J.G. Williamson

Bio: J.G. Williamson is an academic researcher from Philips. The author has contributed to research in topics: Quantum point contact & Muon. The author has an hindex of 30, co-authored 59 publications receiving 6450 citations. Previous affiliations of J.G. Williamson include Rutherford Appleton Laboratory.

Quantized conductance of point contacts in a two-dimensional electron gas.

Abstract: As a result of the high mobihty attamable in the twodimensional electron gas (2DEG) in GaAs-AlGaAs heterostructures it is now becoming feasible to study ballistic transport in small devices '"6 In metals ideal tools for such studies are constnctions havng a width W and length L much smaller than the mean free path le These are known as Sharvin pomt contacts 7 Because of the ballistic transport through these constnctions, the resistance is determmed by the pomt-contact geometry only Point contacts have been used extensively for the study of elastic and melastic electron scattermg With use of biased pomt contacts, electrons can be mjected mto metals at energies above the Fermi level This allows the study of the energy dependence of the scattermg mechamsms 8 With the use of a geometry containmg two pomt contacts, with Separation smaller than le, electrons mjected by a pomt contact can be focused mto the other contact, by the application of a magnetic field This technique (transverse electron focusmg) has been applied to the detailed study of Fermi surfaces 9 In this Letter we report the first expenmental study of the resistance of ballistic pomt contacts m the 2DEG of high-mobihty GaAs-AlGaAs heterostructures The smgle-pomt contacts discussed m this paper are part of a double-pomt-contact device The results of transverse electron focusmg m these devices will be published elsewhere '° The pomt contacts are dehned by electrostatic depletion of the 2DEG underneath a gate This method, which has been used by several authors for the study of l D conduction,'1 offers the possibility to control the width of the pomt contact by the gate voltage Control of the width is not feasible in metal pomt contacts

Coherent electron focusing with quantum point contacts in a two-dimensional electron gas

Abstract: Transverse electron focusing in a two-dimensional electron gas is investigated experimentally and theoretically for the first time. A split Schottky gate on top of a GaAs-${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$As heterostructure defines two point contacts of variable width, which are used as injector and collector of ballistic electrons. As evidenced by their quantized conductance, these are quantum point contacts with a width comparable to the Fermi wavelength. At low magnetic fields, skipping orbits at the electron-gas boundary are directly observed, thereby establishing that boundary scattering is highly specular. Large additional oscillatory structure in the focusing spectra is observed at low temperatures and for small point-contact size. This new phenomenon is interpreted in terms of interference of coherently excited magnetic edge states in a two-dimensional electron gas. A theory for this effect is given, and the relation with nonlocal resistance measurements in quantum ballistic transport is discussed. It is pointed out, and experimentally demonstrated, that four-terminal transport measurements in the electron-focusing geometry constitute a determination of either a generalized longitudinal resistance or a Hall resistance. At high magnetic fields the electron-focusing peaks are suppressed, and a transition is observed to the quantum Hall regime. The anomalous quantum Hall effect in this geometry is discussed in light of a four-terminal resistance formula.

Observation of zero-dimensional states in a one-dimensional electron interferometer.

Abstract: We have studied the electron transport in a one-dimensional electron interferometer. It consists of a disk-shaped two-dimensional electron gas, to which quantum point contacts are attached. Discrete zero-dimensional states are formed due to constructive interference of electron waves traveling along the circumference of the disk in one-dimensional magnetic edge channels. The conductance shows pronounced Aharonov-Bohm–type oscillations, with maxima occurring whenever the energy of a zero-dimensional state coincides with the Fermi energy. Good agreement with theory is found, taking energy averaging into account.

Quantum ballistic and adiabatic electron transport studied with quantum point contacts

Abstract: We present an experimental and theoretical study of quantum ballistic transport in single quantum point contacts (QPC's), defined in the two-dimensional electron gas (2DEG) of a high-mobility GaAs/${\mathrm{Al}}_{0.33}$${\mathrm{Ga}}_{0.67}$As heterojunction. In zero magnetic field the conductance of quantum point contacts shows the formation of quantized plateaus at multiples of 2${\mathit{e}}^{2}$/h. The experimental results are explained with a simple model. Deviations from ideal quantization are discussed. The experimental results are compared with model calculations. Energy averaging of the conductance has been studied, both as a function of temperature and voltage across the device. The application of a magnetic field leads to the magnetic depopulation of the one-dimensional subbands in the QPC. It is shown that the zero-field quantization and quantization in high magnetic fields are two limiting cases of a more general quantization phenomenon. We use quantum point contacts to study the high-magnetic-field transport in a 2DEG. Quantum point contacts are used to selectively populate and detect edge channels. The experiments show that scattering between adjacent edge channels can be very weak, under certain circumstances even on length scales longer than 200 \ensuremath{\mu}m. This adiabatic transport has resulted in the observation of an anomalous integer quantum Hall effect, in which the quantization of the Hall conductance is not determined by the number of Landau levels in the bulk 2DEG, but by the number of Landau levels in the QPC's instead. Related effects are the anomalous quantization of the longitudinal resistance and the adiabatic transport through QPC's in series. A theoretical description for transport in the presence of Shubnikov--de Haas (SdH) backscattering is given. This model explains the experimentally observed suppression of the SdH oscillations due to the selective population or detection of edge channels. Finally, we demonstrate that the combination of a QPC and a bulk Ohmic contact can act as a controllable edge-channel mixer.

Spintronics: Fundamentals and applications

Abstract: Spintronics, or spin electronics, involves the study of active control and manipulation of spin degrees of freedom in solid-state systems. This article reviews the current status of this subject, including both recent advances and well-established results. The primary focus is on the basic physical principles underlying the generation of carrier spin polarization, spin dynamics, and spin-polarized transport in semiconductors and metals. Spin transport differs from charge transport in that spin is a nonconserved quantity in solids due to spin-orbit and hyperfine coupling. The authors discuss in detail spin decoherence mechanisms in metals and semiconductors. Various theories of spin injection and spin-polarized transport are applied to hybrid structures relevant to spin-based devices and fundamental studies of materials properties. Experimental work is reviewed with the emphasis on projected applications, in which external electric and magnetic fields and illumination by light will be used to control spin and charge dynamics to create new functionalities not feasible or ineffective with conventional electronics.

PYTHIA 6.4 Physics and Manual

Abstract: The Pythia program can be used to generate high-energy-physics ''events'', i.e. sets of outgoing particles produced in the interactions between two incoming particles. The objective is to provide as accurate as possible a representation of event properties in a wide range of reactions, within and beyond the Standard Model, with emphasis on those where strong interactions play a role, directly or indirectly, and therefore multihadronic final states are produced. The physics is then not understood well enough to give an exact description; instead the program has to be based on a combination of analytical results and various QCD-based models. This physics input is summarized here, for areas such as hard subprocesses, initial- and final-state parton showers, underlying events and beam remnants, fragmentation and decays, and much more. Furthermore, extensive information is provided on all program elements: subroutines and functions, switches and parameters, and particle and process data. This should allow the user to tailor the generation task to the topics of interest.

Parton distributions for the LHC

Abstract: We present updated leading-order, next-to-leading order and next-to-next-to-leading order parton distribution functions (“MSTW 2008”) determined from global analysis of hard-scattering data within the standard framework of leading-twist fixed-order collinear factorisation in the $\overline{\mathrm{MS}}$ scheme. These parton distributions supersede the previously available “MRST” sets and should be used for the first LHC data taking and for the associated theoretical calculations. New data sets fitted include CCFR/NuTeV dimuon cross sections, which constrain the strange-quark and -antiquark distributions, and Tevatron Run II data on inclusive jet production, the lepton charge asymmetry from W decays and the Z rapidity distribution. Uncertainties are propagated from the experimental errors on the fitted data points using a new dynamic procedure for each eigenvector of the covariance matrix. We discuss the major changes compared to previous MRST fits, briefly compare to parton distributions obtained by other fitting groups, and give predictions for the W and Z total cross sections at the Tevatron and LHC.

Berry phase effects on electronic properties

Abstract: Ever since its discovery, the Berry phase has permeated through all branches of physics. Over the last three decades, it was gradually realized that the Berry phase of the electronic wave function can have a profound effect on material properties and is responsible for a spectrum of phenomena, such as ferroelectricity, orbital magnetism, various (quantum/anomalous/spin) Hall effects, and quantum charge pumping. This progress is summarized in a pedagogical manner in this review. We start with a brief summary of necessary background, followed by a detailed discussion of the Berry phase effect in a variety of solid state applications. A common thread of the review is the semiclassical formulation of electron dynamics, which is a versatile tool in the study of electron dynamics in the presence of electromagnetic fields and more general perturbations. Finally, we demonstrate a re-quantization method that converts a semiclassical theory to an effective quantum theory. It is clear that the Berry phase should be added as a basic ingredient to our understanding of basic material properties.