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Vol. 42 No. 4 - Highlights

Electroweak model without a Higgs particle (Vol. 42, No. 4)

Thanks to the great accuracy in predicting experimental data, the standard model of particle physics is widely considered to be a building block of our current knowledge of the structure of matter. In spite of this success, we are still lacking an essential piece of evidence, namely the detection of the Higgs boson, a hypothetical massive elementary particle whose existence makes it possible to explain how most of the known elementary particles become massive. In this paper, an alternative electroweak model is presented that assumes running coupling constants described by energy-dependent entire functions. Contrary to the conventional formulation the action contains no physical scalar fields and no Higgs particle, even if the foreseen masses for particles are compatible with known experimental values. In addition the vertex couplings possess an energy scale for predicting scattering amplitudes that can be tested in current particle accelerators. As a result the paper provides an essential alternative to the current established knowledge in the field and addresses an issue that might soon be resolved, as the Large Hadron Collider could provide the experimental evidence of the existence or non-existence of the Higgs boson.

Ultraviolet complete electroweak model without a Higgs particle
J.W. Moffat, Eur. Phys. J. Plus, 126, 53 (2011)
[Abstract]

Atomic photoionization: When does it actually begin? (Vol. 42, No. 4)

image The crest position of the electron wave packet after the end of the XUV pulse is fitted with the straight line, which corresponds to the free propagation. In the inset, extrapolation of the free propagation inside the atom is shown. The XUV pulse is over-plotted with the black dotted line.

Among other spectacular applications of the attosecond streaking technique, it has become possible to determine the time delay between subjecting an atom to a short XUV pulse and subsequent emission of the photoelectron. This observation opened up a question as to when does atomic photoionization actually begin.

We address this question by solving the time dependent Schrödinger equation and by carefully examining the time evolution of the photoelectron wave packet. In this way we establish the apparent "time zero" when the photoelectron leaves the atom. At the same time, we provide a stationary treatment to the photoionization process and connect the observed time delay with the quantum phase of the dipole transition matrix element, the energy dependence of which defines the emission timing.

As an illustration of our approach, we consider the valence shell photoionization of Ne and double photoionization (DPI) of He. In Ne, we relate the opposite signs of the time delays t0(2s)<0 and t0(2p)<0 (Figure) with energy dependence of the p and d scattering phases which is governed by the Levinson-Seaton theorem. In He, we demonstrate that an attosecond time delay measurement can distinguish between the two leading mechanisms of DPI: the fast shake-off (SO) and the slow knockout (KO) processes. The SO mechanism is driven by a fast rearrangement of the atomic core after departure of the primary photoelectron. The KO mechanism involves repeated interaction of the primary photoelectron with the remaining electron bound to the singly charged ion.

Timing analysis of two-electron photoemission
A.S. Kheifets, I.A. Ivanov and Igor Bray, J. Phys. B: At. Mol. Opt. Phys. 44, 101003 (2011)
[Abstract]

Practical limits for detection of ferromagnetism (Vol. 42, No. 4)

image Ferromagnetic saturation moment of a ZnO substrate measured in five consecutive stages, exemplifying two of the most common sources of ferromagnetic contamination and showing a type of reversibility upon annealing under different atmospheres, which is often observed in some of the recently discovered nanomagnets mentioned in the text (the detection of ferromagnetism below 5 10-7 emu is hindered by setup-related artefacts).

Over the last ten years, signatures of room-temperature ferromagnetism have been found in thin films and nanoparticles of various materials that are non-ferromagnetic in bulk. The implications of such high temperature ferromagnetism are in some cases so extraordinary, e.g. dilute magnetic semiconductors (DMS) with carrier-mediated ferromagnetism well above room temperature would revolutionize semiconductor-based spintronics, that they triggered an enormous volume of materials research and development. However, the magnetics community soon started realizing the dangers of measuring the very small magnetic moments of these nanomagnets (nanometer sized materials with nano-emu magnetic moments). Pushing state-of-the-art magnetometers to their sensitivity limits, where extrinsic ferromagnetic signals originating from magnetic contamination and measurement artefacts are non-negligible, these new nanomagnets raise a number of challenges to magnetometry techniques and, most of all, to its users' methods and procedures. While new nanomagnets continue being "discovered" based on magnetometry measurements, the general opinion is moving towards the notion that finding a signature of ferromagnetism by means of magnetometry, i.e. a magnetic hysteresis, is only necessary but not sufficient to claim its existence.

Through an extensive analysis of various materials subject to different experimental conditions, the authors aim at re-establishing the reliability limits for detection of ferromagnetism using high sensitivity magnetometry. The paper provides a roadmap describing how extrinsic ferromagnetism can be avoided or otherwise removed, its magnitude when such optimum conditions cannot be guaranteed, and to what extent its characteristics may or may not be used as criteria to distinguish it from intrinsic ferromagnetism.

Practical limits for detection of ferromagnetism using highly sensitive magnetometry techniques
L.M.C. Pereira, J.P. Araújo, M.J. Van Bael, K. Temst and A. Vantomme, J. Phys. D: Appl. Phys. 44, 215001 (2011)
[Abstract]

Classical and quantum approaches to the photon mass (Vol. 42, No. 4)

image In new effects of the Aharonov-Bohm type, coherent superpositions of particles possessing opposite electromagnetic properties are used. For the one shown in this figure, charged particles interact with the magnetic vector potential A of a solenoid. If the photon mass is not zero, the electromagnetic interaction is modified. Measuring the corresponding change of quantum phase shift with an interferometer leads to an estimate of mγ.

Since Proca's prediction in 1936 that the rest mass of the photon, mγ, may not be zero, there have been several searches for evidence for a possible finite photon mass. In fact, for even a very small value of mγ, fascinating physical implications arise such as breakdowns of Coulomb's law, wavelength dependence of the speed of light in free space, existence of longitudinal electromagnetic waves, presence of an additional Yukawa potential for magnetic dipole fields, and effects that a photon mass may have during early-universe inflation and the resulting magnetic fields on a cosmological scale.

Traditionally, limits on mγ of < 10-49g have been obtained by means of classical approaches, such as searches for departures from Coulomb's law. What happens if we instead exploit quantum approaches? Could better limits be achieved? This is the novel objective of the present work, in which quantum physics is applied to the photon mass question. We first examine the implications that the Aharonov-Bohm class of quantum effects (Figure) have on searches for mγ, and then move on to explore the quantum electrodynamics scenario with an approach that employs measurements of the electron's g-factor. Within the quantum framework, we show that competitive new lower limits on the photon mass may reach the range 10-54 < mγ < 10-53g. We provide an assessment of the state of the art in these areas and a prognosis for future work.

A survey of existing and proposed classical and quantum approaches to the photon mass
G. Spavieri, J. Quintero, G.T. Gillies and M. Rodriguez, Eur. Phys. J. D 61, 531 (2011)
[Abstract]

UV absorption spectroscopy to monitor reactive plasma (Vol. 42, No. 4)

image Absorbance of the HBr gas at three pressures, as used in silicon gate etching processes.

A new high sensitivity technique is developed by extending the broad-band absorption spectroscopy to the vacuum ultraviolet (VUV) spectral region. It is well adapted for the detection and density measurement of closed-shell molecules that have strong electronic transitions in the 110-200 nm range. Among them, molecules such as Cl2, HBr, BrCl, Br2, HCl, BCl3, SiCl4, SiF4, CCl4, SF6, CH2F2 and O2, used in the microelectronics industry for etching or deposition processes, are of prime interest. In our system, the light of a deuterium lamp crosses a 50 cm diameter industrial etch reactor containing the gas of interest. The transmitted light is recorded with a 20 cm focal length VUV scanning spectrometer backed with a photomultiplier tube (PMT). The attached figure shows the absorbance at three pressures of the HBr gas, which is used in silicon gate etching processes. Peaks at 137, 143 and 150 nm, which show a non-linear, but very strong absorbance, correspond to transitions to Rydberg states of the molecule and can be used for the detection of very small HBr densities. In our present experiment, an absorption rate of 2%, corresponding to about 0.03 mTorr of HBr, can be easily detected on the 143 nm absorption peak. Replacing the PMT detector by a VUV sensitive CCD camera, would permit to reach the same signal to noise ratio with a few seconds acquisition time. For HBr pressures in the 1 to 100 mTorr range, the continuum part of the absorption spectrum (160-200 nm), which shows a weak but linear absorbance can be used. The technique is applied to monitor in Cl2-HBr mixture the dissociation rate of HBr and the amount of Br2 molecule formation at different plasma conditions.

Vacuum UV broad-band absorption spectroscopy: a powerful diagnostic tool for reactive plasma monitoring
G Cunge, M Fouchier, M Brihoum, P Bodart, M Touzeau and N Sadeghi, J. Phys. D: Appl. Phys. 44, 122001 (2011)
[Abstract]

Flexibility and phase transitions in zeolite frameworks (Vol. 42, No. 4)

image Detail of a zeolite structure built from corner-sharing tetrahedral units.

The zeolites are a group of minerals whose complex and beautiful atomic structures are formed by different arrangements of a very simple building block- a group of four oxygen atoms forming a tetrahedron, with a silicon or aluminium atom at the centre. Each oxygen atom belongs to two tetrahedra, so the structure can be viewed as a network of tetrahedra linked at the corners.

Zeolites have found widespread applications in chemical industry, particularly as catalysts. Their chemical properties depend on the shape of the pores and channels that run through the structure, containing water molecules, ions and even small organic molecules. More than a hundred different frameworks are known to exist in natural minerals or have been synthesised by chemists.

A fundamental geometric question is whether it is possible for the tetrahedra of the framework to exist in an undistorted, geometrically ideal form, or whether distortions are inevitably caused by the linking together of the tetrahedral units to form the structure. A new study links this question to the compression behaviour of zeolites in the analcime group. Four different structures display a common behaviour: they exist in a high-symmetry form at low pressures when the tetrahedra can exist without distortions, but transform to low-symmetry forms under pressure when distortions become inevitable. A deeper understanding of the rules governing the formation of zeolite structures may one day allow us to synthesise structures with specific properties on demand. New insights into the physics and geometry of frameworks are an important step in this direction.

Flexibility windows and phase transitions of ordered and disordered ANA framework zeolites
S. A. Wells, A. Sartbaeva and G. D. Gatta, EPL, 94, 56001 (2011)
[Abstract]

Molecular motors in the rigid and crossbridge models (Vol. 42, No. 4)

image Examples of spontaneous oscillations of motor assemblies in the crossbridge model (red) and the rigid model (blue).

In cells, motor proteins use chemical energy to generate motion and forces. Motors often interact and form clusters because they are connected to a single rigid backbone. In a muscle the backbone is made by association of the motor tails. The backbone motion results from the action of all the motors, and feeds back on each motor. Previous works suggest that motor assemblies are endowed with complex dynamical properties, including dynamic instabilities and spontaneous oscillations, which may play a role in the mechanisms of heartbeat, flagellar beating, or hearing. In this paper, we study two models of motor assemblies: the rigid two-state model and the classical crossbridge model widely used in muscle physiology.

Both models predict spontaneous oscillations. In the rigid two-state model, they can have a "rectangular'' shape or a characteristic "cusp-like'' shape that resembles cardiac sarcomere and "stick-slip'' oscillations. The oscillations in the vicinity of the Hopf bifurcation threshold can be much faster than the chemical cycle. This property, not found in the crossbridge model where protein friction slows down the motion, could be important for the description of high frequency oscillations, such as insect wingbeat. Experiments based on the response of a motor assembly to a step displacement are also well described by both theories, which predict non-linear force displacement relations, delayed rise in tension and "sarcomere give''. This suggests that these effects are not directly dependent on molecular details. We also relate the collective properties of the motors to their microscopic properties accessible in single molecule experiments: we show that a three state state crossbridge model predicts the existence of instabilities even in the case of an apparent load decelerated detachment rate.

Dynamical behaviour of molecular motor assemblies in the rigid and crossbridge models
T. Guérin, J. Prost and J-F. Joanny, Eur. Phys. J. E, 34, 60 (2011)
[Abstract]