Denis Jerome, Université Paris-Sud, Orsay

The author  at the University of California

La Jolla, in 1966

 Some decades later

For an extensive CV of Denis Jerome,  see also Academy of Sciences

Denis Jerome has graduated at the University Paris-Sorbonnes in 1961. He has obtained a PhD in Physics at CEN-Saclay in 1965 under the supervision of Professor A. Abragam, then moved to the University of California for a post doctoral visit under Professor W. Kohn. He began his research activity at the University of Paris-Sud, Orsay in 1967 in the Solid State Physics Laboratory of Profs R. Castaing, A. Guinier and J. Friedel where he has developed high pressure measurements for the study of metals and alloys at low temperature.

He is  Emeritus Director of Research at CNRS and became a corresponding member of the French Academy of Sciences in 1990 and full member in 2005.

The major contribution of Denis Jerome to the advancement of  physics is the discovery of Organic Superconductivity in the  One Dimensional Organic Bechgaard salt (TMTSF2PF6) in 1979 at the University of Paris-Sud, Orsay.

He has also contributed to the study of the physical properties of a vast  number of one dimensional and two dimensional organic conductors and established a generic phase diagram for these families.

He has been active in the  experimental investigation of the metal-insulator Mott transition (doped silicon, transition metal oxides and 2D organic conductors).

Reviews and book chapters on organic condcuctors and organic superconductivity

Here can be found a large collection of review articles and book chapters on the field of organic one dimensional conductors and superconductors.see papers

Jumping over the Mott-Hubbard gap

Mott transition in impurity doped silicon, Si-P

The Metal-Insulator transition occurring in pure silicon as doping is increased is a Mott transition in a disordered electronic system.

This phenomenon has been studied in phosporous doped silicon using double resonance ENDOR techniques leading to :

  1. The determination of the antiferromagnetic nature of the exchange interaction between spins localized on near neighbour donor sites for the concentration near to 8x1016 cm-3
  2. The evidence for an electron delocalization giving rise to an impurity band around 3x1018 cm-3 , studied via 29Si resonance.

On the insulating side, N<3x1018 cm-3, the nuclear relaxation is due to the modulation of the dipolar electron nucleus interaction through the hopping of carriers between neighbouring sites.

On the metallic side, N>3x1018 cm-3, the fast jump of carriers between sites within the impurity band is providing the nuclear relaxation leading in  turn to a very narrow electron resonance line at low temperature.

The electron wave function does reveal sharp peaks around impurity sites even in the metallic state.see papers

Excitonic instability

An excitonic transition is expected to take place in semi-metals when conditions for perfect nesting of electron and hole pockets are achieved.

Response functions have been determined and possible materials such as Bismuth or divalent metals like Strontium or Ytterbium have been considered for the experimental observation of this new phase. see papers

Helium gas high pressure and hydrostatic pressure up to 35 kbar

A description of some of the high pressure set up developed at Orsay  using helium gas pressure media enabling hydrostatic pressure down to low temperatures, teflon cells techniques with either a capillary tubing and a low temperature intensifier or clamp cells using berylium –copper and nickel-chromium- aluminum alloys.see papers


Layered metallic compounds, transition metal dichalcogenides

Transition metal dichalcogenides such as TX2 compound where T=V, Nb,Ta, Tl,etc,.. and X= S, Se, etc,..are two dimensional conductors also known as layered compounds.

They often undergo a lattice distortion at low temperature leading to a structure which is  incommensurate with the original lattice.

The distorted phases have been studied via transport and NMR investigations under pressure. These studies have clearly established a relation existing between the onset of a periodic distortion at To and the onset of superconductivity at a lower temperature Tc .

Studies on 2H-NbSe2, 4Hb-TaS2, 2H-NbS2, 2H-TaS2 have shown that the drop of the density of states at Fermi level occuring below To becoming suppressed under pressure is responsible in turn for the increase of Tc under pressure.

2H-NbSe2 has been the first single crystal where NMR experiments have shown the incommensurate nature of the periodic distortion.see papers

Organic superconductivity in quasi-one dimensional organic salts, Bechgaard and Fabre salts, TM2X family

Superconductivity of organic matter has been discovered in December 1979 at Orsay by D. Jerome, A. Mazaud, M. Ribault and K. Bechgaard and published in 1980.

The first organic material revealing the essential features for superconductivity, (zero resistance state and Meissner expulsion of the magnetic flux) is the organic salt (TMTSF)2PF6 namely (tetramethyl tetra selenafulvalene)2 hexafluorophosphate under an hydrostatic pressure of 9 kbar at Tc=1K.

The peculiarity of this salt stems from the design of these materials made of flat, conjugated fulvalene electron donating molecules, packed in columns along the a-axis. This direction has a high, metallic conductivity due to the intrachain overlap of pi-molecular orbitals  ; the transverse directions have much weaker intermolecular couplings and thus much lower conductivity. It is the reason why they are often called quasi one dimensional conductors (and superconductors).

Following the inital discovery of superconductivity in (TMTSF)2PF6  intense experimental activity in physics and chemistry has shown that the PF6 compound is only a member of a much broader family of compounds, TM2X, where the mono anion X can also be ClO4, NO3, AsF6, etc,..and the cation molecule is either the 4-selenium or the 4-sulfur atom molecule TMTTF.

TM2X compounds belong to the same isostructural family (Bechgaard-Fabre family) all (but NO3) undergoing a superconducting instability at low temperature under pressure with the exception of ClO4 which becomes superconducting at ambient pressure with Tc=1.2K.

The generic phase diagram that was gradually built over the years around these materials stands out as a model of unity for experimental and theoretical physics of low dimensional correlated systems and chemistry.

Main features of the TM2X generic diagram are Mott localization at high temperature and charge ordering, spin-Peierls , spin density wave phases and superconductivity at low temperature. In addition, they have revealed a new effect of the magnetic field whenever a metallic phase is stable at low temperature namely, the stabilization of field induced spin density wave phases (FISDW) in which the Hall effect is quantized.

Recent transport, NMR and thermodynamic investigations under oriented magnetic field support the existence of d-wave coupling for the superconducting pairing mechanism with line nodes on the Fermi surface. This unconventional superconductivity is likely due to the interchain exchange of antiferromagnetic fluctuations following the suggestion of Caron and Bourbonnais in 1986.see papers


Two dimensional Organic Superconductors

The BEDT-TTF (ET) donor molecule is the main for several series of organic salts in which a strong interchain coupling along a transverse axis give rise to a large conductivity along both a and b axis. Very much like TM2X compounds, the  ET2X  two dimensional compounds form isotructural families where the Mott insulating state shares a common border with superconductivity. The superconducting Tc’s of these materials can be raised up to 13K in the ET kappa-phases. Furthermore, kappa-phase compounds have provided evidences for d-wave superconductivity according to NMR Knight shift and relaxation data.see papers

Charge Transfer Q1D compounds

The work on charge transfer organic conductors started in October 1973 shortly after the announcement by the Philadelphia group of a huge conductivity peak around 59K in tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ). The use of high pressure was justified by a work of M. Weger and co-workers suggesting the possibility for superconductivity in this charge transfer compound once the Peierls Metal-insulator transition at 56K is suppressed under a high enough pressure.

High pressure studies have led to several important results:

  1. The charge transfer between TTF and TCNQ chains, incommensurate at ambiant pressure becomes commensurate around 19 kbar.
  2. A very large fraction of conductivity giving rise to the huge peak at 59K is due to sliding Froehlich fluctuations and rules out the proposal of superconducting fluctuations made intitially by the Philadelphia group.
  3. The Peierls ground state is still stable under the pressure of 36kbar and precludes the stabilization of superconductivity in this charge transfer compound.
  4. Unlike TTF-TCNQ, the charge transfer TMTSF-DMTCNQ can be stabilized in a highly conducting metallic phase under a pressure of 9 kbar.

Although no superconductivity could be found in the TMTSF-DMTCNQ charge transfer compound under pressure, the study of its phase diagram has deeply contributed to the discovery of superconductivity in Bechgaard salts containing the same cation molecule TMTSF.see papers

Spin ladders

Superconductivity in the two leg spin ladders Sr2Ca12Cu24O41 (the so-called telephone number compound) under  a pressure of 31 kbar, studied  by transport and 63Cu NMR. (Sr/Ca)14Cu24041 spin ladders have also been studied by 63Cu and 17O NMR under pressure. Hole redistribution and spin excitations in Sr14-x CaxCu24O41(x=0, 12) spin ladder compounds are derived.see papers

Solid electrolytes, beta-alumina see papers

Fullerenes see papers

Platino-cyanate (KCP) 1D conductors and inorganic superconducing polymer SNx see papers

Mott transition in V2O3 see papers

High Tc cuprates see papers

Miscellanous see papers

Dossier de Test see paper