Research

It is well known that the thermalisation of laser excitations in metals and semiconductors is importantly different. In metals, the excited electrons frequently scatter with the other carriers and are quickly brought down in energy. This manifests as a rather fast thermalisation. On the other hand excited electrons-hole pairs across the semiconducting bandgap are known to recombine over importantly longer timescales, due top the lack of efficient scattering channels for their recombination.

Half metals have a peculiar band structure with one spin channel displaying metallic character, while the other presenting a bandgap. It is a question what fingerprint this will leave on the thermalisation. We have predicted theoretically and experimentally confirmed that the thermalization process in HMs leads to a long living partially thermalized configuration characterized by three Fermi-Dirac distributions for the minority, majority conduction, and majority valence electrons, respectively [1]. Remarkably, these distributions have the same temperature but different chemical potentials. This unusual thermodynamic state is causing a persistent nonequilibrium spin polarization only well above the Fermi energy.

Very interestingly such spin dynamics is extremely robust even in the presence of fully metallised surfaces and can be reliably used to probe bulk half metallicity in cases where alternative methods fail [2-6].

[1] M. Battiato, J. Minár, W. Wang, W. Ndiaye, M. C. Richter, O. Heckmann, J.-M. Mariot, F. Parmigiani, K. Hricovini, and C. Cacho, Phys. Rev. Lett. 121, 077205 (2018).

[2] B. S. D. Ch. S. Varaprasad, A. Rajanikanth, Y. K. Takahashi, and K. Hono, Appl. Phys. Express 3, 023002 (2010).

[3] G. T. Woods, R. J. Soulen, I. I. Mazin, B. Nadgorny, M. S. Osofsky, J. Sanders, H. Srikanth, W. F. Egelhoff, and R. Datla, Phys. Rev. B 70, 054416 (2004).

[4] E. A. Seddon, in Handbook of Spintronics, Vol. 1, Chap. 22, p. 831.

[5] M. Jourdan et al., Nat. Commun. 5, 3974 (2014).

[6] W. Wang et al., Phys. Rev. B 87, 085118 (2013).

Interested? Want to know more?

• Distinctive Picosecond Spin Polarization Dynamics in Bulk Half Metals (M. Battiato et al. Phys. Rev. Lett. 121, 077205 (2018))

Modern electronics is pushing its frequencies to the limits of the currently used physical processes. The discovery of the ultrafast magnetisation dynamics [1] opened the route to magnetic storage working thousand times faster than current technology. However much remains ununderstood in these timescales, where new phenomena appear inextricably intertwined to the out-of-equilibrium state of the system.

I explained one of the microscopic mechanisms leading to the ultrafast demagnetisation [2-4]. The laser penetrates within the ferromagnetic layer only for few tens of nanometers and excites electrons in both spin channels. The excited electrons then undergo a very efficient diffusion towards the substrate (the effect is conceptually the strongly out-of-equilibrium equivalent of heat diffusion). However within the ferromagnetic metal, the two spin channels have strongly different transport properties. This leads to a spin dependent diffusion away from the laser excited region, which in turn triggers a spin diffusion.

In ferromagnetic metals majority electrons always tend to have better transport properties and therefore a more efficient diffusion compared to minority ones. This asymmetry in the diffusion efficiency leads to a demagnetisation of the ferromagnetic metal after a laser excitation.

[1] E. Beaurepaire, J. C. Merle, A. Daunois, and J.-Y. Bigot, Phys. Rev. Lett. 76, 4250 (1996).

[2] M. Battiato, K. Carva, and P.M. Oppeneer, Superdiffusive Spin Transport as a Mechanism of Ultrafast Demagnetization, Phys. Rev. Lett. 105, 027203 (2010).

[3] M. Battiato, K. Carva, and P.M. Oppeneer, Theory of laser-induced ultrafast superdiffusive spin transport in layered heterostructures, Phys. Rev. B 86, 024404 (2012).

[4] M. Battiato, P. Maldonado, and P.M. Oppeneer, Treating the effect of interface reflections on superdiffusive spin transport in multilayer samples (invited), J. Appl. Phys. 115, 172611 (2014).

Interested? Want to know more?

• Article proposing the superdiffusive spin transport model (M. Battiato, K. Carva, and P.M. Oppeneer, Phys. Rev. Lett. 105, 027203 (2010))

• Ultrafast demagnetisation triggered in Ni by non spin polarised electrons diffusing from a neighbouring Au layer (A. Eschenlohr,* M. Battiato,* et al., Nature Mater. 12, 332 (2013))

• Superdiffusive spin transport triggering an ultrafast increase of the magnetisation (D. Rudolf,* C. La-O-Vorakiat,* M. Battiato,* et al, Nature Comm. 3, 1037 (2012))

• How to control the sub picosecond spin currents (T. Kampfrath, M. Battiato, et al, Nature Nanotechnol. 8, 256 (2013))

• Details on the numerics of the solver of the superdiffusive spin transport model (M. Battiato, K. Carva, and P.M. Oppeneer, Phys. Rev. B 86, 024404 (2012))

• How multiple partial reflections are treated numerically within the superdiffusive spin transport model (M. Battiato, P. Maldonado, and P.M. Oppeneer, J. Appl. Phys. 115, 172611 (2014))

The discovery of the possibility of, not only modify magnetisation locally, but transfer it in the sub picosecond timescale [1-4] is a milestone in the field of ultrafast magnetisation dynamics. It was proven that ultrashort spin current pulses can be created and transported [5] and that they can strongly affect the magnetisation of neighbouring layers [6-7]. These spin pulses have an extremely compressed temporal profile of just a few hundreds of femtoseconds. If used to transport (and process) information, one could imagine pushing the frequency of spintronics up to the THz regime, thousand times faster than modern electronics.

Unfortunately these ultrashort spin pulses do not travel efficiently in metals: the information can be transported only up to few hundreds of nanometers, a distance that is too short to be useful in electronics. The reason why their transport is so inefficient in metals is that these highly excited electrons experience too many scatterings. One way to circumvent this problem is to let the ultrashort spin pulses to propagate in materials where they experience few electron-electron scatterings, like semiconductors. However to achieve this, one has to inject these current pulses from the ferromagnetic metal where they are generated in the semiconductor.

We have described [8-9] the very complex process of injection of highly out-of-equilibrium carriers from metals into semiconductors, and how the strong electric fields generated at the interface actually help increasing the spin polarisation and compacting the time envelope of the spin pulse.

We have recently verified the prediction above in collaboration with experimental groups [10]. We demonstrated this by producing ultrashort spin current pulses into cobalt and injecting them into monolayer MoS2. The semiconducting MoS2 layer also acts as a selective converter of the spin current into a charge current, whose THz emission is then measured. As predicted, we measured a giant spin current, orders of magnitude larger than typical spin current densities injected in semiconductors in modern devices!

[1] G. Malinowski, F. Dalla Longa, J. H. H. Rietjens, P. V. Paluskar, R. Huijink, H. J. M.Swagten, and B. Koopmans, Nature Phys. 4, 855 (2008).

[2] M. Battiato, K. Carva, and P.M. Oppeneer, Superdiffusive Spin Transport as a Mechanism of Ultrafast Demagnetization, Phys. Rev. Lett. 105, 027203 (2010).

[3] M. Battiato, K. Carva, and P.M. Oppeneer, Theory of laser-induced ultrafast superdiffusive spin transport in layered heterostructures, Phys. Rev. B 86, 024404 (2012).

[4] M. Battiato, P. Maldonado, and P.M. Oppeneer, Treating the effect of interface reflections on superdiffusive spin transport in multilayer samples (invited), J. Appl. Phys. 115, 172611 (2014).

[5] T. Kampfrath, M. Battiato, P. Maldonado, G. Eilers, J. Notzold, S. Mahrlein, V. Zbarsky, F. Freimuth, Y. Mokrousov, S. Blugel, M. Wolf, I. Radu, P. M. Oppeneer, and M. Munzenberg, Terahertz spin current pulses controlled by magnetic heterostructures, Nature Nanotechnol. 8, 256 (2013).

[6] D. Rudolf,* C. La-O-Vorakiat,* M. Battiato,* R. Adam, J. M. Shaw, E. Turgut, P. Maldonado, S. Mathias, P. Grychtol, H. T. Nembach, T. J. Silva, M. Aeschlimann, H. C. Kapteyn, M. M. Murnane, C. M. Schneider, and P. M. Oppeneer, Ultrafast magnetization enhancement in metallic multilayers driven by superdiffusive spin current, Nature Comm. 3, 1037 (2012).

[7] A. Eschenlohr,* M. Battiato,* P. Maldonado, N. Pontius, T. Kachel, K. Holldack, R. Mitzner, A. Fohlisch, P. M. Oppeneer, and C. Stamm, Ultrafast spin transport as key to femtosecond demagnetization, Nature Mater. 12, 332 (2013).

[8] M. Battiato, K. Held, Ultrafast and Gigantic Spin Injection in Semiconductors, Phys. Rev. Lett. 116, 196601 (2016).

[9] M. Battiato, Spin polarisation of ultrashort spin current pulses injected in semiconductors, J. Phys. Condens. Matter 29, 174001 (2017).

[10] L. Cheng, X. Wang, W. Yang, J. Chai, M. Yang, M. Chen, Y. Wu, X. Chen, D. Chi, K. E. J. Goh, J.-X. Zhu, H. Sun, S. Wang, J. C. W. Song, M. Battiato, H. Yang, E. E. M. Chia, Far out-of-equilibrium spin populations trigger giant spin injection into atomically thin MoS2, Nature Physics (2019)

Interested? Want to know more?

• Injection of spin currents in semiconductors (M. Battiato, K. Held, Phys. Rev. Lett. 116, 196601 (2016))

• Effects of different materials on the injection (M. Battiato, J. Phys. Condens. Matter 29, 174001 (2017))

• Experimental verification (L. Cheng, et al Nature Physics 15, 347 (2019))

One of the mysteries around the ultrafast demagnetisation was the fact that it was always accompanied by the emission of THz radiation [1]. The THz emission was initially ascribed to the dynamics of the magnetisation, however we showed that the measured THz emission was one order of magnitude larger than what the magnetisation dynamics could produce in the most optimistic configuration [2].

We were then able to identify the microscopic mechanism of the THz emission. The laser excitation generates a spin current orthogonal to the sample surface. In presence of spin-orbit interaction, the spin current, through an effect known as the inverse spin Hall effect, generates an orthogonal charge current. The latter switches on and off in less than a picosecond. This turns the sample into an ultrafast antenna emitting in the THz regime.

Very interestingly by engineering the samples it is possible to fabricate efficient broad-band THz emitters.

Recently we have used this technique to measure the injection of spin currents into semiconductors [3] or study ultrafast spin transport through topological insulators [4].

[1] E. Beaurepaire, G. M. Turner, S. M. Harrel, M. C. Beard, J.-Y. Bigot, and C. A. Schmuttenmaer, 2004, Appl. Phys. Lett. 84, 3465.

[2] T. Kampfrath, M. Battiato, P. Maldonado, G. Eilers, J. Notzold, S. Mahrlein, V. Zbarsky, F. Freimuth, Y. Mokrousov, S. Blugel, M. Wolf, I. Radu, P. M. Oppeneer, and M. Munzenberg, Terahertz spin current pulses controlled by magnetic heterostructures, Nature Nanotechnol. 8, 256 (2013).

[3] L. Cheng, X. Wang, W. Yang, J. Chai, M. Yang, M. Chen, Y. Wu, X. Chen, D. Chi, K. E. J. Goh, J.-X. Zhu, H. Sun, S. Wang, J. C. W. Song, M. Battiato, H. Yang, E. E. M. Chia, Far out-of-equilibrium spin populations trigger giant spin injection into atomically thin MoS2, Nature Physics (2019)

[4] X. Wang, L. Cheng, D. Zhu, Y. Wu, M. Chen, Y. Wang, D. Zhao, C. B. Boothroyd, Y. M. Lam, J.‐X. Zhu, M. Battiato, J. C. W. Song, H. Yang, E. E. M. Chia, Ultrafast Spin‐to‐Charge Conversion at the Surface of Topological Insulator Thin Films, Adv. Mater. 2018, 1802356 (2018).

Interested? Want to know more?

• Identification of mechanism behind the THz emission during the ultrafast demagnetisation (T. Kampfrath, M. Battiato, et al, Nature Nanotechnol. 8, 256 (2013))

• Experimental observation of ultrafast and gigantic spin injection in semiconductors (L. Cheng,et al Nature Physics, (2019))

• Thz emission from topological insulators (X. Wang, et al, Adv. Mater. 2018, 1802356 (2018))

Topological insulators [1-4] present outstanding properties that make them particularly interesting for future ultra-thin electronics. However there is no reason to limit the use of these materials to standard timescales. In view of creating ultrafast spintronics, the ultrafast dynamics in topological insulators and the generation and confinement of ultrafast spin currents along surfaces and interfaces is critically important.

In particular it is imperative to understand the picosecond and sub-picosecond relaxation mechanisms. Critically we have recently shown that we can disentangle in the ultrafast timescale the dynamics, and therefore the transport, at the surface of Bi2Se3 from that in the bulk [5].

We have also identified the sub-picosecond scattering channels in Bi2Te3, and followed the dynamics of the momentum redistribution. This is critical to the creation and transport of sub-picosecond spin current pulses in topological insulators, since the main contribution to resistivity comes from the momentum dissipation [6].

[1] D. J. Thouless, M. Kohmoto, M. P. Nightingale, and M. den Nijs, Phys. Rev. Lett. 49, 405 (1982).

[2] L. Fu, C. L. Kane, and E. J. Mele, Phys. Rev. Lett. 98, 106803 (2007).

[3] J. E. Moore and L. Balents, Phys. Rev. B 75, 121306(R) (2007).

[4] R. Roy, Phys. Rev. B 79, 195322 (2009).

[5] C. Cacho, A. Crepaldi, M. Battiato, J. Braun, F. Cilento, M. Zacchigna, M. C. Richter, O. Heckmann, Y. Liu, S. Dhesi, H. Berger, Ph. Bugnon, M. Grioni, H. Ebert, K. Held, K., Hricovini, J. Minar, and F. Parmigiani , Momentum-resolved spin dynamics of bulk and surface excited States in the topological insulator Bi2Se3, Phys. Rev. Lett. 114, 097401 (2015).

[6] J. Sanchez-Barriga, M. Battiato, M. Krivenkov, E. Golias, A. Varykhalov, A. Romualdi, L. V. Yashina, J. Minar, H. Ebert, O. Kornilov, K. Held, and J. Braun, Sub-picosecond spin dynamics of excited states in the topological insulator Bi2Te3, Phys. Rev. B 95, 125405 (2017).

Interested? Want to know more?

• Disentanglement of surface and bulk dynamics in Bi2Se3 (C. Cacho, A. Crepaldi, M. Battiato, et al., Phys. Rev. Lett. 114, 097401 (2015))

• Trailing the momentum transfer during scatterings in Bi2Te3 (J. Sanchez-Barriga, M. Battiato, et al., Phys. Rev. B 95, 125405 (2017))

The most common way of producing energy is the conversion of temperature gradients into work. The discovery of the steam engine has fuelled (pun intended) the industrial revolution. However the complexity of the machinery and the presence of many moving parts decreases its reliability and requires human supervision. Using the Seebeck effect instead provides a way of turning simply a slab of material into an engine, making for an extremely reliable and compact machine.

It is however a challenge to find materials with high Seebeck coefficients (required to have high conversion efficiencies). FeSb2 was discovered to have a record high Seebeck coefficient [1], but its origin was not understood. We solved the puzzle of the microscopic mechanism behind the colossal thermoelectric response by analysing the rather unusual, yet outstanding, correlation between the temperature dependence of several transport properties of FeSb2 [2].

[1] A. Bentien, S. Johnsen, G. K. H. Madsen, B. B. Iversen and F. Steglich, Colossal Seebeck coefficient in strongly correlated semiconductor FeSb2, EPL 80, 17008 (2007).

[2] M. Battiato, J. Tomczak, Z. Zhong and K. Held, Unified picture for the colossal thermopower compound FeSb2, Phys. Rev. Lett. 114, 236603 (2015).

Interested? Want to know more?

Unified picture for the colossal thermopower compound FeSb2 (Battiato, et al, Phys. Rev. Lett. 114, 236603 (2015))