# Festkörperkolloquium Sommersemester 2014

Donnerstag, 16-18 Uhr, Raum 3701-268 (Appelstraße 2)

Termin | Redner/in | Einladender | Thema |
---|---|---|---|

08.05. | Stefan Förster (Uni Halle) | Christoph Tegenkamp | Two-dimensional oxide quasicrystals: a new class of materials? |

15.05. | Simone Sanna (Uni Paderborn) | Eric Jeckelmann | Understanding the puzzling behavior of LiNbO_{3} surfaces from first-principles |

22.05. | Tobias Kießling (Uni Würzburg) | Michael Oestreich | The significance of hot carrier effects for low temperature spin-diffusion studies in GaAs |

19.06. | Martin Weinelt (FU Berlin) | Ultrafast dynamics in spin systems – studying the strongest force in magnetism | |

10.07. | Patrik Recher (Uni Braunschweig) | Rolf Haug | Transport through topological states of matter |

# 08.05.2014 - Two-dimensional oxide quasicrystals: a new class of materials?

**Stefan Förster**^{1} , Klaus Meinel^{1} , René Hammer^{1} , Martin Trautmann^{1} , and Wolf Widdra^{1,2}

^{1} Institute of Physics, Martin-Luther-Universität Halle-Wittenberg^{2} Max-Planck-Institut für Mikrostrukturphysik, Halle

We report here the first observation of a two-dimensional oxide quasicrystal (QC), a new member in the family of 2D materials [1]. The QC is derived from BaTiO 3 thin films on a hexagonal Pt(111) substrate. Low-energy electron diffraction (LEED) reveals a twelve-fold rotational symmetry. Scanning tunneling microscopy (STM) at room temperature as well as at low temperatures (80 K) allow to resolve the atomic structure. The aperiodic structure is formed by primitive atomic arrangements in squares, triangles, and rhombi with a universal edge length of 0.69 nm. In addition to this dodecagonal atomic arrangement, building blocks of squares, triangles, and rhombi are also found on (2+√3) and (2+√3)² larger scales indicating the characteristic self-similarity of an ordered QC. The high-resolution STM measurements allow furthermore to identify atomic flips in the structure indicating lattice excitations in the quasicrystal called phasons. The observed interface-driven formation of a 2D QC from a perovskite oxide in contact with a hexagonal substrate is expected to be a general phenomenon.

[1] S. Förster, K. Meinel, R. Hammer, M. Trautmann, and W. Widdra, Nature 502 (2013), 215.

# 15.05.2014 - Understanding the puzzling behavior of LiNbO3 surfaces form first-principles

**Simone Sanna**

*Lehrstuhl für Theoretische Physik, Universität Paderborn*

The hallmark of ferroelectric surfaces is the possibility to switch their surface chemistry and physics by switching the bulk polarization. This is a unique feature, successfully exploited in modern applications, such as molecular self-assembly or molecular detectors. At the same time, however, the polarization charge gives rise to many exciting surface phenomena such as reconstructions and relaxations, which are not fully understood. Striking differences in the evaporation rates, work functions, chemical reactivity, etching rates and water freezing temperatures at differently polarized LiNbO3 (0001) surfaces are further puzzling examples of a peculiar behavior, whose origin is still unclear. Unfortunately, the atomic structure of the LiNbO3 surfaces remained for a long time experimentally inaccessible, as the unscreened surface charges hinder atomic force microscopy (AFM). In this work we present theoretical models of the technologically relevant (0001) and (2110) lithium niobate surfaces, commonly referred to as Z-cut and X-cut. In a first step, the microscopic structure of the stable surface terminations is determined by ab initio thermodynamics. In a second step, the out- come of available experiments is explained on the basis of our density functional theory models. Particular emphasis is given to the interpretation of available AFM-images and to the explanation of several uncommon properties.

# 22.05.2014 - The significance of hot carrier effects for low temperature spin-diffusion studies in GaAs

**Tobias Kießling**

*Physikalisches Institut, Julius-Maximilians-Universität Würzburg*

Pump-probe MOKE microscopy is a widely used tool for the investigation of electron spin transport in semiconductors. However, while the significance of pump-induced electron heating for the diffusion of photoexcited carriers has been understood for a long time in the related context of exciton transport, such hot electron effects have been neglected in the interpretation of early results on electron spin diffusion obtained by MOKE microscopy.

We show that in case of the widely applied optical detection by means of continuous wave magneto-optical Kerr spectroscopy one has to carefully take into account the impact of hot carrier effects on the magneto-optical response function, which then is spatially non-uniform [1]. Moreover, the thermal gradients in the electron system result in spatially non-uniform

diffusion [2]. We present the first direct measurements of the strong influence of photocarrier heating on electron spin diffusion, using time-resolved two-color Kerr microscopy with picosecond time and micrometer spatial resolution [3]. This method enables real-space imaging of the advancing spread of an optically excited electron spin packet by spin diffusion. We observe a high initial expansion rate of the spin packet which is strongly reduced with increasing time [4]. By comparison with continuous-wave Kerr spectroscopy we demonstrate that this decrease is caused by the influence of the transient cooling of hot photocarriers on the electron spin diffusion coefficient.

[1] T. Henn et al., Phys. Rev. B 88, 085303 (2013).

[2] J.-H. Quast et al., Phys. Rev. B 87, 205203 (2013).

[3] T. Henn et al., Rev. Sci. Instrum. 84, 123903 (2013).

[4] T. Henn et al., Phys. Rev. B 88, 195202 (2013).

# 19.06.2014 - Ultrafast dynamics in spin systems – studying the strongest force in magnetism

**Martin Weinelt**

*Fachbereich Physik, Freie Universität Berlin*

Time-resolved photoelectron spectroscopy has evolved into a powerful and versatile tool to study spin-dependent electron dynamics at surfaces. Today we can detect the spin polarization of photo-excited electrons and thus study lifetimes and scattering of majority and minority spin carriers. Here I will report on fundamental investigations of spin-dependent electron dynamics related to the field of ultrafast magnetism. Femtosecond VUV light pulses generated as higher-order harmonics allow us to map the transient electronic band structure of magnetic materials. Thereby we can follow inter- and intra-atomic exchange in itinerant and localized spin systems. Exciting the lanthanide metal Gadolinium by a femtosecond laser pulse, we observe an unexpected breakdown of the intra-atomic 5d-4f exchange, which lasts for several picoseconds.

# 10.07.2014 - Transport through topological states of matter

**Patrik Recher**

*Institut für Mathematische Physik, Technische Universität BraunschweigLaboratory for Emerging Nanometrology Braunschweig*

Topological states of matter represent a fascinating new field in condensed matter physics. These states appear as surface states of a suitable bulk material and therefore can be probed by electron transport [1,2]. In two-dimensional (2D) Topological Insulators (TIs), these surface states are one-dimensional helical edge states with spin-momentum locking. In the absence of magnetic fields, they are protected by the time-reversal symmetry and exhibit the quantum spin Hall effect [3,4]. In this colloquium talk, I will discuss how transport in helical edge states are influenced by various types of (inelastic) backscattering processes. I also present a proposal to tune the backscattering current electrically and to identify the direction of spin-polarization of helical edge states, when the helical edge mode is biased by a voltage and tunnel-coupled to a quantum dot in the Coulomb blockade regime which is subjected to a small Zeeman splitting [5].

[1] M.Z. Hasan and C.L. Kane, Rev. Mod. Phys. 82, 3045 (2010).

[2] X.-L. Qi and S.-C. Zhang, Rev. Mod. Phys. 83, 1057 (2011).

[3] M. König et al., Science 318, 766 (2007).

[4] I. Knez, R.-R. Du, and G. Sullivan, Phys. Rev. Lett. 107, 136603 (2011).

[5] B. Probst, P. Virtanen, and P. Recher, in preparation.

Webseite: https://www.tu-braunschweig.de/imaph