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Université de Fribourg

Collective iteration behavior for online social networks

Liu, Jian-Guo ; Li, Ren-De ; Guo, Qiang ; Zhang, Yi-Cheng

In: Physica A: Statistical Mechanics and its Applications, 2018, vol. 499, p. 490–497

Understanding the patterns of collective behavior in online social network (OSNs) is critical to expanding the knowledge of human behavior and tie relationship. In this paper, we investigate a specific pattern called social signature in Facebook and Wiki users’ online communication behaviors, capturing the distribution of frequency of interactions between different alters over time in the...

Université de Fribourg

Ultrafast doublon dynamics in photoexcited $1T$-${\mathrm{TaS}}_{2}$

Ligges, M. ; Avigo, I. ; Gole?, Denis ; Strand, H. ?U.? R. ; Beyazit, Y. ; Hanff, K. ; Diekmann, F. ; Stojchevska, L. ; Kalläne, M. ; Zhou, P. ; Rossnagel, K. ; Eckstein, Martin ; Werner, Philipp ; Bovensiepen, U.

In: Physical Review Letters, 2018, vol. 120, no. 16, p. 166401

Strongly correlated materials exhibit intriguing properties caused by intertwined microscopic interactions that are hard to disentangle in equilibrium. Employing nonequilibrium time-resolved photoemission spectroscopy on the quasi-two- dimensional transition-metal dichalcogenide 1T-TaS2, we identify a spectroscopic signature of doubly occupied sites (doublons) that reflects fundamental Mott...

Université de Fribourg

Nonthermal switching of charge order: Dynamical slowing down and optimal control

Schüler, Michael ; Murakami, Yuta ; Werner, Philipp

In: Physical Review B, 2018, vol. 97, no. 15, p. 155136

We investigate the laser-induced dynamics of electronically driven charge-density- wave (CDW) order. A comprehensive mean-field analysis of the attractive Hubbard model in the weak-coupling regime reveals ultrafast switching and ultrafast melting of the order via a nonthermal pathway. The resulting nonequilibrium phase diagram exhibits multiple distinct regimes of the order parameter dynamics...

Université de Fribourg

Enhanced pairing susceptibility in a photodoped two-orbital Hubbard model

Werner, Philipp ; Strand, Hugo U. R. ; Hoshino, Shintaro ; Murakami, Yuta ; Eckstein, Martin

In: Physical Review B, 2018, vol. 97, no. 16, p. 165119

Local spin fluctuations provide the glue for orbital-singlet spin-triplet pairing in the doped Mott insulating regime of multiorbital Hubbard models. At large Hubbard repulsion U, the pairing susceptibility is nevertheless tiny because the pairing interaction cannot overcome the suppression of charge fluctuations. Using nonequilibrium dynamical mean field simulations of the two-orbital...

Université de Fribourg

Cesium alignment produced by pumping with unpolarized light

Shi, Yongqi ; Weis, Antoine

In: The European Physical Journal D, 2018, vol. 72, no. 4, p. 73

We demonstrate optical pumping on the four hyperfine components of the Cs D1 transition by unpolarized (UPL) resonant laser light. The evidence is based on the reduction of the absorption coefficients κ0 with increasing light power P in an uncoated Cs vapor cell with isotropic spin relaxation. For comparison we perform the same quantitative κ0(P) measurements with linearly-polarized light...

Université de Fribourg

Glass transition of soft colloids

Philippe, Adrian-Marie ; Truzzolillo, Domenico ; Galvan-Myoshi, Julian ; Dieudonné-George, Philippe ; Trappe, Véronique ; Berthier, Ludovic ; Cipelletti, Luca

In: Physical Review E, 2018, vol. 97, no. 4, p. 040601

We explore the glassy dynamics of soft colloids using microgels and charged particles interacting by steric and screened Coulomb interactions, respectively. In the supercooled regime, the structural relaxation time τα of both systems grows steeply with volume fraction, reminiscent of the behavior of colloidal hard spheres. Computer simulations confirm that the growth of τα on approaching...

Université de Fribourg

Brownian dynamics of colloidal microspheres with tunable elastic properties from soft to hard

Yoon, Jiwon ; Cardinaux, Frédéric ; Lapointe, Clayton ; Zhang, Chi ; Mason, Thomas G. ; Ahn, Kyung Hyun ; Scheffold, Frank

In: Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2018, vol. 546, p. 360–365

We study the Brownian thermal motion of a colloidal model system made by emulsifying hot liquid α-eicosene wax into an aqueous surfactant solution of sodium dodecyl sulfate (SDS). When this waxy oil-in-water emulsion is cooled below α- eicosene's melting point of Tc ≃ 25 °C, the microscale emulsion droplets solidify, effectively yielding a dispersed particulate system. So, the...

Université de Fribourg

Optical study of Dirac fermions and related phonon anomalies in the antiferromagnetic compound CaFeAsF

Xu, B. ; Xiao, H. ; Gao, B. ; Ma, Y. H. ; Mu, G. ; Mar?ík, Premysl ; Sheveleva, Evgeniia ; Lyzwa, F. ; Dai, Y. M. ; Lobo, R. P. S. M. ; Bernhard, Christian

In: Physical Review B, 2018, vol. 97, no. 19, p. 195110

We performed optical studies on CaFeAsF single crystals, a parent compound of the 1111-type iron-based superconductors that undergoes a structural phase transition from tetragonal to orthorhombic at Ts=121 K and a magnetic one to a spin density wave (SDW) state at TN=110 K. In the low-temperature optical conductivity spectrum, after the subtraction of a narrow Drude peak, we observe a...

Université de Fribourg

Lattice-mediated magnetic order melting in ${\mathrm{TbMnO}}_{3}$

Baldini, Edoardo ; Kubacka, Teresa ; Mallett, Benjamin P. P. ; Ma, Chao ; Koohpayeh, Seyed M. ; Zhu, Yimei ; Bernhard, Christian ; Johnson, Steven L. ; Carbone, Fabrizio

In: Physical Review B, 2018, vol. 97, no. 12, p. 125149

Recent ultrafast magnetic-sensitive measurements [Johnson et al., Phys. Rev. B 92, 184429 (2015); Bothschafter et al., Phys. Rev. B 96, 184414 (2017)] have revealed a delayed melting of the long-range cycloid spin order in TbMnO3 following photoexcitation across the fundamental Mott-Hubbard gap. The microscopic mechanism behind this slow transfer of energy from the photoexcited carriers to...

Université de Fribourg

Local real-space view of the achiral $1T\text{\ensuremath{-}}{\mathrm{TiSe}}_{2}$ $2\times2\times2$ charge density wave

Hildebrand, Baptiste ; Jaouen, Thomas ; Mottas, Marie-Laure ; Monney, Gaël ; Barreteau, C. ; Giannini, E. ; Bowler, D. R. ; Aebi, Philipp

In: Physical Review Letters, 2018, vol. 120, no. 13, p. 136404

The transition metal dichalcogenide 1T−TiSe2-two-dimensional layered material undergoing a commensurate 2×2×2 charge density wave (CDW) transition with a weak periodic lattice distortion (PLD) below ≈200 K. Scanning tunneling microscopy (STM) combined with intentionally introduced interstitial Ti atoms allows us to go beyond the usual spatial resolution of STM and to intimately probe...