Controlling collisions of ultracold atoms with dc electric fields

It is demonstrated that elastic collisions of ultracold atoms forming a heteronuclear collision complex can be manipulated by laboratory practicable dc electric fields. The mechanism of electric field control is based on the interaction of the instantaneous dipole moment of the collision pair with external electric fields. It is shown that this interaction is dramatically enhanced in the presence of a p-wave shape or Feshbach scattering resonance near the collision threshold, which leads to novel electric-field-induced Feshbach resonances.     

Transverse spin relaxation of spin carriers diffusing in spatially periodic magnetic fields

An exact solution of the Bloch–Torrey equation that covers the entire range of relative diffusivities D between the spin carriers and the magnetic structure (due to, e.g., spin‐density waves) is given for the transverse relaxation of an initally uniformly polarized spin system under the influence of a magnetic field varying sinusoidally in space. Explicit closed‐form results for the short‐time relaxation are obtained making use of Laplace transforms, the three‐term recurrence relations associated with Mathieu’s equation, and novel sum rules. At intermediate diffusivities the transverse polarization exhibits a novel long‐time behaviour as a function of D. This revised version was published online in August 2006 with corrections to the Cover Date.     

Temperature dependence of the electronic spin relaxation of DCNQI salts

For the elucidation of the charge and spin dynamics of the radical anion salts of DCNQI with metallic counterions we have performed cw- and pulsed ESR experiments (βB _pp,T _1e ^?1 andT _2e ^?1 ) between 300 and 4 K at nine salts differing in counterions and sidegroups, respectively. We can explain the relaxation rates by dipolar electron-electron interaction and spin-orbit contribution. In the high temperature range we have a gradual decrease in the number of charge carriers by interband transitions without a slowing down of the mobility. With complete localization of the electron spins (no mobile electrons anymore) exchange interaction governs the spectral density, becoming strongly temperature dependent due to effective spin exchangeJ _eff(T), explained by an extended REHAC-model. This effective spin exchangeJ _eff(T) includes for the first time a contribution by the metallic counterions. For spin-orbit interaction we developed a model based on F. Adrian [1] not depending on the mixture of Bloch and spin states as given by Elliott [2]. This is achieved by the inclusion of the electronic probability on atoms with higher atomic numbers, modulated by phonons. This model explains the drastic changes in the ESR linewidth of different radical ion salts of DCNQI and allows inductively the prediction of the electronic properties of new radical ion systems of which just the molecular and crystal structure is known.     

Out-of-plane spin polarization from in-plane electric and magnetic fields

We show that the joint effect of spin-orbit and magnetic fields leads to a spin polarization perpendicular to the plane of a homogeneous two-dimensional electron system with Rashba spin-orbit coupling and in-plane parallel dc magnetic and electric fields, for angle-dependent impurity scattering or nonparabolic energy spectrum, while only in-plane polarization persists for simplified models. We derive Bloch equations, describing the main features of recent experiments, including the magnetic field dependence of static and dynamic responses.     

Muon spin relaxation by electronic excitations moving in one dimension

The manner in which an electronic spin, executing a linear random walk, e.g. along a polymer chain, depolarizes a muon (or proton) probe spin, is investigated by computer simulation. The essential features of the model are the assumptions of a contact hyperfine interaction with limited range and of loss of coherence between successive encounters. The low dimensionality of the motion is reflected in the shape of the relaxation functions generated, which depart significantly from simple exponentials. Fits to various functional forms are examined for different combinations of hop rate and chain length, hyperfine constant and applied magnetic field. This revised version was published online in August 2006 with corrections to the Cover Date.     

Possibility for separation of polarizable molecules using electric fields

The motion of polarizable molecules in the presence of nonuniform electric fields is analyzed. The separation of molecular flows in the presence of such fields is theoretically demonstrated. Angular dispersions are estimated for endofullerene (Sc@C₈₂, Y@C₈₂, La@C₈₂, and Gd@C₈₂) and fullerene (C₆₀, C₇₀, C₈₂, and C₈₄) molecules in the presence of the field that is generated by a system of parallel charged wires using the known dipole moments and polarizabilities. The numerical simulation shows that a separator of polarizable molecules can be constructed using such fields.     

Two-dimensional trapping of dipolar molecules in time-varying electric fields

Simultaneous two-dimensional trapping of neutral dipolar molecules in low- and high-field seeking states is analyzed. A trapping potential of the order of 20 mK can be produced for molecules such as ND3 with time-dependent electric fields. The analysis is in agreement with an experiment where slow molecules with longitudinal velocities of the order of 20 m/s are guided between four 50 cm long rods driven by an alternating electric potential at a frequency of a few kHz.     

Spectrum of relaxation times for ising spin clusters in random fields

Summary Exact results on the single-spin-flip Glauber dynamics of six-coupled random field Ising spins with the coordination number of four are presented. Two distributions of random fields (RF), binary (BD) and Gaussian (GD) ones, are investigated. The effects of the static magnetic field are discussed. In the zero-magnetic-field case, the number of diverging relaxation times is equal to the number of energy minima minus one. This rule breaks in the presence of a magnetic field. The longest relaxation times in the absence of the field verify the Arrhenius law with the energy barrier determined by the energy needed to invert the ground-state spin configuration. At low temperature, according to the Arrhenius law, the spectrum of relaxation times shows a two-peaked distribution on a logarithmic scale. In the GD case of RF, the energy barrier distribution is continuous, while it is quasi-discrete in the BD case.     

Orientation of adsorbed molecules under the influence of electric fields

The dynamic orientation of vertically adsorbed polar molecules is investigated by switching on electric fields. The rotational properties of molecules are modulated by the electric field and the conical well. The orientation reveals multiple pronounced oscillations, depending on the specific field parameters. The range of oscillation is enhanced even in the presence of conical-well confinement. Furthermore, population concentration and inversion cyclically occur in the system, corresponding to distinctive molecular orientations. Specific field-free orientation is also obtained by turning off the field. The population can be concentrated or reversed for a long time. The thermal effect on the properties of orientation is analysed in detail.     

Nuclear spin lattice relaxation and electric field gradient in liquid indium

We have measured the nuclear spin lattice relaxation time in liquid indium from 130°C to 300°C to be: 1/T ₁=(1.98 × 0.0082T) × 10³ sec^-1. The relaxation rate consists of two significant parts: (1/T ₁) _K from the nuclear magnetic hyperfine interaction, and (1/T ₁) _Q from the nuclear quadrupole interaction. We calculate (1/T ₁) _K from the the modified Korringa relation using a correction factor of order unity for electron-electron interactions. The hyperfine term is linear in T and accounts for the second term in 1/T ₁. Within experimental error the remaining rate, (1/T ₁) _Q , is temperature independent, and theoretically varies as the product of the square of the electric field gradient, q, and τ_c, a typical time between field gradient fluctuations. Making use of the x-ray RDF, we construct a simple model for liquid indium and calculate the ionic and electronic contributions, q _I and q _E, to the electric field gradient, to be q _I=1.4 × 10²⁴/cm³ and q _E=8.5 × 10²⁴/cm³. The calculation of q _E assumes covalent bonding between nearest neighbours. Taking q _I and q _E to be of opposite sign, we find that the correlation time τ_c is 1.6 × 10^-13 sec. When we further identify τ _c with the correlation time for diffusion in a three-dimensional random walk, we are able to calculate the r.m.s. jump distance, Δr _D, involved in self-diffusion, Δr _D=0.38 Å. This value is consistent with the x-ray peak width of 0.38 Å which we used earlier to calculate the electric field gradient.     

Rotational energies of plar symmetric-top molecules in high electric fields

A method based on hypervirial theorems and perturbation theory is developed for calculating rotational energies of polar symmetric-top molecules in uniform electric fields. This procedure also provides expectation values of other physical observables such as the effective dipole moment.     

电场作用下CaS的分子结构和电子光谱

以6-311++G(d,p)为基组,采用密度泛函的B3LYP方法优化得到不同外电场(-0·03—0·045a.u.)下CaS分子的基态结构参数、电偶极矩μ、电荷分布、HOMO能级、LUMO能级、能隙、红外光谱和谐振频率等.结果表明,随着正向电场的增加,分子结构与外电场有着强烈的依赖关系,且对电场方向的依赖呈现出不对称性,基态键长和分子偶极矩μ先减小后增大,在F=0·02a.u.时,键长Re取最小值0·2289nm,电偶极矩取最小值1·5969D,HOMO能级和LUMO能级处于先增大后减小,能隙始终是减小的,占据轨道的电子容易激发到空轨道.外电场对CaS分子的激发能和振子强度有较大的影响,这为进一步研究它的电致发光机理提供了一定的理论基础.     

Muon spin relaxation in a spin glass CuMn observed in finite longitudinal magnetic fields

Muon spin depolarization in a spin glass CuMn (1.1 at.%, T_g = 10.8 K) has been observed in longitudinal external magnetic fields H_L = 0–640 Oe. The depolarization rate did not depend on H_L at T > T_g, reflecting a fast dynamical fluctuation of Mn moments. In contrast, a remarkable field dependence was observed below T_g. Static and dynamical local fields on μ⁺ from Mn moments were found to coexist at T ～ 0.9 T_g. The results suggested a rapid change of spin dynamics around T_g.     

Dipolar interactions in molecules aligned by strong AC electric fields

We observed magnetization transfer and spectroscopic splittings due to dipolar couplings in the solution NMR spectra of neat nitrobenzene aligned using AC electric fields. Weak dipolar splittings have been previously observed for nitrobenzene in a DC electric field (T. M. Plantenga, et al., Chem. Phys. 66, 1-9, 1982); the use of homogeneous pulsed AC fields has allowed us to establish stable experimental conditions, which were more tolerable to sample impurities and required no sample purification, and to carry out multidimensional experiments. A pulse sequence is discussed in which the electric field is present only for the indirect dimension: this sequence records the dipolar splittings for each proton in the indirect dimension; the direct dimension presents the isotropic chemical shift. Another pulse sequence is discussed that uses the applied electric field only in the mixing period to produce cross peaks between dipolar coupled pairs and correlate their isotropic chemical shifts. The order parameter describing molecular alignment was in good agreement with that previously determined from deuterium quadrupolar measurements of deuterated nitrobenzene in a similar range of electric fields: S(mol) approximately 0.025% for a field strength of 7.0 MV/m (rms). The dipolar splittings for ortho-meta, meta-para, and ortho-para protons were in qualitative agreement with the known geometry. Copyright 2000 Academic Press.     

Gating the charge state of single molecules by local electric fields

The electron-acceptor molecule TCNQ is found in either of two distinct integer charge states when embedded into a monolayer of a charge transfer complex on a gold surface. Scanning tunneling spectroscopy measurements identify these states through the presence or absence of a zero-bias Kondo resonance. Increasing the (tip-induced) electric field allows us to reversibly induce the oxidation or reduction of TCNQ species from their anionic or neutral ground state, respectively. We show that the different ground states arise from slight variations in the underlying surface potential, pictured here as the gate of a three-terminal device.     

外电场作用下多嵌段共聚物的电子光吸收性质

在紧束缚模型哈密顿的基础上,研究了由聚乙炔(polyacetylene,简写为PA)和聚对苯撑(poly(p-phenylene),简写为PPP)组成的准一维多嵌段共聚物的光吸收性质,发现沿链方向外加电场的作用,使得体系的带隙变小,电子光吸收出现红移.     

Controlling the orientation of polar molecules with combined electrostatic and pulsed,nonresonant laser fields

We demonstrate that molecules with a moderate permanent dipole moment can be oriented with combined electrostatic and pulsed, nonresonant laser fields. We use OCS molecules as a sample. The degree of orientation can be increased by increasing the magnitude of electrostatic field and the peak intensity of the laser field or by decreasing the rotational temperature of the molecules.