（Project use）

Project No.

Date of Approval 2016/8/3
Approver Ryoichi Kajimoto
Date of Report 2016/8/3
Name of instrument scientist

2014P0201

Ryoichi Kajimoto

Title of experiment

Name of Instrument/（BL No.）

Study of structure and electronic properties of functional

4SEASONS/BL01

materials at BL01

Date(s) of experiment(s)Name of principal investigator

2014/5/5 11:00 – 2014/5/13 9:00

Ryoichi Kajimoto

2015/3/12 11:00 – 2015/3/17 9:00

Experimental Report

Affiliation
Japan Atomic Energy Agency

1. Outline of experimental method and results.

(Experimental method and results should be reported including

sample information such as composition, physical and/or chemical characteristics.)
We conducted the research subject “Study of magnetic excitations and phonons in cuprate
superconductors to explore the hierarchical nature of magnetic excitations as well as the effect of spin-latticecoupling” by S2 Team in the project research group. It is well known that the hole-doped cuprate
superconductors

show

so-called“hour-glass”

magneticexcitations.

The

excitation

appears

atincommensurate positions (0.5, 0.5±δ), (0.5±δ, 0.5) in the two-dimensional reciprocal lattice at low energies.
Its position gets closer to (0.5, 0.5) as energy increases, but spreads as a function of energy toward the zone
boundary like an ordinary antiferromagnetic spin wave above some characteristic energy Ecross. Although thehour-glass magnetic excitation is regarded as one of the important ingredients for the high-Tc
superconductivity, there are still several issues to be clarified. In the present study, we focused on two
remaining issues for the hour-glass magnetic excitations. (1) One is to identify possible spin-charge-phonon
coupled excitation. (2) The other is to characterize the true nature of the hour-glass excitations especiallyaround Ecross.
(1) Study of spin-charge-phonon coupled excitation
One of the interesting features of the hour-glass excitation is that its spectral weight enhances at anenergy (~15 meV) where the excitation crosses optical phonon modes. We expect it is an intrinsic
phenomenon in the superconducting cuprates arising from the coupling of the magnetic excitation and thephonons. On the other hand, it is also possible that the enhancement is just a simple overlap of the two kindsof excitations. Last year (2013P0201), we have measured excitation spectra in superconducting
La1.925Sr0.075CuO4

andnon-doped

antiferromagnet

La2CuO4

at

5K

overthe4-dimensional

momentum-energy space by rotating the single crystals. In the superconducting sample, we have observed
the enhancement in the excitation spectra at ~15 meV independent of Brillouin zones. Although it suggeststhat the enhancement is intrinsic to the superconducting state, to get more convincing evidence, it isimportant to compare the data with that at the normal state and to estimate the phonon contribution to thespectra more correctly.

1. Experimental method and results (continued)
Accordingly, this year, we performed supplementary measurements on La1.925Sr0.075CuO4, heavily
overdoped La1.7Sr0.3CuO4, and insulating La1.67Sr0.33NiO4 which shows stripe spin-charge ordering. All thecrystals were grown by the TSFZ method. They were aligned so that the orthorhombic a and c axes are in
the horizontal plane, and were rotated over 90° with 0.5° steps. Utilized incident energy was 48 meV and55.47meV for the cuprates and nickelate, respectively.
We converted the observed excitation spectra into the dynamical spin susceptibilities χ″(ω) for
La1.925Sr0.075CuO4 (x = 0.075) at 5 K and 200 K, and La2CuO4 (x = 0) at 5 K. The contribution from the simple
overlap of phonon intensity in each of the spectra was removed by subtracting the excitation spectrum ofLa0.7Sr0.3CuO4, because the latter compound does not show observable magnetic excitations. We obtainedχ″(ω) at several L positions to see whether it is affected by the difference in the phonon intensity as afunction of L, but found that χ″(ω) shows essentially same E (= ħω) dependence in all the L zones. Then, weaveraged the χ″(ω) at various L positions, which is shown in Fig. 1. For x = 0.075, thus obtained χ″(ω) showsa clear enhancement at E ~ 15 meV in the superconducting state at 5 K, but the enhancement disappears in
the normal state at 200 K. The χ″(ω) of x = 0 does not show the enhancement even at 5 K. Furthermore, we
did not observe the enhancement in χ″(ω) of La2/3Sr1/3NiO4 (not shown). From these observations, weconclude that the 15-meV enhancement is intrinsic to the hour-glass excitation in the superconducting state.
Since the characteristic energy of the enhancement, ~15 meV, corresponds to that of several optical modes,
the enhancement is possibly caused by the coupling with one of the phonon modes. One of the features inthe superconducting cuprates which can connect spin and lattice degrees of freedom is that the existence ofdynamical stripe-type spin-charge correlations. However, the absence of the enhancement in the excitation
spectrum in stripe-ordered La2/3Sr1/3NiO4 suggests that the static stripe correlations cannot be the origin of
the enhancement. Then, we expect that the enhancement is a result of spin-charge-lattice (phonon) couplingthrough the dynamical stripe correlations. We are now performing phonon calculation to find any opticalphonon mode that can couple the dynamical stripes and can be the origin of the 15-meV enhancement.

Fig. 1. Dynamical spin susceptibility χ″(ω) for La1.925Sr0.075CuO4 at 5 K and 200 K, and La2CuO4 at 5 K.

Please use A4-size paper for further reporting, if necessary.

1. Experimental method and results (continued)(2) Study of hierarchical structures in the hour-glass magnetic excitation
Recent experimental studies suggested that the hour-glass magnetic excitation consists of twocomponents with different origins, whose ratio varies as a function of energy. To confirm the existence of thetwo components, we performed inelastic neutron scattering experiments on superconducting hole-doped
copper oxides La2-xSrxCuO4. In previous inelastic neutron scattering studies, it was difficult to prove theexistence of the two components because of poor statistics of data. In the present study, to investigate
detailed structures in the excitation spectra, inelastic scattering intensity was collected with much higher
statistics than that available before, thanks to the high performance of 4SEASONS.
Measured samples are single crystals of x = 0.10 and 0.16, which were grown by the TSFZ method. The
crystals were aligned so that the c axis is parallel to the incident neutron beam. The excitation spectra asfunctions of the two-dimensional momentum transfers and energy transfer were obtained neglecting the c*dependence of the spectra.
Figures 2(a)-2(c) show constant-E slices of the magnetic excitation spectrum in x = 0.16, while Fig. 2(g)shows the h-E slice of the spectrum. Our high-statistics data clearly shows that the incommensurate
structure in the low-energy part of the hour-glass excitation does not considerably change as a function ofenergy [Fig. 2(g)]. That is, the incommensurability δ of the magnetic excitation does not show energy
dependence. Based on this experimental result, we modeled the excitation spectrum as a combination of
two components: one is incommensurate with a fixed δ and the other is commensurate centered at (0.5,0.5). By using this model, we successfully reproduced the observed spectrum below 60 meV as shown in
Figs. 2(d)-2(f). The result of the present study provides a strong evidence of the two components in magnetic
excitations of superconducting cuprates.

Fig. 2. (a)-(c) Constant-E slices of the magnetic excitation spectrum in La2-xSrxCuO4 (x = 0.16) at 18, 36, and46 meV. (d)-(f) Calculated spectra assuming the combination of incommensurate and commensuratecomponents. (g) Slice of the magnetic excitation spectrum on the (h,0.5)-E plane. The intensity at 300 K issubtracted.

Please use A4-size paper for further reporting, if necessary.