A Space-Time Study of the Electron-Electron Interaction in High-TC Parent Compound La2CuO4: In Search of an Attractive Effective Interaction

Abstract

The superconductivity under doping in the famous cuprate compounds is believed to originate in the CuO2 planes. The mediator of pairing is generally accepted to be induced from electronic degrees of freedom. While many mechanisms, such as spin fluctuations, have been proposed throughout the years, little research has been carried out on the role of the retarded attraction due to electronic overscreening.

In this study, the dynamically screened interaction W(r,r';t) is studied in the CuO2 planes of the parent compound La2CuO4. To this end, a repulsive test charge, representing an electron, is introduced at various r' in the CuO2 plane, and W(r,r';t) is calculated as a function of r and t. The aim is to explore the possibility of the existence of an attractive effective interaction between electrons. The static screened interaction W(r,r';ω=0), which is the time average of W(r,r';t) is also studied. In addition, the effective interactions U1(r,r';t) and U3(r,r';t), corresponding to the well-known one- and three-band models, are investigated using the constrained random-phase approximation (cRPA). Substantial regions of the CuO2 plane do indeed exhibit an attractive effective interaction, with both U1 and U3 being negative. On the other hand, the extent of such regions is significantly smaller in SrVO3, a non-superconducting metal.

The present work suggests that future studies of electronic overscreening as a possible pairing mechanism is worth consideration. The same ab initio parameters as obtained and utilized in this work can be used to construct a Hubbard-Holstein model, within which the gap function can be calculated by solving the Eliashberg equations.

Popular Abstract

The delayed attraction between electrons has been investigated in high-temperature superconductor "LSCO" by simulating the process of electronic shielding.

Lossless power transmission and personal quantum computers may sound like two unreachable clichés of future technology. These are, however, feasible scientific goals to reach since the main key behind both are room-temperature superconductors. Superconductivity is an exotic property of some materials at low temperature where electrons move together in an ordered fashion through the crystal without encountering collisions. This frictionless electronic motion is caused by an effective attraction between electrons, resulting in what is called pairing. Energy dissipation of a single electron cannot occur since it requires breaking the pairing with another electron, which costs too much energy. Currents thus persist after removing an external voltage since the energy of the moving electrons is not converted to heat.

No material has yet been synthesized which keeps its superconductivity at room temperature. Doped cuprates, a class of materials containing two-dimensional crystal planes made up of copper and oxygen, are superconducting up to about halfways to room temperature. Understanding the pairing in the cuprates can thereby provide clues for pushing this higher, ultimately reaching room temperature. In the cuprates, the pairing is known to be confined to the copperoxide planes. What is not known is the actual dominating mechanism behind the pairing, in other words, the main cause of attraction. Our investigation suggests that electronic shielding, more commonly called screening, could have a more important role in pairing than previously thought. This was seen by plotting the screened interaction, a measure of the effective forces between electrons, as a function of space and time.

Two single electrons repel each other, but in a system with a large number of electrons, two electron will not necessarily repel due to the presence of the surrounding interacting electrons. Just like throwing a rock into the sea, imagine throwing an electron into a material and studying the resulting waves in the screened interaction. These propagating waves correspond to attraction between electrons at the troughs and repulsion at the crests. In this work, by taking the time-average of these waves, large regions with an average attraction were found in the copper-oxygen planes of LSCO, a prototypical cuprate. On the contrary, such regions were not seen in the similar vanadium-oxygen planes of SrVO3, a non-superconducting metal.

While this significant difference between LSCO and SrVO3 was for the time-average, studying these screened interaction waves not only as a function of space, but also of time, allowed for further insight. After introducing and removing an electron to the copper-oxygen planes of LSCO, other electrons got repelled, leading to a reduction of electrons in the surrounding region and thus to a delayed attraction. This attraction then reestablished a high density of electrons in the region which then caused similar, but more complex, cycles of repulsion and attraction. These oscillations turned out to decay very quickly. The first few cycles are thus likely to be particularly important for the pairing.

Comparing several different cuprates is a natural next step. The hope is to find a correlation between the extent of electronic attraction and the temperature at which superconductivity disappears. For this, the space-time approach developed in this work will be crucial.

• author: Sjöstrand, Tor
• supervisor: Ferdi Aryasetiawan
• organization: Mathematical Physics; Department of Physics
• course: PHYM01 20161
• year: 2016
• type: H2 - Master's Degree (Two Years)
• subject: Physics and Astronomy
• language: English
• id: 8893725
• date added to LUP: 2016-10-26 16:24:51
• date last changed: 2017-04-10 09:07:25