Browsing by Author "Hradil, K"

Now showing 1 - 12 of 12
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    Development of the magnetic excitations of charge-stripe ordered La2-x Sr (x) NiO4 on doping towards checkerboard charge order
    (Springer, 2013-05-01) Freeman, PG; Giblin, SR; Hradil, K; Mole, RA; Prabhakaran, D
    The magnetic excitation spectrums of charge stripe ordered La2-x Sr (x) NiO4 x = 0.45 and x = 0.4 were studied by inelastic neutron scattering. We found the magnetic excitation spectrum of x = 0.45 from the ordered Ni2+ S = 1 spins to match that of checkerboard charge ordered La1.5Sr0.5NiO4. The distinctive asymmetry in the magnetic excitations above 40meV was observed for both doping levels, but an additional ferromagnetic mode was observed in x = 0.45 and not in the x = 0.4. We discuss the origin of crossover in the excitation spectrum between x = 0.45 and x = 0.4 with respect to discommensurations in the charge stripe structure. © 2013, Springer.
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    Effect of temperature and compositional changes on the phonon properties of Ni-Mn-Ga shape memory alloys
    (Americal Physical Society, 2012-10-11) Ener, S; Neuhaus, J; Petry, W; Mole, RA; Hradil, K; Siewert, M; Gruner, ME; Entel, P; Titov, I; Acet, M
    We report on the vibrational properties of the ferromagnetic shape memory alloy system Ni-Mn-Ga in its stoichiometric Ni(2)MnGa and off-stoichiometric Ni(49)Mn(32)Ga(19) compositions. Elastic and inelastic neutron scattering measurements at different temperatures are presented with a focus on the austenite phase and compared to first-principles calculations. The overall behavior of the full phonon dispersion is similar for both compositions with remarkable exceptions for the TA(2)[xi xi 0] acoustic branch and optical phonon branches. Less dispersion is found in the optical phonons for Ni(49)Mn(32)Ga(19) in the whole reciprocal space when compared to Ni(2)MnGa and is explained by the occupation of regular Ga sites by excess Mn atoms. A pronounced softening in the TA(2)[xi xi 0] phonon branch within the austenite phase is observed in both samples when approaching the martensitic transition. Its location in reciprocal space reveals the martensitic transition mechanism. The austenite L2(1) structure transforms to the tetragonal modulated martensite structure by shuffling (110) planes in the [1 $(1) over bar $0] direction, similarly to what has been observed at the martensitic transitions of the d(1) and d(2) transition metals. Whereas the temperature dependence of the softening of the TA(2)[xi xi 0] phonons in the stoichiometric sample coincides perfectly with the magnetic and structural transitions, this is not the case for the off-stoichiometric sample. Here the relation between the magnetic ordering and the vibrational properties is still an open question. © 2012, American Physical Society.
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    Hidden magnetic excitation in the pseudogap phase of a high-T-c superconductor
    (Natural Publishing Group, 2010-11-11) Li, Y; Baledent, V; Yu, G; Barisic, N; Hradil, K; Mole, RA; Sidis, Y; Steffens, P; Zhao, X; Bourges, P; Greven, M
    The elucidation of the pseudogap phenomenon of the high-transition-temperature (high-T-c) copper oxides-a set of anomalous physical properties below the characteristic temperature T* and above T-c-has been a major challenge in condensed matter physics for the past two decades(1). Following initial indications of broken time-reversal symmetry in photoemission experiments(2), recent polarized neutron diffraction work demonstrated the universal existence of an unusual magnetic order below T* (refs 3, 4). These findings have the profound implication that the pseudogap regime constitutes a genuine new phase of matter rather than a mere crossover phenomenon. They are furthermore consistent with a particular type of order involving circulating orbital currents, and with the notion that the phase diagram is controlled by a quantum critical point(5). Here we report inelastic neutron scattering results for HgBa2CuO4+delta that reveal a fundamental collective magnetic mode associated with the unusual order, and which further support this picture. The mode's intensity rises below the same temperature T* and its dispersion is weak, as expected for an Ising-like order parameter(6). Its energy of 52-56 meV renders it a new candidate for the hitherto unexplained ubiquitous electron-boson coupling features observed in spectroscopic studies(7-10). © 2011, Nature Publishing Group
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    Highly anisotropic anomaly in the dispersion of the copper-oxygen bond-bending phonon in superconducting YBa2Cu3O7 from inelastic neutron scattering
    (American Physical Society, 2011-10-19) Raichle, M; Reznik, D; Lamago, D; Heid, R; Li, Y; Bakr, M; Ulrich, C; Hinkov, V; Hradil, K; Lin, CT; Keimer, B
    Motivated by predictions of a substantial contribution of the "buckling'' vibration of the CuO2 layers to d-wave superconductivity in the cuprates, we have performed an inelastic neutron scattering study of this phonon in an array of untwinned crystals of YBa2Cu3O7. The data reveal a pronounced softening of the phonon at the in-plane wave vector q = (0, 0.3) upon cooling below similar to 105 K, but no corresponding anomaly at q = (0.3,0). Based on the observed in-plane anisotropy, we argue that the electron-phonon interaction responsible for this anomaly supports an electronic instability associated with a uniaxial charge-density modulation and does not mediate d-wave superconductivity. © 2011, American Physical Society.
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    Magnetic excitations of the charge stripe electrons below half doping in La2−xSrxNiO4 (x = 0.45, 0.4)
    (American Physical Society, 2017-02-03) Freeman, PG; Giblin, SR; Hradil, K; Mole, RA; Cermak, P; Prabhakaran, D
    The low energy magnetic excitation spectrum of charge stripe ordered La2-xSrxNiO4, x=0.4 and x=0.45 samples, were studied by neutron scattering. Two excitation modes are observed in both materials, one from the ordered magnetic moments, and a second mode consistent with pseudo-one-dimensional antiferromagnetic excitations of the charge stripe electrons (q-1D). The dispersion of the q-1D excitation follows the same relation as in x=1/3 composition, with even spectral weight in the two counterpropagating branches of the x=0.4 sample, however in the x=0.45 sample only one dispersion branch has any measurable spectral weight. The evolution of the q-1D excitations on doping to the checkerboard charge ordered phase is discussed. ©2017 American Physical Society.
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    Magnetic soft modes in the distorted triangular antiferromagnet alpha-CaCr(2)O(4)
    (American Physical Society, 2012-09-19) Toth, S; Lake, B; Hradil, K; Guidi, T; Rule, KC; Stone, MB; Islam, ATMN
    In this Letter, we explore the phase diagram and excitations of a distorted triangular lattice anti-ferromagnet. The unique two-dimensional distortion considered here is very different from the "isosceles''- type distortion that has been extensively investigated. We show that it is able to stabilize a 120 degrees spin structure for a large range of exchange interaction values, while new structures are found for extreme distortions. A physical realization of this model is alpha-CaCr(2)O(4), which has a 120 degrees structure but lies very close to the phase boundary. This is verified by inelastic neutron scattering which reveals unusual rotonlike minima at reciprocal space points different from those corresponding to the magnetic order. © 2012 The American Physical Society
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    Magnetic soft modes in the distorted triangular antiferromagnet α-CaCr2O4
    (American Physical Society, 2012-09-19) Toth, S; Lake, B; Hradil, K; Guidi, T; Rule, KC; Stone, MB; Islam, ATMN
    In this Letter, we explore the phase diagram and excitations of a distorted triangular lattice anti-ferromagnet. The unique two-dimensional distortion considered here is very different from the "isosceles''- type distortion that has been extensively investigated. We show that it is able to stabilize a 120 degrees spin structure for a large range of exchange interaction values, while new structures are found for extreme distortions. A physical realization of this model is alpha-CaCr2O4, which has a 120 degrees structure but lies very close to the phase boundary. This is verified by inelastic neutron scattering which reveals unusual rotonlike minima at reciprocal space points different from those corresponding to the magnetic order. © 2012, American Physical Society.
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    Observation of phasons in the magnetic shape memory alloy Ni2MnGa
    (IOP Sciences, 2007-03) Shapiro, SM; Vorderwisch, P; Habicht, K; Hradil, K; Schneider, H
    An inelastic neutron scattering study of the lattice dynamics of the martensite phase of the ferromagnetic shape memory alloy, Ni2MnGa, reveals the presence of well-defined phasons associated with the charge density wave (CDW) resulting from Fermi surface (FS) nesting. The velocity and the temperature dependence of the phason are measured as well as the anomalous [110]-TA(2) phonon. © 2007, EPLA
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    Phason mode in the incommensurate martensitic phase of Ni2MnGa - observed by neutron spectroscopy
    (Australian Institute of Physics, 2006-12-04) Vorderwisch, P; Shapiro, SM; Habicht, K; Hradil, K; Schneider, H
    The ferromagnetic shape-memory alloy Ni2MnGa, of technological interest as a magnetically driven actuator, undergoes a martensitic transformation from a cubic austenite to a tetragonally distorted martensite structure. Previous inelastic neutron scattering experiments, performed in the martensite phase, show an anomaly in the phonon dispersion for the [110] transverse acoustic branch TA2 [1]. This anomaly is identified (in ab-initio calculations through examination of Fermi-surface nesting and electron-phonon coupling) as a Kohn anomaly [2] and occurs at a wave vector incommensurate with the underlying lattice. Elastic neutron scattering reveals a modulation of the martensite phase with this incommensurate wave vector [1]. In very recent inelastic experiments, performed with cold- and thermal-neutron triple-axis-spectrometers, in addition to the known acoustic branch a new dispersive branch was found, showing all characteristics of a phason mode. In our interpretation, the new mode is caused by a charge-density-wave in the incommensurately distorted lattice. Inelastic neutron scattering is the only technique able to unambiguously detect a phason branch - and successful examples are rare. [1] P. Vorderwisch and S.M. Shapiro, submitted [2] C. Bungaro et al., Phys. Rev. B 68 (2003) 134104
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    Phonons in a highly-correlated electron system: the heavy-fermion superconductor CeCu2Si2
    (Australian Institute of Physics, 2014-02-04) Loewenhaupt, M; Danilkin, SA; Capogna, L; Schneidewind, A; Stockert, O; Hradil, K
    CeCu2Si2 crystallizes in the tetragonal ThCr2Si2-type structure with 5 atoms in the primitive unit cell. It exhibits non-conventional superconductivity driven by low-energy magnetic excitations [1]. The Ce3+ Hund’s rule J=5/2 ground state is split by the action of the crystalline electric field into 3 doublets, with two excited doublets forming a quasi-quartet at around 30 meV [2]. Except for Raman data [3] no information about the lattice dynamics was available up to date. We therefore performed inelastic neutron scattering at low temperatures (3 K and 10 K) on a large single crystal on the thermal triple-axis spectrometers PUMA (FRM II) and TAIPAN (OPAL) to determine the phonon dispersion relations in the [001/110] plane. The measured dispersion curves will be compared with ab-initio DFT calculations. In addition we could refine the crystal field level scheme resulting in the observation that the excited quasiquartet actually consists of two considerably broadened doublets situated at around 28 meV and 35 meV, respectively.
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    Two characteristic energies in the low-energy magnetic response of the electron-doped high-temperature superconductor Nd(2-x)Ce(x)CuO(4+delta)
    (American Physical Society, 2010-11-11) Yu, G; Li, Y; Motoyama, EM; Hradil, K; Mole, RA; Greven, M
    Neutron scattering for Nd2-xCexCuO4+delta(x approximate to 0.155, T-c=25 K) reveals two distinct magnetic energy scales in the superconducting state: omega(1)approximate to 6.4 meV and omega(2)approximate to 4.5 meV. These magnetic energies agree quantitatively with the B-1g/B-2g and A(1g) features observed in electronic Raman scattering, where the former is believed to indicate the maximum superconducting gap and the origin of the latter has remained unexplained. The data are inconsistent with previous claims of the existence of a magnetic resonance mode near 10 meV, but consistent with a resonance at omega(2) and with the recently established universal ratio of resonance energy to superconducting gap in unconventional superconductors.[G. Yu et al., Nat. Phys. 5, 873 (2009)]. © 2010, American Physical Society
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    Two Ising-like magnetic excitations in a single-layer cuprate superconductor
    (Nature Publishing Group, 2012-05-01) Li, Y; Yu, G; Chan, MK; Baledent, V; Li, Y; Barisic, N; Zhao, X; Hradil, K; Mole, RA; Sidis, Y; Steffens, P; Bourges, P; Greven, M
    There exists increasing evidence that the phase diagram of the high-transition temperature (T(c)) cuprate superconductors is controlled by a quantum critical point. According to one distinct theoretical proposal, on decreasing the hole-carrier concentration a transition occurs to an ordered state with two circulating orbital currents per CuO(2) square. Below the 'pseudogap' temperature T* (T* > T(c)), the theory predicts a discrete order parameter and two weakly-dispersive magnetic excitations in structurally simple compounds which should be measurable by neutron scattering. Indeed, novel magnetic order and one such excitation were recently observed. Here, we demonstrate for tetragonal HgBa(2)CuO(4+delta) the existence of a second excitation with local character, consistent with the theory. The excitations mix with conventional antiferromagnetic fluctuations, which points towards a unifying picture of magnetism in the cuprates that will probably require a multi-band description. © 2012, Nature Publishing Group.