Imperial College London

DrDerekLee

Faculty of Natural SciencesDepartment of Physics

Senior Lecturer
 
 
 
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Contact

 

+44 (0)20 7594 7602dkk.lee Website

 
 
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Location

 

809Blackett LaboratorySouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
to

50 results found

Rider MS, Sokolikova M, Hanham SM, Navarro-Cia M, Haynes P, Lee D, Daniele M, Guidi MC, Mattevi C, Lupi S, Giannini Vet al., 2019, Experimental signature of a topological quantum dot, Publisher: arXiv

Topological insulators (TIs) present a neoteric class of materials, whichsupport delocalised, conducting surface states despite an insulating bulk. Dueto their intriguing electronic properties, their optical properties havereceived relatively less attention. Even less well studied is their behaviourin the nanoregime, with most studies thus far focusing on bulk samples - inpart due to the technical challenges of synthesizing TI nanostructures. Westudy topological insulator nanoparticles (TINPs), for which quantum effectsdominate the behaviour of the surface states and quantum confinement results ina discretized Dirac cone, whose energy levels can be tuned with thenanoparticle size. The presence of these discretized energy levels in turnleads to a new electron-mediated phonon-light coupling in the THz range. Wepresent the experimental realisation of Bi$_2$Te$_3$ TINPs and strong evidenceof this new quantum phenomenon, remarkably observed at room temperature. Thissystem can be considered a topological quantum dot, with applications to roomtemperature THz quantum optics and quantum information technologies.

Working paper

Lieu S, Ho AF, Lee DKK, Coleman Pet al., 2019, Intertwined superfluidity and density wave order in a p-orbital Bose condensate, Physical Review B, Vol: 99, ISSN: 2469-9950

We study a continuum model of the weakly interacting Bose gas in the presence of an external field with minima forming a triangular lattice. The second lowest band of the single-particle spectrum (p band) has three minima at nonzero momenta. We consider a metastable Bose condensate at these momenta and find that, in the presence of interactions that vary slowly over the lattice spacing, the order parameter space is isomorphic to S5. We show that the enlarged symmetry leads to the loss of topologically stable vortices, as well as two extra gapless modes with quadratic dispersion. The former feature implies that this non-Abelian condensate is a “failed superfluid” that does not undergo a Berezinskii-Kosterlitz-Thouless (BKT) transition. Order-by-disorder splitting appears suppressed, implying that signatures of the S5 manifold ought to be observable at low temperatures.

Journal article

Lieu S, Lee DKK, Knolle J, 2018, Disorder protected and induced local zero-modes in longer-range Kitaev chains, PHYSICAL REVIEW B, Vol: 98, ISSN: 2469-9950

Journal article

Galilo B, Lee DKK, Barnett R, 2017, Topological edge state manifestation of interacting 2D boson lattices in a harmonic trap, Physical Review Letters, Vol: 119, ISSN: 0031-9007

In this work, it is shown that interactions can facilitate the emergence oftopological edge states of bosonic systems in the presence of a harmonicpotential. This effect is demonstrated with the concrete model of a hexagonallattice populated by spin-one bosons under a synthetic gauge field. Infermionic or non-interacting systems, the presence of a harmonic trap canobscure the observation of edge states. For our system with weakly interactingbosons in the Thomas-Fermi regime, we can clearly see a topological bandstructure with a band gap traversed by edge states. We also find that thenumber of edge states crossing the gap is increased in the presence of aharmonic trap, and the edge-modes experience an energy shift while traversingthe first Brillouin zone which is related to the topological properties of thesystem. We find an analytical expression for the edge-state energies and ourcomparison with numerical computation shows excellent agreement.

Journal article

Siroki G, Haynes PD, Lee DKK, Giannini Vet al., 2017, Protection of surface states in topological nanoparticles, Physical Review Materials, Vol: 1, ISSN: 2475-9953

opological insulators host protected electronic states at their surface. These states show little sensitivity todisorder. For miniaturization one wants to exploit their robustness at the smallest sizes possible. This is alsobeneficial for optical applications and catalysis, which favor large surface-to-volume ratios. However, it is notknown whether discrete states in particles share the protection of their continuous counterparts in large crystals.Here we study the protection of the states hosted by topological insulator nanoparticles. Using both analyticaland tight-binding simulations, we show that the states benefit from the same level of protection as those on aplanar surface. The results hold for many shapes and sustain surface roughness which may be useful in photonics,spectroscopy, and chemistry. They complement past studies of large crystals—at the other end of possible lengthscales. The protection of the nanoparticles suggests that samples of all intermediate sizes also possess protectedstates.

Journal article

NyƩki J, Phillis A, Ho A, Lee D, Coleman P, Parpia J, Cowan B, Saunders Jet al., 2017, Intertwined superfluid and density wave order in two-dimensional 4He, Nature Physics, Vol: 13, Pages: 455-459, ISSN: 1745-2473

Superfluidity is a manifestation of the operation of the laws of quantum mechanics on a macroscopic scale. The conditions under which superfluidity becomes manifest have been extensively explored experimentally in both quantum liquids (liquid 4He being the canonical example) and ultracold atomic gases1, 2, including as a function of dimensionality3, 4. Of particular interest is the hitherto unresolved question of whether a solid can be superfluid5, 6. Here we report the identification of a new state of quantum matter with intertwined superfluid and density wave order in a system of two-dimensional bosons subject to a triangular lattice potential. Using a torsional oscillator we have measured the superfluid response of the second atomic layer of 4He adsorbed on the surface of graphite, over a wide temperature range down to 2 mK. Superfluidity is observed over a narrow range of film densities, emerging suddenly and subsequently collapsing towards a quantum critical point. The unusual temperature dependence of the superfluid density in the limit of zero temperature and the absence of a clear superfluid onset temperature are explained, self-consistently, by an ansatz for the excitation spectrum, reflecting density wave order, and a quasi-condensate wavefunction breaking both gauge and translational symmetry.

Journal article

Varley JR, Lee DKK, 2016, Structure of exciton condensates in imbalanced electron-hole bilayers, Physical Review B, Vol: 94, ISSN: 1550-235X

We investigate the possibility of excitonic superfluidity in electron-hole bilayers. We calculate the phase diagram of the system for the whole range of electron-hole density imbalance and for different degrees of electrostatic screening, using mean-field theory and a Ginzburg–Landau expansion. We are able to resolve differences on previous work in the literature which concentrated on restricted regions of the parameter space. We also give detailed descriptions of the pairing wave function in the Fulde–Ferrell–Larkin–Ovchinnikov paired state. The Ginzburg–Landau treatment allows us to investigate the energy scales involved in the pairing state and discuss the possible spontaneous breaking of two-dimensional translation symmetry in the ground state.

Journal article

Malone FD, Blunt NS, Brown EW, Lee DKK, Spencer JS, Foulkes WMC, Shepherd JJet al., 2016, Accurate exchange-correlation energies for the warm dense electron gas, Physical Review Letters, Vol: 117, ISSN: 1079-7114

The density matrix quantum Monte Carlo (DMQMC) method is used to sample exact-on-average N-body density matrices for uniform electron gas systems of up to 10124 matrix elements via a stochastic solution of the Bloch equation. The results of these calculations resolve a current debate over the accuracy of the data used to parametrize finite-temperature density functionals. Exchange-correlation energies calculated using the real-space restricted path-integral formalism and the k-space configuration path-integral formalism disagree by up to ∼10% at certain reduced temperatures T/TF≤0.5 and densities rs≤1. Our calculations confirm the accuracy of the configuration path-integral Monte Carlo results available at high density and bridge the gap to lower densities, providing trustworthy data in the regime typical of planetary interiors and solids subject to laser irradiation. We demonstrate that the DMQMC method can calculate free energies directly and present exact free energies for T/TF≥1 and rs≤2.

Journal article

Siroki G, Lee DKK, Haynes PD, Giannini Vet al., 2016, Single-electron induced surface plasmons on a topological nanoparticle, Nature Communications, Vol: 7, ISSN: 2041-1723

It is rarely the case that a single electron affects the behaviour of several hundred thousands of atoms. Here we demonstrate a phenomenon where this happens. The key role is played by topological insulators—materials that have surface states protected by time-reversal symmetry. Such states are delocalized over the surface and are immune to its imperfections in contrast to ordinary insulators. For topological insulators, the effects of these surface states will be more strongly pronounced in the case of nanoparticles. Here we show that under the influence of light a single electron in a topologically protected surface state creates a surface charge density similar to a plasmon in a metallic nanoparticle. Such an electron can act as a screening layer, which suppresses absorption inside the particle. In addition, it can couple phonons and light, giving rise to a previously unreported topological particle polariton mode. These effects may be useful in the areas of plasmonics, cavity electrodynamics and quantum information.

Journal article

Barnett RL, Lee DKK, Galilo B, 2015, Selective Population of Edge States in a 2D Topological Band System, Physical Review Letters, Vol: 115, ISSN: 1079-7114

We consider a system of interacting spin-one atoms in a hexagonal lattice under the presenceof a synthetic gauge field. Quenching the quadratic Zeeman field is shown to lead to a dynamicalinstability of the edge modes. This, in turn, leads to a spin current along the boundary of the systemwhich grows exponentially fast in time following the quench. Tuning the magnitude of the quenchcan be used to selectively populate edge modes of different momenta. Implications of the intrinsicsymmetries of Hamiltonian on the dynamics are discussed. The results hold for atoms with bothantiferromagnetic and ferromagnetic interactions.

Journal article

Malone FDARA, 2015, Interaction picture density matrix quantum Monte Carlo, Journal of Chemical Physics, Vol: 143, ISSN: 1089-7690

The recently developed density matrix quantum Monte Carlo (DMQMC) algorithm stochastically samplesthe N-body thermal density matrix and hence provides access to exact properties of many-particle quantumsystems at arbitrary temperatures. We demonstrate that moving to the interaction picture provides substan-tial benefits when applying DMQMC to interacting fermions. In this first study, we focus on a system ofmuch recent interest: the uniform electron gas in the warm dense regime. The basis set incompleteness errorat finite temperature is investigated and extrapolated via a simple Monte Carlo sampling procedure. Finally,we provide benchmark calculations for a four-electron system, comparing our results to previous work wherepossible.

Journal article

Genway S, Ho AF, Lee DKK, 2013, Dynamics of thermalization and decoherence of a nanoscale system, Physical Review Letters, Vol: 111, ISSN: 1079-7114

Journal article

Genway S, Ho AF, Lee DKK, 2012, Thermalization of local observables in small Hubbard lattices, PHYSICAL REVIEW A, Vol: 86, ISSN: 1050-2947

We present a study of thermalization of a small isolated Hubbard lattice cluster prepared in a pure state with a well-defined energy. We examine how a two-site subsystem of the lattice thermalizes with the rest of the system as its environment. We explore numerically the existence of thermalization over a range of system parameters, such as the interaction strength, system size, and the strength of the coupling between the subsystem and the rest of the lattice. We find thermalization over a wide range of parameters and that interactions are crucial for efficient thermalization of small systems. We relate this thermalization behavior to the eigenstate thermalization hypothesis and quantify numerically the extent to which eigenstate thermalization holds. We also verify our numerical results theoretically with the help of previously established results from random matrix theory for the local density of states, particularly the finite-size scaling for the onset of thermalization.

Journal article

Eastham PR, Cooper NR, Lee DKK, 2012, Diamagnetism and flux creep in bilayer exciton superfluids, PHYSICAL REVIEW B, Vol: 85, ISSN: 2469-9950

Journal article

Lee DKK, Eastham PR, Cooper NR, 2011, Breakdown of Counterflow Superfluidity in a Disordered Quantum Hall Bilayer, ADVANCES IN CONDENSED MATTER PHYSICS, Vol: 2011, ISSN: 1687-8108

Journal article

Genway S, Ho AF, Lee DKK, 2010, Dynamics of Thermalization in Small Hubbard-Model Systems, PHYSICAL REVIEW LETTERS, Vol: 105, ISSN: 0031-9007

Journal article

Eastham PR, Cooper NR, Lee DKK, 2010, Critical Supercurrents and Self-Organization in Quantum Hall Bilayers, PHYSICAL REVIEW LETTERS, Vol: 105, ISSN: 0031-9007

Journal article

Ros OGC, Lee DKK, 2010, Effect of disorder and electron-phonon interaction on interlayer tunneling current in quantum Hall bilayer, PHYSICAL REVIEW B, Vol: 81, ISSN: 1098-0121

Journal article

Duric T, Lee DKK, 2010, Frustrated Bose condensates in optical lattices, PHYSICAL REVIEW B, Vol: 81, ISSN: 1098-0121

We study the Bose-condensed ground states of bosons in a two-dimensional optical lattice in the presence of frustration due to an effective vector potential, for example, due to lattice rotation. We use a mapping to a large-S frustrated magnet to study quantum fluctuations in the condensed state. Quantum effects are introduced by considering a 1/S expansion around the classical ground state. The large-S regime should be relevant to systems with many particles per site. As the system approaches the Mott insulating state, the hole density becomes small. Our large-S results show that, even when the system is very dilute, the holes remain a (partially) condensed system. Moreover, the superfluid density is comparable to the condensate density. In other words, the large-S regime does not display an instability to noncondensed phases. However, for cases with fewer than 1/3 flux quantum per lattice plaquette, we find that the fractional condensate depletion increases as the system approaches the Mott phase, giving rise to the possibility of a noncondensed state before the Mott phase is reached for systems with smaller S.

Journal article

Duric T, Ho AF, Lee DKK, 2009, Feshbach resonant scattering of three fermions in one-dimensional wells, PHYSICAL REVIEW A, Vol: 80, ISSN: 1050-2947

Journal article

Eastham PR, Cooper NR, Lee DKK, 2009, Vortex states of a disordered quantum Hall bilayer, PHYSICAL REVIEW B, Vol: 80, ISSN: 1098-0121

Journal article

Kay A, Lee DKK, Pachos JK, Plenio MB, Reuter ME, Rico Eet al., 2005, Quantum optics and informatics with single atoms and atomic ensembles, Optika i Spektroskopiya, Vol: 99, Pages: 357-374, ISSN: 0030-4034

The regular structures obtained by optical lattice technology and their behaviour are analysed from the quantum information perspective. Initially, we demonstrate that a triangular optical lattice of two atomic species, bosonic or fermionic, can be employed to generate a variety of novel spin-1/2 models that include effective three-spin interactions. Such interactions can be employed to simulate specific one or two dimensional physical systems that are of particular interest for their condensed matter and entanglement properties. In particular, connections between the scaling behaviour of entanglement and the entanglement properties of closely spaced spins are drawn. Moreover, three-spin interactions are well suited to support quantum computing without the need to manipulate individual qubits. By employing Raman transitions or the interaction of the atomic electric dipole moment with magnetic field gradients, one can generate Hamiltonians that can be used for the physical implementation of geometrical or topological objects. This work serves as a review article that also includes many new results.

Journal article

Kay A, Lee DKK, Pachos JK, Plenio MB, Reuter ME, Rico Eet al., 2005, Quantum information and triangular optical lattices, OPTICS AND SPECTROSCOPY, Vol: 99, Pages: 339-356, ISSN: 0030-400X

Journal article

Jack RL, Lee DKK, Cooper NR, 2005, Quantum and classical dissipative effects on tunneling in quantum Hall bilayers, PHYSICAL REVIEW B, Vol: 71, ISSN: 2469-9950

Journal article

Lee DKK, Schofield AJ, 2005, Metals without electrons: the physics of exotic quantum fluids, Visions of the future: Physics and Electronics, Editors: Thompson, Cambridge, Publisher: Cambridge University Press, Pages: 17-38, ISBN: 9780521805384

Book chapter

Jack RL, Lee DKK, Cooper NR, 2004, Dissipation and tunneling in quantum Hall bilayers, PHYSICAL REVIEW LETTERS, Vol: 93, ISSN: 0031-9007

Journal article

Lee DKK, Rapsch S, Chalker JT, 2003, Dirty quantum Hall ferromagnets and quantum Hall spin glasses, PHYSICAL REVIEW B, Vol: 67, ISSN: 2469-9950

Journal article

Jack RL, Lee DKK, 2002, Bosons in fluctuating gauge fields: Bose metal and phase separation, PHYSICAL REVIEW B, Vol: 66, ISSN: 1098-0121

Journal article

Rapsch S, Chalker JT, Lee DKK, 2002, Spin textures, screening, and excitations in dirty quantum Hall ferromagnets, PHYSICAL REVIEW LETTERS, Vol: 88, ISSN: 0031-9007

Journal article

Ng TK, Lee DKK, 2001, Possibility of a metallic phase in granular superconducting films, PHYSICAL REVIEW B, Vol: 63, ISSN: 1098-0121

Journal article

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