Strong Coulomb Coupling Research Articles

Quantum Hall drag of exciton condensate in graphene

Excitons are pairs of electrons and holes bound together by the Coulomb interaction. At low temperatures, excitons can form a Bose-Einstein condensate (BEC), enabling macroscopic phase coherence and superfluidity. An electronic double layer (EDL), in which two parallel conducting layers are separated by an insulator, is an ideal platform to realize a stable exciton BEC. In an EDL under strong magnetic fields, electron-like and hole-like quasi-particles from partially filled Landau levels (LLs) bind into excitons and condense. However, in semiconducting double quantum wells, this magnetic-field-induced exciton BEC has been observed only in sub-Kelvin temperatures due to the relatively strong dielectric screening and large separation of the EDL. Here we report exciton condensation in bilayer graphene EDL separated by a few atomic layers of hexagonal boron nitride (hBN). Driving current in one graphene layer generates a quantized Hall voltage in the other layer, signifying coherent superfluid exciton transport. Owing to the strong Coulomb coupling across the atomically thin dielectric, we find that quantum Hall drag in graphene appears at a temperature an order of magnitude higher than previously observed in GaAs EDL. The wide-range tunability of densities and displacement fields enables exploration of a rich phase diagram of BEC across Landau levels with different filling factors and internal quantum degrees of freedom. The observed robust exciton superfluidity opens up opportunities to investigate various quantum phases of the exciton BEC and design novel electronic devices based on dissipationless transport.

Improved equation of state for finite‐temperature spin‐polarized electron liquids on the basis of Singwi–Tosi–Land–Sjölander approximation

An accurate expression for the exchange‐correlation free energy fxc of homogeneous electron fluids at finite temperatures is presented on the basis of Singwi–Tosi–Land–Sjölander (STLS) approximation. In addition to the derivation for the paramagnetic state, that for the ferromagnetic state is carried out afresh in which a correction in the strong Coulomb coupling regime is incorporated into the construction of the analytic expression as a function of the coupling constant and Fermi degeneracy. The fitting formula for fxc is then extended over any degree of spin polarization with the aid of available interpolation scheme. The proposed equation of state, called the iSTLS formula, shows reasonable agreement with the existing quantum Monte Carlo evaluations at finite temperatures in the paramagnetic state, thus giving a consensus for the thermodynamic functions between many‐body theories and computer simulations. On the other hand, the current status for the agreement between various evaluations of fxc is relatively unsatisfactory in the ferromagnetic state, suggesting the necessity for further investigations.

Finite-temperature quantum effects on confined charges.

A quantum system of N Coulomb charges confined within a harmonic trap is considered over a wide range of densities and temperatures. A recently described construction of an equivalent classical system is applied in order to exploit the rather complete classical description of harmonic confinement via liquid-state theory. Here, the effects of quantum mechanics on that representation are described with attention focused on the origin and nature of shell structure. The analysis extends from the classical strong Coulomb coupling conditions of dusty plasmas to the opposite limit of low temperatures and large densities characteristic of "warm, dense matter."

A DYNAMIC DENSITY FUNCTIONAL THEORY APPROACH TO DIFFUSION IN WHITE DWARFS AND NEUTRON STAR ENVELOPES

ABSTRACT We develop a multicomponent hydrodynamic model based on moments of the Born–Bogolyubov–Green–Kirkwood–Yvon hierarchy equations for physical conditions relevant to astrophysical plasmas. These equations incorporate strong correlations through a density functional theory closure, while transport enters through a relaxation approximation. This approach enables the introduction of Coulomb coupling correction terms into the standard Burgers equations. The diffusive currents for these strongly coupled plasmas is self-consistently derived. The settling of impurities and its impact on cooling can be greatly affected by strong Coulomb coupling, which we show can be quantified using the direct correlation function.

Non-destructive state detection for quantum logic spectroscopy of molecular ions.

Precision laser spectroscopy of cold and trapped molecular ions is a powerful tool in fundamental physics--used, for example, in determining fundamental constants, testing for their possible variation in the laboratory, and searching for a possible electric dipole moment of the electron. However, the absence of cycling transitions in molecules poses a challenge for direct laser cooling of the ions, and for controlling and detecting their quantum states. Previously used state-detection techniques based on photodissociation or chemical reactions are destructive and therefore inefficient, restricting the achievable resolution in laser spectroscopy. Here, we experimentally demonstrate non-destructive detection of the quantum state of a single trapped molecular ion through its strong Coulomb coupling to a well controlled, co-trapped atomic ion. An algorithm based on a state-dependent optical dipole force changes the internal state of the atom according to the internal state of the molecule. We show that individual quantum states in the molecular ion can be distinguished by the strength of their coupling to the optical dipole force. We also observe quantum jumps (induced by black-body radiation) between rotational states of a single molecular ion. Using the detuning dependence of the state-detection signal, we implement a variant of quantum logic spectroscopy of a molecular resonance. Our state-detection technique is relevant to a wide range of molecular ions, and could be applied to state-controlled quantum chemistry and to spectroscopic investigations of molecules that serve as probes for interstellar clouds.

Observation of an unexpected negative isotope shift inTh+229and its theoretical explanation

We have measured the hyperfine structure and isotope shifts of the 402.0 nm and 399.6 nm resonance lines in 229Th+. These transitions could provide pathways towards the 229Th isomeric nuclear state excitation. An unexpected negative isotope shift relative to 232Th+ is observed for the 399.6 nm line, indicating a strong Coulomb coupling of the excited state to the nucleus. We have developed a new all-order approach to the isotope shift calculations that is generally applicable to heavy atoms and ions with several valence electrons. The theoretical calculations provide an explanation for the negative isotope shift of the 399.6 nm transition and yield a corrected classification of the excited state. The calculated isotope shifts are in good agreement with experimental values.

Optically bright p-excitons indicating strong Coulomb coupling in transition-metal dichalcogenides

It is shown that the strong Coulomb coupling in intrinsic suspended semiconducting transition metal dichalcogenides can exceed the critical value needed for an excitonic ground state. The dipole-allowed optical excitations then correspond to intra-excitonic transitions such that the optically bright excitonic transitions near the Dirac points have a p-like symmetry, whereas the s-like states are dipole forbidden. The large intrinsic coupling strength seems to be a generic property of the semiconducting transition metal dichalcogenides and strong Coulomb-coupling signatures in the form of the optical selection rules can be observed even in samples grown on typical substrates like SiO2. For the examples of WS2 and WSe2, excellent agreement of the computed excitonic resonance energies with recent experiments is demonstrated.

Exciton mobility control throughsub−Åpacking modifications in molecular crystals

Exciton mobility in \ensuremath{\pi} stacks of organic chromophores is shown to be highly sensitive to the interference between long-range Coulombic coupling and a short-range coupling due to wave function overlap. A destructive interference, which leads to a compromised exciton bandwidth, can be converted to constructive interference (and an enhanced bandwidth) upon sub-Angstrom transverse displacements between neighboring chromophores. The feasibility of the control scheme is demonstrated theoretically on a derivative of terrylene, where the exciton is essentially immobile despite strong Coulombic coupling. A transverse slip of only 0.5 \AA{} along either the short or the long molecular axis boosts the exciton velocity to $2\ifmmode\times\else\texttimes\fi{}{10}^{4} \mathrm{m}/\mathrm{s}$. Changes in the mobility are correlated to changes in the absorption spectrum, allowing the latter to be used as a screen for high mobility aggregates.

The contact theorem for charged fluids: from planar to curved geometries

When a Coulombic fluid is confined between two parallel charged plates, an exact relation links the difference of ionic densities at contact with the plates to the surface charges of these boundaries. It no longer applies when the boundaries are curved, and we work out how it generalises when the fluid is confined between two concentric spheres (or cylinders), in two and in three space dimensions. The analysis is thus performed within the cell model picture. The generalised contact relation opens the possibility to derive new exact expressions, of particular interest in the regime of strong Coulombic couplings. Some emphasis is put on cylindrical geometry, for which we discuss in depth the phenomenon of counterion evaporation/condensation, and obtain novel results. Good agreement is found with Monte Carlo simulation data.

Effective potential and interdiffusion in binary ionic mixtures.

We calculate interdiffusion coefficients in a two-component, weakly or strongly coupled ion plasma (gas or liquid, composed of two ion species immersed into a neutralizing electron background). We use an effective potential method proposed recently by Baalrud and Daligaut [Phys. Rev. Lett. 110, 235001 (2013)]. It allows us to extend the standard Chapman-Enskog procedure of calculating the interdiffusion coefficients to the case of strong Coulomb coupling. We compute binary diffusion coefficients for several ionic mixtures and fit them by convenient expressions in terms of the generalized Coulomb logarithm. These fits cover a wide range of plasma parameters spanning from weak to strong Coulomb couplings. They can be used to simulate diffusion of ions in ordinary stars as well as in white dwarfs and neutron stars.

Dynamics of ultrathin laser targets with optimal parameters

The set of equations describing the motion of a thin (compared to the wavelength) target in the field of a laser pulse that takes into consideration separate motion of the electron and ion layers is derived. In the case of strong Coulomb coupling between the layers, the set of equation of motions of the layers is reduced to the well-known light-sail equation containing a self-consistent coefficient of nonlinear reflection of laser radiation by a moving target. The optimal thickness of the laser target at which the target acquires maximum energy for given laser-pulse parameters is determined. It is shown that this thickness depends not only on laser intensity, but also on laser-pulse duration and the ratio of electron and ion masses. The growth rates of transverse instability of optimal targets under their intense acceleration are analyzed. It is demonstrated that instability does not develop in the currently experimentally accessible range of laser intensities and pulse durations between 100 and 200 fs.

Counterion Density Profile around Charged Cylinders: The Strong-Coupling Needle Limit

Charged rod-like polymers are not able to bind all their neutralizing counterions: a fraction of them evaporates, while the others are said to be condensed. We study here counterion condensation and its ramifications, both numerically by means of Monte Carlo simulations employing a previously introduced powerful logarithmic sampling of radial coordinates and analytically, with special emphasis on the strong-coupling regime. We focus on the thin rod or needle limit that is naturally reached under strong Coulombic couplings, where the typical intercounterion spacing a' along the rod is much larger than its radius R. This regime is complementary and opposite to the simpler thick rod case where a' ≪ R. We show that due account of counterion evaporation, a universal phenomenon in the sense that it occurs in the same clothing for both weakly and strongly coupled systems, allows one to obtain excellent agreement between the numerical simulations and the strong-coupling calculations.

Surface transfer doping can mediate both colloidal stability and self-assembly of nanodiamonds

Although undoped diamond is insulating, hydrogenated bulk diamond surfaces exhibit surface conductivity under air and are electrochemically active in aqueous solutions. Due to their large surface/volume ratio, similar surface effects may exhibit a dramatic impact on the properties of nanodiamonds. Here we show that plasma-hydrogenated detonation nanodiamonds (NDs-H) display a positive zeta potential in water due to charge transfer with a redox couple involving oxygen in water. The transfer doping of NDs-H in water can be modulated by pH. Surprisingly, after acid addition, strong Coulomb coupling between NDs-H and adsorbed counterions induces the self-assembly of NDs-H into organized macro-structures reaching millimeter scale.

Near-field spatial mapping of strongly interacting multiple plasmonic infrared antennas

Near-field dipolar plasmon interactions of multiple infrared antenna structures in the strong coupling limit are studied using scattering-type scanning near-field optical microscope (s-SNOM) and theoretical finite-difference time-domain (FDTD) calculations. We monitor in real-space the evolution of plasmon dipolar mode of a stationary antenna structure as multiple resonantly matched dipolar plasmon particles are closely approaching it. Interparticle separation, length and polarization dependent studies show that the cross geometry structure favors strong interparticle charge-charge, dipole-dipole and charge-dipole Coulomb interactions in the nanometer scale gap region, which results in strong field enhancement in cross-bowties and further allows these structures to be used as polarization filters. The nanoscale local field amplitude and phase maps show that due to strong interparticle Coulomb coupling, cross-bowtie structures redistribute and highly enhance the out-of-plane (perpendicular to the plane of the sample) plasmon near-field component at the gap region relative to ordinary bowties.

Do Halo Nuclei Follow Rutherford Elastic Scattering at Energies Below the Barrier? The Case ofLi11

The first measurement of the elastic scattering of the halo nucleus 11Li and its core 9Li on 208Pb at energies near the Coulomb barrier is presented. The 11Li+208Pb elastic scattering shows a strong reduction with respect to the Rutherford cross section, even at energies well below the barrier and down to very small scattering angles. This drastic change of the elastic differential cross section observed in 11Li+208Pb is the consequence of the halo structure of 11Li, as it is not observed in the elastic scattering of its core 9Li at the same energies. Four-body continuum-discretized coupled-channels calculations, based on a three-body model of the 11Li projectile, are found to explain the measured angular distributions and confirm that the observed reduction is mainly due to the strong Coulomb coupling to the dipole states in the low-lying continuum of 11Li. These calculations suggest the presence of a low-lying dipole resonance in 11Li close to the breakup threshold.

Wannier excitons signalling strong Coulomb coupling in graphene

The Wannier equation for quasiparticles with a linear dispersion is investigated. It is shown that Coulomb bound compounds of mass less electron-hole pairs can only exist if the effective coupling strength exceeds the critical value of approximately 0.46. At the critical value, a second-order transition to a weak Coulomb regime is found. It is shown that the existence of bound excitons indicates an instability of the noninteracting ground state.

Scattering of the halo nucleus 11Li and its core 9Li on 208Pb at energies around the Coulomb barrier

The first measurement of the elastic scattering of the halo nucleus 11Li and its core 9Li on 208Pb at energies around the Coulomb barrier is presented. The 11Li reaction showed a large cross section for the breakup channel, even at energies well below the barrier. The analysis of the 11Li + 208Pb scattering data in terms of the continuum-discretized coupled-channel calculations indicates that the effect of the coupling to the breakup channels produces a strong suppression of the elastic cross section at energies above and below the barrier. This effect is mainly due to the strong Coulomb coupling to the dipole states in the low-lying continuum of 11Li.

Experiments and Simulations of Particle Flows in a Magnetized Dust Torus

AbstractAn overview is given of the confinement and dynamical phenomena observed in experiments and simulations of magnetized dust tori. Due to the presence of gravity, a strongly inhomogeneous velocity field is found along the circumference of the torus. The simulations show that the dust flow, which is unsheared and nearly incompressible, exhibits a distinct shell structure, which can be understood by rapid frictional cooling and strong Coulomb coupling. At lower frictional damping, the symmetry of the flow can be spontaneously broken, leading to a region of strong velocity shear and excitation of Kelvin‐Helmholtz instabilities. New experimental evidence of counterflows is found (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Excitonic resonances as fingerprint of strong Coulomb coupling in graphene

Within a self-consistent microscopic theory, the conditions for the existence of a strongly Coulomb-correlated phase in graphene is explored, and its fingerprints in the optical spectra are investigated. A second-order semimetal-to-insulator transition is predicted if the effective fine-structure constant exceeds the critical value 1/2. Above this value, the Coulomb interaction opens a gap in the quasiparticle spectrum that increases rapidly with increasing coupling strength. Energetically below the gap, the optical spectra are predicted to exhibit pronounced excitonic resonances that are superimposed on the Drude-like response of the filled graphene π-band. Experimental observation of these excitons could serve as a fingerprint for the existence of the Coulomb-correlated phase. Increasing the coupling constant results in a blueshift and increasing oscillator strength of the dominant resonance.

Theoretical description of spherically confined, strongly correlated Yukawa plasmas

A theoretical description of the radial density profile for charged particles with Yukawa interaction in a harmonic trap is described. At strong Coulomb coupling shell structure is observed in both computer simulations and experiments. Correlations responsible for such shell structure are described here using a recently developed model based in density functional theory. A wide range of particle number, Coulomb coupling, and screening lengths is considered within the fluid phase. A hypernetted chain approximation shows the formation of shell structure, but fails to give quantitative agreement with Monte Carlo simulation results at strong coupling. Significantly better agreement is obtained within the hypernetted chain structure using a renormalized coupling constant, representing bridge function corrections.