Download CBT - Coulomb Blockade Thermometer

CBT - Coulomb
Blockade Thermometer
Contents
Background
2. Operation principle
3. Measurements
4. Fabrication and the structure
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Coulomb Blockade Thermometer
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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)
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Tunnelling
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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
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What is Coulomb Blockade?
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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.
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Operation principle (1/2 )
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Relation between
thermodynamic temperature
& conductance of the
junctions
The first derivative of the
current-voltage
charactreristics (I-V curve) is
plotted
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Operation principle (2/2)
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Differential
conductance
G=dI/dV vs. V
Æ bell shaped dip in
conductance around
zero bias
Full width at half
minimum, V1/2
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Primary thermometer
The half width V1/2
depends only on
constants of nature
and known parameter
N
Æ primary thermometer
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Secondary thermometer
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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
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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
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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
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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
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Shadow angle evaporation
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The tunnel junction
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Al/Al2O3/Al
e.g. 1,5 um2 area
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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
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PLTS-2000
¾ The traditional PLTS-2000 primary
thermometers: very difficult to use
¾ CBT: easier realisation method
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Summary
¾ Insensitive to magnetic fields: 50 mK, 27 T
¾ Simple electrical measurement
¾ A primary thermometer
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Thank You!
¾ Thank You for your attention
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