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Headlines 2009

29-09-2009

A. Cassimi, D. Hennecart, J. Rangama, T. Been,  D. Lelièvre, J.-M. Ramillon,  -  CIMAP, CEA-CNRS, Caen,
T. Muranaka CEA-IRFU Saclay
A. Leredde, X. Fléchard, LPC
H. Shiromaru, J. Matsumoto,  K.-I. Hayakawa Tokyo Metropolitan University

By exploring matter with energetics ions, the charge distribution between the ionic fragments, informs us about the nature of atomic bonds. It has thus been observed that the charge induced by ionisation is more evenly distributed between the fragments in the case of covalent molecules, where the electrons are delocalized over all atoms, compared to rare gas clusters, where electrons remain localized on their individual atom.

Removing several electrons from a polyatomic structure, such as a molecule or a cluster of atoms, usually leads to its dissociation. Indeed, the cohesion of these systems results from the exchange of electrons between the constituent atoms. Removing some of these electrons destabilizes the bonds in the atomic structure, which dissociates upon relaxation. This includes molecules with covalent bonds or, even more delocalized, metallic clusters.

26-01-2009
P. Camy, A. Benayad, J.L. Doualan, V. Ménard, R. Moncorgé (IRAMIS-CIMAP/MIL)

A major development to improve high power laser chains,  is to achieve both higher yields and better tunability. For future high power lasers that operates by diode-pumping at high rate, several materials likely to meet this double requirement (single crystals, glass and ceramics doped) are studied.

Among these materials, calcium fluoride (CaF2)  doped by Yb3+ cristals, like those manufactured and studied by the "Materials and Instrumentation Laser (MIL)" group of the CIMAP Laboratory, are one of the most serious candidates. Under its single crystal form and doped with trivalent ytterbium ions (Yb3+), it has remarkable properties: broad absorption and emission bands, high radiative life time, good transition cross-sections and excellent thermo-mechanical properties.

06-07-2009
Contact CEA : Hamed Merdji

In photography, the scattered light from an illuminated object is recorded with a detector and one get an image of it. If the image is formed with an objective, the optics imposes many limitations (resolution, aberrations ...). To achieve the ultimate resolution, spatially (function of the wavelength of the radiation used) and temporally (function of the "flash" duration), one possible technique (without any optics) is the coherent diffraction. Using a coherent beam like a laser to illuminate the object, a signal modulation due to interference is present and allows digitally reconstructing the exact image of the object with an unprecedented precision. To achieve nanometer or even atomic resolution, we therefore enlighten with a beam of coherent X-rays (radiation laser wavelength nanometer) and record the image. The usually low average illumination requires long accumulations over several laser shots. Recent advances have yielded images with a single shot femtosecond (10-15 s) from a laser laboratory, opening the way for time resolved studies.

For regular arrangements of elementary objects, the Bragg diffraction of X-rays is a powerful technique for characterization of matter at the atomic scale. It is the primary tool for crystallography. The information contained in the Bragg diffraction is rich: if a is the characteristic size of the elementary object, the Bragg peaks are spaced by 1/a in reciprocal space. However, some information is lost: indeed, the maximum frequency at which the diffraction pattern can be sampled is less than the Nyquist frequency (2a). In particular, if the elementary object has an amplitude and a phase, the phase is lost in the Bragg diffraction.

20-12-2009
F .Onufrieva and P. Pfeuty, Laboratoire Léon Brillouin

Superconducting pairing and electronic anomalies induced by the spin collective mode in high Tc cuprate superconductors

Discovered in 1986, superconductivity at high critical temperature remains today one of the great questions in solid state physics and material studies. Several theoretical models are today in competition and regularly confronted with experimental results. Among possible theories, those involving the magnetism in the coupling interaction forming the superconducting electron pairs are often preferred today (instead of the electron-phonon coupling for conventional superconductors within the BCS theory - J. Bardeen, L . Cooper and R. Schrieffer). In particular, the magnetic excitations in the form of spin waves fluctuations, observed experimentally in some superconductors (eg cuprate high Tc superconductors: YBa2Cu3O6+x, and also new non-conventional superconductors based on iron and arsenic, Tc ~ 40 K) must be considered, as shown by the theory developed below.

04-12-2009

Scientists at the "Laboratoire de chimie et biologie des métaux" (CEA-CNRS-Université J. Fourier, CEA-Grenoble), "Laboratoire de chimie des surfaces et interfaces" (CEA-Saclay) and a team at the Laboratoire d'innovation pour les technologies des énergies nouvelles et les nanomatériaux" (CEA Grenoble) have combined nanoscience and bio-inspired chemistry to develop, for the 1st time, a paltinum free material capable of catalyzing both production of hydrogen and its use in a fuel cell.

This result, important in view of a more competitive hydrogen economy is being published in the journal Science.

17-11-2009

Making still smaller and less power consuming digital memories for mobile electronics? Scientists from CNRS and University Paris Sud XI (Laboratory of Solid State Physics, CNRS / Univ. Paris-Sud 11 and Institut Néel) and CEA-IRAMIS come to demonstrate the feasibility, thanks to a new class, said multiferroic, of materials combining unusual electrical and magnetic properties.


This text is a translation of the joint press release from CEA-CNRS-Univ. Paris XI

In a study published in Physical Review Letters, scientists from the Laboratory of Solid State Physics (CNRS / Université Paris-Sud XI), the "Institut Rayonnement-matière Saclay (CEA IRAMIS)"  and the "Institut Néel" (CNRS) validate the concept of data writing and storage via an electric field, an advantageous technological way for miniaturization memories.

At the microscopic level, atoms and molecules produce electrical and magnetic fields. At our level, in most crystals, the electrical and magnetic properties of the atoms are balanced and cancel each other. Sometimes this is not the case, and for some compounds, known as ferromagnetic, the magnetic properties remain at the macroscopic scale: they may well serve as a magnet. More rarely, for compounds called ferroelectric, an electric order remains at the macroscopic scale. More rarely, both electric and magnetic exist together: this is the case of multiferroic materials. Moreover, in these materials, electrical and magnetic orders interact. Such interaction provides an opportunity to control the spins (magnetic moments of atoms) via an electric field, which represents a considerable challenge especially for information storage.

16-11-2009
O.Bezencenet, A.Barbier, D.Bonamy, R.Belkhou (SOLEIL), P.Ohresser (SOLEIL)

With the advent of spin electronics (spintronics), it became particularly important to visualize and understand how magnetic domains form in magnetic structures. This is particularly tricky for antiferromagnetic layers, which show no macroscopic magnetization and therefore interact weakly with probes.

Antiferromagnetic layers are essential elements in magnetic heterostructures where they are used to pin the magnetization of adjacent layers via magnetic exchange coupling. This type of structure is the basis of operation of modern magnetic sensors (giant magnetoresistance sensors, spin valves and tunnel junctions) and will most likely be incorporated into non-volatile magnetic memories of the future as well as in genuine multiferroic structures. The magnetic domain walls determine the level of noise in electronic devices. It is therefore particularly important and challenging to master them.

16-11-2009

- J. Kermorvant, Laboratoire des Solides Irradiés & Unité Mixte CNRS-Thalès (Palaiseau)
- C.J. van der Beek, Laboratoire des Solides Irradiés, Ecole Polytechnique
- Collaboration J. Briatico, B. Marcilhac, J.C Mage, Unité Mixte CNRS-Thalès (Palaiseau), ANR Blanc grant 07-1-193024 "SURF"

 

The use of high temperature superconductors remains a critical challenge. For applications in telecommunications, it is shown here that Joule heating is responsible for the deteriorated performance of superconducting devices for high microwave power  [1]. Several tracks are proposed to eliminate this undesirable effect.
09-11-2009
Emmanuel Betranhandy, Nathalie Vast and Jelena Sjakste

The mechanical properties of boron carbide at high pressure, explained from first principles.

By theoretical methods based on the density functional theory (DFT and DFPT) [1] [2], the physical properties of boron carbide have been studied for different carbon concentrations and versus temperature and pressure. A type of defect is identified, whose appearance at high pressure and high temperature, can explain the poor mechanical strength upon impact of the material . These results point the way to synthesize a superconducting material combining good mechanical strength at high pressure.

30-09-2009
F. Mallet, F. Ong, A. Palacios, F. Nguyen, P. Bertet, Denis Vion and D. Esteve

After the realization in 2002 of one of the first solid state quantum bits (qubits), scientists from the Quantronique IRAMIS-SPEC research group have performed a further step towards the realization of  an elementary quantum processor: the accurate and non destructive readout of such a qubit.

29-09-2009

A. Cassimi, D. Hennecart, J. Rangama, T. Been,  D. Lelièvre, J.-M. Ramillon,  -  CIMAP, CEA-CNRS, Caen,
T. Muranaka CEA-IRFU Saclay
A. Leredde, X. Fléchard, LPC
H. Shiromaru, J. Matsumoto,  K.-I. Hayakawa Tokyo Metropolitan University

By exploring matter with energetics ions, the charge distribution between the ionic fragments, informs us about the nature of atomic bonds. It has thus been observed that the charge induced by ionisation is more evenly distributed between the fragments in the case of covalent molecules, where the electrons are delocalized over all atoms, compared to rare gas clusters, where electrons remain localized on their individual atom.

Removing several electrons from a polyatomic structure, such as a molecule or a cluster of atoms, usually leads to its dissociation. Indeed, the cohesion of these systems results from the exchange of electrons between the constituent atoms. Removing some of these electrons destabilizes the bonds in the atomic structure, which dissociates upon relaxation. This includes molecules with covalent bonds or, even more delocalized, metallic clusters.

22-09-2009
D. Kopetzki, Y. Michina, T. Gustavsson, D. Carrière

Amphiphilic molecules have a hydrophilic head and a hydrophobic chain. Under certain conditions, they can self-organize into hollow spherical hollow vesicles with a surfactant bilayer enclosing an aqueous core, the overall diameter ranging from a few tens of nanometers to several microns. By carefully choosing specific preparation conditions, the scientists of SIS2M have shown that extremely robust aggregates can be synthesized with an interesting potential for encapsulation.

The vesicles are widely studied for many fundamental issues (mechanism of self-assembly, physical properties of the membrane, etc ...) which understanding may open new perspectives (controlled release of active molecules, chemical nanoreactors, energy conversion, etc...). Usually, these vesicles (in this case also known as "liposomes") are prepared from phospholipids, that also form cell membranes. It would be advantageous to replace the phospholipids with molecules with similar properties but with more accessible or easily modified chemical functions, such as fatty acids molecules. It was already possible to synthesize such vesicles of fatty acids in specific conditions of temperature and pH, but the way to stabilize them was yet to be found: they are indeed very sensitive to external conditions and are destroyed easily to give micelles or crystals.

07-09-2009
D. Dulić, P. Lavie, S. Campidelli, A. Filoramo, collaboration : F. Pump et G. Cuniberti (Université de Dresde)

 

 

Scientists at the Laboratory of Molecular Electronics (IRAMIS / SPEC) recently published a paper entitled  "Controlled Stability of Molecular Junctions" in the prestigious international journal Angewandte Chemie. The work done in collaboration with the group of Prof. Gianaurelio Cuniberti of the University of Dresden shows the influence of the contact interfaces on transport through single molecules by means of the break-junction method.
06-07-2009
Contact CEA : Hamed Merdji

In photography, the scattered light from an illuminated object is recorded with a detector and one get an image of it. If the image is formed with an objective, the optics imposes many limitations (resolution, aberrations ...). To achieve the ultimate resolution, spatially (function of the wavelength of the radiation used) and temporally (function of the "flash" duration), one possible technique (without any optics) is the coherent diffraction. Using a coherent beam like a laser to illuminate the object, a signal modulation due to interference is present and allows digitally reconstructing the exact image of the object with an unprecedented precision. To achieve nanometer or even atomic resolution, we therefore enlighten with a beam of coherent X-rays (radiation laser wavelength nanometer) and record the image. The usually low average illumination requires long accumulations over several laser shots. Recent advances have yielded images with a single shot femtosecond (10-15 s) from a laser laboratory, opening the way for time resolved studies.

For regular arrangements of elementary objects, the Bragg diffraction of X-rays is a powerful technique for characterization of matter at the atomic scale. It is the primary tool for crystallography. The information contained in the Bragg diffraction is rich: if a is the characteristic size of the elementary object, the Bragg peaks are spaced by 1/a in reciprocal space. However, some information is lost: indeed, the maximum frequency at which the diffraction pattern can be sampled is less than the Nyquist frequency (2a). In particular, if the elementary object has an amplitude and a phase, the phase is lost in the Bragg diffraction.

09-06-2009

(french version Version française)

We thought we knew transistor physics fairly well and in particular the millions of MOSFETs (Metal Oxyde Semi-conducteur Field Effect Transistor) which can be found at the core of our computers. However, as early as 1994, a new generation of very high mobility MOSFETs lead to experiments where one could study the regime of extremely low density (Kravchenko et al 1994). The experiments performed on these “extreme transistors” showed very spectacular behaviors: at low density, instead of the expected insulator, one observes a good metallic behavior. (Instead of a divergence of the resistivity at low temperature, one finds that the resistivity decreases by an order of magnitude in a rather narrow window of temperature).

No metal in 2 dimensions. In order to understand why these experimental results were at the origin of a very important research activity, both theoretical and experimental, one must go back to the celebrated article of P.W. Anderson (« Absence of diffusion in certain random lattices », Anderson 1958) for which he was later given the Nobel prize. Anderson studies the effect of a very weak disorder in metals and concludes that at very low temperature (i.e. for a system with quantum coherence), the multiple interferences generated by the reflections on this disorder lead to the localization of the metal wave functions, hence to an insulating behavior. In particular, the effect is very strong in one and two dimensions (as in our MOSFETs) where one finds that an arbitrary weak disorder is enough to drive the system to the insulating limit. During the next 20 years, the scientific community studied localization and verified this paradigm both experimentally and theoretically: there is no metal in two dimensions. The SPEC laboratory was at the origin of key contributions, through in particular, the work of M. Sanquer and J-L Pichard.

The ideal suspect, electronic correlations. The Kravchenko experiments, later reproduced in various experimental groups (including at SPEC in D. L’hôte group) were at the origin of a small revolution in the quantum transport community. Models and physical mechanisms to explain the experimental data came from everywhere to try and conciliate the theory with the observed metallic behavior. Some were looking for experimental artifacts why others came up with weird or ad hoc models. There was only one point on which everybody agreed on : at very low density, electron-electron correlations due to Coulomb repulsion are huge and should be seriously taken into account in the theory. The bad news was that these correlations resist to most theoretical approaches.

Localization and Corelations. During the PhD thesis of Geneviève Fleury, we have developed a numerical approach which uses quantum Monte-Carlo to solve the quantum many-body problem and scaling theory to extract the thermodynamic limit. With this approach, we were able to study, for the first time, the problem of the interplay between Anderson localization and electron-electron correlations. The phase diagram that we have obtained ab-initio (without any adjustable parameters) does show a metallic phase in the experimental regime where it was observed (see figure). The scenario that emerges from our calculations is mixed: On one hand correlations are at the origin of the experimental observations so that new physics arises from the correlations. On the other hand the system is still an insulator in the thermodynamical sense. In other words, we predict that at even lower temperatures the resistivity must diverge in accordance with Anderson localization paradigm.

31-03-2009
H. Desvaux, D. J.Y. Marion, G. Huber, P. Berthault

In nuclear magnetic resonance (NMR) whose medical application is known as MRI, it is the voltage at the extremities of a coil that is usually monitored, since it reflects the variation of the induction created by the precession of the nuclear magnetization after a radiofrequency (RF) excitation pulse. The nuclear magnetization results from the difference in population between the spin states, induced by the strong static magnetic field.

This spectroscopy undoubtely suffers from low sensitivity because this difference remains always low. Thus, at 10 T and room temperature only one nucleus over 100 000 contributes to the signal, defining the detection threshold associated to the radiofrequency excitation technique at typically 1017 spins.

26-01-2009
P. Camy, A. Benayad, J.L. Doualan, V. Ménard, R. Moncorgé (IRAMIS-CIMAP/MIL)

A major development to improve high power laser chains,  is to achieve both higher yields and better tunability. For future high power lasers that operates by diode-pumping at high rate, several materials likely to meet this double requirement (single crystals, glass and ceramics doped) are studied.

Among these materials, calcium fluoride (CaF2)  doped by Yb3+ cristals, like those manufactured and studied by the "Materials and Instrumentation Laser (MIL)" group of the CIMAP Laboratory, are one of the most serious candidates. Under its single crystal form and doped with trivalent ytterbium ions (Yb3+), it has remarkable properties: broad absorption and emission bands, high radiative life time, good transition cross-sections and excellent thermo-mechanical properties.

16-11-2009

- J. Kermorvant, Laboratoire des Solides Irradiés & Unité Mixte CNRS-Thalès (Palaiseau)
- C.J. van der Beek, Laboratoire des Solides Irradiés, Ecole Polytechnique
- Collaboration J. Briatico, B. Marcilhac, J.C Mage, Unité Mixte CNRS-Thalès (Palaiseau), ANR Blanc grant 07-1-193024 "SURF"

 

The use of high temperature superconductors remains a critical challenge. For applications in telecommunications, it is shown here that Joule heating is responsible for the deteriorated performance of superconducting devices for high microwave power  [1]. Several tracks are proposed to eliminate this undesirable effect.
09-11-2009
Emmanuel Betranhandy, Nathalie Vast and Jelena Sjakste

The mechanical properties of boron carbide at high pressure, explained from first principles.

By theoretical methods based on the density functional theory (DFT and DFPT) [1] [2], the physical properties of boron carbide have been studied for different carbon concentrations and versus temperature and pressure. A type of defect is identified, whose appearance at high pressure and high temperature, can explain the poor mechanical strength upon impact of the material . These results point the way to synthesize a superconducting material combining good mechanical strength at high pressure.

04-12-2009

Scientists at the "Laboratoire de chimie et biologie des métaux" (CEA-CNRS-Université J. Fourier, CEA-Grenoble), "Laboratoire de chimie des surfaces et interfaces" (CEA-Saclay) and a team at the Laboratoire d'innovation pour les technologies des énergies nouvelles et les nanomatériaux" (CEA Grenoble) have combined nanoscience and bio-inspired chemistry to develop, for the 1st time, a paltinum free material capable of catalyzing both production of hydrogen and its use in a fuel cell.

This result, important in view of a more competitive hydrogen economy is being published in the journal Science.

16-11-2009
O.Bezencenet, A.Barbier, D.Bonamy, R.Belkhou (SOLEIL), P.Ohresser (SOLEIL)

With the advent of spin electronics (spintronics), it became particularly important to visualize and understand how magnetic domains form in magnetic structures. This is particularly tricky for antiferromagnetic layers, which show no macroscopic magnetization and therefore interact weakly with probes.

Antiferromagnetic layers are essential elements in magnetic heterostructures where they are used to pin the magnetization of adjacent layers via magnetic exchange coupling. This type of structure is the basis of operation of modern magnetic sensors (giant magnetoresistance sensors, spin valves and tunnel junctions) and will most likely be incorporated into non-volatile magnetic memories of the future as well as in genuine multiferroic structures. The magnetic domain walls determine the level of noise in electronic devices. It is therefore particularly important and challenging to master them.

20-12-2009
F .Onufrieva and P. Pfeuty, Laboratoire Léon Brillouin

Superconducting pairing and electronic anomalies induced by the spin collective mode in high Tc cuprate superconductors

Discovered in 1986, superconductivity at high critical temperature remains today one of the great questions in solid state physics and material studies. Several theoretical models are today in competition and regularly confronted with experimental results. Among possible theories, those involving the magnetism in the coupling interaction forming the superconducting electron pairs are often preferred today (instead of the electron-phonon coupling for conventional superconductors within the BCS theory - J. Bardeen, L . Cooper and R. Schrieffer). In particular, the magnetic excitations in the form of spin waves fluctuations, observed experimentally in some superconductors (eg cuprate high Tc superconductors: YBa2Cu3O6+x, and also new non-conventional superconductors based on iron and arsenic, Tc ~ 40 K) must be considered, as shown by the theory developed below.

06-07-2009
Contact CEA : Hamed Merdji

In photography, the scattered light from an illuminated object is recorded with a detector and one get an image of it. If the image is formed with an objective, the optics imposes many limitations (resolution, aberrations ...). To achieve the ultimate resolution, spatially (function of the wavelength of the radiation used) and temporally (function of the "flash" duration), one possible technique (without any optics) is the coherent diffraction. Using a coherent beam like a laser to illuminate the object, a signal modulation due to interference is present and allows digitally reconstructing the exact image of the object with an unprecedented precision. To achieve nanometer or even atomic resolution, we therefore enlighten with a beam of coherent X-rays (radiation laser wavelength nanometer) and record the image. The usually low average illumination requires long accumulations over several laser shots. Recent advances have yielded images with a single shot femtosecond (10-15 s) from a laser laboratory, opening the way for time resolved studies.

For regular arrangements of elementary objects, the Bragg diffraction of X-rays is a powerful technique for characterization of matter at the atomic scale. It is the primary tool for crystallography. The information contained in the Bragg diffraction is rich: if a is the characteristic size of the elementary object, the Bragg peaks are spaced by 1/a in reciprocal space. However, some information is lost: indeed, the maximum frequency at which the diffraction pattern can be sampled is less than the Nyquist frequency (2a). In particular, if the elementary object has an amplitude and a phase, the phase is lost in the Bragg diffraction.

22-09-2009
D. Kopetzki, Y. Michina, T. Gustavsson, D. Carrière

Amphiphilic molecules have a hydrophilic head and a hydrophobic chain. Under certain conditions, they can self-organize into hollow spherical hollow vesicles with a surfactant bilayer enclosing an aqueous core, the overall diameter ranging from a few tens of nanometers to several microns. By carefully choosing specific preparation conditions, the scientists of SIS2M have shown that extremely robust aggregates can be synthesized with an interesting potential for encapsulation.

The vesicles are widely studied for many fundamental issues (mechanism of self-assembly, physical properties of the membrane, etc ...) which understanding may open new perspectives (controlled release of active molecules, chemical nanoreactors, energy conversion, etc...). Usually, these vesicles (in this case also known as "liposomes") are prepared from phospholipids, that also form cell membranes. It would be advantageous to replace the phospholipids with molecules with similar properties but with more accessible or easily modified chemical functions, such as fatty acids molecules. It was already possible to synthesize such vesicles of fatty acids in specific conditions of temperature and pH, but the way to stabilize them was yet to be found: they are indeed very sensitive to external conditions and are destroyed easily to give micelles or crystals.

31-03-2009
H. Desvaux, D. J.Y. Marion, G. Huber, P. Berthault

In nuclear magnetic resonance (NMR) whose medical application is known as MRI, it is the voltage at the extremities of a coil that is usually monitored, since it reflects the variation of the induction created by the precession of the nuclear magnetization after a radiofrequency (RF) excitation pulse. The nuclear magnetization results from the difference in population between the spin states, induced by the strong static magnetic field.

This spectroscopy undoubtely suffers from low sensitivity because this difference remains always low. Thus, at 10 T and room temperature only one nucleus over 100 000 contributes to the signal, defining the detection threshold associated to the radiofrequency excitation technique at typically 1017 spins.

17-11-2009

Making still smaller and less power consuming digital memories for mobile electronics? Scientists from CNRS and University Paris Sud XI (Laboratory of Solid State Physics, CNRS / Univ. Paris-Sud 11 and Institut Néel) and CEA-IRAMIS come to demonstrate the feasibility, thanks to a new class, said multiferroic, of materials combining unusual electrical and magnetic properties.


This text is a translation of the joint press release from CEA-CNRS-Univ. Paris XI

In a study published in Physical Review Letters, scientists from the Laboratory of Solid State Physics (CNRS / Université Paris-Sud XI), the "Institut Rayonnement-matière Saclay (CEA IRAMIS)"  and the "Institut Néel" (CNRS) validate the concept of data writing and storage via an electric field, an advantageous technological way for miniaturization memories.

At the microscopic level, atoms and molecules produce electrical and magnetic fields. At our level, in most crystals, the electrical and magnetic properties of the atoms are balanced and cancel each other. Sometimes this is not the case, and for some compounds, known as ferromagnetic, the magnetic properties remain at the macroscopic scale: they may well serve as a magnet. More rarely, for compounds called ferroelectric, an electric order remains at the macroscopic scale. More rarely, both electric and magnetic exist together: this is the case of multiferroic materials. Moreover, in these materials, electrical and magnetic orders interact. Such interaction provides an opportunity to control the spins (magnetic moments of atoms) via an electric field, which represents a considerable challenge especially for information storage.

30-09-2009
F. Mallet, F. Ong, A. Palacios, F. Nguyen, P. Bertet, Denis Vion and D. Esteve

After the realization in 2002 of one of the first solid state quantum bits (qubits), scientists from the Quantronique IRAMIS-SPEC research group have performed a further step towards the realization of  an elementary quantum processor: the accurate and non destructive readout of such a qubit.

09-06-2009

(french version Version française)

We thought we knew transistor physics fairly well and in particular the millions of MOSFETs (Metal Oxyde Semi-conducteur Field Effect Transistor) which can be found at the core of our computers. However, as early as 1994, a new generation of very high mobility MOSFETs lead to experiments where one could study the regime of extremely low density (Kravchenko et al 1994). The experiments performed on these “extreme transistors” showed very spectacular behaviors: at low density, instead of the expected insulator, one observes a good metallic behavior. (Instead of a divergence of the resistivity at low temperature, one finds that the resistivity decreases by an order of magnitude in a rather narrow window of temperature).

No metal in 2 dimensions. In order to understand why these experimental results were at the origin of a very important research activity, both theoretical and experimental, one must go back to the celebrated article of P.W. Anderson (« Absence of diffusion in certain random lattices », Anderson 1958) for which he was later given the Nobel prize. Anderson studies the effect of a very weak disorder in metals and concludes that at very low temperature (i.e. for a system with quantum coherence), the multiple interferences generated by the reflections on this disorder lead to the localization of the metal wave functions, hence to an insulating behavior. In particular, the effect is very strong in one and two dimensions (as in our MOSFETs) where one finds that an arbitrary weak disorder is enough to drive the system to the insulating limit. During the next 20 years, the scientific community studied localization and verified this paradigm both experimentally and theoretically: there is no metal in two dimensions. The SPEC laboratory was at the origin of key contributions, through in particular, the work of M. Sanquer and J-L Pichard.

The ideal suspect, electronic correlations. The Kravchenko experiments, later reproduced in various experimental groups (including at SPEC in D. L’hôte group) were at the origin of a small revolution in the quantum transport community. Models and physical mechanisms to explain the experimental data came from everywhere to try and conciliate the theory with the observed metallic behavior. Some were looking for experimental artifacts why others came up with weird or ad hoc models. There was only one point on which everybody agreed on : at very low density, electron-electron correlations due to Coulomb repulsion are huge and should be seriously taken into account in the theory. The bad news was that these correlations resist to most theoretical approaches.

Localization and Corelations. During the PhD thesis of Geneviève Fleury, we have developed a numerical approach which uses quantum Monte-Carlo to solve the quantum many-body problem and scaling theory to extract the thermodynamic limit. With this approach, we were able to study, for the first time, the problem of the interplay between Anderson localization and electron-electron correlations. The phase diagram that we have obtained ab-initio (without any adjustable parameters) does show a metallic phase in the experimental regime where it was observed (see figure). The scenario that emerges from our calculations is mixed: On one hand correlations are at the origin of the experimental observations so that new physics arises from the correlations. On the other hand the system is still an insulator in the thermodynamical sense. In other words, we predict that at even lower temperatures the resistivity must diverge in accordance with Anderson localization paradigm.

 

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