2023 Impact factor 2.6
Special Topics

## EPJ B Topical review - Twenty-five years of random asset exchange modeling

Over the last twenty-five years, there has emerged within the subfield of econophysics a sizeable and important literature (hereinafter the “random asset exchange” literature) concerned with the application of stochastic processes to model wealth and income distributions. In a new Topical Review published in EPJ B, written by Max Greenberg (University of Massachusetts Amherst, USA) and H. Oliver Gao (Cornell University, USA), the random asset exchange literature as a whole is comprehensively exposited for the first time.

## EPJ B Highlight - A Mathematical Understanding of Project Schedules

Complex projects are made up of many activities, the duration of which vary according to a power law; this model can be used to predict overall project duration and delay.

We have all been frustrated when a project is delayed because one sub-task cannot begin before another ends. It is less well known that the process of scheduling projects efficiently can be described in mathematical terms. Now, Alexei Vazquez, of technology company Nodes & Links and based in Cambridge, UK, has shown that the distribution of activity lengths in a project follows the mathematical relationship of power law scaling. He has published his findings in the journal EPJ B.

## EPJ B Highlight - Coalescence-fragmentation cycles based on Human conflict

Inspired by insurgency warfare dynamics, a model predicts patterns of how groups gel and shatter

In 1960, Lewis Fry Richardson famously observed that the severity of a wartime event is described by a simple power law distribution that scales according to the size of the conflict. Statisticians since have since proposed various modifications, but they continue to agree that casualty count in a violent conflict tends to scale with the size of the insurgent group that caused the conflict. In a study published in EPJ B, Brennen Fagan, of the University of York, UK, and his colleagues analyze models of how complex systems coalesce and fragment based on these warfare dynamics. Their work evaluates the robustness of these models and elucidates the relationship between microscopic dynamics and observed phenomena.

## EPJ B Highlight - Exploring exotic behaviours in population-imbalanced fermionic systems

New studies show that oscillations in the quantum states of composite particles in trapped systems can be adjusted using an external magnetic field.

Over the past 20 years, many physicists have studied ultra-cold fermionic systems contained in magnetic or optical traps. When an external magnetic field is applied to a two-species fermionic system, the particles can pair together to form composite ‘bosonic molecules’ with a full-integer spin. These molecules undergo “Bose-Einstein Condensation” when cooled, where all the particles accumulate in the lowest-energy quantum state. The precision of these experiments has now been improved by trapping the particles inside optical lattices: periodic patterns formed by counter-propagating laser beams.

Through new research published in EPJ B, Avinaba Mukherjee and Raka Dasgupta at the University of Calcutta, India, have theoretically predicted a distinctive trend in the oscillations of Bose-Einstein condensates formed from these fermions – which can be adjusted using an external magnetic field. They specifically addressed systems where the two species have unequal population (creating leftover unpaired fermions): leading to exotic new phases. Their result could help physicists to detect such novel phases of matter in imbalanced fermionic systems and could open up new opportunities for quantum technologies.

## EPJ B Highlight - Harvesting vibrational energy from coloured noise

Two engineers from Beijing Institute of Technology in China have shown how to optimise the output of a device that can convert ambient vibrational energy into useful electric power.

The energy demands of today’s ubiquitous small electronic devices – including sensors, data transmitters, medical implants and ‘wearable’ consumer products such as Fitbits – can no longer be met by chemical batteries alone. This gap can be filled by energy harvesters, which turn ordinary, ambient vibrational energy into electrical energy. The most efficient types of harvester are tri-stable energy harvesters, which can convert even low-frequency random vibrations into alternating current (AC) and thence into direct current (DC). Tingting Zhang and Yanfei Jin from Beijing Institute of Technology in China have now investigated how the properties of these systems can be altered to optimise the power output; their findings are published in EPJ B.

## EPJ B Highlight - Investigating the role of random walks in particle diffusion

Theoretical analysis reveals new insights into unusual patterns displayed by diffusing particles in recent experiments.

Several recent experiments identify unusual patterns in particle diffusion, hinting at some underlying complexity in the process which physicists have yet to discover. Through new analysis published in EPJ B, Adrian Pacheco-Pozo and Igor Sokolov at Humboldt University of Berlin show how this behaviour emerges through strong correlations between the positions of diffusing particles travelling along similar trajectories. Their results could help researchers to create better models of the diffusion process – ultimately drawing deeper insights into how fluids behave.

## EPJ B: Prof. Dr. Reinhold Egger new Editor-in-Chief for the Condensed Matter section as of 1 January 2024

The publishers of The European Physical Journal B are pleased to announce the appointment of Prof. Dr. Reinhold Egger as new Editor-in-Chief for the Condensed Matter section of the journal as of January 1st, 2024.

Reinhold Egger has been a full professor of theoretical physics at Heinrich-Heine-Universität Düsseldorf since 2001. He has worked in many different areas of condensed matter physics, including nonequilibrium quantum transport, topological quantum matter, superconductivity and low-dimensional quantum field theory. He has been part of the board of EPJB since 2011 and he is recipient of the Gerhard-Hess-Preis of the DFG and of the Physics Prize of the Akademie der Wissenschaften zu Göttingen.

The publishers take the opportunity to thank wholeheartedly Dr. Eduardo Hernandez, for his dedicated work and leadership during his term as Editor-in-Chief of EPJB.

## EPJ B Highlight - Testing particle scattering and reflection in graphene

Testing the quantum effects of Andreev reflection in the wonder material could have positive implications for quantum technology

Humanity stands on the verge of two major revolutions: the boom in 2-dimensional supermaterials like graphene with incredible properties and the introduction of quantum computers with processing power that vastly outstrips standard computers.

Understanding materials like graphene, made of single sheets of atoms, means better investigations of the properties they display at an atomic level. This includes how electrons behave around superconductors — materials that, when cooled to temperatures near absolute zero, can conduct electricity without energy loss.

When a superconductor is sandwiched between metal materials, a type of scattering called crossed Andreev reflection may appear, and in an s-wave superconductor junction, the Andreev reflection usually induces correlated opposite spin in electrons. This can be used to induce entanglement, a quantum phenomenon that is critical for quantum computers.

In a new paper in EPJ B, author Rui Shen, from the National Laboratory of Solid State Microstructures and School of Physics at Nanjing University, China, and his co-authors theoretically assess nonlocal transport and crossed Andreev reflection in a ferromagnetic s-wave superconductor junction composed of the gapped graphene lattices.

## EPJ B Colloquium - Density-matrix renormalization group: a pedagogical introduction

The physical properties of a quantum many-body system can, in principle, be determined by diagonalizing the respective Hamiltonian, but the dimensions of its matrix representation scale exponentially with the number of degrees of freedom. Hence, only small systems that are described through simple models can be tackled via exact diagonalization. To overcome this limitation, numerical methods based on the renormalization group paradigm have been put forth, that restrict the quantum many-body problem to a manageable subspace of the exponentially large full Hilbert space. A striking example is the density-matrix renormalization group (DMRG), which has become the reference numerical method to obtain the low-energy properties of one-dimensional quantum systems with short-range interactions.

## EPJ B Highlight - How a transparent conductor responds to strain

First-principles calculations show how to manipulate some transition metal oxides’ optical and electronic properties for use in thin-film devices.

Liquid crystal displays, touchscreens, and many solar cells rely on thin-film crystalline materials that are both electrically conductive and optically transparent. But the material most widely used in these applications, indium tin oxide (ITO), is brittle and susceptible to cracking. Researchers seeking alternatives have set their sights on strontium vanadate (SrVO3), a material that ticks all the boxes for a transparent conductor. In a study published in EPJ B, Debolina Misra, of the Indian Institute of Information Technology, Design and Manufacturing, Kancheepuram, India, and her colleagues now calculate how SrVO3‘s optical and electron transport properties vary in response to strain. Their simulations provide a detailed mechanism for tuning these properties to optimize the material’s utility in different devices and applications.

Dear Sabine,
On this occasion, may I also thank you for your support: collaboration with you is always very pleasant and effective. Have a nice day, yours, Yurij
ISSN: 1951-6355 (Print Edition)
ISSN: 1951-6401 (Electronic Edition)