Studies on the Effect of Strain on the Electronic Properties of Ultra Thin Films of Superconducting Cuprates

High Tc superconductivity in cuprates is still an unresolved topic, and one particular challenge is why in-plane compressive strain enhances Tc in ultra thin LSCO-214 films (< 40nm), whilst tensile strain diminishes it. Especially intriguing is whether altering this strain at a given doping has an effect on T* (pseudogap) or Tc (condensation/phase coherence). As the ratio between T* and Tc is so high, we systematically grew a series of BSCCO-2201 films by in house pulsed laser deposition (PLD), and then thoroughly analysed these films with X-Rays, four point resistivity measurements and XPS. We succeeded to induce strain into the films; however, the strain was lost in the annealing process necessary for superconductivity. Subsequently, we focused on LSCO-214 films, and grew a series of strained ultra thin films of this compound. While the films were superconducting, none of our samples showed enhanced superconduc- tivity, probably due to growth disorder.Therefore, we eventually studied high quality films grown by MBE from BNL. We used the low energy muon beamline at PSI to perform μSR studies on two different com- pressive strain states. The successful measurements of the penetration depth and Ginzburg- Landau parameter κ on ultra thin films of this type using low energy muons showed the benefits of this technique. As the 2 films were of different thicknesses, further studies need to be done to disentangle which observed effects are due to strain and/or variation in c-axis, and which are due solely to thickness. Our accumulated results and those of other groups, (notably Naito et. al. at NTT) on strain in LSCO-214 films, and the ensemble of data, including our study, indicate that most likely it is a phase coherence enhancement due to the in-plane compressive strain that raises the Tc. In summary, in addition to the succesful implementation of PLD heteroepitaxy, XRD analysis and ρ(T) studies of LSCO-214 films we were able to establish that: 1. Phase coherence is improved by in plane compressive strain, hence Tc enhancement 2. Tc also seems to scale with apical oxygen, as previously reported by H. Keller et. al., and local distortions around the copper-oxygen tetrahedra 3. Last but not least, we were even able to induce strain into BSCCO-2201 films, yet at present the full electronic properties of this system are neither measured nor under- stood. Finally, further systematic studies on μSR are proposed, and are indeed underway at PSI, to further elucidate the microscopic role of strain in the mechanism of high Tc superconductivity.

The Interplay Between Structural and Electronic Degrees of Freedom in Superconductors

Mathieu Julien Gino Cottet

More than one hundred years after the discovery of superconductivity in Leiden, the intriguing physics of several unconventional classes of superconductors continue to fascinate and challenge scientists from all over the world. The majority of these compounds belongs to a major class of materials known as strongly correlated systems that exhibit interesting and diverse physical properties, like the high-temperature superconductivity, the Mott-insulator transition, the colossal magnetoresistance or the structurally-driven electronic phase transitions. Rigorous and deep investigations of the electronic properties, as well as their tunability, should lead to new attractive commercial applications. With the use of time-resolved experiments or bulk sensitive techniques, which represent the main subject of this thesis, the electronic and magnetic properties of strongly correlated type-II superconductors and high-temperature copper-oxide superconductors are accessible. These techniques take advantages of the photon tunability in the case of optical pump-probe by allowing one to record the transient response of the material over the whole visible spectrum, or in the case of X-ray diffraction, the tunability of the synchrotron radiation thanks to the slicing operating at the SLS (Swiss Light Source) in Villigen, Switzerland. Cryo-LTEM allows one to work across the (H ,T ) phase diagram of type-II superconductors, thus enabling the study of the coexistence between normal and superconducting state by careful regulation of the applied magnetic field near the specimen. The Chapter 1 (1) is a general introduction dedicated to the historical evolution of our understanding of superconductivity, according to the theories and general concepts used and developed is this thesis. Chapter 2 (3) is dedicated to the experimental techniques. A careful description of the different setups, as well as a theoretical development needed for the calculations is presented. In Chapter 3 (4), the investigation of magnesium diboride superconductor by means of cryo-LTEM is presented. By observing the field-dependent vortex lattices in this system and performing calculations using the theoretical frameworks developed by Ginzburg-Landau and Abrikosov, I found that this compound belongs to the type-II family [1], and not to the novel type 1.5 class of superconductor. In Chapter 4 (5), I present the investigation related the charge density waves compound Lu5Ir4Si10. The use of ultrafast time-resolved optical pump-probe experiments, coupled with ab-anitio theoretical calculations of the electron-phonon coupling and the electronic structure, permits to shine new light on the nature of this transition which we identified as a structurally-driven Peierls transition [2]. The Chapter 5 (6) is dedicated to the investigation of the high-temperature copper-oxide superconductor, La2−x Srx CuO4, for different doping levels; the experiment was carrying out at the SLS on the Micro-XAS beamline, By using a transient temperature model and comparing our experimental results with temperature-dependent density of states calculations, we report on the electronic temperature dependence of the electron-phonon coupling constant (submitted to Phys. Rev. B).

EPFL2013

Theory of superconductivity in a three-orbital model of Sr2RuO4

Ronny Thomale, Yong Yang

In conventional and high transition temperature copper oxide and iron pnictide superconductors, the Cooper pairs all have even parity. As a rare exception, Sr2RuO4 is the first prime candidate for topological chiral p-wave superconductivity, which has time-reversal breaking odd-parity Cooper pairs known to exist before only in the neutral superfluid He-3. However, there are several key unresolved issues hampering the microscopic description of the unconventional superconductivity. Spin fluctuations at both large and small wave vectors are present in experiments, but how they arise and drive superconductivity is not yet clear. Spontaneous edge current is expected but not observed conclusively. Specific experiments point to highly band- and/or momentum-dependent energy gaps for quasiparticle excitations in the superconducting state. Here, by comprehensive functional renormalization group calculations with all relevant bands, we disentangle the various competing possibilities. In particular, we show the small wave vector spin fluctuations, driven by a single two-dimensional band, trigger p-wave superconductivity with quasi-nodal energy gaps. Copyright (C) EPLA, 2013

Epl Association, European Physical Society2013

Contribution to the Production and Characterization of W-Y, W-Y2O3 and W-TiC Materials for Fusion Reactors

Lyubomira Veleva

This PhD Thesis work was aimed at investigating the potentiality of tungsten-base materials as structural materials for the future thermonuclear fusion reactors in attempting to develop reduced activation tungsten-base materials with high strength and sufficient ductility, especially in terms of fracture toughness and ductile-to-brittle transition temperature, on the basis of the following ideas: (1) nano-grained materials are expected to show an improved ductility with respect to normal grain-sized materials, and (2) nano-grained materials and materials reinforced with either oxide or carbide particles are expected to show improved strength and radiation resistance, as (i) the particles should act as obstacles for the propagation of mobile dislocations, (ii) the numerous grain boundaries and interfaces between the matrix and the particles should act as sinks for the irradiation-induced defects, and (iii) the particles should also help stabilizing the numerous grain boundaries upon thermal annealing and/or irradiation. A variety of W-base materials, namely W-(0.3-1.0-2.0)Y2O3, W-(0.3-1.0-2.0)Y and W-(0.3-0.9-1.7)TiC materials (in weight percent), have been successfully produced at the laboratory scale by powder metallurgy techniques including mechanical alloying followed by either cold pressing and sintering or hot isostatic pressing (HIPping). X-ray diffractometry and scanning and transmission electron microscopy observations combined with density and microhardness measurements as well as with tensile, Charpy impact, fracture toughness and non-standard three point bending tests allowed the identification of optimal chemical compositions and manufacturing conditions, among those investigated and on the basis of the experimental devices at disposal. Microscopic investigations showed that the materials produced at the laboratory scale are composed of small grains, with sizes between a few tens and a few hundreds nanometers, and contain a high density of small Y2O3 or TiC particles, with sizes between 1 and 50 nm. In the case of W-Y materials, all the Y transformed into Y2O3 during mechanical alloying, due to the high amount of O (around 1 wt.%) present in the milled powders, which is beneficial for reducing the excess O content in the materials. All materials also contain a residual porosity of a few percents, which is typical of materials compacted by HIPping. Macroscopic investigations showed that the HIPped materials exhibit high strength values and a promising resistance to irradiation but poor fracture properties up to a very high temperature of about 1100 °C, due mostly to air contamination of the mechanically alloyed powders and to residual porosity of the compacted materials. The materials produced entirely at the laboratory scale by mechanical alloying and HIPping are of better quality than a W-2Y material that was produced by mechanical alloying at the laboratory scale and compacted by sintering and forging at the Plansee company (Austria). Further studies should focus on reducing the O content in the mechanically alloyed powders and the residual porosity of the compacted materials. The grain size should be also optimized as well as the size distribution, number density and crystallographic features of the oxide and carbide particles.