outputs found
Vibrational cooling and thermoelectric response of nanoelectromechanical systems
An important goal in nanoelectromechanics is to cool the vibrational motion,
ideally to its quantum ground state. Cooling by an applied charge current is a
particularly simple and hence attractive strategy to this effect. Here, we
explore this phenomenon in the context of the general theory of
thermoelectrics. In linear response, this theory describes thermoelectric
refrigerators in terms of their cooling efficiency and figure of merit ZT. We
show that both concepts carry over to phonon cooling in nanoelectromechanical
systems. As an important consequence, this allows us to discuss the efficiency
of phonon refrigerators in relation to the fundamental Carnot efficiency. We
illustrate these general concepts by thoroughly investigating a simple
double-quantum-dot model with the dual advantage of being quite realistic
experimentally and amenable to a largely analytical analysis theoretically.
Specifically, we obtain results for the efficiency, the figure of merit, and
the effective temperature of the vibrational motion in two regimes. In the
quantum regime in which the vibrational motion is fast compared to the
electronic degrees of freedom, we can describe the electronic and phononic
dynamics of the model in terms of master equations. In the complementary
classical regime of slow vibrational motion, the dynamics is described in terms
of an appropriate Langevin equation. Remarkably, we find that the efficiency
can approach the maximal Carnot value in the quantum regime, with large
associated figures of merit. In contrast, the efficiencies are typically far
from the Carnot limit in the classical regime. Our theoretical results should
provide guidance to implementing efficient vibrational cooling of
nanoelectromechanical systems in the laboratory.Comment: 14 pages, 11 figure
Transport properties of graphene functionalized with molecular switches
We provide a theory of the electronic transport properties of a graphene layer
functionalized with molecular switches. Our considerations are motivated by
the spiropyran?merocyanine system which is non-polar in its ring-closed
spiropyran form and zwitterionic in its ring-open merocyanine form. The
reversible switching between these two isomers affects the carriers in
graphene through the associated change in the molecular dipole moment, turning
the graphene layer into a sensor of the molecular switching state. We present
results for both the quasiclassical (Boltzmann) and the quantum coherent
regimes of transport. Quite generally, we find a linear sensitivity of the
conductance on the molecular dipole moment whenever quantum interference
effects play an essential role which contrasts with a quadratic (and typically
weaker) dependence when quantum interference is absent
Scattering theory of current-induced forces in mesoscopic systems
We develop a scattering theory of current-induced forces exerted by the
conduction electrons of a general mesoscopic conductor on slow "mechanical"
degrees of freedom. Our theory describes the current-induced forces both in and
out of equilibrium in terms of the scattering matrix of the phase-coherent
conductor. Under general nonequilibrium conditions, the resulting mechanical
Langevin dynamics is subject to both non-conservative and velocity-dependent
Lorentz-like forces, in addition to (possibly negative) friction. We illustrate
our results with a two-mode model inspired by hydrogen molecules in a break
junction which exhibits limit-cycle dynamics of the mechanical modes.Comment: 4+ pages, 1 figure; v2: minor modification
a scattering-matrix approach
Nanoelectromechanical systems are characterized by an intimate connection
between electronic and mechanical degrees of freedom. Due to the nanoscopic
scale, current flowing through the system noticeably impacts upons the
vibrational dynamics of the device, complementing the effect of the
vibrational modes on the electronic dynamics. We employ the scattering-matrix
approach to quantum transport in order to develop a unified theory of
nanoelectromechanical systems out of equilibrium. For a slow mechanical mode
the current can be obtained from the Landauer–Büttiker formula in the strictly
adiabatic limit. The leading correction to the adiabatic limit reduces to
Brouwer’s formula for the current of a quantum pump in the absence of a bias
voltage. The principal results of the present paper are the scattering-matrix
expressions for the current-induced forces acting on the mechanical degrees of
freedom. These forces control the Langevin dynamics of the mechanical modes.
Specifically, we derive expressions for the (typically nonconservative) mean
force, for the (possibly negative) damping force, an effective “Lorentz” force
that exists even for time-reversal-invariant systems, and the fluctuating
Langevin force originating from Nyquist and shot noise of the current flow. We
apply our general formalism to several simple models that illustrate the
peculiar nature of the current-induced forces. Specifically, we find that in
out-of-equilibrium situations the current-induced forces can destabilize the
mechanical vibrations and cause limit-cycle dynamics
Current-induced forces in mesoscopic systems: a scattering matrix approach
Nanoelectromechanical systems are characterized by an intimate connection
between electronic and mechanical degrees of freedom. Due to the nanoscopic
scale, current flowing through the system noticeably impacts the vibrational
dynamics of the device, complementing the effect of the vibrational modes on
the electronic dynamics. We employ the scattering matrix approach to quantum
transport to develop a unified theory of nanoelectromechanical systems out of
equilibrium. For a slow mechanical mode, the current can be obtained from the
Landauer-B\"uttiker formula in the strictly adiabatic limit. The leading
correction to the adiabatic limit reduces to Brouwer's formula for the current
of a quantum pump in the absence of the bias voltage. The principal result of
the present paper are scattering matrix expressions for the current-induced
forces acting on the mechanical degrees of freedom. These forces control the
Langevin dynamics of the mechanical modes. Specifically, we derive expressions
for the (typically nonconservative) mean force, for the (possibly negative)
damping force, an effective "Lorentz" force which exists even for time reversal
invariant systems, and the fluctuating Langevin force originating from Nyquist
and shot noise of the current flow. We apply our general formalism to several
simple models which illustrate the peculiar nature of the current-induced
forces. Specifically, we find that in out of equilibrium situations the current
induced forces can destabilize the mechanical vibrations and cause limit-cycle
dynamics.Comment: "Applications" section considerably extended. Changes in layout.
Version as publishe
Current-induced switching in transport through anisotropic magnetic molecules
Anisotropic single-molecule magnets may be thought of as molecular switches,
with possible applications to molecular spintronics. In this paper, we
consider current-induced switching in single-molecule junctions containing an
anisotropic magnetic molecule. We assume that the carriers interact with the
magnetic molecule through the exchange interaction and focus on the regime of
high currents in which the molecular spin dynamics is slow compared to the
time which the electrons spend on the molecule. In this limit, the molecular
spin obeys a nonequilibrium Langevin equation which takes the form of a
generalized Landau-Lifshitz-Gilbert equation and which we derive
microscopically by means of a nonequilibrium Born-Oppenheimer approximation.
We exploit this Langevin equation to identify the relevant switching
mechanisms and to derive the current-induced switching rates. As a by-product,
we also derive S-matrix expressions for the various torques entering into the
Landau-Lifshitz-Gilbert equation which generalize previous expressions in the
literature to nonequilibrium situations.1\. Auflag
The Milan system for reporting salivary gland cytopathology—A single‐center study of 2156 cases
Antiviral signaling by a cyclic nucleotide activated CRISPR protease
Funding information: M.G. and J.L.S.B. are funded by the Deutsche Forschungsgemeinschaft under Germany’s Excellence Strategy–EXC2151–390873048. M.F.W. acknowledges a European Research Council Advanced Grant (grant number 101018608) and the China Scholarship Council (REF: 202008420207 to H.C.). G.H. is grateful for funding by the Deutsche Forschungsgemeinschaft (grant number HA6805/6-1).CRISPR defense systems such as the well-known DNA-targeting Cas9 and the RNA-targeting type III systems are widespread in prokaryotes1,2. The latter can orchestrate a complex antiviral response that is initiated by the synthesis of cyclic oligoadenylates (cOAs) upon foreign RNA recognition3-5. Among a large set of proteins that were linked to type III systems and predicted to bind cOAs6,7, a CRISPR associated Lon protease (CalpL) stood out to us. The protein contains a sensor domain of the SAVED (SMODS-associated and fused to various effector domains) family7, fused to a Lon protease effector domain. However, the mode of action of this effector was unknown. Here, we report the structure and function of CalpL and show that the soluble protein forms a stable tripartite complex with two further proteins, CalpT and CalpS, that are encoded in the same operon. Upon activation by cA4, CalpL oligomerizes and specifically cleaves the MazF-homolog CalpT, releasing the extracytoplasmic function (ECF) sigma factor CalpS from the complex. This provides a direct connection between CRISPR-based foreign nucleic acid detection and transcriptional regulation. Furthermore, the presence of a cA4-binding SAVED domain in a CRISPR effector reveals an unexpected link to the cyclic oligonucleotide-based antiphage signaling system (CBASS).PostprintPeer reviewe
Balance between matrix metalloproteinases (MMP) and tissue inhibitors of metalloproteinases (TIMP) in the cervical mucus plug estimated by determination of free non-complexed TIMP
<p>Abstract</p> <p>Background</p> <p>The cervical mucus plug (CMP) is a semi-solid structure with antibacterial properties positioned in the cervical canal during pregnancy. The CMP contains high concentrations of matrix metalloproteinase 8 and 9 (MMP-8, MMP-9) and tissue inhibitor of metalloproteinase 1 (TIMP-1). This indicates a potential to degrade extracellular matrix components depending on the balance between free non-complexed inhibitors and active enzymes.</p> <p>Methods</p> <p>Thirty-two CMPs collected during active labor at term were analyzed. Twelve CMPs were separated into a cellular and an extracellular/fluid phase and analyzed by gelatin and reverse zymography to reveal MMP and TIMP location. Twenty samples were homogenized, extracted and studied by the TIMP activity assay based on gelatin zymography. Enzyme-linked immunosorbent assay (ELISA) was used to determine TIMP-1, MMP-8 and MMP-9 protein concentrations, and gelatin and reverse zymography used to identify gelatinases and TIMPs, respectively. The Western blotting technique was applied for semi-quantification of alpha2-macroglobulin. An ELISA activity assay was used to detect MMP-8 and MMP-9 activity.</p> <p>Results</p> <p>ProMMP-2, proMMP-9, TIMP-1 and TIMP-2 were almost exclusively located in the fluid phase compared to the cellular phase of the CMP. All the extracted samples contained MMP-8, MMP-9, TIMP-1, TIMP-2 and alpha2-macroglobulin. Free non-complexed TIMP was detected in all the samples analyzed by the TIMP activity assay and was associated with TIMP-1 protein (R = 0.71, p < 0.001) and with the TIMP/MMP molar ratio (1.7 (1.1–2.5) (mean (95% confidence interval)) (R = 0.65, p = 0.002). The ELISA activity assay showed no activity from MMP-8 or MMP-9.</p> <p>Conclusion</p> <p>Due to their extracellular location, potential proteolytic activity from neutrophil-derived MMPs in the CMP could exert a biological impact on cervical dilatation and fetal membrane rupture at term. The functional TIMP activity assay, revealing excess non-complexed TIMP, and a molar inhibitor/enzyme ratio above unity, indicate that refined MMP control prevents CMP-originated proteolytic activity in the surrounding tissue.</p
Antimicrobial resistance among migrants in Europe: a systematic review and meta-analysis
BACKGROUND: Rates of antimicrobial resistance (AMR) are rising globally and there is concern that increased migration is contributing to the burden of antibiotic resistance in Europe. However, the effect of migration on the burden of AMR in Europe has not yet been comprehensively examined. Therefore, we did a systematic review and meta-analysis to identify and synthesise data for AMR carriage or infection in migrants to Europe to examine differences in patterns of AMR across migrant groups and in different settings. METHODS: For this systematic review and meta-analysis, we searched MEDLINE, Embase, PubMed, and Scopus with no language restrictions from Jan 1, 2000, to Jan 18, 2017, for primary data from observational studies reporting antibacterial resistance in common bacterial pathogens among migrants to 21 European Union-15 and European Economic Area countries. To be eligible for inclusion, studies had to report data on carriage or infection with laboratory-confirmed antibiotic-resistant organisms in migrant populations. We extracted data from eligible studies and assessed quality using piloted, standardised forms. We did not examine drug resistance in tuberculosis and excluded articles solely reporting on this parameter. We also excluded articles in which migrant status was determined by ethnicity, country of birth of participants' parents, or was not defined, and articles in which data were not disaggregated by migrant status. Outcomes were carriage of or infection with antibiotic-resistant organisms. We used random-effects models to calculate the pooled prevalence of each outcome. The study protocol is registered with PROSPERO, number CRD42016043681. FINDINGS: We identified 2274 articles, of which 23 observational studies reporting on antibiotic resistance in 2319 migrants were included. The pooled prevalence of any AMR carriage or AMR infection in migrants was 25·4% (95% CI 19·1-31·8; I2 =98%), including meticillin-resistant Staphylococcus aureus (7·8%, 4·8-10·7; I2 =92%) and antibiotic-resistant Gram-negative bacteria (27·2%, 17·6-36·8; I2 =94%). The pooled prevalence of any AMR carriage or infection was higher in refugees and asylum seekers (33·0%, 18·3-47·6; I2 =98%) than in other migrant groups (6·6%, 1·8-11·3; I2 =92%). The pooled prevalence of antibiotic-resistant organisms was slightly higher in high-migrant community settings (33·1%, 11·1-55·1; I2 =96%) than in migrants in hospitals (24·3%, 16·1-32·6; I2 =98%). We did not find evidence of high rates of transmission of AMR from migrant to host populations. INTERPRETATION: Migrants are exposed to conditions favouring the emergence of drug resistance during transit and in host countries in Europe. Increased antibiotic resistance among refugees and asylum seekers and in high-migrant community settings (such as refugee camps and detention facilities) highlights the need for improved living conditions, access to health care, and initiatives to facilitate detection of and appropriate high-quality treatment for antibiotic-resistant infections during transit and in host countries. Protocols for the prevention and control of infection and for antibiotic surveillance need to be integrated in all aspects of health care, which should be accessible for all migrant groups, and should target determinants of AMR before, during, and after migration. FUNDING: UK National Institute for Health Research Imperial Biomedical Research Centre, Imperial College Healthcare Charity, the Wellcome Trust, and UK National Institute for Health Research Health Protection Research Unit in Healthcare-associated Infections and Antimictobial Resistance at Imperial College London
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