Condensed Matter Theory

Condensed matter theorists at Lancaster employ quantum-mechanical methods to uncover phenomena in electronic, atomic and photonic systems, and determine the characteristics of novel and artificial materials.

The group is renowned for its comprehensive research portfolio in quantum transport, dynamics and material modelling, ranging from ultracold atoms over low-dimensional electronic structures to photonic and quantum-optical systems.

Key Research

  • First principle studies of low-dimensional materials using Monte Carlo techniques and Density Functional Theory
  • Quantum transport in nanostructures including novel materials such as graphene and topological insulators
  • Topological phases of matter in nanostructures 
  • Mesoscopic hybrid systems including superconducting components
  • Quantum measurement and control of electronic nanostructures
  • Disordered interacting quantum systems
  • Superfluids and quantum-Hall liquids
  • Ultracold atomic systems and Bose-Einstein condensates
  • Quantum optics, light-matter interactions and polaritonics
  • Metamaterials and topological photonics

People

Condensed Matter Theory

Condensed Matter Theory

Condensed Matter Theory, Quantum Technology Centre

+44 (0)1524 592258 B075, B - Floor, Physics Building

Condensed Matter Theory

Condensed Matter Theory, Quantum Nanotechnology, Quantum Technology Centre

+44 (0)1524 593210

Condensed Matter Theory, North West Nanoscience Doctoral Training Centre , Quantum Technology Centre

+44 (0)1524 593930 B069, B - Floor, Physics Building

Condensed Matter Theory

+44 (0)1524 593288 B072, B - Floor, Physics Building

+44 (0)1524 595244 C017, C - Floor, Physics Building

Condensed Matter Theory, North West Nanoscience Doctoral Training Centre , Quantum Technology Centre

+44 (0)1524 593281 C015, C - Floor, Physics Building

Condensed Matter Theory

+44 (0)1524 592639 C008, C - Floor, Physics Building

Publications

Current PhD Opportunities

  • Quantum Monte Carlo study of excitons and trions in doped transition metal dichalcogenide semiconductors

    Supervisor

    Neil Drummond

    Description

    In this project quantum Monte Carlo methods will be used to investigate the behaviour of interacting charge carriers in heterostructures of two-dimensional transition-metal dichalcogenide semiconductors, and hence to understand the photoluminescence spectra of these fascinating materials.  In particular, the experimentally relevant situation in which there is a finite, low concentration of electrons will be investigated.  The project is computational in nature and good programming skills are highly desirable.

  • Optical properties of indium and gallium chalcogenides from first-principles calculations

    Supervisor

    Neil Drummond

    Description

    In this project quantum Monte Carlo and density functional theory methods will be used to understand the optical and excitonic properties of two-dimensional crystals of indium selenide and gallium selenide.  The valence band exhibits a non-quadratic dispersion so that holes cannot be treated straightforwardly within an effective mass approximation.  The effects of nonquadratic dispersion will be implemented within the variational quantum Monte Carlo method, requiring good programming skills.

  • Topological concepts for robust lasers and condensates

    Supervisor

    Henning Schomerus

    Description

    Quantum systems can display robust features related to topological properties. These attain precise values that can only change in phase transitions where the states change their topological properties. While the scope of these effects is well understood for electronic and superconducting systems, a much richer range is accounted for photonic and in general bosonic systems. In these systems particles can be created and annihilated, which results in loss, gain, and nonlinearity. Recent years have seen a surge of activity to tailor these bosonic systems to their electronic counterparts, mostly by eliminating the mentioned differences. Going beyond these efforts, work of the supervisor and collaborators has demonstrated that topological physics extends beyond these mere analogies, leading to experimental demonstrations for laser, microwave resonator arrays, and polaritonic condensates.

    What is missing is a detailed understanding of the actual scope of these extensions - how to systematically define the topological invariants, and classify systems in the manner achieved in the electronic context. This project tackles this question both generally, as well as practically by examining specific photonic and polaritonic model systems of experimental interest, and inquire how to increase their robustness for possible applications. This project develops both analytical skills in quantum mechanics as well as numerical modelling skills.

  • Statistical descriptions of interacting disordered quantum systems

    Supervisor

    Henning Schomerus

    Description

    Quantum systems can encode information, but this information quickly becomes inaccessible if the associated degrees of freedom coupled with the environment. A key recent realization points towards a mechanism whereby quantum information can be localised by combining interactions with generic disorder. This turns previously undesired artefacts into a highly valuable resource.

    In previous work, we developed an efficient description of these so-called many-body localised systems based on a simple single-particle picture. This project aims to transfer this picture to a wider context, such as interacting spins or systems with additional internal degrees of freedom or dimensions. The project develops highly advanced numerical skills, such as DMRG, exact diagonalisation, and tensor network approaches. These will be applied to a range of model systems designed to yield conceptual insights that transfer to a wide range of systems.

  • Waveoptical control and dynamics in complex microresonator arrays and metamaterials

    Supervisor

    Henning Schomerus

    Description

    The choice of geometry has dramatic consequences on the features of wave-optical systems. Two paradigms of this situation are optical microresonators with irregular shape whose classical ray dynamics connects to the notion of classical chaos, and metamaterials that acquire unconventional properties from sub-wavelength design features. This project explores the combination of these features with new a class of symmetries that involves the loss and gain in the system. The project aims to identify mechanisms for switching, mode guiding, and directed emission. The project develops both numerical skills for the modelling of optical and photonic systems, as well as analytical and conceptual skills for the design of novel structures that display new physical phenomena.

  • Long-range dipolar interactions in cold atom systems

    Supervisor

    Janne Ruostekoski

    Description

    Applications are invited for a PhD studentship in theoretical cold atom physics, at the Department of Physics, Lancaster University. Cold atomic gases cooperatively coupled with light provide a rich strongly interacting quantum many-body system. The aim of the project is to study long-range dipole-dipole interactions between the atoms and their cooperative behaviour. The light-mediated interactions can also be engineered and manipulated for applications in quantum technologies and to simulate novel strongly interacting quantum systems, e.g., in the context of hybrid systems of atoms and nanophotonic structures. The project involves both numerical and analytic modelling is related to areas, such as quantum optics, optical physics and many-body physics. 

  • Numerical and theoretical studies of topological objects in ultracold atomic systems

    Supervisor

    Janne Ruostekoski

    Description

    Understanding the behaviour of a collection of particles is a challenging problem in physics, in particular in quantum mechanics, where the interaction between a pair of constituents is not necessarily a sufficient guide to predict many-particle dynamics. In many areas of physics, such as in typical relativistic quantum field-theoretical, elementary particle physics or cosmological systems direct observations or controlled laboratory experiments may not be possible. Many dynamical effects involving the emergence of topological defects and textures are so complex that even numerical treatment becomes unfeasible for their accurate description. Ultracold atom systems have been discussed as candidates for experimentally accessible laboratory testing grounds for theories in other areas of physics. Symmetry breaking in a phase transition to an ordered phase provides an important example.

    The project concerns of a study of ultracold atomic gases with spin degrees of freedom as a laboratory system for topological defects and textures that emulates, e.g., stability properties of field-theoretical vacuum states of particularly rich phenomenology, such as knotted solutions. The project combines in unique and ambitious ways interdisciplinary ideas from optical and atomic physics and modern quantum field theories for state engineering, exploiting their generic features.

  • Modelling topological surface states in rhombohedral graphene

    Supervisor

    Edward McCann

    Description

    The aim of this theoretical project is to model the electronic properties of surface states in rhombohedrally-stacked multilayer graphene which are unusual because their topological nature gives rise to flat, degenerate electronic bands near the Fermi level. Their origin can be understood by noting the similarity between the lattice of rhombohedral graphene and that of a one-dimensional chain with alternating bond strengths as described by the Su-Schrieffer-Heeger (SSH) model which describes a one-dimensional topological insulator. This means that two surface states in rhombohedral graphene (each of which is localised near one of the outer two graphene layers) are almost degenerate, giving flat bands fixed to the middle of the bulk band gap for a broad range of in-plane wavevectors. The aim of the project is to model their properties taking into account symmetry-breaking effects, developing both analytical skills in quantum mechanics as well as numerical modelling skills.

  • Quantum measurement induced geometric dynamical features

    Supervisor

    Alessandro Romito

    Description

    Quantum measurement is a pillar of quantum mechanics. Recent technological make it possible to employ quantum measurements in a variety of protocols for precision measurement, quantum control, quantum feedback, and even quantum thermodynamics. With such a level of control, it is possible to use quantum measurements to induce geometric and topologically protected features in the quantum dynamics, as recently proposed.

    The project aim is to set the theoretical basis for this measurement induced geometric dynamics. It will move from the formulation of a general theory that accounts for the effects of different and Hamiltonian dynamics to the proposal and modelling of experiments in electronic nanostructures. The student will make extensive use of theoretical tools of quantum mechanics and numerical simulation of quantum trajectories and will gain experience in the physics of electronic nanostructures.

    Funding

    The PhD starting date is 1 October 2019.  Funding is for 3.5 years and is available to citizens of the UK and the European Union.

  • Heat control via topologically protected excitations.

    Supervisor

    Alessandro Romito

    Description

    Superconductors can exist in a phase that supports topologically protected excitations, Majorana zero modes, which can be controlled to perform certain quantum operations with exponentially good accuracy. The potential of Majorana zero modes for error-free quantum computation has triggered extensive experimental work, and experiments have successfully engineered Majorana zero modes in superconducting nanostructures. Due to their neutral electric charge, Majorana zero modes signatures are limited in electric probing but are expected to be more sensitive to energy probes.

    The project aims at characterising the energetics and thermodynamic properties of driven Majorana based devices. The project will develop the theory of the topologically protected contribution to basic thermodynamic relations at the quantum level, and will then consider the implication of the results for quantum heat pumps and thermal machines. The student will work with the theory of topological superconductors in conjunction with quantum mechanics and thermodynamics. This will require the development of both analytical and numerical skills.

  • Theoretical studies of active holograms with arrayed 2D resonators

    Supervisor

    Janne Ruostekoski

    Description

    Background

    The interaction of light with resonant scattering centres is increasing in importance, for both fundamental research and technological applications, as experimentalists realise a growing number of such systems. Together with ready access to massive computer clusters, this has created the interesting confluence that we now have both the motive and the opportunity to study strong light coupling by means of microscopic numerical simulations. Advances in nanofabrication now allow us to reach high sensitivities with longer coherence times and an enhanced optical thickness (which characterises the light-matter coupling) in miniaturised devices. This results in a cooperative response and strong light-mediated interactions between the excitations of the scattering centres that poses a theoretical challenge in many-body physics with the eventual goal of reaching the quantum regime.

    3D solid-state media as advanced resonance optical devices have encountered fundamental problems due to photon loss and fabrication challenges. These are being replaced by 2D metasurfaces, providing realisations of ultrathin, lightweight flat lenses with unprecedented functionalities.

    Project Outline

    In this project, the student will theoretically analyse and numerically simulate the collective responses of 2D arrays of subwavelength-spaced resonant scattering centres. Together with the research group they will develop methods for designing novel cooperative responses that can be utilised for optical manipulation and functionalities of ultrathin devices. By locally varying the properties of the system it is possible to build optical holograms and phenomena reminiscent of chirality that can be utilised for directed emission of light, as well as potentially for quantum devices, allowing the design of active optical media. The advantage of strong interactions is that the sensitivity of the optical devices is no longer limited by the resonance linewidth of the isolated scatterer, but by the collective resonance linewidth [1], which is potentially several times narrower. The project involves a close collaboration with experimental effort to build such structures in thin semiconductor layers of AlAs and GaAs [2] or MoSe [3], in photonic crystal and silicon arrays.

    The student will join a theory group studying cooperative optical phenomena in different physical systems and will gain experience in large-scale numerical simulations and high-performance computing. The co-supervisor will be performing experiments on these systems, providing guidance on the suitability of the models for laboratory realisations.

    Supervisor  Prof. Janne Ruostekoski, Physics Dept, Lancaster, j.ruostekoski@lancaster.ac.uk

    Co-supervisor  Prof. Rob Young, Physics Dept, Lancaster, r.j.young@lancaster.ac.uk

    [1] S. D. Jenkins, J. Ruostekoski, N. Papasimakis, S. Savo, and N. I. Zheludev, Phys. Rev. Lett. 119, 053901 (2017). [2] G. Scuri et al., Phys. Rev. Lett. 120, 037402 (2018). [3] Patrick Back, Sina Zeytinoglu, Aroosa Ijaz, Martin Kroner, and Atac Imamo─člu, Phys. Rev. Lett. 120, 037401 (2018).

     

Postgraduate Training

The Condensed Matter Theory group provides extensive research and training opportunities for postgraduate students, covering subject-specific and general research skills.

Individual training involves supervision and advice that account for the students’ project requirements and the skills needed to make progress in their work.

This is supplemented by weekly group meetings involving PhD students, MPhys/MSci students, postdocs and academics, including formal lectures, informal presentations and dice seminars on new and emerging topics.

Our students also attend the weekly departmental condensed-matter seminars and participate in the North West England Solid State lectures organised in conjunction with Manchester University.

Our students regularly attend national and international summer schools, including the Windsor Summer School Physics by the Lake, and present their work at a variety of scientific conferences. They also have the opportunity to develop their presentation skills via participation in the departmental outreach programme. Additional training is offered by the Faculty of Science and Technology, Information System Services, and the Library.

Training resources