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- ItemTwo-dimensional ferromagnetic extension of a topological insulator(College Park, MD : APS, 2023) Kagerer, P.; Fornari, C. I.; Buchberger, S.; Tschirner, T.; Veyrat, L.; Kamp, M.; Tcakaev, A. V.; Zabolotnyy, V.; Morelhão, S. L.; Geldiyev, B.; Müller, S.; Fedorov, A.; Rienks, E.; Gargiani, P.; Valvidares, M.; Folkers, L. C.; Isaeva, A.; Büchner, B.; Hinkov, V.; Claessen, R.; Bentmann, H.; Reinert, F.Inducing a magnetic gap at the Dirac point of the topological surface state (TSS) in a three-dimensional (3D) topological insulator (TI) is a route to dissipationless charge and spin currents. Ideally, magnetic order is present only at the surface, as through proximity of a ferromagnetic (FM) layer. However, experimental evidence of such a proximity-induced Dirac mass gap is missing, likely due to an insufficient overlap of TSS and the FM subsystem. Here, we take a different approach, namely ferromagnetic extension (FME), using a thin film of the 3D TI Bi2Te3, interfaced with a monolayer of the lattice-matched van der Waals ferromagnet MnBi2Te4. Robust 2D ferromagnetism with out-of-plane anisotropy and a critical temperature of Tc≈15 K is demonstrated by x-ray magnetic dichroism and electrical transport measurements. Using angle-resolved photoelectron spectroscopy, we observe the opening of a sizable magnetic gap in the 2D FM phase, while the surface remains gapless in the paramagnetic phase above Tc. Ferromagnetic extension paves the way to explore the interplay of strictly 2D magnetism and topological surface states, providing perspectives for realizing robust quantum anomalous Hall and chiral Majorana states.
- ItemIntercalant-mediated Kitaev exchange in Ag3LiIr2O6(College Park, MD : APS, 2022) Yadav, Ravi; Reja, Sahinur; Ray, Rajyavardhan; van den Brink, Jeroen; Nishimoto, Satoshi; Yazyev, Oleg V.The recently synthesized Ag3LiIr2O6 has been proposed as a Kitaev magnet in proximity to the quantum spin liquid phase. We explore its microscopic Hamiltonian and magnetic ground state using many-body quantum chemistry methods and exact diagonalization techniques. Our calculations establish a dominant bond dependent ferromagnetic Kitaev exchange between Ir sites and find that the inclusion of Ag 4d orbitals in the configuration interaction calculations strikingly enhances the Kitaev exchange. Furthermore, using exact diagonalization of the nearest-neighbor fully anisotropic J−K−Γ Hamiltonian, we obtain the magnetic phase diagram as a function of further neighbor couplings. We find that the antiferromagnetic off-diagonal coupling stabilizes long range order, but the structure factor calculations suggest that the material is very close to the quantum spin liquid phase and the ordered state can easily collapse into a liquid by small perturbations such as structural distortion or bond disorder.
- ItemCorrelation induced magnetic topological phases in the mixed-valence compound SmB6(College Park, MD : APS, 2023) Liu, Huimei; Hirschmann, Moritz M.; Sawatzky, George A.; Khaliullin, Giniyat; Schnyder, Andreas P.SmB6 is a mixed-valence compound with flat f-electron bands that have a propensity to magnetism. Here, using a realistic Γ8 quartet model, we investigate the dynamical spin susceptibility and describe the in-gap collective mode observed in neutron scattering experiments. We show that as the Sm valence increases with pressure, the magnetic correlations enhance and SmB6 undergoes a first-order phase transition into a metallic antiferromagnetic state, whose symmetry depends on the model parameters. The magnetic orderings give rise to distinct band topologies: while the A-type order leads to an overlap between valence and conduction bands in the form of Dirac nodal lines, the G-type order has a negative indirect gap with weak Z2 indices. We also consider the spin polarized phase under a strong magnetic field, and find that it exhibits Weyl points as well as nodal lines close to the Fermi level. The magnetic phases show markedly different surface states and tunable bulk transport properties, with important implications for experiments. Our theory predicts that a magnetic order can be stabilized also by lifting the Γ8 cubic symmetry, thus explaining the surface magnetism reported in SmB6.
- ItemObservation of orbital order in the van der Waals material 1T−TiSe2(College Park, MD : APS, 2022) Peng, Yingying; Guo, Xuefei; Xiao, Qian; Li, Qizhi; Strempfer, Jörg; Choi, Yongseong; Yan, Dong; Luo, Huixia; Huang, Yuqing; Jia, Shuang; Janson, Oleg; Abbamonte, Peter; van den Brink, Jeroen; van Wezel, JasperBesides magnetic and charge order, regular arrangements of orbital occupation constitute a fundamental order parameter of condensed matter physics. Even though orbital order is difficult to identify directly in experiments, its presence was firmly established in a number of strongly correlated, three-dimensional Mott insulators. Here, reporting resonant x-ray-scattering experiments on the layered van der Waals compound 1T-TiSe2, we establish that the known charge density wave in this weakly correlated, quasi-two-dimensional material corresponds to an orbital ordered phase. Our experimental scattering results are consistent with first-principles calculations that bring to the fore a generic mechanism of close interplay between charge redistribution, lattice displacements, and orbital order. It demonstrates the essential role that orbital degrees of freedom play in TiSe2, and their importance throughout the family of correlated van der Waals materials.
- ItemStrong effects of uniaxial pressure and short-range correlations in Cr2Ge2Te6(College Park, MD : APS, 2022) Spachmann, S.; Elghandour, A.; Selter, S.; Büchner, B.; Aswartham, S.; Klingeler, R.Cr2Ge2Te6 is a quasi-two-dimensional semiconducting van der Waals ferromagnet down to the bilayer with great potential for technological applications. Engineering the critical temperature to achieve room-temperature applications is one of the critical next steps on this path. Here, we report high-resolution capacitance dilatometry studies on Cr2Ge2Te6 single crystals which directly prove significant magnetoelastic coupling and provide quantitative values of the large uniaxial pressure effects on long-range magnetic order (∂TC/∂pc=24.7 K/GPa and ∂TC/∂pab=−15.6 K/GPa) derived from thermodynamic relations. Moderate in-plane strain is thus sufficient to strongly enhance ferromagnetism in Cr2Ge2Te6 up to room temperature. Moreover, unambiguous signs of short-range magnetic order up to 200 K are found.
- ItemTopological boundaries between helical domains as a nucleation source of skyrmions in the bulk cubic helimagnet Cu2OSeO3(College Park, MD : APS, 2022) Leonov, A.O.; Pappas, C.Cu2OSeO3 represents a unique example in the family of B20 cubic helimagnets with a tilted spiral and a low-temperature skyrmion phase arising for magnetic fields applied along the easy crystallographic (100) axes. Although the stabilization mechanism of these phases can be accounted for by cubic magnetic anisotropy, the skyrmion nucleation process is still an open question, since the stability region of the skyrmion phase displays strongly hysteretic behavior with different phase boundaries for increasing and decreasing magnetic fields. Here, we address this important point using micromagnetic simulations and come to the conclusion that skyrmion nucleation is underpinned by the reorientation of spiral domains occurring near the critical magnetic fields of the phase diagrams: HC1, the critical field of the transition between the helical and conical/tiled spiral phase, and HC2, the critical field between the conical/tiled spiral and the homogenous phase. By studying a wide variety of cases we show that domain walls may have a 3D structure. Moreover, they can carry a finite topological charge stemming from half-skyrmions (merons) also permitting along-the-field and perpendicular-to-the-field orientation. Thus, domain walls may be envisioned as nucleation source of skyrmions that can form thermodynamically stable and metastable lattices as well as skyrmion networks with misaligned skyrmion tubes. The results of numerical simulations are discussed in view of recent experimental data on chiral magnets, in particular, for the bulk cubic helimagnet Cu2OSeO3.
- ItemSuppression of nematicity by tensile strain in multilayer FeSe/SrTiO3 films(College Park, MD : APS, 2023) Lou, Rui; Suvorov, Oleksandr; Grafe, Hans-Joachim; Kuibarov, Andrii; Krivenkov, Maxim; Rader, Oliver; Büchner, Bernd; Borisenko, Sergey; Fedorov, AlexanderThe nematicity in multilayer FeSe/SrTiO3 films has been previously suggested to be enhanced with decreasing film thickness. Motivated by this, there have been many discussions about the competing relation between nematicity and superconductivity. However, the criterion for determining the nematicity strength in FeSe remains highly debated. The understanding of nematicity as well as its relation to superconductivity in FeSe films is therefore still controversial. Here, we fabricate multilayer FeSe/SrTiO3 films using molecular beam epitaxy and study the nematic properties by combining angle-resolved photoemission spectroscopy, Se77 nuclear magnetic resonance, and scanning tunneling microscopy experiments. We unambiguously demonstrate that, near the interface, the nematic order is suppressed by the SrTiO3-induced tensile strain; in the bulk region further away from the interface, the strength of nematicity recovers to the bulk value. Our results not only solve the recent controversy about the nematicity in multilayer FeSe films, but also offer valuable insights into the relationship between nematicity and superconductivity.
- ItemChirality flip of Weyl nodes and its manifestation in strained MoTe2(College Park, MD : APS, 2021) Könye, Viktor; Bouhon, Adrien; Fulga, Ion Cosma; Slager, Robert-Jan; van den Brink, Jeroen; Facio, Jorge I.Due to their topological charge, or chirality, the Weyl cones present in topological semimetals are considered robust against arbitrary perturbations. One well-understood exception to this robustness is the pairwise creation or annihilation of Weyl cones, which involves the overlap of two oppositely charged nodes in energy and momentum. Here we show that their topological charge can in fact change sign, in a process that involves the merging of not two, but three Weyl nodes. This is facilitated by the presence of rotation and time-reversal symmetries, which constrain the relative positions of Weyl cones in momentum space. We analyze the chirality flip process, showing that transport properties distinguish it from the conventional, double Weyl merging. Moreover, we predict that the chirality flip occurs in MoTe$_2$, where experimentally accessible strain leads to the merging of three Weyl cones close to the Fermi level. Our work sets the stage to further investigate and observe such chirality flipping processes in different topological materials.
- ItemMultistate current-induced magnetization switching in Au/Fe/MgO(001) epitaxial heterostructures(College Park, MD : APS, 2021) Gospodarič, P.; Młyńczak, E.; Soldatov, I.; Kákay, A.; Bürgler, D.E.; Plucinski, L.; Schäfer, R.; Fassbender, J.; Schneider, C.M.Magnetization switching using in-plane charge current recently has been widely investigated in heavy metal/ferromagnet bilayers with the switching mechanism usually attributed to the action of the spin-orbit coupling. Here we study in-plane current induced magnetization switching in model epitaxial bilayers that consist of Au(001) and Fe(001) grown on MgO(001). We use the planar Hall effect combined with magnetooptical Kerr effect (MOKE) microscopy to investigate magnetic properties of the bilayers and current-induced switching. We show that a current density beyond 1.4×107 A/cm can be employed for reproducible electrical switching of the magnetization between multiple stable states that correspond to different arrangements of magnetic domains with magnetization direction along one of the in-plane easy magnetization axes of the Fe(001) film. Lower current densities result in stable intermediate transversal resistances which are interpreted based on MOKE-microscopy investigations as resulting from the current-induced magnetic domain structure that is formed in the area of the Hall cross. We find that the physical mechanism of the current-induced magnetization switching of the Au/Fe/MgO(001) system at room temperature can be fully explained by the Oersted field, which is generated by the charge current flowing mostly through the Au layer.
- ItemInhomogeneous ferromagnetism mimics signatures of the topological Hall effect in SrRuO3 films(College Park, MD : APS, 2020) Kim, Gideok; Son, K.; Suyolcu, Y.E.; Miao, L.; Schreiber, N.J.; Nair, H.P.; Putzky, D.; Minola, M.; Christiani, G.; van Aken, P.A.; Shen, K.M.; Schlom, D.G.; Logvenov, G.; Keimer, B.Topological transport phenomena in magnetic materials are a major topic of current condensed matter research. One of the most widely studied phenomena is the topological Hall effect (THE), which is generated via spin-orbit interactions between conduction electrons and topological spin textures such as skyrmions. We report a comprehensive set of Hall effect and magnetization measurements on epitaxial films of the prototypical ferromagnetic metal SrRuO3 the magnetic and transport properties of which were systematically modulated by varying the concentration of Ru vacancies. We observe Hall effect anomalies that closely resemble signatures of the THE, but a quantitative analysis demonstrates that they result from inhomogeneities in the ferromagnetic magnetization caused by a nonrandom distribution of Ru vacancies. As such inhomogeneities are difficult to avoid and are rarely characterized independently, our results call into question the identification of topological spin textures in numerous prior transport studies of quantum materials, heterostructures, and devices. Firm conclusions regarding the presence of such textures must meet stringent conditions such as probes that couple directly to the noncollinear magnetization on the atomic scale.
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