The Extremely Brilliant Source (EBS), the first high-energy 4th-generation synchrotron radiation source, constructed at the ESRF and based upon the novel concept of a Hybrid Multi-Bend Achromat (HMBA), has started user operation on August 25th, 2020. We report here on selected recent scientific results exploiting the greatly improved performances of this novel X-ray source.

A topological theory of the diabolical points (degeneracies) of quantum magnets is presented. Diabolical points are characterized by their diabolicity index, for which topological sum rules are derived. The paradox of the the missing diabolical points for Fe8 molecular magnets is clarified. A new method is also developed to provide a simple interpretation, in terms of destructive interferences due to the Berry phase, of the complete set of diabolical points found in biaxial systems such as Fe8.

W estudy theenergy spectrum ofm agnonsin a ferrom agnetwith topologically nontrivialm agnetization pro le.In thecaseofinhom ogeneousm agnetization corresponding to a m etastable stateof ferrom agnet,the spin-wave equation ofm otion acquiresa gauge potentialleading to a Berry phase for the m agnons propagating along a closed contour. The e ect ofm agnetic anisotropy is crucial fortheBerry phase:we show thattheanisotropy suppressesitsm agnitude,which m akestheBerry phase observable in som e cases,sim ilar to the Aharonov-Bohm e ect for electrons. For exam ple, it can be observed in the interference ofspin waves propagating in m esoscopic rings. W e discuss thee ectofdom ain wallson theinterferencein ferrom agnetic rings,and propose som eexperim ents with a certain geom etry ofm agnetization.W e also show thatthe nonvanishing average topological eld actson them agnonslike a uniform m agnetic eld on electrons.Itleadsto thequantization of the m agnon spectrum in the topological eld.

The local polarization of electromagnetic (EMW) and gravitational waves (GW) is discussed from an operational point of view, in which all the relevant mathematical framework is constructed in terms of measurements of the power absorbed by a local detector. The intrinsic dependence of the observations upon the nature of the detector is emphasized. In particular, the benefit of using a dual-symmetric detector, equally sensitive to the electric and magnetic fields of the EMW (resp. gravito-electric and gravito-magnetic tensor in the GW case) is pointed out. The Majorana stellar representation of the polarization is introduced, and its physical interpretation is highlighted. Finally, expressions for the energy density, linear momentum density, helicity and spin density of the wave in terms of the Majorana representation are presented.

A geometric theory of the irreducible tensor operators of quantum spin systems. It is based upon the Maxwell-Sylvester geometric representation of the multipolar electrostatic potential. In the latter, an order-ℓ multipolar potential is represented by a collection of ℓ equal length vectors, i.e. by ℓ points on a sphere, instead of by its components on some fixed (but arbitrary) basis. The geometric representation offers a much more appropriate tool for getting physical insight on specific characteristics of a multipole, such as its symmetries, or its departure from ideal symmetry. We derive explicit expressions enabling to perform any calculations we may need to perform on multipoles. All relevant quantities are eventually expressed in terms of scalar products of pairs of vectors (i.e., in terms of geometric quantities such as lengths and angles). The whole formalism is entirely independent of any particular choice of coordinate, and needs no use of the somehow abstract formalism traditionally used when dealing with angular momenta. The formalism is then applied to treat the problem of the irreducible tensor operators of quantum spin systems. It enables to completely dispense with the calculation and use of the Stevens operators, which can be quite complicated even for moderate values of ℓ. Explicit expressions for the calculation of expectations values of physical observables are derived. They essentially consist in combinations of scalar products of vector pairs. Together with the coherent state representation of the quantum states of spin systems, this provides a complete geometric, coordinate-free, description of the states, dynamics and physical properties of these systems.

This is an introduction to the theoretical physics of metals for students and physicists from other specialities. Certain simple consequences of the Fermi statistics in pure metals are first addressed, namely the Peierls distortion, Kohn anomalies and the Labbe-Friedel distortion. Then the physics of dilute alloys is discussed. The analogy with nuclear collisions was a fruitful starting point, which suggested one should analyze the effects of impurities in terms of a scattering problem with the introduction of phase shifts. Starting from these concepts, Friedel derived a theory of the resistivity of alloys, and a celebrated sum rule relating the phase shifts at the Fermi level to the number of electrons in the impurity, which turned out to play a prominent role later in the context of correlated impurities, as for instance in the Kondo effect. Friedel oscillations are also an important result, related to incommensurate magnetic structures. It is shown how they can be derived in various ways: from collision theory, perturbation theory, self-consistent approximations and Green's function methods. While collision theory does not permit to take the crystal structure into account, which is responsible for electronic bands, those effects can be included in other descriptions, using for instance the tight binding approximation. (C) 2015 The Authors. Published by Elsevier Masson SAS on behalf of Academie des sciences.

The magnetic and structural properties of cobalt were investigated under high pressure (160 GPa) and low temperature (50 K), by synchrotron K-edge x-ray magnetic circular dichroism and x-ray diffraction. A quasihydrostatic equation of state was measured up to 160 GPa. We found that uniaxial stress plays a role in the hexagonal close packed-face centered cubic (hcp-fcc) structural transition pressure. Also, our data provide the first experimental evidence that changes of the c/a ratio pressure derivative are related to the magnetic behavior. The complete extinction of ferromagnetism is observed above 130 GPa in a mixed hcp-fcc phase with no recovery upon cooling to 50 K, indicating that cobalt at 150 GPa is very likely nonmagnetic, i.e., characterized by zero local spin polarization. Density functional theory calculations point out that the K-edge x-ray magnetic circular dichroism (XMCD) signal is related to the 4p orbital moment rather than to the total spin moment and allow us to get a deeper insight into the K-edge XMCD measurements interpretation. The combination of novel theoretical results and experimental outputs provides a detailed scenario of the structural and magnetic properties of cobalt at these extreme conditions answering some previously unsolved issues.

The magnetic and structural phase diagram of equiatomic FeCo has been studied up to 45 GPa using K-edge x-ray magnetic circular dichroism, x-ray absorption near edge spectroscopy, x-ray diffraction, and supporting density-functional-theory-based calculations. FeCo foils with different degrees of chemical order were obtained by magnetron sputtering. Our results show that Fe0.5Co0.5 undergo the bcc ferromagnetic to hcp nonferromagnetic transition in the 30-45 GPa pressure range. Interestingly, the chemical order, i.e., the relative arrangements of Fe and Co atoms, plays a major role in affecting the high-pressure structural and magnetic phase diagram of these alloys. This result is confirmed by first-principles modeling of different structures of equiatomic FeCo alloy. Moreover, the total-energy analysis reveals a strong competition between different magnetic hcp states upon compression. A possible emergence of antiferromagnetism is emphasized and requires further experimental investigation.

I present arguments indicating the impossibility of spontaneously rotating "quantum time crystals," as recently proposed by Frank Wilczek. In particular, I prove a "no-go theorem," rigorously ruling out the possibility of spontaneous ground-state (or thermal equilibrium) rotation for a broad class of systems.

A Comment on "Space-Time Crystals of Trapped ions", by Tongcang Li et al. (PRL 109, 163001 (2012); arXiv:1206.4772

A Comment on Frank Wilczek's paper "Quantum Time Crystals" (Phys. Rev. Lett. 109, 160401 (2012); arXiv:1202.2539).

The (Berry-Aharonov-Anandan) geometric phase acquired during a cyclic quantum evolution of finite-dimensional quantum systems is studied. It is shown that a pure quantum state in a ($2J+1$)-dimensional Hilbert space (or, equivalently, of a spin-$J$ system) can be mapped onto the partition function of a gas of independent Dirac strings moving on a sphere and subject to the Coulomb repulsion of $2J$ fixed test charges (the Majorana stars) characterizing the quantum state. The geometric phase may be viewed as the Aharonov-Bohm phase acquired by the Majorana stars as they move through the gas of Dirac strings. Expressions for the geometric connection and curvature, for the metric tensor, as well as for the multipole moments (dipole, quadrupole, etc.), are given in terms of the Majorana stars. Finally, the geometric formulation of the quantum dynamics is presented and its application to systems with exotic ordering such as spin nematics is outlined.

The structural stability of fcc Ni over a very large pressure range offers a unique opportunity to experimentally investigate how magnetism is modified by simple compression. K-edge x-ray magnetic circular dichroism (XMCD) shows that fcc Ni is ferromagnetic up to 200 GPa, contradicting recent predictions of an abrupt transition to a paramagnetic state at 160 GPa. Density functional theory calculations point out that the pressure evolution of the K-edge XMCD closely follows that of the p projected orbital moment rather than that of the total spin moment. The disappearance of magnetism in Ni is predicted to occur above 400 GPa.

The dynamic magnetic susceptibility is calculated for bulk bcc iron, a supported monolayer Fe/W(110) and corresponding free standing Fe monolayer. The calculational approach is based on linear response density functional theory formulated within the framework of Korringa–Kohn–Rostoker Green's function method. The calculated susceptibilities are analyzed in terms of dispersion and damping of spin-waves. The results for bulk Fe are in good agreement with experiment and previous theoretical studies. In contrast to the bulk phase, the spin-wave excitations in thin films are well defined in the whole two-dimensional Brillouin zone. It is shown that a non-magnetic substrate changes strongly both the dispersion and damping of the spin-waves.

We consider the Casimir interaction between a ferromagnetic and a nonmagnetic mirror and show how the Casimir effect gives rise to a magnetic anisotropy in the ferromagnetic layer. The anisotropy is out of plane if the nonmagnetic plate is optically isotropic. If the nonmagnetic plate shows a uniaxial optical anisotropy (with optical axis in the plate plane), we find an in-plane magnetic anisotropy. In both cases, the energetically most favorable magnetization orientation is given by the competition between polar, longitudinal, and transverse contributions to the magneto-optical Kerr effect and will therefore depend on the interplate distance. Numerical results will be presented for a magnetic plate made out of Fe and nonmagnetic plates of Au (optically isotropic), quartz, calcite, and barium titanate (all uniaxially birefringent).

An ab initio study of magnetic exchange interactions in antiferromagnetic and strongly correlated 3d transition metal monoxides is presented. Their electronic structure is calculated using the local self-interaction correction approach, implemented within the Korringa-Kohn-Rostoker band structure method, which is based on multiple scattering theory. The Heisenberg exchange constants are evaluated with the magnetic force theorem. Based on these the corresponding Neel temperatures T_N and spin wave dispersions are calculated. The Neel temperatures are obtained using mean field approximation, random phase approximation and Monte Carlo simulations. The pressure dependence of T_N is investigated using exchange constants calculated for different lattice constants. All the calculated results are compared to experimental data.

The energies and lifetimes of magnons in several Mn-based Heusler alloys are studied using linear response density functional theory. The number of the spin wave branches in Co(2)MnSi corresponds to the number of its magnetic sublattices in contrast with the NiMnSb case in which the induced Ni sublattice cannot support optical magnons. The half-metallicity of these systems results in long-living acoustic spin waves. The example of non-half-metallic Cu(2)MnAl shows that the hybridization with Stoner continuum leads not only to the damping of magnons but also to a renormalization of their energies.

Single Ni, Co, and Cu atoms deposited in the center of cobalt nanoislands grown on Cu(111) are investigated by low-temperature scanning tunneling spectroscopy. The surface states of this model magnetic nanolead are unveiled and assigned to arise from atomiclike and surface-induced states by ab initio calculations. Contrary to the first, the second contribution is predicted by calculations to favor a change in sign of the spin polarization with respect to the pristine lead.