PHYSICAL REVIEW B 105, 165201 (2022)
Editors’ Suggestion

Negatively charged boron vacancy center in diamond
T. Umeda ,1,* K. Watanabe ,2 H. Hara,3 H. Sumiya ,4 S. Onoda ,5 A. Uedono,1 I. Chuprina,6 P. Siyushev,6 F. Jelezko,6
J. Wrachtrup,7 and J. Isoya 1
1Faculty of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8573, Japan
2
National Institute for Material Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan
3
Bruker Japan K. K., 3-9 Moriya, Yokohama, Kanagawa 221-0022, Japan
4
Sumitomo Electric Industries, Ltd., Itami, Hyogo 664-0016, Japan
5
Takasaki Advanced Radiation Research Institute, National Institutes for Quantum Science and Technology,
1233 Watanuki, Takasaki, Gunma 370-1292, Japan
6
Institute for Quantum Optics and IQST, Ulm University, Albert-Einstein-Allee 11, D-89081 Ulm, Germany
7
3rd Institute of Physics, Research Center SCoPE and IQST, University of Stuttgart, D-70569 Stuttgart, Germany(Received 26 October 2021; accepted 25 February 2022; published 4 April 2022)
Impurity-vacancy complexes in diamond are an attractive family of spin defects since NV – , SiV – , GeV – ,and SnV – have emerged as promising platforms for quantum applications. Although boron is most easily
incorporated into diamond, a boron-vacancy complex in the negative charge state (BV – ) has eluded experimental observation. This center was theoretically predicted as another promising spin qubit. In this work, we
experimentally observed an electron paramagnetic resonance (EPR) spectrum identified as BV – in synthetic
diamonds via a Fermi-level tuning. Fingerprints of BV – such as the spin multiplicity of S = 1, C3v symmetry,
and the zero-field splitting (D = 2913 MHz), in addition to 10 B and 11 B hyperfine (HF) interactions, havebeen confirmed. Moreover, optically pumped spin polarization has been observed with 3.0–3.6 eV excitation.
However, unlike the NV – center, the photoluminescence as well as optically detected magnetic resonance from
BV – have not been confirmed even at low temperatures. We speculate that the Jahn-Teller instability in the triplet
excited states of the NV – and BV – centers results in different optical properties.
DOI: 10.1103/PhysRevB.105.165201

I. INTRODUCTION

With its excellent properties as a solid-state spin qubit[1], the nitrogen-vacancy center (NV – ) in diamond opens up
new avenues of applications to quantum information processing [2–7], quantum communication [8–12], and quantum
sensing of magnetic fields [13–15], electric fields [16,17],and temperature [18], including ultrahigh-sensitivity nuclearmagnetic-resonance (NMR) spectroscopy [19,20].
In addition to the NV – center, various other defects
have been explored in diamond, such as silicon-vacancy(SiV – ) [21–25], germanium-vacancy (GeV – ) [26,27], and tinvacancy (SnV – ) centers [28–30]. These group-IV impurityvacancy centers having D3d symmetry with a split-vacancy
configuration and a spin-1/2 ground state exhibited opticalproperties superior to the NV – center, enabling us to expand
the limit of diamond-based spin qubits [12,31].
On the other hand, a new family of NV – -like defects possessing C3v symmetry with a pair of substitutional impurity
and adjacent vacancy and triplet (S = 1) ground states has
been searched for by replacing the nitrogen atom with a firstrow element such as oxygen [32–36] and boron [32,37,38].
Figure 1(a) to 1(c) show schematic views of the one-electronstate levels for the family of NV – , OV 0 , and BV – , respectively*

Corresponding author: umeda.takahide.fm@u.tsukuba.ac.jp2469-9950/2022/105(16)/165201(13)(see Appendix A for details). Although the number of valence
electrons and the number of states in the gap are different,
these three centers have a common triplet ground state (3 A2many-electron state) originating from two unpaired electrons
in doubly degenerate e states (denoted by “ex ” and “ey ”) [1]which are localized on the dangling bonds of carbon atoms
surrounding the vacancy. In fact, an electron paramagnetic
resonance (EPR) center with S = 1 and C3v symmetry, labeled“WAR5,” arises from OV 0 [33,34], exhibiting a zero-field
splitting (D) and 13 C hyperfine (HF) couplings quite similar to those of the NV – center. First-principles calculations[35] also supported the consistency of the zero-field splittings
between OV 0 (2888 MHz) [33] and NV – (2872 MHz) [39].
On the other hand, the triplet excited state (3 E many-electronstate) originates from different configurations, i.e., (a1 )1 e3
for NV – , e1 (3a1 )1 for OV 0 , and e1 (a1 )1 for BV – . As a result, the OV 0 and NV – centers have quite different opticalresponses [33–35]. For NV – , an optical excitation above 1.945
eV (conventionally 532-nm or 2.33-eV excitation is used)easily enhances its EPR signal due to spin polarization. This
optical-pumping ability enables the spin qubit initializationof the NV – center. Unfortunately, no spin polarization wasobserved for OV 0 [33,34]. First-principles calculations predicted a fast nonradiative decay caused by a distortion of theexcited state to a configuration of two covalent C-O bonds
and one elongated C-O bond, making OV 0 nonluminescentor weakly luminescent [35]. Accordingly, the OV 0 center has

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PHYSICAL REVIEW B 105, 165201 (2022)

center. Such different formation behaviors will provide a hint
as to the selective formation of the BV – and NV – centers.
Using above-3.0-eV optical pumping, optical absorption with a spin-conserving transition can be activated for
BV – . Subsequently, we have performed confocal microscope(CFM) experiments to identify PL lines of BV – , which isessential for optically addressing individual spin qubits. This
trial, however, was not successful due to too weak luminescence from the BV – centers. Our experimental results suggest
that, even though the BV – and NV – centers have the analog
ground states, radiative and nonradiative decays from their
triplet excited states are quite different.
II. EXPERIMENT

FIG. 1. Electronic configurations of three analog color centers indiamond. (a) NV – center with six valence electrons. (b) OV 0 centerwith six valence electrons. (c) BV – center with four valence electrons. Within a defect-molecular-orbital model (see Appendix A),
one-electron states a1 (black color) and a1  (color of each impurity)
dominantly originate from atomic orbitals of carbon and impurity
atoms, respectively [1]. Doubly degenerate e states also comprise two
carbon-related orbitals (denoted by ex and ey ) adjacent to a vacancy[1]. In the OV 0 center, a1  and a1 are rewritten by 1a1 and 2a1 ,respectively, and another a1  state (denoted by 3a1 ) is generated froman antibonding state of C-O bonds with three back-bonded carbon
atoms [35]. Their energy positions are approximately shown in aband diagram [1,32,35–37], where EC and EV are the conductionband top and the valence-band bottom, respectively.

presumably been excluded from the candidates of NV -akinsolid-state spin qubits [35].
As shown in Fig. 1(c), the BV – center with four valence
electrons also realizes a spin-1 system with the ex and ey levels[32,37,38], which is exactly identical to those of NV – andOV 0 . First-principles calculations predicted that the BV – center is optically active with a triplet ground state and a triplet
excited state in the band gap, suggesting it is a promising spin
qubit [37,38]. Since both boron and nitrogen atoms are themost major impurities in diamond, the BV – center might havepotential comparable to that of the promising NV – center.
Despite B-doped diamonds being as widely used as N-doped
diamonds, there are no reports of the BV – center being foundin diamond.
Here, we experimentally demonstrate that the BV – center issuccessfully formed in electron-irradiated and annealed highpressure–high-temperature (HPHT) grown diamond crystals
with N and B codoping, and that it even exhibits a desired spin-1 system (D = 2913 MHz and C3v symmetry)
resembling that of NV – . Moreover, the BV – center shows
an optically induced spin polarization with 3.0-eV opticalpumping. Using a combination of EPR imaging, photoluminescence (PL), and cathodoluminescence (CL) microscopy,
we suggest a preferential formation of the BV – center in a
specific area of diamond, distributing separately from the NV –We prepared two N and B codoped HPHT diamonds. Two(100)-oriented plates weighing 76.7 mg (plate-I) and 78.9
mg (plate-II) were prepared by laser-cutting of two HPHT
type-IIa diamond crystals. Both crystals were grown by thetemperature gradient method at high pressure (5.5 GPa) andhigh temperature (1350 °C) using a metal solvent of Fe-Co
alloy with Ti added as a nitrogen getter. The boron impurities were incorporated into the crystals via the solid carbon
source [40]. The two crystals were irradiated with 3 MeV
electrons at room temperature to total fluences of 2 × 1017
and 4 × 1017 e/cm2 , respectively, and they were annealed at
850 °C for 2 h in vacuum, converting small fractions of the
impurities into the NV – and BV – centers.
The two diamond samples were characterized by EPR
spectroscopy. Continuous-wave (cw) EPR spectra were measured by a Bruker E500 X-band spectrometer with a Bruker
ER4122SHQ cavity (the loaded Q-factor was QL ∼ 10 000)
and with an Oxford Instruments ESR900 He cryostat. In thecase of paramagnetic centers studied in this paper [i.e., theP1 center or Ns 0 (S = 1/2), the NV – center (S = 1), and the
BV – center (S = 1) with low concentrations], the standard
slow-passage measurements (saturation-free measurements)
were not suitable because of a severe drawback in the signal
sensitivity. Therefore, we used rapid-passage (RP) measurements [41], which allow a stronger microwave excitation,achieving a much higher signal sensitivity. This techniqueutilized the passage effect, which is induced by a rapid scan of
the magnetic field faster than the electron spin relaxation rates[41].
The RP technique has been developed by the group at
the University of Warwick for quantifying low concentrationsof Ns 0 in diamond. It combines a rapid scan of the external
field (∼10 mT/s) with an out-of-phase detection with respect
to 100-kHz field modulation [33,42]. By using this technique, absorption-form EPR spectra are obtained as shownin Fig. 2(a). The RP measurements often give us a muchhigher sensitivity: for instance, the detection limit for Ns 0
was extended to 0.02 ppb for a sample of ∼10 mm3 volume
after appropriate signal averaging [42]. Thus, we used thistechnique for quantifying spin concentrations or observing
weak 10 B HF structures.
Moreover, we used another technique, denoted the “out-ofphase method,” which records an out-of-phase first-harmonic
signal with respect to 100-kHz field modulation under a slow
scan of the external field [43,44]. This technique generates

165201-2

(a)RP-EPR signal (a.u.)

NEGATIVELY CHARGED BORON VACANCY CENTER IN …
P1

9.428 GHz
295K

NV¯

NV¯

280

(b)

300

54.75
Site-1

320

340

NV¯

360

NV¯
BV¯

(c)

EPR signal (a.u.)

BV¯

232

233

35.25
Site-2

234

438

439

279

440

441

NV¯
NV¯
BV¯

BV¯

278

380

280

281

383

384

385

386

Magne c eld (mT)FIG. 2. EPR spectra of N and B codoped diamond. (a) Rapidpassage (RP) EPR spectrum shows EPR signals of NV – (S = 1),
in addition to strong primary signals of P1 (Ns 0 , S = 1/2). This
spectrum was recorded for a magnetic-field angle θ = 90◦ . (b), (c)Magnified EPR spectra show low- and high-field side EPR signalsof NV – as well as those of BV – with a fourfold splitting. These
spectra were recorded at two principal-axis directions. At 54.75°
with B // [111], a site-1 center [cf., Fig. 3(a)] of BV – or NV – shows
its symmetry axis parallel to B. On the other hand, at 35.25°, B
is perpendicular to a symmetry axis of site-2-type BV – and NV –centers [cf., Fig. 3(a)]. Out-of-phase EPR spectra (slow passage)
were recorded by using a field-modulation amplitude of 0.004 mT,
microwave power of 0.1 mW, and an accumulation of 8–13 h.

first-derivative EPR spectra as shown in Figs. 2(b) and 2(c).
Because of the slow field scan, the out-of-phase method enables us to measure resonant positions accurately, which isbeneficial in determining the spin-Hamiltonian (SH) parameters.
By using the RP method, the average concentrations of the
P1, NV – , and BV – centers in each crystal (labeled [Ns 0 ]avg ,[NV – ]avg , and [BV – ]avg , respectively) are estimated to be 90,
9, and 1.5 ppb for plate-I, and 110, 15, and 2 ppb for plate-II,respectively. The subscript “avg” indicates that these HPHT
plates have nonuniform distributions of impurities, which areproven by EPR imaging.
In addition to conventional EPR measurements, photo-EPR
and EPR imaging have been performed. Photo-EPR measurements were performed by irradiating a monochromatic
light (0.50–5.50 eV or 2480–225 nm) extracted from a 150W xenon lamp source. An energy step was set to 0.1 eV,
which is larger than the resolution of our monochromator.
In photo-EPR measurements, EPR spectra under illumination
are recorded as a function of the photon energy. A change
of EPR signal is caused by either optical spin polarization[45,46] or the change of the charge state [47,48]. In the former change, the threshold of the optical pumping matches
the zero-phonon line (ZPL) of the optical absorption band

PHYSICAL REVIEW B 105, 165201 (2022)[45,46]. When the latter change is caused by a transition
between the defect level and the valence band (or the conduction band), the photoexcitation threshold indicates the positionof the defect level or the ionization energy of the defect[47,48].
EPR imaging was performed by using a high-power gradient accessory Bruker E540 GCX2 [two-dimensional (2D)
gradients with 20 mT/cm] and a Bruker ER4108 TMHS resonator. In the present 2D EPR imaging, field-swept cw-EPR
spectra are taken under the main external magnetic field (Bext )along the z axis with superimposing the magnetic field gradient (MFG) parallel to the yz plane. The MFG is generated
by two gradient coils along the y and z axes. Then, the same
paramagnetic species at different yz positions are resonant at
different Bext values. A 2D EPR image is reconstructed froma set of projections acquired at different directions of constant
MFG. The narrower linewidth and stronger MFG provide
a higher spatial resolution. EPR imaging visualizes a spatial distribution of paramagnetic species in a sample [49,50].
Thus, EPR imaging is useful in revealing the growth sector
dependence of impurity incorporation and the formation ofradiation defects in diamond [51,52]. We studied the spatial
distribution of Ns 0 (S = 1/2) and NV – (S = 1) in plate-II. For
recording 2D EPR imaging, the main uniform field, Bext , wasset to be the [011] direction, where the P1 spectrum consists offive split lines and the NV – spectrum (low-field side) reveals
two split lines, as shown in Fig. 2(a). Since their EPR signals
are very well separated, individual distributions are easily
obtained. Full five-line spectra were used for reconstructing
the 2D distribution of Ns 0 , and the low-field-side spectra werecollected for the NV – center. The NV – signals were enhanced∼100 times under illumination of a 532-nm laser light. RP
cw-EPR spectra were obtained by sweeping the main uniform
field (Bext //z) and were collected by varying a constant MFG
in the yz plane stepped for 196 directions spanning 180°. The
MFG started from the z axis (the [011] direction) to the −zaxis via the y axis (the [01̄1] direction). The field gradientsused were 2.5 mT/cm for Ns 0 and 2.0 mT/cm for NV – . A setof projection spectra were collected in ∼75 min.
We also carried out PL and CL measurements to further
reveal inhomogeneous distributions of the impurities and defects. The PL spectra were measured at room temperature withexcitation by the 514.5 nm line of an Ar-ion laser. The signals
were dispersed with a monochromator (Jobin Yvon, HR320) and detected by a liquid-nitrogen-cooled charge-coupleddevice (CCD) (Roper, LN/CCD-400EB-GI). The spectral resolution of the monochromator is less than 0.5 nm. The CL
spectra were measured at 83 K by using a system consisting ofa scanning electron microscope (Topcon, SM-350) equippedwith an optical window, optics coupled to a spectrograph(Photon Design Co., PDP-320), and a liquid-nitrogen-cooled
CCD (Roper, LN/CCD-400EB-GI). The spectral resolution of
the spectrograph is less than 0.5 nm.
Additionally, attempts to identify PL of the BV – centers were carried out using CFM. We utilized a home-built
CFM combined with a continuous-flow He cryostat (CryoVac)
operating at 4 K. To match the absorption band found in
EPR spin-polarization experiments, laser excitation at 375
nm (3.31 eV) was used. The PL collected from the sample was sent either to the photomultiplier tube (Hamamatsu

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PHYSICAL REVIEW B 105, 165201 (2022)

RP-EPR signal (a.u.)

T. UMEDA et al.

exp
t

NV

BV
11B (I = 3/2, 80.2%)

10B (I = 3, 19.8%)

FIG. 3. (a) Experimental coordinate and corresponding atomic
configurations of the BV – and NV – centers. A single impurity atom islocated at one of the site-1 to site-4 positions, and a monovacancy isinserted into the origin of the coordinate. In EPR experiments, the external magnetic field (B) was rotated in the (01̄1) plane from θ = 0◦
at B // [100] to θ = 90◦ at B // [011]. (b) Angular dependences of
EPR signal positions of BV – and NV – . “+” symbols and solid lines
express experimental data and theoretical curves, respectively. Bothof the spin-1 systems of BV – and NV – show the same C3v symmetry
and very similar D tensors. Numbers in the figure denote the site
number. The site-3 and -4 centers are always indistinguishable within
the B rotation in the (01̄1) plane.

H10682-210) for imaging or to the spectrometer (Princeton
Instruments) for the light PL analyses. To reject the reflected
laser light in the detection channel, a long pass filter with
cut-on wavelength at 380 nm was used, so that the expected
BV – luminescence is supposed to be transmitted.
III. RESULTS AND DISCUSSIONS
A. EPR identification of BV – center

Figure 2(a) shows a typical EPR spectrum of N and
B codoped HPHT diamonds, named “plate-I” and “plateII.” This spectrum was obtained by a RP-EPR measurement[33,42,43], which focuses on low-concentration paramagnetic
centers with long spin relaxation times. Major and minor paramagnetic centers in the crystals were neutral substitutional
nitrogen (Ns 0 or the P1 center, S = 1/2) [53,54] and the NV –center (S = 1) [39,45], respectively. When magnifying the
NV – EPR spectrum, another spin-1 center can be observed
besides the low- and high-field-side NV – signals, as shownin Figs. 2(b) and 2(c). This novel EPR signal originates fromthe BV – center, showing a fourfold HF structure due to a 11 B
nuclear spin (I = 3/2). Its concentration was estimated to be
1.5–2 ppb.
In Fig. 3, the spin-1 systems of the BV – and NV – centers
are examined as a function of the magnetic-field angle θ . The
definition of θ and the corresponding experimental coordinate
are shown in Fig. 3(a) . As is clear in Fig. 3(b), both spin-1
systems are quite similar, displaying very similar zero-field
splittings and the same C3v symmetry (111-axial symmetry).
Solid lines for NV – were simulated using the known SH
parameters of NV – [39]. By analogy, solid lines for BV – werealso calculated using the SH parameters determined in thisstudy. The SH parameters for the spin-1 BV – and NV – centers
are defined by the following SH [55]:H = μB BT · g · S + ST · D · S + I T · A · S,

232

233

234

Magne c eld (mT)FIG. 4. Hyperfine (HF) interactions of 10 B and 11 B in the BV –center, which generate fourfold and sevenfold HF splittings, respectively. A fitted curve was calculated by a combination of fourfold andsevenfold HF split structures with the given natural abundances. The
RP-EPR spectra was recorded by using a field-modulation amplitudeof 0.002 mT, microwave power of 0.2 mW, and an accumulation
period of 16 000 s.

where μB is the Bohr magneton, B is a magnetic field vector(superscript “T” denotes a transposed vector), g is an electron
gyromagnetic tensor (g tensor), S is an electron-spin operator, D is an electron spin-electron spin interaction tensor (Dtensor), I is a nuclear-spin operator for 14 N or 10,11 B, and A
is a hyperfine (HF) interaction tensor (A tensor). We omitted
a nuclear Zeeman term and a nuclear quadrupole interaction
term because they did not explicitly appear within the present
angular-map analyses. By giving B, g, D, and A, we numerically simulated the above SH by using an “EPR-NMR” SH
simulator [56]. Since the spin-1 system of BV – shows a C3v
symmetry (111-axial symmetry), its g, D, and A tensors canbe determined from a pair of EPR spectra shown in Figs. 2(b)and 2(c). At θ = 54.75◦ and 35.25°, one can directly findprincipal values of each tensor (e.g., D// and D⊥ for the D
tensor) parallel to and perpendicular to the symmetry axis,
respectively. The determined SH parameters are summarized
in . Solid lines for BV – in Fig. 2(b) were simulated using theseSH parameters.
The zero-field splitting constant D for BV – is obtained
to be D = 3D// /2 = 2913 MHz, which is close to those of
NV – (2872 MHz) [39,45] and OV 0 (2888 MHz) [33]. This
excellent consistency indicates that all these centers establish
the common spin-1 system with the ex and ey levels as shownin Fig. 1. Previous first-principles calculations predicted consistent values of D for NV – (2848 MHz) [35] and OV 0 (2989
MHz) [35], but a much smaller value for BV – (D = 1882
MHz) [32]. The former two values were obtained using a
512-atom supercell [35], while the latter value for BV – wascalculated using a 64-atom supercell [32].
We further evidence the presence of a single
boron atom in the BV – center, which is shownin Fig. 4. In addition to the fourfold HF splitting due to 11 B (I = 3/2, natural abundance =80.2%) in the BV – spectrum, there is a weak sevenfold
HF splitting, which arises from HF interactions with a 10 B
nuclear spin (I = 3, natural abundance = 19.8%). In fact, as
is shown in the figure, the experimental spectra can be fitted
by a combination of 10 B and 11 B HF structures with their

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NEGATIVELY CHARGED BORON VACANCY CENTER IN …

PHYSICAL REVIEW B 105, 165201 (2022)

TABLE I. SH parameters of the BV – center in comparison with the known parameters of the NV – and OV 0 centers. Principal values of D
and A are expressed in MHz.
g tensor (g)Spin-1 system
BV

–

g//
2.0030D tensor (D)g⊥

2.0038A tensor (A)

D//

D⊥

1942.1−971.0nuclear spin

A//

A⊥

Ref.

B (I = 3/2)
B (I = 3)
14
N (I = 1)

7.0
2.3
−2.17

5.5
1.9
−2.63

this work

11

10

NV –
OV 0

2.00292.00292.00312.0026

1914.66
1925.3−957.33−962.7natural abundances. The principal HF constants (A// or A⊥ )for 10 B are 1/3 of those for 11 B (Table I). This factor simply
coincides with the ratio (1/2.98) of their nuclear g factors(gn = 0.602 20 for 11 B /gn = 1.792 437 for 10 B).
When the magnetic field is apart from the principal-axis
directions, the fourfold split 11 B HF structure further splits
into more than five lines. This is due to a nuclear quadrupole
interaction. The determination of the nuclear quadrupole interaction for BV – will be saved for future work.

B. Boron hyperfine interaction of BV – center

For a 11 B atom, isotropic and anisotropic HF constants, a and b, are known to be a = 2547 MHz and b =63.64 MHz, respectively [55]. The experimental isotropic
and anisotropic 11 B HF couplings, Aiso = (A// + 2A⊥ )/3 andAaniso = (A// –A⊥ )/3 [55], are estimated to be 6.2 and 0.65
MHz, respectively. Within a simple linear combination of
atomic orbitals (LCAO) approximation, Aiso and Aaniso should
originate from 2s- and 2p-orbitals of an electron spin on a11
B site. Thus, the 11 BV – center has 2s-orbital fractions of|Aiso /a| = 0.2% and 2p-orbital fractions of |Aaniso /b| = 1.0%
on its boron atom. These very small fractions ensure that the
spin-1 system consists of carbon orbitals (the ex and ey states)on the vacancy side, supporting the picture in Fig. 1(c). Thisnature is exactly common to the case of the NV – center [57].
Considering the symmetry for both BV – and NV – , a dangling bond of each impurity atom should not be mixed into
their e state. In fact, the wave function of NV – is mostly
localized on three C atoms, and its remainder spreads overother far C atoms mostly in the plane perpendicular to
the N-V axis [57]. The very weak boron HF interactions
point towards a similar situation for the BV – center. In sucha case, the anisotropic part of the HF interaction (Aaniso )can be mainly contributed by a dipolar interaction between
an impurity’s nuclear spin and an electron spin on otheratoms. Actually, in the NV – center, the observed anisotropic
part, Aaniso = 0.19 MHz, agrees with the dipolar interaction(0.18 MHz) between a 14 N nucleus and spin densities on
three nearest-neighbor C atoms calculated with N-C = 2.51 Å[39]. If we replace the 14 N nucleus (gn = 0.403 763 7) withthe 11 B nucleus (gn = 1.792 437), the dipolar interaction of
0.18 × 1.7924/0.403 76 = 0.80 MHz is estimated with thesame symmetrical axis, which is similar to the observed Aaniso
value (0.65 MHz). This also supports the correspondence between BV – and NV – .

Ref. [39]Ref. [33]

C. Optically induced spin polarization of BV – center

One of the greatest advantages of the NV – center is its
spin-selective optical responses, which are crucial in spinpolarization and readout of its spin qubit. With an opticalpumping at ∼532 nm, the triplet spin system of NV – can bepolarized into the mS = 0 state via a spin-dependent branching path in the decay process depicted in Fig. 5(a). After astrong spin polarization as shown in the diagram of Fig. 5(b),pairs of absorption and emission lines are observable at thelow- and high-field sides, respectively, and simultaneously
EPR signal intensities are enhanced. Such a spin-polarization
behavior was absent for the spin-1 OV 0 center [33–35].
In contrast, the spin-1 BV – center clearly shows the spinpolarization behavior, which appears in the high-field-side
spectra in Fig. 5(c). For this experiment, we used a white light(150-W xenon lamp) + band-pass filters (365 ± 15 nm). Unfortunately, optical responses of BV – were not intense at room
temperature, due to a weak filtered light (<1 mW). Within
this setup, the spin polarization was almost absent even for
the NV – center. However, at lower temperatures, the spin polarization became stronger for both the NV – and BV – centers.
Eventually, at 150 K or lower. we clearly detected enhanced
high-field-side emission lines (also enhanced low-field-side
absorption lines) for BV – , demonstrating its optically induced
spin polarization. When we used an optimized wavelength for
NV – (e.g., 530 nm), its spin polarization was further strongly
enhanced.
The optical spin polarization loop of NV – originates fromits four-level structure consisting of the 3 A2 , 3 E , 1 A1 , and 1 E
states [Fig. 5(a)]. Spin-selective intersystem crossing (ISC)from 3 E to 1 A1 caused by the phonon-mediated spin-orbit
coupling is crucial in its efficient optical spin polarization[58,59]. First-principles calculations on BV – predicted thespin-conserving transition from a 3 A2 triplet ground state to a3
E triplet excited state; however, its intermediate singlet states
were not calculated [37,38]. Our observation indicates that the
BV – center has intermediate singlet state(s), common to the
NV – system.
We further studied optical thresholds of the BV – and coexisting NV – centers by means of photo-EPR spectroscopy[47,48]. Figures 6(a) and 6(b) show sets of photo-EPR spectra
for NV – and BV – , respectively, in the energy range between
0.50 and 5.50 eV. In Fig. 6(a), we can find the establishedoptical responses of the NV – center [62]. The spin polarizationof NV – was observable in the range between 1.95 and 2.25 eV(636 and 551 nm), where the low- and high-field-side NV –165201-5

T. UMEDA et al.

PHYSICAL REVIEW B 105, 165201 (2022)

(c)

(a)

opcalpumping
2.0 eV (NV )3.0 eV (BV )ground state 3A(a2e2), (ee)

NV

mS
1
0

2

1
0

ISC

×5

intermediate
state 1A1
(a2e2), (ee)

296K

BV

200K
×5
1E
(a2e2), (ee)PL
1.945 eV (NV )?? (BV )ESR signal (a.u.)

photo-excitedstate 3E
(a1e3), (ae)High-field side

150K

dark
photo

100K

×1.5
Zeemanspling

(b)
mS
1
0

×0.5
60K

×1.5

absorpon

×0.5

Zero ﬁeld
spling (D)emission

438

439

440

441

Magne c eld (mT)FIG. 5. (a) Optical spin-polarization loop of NV – . Electronic
configurations corresponding to 3 A2 , 3 E , 1 A1 , and 1 E are shownin terms of electron representation (black) and hole representation(red) [60,61]. For instance, (a1 )2 e2 and (a1 )1 e3 for 3 A2 and 3 E in
the electron representation are rewritten as (ee) and (ae) in the hole
representation, respectively. Intermediate states of 1 A1 and 1 E are
assigned to (ee) in the hole representation. Likewise NV – system, 3 A2ground state and 3 E excited state of BV – are assigned to (ee) and (ae)in the electron representation, respectively. We assumed that BV –has a similar four-level scheme, judging from its spin-polarization
behavior similar to that of NV – . Their ground and photoexcitedstates commonly have triplet spin states with mS = −1, 0, +1. Greenline expresses an optical pumping from the ground 3 A2 state to theexcited 3 E state. For NV – , the optical-pumping threshold is 1.945 eV[1,45], and that of 3.0 eV for BV – was determined in this study. The
photoexcited state is decayed into the ground state via two pathways:
a PL emission (red line) or a nonradiative decay via an intermediate
state (black lines). The latter process converts mS = ±1 states into
an mS = 0 state, causing spin polarization such that the mS = 0 state
has a nonequilibrium large population. The PL line of NV – appears
at 1.945 eV, while that of BV – is still unclear. (b) Spin polarization of
the mS = 0 level via optical pumping, which generates a high-fieldside emission line in the EPR spectrum. (c) Temperature dependenceof spin polarization seen in high-field-side EPR spectra before andafter optical pumping by a filtered light of 365 ± 15 nm (3.40 ± 0.15eV). Light power was ∼1 mW just at the end of a fiber guide. The
actual power at the sample was rather decreased due to a transmission
loss from fiber end to microwave cavity. Due to spin polarization,
low-field-side lines can be enhanced, while high-field-side lines canbe reduced or reversed, as shown here.

signals revealed nonequilibrium signal intensities due to theoptical pumping [see also Fig. 6(c)]. This range covers the
optimal excitation wavelength (510–540 nm) for studying the
NV – luminescence. Above this range, the spin polarizationof NV – was canceled because the single-photon ionizationof NV – to NV 0 started (ionization threshold = 2.6 eV) [62].
Further increasing the excitation energy up to 2.9 eV, theNV – center reexhibited the spin polarization. This threshold isconsistent with the known recombination energy of 2.94 eV,
causing the recovery of NV 0 to NV – states [62].
Likewise, in Fig. 6(b), we also found a clear signature of
the spin polarization for BV – in the range between 3.0 and 3.6
eV (413 and 344 nm), where the high-field-side emission lines
are observed. The optical pumping may start from ∼2.8 eV
for BV – [cf., Fig. 6(c)], above which it can start an opticaltransition from a triplet ground state to a triplet excited state.
This threshold is similar to a calculated ZPL (3.22 eV) of thespin-conserving transition of BV – [38].
Above 3.7 eV, the BV – signal was quenched, suggesting that the single-photon ionization energy of BV – (BV – +hν → BV 0 + e– CB ) is 3.7 eV. Thus, the ground state of BV – is
supposedly located at around EC –3.7 eV (EV + 1.8 eV). Thisvalence-band-side level matches the calculated (−/0) levelsof BV : EV + 1.5–1.6 eV [32,37,38], supporting our identification from the viewpoint of energy levels.
We also observed other optical quenching in the range
between 1.4 and 2.3 eV (886 and 539 nm) for BV – . These
thresholds may be related to charge transfers between BV –and the valence band or other charge traps including in thespecimen. Further works are required to elucidate the originsof these thresholds.
It is also noted that we have not detected an EPR signal of
BV 0 (S = 1/2) in the photo-EPR experiments. The absence ofthe spin-1/2 center is ascribed as a signal-broadening mechanism just the same as the case of the NV 0 center (S = 1/2)
[46].
D. Optical and EPR imaging on N and B codoped diamonds

First-principles calculations indicate that the charge stateof the BV center varies with the Fermi level (EF ) such as BV +(EF –EV < 0.6 eV [38] or EF –EV < 1.2 eV [37]), BV 0 (0.6 <EF –EV < 1.6 eV [38] or 1.2 < EF –EV < 1.5 eV [37]), BV –(1.6 < EF –EV < 3.6 eV [38] or 1.5 < EF –EV < 2.7 eV [37]),
and BV 2– (EF –EV > 3.6 eV [38] or EF –EV > 2.7 eV [37]).
Thus, we expect that the BV – center is stable with EF in the
midgap range. Since the neutral boron has an acceptor level at
EV + 0.37 eV [63], B-doped diamonds after irradiation may
form the BV + and/or BV 0 centers predominantly. To create
BV – associated with a proper EF , N and B codoping may benecessary. In this section, we conclude that the BV – centers
are preferentially formed in {111} growth sectors, which arerevealed by EPR imaging and luminescence microscopy.
Figure 7(a) illustrates the preparation of a diamond plate(plate-II) from a HPHT crystal. Typical HPHT crystals have a
habit of cubo-octahedron consisting of as-grown {100} and{111} facets. Internal morphology exhibits growth sectors,
which show different uptake rates for impurities, since different growth sectors have different growth planes and different
growth rates [64]. Therefore, the concentrations of N and B
should be inhomogeneous in the plate, which is the key forcreating BV – . The distribution of the growth sectors is clearlyseen in the “DiamondView” image shown in Fig. 7(b). The
DiamondView provides a fluorescence image under excitationof UV at wavelengths shorter than ∼230 nm from a filteredxenon arc lamp source. The DiamondView image only reveals
a near-surface information, because of the short penetration
depth of the UV light. In Fig. 7(b), the {100} growth sectors

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FIG. 6. Photo-EPR measurements on BV – and NV – centers at 60 K for B // [111]. Light power was 5–10 μW at the end of a fiber
guide. Highlighted parts in (a) and (b) indicate optical pumping ranges for spin polarizations of NV – and BV – , respectively. Most outerweak resonances in (b) arise from 13 C HF satellites of NV – . (c) EPR intensity profiles of NV – and BV – centers (averaged intensities of
their threefold-split and fourfold-split lines, respectively). Solid and dashed lines are calculated using high-field-side and low-field-side lines,
respectively. Optical thresholds are also indicated.

(at a center of both faces and four corners of seed-side face)show bright red fluorescence, indicating the formation of theNV centers (NV − and NV 0 ). In contrast, four {111} growth
sectors surrounding the central {100} sector appear as darker

FIG. 7. Growth sectors in N and B codoped diamond. (a) Preparation of a (100) diamond plate for EPR imaging and PL/CL study.
Plate-II (1.34 mm thick) was cut out from the upper middle of a
HPHT crystal by laser-cutting. (b) A “DiamondView” image of theplate. Image of the seed-side face (right) is mirrored to make the view
direction the same as that of the growth-side face (left). Brighter anddarker areas indicate {100} and {111} growth sectors, respectively.

areas, suggesting low NV concentrations. Our samples consist
predominantly of {100} and {111} growth sectors together
with very minor {113} and {110} sectors. Since these sectors
show different impurity concentrations, our plate is like aninhomogeneous semiconductor. Therefore, for each sector, theEF position relative to EV can vary with its impurity concentration.
Using optical microscopy and EPR imaging (Fig. 8), we
further reveal the charge states of N and B impurities andexamine the relative EF positions in these growth sectors.
From CL microscopy shown in Figs. 8(c) and 8(d), the concentration of neutral boron acceptor [Bs 0 ] is determined foreach sector from the intensity ratio of excitonic recombination assisted by a transverse optical (TO) phonon between
the neutral-boron bound exciton (BE) and the free exciton(FE) [65,66]. In the {111} growth sectors, [Bs 0 ] estimatedfrom the ratio of BETO /FETO are 6.4 × 1016 cm–3 (0.36 ppm)
and 7.2 × 1016 cm–3 (0.41 ppm) at positions 3 and 4, respectively. On the contrary, in the {100} growth sectors,[Bs 0 ] was below the detection limit. The concentration of
Ns 0 is also estimated from the optical absorption band at
270 nm at room temperature [67]. The [Ns 0 ] of the {100}and {111} growth sectors of plate-II obtained from the UV
absorption are 1.4 ppm and below 0.1 ppm (detection limit),
respectively.
The EPR imaging visualizes two-dimensional spatial distributions of Ns 0 and the NV – center in the plate, which
are shown in Figs. 8(a) and 8(b), respectively. EPR imaging
reveals that both Ns 0 and NV – are preferentially present in the{100} growth sectors. Neutral nitrogen Ns 0 has a deep donor
level at EC –1.7 eV (EV + 3.8 eV) [68]. The ground state of
NV – is located at EC –2.6 eV (EV + 2.9 eV) [69]. Thus, theFermi level in the {100} growth sectors should be higher
than the midgap (EC –2.8 eV or EV + 2.7 eV). PL microscopy

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FIG. 8. Two-dimensional EPR imaging of (a) P1 distribution and (b) NV – distribution. Both P1 and NV – are revealed to be dominantly
distributed in the {100} growth sector. NV – distribution was recorded under 532-nm optical excitation. (c), (d) Room-temperature PL spectrausing 514.5-nm excitation and CL spectra at 83 K obtained for {100} and {111} growth sectors, respectively. In CL spectra, luminescence froma neutral-boron bound exciton (BETO ) was only observed for the {111} growth sector. These CL spectra were normalized by each free-exciton
luminescence (FETO ) intensity. PL spectra indicate that a nitrogen vacancy exists in the {111} growth sector, mostly in the form of the NV 0
center (ZPL: 575 nm).

[Fig. 8(c)] supports the presence of NV 0 and NV – , whichappears as zero phonon lines (ZPLs) at 575 and 637 nm,
respectively [1]. Judging from these observations, we expectthat a dominant charge state is BV 2– if the BV centers wereformed in this sector.
Furthermore, we suggest that they are barely formed inthe {100} sectors, because of low boron concentration. It
is already known that [Bs 0 ]{111} > [Bs 0 ]{110} > [Bs 0 ]{113} >[Bs 0 ]{100} for B-doped diamonds [64,70], where the suffixof {hkl} refers to the {hkl} growth sector. Accordingly, theboron incorporation is normally much less (∼1/100) in the{100} sectors than in the {111} sectors [70,71], reducing thenumber of BV in the {100} sectors of our crystal.
In contrast with the {100} growth sectors, the {111} sectors expectedly generate the BV – center after the electron
irradiation and subsequent annealing, as described below.
First, a much larger uptake of boron (the BETO peak) is evidenced by CL microscopy [Fig. 8(d)], which detected Bs 0 of
∼0.4 ppm. On the other hand, the EPR imaging [Fig. 8(a)] andthe optical absorption reveal the absence of Ns 0 , although it
is generally known that [Ns 0 ]{111} > [Ns 0 ]{100} > [Ns 0 ]{113} >[Ns 0 ]{110} [64]. We deduce that the absence of Ns 0 is mainly
due to the B-doping. Nitrogen and boron impurities compen-sate each other:+−Ns 0 + Bs 0 →← Ns + Bs .

Adding boron impurities changes Ns 0 into Ns + (S = 0).
Moreover, it was reported that the ratio of [Ns 0 ] between the{111} and {100} sectors depends on the growth temperature[72]. At a low growth temperature, [Ns 0 ] in the {111} sectors
became lower (∼0.76 times) than in the {100} sectors. With
these factors, Ns 0 are absent in the {111} sectors of our sample. The PL spectra [Fig. 8(d)] indicate that the NV centers aremostly in the NV 0 state, suggesting a lower EF in the {111}sectors. First-principles calculations estimated that the NV 0
state is stable when 1.1 < EF –EV < 2.6 eV [69]. This range
overlaps with the range in which the BV – state is stable. Thus,
the EF position of the {111} sectors is most probably tuned tostabilize the BV – state (stable when EF –EV > 1.6 eV) [37,38]
via N and B codoping.
It might be puzzling that Bs 0 (the acceptor level of EV +0.37 eV) and BV – coexisted in the {111} sectors of oursample. In diamond, it was pointed out that a defect charge
state is not uniquely determined by the Fermi level [73]. As
a result, two different charge states of the same defect cancoexist in a single crystal. When impurity concentrations are

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FIG. 9. (a) Confocal microscope image of N and B codoped diamond. “MW wire” is used for microwave radiation when ODMR isrecorded. The excitation wavelength was 375 nm, which can generate triplet-to-triplet excitation and spin polarization for BV – . In the {100}growth sectors, a strong luminescence was observed due to ensemble NV – centers, while in the {111} sectors, no strong luminescence was
detected in spite of the presence of BV – . (b) Typical PL spectra from {100} and {111} growth sectors (acquisition times are the same). In theregion around 414 nm, luminescence from the {100} area remains stronger compared to the {111} area. Peak intensity of the 389-nm line is∼1/50 times smaller than that of the 575-nm line of NV 0 under 375-nm excitation. The 389-nm center is a nitrogen-related radiation-induced
defect. Sharp peaks around 412 nm are likely to be local mode replicas of 389-nm ZPL [74].

< 10 ppm, the distance between a defect and a donor (or an
acceptor) has a wide variation. The charge state of a defectdepends on such a distance [73]. Accordingly, the coexistenceof Bs 0 and BV – becomes possible in the {111} sectors of ourcrystals.
E. CFM imaging

As was shown in Sec. III C, the BV – center allows thetriplet-to-triplet optical excitation above ∼2.8 eV, leading to
the spin polarization. We tried to identify PL of this centerusing a 375-nm (3.3-eV) excitation laser in a special lowtemperature CFM apparatus. We note again that this excitation
wavelength fits with the calculated ZPL of BV – (3.22 eV or
385 nm) [38] as well as with the beginning of a strong spinpolarization observed in our photo-EPR studies (threshold ∼3.0 eV).
Figure 9(a) shows a typical CFM raster scan obtained at
4 K with a 380-nm-long pass filter, visualizing the borderof the {111} and {100} growth sectors. As discussed above,the {100} sector contains a much higher concentration of theNV centers, resulting in much brighter luminescence. However, even restricting the detection window to the region of
390–440 nm, the {100} growth sector remains brighter as
indicated in Fig. 9(b), due to the appearance of the 389-nm
line and its replicas [74]. The spectrum obtained from the{111} area did not exhibit any specific features assignable
to the BV – luminescence. Moreover, we applied a microwave
field swept in the range of 2–3 GHz under constant 375-nm
illumination, trying to observe optically detected magnetic
resonance (ODMR). Limiting the detection window from 391
to 450 nm, we could not observe ODMR either from the {111}or {100} area. As a control measurement, using a 532-nm
excitation laser and detecting light from the {100} sector witha 650-nm-long pass filter, an ODMR signal corresponding

to the NV − centers was instantly observed. At present, weconclude that the luminescence from the BV – center is much
weaker compared to the NV − center.
Although the three centers, NV – , OV 0 , and BV – , exhibita close similarity in their ground states (3 A2 ), quite different
decays are observed from their photoexcited states. We deducethat such a difference originates mainly from the nature of
their excited states (3 E ). The 3 A2 ground state is an orbital
singlet that is not subject to the Jahn-Teller distortion. On theother hand, the 3 E triplet excited state, which is Jahn-Teller
unstable, originates from different electronic configurations
for the three centers: (a1 )1 e3 for NV – , e1 (3a1 )1 for OV 0 , ande1 (a1 )1 for BV – . In the NV – center, an electron is excitedfrom a1 into the e level which consists of pure carbon orbitals(see Appendix A). At room temperature, the excited state of
NV – maintains the C3v symmetry, since NV – is a dynamic
Jahn-Teller system with a small barrier energy of reorientation [1]. In the case of OV 0 , an electron is promoted fromthe e state to the antibonding 3a1 state of C-O bonds withthree back-bonded carbon atoms (Appendix A). From the 3 E
excited state of OV 0 , a very fast nonradiative decay leads to ametastable state (Cs symmetry) with a reconstructed structure
having two C-O covalent bonds and one elongated C-O bond[35]. The symmetry lowering from C3v to Cs is the outcomeof the Jahn-Teller distortion. As a result, OV 0 is expected tohave a very weak or no luminescence [35]. In the case of BV – ,an electron is promoted from the e state to the a1 state, which
is high in energy due to a large involvement of the dangling
bond of boron less electronegative than carbon [32,38]. It may
result in a large Jahn-Teller distortion for BV – . Accordingly,
a different manifestation of the Jahn-Teller effect and thespin-orbit coupling may be expected for BV – as comparedto the NV – system. The exact influence on the BV – luminescence by such factors remains an open question, and will beclarified via sophisticated first-principles calculations in the

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future. The theoretical studies will also help us to understandan association between the weak luminescence and the spinpolarization.

derived:a1  = σX − λ (s1 + s2 + s3 ),a1 = (σ1 + σ2 + σ3 ) + λσX ,ex = 2σ1 − σ2 − σ3 ,IV. SUMMARY

We have found a novel EPR center of BV – in N and B
codoped diamonds. Our EPR identification showed that the
BV – center has a very similar triplet ground state (S = 1, C3v
symmetry, and D = 2913 MHz) to the NV – center (S = 1, C3v
symmetry, and D = 2872 MHz). The involvement of a single
boron atom was evidenced by its HF interaction due to 10 B or
11
B nuclear spin.
Furthermore, we confirmed that the triplet-to-triplet opticalexcitation with the threshold of ∼3.0 eV (413 nm) produces
the spin polarization for this center. This optical-pumping
behavior was similar to the NV – center, and was apart fromanother analog center of OV 0 (S = 1, C3v symmetry, andD = 2888 MHz). However, unlike the NV – center, the BV –center did not show detectable luminescence after the tripletto-triplet optical excitation even at low temperatures (e.g.,4 K). Accordingly, we have not yet identified PL from the
BV – centers. At least, our experimental results indicated that,among the three analog centers of NV – , OV 0 , and BV – , only
the NV – center is an extraordinary luminescent center. Understanding the reason why the BV – luminescence is strongly
weakened remains a future work.
For creating the BV – center, it is the key to tuning the Fermi
level to a specific position by taking advantage of N and B
codoping. We demonstrated using a combination of optical
microscopy and EPR imaging that inhomogeneous impurity
distributions in N and B codoped diamonds make it possible tocreate the BV – center preferentially within the {111} growth
sectors.
ACKNOWLEDGMENTS

This work was carried out as a joint project of Universityof Tsukuba, Ulm University, and University of Stuttgart. This
work was partly supported by the Japan Society of the Promotion of Science (JSPS) KAKENHI, Grants No. JP20H00340,
and K.W. was supported from JSPS KAKENHI Grant No.
JP17H02748. J.W. acknowledges support by the Max Planck
Society, the EU via ERC grant SMeL, and the project QIA
and ASTERIQS as well as the DFG via FOR 2724. F.J. acknowledges support by EU via ERC and projects ASTERIQS
and HYPERDIAMOND, DFG, BMBF, and VW Stiftung. P.S.
acknowledges Baden-Württemberg Stiftung for the financial
support via Eliteprogramme for Postdocs.
APPENDIX A: A SIMPLE MODEL ON ELECTRONICSTATES OF NV – , OV 0 , AND BV –Using a simple defect-molecule model consisting of an
impurity atom X (= N, O, and B) and three carbon atoms(C1 , C2 , and C3 ) surrounding a vacancy, molecular orbitals(MOs) of one-electron states are given by a linear combinationof atomic orbitals (LCAO) of four dangling-bond orbitals(σX , σ1 , σ2 , and σ3 ). Under the C3v symmetry, four MOs areey = σ2 − σ3 ,where λ and λ are LCAO coefficients [1]. In the NV – andOV 0 centers, the a1  and 1a1 MO approximately expresses
a nitrogen lone pair and an oxygen lone pair, respectively[see Figs. 1(a) and 1(b)], since nitrogen and oxygen are more
electronegative than carbon. On the other hand, the a1 and 2a1
MO approximately expresses a combination of three carbon
dangling bonds.
Since boron is less electronegative than carbon, a boronrelated a1 state in the BV – center is energetically located muchhigher than the ex and ey states [Fig. 1(c)], contrary to thecases of NV – and OV 0 [32]. First-principles calculation predicted that the unoccupied a1 state located higher in the band
gap originates mainly from a boron dangling bond [37,38].
In the OV 0 center, there is an unoccupied a1 state labeled“3a1 ” [Fig. 1(b)] in the upper side of the band gap, in addition
to the vacancy-related states [32,35]. The 3a1 MO is an antibonding orbital of oxygen and three back-bond carbon atoms[32,35].
All three centers have a common triplet ground state (S =1, 3A2 many-electron state) arising from two parallel-spin
electrons occupying doubly degenerate e orbitals. The two e
orbitals (ex and ey ) consist of pure carbon orbitals, as is seenin the above relations.
APPENDIX B: COHERENCE TIME AND ENGINEERINGCONSIDERATION OF BV –We expect that the coherence time of BV – can be engineered to be as long as that of NV – . The long coherencetime of NV – benefits from the uniqueness of diamond [1].
The high Debye temperature and weak spin-orbit couplingextremely elongate the spin-lattice relaxation time (T1 ) of
NV – . Secondly, the sources of local-field fluctuations can be
extremely reduced by 12 C-isotope enrichment. BV – can gain
these benefits similarly to NV – .
When NV – is fabricated by a conversion between N and
NV (via electron irradiation and subsequent annealing, etc.),
residual substitutional nitrogen atoms, Ns 0 (S = 1/2), play amajor role in shortening the spin coherence times, T2 ∗ and T2 ,of NV – . For example, our EPR imaging of Figs. 8(a) and 8(b)indicates such a situation. On the contrary, EPR imaging onthe {111} sectors, where BV – is fabricated, did not exhibit
the Ns 0 signal. Thus, the Fermi-level tuning creating BV –makes a nonparamagnetic state of substitutional nitrogen, Ns +(S = 0). Accordingly, the sources of local-field fluctuations
were greatly reduced. Instead of Ns 0 , a low concentrationof Bs 0 (S = 1/2) is present in the {111} sectors. However,Bs 0 having a short spin-lattice relaxation time is unlikely toshorten the coherence time of BV – .
It should be noted that the EPR signal of BV – was measured with a high sensitivity by using the RP-EPR technique.
This technique is only effective in a paramagnetic species
having a long relaxation time.

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photon-spin interfaces and quantum communication. A similar weakness will be found for BV – . In contrast with NV – and
BV – , impurity-vacancy defects having D3d symmetry such as
SiV – reveal a vanishing permanent electric dipole momentof the ground and excited states, which makes their opticaltransition insensitive to electric-field noise in nanostructures(e.g., in photonic cavity). For NV – , however, nanostructures
such as a solid immersion lens and a nanopillar are widely
useful for enhancing the photon collection [75]. A similar
strategy will also be useful for BV – if its photoluminescenceis visible.

{100}

503 nm

470 nm389 nm

PL intensity (a.u.)

NEGATIVELY CHARGED BORON VACANCY CENTER IN …

{111}

APPENDIX C: WIDE-RANGE PL SPECTRA

It is also expected that BV – has the same weak points
as those of NV – . The weak points of NV – are as follows:(i) only ∼4% of the emission is extracted as a ZPL, and(ii) a lack of the inversion symmetry makes ZPL frequencies
highly sensitive to local electric fields and local strain. Theseweaknesses add a non-negligible difficulty in applying NV – to

The PL spectra shown in Fig. 9(b) were taken with thegrating 1596 grooves/mm. PL spectra for a wide wavelength
range taken with a lower resolution (grating 150 grooves/mm)
are shown in Fig. 10. No PL features assignable to BV – wereobserved in this range.
The center with ZPL at 470nm is “TR12” which is aradiation-induced center observed in all types of diamonds[76]. The PL feature with ZPL at 503 nm and a phonon
replica of 41 meV arises from the H3 center, [N-V-N]0 , according to the literatures [e.g., [77]]. The PL with ZPL at
517.5 nm and a phonon replica of ∼51 meV was observedin boron-doped HPHT diamond crystal with the low nitrogenconcentration [78].

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FIG. 10. Typical wide-range PL spectra measured with 375-nm
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times are the same).

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