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A New High for Magnetically Doped Topological Insulators (Vol. 47 No. 5-6)

Temperature dependence of the magnetization, M(T), of CrxSb2-xTe3 thin film samples with varying Cr concentration, x. The most highly doped and structurally uncompromised film shows a transition temperature of 125 K

Topological insulators (TIs) are a new phase of quantum matter whose conducting surface states are a result of the topology of their bulk band structure. Their spin-momentum locked topological surface states are resilient to backscattering owing to their protection by time-reversal symmetry (TRS). These properties make them intriguing candidates for low-power devices, spintronics and quantum computation. Breaking TRS by introducing magnetic dopants, and introducing a gap in the topological surface states, unlocks exotic quantum phenomena such as the quantum anomalous Hall state. Doping TIs with magnetic impurities is an experimentally challenging process and most TI materials only exhibit magnetic ordering at low temperatures.

In this study, using a variety of complementary structural, electronic and magnetic characterisation techniques, we demonstrate the synthesis of magnetically doped TI thin films with high structural quality. The Cr-doped Sb2Te3 thin films were grown on sapphire using low-temperature molecular beam epitaxy. We show that this particular system exhibits uniform ferromagnetic ordering up to ~125 K – a step forward towards device-friendly TI materials.

L. J. Collins-McYntire + 13 co-authors, Structural, electronic, and magnetic investigation of magnetic ordering in MBE-grown CrxSb2-xTe3 thin films, EPL 115, 27006 (2016)
[Abstract]

A note on Pöschl-Teller black holes (Vol. 43 No. 4)

An interesting feature of black holes is the existence of quasi-normal modes, arising because the system has a peak in the wave potential (scalar, electromagnetic, or gravitational waves). The quasi-normal mode is excited when a disturbance is put in the field near but outside the black hole, (like a wave packet roughly in a circular orbit near the peak).

The excitation then propagates outward and inward and decays. An excitation “mode” has a definite complex frequency: a given oscillation rate in time, and a corresponding decay rate. For gravitational radiation from a spherical (Schwarzschild) black hole, the least damped mode is: ei 0.747t/tH e−0.178t/tH with tH the time for light to travel a distance equal to the radius of the black hole [S. Chandrasekhar and S. Detweiler, Proc. Roy. Soc. London, 344 (1975) 441].

To calculate these modes is typically a computational problem, with attendant difficulties in controlling errors and convergence. A partial step to ameliorate these difficulties has been to substitute the black hole potential (long range, polynomial decay to infinity), with more localized potentials decaying exponentially at infinity. Pöschl and Teller [G. Pöschl and E. Teller, Z. Phys. 83 (1933) 143] suggested one such potential in another context: 1/cosh2 α(r- r0).

This is much simpler – and decays more rapidly – than the correct gravitational potential, but to date even this potential has required numerical/computational treatment. Now, however, Zarrinkamar, Hassanabadi and Eskolaki have found an ingenious analytic transformation of the Pöschl-Teller wave equation with immediate solution in terms of Jacobi polynomials. Jacobi polynomials are well studied and characterized classical “special functions”. Thus questions of accuracy and convergence are now under control, and Zarrinkamar et al. have completely solved the quasi-normal mode problem for the Pöschl-Teller black hole.

A note on Pöschl-Teller black holes
S. Zarrinkamar, H. Hassanabadi, A.A. Rajabi and P. Ghalandari Eskolaki, Eur. Phys. J. Plus 127, 56 (2012) [Abstract]

A possible source of ultra-high-energy cosmic rays (Vol. 44 No. 1)

The origin of ultra-high-energy cosmic rays, with energies around the GZK (Greisen-Zatsepin-Kuzmin) cut-off, remains an unsolved mystery. According to this cut-off, the mean free path of very energetic particles in the Universe does not exceed 50 megaparsec, due to their scattering on the cosmic microwave background radiation. However, there are no conventional sources of ultra-high-energy cosmic rays inside this radius. Hence some new sources seem to be necessary.

In the present letter a novel and intriguing explanation is suggested that links far-reaching fundamental aspects of F(R) modified theories to an efficient production of highly energetic cosmic rays during the recent history of the Universe (let us recall that F(R) theories present a modification of the usual General Relativity by an addition of a non-linear function F(R) of the scalar curvature R. This function is chosen in such a way that it leads to accelerated cosmological expansion indicated by the recent astronomical data).

At the core of this work lies the proof that in cosmological and astrophysical systems with rising energy densities, the F(R) modified theories of gravity exhibit powerful oscillations of the curvature scalar R, with an amplitude much larger than the standard value of curvature predicted by the General Relativity. These oscillations are strongly anharmonic, with frequencies that can be as large as billions of GeV. This striking and rather unexpected oscillatory behaviour of R lends support to the idea that ultra-high energy cosmic rays can be generated by such curvature oscillations at the appropriate cosmological redshifts.

E.V. Arbuzova, A.D. Dolgov and L. Reverberi, ‘Curvature oscillations in modified gravity theories as possible source of ultra-high-energy cosmic rays’, Eur. Phys. J. C, 72, 2247 (2012)
[Abstract]

A question of reality (Vol. 51, No. 5)

Spooky action at a distance: Bell’s Theorem in sketch form (Figure 12 in the paper). Credit: Reinhold Bertlmann (1988). Original presented to Bell on the occasion of his 60th birthday by R. Bertlmann

John Stewart Bell’s eponymous theorem and inequalities set out, mathematically, the contrast between quantum mechanical theories and local realism. They are used in quantum information, which has evolving applications in security, cryptography and quantum computing.

The distinguished quantum physicist John Stewart Bell (1928-1990) is best known for the eponymous theorem that proved current understanding of quantum mechanics to be incompatible with local hidden variable theories. Thirty years after his death, his long-standing collaborator Reinhold Bertlmann of the University of Vienna, Austria, has reviewed his thinking in a paper for EPJ H, ‘Real or Not Real: That is the question’. In this historical and personal account, Bertlmann aims to introduce his readers to Bell’s concepts of reality and contrast them with some of his own ideas of virtuality.

R. Bertlmann, Real or Not Real: that is the question, Eur. Phys. J. H 45, 205–236 (2020)
[Abstract]

A step closer to composite-based electronics (Vol. 45 No. 1)

An illustration of a small portion of a square lattice.

A new study demonstrates that electrical resistivity obeys a staircase-like dependence on the conducting particle concentration in composite materials. These materials are attractive because they have a controllable electrical resistivity combined with their light and flexible properties. This makes them suited for applications in flexible electronics. Now, a theoretical model, confirmed experimentally, elucidates how electrical resistivity varies with the concentration of the particles in these composite materials.

The authors made the theoretical prediction - and proved experimentally using granular metal and carbon-black composites – that the dependence of the electrical resistance on the conducting particle concentration is manifested by a staircase. This was particularly obvious in nanometric scale systems, in which there is a well-defined discrete series of distances between a particle and its neighbours. Each stair exhibits a universal behaviour— independent of the details of the system—predicted by percolation theory. The electrical resistivity associated with subsequent stairs decreases as the concentration of the conducting particles increases.

I. Balberg, D. Azulay, Y. Goldstein, J. Jedrzejewski, G. Ravid and E. Savir, ‘The percolation staircase model and its manifestation in composite materials’, Eur. Phys. J. B, 86, 428 (2013)
[Abstract]

A topological analysis of plasma flow structures (Vol. 45 No. 1)

Filamentary structure of plasma turbulence.

In toroidally confined plasmas and from a fluid perspective, plasma turbulence is characterized by the existence of multiple vortices located at the magnetic surfaces where the magnetic field lines close on themselves after a finite number of turns around the torus. When we look at transport in such systems, we see that these vortices may cause the trapping of particles, while large scale flows may carry them from vortex to vortex. We develop an analysis approach that has allowed us a complete characterization of the structures of the vortices, determining which ones form close loops, cycles, and which ones have just a finite length, filaments, and make a determination of their length. By comparing these structures at different times we also can determine the life times of the cycles. We have found that both life times of the cycles and lengths of the filaments are well described by lognormal distributions. Having the distribution of the life times of the cycles and lengths of the filaments, we can connect them to the trapping time of particles moving with the turbulence.

B. A. Carreras, I. Llerena Rodríguez and L. García, “A topological analysis of plasma flow structures”, J. Phys. A: Mth. Theor., 46, 375501 (2013)
[Abstract]

A tunnelling probe for molecular currents (Vol. 42, No. 2)

image Laser-induced tunnelling ionization of a multi-electron system (blue wavefunction) triggers charge oscillations of the created electron vacancy (shown in red). A second tunnelling step probes the temporal rearrangement of the vacancy, enabling its characterization.

Internal electron currents in molecules play a crucial role in chemical and biological processes, like charge transport in cellular respiration and in photosynthesis. Since electron currents can be ultrafast and escape most traditional probes, they are hard to capture. We show that laser-induced tunnel ionization is a powerful probe of internal currents.

An intense infrared laser field acts on a molecule essentially as the tip of a scanning tunnelling microscope (STM): it extracts a weakly bound electron through a tunnelling barrier. The electron is not equally likely to tunnel out in any direction when the orbital has an asymmetric shape - this has led to the development of the molecular STM, a probe of the static electronic structure.

Unlike in the STM, tunnelling in a laser field is an attosecond phenomenon and therefore potentially launches attosecond electron dynamics. Such dynamics has also been inferred from high-harmonic generation, but that method is insensitive to the actual degree of electronic coherence. Here, we show that laser-induced tunnelling directly probes time-dependent deformations of the electron cloud and maps them on two complementary observables: the total tunnelling current and its momentum distribution.

We study spin-orbit dynamics in a rare gas ion, the simplest example of an internal electron current launched by ionization. Such currents are ubiquitous in molecules, where the sudden departure of an electron triggers an internal rearrangement. Laser-induced tunnelling is a powerful probe of such fundamental events.

A new aspect offered by the proposed concept is the control over electronic dynamics and double ionization. We show that the spin state (i.e. the entanglement) of the ejected electron pair can be controlled offering interesting opportunities for quantum control.

Imaging and controlling multi-electron dynamics by laser-induced tunnel ionization
H. J. Wörner and P.B. Corkum, J. Phys. B: At. Mol. Opt. Phys. 44, 041001 (2011) [Abstract] | [PDF]

A unified approach for extrinsic loss in optical waveguides (Vol. 45 No.3)

Roughness-induced radiative losses and backscattering in real optical waveguides.

In real optical waveguides, fabrication tolerances cause the unavoidable appearance of sidewall roughness, that is a local and random deviation of the waveguide width from its nominal value. The interaction of the light propagating in the waveguide with sidewall roughness induces a coupling mechanism which transfers part of the optical power from the propagating mode(s) to other guided modes (propagating and counter-propagating) and radiative modes. Backscattered and radiated light result in what is usually referred to as extrinsic loss, which is typically the dominant loss contribution of optical waveguides.

In this paper, we formulate a novel unified vision for these roughness-related impairments (referred to as the nw model), revealing for the first time that, given the roughness properties at the waveguide interface, both backscattering and radiative losses depend only on the sensitivity of the mode effective index to waveguide width perturbation. This result finds general application to both 2D and 3D waveguide structures and is not related to any particular technology or waveguide shape. Further, it provides a key instrument for a deeper understanding of roughness induced scattering as well as simple design rules for the mitigation of waveguide extrinsic loss.

D. Melati, F. Morichetti and A. Melloni, “A unified approach for radiative losses and backscattering in optical waveguides”, J. Opt., 16, 055502 (2014)
[Abstract]

A very clean Fe-based superconductor might show up new ground states (Vol. 42, No. 1)

image The ab-plane resistivity under magnetic fields for two different crystallographic directions of a LiFeAs single crystal grown by the Sn-flux method with Tc=18.2 K. The crystal structure and a typical picture of the grown crystals are also displayed.

Iron-based superconductors, discovered in early 2008 and exhibiting superconducting transition temperatures Tc as high as 57 K, currently draw focused attention in the condensed matter community. Among various structural forms known, LiFeAs represents the prototype of the "111'' compounds with several unique properties: superconductivity without chemical doping, thus being subject to the least disorder effect, no Fermi surface nesting for inducing an antiferromagnetic spin density wave fluctuation as a possible pairing glue for superconductivity. Moreover, the structure contains a FeAs layer sandwiched by the double Li layers so that the cleaving can be easily done in the Li surface without having surface reconstruction, an important favorable condition for surface-sensitive investigations. Due to the highly volatile nature of Li, the high quality single crystal growth has only recently been achieved. The present research reports the first successful growth of a large area single crystal of LiFeAs by using Sn flux, producing Tc = 18.2 K with a narrow transition width ΔTc = 1.1 K as well as a relatively large residual resistivity ratio, ~ 22-35. Upon measuring transport under high magnetic fields for two crystallographic directions, H // ab-plane and // c-axis, the system exhibits a moderate anisotropy of 2.3 near Tc, consistent with a prediction of a reduced anisotropy caused by correlation effects. Based on several recent proposals which point out the possible realization of a p-wave pairing symmetry and a Fulde, Ferrell, Larkin & Ovchinnikov state, it is envisioned that the present cleanest, large LiFeAs single crystal offers an unprecedented opportunity to find new, exotic ground states of correlated electron systems in the Fe-based superconductors.

Single-crystal growth and superconducting properties of LiFeAs
Bumsung Lee, Seunghyun Khim, Jung Soo Kim, G. R. Stewart and Kee Hoon Kim, EPL 91, 67002 (2010)
[Abstract]

A vortex of eigenvalues (Vol. 45 No.4)

Rotating vortex patches behave like clumps of eigenvalues for random matrices.

As the size of a random normal matrix grows, so does the number of its eigenvalues. As this number tends to infinity (with the mutual “repulsion” of eigenvalues reducing simultaneously) the eigenvalues “clump together” into a finite collection of dense, uniform, regions. Here we demonstrate the surprising result that exactly the same phenomenon pertains to rotating equilibrium arrangements of vorticity - so-called “vortex patches” or “V-states” – whose dynamics are governed by the famous Euler equations for ideal fluids. The underlying mathematical structure in these quite distinct areas of physics turns out to be identical.

The connection is made via an inverse moment problem in which the geometry (of either the limiting eigenvalue distribution, or the vortex patches) is dictated by an imposed background potential, but in an indirect way that must be decoded. This analogy is significant not only because it links two erstwhile unconnected areas of study, but also because it affords valuable mathematical “technology transfer”, especially with respect to decoding the shape of what we might now call the limiting “vortex of eigenvalues” in random matrix theory.

D. G. Crowdy, “Vortex patch equilibria of the Euler equation and random normal matrices”, J. Phys. A: Math. Theor., 47, 212002 (2014)
[Abstract]

Accurate dating requires calibration down to the last ion (Vol. 46 No. 3)

Accurate dating requires calibration down to the last ion
Illustration of the experimental setup used for calibrating irradiation

A new solution accurately counting the exact amounts of ions from laboratory radiation exposure helps to simulate the natural radiation of quartz samples used for thermoluminescence dating.

Thermoluminescence is used extensively in archaeology and the earth sciences to date artefacts and rocks. When exposed to radiation quartz, a material found in nature, emits light proportional to the energy it absorbs. Replicating the very low dose of background radiation from natural sources present in quartz is a key precondition for precise and accurate dating results. The authors have now developed a method to control the accuracy of the dose calibrations delivered to the samples during laboratory irradiation with heavy particles, replicating natural radiation exposure. Using oxygen and lithium ions from the Tandem accelerator at INFN LABEC in Florence, they found that their measurements were accurate to within 1%, despite large fluctuations in the irradiation beam.

L. Palla, C. Czelusniak, F. Taccetti, L. Carraresi, L. Castelli, M.E. Fedi, L. Giuntini, P. R. Maurenzig, L. Sottili, and N.Taccetti,, Accurate on line measurements of low fluences of charged particles, Eur. Phys. J. Plus 130, 39 (2015)
[Abstract]

Accurate determination of Curie temperature in helimagnet FeGe (Vol. 48, No. 4)

Accurate determination of Curie temperature in helimagnet FeGe (Vol. 48, No. 4)
Magnetic entropy change (△S) dependence on magnetic order exponent (n) at external magnetic field 3.0 T

Cubic helimagnet FeGe, the prototype of skyrmion materials near room temperature, has emerged and may impact future information technology. The magnetic entropy change (MEC) of helimagnet FeGe and the close relationship between the MEC and critical exponents of a second-order phase transition were studied. A relatively small MEC under external high magnetic field indicates the coexistence and competition between exchange anisotropy and magneto-crystalline anisotropy, and a stable balance is formed in the precursor region when the applied magnetic field cannot completely transform FeGe into a single magnetic structure phase. Based on the obtained magnetic entropy change and critical exponents, an accurate Curie temperature of helimagnet FeGe under zero magnetic field is confirmed to be 279.1 K, lower than 282 K deduced directly from the derivative magnetic susceptibility and higher than 278.2 K previously reported. So，the accurate determination of Curie temperature is conducive to reconsider the inhomogeneous chiral-spin state and reconstruct the magnetic phase diagram in the precursor region of helimagnet FeG.

L. Xu, H. Han, J. Fan, D. Shi, D. Hu, H. Du, L. Zhang, Y. Zhang and H. Yang, Magnetic entropy change and accurate determination of Curie temperature in single-crystalline helimagnet FeGe, EPL 117, 47004 (2017)
[Abstract]

Accurately evaluating on 40Ca+ optical clock BBR temperature (Vol. 48 No. 2)

Modelled temperature distribution of the miniature Paul trap

Optical clock based on 40Ca+ single-ion is a promising option in the program of transportable optical clocks. In such system, one of the largest contributions to the systematic uncertainty is blackbody radiation (BBR) shift. The uncertainty of BBR shift is basically dependent on the uncertainty of the BBR shift coefficient and the uncertainty of temperature measurement on the trap environment which both have a contribution at 10-17 level in fractional frequency units. We report a careful evaluation of BBR temperature rise seen by 40Ca+ ion confined in a miniature Paul trap via FEM modelling. The result indicates that the uncertainty of the BBR shift due to temperature has a contribution of 5.4 × 10-18 to the systematic uncertainty, and it allows improving the clock’s overall accuracy in the future. Moreover, an interesting work has been reported on validating the finite-element temperature model by comparison with thermal camera measurements calibrated against PT1000 thermometers. This work can be used to validate the FEM model of other optical clock systems and to evaluate the temperature in a vacuum chamber measured by thermal camera.

P. Zhang, J. Cao, H. Shu, J. Yuan, J. Shang, K. Cui, S. Chao, S. Wang, D. Liu and X. Huang, Evaluation of blackbody radiation shift with temperature-associated fractional uncertainty at 10-18 level for 40Ca+ ion optical clock, J. Phys. B: At. Mol. Opt. Phys. 50, 015002 (2017).
[Abstract]

Achille’s heel of simulations (Vol. 44 No. 2)

Example of periodic repetition of a small system.

This paper studies what can go wrong when computer simulations commonly used to model physical phenomena influencing molecular behaviour are applied outside their original context. It outlines the many pitfalls associated with simulation methods such as Monte Carlo algorithms or other commonly used molecular dynamics approaches. The context of this paper is the exponential development of computing power in the past 60 years, estimated to have increased by a factor of 1015, in line with Moore’s law. Today, short simulations can reproduce a system the size of a bacterium.

The author outlines diverse examples of issues arising when seemingly simple simulation methods are not applied with the due level of care. For example, simulations of small-scale systems, such as cubic boxes representing a unit cell as part of a crystal or liquid crystal, display effects that are linked to the fact that the sample is of finite size. Therefore, these simulations can only imitate, not reproduce, macroscopic effects unless effects that occur at microscopic scale, such as surface effects, are effectively removed.

The work also focuses on methods that, at first blush, appear reasonable, but are flawed and are akin to attempting to compare apples and oranges. For example, computing a mechanical property of a system—say the potential energy—using a Monte Carlo simulation, which can be based on thermal averages, does not allow to compute the thermal properties of such a system—such as entropy—in terms of thermal averages. Finally, the article also takes great care to debunk common myths and misconceptions pertaining to simulations.

D. Frenkel, ‘Simulations: The dark side’, Eur. Phys. J. Plus 128, 10 (2013)
[Abstract]

Acoustic surface plasmon on Cu(111) (Vol. 41, No. 5)

image Theoretically simulated electron surface wave patterns created by a point charge located close to a metal surface: The conventional Friedel oscillations (bottom) and a snapshot of the dynamical ASP wave (top) propagating from the center.

An acoustic surface plasmon (ASP) is a novel collective electronic excitation at metal surfaces. This new mode has a linear (or acoustic-like) dispersion, i.e. it can be excited at very low energy and wavelength, allowing it to participate in many dynamical processes, such as chemical reactions and nano-sensors at surfaces and sub-wavelength optics and photonic devices as well as new microscopy techniques.

After the original discovery of an ASP on the close-packed surface of beryllium it now has also been excited and detected on Cu(111). Thus, the ASP is indeed a general phenomenon on metal surfaces that support a partially occupied surface state within a wide bulk energy gap. Non-local screening of the surface electrons due to bulk electrons creates the ASP.

Of particular interest is the interaction of the ASP with light: nm-size objects at surfaces, such as atomic steps or molecular structures, can provide coupling between light and ASPs of much lower wavelength than conventional SPs. In this way, the new mode can serve as a tool to confine light in a broad frequency range up to optical frequencies on surface areas of a few nanometers, thus facilitating control of events at metal surfaces with both high spatial (nm) and high temporal (fs) resolution. Another consequence of the acoustic-like character of the ASP dispersion is that both phase and group velocities are the same, so signals can be transmitted undistorted along the surface. The theoretically estimated ASP decay lengths of 100 ∼ 1,000 nm for medium (100 meV) to far (10 meV) infrared are an appealing prospect for the field of nano-optics.

Acoustic surface plasmon on Cu(111)
K. Pohl, B. Diaconescu, G. Vercelli, L. Vattuone, V. M. Silkin, E. V. Chulkov, P. M. Echenique and M. Rocca, EPL 90, 57006 (2010) [Abstract]

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