The winners of the SPS Awards 2014

The SPS Award committee, presided by Prof. Louis Schlapbach, had again the great pleasure to select the SPS award winners 2014 from many submitted papers of excellent scientific quality. For the first time we were able to award the new METAS prize.
The winners presented their work at the annual meeting in Fribourg. Please find in the following the laudationes written by L. Schlapbach, and the summaries written by the authors.



SPS Award in General Physics, sponsored by ABB

SPS President Andreas Schopper and Philip Moll

The SPS 2014 Prize in General Physics is awarded to Philip Moll for his excellent PhD- and postdoctoral work in the field of iron based high temperature superconductors entitled "High magnetic-field scales and critical currents in SmFeAs(O,F) crystals", and "Transition from slow Abrikosov to fast moving Josephson vortices in iron pnictide superconductors", both published in Nature Materials (2010, 2013).

Based on pioneering use of microfabrication techniques to shape highest quality small samples allowing measurements with unprecedented signal to noise ratios, he discovered surprisingly large intrinsic upper critical magnetic fields exceeding 65 T and isotropic critical current density in iron-based high-temperature superconductor of the LnFeAs(O,F) family. Furthermore, he detected the transition from Abrikosov to Josephson vortices, two different types of magnetic vortices.


High magnetic-field scales and critical currents in SmFeAs(O,F) crystals

Transition from slow Abrikosov to fast moving Josephson vortices in iron pnictide superconductors

Materials of interest in modern condensed matter physics are often challenging to characterize electrically due to the small size, unfavorable shape and overall homogeneity of available samples. We use Focused Ion Beam (FIB) based techniques to shape, manipulate and electrically contact even smallest crystallites, to address a wide range of questions in high-temperature superconductivity, heavy fermions and charge-order compounds. These samples perform well even under the most extreme experimental conditions, from high pressures (> 40 kBar) and the most powerful pulsed magnetic fields in excess of 100 T. Using these FIB techniques on microcrystals, we have investigated the interplay of strong magnetic fields with the unconventional superconductivity in the iron-pnictide SmFeAs(O,F) (Tc ~ 50 K). The superconducting properties, such as upper critical fields and critical currents, show a tendency towards isotropic behavior at low temperatures. While this behavior is favorable for applications, it comes as a surprise: The low dimensional character of these layered compounds is believed to be essential for high Tc and is reflected by the strongly two-dimensional character of the electronic band structure [1]. This peculiar dichotomy of isotropic and anisotropic characters lead to new phenomena in the vortex matter arising from the competition of two important length scales: The interlayer coherence length ξc(T) and the distance dc between adjacent FeAs-layers. ξc(T) crosses dc at a temperature T* ~ 0.8 Tc, separating two regimes well pinned Abrikosov-like vortices at high temperature (ξc(T) > dc) and highly mobile Josephson-like vortices at low temperature (ξc(T) < dc). The transition is evidenced by a sudden jump in flux flow voltage over many orders of magnitude [2].

[1] Philip J. W. Moll, Roman Puzniak, Fedor Balakirev, Krzysztof Rogacki, Janusz Karpinski, Nikolai D. Zhigadlo, Bertram Batlogg. Nature Materials 9, 628-633 (2010)
[2] Philip J. W. Moll, Luis Balicas, Vadim Geshkenbein, Gianni Blatter, Janusz Karpinski, Nikolai D. Zhigadlo, Bertram Batlogg. Nature Materials 12, 134-138 (2013)



SPS Award in Applied Physics, sponsored by OC Oerlikon

Stefan Abel is awarded with the SPS 2014 Prize in Applied Physics for his excellent contribution to the advancement of silicon photonics reported in "A strong electro-optically active lead-free ferroelectric integrated on silicon" published in Nature Communications.

Silicon microfabrication technologies are advanced, however, silicon does not exhibit an electro-optic effect. Stefan Abel has grown a thin film layer of ferroelectric barium titanate directly onto silicon. Because of the high structural quality of the material, an electro-optic coefficient five times as large as the one in the current material of choice for electro-optic devices (lithium niobate) was reached in these layers. Silicon photonics is becoming a base technology for data communication and quantum technology. This result is therefore a key to many important quantum electronic devices such as modulators, phase-shifters, frequency shifters etc., that are the building blocks of this emerging areas of applied physics.


A strong electro-optically active lead-free ferroelectric integrated on silicon

Silicon photonics is an emerging area of applied physics. This new technology of integrated photonics is to be intimately integrated with future generations of high-performance microprocessors, overcoming current bottlenecks related to the excessive power consumption of the computing devices. The use of silicon micro-fabrication technologies will allow high volume and low cost production, thus ensuring ubiquity of this novel technology. But silicon itself cannot deliver all necessary optical functions, and these limitations are purely physical. One of missing functions is an efficient electro-optic effect necessary to modulate and route light through complex optical circuitry. The heart of our work is to introduce ferroelectric oxides such as BaTiO3 (BTO) into silicon photonic processes, and to take advantage of the known large Pockels coefficient of BTO. Because of the lower refractive index of BTO with respect to silicon, slot waveguides were proposed where a thin BTO film is sandwiched between a lower and upper Si slab. This approach results in the enhancement of the optical field in BTO, where the Pockels effect is present. It requires however controlling the deposition of thin, high-quality, well oriented films to exploit the effect associated with the largest element in the electro-optic tensor. In a first step, new growth methodologies have been studied to obtain single crystalline BTO films in a slot waveguide configuration [1]. After having developed a specific characterization technique to measure polarization rotation down to 10-5 degrees, the Pockels effect in epitaxial BTO films grown on silicon could be evidenced [2]. In a second step, BTO stacks were used to fabricate different optical devices, in particular racetrack ring resonators. When applying a DC field across the waveguide, a clear shift in wavelength in the optical resonance could be measured [3]. The magnitude of this shift, depending on the orientation of the electrodes with respect to the crystalline axis of the BTO films, underpins that the true Pockels effect is indeed exploited in his structures. This work clearly paves the way for the fabrication of new type of devices, such as low power modulators that could replace Si-based modulators using the plasma dispersion effect. Low power devices can be envisioned to compensate for the variability in the fabrication process or to compensate for temperature fluctuations. Other device concepts could be proposed such as modulators working at cryogenic temperatures, optical memories and sensors, optical diodes, etc.

[1] S. Abel, M. Sousa, C. Rossel, D. Caimi, M. D. Rossell, R. Erni, J. Fompeyrine, C. Marchiori; Nanotechnology 24(28), 285701 (2013)
[2] S. Abel, T. Stöferle, C. Marchiori, C. Rossel, M. D. Rossell, R. Erni, D. Caimi, M. Sousa, A. Chelnokov, B. Offrein, J. Fompeyrine; Nature Communications 4, 1671 (2013)
[3] S. Abel, T. Stöferle, C. Marchiori, D. Caimi, L. Czornomaz, C. Rossel, M. Rossell, R. Erni, M. Sousa, H. Siegwart, J. Hofrichter, M. Stuckelberger, A. Chelnokov, B. J. Offrein, J. Fompeyrine; Integrated Photonics Research “IPR,” Silicon and Nanophotonics, Rio Grande, Puerto Rico (OSA, July 2013), 10.1364/IPRSN.2013.IW4A.5



SPS Award in Condensed Matter Physics, sponsored by IBM

The SPS 2014 Prize in Condensed Matter Physics is awarded to Andreas Kuhlmann for his excellent PhD-work in the field of semiconductor quantum dots. He developed a new dark-field microscope for the detection of resonance fluorescence from single semiconductor quantum dots (Review of Scientific Instruments 2013, Phys. Rev. Lett. 2012) and discovered a spectroscopic technique to distinguish between the two noise sources in semiconductor quantum dots, charge noise and spin noise, published in Nature Physics (2013) under the title "Charge noise and spin noise in a semiconductor quantum device". Based on this new understanding, he went on to develop a frequency-stabilized source of single photons using a single quantum dot as emitter (Phys. Rev. X 2013).

Noise is a central theme in solid-state qubits such as single quantum dots as it limits their performance. Qubits need to be robust against noise; on the other hand, much of the interesting physics lies in the noise and by operating at the single electron - single photon level with contemporary experimental techniques, powerful new insights can be gleaned into core themes: electron localization, the central spin problem and electron-electron interactions, etc. Andreas Kuhlmann has made a very significant contribution to the understanding of noise in a semiconductor device and ways to circumvent it.


Charge noise and spin noise in a self-assembled semiconductor quantum dot

Self-assembled quantum dots are potentially excellent single-photon sources. A single quantum dot is a robust, fast, bright and narrow-linewidth emitter of single photons. A coherent spin qubit in the solid state is realized by a single hole spin confined to a quantum dot. Both optical ("exciton") and spin qubit coherence are limited by intrinsic sources of noise. Optimizing performance demands an understanding of noise and a strategy to circumvent its deleterious effects.
There are two main sources of noise inherent to the semiconductor: charge noise and spin noise [1]. Charge noise arises from occupation fluctuations of the available states in the semiconductor and leads to fluctuations in the local electric field [1, 2]. Spin noise arises from fluctuations in the nuclear spins of the host material and, on account of the hyperfine interaction, results in a fluctuating magnetic field (the Overhauser field) experienced by an electron spin.

We have investigated noise in an ultra-clean semiconductor quantum device at low temperature using a minimally-invasive, ultra-sensitive, local probe: resonance fluorescence from a single quantum dot [3]. This yields noise spectra with 6 decades of resolution in the noise power over 6 decades of frequency, from 0.1 Hz to 100 kHz [1]. Significantly, we have discovered a spectroscopic way to distinguish charge noise from spin noise. We find that the charge noise is concentrated at low frequencies and gives a large noise power but only in a small bandwidth. The spin noise lies at higher frequencies and gives much weaker noise powers but over a much larger bandwidth. We present a dynamic feedback technique to remove charge noise from the device [2]. We show that nuclear spin noise is the dominant dephasing mechanism that limits performance as a single-photon source. For the neutral exciton, we demonstrate an increase in the spin noise with increasing resonant laser power. Conversely for the charged exciton, we demonstrate a significant decrease in the spin noise with resonant laser excitation. This noise reduction for the charged exciton is exploited to demonstrate transform-limited optical linewidths even when the measurement is performed very slowly [4].

[1] A. V. Kuhlmann et al., Nature Phys. 9, 570 (2013).
[2] J. H. Prechtel et al., Phys. Rev. X 3, 041006 (2013).
[3] A. V. Kuhlmann et al., Rev. Sci. Instrum. 84, 073905 (2013).
[4] A. V. Kuhlmann et al., arXiv:1307.7109 (2013).



SPS Award related to Metrology, sponsored by METAS

Louis Schlapbach, President of the Award Committee, Andreas Schopper, Giorgio Signorello and Philippe Richard, Vice-Director of METAS, which sponsors the new award for the first time.

The SPS 2014 Prize related to Metrology is awarded to Giorgio Signorello for his excellent PhD-work entitled "Uniaxial Stress Effects in Zincblende and Wurtzite GaAs Nanowires: an Optical Spectroscopy Study" and his follow-up publications "Tuning the Light Emission from GaAs Nanowires over 290 meV with Uniaxial Strain" in Nano Letters (2013), and "Inducing a Direct-to-Pseudodirect Bandgap Transition in Wurtzite GaAs Nanowires with Uniaxial Stress" (Nature Communications 2014).

Giorgio Signorello successfully demonstrated that the intensity as well as the colour of the emitted light by GaAs nanowires of the Wurtzite structure (not obtainable in bulk or thin film form) can be varied over a broad range by applying strain. Combining experimental with theoretical skills, he showed that this remarkable finding results from a novel bandstructure transition that has never been observed before in a one-dimensional semiconductor structure, and has a high potential for future applications in the field of electronic and optoelectronic devices, including measurement tools.


Uniaxial Stress Effects in Zincblende and Wurtzite GaAs Nanowires: an Optical Spectroscopy Study

Inspired by possibility to boost the performance of future transistors and optoelectronic devices, we have explored the effect of strain in III-V nanowires. GaAs nanowires are characterized by large yield strength and the exceptional mechanics, which makes them an attractive system to study the enhancement of strain effects. At nanoscale dimensions it is possible to achieve controllable growth of different crystal structures like Zincblende or Wurtzite, enabling new degrees of freedom to tailor electronic and optoelectonic properties.
We show that the photoluminescence (PL) of Zincblende GaAs nanowires can be red-shifted by 290 meV by axially elongating Zincblende GaAs nanowires by up to 3.5%, from tension to compression. Fingerprints of symmetry breaking due to the anisotropic nature of the nanowire deformation are found in the Raman spectra, where the phonon-lifted degeneracy is resolved, and in the PL, which undergoes a more pronounced shift in tension than in compression because of the different symmetry character (heavy or light hole) of the top valence band [1].

In Wurtzite GaAs nanowires, we demonstrate a remarkable energy shift of the PL up to 345 meV by varying the axial strain over a range of 2%, in tension and compression. For the first time, we show spectroscopic evidence of a direct-to-pseudodirect bandgap transition and demonstrate that light emission can be suppressed by more than three orders of magnitude. Using the Raman scattering spectra as relative strain gauge and fitting the optical transition energies to a k·p model, we determine all band-structure parameters of Wurtzite GaAs in unstrained conditions, clarifying once and for all its band structure. Quantities like the Poisson ratio along the c-axis and the phonon deformation potentials of the GaAs and AlGaAs optical phonons have also been determined [2].
This body of results constitutes a solid foundation for understanding strain effects on the optical and electronic properties of III-V nanowires.

[1] Signorello, G., Karg, S., Björk, M. T., Gotsmann, B. & Riel, H. Tuning the light emission from GaAs nanowires over 290 meV with uniaxial strain. Nano Lett. 13, 917–24 (2013).
[2] Signorello, G. et al. Inducing a direct-to-pseudodirect bandgap transition in wurtzite GaAs nanowires with uniaxial stress. Nat. Commun. 5, article 3655 (2014).