Laboratory of microwave superconductivity

Linear microwave response

Microwave (mw) properties of superconductors in the linear regime are investigated by using superconducting cavity resonators. We measure the microwave surface resistance, R_s, of the sample by the “hot-finger cavity perturbation” technique: a small superconducting sample is placed into a copper cavity, resonating at about 9.6 GHz, through a sapphire rod and, by using a hp-8719D Network Analyzer (NA), operating in the frequency range 50 MHz – 13.5 GHz, we measure the variations of the quality factor Q_L of the cavity induced by the sample. By sweeping in opportune ranges the frequency of the CW generated by the NA, we acquire the resonance curve of the cavity in one fixed resonant mode. By Lorentzian fits we find the central frequency and the half-height width of the resonance curves, from which we determine Q_L and then the microwave surface resistance of the superconducting material. The measurements can be performed as a function of the external DC magnetic field (up to 1 Tesla), at fixed temperatures, and as a function of the temperature, at fixed DC magnetic field. In both the cases, the cavity is maintained in a thermal bath (at the liquid helium, or nitrogen, temperature) and the temperature of the sample is increased by warming up the sapphire rod at which the sample is attached.

Non linear microwave response

To investigate the microwave response of superconductors, we feed a superconducting cavity by a train of mw pulses with pulse width 10 μs, pulse repetition rate 10 Hz and maximum input peak power 44 dBm. The decay time of the transmitted power allows us to determine the loaded quality factor as Q_L = 2πfτ, from which we can investigate the non linear mw response of the superconducting material.

Microwave harmonic emission

To investigate mw harmonic emission of superconductors, we use a non-linear mw spectrometer. The basic element of the apparatus is a bimodal cavity oscillating at the two frequencies ω and nω, with n = 2, 3 for the second harmonic (SH) and the third harmonic (TH) detection respectively. The resonance at the fundamental frequency is obtained by inserting a metallic rod into a rectangular copper cavity. The intensity of the magnetic field of the ω-mode varies along the rod as H(ω) = H₁cos(πz/2L), where L is the length of the rod. The harmonic mode is the TE102 mode of the rectangular cavity, resonating at 6 GHz, for both SH and TH detection. The sample can be located in a region in which the field distribution is known. To study magnetic properties the sample is located in a region where the fields H(ω) and H(nω) are maximal and parallel to each other. The fundamental mode of the cavity is fed by a train of microwave pulses, with repetition rate ranging from 1 to 200 pps and pulse width ranging from 1 μs to 1 ms. High input power levels can be used (0.1 - 1 kW) provided that the ratio between the pulse width and the repetition rate does not overcome the value of about 10^-3. A low pass filter at the input of the cavity cuts any harmonic content of the oscillator by more than 60 dB. The harmonic signals generated by the sample are filtered by a band pass filter, with more than 60-dB rejection at the fundamental frequency, and are detected by a superheterodyne receiver. The superheterodyne receiver consists of a mixer, a local oscillator at ω_L = nω + 30 MHz and an IF amplifier at 30 MHz with 1 MHz bandwidth. It allows detecting a 30 MHz signal whose intensity is proportional to the harmonic power emitted by the sample. The signal is displayed by an oscilloscope and recorded on a personal computer. Harmonic signals can be investigated as a function of the temperature, static magnetic field and input power level.

Characterization of microwave devices

Our experimental apparatus allows one to characterize microwave devices such as resonant cavities. We have assembled and characterized different cavities resonating at microwave frequencies made up, fully or partially, of superconducting materials. In particular, we have assembled a coaxial cavity composed by a external Cu cylinder and an inner BISCCO rod and, with the collaboration of EDISON SpA, a cylindrical cavity entirely made of MgB2 and coaxial cavities made of Cu/MgB2 and MgB2/MgB2. Measurements of the microwave response of these cavities allowed us to investigate the properties of a BISCCO rod, different MgB2 cylinders and different MgB2 rods.

AC magnetic susceptibility

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