Large Biaxial Compressive Strain Tuning of Neutral and Charged Excitons in Single-Layer Transition Metal Dichalcogenides

The absorption and emission of light in single-layer transition metal dichalcogenides are governed by the formation of excitonic quasiparticles. Strain provides a powerful technique to tune the optoelectronic properties of two-dimensional materials and thus to adjust their exciton energies. The effects of large compressive strain in the optical spectrum of two-dimensional (2D) semiconductors remain rather unexplored compared to those of tensile strain, mainly due to experimental constraints. Here, we induced large, uniform, biaxial compressive strain (∼1.2%) by cooling, down to 10 K, single-layer WS2, MoS2, WSe2, and MoSe2 deposited on polycarbonate substrates. We observed a significant strain-induced modulation of neutral exciton energies, with blue shifts up to 160 meV, larger than in any previous experiments. Our results indicate a remarkably efficient transfer of compressive strain, demonstrated by gauge factor values exceeding previous results and approaching theoretical expectations. At low temperatures, we investigated the effect of compressive strain on the resonances associated with the formation of charged excitons. In WS2, a notable reduction of gauge factors for charged compared to neutral excitons suggests an increase in their binding energy, which likely results from the effects of strain added to the influence of the polymeric substrate.The authors acknowledge funding from the Generalitat Valenciana through grants IDIFEDER/2020/005 and IDIFEDER/2021/016 and support from the Plan Gen-T of Excellence for M.R.C (CideGenT2018004) and from the Spanish MCINN through grants PLASTOP PID2020-119124RB-I00, TED2021-131641B-C43, PID2020-115566RB-I00, TED2021-132267B-I00, and PID2020-112811GB-I00. This work was funded by the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (grant agreement no. 755655, ERC-StG 2017 project 2D-TOPSENSE). The authors also acknowledge funding from the EU FLAG-ERA project To2Dox (JTC-2019-009) and the Comunidad de Madrid through the CAIRO-CM project (Y2020/NMT-6661). H.L. acknowledges support from China Scholarship Council (CSC) under grant no. 201907040070. D.G.-M. thanks the Maria Zambrano Program at the University of Alicante founded by the European Union-Next Generation EU. R.D’A. acknowledges support from the Grant No. IT1453- 22 “Grupos Consolidados UPV/EHU del Gobierno Vasco”

Serum PCB levels in a representative sample of the Spanish adult population: the BIOAMBIENT.ES project

This manuscript presents the levels of six indicator polychlorinated biphenyl (PCB) congeners (IUPAC nos. 28, 52, 101, 138, 153 and 180) in the serum of 1880 individuals from a representative sample of the Spanish working population recruited between March 2009 and July 2010. Three out of the six PCBs studied (180, 153 and 138) were quantified in more than 99% of participants. PCB 180 was the highest contributor, followed by PCBs 153 and 138, with relative abundances of 42.6%, 33.2% and 24.2%, respectively. In contrast, PCBs 28 and 52 were detected in only 1% of samples, whereas PCB 101 was detectable in 6% of samples. The geometric mean (GM) for ΣPCBs138/153/180 was 135.4 ng/g lipid (95% CI: 121.3-151.2 ng/g lipid) and the 95th percentile was 482.2 ng/g lipid. Men had higher PCB blood concentrations than women (GMs 138.9 and 129.9 ng/g lipid respectively). As expected, serum PCB levels increased with age and frequency of fish consumption, particularly in those participants younger than 30 years of age. The highest levels we found were for participants from the Basque Country, whereas the lowest concentrations were found for those from the Canary Islands. The Spanish population studied herein had similar levels to those found previously in Greece and southern Italy, lower levels than those in France and central Europe, and higher PCB levels than those in the USA, Canada and New Zealand. This paper provides the first baseline information regarding PCB exposure in the Spanish adult population on a national scale. The results will allow us to establish reference levels, follow temporal trends and identify high-exposure groups, as well as monitor implementation of the Stockholm Convention in Spain.This work was funded as part of a research agreement between the Spanish Ministry of Agriculture, Food and the Environment n° EG042007 and the Institute of Health Carlos III (project nos. SEG 1251/07 and 1210/10). The authors would like to thank S. González for the technical assistance, F. Cutanda for the helpful advice, and Silvia Gómez; the volunteers of BIOAMBIENT.ES and healthcare staff from the Societies for Prevention of IBERMUTUAMUR, MUTUALIA, MCPREVENCIÓN, MUGATRA, UNIMAT PREVENCIÓN, and PREVIMAC

Angular moments of the decay Λb 0 → Λμ + μ − at low hadronic recoil

An analysis of the angular distribution of the decay Λ0 b → Λµ +µ − is presented, using data collected with the LHCb detector between 2011 and 2016 and corresponding to an integrated luminosity of approximately 5 fb−1 . Angular observables are determined using a moment analysis of the angular distribution at low hadronic recoil, corresponding to the dimuon invariant mass squared range 15 < q2 < 20 GeV2/c4 . The full basis of observables is measured for the first time. The lepton-side, hadron-side and combined forward-backward asymmetries of the decay are determined to b

Observation of the decay Bs0→D¯0K+K−

The first observation of the B0 s → D¯ 0KþK− decay is reported, together with the most precise branching fraction measurement of the mode B0 → D¯ 0KþK−. The results are obtained from an analysis of pp collision data corresponding to an integrated luminosity of 3.0 fb−1. The data were collected with the LHCb detector at center-of-mass energies of 7 and 8 TeV. The branching fraction of the B0 → D¯ 0KþK− decay is measured relative to that of the decay B0 → D¯ 0πþπ− to be BðB0→D¯ 0KþK−Þ BðB0→D¯ 0πþπ−Þ ¼ ð6.9 0.4 0.3Þ%, where the first uncertainty is statistical and the second is systematic. The measured branching fraction of the B0 s → D¯ 0KþK− decay mode relative to that of the corresponding B0 decay is BðB0 s→D¯ 0KþK−Þ BðB0→D¯ 0KþK−Þ ¼ ð93.0 8.9 6.9Þ%. Using the known branching fraction of B0 → D¯ 0πþπ−, the values of BðB0 →D¯ 0KþK−Þ¼ð6.10.40.30.3Þ×10−5 and BðB0 s →D¯ 0KþK−Þ¼ð5.70.50.40.5Þ×10−5 are obtained, where the third uncertainties arise from the branching fraction of the decay modes B0 → D¯ 0πþπ− and B0 → D¯ 0KþK−, respectively

Measurement of Z → τ + τ − production in proton-proton collisions at √s=8 TeV

A measurement of Z → τ +τ − production cross-section is presented using data, corresponding to an integrated luminosity of 2 fb−1 , from pp collisions at √ s = 8 TeV collected by the LHCb experiment. The τ +τ − candidates are reconstructed in final states with the first tau lepton decaying leptonically, and the second decaying either leptonically or to one or three charged hadrons. The production cross-section is measured for Z bosons with invariant mass between 60 and 120 GeV/c2 , which decay to tau leptons with transverse momenta greater than 20 GeV/c and pseudorapidities between 2.0 and 4.5. The crosssection is determined to be σpp→Z→τ+τ− = 95.8 ± 2.1 ± 4.6 ± 0.2 ± 1.1 pb, where the first uncertainty is statistical, the second is systematic, the third is due to the LHC beam energy uncertainty, and the fourth to the integrated luminosity uncertainty. This result is compatible with NNLO Standard model predictions. The ratio of the cross-sections for Z → τ +τ − to Z → µ +µ − (Z → e +e −), determined to be 1.01 ± 0.05 (1.02 ± 0.06), is consistent with the lepton-universality hypothesis in Z decays

Search for the rare decay Λc+ →pμ+μ-

The flavor-changing neutral-current (FCNC) decay Λþ c → pμþμ− (inclusion of the charge-conjugate processes is implied throughout) is expected to be heavily suppressed in the Standard Model (SM) by the Glashow-IliopoulosMaiani mechanism [1]. The branching fractions for shortdistance c → ulþl− contributions to the transition are expected to be of Oð10−9Þ in the SM but can be enhanced by effects beyond the SM. However, long-distance contributions proceeding via a tree-level amplitude, with an intermediate meson resonance decaying into a dimuon pair [2,3], can increase the branching fraction up to Oð10−6Þ [4]. The short-distance and hadronic contributions can be separated by splitting the data set into relevant regions of dimuon mass. The Λþ c → pμþμ− decay has been previously searched for by the BABAR Collaboration [5], yielding 11.1 5.0 2.5 events and an upper limit on the branching fraction of 4.4 × 10−5 at 90% C.L. Similar FCNC transitions for the b-quark system (b → slþl−) exhibit a pattern of consistent deviations from the current SM predictions both in branching fractions [6] and angular observables [7], with the combined significance reaching 4 to 5 standard deviations [8,9]. Processes involving c → ulþl− transitions are far less explored at both the experimental and theoretical levels, which makes such measurements desirable. Similar analyses of the D system have reported evidence for the longdistance contribution [10]; however, the short-distance contributions have not been established [11]

Ultra-broad spectral photo-response in FePS3 air-stable devices

Van der Waals materials with narrow energy gaps and efficient response over a broadband optical spectral range are key to widen the energy window of nanoscale optoelectronic devices. Here, we characterize FePS3 as an appealing narrow-gap p-type semiconductor with an efficient broadband photo-response, a high refractive index, and a remarkable resilience against air and light exposure. To enable fast prototyping, we provide a straightforward guideline to determine the thickness of few-layered FePS3 nanosheets extracted from the optical transmission characteristics of several flakes. The analysis of the electrical photo-response of FePS3 devices as a function of the excitation energy confirms a narrow gap suitable for near IR detection (1.23 eV) and, more interestingly, reveals a broad spectral responsivity up to the ultraviolet region. The experimental estimate for the gap energy is corroborated by ab-initio calculations. An analysis of photocurrent as a function of gate voltage and incident power reveals a photo-response dominated by photogating effects. Finally, aging studies of FePS3 nanosheets under ambient conditions show a limited reactivity of the outermost layers of flakes in long exposures to air

A measurement of the CP asymmetry difference between Λc + → pK − K + and pπ−π+ decays

The difference between the CP asymmetries in the decays Λ + c → pK−K+ and Λ + c → pπ−π + is presented. Proton-proton collision data taken at centre-of-mass energies of 7 and 8 TeV collected by the LHCb detector in 2011 and 2012 are used, corresponding to an integrated luminosity of 3 fb−1 . The Λ + c candidates are reconstructed as part of the Λ 0 b → Λ + c µ −X decay chain. In order to maximize the cancellation of production and detection asymmetries in the difference, the final-state kinematic distributions of the two samples are aligned by applying phase-space-dependent weights to the Λ + c → pπ−π + sample. This alters the definition of the integrated CP asymmetry to A wgt CP (pπ−π +). Both samples are corrected for reconstruction and selection efficiencies across the five-dimensional Λ + c decay phase space. The difference in CP asymmetries is found to be ∆A wgt CP = ACP (pK−K+) − A wgt CP (pπ−π +) = (0.30 ± 0.91 ± 0.61) %, where the first uncertainty is statistical and the second is systemati