Photoinduced absorption and anomalous dichroism in NaCa₂Mn₂V₃O₁₂ garnet as an evidence for the formation of oxygen holes dynamics
It is shown that long-lived photoinduced dichroism in garnets is caused by photoproduced charges with anisotropic structure, keeping long memory of the pumping light polarization, while photoinduced absorption is due to all photoproduced charges irrespective of their intrinsic structure. The char...
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| Zitieren: | Photoinduced absorption and anomalous dichroism in NaCa₂Mn₂V₃O₁₂ garnet as an evidence for the formation of oxygen holes dynamics / V.V. Eremenko, S.L. Gnatchenko, I.S. Kachur, V.G. Piryatinskaya, A.M. Ratner, V.V. Shapiro, M.B. Kosmyna, B.P. Nazarenko, V.M. Puzikov // Физика низких температур. — 2005. — Т. 31, № 11. — С. 1293-1301. — Бібліогр.: 17 назв. — англ. |
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irk-123456789-1217292017-06-16T03:03:05Z Photoinduced absorption and anomalous dichroism in NaCa₂Mn₂V₃O₁₂ garnet as an evidence for the formation of oxygen holes dynamics Eremenko, V.V. Gnatchenko, S.L. Kachur, I.S. Piryatinskaya, V.G. Ratner, A.M. Shapiro, V.V. Kosmyna, M.B. Nazarenko, B.P. Puzikov, V.M. Квантовые эффекты в полупpоводниках и диэлектриках It is shown that long-lived photoinduced dichroism in garnets is caused by photoproduced charges with anisotropic structure, keeping long memory of the pumping light polarization, while photoinduced absorption is due to all photoproduced charges irrespective of their intrinsic structure. The charges with anisotropic structure are identified as two-center oxygen holes. The formation of an oxygen hole is preceded by the excitation of a charge-transfer state with electron partially transferred to a cation C (V⁵⁺ for NaCa₂Mn₂V₃O₁₂ garnet) from an adjacent oxygen anion. To turn this excited state into a free hole state requires some time τhole during which the hole axis can be reoriented resulting in a diminution of dichroism. The time τhole shortens with increasing ionization potential of the cation C (very high for V⁵⁺). Such a mechanism explains qualitatively a set of unusual experimental facts, in particular, a very strong dichroism observed just in the NaCa₂Mn₂V₃O₁₂ garnet, where photoinduced changes of all optical properties disappear after switching off of the irradiation significantly faster than those in other garnets examined. 2005 Article Photoinduced absorption and anomalous dichroism in NaCa₂Mn₂V₃O₁₂ garnet as an evidence for the formation of oxygen holes dynamics / V.V. Eremenko, S.L. Gnatchenko, I.S. Kachur, V.G. Piryatinskaya, A.M. Ratner, V.V. Shapiro, M.B. Kosmyna, B.P. Nazarenko, V.M. Puzikov // Физика низких температур. — 2005. — Т. 31, № 11. — С. 1293-1301. — Бібліогр.: 17 назв. — англ. 0132-6414 PASC: 78.40.-q http://dspace.nbuv.gov.ua/handle/123456789/121729 en Физика низких температур Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України |
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Digital Library of Periodicals of National Academy of Sciences of Ukraine |
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DSpace DC |
| language |
English |
| topic |
Квантовые эффекты в полупpоводниках и диэлектриках Квантовые эффекты в полупpоводниках и диэлектриках |
| spellingShingle |
Квантовые эффекты в полупpоводниках и диэлектриках Квантовые эффекты в полупpоводниках и диэлектриках Eremenko, V.V. Gnatchenko, S.L. Kachur, I.S. Piryatinskaya, V.G. Ratner, A.M. Shapiro, V.V. Kosmyna, M.B. Nazarenko, B.P. Puzikov, V.M. Photoinduced absorption and anomalous dichroism in NaCa₂Mn₂V₃O₁₂ garnet as an evidence for the formation of oxygen holes dynamics Физика низких температур |
| description |
It is shown that long-lived photoinduced dichroism in garnets is caused by photoproduced
charges with anisotropic structure, keeping long memory of the pumping light polarization, while
photoinduced absorption is due to all photoproduced charges irrespective of their intrinsic structure.
The charges with anisotropic structure are identified as two-center oxygen holes. The formation
of an oxygen hole is preceded by the excitation of a charge-transfer state with electron partially
transferred to a cation C (V⁵⁺ for NaCa₂Mn₂V₃O₁₂ garnet) from an adjacent oxygen anion.
To turn this excited state into a free hole state requires some time τhole during which the hole axis
can be reoriented resulting in a diminution of dichroism. The time τhole shortens with increasing
ionization potential of the cation C (very high for V⁵⁺). Such a mechanism explains qualitatively a
set of unusual experimental facts, in particular, a very strong dichroism observed just in the
NaCa₂Mn₂V₃O₁₂ garnet, where photoinduced changes of all optical properties disappear after
switching off of the irradiation significantly faster than those in other garnets examined. |
| format |
Article |
| author |
Eremenko, V.V. Gnatchenko, S.L. Kachur, I.S. Piryatinskaya, V.G. Ratner, A.M. Shapiro, V.V. Kosmyna, M.B. Nazarenko, B.P. Puzikov, V.M. |
| author_facet |
Eremenko, V.V. Gnatchenko, S.L. Kachur, I.S. Piryatinskaya, V.G. Ratner, A.M. Shapiro, V.V. Kosmyna, M.B. Nazarenko, B.P. Puzikov, V.M. |
| author_sort |
Eremenko, V.V. |
| title |
Photoinduced absorption and anomalous dichroism in NaCa₂Mn₂V₃O₁₂ garnet as an evidence for the formation of oxygen holes dynamics |
| title_short |
Photoinduced absorption and anomalous dichroism in NaCa₂Mn₂V₃O₁₂ garnet as an evidence for the formation of oxygen holes dynamics |
| title_full |
Photoinduced absorption and anomalous dichroism in NaCa₂Mn₂V₃O₁₂ garnet as an evidence for the formation of oxygen holes dynamics |
| title_fullStr |
Photoinduced absorption and anomalous dichroism in NaCa₂Mn₂V₃O₁₂ garnet as an evidence for the formation of oxygen holes dynamics |
| title_full_unstemmed |
Photoinduced absorption and anomalous dichroism in NaCa₂Mn₂V₃O₁₂ garnet as an evidence for the formation of oxygen holes dynamics |
| title_sort |
photoinduced absorption and anomalous dichroism in naca₂mn₂v₃o₁₂ garnet as an evidence for the formation of oxygen holes dynamics |
| publisher |
Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України |
| publishDate |
2005 |
| topic_facet |
Квантовые эффекты в полупpоводниках и диэлектриках |
| url |
http://dspace.nbuv.gov.ua/handle/123456789/121729 |
| citation_txt |
Photoinduced absorption and anomalous dichroism in NaCa₂Mn₂V₃O₁₂ garnet as an evidence for the formation of oxygen holes dynamics / V.V. Eremenko, S.L. Gnatchenko, I.S. Kachur, V.G. Piryatinskaya, A.M. Ratner, V.V. Shapiro, M.B. Kosmyna, B.P. Nazarenko, V.M. Puzikov // Физика низких температур. — 2005. — Т. 31, № 11. — С. 1293-1301. — Бібліогр.: 17 назв. — англ. |
| series |
Физика низких температур |
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| first_indexed |
2025-07-08T20:25:48Z |
| last_indexed |
2025-07-08T20:25:48Z |
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| fulltext |
Fizika Nizkikh Temperatur, 2005, v. 31, No. 11, p. 1293–1301
Photoinduced absorption and anomalous dichroism in
NaCa2Mn2V3O12 garnet as an evidence for the formation
of oxygen holes dynamics
V.V. Eremenko, S.L. Gnatchenko, I.S. Kachur, V.G. Piryatinskaya,
A.M. Ratner, and V.V. Shapiro
B. Verkin Institute for Low Temperature Physics and Engineering of the National Academy
of Sciences of Ukraine, 47 Lenin Ave., Kharkov 61103, Ukraine
E-mail: piryatinskaya@ilt.kharkov.ua
M.B. Kosmyna, B.P. Nazarenko, and V.M. Puzikov
STC «Institute for Single Crystals» of the National Academy of Sciences of Ukraine
60 Lenin Ave., Kharkov 61001, Ukraine
Received April 27, 2005, revised May 25, 2005
It is shown that long-lived photoinduced dichroism in garnets is caused by photoproduced
charges with anisotropic structure, keeping long memory of the pumping light polarization, while
photoinduced absorption is due to all photoproduced charges irrespective of their intrinsic struc-
ture. The charges with anisotropic structure are identified as two-center oxygen holes. The forma-
tion of an oxygen hole is preceded by the excitation of a charge-transfer state with electron par-
tially transferred to a cation C (V5+ for NaCa2Mn2V3O12 garnet) from an adjacent oxygen anion.
To turn this excited state into a free hole state requires some time �hole during which the hole axis
can be reoriented resulting in a diminution of dichroism. The time �hole shortens with increasing
ionization potential of the cation C (very high for V5+). Such a mechanism explains qualitatively a
set of unusual experimental facts, in particular, a very strong dichroism observed just in the
NaCa2Mn2V3O12 garnet, where photoinduced changes of all optical properties disappear after
switching off of the irradiation significantly faster than those in other garnets examined.
PASC: 78.40.-q
1. Introduction
Long-lived photoinduced phenomena in magnetic
insulators have been extensively studied (e.g.,
[1–10]) and associated with the photoinduced transfer
of charges. In ferromagnets or ferrimagnets, whose
Curie temperature exceeds the upper temperature
limit of the existence of long-lived photoinduced phe-
nomena, photoinduced changes in optical properties
can be observed only simultaneously with changes in
magnetic structure (in particular, illumination of the
yttrium iron garnet Y3Fe5O12 with linearly polarized
light affects the magnetic anisotropy [1–3], domain
structure [4], and optical dichroism [3]). The
photoinduced changes in optical and magnetic proper-
ties are deeply interconnected, which strongly compli-
cates their mechanism and hampers its elucidation.
To separate photoinduced changes of optical prop-
erties from those of magnetic characteristics, the au-
thors [5–10] recently examined antiferromagnetic gar-
nets (Ca3Mn2Ge3O12, NaCa2Mn2V3O12 with a low
N�el temperature TN � 20 K) and the paramagnetic
garnet Ca3Ga2–xMnxGe3O12 in a broad temperature
region below the temperature of disappearance of
long-lived photoinduced phenomena (of about 150 K).
In the absence of magnetic ordering, photoinduced op-
tical phenomena in the set of garnets mentioned have
common well-pronounced features, which enabled us
to elucidate their nature [5–8].
For all the garnets examined, Ca3Mn2Ge3O12,
NaCa2Mn2V3O12 and Ca3Ga2–xMnxGe3O12 (x = 0.01
and 0.02), a photoinduced addition to absorption, �K,
and photoinduced dichroism were observed and ana-
© V.V. Eremenko, S.L. Gnatchenko, I.S. Kachur, V.G. Piryatinskaya, A.M. Ratner, V.V. Shapiro, M.B. Kosmyna, B.P. Nazarenko, and
V.M. Puzikov, 2005
lyzed. It was found that the corresponding relaxation
curves, recorded after switching off the irradiation,
are of similar character. Each relaxation curve con-
tains a continuous set of exponential decay compo-
nents with decay time varying in a range from a min-
ute up to many hours (a special analysis proved that
decay times fill in this interval continuously [7]). As
temperature rises, the observed decay kinetics does
not change noticeably, while �K diminishes by an or-
der of magnitude. Such decay kinetics cannot be as-
cribed to some new irradiation-produced optical cen-
ters but is naturally explained by random electric
fields of photoproduced localized charges. These fields
play a dual role: they enhance the optical transition
observed and strongly promote delocalization of holes,
thus accelerating their recombination with negative
charges (localized electrons). A broad continuous set
of decay times is conditioned by a continuous distribu-
tion of random electric fields over magnitude. Such
notion was quantitatively corroborated by the solu-
tion of the corresponding kinetic equation with realis-
tic values of parameters [5,6].
Photoinduced dichroism, observed simultaneously
with photoinduced absorption �K, has the same relaxa-
tion kinetics but, unlike �K, sharply grows when chang-
ing from Ca3Mn2Ge3O12 or Ca3Ga2–xMnxGe3O12 to
NaCa2Mn2V3O12 (when varying the polarization
direction of the pump light, the difference �Kmax �
� �Kmin between the maximum and minimum values
of �K amounts to 90 % of �Kmax for NaCa2Mn2V3O12
and 10 to 20 % only for other mentioned garnets).
These and other facts inspired a mechanism of
long-lived photoinduced dichroism caused by two-cen-
ter oxygen holes whose axis direction keeps long mem-
ory of the pump polarization direction [7,8]. This
mechanism is stated in Sec. 2.
The purpose of the present paper is to corroborate
the proposed mechanism by new experimental data de-
scribed in Sec. 3. The main experimental result con-
sists in separating the optical manifestations of
photoproduced charges of opposite signs: electrons lo-
calized on lattice cations cause photoinduced absorp-
tion only, while two-center oxygen holes are responsi-
ble both for photoinduced dichroism and absorption.
The comparison of dichroism and absorption, caused
by opposite-sign charges, makes it possible to specify
and confirm the proposed mechanism of dichroism re-
corded through two-center holes when pumping a
crystal with polarized light. Such analysis is carried
out in Sec. 4.
2. Main notion to be corroborated by experiment
Prior to the description of experiment, in order to
elucidate its goals, it seems helpful to summarize the
main relevant notion that is based on our previous
results [7,8] and has to be corroborated by new experi-
ments stated in Sec. 3. This notion consists in the fol-
lowing.
(i) The role of oxygen holes as photoproduced
charges in the formation of photoinduced absorption
and dichroism follows from different independent con-
siderations [7,8]. First, the motion and subsequent re-
combination of photoproduced charges is mirrored by
the relaxation kinetics of photoinduced changes after
switching off of the irradiation. A similarity of relax-
ation curves, observed for garnets Ca3Mn2Ge3O12,
Ca3Ga2—xMnxGe3O12 , NaCa2Mn2V3O12 with differ-
ent cation composition and the same anion group O12 ,
suggests a predominant role of the O2� anion subsys-
tem in the motion of photoproduced charges. This
means that irradiation creates holes (O�) moving in
the oxygen subsystem.
Second, there is no alternative for the nature of
photoproduced positive charges. Indeed, in all the gar-
nets examined photoinduced absorption can be excited
by red light with photon energy (of about 2 ev) much
less than the ionization potential of every lattice cat-
ion (> 30 eV) but comparable with the dielectric gap,
Eg, of the O2� sublattice (an estimate Eg � 2 eV follows
from the absorption spectrum given below in Fig. 1).
(ii) The structural features of holes in the oxygen
subsystem of garnets are predetermined by a clo-
sed-shell configuration of O2� ion identical with that of
Ne. It was well established that in solid neon, as well as
in other rare-gas solids, the stable lowest state of the
hole ns2np5 is a two-atom molecule ns2np5
�ns2np6
formed on adjacent lattice sites. The atomic valence
p-hole, distributed between two atoms, realizes a
strong exchange binding of the scale of 1 eV [11,12].
The magnitude of such exchange binding mainly de-
pends on the ratio of the valence p-state radius to the
interatomic distance in the ideal lattice. This ratio
amounts to about 0.3 for the garnets considered, to
0.17 for solid neon, and to 0.25 for solid argon. For al-
kali halides (KCl, KI, NaI), where the existence of
stable two-center holes in the anion subsystem is also
reliably established, this ratio varies from 0.20 to
0.27. Hence, stable two-atom hole polarons must exist
also in the O2�-subsystem of garnets.
(iii) Only charges with anisotropic structure can
retain a long memory of the pumping polarization di-
rection and, hence, be responsible for long-lived
photoinduced dichroism observed. A two-center hole
in the oxygen subsystem is the only realistic version of
charge with anisotropic structure in garnets.
It is easy to trace how two-center holes, created by
polarized illumination, cause dichroism. The latter is
described by the difference, �K� � �K||, in photo-
1294 Fizika Nizkikh Temperatur, 2005, v. 31, No. 11
V.V. Eremenko et al.
induced absorption measured with the probe light po-
larized perpendicular to and parallel to the pump light
polarization. The photoinduced addition to absorp-
tion, �K, is determined by the photoinduced field F
enhancing a weak optical transition in the lattice ions
(obviously, Mn3+ or Mn4+) serving as probe optical
centers. The field F is produced mainly by a two-cen-
ter hole, lying in the first coordination sphere of the
probe ion A, and is perpendicular to the two-center
hole axis (Fig. 2). Since the axis is oriented predomi-
nantly parallel to the pump polarization direction,
probe light experiences a stronger additional absorp-
tion if polarized perpendicularly to the pump polariza-
tion. Hence, �K�>�K|| in accordance with experiment
(see Sec. 3).
(iv) The magnitude of dichroism is obviously dic-
tated by the reorientation of the axes of two-center
holes created by polarized light. Experiment shows
that the reorientation of the axes of two-center holes
after the completion of their creation is not of great
importance. Indeed, a very strong dichroism, much
greater than that in other garnets examined, is ob-
served in NaCa2Mn2V3O12 garnet, where dichroism
disappears after switching off of the irradiation signi-
ficantly faster than in other garnets. Thus, of the most
importance is the reorientation of the axes of two-cen-
ter holes in the course of their creation by polarized
light that occurs in the following way.
The initial excitation, caused by pump light polar-
ized in the z direction, rearranges the binding between
a lattice cation C (identified below as vanadium for
NaCa2Mn2V3O12) and an adjacent anion O2� lying on
the z axis with respect to ion C. Such excitation is of
charge-transfer character: an electron is partially
transferred from O2� to cation C having a high ioniza-
tion potential. Such excited state, denoted as <j = 1|,
is one of 6 degenerate states related to 6 oxygen ions in
the first coordination sphere of ion C. Every excited
state <j| can pass during a time �hole to another type of
excitation, energetically positioned somewhat lower,
with the completed electron transfer: a free hole ap-
pears in the oxygen sublattice and the cation C in-
volved has some charge less by unity than that in regu-
lar sites. Along with this, every excited state <j| can
pass during a time �j to another of 6 degenerate states
<j’| (such transitions are caused by random photo-
induced electric fields absent in the ideal lattice
Hamiltonian with orthogonal eigenstates <j|). Under
pumping with polarized light, the strongest possible
dichroism takes place if �hole<<�j. In the opposite case
�hole>>�j, all the states <j| become equally populated
(as under unpolarized pumping), so that dichroism
vanishes. Thus, the cause of the anomalous dichroism
observed in NaCa2Mn2V3O12 sought be searched in a
small magnitude of the time �hole required for
charge-transfer excitation to be transformed into an
oxygen hole (the time �j, dictated by random fields, is
nearly the same for all garnets). In Sec. 4 it will be
elucidated why �hole takes on the lowest value in just
the NaCa2Mn2V3O12 garnet.
Photoinduced absorption and anomalous dichroism in NaCa2Mn2V3O12
Fizika Nizkikh Temperatur, 2005, v. 31, No. 11 1295
12 14 16 188 10
10
20
0
�
K
, c
m
–
1
30
8 7 6
0
1012
5
10
15
1
2
3 p
u
m
p
in
g
�, 10 nm
2
11 9
�, 10 cm
–13
–
1
K
,1
0
cm
2
Fig. 1. Absorption spectrum of garnet NaCa2Mn2V3O12
with five-valence vanadium at 30 K: without pumping
(curve 1) and the photoinduced addition to absorption un-
der pumping (curve 2); the pumping light frequency is
shown by an arrow, and the small spectral gap near it is
caused by a light filter suppressing the scattered pump
light) [9]. For comparison, dotted line 3 shows the ab-
sorption spectrum of garnet NaCa2Mg2V3O12 with four-va-
lence vanadium [13]. The comparison of the curves 2 and
3 indicates that the photoinduced absorption of
NaCa2Mn2V3O12 at � < 16000 cm�1 is due to V5+-ions tur-
ned to V4+ via the taking away of an electron from O2�.
p
u
m
p
in
g
p
o
la
ri
za
ti
o
n
h
o
lefield
A
Fig. 2. Origin of photoinduced dichroism: a two-center
oxygen hole, allocated in the first coordination sphere of
the probe manganese ion A, creates at the point A electric
field perpendicular to the two-center hole axis and to the
polarization direction of the pump.
3. Experiment
3.1. Experimental technique
Single crystals of NaCa2Mn2V3O12 garnet were
grown from melt solution by the method of spontane-
ous crystallization [10]. A sample was cut in the form
of a (50±10) �m thick plane-parallel plate perpendic-
ular to the [100] direction. Photoinduced phenomena
were examined with an optical double-beam setup.
The sample was illuminated by a He–Ne laser (with
light wavelength of � = 633 nm and flux density of
0.13 W/cm2). A stable wide-band emission of an arc
xenon lamp, dispersed through a monochromator,
served as a probe light. The intensity of the probe
beam was low enough to cause no photoinduced phe-
nomena. A special light filter was applied to suppress
scattered illumination from the laser. The intensity of
probe light passed through the sample was detected by
a photoelectron multiplier. The photoinduced absorp-
tion coefficient is defined as �K = (1/d) ln(I0/I),
where d is the plate thickness, and I0 and I denote the
intensity of the probe beam passed through the plate
being in the ground state or exposed to irradiation, re-
spectively (photoinduced changes in reflection coeffi-
cient are not observed).
The absorption spectrum of the illuminated sample
was registered under pumping lasting 15 min (such
time interval is sufficient for the photoinduced effect
to reach saturation).
To examine photoinduced dichroism, probe light
was polarized in the lattice direction [110] and the
photoinduced addition to absorption coefficient was
measured under irradiation with light polarized paral-
lel (�K||) and perpendicular (�K�) to the probe light
polarization. Photoinduced dichroism is defined as the
difference �K���K||.
3.2. Spectra of photoinduced absorption and
dichroism
The absorption spectrum of the garnet
NaCa2Mn2V3O12, measured in the absence of pump-
ing, is shown in Fig. 1 (curve 1). The figure presents a
long-wavelength tail of a strong absorption that is fast
growing with increasing frequency (the position of the
absorption band maximum, lying in the region of a
very strong absorption, could not be determined). To
all appearance, this absorption band is formed with
the participation of manganese ions. Indeed, the
garnet NaCa2Mg2V3O12, differing from the
NaCa2Mn2V3O12 garnet considered by the replace-
ment of Mn by Mg only, is transparent in the same
spectral region [13]. On the other hand, such a broad
absorption band cannot be assigned to d–d transitions
inside Mn subsystem, which manifest themselves as
narrow absorption bands. The broad absorption band
observed can be attributed to charge transfer transi-
tions between Mn ion and its crystalline surroundings
(probably, adjacent oxygen anions). Note that
photoinduced dichroism considered below is con-
nected with this absorption band.
Figure 1 also presents the spectrum of additional
absorption caused by illumination (curve 2) [9]. For
comparison, the dotted curve shows the absorption
spectrum of the garnet NaCa2Mg2V3O12 exposed to
thermal quenching that lowers the valence of a part of
V5+ cations from 5 down to 4 [13] (in regular crystals
NaCa2Mn2V3O12 and NaCa2Mg2V3O12, vanadium is
present in the form of V5+). According to [13], transi-
tions in V4+ ions manifest themselves in the absorp-
tion band shown by the dotted curve in Fig. 1. A close
similarity of the photoinduced absorption band, ob-
served in NaCa2Mn2V3O12 near � = 14300 cm�1, with
the absorption band of V4+ ions in NaCa2Mg2V3O12,
leads to a conclusion that in the former case this
photoinduced absorption band belongs to photopro-
duced V4+ ions [9]. Thus, the photoproduction of an ox-
ygen hole is realized through the localization of an elec-
tron, taken away from an O2� anion, on a V5+ cation.
Figure 3 presents the spectrum of photoinduced
dichroism described by the difference, �K�� �K||, be-
tween absorption measured with probe light polarized
perpendicular to and parallel to the pump light polar-
ization. As can be seen from the figure, this dichroism
has a maximum at the point �=16700 cm�1 lying
within the absorption band of manganese. Near this
maximum, �K� exceeds �K|| by an order of magnitude.
This provides direct evidence that photoinduced ab-
sorption of probe light by manganese ions is highly
sensitive to the angle between the polarizations of the
probe and pump lights (Sec. 2, Item iii, Fig. 2).
On the contrary, the absorption band of V4+ ions
(shown in Fig. 1) is not sensitive to the polarizations
of the pump and probe rays. Indeed, although the ab-
sorption of V4+ ions makes noticeable contributions to
the curves �K� and �K|| presented in Fig. 3,a, these
contributions are equal and disappear from the differ-
ence curve shown in Fig. 3,b. Really, this difference
curve has no maximum at the point � = 14000 cm�1 re-
lated to the absorption of V4+ ions and distinctly seen
in Fig. 1 (curves 2 and 3). A noticeable dichroism, ob-
served at � � 14000 cm�1, bears no relation to the ab-
sorption of V4+ ions and is due to the long-wavelength
tail of the manganese absorption sensitive to the pump
polarization. We will return to this fact in Sec. 4.
1296 Fizika Nizkikh Temperatur, 2005, v. 31, No. 11
V.V. Eremenko et al.
3.3. Kinetics of photoinduced dichroism
and absorption
Figure 4,a demonstrates the measured dichroism and
its kinetics under changing conditions of pumping. At
first, the photoinduced addition to absorption, �K�, was
measured under pumping polarized perpendicularly to
the probe light polarization. Then, at t = 15 min when
photoinduced dichroism nears its saturation value, the
pump was switched off and the relaxation of �K� was
observed during the next 15 min. At t = 30 min the
perpendicularly polarized pump was switched on
again for 5 min, which was sufficient to achieve the
same level of �K� as before the pump had been
switched off. At t = 35 min the pump polarization di-
rection was switched parallel to the probe light polar-
ization, causing the diminution of �K down to a small
value �K || � 0.1�K�; this indicates on a high degree of
dichroism �K�/ �K|| �10.
For comparison, Fig. 4,b shows the time depend-
ence of the photoinduced absorption observed under
unpolarized pumping at the frequency � = 14300 cm�1,
i.e., at the maximum of the photoinduced absorption
band of V4+ centers insensitive to the polarization of
the pump (see Sec. 2). As seen from the comparison of
Figs. 4,b and 4,a, the polarization of the pump influ-
ences only the magnitude of photoinduced absorption
but not its kinetics under irradiation or after the irra-
diation is switched off. In more detail, such compari-
son will be carried out in Sec. 4.
It is also helpful to compare the relaxation rate of
photoinduced absorption, observed at a low tempera-
ture after switching off of the irradiation in two gar-
nets: in the garnet NaCa2Mn2V3O12 with a strong
dichroism, and in the garnet Ca3Mn2Ge3O12 with a
weak dichroism [5]. To that end, making allowance
for the close similarity between the relaxation kinetics
of photoinduced absorption and dichroism, let us de-
fine the mean relaxation rate as
Rrelax= [�K(0) – �K(20 min)]/ �K(0) (1)
Photoinduced absorption and anomalous dichroism in NaCa2Mn2V3O12
Fizika Nizkikh Temperatur, 2005, v. 31, No. 11 1297
12 14 168 10
�
�
K
–
K
, c
m
–
1
10
20
0
30
40 b
8 7 691012
�
K
, c
m
–
1
10
20
0
30
40 a �K
�K
�, 10 nm
2
11
�, 10 cm–13
Fig. 3. The spectrum of photoinduced addition to absorp-
tion NaCa2Mn2V3O12 under irradiation with light polar-
ized perpendicular and parallel to the probe light polariza-
tion (�K� and �K||, respectively) at 30 K (a). The
difference spectrum, �K�– �K||, describing the dichroism
of NaCa2Mn2V3O12 (b).
unpolarized
pumping
relaxation
4010 20 300
�
K
,c
m
–
1
0
10
20
30
40
50
0
5
10
15
t, min
–1� = 17000 cm
a
b
60
20
�
K
,c
m
–
1
relaxation
–1� = 14300 cm
Fig. 4. Time dependence of the photoinduced addition to
absorption coefficient, �K, at T = 40 K measured at dif-
ferent frequencies: 17000 cm�1 (a) and 14300 cm�1 (b) un-
der pumping conditions changed as indicated in the figure.
The symbol � or || denotes a time interval when the sam-
ple was irradiated with light polarized perpendicular to or
parallel to the probe light polarization. The relaxation re-
gime in the absence of pumping is also shown.
where time is counted from the switching off of the
irradiation. Fig. 4 gives Rrelax � 0.35 for �K
�
in NaCa2Mn2V3O12. The value of Rrelax in
Ca3Mn2Ge3O12, equal to about 0.12 [5,6], indicates
on a considerably slower relaxation.
3.4. Temperature dependence of photoinduced
dichroism and absorption
Figure 5 shows the temperature dependence of
dichroism Kdich �K� – �K|| (�K� and �K|| were mea-
sured under irradiation lasting long enough that �K�
and �K|| become independent of time). For compari-
son, the temperature dependence of the same quantity
for Ca3Mn2Ge3O12 [5] is plotted by the dashed the
line. Figure 5 demonstrates different temperature
behavior of dichroism for NaCa2Mn2V3O12 and
Ca3Mn2Ge3O12. For NaCa2Mn2V3O12, Kdich decreases
with temperature within the total temperature inter-
val examined, quite similarly to photoinduced absorp-
tion �K observed under unpolarized pumping. For
Ca3Mn2Ge3O12, Kdich diminishes with increasing
temperature above the point Tdim = 90 K only, while
the temperature behavior of �K is quite similar for
both garnets. For these garnets, Table 1 presents Tdim
together with the degree of dichroism �K�/ �K|| and
relaxation rate (1).
Table 1. Characteristics of dichroism for the two garnets:
relative degree of dichroism, the commencement Tdim of its
temperature diminution, and the relaxation rate (1).
Garnet �K
�
/�K
||
– 1 T
dim
, K R
relax
NaCa
2
Mn
2
V
3
O
12 9 * 0.35
Ca
3
Mn
2
Ge
3
O
12 0.2 90 0.12
C o m m e n t: *Dichroism decreases with temperature in all of
the region examined
4. Corroboration of the mechanism of
photoinduced dichroism and of the role of
two-center holes
4.1. General conception of the enhancement
of optical transitions by electric fields of
photoproduced charges
As was mentioned in Introduction, long-lived pho-
toinduced phenomena in garnets are generally caused
by the enhancement of optical transitions in the man-
ganese subsystem by the electric field of photo-
produced charges [5,6]. In NaCa2Mn2V3O12 garnet,
along with such photoinduced contribution to absorp-
tion at �
16000 cm�1 (Fig. 3), irradiation creates V4+
centers which give rise to the photoinduced absorption
band near 14000 cm�1 (Fig. 1). The proposed mecha-
nism of photoinduced absorption is illustratively cor-
roborated by a comparison of these photoinduced con-
tributions to absorption in NaCa2Mn2V3O12 garnet.
Under irradiation with polarized light, an
anisotropic electric field of two-center holes, influenc-
ing optical transitions in the manganese subsystem,
causes a sharp dichroism of photoinduced absorption
at � � 17000 cm�1 (Fig. 3). Note that these transitions
can occur in all manganese ions being, enhanced by
the applied field by a value proportional to the num-
ber of charges. But the V4+-absorption, proportional
to a small photoproduced portion of V4+ ions, is prac-
tically insensitive to the field of photoproduced
charges: this field changes the V4+-absorption by a
negligibly small value quadric in the number of
charges. Being insensitive to the anisotropic field of
photoproduced two-center holes, the V4+-absorption
exhibits no dichroism, is was spectroscopically evi-
denced in Sec. 3.2.
Thus, two different ions, labeled as C and A, are in-
volved in photoinduced phenomena. Ion C (V5+ for
NaCa2Mn2V3O12 garnet) participates in the creation
of oxygen hole O� via the taking away of an electron
from an adjacent O2� cation. The created hole O� un-
dergoes two-site self-trapping and turns to a two-cen-
ter hole with the initial orientation of the axis. Then
1298 Fizika Nizkikh Temperatur, 2005, v. 31, No. 11
V.V. Eremenko et al.
0 50 100 150
T, K
10
20
0
30
40
50
60
70
80
90
�
K
, c
m
–
1
Fig. 5. Temperature dependence of photoinduced dichroism
�K�� �K|| measured for NaCa2Mn2V3O12 under polarized
pumping (squares). The temperature dependence of
photoinduced absorption measured under unpolarized
pumping (circles) is of the same character. For comparison,
the corresponding dependences of photoinduced dichroism
and photoinduced absorption for garnet Ca3Mn2Ge3O12 [5]
are shown by dashed and solid lines, respectively.
the two-center hole, retaining its axis direction, gets to
the first coordination sphere of the probe ion A ( Mn)
sensitive to the electric field of the hole and, hence, to
its axis direction, which conditions dichroism. Note that
V5+ ions with a very high ionization potential cannot
play the role of probe ion A, since the oxygen holes
created are more strongly attracted by Mn ions, with a
much lower ionization potential.
4.2. Creation of oxygen holes through complete
transfer of electron from O2� anion to V5+ cation
As was shown in Sec. 3.2, an oxygen hole is created
through the taking away of an electron from a O2� an-
ion and localizing it on a V5+ cation. Hence it follows
that the number of photoproduced oxygen holes coin-
cides with that of V4+ ions. This coincidence can be
proved by the comparison of the absorption band of
V4+ ions, created by unpolarized pumping with maxi-
mum at � � 14000 cm�1 (Fig. 1), with the absorption
band of two-center oxygen holes created by pumping
with light polarized perpendicularly to the polarization
of the probe light (the reorientation of two-center holes
after their creation can be neglected; see Sec. 4.3). The
ratio of the maximum ordinates of these absorption
bands, � = �K�(17000)/�K (14300), must be inde-
pendent of the number of holes and, hence, of temper-
ature and irradiation time. Figure 6 presents the ratio
� = �K�(17000)/�K (14300) observed under pump-
ing and in the course of the subsequent relaxation.
This ratio, measured under pumping with the same in-
tensity, was found, within the accuracy of measure-
ments, to be independent of temperature in the region
examined, 40K � T � 80 K. After switching off of the
irradiation, � increases slightly, which provides an ad-
ditional evidence for the reorientation of holes on the
stage of their creation (see Sec. 4.4, Item ii).
The constancy of the ratio � confirms that oxygen
holes are created through the trapping of an electron,
taken away from an O2�-ion, by an adjacent vanadium
cation of the lattice.
4.3. Conservation of the orientation of two-center
holes after their formation
Experiment does not detect the reorientation of the
axes of two-center holes after the completion of their
formation. Indeed, as seen from Fig. 6, after switching
off of the irradiation, the dichroism relaxes (disap-
pears) with the same rate (or even slightly slower)
than the number of photoproduced charges. If this re-
laxation were accompanied by the reorientation of the
axes of two-center holes, the dichroism would dimin-
ish faster than the number of photoproduced charges,
and the ratio � = �K� (17000)/�K (14300), pre-
sented in Fig. 6, would be a decreasing function of
time after the switching off of the irradiation.
Thus, Fig. 6 demonstrates that the time, �reor, re-
quired for a two-center hole to be reoriented, greates
exceeds the time of the hole–electron recombination
dictated by the hole hopping time �hop:
�reor>> �hop. (2)
Note that for two-center holes observed in alkali
halide crystals the inequality (2) was experimentally
established as well [14–16].
The physical reason for the relation (2) is eluci-
dated by Fig. 7. Initially, a two-center hole was lo-
cated on atoms A and B indicated by bold circles; the
orbital of the p-hole at every atom (the absent p-elec-
tron) is schematically shown by thin line. Immedi-
ately after a hop, the two-center hole occupies a new
position BC or BD with the same orientation of the
axes of the atomic p-holes. (In the latter case, the sub-
sequent reorientation of the hole axis in the direction
BD does not affect the hopping probability). The hop
creating hole BD is much less probable than the hop
Photoinduced absorption and anomalous dichroism in NaCa2Mn2V3O12
Fizika Nizkikh Temperatur, 2005, v. 31, No. 11 1299
40 K
60 K
80 K
10 20 300
t, min
5 15 25
0
2
4
0
2
4
0
2
4
�
Fig. 6. Ratio � = �K� (17000 cm�1)/�K(14300 cm�1)
measured under pumping and in the course of the subse-
quent relaxation at different temperatures (the moment of
switching off of the irradiation is indicated by an arrow).
This ratio mainly reproduces the ratio of the number of
photoproduced oxygen holes to that of photoproduced
V4+-ions. The approximate constancy of � confirms that
oxygen holes are created through the trapping of an elec-
tron, taken away from an O2�-ion, by an adjacent V5+-cat-
ion (a slight enhancement of � after the switching off of
the irradiation is explained in Sec. 4.4, Item (ii) ).
creating hole BC. Indeed, the probability of hopping
sharply depends on the exchange interaction between
atom B and the adjacent atom C or D [17], the ex-
change BC being much stronger due to a greater over-
lap of wave-functions.
4.4. Reorientation of the axes of oxygen holes in the
course of their creation by polarized light
Experimental data corroborate the reorientation
mechanism, stated in Sec. 2 (Item 4). This mechanism
involves excited states <j| (j = 1,..., 6) formed by a
partial electron transfer to cation C from any of 6 ad-
jacent O2� ions. The efficiency of this mechanism is
characterized by the time ratio
u / j� � �hole (3)
where �j is the time of the transition between degener-
ate states <j|; the time �hole is required for a complete
electron transfer to ion C, resulting in the formation of
a free oxygen hole. (As was shown in Sec. 4.2, for
NaCa2Mn2V3O12 the ion C should be identified with
V5+). Let us trace how the variation of the numerator
and denominator in (3) affects the dichroism.
(i) The reorientation of two-center holes is due to
transitions between its excited states caused by ran-
dom electric fields of photoproduced charges (in the
ideal lattice, the degenerate states <j| are orthogonal
to one another). These fields have a large straggling in
magnitude, which is mirrored by the relaxation kinet-
ics after switching off of the irradiation (Fig. 4):
two-center holes, formed at the places of a strong
field, disappear rapidly and form the initial steep part
of the relaxation curve, while its gently sloping part
corresponds to holes formed at the places of a weak
field. In the latter case, the reorientation rate, com-
mensurable with the magnitude of random fields, is
significantly lower. Thus, the gently sloping part of
the relaxation curve pertains to holes that have under-
gone a weak reorientation in their creation stage. This
is mirrored by a slight enhancement of the ratio � =
� �K� (17000)/�K (14300) with an increase of time
counted from the moment of switching off of the irra-
diation (Fig. 6).
(ii) Figure 5 presents the temperature dependences
of photoinduced absorption and dichroism for
NaCa2Mn2V3O12 (marks) and Ca3Mn2Ge3O12 (lines).
As seen from the figure, photoinduced absorption and
dichroism vary with temperature in a similar way for
NaCa2Mn2V3O12 and in a different manner for
Ca3Mn2Ge3O12 . Such a distinction between two gar-
nets can be explained in terms of the ratio (3). An in-
crease of temperature promotes overcoming the energy
barrier between the initial excited state <j| and the
state with a free hole, so that the hole creation time
�hole must shorten with increasing temperature. On
the contrary, the time, �j , of the transitions between
the degenerate excited states is determined by random
fields which cannot noticeably depend on tempera-
ture. Hence, an increase of temperature entails a
decrease of the ratio (3) and a diminution of the reori-
entation of two-center holes in the stage of their cre-
ation, which compensates the temperature accelera-
tion of the recombination of photoproduced charges
after their creation. In the case of garnet
Ca3Mn2Ge3O12, where the reorientation mechanism
acts very efficiently (see Table 1), this effect is well
pronounced and results in a weakened temperature
dependence of dichroism within a rather broad tem-
perature interval T < Tdim = 90 K (Fig. 5). But for
NaCa2Mn2V3O12, the reorientation mechanism ma-
nifests itself very weakly and cannot cause a notice-
able difference between the temperature dependences
of dichroism and photoinduced absorption. Thus,
the comparison of photoinduced phenomena in
NaCa2Mn2V3O12 and Ca3Mn2Ge3O12 (a different tem-
perature behavior of dichroism explainable only in
terms of the initial stage of the oxygen hole formation)
corroborates the mechanism of the creation and reorien-
tation of two-center holes stated in Sec. 2, Item iv.
(iii) A sharp difference in dichroism, observed in
NaCa2Mn2V3O12 and Ca3Mn2Ge3O12 (see Table 1),
can be also understood in terms of relation (3) involv-
ing excited states <j| with a partial electron transfer to
the ion C from adjacent O2� ions. The degree of this
electron transfer is dictated by the attractive potential
of the ion C, which is commensurate with the ioniza-
tion potential IC of the separate ion C. On the other
hand, the greater is the portion of an electron trans-
ferred to the cation C from an adjacent O2� anion in
an excited state, the easier the electron can be comp-
1300 Fizika Nizkikh Temperatur, 2005, v. 31, No. 11
V.V. Eremenko et al.
A B C
D
Fig. 7. Motion of a two-center hole, initially allocated on
the atoms A and B, to a next position BC or BD. The mo-
tion rate is determined by the overlap of the atomic p-or-
bital not occupied at the atom B (thin line) with its or-
bital occupied at adjacent atoms (bold line); hence, the
hop AB–BD has much lower probability. In the latter case
the hop is followed (in order to lower the resonance en-
ergy) by a reorientation the axes of the atomic p-holes
and, hence, of the axis of the two-center hole.
letely localized at the ion C, resulting in the genera-
tion of a free oxygen hole. Thus, with an increase of IC
the hole formation time �hole shortens, which leads to
a diminution of the oxygen hole reorientation and to
the corresponding enhancement of dichroism. For gar-
nets Ca3Mn2Ge3O12 and NaCa2Mn2V3O12, Table 2
presents the nth ionization potential of each n-valence
cation taken in a free state. The cation V5+, playing
the role of the ion C in NaCa2Mn2V3O12, has ioni-
zation potential of 65 eV, significantly exceeding
the ionization potential of every cation of
Ca3Mn2Ge3O12. The corresponding difference in the
hole formation time �hole explains the strong differ-
ence in dichroism observed in NaCa2Mn2V3O12 and
Ca3Mn2Ge3O12.
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Photoinduced absorption and anomalous dichroism in NaCa2Mn2V3O12
Fizika Nizkikh Temperatur, 2005, v. 31, No. 11 1301
Table 2. Ionization potentials of free ions of garnet constituents
Garnet Ca
3
Mn
2
Ge
3
O
12 NaCa
2
Mn
2
V
3
O
12
Cation Ca2+ Mn3+ Ge4+ Na+ Ca2+ Mn2+ C=V5+
Ionization potential (eV) I
2
=11.9 I
3
=33.7 I
4
=45.7 I
1
=5.1 I
2
=11.9 I
2
=15.6 I
5
=65.3
|