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On Electrons and Solids

The LPS,(1) a renowned French solid-state physics laboratory, turned 50 last year. Alongside fascinating experiments, the laboratory continues to make important discoveries, particularly regarding the intriguing electronic properties of matter.

It looks much like any other control room: two computers, a panel with screens and buttons, electric cables and pipes that run from the ceiling to the floor—and even a few tools, including a soldering iron in the corner. But the real “attraction”—and one of the solid-state physics laboratory's latest acquisitions—lies behind the glass panel: a super-powerful 14-tesla2 magnet that could pull the keys right out of your pocket.
LPS researchers are expecting wonders from this magnet that should expose undiscovered quantum properties of matter in nuclear magnetic resonance (NMR) experiments, a technique used to determine the properties of each electron in a sample. “We finished setting up this new lab two months ago,” explains LPS researcher Julien Bobroff. “Apart from the magnet, everything was made in-house: the cryostat, the spectrometer, the computer software... And with remote access, experiments can continue round the clock!”
For LPS Director Jean-Paul Pouget, “developing original instrumentation is one of our trademarks. What's more, our aim is to tackle all aspects of solid-state physics, from the study of the structure of materials to their electronic properties, and to do this via a strong link-up between experiment and theory.” This has been the methodology of the LPS since its creation, exactly 50 years ago. And it soon produced results. Not a decade had passed before it received funding from two major scientific awards given to Raimond Castaing in 1966, and to André Guinier, Jacques Friedel, and Pierre-Gilles de Gennes in 1967. In fact, two of the last four French Nobel prize-winners in physics—Pierre-Gilles de Gennes (1991) and Albert Fert (2007)—worked at the LPS at one point in their careers. But Pouget prefers to focus on the lab's present activities.


© F. Restagno, J. Bobroff/CNRS Photothèque

The superconductor state of this oxide (in black) is demonstrated in a magnet levitation experiment. In fact, superconductivity is characterized by the expulsion of any magnetic field outside of the sample. This is known as the Meissner effect.

Magnetization, Conduction, or Both
Solid-state physics, which ranges from electronics to nanotechnology and also encompasses biology, is flourishing. And renewed interest in the physics of electrons in materials is the perfect illustration.
“The general area of study is materials in which electrons have difficulty moving about because they are constrained to one or two dimensions, and thus hinder each other. At first sight, these materials should be poor electrical conductors and therefore of little interest. But paradoxically, it is in these materials that the most surprising properties have been observed in the past few years,” Bobroff explains.
The archetype of these new states of matter, in which the LPS specializes, is “high temperature superconductivity,” observed in materials composed of multiple layers of copper oxide (cuprates) or more recently, iron pnictides. This complex name hides one of the greatest mysteries in physics today: superconductivity. It occurs in metal when the temperature approaches absolute zero (-273.15 °C) and is characterized by the disappearance of resistance to the flow of electric currents.
Since the 1960s, physicists have known that superconductivity results from the interaction between electrons and vibrations in the crystalline matrix in which they circulate.
In the case of cuprate oxides, superconductivity takes place at a much higher temperature, with the current record observed in one oxide standing at -135 °C, which is still cold, but much less than for metals. This makes scientists believe that superconductivity could one day be achieved at room temperature, leading to materials that could convey electricity over great distances without any loss. To this day, the origin of superconductivity in oxides has not been elucidated. “What's even more mysterious is that you only need to slightly tweak an oxide to turn it from a superconductor to a magnetic insulating material,” says Bobroff.
The physicist and his team recently discovered something surprising. In an NMR experiment on a pnictide, the researchers discovered that the electrons exhibited both properties of magnetism3 and superconductivity at the same time. “It's fascinating because, in principle, magnetism and superconduction have a tendency to be mutually exclusive. Yet our results suggest that they could have a shared origin in the case of pnictides,” exclaims the researcher.
To get these results, the LPS researchers worked in close collaboration with physicists and chemists from the SPEC,4 a division of condensed matter physics of the French atomic energy commission (CEA). As Pouget explains: “With the CEA, the University of Orsay, and the different laboratories located in Palaiseau, the LPS has access to an extraordinary scientific environment. To make the most of this, we initiated in 2006 an advanced research network called the 'triangle of physics,' which enables funding for common projects among the different associated laboratories.” Véronique Brouet, another specialist of high critical temperature superconduction at the LPS, is preparing to work with another neighbor, Soleil, the national synchrotron radiation source,5 on what are known as photoemission experiments. “The laboratory has built some of the equipment used on certain lines of Soleil, particularly the photoemission line,” explains the physicist. “This technique provides insights into the relationship between the energy of an electron and the direction in which it propagates within a material. It is thus a complementary technique to NMR which, for its part, provides information on the spatial distribution of electrons.”

low temperature manipulator

© V. Brouet

This low-temperature manipulator, used on the Soleil synchrotron, was made at the LPS, and illustrates the laboratory's ability to produce original instrumentation.

From Disorder to Order
Using NMR technology, LPS researchers have also obtained remarkable results on what are known as spin ladder materials, in which the crystal lattice forms the rungs of a ladder, at the end of which the electrons are positioned. In a standard material, at very low temperature, the spins of all the electrons are either all lined up in the same direction or oriented head to tail. The material is then said to have magnetic order. Conversely, in a spin ladder, the electrons are oriented in all directions. The material is thus magnetically disordered.
“Collaborating with Nicolas Laflorencie, a young theoretician working in the laboratory, we have shown that by randomly removing one magnetic atom out of 100, the system, until then disordered, becomes ordered,” explains Bobroff. It would be similar to having one's bookshelves reorganized alphabetically after randomly removing a few books.
Are these strange quantum effects simply good fun for a few fringe specialists? Maybe, but it was also at the LPS in 1988 that the Nobel prizewinner in physics, Albert Fert, discovered another strange property of condensed matter known as magnetoresistance, the phenomenon behind an emerging type of electronics based on magnetism, now known as spintronics. “The important thing is to ensure that the laboratory does not become an ivory tower,” says Pouget. “For instance, we encourage collaboration with other institutions, with industry, and even between our different research teams because it is at the very interface between existing disciplines that new ones appear.” The researchers of the LPS are thus active on all fronts of condensed matter. With their new NMR-dedicated magnet, it is unlikely that the electron specialists will be left behind.

Mathieu Grousson

The LPS is one of the largest French research centers specialized in condensed matter. Between 200 and 250 people work at the LPS on a daily basis, including 61 CNRS researchers, 35 academics, 50 engineers, technicians, and administrative staff, and 35 PhD students.
Eighteen research teams are grouped together around three main research topics: “New electronic states of matter,” “Physical phenomena with reduced dimensions,” and “Soft matter and physics-biology interface.” The topics of research thus extend from superconductors to living tissues, while encompassing liquid crystals and nano-objects.
Between 2005 and 2008, the LPS produced 650 publications, including 500 articles in international scientific journals with peer-review committees.


Notes :

1. Laboratoire de physique des solides (CNRS / Université Paris-XI).
2. The tesla is the unit used to measure magnetic fields. By way of comparison, the magnetic field of the Earth is around 50 microteslas.
3. In other words their spin (internal magnetization) is oriented in a favored direction.
4. Service de physique de l'état condensé (CNRS/ CEA).
5. Synchrotron radiation is an electromagnetic radiation emitted by electrons in movement.

Contacts :

Jean-Paul Pouget,
Julien Bobroff,
Véronique Brouet,


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