Behaviour of intermolecular interactions at extreme pressures

In organic solids, pressures of only a few gigapascals modify and rearrange intermolecular contacts such as H-bonds and van der Waals contacts leading to extensive phase diversity. Applications in this rich area of research include searches for new phases and solvates of pharmaceutical materials; modelling of detonation mechanisms of energetic materials, and modelling of the driving forces of phase transitions. The overarching theme of this PhD thesis is to obtain new, often difficult to isolate, high-pressure polymorphs of small molecules and elucidate the role of intermolecular interactions in their phase stabilities. The need to obtain precise structural information at atomic resolution demands the use of single crystal diffraction methods but scattering intensities are typically low, and the pressure apparatus used in these studies (the diamond anvil cell) results in incomplete data. This can make direct structure determinations for some materials difficult or even impossible. Third generation synchrotron X-ray sources are therefore used for their brightness, high energies, and small focused beams to extract as much structural information from samples as possible. The amino acid L-threonine, characterised by its hydrogen bond network, has been structurally characterised at 22 GPa which is an unusually high-pressure for a complex organic molecule. L-threonine undergoes two isosymmetric phase transitions at ca. 2 and ca. 9 GPa, and a phase transition at ca. 18 GPa that results in a loss of crystal symmetry. Structures of L-threonine were determined by single-crystal X-ray diffraction to 22 GPa; which is the highest-pressure structure ever reported for an amino acid. High-pressure polymorphism in pyridine was studied extensively by single-crystal X-ray diffraction, Raman spectroscopy and neutron powder diffraction. Pyridine has at least three polymorphs in the narrow pressure range of ca. 1 to ca. 2 GPa but the sluggish nature of the phase transitions has made isolating and characterising one of the phases difficult, until now. Here, we used in situ crystal growth in the diamond anvil cell to obtain a stable, diffraction quality single crystal of the elusive phase III and determined its crystal structure for the first time. A mechanism for the transformation is also proposed. The halogen bonded molecule, 4-iodobenzonitrile was studied experimentally by single-crystal X-ray diffraction and Raman spectroscopy up to 10 GPa. 4-iodobenzonitrile undergoes a reconstructive phase change above 5 GPa that results in crystals breaking apart, making it difficult to obtain meaningful diffraction data. Nevertheless, the structure of the new high-pressure phase was determined for the first time by rapidly pressurising a crystal grown in situ to 8 GPa. Crystal lattice and intermolecular PIXEL energy calculations have been validated for use with small organics to 22 GPa, as well as for halogen containing molecules at very high pressures; allowing the roles of stabilising, or destabilising, molecular interactions to be probed in high-pressure polymorphs for a range of organic molecules. Finally, a neon co-crystal was obtained on compression of a Cu2 Pacman complex. This single-crystal structure represents one of only a few published neon containing organometallic structures. Neon resides within the interstitial voids as a result of the Pacman complex reconfiguring to allow neon-uptake. The study shows the interplay between the pressure transmitting medium and crystal structure and we discuss the potential applications of pressure mediated guest-uptake in the Pacman complexes

Structural and magnetic study of Yb3+ in the perovskites Sr2YbMO6 (M = Nb, Ta, Sb)

The compounds Sr2YbNbO6, Sr2YbTaO6 and Sr2YbSbO6 have been prepared using solid state methods by heating pelleted reagents in air at temperatures up to 1400°C. Rietveld refinement against room temperature neutron powder diffraction data show that all three compounds crystallise with a cationordered variant of the perovskite structure in the P21/n space group. Complete cation ordering occurs between M5+ and Yb3+ over two octahedrally-coordinated sites in the structure and all compounds are stoichiometric in oxygen. The Sb-O bond lengths are similar to related perovskite compounds but differ slightly from those indicated by bond valence sums. Magnetic susceptibility data resemble Curie-Weiss paramagnetic behaviour, but can be better understood as arising from the effect of the octahedral crystal field on the 2F5/2 ground state of Yb3+ leading to a temperature dependent magnetic moment on this ion below 100 K

Pressure-induced inclusion of neon in the crystal structure of a molecular Cu2(pacman) complex at 4.67 GPa

Crystals of a Cu complex of the macrocyclic Schiff-base calixpyrrole or 'Pacman' ligand, Cu2(L), do not contain any solvent-accessible void space at ambient pressure, but adsorb neon at 4.67 GPa, forming Cu2(L)·3.5Ne

High-pressure polymorphism in L-threonine between ambient pressure and 22 GPa

The crystal structure of l-threonine has been studied to a maximum pressure of 22.3 GPa using single-crystal X-ray and neutron powder diffraction. The data have been interpreted in the light of previous Raman spectroscopic data by Holanda et al. (J. Mol. Struct. (2015), 1092, 160-165) in which it is suggested that three phase transitions occur at ca. 2 GPa, between 8.2 and 9.2 GPa and between 14.0 and 15.5 GPa. In the first two of these transitions the crystal retains its P212121 symmetry, in the third, although the unit cell dimensions are similar either side of the transition, the space group symmetry drops to P21. The ambient pressure form is labelled phase I, with the successive high-pressure forms designated I′, II and III, respectively. Phases I and I′ are very similar, the transition being manifested by a slight rotation of the carboxylate group. Phase II, which was found to form between 8.5 and 9.2 GPa, follows the gradual transformation of a long-range electrostatic contact becoming a hydrogen bond between 2.0 and 8.5 GPa, so that the transformation reflects a change in the way the structure accommodates compression rather than a gross change of structure. Phase III, which was found to form above 18.2 GPa in this work, is characterised by the bifurcation of a hydroxyl group in half of the molecules in the unit cell. Density functional theory (DFT) geometry optimisations were used to validate high-pressure structural models and PIXEL crystal lattice and intermolecular interaction energies are used to explain phase stabilities in terms of the intermolecular interactions

High-pressure polymorphism in pyridine

Single crystals of the high-pressure phases II and III of pyridine have been obtained by in situ crystallization at 1.09 and 1.69 GPa, revealing the crystal structure of phase III for the first time using X-ray diffraction. Phase II crystallizes in P212121 with Z' = 1 and phase III in P41212 with Z' = ½. Neutron powder diffraction experiments using pyridine-d5 establish approximate equations of state of both phases. The space group and unit-cell dimensions of phase III are similar to the structures of other simple compounds with C 2v molecular symmetry, and the phase becomes stable at high pressure because it is topologically close-packed, resulting in a lower molar volume than the topologically body-centred cubic phase II. Phases II and III have been observed previously by Raman spectroscopy, but have been mis-identified or inconsistently named. Raman spectra collected on the same samples as used in the X-ray experiments establish the vibrational characteristics of both phases unambiguously. The pyridine molecules interact in both phases through CH⋯π and CH⋯N interactions. The nature of individual contacts is preserved through the phase transition between phases III and II, which occurs on decompression. A combination of rigid-body symmetry mode analysis and density functional theory calculations enables the soft vibrational lattice mode which governs the transformation to be identified

Electrical operation of planar Ge hole spin qubits in an in-plane magnetic field

In this work we present a comprehensive theory of spin physics in planar Ge hole quantum dots in an in-plane magnetic field, where the orbital terms play a dominant role in qubit physics, and provide a brief comparison with experimental measurements of the angular dependence of electrically driven spin resonance. We focus the theoretical analysis on electrical spin operation, phonon-induced relaxation, and the existence of coherence sweet spots. We find that the choice of magnetic field orientation makes a substantial difference for the properties of hole spin qubits. Furthermore, although the Schrieffer-Wolff approximation can describe electron dipole spin resonance (EDSR), it does not capture the fundamental spin dynamics underlying qubit coherence. Specifically, we find that: (i) EDSR for in-plane magnetic fields varies non-linearly with the field strength and weaker than for perpendicular magnetic fields; (ii) The EDSR Rabi frequency is maximized when the a.c. electric field is aligned parallel to the magnetic field, and vanishes when the two are perpendicular; (iii) The Rabi ratio $T_1/T_\pi$, i.e. the number of EDSR gate operation per unit relaxation time, is expected to be as large as $5{\times}10^5$ at the magnetic fields used experimentally; (iv) The orbital magnetic field terms make the in-plane $g$-factor strongly anisotropic in a squeezed dot, in excellent agreement with experimental measurements; (v) The coherence sweet spots do not exist in an in-plane magnetic field, as the orbital magnetic field terms expose the qubit to all components of the defect electric field. These findings will provide a guideline for experiments to design ultrafast, highly coherent hole spin qubits in Ge

Electrical control of uniformity in quantum dot devices

Highly uniform quantum systems are essential for the practical implementation of scalable quantum processors. While quantum dot spin qubits based on semiconductor technology are a promising platform for large-scale quantum computing, their small size makes them particularly sensitive to their local environment. Here, we present a method to electrically obtain a high degree of uniformity in the intrinsic potential landscape using hysteretic shifts of the gate voltage characteristics. We demonstrate the tuning of pinch-off voltages in quantum dot devices over hundreds of millivolts that then remain stable at least for hours. Applying our method, we homogenize the pinch-off voltages of the plunger gates in a linear array for four quantum dots reducing the spread in pinch-off voltage by one order of magnitude. This work provides a new tool for the tuning of quantum dot devices and offers new perspectives for the implementation of scalable spin qubit arrays

Could MicroRNAs be Useful Tools to Improve the Diagnosis and Treatment of Rare Gynecological Cancers? A Brief Overview

Gynecological cancers pose an important public health issue, with a high incidence among women of all ages. Gynecological cancers such as malignant germ-cell tumors, sex-cord-stromal tumors, uterine sarcomas and carcinosarcomas, gestational trophoblastic neoplasia, vulvar carcinoma and melanoma of the female genital tract, are defined as rare with an annual incidence of <6 per 100,000 women. Rare gynecological cancers (RGCs) are associated with poor prognosis, and given the low incidence of each entity, there is the risk of delayed diagnosis due to clinical inexperience and limited therapeutic options. There has been a growing interest in the field of microRNAs (miRNAs), a class of small non-coding RNAs of 22 nucleotides in length, because of their potential to regulate diverse biological processes. miRNAs usually induce mRNA degradation and translational repression by interacting with the 30 untranslated region (30-UTR) of target mRNAs, as well as other regions and gene promoters, as well as activating translation or regulating transcription under certain conditions. Recent research has revealed the enormous promise of miRNAs for improving the diagnosis, therapy and prognosis of all major gynecological cancers. However, to date, only a few studies have been performed on RGCs. In this review, we summarize the data currently available regarding RGCs.peer-reviewe

Nanoscale Mapping of the 3D Strain Tensor in a Germanium Quantum Well Hosting a Functional Spin Qubit Device

A strained Ge quantum well, grown on a SiGe/Si virtual substrate and hosting two electrostatically defined hole spin qubits, is nondestructively investigated by synchrotron-based scanning X-ray diffraction microscopy to determine all its Bravais lattice parameters. This allows rendering the three-dimensional spatial dependence of the six strain tensor components with a lateral resolution of approximately 50 nm. Two different spatial scales governing the strain field fluctuations in proximity of the qubits are observed at 1 μm, respectively. The short-ranged fluctuations have a typical bandwidth of 2 × 10-4 and can be quantitatively linked to the compressive stressing action of the metal electrodes defining the qubits. By finite element mechanical simulations, it is estimated that this strain fluctuation is increased up to 6 × 10-4 at cryogenic temperature. The longer-ranged fluctuations are of the 10-3 order and are associated with misfit dislocations in the plastically relaxed virtual substrate. From this, energy variations of the light and heavy-hole energy maxima of the order of several 100 μeV and 1 meV are calculated for electrodes and dislocations, respectively. These insights over material-related inhomogeneities may feed into further modeling for optimization and design of large-scale quantum processors manufactured using the mainstream Si-based microelectronics technology