ELSEVIER
Physica B 201 (1994) 178-181
High field ESR of the spin-Peierls system CuGeO*3 S. Imagawa a, H. Ohta a'*, M. Motokawa a, O. Fujita b, J.
Akimitsu b
aDepartment of Physics, Faculty of Science, Kobe University, Rokkodai, Nada, Kobe 657, Japan b Department of Physics, Aoyama-Gakuin University, 6-16-1 Chitosedai, Setagaya, Tokyo 157, Japan
Abstract Submillimeter wave ESR measurements of single crystal CuGeO3, which has recently been found as the first inorganic compound which shows a spin-Peierls transition, have been performed in the frequency region from 90 to 380 GHz using a pulsed magnetic field up to 16 T. The temperature dependences of the intensity and the line width in the temperature range from 4.2 to 300 K have been observed and these results will be discussed in connection with the well-known organic spin-Peierls system.
1. Introduction Recently Hase et al. [1] observed the rapid decrease of the magnetic susceptibility to zero below Tsp = 14 K for all three directions in CuGeO3 and suggested that it is in the spin-Peierls state below Tsa. The spin-Peierls transition is the formation of singlet ground state due to the dimerizations of spin sites in a one-dimensional antiferromagnetic Heisenberg chain which are induced by the spin-phonon coupling between the one-dimensional spin system and the three-dimensional phonon system. The examples of such a transition were known so far in some organic compounds such as TTF-CuBDT, TTF-AuBDT [2], TTF-CuBDSe [3], MEM(TCNQ) 2 [4] and DEM(TCNQ)2 [5]. However, CuGeO 3 is the first inorganic compound which shows the spin-Peierls transition. Therefore, it will be very interesting to investigate the magnetic properties of CuGeO 3 and compare those of CuGeO3 with other known organic spin-Peierls systems.
* Corresponding author. *This work was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Scienceand Culture.
The existence of high field incommensurate phase above Hc in spin-Peierls systems has been predicted by some theories [6-8]. Also there has been one attempt to investigate the nature of this phase in the case of MEM(TCNQ)2 by far infrared ESR [9]. The purpose of this paper is to investigate the nature of spin-Peierls phase below Hc and high field incommensurate phase above Hc through submillimeter wave ESR by varying the frequency in a wide frequency range. The results obtained are discussed in relation with the other known organic spin-Peierls systems.
2. Experimental Submillimeter wave ESR measurements of single crystal CuGeO 3 were performed by using 90GHz G u n n oscillator, and 220 and 370 GHz bands backward travelling wave tubes as light sources. The pulsed magnetic field was applied parallel to the three principle axes of CuGeO3. Two types of cryostats are used: one for the temperature range from 4.2 to 60K with 16T magnet, and the other for the temperature range from 86 to 300K with 30T magnet. The transmitted light was detected by the InSb detector. The details of our
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S. Imagawa et al./ Physica B 201 (1994) 178-181
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Fig. 2. Typical examples of the temperature dependence of the ESR absorption lines of single crystal CuGeO3 taken at 90 GHz. The direction of the applied magnetic field is parallel to the a-axis. DPPH is the standard of # = 2.
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.~ 0.1 F r o m the frequency dependence of the resonance field as shown in Fig. 1, the g-value of each direction was determined by our submillimeter wave ESR at 80 K. F o r Hna, HlJb and nllc, they were determined to be 2.15, 2.26 and 2.06, respectively. They are rather consistent with the g-values obtained by the X-band ESR of Petrakovskii et al. [12], which is equal to 2.19, 2.266 and 2.083 for HJla, nHb and HIIc, respectively, at r o o m temperature. Fig. 2 shows the examples of the ESR absorption at 90 G H z for Hlla. F r o m the structural consideration, onedimensional antiferromagnetic chains of Cu 2 ÷ ions seem to be formed along the c-axis. Therefore, Hlla means applying the field perpendicular to the chain direction. Almost no ESR absorption is observed at 4.2 K, but it starts to appear as the temperature is raised and it becomes strong and sharp at around Tsp = 14 K. Then the ESR absorption becomes broader as the temperature is raised to higher temperature. Almost no shift of resonance field was observed through entire temperature range. The same kind of tendency was observed for 232.3GHz. However, the resonance field of the ESR
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Temperature (K) Fig. 3. Temperature dependence of the observed line width of single crystal CuGeO3 at 90 (closed circle), 232.2 (triangle) and 370.4 GHz (open circle) when the magnetic field is applied parallel to the a-axis. The~solid line shows xTtaken from the results of Hase et al. [1].
absorption at 370.4 G H z is just above the criticalfield He, and it shows different temperature dependence. Fig. 3 shows the temperature dependence of the observed line width. At 90 and 232.3 GHz, where the resonance fields are below the critical field He, the temperature dependence of the line width has a m i n i m u m a t low temperature and increases as the temperature is
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Fig. 4. Temperature dependence of the integrated intensity of single crystal CuGeO3 ESR absorption at 90 (closed circle), 232.3 (triangle) and 370.4 GHz (open circle) when the magnetic field is applied parallel to the a-axis. The solid line shows Xtaken from the results of Hase et al. [1]. increased. The minimum point is about 8.5 K which is about half of Tse. The temperature dependence of the line width observed at 90 GHz seems to be proportional to zT, which is shown by the solid line in Fig. 3, above T~p. The data of magnetic susceptibility Z are taken from Ref [1], but it does not obey the Bonnet-Fisher calculation for one-dimensional Heisenberg antiferromagnetic system (S = ½). This temperature dependence observed for CuGeO3 is rather similar to that of the well-known spin-Peierls system MEM(TCNQ)2 observed at the Xband [9]. However, the minimum point was about Tsa in the case of MEM(TCNQh. This difference between CuGeO3 and MEM(TCNQ)2 may be due to the difference of frequency used for the ESR measurement, and it should be checked by observing the X-band ESR for CuGeO3. On the other hand, the line width observed at 370.4GHz, where the resonance field is just above the critical field H c, simply increases as the temperature is increased. In the case of MEM(TCNQ)2, the temperature dependence of the line width stays almost constant at the resonance field which is above the critical field H,. The temperature dependence of the ESR integrated intensity of CuGeO 3 is shown in Fig. 4. The data of Z taken from Ref. [1] are also shown by the solid line. At 370.4 GHz it is observed down to 4.2 K and no abrupt change is observed at Tse. On the other hand, the intensity of ESR absorption starts to decrease exponentially at around T~p and becomes almost zero at 4.2 K for 90 and 232.3 GHz. The temperature dependence of the integrated intensity at these frequencies seem to obey the temperature dependence of magnetic suscepti-
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1/T Fig. 5. The estimation of the spin-Peierls gaps at 90 (closed circle) and 232.3GHz (triangle).The gap in zero field was determined from the result of susceptibility measurement by Akimitsu et al. (cross).
bility Z as in the case of other spin-Peierls systems MEM(TCNQ)2 [9] and TTF-CuBDT [2]. We estimated the spin-Peierls gap in the finite field from the results of the temperature dependence of the ESR integrated intensity. Below the transition temperature Tsp, the absorption intensity should be proportional to exp[ - A(T)/T], where A(T) is the excitation gap. The resonance fields at 90 and 232.3 GHz are 3.0 and 7.7 T, respectively. As they are below He, the resonances observed at these frequencies below Tsp are in the spin-Peierls phase. As it is reasonable to think that A(T) ~ A(0) at about T <~ Tsp/2 from the BCS theory, we can estimate the excitation gap A(0) in each magnetic field using the data below 9 K. Fig. 5 shows the results of the fitting of the data taken at 90 and 232.3 GHz, the gap A(0) was determined to be 15 K for both frequencies. The same gap values for both frequencies are strange because the gap value obtained at 232.3 GHz, whose resonance field is higher, should be smaller than that obtained at 90GHz. This may be due to the poor data quality. Therefore, more precise measurement is required for this point. As the magnetic susceptibility is also proportional to exp[ - A ( T ) / T ] the value in zero field was also estimated to be 23 K in the same way from the results of low temperature magnetic susceptibility measurements for Hlla done by Akimitsu et al. for the same series of CuGeOa single crystal used in this submillimeter wave ESR measurement. This value is consistent with the value of A(0) = 24K obtained by Hase et al. [1]. It is reasonable that the excitation gap becomes smaller than the zero field value when the external magnetic field is applied. But for one more detailed discussion, more precise
S. lmagawa et al./ Physica B 201 (1994) 178-181 measurement and the measurements for n[Ib nllc are required. The measurements are still continuing and more detailed results will appear in the forthcoming paper.
References [1] M. Hase, I. Tesasaki and K. Uchinokura, Phys. Rev. Lett. 70 (1993) 3651. 1-2] J.W. Bray, H.R. Hart Jr., L.V. Interrante, I.S. Jacobs, J.S. Kasper, G.D. Watkins, S.H. Wei and J.C. Bonner, Phys. Rev. Lett. 35 (1975) 744. [3] L.V. Interrante, J.W. Bray, H.R. Hart Jr., I.S. Jacobs, J.S. Kasper and P.A. Piacente, in: Lecture Notes in Physics, Vol. 96 (Springer, Berlin, 1979) p.55. [4] S. Huizinga, J. Kommandeur, G.A. Sauatzky, B.T. Thole, K. Kopinga, W.J. de Jonge and J. Roos, Phys. Rev. B 19 (1979) 4723.
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