Search

A gas of ultracold atoms probed with laser light is a nearly-ideal experimental realization of a medium of resonant point-like scatterers, a key problem from condensed matter to biology or photonics. Yet, several recent experiments have reported large discrepancies with theory. In this work, we measure the complex transmission through a slab of ultracold two-level atoms with an interferometric technique. We find good agreement with first-principles simulations of mutually-coupled, laser-driven dipoles, and provide an explanation for the discrepancies in earlier measurements., Comment: Supplementary informations available as ancillary file

The aim of this paper is to illustrate that generalized two-dimensional XY models (proposed by Romano and Zagrebnov) may also support a first-order phase transition. Two approaches are employed to accurately determine the critical parameter $q$ at which such a transition takes place. Furthermore, we show that the model is characterized by three distinct regions concerning both first-order and Berezinskii-Kosterlitz-Thouless phase transitions. Finally, the underlying mechanisms governing such transitions are presented, along with an estimation of the critical temperatures.

We study the Bisognano-Wichmann Hamiltonian for fractional quantum Hall states defined on a sphere and explore its relationship with the entanglement Hamiltonian associated to the state. We present results for several examples, namely the bosonic Laughlin state stabilized by contact two-body interactions and the bosonic Moore-Read state by either three- or two-body interactions. Our findings demonstrate that the Bisognano-Wichmann Hamiltonian provides a reliable approximation of the entanglement Hamiltonian as a fully-local operator that can be written without any prior knowledge of the specific state under consideration.

Magnetic analogue of an isolated free electric charge, i.e., a magnet with a single north or south pole, is a long sought-after particle which remains elusive so far. In magnetically frustrated pyrochlore solids, a classical analogue of monopole was observed as a result of excitation of spin ice vertices. Direct visualization of such excitations were proposed and later confirmed in analogous artificial spin ice (ASI) systems of square as well as Kagome geometries. However, such charged vertices are randomly created as they are thermally driven and are always associated with corresponding emergent antimonopoles of equal and opposite charges connected by observable strings. Here, we demonstrate a controlled stabilisation of a robust isolated emergent monopole state in individual square ASI vertices by application of an external magnetic field. The excitation conserves the magnetic charge without the involvement of a corresponding antimonopole. Well supported by Monte Carlo simulations our experimental results enable, in absence of a true elemental magnetic monopole, creation of electron vortices and studying electrodynamics in presence of a monopole field in a solid state environment., Comment: 6 pages, 7 figures

The design of a primary electron beam facility at CERN is described. The study has been carried out within the framework of the wider Physics Beyond Colliders study. It re-enables the Super Proton Synchrotron (SPS) as an electron accelerator, and leverages the development invested in Compact Linear Collider (CLIC) technology for its injector and as an accelerator research and development infrastructure. The facility would be relevant for several of the key priorities in the 2020 update of the European Strategy for Particle Physics, such as an electron-positron Higgs factory, accelerator R\&D, dark sector physics, and neutrino physics. In addition, it could serve experiments in nuclear physics. The electron beam delivered by this facility would provide access to light dark matter production significantly beyond the targets predicted by a thermal dark matter origin, and for natures of dark matter particles that are not accessible by direct detection experiments. It would also enable electro-nuclear measurements crucial for precise modelling the energy dependence of neutrino-nucleus interactions, which is needed to precisely measure neutrino oscillations as a function of energy. The implementation of the facility is the natural next step in the development of X-band high-gradient acceleration technology, a key technology for compact and cost-effective electron/positron linacs. It would also become the only facility with multi-GeV drive bunches and truly independent electron witness bunches for plasma wakefield acceleration. A second phase capable to deliver positron witness bunches would make it a complete facility for plasma wakefield collider studies. [...]

During the last years, topologically protected collective modes of the magnetization have called much attention. Among these, skyrmions and merons have been the object of intense study. In particular, topological skyrmions are objects with an integer skyrmion number $Q$ while merons have a half-integer skyrmion charge $q$. In this work, we consider a $Q=1$ skyrmion, composed by a meron and an antimeron (bimeron), displacing in a ferromagnetic racetrack, disputing a long-distance competition with its more famous counterpart, the typical $Q=1$ cylindrically symmetrical skyrmion. Both types of topological structures induce a Magnus force and then are subject to the Hall effect. The influence of the Dzyaloshinskii-Moriya interaction ($DMI$) present in certain materials and able to induces $DMI$-skyrmions is also analyzed. Our main aim is to compare the motions (induced by a spin-polarized current) of these objects along with their own specific racetracks. We also investigate some favorable factors which are able to give breath to the competitors, impelling them to remain in the race for longer distances before their annihilation at the racetrack lateral border. An interesting result is that the $DMI$-skyrmion loses this hypothetical race due to its larger rigidity., Comment: 6 pages, 5 figures

In this letter, we propose a Voronoi shaping method with reduced encoding complexity. The method works for integer shaping and coding lattices satisfying the chain $\Lambda_s \subseteq \textbf{K}\mathbb{Z}^n \subseteq \Lambda_c$, with $\textbf{K}$ an integer diagonal matrix. This assumption is easily satisfied for lattices obtained from error-correcting codes. For those lattices, using this strategy, an explicit set of coset representatives is obtained and lattice encoding complexity is reduced to the linear code encoding complexity. Our proposal is illustrated in construction-D lattices with Gosset ($E_8$) and Leech ($\Lambda_{24}$) lattices as shaping lattices.

The robust adjustment of nonlinear models to data is considered in this paper. When data comes from real experiments, it is possible that measurement errors cause the appearance of discrepant values, which should be ignored when adjusting models to them. This work presents a Lower Order-value Optimization (LOVO) version of the Levenberg-Marquardt algorithm, which is well suited to deal with outliers in fitting problems. A general algorithm is presented and convergence to stationary points is demonstrated. Numerical results show that the algorithm is successfully able to detect and ignore outliers without too many specific parameters. Parallel and distributed executions of the algorithm are also possible, allowing for the use of larger datasets. Comparison against publicly available robust algorithms shows that the present approach is able to find better adjustments in well known statistical models.

An in-field interaction plot, delta-M_R, has been recently introduced, presenting important advantages over the classical remanence plots. Here a general delta-M_R is proposed, allowing to assess interactions even in systems with shifted and asymmetric major loops. To construct such a plot, a recoil loop and the position of the center of the major loop are only needed. Applying the method on exchange-biased Co/IrMn bilayer gives two types of delta-M_R obtained for measuring field either parallel or antiparallel to the exchange-bias direction. This provides valuable information on the reversal mechanism and allows distinguishing between effects coming from magnetic coupling into the ferromagnet (Co) and those stemming from interactions in its interface with the antiferromagnet (IrMn). The essentially nonzero general delta-M_R plot obtained from the major loop revealed to be a precise measure of the reversal asymmetry. The technique can readily be adjusted for use in other scientific fields where hysteresis is observed. We provide free software which generates such delta-M_R plot(s)., Comment: 7 pages, 6 figures

We investigate vortex configuration in antiferromagnetic thin discs. It is shown that the vortex acquires oscillatory dynamics with well-defined amplitude and frequency which may be controlled on demand by an alternating spin polarized current. These findings may be useful for the emerging field of antiferromagnetic topological spintronics, once vortex dynamics may be controlled by purely electric means., Comment: 6 pages (including references); 4 figures

Pinor and spinor fields are sections of the subbundles whose fibers are the representation spaces of the Clifford algebra of the forms, equipped with the Graf product. In this context, pinors and spinors are here considered and the geometric generalized Fierz identities provide the necessary framework to derive and construct new spinor classes on the space of smooth sections of the exterior bundle, endowed with the Graf product, for prominent specific signatures, whose applications are discussed., Comment: 21 pages, matches the published version

The geometric product, defined by Graf on the space of differential forms, endows the sections of the exterior bundle by a structure that is necessary to construct a Clifford algebra. The Graf product is introduced and revisited with a suitable underlying framework that naturally encompasses a coframe in the cotangent bundle, besides the volume element centrality, the Hodge operator and the so called truncated subalgebra as well., Comment: 16 pages, improved version matching the published one

A $(k_1+k_2)$-bispindle is the union of $k_1$ $(x,y)$-dipaths and $k_2$ $(y,x)$-dipaths, all these dipaths being pairwise internally disjoint. Recently, Cohen et al. showed that for every $(1,1)$- bispindle $B$, there exists an integer $k$ such that every strongly connected digraph with chromatic number greater than $k$ contains a subdivision of $B$. We investigate generalisations of this result by first showing constructions of strongly connected digraphs with large chromatic number without any $(3,0)$-bispindle or $(2,2)$-bispindle. Then we show that strongly connected digraphs with large chromatic number contains a $(2,1)$-bispindle, where at least one of the $(x,y)$-dipaths and the $(y,x)$-dipath are long.

We report on magneto-optical imaging, magnetization, Hall effect and magneto-resistance experiments in Nb/Al_2O_3/Co thin film heterostructures. The magnetic field is applied perpendicularly to the plane of the film and gives rise to abrupt flux penetration of dendritic form. A magnetization texture is imprinted in the Co layer in perfect coincidence with these ramifications. The spin domains that mimic the vortex dendrites are stable upon the field removal. Moreover, the imprinted spin structure remains visible up to room temperature. Complementary magnetization, Hall effect and magneto-resistance experiments were performed in a similar sample where electrical contacts were placed on the Co layer. In the region of the field - temperature diagram where flux instabilities are known to occur in Nb films, irregular jumps are observed in the magnetic hysteresis and large amplitude noise is detected in the magneto-resistance and Hall resistivity data when measured as a function of the field., Comment: 13 pages, 8 figures