Metal-Insulator Transitionin Single Crystal Spinel ...

3 downloads 0 Views 963KB Size Report
takes place at TMI. ~ 226 K in CuIr2S4 [3], This MIT is accompanied with a structural transition from cubic to tetragonal (triclinic) symmetry with volume contrac-.
Metal-Insulator Transition in Single Crystal Spinel CuIr2S4 Studied by 63Cu-NMR K. Kumagai*, M. Sasaki*, N. Yatsif, T. Wazumi*, N. Matsumoto1 and S. Nagataf * Division of Physics, Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan ^Dept. of Materials Science and Engineering, Muroran Institute of Technology, Muroran 050-8585, Japan Abstract. We have investigated 63Cu-NMR in a single crystal of CuIr2S4 by a high resolution NMR measurement. The evolution of the electronic state associated with the metal-insulator transition is caused by a charge ordering and a spin dimer formation, as consistent with the recent x-ray study. Keywords: NMR, spin dimer, metal-insulator transition, spinel compound, CuIr2S4 PACS: 76.60.-k,76.60.Cq,71.30.+h Spinel compounds C11M2X4 (M=a transition metal, and X= a chalcogen) exhibit very interesting physical properties. In addition to superconductivity in CURI12S4 and CuRh 2 Se4 [1, 2], a metal-insulator transition (MIT) takes place at TMI ~ 226 K in CuIr 2 S 4 [3], This MIT is accompanied with a structural transition from cubic to tetragonal (triclinic) symmetry with volume contraction of 0.7% [4]. As NMR [5] and photoemission measurements [6] show that the Cu ions are monovalent, the nominal valence of the Ir atoms is expected to be 3.5. Thus, the nonmagnetic ground state revealed by the magnetic susceptibility measurement [3] remains as puzzling problem for possible Ir 3 + (S=0) and Ir 4 + (5=1/2) configurations in the insulating state of CuIr 2 S4. Recently, a precise x-ray measurement has brought an interesting progress for this issue, namely, the analysis of the structural results reveals that a charge ordering of Ir 3 + and Ir 4 + and spin dimerization of Ir 4 + ions occurs simultaneously below TMI [7]. Successively, interesting electric transport anomalies associated with the x-ray induced disordering are observed at low temperature [8, 9]. Thus, in order to elucidate the origin of the MIT in the Cu-spinels, it is important to clarify the valence state and magnetic properties from a microscopic point of view. So far, we have investigated the evolution of the electronic state associated with MIT of CuIr 2 S4 by a Cu-NMR study with a high resolution spectroscopy [10]. Here we present 6 3 Cu-NMR results for the single crystal of CuIr 2 S4 for the first time. The preparation method of single crystal samples was described elsewhere [11]. NMR of a tiny crystal of 1 ^ m g was measured by the conventional phase coherent pulse method with a highly-homogeneous superconducting magnet. We measure angle dependence of the NMR spectrum with rotating the sample. The spectrum was obtained by the Fourier transform method (FT) with a fixed magnetic field (9.4 T).

In order to give insight into over-all features of the NMR for this system, the spectra of 6 3 Cu of powder sample [12] are shown both below and above TMI in Fig.l. Very narrow 6 3 Cu spectra with a line width less than 5 kHz are observed without any notable anisotropic Knight shift and nuclear quadrupole interactions above TMI. The spectrum becomes broad and shows several peaks within the range of 100 kHz below TMI. A S the MIT is the first order transition [3], NMR signals in the metallic and insulating phase coexist between 230 K and 210 K. For the single crystal, the Cu NMR spectrum is quite similar to that of the powder sample. In the metallic state, an extremely narrow spectrum is observed as shown in the inset of Fig. 2. The resonance position and the line width of ~ 2 kHz are independent of the crystal direction with respect to external field, indicating again that the anisotropy of the Knight shift is completely absent in the metallic state. The spectrum below TMI shows at least 12 peaks within the range of 100 kHz as shown in Fig. 2. In comparison with the spectrum of the powder sample, well-separated and narrow peaks are observed. The absence of apparent temperature dependence of the Knight shift below TMI indicates that the electronic state is basically non-magnetic in the insulating state, suggesting that both Cu and the Ir atoms in CuIr 2 S4 do not carry any magnetic moments below TMI. The peak frequencies change with changing crystal direction with respect to magnetic field, showing that the split of the spectrum is mainly associated with the quadrupole interaction. Because of the lattice distortion from cubic to triclinic symmetry below TMI, an appreciable electric field gradient (EFG) at the Cu site is expected. Thus, the nuclear quadrupole interaction for the Cu nuclei (1=3/2) is not negligible below TMI. We analyze the spectrum with including nuclear electric quadrupole interaction which is sensitive to the distribution of the charges around Cu

CP850, Low Temperature Physics: 24th International Conference on Low Temperature Physics; edited by Y. Takano, S. P. Hershfield, S. O. Hill, P. J. Hirschfeld, and A. M. Goldman © 2006 American Institute of Physics 0-7354-0347-3/06/$23.00 1450

CuIr 2 S 4 single crystal #1 |

T=90K

•M

J

106.20

106.1

106.2

Inten ity (arb . unit)

[

106.120

A «

^ 106.24

Ai

f[

T=240K

106.128 106.136 f ( MHz )

v^w v ^ w y^ 106.28

f (MHz)

FIGURE 2. w Cu-NMR spectrum in the insulating state for the single crystal of Cuh^Szt. The inset shows the NMR spectrum at T=240 K (in the metallic state). External field is applied along [100].

106.3 106.4 f (MHz)

FIGURE 1. 63Cu-NMR spectra as a function of frequency for the powder sample of CuIr2S4 [12] for various temperatures.

atoms. We have calculated the EFG at the Cu site with a point charge model. For this purpose, we use atomic positions and ionic configurations of Ir 3 + and Ir 4 + obtained by the recent precise x-ray measurement [13]. The calculation reveals that only one equivalent Cu site in the cubic symmetry above TMI changes to at least four nonequivalent Cu sites. For the next step, we simulate the nuclear quadrupole split spectrum by taking into account the perturbation of the nuclear quadrupole interaction for the Zeeman field. In this case, we expect more than 12 peaks of the central and satellite lines for four non-equivalent Cu sites. Be sure that the spectrum becomes complicate with a large anisotropic parameter of 7]. The fitting is achieved with superposition of possible components of the spectrum with the anisotropic Knight shift. The observed angel dependence of the 6 3 Cu-NMR spectrum can be well fitted by a simulation based on the atomic coordination determined by the x-ray measurement. Although detail analysis will be presented in a forthcoming paper, the present preliminary trial of the fitting indicates that the charge separation and the dimerization of Ir 4 + spins proposed by the x-ray study [7] are supported by the NMR study at a microscopic point view. In summary, we have investigated Cu-NMR in the single crystal CuL^Szt. 6 3 Cu-NMR spectrum is reasonably narrow under the cubic symmetry in the metallic state. Below TMI, the spectrum consists of four components of nonequivalent Cu signals. Each of them is split by non-

vanishing nuclear quadrupole interactions in the insulating state. The charge ordering in Ir 3 + and Ir 4 + , and the dimerization of Ir 4 + spins in Q1L-2S4 can be supported with taking into account the nuclear quadrupole interaction and the anisotropic Knight shift based on the atomic coordination obtained by the x-ray measurement [7, 13]. The authors would like to thank Dr. Y. Horibe for fruitful discussion and kind informing the atomic coordination.

REFERENCES

6. 7.

1451

10. 11. 12. 13.

T. Bitoh, et al, J. Phys. Soc. Jpn. 61 (1992) 3011. T. Hagino etal, Phys. Rev. B 51 (1995) 12673. S. Nagata et al, Physica B 194-196 (1994) 1077, and T. Hagino et al, Philos. Mag. B 71 (1995) 881. T. Furubayashi, etal. J. Phys. Soc. Jpn. 63 (1994) 3333. K. Kumagai et al, Spectroscopy ofMott Insulator and Correlated Metals, eds. by A. Fujimori and Y. Tokura, (Springer-Verlag, 1995) p255. J. Matsuno et al, Phys. Rev. B 55 (1997) R15979. P. G. Radaelli et al, Nature 416 (2002) 155. H. Ishibashi et al, Phys. Rev. B 66 (2002) 144424. T. Furubayashi et al, Solid State Commun. 126 (2003) 617. S. Tsuji et al, Physica C 282-287 (1997) 1107, S. Tsuji et al, Physica B 237-238 (1997) 156, and K. Kumagai et al, Physica C341-348 (2000) 741. N. Matsumoto and S. Nagata, /. Crystal Growth, 210 (2000) 772. M. Sasaki et al, Physica C 408-410 (2004) 822. Y Horibe, private communications.