Dilute Quantum Research Articles

Towards a quantum Monte Carlo–based density functional including finite-range effects: Excitation modes of a K39 quantum droplet

Some discrepancies between experimental results on quantum droplets made of a mixture of $^{39}\mathrm{K}$ atoms in different hyperfine states and their analysis within extended Gross-Pitaevskii theory (which incorporates beyond mean-field corrections) have been recently solved by introducing finite-range effects into the theory. Here we study the influence of these effects on the monopole and quadrupole excitation spectrum of extremely dilute quantum droplets using a density functional built from first-principles quantum Monte Carlo calculations, which can be easily introduced in the existing Gross-Pitaevskii numerical solvers. Our results show differences of up to $20%$ with those obtained within the extended Gross-Pitaevskii theory, likely providing another way to observe finite-range effects in mixed quantum droplets by measuring their lowest excitation frequencies.

Spin Dynamics in Mn:ZnSe Quantum Dots: A Pulsed High-Frequency EPR Study

Dilute magnetic semiconductor quantum dots (DMSQDs) have emerged as potential materials for future spin-based technologies. A critical parameter for the development of this technology is a relative...

Static-response theory and the roton-maxon spectrum of a flattened dipolar Bose-Einstein condensate

Dilute quantum gases offer a controllable platform for investigating quantum liquids, such as superfluid helium, in which correlations and fluctuations are encoded in the form of a quasiparticle energy (i.e., roton-maxon) dispersion. An approach to extracting the roton-maxon spectrum of a Bose-Einstein condensate is theoretically developed: the method relies on using an external one-dimensional optical lattice that effectively controls the response of the system, giving access to its elementary excitations.

Exact surface-wave spectrum of a dilute quantum liquid

We consider a dilute gas of bosons with repulsive contact interactions, described on the mean-field level by the Gross-Pitaevski\ifmmode \check{i}\else \v{i}\fi{} equation, and bounded by an impenetrable ``hard'' wall (either rigid or flexible). We solve the Bogoliubov-de Gennes equations for excitations on top of the Bose-Einstein condensate analytically, by using matrix-valued hypergeometric functions. This leads to the exact spectrum of gapless Bogoliubov excitations localized near the boundary. The dispersion relation for the surface excitations represents for small wave numbers $k$ a ripplon mode with fractional power law dispersion for a flexible wall, and a phonon mode (linear dispersion) for a rigid wall. For both types of excitation we provide, for the first time, the exact dispersion relations of the dilute quantum liquid for all $k$ along the surface, extending to $k\ensuremath{\rightarrow}\ensuremath{\infty}$. The small wavelength excitations are shown to be bound to the surface with a maximal binding energy $\mathrm{\ensuremath{\Delta}}=\frac{1}{8}{(\sqrt{17}\ensuremath{-}3)}^{2}m{c}^{2}\ensuremath{\simeq}0.158\phantom{\rule{0.16em}{0ex}}m{c}^{2}$, which both types of excitation asymptotically approach, where $m$ is mass of bosons and $c$ bulk speed of sound. We demonstrate that this binding energy is close to the experimental value obtained for surface excitations of helium II confined in nanopores, reported in Phys. Rev. B 88, 014521 (2013).

Modeling of the tandem III–V dilute nitride bulk or quantum engineered/silicon solar cells

Tandem silicon solar cells are designed in order to lower the levelized cost of solar electricity by increasing the cell’s efficiency to more than 30%. Dual junction thin film III–V/Si cell efficiencies are limited to 20% because of high dislocation densities (>106 cm−2) due to lattice mismatch between them. GaAsyP1−x−yNx has a lattice constant matched with silicon at y = 4.7*x − 0.1 and has an optimal bandgap i.e. between 1.7 and 1.9 eV for tandem III–V’s with Si. This work designs a p-i-n GaAsPN/GaP symmetric or asymmetric (resonant thermos tunneling) multi-quantum well (MQW) device for a top sub-cell of a dual junction silicon tandem to minimize the effect of the degraded minority carrier transport properties of bulk dilute nitride layers and presents the simulated solar cell properties using a realistic drift-diffusion approach. Using extracted experimental spectral responses as well as other solar cell parameters of various silicon solar cell like: HIT, PERC, IBC etc, the tandem efficiencies of designed p-i-n MQWs devices in conjunction with silicon solar cells have been evaluated. The tandem efficiency of an optimized p-i-n MQWs device under AM 1.5 G spectrum can exceed 33% when in conjunction with a 25.6% efficient HIT silicon solar cell.

Plasmon-assisted bandgap engineering in dilute nitrides

Abstract The inherent ability of plasmonic bowtie nanoapertures (NAs) to localize the electromagnetic field at a subwavelength scale was exploited to engineer the H removal process in dilute nitrides at the nanometer level. Dilute nitride semiconductor alloys (e.g. GaAsN with a small percentage of nitrogen) are characterized by peculiar optoelectronic properties and, most importantly, by an even more peculiar response to hydrogen incorporation. In this class of materials, it is indeed possible to tune post-growth the alloy bandgap energy by a controlled incorporation of hydrogen atoms. The formation of N-H complexes neutralizes all the effects N has on the host matrix, among which is the strong narrowing of bandgap energy. In the present work, bowtie NAs resonant to the N-H complex dissociation energy were numerically modeled by finite element method simulations, realized by a lithographic approach, and characterized by scanning probe microscopy and resonant scattering spectroscopies. The conditions to get the maximum field enhancement at a specific position below the metal/semiconductor interface, namely at the dilute nitride quantum well position, were identified, demonstrating the ability to achieve a plasmon-assisted spatially selective hydrogen removal in a GaAsN/GaAs quantum well sample. Hydrogen removal through bowtie NAs turns out to be way more efficient (approximately two orders of magnitude) than through the plain surface, thus indicating that bandgap engineering through plasmonic nanostructures can be optimized for future efficient realization of site-controlled single-photon emitters and for their deterministic integration in plasmonic devices.

Transport phenomena in dilute quantum gases

Transport measurement is one of the most important ways to study the properties of matter. In this article, we discuss recent experiments in ultracold atomic gases where the analog of Landauer transport in mesoscopic devices is realized and spin dynamics in a strongly interacting Fermi gas is probed. In the latter case, we pay special attention to the peculiarity of spin dynamics due to identical spin rotation effect which leads to a novel form of spin diffusion current. This modifies the usual diffusion equation into a more complicated form and leads to important consequence for, in particular, transverse spin diffusion in ultracold Fermi gases.

Data transmission at 1300 nm using optical interposer comprising hybrid integrated silicon waveguide and dilute nitride electroabsorption modulator

High speed back-to-back transmission of NRZ data at 12.5 Gbit/s was achieved over a repeaterless optical network without the use of forward error correction or optical clock recovery using a hybrid integrated silicon photonics optical interconnect. The interconnect comprises an electroabsorption modulator based on dilute nitride multiple quantum well material on GaAs substrate optically coupled to large core silicon waveguide using passive alignment and flip-chip bonding.

Observation of universal dynamics in a spinor Bose gas far from equilibrium.

Predicting the dynamics of quantum systems far from equilibrium represents one of the most challenging problems in theoretical many-body physics1,2. While the evolution of a many-body system is in general intractable in all its details, relevant observables can become insensitive to microscopic system parameters and initial conditions. This is the basis of the phenomenon of universality. Far from equilibrium, universality is identified through the scaling of the spatio-temporal evolution of the system, captured by universal exponents and functions. Theoretically, this has been studied in examples as different as the reheating process in inflationary Universe cosmology3,4, the dynamics of nuclear collision experiments described by quantum chromodynamics5,6, and the post-quench dynamics in dilute quantum gases in non-relativistic quantum field theory7-11. However, an experimental demonstration of such scaling evolution in space and time in a quantum many-body system has been lacking. Here we observe the emergence of universal dynamics by evaluating spatially resolved spin correlations in a quasi-one-dimensional spinor Bose-Einstein condensate12-16. For long evolution times we extract the scaling properties from the spatial correlations of the spin excitations. From this we find the dynamics to be governed by an emergent conserved quantityand the transport of spin excitations towards low momentum scales. Our results establish an important class of non-stationary systems whose dynamics is encoded in time-independent scaling exponents and functions, signalling the existence of non-thermal fixed points10,17,18. We confirm that the non-thermal scaling phenomenon involves no fine-tuningof parameters, by preparing different initial conditions and observing the same scaling behaviour. Our analogue quantum simulation approach provides the basis with which to reveal the underlying mechanisms and characteristics of non-thermal universality classes. One may use this universality to learn, from experiments with ultracold gases, about fundamental aspects of dynamics studied in cosmology and quantum chromodynamics.

Three-dimensional droplets of swirling superfluids

A new method for the creation of 3D solitary topological modes, corresponding to vortical droplets of a two-component dilute superfluid, is presented. We use the recently introduced system of nonlinearly coupled Gross-Pitaevskii equations, which include contact attraction between the components, and quartic repulsion stemming from the Lee-Huang-Yang correction to the mean-field energy. Self-trapped vortex tori, carrying the topological charges m1=m2=1 or m1=m2=2 in their components, are constructed by means of numerical and approximate analytical methods. The analysis reveals stability regions for the vortex droplets (in broad and relatively narrow parameter regions for m1=m2=1 and m1=m2=2, respectively). The results provide the first example of stable 3D self-trapped states with the double vorticity, in any physical setting. The stable modes are shaped as flat-top ones, with the space between the inner hole, induced by the vorticity, and the outer boundary filled by a nearly constant density. On the other hand, all modes with hidden vorticity, i.e., topological charges of the two components m1=-m2=1, are unstable. The stability of the droplets with m1=m2=1 against splitting (which is the main scenario of possible instability) is explained by estimating analytically the energy of the split and un-split states. The predicted results may be implemented, exploiting dilute quantum droplets in mixtures of Bose-Einstein condensates.

Quantum liquid droplets in a mixture of Bose-Einstein condensates.

Quantum droplets are small clusters of atoms self-bound by the balance of attractive and repulsive forces. Here, we report on the observation of droplets solely stabilized by contact interactions in a mixture of two Bose-Einstein condensates. We demonstrate that they are several orders of magnitude more dilute than liquid helium by directly measuring their size and density via in situ imaging. We show that the droplets are stablized against collapse by quantum fluctuations and that they require a minimum atom number to be stable. Below that number, quantum pressure drives a liquid-to-gas transition that we map out as a function of interaction strength. These ultradilute isotropic liquids remain weakly interacting and constitute an ideal platform to benchmark quantum many-body theories.

Radial vortex core oscillations in Bose-Einstein condensates

Dilute ultracold quantum gases form an ideal and highly tunable system in which superuidity can be studied. Recently quantum turbulence in Bose-Einstein condensates was reported [PRL 103, 045310 (2009)], opening up a new experimental system that can be used to study quantum turbulence. A novel feature of this system is that vortex cores now have a finite size. This means that the vortices are no longer one dimensional features in the condensate, but that the radial behaviour and excitations might also play an important role in the study of quantum turbulence in Bose-Einstein condensates. In this paper we investigate these radial modes using a simplified variational model for the vortex core. This study results in the frequencies of the radial modes, which can be compared with the frequencies of the thoroughly studied Kelvin modes. From this comparison we find that the lowest (l=0) radial mode has a frequency in the same order of magnitude as the Kelvin modes. However the radial modes still have a larger energy than the Kelvin modes, meaning that the Kelvin modes will still constitute the preferred channel for energy decay in quantum turbulence.

Contact matrix in dilute quantum systems

Contact has been well established as an important quantity to govern dilute quantum systems, in which the pairwise correlation at short distance traces a broad range of thermodynamic properties. So far, studies have been focusing on contact in individual angular momentum channels. Here, we point out that, to have a complete description of the pairwise correlation in a general dilute quantum systems, contact should be defined as a matrix. Whereas the diagonal terms of such matrix include contact of all partial wave scatterings, the off-diagonal terms, which elude previous studies in the literature, characterise the coherence of the asymptotic pairwise wavefunction in the angular momentum space and determine important thermodynamic quantities including the momentum distribution. Contact matrix allows physicists to access unexplored connections between short-range correlations and macroscopic quantum phenomena. As an example, we show the direct connection between contact matrix and order parameters of a superfluid with mixed partial waves.

Pump-and-probe optical transmission phase shift as a quantitative probe of the Bogoliubov dispersion relation in a nonlinear channel waveguide

We theoretically investigate the dispersion relation of small-amplitude optical waves superimposing upon a beam of polarized monochromatic light propagating along a single-mode channel waveguide characterized by an instantaneous and spatially local Kerr nonlinearity. These small luminous fluctuations propagate along the waveguide as Bogoliubov elementary excitations on top of a one-dimensional dilute Bose quantum fluid evolve in time. They consequently display a strongly renormalized dispersion law, of Bogoliubov type. Analytical and numerical results are found in both the absence and the presence of one- and two-photon losses. Silicon and silicon-nitride waveguides are used as examples. We finally propose an experiment to measure this Bogoliubov dispersion relation, based on a stimulated four-wave mixing and interference spectroscopy techniques.

Active Region Design and Gain Characteristics of InP-Based Dilute Bismide Type-II Quantum Wells for Mid-IR Lasers

The electronic and optical properties of InP-based InGaAs/GaAsSbBi type-II quantum well (QW) structures are investigated theoretically. The detailed analyses of the InGaAs/GaAsSbBi type-II QW laser structures were based on a 14-band $\text {k}\cdot \text {p}$ model. The theoretical results indicate that adding Bi into InGaAs/GaAsSb type-II active regions on InP allows wavelength extension, but with minimal impact to transition matrix element. Moreover, the gain characteristics of the active region of laser were studied as a function of the Bi composition. It is shown that these type-II QW structures are suitable for mid-infrared (2– $4~{\mu }\text{m}$ ) operation at room temperature.

Thermodynamics and renormalized quasiparticles in the vicinity of the dilute Bose gas quantum critical point in two dimensions

We use the functional renormalization group (FRG) to derive analytical expressions for thermodynamic observables (density, pressure, entropy, and compressibility) as well as for single-particle properties (wavefunction renormalization and effective mass) of interacting bosons in two dimensions as a function of temperature $T$ and chemical potential $\mu$. We focus on the quantum disordered and the quantum critical regime close to the dilute Bose gas quantum critical point. Our approach is based on a truncated vertex expansion of the hierarchy of FRG flow equations and the decoupling of the two-body contact interaction in the particle-particle channel using a suitable Hubbard-Stratonovich transformation. Our analytic FRG results extend previous analytical renormalization group calculations for thermodynamic observables at $\mu =0$ to finite values of $\mu$. To confirm the validity of our FRG approach, we have also performed quantum Monte Carlo simulations to obtain the magnetization, the susceptibility, and the correlation length of the two-dimensional spin-$1/2$ quantum $XY$ model with coupling $J$ in a regime where its quantum critical behavior is controlled by the dilute Bose gas quantum critical point. We find that our analytical results describe the Monte Carlo data for $\mu \leq 0$ rather accurately up to relatively high temperatures $T \lesssim 0.1 J$.

Diffusion doping in quantum dots: bond strength and diffusivity.

Semiconducting materials uniformly doped with optical or magnetic impurities have been useful in a number of potential applications. However, clustering or phase separation during synthesis has made this job challenging. Recently the "inside out" diffusion doping was proposed to be successful in obtaining large sized quantum dots (QDs) uniformly doped with a dilute percentage of dopant atoms. Herein, we demonstrate the use of basic physical chemistry of diffusion to control the size and concentration of the dopants within the QDs for a given transition metal ion. We have studied three parameters; the bond strength of the core molecules and the diffusion coefficient of the diffusing metal ion are found to be important while the ease of cation exchange was not highly influential in the control of size and concentration of the single domain dilute magnetic semiconductor quantum dots (DMSQDs) with diverse dopant ions M2+ (Fe2+, Ni2+, Co2+, Mn2+). Steady state optical emission spectra reveal that the dopants are incorporated inside the semiconducting CdS and the emission can be tuned during shell growth. We have shown that this method enables control over doping percentage and the QDs show a superior ferromagnetic response at room temperature as compared to previously reported systems.

Fast and accurate calculation of dilute quantum gas using Uehling–Uhlenbeck model equation

The Uehling–Uhlenbeck (U–U) model equation is studied for the fast and accurate calculation of a dilute quantum gas. In particular, the direct simulation Monte Carlo (DSMC) method is used to solve the U–U model equation. DSMC analysis based on the U–U model equation is expected to enable the thermalization to be accurately obtained using a small number of sample particles and the dilute quantum gas dynamics to be calculated in a practical time. Finally, the applicability of DSMC analysis based on the U–U model equation to the fast and accurate calculation of a dilute quantum gas is confirmed by calculating the viscosity coefficient of a Bose gas on the basis of the Green–Kubo expression and the shock layer of a dilute Bose gas around a cylinder.

Self-bound droplets of a dilute magnetic quantum liquid.

Self-bound many-body systems are formed through a balance of attractive and repulsive forces and occur in many physical scenarios. Liquid droplets are an example of a self-bound system, formed by a balance of the mutual attractive and repulsive forces that derive from different components of the inter-particle potential. It has been suggested that self-bound ensembles of ultracold atoms should exist for atom number densities that are 108 times lower than in a helium droplet, which is formed from a dense quantum liquid. However, such ensembles have been elusive up to now because they require forces other than the usual zero-range contact interaction, which is either attractive or repulsive but never both. On the basis of the recent finding that an unstable bosonic dipolar gas can be stabilized by a repulsive many-body term, it was predicted that three-dimensional self-bound quantum droplets of magnetic atoms should exist. Here we report the observation of such droplets in a trap-free levitation field. We find that this dilute magnetic quantum liquid requires a minimum, critical number of atoms, below which the liquid evaporates into an expanding gas as a result of the quantum pressure of the individual constituents. Consequently, around this critical atom number we observe an interaction-driven phase transition between a gas and a self-bound liquid in the quantum degenerate regime with ultracold atoms. These droplets are the dilute counterpart of strongly correlated self-bound systems such as atomic nuclei and helium droplets.

Electronic bandstructure and optical gain of lattice matched III-V dilute nitride bismide quantum wells for 1.55 μm optical communication systems

Dilute nitride bismide GaNBiAs is a potential semiconductor alloy for near- and mid-infrared applications, particularly in 1.55 μm optical communication systems. Incorporating dilute amounts of bismuth (Bi) into GaAs reduces the effective bandgap rapidly, while significantly increasing the spin-orbit-splitting energy. Additional incorporation of dilute amounts of nitrogen (N) helps to attain lattice matching with GaAs, while providing a route for flexible bandgap tuning. Here we present a study of the electronic bandstructure and optical gain of the lattice matched GaNxBiyAs1−x−y/GaAs quaternary alloy quantum well (QW) based on the 16-band k·p model. We have taken into consideration the interactions between the N and Bi impurity states with the host material based on the band anticrossing and valence band anticrossing model. The optical gain calculation is based on the density matrix theory. We have considered different lattice matched GaNBiAs QW cases and studied their energy dispersion curves, optical gain spectrum, maximum optical gain, and differential gain and compared their performances based on these factors. The thickness and composition of these QWs were varied in order to keep the emission peak fixed at 1.55 μm. The well thickness has an effect on the spectral width of the gain curves. On the other hand, a variation in the injection carrier density has different effects on the maximum gain and differential gain of QWs of varying thicknesses. Among the cases studied, we found that the 6.3 nm thick GaN3Bi5.17As91.83 lattice matched QW was most suited for 1.55 μm (0.8 eV) GaAs-based photonic applications.