The spectral density of Nyquist noise current in a tuned circuit coupled to a sample of nuclear spins has been measured at 4 He temperatures with a dc SQUID used as a rf amplifier. When the sample is in thermal equilibrium, a dip is observed in the spectral density at the Larmor frequency. For zero spin polarization, on the other hand, a bump in the spectral density is observed. This bump is due to temperature-independent fluctuations in the transverse component of magnetization, and represents spontaneous emission from the spins into the circuit.PACS numbers: 33.25.Fs, 05.40. + j, 74.50. + r, 78.90. + t In his pioneering paper 1 on nuclear induction, Bloch noted that in the absence of any external radiofrequency (rf) driving field a sample of N spins of magnetic moment fi contained in a pickup coil would induce very small voltage fluctuations proportional to N 1/2 fi.In this Letter, we report the observation of these temperature-independent fluctuations at liquid-4 He temperatures arising from the 35 C1 nuclei in NaC10 3 at the nuclear quadrupole resonance (NQR) frequency of about 30 MHz.In the experiment, a sample of nuclear spins is placed in the inductor L p of a tuned LCR circuit and the spectral density of the current fluctuations is measured over the bandwidth of the circuit. The circuit resistance R; produces a Nyquist voltage noise and therefore a current noise that, in the absence of a sample, has a Lorentzian spectral density. The presence of the sample is found to modify the shape of this noise-power spectrum in the region of the NQF frequency. The influence of the sample is determined from its complex spin susceptibility 2 X(co)==X'(co)~jX"(ca), where X' and X" are the dispersion and absorption. The complex impedance of the coil in the presence of the sample is written aswhere £=2^/3^ is the sample filling factor; 7T S and f c are the volumes of the sample and the pickup coil. The added spin inductance L S =4TT%L P X' shifts the circuit resonant frequency, while the added spin resistance R s =4TrgcoL p X" modifies the damping of the circuit and acts as a source of Nyquist noise. This noise is due to spin fluctuations in the transverse direction. To observe these fluctuations in a reasonable averaging time one requires, first, that R s /R t be not too small, and second, that the noise current be measured by an amplifier with a noise temperature comparable with or smaller than the bath temperature T.We can compute the Nyquist noise generated by the spins in terms of the microscopic parameters of the sample. Since the NQR sample is equivalent to a two-level system, 3 we take as a model an ensemble of spins in an external magnetic field H z z with spin / = y, spin density n =N/Tly and Larmor frequency CO S /2TT=YH Z /2 9 where y is the gyromagnetic ratio. The axis of the pickup coil is along the x direction. We ascribe a spin temperature T s to the magnetizationWe assume that Bloch's equations apply, so that X" =X'/AcoT 2 is given bywhere Aco=co s -co, and the linewidth is given by Af s = l/TrT 2 . Th...

The spontaneous emission from nuclear spins has been observed at liquid-He temperatures. The spins, 'Cl nuclei, are placed in the inductor of a tuned LCR circuit coupled to a dc superconducting quantum interference device used as a radio-frequency amplifier. 'When the spins are saturated and have zero polarization, the emission is observed at the nuclear quadrupole Larmor frequency as a bump in the spectral density of the Nyquist noise current in the tuned circuit. This bump arises from the temperature-independent fluctuations in the transverse component of the nuclear magnetization. When the spins are in thermal equilibrium, on the other hand, a dip in the spectral density of the current noise is observed, arising from an induced absorption of noise power from the circuit at the Larmor frequency. The standard circuit-coupled Bloch's equation, modified to take into account radiation damping and transverse spin fluctuations, is consistent with the predictions of the Nyquist theorem and the Einstein equation for spontaneous emission. A spin-pendulum model for spin noise is described. The signal-to-noise ratio obtainable in a spin-noise measurement is discussed.

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Deep-submicron trench profile control using a magnetron enhanced reactive ion etching system for shallow trench isolation Deep and narrow anisotropic etching of silicon structures has been investigated in a low-pressure high density plasma reactor working with a cryogenic chuck. We have previously demonstrated the feasibility of this technique on such structures. Improvement of etch rate and profiles has been studied and new results show 2 m wide trenches etched to a depth of 50 m at an average etch rate of 5 m/min with highly anisotropic profiles and very high selectivity ͑Ͼ500:1͒ toward the SiO 2 mask. An evaluation of a commercially available reactor from Alcatel has been carried out and similar results are obtained. A phosphosilicate glass mask has been used to study the effect on profiles. It is shown that undercut is reduced while bowing is independent of the mask material. Since surface temperature strongly affects the profiles, wafer deformations in our cryogenic chuck have been measured and temperature evolution across the wafer has been estimated. A significant temperature difference of 10°C between the chuck and the wafer is expected for thin wafers ͑210 m͒.