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Application Notes

Application Notes written by staff and institutional users for the various instruments.

Gate Voltage Control and I-V spectroscopy in fractional Hall Systems using Isolated Voltage Source             Application Note by: Prof. Biswajit Karmakar

Condensed Matter Physics Divison,  SINP, Kolkata

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We have procured 16-bit digital to analog isolated voltage source (IVS) from Cryonano Labs. It has four truly bipolar +/- 10 V output that is controlled by computer interface/Labview. It can give out put current 10 mA max. We are extensively using the IVS for our ongoing research work as described below:

1. Gate voltage control:

 

We work on multi-terminal quantum Hall system, where gates on a two dimensional electron system is used for separately contacting the quantum Hall edge modes as shown in figure.1. In our experiments equilibration length of fractional and integer quantum Hall edge modes are studied. Using the IVS we have carried out several experiments and published two papers (Phys. Rev. Lett. 125, 076802 – Published 12 August 2020; Phys. Rev. B 104, 085304 – Published 9 August 2021).

2. Gate voltage control on graphene devices:

 

In general graphene devices are made on SiO2/n-Si substrate. To tune the carrier density in graphene back gate voltage (on n-Si) is controlled. The IVS can be a suitable voltage source for the back gate in graphene devices. Since IVS has isolated truly bipolar four sources, it can deliver output +/- 40 V by adding the sources in series. We found IVS is very handy in back gating purpose of the graphene devices (figure.2). Common example of graphene devices can be found in ACS Appl. Nano Mater. 2022, 5, 8, 10941–10950 Publication Date: July 20, 2022.

3. I-V spectroscopy of fractional quantum Hall system:

 

enerally, a DC and an AC voltage excitation are used to measure dI/dV conductance with DC voltage excitation. In such measurement IVS is used as a DC voltage source. Since IVS is low noise 16-bit digital to analog converter, it is very useful for our experiments without any ground-loop problem because of isolation. Some representative data is given in figure.3.

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Figure.1: Gates g1-g4 are controlled by IVS for separately contacting edge modes

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Figure.2: Graphene device for H2 gas sensing

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Figure.3: Differential transmittance conductance (e2/h) versus DC voltage bias.

Helium Dipstick with DC,RF, DC Magnet for characterizing Josephson Junction                                                           Fabrication Complexities: Dr. Deep Talukdar

Cryonano LABS

Objective: The dipstick would be used to characterize a Josephson Junction at 4K using I-V measurements. The environment would be in presence of a small magnetic Field 0.1 Tesla or an RF field suplied externally from top.

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Measurements required: The dipstick would have 24 pin DC measurements with 24 in-line Cryogenic low pass DC-RF filters. The sample space would have chip carriers with in-plane and out of plane measurements. The connections would come out in a breakout box with fisher connectors breaking out into 24 BNC connectors.  It would also have room temperature RF filters built in. 

What we did: We made stainless steel dipstick with three separate stainless steel tubes to isolate the DC lines, heater and Cernox Sensor Lines, Magnet DC lines. A separate RF semi rigid cable was used from the top as an antenna to send RF signal up to 18 GHz. A 24 line cryogenic low pass filter was incorporated inline and wire thermalization bobbins were used in-line. The sample holder was gold plated OFC with in-plane and out of plane chip carrier holders. BeCu Wire looms were used for sample connections with appropriate breaks. A  mumetal shield was  placed outside the sample box sealed with indium seals to guard for earth's magnetic Field. 

Some images and videos of the whole project is shown below.

2 D Transfer of materials - Process and Dynamics                                                                                                                     Short Application Note by: Mr Sourav Paul with inputs from Mr Vineet Pandey 

Research Scholar, Materials Science Centre, IIT KHARAGPUR

We have successfully fabricated a lot of 2D heterostructures precisely with well control using 2D transfer system by Cryonano. We have published a research article where we described a modified transfer method using the setup. Here we write a short note on the procedure of 2D dry transfer (universal dry transfer).

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In the The 2D transfer setup there are two type of stage (i) XYƟ and (ii) XYZ stage. XYZ stage is having a facility to hold a cantilever type plate. (fig: a,b) The whole process has been done using dry transfer method by Polydimethylsiloxane / Poly-propylene carbonate (PDMS/PPC).

Step 1: First we have prepared hemisphere structure on a cleaned coverslip using PDMS. PPC has been spin coated on PDMS. (fig: c) Under the microscope, upside down PDMS/PPC on Coverslip has been aligned on the sample which is placed on sample holder of our 2D transfer system.

Step 2: The PDMS/PPC picks up the few layer mechanically exfoliated h-BN flake from substrate at 650c first. We use SiO2/Si as a substrate. Figs d-f show how it looks through the microscope of this 2D transfer system when the 2D flake is on substrate (fig: d), when the PDMS/PPC touches the substrate (yellow interface) (fig: e) and after picking up how picked up 2D flake on PDMS looks (transparent interface) (fig: f).

Step 3: To pick up any exfoliated 2D materials including Graphene and TMDs using PDMS/PPC/hBN stack, we follow the same procedure of step 2 after precisely aligned the 2D flake with hBN which is now on PDMS hemisphere (fig: g). This time pick up temperature is 55 Deg C.

Step 4: Final transfer we generally do at high temperature.  In this step, the PDMS/PPC/hBN/2D flake is aligned and touched on another hBN which is exfoliated on an cleaned substrate at 85 Deg C. Then we slowly move the cantilever plate upward direction. The PDMS/PPC will be de touch from the heterostucture (hBN/2D flake/hBN). The heterostucture needs to be cleaned in acetone for few minutes followed by IPA to remove the residue which may come from PPC.

The full detail of the transfer process can be found in the publication mentioned below:

Raman spectroscopic studies on the evolution of interlayer coupling and stacking order in twisted bilayers and polytypes of WSe2 

Sourav Paul  et. al. ,  Journal of Applied Physics 133, 114301 (2023)

To make a successful transfer it is important to understand the  live optical micro-graphs during the time the sample is about to touch another sample on stamp and when it has finally touched to control the process carefully. Below are the actual snapshots of the live  images of the situation when the flakes are about to touch - notice the interference fringes - and the moment when they have fully touched.

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Electromagnet with optical access installation at INST Mohali           

Complexity: The complexity of the electromagnet involved drilling of a bore at the center of the Pole pieces for optical access. The user wanted to send laser through the center holes for Magneto Optic experiments.

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Fig 1 Top: datasheet being made with Gaussmeter and transverse probes for checking the Field uniformity.

 

Fig 2 Left: Video of the electromagnet showing the straight perfectly aligned holes along the axis of the Magnetic Pole Pieces.  

Fig 3: Final Installation of the electromagnet along with the water chiller for cooling of the coils installed.

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Document 1:  Manual of EMCT2 with optical access compatibility.

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