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Physical Review Letters

Jan Maarten van Doorn, Jochem Bronkhorst, Ruben Higler, Ties van de Laar, Joris Sprakel
Colloidal gels are a prototypical example of a heterogeneous network solid whose complex properties are governed by thermally activated dynamics. In this Letter we experimentally establish the connection between the intermittent dynamics of individual particles and their local connectivity. We interpret our experiments with a model that describes single-particle dynamics based on highly cooperative thermal debonding. The model, in quantitative agreement with experiments, provides a microscopic picture for the structural origin of dynamical heterogeneity in colloidal gels and sheds new light on the link between structure and the complex mechanics of these heterogeneous solids...
May 5, 2017: Physical Review Letters
Wenjuan Fang, Baojiu Li, Gong-Bo Zhao
The morphological properties of the large scale structure of the Universe can be fully described by four Minkowski functionals (MFs), which provide important complementary information to other statistical observables such as the widely used 2-point statistics in configuration and Fourier spaces. In this work, for the first time, we present the differences in the morphology of the large scale structure caused by modifications to general relativity (to address the cosmic acceleration problem), by measuring the MFs from N-body simulations of modified gravity and general relativity...
May 5, 2017: Physical Review Letters
Zvi Bern, John Joseph Carrasco, Wei-Ming Chen, Henrik Johansson, Radu Roiban
Whenever the integrand of a gauge-theory loop amplitude can be arranged into a form where the Bern-Carrasco-Johansson duality between color and kinematics is manifest, a corresponding gravity integrand can be obtained simply via the double-copy procedure. However, finding such gauge-theory representations can be challenging, especially at high loop orders. Here, we show that we can, instead, start from generic gauge-theory integrands, where the duality is not manifest, and apply a modified double-copy procedure to obtain gravity integrands that include contact terms generated by violations of dual Jacobi identities...
May 5, 2017: Physical Review Letters
Volodymyr Vovchenko, Mark I Gorenstein, Horst Stoecker
An extension of the ideal hadron resonance gas (HRG) model is constructed which includes the attractive and repulsive van der Waals (VDW) interactions between baryons. This VDW-HRG model yields the nuclear liquid-gas transition at low temperatures and high baryon densities. The VDW parameters a and b are fixed by the ground state properties of nuclear matter, and the temperature dependence of various thermodynamic observables at zero chemical potential are calculated within the VDW-HRG model. Compared to the ideal HRG model, the inclusion of VDW interactions between baryons leads to a qualitatively different behavior of second and higher moments of fluctuations of conserved charges, in particular in the so-called crossover region T∼140-190  MeV...
May 5, 2017: Physical Review Letters
Yonghao Mi, Nicolas Camus, Lutz Fechner, Martin Laux, Robert Moshammer, Thomas Pfeifer
Channel-selective electron emission from strong-field photoionization of H_{2} molecules is experimentally investigated by using ultrashort laser pulses and a reaction microscope. The electron momenta and energy spectra in coincidence with bound and dissociative ionization channels are compared. Surprisingly, we observed an enhancement of the photoelectron yield in the low-energy region for the bound ionization channel. By further investigation of asymmetrical electron emission using two-color laser pulses, this enhancement is understood as the population of the autoionizing states of H_{2} molecules in which vibrational energy is transferred to electronic energy...
May 5, 2017: Physical Review Letters
Ce Wang, Pengfei Zhang, Xin Chen, Jinlong Yu, Hui Zhai
We show how the topological number of a static Hamiltonian can be measured from a dynamical quench process. We focus on a two-band Chern insulator in two dimension, for instance, the Haldane model, whose dynamical process can be described by a mapping from the [k_{x},k_{y},t] space to the Bloch sphere, characterized by the Hopf invariant. Such a mapping has been constructed experimentally by measurements in cold atom systems. We show that, taking any two constant vectors on the Bloch sphere, their inverse images of this mapping are two trajectories in the [k_{x},k_{y},t] space, and the linking number of these two trajectories exactly equals the Chern number of the static Hamiltonian...
May 5, 2017: Physical Review Letters
Jonathan D'Emidio, Ribhu K Kaul
We study fixed points of the easy-plane CP^{N-1} field theory by combining quantum Monte Carlo simulations of lattice models of easy-plane SU(N) superfluids with field theoretic renormalization group calculations, by using ideas of deconfined criticality. From our simulations, we present evidence that at small N our lattice model has a first-order phase transition which progressively weakens as N increases, eventually becoming continuous for large values of N. Renormalization group calculations in 4-ε dimensions provide an explanation of these results as arising due to the existence of an N_{ep} that separates the fate of the flows with easy-plane anisotropy...
May 5, 2017: Physical Review Letters
Abhishek Bagusetty, Pabitra Choudhury, Wisssam A Saidi, Bridget Derksen, Elizabeth Gatto, J Karl Johnson
Graphane functionalized with hydroxyl groups is shown to rapidly conduct protons under anhydrous conditions through a contiguous network of hydrogen bonds. Density functional theory calculations predict remarkably low barriers to diffusion of protons along a 1D chain of surface hydroxyls. Diffusion is controlled by the local rotation of hydroxyl groups, a mechanism that is very different from that found in 1D water wires in confined nanopores or in bulk water. The proton mean square displacement in the 1D chain was observed to follow Fickian diffusion rather than the expected single-file mobility...
May 5, 2017: Physical Review Letters
Stephen R Taylor, Joseph Simon, Laura Sampson
We introduce a technique for gravitational-wave analysis, where Gaussian process regression is used to emulate the strain spectrum of a stochastic background by training on population-synthesis simulations. This leads to direct Bayesian inference on astrophysical parameters. For pulsar timing arrays specifically, we interpolate over the parameter space of supermassive black-hole binary environments, including three-body stellar scattering, and evolving orbital eccentricity. We illustrate our approach on mock data, and assess the prospects for inference with data similar to the NANOGrav 9-yr data release...
May 5, 2017: Physical Review Letters
J A Alonso, E Sánchez, I Calvo, J L Velasco, K J McCarthy, A Chmyga, L G Eliseev, T Estrada, R Kleiber, L I Krupnik, A V Melnikov, P Monreal, F I Parra, S Perfilov, A I Zhezhera
Measurements of the relaxation of a zonal electrostatic potential perturbation in a nonaxisymmetric magnetically confined plasma are presented. A sudden perturbation of the plasma equilibrium is induced by the injection of a cryogenic hydrogen pellet in the TJ-II stellarator, which is observed to be followed by a damped oscillation in the electrostatic potential. The waveform of the relaxation is consistent with theoretical calculations of zonal potential relaxation in a nonaxisymmetric magnetic geometry. The turbulent transport properties of a magnetic confinement configuration are expected to depend on the features of the collisionless damping of zonal flows, of which the present Letter is the first direct observation...
May 5, 2017: Physical Review Letters
Ian A Leahy, Christopher A Pocs, Peter E Siegfried, David Graf, S-H Do, Kwang-Yong Choi, B Normand, Minhyea Lee
We report on the unusual behavior of the in-plane thermal conductivity κ and torque τ response in the Kitaev-Heisenberg material α-RuCl_{3}. κ shows a striking enhancement with linear growth beyond H=7  T, where magnetic order disappears, while τ for both of the in-plane symmetry directions shows an anomaly at the same field. The temperature and field dependence of κ are far more complex than conventional phonon and magnon contributions, and require us to invoke the presence of unconventional spin excitations whose properties are characteristic of a field-induced spin-liquid phase related to the enigmatic physics of the Kitaev model in an applied magnetic field...
May 5, 2017: Physical Review Letters
Simranjeet Singh, Jyoti Katoch, Tiancong Zhu, Keng-Yuan Meng, Tianyu Liu, Jack T Brangham, Fengyuan Yang, Michael E Flatté, Roland K Kawakami
Two-dimensional materials provide a unique platform to explore the full potential of magnetic proximity-driven phenomena, which can be further used for applications in next-generation spintronic devices. Of particular interest is to understand and control spin currents in graphene by the magnetic exchange field of a nearby ferromagnetic material in graphene-ferromagnetic-insulator (FMI) heterostructures. Here, we present the experimental study showing the strong modulation of spin currents in graphene layers by controlling the direction of the exchange field due to FMI magnetization...
May 5, 2017: Physical Review Letters
J P Eisenstein, L N Pfeiffer, K W West
Double layer two-dimensional electron systems at high perpendicular magnetic field are used to realize magnetic tunnel junctions in which the electrons at the Fermi level in the two layers have either parallel or antiparallel spin magnetizations. In the antiparallel case the tunnel junction, at low temperatures, behaves as a nearly ideal spin diode. At elevated temperatures the diode character degrades as long-wavelength spin waves are thermally excited. These tunnel junctions provide a demonstration that the spin polarization of the electrons in the N=1 Landau level at filling factors ν=5/2 and 7/2 is essentially complete, and, with the aid of an in-plane magnetic field component, that Landau level mixing at these filling factors is weak in the samples studied...
May 5, 2017: Physical Review Letters
Clément Hainaut, Isam Manai, Radu Chicireanu, Jean-François Clément, Samir Zemmouri, Jean Claude Garreau, Pascal Szriftgiser, Gabriel Lemarié, Nicolas Cherroret, Dominique Delande
We report on the observation of the coherent enhancement of the return probability ["enhanced return to the origin" (ERO)] in a periodically kicked cold-atom gas. By submitting an atomic wave packet to a pulsed, periodically shifted, laser standing wave, we induce an oscillation of ERO in time that is explained in terms of a periodic, reversible dephasing in the weak-localization interference sequences responsible for ERO. Monitoring the temporal decay of ERO, we exploit its quantum-coherent nature to quantify the decoherence rate of the atomic system...
May 5, 2017: Physical Review Letters
Toby Crisford, Jorge E Santos
We consider time-dependent solutions of the Einstein-Maxwell equations using anti-de Sitter (AdS) boundary conditions, and provide the first counterexample to the weak cosmic censorship conjecture in four spacetime dimensions. Our counterexample is entirely formulated in the Poincaré patch of AdS. We claim that our results have important consequences for quantum gravity, most notably to the weak gravity conjecture.
May 5, 2017: Physical Review Letters
Peter Asenbaum, Chris Overstreet, Tim Kovachy, Daniel D Brown, Jason M Hogan, Mark A Kasevich
Spacetime curvature induces tidal forces on the wave function of a single quantum system. Using a dual light-pulse atom interferometer, we measure a phase shift associated with such tidal forces. The macroscopic spatial superposition state in each interferometer (extending over 16 cm) acts as a nonlocal probe of the spacetime manifold. Additionally, we utilize the dual atom interferometer as a gradiometer for precise gravitational measurements.
May 5, 2017: Physical Review Letters
Mahdi Hosseini, Yiheng Duan, Kristin M Beck, Yu-Ting Chen, Vladan Vuletić
We demonstrate cavity cooling of all motional degrees of freedom of an atomic ensemble using light that is far detuned from the atomic transitions by several gigahertz. The cooling is achieved by cavity-induced frequency-dependent asymmetric enhancement of the atomic emission spectrum, thereby extracting thermal kinetic energy from the atomic system. Within 100 ms, the atomic temperature is reduced from 200 to 10  μK, where the final temperature is mainly limited by the linewidth of the cavity. In principle, the technique can be applied to molecules and atoms with complex internal energy structure...
May 5, 2017: Physical Review Letters
J S Ross, D P Higginson, D Ryutov, F Fiuza, R Hatarik, C M Huntington, D H Kalantar, A Link, B B Pollock, B A Remington, H G Rinderknecht, G F Swadling, D P Turnbull, S Weber, S Wilks, D H Froula, M J Rosenberg, T Morita, Y Sakawa, H Takabe, R P Drake, C Kuranz, G Gregori, J Meinecke, M C Levy, M Koenig, A Spitkovsky, R D Petrasso, C K Li, H Sio, B Lahmann, A B Zylstra, H-S Park
A study of the transition from collisional to collisionless plasma flows has been carried out at the National Ignition Facility using high Mach number (M>4) counterstreaming plasmas. In these experiments, CD-CD and CD-CH planar foils separated by 6-10 mm are irradiated with laser energies of 250 kJ per foil, generating ∼1000  km/s plasma flows. Varying the foil separation distance scales the ion density and average bulk velocity and, therefore, the ion-ion Coulomb mean free path, at the interaction region at the midplane...
May 5, 2017: Physical Review Letters
Steve M Young, Benjamin J Wieder
Filling-enforced Dirac semimetals, or those required at specific fillings by the combination of crystalline and time-reversal symmetries, have been proposed in numerous materials. However, Dirac points in these materials are not generally robust against breaking or modifying time-reversal symmetry. We present a new class of two-dimensional Dirac semimetal protected by the combination of crystal symmetries and a special, antiferromagnetic time-reversal symmetry. Systems in this class of magnetic layer groups, while having broken time-reversal symmetry, still respect the operation of time-reversal followed by a half-lattice translation...
May 5, 2017: Physical Review Letters
Wenchao Ma, Bin Chen, Ying Liu, Mengqi Wang, Xiangyu Ye, Fei Kong, Fazhan Shi, Shao-Ming Fei, Jiangfeng Du
The uncertainty principle is considered to be one of the most striking features in quantum mechanics. In the textbook literature, uncertainty relations usually refer to the preparation uncertainty which imposes a limitation on the spread of measurement outcomes for a pair of noncommuting observables. In this work, we study the preparation uncertainty for the angular momentum, especially for spin-1/2. We derive uncertainty relations encompassing the triple components of angular momentum and show that, compared with the relations involving only two components, a triple constant 2/sqrt[3] often arises...
May 5, 2017: Physical Review Letters
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