to appear in Supercond. Sci. Technol. 18 (2005) R1-R8 1 ______

TOPICAL REVIEW Assembling the puzzle of superconducting elements: A Review

Cristina Buzea 1,2 * and Kevin Robbie 1,2

1 Physics Department, Queen's University, Kingston K7L 3N6, Canada 2 Centre for Manufacturing of Advanced Ceramics & Nanomaterials, Queen's University

ABSTRACT: in the simple elements is of both technological relevance and fundamental scientific interest in the investigation of superconductivity phenomena. Recent advances in the instrumentation of physics under pressure have enabled the observation of superconductivity in many elements not previously known to superconduct, and at steadily increasing temperatures. This article offers a review of the state of the art in the superconductivity of elements, highlighting underlying correlations and general trends.

PACS: 74.70.Ad, 74.62.-c, 74.62.Fj, 74.62.Yb

(Some figures in this article are in colour only in the electronic version) ______

1. Introduction Superconductivity is a collective state occurring in the electron population of a material, and its study is an After two decades of focused research on important discipline of condensed matter physics. The superconductivity in copper oxides or cuprates, interest has occurrence of superconductivity is not restricted to recently veered toward simpler materials, such as conventional materials, however, but is believed to be a magnesium diboride and basic elements. Simple materials universal phenomenon, arising at all scales from were recently reported to superconduct at, surprisingly high superconducting cosmic strings generated as topological critical temperatures (Tc), 40 K for MgB2 - a binary defects in the early universe [9] or superconducting cores compound [1], and 20 K for lithium under pressure - the in neutron stars [10], down to the fascinating phenomenon highest Tc for a simple element [2]. During the last few of nuclear size superconductivity [11]. years, the physics community has been delighted with observation of superconductivity in many new elements, some of them under pressure, such as sulphur 17 K [3], 0.5 K [4], carbon in nanotube 15 K [5] and diamond forms 4K [6], a non-magnetic state of iron 1 K [7], and the light elements lithium 20 K [2] and boron 11 K [8]. While we will soon celebrate a century of superconductivity (Figure 1), these recent discoveries highlight answers to fundamental questions that the elements of the periodic table still conceal. Though thousands of papers have been published on the topic, a general predictive model of superconductivity has eluded us. ______

* electronic mail: [email protected] Figure 1. Historical development of the critical temperature of simple elements to appear in Supercond. Sci. Technol. 18 (2005) R1-R8 2 ______

Figure 2. Periodic table of superconducting elements

Recently experiments and theories on atomic condensation If a given element is not superconducting at normal [12] and superconductivity in a two-dimensional electron pressure, there are several ways of causing it to gas [13] have demonstrated a great variety of correlated superconduct, illustrated schematically in Fig. 3. Transport states related to superconductivity. First observed in of electron pairs from a superconductor in close proximity mercury almost a century ago [14], condensed-matter creates a small superconducting interface layer in some superconductivity remains among the most studied materials, a phenomenon called proximity induced subfields of physics. superconductivity. Crystal strain or non-equilibrium crystal This review is organized as a systematic summary of textures can also induce superconductivity, through applied the superconducting behaviour of the chemical elements, pressure, quenched growth on a cold substrate, or epitaxial referencing the periodic table of Fig. 2. Beginning from templating in the form of thin films. Chemical doping that hydrogen, ending with iodine, the discussion surveys the affects electron occupancy (charge doping) produces table, grouping elements by electronic structure to shed superconductivity in many materials, including diamond. light on the puzzle of superconductivity. Finally, irradiation induced lattice disorder can also lead to superconductivity through the suppression of spin fluctuations. 2. Inducing Superconductivity The atomic structure of a material determines its superconducting characteristics with superconductivity Once thought a rare anomaly, it now appears that arising as a collective transport behavior through the superconducting states can be formed in most materials, electronic potential of the crystal. An illustration of this is including thousands of inorganic compounds, most of the the existence of different critical temperature values for natural elements, alloys, oxides, and even organic materials polymorphic forms of certain elements, such as lanthanum, and, significantly, the double-stranded molecules of DNA which superconducts at Tc = 4.8 K when double hexagonal [15]. closed packed, dhcp, compared to Tc = 6 K when face centered cubic, fcc [16]. to appear in Supercond. Sci. Technol. 18 (2005) R1-R8 3 ______

Figure 3. Techniques used to transform normal elements into superconductors

3. Superconductivity under Pressure pressure into a monoatomic metallic solid [22], then that Applying pressure is often the simplest way of modifying metallic H will superconduct in the atomic [19], with the crystalline structure of a material, where the material critical temperatures reaching high values, perhaps to room can undergo structural phase transitions, each structure temperature. The main technological problem encountered being characterized by a unique electronic configuration. in the study of H, the smallest of atoms, is its high Phase transformations occur in many elements under diffusivity and chemical reactivity, which make it the most compression and some high-pressure phases exhibit difficult material to contain at high pressure, tending to superconductivity. embrittle and weaken gaskets and anvils. Despite these The renaissance in high-pressure research on difficulties, the study of H has generated several interesting superconductivity is primarily the result of recent results, among which is its transformation into a metallic development of high-pressure diamond anvil cells [17]. fluid at 140 GPa [23], and the modification of its optical While the highest attainable pressure in 1968 was properties (darkening) at higher pressures [24]. All approximately 25 GPa employing Bridgman type anvils at attempts at demonstrating its superconductivity, however, liquid helium temperatures [18], today measurements can have had negative results. be performed to a maximum pressure of 260 GPa [8]. This made possible the surprise observation of higher-pressure emergent phases of simple elements that exhibit 4.2. Alkali and alkaline metals superconducting properties. Of particular interest is the research of the light elements, as they are predicted to Among the alkali metals (Li, Na, K, Rb, Cs, Fr) only Li superconduct at record high temperatures [19]. and Cs were found to superconduct under pressure. Superconductivity under pressure is seen in many Calculations predicted superconductivity of Li at high elements, as is summarized in the periodic table of Fig. 2. pressure [25] with a Tc of up to 80 K [26]. Early Some elements transformed to a metallic state without measurements of Li resistivity under pressure revealed a showing signs of superconductivity, e.g. xenon [20], others accompanied by a sudden drop of to semiconductors, e.g. nitrogen [21]. resistivity around 7 K, attributed to possible superconductivity [27]. A clear demonstration of Li superconductivity was recently published [2, 28], with Tc 4. Superconducting elements increasing with pressure and reaching an unprecedented 20 K for a simple element, under 50 GPa [2]. A plot of its 4.1. Hydrogen critical temperature dependence on pressure is shown in Fig. 4 together with the behavior of superconducting As the first element in the periodic table, hydrogen attracts alkaline earth metals. The experimental behavior of Li is much attention, especially due to the predictions of its not adequately resolved, and agreement has not been high-pressure superconductivity at very high temperatures reached on its Tc dependence on pressure [2, 28, 29]. Data [19]. At moderate pressures and temperatures, H occurs in measured under almost hydrostatic pressures [29] an insulating molecular phase. Almost seven decades ago (compressed in an anvil while surrounded by helium to hydrogen was predicted to transform under extreme provide uniform pressure without shear) shows an increase to appear in Supercond. Sci. Technol. 18 (2005) R1-R8 4 ______

2 1 Figure 4. Critical temperature dependence on pressure for alkali Figure 5. Critical temperature dependence on pressure for s d and alkaline metals, Li [2, 28, 29], Ca [36], Ba [37-43], Sr [37]. transition metals, La [16], Lu [44], Sc [44], Y [30].

then decrease of Tc with pressure, evidence that 4.4. Transition metals superconductivity competes with symmetry breaking structural phase transitions. Most of the transition metals [27 metals from the area Cesium under pressure (not shown in Fig. 4) has a cornered by Ti, Hf, Hg, and Zn) are superconducting at much lower critical temperature than Li, with Tc decreasing normal pressure, the highest critical temperatures being for monotonically from 1.66 K for increasing pressure [30]. elements in the fifth group: Nb at 9.25 K [45], V at 5.4 K For the other alkalis, preliminary calculations suggest that [46], and Ta at 4.4 K [47]. Niobium also holds the record Na might become superconducting under pressure [31], but for the highest critical temperature of an element at normal has yet to be experimentally observed. pressure. No structural phase transitions are observed in Nb Among alkaline earth metals (Be, Mg, Ca, Sr, Ba, Ra) and Ta, so changes in Tc are thought to be the result of several superconduct: Be, Ca, Sr, and Ba. Beryllium, has a changes in electronic structure alone, creating a Tc(p) that is nearly constant in Nb from 10 Gpa to 70 GPa, and from modest Tc of 26 mK in bulk form [32], but attains a Tc of 9.95 K when deposited as a quenched-condensed film [33]. ambient to 45 GPa in Ta [45], as seen in Fig. 6. Unlike The heavier alkaline earth metals, Ca, Ba, and Sr transform niobium and tantalum, vanadium has a large positive into superconductors only under pressure, explained as an pressure coefficient of the critical temperature [48], its s-d electron transition arising from ion core orthogonality critical temperature under pressure reaching 17.2 K at 120 requirements [34]. Calcium [35, 36] and strontium [37] GPa, among the highest for simple elements [49]. The increase of T is thought to be due to suppression of have a positive dTc/dp curve, while the critical temperature c of Ba increases and then decreases [37-42], as seen in Fig. electron spin fluctuations through broadening of the d-band 4. From electronic band structure calculations for Ba [43] it width [50]. was concluded that the 5d component in the conduction- A material less studied since its superconductivity electron wave function increases strongly under pressure, discovery [51, 52], is zirconium, with two superconducting transforming Ba into a 5d transition metal through s-d phases, hcp and bcc (body center cubic), reaching a critical hybridization at the Fermi level. temperature of 11 K at 30 GPa in its bcc form [53]. Chromium superconducts at 3 K in an fcc phase fabricated by epitaxially sandwiching Cr between gold 4.3. s2d1 transition metals layers [54]. Palladium is a metal with particularly interesting Yttrium [30] and its related s2d1 transition metals Sc, La, properties. From its high electronic density of states at the Fermi surface and its phonon spectrum, one would expect and Lu [44], have a positive dTc/dp slope, as shown in Fig. 5. Lanthanum has two superconducting phases at normal strong electron-phonon coupling, and therefore a pressure, dhcp at 4.8 K and fcc at 6 K [16]. reasonable high superconducting transition temperature. Experimentally, however, superconductivity does not occur down to 1.7 mK [55] due to spin fluctuations, though to appear in Supercond. Sci. Technol. 18 (2005) R1-R8 5 ______

Figure 7. Normalized critical temperature variation with Figure 6. Critical temperature dependence on pressure for pressure and doping level for materials with ferromagnetic or transition metals, Zr [52, 53], V [48-50], Nb [45], Ta [45, 67], and antiferromagnetic behavior. Data are taken from references [59], Fe [7]. [58] and [7] for UGe2, La-Sr-Cu-O, and Fe, respectively.

palladium can be transformed into a superconductor with a superconductivity was observed in UGe2, on the border of ferromagnetism within the same electrons that produce Tc of 3.2 K through the introduction of lattice disorder via low-temperature irradiation with He+ ions [56], in band magnetism [59]. These recent discoveries of qualitative agreement with theoretical predictions for superconductivity in iron [7], as well as in cobalt superconductivity in palladium without spin fluctuations compounds [60, 61], re-focuses attention on magnetic [56]. mediated superconductivity as an alternative to phonon mediated.

4.5. Superconductivity and magnetism 4.6. Metals The correlation between superconductivity and magnetism makes the study of ferromagnetic metals (Fe, Co, Ni, Gd) Metals from groups 13-15 (Al, Ga, In, Tl, Sn, Pb, Bi) particularly important. Iron is the classic magnetic metal, situated on the right edge of the transition metals in the being strongly ferromagnetic at standard conditions in its periodic table show several common characteristics. First, bcc phase. In this ferromagnetic form iron does not they all superconduct at normal pressure, except bismuth, superconduct to the lowest temperatures attainable. At which is almost a semi-metal. Second, the Tcs of the pressures above 10 GPa iron assumes an hcp structure, elements decrease with increasing pressure, as shown in believed to be non-magnetic. The superconductivity of the Fig. 8 for Al [62], Sn [63], Pb [40, 64-67] and Bi [39, 40], iron hcp phase was predicted [57] and was recently the derivative dTc/dp having similar values. In addition, the confirmed experimentally [7]. As illustrated in Fig. 6, the critical temperature seems to increase from group 13 to 15. Aluminum superconducts below 1.18 K [62], and when Tc(p) diagram for iron has a specific bell-shaped curve. Interestingly, this for iron [7] shows a in mesoscopic wire form shows a peculiar resistance peak striking resemblance to the critical temperature versus in the superconducting state near Tc, attributed to thermal doping or carrier concentration of superconductors with fluctuations producing phase slips of the superconducting antiferromagnetic parent compounds - La Sr Cu O [58], order parameter [68]. 2-x x 2 4 Gallium has a critical temperature of 1.08 K in bulk or UGe2 [59], as illustrated in Fig. 7. While their relationship is not fully understood, it was believed that form, and in amorphous film form, quench-condensed on ferromagnetism and superconductivity were competing liquid-helium cooled substrates, attains 8.6 K [69]. mechanisms, and the superconductivity of iron came as a Bismuth is an example of those elements that do not surprise. Perhaps the explanation lies in theoretical model display superconductivity under ordinary circumstances, of magnetism mediated superconductivity [59]. The but undergo superconducting phase transitions in a fcc validity of this model was questioned until recently, when phase [70] after being subjected to hydrostatic pressure. Bi to appear in Supercond. Sci. Technol. 18 (2005) R1-R8 6 ______

Figure 8. Critical temperature dependence on pressure for Figure 9. Critical temperature dependence on pressure for semi- metals, Al [62], Sn [63], Pb [40, 64-67], Bi [39, 40]. metals, B [8], Si [72-74], As [75, 76], Te [77-79].

superconducts with a Tc of 8.7 K at 9 GPa [40], with a Tc = Carbon likely occurs in the widest variety of elemental 4 K [70] in thin film form on Ni substrates, and with Tc = forms, the most known allotropes being graphite, diamond, 6.2 K [71] when amorphous. C60 carbon spheres (Buckminster-fullerenes), and carbon nanotubes derived from curved graphene sheets. The possibility of superconductivity in carbon nanotubes was 4.7. Semi-metals predicted in 1995 with Tc expected to be inversely proportional to the nanowire diameter, an effect related to A general characteristic of semi-metals is that they do not electron-phonon interactions [81]. In 2001 Kociak et al. show superconductivity at normal pressure, but [82] reported superconductivity below 0.55 K in ropes of superconduct when compressed. Most of them have single-walled carbon nanotubes with low-resistance structural phase transitions within the studied pressure contacts to non-superconducting metallic pads. Later the range, and exhibit different Tcs for different phases. As a same year Tang et al. [5] succeeded in observing common feature of Si [72-74], As [75, 76], and Te [77-79], superconductivity in one-dimensional 0.4 nm diameter the critical temperature increases with pressure, followed single-walled carbon nanotubes encapsulated in channels by saturation and a decline (Fig. 9). The recent research of of zeolite crystals with a record high Tc of 15 K. The latest superconductivity in the lightest semi-metal - boron, surprise from superconductivity in carbon-based materials probably triggered by the announcement of MgB2 came from boron doped diamond [6]. The discovery of superconductivity at nearly 40 K [1], spurred interest in superconductivity in a diamond-type structure suggests that superconducting physics on boron-related compounds [80]. Si and Ge, also having a diamond , might Boron superconductivity was observed with a maximum Tc superconduct under appropriate doping conditions. of 11.2 K at 250 GPa [8], as shown in Fig. 9. Since the discovery of superconductivity in phosphorous [83] at 5.8 K under 17 GPa its critical temperature was increased under 30 GPa pressure to 18 K 4.8. Non-metals [84]. The superconducting transition temperature of phosphorus under pressure depends on the path in the Among non-metals, carbon (in the form of diamond and pressure-temperature diagram [84-86]. Black insulating nanotubes), oxygen, bromine, and iodine have recently phosphorous with an orthorhombic structure is the most been transformed into superconductors, while the stable form at ambient conditions. When pressure is superconductivity under pressure of phosphorus, sulfur, increased at liquid-helium temperature black phosphorus and selenium was observed much earlier (Fig. 10). transforms to a metallic phase with a simple cubic structure Nitrogen was transformed into a semiconductor under with a Tc up to 10.5 K [85]. If red phosphorus is used as the pressure without exhibiting superconductivity [21]. starting material and pressurized at 4.2 K, the Tc increases up to 13 K with the onset of transition at 18 K under 30 to appear in Supercond. Sci. Technol. 18 (2005) R1-R8 7 ______

Figure 10. Critical temperature dependence on pressure for non- Figure 11. Highest critical temperature of simple elements as a metals, P [86], S [3, 93], Se [77], Br [93], I [92]. function of the atomic number.

Gpa. The resistance transition versus temperature exhibits a below 0.6 K, as revealed by resistive and magnetic drop with a finite temperature width, with onset at 18 K measurements [4]. It seems that the oxygen and zero resistance at 8 K. A new simple bcc structure has superconductivity is present even in the molecular metallic been observed above 262 GPa [87], giving hope that a state, in contrast with iodine and bromine. The transition similar or higher Tc could be achieved in this phase. It was temperature of oxygen is rather low compared to other suggested that this bcc phase of phosphorous might be elements in the 16th series: S, Se, and Te which stabilized at ambient conditions using suitable templates superconduct at 17, 7, and 7.4 K, respectively, in spite of such as V(100), Fe(100), or Cr(100) substrates [88]. This all expectations from both the structural sequence and the could lead to a breakthrough in technological spintronic increase of Tc for this group. applications, where the combined spin and superconducting None of the noble gases (He, Ne, Ar, Kr, Xe, Rn) are degrees of freedom will provide a new level of known to superconduct. A report on xenon indicates its functionality for microelectronic devices. transforms into a metallic state but without showing signs The first studies of superconductivity of sulfur were of superconductivity [20]. performed in 1978 with reports of transitions into the superconducting state at 5.7 K [89] and 9.8 K [90]. Recent studies of compressed sulfur [3, 91] show that the element transforms to a superconductor with Tc increasing with 5. Critical temperature correlations pressure to 17 K at 160 Gpa (Fig. 10). This is the third highest reported transition temperature for an elemental As a general trend, the maximum critical temperature of solid. This behavior contrasts with the negative dTc/dp simple elements scales with the atomic number Z, the observed at much lower pressures in the heavier highest Tc's being achieved for low Z (Fig. 11), implying superconducting Se and Te [77]. that light elements are the best candidates for the utmost The successful searches for superconductivity in the critical temperatures. Other correlations of Tc with normal halogen group were iodine [92] and bromine [93]. In the state properties [97], such as bulk modulus, work function, case of bromine under pressure, a molecular to Hall coefficient, Debye temperature, etc. should be re- monoatomic phase transition takes place at 80 GPa. At evaluated in the light of the most recent findings in the pressures higher than 90 GPa bromine becomes a critical temperature of simple elements. An interesting path superconductor [93]. would be a search for possible correlation of Tc with Hall Oxygen is known to show magnetic properties at low coefficient changes under pressure. temperatures, so was not expected to superconduct. At pressures exceeding 95 GPa, solid molecular oxygen undergoes a phase transition and becomes metallic. At around 100 GPa solid oxygen becomes superconducting to appear in Supercond. Sci. Technol. 18 (2005) R1-R8 8 ______

6. Conclusions 2001 Superconductivity in boron Science 293 272-274 [9] Vilenkin A and Shellard E P S 1994 Cosmic Strings and As future directions, further theoretical and experimental Other Topological Defects (Cambridge, Cambridge studies of superconductivity at pressures exceeding 50 GPa University Press) for most of the elements are needed. [10] Hoffberg M, Glassgold A E, Richardson R W and While applying pressure to a material is an established Ruderman M 1970 Anisotropic superfluidity in neutron way of exploring new crystalline structures, recent thin star matter Phys. Rev. Lett. 24 775-777 film fabrication methods have shown great utility in [11] Sharma S S and Sharma N K 1994 Q-deformed pairing growing unique and non-equilibrium crystals through vibrations Phys. Rev. C 50 2323-2331. [12] Greiner M, Regal CA and Jin DS 2003 Emergence of a sandwiched epitaxy [54] or control of geometrical effects molecular Bose-Einstein condensate from a Fermi gas in vapor aggregation [94, 95]. These methods should be Nature 426 537-540 further employed in the search for new superconducting [13] Phillips P et al 1998 Superconductivity in a two- phases. dimensional electron gas Nature 395 253-257 The study of elemental superconductors has an [14] Onnes H K 1911 The superconductivity of mercury enormous impact on our understanding of Comm. Phys. Lab. Univ. Leiden 122 124. superconductivity. Current work is laying the foundations [15] Kasumov A Yu et al 2001 Proximity-induced of elemental superconductor nanowires applications and superconductivity in DNA Science 291 280-282 further elucidation of the low dimensionality effect on their [16] Maple M B, Wittig J and Kim K S 1969 Pressure-induced properties. Advances in materials and nanomaterials magnetic-nonmagnetic transition of Ce impurities in La fabrication and characterization will certainly lead to Phys. Rev. Lett. 23 1375-1377 observation of superconductivity in elements at higher [17] Timofeev Y A et al 2002 Improved techniques for temperatures, creating new opportunities for applications measurement of superconductivity in diamond anvil cells such as spintronics [88] or DNA integrated molecular by magnetic susceptibility Rev. Sci. Instrum. 73 371-377 electronics [96]. As suggested by the fast pace of [18] Brandt N B and Berman I V 1968 Superconductivity of discoveries in this field of superconductivity, it seems the phosphorus at high pressures JETP Lett. 7 323 periodic table of superconducting elements will likely need [19] Ashcroft N W 1968 Mettalic hydrogen: a high- updating in the near future. temperature superconductor? Phys. Rev. Lett. 21 1748- 1749 [20] Eremets M I et al 2000 Electrical Conductivity of Xenon at Megabar Pressures Phys. Rev. Lett. 85 2797-2800 Acknowledgements [21] Eremets M I, Hemley R J, Mao H K and Gregoryanz E 2001 Semiconducting non-molecular nitrogen up to 240 We thank I. Pacheco for stimulating discussions and GPa and its low-pressure stability Nature 411 170-174 invaluable suggestions, and J. E. Hirsch, J. M. Fraser, M. [22] Wigner E and Huntington H B 1935 On the possibility of J. Stott, and K. Shimizu for helpful comments on the text. a metallic modification of hydrogen J. Chem. Phys. 3 764 This research was supported in part by the Natural [23] Nellis W J, Weir S T and Mitchell A C 1999 Minimum Sciences and Engineering Research Council of Canada metallic conductivity of fluid hydrogen at 140GPa (NSERC). (1.4Mbar) Phys. Rev. B 59 3434-3449 [24] Loubeyre P, Occelli F and LeToullec R 2002 Optical studies of solid hydrogen to 320 GPa and evidence for References black hydrogen Nature 416 613-617 [25] Neaton J B and Ashcroft N W 1999 Pairing in dense [1] Nagamatsu J et al 2001 Superconductivity at 39 K in lithium Nature 400 141-145 magnesium diboride Nature 410 63-64. [26] Christensen N E and Novikov D L 2001 Predicted [2] Shimizu K et al 2002 Superconductivity in compressed superconductive properties of lithium under pressure Phys. lithium at 20 K Nature 419 597-599 Rev. Lett. 86 1861-1864 [3] Struzhkin V V, Hemley R J, Mao H K and Timofeev Y A [27] Lin T H and Dunn K J 1986 High-pressure and low- 1997 Superconductivity at 10-17 K in compressed sulphur temperature study of electrical resistance of lithium Phys. Nature 390 382-384 Rev. B 33 807-811 [4] Shimizu K et al 1998 Superconductivity in oxygen Nature [28] Eremets M I, Struzhkin V V, Mao H K and Hemley R L 393 767-769. 2003 Exploring superconductivity in low-Z materials at [5] Tang Z K et al 2001 Superconductivity in 4 angstrom megabar pressures Physica B 329–333 1312-1316 single-walled carbon nanotubes Science 292 2462-2465 [29] Deemyad S and Schilling J S 2003 Superconducting [6] Ekimov E A et al 2004 Superconductivity in diamond Phase Diagram of Li Metal in Nearly Hydrostatic Nature 428 542-545 Pressures up to 67 GPa Phys. Rev. B 91 167001 [7] Shimizu K et al 2001 Superconductivity in the non- [30] Wittig, J 1970 Pressure-induced superconductivity in magnetic state of iron under pressure Nature 412 316-318 cesium and yttrium Phys. Rev. Lett. 24 812-815 [8] Eremets M I, Struzhkin V V, Mao H K and Hemley R J [31] Neaton J B and Ashcroft N W 2001 On the constitution to appear in Supercond. Sci. Technol. 18 (2005) R1-R8 9 ______

of sodium at higher densities Phys. Rev. Lett. 86 2830- pressure on the superconducting properties of zirconium 2833 Sov. Phys. JETP 19 823-825 [32] Falge R L Jr 1967 Superconductivity of hexagonal [53] Akahama Y, Kobayashi M and Kawamura H 1990 beryllium Phys. Lett. A 24 579-580 Superconductivity and phase transition of zirconium under [33] Grandqvist C G and Claeson T 1974 Superconducting high pressure up to 50 GPa J. Phys. Soc. Japan 59 3843- transition temperatures of vapour quenched Beryllium 3845 Phys. Lett. A 47 97-98 [54] Brodsky M S et al 1982 Superconductivity in Au/Cr/Au [34] McMahan A K 1986 Pressure-induced changes in the epitaxial metal film sandwiches (EMFS) Solid State electronic structure of solids, Physica B&C 139-140 31-41 Commun. 42 675-678 [35] Dunn K J and Bundy F P 1981 Electrical-resistance [55] Webb R A et al 1978 Very low temperature search for behavior of Ca at high pressures and low temperatures superconductivity in Pd, Pt, and Rh J. Low Temp. Phys. 32 Phys. Rev. B 24 1643-1650 659-664 [36] Okada S et al 1996 Superconductivity of calcium under [56] Stritzker B 1979 Superconductivity in irradiated high pressures J. Phys. Soc. Japan 65 1924-1926 palladium Phys. Rev. Lett. 41 1769-1773 [37] Dunn K J and Bundy F P 1982 Pressure-induced [57] Wohlfarth E. P. 1979 The possibility that e-Fe is a low superconductivity in strontium and barium Phys. Rev. B 25 temperature superconductor Phys. Lett. A 75 141-143 194-197 [58] Dabrowski B et al 1996 Dependence of Superconducting [38] Wittig J and Matthias B T 1969 Superconductivity of Transition Temperature on Doping and Structural barium under pressure Phys. Rev. Lett. 22 634-636 Distortion of the CuO2 Planes in La2-xMxCuO4 (M = Nd, [39] Il'ina M A and Itskevich E S 1970 Superconductivity of Ca, Sr) Phys. Rev. Lett. 76 1348-1351 barium at high pressures JETP Lett 11 15-17 [59] Saxena S S et al 2000 Superconductivity on the border of [40] Il'ina M A, Itskevich E S and Dizhur E M 1972 itinerant-electron ferromagnetism in UGe2 Nature 406 Superconductivity of bismuth, barium, and lead at 588-592 pressures exceeding 100 kbar Soviet Physics JETP 34 [60] Takada K et al 2003 Superconductivity in two- 1263-1265 dimensional CoO2 layers Nature 422 53-55 [41] Moodenbaugh A R and Wittig J 1973 Superconductivity [61] Sarrao J L et al 2002 Plutonium-based superconductivity in the high-pressure phases of barium J. Low. Temp. Phys. with a transition temperature above 18 K Nature 420 297- 10 203-206 299 [42] Probst C and Wittig J 1977 Superconductivity of bcc [62] Gubser D U and Webb A W 1975 High-pressure effects barium under pressure Phys. Rev. Lett. 39 1161-1163 on the superconducting transition temperature of [43] Vasvari B, Animalu A O E and Heine V 1967 Electronic aluminum Phys. Rev. Lett. 35 104-107 Structure of Ca, Sr, and Ba under Pressure Phys. Rev. Lett. [63] Jennings L D and Swenson C A 1958 Effects of pressure 154 535–539 on the superconducting transition temperatures of Sn, In, [44] Wittig J, Probst C, Schmidt F A and Gschneidner K A Jr Ta, Tl, and Hg Phys. Rev. 112 31-43 1979 Superconductivity in a new high-pressure phase of [64] Wittig J 1966 Super conductivity of germanium and scandium Phys. Rev. Lett. 42 469-472 silicon at high pressure Z. Phys. 195 215-227 [45] Struzhkin V V, Timofeev Y A, Hemley R J and Mao H K [65] Wittig J 1966 Superconductivity of tin and lead under 1997 Superconducting Tc and electron-phonon coupling in very high pressure Z. Phys. 195 228-238 Nb to 132 GPa: magnetic susceptibility at megabar [66] Smith T F and Chu C W 1967 Will pressure destroy pressures Phys. Rev. Lett. 79 4262-4265 superconductivity? Phys. Rev. 159 353-358 [46] Webber R T, Reynolds J M and McGuire T R 1949 [67] Kohnlein D 1968 Supraleitung von vanadium, niob und Superconductors in alternating magnetic fields Phys. Rev. tantal unter hohem druck Z. Phys. 208 142 76 293-295 [68] Moshchalkov V V et al 1994 Intrinsic resistance [47] Worley R D, Zemansky M W and Boorse H A 1955 Heat fluctuations in mesoscopic superconducting wires Phys. capacities of vanadium and tantalum in the normal and Rev. B 49 15412-15415 superconducting phases Phys. Rev. 99 447-458 [69] Buckel W, Bulow H and Hilsh R 1954 [48] Brandt N B and Zarubina O A 1973 Superconductivity of Elektronenbeugungs-aufnahmen von dünnen vanadium at pressures up to 250 kbar Sov. Phys. Solid metallschichten bei tifen temperaturen Z. Phys. 138 136 State 15, 3423-3425 [70] Moodera J S and Meservey R 1990 Superconducting [49] Ishizuka M, Iketani M and Endo S 2000 Pressure effect phases of Bi and ga induced by deposition on a Ni on superconductivity of vanadium at megabar pressures sublayer Phys. Rev. B 42 179-183 Phys. Rev. B 61 R3823–R3825 [71] Chen T T, Chen J T, Leslie J D and Smith H J T 1969 [50] Akahama Y, Kobayashi M and Kawamura H 1995 Phonon Spectrum of Superconducting Amorphous Pressure effect on superconductivity of V and V-Cr alloys Bismuth and Gallium by Electron Tunneling Phys. Rev. up to 50 GPa J. Phys. Soc. Japan 64 4049-4050 Lett. 22 526-530 [51] Smith T S and Daunt J G 1952 Some properties of [72] Il'ina M A and Itskevich E S Superconductivity of high- superconductors below 1K. III. Zr, Hf, Cd, and Ti Phys. pressure phases of silicon in the pressure range up to 14 Rev. 88 1172-1176 GPa Sov. Phys. Solid State 22 1833-1835 [52] Brandt N B and Ginzburg N I 1964 Effect of high [73] Chang K J et al 1985 Superconductivity in high-pressure to appear in Supercond. Sci. Technol. 18 (2005) R1-R8 10 ______

metallic phases of Si Phys. Rev. Lett. 54 2375-2378 arsenic alloys at low temperatures and high pressures [74] Lin T H et al 1986 Pressure-induced superconductivity in Phys. Rev. B 50 16274–16278 high-pressure phases of Si Phys. Rev. B 33 7820-7822 [87] Akahama Y et al 2000 Structural stability and equation of [75] Chen A L et al 1992 Superconductivity in arsenic at high state of simple-hexagonal phosphorus to 280 GPa: Phase pressures Phys. Rev. B 46 5523-5527 transition at 262 GPa Phys. Rev. B 61 3139-3142 [76] Bermen I V and Brandt N B 1969 Superconductivity of [88] Ostanin S, Trubitsin V, Staunton J B and Savrasov S Y arsenic at high pressures JETP Lett. 10 55-57 2003 Density functional study of the phase diagram and [77] Akahama Y, Kobayashi M and Kawamura H 1992 pressure-induced superconductivity in P: implications for Pressure induced superconductivity and phase transition in spintronics Phys. Rev. Lett. 91 087002 selenium and tellurium Solid State Commun. 84 803-806 [89] Evdokimova V V and Kuzemskaya I G 1978 [78] Berman I V, Binzarov Z I and Zhurkin P 1973 Study of Superconductivity in S at high pressure JETP Lett. 28 390- superconductivity properties of Te under pressure up to 392 260 kbar Sov. Phys. State 14 2192-2194 [90] Yakovlev E N, Stepanov G N, Timofeev Y A and [79] Bundy F P and Dunn K J 1980 Pressure dependence of Vinogradov B V 1978 Superconductivity of sulfur at high superconducting transition temperature of high-pressure pressures JETP Lett. 28 340-342 metallic Te Phys. Rev. Lett. 44 1623-1626 [91] Kometani S et al 1997 Observation of pressure induced [80] for a review see Buzea C and Yamashita T 2001 Review superconductivity in sulfur J. Phys. Soc. Japan 66 2564- of the superconducting properties of MgB2 Supercond. Sci. 2565 Technol. 14 R115–R146 and references therein [92] Shimizu K et al 1994 The pressure induced [81] Benedict L X, Crespi V H, Louie S G and Cohen M L superconductivity of iodine J. Superconductivity 7 921- 1995 Static conductivity and superconductivity of carbon 924 nanotubes: Relations between tubes and sheets Phys. Rev. [93] Amaya, K et al 1998 Observation of pressure-induced B 52 14935-14940 superconductivity in the megabar region J. Phys.: [82] Kociak M et al 2001 Superconductivity in ropes of Condens. Matter. 10 111179-111190 single-walled carbon nanotubes Phys. Rev. Lett. 86 2416- [94] Robbie K et al 2004 Ultrahigh vacuum glancing angle 2419 deposition system for thin films with controlled three- [83] Wittig J and Matthias B T 1968 Superconducting dimensional nanoscale structure Rev. Sci. Instrum. 75 phosphorous Science 160 994-996 1089-1097 and references therein [84] Shirotani I, Kawamura H, Tsuburaya K and Tachikawa K [95] Karabacak T et al 2003 Beta-phase tungsten nanorod 1987 Superconductivity of phosphorus and phosphorus- formation by oblique-angle sputter deposition Appl. Phys. arsenic alloy under high pressures Jpn. J. Appl. Phys. Lett. 83 3096-3098 Suppl. 26 921-922 [96] Shigematsu T et al 2003 Transport properties of carrier- [85] Kawamura H, Shirotani I and Tachikawa K 1984 injected DNA J. Chem. Phys. 118 4245-4252 Anomalous superconductivity in black phosphors under [97] Hirsch J E 1997 Correlations between normal-state high pressures Solid State Commun. 49 879-881 properties and superconductivity Phys. Rev. B 55 9007- [86] Shirotani I et al 1994 Phase transitions and 9023 superconductivity of black phosphorus and phosphorus-