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- ItemSingle Molecule Magnetism with Strong Magnetic Anisotropy and Enhanced Dy∙∙∙Dy Coupling in Three Isomers of Dy-Oxide Clusterfullerene Dy2O@C82(Chichester : John Wiley and Sons Ltd, 2019) Yang, W.; Velkos, G.; Liu, F.; Sudarkova, S.M.; Wang, Y.; Zhuang, J.; Zhang, H.; Li, X.; Zhang, X.; Büchner, B.; Avdoshenko, S.M.; Popov, A.A.; Chen, N.A new class of single-molecule magnets (SMMs) based on Dy-oxide clusterfullerenes is synthesized. Three isomers of Dy2O@C82 with Cs(6), C3v(8), and C2v(9) cage symmetries are characterized by single-crystal X-ray diffraction, which shows that the endohedral Dy−(µ2-O)−Dy cluster has bent shape with very short Dy−O bonds. Dy2O@C82 isomers show SMM behavior with broad magnetic hysteresis, but the temperature and magnetization relaxation depend strongly on the fullerene cage. The short Dy−O distances and the large negative charge of the oxide ion in Dy2O@C82 result in the very strong magnetic anisotropy of Dy ions. Their magnetic moments are aligned along the Dy−O bonds and are antiferromagnetically (AFM) coupled. At low temperatures, relaxation of magnetization in Dy2O@C82 proceeds via the ferromagnetically (FM)-coupled excited state, giving Arrhenius behavior with the effective barriers equal to the AFM-FM energy difference. The AFM-FM energy differences of 5.4–12.9 cm−1 in Dy2O@C82 are considerably larger than in SMMs with {Dy2O2} bridges, and the Dy∙∙∙Dy exchange coupling in Dy2O@C82 is the strongest among all dinuclear Dy SMMs with diamagnetic bridges. Dy-oxide clusterfullerenes provide a playground for the further tuning of molecular magnetism via variation of the size and shape of the fullerene cage.
- ItemSpectromicroscopic measurements of electronic structure variations in atomically thin WSe2(2020) Klaproth, T.; Habenicht, C.; Schuster, R.; Büchner, B.; Knupfer, M.; Koitzsch, A.Atomically thin transition metal dichalcogenides (TMDCs) are promising candidates for implementation in next generation semiconducting devices, for which laterally homogeneous behavior is needed. Here, we study the electronic structure of atomically thin exfoliated WSe2, a prototypical TMDC with large spin–orbit coupling, by photoemission electron microscopy, electron energy-loss spectroscopy, and density functional theory. We resolve the inhomogeneities of the doping level by the varying energy positions of the valence band. There appear to be different types of inhomogeneities that respond differently to electron doping, introduced by potassium intercalation. In addition, we find that the doping process itself is more complex than previously anticipated and entails a distinct orbital and thickness dependence that needs to be considered for effective band engineering. In particular, the density of selenium vs tungsten states depends on the doping level, which leads to changes in the optical response beyond increased dielectric screening. Our work gives insight into the inhomogeneity of the electron structure of WSe2 and the effects of electron doping, provides microscopic understanding thereof, and improves the basis for property engineering of 2D materials.
- ItemShape-adaptive single-molecule magnetism and hysteresis up to 14 K in oxide clusterfullerenes Dy2O@C72 and Dy2O@C74 with fused pentagon pairs and flexible Dy-(μ2-O)-Dy angle(Cambridge : Royal Society of Chemistry, 2020) Velkos, G.; Yang, W.; Yao, Y.-R.; Sudarkova, S.M.; Liu, X.; Büchner, B.; Avdoshenko, S.M.; Chen, N.; Popov, A.A.Dysprosium oxide clusterfullerenes Dy2O@Cs(10528)-C72 and Dy2O@C2(13333)-C74 are synthesized and characterized by single-crystal X-ray diffraction. Carbon cages of both molecules feature two adjacent pentagon pairs. These pentalene units determine positions of endohedral Dy ions hence the shape of the Dy2O cluster, which is bent in Dy2O@C72 but linear in Dy2O@C74. Both compounds show slow relaxation of magnetization and magnetic hysteresis. Nearly complete cancelation of ferromagnetic dipolar and antiferromagnetic exchange Dy⋯Dy interactions leads to unusual magnetic properties. Dy2O@C74 exhibits zero-field quantum tunneling of magnetization and magnetic hysteresis up to 14 K, the highest temperature among Dy-clusterfullerenes.
- ItemMomentum dependent dxz/yz band splitting in LaFeAsO(Berlin : Springer Nature, 2020) Huh, S.S.; Kim, Y.S.; Kyung, W.S.; Jung, J.K.; Kappenberger, R.; Aswartham, S.; Büchner, B.; Ok, J.M.; Kim, J.S.; Dong, C.; Hu, J.P.; Cho, S.H.; Shen, D.W.; Denlinger, J.D.; Kim, Y.K.; Kim, C.The nematic phase in iron based superconductors (IBSs) has attracted attention with a notion that it may provide important clue to the superconductivity. A series of angle-resolved photoemission spectroscopy (ARPES) studies were performed to understand the origin of the nematic phase. However, there is lack of ARPES study on LaFeAsO nematic phase. Here, we report the results of ARPES studies of the nematic phase in LaFeAsO. Degeneracy breaking between the dxz and dyz hole bands near the Γ and M point is observed in the nematic phase. Different temperature dependent band splitting behaviors are observed at the Γ and M points. The energy of the band splitting near the M point decreases as the temperature decreases while it has little temperature dependence near the Γ point. The nematic nature of the band shift near the M point is confirmed through a detwin experiment using a piezo device. Since a momentum dependent splitting behavior has been observed in other iron based superconductors, our observation confirms that the behavior is a universal one among iron based superconductors.
- ItemCoupling of lattice, spin, and intraconfigurational excitations of Eu3+ in Eu2ZnIrO6(Washington, DC : American Association for the Advancement of Science, 2020) Singh, Birender; Vogl, M.; Wurmehl, S.; Aswartham, S.; Büchner, B.; Kumar, PradeepIn Eu2ZnIrO6, effectively two atoms are active; i.e., Ir is magnetically active, which results in complex magnetic ordering within the Ir sublattice at low temperature. On the other hand, although Eu is a Van Vleck paramagnet, it is active in the electronic channels involving 4f6 crystal-field split levels. Phonons, quanta of lattice vibration involving vibration of atoms in the unit cell, are intimately coupled with both magnetic and electronic degrees of freedom (DOF). Here, we report a comprehensive study focusing on the phonons as well as intraconfigurational excitations in double-perovskite Eu2ZnIrO6. Our studies reveal strong coupling of phonons with the underlying magnetic DOF reflected in the renormalization of the phonon self-energy parameters well above the spin-solid phase (TN∼12K) until temperature as high as ∼3TN evidences broken spin rotational symmetry deep into the paramagnetic phase. In particular, all the observed first-order phonon modes show softening of varying degree below ∼3TN, and low-frequency phonons become sharper, while the high-frequency phonons show broadening attributed to the additional available magnetic damping channels. We also observed a large number of high-energy modes, 39 in total, attributed to the electronic transitions between 4f levels of the rare-earth Eu3+ ion and these modes shows anomalous temperature evolution as well as mixing of the crystal-field split levels attributed to the strong coupling of electronic and lattice DOF.
- ItemConfined crystals of the smallest phase-change material(Washington, DC : American Chemical Society, 2013) Giusca, C.E.; Stolojan, V.; Sloan, J.; Börrnert, F.; Shiozawa, H.; Sader, K.; Rümmeli, M.H.; Büchner, B.; Silva, S.R.P.The demand for high-density memory in tandem with limitations imposed by the minimum feature size of current storage devices has created a need for new materials that can store information in smaller volumes than currently possible. Successfully employed in commercial optical data storage products, phase-change materials, that can reversibly and rapidly change from an amorphous phase to a crystalline phase when subject to heating or cooling have been identified for the development of the next generation electronic memories. There are limitations to the miniaturization of these devices due to current synthesis and theoretical considerations that place a lower limit of 2 nm on the minimum bit size, below which the material does not transform in the structural phase. We show here that by using carbon nanotubes of less than 2 nm diameter as templates phase-change nanowires confined to their smallest conceivable scale are obtained. Contrary to previous experimental evidence and theoretical expectations, the nanowires are found to crystallize at this scale and display amorphous-to-crystalline phase changes, fulfilling an important prerequisite of a memory element. We show evidence for the smallest phase-change material, extending thus the size limit to explore phase-change memory devices at extreme scales.
- ItemImpact of Mn-Pn intermixing on magnetic properties of an intrinsic magnetic topological insulator: the µSR perspective(Bristol : IOP Publ., 2023) Sahoo, M.; Salman, Z.; Allodi, G.; Isaeva, A.; Folkers, L.; Wolter, A.U.B.; Büchner, B.; De Renzi, R.We investigated the magnetic properties of polycrystalline samples of the intrinsic magnetic topological insulators MnPn2Te4, with pnictogen Pn = Sb, Bi, by bulk magnetization and μSR. DC susceptibility detects the onset of magnetic ordering at TN = 27 K and 24 K and a field dependence of the macroscopic magnetization compatible with ferri- (or ferro-) and atiferro- magnetic ordering, respectively. Weak transverse field (wTF) Muon Spin Rotation (μSR) confirms the homogeneous bulk nature of magnetic ordering at the same two distinct transition temperatures. Zero Field (ZF) μSR shows that the Sb based material displays a broader distribution of internal field at the muon, in accordance with a larger deviation from the stoichiomectric composition and a higher degree of positional disorder (Mn at the Pn(6c) site), which however does not affect significantly the sharpness of the thermodynamic transition, as detected by the muon magnetic volume fraction and the observability of a critical divergence in the longitudinal and transverse muon relaxation rates.
- 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.
- 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.
- ItemTailoring electron beams with high-frequency self-assembled magnetic charged particle micro optics([London] : Nature Publishing Group UK, 2022) Huber, R.; Kern, F.; Karnaushenko, D.D.; Eisner, E.; Lepucki, P.; Thampi, A.; Mirhajivarzaneh, A.; Becker, C.; Kang, T.; Baunack, S.; Büchner, B.; Karnaushenko, D.; Schmidt, O.G.; Lubk, A.Tunable electromagnets and corresponding devices, such as magnetic lenses or stigmators, are the backbone of high-energy charged particle optical instruments, such as electron microscopes, because they provide higher optical power, stability, and lower aberrations compared to their electric counterparts. However, electromagnets are typically macroscopic (super-)conducting coils, which cannot generate swiftly changing magnetic fields, require active cooling, and are structurally bulky, making them unsuitable for fast beam manipulation, multibeam instruments, and miniaturized applications. Here, we present an on-chip microsized magnetic charged particle optics realized via a self-assembling micro-origami process. These micro-electromagnets can generate alternating magnetic fields of about ±100 mT up to a hundred MHz, supplying sufficiently large optical power for a large number of charged particle optics applications. That particular includes fast spatiotemporal electron beam modulation such as electron beam deflection, focusing, and wave front shaping as required for stroboscopic imaging.