We observe anomalous magnetic field dependence of the microwave vortex response of epitaxial YBazCusO?-^ films, which we identify as a crossover from flux pinning to flux flow, occurring near a characteristic field H*(T). We show that the presence of the crossover depends only on adequate softening of the pinning potential U{T,H) (ie., it is independent of the nature of the vortex transition). However, the data are in good agreement with a thermal activation model in which U(T,H) =A(T)/H. A separate analysis of the high-field, high-frequency superconducting transition yields a measure of the upper critical field Hci^ PACS numbers: 74.30. Gn, 74.60.Ec, 74.60.Ge, 74.75.+t Probing vortex dynamics in superconductors at high frequencies is well known to provide important information regarding the mixed state. In the low-T^ superconductors, it was shown by Gittleman and Rosenblum [1] that, at fixed magnetic field, a crossover occurs from pinned vortices to free flux flow, as the probe frequency crosses a characteristic frequency (-100 MHz typically). An understanding of these results was developed in terms of the viscoelastic response of individual vortices, in which the frequency scale is the viscoelastic relaxation frequency, and the only field scale which enters is the upper critical field HdIn this paper we present experimental evidence, based on measurements of the 10-GHz microwave surface resistance RsiT.H), that this description is inadequate at high fields in YBa2Cu307-5 films. We observe, for the first time, a crossover as a function of field at fixed frequency, from a pinning dominated regime to a flow dominated regime. This occurs near a characteristic field which we call H*(T), which is comparable to the socalled ^irreversibility" line [2]. We show that the experimental results are well described by a theory [3] by Coffey and Clem which includes a field-dependent pinning potential and the influence of thermal fluctuations on the surface impedance, and we connect the observed crossover field with the pinning potential (7(7,//). However, we also show that this crossover is model independent, and requires only an adequate softening of the pinning potential or force constant caused by the magnetic field. The analysis of the data yields experimental results for the flux viscosity ri(T), the flux pinning force constant ap(T), and the pinning potential U{T,H).The T dependence of rf(T) suggests a rapid decrease of the quasiparticle scattering rate below Tc. Our work demonstrates significantly new phenomenology of vortex electroydnamics in YBa2Cu307-5, which were not recognized or are absent in low-T^ superconductors.Near the (high frequency, high field) superconducting transition, the dominant field scale is the upper critical field Hci. We analyze the transition in terms of the temperature derivative {BRS/QT)H, from which we obtain a measure of HdEpitaxial (c-axis oriented) YBa2Cu307-5 films (0.5 jum thick) were deposited onto LaAlOs substrates (nominally 0.5 in. square) using standard off-axis sputtering techn...
The Cu02-plane optical reflectance of superconducting La2 Sr"Cu04thin films (T, =31 K) has been measured over a wide frequency and temperature range. The optical conductivity in the normal state is well described by a temperature-dependent weak-coupling (X=0.25) free-carrier term plus an overdamped, weakly temperature-dependent, midinfrared component. The free-carrier plasma frequency is nearly constant, co~a =6300 cm, whereas the relaxation rate varies linearly with temperature above T, .In the superconducting state, according to our two-component approach, most of the Drude oscillator strength condenses to a 6(co) function. A two-fluid analysis gives a rapid drop in the quasiparticle damp-0 ing rate below T, . A reasonable estimate (-2750 A) for the ab-plane London penetration depth is obtained from the superfluid density. We observe that the midinfrared strength increases below T"suggesting that some (-15%%uo) of the free carriers do not condense into superconducting pairs and may have a strong interaction with pair-breaking excitations. Two absorption edges around 80 cm (3.7 k~T, ) and 400 cm (18 kz T, ) are seen but neither is assigned to the superconducting gap. Comparisons with a one-component picture described by a frequency-dependent scattering rate and effective mass are made and discussed. The far-infrared ab-plane phonons show systematic changes with temperature, which are associated with the structural transition near 250 K.
Abstract-Ultrathin dielectric tunneling barriers are critical to Josephson junction (JJ) based superconducting quantum bits (qubits). However, the prevailing technique of thermally oxidizing aluminum via oxygen diffusion produces problematic defects, such as oxygen vacancies, which are believed to be a primary source of the two-level fluctuators and contribute to the decoherence of the qubits. Development of alternative approaches for improved tunneling barriers becomes urgent and imperative. Atomic Layer Deposition (ALD) of aluminum oxide (Al 2 O 3 ) is a promising alternative to resolve the issue of oxygen vacancies in the Al 2 O 3 tunneling barrier, and its self-limiting growth mechanism provides atomic-scale precision in tunneling barrier thickness control. A critical issue in ALD of Al 2 O 3 on metals is the lack of hydroxyl groups on metal surface, which prevents nucleation of the trimethylaluminum (TMA). In this work, we explore modifications of the aluminum surface with water pulse exposures followed by TMA pulse exposures to assess the feasibility of ALD as a viable technique for JJ qubits. ALD Al 2 O 3 films from 40 Å to 100 Å were grown on 1.4 Å to 500 Å of Al and were characterized with ellipsometry and atomic force microscopy. A growth rate of 1.2 Å/cycle was measured, and an interfacial layer (IL) was observed. Since the IL thickness depends on the availability of Al and saturated at 2 nm, choosing ultrathin Al wetting layers may lead to ultrathin ALD Al 2 O 3 tunneling barriers.
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