A study of temperature-related non-linearity at the metal-silicon interface

In this paper, we investigate the temperature dependencies of metal-semiconductor interfaces in an effort to better reproduce the current-voltage-temperature (IVT) characteristics of any Schottky diode, regardless of homogeneity. Four silicon Schottky diodes were fabricated for this work, each displaying different degrees of inhomogeneity; a relatively homogeneous NiV/Si diode, a Ti/Si and Cr/Si diode with double bumps at only the lowest temperatures, and a Nb/Si diode displaying extensive non-linearity. The 77–300 K IVT responses are modelled using a semi-automated implementation of Tung's electron transport model, and each of the diodes are well reproduced. However, in achieving this, it is revealed that each of the three key fitting parameters within the model display a significant temperature dependency. In analysing these dependencies, we reveal how a rise in thermal energy “activates” exponentially more interfacial patches, the activation rate being dependent on the carrier concentration at the patch saddle point (the patch's maximum barrier height), which in turn is linked to the relative homogeneity of each diode. Finally, in a review of Tung's model, problems in the divergence of the current paths at low temperature are explained to be inherent due to the simplification of an interface that will contain competing defects and inhomogeneities

Tunable coaxial cavity resonator for linear and nonlinear microwave characterization of superconducting wires

We discuss experimental results obtained using a tunable cylindrical coaxial cavity constituted by an outer Cu cylinder and an inner Pb-BSCCO wire. We have used this device for investigating the microwave response of the superconducting wire, both in the linear and nonlinear regimes. In particular, by tuning the different modes of the cavity to make them resonant at exactly harmonic frequencies, we have detected the power emitted by the superconducting inner wire at the second- and third-harmonic frequency of the driving field. The results obtained in the nonlinear regime, whether for the microwave surface impedance or the harmonic emission, are qualitatively accounted for considering intergrain fluxon dynamics. The use of this kind of device can be of strong interest to investigate and characterise wires of large dimensions to be used for implementing superconducting-based microwave devices.Comment: 14 pages, 6 embedded figures, accepted for publication in Supercond. Sci. Techno

Continuous-wave room-temperature diamond maser

The maser, older sibling of the laser, has been confined to relative obscurity due to its reliance on cryogenic refrigeration and high-vacuum systems. Despite this it has found application in deep-space communications and radio astronomy due to its unparalleled performance as a low-noise amplifier and oscillator. The recent demonstration of a room-temperature solid- state maser exploiting photo-excited triplet states in organic pentacene molecules paves the way for a new class of maser that could find applications in medicine, security and sensing, taking advantage of its sensitivity and low noise. However, to date, only pulsed operation has been observed in this system. Furthermore, organic maser molecules have poor thermal and mechanical properties, and their triplet sub-level decay rates make continuous emission challenging: alternative materials are therefore required. Therefore, inorganic materials containing spin-defects such as diamond and silicon carbide have been proposed. Here we report a continuous-wave (CW) room-temperature maser oscillator using optically pumped charged nitrogen-vacancy (NV) defect centres in diamond. This demonstration unlocks the potential of room-temperature solid-state masers for use in a new generation of microwave devices.Comment: 7 pages, 4 figure

A Twist in the Dyon Partition Function

In four dimensional string theories with N=4 and N=8 supersymmetries one can often define twisted index in a subspace of the moduli space which captures additional information on the partition function than the ones contained in the usual helicity trace index. We compute several such indices in type IIB string theory on K3 x T^2 and T^6, and find that they share many properties with the usual helicity trace index that captures the spectrum of quarter BPS states in N=4 supersymmetric string theories. In particular the partition function is a modular form of a subgroup of Sp(2,Z) and the jumps across the walls of marginal stability are controlled by the residues at the poles of the partition function. However for large charges the logarithm of this index grows as 1/n times the entropy of a black hole carrying the same charges where n is the order of the symmetry generator that is used to define the twisted index. We provide a macroscopic explanation of this phenomenon using quantum entropy function formalism. The leading saddle point corresponding to the attractor geometry fails to contribute to the twisted index, but a Z_n orbifold of the attractor geometry produces the desired contribution.Comment: LaTeX file, 35 pages; v2: references adde

Discrete Information from CHL Black Holes

AdS_2/CFT_1 correspondence predicts that the logarithm of a Z_N twisted index over states carrying a fixed set of charges grows as 1/N times the entropy of the black hole carrying the same set of charges. In this paper we verify this explicitly by calculating the microscopic Z_N twisted index for a class of states in the CHL models. This demonstrates that black holes carry more information about the microstates than just the total degeneracy.Comment: LaTeX file, 24 pages; v2: references adde

Temperature compensated niobate microwave ceramics with the columbite structure, M2+Nb2O6

Of the niobate ceramics with the formula M2+Nb2 O6, several compounds (M2+ = Zn, Mg, Ca and Co) have Qf values (at f = 1-10 GHz) between 40 000 and 90 000 GHz, offering potential in dielectric resonator applications. However, their temperature coefficient of resonant frequency (τf) values are too high for commercial development, at between -50 and -90 ppm. This paper details the doping of these materials with dielectric ceramics having a large positive τf, in an attempt to reduce the overall τf to zero, whilst maintaining a high quality factor (Q). It was also found that doping increased the relative permitivity (εr) of the niobates. Several materials have been made with near-zero τf, such as 90% CoNb2O6/10% CaTiO3 (τf = + 2.0 ppm, εr = 25.2 and Qf = 21 700 GHz), and 94% CoNb2O6/6% TiO2 (τf = +4.4 ppm, εr = 29.6 and Qf = 20 300 GHz) © 2003 Elsevier Ltd. All rights reserved

The effects of sintering aids upon dielectric microwave properties of columbite niobates, M2+Nb2O6

The columbite niobate ceramics ZnNb2O6, MgNb 2O6, CaNb2O6 and CoNb 2O6 have low dielectric losses at microwave (1-10 GHz) frequencies, resulting in Qf values between 40 000 and 90 000 GHz, making them suitable materials for use in dielectric resonator applications. However, their temperature coefficient of resonant frequency (τf) values were high, at between -50 and -90 ppm, and the optimum sintering temperatures were found to be between 1150°C and 1350°C. This paper details the doping of these ceramics with 1 wt%V2O5, 1 wt% CeO2, 2 wt% WO3 and 0.5 wt% CuO, in an attempt to reduce the sintering temperature. It was found that in many cases the dopants also had an extremely beneficial effect upon microwave properties, increasing ψr, and decreasing τf considerably. Although the dopants often had a deleterious effect upon the quality factor (Q), in some cases they caused an increase in Q. Qf values of over 20 000 GHz were often obtained at lower temperatures, even in poorly sintered niobates, and CuO-doped CaNb 2O6 yielded Qf values in excess of 65 000 GHz. CoNb 2O6 + V2O5 or CuO gave 90% sintered and poorly sintered materials with Qf over 10 000 GHz and 25 000 GHz, respectively, at temperatures within low temperature Co-fired ceramic limits

Techniques for microwave measurements of ferroelectric thin films and their associated error and limitations

This paper examines the problem of evaluating the microwave properties of thin ferroelectric films patterned as planar capacitors. Two types of microwave measurements of ferroelectric thin films are considered: reflection- and resonance-type measurements. Algorithms are presented for evaluation of capacitance-permittivity and dielectric loss. Using sensitivity analysis, the error and limitations associated with these measurements are estimated. The end result is a series of formulae that use the network analyser's measurement data to calculate the capacitance-permittivity, the dielectric loss and the associated error