Higher order gaps in the renormalized band structure of doubly aligned hBN/bilayer graphene moiré superlattice

This paper presents our findings on the recursive band gap engineering of chiral fermions in bilayer graphene doubly aligned with hBN. Using two interfering moiré potentials, we generate a supermoiré pattern that renormalizes the electronic bands of the pristine bilayer graphene, resulting in higher order fractal gaps even at very low energies. These Bragg gaps can be mapped using a unique linear combination of periodic areas within the system. To validate our findings, we use electronic transport measurements to identify the position of these gaps as a function of the carrier density. We establish their agreement with the predicted carrier densities and corresponding quantum numbers obtained using the continuum model. Our study provides strong evidence of the quantization of the momentum-space area of quasi-Brillouin zones in a minimally incommensurate lattice. It fills important gaps in the understanding of band structure engineering of Dirac fermions with a doubly periodic superlattice spinor potential.

Controlling Umklapp Scattering in a Bilayer Graphene Moiré Superlattice

We present experimental findings on electron-electron scattering in two-dimensional moiré heterostructures with a tunable Fermi wave vector, reciprocal lattice vector, and band gap. We achieve this in high-mobility aligned heterostructures of bilayer graphene (BLG) and hBN. Around the half-full point, the primary contribution to the resistance of these devices arises from Umklapp electron-electron (Uee) scattering, making the resistance of graphene/hBN moiré devices significantly larger than that of non-aligned devices (where Uee is forbidden). We find that the strength of Uee scattering follows a universal scaling with Fermi energy and is nonmonotonically dependent on the superlattice period. The Uee scattering can be tuned with the electric field and is affected by layer polarization of BLG. It has a strong particle-hole asymmetry; the resistance when the chemical potential is in the conduction band is significantly lower than when it is in the valence band, making the electron-doped regime more practical for potential applications.

Observation of the Time-Reversal Symmetric Hall Effect in Graphene-WSe2 Heterostructures at Room Temperature

In this Letter, we provide experimental evidence of the time-reversal symmetric Hall effect in a mesoscopic system, namely, high-mobility graphene-WSe2 heterostructures. This linear, dissipative Hall effect, whose sign depends on the sign of the charge carriers, persists up to room temperature. The magnitude and the sign of the Hall signal can be tuned using an external perpendicular electric field. Our joint experimental and theoretical study establishes that the strain induced by lattice mismatch, or alignment angle inhomogeneity, produces anisotropic bands in graphene while simultaneously breaking the inversion symmetry. The band anisotropy and reduced spatial symmetry lead to the appearance of a time-reversal symmetric Hall effect. Our study establishes graphene-transition metal dichalcogenide-based heterostructures as an excellent platform for studying the effects of broken symmetry on the physical properties of band-engineered two-dimensional systems.

Large-Area 3D Printable Soft Electronic Skin for Biomedical Applications

Soft electronic skin (soft-e-skin) capable of sensing touch and pressure similar to human skin is essential in many applications, including robotics, healthcare, and augmented reality. However, most of the research effort on soft-e-skin was confined to the lab-scale demonstration. Several hurdles remain challenging, such as highly complex and expensive fabrication processes, instability in long-term use, and difficulty producing large areas and mass production. Here, we present a robust 3D printable large-area electronic skin made of a soft and resilient polymer capable of detecting touch and load, and bending with extreme sensitivity (up to 150 kPa^-1) to touch and load, 750 times higher than earlier work. The soft-e-skin shows excellent long-term stability and consistent performance up to almost a year. In addition, we describe a fabrication process capable of producing large areas and in large numbers, yet is cost-effective. The soft-e-skin consists of a uniquely designed optical waveguide and a layer of a soft membrane with an array of soft structures which work as passive sensing nodes. The use of a soft structure gives the liberty of stretching to the soft-e-skin without considering the disjoints among the sensing nodes. We have shown the functioning of the soft-e-skin under various conditions.

Multifractal Conductance Fluctuations in High-Mobility Graphene in the Integer Quantum Hall Regime

We present the first experimental evidence for the multifractality of a transport property at a topological phase transition. In particular, we show that conductance fluctuations display multifractality at the integer quantum Hall plateau-to-plateau transitions in high-mobility mesoscopic graphene devices. The multifractality gets rapidly suppressed as the chemical potential moves away from these critical points. Our combination of experimental study and multifractal analysis provides a novel method for probing the criticality of wave functions at phase transitions in mesoscopic systems, and quantum criticality in several condensed-matter systems.

Electric-Field-Tunable Valley Zeeman Effect in Bilayer Graphene Heterostructures: Realization of the Spin-Orbit Valve Effect

We report the discovery of electric-field-induced transition from a topologically trivial to a topologically nontrivial band structure in an atomically sharp heterostructure of bilayer graphene (BLG) and single-layer WSe_{2} per the theoretical predictions of Gmitra and Fabian [Phys. Rev. Lett. 119, 146401 (2017)PRLTAO0031-900710.1103/PhysRevLett.119.146401]. Through detailed studies of the quantum correction to the conductance in the BLG, we establish that the band-structure evolution arises from an interplay between proximity-induced strong spin-orbit interaction (SOI) and the layer polarizability in BLG. The low-energy carriers in the BLG experience an effective valley Zeeman SOI that is completely gate tunable to the extent that it can be switched on or off by applying a transverse displacement field or can be controllably transferred between the valence and the conduction band. We demonstrate that this results in the evolution from weak localization to weak antilocalization at a constant electronic density as the net displacement field is tuned from a positive to a negative value with a concomitant SOI-induced splitting of the low-energy bands of the BLG near the K(K^{'}) valley, which is a unique signature of the theoretically predicted spin-orbit valve effect. Our analysis shows that quantum correction to the Drude conductance in Dirac materials with strong induced SOI can only be explained satisfactorily by a theory that accounts for the SOI-induced spin splitting of the BLG low-energy bands. Our results demonstrate the potential for achieving highly tunable devices based on the valley Zeeman effect in dual-gated two-dimensional materials.

Observation of Time-Reversal Invariant Helical Edge-Modes in Bilayer Graphene/WSe2 Heterostructure

Topological insulators, along with Chern insulators and quantum Hall insulator phases, are considered as paradigms for symmetry protected topological phases of matter. This article reports the experimental realization of the time-reversal invariant helical edge-modes in bilayer graphene/monolayer WSe₂-based heterostructures-a phase generally considered as a precursor to the field of generic topological insulators. Our observation of this elusive phase depended crucially on our ability to create mesoscopic devices comprising both a moiré superlattice potential and strong spin-orbit coupling; this resulted in materials whose electronic band structure could be tuned from trivial to topological by an external displacement field. We find that the topological phase is characterized by a bulk bandgap and by helical edge-modes with electrical conductance quantized exactly to 2e²/h in zero external magnetic field. We put the helical edge-modes on firm ground through supporting experiments, including the verification of predictions of the Landauer-Büttiker model for quantum transport in multiterminal mesoscopic devices. Our nonlocal transport properties measurements show that the helical edge-modes are dissipationless and equilibrate at the contact probes. We achieved the tunability of the different topological phases with electric and magnetic fields, which allowed us to achieve topological phase transitions between trivial and multiple, distinct topological phases. We also present results of a theoretical study of a realistic model which, in addition to replicating our experimental results, explains the origin of the topological insulating bulk and helical edge-modes. Our experimental and theoretical results establish a viable route to realizing the time-reversal invariant Z2 topological phase of matter.

Strong suppression of emission quenching in core quantum dots coupled to monolayer MoS2

Non-radiative processes like energy and charge transfer in 0D-2D semiconductor quantum dot (QD)-transition metal dichalcogenides (TMDs) and other two-dimensional layered materials, like graphene and analogs, leading to strong quenching of the photoluminescence (PL) of the usually highly emissive QDs, have been widely studied. Here we report control of the emission efficiency of core QDs placed in close proximity to the monolayers of MoS₂. The QDs are transferred in the form of a high-density compact monolayer with the dot-dot separation, δ as well as the MoS₂-QD separation, d, being controlled through chemical methods. While at larger separations we observe some quenching due to non-radiative processes, at smaller separations we observe enhanced emission from QDs on MoS₂ as compared to the reference despite the presence of significant non-radiative charge transfer. Interestingly, at small separations δ, we see evidence of strong dot-dot interactions and a significant red shift of QD PL which enhances spectral overlap with the B exciton of MoS₂. However, we observe significant reduction of PL quenching of QDs relative to longer δ and d cases, despite increased probability of non-radiative resonant energy transfer to MoS₂, due to the enhanced spectral overlap, as well as charge transfer. Significantly we observe that simultaneously the intensity of the B exciton of MoS₂ increases significantly suggesting the possibility of coherent and resonant radiative energy exchange between the 0D excitons in QDs and the 2D B exciton in MoS₂. Our study reveals interesting nanoscale light-matter interaction effects which can suppress quenching of QDs leading to potential applications of these nanoscale materials in light emitting and photonic devices.

Evolution of inter-layer coupling in artificially stacked bilayer MoS2

In this paper, we show experimentally that for van der Waals heterostructures (vdWh) of atomically-thin materials, the hybridization of bands of adjacent layers is possible only for ultra-clean interfaces. This we achieve through a detailed experimental study of the effect of interfacial separation and adsorbate content on the photoluminescence emission and Raman spectra of ultra-thin vdWh. For vdWh with atomically-clean interfaces, we find the emergence of novel vibrational Raman-active modes whose optical signatures differ significantly from that of the constituent layers. Additionally, we find for such systems a significant modification of the photoluminescence emission spectra with the appearance of peaks whose strength and intensity directly correlate with the inter-layer coupling strength. Our ability to control the intensity of the photoluminescence emission led to the observation of detailed optical features like indirect-band peaks. Our study establishes that it is possible to engineer atomically-thin van der Waals heterostructures with desired optical properties by controlling the inter-layer spacing, and consequently the inter-layer coupling between the constituent layers.

Continuous transition from weakly localized regime to strong localization regime in Nd0.7La0.3NiO3 films

We report an investigation of metal-insulator transition (MIT) using conductivity and magnetoconductance (MC) measurements down to 0.3 K in Nd_0.7La_0.3NiO₃ films grown on crystalline substrates of LaAlO₃ (LAO), SrTiO₃ (STO), and NdGaO₃ (NGO) by pulsed laser deposition. The film grown on LAO experiences a compressive strain and shows metallic behavior with the onset of a weak resistivity upturn below 2 K which is linked to the onset of weak localization contribution. Films grown on STO and NGO show a cross-over from a positive temperature coefficient (PTC) resistance regime to negative temperature coefficient (NTC) resistance regime at definite temperatures. We establish that a cross-over from PTC to NTC on cooling does not necessarily constitute a MIT because the extrapolated conductivity at zero temperature [Formula: see text] though small (<10 S cm^-1) is finite, signaling the existence of a bad metallic state and absence of an activated transport. The value of [Formula: see text] for films grown on NGO is reduced by a factor of 40 compared to that for films grown on STO. We show that a combination of certain physical factors makes substituted nickelate (that are known to exhibit first-order Mott type transition), undergo a continuous transition as seen in systems undergoing disorder/composition driven Anderson transition. The MC measurement also supports the above observation and shows that at low temperatures, there exists a positive MC that arises from the quantum interference which co-exists with a spin-related negative MC that becomes progressively stronger as the electrons approach a strongly localized state in the film grown on NGO.

Quantum Phase Transition in Few-Layer NbSe_{2} Probed through Quantized Conductance Fluctuations

We present the first observation of dynamically modulated quantum phase transition between two distinct charge density wave (CDW) phases in two-dimensional 2H-NbSe_{2}. There is recent spectroscopic evidence for the presence of these two quantum phases, but its evidence in bulk measurements remained elusive. We studied suspended, ultrathin 2H-NbSe_{2} devices fabricated on piezoelectric substrates-with tunable flakes thickness, disorder level, and strain. We find a surprising evolution of the conductance fluctuation spectra across the CDW temperature: the conductance fluctuates between two precise values, separated by a quantum of conductance. These quantized fluctuations disappear for disordered and on-substrate devices. With the help of mean-field calculations, these observations can be explained as to arise from dynamical phase transition between the two CDW states. To affirm this idea, we vary the lateral strain across the device via piezoelectric medium and map out the phase diagram near the quantum critical point. The results resolve a long-standing mystery of the anomalously large spectroscopic gap in NbSe_{2}.

Crumpled sheets of reduced graphene oxide as a highly sensitive, robust and versatile strain/pressure sensor

Sensing of mechanical stimuli forms an important communication pathway between humans/environment and machines. The progress in such sensing technology has possible impacts on the functioning of automated systems, human machine interfacing, health-care monitoring, prosthetics and safety systems. The challenges in this field range from attaining high sensitivity to extreme robustness. In this article, sensing of complex mechanical stimuli with a patch of taped crumpled reduced graphene oxide (rGO) has been reported which can typically be assembled under household conditions. The ability of this sensor to detect a wide variety of pressures and strains in conventional day-to-day applications has been demonstrated. An extremely high gauge factor (∼10³) at ultralow strains (∼10^-4) with fast response times (<20.4 ms) could be achieved with such sensors. Pressure resulting from a gentle touch to over human body weight could be sensed successfully. The capability of the sensor to respond in a variety of environments could be exploited in the detection of water and air pressures both below and above atmospheric, with a single device.

Manipulation of Optoelectronic Properties and Band Structure Engineering of Ultrathin Te Nanowires by Chemical Adsorption

Band structure engineering is a powerful technique both for the design of new semiconductor materials and for imparting new functionalities to existing ones. In this article, we present a novel and versatile technique to achieve this by surface adsorption on low dimensional systems. As a specific example, we demonstrate, through detailed experiments and ab initio simulations, the controlled modification of band structure in ultrathin Te nanowires due to NO₂ adsorption. Measurements of the temperature dependence of resistivity of single ultrathin Te nanowire field-effect transistor (FET) devices exposed to increasing amounts of NO₂ reveal a gradual transition from a semiconducting to a metallic state. Gradual quenching of vibrational Raman modes of Te with increasing concentration of NO₂ supports the appearance of a metallic state in NO₂ adsorbed Te. Ab initio simulations attribute these observations to the appearance of midgap states in NO₂ adsorbed Te nanowires. Our results provide fundamental insights into the effects of ambient on the electronic structures of low-dimensional materials and can be exploited for designing novel chemical sensors.

Structural instability and phase co-existence driven non-Gaussian resistance fluctuations in metal nanowires at low temperatures

We report a detailed experimental study of the resistance fluctuations measured at low temperatures in high quality metal nanowires ranging in diameter from 15-200 nm. The wires exhibit co-existing face-centered-cubic and 4H hcp phases of varying degrees as determined from the x-ray diffraction data. We observe the appearance of a large non-Gaussian noise for nanowires of diameter smaller than 50 nm over a certain temperature range around ≈30 K. The diameter range ∼30 nm, where the noise has maxima coincides with the maximum volume fraction of the co-existing 4H hcp phase thus establishing a strong link between the fluctuation and the phase co-existence. The resistance fluctuation in the same temperature range also shows a deviation of [Formula: see text] behavior at low frequency with appearance of single frequency Lorentzian type contribution in the spectral power density. The fluctuations are thermally activated with an activation energy [Formula: see text] meV, which is of same order as the activation energy of creation of stacking fault in FCC metals that leads to the co-existing crystallographic phases. Combining the results of crystallographic studies of the nanowires and analysis of the resistance fluctuations we could establish the correlation between the appearance of the large resistance noise and the onset of phase co-existence in these nanowires.

Role of different scattering mechanisms on the temperature dependence of transport in graphene

Detailed experimental and theoretical studies of the temperature dependence of the effect of different scattering mechanisms on electrical transport properties of graphene devices are presented. We find that for high mobility devices the transport properties are mainly governed by completely screened short range impurity scattering. On the other hand, for the low mobility devices transport properties are determined by both types of scattering potentials - long range due to ionized impurities and short range due to completely screened charged impurities. The results could be explained in the framework of Boltzmann transport equations involving the two independent scattering mechanisms.

High-Performance Sensors Based on Resistance Fluctuations of Single-Layer-Graphene Transistors

One of the most interesting predicted applications of graphene-monolayer-based devices is as high-quality sensors. In this article, we show, through systematic experiments, a chemical vapor sensor based on the measurement of low-frequency resistance fluctuations of single-layer-graphene field-effect-transistor devices. The sensor has extremely high sensitivity, very high specificity, high fidelity, and fast response times. The performance of the device using this scheme of measurement (which uses resistance fluctuations as the detection parameter) is more than 2 orders of magnitude better than a detection scheme in which changes in the average value of the resistance is monitored. We propose a number-density-fluctuation-based model to explain the superior characteristics of a noise-measurement-based detection scheme presented in this article.

Correlated conductance fluctuations close to the Berezinskii-Kosterlitz-Thouless transition in ultrathin NbN films

We probe the presence of long-range correlations in phase fluctuations by analyzing the higher-order spectrum of resistance fluctuations in ultrathin NbN superconducting films. The non-Gaussian component of resistance fluctuations is found to be sensitive to film thickness close to the transition, which allows us to distinguish between mean field and Berezinskii-Kosterlitz-Thouless (BKT) type superconducting transitions. The extent of non-Gaussianity was found to be bounded by the BKT and mean field transition temperatures and depends strongly on the roughness and structural inhomogeneity of the superconducting films. Our experiment outlines a novel fluctuation-based kinetic probe in detecting the nature of superconductivity in disordered low-dimensional materials.

Shot noise and charge at the 2/3 composite fractional quantum Hall state

The exact structure of edge modes in "hole conjugate" fractional quantum Hall states remains an unsolved issue despite significant experimental and theoretical efforts devoted to their understanding. Recently, there has been a surge of interest in such studies led by the search for neutral modes, which in some cases may lead to exotic statistical properties of the excitations. In this Letter, we report on detailed measurements of shot noise, produced by partitioning of the more familiar 2/3 state. We find a fractional charge of (2/3)e at the lowest temperature, decreasing to e/3 at an elevated temperature. Surprisingly, strong shot noise had been measured on a clear 1/3 plateau upon partitioning the 2/3 state. This behavior suggests an uncommon picture of the composite edge channels quite different from the accepted one.

Transmission phase of a singly occupied quantum dot in the Kondo regime

We report on the phase measurements on a quantum dot containing a single electron in the Kondo regime. Transport takes place through a single orbital state. Although the conductance is far from the unitary limit, we measure directly, for the first time, a transmission phase as theoretically predicted of pi/2. As the dot's coupling to the leads is decreased, with the dot entering the Coulomb blockade regime, the phase reaches a value of pi. Temperature shows little effect on the phase behavior in the range 30-600 mK, even though both the two-terminal conductance and amplitude of the Aharonov-Bohm oscillations are strongly affected. These results also suggest that previous phase measurements involved transport through more than a single level.

Stability of metal nanowires (d > or = 15 nm) against electromigration

We have studied stability of metal nanowires against electromigration damage when they are subjected to high current density. Silver nanowires of diameter ranging from 15 nm to 100 nm were synthesized with electrochemical method using template route. Electromigration study was accompanied with in-situ low frequency electric resistance fluctuation or 1/f noise measurement in the nanowires. It was observed that nanowires with smaller diameter stayed without substantial damage even after 10 to 14 days of continuous stressing with a current of density of the order of 1012 A/m2, while those with larger diameter get damaged much sooner. Noise measurements show that for nanowires with smaller diameter, noise spectra remains almost same for the entire duration of current stressing whereas in case of nanowires of larger diameter, spectral power of noise (SV(f) infinity 1/falpha) as well as the frequency exponent, alpha, increases as the electromigration process progresses. This is similar to that seen in metal films.

Debye temperature of metallic nanowires--an experimental determination from the resistance of metallic nanowires in the temperature range 4.2 K-300 K

We have studied the resistance of metallic nanowires (silver and copper) as a function of the wire diameter in the temperature range 4.2 K-300 K. The nanowires with an average diameter of 15 nm-200 nm and length 6 microm were electrochemically deposited using polycarbonate membranes as template from AgNO3 and CuSO4, respectively. The wires after growth were removed from the membranes by dissolving the polymer in dichloromethane and their crystalline nature confirmed by XRD and TEM studies. The TEM study establishes that the nanowires are single crystalline and can have twin in them. The resistivity data was fitted to Bloch-Gruneisen theorem with the values of Debye temperature and the electron-acoustic phonon coupling constant as the two fit variables. The value of the Debye temperature obtained for the Ag wires was seen to match well with that of the bulk while for Cu wires a significant reduction was observed. The observed increase in resistivity with a decrease in the wire diameter could be explained as due to diffuse surface scattering of the conduction electrons.