Thermal electron spin flip in quantum dots

Abstract

We study a thermally induced spin flip of an electron spin located in a semiconductor quantum dot. This interesting effect arises from an intriguing interplay between the Zeeman coupling to an external magnetic field and the hyperfine interaction with the surrounding nuclear spins. By considering a minimal model, we explain the main mechanism driving this spin flip and analyze its dependence on the strength of the external magnetic field, the number of nuclear spins and the ratio of the electron and nuclear Zeeman energies, respectively. Finally we show, that this minimal model can be applied to experimentally relevant QDs in III-V heterostructures, where we explicitly predict the temperature at which the spin flip occurs.Comment: 9 pages, 5 figures; included generalized calculations, which additionally consider the so-called flip-flop terms; three additional appendices; two additional figures; changes in the main text in order to include our new result

26‐2: Extremely High‐Efficient OLED Achieving External Quantum Efficiency over 40% by Carrier Injection Layer with Super‐Low Refractive Index

“…In terms of an external quantum efficiency (EQE), a phosphorescent bottom-emission device adopting our ExTET technology exhibits an EQE greater than 30% [10,11]. On the other hand, the outcoupling efficiency of typical OLEDs has been reported to be approximately 25%-30% because of a high-refractiveindex organic film (n = 1.7-1.8) [14,15]. These facts indicate that the internal quantum efficiency of our phosphorescent device approaches 100%.…”

“…Specifically, a decrease in the refractive index of the EIL to 1.5 will increase the efficiency by approximately 1.1 times, and a decrease in the refractive index of the HIL to 1.5 will increase the efficiency by approximately 1.15 times. Previously reported studies have stated that an increased efficiency corresponding to the low-refractive-index layers is caused by the suppression of a waveguide mode and a surface plasmon polariton mode [14,23].…”

“…The techniques for decreasing the refractive index of the organic layer to increase the outcoupling efficiency of the OLED have been researched for a long time [20][21][22][23]. Recently, it has been reported that the outcoupling efficiency increases with the decrease in the refractive indices of an electron-injection layer (EIL) and a hole-injection layer (HIL), which are away from a light-emitting layer (EML) [14,24]. Low-refractive-index techniques will contribute to the further reduction in the power consumption of OLED displays.…”

“…The EQE of Device 1 (reference) is 30.5%. The EQE of a typical bottom-emission phosphorescent OLED has been reported to be 25%-30% because of the outcoupling efficiency limit [14,15], implying that Device 1 already exhibits a maximum EQE as a conventional OLED device. However, Devices 2-4 with the low-refractive-index layers exhibit higher efficiencies than Device1.…”

20‐3: Ultra‐High‐Performance Blue Fluorescent OLED with Efficiency Over 250 cd/A/CIEy Using Organic Carrier‐Transport Material with Low Refractive Index

“…Meanwhile, we have worked on improvement of outcoupling, which is a fundamental issue of OLEDs, by controlling the refractive indices of OLED materials [8]. The outcoupling of OLEDs greatly affects the external quantum efficiency (EQE) and is typically as low as 20 to 30 [9,10]. In other words, a large amount of light emitted from light-emitting layers attenuates in devices or substrates and is not extracted outside of the device.…”