Spin-Nano

Institutionen
  • AG Burkard (Theoretische Physik mit SP Festkörperphysik und Quanteninformation)
Publikationen
  Sortino, Luca; Brooks, Matthew; Zotev, Panaiot G.; Genco, Armando; Cambiasso, Javier; Mignuzzi, Sandro; Maier, Stefan A.; Burkard, Guido; Sapienza, Riccardo; Tartakovskii, Alexander I. (2020): Dielectric Nanoantennas for Strain Engineering in Atomically Thin Two-Dimensional Semiconductors ACS Photonics. ACS Publications. 2020, 7(9), pp. 2413-2422. eISSN 2330-4022. Available under: doi: 10.1021/acsphotonics.0c00294

Dielectric Nanoantennas for Strain Engineering in Atomically Thin Two-Dimensional Semiconductors

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Atomically thin two-dimensional semiconducting transition metal dichalcogenides (TMDs) can withstand large levels of strain before their irreversible damage occurs. This unique property offers a promising route for control of the optical and electronic properties of TMDs, for instance, by depositing them on nanostructured surfaces, where position-dependent strain can be produced on the nanoscale. Here, we demonstrate strain-induced modifications of the optical properties of mono- and bilayer TMD WSe2 placed on photonic nanoantennas made from gallium phosphide (GaP). Photoluminescence (PL) from the strained areas of the TMD layer is enhanced owing to the efficient coupling with the confined optical mode of the nanoantenna. Thus, by following the shift of the PL peak, we deduce the changes in the strain in WSe2 deposited on the nanoantennas of different radii. In agreement with the presented theory, strain up to ≈1.4% is observed for WSe2 monolayers. We also estimate that >3% strain is achieved in bilayers, accompanied by the emergence of a direct bandgap in this normally indirect-bandgap semiconductor. At cryogenic temperatures, we find evidence of the exciton confinement in the most strained nanoscale parts of the WSe2 layers, as also predicted by our theoretical model. Our results of direct relevance for both dielectric and plasmonic nanoantennas, show that strain in atomically thin semiconductors can be used as an additional parameter for engineering light–matter interaction in nanophotonic devices.

Forschungszusammenhang (Projekte)

  (2020): Electric dipole spin resonance of two-dimensional semiconductor spin qubits Physical Review B. American Physical Society. 2020, 101(3), 035204. ISSN 2469-9950. eISSN 2469-9969. Available under: doi: 10.1103/PhysRevB.101.035204

Electric dipole spin resonance of two-dimensional semiconductor spin qubits

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Monolayer transition metal dichalcogenides (TMDs) offer a novel two-dimensional platform for semiconductor devices. One such application, whereby the added low dimensional crystal physics (i.e. optical spin selection rules) may prove TMDs a competitive candidate, are quantum dots as qubits. The band structure of TMD monolayers offers a number of different degrees of freedom and combinations thereof as potential qubit bases, primarily electron spin, valley isospin and the combination of the two due to the strong spin-orbit coupling known as a Kramers qubit. Pure spin qubits in monolayer MoX2 (where X= S or Se) can be achieved by energetically isolating a single valley and tuning to a spin degenerate regime within that valley by a combination of a sufficiently small quantum dot radius and large perpendicular magnetic field. Within such a TMD spin qubit, we theoretically analyse single qubit rotations induced by electric dipole spin resonance. We employ a rotating wave approximation (RWA) within a second order time dependent Schrieffer-Wolf effective Hamiltonian to derive analytic expressions for the Rabi frequency of single qubit oscillations, and optimise the mechanism or the parameters to show oscillations up to \unit[250]MHz. This is significantly faster than similar predictions found for TMD qubits in the Kramers pair spin-valley or valley-only basis as well as experimental results for conventional semiconductor devices.

Forschungszusammenhang (Projekte)

    Lyons, Thomas P.; Dufferwiel, Scott; Brooks, Matthew; Withers, Freddie; Taniguchi, T.; Watanabe, Kenji; Novoselov, K. S.; Burkard, Guido; Tartakovskii, Alexander I. (2019): The valley Zeeman effect in inter- and intra-valley trions in monolayer WSe2 Nature communications. 2019, 10(1), 2330. eISSN 2041-1723. Available under: doi: 10.1038/s41467-019-10228-7

The valley Zeeman effect in inter- and intra-valley trions in monolayer WSe2

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Monolayer transition metal dichalcogenides (TMDs) hold great promise for future information processing applications utilizing a combination of electron spin and valley pseudospin. This unique spin system has led to observation of the valley Zeeman effect in neutral and charged excitonic resonances under applied magnetic fields. However, reported values of the trion valley Zeeman splitting remain highly inconsistent across studies. Here, we utilize high quality hBN encapsulated monolayer WSe2 to enable simultaneous measurement of both intervalley and intravalley trion photoluminescence. We find the valley Zeeman splitting of each trion state to be describable only by a combination of three distinct g-factors, one arising from the exciton-like valley Zeeman effect, the other two, trion specific, g-factors associated with recoil of the excess electron. This complex picture goes significantly beyond the valley Zeeman effect reported for neutral excitons, and eliminates the ambiguity surrounding the magneto-optical response of trions in tungsten based TMD monolayers.

Forschungszusammenhang (Projekte)

  (2018): Theory of strain-induced confinement in transition metal dichalcogenide monolayers Physical Review B. 2018, 97(19), 195454. ISSN 2469-9950. eISSN 2469-9969. Available under: doi: 10.1103/PhysRevB.97.195454

Theory of strain-induced confinement in transition metal dichalcogenide monolayers

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Recent experimental studies of out-of-plane straining geometries of transition metal dichalchogenide (TMD) monolayers have demonstrated sufficient band-gap renormalization for device application, such as single-photon emitters. Here, a simple continuum-mechanical plate-theory approach is used to estimate the topography of TMD monolayers layered atop nanopillar arrays. From such geometries, the induced conduction-band potential and band-gap renormalization are given, demonstrating a curvature of the potential that is independent of the height of the deforming nanopillar. Additionally, with a semiclassical WKB approximation, the expected escape rate of electrons in the strain potential may be calculated as a function of the height of the deforming nanopillar. This approach is in accordance with experiment, supporting recent findings suggesting that increasing nanopillar height decreases the linewidth of the single-photon emitters observed at the tip of the pillar and predicting the shift in photon energy with nanopillar height for systems with consistent topography.

Forschungszusammenhang (Projekte)

  Pisoni, Riccardo; Kormányos, Andor; Brooks, Matthew; Lei, Zijin; Back, Patrick; Eich, Marius; Overweg, Hiske; Lee, Yongjin; Rickhaus, Peter; Burkard, Guido (2018): Interactions and Magnetotransport through Spin-Valley Coupled Landau Levels in Monolayer MoS2 Physical Review Letters. 2018, 121(24), 247701. ISSN 0031-9007. eISSN 1079-7114. Available under: doi: 10.1103/PhysRevLett.121.247701

Interactions and Magnetotransport through Spin-Valley Coupled Landau Levels in Monolayer MoS2

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The strong spin-orbit coupling and the broken inversion symmetry in monolayer transition metal dichalcogenides results in spin-valley coupled band structures. Such a band structure leads to novel applications in the fields of electronics and optoelectronics. Density functional theory calculations as well as optical experiments have focused on spin-valley coupling in the valence band. Here we present magnetotransport experiments on high-quality n-type monolayer molybdenum disulphide (MoS2) samples, displaying highly resolved Shubnikov–de Haas oscillations at magnetic fields as low as 2 T. We find the effective mass 0.7me, about twice as large as theoretically predicted and almost independent of magnetic field and carrier density. We further detect the occupation of the second spin-orbit split band at an energy of about 15 meV, i.e., about a factor of 5 larger than predicted. In addition, we demonstrate an intricate Landau level spectrum arising from a complex interplay between a density-dependent Zeeman splitting and spin- and valley-split Landau levels. These observations, enabled by the high electronic quality of our samples, testify to the importance of interaction effects in the conduction band of monolayer MoS2.

Forschungszusammenhang (Projekte)

    (2017): Electron spin relaxation in a transition-metal dichalcogenide quantum dot 2D Materials. 2017, 4(2), 025114. eISSN 2053-1583. Available under: doi: 10.1088/2053-1583/aa7364

Electron spin relaxation in a transition-metal dichalcogenide quantum dot

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We study the relaxation of a single electron spin in a circular quantum dot in a transition-metal dichalcogenide monolayer defined by electrostatic gating. Transition-metal dichalcogenides provide an interesting and promising arena for quantum dot nano-structures due to the combination of a band gap, spin-valley physics and strong spin–orbit coupling. First we will discuss which bound state solutions in different B-field regimes can be used as the basis for qubits states. We find that at low B-fields combined spin-valley Kramers qubits to be suitable, while at large magnetic fields pure spin or valley qubits can be envisioned. Then we present a discussion of the relaxation of a single electron spin mediated by electron–phonon interaction via various different relaxation channels. In the low B-field regime we consider the spin-valley Kramers qubits and include impurity mediated valley mixing which will arise in disordered quantum dots. Rashba spin–orbit admixture mechanisms allow for relaxation by in-plane phonons either via the deformation potential or by piezoelectric coupling, additionally direct spin-phonon mechanisms involving out-of-plane phonons give rise to relaxation. We find that the relaxation rates scale as α B6 for both in-plane phonons coupling via deformation potential and the piezoelectric effect, while relaxation due to the direct spin-phonon coupling scales independant to B-field to lowest order but depends strongly on device mechanical tension. We will also discuss the relaxation mechanisms for pure spin or valley qubits formed in the large B-field regime.

Forschungszusammenhang (Projekte)

  (2017): Ultracoherent operation of spin qubits with superexchange coupling Physical Review B. 2017, 96(20), 201304. ISSN 2469-9950. eISSN 2469-9969. Available under: doi: 10.1103/PhysRevB.96.201304

Ultracoherent operation of spin qubits with superexchange coupling

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With the use of nuclear-spin-free materials such as silicon and germanium, spin-based quantum bits (qubits) have evolved to become among the most coherent systems for quantum information processing. The new frontier for spin qubits has therefore shifted to the ubiquitous charge noise and spin-orbit interaction, which are limiting the coherence times and gate fidelities of solid-state qubits. In this paper we investigate superexchange, as a means of indirect exchange interaction between two single electron spin qubits, each embedded in a single semiconductor quantum dot (QD), mediated by an intermediate, empty QD. Our results suggest the existence of “supersweet spots”, in which the qubit operations implemented by superexchange interaction are simultaneously first-order-insensitive to charge noise and to errors due to spin-orbit interaction. The proposed spin-qubit architecture is scalable and within the manufacturing capabilities of semiconductor industry.

Forschungszusammenhang (Projekte)

  (2017): Spin-degenerate regimes for single quantum dots in transition metal dichalcogenide monolayers Physical Review B. 2017, 95(24), 245411. ISSN 2469-9950. eISSN 2469-9969. Available under: doi: 10.1103/PhysRevB.95.245411

Spin-degenerate regimes for single quantum dots in transition metal dichalcogenide monolayers

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Forschungszusammenhang (Projekte)

  Pearce, Alexander J.; Mariani, Eros; Burkard, Guido (2016): Tight-Binding Approach to Strain and Curvature in Monolayer Transition-Metal Dichalcogenides Physical Review B. 2016, 94(15), 155416. ISSN 2469-9950. eISSN 2469-9969. Available under: doi: 10.1103/PhysRevB.94.155416

Tight-Binding Approach to Strain and Curvature in Monolayer Transition-Metal Dichalcogenides

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We present a model of the electronic properties of monolayer transition-metal dichalcogenides based on a tight binding approach which includes the effects of strain and curvature of the crystal lattice. Mechanical deformations of the lattice offer a powerful route for tuning the electronic structure of the transition-metal dichalcogenides, as changes to bond lengths lead directly to corrections in the electronic Hamiltonian while curvature of the crystal lattice mixes the orbital structure of the electronic Bloch bands. We first present an effective low energy Hamiltonian describing the electronic properties near the K point in the Brillouin zone, then present the corrections to this Hamiltonian due to arbitrary mechanical deformations and curvature in a way which treats both effects on an equal footing. This analysis finds that local area variations of the lattice allow for tuning of the band gap and effective masses, while the application of uniaxial strain decreases the magnitude of the direct band gap at the K point. Additionally, strain induced bond length modifications create a fictitious gauge field with a coupling strength that is smaller than that seen in related materials like graphene. We also find that curvature of the lattice leads to the appearance of both an effective in-plane magnetic field which couples to spin degrees of freedom and a Rashba-like spin-orbit coupling due to broken mirror inversion symmetry.

Forschungszusammenhang (Projekte)

Mittelgeber
Name Finanzierungstyp Kategorie Kennziffer
Europäische Union Drittmittel Forschungsförderprogramm 435/16
Weitere Informationen
Laufzeit: 01.01.2016 – 31.12.2017