Publications
Superheating field of superconductors within Ginzburg-Landau theory
We study the superheating field of a bulk superconductor within Ginzburg-Landau theory, which is valid near the critical temperature. We calculate, as functions of the Ginzburg-Landau parameter κ, the superheating field Hsh and the critical momentum kc characterizing the wavelength of the instability of the Meissner state to flux penetration. By mapping the two-dimensional linear stability theory into a one-dimensional eigenfunction problem for an ordinary differential equation, we solve the problem numerically.
Geometry of nonlinear least squares with applications to sloppy models and optimization
Parameter estimation by nonlinear least-squares minimization is a common problem that has an elegant geometric interpretation: the possible parameter values of a model induce a manifold within the space of data predictions. The minimization problem is then to find the point on the manifold closest to the experimental data. We show that the model manifolds of a large class of models, known as sloppy models, have many universal features; they are characterized by a geometric series of widths, extrinsic curvatures, and parameter-effect curvatures, which we describe as a hyper-ribbon.
Angle dependence of quantum oscillations in YBa 2 Cu 3 O 6.59 shows free-spin behaviour of quasiparticles
Measurements of quantum oscillations in the cuprate superconductors afford an opportunity to assess the extent to which their electronic properties yield to a description rooted in Fermi-liquid theory. However, such an analysis is hampered by the small number of oscillatory periods observed in the accessible magnetic field range. Here we employ a genetic algorithm to globally model the field, angular and temperature dependence of the quantum oscillations observed in the resistivity of YBa 2 Cu 3 O 6.59 .
Scatterometry measurement for gate ADI and AEI critical dimension of 28-nm metal gate technology
This paper discusses the scatterometry-based metrology measurement of 28nm high k metal gate after-develop inspection (ADI) and after-etch inspection (AEI) layer structures. For these structures, the critical measurement parameters include side wall angle (SWA) and critical dimension (CD). For production process control of these structures, a metrology tool must utilize a non-destructive measurement technique, and have high sensitivity, precision and throughput.
Small-angle solution scattering using the mixed-mode pixel array detector
Solution small-angle X-ray scattering (SAXS) measurements were obtained using a 128 X 128 pixel X-ray mixed-mode pixel array detector (MMPAD) with an 860 μs readout time. The MMPAD offers advantages for SAXS experiments: a pixel full-well of >2 × 107 10 keV X-rays, a maximum flux rate of 108 X-rays pixel-1 s-1, and a sub-pixel point-spread function. Data from the MMPAD were quantitatively compared with data from a charge-coupled device (CCD) fiber-optically coupled to a phosphor screen.
X-ray analog pixel array detector for single synchrotron bunch time-resolved imaging
Dynamic X-ray studies can reach temporal resolutions limited by only the X-ray pulse duration if the detector is fast enough to segregate synchrotron pulses. An analog integrating pixel array detector with in-pixel storage and temporal resolution of around 150 ns, sufficient to isolate pulses, is presented. Analog integration minimizes count-rate limitations and in-pixel storage captures successive pulses. Fundamental tests of noise and linearity as well as high-speed laser measurements are shown.
Quantum Monte Carlo with Jastrow-valence-bond wave functions
We consider the use in quantum Monte Carlo calculations of two types of valence bond wave functions based on strictly localized active orbitals, namely valence bond self-consistent-field and breathing-orbital valence bond wave functions. Complemented by a Jastrow factor, these Jastrow-valence-bond wave functions are tested by computing the equilibrium well depths of the four diatomic molecules C2, N2, O2, and F 2 in both variational Monte Carlo and diffusion Monte Carlo.
Basis set construction for molecular electronic structure theory: Natural orbital and Gauss-Slater basis for smooth pseudopotentials
A simple yet general method for constructing basis sets for molecular electronic structure calculations is presented. These basis sets consist of atomic natural orbitals from a multiconfigurational self-consistent field calculation supplemented with primitive functions, chosen such that the asymptotics are appropriate for the potential of the system. Primitives are optimized for the homonuclear diatomic molecule to produce a balanced basis set. Two general features that facilitate this basis construction are demonstrated.
Lifshitz critical point in the cuprate superconductor YBa 2Cu3Oy from high-field Hall effect measurements
The Hall coefficient RH of the cuprate superconductor YBa 2Cu3Oy was measured in magnetic fields up to 60 T for a hole concentration p from 0.078 to 0.152 in the underdoped regime. In fields large enough to suppress superconductivity, RH(T) is seen to go from positive at high temperature to negative at low temperature, for p0.08. This change of sign is attributed to the emergence of an electron pocket in the Fermi surface at low temperature. At p<0.08, the normal-state R H(T) remains positive at all temperatures, increasing monotonically as T→0.
Mass coupling and Q -1 of impurity-limited normal 3He in a torsion pendulum
We present results of the Q -1 and period shift, ΔP, for 3He confined in a 98% nominal open aerogel on a torsion pendulum. The aerogel is compressed uniaxially by 10% along a direction aligned to the torsion pendulum axis and was grown within a 400 μm tall pancake (after compression) similar to an Andronikashvili geometry. The result is a high Q pendulum able to resolve Q -1 and mass coupling of the impurity-limited 3He over the whole temperature range.