Research summary

My primary research interest is looking into the fundamental aspects of how carrier dynamics evolve in two dimensional electronic systems, primarily graphene, a single sheet of carbon atoms. The ease with which average carrier concentration can be tuned in these systems is accompanied with its own set of complications; for example being sensitive to practically any voltage source in its immediate environment. The result is charge disorder which can have important consequences in carrier transport. In a relativistic Dirac system like graphene, where the interactions and screening properties are highly dependent on the average carrier concentration in the system, the problem seems to be far from trivial. Moreover, when the system is tuned very close to charge neutrality, the standard Fermi liquid model breaks down as quantum criticality is predicted to set in. This only means it is not just the extent of charge disorder that changes with average carrier concentration, but also that we are actually triggering a transition between different regimes of carrier transport, that may well range from ballistic to diffusive and even turn viscous close to charge neutrality, and this might be observable even at room temperature.

As part of my PhD, my main work dealt with exploring charge disorder in graphene. Firstly, looking into the origin of this disorder for graphene on a metallic substrate of Ir(111) which is supposed to offer a highly screened environment; and then later on, in graphene FETs on insulating SiO2/Si substrates where carrier concentration and screening is tunable with an externally controlled gate voltage. How this disorder landscape evolves with carrier concentration on a local scale and how it compares with the global transport properties of the device- these were some of the key questions that I tried to answer. This involved use of a combined AFM/STM setup at cryogenic temperatures as low as ~100 mK.

As part of my postdoc, I moved on to look at how carrier dynamics, i.e. charge flow, is affected locally with average carrier concentration. To realize this, I used Kelvin Probe Force Microscopy and Electrostatic Force Microscopy in combination with transport measurements which enables visualization of local in-plane electric fields due to carrier transport. This has resulted in some rather surprising and interesting results, the key among which seems to be observation of local spots of viscous electron flow in relatively disordered samples at room temperature.

As a Higher Research Scientist in NPL, I continue to explore the fundamental aspects of electronic systems not only in graphene, but also other two-dimensional systems like transition metal dichalcogenides (TMDs) as well as van der Waals heterostructures involving interesting combinations of these. The subtle details of the atomic interfaces in such systems like strain, defect distribution or twist angle, influences the charge transfer and hence the resulting interactions. This leads to intriguing effects like two dimensional ferromagnetism, piezoelectricity, superconductivity to name a few. Such This is enabled using powerful characterization tools like a multi-tip combined AFM/STM setup, KPFM, Raman microscope etc. Additionally, working with industrial partners to realize commercial applications for such physical systems as well as the full commercial potential of the scanning probe microscopes, would be another key target

Key Publications

Evidence for Local Spots of Viscous Electron Flow in Graphene at Moderate Mobility

Dominating electron–electron scattering enables viscous electron flow exhibiting hydrodynamic current density patterns, such as Poiseuille profiles or vortices. The viscous regime has recently been observed in graphene by nonlocal transport experiments and mapping of the Poiseuille profile. Herein, we probe the current-induced surface potential maps of graphene field-effect transistors with moderate mobility using scanning probe microscopy at room temperature. We discover micrometer-sized large areas appearing close to charge neutrality that show current-induced electric fields opposing the externally applied field.

S. Samaddar et al., Nano Letters 21, 9365-9373 (2021)

By estimating the local scattering lengths from the gate dependence of local in-plane electric fields, we find that electron–electron scattering dominates in these areas as expected for viscous flow. Moreover, we suppress the inverted fields by artificially decreasing the electron-disorder scattering length via mild ion bombardment. These results imply that viscous electron flow is omnipresent in graphene devices, even at moderate mobility.

Charge Puddles in Graphene Near the Dirac Point

The charge carrier density in graphene on a dielectric substrate such as SiO2 displays inhomogeneities, the so-called charge puddles. Because of the linear dispersion relation in monolayer graphene, the puddles are predicted to grow near charge neutrality, a markedly distinct property from conventional two-dimensional electron gases. By performing scanning tunneling microscopy/spectroscopy on a mesoscopic graphene device, we directly observe the puddles’ growth, both in spatial extent and in amplitude, as the Fermi level approaches the Dirac point. Selfconsistent screening theory provides a unified description of both the macroscopic transport properties and the microscopically observed charge disorder

S. Samaddar et al., Phys. Rev. Lett 116, 126804 (2016). [Editor’s Suggestion]

Equal variations of the Fermi level and work function in graphene at the nanoscale

If surface effects are neglected, any change of the Fermi level in a semiconductor is expected to result in an equal and opposite change of the work function. However, this is in general not observed in three-dimensional semiconductors, because of Fermi level pinning at the surface. By combining Kelvin probe force microscopy and scanning tunneling spectroscopy on single layer graphene, we measure both the local work function and the charge carrier density. The one-to-one equivalence of changes in the Fermi level and the work function is demonstrated to accurately hold in single layer graphene down to the nanometer scale.

S. Samaddar et al., Nanoscale 8, 15162 (2016).

Niobium-based superconducting nano-device fabrication using all-metal suspended masks

We report a novel method for the fabrication of superconducting nano-devices based on niobium. The well-known difficulties of lithographic patterning of high-quality niobium are overcome by replacing the usual organic resist mask by a metallic one. The quality of the fabrication procedure is demonstrated by the realization and characterization of long and narrow superconducting lines and niobium–gold–niobium proximity SQUIDs.

S. Samaddar et al, Nanotechnology 24, 375304 (2013).

Other Topics from Postdoctoral research

Study of electronic phases and edge states in Graphene in the Quantum Hall regime by combining scanning probe microscopy (STM+AFM) with transport measurements

A system pushed to the quantum Hall (QH) regime offers an appealing example to study phases of interacting electrons confined to two dimensions. Graphene provides the unique possibility to probe this QH physics for relativistic carriers. Being atomically thin, interaction effects are more pronounced in graphene as compared to other two dimensional electron systems. Further, the four-fold spin valley degeneracy of the graphene Landau levels (LLs) enable the development of a rich variety of broken symmetry states.

Using low temperature STM at tunable magnetic fields up to 14 T, we look at how the Landau levels evolve with magnetic field and carrier concentration locally, how they are affected by disorder or the presence of local impurities. We observe a degeneracy lifting in the lower LLs as well as Coulomb diamond like charging patterns implying the spatial confinement of LLs leading to quantum dot systems. We are also able to measure at the interface of two differently gated graphene regions where we can see the expected bending of LLs leading to edge states, the number and properties of which can be tuned by changing the gate voltages.

Results from this work are being analysed presently.

Role: contributing in data analysis and general discussions

Fabrication of high mobility graphene FETs with ultra clean surfaces ensuring scanning probe compatibility

Providing large, clean surfaces of gated graphene and other 2D materials is the key challenge for investigating them by Scanning tunnelling spectroscopy (STS). Most of the fabrication processes that have resulted in exceptionally high mobility devices yet, have targeted transport measurements such that the 2D material is encapsulated or the surface cleanliness is not ensured. In this work, we assemble boron nitride/graphene stacks with a graphite back-gate on SiO2/Si substrate, with all contacts realized prior to the graphene transfer i.e. we transfer the graphene as the last step over a centrally placed graphite (gate)/hBN stack. The final PMMA based dry transfer, ensures clean surfaces free of any resist and the possibility of parallel probing by STS and electronic transport at low temperature. In future application, this technique would allow transfer of air sensitive 2D materials inside argon glove boxes or even in UHV conditions.

The process is still being optimized and tested.

Role: contributing in fabrication, measurement, data analysis and general discussions

Combining Q-Plus sensor based AFM with STM for device location at low temperatures

For scanning probe investigation of gated graphene/other 2D materials, one of the key challenges seems to be 'finding the sample' i.e. positioning the tip above the region of interest. In low temperature STM setups, often the best optical arrangements allow a precision of ~ 20 µm which is not enough as the sample dimensions are smaller in most cases. Combining atomic force microscopy with STM can offer a way out as unlike the latter technique, having a conducting surface is not a pre-requisite.

Using my PhD expertise, I integrated AFM functionality to a pre-existing low temperature STM setup. This involved designing and fabricating suitable scanning probes based on q-plus sensors as well as as optimizing/modifying existing electronics to operate these.

The present status is that the AFM part is fully functional i.e. we are able to locate our target measurement position in the sample using a scan range of just 1 µm x 1 µm. It is possible to repeatedly pulse the tip in order to prepare it for STM measurements without compromising on its AFM functionality and vice versa i.e. preserve the STM conditions after making large area scans in AFM mode.

Role: fabrication of the Q-Plus sensors, contributed towards optimization of the electronics and implementation

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