UMR 8502 Université Paris Sud

Bat 510, 91405 Orsay cedex

2020

P. Stoliar, Schneegans, O., and Rozenberg, M. J., “Biologically Relevant Dynamical Behaviors Realized in an Ultra-Compact Neuron Model”, Frontiers in Neuroscience, vol. 14, p. 421, 2020. WebsiteAbstract

We demonstrate a variety of biologically relevant dynamical behaviors building on a recently introduced ultra-compact neuron (UCN) model. We provide the detailed circuits which all share a common basic block that realizes the leaky-integrate-and-fire (LIF) spiking behavior. All circuits have a small number of active components and the basic block has only three, two transistors and a silicon controlled rectifier (SCR). We also demonstrate that numerical simulations can faithfully represent the variety of spiking behavior and can be used for further exploration of dynamical behaviors. Taking Izhikevich’s set of biologically relevant behaviors as a reference, our work demonstrates that a circuit of a LIF neuron model can be used as a basis to implement a large variety of relevant spiking patterns. These behaviors may be useful to construct neural networks that can capture complex brain dynamics or may also be useful for artificial intelligence applications. Our UCN model can therefore be considered the electronic circuit counterpart of Izhikevich’s (2003) mathematical neuron model, sharing its two seemingly contradicting features, extreme simplicity and rich dynamical behavior.

B. van der Meer, Smallenburg, F., Dijkstra, M., and Filion, L., “High antisite defect concentrations in hard-sphere colloidal Laves phases”, Soft matter, vol. 16, p. 4155—4161, 2020. Website

J. Trastoy, et al., “Magnetic field frustration of the metal-insulator transition in ${\mathrm{V}}_{2}{\mathrm{O}}_{3}$”, Phys. Rev. B, vol. 101, p. 245109, 2020. Website

S. Polatkan, et al., “Magneto-Optics of a Weyl Semimetal beyond the Conical Band Approximation: Case Study of TaP”, Phys. Rev. Lett., vol. 124, p. 176402, 2020. Website

G. M. Thurston, Hayden, D. L., Foffi, G., Ross, D. S., and Hamilton, J. F., “On Non-Monotonic Dependence of Phase Separation Properties on Molecular Interaction Parameters”, Biophysical JournalBiophysical Journal, vol. 118, no. 3, p. 517a, 2020. Website

Y. Kalcheim, Camjayi, A., del Valle, J., Salev, P., Rozenberg, M., and Schuller, I. K., “Non-thermal resistive switching in Mott insulator nanowires”, vol. 11, no. 1, p. 2985, 2020. WebsiteAbstract

Resistive switching can be achieved in a Mott insulator by applying current/voltage, which triggers an insulator-metal transition (IMT). This phenomenon is key for understanding IMT physics and developing novel memory elements and brain-inspired technology. Despite this, the roles of electric field and Joule heating in the switching process remain controversial. Using nanowires of two archetypal Mott insulators—VO2 and V2O3 we unequivocally show that a purely non-thermal electrical IMT can occur in both materials. The mechanism behind this effect is identified as field-assisted carrier generation leading to a doping driven IMT. This effect can be controlled by similar means in both VO2 and V2O3, suggesting that the proposed mechanism is generally applicable to Mott insulators. The energy consumption associated with the non-thermal IMT is extremely low, rivaling that of state-of-the-art electronics and biological neurons. These findings pave the way towards highly energy-efficient applications of Mott insulators.

N. Ehlen, et al., “Origin of the Flat Band in Heavily Cs-Doped Graphene”, ACS Nano, vol. 14, p. 1055-1069, 2020. Website

G. Bareigts, et al., “Packing Polydisperse Colloids into Crystals: When Charge-Dispersity Matters”, Phys. Rev. Lett., vol. 124, p. 058003, 2020. Website

A. } {Jagannathan and {Tarzia, M. }, “Re-entrance and localization phenomena in disordered Fibonacci chains - Disorder induced crossover from critical states to Anderson localized states”, Eur. Phys. J. B, vol. 93, p. 46, 2020. Website

S. Mar\'ın-Aguilar, Wensink, H. H., Foffi, G., and Smallenburg, F., “Tetrahedrality Dictates Dynamics in Hard Sphere Mixtures”, Phys. Rev. Lett., vol. 124, p. 208005, 2020. Website

B. Loret, et al., “Universal relationship between the energy scales of the pseudogap phase, the superconducting state, and the charge-density-wave order in copper oxide superconductors”, Phys. Rev. B, vol. 101, p. 214520, 2020. Website

2019

A. M. R. V. L. Monteiro, et al., “Band inversion driven by electronic correlations at the (111) ${\mathrm{LaAlO}}_{3}/{\mathrm{SrTiO}}_{3}$ interface”, Phys. Rev. B, vol. 99, p. 201102, 2019. Website

N. Manca, et al., “Bimodal Phase Diagram of the Superfluid Density in LaAlO3/SrTiO3 Revealed by an Interfacial Waveguide Resonator”, PHYSICAL REVIEW LETTERS, vol. 122, p. 036801, 2019.Abstract

We explore the superconducting phase diagram of the two-dimensional electron system at the LaAlO3/SrTiO3 interface by monitoring the frequencies of the cavity modes of a coplanar waveguide resonator fabricated in the interface itself. We determine the phase diagram of the superconducting transition as a function of the temperature and electrostatic gating, finding that both the superfluid density and the transition temperature follow a dome shape but that the two are not monotonically related. The ground state of this two-dimensional electron system is interpreted as a Josephson junction array, where a transition from long-to short-range order occurs as a function of the electronic doping. The synergy between correlated oxides and superconducting circuits is revealed to be a promising route to investigate these exotic compounds, complementary to standard magnetotransport measurements.

M. Trif and Simon, P., “Braiding of Majorana Fermions in a Cavity”, Phys. Rev. Lett., vol. 122, p. 236803, 2019. Website

A. Hichri, Jaziri, S., and Goerbig, M. O., “Charged excitons in two-dimensional transition metal dichalcogenides: Semiclassical calculation of Berry curvature effects”, Phys. Rev. B, vol. 100, p. 115426, 2019. Website

S. Chen, et al., “Competing Fractional Quantum Hall and Electron Solid Phases in Graphene”, PHYSICAL REVIEW LETTERS, vol. 122, p. 026802, 2019.Abstract

We report experimental observation of the reentrant integer quantum Hall effect in graphene, appearing in the N = 2 Landau level. Similar to high-mobility GaAs/AlGaAs heterostructures, the effect is due to a competition between incompressible fractional quantum Hall states, and electron solid phases. The tunability of graphene allows us to measure the B-T phase diagram of the electron solid phase. The hierarchy of reentrant states suggests spin and valley degrees of freedom play a role in determining the ground state energy. We find that the melting temperature scales with magnetic field, and construct a phase diagram of the electron liquid-solid transition.

A. Borin, Safi, I., and Sukhorukov, E., “Coulomb drag effect induced by the third cumulant of current”, Phys. Rev. B, vol. 99, p. 165404, 2019. Website

B. Sung, Wensink, H. H., and Grelet, E., “Depletion-driven morphological transitions in hexagonal crystallites of virus rods”, Soft Matter, vol. 15, p. 9520-9527, 2019. WebsiteAbstract

The assembly of nanometer-sized building blocks into complex morphologies is not only of fundamental interest but also plays a key role in material science and nanotechnology. We show that the shape of self-assembled superstructures formed by rod-like viruses can be controlled by tuning the attraction via the depletion interaction between the rods. Using non-adsorbing polymers as a depleting agent{,} we demonstrate that a hierarchical unidimensional self-organization into crystalline clusters emerges progressively upon increasing depletion attraction and enhanced growth kinetics. We observe a polymorphic change proceeding from two-dimensional (2D) crystalline monolayers at weak depletion to one-dimensional (1D) columnar fibers at strong depletion{,} via the formation of smectic fibrils at intermediate depletion strength. A simple theory for reversible polymerization enables us to determine the typical bond energy between monomeric units making up the smectic fibrils. We also demonstrate that gentle flow-assistance can be used to template filament-like structures into highly aligned supported films. Our results showcase a generic bottom-up approach for tuning the morphology of crystalline superstructures through modification of the interaction between non-spherical building blocks. This provides a convenient pathway for controlling self-organization{,} dimensionality and structure-formation of anisotropic nanoparticles for use in nanotechnology and functional materials.

A. A. Zyuzin and Simon, P., “Disorder-induced exceptional points and nodal lines in Dirac superconductors”, Phys. Rev. B, vol. 99, p. 165145, 2019. Website

I. Safi, “Driven quantum circuits and conductors: A unifying perturbative approach”, PHYSICAL REVIEW B, vol. 99, p. 045101, 2019.Abstract

We develop and exploit an out-of-equilibrium theory, valid in arbitrary dimensions, which does not require initial thermalization. It is perturbative with respect to a weak time-dependent (TD) Hamiltonian term, but is nonperturbative with respect to strong coupling to an electromagnetic environment or to Coulomb or superconducting correlations. We derive unifying relations between the current generated by coherent radiation or statistical mixture of radiations, superimposed on a dc voltage V-dc, and the out-of-equilibrium dc current which encodes the effects of interactions. We extend fully the lateral band-transmission picture, thus quantum superposition, to coherent many-body correlated states. This provides methods for a determination of the carrier's charge q free from unknown parameters through the robustness of the Josephson-like frequency. We have derived similar relations for noise (I. Safi, arXiv:1401.5950) which have been exploited, recently, to determine the fractional charge in the fractional quantum Hall effect (FQHE) within the Jain series {[}M. Kapfer et al., Science (to be published)]. The present theory allows for breakdown of inversion symmetry and for asymmetric rates for emission and absorption of radiations. This generates rectification exploited here to propose methods to measure the charge q, as well as spectroscopical analysis of the out-of-equilibrium dc current and the third cumulant of non-Gaussian source of noise. We also apply the theory to the Tomonaga-Luttinger liquid (TLL), showing a counterintuitive feature: A Lorentzian pulse superimposed on V-dc can reduce the current compared to its dc value, at the same V-dc, questioning the terminology ``photoassisted.{''} Beyond a charge current, the theory applies to operators such as spin current in the spin Hall effect or voltage drop across a phase-slip Josephson junction.

R. Botet and Kuratsuji, H., “The duality between a non-Hermitian two-state quantum system and a massless charged particle”, JOURNAL OF PHYSICS A-MATHEMATICAL AND THEORETICAL, vol. 52, p. 035303, 2019.Abstract

We show that the equations for the dynamics of a non-Hermitian two-state quantum system arc the same as the equations of motion for a massless charged particle in an electromagnetic field. Using simple analytical arguments to prove this unexpected duality between two very different domains in physics, we further exemplify it through a case-study of polarization of light propagating in a dichroic medium with magneto-optic activity.

H. H. Wensink, “Effect of Size Polydispersity on the Pitch of Nanorod Cholesterics”, Crystals, vol. 9, 2019. WebsiteAbstract

Many nanoparticle-based chiral liquid crystals are composed of polydisperse rod-shaped particles with considerable spread in size or shape, affecting the mesoscale chiral properties in, as yet, unknown ways. Using an algebraic interpretation of Onsager-Straley theory for twisted nematics, we investigate the role of length polydispersity on the pitch of nanorod-based cholesterics with a continuous length polydispersity, and find that polydispersity enhances the twist elastic modulus, K 2 , of the cholesteric material without affecting the effective helical amplitude, K t . In addition, for the infinitely large average aspect ratios considered here, the dependence of the pitch on the overall rod concentration is completely unaffected by polydispersity. For a given concentration, the increase in twist elastic modulus (and reduction of the helical twist) may be up to 50% for strong size polydispersity, irrespective of the shape of the unimodal length distribution. We also demonstrate that the twist reduction is reinforced in bimodal distributions, obtained by doping a polydisperse cholesteric with very long rods. Finally, we identify a subtle, non-monotonic change of the pitch across the isotropic-cholesteric biphasic region.

S. Brazovskii and Kirova, N., “From chiral anomaly to two-fluid hydrodynamics for electronic vortices”, Annals of Physics, vol. 403, p. 184 - 197, 2019. WebsiteAbstract

Many recent experiments addressed manifestations of electronic crystals, particularly the charge density waves, in nano-junctions, under electric field effect, at high magnetic fields, together with real space visualizations by STM and micro X-ray diffraction. This activity returns the interest to stationary or transient states with static and dynamic topologically nontrivial configurations: electronic vortices as dislocations, instantons as phase slip centers, and ensembles of microscopic solitons. Describing and modeling these states and processes calls for an efficient phenomenological theory which should take into account the degenerate order parameter, various kinds of normal carriers and the electric field. Here we notice that the commonly employed time-depend Ginzburg–Landau approach suffers with violation of the charge conservation law resulting in unphysical generation of particles which is particularly strong for nucleating or moving electronic vortices. We present a consistent theory which exploits the chiral transformations taking into account the principle contribution of the fermionic chiral anomaly to the effective action. The resulting equations clarify partitions of charges, currents and rigidity among subsystems of the condensate and normal carriers. On this basis we perform the numerical modeling of a spontaneously generated coherent sequence of phase slips – the spacetime vortices – serving for the conversion among the injected normal current and the collective one.

B. Loret, et al., “Intimate link between charge density wave, pseudogap and superconducting energy scales in cuprates”, Nature Physics, vol. 15, p. 771-775, 2019. WebsiteAbstract

The cuprate high-temperature superconductors develop spontaneous charge density wave (CDW) order below a temperature TCDW and over a wide range of hole doping (p). An outstanding challenge in the field is to understand whether this modulated phase is related to the more exhaustively studied pseudogap and superconducting phases1,2. To address this issue, it is important to extract the energy scale DCDW associated with the CDW order, and to compare it with the pseudogap DPG and with the superconducting gap DSC. However, while TCDW is well characterized from earlier work3, little is currently known about DCDW. Here, we report the extraction of DCDW for several cuprates using electronic Raman spectroscopy. We find that on approaching the parent Mott state by lowering p, DCDW increases in a manner similar to the doping dependence of DPG and DSC. This reveals that these three phases have a common microscopic origin. In addition, we find that DCDW [?] DSC over a substantial doping range, which suggests that CDW and superconducting phases are intimately related; for example, they may be intertwined or connected by an emergent symmetry1,4-9.

G. C. Ménard, et al., “Isolated pairs of Majorana zero modes in a disordered superconducting lead monolayer”, Nature Communications, vol. 10, p. 2587, 2019. WebsiteAbstract

Majorana zero modes are fractional quantum excitations appearing in pairs, each pair being a building block for quantum computation. Some signatures of Majorana zero modes have been reported at endpoints of one-dimensional systems, which are however required to be extremely clean. An alternative are two-dimensional topological superconductors, such as the Pb/Co/Si(111) system shown recently to be immune to local disorder. Here, we use scanning tunneling spectroscopy to characterize a disordered superconducting monolayer of Pb coupled to underlying Co-Si magnetic islands. We show that pairs of zero modes are stabilized: one zero mode positioned in the middle of the magnetic domain and its partner extended all around the domain. The zero mode pair is remarkably robust, isolated within a hard superconducting gap. Our theoretical scenario supports the protected Majorana nature of this zero mode pair, highlighting the role of magnetic or spin-orbit coupling textures.

J. - N. Fuchs, Mosseri, R., and Vidal, J., “Landau level broadening, hyperuniformity, and discrete scale invariance”, Phys. Rev. B, vol. 100, p. 125118, 2019. Website

S. Rostamzadeh, Adagideli, ıfmmode \dot{I. }\else İ. \fi{}nanıfmmode \mboxç\else ç\fi{}, and Goerbig, M. O., “Large enhancement of conductivity in Weyl semimetals with tilted cones: Pseudorelativity and linear response”, Phys. Rev. B, vol. 100, p. 075438, 2019. Website

Y. Zhang, et al., “Machine learning in electronic-quantum-matter imaging experiments”, Nature, vol. 570, p. 484-490, 2019. WebsiteAbstract

For centuries, the scientific discovery process has been based on systematic human observation and analysis of natural phenomena1. Today, however, automated instrumentation and large-scale data acquisition are generating datasets of such large volume and complexity as to defy conventional scientific methodology. Radically different scientific approaches are needed, and machine learning (ML) shows great promise for research fields such as materials science2-5. Given the success of ML in the analysis of synthetic data representing electronic quantum matter (EQM)6-16, the next challenge is to apply this approach to experimental data–for example, to the arrays of complex electronic-structure images17 obtained from atomic-scale visualization of EQM. Here we report the development and training of a suite of artificial neural networks (ANNs) designed to recognize different types of order hidden in such EQM image arrays. These ANNs are used to analyse an archive of experimentally derived EQM image arrays from carrier-doped copper oxide Mott insulators. In these noisy and complex data, the ANNs discover the existence of a lattice-commensurate, four-unit-cell periodic, translational-symmetry-breaking EQM state. Further, the ANNs determine that this state is unidirectional, revealing a coincident nematic EQM state. Strong-coupling theories of electronic liquid crystals18,19 are consistent with these observations.

X. } {Lu and {Goerbig, M. O. }, “Magneto-optical signatures of Volkov-Pankratov states in topological insulators”, EPL, vol. 126, p. 67004, 2019. Website

X. Lu and Goerbig, M. O., “Magneto-optical signatures of Volkov-Pankratov states in topological insulators”, {EPL} (Europhysics Letters), vol. 126, p. 67004, 2019. WebsiteAbstract

In addition to the usual chiral surface states, massive surface states can arise at a smooth interface between a topological and a trivial bulk insulator. While not subject to topological protection as the chiral states, these massive states, theorized by Volkov and Pankratov in the 1980s, reflect nevertheless emergent Dirac physics at the interface. We study theoretically the magneto-optical response of these surface states, which is strikingly different from that of the bulk states. Most saliently, we show that these states can be identified clearly in the presence of a magnetic field and its orientation with respect to the interface.

D. C. Vaz, et al., “Mapping spin–charge conversion to the band structure in a topological oxide two-dimensional electron gas”, vol. 18, no. 11, p. 1187 - 1193, 2019. WebsiteAbstract

While spintronics has traditionally relied on ferromagnetic metals as spin generators and detectors, spin–orbitronics exploits the efficient spin–charge interconversion enabled by spin–orbit coupling in non-magnetic systems. Although the Rashba picture of split parabolic bands is often used to interpret such experiments, it fails to explain the largest conversion effects and their relationship with the electronic structure. Here, we demonstrate a very large spin-to-charge conversion effect in an interface-engineered, high-carrier-density SrTiO3 two-dimensional electron gas and map its gate dependence on the band structure. We show that the conversion process is amplified by enhanced Rashba-like splitting due to orbital mixing and in the vicinity of avoided band crossings with topologically non-trivial order. Our results indicate that oxide two-dimensional electron gases are strong candidates for spin-based information readout in new memory and transistor designs. Our results also emphasize the promise of topology as a new ingredient to expand the scope of complex oxides for spintronics.

R. Bisognin, et al., “Microwave photons emitted by fractionally charged quasiparticles”, Nature Communications, vol. 10, no. 1, p. 1708, 2019. WebsiteAbstract

Strongly correlated low-dimensional systems can host exotic elementary excitations carrying a fractional charge q and potentially obeying anyonic statistics. In the fractional quantum Hall effect, their fractional charge has been successfully determined owing to low frequency shot noise measurements. However, a universal method for sensing them unambiguously and unraveling their intricate dynamics was still lacking. Here, we demonstrate that this can be achieved by measuring the microwave photons emitted by such excitations when they are transferred through a potential barrier biased with a dc voltage Vdc. We observe that only photons at frequencies f below qVdc/h are emitted. This threshold provides a direct and unambiguous determination of the charge q, and a signature of exclusion statistics. Derived initially within the Luttinger model, this feature is also predicted by universal non-equilibrium fluctuation relations which agree fully with our measurements. Our work paves the way for further exploration of anyonic statistics using microwave measurements.

R. Bisognin, et al., “Microwave photons emitted by fractionally charged quasiparticles”, Nature Communications, vol. 10, p. 1708, 2019. WebsiteAbstract

Strongly correlated low-dimensional systems can host exotic elementary excitations carrying a fractional charge q and potentially obeying anyonic statistics. In the fractional quantum Hall effect, their fractional charge has been successfully determined owing to low frequency shot noise measurements. However, a universal method for sensing them unambiguously and unraveling their intricate dynamics was still lacking. Here, we demonstrate that this can be achieved by measuring the microwave photons emitted by such excitations when they are transferred through a potential barrier biased with a dc voltage Vdc. We observe that only photons at frequencies f below qVdc/h are emitted. This threshold provides a direct and unambiguous determination of the charge q, and a signature of exclusion statistics. Derived initially within the Luttinger model, this feature is also predicted by universal non-equilibrium fluctuation relations which agree fully with our measurements. Our work paves the way for further exploration of anyonic statistics using microwave measurements.

A. Jagannathan, Jeena, P., and Tarzia, M., “Nonmonotonic crossover and scaling behavior in a disordered one-dimensional quasicrystal”, PHYSICAL REVIEW B, vol. 99, p. 054203, 2019.Abstract

We consider a noninteracting disordered 1D quasicrystal in the weak-disorder regime. We show that the critical states of the pure model approach strong localization in strikingly different ways, depending on their renormalization properties. A finite-size scaling analysis of the inverse participation ratios of states (IPR) of the quasicrystal shows that they are described by several kinds of scaling functions. While most states show a progressively increasing IPR as a function of the scaling variable, other states exhibit a nonmonotonic ``reentrant{''} behavior wherein the IPR first decreases, and passes through a minimum, before increasing. This surprising behavior is explained in the framework of perturbation renormalization group treatment, where wave functions can be computed analytically as a function of the hopping amplitude ratio and the disorder, however it is not specific to this model. Our results should help to clarify results of recent studies of localization due to random and quasiperiodic potentials.

H. H. Wensink, “Polymeric Nematics of Associating Rods: Phase Behavior, Chiral Propagation, and Elasticity”, MacromoleculesMacromolecules, vol. 52, no. 21, p. 7994 - 8005, 2019. Website

G. Rai, Haas, S., and Jagannathan, A., “Proximity effect in a superconductor-quasicrystal hybrid ring”, Phys. Rev. B, vol. 100, p. 165121, 2019. Website

V. Kaladzhyan, Zyuzin, A. A., and Simon, P., “RKKY interaction on the surface of three-dimensional Dirac semimetals”, Phys. Rev. B, vol. 99, p. 165302, 2019. Website

S. Marín-Aguilar, Wensink, H. H., Foffi, G., and Smallenburg, F., “Slowing down supercooled liquids by manipulating their local structure”, Soft Matter, vol. 15, p. 9886-9893, 2019. WebsiteAbstract

Glasses remain an elusive and poorly understood state of matter. It is not clear how we can control the macroscopic dynamics of glassy systems by tuning the properties of their microscopic building blocks. In this paper{,} we propose a simple directional colloidal model that reinforces the optimal icosahedral local structure of binary hard-sphere glasses. We show that this specific symmetry results in a dramatic slowing down of the dynamics. Our results open the door to controlling the dynamics of dense glassy systems by selectively promoting specific local structural environments.

S. Gariglio, Caviglia, A. D., Triscone, J. - M., and Gabay, M., “A spin-orbit playground: surfaces and interfaces of transition metal oxides”, REPORTS ON PROGRESS IN PHYSICS, vol. 82, p. 012501, 2019.Abstract

Within the last twenty years, the status of the spin-orbit interaction has evolved from that of a simple atomic contribution to a key effect that modifies the electronic band structure of materials. It is regarded as one of the basic ingredients for spintronics, locking together charge and spin degrees of freedom and recently it is instrumental in promoting a new class of compounds, the topological insulators. In this review, we present the current status of the research on the spin-orbit coupling in transition metal oxides, discussing the case of two semiconducting compounds, SrTiO3 and KTaO3, and the properties of surface and interfaces based on these. We conclude with the investigation of topological effects predicted to occur in different complex oxides.

J. del Valle, et al., “Subthreshold firing in Mott nanodevices”, Nature, vol. 569, p. 388-392, 2019. WebsiteAbstract

Resistive switching, a phenomenon in which the resistance of a device can be modified by applying an electric field1-5, is at the core of emerging technologies such as neuromorphic computing and resistive memories6-9. Among the different types of resistive switching, threshold firing10-14 is one of the most promising, as it may enable the implementation of artificial spiking neurons7,13,14. Threshold firing is observed in Mott insulators featuring an insulator-to-metal transition15,16, which can be triggered by applying an external voltage: the material becomes conducting ('fires') if a threshold voltage is exceeded7,10-12. The dynamics of this induced transition have been thoroughly studied, and its underlying mechanism and characteristic time are well documented10,12,17,18. By contrast, there is little knowledge regarding the opposite transition: the process by which the system returns to the insulating state after the voltage is removed. Here we show that Mott nanodevices retain a memory of previous resistive switching events long after the insulating resistance has recovered. We demonstrate that, although the device returns to its insulating state within 50 to 150?nanoseconds, it is possible to re-trigger the insulator-to-metal transition by using subthreshold voltages for a much longer time (up to several milliseconds). We find that the intrinsic metastability of first-order phase transitions is the origin of this phenomenon, and so it is potentially present in all Mott systems. This effect constitutes a new type of volatile memory in Mott-based devices, with potential applications in resistive memories, solid-state frequency discriminators and neuromorphic circuits.

B. Brun, et al., “Thermoelectric Scanning-Gate Interferometry on a Quantum Point Contact”, Phys. Rev. Applied, vol. 11, p. 034069, 2019. Website

M. Garnier, Mesaros, A., and Simon, P., “Topological superconductivity with deformable magnetic skyrmions”, Communications Physics, vol. 2, p. 126, 2019. WebsiteAbstract

Magnetic skyrmions are nanoscale spin configurations that are efficiently created and manipulated. They hold great promises for next-generation spintronics applications. In parallel, the interplay of magnetism, superconductivity and spin-orbit coupling has proved to be a versatile platform for engineering topological superconductivity predicted to host non-abelian excitations, Majorana zero modes. We show that topological superconductivity can be induced by proximitizing skyrmions and conventional superconductors, without need for additional ingredients. Apart from a previously reported Majorana zero mode in the core of the skyrmion, we find a more universal chiral band of Majorana modes on the edge of the skyrmion. We show that the chiral Majorana band is effectively flat in the physically relevant parameter regime, leading to interesting robustness and scaling properties. In particular, the number of Majorana modes in the (nearly-)flat band scales with the perimeter length of the system, while being robust to local disorder.

M. Miliıfmmode \acute{c}\else ć\fi{}eviıfmmode \acute{c}\else ć\fi{}, et al., “Type-III and Tilted Dirac Cones Emerging from Flat Bands in Photonic Orbital Graphene”, Phys. Rev. X, vol. 9, p. 031010, 2019. Website

M. J. Rozenberg, Schneegans, O., and Stoliar, P., “An ultra-compact leaky-integrate-and-fire model for building spiking neural networks”, Scientific Reports, vol. 9, p. 11123, 2019. WebsiteAbstract

We introduce an ultra-compact electronic circuit that realizes the leaky-integrate-and-fire model of artificial neurons. Our circuit has only three active devices, two transistors and a silicon controlled rectifier (SCR). We demonstrate the implementation of biologically realistic features, such as spike-frequency adaptation, a refractory period and voltage modulation of spiking rate. All characteristic times can be controlled by the resistive parameters of the circuit. We built the circuit with out-of-the-shelf components and demonstrate that our ultra-compact neuron is a modular block that can be associated to build multi-layer deep neural networks. We also argue that our circuit has low power requirements, as it is normally off except during spike generation. Finally, we discuss the ultimate ultra-compact limit, which may be achieved by further replacing the SCR circuit with Mott materials.

G. C. Ménard, et al., “Yu-Shiba-Rusinov bound states versus topological edge states in Pb/Si(111)”, The European Physical Journal Special Topics, vol. 227, p. 2303–2313, 2019. WebsiteAbstract

There is presently a tremendous activity around the field of topological superconductivity and Majorana fermions. Among the many questions raised, it has become increasingly important to establish the topological or non-topological origin of features associated with Majorana fermions such as zero-bias peaks. Here, we compare in-gap features associated either with isolated magnetic impurities or with magnetic clusters strongly coupled to the atomically thin superconductor Pb/Si(111). We study this system by means of scanning tunneling microscopy and spectroscopy (STM/STS). We take advantage of the fact that the Pb/Si(111) monolayer can exist either in a crystal-ordered phase or in an incommensurate disordered phase to compare the observed spectroscopic features in both phases. This allows us to demonstrate that the strongly resolved in-gap states we found around the magnetic clusters in the disordered phase of Pb have a clear topological origin.

2018

G. Montambaux, “Artificial graphenes: Dirac matter beyond condensed matter”, COMPTES RENDUS PHYSIQUE, vol. 19, p. 285-305, 2018.Abstract

After the discovery of graphene and of its many fascinating properties, there has been a growing interest for the study of ``artificial graphenes{''}. These are totally different and novel systems that bear exciting similarities with graphene. Among them are lattices of ultracold atoms, microwave or photonic lattices, ``molecular graphene{''} or new compounds like phosphorene. The advantage of these structures is that they serve as new playgrounds for measuring and testing physical phenomena that may not be reachable in graphene, in particular the possibility of controlling the existence of Dirac points (or Dirac cones) existing in the electronic spectrum of graphene, of performing interference experiments in reciprocal space, of probing geometrical properties of the wave functions, of manipulating edge states, etc. These cones, which describe the band structure in the vicinity of the two connected energy bands, are characterized by a topological ``charge{''}. They can be moved in the reciprocal space by appropriate modification of external parameters (pressure, twist, sliding, stress, etc.). They can be manipulated, created or suppressed under the condition that the total topological charge be conserved. In this short review, I discuss several aspects of the scenarios of merging or emergence of Dirac points as well as the experimental investigations of these scenarios in condensed matter and beyond. (C) 2018 Published by Elsevier Masson SAS on behalf of Academie des sciences.

J. del Valle, Gabriel Ramirez, J., Rozenberg, M. J., and Schuller, I. K., “Challenges in materials and devices for resistive-switching-based neuromorphic computing”, JOURNAL OF APPLIED PHYSICS, vol. 124, p. 211101, 2018.Abstract

This tutorial describes challenges and possible avenues for the implementation of the components of a solid-state system, which emulates a biological brain. The tutorial is devoted mostly to a charge-based (i.e. electric controlled) implementation using transition metal oxide materials, which exhibit unique properties that emulate key functionalities needed for this application. In Sec. I, we compare the main differences between a conventional computational machine, based on the Turing-von Neumann paradigm, and a neuromorphic machine, which tries to emulate important functionalities of a biological brain. We also describe the main electrical properties of biological systems, which would be useful to implement in a charge-based system. In Sec. II, we describe the main components of a possible solid-state implementation. In Sec. III, we describe a variety of Resistive Switching phenomena, which may serve as the functional basis for the implementation of key devices for neuromorphic computing. In Sec. IV, we describe why transition metal oxides are promising materials for future neuromorphic machines. Theoretical models describing different resistive switching mechanisms are discussed in Sec. V, while existing implementations are described in Sec. VI. Section VII presents applications to practical problems. We list in Sec. VIII important basic research challenges and open issues. We discuss issues related to specific implementations, novel materials, devices, and phenomena. The development of reliable, fault tolerant, energy efficient devices, their scaling, and integration into a neuromorphic computer may bring us closer to the development of a machine that rivals the brain. Published by AIP Publishing.

H. Braganca, Sakai, S., Aguiar, M. C. O., and Civelli, M., “Correlation-Driven Lifshitz Transition at the Emergence of the Pseudogap Phase in the Two-Dimensional Hubbard Model”, PHYSICAL REVIEW LETTERS, vol. 120, p. 067002, 2018.Abstract

We study the relationship between the pseudogap and Fermi-surface topology in the two-dimensional Hubbard model by means of the cellular dynamical mean-field theory. We find two possible mean-field metallic solutions on a broad range of interactions, doping, and frustration: a conventional renormalized metal and an unconventional pseudogap metal. At half filling, the conventional metal is more stable and displays an interaction-driven Mott metal-insulator transition. However, for large interactions and small doping, a region that is relevant for cuprates, the pseudogap phase becomes the ground state. By increasing doping, we show that a first-order transition from the pseudogap to the conventional metal is tied to a change of the Fermi surface from hole- to electronlike, unveiling a correlation-driven mechanism for a Lifshitz transition. This explains the puzzling link between the pseudogap phase and Fermi surface topology that has been pointed out in recent experiments.

S. Sakai, Civelli, M., and Imada, M., “Direct connection between Mott insulators and d-wave high-temperature superconductors revealed by continuous evolution of self-energy poles”, PHYSICAL REVIEW B, vol. 98, p. 195109, 2018.Abstract

The high-temperature superconductivity in copper oxides emerges when carriers are doped into the parent Mott insulator. This well-established fact has, however, eluded a microscopic explanation. Here we show that the missing link is the self-energy pole in the energy-momentum space. Its continuous evolution with doping directly connects the Mott insulator and high-temperature superconductivity. We show this by numerically studying the extremely small doping region close to the Mott insulating phase in a standard model for cuprates, the two-dimensional Hubbard model. We first identify two relevant self-energy structures in the Mott insulator: the pole generating the Mott gap and a relatively broad peak generating the so-called waterfall structure, which is another consequence of strong correlations present in the Mott insulator. We next reveal that either the Mott-gap pole or the waterfall structure (the feature at the energy closer to the Fermi level) directly transforms itself into another self-energy pole at the same energy and momentum when the system is doped with carriers. The anomalous self-energy yielding the superconductivity is simultaneously born exactly at this energy-momentum point. Thus created self-energy pole, interpreted as arising from a hidden fermionic excitation, continuously evolves upon further doping and considerably enhances the superconductivity. Above the critical temperature, the same self-energy pole generates a pseudogap in the normal state. We thus elucidate a unified Mott-physics mechanism, where the self-energy structure inherent to the Mott insulator directly gives birth to both the high critical superconducting temperature and pseudogap.

V. S. Nguyen, et al., “Direct Evidence of Lithium Ion Migration in Resistive Switching of Lithium Cobalt Oxide Nanobatteries”, SMALL, vol. 14, p. 1801038, 2018.Abstract

Lithium cobalt oxide nanobatteries offer exciting prospects in the field of nonvolatile memories and neuromorphic circuits. However, the precise underlying resistive switching (RS) mechanism remains a matter of debate in two-terminal cells. Herein, intriguing results, obtained by secondary ion mass spectroscopy (SIMS) 3D imaging, clearly demonstrate that the RS mechanism corresponds to lithium migration toward the outside of the LixCoO2 layer. These observations are very well correlated with the observed insulator-to-metal transition of the oxide. Besides, smaller device area experimentally yields much faster switching kinetics, which is qualitatively well accounted for by a simple numerical simulation. Write/erase endurance is also highly improved with downscaling - much further than the present cycling life of usual lithium-ion batteries. Hence very attractive possibilities can be envisaged for this class of materials in nanoelectronics.

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