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Twist angle modifies the band alignment, screening, and interlayer (IL) coupling in twisted bilayers (tBLs) of transition metal dichalcogenides. Intermediate misorientation (twist angles > 15 degrees) bilayers (BLs) offer a unique opportunity to tune excitonic behavior within these concurrent physical mechanisms but are seldom studied. In this paper, we measure many-body excitonic complexes in monolayer (ML), natural BL, and tBL WSe2. Neutral biexciton (XX) is observed in tBL, while being undetected in nonencapsulated ML and BL, demonstrating unique effects of disorder screening in tBLs. The XX as well as charged biexciton are robust to thermal dissociation and are controllable by electrostatic doping. Vanishing of momentum-indirect IL excitons with increasing electron doping is demonstrated in tBL, resulting from the near alignment of Q-K and K-K valleys. Intermediate misorientation samples offer a high degree of control of excitonic complexes while offering possibilities for studying exciton-phonon coupling, band alignment, and screening.

The intriguing electronic properties of graphene and the strong light-matter interaction in layered transition-metal dichalcogenide (TMDC) make them a natural partner for hybrid devices, not only for optoelectronic device applications, but also to understand the conversion of light to electricity in this atomic scale prototype of a donor-acceptor complex. Here, we describe graphene-on-${\mathrm{WS}}_{2}$ binary heterostructure FET device, displaying gate-tunable persistent photoconductivity. Our time-dependent photovoltage relaxation experiments suggest that the charge-transfer time scale in this heterostructure is dependent on the input optical power, contrary to what is observed for the bare TMDC and is orders of magnitude slower than that observed for various other vdW hybrids. The optoelectronic responsivity of this device at low optical power is found to be as high as ${10}^{10}\phantom{\rule{0.1em}{0ex}}\mathrm{V/W}$, and thus shows the potential to be one of the most sensitive visible range photodetectors, while the gate tunability of the persistent photoconductivity can be utilized in the memory device applications. We identify that the photoresponse is the outcome of a photogating mechanism, due to the exciton dissociation under optical excitation, followed by the trapping of holes in ${\mathrm{WS}}_{2}$ and subsequent electron transfer to graphene.

The conformational flexibility and dynamics of unfolded peptide chains is of major interest in the context of protein folding and protein functioning. The rate with which amino acids at different positions along the peptide chain meet sets an upper speed limit for protein folding. By using single-molecule photo-induced energy transfer spectroscopy, we have systematically measured end-to-end and end-to-internal site contact formation rates for several intrinsically disordered protein fragments (IDPs) (11 to 41 amino acids) and have also determined their hydrodynamic radius using dual-focus fluorescence correlation spectroscopy. For interpreting the measured values, we have developed a Brownian dynamics model (based on bead-rod chain dynamics in a thermal bath including hydrodynamic interactions) which quantitatively reproduces all measured data surprisingly well while requiring only two fit parameters. The model provides a complete picture of the peptides' dynamics and allows us to translate the experimental rates and radii into molecular properties of the peptides: We find a persistence length of ${l}_{P}=5.2\ifmmode\pm\else\textpm\fi{}1.9\phantom{\rule{3.33333pt}{0ex}}\text{\AA{}}$, a hydrodynamic radius of $a=3.5\ifmmode\pm\else\textpm\fi{}0.7\phantom{\rule{3.33333pt}{0ex}}\text{\AA{}}$ per amino acid, and that excluded volume effects play an important role in the dynamics of IDPs.

The interaction between the Fermi sea of conduction electrons and a non-adiabatic attractive impurity potential can lead to a power-law divergence in the tunneling probability of charge through the impurity. The resulting effect, known as the Fermi edge singularity (FES), constitutes one of the most fundamental many-body phenomena in quantum solid state physics. Here we report the first observation of FES for Dirac Fermions in graphene driven by isolated Coulomb impurities in the conduction channel. In high-mobility graphene devices on hexagonal boron nitride substrates, the FES manifests in abrupt changes in conductance with a large magnitude $\approx e^{2}/h$ at resonance, indicating total many-body screening of a local Coulomb impurity with fluctuating charge occupancy. Furthermore, we exploit the extreme sensitivity of graphene to individual Coulomb impurities, and demonstrate a new defect-spectroscopy tool to investigate strongly correlated phases in graphene in the quantum Hall regime., 6 pages, 4 figures. To appear in Physical Review Letters

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.

In this paper we report the measurement of conductance fluctuations in single crystals of Si made metallic by heavy doping (n \approx 2-2.5n_c, n_c being critical composition at Metal-Insulator transition). Since all dimensions (L) of the samples are much larger than the electron phase coherent length L_\phi (L/L_\phi \sim 10^3), our system is truly three dimensional. Temperature and magnetic field dependence of noise strongly indicate the universal conductance fluctuations (UCF) as predominant source of the observed magnitude of noise. Conductance fluctuations within a single phase coherent region of L_\phi^3 was found to be saturated at \approx (e^2/h)^2. An accurate knowledge of the level of disorder, enables us to calculate the change in conductance \delta G_1 due to movement of a single scatterer as \delta G_1 \sim e^2/h, which is \sim 2 orders of magnitude higher than its theoretically expected value in 3D systems., Comment: Text revised version. 4 eps figs unchanged

We have explored the mechanism of spin-torque-driven domain-wall (DW) depinning in cylindrical nanowires of nickel using noise in electrical resistance. We find that the spectral slope of noise is a sensitive probe to the DW kinetics that reveals a creeplike behavior of the DWs at the depinning threshold, and diffusive DW motion at higher spin-torque drive. Different regimes of DW kinetics were characterized by universal kinetic exponents.

We report low-frequency 1/f noise measurements of degenerately doped Si:P \delta-layers at 4.2K. The noise was found to be over six orders of magnitude lower than that of bulk Si:P systems in the metallic regime and is one of the lowest values reported for doped semiconductors. The noise was found to be nearly independent of magnetic field at low fields, indicating negligible contribution from universal conductance fluctuations. Instead interaction of electrons with very few active structural two-level systems may explain the observed noise magnitude, Comment: 4 pages, 4 figures

We investigate the Nernst effect in a mesoscopic two-dimensional electron system (2DES) at low magnetic fields, before the onset of Landau level quantization. The overall magnitude of the Nernst signal agrees well with semi-classical predictions. We observe reproducible mesoscopic fluctuations in the signal which diminish significantly with an increase in temperature. We also show that the Nernst effect exhibits an anomalous component which is correlated with an oscillatory Hall effect. This behavior may be able to distinguish between different spin-correlated states in the 2DES., Accepted for publication in Phys. Rev. B (Brief Reports)

Driven non-equilibrium structural phase transformation has been probed using time varying resistance fluctuations or noise. We demonstrate that the non-Gaussian component (NGC) of noise obtained by evaluating the higher order statistics of fluctuations, serves as a simple kinetic detector of these phase transitions. Using the martensite transformation in free-standing wires of nickel-titanium binary alloys as a prototype, we observe clear deviations from the Gaussian background in the transformation zone, indicative of the long range correlations in the system as the phase transforms. The viability of non- Gaussian statistics as a robust probe to structural phase transition was also confirmed by comparing the results from differential scanning calorimetry measurements. We further studied the response of the NGC to the modifications in the microstructure on repeated thermal cycling, as well as the variations in the temperature drive rate, and explained the results using established simplistic models based on the different competing time scales. Our experiments (i) suggest an alternative method to estimate the transformation temperature scales with high accuracy, and (ii) establish a connection between the material-specific evolution of microstructure to the statistics of its linear response. Since the method depends on an in-built long-range correlation during transformation, it could be portable to other structural transitions, as well as to materials of different physical origin and size., Comment: 8 pages, 10 figures

The low-energy electronic density of states (DOS) in GaAs/AlGaAs-based two-dimensional systems is investigated at mesoscopic length scales. Using nonequilibrium magnetotransport spectroscopy we find resonances in the DOS to be extremely sensitive to perpendicular magnetic fields as small as a few millitesla and to oscillate reproducibly as a function of the field. Since these resonances (also called zero-bias anomalies) are discussed as an indication of spontaneous spin polarization at low temperature, this Brief Report demonstrates a strong sensitivity of such possible polarization to magnetic fields.

We report an unexpected anomaly in the zero-field Hall coefficient of two-dimensional electron systems in high-mobility GaAs/AlGaAs heterostructures. Our device layout allows the investigation of mesoscopic systems with variable Fermi energy as well as with tunable Hall probes. At very low temperature, both positive and negative deviations from the noninteracting Hall coefficient ${\ensuremath{\gamma}}_{\text{H}}^{0}$ are observed, which can be twice as large as ${\ensuremath{\gamma}}_{\text{H}}^{0}$ itself. A distinct regularity in the deviations and their temperature dependence are interpreted as the spontaneous formation of localized spins and their indirect exchange interaction.

We report the experimental realization of a correlated insulating phase in 2D GaAs/AlGaAs heterostructures at low electron densities in a limited window of background disorder. This has been achieved at mesoscopic length scales, where the insulating phase is characterized by a universal hopping transport mechanism. Transport in this regime is determined only by the average electron separation, independent of the topology of background disorder. We have discussed this observation in terms of a pinned electron solid ground state, stabilized by mutual interplay of disorder and Coulomb interaction., Comment: 4+delta pages, 4 figures, To appear in the Physical Review B (Rapid Comm)

We have investigated the local magnetotransport in high quality two-dimensional electron systems at low carrier densities. The positive magnetoresistance in perpendicular magnetic field in the strongly insulating regime has been measured to evaluate the spatial concentration of electron trap sites within a mesoscopic region of the samples. A simultaneous measurement of the electron density within the same region shows an unexpected correspondence between the density of electrons in the metallic regime and that of the localizing sites in the insulating phase. We have shown this to imply a self-localized many-body ground state in the interacting regime, stabilized by optimal disorder.

We have experimentally studied the nonequilibrium transport in low-density clean two-dimensional (2D) electron systems at mesoscopic length scales. At zero magnetic field (B), a double-peak structure in the nonlinear conductance was observed close to the Fermi energy in the localized regime. From the behavior of these peaks at nonzero B, we could associate them with the opposite spin states of the system, indicating a spontaneous spin polarization at B=0. Detailed temperature and disorder dependence of the structure shows that such a splitting is a ground-state property of low-density 2D systems.

We have investigated the. kinetics of current-induced change of resistance and conductivity noise in thin epitaxial metallic films of LaNiO3. The resistance of the film changes at a very low current (threshold current density J(th)similar to 10(3) A/cm(2)). We find that the time dependence associated with the change of resistance shows a stretched-exponential-type dependence at lower temperature. Above a certain temperature scale T* (approximate to 350 K), this crosses over to a creep-type behavior. At T similar toT*, the time scale shows a drastic drop in the magnitude, and a long-range diffusion sets in, which leads to an increase in the conductivity noise. The phenomenon is like a "glass-transition" in the random lattice. of oxygen ions. We observe that the stretched exponential relaxation function, as obtained from time dependence of resistivity change, can explain the spectral structure as well as the temperature dependence of the low-frequency conductivity noise. The frequency and temperature dependence of noise could clearly identify the various processes, which had been seen in the current-stressing experiments carried out in the time domain. This establishes a quantitative link between the dynamics of current-induced resistivity changes and the conductivity noise. Both the phenomena are direct consequences of the low-frequency dynamics associated with the migration of the oxygen ions. Though done in the specific context of oxide films (used in oxide electronics), this observation has a generic aspect, and the treatments developed here can be used for establishing a quantitative link between electromigration current stressing and the conductivity noise in other metallic interconnects as well.