Seminars 2017

In our information-everywhere society IT is a major player for energy consumption. Novel spintronic devices can play a role in the quest for GreenIT if they are stable and can transport and manipulate spin with low power. Devices have been proposed, where switching by energy-efficient approaches, such as spin-polarized currents is used^1 , for which we develop new highly spin-polarized materials and characterize the spin transport using THz spectroscopy^2 . Firstly to obtain ultimate stability, topological spin structures that emerge due to the Dzyaloshinskii-Moriya interaction (DMI) at structurally asymmetric interfaces, such as chiral domain walls and skyrmions with enhanced topological protection can be used^3,4,5. We have investigated in detail their dynamics and find that it is governed by the topology of their spin structures^3 . By designing the materials, we can even obtain a skyrmion lattice phase as the ground state of the thin films^4 . Secondly, for ultimately efficient spin manipulation, we use spin-orbit torques, that can transfer more than 1ħ per electron by transferring not only spin but also orbital angular momentum. We combine ultimately stable skyrmions with spin orbit torques into a skyrmion racetrack device^4 , where the real time imaging of the trajectories allows us to quantify the novel skyrmion Hall effect^4 . Finally to obtain efficient spin transport, we study graphene and low damping ferro- and antiferromagnetic insulators as spin conduits for long distance spin transport6 .

1. Reviews: O. Boulle et al., Mater. Sci. Eng. R 72, 159 (2011); G. Finocchio et al., J. Phys. D: Appl. Phys. 49, 423001 (2016); A. Bisig et al., Phys. Rev. Lett (in press (2016))
2. M. Jourdan et al., Nature Commun. 5, 3974 (2014); Z. Jin et al., Nature Phys. 11, 761 (2015). 3. F. Büttner et al., Nature Phys. 11, 225 (2015).
4. S. Woo et al, Nature Mater. 15, 501 (2016).
5. K. Litzius et al., Nature Phys. 13, 170 (2017).
6. A. Kehlberger et al., Phys. Rev. Lett. 115, 096602 (2015); S. Geprägs et al., Nature Commun. 7, 10452 (2016).

Seminar title: Anomalous sign change in the Seebeck Coefficient of few layer MoS2
Speaker: Wu Jing

Thermoelectrics provides a means to harvest energy sustainably from the environment and also plays an important role in materials science. Such environmentally friendly energy conversion sources are considered to be promising for supplementing future global energy demands. In recent years, low dimensionality (one-dimensional and two-dimensional) has opened up new routes to achieve high efficiency thermoelectric devices. High mobility two-dimensional (2D) transition metal dichalcogenides semiconductors represent a new class of thermoelectric materials due to their enhanced density of states of confined carriers, as well as their large effective masses and valley degeneracies. In most cases, the Seebeck coefficient is determined by a normal energy-dependent electronic density of states near the Fermi level and the charge carrier type (electrons or holes). Thus in previous studies, only a negative Seebeck coefficient due to conduction electrons was observed in MoS2 based devices. In this study, we observe for the first time, a positive Seebeck coefficient in n-doped six-layer MoS2 at temperatures below ~70K, which indicates that the Seebeck effect originates from the change in energy dependence of the charge-carrier relaxation times. The measured mobility undergoes a concomitant change in the slope at the same temperature, which corroborates a change in the relaxation time as a function of temperature and results in a sizable positive addition to the Seebeck coefficient. At low temperatures, we observe a large positive Seebeck (~ 1.5 mV/K at 50K). This new finding advances the study of thermoelectric physics in 2D materials and demonstrates a new avenue for superior thermoelectric performance by tuning the energy-dependent relaxation time.


Title: Realistic lattice model for electron-electron interactions in undoped graphene
Speaker: Joao Rodriguez

This talk will present a study of the effects of realistic electron-electron interactions in graphene at half-filling. In this study we use projective quantum Monte Carlo simulations of electrons living on the honeycomb lattice and interacting through an effective Coulomb potential. We compute the antiferromagnetic ordering, the renormalized Fermi velocity and the quasiparticle residue as a function of the strength of the short- and the long-range components of the effective Coulomb potential. We find that the short-range part of the potential is more efficient in driving the semi-metal to Mott insulator transition than the long-range part. We then argue that isotropic strain may bring graphene close to such phase transition. This transition is consistent with the Gross-Neveu-Yukawa critical theory. Far from the critical point, the Fermi velocity renormalization is dominated by the long-range part of the interaction, being compatible with the predictions from perturbative theory for massless Dirac fermions interacting through a bare Coulomb potential. In contrast, close to the Mott insulator transition, the Fermi velocity behavior is modified by a competition between spin density wave and charge density wave fluctuations. Interestingly, real graphene samples are generally in between these two limits. Since finite system sizes restrict the QMC results to large momentum scales, we perform a phenomenologial reconstruction of the renomalization group flow of the Fermi velocity, so that we can make predictions testable against recent experimental observations. Finally, we look at the quasi-particle residue, which is found to interpolate between unity and zero as we move from the weakly interacting regime into the close vicinity of the phase transition.