Real-Time Response and Phase-Sensitive Detection to Demonstrate the Validity of ESR-STM Results
✍ Scribed by Yishay Manassen
- Book ID
- 102600349
- Publisher
- Elsevier Science
- Year
- 1997
- Tongue
- English
- Weight
- 304 KB
- Volume
- 126
- Category
- Article
- ISSN
- 1090-7807
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✦ Synopsis
of v i to obtain the phase of the signal. Fourier expansion of In ESR-STM (1-3), the tip of a scanning tunneling mithis signal gives croscope (STM) scans a surface which contains isolated paramagnetic spin centers. It was found previously that, in the presence of an external magnetic field, an AC component F(t) Å A{J 0 (m v )sin(v c t) / J 1 (m v )[sin(v c / v m )t at the Larmor frequency appears in addition to the DC tun-
m )t neling current and that this RF signal is spatially localized to a region with a radius of 0.5-1 nm. The AC signal was / sin(v c 0 2v m )t] / rrr}, detected, after amplification, by a spectrum analyzer. An impedance-matching circuit is added to match the output where J n (m v ) are nth-order Bessel functions of the first kind. impedance of the STM to 50 V and to optimize the sensitiv-For a large modulation index, there are many significant ity. The sample which was used for this observation was frequency terms. A spectral analysis results in a set of a silicon surface covered with several monolayers of SiO 2 equally spaced sidebands, each separated by v m from a produced by thermal oxidation, and the paramagnetic centers neighbor, and the sideband spectrum looks as in Fig. 1, are isolated silicon dangling bonds located at the Si/SiO 2 bottom. The intensity of each sideband is given by J n . The interface (P b centers).
total number of sidebands of significant intensity in this Although it was shown that the signal observed was spectrum is 2m v ; they are too close to each other to be detected at the proper frequencies for different magnetic distinguished in Fig. 1. The frequency difference between fields, this observation remained somewhat controversial. the two largest sidebands at the two edges of this spectrum One claim was that there was no way of distinguishing is approximately 2Dv (4). between spurious signals which might accidently exist at
In order to check the real-time response of ESR-STM the frequency examined and the real signals. In this Note, signals, a coil was added to the STM that can generate a this concern is eliminated by showing that the signals small oscillatory field component parallel to the DC magrespond in real time to changes in the magnetic field: In netic field. Both fields are applied parallel to the tip. The addition to the static DC magnetic field, a small time-DC magnetic field was 150.5 { 0.5 G. The error of 0.5 G dependent sinusoidal field is applied. This field modulais due to the inhomogeneous component of the magnetic tion causes a modulation of the signal. The response to the field which creates a slight variation in the intensity of the field modulation enables phase-sensitive detection ( PSD ) . field. This occurs mainly because of small changes in the The previous observation of the two-dimensional spatial precise position of the tunneling region (for example, belocalization is reconfirmed; this time the signal is detected cause of tips with different lengths). The precision of the by PSD when the magnetic field is modulated. This obsergaussmeter is also {0.5 G. The modulation intensity DH vation eliminates concerns that the previous observations was 27 mG. This was measured by applying a DC current were due to traps or any other species on the surface which to the coil and monitoring the change in the DC field. A could, in principle, create a local increase in the highchange of the field by 2-3 G was measured. Then, by extrapfrequency noise level.
olation, the field intensity for smaller current values was Suppose that the signal is driven by a field (in frequency estimated. The error in the value of the modulation intensity units) of the form v i Å v c / Dv cos(v m t), where v c is the is {10%. The modulation frequency was 300 Hz, the tunnelcarrier frequency, v m is the modulation frequency, and Dv ing current was 1 nA, and the tip to sample bias voltage was is the modulation intensity. This signal can be expressed as 0.32 V. The static magnetic field, the tunneling current, and F(t) Å A sin[v c t / m v sin(v m t)], where m v is the modulathe tip to sample bias voltage were the same for all the measurements reported in this Note. tion index m v Å Dv/v m .