Experimental Report

Date of Approval 2017/06/07
Approver Ryoichi Kajimoto
Date of Report 2017/06/07

Project No.

Name of Instrument scientist

2016B0116

Kazuki Iida, Kazuya Kamazawa
Title of experiment

Name of Instrument/ BL No.

Investigation of magnetic excitations in spin-web compound Cu3TeO6Name of principal investigator
Yuan Li

4SEASONS(BL01)
Date of Experiment
19 Feb 2017 to 28 Feb 2017

Affiliation
School of Physics, Peking University

Please report your samples, experimental method and results, discussion and conclusions. Please add figures andtables for better explanation.
1.
Name of sample(s) and chemical formula, or compositions including physical form.
The sample is a copper tellurate, Cu3TeO6. It is an antiferromagnetic insulator with the transition temperature
TN at around 61K.
We have used about 16.9 grams coaligned Cu3TeO6 single crystal in the inelastic neutron scatteringexperiment.

2.
Experimental method and results.

If you failed to conduct experiment as planned, please describe reasons.

We used the 4SEASONS neutron spectrometer based on the technique of time of flight (TOF).
The experiment basically consists of three parts. In the first part, we set the temperature at 4K to study spin
dynamics in Cu3TeO6. For the second part, we raised the temperature to 73K to see its high temperature
behavior. And finally, we went back to 4K for detailed study of the magnetic excitation. We have obtained thefull spin wave spectrum in Cu3TeO6 from this experiment as the primary result.
Most of our data analysis work and the results represented below are based on the Horace software package.
We combined the data at symmetrically equivalent wave vectors in reciprocal space with the help of Horace.
We carried out the experiment with the sample mounted in the [H H L] plane. During the experiment werotated the sample every 0.5 degree and a total coverage of nearly 180 degrees was obtained. The incident
energy we mainly used is 28.18meV. And thanks to the multiple-Ei method, additional data with several otherincident energies such as 55.05 and 8.21meV are also available.

2.

Experimental method and results (continued)

We choose the high symmetry directions (H-P- -H-N- -P-N) in the reciprocal space to represent our data as
shown in FIG 1.From the low temperature measurement (T=4K), we find that the magnetic excitation extends
as high as 20meV and shows semiclassical spin wave spectrum features (FIG 1(a)). Six branches of spin waves
are clearly discerned, each of which is two-fold degenerate due to symmetries in this magnetic system. The
linear dispersion of the acoustic mode near

point is consistent with antiferromagnetic spin wave theory atzone center. If we scrutinize carefully in the data with Ei=8.21meV, we can find an energy gap at the(FIG 2), although it seems gapless from the full map. And the energy scan at thepoint

point shows that themagnitude of the energy gap is around 2.5meV (FIG 2 inset). This gap is due to certain anisotropic effects inthis material, most likely in the form of Dzyaloshinsky-Moriya interactions. The clear spin waves discussed
above suggest that linear spin wave theory constitute an excellent description of the system. This is further
confirmed in our measurement at 73K, which is higher than TN. No spin wave is observed at this temperature(FIG 1(b)). But if we integrate the intensity over all reciprocal space and the energy range from 3meV to22meV, we find the total spectrum weight is basically the same as in the low temperature case (not show).
This result is consistent with the neutron scattering theory.
In order to study the optical branches in detail, we increased the frequency of the Fermi chopper from 250Hz
to 500Hz, and measured the magnetic excitations at 4K with a higher energy resolution. We get an almost
complete dispersion map with the help of Horace (FIG 3) despite of a smaller reciprocal space coverage.
The clear spin wave dispersion we got will be variable information for further study of the magnetism in thiscompound and set limitations for possible theoretical models.
(c)

FIG 1: (a) A global spin wave dispersion alonghigh symmetry lines in the first Brillouin zone
at T=4K; (b) No clear feature shows at T=73K
along the same trajectory. The intensity is inarbitrary unit. (c) Brillouin zone of the bcc
lattice with the thick red lines indicating thehigh symmetry directions, along which the data
are presented in (a) and (b).

FIG 2: A colormap along (0,1L) direction from the datawith the incident energy Ei=8.21meV. An energy can beseen in the Brillouin zone center (0,1,1). The inset
shows the energy at this point.

FIG 3: The spin wave dispersion measured with theincreased Fermi chopper frequency. More details canbe discerned comparing with FIG 1(a).