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CBT - Coulomb Blockade Thermometer Contents Background 2. Operation principle 3. Measurements 4. Fabrication and the structure 1. 2 Coulomb Blockade Thermometer ¾ ¾ ¾ ¾ ¾ ¾ ¾ Primary thermometer Cryogenic temperatures 20mK…30K Insensitive to magnetic fields (50 mK, 27 T) An array of tunnel junctions Invented in 1994 in Jyväskylä Commercialized in 2001 (Nanoway Oy) 3 Tunnelling ¾ ¾ ¾ ¾ Thin insulating barrier between two electrodes QM: propability for electron to move to the other side Behaves like an ohmic resistor The junction has a capacitance 4 What is Coulomb Blockade? ¾ ¾ Current can flow through a tunnel junction when exactly one electron passes through the tunnel. The electron charges the capacitor, causing a buildup voltage U=e/C. If the capacitance is small the buildup voltage can be large enough to prevent another electron from tunnelling Æ The electrical current is then suppressed at low bias voltages, and the resistance of the device is no longer constant. The increase of the differential resistance around zero bias is called the coulomb blockade. 5 Operation principle (1/2 ) ¾ ¾ Relation between thermodynamic temperature & conductance of the junctions The first derivative of the current-voltage charactreristics (I-V curve) is plotted 6 Operation principle (2/2) ¾ ¾ ¾ ¾ Differential conductance G=dI/dV vs. V Æ bell shaped dip in conductance around zero bias Full width at half minimum, V1/2 7 Primary thermometer The half width V1/2 depends only on constants of nature and known parameter N Æ primary thermometer ¾ 8 Secondary thermometer ¾ ¾ ¾ ¾ ¾ ¾ A faster temperature reading ∆G/GT Needs to be calibrated Capacitance Æ secondary thermometer Dependence on the capasitances of the device Æ variation due to the production process Rate of about 1 sample/second 9 Measurements ¾ Cryostat: 3He/4He dilution refrigerator ¾ 8 mK…1K ¾ CBT measured against a 3He melting curve thermometer (PLTS-2000) ¾ A standard measurement of electrical resistance with only slightly modified resistance bridges gives a voltage reading directly related to temperature 10 Temperature range and the limits decoupling of the electron system from the underlying lattice. ¾ At high end: The charging dip gets increasingly small at high temperature ¾ At low end: The resolution of lithography sets the upper temperature limit. The finite height of the tunnel barrier Signal to noise ratio to detect small changes of G 11 Fabrication and structure 1. 2. 3. 4. Oxidised Si-wafer EBL Æ areas of junctions, cooling fins PMMA, Shadow angle evaporation for the deposition of: Al/Al2O3/Al Cooling blocks of copper 12 Shadow angle evaporation 13 The tunnel junction ¾ ¾ Al/Al2O3/Al e.g. 1,5 um2 area 14 ITS-90 and PLTS-2000 ¾ ITS-90: fixed points, interpolating thermometers ¾ 0,65 K at the low end ¾ PLTS-2000 0,9 mK…1K ¾ PLTS-2000: defined by an equation for the melting pressure of 3He 15 PLTS-2000 ¾ The traditional PLTS-2000 primary thermometers: very difficult to use ¾ CBT: easier realisation method 16 Summary ¾ Insensitive to magnetic fields: 50 mK, 27 T ¾ Simple electrical measurement ¾ A primary thermometer 17 Thank You! ¾ Thank You for your attention 18