Condensed Matter

Gebhart, Valentin;Snizhko, Kyrylo;Wellens, Thomas;Buchleitner, Andreas;Romito, Alessandro;Gefen, Yuval

doi: 10.1073/pnas.1911620117pmid: 32123099

Abstract: The state of a quantum system, adiabatically driven in a cycle, may acquire a measurable phase depending only on the closed trajectory in parameter space. Such geometric phases are ubiquitous, and also underline the physics of robust topological phenomena such as the quantum Hall effect. Equivalently, a geometric phase may be induced through a cyclic sequence of quantum measurements. We show that the application of a sequence of weak measurements renders the closed trajectories, hence the geometric phase, stochastic. We study the concomitant probability distribution and show that, when varying the measurement strength, the mapping between the measurement sequence and the geometric phase undergoes a topological transition. Our finding may impact measurement-induced control and manipulation of quantum states---a promising approach to quantum information processing. It also has repercussions on understanding the foundations of quantum measurement.

Rafsanjani, Ahmad;Jin, Lishuai;Deng, Bolei;Bertoldi, Katia

doi: 10.1073/pnas.1817763116pmid: 30962388

Abstract: Kirigami-inspired metamaterials are attracting increasing interest because of their ability to achieve extremely large strains and shape changes via out-of-plane buckling. While in flat kirigami sheets the ligaments buckle simultaneously as Euler columns leading to a continuous phase transition, here we demonstrate that kirigami shells can also support discontinuous phase transitions. Specifically, we show via a combination of experiments, numerical simulations and theoretical analysis that in cylindrical kirigami shells the snapping-induced curvature inversion of the initially bent ligaments results in a pop-up process that first localizes near an imperfection and then, as the deformation is increased, progressively spreads through the structure. Notably, we find that the width of the transition zone as well as the stress at which propagation of the instability is triggered can be controlled by carefully selecting the geometry of the cuts and the curvature of the shell. Our study significantly expands the ability of existing kirigami metamaterials and opens avenues for the design of the next generation of responsive surfaces, as demonstrated by the design of a smart skin that significantly enhance the crawling efficiency of a simple linear actuator.

Brumley, Douglas R.;Carrara, Francesco;Hein, Andrew M.;Yawata, Yutaka;Levin, Simon A.;Stocker, Roman

doi: 10.1073/pnas.1816621116pmid: 31097577

Abstract: Ephemeral aggregations of bacteria are ubiquitous in the environment, where they serve as hotbeds of metabolic activity, nutrient cycling, and horizontal gene transfer. In many cases, these regions of high bacterial concentration are thought to form when motile cells use chemotaxis to navigate to chemical hotspots. However, what governs the dynamics of bacterial aggregations is unclear. Here, we use a novel experimental platform to create realistic sub-millimeter scale nutrient pulses with controlled nutrient concentrations. By combining experiments, mathematical theory and agent-based simulations, we show that individual \textit{Vibrio ordalii} bacteria begin chemotaxis toward hotspots of dissolved organic matter (DOM) when the magnitude of the chemical gradient rises sufficiently far above the sensory noise that is generated by stochastic encounters with chemoattractant molecules. Each DOM hotspot is surrounded by a dynamic ring of chemotaxing cells, which congregate in regions of high DOM concentration before dispersing as DOM diffuses and gradients become too noisy for cells to respond to. We demonstrate that \textit{V. ordalii} operates close to the theoretical limits on chemotactic precision. Numerical simulations of chemotactic bacteria, in which molecule counting noise is explicitly taken into account, point at a tradeoff between nutrient acquisition and the cost of chemotactic precision. More generally, our results illustrate how limits on sensory precision can be used to understand the location, spatial extent, and lifespan of bacterial behavioral responses in ecologically relevant environments.

Dhuri, Dattaraj B.;Hanasoge, Shravan M.;Cheung, Mark C. M.

doi: 10.1073/pnas.1820244116pmid: 31110008

Abstract: Solar flares - bursts of high-energy radiation responsible for severe space-weather effects - are a consequence of the occasional destabilization of magnetic fields rooted in active regions (ARs). The complexity of AR evolution is a barrier to a comprehensive understanding of flaring processes and accurate prediction. Though machine learning (ML) has been used to improve flare predictions, the potential for revealing precursors and associated physics has been underexploited. Here, we train ML algorithms to classify between vector-magnetic-field observations from flaring ARs, producing at least one M-/X-class flare, and non-flaring ARs. Analysis of magnetic-field observations accurately classified by the machine presents statistical evidence for (1) ARs persisting in flare-productive states --- characterized by AR area --- for days, before and after M- and X-class flare events, (2) systematic pre-flare build-up of free energy in the form of electric currents, suggesting that associated subsurface magnetic field is twisted, (3) intensification of Maxwell stresses in the corona above newly emerging ARs, days before first flares. These results provide new insights into flare physics and improving flare forecasting.

Head-Gordon, Teresa;Paesani, Francesco

doi: 10.1073/pnas.1902031116pmid: 31239338

Abstract: The contributed paper by Naserifar and Goddard reports that their RexPoN water model under ambient conditions simulates liquid water as a dynamic polydisperse branched polymer, which they speculate explains the existence of the liquid-liquid critical point (LLCP) in the supercooled region. Our work addresses several serious factual errors and needless speculation in their paper about their interpretation of their model and its implication for the LLCP in supercooled water.

Choukroun, Mathieu;Vu, Tuan H.;Fayolle, Edith C.

doi: 10.1073/pnas.1902381116pmid: 31270233

Abstract: A recent article reported experimental observations of methane and CO2 clathrate formation at conditions similar to the interstellar medium (ISM), namely 10-30 K and 10-10 mbar. The authors conducted time-dependent reflection-absorption infrared spectroscopy (RAIRS) of vapor-deposited H2O:CH4 and H2O:CO2 mixtures and interpreted new blue and red -shifted peaks from those of trapped CH4 and CO2 in amorphous ice, respectively, as indicative of clathrate formation. In this Letter to the Editor, we point out potential pitfalls and caution against the implications drawn for the ISM.

Zhong, Ruidan;Guo, Shu;Xu, Guangyong;Xu, Zhijun;Cava, Robert J.

doi: 10.1073/pnas.1906483116pmid: 31266895

Abstract: Currently under active study in condensed matter physics, both theoretically and experimentally, are quantum spin liquid (QSL) states, in which no long-range magnetic ordering appears at low temperatures due to strong quantum fluctuations of the magnetic moments. The existing QSL candidates all have their intrinsic disadvantages, however, and solid evidence for quantum fluctuations is scarce. Here we report a new compound, Na$_{2}$BaCo(PO$_{4}$)$_{2}$, a geometrically frustrated system with effective spin-1/2 local moments for Co$^{2+}$ ions on an isotropic two-dimensional triangular lattice. Magnetic susceptibility and neutron scattering experiments show no magnetic ordering down to 0.05 K. Thermodynamic measurements show that there is a tremendous amount of magnetic entropy present below 1 K in zero applied magnetic field. The presence of localized low-energy spin fluctuations is revealed by inelastic neutron measurements. At low applied fields, these spin excitations are confined to low energy and contribute to the anomalously large specific heat. In larger applied fields, the system reverts to normal behavior as evident by both neutron and thermodynamic results. Our experimental characterization thus reveals that this new material is an excellent candidate for the experimental realization of a quantum spin liquid state.

Duclut, Charlie;Sarkar, Niladri;Prost, Jacques;Jülicher, Frank

doi: 10.1073/pnas.1908481116pmid: 31492815

Abstract: We discuss the physical mechanisms that promote or suppress the nucleation of a fluid-filled lumen inside a cell assembly or a tissue. We discuss lumen formation in a continuum theory of tissue material properties in which the tissue is described as a two-fluid system to account for its permeation by the interstitial fluid, and we include fluid pumping as well as active electric effects. Considering a spherical geometry and a polarized tissue, our work shows that fluid pumping and tissue flexoelectricity play a crucial role in lumen formation. We furthermore explore the large variety of long-time states that are accessible for the cell aggregate and its lumen. Our work reveals a role of the coupling of mechanical, electrical and hydraulic phenomena in tissue lumen formation.

Lewandowski, Cyprian;Levitov, Leonid

doi: 10.1073/pnas.1909069116pmid: 31562197

Abstract: Surface plasmons in 2-dimensional electron systems with narrow Bloch bands feature an interesting regime in which Landau damping (dissipation via electron-hole pair excitation) is completely quenched. This surprising behavior is made possible by strong coupling in narrow-band systems characterized by large values of the "fine structure" constant $\alpha=e^2/\hbar \kappa v_{\rm F}$. Dissipation quenching occurs when dispersing plasmon modes rise above the particle-hole continuum, extending into the forbidden energy gap that is free from particle-hole excitations. The effect is predicted to be prominent in moiré graphene, where at magic twist-angle values, flat bands feature $\alpha\gg1$. The extinction of Landau damping enhances spatial optical coherence. Speckle-like interference, arising in the presence of disorder scattering, can serve as a telltale signature of undamped plasmons directly accessible in near-field imaging experiments.

Hirshberg, Barak;Rizzi, Valerio;Parrinello, Michele

doi: 10.1073/pnas.1913365116pmid: 31591226

Abstract: Trapped Bosons exhibit fundamental physical phenomena and are potentially useful for quantum technologies. We present a method for simulating Bosons using path integral molecular dynamics. A main challenge for simulations is including all permutations due to exchange symmetry. We show that evaluation of the potential can be done recursively, avoiding explicit enumeration of permutations, and scales cubically with system size. The method is applied to Bosons in a 2D trap and agrees with essentially exact results. An analysis of the role of exchange with decreasing temperature is also presented.