Probing the effects of graphene under strain using a microelectromechanical systems device

SEM Images: Special thanks to our collaborators, Qin Zhou group at UNL

Different theoretical studies have motivated experiments on strained graphene, predicting exotic behaviors such as superconductivity or the induction of gauge fields that act effectively as large magnetic fields. Up to now the study of strain in graphene has been limited to the use of substrates where wrinkles or bubbles create strain or to the use of flexible substrates that create strain when they are bent. Here we present preliminary electronic transport experiments at low temperatures on a suspended graphene where strain is applied through a sophisticated microelectromechanical systems (MEMS). 

I tested first the setup using a Niobium (Nb) thin film superconductor. This allowed me to tune the experiment for superconductivity measurements and test my automated python routines without risk of damage to the MEMS. The setup consists of a lock-in amplifier which is used to superimpose a DC bias with an AC signal. The AC excitation is set to a frequency range between 70Hz-90Hz and allows the lock-in to discriminate signal from noise. The lock-in input is AC coupled and a voltage drop is measured across the sample. The DC component alters the electronic properties of the sample without contributing to the reference signal measurement. I measured a supercurrent in the Nb thin film for temperatures less than 4.4K and between ±20mA of DC current. As the temperature increased, the superconducting gap began closing. Similarly, the gap was closed by applying a magnetic field, due to the breaking of cooper pairs.

The experimental setup consists of a 48 pin breakout box, lock-in amplifier, source meters, pre-amplifier, and a cryostat with a superconducting magnet. The 48 pin breakout box was built by myself and another graduate student. The pins are configured such that a make-before-break connection is made in order to protect the sample from electrostatic discharge. The lock-in, source meters, cryostat, and data acquisition process are automated via Python programming. The source meters are used to actuate the MEMS device and measure leakage current, while the lock-in measures the suspended graphene. The preamp is used to DC couple the AC and DC component of the lock-in. The measurement probe is set to 1.5K and the magnet can reach fields strengths of up to 12T.

The MEMS are provided by Qin Zhou from University of Nebraska and are fabricated such that two probes are in poor contact with the graphene, causing them to behave like tunnel barriers. A differential conductance measurement can therefore access the density of states in graphene, allowing me to potentially track the opening of a band gap with strain. In recent experiments, I have observed a clear magnetoresistance with features related to quantum Hall effect. In the presence of both an external field and strain, a superposition of Landau levels related to the real magnetic field and pseudo magnetic field is expected to emerge. My results so far agree with this theory (BitanRoy, Zi-Xiang Hu, and Kun Yang). Under the guidance of Claudia Ojeda-Aristizabal, I perform the experiments and analyze the data. We are currently collaborating with Qin Zhou (UNL) who fabricates the MEMS and Stephan Haas (USC) who is performing calculations to explain my experimental results.

To analyze my data, I have generated a filling factor simulation superimposed with the magnetoresistance data to quantify potential Hall plateaus. Here is a screen shot of the application I developed using Macros in Excel programmed with Visual Basic for Applications (VBA):

I am scheduled to present my results at the American Physical Society (APS) March meeting in Boston, MA. on Tuesday March 5th, 2019. For more info on this session, please visit: http://meetings.aps.org/Meeting/MAR19/Session/E14.11