Detecting superconductivity in a thin crystal
Cool a crystal and its resistance may drop sharply over a small temperature interval. To interpret the drop, it helps to know how resistance is measured, which point defines the transition temperature, and what a magnetic measurement can tell us.
Which point on the curve defines the transition?
In a four-terminal measurement one pair of contacts supplies a small current, while another measures the voltage drop across the sample. The voltmeter draws very little current, so the resistance of its contacts contributes little to the reading. Dividing the measured voltage by current at each temperature gives the curve R(T).
The onset of the drop, the crossing of a chosen fraction of the normal resistance and the point below the instrument’s resolution can occur at different temperatures. When a transition is broad, read the criterion before comparing critical temperatures across flakes.
Can current flow, and can magnetic flux enter?
Zero resistance indicates a dissipationless path between the voltage contacts. The Meissner effect is a separate test: when a sample enters the superconducting state in a sufficiently weak field, equilibrium screening currents expel flux from its interior. The field decays near the surface over the penetration depth. A resistance measurement probes a path; a magnetization measurement probes the sample’s response to an applied field.
Between the lower and upper critical fields of a type-II superconductor, flux enters in vortices while superconductivity persists. Interpreting a thin crystal in a field therefore requires the temperature, field strength and field direction relative to the layers.
Measurement schematic and relation
One transition criterion used in the paper. For example, if the resistance at 10 K were 100 Ω, T₉₀ would be the temperature at which the measured resistance reached 90 Ω.
Reading a plot: what does 5.3 K mean for MoTe₂?
In Fig. 2 of their 2019 paper, Gan and colleagues compared MoTe₂ flakes using thresholds of 90% and 60% of the resistance at 10 K. The 90% criterion gave 5.3 K for a 4 nm flake. The cryostat’s base temperature limited the measurement before full zero resistance was reached. In Fig. 3, check the field direction: a perpendicular field and a parallel field suppress superconductivity at different strengths.
Open the original experiment ↗Papers from the group
- Giant enhancement of superconductivity in few layers MoTe₂Chinese Physics B · 2019
- Ambipolar Superconductivity with Strong Pairing Interaction in Monolayer 1T′-MoTe₂Nano Letters · 2023