144 research outputs found
Organic polaritons : modelling the effect of vibrational dressing
No full textThis thesis is a theoretical study of the effects of vibrational degrees of freedom on the polariton physics. The work is motivated by recent experiments, which show that by allowing light to strongly couple with organic matter (inside a microcavity), polariton condensation can occur at room temperature. We begin by introducing a model, which describes localised electronic excitations of molecules coupled to cavity photon modes. The additional feature is the coupling between electronic excitations and local vibrational modes of molecules.
Investigations of equilibrium phase diagrams and absorption spectra of the system (with a single cavity mode and without disorder) have revealed that coupling to vibrational modes acts to suppress the effective light-matter strength, can give rise to a sequence of normal-condensed-normal transitions as a function of temperature, and can drive the phase transition first order. We have also found that despite the vibrational sidebands existing at energies below the lower polariton, they cannot result in condensation, though their admixture has been found in the state which acquires macroscopic occupation.
Secondly, we focused on the effects of excitonic disorder and the possibility of the ground state reconfiguration in ultra-strong coupling regime, with the aim to explain the temperature dependence of absorption spectra published in [A. Canaguier-Durand et al. Angew. Chem. Int. Ed. 52, 10533 (2013)]. We have found that the latter mechanism, although not impossible, could not result in any observable changes as, for the experimental parameters, it is too weak. The study of absorption spectra in the presence of disorder has revealed that the temperature dependence can be accounted for by the vibrational dressing of electronic transitions
Strong matter-light coupling with organic molecules and inorganic semiconductors
No full textThis dissertation studies the effects of strong matter-light coupling on properties of organic molecules and inorganic semiconductors. The interplay of complex intramolecular dynamics and strong coupling of a photon to molecular transitions results in new physics having no counterparts in other systems. In contrast, low-energy optically active excitations of semiconductors (excitons) usually do not feature such complexity. However, the combination of strong electronic correlations and strong matter-light coupling leads to new physics.
Firstly, the effect of strong coupling between molecular vibrations and infrared photons on Raman scattering (RS) is considered. This is motivated by the experiment of Ref. [1] showing up to 10³ enhancement of RS signal under strong coupling. While the exact analytical results of this dissertation predict around 100% enhancement of total RS signal, they cannot explain orders of magnitude enhancement, leaving the question open for further studies.
Next, the effects of strong coupling of an optical photon and a molecular electronic transition on molecular lasing properties are discussed. Starting from a microscopic description of a driven-dissipative system, an exact (in the thermodynamic limit) mean-field solution is developed. It allows to uncover the mechanism of molecular lasing in the weak and strong coupling regime and to obtain a non-equilibrium lasing phase diagram.
Finally, a semiconductor with different densities of electrons and holes, strongly coupled to a microcavity photon, is studied. While finite electron-hole density imbalance is detrimental for excitonic condensation, it may still lead to a condensed state of excitons with finite centre of mass momentum coexisting with unpaired electrons. On the other hand, due to its low mass, a photon favours zero center of mass momentum condensation. The variational mean-field calculations reveal that the interplay of these effects leads to a variety of novel states with coexisting polariton condensate and unpaired electrons."This work was supported by the Engineering and Physical Sciences Research
Council, the Scottish Doctoral Training Centre in Condensed Matter Physics
(grant number EP/L015110/1)." -- Fundin
Data sets for the Journal of Physics: Condensed Matter article entitled “Structure of rare-earth chalcogenide glasses by neutron and x-ray diffraction”
No full textData sets used to prepare Figures 1 – 10 in the Journal of Physics: Condensed Matter article entitled “Structure of rare-earth chalcogenide glasses by neutron and x-ray diffraction.” The files are labelled according to the figure numbers. The data sets were created using the methodology described in the manuscript. Each of the plots was drawn using Origin (http://www.originlab.com/). The data set corresponding to a plotted curve within an Origin file can be identified by clicking on that curve. The units for each axis are identified on the plots
Phase transitions in two-dimensional itinerant electron systems
No full textThe aim of this thesis it to contribute to three open problems in the theory of itinerant electron systems in two spatial dimensions. Firstly, the mechanism for charge density wave formation in the transition metal dichalcogenides is a debated subject. In this thesis it is shown that charge density wave formation is possible via a purely electronic mechanism in monolayer vanadium diselenide. The competition of superconductivity and density wave formation is taken into account using the renormalisation group. As the Fermi surface is tuned to perfect nesting, a charge density wave phase emerges when the Heisenberg exchange interaction is of the order of the contact Coulomb repulsion. Secondly, the search for materials which exhibit topological superconductivity is ongoing. Possible candidates are strongly spin-orbit-coupled metals. In this thesis a square-lattice Hubbard model with strong Rashba spin-orbit coupling and one of the Fermi surfaces close to a Lifshitz transition is examined. The metal is shown to be generically unstable to the formation of mixed-parity superconductivity with a helical triplet component via a renormalisation group analysis. Thirdly, the breakdown of Fermi liquid theory close to a quantum critical point is still not well understood. In this thesis a functional renormalisation group analysis is presented using a soft frequency cutoff, investigating a general class of Pomeranchuk instabilities with _ flavours of boson. At small _ the theory is characterised by weakly non-Fermi-liquid behaviour of the electrons and ≈2 dynamics for the order parameter fluctuations. For large _, the theory crosses over to ≈1 scaling and non-Fermi-liquid behaviour."The work in this thesis was supported by the Engineering and
Physical Sciences Research Council (UK) Centre for Doctoral Training in Condensed
Matter Physics [grant number EP/L015110/1]." -- Acknowledgement
Simulating and understanding quantum processes using tensor networks
No full textSimulation of quantum processes is essential to both furthering our understanding of the microscopic world and also in developing quantum technologies. For accuracy we must take into consideration the environment in which they take place. In this thesis we begin by introducing an existing method that utilises tensor networks to efficiently represent the time non-local evolution of the system induced by interaction with its environment. Previously reliant on a linear interaction and a Gaussian environment, it is presented with no assumptions made on the form of interaction or environment statistics. The versatility is then showcased by first simulating the dynamics of a system coupled to a structured environment before then considering a pair of spatially separated systems coupled to the same environment. Here we see that the separation can be tuned to screen the interaction with dominant modes in the environment. Moving beyond system dynamics we show how correlation functions can be efficiently calculated and used to infer the dynamics of the bath. This result is then applied to study the dynamics of heat exchange between a system and regions of the environment; in particular a time dependent drive is employed to move heat between specific regions as desired. Finally, we see how the dynamics of a dimer coupled to both photons and phonons can be exactly captured and its non-equilibrium steady state is explored in all coupling regimes. This required extending the method to be able to combine the memory effects from each environment. The occurrence of population inversion is confirmed at weak light-matter coupling and then shown to disappear as the coupling is increased before entering a regime characterised by quantum Zeno physics."This work was supported by the CM-CDT under the EPSRC [grant number EP/L015110/1]."--Fundin
Non-equilibrium quantum dynamics : interplay of disorder, interactions and confinement
No full textThe study of collective behaviour in many-body systems often explores fundamentally new ideas absent from the mere constituents of such a system. A paradigmatic model for these studies is the spin-1/2 XXZ chain and its fermionic equivalent. This thesis can be broadly divided into the study of two fundamental aspects of this model. Firstly, we discuss localisation phenomena in one dimensional lattices as often experimentally realised in cold atom systems. Secondly, we investigate how disorder and symmetry influence heat transport in spin chains.
More specifically, in the first part we consider a system of non-interacting fermions in one dimension subject to a single-particle potential consisting of a strong optical lattice, a harmonic trap, and uncorrelated on-site disorder. We investigate a global inhomogeneous quantum quench and present numerical and analytical results for static and dynamical properties. We show that the approach to the non-thermal equilibrium state is extremely slow and that it implies a sensitivity to disorder parametrically stronger than that expected from Anderson localisation.
We also consider the above system in a strong non-uniform electric field. In the non-interacting case, due to Wannier-Stark localisation, the single-particle wave functions are exponentially localised without quenched disorder. We show that this system remains localised in the presence of nearest-neighbour interactions and exhibits physics analogous to models of conventional many-body localisation.
The second part explores the hydrodynamics of the disordered XYZ spin chain. Using time-evolving block decimation on open chains of up to 400 spins attached to thermal baths, we probe the energy transport of this system. Our principal findings are as follows. For weak disorder there is a stable diffusive region that persists up to a critical disorder strength that depends on the XY anisotropy. Then, for disorder strengths above this critical value energy transport becomes increasingly subdiffusive.Funded by CM-CDT and EPSRC (UK) under grants EP/G03673X/1 and EP/L015110/1
Electrical transport properties of URhGe and BiPd at very low temperature
No full textURhGe has garnered interest recently as one of the few known ferromagnetic superconductors. The superconductivity in this material appears to arise from magnetic fluctuations rather than phonons, and take a triplet form which is remarkably resistant to field. In this thesis, a number of measurements on the material are presented.
Some probe the Fermiology, with strong evidence appearing for a model which has both light open sheets and heavy, small, closed pockets. The open sheets, associated with chains of real-space electron density running along the axis, dominate the conductivity in most circumstances. Evidence for their existence arises from the general large and non-saturating magnetoresistance, and from the unusual observation of negative temperature coefficient of resistance at high fields. The closed pockets have provided a few Shubnikov-de Haas oscillations, but mostly they remain inferred from the high specific heat and their role in the magnetism.
In order to better probe the superconductivity, a high precision low noise DC resistance measurement bridge was built using a SQUID. Along with conventional measurements, this provides evidence that the two pockets of superconductivity on the phase diagram are the same phase. The re-entrance can be understood simply as a result of magnetic field being a tuning parameter, but also suppressing bulk superconductivity through orbital limiting. The SQUID bridge allowed the detection of domain wall superconductivity linking up these two pockets.
The SQUID bridge was also used to study the highly structured superconducting transition in BiPd. This material lacks inversion symmetry in its crystal structure, so is a good candidate for unusual forms of superconductivity. Here again non-bulk superconductivity is considered the most likely cause for the structure. Unusual and distinctive IV curves have been measured, and a simple model of inhomogeneous conductivity channels with different critical currents is proposed as an explanation
Data sets for "Structure of amorphous materials in the NASICON system Na_{1+x}Ti_2Si_xP_{3-x}O_{12}"
No full textData sets used to prepare Figures 3, 5 and 6 in the Journal of Physics: Condensed Matter article entitled "Structure of amorphous materials in the NASICON system Na_{1+x}Ti_2Si_xP_{3-x}O_{12}". The data sets refer to the glass structure for the compositions x = 0.8 and x = 1.0, as measured using neutron and x-ray diffraction. The diffraction results were combined with those from 29Si, 31P and 23Na solid-state nuclear magnetic resonance experiments to obtain a more complete picture of the atomic structure. NASICON is an acronym for sodium (Na) super-ionic conductor and NASICON materials are of interest as solid electrolytes and electrode materials for electrical storage energy devices. The crystalline phase can be prepared via the glass-ceramic route, leading to basic questions about the structure of the glass and how it evolves during the process of crystallisation
Heisenberg spins on an anisotropic triangular lattice : PdCrO2 under uniaxial stress
No full textExperiments at the ISIS Pulsed Neutron and Muon Source were supported by a beam time allocation from the Science and Technology Facilities Council under Expt. No. RB1820290. Financial support from the Deutsche Forschungsgemeinschaft through SFB 1143 (Project ID 247310070) and the Max Planck Society is gratefully acknowledged. RW acknowledges funding from the Engineering and Physical Sciences Research Council (EPSRC) Centre for Doctoral Training in Condensed Matter Physics (CDT-CMP), Grant No. EP/L015544/1.When Heisenberg spins interact antiferromagnetically on a triangular lattice and nearest-neighbor interactions dominate, the ground state is 120° antiferromagnetism. In this work, we probe the response of this state to lifting the triangular symmetry, through investigation of the triangular antiferromagnet PdCrO2 under uniaxial stress by neutron diffraction and resistivity measurements. The periodicity of the magnetic order is found to change rapidly with applied stress; the rate of change indicates that the magnetic anisotropy is roughly forty times the stress-induced bond length anisotropy. At low stress, the incommensuration period becomes extremely long, on the order of 1000 lattice spacings; no locking of the magnetism to commensurate periodicity is detected. Separately, the magnetic structure is found to undergo a first-order transition at a compressive stress of ∼0.4 GPa, at which the interlayer ordering switches from a double-to a single-q structure.Peer reviewe
Atom-only theories for U(1) symmetric cavity-QED models
No full textFunding: R.P. was supported by the EPSRC Scottish Doctoral Training Centre in Condensed Matter Physics (CM-CDT), Grant No. EP/L015110/1.We consider a generalized Dicke model with U(1) symmetry, which can undergo a transition to a superradiant state that spontaneously breaks this symmetry. By exploiting the difference in timescale between atomic and cavity dynamics, one may eliminate the cavity dynamics, providing an atom-only theory. We show that the standard Redfield theory cannot describe the transition to the superradiant state, but including higher-order corrections does recover the transition. Our work reveals how the forms of effective theories must vary for models with continuous symmetry, and provides a template to develop effective theories of more complex models.Peer reviewe
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