Blog here relevant events and changes. Use format: date Content of the report.

05-2007
During the first “real attempt” to change sample with the wobble stick, the sample holder-holder comes loose from the piezo-stack. Repaired tip and sample holders, as also the tip holder came loose by hand. Apparently they were both not glued well (white stycast was used before). Used EpoTek H70E this time (thermally conducting, electrically insulating 1:1 two component epoxy).
Federica

May-2007
COARSE APPROACH: This STM employs a compact design where the Z-coarse and Z-fine motion of the tip is done by the same set of (shear) piezos. The same HV amplifier is used for Z-coarse and Z-fine. This means that the available voltage range has/can to be reserved for the Z-coarse and Z-fine motion, as depicted in the figure here under:

This has the limitation that the Z-range for current probing would be too small. Furthermore, this can only be realized using the “Analog Tip Ramp” method with the RHK electronics, and with this method the 2nd DAC will have to be used as well and piped into Z-modulation input nr.2 which mixes in the DAC input with the Z-scan signal with NO amplification. To have an amplification different from 1, the RHK box would have to be hacked and a gain resistor changed. Possible but possibly unwanted. DAC 2 should also remain free for spectroscopy as well.

A solution is to use the whole range for tip fine motion during approach and use the time-reversal method to the waveform to walk back, instead of inverting the polarity, as sketched here under:

HOWTO: In Approach Control, Main: use always “Tip Retract” method, with TTL-Feedback control, Tip back in motion ON.

1. APPROACH: Waveform: Parabola, Asymmetry: 0%
2. RETRACT: Waveform: Sawtooth, Asymmetry: 100%
   

After the tip has been retracted sufficiently far using above method, there is a way to move the slider faster down (to make tip/sample exchange for example):

• invert Z piezo polarity by inverting the switch on the back side of the RHK controller
• Choose: Waveform: Parabola
  
• Click “Retract” a number of times until the slider moved back sufficiently.

Federica

End May-2007

Changed the Z-motor spring spacers into a spacer-frame. With this new design, two bars bridging (with some clearance) over the top piezo beam plate act as mechanical stop to excessive motion of the beam plate during tip-exchange (when pulling out the tip holder). This avoids: 1. accidental loss of the ruby ball when in UHV, 2. deformation of the leaf spring which would loosen the motor.
Federica

Beginning June-2007

Start another cool down attempt…(see logs).
Repaired leak in He3 capillary.
Operators: Federica

January-2008

Many changes:

• Re-serviced CuBe nut (it was jammed)
• Straightned insert that was in the past bent badly. This hindered vertical motion of the insert and proper engagement in the pin at the bottom.
• Made STM-1K-shield window a bit bigger (wider) for easier tip/sample exchange
• Straightened STM-1K-inner-shield (it was touching and jamming the 1K shield)
• Improved 1K shield with a side set screw to avoid that it rotates and loosens itself!
• CRITICALLY aligned the insert so that the window faces the wobble-stick optimally
• Removed funnel (not giving problems really, but not helping either).

Federica

February 2008 - April 2008

First fully successful round where low-T was reached and high magnetic field was used (see logs).
Operators: Federica

Mid April 2008 - Beginning July 2008

Second fully successful round where low-T was reached and high magnetic field was used (see logs). Concluded most of the measurements on the CMR LCMO/STO. A lot of STS at all temperatures between 50K and 180K (used He3-pot heater with 1K-flow in the 1K-capillary to increase the temperature to max. 180K). “Dead layers” on CMR thin films, see http://arxiv.org/abs/0902.0626, arXiv:0902.0626v1 [cond-mat.str-el]. Submitted to PRB.
Operators: Federica, sometimes with Simon Kelly

Beginning October 2008 - End December 2008

Third fully successful round where system was operated all the time between 2 and 0.3K.

Superconducting gap of MoGe thin film covered by a Au capping layer (T_c~6K, Δ ~ 2 mV). Vortices in 0.5 T (image skew due to temporary XY scan problem - due to external faulty wiring).
Operators: Federica

Superconducting gap of NbGe thin film covered by a Au capping layer (T_c~3K, Δ ~ 1 mV). Vortices in 0.5 T (image skew due to temporary XY scan problem - due to external faulty wiring).

Zero Bias Peak (due to Andreev Bound States) in YBCO(50nm) grown epitaxially on STO(5.0.3). Sample from Twente, topography also shown.

January 2009

Improved signal by replacing the grounded BNC (signal and bias) CF flange with isolated connectors flange. In this way electrical connection between the signal/bias-ground and the UHV chamber is removed, which makes the whole signal more clean and less critically affected (with respect to interference from 50 Hz et al.) by the UHV chamber electrical connections (pumps etc.).
Federica

End January 2009 - End April 2009

4th successful round: ZBA in YBCO, ZBA in H-HOPG and atomic res. Also a flicker of pi-state in Nb/PdFe.

Absent ZBP on the same sample but with a Au thin (4nm) layer on top. Note Coulomb Blockades seen occasionally at low bias settings, indicating weakly coupled grains on the surface.

Some really nice images with atomic resolution and Moire'-pattern on H-HOPG (measurements at 300mK):

Some of the first results of STS on HHOPG in high magnetic fields:

Some very nice atomic resolution images on H-HOPG (and more Moire'-patterns). Measurements at 300mK.

Run has to stop at the end of April because of a problem with the coarse approach.
Operators: Federica, Simon Kelly

Beg. May 2009 - Mid May 2009

Fix coarse approach. TiC coated Ti slider had to be changed due to severe wear of the coating due to the rubbing of the alumina pads. A spare new slider was available. Gluing a new sample/tip holder on the top of it was required (used Epo Tek H70E electrically insulating and thermally conducting epoxy).
Federica

Mid May 2009 - Mid August 2009

5th successful run. Base T and high magnetic field for regular measurements. Observed very nice Nb-gap (after surface etching in load-lock) and continued S/F project with Nb/Py bylayers. Run interrupted for a leak at the rotary feed-through of the STM “lift”.
Operators: Federica, Simon Kelly

BCS-fit of Nb-gap measured at 1.2K gives an effective temperature of 1.5K.

End August 2009 - Mid September 2009

Replace leaky feed-through with new one. Re-cool STM.
Federica, Simon Kelly

Mid September 2009 - Mid October 2009

Run stops because of problems with 1K pot (not able to sustain operational flow for more than 15-30 minutes) Operators: Simon Kelly

Mid October 2009 - 12 January 2011

STM was not cooled down for 1.5 years because of contamination of the internal liquid helium production system, which turned out to be the cause of the 1K pot issues.

Some nice images taken at room temperature in HV (10-9 mbar) in 1 year…

Very beautiful and clean steps on HOPG with some interesting artifacts (??) between forward and backward (and some atomic resolution):

And some atomic resolution (still HOPG):

Very nice flat terraces on Au<111> single crystal from Surface Preparation Lab, without any in-situ treatment (RT, High Vacuum):

And atomic resolution on Au<111> single crystal from Surface Preparation Lab, without any in-situ treatment (RT, High Vacuum):

Insert here changes/modifications Federica, Marius Trowuborst

Insert here atomic res on untreated Au<111> Federica

12 January 2011 - 24 March 2011
Operators: Federica, Marius Trowuborst

Cool down cryogenically speaking was successful. The 1K pot could run continuously several weeks, therefore the contamination was successfully removed from the liquid helium. The system was operated regularly at 300mK. No magnetic field was used.

Issues that lead to warming up:

1. Problem with wobble stick (gripping mouth was mounted on a too bent rod so there was too much friction).
2. A low-ohmic short between the bias-signal and the bias-shield (later found, in the SSMC connector down at the STM, insert side).
3. No energy resolution for Nb-gaps and smaller. Probably related to previous point (to be able to continue the experiments the bias-shield was interrupted just before it entered the cryostat, maybe introducing too much bias noise).
4. Problem with Au surface preparation (later we found it was related to accidental sample swap between <110> and <111>.)

Federica, Marius Trouwborst

20 April 2011 -

• Repaired the wobble-stick (Chris Pen)
• Repaired the short (Marius, Federica). The short was inside one of the 2 SSMC connectors at the end of the refrigerator, in the macor-piece. Note that the macor-piece broke during the repair, so we isolated and fixed the SSMC inside glass-fiber sleeves.
• Did a number of experiments at RT primarily to be sort out the Au <110> surface treatment for the missing-row reconstruction. Raphael Mueller, bachelor student from Germany, sorted out the procedure successfully.
  

Really nice STM images at room temperature in HV (~2×10-8 mbar) of Au<111> surface (herring-bone reconstruction) (V_b~100mV, I_t~4nA):

Really nice STM images at room temperature in HV (~2×10-8 mbar) of Au<110> surface (missing-row reconstruction) (V_b~100mV, I_t~4nA):

Initially, after the 1st cycle of 2 x sputter/anneal the surface is still a bit dirty (note absorbates/particles pinned by the step-edges)

But after more sputter/anneal cycles the surface becomes clearly more clean and the atomic chains become more clear: