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Microprocessor designs are increasingly moving towards multiple cores on a single die. Validating memory consistency, coherency, ordering, and atomicity is crucial. X86 microprocessors are prevalent at most levels of computing. Thus, new x86 microprocessors undergo extensive compatibility testing. Being a high volume product, the economic and logistical repercussions of a functional deficiency escaping into the production cycle and beyond are humbling. The first silicon functional validation and debug of multicore microprocessors are constrained by design complexity, compatibility with existing hardware and software, and time-to-market pressures. This paper describes microprocessor debug features and their use in debugging functional failures. An encompassing overview of the microprocessor's first silicon validation is presented. Emphasis is put on validation and debug of multicore microprocessors targeting multinode systems. This paper presents a novel method to validate and debug intra-node and inter-node communication traffic. This paper also develops an analysis to determine optimal on die debug resources. Finally, data from an 8-node system is presented to demonstrate the extent of intrusiveness of a coherent and noncoherent traffic debug feature

The d.c. and a.c. characteristics of double-barrier diodes have been used to probe the large-scale annealing of defects within ion-implanted GaAs. Multi-stage annealing of defects was elucidated by the staged recovery of resonant tunnelling through ion-implanted diodes. Surprisingly, the current - voltage characteristics of some ion-implanted diodes after very rapid anneals were qualitatively identical to the as-grown characteristics, but the peak-current density was two orders of magnitude lower. This is explained by the creation of small percolation paths of as-grown material during the initial stages of annealing, where these paths surrounded clusters of defects. A simple capacitance model based on the creation of these percolation paths is described, and is consistent with the capacitance - voltage data. The idea that small-area percolation paths of as-grown material were created within the ion-implanted DBDs during very rapid annealing was supported by the observation of very-low bias resonant tunnelling and single-electron switching at 4.2 K. Both these phenomena are usually observed only within small-area pristine DBDs.

We present a systematic magnetotransport study of high mobility (3 1 1)A strongly coupled p-type double quantum wells which demonstrate the existence of a correlated v = 1 quantum Hall state in the limit of weak tunnelling. Our results are in good agreement with the finite temperature phase transition predicted for this state.

We have measured the magnetoresistance measurements of a 2D electron gas subjected to a periodic magnetic field created by the presence of type II superconducting stripes on the surface of the heterostructure. We observe oscillatory behaviour due to a commensurability effect between the cyclotron radius and the period of the magnetic field. The results can be understood in terms of a simple semi-classical theory.

A donor-related resonance is observed in double-barrier resonant tunneling devices with Si donors incorporated in the quantum well. In high magnetic fields the resonance becomes dominant over the Is resonance associated with the ground state of a single donor. The bias position of the donor resonance, its magnetic field dependence, and large amplitude indicate unambiguously that the resonance is due to tunneling through the ground state of a shallow donor with two bound electrons (D- level).

We report on the motion of two-dimensional electrons in a magnetic field which is periodic on the scale of a few hundred nanometres. Such a system exhibits novel magnetoresistance oscillations that we unambiguously identify as due to a commensurability effect between the period of the magnetic field and the diameter of the cyclotron orbit. We find that a semiclassical picture of electron motion can quantitatively describe all of the observed experimental features. We also propose methods of optimizing the effects of the magnetic modulation in hybrid semiconductor-superconductor devices.

We report on tunnelling transport through a novel quantum device consisting of a double barrier structure fabricated as a submicron square loop in the plane of growth. We find two kinds of quantum confinement coexist in such a device: near the current onset, tunelling starts through zero-dimensional states located in the four corners of the loop, whereas at higher bias the resonant current exhibits kinks due to one-dimensional confinement in the arms of the loop. The use of a high magnetic field permits us to measure the binding energy of the dot ground state to the first one-dimensional subband in the connecting wires. This value is then compared to existing theory. Moreover, we show that the different kinds of lateral confinement are not only evidenced in the resonant structure, but also in the scattering assisted replica peaks.

We have studied fluctuations in the differential conductance of resonant tunnelling diodes which incorporate a δ-layer of donor impurities in the centre plane of the quantum well. The system is well-suited to studying this type of conductance fluctuation, as we can control the number and position of the impurity states and also vary their effective linewidth by the application of a magnetic field perpendicular to the current direction.

Tunnelling through highly localised impurity states in the quantum well of a resonant tunnelling diode allows us to study the properties of the two-dimensional electron gas formed at the emitter barrier of the device. In high magnetic fields, the electrons form quantum dots in the disordered potential due to unscreened donors in the depleted collector contact.

Electron tunnelling through donor-related states is discussed. This tunnelling process, which occurs well below the threshold voltage for conventional resonant tunnelling into the two-dimensional continuum states of the quantum well, reveals a new type of Fermi edge singularity effect which arises from the Coulomb interaction between the tunnelling electron on the localized site and the Fermi sea of electrons in the emitter layer. A new means of forming laterally confined resonant tunnelling devices is also described. By studying the effect of an applied magnetic field, the additional structure that appears in the current-voltage characteristics of these devices can be unambiguously associated with a lateral quantum mechanical confinement effect.

We discuss the origin of zero-dimensional states, which give rise to resonant structure at the current onset in tunneling devices. The states can be identified as being due to random pairs of shallow donors.

We have measured the low-temperature magnetoresistance of long (>50 mu m), narrow (similar to 1 mu m) quantum wires fabricated from a high-mobility two-dimensional electron gas. In the quantum Hall regime at high magnetic fields we observe that the Shubnikov-de Haas oscillations split into a series of aperiodic sharp peaks. Both the amplitude and sharpness of the peaks are strongly dependent on temperature below 1 K. We attribute the peaks to resonant tunneling between edge states through localized states in the bulk. The results are in quantitative agreement with the theoretical model of Jain and Kivelson.

We have investigated mesoscopic conductance fluctuations in a GaAs quantum well with five electrically quantized two-dimensional (2D) subbands occupied. In contrast to the case of a single occupied subband, where the fluctuations depend only on the component of magnetic field perpendicular to the plane of the electrons, in our multi-subband structures the conductance fluctuations occur even in a magnetic field parallel to the plane. We attribute the fluctuations to rapid electron-impurity scattering between different 2D subbands which leads to quasi-3D motion of electrons within the quantum well. We are able to confirm the 3D character of the fluctuations by correlations between the magnetoresistances observed when the magnetic field is applied at various angles relative to the plane of the quantum well.

We have investigated resonant tunnelling in GaAs/(AlGa)As heterostructures which have been fabricated into square mesas 6 x 6 mum. A delta-layer of donors (n approximately 2 x 10(9) cm-2) has been incorporated at the centre of the quantum well which is 9 nm wide. The I(V) characteristics show a feature at approximately 70 mV, which is below the threshold for the main resonance and is due to resonant tunnelling through single donor states in the well. This feature is also present in large area mesas. At lower biases and at low temperatures we see a new set of resonances which, although they occur in all small area mesas, differ in detail between devices with regard to their strength and bias position. The form of the low-bias structure is strongly dependent on temperature, T, below 4 K where several very sharp steps appear, becoming sharper as T is decreased. We have also investigated the dependence of the new structure on magnetic field, B, parallel to the current direction. We attribute the new features to tunnelling through potential fluctuations on the mesoscopic scale due to donor clustering.

We have observed a Fermi-edge singularity in the tunneling current between a two-dimensional electron gas (2DEG) and a zero-dimensional localized state. A sharp peak in the tunnel current is observed when the energy of the localized state matches the Fermi energy of the 2DEG. The peak grows and becomes sharper as the temperature is decreased to our lowest temperature of 70 mK. We attribute the singularity to the Coulomb interaction between the tunneling electron on the localized site and the Fermi sea.

We report measurements of the magnetoresistance of long (> 50 mum), narrow (< 1 mum) quantum wires fabricated from a high mobility two-dimensional electron gas. We observe that, at low filling factors, the Shubnikov-de Haas oscillations split into a series of sharp peaks which we attribute to resonant tunnelling between edge states through localized states in the bulk. The results are in excellent agreement with the theoretical model of Jain and Kivelson.

Staphylococcus aureus is a commensal and opportunistic pathogen of humans. It can colonize and infect the mammary gland of cows. Adherence to the moist squamous epithelium is crucial to colonization. This is mediated by several different surface proteins that interact with the surface of cornified squamous epithelial cells. Staphylococcus aureus can express a number of surface and secreted proteins that contribute to the process of infection. Surface proteins promote adhesion to the extracellular matrix, evasion of immune responses, and invasion of epithelial cells. Secreted proteins inhibit neutrophil migration and complement fixation, and damage the membranes of host cells including phagocytes. If ingested by a neutrophil or macrophage the bacterium has several mechanisms to avoid being killed. Secreted enterotoxins can cause food poisoning if ingested. Their superantigen activity compromises adaptive immunity. Small colony variants can survive in a semidormant state after invasion of epithelial cells and contribute to chronic infections. MRSA (methicillin-resistant Staphylococcus aureus) strains are resistant to multiple antibiotics including all β-lactams. They cause infections in hospitalized patients and more recently virulent strains of MRSA have emerged in the community. New antibiotics have been introduced to combat MRSA and experimental subunit vaccines are in development.

We have investigated the I ( V ) characteristics of a sub-micron gated GaAs/ (AlGa)As resonant tunnelling diode. We find that, relative to a large area device, there is additional structure at both high and low source-drain bias. The low bias features are shown to be due to the presence of impurities in the quantum well. This is confirmed by experiments on δ-doped large area devices. Using the gate on the sub-micron devices we are able to control the current through a single donor state. The high voltage structure occurs within the main resonance and only in forward bias where the peak-to-valley ratio (PVR) is poor. We ascribe the poor PVR to a lateral variation of the resonance condition and discuss the high voltage structure in these devices, and in similar two-terminal devices, in the light of this hypothesis.

We have studied universal conductance fluctuations in the magnetoresistance of n+-type GaAs submicrometer wires, which represent an orthodox mesoscopic system but also allow us to reach the high-magnetic-field regime, omega(c)tau > 1, where omega(c) is the cyclotron frequency and tau the electron scattering time. The Lee-Stone correlation field B(c) increases by more than an order of magnitude as the magnetic field increases from 0 to 18 T, but the amplitude of the fluctuations remains unchanged. This implies that the universal scaling of conductance fluctuations is not valid in high magnetic fields, in strong disagreement with theoretical predictions. We show that this behavior is not specific to the nonlocal geometry of measurements, where the breakdown has been reported earlier, but that it also occurs in the local magnetoresistance and rectification fluctuations. The violation of universal scaling is attributed to the appearance of a second phase-breaking length, in the regime omega(c)tau > 1, due to extended electron diffusion near the sample boundaries.

We report the observation of the fractional quantum Hall effect in single GaAs-(Ga,Al)As quantum well structures with well widths of 102 A and 68 A. In both samples we have observed a strong v= 4 3 fraction in both the longitudinal resistivity ϱ xx and the Hall resistivity ϱ xy . The surprizing result in our data (in comparison with the FQHE in conventional single heterojunctions) is that the 5 3 is heavily suppressed or possibly absent. In a tilted magnetic field the energy gap of 4 3 depends only on the perpendicular field. We associate this difference between our samples and the conventional heterojunctions with the energy (well width) dependence of the Lande g ∗ factor and the influence of the Zeeman energy on the (partially) spin-polarized energy levels and hence the activation energies.

We have observed a high-field transition into a high-resistance state at filling factor v just below 2/7 in a high -density, high-mobility two-dimensional hole gas (2DHG). The state shows clear threshold behaviour consistent with a strongly pinned Wigner solid. The transition is at lower filling factor than the value of ∼0.37 recently reported by Santos et al. [7] for a low -density 2DHG, and higher than the value of ∼0.22 usually found in equivalent 2D electron systems. Our experiment shows no indication of a re-entrant behaviour of the insulating phase. We believe that this may be the highest-carrier-density Wigner-transition observed.

A series of sub-threshold resonances have been observed in the source-drain I ( V ) characteristics of a symmetric double harrier gated resonant tunnelling device. The structure is unaffected when the area of the device is reduced by as much as 50% and is independent of temperature between 35 mK and 10 K. The origin of this structure is neither lateral quantisation nor Coulomb blockade. The amplitude of the first peak in this structure (∼ 18 pA) implies that it is due to single electron tunnelling. We propose a model based upon tunnelling through the hound states of a single donor in the active region of the device.

We have observed remarkable new structure in the source-drain I(V) characteristics of a symmetric double barrier resonant tunneling device in which the cross sectional area may be varied from ≅ 1μm2 to ≅0.1 μm2 by applying a voltage to a gate. In the source-drain voltage range close to, but slightly below, the threshold for current flow, peaks are observed with a peak value of ≅ 20 pA, and peak/valley ratio up to 10 1 . The structure in I(V) is independent of temperature between 35 mK and 10 K, and of gate voltage between 0 and −2 V. The first peak in I(V) in each polarity is unaffected by a magnetic field applied parallel to the current flow for fields up to 6 T. This structure cannot be explained by lateral quantisation or Coulomb blockade arising from the confinement. We propose that it is due to resonant tunneling between zero-dimensional states formed in the active region by the Coulombic potential of a single ionised donor, combined with the barrier potential.

Hydrostatic pressure is used to investigate two types of device based upon the n-type GaAs/(AlGa)As material system: an asymmetric double barrier resonant tunnelling device and a device incorporating two superlattices separated by a single barrier. In the double barrier structure, the pressure-induced increase in Gamma -X tunnelling and scattering reduces the build-up of electronic space charge in the GaAs quantum well. This allows one to study the transition from intrinsic bistability to ordinary negative differential conductivity in I(V). In the superlattice/barrier/superlattice structure the authors have evidence for a pressure-induced decrease in the free-electron density of the heavily doped n+ contact regions due to DX centres.

A new process based on photolithography and selective wet etching has been used to fabricate small area resonant tunnelling diodes. The low‐temperature current‐voltage (I‐V) characteristics of diodes with conducting widths less than 0.1 μm show additional peaks due to 1D lateral quantum confinement. We observe a pronounced asymmetry in I‐V which we explain in terms of tunnelling from emitter states which have a different degree of lateral quantum confinement.

We have investigated the current‐voltage [I(V)] characteristics of a gated GaAs/(AlGa)As resonant tunneling diode. As the negative gate voltage is progressively increased I(V) becomes asymmetric. In particular the peak‐to‐valley ratio in forward bias is decreased from ≂20 to ≂1, but in reverse bias remains constant ≂20. This arises from a lateral variation of the voltage drop across the emitter tunnel barrier, which in forward bias leads to a smearing of the resonance. We discuss the relationship between our experiment and the low peak‐to‐valley ratios of two‐terminal submicron resonant tunneling diodes observed by other groups.

Hole resonant tunnelling is investigated in an asymmetric p-type RTD which exhibits intrinsic multistability on the first light-hole (LH1) resonant peak. This device exhibits two previously unseen satellite peaks just beyond the LH1 resonant peak. Both satellites are strongly dependent on temperature and quench above 18K. We have modeled the tunneling transitions in our devices and show that these features are due to plasmon-assisted resonant tunneling of holes.

The classical phase space for hot electrons resonantly injected into a wide quantum well (QW) exhibits a transition from stable regular motion to strong classical chaos when an applied magnetic field B is tilted away from the normal to the well walls1. The corresponding quantised energy level spectra and eigenfunctions of this experimentally accessible system have been investigated in the regime of strong classical chaos using resonant tunneling spectroscopy of very wide QWs (~ 100 nm)1,2. In this domain, distinct series of periodic resonant peaks can be identified and related to periodic fluctuations3 in the density of levels in the QW and periodic patterning of the wavefunctions4.

We have investigated reflection of edge states from the boundary between a disordered region and a high-mobility region of a two-dimensional electron gas. \ensuremath{\alpha}-particle bombardment has been employed to reduce selectively the mobility. We observe an exponential increase in reflection of the innermost edge state from the disordered region as the temperature decreases. The results are explained in terms of the Milne reflection of electrons which are elastically scattered back before thermalization can occur.

Hydrostatic pressure and high magnetic fields are used to study space charge buildup under resonant tunneling conditions in an asymmetric double barrier structure based on n-type GaAs/(AlGa)As. The space charge buildup gives rise to an intrinsic bistability effect in the current-voltage characteristics. The pressure-induced increase in Γ-X tunneling and scattering reduces the resonant space charge buildup and allows us to study the transition from intrinsic bistablity to ordinary negative differential conductivity in I(V). Over a certain pressure range a sharp resonance is observed in which electrons tunnel into the well via Γ-states and scatter out mainly via X-states, a clear example of sequential tunneling. The final part of the paper describes a remarkable enhancement of the intrinsic bistability when a quantizing magnetic field is applied perpendicular to the barriers. This is associated with the strong degeneracy of Landau levels at high magnetic fields.

Intense, pulsed magnetic field studies of 2D hole gas systems at low temperatures examine competition of fractional QHE ground states with an insulating phase in the region ν

A modified molecular beam epitaxy technique has been used to grow single barrier GaAs/GaN/GaAs heterostructures of both n‐ and p‐type. The temperature‐dependent I(V) measurements show thermally activated conduction over the barrier above about 200 K. A Type I band alignment is indicated by significant offsets in both the conduction and valence bands at the GaAs/GaN heterointerface.

We have used resonant tunnelling spectroscopy to investigate the energy level spectrum of a wide 60 nm potential well with a strong magnetic field applied at an angle θ to the normal to the barriers. In this geometry, the current-voltage characteristic I(V) reveal distinct series of resonances which change dramatically with both V and θ. In a classical picture, the electron orbits in the well are strongly chaotic for 15° ≤ θ ≤ 75° and voltages V ≤ 1.2 V. However, we have identified a hierarchy of unstable but periodic orbits whose calculated properties explain the origin and θ-dependence of the resonant structure. We show that the classical motion becomes non-chaotic at high bias voltages and that the changeover to stable orbits is characteriaed by the appearance of widely-spaced resonances in I(V) .

The conductance of a mesoscopic device is usually described in terms of the transmission of electrons between electrical contacts. Implicit in this description is the assumption that contacts can be approximated as ideal thermal reservoirs which act solely to thermalise incident electrons before re-injecting them into the device. We describe two classes of experiment where this assumption is not valid. First, when the dissipation in the system is strongly dependent on an external parameter, such as temperature or magnetic field, the physical position of the contact may itself depend the parameter. We study this by investigating the transport properties of quantum wires with disordered contacts. Second, when there is the possibility of some sort of interaction between the electrons in the contact and those in the device, as we find in resonant tunnelling from a 2D contact into a 0D state. In both cases we find that the conductance properties of the device are strongly modified by the nature of the contact. PACS: 73.40.Kp, 73.40.Hm, 73.40Gk

Magnetoquantum oscillations in the tunnel current of double barrier n-GaAs/(AlGa)As/GaAs/ (AlGa)As/GaAs resonant tunnelling devices are shown to contain both resonant and non-resonant contributions. The former demonstrates the build-up of charge in the GaAs quantum well when electrons are resonant with the first subband of the well. A theoretical analysis of resonant tunnelling is in excellent agreement with the experimental data. Evidence of sequential tunnelling is found in the voltage range corresponding to the resonance when electrons tunnel into the second subband of the well. Near this resonance, the charge build-up in the well is estimated from the magnetotunnelling oscillations.

Shubnikov-de Haas and persistent photoconductivity measurements are used to study the effect of hydrostatic pressure on the free electron concentration, mobility, and the occupancy of the DX centre in MBE grown n + GaAs heavily doped with either Si or Sn. The results show that the DX centre produces a resonant donor level between the Γ- and L- conduction band minima at a concentration comparable with the doping level. The position and occupancy of the DX centre are calculated using Fermi-Dirac statistics. For the Si-doped samples comparison with local vibrational mode measurements indicate that the DX level can be identified with Si Ga .

Hydrostatic pressure is used to investigate band structure effects in double barrier resonant tunnelling devices based on n-type (AlGa)As/GaAs. The pressure-induced increase in Γ-X tunnelling and scattering reduces the space charge build-up in the quantum well at resonance and allows us to study the transition from intrinsic bistability to ordinary negative differential conductivity in the I(V) characteristics. In the limit of fast Γ-X transfer the resonant peak in I(V) is lost.

Shubnikov-de Haas and persistent photoconductivity measurements are used to study the mobility, free electron density (n) and the occupancy of the DX centre in heavily doped n-GaAs as a function of doping level and hydrostatic pressure. The results show that the DX centre produces a resonant donor level between the Γ - and L-conduction band minima at a concentration comparable with the doping level. For the Si-doped samples, comparison with local vibration mode measurements indicates that the DX level can be identified with SiGa. The level acts to pin the Fermi energy at electron concentrations around 1.8 × 1019 cm−3. Analysis of the results indicates that macroscopic charge separation is not responsible for persistent photoconductivity in these samples.

Planar geared linkages readily lend themselves to function, path and motion generation. Function generation includes any problems in which rotations or sliding motion of input and output elements (either links or gears) must be correlated. In some cases, the designer may want to produce a formal functional relationship between the input and output. In these cases, the input and output rotations can be used as the linear analogs of the independent and dependent variables. In other cases, the designer has no particular functional relationship in mind, but merely wants to produce specific relationships between the input and output at certain “precision points”. Path generation involves moving a tracer point along a specified path. A point attached to any “floating” link (such as a coupler) may be used as the tracer point. Motion generation requires that an entire body be guided through a prescribed motion sequence. The body to be guided can be attached to any floating link.

We show that the bistability effect observed in the current-voltage characteristics of a double-barrier resonant-tunneling device can be removed by connecting a suitable capacitor across the device. These measurements cast serious doubt on the recent interpretation of the bistability as an intrinsic space-charge effect.