“Superfluid” describes a phase of matter. In this
phase, a liquid has no viscosity and may exhibit
several “unusual” effects. Liquid Helium-4
transitions into a superfluid state at
temperatures below 2.17 K, and is a good
subject because of its weak intermolecular
“Strange” Superfluid Effects
The primary effect in which we are interested is
“second sound.” This is a phenomenon in which
heat travels as a compressional wave (as sound
does), rather than through diffusion.
The heat wave can reach speeds of ~20 m/s!*
*Lane, C.T., Fairbank, H.A., and Fairbank, W.M. Second Sound in Liquid
Helium II (1947) Phys. Rev. 71, 600 - 605
The Fountain Effect
A fine filter that only superfluid
can permeate covers the bottom
of a chamber with a heater
(resistor). The heated superfluid
transitions to normal liquid,
maintaining a gradient that drives
superfluid into the chamber. The
normal liquid builds up and can’t
escape through the bottom filter,
so it “fountains” out the top of the
• To observe second sound, we started with a thin
stainless steel tube, and wired a resistor into the
bottom of the tube. This resistor will serve as the
• We then installed two Cernox thermistors 7 cm
and 14 cm along the tube from the heater. Their
resistance increases as temperature decreases,
so temperature changes can be measured by
running a constant current and noting the
change in voltage.
Getting Everything Cold
Our Dewar has four chambers:
• The outer chamber is a vacuum.
• The next chamber holds liquid nitrogen.
• The third chamber also is evacuated.
• In the inner chamber, there is liquid Helium!
To change the normal liquid Helium into
superfluid, we pumped on it to lower the vapor
pressure above the Helium, effectively cooling it
to temperatures as low as 1.5 K.
We used a voltage source to drive the resistor at
a variable frequency, and used an oscilloscope
to find the time lag between the responses of the
two detectors. We read out the voltage of the
two temperature sensors and used a signal
analyzer to take the Fourier transform of their
voltages to see if the superfluid’s temperature
was varying at the same frequency as the
What’s going on here?
– We see a much stronger response once the
temperature drops below the λ point (2.17 K)
– This is expected, as superfluid helium conducts heat
much more quickly. (Waves vs diffusion)
Temperature (K) Frequency (Hz) Response Amplitude (mV/√Hz)
4.1 80 13
4.1 100 8
1.5 80 22
1.5 160 33
1.5 210 18
What’s going on here?
– Maybe the tube is interfering with the propagation of
the heat wave at longer wavelengths
– Maybe for the higher frequencies, the time delay is
greater than the period of the driving frequency;
perhaps we are measuring the delay between a given
pulse and the arrival of an earlier pulse, instead of the
arrival of the given pulse.
Frequency (Hz) Time Delay at 1.54K for Sine Wave Driving Voltage (ms)
Heater Thermistor 1 Heater Thermistor 2
80 7 13
690 1 1.3
1200 0.2 0.5
The Fountain Effect Set-up
This experiment was done by a former class.
able to use
Where you should look
• Heat travels faster in superfluid, which agrees
with the theory of second sound.
• We were not able to experimentally measure the
exact speed of second sound.
• The fountain effect was observed.