Potassium-doped Picene Research Articles

Scanning Tunneling Microscopy Studies of Potassium-Doped Picene Films on Au(111) Surface

Potassium-doped picene films grown on an Au(111) surface have been investigated using low-temperature scanning tunneling microscopy. The evolution of both the morphologies and the electronic proper...

Chemical analysis of superconducting phase in K-doped picene

Potassium-doped picene (K3.0picene) with a superconducting transition temperature (TC) as high as 14 K at ambient pressure has been prepared using an annealing technique. The shielding fraction of this sample was 5.4% at 0 GPa. The TC showed a positive pressure-dependence and reached 19 K at 1.13 GPa. The shielding fraction also reached 18.5%. To investigate the chemical composition and the state of the picene skeleton in the superconducting sample, we used energy-dispersive x-ray (EDX) spectroscopy, MALDI-time-of-flight (MALDI-TOF) mass spectroscopy and x-ray diffraction (XRD). Both EDX and MALDI-TOF indicated no contamination with materials other than K-doped picene or K-doped picene fragments, and supported the preservation of the picene skeleton. However, it was also found that a magnetic K-doped picene sample consisted mainly of picene fragments or K-doped picene fragments. Thus, removal of the component contributing the magnetic quality to a superconducting sample should enhance the volume fraction.

Searching superconductivity in potassium-doped p-terphenyl

Searching new superconducting materials and understanding their superconducting mechanisms are the important research directions in the condensed matter physics study. The recent discovery of aromatic hydrocarbon superconductors, including potassium-doped picene, phenanthrene and dibenzopentacene, has aroused considerable research interest of physicists and materials scientists. In this work, potassium-doped p-terphenyl is grown by sealing potassium and p-terphenyl with a mole ratio of 3 : 1 in high-vacuum glass tube and then annealed at 170 ℃ for 7 days or at 240 and 260 ℃ for 24 h. The crystal structure, molecular vibration, and magnetic property are characterized by using X-ray diffraction, Raman scattering, and superconducting quantum interference device. The combination of X-ray diffraction spectrum and Raman spectrum shows that besides potassium-doped p-terphenyl and KH, there exist C60 and graphite in annealed sample, which are found for the first time in the metal-doped aromatic hydrocarbon. Owing to the presence of potassium with high chemical activity, the C-H bond can be broken, resulting in dehydrogenated p-terphenyl with dangling bonds. Consequently, the recombination of dehydrogenated p-terphenyl will form graphite and C60. In addition, the red-shifts of partial peaks of p-terphenyl in Raman spectrum demonstrate that 4 s electron of doped potassium is transferred to C atom. For the samples annealed at 170 and 240 ℃, Curie paramagnetic behaviors are observed in the whole temperature region. On the other hand, in one of the samples annealed at 260 ℃, there exist three anomalous sharp decreases respectively at 17.86, 10.00 and 6.42 K in the zero-field cooling magnetic measurement. Previous studies indicated that the superconducting transition temperatures of potassium-doped C60 and potassium-doped graphite are about 18 K and 3 K. Therefore, it is reasonable to attribute the anomalous sharp decrease at 17.86 K to being produced by potassium-doped C60, while the anomalous sharp decreases at 10.00 and 6.42 K, which have not been reported yet, may be produced by potassium-doped p-terphenyl. The first principles calculations show that potassium-doped p-terphenyl lies in the metallic state, which can form superconductivity due to the electron-phonon interaction. Our results are useful for understanding the crystal growth and physical properties of metal-doped aromatic hydrocarbon organic superconductors. Furthermore, our findings provide a new routine to synthesizing C60 and graphite at low temperature.

Downfolding electron-phonon Hamiltonians fromab initiocalculations: Application toK3picene

We propose an electron-phonon parametrization which is constructed to reproduce target geometry and harmonic frequencies taken from first principles calculations or experiment. With respect to standard electron-phonon models, it adds a ``double-counting'' correction, which takes into account the lattice deformation as the system is dressed by low-energy electron-phonon processes. We show the importance of this correction by studying potassium-doped picene (${\mathrm{K}}_{3}$ picene), recently claimed to be a superconductor with a ${T}_{c}$ of up to 18 K. The Hamiltonian parameters are derived from ab initio density functional theory, and the lattice model is solved by dynamical mean-field theory. Our calculations include the effects of electron-electron interactions and local electron-phonon couplings. Even with the inclusion of a strongly coupled molecular phonon, the Hubbard repulsion prevails and the system is an insulator with a small Mott gap of $\ensuremath{\approx}0.2$ eV.

Absence of Metallicity in K-doped Picene: Importance of Electronic Correlations

Potassium-doped picene (K(x)picene) has recently been reported to be a superconductor at x=3 with critical temperatures up to 18 K. Here we study the electronic structure of K-doped picene films by photoelectron spectroscopy and ab initio density functional theory combined with dynamical mean-field theory (DFT+DMFT). Experimentally we observe that, except for spurious spectral weight due to the lack of a homogeneous chemical potential at low K concentrations (x≈1), the spectra always display a finite energy gap. This result is supported by our DFT+DMFT calculations which provide clear evidence that K(x)picene is a Mott insulator for integer doping concentrations x=1, 2, and 3. We discuss various scenarios to understand the discrepancies with previous reports of superconductivity and metallic behavior.

Hybridized electronic states in potassium-doped picene probed by soft x-ray spectroscopies

The electronic structure of the unoccupied and occupied states of potassium (K)-doped and undoped picene crystalline films has been investigated by using the element-selective and bulk-sensitive photon-detection methods of X-ray absorption and emission spectroscopies. We observed the formation of the doping-induced unoccupied and occupied electronic states in K-doped picene. By applying the inner-shell resonant-excitation experiments, we observed the evidence for the orbital hybridization between K and picene near the Fermi energy. Furthermore, the resonant X-ray emission experiment suggests the presence of the Raman-active vibronic interaction in K-doped picene. These experimental evidences play a crucial role in the superconductivity of K-doped picene.

A Theoretical Study Showing K2picene as a Parent Semiconductor for Organic Superconductivity

Based on the density functional theory (DFT), a theoretical determination method is applied to find structural and electronic properties of potassium-doped picene. In a unit cell of K 2 picene observed experimentally, the atomic positions and the electronic state are determined within a generalized-gradient approximation of DFT using the plane-wave expansion method. Some metastable structures of K 2 picene are found in the optimization simulations. Notable properties of a structure with AB stacking, which is the lowest in the energy than others, are 1) appearance of electron doped molecular bands with the band width of 0.1–0.4 eV, 2) gap formation with a gap of 0.04–0.1 eV at the Fermi energy, and 3) narrow conduction bands with the band width of ∼0.1 eV. In all meta-stable structures determined in this study, these characteristics are commonly found. Assuming further electron doping in the conduction bands of K 2 picene, electron–correlation effects may appear as the pair-hopping processes between doped ...

Plasmon dispersion in molecular solids: Picene and potassium-doped picene

We investigate the dynamic response of pristine and potassium-doped picene, the first example of a new family of organic molecular superconductors, by combining first-principles calculations and state-of-the-art experimental tools. We find that charge-carrier plasmons in K${}_{3}$ picene have a negative or almost negligible dispersion, which deviates from the traditional picture of metals based on the homogeneous electron gas. We show how this finding is the result of the competition between metallicity and electronic localization on the molecular units. Conduction electrons alone give rise to the negative dispersion, which is reduced by molecular polarization and crystal local-field effects. This analysis allows us to obtain a general picture of the plasmon dispersion in metallic molecular crystals.

Electronic correlation effects in superconducting picene fromab initiocalculations

We show, by means of ab-initio calculations, that electron-electron correlations play an important role in potassium-doped picene ($K_x$-picene), recently characterized as a superconductor with $T_c = 18K$. The inclusion of exchange interactions by means of hybrid functionals reproduces the correct gap for the undoped compound and predicts an antiferromagnetic state for $x=3$, where superconductivity has been observed. The latter finding is compatible with a sizable value of the correlation strength, in agreement with simple estimates. Our results highlight the similarity between potassium-doped picene and alkali-doped fulleride superconductors.

Dynamic response and electronic structure of potassium-doped picene investigated by electron energy-loss spectroscopy

We performed electron energy-loss spectroscopy studies to get a deeper insight into the electronic properties of potassium intercalated picene, a recently discovered superconductor. A comparison of the loss function of the undoped and doped compound shows the appearance of a new peak in the optical gap, which we attribute to the charge carrier plasmon. We find a dramatic increase for the value of the background dielectric constant ${\ensuremath{\epsilon}}_{\ensuremath{\infty}}$ upon doping. Our core level excitation data clearly signal filling of the conduction bands with electrons upon potassium addition.