NMR imaging of the soliton lattice profile in the spin-Peierls compound CuGeO_3

In the spin-Peierls compound CuGeO$_{3}$, the commensurate-incommensurate transition concerning the modulation of atomic position and the local spin-polarization is fully monitored at T=0 by the application of an external magnetic field ($H$) above a threshold value $H_{c}\simeq$ 13 Tesla. The solitonic profile of the spin-polarization, as well as its absolute magnitude, has been precisely imaged from $^{65}Cu$ NMR lineshapes obtained for $h=(H-H_{c})/H_{c}$ varying from 0.0015 to 2. This offers a unique possibility to test quantitatively the various numerical and analytical methods developed to solve a generic Hamiltonian in 1-D physics, namely strongly interacting fermions in presence of electron-phonon coupling at arbitrary band filling.Comment: 3 pages, 4 eps figures, RevTeX, submitted to Physical Review Lette

Zero Temperature Phase Transition in Spin-ladders: Phase Diagram and Dynamical studies of Cu(Hp)Cl

In a magnetic field, spin-ladders undergo two zero-temperature phase transitions at the critical fields Hc1 and Hc2. An experimental review of static and dynamical properties of spin-ladders close to these critical points is presented. The scaling functions, universal to all quantum critical points in one-dimension, are extracted from (a) the thermodynamic quantities (magnetization) and (b) the dynamical functions (NMR relaxation). A simple mapping of strongly coupled spin ladders in a magnetic field on the exactly solvable XXZ model enables to make detailed fits and gives an overall understanding of a broad class of quantum magnets in their gapless phase (between Hc1 and Hc2). In this phase, the low temperature divergence of the NMR relaxation demonstrates its Luttinger liquid nature as well as the novel quantum critical regime at higher temperature. The general behaviour close these quantum critical points can be tied to known models of quantum magnetism.Comment: few corrections made, 15 pages, to be published in European Journal of Physics

Atomic and Electronic Structure of a Rashba pp-nn Junction at the BiTeI Surface

The non-centrosymmetric semiconductor BiTeI exhibits two distinct surface terminations that support spin-split Rashba surface states. Their ambipolarity can be exploited for creating spin-polarized $p$-$n$ junctions at the boundaries between domains with different surface terminations. We use scanning tunneling microscopy/spectroscopy (STM/STS) to locate such junctions and investigate their atomic and electronic properties. The Te- and I-terminated surfaces are identified owing to their distinct chemical reactivity, and an apparent height mismatch of electronic origin. The Rashba surface states are revealed in the STS spectra by the onset of a van Hove singularity at the band edge. Eventually, an electronic depletion is found on interfacial Te atoms, consistent with the formation of a space charge area in typical $p$-$n$ junctions.Comment: 5 pages, 4 figure

Giant alkali-metal-induced lattice relaxation as the driving force of the insulating phase of alkali-metal/Si(111):B

Ab initio density-functional theory calculations, photoemission spectroscopy (PES), scanning tunneling microscopy, and spectroscopy (STM, STS) have been used to solve the 2√3 x 2√3R30 surface reconstruction observed previously by LEED on 0.5 ML K/Si:B. A large K-induced vertical lattice relaxation occurring only for 3/4 of Si adatoms is shown to quantitatively explain both the chemical shift of 1.14 eV and the ratio 1/3 measured on the two distinct B 1s core levels. A gap is observed between valence and conduction surface bands by ARPES and STS which is shown to have mainly a Si-B character. Finally, the calculated STM images agree with our experimental results. This work solves the controversy about the origin of the insulating ground state of alkali-metal/Si(111):B semiconducting interfaces which were believed previously to be related to many-body effectsThis work has received the financial support of the French ANR SURMOTT program (ANR-09-BLAN- 0210-01) and the Spanish MICIIN under Project No. FIS2010-1604

Charge Order Driven spin-Peierls Transition in NaV2O5

We conclude from 23Na and 51V NMR measurements in NaxV2O5(x=0.996) a charge ordering transition starting at T=37 K and preceding the lattice distortion and the formation of a spin gap Delta=106 K at Tc=34.7 K. Above Tc, only a single Na site is observed in agreement with the Pmmn space group of this first 1/4-filled ladder system. Below Tc=34.7 K, this line evolves into eight distinct 23Na quadrupolar split lines, which evidences a lattice distortion with, at least, a doubling of the unit cell in the (a,b) plane. A model for this unique transition implying both charge density wave and spin-Peierls order is discussed.Comment: 4 pages, 3 figures, submitted to Phys. Rev. Let

Geriatric Patient Safety Indicators Based on Linked Administrative Health Data to Assess Anticoagulant-Related Thromboembolic and Hemorrhagic Adverse Events in Older Inpatients: A Study Proposal.

Frail older people with multiple interacting conditions, polypharmacy, and complex care needs are particularly exposed to health care-related adverse events. Among these, anticoagulant-related thromboembolic and hemorrhagic events are particularly frequent and serious in older inpatients. The growing use of anticoagulants in this population and their substantial risk of toxicity and inefficacy have therefore become an important patient safety and public health concern worldwide. Anticoagulant-related adverse events and the quality of anticoagulation management should thus be routinely assessed to improve patient safety in vulnerable older inpatients. This project aims to develop and validate a set of outcome and process indicators based on linked administrative health data (ie, insurance claims data linked to hospital discharge data) assessing older inpatient safety related to anticoagulation in both Switzerland and France, and enabling comparisons across time and among hospitals, health territories, and countries. Geriatric patient safety indicators (GPSIs) will assess anticoagulant-related adverse events. Geriatric quality indicators (GQIs) will evaluate the management of anticoagulants for the prevention and treatment of arterial or venous thromboembolism in older inpatients. GPSIs will measure cumulative incidences of thromboembolic and bleeding adverse events based on hospital discharge data linked to insurance claims data. Using linked administrative health data will improve GPSI risk adjustment on patients' conditions that are present at admission and will capture in-hospital and postdischarge adverse events. GQIs will estimate the proportion of index hospital stays resulting in recommended anticoagulation at discharge and up to various time frames based on the same electronic health data. The GPSI and GQI development and validation process will comprise 6 stages: (1) selection and specification of candidate indicators, (2) definition of administrative data-based algorithms, (3) empirical measurement of indicators using linked administrative health data, (4) validation of indicators, (5) analyses of geographic and temporal variations for reliable and valid indicators, and (6) data visualization. Study populations will consist of 166,670 Swiss and 5,902,037 French residents aged 65 years and older admitted to an acute care hospital at least once during the 2012-2014 period and insured for at least 1 year before admission and 1 year after discharge. We will extract Swiss data from the Helsana Group data warehouse and French data from the national health insurance information system (SNIIR-AM). The study has been approved by Swiss and French ethics committees and regulatory organizations for data protection. Validated GPSIs and GQIs should help support and drive quality and safety improvement in older inpatients, inform health care stakeholders, and enable international comparisons. We discuss several limitations relating to the representativeness of study populations, accuracy of administrative health data, methods used for GPSI criterion validity assessment, and potential confounding bias in comparisons based on GQIs, and we address these limitations to strengthen study feasibility and validity

Possible Localized Modes in the Uniform Quantum Heisenberg Chains of Sr2CuO3

A model of mobile-bond defects is tentatively proposed to analyze the "anomalies" observed on the NMR spectrum of the quantum Heisenberg chains of Sr2CuO3. A bond-defect is a local change in the exchange coupling. It results in a local alternating magnetization (LAM), which when the defect moves, creates a flipping process of the local field seen by each nuclear spin. At low temperature, when the overlap of the LAM becomes large, the defects form a periodic structure, which extends over almost all the chains. In that regime, the density of bond-defects decreases linearly with T.Comment: 4 pages + 3 figures. To appear in Physical Review

Ab initio evaluation of the charge-ordering in α′NaV2O5\alpha^\prime NaV_2O_5

We report {\it ab initio} calculations of the charge ordering in $\alpha^\prime NaV_2O_5$ using large configurations interaction methods on embedded fragments. Our major result is that the $2p_y$ electrons of the bridging oxygen of the rungs present a very strong magnetic character and should thus be explicitly considered in any relevant effective model. The most striking consequence of this result is that the spin and charge ordering differ substantially, as differ the experimental results depending on whether they are sensitive to the spin or charge density.Comment: 4 page

A 25,000 years climate record from the East African equator: Half-precessional climate forcing and the history of temperature and hydrological change

Particularly in tropical and subtropical regions, high-resolution climate records have demonstrated major variability in the hydrological cycle at orbital, millennial and sub-millennial time scales in the last 25,000 years. Geographical patterns of past climate change (from the equator to both poles, and among continents and ocean basins at similar latitude) at these various time scales hold the key to understanding the climate-dynamical processes governing them, and to resolution of longstanding questions about the relative importance of tropical and high-latitude climate dynamics in translating external climate-forcing mechanisms into regional climate variability. Currently, dynamical analysis of past tropical climate variability is being hampered by the fragmentary or poorly dated nature of available records from key continental regions, debate to what extent marine records are representative for climate history on the continents, and uncertainty about whether traditional isotopic tracers applied to tropical continental records mainly reflect temperature or hydrological change. What has been missing is a continuous, high-resolution climate record from the tropics that adequately separates the evolution of temperature and hydrological change, and covers the period from the Last Glacial Maximum until the present with sufficient age control to establish supra-regional phase relationships in past climate anomalies at millennial and century time scales.In this context, the EuroCLIMATE project CHALLACEA here presents a reconstruction of climate history on the East African equator, based on multiple proxy-indicator analyses in the sediment record of a permanently stratified crater lake (Lake Challa, a 4.2 km2, 92 m deep crater lake on the lower East slope of Mt. Kilimanjaro, Tanzania) with stable physical limnology and sedimentation dynamics over the past 25,000 years. This has resulted in a unique combination of high temporal resolution, excellent radiometric (210Pb, 14C) age control, and confidence that the recording parameters of the climatic proxies (i.e. the relationship between climate change and its proxy signals extracted from the sediment record) have remained constant through time. The equatorial (3° S) location of our study site in East Africa, where seasonal migration of the Intertropical Convergence Zone spans the widest latitude range, provides unique information on how varying rainfall contributions from the Indian Ocean monsoons have shaped the region’s climate history. The detailed reconstruction of the temperature and moisture-balance history of equatorial East Africa from before the Last Glacial Maximum to the present uniquely weaves together tropical climate variability at orbital, millennial and century time scales. The temporal pattern of reconstructed climate changes bears the clear signature of half-precessional insolation forcing of tropical monsoon dynamics on the East African equator, modified by high southern latitude influence on the timing of post-glacial temperature rise, and by high northern latitude influence on tropical hydrological variability at millennial and century time scales