arXiv subjects

P. Zhang

Publications and source records attributed to P. Zhang.

At least 181 records · Page 10Linked to original sources

arXiv2009

3D visualization of two-phase flow in the micro-tube by a simple but effective method

The present study presents a simple but effective method for 3D visualization of the two-phase flow in the micro-tube. An isosceles right-angle prism combined with a mirror located 45^o bevel to the prism is employed to obtain synchronously the front and side views of the flow patterns with a single camera, where the locations of the prism and the micro-tube for clearly imaging should satisfy a fixed relationship which is specified in the present study. The optical design is proven successfully by the tough visualization work at the cryogenic temperature range. The image deformation due to the refraction and geometrical configuration of the test section is quantitatively investigated. It is calculated that the image is enlarged by about 20% in inner diameter compared to the real object, which is validated by the experimental results. Meanwhile, the image deformation by adding a rectangular optical correction box outside the circular tube is comparatively investigated. It is calculated that the image is reduced by about 20% in inner diameter with a rectangular optical correction box compared to the real object. The 3D re-construction process based on the two views is conducted through three steps, which shows that 3D visualization method can be easily applied for two-phase flow research in micro-scale channels and improves the measurement accuracy of some important parameters of two-phase flow such as void fraction, spatial distribution of bubbles, etc..

physics.flu-dyn↗

arXiv2009

Spin diffusion in Si/SiGe quantum wells: spin relaxation in the absence of D'yakonov-Perel' relaxation mechanism

In this work, the spin relaxation accompanying the spin diffusion in symmetric Si/SiGe quantum wells without the D'yakonov-Perel' spin-relaxation mechanism is calculated from a fully microscopic approach. The spin relaxation is caused by the inhomogeneous broadening from the momentum-dependent spin precessions in spatial domain under a magnetic field in the Voigt configuration. In fact, this inhomogeneous broadening together with the scattering lead to an irreversible spin relaxation along the spin diffusion. The effects of scattering, magnetic field and electron density on spin diffusion are investigated. Unlike the case of spin diffusion in the system with the D'yakonov-Perel' spin-orbit coupling such as GaAs quantum wells where the scattering can either enhance or reduce spin diffusion depending on whether the system is in strong or weak scattering limit, the scattering in the present system has no counter-effect on the inhomogeneous broadening and suppresses the spin diffusion monotonically. The increase of magnetic field reduces the spin diffusion, while the increase of electron density enhances the spin diffusion when the electrons are degenerate but has marginal effect when the electrons are nondegenerate.

cond-mat.mtrl-sci↗

arXiv2009

Continuous vortex pumping into a spinor condensate with magnetic fields

We study the mechanisms and the limits of pumping vorticity into a spinor condensate through manipulations of magnetic (B-) fields. We discover a fundamental connection between the geometrical properties of the magnetic fields and the quantized circulation of magnetically trapped atoms, a result which generalizes several recent experimental and theoretical studies. The optimal procedures are devised that are capable of continuously increasing or decreasing a condensate's vorticity by repeating certain two step B-field manipulation protocols. We carry out detailed numerical simulations that support the claim that our protocols are highly efficient, stable, and robust against small imperfections of all types. Our protocols can be implemented experimentally within current technologies.

cond-mat.other↗

arXiv2008

Density dependence of spin relaxation in GaAs quantum well at room temperature

Carrier density dependence of electron spin relaxation in an intrinsic GaAs quantum well is investigated at room temperature using time-resolved circularly polarized pump-probe spectroscopy. It is revealed that the spin relaxation time first increases with density in the relatively low density regime where the linear D'yakonov-Perel' spin-orbit coupling terms are dominant, and then tends to decrease when the density is large and the cubic D'yakonov-Perel' spin-orbit coupling terms become important. These features are in good agreement with theoritical predictions by Lü {\em et al.} [Phys. Rev. B {\bf 73}, 125314 (2006)]. A fully microscopic calculation based on numerically solving the kinetic spin Bloch equations with both the D'yakonov-Perel' and the Bir-Aronov-Pikus mechanisms included, reproduces the density dependence of spin relaxation very well.

cond-mat.mtrl-sci↗

arXiv2008

Multi-valley spin relaxation in the presence of high in-plane electric fields in $n$-type GaAs quantum wells

Multi-valley spin relaxation in $n$-type GaAs quantum wells with in-plane electric field is investigated at high temperature by means of kinetic spin Bloch equation approach. The spin relaxation time first increases and then decreases with electric field, especially when the temperature is relatively low. We show that $L$ valleys play the role of a ``drain'' of the total spin polarization due to the large spin-orbit coupling in $L$ valleys and the strong $Γ$-$L$ inter-valley scattering, and thus can enhance spin relaxation of the total system effectively when the in-plane electric field is high. Under electric field, spin precession resulting from the electric-field-induced magnetic field is observed. Meanwhile, due to the strong $Γ$-$L$ inter-valley scattering as well as the strong inhomogeneous broadening in $L$ valleys, electron spins in $L$ valleys possess almost the same damping rate and precession frequency as those in $Γ$ valley. This feature still holds when a finite static magnetic field is applied in Voigt configuration, despite that the $g$-factor of $L$ valleys is much larger than that of $Γ$ valley. Moreover, it is shown that the property of spin precession of the whole system is dominated by electrons in $Γ$ valley. Temperature, magnetic field, and impurity can affect spin relaxation in low electric field regime. However, they are shown to have marginal influence in high electric field regime.

cond-mat.mtrl-sci↗

arXiv2007

Non-Markovian hole spin kinetics in $p$-type GaAs quantum wells

Based on fully microscopic kinetic spin Bloch equation approach, we show the non-Markovian effect in spin dephasing of heavy holes in $p$-type GaAs quantum wells. The non-Markovian effect manifests itself in spin dephasing when the mean spin precession time is shorter than the momentum relaxation time. The spin dephasing becomes slower when the non-Markovian effect is important. Moreover, quantum spin beats due to the memory effect of the non-Markovian hole-longitudinal optical phonon scattering are predicted.

cond-mat.mtrl-sci↗

arXiv2007

Low energy H+CO scattering revisited: CO rotational excitation with new potential surfaces

A recent modeling study of brightness ratios for CO rotational transitions in gas typical of the diffuse ISM by Liszt found the role of H collisions to be more important than previously assumed. This conclusion was based on quantum scattering calculations using the so-called WKS potential energy surface (PES) which reported a large cross section for the important 0->1 rotational transition. New close-coupling (CC) rigid-rotor calculations for CO(v=0,J=0) excitation by H are performed on four different PESs. Two of the PESs are obtained in this work using state-of-the-art quantum chemistry techniques at the CCSD(T) and MRCI levels of theory. Cross sections for the J=0->1, as well as other odd Delta J, transitions are significantly suppressed compared to even Delta J transitions in thermal energy CC calculations using the CCSD(T) and MRCI surfaces. This is consistent with a expected even Delta J propensity and in contrast to CC calculations using the earlier WKS PES which predict a dominating 0->1 transition. The current results suggest that the original astrophysical assumption that excitation of CO by H_2 dominates the kinetics of CO in diffuse ISM gas is likely to remain valid.

astro-ph↗

arXiv2007

Geometric phase of an atom inside an adiabatic radio frequency potential

We investigate the geometric phase of an atom inside an adiabatic radio frequency (rf) potential created from a static magnetic field (B-field) and a time dependent rf field. The spatial motion of the atomic center of mass is shown to give rise to a geometric phase, or Berry's phase, to the adiabatically evolving atomic hyperfine spin along the local B-field. This phase is found to depend on both the static B-field along the semi-classical trajectory of the atomic center of mass and an ``effective magnetic field'' of the total B-field, including the oscillating rf field. Specific calculations are provided for several recent atom interferometry experiments and proposals utilizing adiabatic rf potentials.

quant-ph↗

arXiv2007

Geometric phase of an atom in a magnetic storage ring

A magnetically trapped atom experiences an adiabatic geometric (Berry's) phase due to changing field direction. We investigate theoretically such an Aharonov-Bohm-like geometric phase for atoms adiabatically moving inside a storage ring as demonstrated in several recent experiments. Our result shows that this phase shift is easily observable in a closed loop interference experiment, and thus the shift has to be accounted for in the proposed inertial sensing applications. The spread in phase shift due to the atom transverse distribution is quantified through numerical simulations.

quant-ph↗

arXiv2007

The angular momentum of a magnetically trapped atomic condensate

For an atomic condensate in an axially symmetric magnetic trap, the sum of the axial components of the orbital angular momentum and the hyperfine spin is conserved. Inside an Ioffe-Pritchard trap (IPT) whose magnetic field (B-field) is not axially symmetric, the difference of the two becomes surprisingly conserved. In this paper we investigate the relationship between the values of the sum/difference angular momentums for an atomic condensate inside a magnetic trap and the associated gauge potential induced by the adiabatic approximation. Our result provides significant new insight into the vorticity of magnetically trapped atomic quantum gases.

cond-mat.other↗

arXiv2006

Controlled splitting of an atomic wave packet

We propose a simple scheme capable of adiabatically splitting an atomic wave packet using two independent translating traps. Implemented with optical dipole traps, our scheme allows a high degree of flexibility for atom interferometry arrangements and highlights its potential as an efficient and high fidelity atom optical beam splitter.

quant-ph↗

arXiv2006

Three dimensionality of pulsed second-sound waves in He II

Three dimensionality of 3D pulsed second sound wave in He II emitted from a finite size heater is experimentally investigated and theoretically studied based on two-fluid model in this study. The detailed propagation of 3D pulsed second sound wave is presented and reasonable agreement between the experimental and theoretical results is obtained. Heater size has a big influence on the profile of 3D second sound wave. The counterflow between the superfluid and normal fluid components becomes inverse in the rarefaction of 3D second sound wave. The amplitude of rarefaction decreases due to the interaction between second sound wave and quantized vortices, which explains the experimental results about second sound wave near [Phys. Rev. Lett. 73, 2480 (1994)]. The accumulation of dense quantized vortices in the vicinity of heater surface leads to the formation of a thermal boundary layer, and further increase of heating duration results in the occurrence of boiling phenomena. PACS numbers: 67.40.Pm 43.25.+y 67.40.Bz

cond-mat.supr-con↗

arXiv2006

Quantum-Classical Transition of Photon-Carnot Engine Induced by Quantum Decoherence

We study the physical implementation of the Photon Carnot engine (PCE) based on the cavity QED system [M. Scully et al, Science, \textbf{299}, 862 (2003)]. Here, we analyze two decoherence mechanisms for the more practical systems of PCE, the dissipation of photon field and the pure dephasing of the input atoms. As a result we find that (I) the PCE can work well to some extent even in the existence of the cavity loss (photon dissipation); and (II) the short-time atomic dephasing, which can destroy the PCE, is a fatal problem to be overcome.

quant-ph↗

arXiv2006

Nonadiabatic production of spinor condensates with a QUIC trap

Motivated by the recent experimental observation of multi-component spinor condensates via a time-dependent quadrupole-Ioffe-configuration trap (QUIC trap), we provide a general framework for the investigation of nonadiabatic Landau-Zener dynamics of a hyperfine spin, e.g., from an atomic magnetic dipole moment coupled to a weak time-dependent magnetic (B-) field. The spin flipped population distribution, or the so-called Majorona formula is expressed in terms of system parameters and experimental observables; thus, provides much needed insight into the underlying mechanism for the production of spinor condensates due to nonadiabatic level crossings.

cond-mat.stat-mech↗

arXiv2006

Demonstration of Temporal Distinguishability in a Four-Photon State and a Six-Photon State

An experiment is performed to demonstrate the temporal distinguishability of a four-photon state and a six-photon state, both from parametric down-conversion. The experiment is based on a multi-photon interference scheme in a recent discovered NOON-state projection measurement. By measuring the visibility of the interference dip, we can distinguish the various scenarios in the temporal distribution of the pairs and thus quantitatively determine the degree of temporal (in)distinguishability of a multi-photon state.

quant-ph↗

arXiv2005

Cooling of Nanomechanical Resonator Based on Periodical Coupling to Cooper Pair Box

We propose and study an active cooling mechanism for the nanomechanical resonator (NAMR) based on periodical coupling to a Cooper pair box (CPB), which is implemented by a designed series of magnetic flux pluses threading through the CPB. When the initial phonon number of the NAMR is not too large, this cooling protocol is efficient in decreasing the phonon number by two to three orders of magnitude. Our proposal is theoretically universal in cooling various boson systems of single mode. It can be specifically generalized to prepare the nonclassical state of the NAMR.

quant-ph↗

arXiv2005

Quantum Heat Engine With Multi-Level Quantum Systems

By reformulating the first law of thermodynamics in the fashion of quantum-mechanical operators on the parameter manifold, we propose a universal class of quantum heat engines (QHE) using the multi-level quantum system as the working substance. We obtain a general expression of work for the thermodynamic cycle with two thermodynamic adiabatic processes, which are microscopically quantum adiabatic processes. We also classify the conditions for a 3-level QHE to extract positive work from a heat bath. Our result is counter-intuitively different from that of a 2-level system. As a more realistic illustration, a 3-level atom system with dark state configuration manipulated by classical light is used to demonstrate our central idea.

quant-ph↗

arXiv2005

Second Order Geodesic Corrections to Cosmic Shear

We consider the impact of second order corrections to the geodesic equation governing gravitational lensing. We start from the full second order metric, including scalar, vector and tensor perturbations, and retain all relevant contributions to the cosmic shear corrections that are second order in the gravitational potential. The relevant terms are: the nonlinear evolution of the scalar gravitational potential, the Born correction, and lens-lens coupling. No other second order terms contribute appreciably to the lensing signal. Since ray-tracing algorithims currently include these three effects, this derivation serves as rigorous justification for the numerical predictions.

astro-ph↗