Frustration effect and possibility of spin quantum liquid state in a tetrahedral spin ½ molecule

In this work, we studied the magnetic properties of a regularly tetrahedral molecule made of four spin-½, interconnected by exchange. For this purpose, we used the Heisenberg model and performed an exact resolution. In the case of a ferromagnetic coupling among the spins, the system orders itself under a magnetic field without displaying any net spontaneous magnetization. It behaves as a high spin-2 unique magnet.Under antiferromagnetic exchange, some exotic behaviour has been revealed due to the frustration governing the spin edifice. Thus, the magnetization is quantized into plateaus, which are separated by quantum phase transitions. In particular, we have argued the possibility of a quantum spin liquid (QSL) state that can occur at low temperature.This contribution intends mainly to bring single-molecular magnets to the front line of innovative nanoscale applications.     

Realization of a Ramsey-Bordé matter wave interferometer on the molecule

We show first results and systematic investigations of a matter wave interferometer with the K₂ molecule, using a transition between the electronic ground state X and the state b . This spin forbidden transition is observable due to spin-orbit coupling between the states b and A . The experimental results are compared with numerical simulations, which show the power of the interferometer to observe small phase shifts by weak interactions. Received 13 March 2000     

Does molecular environment cause a substructure underlying the expected eigenstates of a molecule?

The paper reports on Hanle effect experiments in NO₂ which indicate that the environment of the molecule may add a substructure to the expected eigenstates of NO₂. It is concluded that isolation of the molecule is presumably not possible.     

In search of the ‘impenetrable’ volume of a molecule in a noncovalent complex

ABSTRACT

We propose to characterise the “impenetrable” volumes of molecules A and B in a complex A---B by finding that contour of its electronic density that separates the molecular surfaces of A and B but leaves them almost touching. The volume of the complex within that contour is always less than within the 0.001 au contour. The percent difference measures the interpenetration of the two molecules at equilibrium, and is found to directly correlate with the binding energy of the complex. We interpret the volume of each molecule that is enclosed by the almost-touching contour as that molecule's impenetrable volume relative to its particular partner. The percents by which the molecules' relative impenetrable volumes differ from their 0.001 au volumes in the free states also correlate with the strengths of the interactions. This allows the “absolute” impenetrable volume of any molecule to be estimated as ～25% of its 0.001 au volume in the free state. However this absolute impenetrable volume is only approached by the molecule in a relatively strong interaction.     

Improved adiabatic corrections for the a3Σg+ state of the hydrogen molecule

Adiabatic corrections have been calculated for a wide range of internuclear distances for the lowest bound triplet state of molecular hydrogen. It is shown that the existing small discrepancy between theoretical and experimental D₀ values can be removed if an additional correction to the Born-Oppenheimer energy on the order of 0.4 cm⁻¹ is taken into account.     

Is there a stable B2Π state for the CNO molecule?

We report MRD-CI calculations on the ground state X²Π and the excited states A²Σ⁺ and B²Π of the CNO molecule in linear geometry. The surfaces for oxygen and carbon extraction are calculated using a limited CI expansion of 47 configuration state functions; in the vicinity of the minima obtained with this procedure large-scale CI calculations are carried out including deter-mination of the spin-orbit splitting of the ²Π states of the minima. We find that the B²Π state will be difficult to detect spectroscopically due to an avoided crossing just at the equilibrium geometry of the ground state at R_CN = 2.25 a.u., R_NO = 2.30 a.u. Accordingly we find two shallow minima for B²Π at R_CN = 2.33 a.u., R_NO = 2.91 a.u. and R_CN = 2.78 a.u., R_NO = 2.28 a.u., respectively.     

Radiative decay of the Ξ(1620) in a hadronic molecule picture

Last year,the Ξ(1620) state,which is cataloged in the Particle Data Group(PDG) with only one star,was reported again in the Ξ~-π~+final state by the Belle Collaboration.Its properties,such as the spectroscopy and decay width,cannot be simply explained in the context of conventional constituent quark models.This inspires an active discussion on the structure of this resonance.In this paper,we study the radiative decays of the newly observed Ξ(1620) assuming that it is a meson-baryon molecular state of AK and with spin-parity J~P=1/2~-developed in our previous study.The partial decay widths of the AK-ΣK molecular state into Ξγ and Ξπγ final states through hadronic loops are evaluated using effective Lagrangians.The partial widths for Ξ(1620)~0→γΞ is evaluated to be approximately 118.76-174.21 keV,which may be accessible for the LHCb experiment.If Ξ(1620) is aΛK-∑K molecule,the radiative transition strength Ξ(1620)~0→γKA is considerably small and the decay width is of the order of 0.01 eV.Future experimental measurements of these processes can be useful to test the molecule interpretations of the Ξ(1620).     

Geometric model of a fullerene molecule in the presence of Aharonov–Bohm flux

In this study, we describe a geometric model of a fullerene molecule with Ih symmetry. We combine the well known non-Abelian monopole approach and the geometric theory of defects, where every topological defect is associated with curvature and torsion, to describe a fullerene molecule. The geometric theory of defects in solids is used to consider the topological defects that allow this molecule to form and we apply a continuum formulation to describe this spherical geometry in the presence of an external Aharonov–Bohm flux. We solve a Dirac equation for this model and obtain the eigenvalues and eigenfunction of the Hamiltonian, and we obtain the persistent current for this model and show that it depends on the geometrical and topological properties of the fullerene.     

Hyper-radial adiabatic expansion for a muonic molecule dtμ

By using the hyper-radius, adiabatic potential energy curves with correct asymptotic energies are obtained for the Coulomb three-body problem. The bound state energies of the muonic molecules dtμ with total angular momentum J=0 calculated adopting the three lowest adiabatic potential energy curves are −318.72 and −34.36 eV for vibrational quantum numbers ν=0 and 1, respectively.     

The quadrupole–quadrupole polarizability of a non-magnetic molecule

The quadrupole–quadrupole polarizability of a molecule introduced by Buckingham is not a unique molecular property, in the sense that it depends on an arbitrary choice of the origin of the molecular coordinates. To obtain a physically acceptable form of this property for a non-magnetic molecule, a linear combination is taken of molecular polarizabilities of the relevant multipole order (electric octopole-magnetic quadrupole). By requiring this combination to possess the desired space–time and symmetry properties, as well as origin independence, an acceptable quadrupole–quadrupole polarizability is obtained. Expressions are presented for the components of this polarizability for molecules of certain symmetries.     

Is the Free Electromagnetic Field a Consequence of Maxwell's Equations or a Postulate?

It is generally accepted that solutions of so called free Maxwell equations for = 0 (null charge density at every point of the whole space) describe a free electromagnetic field for which flux lines neither begin nor end in a charge). In order to avoid ambiguities and unacceptable approximation which have place in the conventional approach in respect to the free field concept, we explicitly consider three possible types of space regions: (i) isolated charge-free region, where a resultant electric field with the flux lines which either begin or end in a charge is zero in every point, for example, inside a hollow conductor of any shape or in a free-charge universe; (ii) non-isolated charge-free region, where this electric [see (i)] field is not zero in every point; and (iii) charge-neutral region, where point charges exist but their algebraic sum is zero. According to these definitions a strict mathematical interpretation of Maxwell's equations gives following conclusions: (1) In isolated charge-free regions electric free field cannot be unconditionally understood neither as a direct consequence of Maxwell's equations nor as a valid approximation: it may be introduced only as a postulate; nevertheless, this case is compatible is the existence of a time-independent background magnetic field. (2) In both charge-neutral and non-isolated charge-free regions, where the condition = function or = 0 respectively holds, Maxwell's equation for the total electric field have non-zero solutions, as in the conventional approach. However, these solution cannot be strictly identified with the electric free field. This analysis gives rise to the reconsideration of the free-electromagnetic field concept and leads to the simplest implications in respect to charge-neutral universe.     

Absolute population of the A 3 Σ u + metastable state of a nitrogen molecule in a glow discharge plasma

The absolute populations of seven vibrational levels of the B ³Π_g state of a nitrogen molecule in a negative glow are determined at a pressure of 0.01–5.00 Torr and a current of 100–400 mA from the absolute intensity of the lines of the nitrogen first positive system. The population of the A ³Σ _u ⁺ metastable state at different discharge parameters is found using the balance equation for this state.     

The effect of an external electric field on the optical properties of a quantum-dot molecule with a resonant state of the D 2 − center

The effect of an external electric field on the probability of the electron radiative transition from a resonant u-state to the localized g-state of the D ₂ ^? center in the presence of dissipative tunneling has been investigated in the model of zero-radius potential. It is shown that the probability of radiative transition increases by about two orders of magnitude in the case of the external electric field intensity for which the initially asymmetric double-well oscillator potential simulating a quantum-dot molecule becomes symmetrical.     

Tunneling-induced π-phase shift of a quantum-dot molecule coupled to a single-sided cavity

We study optical dispersive properties of a combined system which is composed of a single-sided cavity with low quality factor Q and a quantum-dot molecule (QDM) embedded in cavity. It is shown that, with a voltage-controlled tunneling, an input signal after reflection from cavity experiences a π phase shift while not suffering from absorption in the experimentally available parameter range.     

The study of the valence bond property in a two—different—quantum—dot molecule

The electronic energy spectrum and wavefunction of a quantum-dot molecule are studied by means of the finite-element solution of the single electron Schr?dinger equation. We find that the nature of the coupling can be covalent, ionic, or "intermediate" new mixed states, depending on various parameters such as the separation distance between two dots, the height of potential barrier, matching of the energies and parities of the orbital localized on each dot. The bond property can be used to explain the experimental result obtained by Oosterkamp et al. (1998 Nature 395 873).     

Probing the role of metal cations on the aggregation behavior of amyloid β-peptide at a single molecule level by AFM

With the development of nanotechnology, understanding of intermolecular interactions on a single molecule level by atomic force spectroscopy (AFM) has played an important role in molecular biology and biomedical science. In recent years, some research suggested that the presence of metal cations is an important regulator in the processes of misfolding and aggregation of the amyloid β-protein (Aβ), which may be an important etiological factor of Alzheimer’s disease. However, the knowledge on the principle of interactions between Aβ and metal cations at the single molecule level is still poor understood. In this paper, the amyloid β-protein (Aβ) was fabricated on substrate of mixed thiol-modified gold nanoparticles using self-assembled monolayer method and the adhesion force in the longitudinal direction between metal cations and Aβ42 were investigated by AFM. The role of metal ions on Aβ aggregation is discussed from the perspective of single molecular force. The force results showed that the specific adhesion force F _i and the nonspecific force F ₀ between a single Aβ–Aβ pair in control experiment were calculated as 42 ± 3 and 80 pN, respectively. However, F _i between a single Aβ–Aβ pair in the presence of Cu²⁺, Zn²⁺, Ca²⁺ and Al³⁺ increased dramatically to 84 ± 6, 89 ± 3, 73 ± 5, 95 ± 5 pN successively, which indicated that unbinding between Aβ proteins is accelerated in the presence of metal cations. What is more, the imaging results showed that substoichiometric copper cations accelerate the formation of fibrils within 3 days. The combined atomic force spectroscopy and imaging analysis indicate that metal cations play a role in promoting the aggregating behavior of Aβ42.     

Ab initio multi-reference configuration interaction of the low-lying states of the AsP molecule

Nine low-lying electronic states of the AsP molecule,includingΣ~ ,Π,andΔsymmetries with singlet, triplet,and quintet spin multiplicities,are studied using multi-reference configuration interaction method. The potential energy curves and the spectroscopic constants of these nine states are determined,and compared with the experimental observed data as well as other theoretical works available at present. Three quintet states are reported for the first time.Furthermore,the analytical potential energy functions of these states are fitted using Murrell-Sorbie function and least square fitting method.     

Peculiarities of the SEHRS spectrum of 4,4′-bipyridine molecule

The SEHRS spectrum of 4,4′-bipyridine is analyzed on the base of the Dipole-Quadrupole theory. It is demonstrated that there appear strong lines caused by vibrations transforming after the unit irreducible representation of the D₂ symmetry group, which is most probably describes the symmetry properties of the molecule. These lines are nearly forbidden for the molecule, adsorbed on rough metal surfaces. Appearance of these lines is associated with a strong quadrupole light–molecule interaction, which exists in this system. In addition, there are lines caused by contributions from both the vibrations transforming after the unit irreducible representation A and the representation B₁, which describes transformational properties of the d_z component of the dipole moment, which is perpendicular to the surface. This result is associated with the specific geometry of the molecule, when the indicated vibrations can be nearly degenerate and cannot be resolved by the SEHRS spectra analysis. Analysis of the SEHRS spectra for the possible geometry of the molecule with the D_2h symmetry group leads to similar results. This issue is in a full coincidence with the results of the SEHRS Dipole-Quadrupole theory.     

Conditional stability of diatomic molecule driven by a weak laser field

Using a direct perturbation method, we investigate the stability of a diatomic molecule modelled by a weakly laser-driven Morse oscillator. It is shown that stationary state solution of the system is stable in the sense of Lyapunov and the periodical one possesses conditional stability, namely its stability depends on the initial conditions and system parameters. The corresponding sufficient and necessary conditions are established that indicate the stable states associated with some discrete energies. The results reveal how a diatomic molecule can be stabilized or dissociated with a weak laser, and demonstrate that the mathematical conditional stability works in the considered physical system.