Thomas Palstra

Effect of impurities on the mobility of single crystal pentacene

We have obtained a hole mobility for the organic conductor pentacene of μ=35 cm2/V s at room temperature increasing to μ=58 cm2/V s at 225 K. These high mobilities result from a purification process in which 6,13-pentacenequinone was removed by vacuum sublimation. The number of traps is reduced by two orders of magnitude compared with conventional methods. The temperature dependence of the mobility is consistent with the band model for electronic transport.

C60 thin film transistors

N‐channel field effect transistors with excellent device characteristics have been fabricated by utilizing C60 as the active element. Measurements on C60 thin films in ultrahigh vacuum show on‐off ratios as high as 106 and field effect mobilities up to 0.08 cm2/V s.

Critical currents and thermally activated flux motion in high-temperature superconductors

We have measured the resistance below Tc of single crystals of the high‐temperature superconductors Ba2YCu3O7 and Bi2.2Sr2Ca0.8Cu2O8+δ in magnetic fields up to 12 T. The resistive transition of both compounds is dominated by intrinsic dissipation which is thermally activated, resulting in an exponential temperature dependence of the resistivity well below Tc. The dissipation is significantly larger and of different character in the Bi‐Cu compound than in Ba2YCu3O7. The relation between the activated behavior and the depinning critical current is discussed.

Identification of polymorphs of pentacene

Pentacene crystallizes in a layered structure with a herringbone arrangement within the layers. The electronic properties depend strongly on the stacking of the molecules within the layers [J. Phys. Chem. B. 106 (2002) 8288]. We have synthesized four different polymorphs of pentacene, identified by their layer periodicity, d(001): 14.1, 14.4, 15.0 and 15.4 Angstrom. Single crystals commonly adopt the 14.1 Angstrom structure, whereas all four polymorphs can be synthesized in thin film form, depending on growth conditions. We have identified part of the unit cell parameters of these polymorphs by X-ray and electron diffraction (ED). The 15.0 and 15.4 X polymorphs transform at elevated temperature to the 14.1 and 14.4 Angstrom polymorphs, respectively. Using SCLC measurements, we determined the mobility of the 14.1 Angstrom polymorph to be 0.2 cm(2)/V s at room temperature

Polymorphism in pentacene

Pentacene, C(22)H(14), crystallizes in different morphologies characterized by their d(001)-spacings of 14.1, 14.5, 15.0 and 15.4 A. We have studied the crystal structure of the 14.1 and 14.5 A d-spacing morphologies grown by vapour transport and from solution. We find a close correspondence between the 14.1 A structure reported by Holmes, Kumaraswamy, Matzeger & Vollhardt [Chem. Eur. J. (1999), 5, 3399-3412] and the 14.5 A structure reported by Campbell, Monteath Robertson & Trotter [Acta Cryst. (1961), 14, 705-711]. Single crystals commonly adopt the 14.1 A d-spacing morphology with an inversion centre on both molecules in the unit cell. Thin films grown on SiO(2) substrates above 350 K preferentially adopt the 14.5 A d-spacing morphology, with a slightly smaller unit-cell volume.

Low-temperature structure of rubrene single crystals grown by vapor transport.

We report the crystal structure of rubrene, C42H28 (5,6,11,12-tetraphenyltetracene), in the temperature interval 100-300 K. The crystals are grown by physical vapor transport in an open system. The crystal structure is orthorhombic over the entire temperature range.

Study of the critical behaviour of the magnetization and electrical resistivity in cubic La(Fe, Si)13 compounds

Abstract The magnetic properties of the cubic NaZn 13 type pseudobinary compounds La(T x Si 1- x ) 13 were studied for T = Fe, Co and Ni in the temperature range 4.2–300 K. 57 Fe Mossbauer spectroscopy was performed on LaFe 11 Si 2 and 57 Fe-doped LaCo 11 Si 2 . The compounds with T = Fe or Co are ferromagnetic, while the compound LaNi 11 Si 2 is a Pauli paramagnet. The critical behaviour around the Curie temperature T c was studied in La(Fe x Si 1- x ) 13 by measuring the temperature dependences on the zero-field susceptibility and the electrical resistivity. The critical exponent γ in the expression χ ∝ ( T - T c ) -γ was found to be close to 1.38 corresponding to an isotropic Heisenberg ferromagnet. The anomalous critical behaviour shown by electrical resistivity can be explained in terms of lattice softening associated with the Invar effect.

Superconductivity at 40K in Cesium Doped C60

We report superconductivity in Cs3C60 at 40K using ac susceptibility measurements under hydrostatic conditions up to 15 kbar. Cs3C60 was prepared by reaction of C60 with Cs in liquid ammonia, followed by heating at 150°C. This route circumvents formation of the energetically more stable Cs1C60 and Cs4C60 phases. We have studied the synthesis and phase formation by measuring the symmetric Ag pentagonal pinch mode of C60 using Raman spectroscopy. Whereas Raman spectroscopy indicates homogeneous charge transfer of three electrons, x-ray diffraction indicates two phases other than the commonly observed fcc structure. This superconducting transition temperature is considerably higher than for known doped C60 compounds and intermetallic compounds.

Temperature-induced magnetization reversal in a YVO3 single crystal

The total energy of a magnet in a magnetic field is lowest when the magnetic moment is aligned parallel to the magnetic field. Once aligned, the magnetic moment can be reversed by applying a sufficiently large field in the opposite direction. These properties form the basis of most magnetic recording and storage devices. But the phenomenon of magnetization reversal in response to a change in temperature (in a small magnetic field) is rarer. This effect occurs in some ferrimagnetic materials consisting of two or more types of antiferromagnetically ordered magnetic ions, and forms the operational basis of ferrimagnetic insulators. Here we report the observation of multiple temperature-induced magnetization reversals in YVO3. The net magnetic moment is caused by a tilting of the antiferromagnetically aligned moments of (crystallographically identical) V3+ ions, due to orthorhombic distortion in the crystal structure. We observe an abrupt switching at 77 K associated with a first-order structural phase transition, and a gradual reversal at ∼95 K without an accompanying structural change. The magnetization always reverses if the crystal is cooled or warmed through these two temperatures in modest fields. We propose a possible mechanism involving a change in orbital ordering which may be generic to a broad class of transition metal oxides.

Electronic transport properties of pentacene single crystals upon exposure to air

We report the effect of air exposure on the electronic properties of pentacene single crystals. Air can diffuse reversibly in and out of the crystals and influences the physical properties. We discern two competing mechanisms that modulate the electronic transport. The presence of oxygen increases the hole conduction, as in dark four O2 molecules introduce one charge carrier. This effect is enhanced by the presence of visible light. Contrarily, water, present in ambient air, is incorporated in the crystal lattice and forms trapping sites for injected charges.