Schematics of a typical SPM setup for cryogenic environment (left panel) and a drawing of one possible design (right panel). A piezotube or a series of piezostacks (A in left panel) are used for fine scanning and positioning of a local probe sensing arrangement down to subatomic distances and at ranges up to some micron at low temperatures. In the right panel, the piezotube is not shown. It is mounted within its frame (the prism B), with the free end for scanning at the bottom. As a local probe we show in the left panel a tip for a tunneling microscope (STM, F in left panel, not shown in right panel), but this may carry another device, as, e.g., a tuning fork resonator for AFM (Ref. 13). The frame (B) is itself connected through a coarse Z approach (D) to another frame (C). Eventually, the sample holder is mounted on another coarse positioning system (E), which may allow for X or X–Y positioning of the tip.
In (a), an overall schematics of the STM within the dilution refrigerator (A) and magnetic field coil (B) is shown. Corresponding photography is shown in (b). The dilution refrigerator has a sliding seal arrangement (C), which allows for a fast turn around time. In (c), we show a photograph of the low temperature STM head (D), which is thermally linked to the coldest point of the dilution fridge.
In (a), a representation of the overall setup used to prove the concept of the new motion unit. The tip (A) is hanging on the bottom of the piezotube, which is mounted on a prism (B, see also Fig. 1, right panel). The spring (C) fixes the prism to the main frame (D), and piezostacks are glued on the support, to make an inertial motor used to approach the tip to the sample. The positioning unit to move in the scanning plane is screwed at the bottom [a schematic lateral view is shown in (b)]. It consists of a slider (E), fixed using springs (F and G) to the support (D). The slider moves on a track, constructed as a wedge directed to the bottom (into D) to remove degrees of freedom perpendicular to the motion. Alumina covered with graphite are located within the track and at the slider, in the contact points between both. The sample holder is screwed to the slider. Additional screws (H) serve to position the slider in the scanning plane prior to cool down. The Kevlar rope (I) serves to pulls on the slider (D), and a stainless steel wire [K, in (c)] pulls on the Kevlar rope. The stainless steel wire is attached to a support [M, in (c)] below the mixing chamber of the dilution refrigerator with a spring [J, in (c)] and then thermalized at different positions (L). Mixing chamber, still and 1 K pot are schematically represented by the grey cylinders. Overall dimensions are given.
Atomic scale topography taken at zero field (a) and under a magnetic field of 8 T (b) in the compound NbSe2. These images have been taken at 100 mK with the device described here. The grey scale in (a) corresponds to a corrugation of 0.15 nm. Tunneling current was of 10 nA and the voltage bias 50 mV. In the right panels, we show a Fourier transform of the topography, which shows the characteristic sixfold pattern of atomic corrugation at wavevectors corresponding to the inverse of interatomic spacing (a). Around each Bragg peak, small sixfold peaks appear due to the charge density wave modulation at 1/. Arrows point to the lattice Bragg peaks (1/a) and the CDW modulation (1/).
In (a), we show a SEM image of a pattern nanofabricated with a focused ion beam on top of an Au film. In (b) we show several consecutive images taken with the STM when moving the sample holder with the sliding assembly at 100 mK. Tunneling current and voltage bias are of 8 nA and 5 mV. Height changes represented in the grey scale correspond to a corrugation of 20 nm. Note that the typically grain like topography in gold changes along the direction of the arrow. This is the result of the exposure of the Au film to the focused ion beam, which disappears when going out of the patterned region (image at the right).
Fourier transform of a record of the tunneling current as a function of time during 1 s, with the feedback loop switched off at 100 mK and with a magnetic field of 5 T (mean value of tunneling current is of 1.2 nA, and bias voltage is of 100 mV, tip and sample are of Au). Line in upper panel is measured on the STM setup without the positioning setup described here, and line in bottom panel is taken with the macroscopic positioning device and the slider at an intermediate position, with the pulling rope on tension.
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