Elementary particles are the building blocks of matter, but there is also a zoo of quasiparticles that are crucial for understanding how this matter behaves.

]]>Author: Mark Buchanan

]]>Authors: Liesbeth Venema, Bart Verberck, Iulia Georgescu, Giacomo Prando, Elsa Couderc, Silvia Milana, Maria Maragkou, Lina Persechini, Giulia Pacchioni & Luke Fleet

Quasiparticles are an extremely useful concept that provides a more intuitive understanding of complex phenomena in many-body physics. As such, they appear in various contexts, linking ideas across different fields and supplying a common language.

]]>Author: Iulia Georgescu

]]>Author: Andreas H. Trabesinger

]]>Author: Luke Fleet

]]>Author: Abigail Klopper

]]>Author: Bart Verberck

]]>Author: Herbert Levine

Dendritic cells use components of their cytoskeleton to both move and ingest pieces of infected cells. This competition for protein resources can give rise to a complex set of states that may be understood with an advection–diffusion model.

]]>Author: Alessandro Retinò

Direct satellite observations of energy transfer between large and small space plasma scales contribute to our understanding of how matter in the Universe gets hot.

]]>Author: Bart Verberck

]]>Authors: Johannes Zeiher, Rick van Bijnen, Peter Schauß, Sebastian Hild, Jae-yoon Choi, Thomas Pohl, Immanuel Bloch & Christian Gross

Ultracold atoms in optical lattices are ideal to study fundamentally new quantum many-body systems including frustrated or topological magnetic phases and supersolids. However, the necessary control of strong long-range interactions between distant ground state atoms has remained a long-standing goal. Optical dressing of ground state atoms via off-resonant laser coupling to Rydberg states is one way to tailor such interactions. Here we report the realization of coherent Rydberg dressing to implement a two-dimensional synthetic spin lattice. Our single-atom-resolved interferometric measurements of the many-body dynamics enable the microscopic probing of the interactions and reveal their highly tunable range and anisotropy. Our work marks the first step towards the use of laser-controlled Rydberg interactions for the study of exotic quantum magnets in optical lattices.

]]>Authors: Peizhe Tang, Quan Zhou, Gang Xu & Shou-Cheng Zhang

Analogues of the elementary particles have been extensively searched for in condensed-matter systems for both scientific interest and technological applications. Recently, massless Dirac fermions were found to emerge as low-energy excitations in materials now known as Dirac semimetals. All of the currently known Dirac semimetals are non-magnetic with both time-reversal symmetry nphys3839-m1gif1251211 and inversion symmetry nphys3839-m2gif1371111. Here we show that Dirac fermions can exist in one type of antiferromagnetic system, where both nphys3839-m3gif1251211 and nphys3839-m4gif1371111 are broken but their combination nphys3839-m5gif1712211 is respected. We propose orthorhombic antiferromagnet CuMnAs as a candidate, analyse the robustness of the Dirac points under symmetry protections and demonstrate its distinctive bulk dispersions, as well as the corresponding surface states, by ab initio calculations. Our results provide a possible platform to study the interplay of Dirac fermion physics and magnetism.

]]>Authors: Ke Deng, Guoliang Wan, Peng Deng, Kenan Zhang, Shijie Ding, Eryin Wang, Mingzhe Yan, Huaqing Huang, Hongyun Zhang, Zhilin Xu, Jonathan Denlinger, Alexei Fedorov, Haitao Yang, Wenhui Duan, Hong Yao, Yang Wu, Shoushan Fan, Haijun Zhang, Xi Chen & Shuyun Zhou

Weyl semimetal is a new quantum state of matter hosting the condensed matter physics counterpart of the relativistic Weyl fermions originally introduced in high-energy physics. The Weyl semimetal phase realized in the TaAs class of materials features multiple Fermi arcs arising from topological surface states and exhibits novel quantum phenomena, such as a chiral anomaly-induced negative magnetoresistance and possibly emergent supersymmetry. Recently it was proposed theoretically that a new type (type-II) of Weyl fermion that arises due to the breaking of Lorentz invariance, which does not have a counterpart in high-energy physics, can emerge as topologically protected touching between electron and hole pockets. Here, we report direct experimental evidence of topological Fermi arcs in the predicted type-II Weyl semimetal MoTe2 (refs ,,). The topological surface states are confirmed by directly observing the surface states using bulk- and surface-sensitive angle-resolved photoemission spectroscopy, and the quasi-particle interference pattern between the putative topological Fermi arcs in scanning tunnelling microscopy. By establishing MoTe2 as an experimental realization of a type-II Weyl semimetal, our work opens up opportunities for probing the physical properties of this exciting new state.

]]>Authors: Eryin Wang, Xiaobo Lu, Shijie Ding, Wei Yao, Mingzhe Yan, Guoliang Wan, Ke Deng, Shuopei Wang, Guorui Chen, Liguo Ma, Jeil Jung, Alexei V. Fedorov, Yuanbo Zhang, Guangyu Zhang & Shuyun Zhou

Graphene/hexagonal boron nitride (h-BN) has emerged as a model van der Waals heterostructure as the superlattice potential, which is induced by lattice mismatch and crystal orientation, gives rise to various novel quantum phenomena, such as the self-similar Hofstadter butterfly states. Although the newly generated second-generation Dirac cones (SDCs) are believed to be crucial for understanding such intriguing phenomena, fundamental knowledge of SDCs, such as locations and dispersion, and the effect of inversion symmetry breaking on the gap opening, still remains highly debated due to the lack of direct experimental results. Here we report direct experimental results on the dispersion of SDCs in 0°-aligned graphene/h-BN heterostructures using angle-resolved photoemission spectroscopy. Our data unambiguously reveal SDCs at the corners of the superlattice Brillouin zone, and at only one of the two superlattice valleys. Moreover, gaps of approximately 100 meV and approximately 160 meV are observed at the SDCs and the original graphene Dirac cone, respectively. Our work highlights the important role of a strong inversion-symmetry-breaking perturbation potential in the physics of graphene/h-BN, and fills critical knowledge gaps in the band structure engineering of Dirac fermions by a superlattice potential.

]]>Authors: Xiawei Wang & Abraham Loeb

The origin of the extragalactic γ-ray background permeating throughout the Universe remains a mystery forty years after its discovery. The extrapolated population of blazars can account for only half of the background radiation in the energy range of ∼0.1–10 GeV (refs ,). Here we show that quasar-driven outflows generate relativistic protons that produce the missing component of the extragalactic γ-ray background and naturally match its spectral fingerprint, with a generic break above ∼1 GeV. The associated γ-ray sources are too faint to be detected individually, explaining why they had not been identified so far. However, future radio observations may image their shock fronts directly. Our best fit to the Fermi-LAT observations of the extragalactic γ-ray background spectrum provides constraints on the outflow parameters that agree with observations of these outflows and theoretical predictions. Although our model explains the data, there might be additional contributing sources.

]]>Authors: T. Suzuki, R. Chisnell, A. Devarakonda, Y.-T. Liu, W. Feng, D. Xiao, J. W. Lynn & J. G. Checkelsky

The quantum mechanical (Berry) phase of the electronic wavefunction plays a critical role in the anomalous and spin Hall effects, including their quantized limits. While progress has been made in understanding these effects in ferromagnets, less is known in antiferromagnetic systems. Here we present a study of antiferromagnet GdPtBi, whose electronic structure is similar to that of the topologically non-trivial HgTe (refs ,,), and where the Gd ions offer the possibility to tune the Berry phase via control of the spin texture. We show that this system supports an anomalous Hall angle ΘAH > 0.1, comparable to the largest observed in bulk ferromagnets and significantly larger than in other antiferromagnets. Neutron scattering measurements and electronic structure calculations suggest that this effect originates from avoided crossing or Weyl points that develop near the Fermi level due to a breaking of combined time-reversal and lattice symmetries. Berry phase effects associated with such symmetry breaking have recently been explored in kagome networks; our results extend this to half-Heusler systems with non-trivial band topology. The magnetic textures indicated here may also provide pathways towards realizing the topological insulating and semimetallic states predicted in this material class.

]]>Authors: Cheng He, Xu Ni, Hao Ge, Xiao-Chen Sun, Yan-Bin Chen, Ming-Hui Lu, Xiao-Ping Liu & Yan-Feng Chen

Topological design of materials enables topological symmetries and facilitates unique backscattering-immune wave transport. In airborne acoustics, however, the intrinsic longitudinal nature of sound polarization makes the use of the conventional spin–orbital interaction mechanism impossible for achieving band inversion. The topological gauge flux is then typically introduced with a moving background in theoretical models. Its practical implementation is a serious challenge, though, due to inherent dynamic instabilities and noise. Here we realize the inversion of acoustic energy bands at a double Dirac cone and provide an experimental demonstration of an acoustic topological insulator. By manipulating the hopping interaction of neighbouring ’atoms’ in this new topological material, we successfully demonstrate the acoustic quantum spin Hall effect, characterized by robust pseudospin-dependent one-way edge sound transport. Our results are promising for the exploration of new routes for experimentally studying topological phenomena and related applications, for example, sound-noise reduction.

]]>Authors: Giulio Biroli & Pierfrancesco Urbani

What characterizes a solid is the way that it responds to external stresses. Ordered solids, such as crystals, exhibit an elastic regime followed by a plastic regime, both understood microscopically in terms of lattice distortion and dislocations. For amorphous solids the situation is instead less clear, and the microscopic understanding of the response to deformation and stress is a very active research topic. Several studies have revealed that even in the elastic regime the response is very jerky at low temperature, resembling very much the response of disordered magnetic materials. Here we show that in a very large class of amorphous solids this behaviour emerges upon decreasing temperature, as a phase transition, where standard elastic behaviour breaks down. At the transition all nonlinear elastic moduli diverge and standard elasticity theory no longer holds. Below the transition, the response to deformation becomes history- and time-dependent.

]]>Authors: Sudeesh Krishnamurthy, Subho Ghosh, Dipankar Chatterji, Rajesh Ganapathy & A. K. Sood

Artificial microscale heat engines are prototypical models to explore the mechanisms of energy transduction in a fluctuation-dominated regime. The heat engines realized so far on this scale have operated between thermal reservoirs, such that stochastic thermodynamics provides a precise framework for quantifying their performance. It remains to be seen whether these concepts readily carry over to situations where the reservoirs are out of equilibrium, a scenario of particular importance to the functioning of synthetic and biological microscale engines and motors. Here, we experimentally realize a micrometre-sized active Stirling engine by periodically cycling a colloidal particle in a time-varying optical potential across bacterial baths characterized by different degrees of activity. We find that the displacement statistics of the trapped particle becomes increasingly non-Gaussian with activity and contributes substantially to the overall power output and the efficiency. Remarkably, even for engines with the same energy input, differences in non-Gaussianity of reservoir noise results in distinct performances. At high activities, the efficiency of our engines surpasses the equilibrium saturation limit of Stirling efficiency, the maximum efficiency of a Stirling engine where the ratio of cold to hot reservoir temperatures is vanishingly small. Our experiments provide fundamental insights into the functioning of micromotors and engines operating out of equilibrium.

]]>Authors: Peng Peng, Wanxia Cao, Ce Shen, Weizhi Qu, Jianming Wen, Liang Jiang & Yanhong Xiao

]]>Authors: Ido Lavi, Matthieu Piel, Ana-Maria Lennon-Duménil, Raphaël Voituriez & Nir S. Gov

]]>Authors: Thierry Mora, Aleksandra M. Walczak, Lorenzo Del Castello, Francesco Ginelli, Stefania Melillo, Leonardo Parisi, Massimiliano Viale, Andrea Cavagna & Irene Giardina

]]>Authors: E. V. Panov, W. Baumjohann, R. A. Wolf, R. Nakamura, V. Angelopoulos, J. M. Weygand & M. V. Kubyshkina

]]>Authors: T. W. Moore, K. Nykyri & A. P. Dimmock

]]>Authors: D. Carollo, T. C. Beers, V. M. Placco, R. M. Santucci, P. Denissenkov, P. B. Tissera, G. Lentner, S. Rossi, Y. S. Lee & J. Tumlinson

]]>Author: Felicitas Arias

Every now and then, an extra second is added to an earthly year — a cause for trouble and debate, as Felicitas Arias has been witnessing.

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