Thesis advisor: Hong Ding / Thesis advisor: Vidya Madhavan / High temperature superconductivity has been one of the most challenging problems in condensed matter physics since its discovery. This dissertation presents systematic studies on the single layer high temperature superconductor Bi2Sr2-xLaxCuO6+delta by scanning tunneling microscopy. The STM results have been compared to Angle-resolved photoemission spectroscopy (ARPES) data. Using STM spectroscopy and ARPES we observed two distinct gaps that coexist both in real space and in the antinodal region of momentum space, below the superconducting transition temperature. By looking at the energy scale of these two gaps along with the temperature dependence data, we fnd that the small gap is associated with superconductivity. The large gap persists above Tc, and seems linked to observed charge ordering. We also find a strong correlation between the large and small gaps suggesting that they are affected by similar physical processes. This is the first time that two coexisting and competing energy scales have been directly observed in STM spectroscopy. Combining this with ARPES data, we show that the pseudogap may be a diffferent order parameter from the superconducting phase. This provides support to the recently proposed "two gaps scenario" and should lead to more experimental discovery and theoretical discussions. In this dissertation we also discuss the spatial properties of the scanning tunneling microscopy conductance maps, as well as the charge ordering pattern at high energies. We observe interesting periodic patterns at low energies which can not be explained by a simple charge density wave picture. We also fnd the surprising bias dependence in terms of the contrast reversal. We propose a model of STM measuring effect to explain these phenomena. / Thesis (PhD) — Boston College, 2009. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

Title: Type-II thin film superconductors studied by terahertz radiation Author: RNDr. Roman Tesař Department: Department of Low Temperature Physics Supervisor: prof. RNDr. Ladislav Skrbek, DrSc. Consultant: RNDr. Jan Koláček, CSc. Abstract: Utilization of type-II superconductors for future practical applications such as fluxonics requires detailed knowledge of their physical properties, espe- cially at high frequencies within the THz spectral region. We have investigated interactions of thin-film NbN samples deposited on Si substrate and of a high quality epitaxial film of the NbN superconductor grown on a birefringent R-cut sapphire substrate with monochromatic linearly polarized laser beam both below and above the critical temperature Tc. For photon energies lower than the optical gap, detailed measurements of transmission in zero field provide BCS-like tem- perature curves with a pronounced peak below Tc which disappears as the energy of incident radiation is increased above the gap. In externally applied magnetic fields up to 10 T oriented perpendicularly to the sample, i.e., in the Faraday exper- imental geometry, the temperature behavior of transmission is modified because the gap is suppressed and vanishes at the upper critical field and, additionally, the presence of quantized vortices changes the shape...

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Made available in DSpace on 2017-07-21T19:26:02Z (GMT). No. of bitstreams: 1 Marlon Luiz Hneda.pdf: 6704011 bytes, checksum: b29186e60beebe411525b53a8023ea5d (MD5) Previous issue date: 2012-03-09 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / In this work we report an experimental study about the effects of Fe doping, the variation of the current density and the application of low magnetic fields on the resistive transition and normal phase of the polycrystal ine and monocrystalline Ba2Cu2:985Fe0:015O7 samples. Three polycrystalline samples were produced, one without oxygenation and two that were oxygenated during 24 and 48 hours. Was produced, also, one well oxygenated monocrystalline sample. In the polycrystalline samples and in the single crystal sample were applied electrical currents of 5, 10 and 15 mA. The X-ray diffraction, with Rietveld refinement, shows a decrease in the lattice parameters when related with Y Ba2Cu3O7. Raman spectroscopy measurements of the polycrystalline samples reveal the displacement of bands at 150 and 500 cm1 when related with Ba2Cu3O7. The obtained results for the polycrystalline samples indicate that, in a general manner, the Fe addition and the oxygenation degree strongly affect the electronic transport properties in the normal state and in the resistive transition. Were not observed effects of the current intensity in the normal phase. In the resistive transition the current has moderate effects, similar to those produced by the application of low magnetic fields. Using the temperature derivative of the resistivity and the logarithmic derivative of the conductivity plotted as a function of the temperature, it is shown that the resistive transition is a two-step process, typical of disordered systems. In the normal phase Gaussian and critical fluctuations were observed. There were no effects of doping, oxynenation or electrical current on the paraconductivity. The paracoherent phase is strongly affected by the degree of oxygenation, but no effects were observed by the Fe doping. The effect of the electrical current was more intense in the poorly oxygenated sample. At temperatures close to the state of zero resistance it was observed, for the sample without oxygenation, a critical regime whose exponent is current dependent. For the samples oxygenated during 24 and 48 hours, the exponent was independent of the degree of oxygenation, doping with Fe, and the current intensity. The obtained values for the exponent suggest that the universality class for the coherence transition is that of the 3D-XY model where the non-trivial disorder is relevant. In the single crystal sample, were applied low magnetic fields between 0 and 500 Oe. The results showed that the Fe doping affects the electronic transport properties of both normal phase and resistive transition. There were no effects of the fields or the currents in the normal phase. The derivative of the resistiviry shows a structure of multiple peaks, suggesting the existence of a distribution of critical temperatures which can be connected with a multiple and subtle phase separation related to the presence of regions of the sample with different concentrations of Fe and/or O. In the study of the fluctuations for T TC it was observed the super-critical regime. The regime was not affected by the current intensity variation or magnetic field application. Resistivity measurements of greater accuracy, made by applying magnetic fields, also revealed the genuinely critical regime, that proved to be stable under the applied fields. / No presente trabalho é apresentado um estudo experimental dos efeitos da adição de Fe, da variação da intensidade de corrente e da aplicação de baixos campos magnéticos na transição resistiva e na fase normal do sistema Y Ba2Cu2:985Fe0:015O7 nas formas policristalina e monocristalina. Foram produzidas três amostras policristalinas, uma sem oxigenação e outras duas que foram oxigenadas durante 24 e 48 horas, e uma amostra monocristalina bem oxigenada. Tanto nas amostras policristalinas quanto na amostra monocristalina foram aplicadas correntes de 5, 10 e 15 mA. A difratometria de raios X, juntamente com refinamento Rietveld, mostram uma diminuição dos parâmetros de rede em relação ao Y Ba2Cu3O7. Medidas de espectroscopia Raman das amostras policristalinas revelam um deslocamento das bandas em 150 e 500 cm1 em relação ao Y Ba2Cu3O7. Os resultados obtidos para as amostras policristalinas indicam que, de maneira geral, a adição de Fe e o grau de oxigenação afetam fortemente as propriedades de transporte eletrônico no estado normal e na transição resistiva. Não foram observados efeitos da intensidade de corrente na fase normal. Na transição resistiva a corrente tem efeitos moderados, semelhantes aos produzidos pela aplicação de baixos campos magnéticos. A partir da derivada da resistividade e da derivada logarítmica da condutividade em função da temperatura mostra-se que a transição resistiva das amostras policristalinas é um processo com duas etapas, geralmente observado em sistemas desordenados. Na fase normal foram identificadas flutuações Gaussianas e críticas. Não foram observados efeitos da dopagem, oxigenação ou corrente na paracondutividade. A fase paracoerente se mostrou fortemente afetada pelo grau de oxigenação, mas não foram observados efeitos da dopagem com Fe. O efeito da variação da corrente foi mais intenso na amostra pouco oxigenada. Em temperaturas próximas ao estado de resistência nula foi observado um regime crítico com expoente dependente de corrente para a amostra sem oxigenação. Para as amostras oxigenadas em 24 e 48 h, o expoente se mostrou independente do grau de oxigenação, da dopagem com Fe e da intensidade de corrente. Os valores obtidos para o expoente sugerem que a classe de universalidade da transição de coerência é a mesma de um modelo 3D-XY onde a desordem não trivial é relevante. Na amostra monocristalina foram aplicados campos magnéticos fracos, entre 0 e 500 Oe. A análise dos resultados revelou que a dopagem com Fe afeta as propriedades de transporte eletrônico tanto da fase normal quanto da transição resistiva. Não foram observados efeitos dos campos nem das correntes na fase normal. A derivada da resistividade mostra uma estrutura de múltiplos picos, que sugere a existência de uma distribuição de temperaturas críticas que pode estar ligada com uma múltipla e sutil separação de fases relacionada com a presença de regiões da amostra com diferentes concentrações de Fe e/ou O. No estudo das flutuações para T > TC foi identificado o regime super-crítico. O regime se mostrou estável frente às variações de corrente e de campo magnético. Medidas de resistividade de maior precisão experimental, feitas com a aplicação de campos magnéticos, mostraram também o regime genuinamente crítico, que se revelou estável frente a aplicação dos campos estudados.

We report changes in the intergranular material and grain morphology of YBa2Cu3O7-x during cyclic anneals between 780 and 980 ºC in oxygen at atmospheric pressure. Two endothermic reactions were detected: (a) the eutectic reaction of YBa2Cu3O7-x with CuO and BaCuO2 at 900 ºC (enthalpy ΔHa) and (b) the peritectic reaction of YBa2Cu3O7-x with CuO at 950 ºC (ΔHb). During the first anneal, only reaction (b) is detected, and although it should only occur if there is an excess of CuO, its signature is present in all published data. Cyclic annealing causes a monotonic decrease in ΔHb and an increase in ΔHa, larger average grain sizes, and greater volume fraction of the superconducting phase. A steady state is reached after 10 cycles at which point ΔHb = 0. We propose a model that explains the origin of the intergranular CuO and the changes in the intergranular material composition with cyclic annealing.

Growth Mechanism and Infrared Detection of High-temperature Superconducting and Colossal Magnetoresistance Films Department of Electrical Engineering, National Sun Yat-Sen University Meng-Tsong Hong* Ying-Chung Chen**, Hsiung Chou** -------------------------------------------------- Abstract---- The growth mechanism of YBa2Cu3O7-d (YBCO) films grown by RF sputtering has been investigated. When growing films by RF sputtering, the shape of the plasma and the degree of resputtering effect were varied by setting different relative positions of the heater to the gun. As the substrate was near the plasma, the negative oxygen ions resputtered part of the mobile atoms from the surface of film back into the plasma, which caused the composition distortion, delayed the merge of grains and left uncovered holes. Setting a longer relative distance, the resputtering effect was suppressed and the precipitates appeared on the surface of films resulting in a rough surface. At an optimum relative position between heater and gun, the function of resputtering produced a polishing effect on the surface of films. This polishing effect suppressed the growth of precipitates without slowing down the growth of grains, a smooth and precipitate-free YBCO film might obtain. We also found that the film with smooth and precipitate-free morphology exhibited suppressed superconductivity. The most direct way to enhance the photoresponse of a bolometer is by modifying the microbridge from a single straight bridge to a meander or change the thermal coupling configuration between bolometer and heat sink. In the study of high-temperature superconducting (HTSC) bolometers, it is found that the geometry and thermal coupling configuration play very important roles on the behavior of heat conduction, which alter the thermal conversion efficiency, DT/WD. Actually, DT/WD is a matter of the absorption of the AC thermal irradiation and the dissipation of both the irradiation and the DC joule heat generated by the bias current. The competition between the capability of heat dissipation and the thermal generation determined the magnitude of DT/WD. The La0.67Ca0.33MnO3-y (LCMO) thin films with epitaxial structure and smooth surface morphology have been deposited. A LCMO thin-film microbridge was fabricated into a microbridge by conventional photolithography and dry etching for optical detection. The measured photoresponse, DV, of this LCMO thin-film microbridge reveals that it is bolometric in nature. The photoresponse is linearly proportional to the bias current Ib and the power density of irradiation WD, which strongly suggests the applicability of an LCMO thin-film microbridge to a linear optical detector. The ratio of the photoresponse to the irradiated power density, DV/WD, is independent of the incident-light wavelength l from 0.633 to 3.5 mm. The dependence of the photoresponse on modulated frequency f, follows the DV µ f -0.21 relation. Under Ib = 100 mA and f = 5 Hz at an operating temperature Top = 223 K, the responsivity S and noise voltage Vn are 685 V/W and 20 nV¡ÑHz -0.5, respectively, for this LCMO thin-film microbridge. From the measured S and Vn, the noise equivalent power (NEP) and detectivity D* were be calculated to be 2.92´10-11 W¡ÑHz -0.5 and 2.76´109 cm¡ÑHz 0.5¡ÑW -1, respectively, for this LCMO thin-film microbridge. The experimental results from this LCMO thin-film microbridge show the practical applicability of this new detector system compared to other established detectors. *Student **Advisor

The work described in this thesis consists of an investigation into the behaviour of the critical current density (Jc) of several different high temperature superconductors (HTSCs) as a function of temperature and applied magnetic field. The focus of this research has been to investigate the discrepancies which generally exist between magnetic and transport measurements on HTSCs. In order to do this a number of systems were selected with different weak link strengths, overall alignment and pinning characteristics. Systematic studies were carried out on these systems using both transport and magnetic techniques. The results obtained were compared to obtain a coherent picture of the relation between critical currents in HTSCs, their granularity and structure, and how this accounts for the differences in magnetic and transport measurements. The Jc of granular bulk sintered YBCO was measured as a function of magnetic field and orientation, and attempts made to fit the results obtained to theory. The results obtained indicate that hysteresis of Jc with field cannot be explained by flux trapping alone. Jc measurements were carried out on the 'hub-and-spoke' (H-S) grains of melt-processed YBCO thick films. These indicated that within each H-S grain the current is constrained to radial paths through the centre of the grain. This has major implications for the analysis of any measurements carried out on these samples, as well as for applications, as only a small fraction of the sample carries the applied current. Silver-clad tapes of Tl:1223 and Tl:2223 were measured in different temperatures, applied magnetic fields and orientations. The variation of Jc along the tape length was also measured to provide an estimate of their homogeneity. It was found that the processing route used in the production of these tapes had not aligned the superconductor within them, and also that the tape properties were not homogenous along their length. This probably arises from the crystal structure of the thallium materials. The Jcs of melt-processed thick films of Bi:2212 on silver substrates were measured as a function of temperature and applied magnetic field, and compared with results obtained from magnetic measurements obtained from a VSM. Attempts were made to fit these results to theory. The results obtained show clear scaling behaviour of Jc with T at constant B and imply that the variation in the form of the Jc versus T curves is due simply to the suppression of Tc with increasing applied field.

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While in conventional superconductors coupling between electrons and phonons is known to be responsible for the electron pairing, for the high temperature superconductors the pairing media remains under debates. Since the interactions of electrons with other degrees of freedom (phonons, magnetic excitations, etc) manifest themselves by an additional renormalization in the electronic dispersion, they can be investigated by means of Angle Resolved Photoelectron Spectroscopy. In the work renormalization in two families of high Tc cuprates have been studied. Along the diagonal of the two-dimensional BZ, the renormalization effects are represented by an unusual band dispersion that develops a so-called ‘‘kink’’. In the vicinity of the (pi, 0) point of the BZ, where the order parameter reaches its maximum, the renormalization is noticeably stronger and makes itself evident even in the shape of a single spectral line measured for a fixed momentum. It was shown that for the Bi-2212 samples substitution of Cu atoms in Cu-O plane changes renormalization features in ARPES spectra both in nodal and antinodal parts of the Brillouin zone. The smearing of the dip in the in the spectral line shape measured at (pi; 0) point can be well explained by coupling of electrons to the magnetic resonance mode. The effect of Zn and Ni substitution on the antinodal ARPES spectra was shown to be in good agreement with the influence of these impurities on magnetic resonance mode seen in inelastic neutron scattering experiments. This, in addition to the previous ARPES studies of temperature and doping dependence of peak-dip-hump structure, mass renormalization near antinodal region and a kink in the nodal part of Brillouin zone, provides further evidence that the coupling to magnetic excitations, rather than to phonons, is responsible for the observed unusual renormalization. Unlike the well studied Bi-2212 family of cuprates, photoemission on YBCO-123 turns out to be much more complicated. The observed spectra have a strong contribution from a heavily overdoped surface component with the hole doping level of about x~0.30, which is weakly dependent on the sample stochiometry. Absence of any signs of superconductivity in the spectra of the overdoped component was argued to result from the unusually high doping level. This conclusion is supported by the fact that the overdoped bands give rise to the Fermi surface and band structure consistent with the predictions of the LDA calculations, as well as, by the dependence of the photoemission matrix element on the excitation energy, which closely follows that of the superconducting bulk component. Specific experimental geometry was used to enhance the signal coming from the superconducting component. In particular, experiments with circularly polarized light bundled with simple theoretical considerations enabled better separation of the surface and the bulk components. This type of experiments also suggests that the overdoped component is mainly localized in the topmost CuO2 bilayer, while the next bilayers in the YBCO-123 structure already represent bulk properties and retain superconductivity. Using partially Ca substituted samples it was possible to obtain spectra with a suppressed overdoped component. The likely reason for the suppression is a shift of the most probable cleavage plane from the Ba–O interface to the Y layer. Spectra from the Ca substituted sample clearly reveal a sizable superconducting gap, and strong renormalization effects in the vicinity of the antinodal point. The fact that the renormalization vanishes above Tc and has strong momentum dependence, diminishing away from the (pi; 0)/(0; pi) point, strongly suggests that the reason for this renormalization in YBCO-123 is coupling of the electronic subsystem to spin resonance, similar to the case of Bi-2212.

While in conventional superconductors coupling between electrons and phonons is known to be responsible for the electron pairing, for the high temperature superconductors the pairing media remains under debates. Since the interactions of electrons with other degrees of freedom (phonons, magnetic excitations, etc) manifest themselves by an additional renormalization in the electronic dispersion, they can be investigated by means of Angle Resolved Photoelectron Spectroscopy. In the work renormalization in two families of high Tc cuprates have been studied. Along the diagonal of the two-dimensional BZ, the renormalization effects are represented by an unusual band dispersion that develops a so-called ‘‘kink’’. In the vicinity of the (pi, 0) point of the BZ, where the order parameter reaches its maximum, the renormalization is noticeably stronger and makes itself evident even in the shape of a single spectral line measured for a fixed momentum. It was shown that for the Bi-2212 samples substitution of Cu atoms in Cu-O plane changes renormalization features in ARPES spectra both in nodal and antinodal parts of the Brillouin zone. The smearing of the dip in the in the spectral line shape measured at (pi; 0) point can be well explained by coupling of electrons to the magnetic resonance mode. The effect of Zn and Ni substitution on the antinodal ARPES spectra was shown to be in good agreement with the influence of these impurities on magnetic resonance mode seen in inelastic neutron scattering experiments. This, in addition to the previous ARPES studies of temperature and doping dependence of peak-dip-hump structure, mass renormalization near antinodal region and a kink in the nodal part of Brillouin zone, provides further evidence that the coupling to magnetic excitations, rather than to phonons, is responsible for the observed unusual renormalization. Unlike the well studied Bi-2212 family of cuprates, photoemission on YBCO-123 turns out to be much more complicated. The observed spectra have a strong contribution from a heavily overdoped surface component with the hole doping level of about x~0.30, which is weakly dependent on the sample stochiometry. Absence of any signs of superconductivity in the spectra of the overdoped component was argued to result from the unusually high doping level. This conclusion is supported by the fact that the overdoped bands give rise to the Fermi surface and band structure consistent with the predictions of the LDA calculations, as well as, by the dependence of the photoemission matrix element on the excitation energy, which closely follows that of the superconducting bulk component. Specific experimental geometry was used to enhance the signal coming from the superconducting component. In particular, experiments with circularly polarized light bundled with simple theoretical considerations enabled better separation of the surface and the bulk components. This type of experiments also suggests that the overdoped component is mainly localized in the topmost CuO2 bilayer, while the next bilayers in the YBCO-123 structure already represent bulk properties and retain superconductivity. Using partially Ca substituted samples it was possible to obtain spectra with a suppressed overdoped component. The likely reason for the suppression is a shift of the most probable cleavage plane from the Ba–O interface to the Y layer. Spectra from the Ca substituted sample clearly reveal a sizable superconducting gap, and strong renormalization effects in the vicinity of the antinodal point. The fact that the renormalization vanishes above Tc and has strong momentum dependence, diminishing away from the (pi; 0)/(0; pi) point, strongly suggests that the reason for this renormalization in YBCO-123 is coupling of the electronic subsystem to spin resonance, similar to the case of Bi-2212.