arXiv subjects

K. Chen

Publications and source records attributed to K. Chen.

68 records · Page 4Linked to original sources

arXiv2016

Development of a modular test system for the silicon sensor R&D of the ATLAS Upgrade

High Voltage CMOS sensors are a promising technology for tracking detectors in collider experiments. Extensive R&D studies are being carried out by the ATLAS Collaboration for a possible use of HV-CMOS in the High Luminosity LHC upgrade of the Inner Tracker detector. CaRIBOu (Control and Readout Itk BOard) is a modular test system developed to test Silicon based detectors. It currently includes five custom designed boards, a Xilinx ZC706 development board, FELIX (Front-End LInk eXchange) PCIe card and a host computer. A software program has been developed in Python to control the CaRIBOu hardware. CaRIBOu has been used in the testbeam of the HV-CMOS sensor CCPDv4 at CERN. Preliminary results have shown that the test system is very versatile. Further development is ongoing to adapt to different sensors, and to make it available to various lab test stands.

physics.ins-det↗

arXiv2016

Results of the 2015 testbeam of a 180 nm AMS High-Voltage CMOS sensor prototype

Active pixel sensors based on the High-Voltage CMOS technology are being investigated as a viable option for the future pixel tracker of the ATLAS experiment at the High-Luminosity LHC. This paper reports on the testbeam measurements performed at the H8 beamline of the CERN Super Proton Synchrotron on a High-Voltage CMOS sensor prototype produced in 180 nm AMS technology. Results in terms of tracking efficiency and timing performance, for different threshold and bias conditions, are shown.

physics.ins-det↗

arXiv2016

Termination control of magnetic coupling at a complex oxide interface

Atomically flat interfaces between ternary oxides have chemically different variants, depending on the terminating lattice planes of both oxides. Electronic properties change with the interface termination which affects, for instance, charge accumulation and magnetic interactions at the interface. Well-defined terminations have yet rarely been achieved for oxides of ABO3 type (with metals A, B). Here, we report on a strategy of thin film growth and interface characterization applied to fabricate the La0.7Sr0.3MnO3-SrRuO3 interface with controlled termination. Ultra-strong antiferromagnetic coupling between the ferromagnets occurs at the MnO2-SrO interface, but not for the other termination, in agreement with density functional calculations. X-ray magnetic circular dichroism measurements reveal coupled reversal of Mn and Ru magnetic moments at the MnO2-SrO interface. Our results demonstrate termination control of magnetic coupling across a complex oxide interface and provide a pathway for theoretical and experimental explorations of novel electronic interface states with engineered magnetic properties.

cond-mat.mtrl-sci↗

arXiv2014

A note on the Manin-Mumford conjecture

In this paper we prove a special case of the André-Oort conjecture for Kuga varieties. If $M$ is a Kuga variety fibred over a pure Shimura variety $S$ as an abelian scheme, and $(M_n)$ is a sequence of special subvarieties in $M$ which are faithfully flat over $S$, then the Zariski closure of the union of the $M_n$'s is a finite union of special subvarieties faithfully flat over $S$. The proof is reduced to a variant of the Manin-Mumford conjecture for abelian schemes.

math.NT↗

arXiv2014

Bounded equidistribution of special subvarieties I

In this paper, we prove the bounded case of the Andre-Oort conjecture for special subvarieties in a mixed Shimura variety. This generalizes previous results of L. Clozel, E. Ullmo, and A. Yafaev. The proof is reduced to a special case of the equidistribution conjecture for mixed Shimura varieties. The main tool is a theorem of S. Mozes and N. Shah for certain measures on lattice spaces.

math.NT↗

arXiv2014

Bounded equidistribution of special subvarieties II

In this paper, we prove a lower bound for the Galois orbits of a pure special subvariety in a general mixed Shimura variety. For special subvarieties that are not pure, we propose the notion of test invariants as a substitute for the lower bound estimation, and prove the bounded equidistribution for sequences of special subvarieties with uniformly bounded test invariants in a given mixed Shimura variety. Both the estimation and the bounded equidistribution rely on the Generalized Riemann Hypothesis for CM fields, similar to the pure case treated by E. Ullmo and A. Yafaev as part of the ergodic-Galois alternative for the André-Oort conjecture for pure Shimura varieties.

math.NT↗

arXiv2012

On Special Subvarieties of Kuga Varieties

In this paper we prove the equidistribution of certain families of special subvarieties in Kuga varieties, which is a special case of the general Andre-Oort conjecture formulated for mixed Shimura varieties proposed by R.Pink. Our approach is parallel to the pure case treated in the works of L.Clozel, E.Ullmo, and A.Yafaev using tools from ergodic theory.

math.NT↗

arXiv2011

Properties of MgB2 Films Grown at Various Temperatures by Hybrid Physical-Chemical Vapour Deposition

A Hybrid Physical-Chemical Vapour Deposition (HPCVD) system consisting of separately controlled Mg-source heater and substrate heater is used to grow MgB2 thin films and thick films at various temperatures. We are able to grow superconducting MgB2 thin films at temperatures as low as 350 C with a Tc0 of 35.5 K. MgB2 films up to 4 um in thickness grown at 550 C have Jc over 10E6 A/cm2 at 5 K and zero applied field. The low deposition temperature of MgB2 films is desirable for all-MgB2 tunnel junctions and MgB2 thick films are important for applications in coated conductors.

cond-mat.supr-con↗

arXiv2011

On special subvarieties of Kuga varieties II

In this paper we prove the equidistribution of $\Cbf$-special subvarieties in certain Kuga varieties, which implies a special case of the general André-Oort conjecture formulated for mixed Shimura varieties proposed by R.Pink. The main idea is to reduce the equidistribution to a theorem of Szpiro-Ullmo-Zhang on small points of abelian varieties and a theorem on the equiditribution of $C$-special subvarieties of Kuga varieties of rigid type treated by the author in a previous paper.

math.NT↗

arXiv2011

Measurement of correlations between low-frequency vibrational modes and particle rearrangements in quasi-two-dimensional colloidal glasses

We investigate correlations between low-frequency vibrational modes and rearrangements in two-dimensional colloidal glasses composed of thermosensitive microgel particles which readily permit variation of sample packing fraction. At each packing fraction, the particle displacement covariance matrix is measured and used to extract the vibrational spectrum of the "shadow" colloidal glass (i.e., the particle network with the same geometry and interactions as the sample colloid but absent damping). Rearrangements are induced by successive, small reductions in packing fraction. The experimental results suggest that low-frequency quasi-localized phonon modes in colloidal glasses, i.e., modes that present low energy barriers for system rearrangements, are spatially correlated with rearrangements in this thermal system.

cond-mat.soft↗

arXiv2011

μSR Study of Spin Dynamics in LiY$_{1-x}$Ho$_{x}$F$_{4}$

We present zero-field μSR measurements for LiY$_{1-x}$Ho$_{x}$F$_{4}$ samples with x = 0.0017, 0.0085, 0.0406, and 0.0855. We characterize the dynamics associated with the formation of the (F-μ-F)$^{-1}$ complex by comparing our data to Monte Carlo simulations to determine the concentration range over which the spin dynamics are determined primarily by the Ho$^{3+}$-μ interaction rather than the F-μ interaction. Simulations show that F-μ-F oscillations should evolve into a Lorentzian Kubo-Toyabe decay for an increasing static magnetic field distribution Γ (i.e., increasing x), but the data do not show this behavior, consistent with the recently reported existence of strong magnetic fluctuations in this system at low temperatures. Anisotropy in the field distribution is shown to cause small errors of order 10% from behavior predicted for an isotropic distribution. Finally, numerical calculations show that values of Γ calculated in the single ion limit greatly exceed the values extracted from curve fits, suggesting that strong correlations play an important role in this system.

cond-mat.str-el↗

arXiv2006

Packing grains by thermally cycling

One of the oldest and most intriguing problems in the handling of materials is how a collection of solid grains packs together. While granular packing is normally determined by how grains are poured or shaken, we find that a systematic and controllable increase in packing is induced by simply raising and lowering the temperature, e.g., without the input of mechanical energy. The results demonstrate that thermal processing provides a largely unexplored mechanism of grain dynamics, as well as an important practical consideration in the handling and storage of granular materials.

cond-mat.soft↗

arXiv2006

Flux through a hole from a shaken granular medium

We have measured the flux of grains from a hole in the bottom of a shaken container of grains. We find that the peak velocity of the vibration, vmax, controls the flux, i.e., the flux is nearly independent of the frequency and acceleration amplitude for a given value of vmax. The flux decreases with increasing peak velocity and then becomes almost constant for the largest values of vmax. The data at low peak velocity can be quantitatively described by a simple model, but the crossover to nearly constant flux at larger peak velocity suggests a regime in which the granular density near the container bottom is independent of the energy input to the system.

cond-mat.soft↗

arXiv2003

Generalized reduction criterion for separability of quantum states

A new necessary separability criterion that relates the structures of the total density matrix and its reductions is given. The method used is based on the realignment method [K. Chen and L.A. Wu, Quant. Inf. Comput. 3, 193 (2003)]. The new separability criterion naturally generalizes the reduction separability criterion introduced independently in previous work of [M. Horodecki and P. Horodecki, Phys. Rev. A 59, 4206 (1999)] and [N.J. Cerf, C. Adami and R.M. Gingrich, Phys. Rev. A 60, 898 (1999)]. In special cases, it recovers the previous reduction criterion and the recent generalized partial transposition criterion [K. Chen and L.A. Wu, Phys. Lett. A 306, 14 (2002)]. The criterion involves only simple matrix manipulations and can therefore be easily applied.

quant-ph↗