Cooper pair | EPFL Graph Search

In condensed matter physics, a Cooper pair or BCS pair (Bardeen–Cooper–Schrieffer pair) is a pair of electrons (or other fermions) bound together at low temperatures in a certain manner first described in 1956 by American physicist Leon Cooper. Cooper pair Cooper showed that an arbitrarily small attraction between electrons in a metal can cause a paired state of electrons to have a lower energy than the Fermi energy, which implies that the pair is bound. In conventional superconductors, this attraction is due to the electron–phonon interaction. The Cooper pair state is responsible for superconductivity, as described in the BCS theory developed by John Bardeen, Leon Cooper, and John Schrieffer for which they shared the 1972 Nobel Prize. Although Cooper pairing is a quantum effect, the reason for the pairing can be seen from a simplified classical explanation. An electron in a metal normally behaves as a free particle. The electron is repelled from o

Kinks in the angle resolved photoemission and inelastic x-ray spectra of high temperature superconductors

Jeff Graf

Though the high superconducting transition temperature (Tc) is the most interesting technological aspect of high temperature superconductors, the complex way in which the spin, lattice and electronic degrees of freedom interplay, makes them of the highest scientific interest. This is illustrated by their rich phase diagram characterized by a variety of exotic groundstate including; Mott insulating, pseudogap and high temperature superconductivity. One of the most serious barriers that has prevented our understanding of the mechanism of high temperature superconductors thus far is the lack of information on how low energy Landau quasiparticles result from an organization of real particles. This organization, often colloquially referred to as "dressing", is a fundamental general concept in physics that explains a variety of physical phenomena, such as exotic particle formations and phase transitions. The role of the lattice in this dressing is particularly controversial. Phonons are quanta of lattice vibration energy, and play a crucial role in conventional superconductivity. They provide an attractive interaction allowing the electrons to condensate in superconducting Cooper pairs. However, high temperature superconductivity in the cuprates in achieved through hole doping in an antiferromagnetic Mott insulator. In this case, the antiferromagnetic background, the strong coulomb repulsion and the anisotropic superconducting gap all suggest a marginal role of the phonons. In order to assess experimentally the exact role of the phonon, angle resolved photoemission spectroscopy (ARPES) is the weapon of choice given its unique ability to probe the electronic structure of solids in an energy- and momentum-resolved manner. In this thesis, I will use ARPES to characterize the various form of dressing of the quasiparticles in the high temperature superconductor from the Fermi level all the way to the valence band complex. I'll show that when combined with isotope substitution and inelastic x-rays scattering, the strong electron-phonon interaction can be probed in vivid details. This thesis presents three fundamental results. 1) Using the isotope effect, the important and unusual role of the phonon is demonstrated, as well as the strong interplay between the magnetic and phonic degrees of freedom. 2) Using inelastic scattering, the intriguing interplay between the Fermi surface and the bond stretching phonon is exposed. And 3) By exploring the ARPES data up to high energy, two new energy scales at high binding energy as well as the spectral waterfalls phenomenon are revealed. When all these new elements are considered together, they clearly show the need to consider simultaneously the spin, lattice, Fermi surface topology and electronic degrees of freedom. The spectacular progress in ARPES techniques over the past two decades was critical for these results, and I will present in this thesis our current effort in pushing the techniques even further. In particular I'll present my efforts in building a laser based ARPES setup with a hemispherical analyzer and a spin resolved time of flight analyzer.

EPFL2008

Josephson Tunneling at the Atomic Scale

Berthold Jäck

The aim of this thesis is to characterize the properties of a Josephson junction in a Scanning Tunneling Microscope (STM) at millikelvin temperatures and to implement Josephson STM (JSTM) as a versatile probe at the atomic scale. To this end we investigate the I(V) tunneling characteristics of the Josephson junction in our STM at a base temperature of 15 mK by means of current-biased and voltage-biased experiments. We find that in the tunneling regime, the Josephson junction is operated in the dynamical Coulomb blockade (DCB) regime in which the sequential tunneling of Cooper pairs dominates the tunneling current. Employing P(E)-theory allows us to model experimental I(V) characteristics from voltage-biased experiments and determine experimental values of the Josephson critical current in agreement to theory. Moreover, we observe a breakdown of P(E)-theory for experiments at large values of the tunneling conductance on the order of the quantum of conductance, which could indicate that the coherent tunneling of Cooper pairs strongly contributes to the tunneling current in this limit. We also observe that the Josephson junction in an STM at temperatures well below 100 mK is highly sensitive to its electromagnetic environment that results from its tiny junction capacitance of a few femtofarads. The combination of the experimental results with numerical simulations reveals that the immediate environment of the Josephson junction in an STM is frequency-dependent and, additionally, that a typical STM geometry shares the electromagnetic properties of a monopole antenna with the STM tip acting as the antenna. Comparing the I(V) curves of voltage-biased and current-biased experiments, we observe that the time evolution of the phase is strongly effected by dissipation due to quasiparticle excitations. From investigations on the retrapping current we show first that the temporal evolution of the junction phase in our STM satisfies a classical equation of motion. Second, we can determine two different channels for energy dissipation of the junction phase. For tunneling resistance values RN< 150 kOhm the junction dissipates via Andreev reflections whereas for larger values of RN

EPFL2015

Resonant Inelastic X-Ray Scattering Study of Electron-Exciton Coupling in High-T-c Cuprates

Francesco Barantani, Christophe Berthod, Fabrizio Carbone, Ivan Madan, Thorsten Schmitt, Yi Tseng, Dirk Van der Marel

Explaining the mechanism of superconductivity in the high-T-c cuprates requires an understanding of what causes electrons to form Cooper pairs. Pairing can be mediated by phonons, the screened Coulomb force, spin or charge fluctuations, excitons, or by a combination of these. An excitonic pairing mechanism has been postulated, but experimental evidence for coupling between conduction electrons and excitons in the cuprates is sporadic. Here we use resonant inelastic x-ray scattering to monitor the temperature dependence of the dd exciton spectrum of Bi2Sr2CaCu2O8-x crystals with different charge carrier concentrations. We observe a significant change of the dd exciton spectra when the materials pass from the normal state into the superconductor state. Our observations show that the dd excitons start to shift up (down) in the overdoped (underdoped) sample when the material enters the superconducting phase. We attribute the superconductivity-induced effect and its sign reversal from underdoped to overdoped to the exchange coupling of the site of the dd exciton to the surrounding copper spins.