Articles |
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Superconductivity in Graphite Intercalation Compounds (GIC) |
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| Superconductivity
in metal-graphite Masatsugu Suzuki, Itsuko
S. Suzuki (Department of Physics, SUNY-Binghamton),
Jurgen Walter (Department of Materials Science and
Processing, Osaka University) Metal-graphites (Ta-graphite and Pd-graphite) provide a model system for studying the physical properties of metal atoms in a two-dimensionally restricted space. These compounds have been synthesized by reduction of the metal chloride graphite intercalation compound (GIC) precursors at temperatures above a threshold temperature. In metal chloride GIC intercalate layers are formed of metal intercalate layers sandwiched between adjacent Cl layers. The Cl layers provide acceptor sites to charges transferred from graphite layers. Because of the attractive Coulomb interaction between metal and graphite layers, metal atoms form a regular lattice. In contrast, there is no Cl layer in metal-graphite. The absence of interplanar interaction between metal and graphite layers may give rise to the generation of metal nanoparticles encapsulated in the form of either a one dimensional chain or two-dimensional platelets in the interlamellar space between graphite layers. We will report experimental results of SQUID DC magnetization and SQUID AC magnetic susceptibility measurements on Ta- and Pd-graphites. Ta-graphite shows superconductivity at either 160 K or 10.2 K depending on sample. Pd- graphite shows either superconductivity at 3.8 K or an antiferromagnetic phase transition at 14 K depending on sample. The superconductivity of Ta-graphite at 10.2 K and Pd-graphite at 3.8 K may be related to possible formation of TaC and PdH in the intercalate layer, respectively. The superconducting properties of these compounds are characterized by glassy states reflecting the nature of granular systems. From : http://www.eps.org/aps/meet/MAR00/baps/abs/S6490009.html ------------------------------------------------------------ Enhanced Superconductivity in Hydrogenated Potassium-Mercury Graphite Intercalation Compounds G. Roth, A. Chaiken, T. Enoki, and N.C. Yeh, G. Dresselhaus, and P.M. Tedrow, AbstractStage 1 potassium--mercury graphite intercalation
compounds with markedly different superconducting
transition temperatures, Tc, and transition
widths, delta Tc, are found to have uniformly
high Tc and narrow transitions after exposure
to hydrogen. These hydrogen--doping experiments suggest a
schematic model for the density of states in stage 1 KHg--GIC's.
IntroductionOne particularly active area of research in recent years has been the study of the effects of hydrogen absorption on metals.[1] Among the many results of hydrogen uptake are changes in superconductivity. For example, in some transition metals hydrogen absorption causes an increase in the superconducting transition temperature, Tc, while in others Tc decreases upon hydrogenation.[2] The introduction of hydrogen has previously been shown to increase the transition temperature of one graphite intercalation compound, C8K.[3] The transition temperature depends strongly on the details of the band structure near the Fermi level. Therefore one cannot generalize from a single example about the effect of hydrogenation on other graphite intercalation compounds, just as it was difficult to generalize about the transition metals. The stage 1 potassium-mercury graphite intercalation compounds (KHg-GIC's) are especially interesting from the standpoint of superconductivity. Discussions of the nature of the superconductivity in these compounds have been greatly complicated by widely varying values reported for the superconducting transition temperature, Tc, which have ranged from the 0.8 K initially found[4] up to 1.5 K both with[5] and without[6] the application of external pressure. Structural and stoichiometric distinctions between samples have been cited as the source of variation among them. Previous attempts to understand the differences between samples focused on their intercalation conditions.[7] These experiments were limited by the indirect nature of the connection between the preparation parameters and the final properties of the GIC. In this work we describe the application of hydrogen-doping as a new probe of the relationship between structure and superconductivity in stage 1 KHg-GIC's. In contrast to other experiments relevant to the superconductivity of this compound, exposure to hydrogen yields a result which is reproducible not only among similarly prepared samples, but among stage 1 KHg-GIC's with a wide range of initial properties. Therefore this technique may properly be used to study the characteristics of stage 1 KHg-GIC's (denoted by C4KHg when stoichiometric[8]) without reference to the history of a particular sample. It is remarkable in this context to note that the uniformity of superconducting properties is achieved not by degrading all compounds to the level of the most disordered GIC, but by elevating all of them to the quality of the best ordered ones. Generally the stage 1 KHg-GIC has been discussed in conjunction with the corresponding stage 2 compound because of the surprising finding that stage 2 has a higher Tc than stage 1, despite specific heat measurements which indicate that stage 2 has a lower density of states at the Fermi level.[9] In this context, a comparison with the potassium hydride graphite intercalation compounds should also prove instructive. One reason is that C8KH2/3 has a triple-intercalate-layer structure[10] similar to that of potassium-mercury GIC's.[8] Also significant as far as superconductivity is concerned is that the Tc of C8KH0.19 is 0.22 K,[3] fifty percent higher than that of its parent compound, C8K, with a Tc of 0.15 K.[11] ExperimentalThe hydrogen-doped stage 1 KHg-GIC's described here were prepared in a two-step process, analogous with the preparation of C8KHx.[10] First, three types of stage 1 KHg-GIC's were prepared by the usual two-zone method. Two batches were prepared isothermally at temperatures of 200 and 260°C, respectively, and one was prepared at 200°C with a temperature difference of 4°C. As was described previously,[7] the two batches of GIC's which were prepared at 200°C showed one repeat distance, Ic, of (10.24 ± 0.03)? in (00l) x-ray scans, while those prepared at 260°C showed two sets of Bragg peaks, one corresponding to Ic= (10.22 ± 0.03)?, and one to Ic= (10.83 ± 0.03)?. In order to perform the hydrogen doping, the same samples that were characterized without hydrogen were transferred under vacuum to an ampoule containing 200 mbar of highly purified hydrogen gas. The two isothermally prepared GIC's, which were initially pink, became blue after about five minutes of exposure, and then turned a dark purple. The GIC which was prepared with a temperature difference was initially gold, but remained blue-violet indefinitely afterward. The superconducting transitions of all three types of samples were measured inductively (see Fig. 1), while the temperature was monitored by observation of the 3He vapor pressure. Tc was determined from the intersection of a tangent drawn to the transition curve with the level upper portion of the trace, and delta Tc was determined by measuring the difference in temperature between the points where the tangent intersected the upper and lower levels of the traces. Subsequent (00l) scans showed that the samples' repeat distances were unchanged upon hydrogen doping, but the effect on the superconductivity was dramatic. The superconducting transitions of the three types of samples before and after hydrogen doping are shown in Fig. 1. The addition of hydrogen is seen to have produced two effects. One is a marked narrowing of the transition width delta Tc which occurred in all samples measured, especially in the one intercalated at 260°C in which delta Tc went from 0.25 K to 0.03 K (Fig. 1c). The other notable effect was the increase of the Tc of all the samples to 1.5 K, except for the GIC prepared at 200°C, whose initial Tc of 1.53 K was unchanged by hydrogen uptake (Fig. 1b). Tc increased eighty percent in the compound prepared with a temperature difference, rising 0.66 K (Fig. 1a). The most significant result overall, though, is that the transitions of all three samples after hydrogen addition strongly resemble one another, whereas before they were quite distinct. Previous work has correlated the existence of a narrow transition in stage 1 KHg-GIC samples with the presence of a well-ordered in-plane structure, probably a (2 × 2) R0° phase.[6,7] The initially broad transition of the GIC with two repeat distances is then connected with the presence of a second in-plane phase, possibly a (cube-root × cube-root) R30° or a (cube-root × 2) R(30°,0°) structure,[7] or a disordered structure. Experiments with the application of external pressure to KHg-GIC's have produced a sharpening of the superconducting transition[5] apparently similar to that produced by hydrogen doping. The effect of pressure seems to be the formation of a more homogeneous intercalate structure. The transition-narrowing seen in association with hydrogen exposure must also be due to greater uniformity, probably caused by hydrogen diffusion into mercury vacancies associated with disorder. Rapid diffusion of hydrogen atoms is very plausible because of their small size, and is well known to occur in other metallic systems.[12] The total hydrogen uptake may then be limited by the number of available vacant Hg sites, or it may be determined by electronic effects, as in KHx-GIC's, where the maximum possible value of x=0.8 is limited by the finite number of electrons available for transfer to the hydrogen.[13] Charge transfer to hydrogen is also to be expected in doped stage 1 KHg-GIC samples because of hydrogen's high electron affinity. The filling of low-lying hydrogen states must remove electrons from intercalate and graphite bands and thus lower the Fermi energy. In the KH-GIC's, the transfer of electrons from potassium to hydrogenic states has already been directly observed using ESR[14] and 13C NMR.[15] Moreover, recent Shubnikov-de Haas measurements on these compounds show them to have a lower Fermi energy than the parent compound, C8K.[13] To see how the change in electronic occupation might affect the transition temperature of the KHg-GIC's, we can refer to the BCS theory of superconductivity, in which the superconducting transition temperature increases with increasing density of states at the Fermi level according to Tc =1.14 thetaD exp [- 1/N(Ef)V]. Here thetaD is the Debye temperature, N(Ef) is the density of states at the Fermi level, and V is the electron-phonon coupling matrix element. We see that if the rise in the transition temperature with the uptake of hydrogen is due to a change in the density of states, then the experimentally observed decrease in Ef must lead to an increase in N(Ef). The effect of hydrogen on the three types of samples can then be explained using the schematic density of states curves for the normal state compound shown in Fig. 2. Figure 2a displays the form proposed for the density
of states of a KHg-GIC with Tc less than 1.5 K.
There are small contributions from the graphite bonding
pi and antibonding pi* bands, but the density
of states is dominated by s-like and p-like bands due to
potassium and mercury. Note that the Fermi level is above
a local maximum in the schematic density of states of the
intercalate-derived bands. The basic shape of the density
of states is consistent with recent EELS experiments
which found strong evidence for the presence of mercury-like
and potassium-like contributions to N(Ef).[16]
Figure 2b shows the final configuration for a
hydrogenated stage 1 KHg-GIC. Now the Fermi level is
lower because of electron transfer from the intercalate
and graphite bands into the hydrogen states, but the
density of states at the Fermi level is higher. A GIC
whose Tc was unchanged from 1.5 K by
hydrogenation must have had its N(Ef) near
that indicated in Fig. 2b before doping. Therefore one
concludes that either its hydrogen absorption and
associated Fermi level shift must have been small, or
that its Fermi level lies near a broad local maximum in
the density of states. Low hydrogen uptake for samples
with an undoped Tc of 1.5 K is consistent with
the observation that their color change after
hydrogenation was less noticeable than that of the GIC's
with a lower undoped Tc. DiscussionFurther evidence that the role of hydrogen in stage 1 KHg-GIC's is to accept electrons comes from an examination of the effect of stoichiometry on the superconductivity of KHg alloys. In these compounds, Tc decreases monotonically with increasing potassium content, from 4.2 K in pure mercury to 0.9 K in KHg itself.[17,18] These data provide strong support for the idea that adding electrons to Hg decreases rather than increases the transition temperature. One can then think of the carbon layers in the potassium-mercury graphite intercalation compounds as a sink for some of the intercalant electrons, and use this idea to explain why Tc is higher in stage 1 KHg-GIC's than it is in the KHg alloy. The hypothesis that hydrogen increases the transition temperature by removing electrons from the intercalate bands is then fully consistent. The density of states model described above could also be extended to treat stage 2 KHg-GIC's. Several different experiments show that the Fermi level in stage 2 is about 0.5 eV lower than in stage 1.[15,16,19] The finding that Tc = 1.9 K is higher in stage 2 than in stage 1 may indicate that the same type of density of states shift that occurs upon hydrogenation also occurs from stage 1 to stage 2. This conclusion is supported by EELS data which indicates that the density of intercalate states is higher in stage 2.[16] The EELS result is apparently in conflict with the finding from specific heat measurements that the total N(Ef) decreases substantially from stage 1 to stage 2.[9] The conflict stems from the fact that for the two results to be consistent, one must assign a contribution to N(Ef) from the graphite pi bands which is an order of magnitude larger than that expected for the graphite pi bands at this Fermi level.[19] The seeming discrepancy between the density of states model and the specific heat measurements can be resolved by noting that the stage 1 GIC used for the specific heat measurement was found to be not superconducting down to 0.8 K.[9] Therefore the upper limit on the transition temperature of the GIC used in the specific heat experiment is below the lowest Tc measured for any of the samples described in the present work, which implies that the electronic structure of that compound may well have been different from that of the GIC's described here. If this assertion be correct, then other experiments present a unified picture of the behavior of the density of states in these compounds. ConclusionsIn summary, we have presented the results of a new experiment with many implications for the relationship between electronic structure and superconductivity in the stage 1 KHg-GIC's. The unique property of the experiment is that hydrogen doping produces the same effect on samples with very different initial properties, in contrast to many other methods of studying these compounds. A density of states model suggested by the hydrogen-doping results has been described. Extension of the hydrogenation studies to different stages and other compounds should prove to be of great interest. AcknowledgmentsThe authors G.R., A.C., N.C.Y. and G.D. gratefully acknowledge support by AFOSR #F49620-83-C-0011. T.E. was supported by DOE Grant #DE-AC02-83ER45041 and a grant from the Japanese Ministry of Education. T.E.'s permanent address is Institute for Molecular Science, Okazaki 444, Japan. The Francis Bitter National Magnet Laboratory is supported by the NSF. We would also like to thank M.S. Dresselhaus for useful discussions, the staff of the National Magnet Laboratory for help with the 3He experiments, and Mar?a Kudisch for assistance with sample preparation and x-rays. References
Figure CaptionsFigure 1: The effect of hydrogen doping on the superconducting transitions of three types of stage 1 KHg-GIC's. (a) Tc increases from 0.85 K to 1.51 K, and delta Tc decreases from 0.035 K to 0.011 K. (b) Tc remains constant at 1.53 K, and delta Tc decreases the same amount as in (a). (c) Tc increases from 1.32 K to 1.49 K, and delta Tc decreases from 0.25 K to 0.03 K. Figure 2: Schematic model for the density of states in (a) undoped and (b) hydrogenated stage 1 KHg-GIC's. (a) N(Ef) for an undoped compound with Tc less than 1.5 K. Ef lies above the maximum in the density of states in the intercalate bands. (b) N(Ef) for the hydrogen-doped KHg-GIC. The transfer of electrons from intercalate-derived bands to hydrogen levels has lowered the Fermi energy and increased N(Ef). --------------------------------------------------- |
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