arXiv subjects

Ken W. West

Publications and source records attributed to Ken W. West.

At least 37 records · Page 2Linked to original sources

arXiv2011

Vertical Field-Effect Transistor Based on Wavefunction Extension

We demonstrate a mechanism for a dual layer, vertical field-effect transistor, in which nearly-depleting one layer will extend its wavefunction to overlap the other layer and increase tunnel current. We characterize this effect in a specially designed GaAs/AlGaAs device, observing a tunnel current increase of two orders of magnitude at cryogenic temperatures, and we suggest extrapolations of the design to other material systems such as graphene.

cond-mat.mes-hall↗

arXiv2011

Degenerate versus semi-degenerate transport in a correlated 2D hole system

It has been puzzling that the resistivity of high mobility two-dimensional(2D) carrier systems in semiconductors with low carrier density often exhibits a large increase followed by a decrease when the temperature ($T$) is raised above a characteristic temperature comparable with the Fermi temperature ($T_F$). We find that the metallic 2D hole system (2DHS) in GaAs quantum well (QW) has a linear density ($p$) dependent conductivity, $σ\approx eμ^*(p-p_0)$, in both the degenerate (T< T_F$) is originated from the reduced $p_0$, the density of immobile carriers in a two-phase picture.

cond-mat.str-el↗

arXiv2010

Spin reconstruction in quantum wires subject to a perpendicular magnetic field

We study the effects of a perpendicular magnetic field on the spin and charge distribution across a quantum wire. Using momentum resolved tunneling between two parallel wires we measure the dispersion relation for different perpendicular magnetic fields. We find that as the magnetic field increases, a strip of spin polarized electrons separates in the cross section of the wire. We provide a quantitative description of the emerging structure within a Hartree-Fock approximation, showing that it results from exchange interaction between electrons in the wire. We discuss the relation of these results to the structure of Quantum Hall edge states.

cond-mat.str-el↗

arXiv2010

Correlated electrons in optically-tunable quantum dots: Building an electron dimer molecule

We observe the low-lying excitations of a molecular dimer formed by two electrons in a GaAs semiconductor quantum dot in which the number of confined electrons is tuned by optical illumination. By employing inelastic light scattering we identify the inter-shell excitations in the one-electron regime and the distinct spin and charge modes in the interacting few-body configuration. In the case of two electrons a comparison with configuration-interaction calculations allows us to link the observed excitations with the breathing mode of the molecular dimer and to determine the singlet-triplet energy splitting.

cond-mat.mes-hall↗

arXiv2009

A Phase Diagram for Quantum Hall Bilayers with Strong Inter-layer Correlation

Inter-layer excitonic coherence in a quantum Hall bilayer with negligible tunneling is monitored by measurements of low-lying spin excitations. At $ν_T =1$ new quasiparticle excitations are observed above a transition temperature revealing a competing metallic phase. For magnetic fields above an onset Zeeman energy this metallic phase has full spin polarization. A phase diagram in the parameter space of temperature and Zeeman energy reveals that the transition temperature increases at higher fields. This unexpected result suggests intriguing impacts of spin polarization in the highly correlated phases.

cond-mat.other↗

arXiv2009

A molecular state of correlated electrons in a quantum dot

Correlation among particles in finite quantum systems leads to complex behaviour and novel states of matter. One remarkable example is predicted to occur in a semiconductor quantum dot (QD) where at vanishing density the Coulomb correlation among electrons rigidly fixes their relative position as that of the nuclei in a molecule. In this limit, the neutral few-body excitations are roto-vibrations, which have either rigid-rotor or relative-motion character. In the weak-correlation regime, on the contrary, the Coriolis force mixes rotational and vibrational motions. Here we report evidence of roto-vibrational modes of an electron molecular state at densities for which electron localization is not yet fully achieved. We probe these collective modes by inelastic light scattering in QDs containing four electrons. Spectra of low-lying excitations associated to changes of the relative-motion wave function -the analogues of the vibration modes of a conventional molecule- do not depend on the rotational state represented by the total angular momentum. Theoretical simulations via the configuration-interaction (CI) method are in agreement with the observed roto-vibrational modes and indicate that such molecular excitations develop at the onset of short-range correlation.

cond-mat.mes-hall↗

arXiv2008

Optical Anisotropy of Electronic Excitations in Elliptical Quantum Dots

The authors report that anisotropic confining potentials in laterally-coupled semiconductor quantum dots (QDs) have large impacts in optical transitions and energies of inter-shell collective electronic excitations. The observed anisotropies are revealed by inelastic light scattering as a function of the in-plane direction of light polarization and can be finely controlled by modifying the geometrical shape of the QDs. These experiments show that the tuning of the QD confinement potential offers a powerful method to manipulate electronic states and far-infrared inter-shell optical transitions in quantum dots.

cond-mat.mes-hall↗

arXiv2008

Evidence of a first-order quantum phase transition of excitons in electron double layers

Complexity in many-particle systems occurs through processes of qualitative differentiation. These are described by concepts such as emerging states with specific symmetries that are linked to order parameters. In quantum Hall phases of electrons in semiconductor double layers with large inter-layer electron correlation there is an emergent many body exciton phase with an order parameter that measures the condensate fraction of excitons across the tunneling gap. As the inter-layer coupling is reduced by application of an in-plane magnetic field, this excitonic insulating state is brought in competition with a Fermi-metal phase of composite fermions (loosely, electrons with two magnetic flux quanta attached) stabilized by intra-layer electron correlation. Here we show that the quantum phase transformation between metallic and excitonic insulating states in the coupled bilayers becomes discontinuous (first-order) by impacts of different terms of the electron-electron interactions that prevail on weak residual disorder. The evidence is based on precise determinations of the excitonic order parameter by inelastic light scattering measurements close to the phase boundary. While there is marked softening of low-lying excitations, our experiments underpin the roles of competing orders linked to quasi-particle correlations in removing the divergence of quantum fluctuations.

cond-mat.mes-hall↗

arXiv2007

Spin-texture and magneto-roton excitations at nu=1/3

Neutral spin texture ST excitations at nu=1/3 are directly observed for the first time by resonant inelastic light scattering. They are determined to involve two simultaneous spin- flips. At low magnetic fields, the ST energy is below that of the magneto-roton minimum. With increasing in-plane magnetic field these modes energies cross at a critical ratio of the Zeeman and Coulomb energies of eta_c=0.020 +- 0.001. Surprisingly, the intensity of the ST mode grows with temperature in the range in which the magneto-roton modes collapse. The temperature dependence is interpreted in terms of a competition between coexisting phases supporting different excitations. We consider the role of the ST excitations in activated transport at nu=1/3.

cond-mat.str-el↗

arXiv2007

Control of optical emission in doped GaAs/AlGaAs nanofabricated quantum dots

Dilute arrays of GaAs/AlGaAs modulation-doped quantum dots (QDs) fabricated by electron-beam lithography and low impact reactive-ion etching exhibit highly homogeneous luminescence. Single quantum dots display spectral emission with peak energies and linewidths linked largely to the geometrical diameter of the dot and to the built-in electron population. Excitonic-like and biexcitonic-like emission intensities have activation energy of about 2 meV. These results highlight the potential of high quality nanofabricated QDs for applications in areas that require fine control of optical emission.

cond-mat.other↗

arXiv2007

Metamorphosis of a Quantum Hall Bilayer State into a Composite Fermion Metal

Composite fermion metal states emerge in quantum Hall bilayers at total Landau level filling factor $ν_T$=1 when the tunneling gap collapses by application of in-plane components of the external magnetic field. Evidence of this transformation is found in the continua of spin excitations observed by inelastic light scattering below the spin-wave mode at the Zeeman energy. The low-lying spin modes are interpreted as quasiparticle excitations with simultaneous changes in spin orientation and composite fermion Landau level index.

cond-mat.mes-hall↗

arXiv2005

Observation of low-lying excitations of electrons in coupled quantum dots

Tunneling excitations of electrons in dry-etched modulation-doped AlGaAs/GaAs coupled quantum dots (QDs) are probed by resonant inelastic light scattering. A sequence of intra- and intershell excitations are found at energies determined by the interplay between the QD confinement energy $\hbar ω_0$ and the tunneling gap $Δ_{SAS}$, the splitting between the symmetric and anti-symmetric delocalized single particle molecular states. The narrow line-widths displayed by electronic excitations in these nanostructures indicate promising venues for the spectroscopic investigation of entanglement of electron states in these artificial molecules.

cond-mat.mes-hall↗

arXiv2005

Evidence of correlation in spin excitations of few-electron quantum dots

We report inelastic light scattering measurements of spin and charge excitations in nanofabricated AlGaAs/GaAs quantum dots with few electrons. A narrow spin excitation peak is observed and assigned to the intershell triplet-to-singlet monopole mode of dots with four electrons. Configurationinteraction theory provides precise quantitative interpretations that uncover large correlation effects that are comparable to exchange Coulomb interactions.

cond-mat.mes-hall↗

arXiv2005

Room-temperature excitonic absorption in quantum wires

We measured absorption spectra of T-shaped quantum wires at room temperature using waveguide-transmission spectroscopy. Strong and narrow room-temperature one-dimensional-exciton absorption peak was observed, which shows peak modal absorption coefficient of 160 cm$^{-1}$ per 20 wires with $Γ$-factor of $4.3\times10^{-3}$, width of 7.2 meV, and strong polarization anisotropy.

cond-mat.mes-hall↗

arXiv2005

Evolution of excitons via biexcitons to an electron-hole plasma without level crossing between band edge and exciton in a quantum wire

A recent single quantum wire is of sufficient quality to reveal new details of the photoluminescence (PL) evolution with increasing electron--hole (e--h) pair density. At a pair density of 3.6 $\times$ 10$^{3}$ cm$^{-1}$, the PL is characteristic of biexcitons shifted below the exciton peak by the 2.8-meV biexciton binding. At the pair density of 1.2 $\times$ 10$^{5}$ cm$^{-1}$, the biexciton peak broadens without energy shift to an e--h plasma. At all pair densities up to 30 K, neither the exciton peak nor the one-dimensional (1D) continuum edge shows any shift. In contrast to prevailing theories, the low-energy edge of the plasma PL line never crosses the exciton peak and never makes contact with the 1D e--h continuum.

cond-mat.mes-hall↗

arXiv2005

Strong photo-absorption by a single quantum wire in waveguide-transmission spectroscopy

We measured the absorption spectrum of a single T-shaped, 14x6 nm lateral-sized quantum wire embedded in an optical waveguide using waveguide-transmission spectroscopy at 5 K. In spite of its small volume, the one-dimensional-exciton ground state shows a large absorption coefficient of 80 /cm, or a 98 % absorption probability for a single pass of the 500-um-long waveguide.

cond-mat.other↗

arXiv2005

Particle-hole symmetric Luttinger liquids in a quantum Hall circuit

We report current transmission data through a split-gate constriction fabricated onto a two-dimensional electron system in the integer quantum Hall (QH) regime. Split-gate biasing drives inter-edge backscattering and is shown to lead to suppressed or enhanced transmission, in marked contrast with the expected linear Fermi-liquid behavior. This evolution is described in terms of particle-hole symmetry and allows us to conclude that an unexpected class of gate-controlled particle-hole-symmetric chiral Luttinger Liquids (CLLs) can exist at the edges of our QH circuit. These results highlight the role of particle-hole symmetry on the properties of CLL edge states.

cond-mat.mes-hall↗

arXiv2004

Observation of collapse of pseudospin order in bilayer quantum Hall ferromagnets

The Hartree-Fock paradigm of bilayer quantum Hall states with finite tunneling at filling factor $ν$=1 has full pseudospin ferromagnetic order with all the electrons in the lowest symmetric Landau level. Inelastic light scattering measurements of low energy spin excitations reveal major departures from the paradigm at relatively large tunneling gaps. The results indicate the emergence of a novel correlated quantum Hall state at $ν$=1 characterized by reduced pseudospin order. Marked anomalies occur in spin excitations when pseudospin polarization collapses by application of in-plane magnetic fields.

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