BNL SI-STM PROGRAM
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Researchers
Dr. Jinho Lee

Introduction

Instrument

Experimental Room

Our SI-STM and fridge are held from a vibration-isolation table, which is installed in a radio-frequency (RF) shielded room. We use a gas panel for our vacuum system and a dilution refrigeration unit to reach milliKelvin temperatures. The RF shield room, together with its foundation consisting of 26 tons of concrete, is floated by 6 air springs. Vacuum lines are anchored to the concrete block and wall before going into the RF shield room. All these instruments are installed inside an acoustic shielded room which is isolated from an external control room where the electronics are placed. Vacuum pumps are located underneath the floor of the control room.

RF-shield room Insert, vibration-isolation table, gas panel Gas panel, RF-tight box Gas panels Vacuum lines Control room

Click to enlarge pictures.
See also William Wilson Associated Architects Inc. site.

 

Dilution Refrigerator

The dilution refrigerator system, shown on the right, was purchased from Oxford Instruments. The highly modified design included decreased circulation rate at the base temperature of 10mK, decreased helium boiloff, and an off-axis top loading port. The refrigerator itself consists of a 1K pot (pumped liquid 4He), a condenser (to liquefy 3He coming from the room temperature pumps), the still (to pump on the dilute 3He-4He mixture), two sets of heat exchangers (one side of each heat exchanger contains 3He from the condenser while the other side has dilute mixture headed toward the still), and the mixing chamber (which contains the phase boundary between the pure 3He coming from the condenser and the 94% 4He - 6% 3He, the "dilute" phase, going to the still).

The modifications are shown on the right side of the figure. A system for in situ cryo-cleaving of crystalline samples is bolted to the 4K plate. This cleaving stage must also act as a radiation shield to prevent 300K black body radiation (from the room temperature vaccum port) from impinging directly on the dilution refrigerator and SI-STM. [Empirically, if we leave the door to the cleaving system open, the refrigerator warms up to roughly 1K.] The cooling system (rather difficult to see in this photo) consists of an extra pair of stainless steel tubes which are heat sunk to each stage of the refrigerator. Cryogen (either LN2 or LHe) is forced through the tubes from a storage dewar outside the cryostat, returning along the second tube back into the room. This allows us to cool the entire system from room temperature to 4K without using exchange gas. Two sample guides are used to, well, guide the sample from the cleaving stage into the SI-STM receptacle. Finally, the SI-STM is located at the bottom of the cryostat. Thermometers, heaters, and electrical connections are not shown for clarity (or minimal clutter, in this case).


Head Compartment

Top view Side view After assembly
STM diagram -- top view STM diagram -- side view
  1. Sample Receptacle
  2. Sample Holder
  3. Tip
  4. Tube Scanner
  5. Scanner Holder
  6. Sapphire Prism
  7. Shear Piezo Stacks
  8. Macor Body
  9. Spring Plate
  10. Capacitive Sensor

Results
[1] T. Valla et al, Science 314, 1914 (2006) (PDF format)
[2] Y. Kohsaka et al, Science 315, 1380 (2007) (PDF format)
[3] T. Hanaguri et al, Nature Physics 3, 865 (2007) (PDF format)
[4] J. Slezak et al, Proc. Nat'l Acad. Sci. 105, 3203 (2008) (PDF format)

Collaborators
S. Uchida - Tokyo University
H. Takagi - RIKEN
G. Gu - Brookhaven National Lab