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Charge and spin density in helical Luttinger liquids (Vol. 47 No. 3)
Often an electron confined to one spatial dimension does not have many options. It can have spin up or down and it can go right or left. In helical systems the possibilities are further reduced: the spin projection is tied to the direction of motion. This phenomenon is called spin-momentum locking and takes place at the edges of two-dimensional topological insulators. Its consequences are exciting for spintronics: since spin up and down electrons counter-propagate, spin transport can be easily generated by charge currents. In our work, we identify peculiar properties of the weakly interacting helical Luttinger liquid. We address the interesting question: Are the charge density and spin correlations influenced by spin-momentum locking? While for strong interactions the system becomes a Wigner crystal of fractional charges e/2 built on a strongly anisotropic spin wave (which we have shown in a previous publication), in this work, we demonstrate that, for weak interactions, density correlations are featureless, i.e. the density is not affected by impurities. However, spin correlations are well represented by a planar spin helix that can be pinned by magnetic impurities.
N. Traverso Ziani, C. Fleckenstein, F. Crépin and B. Trauzettel, Charge and spin density in the helical Luttinger liquid, EPL 113, 37002 (2016)
[Abstract]
Analysing Coulomb-excitation experiments with exotic beams (Vol. 47 No. 3)
This paper presents a number of novel and alternative analysis techniques to extract transition strengths and quadrupole moments from Coulomb excitation data with Radioactive Ion Beams (RIBs) using the GOSIA code. It is anticipated that related approaches and techniques will gain an even greater importance as a wider range of post-accelerated RIBs becomes available at the next generation of ISOL facilities.
Indeed, recent advances in RIB technology, in particular the increasing range of species and post-acceleration energies available from ISOL facilities, have led to a resurgence of the use of nuclear reactions to study the structure of nuclei. Specifically, Coulomb excitation at safe energies, where the surfaces of the colliding nuclei are kept apart (typically 2-5 MeV/A), is now giving unrivalled information on the electromagnetic properties of exotic nuclei, leading to knowledge of the nuclear shape or, more precisely, the charge distribution of individual states. The usefulness of the technique to extract key nuclear-structure information has been demonstrated since the 1960's with stable beam and target combinations.
The paper addresses the situation where new challenges present themselves when studying exotic nuclei, including dealing with low statistics or number of data points, absolute and relative normalization of the measured cross sections and a lack of complementary experimental data, such as excited-state lifetimes, mixing and branching ratios.
M. Zielińska + 9 co-authors, Analysis methods of safe Coulomb-excitation experiments with
radioactive ion beams using the GOSIA code, Eur. Phys. J. A 52, 99 (2016)
[Abstract]
Timeless thoughts on the definition of time (Vol. 47 No. 3)
On the evolution of how we have defined time, time interval and frequency since antiquity.
The earliest definitions of time and time-interval quantities were based on observed astronomical phenomena, such as apparent solar or lunar time, and as such, time as measured by clocks, and frequency, as measured by devices were derived quantities. In contrast, time is now based on the properties of atoms, making time and time intervals themselves derived quantities. Today’s definition of time uses a combination of atomic and astronomical time. However, their connection could be modified in the future to reconcile the divergence between the astronomic and atomic definitions. These are some of the observations made by the author of this riveting work published recently, which provides unprecedented insights into the nature of time and its historical evolution.
J. Levine, The history of time and frequency from antiquity to the present day, Eur. Phys. J. H 41, 1 (2016)
[Abstract]
Repulsive Casimir forces at quantum criticality (Vol. 47 No. 3)
Casimir forces act between macroscopic objects immersed in a fluctuating entity, which may be the quantum vacuum or material medium in a state hosting sizable fluctuations. These forces were first discussed in 1948 as an observable manifestation of the quantum nature of the vacuum, lying at the heart of quantum electrodynamics. At a somewhat later stage it was realized that a material medium brought to the vicinity of a critical state induces analogous interactions once some macroscopic bodies become immersed therein.
In most of the known cases the Casimir force is attractive in situations where the bodies in question are identical. This however turns out not to be a general rule.
In our theoretical work we addressed a system of bosonic particles in the vicinity of a quantum critical state, where both thermal and quantum fluctuations are strong. As our exact analysis indicates, the sign of the Casimir force between two bodies immersed in such a medium may be changed by varying the ratio between their separation D and the thermal de Broglie length λ. In the thermal regime D>>λ the force in question is attractive, however, by varying the system setup so that D<<λ one crosses over to a regime admitting repulsive Casimir interactions.
P. Jakubczyk, M. Napiórkowski and T. Sęk, Repulsive Casimir forces at quantum criticality, EPL 113, 30006 (2016)
[Abstract]
