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The 15th International Conference on

Vibrations at Surfaces

June 22-26, 2015 ▪ Donostia-San Sebastián, Spain

Donostia Igeldotik

Program

OverviewMondayTuesdayWednesdayThursdayFriday

Tuesday June 23

09:00-10:40 Tu1: Transport in electronic devices
10:40-11:20 Coffee break
11:20-13:00 Tu2: Surface diffusion and migration
13:00-15:30 Lunch break (on your own)
15:30-16:40 Tu3: Chemistry and growth of graphene
16:40-17:20 Coffee break
17:20-18:50 Tu4: Electron-phonon coupling in graphene
19:00-21:30 Poster session A

Tu4: Electron-phonon coupling in graphene

Chair: W. Widdra, Halle, Germany

17:20-17:50 J. W. Wells, Trondheim, Norway
Electron phonon mediated transitions in 2D materials; towards a designer superconductor?
17:50-18:10 S. Tanaka, Osaka, Japan
Momentum-resolved direct-observation of the electron-phonon scattering for graphite and graphene by using ARPES and HREELS
18:10-18:30 V. De Renzi, Modena, Italy
Terahertz optical modes of supported graphene bilayer
18:30-18:50 D. Stradi, Lyngby, Denmark
Nanostructured phonons in epitaxial graphene on ruthenium surfaces

Contributed talk

Terahertz optical modes of supported graphene bilayer

A. Lodi Rizzini1,2, V. De Renzi1,2, R. Biagi1,2, V. Corradini2, D. Pacilè3, A. Politano3, U. del Pennino1,2

1Dipartimento di Scienze Fisiche, Informatiche e Matematiche dell'Università di Modena e Reggio Emilia, V. Campi 213/a 41125 Modena, Italy

2CNR-Nanoscience Institute, S3 Center, 41125 Modena, Italy

3Dipartimento di Fisica, Universitá della Calabria, 87036 Arcavacata di Rende (Cs), Italy

The peculiar and promising thermal properties of graphene are strongly influenced by interactions with the substrate, presence of defects and lattice distortions. At low temperature, thermal capacity and conduction are mainly related to the properties, i.e. dispersion and density of states, of the acustic modes. The ability to tune these modes can therefore open the way to a fine control on the thermal properties of graphene-based systems, through the so-called phonon engineering [1]. Moreover, ultra-low frequency vibrations could have interesting applications such as the design of novel electromechanical nanoresonators operating in the THz range [2].

In this work, we investigate by HREELS the vibrational properties of a graphene bilayer, grown on the Ru(1000) surface (BLG@Ru) [3], unraveling the details of phonon dispersions in the THz region.

As shown in the inset of Fig. 1 a typical HREEL spectrum is built by several peaks, observed in both loss and gain sides. Spectra are fitted with a sum of gain-loss Voigt doublets, thus allowing a precise determination of the peak energy. In Fig.1 the dispersion curves are shown and compared with the theoretical dispersion of the freestanding bilayer of graphene (BLG) [4]. In the BLG case, the ZA branch splits, giving origin to the optical mode ZO', i.e. a breathing mode of the two layers [4,5]. Moreover, the TA and LA modes are expected to give origin to an optical ultra-low frequency shear mode [6,7]. In the case of BLG@Ru, we observe three optical modes, which near the Γ point are located at 4.7 (38), 8.2 (66) and 12.5 (101) meV (cm-1), respectively. Interestingly, these modes survive also at higher momentum, displaying little dispersion. These findings can be rationalized taking into account the role of the mismatch-induced layer corrugation on the phonon dispersions. In particular, the coexistence of nano-regions characterized by different interlayer distances could explain both the splitting of the optical branches and their partial localization.

DeRenzi.jpg

Figure 1: Phonon dispersion curves of BLG@Ru (open and filled symbols). The dotted lines are guidelines for the eyes. The solid-line black curves represents the free BLG dispersions [4]. In the inset, a typical HREELS spectrum is displayed, together with the fitting curves.

[1] E. Pop, V. Varshney, and A. K. Roy, MRS Bulletin 37, 1273 (2012)

[2] J. S. Bunch, A. M. van der Zande, S. S. Verbridge, I. W. Frank, D. M. Tanenbaum, J. M. Parpia, H. G. Craghead, P. L. McEuen, Science 315, 490–493 (2007)

[3] P. W. Sutter, J. I. Flege, E. A. Sutter, Nat. Mater. 7, 406 (2008)

[4] J.-A. Yan, W. Y. Ruan, and M. Y. Chou, Phys. Rev B 77, 125401 (2008)

[5] P. T. Araujo, D. L. Mafra, K. Sato, R. Saito, J. Kong and M. S. Dresselhaus, Sci. Rep. 2, 1017 (2012)

[6] P. H. Tan et al., Nat. Mater. 11, 294 (2012)

[7] C. Cong and T. Yu Nat. Comm. 5, 4709, 201