# Publications

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.
, “Braiding of Majorana Fermions in a Cavity”, Phys. Rev. Lett., vol. 122, p. 236803, 2019. Website
, “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.
, “Coulomb drag effect induced by the third cumulant of current”, Phys. Rev. B, vol. 99, p. 165404, 2019. Website
, “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.
, “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.
, “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.
, “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.
, “Landau level broadening, hyperuniformity, and discrete scale invariance”, Phys. Rev. B, vol. 100, p. 125118, 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.
, “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, 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, 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.
, “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
, “Proximity effect in a superconductor-quasicrystal hybrid ring”, Phys. Rev. B, vol. 100, p. 165121, 2019. Website
, “RKKY interaction on the surface of three-dimensional Dirac semimetals”, Phys. Rev. B, vol. 99, p. 165302, 2019. Website
, “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.
, “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
, “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.
, “Type-III and Tilted Dirac Cones Emerging from Flat Bands in Photonic Orbital Graphene”, Phys. Rev. X, vol. 9, p. 031010, 2019. Website
, “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.
, “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.
, “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.
, “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.
, “Dynamic current susceptibility as a probe of Majorana bound states in nanowire-based Josephson junctions”, PHYSICAL REVIEW B, vol. 97, p. 041415, 2018.Abstract
We theoretically study a Josephson junction based on a semiconducting nanowire subject to a time-dependent flux bias. We establish a general density-matrix approach for the dynamical response of the Majorana junction and calculate the resulting flux-dependent susceptibility using both microscopic and effective low-energy descriptions for the nanowire. We find that the diagonal component of the susceptibility, associated with the dynamics of the Majorana state populations, dominates over the standard Kubo contribution for a wide range of experimentally relevant parameters. The diagonal term, explored, in this Rapid Communication, in the context of Majorana physics, allows probing accurately the presence of Majorana bound states in the junction.
, “Elastic moduli of a smectic membrane: a rod-level scaling analysis”, JOURNAL OF PHYSICS-CONDENSED MATTER, vol. 30, p. 075101, 2018.Abstract
Chiral rodlike colloids exposed to strong depletion attraction may self-assemble into chiral membranes whose twisted director field differs from that of a 3D bulk chiral nematic. We formulate a simple microscopic variational theory to determine the elastic moduli of rods assembled into a bidimensional smectic membrane. The approach is based on a simple Onsager-Straley theory for a non-uniform director field that we apply to describe rod twist within the membrane. A microscopic approach enables a detailed estimate of the individual Frank elastic moduli (splay, twist and bend) as well as the twist penetration depth of the smectic membrane in relation to the rod density and shape. We find that the elastic moduli are distinctly different from those of a bulk nematic fluid, with the splay elasticity being much stronger and the curvature elasticity much weaker than for rods assembled in a three-dimensional nematic fluid. We argue that the use of the simplistic one-constant approximation in which all moduli are assumed to be of equal magnitude is not appropriate for modelling the structure-property relation of smectic membranes.
M. Hakl, et al., “Energy scale of Dirac electrons in Cd3As2”, PHYSICAL REVIEW B, vol. 97, p. 115206, 2018.Abstract
Cadmium arsenide (Cd3As2) has recently became conspicuous in solid-state physics due to several reports proposing that it hosts a pair of symmetry-protected 3D Dirac cones. Despite vast investigations, a solid experimental insight into the band structure of this material is still missing. Here we fill one of the existing gaps in our understanding of Cd3As2, and based on our Landau-level spectroscopy study, we provide an estimate for the energy scale of 3D Dirac electrons in this system. We find that the appearance of such charge carriers is limited-contrary to a widespread belief in the solid-state community-to a relatively small energy scale (below 40 meV).
C. Adda, et al., “First demonstration of Leaky Integrate and Fire{''} artificial neuron behavior on (V0.95Cr0.05)(2)O-3 thin film”, MRS COMMUNICATIONS, vol. 8, p. 835-841, 2018.Abstract
A great challenge in the field of neurocomputing is to mimic the brain behavior by implementing artificial synapses and neurons directly in hardware. This work shows that a Leaky Integrate and Fire (LIF) artificial neuron can be realized with a two-terminal device made of Mott insulator thin films. Polycrystalline thin films of the well-known Mott insulator oxide (V0.95Cr0.05)(2)O-3 were deposited by magnetron sputtering and patterned with micron-scale TIN electrodes. These devices exhibit a volatile resistive switching and a remarkable LIF behavior under a train of pulses suggesting that LIF artificial neurons may be realized from (V0.95Cr0.05)(2)O-3 thin films.
H. H. Wensink, “Frank elasticity of composite colloidal nematics with anti-nematic order”, SOFT MATTER, vol. 14, p. 8935-8944, 2018.Abstract
Mixing colloid shapes with distinctly different anisotropy generates composite nematics in which the order of the individual components can be fundamentally different. In colloidal rod-disk mixtures or hybrid nematics composed of anisotropic colloids immersed in a thermotropic liquid crystal, one of the components may adopt so-called anti-nematic order while the other exhibits conventional nematic alignment. Focussing on simple models for hard rods and disks, we employ Onsager-Straley's second-virial theory to derive scaling expressions for the elastic moduli of rods and disks in both nematic and anti-nematic configurations and identify their explicit dependence on particle concentration and shape. We demonstrate that the splay, bend and twist elasticity of anti-nematically ordered particles scale logarithmically with the degree of anti-nematic order, with the bend-splay ratio for anti-nematic discotic nematics being far greater than for conventional nematic systems. The impact of surface anchoring on the elastic properties of hybrid nematics will also be discussed in detail. We further demonstrate that the elasticity of mixed uniaxial rod-disk nematics depends exquisitely on the shape of the components and we provide simple scaling expressions that could help engineer the elastic properties of composite nematic liquid crystals.
G. Montambaux, “Generalized Stefan-Boltzmann Law”, FOUNDATIONS OF PHYSICS, vol. 48, p. 395-410, 2018.Abstract
We reconsider the thermodynamic derivation by L. Boltzmann of the Stefan law and we generalize it for various different physical systems whose chemical potential vanishes. Being only based on classical arguments, therefore independent of the quantum statistics, this derivation applies as well to the saturated Bose gas in various geometries as to compensated{''} Fermi gas near a neutrality point, such as a gas of Weyl Fermions. It unifies in the same framework the thermodynamics of many different bosonic or fermionic non-interacting gases which were until now described in completely different contexts.
, “Hamiltonian theory for quantum Hall systems in a tilted magnetic field: Composite-fermion geometry and robustness of activation gaps”, PHYSICAL REVIEW B, vol. 98, p. 205150, 2018.Abstract
We use the Hamiltonian theory developed by Shankar and Murthy to study a quantum Hall system in a tilted magnetic field. With a finite width of the system in the z direction, the parallel component of the magnetic field introduces anisotropy into the effective two-dimensional interactions. The effects of such anisotropy can be effectively captured by the recently proposed generalized pseudopotentials. We find that the off-diagonal components of the pseudopotentials lead to mixing of composite fermions Landau levels, which is a perturbation to the picture of p filled Landau levels in composite-fermion theory. By changing the internal geometry of the composite fermions, such a perturbation can be minimized and one can find the corresponding activation gaps for different tilting angles, and we calculate the associated optimal metric. Our results show that the activation gap is remarkably robust against the in-plane magnetic field in the lowest and first Landau levels.
P. Vodnala, et al., “Hard-sphere-like dynamics in highly concentrated alpha-crystallin suspensions”, PHYSICAL REVIEW E, vol. 97, p. 020601, 2018.Abstract
The dynamics of concentrated suspensions of the eye-lens protein alpha crystallin have been measured using x-ray photon correlation spectroscopy. Measurements were made at wave vectors corresponding to the first peak in the hard-sphere structure factor and volume fractions close to the critical volume fraction for the glass transition. Langevin dynamics simulations were also performed in parallel to the experiments. The intermediate scattering function f (q, tau) could be fit using a stretched exponential decay for both experiments and numerical simulations. The measured relaxation times show good agreement with simulations for polydisperse hard-sphere colloids.
P. Diener, et al., “How a dc Electric Field Drives Mott Insulators Out of Equilibrium”, PHYSICAL REVIEW LETTERS, vol. 121, p. 016601, 2018.Abstract
Out of equilibrium phenomena are a major issue of modern physics. In particular, correlated materials such as Mott insulators experience fascinating long-lived exotic states under a strong electric field. Yet, the origin of their destabilization by the electric field is not elucidated. Here we present a comprehensive study of the electrical response of canonical Mott insulators GaM(4)Q(8) (M = V, Nb, Ta, Mo; Q = S, Se) in the context of a microscopic theory of electrical breakdown where in-gap states allow for a description in terms of a two-temperature model. Our results show how the nonlinearities and the resistive transition originate from a massive creation of hot electrons under an electric field. These results give new insights for the control of the long-lived states reached under an electric field in these systems which has recently open the way to new functionalities used in neuromorphic applications.
, “Hybrid molecular-colloidal liquid crystals”, SCIENCE, vol. 360, p. 768-771, 2018.Abstract
Order and fluidity often coexist, with examples ranging from biological membranes to liquid crystals, but the symmetry of these soft-matter systems is typically higher than that of the constituent building blocks. We dispersed micrometer-long inorganic colloidal rods in a nematic liquid crystalline fluid of molecular rods. Both types of uniaxial building blocks, while freely diffusing, interact to form an orthorhombic nematic fluid, in which like-sized rods are roughly parallel to each other and the molecular ordering direction is orthogonal to that of colloidal rods. A coarse-grained model explains the experimental temperature-concentration phase diagram with one biaxial and two uniaxial nematic phases, as well as the orientational distributions of rods. Displaying properties of biaxial optical crystals, these hybrid molecular-colloidal fluids can be switched by electric and magnetic fields.
H. T. Stinson, et al., “Imaging the nanoscale phase separation in vanadium dioxide thin films at terahertz frequencies”, NATURE COMMUNICATIONS, vol. 9, p. 3604, 2018.Abstract
Vanadium dioxide (VO2) is a material that undergoes an insulator-metal transition upon heating above 340 K. It remains debated as to whether this electronic transition is driven by a corresponding structural transition or by strong electron-electron correlations. Here, we use apertureless scattering near-field optical microscopy to compare nanoscale images of the transition in VO2 thin films acquired at both mid-infrared and terahertz frequencies, using a home-built terahertz near-field microscope. We observe a much more gradual transition when THz frequencies are utilized as a probe, in contrast to the assumptions of a classical first-order phase transition. We discuss these results in light of dynamical mean-field theory calculations of the dimer Hubbard model recently applied to VO2, which account for a continuous temperature dependence of the optical response of the VO2 in the insulating state.
, “Landau levels in quasicrystals”, PHYSICAL REVIEW B, vol. 98, p. 165427, 2018.Abstract
Two-dimensional tight-binding models for quasicrystals made of plaquettes with commensurate areas are considered. Their energy spectrum is computed as a function of an applied perpendicular magnetic field. Landau levels are found to emerge near band edges in the zero-field limit. Their existence is related to an effective zero-field dispersion relation valid in the continuum limit. For quasicrystals studied here, an underlying periodic crystal exists and provides a natural interpretation to this dispersion relation. In addition to the slope (effective mass) of Landau levels, we also study their width as a function of the magnetic flux and identify two fundamental broadening mechanisms: (i) tunneling between closed cyclotron orbits and (ii) individual energy displacement of states within a Landau level. Interestingly, the typical broadening of the Landau levels is found to behave algebraically with the magnetic field with a nonuniversal exponent.