Jump to content

Spin phenomena in quantum dots

| Opportunities | Research | Team | Collaborations | Results |

Outlook: Spin in nanometer-scale semiconductors

The successful utilisation of nanoscale systems in quantum devices requires the development of novel techniques for controlling the coupling between individual nano-objects and the interfacing of these systems to the outside world. These topics are central to our present research, which is based around semiconductor quantum dot (QD) nano-structures containing controllable atomic and electronic spin nano-systems. Quantum dots have already been widely exploited to create novel opto-electronic devices. A little explored aspect is that they may also provide numerous opportunities for construction of composite nano-magnetic materials with controlled interconnects between the component parts to be exploited for a new generation of quantum devices.

Researchers

Academic Staff: Alexander Tartakovskii
Post-Docs: Evgeny Chekhovich
Visitors: Dr Kirill Kavokin
PhD Students: Jorge Puebla
Daniel Sercombe
Romain Toro
Osvaldo Del Pozo

Nanostructures for optics

Opportunities in our group

We specialise in optics and electron transport of quantum dots. The group possesses state of the art experimental equipment to study individual quantum dots at ultra-low temperatures (<4 Kelvin) and very high magnetic fields (up to 10 Tesla). The type of experiments we carry out can be best described as optical magneto-microscopy at ultra-low temperatures.

Our labs are modern and high spec, one of the best in the UK for solid state physics research. We have double spectrometers including the world-best U1000 system from Jobin Yvon (France); a state-of-the-art unique magneto-microscopy system from AttoCube (Germany); many tunable laser sources from Coherent (USA, Scotland) and Spectra Physics (USA) including single-frequency Ti-Sapphire, femto-second Mira and Tsunami.

The samples we study are grown by advanced crystal growth techniques such as MBE and MOVPE either here in Sheffield at EPSRC National Centre for III-V Technologies or by our collaborators in Nottingham (UK), Marcoussis (France) and other places.

In addition to laboratory work, all PhD students are trained to use state-of-the-art clean room facilities at the National Centre. There they learn how to do electron-beam and optical lithography, deposition of thin metal and dielectric films, various etching and microscopy techniques. In the lab students receive full support of experienced post-doctoral researchers.

Students regularly attend International Conferences and Summer and Winter Schools that take place in such exotic places as Korea (QD2008, ICPS2010), Brazil (ICPS2008) or closer to home in Chamonix in France (QD2006) and Genoa in Italy (MSS2007). There is a range of smaller meeting which are attended by members of our group, examples are recent schools and project meetings in Les Houches (French Alps, March 2010) and Crete (October 2010).

Attocube

Experimental set-up for measurements of the nuclear spin dynamics in individual quantum dots. The picture shows optics assembled above the optical port of the AttoCube magneto-cryostat system.

Our research

Each individual QD can physically isolate three key spin nano-systems forming a composite QD nano-magnet: electron or hole spins, a small ensemble of nuclear spins and a single magnetic impurity atom. Separately, these systems exhibit a range of favourable properties. By combining them by efficient interconnection between the individual elements, novel sophisticated functionality on the nano-scale will be achieved. We explore possibilities to take advantage of ultra-long spin life-times (up to several minutes) accessible in the QD nano-magnets. This provides a natural route for controlled communication between individual components of the entire system via engineered spin interactions.

Nuclear spin - sub-nanometer magnet

Each individual component of the QD nano-magnet is sensitive to a specific type of external perturbation including ultra-fast optical pulses, radio-frequency excitation, electric fields etc. As a result, a range of techniques will be available to build the essential link to the outside world to access the desirable properties of the QD nano-magnet. Combining the properties of the QD nano-magnet with newly developed methods for engineering and precisely coupling several quantum nano-dots, provides a way to achieve spin interconnection between nano-magnets separated by several nm or more, with the potential to lead to nano-magnetic networks within synthetic solid state materials. These opportunities are being explored in our laboratories.

Evgeny Chekhovich

Evgeny is about to launch another series of measurements: In about 20 hrs he'll have another few thousand spectra to analyse. At the core of this experiment is the ultra-stable magneto-optics system supplied by Attocube.

The team

A well-balanced team of PhD students, post-doctoral and visiting researchers and more senior staff. We are closely linked with other quantum dot and photonics activities in the larger group led by Prof M Skolnick.

The Team

Daniel, Odilon and Evgeny are setting up a new magneto-spectroscopy lab.

Collaborations

We have established a range of close collaborations with research groups in the UK and overseas. Among the most successful are collaboration with the theory group of Prof V Falko (Lancaster); crystal growers in Nottingham (Dr R Campion, Dr T Foxon); an industrial partner Toshiba Research Europe (Dr Mark Stevenson, Cambridge); semiconductor physics groups: Prof V Kulakovskii and Prof I Kukushkin in Chernogolovka (Russia), Dr K Kavokin and Dr V Korenev from St Petersburg (Russia), Dr A Lemaitre and Dr P Senellart from LPN (Marcoussis, France). We have other close links with groups in Germany (Munich, Dortmund), France (Grenoble, Clermont-Ferrand), Russia (Novosibirsk). Our PhD students and postdocs are closely involved in these collaborations and participate in projects meetings in as well as outside the UK.

Our most recent results

  1. 1. Fast control of nuclear spin polarization in an optically pumped single quantum dot
    M. N. Makhonin, K. V. Kavokin, P. Senellart, A. Lemaître, A. J. Ramsay, M. S. Skolnick, A. I. Tartakovskii
    Nature Materials 10 844-848 (2011) http://www.nature.com/nmat/journal/v10/n11/full/nmat3102.html
  1. 2. Charge control in InP/(Ga,In)P single quantum dots embedded in Schottky diodes
    O. D. D. Couto Jr., J. Puebla, E. A. Chekhovich, I. J. Luxmoore, C. J. Elliott, N. Babazadeh, M. S. Skolnick, A. I. Tartakovskii
    Physical Review B 84 125301 (2011) http://prb.aps.org/abstract/PRB/v84/i12/e125301
  1. 3. Light-polarization-independent nuclear spin alignment in a quantum dot
    E. A. Chekhovich, A. B. Krysa, M. S. Skolnick, A. I. Tartakovskii
    Physical Review B 83 125318 (2011) http://link.aps.org/doi/10.1103/PhysRevB.83.125318
  1. 4. Direct Measurement of the Hole-Nuclear Spin Interaction in Single InP/GaInP Quantum Dots Using Photoluminescence Spectroscopy
    E. A. Chekhovich, A. B. Krysa, M. S. Skolnick, and A. I. Tartakovskii
    Physical Review Letters 106 027402 (2011) http://prl.aps.org/abstract/PRL/v106/i2/e027402
  1. 4. Optically tunable nuclear magnetic resonance in a single quantum dot
    M. N. Makhonin, E. A. Chekhovich, P. Senellart, A. Lemaître, M. S. Skolnick, A. I. Tartakovskii
    Physical Review B 82 161309(R) (2010) http://link.aps.org/doi/10.1103/PhysRevB.82.161309
  1. 5. Dynamics of optically induced nuclear spin polarization in individual InP/GaInP quantum dots
    E. A. Chekhovich, M. N. Makhonin, J. Skiba-Szymanska, A. B. Krysa, V. D. Kulakovskii, M. S. Skolnick, A. I. Tartakovskii
    Physical Review B 81 245308 (2010) http://link.aps.org/doi/10.1103/PhysRevB.81.245308
  1. 6. Pumping of Nuclear Spins by Optical Excitation of Spin-Forbidden Transitions in a Quantum Dot
    E. A. Chekhovich, M. N. Makhonin, K. V. Kavokin, A. B. Krysa, M. S. Skolnick, A. I. Tartakovskii
    Physical Review Letters 104 066804 (2010) http://link.aps.org/doi/10.1103/PhysRevLett.104.066804
  1. 7. Suppression of nuclear spin diffusion at a GaAs/AlGaAs interface measured with a single quantum-dot nanoprobe
    A. E. Nikolaenko, E. A. Chekhovich, M. N. Makhonin, I. W. Drouzas, A. B. Van’kov, J. Skiba-Szymanska, M. S. Skolnick, P. Senellart, D. Martrou, A. Lemaître, and A. I. Tartakovskii
    Physical Review B 79 081303(R) (2009) http://dx.doi.org/10.1103/PhysRevB.79.081303
  1. 8. Voltage-controlled nuclear polarization switching in a single InGaAs quantum dot
    M. N. Makhonin, J. Skiba-Szymanska, M. S. Skolnick, H.-Y. Liu, M. Hopkinson, and A. I. Tartakovskii
    Physical Review B 79 125318 (2009) http://dx.doi.org/10.1103/PhysRevB.79.125318
  1. 9. Nuclear spin pumping under resonant optical excitation in a quantum dot
    M. N. Makhonin, A. I. Tartakovskii,A. Ebbens, M. S. Skolnick, A. Russell, V. I. Fal’ko and M. Hopkinson
    Applied Physics Letters 93 073113 (2008) http://dx.doi.org/10.1063/1.2958221
  1. 10. Overhauser effect in individual InP/GaxIn1−xP dots
    J. Skiba-Szymanska, E. A. Chekhovich, A. E. Nikolaenko, A. I. Tartakovskii, M. N. Makhonin, I. Drouzas, M. S. Skolnick, and A. B. Krysa
    Physical Review B 77 165338 (2008) http://link.aps.org/abstract/PRB/v77/e165338
  1. 11. Long nuclear spin polarization decay times controlled by optical pumping in individual quantum dots
    M. N. Makhonin, A. I. Tartakovskii, A. B. Van’kov, I. Drouzas, T. Wright, J. Skiba-Szymanska, A. Russell, V. I. Fal’ko, M. S. Skolnick, H.-Y. Liu, M. Hopkinson
    Physical Review B 77 125307 (2008) http://link.aps.org/doi/10.1103/PhysRevB.77.125307
  1. 12. Bistability of optically induced nuclear spin orientation in quantum dots
    A. Russell, Vladimir I. Fal’ko, A. I. Tartakovskii, M. S. Skolnick
    Physical Review B 76 195310 (2007) http://link.aps.org/doi/10.1103/PhysRevB.76.195310
  1. 13. Nuclear Spin Switch in Semiconductor Quantum Dots
    A. I. Tartakovskii, T. Wright, A. Russell, V. I. Fal'ko, A. B. Van'kov, J. Skiba-Szymanska, I. Drouzas, R. S. Kolodka, M. S. Skolnick, P. W. Fry, A. Tahraoui, H.-Y. Liu, and M. Hopkinson
    Physical Review Letters 98 026806 (2007) http://link.aps.org/abstract/PRL/v98/e026806

Fast control of nuclear spin polarization in an optically pumped single quantum dot

  1. Fast control of nuclear spin polarization in an optically pumped single quantum dot
    M. N. Makhonin, K. V. Kavokin, P. Senellart, A. Lemaître, A. J. Ramsay, M. S. Skolnick, A. I. Tartakovskii
    Nature Materials 10 844-848 (2011) http://www.nature.com/nmat/journal/v10/n11/full/nmat3102.html

Highly polarized nuclear spins within a semiconductor quantum dot induce effective magnetic (Overhauser) fields of up to several Tesla acting on the electron spin, or up to a few hundred mT for the hole spin. Recently this has been recognized as a resource for intrinsic control of quantum dot- based spin quantum bits. However, only static long-lived Overhauser fields could be used. Here we demonstrate fast redirection on the microsecond timescale of Overhauser fields on the order of 0.5T experienced by a single electron spin in an optically pumped GaAs quantum dot. This has been achieved using coherent control of an ensemble of 105 optically polarized nuclear spins by sequences of short radiofrequency pulses. These results open the way to a new class of experiments using radiofrequency techniques to achieve highly correlated nuclear spins in quantum dots, such as adiabatic demagnetization in the rotating frame leading to sub-µK nuclear spin temperatures, rapid adiabatic passage, and spin squeezing.

Diagram 1

Diagram 2

Charge control in InP/(Ga,In)P single quantum dots embedded in Schottky diodes

  1. Charge control in InP/(Ga,In)P single quantum dots embedded in Schottky diodes
    O. D. D. Couto Jr., J. Puebla, E. A. Chekhovich, I. J. Luxmoore, C. J. Elliott, N. Babazadeh, M. S. Skolnick, A. I. Tartakovskii
    Physical Review B 84 125301 (2011) http://prb.aps.org/abstract/PRB/v84/i12/e125301
Diagram 1

We demonstrate control by applied electric field of the charge states in single self-assembled InP quantum dots placed in GaInP Schottky structures grown by metalorganic vapor phase epitaxy. This has been enabled by growth optimization leading to suppression of formation of large dots uncontrollably accumulating charge. Using bias- and polarization-dependent micro-photoluminescence, we identify the exciton multiparticle states and carry out a systematic study of the neutral exciton state dipole moment and polarizability. This analysis allows for the characterization of the exciton wave-function properties at the single-dot level for this type of quantum dot. Photocurrent measurements allow further characterization of exciton properties by electrical means, opening new possibilities for resonant excitation studies for such systems.

Light-polarization-independent nuclear spin alignment in a quantum dot

  1. Light-polarization-independent nuclear spin alignment in a quantum dot
    E. A. Chekhovich, A. B. Krysa, M. S. Skolnick, A. I. Tartakovskii
    Physical Review B 83 125318 (2011) http://link.aps.org/doi/10.1103/PhysRevB.83.125318

We study experimentally nuclear spin-pumping mechanisms in neutral InP/GaInP quantum dots under nonresonant optical excitation. We find two distinct regimes of dynamic nuclear polarization. At low optical powers when the dot is populated with "dark" excitons we observe nuclear spin polarization up to ~10% with direction insensitive to polarization and wavelength of light. Measurements of photoluminescence of both "dark" and "bright" excitons in single dots reveal that at low optical power nuclear spin pumping occurs via a virtual spin-flip transition between these states accompanied by photon emission. Under these conditions the sign of the nuclear spin polarization is determined by asymmetry in the exciton energy spectrum rather than by the sign of the exciton spin polarization. By contrast at high optical powers resulting in saturation of the quantum dot and suppression of exciton photoluminescence we detect nuclear spin polarization with direction and degree (up to ~50%) determined by the polarization of light.

Diagram 1 Diagram 2

Direct Measurement of the Hole-Nuclear Spin Interaction in Single InP/GaInP Quantum Dots Using Photoluminescence Spectroscopy

  1. Direct Measurement of the Hole-Nuclear Spin Interaction in Single InP/GaInP Quantum Dots Using Photoluminescence Spectroscopy
    E. A. Chekhovich, A. B. Krysa, M. S. Skolnick, and A. I. Tartakovskii
    Physical Review Letters 106 027402 (2011) http://prl.aps.org/abstract/PRL/v106/i2/e027402

In this work we directly measure hole Overhauser shifts in individual self-assembled InP/GaInP dots, which allows us to deduce the magnitude and sign of the hole hyperfine interaction constant. We use non-resonant laser excitation and a pump-probe method to achieve two crucial ingredients of this measurement: (i) nuclear spin polarization on the dot variable in a wide range by altering the polarization of the high power pump and (ii) detection of both `bright' and `dark' excitons in photoluminescence (PL) excited by a low intensity probe. This technique enables the measurement of energy shifts of the four excitonic states with all possible electron and heavy-hole spin projections at different magnitudes of optically induced nuclear spin polarization. This allows simultaneous detection of the electron and hole Overhauser shifts, and as a result the ratio of the hyperfine constants of the heavy-hole (C) and the electron (A) can be measured. We find that, in good agreement with recent calculations for the hole-nuclear spin coupling induced by the dipole-dipole interaction, this ratio is negative and on average C/A=-0.11.

Diagram 1 Diagram 2

Optically tunable nuclear magnetic resonance in a single quantum dot

  1. Optically tunable nuclear magnetic resonance in a single quantum dot
    M. N. Makhonin, E. A. Chekhovich, P. Senellart, A. Lemaître, M. S. Skolnick, A. I. Tartakovskii
    Physical Review B 82 161309(R) (2010) http://link.aps.org/doi/10.1103/PhysRevB.82.161309

We report optically detected nuclear magnetic resonance ODNMR measurements on small ensembles of nuclear spins in single GaAs quantum dots. Using ODNMR we make direct measurements of the inhomogeneous Knight field from a photoexcited electron which acts on the nuclei in the dot. The resulting shifts of the NMR peak can be optically controlled by varying the electron occupancy and its spin orientation, and lead to strongly asymmetric line shapes at high optical excitation. The all-optical control of the NMR line shape will enable position-selective control of small groups of nuclear spins inside a dot.

Diagram 1 Diagram 2

In this work we take advantage of the strong gradients of the Knight field inside a quantum dot produced by the localized electron spin and enter a new regime of nano-ODNMR. By employing ODNMR techniques, we measure with high precision the Knight shifts in the resonant frequencies of each individual isotope spin subsystem in individual GaAs/AlGaAs interface dots and find their dependence on the polarization and power of optical excitation. By varying the optical power, we find striking modifications of the line shape of the NMR spectrum of the dot. These arise from the Knight field variation across the dot determined by the spatial distribution of the electron wave function. The interpretations are supported by calculations, which further demonstrate that by employing the inhomogeneities of the Knight shifts, it becomes possible to access selectively, by appropriate resonant frequencies, small groups of nuclear spins located in different regions within the dot hence the term nano-ODNMR. This may be used for spatially selective control of the nuclear spins in nanometer-sized semiconductor structures.

Dynamics of optically induced nuclear spin polarization in individual InP/GaxIn1-xP quantum dots

  1. Dynamics of optically induced nuclear spin polarization in individual InP/GaInP quantum dots
    E. A. Chekhovich, M. N. Makhonin, J. Skiba-Szymanska, A. B. Krysa, V. D. Kulakovskii, M. S. Skolnick, A. I. Tartakovskii
    Physical Review B 81 245308 (2010) http://link.aps.org/doi/10.1103/PhysRevB.81.245308

We report on dynamics of optically induced nuclear spin polarization in individual InP/GaInP quantum dots at T=4.2 K. Dots with different charge states arising from residual doping in a nominally undoped sample have been studied. In the same sample, we find strong dot-to-dot variation in the nuclear spin decay times in the dark from ~85 to ~6000 s. The longest decay times measured are comparable to those previously measured in bulk InP and correspond to almost complete suppression of nuclear spin diffusion out of the dot. In the negatively charged dots, the spin decay times exceed 300 s (with the slowest decay of ~6000 s), about 105 times longer than those reported previously in electron charged dots in gated structures. We discuss possible mechanisms responsible for suppression of nuclear spin diffusion, including inhomogeneous quadrupolar shifts and stabilizing effect of the hyperfine interaction with the electron confined in the dot.

Diagram 9a

(a) Time diagram of the measurement of the nuclear polarization buildup dynamics in a single dot. In this experiment tpump is varied as an independent variable while terase=8tpump, tdet=tpump/4, and excitation power of both pulses is ≈70 µW. [(c) and (d)] Overhauser field BN buildup dynamics in the negatively charged dot X- N4 at Bz=0 (circles), 0.4 T (triangles), and 2 T (squares). Open (solid) symbols correspond to σ- erase/σ+ pump (σ+ erase/σ- pump) experiments. Dashed arrow shows initial increase in BN not resolved in the experiment.

Diagram 9b

(a) Schematic representation of the effect of a trapped electron on nuclear spin on the dot: inhomogeneous Knight field causes energy splitting mismatch between different nuclei, leading to suppression of nuclear spin diffusion out of the dot. (b) Magnetic field dependence of the nuclear polarization |BN| on the negatively charged dot N4 under σ+ (squares) and σ- (circles) excitations. The observed minima of nuclear polarization correspond to the situation when the external field Bz compensates electron Knight field ±Be induced by σ± excitation, resulting in partial depolarization of nuclei. The value of <Be>≈3 mT is estimated from splitting between the minima. [(c) and (d)] PL spectrum from the aperture containing two positively charged dots (X+ N2 and N5) (c) at a low excitation power 10 µW and (d) its time dependence under high power (200 µW) excitation. Dot N2 and additional spectral features observed at high power demonstrate a large random spectral drift while PL from the dot N5 is stable in time.

Pumping of Nuclear Spins by Optical Excitation of Spin-Forbidden Transitions in a Quantum Dot

  1. Pumping of Nuclear Spins by Optical Excitation of Spin-Forbidden Transitions in a Quantum Dot
    E. A. Chekhovich, M. N. Makhonin, K. V. Kavokin, A. B. Krysa, M. S. Skolnick, A. I. Tartakovskii
    Physical Review Letters 104 066804 (2010) http://link.aps.org/doi/10.1103/PhysRevLett.104.066804

Diagram 8a

a) Pulse sequence used in the resonant nuclear spin pump-probe experiment. (b) Energy level diagram of a positively charged dot in magnetic field Bz. Electron and hole spin up (down) states are shown by ↑ (↓) and ⇑ (⇓), respectively. Long thick (thin) arrows show “allowed” (“forbidden”) optical transitions. Dotted arrow shows hole spin relaxation. (c) PL spectra of a positively charged dot measured with the probe pulse at Bz=2.5 T for BN ≈ 0 (solid symbols) and at BN ≈−1.5 T (open symbols). Here the optics were optimized to maximize the PL signal, leading to differing sensitivity in detection of light with σ+ and σ- polarizations and effectively unpolarized PL spectra.

We demonstrate that efficient optical pumping of nuclear spins in semiconductor quantum dots (QDs) can be achieved by resonant pumping of optically forbidden transitions. This process corresponds to one-to-one conversion of a photon absorbed by the dot into a polarized nuclear spin, and also has potential for initialization of hole spin in QDs. We find that by employing this spin-forbidden process, nuclear polarization of 65% can be achieved, markedly higher than from pumping the allowed transition, which saturates due to the low probability of electron-nuclear spin flip-flop.

Diagram 8a

Overhauser field BN on the dot (a),(c) and PL transitions energies EPL (b),(d) as a function of the energy El of the σ+ polarized resonant laser with excitation power Pres=15 µW at Bz=2.5 T (a),(b) and Bz=0 (c),(d). The full lines on panels (b) and (d) show the laser energy.

Suppression of nuclear spin diffusion at a GaAs/AlxGa1-xAs interface measured with a single quantum-dot nanoprobe

  1. Suppression of nuclear spin diffusion at a GaAs/AlGaAs interface measured with a single quantum-dot nanoprobe
    A. E. Nikolaenko, E. A. Chekhovich, M. N. Makhonin, I. W. Drouzas, A. B. Van’kov, J. Skiba-Szymanska, M. S. Skolnick, P. Senellart, D. Martrou, A. Lemaître, and A. I. Tartakovskii
    Physical Review B 79 081303(R) (2009) http://dx.doi.org/10.1103/PhysRevB.79.081303

Diagram 5b

Nuclear polarization decay curves measured for GaAs/AlxGa1-xAs interface QDs for two polarizations of the pump pulse in external magnetic field of 2 T (circles). Lines show polarization decay curves calculated using Eq. (1) with DQD =2×10-15 (thick black), 10-13 (thin black), and 10-12 (gray).

Nuclear spin polarization dynamics are measured in optically pumped individual GaAs/AlxGa1-xAs interface quantum dots by detecting the time dependence of the Overhauser shift in photoluminescence spectra. Long nuclear polarization decay times of ≈1 min have been found indicating inefficient nuclear spin diffusion from the GaAs dot into the surrounding AlGaAs matrix in externally applied magnetic field. A spin-diffusion coefficient two orders lower than that previously found in bulk GaAs is deduced.

Voltage controlled nuclear polarization switching in a single InGaAs quantum dot

  1. Voltage-controlled nuclear polarization switching in a single InGaAs quantum dot
    M. N. Makhonin, J. Skiba-Szymanska, M. S. Skolnick, H.-Y. Liu, M. Hopkinson, and A. I. Tartakovskii
    Physical Review B 79 125318 (2009) http://dx.doi.org/10.1103/PhysRevB.79.125318

Diagram 7b

(a) Voltage scans at Bz=2.1 T and power 0.4 mW under σ- excitation. Vthr1 (Vthr2) denotes the bias where the nuclear-spin switching occurs when reducing (increasing) the bias. Directions of the bias scans are shown with arrows. (b) The bias dependence of the switching threshold power Pthr. [(c) and (d)] Bias-dependence of the integrated PL intensity measured in both circular polarizations at Bz=2.1 T under σ- excitation with powers (c) 0.2 mW and (d) 0.5 mW for X0 (electron and hole pairs or eh state), XX0 (eehh), X+ (ehh), and X- (eeh) peaks. Horizontal dashed lines show the level of noise in PL, below which the line intensities cannot be measured.

Sharp thresholdlike transitions between two stable nuclear-spin polarizations are observed in optically pumped individual InxGa1-xAs self-assembled quantum dots embedded in a Schottky diode when the bias applied to the diode is tuned. The abrupt transitions lead to the switching of the Overhauser field in the dot by up to 3 T. The bias-dependent photoluminescence measurements reveal the importance of the electrontunneling-assisted nuclear-spin pumping. We also find evidence for the resonant LO-phonon-mediated electron cotunneling, the effect controlled by the applied bias and leading to the reduction of the nuclear-spin pumping rate.

Diagram 7a

(a) Bias dependence of the QD PL recorded at Bz=1.5 T under σ- excitation. (b) Power dependence of the X+ Zeeman splitting measured at Bz=2 T and bias -0.45 V for σ- polarized excitation. Pthr denotes the power where the nuclear spin "switch" is observed in the scan where the power is increased. Arrows at the switching thresholds indicate the directions of the power scans.

Nuclear spin pumping under resonant optical excitation in a quantum dot

  1. Nuclear spin pumping under resonant optical excitation in a quantum dot
    M. N. Makhonin, A. I. Tartakovskii,A. Ebbens, M. S. Skolnick, A. Russell, V. I. Fal’ko and M. Hopkinson
    Applied Physics Letters 93 073113 (2008) http://dx.doi.org/10.1063/1.2958221

Diagram 1a

(a) Stark shift of the ground state exciton peak. (b) Exciton Zeeman peak energies as functions of Bext. (c) PC spectra measured with circularly polarized excitation into the ground exciton state of an individual dot at Bext=5 T. The Zeeman doublet excited with σ+(σ-) polarized light is shown with solid (open) symbols and ΔE+(ΔE-) denotes the corresponding exciton Zeeman splitting.

We demonstrate nuclear spin pumping in a single InGaAs/GaAs dot embedded in a p-i-n diode in the regime of resonant optical excitation of spin-polarized electron-hole pairs in the lowest energy states of the dot. A nuclear spin pumping mechanism is proposed relevant to the regime of high electric field where carriers escape from the dot by tunneling. The degree of nuclear spin polarization is shown to increase strongly with the applied electric field, controlling the carrier tunneling from the dot, since at low electric fields the dot is blocked for re-excitation due to the slow hole escape.

Overhauser effect in individual InP/GaInP dots

  1. Overhauser effect in individual InP/GaxIn1−xP dots
    J. Skiba-Szymanska, E. A. Chekhovich, A. E. Nikolaenko, A. I. Tartakovskii, M. N. Makhonin, I. Drouzas, M. S. Skolnick, and A. B. Krysa
    Physical Review B 77 165338 (2008) http://link.aps.org/abstract/PRB/v77/e165338

Diagram 1a

Sizable nuclear spin polarization is pumped in individual electron-charged InP/GaInP dots in a wide range of external magnetic fields B=0-5T by circularly polarized optical excitation. We observe nuclear polarization of up to 40% at B=1.5T corresponding to an Overhauser field of ~1.2T. We find a strong feedback of the nuclear spin on the spin pumping efficiency.

Diagram 1a

This feedback, produced by the Overhauser field, leads to nuclear spin bi-stability at low magnetic fields of B~0.3-1T. We find that the splitting in magnetic field between the trion radiative recombination peaks increases markedly, when the Overhauser field in the dot cancels the external field. This counter-intuitive result is shown to arise from the opposite contribution of the electron and hole Zeeman splittings to the transition energies.

Long nuclear spin polarization decay times controlled by optical pumping in individual quantum dots

  1. Long nuclear spin polarization decay times controlled by optical pumping in individual quantum dots
    M. N. Makhonin, A. I. Tartakovskii, A. B. Van’kov, I. Drouzas, T. Wright, J. Skiba-Szymanska, A. Russell, V. I. Fal’ko, M. S. Skolnick, H.-Y. Liu, M. Hopkinson
    Physical Review B 77 125307 (2008) http://link.aps.org/doi/10.1103/PhysRevB.77.125307

Diagram 1a

Nuclear polarization dynamics are measured in the nuclear spin bistability regime in a single optically pumped InGaAs/GaAs quantum dot. The controlling role of nuclear spin diffusion from the dot into the surrounding material is revealed in pump-probe measurements of the nonlinear nuclear spin dynamics.

Diagram 1a

We measure nuclear spin polarization decay times in the range of 0.2 s to 5 s, strongly dependent on the optical pumping time. The long nuclear spin decay arises from polarization of the material surrounding the dot by spin diffusion for long >5 s pumping times. The time-resolved methods allow the detection of the unstable nuclear polarization state in the bistability regime otherwise undetectable in cw experiments.

Bistability of optically induced nuclear spin orientation in quantum dots

See a theory paper we published on this topic in collaboration with Lancaster group:

  1. Bistability of optically induced nuclear spin orientation in quantum dots
    A. Russell, Vladimir I. Fal’ko, A. I. Tartakovskii, M. S. Skolnick
    Physical Review B 76 195310 (2007) http://link.aps.org/doi/10.1103/PhysRevB.76.195310

Diagram 1a

We demonstrate that bistability of the nuclear spin polarization in optically pumped semiconductor quantum dots is a general phenomenon possible in dots with a wide range of parameters. In experiment, this bistability manifests itself via the hysteresis behavior of the electron Zeeman splitting as a function of either pump power or external magnetic field. In addition, our theory predicts that the nuclear polarization can strongly influence the charge dynamics in the dot leading to bistability in the average dot charge.

Diagram 1a

Nuclear Spin Switch in Semiconductor Quantum Dots

  1. Nuclear Spin Switch in Semiconductor Quantum Dots
    A. I. Tartakovskii, T. Wright, A. Russell, V. I. Fal'ko, A. B. Van'kov, J. Skiba-Szymanska, I. Drouzas, R. S. Kolodka, M. S. Skolnick, P. W. Fry, A. Tahraoui, H.-Y. Liu, and M. Hopkinson
    Physical Review Letters 98 026806 (2007) http://link.aps.org/abstract/PRL/v98/e026806

Diagram 1a

We show that by illuminating an InGaAs/GaAs self-assembled quantum dot with circularly polarised light, the nuclei of the thousands of atoms constituting a 20 nm InGaAs island can be driven into a bistable regime, in which either a threshold-like enhancement or reduction of the local nuclear field, BN, by up to 3 Tesla can be generated.

Diagram 1a

We refer to the threshold-like changes of BN as a nuclear spin switch. The nuclear field in the bistability regime can be controlled by varying the intensity of the exciting circularly polarised light and/or magnitudes of the external magnetic and electric field. Surprisingly large magnitudes of BN in a single QD can be switched on and off by the bias applied to the Schottky diode containing the dot.

Polarization freezing of 104 optically-cooled nuclear spins by coupling to a single electron

Here you may view the pre-publication PDF iconArxiv copy.

Diagram 6a

The nature of the nano-scale environment presents a major challenge for solid-state implementation of spin-based qubits. In this work, a single electron spin in an optically pumped nanometer-sized III-V semiconductor quantum dot is used to control a macroscopic nuclear spin of several thousand nuclei, freezing its decay and leading to spin life-times exceeding 100 seconds at low temperatures. Few-millisecond-fast optical initialization of the nuclear spin is followed by a slow decay exhibiting random telegraph signals at long delay times, arising from low probability electron jumps out of the dot. The remarkably long spin life-time in a dot surrounded by a densely-packed nuclear spin environment arises from the Knight field created by the resident electron, which leads to suppression of nuclear spin depolarization.

Diagram 1a

Last updated Friday, 23rd March 2012