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A 10 mK scanning tunneling microscope operating in ultra high vacuum and high magnetic fields
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Image of FIG. 1.
FIG. 1.

STM scan head. (a) Overview of the STM scan head. The outer housing is cut open in the image. (b) View to the interior of the STM unit. (c) Perspective view of the STM unit connected to the four posts of the link structure to the mixing chamber.

Image of FIG. 2.
FIG. 2.

Scan assembly. (a) Scanning piezo is attached to the adapter plate via an insulation ceramic. It carries the receptacle for the tip holder. The whole scan assembly is mounted to an attocube coarse motor (not shown). (b) Close up of the tip holder.

Image of FIG. 3.
FIG. 3.

The sample section which is mounted to the outer housing of the STM scan head contains an inner thread for the sample holder. An electrically insulating contact ring is mounted to the sample stage with three springs to contact the sample.

Image of FIG. 4.
FIG. 4.

Sample holder. (a) Completely assembled sample holder for e-beam heating. The base of the sample holder is a body containing an outer thread fitting into the sample section. An e-beam collector and the sample are clamped to the base body by two half discs made of thermoelement material. (b) One half disc is removed. A sapphire plate is used to insulate the sample and e-beam collector from the base body. (c) Cross section through the sample holder. (d) Perspective view of the sample holder adapted for semiconducting samples. The half discs fixing the sample to the base body are made of molybdenum. (e) Spacer platelets compensate for different wafer thickness.

Image of FIG. 5.
FIG. 5.

Tip and sample transfer. (a) The transfer tool is moved into the STM unit through the guiding cone. The transfer head is an Allen key fitting into the back of the sample holder. A threaded hole on top is used for accepting the tip holder. Two alignment rings slide into the transfer guiding cone ensuring alignment of the axes. (b) The sample is transferred to the STM. (c) The tip is removed from the STM.

Image of FIG. 6.
FIG. 6.

Cross sectional view through the complete cryostat.

Image of FIG. 7.
FIG. 7.

Calibration curve of the CMN magnetization thermometer. Magnetization values are plotted against the inverse transition temperature of the superconducting compounds in the fixed point device (see text for details).

Image of FIG. 8.
FIG. 8.

Calibration curves of the RuO temperature sensors. (a) Temperature calibration curves. Red: data from RuO sensor at MC, blue: data for RuO sensor at the interface plate, black: fit to the data. (b) Relative deviations between fitting curves and measured data. The fits lie very well on the data below 500 mK with less than 5% of error.

Image of FIG. 9.
FIG. 9.

MC temperature as a function of the 3He circulation. There is a clear minimum for circulation rates between about 130 μmol/s and 200 μmol/s. The cryostat is normally operated at lowest circulation rate at a temperature of about 15 mK.

Image of FIG. 10.
FIG. 10.

Sample preparation stages for electron beam and direct current heating. (a) Perspective view and (b) side view of the preparation stage for electron beam heating. The stage is connected through an electric insulator to the manipulator. The sample is in the stage. For temperature measurements the thermoelement half discs of the sample holder are used. The filament for sample annealing is transferred to the operating position from below. (c) Perspective view of the filament with the two banana plug pins for the electrical contacts on the side, which also hold the filament in place. (d) Perspective view and (e) side view of the preparation stage for direct current heating. Two tip holders in parking positions are shown as well.

Image of FIG. 11.
FIG. 11.

Cool down times after tip and sample transfer. Red curve: temperature at the MC. Blue curve: temperature at interface plate. (a) Cool down after tip transfer to approximately 1 K with an initial tip temperature of 300 K. (b) Cool down after sample transfer with an initial sample temperature of 100 K. (c) Cool down from 1 K to base temperature using the dilution cycle.

Image of FIG. 12.
FIG. 12.

Schematic of the different vibration levels. For damping the cryostat a combination of active controlled and passive dampers are used. The colors are chosen to illustrate the different levels of vibration isolation. Vibrations transmitted through the pumping lines are eliminated by attaching the lines to a massive granite block. For the still pumping line soft bellow segments are used to reduce the coupling among the various stages.

Image of FIG. 13.
FIG. 13.

Overview of the mK-STM setup. The cryostat rests on a vibration isolation stage in the upper level lab. It hangs through the ceiling into the lower level lab with the UHV preparation chamber attached at the bottom. The pumps are located in a separate room hanging from the ceiling to avoid mechanical disturbances on the floor, on which the vibration isolation is placed.

Image of FIG. 14.
FIG. 14.

(a) Topography of Au(111) measured at T = 800 mK. (b) Topography of Cu(111) with standing wave pattern of the scattered surface state measured at T = 15 mK. V = 50 mV, I = 50 pA.

Image of FIG. 15.
FIG. 15.

Temperature measured directly at tip and sample position of the STM. The blue graph shows the sample temperature. The red graph shows the tip temperature. The tip slightly warms up when a sine voltage (V PP = 5 V) is applied to the scanning piezo.

Image of FIG. 16.
FIG. 16.

Measurement of the tunneling conductance between a superconducting Al tip and a Cu(111) surface. (a) The conductance spectrum is shown in red and the fit to the Maki equation is shown in blue. For comparison, a fit at 0 mK is shown in green. (b) Zoom into the corners of the gap. The 0 mK spectrum illustrates that the rounding of the corner is only due to an effective thermal broadening. For comparison, the dotted spectra show a fit to the data for different effective temperatures demonstrating the sensitivity of the fit.


Generic image for table
Table I.

Some of the superconducting compounds and the corresponding superconducting transition temperatures in the FPD temperature sensor.

Generic image for table
Table II.

Fitting coefficients for the temperature calibration of the resistive temperature sensors at MC and STM support.

Generic image for table
Table III.

Fitting coefficients of the tunneling conductance between an Al tip and a Cu(111) surface. The DOS was calculated using the Maki equation which is explained in the text. Here Δ is the superconducting gap, ξ is the pair-braking parameter originating from the Maki theory, and T is the effective temperature. The energy resolution ΔE = 3.5k B T eff has been calculated from the effective temperature.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: A 10 mK scanning tunneling microscope operating in ultra high vacuum and high magnetic fields