Resistive detection of nuclear spins in a single quantum dot under Kondo effect regime

0.75 MB·English

by  Minoru Kawamura

Resistive detection of nuclear spins in a single quantum dot
under Kondo effect regime
M. Kawamura,1,2 D. Gottwald,1 K. Ono,1 T. Machida,3,4 and K. Kono1
1RIKEN Advanced Science Institute, Saitama 351-0198, Japan∗
2PRESTO, Japan Science and Technology Agency, Saitama 333-0012,Japan
3Institute of Industrial Science, University of Tokyo, Tokyo 153-8505, Japan
4Institute for Nano Quantum Information Electronics, University of Tokyo, Tokyo 153-8505, Japan
(Dated: February 1, 2013)
3
1 Westudydynamicpolarization and resistivedetection of nuclearspinsin asemiconductor quan-
0 tum dot (QD) under the Kondo effect regime. We find that the differential conductance spectra
2 of the QD exhibit hysteresis under the Kondo effect regime in magnetic fields. Relevance of nu-
n clear spins to the hysteresis is confirmed by the detection of nuclear magnetic resonance signals by
a monitoring the differential conductance. We attribute the origin of the hysteresis to the dynamic
J nuclear spin polarization (DNP) induced in the QD. Using the DNP, we demonstrate nuclear spin
1 relaxation rate measurements in theQD undertheKondo effect regime.
3
PACSnumbers: 73.63.Kv,72.10.Fk,76.60.-k

]
l
l
a TheKondoeffect,arisingfromtheinteractionbetween
h alocalizedspinanditinerantspinsattheFermienergy,is (a) (b)ce
s- oneofthe mostfundamentalmany-bodyphenomenaob- U ctan
e served in semiconductor quantum dot (QD) systems1–4. du
t.m QThDeedxihffiebrietsntaiaslhacorpndzuercota-bnicaesc(donI/dduVctsda)ncseppecetarkum(ZBofCtPh)e, ntial con B = 0
ma wInhtichherperfleescetnsctehoefKanonedxoterrensaolnamnacgenaettitchfieeFlderBm,i tehneeregny-. BN Differe B = 0
- ergyforthe KondoresonanceshiftsawayfromtheFermi
d ∗ 0
energy by ±|g |µ B due to the Zeeman splitting of the
B Bias voltage
n electronic state in the QD1, where g∗ is the effective g-
o FIG. 1: (color online). (a) Schematic of a QD with nuclear
factor for electrons and µ is the Bohr magneton. Con-
c B spins. An electron spin in the QD is influenced by an effec-
[ sequently, the dI/dVsd spectrum splits into two peaks at tive magnetic field BN produced by the nuclear spins. (b)
1 finite bias voltages. The peak-to-peak voltage Vp−p, de- Schematic of differential conductance spectra in a QD under
fined as the difference in the bias voltage between the the Kondo effect regime. Application of an external mag-
v
0 splitdifferentialconductancepeaks,isgivenbytwicethe netic field B causes the splitting of the dI/dVsd spectrum.
3 Zeeman energy1,3 Vp−p =2|g∗|µBB/e. Thepeak-to-peak voltage between thesplit dI/dVsd peaksis
7 furthermodified by BN.
7 Because an electron spin S~ in a semiconductor device
1. can interact with a nuclear spin I~ of the host mate-
0 rial through the contact hyperfine interaction Hhyp =
3 AI~·S~ =A(I+S−+I−S+)/2+AI S ,whereAisthehy- properties in a QD through nuclear magnetic resonance
z z
1 (NMR) measurements.
perfine coupling constant, a dynamic nuclear spin polar-
v: ization (DNP) can be build by driving electronspins us- In this paper, we report dynamic polarization and re-
i ing optical or electrical means5,6. The polarized nuclear sistive detection of nuclear spins in a single QD under
X
spinsmodifytheZeemanenergyforelectronsthroughan the Kondoeffect regime. We findthat the dI/dVsd spec-
ar effective magnetic field BN =AhIzi/g∗µB [Fig. 1(a)]. In tra exhibit hysteresis under the Kondo effect regime in
a QD under the Kondo effect regime, the modification magnetic fields. We also find that the value of dI/dVsd
of the Zeeman energy is expected to change the peak- evolves slowly at the bias voltages between the split
to-peakvoltageVp−p betweenthe splitdI/dVsd peaks to dI/dVsd peaks. Relevance of nuclear spins to the hys-
Vp−p = 2|g∗|µB(B +BN)/e, as shown schematically in teresisandthe slowevolutionofdI/dVsd is confirmedby
Fig. 1(b). Therefore a small change in the nuclear spin thedetectionofNMRsignalsbymonitoringdI/dVsd. We
polarizationcanbesensitivelydetectedbymeasuringthe attribute the origin of the hysteresis and the slow evolu-
dI/dVsd spectrum. However, to the best of our knowl- tion of dI/dVsd to the DNP induced in the QD. Using
edge, effects of nuclear spins on transport properties of the DNP, we demonstrate nuclear spin relaxation rate
a QD under the Kondo effect regime have not been ad- measurementsinthe QDunder the Kondoeffect regime.
dressed experimentally. In addition, establishment of a WepreparedQDsusingawaferofGaAs/Al Ga As
0.3 0.7
noveltechniquefortheresistivedetectionofnuclearspins single heterostructure with a two-dimensional electron
has a potential to open a way for studying electron spin gas (2DEG) at the interface. The carrier density and
2
mobility of the 2DEG are n = 2.3 ×1015 m−2 and µ =
1gidn7aevtmReice2def/sesV.vnis7ce,earsarens[tidphneesc8etp;tiivonQefclDhyF.-siogTfff.ohrce2mo(Qne)dDo]i,ctscifowoanlselisoroewninaimnltlgayhdteeihnpuerssaeipnsplegpintrs-cogpeaalictothe-f 2 dI/dV (2e/h)10sd ..05 (a) 2 6800 mmKK 635700 mmKK 2 0dI/dV (2e/h)sd0..43 (b)
disorder9, and some of them exhibit the Kondo effect7,8. 1500 mK
0.0 0.2
Thedeviceswerecooledusingadilutionrefrigeratorand -0.70 -0.65 -0.60 -200 0 200
magnetic fields were applied parallel to the plane of the Vg (V)
2vmoDelEatasGgue.reAodfIs1t/0adnVµdsVdaradapnpldolycakin-figrneaqtuebceiahnsncyivqouoltefa1wg8eitVHhszdanwbeaetsxwcueiseteandttiohtnoe V (V)g---000...666024 (c) B = 0.00 T (d) B = 2.88 T d(2I/ed02.V/6hs)d
source and drain electrodes. 0.4
-0.66
0.2
Figure 2(a) shows the gate voltage V dependence of
g -0.68 0
the conductance of one of the devices. The conductance -200 0 200 -200 0 200
decreases with decreasing V , and several conductance
g
pofeaakQsDapapteathrebceofonrsetrtihcteiopni.ncThh-oefft,rainndspicoarttinpgrofpoermrtiaetsioonf 2 e/h (e) scan directions
2
thedeviceexhibitbehaviortypicaloftheKondoeffectin x
a QD2,3. (1) The conductance shows an increase with 1
0.
decreasing temperature T at a conductance minimum 0.00 T
in the dI/dV –V curve and the opposite temperature d
sd g Vs 1.15 T
dependence at the neighboring conductance minimum d
[Fig. 2(a)]. (2) A pronounced ZBCP in the dI/dVsd– dI/ 1.73 T
V curve develops with decreasing temperature below
sd
2.88 T
700 mK [Fig. 2(b)]. The ZBCPs appear as a bright col-
oredridgeinthe gray-scaleplotofdI/dV asa function
sd 3.46 T
of V and V [Fig. 2(c)]. (3) The dI/dV spectrum
g sd sd
splits into two peaks under in-plane magnetic fields B
[Fig. 2(e)], accompanied by the splitting of the dI/dV
sd
ridge[Fig.2(d)]. Thepeak-to-peakvoltageVp−pbetween -200 -100 0 100 200
the split dI/dV peaks is almost independent of V as
sd g
expected from the theory of the Kondo effect in a QD1.
The value of Vp−p increases with increasing B, show- VFIG=. 2:0 µ(Vcoluonrdoenrlitneem).pe(raa)tDureepseTnd=enc6e0,of28d0I,/3d7V0sd, 6o5n0,Vganadt
sd
ing an agreement with theoretically expected values of
∗ ∗ 1500mK.(b)DependenceofdI/dVsd onVsd atVg =−0.66V
Vp−p =2|g |µBB/ewith|g |=0.44forGaAs,asmarked under various temperatures. The temperatures are the same
by triangles in Fig. 2(e). asin(a). (c)and(d)Gray-scaleplotofdI/dV asafunction
sd
We find that remarkable hysteresis appears in the ofVg andVsd forB=0.00T(c)and2.88T(d). Dashedlines
dI/dVsd spectra under magnetic fields; the dI/dVsd–Vsd in(d)indi∗catethetheoretically expectedposition ofdI/dVsd
curve of the positive scan (dV /dt > 0) appears above peaks±|g |µBB/e. (e)DependenceofdI/dVsdonVsdatVg=
sd
−0.66VandT =30mKundermagneticfieldsB=0.00,1.15,
that of the negative scan (dV /dt<0) as shown in Fig.
sd 1.73, 2.88, and 3.46 T.Thecurvesareoffset by0.06×2e2/h.
2(e). The dI/dV –V curves shown in Fig. 2(e) are ob-
sd sd The expected voltage positions of dI/dV peaks are marked
tainedbyscanningV between−500µVand+500µVin sd
sd by triangles. The solid and dashed curves represent the data
positiveandnegativedirectionsatarateof5µV/s. The
for the positive and negative scans, respectively. Inset shows
hysteresis appears in a particular range of Vsd between scanning electron micrograph of the split-gate device.
the dI/dV peaksateachmagneticfield. Thehysteresis
sd
inthedI/dV –V curvesindicatesinvolvementofaphe-
sd sd
nomenon with a long relaxation time in the mechanism
of electron transport.
the slowevolutionofdI/dV is confirmedby the follow-
sd
Figure 3(a) shows representative data describing the ingNMR spectroscopymeasurement. We applycontinu-
temporal change in dI/dVsd at B = 2.88 T after a rapid ous waves of radio-frequency (rf) magnetic fields using a
changeofVg andVsd fromthepinch-offconditionto(Vg, single-turncoilwoundaroundthe device. The frequency
Vsd) = (−0.65 V, −40 µV) on the slope of the dI/dVsd f of the rf-magnetic field is scanned after the saturation
peak. The value of dI/dVsd increases continuously over of dI/dVsd at (Vg, Vsd) = (−0.65 V, −40 µV). As shown
a period of 150 s. The long time required to change in Fig. 3(b), the value of dI/dV decreases when the
sd
dI/dVsd in Fig. 3(a) is in accordance with the typical frequency matches the NMR frequencies of 75As (gyro-
time scale for building up a DNP in GaAs10–17, suggest- magnetic ratio γ = 45.82 rad·MHz/T). The NMR spec-
ing the occurrence of the DNP in the present QD. trum of 75As is obtained18 by monitoring dI/dV . The
sd
Relevance of the nuclear spins to the hysteresis and NMR spectra of 69Ga and 71Ga are also obtained (not
3
2 dI/dV (2e/h)0sd .39 (a) D 2 0.02 x 2e/h (a) 10312.....1047850638 TTTTT 10S (arb. unit)..05 (b) 000...432sddI/dV (2e/h) 2
0.0
0.38 -200 0 200 0 1 2 3
0 50 100 150
t (s) B (T)
V sd h) (b) unit)1.5 (c) 1.0dI/dV
dI/d -42 x 10 (2e/ S (arb. 10..05 0.5sd (2e/h 2
5 75 )
As 0.0 0.0
20.8 20.9 21.0 21.1 -0.70 -0.68 -0.66 -0.64 -0.62 -0.60
f (MHz) Vg (V)
F(VIgG,.V3sd:)=(co(−lo0r.6o5nlVin,e−).4(0aµ)VT)emanpdorBal=ch2a.n8g8eTofafdteIr/daVrsadpaidt Vsd (d) 1unit).5 (e) 0.37dI/dV
=mc=hoa(2nn−.i8gt08eo.r7Tii0nnguVVndg,dI0ae/rndµdViVrsr)Vda.sdad(tibaf)(trVioNomgnM,VotRshfder)sfpp-=meinc(atc−rghun0-moe.6fft5ioccfVofi7n,e5d−lAdi4t.si0ooBnµblaVt(acV)iknga,ecnduVdrsbvdBye) dI/d 2 0.1 x 2e/h 10S (arb. ..05 0.35sd (2e/h) 2
istheLorentzianfittingresult. Thesplittingofthespectrum
0.0
is dueto theelectric quadrupoleinteraction of thenuclei. -200 0 200 100 200 300
T (mK)
FIG.4: (coloronline). (a)Hysteresis∆definedasthediffer-
enceindI/dV betweenthepositiveandnegativescansofV
sd sd
shown). The observed decreases in dI/dVsd correspond inFig.2(e). The∆–Vsd curvesforVg =−0.66VatB=0.00,
to the decreases in the nuclear spin polarization due to 1.15, 1.73, 2.88, and 3.46 T are shown. The curves are offset
the absorptionofrf-magneticfieldsatthe NMRfrequen- by 0.02×2e2/h. (b) Magnetic field dependences of the area
cies. Thus, the changes in the nuclear spin polarization S ofthehysteresis(opencircles,leftaxis)anddI/dVsd(Vsd =
are detected by measuring dI/dVsd under the Kondo ef- 0 µV) (dashed curve, right axis) at Vg = −0.66 V and T =
fect regime as expected [Fig. 1(b)]. 30 mK. (c) Gate voltage dependences of S (open circles, left
axis) and dI/dV (V = 0 µV) (dashed curve, right axis) at
Because dI/dV reflects changes in the nuclear spin sd sd
sd
B =2.88 TandT =30mK.(d)ThedI/dV –V curvesfor
polarization, the slow increase of dI/dV in Fig. 3(a) sd sd
is interpreted as the development of thsed DNP in the Vg =−0.65VandB =2.88 TundervarioustemperaturesT
= 60, 120, 170, 190, 280, and 360 mK from bottom to top.
present QD. The possibilities of the electron heating or
The solid and dashed curves represent the data for the posi-
the drift of the gate voltages are ruled out from the ori- tiveandnegativescans,respectively. Thecurvesareoffsetby
gin of the hysteresis and the slow evolution of dI/dVsd 0.04×2e2/h. (e)TemperaturedependencesofS(opencircles,
because of the absence of the hysteresis at B = 0.00 T. leftaxis)anddI/dV (V =0µV)(dashedcurve,rightaxis)
sd sd
The induced DNP modifies the dI/dVsd spectra through at Vg = −0.65 V and B = 2.88 T.
the Zeeman energy, changing the peak-to-peak voltage
∗
to Vp−p =2|g |µB(B+BN)/e. Because the DNP devel-
ops during the scans of V in the dI/dV -V measure-
sd sd sd
ments inFig.2(e),the resultantdifference inthe nuclear the hysteresis∆,definedasthedifference indI/dV be-
sd
spin polarization at the same V causes the hysteresis tween the positive and negative scans of V shown in
sd sd
in the dI/dV -V curves. We think that coexistence of Fig. 2(e). To discuss the conditions for the hysteresis to
sd sd
the DNP and the Kondo effect is allowed because the occur, we introduce the area S of the hysteresis by inte-
electron-nuclear flip-flop process of the DNP does not grating the hysteresis ∆ over a range −200 µV ≤ V ≤
sd
interfere with the formation of the Kondo singlet sig- 200 µV. The area S, which is zero at B = 0.00 T, in-
nificantly due to the small characteristic energy for the creaseswithincreasingBandbecomessaturatedaboveB
flip-flop process (∼ 1 neV)19,20 comparedto that for the = 2 T [Fig. 4(b)]. The dependence of S on V [Fig. 4(c)]
g
Kondo effect (k T ∼ 80 µeV). reveals that the hysteresis is remarkable in a range of
B K
The above described mechanism of the hysteresis is V where the Kondo effect appears (−0.67 V < V <
g g
based on the sensitivity of the dI/dV spectra to the −0.64 V). The values of S are almost zero at the other
sd
changes in the Zeeman energy. Therefore the conditions gate voltages. With increasing temperature T under a
for the hysteresis to occur is expected to be related to fixed magnetic field, the hysteresis becomes small and
the appearance of the Kondo effect. Figure 4(a) shows the value of S decreases as shown in Figs. 4(d) and 4(e).
4
is smaller than that without the DNP pumping. With
2 e/h)0.25 (a) 195 (b) idneccrreeaassiensggtrhaedupaelrliyo,daosfsthhoewDnNinPFpiugm. p5i(nbg).tpTumhpe,gVrpa−dp-
2
dI/dV (0sd .20 NDoN PD NatP 119805 udshaitilifotcnh∆aalnVgepve−ipdinemnVcepea−sfpuorrwedtithhefroionmcccruetrahrseeinngvceatlupouefmtophfegVivDpe−NspPa.nwTiathhde--
180 out the DNP pumping allows us to evaluate the ef-
0.15
fective magnetic field B using a proportional relation
N
-100 0 100 0 200 400 600 ∆Vp−p/Vp−p = BN/(B + BN), which is yielded based
tpump (s) on an assumption Vp−p = 2|g∗|µB(B +BN)/e. A rep-
(c) 0.4 resentative value of ∆Vp−p = −17.9 µV obtained from
10 the difference between the dashed and solid curves in
0 0 Fig. 5(a) corresponds to B = −0.26 T. This value is
N
-10 Vg, pump = -0.650 V d equivalenttothenuclearspinpolarizationof4.9%using
10 (d) 0.4sdI/dV theFiegxuprreess5si(ocn)-5fo(re)BsNhoiwntGhaeAdespoebntdaeinnecdeoinf∆RVepf.−p21o.nthe
0 0 (2 DNP pumping voltages (Vg,pump, Vsd,pump). The values
-10 Vg, pump = -0.655 V e 2 of∆Vp−p,whichareproportionalto BN,arealmostzero
/h) at around Vsd,pump = 0, while large values of |∆Vp−p|
(e) 0.4 are obtained at |V | > 100 µV. In addition, the sign
10 sd
of ∆Vp−p, which indicates the DNP polarity, strongly
0 0
depends on the DNP pumping voltage. The negative
-10 Vg, pump = -0.665 V ∆Vp−p (BN < 0) is remarkable at Vg,pump = −0.650 V
-400 -200 0 200 400 with Vsd,pump < 0 [Fig. 5(c)], while the positive ∆Vp−p
(B > 0) is remarkable at V = −0.665 V with
N g,pump
V > 0 [Fig. 5(e)]. At the gate voltage between
sd,pump
FIG. 5: (color online). (a) dI/dVsd-Vsd curves at Vg = them (Vg,pump = −0.655 V), both polarities appear de-
−0.650 V obtained without the DNP pumping (dashed) and pending on the sign of V [Fig. 5(d)].
sd,pump
after the DNP pumping (solid) for tpump = 600 s. The The optimum values of |V | for the DNP pump-
DNP pumping voltages (Vg,pump, Vsd,pump) = (−0.650 V, ingarelargerthanthe|V |vsadl,puuemspforthedI/dV peaks.
−320 µV). (b) Dependence of Vp−p on the DNP pumping sd sd
Moreover,the V rangewherethe DNPisachieved
period tpump. The DNP pumping voltages are the same as sd,pump
in (a). The DNP is relaxed between each measurement by doesnotcoincidewiththeVsd rangewherethehysteresis
waiting more than 20 minutes under the pinch-off condition. is observed. Both facts suggest that the mechanism for
(c)-(e) Dependence of ∆Vp−p on the DNP pumping voltages the DNP pumping is not directly related to that of the
(Vg,pump, Vsd,pump) for tpump = 600 s. The Vsd dependences Kondo effect. In addition, the dependence of the DNP
of dI/dVsd (solid curve)are plotted on theright axis. polarityonthesignofVsd,pump indicatesthatthepresent
QD is asymmetric and that the asymmetry plays an im-
portant role in the DNP pumping mechanism. We think
that the positive B could be attributed to the electron
N
ThehysteresisalmostdisappearsataroundT =360mK, spin relaxation in the QD. When adding an electron to
which is considerably lower than the onset temperature the empty QD, both up-spin and down-spinstate are al-
of the Kondo effect (about 700 mK). The temperature lowed. If the added electron is in the down-spin state,
where the hysteresis disappears is close to the temper- it can relax to the up-spin state in the QD before going
ature where the splitting of the dI/dVsd spectrum be- to the electrode. The down-to-up flips of electron spins
comes unclear [Fig. 4(d)]. These observations show that cause to decrease the nuclear spin polarization hI i via
z
the hysteresis is one of the unique features of the Kondo the electron-nuclear flip-flop process of the hyperfine in-
effectinthemagneticfieldsandthatthedifferentialcon- teraction,buildingupthepositiveB inFig. 5(e)22. Be-
N
ductance under the Kondo effect regime is sensitive to cause of the asymmetric couplings between the QD and
the changes in the effective magnetic field B+BN. theelectrodes,theaddedelectronprobablygoesthrough
The effect of the DNP on the dI/dVsd spectra can be the QD before relaxing its spin when Vsd,pump <0. The
seenmoredirectlyinthefollowingpump-probemeasure- negativeBN inFig. 5(c)indicatesinvolvementoftheup-
ment. The dashedandsolidcurvesin Fig.5(a)show the to-downflips of electronspins in the QD. At the present
dI/dV –V curves at the same V . The dashed curve stage, we do not have a comprehensive explanation why
sd sd g
is obtained immediately after relaxing the nuclear spin the up-to-down flips become dominant as the gate volt-
polarization under the pinch-off condition and the solid age is increased. Further studies will be needed to fully
curve is obtained after the DNP pumping at (V , understand the DNP pumping mechanism.
g,pump
V ) = (−0.650 V, −320 µV) for t = 600 s. Impact of the newly developed techniques of the DNP
sd,pump pump
The peak-to-peak voltage Vp−p after the DNP pumping andthe resistivedetectionofnuclearspinsisfurtherem-
5
read out by monitoring dI/dV at (−0.650 V, 0 µV).
-1 s)10 (a)dI/dVsd 0.02 x 2e2/h 20 (c) Bweyorebpteaaintinthgethdiescpayroccuedrvuereowf ittshhdedniuffcelreeanrtsppeirniopdoslatrreilzaax-,
-3 0 0 5001000 15 tion as shown in the inset of Fig. 6(a). The value of
T(11 8 trelax (s) 1/T1 is evaluated by fitting the decay curve to the form
1/ 10 dI/dVsd =a+bexp(−trelax/T1).
6
Figure 6(a) shows the dependence of 1/T on V
1 g,temp
h) 4 5 at B = 2.88 T for Vsd,temp = 0 µV. The relaxation rate
2 2e/ (b) 0.25 (d) 1/T1 = 4.4 × 10−3 s−1 for Vg,temp = −0.70 V (the QD
(V0sd .5 0.20 is depletedduring the relaxation)is inthe same orderas
dI/d 0.15 the reported1/T1 values in GaAs13. The relaxationrate
0 increases as V approaches the conductance peaks
-0.68 -0.64 -200 -100 0 100 200 g,temp
Vg, temp (V) [Fig. 6(b)]. We think that electron spin fluctuations
introduced by the electron tunneling cause the increase
FIG.6: (coloronline). (a)Dependenceof1/T1onVg,tempfor in 1/T1. The small values of 1/T1 at around Vg,temp =
Vsd,temp =0µVandB=2.88T.Insetshowsarepresentative −0.66Vcanbeattributedtothesuppressionofthe elec-
decay curve of the nuclear spin polarization at the pinch-off
tron spin fluctuation due to the Zeeman effect. When
condition(Vg,temp,Vsd,temp)=(−0.70V,0µV).(b)dI/dVsd- a bias voltage V is applied during the relaxation
Vg curve at Vsd = 0 µV (c) Dependence of 1/T1 on Vsd,temp sd,temp
[Fig. 6(c)], the values of 1/T do not increase largely at
forVg,temp =−0.66VandB =2.88T.(d)dI/dVsd-Vsd curve 1
|V |<40µV.1/T becomeslargeat|V |>40
at Vg = −0.66 V. sd,temp 1 sd,temp
µV associated with the increase in dI/dV [Fig. 6(d)].
sd
The increase in 1/T is probably related to the electron
1
transport through the Kondo resonance state. We think
phasized by the following nuclear spin relaxation rate that the 1/T measurement using the DNP technique is
1
1/T1 measurements. Because 1/T1 is enhanced by the useful to understand spin properties of the Kondo effect
electron spin fluctuation, the electron spin dynamics in under the non-equilibrium regime.
the QD can be studied through the 1/T measurements.
1
The decay curve of nuclear spin polarization is obtained This work was supported by the PRESTO of JST,
by the following procedure: First, the DNP is pumped Grant-in-Aid for Scientific Research from the MEXT,
at (V , V ) = (−0.650 V, −320 µV). Then, the nu- andtheProjectforDevelopingInnovationSystemsofthe
g sd
clear spin polarization is relaxed under a certain volt- MEXT. We thank M. Eto, T. Kubo, S. Amaha, and E.
age condition (V , V ) for a given time t . Minamitani for helpful discussions. D. G. acknowledges
g,temp sd,temp relax
Finally, the change in the nuclear spin polarization is the RIKEN-Konstantz Univ. internship program.
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