Wednesday 10 July 2013, Strathblane & Cromdale Halls, 16:30-18:30
Poster session B - Pnictides, cuprates and ruthenates
P.171 Direct evidence of nuclear spin waves in Nd2CuO4 by neutron spin-echo spectroscopy
T Chatterji1, O Holderer2 and H Schneider2
1
Institut Laue-Langevin, France, 2Forschungszentrum Juelich, Germany
The coupling of nuclear spins through Suhl-Nakamura indirect interaction gives rise to nuclear spin excitations. Each
nuclear spin feels the presence of the electronic spin on its own ion through the effective hyperfine coupling AI.S,
where A is the hyperfine constant and I and S are nuclear and electronic spins. The electronic spins of the ions are
coupled by exchange interaction. An interaction between the nuclear spins therefore arises via the low-lying excited
states (spin waves) of the electronic systems as intermediate states. We have investigated the dispersion of
nuclear spin waves in Nd2CuO4with the spin-echo spectrometer at millikelvin temperatures. We observed oscillating
signals of S(q,t)/S(q,0) corresponding to the inelastic scattering from the nuclear spin waves with energy of the
order of 1.5 μeV. Our results show the existence of considerable dispersion of the nuclear spin waves at T = 40 mK.
A fit of the data with the spin wave dispersion formula gave the Suhl-Nakamura interaction range to be of the order
of 80Å.
P.172 Impact of sulfur substitution on magnetic order in quasi two-dimensional Fe0.5TiS2-ySey intercalated
compounds
A Gubkin1, E Sherokalova2, N Selezneva2, L Keller3 and N Baranov4
1
Institute for metal physics, UB RAS, Russia 2Ural Federal University, Russia, 3Paul Scherrer Institut,
Switzerland, 4Ural Federal University and Institute for metal physics, UB RAS, Russia
The transition (T) metal dichalcogenides MxTX2 (X = S, Se, Te) intercalated by magnetoactive (M) atoms can be
considered as an analogue of artificial multi-layer structures since the M layers are separated by non-magnetic
layers [1]. We present the results of the neutron diffraction study on the Fe0.5TiS2-ySey powder samples (y = 0, 0.5,
1, 1.5, 2) in order to answer the question of how the chalcogen type in TX2 matrixes influences the magnetic order
within the subsystem of intercalated Fe atoms. It has been found that all the compounds investigated exhibit an
antiferromagnetic (AF) order below their Neel temperatures. The Fe0.5TiS2 compound exhibit the incommensurate
spin wave structure with the propagation vector k = (μ 0 ν), where μ ≈ 0.26 and ν ≈ 0.23 just below the Neel
temperature at T=130 K. This magnetic structure locks-in to the commensurate one with the propagation vector k =
(0.25 0 0.25) on cooling below T = 110 K. Unlike Fe0.5TiS2 having a quadruplicated AF magnetic structure, the
compounds with a high Se content (x ≥ 1) exhibit the AF structure with the doubled magnetic unit cell along a and c
crystallographic directions of the monoclinic crystal lattice (I2/m space group). The dramatic changes of the AF
structure with the substitution in Fe0.5TiS2-ySey are observed to occur at the Se concentrations around x = 0.5, which
can be associated mainly with the change in exchange interactions owing to the growth of interatomic distances
with increasing Se content. This work was supported by the RFBR (projects No 13-02-00364 and 13-02-96038)
and by the program of the Ural Branch of RAS (project No 12-T-2-1012).
[1]
Inoue M, Hughes H P and Yoffe A D 1989 Adv. Phys. 38 565
ICNS 2013 International Conference on Neutron Scattering
P.173 Phonons and magnetic excitations of a large superconducting crystal of Ca10Pt4As8 (Fe1-xPtxAs)10
K Ikeuchi1, R Kajimoto1, M Sato1, Y Kobayashi2, K Suzuki2, M Itoh2, P Bourges3, A Christianson4, H Nakamura5 and M
Machida5
1
Comprehensive Research Organization for Science and Society, Japan, 2Nagoya University, Japan, 3Laboratoire
Léon Brillouin, France, 4Oak Ridge National Laboratory, USA 5Japan Atomic Energy Agency, Japan
By means of neutron inelastic scattering, magnetic excitation spectra χ"(Q,ω) and phonons were studied on a large
single crystal of Ca10Pt4As8 (Fe1-xPtxAs)10 with superconducting transition temperature Tc of ~33 K. The peak profile
of χ"(Q,ω) around the reciprocal point Q = (1/2, 1/2, 0) of the pseudo tetragonal cell was observed at several
fixed values of w and temperature T. The ω dependence of the peak intensity was also studied at several T values.
We also measured the ω dependence of the peak intensity of χ"(Q,ω) at Q =(1/2, 1/2, 2). From the
data, χ"(Q,ω) is enhanced by superconductivity. The T dependence of the in-plane TA phonon, which corresponds
to the elastic constant C66, indicates softening similar to that observed for a slightly overdoped superconducting
Ba(Fe1-xCox)2As2. The phonons measured at the M points of the pseudo tetragonal lattice under a condition to see
mainly in-plane optical phonons, we observed significant enhancement of the phonon density of states at low
temperatures in the region 35<ω<40 meV, suggesting that the in-plane phonons possibly related to the FeAs inplane motions behave in an anomalous way. Based on these data, we present arguments from the view point which
of the spin fluctuation and orbital fluctuation is primarily relevant to the occurrence of the superconductivity.
P.174 Spin resonance mode in K0.8Fe1.6Se2 by neutron inelastic scattering
R Ishii1, H Furukawa1, A Cameron2, L Lemberger2, E Blackburn2, E Forgan2, L Debeer-Schmitt3, C-H Lee4, K Kihou4, H
Eisaki4, M Nakajima5, S Uchida5, J White6 and K Littrell3
1
Ochanomizu University, Japan, 2University of Birmingham, UK, 3Oak Ridge National Laboratory, USA, 4 National
Institute of Advanced Industrial Science, Japan 5University of Tokyo, Japan, 6Paul Scherrer Institute, Switzerland
To determine a mechanism of unconventional superconductivity, it is important to investigate the role of spins in the
system. Presently, for iron-based superconductors (SCs), s±-wave pairing symmetry originating from interband
scattering is widely accepted theoretically and experimentally. More specifically, the system has hole and electron
Fermi surfaces (FSs) at the zone center and the corner of the Brillion zone. Then, interband scattering between those
FSs is believed to play a key role for realization of its superconductivity.
Recently, K0.8Fe1.6Se2 (KFS) was discovered as a new iron-based SC and it has simulated much activity because of
the absence of arsenic. However, conflicting evidence for the superconducting symmetry of KFS comes from
experimental results by angle-resolved photoemission spectroscopy (ARPES) and NMR measurements. If interband
scattering could be detected in KFS by inelastic scattering measurements, s±-wave pairing symmetry becomes a
more reasonable scenario for this system, and maintains the idea that interband scattering is the key mechanism
for all iron-based SCs.
We succeeded in growing single crystals of KFS with a total mass of 5g and Tc 32 K, and performed inelastic
neutron scattering measurements at IN22, ILL in France. We observed interband scattering in KFS at q (0.25, 0.5,
0.5) andΔE 12.7 meV, while no peaks were observed at q=(0, 0.5, 0.5) and q=(0.5, 0.5, 0.5) in agreement with
Rb2Fe4Se5. This resonant feature disappears around Tc, which reveals that this feature related to its superconducting
state.
ICNS 2013 International Conference on Neutron Scattering
P.175 Vortex lattice measurements on BaFe2(As,P)2 by the SANS technique
R Ishii1, H Furukawa1, A Cameron2, L Lemberger2, E Blackburn2, E Forgan2, L Debeer-Schmitt3, K Littrell3, J White4, M
Nakajima5, S Uchida5, Ko Kihou6, C-H Lee6, A Iyo6 and H Eisaki6
1
Ochanomizu University, Japan, 2University of Birmingham, UK, 3Oak Ridge National Laboratory, USA, 4Paul Scherrer
Institute, Switzerland, 5University of Tokyo, Japan 6 National Institute of Advanced Industrial Science, Japan
Small angle neutron scattering (SANS) experiments on vortex lattice (VL) have been proven to be a useful tool to
investigate the superconducting state by imaging the VL state. The VL state is much affected by the underlying
superconducting properties of host crystal, so SANS results can give information about the superconducting gap
structure and pairing state. Recently, the notable exception showing ordered diffraction patterns was obtained from
KFe2As2 and BaFe2(As/P)2 (BFAP), which have provided much clear physical pictures of the pnictides than those
have been obtained from ring-like diffraction patterns due to disorder. Here, we will report an observation of VL on
BFAP by SANS measurements under H//c up to 17 T, and discuss its superconducting gap structure and pairing
symmetry.
Figures 1 (a)-(f) show VL patterns observed at 2 K. The double hexagonal spots were observed above 3 T, which
were derived from two domain hexagonal VLs on tetragonal symmetry. On the other hand, strong 4 spots appeared
along diagonal directions below 1 T. We speculate a possibility of a phase transition between 1 T and 3 T, because
the mirror axis of VL changes from diagonal direction to horizontal/vertical direction. T-dependence of integrated
intensity shows nodal gap behavior at 0.3 T. As increasing fields, however, we found that the s + nodal gap behavior
has disappeared at 5 T and the full gap component is left.
P.176 Superstructures in superoxygenated La2CuO4+y
P Jensen1, L Udby1, N H Andersen2, M J Gutmann3, M Frontzek4, J-C Grivel2, B O Wells5 and K Lefmann1
1
University of Copenhagen, Denmark, 2Technical University of Denmark, Denmark, 3ISIS Facility, STFC Rutherford
Appleton Laboratory, UK 4Paul Scherrer Institute, Switzerland, 5University of Connecticut, USA
The super-oxygenated high-TC superconductor (HTSC) La2CuO4+y may be a key model system to understand the
complex interplay between the superconducting and magnetic phases in the HTSC cuprates. The interstitial oxygen
(y) are mobile even at low temperature, allowing the system to reach different equilibrium configurations with
respect to the charge carrying doped holes [1].
A high degree of oxygen order seems to promote superconductivity up to an onset TC above 40 K, but at the same
time demote magnetism [2]. The intercalation process creates rich superstructure patterns as seen by diffraction.
Further, very recent evidence from X-ray microstructure diffraction points to particularly two types of superstructures
which influence the TC [3].
Here we present novel results of the superstructures in La2CuO4+y. Measurements have been done on a single crystal
using the neutron time-of-flight single crystal diffractometer SXD at ISIS, as well as the cold neutron powder
diffractometer DMC at PSI. Neutron diffraction was used because of its inherent sensitivity to the light interstitial
oxygen in contrast to X-ray diffraction. The data will be used to refine the ordering of the interstitial oxygen within the
crystal lattice, which will improve the possibilities for testing theories describing the coupling between magnetism
and HTSC.
[1]
[2]
[3]
Mohottala et al, Nat Mat 5 (2006)
Lee et al, PRB 69 (2004)
Poccia et al, PNAS 109 (2012)
ICNS 2013 International Conference on Neutron Scattering
P.177 Magnetism and magnetic structure of the quasi-2D layered Ru oxide Sr3-xCaxRu2O7
S Katano1, K Iwata1, Y Yoshida2 and N Aso3
1
Saitama University 2National Institute of Advanced Industrial Science and Technology (AIST), 3University of the
Ryukyus, Japan
Ruddlesden-Popper type perovskite ruthenates (Sr,Ca)n + 1RunO3n + 1 have provided a wide variety of novel physical
properties. Sr3-xCaxRu2O7, the system n=2, exhibits enhanced paramagnetic metals (x<0.5), ferromagnetically
correlated metals (0.5<x<1.2), and antiferromagnetic two-dimensional (2D) metals (1.2<x< 3). In the
antiferromagnetic phase with x≈1-2, the magnetization data at low temperatures show a clear hysteresis, suggesting
that the ground state is canted antiferromagnetic. The magnetic susceptibility indicates that the magnetic easy axis
continuously changes from the ab-plane to the c-axis with increasing Sr content. By neutron diffraction experiments
these magnetic structures have been determined. In Ca3Ru2O7 (x=3) the magnetic moments align ferromagnetcally
in the double layers and antiferromagnetically between the layers below the Néel temperature TN of 56 K. The
magnetic moment orients along the a-axis between 48 K and 56 K, but changes to the b-axis below 48 K. At this
temperature of 48 K, the transition from the normal metal to the 2D metal and the crystal structural phase transition
occur simultaneously. With Sr content, the ferromagnetic intensities grow rather, corresponding to the results of the
magnetic susceptibility and the magnetization. The enhanced intensities along the c-axis observed in x=1.25
indicate that the canted antiferromagnetism is certainly induced in the compounds around this concentration.
P.178 Crystal structures and physical properties of the quasi-2D layered Ru oxide (Sr,Ca)3Ru2O7
S Katano1, K Iwata1 and Y Yoshida2
1
Saitama University, 2National Institute of Advanced Industrial Science and Technology (AIST), Japan
(Sr,Ca)n + 1RunO3n + 1, Ruddlesden-Popper type perovskite ruthenates, have provided many novel physical properties ferromagnetic metal, antiferromagnetic insulator, superconductivity and so forth. The system of n=2, Sr3-xCaxRu2O7
exhibits enhanced paramagnetic metals (x<0.5), ferromagnetically correlated metals (0.5<x<1.2), and
antiferromagnetic two-dimensional (2D) metals (1.2<x< 3). Crystal structures of compounds with x=1.0, 1.5 and
2.0 have been studied in detail with high-resolution neutron powder diffraction from room temperature to 12
K. These data can be analyzed quite well by a Rietveld analysis with the space groups of Bbcb for x=1.0, and
Bb21m for x=1.5 and 2.0. For x=1.0, all lattice constants decrease with decreasing temperature in contrast with
antiferromagnetic metals with higher Ca contents. The rotation angle of its RuO6 octahedrons increase from that of
paramagnetic Sr3Ru2O7, indicating an enhancement of ferromagnetic correlations. For x=1.5 and 2.0, the lattice
constants a and b increase at low temperatures, corresponding with their antiferromagnetic transition. In these
compounds the RuO6 octahedrons change from rotation to tilting, which also correlates to their antiferromagnetic
ordering. Characteristics of the crystal structure of the system are discussed in relation to their fascinating physical
properties.
ICNS 2013 International Conference on Neutron Scattering
P.179 Field angle dependence of vortex lattice structure and anisotropic penetration depth on Kfe2As2 studied by
SANS technique
H Kikuchi1, A Cameron2, A Holmes2, R Heslop2, E Forgan2, L Debeer-Schmitt3, J White4, J Gavilano4, K Kihou5, Chul-H
Lee5, A Iyo5, H Eisaki5, H Fukazawa6, Y Kohori6 and H Kawano-Furukawa1
1
Ochanomizu University, Japan, 2University of Birmingham, UK, 3Oak Ridge National Laboratory, USA, 4ETH Zurich
and Paul Scherrer Institute, Switzerland, 5National Institute of Advanced Industrial Science and Technology and
Japan Science and Technology Agency, Japan, 6Chiba University and Japan Science and Technology Agency, Japan
Since the discovery of the new Fe-based superconductors, much effort has been expended to reveal the nature of
these exotic materials. Kfe2As2 is the end member of the hole-doped 122 Fe-based superconductor (Ba,K)Fe2As2
where the Ba is completely replaced by K. All Fe-based superconductors are type-II materials, so that over a wide
range of applied magnetic fields, the magnetic flux enters as vortex lines. Due to their mutual repulsion, the vortices
form a periodic array: the vortex lattice (VL). Small angle neutron scattering (SANS) is an ideal technique for
studying the magnetic field profile of the superconducting vortices and it gives some important information about
the characteristic properties of the superconductor such as penetration depth, coherence length and nodal state
etc.
Using SANS, the anisotropy of the VL in Kfe2As2 was studied. Well-ordered VL scattering patterns were measured
with fields applied in directions between H ∥c and the basal plane of the tetragonal crystal structure, rotating either
towards [100] or [110]. Slightly distorted hexagonal patterns were observed when H ∥c. However, the scattering
pattern distorted strongly as the field was rotated away from the c-axis. In low field, the arrangement of vortices is
strongly affected by the anisotropy of penetration depth in the plane perpendicular to the field. By fitting the
distortion using the anisotropic London model, we obtained an estimate of ∼3-4 for the anisotropy factor, γ,
between the in-plane and c-axis penetration depths. The results further reveal VL phase transitions as a function of
field direction. We discuss these transitions using the “Hairy Ball” theorem.
P.180 Magnetic Structure of La2O3FeMnSe2: Neutron Diffraction and Physical Property Measurements
S Landsgesell and K Prokes
Helmholtz-Zentrum, Germany
In the superconducting (SC) iron pnictide systems, the parent’s phase ground state is metallic and the Fe ion
resides in a tetrahedrally coordinated site[1]. Indeed, recent work has shown that the oxy-selenide compound
La2O3Fe2Se2 provides for a similar local environment of the Fe-ion to that found in the Fe superconductors [2,3].
However, the doping attempts so far, like recently synthesized La2O3Fe2−xMnxSe2[4], did not give a hint of a new SC
class but showed unexpected and interesting behaviour.
The susceptibility data of La2O3FeMnSe2 shows a magnetic transition temperature of 76 K and a strong difference
between field cooled and zero field cooled (ZFC) data at low fields. While the ZFC magnetization curve exhibits
negative values below about 45K, hysteresis measurement reveals after an initial negative magnetic moment a
hysteresis loop typical for ferromagnetic material, pointing to competing ferromagnetic and antiferromagnetic
interactions. Resistivity and dielectric permitivity measurements indicate the La 2O3FeMnSe2 is a semiconductor. We
performed X-ray diffraction at 295 K and neutron diffraction at 90 and 1.7 K. We did not detect a structural
distortion and the Fe and Mn atoms were randomly distributed. The magnetic order was found to be
antiferromagnetic, with magnetic moments of 3.44 (5) μB per Fe/Mn atom aligned within the a-b plane. This
magnetic order is different with respect to the pure Fe or Mn compositions reported in other studies.
[1]
[2]
[3]
[4]
Y. Kamihara et al., J. Am. Chem. Soc. 130, 3296 (2008).
E. Manousakis et al., Solid State Comm. 150, 62 (2010).
D. G. Free and John S. O. Evans, Phys. Rev. B 81, 214433 (2010).
H. Lei et al. Phys. Rev. B 86, 125122 (2012).
ICNS 2013 International Conference on Neutron Scattering
P.181 Anisotropy of the ( , ) dynamic susceptibility in magnetically ordered (x = 0.05) and superconducting (x =
0.40) Fe1.02Te1−xSex
S Landsgesell and K Prokes
Helmholtz-Zentrum, Germany
The phase diagrams of the iron-based superconductors show typically a suppression of the AF order and an
appearance of SC with doping or pressure.[1] The spin dynamics is known to change on entering the SC state.
Similar to the cuprate supercondcutors, spin resonance develops also in iron-based superconductors and it has
been shown to be related to the superconducting (SC) gap symmetry.[2,3].
Polarized inelastic neutron scattering on magnetically ordered Fe1.02Te0.95Se0.05 (FTS5) and SC Fe1.02Te0.60Se0.40
(FTS40) has been used to elucidate the anisotropy of the magnetic dynamical susceptibility at or near the ( , )
momentum space position. FTS40 in its SC state at 2 K, with a spin resonance at about 6 meV, shows the
susceptibility perpendicular to the c axis is 35% larger than for the direction along the c axis suggesting dominant
singlet pairing of s ±type. At higher temperatures this type of anisotropy is present in the spin dynamics of the
normal state. The FTS5 sample shows an almost isotropic response in the paramagnetic state. When the compound
is magnetically ordered at ( , 0), the response near ( , ) persists, but with reduced intensity. In conclusion, for
FTS40, the spin resonance emerges out of an anisotropic normal state spin systemwhich favors in-plane over c-axis
polarized spin excitations. This points to the fact that in FTS5sample the occurrence of AF order at Q = (0.5, 0, 0) =
( ,0)appears to reduce this in-plane spin dynamics greatly at Q = (0.5,0.5, 0) = ( , ).
[1]
[2]
[3]
F.-C. Hsu et.al. Proc. Natl. Acad. Sci. USA 105, 14262 (2008).
T. A. Maier and D. J. Scalapino, Phys. Rev. B 78, 020514(R) (2008).
M. M. Korshunov and I. Eremin, Phys. Rev. B 78, 140509(R) (2008).
P.182 Spin fluctuations of BaFe2(As,P)2 studied by neutron scattering
C-H Lee1, P Steffens2, N Qureshi3, M Nakajima1, K Kihou1, A Iyo1, H Eisaki1 and M Braden3
National Institute of Advanced Industrial Science and Technology, Japan, 2ILL, France, 3Universität zu Köln, Germany
Superconductivity can be induced in parent compounds of iron-based superconductors by several methods: carrier
doping, external pressure and chemical pressure. To understand the superconducting mechanism, it is crucial to
clarify what is the common property among them. To date, studies on spin fluctuations have been mainly performed
on carrier doped samples. On the other hand, studies in samples whose superconductivity is induced by other
methods have been restricted. Therefore, in this work, we studied spin fluctuations of P-doped BaFe2(As,P)2
(Tc 29.5K) using single crystal samples whose superconductivity is induced by chemical pressure. Inelastic
neutron scattering measurements were conducted using triple axis spectrometer IN8 of ILL.
We observed well-defined commensurate peaks at (0.5,0.5,L) in tetragonal notation, which is consistent with the
nesting vector of the Fermi surface. The result indicates that three-dimensional long range antiferromagnetic signals
observed in nondoped BaFe2As2 can be developed to short range magnetic signals by not only carrier doping but
also by applying chemical pressure. Spin fluctuations at the normal state consist of two- and three-dimensional
antiferromagnetic components. Clear spin gap is observed in the superconducting phase with resonance energies
depending on L. Universal relationship among other iron-based superconductors is found on the range of
dispersion, which indicates that the dispersive feature can be attributed to the presence of three-dimensional
antiferromagnetic correlation.
ICNS 2013 International Conference on Neutron Scattering
P.183 Field-induced c-axis correlation in the high-temperature superconductor LSCO
K Lefmann1, A Tranum-Rømer1, S Holm1, H Jacobsen1, N Bech Christensen2, L Udby1, P Jensen1, A Schneidewind3, P
Link3, R Toft-Petersen4, J Lynn5 and N Momono6
1
University of Copenhagen, Denmark, 2Technical University of Denmark, Denmark, 3FRM-2, Germany, 4Helmholtz
Center Berlin, Germany, 5NIST, USA, 6Hokkaido University, Japan
We present elastic neutron scattering studies of the interplanar magnetic correlations in the high-temperature
superconductor LSCO. The Sr-doping is x=0.12, close to 1/8 which denotes the regime of suppressed
superconductivity (in our sample Tc=27 K) and enhanced magnetic order.
In zero field, the interplanar spin correlation length is either absent or very short (5 Å), depending upon cooling
history. The short range of spin correlations is expected from the weak interplanar exchange coupling of the mother
compound LCO.
In contrast, new data from a similar experiment in which a magnetic field is applied perpendicular to the CuO2
planes show an enhanced correlation length, of the order 10-15 Å. This is consistent with earlier results by Lake et
al for a doping value in the underdoped range, x=0.105. It is therefore plausible that the field-induced c-axis
correlation in the static correlation function is universal for LSCO for dopings less than or equal to 1/8. In addition,
our observation lead us to conclude that the usual interpretation of the field-enhanced elastic signal as resulting
from an increased ordered moment must be reconciled.
P.184 Spin-Lattice Coupling in K0.8Fe1.6Se2 and KFe2Se2: Inelastic Neutron Scattering and ab-initio Phonon
Calculations
R Mittal1, M K Gupta1, S L Chaplot1, M Zbiri2, S Rols2, H Schober2, Y Su3, Th Brueckel4 and T Wolf5
1
Bhabha Atomic Research Centre India, 2Institut Laue-Langevin, France, 3Juelich Centre for Neutron Science JCNSFRM II, Germany, 4Juelich Centre for Neutron Science and Peter Gruenberg Institut, Germany, 5Karlsruhe Institute of
Technology, Germany
Adding to the excitement generated by the discovery of iron-based compounds the newly discovered alkali-doped
iron selenide compounds exhibit several unique characters that are noticeably absent in other iron-based
superconductors, such as antiferromagnetically ordered insulating phases and extremely high Neel transition
temperatures. We have been extensively studying (Phys. Rev. Lett.102, 217001 (2009); Phys. Rev. B 78, 224518
(2008); Phys. Rev. B 79, 214514 (2009); Phys. Rev. B 81, 144502 (2010)) phonon properties of FeAs based
compounds. Here we report measurements of the temperature dependence of phonon densities of states in
K0.8Fe1.6Se2using IN4 spectrometer at ILL, Grenoble.While cooling down to 150 K, a phonon peak splitting around
25 meV is observed and a new peak appears at 31 meV. The measurements support the recent Raman and infrared measurements indicating a lowering of symmetry of K0.8Fe1.6Se2 upon cooling below 250 K. Ab-initio phonon
calculations have been carried out for K0.8Fe1.6Se2 and Kfe2Se2. The comparison of the phonon spectra as obtained
from the magnetic as well as non magnetic calculations show pronounced differences. We conclude that Fe
magnetism is correlated to the phonon dynamics and it plays an important role in stabilizing the structure of
K0.8Fe1.6Se2 as well as that of Kfe2Se2. The calculations highlight the presence of low energy librational modes in
K0.8Fe1.6Se2 as compared to Kfe2Se2.
ICNS 2013 International Conference on Neutron Scattering
P.185 Four-Dimensional S(Q,ω) Using the Sweep Mode
R Osborn1, S Rosenkranz1, J-P Castellan1,2, F Weber2, D L Abernathy3 and M B Stone3
1
Argonne National Laboratory, USA, 2Karlsruhe Institute of Technology, Germany 3Oak Ridge National Laboratory,
USA
Time-of-flight spectrometers have proved to be effective probes of coherent excitations but less effective in
measuring strongly Q-dependent fluctuations with a broad frequency response. However, the new method of
measuring four-dimensional S(Q,ω) by rotating the sample about a vertical axis in discrete angular steps (the socalled "Horace" mode), pioneered at ISIS by Perring and Ewings, overcomes this limitation. We will describe the
use of event-mode data acquisition at the Spallation Neutron Source to perform such measurements by rotating the
sample continuously during a measurement (“Sweep” mode), correlating each neutron event with the rotation
angle. This has the advantage that the complete angular range is covered within a few minutes, allowing an
evaluation of the complete data set as the neutron events are collected, and that the results represent true
histogram distributions. We will review recent experiments on the ARCS spectrometer, including measurement of the
linewidths of high energy phonons in YB2Ni2C and bilayer manganites, the dispersion of coupled dimers in
La4Ru2O10, and the measurement of coherent inter-band excitations in CePd3.
P.186 Intrinsic crystal phase separation in the antiferromagnetic superconductor RbyFe2-xSe2: a diffraction study
V Pomjakushin1, A Krzton-Maziopa2, E Pomjakushina1, K Conder1, D Chernyshov3, V Svitlyk4 and A Bosak5
1
Paul Scherrer Institute, Switzerland, 2Warsaw University of Technology, Poland, 3Swiss-Norwegian Beam Lines at
ESRF, France, 4WWU Münster Institut für Anorganische und Analytische Chemie, Germany, 5European Synchrotron
Radiation Facility, France
The crystal and magnetic structures of the superconducting iron-based chalcogenides RbyFe2-xSe2 have been studied
by means of single-crystal synchrotron x-ray and high-resolution neutron powder diffraction in the temperature
range 2-570 K. The ground state of the crystal is an intrinsically phase-separated state with two distinct-bysymmetry phases. The main phase has the iron vacancy ordered √5 x √5 superstructure (I4/m space group) with
AFM ordered Fe spins. The minority phase does not have √5 x √5-type of ordering and has a smaller in-plane
lattice constant a and larger tetragonal c-axis and can be well described by assuming the parent average vacancy
disordered structure (I4/mmm space group) with the refined stoichiometry Rb0.60(5)(Fe1.10(5)Se)2. The minority phase
amounts to 8-10% mass fraction. The unit cell volume of the minority phase is 3.2% smaller than the one of the
main phase at T = 2 K and has quite different temperature dependence. The minority phase merges with the main
vacancy ordered phase on heating above the phase separation temperature TP = 475 K. The spatial dimensions of
the phase domains strongly increase above TP from 1000 to > 2500 Å due to the integration of the regions of the
main phase that were separated by the second phase at low temperatures. Additional annealing of the crystals at a
temperature T = 488 K, close to TP for a long time drastically reduces the amount of the minority phase.
[1]
V Yu Pomjakushin, A Krzton-Maziopa E V Pomjakushina, K Conder, D Chernyshov, V Svitlyk and A Bosak , J.
Phys.: Condens. Matter 24 435701 (2012)
P.187 AF structure in Ca(Fe1−xCox)2As2 under hydrostatic pressure
K Prokes1, E Ressouche2, S Landsgesell1, B Ouladdiaf3, L Harnagea4, S Wurmehl4, B Buechner4
1
Helmholtz Zentrum Berlin, Germany, 2CEA Grenoble, France, 3ILL Grenoble, France, 4IFW Dresden, Germany
At ambient pressure I2As2 system undergoes a first order transition from a high temperature tetragonal (T) phase to
a structure with orthorhombic (O) symmetry at around 172 K concomitant with a first order AF transition [1]. Upon
the application of a pressure, the structural and AF transitions are rapidly suppressed and superconductivity (SC) is
ICNS 2013 International Conference on Neutron Scattering
observed. Neutron diffraction measurements, revealed a new collapsed tetragonal (cT) phase appearing above 0.5
Gpa below 100 K, with a dramatic 9 % decrease in the c-lattice constant. All measurements demonstrated that
the electronic, magnetic and structural transitions are sharp and clearly defined under hydrostatic pressure.
Chemical doping with Co yield to SC as well. The phase diagram shows a region of coexistence of magnetism and
SC from 4% to 6% Co doping, while reaching a maximum TC of 20 K [2]. In this contribution we report on
neutron diffraction studies under hydrostatic pressure using Ca(Fe1−xCox)2As2 single crystals with concentrations
between x = 0.032 and 0.063. p-T phase diagrams were constructed for different x [3]. T-cT phase transition that is
connected with the destruction of the AF order is highly hysteretic and the differences in critical pressures with
increasing and decreasing branches eventually makes the collapsed tetragonal phase stable down to ambient
pressure and enables structural studies under identical conditions. Within the error bars on the forth digit, no
modification of the structural positional parameters is found.
[1]
[2]
[3]
A. I. Goldman et al., Phys. Rev. B 79, 024513 (2009)
L. Harnagea et al.,, Phys. Rev. B 83, 094523 (2011)
K. Prokes et al., Phys. Rev. B 85, (2012) 104523
P.188 Nodal superconductivity and spin excitation on Iron-based superconductors
S Shamoto1, M Ishikado2, K Kodama1, R Kajimoto1, M Nakamura1, Y Inamura4, A Iyo3, H Eisaki3, T Hong6, H
Mutka7and M Arai1
1
Japan Atomic Energy Agency, Japan, 2Comprehensive Research Organization for Science and Society,
Japan 3National Institute of Advanced Industrial Science and Technology, Japan, 6Oak Ridge National
Laboratory, USA, 7Institute Laue-Langevin, France
Nodal superconducting gap symmetry has been reported on some iron-based superconductors by thermal
conductivity and penetration depth measurements at low temperature [1]. From the early stage of iron-based
superconductor study, a spin fluctuation based theory successfully explains the nodal gap symmetry of LaFePO [2].
Up to now, number of the type of iron-based superconductors increases, possibly due to multi-orbital effect to the
electronic properties. Some of them are LaFePO [3], BaFe2(As1-xPx)2 [4], and LaFeAsO1-xFx [5]. Their spin excitation
spectra will be discussed on those powder samples in relation to their superconducting gap symmetries. At present,
no one grows these large single crystals. Therefore, it would still be meaningful to study spin excitation spectra of
these powder samples.
[1]
[2]
[3]
[4]
[5]
For example, K. Hashimoto et al., Phys. Rev. B 81, 220501I (2010).
K. Kuroki et al., Phys. Rev. B 79, 224511 (2009).
M. Ishikado,K. Kodama et al., in preparation.
M. Ishikado et al., Phys. Rev. B 84, 144517 (2011).
S. Wakimoto et al., J. Phys. Soc. Jpn., 79, 074715 (2010).
P.189 Common magnetic order in high-Tc Sr/O co-doped La2-xSrxCuO4+y for x=0.04,0.065,0.09
L Udby1, J Larsen2, M Boehm3, N B Christensen2, C Niedermayer4, H E Mohottala5, T B S Jensen2, R Toft-Petersen6, N
H Andersen2, B O Wells7 and K Lefmann1
1
University of Copenhagen, Denmark, 2Technical University of Denmark, Denmark, 3Institut Laue-Langevin,
France, 4Paul Scherrer Institute, Switzerland, 5University of Hartford, USA, 6Helmholtz Zentrum Berlin für Materialien
und Energie, Germany and Technical University of Denmark, Denmark, 7University of Connecticut, USA
We present results of muon spin rotation and elastic magnetic neutron scattering experiments on the co-doped
super-oxygenated superconductor La(2-x)Sr(x)CuO(4+y) with x=0.04,0.065 and 0.09. We find incommensurate
antiferromagnetic order below T_N coinciding with the superconducting ordering temperature T_c 38 K for all
ICNS 2013 International Conference on Neutron Scattering
dopings. For all x, we find the size of the magnetic moment to be the same and correlated over long distances with
an incommensurability value that is consistent with a hole-doping of at least 1/8.
P.190 Magnetic phases in the co-doped high-temperature superconductor LSCO+O
L Udby1, S L Holm1, J Larsen2, U B Hansen1, A Schneidewind3, C Niedermayer4, M Laver4, N B Christensen2, J J
Chang5, N H Andersen2, B Wells6 and L Kim1
1
University of Copenhagen, Denmark, 2Technical University of Denmark, Denmark, 3FRM-2, Germany, 4Paul Scherrer
Institute, Switzerland, 5EPFL, Switzerland, 6University of Connecticut, USA
In cuprate superconductors, doping with mobile oxygen atoms may lead to a more annealed crystal structure, with
profound consequences on the electronic ground state and on superconductivity itself. We here present detailed
neutron scattering studies of magnetic structure and dynamics in the co-doped superconductor La(2x)Sr(x)CuO(4+y), with x=0.06.
Magnetic diffraction shows in zero field a total absence of incommensurate (IC) ordering, supported by the lack of
magnetic order observed in muon experiments. This lack of order is very unusual for oxygen-doped LCO or LSCO.
However, in applied fields along the c-axis, a sharp IC signal emerges, its intensity being linear in the field. This
suggests that our sample at zero field lies at the quantum critical point between the pure superconducting phase
and the mixed magnetic-superconducting phase.
Surprisingly, inelastic scattering on the IC positions in zero field reveal a well-developed spin gap. This is a signature
of the optimally doped superconductor and in contrast to the gap closing one would expect at a quantum phase
transition. To elucidate this apparant discrepancy, we have performed inelastic scattering at applied fields and seen
signatures of gap closing at higher fields.
Our observations of the elastic and inelastic signal cannot be reconciled within a simple model, and speculate that
we see the signatures of two different phases in the system. We will discuss our findings in the light of recent
theoretical work.
P.191 Investigation of the metal-insulator transition in Hg2Ru2O7
J van Duijn1, R Ruiz-Bustos2, A Dauod-Aladine3, A Hillier3 and P Deen4
1
Universidad de Castilla-La Mancha, Spain, 2Instituto de Investigación en Energías Renovables, Mexico, 3ISIS
Facility, UK 4ILL, France
Hg2Ru2O7 is a pyrochlore material (containing Ru5+) that undergoes a first order metal to Mott insulator transition
(MIT) at T= 107 K.[1] In the insulating phase the magnetic susceptibility is significantly reduced and nearly Tindependent, similar as to what is observed in Tl2Ru2O7 (containing Ru4+) suggesting that the Ru moments are
quenched into an antiferromagnetic spin-singlet ground-state.[2] In order to understand the magnetic ground-state
of Hg2Ru2O7 we have performed both non-polarized and polarized neutron powder diffraction and muon spin
relaxation (μSR) experiments.
Our muon and polarized neutron diffraction experiments revealed the onset of long-range magnetic ordering below
the MIT. This suggests that instead of discrete singlets the Ru moments form composite spin degrees of freedom, as
has previously been observed in the chromate spinels.[3] To shed further light on the magnetic interactions that give
rise to the observed long-range ordering we have performed high resolution powder neutron diffraction experiments
to determine the low temperature structure of Hg2Ru2O7.
ICNS 2013 International Conference on Neutron Scattering
Below the MIT the symmetry is lowered from cubic to monoclinic and the Ru-Ru bonds, which are equal in the
pyrochlore phase, become split into short, medium and long bonds. As a result the exchange interactions between
the Ru atoms become more two dimensional. The short and medium bonds form layers, which are separated by the
long bonds that run parallel to the monoclinic ab plane. The low temperature structure can best be described as a
stacking of Kagome-like layers.
[1]
[2]
[3]
A. Yamamoto et al., J. Phys. Soc. Japan 76, 043703 (2007).
S. Lee et al., Nat. Mater. 5, 471 (2006).
S. H. Lee et al., Nature 418 , 856 (2002).
P.192 Two dimensional incommensurate spin excitations and lattice fluctuations in La2-xBaxCuO4
J Wagman1, J Carlo2, G Van Gastel1, Y Zhao3, A Kallin4, E Mazurek1, H Dabkowska1, A Savici5, Gt Granroth5, Z
Yamani6, Z Tun6 and B Gaulin1
1
McMaster University, Canada, 2Villanova University, USA, 3National Institute of Standards and Technology,
USA, 4University of Waterloo, Canada, 5Oak Ridge National Laboratories, USA, 6Canadian Neutron Beam Centre,
Canada
‘Hour-glass’ shaped dispersions of antiferromagnetic (AF) spin fluctuations are a robust feature common to many
high temperature superconductors. In 214 cuprates, these phenomena are well known to display a strong
dependence on the concentration of holes that are introduced into the copper oxide planes by doping. The
incommensurability (IC) of the two dimensional magnetic order in this system is sensitive to hole concentration.
Here, we present a series of neutron scattering measurements on single crystals of La2-xBaxCuO4 (LBCO), with 0.035
< x < 0.095, a doping range that spans the transition from diagonal to parallel IC ordering wavevectors, and from
non-superconducting to superconducting ground states. Our measurements map out the evolution of the spin
excitations for energies below 50 meV, and focus on an enhancement in the scattered intensity centered in the
17-20 meV at the AF IC positions. This regime corresponds to the approximate crossing of very dispersive spin
excitations and weakly dispersive low lying optic phonons in LBCO.
P.193 Clarifying the origin of chain superconductivity in YBCO cuprates
J S White1, C J Bowell2, A S Cameron2, R W Heslop2, J Mesot1, J L Gavilano1, S Strässle3, R Khasanov1, C D
Dewhurst4, J Karpinski5 and E M Forgan2
1
Paul Scherrer Institut, Switzerland, 2University of Birmingham, UK, 3Physik-Institut der Universität Zürich,
Switzerland, 4Institut Laue-Langevin, France, 5ETH Zürich, Switzerland
YBa2Cu4O8 (Y-124) is a stoichiometric and double-chained member of the YBCO cuprates that display a diverse
range of low temperature (T) behaviour including superconducting, spin and charge orders. We report the first SANS
study of the superconducting vortex lattice (VL) in Y-124, and uncover surprising differences in the VL properties
when compared with the case of fully-oxygenated, and single-chained, YBa2Cu3O7 (Y-123). For magnetic fields
applied parallel to the crystal c-axis, and at T = 2 K, we find that the VL structure in Y-124 is composed of just a
single type of distorted hexagonal domain over the observable field range up to 6 T. The measure of the VL distortion
is proportional to the in-plane electronic anisotropy, and is larger in Y-124 than in Y-123 at comparably low fields.
Also unlike Y-123, no low field VL structure transitions are observed in Y-124, which likely evidences a pronounced
low-field Fermi surface anisotropy derived from the plane-chain hybridisation. Further differences are seen in the Tdependent Bragg spot intensity, which is proportional to the superfluid density. In particular for Y-124, the
superfluid density along the a-axis is well-described by a simple d-wave model, while that along the b-axis displays
a form consistent with model calculations that include chain superconductivity which is easily suppressed by
magnetic field and temoerature. We conclude that while the chains in Y-124 are superconducting, likely by the
proximity effect, those in Y-123 are either not, or display a more robust coupling to the planes.
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P.194 Coexistence of cuprate magnetism and superconductivity in GdSr2Cu2RuO8
W Yelon1 and H Blackstead2
1
Missouri University of Science and Technology, USA 2University of Notre Dame, USA
Neutron diffraction studies on YSr2Cu2RuO8 have shown that the Ru spins are ordered antiferromagnetically along
the c-axis, while the cuprate planes are ordered ferromagnetically in the a-b plane and are stacked antiferromagnetically[1]. However, the small Cu moment (~0.5μB) produces only a small signal, and the results have not
been generally appreciated. Powerful confirmation of this result is obtained from the analysis of two independent
muon spin rotation experiments on homologous superconducting GdSr2Cu2RuO8[2-3]. A magnetic induction of 720740G was observed at one muon stopping site, and the data strongly suggest a second stopping cite with zero
induction. Using the spin configuration from the neutron experiment, with the refined spin moments for YSr2Cu2RuO8,
we have calculated the magnetic induction at potential muon stopping sites. Muons stopping in the center of the
ferromagnetic CuO2 planes will experience an induction of ~725G, while those stopping in the center of the Ru
planes experience zero field, The excellent agreement with experiment provides strong support for the neutron
results and shows that superconductivity can coexist with magnetically ordered Cu in the ruthenocuprates.
[1]
[2]
[3]
W.B. Yelon et al. J. Appl. Phys.101, (2007) 09G104-1 -09G104-3.
C. Bernhard et al. Phys. Rev. B 59, (1999) 14099-14107.
D.R. Harshman, et al. Phil. Mag. 83, (2003) 3055-3064.
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