Deviations from the London model in superconductors

Using Small-Angle Neutron Scattering (SANS) we have studied the flux line lattices of the first heavy-fermion superconductor to be discovered, CeCu2Si2, the well-known high-temperature superconductor YBa2Cu3O7, along with Ca-doped variants of this compound, and several topological superconductor candidates that have attracted a lot of attention in the last few years, such as PbTaSe2, Au2Pb and β-Bi2Pd.Unconventional superconductors, with pairing mechanisms and properties that are not accounted for within BCS theory, are a subject of continuing interest. One example is heavy-fermion materials, in which the charge carriers respond as if they had a mass many times that of a free electron. We find that CeCu2Si2 shows a strong increase of scattered intensity from the flux lines as the applied magnetic field is increased towards the edge of the superconducting phase. In the standard theories, this intensity should drop steadily towards zero. This indicates that the superconductivity in this material is not destroyed by the same mechanism as in most all other superconductors but is instead destroyed by Pauli paramagnetic effects (PPE), which act to break up the Cooper pairs by favouring parallel alignment of the spins, increasing the relative strength of the magnetization in the vortex cores. This behaviour had previously been observed in CeCoIn5. To better understand the behaviour of CeCu2Si2 and other Pauli limited superconductors, we present a physically-based expression for the size and magnetization of the vortex cores in Pauli-limited superconductors.For many years, high-temperature superconductors have attracted significant attention as they may be key for the use of superconductors in our everyday life. In the high-Tc superconductors, like YBa2Cu3O7−δ (YBCO), the doping level can be used to tune the observed properties. Usually, δ is the doping control parameter in YBCO, which varies the occupancy of the oxygen chains running along the b-axis of the crystal. The (d-wave) superconductivity in YBCO develops primarily from the CuO2 planes, but s-wave superconductivity also develops from the chains, which can be seen in the distortion of the vortex lattice. We study the temperature and field dependence of this anisotropy and SANS intensity from the vortex lattice up to 25 T using a newly developed formula for SANS time-of-flight (TOF) experiments. Furthermore, we can dope the yttrium site with calcium instead, resulting in a higher hole concentration than fully over-doped YBCO, pushing the field scale down closer to the magnetic fields we can apply in experiment. We include in this work some of the specific effects of doping YBCO with calcium and how it lowers the characteristic critical fields.Topological materials have attracted a lot of attention due to their novel quantum states and their possible applications to quantum computation. A key property is the appearance of Majorana bound states in the vortex core. When the Bogoliubov quasiparticles associated with the superconducting state can be constructed as superpositions of electron and hole states such that the bound quasiparticle is its own anti-quasiparticle (i.e. a Majorana particle), in the bulk, these quasiparticles are dispersive Majorana fermions, and in the vortex core they lead to bound states that obey non-Abelian statistics, and can (in theory) be used as qubits. In this thesis we report SANS experiments performed on three different topological superconductor candidates, PbTaSe2, Au2Pb and β-Bi2Pd. For the latter, we discuss the anisotropy of the vortex lattice and the temperature dependence of the SANS intensity by rotating the sample with respect to the magnetic field direction and shed some light on the superconducting gap nature of β-Bi2Pd

Unconventional superconductivity in the nickel-chalcogenide superconductor, TlNi2_2Se2_2

We present the results of a study of the vortex lattice (VL) of the nickel chalcogenide superconductor TlNi2Se2, using small angle neutron scattering. This superconductor has the same crystal symmetry as the iron arsenide materials. Previous work points to it being a two-gap superconductor, with an unknown pairing mechanism. No structural transitions in the vortex lattice are seen in the phase diagram, arguing against d-wave gap symmetry. Empirical fits of the temperature-dependence of the form factor and penetration depth rule out a simple s-wave model, supporting the presence of nodes in the gap function. The variation of the VL opening angle with field is consistent with earlier reports of of multiple gaps

Role of magnetic ions in the thermal Hall effect of the paramagnetic insulator TmVO4_{4}

In a growing number of materials, phonons have been found to generate a thermal Hall effect, but the underlying mechanism remains unclear. Inspired by previous studies that revealed the importance of Tb$^{3+}$ ions in generating the thermal Hall effect of Tb$_{2}$Ti$_{2}$O$_{7}$, we investigated the role of Tm$^{3+}$ ions in TmVO$_{4}$, a paramagnetic insulator with a different crystal structure. We observe a negative thermal Hall conductivity in TmVO$_{4}$ with a magnitude such that the Hall angle, $|\kappa_{xy}$/$\kappa_{xx}|$, is approximately 1 x 10$^{-3}$ at $H$ = 15 T and $T$ = 20 K, typical for a phonon-generated thermal Hall effect. In contrast to the negligible $\kappa_{xy}$ found in Y$_{2}$Ti$_{2}$O$_{7}$, we observe a negative $\kappa_{xy}$ in YVO$_{4}$ with a Hall angle of magnitude comparable to that of TmVO$_{4}$. This shows that the Tm$^{3+}$ ions are not essential for the thermal Hall effect in this family of materials. Interestingly, at an intermediate Y concentration of 30 % in Tm$_{1-x}$Y$_{x}$VO$_{4}$, $\kappa_{xy}$ was found to have a positive sign, pointing to the possible importance of impurities in the thermal Hall effect of phonons

Searching for the signature of a pair density wave in YBa2_2Cu3_3O6.67_{6.67} using high energy X-ray diffraction

We have carried out a search for a pair density wave signature using high-energy X-ray diffraction in fields up to 16 T. We do not see evidence for a signal at the predicted wavevector. This is a report on the details of our experiment, with information on where in reciprocal space we looked.Comment: 5 pages, report on experimental result

Effect of plyometric training on handspring vault performance and functional power in youth female gymnasts

This study aimed to determine the effect of plyometric training (PT) when added to habitual gymnastic training (HT) on handspring vault (HV) performance variables. Twenty youth female competitive gymnasts (Age: 12.5 ± 1.67 y) volunteered to participate and were randomly assigned to two independent groups. The experimental plyometric training group (PTG) undertook a six-week plyometric program, involving two additional 45 min PT sessions a week, alongside their HT, while the control group (CG) performed regular HT only. Videography was used (120 Hz) in the sagittal plane to record both groups performing three HVs for both the baseline and post-intervention trials. Furthermore, participants completed a countermovement jump test (CMJ) to assess the effect of PT on functional power. Through the use of Quintic biomechanics software, significant improvements (P < 0.05) were found for the PTG for run-up velocity, take-off velocity, hurdle to board distance, board contact time, table contact time and post-flight time and CMJ height. However, there were no significant improvements on pre-flight time, shoulder angle or hip angle on the vault for the PTG. The CG demonstrated no improvement for all HV measures. A sport-specific PT intervention improved handspring vault performance measures and functional power when added to the habitual training of youth female gymnasts. The additional two hours plyometric training seemingly improved the power generating capacity of movement-specific musculature, which consequently improved aspects of vaulting performance. Future research is required to examine the whether the improvements are as a consequence of the additional volume of sprinting and jumping activities, as a result of the specific PT method or a combination of these factors

Effects of Plyometric Training and Beta-Alanine Supplementation on Maximal-Intensity Exercise and Endurance in Female Soccer Players.

Plyometric training and beta-alanine supplementation are common among soccer players, although its combined use had never been tested. Therefore, a randomized, double-blind, placebo-controlled trial was conducted to compare the effects of a plyometric training program, with or without beta-alanine supplementation, on maximal-intensity and endurance performance in female soccer players during an in-season training period. Athletes (23.7 ± 2.4 years) were assigned to either a plyometric training group receiving a placebo (PLACEBO, n = 8), a plyometric training group receiving beta-alanine supplementation (BA, n = 8), or a control group receiving placebo without following a plyometric training program (CONTROL, n = 9). Athletes were evaluated for single and repeated jumps and sprints, endurance, and change-of-direction speed performance before and after the intervention. Both plyometric training groups improved in explosive jumping (ES = 0.27 to 1.0), sprinting (ES = 0.31 to 0.78), repeated sprinting (ES = 0.39 to 0.91), 60 s repeated jumping (ES = 0.32 to 0.45), endurance (ES = 0.35 to 0.37), and change-of-direction speed performance (ES = 0.36 to 0.58), whereas no significant changes were observed for the CONTROL group. Nevertheless, compared to the CONTROL group, only the BA group showed greater improvements in endurance, repeated sprinting and repeated jumping performances. It was concluded that beta-alanine supplementation during plyometric training may add further adaptive changes related to endurance, repeated sprinting and jumping ability

Analysis of time-of-flight small-angle neutron scattering data on mesoscopic crystals such as magnetic vortex lattices

Bragg diffracted intensities and q values for crystalline structures with long repeat distances may be obtained by small-angle neutron scattering (SANS) investigations. An account is given of the methods, advantages and disadvantages of obtaining such data by the multichromatic time-of-flight method, compared with the more traditional quasi-monochromatic SANS method. This is illustrated with data obtained from high-magnetic-field measurements on magnetic vortex line lattices in superconductors on the former HFM/EXED instrument at Helmholtz-Zentrum Berlin. The methods have application to other mesoscopic crystalline structures investigated by SANS instruments at pulsed sources