Vol. 42 No. 3 - Highlights

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 (ρba) of the dc resistivity.

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

Quantitative interpretation of the excitonic splittings in aluminium nitride (Vol. 42, No. 3)

image locus of the values of the hole effective masses compatible with the 38meV experimental value of the 1s-2s splitting (purple line). Similar plots for alternative values of the dielectric constant (blue and green plots). The predictions of several first principle calculations are plotted using red dots.

Properties of free exciton are used to address the first self consistent, all-optical determination of hole effective masses in aluminium nitride.

AlN is a wurtzite semiconductor, which appears to be very promising as a substrate or as a buffer layer in many heteroepitaxial growths of devices like UV light emitting diodes, solar blind light detectors, high power and/or high frequency operating field-effect transistors. Its thermal conductivity is superior to that of GaN, which is today, word widely used as a substrate for pulling forward the solid state lighting and the Blue Ray laser technologies.

The growth of AlN was only very recently achieved as bulky bowls or as hetero-epitaxial films. The band gap of AlN changes with the growth conditions, which is interpreted in terms of residual strain fields existing in hetero-epitaxies. Here, the model leading to the quantitative interpretation of the evolution of the band gap of GaN under strain (B.Gil et al., PRB, 52, R17028, (1995)) is extended to AlN.

Improved crystalline quality and residual doping now allow high-resolution optical spectroscopy. The origin of the experimental value of the 1s-2s excitonic splitting is analyzed using a model of H atom adapted to anisotropic masses and dielectric constants. This analysis permits to extract from the experimental data, the couple of relevant values of the dielectric constant, to find the locus for values for the on-axis and in-plane hole effective masses (purple line in the figure) fully compatible with the measured value of the 1s-2s splitting.

Quantitative interpretation of the excitonic splittings in aluminum nitride
B. Gil, et al., Eur. Phys. J. Appl. Phys 53, 20303 (2011)

Particle accelerators for the study of Cultural Heritage (Vol. 42, No. 3)

image Details of the painting by Mantegna (above, part of the veil of the Virgin; below, the eye of the Virgin), and corresponding X ray distribution maps obtained by PIXE at the external scanning micro-beam set-up at INFN-LABEC.

This paper is a first critical discussion of the contemporary role that small particle accelerators play in the field of applications related to Cultural Heritage for non-destructive materials analysis and dating, such as ion beam analysis (IBA) and accelerator mass spectroscopy (AMS). This analysis is put in perspective by comparing accelerator-based methods to other techniques, pointing out the corresponding pros and cons. It is stressed that IBA can address questions that cannot be answered by conventional techniques like X-ray fluorescence (the latter having however the advantage of portability). It is shown in particular that IBA can still yield unrivalled results thanks to novel uses of its potential, such as providing elementalmaps and resolving layer structures. This was strikingly demonstrated in a recent PIXE (Particle Induced X-ray Emission) analysis at LABEC of Mantegna’s famous painting Madonna con Bambino (see figure), which used the scanning external proton microbeam set-up to produce a high-resolution elementalmapping.The use of different, even precious, pigments to paint tiny details is thus pointed out, which can be of great interest to art historians and restorers. In addition, by utilizing the different absorption rates of different X-ray energies, even the paint layer structure can sometimes be reconstructed: for example, in the case shown in the upper part of the figure, it is shown that the Virgin’s veil was made with a gold paint over a background of ultramarine blue, lightened with lead-white. No other technique can provide such information in a totally non-destructive way and with no pick-up of samples

The present role of small particle accelerators for the study of Cultural Heritage
P.A. Mandò et al., Eur. Phys. J. Plus 126, 41 (2011)

Suppression of resonant two-photon ionization rate by Rabi oscillation (Vol. 42, No. 3)

image Correlation between rabi oscillations and the time-dependence of the ionization probability for the resonant two-photon ionization of Ar+: (a) envelop of the laser field, (b) and (c) the square normof the probability amplitudes of the ground g state (blue) and of the intermediate i state (red) for aweak and a strong field case. The time-dependent ionization probability i.P. (green), is suppressed during the second half cycle of the rabi oscillations for the strong field case (c) due to the decrease in the probability amplitude of the intermediate state i.

In the form of resonantly enhanced multiphoton ionization (REMPI), few-photon spectroscopy has become a tool in a number of applications. This is because multiphoton ionization (n+1) rate increases in proportion to the (n+1)-th power of laser intensity, where n is the number of photons for the resonance excitation and the resonance enhances excitation probability, compared to non-resonance channel. Unlike REMPI in optical laser frequency regime, the intermediate resonance is reached by absorbing one photon in VUV laser frequency regime. In such case, the resonance state emits a photon with the same phase and frequency as the laser light. This cycling process of absorbing and emitting is called Rabi oscillations, which might affect the multiphoton ionization rate.

In fact, we have revealed the strong suppression of the resonant two-photon ionization (1+1) rate of Ar+ ion that increases according to the second power of laser intensity within the framework of perturbation theory. Our interpretation of the new results, the Rabi oscillation on the course of the duration of a single light pulse of the VUV free-electron laser plays the crucial role to determine the resonant two-photon ionization rate, bridges a gap between multiphoton physics and quantum optics.

Three-photon double ionization of Ar studied by photoelectron spectroscopy using extreme ultraviolet free-electron laser:Manifestation of resonance states of intermediate Ar+ ion
N. Miyauchi et al., J. Phys. B: At.Mol. Opt. Phys. 44, 07100 (2011)

Interaction control in ultracold Fermi-Fermimixtures (Vol. 42, No. 3)

image Interaction control in a trapped fermi-fermimixture of 6li and 40K atoms. a feshbach resonancemanifests itself in a pronounced dependence of the scattering length a on themagnetic field strength b. This facilitateswide interaction tuning between the trapped species.

Many present-day experiments on ultracold quantum gases crucially rely on the ability to control the interparticle interaction via Feshbach resonances [C. Chin et al., Rev. Mod. Phys. 82, 1225 (2010)]. Such resonances occur when colliding atoms couple to a bound molecular state and manifest themselves in a pronounced dependence of the scattering length on the magnetic field (Fig.). This phenomenon provides experimentalists with a unique "tuning knob" to control the two-body, few-body, and many-body properties of the system.

Ultracold mixtures of two fermionic species hold great promise for synthesizing novel types of few- and many body quantum states, including exotic types of superfluids. The investigation of such systems has been pursued in the collaborative project FerMix in the network EuroQUAM (an ESF EUROCORE). The prime candidate for experimental realizations is a combination of the fermionic isotopes 6Li and 40K. The article presents a state-of-the-art characterization of Feshbach resonances in a mixture of two Fermi gases of 6Li and 40K. A thorough case study is reported for a particularly promising resonance near a magnetic field strength of 155G, and the comparison of experimental results with theory highlights the high understanding gained for the system. A survey of other resonances allows experimentalists to identify the most suited “tuning knobs” in such Fermi-Fermi systems.

The deep understanding of the collisional properties in the ultracold domain opens towards all applications concerning complex quantum states of strongly interacting fermionic matter.

Feshbach resonances in the 6Li-40K Fermi-Fermimixture: Elastic versus inelastic Interactions
D. Naik et al., Eur. Phys. J. D 61, Special issue "Cold quantummatter …" (2011)

Shortcut to adiabaticity with ultracold atoms (Vol. 42, No. 3)

image Top: Principle of shortcut to adiabaticity. The blue and red curves represent the BEC wave function and trapping potential respectively. below: comparison of BEC excitations produced by standard and shortcut decompressions.

Adiabatic transformations are widely used in physics, and for instance are at the heart of the manipulation of quantum states. In such transformations, the Hamiltonian should typically vary slowly with time, such that the system always remains close to equilibrium. The drawback is the long transition time, which in some situations becomes unpractical due to finite lifetime or coherence time of the state under study. Alternatively, a "shortcut to adiabaticity" is a specifically designed temporal trajectory of the Hamiltonian, which connects the initial to the final state in a shorter time. The system is out of equilibrium during the transition, but the final state is identical to that obtained via an adiabatic transition (Fig., upper panel).

We have applied this strategy for the first time, to rapidly decompress an interacting Bose-Einstein condensate (BEC) held in a magnetic trap. Reducing the trap confinement also shifts the cloud vertically by a large amount, due to gravity. We implemented a 30ms-long trajectory designed for a 10-fold reduction of the trap frequency. Because of experimental imperfections, the final state we obtained is not an equilibrium one. However, we were able to demonstrate a large reduction of BEC excitations (dipole and breathing modes) when comparing the shortcut to other standard decompression schemes (Fig., lower panel). This trajectory was also shown to work for a thermal cloud with negligible interactions, hinting at the broad range of application of this technique.

Shortcut to adiabaticity for an interacting Bose-Einstein condensate
J.-F. Schaff et al., EPL 93, 23001 (2011)

Statistical physics approach to graphical games (Vol. 42, No. 3)

image network of agents playing a graphical game. The payoff matrices depend both on the local neighbourhoods and on some aggregate signal. The equilibriums are reached by exchanging localmessages and their overall geometrical structure can be analyzed by statistical physics techniques.

A graphical game is a mathematical framework to analyse strategic interactions among self-interested agents who play only with their neighbours in a graph. It usually makes predictions in terms of equilibrium concepts, chief among which is the Nash equilibrium: a configuration of strategies where nobody has a unilateral incentive to deviate. The problem of finding a Nash equilibrium is believed to be computationally intractable. In this work we show how to use methods from statistical physics of disordered systems to develop efficient and fully distributed (approximate) algorithms to tackle this problem. Furthermore, motivated by the recent interest in the study of the interplay between local and global interactions in multi-agent systems, we propose a new compact representation of games that extends over graphical games to deal conveniently with a global interaction and show how to extend the methods to this new case. We also derive the phase diagrams of different ensembles of random graphical games and study the structure of the corresponding space of solutions. Evidence of HARD/EASY phase transitions is found in some cases.

Statistical physics approach to graphical games: local and global interactions
A. Ramezanpour et al., Eur. Phys. J. B 80, 555 (2011)

Spin-charge locking and tunnelling into a helicalmetal (Vol. 42, No. 3)

image a ferromagnet (green) that is coupled to a topological insulator (grey) such that electrons can tunnel between the two materials. it is found that a current flowin the surface states of the topological insulator strongly modifies the tunnelling current.

Spintronics aims at exploiting the electron spin for new device functionalities. In recent years a new spintronic paradigm, based on the spin-orbit interaction, has been proposed aiming to gain spin control by electric fields. In this respect, topological insulators (TI) appear as a very promising opportunity. At the surface of aTI gapless excitations occur with extraordinarily strong spinorbit coupling: a given surface momentum is associated with a single spin direction, such that the states on the Fermi surface have a well-defined helicity.

In this paper we present a theoretical study of the dynamics of the electrons moving on the surface of a three-dimensional TI, i.e. in a two-dimensional helical metal (HM). When the HM is brought into contact with a ferromagnet, there arises an unconventional magnetoresistance. The origin of the effect is the spin-orbit coupling: since the electron momentum is connected to a single spin state, a current flow creates a nonequilibrium spin polarization. This current-induced spin polarization increases or decreases the spin dependent voltage difference between the helicalmetal and themajority or minority carriers in the FM and thus modifies the tunnelling current. By reversing the flow of the current in the helical metal the two spin species exchange their role. In the ideal case the tunnelling current between the FM and the HM can be switched on and off depending on the relative orientation of the magnetization of the FM with respect to the direction of the current flow in the HM.

Spin-Charge Locking and Tunnelling into a HelicalMetal
P. Schwab, R. Raimondi and C. Gorini, EPL 93, 67004 (2011)

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)