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D. S. Inosov,
G. Friemel,
J. T. Park,
A. C. Walters,
Y. Texier,
Y. Laplace,
J. Bobroff,
V. Hinkov,
D. L. Sun,
Y. Liu,
R. Khasanov,
K. Sedlak,
Ph. Bourges, Y. Sidis,
A. Ivanov,
C. T. Lin,
T. Keller,
B. Keimer
[show abstract]
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ABSTRACT: We investigate magnetic ordering in metallic Ba[Fe(1-x)Mn(x)](2)As(2) and
discuss the unusual magnetic phase, which was recently discovered for Mn
concentrations x > 10%. We argue that it can be understood as a Griffiths-type
phase that forms above the quantum critical point associated with the
suppression of the stripe-antiferromagnetic spin-density-wave (SDW) order in
BaFe2As2 by the randomly introduced localized Mn moments acting as strong
magnetic impurities. While the SDW transition at x = 0, 2.5% and 5% remains
equally sharp, in the x = 12% sample we observe an abrupt smearing of the
antiferromagnetic transition in temperature and a considerable suppression of
the spin gap in the magnetic excitation spectrum. According to our
muon-spin-relaxation, nuclear magnetic resonance and neutron-scattering data,
antiferromagnetically ordered rare regions start forming in the x = 12% sample
significantly above the N\'eel temperature of the parent compound. Upon
cooling, their volume grows continuously, leading to an increase in the
magnetic Bragg intensity and to the gradual opening of a partial spin gap in
the magnetic excitation spectrum. Using neutron Larmor diffraction, we also
demonstrate that the magnetically ordered volume is characterized by a finite
orthorhombic distortion, which could not be resolved in previous diffraction
studies most probably due to its coexistence with the tetragonal phase and a
microstrain-induced broadening of the Bragg reflections. We argue that
Ba[Fe(1-x)Mn(x)](2)As(2) could represent an interesting model spin-glass
system, in which localized magnetic moments are randomly embedded into a SDW
metal with Fermi surface nesting.
05/2013;
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[show abstract]
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ABSTRACT: The magnon dispersion in the charge, orbital and spin ordered phase in La(0.5)Sr(1.5)MnO(4) has been studied by means of inelastic neutron scattering. We find an excellent agreement with a magnetic interaction model basing on the CE-type superstructure. The magnetic excitations are dominated by ferromagnetic exchange parameters revealing a nearly-one dimensional character at high energies. The nearest neighbor ferromagnetic interaction in La(0.5)Sr(1.5)MnO(4) is significantly larger than the one in the metallic ferromagnetically ordered manganites. The large ferromagnetic interaction in the charge/orbital ordered phase appears to be essential for the capability of manganites to switch between metallic and insulating phases. Comment: 4 pages, 3 figures
Physical Review Letters 02/2013; 96:257201. · 7.37 Impact Factor
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[show abstract]
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ABSTRACT: We present elastic and inelastic neutron scattering data on LaSrFeO$_4$. We
confirm the known magnetic structure with the magnetic moments lying in the
tetragonal basal plane, but contrarily to previous reports our macroscopic and
neutron diffraction data do not reveal any additional magnetic phase transition
connected to a spin reorientation or to a redistribution of two irreducible
presentations. Our inelastic neutron scattering data reveals the magnon
dispersion along the main-symmetry directions [0 $\xi$ 0] and [$\xi$ -$\xi$ 0].
The dispersion can be explained within linear spin-wave theory yielding an
antiferromagnetic nearest-neighbour interaction parameter $J_{1}=7.4(1)$ meV
and a next-nearest neighbour interaction parameter $J_{2}=0.4(1)$ meV. The
dispersion is gapped with the out-of-plane anisotropy gap found at
$\Delta_{out}=5.26(2)$ meV, while evidence is present that the in-plane
anisotropy gap lies at lower energies, where it cannot be determined due to
limited instrument resolution.
01/2013;
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Yuan Li,
G. Yu,
M. K. Chan,
V. Baledent,
Yangmu Li,
N. Barisic,
X. Zhao,
K. Hradil,
R. A. Mole, Y. Sidis,
P. Steffens,
P. Bourges,
M. Greven
[show abstract]
[hide abstract]
ABSTRACT: There exists increasing evidence that the phase diagram of the
high-transition temperature (Tc) cuprate superconductors is controlled by a
quantum critical point. One distinct theoretical proposal is that, with
decreasing hole-carrier concentration, a transition occurs to an ordered state
with two circulating orbital currents per CuO2 square. Below the 'pseudogap'
temperature T* (T* > Tc), the theory predicts a discrete order parameter and
two weakly-dispersive magnetic excitations in structurally simple compounds
that should be measurable by neutron scattering. Indeed, novel magnetic order
and one such excitation were recently observed. Here, we demonstrate for
tetragonal HgBa2CuO4+d the existence of a second excitation with local
character, consistent with the theory. The excitations mix with conventional
antiferromagnetic fluctuations, which points toward a unifying picture of
magnetism in the cuprates that will likely require a multi-band description.
10/2012;
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[show abstract]
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ABSTRACT: Polarized elastic neutron scattering measurements have been performed in the
bilayer copper oxide system Bi2Sr2CaCu2O8+d, providing evidence for an intra
unit cell (IUC) magnetic order inside the pseudo-gap state. That shows time
reversal symmetry breaking in that state as already reported in Bi2Sr2CaCu2O8+d
through dichroism in circularly polarized photoemission experiments. The
magnetic order displays the same characteristic features as the one previously
reported for monolayer HgBa2CuO4+d and bilayer YBa2Cu3O6+x, demonstrating that
this genuine phase is ubiquitous of the pseudo-gap of high temperature copper
oxide materials.
07/2012;
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[show abstract]
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ABSTRACT: In this paper, we first give a concise overview of recent experimental and theoretical work dealing with “electronic liquid-crystal
states” which spontaneously break different symmetries of the CuO2 layers of high-T
c cuprates, with an emphasis on evidence in the spin excitation spectrum. Then we describe the importance of using twin-free
samples to look for evidence for fourfold symmetry breaking in the spectrum and explain the preparation procedure to obtain
such samples. We present inelastic neutron scattering results for moderately underdoped YBa2Cu3O6.6(T
c = 61 K) and nearly optimally doped YBa2Cu3O6.85(T
c = 89 K). In YBa2Cu3O6.6, the dispersion topology changes when heating above T
c from an hourglass shape with constricted, commensurate resonance peak to a “Y”-shape without resonance anomaly. This change,
and the fact that the low-energy signal above T
c can be described by an incommensurate, quasi-one-dimensional distribution, indicates a competition of superconductivity with
an electronic liquid-crystal state. We then show a striking analogy between the difference signal I(5 K) − I(70 K) and the downward dispersing resonance mode in YBa2Cu3O6.85. We therefore argue that a resonance mode only emerges below T
c, irrespective of the doping level. We finally discuss the implications of our results for the different scenarios invoked
to explain the electronic liquid-crystal state in cuprates.
The European Physical Journal Special Topics 04/2012; 188(1):113-129. · 1.56 Impact Factor
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N Qureshi,
P Steffens,
Y Drees,
A C Komarek,
D Lamago, Y Sidis,
L Harnagea,
H-J Grafe,
S Wurmehl,
B Büchner,
M Braden
[show abstract]
[hide abstract]
ABSTRACT: Magnetic correlations in superconducting LiFeAs were studied by elastic and by inelastic neutron-scattering experiments. There is no indication for static magnetic ordering, but inelastic correlations appear at the incommensurate wave vector (0.5±δ,0.5-/+δ,0) with δ~0.07 slightly shifted from the commensurate ordering observed in other FeAs-based compounds. The incommensurate magnetic excitations respond to the opening of the superconducting gap by a transfer of spectral weight.
Physical Review Letters 03/2012; 108(11):117001. · 7.37 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: The excitation spectrum in single-layered Nd(0.33)Sr(1.67)MnO4 and
Pr(0.33)Ca(1.67)MnO4 resembles the hourglass-like excitation dispersion seen in
various cuprates superconductors. However, our spin-wave dispersion in
Pr(0.33)Ca(1.67)MnO4, which exhibits a large correlation length of the magnetic
order, showing outward-dispersing branches starting from the incommensurate
zone-centres. The magnetic correlation length is identified as the decisive
parameter to suppress this branches and generating an correct hourglass shape.
12/2011;
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N. Qureshi,
P. Steffens,
Y. Drees,
A. C. Komarek,
D. Lamago, Y. Sidis,
L. Harnagea,
H. -J. Grafe,
S. Wurmehl,
B. Büchner,
M. Braden
[show abstract]
[hide abstract]
ABSTRACT: Magnetic correlations in superconducting LiFeAs were studied by elastic and
by inelastic neutron scattering experiments. There is no indication for static
magnetic ordering but inelastic correlations appear at the incommensurate wave
vector (0:5+-d 0:-+d 0) with d=0.07 slightly shifted from the commensurate
ordering observed in other FeAs-based compounds. The incommensurate magnetic
excitations respond to the opening of the superconducting gap by a transfer of
spectral weight.
08/2011;
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[show abstract]
[hide abstract]
ABSTRACT: Inelastic neutron scattering experiments have been performed on single
crystals of Nd$_{1-x}$Sr$_{x}$MnO$_{3}$ with x$\approx$0.5. Colossal
magnetoresistance (CMR) in the manganites arises from the interplay between a
ferromagnetic metallic and antiferromagnetic charge and orbital ordered
insulating state. Therefore, it appears important to compare these phases
concerning their underlying magnetic interaction parameters. Our investigations
of the spin-wave disperion in the AFM ordered state of
Nd$_{0.5}$Sr$_{0.5}$MnO$_{3}$ exhibits a strongly anisotropic stiffness. The
sign of the anisotropy is characteristic for the site-centered model for charge
and orbital ordering in half-doped manganites. Within this model, linear
spin-wave theory yields a perfect description of the experimental dispersion.
Furthermore, magnetic excitations in the ferromagnetic metallic state of
Nd$_{1-x}$Sr$_{x}$MnO$_{3}$ with x=0.49 and x=0.50 exhibit nearly the same
magnon dispersion which can be described with a Heisenberg model including
nearest-neighbor interactions.
07/2011;
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[show abstract]
[hide abstract]
ABSTRACT: Overdoped La0.42Sr1.58MnO4 exhibits a complex ordering of charges, orbitals, and spins. Neutron diffraction experiments reveal three incommensurate and one commensurate order parameters to be tightly coupled. The position and the shape of the distinct superstructure scattering as well as higher-order signals are inconsistent with a harmonic charge and spin-density-wave picture but point to a stripe arrangement in which ferromagnetic zigzag chains are disrupted by excess Mn(4+).
Physical Review Letters 04/2011; 106(15):157201. · 7.37 Impact Factor
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[show abstract]
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ABSTRACT: We report a polarized neutron scattering study of the orbital-like magnetic
order in strongly underdoped ${\rm YBa_2Cu_3O_{6.45}}$ and ${\rm
YBa_2(Cu_{0.98}Zn_{0.02})_3O_{6.6}}$. Their hole doping levels are located on
both sides of the critical doping $p_{MI}$ of a metal-insulator transition
inferred from transport measurements. Our study reveals a drop down of the
orbital-like order slightly below $p_{MI}$ with a steep decrease of both the
ordering temperature $T_{mag}$ and the ordered moment. Above $p_{MI}$,
substitution of quantum impurities does not change $T_{mag}$, whereas it lowers
significantly the bulk ordered moment. The modifications of the orbital-like
magnetic order are interpreted in terms of a competition with electronic liquid
crystal phases around $p_{MI}$. This competition gives rise to a mixed magnetic
state in ${\rm YBa_2Cu_3O_{6.45}}$ and a phase separation in ${\rm
YBa_2(Cu_{0.98}Zn_{0.02})_3O_{6.6}}$.
12/2010;
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Yuan Li,
V Balédent,
G Yu,
N Barišić,
K Hradil,
R A Mole, Y Sidis,
P Steffens,
X Zhao,
P Bourges,
M Greven
[show abstract]
[hide abstract]
ABSTRACT: 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. Following initial indications of broken time-reversal symmetry in photoemission experiments, 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. Here we report inelastic neutron scattering results for HgBa(2)CuO(4+δ) 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. Its energy of 52-56 meV renders it a new candidate for the hitherto unexplained ubiquitous electron-boson coupling features observed in spectroscopic studies.
Nature 11/2010; 468(7321):283-5. · 36.28 Impact Factor
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[show abstract]
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ABSTRACT: We present a neutron triple-axis and resonant spin-echo spectroscopy study of the spin correlations in untwinned YBCO crystals with x= 0.3, 0.35, and 0.45 as a function of temperature and magnetic field. As the temperature T approaches 0, all samples exhibit static incommensurate magnetic order with propagation vector along the a-direction in the CuO2 planes. The incommensurability delta increases monotonically with hole concentration, as it does in LSCO. However, delta is generally smaller than in LSCO at the same doping level. The intensity of the incommensurate Bragg reflections increases with magnetic field for YBCO(6.45) (superconducting Tc = 35 K), whereas it is field-independent for YBCO(6.35) (Tc = 10 K). These results suggest that YBCO samples with x ~ 0.5 exhibit incommensurate magnetic order in the high fields used for the recent quantum oscillation experiments on this system, which likely induces a reconstruction of the Fermi surface. We present neutron spin-echo measurements (with energy resolution ~ 1 micro-eV) for T > 0 that demonstrate a continuous thermal broadening of the incommensurate magnetic Bragg reflections into a quasielastic peak centered at excitation energy E = 0, consistent with the zero-temperature transition expected for a two-dimensional spin system with full spin-rotation symmetry. Measurements on YBCO(6.45) with a triple-axis spectrometer (with energy resolution ~ 100 micro-eV) yield a crossover temperature T_SDW ~ 30 K for the onset of quasi-static magnetic order. Upon further heating, the wavevector characterizing low-energy spin excitations approaches the commensurate antiferromagnetic wave vector, and the incommensurability vanishes in an order-parameter-like fashion at an "electronic liquid-crystal" onset temperature T_ELC ~ 150 K. Both T_SDW and T_ELC increase continuously as the Mott-insulating phase is approached with decreasing doping level. Comment: to appear in a special issue on "Fermiology of Cuprates" of the New Journal of Physics
08/2010;
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J. T. Park,
D. S. Inosov,
A. Yaresko,
S. Graser,
D. L. Sun,
Ph. Bourges, Y. Sidis,
Yuan Li,
J.-H. Kim,
D. Haug,
A. Ivanov,
K. Hradil,
A. Schneidewind,
P. Link,
E. Faulhaber,
I. Glavatskyy,
C. T. Lin,
B. Keimer,
V. Hinkov
[show abstract]
[hide abstract]
ABSTRACT: We study the symmetry of spin excitation spectra in 122-ferropnictide superconductors by comparing the results of first-principles calculations with inelastic neutron-scattering (INS) measurements on BaFe1.85Co0.15As2 and BaFe1.91Ni0.09As2 samples that exhibit neither static magnetic phases nor structural phase transitions. In both the normal and superconducting (SC) states, the spectrum lacks the three-dimensional 42/m screw symmetry around the (1/21/2L) axis that is implied by the I4/mmm space group. This is manifest both in the in-plane anisotropy of the normal- and SC-state spin dynamics and in the out-of-plane dispersion of the spin-resonance mode. We show that this effect originates from the higher symmetry of the magnetic Fe sublattice with respect to the crystal itself, hence the INS signal inherits the symmetry of the unfolded Brillouin zone (BZ) of the Fe sublattice. The in-plane anisotropy is temperature independent and can be qualitatively reproduced in normal-state density-functional-theory calculations without invoking a symmetry-broken (“nematic”) ground state that was previously proposed as an explanation for this effect. Below the SC transition, the energy of the magnetic resonant mode ωres, as well as its intensity and the SC spin gap inherit the normal-state intensity modulation along the out-of-plane direction L with a period twice larger than expected from the body-centered-tetragonal BZ symmetry. The amplitude of this modulation decreases at higher doping, providing an analogy to the splitting between even and odd resonant modes in bilayer cuprates. Combining our and previous data, we show that at odd L a universal linear relationship ℏωres≈4.3 kBTc holds for all the studied Fe-based superconductors, independent of their carrier type. Its validity down to the lowest doping levels is consistent with weaker electron correlations in ferropnictides as compared to the underdoped cuprates.
Phys. Rev. B. 07/2010; 82(13).
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A Suchaneck,
V Hinkov,
D Haug,
L Schulz,
C Bernhard,
A Ivanov,
K Hradil,
C T Lin,
P Bourges,
B Keimer, Y Sidis
[show abstract]
[hide abstract]
ABSTRACT: We report an inelastic-neutron-scattering and muon-spin-relaxation study of the effect of 2% spinless (Zn) impurities on the magnetic order and dynamics of YBa(2)Cu(3)O(6.6), an underdoped high-temperature superconductor that exhibits a prominent spin pseudogap in its normal state. Zn substitution induces static magnetic order at low temperatures and triggers a large-scale spectral-weight redistribution from the magnetic resonant mode at 38 meV into uniaxial, incommensurate spin excitations with energies well below the spin pseudogap. These observations indicate a competition between incommensurate magnetic order and superconductivity close to a quantum critical point. Comparison to prior data on La(2-x)Sr(x)CuO(4) suggests that this behavior is universal for the layered copper oxides and analogous to impurity-induced magnetic order in one-dimensional quantum magnets.
Physical Review Letters 07/2010; 105(3):037207. · 7.37 Impact Factor
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ABSTRACT: In high-temperature copper oxide superconductors, a novel magnetic order associated with the pseudogap phase has been identified in two different cuprate families over a wide region of temperature and doping. We report here the observation below 120 K of a similar magnetic ordering in the archetypal cuprate La(2-x)Sr(x)CuO4 (LSCO) system for x=0.085. In contrast with the previous reports, the magnetic ordering in LSCO is only short range with an in-plane correlation length of ∼10 A and is bidimensional (2D). Such a less pronounced order suggests an interaction with other electronic instabilities. In particular, LSCO also exhibits a strong tendency towards stripes ordering at the expense of the superconducting state.
Physical Review Letters 07/2010; 105(2):027004. · 7.37 Impact Factor
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[show abstract]
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ABSTRACT: We present a comprehensive inelastic neutron scattering study of the magnetic excitations in twin-free YBa(2)Cu(3)O(6.6) (Tc=61 K) for 5 K < T < 290 K. Taking full account of the instrumental resolution, we derive analytical model functions for the magnetic susceptibility chi''(Q,omega) at T = 5 K and 70 K in absolute units. Our models are supported by previous results on similar samples and are valid at least up to excitation energies of omega = 100 meV. The detailed knowledge of chi''(Q,omega) permits quantitative comparison to the results of complementary techniques including angle-resolved photoemission spectroscopy (ARPES), as demonstrated in Dahm et al., Nature Phys. 5, 217, (2009). Based on accurate modeling of the effect of the resolution function on the detected intensity, we determine important intrinsic features of the spin excitation spectrum, with a focus on the differences above and below Tc. In particular, at T = 70 K the spectrum exhibits a pronounced twofold in-plane anisotropy at low energies, which evolves towards fourfold rotational symmetry at high energies, and the relation dispersion is "Y"-shaped. At T = 5 K, on the other hand, the spectrum develops a continuous, downward-dispersing "resonant" mode with weaker in-plane anisotropy. We understand this topology change as arising from the competition between superconductivity and the same electronic liquid-crystal state as observed in YBa(2)Cu(3)O(6.45). We discuss our data in the context of different theoretical scenarios suggested to explain this state. Comment: 19 pages, 19 figures
06/2010;
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[show abstract]
[hide abstract]
ABSTRACT: In high temperature copper oxides superconductors, a novel magnetic order associated with the pseudogap phase has been identified in two different cuprate families over a wide region of temperature and doping. We here report the observation below 120 K of a similar magnetic ordering in the archetypal cuprate ${\rm La_{2-x}Sr_xCuO_4}$ (LSCO) system for x=0.085. In contrast to the previous reports, the magnetic ordering in LSCO is {\it\bf only} short range with an in-plane correlation length of $\sim$ 10 \AA\ and is bidimensional (2D). Such a less pronounced order suggests an interaction with other electronic instabilities. In particular, LSCO also exhibits a strong tendency towards stripes ordering at the expense of the superconducting state. Comment: 4 figures, submitted to Phys. Rev. Lett
05/2010;
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[show abstract]
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ABSTRACT: By inelastic neutron scattering, we have analyzed the magnetic correlations in the paramagnetic metallic region of the series Ca2-xSrxRuO4, 0.2⩽x⩽0.62. We find different contributions that correspond to two-dimensional ferromagnetic fluctuations and to fluctuations at incommensurate wave vectors Q1IC=(0.11,0,0), Q2IC=(0.26,0,0), and QαβIC=(0.3,0.3,0). These components constitute the measured response as a function of the Sr concentration x, of the magnetic field, and of the temperature. A generic model is applicable to metallic Ca2-xSrxRuO4 close to the Mott transition, in spite of their strongly varying physical properties. The amplitude, characteristic energy, and width of the incommensurate components vary only slightly as functions of x, but the ferromagnetic component depends sensitively on concentration, temperature, and magnetic field. While ferromagnetic fluctuations are very strong in Ca1.38Sr0.62RuO4 with a low characteristic energy of 0.2 meV at T=1.5 K, they are strongly suppressed in Ca1.8Sr0.2RuO4, but reappear upon the application of a magnetic field, and form a magnon mode above the metamagnetic transition. The inelastic neutron scattering results document how the competition between ferromagnetic and incommensurate antiferromagnetic instabilities governs the physics of this system.
Phys. Rev. B. 04/2010; 83(5).
