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J. Bobroff

Publications and source records attributed to J. Bobroff.

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↗
arXiv2013

NMR study of electronic correlations in Mn-doped Ba(Fe1-xCox)2As2 and BaFe(As1-xPx)2

We probe the real space electronic response to a local magnetic impurity in isovalent and heterovalent doped BaFe2As2 (122) using Nuclear Magnetic Resonance (NMR). The local moments carried by Mn impurities doped into Ba(Fe1-xCox)2As2(Co-122) and BaFe(As1-xPx)2(P-122) at optimal doping induce a spin polarization in the vicinity of the impurity. The amplitude, shape and extension of this polarisation is given by the real part of the susceptibility chi'(r) of FeAs layers, and is consequently related to the nature and strength of the electronic correlations present in the system. We study this polarisation using 75As NMR in Co-122 and both 75As and 31P NMR in P-122. The NMR spectra of Mn-doped materials is made of two essential features. First is a satellite line associated with nuclei located as nearest neighbor of Mn impurities. The analysis of the temperature dependence of the shift of this satellite line shows that Mn local moments behave as isolated Curie moments. The second feature is a temperature dependent broadening of the central line. We show that the broadening of the central line follows the susceptibility of Mn local moments, as expected from typical RKKY-like interactions. This demonstrates that the susceptibility chi'(r) of FeAs layers does not make significant contribution to the temperature dependent broadening of the central line. chi'(r) is consequently only weakly temperature dependent in optimally doped Co-122 and P-122. This behaviour is in contrast with that of strongly correlated materials such as underdoped cuprate high-Tc superconductors where the central line broadens faster than the impurity susceptibility grows, because of the development of strong magnetic correlations when T is lowered. Moreover, the FeAs layer susceptibility is found quantitatively similar in both heterovalent doped and isolvalent doped BaFe2As2.

cond-mat.supr-con↗
arXiv2013

Possible realization of an antiferromagnetic Griffiths phase in Ba[Fe(1-x)Mn(x)](2)As(2)

We investigate magnetic ordering in metallic Ba[Fe(1-x)Mn(x)](2)As(2) and discuss the unusual magnetic phase, which was recently discovered for Mn concentrations x > 10%. We argue that it can be understood as a Griffiths-type phase that forms above the quantum critical point associated with the suppression of the stripe-antiferromagnetic spin-density-wave (SDW) order in BaFe2As2 by the randomly introduced localized Mn moments acting as strong magnetic impurities. While the SDW transition at x = 0, 2.5% and 5% remains equally sharp, in the x = 12% sample we observe an abrupt smearing of the antiferromagnetic transition in temperature and a considerable suppression of the spin gap in the magnetic excitation spectrum. According to our muon-spin-relaxation, nuclear magnetic resonance and neutron-scattering data, antiferromagnetically ordered rare regions start forming in the x = 12% sample significantly above the Néel temperature of the parent compound. Upon cooling, their volume grows continuously, leading to an increase in the magnetic Bragg intensity and to the gradual opening of a partial spin gap in the magnetic excitation spectrum. Using neutron Larmor diffraction, we also demonstrate that the magnetically ordered volume is characterized by a finite orthorhombic distortion, which could not be resolved in previous diffraction studies most probably due to its coexistence with the tetragonal phase and a microstrain-induced broadening of the Bragg reflections. We argue that Ba[Fe(1-x)Mn(x)](2)As(2) could represent an interesting model spin-glass system, in which localized magnetic moments are randomly embedded into a SDW metal with Fermi surface nesting.

cond-mat.supr-con↗
arXiv2013

Large temperature dependence of the number of carriers in Co-doped BaFe2As2

Using angle-resolved photoemission spectroscopy, we study the evolution of the number of carriers in Ba(Fe(1-x)Cox)2As2 as a function of Co content and temperature. We show that there is a k-dependent energy shift compared to density functional calculations, which is large at low Co contents and low temperatures and reduces the volume of hole and electron pockets by a factor 2. This k-shift becomes negligible at high Co content and could be due to interband charge or spin fluctuations. We further reveal that the bands shift with temperature, changing significantly the number of carriers they contain (up to 50%). We explain this evolution by thermal excitations of carriers among the narrow bands, possibly combined with a temperature evolution of the k-dependent fluctuations.

cond-mat.str-el↗
arXiv2012

Mn local moments prevent superconductivity in iron-pnictides Ba(Fe 1-x Mn x)2As2

75As nuclear magnetic resonance (NMR) experiments were performed on Ba(Fe1-xMnx)2As2 (xMn = 2.5%, 5% and 12%) single crystals. The Fe layer magnetic susceptibility far from Mn atoms is probed by the75As NMR line shift and is found similar to that of BaFe2As2, implying that Mn does not induce charge doping. A satellite line associated with the Mn nearest neighbours (n.n.) of 75As displays a Curie-Weiss shift which demonstrates that Mn carries a local magnetic moment. This is confirmed by the main line broadening typical of a RKKY-like Mn-induced staggered spin polarization. The Mn moment is due to the localization of the additional Mn hole. These findings explain why Mn does not induce superconductivity in the pnictides contrary to other dopants such as Co, Ni, Ru or K.

cond-mat.supr-con↗
arXiv2012

NMR Study in the Iron-Selenide Rb0.74Fe1.6Se2: Determination of the Superconducting Phase as Iron Vacancy-Free Rb0.3Fe2Se2

77Se and 87Rb nuclear magnetic resonance (NMR) experiments on Rb0.74Fe1.6Se2 reveal clearly distinct spectra originating from a majority antiferromagnetic (AF) and a minority metallic-superconducting (SC) phase. The very narrow NMR line of the SC phase evidences the absence of Fe vacancies and any trace of AF order. The Rb content of the SC phase is deduced from intensity measurements identifying Rb0.3(1)Fe2Se2 as the actual composition of the SC fraction. The resulting estimate of 0:15 electrons/Fe brings this class of superconductors 245 family closer to the other Fe-based superconductor families.

cond-mat.supr-con↗
arXiv2010

Impurity effects in coupled-ladder BiCu2PO6 studied by NMR and quantum Monte Carlo simulations

We present a 31P NMR study of the coupled spin 1/2 ladder compound BiCu2PO6. In the pure material, intrinsic susceptibility and dynamics show a spin gap of about . Substitution of non magnetic Zn or magnetic Ni impurity at Cu site induces a staggered magnetization which results in a broadening of the 31P NMR line, while susceptibility far from the defects is unaffected. The effect of Ni on the NMR line broadening is twice that of Zn, which is consistent with Quantum Monte Carlo (QMC) calculations assuming that Ni couples ferromagnetically to its adjacent Cu. The induced moment follows a 1/T temperature dependence due to the Curie-like development of the moment amplitude while its extension saturates and does not depend on impurity content or nature. This allow us to verify the generically expected scenario for impurity doping and to extend it to magnetic impurity case: in an antiferromagnetically correlated low dimensional spin system with antiferromagnetic correlations, any type of impurity induces a staggered moment at low temperature, whose extension is not linked to the impurity nature but to the intrinsic physics at play in the undoped pure system, from 1D to 2D systems.

cond-mat.str-el↗
arXiv2010

Absence of large nanoscale electronic inhomogeneities in the Ba(Fe1-xCox)2As2 pnictide

75As NMR and susceptiblity were measured in a Ba(Fe1-xCox)2As2 single crystal for x=6% for various field H values and orientations. The sharpness of the superconducting and magnetic transitions demonstrates a homogeneity of the Co doping x better than +-0.25%. On the nanometer scale, the paramagnetic part of the NMR spectra is found very anisotropic and very narrow for H//ab which allows to rule out the interpretation of Ref.[6] in terms of strong Co induced electronic inhomogeneities. We propose that a distribution of hyperfine couplings and chemical shifts due to the Co effect on its nearest As explains the observed linewidths and relaxations. All these measurements show that Co substitution induces a very homogeneous electronic doping in BaFe2As2, from nano to micrometer lengthscales, on the contrary to the K doping.

cond-mat.supr-con↗
arXiv2009

Atomic coexistence of superconductivity and incommensurate magnetic order in the Ba(Fe1-xCox)2As2 pnictide

75As NMR and susceptiblity were measured in a Ba(Fe1-xCox)2As2 single crystal for x=6%. Nuclear Magnetic Resonance (NMR) spectra and relaxation rates allow to show that all Fe sites experience an incommensurate magnetic ordering below T=31K. Comparison with undoped compound allows to estimate a typical moment of 0.05 muB. Anisotropy of the NMR widths can be interpreted using a model of incommensurability with a wavevector (1/2-eps,0,l) with eps of the order of 0.04. Below TC=21.8K, a full volume superconductivity develops as shown by susceptibility and relaxation rate, and magnetic order remains unaffected, demonstrating coexistence of both states on each Fe site.

cond-mat.supr-con↗
arXiv2009

Doping effects in the coupled, two-leg spin ladder BiCu2PO6

We report preparation, x-ray diffraction, magnetic susceptibility chi(T) and heat capacity Cp(T) measurements on the undoped samples as also samples with Zn-doped (S = 0) at Cu site, Ni doped (S = 1) at Cu site, and Ca-doped (holes) at Bi site in the coupled two-leg spin ladder system BiCu2PO6. While, Zn shows complete solid solubility, Ni could be doped to about 20% and Ca to about 15%. Magnetization and heat capacity data in the undoped compound point towards the existence of frustration effects. In all the samples, the chi(T) at low temperature increases with doping content. The Zn-induced susceptibility is smaller than that due to effective S=1/2 moments possibly due to frustrating next-nearest-neighbor interactions along the leg. For Zn content x > 0.01, chi(T) deviates from the Curie-law at low temperatures. The magnetic specific heat data Cm(T) for the Zn-doped samples show weak anomalies at low temperature in agreement with chi(T) behavior. The anomalies are suggestive of spin freezing at low-T. In contrast, prominent effects are observed in chi(T) and Cm(T) on Ni-doped samples. The zero-field-cooled (ZFC) and field-cooled (FC) chi(T) data are different from each other at low temperature unlike that for Zn doped samples, clearly indicating a transition to a spin-glass like phase. No anomalies were found in Ca- or Pb-doped samples.

cond-mat.str-el↗
arXiv2009

Impurity-induced magnetic order in low dimensional spin gapped materials

We have studied the effect of non-magnetic Zn impurities in the coupled spin-ladder Bi(Cu1xZnx)2PO6 using 31P NMR, muSR and Quantum Monte Carlo simulations. Our results show that the impurities induce in their vicinity antiferromagnetic polarizations, extending over a few unit cells. At low temperature, these extended moments freeze in a process which is found universal among various other spin-gapped compounds: isolated ladders, Haldane or Spin-Peierls chains. This allows us to propose a simple common framework to explain the generic low-temperature impurity induced freezings observed in low dimensional spin-gapped materials.

cond-mat.str-el↗
arXiv2008

Spin correlations and cobalt charge states: A new phase diagram of sodium cobaltates

Using 23Na NMR measurements on sodium cobaltates at intermediate dopings (0.44<=x<=0.62), we establish the qualitative change of behavior of the local magnetic susceptibility at x*=0.63-0.65, from a low x Pauli-like regime to the high x Curie-Weiss regime. For 0.5<=x<=0.62, the presence of a maximum T* in the temperature dependence of the susceptibility shows the existence of an x-dependent energy scale. T_1 relaxation measurements establish the predominantly antiferromagnetic character of spin correlations for x x*. It is suggested that at a given x the ferromagnetic correlations might dominate the antiferromagnetic ones above T*. From 59Co NMR data, it is shown that moving towards higher x away from x=0.5 results in the progressive appearance of nonmagnetic Co3+ sites, breaking the homogeneity of Co states encountered for x<=0.5. The main features of the NMR-detected 59Co quadrupolar effects, together with indications from the powder x-ray diffraction data, lead us to sketch a possible structural origin for the Co3+ sites. In light of this ensemble of new experimental observations, a new phase diagram is proposed, taking into account the systematic presence of correlations and their x-dependence.

cond-mat.str-el↗
arXiv2007

Defects in correlated metals and superconductors

In materials with strong local Coulomb interactions, simple defects such as atomic substitutions strongly affect both macroscopic and local properties of the system. A nonmagnetic impurity, for instance, is seen to induce magnetism nearby. Even without disorder, models of such correlated systems are generally not soluble in 2 or 3 dimensions, and so few exact results are known for the properties of such impurities. Nevertheless, some simple physical ideas have emerged from experiments and approximate theories. Here, we first review what we can learn about this problem from 1D antiferromagnetically correlated systems. We then discuss experiments on the high Tc cuprate normal state which probe the effect of impurities on local charge and spin degrees of freedom, and compare with theories of single impurities in correlated hosts, as well as phenomenological effective Kondo descriptions. Subsequently, we review theories of impurities in d-wave superconductors including residual quasiparticle interactions, and compare with experiments in the superconducting state. We argue that existing data exhibit a remarkable similarity to impurity-induced magnetism in the 1D case, implying the importance of electronic correlations for the understanding of these phenomena, and suggesting that impurities may provide excellent probes of the still poorly understood ground state of the cuprates.

cond-mat.str-el↗
arXiv2007

Interplay between magnetic properties and thermoelectricity in misfit and Na cobaltates

We present a comparative study of CoO2 layers in the Bi-misfit and NaxCoO2 cobaltates. Co NMR measures the intrinsic susceptibility of the Co layers and is not affected by spurious contributions. At low dopings where room-temperature thermopower (TEP) is large, Curie-Weiss susceptibilities are observed in both materials. But NMR and muSR experiments find neither charge nor spin order down to low temperatures in Bi-misfit, in contrast to the case of NaxCoO2. This demonstrates that metallicity, charge and magnetic orders are specific of the Na layers in NaxCoO2 whereas strong correlations are generic of the cobaltates physics and could explain the large TEP.

cond-mat.str-el↗