arXiv subjects

J. Zhao

Publications and source records attributed to J. Zhao.

154 records · Page 9Linked to original sources

arXiv2003

Magnon squeezing in an antiferromagnet: reducing the spin noise below the standard quantum limit

At absolute zero temperature, thermal noise vanishes when a physical system is in its ground state, but quantum noise remains as a fundamental limit to the accuracy of experimental measurements. Such a limitation, however, can be mitigated by the formation of squeezed states. Quantum mechanically, a squeezed state is a time-varying superposition of states for which the noise of a particular observable is reduced below that of the ground state at certain times. Quantum squeezing has been achieved for a variety of systems, including the electromagnetic field, atomic vibrations in solids and molecules, and atomic spins, but not so far for magnetic systems. Here we report on an experimental demonstration of spin wave (i.e., magnon) squeezing. Our method uses femtosecond optical pulses to generate correlations involving pairs of magnons in an antiferromagnetic insulator, MnF2. These correlations lead to quantum squeezing in which the fluctuations of the magnetization of a crystallographic unit cell vary periodically in time and are reduced below that of the ground state quantum noise. The mechanism responsible for this squeezing is stimulated second order Raman scattering by magnon pairs. Such squeezed states have important ramifications in the emerging fields of spintronics and quantum computing involving magnetic spin states or the spin-orbit coupling mechanism.

cond-mat↗

arXiv2003

Ultrafast optical generation of coherent phonons in CdTe1-xSex quantum dots

We report on the impulsive generation of coherent optical phonons in CdTe0.68Se0.32 nanocrystallites embedded in a glass matrix. Pump probe experiments using femtosecond laser pulses were performed by tuning the laser central energy to resonate with the absorption edge of the nanocrystals. We identify two longitudinal optical phonons, one longitudinal acoustic phonon and a fourth mode of a mixed longitudinal-transverse nature. The amplitude of the optical phonons as a function of the laser central energy exhibits a resonance that is well described by a model based on impulsive stimulated Raman scattering. The phases of the coherent phonons reveal coupling between different modes. At low power density excitations, the frequency of the optical coherent phonons deviates from values obtained from spontaneous Raman scattering. This behavior is ascribed to the presence of electronic impurity states which modify the nanocrystal dielectric function and, thereby, the frequency of the infrared-active phonons.

cond-mat.mes-hall↗

arXiv2003

A Diagram for Bianchi A-Types

A diagram for Bianchi spaces with vanishing vector of structure constants (type A in the Ellis-MacCallum classification) illustrates the relations among their different types under similarity transformations. The Ricci coefficients and the Ricci tensor are related by a Cremona transformation.

gr-qc↗

arXiv2002

Measurement of the R$_{LT}$ response function for $π^0$ electroproduction at $Q^2$ = 0.070 (GeV/c)$^2$ in the $N\toΔ$ transition}

Quadrupole amplitudes in the $γ^{*}N\toΔ$ transition are associated with the issue of nucleon deformation. A search for these small amplitudes has been the focus of a series of measurements undertaken at Bates/MIT by the OOPS collaboration. We report on results from H$(e,e^\prime p)π^0$ data obtained at $Q^2= 0.070$ (GeV/c)$^2$ and invariant mass of W=1155 MeV using the out-of-plane detection technique with the OOPS spectrometers. The $σ_{LT}$ and $σ_{T}+ε\cdot$ $σ_{L}$ response functions were isolated. These results, along with those of previous measurements at $W$=1172 MeV and $Q^2= 0.127$ (GeV/c)$^2$, aim in elucidating the interplay between resonant and non resonant amplitudes.

nucl-ex↗

arXiv2001

A Prototype of Upgrading Bepc Control System

We will use EPICS toolkit [1] to build a prototype for upgrading BEPC control system. The purposes are for the following three aspects: (1) Setup a network based distributed control system with EPICS. (2) Study some front-end control technology. (3) Merge the existing control systems into EPICS. The hardware architecture consists of three physical layers. The front-end layer is VME crate, PowerPC, VME I/O modules and interface boards (VME-CAMAC and Fieldbus). The back-end layer is SUN workstation, which is used as application development. The two layers are connected via the middle layer which is 100M Ethernet. In order to preserve our investment in the hardware, the existing CAMAC hardware will remain. The CAMAC will be connected to the VME via CAMAC serial highway [2]. The operator interface will be developed with EPICS DM2K. High-level accelerator control applications will be studied as well with EPICS channel access and archive access.

physics.acc-ph↗

arXiv2001

Control System of the Bepcii

Recently the Chinese Academy of Sciences has chosen BEPCII as the future development of the BEPC, i.e. upgrade of both the machine and detector. The luminosity of the machine is expected to increase to 1.0x1033cm-2s-1. The project will be started at the beginning of 2002 and finished with in 3-4 years. The BEPC control system was built in 1987 and upgraded in 1994. According to the design of the BEPCII, a double ring schema will be adopted and a number of new devices will be added in the system. The existing control system has to be upgraded. The BEPCII will be distributed architecture and developed by EPICS. We are going to apply the standard hardware interfaces and mature technologies in the system. A number of VME IOCs will be added in the system and the feildbus, PLCs will be used as device control for some kind of equipment. We will keep the existing system in use, such as CAMAC modules and PC front-ends, and merge it into EPICS system. Recently the development of the prototype is in progress. This paper will describe the system design and development issues.

hep-ph↗

arXiv2001

x- and xi-scaling of the Nuclear Structure Function at Large x

Inclusive electron scattering data are presented for ^2H and Fe targets at an incident electron energy of 4.045 GeV for a range of momentum transfers from Q^2 = 1 to 7 (GeV/c)^2. Data were taken at Jefferson Laboratory for low values of energy loss, corresponding to values of Bjorken x greater than or near 1. The structure functions do not show scaling in x in this range, where inelastic scattering is not expected to dominate the cross section. The data do show scaling, however, in the Nachtmann variable ξ. This scaling may be the result of Bloom Gilman duality in the nucleon structure function combined with the Fermi motion of the nucleons in the nucleus. The resulting extension of scaling to larger values of ξopens up the possibility of accessing nuclear structure functions in the high-x region at lower values of Q^2 than previously believed.

nucl-ex↗

arXiv2000

Grassmannian trilogarithms

Some time ago the first author suggested two different constructions of Grassmannian n-logarithms. One of them works for all n, but its motivic nature is hidden. The other is a motivic but so far available only for n=2,3,4. We derived some general results about the first construction of Grassmannian polylogarithms, and found an explicit relation between the two constructions of the Grassmannian trilogarithms.

math.AG↗

arXiv1999

A high-precision polarimeter

We have built a polarimeter in order to measure the electron beam polarization in hall C at JLAB. Using a superconducting solenoid to drive the pure-iron target foil into saturation, and a symmetrical setup to detect the Moller electrons in coincidence, we achieve an accuracy of <1%. This sets a new standard for Moller polarimeters.

nucl-ex↗

arXiv1999

High-precision Studies of the $^{\bf{3}}$He(e,e$^{\bf{\prime}}$p) Reaction at the Quasielastic Peak

Precision studies of the reaction $^{3}$He(e,e$^\prime$p) using the three-spectrometer facility at the Mainz microtron MAMI are presented. All data are for quasielastic kinematics at $|\vec{q} | =685$ MeV/c. Absolute cross sections were measured at three electron kinematics. For the measured missing momenta range from 10 to 165 MeV/c, no strength is observed for missing energies higher than 20 MeV. Distorted momentum distributions were extracted for the two-body breakup and the continuum. The longitudinal and transverse behavior was studied by measuring the cross section for three photon polarizations. The longitudinal and transverse nature of the cross sections is well described by a currently accepted and widely used prescription of the off-shell electron-nucleon cross-section. The results are compared to modern three-body calculations and to previous data.

nucl-ex↗