arXiv subjects

R. Tourbot

Publications and source records attributed to R. Tourbot.

15 recordsLinked to original sources

arXiv2017

Unifying different interpretations of the nonlinear response in glass-forming liquids

This work aims at reconsidering several interpretations coexisting in the recent literature concerning non-linear susceptibilities in supercooled liquids. We present experimental results on glycerol and propylene carbonate showing that the three independent cubic susceptibilities have very similar frequency and temperature dependences, both for their amplitudes and phases. This strongly suggests a unique physical mechanism responsible for the growth of these non-linear susceptibilities. We show that the framework proposed by two of us [BB, Phys. Rev. B 72, 064204 (2005)], where the growth of non-linear susceptibilities is intimately related to the growth of "glassy domains", accounts for all the salient experimental features. We then review several complementary and/or alternative models, and show that the notion of cooperatively rearranging glassy domains is a key (implicit or explicit) ingredient to all of them. This paves the way for future experiments which should deepen our understanding of glasses.

cond-mat.dis-nn↗

arXiv2016

Fifth-order susceptibility unveils growth of thermodynamic amorphous order in glass-formers

Glasses are ubiquitous in daily life and technology. However the microscopic mechanisms generating this state of matter remain subject to debate: Glasses are considered either as merely hyper-viscous liquids or as resulting from a genuine thermodynamic phase transition towards a rigid state. We show that third- and fifth-order susceptibilities provide a definite answer to this longstanding controversy. Performing the corresponding high-precision nonlinear dielectric experiments for supercooled glycerol and propylene carbonate, we find strong support for theories based upon thermodynamic amorphous order. Moreover, when lowering temperature, we find that the growing transient domains are compact - that is their fractal dimension d_f = 3. The glass transition may thus represent a class of critical phenomena different from canonical second-order phase transitions for which d_f < 3.

cond-mat.dis-nn↗

arXiv2009

A method for measuring the nonlinear response in dielectric spectroscopy through third harmonics detection

We present a high sensitivity method allowing the measurement of the non linear dielectric susceptibility of an insulating material at finite frequency. It has been developped for the study of dynamic heterogeneities in supercooled liquids using dielectric spectroscopy at frequencies 0.05 Hz < f < 30000 Hz . It relies on the measurement of the third harmonics component of the current flowing out of a capacitor. We first show that standard laboratory electronics (amplifiers and voltage sources) nonlinearities lead to limits on the third harmonics measurements that preclude reaching the level needed by our physical goal, a ratio of the third harmonics to the fundamental signal about 7 orders of magnitude lower than 1. We show that reaching such a sensitivity needs a method able to get rid of the nonlinear contributions both of the measuring device (lock-in amplifier) and of the excitation voltage source. A bridge using two sources fulfills only the first of these two requirements, but allows to measure the nonlinearities of the sources. Our final method is based on a bridge with two plane capacitors characterized by different dielectric layer thicknesses. It gets rid of the source and amplifier nonlinearities because in spite of a strong frequency dependence of the capacitors impedance, it is equilibrated at any frequency. We present the first measurements of the physical nonlinear response using our method. Two extensions of the method are suggested.

cond-mat.mtrl-sci↗

arXiv2009

Magnetization and magneto-electric effect in La-doped BiFeO3

We report on magnetisation and magneto-capacitance measurements in the Bi1-xLaxFeO3 series for 0 < x < 0.15. We confirm that doping with La reduces the threshold magnetic field Hc for cancelling the magnetic spiral phase, and we show that Hc decreases as the La content increases up to x=0.15, which is the highest concentration for maintaining the non-centrosymmetric rhombohedral structure of BiFeO3. Measurements of the dielectric constant as a function of magnetic field in the series also show a maximum magneto-capacitance for x=0.15.

cond-mat.str-el↗

arXiv2006

Nernst effect and disorder in the normal state of high-T_{c} cuprates

We have studied the influence of disorder induced by electron irradiation on the Nernst effect in optimally and underdoped YBa2Cu3O(7-d) single crystals. The fluctuation regime above T_{c} expands significantly with disorder, indicating that the T_{c} decrease is partly due to the induced loss of phase coherence. In pure crystals the temperature extension of the Nernst signal is found to be narrow whatever the hole doping, contrary to data reported in the low-T_{c} cuprates families. Our results show that the presence of "intrinsic" disorder can explain the enhanced range of Nernst signal found in the pseudogap phase of the latter compounds.

cond-mat.supr-con↗

arXiv2005

1/f noise in a dilute GaAs two-dimensional hole system in the insulating phase

We have measured the resistance and the 1/f resistance noise of a two-dimensional low density hole system in a high mobility GaAs quantum well at low temperature. At densities lower than the metal-insulator transition one, the temperature dependence of the resistance is either power-like or simply activated. The noise decreases when the temperature or the density increase. These results contradict the standard description of independent particles in the strong localization regime. On the contrary, they agree with the percolation picture suggested by higher density results. The physical nature of the system could be a mixture of a conducting and an insulating phase. We compare our results with those of composite thin films.

cond-mat.dis-nn↗

arXiv2005

1/f Noise In Low Density Two-Dimensional Hole Systems In GaAs

Two-dimensional electron or hole systems in semiconductors offer the unique opportunity to investigate the physics of strongly interacting fermions. We have measured the 1/f resistance noise of two-dimensional hole systems in high mobility GaAs quantum wells, at densities below that of the metal-insulator transition (MIT) at zero magnetic field. Two techniques voltage and current fluctuations were used. The normalized noise power SR/R2 increases strongly when the hole density or the temperature are decreased. The temperature dependence is steeper at the lowest densities. This contradicts the predictions of the modulation approach in the strong localization hopping transport regime. The hypothesis of a second order phase transition or percolation transition at a density below that of the MIT is thus reinforced.

cond-mat.dis-nn↗

arXiv2004

Resistance noise scaling in a 2D system in GaAs

The 1/f resistance noise of a two-dimensional (2D) hole system in a high mobility GaAs quantum well has been measured on both sides of the 2D metal-insulator transition (MIT) at zero magnetic field (B=0), and deep in the insulating regime. The two measurement methods used are described: I or V fixed, and measurement of resp. V or I fluctuations. The normalized noise magnitude SR/R^2 increases strongly when the hole density is decreased, and its temperature (T) dependence goes from a slight increase with T at the largest densities, to a strong decrease at low density. We find that the noise magnitude scales with the resistance, SR /R^2 ~ R^2.4. Such a scaling is expected for a second order phase transition or a percolation transition. The possible presence of such a transition is investigated by studying the dependence of the conductivity as a function of the density. This dependence is consistent with a critical behavior close to a critical density p* lower than the usual MIT critical density pc.

cond-mat.str-el↗

arXiv2003

Resistance Noise Scaling in a Dilute Two-Dimensional Hole System in GaAs

We have measured the resistance noise of a two-dimensional (2D)hole system in a high mobility GaAs quantum well, around the 2D metal-insulator transition (MIT) at zero magnetic field. The normalized noise power $S_R/R^2$ increases strongly when the hole density p_s is decreased, increases slightly with temperature (T) at the largest densities, and decreases strongly with T at low p_s. The noise scales with the resistance, $S_R/R^2 \sim R^{2.4}$, as for a second order phase transition such as a percolation transition. The p_s dependence of the conductivity is consistent with a critical behavior for such a transition, near a density p* which is lower than the observed MIT critical density p_c.

cond-mat.str-el↗

arXiv2003

Respective influences of pair breaking and phase fluctuations in disordered high Tc superconductors

Electron irradiation has been used to introduce point defects in a controlled way in the CuO2 planes of underdoped and optimally doped YBCO. This technique allows us to perform very accurate measurements of Tc and of the residual resistivity in a wide range of defect contents xd down to Tc=0. The Tc decrease does not follow the variation expected from pair breaking theories. The evolutions of Tc and of the transition width with xd emphasize the importance of phase fluctuations, at least for the highly damaged regime. These results open new questions about the evolution of the defect induced Tc depression over the phase diagram of the cuprates

cond-mat.supr-con↗

arXiv2001

Heating of Two-Dimensional Holes in SiGe and the B = 0 Metal-Insulator Transition

We study the resistivity vs. electric field dependence $\rho(E)$ of a 2D hole system in SiGe close to the B=0 metal-insulator transition. Using $\rho$ as a ``thermometer'' to obtain the effective temperature of the holes $T_e(E)$, we find that the $\rho(E)$ dependence can be attributed to hole heating. The hole-phonon coupling involves weakly screened piezoelectric and deformation potentials compatible with previous measurements. The damping of the Shubnikov-de Haas oscillations gives the same $T_e$ values. Thus the $\rho(E)$ dependence and the $E$-field ``scaling'' do not provide additional evidence for a quantum phase transition (QPT). We discuss how to study, in general, true $E$-field scaling and extract the ratio of the QPT characteristic lengths.

cond-mat↗

arXiv2001

Disorder and transport in cuprates: weak localization and magnetic contributions

We report resistivity measurements in underdoped YBa$_{2}$Cu$_{3}$O$_{6.6} $and overdoped Tl$_{2}$Ba$_{2}$CuO$_{6+x}$ single crystals in which the concentration of defects in the CuO$_{2}$ planes is controlled by electron irradiation. Low $T$ upturns of the resistivity are observed in both cases for large defect content. In the Tl compound the decrease of conductivity scales as expected from weak localization theory. On the contrary in YBa$_{2}$Cu$_{3}$O$_{6.6}$ the much larger low T contribution to the resistivity is proportional to the defect content and might then be associated to a Kondo like spin flip scattering term. This would be consistent with the results on the magnetic properties induced by spinless defects.

cond-mat.supr-con↗

arXiv2000

Background discrimination capabilities of a heat and ionization germanium cryogenic detector

The discrimination capabilities of a 70 g heat and ionization Ge bolometer are studied. This first prototype has been used by the EDELWEISS Dark Matter experiment, installed in the Laboratoire Souterrain de Modane, for direct detection of WIMPs. Gamma and neutron calibrations demonstrate that this type of detector is able to reject more than 99.6% of the background while retaining 95% of the signal, provided that the background events distribution is not biased towards the surface of the Ge crystal. However, the 1.17 kg.day of data taken in a relatively important radioactive environment show an extra population slightly overlapping the signal. This background is likely due to interactions of low energy photons or electrons near the surface of the crystal, and is somewhat reduced by applying a higher charge-collecting inverse bias voltage (-6 V instead of -2 V) to the Ge diode. Despite this contamination, more than 98% of the background can be rejected while retaining 50% of the signal. This yields a conservative upper limit of 0.7 event.day^{-1}.kg^{-1}.keV^{-1}_{recoil} at 90% confidence level in the 15-45 keV recoil energy interval; the present sensitivity appears to be limited by the fast ambient neutrons. Upgrades in progress on the installation are summarized.

astro-ph↗

arXiv1999

Non-ohmic hopping transport in a-YSi: from isotropic to directed percolation

Variable range hopping transport has been investigated in amorphous YSi from 30 mK to 2 K as a function of the temperature T and electric field F. For F=0, where conduction is along sinuous paths (isotropic percolation), the conductance G depends on T according to an Efros-Shklovskii law. The nonlinear I-V's were studied up to very high fields for which G no longer depends on T (directed percolation : current paths along F). We show that heating effects are negligible. Then, we show that for low F values, ln(G(F,T))=ln(G(0,T))-eFL/kT. The order of magnitude (5-10 nm) and the T dependence of L agree with theoretical predictions. From L, we extract the dielectric constant. The very high electric field data do not agree with the theoretical predictions, which could be due to tunneling across the mobility edge. For intermediate electric fields our data evidence the influence of the straightening of the paths (transition from isotropic to directed percolation). The statistical properties of the segments of the paths where the current flows against the electrical force are extracted for the first time.

cond-mat.dis-nn↗

arXiv1998

Status of the EDELWEISS Experiment

The status of the EDELWEISS experiment (underground dark matter search with heat-ionisation bolometers) is reviewed. Auspicious results achieved with a prototype 70 g Ge heat-ionisation detector under a 2 V reverse bias tension are discussed. Based on gamma and neutron calibrations, a best-case rejection factor, over the 15-45 keV range, of 99.7 % for gammas, with an acceptance of 94 % for neutrons, is presented first. Some operational results of physical interest obtained under poor low radioactivity conditions follow. They include a raw event rate of around 30 events/day/kg/keV over the same energy range, and, after rejection of part of the background, lead to a conservative upper limit on the signal of approximately 1.6 events/day/kg/keV at a 90 % confidence level. Performance degrading surface effects of the detector are speculated upon; and planned upgrades are summarized.

astro-ph↗