Superconductivity at 5 K in Alkali-Metal-Doped Phenanthrene

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Received 13 Jul 2011 | Accepted 21 Sep 2011 | Published 18 Oct 2011 DOI: 10.1038/ncomms1513 Superconductivity at 5 K in alkali-metal-doped phenanthrene

X.F. Wang1, R.H. Liu1, Z. Gui2, Y.L. Xie1, Y.J. Yan1, J.J. Ying1, X.G. Luo1 & X.H. Chen1

Organic superconductors have π-molecular orbitals, from which electrons can become delocalized, giving rise to metallic conductivity due to orbital overlap between adjacent molecules. Here we report the discovery of superconductivity at a transition temperature (Tc) of ~5 K in alkali-metal-doped phenanthrene. A 1-GPa pressure leads to a 20% increase of Tc, suggesting that alkali-metal-doped phenanthrene shows unconventional superconductivity. Raman spectra indicate that alkali-metal doping injects charge into the system to realize the superconductivity. The discovery of superconductivity in A3phenanthrene (where A can be either K or Rb) produces a novel broad class of superconductors consisting of fused hydrocarbon benzene rings with π-electron networks. An increase of Tc with increasing number of benzene rings from three to five suggests that organic hydrocarbons with long chains of benzene rings are potential superconductors with high Tc.

1 Hefei National Laboratory for Physical Sciences at Microscale and Department of Physics, University of Science and Technology of China, Hefei, Anhui 230026, China. 2 State Key Laboratory of Fire Science, University of Science and Technology of China, Hefei, Anhui 230026, China. Correspondence and requests for materials should be addressed to X.H.C. (email: [email protected]). nature communications | 2:507 | DOI: 10.1038/ncomms1513 | www.nature.com/naturecommunications © 2011 Macmillan Publishers Limited. All rights reserved. ARTICLE nature communications | DOI: 10.1038/ncomms1513

rganic superconductors mainly belong to one of two fami- c o b a lies: the quasi-one-dimensional (TMTSF)2X and two- Odimensional (BEDT–TTF)2X (refs 1,2), in which TMTSF C H is tetramethyltetraselenafulvalene (C10H12Se4), and BEDT–TTF is 14 10 bis(ethylenedithio)tetrathiafulvalene (C10H8S8). The introduction of charge into C60 solids and graphites with π-electron networks by 3,4 doping to realize superconductivity has been reported . Organic Phenanthrene superconductors exhibit many interesting phenomena, including Intensity (a.u.) low dimensionality, strong electron–electron and electron–phonon interactions and the proximity of antiferromagnetism, insulator states and superconductivity. They consist of open-shell molecular units that are the result of a partial oxidation and reduction of the 10 20 30 40 50 60 donor and acceptor molecules in the crystal-growth process. The unpaired electrons residing in the π-molecular orbital of the donor unit are responsible for the electronic properties of these charge– transfer salts. π-Molecular orbitals also have an important role in the superconductivity of C60 solids and graphite superconductors. Key features of organic superconductors are the low dimension- K phenanthrene ality of the materials and a strong coupling of the charge carriers 3 Rb phenanthrene to the lattices. Organic superconductors provide a good system to 3 study the interplay of strong electron–electron and electron–pho- Intensity (a.u.) non interactions in low-dimensional system. It is striking that the close proximity of superconductivity to a magnetically ordered state occurs in organic superconductors5,6, similar to the other strongly correlated electron systems: the heavy fermion metals7 and high- 10 20 30 40 50 60 temperature superconductors including cuprates8 and pnictides9,10. 2� (degree) Therefore, it challenges the understanding of superconductivity. It Figure 1 | X-ray diffraction patterns for phenanthrene and is found that the Tc increases with the expansion of the lattice in A3phenanthrene. (a) X-ray diffraction pattern for the phenanthrene alkali-metal-doped C60. This behaviour is expected by the Bardeen, Cooper and Schrieffer (BCS) theory, because the expansion of lat- purchased from Alfa Aesar. The molecular structure and crystal structure tice leads to an enhancement of the density of states on the Fermi of phenanthrene are shown in the inset. (b) X-ray diffraction patterns for the superconducting samples of K3phenanthrene and Rb3phenanthrene, surface. However, the body-centered-cubic fulleride Cs3C60 is found to be a true Mott–Jahn–Teller insulator, and it possesses both the respectively. The inset shows the appearance for the pristine phenanthrene (white) and K phenanthrene (black). ingredients required by the strongly correlated theory11,12. The insu- 3 13 lator Cs3C60 can be turned into superconductor by pressure . The superconductivity induced by pressure in Cs3C60 is related to the antiferromagnetic Mott insulator11,12, similar to the cuprate. For the graphite superconductors with π-electron networks, C6Yb and C6Ca in P21 symmetry with lattice parameters: a = 8.453 Å, b = 6.175 Å, show superconductivity at 6.5 and 11.5 K (ref. 14), respectively. Such c = 9.477 Å, β = 98.28°. All peaks in X-ray diffraction pattern can high transition temperatures are unprecedented although it is char- be well indexed with the crystal structure mentioned above. Miller acterized by single-gap s-wave superconductivity15 and the coupling indices are marked in the pattern. The lattice constants obtained with intercalant phonon is believed to be main force for supercon- here are consistent with the results reported before18. The crystal ductivity16. Therefore, it indicates that superconductivity for the that comprises layers stacked in c direction, where each layer (in a–b superconductors with π-electron networks is not simply explained plane) contains phenanthrene molecules arranged in a herringbone by BCS theory. Recently, a potassium-doped picene (C22H14) with structure, as shown in the inset of Figure 1a. Figure 1b shows the five fused benzene rings was shown to display superconductivity at X-ray diffraction patterns of K3phenanthrene and Rb3phenanthrene a temperature as high as 18 K (ref. 17). with all reflections indexed with theP2 1 symmetry, and no impurity Here we report the discovery of superconductivity in alkali- phase is observed for both K3phenanthrene and Rb3phenanthrene. metal-doped phenanthrene (C14H10). The work raises the prospect Figure 2 shows the X-ray diffraction pattern taken after exposing of producing a new family of superconductor based on aromatic the superconducting K3phenanthrene to air for 5 min. As a com- hydrocarbons. The fact thatT c increases from 5 K for A3phenanthrene parison, the X-ray diffraction pattern for the as-grown 3K phenan- (A = K and Rb) with three benzene rings to 18 K for Kxpicene with threne is also displayed in Figure 2. As exposed to air, the black five benzene rings suggests that organic hydrocarbons with long powder products changed to white quickly. But, it should be noted chains of benzene rings could be superconductors with high Tc. Our that no fierce reaction, such as combustion, can be observed as results indicate that such aromatic hydrocarbons may be unconven- exposing the as-grown K3phenanthrene to air, suggesting that no tional superconductors. elementary substance of potassium remains after the reaction of potassium and phenanthrene. Figure 2 shows that the diffraction

Results peaks of KOH·H2O can obviously be observed, indicating that the Crystal structure of phenanthrene and A3phenanthrene. The de-intercalation of K3phenantrene occurs as exposed to air, and K molecular structure and crystal structure of phenanthrene were reacts with water to form KOH·H2O. The potassium de-intercalat- shown in the inset of Figure 1a. Both phenanthrene and picene ing from K3phenantrene in air is consistent with the fact that the molecules are phenanthrene-like structure motif. The phenanthrene K3phenantrene loses the superconductivity quickly for exposing to contains three fused benzene rings, whereas picene contains five. air. Such instability of K3phenantrene in air is quite similar to that of 19–21 17 The ribbon-like molecular structure can be regarded as part of alkali-metal-intercalated graphite and Kxpicene . sheet of graphene. Figure 1a shows the X-ray diffraction pattern of With potassium intercalated into the phenanthrene, the lattice phenanthrene. As shown in top panel, the phenanthrene crystallizes constants of K3phenantrene change to a = 8.430 Å, b = 6.134 Å,

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0.0

0 0.0 K phenanthrene ) T = 2.5 K 2.5 –1

l FC –20 –0.2

mo –0.5

–1 –40 H c1 H=10 Oe K phenanthrene –1.0 3 –0.4 –60 0 150 300 450 (emu Oe Intensity (a.u.) H (Oe) # KOH•H O � –1.5 2 ZFC Exposed to air for 5 min K3phenanthrene –0.6 –2.0 Kb phenanthrene 3 4 5 3 T (K)

10 20 30 40 50 60 3 4 5 6 7 2� (degree) T (K)

Figure 2 | X-ray diffraction patterns for the K3phenanthrene before 2,000 Oe and after exposing to air. Black curve was taken by putting the powder 0.0 1,000 Oe 500 Oe sample in Mylar film after exposing to air for 5min to avoid more water 200 Oe 100 Oe being absorbed. The vertical short lines indicate the Bragg reflections of ) 50 Oe –1

l 20 Oe –0.2 10 Oe

KOH·H O, which appears after exposing the sample to air for 5 min. The mo

2 –1 X-ray diffraction pattern for the non-superconducting K2.5phenanthrene 1,200

(green) is also shown, and all peaks can be indexed with lattice parameters ZFC 800 (Oe)

3 –0.4 H

a = 8.449 Å, b = 6.144 Å, c = 9.432 Å, β = 98.20°, Vcell = 484.6 Å . It seems (emu Oe 400 �

that no big difference in structure between the superconducting and non- 0 superconducting samples is observed. 3.5 4.0 4.5 –0.6 T (K)

2 3 4 5 6 7 8 T (K)

c = 9.417 Å, β = 98.18 ° and the unit cell volume contracts to 482.0 Å3 Figure 3 | Temperature dependence of magnetization (χ) for 3 A3phenanthrene. (a) χ versus T plots for powder samples of from 489.6 Å . For Rb3phenanthrene, the lattice constants are K phenanthrene with T = 4.95 K and Rb phenanthrene with T = 4.75 K in a = 8.450 Å, b = 6.139 Å, c = 9.442 Å, β = 97.99° and the unit cell vol- 3 c 3 c ume also contracts to 485.06 Å3 from 489.6 Å3. Such contraction of the ZFC and FC measurements under 10 Oe, and the shield fractions are lattice parameters could arise from strong interaction between the 5.3% and 6.7%, respectively. The right inset exhibits the χ versus T plot of the K phenanthrene pellet sample by pressing powder, and a shielding benzene ring layers induced by K or Rb doping that leads to charge 3 fraction of 15.5% was achieved. M versus H plot for K phenanthrene transfer from dopant to the benzene rings, which is confirmed by 3 at 2.5 K was shown in the left inset. (b) χ versus T plots for the Raman spectra. The lattice parameters of the Rb3phenanthrene K3phenanthrene powder sample in ZFC measurements under different H. are slightly larger than that of the K3phenanthrene, which arises The H versus T plot is shown in the inset. The onset temperature (T onset) from the fact that the ionic radius of Rb is larger than that of K. c c is defined asT . The evolution of lattice constants with K doping is quite different c 17 from the case of Kxpicene reported by Mitsuhashi et al. , in which the remarkable feature is that the lattice constants c and b shrink, whereas the a expands with intercalating K. This is also contrary to alkali-atoms-intercalated pentacene22, where the lattice constant of c pellet, the shielding fraction is increased to 15.5%. The shielding 17 axis is significantly expanded because of alkali metal atoms interca- fraction of the K3phenanthrene is larger than that of Kxpicene . lating between the molecular layers of pentacene. The intercalations The fact that the shielding fraction can be increased when the pow-

in the K3phenanthrene and Rb3phenanthrene are also quite differ- der sample was pressed to form a pellet suggests that the super- ent from (BEDT–TTF)2I3 (ref. 23) and (TMTSF)2PF6 (ref. 24) superconductivity is a bulk behaviour. The lower critical magnetic field conductors. In superconducting (BEDT–TTF)2I3 and (TMTSF)2PF6, HC1~175 Oe was estimated from the M versus H plot at 2.5 K (see the electron donor and acceptor layers form quasi-two-dimensional inset of Figure 3a), less than HC1~380 Oe in K3.3picene with Tc~18 K layers that is layer-by-layer stacked. at 5 K (ref. 17). As shown in Figure 3a, a superconducting transition at 4.75 K is observed in the susceptibility measured in ZFC

Susceptibility of A3phenanthrene. Figure 3a shows the magnetic and FC procedures for the powder sample Rb3phenanthrene with susceptibility χ as a function of temperature for the powder samples the shielding fraction of about 6.7%. It should be pointed out that

of K3phenanthrene and Rb3phenanthrene in the zero-field cooling Tc of Rb3phenanthrene is about 0.20 K less than that of K3phenan- (ZFC) and field cooling (FC) measurement procedures under mag- threne. This is consistent with the case of picene in which the Tc is netic field H of 10 Oe. The magnetic susceptibility χ versus T plot about 7.1 K for K doping, and shifts to 6.9 K for Rb doping. Figure shows a sharp decrease below 4.95 K in ZFC and FC measurements 3b shows temperature dependence of χ under different magnetic

for the K3phenanthrene, and here the temperature corresponding fields for K3phenanthrene in the ZFC measurement. One can see to the sharp decrease is defined as the superconducting transition that the diamagnetic signal disappears distinctly with increasing H,

temperature Tc. One can see that the superconducting transition is but there was an obvious drop of χ at 3.7 K even at 1,000 Oe. On very sharp and transition width is less than 0.5 K. The diamagnetic the basis of data shown in Figure 3b, we obtained magnetic-field

χ in the ZFC measurements can be assigned to the shielding effect, dependence of Tc, and plot the upper critical field HC2 versus T in and the shielding fraction is about 5.3%. As discussed by Mitsu- the inset of Figure 3b. 17 hashi et al. , the small shielding fraction in powder sample could The samples Kxphenanthrene with the nominal compositions arise from the penetration of magnetic field into superconduct- (x = 2.5, 2.8, 2.9, 3, 3.1, 3.2 and 3.5) were also synthesized in the ing phase because of smaller size of crystallites than the London same condition. It is found that only the sample with nominal com-

penetration depth. When the K3phenanthren was pressed into a position K3phenanthrene shows superconductivity. To confirm it,

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Table 1 | Kxphenenthrene samples.

No. Nominal composition Annealing Annealing time (h) Onset SC transition Shielding fraction temperature (°C)

Sample A K3ph 200 20 4.95 K 5.3%

Sample B K3ph 200 20 4.8 K 2.4%

Sample C K3ph 200 20 4.9 K 3.7%

Sample D K2.5ph 200 20 No SC —

Sample E K2.8ph 200 20 No SC —

Sample F K2.9ph 200 20 No SC —

Sample G K3.1ph 200 20 No SC —

Sample H K3.2ph 200 20 No SC —

Sample I K3.5ph 200 20 No SC —

we prepared five batches of samples of Kxphenanthrene for the each nominal composition with x = 2.5, 2.8, 2.9, 3, 3.1, 3.2 and 3.5. The 0.0 0 GPa parameters of superconductivity for those Kxphenanthrene are dis- )

–1 0.2 GPa played in Table 1, where T and shielding fractions were obtained c 0.4 GPa from the magnetic susceptibility measurements. In the five batches mol

–1 0.6 GPa of samples for each x, only the sample Kxphenanthrene with the –0.1 nominal composition of x = 3 shows superconducting transition. 0.8 GPa Although the superconducting fraction is quite different from batch 1.0 GPa (emu Oe

to batch among the superconducting samples, the superconducting � critical temperature Tc changes less than 5%. The difference of the –0.2 superconducting fraction may arise from the different sizes of the crystallite. The shielding fraction from zero-field-cooled suscepti- 3 4 5 6 bility is quite small, and the field-cooled susceptibility shows the T (K) small Meissner effect. These results could arise from the quite small size of crystallites for the powder samples, so that the penetration depth is much larger than the size of crystallites. 5.7 Effect of pressure on Tc for K3phenanthrene. Figure 4a presents the temperature-dependent magnetization for the powder sample

Kxphenanthrene under the pressures ranging from ambient pres- 5.4

sure to 1 GPa. The most notable result is that both Tc and shielding (K) c fraction increase rapidly with increasing the pressure. Tc is enhanced T 5.1 from 4.7 K at ambient pressure, to 5.9 K at 1 GPa. The pressure dependence of Tc is shown in Figure 4b. The positive d(Tc/Tc(0))/ − 1 4.8 dP~0.26 GPa at low pressure range (less than 1 GPa) in K3phen- anthrene is similar to graphite intercalation compounds25. Such positive dependence would not be expected within a conventional 0.0 0.2 0.4 0.6 0.8 1.0 weak coupling phonon model because Tc is usually suppressed by P (GPa) pressure for conventional superconductivity.

Figure 4 | Pressure dependence of the superconducting transition Tc for

Raman spectra of phenanthrene and A3phenanthrene. Figure 5 K3phenanthrene. (a) Magnetization χ as a function of temperature for shows the room-temperature Raman spectra for K3phenanthrene the powder sample of K3phenanthrene under the pressures of P = 0, 0.2, and Rb3phenanthrene as well as pure phenanthrene. There are 0.4, 0.6, 0.8, 1.0 GPa in ZFC measurements. The sample used here was seven major Raman peaks, which can be conveniently compared obtained by reacting phenanthrene purchased from Alfa Aesar with K. between pure and intercalated phenanthrene: 1,524, 1,441, 1,350, (b) Evolution of Tc with pressure for the superconducting K3phenanthrene. 1,037, 830, 411 and 250 cm − 1, as listed in Table 2. All of these peaks belong to the A1 mode, and arise mainly from the C–C stretching vibration26,27. Compared with the pure phenanthrene, alkali-metal- − 1 doped phenanthrene displays obvious downshift, suggesting the show that the C–C stretching A1 mode of 1,434 cm (in monoan- phonon-mode softening effect. Such softening of the phonon modes ion of phenanthrene, 7 cm − 1 downshift from 1441 cm − 1 in the pure should arise from the charge–transfer effect due to the intercalation phenanthrene) strongly coupled to the a2 LUMO in phenanthrene of the alkali-metal into between phenanthrene molecules. Nomi- and has large electron–phonon coupling constants in the monoan- nally, three electrons may transfer to a phenanthrene molecule for ion of phenanthrene29. The linewidth of the modes are obviously − 1 K3phenanthrene and Rb3phenanthrene. There are 17 and 21 cm increased in K- and Rb-doped samples. Similar behaviour has − 1 − 1 downshifts for the 1,441 cm mode in K3phenanthrene (1424 cm ) been observed in potassium-doped fullerides. Such a broadening − 1 − 1 28 and Rb3phenanthrene (1,419 cm ), respectively. It suggests 6 cm of the modes arises from the electron–phonon interaction . Kato per electron and 7 cm − 1 per electron redshift with the complete et al. have studied the electron–phonon coupling and predicted transfer of charge. This is consistent with that observed in super- occurrence of possible superconductivity in negatively charged 28 − 1 conducting A3C60 (A = K, Rb) , in which the redshift is also 6 cm phenanthrene-edge-type hydrocarbon crystals based on the BCS per electron for the mode at 1,460 cm − 1. Theoretical calculations theory with strong electron–phonon coupling29. They found that

Rb3phenanthrene ) –1.12 Phenanthrene

K phenanthrene –1 3 Fitting curve

) Phenanthrene mol

–1 –1.16

Intensity (a.u. –1.20 emu Oe –4 (10

� –1.24

400 800 1,200 1,600 0 30 60 90 Raman shift (cm–1) T (K)

Figure 5 | Room temperature Raman spectra for phenanthrene and

A3phenanthrene. The black arrows point to the major Raman modes with − 1 − 1 − 1 − 1 − 1 the wavelength of 1,524 cm , 1441 cm , 1,350 cm , 1,037 cm , 830 cm , K3phenanthrene ) − 1 − 1 12 411 cm and 250 cm , respectively, and obvious downshift can be –1 Fitting curve for K3phenanthrene observed for these modes. A strong mode at 771 cm − 1 in pure phenanthrene mol Rb3phenanthrene becomes invisible with the potassium and rubidium intercalation. –1 Fitting curve for Rb3phenanthrene 8 emu Oe –4 0 (1

� 4 Table 2 | The position of major Raman peaks in phenanthrene 0 30 60 90 and A phenanthrene. 3 T (K)

33 Phenathrene K3Ph Rb3Ph Assignments Figure 6 | Susceptibility of normal state for phenanthrene and

1,524 1,502 1,497 C–C stretching, C–H rocking A3phenanthrene. The magnetic susceptibility as a function of 1,441 1,424 1,419 C–C stretching, C-H rocking temperature was measured under 5 T. The fitting curves comes from 1,350 1,326 1,322 C–C stretching the formula χ = χ + C/(T + θ), where χ is a constant, C is Curie constant, 1,037 1,006 1,002 C–C stretching 0 0 and θ is the paramagnetic Curie temperature. χ is negative for pure 830 812 808 Ring bending 0 phenanthrene while positive in the superconducting K phenanthrene 711 — — Ring bending, C–C stretching 3 411 398 393 Ring bending and Rb3phenanthrene. The fitting results between 4 and 80 K are listed 250 238 232 Ring bending in Table 3.

the strength of electron–phonon coupling and Tcs have relevance to the molecular size and the molecular edge structures, and Tc and implies that there exist local spins in these superconducting mate- electron–phonon coupling constants decrease with increasing the rials. It is well known that a positive pressure effect on supercon- molecular size. ductivity is widely observed in Cs3C60 (ref. 11), high-Tc cuprate compounds30 and iron-based superconductors31 as a consequence

Susceptibility of normal state for phenanthrene and A3phen- of the suppression of antiferromagnetism. Similar positive pres- anthrene. We measured the magnetic susceptibility of pure phen- sure effect on Tc in aromatic hydrocarbon superconductors may anthrene, and superconducting K3phenanthrene and Rb3phen- also be relevant to the suppression of the localized spin interaction anthrene, as shown in Figure 6. The magnetic susceptibilities by pressure. in normal states for K3phenanthrene and Rb3phenanthrene are considerably large, and show the Curie–Weiss behaviour with the Discussion superconductivity completely suppressed under the magnetic field We prepared a series of Kxphenanthrene samples with different of 5 T. Pure phenanthrene shows diamagnetic behaviour, and the potassium content, and found that only the sample with nominal temperature-dependent negative magnetic susceptibility follows composition of K3phenanthrene shows superconductivity: all other Curie–Weiss behaviour in the low-temperature regime. We fit the samples with x deviation from 3 do not show superconductivity. We data of magnetic susceptibility with the formula χ = χ0 + C/(T + θ), observed only a superconducting transition for the superconduct- and the fitting results are listed in the Table 3. The values at 10 K ing K3phenanthrene and Rb3phenanthrene; this is different from the for the alkali-intercalated phenanthrenes are much smaller than case of potassium-doped picene in which there are two supercon- 17 those reported in the superconducting Kxpiecene . In the pure ducting phases with Tc = 7 and 18 K, respectively. There is a large phenanthrene, only small magnetic moment of 0.06 µB per mole family of polycyclic aromatic compounds with an extended phen- is obtained from the fitting, indicating that almost no local spin is anthrene-like structural motif designated as [n]phenacene, where n present. While in the K3phenanthrene and Rb3phenanthrene, the is the number of fused benzene rings. [n]Phenacene molecules are magnetic moment increases to 0.22 µB per mole. It suggests that related to layers of graphene in the way that ribbons are related to the charge transfer by the alkali intercalation into phenanthrene sheets. Therefore, discovery of superconductivity inA 3phenanthrene not only injects conduction electrons, but also induces local spins. (A3C14H10, A = K and Rb) besides the superconductor Kxpicene This may hint an unconventional nature of the superconductivity opens a new broad family of superconductors that consists of aro- in the alkali-metal-intercalated phenanthrene-edge-type hydro- matic hydrocarbons. To search for superconductors in such a family carbons. Similar behaviour has been observed in Kxpicene. It of [n]phenacene molecules and to study their physical properties is nature communications | 2:507 | DOI: 10.1038/ncomms1513 | www.nature.com/naturecommunications © 2011 Macmillan Publishers Limited. All rights reserved. ARTICLE nature communications | DOI: 10.1038/ncomms1513

Table 3 | Fitting parameters of Curie–Weiss formula for phenanthrene and A3phenanthrene.

− 1 − 1 − 1 − 1 − 1 Sample χ0 (emu mol Oe ) θ (K) C (emu K mol Oe ) Magnetic moment (B mol ) Phenanthrene − 1.27195e − 4 34.1230 5.47058e-4 0.06

K3phenanthrene 3.70571e − 4 3.63466 0.0062 0.22

Rb3phenanthrene 3.63113e − 4 3.99576 0.0057 0.21

The fitting results for the normal-state magnetic susceptibility at 5 T and between 4 and 80 K of phenanthrene, superconducting K3phenenthrene and Rb3phenenthrene by using the formula

χ=χ0 + C/(T + θ), where C is the Curie constant, and θ is the Curie temperature.

powder sample shows uniform dark black colour, which is totally different from important to understand the mechanism of superconductivity. Tc of the pure white colour of phenanthrene. The appearance of samples before and 5 K is relatively low for A3phenanthrene (A = K, Rb) with three ben- after annealing was shown as the inset ofFigure 1b. In the Table 1, all samples of K phenanthrene are obtained by reacting the purified phenanthrene by sublimation zene rings, compared with 7 and 18 K for Kxpicene with five benzene x rings, suggesting that such organic hydrocarbons with long benzene method with K. The impurity phase of KH can be observed in X-ray diffraction pattern for the samples prepared by reacting the phenanthrene purchased from rings are potential superconductors with high T . c company with K. Axphenanthrene is quite sensitive to oxygen and moisture, and it 29 A theoretical study by Kato et al. indicates that the electron– will be decomposed in air within few minutes. All the processes except for anneal- phonon coupling constants decrease with an increase in the number ing were done in glove box with the oxygen and moisture level less than 1 p.p.m. of carbon atoms in phenanthrene edge-type hydrocarbons. They Characterization of structure and susceptibility. Before the X-ray diffraction estimated Tc for the hydrocarbon monoanions by using the approxi- measurement, the samples were first ground into fine powder in a glove box, and mate solution of the Eliashberg equation, and found that Tc decreases then loaded into the capillaries (purchased from Hilgenberg GmbH, made of with increasing the number of carbon atoms in phenanthrene edge- special glass no. 10. L = 80 mm, OD = 0.70 mm, Wall = 0.01 mm); finally, the capil- type hydrocarbons. However, such theoretical prediction is in con- lary was sealed to keep the sample in an inert atmosphere during X-ray diffraction trast to experimental results, which shows that T increases from ~5 measurement. The capillaries were fixed to the sample shelf of X-ray diffractometer c (TTR-III theta/theta rotating anode X-ray Diffractometer) by plasticine. X-ray to 7 K/18 K when increasing the number of benzene rings from 3 diffraction pattern was obtained in the 2-theta range of 5°–65° with a scanning rate in phenanthrene to 5 in picene. It suggests that superconductivity of 0.5° per minute. The magnetization measurement was made by SQUID MPMS in the phenanthrene edge-type hydrocarbons could be unconven- (Quantum Design). The sample was placed into Polypropylene powder holder tional, and cannot be explained only by electron–phonon coupling. (Quantum Design). Enhancement of T with increasing pressure and coexistence of c Susceptibility under pressure. The magnetization under pressure was measured superconductivity and local spins are also the indications of uncon- by incorporating a copper–beryllium pressure cell (EasyLab) into SQUID MPMS ventional superconductivity, because the existence of local spin (Quantum Design). The sample was firstly placed in a teflon cell (EasyLab) with destroys the superconductivity in the frame of BCS theory. Further coal oil (EasyLab) as the pressure media. Then, the teflon cell was set in the copper–beryllium pressure cell for magnetization measurement. The contribution evidence for unconventional superconductivity is the decrease of Tc by 0.2 K for Rb doped both phenanthrene and picene relative to K- of background magnetization against the sample at 2 K was less than 5%. doped samples, respectively, although the lattice constants slightly Raman spectrum. Raman-scattering experiments were carried out using the increase with change of dopant from K to Rb. This result is consist- 780-nm laser line in DXR Raman Microscope (Thermo Scientific). The scattering ent with the pressure effect on Tc. This is totally different from the light was captured with a single exposure of the CCD with a spectral resolution of case of superconducting fullerides in which T increases with the 1 cm − 1. The powder samples were loaded in the sealed capillary with highly pure c Ar gas during Raman measurements. It should be pointed out that the X-ray dif- expansion of the lattice in alkali-metal-doped C60 (ref. 32), which is fraction and Raman measurements were carried out on the powder samples in the expected by the BCS theory, because the expansion of lattice leads to same capillary. an enhancement of the density of states at the Fermi level. To confirm the proposed unconventional superconductivity, a References high-quality superconducting sample, even a single crystal, may 1. 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