Quantum Shell Research Articles

Fission hindrances in transfermium nuclei

Very heavy nuclei owe their stability against spontaneous fission to quantum shell effects, which depend on the local density of single-particle states. The height but also the width and the structure of the barrier in multi-dimensional deformation space determine the fission half-lives. Other effects come into play, such as the conservation of quantum numbers and superfluidity or stiffness of the system in the fission process. This is why odd nuclei have longer fission partial half-lives with respect to their even neighbours and also why multi-quasi-particle states, such as high-K states, are thought to be more stable against fission than the ground state. We will report here on two different fission studies carried out with the GABRIELA detector array at the focal plane of the recoil separator SHELS. The first study concerns the fission properties of 253Rf, the most neutron deficient Rf isotope known to date. The second study focusses on a new measurement of the fission hindrance of the known 8- isomer in 254No.

Experimental investigation of the role of shell structure in quasifission mass distributions

Background: To understand superheavy element synthesis reactions, quantifying the role of quantum shells in quasifission dynamics is important. In reactions with actinide nuclides, a wide peak in the binary quasifission mass yield is seen, centered close to the $^{208}\mathrm{Pb}$ mass. It is generally attributed to the $^{208}\mathrm{Pb}$ spherical closed shells causing a valley in the potential-energy surface, attracting flux to these mass splits. However, an early experiment studying $^{48}\mathrm{Ca}, ^{50}\mathrm{Ti}+^{238}\mathrm{U}$ reactions showed strong evidence that sequential fission plays an important role in generating the observed peak. These conflicting interpretations have not been resolved up to now.Purpose: This work aims to measure quasifission mass spectra for reactions with nuclei lighter than $^{208}\mathrm{Pb}$, having negligible sequential fission, to search for systematic features correlated with the proton shells known to affect low-energy fission mass distributions of the same actinide elements.Methods: Systematic measurements have been made at energies near and below the capture barriers (where quasifission is most prominent) of mass-angle distributions for fission following collisions of $^{48}\mathrm{Ti}$ projectiles with even-even nuclides from $^{154}\mathrm{Sm}$ to $^{200}\mathrm{Hg}$. Mean excitation energies above the ground-states ranged from 51 to 33 MeV, respectively.Results: With increasing compound nucleus atomic number ${\mathrm{Z}}_{CN}$, a rapid transition occurs from fission having characteristics of fusion-fission to fast quasifission. The heaviest reactions form $^{240}\mathrm{Cf}, ^{244}\mathrm{Fm}$, and $^{248}\mathrm{No}$. Low-energy fission of neighboring isotopes is mass asymmetric, correlated with proton number $Z=56$. However, peak quasifission yields are at mass-symmetry for all reactions. There appears to be a very small ($\ensuremath{\approx}3$%) systematic excess of yield correlated with $Z=56$, however this is at the limit of sensitivity of the experiment.Conclusions: No significant ($>3%$) systematic features are seen in the quasifission mass spectra that can be unambiguously identified as resulting from shells. This small influence may result from attenuation of shell effects due to the excitation energy introduced, even in these near-barrier reactions giving low excitation energies typical of superheavy element synthesis reactions.

(Keynote) Colloidal Quantum Shells: An Emerging Class of 2D Semiconductors

Low-dimensional colloidal semiconductors are inexpensive to process and offer a broad spectrum of attractive quantum-mechanical properties. The notorious problem of these nanostructures lies in their limited performance under high energetic loads, when more than one exciton per particle is created. Multiple excitons undergo fast Auger recombination, causing efficiently roll-off in energy-intensive applications, including high-brightness LEDs, X-ray scintillators, and solar cells. This presentation will discuss an emerging type of low-dimensional semiconductors, known as colloidal quantum shells that allow avoiding such multi-exciton (MX) energy losses. The geometry of quantum shells benefits from the spatial separation of multiple excitons, which leads to extraordinary improvements to MX lifetimes and MX quantum yield. We will compare quantum shells with other nanoscale geometries in three categories: Auger suppression, multi-exciton level structure, and solution-processed film conductivity, highlighting the potential benefits quantum shells can contribute to various applications. Theoretical analysis discussing the role of the quantum shell geometry in suppressing Auger recombination will also be presented.

(Invited) Quantum Well Architecture to Realize High Optical Gain Lasing Media

Photonic applications of semiconductor quantum dots (QDs) had significant drawbacks due to their ultrafast Auger decay of multiple excitons. These nonradiative processes have hindered the performance of electroluminescent and lasing devices. The emergence of the colloidal quantum well (QW) architecture where semiconductor quantum shell with an inverted QD geometry has led to suppressed Auger recombination. Ultra long biexciton lifetime (>10 ns) and large biexciton quantum yields has been achieved through spatial separation promoted by the QW geometry. Moreover, architecture induced exciton-exciton repulsions have led to the splitting of exciton and biexciton optical transitions which resulted in the single exciton gain mode along with the biexciton gain mode. High biexciton lifetime and the longest reported single exciton gain lifetime (>6 ns) has rendered this geometry a worthy candidate for the development of optically and electrically pumped gain media

Quantum thin shell surrounding a black hole

In a previous work we obtained exact solutions for the proper time quantum mechanics of a thin dust shell, collapsing in a vacuum. We extend these results to the quantum collapse of a dust shell surrounding a preexisting black hole. In lieu of exact solutions, which have so far proved difficult to obtain for this system, we establish the essential features of the quantum shell through a Wentzel-Kramers-Brillouin approximation, which is valid only when the mass of the shell is much greater than the Planck mass. There are many similarities with the vacuum collapse: only bound states exist and the proper energy spectrum of the shell is unaffected by the presence of the central black hole to this order. There are no peculiar or distinguishing features of the wave function near the black hole horizon. It vanishes at the center and oscillates between the origin and the classically forbidden region, beyond which it decays exponentially.

Impact of tensor force on quantum shell effects in quasifission reactions

Quantum shell effects drive many aspects of many-body quantal systems and their interactions. Among these are the quasifission reactions that impede the formation of a compound nucleus in superheavy element (SHE) searches. Fragment production in quasifission is influenced by shell effects as a nontrivial manifestation of microscopic dynamics hindering the full equilibration of the composite system to form the compound nucleus. In this Letter, we use the microscopic time-dependent Hartree-Fock (TDHF) theory to study 48Ca+249Bk collisions to investigate the influence of the tensor component of the effective nucleon-nucleon interaction. The results show that the inclusion of the tensor force causes the spherical shell effect to become more prominent, particularly for the neutron number yield whose peak is exactly at magic number N=126. This suggests that the tensor force plays a compelling role in the evolution of dynamical shell effects in nuclear reactions, influencing the competition between spherical and deformed shell gaps.

Quantum Shells Boost the Optical Gain of Lasing Media.

Auger decay of multiple excitons represents a significant obstacle to photonic applications of semiconductor quantum dots (QDs). This nonradiative process is particularly detrimental to the performance of QD-based electroluminescent and lasing devices. Here, we demonstrate that semiconductor quantum shells with an "inverted" QD geometry inhibit Auger recombination, allowing substantial improvements to their multiexciton characteristics. By promoting a spatial separation between multiple excitons, the quantum shell geometry leads to ultralong biexciton lifetimes (>10 ns) and a large biexciton quantum yield. Furthermore, the architecture of quantum shells induces an exciton-exciton repulsion, which splits exciton and biexciton optical transitions, giving rise to an Auger-inactive single-exciton gain mode. In this regime, quantum shells exhibit the longest optical gain lifetime reported for colloidal QDs to date (>6 ns), which makes this geometry an attractive candidate for the development of optically and electrically pumped gain media.

Morphology Control and Crystalline Quality of p-Type GaN Shells Grown on Coaxial GaInN/GaN Multiple Quantum Shell Nanowires.

The morphology and crystalline quality of p-GaN shells on coaxial GaInN/GaN multiple quantum shell (MQS) nanowires (NWs) were investigated using metal-organic chemical vapor deposition. By varying the trimethylgallium (TMG) flow rate, Mg doping, and growth temperature, it was verified that the TMG supply and growth temperature were the dominant parameters in the control of the p-GaN shape on NWs. Specifically, a sufficiently high TMG supply enabled the formation of a pyramid-shaped NW structure with a uniform p-GaN shell. The ratio of the growth rate between the c- and m-planes on the NWs was calculated to be approximately 0.4545. High-angle annular dark-field scanning transmission electron microscopy characterization confirmed that no clear extended defects were present in the n-GaN core and MQS/p-GaN shells on the sidewall. Regarding the p-GaN shell above the c-plane MQS region, only a few screw dislocations and Frank-type partial dislocations appeared at the interface between the serpentine c-plane MQS and the p-GaN shell near the tips. This suggested that the crystalline quality of the MQS structure can trigger the formation of screw dislocations and Frank-type partial dislocations during the p-GaN growth. The growth mechanism of the p-GaN shell on NWs was also discussed. To inspect the electronic properties, a prototype of a micro light-emitting diode (LED) with a chip size of 50 × 50 μm2 was demonstrated in the NWs with optimal growth. By correlating the light output curve with the electroluminescence spectra, three different emission peaks (450, 470, and 510 nm) were assignable to the emission from the m-, r-, and c-planes, respectively.

Comparison of fission and quasi-fission modes

Quantum shell effects are known to affect the formation of fragments in nuclear fission. Shell effects also affect quasi-fission reactions occurring in heavy-ion collisions. Systematic time-dependent Hartree-Fock simulations of 50Ca+176Yb collisions show that the mass equilibration between the fragments in quasi-fission is stopped when they reach similar properties to those in the asymmetric fission mode of the 226Th compound nucleus. Similar shell effects are then expected to determine the final repartition of nucleons between the nascent fragments in both mechanisms. Future experimental studies that could test these observations are discussed.

Emission characteristics of GaInN/GaN multiple quantum shell nanowire-based LEDs with different p-GaN growth conditions

Abstract Improving current injection into r- and m-planes of nanowires (NWs) is essential to realizing efficient GaInN/GaN multiple quantum shell (MQS) NW-based light-emitting diodes (LEDs). Here, we present the effects of different p-GaN shell growth conditions on the emission characteristics of MQS NW-LEDs. Firstly, a comparison between cathodoluminescence (CL) and electroluminescence (EL) spectra indicates that the emission in NW-LEDs originates from the top region of the NWs. By growing thick p-GaN shells, the variable emission peak at around 600 nm and degradation of the light output of the NW-LEDs are elaborated, which is attributable to the localization of current in the c-plane region with various In-rich clusters and deep-level defects. Utilizing a high growth rate of p-GaN shell, an increased r-plane and a reduced c-plane region promote the deposition of indium tin oxide layer over the entire NW. Therefore, the current is effectively injected into both the r- and m-planes of the NW structures. Consequently, the light output and EL peak intensity of the NW-LEDs are enhanced by factors of 4.3 and 13.8, respectively, under an injection current of 100 mA. Furthermore, scanning transmission electron microscope images demonstrate the suppression of dislocations, triangular defects, and stacking faults at the apex of the p-GaN shell with a high growth rate. Therefore, localization of current injection in nonradiative recombination centers near the c-plane was also inhibited. Our results emphasize the possibility of realizing high efficacy in NW-LEDs via optimal p-GaN shell growth conditions, which is quite promising for application in the long-wavelength region.

Crystal Growth and Characterization of n-GaN in a Multiple Quantum Shell Nanowire-Based Light Emitter with a Tunnel Junction.

Here, we systematically investigated the growth conditions of an n-GaN cap layer for nanowire-based light emitters with a tunnel junction. Selective-area growth of multiple quantum shell (MQS)/nanowire core-shell structures on a patterned n-GaN/sapphire substrate was performed by metal-organic vapor phase epitaxy, followed by the growth of a p-GaN, an n++/ p++-GaN tunnel junction, and an n-GaN cap layer. Specifically, two-step growth of the n-GaN cap layer was carried out under various growth conditions to determine the optimal conditions for a flat n-GaN cap layer. Scanning transmission electron microscopy characterization revealed that n++-GaN can be uniformly grown on the m-plane sidewall of MQS nanowires. A clear tunnel junction, involving 10-nm-thick p++-GaN and 3-nm-thick n++-GaN, was confirmed on the nonpolar m-planes of the nanowires. The Mg doping concentration and distribution profile of the p++-GaN shell were inspected using three-dimensional atom probe tomography. Afterward, the reconstructed isoconcentration mapping was applied to identify Mg-rich clusters. The density and average size of the Mg clusters were estimated to be approximately 4.3 × 1017 cm-3 and 5 nm, respectively. Excluding the Mg atoms contained in the clusters, the remaining Mg doping concentration in the p++-GaN region was calculated to be 1.1 × 1020 cm-3. Despite the lack of effective activation, a reasonably low operating voltage and distinct light emissions were preliminarily observed in MQS nanowire-based LEDs under the optimal n-GaN cap growth conditions. In the fabricated MQS-nanowire devices, carriers were injected into both the r-plane and m-plane of the nanowires without a clear quantum confinement Stark effect.

Photoionization cross-section in a GaAs spherical quantum shell: the effect of parabolic confining electric potentials

Photoionization cross-section in a GaAs spherical quantum shell: the effect of parabolic confining electric potentials

Hyperheavy spherical and toroidal nuclei: The role of shell structure

The properties of toroidal hyperheavy even-even nuclei and the role of toroidal shell structure are extensively studied within covariant density functional theory. The general trends in the evolution of toroidal shapes in the $Z\approx 130-180$ region of nuclear chart are established for the first time. These nuclei are stable with respect of breathing deformations. The most compact fat toroidal nuclei are located in the $Z\approx 136, N\approx 206$ region of nuclear chart, but thin toroidal nuclei become dominant with increasing proton number and on moving towards proton and neutron drip lines. The role of toroidal shell structure, its regularity, supershell structure, shell gaps as well as the role of different groups of the pairs of the orbitals in its formation are investigated in detail. The lowest in energy solutions at axial symmetry are characterized either by large shell gaps or low density of the single-particle states in the vicinity of the Fermi level in at least one of the subsystems (proton or neutron). Related quantum shell effects are expected to act against the instabilities in breathing and sausage deformations for these subsystems. The investigation with large set of covariant energy density functionals reveals that substantial proton $Z=154$ and 186 and neutron $N=228$, 308 and 406 spherical shell gaps exist in all functionals. The nuclei in the vicinity of the combination of these particle numbers form the islands of stability of spherical hyperheavy nuclei. The study suggests that the $N=210$ toroidal shell gap plays a substantial role in the stabilization of fat toroidal nuclei.

Semiclassical level density parameter with collective enhancements in nuclei

In this work, we calculate the level density parameter for several nuclei by deploying the semiclassical trace formula for spherically symmetric harmonic oscillator potential with spin-orbit interactions, which describes the quantum shell effects in terms of the classical periodic orbits. With the usage of recently proposed Laplace like formula of level density parameter by Canbula et al., the effects of collective rotations and vibrations as well as deformations in nuclei are incorporated. The comparison of our results with the experimental ones show a good agreement. Later on, the study is also extended to the calculation of nuclear level density and survival probability in several superheavy nuclei (SHN).

Correlation between Optical and Structural Characteristics in Coaxial GaInN/GaN Multiple Quantum Shell Nanowires with AlGaN Spacers

High crystalline quality coaxial GaInN/GaN multiple quantum shells (MQSs) grown on dislocation-free nanowires are highly in demand for efficient white-/micro-light-emitting diodes (LEDs). Here, we propose an effective approach to improve the MQS quality during the selective growth by metal-organic chemical vapor deposition. By increasing the growth temperature of GaN barriers, the cathodoluminescent intensity yielded enhancements of 0.7 and 3.9 times in the samples with GaN and AlGaN spacers, respectively. Using an AlGaN spacer before increasing the barrier temperature, the decomposition of GaInN quantum wells was suppressed on all planes, resulting in a high internal quantum efficiency up to 69%. As revealed by scanning transmission electron microscopy (STEM) characterization, the high barrier growth temperature allowed to achieve a clear interface between GaInN quantum wells and GaN quantum barriers on the c-, r-, and m-planes of the nanowires. Moreover, the correlation between the In incorporation and structure features in MQS was quantitatively assessed based on the STEM energy-dispersive X-ray spectroscopy mapping and line-scan profiles of In and Al fractions. Ultimately, it was demonstrated that the unintentional In incorporation in GaN barriers was induced by the evaporation of predeposited In-rich particles during low-temperature growth of GaInN wells. Such residual In contamination was sufficiently inhibited by inserting low Al fraction (∼6%) AlGaN spacers after each GaInN well. During the growth of AlGaN spacers, AlN polycrystalline particles were deposited on the surrounding dummy substrate, which suppressed the evaporation of the predeposited In-rich particles. Thus, the presence of AlGaN spacers certainly improved the uniformity of In fraction through five GaInN quantum wells and reduced the diffusion of point defects from n-core to MQS active structures. The superior coaxial GaInN/GaN MQS structures with the AlGaN spacer are supposed to improve the emission efficiency in white-/micro-LEDs.

Color-tunable emission in coaxial GaInN/GaN multiple quantum shells grown on three-dimensional nanostructures

Tunable emission wavelengths from 415 nm to 600 nm were demonstrated in coaxial GaInN/GaN multiple quantum shells (MQS) grown on three-dimensional nanostructures. Uniform aperture arrays with different diameters and pitches were patterned on GaN/sapphire substrates by nanoimprint and e-beam lithography. Thereafter, the coaxial GaInN/GaN MQS structures were selectively grown on the nanostructures using metalorganic chemical vapor deposition (MOCVD). To achieve long wavelength emissions, a detailed investigation in terms of height, filling factor, diameter of apertures, and pitch was performed for clarifying the In incorporation in nanostructures. Scanning transmission electron microscopy (STEM) features demonstrate that the diffusion of precursors on SiO2 mask area plays an important role in the coaxially growth of MQS structures on naonopyramids. A growth model based on the diffusion length and pitch was proposed to explain the experimental results. It was clarified that the probability of In incorporation was promoted in nanopyramids with a small pitch (less than 3 times of the diffusion length), wherein direct impinged adatoms on nanopyramids also diffused and participated in the growth on the neighbor. With a decrease in the intervals of aperture patterns, emission longer than 600 nm can be achieved for white and micro light-emitting diodes.

Development of Monolithically Grown Coaxial GaInN/GaN Multiple Quantum Shell Nanowires by MOCVD.

Broadened emission was demonstrated in coaxial GaInN/GaN multiple quantum shell (MQS) nanowires that were monolithically grown by metalorganic chemical vapor deposition. The non-polar GaInN/GaN structures were coaxially grown on n-core nanowires with combinations of three different diameters and pitches. To broaden the emission band in these three nanowire patterns, we varied the triethylgallium (TEG) flow rate and the growth temperature of the quantum barriers and wells, and investigated their effects on the In incorporation rate during MQS growth. At higher TEG flow rates, the growth rate of MQS and the In incorporation rate were promoted, resulting in slightly higher cathodoluminescence (CL) intensity. An enhancement up to 2–3 times of CL intensity was observed by escalating the growth temperature of the quantum barriers to 800 °C. Furthermore, decreasing the growth temperature of the quantum wells redshifted the peak wavelength without reducing the MQS quality. Under the modified growth sequence, monolithically grown nanowires with a broaden emission was achieved. Moreover, it verified that reducing the filling factor (pitch) can further promote the In incorporation probability on the nanowires. Compared with the conventional film-based quantum well LEDs, the demonstrated monolithic coaxial GaInN/GaN nanowires are promising candidates for phosphor-free white and micro light-emitting diodes (LEDs).

Core electrons and specific heat capacity in the fast electron heating of solids

The accuracy with which the Thomas–Fermi (TF) model can provide electronic specific heat capacities for use in calculations relevant to fast electron transport in laser-irradiated solids is examined. It is argued that the TF model, since it neglects the quantum shell structure, is likely to be significantly inaccurate for low- and intermediate-Z materials. This argument is supported by examining the results of calculations using more sophisticated methods that account for degeneracy, the quantum shell structure, and other non-ideal corrections. It is further shown that the specific heat capacity curve generated by this more advanced treatment leads to substantial (factor of two) changes in fast electron transport simulations relative to similar modeling based upon the TF model.

Effect of shell structure on the fission of sub-lead nuclei

Fission of atomic nuclei often produces mass asymmetric fragments. However, the origin of this asymmetry was believed to be different in actinides and in the sub-lead region [A. Andreyev {\it et al.}, Phys. Rev. Lett. {\bf 105}, 252502 (2010)]. It has recently been argued that quantum shell effects stabilising pear shapes of the fission fragments could explain the observed asymmetries in fission of actinides[G. Scamps and C. Simenel, Nature {\bf 564}, 382 (2018)]. This interpretation is tested in the sub-lead region using microscopic mean-field calculations of fission based on the Hartree-Fock approach with BCS pairing correlations. The evolution of the number of protons and neutrons in asymmetric fragments of mercury isotope fissions is interpreted in terms of deformed shell gaps in the fragments. A new method is proposed to investigate the dominant shell effects in the pre-fragments at scission. We conclude that the mechanisms responsible for asymmetric fissions in the sub-lead region are the same as in the actinide region, which is a strong indication of their universality.

Modified hydrodynamic equations of quantum liquid, taking into account quantum shell effects

In this paper, the modified hydrodynamic equations of a quantum system consisting of a degenerate electron gas and a classical ion gas, which take into account quantum shell effects, are derived. Two approaches are considered. The first is based on the modification of hydrodynamic equations. The modified equations take into account the shell effects by introducing an additional external field into the equations of motion for the electronic component. The second approach is based on a simplified description of the DFT modeling of the electron distribution for a given ionic distribution. In deriving the self-consistent system of equations, the Green’s function method is used in the coordinate representation, previously successfully applied in analyzing quantum shell effects in a gas bubble of submicron size.