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- ItemMagnetic quantum oscillations of diagonal conductivity in a two-dimensional conductor with a weak square superlattice modulation under conditions of the integer quantum Hall effect(Milton Park : Taylor & Francis, 2009) Gvozdikov, V.M.; Taut, M.We report on analytical and numerical studies of the magnetic quantum oscillations of the diagonal conductivity σxx in a two-dimensional conductor with a weak square superlattice modulation under conditions of the integer quantum Hall (IQHE) effect. The quantum Hall effect in such a system differs from the conventional IQHE, in which the finite width of the Landau bands is due to disorder only. The superlattice modulation potential yields a fractal splitting of the Landau levels into Hofstadter minibands. For rational flux through a unit cell, the minibands have a finite width and intrinsic dispersion relations. We consider a regime, now accessible experimentally, in which disorder does not wash out the fractal internal gap structure of the Landau bands completely. We found the following distinctions from the conventional IQHE produced by the superlattice: (i) the peaks in diagonal conductivity are split due to the Hofstadter miniband structure of Landau bands; (ii) the number of split peaks in the bunch, their positions and heights depend irregularly on the magnetic field and the Fermi energy; (iii) the gaps between the split Landau bands (and related quantum Hall plateaus) become narrower with the superlattice modulation than without it.
- ItemIntroducing artificial length scales to tailor magnetic properties(Milton Park : Taylor & Francis, 2009) Fassbender, J.; Strache, T.; Liedke, M.O.; Markó, D.; Wintz, S.; Lenz, K.; Keller, A.; Facsko, S.; Mönch, I.; McCord, J.Magnetism is a collective phenomenon. Hence, a local variation on the nanoscale of material properties, which act on the magnetic properties, affects the overall magnetism in an intriguing way. Of particular importance are the length scales on which a material property changes. These might be related to the exchange length, the domain wall width, a typical roughness correlation length, or a length scale introduced by patterning of the material. Here we report on the influence of two artificially created length scales: (i) ion erosion templates that serve as a source of a predefined surface morphology (ripple structure) and hence allow for the investigation of roughness phenomena. It is demonstrated that the ripple wave length can be easily tuned over a wide range (25–175 nm) by varying the primary ion erosion energy. The effect of this ripple morphology on the induced uniaxial magnetic anisotropy in soft magnetic Permalloy films is studied. Only below a ripple wavelength threshold (≈60 nm) is a significant induced magnetic anisotropy found. Above this threshold the corrugated Permalloy film acts as a flat film. This cross-over is discussed in the frame of dipolar interactions giving rise to the induced anisotropies. (ii) Ion implantation through a lithographically defined mask, which is used for a magnetic property patterning on various length scales. The resulting magnetic properties are neither present in non-implanted nor in homogeneously implanted films. Here new insight is gained by the comparison of different stripe patterning widths ranging from 1 to 10 μm. In addition, the appearance of more complicated magnetic domain structures, i.e. spin-flop domain configurations and head-on domain walls, during hard axis magnetization reversal is demonstrated. In both cases the magnetic properties, the magnetization reversal process as well as the magnetic domain configurations depend sensitively on the artificially introduced length scale.
- ItemOrbital and spin effects for the upper critical field in As-deficient disordered Fe pnictide superconductors(Milton Park : Taylor & Francis, 2009) Fuchs, G.; Drechsler, S.-L.; Kozlova, N.; Bartkowiak, M.; Hamann-Borrero, J.E.; Behr, G.; Nenkov, K.; Klauss, H.-H.; Maeter, H.; Amato, A.; Luetkens, H.; Kwadrin, A.; Khasanov, R.; Freudenberger, J.; Köhler, A.; Knupfer, M.; Arushanov, E.; Rosner, H.; Büchner, B.; Schultz, L.We report upper critical field Bc2(T) data for LaO0.9F0.1FeAs1- δ in a wide temperature and field range up to 60 T. The large slope of Bc2≈- 5.4 to -6.6 T K-1 near an improved Tc≈28.5 K of the in-plane Bc2(T) contrasts with a flattening starting near 23 K above 30 T we regard as the onset of Pauli-limited behaviour (PLB) with Bc2(0)≈63–68 T. We interpret a similar hitherto unexplained flattening of the Bc2(T) curves reported for at least three other disordered closely related systems, Co-doped BaFe2As2, (Ba,K) Fe2As2 and NdO0.7F0.3FeAs (all single crystals), for applied fields H∥(a,b), also as a manifestation of PLB. Their Maki parameters have been estimated by analysing their Bc2(T) data within the Werthamer–Helfand–Hohenberg approach. The pronounced PLB of (Ba, K)Fe2As2 single crystals obtained from an Sn flux is attributed also to a significant As deficiency detected by wavelength dispersive x-ray spectroscopy as reported by Ni et al (2008 Phys. Rev. B 78 014507). Consequences of our results are discussed in terms of disorder effects within conventional superconductivity (CSC) and unconventional superconductivity (USC). USC scenarios with nodes on individual Fermi surface sheets (FSS), e.g. p- and d-wave SC, can be discarded for our samples. The increase of dBc2/dT|Tc by sizeable disorder provides evidence for an important intraband (intra-FSS) contribution to the orbital upper critical field. We suggest that it can be ascribed either to an impurity-driven transition from s± USC to CSC of an extended s++-wave state or to a stabilized s±-state provided As-vacancies cause predominantly strong intraband scattering in the unitary limit. We compare our results with Bc2 data from the literature, which often show no PLB for fields below 60–70 T probed so far. A novel disorder-related scenario of a complex interplay of SC with two different competing magnetic instabilities is suggested.
- ItemTuning functional properties by plastic deformation(Milton Park : Taylor & Francis, 2009) Kwon, A.R.; Neu, V.; Matias, V.; Hänisch, J.; Hühne, R.; Freudenberger, J.; Holzapfel, B.; Schultz, L.; Fähler, S.It is well known that a variation of lattice constants can strongly influence the functional properties of materials. Lattice constants can be influenced by external forces; however, most experiments are limited to hydrostatic pressure or biaxial stress. Here, we present an experimental approach that imposes a large uniaxial strain on epitaxially grown films in order to tune their functional properties. A substrate made of a ductile metal alloy covered with a biaxially oriented MgO layer is used as a template for growth of epitaxial films. By applying an external plastic strain, we break the symmetry within the substrate plane compared to the as-deposited state. The consequences of 2% plastic strain are examined for an epitaxial hard magnetic Nd2Fe14B film and are found to result in an elliptical distortion of the in-plane anisotropy below the spin-reorientation temperature. Our approach is a versatile method to study the influence of large plastic strain on various materials, as the MgO(001) layer used is a common substrate for epitaxial growth.
- ItemMagnetically induced reorientation of martensite variants in constrained epitaxial Ni-Mn-Ga films grown on MgO(001)(Milton Park : Taylor & Francis, 2008) Thomas, M.; Heczko, O.; Buschbeck, J.; Rößler, U.K.; McCord, J.; Scheerbaum, N.; Schultz, L.; Fähler, S.Magnetically induced reorientation (MIR) is observed in epitaxial orthorhombic Ni-Mn-Ga films. Ni-Mn-Ga films have been grown epitaxially on heated MgO(001) substrates in the cubic austenite state. The unit cell is rotated by 45° relative to the MgO cell. The growth, structure texture and anisotropic magnetic properties of these films are described. The crystallographic analysis of the martensitic transition reveals variant selection dominated by the substrate constraint. The austenite state has low magnetocrystalline anisotropy. In the martensitic state, the magnetization curves reveal an orthorhombic symmetry having three magnetically non-equivalent axes. The existence of MIR is deduced from the typical hysteresis within the first quadrant in magnetization curves and independently by texture measurement without and in the presence of a magnetic field probing micro structural changes. An analytical model is presented, which describes MIR in films with constrained overall extension by the additional degree of freedom of an orthorhombic structure compared to the tetragonal structure used in the standard model.
- ItemMechanisms of stress generation and relaxation during pulsed laser deposition of epitaxial Fe-Pd magnetic shape memory alloy films on MgO(Milton Park : Taylor & Francis, 2008) Edler, Tobias; Buschbeck, Jörg; Mickel, Christine; Fähler, Sebastian; Mayr, S.G.Mechanical stress generation during epitaxial growth of Fe–Pd thin films on MgO from pulsed laser deposition is a key parameter for the suitability in shape memory applications. By employing in situ substrate curvature measurements, we determine the stress states as a function of film thickness and composition. Depending on composition, different stress states are observed during initial film growth, which can be attributed to different misfits. Compressive stress generation by atomic peening is observed in the later stages of growth. Comparison with ex situ x-ray based strain measurements allows integral and local stress to be distinguished and yields heterogeneities of the stress state between coherent and incoherent regions. In combination with cross-sectional TEM measurements the relevant stress relaxation mechanism is identified to be stress-induced martensite formation with (111) twinning.
- ItemEnhanced spin coherence via mesoscopic confinement during acoustically induced transport(Milton Park : Taylor & Francis, 2008) Stotz, J.A.H.; Hey, R.; Santos, P.V.; Ploog, K.H.Long coherence lifetimes of electron spins transported using moving potential dots are shown to result from the mesoscopic confinement of the spin vector. The confinement condition to control electron spin dephasing is governed by the relation between the characteristic spin–orbit length of the electron spins and the dimensions of the dot potential, which governs the electron spin coherence lifetime. The spin–orbit length is a sample-dependent parameter determined by the Dresselhaus contribution to the spin–orbit coupling and can be predictably controlled by varying the sample geometry. We further show that the coherence lifetime of the electron spins is independent of the local carrier densities within each potential dot, which suggests the possibility of coherent, long-range transport of single electron spins.
- ItemFermi surface nesting in several transition metal dichalcogenides(Milton Park : Taylor & Francis, 2008) Inosov, D.S.; Zabolotnyy, V.B.; Evtushinsky, D.V.; Kordyuk, A.A.; Büchner, B.; Follath, R.; Berger, H.; Borisenko, S.V.By means of high-resolution angle-resolved photoelectron spectroscopy (ARPES), we have studied the fermiology of 2H transition metal dichalcogenide polytypes TaSe2, NbSe2 and Cu0.2NbS 2. The tight-binding model of the electronic structure, extracted from ARPES spectra for all three compounds, was used to calculate the Lindhard function (bare spin susceptibility), which reflects the propensity to charge density wave (CDW) instabilities observed in TaSe2 and NbSe 2. We show that though the Fermi surfaces of all three compounds possess an incommensurate nesting vector in the close vicinity of the CDW wave vector, the nesting and ordering wave vectors do not exactly coincide, and there is no direct relationship between the magnitude of the susceptibility at the nesting vector and the CDW transition temperature. The nesting vector persists across the incommensurate CDW transition in TaSe2 as a function of temperature despite the observable variations of the Fermi surface geometry in this temperature range. In Cu0.2NbS2, the nesting vector is present despite different doping levels, which leads us to expect a possible enhancement of the CDW instability with Cu intercalation in the Cu xNbS2 family of materials.
- ItemElectronic structure and magnetic properties of the spin-1/2 Heisenberg system CuSe2O5(Milton Park : Taylor & Francis, 2009) Janson, O.; Schnelle, W.; Schmidt, M.; Prots, Yu; Drechsler, S.-L.; Filatov, S.K.; Rosner, H.A microscopic magnetic model for the spin-1/2 Heisenberg chain compound CuSe2O5 is developed based on the results of a joint experimental and theoretical study. Magnetic susceptibility and specific heat data give evidence for quasi-one-dimensional (1D) magnetism with leading antiferromagnetic (AFM) couplings and an AFM ordering temperature of 17 K. For microscopic insight, full-potential density functional theory (DFT) calculations within the local density approximation (LDA) were performed. Using the resulting band structure, a consistent set of transfer integrals for an effective one-band tight-binding model was obtained. Electronic correlations were treated on a mean-field level starting from LDA (LSDA+U method) and on a model level (Hubbard model). With excellent agreement between experiment and theory, we find that only two couplings in CuSe2O5 are relevant: the nearest-neighbour intra-chain interaction of 165 K and a non-frustrated inter-chain (IC) coupling of 20 K. From a comparison with structurally related systems (Sr2Cu(PO4)2, Bi2CuO4), general implications for a magnetic ordering in presence of IC frustration are made.
- ItemDynamics of graphene growth on a metal surface: A time-dependent photoemission study(Milton Park : Taylor & Francis, 2009) Grüneis, Alexander; Kummer, Kurt; Vyalikh, Denis V.Applying time-dependent photoemission we unravel the graphene growth process on a metallic surface by chemical vapor deposition (CVD). Graphene CVD growth is in stark contrast to the standard growth process of two-dimensional films because it is self-limiting and stops as soon as a monolayer of graphene has been synthesized. Most importantly, a novel phase of metastable graphene was discovered that is characterized by permanent and simultaneous construction and deconstruction. The high quality and large area graphene flakes are characterized by angle-resolved photoemission, proving that they are indeed monolayer and cover the whole 1×1 cm Ni(111) substrate. These findings are of high relevance to the intensive search for reliable synthesis methods for large graphene flakes of controlled layer number.