38 research outputs found
Fabrication of mesa structures on superconducting Bi2Sr2CaCu2O8+& single crystals
Thesis (Master)--Izmir Institute of Technology, Physics, Izmir, 2005Includes bibliographical references (leaves: 109-114)Text in English; Abstract: Turkish and Englishxii, 114 leavesThere have been tremendous efforts to understand the relatively much more sophisticated mechanism of superconductivity in high temperature superconductors (HTSC). In order to investigate the inherent features and tunneling characteristics just only peculiar to HTSC, micron-sized mesa structures were fabricated on the surfaces of both pristine optimally doped and HgBr2 intercalated Bi2Sr2CaCu2O8+& (Bi-2212) single crystals using photolithography and Argon ion beam etching techniques. The surface topography and heights of the mesas were examined with atomic force microscopy.Hysteretic I-V curves with multiple branches and temperature dependence of tunneling characteristics were investigated by means of a novel technique, point contact tunneling (PCT) and experiments were carried out in a large range of temperatures from 4.2 K to 300 K. The results of SIN single junctions and SIS break junctions obtained by tunneling measurements using PCT technique on bulk crystals were compared withtunneling measurements using PCT technique on bulk crystals were compared with intrinsic Josephson junction quasiparticle spectra generally showing sharp peaks at the gap voltages and no dip/hump structures; which are reconciled with overheating in the mesa. The IJJ measurements performed with HgBr2 intercalated Bi-2212 samples showed far more enhanced characteristics indicating less heating. The zero bias conductance versus temperature plots were examined to scrutinize the existence of pseudogap in electronic excitation spectra of investigated samples. Besides, the normalized gap voltages were plotted against normalized temperature to show the deviation from BCS fit, which displays the novelty of HTSC
Unconventional rf photoresponse from a superconducting spiral resonator
Superconducting thin film resonators employing strip geometries show great
promise in rf/microwave applications due to their low loss and compact nature.
However, their functionality is limited by nonlinear effects at elevated
rf/microwave powers. Here, we show that by using a planar spiral geometry
carrying parallel currents in adjacent turns, this limitation can be minimized.
We investigate the rf current distributions in spiral resonators implemented
with Nb thin films via laser scanning microscopy. The rf current density
profile along the width of the individual turns of the resonators reveals an
unconventional trend: maximum current in the middle of the structure and
decaying toward its edges. This unusual behavior is associated with the
circular nature of the geometry and the cancellation of magnetic field between
the turns, which is favorable for handling high powers since it allows the
linear characteristics to persist at high rf current densities.Comment: 8 pages, 7 figure
Miniaturized Superconducting Metamaterials for Radio Frequencies
We have developed a low-loss, ultra-small radio frequency metamaterial
operating at 76 MHz. This miniaturized medium is made up of planar spiral
elements with diameter as small as /658 ( is the free
space wavelength), fashioned from Nb thin films on quartz substrates. The
transmission data are examined below and above the superconducting transition
temperature of Nb for both a single spiral and a one dimensional array. The
validity of the design is tested through numerical simulations and good
agreement is found. We discuss how superconductors enable such a compact design
in the with high loaded-quality factor (in excess of 5000), which is in
fact difficult to realize with ordinary metals.Comment: 3 pages, 4 figure
Large Energy Gaps in CaC₆ from Tunneling Spectroscopy: Possible Evidence of Strong-Coupling Superconductivity
Tunneling in CaC6 crystals reproducibly reveals superconducting gaps Δ of 2.3±0.2 meV that are ~40% larger than reported earlier. In an isotropic s -wave scenario, that puts CaC6 into the class of very strongly coupled superconductors, since 2Δ k Tc ~4.6, implying that soft Ca phonons are primarily involved in the superconductivity. This conclusion explains the relatively large Ca isotope effect found recently for CaC6, but it could also signal a strong anisotropy in the electron-phonon interaction
Modeling Study of the Dip-Hump Feature in Bi₂ Sr₂ CaCu₂ O\u3csub\u3e8+δ\u3c/sub\u3e Tunneling Spectroscopy
The tunneling spectra of high-temperature superconductors on Bi2 Sr2 CaCu2 O8+δ (Bi-2212) reproducibly show a high-bias structure in the form of a dip-hump at voltages higher than the gap voltage. Of central concern is whether this feature originates from the normal state background or is intrinsic to the superconducting mechanism. We address this issue by generating a set of model conductance curves-a normal state conductance that takes into account effects such as the band structure and a possible pseudogap, and a pure superconducting state conductance. When combined, the result shows that the dip-hump feature present in the experimental conductance curves cannot be naively attributed to a normal state effect. In particular, strong dip features found in superconductor-insulator-superconductor data on optimally doped Bi-2212, including negative dI /dV, cannot be a consequence of an extrinsic pseudogap. However, such features can easily arise from state-conserving deviations in the superconducting density of states, e.g., from strong-coupling effects
Microscopic examination of hot spots giving rise to nonlinearity in superconducting resonators
We investigate the microscopic origins of nonlinear rf response in
superconducting electromagnetic resonators. Strong nonlinearity appearing in
the transmission spectra at high input powers manifests itself through the
emergence of jumplike features near the resonant frequency that evolve toward
lower quality factor with higher insertion loss as the rf input power is
increased. We directly relate these characteristics to the dynamics of
localized normal regions (hot spots) caused by microscopic features in the
superconducting material making up the resonator. A clear observation of
hot-spot formation inside a Nb thin film self-resonant structure is presented
by employing the microwave laser scanning microscope, and a direct link between
microscopic and macroscopic manifestations of nonlinearity is established.Comment: 5 pages, 4 figure
Self-heating effect in intrinsic tunneling spectroscopy of HgBr2 intercalated Bi2.1Sr1.4Ca1.5Cu 2O8+δ single crystals
We report tunneling results in intrinsic Josephson junction (IJJ) stacks fabricated in the form of square micromesas on HgBr2 intercalated Bi2.1Sr1.4Ca1.5Cu2O 8+δ (Bi2212) single crystals using photolithography and Ar ion milling techniques. Self-heating is the most common problem encountered in interlayer tunneling and it is likely to reduce the reliability of IJJ data. Although intercalation reduces heating a hundredfold, it still needs to be minimized substantially in order to approach the authentic superconducting energy gap observed by tunneling using more conventional junctions. We report tunneling characteristics of two mesas with the same height but different sizes (5 × 5 μm2 and 10 × 10 μm2) to show that heating effects are strongly related to IJJ stack size. For the smaller mesa, we observed an energy gap close to that seen in single SIN (S: superconductor, I: insulator, N: normal metal) and SIS break junctions as well as the dip and hump structures at high bias. The subgap data of 5 × 5 μm2 mesa were successfully fit with a momentum averaged d-wave model using convenient parameters. Thus our data is consistent with the predominant pairing symmetry suggested by point contact tunneling, break junction, scanning tunneling microscopy/spectroscopy and angle resolved photoemission measurements in Bi2Sr2CaCu2O8+δ
Classical Analogue of Electromagnetically Induced Transparency with a Metal-Superconductor Hybrid Metamaterial
Metamaterials are engineered materials composed of small electrical circuits
producing novel interactions with electromagnetic waves. Recently, a new class
of metamaterials has been created to mimic the behavior of media displaying
electromagnetically induced transparency (EIT). Here we introduce a planar EIT
metamaterial that creates a very large loss contrast between the dark and
radiative resonators by employing a superconducting Nb film in the dark element
and a normal-metal Au film in the radiative element. Below the critical
temperature of Nb, the resistance contrast opens up a transparency window along
with a large enhancement in group delay, enabling a significant slowdown of
waves. We further demonstrate precise control of the EIT response through
changes in the superfluid density. Such tunable metamaterials may be useful for
telecommunication because of their large delay-bandwidth products.Comment: 4 pages, 4 figure
Large Group Delay in a Microwave Metamaterial Analog of Electromagnetically Induced Transparency
We report on our experimental work concerning a planar metamaterial exhibiting classical electromagnetically induced transparency (EIT). Using a structure with two mirrored split-ring resonators as the dark element and a cut wire as the radiative element, we demonstrate that an EIT-like resonance can be achieved without breaking the symmetry of the structure. The mirror symmetry of the metamaterial\u27s structural element results in a selection rule inhibiting magnetic dipole radiation for the dark element, and the increased quality factor leads to low absorption (\u3c10%) and large group index (of the order of 30)
Large Group Delay in a Microwave Metamaterial Analog of Electromagnetically Induced Transparency
We report on our experimental work concerning a planar metamaterial exhibiting classical electromagnetically induced transparency (EIT). Using a structure with two mirrored split-ring resonators as the dark element and a cut wire as the radiative element, we demonstrate that an EIT-like resonance can be achieved without breaking the symmetry of the structure. The mirror symmetry of the metamaterial\u27s structural element results in a selection rule inhibiting magnetic dipole radiation for the dark element, and the increased quality factor leads to low absorption (\u3c10%) and large group index (of the order of 30)