Controlling electronic spin relaxation of cold molecules with electric fields

We present a theoretical study of atom-molecule collisions in superimposed electric and magnetic fields and show that dynamics of electronic spin relaxation in molecules at temperatures below 0.5 K can be manipulated by varying the strength and the relative orientation of the applied fields. The mechanism of electric field control of Zeeman transitions is based on an intricate interplay between intramolecular spin-rotation couplings and molecule-field interactions. We suggest that electric fields may affect chemical reactions through inducing nonadiabatic spin transitions and facilitate evaporative cooling of molecules in a magnetic trap.     

Formation of high-density cold molecules via electromagnetic trap

Preparation and control of cold molecules are advancing rapidly, motivated by many exciting applications ranging from tests of fundamental physics to quantum information processing. Here, we propose a trapping scheme to create high-density cold molecular samples by using a combination of electric and magnetic fields. In our theoretical analysis and numerical calculations, a typical alkaline-earth monofluoride, MgF, is used to test the feasibility of our proposal. A cold MgF molecular beam is first produced via an electrostatic Stark decelerator and then loaded into the proposed electromagnetic trap, which is composed of an anti-Helmholtz coil, an octupole, and two disk electrodes. Following that, a huge magnetic force is applied to the molecular sample at an appropriate time, which enables further compressing of the spatial distribution of the cold sample. Molecular samples with both higher number density and smaller volume are quite suitable for the laser confinement and other molecular experiments such as cold collisions in the next step.     

强磁场与涡旋场中的夸克胶子物质

相对论重离子碰撞可以产生高温的夸克胶子物质,同时也产生极强的电磁场和流体涡旋。在强电磁场和涡旋场中的夸克胶子物质呈现出新奇的宏观量子现象,比如手征磁效应、手征涡效应、手征分离效应、手征电分离效应、自旋极化现象等。它们一方面给我们提供了可以探测高温下量子色动力学的非平庸规范场拓扑结构、强相互作用的宇称破坏、夸克胶子物质中的自旋动力学等的实验手段,另一方面也与物理学其他分支,比如粒子物理、凝聚态物理、天体物理、冷原子物理等发生紧密联系,形成新的交叉研究领域。本文旨在对这些宏观量子现象的产生机制以及它们在相对论重离子碰撞中的探测等做一回顾和展望。特别地,我们揭示出重离子碰撞的磁场强度可以达到$10^{18}\sim 10^{20}$ G,流体涡旋可以达到$10^{22}$ s–1;这是我们已知当前宇宙中最强的磁场和流体涡旋。我们定量地对同量素碰撞实验做了分析,发现即便背景比例达到93%以上,当前的同量素碰撞实验仍然可在大约$3\sigma$的显著性水平上判断是否有手征磁效应的发生。我们系统地给出了满足因果律的自旋流体力学方程,并推导了其中的集体激发模式,这将有助于理解超子自旋极化中出现的符号问题。     

Anisotropic hyperfine interactions limit the efficiency of spin-exchange optical pumping of ³He nuclei

We use accurate ab initio and quantum scattering calculations to demonstrate that the maximum 3He spin polarization that can be achieved in spin-exchange collisions with potassium (3?K) and silver (1??Ag) atoms is limited by the anisotropic hyperfine interaction. We find that spin exchange in Ag-He collisions occurs much faster than in K-He collisions over a wide range of temperatures (10-600 K). Our analysis indicates that measurements of trap loss rates of 2S atoms in the presence of cold 3He gas may be used to probe anisotropic spin-dependent interactions in atom-He collisions.     

Suppression of inelastic collisions between polar molecules with a repulsive shield

We propose and analyze a technique that allows one to suppress inelastic collisions and simultaneously enhance elastic interactions between cold polar molecules. The main idea is to cancel the leading dipole-dipole interaction with a suitable combination of static electric and microwave fields in such a way that the remaining van der Waals-type potential forms a three-dimensional repulsive shield. We analyze the elastic and inelastic scattering cross sections relevant for evaporative cooling of polar molecules and discuss the prospect for the creation of stable crystalline structures.     

Novel electrostatic trap for cold polar molecules

We propose a novel scheme in which cold polar molecules are trapped by an electrostatic field generated by the combination of a pair of parallel transparent electrodes (i.e., two infinite transparent plates) and a ring electrode (i.e., a ring wire). The spatial distributions of the electrostatic fields from the above charged wire and the charged plates and the corresponding Stark potentials for cold CO molecules are calculated; the dependences of the trap centre position on the geometric parameters of the electrode are analysed. We also discuss the loading process of cold molecules from a cold molecular beam into our trap. This study shows that the proposed scheme is not only simple and convenient to trap, manipulate and control cold polar molecules in weak-field-seeking states, but also provides an opportunity to study cold collisions and collective quantum effects in a variety of cold molecular systems, etc.     

A driven three-dimensional electric lattice for polar molecules

Three-dimensional (3D) driven optical lattices have attained great attention for their wide applications in the quest to engineer new and exotic quantum phases. Here we propose a 3D driven electric lattice (3D-DEL) for cold polar molecules as a natural extension. Our 3D electric lattice is composed of a series of thin metal plates in which two-dimensional square hole arrays are distributed. When suitable modulated voltages are applied to these metal plates, a 3D potential well array for polar molecules can be generated and can move smoothly back and forth in the lattice. Thus, it can drive cold polar molecules confined in the 3D electric lattice. Theoretical analyses and trajectory calculations using two types of molecules, ND₃ and PbF, are performed to justify the possibility of our scheme. The 3D-DEL offers a platform for investigating cold molecules in periodic driven potentials, such as quantum computing science, quantum information processing, and some other possible applications amenable to the driven optical lattices.     

Cold N+NH collisions in a magnetic trap

We present an experimental and theoretical study of atom-molecule collisions in a mixture of cold, trapped N atoms and NH molecules at a temperature of ～600 mK. We measure a small N+NH trap loss rate coefficient of k(loss)(N+NH)=9(5)(3)×10(-13) cm(3)?s(-1). Accurate quantum scattering calculations based on ab initio interaction potentials are in agreement with experiment and indicate the magnetic dipole interaction to be the dominant loss mechanism. Our theory further indicates the ratio of N+NH elastic-to-inelastic collisions remains large (>100) into the mK regime.     

一种可控的Ioffe型冷分子表面微电阱

囚禁于阱中的粒子(原子或分子)可获得更长的相互作用时间,因而在精密测量中可获得更高的分辨率.阱中的粒子与外界隔离,从而可以被冷却到更低的温度.因此原子(或分子)阱已广泛应用到许多研究领域.然而中心电场强度为零的势阱会导致粒子发生非绝热跃迁,这是原子或分子损失的主要来源.该损失曾是制备原子玻色-爱因斯坦凝聚的最后一道障碍.本文提出了一种可控的Ioffe型表面微电阱,其电场强度处处不为零,可有效避免分子的非绝热损失.另外,通过调节电压等参数,势阱中心电场强度以及势阱中心距芯片表面的高度可以在较大范围内调节,例如在本文参数下,势阱中心电场强度可在0.15—5.5 kV/cm变化,势阱中心高度可在6.0—17.0μm变化.本文通过有限元软件计算了芯片表面微电阱的电场分布,并用Monte Carlo模拟验证了该方案的可行性.该表面微电阱不仅可用于分子芯片的集成,而且可用于表面量子简并气体的制备.为精密测量、量子计算、表面冷碰撞和冷化学等领域提供了一个平台.     

Sympathetic cooling of polyatomic molecules with S-state atoms in a magnetic trap

We present a rigorous theoretical study of low-temperature collisions of polyatomic molecular radicals with (1)S(0) atoms in the presence of an external magnetic field. Accurate quantum scattering calculations based on ab initio and scaled interaction potentials show that collision-induced spin relaxation of the prototypical organic molecule CH(2)(X(3)B(1)) (methylene) and nine other triatomic radicals in cold (3)He gas occurs at a slow rate, demonstrating that cryogenic buffer-gas cooling and magnetic trapping of these molecules is feasible with current technology. Our calculations further suggest that it may be possible to create ultracold gases of polyatomic molecules by sympathetic cooling with alkaline-earth atoms in a magnetic trap.     

Electronic molecule-like spectrum of quantum wells in crossed fields

We show that an electron confined to a single finite parabolic quantum well in crossed electric and magnetic fields can behave as a double quantum well system. The magnetic field is parallel to the heterostructure layers and the electric field is perpendicular to those. For a suitable choice of both fields and quantum well width, the electron can be confined to a double quantum well effective potential that is very similar to the electronic potential model for diatomic molecules. The double quantum well spectrum is calculated using a numerical algorithm based on semiclassical methods. A physical interpretation of this quantum system is given based on the analogy to the electrons bound to diatomic molecules.     

Linking ultracold polar molecules

We predict that pairs of polar molecules can be weakly bound together in an ultracold environment, provided that a dc electric field is present. The field that links the molecules together also strongly influences the basic properties of the resulting dimer, such as its binding energy and predissociation lifetime. Because of their long-range character, these dimers will be useful in disentangling cold collision dynamics of polar molecules. As an example, we estimate the microwave photoassociation yield for OH-OH cold collisions.     

Effect of Giant Resonances on Fluctuations of Electromagnetic Fields in Heavy Ion Collisions

JETP Letters - We perform quantum calculations of fluctuations of the electromagnetic fields in AA collisions at RHIC and LHC energies. Calculations are performed with the help of the...     

On the role of the magnetic dipolar interaction in cold and ultracold collisions: numerical and analytical results for NH(3Σ−) + NH(3Σ−)

We present a detailed analysis of the role of the magnetic dipole-dipole interaction in cold and ultracold collisions. We focus on collisions between magnetically trapped NH molecules, but the theory is general for any two paramagnetic species for which the electronic spin and its space-fixed projection are (approximately) good quantum numbers. It is shown that dipolar spin relaxation is directly associated with magnetic-dipole induced avoided crossings that occur between different adiabatic potential curves. For a given collision energy and magnetic field strength, the cross-section contributions from different scattering channels depend strongly on whether or not the corresponding avoided crossings are energetically accessible. We find that the crossings become lower in energy as the magnetic field decreases, so that higher partial-wave scattering becomes increasingly important below a certain magnetic field strength. In addition, we derive analytical cross-section expressions for dipolar spin relaxation based on the Born approximation and distorted-wave Born approximation. The validity regions of these analytical expressions are determined by comparison with the NH + NH cross sections obtained from full coupled-channel calculations. We find that the Born approximation is accurate over a wide range of energies and field strengths, but breaks down at high energies and high magnetic fields. The analytical distorted-wave Born approximation gives more accurate results in the case of s-wave scattering, but shows some significant discrepancies for the higher partial-wave channels. We thus conclude that the Born approximation gives generally more meaningful results than the distorted-wave Born approximation at the collision energies and fields considered in this work.     

Mg-CO(X1Σ+)体系的冷碰撞动力学

应用量子散射动力学方法, 研究了电场条件下Mg-CO体系的冷碰撞动力学性质, 探索了外电场对碰撞体系低场追索态的弹性和非弹性截面的影响, 为碱土金属Mg原子感应冷却CO分子提供理论预测.     

Resonances in electron scattering by molecules

The main features of resonance scattering of electrons by molecules are described and resonances are determined on the basis of the theory of collisions in a two-body system, as well as resonances emerging as a result of collisions in a few-body system. Regularities in the emergence of such resonances and their characteristics are analyzed. The results of calculations of these resonant processes occurring during collisions of electrons with diatomic molecules, made on the basis of the quantum theory of scattering in a few-body system, are presented. The results of calculating the cross sections of resonant processes of electron collisions with molecules are compared with the available experimental data and with the results of calculations based on other approximations.     

Electrostatic surface guiding of cold polar molecules with double charged wires

We propose a novel scheme to guide cold polar molecules on the surface of an insulating substrate (i.e., a chip) using a static electric field generated by the combination of a pair of parallel charged wires and a grounded metal plate. We calculate the spatial distributions of the electric fields from the above chargedwire layout and their Stark potentials for cold CO molecules, and analyze the relationships between the electric field and the parameters of the charged-wire layout. The result shows that this charged-wire scheme can be used to guide cold polar molecules in the weak-field-seeking state and to form various molecule-optical elements, even to realize a single-mode molecular waveguide on a molecule chip under certain conditions.     

Quantum well excitons in an electric field: two versus three dimensional behavior

Previous calculations have shown a transition between two dimensional and three dimensional behavior of excitons confined in a semiconducting quantum well structure as a function of electric field. We here present calculations of the exciton binding energy as a function of electric field using a two parameter variational wave function of the form used in the absence of the electric field by Matsuura and Shinozuka. Our calculations were performed using a finite potential barrier model for the confinement of the exciton in the quantum well. The results of our calculations confirm the validity of the conclusion that the variational exciton wave function goes from being of a purely 2D hydrogenic type at small well widths and/or low electric fields to a 3D hydrogenic type in wide wells and/or high electric fields.     

Initial state effects on J/ψ production at RHIC energy: from d+Au to Au+Au

We present a calculation of the cold nuclear matter effect on inclusive production of J/ψ in d+A and A+A collisions in the framework of the gluon saturation/CGC approach. Our model is based on the observation that the leading production mechanism involves odd number of inelastic interactions with the nuclei. Our numerical calculations are in good agreement with the experimental data in the case of d+Au collisions. However, in Au+Au collisions the cold nuclear matter effect is not suffcient to describe the data.     

Collisions of electrons with molecules in excited vibrational-rotational states

Results are presented of calculations of cross sections for scattering of electrons by diatomic molecules in specific excited vibrational-rotational states. The calculations were made using an approximation based on a quantum theory of scattering in a system of several bodies which can be applied to calculations of direct reactions and reactions involving the formation of an intermediate transition complex. Results of calculations of cross sections for collisions of electrons with hydrogen, nitrogen, lithium, sodium, and hydrogen halide molecules are compared with existing experimental data and the results of calculations made by other authors.