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F. Bouquet

Publications and source records attributed to F. Bouquet.

At least 19 recordsLinked to original sources
arXiv2021

Enhance your smartphone with a Bluetooth Arduino nano board

Using smartphones in experimental physics teachings offers many advantages in term of engagement, pedagogy and flexibility. But it presents the drawbacks of possibly endangering the device and also facing the heterogeneity of available sensors on different smartphones. We present a low-cost alternative that preserves the advantages of smartphones: using a microcontroller equipped with a large variety of sensors that transmits data to a smartphone using Bluetooth Low-Energy protocol. This device can be lent to students with little risks and used to perform a wide range of experiments. It opens the way to new types of physics teachings.

physics.ed-ph↗
arXiv2019

Low-Cost Experiments with Everyday Objects for Homework Assignments

We describe four classical undergraduate physics experiments that were done with everyday objects and low-cost sensors: mechanical oscillations, transmittance of light through a slab of matter, beam deformation under load, and thermal relaxation due to heat loss. We used these experiments to train students for experimental homework projects but they could be used and expanded in a variety of contexts: lecture demonstrations, low cost students' labs, science projects, distance learning courses...

physics.ed-ph↗
arXiv2018

Temperature-dependent transport measurements with Arduino

The current performances of single-board microcontrollers render them attractive not only for basic applications but also for more elaborate projects, amongst which physics teaching or research. In this article, we show how temperature-dependent transport measurements can be performed using an Arduino microcontroller, from cryogenic temperatures up to room temperature or above. We focus on two of the main issues for this type of experiments: the determination of the sample temperature and the measurement of its resistance. We also detail two student-led experiments: evidencing the magnetocaloric effect in Gadolinium and measuring the resistive transition of a high critical temperature superconductor.

physics.ed-ph↗
arXiv2016

Project-based physics labs using low-cost open-source hardware

We describe a project-based physics lab, which we proposed to third-year university students. Theses labs are based on new open-source low-cost equipment (Arduino microcontrollers and compatible sensors). Students are given complete autonomy: they develop their own experimental setup and study the physics topic of their choice. The goal of these projects is to let students discover the reality of experimental physics. Technical specifications of the acquisition material and case studies are presented for practical implementation in other universities.

physics.ed-ph↗
arXiv2014

Spin dynamics and disorder effects in the S=1/2 kagome Heisenberg spin liquid phase of kapellasite

We report $^{35}$Cl NMR, ESR, $μ$SR and specific heat measurements on the $S=1/2$ frustrated kagomé magnet kapellasite, $α-$Cu$_3$Zn(OH)$_6$Cl$_2$, where a gapless spin liquid phase is stabilized by a set of competing exchange interactions. Our measurements confirm the ferromagnetic character of the nearest-neighbour exchange interaction $J_1$ and give an energy scale for the competing interactions $|J| \sim 10$ K. The study of the temperature-dependent ESR lineshift reveals a moderate symmetric exchange anisotropy term $D$, with $|D/J|\sim 3$%. These findings validate a posteriori the use of the $J_1 - J_2 - J_d$ Heisenberg model to describe the magnetic properties of kapellasite [Bernu et al., Phys. Rev. B 87, 155107 (2013)]. We further confirm that the main deviation from this model is the severe random depletion of the magnetic kagomé lattice by 27%, due to Cu/Zn site mixing, and specifically address the effect of this disorder by $^{35}$Cl NMR, performed on an oriented polycrystalline sample. Surprisingly, while being very sensitive to local structural deformations, our NMR measurements demonstrate that the system remains homogeneous with a unique spin susceptibility at high temperature, despite a variety of magnetic environments. Unconventional spin dynamics is further revealed by NMR and $μ$SR in the low-$T$, correlated, spin liquid regime, where a broad distribution of spin-lattice relaxation times is observed. We ascribe this to the presence of local low-energy modes.

cond-mat.str-el↗
arXiv2012

Kapellasite: a kagome quantum spin liquid

Magnetic susceptibility, NMR, muSR, and inelastic neutron scattering measurements show that kapellasite, Cu3Zn(OH)6Cl2, a geometrically frustrated spin-1/2 kagome antiferromagnet polymorphous with the herbertsmithite mineral, is a gapless spin liquid with frustrated interactions showing unusual dynamic short-range correlations of non-coplanar cuboc2 type which persist down to 20 mK. The Hamiltonian is determined from a fit of a high-temperature series expansion to thermodynamical data. The experimental data are compared to theoretical calculations using the Schwinger-boson approach.

cond-mat.str-el↗
arXiv2012

Zero-bias anomalies on Sr$_{0.88}$La$_{0.12}$CuO$_2$ thin films

High-impedance contacts made on the surface of Sr$_{0.88}$La$_{0.12}$CuO$_2$ superconducting thin films systematically display a zero-bias anomaly. We consider two-level systems (TLS) as the origin of this anomaly. We observe that the contribution of some TLS to the contact resistance is weakened by a magnetic field. We show that this could result from the increase of the TLS relaxation rate in the superconducting state, due to its ability to create pairs of quasiparticles out of the condensate, when located close to the surface of the film.

cond-mat.supr-con↗
arXiv2011

Electric--field effect on electron-doped infinite-layer Sr$_{0.88}$La$_{0.12}$CuO$_{2+x}$ thin films

We have used the electric--field effect to modulate the resistivity of the surface of underdoped Sr$_{0.88}$La$_{0.12}$CuO$_{2+x}$ thin films, allowing opposite modifications of the electron and hole density in the CuO$_2$ planes, an original situation with respect to conventional chemical doping in electron-doped materials. When the Hall effect indicates a large contribution of a hole band, the electric--field effect on the normal state resistivity is however dominated by the electrons, and the superconducting transition temperature increases when carriers are transfered from holes to electrons.

cond-mat.supr-con↗
arXiv2010

Penetration depth of electron-doped-infinite-layer Sr$_{0.88}$La$_{0.12}$CuO$_{2+x}$ thin films

The in-plane penetration depth of Sr$_{0.88}$La$_{0.12}$CuO$_{2+x}$ thin films at various doping obtained from oxygen reduction has been measured, using AC susceptibility measurements. For the higher doping samples, the superfluid density deviates strongly from the s-wave behavior, suggesting, in analogy with other electron-doped cuprates, a contribution from a nodal hole pocket, or a small gap on the Fermi surface such as an anisotropic s-wave order parameter. The low value of the superfluid densities, likely due to a strong doping-induced disorder, places the superconducting transition of our samples in the phase-fluctuation regime.

cond-mat.supr-con↗
arXiv2010

Ultrafast transient response and electron-phonon coupling in the iron-pnictide superconductor Ba(Fe(1-x)Co(x))2As2

The transient response of Ba(Fe(1-x)Co(x))2As2, x=0.08 was studied by pump-probe optical reflectivity. After ultrafast photoexcitation, hot electrons were found to relax with two different characteristic times, indicating the presence of two distinct decay channels: a faster one, of less than 1 ps in the considered pump fluence range, and a slower one, corresponding to lattice thermalization and lasting ~6 ps. Our analysis indicates that the fast relaxation should be attributed to preferential scattering of the electrons with only a subset of the lattice vibration modes, with a second moment of the Eliashberg function lambda(omega^2)~64 meV^2. The simultaneous excitation of a strong fully symmetric A(1g) optical phonon corroborates this conclusion and makes it possible to deduce the value of lambda~0.12. This small value for the electron-phonon coupling confirms that a phonon mediated process cannot be the only mechanism leading to the formation of superconducting pairs in this family of pnictides.

cond-mat.supr-con↗
arXiv2008

Magnetoresistance and Hall effect in e-doped superconducting SrLaCuO thin films

We have epitaxially grown c-axis oriented SrxLa1-xCuO2 thin films by rf sputtering on KTaO3 substrates with x = 0.12. The as-grown deposits are insulating and a series of superconducting films with various Tc(R=0) up to 26 K have been obtained by in-situ oxygen reduction. Transport measurements in the ab plane of these samples have been undertaken. We report original results on the temperature dependence of the Hall effect and on the anisotropic magnetoresistance (T > Tc). We discuss the magnitude of upper critical fields and anisotropy, the Hall effect, which presents changes of sign indicative of the existence of two types of carriers, the normal state magnetoresistance, negative in parallel magnetic field, a possible signature of spin scattering. These properties are compared to those of hole-doped cuprates, such as BiSr(La)CuO with comparable Tc.

cond-mat.supr-con↗
arXiv2007

Influence of Disorder on the Hall Effect in Bi$_2$Sr$_2$CuO$_{6+δ}$

The in-plane resistivity and Hall coefficient have been measured for the single-layer compound Bi$_2$Sr$_2$CuO$_{6+δ}$ for the whole range of doping states. The deviation of the Hall coefficient, $R_H$, from a high-temperature linear behavior and the temperature dependence of the Hall angle are both only weakly dependent upon doping, contrasting with Bi$_2$Sr$_{2-x}$La$_x$CuO$_{6+δ}$ and Bi$_2$Sr$_2$CaCuO$_{8+δ}$. This is in contradiction with former proposals that the transverse transport detects the formation of incoherent Cooper pairs in the pseudogap state. Conversely, the analysis of the data using a phenomenological angular dependent scattering rate clearly allows to distinguish between underdoped and overdoped states, and we propose that the maximum in $R_H(T)$ simply arises due to the combination of a large isotropic scattering rate and an anisotropic temperature dependent one.

cond-mat.supr-con↗
arXiv2006

Upper critical field from normal state fluctuations in Bi$_2$Sr$_2$CuO$_{6+δ}$

The in-plane magnetoresistance of an epitaxial Bi$_2$Sr$_2$CuO$_{6+δ}$ thin film was systematically investigated as a function of doping, above $T_c$. The orbital magnetoconductance is used to extract the crossover field line $H_{c2}^*(T)$ in the fluctuation regime. This field is found in good agreement with the upper critical field obtained from resistivity data below $T_c$, and exhibits a similar upward curvature, thus pointing toward the existence of a critical correlation length. The consequences regarding the nature of the resistive transition are discussed.

cond-mat.supr-con↗
arXiv2005

Current dependent fluctuations in a Bi$_2$Sr$_2$CuO$_{6+δ}$ thin film

The current dependence of the excess conductivity is measured up to $\simeq 3 T_c$ for a Bi$_2$Sr$_2$CuO$_{6+δ}$ thin film, as a function of doping. It is found to be anomalously sensitive to the transport current and to behave as a universal function of $T/T_c$ in the whole doping range. We discuss these results in the perspective of a granular superconductor with a gapless-like behavior.

cond-mat.supr-con↗
arXiv2004

Specific heat of Nb_3Sn: The case for a second energy gap

We present new specific heat data for Nb3Sn, a well-known technically applied superconductor with a critical temperature T_c ~ 18 K, in the temperature range from 1.2 to 200 K in zero magnetic field, and from 1.5 to 22 K in fields H <= 16 T. The particularly dense and homogeneous polycrystalline sample used for this study is characterized in detail. We determine the bulk upper critical field H_c2(T) from specific heat data, and the Sommerfeld constant Gamma from the entropy S(T). We investigate in detail a low-temperature anomaly already noticed in previous investigations in zero field, and find that this feature can be quantitatively ascribed to the presence of a second superconducting gap 2 Delta_S(0) ~ 0.8 k_B T_c, in addition to the main one 2 Delta_L(0) ~ 4.9 k_B T_c. The signature of this minor gap, which affects 7.5% of the electronic density-of-states, vanishes in fields above ~ 7 T.

cond-mat.supr-con↗
arXiv2004

Non linear excess conductivity of Bi$_2$Sr$_2$Ca$_{n-1}$Cu$_n$O$_{2n+4+x}$ (n = 1,2), thin films

The suppression of excess conductivity with electric field is studied for Bi$_2$Sr$_2$Ca$_{n-1}$Cu$_n$O$_{2n+4+x}$ ($n$ = 1, 2) thin films. A pulse-probe technique is used, which allows for an estimate of the sample temperature. The characteristic electric field for fluctuations suppression is found well below the expected value for all samples. For the $n=1$ material, a scaling of the excess conductivity with electric field and temperature is obtained, similar to the scaling under strong magnetic field.

cond-mat.supr-con↗
arXiv2002

Unusual effects of anisotropy on the specific heat of ceramic and single crystal MgB2

The two-gap structure in the superconducting state of MgB_2 gives rise to unusual thermodynamic properties which depart markedly from the isotropic single-band BCS model, both in their temperature- and field dependence. We report and discuss measurements of the specific heat up to 16 T on ceramic, and up to 14 T on single crystal samples, which demonstrate these effects in the bulk. The behavior in zero field is described in terms of two characteristic temperatures, a crossover temperature Tc_pi ~ 13 K, and a critical temperature Tc = Tc_sigma ~ 38 K, whereas the mixed-state specific heat requires three characteristic fields, an isotropic crossover field Hc2_pi ~ 0.35 T, and an anisotropic upper critical field with extreme values Hc2_sigma_c ~ 3.5 T and Hc2_sigma_ab ~ 19 T, where the indexes πand σrefer to the 3D and 2D sheets of the Fermi surface. Irradiation-induced interband scattering tends to move the gaps toward a common value, and increases the upper critical field up to ~ 28 T when Tc = 30 K.

cond-mat.supr-con↗