Figure 1 (a) Voltage drop across the NbSe2 layer with respect to the current and magnetic field at T=1.55 K. The nonreciprocity and magnetochirality appear as point symmetry, as indicated by the purple dotted line. (b) I-V line cuts of Fig. 3a at 10, 20, 30, and 40 mT. The solid and dotted lines indicate the positive and negative transports direction, respectively. The line cuts are plotted near Ic to highlight the nonreciprocity. The location of each curve in the colormap of Fig. 3a is marked with arrows of the same color. The inset shows the current-voltage characteristic with a different current sweep direction at B=0 T, where the arrows with the corresponding color indicate the current sweep direction. (c) Dependence of ΔIc on the applied magnetic field. The measurement was performed by sweeping the magnetic field from positive to negative. The inset shows the dependence of %Ic on the applied magnetic field. A maximum value of 16% was observed. (d) Magnetic-field dependence of the second-harmonic resistance. The magnetic field was swept from positive to negative. (e)Magnetochiral anisotropy γ was calculated from Fig. 2d with the corresponding error bar. The inset shows the temperature dependence of the resistance of the device.
Figure 2 (a) An optical microscope image of the device, where the NbSe2 and CrPS4 layers are demarcated by purple and green lines, respectively. The black and red arrows indicate the current paths with and without the adjacent CrPS4. (b) The critical current Ic of the device when the current flows through NbSe2 layers without CrPS4. The current direction is indicated by the black arrow in (a). No superconducting diode effect is observed. (c) The Ic of the device when the current flows through NbSe2 in close contact with CrPS4 layers. The current direction is indicated by the red arrow in (a). A superconducting diode effect is observed. (d) The critical current difference ΔIc of the device when CrPS4 is present (red circle) and absent (black square). The correlation between the CrPS4 layer and the superconducting diode effect of NbSe2 can be clearly identified.
Superconducting Diode Effect in a vdW heterostructure Nov. 2020 – Nov. 2021
⌜Observed a magnetic proximity-induced superconducting diode effect (SDE) and infinite magnetoresistance in a van der Waals (vdW) heterostructure. Characterized nonreciprocal critical currents with efficiencies up to 40% in a NbSe2/CrPS4-based spin-valve device. Gained comprehensive skills in low-temperature transport measurements using a Teslatron PT cryostat and various low-noise electronics.⌟
This project was my first project to advanced low-temperature transport measurements as a new graduate student.
The primary goal was to master the essential toolkit for characterizing novel quantum materials.
During this work, I became proficient in operating an Oxford Instruments Teslatron PT cryostat, including managing its superconducting magnet and implementing safety protocols against quenching.
I gained extensive hands-on experience with a suite of low-noise measurement instruments, including lock-in amplifiers, Yokogawa DC sources, and Keithley nanovoltmeters, as well as practical skills like wirebonding.
The devices themselves were expertly fabricated by my collaborator, Dr. S. Son.
The scientific goal was to investigate the superconducting diode effect (SDE), a fascinating nonreciprocal phenomenon where a material's critical supercurrent depends on the direction of current flow.
This effect requires the simultaneous breaking of both inversion and time-reversal symmetries.
A vdW heterostructure, which interfaces a 2D superconductor (NbSe2) with a layered antiferromagnetic insulator (CrPS4), provides an ideal platform to engineer these broken symmetries via the magnetic proximity effect.
In a bilayer NbSe2/CrPS4 device, my measurements revealed a clear nonreciprocal critical current with a diode efficiency of up to 16%.
To further enhance the effect, I characterized a trilayer spin-valve device (CrPS4/NbSe2/CrPS4), which exhibited a significantly larger efficiency of up to 40%.
Furthermore, this spin-valve structure displayed an infinite magnetoresistance ratio that was switchable depending on the magnetic field sweep direction.
These results, published in Physical Review Research, demonstrate a powerful method for realizing and controlling nonreciprocal phenomena in 2D materials via magnetic proximity effects.