Showing papers in "Annalen der Physik in 2020"

Atomic Physics Studies at the Gamma Factory at CERN

Abstract: The Gamma Factory initiative proposes to develop novel research tools at CERN by producing, accelerating and storing highly relativistic, partially stripped ion beams in the SPS and LHC storage rings. By exciting the electronic degrees of freedom of the stored ions with lasers, high-energy narrow-band photon beams will be produced by properly collimating the secondary radiation that is peaked in the direction of ions' propagation. Their intensities, up to $10^{17}$ photons per second, will be several orders of magnitude higher than those of the presently operating light sources in the particularly interesting $\gamma$--ray energy domain reaching up to 400 MeV. This article reviews opportunities that may be afforded by utilizing the primary beams for spectroscopy of partially stripped ions circulating in the storage ring, as well as the atomic-physics opportunities afforded by the use of the secondary high-energy photon beams. The Gamma Factory will enable ground breaking experiments in spectroscopy and novel ways of testing fundamental symmetries of nature.

The Role of Odd-Frequency Pairing in Multiband Superconductors

Abstract: In this article we review recent progress in the understanding of multiband superconductivity and its relationship to odd-frequency pairing. We begin our discussion by reviewing the emergence of odd-frequency pairing in a simple two-band model, providing a brief pedagogical overview of the formalism. We then examine several examples of multiband superconducting systems in each case describing, both, the origin of the band degree of freedom and the nature of the odd-frequency pairing. Throughout, we attempt to convey a unified picture of how odd-frequency pairing emerges in these materials and propose that similar mechanisms are responsible for odd-frequency pairing in several analogous systems: layered two-dimensional heterostructures, double quantum dots, double nanowires, Josephson junctions, and systems described by isolated valleys in momentum space. We also review experimental probes of odd-frequency pairing in multiband systems, focusing on hybridization gaps in the electronic density of states, paramagnetic Meissner effect, and Kerr effect.

Magnon Blockade in a PT‐Symmetric‐Like Cavity Magnomechanical System

Abstract: We investigate the magnon blockade effect in a parity-time (PT) symmetric-like three-mode cavity magnomechanical system involving the magnon-photon and magnon-phonon interactions. In the broken and unbroken PT-symmetric regions, we respectively calculate the second-order correlation function analytically and numerically and further determine the optimal value of detuning. By adjusting different system parameters, we study the different blockade mechanisms and find that the perfect magnon blockade effect can be observed under the weak parameter mechanism. Our work paves a way to achieve the magnon blockade in experiment.

Out‐of‐Time‐Order Correlators and Quantum Phase Transitions in the Rabi and Dicke Models

Abstract: The out‐of‐time‐order correlators (OTOCs) is used to study the quantum phase transitions (QPTs) between the normal phase and the superradiant phase in the Rabi and few‐body Dicke models with large frequency ratio of the atomic level splitting to the single‐mode electromagnetic radiation field frequency. The focus is on the OTOC thermally averaged with infinite temperature, which is an experimentally feasible quantity. It is shown that the critical points can be identified by long‐time averaging of the OTOC via observing its local minimum behavior. More importantly, the scaling laws of the OTOC for QPTs are revealed by studying the experimentally accessible conditions with finite frequency ratio and finite number of atoms in the studied models. The critical exponents extracted from the scaling laws of OTOC indicate that the QPTs in the Rabi and Dicke models belong to the same universality class.

Protecting Quantum Fisher Information in Correlated Quantum Channels

Abstract: Quantum Fisher information (QFI) has potential applications in quantum metrology tasks. We investigate QFI when the consecutive actions of a quantum channel on the sequence of qubits have partial classical correlations. Our results showed that while the decoherence effect is detrimental to QFI, effects of such classical correlations on QFI are channel-dependent. For the Bell-type probe states, the classical correlations on consecutive actions of the depolarizing and phase flip channels can be harnessed to improve QFI, while the classical correlations in the bit flip and bit-phase flip channels induce a slight decrease of QFI. For a more general parameterization form of the probe states, the advantage of using initial correlated system on improving QFI can also be remained in a wide regime of the correlated quantum channels.

Dynamically Induced Excitonic Instability in Pumped Dirac Materials

Abstract: Driven and non-equilibrium quantum states of matter have attracted growing interest in both theoretical and experimental studies in condensed matter physics. Recent progress in realizing transient ...

Hydrostatic and Uniaxial Pressure Tuning of Iron‐Based Superconductors: Insights into Superconductivity, Magnetism, Nematicity, and Collapsed Tetragonal Transitions

Abstract: Iron-based superconductors are well-known for their intriguing phase diagrams, which manifest a complex interplay of electronic, magnetic and structural degrees of freedom. Among the phase transitions observed are superconducting, magnetic, and several types of structural transitions, including a tetragonal-to-orthorhombic and a collapsed-tetragonal transition. In particular, the widely-observed tetragonal-to-orthorhombic transition is believed to be a result of an electronic order that is coupled to the crystalline lattice and is, thus, referred to as nematic transition. Nematicity is therefore a prominent feature of these materials, which signals the importance of the coupling of electronic and lattice properties. Correspondingly, these systems are particularly susceptible to tuning via pressure (hydrostatic, uniaxial, or some combination). We review efforts to probe the phase diagrams of pressure-tuned iron-based superconductors, with a strong focus on our own recent insights into the phase diagrams of several members of this material class under hydrostatic pressure. These studies on FeSe, Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$, Ca(Fe$_{1-x}$Co$_x$)$_2$As$_2$ and CaK(Fe$_{1-x}$Ni$_x$)$_4$As$_4$ were, to a significant extent, made possible by advances of what measurements can be adapted to the use under differing pressure environments. We point out the potential impact of these tools for the study of the wider class of strongly correlated electron systems.

Magnetic Textures and Dynamics in Magnetic Weyl Semimetals

Abstract: Recent theoretical and experimental attemps have been successful in finding magnetic Weyl semimetal phases, which show both nodal-point structure in the electronic bands and magnetic orders. Beyond uniform ferromagnetic or antiferromagnetic orders, nonuniform magnetic textures, such as domain walls and skyrmions, may even more enrich the properties of the Weyl electrons in such materials. This article gives a topical review on interplay between Weyl electrons and magnetic textures in those magnetic Weyl semimetals. The basics of magnetic textures in non-topological magnetic metals are reviewed first, and then the effect of magnetic textures in Weyl semimetals is discussed, regarding the recent theoretical and experimental progress therein. The idea of the fictitious "axial gauge fields" is pointed out, which effectively describes the effect of magnetic textures on the Weyl electrons and can well account for the properties of the electrons localized around magnetic domain walls.

Full Manipulation of the Power Intensity Pattern in a Large Space-Time Digital Metasurface: From Arbitrary Multibeam Generation to Harmonic Beam Steering Scheme

Abstract: Beyond the scope of space-coding metasurfaces, space-time digital metasurfaces can substantially expand the application scope of digital metamaterials in which simultaneous manipulation of electromagnetic waves in both space and frequency domains would be feasible. In this paper, by adopting a superposition operation of terms with unequal coefficient, Huygens principle, and a proper time-varying biasing mechanism, some useful closed-form formulas in the class of large digital metasurfaces were presented for predicting the absolute directivity of scatted beams. Moreover, in the harmonic beam steering scheme, by applying several suitable assumptions, we have derived two separate expressions for calculating the exact total radiated power at harmonic frequencies and total radiated power for scattered beams located at the end-fire direction. Despite the simplifying assumptions we have applied, we have proved that the provided formulas can still be a good and fast estimate for developing a large digital metasurface with a predetermined power intensity pattern. The effect of quantization level and metasurface dimensions on the performance of power manipulating as well as the limitation on the maximum scan angle in harmonic beam steering have been addressed. Several demonstrative examples numerically demonstrated through MATLAB software and the good agreement between simulations and theoretical predictions have been observed. By considering the introduced restrictions in the manuscript, this method can be implemented in any desired frequency just by employing phase-only meta-particles as physical coding elements. The author believes that the proposed straightforward approach discloses a new opportunity for various applications such as multiple-target radar systems and THz communication.

Van Der Waals Heterostructures with Spin‐Orbit Coupling

Abstract: In this article we review recent work on van der Waals (vdW) systems in which at least one of the components has strong spin-orbit coupling. We focus on a selection of vdW heterostructures to exemplify the type of interesting electronic properties that can arise in these systems. We first present a general effective model to describe the low energy electronic degrees of freedom in these systems. We apply the model to study the case of (vdW) systems formed by a graphene sheet and a topological insulator. We discuss the electronic transport properties of such systems and show how they exhibit much stronger spin-dependent transport effects than isolated topological insulators. We then consider vdW systems in which the layer with strong spin-orbit coupling is a monolayer transition metal dichalcogenide (TMD) and briefly discuss graphene-TMD systems. In the second part of the article we discuss the case in which the vdW system includes a superconducting layer in addition to the layer with strong spin-orbit coupling. We show in detail how these systems can be designed to realize odd-frequency superconducting pair correlations. Finally, we discuss twisted graphene-NbSe2 bilayer systems as an example in which the strength of the proximity-induced superconducting pairing in the normal layer, and its Ising character, can be tuned via the relative twist angle between the two layers forming the heterostructure.