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Thierry Klein

Publications and source records attributed to Thierry Klein.

53 records · Page 3Linked to original sources
arXiv2013

New sensitivity analysis subordinated to a contrast

In a model of the form $Y=h(X_1,\ldots,X_d)$ where the goal is to estimate a parameter of the probability distribution of $Y$, we define new sensitivity indices which quantify the importance of each variable $X_i$ with respect to this parameter of interest. The aim of this paper is to define {\it goal oriented sensitivity indices} and we will show that Sobol indices are sensitivity indices associated to a particular characteristic of the distribution $Y$. We name the framework we present as {\it Goal Oriented Sensitivity Analysis} (GOSA).

stat.ME↗
arXiv2013

Statistical inference for Sobol pick freeze Monte Carlo method

Many mathematical models involve input parameters, which are not precisely known. Global sensitivity analysis aims to identify the parameters whose uncertainty has the largest impact on the variability of a quantity of interest (output of the model). One of the statistical tools used to quantify the influence of each input variable on the output is the Sobol sensitivity index. We consider the statistical estimation of this index from a finite sample of model outputs. We study asymptotic and non-asymptotic properties of two estimators of Sobol indices. These properties are applied to significance tests and estimation by confidence intervals.

stat.AP↗
arXiv2013

Asymptotic normality and efficiency of two Sobol index estimators

Many mathematical models involve input parameters, which are not precisely known. Global sensitivity analysis aims to identify the parameters whose uncertainty has the largest impact on the variability of a quantity of interest (output of the model). One of the statistical tools used to quantify the influence of each input variable on the output is the Sobol sensitivity index. We consider the statistical estimation of this index from a finite sample of model outputs: we present two estimators and state a central limit theorem for each. We show that one of these estimators has an optimal asymptotic variance. We also generalize our results to the case where the true output is not observable, and is replaced by a noisy version.

math.ST↗
arXiv2013

S-wave superconductivity probed by measuring magnetic penetration depth and lower critical field of MgCNi$_{3}$ single crystals

The magnetic penetration depth $λ$ has been measured in MgCNi$_{3}$ single crystals using both a high precision Tunnel Diode Oscillator technique (TDO) and Hall probe magnetization (HPM). In striking contrast to previous measurements in powders, $δλ$(T) deduced from TDO measurements increases exponentially at low temperature, clearly showing that the superconducting gap is fully open over the whole Fermi surface. An absolute value at zero temperature $λ(0)=230 $nm is found from the lower critical field measured by HPM. We also discuss the observed difference of the superfluid density deduced from both techniques. A possible explanation could be due to a systematic decrease of the critical temperature at the sample surface.

cond-mat.supr-con↗
arXiv2013

In-plane magnetic penetration depth in NbS$_{2}$

We report on the temperature dependence of the in plane magnetic penetration depth ($λ_{ab}$) and first penetration field ($H_f\propto 1/λ_{ab}^2(T)$ for $H\|c$) in 2H-NbS$_2$ single crystals. An exponential temperature dependence is clearly observed in $λ_{ab}$(T) at low temperature, signing the presence of a fully open superconducting gap. This compound is the only superconducting 2H-dichalcogenide which does not develop a charge density wave (CDW). However as previously observed in 2H-NbSe$_2$, this gap ($Δ_{1}=1.1\, k_{B}T_{c}$) is significantly smaller than the standard BCS weak coupling value. At higher temperature, a larger gap ($Δ_{2}=1.8\, k_{B}T_{c}$) has to be introduced to describe the data which are compatible with a two gap model. The superconducting gaps are hence very similar in NbS$_2$ and NbSe$_2$ and we show here that both of them open in the strongly coupled Nb tubular sheets independently of the presence of a CDW or not.

cond-mat.supr-con↗
arXiv2012

Modeling Network Coded TCP: Analysis of Throughput and Energy Cost

We analyze the performance of TCP and TCP with network coding (TCP/NC) in lossy networks. We build upon the framework introduced by Padhye et al. and characterize the throughput behavior of classical TCP and TCP/NC as a function of erasure probability, round-trip time, maximum window size, and duration of the connection. Our analytical results show that network coding masks random erasures from TCP, thus preventing TCP's performance degradation in lossy networks. It is further seen that TCP/NC has significant throughput gains over TCP. In addition, we show that TCP/NC may lead to cost reduction for wireless network providers while maintaining a certain quality of service to their users. We measure the cost in terms of number of base stations, which is highly correlated to the energy, capital, and operational costs of a network provider. We show that increasing the available bandwidth may not necessarily lead to increase in throughput, particularly in lossy networks in which TCP does not perform well. We show that using protocols such as TCP/NC, which are more resilient to erasures, may lead to a throughput commensurate the bandwidth dedicated to each user.

cs.NI↗
arXiv2012

Trade-off between cost and goodput in wireless: Replacing transmitters with coding

We study the cost of improving the goodput, or the useful data rate, to user in a wireless network. We measure the cost in terms of number of base stations, which is highly correlated to the energy cost as well as capital and operational costs of a network provider.We show that increasing the available bandwidth, or throughput, may not necessarily lead to increase in goodput, particularly in lossy wireless networks in which TCP does not perform well. As a result, much of the resources dedicated to the user may not translate to high goodput, resulting in an inefficient use of the network resources. We show that using protocols such as TCP/NC, which are more resilient to erasures and failures in the network, may lead to a goodput commensurate the throughput dedicated to each user. By increasing goodput, users' transactions are completed faster; thus, the resources dedicated to these users can be released to serve other requests or transactions. Consequently, we show that translating efficiently throughput to goodput may bring forth better connection to users while reducing the cost for the network providers.

cs.NI↗
arXiv2011

Intensity estimation of non-homogeneous Poisson processes from shifted trajectories

This paper considers the problem of adaptive estimation of a non-homogeneous intensity function from the observation of n independent Poisson processes having a common intensity that is randomly shifted for each observed trajectory. We show that estimating this intensity is a deconvolution problem for which the density of the random shifts plays the role of the convolution operator. In an asymptotic setting where the number n of observed trajectories tends to infinity, we derive upper and lower bounds for the minimax quadratic risk over Besov balls. Non-linear thresholding in a Meyer wavelet basis is used to derive an adaptive estimator of the intensity. The proposed estimator is shown to achieve a near-minimax rate of convergence. This rate depends both on the smoothness of the intensity function and the density of the random shifts, which makes a connection between the classical deconvolution problem in nonparametric statistics and the estimation of a mean intensity from the observations of independent Poisson processes.

math.ST↗
arXiv2010

Flux pinning in (1111) iron-pnictide superconducting crystals

Local magnetic measurements are used to quantitatively characterize heterogeneity and flux line pinning in PrFeAsO_1-y and NdFeAs(O,F) superconducting single crystals. In spite of spatial fluctuations of the critical current density on the macroscopic scale, it is shown that the major contribution comes from collective pinning of vortex lines by microscopic defects by the mean-free path fluctuation mechanism. The defect density extracted from experiment corresponds to the dopant atom density, which means that dopant atoms play an important role both in vortex pinning and in quasiparticle scattering. In the studied underdoped PrFeAsO_1-y and NdFeAs(O,F) crystals, there is a background of strong pinning, which we attribute to spatial variations of the dopant atom density on the scale of a few dozen to one hundred nm. These variations do not go beyond 5% - we therefore do not find any evidence for coexistence of the superconducting and the antiferromagnetic phase. The critical current density in sub-T fields is characterized by the presence of a peak effect, the location of which in the (B,T)-plane is consistent with an order-disorder transition of the vortex lattice.

cond-mat.supr-con↗
arXiv2010

AC Heat capacity and magnetocaloric effect measurements for pulsed magnetic fields

A new calorimeter for measurements of the AC heat capacity and magnetocaloric effect of small samples in pulsed magnetic fields is discussed for the exploration of thermal and thermodynamic properties at temperatures down to 2 K. We tested the method up to μ0H = 50 Tesla, but it could be extended to higher fields. For these measurements we used carefully calibrated bare chip Cernox\textregistered and RuO2 thermometers, and we present a comparison of their performance. The monotonic temperature and magnetic field dependences of the magneto resistance of RuO2 allow us to carry on precise thermometry with a precision as good as \pm 1mK at T = 2 K. To test the performance of our calorimeter, AC heat capacity and magnetocaloric effect for the spin-dimer compound Sr3Cr2O8 and the triangular lattice antiferromagnet RbFe(MoO4)2 are presented.

cond-mat.mtrl-sci↗
arXiv2007

Thermodynamics of a Heavy Ion-Irradiated Superconductor: the Zero-Field Transition

Specific heat measurements show that the introduction of amorphous columnar defects considerably affects the transition from the normal to the superconducting state in zero magnetic field. Experimental results are compared to numerical simulations of the 3D XY model for both the pure system and the system containing random columnar disorder. The numerics reproduce the salient features of experiment, showing in particular that the specific heat peak changes from cusp-like to smoothly rounded when columnar defects are added. By considering the specific heat critical exponent alpha, we argue that such behavior is consistent with recent numerical work [Vestergren et al., PRB 70, 054508 (2004)] showing that the introduction of columnar defects changes the universality class of the transition.

cond-mat.supr-con↗
arXiv2005

Thermodynamics of the vortex liquid in heavy ion-irradiated superconductors

It is shown that the large effect of heavy ion-irradiation on the thermodynamical properties of the anisotropic superconductor YBa$_{2}$Cu$_{3}$O$_{7-δ}$ extends well into the superconducting fluctuation regime. The presence of the induced amorphous columnar defects shifts the specific heat maximum at the normal-to-superconducting transition. This effect is similar to that recently put into evidence in cubic K$_{x}$Ba$_{1-x}$BiO$_{3}$ ($x \simeq 0.35$). In both compounds, vortex pinning manifests itself as a sharp angular dependence of the \em equilibrium \rm torque. In YBa$_{2}$Cu$_{3}$O$_{7-δ}$, pinning by the defects appears at the temperature $T_{C_{p}}^{max}$ of the specific heat maximum, well above the magnetic irreversibility line $T_{irr}(H)$. In isotropic K$_{x}$Ba$_{1-x}$BiO$_{3}$, the onset of the pinning-related torque anomaly tracks the onset of the specific heat anomaly and the irreversibility line. In YBa$_{2}$Cu$_{3}$O$_{7-δ}$, fluctuations of the amplitude of the order parameter (and not vortex line wandering) are ultimately responsible for the vanishing of pinning. In K$_{x}$Ba$_{1-x}$BiO$_{3}$, vortex pinning disappears only at the superconducting-to-normal transition. The results indicate that in both compounds, the pinning energy at the ``Bose glass'' transition is large with respect to the total free energy gain in the superconducting state. By implication, the mechanism of this latter transition should be reconsidered.

cond-mat.supr-con↗
arXiv2004

Martingales and Profile of Binary Search Trees

We are interested in the asymptotic analysis of the binary search tree (BST) under the random permutation model. Via an embedding in a continuous time model, we get new results, in particular the asymptotic behavior of the profile.

math.PR↗
arXiv2004

Dependence of the superconducting transition temperature on the doping level in single crystalline diamond films

Homoepitaxial diamond layers doped with boron in the 10^20-10^21 /cm3 range are shown to be type II superconductors with sharp transitions (~0.2K) at temperatures increasing from 0 to 2.1 K with boron contents. The critical concentration for the onset of superconductivity is about 5-7 10^20 /cm3, close to the metal-insulator transition. The H-T phase diagram has been obtained from transport and a.c. susceptibility measurements down to 300mK. These results bring new quantitative constraints on the theoretical models proposed for superconductivity in diamond.

cond-mat.supr-con↗