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Electron Spin Noise Scanning Tunneling Microscopy

Final Report Summary - ESNSTM (Electron Spin Noise Scanning Tunneling Microscopy)

The ESNSTM project was executed as a collaborative and complementary effort by the consortium in two different directions according to the research plan. The first effort was in the direction of technical improvement of the different aspects of the technique, and the second one was in the direction of demonstration of the new abilities of the ESNSTM when these improvements were implemented.

The impedance matching problem was the first one to be solved. In a detector, the signal must be above the thermal Johnson noise given by: P=4kBTΔf. In units of dBm we get: PdBm = -174+10log(Δf). Namely in the usual bandwidth (Δf) of 30kHz we get noise power of -129dBm. For comparison, the noise power of 1 nA rf current with a 50 ohm detector is -133dBm. Our signal is of the order of 10pA. We applied several techniques to improve the sensitivity: Working at a small bandwidth; modulating the magnetic field and using a lock in amplifier; averaging over many spectra; working at cryogenic temperature to decrease the thermal noise; working in time domain, to shorten the acquisition time and impedance matching.

The first impedance matching circuit was an active one, where the current from the high impedance source is not dissipated in the low impedance detector but is used to create voltage oscillation by gating a JFET (field effect transistor) giving a much larger power in the detector.

This matching solution, together with phase sensitive detection and averaging enabled an experiment observing hyperfine coupling of vacancies in SiC surfaces. This hyperfine spectrum observed as a result from the presence of 29Si atoms. We have done such measurements in frequency sweep, field sweep and in different magnetic fields. The hyperfine coupling is of the order of 10MHz, in agreement with the macroscopic spectrum.

Until now the signals observed were in the frequency domain. Yet it was shown that detecting the signals in time domain offers a significant reduction of the acquisition time, since all the signals are observed simultaneously, using a fast oscilloscope – followed by FT and taking the power spectrum, gave detectable signals within a time scale of several tens of microseconds, which consists an improvement of signal to noise ratio of several orders of magnitude.

The main limitation of the active matching circuit is the parasitic capacitance of the JFET. Therefore we applied a passive matching circuit by adding in series and is parallel reactive components between the rf source (the STM) and the load (the detector). In addition, both active and passive matching circuits were designed to work also at cryogenic temperatures and UHV conditions. Cryogenic ESNTM microscopes were constructed and tested,

For a LT microscope, A SQUID (superconducting quantum interference device) detector can detect the current only, through the magnetic flux that is induced by the magnetic moment of the electron, eliminating altogether the mismatch problem. The sensitivity in this device is beyond any other alternative technique. It was demonstrated that in a system that emulates the STM current (namely high impedance source) the SQUID detector can detect a signal with an intensity of 10 pA up to 50MHz. This enables the use of this detector for ESNSTM experiments. Further work is in the direction of increasing the speed of the SQUID detector up to several hundreds of MHz.

Another approach to handle this problem is a top-down approach, assuming that ESNSTM results from capacitive coupling of the tip to the conductive sample. It was shown that when a macroscopic conductor is approaching to a conductive surface covered with paramagnetic molecules, the capacitive coupling of the rf magnetic field to the sample is similar to the coupling observed using an coil in a regular ESR spectrometer and a spin resonance signal is clearly observed. The technique was shown to be able to observe spatial information with resolution of 100 microns) when the rf source is raster scanned parallel to the samples. In ESNSTM there are several facts (such as the spatial dependence of the signal) pointing to the relevance of capacitive coupling also in ESNSTM.

Sample preparation is also an important ingredient of the research plan. By putting paramagnetic molecules on the surface it is possible to investigate a well defined spin system with ESNSTM. Two approaches were adopted. The first is to synthesize paramagnetic molecules with chemical binding groups such as thiol – to immobilize the molecule on Au surface. Such molecules can be drop casted on the surface. The second is to put a polyaromatic group attached to the paramagnetic molecules. Such molecules can be sublimed in UHV on the surface, creating a cleaner sample, which can be imaged with higher resolution. The immobilization of the molecules was done through the van der Waals interaction between the aromatic group and the surface.

The observation of hyperfine spectrum on several spin systems (such as vacancies in SiC, Cu atoms on Si(111)7x7, and tempo molecules) was demonstrated. The hyperfine spectrum was used to observe STM ENDOR spectra, where the nuclear transitions are irradiated and the change in the intensity of the ESNSTM electronic signal is recorded. The spectral resolution in these experiments was found to be significantly better. More precise information was observed on hyperfine components, distinguishing between different isotopes, quadruple spectrum (I=3/2), and even nuclear Zeeman transitions were detected opening many possibilities in nm chemical analysis.

In the STM there can be also a spin on the tip. An experiment on TEMPO molecules on Au(111) was done with PtIr tip. In addition to signals at frequencies g2H– from Tempo, there are additional frequencies g1H – from the spins on the tip. Additional much stronger signals are observed at the frequencies of ½|g2H – g1H| and ½(g2H + g1H) – in agreement with a model of interference between propagation through spin on the surface and the tip. Similar results were observed also with W tips (on SiC and Cu@Si(111)7x7). This is a significant step forward to understanding ESNSTM.

Finally some preliminary results were observed on the connection between the spectrum and the STM topography. The ESR spectrum of Tempo molecules is known to be dependent on the stochastic dynamics of the molecules. In STM it is possible to locally affect this dynamics which is affecting the ESNSTM spectrum, Experiments that are carried now are LT experiments, and experiments with magnetic tips (Cr tips)
Further questions that were investigated in the project and will be further explored afterwards are to observe signals from radical ions, enabling ESNSTM also to non paramagnetic systems: Enabling coherent ESTSTM in time domain: Testing schemes for single spin NMR and developing applications in spin related phenomena (Kondo physics and quantum information).

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