Delocalization Of Electron Wave Function Research Articles

Systemic consequences of disorder in magnetically self-organized topological MnBi2Te4/(Bi2Te3) n superlattices

MnBi2Te4/(Bi2Te3) n materials system has recently generated strong interest as a natural platform for the realization of the quantum anomalous Hall (QAH) state. The system is magnetically much better ordered than substitutionally doped materials, however, the detrimental effects of certain disorders are becoming increasingly acknowledged. Here, from compiling structural, compositional, and magnetic metrics of disorder in ferromagnetic (FM) MnBi2Te4/(Bi2Te3) n it is found that migration of Mn between MnBi2Te4 septuple layers (SLs) and otherwise non-magnetic Bi2Te3 quintuple layers (QLs) has systemic consequences—it induces FM coupling of Mn-depleted SLs with Mn-doped QLs, seen in ferromagnetic resonance as an acoustic and optical resonance mode of the two coupled spin subsystems. Even for a large SL separation (n 4 QLs) the structure cannot be considered as a stack of uncoupled two-dimensional layers. Angle-resolved photoemission spectroscopy and density functional theory studies show that Mn disorder within an SL causes delocalization of electron wave functions and a change of the surface band structure as compared to the ideal MnBi2Te4/(Bi2Te3) n . These findings highlight the critical importance of inter- and intra-SL disorder towards achieving new QAH platforms as well as exploring novel axion physics in intrinsic topological magnets.

Photoluminescence Blinking and Biexciton Auger Recombination in Single Colloidal Quantum Dots with Sharp and Smooth Core/Shell Interfaces.

There is an inconsistence on whether a smooth core/shell interface can reduce Auger recombination and suppress photoluminescence (PL) blinking in single colloidal quantum dots (QDs). Here, we investigate the influence of a core/shell interface on PL blinking and biexciton Auger recombination by comparing the single-dot PL spectra of CdxZn1-xSeyS1-y/ZnS core/shell QDs with sharp and smooth interfaces. The inconsistence can be clarified when considering different PL blinking mechanisms. For the single QDs showing Auger blinking, a smooth core/shell interface potential can suppress PL blinking through reducing the Auger recombination. In contrast, we find slightly reduced biexciton Auger recombination rates but increased PL blinking activities in the band-edge carrier (BC)-blinking QDs with the smooth core/shell interface. This is because the smooth interface potential cannot reduce the PL blinking caused by the transfer of electrons to the surface states; however, there is potential to increase electron wave function delocalization for reducing the biexciton Auger recombination rate.

Delocalization of exciton and electron wavefunction in non-fullerene acceptor molecules enables efficient organic solar cells

A major challenge for organic solar cell (OSC) research is how to minimize the tradeoff between voltage loss and charge generation. In early 2019, we reported a non-fullerene acceptor (named Y6) that can simultaneously achieve high external quantum efficiency and low voltage loss for OSC. Here, we use a combination of experimental and theoretical modeling to reveal the structure-property-performance relationships of this state-of-the-art OSC system. We find that the distinctive π–π molecular packing of Y6 not only exists in molecular single crystals but also in thin films. Importantly, such molecular packing leads to (i) the formation of delocalized and emissive excitons that enable small non-radiative voltage loss, and (ii) delocalization of electron wavefunctions at donor/acceptor interfaces that significantly reduces the Coulomb attraction between interfacial electron-hole pairs. These properties are critical in enabling highly efficient charge generation in OSC systems with negligible donor-acceptor energy offset.

Effects of interface-potential smoothness and wavefunction delocalization on Auger recombination in colloidal CdSe-based core/shell quantum dots.

Auger nonradiative recombination dominates decay of multicarrier states in high quality colloidal quantum dots (QDs) and thus is critical for many of their optical and optoelectronic applications. Controlling interface-potential smoothness and wavefunction delocalization are proposed as two main strategies for Auger engineering in core/shell QDs. Here, a series of CdSe-based core/shell QDs with nearly ideal optical quality of their single-exciton states are developed and applied for studying biexciton quantum yields and Auger nonradiative recombination rates. Comparative experiments find that the interface-potential smoothness has little influence on biexciton quantum yield and Auger rates of these core/shell QDs with the same CdS outer shells. In contrast, with a fixed total size of the series of QDs, the decreasing hole wavefunction delocalization can increase the Auger rates of positive trions by ∼400%. A mild decrease in electron wavefunction delocalization among the series of QDs results in a small increase in the Auger rates of negative trions (∼50%). Smoothing the core/shell interface can indeed affect the Auger rates, but this is by the way of altering wavefunction delocalization. These findings highlight the importance of control of wavefunction delocalization among the strategies of Auger engineering and provide guidelines for rational design QDs for applications.

Highly Responsive Ultraviolet Sensor Based on ZnS Quantum Dot Solid with Enhanced Photocurrent

Detection of visible blind UV radiation is not only interesting but also of technologically important. Herein, we demonstrate the efficient detection of UV radiation by using cluster like ZnS quantum dot solid nanostructures prepared by simple reflux condensation technique. The short-chain ligand 3-mercaptopropionic acid (MPA) involved in the synthesis lead to the cluster like formation of ZnS quantum dots into solids upon prolonged synthesis conditions. The ZnS QD solid formation resulted in the strong delocalization of electronic wave function between the neighboring quantum dots. It increases the photocurrent value, which can be further confirmed by the decrease in the average lifetime values from 64 to 4.6 ns upon ZnS cluster like QD solid formation from ZnS QDs. The ZnS quantum dot solid based UV sensor shows good photocurrent response and a maximum responsivity of 0.31 (A/W) at a wavelength of 390 nm, is not only competitive when compared with previous reports but also better than ZnS and metal oxide-based photodetectors. The device exhibits a high current value under low-intensity UV light source and an on/off ratio of IUV/Idark = 413 at zero biasing voltage with a fast response. Further, photocurrent device has been constructed using ZnS quantum dot solid nanostructures with graphene hybrids as an active layer to improve the enhancement of photoresponsivity.

Exploring the Effect of Band Alignment and Surface States on Photoinduced Electron Transfer from CuInS2/CdS Core/Shell Quantum Dots to TiO2 Electrodes

Photoinduced electron transfer (ET) processes from CuInS2/CdS core/shell quantum dots (QDs) with different core sizes and shell thicknesses to TiO2 electrodes were investigated by time-resolved photoluminescence (PL) spectroscopy. The ET rates and efficiencies from CuInS2/CdS QDs to TiO2 were superior to those of CuInS2/ZnS QDs. An enhanced ET efficiency was surprisingly observed for 2.0 nm CuInS2 core QDs after growth of the CdS shell. On the basis of the experimental and theoretical analysis, the improved performances of CuInS2/CdS QDs were attributed to the passivation of nonradiative traps by overcoating shell and enhanced delocalization of electron wave function from core to CdS shell due to lower conduction band offset. These results indicated that the electron distribution regulated by the band alignment between core and shell of QDs and the passivation of surface defect states could improve ET performance between donor and acceptor.

Magnetotransport in iodine-doped single-walled carbon nanotubes

Magnetoresistance (MR) and magnetothermoelectric power (MTEP) of iodine-doped single-walled carbon nanotubes (I@SWNT) under magnetic fields up to 14 T are investigated from room temperature (300 K) down to 1.6 K. Our results on resistivity and thermoelectricpower (TEP) in a zero magnetic field are similar to those reported by Grigorian et al. [Phys. Rev. Lett. 80, 5560 (1998)] The positive sign of the TEP values indicates that the majority of the carriers in the I@SWNT are holes. The broad enhancement of TEP at temperatures of 30\char21{}200 K shows quasilinear temperature dependence and is consistent with sharply varying density of states near the Fermi level with additional contribution from the spin-orbit scattering in the normal metallic characteristics of the I@SWNT. For $Tl7\text{ }\text{K}$, MR is negative and it decreases with ${H}^{2}$ followed by the ${H}^{1/2}$ dependence at around $H=2\text{ }\text{T}$ which is characteristic for the weak localization. In the range $7\text{ }\text{K}lTl70\text{ }\text{K}$, MR is positive at low magnetic field and becomes negative at higher magnetic field. The negative MR in the high magnetic fields decreases linearly. At $T\ensuremath{\ge}\ensuremath{\sim}100\text{ }\text{K}$, MR is positive up to 14 T, which could be the result of spin-orbit scattering in the I@SWNT. The MTEP decreases under magnetic field at $Tl90\text{ }\text{K}$. The reduction in MTEP is originated from the delocalization of electron wave functions under the magnetic field. At $Tg90\text{ }\text{K}$, the thermal fluctuation dominates the effect of magnetic fields resulting the MTEP to be the same as the zero magnetic field TEP.

Evidence of electron wave function delocalization in CdSe/CdS asymmetric nanocrystals

We studied the delocalization of electron wave function in asymmetric CdSe/CdS nanocrystals, consisting of a spherical CdSe dot embedded in an elongated CdS shell, by means of a pump–probe technique. By comparing the transient spectra obtained upon pumping the band edge transition of the CdSe in CdSe/CdS heterostructure and in a bare CdSe dot, we observed the delocalization of electron wave function at the CdSe/CdS interface.

Structure and bonding ofAu5M(M=Na, Mg, Al, Si, P, and S) clusters

The atomic and electronic structure of ${\mathrm{Au}}_{5}M$ ($M=\mathrm{Na}$, Mg, Al, Si, P, S, and Au) clusters have been investigated using generalized gradient approximation to the density functional theory. Depending on the nature of interaction with different impurity elements a structural transition from planar to nonplanar configuration has been observed in ${\mathrm{Au}}_{5}M$. With the exception of S, impurities with $p$ electrons (Al, Si, P) yield nonplanar geometries of ${\mathrm{Au}}_{5}M$ clusters, while those with $s$ electrons (Na, Mg) yield planar geometries. The properties of ${\mathrm{Au}}_{5}\mathrm{S}$ cluster are anomalous: The cluster not only has a planar geometry, but also is chemically most stable with the highest vertical ionization potential among all the clusters studied. The origin of these anomalous properties of ${\mathrm{Au}}_{5}\mathrm{S}$ cluster is attributed to the delocalization of electronic wave function associated with the highest occupied molecular orbital.

Dynamic nuclear polarization and nuclear magnetic resonance in the vicinity of edge states of a 2DES in GaAs quantum wells

Nuclear magnetic resonance is detected via the in-plane conductivity of a two-dimensional electron system at unity Landau level filling factor in the regime of the quantum Hall effect in narrow and wide quantum wells. The NMR is spatially selective to nuclei with a coupling to electrons in the current carrying edge states at the perimeter of the 2DES. Interpretation of the electron-nuclear double resonance signals is facilitated by numerical simulations. A new RF swept method for conductivity-detected NMR is introduced which offers more efficient signal averaging. The method is applied to the study of electric quadrupole interactions, weakly allowed overtone transitions, and evaluation of the extent of electron wave function delocalization in the wide quantum well.

Design and operation of mid-infrared light-emitting devices ( 11 µm) based on a chirped superlattice

The optical and electrical characteristics of a set of devices in which the periodicity of the superlattice layers has been chirped are presented. The operation of these devices can be explained through the delocalization of electron wavefunctions which satisfy the Bragg transmission condition across a portion of our structures, under the influence of an electric field. This groups together discrete states in sets of manifolds, each manifold separated by an energy gap of the order of the emitted photon energy under bias. Thus a quantum cascade scheme of electrons jumping between each manifold of states forms the basis of these structures. The operation of electroluminescent devices working at 11 µm has been measured over the temperature range from 77 to 300 K. Line widths of 13 meV and slope efficiencies of 15 nW A-1 have been measured at 77 K. Limiting factors in the operation of these devices can be understood from carrier overflow into higher-energy states, an effect that can be seen from light-current (L-I), current-voltage (I-V) and spectra characteristics.

Suppression of the observation of Stark ladders in optical measurements on superlattices by excitonic effects.

We investigate experimentally and theoretically how excitonic effects influence the optical properties of semiconductor superlattices in the Stark-ladder regime. Excitonic effects are particularly important when the superlattice miniband width is comparable to the exciton binding energy. In order to observe a Stark ladder it is necessary that either the electron or hole wave function (or both) be at least partially delocalized. In an optical experiment the delocalization of the wave functions is affected by the electron-hole Coulomb interaction. Excitonic effects can therefore prevent the observation of the Stark ladder if the Coulomb interaction is strong enough to localize the wave functions completely. We have studied three GaAs/${\mathrm{Al}}_{0.3}$${\mathrm{Ga}}_{0.7}$As superlattices, with calculated conduction-band miniband widths \ensuremath{\Delta}E of 6, 11, and 23 meV. Experimentally, we observe a heavy-hole Stark-ladder fan diagram in the sample with the 23-meV miniband width, which indicates an electron wave-function delocalization over several superlattice periods. However, in the other two samples in which \ensuremath{\Delta}E is comparable to the exciton binding energy, we do not observe a fan diagram, which indicates much stronger wave-function localization. Instead, we observe an anticrossing at a field strength of \ensuremath{\sim}5 kV ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. In these conditions, the multiwell structure behaves more like many repeated pairs of coupled double wells rather than a superlattice. We interpret the observed anticrossings at \ensuremath{\sim}5 kV ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ in the samples with the smaller miniband widths as an excitonic degeneracy similar to that observed previously at higher fields [A.M. Fox et al., Phys. Rev. B 44, 6231 (1991)]. We have been able to explain this behavior using both a variational exciton model based on three coupled quantum wells and a full Green's-function solution for the excitons assuming a double-quantum-well structure.

Dynamical properties and Wigner transitions of two-dimensional electron lattices on dielectric substrates.

The dynamical properties (phonon spectrum, sound velocities, vibrational energy) and phase transitions of a two-dimensional Wigner lattice are discussed in a model that takes into account the perpendicular delocalization of electron wave functions. The lattice floats above the dielectric substrate (He,Ar), with the dielectric constant determining the image force on the lattice. Those two effects, the perpendicular electron delocalization and the dielectric image force, which are obviously correlated, were usually neglected in the previous discussions of the Wigner-lattice dynamics. Here we show their influence on the electron-electron interaction and consequently on the dynamical properties of the Wigner lattice. We also suggest an experimental situation where those effects in the phase diagram could be eventually studied and detected.

Coulomb interaction effect from the atomic limit to the band limit

The effect of the electron-electron interaction on the delocalization of electron wavefunctions is investigated when the atoms are packed to form solids. The authors propose a one-dimensional square-well potential model which contains two parameters; V for the well depth and U for the electron correlation energy. When the interatomic distance is reduced from the atomic limit, instead of forming the Bloch states, the electron correlation effect breaks the translational symmetry and therefore prevents a complete delocalization of the electrons. In the one-electron picture, it can be viewed as each electron occupying an impurity state. Since the electron correlation energy and the electron screening depend on the 'impurity' state wavefunctions, the problem is solved self-consistently. The authors have investigated the localization of electrons, the electron correlation energy, the electron screening strength, the band narrowing effect and the U/W (W is the bandwidth) ratio to show a transition from the atomic model to the band model. It is found that the sharpness of the transition depends on the correlation energy, while the outset of the transition is determined by the strength of the atomic potential.