Showing papers by "Aron Pinczuk published in 2008"

Observation of anomalous phonon softening in bilayer graphene.

Abstract: The interaction of electron-hole pairs with lattice vibrations exhibits a wealth of intriguing physical phenomena such as the renowned Kohn anomaly. Here we report the observation in bilayer graphene of an unusual phonon softening that provides the first experimental proof for another type of phonon anomaly. Similar to the Kohn anomaly, which is a logarithmic singularity in the phonon group velocity [W. Kohn, Phys. Rev. Lett. 2, 393 (1959)], the observed phonon anomaly exhibits a logarithmic singularity in the optical-phonon energy. Arising from a resonant electron-phonon coupling effect, the anomaly was also expected, albeit not observed, in monolayer graphene. We propose an explanation for why it is easier to observe in bilayer samples.

A molecular state of correlated electrons in a quantum dot

Abstract: Four electrons in a semiconductor quantum dot exhibit similar correlation effects to those found in a molecule. Excitations of these electrons can be probed by inelastic light scattering, which reveals a decoupling of their rigid rotational motion from their spin excitations. 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 dot1,2,3, where at vanishing electron density the Coulomb interaction between electrons rigidly fixes their relative positions as those of the nuclei in a molecule4,5,6,7,8,9,10,11,12,13,14. In this limit, the neutral few-body excitations are roto-vibrations, which have either rigid-rotor or relative-motion character15. In the weak correlation regime, on the contrary, the Coriolis force mixes rotational and vibrational motions. Here, we report evidence for 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 using inelastic light scattering16,17,18 in quantum dots containing four electrons19. Spectra of low-lying excitations associated with changes of the relative-motion wavefunction—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 by the configuration-interaction method20 are in agreement with the observed roto-vibrational modes and indicate that such molecular excitations develop at the onset of short-range correlation.

Soft spin wave near nu=1: evidence for a magnetic instability in Skyrmion systems.

Abstract: The ground state of the two-dimensional electron gas near nu=1 is investigated by inelastic light scattering measurements carried down to very low temperatures. Away from nu=1, the ferromagnetic spin wave collapses and a new low-energy spin wave emerges below the Zeeman gap. The emergent spin wave shows soft behavior as its energy increases with temperature and reaches the Zeeman energy for temperatures above 2 K. The observed softening indicates an instability of the two-dimensional electron gas towards a magnetic order that breaks spin rotational symmetry. We discuss our findings in light of the possible existence of a Skyrme crystal.

Spin Texture and Magnetoroton Excitations at ν = 1 / 3

Abstract: Neutral spin texture (ST) excitations at $\ensuremath{ u}=1/3$ 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 magnetoroton minimum. With increasing in-plane magnetic field these mode energies cross at a critical ratio of the Zeeman and Coulomb energies of ${\ensuremath{\eta}}_{c}=0.020\ifmmode\pm\else\textpm\fi{}0.001$. Surprisingly, the intensity of the ST mode grows with temperature in the range in which the magnetoroton 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 $\ensuremath{ u}=1/3$.

Optical Anisotropy of Electronic Excitations in Elliptical Quantum Dots

Abstract: 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.

Optical control of energy-level structure of few electrons in AlGaAs/GaAs quantum dots

Abstract: Optical control of the lateral quantum confinement and number of electrons confined in nanofabricated GaAs/AlGaAs quantum dots is achieved by illumination with a weak laser beam that is absorbed in the AlGaAs barrier. Precise tuning of energy-level structure and electron population is demonstrated by monitoring the low-lying transitions of the electrons from the lowest quantum-dot energy shells by resonant inelastic light scattering. These findings open the way to the manipulation of single electrons in these quantum dots without the need of external metallic gates.

Optical Control of Energy-Level Structure of Few Electrons in AlGaAs/GaAs Quantum Dots

Abstract: Optical control of the lateral quantum confinement and number of electrons confined in nanofabricated GaAs/AlGaAs quantum dots is achieved by illumination with a weak laser beam that is absorbed in the AlGaAs barrier. Precise tuning of energy-level structure and electron population is demonstrated by monitoring the low-lying transitions of the electrons from the lowest quantum-dot energy shells by resonant inelastic light scattering. These findings open the way to the manipulation of single electrons in these quantum dots without the need of external metallic gates.

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

Abstract: 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.

Correlated states and spin transitions in nanofabricated AlGaAs/GaAs few-electron quantum dots probed by inelastic light scattering

Abstract: Spin transitions and correlated few-electron states are investigated by resonant inelastic light scattering in dilute arrays of GaAs/AlGaAs modulation-doped quantum dots (QDs) fabricated by electron-beam lithography and low impact reactive-ion etching We focus on QDs with four electrons We show that at moderate magnetic fields, the ground state is a singlet with total spin S=0 A rich spectrum of distinct spin and charge inter-shell excitations is found, which cannot be described by a mean-field Hartree–Fock framework based on the quantum description of Fock–Darwin energy levels Instead, the experimental results are well modeled by numerical evaluations within a full configuration interaction approach that highlights the impact of correlation effects in this configuration This work demonstrates that the sensitivity reached by resonant inelastic light scattering enables the study of few-electron effects in QDs formed by state-of-the-art nanofabrication processes under the extreme conditions of low temperatures and high magnetic fields

Signatures of composite-fermion metals in electron bilayers at νT = 1

Abstract: Composite-fermion metals occur in the quantum Hall bilayers at total Landau level filling fraction ν T =1 as the tunneling gap Δ SAS collapses by application of in-plane magnetic fields. Experimental evidence is obtained from the observation of a spin continuum below the Zeeman energy by resonant inelastic light scattering. These low-lying spin modes are assigned to quasi-particle excitations, where spin and composite-fermion Landau level index change simultaneously.

Inelastic light scattering measurements of spin excitations in Skyrmion systems

Abstract: The magnetic properties of the 2DES close to ν = 1 are studied by the direct measurement of the low-lying spin excitation spectrum by inelastic light scattering at sub-Kelvin temperatures. The results suggest an instability of the two-dimensional electron systems (2DES) toward magnetic order at low temperatures. The spin excitation spectra are consistent with the ordering of the in-plane components of spin in a square crystal phase proposed in theoretical evaluations. Our experiments create venues for the determination of spin phases from measurements of low-lying spin excitations by inelastic light scattering.