Publications
Pavlov S. G., Hübers H. -W. ., Shuman V. B., Portsel L. M., Lodygin A. N., LI JUERONG, LE HUY NGUYEN, Fisher A., Astrov A., Abrosimov N. V., Pidgeon C. R., Zeng Zaiping, Niquet Y. -M., MURDIN BENEDICT NEIL, Engelkamp H., LITVINENKO KONSTANTIN, CLOWES STEVEN K (2018)Radii of Rydberg states of isolated silicon donors, In: Physical Review B98085423 American Physical Society
We have performed a high field magneto-absorption spectroscopy on silicon doped with a variety of single and double donor species. The magnetic field provides access to an experimental magnetic length, and the quadratic Zeeman effect in particular may~be used to extract the wavefunction radius without reliance on previously determined effective mass parameters. We were therefore able to determine the limits of validity for the standard one-band anisotropic effective mass model. We also provide improved parameters and use them for an independent check on the accuracy of effective mass theory. Finally, we show that the optically accessible excited state wavefunctions have the attractive property that interactions with neighbours are far more forgiving of position errors than (say) the ground state.
This study reports the effect of an increasing ion dose on both the electrical activation yield and the characteristic properties of implanted bismuth donors in silicon. A strong dependence of implant fluence is observed on both the yield of bismuth donors and the measured impurity diffusion. This is such that higher ion concentrations result in both a decrease in activation and an enhancement in donor migration through interactions with mobile silicon lattice vacancies and interstitials. Furthermore, the effect of implant fluence on the properties of the Si:Bi donor bound exciton, D0X, is also explored using photoluminescence (PL) measurements. In the highest density sample, centers corresponding to the PL of bismuth D0Xs within both the high density region and the lower concentration diffused tail of the implanted donor profile are identifiable.
An innovative, extrudable material with the ability to filter the sun’s energy has been developed for the mass manufacture of high performance swimming pool covers. Solar radiation in the visible spectrum ( nm) is absorbed by the material so that minimal visible light enters the pool water which inhibits photosynthesis to prevent algae growth. Furthermore, the material has high transmission properties in the near infrared that can be efficiently absorbed by the water allowing for a higher temperature increase compared to a standard non-selective opaque cover. We have developed a model to enable the cover efficiency to convert solar energy to heat a swimming pool, calculated based on the wavelength dependent absorption and transmission properties of the cover. We have validated this model using dedicated full-scale test-facility. Our results demonstrate that a selective transmission cover can increase the absolute heating efficiencies by approximately 12% compared to the fully opaque equivalent.
Group V donors in silicon are extremely promising candidates for applications within quantum technologies due to their long spin coherence lifetimes and existing compatibility within the microelectronics industry. However, there are various technical challenges which need to be overcome in order to achieve this potential. This body of work aims to address two such fabrication challenges and attempt to provide solutions which will aid the realisation of silicon and donor based quantum architectures. Bismuth donors display a vast potential for the implementation as spin qubits however there are limited techniques to controllably fabricate a high quality Si:Bi doped system. Ion implantation is the leading candidate to achieve this although there are concerns that the violent mechanism of Bi+ bombardment into the silicon lattice will significantly degrade the condition of the crystal environment. Therefore, the formation of bismuth donor states in silicon using ion implantation is studied with a specific emphasis on the quality of dopant incorporation. Using a combination of electrical measurements and donor bound exciton spectroscopy it is determined that, under the appropriate annealing conditions, it is possible to produce ion implanted bismuth donors in an environment free from the effects of lattice strain. This therefore motivates the use of ion implanted samples as an alternative to bulk doped Si:Bi for applications within quantum technologies. Implanted samples are then fabricated into simple two terminal devices and used in electrically detected THz spectroscopy measurements. Using a free electron laser, the resonant excitation of implanted bismuth donors is observed, however the characteristic properties of photoconductivity spectra strongly suggest that devices are being heated significantly. Finally, an alternative to bulk silicon is studied as a vehicle for quantum technologies. Specifically, the fabrication requirements to controllably align single crystal silicon nanowires is studied using a process known as dielectrophoresis, DEP. Here it is demonstrated that, under the appropriate experimental conditions, it is possible to utilise DEP to manipulate single silicon nanowires to fabricate a wide range of devices. This could prove highly beneficial for the integration of this material within quantum technologies.
Branford WR, Gilbertson AM, Buckle PD, Buckle L, Ashley T, Magnus F, Clowes SK, Harris JJ, Cohen LF (2008)Gate dependence of spin-splitting in an InSb/InAlSb quantum well, In: Clowes S, Murdin BN, (eds.), NARROW GAP SEMICONDUCTORS 2007119pp. 3-5 Zhang T, Clowes S K, Branford W R, Harris J, Cohen L F, Pidgeon C R, Murdin B N, Litvinenko K, Allam J, Bird M, Morrison K (2005)Temperature and Doping Dependence of Spin Relaxation in n-InAs, In: Physical Review B72(8) We have used time-resolved spectroscopy to measure the relaxation of spin polarizations in the narrow gap semiconductor material n-InAs as a function of temperature, doping, and pump wavelength. The results are consistent with the D'Yakonov-Perel mechanism for temperatures between 77 and 300 K. However, the data suggest that electron-electron scattering should be taken into account in determining the dependence of the spin lifetime on the carrier concentration in the range 5.2x10(16)-8.8x10(17) cm(-3). For a sample with doping of 1.22x10(17) cm(-3) the spin lifetime was 24 ps at room temperature. By applying a magnetic field in the sample plane we also observed coherent precession of the spins in the time domain, with a g factor g(*)=-13, also at room temperature.
Branford W R, Clowes S K, Bugoslavsky Y V, Gardelis S, Androulakis J, Giapintzakis J, Grigorescu C E A, Manea S A, Freitas R S, Roy S B, Cohen L F (2004)Thickness Dependence of Hall Transport in Ni1.15Mn0.85Sb Thin Films on Silicon, In: Physical Review B69(20) Highly spin polarized Heusler alloys, NiMnSb and Co2MnSi, attract a great deal of interest as potential spin injectors for spintronic applications. Spintronic devices require control of interfacial properties at the ferromagnet:semiconductor contact. To address this issue we report a systematic study of the ordinary and anomalous Hall effect, in Ni1.15Mn0.85Sb films on silicon, as a function of film thickness. In contrast to the bulk stoichiometric material, the Hall carriers in these films become increasingly electron-like as the film thickness decreases, and as the temperature increases from 50 K toward room temperature. High field Hall measurements confirm that this is representative of the majority transport carriers. This suggests that current injected from a NiMnSb:semiconductor interface may not necessarily carry the bulk spin polarization. The films also show a low temperature upturn in the resistivity, which is linked to a discontinuity in the anomalous Hall coefficient. Overall these trends indicate that the application of Heusler alloys as spin injectors will require strictly controlled interfacial engineering, which is likely to be demanding in these ternary alloys.
Murdin BN, Merrick M, Litvinenko K, Murzyn P, Phillips PJ, Pidgeon CR, Zhang T, Clowes SK, Cohen LF, Buckle P, Ashley T (2004)Spin lifetimes in narrow gap semiconductors measured with free-electron and solid-state lasers, In: Buckley DN, Chang PC, Fox PD, Chan WK, Shiojima K, (eds.), STATE-OF-THE-ART PROGRAM ON COMPOUND SEMICONDUCTORS XL (SOTAPOCS XL) AND NARROW BANDGAP OPTOELECTRONIC MATERIALS AND DEVICES II2004(2)pp. 253-258 Zhang T, Clowes S K, Debnath M, Bennett A, Roberts C, Harris J J, Stradling R A, Cohen L F, Lyford T, Fewster P F (2004)High-Mobility Thin InSb Films Grown by Molecular Beam Epitaxy, In: Applied Physics Letters84(22) The problem of preparing high-mobility thin InSb films is revisited for magnetoresistive and spintronic sensor applications. We introduce a growth process that significantly improves the electrical properties of thin unintentionally doped InSb layers (60-300 nm) epitaxially grown on GaAs(100) substrates by reducing the density of dislocations within the interfacial layer. The epilayer properties are well described by a differential two-layer model. This model confirms that the contribution of the interface can only be donor-like. Moreover, the electrical properties of the InSb layers change continuously away from the interface up to sample thickness of the order of 1 mum.
Clowes SK, Miyoshi Y, Johannson O, Hickey BJ, Marrows CH, Blamire M, Branford WR, Bugoslavsky YV, Cohen LF (2004)Using PCAR to study Cu/Co bilayers, In: JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS272pp. E1471-E1473 ELSEVIER SCIENCE BV
Stachel S, Olbrich P, Zoth C, Hagner U, Stangl T, Karl C, Lutz P, Bel'kov VV, Clowes Steven, Ashley T, Gilbertson AM, Ganichev SD (2012)Interplay of spin and orbital magnetogyrotropic photogalvanic effects in InSb/AlInSb quantum well structures, In: Physical Review B (Condensed Matter and Materials Physics)85045305 American Physical Society
We report on the observation of linear and circular magnetogyrotropic photogalvanic effects in InSb/AlInSb quantum well structures. We show that intraband (Drude-like) absorption of terahertz radiation in the heterostructures causes a dc electric current in the presence of an in-plane magnetic field. The photocurrent behavior upon variation of the magnetic field strength, temperature and wavelength is studied. We show that at moderate magnetic fields the photocurrent exhibits a typical linear field dependence. At high magnetic fields, however, it becomes nonlinear and inverses its sign. The experimental results are analyzed in terms of the microscopic models based on asymmetric relaxation of carriers in the momentum space. We demonstrate that the observed nonlinearity of the photocurrent is caused by the large Zeeman spin splitting in InSb/AlInSb structures and an interplay of the spin-related and spin-independent roots of the magnetogyrotropic photogalvanic effect.
Magnus F, Yates KA, Clowes SK, Miyoshi Y, Bugoslavsky Y, Cohen LF, Aziz A, Burnell G, Blamire MG, Josephs-Franks PW (2008)Interface properties of Pb/InAs planar structures for Andreev spectroscopy, In: APPLIED PHYSICS LETTERS92(1)ARTN 0pp. ?-? AMER INST PHYSICS
Electron spin relaxation times have been measured in InSb and InAs quantum wells and epi-layers in a moderate (<4T) external magnetic field by means of time-resolved optical orientation using circularly polarised light. A strong and opposite field dependence of the spin lifetime was observed for longitudinal (Faraday) and transverse (Voigt) configuration. In the Faraday configuration the spin lifetime increases because the DP dephasing process is suppressed. At the high field limit the EY relaxation process dominates, enabling its direct determination. Conversely, in the Voigt configuration an additional efficient spin dephasing mechanism dominates and shortens the electron spin lifetime considerably with B-2. We demonstrate that for narrow gap semiconductors simply changing the direction of the magnetic field of 1T can alter the electron spin lifetime by one order of magnitude.
A comprehensive study of the optical properties of PbS nanocrystals (NCs) is reported that includes the temperature dependent absorption, photoluminescence (PL) and PL lifetime in the range of 3-300 K. The absorption and PL are found to display different temperature dependent behaviour though both redshift as temperature is reduced. This results in a temperature dependent Stokes shift which increases from ∼75 meV at 300 K with reducing temperature until saturating at ∼130 meV below ∼150 K prior to a small reduction to 125 meV upon cooling from 25 to 3 K. The PL lifetime is found to be single exponential at 3 K with a lifetime of τ(1) = 6.5 μs. Above 3 K biexponential behaviour is observed with the lifetime for each process displaying a different temperature dependence. The Stokes shift is modelled using a three-level rate equation model incorporating temperature dependent parameter values obtained via fitting phenomenological relationships to the observed absorption and PL behaviour. This results in a predicted energy difference between the two emitting states of ∼6 meV which is close to the excitonic exchange energy splitting predicted theoretically for these systems.
Litvinenko KL, Murdin BN, Clowes SK, Nikzad L, Allam J, Pidgeon CR, Branford W, Cohen LF, Ashley T, Buckle L (2008)Density and well-width dependence of the spin relaxation in n-InSb/AlInSb quantum wells, In: Murdin BN, Clowes S, (eds.), NARROW GAP SEMICONDUCTORS 2007119pp. 19-21 Stachel S, Budkin GV, Hagner U, Bel'kov VV, Glazov MM, Tarasenko SA, Clowes SK, Ashley T, Gilbertson AM, Ganichev SD (2014)Cyclotron-resonance-assisted photon drag effect in InSb/InAlSb quantum wells excited by terahertz radiation, In: PHYSICAL REVIEW B89(11)ARTN 1pp. ?-? AMER PHYSICAL SOC
Three terminal measurements on a carbon nanotube field effect transistor (CNTFET) were carried out in high vacuum and the ambient, and its performance compared. The on-off current ratio, ION/IOFF, were 102 and 105 for devices operated in high vacuum and in ambient air, respectively. Here, we show that the conversion of p-type to ambipolar behavior may largely be attributed to the O2 in ambient doping the single walled carbon nanotubes (SWCNTs) in the active channel which consists of bundles of SWCNTs. Switching behaviour of these devices, with respect to constituent types of SWCNTs in the bundles will be discussed.
Leontiadou MA, Litvinenko KL, Gilbertson AM, Pidgeon CR, Branford WR, Cohen LF, Fearn M, Ashley T, Emeny MT, Murdin BN, Clowes SK (2011)Experimental determination of the Rashba coefficient in InSb/InAlSb quantum wells at zero magnetic field and elevated temperatures, In: JOURNAL OF PHYSICS-CONDENSED MATTER23(3)ARTN 0pp. ?-? IOP PUBLISHING LTD
Branford W R, Clowes S K, Syed M H, Bugoslavsky Y V, Gardelis S, Androulakis J, Giapintzakis J, Grigorescu C E, Berenov A V, Roy S B, Cohen L F (2004)Large Positive Magnetoresistance in Nonstoichiometric NiMnSb Thin Films on Silicon, In: Applied Physics Letters84(13) We report a systematic study of the transport properties of pulsed-laser-deposited NiMnSb films on silicon as a function of film thickness. A low-temperature upturn is observed in the resistivity for film thicknesses of 130 nm and below. The resistivity minimum corresponds to the maximum in the positive magnetoresistance for all samples. As the film thickness decreases, the magnitude of both the resistivity upturn and the. magnetoresistance increase. There is no feature associated with the upturn in the low-field Hall resistivity, which becomes systematically more electron dominated as the film thickness decreases and the temperature increases. This has implications for the use of NiMnSb as a spin injector for spintronic applications. The positive magnetoresistance of the 5 nm sample is greater than 100% at 200 K in 8 T. Further enhancement of the magnetoresistance occurs for field parallel, rather than perpendicular, to the film surface. The magnetoresistance behavior is compared to various model systems, including the band-gap tuning found in the silver chalcogenides, disorder-induced weak localization, and the, emerging class of "bad metal" ferromagnets.
We measure transverse magnetically focused photocurrent signals in an InSb/InAlSb quantum well device. Using optical spin orientation by modulated circularly polarized light an electron spin-dependent signal is observed due to the spin-orbit interaction. Simulations of the focusing signal are performed using a classical billiard ball model, which includes both spin precession and a spin-dependent electron energy. The simulated data suggest that a signal dependent on the helicity of the incident light is expected for a Rashba parameter α > 0.1 eVÅ and that a splitting of the focusing signal is not expected to be observed in linear polarized photocurrent and purely electrical measurements.
Seddon EA, Malins AER, Petty M, Crapper MD, Wardell IRM, Hide AK, Hardiman M, Xu YB, Hucknall P, Greig D, Clowes S, McCash EM (1999)Amorphous alloys as secondary standards for electron spin polarimetry, In: MEASUREMENT SCIENCE AND TECHNOLOGY10(4)pp. 269-274 IOP PUBLISHING LTD
Characterization of the Ge concentration in a Si1-xGe x heterojunction for x varying from 5% to 40% in steps of 5% is done by beveling and wet thermal oxidation of the exposed layers. The resulting difference in oxide thickness as a function of Ge concentration is visualized due to light interference. Different Ge concentrations are seen as different colors through an optical microscope. CABOOM - Characterization of the Alloy concentration by Beveling, Oxidation and Optical Microscopy - in combination with AFM - Atomic Force Microscopy, is used as a tool to study the oxidation kinetics of unstrained, crystalline Si1-xGex by wet thermal oxidation.
Abrosimov NV, Riemann H, Pavlov SG, Hübers HW, Murdin PG, Murdin BN, Li J, Pang ML, Bowyer ET, Litvinenko KL, Clowes SK, Engelkamp H, Pidgeon CR, Galbraith I (2013)Si:P as a laboratory analogue for hydrogen on high magnetic field white dwarf stars., In: Nat Commun4pp. 1469-? Nature Publishing Group
Laboratory spectroscopy of atomic hydrogen in a magnetic flux density of 10(5) T (1 gigagauss), the maximum observed on high-field magnetic white dwarfs, is impossible because practically available fields are about a thousand times less. In this regime, the cyclotron and binding energies become equal. Here we demonstrate Lyman series spectra for phosphorus impurities in silicon up to the equivalent field, which is scaled to 32.8 T by the effective mass and dielectric constant. The spectra reproduce the high-field theory for free hydrogen, with quadratic Zeeman splitting and strong mixing of spherical harmonics. They show the way for experiments on He and H(2) analogues, and for investigation of He(2), a bound molecule predicted under extreme field conditions.
This book contains the invited and contributed papers which were presented at this meeting and serves to provide a broad overview of the current worldwide ...
Eustace DA, McComb DW, Buckle L, Buckle P, Ashley T, Singh LJ, Barber ZH, Gilbertson AM, Branford WR, Clowes SK, Cohen LF (2008)(S)TEM Characterisation of InAs/MgO/Co Multilayers, In: Cullis AG, Midgley PA, (eds.), MICROSCOPY OF SEMICONDUCTING MATERIALS 2007120pp. 153-156 Magnus F, Clowes SK, Gilbertson AM, Branford WR, Barkhoudarov ED, Cohen LF, Singh LJ, Barber ZH, Blamire MG, Buckle PD, Buckle L, Ashley T, Eustace DA, McComb DW (2007)Electrical characterization of MgO tunnel barriers grown on InAs (001) epilayers, In: APPLIED PHYSICS LETTERS91(12)ARTN 1pp. ?-? AMER INST PHYSICS
We report on the electrical detection of spin dependent photoconductivity in 500 nm wide InSb quantum well nanowires using the optical orientation of electron spins. By applying weak magnetic fields (≈ 200 mT), we observe a spin filtering effect of classical origin caused by spin dependent back scattering of electrons from the sidewalls. Spin dependent features in the longitudinal photovoltage decay with temperature and disappears at characteristic energy (≈ 50 K) consistent with the theoretical spin splitting and the thermal level broadening. We show that the observed signal is due to the inversion asymmetry of the quantum well, with an additional Zeeman contribution. © 2012 American Institute of Physics.
Peach Tomas, Homewood Kevin, Lourenco Manon, Hughes M, Saeedi Kaymar, Stavrias Nikolaos, Li Juerong, Chick Steven, Murdin Benedict, Clowes Steven (2018)The Effect of Lattice Damage and Annealing Conditions on the Hyperfine Structure of Ion Implanted Bismuth Donors in Silicon, In: Advanced Quantum Technologies1(2)1800038 Wiley
This study reports on high energy bismuth ion implantation into silicon with a particular emphasis on the effect that annealing conditions have on the observed hyperfine structure of the Si:Bi donor state. A suppression of donor bound exciton, D0X, photoluminescence is observed in implanted samples which have been annealed at 700 °C relating to the presence of a dense layer of lattice defects that is formed during the implantation process. Hall measurments at 10 K show that this implant damage manifests itself at low temperatures as an abundance of p‐type charge carriers, the density of which is observed to have a strong dependence on annealing temperature. Using resonant D0X photoconductivity, we are able to identify the presence of a hyperfine structure in samples annealed at a minimum temperature of 800 °C; however, higher temperatures are required to eliminate effects of implantation strain.
Branford WR, Clowes SK, Bugoslavsky YV, Miyoshi Y, Cohen LF, Berenov AV, MacManus-Driscoll JL, Rager J, Roy SB (2003)Effect of chemical substitution on the electronic properties of highly aligned thin films of Sr(2-x)A(x)FeMoO(6) (A=Ca, Ba, La; x=0, 0.1), In: JOURNAL OF APPLIED PHYSICS94(7)pp. 4714-4716 AMER INST PHYSICS
Branford WR, Singh LJ, Barber ZH, Kohn A, Petford-Long AK, Van Roy W, Magnus F, Morrison K, Clowes SK, Bugoslavsky YV, Cohen LF (2007)Temperature insensitivity of the spin-polarization in Co2MnSi films on GaAs (001), In: NEW JOURNAL OF PHYSICS9PII S1pp. ?-? IOP PUBLISHING LTD
Litvinenko K L, Murdin B N, Allam J, Pidgeon C R, Clowes S K, Bird M, Morris K, Branford W, Cohen L F, Ashley T, Buckle L (2006)Spin Relaxation in N-InSb/AlInSb Quantum Wells, In: New Journal of Physics8(4) We have used time resolved spectroscopy to measure the relaxation of spin polarization in InSb/AlInSb quantum wells (QWs) as a function of temperature and mobility. The results are consistent with the D'yakonov - Perel (DP) mechanism for high mobility samples over the temperature range from 50 to 300 K. For low mobility samples at high temperature the Elliott - Yafet and DP mechanisms become comparable. We show that the mobility can in certain circumstances determine which mechanism is dominant, and that above 1 m(2) V-1 s(-1) in 20 nm wide InSb QWs it is the DP mechanism. We also give a criterion for the maximum spin lifetime in terms of mobility and temperature, and show that for our 20 nm wide QWs this corresponds to 0.5 ps at 300 K and mobility 1 m(2) V-1 s(-1).
Several polycrystalline samples of the half-Heusler alloy NiMnSb were grown by arc melting of stoichiometric and nonstoichiometric amounts of high-purity constituent elements. The structure and the phase-purity of the prepared samples were examined systematically by powder x-ray diffraction. The transport properties of the best sample, with saturation magnetization M-s(5 K)=4 mu(B)/formula unit, were studied by measuring electrical resistivity, thermal conductivity, and thermopower. Features in both magnetic and transport data are consistent with NiMnSb being in a half-metallic state at low temperatures, i.e., the conduction electrons are fully spin polarized. However, point-contact Andreev reflection measurements on the same sample at 4.2 K demonstrate only similar to45% spin polarization.
Zhang T, Harris JJ, Branford WR, Clowes SK, Cohen LF, Solin SA (2007)Inherent magnetoresistance and surface morphology of InSb thin films, In: Jantsch W, Schaffler F, (eds.), Physics of Semiconductors, Pts A and B893pp. 561-562 Murdin B N, Litvinenko K, Clarke D G, Pidgeon C R, Murzyn P, Phillips P J, Carder D, Berden G, Redlich B, van der Meer A F, Clowes S, Harris J J, Cohen L F, Ashley T, Buckle L (2006)Spin Relaxation by Transient Monopolar and Bipolar Optical Orientation, In: Physical Review Letters96(9) We have used two-color time-resolved spectroscopy to measure the relaxation of electron spin polarizations in a bulk semiconductor. The circularly polarized pump beam induces a polarization either by direct excitation from the valence band, or by free-carrier (Drude) absorption when tuned to an energy below the band gap. We find that the spin relaxation time, measured with picosecond time resolution by resonant induced Faraday rotation in both cases, increases in the presence of photogenerated holes. In the case of the material chosen, n-InSb, the increase was from 14 to 38 ps.
The point contact Andreev reflection employing niobium tips was used to determine the degree of transport current spin polarization ( Pt ) at the free surface of bulk NiMnSb at 4.2 K . The data was analyzed within the framework of a modified version of the Blonder, Tinkham, and Klapwijk formulism taking into account the two spin polarized channels in the ferromagnet and treating the interface as a planar delta function barrier of height Z between free electron materials. We find that the measured spin polarization is rather insensitive to different surface preparations and magnetic domain structure, and the maximal value of the Pt at Z=0 is 44% , consistent with recent calculations of the surface reconstruction of NiMnSb . Saturation magnetization of the samples was found to be 3.6 ìB per formula unit indicative of a small amount of atomic disorder.
Clowes SK, Branford WR, Gilbertson AM, Zhang T, Buckle L, Husmann A, Solin SA, Cohen LF (2007)Improved geometric control of the high-field linear magnetoresistance by metallic bridges in InSb arrays, In: Jantsch W, Schaffler F, (eds.), Physics of Semiconductors, Pts A and B893pp. 1383-1384 Singh LJ, Oliver RA, Barber ZH, Eustace DA, McComb DW, Clowes SK, Cohen LF, Buckle L, Buckle PD, Ashley T, Gilbertson AM, Magnus F, Branford WR (2007)Preparation of InAs(001) surface for spin injection via a chemical route, In: JOURNAL OF PHYSICS D-APPLIED PHYSICS40(10)pp. 3190-3193 IOP PUBLISHING LTD
Pidgeon CR, Murzyn P, Phillips PJ, Murdin BN, Litvinenko K, Merrick M, Cohen LF, Zhang T, Clowes SK, Buckle P, Ashley T, Thumm M, Wiesbeck W (2004)Spin relaxation in InSb and InAs by 1-and 2-colour spectroscopy with free electron and solid state lasers, In: CONFERENCE DIGEST OF THE 2004 JOINT 29TH INTERNATIONAL CONFERENCE ON INFRARED AND MILLIMETER WAVES AND 12TH INTERNATIONAL CONFERENCE ON TERAHERTZ ELECTRONICSpp. 207-208 Murdin B. N., Litvinenko K., Allam J., Pidgeon C. R., Bird M., Morrison K., Zhang T., Clowes S. K., Branford W. R., Harris J., Cohen L. F. (2005)Temperature and doping dependence of spin relaxation in n-InAs, In: Physical Review B72(085346) We have used time-resolved spectroscopy to measure the relaxation of spin polarizations in the narrow gap semiconductor material n-InAs as a function of temperature, doping, and pump wavelength. The results are consistent with the D'Yakonov-Perel mechanism for temperatures between 77 and 300 K. However, the data suggest that electron-electron scattering should be taken into account in determining the dependence of the spin lifetime on the carrier concentration in the range 5.2×1016–8.8×1017 cm–3. For a sample with doping of 1.22×1017 cm–3 the spin lifetime was 24 ps at room temperature. By applying a magnetic field in the sample plane we also observed coherent precession of the spins in the time domain, with a g>/i> factor g*=–13, also at room temperature.
Magnus F, Burnell G, Miyoshi Y, Yates KA, Bugoslavsky Y, Clowes SK, Josephs-Franks PW, Blamire MG, Cohen LF (2007)Planar Andreev spectroscopy in InAs, In: Jantsch W, Schaffler F, (eds.), PHYSICS OF SEMICONDUCTORS, PTS A AND B893pp. 1281-1282 We report significant temperature dependence of the transverse electron g∗‐factor in symmetric lead chalcogenide multi‐quantum wells (MQWs). The g∗‐factor values were extracted from the electron Larmor precessions recorded by means of a circularly polarized pump probe technique under the influence of transverse external magnetic field (Voigt geometry) in the temperature range between 10 and 150K. The reported g∗‐factor values are in good agreement with theoretical predictions and available low temperature experimental data. Although temperature tuning of lead salt laser emission wavelengths has been the method of choice in these systems for many years, we demonstrate that temperature can also be used to modulate g∗, and hence the spin lifetime in lead salt QW spintronic devices.
This paper reports on the predicted increase in the Rashba interaction due to the incorporation of Bi in GaAs/AlGaAs heterostructures. Band structure parameters obtained from the band anti-crossing theory have been used in combination with self-consistent Schrödinger-Poisson calculations and k.p models to determine the electron spin-splitting caused by structural inversion asymmetry and increased spin-orbit interaction. A near linear seven fold increase in the strength of the Rashba interaction is predicted for a 10% concentration of Bi in a GaAsBi/AlGaAs quantum well heterostructure.
Itskos G, Harbord E, Clowes SK, Clarke E, Van Dorpe P, Van Roy W, Cohen LF, Murray R (2007)Oblique Hanle measurements of InAs/GaAs quantum dot spin-light emitting diodes, In: Jantsch W, Schaffler F, (eds.), Physics of Semiconductors, Pts A and B893pp. 1289-1290 Litvinenko K. L., Murdin B. N., Allam J., Pidgeon C. R., Bird M., Morris K., Branford W., Clowes S. K., Cohen L. F., Ashley T., Buckle L. (2006)Spin relaxation in n-InSb/AlInSb quantum wells, In: New Journal of Physics8(4) We have used time resolved spectroscopy to measure the relaxation of spin polarization in InSb/AlInSb quantum wells (QWs) as a function of temperature and mobility. The results are consistent with the D'yakonov–Perel (DP) mechanism for high mobility samples over the temperature range from 50 to 300 K. For low mobility samples at high temperature the Elliott–Yafet and DP mechanisms become comparable. We show that the mobility can in certain circumstances determine which mechanism is dominant, and that above 1 m2 V-1 s-1 in 20 nm wide InSb QWs it is the DP mechanism. We also give a criterion for the maximum spin lifetime in terms of mobility and temperature, and show that for our 20 nm wide QWs this corresponds to 0.5 ps at 300 K and mobility 1 m2 V-1 s-1.
We address the inherent high-field magnetoresistance (MR) of indium antimonide epilayers on GaAs (001), studying the modification of the MR when processed into a set of geometries. The changes produced by the geometries are quite subtle. The extraordinary MR geometry produces the highest low-field MR while the Corbino geometry produces the largest high-field magnetoresistance. We demonstrate that any material with an unsaturating linear intrinsic MR, will also have an unsaturating linear Corbino MR, and that the ideal material for linear MR sensors in conventional geometries would have a high mobility and a small, linear intrinsic MR.
Redlich B., van der Meer A. F. G., Clowes S., Harris J. J., Cohen L. F., Ashley T., Buckle L., Murdin B. N., Litvinenko K., Clarke D. G., Pidgeon C. R., Murzyn P., Phillips P. J., Carder D., Berden G. (2006)Spin Relaxation by Transient Monopolar and Bipolar Optical Orientation, In: Physical Review Letters96(096603) We have used two-color time-resolved spectroscopy to measure the relaxation of electron spin polarizations in a bulk semiconductor. The circularly polarized pump beam induces a polarization either by direct excitation from the valence band, or by free-carrier (Drude) absorption when tuned to an energy below the band gap. We find that the spin relaxation time, measured with picosecond time resolution by resonant induced Faraday rotation in both cases, increases in the presence of photogenerated holes. In the case of the material chosen, n-InSb, the increase was from 14 to 38 ps.
Post-synthesis separation of metallic (m-SWNTs) and semiconducting (s-SWNTs) single-wall carbon nanotubes (SWNTs) remains a challenging process. Gel agarose chromatography is emerging as an efficient and large scale separation technique. However, the full (100%) separation has not been achieved yet, mainly due to the lack of understanding of the underlying mechanism. Here, we study the temperature effect on the SWNTs separation via gel agarose chromatography, for four different SWNT sources. Exploiting a gel agarose micro-beads filtration technique we achieve up to 70% m-SWNTs and over 90% s-SWNTs, independent of the source material. The process is temperature dependent, with yields up to 95% for s-SWNT (HiPco) at 6 °C. Temperature affects the sodium dodecyl sulfate surfactant-micelle distribution along the SWNT sidewalls, thus determining the effectiveness of the SWNTs sorting by electronic type. The sorted SWNTs are then used to fabricate transistors with very low OFF-currents (∼10−13 A), high ON/OFF current ratio (>106) and charge carriers mobility ∼ 40 cm2 V−1 s−1.
Quantum computation holds the promise of efficient solutions to currently formidable problems. A prospective technology is that of computing using quantum states, which may dramatically speed up or otherwise reduce the complexity for the solution of some hard problems. Of the many different schemes proposed, shallow substitutional donors in Silicon hold great attractiveness for the detailed study of the medium, high industrial capacity, and ubiquity of raw materials. We develop control over the quantum orbital and spin states of bound electrons using the THz ultrafast laser FELIX. Manipulation of orbital states is studied using interferometric methods to produce Ramsey fringes, read out using optical and electrical methods. Contactless electrical measurement of free charge carriers is implemented, and the details of its advantages and limitations are explored. Both types of readout are used to demonstrate a coherent 3-level orbital manipulation, otherwise known as a quantum beat. The work constitutes a nontrivial control over the spatial distribution of the wavefunction of the atom which may enable error correcting surface code implementations. Spin dynamics are then explored in the presence of FELIX using donor-bound exciton techniques, which allow a sensitive and fast control over the electron spin states. The implementation is used to probe whether orbital excitation has a strong effect upon the spin states of the donor electrons, and it is shown that any modification of the spin is negligible. Experimental measurements of optically gated spin-exchange coupling are enabled by this methodology, and a roadmap to future implementation is discussed in the context of the present work. Overall, the work advances the state of spin and orbital control of neutral donor states for the purposes of optically gated quantum computing. Combined spin and orbital manipulations are now possible in the system, which will allow a more advanced implementation of quantum computation using orbital states than was previously possible.
