Density functional theory and density functional perturbation theory are used to investigate the electronic and vibrational properties of TiS$_2$. Within the local density approximation the material is a semi-metal both in the bulk and in the monolay
er form. Most interestingly we observe a Kohn anomaly in the bulk phonon dispersion, which turns into a charge density wave instability when TiS$_2$ is thinned to less than four monolayers. Such charge density wave phase can be tuned by compressive strain, which appears to be the control parameter of the instability.
Scanning tunneling microscopy (STM) can be used to detect inelastic spin transitions in magnetic nano-structures comprising only a handful of atoms. Here we demonstrate that STM can uniquely identify the electrostatic spin crossover effect, whereby t
he exchange interaction between two magnetic centers in a magnetic molecule changes sign as a function of an external electric field. The fingerprint of such effect is a large drop in the differential conductance as the bias increases. Crucially in the case of a magnetic dimer the spin crossover transition inverts the order between the ground state and the first excited state, but does not change their symmetry. This means that at both sides of the conductance drop associated to the spin crossover transition there are two inelastic transition between the same states. The corresponding conductance steps split identically in a magnetic field and provide a unique way to identify the electrostatic spin crossover.
Spin crossover molecules have recently emerged as a family of compounds potentially useful for implementing molecular spintronics devices. The calculations of the electronic properties of such molecules is a formidable theoretical challenge as one ha
s to describe the spin ground state of a transition metal as the legand field changes. The problem is dominated by the interplay between strong electron correlation at the transition metal site and charge delocalization over the ligands, and thus it fits into a class of problems where density functional theory may be inadequate. Furthermore, the crossover activity is extremely sensitive to environmental conditions, which are difficult to fully characterize. Here we discuss the phase transition of a prototypical spin crossover molecule as obtained with diffusion Monte Carlo simulations. We demonstrate that the ground state changes depending on whether the molecule is in the gas or in the solid phase. As our calculation provides a solid benchmark for the theory we then assess the performances of density functional theory. We find that the low spin state is always over-stabilized, not only by the (semi-)local functionals, but even by the most commonly used hybrids (such as B3LYP and PBE0). We then propose that reliable results can be obtained by using hybrid functionals containing about 50% of exact-exchange.
The conductance profiles of magnetic transition metal atoms, such as Fe, Co and Mn, deposited on surfaces and probed by a scanning tunneling microscope (STM), provide detailed information on the magnetic excitations of such nano-magnets. In general t
he profiles are symmetric with respect to the applied bias. However a set of recent experiments has shown evidence for inherent asymmetries when either a normal or a spin-polarized STM tip is used. In order to explain such asymmetries here we expand our previously developed perturbative approach to electron-spin scattering to the spin- polarized case and to the inclusion of out of equilibrium spin populations. In the case of a magnetic STM tip we demonstrate that the asymmetries are driven by the non-equilibrium occupation of the various atomic spin-levels, an effect that reminds closely that electron spin-transfer. In contrast when the tip is not spin-polarized such non-equilibrium population cannot be build up. In this circumstance we propose that the asymmetry simply originates from the transition metal ion density of state, which is included here as a non-vanishing real component to the spin-scattering self-energy.
We report on a density functional theory study demonstrating the coexistence of weak ferromagnetism and antiferroelectricity in boron-deficient MgB6. A boron vacancy produces an almost one dimensional extended molecular orbital, which is responsible
for the magnetic moment formation. Then, long-range magnetic order can emerge from the overlap of such orbitals above percolation threshold. Although there is a finite density of states at the Fermi level, the localized nature of the charge density causes an inefficient electron screening. We find that the Mg ions can displace from the center of their cubic cage, thus generating electrical dipoles. In the ground state these order in an antiferroelectric configuration. If proved experimentally, this will be the first material without d or f electrons displaying the coexistence of magnetic and electric order.
Recent experimental advances in scanning tunneling microscopy make the measurement of the conductance spectra of isolated and magnetically coupled atoms on nonmagnetic substrates possible. Notably these spectra are characterized by a competition betw
een the Kondo effect and spin-flip inelastic electron tunneling. In particular they include Kondo resonances and a logarithmic enhancement of the conductance at voltages corresponding to magnetic excitations, two features that cannot be captured by second order perturbation theory in the electron-spin coupling. We have now derived a third order analytic expression for the electron-spin self-energy, which can be readily used in combination with the non-equilibrium Greens function scheme for electron transport at finite bias. We demonstrate that our method is capable of quantitative description the competition between Kondo resonances and spin-flip inelastic electron tunneling at a computational cost significantly lower than that of other approaches. The examples of Co and Fe on CuN are discussed in detail.
We present a theoretical study of the spin transport properties of mono-atomic magnetic chains with a focus on the spectroscopical features of the I-V curve associated to spin-flip processes. Our calculations are based on the s-d model for magnetism
with the electron transport treated at the level of the non-equilibrium Greens function formalism. Inelastic spin-flip scattering processes are introduced perturbatively via the first Born approximation and an expression for the associated self-energy is derived. The computational method is then applied to describe the I-V characteristics and its derivatives of one dimensional chains of Mn atoms and the results are then compared to available experimental data. We find a qualitative and quantitative agreement between the calculated and the experimental conductance spectra. Significantly we are able to describe the relative intensities of the spin excitation features in the I-V curve, by means of a careful analysis of the spin transition selection rules associated to the atomic chains.
We demonstrate that the critical temperature for valence tautomeric interconversion in Cobalt dioxolene complexes can be significantly changed when a static electric field is applied to the molecule. This is achieved by effectively manipulating the r
edox potential of the metallic acceptor forming the molecule. Importantly our accurate density functional theory calculations demonstrate that already a field of 0.1 V/nm, achievable in Stark spectroscopy experiments, can produce a change in the critical temperature for the interconversion of 20 K. Our results indicate a new way for switching on and off the magnetism in a magnetic molecule. This offers the unique chance of controlling magnetism at the atomic scale by electrical means.
The possibility that the apparent room temperature ferromagnetism, often measured in Co-doped ZnO, is due to uncompensated spins at the surface of wurtzite CoO nanoclusters is investigated by means of a combination of density functional theory and Mo
nte Carlo simulations. We find that the critical temperature extracted from the specific heat systematically drops as the cluster size is reduced, regardless of the particular cluster shape. Furthermore the presence of defects, in the form of missing magnetic sites, further reduces $T_mathrm{C}$. This suggests that even a spinodal decomposed phase is unlikely to sustain room temperature ferromagnetism in ZnO:Co.
We investigate the electron transport properties of a model magnetic molecule formed by two magnetic centers whose exchange coupling can be altered with a longitudinal electric field. In general we find a negative differential conductance at low temp
eratures originating from the different scattering amplitudes of the singlet and triplet states. More interestingly, when the molecule is strongly coupled to the leads and the potential drop at the magnetic centers is only weakly dependent on the magnetic configuration, we find that there is a critical voltage V_C at which the current becomes independent of the temperature. This corresponds to a peak in the low temperature current noise. In such limit we demonstrate that the quadratic current fluctuations are proportional to the product between the conductance fluctuations and the temperature.
