[Seminar] Unravelling electronic correlations in atomic chains with photoemission by Chris W. Nicholson

Speaker: Chris W. Nicholson, Fritz Haber Institute, Berlin

Date and Time: June 2nd 2017,  8:30-10:00 

Venue: C700 (Lab3)

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Unravelling electronic correlations in atomic chains with photoemission

The remarkable success of non-interacting electron models such as the Fermi liquid theory is underpinned by the reduction of Coulomb interactions via screening in many materials. The fact that electronic interactions may be strongly enhanced by confining electrons to reduced dimensions means that low dimensional materials are a fertile environment for emergent phenomena. This is particularly acute in one-dimensional (1D) or quasi-1D systems where electrons are effectively confined to move along individual chains. In this talk I will present studies of two model quasi-1D systems using angle-resolved photoemission spectroscopy (ARPES) and time-resolved ARPES, respectively, to obtain detailed insights into their electronic behavior.

A fundamental concept in quasi-1D systems is the energy scale on which inter-chain coupling becomes relevant. Given that quantum and thermal fluctuations destroy long-range ordering in 1D, the existence of ordered phases such as superconductivity and charge density waves in quasi-1D actually require this higher dimensional coupling. At energies or temperature above this dimensional crossover scale, E_C, excitations exhibit 1D character, while low-energy excitations behave as in a Fermi liquid. To date, the properties of the low-temperature phase, in particular how strong one-dimensional correlations are imprinted on it, remain poorly understood [1]. We utilize high-resolution ARPES to reveal a dimensional crossover in the transition metal chalcogenide NbSe₃ [2]. A crossover energy scale of E_C ~ 110 meV is extracted based on a tight-binding model and corroborated by an analysis of the density of states which reveals 1D behavior only above E_C. The observed anomalous depletion of spectral weight in the crossover regime suggests that the strong correlations of the 1D phase survive and play an important role even in the presence of higher dimensional coupling.

In the second study I will present a time-resolved ARPES (trARPES) investigation of the quasi-1D metallic nanowire system In/Si(111) during a photoinduced phase transition. Progress in time-resolved spectroscopies has led to the observation of real-time atomic motion [3], the formation of precursor states during chemical reactions at surfaces [4], and bond stretching dynamics during optically driven phase transitions [5] occurring on ultrafast time scales. However an enduring goal is to follow the complete evolution of individual electronic states during a reaction or phase transition in order to obtain microscopic insights into the reaction mechanism. We track the gradual evolution of the In/Si(111) system from its insulating ground state into a metallic state, which develops on three distinct timescales. Calculations reveal that the temporal evolution of particular electronic bands can be assigned to specific atomic displacements, allowing access to the full dynamical evolution of the system during the phase transition. In addition we map out the distribution of excited electronic states across multiple Brillouin zones and reveal that while the electrons are widely distributed, the holes are more strongly localized. Indeed, molecular dynamics simulations suggest the holes play a prominent role in the transition mechanism. I will additionally introduce the trARPES setup used to obtain these results, which has been developed at the Fritz Haber Institute in Berlin with an output of 22 eV at 500 kHz and a time resolution < 50 fs. This system opens up new opportunities for both excited state mapping and dynamical studies.

[1]       T. Giamarchi, “Theoretical framework for quasi-one dimensional systems.,” Chem. Rev., vol. 104, no. 11, pp. 5037–56, Nov. 2004.

[2]       C. W. Nicholson, C. Berthod, M. Puppin, H. Berger, M. Wolf, M. Hoesch, and C. Monney, “Dimensional Crossover in a Charge Density Wave Material Probed by Angle-Resolved Photoemission Spectroscopy,” Phys. Rev. Lett., vol. 118, p. 206401, 2017.

[3]       H. Petek, M. J. Weida, H. Nagano, and S. Ogawa, “Real-Time Observation of Adsorbate Atom Motion Above a Metal Surface,” Science (80-. )., vol. 288, no. 5470, pp. 1402–1404, 2000.

[4]       H. Öström, H. Oberg, H. Xin, J. LaRue, M. Beye, M. Dell’Angela, J. Gladh, M. L. Ng, J. A. Sellberg, S. Kaya, G. Mercurio, D. Nordlund, M. Hantschmann, F. Hieke, D. Kuhn, W. F. Schlotter, G. L. Dakovski, J. J. Turner, M. P. Minitti, A. Mitra, S. P. Moeller, A. Fohlisch, M. Wolf, W. Wurth, M. Persson, J. K. Norskov, F. Abild-Pedersen, H. Ogasawara, L. G. M. Pettersson, and A. Nilsson, “Probing the transition state region in catalytic CO oxidation on Ru,” Science (80-. )., vol. 347, no. 6225, pp. 978–982, 2015.

[5]       F. Schmitt, P. S. Kirchmann, U. Bovensiepen, R. G. Moore, L. Rettig, M. Krenz, J.-H. Chu, N. Ru, L. Perfetti, D. H. Lu, M. Wolf, I. R. Fisher, and Z.-X. Shen, “Transient electronic structure and melting of a charge density wave in TbTe3.,” Science, vol. 321, no. 5896, pp. 1649–52, Sep. 2008.

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Chris W. Nicholson

Chris Nicholson received a Master in physics from the University of St Andrews in the UK in 2007. During his undergraduate time he predominantly worked with lab-based high-energy resolution laser photoemission, but also undertook an internship at the Paul Scherrer Institute in Switzerland working on small angle x-ray scattering analysis of samples produced by nano-lithography. He also received a summer scholarship to work at Fermilab in the USA modelling fast particle detectors.

After graduating from St Andrews he started a PhD at the Fritz Haber Institute in Berlin, where he is currently based, to work on femtosecond time-resolved photoemission of low dimensional and correlated materials. His work there has focussed on the electronic structure and dynamics of quasi-1D systems and in situ MBE growth of metallic atomic-scale nanowires. Other areas of interest during his PhD time have been ultrafast studies of 2D transition metal dichalcogenides and photoinduced phase transitions. He additionally continues to work with high-energy resolution ARPES at synchrotron facilities.

Hosted by Femtosecond Spectroscopy Unit (Dani Unit)