This year the discovery of femtosecond demagnetization by laser pulses is 20 years old. For the first time, this milestone work by Bigot and coworkers gave insight directly into the time scales of microscopic interactions that connect the spin and electron system. While intense discussions in the field were fueled by the complexity of the processes in the past, it now became evident that it is a puzzle of many different parts. Rather than providing an overview that has been presented in previous reviews on ultrafast processes in ferromagnets, this perspective will show that with our current depth of knowledge the first applications are developed: THz spintronics and all-optical spin manipulation are becoming more and more feasible. The aim of this perspective is to point out where we can connect the different puzzle pieces of understanding gathered over 20 years to develop novel applications. Based on many observations in a large number of experiments. Differences in the theoretical models arise from the localized and delocalized nature of ferromagnetism. Transport effects are intrinsically non-local in spintronic devices and at interfaces. We review the need for multiscale modeling to address the processes starting from electronic excitation of the spin system on the picometer length scale and sub-femtosecond time scale, to spin wave generation, and towards the modeling of ultrafast phase transitions that altogether determine the response time of the ferromagnetic system. Today, our current understanding gives rise to the first usage of ultrafast spin physics for ultrafast magnetism control: THz spintronic devices. This makes the field of ultrafast spin-dynamics an emerging topic open for many researchers right now.

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Perspective: Ultrafast magnetism and THz spintronics

This year the discovery of femtosecond demagnetization by laser pulses is 20 years old. For the first time this milestone work by Bigot and coworkers gave insight in a very direct way into the time scales of microscopic interactions that connect the spin and electron system. While intense discussions in the field were fueled by the complexity of the processes in the past, it now became evident that it is a puzzle of many different parts. Rather than giving an overview that has been presented in previous reviews on ultrafast processes in ferromagnets, this perspective will show that with our current depth of knowledge the first real applications are on their way: THz spintronics and all-optical spin manipulation are becoming more and more feasible. The aim of this perspective is to point out where we can connect the different puzzle pieces of understanding gathered over 20 years to develop novel applications. based on many observations in a large number of experiments. Differences in the theoretical models arise from the localized and delocalized nature of ferromagnetism. Transport effects are intrinsically non-local in spintronic devices and at interfaces. We review the need for multiscale modeling to address processes starting from electronic excitation of the spin system on the picometer length scale and sub-femtosecond time scale, to spin wave generation, and towards the modeling of ultrafast phase transitions that altogether determine the response time of the ferromagnetic system. Today, our current understanding gives rise to the first real applications of ultrafast spin physics for ultrafast magnetism control: THz spintronic devices. This makes the field of ultrafast spin-dynamics an emerging topic open for many researchers right now.

Spin currents can be generated by passing electric currents through ferromagnets, but the process is too slow for ultrafast spintronics. Here, the authors show an approach for laser-driven thermal spin generation that has the potential to attain much higher speeds.

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Spin current generated by thermally driven ultrafast demagnetization

The spin-dependent Seebeck effect converts thermal gradients into spin currents. It is now shown that this effect can be used to drive spin-transfer torques on picosecond timescales using the heat currents created by ultrafast pulses of laser light.

Thermal spin-transfer torque driven by the spin-dependent Seebeck effect in metallic spin-valves

Advances in nano-electronics, nano-optics, energy harvesting materials, and nanoparticle-based photothermal therapies are motivating studies of the thermal properties of micro/nanostructures. Thus, the demands for highly sensitive and accurate thermal measurement techniques are encouraged for both fundamental studies and industrial applications. The time-domain thermoreflectance (TDTR) method, based on an ultrafast pump-probe technique, enables high-fidelity thermal measurements at the micro/nanoscale and the observation of dynamic processes with sub-picosecond time resolution. TDTR is an optical technique, capable of measuring the thermal properties of micro/nanostructures, including thermal conductivity and interfacial thermal conductance of bulk substrates, thin films, and nanoparticles, among others. Here we review some recent developments in the state-of-the-art ultrafast pump-probe method applied to study the thermal and magnetic properties of materials at the micro- and nanometer scales. We...

The Ultrafast Laser Pump-Probe Technique for Thermal Characterization of Materials With Micro/Nanostructures

Laser-induced demagnetization is theoretically studied by explicitly taking into account interactions among electrons, spins, and lattice. Assuming that the demagnetization processes take place during the thermalization of the subsystems, the temperature dynamics is given by the energy transfer between the thermalized interacting baths. These energy transfers are accounted for explicitly through electron-magnon and electron-phonon interactions, which govern the demagnetization time scale. By properly treating the spin system in a self-consistent random phase approximation, we derive magnetization dynamic equations for a broad range of temperature. The dependence of demagnetization on the temperature and pumping laser intensity is calculated in detail. In particular, we show several salient features for understanding magnetization dynamics near the Curie temperature. While the critical slowdown in dynamics occurs, we find that an external magnetic field can restore the fast dynamics. We discuss the implication of the fast dynamics in the application of heat-assisted magnetic recording.

Theory of laser-induced demagnetization at high temperatures

Abstract The quantum limit (QL) of an electron liquid, realised at strong magnetic fields, has long been proposed to host a wealth of strongly correlated states of matter. Electronic states in the QL are, for example, quasi-one dimensional (1D), which implies perfectly nested Fermi surfaces prone to instabilities. Whereas the QL typically requires unreachably strong magnetic fields, the topological semimetal ZrTe 5 has been shown to reach the QL at fields of only a few Tesla. Here, we characterize the QL of ZrTe 5 at fields up to 64 T by a combination of electrical-transport and ultrasound measurements. We find that the Zeeman effect in ZrTe 5 enables an efficient tuning of the 1D Landau band structure with magnetic field. This results in a Lifshitz transition to a 1D Weyl regime in which perfect charge neutrality can be achieved. Since no instability-driven phase transitions destabilise the 1D electron liquid for the investigated field strengths and temperatures, our analysis establishes ZrTe 5 as a thoroughly understood platform for potentially inducing more exotic interaction-driven phases at lower temperatures. 

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Signatures of a magnetic-field-induced Lifshitz transition in the ultra-quantum limit of the topological semimetal ZrTe5

The strange metal phase, which dominates large part of the phase diagram of the cuprates 1,2,3,4,5 , has been found in many quantum materials, including pnictides 6 , heavy fer-mions 7 and magic angle double layer graphene 8 . One suggested origin of this universality is that it stems from the presence of a quantum critical point 9 (QCP), i.e., a phase transition at zero temperature ruled by quantum fluctuations, in contrast to ordinary phase transitions ruled by thermal fluctuations. Although in cuprates superconductivity hinders the direct observation of the QCP phenomenology at low temperatures, indirect evidence comes from the identification of fluctuations compatible with the strange metal phase at higher temperatures. Here we show that the recently discovered charge density fluctuations 10,11 (CDF) possess the right properties to be associated to a quantum phase transition. By using resonant x-ray scattering, we have studied the CDF in two families of cuprate superconductors over a wide range of doping p , up to p = 0.22. We show that at p * ≈ 0.19, corresponding to the putative QCP, the CDF intensity is the strongest, and the characteristic energy Δ is the smallest, marking a wedge-shaped region in the phase diagram indicative of a quantum critical behavior, although with anomalous properties. These results support the leading role of charge order in driving the unconventional phenomenology of the strange metal and add new elements for the understanding of high critical temperature superconductivity.

Signature of quantum criticality in cuprates by charge density fluctuations

Using Landau level spectroscopy, we determine the temperature dependence of the energy band gap in zirconium pentatelluride (ZrTe$_5$). We find that the band gap reaches $E_g=(5 \pm 1)$ meV at low temperatures and increases monotonously when the temperature is raised. This implies that ZrTe$_5$ is a weak topological insulator, with non-inverted ordering of electronic bands in the center of the Brillouin zone. Our magneto-transport experiments performed in parallel show that the resistivity anomaly in ZrTe$_5$ is not connected with the temperature dependence of the band gap.

Temperature dependence of the energy band gap in 
ZrTe5
: Implications for the topological phase

Sm-doped YBa2Cu3O7–δ samples were prepared by conventional solid state reaction method in air. The studies of X-ray photoemission spectroscopy and X-ray absorption spectroscopy indicate that the extra electrons fill the holes in CuO2 planes, so that the hole carrier concentration decrease under Sm substitution. The X-ray absorption spectroscopy of Cu L-edge and O K-edge demonstrates that the hybridization strength between Cu 3 d and O 2 p decreases with increasing content of Sm, but the hybridization strength between Y 4 d, Ba 5 d, Sm 4 f and O 2 p increases with the increasing of Sm content, while the superconductivity of the Sm-doped YBa2Cu3O7–δ samples is suppressed, i.e., the superconducting transition temperature of the samples was lowered. Thus, our experimental results suggest a close relationship between the superconductivity and orbital hybridization.

Q. Li

Papers

Theory of laser-induced demagnetization at high temperatures

Signatures of a magnetic-field-induced Lifshitz transition in the ultra-quantum limit of the topological semimetal ZrTe5

Signature of quantum criticality in cuprates by charge density fluctuations

Temperature dependence of the energy band gap in ZrTe5 : Implications for the topological phase

The relationship between orbital hybridization and superconductivity of sm-doped YBa2Cu3O7–δ studied by x-ray spectroscopy