A Novel Design of a Low Temperature Preamplifier for Pulsed NMR Experiments of dilute 3 He in Solid 4 He

Transcription

1 Journal of Low Temperature Physics manuscript No. (will be inserted by the editor) A Novel Design of a Low Temperature Preamplifier for Pulsed NMR Experiments of dilute 3 He in Solid 4 He C. Huan 1,2, S. S. Kim 1,2, L. Phelps 1, J. S. Xia 1,2, D. Candela 3 and N. S. Sullivan 1, Keywords NMR, supersolid, defects, preamplifier Abstract Recent experimental studies of solid 4 He indicate a strong correlation between the crystal defects and the onset of a possible supersolid state. We use pulsed NMR techniques to explore the quantum dynamics of 3 He impurities in solid 4 He in order to examine certain theoretical models that describe how the disordered states are related to supersolidity. Because of the very small signal-to-noise ratio at low 3 He concentration and the long spin-lattice relaxation time (T 1 ), it is essential to significantly enhance the NMR sensitivity to be able to carry out the experiments. Here we present the design of a novel low temperature preamplifier which is built with a low noise pseudomorphic HEMT transistor that is embedded into a cross-coil NMR probe. With a low power dissipation of about 0.7 mw, the preamplifier is capable of providing a power gain of 30 db. By deploying the preamplifier near the NMR coil below 4 K, the noise temperature of the receiver is reduced to approximately 1 K. This preamplifier design also has the potential to be adapted into a low temperature amplifier with both input and output impedance at 50 Ω or a low temperature oscillator. PACS numbers: Pt, D, Le 1 Introduction While NMR techniques are of great value and one of the few microscopic probes available to study dynamics of atoms and molecules at very low temperatures, their use raises a number of challenges at millikelvin temperatures. Firstly, the RF pulses needed to study diffusion and kinetics through the use of echo techniques or relaxation time measurements 1:Department of Physics, University of Florida, Gainesville, FL 32611, USA Tel.: Fax: huan@phys.ufl.edu 2: High B/T Facility, National High Magnetic Field Laboratory, University of Florida, Gainesville, FL USA. 3: Department of Physics, University of Massachusetts, Amherst, MA 01003, USA.

2 2 Fig. 1 (Color online) Circuit diagram of the low temperature preamplifier. The transistor is a phemt, Aglient type ATF-35143, operated as a source follower to match the NMR resonant circuit (LC 2 ) to a 50Ω RF cable. The operating point of the amplifier is controlled by the bias voltage V which is adjusted externally. Metal film chip resistors and ceramic chip capacitors are used throughout. result in non-negligible RF heating, especially for solid state studies where the power spectrum of the pulses needs to be relatively high to cover broad NMR lines. Secondly, at very low temperatures, the nuclear spin-lattice relaxation times can become very long, especially in high magnetic fields, thereby limiting the ability to signal average except over long periods of time, and often restricting studies to low magnetic fields. The low field values lead to a further loss in signal amplitude. These challenges were particularly acute for experiments designed to study the motion of 3 He impurities in solid 4 He in the region where non-classical rotational inertia fractions are observed. 1,2,3 It is known that 3 He impurities suppress the supersolid behavior, while at high temperatures T>400 mk, previous NMR studies show that 3 He impurities travel through solid 4 He as a diffusing impuriton gas driven by the quantum exchange of 3 He atoms with neighboring 4 He atoms. 4 Careful studies of this impurity motion is needed to test models 5,6 for the supersolid behavior and to determine if pinning of vortices or other excitations at impurity sites blocks superflow properties. These pinning effects would lead to an increase in NMR linewidth and nuclear spin-spin relaxation rates due to the reduction of the power spectrum for the spin-spin intteractions as the time dependence of the magnetic interactions of the 3 He atoms decreases with pinning. Previous studies of the quantum mobility have been limited to fairly high concentrations of the order of 1000 ppm 4,7 but we needed to be able to observe NMR signals from samples with 10 ppm 3 He and for temperatures T 10 mk for which the spin-lattice relaxation times could according to some models exceed 10 3 s. 8 2 Design considerations The primary goal of the design of the preamplifier was to provide optimized matching of NMR signals induced in a tuned LC resonant circuit operating at ultra-low temperatures to a low loss coaxial cable that extended from the low temperature region to room temperature. We chose to use a simple source follower circuit with high input impedance and biased to provide the required transfer admittance to match to a 50Ω low loss cryogenic cable. This design has the advantage of minimizing the tendency of the circuit to oscillate because of the high gain of the devices used. The circuit diagram is shown in Fig.1. The resistors were

3 3 Preamp Sample NMR Receiving Coil NMR Transmit Fig. 2 (Color online) Schemetic diagram of the low temperature experimental region, showing the NMR cell, amplifier, and solid silver thermal link to the nuclear refrigerator. miniature metal-film resistors and the capacitors miniature ceramic chip capacitors tested prior to assembly at liquid nitrogen temperatures for integrity on thermal cycling. The device used was an Aglient type ATF pseudomorhic high electron mobility field effect transistor. 9 This device has low power dissipation compared to others in the same class. The gate bias was supplied by an external supply to limit the total power dissipation of 0.5 mw and with the source resistance determined the operating point and thus the transconductance of the phemt. Optimum matching of the output can be achieved by fine adjustment of the bias voltage. The circuit was operated at 2.1MHz as this corresponded to estimated low temperature (T 10 mk) nuclear spin-lattice relaxation times of 10 3 s. which was the maximum for which tolerable signal/noise ratios in reasonable averaging times could be realized. The circuit was mounted on a standard printed circuit 1 cm by 1 cm that could be mounted in close proximity to the experimental cell and oriented so that the plane of the phemt was parallel to the applied field to minimize Hall effects. 10 The physical arrangement of the amplifier and the NMR cell is shown in Fig. 2. A crossed-coil arrangement was used with the receiving coil oriented perpendicular to the transmitting coil. The latter is mounted on a polycarbonate former that slides onto the outer wall of the former for the receiving coil (100µH.) This arrangement minimizes the transmission of unwanted RF to the receiving coil during pulse transmission, and because of the poor thermal conductivity of the polycarbonate provides good thermal isolation of the RF coils and the amplifier from the solid 4 He sample. Thermal contact to the sample is assured by a sintered silver plug that is cold pressed to the end of a solid silver cold finger that extends from the tail of a nuclear demagnetization refrigerator. 11

4 4 Standoff Tuning capacitor Silver thermal link ] RF amplifier Fill line RF transmission coil Fig. 3 (Color online) Photograph of the layout of the NMR cell and RF amplifier. The amplifier is mounted rigidly with maple wood standoffs attached to the silver post that connects the nuclear demagnetization stage to the sample cell. The NMR cols are wound on polycarbonate formers that surround the NMR cell. 3 Performance The complete system operated satisfactorily down to a temperature of 250 mk. The observed signal/noise for a sample cotianing 500 ppm of 3 He in solid 4 He was 110 after signal averaging (100 pulse sequences) for a bandwidth of 35kHz. From the calibrated gain this corresponded to a noise temperature of approximately 1.1 K. A small cross-coupling from the transmssion coil led to an uncertainty in the gain calibration and the quoted noise temperature is an upper limit. While good thermal isolation of the amplifier and NMR detection circuit was achieved, a weak thermal contact between the RF cables and the nuclear refrigerator several feet from the NMR cell resulted in a fraction of the amplifier heating reaching the cooling stage and limited the temperature to 250 mk. This can be alleviated with improved isolation at the refrigerator. 4 Conclusion We have demonstrated that low power pseudomorhpic amplifiers can be used to realize low noise temperatues, T N < 1.1 K, for pulsed NMR experiments at millikelvin temperatures by isolating the amplifier and and NMR coils from the sample cell itself. The circuit is easy to construct and in principal can operate up to UHF frequencies. The design allows the use of high power pulses for straightforward pulse sequences. The improvement over simple passive matching using a capacitive trnsformer to match a resonant coil to a 50 Ω transmission line is better than a factor of 260. Acknowledgements This research was carried out at the NHMFL High B/T Facility which is supported by NSF Grant DMR and by the State of Florida. This project was supported in part by an award from the Collaborative Users Grant Program of the NHMFL.

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