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    Extremely well isolated two-dimensional spin-1/2 antiferromagnetic Heisenberg layers with a small exchange coupling in the molecular-based magnet CuPOF
    (Woodbury, NY : Inst., 2020) Opherden, D.; Nizar, N.; Richardson, K.; Monroe, J.C.; Turnbull, M.M.; Polson, M.; Vela, S.; Blackmore, W.J.A.; Goddard, P.A.; Singleton, J.; Choi, E.S.; Xia, F.; Williams, R.C.; Lancaster, T.; Pratt, F.L.; Blundell, S.J.; Skourski, Y.; Uhlarz, M.; Ponomaryov, A.N.; Zvyagin, S.A.; Wosnitza, J.; Baenitz, M.; Heinmaa, I.; Stern, R.; Kühne, H.; Landee, C.P.
    We report on a comprehensive characterization of the newly synthesized Cu2+-based molecular magnet [Cu(pz)2(2-HOpy)2](PF6)2 (CuPOF), where pz=C4H4N2 and 2-HOpy=C5H4NHO. From a comparison of theoretical modeling to results of bulk magnetometry, specific heat, μ+SR, ESR, and NMR spectroscopy, this material is determined as an excellent realization of the two dimensional square-lattice S=12 antiferromagnetic Heisenberg model with a moderate intraplane nearest-neighbor exchange coupling of J/kB=6.80(5) K, and an extremely small interlayer interaction of about 1 mK. At zero field, the bulk magnetometry reveals a temperature-driven crossover of spin correlations from isotropic to XY type, caused by the presence of a weak intrinsic easy-plane anisotropy. A transition to long-range order, driven by the low-temperature XY anisotropy under the influence of the interlayer coupling, occurs at TN=1.38(2) K, as revealed by μ+SR. In applied magnetic fields, our H1-NMR data reveal a strong increase of the magnetic anisotropy, manifested by a pronounced enhancement of the transition temperature to commensurate long-range order at TN=2.8 K and 7 T. © 2020 authors.
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    Splitting of the magnetic monopole pair-creation energy in spin ice
    (Bristol : IOP Publ., 2020) Hornung, J.; Gottschall, T.; Opherden, L.; Antlauf, M.; Schwarz, M.; Kroke, E.; Herrmannsdörfer, T.; Wosnitza, J.
    The thermodynamics in spin-ice systems are governed by emergent magnetic monopole excitations and, until now, the creation of a pair of these topological defects was associated with one specific pair-creation energy. Here, we show that the electric dipole moments inherent to the magnetic monopoles lift the degeneracy of their creation process and lead to a splitting of the pair-creation energy. We consider this finding to extend the model of magnetic relaxation in spin-ice systems and show that an electric dipole interaction in the theoretically estimated order of magnitude leads to a splitting which can explain the controversially discussed discrepancies between the measured temperature dependence of the magnetic relaxation times and previous theory. By applying our extended model to experimental data of, various spin-ice systems, we show its universal applicability and determine a dependence of the electric dipole interaction on the system parameters, which is in accordance with the theoretical model of electric dipole formation. © 2020 The Author(s). Published by IOP Publishing Ltd.
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    Observation of giant spin-split Fermi-arc with maximal Chern number in the chiral topological semimetal PtGa
    (London : Nature Publishing Group, 2020) Yao, M.; Manna, K.; Yang, Q.; Fedorov, A.; Voroshnin, V.; Valentin Schwarze, B.; Hornung, J.; Chattopadhyay, S.; Sun, Z.; Guin, S.N.; Wosnitza, J.; Borrmann, H.; Shekhar, C.; Kumar, N.; Fink, J.; Sun, Y.; Felser, C.
    Non-symmorphic chiral topological crystals host exotic multifold fermions, and their associated Fermi arcs helically wrap around and expand throughout the Brillouin zone between the high-symmetry center and surface-corner momenta. However, Fermi-arc splitting and realization of the theoretically proposed maximal Chern number rely heavily on the spin-orbit coupling (SOC) strength. In the present work, we investigate the topological states of a new chiral crystal, PtGa, which has the strongest SOC among all chiral crystals reported to date. With a comprehensive investigation using high-resolution angle-resolved photoemission spectroscopy, quantum-oscillation measurements, and state-of-the-art ab initio calculations, we report a giant SOC-induced splitting of both Fermi arcs and bulk states. Consequently, this study experimentally confirms the realization of a maximal Chern number equal to ±4 in multifold fermionic systems, thereby providing a platform to observe large-quantized photogalvanic currents in optical experiments.
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    Magnetic-field- and temperature-dependent fermi surface of CeBiPt
    (Milton Park : Taylor & Francis, 2006) Wosnitza, J.; Goll, G.; Bianchi, A.D.; Bergk, B.; Kozlova, N.; Opahle, I.; Elgazzar, S.; Richter, Manuel; Stockert, O.; Löhneysen, H.V.; Yoshino, T.; Takabatake, T.
    The half-Heusler compounds CeBiPt and LaBiPt are semimetals with very low charge-carrier concentrations as evidenced by Shubnikov–de Haas (SdH) and Hall-effect measurements. Neutron-scattering results reveal a simple antiferromagnetic structure in CeBiPt below TN = 1.15 K. The band structure of CeBiPt sensitively depends on temperature, magnetic field and stoichiometry. Above a certain, sample-dependent, threshold field (B>25 T), the SdH signal disappears and the Hall coefficient reduces significantly. These effects are absent in the non-4f compound LaBiPt. Electronic-band-structure calculations can well explain the observed behaviour by a 4f-polarization-induced Fermi-surface modification.