XMCD of a single layer of single molecule magnets

A detailed report on the X-ray Magnetic Circular Dichroism (XMCD) investigation of monolayers of Mn12-based single molecule magnets (SMMs) deposited on gold Au(111) is presented. A semi-quantitative analysis of data is provided in order to extract chemical and magnetic information on Mn ions, by comparison with XMCD on bulk samples. This work points that XMCD is a key-tool for the characterization of SMMs-based nanostructured systems. XMCD surface sensitivity and element-specificity will play a fundamental role in the identification of good candidates for SMMs based devices.     

Coupling single molecule magnets to ferromagnetic substrates

We investigate the interaction of TbPc(2) single molecule magnets (SMMs) with ferromagnetic Ni substrates. Using element-resolved x-ray magnetic circular dichroism, we show that TbPc(2) couples antiferromagnetically to Ni films through ligand-mediated superexchange. This coupling is strongly anisotropic and can be manipulated by doping the interface with electron acceptor or donor atoms. We observe that the relative orientation of the substrate and molecule anisotropy axes critically affects the SMM magnetic behavior. TbPc(2) complexes deposited on perpendicularly magnetized Ni films exhibit enhanced magnetic remanence compared to SMMs in the bulk. Contrary to paramagnetic molecules pinned to a ferromagnetic support layer, we find that TbPc(2) can be magnetized parallel or antiparallel to the substrate, opening the possibility to exploit SMMs in spin valve devices.     

Low-temperature phonoemissive tunneling rates in single molecule magnets

Tunneling between the two lowest energy levels of single molecule magnets with Ising type anisotropy, accompanied by the emission or absorption of phonons, is considered. Quantitatively accurate calculations of the rates for such tunneling are performed for a model Hamiltonian especially relevant to the best studied example, Fe₈. Two different methods are used: high-order perturbation theory in the spin–phonon interaction and the non-Ising-symmetric parts of the spin Hamiltonian, and a novel semiclassical approach based on spin-coherent-state-path-integral instantons. The methods are found to be in good quantitative agreement with other, and consistent with previous approaches to the problem. The implications of these results for magnetization of molecular solids of these molecules are discussed briefly.     

Magnetization process of single molecule magnets at low temperatures

We show that correlations established before quenching to very low temperatures later drive the magnetization process of systems of single molecule magnets, after a magnetic field is applied at t=0. We also show that in simple cubic lattices m proportional, variant sqrt[t], as observed in Fe8, but only for 1+2log((10)(h(d)/h(w)) time decades, where h(d) is a nearest neighbor dipolar magnetic field and a spin reversal can occur only if the field on it is within (-h(w),h(w)). However, the sqrt[t] behavior is not universal. For bcc and fcc lattices, m proportional, variant t(p), but p approximately 0.7. The value to which m finally levels off is also given.     

Single-photon spectroscopy of a single molecule

Efficient interaction of light and matter at the ultimate limit of single photons and single emitters is of great interest from a fundamental point of view and for emerging applications in quantum engineering. However, the difficulty of generating single-photon streams with specific wavelengths, bandwidths, and power as well as the weak interaction probability of a single photon with an optical emitter pose a formidable challenge toward this goal. Here, we demonstrate a general approach based on the creation of single photons from a single emitter and their use for performing spectroscopy on a second emitter situated at a distance. While this first proof of principle realization uses organic molecules as emitters, the scheme is readily extendable to quantum dots and color centers. Our work ushers in a new line of experiments that provide access to the coherent and nonlinear couplings of few emitters and few propagating photons.     

Nanoparticle-free single molecule anti-stokes Raman spectroscopy

In the absence of large, plasmon-supporting nanoparticles, biocompatible dendrimer- and peptide-encapsulated few-atom Ag nanoclusters produce scaffold-specific single molecule (SM) Stokes and anti-Stokes Raman scattering. The strong SM vibrational signatures are enhanced by the Agn transitions in nanoparticle-free samples and cannot arise from plasmon enhancement. Characteristic SM-Raman intermittency is observed, with antibunching of the underlying Agn emission directly confirming the SM nature of the emissive species.     

Theory of single molecule vibrational spectroscopy and microscopy

We have carried out a density functional study of vibrationally inelastic tunneling in the scanning tunneling microscope of acetylene on copper. Our approach is based on a many-body generalization of the Tersoff-Hamann theory. We explain why only the carbon-hydrogen stretch modes are observed in terms of inelastic and elastic contributions to the tunneling conductance. The inelastic tunneling is found to be efficient and highly localized in space without any resonant interaction and to be governed by a vibration-induced change in tunneling amplitude.     

Investigation of the stability of Mn<Subscript>12</Subscript> single molecule magnets

The stability of single crystals and monolayers of Mn₁₂ single molecule magnets under the influence of X-ray radiation and other possibly disruptive influences has been investigated by means of synchrotron radiation. Clear evidence for radiation induced sample degradation was found for both single crystals and monolayers. The comparison with spectra obtained after damaging the molecules by Ar⁺ sputtering, metal evaporation or water moistening indicates a possibility to distinguish between radiation damage and other external influences. The results clarify some of the previous conflicting reports on the integrity of Mn₁₂ molecules deposited on surfaces and are linked to the investigations aiming at studies of the electronic and magnetic properties of individual Mn₁₂ clusters.     

Effect of disorder on ultrafast exciton dynamics probed by single molecule spectroscopy

We present a single-molecule study unraveling the effect of static disorder on the vibrational-assisted ultrafast exciton dynamics in multichromophoric systems. For every single complex, we probe the initial exciton relaxation process by an ultrafast pump-probe approach and the coupling to vibrational modes by emission spectra, while fluorescence lifetime analysis measures the amount of static disorder. Exploiting the wide range of disorder found from complex to complex, we demonstrate that static disorder accelerates the dephasing and energy relaxation rate of the exciton.     

A dynamic approach to the sudden jump model in single molecule spectroscopy

Various theories that use the sudden jump model to explain the time dependence of the optical band of a single molecule in a fluctuating environment are analyzed. It is shown that the dynamic theory of the optical band, based on the Hamiltonian of the system, should necessarily include the initial conditions for the quantum system. We show how these initial conditions should be chosen for the dynamic theory to properly describe the quantum jumps of the optical band of a single molecule. Unlike stochastic theories based on the sudden jump model, the dynamic theory predicts several absorptances of a single molecule and directly connects the absorptance fluctuations with this fact. The theory is used to account for the time dependence of the broadening of experimentally measured lines of individual molecules.     

Spin quantum tunneling in single molecular magnets: fingerprints in transport spectroscopy of current and noise

We demonstrate that transport spectroscopy of single molecular magnets shows signatures of quantum tunneling at low temperatures. We find current and noise oscillations as a function of bias voltage due to a weak violation of spin-selection rules by quantum tunneling processes. The interplay with Boltzmann suppression factors leads to fake resonances with temperature-dependent position which do not correspond to any charge excitation energy. Furthermore, we find that quantum tunneling can completely suppress transport if the transverse anisotropy has a high symmetry.     

Scaling exponent and Kuhn length of pinned polymers by single molecule force spectroscopy

The end-to-end distance and the contour length of single polymers in dynamic adsorbate layers were measured with a mechanical approach. Individual polysaccharide chains were covalently pinned to the surface with one segment and picked up randomly with an atomic force microscope tip. The polymer section between pinpoint and the pickup point was stretched by retracting the tip from the surface. The pinpoint was derived by measuring the normal force while laterally scanning the surface at constant height. For carboxy-methyl-amylose, a Kuhn length of 0.44 nm and a scaling exponent of 0.74 were found.     

单分子操纵与单分子生物物理

文章介绍了近年来发展起来的一些单分子操纵实验技术如光镊、磁镊、微针、斯托克斯拖曳技术，以及应用这些技术拉伸、旋转、解链DNA分子，从而研究其力学性质所取得的研究进展．各种蛋白质如T7 DNA聚合酶、拓扑异构酶，SWI／SNF染色质重建复合体、RNA聚合酶与DNA的作用在生化过程中十分重要，因此，文章也介绍了这些蛋白质与DNA在单分子的水平上相互作用所取得的研究进展．     

Quantum-tunneling-induced Kondo effect in single molecular magnets

We consider transport through a single-molecule magnet strongly coupled to metallic electrodes. We demonstrate that, for a half-integer spin of the molecule, electron and spin tunneling cooperate to produce both quantum tunneling of the magnetic moment and a Kondo effect in the linear conductance. The Kondo temperature depends sensitively on the ratio of the transverse and easy-axis anisotropies in a nonmonotonic way. The magnetic symmetry of the transverse anisotropy imposes a selection rule on the total spin for the occurrence of the Kondo effect which deviates from the usual even-odd alternation.     

单分子操纵与单分子生物物理

文章介绍了近年来发展起来的一些单分子操纵实验技术如光镊、磁镊、微针、斯托克斯拖曳技术,以及应用这些技术拉伸、旋转、解链DNA分子,从而研究其力学性质所取得的研究进展.各种蛋白质如T7 DNA聚合酶、拓扑异构酶,SWI/SNF染色质重建复合体、RNA 聚合酶与DNA的作用在生化过程中十分重要,因此,文章也介绍了这些蛋白质与DNA在单分子的水平上相互作用所取得的研究进展.     

Matrix embedded cobalt-carbon nano-cluster magnets: behavior as room temperature single domain magnets

A new type of Co-C nanoparticles is synthesized from CH₂Cl₂ solution of Co₄(CO)_{1 2} by heating up to 210 ^°C in a closed vessel. Transmission electron microscope (TEM) and electron energy loss spectroscopy (EELS) observation show that the particles are embedded in amorphous carbon and their average size is 12 nm. The radial structure function obtained from the extended X-ray absorption fine structure (EXAFS) of the Co K-edge absorption of the Co-C nanoparticles provides a Co-C average distance of 2.08 Å and the Co-Co distances of 3.18 Å and 3.9 (±0.2) Å. The particles exhibit the magnetic hysteresis curve with a coercive force of 200 Oe at 20 K and 260 Oe at 300 K. The temperature dependence of the magnetic susceptibility measured under zero-field cooling and 10 Oe field cooling conditions exhibits the behavior characteristic of a set of single magnetic domain nanomagnets in an amorphous carbon matrix.     

Theory of inelastic tunneling spectroscopy of a single molecule - Competition between elastic and inelastic current

A theory of inelastic electron tunneling spectroscopy of a single molecule with scanning tunneling microscope is presented using the Keldysh Green’s function method for an adsorbate-induced resonance coupled to the molecular vibration. It is found that the correction to the tunneling current is expressed in terms of the transmission probability; the correction is negative for the elastic part of the current and positive for the inelastic one. The differential conductance (dI/dV) exhibits an increase or decrease at the threshold corresponding to the opening of inelastic channel depending on the sign of the correction, and the size of this conductance jump is scaled with the vibrational damping due to electron-hole pair excitation. The lineshape of d²I/dV²-spectra calculated using a renormalized adsorbate Green’s function evolves from an antisymmetric dip to a peak through the derivative-like one as the position of the adsorbate resonance recedes from the Fermi level of the substrate.