High temperature quantum transport in graphene/hexagonal-boron nitride heterostructures

The past decade has seen a new paradigm in solid state physics, where a new class of layered crystals can be thinned down to a monolayer and exhibit drastic changes in their electronic and optical properties in comparison to their bulk counterpart. Graphene was the first, and certainly most outstanding, of this set of so called two-dimensional (2D) materials. Aside from its obvious appeal which earnt its discovery the 2010 Nobel Prize, the electronic properties of graphene are truly unique. Perhaps the most familiar is its linear electron dispersion which hosts quasi-particles that obey the Dirac equation. This has enabled the study of a plethora of transport phenomena, as well as the realisation of novel device architectures that will be used in the next generation electronics. In general, experimental signatures of electron transport are most prominent at liquid helium temperatures when lattice vibrations are weak, for example in quantum hall physics. In this Thesis, we explore the regime of intermediate temperatures where the physics of interest is strongest between 100 and 300 K. Equipped with the state of the art high quality graphene samples, we demonstrate novel electron transport unique to graphene. The experimental work consists of two themes. In the first work, we study hydrodynamic electron flow in graphene encapsulated with hexagonal boron nitride devices. At elevated temperatures, electron-electron collisions become significant, and the electron viscosity starts to influence the steady state current distribution in a variety of surprising ways. In the first work, we perform transport experiments on standard graphene hall bars in a unique measurement geometry which allows the detection of negative non-local voltages intrinsic to viscous flow. In another experiment, we study viscous electron flow through graphene nano-constrictions/classical point contacts. Here, we observed anomalous temperature dependence in the conductance measured across the constriction. Specifically, the conductance increases with increasing temperature and even exceeded the semi-classical limit which is expected for single-particle ballistic transport. The underlying mechanism originates from electron-electron collisions, which, counter-intuitively, act to enhance current flow. In the second work, we slightly change our experimental system by studying magneto transport in a graphene/hexagonal boron nitride superlattice. Owed to the large periodicity of the superlattice unit cell, these devices have allowed experimental observation of the long sought Hofstadter butterfly, which addresses the electronic dispersion of electrons in a periodic potential and magnetic field. Here, we again go to elevated temperatures, where all the spectral gaps related to Hofstadter butterflies are completely smeared, and instead find a new type of quantum oscillation. These new oscillations are periodic in 1/B with a frequency corresponding to one flux quantum piercing the superlattice unit cell. Whilst these oscillations are related to Hofstadter physics, they are in fact more primal in origin. The most fascinating feature is their robustness with respect to increasing temperature. The oscillations are easily observable at room temperature in fields as low as 3 T and still remained prominent at 373 K, the boiling point of wate

Fluidity Onset in Graphene

Viscous electron fluids have emerged recently as a new paradigm of strongly-correlated electron transport in solids. Here we report on a direct observation of the transition to this long-sought-for state of matter in a high-mobility electron system in graphene. Unexpectedly, the electron flow is found to be interaction-dominated but non-hydrodynamic (quasiballistic) in a wide temperature range, showing signatures of viscous flows only at relatively high temperatures. The transition between the two regimes is characterized by a sharp maximum of negative resistance, probed in proximity to the current injector. The resistance decreases as the system goes deeper into the hydrodynamic regime. In a perfect darkness-before-daybreak manner, the interaction-dominated negative response is strongest at the transition to the quasiballistic regime. Our work provides the first demonstration of how the viscous fluid behavior emerges in an interacting electron system.Comment: 8pgs, 4fg

Graphene ballistic nano-rectifier with very high responsivity

Although graphene has the longest mean free path of carriers of any known electronic material, very few novel devices have been reported to harness this extraordinary property. Here we demonstrate a ballistic nano-rectifier fabricated by creating an asymmetric cross-junction in single-layer graphene sandwiched between boron nitride flakes. A mobility ∼200,000 cm(2) V(−1) s(−1) is achieved at room temperature, well beyond that required for ballistic transport. This enables a voltage responsivity as high as 23,000 mV mW(−1) with a low-frequency input signal. Taking advantage of the output channels being orthogonal to the input terminals, the noise is found to be not strongly influenced by the input. Hence, the corresponding noise-equivalent power is as low as 0.64 pW Hz(−1/2). Such performance is even comparable to superconducting bolometers, which however need to operate at cryogenic temperatures. Furthermore, output oscillations are observed at low temperatures, the period of which agrees with the lateral size quantization

Scaling approach to tight-binding transport in realistic graphene devices:the case of transverse magnetic focusing

Ultraclean graphene sheets encapsulated between hexagonal boron nitride crystals host two-dimensional electron systems in which low-temperature transport is solely limited by the sample size. We revisit the theoretical problem of carrying out microscopic calculations of nonlocal ballistic transport in such micron-scale devices. By employing the Landauer-Büttiker scattering theory, we propose a scaling approach to tight-binding nonlocal transport in realistic graphene devices. We test our numerical method against experimental data on transverse magnetic focusing (TMF), a textbook example of nonlocal ballistic transport in the presence of a transverse magnetic field. This comparison enables a clear physical interpretation of all the observed features of the TMF signal, including its oscillating sign

Management of Treatment and Prevention of Acute OP Pesticide Poisoning by Medical Informatics, Telemedicine and Nanomedicine

Acute organophosphorous pesticide (OP) poisoning kills a lot of people each year. Treatment of acute OP poisoning is of very difficult task and is a time taking event. Present day informatics methods (telemedicine), bioinformatics methods (data mining, molecular modeling, docking, cheminformatics), and nanotechnology (nanomedicine) should be applied in combination or separately to combat the rise of death rate due to OP poisoning. Use of informatics method such as Java enabled camera mobiles will enable us early detection of insecticidal poisoning. Even the patients who are severely intoxicated (suicidal attempts) can be diagnosed early. Telemedicine can take care for early diagnosis and early treatment. Simultaneously efforts must be taken with regard to nanotechnology to find lesser toxic compounds (use less dose of nanoparticle mediated compounds: nano-malathion) as insecticides and find better efficacy of lesser dose of compounds for treatment (nano-atropine) of OP poisoning. Nano-apitropine (atropine oxide) may be a better choice for OP poisoning treatment as the anticholinergic agent; apitropine and hyoscyamine have exhibited higher binding affinity than atropine sulfate. Synthesis of insecticides (malathion) with an antidote (atropine, apitropine) in nanoscale range will prevent the lethal effect of insecticides

Tuberous sclerosis with visceral leishmaniasis: a case report

<p>Abstract</p> <p>Introduction</p> <p>Visceral leishmaniasis, a tropical infectious disease, is a major public health problem in India. Tuberous sclerosis, a congenital neuro-ectodermosis, is an uncommon disease which requires life long treatment.</p> <p>Case presentation</p> <p>A 15-year-old Indian patient, presented to the outpatient department of our institute with a high-grade fever for two months, splenomegaly and a history of generalized tonic-clonic convulsions since childhood. The clinical and laboratory findings suggested visceral leishmaniasis with tuberous sclerosis. The patient was treated with miltefosine and antiepileptics.</p> <p>Conclusion</p> <p>The patient responded well and in a follow up six months after presentation, she was found free of visceral leishmaniasis and seizures. Diagnosis and treatment of this rare combination of diseases is difficult.</p

Magnetization Signature of Topological Surface States in a Non-Symmorphic Superconductor

Superconductors with nontrivial band structure topology represent a class of materials with unconventional and potentially useful properties. Recent years have seen much success in creating artificial hybrid structures exhibiting the main characteristics of 2D topological superconductors. Yet, bulk materials known to combine inherent superconductivity with nontrivial topology remain scarce, largely because distinguishing their central characteristic—the topological surface states—has proved challenging due to a dominant contribution from the superconducting bulk. In this work, a highly anomalous behavior of surface superconductivity in topologically nontrivial 3D superconductor In2Bi, where the surface states result from its nontrivial band structure, itself a consequence of the non-symmorphic crystal symmetry and strong spin–orbit coupling, is reported. In contrast to smoothly decreasing diamagnetic susceptibility above the bulk critical field, Hc2, as seen in conventional superconductors, a near-perfect, Meissner-like screening of low-frequency magnetic fields well above Hc2 is observed. The enhanced diamagnetism disappears at a new phase transition close to the critical field of surface superconductivity, Hc3. Using theoretical modeling, the anomalous screening is shown to be consistent with modification of surface superconductivity by the topological surface states. The possibility of detecting signatures of the surface states using macroscopic magnetization provides a new tool for the discovery and identification of topological superconductor

Magnetization Signature of Topological Surface States in a Non-Symmorphic Superconductor

Superconductors with nontrivial band structure topology represent a class of materials with unconventional and potentially useful properties. Recent years have seen much success in creating artificial hybrid structures exhibiting the main characteristics of 2D topological superconductors. Yet, bulk materials known to combine inherent superconductivity with nontrivial topology remain scarce, largely because distinguishing their central characteristic—the topological surface states—has proved challenging due to a dominant contribution from the superconducting bulk. In this work, a highly anomalous behavior of surface superconductivity in topologically nontrivial 3D superconductor In2Bi, where the surface states result from its nontrivial band structure, itself a consequence of the non-symmorphic crystal symmetry and strong spin–orbit coupling, is reported. In contrast to smoothly decreasing diamagnetic susceptibility above the bulk critical field, Hc2, as seen in conventional superconductors, a near-perfect, Meissner-like screening of low-frequency magnetic fields well above Hc2 is observed. The enhanced diamagnetism disappears at a new phase transition close to the critical field of surface superconductivity, Hc3. Using theoretical modeling, the anomalous screening is shown to be consistent with modification of surface superconductivity by the topological surface states. The possibility of detecting signatures of the surface states using macroscopic magnetization provides a new tool for the discovery and identification of topological superconductor

Giant ultra-broadband photoconductivity in twisted graphene heterostructures

The requirements for broadband photodetection are becoming exceedingly demanding in hyperspectral imaging. Whilst intrinsic photoconductor arrays based on mercury cadmium telluride represent the most sensitive and suitable technology, their optical spectrum imposes a narrow spectral range with a sharp absorption edge that cuts their operation to < 25 um. Here, we demonstrate a giant ultra-broadband photoconductivity in twisted double bilayer graphene heterostructures spanning a spectral range of 2 - 100 um with internal quantum efficiencies ~ 40 % at speeds of 100 kHz. The giant response originates from unique properties of twist-decoupled heterostructures including pristine, crystal field induced terahertz band gaps, parallel photoactive channels, and strong photoconductivity enhancements caused by interlayer screening of electronic interactions by respective layers acting as sub-atomic spaced proximity screening gates. Our work demonstrates a rare instance of an intrinsic infrared-terahertz photoconductor that is complementary metal-oxide-semiconductor compatible and array integratable, and introduces twist-decoupled graphene heterostructures as a viable route for engineering gapped graphene photodetectors with 3D scalability