Table Of ContentVoltage controlled nuclear polarization switching in a single InGaAs quantum dot
M. N. Makhonin1, J. Skiba-Szymanska1, M. S. Skolnick1, H.-Y. Liu2, M. Hopkinson2, A. I. Tartakovskii1
1 Department of Physics and Astronomy, University of Sheffield, S3 7RH,UK
2 Department of Electronic and Electrical Engineering, University of Sheffield, Sheffield S1 3JD, UK
(Dated: January 15, 2009)
Sharp threshold-like transitions between two stable nuclear spin polarizations are observed in
optically pumped individual InGaAs self-assembled quantum dots embedded in a Schottky diode
9
when the bias applied to the diode is tuned. The abrupt transitions lead to the switching of the
0
Overhauserfieldinthedotbyupto3Tesla. Thebias-dependentphotoluminescencemeasurements
0
reveal the importance of the electron-tunneling-assisted nuclear spin pumping. We also find
2
evidence for the resonant LO-phonon-mediated electron co-tunneling, the effect controlled by the
n applied bias and leading tothe reduction of thenuclear spin pumpingrate.
a
J
5
1 The strong influence of the lattice nuclei on the elec- sameenergy,flopsthespinofasinglenucleusandtunnels
tron spin properties in semiconductor nano-structures out of the dot into a continuum of states in the contact.
] has been identified as a major challenge in the utiliza- However, we also find that a resonant phonon-assisted
h
p tion of desirable electron spin properties for quantum electron co-tunneling can occur, leading to depolariza-
- logic applications [1, 2, 3, 4, 5]. This has sparked re- tion of the electron on the dot and consequent lowering
t
n centintenseresearcheffortstocontrolthenuclearspinin of the nuclear spin pumping rate.
a nano-structures,andparticularlyinIII-Vsemiconductor We present the results for dots grown in the intrin-
u
q quantum dots, where the hyperfine interaction between sic region of an n-type Schottky diode. The device is
[ the magnetic moments of the electron and nuclear spins grown by MBE on an undoped GaAs substrate. It con-
[6] has been shown to limit the electron spin life-time sists of a 50 nm GaAs Si-doped back contact on top of
1
[2, 7, 8, 9, 10] and coherence [2, 3, 4, 11]. whichthefollowingundopedlayersaredeposited: 25nm
v
3 Recently, optical excitation [12, 13, 14, 15, 16, 17, 18, GaAstunnel barrier,InGaAs dots,125nmGaAs,75nm
8 19, 20, 21, 22, 23] and transport [2, 2, 3, 24, 25] of spin- Al0.3Ga0.7As,5nmcapGaAslayer. Thesamplewasthen
2 polarizedelectronsinsemiconductorquantumdots(QD) covered with an opaque metal mask, where sub-micron
2
have been shown to lead to dynamic nuclear polariza- clearapertureswereopenforopticalaccesstoindividual
.
1 tion. The effect is a result of the hyperfine interaction dots. In the micro-photoluminescence (PL) experiment
0
leading to relaxation of electron spin via the ”flip-flop” presentedhere,circularlypolarizedopticalexcitationwas
9
process, in which an electron transfers its spin to a sin- employed to generate e-h pairs in the low energy tail of
0
: gle nucleus in the dot. In optically pumped dots Over- the wetting layer at 1.425eV. Unpolarized PL from the
v
hauser magnetic fields, B , up to a few Tesla have been dots at ≈ 1.32eV was measured by a double spectrome-
i N
X generatedunderexcitationwithcircularlypolarizedlight ter anda CCD.The identificationofPL peaksemployed
r [16, 17, 19, 23]. Strong non-linearities and noise associ- here is carried out following the standard procedure for
a ated with nuclear spin memory effects have been found dots embedded in an n-type Schottky diode [27] and is
in electron transport measurements on GaAs-based dots based on the bias- and power-dependence of their inten-
and quantum point contacts [2, 24, 26]. sities and spectral positions.
Inthispaper,weshowthatlargeOverhauserfields(up Fig.1a shows bias-dependent PL spectra measured for
to 3 T) can be controlled in a single optically active dot high power (0.5 mW) laser excitation and external mag-
grown in the intrinsic region of a semiconductor diode netic field Bz = 1.5T. Observed PL arises from an e-h
byapplyingsmallchangestothediodebias. Heretheef- pair recombination when the following combination of
fectisobservedinInGaAs/GaAsdotsinthenuclearspin charges is confined in the dot: ehh (positively charged
bi-stability regime in external field of 1.5 < B < 3T exciton,X+),eh(neutralexciton,X0),eehh(bi-exciton,
z
[16, 17, 19, 21, 23]. We demonstrate bias-controlled XX), eeh (negatively charged exciton, X−). Note, that
switchingbetweentwodistinctlydifferentandstablecol- in the time averaged spectra measured in PL experi-
lectivenuclearspinstatesinadot. Astrongdependence ments, the relative intensities of the PL peaks represent
of the optically pumped nuclear spin on the dot on the probabilities to find the dot with a certain e-h popula-
probability of the non-radiative escape of the electron tion forming an exciton complex, which can recombine
from the dot to the back contact is demonstrated. In radiatively.
some biasregimesthis processleadsto increasednuclear As seen in Fig.1a at B = 1.5T each of these exci-
z
polarization due to the electron-tunneling-mediated nu- ton states exhibit Zeeman splitting. The electron spin is
clear spin pumping: the photo-excited electron virtually alsosensitivetotheeffectivemagnetic(Overhauser)field
occupies the inverted spin-state while remaining at the B produced by the polarized nuclei, so that the total
N
2
magnetic field seen by the electron is B = B +B . polarization and occurrence of high B ≈B . For pow-
tot z N N z
When circularlypolarizedexcitationis employedto gen- ers (P) above P a weak power-dependence of the nu-
thr
erateelectronswithwell-definedspinonthe dot, nuclear clear polarization is observed. If P is reduced again, the
spin polarizationbuilds up as a result of the electron-to- switching to the low nuclear polarization branch occurs
nuclear spin transfer induced by the hyperfine interac- atP <P ,i.e. ahysteresisisobservedconstitutingob-
thr
tion. B (anti)paralleltoB isobservedfor(σ−)σ+ cir- servationoftheopticallyinducednuclearspinbistability.
N z
cularly polarized excitation. The electron Zeeman split- Note, that in Fig.1b the Overhauser field B abruptly
N
ting can be written as E (σ±)=g µ (B ±B ), where changesbymorethan1Tattheswitchingthresholds. In
e e B z N
g is the electrong-factorand µ is the Bohr magneton. whatfollowswe willconsiderindetailthe dependence of
e B
Note thatthe hyperfineinteractionis negligible forholes the switching behavior in Fig.1b on the bias applied to
due to their p-like wave function. the diode. We will show that the bias tuning produces
The efficiency of the electron-to-nuclei spin transfer, dramatic changes in the nuclear polarization on the dot.
w ∝w |A |2/(∆E2+γ2/4), (1) Fig.2ashowsthe X+ Zeemansplitting EX+ asafunc-
s x hf e tionofappliedbiasmeasuredforafixedpowerof0.4mW
−
depends onthe majorenergycostofthe electron-nuclear of σ polarized excitation at Bz = 2.1T. The bias scan-
spin flip-flopevent[7]- the electronZeemansplitting E ning directions are shown with arrows. The curve de-
e
[14,16,17,19,21,28]. Inthisexpressionw istheoptical picted with circles corresponds to the bias tuned from
x
pumping rate, A is the hyperfine interaction constant forward+0.2Vtoreverse-0.6V.Aslightoscillationofthe
hf
and γ is the electron state broadening [7, 28]. The res- splitting around 240µeV is observed before a threshold-
onant form of the rate ws in Eq.1 assumes a feedback like transition to a notably lower EX+ ≈ 170µeV is de-
between the nuclear spin polarization on the dot, influ- tected at Vthr1 ≈ −0.45V. The transition corresponds
encing∆E ,andw . Inparticular,anabruptbuild-upof to an abrupt build-up of nuclear polarization and oc-
e s
nuclear spin on the dot under optical excitation, the so- currence of BN ≈ 2T. When the bias is scanned in the
callednuclearspin”switch”effect[16,17,19,20,21,23], opposite direction(shown with squares)the high magni-
occurs when Ee ≈ 0 for BN ≈ −Bz leading to a sharp tude of BN is observed up to Vthr2 ≈ −0.1V, where a
increase of ws. threshold-like transition to a low magnitude of BN ≈ 0
An example of this phenomenon is shown in Fig.1b occurs. ApronouncedhysteresisisclearlyseeninFig.2a.
wheretheX+ Zeemansplittingisplottedasafunctionof The observed threshold-like behavior of the Overhauser
thepumpingpowerforσ− polarizedexcitation,B =2T field indicates that large Overhauser fields in the Tesla
z
andbias-0.45V.Asthepowerisincreasedfromzero(cir- rangecanbemanipulatedinasingledotbysmallchanges
cles), a gradual decrease in the X+ Zeeman splitting is ofappliedbias. Thebias-controlledswitchingisobserved
observeddue to the increasing BN and hence decreasing uptothemaximumBz =3T[19],wheretheswitchingof
Ee. AtthepowerPthr =0.3mWathreshold-like”switch- BN ≈ 3T is achieved. The switching thresholds depend
ing” occurs due to a significant build up of the nuclear on the excitation power and magnetic field, since these
two parameters define the nuclear spin pumping rate w
s
as seen from Eq.1: (i) a higher rate is achievedat a high
0.2 240
excitation power due to the increased electron spin flux
)
V
e through the dot and (ii) ws is reduced at higher Bz due
µ
0.0 X- g ( to the increased Ee.
) n200 We have carried out power-dependent measurements
as (V-0.2 X+ spitti athseinnuFcilge.a1rbsfpoirnaswriatncghecoafnbbiaeseasc.hieItveids oubnsdeerrveσd−theaxt-
Bi an Pthr citation in a wide range of voltages -0.6V< Vapp <0V.
-0.4 XX0 em160 The dependence of Pthr on the bias (shown in Fig.2b)
e reveals a notable reduction of P at reverse biases be-
X0 (a) + Z (b) low -0.4V.This bias-dependencethcrorrelateswell with the
-0.6 X
1.315 1.320 0 2 4 strong bias-dependence of EX+ in Fig.2a: the switching
Energy (eV) Power (mW) of EX+ to a lowermagnitude when tuning the bias from
+0.2Vto-0.6Voccursatabiaswhereanotablereduction
ofP ismeasured,i.e. alowerpumpingrateisrequired
FIG.1: (a)Bias-dependenceoftheQDPLrecorded atBz = thr
1.5T under σ− excitation. (b) Power dependence of the X+ toefficientlypolarizethe nuclei. Notethatapronounced
Zeeman splitting measured at Bz = 2T and bias -0.45 V for resonance-like feature with a notable increase of Pthr is
σ− polarized excitation. Pthr denotes the power where the observedaround-0.3Vinthebias-dependenceofPthr (to
nuclearspin”switch”isobservedinthescanwherethepower be discussed below).
is increased. The arrows at theswitching thresholds indicate In order to understand the observed bias-dependent
thedirections of the power scans. behavior, we consider the electron dynamics on the
3
of biases below −0.4V coincides with the regime where
B =2.1T 0T) the most efficient nuclear spin pumping on the dot oc-
250 z ( curs and a marked decrease in the threshold power for
n eV) B N the spin switch is found.
ma (µ 1 2 d The observed bias-sensitivity of the dynamic nuclear
+X Zee splitting 200 Vthr Vthr(a) 1uclear fiel poofuotltahereilzeanctturicoolnneatcruasnnpnienbleipnugu.mndpIenirnsgtthoieondPreLfgroirmmegeismthweeitthmheeacnohpdatniwcisaitmlhlys-
2N pumped carriers escape from the dot via radiative re-
combination. When an e-h pair is excited in an unoc-
0.6
) cupied dot, the nuclear spin pumping occurs via initial
W
m 0.5 formation of a dark exciton (with an electron preserving
P (thr 0.4 (b) ittusalsppirnocaensds ianvdoelvpionlgareizleecdtrhoonl-en)ucalnedaraspcionnflseipq-ufleonpt avnird-
0.3
the dark exciton recombination [12, 19]. In the case of
(c) the dot charging with a hole and X+ formation, a simi-
-
4 X0 X lar spin-flip process is also possible, since the hole states
withthetwoorientationsofthe spinareavailablefore-h
y
nsit 2 X+ XX0 rtheceoemlebcitnraotniosnp.inIinnftohremcaatsioenoifstlhosetbain-edxtchiteonnuecxlecaitrastpioinn
e
nt pumping is suppressed. Finally, in the regime of the dot
L i 0 negative charging the nuclear spin pumping occurs via
d P (d) X+ XX0 the spintransferfromthe residentelectronremainingon
ate 4 the dot after the e-h recombination [14, 15, 17, 18, 23].
gr InSchottkystructuresthishasbeenfoundtoleadtoBN
e anti-paralleltotheOverhauserfieldpumpedwhenaneu-
nt 2
I X0 X- tral and positively charged exciton is generated [14, 17].
We thus can expect a relatively high efficiency of the
0 generation of the Overhauser field with the sign corre-
-0.6 -0.3 0.0 0.3
sponding to the spin switch condition (B anti-parallel
N
Bias (V) B ) in the range of biases where X+ and X0 are strong,
z
but at the same time a weaker spin pumping effect at
voltages where the contribution from XX0 and X− is
FunIGde.r2σ:−(ae)xcVitoalttaiogne.sVcathnrs1a(VttBhrz2)=de2n.1oTtesanthdepboiwaserw0h.e4remtWhe significant. In Fig.2 the X0/X+ regime spreads from
nuclear spin switching occurs when reducing(increasing) the 0V to -0.6V, beyond which PL is almost completely
bias. Directions ofthebias scansare shown with arrows. (b) quenched. On the other hand, as seen from the bias-
The bias-dependence of the switching threshold power Pthr. dependence measured at a high power (Fig.2d), XX0
(c and (d) Bias-dependence of the integrated PL intensity becomes strong at V ≈−0.3V and above 0V and X−
app
measured in both circular polarizations at Bz = 2.1T under appears at V > 0V. This correlates well with the in-
σ− excitation with powers 0.2 (c) and 0.5 (d) mW for X0 app
creasingthresholdpowerforthespinswitchatthesevolt-
(electron and hole pair or eh state), XX0 (eehh), X+ (ehh)
and X− (eeh) peaks. ages as depicted in Fig.2b, and can be explained by the
detrimentaleffectoftheformationofthesee-hcomplexes
on the nuclear spin pumping.
An additional nuclear spin pumping mechanism be-
dot that can be revealed by detailed PL measurements comes possible in the regime where the non-radiativees-
showedin Figs.3a,b. Here the integratedintensity of the cape of the electron from the lowest energy dot states
unpolarizedPL,measuredunderσ− excitationisplotted to the contact becomes possible due to the tunneling at
as a function of applied bias. Fig.2 reveals that both for high applied voltages V ≤ −0.4V. This mechanism
app
powers above and below the switching threshold (Fig.2c has been recently proposed by us in Ref.[32]: the photo-
andd,respectively)anotablequenchingofPLsignaloc- excitedelectronvirtuallyoccupiestheinvertedspin-state
curs at biases below -0.4V. Separately conducted photo- while remaining at the same energy, flops the spin of a
current experiments on this sample, performed with the single nucleus and tunnels out of the dot into a contin-
resonant excitation in the ground state of the dot, also uum of states in the contact. In addition, as seen from
showedthattheonsetofthePCsignal,correspondingto Eq.1, the degree of nuclear spin polarization is depen-
theelectrontunneling fromthe loweststateinthe dotto dent on the rate of re-excitation of the dot with a spin
the contact [29, 30], occurs at ≈ −0.4V [31]. The range polarized electron [19]. In the low voltage regime this is
4
limited by the rate of the e-h radiative recombination, creaseinXX0 andX0 PL anddecreaseinX+ PLoccur
whereas in the regime of high bias, an additional fast in a narrowrange of biases around V ≈−0.3V,which
app
electron escape route appears due to the tunneling. In supportsoursuggestionabouttheresonantnatureofthe
contrasttothecaseoftheresonantexcitationconsidered observed phenomenon, namely the phonon-assisted elec-
in Ref.[32], the re-excitation of the spin-polarized elec- tron co-tunneling.
trons on the dot by the non-resonantoptical pumping is In conclusion, we found an efficient way of controlling
notconditionalonthefastholeescape. Thustheelectron nuclear spin in an individual quantum dot by applying
re-pumping rate and, as a consequence, the nuclear spin smallchangesinthe electric fieldacrossthe device. This
pumping can be considerably enhanced in the regime of canserveasanefficienttechniqueincontrollingthemag-
high voltage compared to the range of V , where the netic environmentof the electronspinon the dotby em-
app
radiative recombination dominates and clear evidence of ploying an externally controlled high-rate switching of
the complete blocking of the dot ground state (observa- the Overhauser field in the range of several Tesla. This
tion of XX0) is found. will be achieved by applying fast bias bursts combined
The same factors influencing the nuclear spin pump- with ms-long optical pulses [18, 22] sufficient to pump
ing on the dot will determine the magnitude of Vthr2, high nuclear spin polarization in an individual dot.
where the switching back to a low nuclear polarization This work has been supported by the EPSRC grants
is observed. In this case, however, a high nuclear spin GR/S76076,EP/G601642/1,the EPSRCIRC for Quan-
pumping rate can be maintained far into the regime of tum Information Processing, and the Royal Society.
radiative recombinationdue to the initial cancellationof AIT acknowledges support from the EPSRC grants
E by the Overhauser field at high bias (see Eq.1). EP/C54563X/1,EP/C545648/1.
e
We now discuss the resonant feature observed at ≈
−0.3Vin the bias-dependence of the switching threshold
power P shown in Fig.2b. Firstly, we note that the
thr
efficient electron charging arising from the tunneling of
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