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Superconductivity in few-layer semiconducting transition metal dichalcogenides (TMDs) can be induced by field-effect doping through ionic-liquid gating. While several experimental observations have been collected over the years, a fully-consistent theoretical picture is still missing. Here we develop a realistic framework that combines the predictive power of first-principles simulations with the versatility and insight of Bardeen-Cooper-Schrieffer gap equations to rationalize such experiments. The multi-valley nature of semiconducting TMDs is taken into account, together with the doping- and momentum-dependent electron-phonon and Coulomb interactions. Consistently with experiments, we find that superconductivity occurs when the electron density is large enough that the Q valleys get occupied, as a result of a large enhancement of electron-phonon interactions. Despite being phonon-driven, the superconducting state is predicted to be sensitive to Coulomb interactions, which can lead to the appearance of a relative sign difference between valleys and thus to a $s_{+-}$ character. We discuss qualitatively how such scenario may account for many of the observed physical phenomena for which no microscopic explanation has been found so far, including in particular the presence of a large subgap density of states, and the sample-dependent dome-shaped dependence of $T_c$ on accumulated electron density. Our results provide a comprehensive analysis of gate-induced superconductivity in semiconducting TMDs, and introduce an approach that will likely be valuable for other multivalley electronic systems, in which superconductivity occurs at relatively low electron density., Comment: 17 pages, 8 figures

Since 2014, we have been conducting experiments based on a multidisciplinary collaboration between specialists in theatrical staging and researchers in virtual reality, digital art, and video games. This team focused its work on the similarities and differencesthat exist between real physical actors (actor-performers) and virtual digital actors (avatars). From this multidisciplinary approach, experimental research-creation projects have emerged and rely on a physical actor playing with the image of an avatar, controlled by another physical actor via the intermediary of a low-cost motion-capture system. In the first part of the paper, we will introduce the scenographic design on which our presentation is based, and the modifications we have made in relation to our previous work. Next, in the second section, we will discuss in detail the impact of augmenting the player's game using an avatar, compared to the scenic limitations of the theatrical stage. In part three of the paper, we will discuss the software-related aspects of the project, focusing on exchanges between the different components of our design and describing the algorithms enabling us to utilize the real-time movement of a player via various capture devices. To conclude, we will examine in detail how our experimental system linking physical actors and avatars profoundly alters the nature of collaboration between directors, actors, and digital artists in terms of actor/avatar direction., Comment: Wiley. Challenges of the Internet of Things. Technology, Use, Ethics, 2018

Plasmons and polar phonons are elementary electrodynamic excitations of matter. In 2d and at long wavelengths, they couple to light and act as the system polaritons. They also dictate the scattering of charged carriers. Van der Waals heterostructures offer the opportunity to couple excitations from different layers via long-range Coulomb interactions, modifying both their dispersion and their scattering of electrons. Even when the excitations do not couple, they are still influenced by the screening from all layers, leading to complex dynamical interactions between electrons, plasmons and polar phonons. We develop an efficient ab initio model to solve the dynamical electric response of Van der Waals heterostructures, accompanied by a formalism to extract relevant spectroscopic and transport quantities. Notably, we obtain scattering rates for electrons of the heterostructure coupling remotely with electrodynamic excitations. We apply those developments to BN-capped graphene, in which polar phonons from BN couple to plasmons in graphene. We study the nature of the coupled excitations, their dispersion and their coupling to graphene's electrons. Regimes driven by either phonons or plasmons are identified, as well as a truly hybrid regime at long wavelengths. Those are studied as a function of the graphene's Fermi level and the number of BN layers. In contrast with descriptions in terms of surface-optical phonons, we find that the electron-phonon interaction stems from different modes. Moreover, the dynamical screening of the coupling between BN's LO phonons and graphene's electrons crosses over from inefficient to metal-like depending on the relative value of the phonons' frequency and the energetic onset of interband transitions. While the coupling is significant in general, the associated scattering of graphene's carriers is found to be negligible in the context of electronic transport., Comment: 32 pages, 18 figures

We theoretically investigate the double-resonance Raman spectrum of monolayer graphene down to infrared laser excitation energies. By using first-principles density functional theory calculations, we improve upon previous theoretical predictions based on conical models or tight-binding approximations, and rigorously justify the evaluation of the electron-phonon enhancement found in Ref. [Venanzi, T., Graziotto, L. et al., Phys. Rev. Lett. 130, 256901 (2023)]. We proceed to discuss the effects of such enhancement on the room temperature graphene resistivity, hinting towards a possible reconciliation of theoretical and experimental discrepancies., Comment: 19 pages, 18 figures

The electronic and vibrational properties and electron-phonon couplings of one-dimensional materials will be key to many prospective applications in nanotechnology. Dimensionality strongly affects these properties and has to be correctly accounted for in first-principles calculations. Here we develop and implement a formulation of density-functional and density-functional perturbation theory that is tailored for one-dimensional systems. A key ingredient is the inclusion of a Coulomb cutoff, a reciprocal-space technique designed to correct for the spurious interactions between periodic images in periodic-boundary conditions. This restores the proper one-dimensional open-boundary conditions, letting the true response of the isolated one-dimensional system emerge. In addition to total energies, forces and stress tensors, phonons and electron-phonon interactions are also properly accounted for. We demonstrate the relevance of the present method on a portfolio of realistic systems: BN atomic chains, BN armchair nanotubes, and GaAs nanowires. Notably, we highlight the critical role of the Coulomb cutoff by studying previously inaccessible polar-optical phonons and Frohlich electron-phonon couplings. We also develop and apply analytical models to support the physical insights derived from the calculations and we discuss their consequences on electronic lifetimes. The present work unlocks the possibility to accurately simulate the linear response properties of one-dimensional systems, sheds light on the transition between dimensionalities and paves the way for further studies in several fields, including charge transport, optical coupling and polaritronics., Comment: 15 pages, 7 figures

The main objective of this work is to investigate non-linear errors and pairwise summation using stochastic rounding (SR) in variance computation algorithms. We estimate the forward error of computations under SR through two methods: the first is based on a bound of the variance and Bienaym{\'e}-Chebyshev inequality, while the second is based on martingales and Azuma-Hoeffding inequality. The study shows that for pairwise summation, using SR results in a probabilistic bound of the forward error proportional to log(n)u rather than the deterministic bound in O(log(n)u) when using the default rounding mode. We examine two algorithms that compute the variance, called ''textbook'' and ''two-pass'', which both exhibit non-linear errors. Using the two methods mentioned above, we show that these algorithms' forward errors have probabilistic bounds under SR in O($\sqrt$ nu) instead of nu for the deterministic bounds. We show that this advantage holds using pairwise summation for both textbook and two-pass, with probabilistic bounds of the forward error proportional to log(n)u.

The diffusion of molecules on interstellar grain surfaces is one of the most important driving forces for the molecular complexity in the interstellar medium. Due to the lack of laboratory measurements, astrochemical modeling of grain surface processes usually assumes a constant ratio between the diffusion energy barrier and the desorption energy. This over-simplification inevitably causes large uncertainty in model predictions. We present a new measurement of the diffusion of CO$_2$ molecules on the surface of non-porous amorphous solid water (np-ASW), an analog of the ice mantle that covers cosmic dust grains. A small coverage of CO$_2$ was deposited onto an np-ASW surface at 40~K, the subsequent warming of the ice activated the diffusion of CO$_2$ molecules, and a transition from isolated CO$_2$ to CO$_2$ clusters was seen in the infrared spectra. To obtain the diffusion energy barrier and pre-exponential factor simultaneously, a set of isothermal experiments were carried out. The values for the diffusion energy barrier and pre-exponential factor were found to be $1300\pm110$~K and $10^{7.6\pm0.8}$~s$^{-1}$. A comparison with prior laboratory measurements on diffusion is discussed., Comment: 13 pages, 6 figures

Electron-phonon interaction and phonon frequencies of doped polar semiconductors are sensitive to long-range Coulomb forces and can be strongly affected by screening effects of free carriers, the latter changing significantly when approaching the two-dimensional limit. We tackle this problem within a linear-response dielectric-matrix formalism, where screening effects can be properly taken into account by generalized effective charge functions and the inverse scalar dielectric function, allowing for controlled approximations in relevant limits. We propose complementary computational methods to evaluate from first principles both effective charges -- encompassing all multipolar components beyond dynamical dipoles and quadrupoles -- and the static dielectric function of doped two-dimensional semiconductors, and provide analytical expressions for the long-range part of the dynamical matrix and the electron-phonon interaction in the long-wavelength limit. As a representative example, we apply our approach to study the impact of doping in disproportionated graphene, showing that optical Fr\"ohlich and acoustic piezoelectric couplings, as well as the slope of optical longitudinal modes, are strongly reduced, with a potential impact on the electronic/intrinsic scattering rates and related transport properties., Comment: 28 pages, 11 figures

Coupling regular topologies with optimised routing algorithms is key in pushing the performance of interconnection networks of supercomputers.In this paper we present Dmodc, a fast deterministic routing algorithm for Parallel Generalised Fat-Trees (PGFTs) which minimises congestion risk even under massive network degradation caused by equipment failure.Dmodc computes forwarding tables with a closed-form arithmetic formula by relying on a fast preprocessing phase.This allows complete re-routing of networks with tens of thousands of nodes in less than a second.In turn, this greatly helps centralised fabric management react to faults with high-quality routing tables and no impact to running applications in current and future very large-scale HPC clusters., Comment: arXiv admin note: text overlap with arXiv:2211.11817

Coupling regular topologies with optimized routing algorithms is key in pushing the performance of interconnection networks of HPC systems. In this paper we present Dmodc, a fast deterministic routing algorithm for Parallel Generalized Fat-Trees (PGFTs) which minimizes congestion risk even under massive topology degradation caused by equipment failure. It applies a modulo-based computation of forwarding tables among switches closer to the destination, using only knowledge of subtrees for pre-modulo division. Dmodc allows complete rerouting of topologies with tens of thousands of nodes in less than a second, which greatly helps centralized fabric management react to faults with high-quality routing tables and no impact to running applications in current and future very large-scale HPC clusters. We compare Dmodc against routing algorithms available in the InfiniBand control software (OpenSM) first for routing execution time to show feasibility at scale, and then for congestion risk under degradation to demonstrate robustness. The latter comparison is done using static analysis of routing tables under random permutation (RP), shift permutation (SP) and all-to-all (A2A) traffic patterns. Results for Dmodc show A2A and RP congestion risks similar under heavy degradation as the most stable algorithms compared, and near-optimal SP congestion risk up to 1% of random degradation.

This paper reviews the entire engineering process of trustworthy Machine Learning (ML) algorithms designed to equip critical systems with advanced analytics and decision functions. We start from the fundamental principles of ML and describe the core elements conditioning its trust, particularly through its design: namely domain specification, data engineering, design of the ML algorithms, their implementation, evaluation and deployment. The latter components are organized in an unique framework for the design of trusted ML systems., Comment: This work has been supported by the French government under the "France 2030" program, as part of the SystemX Technological Research Institute Research Institute

Dimensionality provides a clear fingerprint on the dispersion of infrared-active, polar-optical phonons. For these phonons, the local dipoles parametrized by the Born effective charges drive the LO-TO splitting of bulk materials; this splitting actually breaks down in two-dimensional materials. Here, we extend the existing theory to the one-dimensional (1D) case. Combining an analytical model with the implementation of density-functional perturbation theory in 1D boundary conditions, we show that the dielectric splitting in the dispersion relations collapses logarithmically at the zone center. The dielectric properties and the radius of the 1D materials are linked by the present work to these red shifts, opening novel IR and Raman characterization avenues.

Stochastic rounding (SR) offers an alternative to the deterministic IEEE-754 floating-point rounding modes. In some applications such as PDEs, ODEs and neural networks, SR empirically improves the numerical behavior and convergence to accurate solutions while no sound theoretical background has been provided. Recent works by Ipsen, Zhou, Higham, and Mary have computed SR probabilistic error bounds for basic linear algebra kernels. For example, the inner product SR probabilistic bound of the forward error is proportional to $\sqrt$ nu instead of nu for the default rounding mode. To compute the bounds, these works show that the errors accumulated in computation form a martingale. This paper proposes an alternative framework to characterize SR errors based on the computation of the variance. We pinpoint common error patterns in numerical algorithms and propose a lemma that bounds their variance. For each probability and through Bienaym{\'e}-Chebyshev inequality, this bound leads to better probabilistic error bound in several situations. Our method has the advantage of providing a tight probabilistic bound for all algorithms fitting our model. We show how the method can be applied to give SR error bounds for the inner product and Horner polynomial evaluation.

Recently, stochastic rounding (SR) has been implemented in specialized hardware but most current computing nodes do not yet support this rounding mode. Several works empirically illustrate the benefit of stochastic rounding in various fields such as neural networks and ordinary differential equations. For some algorithms, such as summation, inner product or matrixvector multiplication, it has been proved that SR provides probabilistic error bounds better than the traditional deterministic bounds. In this paper, we extend this theoretical ground for a wider adoption of SR in computer architecture. First, we analyze the biases of the two SR modes: SR-nearness and SR-up-or-down. We demonstrate on a case-study of Euler's forward method that IEEE-754 default rounding modes and SR-up-or-down accumulate rounding errors across iterations and that SR-nearness, being unbiased, does not. Second, we prove a O($\sqrt$ n) probabilistic bound on the forward error of Horner's polynomial evaluation method with SR, improving on the known deterministic O(n) bound.

The transport and optical properties of semiconducting transition metal dichalcogenides around room temperature are dictated by electron-phonon scattering mechanisms within a complex, spin-textured and multi-valley electronic landscape. The relative positions of the valleys are critical, yet they are sensitive to external parameters and very difficult to determine directly. We propose a first-principle model as a function valley positions to calculate carrier mobility and Kerr rotation angles. The model brings valuable insights, as well as quantitative predictions of macroscopic properties for a wide range of carrier density. The doping-dependant mobility displays a characteristic peak, the height depending on the position of the valleys. The Kerr rotation signal is enhanced when same spin-valleys are aligned, and quenched when opposite spin-valleys are populated. We provide guidelines to optimize these quantities with respect to experimental parameters, as well as the theoretical support for \emph{in situ} characterization of the valley positions., Comment: 24 pages, 11 figures

A recent 2D spinFET concept proposes to switch electrostatically between two separate sublayers with strong and opposite intrinsic Rashba effects. This concept exploits the spin-layer locking mechanism present in centrosymmetric materials with local dipole fields, where a weak electric field can easily manipulate just one of the spin channels. Here, we propose a novel monolayer material within this family, lutetium oxide iodide (LuIO). It displays one of the largest Rashba effects among 2D materials (up to $k_R = 0.08$ {\AA}$^{-1}$), leading to a $\pi/2$ rotation of the spins over just 1 nm. The monolayer had been predicted to be exfoliable from its experimentally-known 3D bulk counterpart, with a binding energy even lower than graphene. We characterize and model with first-principles simulations the interplay of the two gate-controlled parameters for such devices: doping and spin channel selection. We show that the ability to split the spin channels in energy diminishes with doping, leading to specific gate-operation guidelines that can apply to all devices based on spin-layer locking., Comment: 11 pages, 9 figures

Many promising optoelectronic devices, such as broadband photodetectors, nonlinear frequency converters, and building blocks for data communication systems, exploit photoexcited charge carriers in graphene. For these systems, it is essential to understand, and eventually control, the cooling dynamics of the photoinduced hot-carrier distribution. There is, however, still an active debate on the different mechanisms that contribute to hot-carrier cooling. In particular, the intrinsic cooling mechanism that ultimately limits the cooling dynamics remains an open question. Here, we address this question by studying two technologically relevant systems, consisting of high-quality graphene with a mobility >10,000 cm$^2$V$^{-1}$s$^{-1}$ and environments that do not efficiently take up electronic heat from graphene: WSe$_2$-encapsulated graphene and suspended graphene. We study the cooling dynamics of these two high-quality graphene systems using ultrafast pump-probe spectroscopy at room temperature. Cooling via disorder-assisted acoustic phonon scattering and out-of-plane heat transfer to the environment is relatively inefficient in these systems, predicting a cooling time of tens of picoseconds. However, we observe much faster cooling, on a timescale of a few picoseconds. We attribute this to an intrinsic cooling mechanism, where carriers in the hot-carrier distribution with enough kinetic energy emit optical phonons. During phonon emission, the electronic system continuously re-thermalizes, re-creating carriers with enough energy to emit optical phonons. We develop an analytical model that explains the observed dynamics, where cooling is eventually limited by optical-to-acoustic phonon coupling. These fundamental insights into the intrinsic cooling mechanism of hot carriers in graphene will play a key role in guiding the development of graphene-based optoelectronic devices.

Van der Waals heterostructures provide a versatile tool to not only protect or control, but also enhance the properties of a 2D material. We use ab initio calculations and semi-analytical models to find strategies which boost the mobility of a current-carrying 2D semiconductor within an heterostructure. Free-carrier screening from a metallic "screener" layer remotely suppresses electron-phonon interactions in the current-carrying layer. This concept is most effective in 2D semiconductors whose scattering is dominated by screenable electron-phonon interactions, and in particular the Fr\"ohlich coupling to polar-optical phonons. Such materials are common and characterised by overall low mobilities in the small doping limit, and much higher ones when the 2D material is doped enough for electron-phonon interactions to be screened by its own free carriers. We use GaSe as a prototype and place it in a heterostructure with doped graphene as the "screener" layer and BN as a separator. We develop an approach to determine the electrostatic response of any heterostructure by combining the responses of the individual layers computed within density-functional perturbation theory. Remote screening from graphene can suppress the long-wavelength Fr\"ohlich interaction, leading to a consistently high mobility around $500$ to $600$ cm$^2$/Vs for carrier densities in GaSe from $10^{11}$ to $10^{13}$ cm$^{-2}$. Notably, the low-doping mobility is enhanced by a factor 2.5. This remote free-carrier screening is more efficient than more conventional manipulation of the dielectric environment, and it is most effective when the separator (BN) is thin., Comment: 20 pages, 14 figures

When complex mechanisms are involved, pinpointing high-performance materials within large databases is a major challenge in materials discovery. We focus here on phonon-limited conductivities, and study 2D semiconductors doped by field effects. Using state-of-the-art density-functional perturbation theory and Boltzmann transport equation, we discuss 11 monolayers with outstanding transport properties. These materials are selected from a computational database of exfoliable materials providing monolayers that are dynamically stable and that do not have more than 6 atoms per unit cell. We first analyse electron-phonon scattering in two well-known systems: electron-doped InSe and hole-doped phosphorene. Both are single-valley systems with weak electron-phonon interactions, but they represent two distinct pathways to fast transport: a steep and deep isotropic valley for the former and strongly anisotropic electron-phonon physics for the latter. We identify similar features in the database and compute the conductivities of the relevant monolayers. This process yields several high-conductivity materials, some of them only very recently emerging in the literature (GaSe, Bi$_2$SeTe$_2$, Bi$_2$Se$_3$, Sb$_2$SeTe$_2$), others never discussed in this context (AlLiTe$_2$, BiClTe, ClGaTe, AuI). Comparing these 11 monolayers in detail, we discuss how the strength and angular dependency of the electron-phonon scattering drives key differences in the transport performance of materials despite similar valley structure. We also discuss the high conductivity of hole-doped WSe$_2$, and how this case study shows the limitations of a selection process that would be based on band properties alone.