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The substitution of scandium in fullerene single‐molecule magnets (SMMs) DySc2N@C80 and Dy2ScN@C80 by lutetium has been studied to explore the influence of the diamagnetic metal on the SMM performance of dysprosium nitride clusterfullerenes. The use of lutetium led to an improved SMM performance of DyLu2N@C80, which shows a higher blocking temperature of magnetization (TB=9.5 K), longer relaxation times, and broader hysteresis than DySc2N@C80 (TB=6.9 K). At the same time, Dy2LuN@C80 was found to have a similar blocking temperature of magnetization to Dy2ScN@C80 (TB=8 K), but substantially different interactions between the magnetic moments of the dysprosium ions in the Dy2MN clusters. Surprisingly, although the intramolecular dipolar interactions in Dy2LuN@C80 and Dy2ScN@C80 are of similar strength, the exchange interactions in Dy2LuN@C80 are close to zero. Analysis of the low‐frequency molecular and lattice vibrations showed strong mixing of the lattice modes and endohedral cluster librations in k‐space. This mixing simplifies the spin–lattice relaxation by conserving the momentum during the spin flip and helping to distribute the moment and energy further into the lattice.

Dysprosium oxide clusterfullerenes Dy2O@Cs(10528)–C72 and Dy2O@C2(13333)–C74 are synthesized and characterized by single-crystal X-ray diffraction. Carbon cages of both molecules feature two adjacent pentagon pairs. These pentalene units determine positions of endohedral Dy ions hence the shape of the Dy2O cluster, which is bent in Dy2O@C72 but linear in Dy2O@C74. Both compounds show slow relaxation of magnetization and magnetic hysteresis. Nearly complete cancelation of ferromagnetic dipolar and antiferromagnetic exchange Dy…Dy interactions leads to unusual magnetic properties. Dy2O@C74 exhibits zero-field quantum tunneling of magnetization and magnetic hysteresis up to 14 K, the highest temperature among Dy-clusterfullerenes.

Cubic $f$-electron compounds commonly exhibit highly anisotropic magnetic phase diagrams consisting of multiple long-range ordered phases. Field-driven metamagnetic transitions between them may depend not only on the magnitude, but also on the direction of the applied magnetic field. Examples of such behavior are plentiful among rare-earth borides, such as $R{\mathrm{B}}_{6}$ or $R{\mathrm{B}}_{12}$ ($R$ = rare earth). In this work, for example, we use torque magnetometry to measure anisotropic field-angular phase diagrams of La-doped cerium hexaborides, ${\mathrm{Ce}}_{1\ensuremath{-}x}{\mathrm{La}}_{x}{\mathrm{B}}_{6}$ ($x=0,0.18,0.28,0.5$). One expects that field-directional anisotropy of phase transitions must be impossible to understand without knowing the magnetic structures of the corresponding competing phases and being able to evaluate their precise thermodynamic energy balance. However, this task is usually beyond the reach of available theoretical approaches, because the ordered phases can be noncollinear, possess large magnetic unit cells, involve higher-order multipoles of $4f$ ions rather than simple dipoles, or just lack sufficient microscopic characterization. Here we demonstrate that the anisotropy under field rotation can be qualitatively understood on a much more basic level of theory, just by considering the crystal-electric-field scheme of a pair of rare-earth ions in the lattice, coupled by a single nearest-neighbor exchange interaction. Transitions between different crystal-field ground states, calculated using this minimal model for the parent compound ${\mathrm{CeB}}_{6}$, possess field-directional anisotropy that strikingly resembles the experimental phase diagrams. This implies that the anisotropy of phase transitions is of local origin and is easier to describe than the ordered phases themselves.

Yb- and Ce-based delafossites were recently identified as effective spin-1/2 antiferromagnets on the triangular lattice. Several Yb-based systems, such as NaYbO2, NaYbS2, and NaYbSe2, exhibit no long-range order down to the lowest measured temperatures and therefore serve as putative candidates for the realization of a quantum spin liquid. However, their isostructural Ce-based counterpart KCeS2 exhibits magnetic order below TN = 400 mK, which was so far identified only in thermodynamic measurements. Here we reveal the magnetic structure of this long-range ordered phase using magnetic neutron diffraction. We show that it represents the so-called 'stripe-yz' type of antiferromagnetic order with spins lying approximately in the triangular-lattice planes orthogonal to the nearest-neighbor Ce–Ce bonds. No structural lattice distortions are revealed below TN, indicating that the triangular lattice of Ce3+ ions remains geometrically perfect down to the lowest temperatures. We propose an effective Hamiltonian for KCeS2, based on a fit to the results of ab initio calculations, and demonstrate that its magnetic ground state matches the experimental spin structure., This project was funded in part by the German Research Foundation (DFG) under the individual research Grant IN 209/9-1, via the project C03 of the Collaborative Research Center SFB 1143 (project-id 247310070) at the TU Dresden, and the Würzburg-Dresden Cluster of Excellence on Complexity and Topology in Quantum Matter—ct.qmat (EXC 2147, project-id 390858490). SA thanks A Popov (IFW, Dresden) and M Vojta (TU Dresden) for fruitful discussions and acknowledges financial support from the German Research Foundation (DFG) under Grant No. AV 169/3-1. We also acknowledge V Joyet and S Djellit for technical assistance and Institut Laue-Langevin, Grenoble (France) for providing neutron beam time.

Recently, several putative quantum spin liquid (QSL) states were discovered in ${\tilde S} = 1/2$ rare-earth based triangular-lattice antiferromagnets (TLAF) with the delafossite structure. A way to clarify the origin of the QSL state in these systems is to identify ways to tune them from the putative QSL state towards long-range magnetic order. Here, we introduce the Ce-based TLAF KCeS$_2$ and show via low-temperature specific heat and $\mu$SR investigations that it yields magnetic order below $T_{\mathrm N} = 0.38$ K despite the same delafossite structure. We identify a well separated ${\tilde S} = 1/2$ ground state for KCeS$_2$ from inelastic neutron scattering and embedded-cluster quantum chemical calculations. Magnetization and electron spin resonance measurements on single crystals indicate a strong easy-plane $g$~factor anisotropy, in agreement with the ab initio calculations. Finally, our specific-heat studies reveal an in-plane anisotropy of the magnetic field-temperature phase diagram which may indicate anisotropic magnetic interactions in KCeS$_2$., Comment: submission to Scipost Physics

Relaxation of magnetization in endohedral metallofullerenes DySc2N@C80 is studied at different temperatures, in different magnetic fields, and in different molecular arrangements. Magnetization behavior and relaxation are analyzed for powder sample, and for DySc2N@C80 diluted in non-magnetic fullerene Lu3N@C80, adsorbed in voids of a metal–organic framework, and dispersed in a polymer. The magnetic field dependence and zero-field relaxation are also studied for single-crystals of DySc2N@C80 co-crystallized with Ni(II) octaethylporphyrin, as well as for the single crystal diluted with Lu3N@C80. Landau–Zener theory is applied to analyze quantum tunneling of magnetization in the crystals. The field dependence of relaxation rates revealed a dramatic dependence of the zero-field tunneling resonance width on the dilution and is explained with the help of an analysis of dipolar field distributions. AC magnetometry is used then to get access to the relaxation of magnetization in a broader temperature range, from 2 to 87 K. Finally, a theoretical framework describing the spin dynamics with dissipation is proposed to study magnetization relaxation phenomena in single molecule magnets.

The hunt for high-performance single-molecule magnets (SMM) revealed that atomic and molecular lanthanide systems combining 4f-shell and valence magnetism (FV-magnetism) may show magnetic bistability up to unexpectedly high temperatures. Here we rationalize the stability of the magnetism in the FV-systems from first principles on the example of LnII(CpiPr5)2 molecules.

The behavior in magnetic field of a paramagnetic center is characterized by its $g$ tensor. An anisotropic form of the latter implies different kind of response along different crystallographic directions. Here we shed light on the anisotropy of the $g$ tensor of Yb$^{3+}$ 4$f^{13}$ ions in NaYbS$_2$ and NaYbO$_2$, layered triangular-lattice materials suggested to host spin-liquid ground states. Using quantum chemical calculations we show that, even if the ligand-cage trigonal distortions are significant in these compounds, the crucial role in realizing strongly anisotropic, `noncubic' $g$ factors is played by inter-layer cation charge imbalance effects. The latter refer to the asymmetry experienced by a given Yb center due to having higher ionic charges at adjacent metal sites within the magnetic $ab$ layer, i.e., 3+ nearest neighbors within the ab plane versus 1+ species between the magnetic layers. According to our results, this should be a rather general feature of 4$f^{13}$ layered compounds: less inter-layer positive charge is associated with stronger in-plane magnetic response., 6 pages, 1 figure

The azafullerene Tb-2@C79N is found to be a single-molecule magnet with a high 100-s blocking temperature of magnetization of 24 K and large coercivity. Tb magnetic moments with an easy-axis single-ion magnetic anisotropy are strongly coupled by the unpaired spin of the single-electron Tb-Tb bond. Relaxation of magnetization in Tb-2@C79N below 15 K proceeds via quantum tunneling of magnetization with the characteristic time tau(QTM)=16462 +/- 1230 s. At higher temperature, relaxation follows the Orbach mechanism with a barrier of 757 +/- 4 K, corresponding to the excited states, in which one of the Tb spins is flipped. European Union's Horizon 2020 research and innovation programme, European Research Council [648295]; European Union's Horizon 2020 research and innovation programme, Marie Sklodowska-Curie action [748635]; Deutsche Forschungsgemeinschaft [PO 1602/4-1, 1602/5-1]; Luther & Alice Hamlett Scholarship We acknowledge funding from the European Union's Horizon 2020 research and innovation programme, European Research Council (grant agreement No 648295 to A.A.P.), and Marie Sklodowska-Curie action (grant agreement No. 748635 to S.M.A.), and the Deutsche Forschungsgemeinschaft (grants PO 1602/4-1 and 1602/5-1 to A.A.P.). K.K. acknowledges financial support from a Luther & Alice Hamlett Scholarship. Computational resources were provided by the Center for High Performance Computing at the TU Dresden. We appreciate the help from Dr. Anja U. B. Wolter and Sebastian Gass in magnetic measurements and technical support with computational resources in IFW Dresden by Ulrike Nitzsche.

The Dy-Sc nitride clusterfullerene Dy2ScN@C80-Ih exhibits slow relaxation of magnetization up to 76 K. Above 60 K, thermally-activated relaxation proceeds via the fifth-excited Kramers doublet with the energy of 1735 ± 21 K, which is the highest barrier ever reported for dinuclear lanthanide single molecule magnets.

Charge carrier transport under reverse voltage conditions is of major relevance in devices like organic photo-detectors, organic solar cells (tandem cells), organic light emitting diodes (generation contacts), and organic Zener diodes. We present organic pin-diodes comprising molecular doped layers of pentacene and C60 with an adjustable and reversible reverse breakdown behavior. We discuss the electric field and temperature dependence of the breakdown mechanism and propose a coherent charge transport scenario to describe the experimental findings. Within this model a field assisted tunneling of charge carriers over a rather large distance from valence to conductance states (and vice versa) governs the breakdown behavior. This is in accordance to experimental observations where charge carriers can overcome a layer thickness of 110 nm in the breakdown regime.

Graphene layers have been targeted in the last years as excellent host materials for sensing a remarkable variety of gases and molecules. Such sensing abilities can also benefit other important scientific fields such as medicine and biology. This has automatically led scientists to probe graphene as a potential platform for sequencing DNA strands. In this work, we use robust numerical tools to model the dynamic and electronic properties of molecular sensor devices composed of a graphene nanopore through which DNA molecules are driven by external electric fields. We performed molecular dynamic simulations to determine the relation between the intensity of the electric field and the translocation time spent by the DNA to pass through the pore. Our results reveal that one can have extra control on the DNA passage when four additional graphene layers are deposited on the top of the main graphene platform containing the pore in a 2 × 2 grid arrangement. In addition to the dynamic analysis, we carried electronic transport calculations on realistic pore structures with diameters reaching nanometer scales. The transmission obtained along the graphene sensor at the Fermi level is affected by the presence of the DNA. However, it is rather hard to distinguish the respective nucleobases. This scenario can be significantly altered when the transport is conducted away from the Fermi level of the graphene platform. Under an energy shift, we observed that the graphene pore manifests selectiveness toward DNA nucleobases.

Graphene|metal|ligand systems open a new realm in surface magnetochemistry. We show that by trapping metal atoms in the two-dimensional potential lattice of a graphene-ligand interface it is possible to build a chemical analogue of an optical lattice, a key setup in quantum information and strongly correlated systems. Employing sophisticated first-principles calculations, we studied electronic and dynamic properties of graphene|metal|ligand assemblies and showed that there is a general principle--spin-charge separation in π-d systems--that underlies the possibility of synthesizing and controlling such systems. We find that ligands can work as a local gate to control the properties of trapped metal atoms and can impose bosonic or fermionic character on such atomic nets, depending on the ligand's nature. Remarkably, the magnetization energy in such systems reaches record-high values of ca. 400 meV, which makes the respective magnetic phenomena utilizable at room temperature. Accompanied by spin polarization of the graphene π-conjugated system it leads to spin-valve materials and brings the realization of quantum computing one step closer.

Electron impact study of isomer S 6 -C 60 (CF 3 ) 12 resulted in the ionization efficiency curves (IEC) of singly and doubly charged positive ions and electron capture resonances for negative ones. The appearance energy (AE) values of the fragment C 60 (CF 3 ) n ions appear to correlate linearly with n down to n = 0 (C 60 ion) and its value for each additional act of CF 3 detachment rises by 2–3 eV. Observations of metastable ions reveal several cases of quasi-simultaneous detachment of two CF 3 -groups. Energy shift between ionization energy of S 6 -C 60 (CF 3 ) 12 and AE of the first fragment C 60 (CF 3 ) 11 ion was observed as 8.8 eV for singly and 8.3 eV for doubly charged ions. To rationalize this energy shift a DFT calculation of the orbital energies for the C 60 (CF 3 ) 12 + cation have been carried out and energy gap between HOMO and LUMO orbitals was estimated. High probability of occupation of the localized σ* orbital and CF 3 -elimination require four electron excitation and can be effected via a multielectron transition from the multitude of the densely distributed occupied π orbitals. The analogies with the fragmentation of C 60 with C 2 -elimination are mentioned.

Electron ionization, thermal surface ionization, matrix-assisted laser desorption/ionization, electrospray ionization, and electron capture mass spectrometric techniques have been applied to the analysis of mixtures of trifluoromethylated C 60 derivatives. For the first time ionization energy and electron affinity values for some of the molecules under investigation have been reported. These ionization characteristics have allowed to explain the differences in the results obtained by means of different mass spectral techniques.

Singly and doubly charged anions of fluorofullerenes were produced by means of electrospray ionization from acetonitrile/dichlorobenzene solutions of C 60 F 36 and C 60 F 48 doped with organic electron donors. These anions can be formally viewed as products of mono- and bis-substitution of fluorine atoms by electrons. Photoelectron spectra of C 60 F 34 2− and C 60 F 46 2− allow estimating the second electron affinity of the corresponding neutral species to be 2.4 (1) and 3.2 (1) eV, respectively. Quantum chemical calculations at the DFT level of theory suggest that the observed dianions form due to fluorine rearrangements on the carbon shell, which is governed by their high relative stability.

Non-equilibrium molecular dynamics simulations reveal that the thermal conductance of Si nanowires with periodic corrugations is lower than that of smooth wires with cross-sections equivalent to the constricted portions. This reduction in conductance is up to 30% and tends to plateau with increasing corrugation height. Spatially resolved temperature and heat current maps provide a microscopic understanding of this effect; we find that 80% of the heat current is carried through the constricted area even for high-amplitude corrugations. More importantly, we show that temperature gradient inversion and heat current vortices at the ridge peaks establish fundamental limits on maximum conductance reduction. (C) 2013 AIP Publishing LLC.

Charge transport characteristics of short double-strand (ds) DNA including mismatches are stud- ied within a methodology combining molecular dynamics (MD) simulations and electronic structure calculations based on a fragment orbital approach. Electronic parameters and transmission prob- abilities are computed along the MD trajectory. We find that in the course of the MD simulation the energetic position of frontier orbitals may be interchanged. As a result, the highest occupied molecular orbital (HOMO) can temporarily have a large weight on the backbones as a function of time. This shows that care must be taken when projecting the electronic structure onto effective low-dimensional model Hamiltonians to calculate transport properties. Further, the transport cal- culations indicate a suppression of the charge migration efficiency when introducing a single GT or AC mismatch in the DNA sequence.

Organic Zener diodes with a precisely adjustable reverse breakdown from -3 V to -15 V without any influence on the forward current-voltage curve are realized. This is accomplished by controlling the width of the charge depletion zone in a pin-diode with an accuracy of one nanometer independently of the doping concentration and the thickness of the intrinsic layer. The breakdown effect with its exponential current voltage behavior and a weak temperature dependence is explained by a tunneling mechanism across the HOMO-LUMO gap of neigh- boring molecules. The experimental data are confirmed by a minimal Hamiltonian model approach, including coherent tunneling and incoherent hopping processes as possible charge transport pathways through the effective device region., to appear in Nano Letters Article ASAP (2010)

In situ low-voltage aberration corrected transmission electron microscopy (TEM) observations of the dynamic entrapment of a C60 molecule in the saddle of a bent double-walled carbon nanotube is presented. The fullerene interaction is non-covalent, suggesting that enhanced π-π interactions (van der Waals forces) are responsible. Classical molecular dynamics calculations confirm that the increased interaction area associated with a buckle is sufficient to trap a fullerene. Moreover, they show hopping behavior in agreement with our experimental observations. Our findings further our understanding of carbon nanostructure interactions, which are important in the rapidly developing field of low-voltage aberration corrected TEM and nano-carbon device fabrication. © The Author(s) 2010.

Understanding and eventually controlling the high-temperature spectral emissivity of ceramic materials is important for a range of applications, including thermal barrier coatings. In this paper, we use ab initio density functional theory simulations to predict the emissivity of silica, zirconia and rare-earth oxides. High-temperature emissivity is dominated by processes with energies lower than the band gap of these materials and we focus on how dynamic and static features in the atomic structure of these materials (including defects, glasses and thermal fluctuations) enable transitions with desired energies. We find that neutral oxygen vacancies contribute significantly to the high emissivity of ZrO2. On the other hand, neutral point defects in α and amorphous silica fail to provide transitions with energies significantly below the band gap, explaining the low emissivity of this material. In the case of Sm2O3, we find that transitions between localized f-electron states as well as point defects contribute to its high emissivity. Interestingly, dynamical changes in electronic structure in samples taken from molecular dynamics simulations of molten materials lead to a significant increase in their emissivity.

Two isomers of metallofullerene Dy2S@C82 with sulfur-bridged Dy ions exhibit broad magnetic hysteresis with sharp steps at sub-Kelvin temperature. Analysis of the level crossing events for different orientations of a magnetic field showed that even in powder samples, the hysteresis steps caused by quantum tunneling of magnetization can provide precise information on the strength of intramolecular Dy⋯Dy inter-actions. A comparison of different methods to determine the energy difference between ferromagnetic and antiferromagnetic states showed that sub-Kelvin hysteresis gives the most robust and reliable values. The ground state in Dy2S@C82 has ferromagnetic coupling of Dy magnetic moments, whereas the state with antiferromagnetic coupling in C s and C 3v cage isomers is 10.7 and 5.1 cm−1 higher, respectively. The value for the C s isomer is among the highest found in metallofullerenes and is considerably larger than that reported in non-fullerene dinuclear molecular magnets. Magnetization relaxation times measured in zero magnetic field at sub-Kelvin temperatures tend to level off near 900 and 3200 s in C s and C 3v isomers. These times correspond to the quantum tunneling relaxation mechanism, in which the whole magnetic moment of the Dy2S@C82 molecule flips at once as a single entity.