Session A39: Coherent Spins in Semiconductors

Spin–photon interface and spin-controlled photon switching in a nanobeam waveguide

Selected by FT organizers   

Off-Resonance Time-Resolved Kerr Rotation Spectroscopy

We show that time-resolved Kerr rotation (TRKR) is a quantitatively accurate tool to measure spin dynamics, even when conducted highly off resonance. We find that TRKR measurements have essentially identical temporal profiles when the probe energy is tuned far below, far above, and on resonance with a semiconducting band gap. We also present energy dependent TRKR spectra, and show they have an odd Lorenztian profile around the semiconducting band gap, at odds with the simplest theoretical model which predicts an even Lorenztian form. We show that this discrepancy is resolved if one accounts for a decrease in the transition linewidth as the optical energy increases above the band-edge transition.   

Identification of multi-photon transitions between magnetic dipole states using electrically detected magnetic resonant excitation with variable drive-field helicities.

For magnetic resonance conditions where the amplitude of the resonant driving field B₁ is close to the static magnetic Zeeman field B₀, quantum-optical effects linked to the two photon helicities, including the Bloch-Siegert shift [1,2] or multiple photon effects [3,4,5], emerge. In order to study the latter, we built a radio frequency domain electron spin resonance setup which allows for driving fields with arbitrary polarization. For the detection of magnetic resonance at B₀ in the mT-range where spin polarization is all but vanishing, we used spin-dependent recombination currents in a fully deuterated form of poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (d-MEH-PPV), in which proton-induced random hyperfine fields are minimized. We report results obtained for circularly, linearly, and elliptically polarized excitation states. [1] J. J. Sakurai, Modern Quantum Mechanics, Revised Edition. Addison-Wesley (1994); [2] J. Romhányi et al., Phys. Rev. B 92(5), 054422 (2015); [3] J. H. Shirley, Phys. Rev. 138 (4B), B979 (1965); [4] M. Göppert-Mayer, Annalen der Physik, 18(7-8), 466 (1931); [5] J. I. Kaplan and S. Meiboom, Phys. Rev. 106(3), 499 (1957); [6] D. P. Waters et al., Nature Phys. 11(11), 910 (2015).   

A charge-tunable quantum dot strongly coupled to a nanophotonic cavity

Quantum dots (QDs) integrated with photonic structures is a promising scalable platform for the development of quantum networks and distributed quantum computing. Probabilistic charging of the QDs with electron/hole spins by adding a doping layer suffers from low charging stability due to carrier tunneling. Sandwiching the QDs into a p-n junction, which allows deterministically changing the charging state of the QDs by changing the external gate voltage can resolve this problem. Many experiments that previously have been achieved in probabilistic charged QDs sample, like cavity assisted QDs spin manipulation, have been successfully demonstrated using those charge tunable p-n junction QDs sample. However, the strong coupling between QDs spin and a microcavity, which is the basis of many QDs-based quantum information processing protocols and quantum optics experiments, hasn't been accomplished yet. Here, we report for the first time the realization of strong coupling between charge tunable QDs spin and a micro cavity, and the demonstration of spin population transfer via optical pumping. The strong coupling combined with deterministic charging paves way for applications of QDs in quantum optics.   

Theory of the circulating current of a single magnetic impurity in a semiconductor

The localized electron spin of a single impurity in a semiconductor is a promising system to realize quantum information schemes [1]. Coherent control of this spin depends on understanding the structure of the magnetic moment that couples the system with external fields. In this work we investigate the spin-orbit induced circulating current associated with the ground state of a single magnetic impurity in zincblende III-V semiconductor. This circulating current is dissipationless and represents an electron moving in a closed trajectory producing an orbital contribution to the magnetic moment [2]. We developed a formalism employing Green’s functions obtained by the Koster-Slater technique [3,4] with a sp³d⁵s^* empirical tight-binding Hamiltonian to describe the host material.

[1] Koenraad,P. M. and Flatté, M. E. , Nature Materials 10, 1038 (2011).
[2] van Bree, J. and Silov, A. Yu and Koenraad,P. M. and Flatté,M. E., Phys. Rev. Lett. 112, 187201 (2014).
[3] Tang,J. M. and Flatté, M.E., Phys. Rev. Lett. 92, 047201 (2004).
[4] Kortan, V. R. and Sahin, C. and Flatté, M. E., PRB 93, 220402(R) (2016)   

Polarization-to-spin conversion and entanglement distribution via coherent interface with semiconductor double quantum dot

Interfacing photonic qubits with a gate-defined spin qubit remains critical challenges in semiconductor devices. In this talk, we discuss the key properties of a coherent interface by two separate experiments, quantum state transfer, and entanglement absorption with a double quantum dot. We report polarizationto-to-spin conversion implemented with single-shot readout of a single electron spin generated in a GaAs quantum dot. The optical spin blockade effect requires in-plane magnetic field and photo-excitation associate to Zeeman-splitted light-holes, but not heavy-holes. This effect, following the optical selection rules, determines the generation efficiency depending on the photonlinear-polarization and the electron number in the dot. Secondly, we discuss the experimental progress of entanglement absorption between an entangled photon pair to a single spin and a photon. With the temporal coincidences and the conditional probabilities of the polarized photon and the electron spin correlation, we plan to detect individual entanglement transferal events.   

Observation of magnetoresistance effect in charge pumping measurements

We report on a new magnetoresistance effect based on spin dependent trapping events at MOSFET gate/substrate interfaces called near zero field spin dependent charge pumping (NZF SDCP). NZF SDCP involves the application of a trapezoidal gate voltage waveform which cycles the substrate Fermi level between the conduction and valence band edges. Interface defects are cyclically filled and emptied, generating a net substrate recombination current proportional to the number of defects. The change in current is measured as a function of magnetic field. We find that: (1) in most cases the NZF SDCP amplitude appears to saturate as a function of waveform frequency, and (2) the NZF SDCP spectrum broadens with increasing frequency. These observations may allow for experimental exploration of several magnetoresistance theories regarding interaction or exchange times between charge carriers and defect spin centers. (3) The addition of N to the 4H-SiC MOSFET interface can have a profound impact on the NZF SDCP response, and (4) we almost certainly resolve electron-nuclear hyperfine interactions from a H-complexed defect. These observations strongly suggest that NZF SDCP could be a powerful tool to obtain atomic-scale physiochemical information about MOSFET interface defects.   

Electrically Detected Magnetic Resonance in Silicon Nitride Thin Films of Widely Varying Stoichiometries

We utilize electrically detected magnetic resonance (EDMR) to identify defects responsible for electronic transport in thin films of silicon nitride. The EDMR response is detected via spin dependent trap assisted tunneling over a range of electric fields. The EDMR measurements are made at both high and low field/frequency combinations. These EDMR measurements are compared with near-zero field magnetoresistance (NZFMR) measurements in which no oscillating magnetic field is applied; we observe an NZFMR response related to the EDMR. A comparison between the EDMR and NZFMR allows us to draw conclusions with regard to the relative analytical power of NZFMR versus EDMR as well as aid in the development of the physical understanding of the NZFMR response.   

Electrically Detected Magnetic Resonance Study of 4H-SiC/SiO2 Transistors with Barium Passivation

We report on electrically detected magnetic resonance measurements on 4H-SiC/SiO₂ metal oxide semiconductor field effect transistors. 4H-SiC/SiO₂ based MOSFETs show great promise for high power and high temperature applications. However, the SiC/SiO₂ interface has a high concentration of interface and near-interface traps which limits effective channel mobility. Passivation decreases the interface state density and thus increases the mobility. Passivation schemes typically including post-oxidation annealing in NO. Recent work suggests promise for a barium interfacial layer (IL) before oxidation¹. We probe the atomic scale defects at the SiC/SiO₂ in NO and barium treated devices and compare the results with those on unpassivated devices. Both the NO anneal and the barium IL greatly reduce the density of near-interface silicon vacancies, but yield somewhat different post-passivation defect structures.

¹ D.J. Lichtenwalner, L. Cheng, S. Dhar, A. Agarwal, and J.W. Palmour, Appl. Phys. Lett. 105, 1 (2014).
  

Embedded quantum dots in (Ga, Al)As semiconductor nanostructure: structural study of current tunneling

We report the tunneling current behavior in a nano-semiconductor structure of (Ga, Al) As / GaAs, which contains the Rashba spin orbital interaction in the presence of embedded InAs quantum dots of different geometries (lens, pyramid and ring) depending on the voltage, magnetic field, and different spin-orbit interaction values K_gL=π/4, π/2, 3π/4. These results show that the intensity of the current presents appreciable changes when the morphology of the quantum dot changes and also when the intensity of the bias voltage and magnetic field increase. For voltages less than or equal to 0.2 V and considering spin up we observed the appearance of the first peaks of current intensity according to the morphology of the following way: Pyramid, lens and ring. However, when the voltage is greater than 0.2 V a change in the current occurs due to the morphology of the quantum dots changing in its order: Ring, lens and pyramid. This phenomenon also occurs when the spin orbit coupling is changed, showing that as the value of the phase increases, a change in the domain of the morphology for similar voltages also occurs.
  

Zeeman-type spin splitting in non-magnetic three-dimensional compounds: Material prediction and electrical control

Besides the Rashba and Dresselhaus effects, another kind of spin discrimination phenomena in non-magnetic inversion asymmetry (IA) compounds is the so-called Zeeman-type spin splitting, which exhibits a spin-texture similar to the one observed in the magnetic Zeeman effect. Despite its potential for device application, this non-magnetic effect has only been predicted and observed in the two-dimensional WSe₂ and MoS₂ materials. Here, we demonstrate that three-dimensional compounds can also exhibit this splitting. The required conditions for this effect are: valence band maximum or conduction band minimum in a non-time-reversal-invariant k-point, inversion asymmetry, and zero magnetic moment. Using these conditions as filters, we perform a material screening and high-throughput ab-initio calculations to systematically search for these materials in the ICSD database. We find 20 candidates featuring this splitting. Our calculated spin splittings can be as large as 433, 510, and 491meV for the compounds WN₂ (P6m1), WS₂ (R3m), and SnTe (F43m), respectively. We also demonstrate that the spin splitting in slabs of these compounds depends on the growth direction and can be controlled by an external electric field.   

DETECTION OF STRONG MAGNETIC RESONANT DRIVE EFFECTS USING SPIN-DEPENDENT ELECTRONIC TRANSITION RATES IN ORGANIC SEMICONDUCTOR MATERIALS

Spin-dependent recombination currents in π-conjugated polymers allow for the detection of charge carrier spin resonance at very weak applied static Zeeman fields B₀ [1]. We have used this effect to study magnetic resonance in the strong driving regime when the amplitude of the driving field B₁ ~ B₀. Technologically, these measurements were carried out by using monolithic thin-film device structures in which a polymer bipolar injection device [an organic light emitting diode] was fabricated directly on top of an RF microwire [2]. We used a fully deuterated form of poly [2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] as active device layer due to its low local hyperfine fields. Under strong-drive, spin collectivity set in [1,2] and a variety of strong drive effects could be observed, including the Bloch-Siegert shift and two photon transitions. The measured dependence of the former on B₁ confirmed theoretical predictions, suggesting that the monolithic nano-layer device stack used in these experiments could serve as probe for ultra-strong light-matter coupling of paramagnetic charge carriers in polymer materials. [1] Waters et al., Nature Phys., 2015, 11, 910; [2] Jamali et al., Nano Lett., 2017, 17, 4648.   

Looking for new layered ferromagnetic semiconductor

Layered magnetic semiconductors have attracted great attention recently since ferromagnetism was demonstrated to persist down to single-molecular-layer level. In this talk, we will show our recent effort in looking for new compounds under this category and report the discovery of a new layered ferromagnetic semiconductor with a van der Waals gap.