Jump To Content

Jump to content University of Sheffield

Low Dimensional Structures and Devices •

Home ○

Group Talks 

○ •

Site Map

Research ○

○

○

Mid-Infrared 

Collaborations



MOVPE Grown QCLs



Antimonide QCLs



Broadband QCLs



QCL Properties



Polarons



QD based devices



Disordered structures



Publications

Quantum Dots 

Spin Phenomena



Coherent Control



QD Lasers



Nitrides



Publications

Microcavities 

○ ○

Publications

Photonic Structures 

•

Archive

Publications

Theory 

Rabi Flops



Photonic Crystals



Microcavity OPO



Publications

Group Members

○

Login [members only]

•

Publications

•

Vacancies

•

Destinations of former group members

•

Internal pages University of Sheffield only

○

Printers

○

Email lists

○

Phone numbers

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 Maxim Makhonin Post-Docs: Andrey Nikolaenko Jacob Lin Visitors: Evgeny Chekhovich Nasser Babazadeh Ilias Drouzas PhD Students: Claire Elliott Joanna Skiba-Szymanska Tim Wright

Opportunities in our group The group is one of the world-leaders in semiconductor nano-science. 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, probably 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), Brazil (ICPS2008) or closer to home in Chamonix in France (QD2006) and Genoa in Italy (MSS2007).

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 nanomagnets. This provides a natural route for controlled communication between individual components of the entire system via engineered spin interactions.

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.

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

Left to right: Sasha Tartakovskii, Andrey Nikolaenko, Nasser Babazadeh, Joanna SkibaSzymanska, Claire Elliot, Jacob Lin, Maxim Makhonin, Evgeny Chekhovich

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), semiconductor physics group of Prof V Kulakovskii and Prof I Kukushkin in Chernogolovka (Russia) and semiconductor group in Marcoussis (Dr A Lemaitre, Dr P Senellart). We have other close links with groups in Germany (Munich, Dortmund), France (Grenoble, Clermont-Ferrand), Poland (Warsaw), Russia (St Petersburg, Novosibirks). Our PhD students and postdocs are closely involved in these collaborations and participate in projects meetings in as well as outside Sheffield.

Our most recent results 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 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. 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.

Bistability of optically induced nuclear spin orientation in quantum dots See also 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.aip.org/link?prb/76/195310

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. SkibaSzymanska, A. Russell, V. I. Fal’ko, M. S. Skolnick, H.-Y. Liu, M. Hopkinson Physical Review B 77 125307 (2008) http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=PRBMDO0000770 00012125307000001&idtype=cvips&gifs=yes 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. 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.

Overhauser effect in individual InP/GaInP dots 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. 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.

Ultra-long nuclear spin life-times in an electron-charged quantum dot Dynamics of optically pumped nuclear polarization is studied in individual electron-charged InP/GaInP dots, where electron population arises from residual doping in the sample. A wide range of external magnetic fields B=0-8T is studied. As the B-field is varied we find a dramatic change of the nuclear spin pumping dynamics with the polarisation rise times changing from ~1ms at 0T to ~1s at 1T. A week variation of the rise time is observed for external fields of >2T. After a few second optical excitation the nuclear spin memory on the dot survives for up to 20 min in fields above 2T. This time becomes as short as ~2s at 0T, still preserving the ratio between spin decay and rise times of up to 1000. A dramatic impact of the resident electron on the nuclear spin dynamics is demonstrated leading to suppression of spin diffusion. At the same time, the

Overhauser field has a profound effect on the dynamics of the resident electron spin enabling efficient optical orientation of the electron spin even at zero external field.

Last updated Tuesday, 22nd April 2008 ^ Top | Site Design: bencarpenter.co.uk | Administrator: Sam Davies |

|

|