Download Josephson junction

Superconducting Quantum Interference Device
SQUID
C. P. Sun
National Sun Yat Sen University
Department of Physics
Outline
Essential mechanism
of SQUID
The way
to measure
Temperature control
system
AC
RSO
Josephson Junction
DC
Introduction
● Essential Mechanism

Josephson junction
A Josephson junction is an superconductor-insulatorsuperconductor (SIS) layer structure placed between two electrodes
DC Josephson : A dc current flows across the junction in the absence of
any electric or magnetic field.
AC Josephson : A dc voltage applied across the junction causes rf current
oscillations across the junction. This effect has been
utilized in a precision determination of the value of  / e
Further, an rf voltage applied with the dc voltage can
then cause a dc current across the junction.
Macroscopic long-range quantum interference:
A dc magnetic field applied through a superconducting
circuit containing two junctions causes the maximum
supercurrent to show interference effect as a function
of magnetic field intensity. Magnetometer
DC Measurement
When the external dc field is applied to the sample, the magnetization
changed with temperature will be obtained due to the spin-spin, spinorbital and orbital-orbital effect interacting with magnetic field.
AC Measurement:
An oscillating AC magnetic field is applied to the sample. The
change in flux seen by the detection circuitry is caused only by the
magnetic moment of the sample as it responses the applied AC field.
Xac= dM/dH obtained from these measurement is described as
having both real and imaginary components X’ and X’’ , where
the imaginary component is proportional to the energy losses in
the sample.
What can be derived from AC Measurement?
Structure details of materials, resonance phenomena, electrical
conductivity by induced currents, relaxation processes such as
flux creep in SC and energy exchange between magnetic spins
and the lattice in the paramagnetic materials.
Advantage of MPMS AC System
Conventional AC suceptometers measure the voltage induced
in an inductive detection coil by an oscillating AC magnetic moment.
The most common systems use mutual inductance bridge to
measure the voltage induced.  High pressure effect
These systems measure only signals with frequencies at or very
near the applied excitation.  The natural constraint
Voltage induced is proportional to the frequencies of the oscillating
drive field
How does MPMS solve it?
★ MPMS AC option combining an AC drive field with a SQUID-based
detection system.
★The SQUID is an extremely sensitive flux-to-voltage converter that
directly measures the change in flux as the sample moves through a
superconducting detection coil coupled to the SQUID circuit.
RSO Measurement (Reciprocating Sample Option)
RSO measure a sample by moving it rapidly and sinusoidally
through the SQUID pickup coil.
The option’s use of a high-quality servo motor and a digital signal
processor (DSP) allow rapid measurement. The servo motor, unlike
the stepper motor performing DC measurements, doesn’t stop sample
movement for each data reading. Lock-in techniques that use the DSP
reduce the contribution of low-frequency noise to the measurement.
Measure consideration of RSO
☆ Center position
To ensure absolutely accurate measurements even when temperature drifts.
To oscillate sample through most or all of pickup coils while taking a high
number of readings.
☆Maximum slope
To take measurements quickly.
To move sample through small section of pickup coils.
To perform hysteresis measurement.
sensitive to position
Temperature Control system
Heater
Needle Valve
Thermal meter
Thank you for attention
Sun 2005.05.18
Conclusion
Tc [Curie Temperature] Ths phase transition between paramagnetic
and ferromagnetic behavior.
TN [Neel’ Temperature] Ths phase transition between paramagnetic
and Antiferromagnetic behavior.
Collect More Data After More Experience
More Experience can help us think more details
The minimum value of magnetic flux
(CGS)
(SI)
Fluxon
 0  2c / 2e  2.0678 10 7 gauss cm 2
 0  2 / 2e  2.0678 10
15
tesla m
2
Spin Glass Material exhibiting a high magnetic frustration and
its magnetic structure is disordered even at low temperature.
Tc [Curie Temperature] Ths phase transition between paramagnetic
and ferromagnetic behavior.
TN [Neel’ Temperature] Ths phase transition between paramagnetic
and Antiferromagnetic behavior.