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T Siegrist

Publications and source records attributed to T Siegrist.

At least 19 recordsLinked to original sources
PubMed2004

Crystallization of charge holes in the spin ladder of Sr14Cu24O41.

Determining the nature of the electronic phases that compete with superconductivity in high-transition-temperature (high-T(c)) superconductors is one of the deepest problems in condensed matter physics. One candidate is the 'stripe' phase, in which the charge carriers (holes) condense into rivers of charge that separate regions of antiferromagnetism. A related but lesser known system is the 'spin ladder', which consists of two coupled chains of magnetic ions forming an array of rungs. A doped ladder can be thought of as a high-T(c) material with lower dimensionality, and has been predicted to exhibit both superconductivity and an insulating 'hole crystal' phase in which the carriers are localized through many-body interactions. The competition between the two resembles that believed to operate between stripes and superconductivity in high-T(c) materials. Here we report the existence of a hole crystal in the doped spin ladder of Sr14Cu24O41 using a resonant X-ray scattering technique. This phase exists without a detectable distortion in the structural lattice, indicating that it arises from many-body electronic effects. Our measurements confirm theoretical predictions, and support the picture that proximity to charge ordered states is a general property of superconductivity in copper oxides.

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PubMed2004

Amorphouslike density of gap states in single-crystal pentacene.

We show that optical and electrical measurements on pentacene single crystals can be used to extract the density of states in the highest occupied molecular orbital-lowest unoccupied molecular orbital band gap. It is found that these highly purified crystals possess band tails broader than those typically observed in inorganic amorphous solids. Results on field-effect transistors fabricated from similar crystals imply that the gap state density is much larger within 5-10 nm of the gate dielectric. Thus, organic thin-film transistors for such applications as flexible displays might be significantly improved by reducing these defects.

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PubMed2004

Bias-dependent generation and quenching of defects in pentacene.

We describe a defect generation phenomenon that is new to organic semiconductors. A defect in pentacene single crystals can be created by bias-stress and persists at room temperature for an hour in the dark but only seconds with 420 nm illumination. The defect gives rise to a hole trap at Ev+0.38 eV and causes metastable transport effects at room temperature. Creation and decay rates of the hole trap have a 0.67 eV activation energy with a small (10(8) s(-1)) prefactor, suggesting that atomic motion plays a key role in the generation and quenching process.

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PubMed2004

Synthesis, structure and physical properties of the first one-dimensional phenalenyl-based neutral radical molecular conductor.

We report the preparation, crystallization, and solid-state characterization of a benzyl-substituted spirobiphenalenyl radical. The crystal structure shows that the radical is monomeric in the solid state, with the molecules packed in an unusual one-dimensional (1-D) fashion that we refer to as a pi-step stack. This particular mode of 1-D stacking is forced on the lattice arrangement by the presence of the orthogonal phenalenyl units that were specifically incorporated to prevent the crystallization of low-dimensional structures. The structure shows that this strategy is effective, and neighboring molecules in the stack can only interact via the overlap of one pair of active (spin-bearing) carbon atoms per phenalenyl unit, leading to the pi-step structure in which the remaining four active carbon atoms per phenalenyl unit do not interact with nearest neighbor molecules. The magnetic susceptibility data in the temperature range 4-360 K may be fit to an antiferromagnetic Heisenberg S = 1/2 linear chain model with intrachain spin coupling J = -52.3 cm(-1). Despite the uniform stacking, the material has a room temperature conductivity of 1.4 x 10(-3) S/cm and is best described as a Mott insulator.

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PubMed2001

Superconductivity in single crystals of the fullerene C70.

The observation of superconductivity in doped C60 has attracted much attention, as these materials represent an entirely new class of superconductors. A maximum transition temperature (Tc) of 40 K has been reported for electron-doped C60 crystals, while a Tc of 52 K has been seen in hole-doped crystals; only the copper oxide superconductors have higher transition temperatures. The results for C60 raise the intriguing questions of whether conventional electron-phonon coupling alone can produce such high transition temperatures, and whether even higher transition temperatures might be observed in other fullerenes. There have, however, been no confirmed reports of superconductivity in other fullerenes, though it has recently been observed in carbon nanotubes. Here we report the observation of superconductivity in single crystals of electric-field-doped C70. The maximum transition temperature of about 7 K is achieved when the sample is doped to approximately four electrons per C70 molecule, which corresponds to a half-filled conduction band. We anticipate superconductivity in smaller fullerenes at temperatures even higher than in C60 if the right charge density can be induced.

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PubMed2000

A soluble and air-stable organic semiconductor with high electron mobility

Electronic devices based on organic semiconductors offer an attractive alternative to conventional inorganic devices due to potentially lower costs, simpler packaging and compatibility with flexible substrates. As is the case for silicon-based microelectronics, the use of complementary logic elements-requiring n- and p-type semiconductors whose majority charge carriers are electrons and holes, respectively-is expected to be crucial to achieving low-power, high-speed performance. Similarly, the electron-segregating domains of photovoltaic assemblies require both n- and p-type semiconductors. Stable organic p-type semiconductors are known, but practically useful n-type semiconductor materials have proved difficult to develop, reflecting the unfavourable electrochemical properties of known, electron-demanding polymers. Although high electron mobilities have been obtained for organic materials, these values are usually obtained for single crystals at low temperatures, whereas practically useful field-effect transistors (FETs) will have to be made of polycrystalline films that remain functional at room temperature. A few organic n-type semiconductors that can be used in FETs are known, but these suffer from low electron mobility, poor stability in air and/or demanding processing conditions. Here we report a crystallographically engineered naphthalenetetracarboxylic diimide derivative that allows us to fabricate solution-cast n-channel FETs with promising performance at ambient conditions. By integrating our n-channel FETs with solution-deposited p-channel FETs, we are able to produce a complementary inverter circuit whose active layers are deposited entirely from the liquid phase. We expect that other complementary circuit designs can be realized by this approach as well.

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