Collective Density Excitations Research Articles

Tunable Transparency and Group Delay in Cavity Optomechanical Systems with Degenerate Fermi Gas

We theoretically investigate the optical response and the propagation of an external probe field in a Fabry–Perot cavity, which consists of a mechanical mode of trapped, ultracold, fermionic atoms inside and simultaneously driven by an optical laser field. We investigate the electromagnetically-induced transparency due to coupling of the optical cavity field with the collective density excitations of the ultracold fermionic atoms via radiation pressure force. Moreover, we discuss the variations in the phase and group delay of the transmitted probe field with respect to effective cavity detuning as well as pumping power. It is observed that the transmitted field is lagging in this fermionic cavity optomechanical system. Our study shall provide a method to control the propagation as well as the speed of the transmitted probe field in this kind of fermionic, ultracold, atom-based, optomechanical cavity system, which might have potential applications in optical communications, signal processing and quantum information processing.

Acoustic plasmons in type-I Weyl semimetals

Massless chiral fermions emergent in inversion symmetry-breaking Weyl semimetals (WSMs) reside in the vicinity of multiple low symmetry nodes and thus acquire strongly anisotropic dispersion. We investigate the longitudinal electromagnetic modes of two-component degenerate Weyl plasma relevant to the realistic band structure of type-I WSM. We show that the actual spectrum of three dimensional collective density excitations in TaAs family of WSM is gaples due to emergence of acoustic plasmons corresponding to out-of-phase oscillations of the plasma components. These modes exist around the [001] crystallographic direction and are weakly damped, thanks to large difference in the Weyl velocities of the $W_1$ and $W_2$ quasiparticles when propagating along [001]. We show that acoustic plasmons can manifest themselves as slow beatings of electric potential superimposed on fast plasmonic oscillations upon charge relaxation. The revealed acoustic modes can stimulate purely electronic superconductivity, collisionless plasmon instabilities, and formation of Weyl soundarons.

Collective spin density excitation of fractional quantum Hall states in dilute ultra-cold Bose atoms

We have studied collective spin density excitation of fractional quantum Hall effect (FQHE) in rotating Bose–Einstein condensation for the three filling fractions of first series of Jain’s composite fermion sequences. We have considered short-ranged contact interactions between the Bose atoms as well as long range Coulomb interactions to compare the nature of the spectra with FQHE of electrons. Using Monte-Carlo method for finite but large number of particles, the lowest order collective modes of spin-reversed sectors is calculated here, by computing the energy differences of the respective excitons from the fully polarized ground states.

Plasmonic Spin-Hall Effect in Surface Plasmon Polariton Focusing

Surface plasmon polaritons (SPPs) are fundamental collective charge density excitations at surfaces of solid-state plasmas. They can energize physical and chemical processes at the nanoscale, but control of the spatiotemporal evolution of their energy, momentum, and spin is necessary to effectively transfer their quantum properties to molecules and other nanoscopic objects. Thus, to design the coupling of SPPs into other modes of an electronic system, it is important to describe and control their nanofemto spatiotemporal distributions. We investigate the spatial SPP field and spin distributions when launched by illuminating a converging lens coupling structure in an Ag film with obliquely incident linearly or circularly polarized femtosecond light pulses. The propagation and focusing of SPPs are recorded as movies by scanning the pump–probe pulse delay and recording the evolving interference patterns between the SPP and optical fields by ultrafast interferometric time-resolved two-photon photoemission ele...

Electromagnetically Induced Absorption in Cavity Optomechanics System with a Bose–Einstein Condensate

We investigate the light propagation in the Bose–Einstein condensate (BEC)-cavity system, where the collective density excitation of the BEC serves as a mechanical oscillator coupled to the optical cavity field. The existence of optomechanically-induced absorption (OMIA) is demonstrated theoretically in the cavity optomechanics system under two-photon detuning driven by two-tone fields. We give the physical origin of the OMIA and its manipulation by adjusting the power of the pump field in the system. Moreover, there will be a transition between the absorption dip and transmission peak via dominating the pump power. The OMIA in this strong coupled BEC-cavity optomechanics system will provide a platform for potential applications in photoswitching.

Hidden multiparticle excitation in a weakly interacting Bose-Einstein condensate

We investigate multiparticle excitation effect on a collective density excitation as well as a single-particle excitation in a weakly interacting Bose--Einstein condensate (BEC). We find that although the weakly interacting BEC offers weak multiparticle excitation spectrum at low temperatures, this multiparticle excitation effect may not remain hidden, but emerges as bimodality in the density response function through the single-particle excitation. Identification of spectra in the BEC between the single-particle excitation and the density excitation is also assessed at nonzero temperatures, which has been known to be unique nature in the BEC at absolute zero temperature.

Spin-orbit-coupling-induced backaction cooling in cavity optomechanics with a Bose-Einstein condensate

We report a spin-orbit coupling induced back-action cooling in an optomechanical system, composed of a spin-orbit coupled Bose-Einstein condensate trapped in an optical cavity with one movable end mirror, by suppressing heating effects of quantum noises. The collective density excitations of the spin-orbit coupling mediated hyperfine states - serving as atomic oscillators equally coupled to the cavity field - trigger strongly driven atomic back-action. We find that the back-action not only revamps low-temperature dynamics of its own but also provides an opportunity to cool the mechanical mirror to its quantum mechanical ground state. Further, we demonstrate that the strength of spin-orbit coupling also superintends dynamic structure factor and squeezes nonlinear quantum noises, like thermo-mechanical and photon shot noise, which enhances optomechanical features of hybrid cavity beyond the previous investigations. Our findings are testable in a realistic setup and enhance the functionality of cavity-optomechanics with spin-orbit coupled hyperfine states in the field of quantum optics and quantum computation.

Emergence of a Metallic Quantum Solid Phase in a Rydberg-Dressed Fermi Gas.

We examine possible low-temperature phases of a repulsively Rydberg-dressed Fermi gas in a three-dimensional free space. It is shown that the collective density excitations develop a roton minimum, which is softened at a wave vector smaller than the Fermi wave vector when the particle density is above a critical value. The mean field calculation shows that, unlike the insulating density wave states often observed in conventional condensed matters, a self-assembled metallic density wave state emerges at low temperatures. In particular, the density wave state supports a Fermi surface and a body-centered-cubic crystal order at the same time with the estimated critical temperature being about one tenth of the noninteracting Fermi energy. Our results suggest the emergence of a fermionic quantum solid that should be observable in the current experimental setup.

Regarding the relation between the photon-roton spectrum and the single-particle excitation spectrum in liquids with Bose-Einstein condensate

An analysis of the existing theoretical ideas as to the relationship between the spectrum of collective density excitations and the spectrum of single-particle excitations in liquids with a Bose-Einstein condensate. Using available experimental results, we examine the problem of the relationship between the collective phonon-roton excitations in different liquids, and the Bose-Einstein condensate.

Static and dynamic properties of liquid Zn, Cd and Hg divalent metals: An orbital free ab initio molecular dynamics study

We report a study on several static and dynamic properties of liquid Zn, Cd, and Hg divalent metals at thermodynamic conditions near their respective triple points. The calculations have been carried out by using the orbital free ab initio molecular dynamics (OF-AIMD) simulation technique which is based on the density functional theory. Results are reported for a range of static structural magnitudes, such as static structure factors, pair distribution functions and coordination numbers. A comparison with the available X-ray diffraction data shows that the OF-AIMD method can provide a reasonable description of the static structure. As for the dynamic properties, results are reported for both single and collective dynamics. The calculated dynamic structure factors show side peaks which point to the existence of collective density excitations. Finally, we report the calculation of some transport coefficients which are compared with the corresponding experimental data. Comparison with other AIMD methods show some discrepancies in dynamic properties, and possible modifications of the approach used in this study are suggested accordingly.

Tunable Nonequilibrium Luttinger Liquid Based on Counterpropagating Edge Channels

We investigate energy transfer between counter-propagating quantum Hall edge channels (ECs) in a two-dimensional electron system at filling factor \nu=1. The ECs are separated by a thin impenetrable potential barrier and Coulomb coupled, thereby constituting a quasi one-dimensional analogue of a spinless Luttinger liquid (LL). We drive one, say hot, EC far from thermal equilibrium and measure the energy transfer rate P into the second, cold, EC using a quantum point contact as a bolometer. The dependence of P on the drive bias indicates breakdown of the momentum conservation, whereas P is almost independent on the length of the region where the ECs interact. Interpreting our results in terms of plasmons (collective density excitations), we find that the energy transfer between the ECs occurs via plasmon backscattering at the boundaries of the LL. The backscattering probability is determined by the LL interaction parameter and can be tuned by changing the width of the electrostatic potential barrier between the ECs.

Quantum Bohm correction to polarization spectrum of graphene

In this paper, by using a quantum hydrodynamic plasma model which incorporates the important quantum statistical pressure and electron diffraction force, we present the corrected plasmon dispersion relation for graphene which includes a k4 quantum term arising from the collective electron density wave interference effects. This correction may well describe the shortcoming of the previous results based on the classical hydrodynamics and confirms that the quantum hydrodynamic model may be as effective as the random phase approximation in successful description of the collective density excitations in quantum plasmas. It is clearly observed that the quantum correction due to the collective interaction of electron waves gives rise to significant contribution in the dispersion behavior of the collective plasmon density waves in a wide range of wavelength, as a fundamental property of the monolayer atom-thick graphene. It is revealed that the plasmon density-perturbation linear phase-speed in graphene possesses some universal minimum characteristic value, in the absence of an external magnetic field. It is further remarked that such correction also has important effect on the dielectric function, hence on the impurity screening, in graphene.

Electromagnetically-Induced Transparency in Optomechanical Systems with Bose–Einstein Condensate

We study theoretically electromagnetically-induced transparency (EIT) in an optomechanical system that consists of a Bose–Einstein condensate (BEC) trapped inside a Fabry–Perot cavity driven by the laser field. The quantized laser field interacts with the collective density excitations (Bogoliubov mode) of the condensate. The phenomenon of electromagnetically-induced transparency is observed in the output of the probe laser field. We show that the probe laser field can efficiently be amplified or attenuated depending on the interaction of the BEC with the pump laser field. Furthermore, we explain the effect of atom–atom interaction on the transparency window and show that for increasing atom–atom interaction the transparency window increases.

An ab initio study of the structure and dynamics of bulk liquid Cd and its liquid–vapor interface

Several static and dynamic properties of bulk liquid Cd at a thermodynamic state near its triple point have been calculated by means of ab initio molecular dynamics simulations. The calculated static structure shows a very good agreement with the available experimental data. The dynamical structure reveals collective density excitations with an associated dispersion relation which points to a small positive dispersion. Results are also reported for several transport coefficients. Additional simulations have also been performed at a slightly higher temperature in order to study the structure of the free liquid surface. The ionic density profile shows an oscillatory behavior with two different wavelengths, as the spacing between the outer and first inner layer is different from that between the other inner layers. The calculated reflectivity shows a marked maximum whose origin is related to the surface layering, along with a shoulder located at a much smaller wavevector transfer.

Orbital free ab initio molecular dynamics simulation study of some static and dynamic properties of liquid noble metals

Several static and dynamic properties of liquid Cu, Ag and Au at thermodynamic states near their respective melting points, have been evaluated by means of the orbital free ab-initio molecular dynamics simulation method. The calculated static structure shows good agreement with the available X-ray and neutron diffraction data. As for the dynamic properties, the calculated dynamic structure factors point to the existence of collective density excitations along with a positive dispersion for l-Cu and l-Ag. Several transport coefficients have been obtained which show a reasonable agreement with the available experimental data.

Cavity optomechanics with a Bose–Einstein condensate: normal mode splitting

We study the normal mode splitting in a system consisting of a Bose–Einstein condensates (BECs) trapped inside a Fabry–Pérot cavity driven by a single mode laser field. We analyze the variations in frequency and damping rate of the collective density excitation of a BEC imparted by the optical field. We study the occurrence of normal mode splitting which appears as consequences of the hybridization of the fluctuations of the intracavity field and the condensate mode. It is shown that normal mode splitting vanishes for weak coupling between the condensate mode and the intracavity field. Moreover, we investigate the normal mode splitting in the transmission spectrum of the cavity field.

Plasmons in a free-standing nanorod with a two-dimensional parabolic quantum well caused by surface states

The collective charge density excitations in a free-standing nanorod with a two-dimensional parabolic quantum well are investigated within the framework of Bohm—Pine's random-phase approximation in the two-subband model. The new simplified analytical expressions of the Coulomb interaction matrix elements and dielectric functions are derived and numerically discussed. In addition, the electron density and temperature dependences of dispersion features are also investigated. We find that in the two-dimensional parabolic quantum well, the intrasubband upper branch is coupled with the intersubband mode, which is quite different from other quasi-one-dimensional systems like a cylindrical quantum wire with an infinite rectangular potential. In addition, we also find that higher temperature results in the intersubband mode (with an energy of 12 meV (∼ 3 THz)) becoming totally damped, which agrees well with the experimental results of Raman scattering in the literature. These interesting properties may provide useful references to the design of free-standing nanorod based devices.

Entangling two Bose–Einstein condensates in a double cavity system

We propose a scheme to transfer the quantum state of light fields to the collective density excitations of a Bose–Einstein condensate (BEC) in a cavity. This scheme allows us to entangle two BECs in a double cavity setup by transferring the quantum entanglement of two light fields produced from a nondegenerate parametric amplifier to the collective density excitations of the two BECs. An EPR state of the collective density excitations can be created by a judicious choice of the system parameters.

Quantum noise reduction using a cavity with a Bose–Einstein condensate

We study an opto-mechanical system in which the collective density excitations (Bogoliubov modes) of a Bose–Einstein condensate (BEC) is coupled to a cavity field. We show that the optical force changes the frequency and the damping constant of the collective density excitations of the BEC. We further analyse the occurrence of normal mode splitting (NMS) due to mixing of the fluctuations of the cavity field and the fluctuations of the condensate with finite atomic two-body interaction. The NMS is found to vanish for small values of the two-body interaction. We further show that the density excitations of the condensate can be used to squeeze the output quantum fluctuations of the light beam. This system may serve as an opto-mechanical control of quantum fluctuations using a BEC.

Hamiltonian chaos in a coupled BEC–optomechanical-cavity system

We study a hybrid optomechanical system consisting of a Bose-Einstein condensate (BEC) trapped inside a single-mode optical cavity with a moving end-mirror. The intracavity light field has a dual role: it excites a momentum side-mode of the condensate, and acts as a nonlinear spring that couples the vibrating mirror to that collective density excitation. We present the dynamics in a regime where the intracavity optical field, the mirror, and the side-mode excitation all display bistable behavior. In this regime we find that the dynamics of the system exhibits Hamiltonian chaos for appropriate initial conditions.