Investigation of superconducting fluctuations and pseudogap in high temperature superconductors microwave method

U ovom radu je tehnikom mikrovalne apsorpcije proučavan doprinos fluktuacijske vodljivosti na temperaturama iznad temperature supravodljivog prijelaza (Tc) u trima porodicama kuprata - YBa2Cu3O6+x, HgBa2CuO4+δ i La2−xSrxCuO4. Iz izračunatih vrijednosti σab(c) određen je raspon temperaturnog prostiranja fluktuacija ab(c)T′. Diskutirana je ovisnost ab(c)T′ o dopiranju i kakve implikacije ono nosi obzirom na trenutna teorijska predviđanja o supravodljivim fluktuacijama i pseudoprocjepu. Za potrebe rada su izračunate raspodjele polja za modove eliptične cilindrične šupljine, raspodjela polja u beskonačno dugom pravokutnom anizotropnom uzorku, te je napravljen jednodimenzionalni numerički model za nelinearno širenje topline. Također, izrađeni su novi dijelovi eksperimentalnog postava kako bi se osigurala bolja stabilnost mjerenja, te su analizirana ranija mjerenja na uzorku Bi2Sr2CaCu2O8+δ.We study the response of superconducting fluctuations by microwave absorption technique, above the transition temperature (Tc) in three families of cuprates - YBa2Cu3O6+x, HgBa2CuO4+δ and La2−xSrxCuO4. From the calculated values of σab(c) with and without applied magnetic field we determine the temperature span of fluctuations ab(c)T′. The doping dependence of ab(c)T′(p) and the implications of the measured data are discussed. Obtained results are compared with current theoretical and experimental findings on superconducting fluctuations and pseudogap. Within the scope of the thesis we calculated the electromagnetic field distributions of our resonating cavity, magnetic field distribution in an infinite rectangular anisotropic rod, and developed a numerical model for one−dimensional nonlinear heat propagation. Also, new parts of the measurement setup were made to ensure a better stability of the measurement process, and earlier measurements of Bi2Sr2CaCu2O8+δ were analyzed

Two-axis goniometer for single-crystal nuclear magnetic resonance measurements

We report the design and construction of a two-axis goniometer capable of any sample orientation with respect to the external magnetic field. The advantage of this design is that it allows free rotations around a single axis independent of the other which minimizes rotational error without reduction of angle range. Goniometer is capable of operating with high precision at both low and high temperatures and in high magnetic fields. It was mounted on the custom made nuclear magnetic resonance probe for use in Oxford Instruments wide-bore variable field superconducting magnet.Comment: 5 pages, 6 figure

Emergence of superconductivity in the cuprates via a universal percolation process

A pivotal step toward understanding unconventional superconductors would be to decipher how superconductivity emerges from the unusual normal state upon cooling. In the cuprates, traces of superconducting pairing appear above the macroscopic transition temperature $T_c$, yet extensive investigation has led to disparate conclusions. The main difficulty has been the separation of superconducting contributions from complex normal state behaviour. Here we avoid this problem by measuring the nonlinear conductivity, an observable that is zero in the normal state. We uncover for several representative cuprates that the nonlinear conductivity vanishes exponentially above $T_c$, both with temperature and magnetic field, and exhibits temperature-scaling characterized by a nearly universal scale $T_0$. Attempts to model the response with the frequently evoked Ginzburg-Landau theory are unsuccessful. Instead, our findings are captured by a simple percolation model that can also explain other properties of the cuprates. We thus resolve a long-standing conundrum by showing that the emergence of superconductivity in the cuprates is dominated by their inherent inhomogeneity

Solid-state NMR/NQR and first-principles study of two niobium halide cluster compounds.

International audience: Two hexanuclear niobium halide cluster compounds with a [Nb6X12](2+) (X=Cl, Br) diamagnetic cluster core, have been studied by a combination of experimental solid-state NMR/NQR techniques and PAW/GIPAW calculations. For niobium sites the NMR parameters were determined by using variable Bo field static broadband NMR measurements and additional NQR measurements. It was found that they possess large positive chemical shifts, contrary to majority of niobium compounds studied so far by solid-state NMR, but in accordance with chemical shifts of (95)Mo nuclei in structurally related compounds containing [Mo6Br8](4+) cluster cores. Experimentally determined δiso((93)Nb) values are in the range from 2400 to 3000ppm. A detailed analysis of geometrical relations between computed electric field gradient (EFG) and chemical shift (CS) tensors with respect to structural features of cluster units was carried out. These tensors on niobium sites are almost axially symmetric with parallel orientation of the largest EFG and the smallest CS principal axes (Vzz and δ33) coinciding with the molecular four-fold axis of the [Nb6X12](2+) unit. Bridging halogen sites are characterized by large asymmetry of EFG and CS tensors, the largest EFG principal axis (Vzz) is perpendicular to the X-Nb bonds, while intermediate EFG principal axis (Vyy) and the largest CS principal axis (δ11) are oriented in the radial direction with respect to the center of the cluster unit. For more symmetrical bromide compound the PAW predictions for EFG parameters are in better correspondence with the NMR/NQR measurements than in the less symmetrical chlorine compound. Theoretically predicted NMR parameters of bridging halogen sites were checked by (79/81)Br NQR and (35)Cl solid-state NMR measurements

Nuclear magnetic resonance study of the magnetic-field-induced ordered phase in the NiCl2-4SC(NH2)2 compound

Nuclear magnetic resonance (NMR) study of the high magnetic field (H) part of the Bose-Einstein condensed (BEC) phase of the quasi-onedimensional (quasi-1D) antiferromagnetic quantum spin-chain compound NiCl2-4SC(NH2)2 (DTN) was performed. We precisely determined the phase boundary, Tc(H), down to 40 mK; the critical boson density, n_c(Tc); and the absolute value of the BEC order parameter S_perp at very low temperature (T = 0.12 K). All results are accurately reproduced by numerical quantum Monte Carlo simulations of a realistic three-dimensional (3D) model Hamiltonian. Approximate analytical predictions based on the 1D Tomonaga-Luttinger liquid description are found to be precise for Tc(H), but less so for S_perp(H), which is more sensitive to the strength of 3D couplings, in particular close to the critical field. A mean-field treatment, based on the Hartree-Fock-Popov description, is found to be valid only up to n_c = 4% (T < 0.3 K), while for higher n_c boson interactions appear to modify the density of states.Comment: Manuscript (6 pages, 3 figures) and the corresponding Supplemental material (5 pages, 6 figures), altogether 11 pages and 9 figure

Microwave measurements of the in-plane and c-axis conductivity in HgBa2CuO4+δ: Discriminating between superconducting fluctuations and pseudogap effects

An approach to microwave measurements is used in order to determine both, the in-plane and out-of-plane conductivity of the high-Tc superconductor HgBa2CuO4+δ near optimal doping. Unlike the ab-plane conductivity, the c-axis conductivity is highly sensitive to superconducting fluctuations. From a single c-axis data set, we can clearly discern the opening of the pseudogap at T∗=185(15) K, the appearance of the superconducting fluctuations at a much lower temperature T′=105(2) K, and the full transition to the superconducting state at the critical temperature Tc=94.3 K. Thus, with the present high sensitivity, we establish that the extent of the superconducting fluctuations is only about 10 K above Tc