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- ItemHigh-field ESR studies of the quantum spin magnet CaCu2O 3(Milton Park : Taylor & Francis, 2006) Goiran, M.; Costes, M.; Broto, J.M.; Chou, F.C.; Klingeler, R.; Arushanov, E.; Drechsler, S.-L.; Büchner, B.; Kataev, V.We report an electron spin resonance (ESR) study of the s = 1/2 Heisenberg pseudo-ladder magnet CaCu2O3 in pulsed magnetic fields up to 40 T. At sub-terahertz frequencies we observe an ESR signal originating from a small amount of uncompensated spins residing presumably at the imperfections of the strongly antiferromagnetically correlated host spin lattice. The data give evidence that these few per cent of 'extra' spin states are coupled strongly to the bulk spins and are involved in the antiferromagnetic (AF) ordering at TN = 25 K. By mapping the frequency/resonance field diagram we have determined a small gap for magnetic excitations below TN of the order of ~0.3–0.8 meV. Such a small value of the gap explains the occurrence of the spin-flop transition in CaCu2O3 at weak magnetic fields μ0Hsf ~ 3 T. Qualitative changes of the ESR response with the increasing field strength give indications that strong magnetic fields reduce the AF correlations and may even suppress the long-range magnetic order in CaCu2O3. ESR data support scenarios with a significant role of the 'extra' spin states for the properties of low-dimensional quantum magnets.
- ItemStructural and optical properties of (112̅2) InGaN quantum wells compared to (0001) and (112̅0)(Milton Park : Taylor & Francis, 2016) Pristovsek, Markus; Han, Yisong; Zhu, Tongtong; Oehler, Fabrice; Tang, Fengzai; Oliver, Rachel A.; Humphreys, Colin J.; Tytko, Darius; Choi, Pyuck-Pa; Raabe, Dierk; Brunner, Frank; Weyers, MarkusWe benchmarked growth, microstructure and photo luminescence (PL) of (112-2) InGaN quantum wells (QWs) against (0001) and (112-0). In incorporation, growth rate and the critical thickness of (112-2) QWs are slightly lower than (0001) QWs, while the In incorporation on (112-0) is reduced by a factor of three. A small step-bunching causes slight fluctuations of the emission wavelength. Transmission electron microscopy as well as atom probe tomography (APT) found very flat interfaces with little In segregation even for 20% In content. APT frequency distribution analysis revealed some deviation from a random InGaN alloy, but not as severe as for (112-0). The slight deviation of (112-2) QWs from an ideal random alloy did not broaden the 300 K PL, the line widths were similar for (112-2) and (0001) while (112-0) QWs were broader. Despite the high structural quality and narrow PL, the integrated PL signal at 300 K was about 4 lower on (112-2) and more than 10 lower on (112-0).
- ItemDoping dependence and electron–boson coupling in the ultrafast relaxation of hot electron populations in Ba(Fe1–x Co x )2As2(Milton Park : Taylor & Francis, 2016) Avigo, I.; Thirupathaiah, S.; Ligges, M.; Wolf, T.; Fink, J.; Bovensiepen, U.Using femtosecond time- and angle-resolved photoemission spectroscopy we investigate the effect of electron doping on the electron dynamics in $\mathrm{Ba}{({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_{x})}_{2}{\mathrm{As}}_{2}$ in a range of $0\leqslant x\lt 0.15$ at temperatures slightly above the Néel temperature. By analyzing the time-dependent photoemission intensity of the pump laser excited population as a function of energy, we found that the relaxation times at $0\lt E-{E}_{{\rm{F}}}\lt 0.2\,\mathrm{eV}$ are doping dependent and about 100 fs shorter at optimal doping than for overdoped and parent compounds. Analysis of the relaxation rates also reveals the presence of a pump fluence dependent step in the relaxation time at $E-{E}_{{\rm{F}}}=200\,\mathrm{meV}$ which we explain by coupling of the excited electronic system to a boson of this energy. We compare our results with static ARPES and transport measurements and find disagreement and agreement concerning the doping-dependence, respectively. We discuss the effect of the electron–boson coupling on the energy-dependent relaxation and assign the origin of the boson to a magnetic excitation.
- ItemThe properties of isolated chiral skyrmions in thin magnetic films(Milton Park : Taylor & Francis, 2016) Leonov, A.O.; Monchesky, T.L.; Romming, N.; Kubetzka, A.; Bogdanov, A.N.; Wiesendanger, R.Axisymmetric solitonic states (chiral skyrmions) were first predicted theoretically more than two decades ago. However, until recently they have been observed in a form of skyrmionic condensates (hexagonal lattices and other mesophases). In this paper we report experimental and theoretical investigations of isolated chiral skyrmions discovered in PdFe/Ir(111) bilayers two years ago by Romming et al (2013 Science 341 636). The results of spin-polarized scanning tunneling microscopy analyzed within the continuum and discrete models provide a consistent description of isolated skyrmions in thin layers. The existence region of chiral skyrmions is restricted by strip-out instabilities at low fields and a collapse at high fields. We demonstrate that the same equations describe axisymmetric localized states in all condensed matter systems with broken mirror symmetry, and thus our findings establish basic properties of isolated skyrmions common for chiral liquid crystals, different classes of noncentrosymmetric magnets, ferroelectrics, and multiferroics.
- ItemStructural investigation of nanocrystalline graphene grown on (6√3 × 6√3)R30°-reconstructed SiC surfaces by molecular beam epitaxy(Milton Park : Taylor & Francis, 2013) Schumann, T.; Dubslaff, M.; Oliveira, M.H.; Hanke, M.; Fromm, F.; Seyller, T.; Nemec, L.; Blum, V.; Scheffler, M.; Lopes, J.M.J.Growth of nanocrystalline graphene films on (6√3 × 6√3)R30°-reconstructed SiC surfaces was achieved by molecular beam epitaxy, enabling the investigation of quasi-homoepitaxial growth. The structural quality of the graphene films, which is investigated by Raman spectroscopy, increases with growth time. X-ray photoelectron spectroscopy proves that the SiC surface reconstruction persists throughout the growth process and that the synthesized films consist of sp2-bonded carbon. Interestingly, grazing incidence x-ray diffraction measurements show that the graphene domains possess one single in-plane orientation, are aligned to the substrate, and offer a noticeably contracted lattice parameter of 2.450 Å. We correlate this contraction with theoretically calculated reference values (all-electron density functional calculations based on the van der Waals corrected Perdew–Burke–Ernzerhof functional) for the lattice parameter contraction induced in ideal, free-standing graphene sheets by: substrate-induced buckling, the edges of limited-size flakes and typical point defects (monovacancies, divacancies, Stone–Wales defects).
- ItemFormation of heavy d-electron quasiparticles in Sr3Ru2O7(Milton Park : Taylor & Francis, 2013) Allan, M.P.; Tamai, A.; Rozbicki, E.; Fischer, M.H.; Voss, J.; King, P.D.C.; Meevasana, W.; Thirupathaiah, S.; Rienks, E.; Fink, J.; Tennant, D.A .; Perry, R.S.; Mercure, J.F.; Wang, M.A.; Lee, Jinho; Fennie, C.J.; Kim, E.A.; Lawler, M.J.; Shen, K.M.; Mackenzie, A.P.; Shen, Z.X.; Baumberger, F.The phase diagram of Sr3Ru2O7 shows hallmarks of strong electron correlations despite the modest Coulomb interaction in the Ru 4d shell. We use angle-resolved photoelectron spectroscopy measurements to provide microscopic insight into the formation of the strongly renormalized heavy d-electron liquid that controls the physics of Sr3Ru2O7. Our data reveal itinerant Ru 4d-states confined over large parts of the Brillouin zone to an energy range of <6 meV, nearly three orders of magnitude lower than the bare band width. We show that this energy scale agrees quantitatively with a characteristic thermodynamic energy scale associated with quantum criticality and illustrate how it arises from a combination of back-folding due to a structural distortion and the hybridization of light and strongly renormalized, heavy quasiparticle bands. The resulting heavy Fermi liquid has a marked k-dependence of the renormalization which we relate to orbital mixing along individual Fermi surface sheets.
- ItemA new look on the two-dimensional Ising model: Thermal artificial spins(Milton Park : Taylor & Francis, 2016) Arnalds, Unnar B.; Chico, Jonathan; Stopfel, Henry; Kapaklis, Vassilios; Bärenbold, Oliver; Verschuuren, Marc A.; Wolff, Ulrike; Neu, Volker; Bergman, Anders; Hjörvarsson, BjörgvinWe present a direct experimental investigation of the thermal ordering in an artificial analogue of an asymmetric two-dimensional Ising system composed of a rectangular array of nano-fabricated magnetostatically interacting islands. During fabrication and below a critical thickness of the magnetic material the islands are thermally fluctuating and thus the system is able to explore its phase space. Above the critical thickness the islands freeze-in resulting in an arrested thermalized state for the array. Determining the magnetic state we demonstrate a genuine artificial two-dimensional Ising system which can be analyzed in the context of nearest neighbor interactions.
- ItemDomain evolution during the spin-reorientation transition in epitaxial NdCo5 thin films(Milton Park : Taylor & Francis, 2013) Seifert, M.; Schultz, L.; Schäfer, R.; Neu, V.; Hankemeier, S.; Rössler, S.; Frömter, R.; Oepen, H.P.The domain structure and its changes with temperature were investigated for an epitaxial NdCo5 thin film with in-plane texture in which a spin-reorientation transition takes place from the easy c-axis via the easy cone to the easy plane. Scanning electron microscopy with polarization analysis reveals a transition from a two-domain state at temperatures above 318 K via a four-domain state back to a 90°-rotated two-domain state at temperatures below 252 K. The transition temperatures correspond well to those determined by global magnetization measurements. The magnetization configuration at the three different regimes of magnetic anisotropy and its transition with temperature were analysed in detail. From the local measurements, the spin-reorientation angle and the magnetocrystalline anisotropy constants of first and second order were derived.
- ItemMagnetic field-induced twin boundary motion in polycrystalline Ni-Mn-Ga fibres(Milton Park : Taylor & Francis, 2008) Scheerbaum, N.; Heczko, O.; Liu, J.; Hinz, D.; Schultz, L.; Gutfleisch, O.Magnetic field-induced twin boundary motion leading to large magnetic field-induced strain of ~1.0% was established in polycrystalline Ni50.9Mn27.1Ga22.0 (at.%) fibres at room temperature (~60–100 μm in diameter and ~3 mm in length). The fibres' grains are as large as the fibre diameter and of random orientation. At room temperature, a ferromagnetic 5M martensite is found. Magnetic field-induced twin boundary motion was indicated by magnetic measurements and validated by electron backscatter diffraction (EBSD). The application of a magnetic field shifts the equilibrium temperature of martensite and austenite by ~0.4 K T−1, which agrees with calculations using the Clapeyron–Clausius approach.
- ItemWide-range strain tunability provided by epitaxial LaAl1−xScxO3 template films(Milton Park : Taylor & Francis, 2010) Herklotz, Andreas; Biegalski, Michael D.; Kim, Hyun-Sik; Schultz, Ludwig; Dörr, Kathrin; Christen, Hans M.The dielectric diamagnetic LaAl1− xScxO3 (LASO) (x=0–1) is proposed for adjusting of the biaxial in-plane lattice parameter of oxide substrates in the wide range from 3.79 to 4.05 Å (6.5%). This range includes the pseudocubic lattice parameters of most of the currently investigated complex oxides. The in-plane lattice parameter of strain-relaxed LASO films depends linearly on the composition, and these films grow with a smooth surface. On several different LASO-buffered substrates, ferromagnetic La0.7Sr0.3MnO3 (LSMO) films have been grown in predetermined strain states. A series of 30 nm thick LSMO films on LASO-buffered LaSrAlO4(001) demonstrates that continuously controlled coherent strains in a wide range, in this case from − 1 to +0.6%, can be obtained for the functional oxide films grown on LASO.