Our NQR apparatus is an hybridization of Oxford instruments continuous flow cryostat, a TECMAG spectrometer, and a home build head. All systems are controled by a PC under windows NT, and experimental parameter such as, temperature, frequency etc. can be modified automatically.
The construction of this instrument was funded by:
- The French Israeli colaboration program AFIRST of the Israeli Minestry of Science.
- Israel Academy of Science, grant for high Tc.
The Oxford instruments continuous flow cryostat (CF1200 daynamic) allows a working temperatures ranging from 1.6 to 450K, and a 62mm sample space diameter. The cryostat is placed in an AD-Vance Magneticsm-metal tube in order to have zero field.
We use TECMAG APPOLO spectrometer with two channels. This spectometer can also be used for double resonance. The power supply is ATM-M3446 1KW capable of working between 10-130 MHz. For tuning and meaching we use the Morris Instruments Model 405NV+ RF Sweeper.
The head is made of series-parallel tank circuit using Cylindrical Teflon capacitors. The capacitance can be varied from the top of the cryostat. In order to load a sample the head must be removed from the cryostat.
We have published the following papers on the subject:
- R.Ofer and A.Keren, Nutation versus angular-dependent NQR spectroscopy and impact of underdoping on charge inhomogeneities in YBa2Cu3Oy, Phys. Rev. B. 80 (2009), 224512.
We describe two different nuclear quadrupole resonance (NQR) based techniques, designed to measure the local asymmetry of the internal electric field gradient η and the tilt angle α of the main NQR principal axis z^^ from the crystallographic axis c^. These techniques use the dependence of the NQR signal on the duration of the radio frequency (rf) pulse and on the direction of the rf field H1 with respect to the crystal axis. The techniques are applied to oriented powder of YBa2Cu3Oy fully enriched with 63Cu. Measurements were performed at different frequencies, corresponding to different in-plane copper sites with respect to the dopant. Combining the results from both techniques, we conclude that oxygen deficiency in the chain layer lead to a rotation of the NQR main principal axis at the nearby Cu on the CuO2 planes by α ≂ 20° ∓ 5°. This occurs with no change to η. The axis rotation associated with oxygen deficiency means that there must be electric field inhomogeneities in the CuO2 planes only in the vicinity of the missing oxygen..
- R.Ofer, S.Levy, A.Kanigel and A.Keren, Charge-inhomogeneity doping relations in YBa2Cu3Oy detected by angle-dependent nuclear quadrupole resonance, Phys. Rev. B. 73, (2006) 012503.
The origin of charge inhomogeneity in YBa2Cu3Oy is investigated using an experimental method designed to determine the nuclear quadrupole resonance (NQR) asymmetry parameter η for very wide NQR lines at different positions on the line. The method is based on the measurement of the echo intensity as a function of the angle between the radio frequency field H1 and the principal axis of the electric field gradient. Static charge inhomogeneity deduced from η>0 is found in this compound, but only in conjunction with oxygen deficiency. This limits considerably the possible forms of charge inhomogeneity in bulk YBa2Cu3Oy.
- S.Levy and A.Keren, Pure nuclear quadrupole resonance determination of the electric field gradient asymmetry for broad lines, J. Mag. Res. 167, (2004) 317-321.
We present an angle dependent nuclear quadrupole resonance (ADNQR) method to determine the electric field gradient asymmetry parameter g in systems where the resonance line is so broad that the radio frequency field can excite only a portion of the nuclear spins. In this situation, the recently developed spectroscopic methods are not applicable. ADNQR is useful for single crystals and oriented powders, and, for small η determines η4. Therefore, it can be used to evaluate fluctuations in η due to inhomogeneities. We demonstrate the application of ADNQR experimentally to oriented superconducting YBa2Cu3O7 powder.