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Observational constraints and astrophysical uncertainties in WIMP detection (Vol. 49, No. 1)

Weakly Interacting Massive Particles (WIMPs) are one of the best candidates for the exotic dark matter that makes up 80% of the matter in the Universe. They are predicted to exist in extensions of the standard model of particle physics and would be produced in the Big Bang in the right amount to account for the dark matter. Lab-based direct detection experiments aim to detect WIMPs via their rare interactions with normal matter. The signals expected in these experiments depend on the local dark matter density and velocity distribution. An accurate understanding of these quantities is therefore required to obtain reliable constraints on the WIMP particle physics properties, i.e. its mass and interaction cross sections. This paper reviews the current status of observational constraints on the local dark matter distribution and also numerical simulations of Milky Way-like galaxies, before discussing the effects of uncertainties in these quantities on the experimental signals. It concludes with an overview of various methods for handling these astrophysical uncertainties, and hence obtaining accurate constraints on the WIMP mass and cross-sections from current and future experimental data.

A. M. Green, Astrophysical uncertainties on the local dark matter distribution and direct detection experiments, J. Phys. G: Nucl. Part. Phys. 44, 084001 (2017)
[Abstract]

Observing different quantum trajectories in cavity QED (Vol. 42, No. 5)

Quantum systems, as isolated as they can be, always interact with their surrounding environment. This interaction can lead to correlations between system and environment and, when the states of the reservoir are inaccessible to observation, to an irreversible loss of information on the system. This deleterious decoherence effect is at the heart of the quantum theory of measurement and plays an essential role in explaining the emergence of classical behaviour in quantum systems.

However, this system-reservoir interaction can be exploited in a completely different way when the environment can be monitored. In this case, extracting information from the environment causes the state of the system to change stochastically, conditioned on the measurement record. This can then be used to manipulate the system dynamics, being an important strategy in quantum dynamical control.

While it is well known that there are infinitely many possible ways of unravelling the decoherence process in terms of stochastic trajectories, it is not always clear how to interpret these trajectories in terms of concrete physical measurements on the environment. In this paper we show how to produce, in a controllable manner, a variety of quantum trajectories in realistic cavity quantum electrodynamics setups. In the microwave regime, we show how the detection of atoms that have crossed a cavity can induce a jump in the field proportional to its quadrature. In this case, the field dynamics is quite different from the usual photodetection monitoring and can be used to produce conditional four-component cat-like states. Alternatively, in the optical domain, the detection of photons can be used to protect entangled states of atoms that have interacted with the cavity field.

This proposal for the implementation of new stochastic trajectories in terms of continuous measurements in realistic systems certainly expands the possibility of engineering quantum states of lights and atoms.

Observing different quantum trajectories in cavity QED
M.F. Santos and A.R.R. Carvalho, EPL, 94, 64003 (2011)
[Abstract]

On-demand conductivity for graphene nanoribbons (Vol. 46 No. 1)

Sketch of a kicked zigzag graphene nanoribbon.
Credit: D. Babajanov et al.

The authors have devised a theoretical model to tune the conductivity of graphene zigzag nanoribbons using ultra-short pulses.

Physicists have, for the first time, explored in detail the time evolution of the conductivity, as well as other quantum-level electron transport characteristics, of a graphene device subjected to periodic ultra-short pulses. To date, the majority of graphene studies have considered the dependency of transport properties on the characteristics of the external pulses, such as field strength, period or frequency. These results may help to develop graphene-based electronic devices that only become conductors when an external ultra-short pulse is applied, and are otherwise insulators.

Specifically, they found that applying external driving force leads to enhancement of electronic transitions within valence and conduction bands. This study thus demonstrates that such transitions allow a dramatic increase in conductivity within a short time, making it possible to tune the electronic properties using short external pulses.

D. Babajanov, D.U. Matrasulov and R. Egger, “Particle Transport in Graphene Nanoribbon Driven by Ultrashort Pulses”, Eur. Phys. J. B 87, 258 (2014)
[Abstract]

One-D neutron-polarization analysis on magnetic nanostructures (Vol. 41, No. 5)

image Neutron non-spin-flip (left) and spin-flip scattering cross section of a nanocrystalline Fe-Cr-based alloy at magnetic saturation (B0 = 1.31 T).

Small-angle neutron scattering (SANS) is a prominent and powerful method to investigate the bulk of magnetic nanostructures on a length scale between a few and a few hundred nanometers. However, up to now, SANS was almost exclusively utilized with an unpolarized or a polarized incident neutron beam (denoted as SANSPOL), and an analysis of the spin state of the neutron after the scattering process is frequently not performed. The recent development of efficient 3He spin filters (for cold neutrons) allows one to perform routinely one-dimensional neutron-polarization analysis (POLARIS) in a SANS experiment. The general equations for the non-spin-flip (nsf) and spin-flip (sf) POLARIS cross sections of a bulk ferromagnet suggest that a variety of angular anisotropies and asymmetries may be observed on a two dimensional detector.

First experiments on an FeCr based two-phase nanocrystalline alloy demonstrate the power of the POLARIS technique for the investigation of magnetic nanostructures. In particular, the analysis of the sf data, which does not contain the coherent nuclear scattering, permits the independent determination of the magnitude-squares of the three vector (Fourier) components of the bulk magnetization. In the figure below, the nsf data (left image) is a superposition of nuclear and magnetic scattering, whereas the sf channel at magnetic saturation (right image) exclusively contains the signal due to longitudinal (Mz) magnetization fluctuations; analysis of the sf data along certain directions in momentum space provides access to the transversal (Mx and My) spin components. Such studies are now feasible at the SANS instrument D22 of the Institut Laue-Langevin, Grenoble.

Longitudinal polarization analysis in small-angle neutron scattering
D. Honecker, A. Ferdinand, F. Döbrich, C.D. Dewhurst, A. Wiedenmann, C. Gómez-Polo, K. Suzuki and A. Michels, Eur. Phys. J. B 76, 209 (2010).
[Abstract] | [PDF]

Operating regimes in an optical rectenna (Vol. 48 No. 2)

Operating regimes in an optical rectenna

Optical rectennas: where Maxwell meets Einstein

Conventional semiconductor solar cells convert the solar spectrum to dc electricity, relying on the photoelectric effect. Their ultimate efficiency is limited to 44% because the entire photon spectrum is used at a voltage equal to the semiconductor bandgap. An unconventional approach is to use optical rectennas, nanoantennas with high-speed diodes. In this work we show how to break the efficiency limit using optical rectennas.

Microwave rectennas are described by classical electromagnetics and have been used for rectifying microwaves with power conversion efficiencies greater than 80%. However, the interaction of high-speed diodes with light is different than with microwaves. Instead, an optical rectenna can operate in one of three different regimes: quantum, transition, and classical.

The quantum regime occurs for weak optical intensities and is subject to the 44% limit because each incoming photon is used to produce an electron at the rectenna operating voltage, as in conventional solar cells. Classical operation occurs when the intensity is strong and the photon energy is low. Here, electrons absorb multiple photons to produce current at higher voltages, as in classical rectennas, resulting in higher solar energy conversion efficiencies that ideally can exceed 80%.

S. Joshi and G. Moddel, Optical rectenna operation: where Maxwell meets Einstein, J. Phys. D: Appl. Phys. 49, 265602 (2016)
[Abstract]

Optical detection of low frequency NQR signals (Vol. 48, No. 3)

Optical detection of NQR signal

Nuclear quadrupole resonance (NQR) is a branch of radiofrequency (RF) spectroscopy. It became a promising tool in detecting illicit substances like explosives, narcotics and counterfeit medicines. Many of these substances contain 14N nuclei and are detectable by the NQR spectrometer. Practically all 14N NQR frequencies are in the range below 5 MHz and correspondingly the spectrometer sensitivity is low. One of possible improvements is a combination of the very sensitive potassium (K) pumped optical magnetometer (KPOM) and the pulsed NQR spectrometer. The linearly polarized probe laser beam detects the magnetic part of the low frequency 14N RF signal. This results in a rotation of the probe beam polarization plane after the beam leaves the K-cell. This rotation is measured and is proportional to the NQR signal. Combination of the classic RF excitation of the sample 14N nuclei and a subsequent optical detection of the sample response leads to a S/N improvement of up to a factor of 10 as it was demonstrated in the case study of some difficult-to-detect illicit substances. An efficient magnetic shielding may be necessary.

S. Begus, J. Pirnat, V. Jazbinsek and Z Trontelj, Optical detection of low frequency NQR signals: a step forward from conventional NQR, J. Phys. D: Appl. Phys. 50, 095601 (2017)
[Abstract]

Optical evidence of nematicity in iron-based superconductors (Vol. 42, No. 3)

image Temperature dependence of the dichroism Δσ1(ω)=σ1(ω,E||a)-σ1(ω,E||b) for x=0.025 at selected frequencies compared to the dc anisotropy ratio (Δρ/ρ). The anisotropy in the dc limit of the optical conductivity (Δρopt) is reported, as well. The vertical dashed and dashed-dotted lines mark the magnetic and structural phase transitions, respectively.

A nematic order recently arose as a robust electronic state describing the nature of the pseudogap phase in the high-temperature superconducting cuprates. In the field of liquidcrystals, a nematic statederives froma transitionbreaking the rotational symmetry of the high temperaturephasebutpreserving the translational one. Besides the cuprates, the novel iron-based superconductors in their parent and underdoped phase recently emerged as an alternative playground for studying an electron nematicity in a correlated system.

The iron-arsenide superconductors harbor indeed an anti ferromagnetic ground state, which is either preceded or accompanied by a structural tetragonal-orthorhombic phase transition at Ts. This structural transition breaks the four-fold symmetry of the high-temperature lattice and leads to an anisotropic conducting state. This broken rotational symmetry has thus a direct impact in the optical properties.We investigate the optical conductivity σ1(ω) with light polarized along the in-plane orthorhombic a- and b-axes of Ba(Fe1-xCox)2As2 for x=0 and 2.5% under uniaxial pressure across their phase transitions. The charge dynamics on these detwinned, single domain samples reveals an in-plane optical anisotropy (i.e., linear dichroism) which extends to relatively high frequencies and at T>Ts. This reveals substantial nematic susceptibility as well as the electronic nature of the structural transition. Another key result consists in the opportunity to disentangle the distinct behaviours of the Drude weights and scattering rates of the itinerant charge carriers,which are both enhanced along the a-axis with respect to the b-axis. Our findings allow us to clarify the long-standing striking anisotropy (ρb>ρa) of the dc resistivity.

Anisotropic charge dynamics in detwinned Ba(Fe1-xCox)2As2
A. Dusza et al., EPL 93, 37002 (2011)
[Abstract]

Optical manipulation of particles of all shapes and sizes (Vol. 46 No. 2)

Ellipsoidal particle levitating at the optical beam waist, with its long axis aligned to the beam axis. Credit: B. M. Mihiretie et al.

A new study of how particles may respond to the mechanical effects of light helps improve optical manipulation and remote sensing of non-spherical particles.

Manipulation of small objects by light has gained in popularity in the past few years. Now, scientists have performed the first systematic analysis of the behaviour of ellipsoidal particles manipulated by laser beams. The work shows that such particles are constantly moving in and out of the reach of an optical beam, creating oscillations. These findings have been obtained by the authors. This work could help understand the unusual behaviour of rod-like particles manipulated by optical tweezers. Ultimately, the theoretical part of this work could contribute numerical models of how complicated shapes and large sizes scatter laser light. Numerous applications exist in fluid engineering and remote sensing methods.

B. M. Mihiretie, P.Snabre, J.-C. Loudet and B. Pouligny, "Optically driven oscillations of ellipsoid particles. Part I: Experimental observations", Eur. Phys. J. E 37, 124 (2014)
[Abstract]

J.C. Loudet, B. M. Mihiretie and B. Pouligny, "Optically driven oscillations of ellipsoidal particles. Part II: Ray-Optics calculations", Eur. Phys. J. E 37, 125 (2014)
[Abstract]

Optical waveguide arrays (Vol. 44 No. 5)

Centre or edge localization in tunable waveguide arrays.

Over the past few years, advances in fabrication have made possible arrangements of coupled optical waveguides with tailored specifications. Such waveguide arrays are indispensible for building a scalable photonic quantum computer or creating novel optical elements.

The present paper summarizes the properties of light propagation in such an array with position-dependent coupling between adjacent waveguides. It shows that such arrays have properties that are markedly different from those of constant-coupling arrays. One such property is the formation of mirror-symmetric intensity profile in an array with a centre-maximum coupling profile. Another, shown in the figure, is the disorder-induced localization of a broad-input light to the centre of the array for centre-maximum coupling profiles and to the array edges, which occurs for centre-minimum coupling profiles. 

These results, along with novel properties of light propagation in the presence of balanced, spatially separated absorption and amplification, indicate the wealth of tunability that is offered by coupled optical waveguides.

Y. N. Joglekar, C. Thompson, D. D. Scott and G. Vemuri , ‘Optical waveguide arrays: quantum effects and PT symmetry breaking’, Eur. Phys. J. AP
[Abstract]

Optimising custody is child’s play for physicists (Vol. 45 No.2)

A graph depicting the children’s custody problem, where grey nodes represent couples

Ensuring that parents in recomposed families see their children regularly is a complex network problem that models developed to study materials may help to solve.

As a diversion from his normal duties as a theoretical physicist, one of the authors set out to finding a suitable weekend for both partners in his recomposed family to see all their children at the same time. This resulted in a study showing that solving this problem equates to minimising the energy in a material model.

The authors assume that people in the network, who are connected, as current or ex-partners, are willing to cooperate and communicate. They attempt to verify whether all parents could see all of their children together every other weekend. The answer is that it is not possible.  However, authors found an algorithm to maximise the number of parents spending time with their own children and those of their current partners. It was akin to minimising the energy of a particular magnetic material called a spin glass.

A. Gomberoff, V. Muñoz and P. P. Romagnoli, “The physics of custody”, Eur. Phys. J. B, 87, 37 (2014)
[Abstract]

Optimising proton beam therapy with mathematical models (Vol. 50, No. 3)

Optimising proton beam therapy with mathematical models

Particle beam therapy is increasingly being used to treat many types of cancer. It consists in subjecting tumours to beams of high-energy charged particles such as protons. Although more targeted than conventional radiotherapy using X-rays, this approach still damages surrounding normal tissue. To design the optimum treatment plan for each patient, it is essential to know the energy of the beam and its effect on tumour and normal tissue alike. In a recent study, a group of researchers put forward a new mathematical model outlining the effects of these beam therapies on patients' tissues, based on new, more complex, parameters. Using these new models, clinicians should be able to predict the effect of proton beams on normal and tumour tissue more precisely, allowing them to prepare more effective treatment plans.

R. Abolfath, Y. Helo, L. Bronk, A. Carabe, D. Grosshans and R. Mohan, Renormalization of radiobiological response functions by energy loss fluctuations and complexities in chromosome aberration induction: deactivation theory for proton therapy from cells to tumor control. Eur. Phys. J. D 73, 64 (2019)
[Abstract]

Optimising structures within complex arrangements of bubbles (Vol. 50, No. 5-6)

Optimising structures within complex arrangements of bubbles
Optimising an arrangement of five bubbles.

Computer simulations reveal the secret to stronger, cheaper structures shaped like bubbly foams.

While structures which emulate foam-like arrangements of bubbles are lightweight and cheap to build, they are also remarkably stable. The bubbles which cover the iconic Beijing Aquatics Centre, for example, each have the same volume, but are arranged in a way which minimises the total area of the structure – optimising the building’s construction. The mathematics underlying this behaviour is now well understood, but if the areas of the bubbles are not equal, the situation becomes more complicated. Ultimately, this makes it harder to make general statements about how the total surface area or, in 2D, edge length, or ‘perimeter’, can be minimised to optimise structural stability. In new research published recently, the authors explore how different numbers of 2D bubbles of two different areas can be arranged within circular discs, in ways which minimise their perimeters.

F. Headley, and S. Cox, Least-perimeter partition of the disc into N bubbles of two different areas, Eur. Phys. J. E 42, 92 (2019)
[Article]

Optimization of MEOP experiment performed at elevated 3He pressures (Vol. 42, No. 3)

image (a) Reshape of the gaussian pump laser beam into the annular profile using a pair of axicons, (b) Photograph of the rf discharge plasma afterglow in a transversal plane of helium cell, (c) Steady-state nuclear polarization of 3He gas vs pressure for two laser beam profiles.

It was demonstrated about fifteen years ago that hyperpolarized 3He gas can be used as an inhaled contrast agent in Magnetic Resonance Imaging (MRI) of human lungs. Since then, the technique has proved successful in anatomical and dynamic ventilation studies, which are not feasible by the standard proton MRI. One of the methods to obtain 3He gas of high polarization is Metastability Exchange Optical Pumping (MEOP),which is usually performed at low operating pressure of 1 mbar and at low magnetic field of 1 mT. Recently it has been shown that the MEOP method can be also performed at elevated 3He gas pressure, up to 260 mbar, provided it is performed at high magnetic field of 1.5 T or higher. Under these operating conditions, one of the factors that limits the efficiency of the method is an inhomogeneous density distribution of metastable state atoms produced by the RF discharge in the optical pumping cell (Fig.b). The paper shows how the situation can be improved by matching the spatial profile of the pumping laser beam to the distribution of metastable atoms. It is achieved by using a pair of axicons to produce an annular, instead of conventional Gaussian beam profile (Fig.a). The obtained nuclear polarization of 3He gas was up to 60% higher for pressures above 67 mbar (Fig.c). This result opens the possibility of producing large quantities of highly polarized 3He gas in a compact polarizer working in highmagnetic field of medical MRI scanner.

Optimization of the pumping laser beamspatial profile in the MEOP experiment performed at elevated 3He pressures
T. Dohnalik et al. Eur. Phys. J. Appl. Phys. 54, 20802 (2011)
[Abstract]

Optimum inertial self-propulsion design for snowman-like nanorobot (Vol. 45 No.5-6)

Streamlines, velocity field, and magnitude of the share of the propelling flow attributed to the low level inertial force, in the case of touching spheres.
Credit: Nadal et al.

A new study investigates the effects of small but finite inertia on the propulsion of micro and nano-scale swimming machines that could have implications for biomedical applications.

Scale plays a major role in locomotion. Swimming microorganisms, such as bacteria and spermatozoa, are subjected to relatively small inertial forces compared to the viscous forces exerted by the surrounding fluid. Such low-level inertia makes self-propulsion a major challenge. Now, the authors have found that the direction of propulsion made possible by such inertia is opposite to that induced by a viscoelastic fluid. This study could help optimise the design of self-propelled micro- and nanoscale artificial swimming machines to improve their mobility in medical applications. The study shows that a rotating dumbbell propels with the large sphere due to intertial forces in the fluid and the small sphere ahead in a pure viscoelastic fluid. The authors then derive the optimal dumbbell geometry for a self-propelling small-scale swimmer.

F. Nadal, O. S. Pak, L. Zhu, L. Brandt and E. Lauga, “Rotational propulsion enabled by inertia”, Eur. Phys. J. E 37, 60 (2014)
[Abstract]

Orbital tomography for highly symmetric adsorbate systems (Vol. 44 No. 2)

Schematics of the PTCDA/Ag(111) herringbone structure and a momentum space representation of its photoelectron angular distribution.

The alignment of electronic energy levels at interfaces strongly influences the electronic properties of multilayer systems like, e.g., organic thin films. One very powerful technique to investigate this level alignment and determine electronic binding energies is angular resolved photoelectron spectroscopy (ARPES). This method also allows assigning the spectroscopic resonances to molecular orbitals, since the angular distribution of the photoelectron yield can be understood as a fingerprint of the orbitals in momentum space. So far the possibilities of this emerging concept have only been demonstrated for low symmetric surfaces and adsorbates with only few molecular orientations.

This approach is extended to highly symmetric surfaces by studying the monolayer structure of the prototypical molecule 3,4,9,10-perylene-tetra-carboxylic-dianhydride (PTCDA) on the Ag(111) surface. This structure is particularly challenging since it contains two non-equivalent molecules per unit cell (Mol. A and B, see Figure) in six rotational domains, i.e., in total six differently oriented molecules of each type. However, the ARPES signal from both molecules could still be separated and an almost perfect match between the experimental data and the calculated moment space distributions was found for the highest occupied and the lowest unoccupied molecular orbital (HOMO and LUMO).

Furthermore, the contributions of both molecules to the ARPES data were analyzed regarding their binding energy by a two-dimensional fitting algorithm called “orbital tomography”. The result, (experimentally obtained) densities of states projected on molecules A and B (PDOS), turned out to agree very well with scanning tunnelling spectroscopy results reported earlier. The present findings clearly indicate that even for complex surface structures containing many differently oriented molecules the orbital tomography technique allows reliable investigations of the electronic structure of individual molecular species in thin organic films.

B. Stadtmüller and 10 co-authors, ‘Orbital tomography for highly symmetric adsorbate systems’, EPL, 100, 26008 (2012)
[Abstract]

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