arXiv subjects

Henri Alloul

Publications and source records attributed to Henri Alloul.

11 recordsLinked to original sources

arXiv2024

What do we learn from impurities and disorder in High Tc cuprates?

We established that the morphology of the phase diagrams is mostly governed by disorder in the various cuprate families. In the minimally disordered YBCO substitutions of Zn or Li spinless impurities on the planar Cu sites control the extent of the spin glass phase and superconducting (SC) dome. NMR data establish that these defects induce in their vicinity a cloud with a Kondo like paramagnetic behavior that differs markedly between the pseudogap and strange metal regimes. We have studied the evolution of transport properties with disorder by introduction of a controlled content of in plane vacancies by electron irradiation. At high T the inelastic scattering of the carriers is surprisingly disorder independent, and governed by the excitations of the correlated electronic state. The low T upturns of the resistivity, that are associated with single impurity Kondo like scattering, agree qualitatively with the local magnetism induced by spinless impurities. Very large defect contents are required to induce Metal Insulator Crossovers. In the SC state, the defect induced reduction of Tc scales quasi linearly with residual resistivity. The paraconductivity due to SC fluctuations, deduced from high field magnetoresistance measurements vanishes beyond a temperature T//c and a field H//c that decrease with increasing defect content. In the pseudogap regime the weaker decrease of T//c with respect to that of Tc reveals a loss of SC phase coherence in presence of disorder. This initiates a discussion of its interplay with pair breaking and allows us to confirm that the family specific dopant induced disorder or order controls the phase diagram. In the ideal disorder free cuprate the 2D SC state would certainly persist at low doping. This ensemble of data provides serious challenges for the theoretical understanding of superconductivity and the normal state in these SC correlated electron systems.

cond-mat.str-el↗

arXiv2016

Iron-based superconductors: Foreword

We introduce the thematic (open access) issue published in Comptes Rendus Physique. This issue is focused on the so-called Fe-based superconductors that, since the discovery of superconductivity in F-doped LaFeAsO by Hosono group in 2008, have become one of the latest hot topics in the field of condensed matter. The idea is to give an up-to-date overview of the fundamental properties of this novel family of high-Tc superconductors. First we briefly recall the scientific context of superconductivity and its relationship to magnetism in metallic compounds, and then we detail the specific aspects of Fe-based superconductors addressed in the issue.

cond-mat.str-el↗

arXiv2015

NMR in strongly correlated materials

Electron-electron interactions are at the origin of many exotic electronic properties of materials which have emerged from recent experimental observations. The main important phenomena discovered are related with electronic magnetic properties, which have been quite accessible to Nuclear Magnetic Resonance techniques. Those specifically permit to distinguish the orbitals or electronic bands responsible for the magnetism, the metallic properties and superconductivity and to reveal the physical properties which are distinct from expectations in an independent electron scheme. The description of some selected experimental cases permits us to underline the importance of the technique and to reveal altogether to the reader a wide range of novel phenomena specific to correlated electron physics.

cond-mat.str-el↗

arXiv2015

NMR studies of electronic properties of solids

The NMR technique is quite essential as it permits atomic scale measurements in materials. The aim of this article is to present the main parameters accessible to NMR experiments and to relate them to the electronic properties of simple solids in which electronic correlations are not dominant. It will be shown that in metals the Pauli paramagnetism, that is the electronic density of states is accessible through NMR shift and spin lattice relaxation data. Those permit as well to determine electronic gaps in some electronic structures, or in the SC state. The capability to reveal by NMR the incidence of impurities and disorder is also underlined. A complementary article will be dedicated to the specific properties of strongly correlated materials which have been revealed and studied by NMR experiments.

cond-mat.other↗

arXiv2015

Strongly Correlated Electrons in Solids

Most emergent properties of the materials discovered since the 1980s are related to the existence of electron-electron interactions which are large with respect to the kinetic energies and could not be thoroughly studied before. The occurrence of metal insulator transitions, of exotic magnetic and/or superconducting properties in many new compounds have stimulated a large series of ex- perimental and theoretical developments to grasp their physical significance. We present here a simple introduction to the elementary aspects of the physics of electron-electron interactions, which could be a starting point for typical undergraduate students.

cond-mat.str-el↗

arXiv2014

What is the simplest model which captures the basic experimental facts of the physics of underdoped cuprates?

I discuss here some well established experimental facts which have been shown to be generic of the pseudogap in underdoped cuprates. Some recent developments on Cellular Dynamic Mean Field Theories of the doped Hubbard model on a square lattice are nowadays allowing one to compute physical properties and k resolved single particle spectra. At high temperatures, recent results present qualitative features which perfectly fit the experimental observations done for more than two decades on the pseudogap in the cuprates. Most various broken symmetry states detected recently at lower temperatures than the pseudogap T*, certainly require more complicated modelling, and are not necessarily generic. I suggest that various perturbations due to the order (or disorder) of chemical dopants, which can hardly be avoided in actual materials, might drive the pseudogap electronic matter into specific ground states which exhibit interesting correlated electronic orders.

cond-mat.str-el↗

arXiv2013

NMR investigation of the pressure induced Mott transition to superconductivity in Cs3C60 isomeric compounds

The discovery in 1991 of high temperature superconductivity (SC) in A3C60 compounds, where A is an alkali ion, has been initially ascribed to a BCS mechanism, with a weak incidence of electron correlations. However various experimental evidences taken for compounds with distinct alkali content established the interplay of strong correlations and Jahn Teller distortions of the C60 ball. The importance of electronic correlations even in A3C60 has been highlighted by the recent discovery of two expanded fulleride Cs3C60 isomeric phases that are Mott insulators at ambient pressure. Both phases undergo a pressure induced first order Mott transition to SC with a (p, T) phase diagram displaying a dome shaped SC, a common situation encountered nowadays in correlated electron systems. NMR experiments allowed us to establish that the bipartite A15 phase displays Néel order at 47K, while magnetic freezing only occurs at lower temperature in the fcc phase. NMR data do permit us to conclude that well above the critical pressure, the singlet superconductivity found for light alkalis is recovered. However deviations from BCS expectations linked with electronic correlations are found near the Mott transition. So, although SC involves an electron-phonon mechanism, correlations have a significant incidence on the electronic properties, as had been anticipated from DMFT calculations.

cond-mat.str-el↗

arXiv2012

Electronic correlations, Jahn-Teller distortions and Mott transition to superconductivity in alkali-C60 compounds

The discovery in 1991 of high temperature superconductivity (SC) in A3C60 compounds, where A is an alkali ion, has been rapidly ascribed to a BCS mechanism, in which the pairing is mediated by on ball optical phonon modes. While this has lead to consider that electronic correlations were not important in these compounds, further studies of various AnC60 with n=1, 2, 4 allowed to evidence that their electronic properties cannot be explained by a simple progressive band filling of the C60 six-fold degenerate t1u molecular level. This could only be ascribed to the simultaneous influence of electron correlations and Jahn-Teller Distortions (JTD) of the C60 ball, which energetically favour evenly charged C60 molecules. This is underlined by the recent discovery of two expanded fulleride Cs3C60 isomeric phases which are Mott insulators at ambient pressure. Both phases undergo a pressure induced first order Mott transition to SC with a (p, T) phase diagram displaying a dome shaped SC, a common situation encountered nowadays in correlated electron systems. NMR experiments allowed us to study the magnetic properties of the Mott phases and to evidence clear deviations from BCS expectations near the Mott transition. So, although SC involves an electron-phonon mechanism, the incidence of electron correlations has an importance on the electronic properties, as had been anticipated from DMFT calculations.

cond-mat.str-el↗

arXiv2012

From Friedel oscillations and Kondo effect to the pseudogap in cuprates

One of the great achievements performed by Jacques Friedel in France has been to promote experimental activities on the electronic properties in condensed matter physics, which has led him, among others, to be an active promoter of the up-rise of the "Laboratoire de Physique des Solides" (LPS). A selective presentation of some of the work achieved in my Nuclear Magnetic Resonance (NMR) group allows me to underline the importance of the initial choices done by J. Friedel for the LPS. I give here an updated review of the work done on Kondo effect in the 1970's which has allowed me to enlighten the real interest of the NMR probe as a local experimental technique to visualize the perturbation brought by impurities in normal metals. In a second part I summarize the early NMR experiments done in the high Tc cuprates which have given evidence for the existence of strong electronic correlations and established the occurrence of the "pseudogap", on which I have initially had some exchanges with J. Friedel. The pseudogap, occurring below a temperature T*, has during the last twenty years raised endless discussions about the incidence of correlations on superconductivity. Many researchers are still advocating today that the pseudogap is due to the existence of preformed pairs above Tc. Its robustness to impurities and disorder that we evidenced from the early days rather suggested that it results from a competing order of the correlated state. This is apparently better accepted nowadays, although various distinct ordered states detected below T* are not yet understood. I also underline the large contrast between the scientific behaviours which prevailed between these two periods of scientific activity. I suggest that these changes might have a negative incidence, in my opinion, on the research and education system, at least in our field of science.

cond-mat.str-el↗

arXiv2010

NMR study of the Mott transitions to superconductivity in the two Cs_3C_60 phases

We report an NMR and magnetometry study on the expanded intercalated fulleride Cs_3C_60 in both its A15 and face centered cubic structures. NMR allowed us to evidence that both exhibit a first-order Mott transition to a superconducting (SC) state, occuring at distinct critical pressures p_c and temperatures T_c. Though the ground state magnetism of the Mott phases differs, their high $T$ paramagnetic and SC properties are found similar, and the phase diagrams versus unit volume per C_60 are superimposed. Thus, as expected for a strongly correlated system, the inter-ball distance is the relvevant parameter driving the electronic behavior and quantum transitions of these systems.

cond-mat.supr-con↗

arXiv2006

Influence of local fullerene orientation on the electronic properties of A3C60 compounds

We have investigated sodium containing fullerene superconductors Na2AC60, A = Cs, Rb, and K, by Na-23 nuclear magnetic resonance (NMR) spectroscopy at 7.5 T in the temperature range of 10 to 400 K. Despite the structural differences from the Rb3C60 class of fullerene superconductors, in these compounds the NMR line of the tetrahedrally coordinated alkali nuclei also splits into two lines (T and T') at low temperature. In Na2CsC60 the splitting occurs at 170 K; in the quenched cubic phase of Na2RbC60 and Na2KC60 we observe split lines at 80 K. Detailed investigations of the spectrum, spin-spin and spin-lattice relaxation as well as spin-echo double resonance (SEDOR) in Na2CsC60 we show that these two different tetrahedral sites are mixed on a microscopic scale. The T and T' sites differ in the orientation of first-neighbor C60 molecules. We present evidence that the orientations of neighboring molecules are uncorrelated. Thermally activated molecular reorientations cause an exchange between the T and T' sites and motional narrowing at high temperature. We infer the same activation energy, 3300 K, in the temperature range 125 to 300 K. The spin lattice relaxation rate is the same for T and T' down to 125 K but different below. Both the spin-lattice relaxation rate and Knight shift are strongly temperature dependent in the whole range investigated. We interpret this temperature variation by the effect of phonon excitations involving the rigid librational motion of the C60 molecules. By extending the understanding of the structure and molecular dynamics of C60 superconductors, these results may help in clarifying the effects of the structure on the superconducting properties.

cond-mat.supr-con↗