(Al,Ga)N quantum dots for deep UV LEDs

Effect of Temperature on Electrical Transport Properties of MBE grown Mg-doped GaN and AlGaN

Photoluminescence study of Mg-doped GaN and AlxGa1-xN (x<0.4) grown by molecular epitaxy

Cooling of nuclear spin system (NSS) in doped semiconductors via dynamic polarization by optical pumping is a powerful method for harnessing ubiquitous fluctuations of nuclear spin. The idea of NSS cooling is based on the hypothesis of spin temperature, which states that NSS reaches an internal thermal equilibrium long before it comes to equilibrium with the external bath (crystal lattice). Although thermodynamic framework has been successfully employed for the description of a variety of the experimental data, a rigorous check of this concept in semiconductors was impossible until recently, in particular at low magnetic field. The reason for that is the lack of experimental techniques allowing nonperturbative optical control over adiabatic transformation of the NSS. We have recently developed such methods, based on off-resonant Faraday rotation and spin noise spectroscopy [1,2]. Using these techniques, combined with photoluminescence spectroscopy, we established a comprehensive picture of the nuclear spin relaxation efficiency, its magnetic field, temperature, and carrier concentration dependence in both n- and p-doped GaAs, a model system in the field of nuclear spin physics in semiconductors [3-5]. We also analyzed the interplay between four relevant relaxation mechanisms: hyperfine interaction, quadrupole interaction, spin diffusion towards paramagnetic impurities, and Korringa mechanisms. Figure 1 illustrates these processes in a nuclei-electron coupled system. Understanding of field dependence of NSS dynamics allowed us to obtain a new insight into the NSS thermodynamics, and verify the spin temperature concept in GaAs bulk material and microcavities [6]. We have demonstrated that NSS exactly follows the predictions of the spin temperature theory, despite the quadrupole interaction that was earlier reported to disrupt nuclear spin thermalization in quantum dots [7]. These results open a way for the deep cooling of nuclear spins in semiconductor structures, with the prospect of realizing nuclear spin-ordered states for high-fidelity spin-photon interfaces. References[1] R. Giri et al, Physical Review Letters, 111, 087603 (2013)[2] I. I. Ryzhov et al, Applied Physics Letters, 106, 242405 (2015)[3] M. Kotur et al, Physical Review B, 94, 081201(R) (2016)[4] M. Vladimirova et al, Physical Review B, 95, 125312 (2017)[5] M. Kotur et al, Physical Review B, 97, 165206 (2018)[6] M. Vladimirova et al, Physical Review B, 97, 041301 (2018)[7] P. Maletinsky et al, Nature Physics, 5, 407 (2009)

The study of open quantum systems often relies on approximate master equations derived under the assumptions of weak coupling to the environment. However when the system is made of several interacting subsystems such a derivation is in many cases very hard. An alternative method, employed especially in the modeling of transport in mesoscopic systems, consists in using local master equations (LMEs) containing Lindblad operators acting locally only on the corresponding subsystem. It has been shown that this approach however generates inconsistencies with the laws of thermodynamics. In this paper we demonstrate that using a microscopic model of LMEs based on repeated collisions all thermodynamic inconsistencies can be resolved by correctly taking into account the breaking of global detailed balance related to the work cost of maintaining the collisions. We provide examples based on a chain of quantum harmonic oscillators whose ends are connected to thermal reservoirs at different temperatures. We prove that this system behaves precisely as a quantum heat engine or refrigerator, with properties that are fully consistent with basic thermodynamics.

Les technologies quantiques

Quantum sensing with diamond