# Publications

2018

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.

J. - N. Fuchs, Mosseri, R., and Vidal, J., “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.

J. - N. Fuchs, Piechon, F., and Montambaux, G., “Landau levels, response functions and magnetic oscillations from a generalized Onsager relation”, SCIPOST PHYSICS, vol. 4, p. 024, 2018.Abstract

A generalized semiclassical quantization condition for cyclotron orbits was recently proposed by Gao and Niu {[}1], that goes beyond the Onsager relation {[}2]. In addition to the integrated density of states, it formally involves magnetic response functions of all orders in the magnetic field. In particular, up to second order, it requires the knowledge of the spontaneous magnetization and the magnetic susceptibility, as was early anticipated by Roth {[}3]. We study three applications of this relation focusing on two-dimensional electrons. First, we obtain magnetic response functions from Landau levels. Second we obtain Landau levels from response functions. Third we study magnetic oscillations in metals and propose a proper way to analyze Landau plots (i.e. the oscillation index n as a function of the inverse magnetic field 1 = B) in order to extract quantities such as a zero-field phase-shift. Whereas the frequency of 1 = B-oscillations depends on the zero-field energy spectrum, the zero-field phase-shift depends on the geometry of the cell-periodic Bloch states via two contributions: the Berry phase and the average orbital magnetic moment on the Fermi surface. We also quantify deviations from linearity in Landau plots (i.e. aperiodic magnetic oscillations), as recently measured in surface states of three-dimensional topological insulators and emphasized by Wright and McKenzie {[}4].

W. Yang, et al., “Landau Velocity for Collective Quantum Hall Breakdown in Bilayer Graphene”, PHYSICAL REVIEW LETTERS, vol. 121, p. 136804, 2018.Abstract

Breakdown of the quantum Hall effect (QHE) is commonly associated with an electric field approaching the inter-Landau-level (LL) Zener field, the ratio of the Landau gap and the cyclotron radius. Eluded in semiconducting heterostructures, in spite of extensive investigation, the intrinsic Zener limit is reported here using high-mobility bilayer graphene and high-frequency current noise. We show that collective excitations arising from electron-electron interactions are essential. Beyond a noiseless ballistic QHE regime a large super-Poissonian shot noise signals the breakdown via inter-LL scattering. The breakdown is ultimately limited by collective excitations in a regime where phonon and impurity scattering are quenched. The breakdown mechanism can be described by a Landau critical velocity as it bears strong similarities with the roton mechanism of superfluids. In addition, we show that breakdown is a precursor of an electric-field induced QHE-metal transition..

C. Ferreiro-Cordova, Toner, J., Loewen, H., and Wensink, H. H., “Long-time anomalous swimmer diffusion in smectic liquid crystals”, PHYSICAL REVIEW E, vol. 97, p. 062606, 2018.Abstract

The dynamics of self-locomotion of active particles in aligned or liquid crystalline fluids strongly deviates from that in simple isotropic media. We explore the long-time dynamics of a swimmer moving in a three-dimensional smectic liquid crystal and find that the mean-square displacement transverse to the director exhibits a distinct logarithmic tail at long times. The scaling is distinctly different from that in an isotropic or nematic fluid and hints at the subtle but important role of the director fluctuation spectrum in governing the long-time motility of active particles. Our findings are based on a generic hydrodynamic theory and Brownian dynamics computer simulation of a three-dimensional soft mesogen model.

M. Thakurathi, Simon, P., Mandal, I., Klinovaja, J., and Loss, D., “Majorana Kramers pairs in Rashba double nanowires with interactions and disorder”, PHYSICAL REVIEW B, vol. 97, p. 045415, 2018.Abstract

We analyze the effects of electron-electron interactions and disorder on a Rashba double-nanowire setup coupled to an s-wave superconductor, which has been recently proposed as a versatile platform to generate Kramers pairs of Majorana bound states in the absence of magnetic fields. We identify the regime of parameters for which these Kramers pairs are stable against interaction and disorder effects. We use bosonization, perturbative renormalization group, and replica techniques to derive the flow equations for various parameters of the model and evaluate the corresponding phase diagram with topological and disorder-dominated phases. We confirm aforementioned results by considering a more microscopic approach, which starts from the tunneling Hamiltonian between the three-dimensional s-wave superconductor and the nanowires. We find again that the interaction drives the system into the topological phase and, as the strength of the source term coming from the tunneling Hamiltonian increases, strong electron-electron interactions are required to reach the topological phase.

M. Trushin, Goerbig, M. O., and Belzig, W., “Model Prediction of Self-Rotating Excitons in Two-Dimensional Transition-Metal Dichalcogenides”, PHYSICAL REVIEW LETTERS, vol. 120, p. 187401, 2018.Abstract

Using the quasiclassical concept of Berry curvature we demonstrate that a Dirac exciton-a pair of Dirac quasiparticles bound by Coulomb interactions-inevitably possesses an intrinsic angular momentum making the exciton effectively self-rotating. The model is applied to excitons in two-dimensional transition metal dichalcogenides, in which the charge carriers are known to be described by a Dirac-like Hamiltonian. We show that the topological self-rotation strongly modifies the exciton spectrum and, as a consequence, resolves the puzzle of the overestimated two-dimensional polarizability employed to fit earlier spectroscopic measurements.

O. Najera, Civelli, M., Dobrosavljevic, V., and Rozenberg, M. J., “Multiple crossovers and coherent states in a Mott-Peierls insulator”, PHYSICAL REVIEW B, vol. 97, p. 045108, 2018.Abstract

We consider the dimer Hubbard model within dynamical mean-field theory to study the interplay and competition between Mott and Peierls physics. We describe the various metal-insulator transition lines of the phase diagram and the breakdown of the different solutions that occur along them. We focus on the specific issue of the debated Mott-Peierls insulator crossover and describe the systematic evolution of the electronic structure across the phase diagram. We found that at low intradimer hopping, the emerging local magnetic moments can unbind above a characteristic singlet temperature T{*}. Upon increasing the interdimer hopping, subtle changes occur in the electronic structure. Notably, we find Hubbard bands of a mix character with coherent and incoherent excitations. We argue that this statemight be relevant formaterials such as VO2 and its signaturesmay be observed in spectroscopic studies, and possibly through pump-probe experiments.

N. Ghenzi, et al., “One-transistor one-resistor (1T1R) cell for large-area electronics”, APPLIED PHYSICS LETTERS, vol. 113, p. 072108, 2018.Abstract

We developed a one-transistor one-resistor cell composed of one TiO2-based resistive switching (RS) device and one ZnO-based thin-film transistor (TFT). We study the electric characteristics of each component individually, and their interplay when both work together. We explored the direct control of bipolar RS devices, using our TFTs to drive current in both directions. We also report striking power implications when we swap the terminals of the RS device. The target of our work is the introduction of RS devices in large-area electronic (LAE) circuits. In this context, RS devices can be beneficial regarding functionality and energy consumption, when compared to other ways to introduce memory cells in LAE circuits. Published by AIP Publishing.

F. Tesler, et al., “Relaxation of a Spiking Mott Artificial Neuron”, PHYSICAL REVIEW APPLIED, vol. 10, p. 054001, 2018.Abstract

We consider the phenomenon of electric Mott transition (EMT), which is an electrically induced insulator-to-metal transition. Experimentally, it is observed that depending on the magnitude of the electric excitation, the final state may show a short-lived or a long-lived resistance change. We extend a previous model for the EMT to include the effect of local structural distortions through an elastic energy term. We find that by strong electric pulsing, the induced metastable phase may become further stabilized by the electroelastic effect. We present a systematic study of the model by numerical simulations and compare the results to experiments in Mott insulators of the AM(4)Q(8) family. Our work significantly extends the scope of our recently introduced leaky-integrate-and-fire Mott neuron {[}P. Stoliar et al., Adv. Funct. Mat. 27, 1604740 (2017)] to provide a better insight into the physical mechanism of its relaxation. This is a key feature for future implementations of neuromorphic circuits.

J. del Valle, et al., “Resistive asymmetry due to spatial confinement in first-order phase transitions”, PHYSICAL REVIEW B, vol. 98, p. 045123, 2018.Abstract

We report an asymmetry in the R vs T characteristics across the first-order metal-insulator transition (MIT) of V2O3 nanowires. The resistance changes in a few, large jumps during cooling through the MIT, while it does it in a smoother way during warming. The asymmetry is greatly enhanced as the width of the nanowire approaches a characteristic domain size. Our results, together with previous reports on VO2 {[}W. Fan et al., Phys. Rev. B 83, 235102 (2011)] and FeRh {[}V. Uhlir et al., Nat. Commun. 7, 13113 (2016)] imply that asymmetry is a generic feature of first-order phase transitions in one-dimensional systems. We show that this behavior is a simple, elegant consequence of the combined effects of the transition hysteresis and the temperature dependence of the insulating gap in this case (and generically, the order parameter relevant for the physical observable). We conclude that our proposed asymmetry mechanism is universally applicable to many electronic first-order phase transitions.

B. van der Meer, van Damme, R., Dijkstra, M., Smallenburg, F., and Filion, L., “Revealing a Vacancy Analog of the Crowdion Interstitial in Simple Cubic Crystals”, PHYSICAL REVIEW LETTERS, vol. 121, p. 258001, 2018.Abstract

Vacancies in simple cubic crystals of hard cubes are known to delocalize over one-dimensional chains of several lattice sites. Here, we use computer simulations to examine the structure and dynamics of vacancies in simple cubic crystals formed by hard cubes, right rhombic prisms (slanted cubes), truncated cubes, and particles interacting via a soft isotropic pair potential. We show that these vacancies form a vacancy analog of the crowdion interstitial, generating a strain field which follows a soliton solution of the sine-Gordon equation, and diffusing via a persistent random walk. Surprisingly, we find that the structure of these ``voidions{''} is not significantly affected by changes in density, vacancy concentration, and even particle interaction. We explain this structure quantitatively using a one-dimensional model that includes the free-energy barrier particles have to overcome to slide between lattice sites and the effective pair interaction along this line. We argue that voidions are a robust phenomenon in systems of repulsive particles forming simple cubic crystals.

M. N. de Biniossek, Loewen, H., Voigtmann, T., and Smallenburg, F., “Static structure of active Brownian hard disks”, JOURNAL OF PHYSICS-CONDENSED MATTER, vol. 30, p. 074001, 2018.Abstract

We explore the changes in static structure of a two-dimensional system of active Brownian particles (ABP) with hard-disk interactions, using event-driven Brownian dynamics simulations. In particular, the effect of the self-propulsion velocity and the rotational diffusivity on the orientationally-averaged fluid structure factor is discussed. Typically activity increases structural ordering and generates a structure factor peak at zero wave vector which is a precursor of motility-induced phase separation. Our results provide reference data to test future statistical theories for the fluid structure of active Brownian systems. This manuscript was submitted for the special issue of the Journal of Physics: Condensed Matter associated with the Liquid Matter Conference 2017.

V. Kaladzhyan, Bena, C., and Simon, P., “Topology from triviality”, PHYSICAL REVIEW B, vol. 97, p. 104512, 2018.Abstract

We show that bringing into proximity two topologically trivial systems can give rise to a topological phase. More specifically, we study a 1D metallic nanowire proximitized by a 2D superconducting substrate with a mixed s-wave and p-wave pairing, and we demonstrate both analytically and numerically that the phase diagram of such a setup can be richer than reported before. Thus apart from the two ``expected{''} well-known phases (i.e., where the substrate and the wire are both simultaneously trivial or topological), we show that there exist two peculiar phases in which the nanowire can be in a topological regime while the substrate is trivial and vice versa.

Z. Iftikhar, et al., “Tunable quantum criticality and super-ballistic transport in a ``charge{''} Kondo circuit”, SCIENCE, vol. 360, p. 1315-1320, 2018.Abstract

Quantum phase transitions (QPTs) are ubiquitous in strongly correlated materials. However, the microscopic complexity of these systems impedes the quantitative understanding of QPTs. We observed and thoroughly analyzed the rich strongly correlated physics in two profoundly dissimilar regimes of quantum criticality. With a circuit implementing a quantum simulator for the three-channel Kondo model, we reveal the universal scalings toward different low-temperature fixed points and along the multiple crossovers from quantum criticality. An unanticipated violation of the maximum conductance for ballistic free electrons is uncovered. The present charge pseudospin implementation of a Kondo impurity opens access to a broad variety of strongly correlated phenomena.

B. Braunecker and Simon, P., “Tunneling spectroscopy between one-dimensional helical conductors”, PHYSICAL REVIEW B, vol. 98, p. 115146, 2018.Abstract

We theoretically investigate the tunneling spectroscopy of a system of two parallel one-dimensional helical conductors in the interacting, Luttinger liquid regime. We calculate the nonlinear differential conductance as a function of the voltage bias between the conductors and the orbital momentum shift induced on tunneling electrons by an orthogonal magnetic field. We show that the conductance map exhibits an interference pattern which is characteristic to the interacting helical liquid. This can be contrasted with the different interference pattern from tunneling between regular Luttinger liquids which is governed by the spin-charge separation of the elementary collective excitations.

G. Montambaux, Lim, L. - K., Fuchs, J. - N., and Piechon, F., “Winding Vector: How to Annihilate Two Dirac Points with the Same Charge”, PHYSICAL REVIEW LETTERS, vol. 121, p. 256402, 2018.Abstract

The merging or emergence of a pair of Dirac points may be classified according to whether the winding numbers which characterize them are opposite (+- scenario) or identical (++ cenario). From the touching point between two parabolic bands (one of them can be flat), two Dirac points with the same winding number emerge under appropriate distortion (interaction, etc.), following the ++ scenario. Under further distortion, these Dirac points merge following the +- scenario, that is corresponding to opposite winding numbers. This apparent contradiction is solved by the fact that the winding number is actually defined around a unit vector on the Bloch sphere and that this vector rotates during the motion of the Dirac points. This is shown here within the simplest two-band lattice model (Mielke) exhibiting a flat band. We argue on several examples that the evolution between the two scenarios is general.

2017

D. J. Groenendijk, et al., “Spin-Orbit Semimetal SrIrO3 in the Two-Dimensional Limit”, PHYSICAL REVIEW LETTERS, vol. 119, p. 256403, 2017.Abstract

We investigate the thickness-dependent electronic properties of ultrathin SrIrO3 and discover a transition from a semimetallic to a correlated insulating state below 4 unit cells. Low-temperature magnetoconductance measurements show that spin fluctuations in the semimetallic state are significantly enhanced while approaching the transition point. The electronic properties are further studied by scanning tunneling spectroscopy, showing that 4 unit cell SrIrO(3)d is on the verge of a gap opening. Our density functional theory calculations reproduce the critical thickness of the transition and show that the opening of a gap in ultrathin SrIrO3 requires antiferromagnetic order.

0

D. K. Mukherjee, Carpentier, D., and Goerbig, M. O., “{Dynamical conductivity of the Fermi arc and the Volkov-Pankratov states on the surface of Weyl semimetals}”, {PHYSICAL REVIEW B}, vol. {100}.Abstract

{Weyl semimetals are known to host massless surface states called Fermi arcs. These Fermi arcs are the manifestation of the bulk-boundary correspondence in topological matter and thus are analogous to the topological chiral surface states of topological insulators. It has been shown that the latter, depending on the smoothness of the surface, host massive Volkov-Pankratov states that coexist with the chiral ones. Here, we investigate these VP states in the framework of Weyl semimetals, namely their density of states and magneto-optical response. We find the selection rules corresponding to optical transitions which lead to anisotropic responses to external fields. In the presence of a magnetic field parallel to the interface, the selection rules and hence the poles of the response functions are mixed.}

S. Brazovskii and Kirova, N., “{Multi-Fluid Hydrodynamics in Charge Density Waves with Collective, Electronic, and Solitonic Densities and Currents}”, {JOURNAL OF EXPERIMENTAL AND THEORETICAL PHYSICS}, vol. {129}, p. {659-668}.Abstract

{We present a general scheme to approach the space-time evolution of deformations, currents, and the electric field in charge density waves related to appearance of intrinsic topological defects: dislocations, their loops or pairs, and solitons. We derive general equations for the multi-fluid hydrodynamics taking into account the collective mode, electric field, normal electrons, and the intrinsic defects. These equations may allow to study the transformation of injected carriers from normal electrons to new periods of the charge density wave, the collective motion in constrained geometry, and the plastic states and flows. As an application, we present analytical and numerical solutions for distributions of fields around an isolated dislocation line in the regime of nonlinear screening by the gas of phase solitons.}