Search

Magnetotransport measurements are crucial for understanding the Fermi surface properties, magnetism, and topology in quantum materials. Here, we report the discovery of giant room temperature odd-parity magnetoresistance (OMR) in a bilayer graphene (BLG) heterostructure interfaced with Cr$_2$Te$_2$Ge$_6$ (CGT). Using magnetotransport measurements, we demonstrate that the BLG/CGT heterostructure exhibits a significant antisymmetric longitudinal magnetoresistance, indicative of intrinsic time-reversal symmetry (TRS) breaking in the system. We show that the OMR is tunable via electrostatic gating. Additionally, the OMR is pronounced near the band edges and diminishes with increasing charge carrier density in graphene. Our theoretical analysis reveals that this phenomenon arises from the coupling of the out-of-plane components of Berry curvature and orbital magnetic moment to the applied magnetic field in a TRS-broken system. Our findings establish OMR as a significant probe for TRS breaking in quantum materials in which the crystal symmetries preclude the appearance of anomalous Hall effect., Comment: 34 pages

Neuromorphic computing has emphasized the need for memristors with non-volatile, multiple conductance levels. This paper demonstrates the potential of hexagonal boron nitride (hBN)/graphene heterostructures to act as memristors with multiple resistance states that can be optically tuned using visible light. The number of resistance levels in graphene can be controlled by modulating doping levels, achieved by varying the electric field strength or adjusting the duration of optical illumination. Our measurements show that this photodoping of graphene results from the optical excitation of charge carriers from the nitrogen-vacancy levels of hBN to its conduction band, with these carriers then being transferred to graphene by the gate-induced electric field. We develop a quantitative model to describe our observations. Additionally, utilizing our device architecture, we propose a memristive crossbar array for vector-matrix multiplications., Comment: 22 pages, Comments welcome

Fractional quantum Hall (FQH) phases emerge due to strong electronic interactions and are characterized by anyonic quasiparticles, each distinguished by unique topological parameters, fractional charge, and statistics. In contrast, the integer quantum Hall (IQH) effects can be understood from the band topology of non-interacting electrons. We report a surprising super-universality of the critical behavior across all FQH and IQH transitions. Contrary to the anticipated state-dependent critical exponents, our findings reveal the same critical scaling exponent $\kappa = 0.41 \pm 0.02$ and localization length exponent $\gamma = 2.4 \pm 0.2$ for fractional and integer quantum Hall transitions. From these, we extract the value of the dynamical exponent $z\approx 1$. We have achieved this in ultra-high mobility trilayer graphene devices with a metallic screening layer close to the conduction channels. The observation of these global critical exponents across various quantum Hall phase transitions was masked in previous studies by significant sample-to-sample variation in the measured values of $\kappa$ in conventional semiconductor heterostructures, where long-range correlated disorder dominates. We show that the robust scaling exponents are valid in the limit of short-range disorder correlations., Comment: 42 pages

In this Letter, we present experimental findings on electron-electron scattering in a two-dimensional moir'e heterostructure with 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 half-filling, the primary contribution to the resistance of BLG/hBN aligned superlattices arises from electron-electron Umklapp (Uee) scattering, making the resistance of graphene/hBN moir'e devices significantly larger than that of non-aligned devices (where Uee is forbidden). We quantify the strength of the Uee scattering and find that it follows a universal scaling with Fermi energy and has a non-monotonic dependence on the charge carrier density. The Uee scattering is strongly electric field tunable and 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 lesser than when it is in the valence band, making the electron-doped regime more practical for potential applications., Comment: Comments and suggestion are welcome

This letter presents our findings on the recursive band gap engineering of chiral fermions in bilayer graphene doubly aligned with hBN. By utilizing two interfering moir\'{e} potentials, we generate a supermoir\'{e} pattern which 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 used electronic transport measurements to identify the position of these gaps as functions of the carrier density and establish their agreement with the predicted carrier densities and corresponding quantum numbers obtained using the continuum model. Our work provides direct experimental evidence of the quantization of the area of quasi-Brillouin zones in supermoir\'{e} systems. It fills essential gaps in understanding the band structure engineering of Dirac fermions by a recursive doubly periodic superlattice potential., Comment: 29 pages (including Supplementary Materials)

The study of Berezinskii-Kosterlitz-Thouless transitions in clean, layered two-dimensional superconductors promises to provide insight into a host of novel phenomena like re-entrant vortex-dynamics, underlying unconventional metallic phases, and topological superconductivity. In this letter, we report the study of charge carrier dynamics in a novel 2-dimensional superconducting van der Waals heterostructure comprising monolayer MoS2 and few-layer NbSe2 (15 nm). Using low-frequency conductance fluctuation spectroscopy, we show that the superconducting transition in the system is percolative. We present a phenomenological picture of different phases across the transition correlating with the evaluated noise. The analysis of the higher-order statistics of fluctuation reveals non-Gaussian components around the transition indicative of long-range correlation in the system., Comment: 24 pages

In this letter, we provide experimental evidence of the time-reversal symmetric Hall effect in a mesoscopic system, namely high-mobility graphene/WSe$_2$ 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 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.

Proximity-induced spin-orbit coupling in graphene has led to the observation of intriguing phenomena like time-reversal invariant $\mathbb{Z}_2$ topological phase and spin-orbital filtering effects. An understanding of the effect of spin-orbit coupling on the band structure of graphene is essential if these exciting observations are to be transformed into real-world applications. In this research article, we report the experimental determination of the band structure of single-layer graphene (SLG) in the presence of strong proximity-induced spin-orbit coupling. We achieve this in high-mobility hBN-encapsulated SLG/WSe2 heterostructures through measurements of quantum oscillations. We observe clear spin-splitting of the graphene bands along with a substantial increase in the Fermi velocity. Using a theoretical model with realistic parameters to fit our experimental data, we uncover evidence of a band gap opening and band inversion in the SLG. Further, we establish that the deviation of the low-energy band structure from pristine SLG is determined primarily by the valley-Zeeman SOC and Rashba SOC, with the Kane-Mele SOC being inconsequential. Despite robust theoretical predictions and observations of band-splitting, a quantitative measure of the spin-splitting of the valence and the conduction bands and the consequent low-energy dispersion relation in SLG was missing -- our combined experimental and theoretical study fills this lacuna., Comment: 27 pages

In this letter, we report our observation of an extraordinarily rich phase diagram of a LaScO$_3$/SrTiO$_3$ heterostructure. Close to the superconducting transition temperature, the system hosts a superconducting critical point of the Infinite-randomness type characterized by an effective dynamical exponent $\nu z$ that diverges logarithmically. At lower temperatures, we find the emergence of a magnetic field-tuned metallic phase that co-exists with a quantum Griffiths phase (QGP). Our study reveals a previously unobserved phenomenon in 2D superconductors -- an unanticipated suppression of the QGP below a crossover temperature in this system. This concealment is accompanied by the destruction of the superconducting quantum critical point signaled by a power-law divergence (in temperature) of the effective dynamical exponent. These observations are entirely at odds with the predictions of the infinite-randomness scenario and challenge the very concept of a vanishing energy scale associated with a quantum critical point. We develop and discuss possible scenarios like smearing of the phase transition that could plausibly explain our observations. Our findings challenge the notion that QGP is the ultimate ground state in two-dimensional superconductors., Comment: 15 pages, 5 figures, Supplementary Material follows main manuscript

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 $\nu=1 \longleftrightarrow \nu=2$ plateau-to-plateau transition in a high-mobility mesoscopic graphene device. We establish that to observe this multifractality, it is crucial to work with very high-mobility devices with a well-defined critical point. This multifractality gets rapidly suppressed as the chemical potential moves away from these critical points. Our combination of multifractal analysis with state-of-the-art transport measurements at a topological phase transition provides a novel method for probing such phase transitions in mesoscopic devices., Comment: 10 pages, 4 figures

Layered van der Waals materials are exciting as they often host multiple, competing electronic phases. This article reports experimental observation of the co-existence of insulating and metallic phases deep within the commensurate charge density wave phase in high-quality devices of few-layer 1T-TaS2. Through detailed conductance fluctuation spectroscopy of the electronic ground state, we establish that the mixed-phase consists of insulating regions surrounded by one-dimensional metallic domain walls. We show that the electronic ground state of 1T-TaS2 can be affected drastically by strain, eventually leading to the collapse of the Mott gap in the commensurate charge density wave phase. Our study resolves an outstanding question, namely the effect of the inter-layer coupling strength on the electronic phases in layered van der Waals materials., Comment: 7 pages, 6 figures

We report the experimental observation of Ising superconductivity in 3-dimensional NbSe2 stacked with single-layer MoS2. The angular dependence of the upper critical magnetic field and the temperature dependence of the upper parallel critical field confirm the appearance of two-dimensional Ising superconductivity in the 3-dimensional NbSe2 with single-layer MoS2 overlay. We show that the superconducting phase has strong Ising spin-orbit correlations which make the holes spin non-degenerate. Our observation of Ising superconductivity in heterostructures of few-layer NbSe2 of thickness ~ 15 nm with single-layer MoS2 raises the interesting prospect of observing topological chiral superconductors with nontrivial Chern numbers in a momentum-space spin-split fermionic system.

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 WSe2 per the theoretical predictions of Gmitra and Fabian [Phys. Rev. Lett. 119, 146401 (2017)]. 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., Comment: 5 pages, 5 figures

In this article, we present evidence for the existence of vortex-solid/glass (VG) to vortex-fluid (VF) transition in a type-II superconductor (SC), NbN. We probed the VG to VF transition in both 2D and 3D films of NbN through studies of magnetoresistance and current-voltage characteristics. The dynamical exponents corresponding to this phase transition were extracted independently from the two sets of measurements. The $H$-$T$ phase diagram for the 2D and 3D SC are found to be significantly different near the critical point. In the case of 3D SC, the exponent values obtained from the two independent measurements show excellent match. On the other hand, for the 2D SC, the exponents obtained from the two experiments were significantly different. We attribute this to the fact that the characteristic length scale diverges near the critical point in a 2D SC in a distinctly different way from its 3D counterpart form scaling behaviour.

We explore the effects of sample dimensionality on vortex pinning in a type-II, low-$T_C$, s-wave superconductor, NbN, in the presence of a perpendicular magnetic field, $H$. We find significant differences in the phase diagrams in the magnetic field--temperature plane between 3-dimensional (3D) and 2-dimensional (2D) NbN films. The differences are most striking close to the normal-superconductor phase transition. We establish that these variances have their origin in the differing pinning properties in two different dimensions. We obtain the pinning strength quantitatively in both the dimensions from two independent transport measurements performed in two different regimes of vortex-motion -- (i) thermally assisted flux-flow (TAFF) regime and (ii) flux flow (FF) regime. Both the measurements consistently show that both the pinning potential and the zero-field free-energy barrier to depinning in the 3D superconductor are at least an order of magnitude stronger than that in the 2D superconductor. Further, we probed the dynamics of pinning in both 2D and 3D superconductor through voltage fluctuation spectroscopy. We find that the mechanism of vortex pinning-depinning is qualitatively similar for the 3D and 2D superconductors. The voltage-fluctuations arising from vortex-motion are found to be correlated only in the 2D superconductor. We establish this to be due to the presence of long-range phase fluctuations near the Berezinskii-Kosterlitz-Thouless (BKT) type superconducting transition in 2-dimensional superconductors.

We have carried out extensive comparative studies of the structural and transport properties of CaRuO$_3$ thin films grown under various oxygen pressure. We find that the preferred orientation and surface roughness of the films are strongly affected by the oxygen partial pressure during growth. This in turn affects the electrical and magnetic properties of the films. Films grown under high oxygen pressure have the least surface roughness and show transport characteristics of a good metal down to the lowest temperature measured. On the other hand, films grown under low oxygen pressures have high degree of surface roughness and show signatures of ferromagnetism. We could verify that the low frequency resistance fluctuations (noise) in these films arise due to thermally activated fluctuations of local defects and that the defect density matches with the level of disorder seen in the films through structural characterizations.

Phonon-phonon anharmonic effects have a strong influence on the phonon spectrum; most prominent manifestation of these effects are the softening (shift in frequency) and broadening (change in FWHM) of the phonon modes at finite temperature. Using Raman spectroscopy, we studied the temperature dependence of the FWHM and Raman shift of $\mathrm{E_{2g}^1}$ and $\mathrm{A_{1g}}$ modes for single-layer and natural bilayer MoS$_2$ over a broad range of temperatures ($8 < $T$ < 300$ K). Both the Raman shift and FWHM of these modes show linear temperature dependence for $T>100$ K, whereas they become independent of temperature for $T<100$ K. Using first-principles calculations, we show that three-phonon anharmonic effects intrinsic to the material can account for the observed temperature-dependence of the line-width of both the modes. It also plays an important role in determining the temperature-dependence of the frequency of the Raman modes. The observed evolution of the line-width of the A$_{1g}$ mode suggests that electron-phonon processes are additionally involved. From the analysis of the temperature-dependent Raman spectra of MoS$_2$ on two different substrates -- SiO$_2$ and hexagonal boron nitride, we disentangle the contributions of external stress and internal impurities to these phonon-related processes. We find that the renormalization of the phonon mode frequencies on different substrates is governed by strain and intrinsic doping. Our work establishes the role of intrinsic phonon anharmonic effects in deciding the Raman shift in MoS$_2$ irrespective of substrate and layer number.

We report an investigation of Metal Insulator Transition (MIT) using conductivity and magnetoconductance (MC) measurements down to 0.3 K in Nd_{0.7}La_{0.3}NiO_{3} films grown on crystalline substrates of LaAlO_{3} (LAO), SrTiO_{3} (STO), and NdGaO_{3}(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 crossover 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 \sigma_{0} though small (<10 S/cm) is finite, signalling the existence of a bad metallic state and absence of an activated transport. The value of \sigma_{0} 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 support the above observation and show that at low temperature 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., Comment: 24 pages

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 WSe$_2$-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 moir\'e 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 $2e^2/h$ in zero external magnetic field. We put the helical edge-modes on firm grounds through supporting experiments, including the verification of predictions of the Landauer-B$\mathrm{\ddot{u}}$ttiker model for quantum transport in multi-terminal mesoscopic devices. Our non-local 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 $\mathbb{Z}_2$ topological phase of matter., Comment: 21 pages, 4 figures

Controlling spin-orbit interaction and its effect on superconductivity has been a long-standing problem in two-dimensional inversion symmetry broken superconductors. An open challenge is to understand the role of various energy scales in shaping the complex phase diagram in these systems. From a combined experimental and theoretical study of resistance fluctuations and its higher order statistics, we propose a phase diagram for the superconducting phase in the magnetic-field--spin orbit interaction energy plane for the quasi-two dimensional electron gas at the interface of LaAlO$_3$/SrTiO$_3$ heterostructures. The relative variance of resistance fluctuations increases by few orders of magnitude below the spin-orbit field B$_{SO}$ and a non-Gaussian component to the fluctuations arises for fields below the upper critical field B$_{C2}$. Theoretical calculations show that the non-Gaussian noise predominantly arises due to percolative nature of the superconducting transition. We quantify the strength and the relative importance of the spin-orbit interaction energy, Zeeman energy and the pairing potential. Our work highlights the important role played by the interplay between these energy scales in framing the fascinating phases seen in two-dimensional inversion-symmetry-broken superconductors., Comment: 9 pages, 11 figures

The metal-insulator transition (MIT) of bulk rare-earth nickelates is accompanied by a simultaneous charge ordering (CO) transition. We have investigated low-frequency resistance fluctuations (noise) across the MIT and magnetic transition of [EuNiO$_3$/LaNiO$_3$] superlattices, where selective suppression of charge ordering has been achieved by mismatching the superlattice periodicity with the periodicity of charge ordering. We have observed that irrespective of the presence/absence of long-range CO, the noise magnitude is enhanced by several orders with strong non-1/$f$ ($f$ = frequency) component when the system undergoes MIT and magnetic transition. The higher order statistics of resistance fluctuations reveal the presence of strong non-Gaussian components in both cases, further indicating inhomogeneous electrical transport arising from the electronic phase separation. Specifically, we find almost three orders of magnitude smaller noise in the insulating phase of the sample without long-range CO compared to the sample with CO. These findings suggest that digital synthesis can be a potential route to implement electronic transitions of complex oxides for device application., Comment: Accepted in Phys. Rev. B

Despite the concerted effort of several research groups, a detailed experimental account of defect dynamics in high-quality single- and few-layer transition metal dichalcogenides remain elusive. In this paper we report an experimental study of the temperature dependence of conductance and conductance-fluctuations on several few-layer MoS$_{2}$ exfoliated on hexagonal boron nitride and covered by a capping layer of high-$\kappa$ dielectric HfO$_{2}$. The presence of the high-$\kappa$ dielectric made the device extremely stable against environmental degradation as well as resistant to changes in device characteristics upon repeated thermal cycling enabling us to obtain reproducible data on the same device over a time-scale of more than one year. Our device architecture helped bring down the conductance fluctuations of the MoS$_2$ channel by orders of magnitude compared to previous reports. The extremely low noise levels in our devices made in possible to detect the generation-recombination noise arising from charge fluctuation between the sulphur-vacancy levels in the band gap and energy-levels at the conductance band-edge. Our work establishes conduction fluctuation spectroscopy as a viable route to quantitatively probe in-gap defect levels in low-dimensional semiconductors., Comment: 9 pages, 11 figures

Multiband transport in superconductors is interesting both from an academic as well as an application point of view. It has been postulated that interband scattering can significantly affect the carrier dynamics in these materials. In this article we present a detailed study of the electrical transport properties of the high-mobility two-dimensional electron gas residing at the interface of LaAlO$_3$/SrTiO$_3$, a prototypical multi-band superconductor. We show, through careful measurements of the gate dependence of the magnetoresistance and resistance fluctuations at ultra-low temperatures, that transport in the superconducting regime of this system has contributions from two bands which host carriers of very different characters. We identify a gate-voltage tunable Lifshitz transition in the system and show that the resistance fluctuations have strikingly different features on either side of it. At low carrier densities, resistance noise is dominated by number-density fluctuations arising from trapping-detrapping of charge carriers from defects in the underlying SrTiO$_3$ substrate, characteristic of a single-band semiconductor. Above the Lifshitz transition, the noise presumably originates from inter-band scattering. Our work highlights the importance of inter-band scattering processes in determining the transport properties of low-dimensional systems and projects resistance fluctuation spectroscopy as a viable technique for probing the charge carrier dynamics across a Lifshitz transition., Comment: 6 pages, 5 figures

In quantum systems, signatures of multifractality are rare. They have been found only in the multiscaling of eigenfunctions at critical points. Here we demonstrate multifractality in the magnetic-field-induced universal conductance fluctuations of the conductance in a quantum condensed-matter system, namely, high-mobility single-layer graphene field-effect transistors. This multifractality decreases as the temperature increases or as doping moves the system away from the Dirac point. Our measurements and analysis present evidence for an incipient Anderson-localization near the Dirac point as the most plausible cause for this multifractality. Our experiments suggest that multifractality in the scaling behaviour of local eigenfunctions are reflected in macroscopic transport coefficients. We conjecture that an incipient Anderson-localization transition may be the origin of this multifractality. It is possible that multifractality is ubiquitous in transport properties of low-dimensional systems. Indeed, our work suggests that we should look for multifractality in transport in other low-dimensional quantum condensed-matter systems.

We present the first observation of dynamically modulated quantum phase transition (QPT) between two distinct charge density wave (CDW) phases in 2-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, ultra-thin \nbse 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 (QCP). The results resolve a long-standing mystery of the anomalously large spectroscopic gap in NbSe$_2$.

We present results of resistance fluctuation spectroscopy on single crystals of the predicted Kondo topological insulator material SmB$_6$. Our measurements show that at low temperatures, transport in this system takes place only through surface states. The measured noise in this temperature range arises due to Universal Conductance Fluctuations whose statistics was found to be consistent with theoretical predictions for that of two-dimensional systems in the Symplectic symmetry class. At higher temperatures, we find signatures of glassy dynamics and establish that the measured noise is caused by mobility fluctuations in the bulk. We find that, unlike the topological insulators of the dichalcogenide family, the noise in surface and bulk conduction channels in SmB$_6$ are completely uncorrelated. Our measurements establish that at sufficiently low temperatures, the bulk has no discernible contribution to electrical transport in SmB$_6$ making it an ideal platform for probing the physics of topological surface states., Comment: 9 pages, 11 figures

We probe the existence of large correlated non-Gaussian phase fluctuations in the vicinity of the superconducting phase transition in the conducting layer residing at the interface of LaAlO$_3$/SrTiO$_3$ heterostructures. The non-Gaussian fluctuations appear between the Berezinskii-Kosterlitz-Thouless transition temperature $T_{BKT}$ and the mean field transition temperature $T_C$. Subsequent theoretical analysis reveals that non-Gaussianity arises predominantly due to the percolative transition of a Josephson coupled network of superconductors. Our results confirm that the superconductivity in this system is confined to two-dimensions. Our study of the non-Gaussian resistance fluctuation spectrum provides a novel means to explore the BKT-transition in two-dimensional inhomogeneous superconductors., Comment: 10 pages, 12 figures

One of the most interesting predicted applications of graphene monolayer based devices is as high quality sensors. In this letter 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 (SLG-FET) 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 two orders of magnitude better than a detection scheme where changes in the average value of the resistance is monitored. We propose a number-density fluctuation based model to explain the superior characteristics of noise measurement based detection scheme presented in this article.

In this letter we present the results of systematic experimental investigations of the effect of different chemical environments on the low frequency resistance fluctuations of single layer graphene field effect transistors (SLG-FET). The shape of the power spectral density of noise was found to be determined by the energetics of the adsorption-desorption of molecules from the graphene surface making it the dominant source of noise in these devices. We also demonstrate a method of quantitatively determining the adsorption energies of chemicals on graphene surface based on noise measurements. We find that the magnitude of noise is extremely sensitive to the nature and amount of the chemical species present. We propose that a chemical sensor based on the measurement of low frequency resistance fluctuations of single layer graphene field effect transistor devices will have extremely high sensitivity, very high specificity, high fidelity and fast response times.

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., Comment: Accepted for Publication in Scientific Reports, 2015

SmB$_6$ has been predicted to be a Kondo Topological Insulator with topologically protected conducting surface states. We have studied quantitatively the electrical transport through surface states in high quality single crystals of SmB$_6$. We observe a large non-local surface signal at temperatures lower than the bulk Kondo gap scale. Measurements and finite element simulations allow us to distinguish unambiguously between the contributions from different transport channels. In contrast to general expectations, the electrical transport properties of the surface channels was found to be insensitive to high magnetic fields. Local and non-local magnetoresistance measurements allowed us to identify definite signatures of helical spin states and strong inter-band scattering at the surface., Comment: 7 pages, 8 figures, 1 table

The complex perovskite oxide SrRuO3 shows intriguing transport properties at low temperatures due to the interplay of spin, charge, and orbital degrees of freedom. One of the open questions in this system is regarding the origin and nature of the low-temperature glassy state. In this paper we report on measurements of higher-order statistics of resistance fluctuations performed in epitaxial thin films of SrRuO3 to probe this issue. We observe large low-frequency non-Gaussian resistance fluctuations over a certain temperature range. Our observations are compatible with that of a spin-glass system with properties described by hierarchical dynamics rather than with that of a simple ferromagnet with a large coercivity.

We report the observation of a magnetic field assisted transient superconducting state in the two dimensional electron gas existing at the interface of LaAlO$_3$/SrTiO$_3$ heterostructures. This metastable state depends critically on the density of charge carriers in the system. It appears concomitantly with a Lifshitz transition as a consequence of the interplay between ferromagnetism and superconductivity and the finite relaxation time of the in-plane magnetization. Our results clearly demonstrate the inherently metastable nature of the superconducting state competing with a magnetic order in these systems. The co-existence of superconductivity and ferromagnetism in the conducting electronic layer formed at the interface of insulating oxides has thrown up several intriguing and as yet unanswered questions. An open question in this field is the energetics of the interplay between these two competing orders and the present observation goes a long way in understanding the underlying mechanism., Comment: 11 pages, 14 figures

We probe the presence of long-range correlations in phase fluctuations by analyzing the higher-order spectrum of resistance fluctuations in ultra-thin 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 depend 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., Comment: submitted to PRL

In the quantum Hall effect regime, taking place in a two-dimensional-electron gas under strong magnetic field, currents flow along the edges of the sample. For some particle-hole conjugate states of the fractional regime, e.g., with filling between 1/2 and 1 of the lowest Landau level; early predictions suggested the presence of counter-propagating edge currents in addition to the expected ones. When this did not agree with the measured conductance, it was suggested that disorder and interactions will lead to counterpropagating modes that carry only energy - the so called neutral modes. In addition, a neutral upstream mode (Majorana mode) was also expected for selected wavefunctions proposed for the even denominator filling 5/2. Here we report on the direct observation of counter-propagating neutral modes in fillings 2/3, 3/5 and 5/2. This was done by injecting such modes and allowing them to impinge on a narrow constriction, which partly reflected them, with two main observed effects: (a) A resultant shot noise proportional to the applied voltage on the injecting contact; (b) With simultaneously injecting also a charge mode, the presence of the neutral mode was found to significantly affect the Fano factor and the temperature of the backscattered charge mode. In particular, such observation for filling 5/2, may single out the non-abelian wavefunctions for the state., Comment: 8 pages, 7 figures

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., Comment: 4 pages, 5 figures, accepted to PRL

Interference of edge channels is expected to be a prominent tool for studying statistics of charged quasiparticles in the quantum Hall effect (QHE) [A. Stern (2008), Ann. Phys. 1:204; C. Chamon et al. (1997), Phys. Rev. B, 55:2331]. We present here a detailed study of an electronic Fabry-Perot interferometer (FPI) operating in the QHE regime [C. Chamon et al. (1997), Phys. Rev. B, 55:2331], with the phase of the interfering quasiparticles controlled by the Aharonov-Bohm (AB) effect. Our main finding is that Coulomb interactions among the electrons dominate the interference, even in a relatively large area FPI, leading to a strong dependence of the area enclosed by the interference loop on the magnetic field. In particular, for a composite edge structure, with a few independent edge channels propagating along the edge, interference of the outmost edge channel (belonging to the lowest Landau level) was insensitive to magnetic field; suggesting a constant enclosed flux. However, when any of the inner edge channels interfered, the enclosed flux decreased when the magnetic field increased. By intentionally varying the enclosed area with a biased metallic gate and observing the periodicity of the interference pattern, charges e (for integer filling factors) and e/3 (for a fractional filling factor) were found to be expelled from the FPI. Moreover, these observations provided also a novel way of detecting the charge of the interfering quasiparticles., Comment: 8 pages, 8 figures

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 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 behaviour 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 confirm that previous phase measurements involved transport through more than a single level., Comment: 4 pages, 5 figures

We have measured the resistances (and resistivities) of Ag and Cu nanowires of diameters ranging from 15nm to 200nm in the temperature range 4.2K-300K with the specific aim to assess the applicability of the Bloch-Gr\"{u}neisen formula for electron phonon resistivity in these nanowires. The wires were grown within polymeric templates by electrodeposition. We find that in all the samples the resistance reaches a residual value at T=4.2K and the temperature dependence of resistance can be fitted to the Bloch-Gr\"{u}neisen formula in the entire temperature range with a well defined transport Debye temperature ($\Theta_{R}$). The value of Debye temperature obtained from the fits lie within 8% of the bulk value for Ag wires of diameter 15nm while for Cu nanowires of the same diameter the Debye temperature is significantly lesser than the bulk value. The electron-phonon coupling constants (measured by $\alpha_{el-ph}$ or $\alpha_{R}$) in the nanowires were found to have the same value as that of the bulk. The resistivities of the wires were seen to increase as the wire diameter was decreased. This increase in the resistivity of the wires may be attributed to surface scattering of conduction electrons. The specularity p was estimated to be about 0.5. The observed results allow us to obtain the resistivities exactly from the resistance and gives us a method of obtaining the exact numbers of wires within the measured array (grown within the template)., Comment: 9 pages, 10 figures

We have measured the low-frequency resistance fluctuations (1 mHz}{R^2} also increases as the diameter d is reduced. We observe that there are new features in the 1/f noise as the size of the wire is reduced and they become more prominent as the diameter of the wires approaches 15nm. It is important to investigate the origin of the new behavior as 1/f noise may become a limiting factor in the use of metal wires of nanometer dimensions as interconnects., Comment: 9 pages, 6 figures, published in Nanotechnology

We have measured the low frequency (1mHz

We describe a set up for measurements of low frequency (1 mHz < f < 20 Hz) conductance fluctuations in a solid conductor. The set-up uses a five probe a.c. measurement technique and extensive digital signal processing to reach a noise floor down to $S_{v}(f) \leq 10^{-20}$ V$^{2}$Hz$^{-1}$. The set up also allows measurement of noise using an a.c. in presence of a superimposed direct current. This feature is desirable in studies of electromigration damage or in systems that show non-linear conductivity. In addition, we describe a scheme which allows us to obtain the probability density function of the conductance fluctuation after subtracting the extraneous noise contributions (background) from the observed noise. The set-up has been used for conductance fluctuation measurement in the temperature range 1.5 K < T < 500 K in the presence of magnetic fields. We present some representative data obtained by this system., Comment: 25 pages 17 figures

We have studied low frequency resistance fluctuations (noise) in a single crystal of the rare earth perovskite manganite Pr$_{0.63}$Ca$_{0.37}$MnO$_3$ which shows a charge ordering transition at a temperature $T_{CO}$ ~ 245K. The noise measurements were made using an ac bias with and without a dc bias current imposed on it. We find that the spectral power $S_V(f)$ contains two components - one broad band 1/f part that exists for all frequency and temperature ranges and a single frequency Lorentzian of frequency $f_c$ which is strongly temperature dependent. The Lorentzian in $S_V(f)$ which appears due to Random telegraphic noise (RTN) as seen in the time series of the fluctuation, is seen in a very narrow temperature window around $T_{CO}$ where it makes the dominating contribution to the fluctuation. When the applied dc bias is increased beyond a certain threshold current density $J_{th}$, the electrical conduction becomes non-linear and one sees appearance of a significant Lorentzian contribution in the spectral power due to RTN. We explain the appearance of the RTN as due to coexisting Charge ordered (CO) and reverse orbitally ordered (ROO) phases which are in dynamical equilibrium over a mesoscopic length scale ($\approx 30nm$) and the kinetics being controlled by an activation barrier $E_{a} ~ 0.45eV. The 1/f noise is low for $T>>T_{CO}$ but increases by nearly two orders in a narrow temperature range as $T_{CO}$ is approached from above and the probability distribution function (PDF) deviates strongly from a Gaussian. We explain this behavior as due to approach of charge localization with correlated fluctuators which make the PDF non-Gaussian., Comment: 23 pages, 14 figures