Evolution of a band insulating phase from a correlated metallic phase
Evolution of a band insulating phase from a correlated metallic phase
Kalobaran Maiti,∗ Ravi Shankar Singh, and V.R.R. Medicherla
Department of Condensed Matter Physics and Materials’ Science,
Tata Institute of Fundamental Research, Homi Bhabha Road, Colaba, Mumbai - 400 005, INDIA
(Dated: October 30, 2018)
We investigate the evolution of the electronic structure in SrRu1−xTixO3 as a function of x using
high resolution photoemission spectroscopy, where SrRuO3 is a weakly correlated metal and SrTiO3
is a band insulator. The surface spectra exhibit a metal-insulator transition at x = 0.5 by opening
up a soft gap. A hard gap appears at higher x values consistent with the transport properties. In
contrast, the bulk spectra reveal a pseudogap at the Fermi level, and unusual evolution exhibiting
an apparent broadening of the coherent feature and subsequent decrease in intensity of the lower
Hubbard band with the increase in x. Interestingly, the first principle approaches are found to be
sufficient to capture anomalous evolutions at high energy scale. Analysis of the spectral lineshape
indicates strong interplay between disorder and electron correlation in the electronic properties of
this system.
PACS numbers: 71.10.Hf, 71.20.-b, 71.30.+h
The investigation of the role of electron correlation in
various electronic properties is a paradigmatic problem
in solid state physics. Numerous experimental and the-
oretical studies are being performed on correlated elec-
tron systems revealing exotic phenomena such as high
temperature superconductivity, giant magnetoresistance
etc. Electron correlation essentially localizes the valence
electrons leading the system towards insulating phase.
Correlation induced insulators, known as Mott insulators
are characterized by a gapped electronic excitations in a
system where effective single particle approaches provide
a metallic ground state. The band insulators represent
insulating phase described within the single particle ap-
proaches. Strikingly, some recent theoretical studies re-
veal a correlation induced metallic ground state in a band
insulator using ionic Hubbard model [1, 2, 3, 4]. Such un-
usual transition has been observed in two dimensions by
tuning effective electron correlation strength, U/W (U
= electron-electron Coulomb repulsion strength, W =
bandwidth) and the local potential, ∆.
In order to realize such effect experimentally, we in-
vestigate the evolution of the electronic structure in
SrRu1−xTixO3 as a function of x, where the end mem-
bers, SrRuO3 and SrTiO3 are correlated ferromagnetic
metal and band insulator, respectively. Ti remains in
tetravalent state in the whole composition range having
no electron in the 3d band[5, 6]. Thus, in addition to
the introduction of disorder in the Ru-O sublattice, Ti-
substitution at the Ru-sites dilutes Ru-O-Ru connectiv-
ity leading to a reduction in Ru 4d bandwidth, W and
hence, U/W will increase. Transport measurements[7]
exhibit plethora of novel phases such as correlated metal
(x ∼ 0.0 ), disordered metal (x ∼ 0.3), Anderson insu-
lator (x ∼ 0.5), soft Coulomb gap insulator (x ∼ 0.6),
disordered correlated insulator (x ∼ 0.8), and band insu-
lator (x = 1.0).
In this study, we have used high resolution photoemis-
sion spectroscopy to probe the density function in the
vicinity of the Fermi level, ǫF and at higher energy scale
as well. Considering the fact that escape depth of the
photoelectrons is small, we have extracted the surface
and bulk spectra in every case by varying the surface
sensitivity of the technique. The surface spectra exhibit
signature of disorder at lower x values in SrRu1−xTixO3,
a metal-insulator transition exhibiting a soft gap at ǫF
for x = 0.5 and a hard gap for higher x. The bulk spec-
tra, on the other hand, reveal an unusual spectral weight
transfer and signature of a pseudogap at ǫF at higher x.
Photoemission measurements were performed using
Gammadata Scienta analyzer, SES2002 and monochro-
matized photon sources. The energy resolution for x-
ray photoemission (XP) and He II photoemission mea-
surements were set at 300 meV and 4 meV, respectively.
High quality samples of SrRu1−xTixO3 with large grain
size were prepared following solid state reaction route
using high purity ingredients[8] followed by a long sin-
tering (for about 72 hours) at the final preparation tem-
perature. Sharp x-ray diffraction patterns reveal single
phase in each composition with no signature of impurity
feature. Magnetic measurements using a high sensitiv-
ity vibrating sample magnetometer exhibit distinct fer-
romagnetic transition at each x up to x = 0.6 studied,
as also evidenced by the Curie-Weiss fits in the param-
agnetic region. The fits provide an estimation of effec-
tive magnetic moment (µ = 2.8 µB , 2.54 µB, 2.45 µB,
2.18 µB, 2.19 µB, 1.95 µB and 1.93 µB) and Curie tem-
perature (θP = 164 K, 156.6 K, 150.6 K, 145.3 K, 139 K,
138.6 K and 100 K) for x = 0.0, 0.15, 0.2, 0.3, 0.4, 0.5
and 0.6, respectively. The values of µ and θP for SrRuO3
are observed to be the largest among those available in
the literature and corresponds to well characterized sin-
gle crystalline materials[9].
In Fig. 1(a), we show the XP valence band spectra
exhibiting 4 distinct features marked by A, B, C and
D. The features C and D appear beyond 2.5 eV and
have large O 2p character as confirmed experimentally
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FIG. 1: (color online) (a) XP valence band spectra of
SrRu1−xTixO3 for various values of x. Solid line represents
the O 2p part for x = 0.6. (b) Ru 4d spectra after the sub-
traction of the O 2p contributions as shown in (a). (c) Ru 4d
band obtained from He II spectra.
by changing photoemission cross-sections [10] and theo-
retically by band structure calculations [11]. The peaks
A and B appear primarily due to the photoemission from
electronic states having Ru 4d character. The O 2p part
remains almost the same in the whole composition range
as expected. While Ru 4d intensity gradually diminishes
with the decrease in Ru-concentrations, the lineshape of
Ru 4d band exhibits significant redistribution in inten-
sity. In order to bring out the clarity, we delineate the
Ru 4d band by subtracting O 2p contributions. The sub-
tracted spectra, normalized by integrated intensity un-
der the curve, exhibit two distinct features as evident in
Fig. 1(b). The feature A corresponds to the delocalized
electronic density of states (DOS) observed in ab initio
results and is termed as coherent feature. The feature B,
absent in the ab initio results[11], is often attributed to
the signature of correlation induced localized electronic
states forming the lower Hubbard band and is known
as incoherent feature. The increase in x leads to a de-
crease in intensity of A and subsequently, the intensity
of B grows gradually. Since the bulk sensitivity of valence
electrons at 1486.6 eV photon energy is high (∼ 60%),
the spectral evolution in Fig. 1(b) manifests primarily
the changes in the bulk electronic structure.
In order to discuss the effect due to the surface elec-
tronic structure, we show the Ru 4d contributions ex-
tracted from the He II spectra in Fig. 1(c), where the
surface sensitivity is about 80%. Interestingly, all the
spectra are dominated by the peak at higher binding en-
ergies (> 1 eV) corresponding to the surface electronic
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FIG. 2: (color online) S(ǫ) obtained from (a) He II and (b)
XP spectra of SrRu1−xTixO3. (b) S(ǫ) in (a) are plotted as
a function of (c) |ǫ− ǫF |
0.5 (d) and |ǫ− ǫF |
1.25. S(ǫ) obtained
from (e) XP and (f) He II spectra of Ca1−xSrxRuO3.
structure as reported in the case of SrRuO3 and the co-
herent feature intensity corresponds essentially to the
bulk electronic structure[10, 12]. The coherent feature
intensity reduces drastically with the increase in x and
becomes almost negligible at x = 0.6. This can be vi-
sualized clearly in the spectral density of states (SDOS)
obtained by symmetrizing (S(ǫ) = I(ǫ) + I(−ǫ); I(ǫ) =
photoemission spectra, ǫ = binding energy) the He II and
XP spectra. The SDOS corresponding to He II spectrum
of SrRuO3 shown in Fig. 2(a) exhibits a sharp dip at ǫF ,
which increases gradually with the increase in x. The
SDOS corresponding to XP spectra in Fig. 2(b), how-
ever, exhibits a peak in SrRuO3 presumably due to large
resolution broadening and intense coherent feature. This
peak loses its intensity and becomes almost flat for x =
0.15 and 0.2. Further increase in x leads to a pseudogap
at ǫF , which gradually increases with the increase in x.
Both these results clearly indicate gradual depletion of
SDOS at ǫF with the increase in Ti-substitution.
The effect of resolution broadening of 4 meV in the He
II spectra is not significant in the energy scale shown in
the figure. The electron and hole lifetime broadening is
also negligible in the vicinity of ǫF . Thus, S(ǫ) in Fig.
2(a) provide a good testing ground to investigate evolu-
tion of the spectral lineshape at ǫF . The lineshape of
S(ǫ) in Fig. 2(a) exhibits significant modification with
the increase in x. We, thus, replot S(ǫ) as a function
of |ǫ − ǫF |
α for various values of α. Two extremal cases
representing α = 0.5 and 1.25 are shown in Fig. 2(c) and
2(d), respectively. It is evident that S(ǫ) of SrRuO3 ex-
hibit a straight line behavior in Fig. 2(c) suggesting sig-
nificant role of disorder in the electronic structure. The
influence of disorder can also be verified by substitutions
at the A-sites in the ABO3 structure. This has been ver-
ified by plotting SDOS obtained from the XP and He II
spectra of Ca1−xSrxRuO3 in Fig. 2(e) and 2(f), respec-
tively. Here, the electronic properties of the end mem-
bers, SrRuO3 and CaRuO3 are known to be strongly in-
fluenced by the disorder[13]. Substitution of Sr at the
Ca-sites is expected to enhance the disorder effect. The
lineshape of S(ǫ) in both Fig. 2(e) and 2(f) remains al-
most the same across the whole composition range. Such
disorder induced spectral dependence is consistent with
the observations in other systems[14, 15] as well.
Interestingly, the lineshape modifies significantly with
the increase in x and becomes 1.25 in the 60% Ti substi-
tuted sample. Ti substitution introduces defects in the
Ru-O network, where Ti4+ having no d-electron, does
not contribute in the valence band. Thus, in addition to
the disorder effects, the reduced degree of Ru-O-Ru con-
nectivity leads to a decrease in bandwidth, W , which in
turn enhances U/W . In systems consisting of localized
electronic states in the vicinity of ǫF , a soft Coulomb gap
opens up due to electron-electron Coulomb repulsion; in
such a situation, the ground state is stable with respect to
single-particle excitations, when SDOS is characterized
by (ǫ − ǫF )
2-dependence [16, 17]. Here, gradual increase
in α with the increase in x in the intermediate compo-
sitions is curious and indicates strong interplay between
correlation effect and disorder in this system.
The extraction of surface and bulk spectra requires
both the XP and He II spectra collected at significantly
different surface sensitivities. Thus, we broaden the He II
spectra upto 300 meV and extract the surface and bulk
spectra analytically using the same parameters as used
before for CaSrRuO3 system[10]. The surface spectra
shown in Fig. 3(b) exhibit a gradual decrease in coherent
feature intensity with the increase in x and subsequently,
the feature around 1.5 eV becomes intense, narrower and
slightly shifted towards higher binding energies. The de-
crease in intensity at ǫF is clearly visible in the sym-
metrized spectra, S(ǫ) shown in Fig. 3(d). Interestingly,
S(ǫ) of x = 0.5 sample exhibits a soft gap at ǫF and
a hard gap appears in S(ǫ) corresponding to higher x.
This spectral evolution is remarkably consistent with the
transport properties[7]. These results corresponding to 2-
dimensional surface states presumably have strong impli-
cation in realizing recent theoretical predictions[1, 2, 3, 4]
and the bulk properties of this system.
The picture is strikingly different in the bulk spectra
where the electronic structure is 3-dimensional. The bulk
spectrum of SrRuO3 exhibits an intense and sharp coher-
ent feature in the vicinity of ǫF and the incoherent fea-
ture appears around 2 eV. The enhancement of U/W due
to Ti substitution is expected to increase the incoherent
feature intensity. In sharp contrast, the intensity of the
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FIG. 3: (color online) Extracted (a) bulk and (b) surface
spectra of SrRu1−xTixO3 for various values of x. The SDOS
obtained from bulk and surface spectra are shown in (c) and
(d), respectively.
2 eV feature reduces significantly and the coherent fea-
ture becomes broad. In addition, the bulk spectra of all
the intermediate compositions appear very similar. The
symmetrized bulk spectra shown in Fig. 3(c) exhibit a
small lowering of intensity at ǫF with the increase in x.
Since, U is weak in these highly extended 4d
systems[10, 12], a perturbative approach may be use-
ful to understand the role of electron correlation in the
spectral lineshape. We have calculated the bare density
of states (DOS) for SrRuO3 and SrRu0.5Ti0.5O3 using
state-of-the-art full potential linearized augmented plane
wave method[11, 18]. The self energy and spectral func-
tions were calculated using this t2g partial DOS as done
before[19]. The real and imaginary parts of the self en-
ergy are shown in Fig. 4(a) and 4(b), and the spectral
functions for different U values are shown in Fig. 4(c) and
4(d) for SrRuO3 and SrRu0.5Ti0.5O3, respectively. The
increase in U leads to a spectral weight transfer outside
the LDA DOS width creating the lower and upper Hub-
bard bands. Subsequently, the total width of the LDA
DOS diminishes gradually. While these results exhibit
similar scenario as that observed in the most sophisti-
cated calculations using dynamical mean field theory, the
separation between the lower and upper Hubbard bands
is significantly larger than the corresponding values of U .
It is important to note here that the band structure cal-
culations include the electron-electron interaction term
within the local density approximations. The perturba-
tion calculations in the present case essentially provide
an estimation of the correction in U already included in
the effective single particle Hamiltonian.
In order to compare with the experimental spectra, the
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FIG. 4: (color online) Real and imaginary parts of the self
energy of (a) SrRuO3 and (b) SrRu0.5Ti0.5O3 obtained by
second order perturbation method following the method of
Treglia et al.[19]. Spectral functions for various values of U of
(c) SrRuO3 and (d) SrRu0.5Ti0.5O3. Calculated experimen-
tal spectra for different values of U of (e) SrRuO3 and (f)
SrRu0.5Ti0.5O3.
calculated spectral functions are convoluted by Fermi-
Dirac distribution function and the gaussian representing
the resolution broadening of 300 meV. The comparison is
shown in Figs. 4(e) and 4(f). Interestingly, the spectral
shape corresponding to U = 0.6 ± 0.1 exhibits remark-
able representation of the experimental bulk spectra in
both the cases. These results clearly establish that per-
turbative approaches and local description of the corre-
lation effects are sufficient to capture electronic structure
of these weakly correlated systems. The overall narrow-
ing of the valence band observed in the substituted com-
pounds are essentially a single particle effect and can be
attributed to the reduced degree of Ru-O-Ru connectiv-
ity in these systems. While the high energy scale features
are reproduced remarkably well within this picture, the
occurrence of a pseudogap at ǫF with increasing x (not
visible in Fig.4 due to large energy scale) suggests in-
creasing role of disorder.
In summary, the high resolution spectra of SrRuO3 ex-
hibit signature of disorder in the vicinity of the Fermi
level. Introduction of the Ti4+ sublattice within the
Ru4+ sublattice provides a paradigmatic example, where
the charge density near Ti4+ sites is close to zero and each
Ru4+ site contributes 4 electrons in the valence band.
Such large charge fluctuation leads to a significant change
in spectral lineshape and a dip appears at ǫF (pseudo-
gap). Interestingly, the effects are much stronger in the
two dimensional (surface) electronic structure leading to
a soft gap at 50% substitution and eventually a hard gap
appears. Bulk electronic structure (3-dimensional), how-
ever, remains less influenced. A theoretical understand-
ing of these effects needs consideration of strong disorder
in addition to the electron correlation effects.
∗ Corresponding author: kbmaiti@tifr.res.in
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