分子磁体中的量子隧穿及宏观量子效应

文章介绍了分子磁体中的量子隧穿和宏观量子效应理论和实验研究的新进展．分子磁体既有宏观磁体特性也呈现纯量子行为，例如磁化矢量的量子隧穿．文章作者解释了如何通过量子隧穿实现宏观量子相干(即薛定谔猫态的相干叠加)和量子态位相干涉．对隧穿率计算的瞬子方法，特别是有限温度隧穿理论及其在分子磁体量子隧穿中的应用也做了简要的阐述．     

Temperature-dependent theory of tunneling in the fractional quantum Hall effect

Recent experiments have studied the tunneling current between two edges of the same fractional quantum Hall liquid as a function of temperature and voltage. The experimental findings for low temperatures are at odds with the model where the edges are described as chiral Luttinger liquids, while the data at high temperatures are quite consistent with the same model. Here, we argue that a temperature dependence of the tunneling amplitude, not foreseen in previous works, can explain this discrepancy.     

The asymmetry of the tunneling time in type II semiconductor heterostructures

The tunneling time asymmetry in type II semiconductor heterostructures is related to the phase difference of the reflection coefficients for the two tunneling directions. Analytical expressions and numerical simulations are given for the difference between the left-to-right and right-to-left tunneling times in asymmetric, single and multiple barrier type II heterostructures.     

A local view of the Kondo effect: Scanning tunneling spectroscopy

The fascinating many-body physics involved in the interaction of a single magnetic impurity with the conduction electrons of its nonmagnetic metallic host is reflected in unconventional phenomena in magnetism, transport properties and the specific heat. Characteristic low-energy excitations, termed the Kondo resonance, are generally believed to be responsible for this striking behaviour. However, in spite of an intense research for over more than 30 years, a direct spectroscopic observation of the Kondo resonance on individual magnetic adatoms withstood experimental efforts hitherto. The development of low-temperature scanning tunneling microscopes (STM) operating under ultrahigh vacuum (UHV) conditions has provided new opportunities for investigating locally the electronic structure at surfaces. At low temperatures, due to the reduced broadening of the Fermi level of the STM tip and the sample, rather high energy resolution (≤ 1 meV) in scanning tunneling spectroscopy (STS) is achievable. Moreover, the absence of diffusion together with the spatial resolution of the STM enables detailed studies of electronic states on and near single adsorbed atoms and other nanoscale structures. Recently, for the first time, two such STS/STM experiments spatially resolved the electronic properties of individual magnetic adatoms displaying the Kondo effect. In particular, the observed Fano lineshape of the Kondo resonance reveals unambiguously the details of the quantum mechanical interference between the localized orbital and the conduction electrons on an atomic length scale [1,2]. This achievement of the detection of individual magnetic atoms with atomic resolution opens new perspectives for probing magnetic nanostructures.     

The critical role of the barrier thickness in spin filter tunneling

Spin filter tunneling is considered in the low bias limit as functions of the temperature dependent barrier parameters. We demonstrate the generation of spin polarized tunneling currents in relation to the magnetic order parameter, and discuss how an interfacially suppressed order parameter leads to a temperature dependent tunneling current asymmetry. Analyzing the full parameter space reveals that the often overlooked barrier thickness plays a critical role in spin filter tunneling. With all else fixed, thicker barriers yield higher spin polarization, and allow a given polarization to be achieved at higher temperatures. This insight may open the door for new materials to serve as spin filter barriers.     

A reaction kinetics model of water sonolysis in the presence of a spin-trap

The time development of the concentration of a spin-trapped OH radical was studied by electron spin resonance at various sound intensities and various 5,5-dimethyl-1-pyrroline N-oxide (DMPO) concentrations in water sonolysis. The lifetime of the spin-trapped OH radical was also studied, and factors governing sonolysis are discussed. We found that the production of spin-trapped OH radical increases with increasing ultrasound intensity. The lifetime of a spin-trapped OH radical decreases linearly with increase in sonication time. This result suggests that an unknown scavenger is produced by ultrasound. Based on the above results, we suggested a model of the reaction kinetics and estimated the production rate of OH radical from this model.     

Sensitivity of current to energy in scanning tunneling microscopy within the vacuum case

Sensitivity of the tunneling current to electron energy is calculated in the context of scanning tunneling microscopy for the vacuum case by using an appropriate parameter. In addition, a mathematical relationship between this parameter and the phase associated with the involved Schrödinger wavefunctions is obtained.     

另类全反射及光学隧穿中的电磁波

讨论了在被称之为受阻全内反射的隧穿过程中发生在由光疏介质到光密介质界面上的一种全反射现象，指出隧穿过程中的隧穿能流密度是隧穿区域内的入射波和反射干涉的结果，进而给出了隧穿区域和透射区域光波的多次反射的表达式。     

Exact calculation of the current in a one-dimension model of a scanning tunneling microscope

Electric current as a function of applied voltage have been calculated between two one-dimensional semi-infinite atomic chains. A tight-binding two band model and the modified tight-binding equations technique have been used. The current derivatives are compared with the surface local density of states. The comparison shows that the current derivatives reproduce the local density of states with a significant distortion. It occurs due to wave vector dependence of the transmission coefficient of electronic waves falling on the interface between the chains.     

Scanning tunneling microscopy and molecular orbital calculation of pentacene molecules adsorbed on the Si(100)2×1 surface

Adsorption of the organic molecule pentacene on Si(100)2×1 surfaces was imaged using scanning tunneling microscopy (STM). The molecular images exhibit distinct shapes corresponding to the expected shapes for adsorption configurations. Semi-empirical molecular orbital (MO) calculations reveal a local surface density of states for the adsorbed pentacene on the Si surface. In the cases where the pentacene molecule is adsorbed on an Si dimer row, the calculated MOs are in good agreement with the molecular images observed in STM. In the case of pentacene adsorbed on two or three Si-dimer rows, however, the MOs of the pentacene do not correlate directly with the observed STM images. It is thus considered that the STM images are produced by a combination of Si dimer states and MO.     

Effects of barrier fluctuation on the tunneling dynamics in the presence of classical chaos in a mixed quantum-classical system

We present a numerical investigation of the tunneling dynamics of a particle moving in a bistable potential with fluctuating barrier which is coupled to a non-integrable classical system and study the interplay between classical chaos and barrier fluctuation in the tunneling dynamics. We found that the coupling of the quantum system with the classical subsystem decreases the tunneling rate irrespective of whether the classical subsystem is regular or chaotic and also irrespective of the fact that whether the barrier fluctuates or not. Presence of classical chaos always enhances the tunneling rate constant. The effect of barrier fluctuation on the tunneling rate in a mixed quantum-classical system is to suppress the tunneling rate. In contrast to the case of regular subsystem, the suppression arising due to barrier fluctuation is more visible when the subsystem is chaotic.      

Entropy is conserved in Hawking radiation as tunneling: A revisit of the black hole information loss paradox

We revisit in detail the paradox of black hole information loss due to Hawking radiation as tunneling. We compute the amount of information encoded in correlations among Hawking radiations for a variety of black holes, including the Schwarzchild black hole, the Reissner–Nordström black hole, the Kerr black hole, and the Kerr–Newman black hole. The special case of tunneling through a quantum horizon is also considered. Within a phenomenological treatment based on the accepted emission probability spectrum from a black hole, we find that information is leaked out hidden in the correlations of Hawking radiation. The recovery of this previously unaccounted for information helps to conserve the total entropy of a system composed of a black hole plus its radiations. We thus conclude, irrespective of the microscopic picture for black hole collapsing, the associated radiation process: Hawking radiation as tunneling, is consistent with unitarity as required by quantum mechanics.     

Space-and time-resolved diffusion-limited binary reaction kinetics in capillaries: experimental observation of segregation,anomalous exponents,and depletion zone

An experimental investigation of one-dimensional, diffusion-limited A+BC chemical reactions is reported. The persistence of reactant segregation and the formation of a depletion zone is observed and expressed in terms of the universal time exponents: (motion of the boundary zone), (width of instantaneous product formation zone), (rate of instantaneous local formation of product), (rate of instantaneous global formation of product), etc. There is good agreement with the recently predicted and/or simulated values:=1/2,=1/6,=2/3,=1/2, in contrast to classical predictions (=0,=1/2,=0,=–1/2). Furthermore, classically the segregation would not be preserved and there would be no formation of a depletion zone and no motion (just dissipation) of the reaction zone. We also discuss the relations to electrode oxidation-reduction reactions, i.e., A+CC where C is a catalyst, electrode, or trap.     

Diffusion-annihilation and the kinetics of the Ising model in one dimension

The relationship between the one-dimensional kinetic Ising model at zero temperature and diffusion-annihilation in one dimension is studied. Explicit asymptotic results for the average domain size, average magnetization squared, and pair-correlation function are derived for the Ising model, for arbitrary initial magnetization. For the case of zero initial magnetization (m ₀=0, a number of recent exact results for diffusion-annihilation with random initial conditions are obtained. However, for the casem ₀ not equal to zero, the asymptotic behavior turns out to be different from diffusion-annihilation with random initial conditions and at a finite density. In addition, in contrast to the case of diffusion-annihilation, the domain-size distribution scaling functionh(x) is found to depend nontrivially on the initial magnetization. The origin of these differences is clarified and the existence of nontrivial correlations in the initial wall distribution for finite initial magnetization is found to be responsible for these differences. Results of Monte Carlo simulations for the domain size distribution function for different initial magnetizations are also presented.This paper is dedicated to George Weiss on the occasion of his 60th birthday.     

MRCI study on spectroscopic and molecular properties of four low-lying electronic states of the BO radical

The potential energy curves (PECs) of four low-lying electronic states of the BO radical, including two ²Σ⁺ and two ²Π states, have been studied using the full valence complete active space self-consistent field (CASSCF) method followed by the highly accurate valence internally contracted multireference configuration interaction (MRCI) approach in combination with the cc-pV5Z basis set for internuclear separations from 0.05 to 2.0 nm. The effect on the PECs by the relativistic correction has been taken into account. With these PECs, the spectroscopic parameters (T_e, D₀, D_e, R_e, ω_e, ω_ex_e, α_e and B_e) of two main isotopologues (¹¹B¹⁶O and ¹⁰B¹⁶O) have been determined. These parameters have been compared in detail with those of previous investigations reported in the literature, and excellent agreement has been found between the available data and the present results. By solving the radial Schrödinger equation of nuclear motion, 60 vibrational states for the ¹¹B¹⁶O(X²Σ⁺), 60 for the ¹⁰B¹⁶O(X²Σ⁺), 66 for the ¹¹B¹⁶O(A²Π) and 64 for the ¹⁰B¹⁶O(A²Π) are predicted for the non-rotating molecule. For each vibrational state of the ¹¹B¹⁶O(X²Σ⁺), ¹⁰B¹⁶O(X²Σ⁺), ¹¹B¹⁶O(A²Π) and ¹⁰B¹⁶O(A²Π), the vibrational level G(υ), inertial rotation constant B_υ and centrifugal distortion constant D_υ have been determined. Comparison with the available data shows that the present molecular constants are reliable and accurate. The ro-vibrational levels have been calculated for the X²Σ⁺ and A²Π states of two main species for future laboratory research.     

Pressure-induced changes in the magnetic properties of the single crystal of Mn3 single-molecule magnet

The magnetic measurements of the single crystal of Mn3 single-molecule magnet under high pressure at T=2 K have been performed.We find both the antiferromagnetic intermolecular coupling parameter J and the effective energy barrier to vary compared with the measurements at low pressure.The increase of|J|is estimated to be 12%at 0.7 GPa when compared with that of0 GPa,whereas the effective energy barrier becomes smaller with increasing pressure.Our results demonstrate that the intermolecular interaction of single-molecule magnet can be changed by pressure.Compared with the normal magnetic alloy,the effect of pressure on the magnetic properties of Mn3 is much more prominent,which implies that Mn3 may have great potential in magnetic multifunctional material.     

Shape of the clusters of a percolative system in the presence of tunneling bonds and a finite electric field

We consider a model correlated percolative system on a 2D square lattice with a finite electric field applied accross its two opposite sides. We study the shape of the clusters formed with the addition of a new kind of bond (we call them tunneling bonds) which respond only above a finite threshold voltage. As expected, the clusters do have an overall elongated shape in the-direction of the applied field. Intuitively, one expects the elongation (with the aspect ratio >1) to increased indefinitely with the field. But, in a previous study, we found the model to belong to the same universality clas in the limits of a zero and an infinite electric field. We explain this behavior by studying the change in these elongated shapes as a function of the applied voltage in finite size samples and find that actually the amount of elongation takes on a maximum value at a size (L)-dependent finite voltageV _m (L) and that asV thje overall deviation from isotropy in the field direction tends to zero (i.e., aspect ratio1) again.     

On the role of electron-hole pair excitation in the kinetics of atom recombination at metal surfaces

The dynamics of energy relaxation of the adspecies in exoergic processes at metal surfaces has been modeled by means of the master equation approach. The effect of energy disposal to the solid via electron-hole (e-h) pair excitation on the rate of adatom recombination, has been investigated in the case of a discrete set of vibrational levels of the adspecies. The kinetics is solved, analytically, for two recombination channels and by taking into account two energy dissipation pathways of the adspecies. It is shown that dissipation pathways, characterized by a sizable energy transfer per scattering event, affect the kinetics leading to enhanced recombination rates. The kinetic model has been applied to describe experimental data on H(D)-adatom abstraction and recombination at metal surfaces. The rate coefficient of the process is shown to be proportional to the energy power transferred to the solid, owing to the reaction exothermicity, and correlates to the surface electron density.     

Decay kinetics of the green emission in tungstates and molybdates

Luminescence characteristics of a number of undoped and variously doped PbWO₄ crystals were studied at 0.4–400 K by the time-resolved spectroscopy and compared with those of ZnWO₄,CdWO₄ and PbMoO₄ crystals. Two types of green emission centres are detected in PbWO₄ crystals. The centres of the first type are responsible for the low-temperature 2.3–2.4 eV emission observed under excitation around 3.90–3.95 eV. The structure and parameters of their relaxed excited states were determined. It was concluded that the origin of defects responsible for the green emission of the first type could vary for different crystals. The centres of the second type with the emission around 2.5 eV appear in crystals containing oxygen vacancies after the thermal destruction of Pb⁺-WO₃ centres at T>180 K. Decomposition of the exciton and various defect-related states was also studied, and activation energies of this process were calculated.     

The importance of imaging conditions in scanning tunneling microscopy for the determination of surface texture and roughness

We show that the control conditions in the feedback loop of a scanning tunneling microscope (STM) affect the values of the surface texture parameters, including fractal characterization, when they are calculated from STM images of the surface. The main surface texture parameters (such as the r.m.s. roughness, kurtosis, skewness, and average wavelength) show a strong dependence on the conditions used in the feedback loop for imaging. Fractal character changes also with feedback parameters. The quality of the STM images can be measured quantitatively by using some of the surface texture parameters.