Project

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Real-time structural investigation of superconductors

Applicant Carbone Fabrizio
Number 126409
Funding scheme Ambizione
Research institution Laboratoire de spectroscopie ultrarapide EPFL - SB - ISIC - LSU
Institution of higher education EPF Lausanne - EPFL
Main discipline Condensed Matter Physics
Start/End 01.11.2009 - 31.08.2010
Approved amount 155'878.00
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All Disciplines (2)

Discipline
Condensed Matter Physics
Physical Chemistry

Keywords (5)

superconductivity; cuprates; Ultrafast electron diffraction; femtosecond; electron-phonon coupling

Lay Summary (English)

Lead
Lay summary
The aim of this project is to observe the structural dynamics that accompany superconductivity by means of ultrafast electron diffraction. This information is important in understanding the mechanism responsible for high temperature superconductivity in cuprates. These materials are promising for application in high capacity cables, magnets for MRI, high precision magnetic sensors and even electronic circuits. However, the inability to engineer higher transition temperatures, limits severely their effective applications. The understanding of their physics is the first step towards a better control of their properties. Ultrafast light pulses can deplete the Cooper-pairs condensate in a cold superconductor, driving the system in its normal metallic state. During the depletion and subsequent re-formation of the condensate, atomic motions can be followed with femtosecond resolution in time and atomic resolution in space by diffracting ultrashort electron pulses. By investigating different chemical compositions, the relation between the structural dynamics and superconductivity will be studied. In particular, these experiments give information about the electron-phonon coupling parameter, its symmetry and temperature dependence. Currently, ultrafast electron diffraction is capable of a time-resolution of few hundreds femtoseconds. During our project we will develop the Radiofrequency streak camera technology, that will allow us to break this limit and push time-resolution below 100 fs. The concept behind this technology is the following: a long pulse of electrons is overlapped on the sample with an excitation light pulse. The electron scattering from the sample before the arrival of the laser pulse see an unperturbed specimen, while those scattering after the laser pulse arrival see the excited state. The sample evolution in time, therefore, is contained in the electron pulse, up to its duration. When such pulse enters the streak camera, the particles arriving first are deflected upwards, while those arriving later are deflected downwards. As a result, the temporal evolution of the sample is projected on a spatial coordinate that can be imaged on a CCD camera. Time resolution is not given anymore by the ability to generate short electron pulses, but rather by the ability of the streaking device to separate in space particles that are very close in time. Thanks to this technological advance, the ultrafast phenomena associated with the electron phonon coupling in superconductors will be studied with unprecedented detail.
Direct link to Lay Summary Last update: 21.02.2013

Responsible applicant and co-applicants

Employees

Associated projects

Number Title Start Funding scheme
128269 The dynamics of chemical bonds by ultrafast electron diffraction 01.09.2010 SNSF Professorships
117088 Proposition d'éxperiences de Diffraction Ultra-rapide d'électrons sur des couches minces organiques au Caltech 01.04.2007 Fellowships for prospective researchers

Abstract

In this proposal we intend to address the issue of structural motions involved in superconductivity. The debate around the pairing mechanism responsible for high temperature superconductivity has been going on for at least two decades. The role of the lattice motions in the formation of Cooper pairs has been elusive to static structural probes. In fact, while anomalies of structural parameter have been detected at the phase transition, their relation to superconductivity is still unclear. Whether the structural changes occurring at the superconducting transition temperature cause superconductivity, or are a consequence of the modified electronic structure below Tc is under intense debate. A notion of time and sequence of causes and effects would be elucidating in this respect, and can be accessible by modern time-resolved techniques, capable of fs (femtosecond) time-resolution, the time-scale of Cooper pair formation and electron-phonon coupling.Our idea is to observe the motion of atoms during the formation of the superconducting condensate, by means of time-resolved electron diffraction. Once Cooper pairs have been broken by a laser pulse, the lattice can be monitored diffracting fs electron pulses.Ultrafast Electron Crystallography (UEC) has been demonstrated to be a powerful tool in the investigation of the electron-phonon coupling of solids. An ultrashort laser pulse is used to induce a temperature imbalance between the lattice and the electrons in the conduction band. The motion of ions in all directions can be followed with fs resolution during the transfer of energy from the electronic subsystem to the bulk by diffraction of fs electron pulses. In a cold superconductor, the initial excitation is used to drive the electronic structure in the normal state, the structural motions are followed in time while the electrons reach their equilibrium with the lattice.This approach allowed the observation of coupling between electrons and a particular vibration of the unit cell (buckling of the Cu-O planes), characteristic of a high temperature superconductor. The time scale for this process was also estimated and found to be comparable with the characteristic time of spin exchange. We intend to extend this preliminary study, by investigating different materials with improved time-resolution. The main questions we want to answer concern the exact nature of the initial laser excitation, for which a resolution in the order of 100 fs is needed (about 5 to 7 times better then currently available), and the exact connection between the crystal structure and its dynamics and the superconducting transition temperature. By studying different compounds with different compositions and lattice arrangement we want to find a correlation between specific atomic motions and superconductivity. Very recently, a new class of materials have been found to superconduct at temperature around 50 K, and the investigation of these systems and the nature of pairing involved is under way. The variety of systems available, exhibiting superconductivity, includes standard metals (Tc less or equal to 10 K) for which BCS theory applies, cuprates ceramics (Tc as high as 130 K), 2 gaps systems such MgB2 (Tc around 40 K), and the new Fe based materials (Tc around 50 K). The exact role of structural dynamics in superconductivity is only known for conventional metals where BCS theory applies. In cuprates, the atomic motions involved in superconductivity were directly observed by means of UEC for the first time. The opportunity to systematically study the structural dynamics in different types of superconductors will provide valuable insight in the chemistry of superconductors and its influence on the transition temperature. For this purpose, a new UEC set-up will be developed in the group of prof. Chergui. A new detection technique based on a radiofrequency streak camera technology, which is currently being developed in collaboration with Caltech and UCLA, will be used in order to achieve better time-resolution.
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