Searcharxiv⌕ Search

arXiv subjects

R. Li

Publications and source records attributed to R. Li.

105 records · Page 6Linked to original sources
arXiv2015

Direct measurement of non-equilibrium phonon occupations in femtosecond laser heated Au films

We use ultrafast electron diffraction to detect the temporal evolution of phonon populations in femtosecond laser-excited ultrathin single-crystalline gold films. From the time-dependence of the Debye-Waller factor we extract a 4.7 ps time-constant for the increase in mean-square atomic displacements. We show from the increase of the diffuse scattering intensity that the population of phonon modes near the X and K points in the Au fcc Brillouin zone grows with timescales of 2.3 and 2.9 ps, respectively, faster than the Debye-Waller average. We find that thermalization continues within the initially non-equilibrium phonon distribution after 10 ps. The observed momentum dependent timescale of phonon populations is in contrast to what is usually predicted in a two-temperature model.

cond-mat.mtrl-sci↗
arXiv2015

Electromagnetic effects on the light hadron spectrum

For some time, the MILC Collaboration has been studying electromagnetic effects on light mesons. These calculations use fully dynamical QCD, but only quenched photons, which suffices to NLO in XPT. That is, the sea quarks are electrically neutral, while the valence quarks carry charge. For the photons we use the non-compact formalism. We have new results with lattice spacing as small as 0.045 fm and a large range of volumes. We consider how well chiral perturbation theory describes these results and the implications for light quark masses

hep-lat↗
arXiv2015

Pauli Spin Blockade of Heavy Holes in a Silicon Double Quantum Dot

In this work, we study hole transport in a planar silicon metal-oxide-semiconductor based double quantum dot. We demonstrate Pauli spin blockade in the few hole regime and map the spin relaxation induced leakage current as a function of inter-dot level spacing and magnetic field. With varied inter-dot tunnel coupling we can identify different dominant spin relaxation mechanisms. Applying a strong out-of-plane magnetic field causes an avoided singlet-triplet level crossing, from which the heavy hole g-factor $\sim$ 0.93, and the strength of spin-orbit interaction $\sim$ 110 $μ$eV, can be obtained. The demonstrated strong spin-orbit interaction of heavy hole promises fast local spin manipulation using only electrical fields, which is of great interest for quantum information processing.

cond-mat.mes-hall↗
arXiv2015

MEIC Design Summary

This document summarizes the design of Jefferson Lab's electron-ion collider, MEIC, as of January 20, 2015, and describes the facility whose cost was estimated for the United States Department of Energy Nuclear Sciences Advisory Committee EIC cost review of January 26-28, 2015. In particular, each of the main technical systems within the collider is presented to the level of the best current information.

physics.acc-ph↗
arXiv2014

Control of Coherent Synchrotron Radiation and Micro-Bunching Effects During Transport of High Brightness Electron Beams

Beam quality preservation during transport of high-brightness electron beams is of general concern in the design of modern accelerators. Methods to manage incoherent synchrotron radiation (ISR) have been in place for decades; as beam brightness has improved coherent synchrotron radiation (CSR) and the microbunching instability (uBI) have emerged as performance limitations. We apply the compensation analysis of diMitri, Cornacchia, and Spampinati - as previously used by Borland - to the design of transport systems for use with low-emittance beams, and find that appropriately configured second order achromats will suppress transverse emittance growth due to CSR and appear to limit uBI gain.

physics.acc-ph↗
arXiv2013

Influence of substitution on the optical properties of functionalized pentacene monomers and crystals: Experiment and theory

The influence of solubilizing substitutional groups on the electronic and optical properties of functionalized pentacene molecules and crystals have been investigated. Density functional theory is used to calculate the electronic and optical properties of pentacene, TIBS-CF$_{3}$-pentacene, and TIPS-pentacene. The results are compared with experimental absorption spectra of solutions and the complex dielectric function of thin films in the 1 eV to 3 eV energy range. In all cases, the band gaps of the isolated molecules are found to be smaller than those of the crystals. The absorption spectra and dielectric function are interpreted in terms of the transitions between the highest occupied molecular orbitals and lowest unoccupied molecular orbitals. The bands associated to C and Si atoms connecting the functional side group to the pentacene in the (6,13) positions are found to be the main contributors to the optical transitions. The calculated dielectric functions of thin films agree with the experimental results. A redshift is observed in crystals as compared to molecules in experiment and theory both, where the amplitude depends on the packing structure.

cond-mat.mtrl-sci↗
arXiv2013

Single Hole Transport in a Silicon Metal-Oxide-Semiconductor Quantum Dot

We describe a planar silicon metal-oxide-semiconductor (MOS) based single hole transistor, which is compatible with conventional Si CMOS fabrication. A multi-layer gate design gives independent control of the carrier density in the dot and reservoirs. Clear Coulomb blockade oscillations are observed, and source-drain biasing measurements show that it is possible to deplete the dot down to the few hole regime, with excited states clearly visible. The architecture is sufficiently versatile that a second hole dot could be induced adjacent to the first one.

cond-mat.mes-hall↗
arXiv2011

Gluons and the quark sea at high energies: distributions, polarization, tomography

This report is based on a ten-week program on "Gluons and the quark sea at high-energies", which took place at the Institute for Nuclear Theory in Seattle in Fall 2010. The principal aim of the program was to develop and sharpen the science case for an Electron-Ion Collider (EIC), a facility that will be able to collide electrons and positrons with polarized protons and with light to heavy nuclei at high energies, offering unprecedented possibilities for in-depth studies of quantum chromodynamics. This report is organized around four major themes: i) the spin and flavor structure of the proton, ii) three-dimensional structure of nucleons and nuclei in momentum and configuration space, iii) QCD matter in nuclei, and iv) Electroweak physics and the search for physics beyond the Standard Model. Beginning with an executive summary, the report contains tables of key measurements, chapter overviews for each of the major scientific themes, and detailed individual contributions on various aspects of the scientific opportunities presented by an EIC.

nucl-th↗
arXiv2011

Distributed cavity phase frequency shifts of the caesium fountain PTB-CSF2

We evaluate the frequency error from distributed cavity phase in the caesium fountain clock PTB-CSF2 at the Physikalisch-Technische Bundesanstalt with a combination of frequency measurements and ab initio calculations. The associated uncertainty is 1.3E-16, with a frequency bias of 0.4E-16. The agreement between the measurements and calculations explains the previously observed frequency shifts at elevated microwave amplitude. We also evaluate the frequency bias and uncertainty due to the microwave lensing of the atomic wavepackets. We report a total PTB-CSF2 systematic uncertainty of 4.1E-16.

physics.atom-ph↗
arXiv2008

High fidelity universal set of quantum gates using non-adiabatic rapid passage

Numerical simulation results are presented which suggest that a class of non-adiabatic rapid passage sweeps first realized experimentally in 1991 should be capable of implementing a universal set of quantum gates G_{u} that operate with high fidelity. The gates constituting G_{u} are the Hadamard and NOT gates, together with variants of the phase, π/8, and controlled-phase gates. The universality of G_{u} is established by showing that it can construct the universal set consisting of Hadamard, phase, π/8, and controlled-NOT gates. Sweep parameter values are provided which simulations indicate will produce the different gates in G_{u}, and for which the gate error probability P_{e} satisfies: (i) P_{e} < 10^{-4} for the one-qubit gates; and (ii) P_{e} < 1.27x 10^{-3} for the modified controlled-phase gate. The sweeps in this class are non-composite and generate controllable quantum interference effects that allow the gates in G_{u} to operate non-adiabatically while maintaining high fidelity. These interference effects have been observed using NMR, and it has previously been shown how these rapid passage sweeps can be applied to atomic systems using electric fields. Here we show how these sweeps can be applied to both superconducting charge and flux qubit systems. The simulations suggest that the universal set of gates G_{u} produced by these rapid passage sweeps shows promise as possible elements of a fault-tolerant scheme for quantum computing.

quant-ph↗
arXiv2006

Pulse tube cryocooler with self-cancellation of cold stage vibration

We experimentally demonstrated a new method for reducing the vibration of the cold stage of a cryocooler. Comparing the RMS amplitude with the case of no phase shift of the driving gas pressure between the two pairs, the longitudinal vibration of the cold stage was reduced by 96.1% at 126 K by supplying gas pressure with 180 degrees of phase shift.

physics.ins-det↗
arXiv2000

Analysis and Simulation on the Enhancement of the CSR Effects

Recent measurements of the coherent synchrotron radiation (CSR) effects indicated that the observed beam emittance growth and energy modulation are often bigger than previous predictions based on Gaussian longitudinal charge distributions. In this paper, by performing a model study, we show both analytically and numerically that when the longitudinal bunch charge distribution involves concentration of charges in a small fraction of the bunch length, enhancement of the CSR self-interaction beyond the Gaussian prediction may occur. The level of this enhancement is sensitive to the level of the local charge concentration.

physics.acc-ph↗
arXiv2000

Coherent Synchrotron Radiation Measurements in the CLIC Test Facility (CTF II)

Bunches of high charge (up to 10 nC) are compressed in length in the CTF II magnetic chicane to less than 0.2 mm rms. The short bunches radiate coherently in the chicane magnetic field, and the horizontal and longitudinal phase space density distributions are affected. This paper reports the results of beam emittance and momentum measurements. Horizontal and vertical emittances and momentum spectra were measured for different bunch compression factors and bunch charges. In particular, for 10 nC bunches, the mean beam momentum decreased by about 5% while the FWHM momentum spread increased from 5% to 19%. The experimental results are compared with simulations made with the code TraFiC4.

physics.acc-ph↗
arXiv2000

Driver Accelerator Design for the 10 kW Upgrade of the Jefferson Lab IR FEL

An upgrade of the Jefferson Lab IR FEL is now under construction. It will provide 10 kW output light power in a wavelength range of 2-10 microns. The FEL will be driven by a modest-sized 80-210 MeV, 10 mA energy-recovering superconducting RF (SRF) linac. Stringent phase space requirements at the wiggler, low beam energy, and high beam current subject the design to numerous constraints. These are imposed by the need for both transverse and longitudinal phase space management, the potential impact of collective phenomena (space charge, wakefields, beam break-up (BBU), and coherent synchrotron radiation (CSR)), and interactions between the FEL and the accelerator RF system. This report addresses these issues and presents an accelerator design solution meeting the requirements imposed by physical phenomena and operational necessities.

physics.acc-ph↗