Deterministic grayscale nanotopography to engineer mobilities in strained MoS2 FETs.

Field-effect transistors (FETs) based on two-dimensional materials (2DMs) with atomically thin channels have emerged as a promising platform for beyond-silicon electronics. However, low carrier mobility in 2DM transistors driven by phonon scattering remains a critical challenge. To address this issue, we propose the controlled introduction of localized tensile strain as an effective means to inhibit electron-phonon scattering in 2DM. Strain is achieved by conformally adhering the 2DM via van der Waals forces to a dielectric layer previously nanoengineered with a gray-tone topography. Our results show that monolayer MoS2 FETs under tensile strain achieve an 8-fold increase in on-state current, reaching mobilities of 185 cm²/Vs at room temperature, in good agreement with theoretical calculations. The present work on nanotopographic grayscale surface engineering and the use of high-quality dielectric materials has the potential to find application in the nanofabrication of photonic and nanoelectronic devices.

GaN Surface Passivation by MoS2 Coating.

In this study, we investigate the impact of two-dimensional MoS2 coating on the optical properties of surface GaN/AlGaN quantum wells (QWs). A strong enhancement in GaN QW light emission is observed with monolayer-MoS2 coating, yielding luminescence intensity comparable to that from a QW capped by an AlGaN barrier. Our results demonstrate that MoS2, despite its quite different nature from III-nitride semiconductors, acts as an effective barrier for surface GaN QWs and suppresses spatially localized intrinsic surface states. This finding provides novel pathways for efficient III-nitride surface passivation.

Unveiling the complex phonon nature and phonon cascades in 1L to 5L WSe2 using multiwavelength excitation Raman scattering.

Tungsten diselenide (WSe2) is a 2D semiconducting material, promising for novel optoelectronic and phononic applications. WSe2 has complex lattice dynamics and phonon structure. Numerous discrepancies in the literature exist regarding the interpretation and identification of phonon modes. This work presents a complete investigation of the vibrational properties of 1L to 5L flakes and bulk WSe2 using multi-wavelength Raman spectroscopy. We especially highlight measurements using 785 nm excitation, which have not been performed before. These allow us to solve inconsistences in the literature in terms of defect-activated non-Γ point single phonon modes and Breit-Wigner-Fano type resonance. We identify 35 Raman peaks per flake thickness, which we attribute to either one-phonon or multi-phonon modes, including two-phonon scattering due to a van Hove singularity (vHs). The measurements are in excellent agreement with the theoretical predictions. Using photoluminescence measurements, we identify photon-exciton coupling leading to resonant Raman scattering, suggesting wavelength laser excitations best suited for further investigations of specific WSe2 flake thicknesses. Finally, we report the observation of phonon-cascades for all WSe2 flake thicknesses, indicating strong phonon-electron interactions during early carrier relaxation processes in WSe2. This research provides a solid foundation and reference for future investigations of the vibrational properties of WSe2, paving the way for further development of this material towards applications.

Combining thermal scanning probe lithography and dry etching for grayscale nanopattern amplification.

Grayscale structured surfaces with nanometer-scale features are used in a growing number of applications in optics and fluidics. Thermal scanning probe lithography achieves a lateral resolution below 10 nm and a vertical resolution below 1 nm, but its maximum depth in polymers is limited. Here, we present an innovative combination of nanowriting in thermal resist and plasma dry etching with substrate cooling, which achieves up to 10-fold amplification of polymer nanopatterns into SiO2 without proportionally increasing surface roughness. Sinusoidal nanopatterns in SiO2 with 400 nm pitch and 150 nm depth are fabricated free of shape distortion after dry etching. To exemplify the possible applications of the proposed method, grayscale dielectric nanostructures are used for scalable manufacturing through nanoimprint lithography and for strain nanoengineering of 2D materials. Such a method for aspect ratio amplification and smooth grayscale nanopatterning has the potential to find application in the fabrication of photonic and nanoelectronic devices.

The fragility of artists' reputations from 1795 to 2020.

This study explores the longevity of artistic reputation. We empirically examine whether artists are more- or less-venerated after their death. We construct a massive historical corpus spanning 1795 to 2020 and build separate word-embedding models for each five-year period to examine how the reputations of over 3,300 famous artists-including painters, architects, composers, musicians, and writers-evolve after their death. We find that most artists gain their highest reputation right before their death, after which it declines, losing nearly one SD every century. This posthumous decline applies to artists in all domains, includes those who died young or unexpectedly, and contradicts the popular view that artists' reputations endure. Contrary to the Matthew effect, the reputational decline is the steepest for those who had the highest reputations while alive. Two mechanisms-artists' reduced visibility and the public's changing taste-are associated with much of the posthumous reputational decline. This study underscores the fragility of human reputation and shows how the collective memory of artists unfolds over time.

Isolated ballistic non-abelian interface channel.

The topological order of a quantum Hall state is mirrored by the gapless edge modes owing to bulk-edge correspondence. The state at the filling of ν = 5/2, predicted to host non-abelian anyons, supports a variety of edge modes (integer, fractional, neutral). To ensure thermal equilibration between the edge modes and thus accurately determine the state's nature, it is advantageous to isolate the fractional channel (1/2 and neutral modes). In this study, we gapped out the integer modes by interfacing the ν = 5/2 state with integer states ν = 2 and ν = 3 and measured the thermal conductance of the isolated-interface channel. Our measured half-quantized thermal conductance confirms the non-abelian nature of the ν = 5/2 state and its particle-hole Pfaffian topological order. Such an isolated channel may be more amenable to braiding experiments.

Melting of Interference in the Fractional Quantum Hall Effect: Appearance of Neutral Modes.

We attempted to measure interference of the outer edge mode in the fractional quantum hall regime with an electronic Mach-zehnder interferometer. The visibility of the interferometer wore off as we approached ν_{B}=1 and the transmission of the quantum point contacts (QPCs) of the interferometer simultaneously developed a v=1/3 conductance plateau accompanied by shot noise. The appearance of shot noise on this plateau indicates the appearance of nontopological neutral modes resulting from edge reconstruction. We have confirmed the presence of upstream neutral modes measuring upstream noise emanating from the QPC. The lack of interference throughout the lowest Landau level was correlated with a proliferation of neutral modes.

Author Correction: Observation of half-integer thermal Hall conductance.

In this Article, the publication details for references 33, 34 and 40 have been corrected online.

Observation of half-integer thermal Hall conductance.

Topological states of matter are characterized by topological invariants, which are physical quantities whose values are quantized and do not depend on the details of the system (such as its shape, size and impurities). Of these quantities, the easiest to probe is the electrical Hall conductance, and fractional values (in units of e2/h, where e is the electronic charge and h is the Planck constant) of this quantity attest to topologically ordered states, which carry quasiparticles with fractional charge and anyonic statistics. Another topological invariant is the thermal Hall conductance, which is harder to measure. For the quantized thermal Hall conductance, a fractional value in units of κ0 (κ0 = π2kB2/(3h), where kB is the Boltzmann constant) proves that the state of matter is non-Abelian. Such non-Abelian states lead to ground-state degeneracy and perform topological unitary transformations when braided, which can be useful for topological quantum computation. Here we report measurements of the thermal Hall conductance of several quantum Hall states in the first excited Landau level and find that the thermal Hall conductance of the 5/2 state is compatible with a half-integer value of 2.5κ0, demonstrating its non-Abelian nature.

Observed quantization of anyonic heat flow.

The quantum of thermal conductance of ballistic (collisionless) one-dimensional channels is a unique fundamental constant. Although the quantization of the electrical conductance of one-dimensional ballistic conductors has long been experimentally established, demonstrating the quantization of thermal conductance has been challenging as it necessitated an accurate measurement of very small temperature increase. It has been accomplished for weakly interacting systems of phonons, photons and electronic Fermi liquids; however, it should theoretically also hold in strongly interacting systems, such as those in which the fractional quantum Hall effect is observed. This effect describes the fractionalization of electrons into anyons and chargeless quasiparticles, which in some cases can be Majorana fermions. Because the bulk is incompressible in the fractional quantum Hall regime, it is not expected to contribute substantially to the thermal conductance, which is instead determined by chiral, one-dimensional edge modes. The thermal conductance thus reflects the topological properties of the fractional quantum Hall electronic system, to which measurements of the electrical conductance give no access. Here we report measurements of thermal conductance in particle-like (Laughlin-Jain series) states and the more complex (and less studied) hole-like states in a high-mobility two-dimensional electron gas in GaAs-AlGaAs heterostructures. Hole-like states, which have fractional Landau-level fillings of 1/2 to 1, support downstream charged modes as well as upstream neutral modes, and are expected to have a thermal conductance that is determined by the net chirality of all of their downstream and upstream edge modes. Our results establish the universality of the quantization of thermal conductance for fractionally charged and neutral modes. Measurements of anyonic heat flow provide access to information that is not easily accessible from measurements of conductance.

Bulk-Induced 1/f Noise at the Surface of Three-Dimensional Topological Insulators.

Slow intrinsic fluctuations of resistance, also known as the flicker noise or 1/f-noise, in the surface transport of strong topological insulators (TIs) is a poorly understood phenomenon. Here, we have systematically explored the 1/f-noise in field-effect transistors (FET) of mechanically exfoliated Bi1.6Sb0.4Te2Se TI films when transport occurs predominantly via the surface states. We find that the slow kinetics of the charge disorder within the bulk of the TI induces mobility fluctuations at the surface, providing a new source of intrinsic 1/f-noise that is unique to bulk TI systems. At small channel thickness, the noise magnitude can be extremely small, corresponding to the phenomenological Hooge parameter γH as low as ≈10(-4), but it increases rapidly when channel thickness exceeds ∼1 µm. From the temperature (T)-dependence of noise, which displayed sharp peaks at characteristic values of T, we identified generation-recombination processes from interband transitions within the TI bulk as the dominant source of the mobility fluctuations in surface transport. Our experiment not only establishes an intrinsic microscopic origin of noise in TI surface channels, but also reveals a unique spectroscopic information on the impurity bands that can be useful in bulk TI systems in general.

Signature effects of spin clustering and distribution of spin couplings on magnetization behaviour in Ni-Fe-Mo and Ni-Fe-W alloys.

Spontaneous magnetization as a function of temperature is investigated for a number of disordered Ni-Fe-Mo and Ni-Fe-W alloys using superconducting quantum interference device magnetometry, with a focus on the low-T behaviour as well as the critical exponents associated with the magnetic phase transition. While the low-T magnetization is found to be well described by Bloch's T(3/2) law, extraordinary enhancements of the spin-wave parameter B and the reduced coefficient B(3/2) = BT(C)(3/2) are observed with increasing Fe dilution as compared to conventional 3d ferromagnets, whereas the critical amplitudes are found to decrease systematically. Recent locally self-consistent calculations of finite-temperature spin dynamics in a generic diluted magnet provide an understanding in terms of two distinct energy scales associated with weakly coupled bulk spins in the ferromagnetic matrix and strongly coupled cluster spins. In view of the similar behaviour observed in diluted magnetic semiconductors and other ferromagnetic alloys, it is proposed that these distinctive features corresponding to the three important temperature regimes provide macroscopic indicators of signature effects of spin clustering on the magnetization behaviour in disordered ferromagnets.

Fe(3.3)Ni(83.2)Mo(13.5): a likely candidate to show spin-glass behaviour at low temperatures.

Unlike other transition metals alloyed with a non-magnetic metal, alloys of Ni behave rather differently. This is because of the fragility of the local magnetic moment on Ni. NiMo and NiW do not show any spin-glass phase. However, addition of Fe can bolster the moment on Ni. We wish to study whether the alloy Fe(3.3)Ni(83.2)Mo(13.5), chosen near a composition where mean-field estimates suggest there could be a spin-glass phase, shows such a phase or not.

Metabolic preconditioning of cells with AICAR-riboside: improved cryopreservation and cell-type specific impacts on energetics and proliferation.

In species whose evolutionary history has provided natural tolerance to dehydration and freezing, metabolic depression is often a pre-requisite for survival. We tested the hypothesis that preconditioning of mammalian cells with 5-aminoimidazole-4-carboxamide-1-b-D-ribofuranoside (AICAR) to achieve metabolic depression will promote greater survivorship during cryopreservation. AICAR is used extensively to stimulate AMP-activated protein kinase (AMPK), which can result in downregulation of biosynthetic processes. We showed that the metabolic interconversion of AICAR was cell-type dependent. Accumulation of 5-aminoimidazole-4-carboxamide-1b-D-ribofuranosyl-5'-monophosphate (ZMP), as well as other metabolites that possess multiple phosphates (i.e., ZDP, ZTP), varied approximately 3.5-fold across the cell lines tested. AICAR treatment also significantly influenced the concentrations of cellular adenylates (ATP, ADP, and AMP). Depression of cell metabolism and proliferation with AICAR treatment differed among cell lines. Proliferation for a given cell line was negatively correlated with the fold-increase achieved in the 'effective adenylate ratio' ([AMP]+[ZMP])/[ATP]) after AICAR treatment. Metabolic preconditioning with AICAR promoted a significant increase in viability post-freezing in J774.A1 macrophages, HepG2/C3A cells and primary hepatocytes but not in NIH/3T3 fibroblasts or OMK cells. The effect of AICAR on viability after freezing was positively correlated (r(2)=0.94) with the fold-increase in the 'effective adenylate ratio'. Thus for each cell line, the greater the depression of metabolism and proliferation due to preconditioning with AICAR, the greater was the survivorship post-freezing.

Phospholipid vesicles increase the survival of freeze-dried human red blood cells.

In a previous report [Z. Török, G. Satpathy, M. Banerjee, R. Bali, E. Little, R. Novaes, H. Van Ly, D. Dwyre, A. Kheirolomoom, F. Tablin, J.H. Crowe, N.M. Tsvetkova, Preservation of trehalose loaded red blood cells by lyophilization, Cell Preservation Technol. 3 (2005) 96-111.], we presented a method for preserving human red blood cells (RBCs) by loading them with trehalose and then freeze-drying. We have now improved that method, based on the discovery that addition of phospholipid vesicles to the lyophilization buffer substantially reduces hemolysis of freeze-dried RBCs after rehydration. The surviving cells synthesize 2,3-DPG, have low levels of methemoglobin, and have preserved morphology. Among the lipid species we studied, unsaturated PCs were found to be most effective in suppressing hemoglobin leakage. RBC-vesicle interactions depend on vesicle size and structure; unilamellar liposomes with average diameter of less than 300 nm were more effective in reducing the hemolysis than multilamellar vesicles. Trehalose loaded RBCs demonstrated high survival and low levels of methemoglobin during 10 weeks of storage at 4 degrees C in the dry state when lyophilized in the presence of liposomes.

The role of thyroid hormone on phenylhydrazine hydrochloride mediated inhibitory effects on blood acetylcholinesterase: an in vivo and in vitro study.

A novel phenomenon of protective counteraction by thyroid hormone has been demonstrated in phenylhydrazine hydrochloride (PHH) induced insult on blood acetylcholinesterase (AChE, EC 3.1.1.7) activity, in both, in vivo and in vitro conditions. Injection of PHH (20 microg/g) to juvenile male rats for three consecutive days caused a 48% decrease (p < 0.001) in the total blood AChE activity on the third day (i.e. 24 h after injections for three consecutive days) in comparison to the control animals. Simultaneous injections of thyroxine (T4) 1 or 2 microg/g with PHH (20 microg/g) showed a recovery in AChE activity by 27% (p < 0.02) and 55% (p < 0.001), respectively, in comparison to the only PHH-injected animals. T4 at 1, 2 and 4 microg/g doses showed unchanged levels in comparison to the untreated controls. In our in vitro system, incubations of the RBCs in PHH (2 mM) containing medium also showed an inhibition of 44% (p < 0.001) of the RBC membrane AChE activity in comparison to the control conditions. A recovery of 23-81% of the enzyme activity was observed after simultaneous use of T4 (1 nM-100 nM) or T3 (0.1 nM-100 nM), or triiodothyroacetic acid (TRIAC) (100 nM) with PHH (2 mM) in a dose-dependent manner with a potency profile of T3 > T4 > TRIAC. Incubation of RBCs only with T4, T3, or TRIAC at 0.1-100 nM concentration did not cause any alteration in the membrane AChE activity in comparison to control conditions. Thus, thyroid hormone distinctly demonstrated a counteraction or protective nature of action on the PHH-induced inhibition of total blood and RBC membrane AChE activity.