116 research outputs found
Temperature dependence of the frequency and noise of superconducting coplanar waveguide resonators
We present measurements of the temperature and power dependence of the resonance frequency and frequency noise of superconducting niobium thin-film coplanar waveguide resonators carried out at temperatures well below the superconducting transition (Tc=9.2 K). The noise decreases by nearly two orders of magnitude as the temperature is increased from 120 to 1200 mK, while the variation of the resonance frequency with temperature over this range agrees well with the standard two-level system (TLS) model for amorphous dielectrics. These results support the hypothesis that TLSs are responsible for the noise in superconducting microresonators and have important implications for resonator applications such as qubits and photon detectors
Noise Properties of Superconducting Coplanar Waveguide Microwave Resonators
We have measured noise in thin-film superconducting coplanar waveguide
resonators. This noise appears entirely as phase noise, equivalent to a jitter
of the resonance frequency. In contrast, amplitude fluctuations are not
observed at the sensitivity of our measurement. The ratio between the noise
power in the phase and amplitude directions is large, in excess of 30 dB. These
results have important implications for resonant readouts of various devices
such as detectors, amplifiers, and qubits. We suggest that the phase noise is
due to two-level systems in dielectric materials.Comment: 4 pages, 3 figures, accepted for publication in Applied Physics
Letter
Experimental evidence for a surface distribution of two-level systems in superconducting lithographed microwave resonators
We present measurements of the temperature-dependent frequency shift of five
niobium superconducting coplanar waveguide microresonators with center strip
widths ranging from 3 m to 50 m, taken at temperatures in the range
100-800 mK, far below the 9.2 K transition temperature of niobium. These data
agree well with the two-level system (TLS) theory. Fits to this theory provide
information on the number of TLS that interact with each resonator geometry.
The geometrical scaling indicates a surface distribution of TLS, and the data
are consistent with a TLS surface layer thickness of order a few nm, as might
be expected for a native oxide layer.Comment: 3 figures, submitted to AP
Characterization and In-situ Monitoring of Sub-stoichiometric Adjustable Tc Titanium Nitride Growth
The structural and electrical properties of Ti-N films deposited by reactive
sputtering depend on their growth parameters, in particular the Ar:N2 gas
ratio. We show that the nitrogen percentage changes the crystallographic phase
of the film progressively from pure \alpha-Ti, through an \alpha-Ti phase with
interstitial nitrogen, to stoichiometric Ti2N, and through a substoichiometric
TiNX to stoichiometric TiN. These changes also affect the superconducting
transition temperature, Tc, allowing, the superconducting properties to be
tailored for specific applications. After decreasing from a Tc of 0.4 K for
pure Ti down to below 50 mK at the Ti2N point, the Tc then increases rapidly up
to nearly 5 K over a narrow range of nitrogen incorporation. This very sharp
increase of Tc makes it difficult to control the properties of the film from
wafer-to-wafer as well as across a given wafer to within acceptable margins for
device fabrication. Here we show that the nitrogen composition and hence the
superconductive properties are related to, and can be determined by,
spectroscopic ellipsometry. Therefore, this technique may be used for process
control and wafer screening prior to investing time in processing devices
Proximity-Coupled Ti/TiN Multilayers for use in Kinetic Inductance Detectors
We apply the superconducting proximity effect in TiN/Ti multi-layer films to
tune the critical temperature, Tc, to within 10 mK with high uniformity (less
than 15 mK spread) across a 75 mm wafer. Reproducible Tc's are obtained from
0.8 - 2.5 K. These films had high resistivities, > 100 uOhm-cm and internal
quality factors for resonators in the GHz range on the order of 100k and
higher. Both trilayers of TiN/Ti/TiN and thicker superlattice films were
prepared, demonstrating a highly controlled process for films over a wide
thickness range. Detectors were fabricated and showed single photon resolution
at 1550 nm. The high uniformity and controllability coupled with the high
quality factor, kinetic inductance, and inertness of TiN make these films ideal
for use in frequency multiplexed kinetic inductance detectors and other
potential applications such as nanowire detectors, transition edge sensors and
associated quantum information applications
Measurement of loss in superconducting microstrip at millimeter-wave frequencies
We have developed a new technique for accurate measurement of the loss of superconducting microstrips at mm-wave frequencies. In this technique, we optically couple power to slot antenna, which is connected to one port of a hybrid coupler. One of the output ports of the hybrid delivers power to a series of mm-wave microstrip resonators which are capacitively coupled to a feedline followed by an MKID (microwave kinetic inductance detector) that measures the transmitted power. Two other MKIDs are connected to the remaining ports of the hybrid to measure the total incident optical power and the power reflected from the mm-wave resonators, allowing |S_(21)|^2 and |S_(11)|^2 to be accurately determined and resonance frequency fr and quality factor Q to be retrieved. We have fabricated such a Nb/SiO_2/Nb microstrip loss test device which contains several mm- wave resonators with f_r~100 GHz and measured it at 30 mK. All the resonators have shown internal quality factor Qi~500–2000, suggesting a loss tangent of ~5×10^(−4)−2×10^(−3) for the SiO_2 in use. For comparison, we have also fabricated a 5 GHz microstrip resonator on the same chip and measured it with a network analyzer. The loss tangent at 5 GHz derived from fitting the f_0 and Q data to the two-level system (TLS) model is 6×10^(−4), about the same as from the mm-wave measurement. This suggests that the loss at both microwave and mm-wave frequencies is probably dominated by the TLS in SiO_2. Our results are of direct interest to mm/submm direct detection applications which use microstrip transmission lines (such as antenna-coupled MKIDs and transition-edge sensors), and other applications (such as on-chip filters). Our measurement technique is applicable up to approximately 1 THz and can be used to investigate a range of dielectrics
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