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Delocalization of edge states in topological phases (Vol. 50, No. 5-6)

Delocalization of edge states in topological phases
Dispersions in a topological system with positive indirect gap and edge states (left); negative indirect gap and no edge states (right).

Topological properties are a hot topic currently. If the bulk of a system is topologically non-trivial (Chern number), the bulk-boundary correspondence predicts in-gap states in finite samples. These states close the energy gap between bands of different topology so that it can change at boundaries. Conventionally, the in-gap states are localized at these boundaries so that they are edge states. We show, however, that this localization only occurs for positive indirect gap. Generically, without indirect gap the in-gap states become extended by mixing with bulk states despite . This is illustrated for two fundamental lattice models (Haldane and checkerboard model) by adding terms to the Hamiltonians proportional to the identity in momentum space. Thus, the dispersions change while the topology remains unchanged. These terms can close the indirect gap and lead to delocalization of edge states in finite geometries. Thus, discrete topological invariants may exist without localized edge modes. This underlines the vital significance of indirect gaps for the existence of topological edge states and puts the bulk-boundary into perspective.

M. Malki and G. S. Uhrich, Delocalization of edge states in topological phases, EPL 127, 27001 (2019)
[Article]

Detecting solar neutrinos with the Borexino experiment (Vol. 52, No. 1)

Distribution of best-fit results of 210Bi and CNO neutrinos interaction rates obtained fitting thousands of simulated datasets under different assumptions of 210Bi backgrounds.

Neutrinos produced by the CNO cycle within the core of the Sun are being hunted by the Borexino experiment so that we may learn more about this important nuclear process. A paper by the Borexino collaboration – including XueFeng Ding, Postdoc Associate of Physics at Princeton University, United States – documents the attempts of the Borexino experiment to measure low-energy neutrinos from the Sun’s carbon-nitrogen-oxygen (CNO) cycle for the first time.

M. Agostini et al. (BOREXINO collaboration), Sensitivity to neutrinos from the solar CNO cycle in Borexino, Eur. Phys. J. C 80, 1091 (2020)
[Abstract]

Determining the shapes of atomic clusters (Vol. 51, No. 1)

Defining the shapes of atomic clusters

By considering the crystal structures of atomic clusters in new ways, researchers may be able to better assess whether the groups have distinctive shapes, or whether they are amorphous.

Too large to be classed as molecules, but too small to be bulk solids, atomic clusters can range in size from a few dozen to several hundred atoms. The structures can be used for a diverse range of applications, which requires a detailed knowledge of their shapes. These are easy to describe using mathematics in some cases; while in others, their morphologies are far more irregular. However, current models typically ignore this level of detail; often defining clusters as simple ball-shaped structures. In research published in EPJ B, José M. Cabrera-Trujillo and colleagues at the Autonomous University of San Luis Potosí in Mexico propose a new method of identifying the morphologies of atomic clusters. They have now confirmed that the distinctive geometric shapes of some clusters, as well as the irregularity of amorphous structures, can be fully identified mathematically.

J.M. Cabrera-Trujillo, J.M. Montejano-Carrizales, C.G Galván, Amorphicity and Structural Changes in Binary Clusters, Eur. Phys. J. B 92, 237 (2019)
[Abstract]

Diffusion in periodic, correlated random forcing landscapes (Vol. 45 No.5-6)

Behaviour of diffusion coefficients

In measurements of physical observables, how should one correctly account for sample-to-sample variation of disorder? A common practice is to replace an averaging over very many different samples by a single measurement for a large enough sample. This process of self-averaging often works well, while there are cases where it breaks down very strongly: rare realizations dominate the average behavior, which therefore does not coincide with the typical behaviour obtained for a single sample. This was explicitly shown by exact analytical and numerical results in a solvable model of transport in random media. The model comprised a Brownian particle moving in a disordered potential given as a periodically-extended fractional Brownian motion with arbitrary Hurst exponent H. It was revealed that the diffusion coefficient of the particle in a potential of period L is strongly non-self-averaging, with moments supported by atypical disorder realizations: while the typical value decays as a stretched exponential in L with exponent H (blue points), the positive and the negative moments have instead the exponents H/(1+H) (red points), and 2H, respectively (see figure). This work underlines the crucial role of disorder in dictating the dynamical behaviour, thereby cautioning against naive implementation of single-sample measurements.

D. S. Dean, S. Gupta, G.Oshanin, A.Rosso and G. Schehr, “Diffusion in periodic, correlated random forcing landscapes”, J. Phys. A: Math. Theor. 47, 372001 (2014)
[Abstract]

Discovering Technicolor (Vol. 42, No. 5)

image Cartoon of the Minimal Walking Technicolor Model extension of the SM

At present there are no known elementary scalar fields. A possible candidate is the as yet undiscovered Higgs particle; however it could well be that this elusive particle is instead composite. This possibility is exhaustively examined in this article, which is both tutorial and extensive review, classifying the diverse technicolor models as extensions of the Standard Model of particle physics. These model extensions are then compared with electroweak precision data, the spectrum of states common to most such models are identified, and their decays and associated experimental signals for the LHC illustrated, including the implementation in event generators important for searches at the LHC. This timely review provides the most complete and up-to-date benchmarks for the potential discovery of technicolor models.

Discovering Technicolor
J.R. Andersen, O. Antipin, G. Azuelos, L. Del Debbio, E. Del Nobile, S. Di Chiara, T. Hapola, M. Järvinen, P.J. Lowdon, Y. Maravin, I. Masina, M. Nardecchia, C. Pica, and F. Sannino, Eur. Phys. J. Plus, 126: 81 (2011)
[Abstract]

Disentangling information from photons (Vol. 43 No. 5)

Disentangling information from photons (Image: © Peter Nguyen, iStockphoto)

This work describes greater chances of accessing more reliable information on applications in quantum computing and cryptography. The authors have found a new method of reliably assessing the information contained in photon pairs used for applications in cryptography and quantum computing.

The findings are so robust that they enable access to the information even when the measurements on photon pairs are imperfect.

The authors focus on photon pairs described as being in a state of quantum entanglement: i.e., made up of many superimposed pairs of states. This means that these photon pairs are intimately linked by common physical characteristics such as a spatial property called orbital angular momentum, which can display a different value for each superimposed state.

They rely on a tool capable of decomposing the photon pairs’ superimposed states onto the multiple dimensions of a Hilbert space, which is a virtual space described by mathematical equations. This approach allows them to understand the level of the photon pairs’ entanglement.

It is shown that the higher the degree of entanglement, the more accessible the information that photon pairs carry. This means that generating entangled photon pairs with a sufficiently high dimension—that is with a high enough number of decomposed photon states that can be measured—could help reveal their information with great certainty.

As a result, even an imperfect measurement of photons’ physical characteristics does not affect the amount of information that can be gained, as long as the level of entanglement was initially strong. These findings could lead to quantum information applications with greater resilience to errors and a higher information density coding per photon pair. They could also lead to cryptography applications where fewer photons carry more information about complex quantum encryption keys.

F.M. Miatto, T. Brougham and A.M. Yao, ‘Cartesian and polar Schmidt bases for down-converted photons’, Eur. Phys. J. D (2012) 66, 183
[Abstract]

Disordered configurations of the Glauber model on two-dimensional networks (Vol. 49, No. 3)

A disordered configuration with two domains comprises a multiclustered state on the lattice.

The Glauber model provides a paradigm for modeling ordering processes in complex systems. The question that we answer is: How is the efficiency of the ordering process in the Glauber model affected if we rewire the links of the two-dimensional host lattice? Our research reveals that the fraction of disordered configurations exhibits a nonlinear dependence on the rewiring probability. In the small-world regime, the Glauber dynamics remains trapped in a metastable configuration that is disordered. In fact, we have observed a stationary state that consists of two intertwined domains of similar size, as shown in the figure. For higher rewiring probabilities, we observe isolated droplets of spins, which emerge due to poorly connected nodes in the network. We have also studied what happens to the ordering process on two-layer networks, in particular comparing outcomes on a multiplex network and on the corresponding network with random inter-layer connections. We have shown that, in this case, the properties of the stationary state are strongly affected by the type of inter-layer connections.

I. Baĉić, I. Franović and M. Perc , Disordered configurations of the Glauber model in two-dimensional networks, EPL, 120, 68001 (2017)
[Abstract]

Disparity in electron and phonon mean-free-paths (Vol. 46 No. 3)

Disparity in electron and phonon mean-free-paths
Electrical and lattice thermal conductivity accumulation with respect to the mean free paths (MFPs).

There has been intensified interest in high performance thermoelectric materials in the last two decades, largely owing to the nanostructuring approaches that significantly reduce heat leakage by phonons. A condition for the nanostructuring approaches to be effective is the phonon mean free path (MFP) much longer than that of electrons so that phonons are more frequently scattered at the interfaces. While there has been a recent significant progress in the first-principles understanding of phonon MFP spectral distributions, the spectral distribution of electron MFPs remains unknown.

In this work the authors compute from first-principles the energy dependent electron scattering and MFPs in silicon. They show that electrons and phonons have very different MFP distributions with phonon MFPs significantly longer than those of electrons. The authors show with a model that silicon 20 nm grains can yield a more than five times enhancement in the thermoelectric figure of merit, since the grain boundaries scatter phonons more significantly than electrons due to their different MFP distributions. This work provides new fundamental insights by quantitatively revealing the disparity in electron and phonon MFP distributions from first principles.

B. Qiu, Z. Tian, A. Vallabhaneni, B. Liao, J. M. Mendoza, O. D. Restrepo, X. Ruan and G. Chen, First-principles simulation of electron mean-free-path spectra and thermoelectric properties in silicon, EPL, 109, 57006 (2015)
[Abstract]

Dissecting Deuteron Compton Scattering I (Vol. 44 No. 5)

Sensitivity of a double-polarisation-asymmetry observable to the E1-M2 spin polarisability.

The electromagnetic polarisabilities of the nucleons characterise their responses to external fields. The simplest are the electric and magnetic polarisabilities that describe the induced dipole moments. For spin-1/2 particles there are also four spin polarisabilities, analogous to rotations of the polarisation of light by optically active media. The best experimental window on them is Compton scattering of photons, which has provided good determinations of the electric and magnetic polarisabilities of the proton. Future experiments with polarised protons will give access to its spin polarisabilities. In contrast, much less is known of about the neutron since it does not exist as a stable target. Nonetheless, its properties can be obtained from Compton scattering on light nuclei, most notably the deuteron -- a weakly bound proton and neutron. A new generation of experiments is planned to provide beams of polarised photons on targets of polarised deuterons. If the spins of the final particles are not observed, there are 18 independent observables. This work provides, for the first time, the complete set of these, which will be needed for the experimental analyses. More importantly, it also examines their sensitivities to the various polarisabilities, which will be crucial for the design of the experiments.

H.W. Grießhamme, 'Dissecting Deuteron Compton Scattering I: The Observables with Polarised Initial States', Eur. Phys. J. A, 49, 100 (2013)
[Abstract]

Distortion isn’t a drag on fluid-straddling particles (Vol. 51, No. 3)

Straddling particles deform fluid interfaces

The drag forces experienced by particles which straddle and distort the interfaces between un-mixable fluids are less influenced by the shape of the distortion than previously thought.

Some intriguing physics can be found at the interfaces between fluids, particularly if they are straddled by particles like proteins or dust grains. When placed between un-mixable fluids such as oil and water, a variety of processes, including inter-molecular interactions, will cause the particles to move around. These motions are characterised by the drag force experienced by the particles, which is itself thought to depend on the extent to which they distort fluid interfaces. In this work it is shown that the drag force experienced by fluid-straddling particles is less affected by interface distortion than previously believed.

J-C Loudet, M. Qiu, J Hemauer, J J Feng, Drag force on a particle straddling a fluid interface: influence of interfacial deformations, Eur. Phys. J. E 43, 13 (2020)
[Abstract]

Does knowing the opponent's strategy guarantee optimal play? (Vol. 46 No. 4)

Does knowing the opponent's strategy guarantee optimal play?
Reentrant phase transition between two cyclically dominant strategy triplets who form alliances against the fourth strategy. The triples are indicated by curved arrows in the inset.

Methods of statistical physics are proving indispensable for the study of evolutionary games in structured populations. The evolution of cooperation and the phase transitions leading to favorable evolutionary outcomes depend sensitively on the structure of the interaction network and the type of interactions, as well as on the number and type of competing strategies. Now, physicists have solved the puzzle of the availability of information in evolutionary games. In a new theoretical model, the authors answer whether knowing the strategy of an opponent is indeed the holy grail of optimal play in social dilemmas, or whether the situation is in fact more complex. It is indeed the latter, as final evolutionary outcomes depend sensitively not just on individual relations between the competitors as determined by payoff elements, but equally strongly on the spatiotemporal dynamics of defensive alliances that emerge spontaneously as a result of strategic complexity. Reentrant phase transitions highlight the fact that the viability of an alliance depends sharply on the invasion speeds between group members who cyclically dominate each other.

A. Szolnoki and M. Perc, Reentrant phase transitions and defensive alliances in social dilemmas with informed strategies, EPL, 110, 38003 (2015)
[Abstract]

Does that “green” plasticiser make my PVC flexible enough for you? (Vol. 46 No. 4)

Does that “green” plasticiser make my PVC flexible enough for you?
A snapshot of the simulated PVC/DOP system at a 30 wt% PVC concentration after minimization. Red, green, yellow and blue spheres represent O, Cl, C, and H atoms, respectively.

A study of an eco-friendly solvent helping to make PVC plastic more flexible reveals the molecular-level interaction of hydrogen bonds between the two ingredients.

What gives plastic objects their flexibility and reduces their brittleness is the concentration of plasticiser. For example, a chemical solvent of the phthalate family called DOP is often used. The trouble is there are concerns that phthalates present health risks. So there is a demand for more alternatives. Now, the authors have examined the effect of using DEHHP, a new eco-friendly plasticiser, used in combination with PVC. For a plasticiser to work, there has to be adequate hydrogen bonding with the plastic. By combining experiments and simulations, the team revealed why the polymer-solvent hydrogen bonding interaction's strength decreases with dilution at a molecular level—which is a phenomenon also observed in the DOP-PVC combination. These findings have been published in the present work.

Y. Liu, R. Zhang, X. Wang, P. Sun, W. Chen, J. Shen and G. Xue, Hydrogenation induced deviation of temperature and concentration dependences of polymer-solvent interactions in poly(vinyl chloride) and a new eco-friendly plasticizer, Eur. Phys. J. Plus 130, 116 (2015)
[Abstract]

Does the universe have a rest frame? (Vol. 48, No. 3)

A simplified diagram showing the basic idea of the experimental design

Experiment aims at resolving divergence between special relativity and standard model of cosmology.

Physics is sometimes closer to philosophy when it comes to understanding the universe. The author attempts to elucidate whether the universe has a resting frame. The results have recently been published. To answer this tricky question, he has developed an experiment to precisely evaluate particle mass. This is designed to test the special theory of relativity that assumes the absence of a rest frame, otherwise it would be possible to determine which inertial frame is stationary and which frame is moving. This assumption, however, appears to diverge from the standard model of cosmology, which assumes that what we see as a vacuum is not an empty space. The assumption is that the energy of our universe comes from the quantum fluctuation in the vacuum. In this study, the author set out to precisely measure the masses of two charged particles moving in opposite directions. The conventional thinking assumes that the inertial frame applies equally to both particles. If that’s the case, no detectable mass difference between these two particles is likely to arise. However, if the contrary is true, and there is a rest frame in the universe, the author expects to see mass difference that is dependent on the orientation of the laboratory frame.

D. C. Chang, Is there a rest frame in the universe? A proposed experimental test based on a precise measurement of particle mass, Eur. Phys. J. Plus 132, 140 (2017)
[Abstract]

Double ionization with absorbers (Vol. 43 No. 1)

image The system is initially described by a two-particle wave function (top). As a particle is ionized and subsequently hits the absorber (Τ), the remaining electron is described by a one-particle density matrix (middle), which may be thought of as an ensemble of several one-particle wave functions. As also the one-particle system may be absorbed, the vacuum state may also be populated (bottom).

In quantum dynamics, unbound systems, such as atoms being ionised, are typically very costly to describe numerically as their extension is not limited. This problem should be reduced if one could settle for a description of the remainder of the system and disregard the escaping particles. Removing the escaping particles may be achieved by introducing absorbers close to the boundary of the numerical grid. The problem is, however, that when such "interactions" are combined with the Schrödinger equation, all information about the system is lost as a particle is absorbed. Thus, if we wish to still describe the remaining particles, a generalization of the formalism is called for. As it turns out, this generalisation is provided by the Lindblad equation.

This generalised formalism has been applied to calculate two-photon double ionisation probabilities for a model helium atom exposed to laser fields. In the simulations, the remaining electron was reconstructed as the first electron was absorbed. Since there was a finite probability for also the second electron to hit the absorber at some point, the system could, with a certain probability, end up in the vacuum state, i.e. the state with no particles. As this probability was seen to converge, it was interpreted as the probability of double ionisation.

The validity of this approach was verified by comparing its predictions with those of a more conventional method applying a large numerical grid.

A master equation approach to double ionization of helium
S. Selstø, T. Birkeland, S. Kvaal, R. Nepstad and M. Førre, J. Phys. B: At. Mol. Opt. Phys. 44, 215003 (2011)
[Abstract]

Drag forces in fluctuating classical field (Vol. 41, No. 5)

image Simulation of the average magnetization profile (in the rest frame of the magnetic field) about a point like magnetic field in an Ising model moving at constant velocity. The resolution in the figure represents the average magnetization of the spin at the centre of the square on the Ising model's square lattice.

Identical objects in thermally fluctuating fields experience a fluctuation induced force between them, examples include the famous critical Casimir force (generated by thermal rather than quantum fluctuations) [Fisher and de Gennes, C. R. Acad. Sci. Paris B 287, 207 (1978)] and forces induced between proteins in lipid membranes via their coupling to membrane height or composition [Goulian, Bruinsma, and Pincus, Europhys. Lett. 22, 145 (1993)] degrees of freedom. Gaussian fields, linearly coupled to the position of a moving inclusion the field can also induce a drag force as studied here.

The underlying physics is similar to that of a polaron - for a stationary inclusion, the polarization of the field is spherically symmetric, however when it moves the polarization field is deformed, see Figure 1, and this deformation yields a drag. The drag force depends on the statics and dynamics of the field and the inclusion's interaction with the field. At low velocities v the drag force is generically linear in v, but for systems with long range correlations, such as fields at critical points (for instance the continuous demixing transition for lipid membranes), the drag force can behave nonanalytically as vΦ, where Φ<1. As the velocity is increased, the drag force increases to a maximum and then decays to zero as 1/v. This is because at high velocity the polarization cloud does not have sufficient time to develop and the drag is thus reduced. These effects could be measured experimentally, for example on membrane proteins dragged in membranes or on colloids dragged through binary liquid mixtures, using optical tweezers.

Drag forces on inclusions in classical fields with dissipative dynamics
V. Démery and D.S. Dean, Eur. Phys. J. E 32, 377-390 (2010)
[Abstract] | [PDF]

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