Dr. Romain Giraud

Journal papers:  6 
Invited talks:  1 
Journal papers
2017
J. Dufouleur, L. Veyrat, B. Dassonneville, E. Xypakis, J.H. Bardarson, C. Nowka, S. Hampel, J. Schumann, B. Eichler, O.G. Schmidt, B. Buechner, R. Giraud: Weaklycoupled quasi1D helical modes in disordered 3D topological insulator quantum wires, Scientific Reports 7 (2017), S. 45276/19 URL2016
J. Dufouleur, L. Veyrat, B. Dassonneville, C. Nowka, S. Hampel, P. Leksin, B. Eichler, O. G. Schmidt, B. Buechner, R. Giraud: Enhanced Mobility of SpinHelical Dirac Fermions in Disordered 3D Topological Insulators, Nano Letters 16 (2016) Nr. 11, S. 67336737 URL2015
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 Bi_{2}Se_{3} Nanostructures, Nano Letters 15 (2015) Nr. 11, S. 75037507 URL2013
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 highmobility twodimensional electron gas at the spinel/ perovskite interface of Y Al2O3/SrTiO3, nature communications 4 (2013), S. 1371/16 URLJ. 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. 186806/15 URL
G. Prando, R. Giraud, S. Aswartham, O. Vakaliuk, M. AbdelHafiez, C. Hess, S. Wurmehl, A.U.B. Wolter, B. Buechner: Evidence for a vortexglass transition in superconducting Ba.Fe0:9Co0:1/2As2, Journal of Physics: Condensed Matter 25 (2013) Nr. 50, S. 505701/110 URL
Invited talks
R. Giraud: Quantum transport of spinchiral 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 quasiparticles. In nanostructures, quantum confinement and/or quantum coherence modify the electrical conductance of socalled "mesoscopic conductors". These properties can be revealed by magnetotransport 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 3Dvector, 6T main field; 1T 2Dvector, 12T main field).
Our research focuses on:
 Topological Insulators
 Oxide Heterostructures
 Semiconducting Nanowires
 Nanomagnetism and Spintronics
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 spinorbit coupling and timereversal symmetry, these electronic states have some peculiar spin symmetry and their band structure is therefore made of spinchiral Dirac fermions. Theory predicts that the dispersion curves are topologically protected by the timereversal symmetry, thus being robust against any kind of nonmagnetic 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 highquality singlecrystalline nanostructures of 3D topological insulators grown by Chemical Vapor Deposition or VaporSolid 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 AhronovBohm interferences or Universal Conductance Fluctuations, down to very low temperatures (T>20mK) allows us to investigate decoherence and to understand how spinpolarized 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 timereversal symmetry breaking by diluted magnetic impurities.
To read more : J. Dufouleur et al., Quasiballistic transport of Dirac fermions in a Bi2Se3 nanowire, Phys. Rev. Lett. 110, 186806 (2013)
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 quasi2D 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 builtin electrostatic potential in a polar epitaxial thin film or the role of Ovacancy 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 γAl2O3/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. Highquality 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 magnetotransport 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 Shubnikovde 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 stronglycorrelated electrons.
To read more : Y. Chen et al., A highmobility twodimensional electron gas at the spinel/perovskite interface of γAl2O3/SrTiO3, Nature Comm. 4, 1371 (2013)