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Rev. High Pressure Sci. Technol., Vol. 7 (1998) 688•`693 Pressure Iinduced Superconductivity in Some Simple Systems K. Amaya a,b,c, K. Shimizu a,b, N. Takeshita a, S. Kometani a, M. I. Eremets b,c,A. Onodera a, T. C. Kobayashi b,d, T. Mizutani d, M. Ishizuka d, S. Endo b,d,M. Takai d, N. Hamaya e, I. Shirotani a Dept. of Material Physics, Faculty of Engineering Science, Osaka University, Toyonaka, Osaka 560 Japanb CREST, Japan Science and Technology Corporation (JST)c Research Group for Quantum Condensed Matter System,Japan Atomic Energy Research Instituted Research Center for Materials Science at Extreme Conditions, Osaka University, Toyonaka Osaka 560, Japane Faculty of Science, Ochanomizu University, Ohtsuka 2-1-1, Bunkyou-ku, Tokyo 112 Japanf Muroran Institute of Technology, Mizuntoto-cho 27-1, Muroran, Hokkaido 050, Japan Experimental techniques on production of ultra-high pressure at very low temperature, electrical resistance and magnetization measurement are developed for the purpose of searching for pressure-induced superconductivity in some simple systems. Previous results of pressure induced superconductivity of I, Br, Ca, SnI4 and C6I4O2are reviewed and remaining problems are discussed shortly. Recent findings of metallization and superconductivity of O and S are also reported. [Ultra high pressure, very low temperature, metallization, superconductivity, electrical resistance] 1. Introduction taking account of the good thermal conductivity as well as the The materials are not so many for which pressure-induced non-magnetism down to temperatures below 1 K. superconductivity are searched under combined extreme For magnetization measurements, standard SQUID conditions of ultra-high pressure up to 150 GPa and very low magnetometer probe is used and instead of conventional do temperature below I K. Since several years, we have developed magnetization detection, we have also tried ac detection by setting high pressure l low temperature apparatus by assembling a moving pick up coils around just above the diamond anvil. This compact diamond anvil cell (DAC) on a powerful 3He/4He ac method is found to have several advantages such as high dilution refrigerator, resulting the production of ultra-high pressure temperature operation, magnetization measurement and so on as exceeding 150 GPa at very low temperature of 30 mK. is described elsewhere [2]. Electrical resistance measurements are Developments are also made for electrical resistance [1] and performed by conventional ac 4 terminal method. The manual magnetizationmeasurements [2] for samples with extremely small setting of 4 metal electrodes needs skills. The difficulty increases volume of the order of less than 10-7cc. towards the ultra-high pressure region where the diameter of the Under those experimental conditions, we have observed pressure surface becomes less than 50ƒÊm. To improve the for the first time, most directly the pressure induced metallization situation, we have tried to draw electrical circuits with the aid of and superconductivityof several elements such as Ca [3], I [4], Br the lithography technique as shown typically in Fig.1, sputtering, [5] and quite recently, S [6] and O [7]. The transition to focused ion beam and so on. Experiments are still going on but superconductivity is also observed for a simple inorganic as well found to be promising for measurements in near future. as organic compound such as tintetra iodide SnI4 [8] and iodanil The cell itself is found to be cooled down to 30 mK by C6I4O2[9], respectively. assembling DAC on 3He/4He dilution refrigerator. The temperature of the pressed sample is expected to be close to the cell 2, Experimentals temperature because the sample heat capacity is negligibly small A compact diamond anvil cell (DAC) is an unique solution because of the size and also the sample is metallic and that keeps for production of ultra-high pressure exceeding 100 GPa at low strong mechanical contact with measuring electrodes. temperatures.Pure BeCu and Cu-Ti are used for our pressure cells, Experimentally, the thermal contact is partly checked by 689 observation of hysteresis of, for example, the transition temperature of the superconductivity in the course of decreasing and increasing mode of temperature. 3. Results Starting with the first sample of iodine I, we have already succeeded for observation of pressure-induced superconductivity of elements of I, Br, Ca, S and O. We have already reported about the first four elements and only point out the remaining problems about them in this report. On the other hand, we observed recently superconductivityas well as metallization by the most direct method of electrical resistance measurementfor the followingtwo elements of S and O, for which we reports rather in detail. Finally we review the results on simple Fig. 1. Four Pt electrodes deposited on the pressure compounds, of inorganic Snl4 and organic iodanil C6I4O2.Both surface of diamond, using lithography-technique. compounds are found to show pressure-inducedamorphization and metallization like sulfur and superconductivity at the amorphous metallic state at low temperature.I and Br Metallic iodine is pointed out to be important as a prototype of metallic hydrogen. One of the most interesting results is that iodine crystal becomes molecular metallic state before the molecular dissociation and that the superconductivityappears only at the monatomic phasejust after the molecular dissociation at 21 GPa. At the highest fcc phase, the transition temperature T, to the superconducting state goes up with increasing pressure as shown in Fig.2. This result contradicts with the theoretical prediction of dTc/dP<0obtained from the first principle band calculation [10]. According to our measurements of Hall effects [11], it becomes definite that the carrier of the iodine is a hole and the carrier density increases rapidly towards molecular dissociation in consistent with the absence of superconductivity at the molecular metallic state. Quite similar properties are observed in the case of bromine, Br, with the higher pressure value of 80 GPa for Fig. 2. Pressure dependence of Tc of solid iodine. molecular dissociation. The superconductivity is observed up to Number I to IV show the crystal phase under pressure and 150 GPa but T is almost pressure independent as shown in Fig.3. the highest IV is fcc phase where the theoretical It is pointed out that there exists scaling laws [ 12] for metallization prediction is given by dotted line. under pressure among solid halogens. For the onset of superconductivity, the transition is observed under pressure 690 region exceeding a critical point Pc where the metallization in the monatomic phase is to be induced but the Tc's of I and Br at each Pc are close to each other and may be difficult to predict in the case of other solid halogen like Cl.Ca Dunn and Bundy [13] observed a small change of electrical resistanceof Ca under pressure at the reached temperature of 2 K and claimedthat it due to the onset of superconductivity.We have examinedthe temperature dependence of the electrical resistance underpressures up to 150 GPa and found the superconductivity at pressuresabove 50 GPa. From the comparison with Ba and Sr, we could support the s-d transformation mechanism for the onset of superconductivity which appears under critical pressure where electrons transfer from s-band to d-band. Figure 4 shows the pressure dependence of dTc/dP>o up to the reached pressure of 150 GPa and Tc of 15 K there. This is compared with the cases for Ba and Sr, in which the maximum of Tc are observed to exists at a certain pressure value. Fig. 3. Temperature dependence of resistance of Br S and O under pressures. The onset of superconductivity is found Sulfur and oxygen belong to the same VI group in the already under 85 GPa. The inset shows the magnetic field periodic table. In the case of sulfur, there exists optical evidence dependence. [14] to show the metallization according to the reflectivity measurementsabove 80 GPa, where new higher pressure phase of orthorhombicstructure begins. On the other hand, about 20 years ago, Stepanov et al. [15] reported the existence of a small resistancedrop at about 10 K and they attributed the result due to the transitionto the superconductivity even if they could not give any idea about the applied pressure value. We tried to confirm the superconductivity definitely through direct measurement of the electrical resistance, giving pressure value definitely by standard ruby-fluorescencemethod. As shown in Figs. 5 and 6, we observed the transition temperature Tc of 12 K and the upper critical field He of 1.2 T under the pressure of 100 GPa. There appears no such a transition under pressures below 60 GPa. The observed small steps at around 15 K and 13 K may reflect the possible pressure Fig 4. Pressure dependence of Ba, Sr and Ca. dTc/dP>0 distribution inside the sample space of the used DAC. We also up to the measured maximum pressure of 150 GPa. confirmed the pressure dependence of Tc is positive, giving the higher Tc than elements of Te and Se, which belong to the same group. Almost at the same time, we also observed the pressure- 691 inducedsuperconductivity of oxygen. Molecular oxygen is unique because of the magnetism with spin S =1. In fact, solid oxygen is well-known to show the antiferromagnetism at low temperature. Under pressures, however, the magnetism of solid oxygen has not yet been examined. Therefore, the magnetism as well as the superconductivityof solid oxygen, if it exists, at the metallic state, is of great interest both theoretical and experimental view points. First of all, we studied the metallization under pressure up to 100 GPa at room temperature by electrical resistance measurements and also by direct observation of reflectivitythrough a microscope. The resistance measured at room temperature is found to drop drastically by several orders of magnitude in between 60 GPa at which the resistance becomes measurable and 100 GPa where significantchange of the resistance seems to stop.
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