• No results found

Spin polarization of the neutral exciton in a single InAs quantum dot at zero magnetic field

N/A
N/A
Protected

Academic year: 2021

Share "Spin polarization of the neutral exciton in a single InAs quantum dot at zero magnetic field"

Copied!
5
0
0

Loading.... (view fulltext now)

Full text

(1)

Linköping University Post Print

Spin polarization of the neutral exciton in a

single InAs quantum dot at zero magnetic field

Evgenii Moskalenko, L A Larsson and Per-Olof Holtz

N.B.: When citing this work, cite the original article.

Original Publication:

Evgenii Moskalenko, L A Larsson and Per-Olof Holtz, Spin polarization of the neutral

exciton in a single InAs quantum dot at zero magnetic field, 2009, PHYSICAL REVIEW B,

(80), 19, 193413.

http://dx.doi.org/10.1103/PhysRevB.80.193413

Copyright: American Physical Society

http://www.aps.org/

Postprint available at: Linköping University Electronic Press

(2)

Spin polarization of the neutral exciton in a single InAs quantum dot at zero magnetic field

E. S. Moskalenko,1,2L. A. Larsson,1 and P. O. Holtz1

1IFM, Material Physics, Linköping University, S-581 83 Linköping, Sweden

2A. F. Ioffe Physical-Technical Institute, Russian Academy of Sciences, Polytechnicheskaya 26, 194021 St. Petersburg, Russia

共Received 24 September 2009; published 30 November 2009兲

A high degree of spin polarization for the neutral exciton in individual quantum dots, at zero external magnetic field, is monitored. While a high polarization degree is commonly observed for the charged ex-citon, a negligible polarization has been predicted for the neutral exciton. The exceptionally high polarization 共⬎60%兲 observed here is explained in terms of a dynamical nuclear polarization field, stabilizing the electron spin. Such polarization of the quantum dot nuclei, in case of the neutral exciton, is possible due to unequal capture time of electrons and holes.

DOI:10.1103/PhysRevB.80.193413 PACS number共s兲: 78.67.Hc, 71.70.Gm, 72.25.Fe, 73.63.Kv

The spin of a single carrier localized in a semiconductor quantum dot共QD兲 has been suggested as a building block for future memory and quantum computer operation.1In particu-lar, the spin of an electron confined in a QD is a good can-didate for these applications because of cancellation of clas-sical spin-relaxation mechanisms.2 The state of the spin of recombining particles can be directly measured by monitor-ing the degree of circular polarization共␳c兲 in

photolumines-cence 共PL兲 experiments. For the case of neutral excitons in QDs, a negligible␳chas been predicted at zero external

mag-netic field 共Bext兲 due to strong anisotropic electron 共e兲-hole

共h兲 exchange interaction.3–7 Conversely, for the case of charged excitons, the anisotropic e-h exchange interaction is suppressed5,8 and an essential

c is expected and has been

confirmed in experiments on individual In共Ga兲As/GaAs QDs. Up to now, a very low ␳cwere recorded at Bext= 0 for

the neutral exciton in experiments with QD ensembles6,9 or individual QDs.3,8,10–12

In the present report, a high degree 共␳c⬇60%兲 of spin

polarization is achieved for the neutral exciton at Bext= 0 in

individual InAs QDs, studied by micro-photoluminescence 共␮-PL兲. The anomalously high␳cis explained by a separate

capture of e’s and h’s into the QD. This provides a time interval, when the QD is occupied with solely an electron, which can polarize the lattice nuclei. In turn, a nuclear mag-netic field 共BN兲, called the Overhauser field, acting upon

electrons will effectively stabilize the electron spin and result in the high␳cobserved in the experiments. Further, the

elec-tron magnetic field 共Be兲, the Knight field, acting upon the

nuclei in the QD has been directly measured to be⬇13 mT. These experiments demonstrate a possible way to bypass the anisotropic e-h exchange interaction in QDs and thereby al-lowing spin conservation of the neutral exciton

The sample studied was grown by MBE with self-assembled InAs QDs positioned between GaAs barriers.13To create spin-polarized carriers, the sample was excited with circular-polarized light共␴+and/or兲. The degree of circular polarization of the PL is given by: ␳c=共Ico− Icross兲/共Ico

+ Icross兲, where Ico共Icross兲 is the spectrally integrated PL

com-ponent of co共cross-兲circular polarization, with respect to the helicity of the excitation light.

Figure 1共a兲shows a ␮-PL spectrum of the wetting layer 共WL兲 emission. In the same panel,␳cof the neutral exciton

as a function of excitation energy 共h␯ex兲 is presented. It

should be stressed that for all hex, the excitation power was

tuned in order to maintain at the same integrated PL intensity from the QD 共to compensate for the change in sample ab-sorption兲. When h␯ex is varied, ␳c remains positive 关Fig.

1共a兲兴, in striking contrast to negatively charged excitons in

1.42 1.44 1.46 1.48 0.2 0.3 0.4 0.5 0.6

a

Bext= 0

b

WL po lar izat ion d egree ρ c Energy (eV) z ωex BΣ IS z ex I S 0 ex B ext 1.3548 1.3552 B ext= 0 det.σY det.σX exc.σX exc.σX det.σ+ det.σ -det.σ -det.σ+ exc.σ -det.σ- det. σ+ exc.σ+

c

P L intensity (C C D counts) Energy (eV)

FIG. 1. 共a兲 The symbols show ␳c as a function of excitation energy measured at ␴+ cw excitation, with Pex= 2 ␮W for h␯ex

⬎EWLand with increased Pexfor hex⬍EWL共see text兲. The line

shows a␮-PL spectrum of the WL measured at h␯ex= 1.684 eV and

at Pex= 40 nW. 共b兲␮-PL spectra 共symbols兲 of a QD measured at

hex= 1.463 eV, Pex= 2 ␮W and at different polarization

configu-rations in excitation and detections paths as indicated in the figure. The solid lines are fit with Lorentzian curves to the measured␮-PL spectra.共c兲 A schematic illustration of the pseudospin model of a neutral exciton exposed to an external magnetic field, as explained in the text. All data in 共a兲 and 共b兲 were recorded at T=4.2 K and

(3)

QDs, which exhibit a negative ␳c.9,12,14–17 A set of typical

␮-PL spectra of the neutral exciton18for h

exin the range of

the WL emission energy共EWL兲, and for different polarization configurations, is shown in Fig. 1共b兲. At circular-polarized excitation, the integrated PL intensity of the cocircular-polarized component appears much stronger than the cross-circular one, illustrating a high and positive degree of ␳c.

The high␳cobserved here is remarkable, since the bright

states 共兩+1典 and 兩−1典兲 of the heavy-hole neutral exciton is known to be mixed due to the in-plane asymmetry of a QD 共兩+1典 and 兩−1典 correspond to spin projections onto the z axis, which are chosen along the growth axis of the sample兲. This mixing creates two linearly polarized dipoles 兩X典=2−1/2共兩 +1典+兩−1兩兲 and 兩Y典=2−1/2共兩+1典−兩−1典兲/i, which in case of InAs/GaAs QDs emit light along the 具110典 and 具11គ0典 crys-tallographic directions.19,20 兩X典 and 兩Y典 are split by the an-isotropic e-h exchange energy共ប␻ex兲 共Ref.21兲 and the bright

states are separated in energy from the dark states. Thus, at

Bext= 0 the neutral exciton is expected to reveal two

orthogo-nal linearly polarized components and only at an elevated external magnetic field共Bext 储z兲, the mixed states 兩X典 and 兩Y典

will transform into “pure”兩+1典 and 兩−1典 states, giving rise to circular-polarized emission. Upon excitation with linearly polarized light 共␴X兲 at Bext= 0, two orthogonal and linearly

polarized PL lines, separated by ប␻ex⬇25 ␮eV, are indeed

monitored关Fig.1共b兲兴.

To explain the high␳cobserved here, we adopt the vector

model for the exciton pseudospin developed in Refs.4and7 关see Fig.1共c兲兴. Here the anisotropic e-h exchange interaction is viewed as an in-plane magnetic field共␻ex兲 and the vector

S0 ex

corresponds to the initial exciton spin. Since the preces-sion time关␶b= 2␲/␻ex⬇165 ps 共Ref.6兲兴 of S0

ex

around␻ex

is smaller than the exciton decay time 关␶d⬇800 ps 共Ref.

19兲兴, the exciton spin 共S0

ex兲 will accomplish many turns

around ␻ex before recombination.20 Hence ␳c共proportional

to the projection of the exciton pseudospin onto the z axis, 兩Sz

ex兩兲 is predicted to be negligible.4,6,7

However, as stated above, an external magnetic field of sufficient strength 兩Bext兩␮Bgex⬎ប␻ex 共␮B⬇58 ␮eV/T is the Bohr magneton

and gex is the neutral exciton g factor兲 applied in Faraday

geometry 共Bext 储z兲 “restores” the polarization of the neutral

exciton because of decoupling of the兩+1典 and 兩−1典 states.6,7 At increasing兩Bext兩, the 兩+1典 and 兩−1典 states are separated in

energy by ⌬E=兵共兩Bext兩␮Bgex兲2+共ប␻ex兲2其1/2,21 which gives

rise to a nonvanishing value of ␳c. In the vector model, an

application of Bext initiates the precession of the exciton

pseudospin around the total magnetic field B= Bext+␻ex

leading to a nonzero value of 兩Sz

ex兩 and hence of c.

From the ␮-PL spectra 关Fig. 1共b兲兴, an obvious energy separation 共⬇50 ␮eV兲 is recorded between the ␴− and ␴+ components at circular-polarized excitation despite Bext= 0.

This is the signature of an effective magnetic field 共BN兲 in

the sample with a projection共BN

z兲 onto the z axis. When the

excitation helicity is reversed, the polarization-resolved PL components exchange their spectral positions, clearly dem-onstrating that BN

z

has reversed its direction. The contribution of兩BN

z兩 to ⌬E, the Overhauser shift 共OHS兲, can be estimated

from OHS=兵共⌬E兲2共ប␻

ex兲2其1/2⬇42 ␮eV.

To further elucidate the existence of a magnetic field in the sample at␴+and/or␴−cw excitation and to measure the build-up time of BN

z

, the QD was excited by a beam with alternating␴+andpolarization with frequency, f. The QD was accordingly exposed to ␴+ light during the time, ⌬t = f−1, followed bylight共of the same power兲. Detection of the two circular-polarized PL components was performed within the time intervals corresponding to only ␴+ 共or excitation windows共see the lower inset of Fig.2兲.

The polarization degree ␳c, recorded with␴+ excitation,

remains approximately the same for f⬍100 Hz, but de-creases progressively for f⬎100 Hz to stabilize at a few percent at f⬎1000 Hz 共Fig. 2兲. ⌬t⬇10 ms is accordingly sufficient for BNto buildup, while for⌬t⬍1 ms, BNis

neg-ligible. The dependence of the averaged polarization degree 共具␳c典兲 共Ref.22兲 on the external magnetic field Bext 储z共upper

inset in Fig. 2兲 shows that 具␳c典 changes symmetrically to

reach about 55% at 兩Bext兩⬇1.5÷2 T. Based on this

experi-ment,兩BN

z兩 determining

cat cw excitation is predicted to be

of the same order. To understand the origin of BN, a concept

of dynamic polarization of lattice nuclei by optically oriented electrons is employed.23This effect originates from the cou-pling of electron and nuclear spins through the hyperfine Fermi interaction, while the corresponding interaction of nuclear spins with a hole is considerably weaker.23 Hence, the nuclear-hole interactions will be excluded from further discussion. The interaction between a single electron and a large number共104– 105兲 of nuclei 共N兲 in the QD results in a dynamical nuclear spin polarization leading to the appear-ance of spin-oriented nuclei, which is equivalent to building up an effective magnetic field acting upon the electron local-ized in the QD. Nuclear fields as large as several Tesla have been detected for In共Ga兲As/GaAs QDs.3,24,25The rise time of

FIG. 2.␳cas a function of the alternating frequency, f, measured during the time windows corresponding to an excitation with␴+, at

Pex= 2 ␮W, h␯ex= 1.463 eV, T = 4.2 K, and at Bext= 0. The upper right inset shows 具␳c典 as a function of Bext 储z measured at Pex

= 2 ␮W, h␯ex= 1.463 eV, T = 4.2 K, and at f = 1050 Hz. The lower left inset illustrates the excitation procedure with light pulses of alternating helicity. The PL signal is only accumulated within the detection time windows corresponding to␴+共or␴−兲 excitation.

BRIEF REPORTS PHYSICAL REVIEW B 80, 193413共2009兲

(4)

the nuclear polarization at Bext= 0 in In共Ga兲As/GaAs QDs

has earlier been experimentally determined to be 9.4 ms.26 This value agrees well with the time scale for the buildup of

BNin our experiments.

Accordingly, the field BN introduced above is identified

as a nuclear field grown up in the QD upon cw excitation with circular-polarized light, which injects spin-oriented electrons into the QD. Consequently, no BN is predicted

in the QD under linearly polarized excitation共i.e., photoex-cited electrons appear nonpolarized兲. Hence, the ␴+ and ␴− PL components are not expected to be split nor exhibit any measurable value of ␳c, as consistent with our

experi-mental observations 关Fig. 1共b兲兴. Since BN influences only

electrons, the Overhauser shift should be defined as OHS =兩BN

z兩␮

B兩ge兩,25,27 where ge is the electron g factor. Adopting

兩ge兩=0.5÷0.6 共Refs.3 and24兲 and OHS⬇42 ␮eV, one can

evaluate 兩BNz兩⬇1.2÷1.4 T, in satisfactory agreement with the predictions made above. The experimentally estimated exciton polarization, ␳c, is entirely determined by the

aver-aged electron spin 共S兲 according toc= 2兩Sz兩,19 where Sz is

the projection of S onto the z axis. This is in agreement with the assumption that, for excitation into the WL, the electron spin is preserved during the capture and relaxation processes in the QD, while the hole spin orientation is lost.5,9,15 It should be emphasized that a buildup of nuclear polarization has been demonstrated earlier for the case of the neutral exciton.27 However, this was achieved at a nonzero field,

Bext, and the nuclear polarization was determined by spin flip

assisted radiative recombination of dark excitons.

To explain the experimentally observed fact that a rela-tively strong nuclear magnetic field共1.2÷1.4 T兲 is builtup in the QD even for the case of a neutral exciton at Bext= 0, the

preceding step to the formation of the exciton is considered as a process of separate capture of e’s and h’s as was dem-onstrated in our previous studies on QDs共Ref.13兲 as well as by others共e.g., Refs.28–30and references therein兲. The pa-rameter,⌬␶e-h, is the difference in capture times between e’s

共␶e兲 and h’s 共␶h兲 into the QD. Since both e’s and h’s are

excited into the WL, these times could be estimated as ␶e共h兲

⬇Le共h兲/Ve共h兲. Here Le共h兲corresponds to the collection length

for individual e’s共h’s兲 into the QD and Ve共h兲is the e’s 共h’s兲

velocity in the WL plane, as was earlier deduced to be 1.6 ⫻107 共3.1⫻106兲 cm/s for the same sample.31 For our ex-perimental conditions共with one QD located within the area of the laser spot兲 Le= Lh⬇1 ␮m can be assumed 共i.e., half

the diameter of the laser spot兲 and, hence ␶e 共␶h

⬇6共32兲 ps resulting in ⌬␶e-h=␶h−␶e⬇26 ps. It should be

noted that ⌬␶e-h represents an expected PL rise time for a

QD, experimentally determined to be 30÷ 50 ps共Refs.9and 32兲 in reasonable agreement with our estimate, ⌬␶e-h ⬇26 ps. Accordingly, before the recombination of an exci-ton, the QD is assumed to be populated with only an electron for ⬇26 ps. The fraction of time 共⌫e兲 with single electron

occupancy in the QD, is defined as⌫e=⌬␶e-h/␶r, where␶ris

the average time between two subsequent exciton formation events. For an excitation power slightly below the biexciton formation level, one can estimate␶dⱖ␶rbecause to form the

biexciton, the formation of a second exciton in the QD is required before the first exciton recombines. Hence, ␶r⬇␶d

⬇800 ps is used giving ⌫e⬇0.0325 which is in reasonable

agreement with other reports.3,25

To check the idea on separate carrier capture times,⌬␶e-h,

determining the possibility for BN to buildup, ⌬␶e-h is

de-creased. This is achieved by exciting directly into the QD, i.e., hex⬍EWL, resulting in a considerable decrease in the

length Le共h兲. The results demonstrate a gradual reduction in

c, down to⬇0.25 共in the range 0.05–0.25 for different QDs兲

upon decreasing hexdown to ⬇1.41 eV 关Fig. 1共a兲兴.

Addi-tionally, experiments on QD ensembles show a monotonous decrease in␳cwith an increasing QD density共i.e., when Le共h兲

is no longer determined by the laser spot size, but rather by the averaged interdot distance兲. These observations support our model with separate carrier capture into the QD deter-mining the nuclear field buildup.

The nuclear magnetic field BNacting upon an electron in

the QD and the electron field Beacting upon each nucleus

are consequences of the same process of dynamical polariza-tion of nuclei by spin-oriented electrons. This circumstance allows BNz to be expressed in the following form:33–35

BN z =␣兵共Bext+ Be兲 · S其兵共Bext+ Be兲2+ BL 2−1共B ext z + Be z兲 共1兲

where ␣ is a proportionality constant, Be= beS, Bext

z 共B

e

z兲 is

the projection of Bext共Be兲 on the z axis, beis to be evaluated

below, and BL is the effective magnetic field caused by the

nuclear spin-spin interactions 关estimated to be ⬇0.3 mT for InAs/GaAs QDs共Ref.14兲兴. Beis related to␳cin the

follow-ing way: 兩Be

z兩=兩b

e兩·兩Sz兩=1/2兩be兩·␳c. The average interaction

energy of an electron spin S with N nuclei of the same spe-cies, assuming that their mean spins 共Iav兲 are equal, is

ex-pressed as: AIavS,23 where A is the hyperfine constant. Tak-ing this quantity as the nuclear spin energy in an electron field, Be, one obtains: AIavS = −Nប␥IavBe and, hence Be=

−SA/共Nប␥兲, whereis the nuclear gyromagnetic ratio. A/␥ is estimated as 1/2⌺j共Aj/␥j兲, where j numerates In and As.

The number of nuclei in a QD is assumed to be N⬇5 ⫻104.3,5,25,26,36With ␥In共␥As兲=5.86共4.58兲⫻107 rad T−1s−1 共Ref. 37兲 and AIn 共AAs兲=56共46兲 ␮eV,3 b

e⬇−30 mT is

-40 -30 -20 -10

0

10

20

30

40

0.1

0.2

0.3

0.4

0.5

0.6

exc.

σ

-exc.

σ

+

po

lar

izat

ion

d

egree

ρ c

B

ext

(mT)

z sample Sz Bez k z sample Sz Bez k

FIG. 3. ␳cas a function of Bext共Bext 储z兲 measured at cw

excita-tion with␴+共black兲 and共gray兲 at P

ex= 2 ␮W, h␯ex= 1.463 eV,

and T = 4.2 K. Positive 共negative兲 Bext correspond to Bextparallel

共antiparallel兲 to the direction of the laser beam. The arrows indicate the directions of the vectors k, Sz, and Be

z

(5)

derived. Hence for fully polarized electron spin共␳c= 1兲 兩Be

z

⬇15 mT and for our experimental conditions 共␳c= 0.55兲 兩Be

z

is estimated as⬇8.3 mT.

It follows from Eq.共1兲 that at Bext= 0,兩Be兩 should

consid-erably exceed BLto achieve a significant value of兩BN兩.

Sec-ond it follows that 兩BN

z兩 should vanish 共and accordingly

c

should decrease兲 at Bext z

= −Be z

. To check this idea, Bext 储z was

applied共see Fig.3兲. In this dependence of ␳con兩Bext兩=Bext

for ␴− and␴+ excitations, distinct minima in ␳c are

moni-tored at兩Bext兩 from 8 up to 18 mT. Thus, the average value

兩Be

z兩=13 mT is chosen as the strength of the electron field, in

satisfactory agreement with the predictions for 兩Be z兩 made

above. It should be noted that the dips in␳care observed at

opposite directions of Bext when the excitation is changed

from␴+to␴−and that the compensation of Be z

by Bext z

takes place at the predicted directions of Be

z

. The␴+共␴兲 excitation creates Sz pointing antiparallel 共parallel兲 to the direction of

the laser beam共k兲, accordingly Be

z↑ ↑共↑↓兲k for these

excita-tion condiexcita-tions. Finally, the experimental observaexcita-tion that 兩Be

z兩ⰇB

Lsupports the idea that an essential nuclear magnetic

field BNcould be achieved even for zero external magnetic

field, as evidenced by the high degree of circular-polarization measured for the neutral exciton.

To conclude, we report on a high degree of circular po-larization for the neutral exciton upon circular-polarized ex-citation, without any external magnetic field applied. This is explained in terms of the buildup of a nuclear magnetic field in the QD, which stabilizes the electron spin. The possibility to polarize the QD nuclei is shown to be due to the unequal capture times of electrons and holes.

The authors would like to thank P. M. Petroff and W. V. Schoenfeld for providing the samples. Acknowledgments also go to V. L. Korenev, V. K. Kalevich, A. S. Yurkov, and K. F. Karlsson for stimulating discussions. This work was supported by grants from the Swedish Research Council 共VR兲 and the Swedish Foundation for Strategic Research 共SSF兲 funded Nanopto consortium.

1Quantum Coherence, Correlation and Decoherence in

Semicon-ductor Nanostructures, edited by T. Takagahara 共Elsevier

Sci-ence, USA, 2003兲.

2A. V. Khaetskii and Y. V. Nazarov, Phys. Rev. B 61, 12639

共2000兲.

3P.-F. Braun et al., Phys. Rev. B 74, 245306共2006兲. 4E. L. Ivchenko, Pure Appl. Chem. 67, 463共1995兲. 5P. F. Braun et al., Phys. Rev. Lett. 94, 116601共2005兲. 6Yu. G. Kusrayev et al., Phys. Rev. B 72, 155301共2005兲. 7R. I. Dzhioev et al., Phys. Rev. B 56, 13405共1997兲. 8B. Eble et al., Phys. Rev. B 74, 081306共R兲 共2006兲. 9S. Laurent et al., Phys. Rev. B 73, 235302共2006兲. 10M. E. Ware et al., Phys. Rev. Lett. 95, 177403共2005兲. 11E. Poem et al., Phys. Rev. B 76, 235304共2007兲. 12E. Poem et al., Solid State Commun. 149, 1493共2009兲. 13E. S. Moskalenko et al., Nano Lett. 9, 353共2009兲. 14R. Oulton et al., Phys. Rev. Lett. 98, 107401共2007兲. 15S. Cortez et al., Phys. Rev. Lett. 89, 207401共2002兲. 16B. Pal et al., Phys. Rev. B 75, 125322共2007兲.

17A. S. Bracker et al., Phys. Rev. Lett. 94, 047402共2005兲. 18E. S. Moskalenko et al., Phys. Rev. B 66, 195332共2002兲. 19M. Paillard et al., Phys. Rev. Lett. 86, 1634共2001兲. 20T. Flissikowski et al., Phys. Rev. Lett. 86, 3172共2001兲. 21M. Bayer et al., Phys. Rev. B 65, 195315共2002兲.

22Parameter具␳

c典 is defined as 1/2兵␳c共␴+兲+␳c共␴−兲其, where␳c共␴+兲

and␳c共␴−兲 are values of

crecorded during␴+and␴−excitation

windows, respectively.

23M. I. Dyakonov and V. I. Perel, in Optical Orientation, edited by

F. Meier and B. P. Zakharchenya 共North-Holland, Amsterdam, 1984兲, Chap. 2.

24A. I. Tartakovskii et al., Phys. Rev. Lett. 98, 026806共2007兲. 25P. Maletinsky et al., Phys. Rev. B 75, 035409共2007兲.

26P. Maletinsky, A. Badolato, and A. Imamoglu, Phys. Rev. Lett.

99, 056804共2007兲.

27D. Gammon et al., Phys. Rev. Lett. 86, 5176共2001兲. 28J. Urayama et al., Phys. Rev. Lett. 86, 4930共2001兲.

29E. C. Le Ru, J. Fack, and R. Murray, Phys. Rev. B 67, 245318

共2003兲.

30M. Grundmann and D. Bimberg, Phys. Status Solidi A 164, 297

共1997兲.

31E. S. Moskalenko et al., Phys. Rev. B 73, 155336共2006兲. 32R. Heitz et al., Phys. Rev. B 56, 10435共1997兲.

33C. W. Lai et al., Phys. Rev. Lett. 96, 167403共2006兲.

34M. I. Dyakonov and V. I. Perel, Zh. Eksp. Teor. Fiz. 68, 1514

共1975兲.

35D. Paget et al., Phys. Rev. B 15, 5780共1977兲.

36M. N. Makhonin et al., Phys. Rev. B 77, 125307共2008兲. 37Encyclopedia of Physical Science and Technology, edited by R.

A. Meyers, 3rd ed. 共Academic, San Diego, 2002兲, Vol. 10, p. 704.

BRIEF REPORTS PHYSICAL REVIEW B 80, 193413共2009兲

References

Related documents

Att det finns några olika hjälpmedel vid arbete med kemiska ämnen eller produkter är något som de intervjuade inte anser att de har nytta av vid rivningsarbeten utan istället är för

Since the optimized trade execution is a multistage decision process (what orders to place at different time steps) and has a clear objective (minimize transaction cost) it can be

The purpose of this study was to compare the Charlson Comorbidity Index and Rx-Risk Index, by applying the methods to data from the Swedish National Patient Register and

The Tree Theme Method in Psychosocial Occupational Therapy: A Case Study Scandinavian Journal of Occupational Therapy 2006 Att beskriva TTM som en interventions- metod

För att systemet ska kunna hämta och lägga till events i personalens kalender krävs det att varje person går in i inställningarna på sitt Gmail-konto och ger systemets servicekonto

Signifikanta skillnader identifierades mellan de olika armaturerna och som i tidigare försök spelade färgtemperaturen en stor roll för flera av de bedömda egenskaperna; inte

In this paper, we presented preliminary results of a fine-tuned Swedish BERT model for focused ter- minology extraction.. The model was devised to discover terms indicative of

Clearly, a hypothesis to test is whether an increase or improvement in finger hydration could improve the dynamic tactile discrimination ability in the elderly group, analogously