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The ultrastrong coupling of single-electron tunneling and nanomechanical motion opens exciting opportunities to explore fundamental questions and develop new platforms for quantum technologies. We have measured and modeled this electromechanical coupling in a fully-suspended carbon nanotube device and report a ratio of $g_\mathrm{m}/\omega_\mathrm{m} = 2.72 \pm 0.14$, where $g_\mathrm{m}/2\pi = 0.80\pm 0.04$ GHz is the coupling strength and $\omega_\mathrm{m}/2\pi=294.5$ MHz is the mechanical resonance frequency. This is well within the ultrastrong coupling regime and the highest among all other electromechanical platforms. We show that, although this regime was present in similar fully-suspended carbon nanotube devices, it went unnoticed. Even higher ratios could be achieved with improvement on device design., Comment: 13 pages, 11 figures This new version contains the same model and analysis but applied to new experimental data compared to the previous version. This is why the coupling ratio is now 2.72 instead of 1.3 and most of the experimental parameters have changed

Electron spin qubits in molecular systems offer high reproducibility and the ability to self-assemble into larger architectures. However, interactions between neighboring qubits are "always on," and although the electron spin coherence times can be several hundred microseconds, these are still much shorter than typical times for nuclear spins. Here we implement an electron-nuclear hybrid scheme which uses coherent transfer between electron and nuclear spin degrees of freedom in order to both effectively turn on or off interqubit coupling mediated by dipolar interactions and benefit from the long nuclear spin decoherence times (T2n). We transfer qubit states between the electron and N15 nuclear spin in N15@C60 with a two-way process fidelity of 88%, using a series of tuned microwave and radio frequency pulses and measure a nuclear spin coherence lifetime of over 100 ms. © 2011 American Physical Society.

We show STM images that display fourfold symmetry hole states around point defects on the NiO (001) surface that are a number of times larger than the unit cell. The observation of these spatially delocalised states demonstrates that a reasonable description of bonding in NiO must go beyond a simple ionic interaction. This is achieved through the implementation of the LSDA + U model in simulating EELS spectra of the oxygen K edge. By using this method an accurate value for the electron-electron interaction parameter U of 6.2 eV can be determined, and hybridisation between O 2p and Ni 3d orbitals can be seen which points to the existence of a significant covalent contribution to bonding in NiO.

We report here on the time evolution of photoconductivity under continuous illumination in nanocrystalline TiO2 samples prepared by a sol gel method, and also on the conductivity decay once the illumination is switched off. We observe strong dependence of the photoconductivity on the illumination intensity for both processes. It is found that the conductivity decay after high-intensity illumination is slower than after low-intensity illumination, and we have attempted to explain these experimental results using a model involving hole trapping-detrapping processes.

The mapping of chemically heterogeneous regions in PEA/PS film was achieved by reacting the film in a low pH environment and analyzing by AFM. In 70:30 blend, the domains were identified as PS rich regions and matrix as the PEA rich regions, based on AFM images, FTIR measurements, and chemical modification study. During the course of hydrolysis of PEA, pits were formed in isolated regions of the matrix, as characterized by AFM.

Photoluminescence spectroscopic measurements have allowed the acquisition of high resolution spectra at low temperature for the endohedral metallofullerenes, Er2 @ C82 (isomer I) and Er2 C2 @ C82 (isomer I). The characteristic emission in the 1.5-1.6 μm region corresponds to the 4 I13 / 2 (m) → 4 I15 / 2 (n) transitions of the Er3 + ion for both molecules. The emission arising from Er2 C2 @ C82 (I) appears acuminated (narrow lines that taper to a point) when compared with that of Er2 @ C82 (I). The Er2 C2 @ C82 (I) emission linewidths are comparable to those found in crystals, making this molecule of interest for applications where accessible, well-defined quantum states are required. © 2009 Elsevier B.V. All rights reserved.

The crystallization process in poly(ethylene terephthalate) (PET) films was followed using both scanning probe microscopy, and FTIR spectroscopy in reflection mode. Nano-meter scale grains of the average size of 10 to 40 nm were observed after annealing of PET films in situ under scanning probe microscope at elevated temperatures above the Tg and identified to be the crystalline phase on the basis of both SPM and IR spectroscopy.

Endohedral fullerenes encapsulating single nitrogen atoms (N@C60 and N@C70) are spin active, due to the presence of unpaired nitrogen electron spins. High reactivity of nitrogen leads to a poor stability of these molecules at elevated temperatures, drastically restricting possibilities of their chemical treatment. In this study, N@C60 and N@C70 in toluene solutions were subjected to high pressure (HP) up to 0.8 GPa and annealed for 1, 2, 10 or 24 h, at either room or elevated temperatures. In each case the number of surviving molecules was evaluated based on the relative spin counts of ex situ EPR spectra obtained before and after the treatment. The enhanced thermal stability of N@C60 and N@C70 under HP conditions is attributed to inhibited formation of azabridges, which under normal pressure facilitate the escape of nitrogen from the fullerene cage. © 2009 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim.

We examine environmental effects of surrounding nuclear spins on the electron spin relaxation of the N@C60 molecule (which consists of a nitrogen atom at the centre of a fullerene cage). Using dilute solutions of N@C60 in regular and deuterated toluene, we observe and model the effect of translational diffusion of nuclear spins of the solvent molecules on the N@C60 electron spin relaxation times. We also study spin relaxation in frozen solutions of N@C60 in CS2, to which small quantities of a glassing agent, S2Cl2 are added. At low temperatures, spin relaxation is caused by spectral diffusion of surrounding nuclear 35Cl and 37Cl spins in the S2Cl2, but nevertheless, at 20 K, T2 times as long as 0.23 ms are observed., Comment: 7 pages, 6 figures

This thesis focuses on fast and sensitive readout of mechanical motion to answer questions in the foundations of physics. By coupling a silicon nitride membrane to different readout cavities I have explored the thermodynamic costs of time keeping and carried out a bench-top experiment to put bounds on the theory of classical channel gravity. In order to reach these aims, I have developed real time displacement measurements of the motion of a nm-thick silicon nitride membrane. For this, a lumped element LC circuit is capacitively coupled to the metallized silicon nitride membrane. This approach allows for the estimation of key parameters without the need for frequency matching or cryogenic cooling. I demonstrate electromechanical characterization of several mechanical modes of the silicon nitride membrane. In addition, I show optomechanically induced transparency on chip in this high loss cavity. The real time displacement measurements of the membrane’s motion allow the device to be operated as a thermomechanical clock and to be used to investigate a thermodynamic trade-off in a nanoscale clock. The system follows a trade-off derived for the idealised quantum setting, indicating a fundamental universality of a relation previously predicted to hold in the quantum regime. This system is compatible with cryogenic temperatures, allowing me to actuate and detect the resonance frequency of a membrane at low temperatures. At these temperatures, one can perform tests of the theory of classical channel gravity which models the gravitational interaction as a classical measurement channel. The theory predicts a density dependent heating effect due to the gravitational interactions within the membrane itself. Bounds could be put on this theory using a membrane capacitively coupled to a 3D superconducting microwave cavity which has a much higher Q factor than the LC circuit. With this cavity, I achieved 10 mHz resolution of the mechanical signal, which is required for an accurate estimation of the mode temperature and thus to put bounds on the theory.

This thesis is concerned with vibrational effects in resonant charge transport through molecular junctions. The first research chapter examines the role of non-equilibrium vibrational dynamics in charge transport through a molecular double quantum dot. We demonstrate that non-equilibrium vibrational effects in such a system can result in current suppression and negative differential conductance. The second research chapter describes how vibrational dynamics can be incorporated into the theoretical description of molecular junctions based on a Pariser-Parr-Pople Hamiltonian. As a case study, we consider a prototypical spiro-conjugated molecular junction. We demonstrate that the transport properties of this system are governed by an interplay between destructive quantum interference and Jahn-Teller distortion (an effect of vibronic origin). In the next chapter, we examine the role of collective vibrational coupling in charge transport through a two-site molecular system akin to the one considered in the first research chapter. We demonstrate that such interactions can significantly enhance the efficiency of transport through such a system, and also take this opportunity to analyse various theoretical descriptions of electron-vibrational interactions commonly used in the transport setting. In the fourth research chapter, we consider a single-site molecular system coupled to a collection of thermalised vibrational modes. We formulate a relatively simple yet powerful theoretical framework describing charge transport through such a system. We recover the Marcus and Landauer-Buttiker theories of transport in the limit of high-temperature and vanishing electron-vibrational coupling, respectively. We further show how lifetime broadening can be consistently incorporated into Marcus theory, and we derive a low-temperature correction to the semi-classical Marcus hopping rates. In the last research chapter, elements of this framework are applied to experimental measurements of charge transport through zinc-porphyrin molecular junctions. This joint experimental-theoretical study provides, for the first time, a full understanding of the resonant transport regime of graphene-based molecular junctions.

Quantum information processing (QIP), which inherently utilizes quantum mechanical phenomena to perform information processing, may outperform its classical counterpart at certain tasks. As one of the physical implementations of QIP, the electron-spin based architecture has recently attracted great interests. Endohedral fullerenes with unpaired electrons, such as N@C60, are promising candidates to embody the qubits because of their long spin decoherence time. This thesis addresses several fundamental aspects of the strategy of engineering the N@C60 molecules for applications in QIP.Chemical functionalization of N@C60 is investigated and several different derivatives of N@C60 are synthesized. These N@C60 derivatives exhibit different stability when they are exposed to ambient light in a degassed solution. The cyclopropane derivative of N@C60 shows comparable stability to pristine N@C60, whereas the pyrrolidine derivatives demonstrate much lower stability. To elucidate the effect of the functional groups on the stability, an escape mechanism of the encapsulated nitrogen atom is proposed based on DFT calculations. The escape of nitrogen is facilitated by a 6-membered ring formed in the decomposition of the pyrrolidine derivatives of N@C60. In contrast, the 4-membered ring formed in the cyclopropane derivative of N@C60 prohibits such an escape through the addends.Two N@C60-porphyrin dyads are synthesized. The dyad with free base porphyrin exhibits typical zero-field splitting (ZFS) features due to functionalization in the solid-state electron spin resonance (ESR) spectrum. However, the nitrogen ESR signal in the second dyad of N@C60 and copper porphyrin is completely suppressed at a wide range of sample concentrations. The dipolar coupling between the copper spin and the nitrogen spins is calculated to be 27.0 MHz. To prove the presence of the encapsulated nitrogen atom in the second dyad, demetallation of the copper porphyrin moiety is carried out. The recovery of approximately 82% of the signal intensity confirms that the dipolar coupling suppresses the ESR signal of N@C60.To prepare ordered structure of N@C60, the nematic matrix MBBA is employed to align the pyrrolidine derivatives of N@C60. Orientations of these derivatives are investigated through simulation of their ESR spectra. The derivatives with a –CH3 or phenyl group derived straightforward from the N-substituent of the pyrrolidine ring are preferentially oriented based on their powder-like ESR spectra in the MBBA matrix. An angle of about is also found between the directors of fullerene derivatives and MBBA. In contrast, the derivatives with a –CH₂ group inserted between the phenyl group and the pyrrolidine ring are nearly randomly distributed in MBBA. These results illustrate the applicability of liquid crystal as a matrix to align N@C60 derivatives for QIP applications.

Information processors that harness quantum mechanics may be able to outperform their classical counterparts at certain tasks. Quantum information processing (QIP) can utilize the quantum mechanical phenomenon of entanglement to implement quantum algorithms. Endohedral fullerenes, where atoms, ions or clusters are trapped in a carbon cage, are a class of nanomaterials that show great promise as the basis for a solid-state QIP architecture. Some endohedral fullerenes are spin–active, and offer the potential to encode information in their spin-states. This thesis addresses the challenges of how to engineer the components of a scalable QIP architecture based on endohedral fullerenes. It focuses on the synthesis and characterization of molecules which may, in the future, permit the demonstration of entanglement; the optical read-out of quantum states; and the creation of quasi-one-dimensional molecular arrays. Due to its long spin decoherence time, N@C60 is the selected as the basic molecular unit for ‘coupled’ fullerene pairs, molecular systems for which it may be possible to demonstrate entanglement. To this end, isolated fullerene pairs, in the form of spin-bearing fullerene dimers, are created. This begins with the processing of N@C60 at the macroscale and leads towards the synthesis of 15N@C60-15N@C60 dimers at the microscale. High throughput processing is introduced as the most efficient technique to obtain high purity N@C60 on a reasonable timescale. A scheme to produce symmetric and asymmetric fullerene dimers is also demonstrated. EPR spectroscopy of the dimers in the solid-state confirms derivatization, whilst permitting the modelling of spin–spin interactions for 'coupled' fullerene pairs. This suggests that the optimum inter–spin separation for which to observe spin–spin coupling in powders is circa 3 nm. Motivated by the properties of the trivalent erbium ion for the optical detection of quantum states, optically–active erbium–doped fullerenes are also investigated. These erbium metallofullerenes are synthesized and isolated as individual isomers. They are characterized by low temperature photoluminescence spectroscopy, emitting in the infra- red at a wavelength of 1.5 μm. The luminescence is markedly different where a C2 cluster is trapped alongside the erbium ions in the fullerene cage. Er2C2@C82 (isomer I) exhibits emission linewidths that are comparable to those observed for Er3+ in crystals. Finally, the discovery of a novel praseodymium-doped fullerene is reported. The balance of evidence favours the structure being assigned as Pr2@C72. This novel endohedral fullerene forms quasi-one-dimensional arrays in carbon nanotubes, which is a useful proof-of-principle of how a scaled fullerene-based architecture may be achieved.