Dr. Romain Giraud

Department:IFF - Chemical Vapor Desposition
Address:IFW Dresden
Helmholtzstraße 20
01069 Dresden
Telephone number:+49-351-4659-769
E-mail:r.giraud(at)ifw-dresden.de



Journal papers:4
Invited talks:1

Journal papers

2015

L. Veyrat, F. Iacovella, J. Dufouleur, C. Nowka, H. Funke, M. Yang, W. Escoffier, M. Goiran, B. Eichler, O.G. Schmidt, B. Buechner, S. Hampel, R. Giraud: Band Bending Inversion in Bi2Se3 Nanostructures, Nano Letters 15 (2015) Nr. 11, S. 7503-7507 URL

2013

Y.Z. Chen, N. Bovet, F. Trier, D.V. Christensen, F.M. Qu, N.H. Andersen, T. Kasama, W. Zhang, R. Giraud, J. Dufouleur, T.S. Jespersen, J.R. Sun, A. Smith, J. Nygard, L. Lu, B. Buechner, B.G. Shen, S. Linderoth, N. Pryds: A high-mobility two-dimensional electron gas at the spinel/ perovskite interface of Y- Al2O3/SrTiO3, nature communications 4 (2013), S. 1371/1-6 URL

J. Dufouleur, L. Veyrat, A. Teichgraeber, S. Neuhaus, C. Nowka, S. Hampel, J. Cayssol, J. Schumann, B. Eichler, O.G. Schmidt, B. Buechner, R. Giraud: Quasiballistic transport of dirac fermions in a Bi2Se3 nanowire, Physical Review Letters 110 (2013) Nr. 18, S. 186806/1-5 URL

G. Prando, R. Giraud, S. Aswartham, O. Vakaliuk, M. Abdel-Hafiez, C. Hess, S. Wurmehl, A.U.B. Wolter, B. Buechner: Evidence for a vortex-glass transition in superconducting Ba.Fe0:9Co0:1/2As2, Journal of Physics - Condensed Matter 25 (2013) Nr. 50, S. 505701/1-10 URL

Invited talks

R. Giraud: Quantum transport of spin-chiral Dirac fermions in Bi2Se3 nanostructures, Workshop on Mesoscopic Physics, Aussois/ France, 9.-12.12.13 (2013)

Welcome to the "mesoSpin" Quantum Transport group page

Our group is studying the quantum properties of new electronic states of matter, such as Dirac fermions or strongly correlated quasi-particles. In nanostructures, quantum confinement and/or quantum coherence modify the electrical conductance of so-called "mesoscopic conductors". These properties can be revealed by magneto-transport measurements at very low temperatures (down to 20 mK). We have expertise in nanofabrication, nanoelectronics and nanomagnetism. Experiments are mainly performed in 3He/4He dilution refrigerators, using high magnetic fields (B<16T) or vector magnets (2T 3D-vector, 6T main field; 1T 2D-vector, 12T main field).

Our research focuses on:

Topological Insulators

Topological insulators are semiconductors that belong to a new class of materials, in which the energy gap vanishes at the interface with any insulator equivalent to a Dirac vacuum insulator. This results in the formation of a topological electronic state where Dirac fermions propagate as edge or surface states, depending on the 2D or 3D dimensionality of the material, respectively. Due to both strong spin-orbit coupling and time-reversal symmetry, these electronic states have some peculiar spin symmetry and their band structure is therefore made of spin-chiral Dirac fermions. Theory predicts that the dispersion curves are topologically protected by the time-reversal symmetry, thus being robust against any kind of non-magnetic disorder, and that backscattering is forbidden. These new properties have a direct consequence on the quantum transport of Dirac fermions, showing a very rich physics which can be revealed in simple geometries of mesoscopic conductors, such as nanowires or nanoribbons. In the Quantum Transport Group, we focus our work on the study of quantum corrections to the classical conductance of high-quality single-crystalline nanostructures of 3D topological insulators grown by Chemical Vapor Deposition or Vapor-Solid Deposition. Such nanostructures give an interesting playground to investigate the quantum coherent transport of Dirac fermions in new regimes, which cannot be studied in grapheme nanostructures. Studying quantum interferences phenomena, like Ahronov-Bohm interferences or Universal Conductance Fluctuations, down to very low temperatures (T>20mK) allows us to investigate decoherence and to understand how spin-polarized Dirac fermions interact with a static and/or dynamic environment. Beyond the study of decoherence, the mesoscopic transport experiments we perform in the QT group are a powerful tool to unveil some fundamental properties of 2D Dirac fermions in a 3D Topological Insulator (Bi2Se3, Bi2Te3, …) such as, for instance, their spin chirality or their sensitivity to time-reversal symmetry breaking by diluted magnetic impurities.    

To read more : J. Dufouleur et al., Quasi-ballistic transport of Dirac fermions in a Bi2Se3 nanowire, Phys. Rev. Lett. 110, 186806 (2013)

Bi2Se3 nanowire with ohmic contacts for the measure of Aharonov-Bohm oscillations (ABO).
a) Fourier transform of ABO measured at 50mK, revealing their many harmonics (as a result of a very long phase-coherence length), b) Temperature dependence of the fundamental harmonics, showing an unusual behavior due to quasi-ballistic transport.

Oxide Heterostructures

Transition metal oxide heterostructures offer interesting opportunities to investigate strongly correlated electronic systems in reduced dimensionality. Among them, the intriguing formation of a quasi-2D metallic state at the interface between two insulating materials gives broad perspectives to study new collective phases induced by electronic interactions. LaAlO3/SrTiO3 is the canonical system in which many properties of the interface 2D electron liquid were discovered (superconductivity, ferromagnetism, and even their possible coexistence). Although a couple of important mechanisms for both electrical doping and charge transfer were clearly identified, such as the built-in electrostatic potential in a polar epitaxial thin film or the role of O-vacancy defects, details of the electrostatics remain a matter of debate. In the Quantum Transport group, we focus our work on the study of a new type of oxide heterostructure γ-Al2O3/SrTiO3, with ordered oxygen vacancies and weak polarity. This results in the formation of a 2D electron system with an enhanced carrier density and/or electron mobility, as compared to optimized LaAlO3/SrTiO3 heterostructures. High-quality heterostructures are grown by Pulsed Laser Deposition by our collaborators in Denmark (Dr. Chen and Dr. Pryds at the DTU Energy Conversion, Denmark) and we study their magneto-transport properties at very low temperatures (T>20mK) and high fields (B<16T). The signature of an enhanced mobility band is clearly evidenced by the observation of Shubnikov-de Haas oscillations at rather low magnetic fields (B>1T), which corresponds to an improvement of about 5 to 10 in the mobility with respect to the case of  LaAlO3/SrTiO3. For this band, we found that the mean free path is of about 100nm, and we expect the phase coherence length to be larger, possibly reaching the micron scale. This makes the study of quantum correction to the classical conductance possible, and therefore it open new directions of research in mesoscopic physics to study strongly-correlated electrons.  

To read more : Y. Chen et al., A high-mobility two-dimensional electron gas at the spinel/perovskite interface of γ-Al2O3/SrTiO3, Nature Comm. 4, 1371 (2013)

Temperature dependence of quantum oscillations of a 2.25uc thick γ-Al2O3 epilayer on SrTiO3. This results shows that a single band has a rather high quantum mobility, in the range of 10^4 cm2V-1s-1, as readily seen from the onset of Shubnikov-deHaas oscillations at 1T.