George Simion

Bio: George Simion is an academic researcher from Katholieke Universiteit Leuven. The author has contributed to research in topics: Quantum Hall effect & Quantum spin Hall effect. The author has an hindex of 9, co-authored 31 publications receiving 230 citations. Previous affiliations of George Simion include IMEC & Purdue University.

Friedel oscillations in a Fermi liquid

Abstract: The problem of the Friedel oscillations in two- and three-dimensional electron Fermi liquids is examined by means of the many-body local fields theory as aided by the most recent results of accurate numerical studies based on the quantum Monte Carlo method. Within linear response, an exact answer is obtained for the amplitude of the electron density distortion due to both short- and long-ranged impurity potentials. It is discussed how a measurement of the local environment of an impurity embedded in an otherwise homogeneous electron liquid can be used to characterize the systems as a Fermi liquid as well as to extract the magnitude of the static many-body corrections beyond the random phase approximation for wave vector equal to $2{k}_{F}$.

Investigation of Microwave Loss Induced by Oxide Regrowth in High- Q Niobium Resonators

Abstract: To improve the performance of state-of-the-art superconducting quantum devices, microwave loss due to defects at amorphous interfacial layers must be reduced, via proper surface treatment. The authors study niobium resonators after removing native oxides by HF etching, which reduces losses about tenfold and yields a quality factor of $7\ifmmode\times\else\texttimes\fi{}{10}^{6}$ in the single-photon limit. Losses reappear as oxides form upon exposure to air; Nb${}_{2}$O${}_{5}$ is the only surface oxide that grows significantly in the first week. These findings are of interest for a panoply of devices, inluding superconducting qubits, quantum-limited amplifiers, microwave kinetic-inductance detectors, and single-photon detectors.

Formation of helical domain walls in the fractional quantum Hall regime as a step toward realization of high-order non-Abelian excitations

Abstract: We propose an experimentally-feasible system based on spin transitions in the fractional quantum Hall effect regime where parafermions, high-order non-abelian excitations, can be potentially realized. We provide a proof-of-concept experiments showing that in specially designed heterostructures spin transitions at a filling factor 2/3 can be induced electrostatically, allowing local control of polarization and on-demand formation of helical domain walls with fractionalized charge excitations, a pre-requisite ingredient for parafermions formation. We also present exact diagonalization numerical studies of domain walls formed between domains with different spin polarization in the fractional quantum Hall effect regime and show that they indeed possess electronic and magnetic structure needed for parafermion formation when coupled to an s-wave superconductor.

The Hierarchy of Incompressible Fractional Quantum Hall States

Abstract: The correlations that give rise to incompressible quantum liquid (IQL) states in fractional quantum Hall systems are determined by the pseudopotential $V(\mathcal R)$ describing the interaction of a pair of Fermions in a degenerate Landau level (LL) as a function of relative pair angular momentum $\mathcal R$. $V(\mathcal R)$ is known for a number of different Fermion systems, e.g. electrons in the lowest Landau level (LL0) or the first excited Landau level (LL1), and for quasiparticles of Laughlin-Jain IQL states. Laughlin correlations, the avoidance of pair states with the smallest values of $\mathcal R$, occur only when $V(\mathcal R)$ satisfies certain conditions. We show that Jain's composite Fermion (CF) picture is valid only if the conditions necessary for Laughlin correlations are satisfied, and we present a rigorous justification of the CF picture without the need of introducing an "irrelevant" mean field energy scale. Electrons in LL1 and quasielectrons in IQL states (e.g. QEs in CF LL1) do not necessarily support Laughlin correlations. Numerical diagonalization studies for small systems of Fermions (electrons in LL0 or in LL1, and QEs in CF LL1), with the use appropriate pseudopotentials $V(\mathcal R)$, show clear evidence for different types of correlations. The relation between LL degeneracy $g=2\ell+1$ and number of Fermions $N$ at which IQL states are found is known for a limited range of $N$ values. However, no simple intuitive models that we have tried satisfactorily describe all of the systems we have studied. Successes and shortcomings of some simple models are discussed, and suggestions for further investigation are made.

Spin phase diagram of the ν e =4/11 composite fermion liquid

Abstract: Spin polarization of the ``second-generation'' ${\ensuremath{ u}}_{e}=4∕11$ fractional quantum Hall state (corresponding to an incompressible liquid in a one-third-filled composite fermion Landau level) is studied by exact diagonalization. A spin phase diagram is determined for GaAs structures of different widths and electron concentrations. Transition between the polarized and partially unpolarized states with distinct composite fermion correlations is predicted for realistic parameters.

Superlattices and microstructures

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Semiconductor science and technology, and Nobel physics prizes

Abstract: Semiconductor science and technology (S T) is a very active branch in the field of natural science, and is also a typical example embodying the development of advanced S T. The Nobel prize for physics is a prize of the highest honour in the world, and there are certain relationships between this award and semiconductor S T. We explore and analyze these inherent relationships which have practical importance for our understanding of the development of semiconductor S T and for predicting its future.

Quantum Hall physics : Hierarchies and conformal field theory techniques

Abstract: The fractional quantum Hall effect, being one of the most studied phenomena in condensed matter physics during the past 30 years, has generated many ground-breaking new ideas and concepts. Very ear ...

Paired Hall States

Abstract: The principle that perturbation in quantum statistics should be accompanied by application of an appropriate magnetic field has been successful in giving a simple understanding of major qualitative features of the fractional quantized Hall states and related anyon superconducting states. In these applications, the starting point is one or more filled Landau levels. Here we consider the question of perturbation around free fermions. We argue that very near this point the statistical interactions are weak and their effects calculable; nevertheless they have the important qualitative consequence that a p-wave BCS pairing instability is triggered. The result is a new line of incompressible states in the (inverse) filling-fraction-statistics plane. This line extrapolates to a state obeying Fermi statistics at filling fractio 12, which is a candidate to describe electron states. A variety of techniques is then employed to elucidate the properties of this state and the unusual quasiparticles it supports. We believe the state is in the same universality class as one Halperin proposed based on grouping electrons into pairs of tightly bound bosonic molecules, which form a correlated state of the Laughlin type. We report the results of extensive numerical work which establishes firmly the existence of an incompressible state with the properties we predict, including the very unusual quasiparticles, for simple model potentials. We also investigate the situation for realistic potentials, and conclude that a paired Hall state of the type investigated here is a good candidate to describe real 2d electron gases, especially for thick samples and higher Landau levels, quite possibly including the state at filling fraction 52 that has already been observed.