Some recent Publications & Papers of the School of Basic Sciences

Force transmission in migrating cells

Maxime F. Fournier, Roger Sauser, Davide Ambrosi, Jean-Jacques Meister, and Alexander B. Verkhovsky

During cell migration, forces generated by the actin cytoskeleton are transmitted through adhesion complexes to the substrate. To investigate the mechanism of force generation and transmission, we analyzed the relationship between actin network velocity and traction forces at the substrate in a model system of persistently migrating fish epidermal keratocytes. Front and lateral sides of the cell exhibited much stronger coupling between actin motion and traction forces than the trailing cell body. Further analysis of the traction-velocity relationship suggested that the force transmission mechanisms were different in different cell regions: at the front, traction was generated by a gripping of the actin network to the substrate, whereas at the sides and back, it was produced by the network's slipping over the substrate. Treatment with inhibitors of the actin-myosin system demonstrated that the cell body translocation could be powered by either of the two different processes, actomyosin contraction or actin assembly, with the former associated with significantly larger traction forces than the latter.

(J. Cell Biol., Jan 2010; 188: 287 - 297 ) scitation - (PDF file)

Theoretical Analysis of Mechanical Displacement Measurement Using a Multiple Cavity Mode Transducer

J. M. Dobrindt  and T. J. Kippenberg
We present an optomechanical displacement transducer that relies on three cavity modes parametrically coupled to a mechanical oscillator and whose frequency spacing matches the mechanical resonance frequency. The additional resonances allow reaching the standard quantum limit at a substantially lower input power (compared to the case of a single cavity mode), as both sensitivity and quantum backaction are enhanced. Furthermore, it is shown that in the case of multiple cavity modes, coupling between the modes is induced via reservoir interaction, e.g., enabling quantum backaction noise cancellation. Experimental implementation of the schemes is discussed in both the optical and microwave domain.
(Phys. Rev. Lett. 104, 033901 (2010) ) scitation - (PDF file)

Dynamics of Long-Range Ordering in an Exciton-Polariton Condensate

Cet après-midi de Noël s'adresse aux enfants des collaboratrices, collaborateurs, étudiantes et étudiants de l'EPFL. Pour la première partie du spectacle (début à 14h30), l'âge minimal admis est de 5 ans.

Les enfants plus jeunes, mais âgé au minimum de 3 ans, sont les bienvenus dès 15h30 pour la collation de l'entracte et pour assister à la deuxième partie du Noël EPFL.

Les inscriptions sont indispensables pour des raisons d'organisation et de sécurité. En confirmation à leur inscription, les parents recevront, à leur adresse professionnelle, un billet d'entrée pour le Noël EPFL. Ce billet devra être présenté le jour du spectacle pour entrer dans la Salle polyvalente.

Les jeunes enfants sont placés sous la responsabilité de leurs parents. Toutefois, en raison de l'affluence et des places limitées de la salle, nous ne pouvons accueillir qu'un seul parent par famille. Merci de votre compréhension.

Sawtooth-Control Mechanism using Toroidally Propagating Ion-Cyclotron-Resonance Waves in Tokamaks

J. P. Graves, I. Chapman, S. Coda, L.-G. Eriksson, and T. Johnson
The sawtooth control mechanism in plasmas employing off-axis toroidally propagating ion cyclotron resonance waves in tokamaks is reinvestigated. The radial drift excursion of energetic passing ions distributed asymmetrically in the velocity parallel to the magnetic field determines stability when the rational q=1 surface resides within a narrow region centered about the shifted fundamental cyclotron resonance
(Phys. Rev. Lett. 102, 065005 (2009) ) scitation - (PDF file)

Pressure Induced Superconductivity in Pristine 1T-TiSe2

A. F. Kusmartseva, B. Sipos, Helmuth Berger, Lazlo Forro , and E. Tutis
The interplay between superconductivity and the charge-density wave (CDW) state in pure 1T-TiSe₂ is examined through a high-pressure study extending up to pressures of 10 GPa between sub-Kelvin and room temperatures. At a critical pressure of 2 GPa a superconducting phase sets in and persists up to pressures of 4 GPa. The maximum superconducting transition temperature is 1.8 K. These findings complement the recent discovery of superconductivity in copper-intercalated 1T-TiSe₂. The comparisons of the normal state and superconducting properties of the two systems reveal the possibility that the emergent electronic state qualitatively depends on the manner in which the CDW state is destabilized, making this a unique example where two different superconducting domes are obtained by two different methods from the same parent compound.
(Phys. Rev. Lett. 103, 236401 (2009) ) scitation - (PDF file)

Observation of Half-Quantum Vortices in an Exciton-Polariton Condensate

K. G. Lagoudakis, T. Ostatnick, A. V. Kavokin, Y. G. Rubo, R. André,  Benoît Deveaud-Plédran
Singly quantized vortices have already been observed in many systems, including the superfluid helium, Bose-Einstein condensates of dilute atomic gases, and condensates of exciton-polaritons in the solid state. Two-dimensional superfluids carrying spin are expected to demonstrate a different type of elementary excitations referred to as half-quantum vortices, characterized by a 180 degrees rotation of the phase and a 180 degrees rotation of the polarization vector when circumventing the vortex core. We detect half-quantum vortices in an exciton-polariton condensate by means of polarization-resolved interferometry, real-space spectroscopy, and phase imaging. Half-quantum vortices coexist with single-quantum vortices in our sample.
(Science 326, 974 - 976, (2009)) citation - (PDF file)
-- EPFL Press Release --

Explanation of Photon Correlations in the Far-Off-Resonance Optical Emission from a Quantum-Dot-Cavity System

Martin Winger, Thomas Volz, Guillaume Tarel, Stefano Portolan, Antonio Badolato, Kevin J. Hennessy, Evelyn L. Hu, Alexios Beveratos, Jonathan Finley, Vincenzo Savona, and Ataç Imamoglu
In a coupled quantum-dot–nanocavity system, the photoluminescence from an off-resonance cavity mode exhibits strong quantum correlations with the quantum-dot transitions, even though its autocorrelation function is classical. Using new pump-power dependent photon-correlation measurements, we demonstrate that this seemingly contradictory observation that has so far defied an explanation stems from cascaded cavity photon emission in transitions between excited multiexciton states. The mesoscopic nature of quantum-dot confinement ensures the presence of a quasicontinuum of excitonic transitions, part of which overlaps with the cavity resonance.
(Phys. Rev. Lett. 103, 207403 (2009) ) scitation - (PDF file)

Spin Amplitude Modulation Driven Magnetoelectric Coupling in the New Multiferroic FeTe2O5Br

M. Pregelj, O. Zaharko, A. Zorko, Z. Kutnjak, P. Jeglic, P. J. Brown, M. Jagodic, Z. Jaglicic, Helmuth Berger, and D. Arcon
The magnetic and ferroelectric properties of the layered geometrically frustrated cluster compound FeTe₂O₅Br were investigated with single-crystal neutron diffraction and dielectric measurements. An incommensurate transverse amplitude modulated magnetic order with the wave vector q=(,0.463,0) develops below T_N=10.6(2) K. Simultaneously, a ferroelectric order due to exchange striction involving polarizable Te⁴⁺ lone-pair electrons develops perpendicular to q and to Fe³⁺ magnetic moments. The observed magnetoelectric coupling is proposed to originate from the temperature dependent phase difference between neighboring amplitude modulation waves.

(Phys. Rev. Lett. 103, 147202 (2009) ) scitation - (PDF file)

Ultrafast Structural Phase Transition Driven by Photoinduced Melting of Charge and Orbital Order

P. Beaud, S. L. Johnson, E. Vorobeva, U. Staub, R. A. De Souza, C. J. Milne, Q. X. Jia, and G. Ingold
We use femtosecond x-ray diffraction to probe directly the structural dynamics of a charge ordered and orbitally ordered thin film of La_0.42Ca_0.58MnO₃ initiated by an ultrafast optical pulse. At low excitation fluences we observe the displacive excitation of a coherent optical A_g phonon. Under high excitation conditions we observe a complete phase transition within 1 ps via the disappearance of a superlattice reflection. The initial step of the phase transition occurs on a time scale significantly faster than the 200 fs time resolution of our experiment.
(Phys. Rev. Lett. 103, 155702 (2009) ) scitation - (PDF file)

Near-field cavity optomechanics with nanomechanical oscillators

G. Anetsberger, O. Arcizet, Q. P. Unterreithmeier, R. Rivière, A. Schliesser, E. M. Weig, J. P. Kotthaus & T. J. Kippenberg
Cavity-enhanced radiation-pressure coupling between optical and mechanical degrees of freedom allows quantum-limited position measurements and gives rise to dynamical backaction, enabling amplification and cooling of mechanical motion. Here, we demonstrate purely dispersive coupling of high-Q nanomechanical oscillators to an ultrahigh-finesse optical microresonator via its evanescent field, extending cavity optomechanics to nanomechanical oscillators. Dynamical backaction mediated by the optical dipole force is observed, leading to laser-like coherent nanomechanical oscillations solely due to radiation pressure. Moreover, sub-fm Hz^-1/2 displacement sensitivity is achieved, with a measurement imprecision equal to the standard quantum limit (SQL), which coincides with the nanomechanical oscillator's zero-point fluctuations. The achievement of an imprecision at the SQL and radiation-pressure dynamical backaction for nanomechanical oscillators may have implications not only for detecting quantum phenomena in mechanical systems, but also for a variety of other precision experiments. Owing to the flexibility of the near-field coupling platform, it can be readily extended to a diverse set of nanomechanical oscillators. In addition, the approach provides a route to experiments where radiation-pressure quantum backaction dominates at room temperature, enabling ponderomotive squeezing or quantum non-demolition measurements.
(Nature Physics online: 11 October 2009  ) scitation - (PDF file)

Coulomb correlations of charged excitons in semiconductor quantum dots

D. Y. Oberli, M. Byszewski, B. Chalupar, E. Pelucchi, A. Rudra, and E. Kapon
The emission pattern of charged excitons in a semiconductor quantum dot is composed of a quadruplet of linearly polarized lines when a magnetic field is applied in a Voigt configuration. The orientation of the linear polarization of exciton emission is controlled by the orientation of the magnetic field in QDs with C3v symmetry while for QDs with C2v symmetry it is not. We demonstrate that the g factor of holes is very sensitive to the dot shape asymmetry, but that of electrons is not. By comparing the effective g factors obtained for the neutral and charged excitons in the same quantum dot, we uncover the role of Coulomb correlations in these excitonic states. We show that the C3v symmetry of pyramidal QDs makes them ideal candidates for implementing all-optical many-qubits gates based on electron spin as a quantum bit.
(Phys. Rev. B 80, 165312 (2009) ) scitation - ( PDF file)

Conformational flexibility of a rotaxane thread probed by electronic spectroscopy in helium nanodroplets

S. Smolarek, A.M. Rijs, J.S. Hannam, D.A. Leigh, M. Drabbels and W.J. Buma

Ultrahigh-resolution spectroscopic studies have been performed to elucidate the conformational landscape of the succinamide-based thread 1 that is frequently employed in mechanically interlocked molecular assemblies. We show how dissolving single molecules into a helium nanodroplet enables us to resolve the broad absorption spectrum—which is normally observed—into the separate contributions of individual conformers that are populated under the employed experimental conditions. Excellent agreement is obtained with the results of molecular dynamics calculations. The absorption spectrum of each conformer reveals a splitting of the zero-phonon resonance that is different for each conformer and could thus serve as a spectral signature.
(Journal of the American Chemical Society  131, 12902 (2009) ) scitation - (PDF file)

Vibrational Coherences and Relaxation in the High-Spin State of Aqueous [FeII(bpy)3]2+

Cristina Consani, Mirabelle Prémont-Schwarz,  Amal El Nahhas , Christian Bressler, Frank van Mourik, Andrea Cannizzo, Majed Chergui.
Dizzy cooling: Femtosecond excitation of the singlet states of aqueous [Fe^II(bpy)₃]²⁺ (bpy=2,2-bipyridine) leads to the formation of a vibrationally hot quintet state that exhibits wave-packet dynamics arising from a chelate-ring and bending mode. The vibrational relaxation involves at least two modes: the FeN stretching mode (see picture) and the coherently excited chelate-ring and bending mode, which relax on different time scales.
(Angew. Chem. Int. Ed. 2009, 48, 7184 –7187) scitation - (PDF file)

Time-resolved lanthanide luminescence for lab-on-a-chip detection of biomarkers on cancerous tissues

Bo Song, Venkataragavalu Sivagnanam, Caroline D. B. Vandevyver, Ilkka Hemmilä, Hans-Anton Lehr, Martin A. M. Gijs and Jean-Claude G. Bünzli
PDMS-based microfluidic devices combined with lanthanide-based immunocomplexes have been successfully tested for the multiplex detection of biomarkers on cancerous tissues, revealing an enhanced sensitivity compared to classical organic dyes.
(Analyst 134, 1991 (2009)) scitation - (PDF file)

Frequency comb assisted diode laser spectroscopy for measurement of microcavity dispersion

P. Del'Haye, O. Arcizet, M. L. Gorodetsky, R. Holzwarth1 & T. J. Kippenberg
Although invented for precision measurements of single atomic transitions, frequency combs have also become a versatile tool for broadband spectroscopy in recent years. Here, we present a novel and simple approach for broadband spectroscopy, combining the accuracy of an optical fibre-laser-based frequency comb with the ease of use of a tunable external cavity diode laser. The scheme enables broadband and fast spectroscopy of more than 4 THz bandwidth at scanning speeds up to 1 THz s^-1 at sub-MHz resolution. We use this method for spectroscopy of microresonator modes and precise measurements of their dispersion, which is relevant in the context of broadband optical frequency comb generation, having recently been demonstrated in these devices. Moreover, we find excellent agreement between measured microresonator dispersion with predicted values from finite element simulations, and we show that microresonator dispersion can be tailored by adjusting their geometrical properties.
(Nature Photonics 3, 529 - 533 (2009)) scitation - (PDF file)

Silicon Surface with Giant Spin Splitting

I. Gierz, T. Suzuki, E. Frantzeskakis, S. Pons, S. Ostanin, A. Ernst, J. Henk, M. Grioni, K. Kern, , and C. R. Ast
We demonstrate a giant Rashba-type spin splitting on a semiconducting substrate by means of a Bi-trimer adlayer on a Si(111) wafer. The in-plane inversion symmetry is broken inducing a giant spin splitting with a Rashba energy of about 140 meV, much larger than what has previously been reported for any semiconductor heterostructure. The separation of the electronic states is larger than their lifetime broadening, which has been directly observed with angular resolved photoemission spectroscopy. The experimental results are confirmed by relativistic first-principles calculations.

(Phys. Rev. Lett. 103, 046803 (2009) ) scitation - (PDF file)

Superfluid to Bose-Glass Transition in a 1D Weakly Interacting Bose Gas

Luca Fontanesi, Michiel Wouters, and Vincenzo Savona
We study the one-dimensional Bose gas in spatially correlated disorder at zero temperature, using an extended density-phase Bogoliubov method. We analyze, in particular, the decay of the one-body density matrix and the behavior of the Bogoliubov excitations across the phase boundary. We observe that the transition to the Bose-glass phase is marked by a power-law divergence of the density of states at low energy. A measure of the localization length displays a power-law energy dependence in both regions, with the exponent equal to -1 at the boundary. We draw the phase diagram of the superfluid-insulator transition in the limit of small interaction strength.
(Phys. Rev. Lett. 103, 030403 (2009) ) scitation - (PDF file)

High-Resolution NMR in Magnetic Fields with Unknown Spatiotemporal Variations

Philippe Pelupessy, Enrico Rennella, Geoffrey Bodenhausen
Nuclear magnetic resonance (NMR) experiments are usually carried out in homogeneous magnetic fields. In many cases, however, high-resolution spectra are virtually impossible to obtain because of the inherent heterogeneity of the samples or living organisms under investigation, as well as the poor homogeneity of the magnets (particularly when bulky samples must be placed outside their bores). Unstable power supplies and vibrations arising from cooling can lead to field fluctuations in time as well as space. We show how high-resolution NMR spectra can be obtained in inhomogeneous fields with unknown spatiotemporal variations. Our method, based on coherence transfer between spins, can accommodate spatial inhomogeneities of at least 11 gauss per centimeter and temporal fluctuations slower than 2 hertz.
(Science 324, 1693 - 1697 (2009)) citation - (PDF file)

Melting of Two-Dimensional Adatom Superlattices Stabilized by Long-Range Electronic Interactions

N. N. Negulyaev, V. S. Stepanyuk, L. Niebergall, P. Bruno, Marina Pivetta, Markus Ternes, François Patthey, and Wolf-Dieter Schneider
The melting transition of Ce adatom superlattices stabilized by long-range substrate-mediated electronic interactions on Cu(111) and Ag(111) noble metal surfaces has been investigated by low-temperature scanning tunneling microscopy, density functional theory calculations, and kinetic Monte Carlo simulations. Intriguingly, owing to the interaction between Ce adatoms and substrate, these superlattices undergo two-dimensional melting to a liquid without transition through the hexatic phase. The crucial parameters for this direct solid to liquid transition are identified.
(Phys. Rev. Lett. 102, 246102 (2009) ) scitation - (PDF file)

Extended Navigability of Small World Networks: Exact Results and New Insights

Cécile Caretta Cartozo and Paolo De Los Rios
Navigability of networks, that is, the ability to find any given destination vertex starting from any other vertex, is crucial to their usefulness. In 2000 Kleinberg showed that optimal navigability could be achieved in small world networks provided that a special recipe was used to establish long range connections, and that a greedy algorithm, that ensures that the destination will be reached, is used. Here we provide an exact solution for the asymptotic behavior of such a greedy algorithm as a function of the system's parameters. Our solution enables us to show that the original claim that only a very special construction is optimal can be relaxed depending on further constraints, such as, for example, cost minimization, that must be satisfied.
(Phys. Rev. Lett. 102, 238703 (2009) ) scitation - (PDF file)

Temperature Dependence of Normal Mode Reconstructions of Protein Dynamics

Francesco Piazza, Paolo De Los Rios and Fabio Cecconi
Normal mode (NM) analysis is a widely used technique for reconstructing conformational changes of proteins from the knowledge of native structures. In this Letter, we investigate to what extent NMs capture the salient features of the dynamics over a range of temperatures from close to T=0 to above unfolding. We show that the use of normal modes at room temperature is justified provided proteins are cooperative, that is, globular and highly structured. On the other hand, it is imperative to consider several modes in order to eliminate the unpredictable temperature dependence of single-mode contributions to the protein fluctuations.
(Phys. Rev. Lett. 102, 218104 (2009) ) scitation - (PDF file)

Reduction of the Superconducting Gap of Ultrathin Pb Islands Grown on Si(111)

Christophe Brun, I-Po Hong, François Patthey, I. Yu. Sklyadneva,2,3 R. Heid,4 P. M. Echenique,2,5 K. P. Bohnen,4 E. V. Chulkov,2,5 and Wolf-Dieter Schneider
The energy gap of superconducting Pb islands grown on Si(111) was probed in situ between 5 and 60 monolayers by low-temperature scanning tunneling spectroscopy. was found to decrease from its bulk value as a function of inverse island thickness. Corresponding T_c values, estimated using bulk gap-to-T_c ratio, are in quantitative agreement with ex situ magnetic susceptibility measurements, however, in strong contrast to previous scanning probe results. Layer-dependent ab initio density functional calculations for freestanding Pb films show that the electron-phonon coupling constant, determining T_c, decreases with diminishing film thickness.
(Phys. Rev. Lett. 102, 207002 (2009) ) scitation - ( PDF file)

Functional electric field changes in photoactivated proteins revealed by ultrafast Stark spectroscopy of the Trp residues

Jérémie Léonarda, Erwin Portuondo-Campa, Andrea Cannizzo, Frank van Mourik, Gert van der Zwan, Jörg Tittor, Stefan Haacke and Majed Chergui
Ultrafast transient absorption spectroscopy of wild-type bacteriorhodopsin (WT bR) and 2 tryptophan mutants (W86F and W182F) is performed with visible light excitation (pump) and UV probe. The aim is to investigate the photoinduced change in the charge distribution with 50-fs time resolution by probing the effects on the tryptophan absorption bands. A systematic, quantitative comparison of the transient absorption of the 3 samples is carried out. The main result is the absence in the W86F mutant of a transient induced absorption band observed at ≈300–310 nm in WT bR and W182F. A simple model describing the dipolar interaction of the retinal moiety with the 2 tryptophan residues of interest allows us to reproduce the dominant features of the transient signals observed in the 3 samples at ultrashort pump-probe delays. In particular, we show that Trp⁸⁶ undergoes a significant Stark shift induced by the transient retinal dipole moment. The corresponding transient signal can be isolated by direct subtraction of experimental data obtained for WT bR and W86F. It shows an instantaneous rise, followed by a decay over ≈500 fs corresponding to the isomerization time. Interestingly, it does not decay back to zero, thus revealing a change in the local electrostatic environment that remains long after isomerization, in the K intermediate state of the protein cycle. The comparison of WT bR and W86F also leads to a revised interpretation of the overall transient UV absorption of bR.
(PNAS 106,  7718 (2009) ) scitation - (PDF file)

Anomalous Magnetic Excitations of Cooperative Tetrahedral Spin Clusters

K. Prša, H. M. Rønnow, O. Zaharko, N. B. Christensen, J. Jensen, J. Chang, S. Streule, M. Jiménez-Ruiz, Helmuth Berger , M. Prester, and J. Mesot
An inelastic neutron scattering study of Cu₂Te₂O₅X₂ (X=Cl, Br) shows strong dispersive modes with large energy gaps persisting far above T_N, notably in Cu₂Te₂O₅Br₂. The anomalous features: a coexisting unusually weak Goldstone-like mode observed in Cu₂Te₂O₅Cl₂ and the size of the energy gaps cannot be explained by existing theories, such as our mean-field or random-phase approximation. We argue that our findings represent a new general type of behavior due to intercluster quantum fluctuations and call for development of a new theoretical approach.
(Phys. Rev. Lett. 102, 177202 (2009) ) scitation - ( PDF file)

Two Energy Gaps and Fermi-Surface ''Arcs'' in NbSe₂

S. V. Borisenko, A. A. Kordyuk, V. B. Zabolotnyy, D. S. Inosov, D. Evtushinsky, B. Büchner, A. N. Yaresko, A. Varykhalov, R. Follath, W. Eberhardt, L. Patthey, and  Helmuth Berger
Using angle-resolved photoemission spectroscopy, we report on the direct observation of the energy gap in 2H-NbSe₂ caused by the charge-density waves (CDW). The gap opens in the regions of the momentum space connected by the CDW vectors, which implies a nesting mechanism of CDW formation. In remarkable analogy with the pseudogap in cuprates, the detected energy gap also exists in the normal state (T>T₀) where it breaks the Fermi surface into “arcs,” it is nonmonotonic as a function of temperature with a local minimum at the CDW transition temperature (T₀), and it forestalls the superconducting gap by excluding the nested portions of the Fermi surface from participating in superconductivity.
(Phys. Rev. Lett. 102, 166402 (2009) ) scitation - ( PDF file)

Topological Frustration in Graphene Nanoflakes: Magnetic Order and Spin Logic Devices

Wei L. Wang, Oleg V. Yazyev, Sheng Meng, and Efthimios Kaxiras

Magnetic order in graphene-related structures can arise from size effects or from topological frustration. We introduce a rigorous classification scheme for the types of finite graphene structures (nanoflakes) which lead to large net spin or to antiferromagnetic coupling between groups of electron spins. Based on this scheme, we propose specific examples of structures that can serve as the fundamental (NOR and NAND) logic gates for the design of high-density ultrafast spintronic devices. We demonstrate, using ab initio electronic structure calculations, that these gates can in principle operate at room temperature with very low and correctable error rates.

Book Review: Physical Biology. From Atoms to Medicine. Edited by Ahmed Zewail.

Majed Chergui

Structural Determination of a Photochemically Active Diplatinum Molecule by Time-Resolved EXAFS Spectroscopy

Renske M. van der Veen, Chris J. Milne, Amal El Nahhas, Frederico A. Lima, Van-Thai Pham, Jonathan Best, Julia A. Weinstein, Camelia N. Borca, Rafael Abela, Christian Bressler, and Majed Chergui
A large contraction of the Pt--Pt bond in the triplet excited state of the photoreactive [Pt₂(P₂O₅H₂)₄]^4- ion is determined by time-resolved X-ray absorption spectroscopy. The strengthening of the Pt--Pt interaction is accompanied by a weakening of the ligand coordination bonds, resulting in an elongation of the platinum-ligand bond that is determined for the first time.

Giant Kohn Anomaly and the Phase Transition in Charge DensityWave ZrTe₃

Moritz Hoesch, Alexey Bosak, Dmitry Chernyshov, Helmuth Berger, and Michael Krisch
A strong Kohn anomaly in ZrTe₃ is identified in the mostly transverse acoustic phonon branch along the modulation vector q_P with polarization along the a* direction. This soft mode freezes to zero frequency at the transition temperature T_P, and the temperature dependence of the frequency is strongly affected by fluctuation effects. Diffuse x-ray scattering of the incommensurate superstructure shows a power-law scaling of the intensity and the correlation length that is compatible with an order parameter of dimension n=2.
(Phys. Rev. Lett. 102, 086402 (2009) ) scitation - (PDF file)

Probing the Electronic Structure of the Hemoglobin Active Center in Physiological Solutions

Emad F. Aziz, Niklas Ottosson, Sébastien Bonhommeau, Nora Bergmann, Wolfgang Eberhardt, and Majed Chergui
Soft-x-ray absorption spectroscopy at the L_2;3 edge of the iron center in bovine hemoglobin and hemin under physiological conditions is reported for the first time. Spectra of the same compounds in solid form are presented for comparison. Striking differences in the electronic structure of the metalloporphyrin are observed between the liquid and solid compounds. We unambiguously show that hemoglobin and hemin are in a high-spin ferric state in solution, and that the 2p spin-orbit coupling decreases for hemin compared to the hemoglobin, while this is not the case in solids. The spectra were simulated using ligand field multiplet theory, in good agreement with the experiment, allowing quantification of the amount of charge transfer between the porphyrin and Fe³⁺ ion in hemoglobin and in hemin.
(Phys. Rev. Lett. 102, 068103 (2009) ) scitation - ( PDF file)

Supramolecular control of the magnetic anisotropy in two-dimensional high-spin Fe arrays at a metal interface.

Gambardella P, Stepanow S, Dmitriev A, Honolka J, de Groot FM, Lingenfelder M, Gupta SS, Sarma DD, Bencok P, Stanescu S, Clair S, Pons S, Lin N, Seitsonen AP, Brune H, Barth JV, Kern K.

Magnetic atoms at surfaces are a rich model system for solid-state magnetic bits exhibiting either classical or quantum behaviour. Individual atoms, however, are difficult to arrange in regular patterns. Moreover, their magnetic properties are dominated by interaction with the substrate, which, as in the case of Kondo systems, often leads to a decrease or quench of their local magnetic moment. Here, we show that the supramolecular assembly of Fe and 1,4-benzenedicarboxylic acid molecules on a Cu surface results in ordered arrays of high-spin mononuclear Fe centres on a 1.5 nm square grid. Lateral coordination with the molecular ligands yields unsaturated yet stable coordination bonds, which enable chemical modification of the electronic and magnetic properties of the Fe atoms independently from the substrate. The easy magnetization direction of the Fe centres can be switched by oxygen adsorption, thus opening a way to control the magnetic a)nisotropy in supramolecular layers akin to that used in metallic thin films.

(Nat. Mater.  [Epub ahead of print] (2009) ) scitation - (PDF file)

Colloidal Characterization and Thermodynamic Stability of Binary Eye Lens Protein Mixtures

N. Dorsaz, G. M. Thurston, A. Stradner, P. Schurtenberger and G. Foffi
We present a study of binary mixtures of eye lens crystallin proteins. A coarse-grained model of aqueous α- and γ-crystallin mixtures based on molecular dynamics simulations and SANS experiments is proposed. Thermodynamic perturbation theory is implemented to obtain the stability boundaries, or spinodal surface, of the binary mixture in the full parameter space. The stability of these high-concentration crystallin mixtures was found to depend on the α−γ attraction in a manner that is both extremely sensitive and nonmonotonic; stronger or weaker attraction resulted in a spectacularly enhanced instability. The relevance of these mechanisms as possible sources of the alteration of the spatial distribution of the lens proteins encountered in cataract disease is discussed.
(J. Phys. Chem. B, (As Soon As Publishable) (2009)) citation - (PDF file)

The microcontainer shape in electropolymerization on bubbles

J. T. Kim, S. K. Seol, J. H. Je, Y. Hwu, and  G. Margaritondo
We investigated the microcontainer shape in electropolymerization of polypyrrole (PPy) on bubbles using real-time microradiography. We revealed the existence of a “deformation force” at the three-phase boundary among gas (bubble), liquid (electrolyte), and solid (PPy). As the deformation force increases, the microcontainer shape gradually changes from spherical to elliptical and then to cylindrical. The force is proportional to the polymerization rate that is enhanced by the applied voltage and monomer concentration.
(Appl. Phys. Lett. 94, 034103 (2009) ) scitation - ( PDF file)

Electron and X-Ray Methods of Ultrafast Structural Dynamics: Advances and Applications

Majed Chergui and Ahmed H. Zewail
In this contribution, we highlight the state of the art in the determination of structures with ultrafast electrons and X-rays. We provide our perspectives and reflections on the principles, techniques and methods, and on applications from different disciplines, with some focus on physical, chemical and biological structures. Although this article is not a survey of all the work done with these techniques, it provides a comprehensive referencing to current research.
(ChemPhysChem  10, 28-43, (2009)) citation - (PDF file)

Femtosecond XANES Study of the Light-Induced Spin Crossover Dynamics in an Iron(II) Complex

Ch. Bressler, C. Milne, V.-T. Pham, A. ElNahhas, R. M. van der Veen, W. Gawelda, S. Johnson, P. Beaud, D. Grolimund, M. Kaiser, C. N. Borca, G. Ingold, R. Abela, M. Chergui
X-ray absorption spectroscopy is a powerful probe of molecular structure, but it has previously been too slow to track the earliest dynamics after photoexcitation. We investigated the ultrafast formation of the lowest quintet state of aqueous iron(II) tris(bipyridine) upon excitation of the singlet metal-to-ligand-charge-transfer (¹MLCT) state by femtosecond optical pump/x-ray probe techniques based on x-ray absorption near-edge structure (XANES). By recording the intensity of a characteristic XANES feature as a function of laser pump/x-ray probe time delay, we find that the quintet state is populated in about 150 femtoseconds. The quintet state is further evidenced by its full XANES spectrum recorded at a 300-femtosecond time delay. These results resolve a long-standing issue about the population mechanism of quintet states in iron(II)-based complexes, which we identify as a simple ¹MLCT→³MLCT→⁵T cascade from the initially excited state. The time scale of the ³MLCT→⁵T relaxation corresponds to the period of the iron-nitrogen stretch vibration.
(Science 323, 489 - 492, (2009)) citation - (PDF file)
-- Communiqué de presse EPFL --

Microwave spin-torque excitation in a template-synthesized nanomagnet

N. Biziere, E. Murè, and J.-Ph. Ansermet

A microwave-compatible lithography-free process is shown to allow the electrical contact of a single nanometric spin valve grown by template synthesis. The complex spin dynamics of a single nanomagnet is revealed by resonant microwave current excitations, detected electrically in the V_dc and second-harmonic voltages. The uniform modes are attributed to spin transfer torque effects, whereas the effect of oersted field may play a role in exciting the nonuniform modes.

(Phys. Rev. B 79, 012404 (2009) ) scitation - ( PDF file)

Fourier space imaging of light localization at a photonic band-edge located below the light cone

N. Le Thomas and R. Houdré, R. Houdré, D. M. Beggs and T. F. Krauss
We observe light localization in a two-dimensional geometry, induced by residual disorder at a photonic band edge located below the light cone. The combination of a spectrally selective illumination and a grating assisted k-space imaging technique allows us to image the equifrequency surfaces associated with such a photonic band, with high accuracy and without aberrations. Thanks to this approach, the impact of the nonideal nature of real planar photonic crystals on the propagation properties of the Bloch wave at the band edge is decorrelated from the contribution of the intrinsic out-of-plane losses. As a by-product, our result demonstrates an immersion free effective numerical aperture as high as 3.5 in k space.
(Phys. Rev. B 79, 033305 (2009) ) scitation - ( PDF file)

Measurement of the Decay Bs0→Ds-π+ and Evidence for Bs0→Ds-/+K± in e+e- Annihilation at √s≈10.87 GeV

R. Louvot, J. Wicht, O. Schneider, T. Aushev, A. Bay, , K. Vervink, N. Zwahlen, in Belle collaboration
We have studied BD⁺ and BDK^± decays using 23.6 fb^-1 of data collected at the (5S) resonance with the Belle detector at the KEKB e⁺e^- collider. This highly pure BD⁺ sample is used to measure the branching fraction, (BD⁺)=[3.67(stat)(syst)±0.49(f_s)]×10^-3 (f_s=N/N) and the fractions of B event types at the (5S) energy, in particular N/N=(90.1±0.2)%. We also determine the masses M(B)=(5364.4±1.3±0.7) MeV/c² and M(B)=(5416.4±0.4±0.5) MeV/c². In addition, we observe BDK^± decays with a significance of 3.5 and measure (BDK^±)=[2.4(stat)±0.3(syst)±0.3(f_s)]×10^-4.

(Phys. Rev. Lett. 102, 021801 (2009) ) scitation - (PDF file)

Observation of Two Nondispersive Magnetic Excitations in NiO by Resonant Inelastic Soft-X-Ray Scatterin

G. Ghiringhelli, A. Piazzalunga, C. Dallera, T. Schmitt, V. N. Strocov, J. Schlappa, L. Patthey, X. Wang, H. Berger, and M. Grioni
We present high resolution (ΔE=120 meV) resonant inelastic x-ray scattering data measured at the Ni L₃ edge (2p_3/2-->3d) on the paradigmatic antiferromagnetic oxide NiO. Spectra reveal clear signatures of magnetic excitations at ~95 and ~190 meV whose energy seems independent from transferred momentum. These spectral features are well reproduced by a single Ni²⁺ ion model in an effective exchange field. Within this local model the two magnetic excitations are characterized by a variation of the atomic magnetic moment along the local ordering direction (ΔS_α)of one and two units. The ΔS_α=2 case has different nature from bimagnons observed in optical Raman spectra, for which ΔS_α=0.

(Phys. Rev. Lett. 102, 027401 (2009) ) scitation - (PDF file)

Morphology Effectively Controls Singlet-Triplet Exciton Relaxation and Charge Transport in Organic Semiconductors

V. K. Thorsmølle, R. D. Averitt, J. Demsar, D. L. Smith, S. Tretiak, R. L. Martin, X. Chi, B. K. Crone, A. P. Ramirez, and A. J. Taylor.

We present a comparative study of ultrafast photoconversion dynamics in tetracene (Tc) and pentacene (Pc) single crystals and Pc films using optical pump-probe spectroscopy. Photoinduced absorption in Tc and Pc crystals is activated and temperature-independent, respectively, demonstrating dominant singlet-triplet exciton fission. In Pc films (as well as C₆₀-doped films) this decay channel is suppressed by electron trapping. These results demonstrate the central role of crystallinity and purity in photogeneration processes and will constrain the design of future photovoltaic devices.

(Phys. Rev. Lett. 102, 017401 (2009) ) scitation - (PDF file)

Gene-mediated Restoration of Normal Myofiber Elasticity in Dystrophic Muscles

Stefania Puttini, Magorzata Lekka, Olivier M Dorchies, Damien Saugy, Tania Incitti, Urs T Ruegg, Irene Bozzoni, Andrzej J Kulik and Nicolas Mermod
Dystrophin mediates a physical link between the cytoskeleton of muscle fibers and the extracellular matrix, and its absence leads to muscle degeneration and dystrophy. In this article, we show that the lack of dystrophin affects the elasticity of individual fibers within muscle tissue explants, as probed using atomic force microscopy (AFM), providing a sensitive and quantitative description of the properties of normal and dystrophic myofibers. The rescue of dystrophin expression by exon skipping or by the ectopic expression of the utrophin analogue normalized the elasticity of dystrophic muscles, and these effects were commensurate to the functional recovery of whole muscle strength. However, a more homogeneous and widespread restoration of normal elasticity was obtained by the exon-skipping approach when comparing individual myofibers. AFM may thus provide a quantification of the functional benefit of gene therapies from live tissues coupled to single-cell resolution.
(Molecular Therapy (2009) 17 1, 19-25 ) scitation - (PDF file)

Dynamics of Trion Formation in In_xGa_1-xAs Quantum Wells

M. T. Portella-Oberli, J. Berney, L. Kappei, F. Morier-Genoud, J. Szczytko, and B. Benoît Deveaud-Plédran

We show a double path mechanism for the formation of charged excitons (trions); they are formed through bi- and trimolecular processes. This directly implies that both negatively and positively charged excitons coexist in a quantum well, even in the absence of excess carriers. The model is substantiated by time-resolved photoluminescence experiments performed on a very high quality In_xGa_1-xAs quantum well sample, in which the photoluminescence contributions at the energy of the trion and exciton and at the band edge can be clearly separated and traced over a broad range of times and densities. The unresolved discrepancy between the theoretical and experimental radiative decay time of the exciton in a doped semiconductor quantum well is explained by the same model.