A numerical study of a method for measuring the effective in situ sound absorption coefficient

The accuracy of a method [Wijnant et al., “Development and applica- tion of a new method for the in-situ measurement of sound absorption”, ISMA 31, Leuven, Belgium (2010).], for measurement of the effective area-averaged in situ sound absorption coefficient is investigated. Based on a local plane wave assump- tion, this method can be applied to sound fields for which a model is not available. Investigations were carried out by means of finite element simulations for a typical case. The results show that the method is a promising method for determining the effective area-averaged in situ sound absorption coefficient in complex sound fields

High-pressure study of the non-Fermi liquid material U_2Pt_2In

The effect of hydrostatic pressure (p<= 1.8 GPa) on the non-Fermi liquid state of U_2Pt_2In is investigated by electrical resistivity measurements in the temperature interval 0.3-300 K. The experiments were carried out on single-crystals with the current along (I||c) and perpendicular (I||a) to the tetragonal axis. The pressure effect is strongly current-direction dependent. For I||a we observe a rapid recovery of the Fermi-liquid T^2-term with pressure. The low-temperature resistivity can be analysed satisfactorily within the magnetotransport theory of Rosch, which provides strong evidence for the location of U_2Pt_2In at an antiferromagnetic quantum critical point. For I||c the resistivity increases under pressure, indicating the enhancement of an additional scattering mechanism. In addition, we have measured the pressure dependence of the antiferromagnetic ordering temperature (T_N= 37.6 K) of the related compound U_2Pd_2In. A simple Doniach-type diagram for U_2Pt_2In and U_2Pd_2In under pressure is presented.Comment: 21 pages (including 5 figures); pdf forma

Organic Single-Crystal Field-Effect Transistors

We present an overview of recent studies of the charge transport in the field effect transistors on the surface of single crystals of organic low-molecular-weight materials. We first discuss in detail the technological progress that has made these investigations possible. Particular attention is devoted to the growth and characterization of single crystals of organic materials and to different techniques that have been developed for device fabrication. We then concentrate on the measurements of the electrical characteristics. In most cases, these characteristics are highly reproducible and demonstrate the quality of the single crystal transistors. Particularly noticeable are the small sub-threshold slope, the non-monotonic temperature dependence of the mobility, and its weak dependence on the gate voltage. In the best rubrene transistors, room-temperature values of $\mu$ as high as 15 cm$^2$/Vs have been observed. This represents an order-of-magnitude increase with respect to the highest mobility previously reported for organic thin film transistors. In addition, the highest-quality single-crystal devices exhibit a significant anisotropy of the conduction properties with respect to the crystallographic direction. These observations indicate that the field effect transistors fabricated on single crystals are suitable for the study of the \textit{intrinsic} electronic properties of organic molecular semiconductors. We conclude by indicating some directions in which near-future work should focus to progress further in this rapidly evolving area of research.Comment: Review article, to appear in special issue of Phys. Stat. Sol. on organic semiconductor

Field-Effect Transistors on Tetracene Single Crystals

We report on the fabrication and electrical characterization of field-effect transistors at the surface of tetracene single crystals. We find that the mobility of these transistors reaches the room-temperature value of $0.4 \ cm^2/Vs$. The non-monotonous temperature dependence of the mobility, its weak gate voltage dependence, as well as the sharpness of the subthreshold slope confirm the high quality of single-crystal devices. This is due to the fabrication process that does not substantially affect the crystal quality.Comment: Accepted by Appl. Phys. Lett, tentatively scheduled for publication in the November 24, 2003 issu

Influence of the gate leakage current on the stability of organic single-crystal field-effect transistors

We investigate the effect of a small leakage current through the gate insulator on the stability of organic single-crystal field-effect transistors (FETs). We find that, irrespective of the specific organic molecule and dielectric used, leakage current flowing through the gate insulator results in an irreversible degradation of the single-crystal FET performance. This degradation occurs even when the leakage current is several orders of magnitude smaller than the source-drain current. The experimental data indicate that a stable operation requires the leakage current to be smaller than $10^{-9} \ \mathrm{A/cm}^2$. Our results also suggest that gate leakage currents may determine the lifetime of thin-film transistors used in applications.Comment: submitted to Appl. Phys. Let

The elliptic genus from split flows and Donaldson-Thomas invariants

We analyze a mixed ensemble of low charge D4-D2-D0 brane states on the quintic and show that these can be successfully enumerated using attractor flow tree techniques and Donaldson-Thomas invariants. In this low charge regime one needs to take into account worldsheet instanton corrections to the central charges, which is accomplished by making use of mirror symmetry. All the charges considered can be realized as fluxed D6-D2-D0 and anti-D6-D2-D0 pairs which we enumerate using DT invariants. Our procedure uses the low charge counterpart of the picture developed Denef and Moore. By establishing the existence of flow trees numerically and refining the index factorization scheme, we reproduce and improve some results obtained by Gaiotto, Strominger and Yin. Our results provide appealing evidence that the strong split flow tree conjecture holds and allows to compute exact results for an important sector of the theory. Our refined scheme for computing indices might shed some light on how to improve index computations for systems with larger charges.Comment: 37 pages, 12 figure

Predictions from a stochastic polymer model for the MinDE dynamics in E.coli

The spatiotemporal oscillations of the Min proteins in the bacterium Escherichia coli play an important role in cell division. A number of different models have been proposed to explain the dynamics from the underlying biochemistry. Here, we extend a previously described discrete polymer model from a deterministic to a stochastic formulation. We express the stochastic evolution of the oscillatory system as a map from the probability distribution of maximum polymer length in one period of the oscillation to the probability distribution of maximum polymer length half a period later and solve for the fixed point of the map with a combined analytical and numerical technique. This solution gives a theoretical prediction of the distributions of both lengths of the polar MinD zones and periods of oscillations -- both of which are experimentally measurable. The model provides an interesting example of a stochastic hybrid system that is, in some limits, analytically tractable.Comment: 16 page

Design Actions for Shifting Conditions

Considering the growing importance that the urban environments assume within contemporary territorial transformations yet little room is offered within market-driven societies to critically reflect the spatial impact that cities face vis-à-vis the urgencies for environmental rebalancing. It embraces a wide range of phenomena we all live and experience nowadays, not only incidentally but more in a structural way, forcing us to rethink our present state and explore futures. Transitions embody the fluctuating form of late capitalism with its raisings and disruptions. Regarding the environmental conditions, they are affecting the world regions’ habitability, the phenomenon of progressive urbanization versus agricultural land abandonment (UN 2018). With these premises, transitions seem to be a proper reference for an interdisciplinary and conceptual frame addressing the transformation of urban and metropolitan contexts, which will be the leading players for the coming decades

Space Charge Limited Transport and Time of Flight Measurements in Tetracene Single Crystals: a Comparative Study

We report on a systematic study of electronic transport in tetracene single crystals by means of space charge limited current spectroscopy and time of flight measurements. Both $I$-$V$ and time of flight measurements show that the room-temperature effective hole-mobility reaches values close to $\mu \simeq 1$ cm$^2$/Vs and show that, within a range of temperatures, the mobility increases with decreasing temperature. The experimental results further allow the characterization of different aspects of the tetracene crystals. In particular, the effects of both deep and shallow traps are clearly visible and can be used to estimate their densities and characteristic energies. The results presented in this paper show that the combination of $I$-$V$ measurements and time of flight spectroscopy is very effective in characterizing several different aspects of electronic transport through organic crystals.Comment: Accepted by J. Appl. Phys.; tentatively scheduled for publication in the January 15, 2004 issue; minor revisions compared to previous cond-mat versio