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- ItemEnhancing laser beam performance by interfering intense laser beamlets([London] : Nature Publishing Group UK, 2019) Morace, A.; Iwata, N.; Sentoku, Y.; Mima, K.; Arikawa, Y.; Yogo, A.; Andreev, A.; Tosaki, S.; Vaisseau, X.; Abe, Y.; Kojima, S.; Sakata, S.; Hata, M.; Lee, S.; Matsuo, K.; Kamitsukasa, N.; Norimatsu, T.; Kawanaka, J.; Tokita, S.; Miyanaga, N.; Shiraga, H.; Sakawa, Y.; Nakai, M.; Nishimura, H.; Azechi, H.; Fujioka, S.; Kodama, R.Increasing the laser energy absorption into energetic particle beams represents a longstanding quest in intense laser-plasma physics. During the interaction with matter, part of the laser energy is converted into relativistic electron beams, which are the origin of secondary sources of energetic ions, γ-rays and neutrons. Here we experimentally demonstrate that using multiple coherent laser beamlets spatially and temporally overlapped, thus producing an interference pattern in the laser focus, significantly improves the laser energy conversion efficiency into hot electrons, compared to one beam with the same energy and nominal intensity as the four beamlets combined. Two-dimensional particle-in-cell simulations support the experimental results, suggesting that beamlet interference pattern induces a periodical shaping of the critical density, ultimately playing a key-role in enhancing the laser-to-electron energy conversion efficiency. This method is rather insensitive to laser pulse contrast and duration, making this approach robust and suitable to many existing facilities.
- ItemImaging and writing magnetic domains in the non-collinear antiferromagnet Mn3Sn([London] : Nature Publishing Group UK, 2019) Reichlova, Helena; Janda, Tomas; Godinho, Joao; Markou, Anastasios; Kriegner, Dominik; Schlitz, Richard; Zelezny, Jakub; Soban, Zbynek; Bejarano, Mauricio; Schultheiss, Helmut; Nemec, Petr; Jungwirth, Tomas; Felser, Claudia; Wunderlich, Joerg; Goennenwein, Sebastian T. B.Non-collinear antiferromagnets are revealing many unexpected phenomena and they became crucial for the field of antiferromagnetic spintronics. To visualize and prepare a well-defined domain structure is of key importance. The spatial magnetic contrast, however, remains extraordinarily difficult to be observed experimentally. Here, we demonstrate a magnetic imaging technique based on a laser induced local thermal gradient combined with detection of the anomalous Nernst effect. We employ this method in one the most actively studied representatives of this class of materials—Mn3Sn. We demonstrate that the observed contrast is of magnetic origin. We further show an algorithm to prepare a well-defined domain pattern at room temperature based on heat assisted recording principle. Our study opens up a prospect to study spintronics phenomena in non-collinear antiferromagnets with spatial resolution.
- ItemDisparate ultrafast dynamics of itinerant and localized magnetic moments in gadolinium metal([London] : Nature Publishing Group UK, 2015) Frietsch, B.; Bowlan, J.; Carley, R.; Teichmann, M.; Wienholdt, S.; Hinzke, D.; Nowak, U.; Carva, K.; Oppeneer, P. M.; Weinelt, M.The Heisenberg–Dirac intra-atomic exchange coupling is responsible for the formation of the atomic spin moment and thus the strongest interaction in magnetism. Therefore, it is generally assumed that intra-atomic exchange leads to a quasi-instantaneous aligning process in the magnetic moment dynamics of spins in separate, on-site atomic orbitals. Following ultrashort optical excitation of gadolinium metal, we concurrently record in photoemission the 4f magnetic linear dichroism and 5d exchange splitting. Their dynamics differ by one order of magnitude, with decay constants of 14 versus 0.8 ps, respectively. Spin dynamics simulations based on an orbital-resolved Heisenberg Hamiltonian combined with first-principles calculations explain the particular dynamics of 5d and 4f spin moments well, and corroborate that the 5d exchange splitting traces closely the 5d spin-moment dynamics. Thus gadolinium shows disparate dynamics of the localized 4f and the itinerant 5d spin moments, demonstrating a breakdown of their intra-atomic exchange alignment on a picosecond timescale.
- ItemGate controlled valley polarizer in bilayer graphene([London] : Nature Publishing Group UK, 2020) Chen, Hao; Zhou, Pinjia; Liu, Jiawei; Qiao, Jiabin; Oezyilmaz, Barbaros; Martin, JensSign reversal of Berry curvature across two oppositely gated regions in bilayer graphene can give rise to counter-propagating 1D channels with opposite valley indices. Considering spin and sub-lattice degeneracy, there are four quantized conduction channels in each direction. Previous experimental work on gate-controlled valley polarizer achieved good contrast only in the presence of an external magnetic field. Yet, with increasing magnetic field the ungated regions of bilayer graphene will transit into the quantum Hall regime, limiting the applications of valley-polarized electrons. Here we present improved performance of a gate-controlled valley polarizer through optimized device geometry and stacking method. Electrical measurements show up to two orders of magnitude difference in conductance between the valley-polarized state and gapped states. The valley-polarized state displays conductance of nearly 4e2/h and produces contrast in a subsequent valley analyzer configuration. These results pave the way to further experiments on valley-polarized electrons in zero magnetic field.
- ItemUnconventional Hall response in the quantum limit of HfTe5([London] : Nature Publishing Group UK, 2020) Galeski, S.; Zhao, X.; Wawrzyńczak, R.; Meng, T.; Förster, T.; Lozano, P.M.; Honnali, S.; Lamba, N.; Ehmcke, T.; Markou, A.; Li., Q.; Gu, G.; Zhu, W.; Wosnitza, J.; Felser, C.; Chen, G.F.; Gooth, J.Interacting electrons confined to their lowest Landau level in a high magnetic field can form a variety of correlated states, some of which manifest themselves in a Hall effect. Although such states have been predicted to occur in three-dimensional semimetals, a corresponding Hall response has not yet been experimentally observed. Here, we report the observation of an unconventional Hall response in the quantum limit of the bulk semimetal HfTe5, adjacent to the three-dimensional quantum Hall effect of a single electron band at low magnetic fields. The additional plateau-like feature in the Hall conductivity of the lowest Landau level is accompanied by a Shubnikov-de Haas minimum in the longitudinal electrical resistivity and its magnitude relates as 3/5 to the height of the last plateau of the three-dimensional quantum Hall effect. Our findings are consistent with strong electron-electron interactions, stabilizing an unconventional variant of the Hall effect in a three-dimensional material in the quantum limit.
- ItemSignatures of a magnetic-field-induced Lifshitz transition in the ultra-quantum limit of the topological semimetal ZrTe5([London] : Nature Publishing Group UK, 2022) Galeski, S.; Legg, H.F.; Wawrzyńczak, R.; Förster, T.; Zherlitsyn, S.; Gorbunov, D.; Uhlarz, M.; Lozano, P.M.; Li, Q.; Gu, G.D.; Felser, C.; Wosnitza, J.; Meng, T.; Gooth, J.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 ZrTe5 has been shown to reach the QL at fields of only a few Tesla. Here, we characterize the QL of ZrTe5 at fields up to 64 T by a combination of electrical-transport and ultrasound measurements. We find that the Zeeman effect in ZrTe5 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 ZrTe5 as a thoroughly understood platform for potentially inducing more exotic interaction-driven phases at lower temperatures.
- ItemObservation of fractional spin textures in a Heusler material([London] : Nature Publishing Group UK, 2022) Jena, Jagannath; Göbel, Börge; Hirosawa, Tomoki; Díaz, Sebastián A.; Wolf, Daniel; Hinokihara, Taichi; Kumar, Vivek; Mertig, Ingrid; Felser, Claudia; Lubk, Axel; Loss, Daniel; Parkin, Stuart S.P.Recently a zoology of non-collinear chiral spin textures has been discovered, most of which, such as skyrmions and antiskyrmions, have integer topological charges. Here we report the experimental real-space observation of the formation and stability of fractional antiskyrmions and fractional elliptical skyrmions in a Heusler material. These fractional objects appear, over a wide range of temperature and magnetic field, at the edges of a sample, whose interior is occupied by an array of nano-objects with integer topological charges, in agreement with our simulations. We explore the evolution of these objects in the presence of magnetic fields and show their interconversion to objects with integer topological charges. This means the topological charge can be varied continuously. These fractional spin textures are not just another type of skyrmion, but are essentially a new state of matter that emerges and lives only at the boundary of a magnetic system. The coexistence of both integer and fractionally charged spin textures in the same material makes the Heusler family of compounds unique for the manipulation of the real-space topology of spin textures and thus an exciting platform for spintronic and magnonic applications.