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The opening of a quantum confinement gap in nanostructured graphene yields extremely sensitive photodetectors, with electrical noise equivalent power lower than 1015 W Hz0.5 at temperatures below 3 K, for detection of radiation in a very broad frequency range, including ultraviolet, visible and terahertz. Here we demonstrate the operation of these detectors in the presence of magnetic field as high as 7 T, paving the way to in situ spectroscopy of molecular nanomagnets.

The opening of a quantum confinement gap in nanostructured graphene yields extremely sensitive photodetectors, with electrical noise equivalent power lower than 1015 W Hz0.5 at temperatures below 3 K, for detection of radiation in a very broad frequency range, including ultraviolet, visible and terahertz. Here we demonstrate the operation of these detectors in the presence of magnetic field as high as 7 T, paving the way to in situ spectroscopy of molecular nanomagnets.

The opening of a quantum confinement gap in nanostructured graphene yields extremely sensitive photodetectors, with electrical noise equivalent power lower than 1015 W Hz0.5 at temperatures below 3 K, for detection of radiation in a very broad frequency range, including ultraviolet, visible and terahertz. Here we demonstrate the operation of these detectors in the presence of magnetic field as high as 7 T, paving the way to in situ spectroscopy of molecular nanomagnets.

This thesis theoretically studies the dynamics of molecular magnets under electrical control. Molecular magnets are nanoscale magnets that can, e.g., consist of single-molecules or single-atoms. In these magnets, the electronically mediated exchange and transport can be controlled by external fields. In this thesis, we study the effect of electrical control and voltage pulses on the transport properties, spin dynamics and the exchange of the molecular magnets. Nonequilibrium Green's functions is the method used to describe the underlying electronic structure of the magnetic molecule. The studied systems consists of single-molecule magnets in a tunnel junction between metallic leads. Here, the transport characteristics are derived for charge, spin and heat currents in the system for time-dependent voltage pulses. Furthermore, a generalized spin equation of motion is derived for the molecular spin moment, using nonequilibrium field theory. The equation of motion incorporates nonequilibrium conditions and is of nonadiabatic character. The effective model for the spin moment can be decomposed into effective magnetic field, isotropic Heisenberg interaction, and anisotropic Ising and Dzyaloshinskii-Moriya interactions. These effective fields depends on the electronic structure of the molecule and can be controlled by, e.g., gate and bias voltages. The thesis encompasses studies on the effect of a sudden on-set of a voltage pulse for a single-molecule magnet and its effect on the spin dynamics and transport properties of the molecule. Different approximations schemes for the spin equation of motion and their regimes of validity are investigated. Moreover, spin-dependent signatures in the heat transport characteristics of the single-molecule magnet are connected to the dynamics of the molecular spin moment. A phase induced switching mechanism of the molecular moment is shown for voltage pulses of varying temporal length. In the stationary limit, it is shown that one can ele

The magnetic instability at the front of the spin avalanche in a crystal of molecular magnets is considered. This phenomenon reveals similar features with the Darrieus-Landau instability, inherent to classical combustion flame fronts. The instability growth rate and the cutoff wavelength are investigated with respect to the strength of the external magnetic field, both analytically in the limit of an infinitely thin front and numerically for finite-width fronts. The presence of quantum tunneling resonances is shown to increase the growth rate significantly, which may lead to a possible transition from deflagration to detonation regimes. Different orientations of the crystal easy axis are shown to exhibit opposite stability properties. In addition, we suggest experimental conditions that could evidence the instability and its influence on the magnetic deflagration velocity.

We propose a Berry phase effect on the chiral degrees of freedom of a triangular magnetic molecule. The phase is induced by adiabatically varying an external electric field in the plane of the molecule via a spin-electric coupling mechanism present in these frustrated magnetic molecules. The Berry phase effect depends on spin-orbit interaction splitting and on the electric dipole moment. By varying the amplitude of the applied electric field, the Berry phase difference between the two spin states can take any arbitrary value between zero and π, which can be measured as a phase shift between the two chiral states by using spin-echo techniques. Our result can be used to realize an electric-field-induced geometric phase-shift gate acting on a chiral qubit encoded in the ground-state manifold of the triangular magnetic molecule.

Geometric manipulation of a quantum system offers a method for fast, universal, and robust quantum information processing. Here, we propose a scheme for universal all-geometric quantum computation using non-adiabatic quantum holonomies. We propose three different realizations of the scheme based on an unconventional use of quantum dot and single-molecule magnet devices, which offer promising scalability and robust efficiency.

Density functional theory calculations on the neutral [Mn-12](0) molecular magnet and super-reduced [Mn-12](8-) cluster were employed to investigate the experimental geometrical changes observed during discharging in a molecular cluster battery. It was found that for relevant low-spin states the eight electrons added in [Mn-12](8-) are mainly added to the outer eight Mn atoms, causing elongation of the bonds between outer Mn and their surrounding O atoms, while the inner Mn-4 cluster is less affected by the reduction. [GRAPHICS] Schematic representation of the spin density of the neutral [Mn-12](0) cluster and its super-reduced state [Mn-12](8-), for which several possible spin states were found.

We present a comprehensive study of Er(trensal) single-ion magnets deposited in ultrahigh vacuum onto metallic surfaces. X-ray photoelectron spectroscopy reveals that the molecular structure is preserved after sublimation, and that the molecules are physisorbed on Au(111) while they are chemisorbed on a Ni thin film on 0(100) single-crystalline surfaces. X-ray magnetic circular dichroism (XMCD) measurements performed on Au(111) samples covered with molecular monolayers held at temperatures down to 4 K suggest that the easy axes of the strongly anisotropic molecules are randomly oriented. Furthermore XMCD indicates a weak antiferromagnetic exchange coupling between the single-ion magnets and the ferromagnetic Ni/Cu(100) substrate. For the latter case, spin-Hamiltonian fits to the XMCD M(H) suggest a significant structural distortion of the molecules. Scanning tunneling microscopy reveals that the molecules are mobile on Au(111) at room temperature, whereas they are more strongly attached on Ni/Cu(100). X-ray photoelectron spectroscopy results provide evidence for the chemical bonding between Er(trensal) molecules and the Ni substrate. Density functional theory calculations support these findings and, in addition, reveal the most stable adsorption configuration on Ni/Cu(100) as well as the Ni-Er exchange path. Our study suggests that the magnetic moment of Er(trensal) can be stabilized via suppression of quantum tunneling of magnetization by exchange coupling to the Ni surface atoms. Moreover, it opens up pathways toward optical addressing of surface-deposited single-ion magnets.

This dissertation investigates theoretically electric control of the magnetic properties of molecular magnets. Two classes of magnetic molecules are considered. The first class consists of molecules that are spin frustrated. As a consequence of the frustration, the ground-state manifold of these molecules is characterized by states of different spin chirality, which can be coupled by an external electric field. Electric control of these spin states can be used to encode and manipulate quantum information. The second class comprises molecules known as single-molecule magnets, which are characterized by a high spin and a large magnetic anisotropy. Here the main goal is to control and manipulate the magnetic properties, such as the anisotropy barriers, by adding and subtracting individual electrons, as achieved in tunneling transport. Papers I, II and III deal with spin-electric coupling in spin frustrated molecules. Spin density functional theory is used to evaluate the parameters that control the strength of this coupling. Paper I reports the electronic and magnetic properties of the triangular antiferromagnet Cu3. It is found that an external electric field couples to the spin chirality of the system. The strength of this coupling is large enough to allow efficient spin-electric manipulation with electric fields generated by a scanning tunneling microscope. Paper II investigates the zero-field splitting in the ground-state manifold of the triangular Cu3 molecular magnet caused by the Dzyaloshinskii-Moriya (DM) interaction. It employs a Hubbard model approach to elucidate the connection between the spin-orbit and the DM interaction. It is shown that the DM interaction constant D can be expressed in terms of the microscopic Hubbard-model parameters, which are calculated by first principles methods. Paper III investigates systematically the spin-electric coupling in several triangular molecular magnets, such as V3 and Cu3O, and its dependence on different types of magn

We report the first band structure calculations of the quasi-one-dimensional [MnTPP][TCNE] compounds (TPP = meso-tetraphenylporphyrinato, TCNE = tetracyanoethylene), based on Density Functional Theory (DFT) methods, in order to interpret the magnetic ordering in these prototypic systems. We compare and contrast the results of broken-symmetry DFT calculations for extended systems, with periodic boundary conditions, and for finite systems, magnetic dimers modeling the actual molecular magnets. By varying systematically the main angles, we are able to determine the geometry dependence of the exchange interaction. Structure-properties correlations in these charge-transfer salts reveal the determinant role of the Mn-(N C) TCNE bond angle on the strength of the ferrimagnetic coupling between the S(1) = 2 spin located on the Mn(III)-porphyrin donor and the S(2) = 1/2 spin positioned on the cyanocarbon acceptor. When the Mn-(N C) TCNE angle is decreased, the intrachain magnetic coupling strengthens, correlated with the increase in the d(z)(2) - pi* orbital overlap. The exchange coupling constants resulting from DFT calculations of extended systems, with periodic boundary conditions, were found to be consistent with those obtained for the dimers, but systematically smaller. The exchange constants vary strongly with the functional used, hybrid functionals such as B3LYP leading to results that better correlate with the experimental mean-field critical temperatures. The coupling constant varies significantly with the type of broken-symmetry approach, depending on the overlap between magnetic orbitals, but weakly on the basis set once polarization effects are included. The electronic structure calculations for the extended systems provide a density of states consistent with the energy spectrum of the corresponding dimer, allowing for an intuitive explanation of the intrachain ferrimagnetic ordering.

This thesis deals with single-electron tunneling in transistor-like devices in which the central electrode is either a metal nanoparticle (possibly ferromagnetic) or a molecular magnet. The investigated systems split into two different categories, depending on the size of the central island. The smaller islands, such as ultrasmall magnetic metal nanoparticles and Mn12 molecular magnets, are studied in the first part of the thesis (Papers I-III). The larger metal islands, both ferromagnetic and nonmagnetic, are studied in the second part (Papers IV-V). Different size regimes result in different types of energy spectra (discrete for the small and continuous for the large islands), and thus in different ways of calculating the electric current through the system. All the systems are investigated within the regime of weak coupling to the external leads. In this regime, quantum transport is characterized by the physics of Coulomb blockade and can be described theoretically by sequential-tunneling rate equations. Papers I-III are purely theoretical, while Papers IV-V consist both of experimental and theoretical parts, the theoretical ones belonging explicitly to this thesis. In Paper I we present a theory of quantum transport through a small ferromagnetic nanoparticle in which particle-hole excitations are coupled to spin collective modes. For strong electron-magnon coupling, we find that the tunneling conductance as a function of bias voltage is characterized by a large and dense set of resonances. Their magnetic field dependence in the large-field regime is linear, with slopes of the same sign. Both features are in agreement with tunneling experiments on similar nanoparticles. Papers II and III deal with transport through a Mn12 molecule. The many-body energy spectrum (composed of spin multiplets) and spin-dependent inter-level transition matrix elements used in transport calculations are determined by means of spin density-functional theory (SDFT). This theory provides

Reaction of GdCl3(H2O)(6) and 1,3-bis(pyridin-4-yl)propane-1,3-dione in methanol with an excess of triethylamine produced a monodimensional polymeric chain [Gd(p-dppd)(3)-(H2O)](infinity), whereas treatment of HoCl3(H2O)(6) with 1,3-bis(pyridin-2-yl)propane-1,3-dione yielded a trinuclear cluster [Ho-3(o-dppd)(3)(mu(3)-OH)(2)(H2O)(4)Cl-2]Cl-2. The compounds were characterised by elemental analysis, IR spectroscopy and magnetism, and their structures were investigated by X-ray crystallography. The 8.20-mu(B) magnetic-moment value of the polymeric [Gd(p-dppd)(3)(H2O)](infinity), between 300 and 20 K, and the magnetisation isotherms (2-20 K; fields 0-5 T), are in agreement with essentially uncoupled single-ion Gd3+ f(7) centres, a small decrease in mu(eff) below 20 K being indicative of zero-field splitting. A temperature-dependent dc-susceptibility and magnetisation investigation of the trinuclear (tri-angular) [Ho-3(o-dppd)(3)(mu(3)-OH)(2)(H2O)(4)Cl-2]Cl-2 revealed that spin-orbit and ligand-field effects on the Ho3+ centres, leading to thermal depopulation of Zeeman levels and consequent decreases in mu(eff) values with decreasing temperature, are occurring rather than weak intra-cluster antiferromagnetic coupling. Frequency- and temperature-dependent ac-susceptibility studies on this homometallic Ho3+ cluster did not show clear evidence for slow magnetisation reversal, characteristic of single-molecule magnetism (SMM), and this contrasts with such behaviour recently reported, elsewhere, for a Dy3+ triangle having the same core structure but with different chelating {O,O} ligands.

This grant supported two postdoctoral fellows who worked on a variety of problems related to the prediction of materials properties from first principles. Projects included optimization of a parallel code for calculating magnetic anisotropy parameters; application of the method to molecule-based magnets; studies of structural and electronic properties of interfaces between ferromagnets and semiconductors; and investigation and quantification of vibrational contributions to the Van der Waals interaction between molecules.

This contribution sununarizes sorne of the theoretical strategies developed in the recent past to study systems with an increasing size and complexity by means of accurate extended CI calculations. Sorne examples ofthe capabilities of these strategies are also given, conceming the evaluation of magnetic exchange constants, as well as many other effective interaction parameters, such as hopping integrals, on-site Coulomb repulsion, four-body exchange term, ... on 2D and spin-ladder cuprates, polyoxovanadates, as well as polynuclear molecular compounds

For V[TCNE](x) y(solvent) (TCNE = tetracyanoethylene), the first room-temperature molecular magnet, the mechanism for the strong exchange coupling in the family M[TCNE](x) y(solvent) has remained an open question. hi an attempt to understand the magnetic ordering in these systems we performed density functional theory calculations on a six coordinated vanadium complex with four TCNE molecules. For this model system we find that the ferrimagnetic spin configuration is energetically more favorable and that the highest single-occupied orbital has pi* character.

We report on the studies of the influence of hydrostatic pressure (up to 1.2 GPa) on dynamic electronic structure phenomena in 3d transition metal coordination compounds by Mossbauer and magnetic susceptibility measurements. We present examples of mononuclear spin crossover compounds of iron(II) and chromium(H), dinuclear complexes of iron(H) exhibiting coexistence of intramolecular antiferromagnetic coupling and thermal spin crossover, ID, 2D and 3D spin crossover coordination polymers of iron(II), a valence tautomeric system of cobalt(II) showing thermal transition from a high-spin [Co-II(semiquinone)] to a low-spin [Co-III(catecholate)] species on lowering the temperature, a photo-magnetically active Prussian blue-type system with temperature- and pressure-induced electron transfer, and finally an iron-TCNE compound with ferromagnetic interaction.

We report quantum chemical calculations providing the spin density of two molecules used as building blocks for molecular magnets: tetracyanoethylene (TCNE) and hexacyanobutadiane (HCBD). Very good electron-acceptors, both molecules are used as building blocks of novel, nanostructured charge-transfer salts with magnetic properties. We investigate by means of perturbation theory (MP2) and density functional theory (DFT) the electronic structure of the neutral molecules as well as of the anionic radicals, and we discuss the roles of the various factors influencing the electron-accepting properties of these molecules. We compare and contrast the charge-acceptor and magnetic properties of both molecules, based on the correlation between their structures, electronic spectrum and spin densities., QC 201111003. Conference: 6th International Balkan Workshop on Applied Physics. Constanta, ROMANIA. JUL 05-07, 2005

The notion of artificial atom relies on the capability to change the number of carriers one by one in semiconductor quantum dots, and the resulting changes in their electronic structure. Organic molecules with transition metal atoms that have a net magnetic moment and display hysteretic behaviour are known as single molecule magnets (SMM). The fabrication of CdTe quantum dots chemically doped with a controlled number of Mn atoms and with a number of carriers controlled either electrically or optically paves the way towards a new concept in nanomagnetism: the artificial single molecule magnet. Here we study the magnetic properties of a Mn-doped CdTe quantum dot for different charge states and show to what extent they behave like a single molecule magnet.

Di-n-propyl dicyanofumarate (DnPrDCF) and di-isopropyl dicyanofumarate (DiPrDCF) have been used as one-electron acceptors in the synthesis of charge-transfer salt magnets with decamethylmetallocenes, MCp*2 (M = Mn, Cr). Salts of each acceptor with each metallocene have been characterized and the structures of the chromium analogues have been solved. The two acceptors are structurally similar to dimethyl dicyanofumarate (DMeDCF) and diethyl dicyanofumarate (DEtDCF), which have been previously studied and found to form charge-transfer salt magnets with the aforementioned decamethylmetallocenes. A typical structural motif is present in these types of charge-transfer salts which allows for the comparison of magnetic properties based on the length or size of the alkyl group of the dialkyl dicyanofumarate. Some trends were established based on the magnetic properties of the homologous series including ordering temperature/bulkiness of the alkyl group and intrastack distances/theta values. Correlation of magnetic and structural properties may give some insight into "through-space" magnetic coupling, of which little is understood.

The funds have been used entirely to support a CUNY Ph.D. student who has conducted extensive measurements of the magnetic properties of molecular magnets. Most of the attention was directed toward experimentally studying quantum tunneling of magnetism in Mn12-acetate.