Effect of strain on the optical properties of LaNiO3: A first-principle study

Computational Materials Science 112 (2016) 113–119

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Effect of strain on the optical properties of LaNiO3: A first-principle study D. Misra, T. K. Kundu ⇑ Department of Metallurgical and Materials Engineering, Indian Institute of Technology Kharagpur, Kharagpur 721302, India

a r t i c l e

i n f o

Article history: Received 18 April 2015 Received in revised form 14 October 2015 Accepted 16 October 2015

Keywords: Density functional theory Strain Strongly correlated Electronic structure Optical properties

a b s t r a c t Optical properties of the pseudo-cubic lanthanum nickel oxide (LaNiO3) have been investigated using first-principle density-functional theory under unstrained and strained condition. To incorporate the effect of strong electron correlation in LaNiO3, the generalized gradient approximation + Hubbard U (GGA+U) approach is used. Electronic structure and the optical properties of pure LaNiO3, namely optical conductivity, refractive index, dielectric function and the reflectance have been studied in detail. The non-vanishing density of states at the Fermi level are found to come from the strong hybridization between Ni 3d and O 2p orbitals, which ascertain the metallic nature of LaNiO3. The optical conductivity spectra have a dominant Drude contribution at low energy, and the high energy region is governed by several inter-band transitions. The changes in the optical properties on application of in-plane tensile and compressive strains are ascribed to the altered electronic structure of the system. Our observation reveals that systems under both tensile and compressive strains are metallic albeit strongly correlated. However LaNiO3 under tensile strain is more strongly correlated than under compressive strain. Ó 2015 Elsevier B.V. All rights reserved.

1. Introduction The rare-earth nickelates family (RNiO3; R = rare-earth element) has been a field under active investigation over the past two decades due to their intriguing structural and electronic properties [1,2]. While most of the nickelates (R – La) display a sharp temperature-controlled metal to insulator transition from a high temperature paramagnetic metal to a low temperature antiferromagnetic insulator at a certain temperature TMI [3,4], LaNiO3 is the only exception in the nickelates series, which remains a paramagnetic metal albeit strongly correlated, down to the low temperatures, and never exhibits any metal–insulator transition [5–7]. Due to its metallic nature at room temperature, LaNiO3 has found a widespread use in the field of oxide electronics as a stable electrode [8]. Other technological interest in this compound extends from ferroelectric thin film devices including non-volatile memories, to opto-electronic and magneto-electronic devices [2,9,10]. A large absorption coefficient of LaNiO3 enables it to be a potential candidate for the thermal absorption layer in infrared detectors also [11]. Efficient application of this material in these broad advanced fields demands a clear insight of its electronic ⇑ Corresponding author. E-mail addresses: [email protected] (D. Misra), [email protected]. ernet.in (T.K. Kundu). http://dx.doi.org/10.1016/j.commatsci.2015.10.021 0927-0256/Ó 2015 Elsevier B.V. All rights reserved.

structure and the ability to control its properties by doping, and by applying electric field, mechanical stress, etc. Recently, experiments performed on thin films of LaNiO3 [12–14] claimed that this compound is very sensitive to strain, doping and electric field. These parameters are very effective in altering the physical properties of LaNiO3, hence helping this compound to have a significant niche in device application. Till date, optical study has been immensely helpful in exploring the electronic properties and structure of a compound, and many experimental studies of the optical properties of LaNiO3 thin films are reported [12,13,15]. However, very little has been known about the theoretical understanding of the variation of the optical properties of LaNiO3 under strain. This paper intends to give a clear insight on all the optical properties of the system including optical conductivity, complex dielectric function, extinction coefficient, reflectance and refractive index, and the effect of strong electron correlation on the optical properties of LaNiO3, under the realm of density functional theory (DFT). We have also studied the effect of strain on the electronic properties of LaNiO3, as in-plane strain is proved to have a key control over the physical properties of LaNiO3 [13,14]. We explained how tensile and compressive strains modify the electronic structure of the system, which is reflected in the altered optical spectra. Our first-principle calculation has revealed the presence of strong electron correlation in LaNiO3 and the remarkable change it undergoes on application of strain. The advantage of using strain

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engineering on LaNiO3 for suitable device applications are discussed thereof. 2. Computational details A first-principle calculation was performed using the MedeAVASP (Vienna abinitio simulation package) software [16] for the pseudo-cubic LaNiO3. It has been well known that LaNiO3 has a minimum distortion and hence the maximum tolerance factor (t) among the nickelates; the crystal tolerance factor in nickelates is defined as the ratio of R–O to Ni–O bond lengths. The maximum value of (t) is unity, which describes an ideal cubic structure and less for the distorted one [1]. A minimum distortion in the LaNiO3 crystal structure prompted us to adopt a pseudo-cubic notation for LaNiO3. In the cubic structures, La atoms sit at the corners, Ni at the body-centre and O atoms at the face-centred positions. The crystal structure with different atomic positions and the related axes used in our calculation are shown in Fig. 1. A correlated metallic oxide with narrow bandwidth is beyond the scope of LDA and GGA

approaches. Even the LSDA technique fails considerably in the regime of strong electron correlation. As LaNiO3 is claimed to be a correlated metal, we went beyond the conventional DFT calculation and used the DFT+U approach using GGA-PBE functional to incorporate the effect of strong electron correlation. Apart from the exchange correlation terms considered in the GGA approach, the energy functional in GGA+U calculation is expanded to include the onsite Hubbard U term as well as the Hunds coupling J. It treats the effective electron interaction U eff to be (U–J) and reproduces the correct ground states [17]. In case of our GGA+U calculation, we used U eff to be 3 eV, as also mentioned in an earlier report [17]. A 600 eV plane wave cut off with a 7
7
7 k-mesh, centred at the C point, was used to optimize the unit cell. We started our calculation using the unit cell information given in an earlier study [18]. The structure optimization was carried out using the tetrahedron method and the convergence of less than 104 eV was achieved. Different in-plane strains, both tensile and compressive in nature, were applied on pseudo-cubic LaNiO3 within DFT framework. The unit cell was kept fixed in each of the calculations and

(a)

(b)

(c)

Fig. 1. (a) Crystal structure, (b) Fermi surface, and (c) band structure of a pseudo-cubic LaNiO3 system.

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the atomic positions were fully optimized. Optical conductivity, absorption index and refractive index for different energies using GGA+U approach, both for the unstrained system, and systems under different tensile and compressive strains were then extensively investigated.

The crystal structure and the band dispersion of cubic LaNiO3 are shown in Fig. 1. The nominal electronic configuration of Ni in LaNiO3 is 3d7, with a filled triply degenerate t2g orbital and a quarter filled doubly degenerate eg orbital. The eg band formed out of the hybridization between Ni 3d and O 2p orbitals are found to be degenerate along the R–C symmetry direction and crossed the Fermi level. A small electron pocket seen to be appeared at the C point is also confirmed from the Fermi surface plot (Fig. 1). The presence of the electron pocket at the C point and a large hole Fermi surface around R point as observed in Fig. 1, has been also reported previously [18,19]. The degenerate eg bands crossing the Fermi level, as shown in Fig. 1, are of utmost importance as they contribute towards the non-vanishing density of states at the Fermi level, as shown in Fig. 3. The inset panel of Fig. 3 shows the total as well as the orbital-resolved density of states (DOS) for unstrained LaNiO3. From Fig. 3 it is evident that there is a strong hybridization between Ni 3d and O 2p orbitals and the population at the Fermi level comes mostly from them. The non-zero DOS at the Fermi level puts LaNiO3 in the category of metallic compounds. 3.1. Optical properties of cubic LaNiO3 in absence of strain

3.1.1. Dielectric function The dielectric function
ðxÞ is consisted of a real part
r ðxÞ and an imaginary part
i ðxÞ, given by [15]

ð1Þ

where e, x;
0 , and V are the electronic charge, frequency, dielectric constant in vacuum, and the unit cell volume respectively, Wck , Eck , and Wvk , Evk are the wave functions and energies describing the conduction and valence bands respectively, and the vector u defines the polarization of the incident electric field. After calculating the imaginary part of the dielectric function, the real part is calculated using the Kramers–Kroning transformation. The inclusion of Drude correction given by

x xðic þ xÞ

20

Real part εr

0

-20

-40

-60

2

4

6

10

12

14

16

18

20

Fig. 2. Real and imaginary parts of the complex dielectric function of LaNiO3.

prediction of dielectric response and explains the conducting nature of LaNiO3. 3.1.2. Optical conductivity To explore the electronic structure of a compound and its response to the external parameters like temperature and strain, optical conductivity calculation has been very useful till date. We calculated the real part of the optical conductivity rðxÞ of LaNiO3, by considering the following relation

rðxÞ ¼

ix ½1 
ðxÞ 4p

ð3Þ

The real part of calculated optical conductivity for different energy values has been shown in the main panel, and the density of states for LaNiO3 is shown in the inset panel of Fig. 3, in order to describe optical properties of LaNiO3 completely. At the low energy region of the optical conductivity spectra, the Drude peak, a typical characteristic of metal is evident. Apart from the Drude like response at the low frequency region, the optical conductivity data also contains several peaks, ranging from 1.5 eV to 5.5 eV, as

8

25

ð2Þ

with c as the damping parameter and xp the plasma frequency, make the expression of the complex dielectric function sufficiently well to describe the metallic nature of a compound [15]. Both real and imaginary parts of the dielectric function, for cubic LaNiO3, were calculated within DFT, and are shown in Fig. 2. Our theoretically obtained data has an excellent agreement with previously reported theoretical [15] and experimental works [9,20]. It can be seen that the real part (
r ) of the dielectric function increases with energy while the imaginary part (
i ) follows the opposite trend. Both
r and
i approach zero in the high energy region, reflecting the presence of strong inter-band electronic transitions in the system. This trend of the dielectric function can be well explained using the classical Drude theory, which clearly states that the real part of the dielectric function approaches to a highly negative value as x tends to 0, while the imaginary part should have a noticeable reduction in the high energy regime and finally approaches zero. Our observation matches exceedingly well with the classical Drude

8

Energy (eV)

DOS (1/eV)


D ¼ 1 

2 p

Imaginary part εi

6

20 Total 15

La 4f

A

(ω)

2e2 p Rk;v ;c jhWck ju^ rjWvk ij2 dðEck  Evk  EÞ V
0

40

Ni 3d

10

4

O 2p

Re


i ðxÞ ¼

Dielectric function

3. Results and discussion

60

5

0 -7

2

-5

-3

-1 0 1 3 Energy (eV)

B 0

0

1

2

D

C

3

5

4

F

7

E

5

6

Energy (eV) Fig. 3. The main panel shows the real part of optical conductivity, and the inset panel depicts the density of states (DOS) for unstrained LaNiO3.

D. Misra, T.K. Kundu / Computational Materials Science 112 (2016) 113–119

shown in the main panel of Fig. 3. A careful study of the density of states of bulk LaNiO3 (Fig. 3 inset panel) reveals that these peaks are the signatures of inter-band electronic transitions in the system. The optical transition peaks of the LaNiO3 systems are located at 1.5(A), 2.8(B), 3.7(C), 4.2(D), 5.0(E), and 5.5(F) eV. Among them the low lying excitations match well with an earlier reported optical spectroscopic study of LaNiO3 [10]. The electronic density of states of LaNiO3 clearly indicates that, the prominent peak at 1.5 eV (A) is due to the electronic transitions from oxygen 2p to Ni 3d, and the peak at 4.2 eV(D) is due to the transition between Ni 3d to La 4f and La 5d bands. The high energy excitations at 5–5.5 eV are in good agreement with U+GW calculations [21] and some earlier experimental observations [22–24]. These peaks are attributed to the transitions between oxygen 2p and La 5d orbitals. In a conventional metal, the low frequency part of the optical spectra is governed by the Drude like responses. The optical conductivity in Drude theory (rD ) is given by

rD

ne2 s 1 ¼ m 1  ixs

ð4Þ

Here the scattering rate (1s) and the electron mass (m) are independent of the frequency x. One needs to go beyond the classical Drude theory and adopt the extended Drude analysis [12], where the scattering rate and the renormalized mass are x dependent. In this framework the scattering rate becomes a function of the highfrequency lattice dielectric constant (
1 ) and the Drude plasma frequency (xp ), via the relation

1

s




¼

x2p 1 Im x
x 
1

 ð5Þ

where the Drude plasma frequency is given by

x2p 8

Z

XD

¼ 0

r1 dx

ð6Þ

The choice of XD was restricted to include only the Drude part of the optical conductivity. Our calculated plasma frequency within GGA+U approach came out to be 6.02 eV which is nearly 1.3 eV higher than the band theory predicted value of 3.97 eV of the plasma frequency value for LaNiO3 [25]. Optical conductivity is highly sensitive to any subtle change in the electronic structure of a crystal due to the effect of strain on LaNiO3. The change in the optical conductivity spectra due to the application of strain and the plasma frequency therein, are discussed in much detail in the subsequent sections. 3.1.3. Refractive index Using the values of the real and imaginary components of the complex dielectric function, various optical parameters like absorption index, optical conductivity, and reflectivity can be easily obtained. Furthermore, the refractive index (n) and the extinction coefficient (k) are derived from the dielectric function by using the following expressions

1 n ¼ pffiffiffi 2 1 k ¼ pffiffiffi 2

rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2r þ
2i þ
r

ð7Þ

rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2r þ
2i 
r

ð8Þ

Fig. 4 shows the refractive index (n) and the extinction coefficient (k) we calculated for LaNiO3, using DFT. Both n and k decreases with increase in energy. This kind of variation of the refractive index and the extinction coefficient has previously been reported [11,26] and agrees well with our findings. We have also performed absorption index and the reflectivity calculations, apart

Refractive index (n) & Extinction coefficient (k)

116

7

6

n

5

k

4

3

2

1

0 1.4

1.6

1.8

2

2.2

2.4

2.6

2.8

3

Energy (eV) Fig. 4. Refractive index (n) and extinction coefficient (k) for cubic LaNiO3.

from the above mentioned calculations. Our absorption spectra show a large absorption coefficient for LaNiO3. A large absorption coefficient value of LaNiO3 can make it a potential infrared detectors material [11,26]. 3.2. Effect of tensile and compressive strain on optical properties of LaNiO3 Strain engineering is proved to be very useful in altering the optical conductivity of LaNiO3, which enables it to have a significant role in device applications [12,14,27]. We have applied various in-plane tensile and compressive strains to study the changes in the optical spectra of LaNiO3, and the effect of strain on the electronic correlation that the system possesses. In both the cases we observe that the value of conductivity increases compared to the unstrained film, in stark contrast to the general notion of the opposite characters of tensile and compressive strains. As tensile strain increases the lattice parameters, respective tensile strains are denoted by a ‘+’ sign and compressive strains are denoted by ‘’ sign as it reduces the lattice parameters. The changes in the optical properties due to the application of strains are discussed below in much detail. 3.2.1. Effect of tensile strain on optical conductivity In plane tensile strains of different values (2–7%) were applied within DFT. Change in the optical conductivity of LaNiO3 under tensile strain values ranging from +2% to +7% are shown in the main panel of Fig. 5. In all the cases, the presence of Drude peak indicates the metallic nature of LaNiO3 under tensile strain. However, due to the change in the electronic structure, some changes in the optical conductivity are evident. From Fig. 5 it can be clearly observed that the features denoted by A–F in the unstrained system (Fig. 3 (main panel)), are now shifted towards the lower energy region. The peak positions and the shifts from the unstrained values due to tensile strain are given in Table 1. The evolution of DOS with tensile strain, in the vicinity of the Fermi level is shown in the inset panel of Fig. 5. The DOS near the Fermi level is seen to increase with tensile strain; this trend of increase in DOS with increase in strain is also seen earlier [14], where it was claimed that a substantial increase in DOS may trigger a ferromagnetic instability in the strongly correlated LaNiO3 system. However, the changes in the optical spectra due to tensile strain clearly reveal that electron correlation plays a

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D. Misra, T.K. Kundu / Computational Materials Science 112 (2016) 113–119

25 1.7

20

DOS (1/eV)

0%

Re

( )

+2% 15

+4%

1.5 1.4

+5%

10

1.6

1.3 -0.2

+7%

-0.1

0

0

0.1

0.2

Energy (eV)

5

0

1

2

3

4

5

6

Energy (eV) Fig. 5. The main panel shows the optical conductivity of LaNiO3 under different tensile strain and the inset panel shows the change in DOS near Fermi level due to application of tensile strain.

Table 1 Positions (in eV) of inter-band transition peaks of the optical conductivity spectra under strain and their shifts in eV (in bracket) from the unstrained values. A

D

E

F

Unstrained LaNiO3

1.5

4.2

5.0

5.5

Tensile strain +2% +4% +5% +7%

0.81(0.69) 0.86(0.64) 0.86(0.64) 0.84(0.66)

3.34(0.86) 4.06(0.14) 4.0(0.2) 3.8(0.4)

4.71(0.3) 4.48(0.52) 4.36(0.64) 4.14(0.86)

5.21(0.4) 4.8(0.7) 4.71(0.79) 4.56(0.94)

44

1.4

0%

1.01(0.49) 1.02(0.48) 1.05(0.45) 1.01(0.49)

5.0(+0.80) 4.57(+0.37) 4.67(+0.47) 4.56(+0.36)

5.46(+0.46) 5.11(+0.11) 5.18(+0.18) 5.34(+0.34)

5.84(+0.34) 5.62(+0.12) 5.71(+0.21) 6.02(+0.52)

major role even in the strained LaNiO3 systems. The correlation is strong enough to hinder the inter-band electron hopping and hence making the system metallic albeit strongly correlated. Shifting of the peaks towards the low energy region implies that tensile strain has a tendency to suppress the energy of interband transitions and hence making the metallic system under consideration strongly correlated. 3.2.2. Effect of compressive strain on optical conductivity Compressive strain of different values ranging from 2% to 7% were applied on the system. The effect of compressive strain on electronic structure of LaNiO3 is shown in Fig. 6. In this case also the Drude peak is present for systems under different compressive strains, indicating that LaNiO3 retains its metallic nature under compressive strain. However the peak positions due to the inter-band transitions in the high energy region seem to deviate from their values in the unstrained case. A comparison between the peak positions due to tensile and compressive strains compared to the unstrained system is given in Table 1. The intensity of the inter-band excitations increases strikingly with application of compressive strain. Additionally, the peaks in the high energy region shift towards higher energy values, indicating that the inter-band electronic transitions are enhanced and hence metallic state is favoured. Hence from the optical spectra it is evident that both tensile and compressive strains enhance the optical conductivity. Our findings match well with some earlier experimental findings [12,14,18]. While the tensile strain seems to favour the electron correlation over the inter-band electronic transitions, compressive strain exhibits an opposite tendency. Compressive strain enhances the inter-band transition remarkably

( )

-2%

Re

Compressive strain 2% 4% 5% 7%

1.5

DOS (1/eV)

Peaks (eV)

55

33

-4% -5%

1.3 1.2 1.1

22

-7% 1 -0.2

11

0

0

1

2

3

4

5

0 Energy (eV)

6

7

0.2

8

Energy (eV) Fig. 6. The main panel shows the optical conductivity of LaNiO3 under different compressive strain and the inset panel shows the change in DOS near Fermi level due to application of compressive strain.

and hence favouring electronic conduction over localization. A noticeable shift of the peak positions in the optical spectra towards the higher energy values, are the signatures of the enhanced metallic nature of the system subjected to compressive strain. 3.2.3. Effect of strain on plasma frequency Drude plasma frequencies (xp ) for tensile and compressive strain were obtained using the same formula as mentioned earlier, and their values are listed in Table 2. It is well known that the kinetic energy of electrons is measured by the area under the coherent Drude part of the optical conductivity [28–30], and is given by,

KðxÞ ¼

2 ha pe2

Z 0

XD

r1 dx

ð9Þ

where a denotes the lattice parameter. Integrating the coherent part up to the cut-off frequency XD , kinetic energy of electron is obtained. To obtain Kinetic energies of electrons theoretically, plasma frequencies for strained LaNiO3 systems (Table 2) were used, as from Eqs. (6) and (9) it can be seen that, kinetic energy of electrons becomes directly proportional to the square of the plasma frequency i.e., KðxÞ / x2p . The Drude plasma frequency

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D. Misra, T.K. Kundu / Computational Materials Science 112 (2016) 113–119

Table 2 Plasma frequencies of LaNiO3 for different tensile and compressive strains. Tensile strain (%)

xp (eV)

Compressive strain (%)

xp (eV)

+2 +4 +5 +7

4.757 4.892 4.739 4.744

2 4 5 7

4.473 4.758 4.272 3.967

1.0 0.9

0% +4%

0.8

+5%

Reflectance

Reflectance

1

0.7

0%

0.6

0.5

0.4

0.3

-5% -7%

0.2 0

0.1

+7%

-2%

2 4 Energy (eV)

1

2

6

3

4

5

6

Energy (eV) Fig. 7. The main panel shows the reflectance of LaNiO3 under tensile strain and the inset panel shows the same under compressive strain.

obtained from experiments for LaNiO3 film grown on the SrTiO3 substrate (+1.7% strain) is reported to be 0.8 eV and for LaAlO3 substrate (1.2% strain) it is 1 eV [25], though no Drude peak was observed in those cases. The difference in the observation may be due to the fact that unlike our case, the experiments were performed on the ultrathin films, and not on bulk LaNiO3. The ratio of the experimentally obtained kinetic energy of electrons (KEXP) to that obtained from theory (KTHEO) is of utmost importance as this ratio is the direct measure of strong correlation in a system. KEXP =K THEO approaches unity when the system is metallic and becomes less when the system under consideration is a strongly correlated system. Our calculations gave KEXP/KTHEO  0.03–0.05, close enough to earlier reported values [25]. This very small value of KEXP/KTHEO proves the presence of high electron correlation even in the presence of tensile and compressive strains, although tensile strain induces more electron correlation than compressive strain. Measuring the strength of correlation by the KEXP/KTHEO ratio, it is apparent that the system under tensile strain has lower value of KEXP/KTHEO compared to the system under compressive strain, which proves that LaNiO3 under tensile strain becomes more strongly correlated compared to that under compressive strain. 3.2.4. Effect of strain on reflectance We have also looked into the reflectance of unstrained as well as strained LaNiO3 to understand the effect of strain on electron correlation. Reflectance spectra of LaNiO3 under tensile and compressive strain are shown as the main and inset panels respectively in Fig. 7. The reflectance spectra of unstrained LaNiO3, as shown by the red1 line in the main panel of Fig. 7, show that the system is metallic. The whole spectra can be divided into two parts. The low 1 For interpretation of color in Fig. 7, the reader is referred to the web version of this article.

energy part up to 0.6 eV falls in the coherent Drude region which comes only from the intra-band electronic transitions within the system. After 0.6 eV the onset of inter-band transitions comes into picture. The high energy region in the reflectance spectra is governed by a broad hump-like feature at 1.5–4 eV and peaks at 5 eV and 5.5 eV. These features reflect strong inter-band transitions in the system. The region of the spectra after 0.6 eV falls under charge-transfer region as reported earlier [26]. While the broad hump like feature appearing at 1.5–4 eV of the spectra is due to the strong electronic transitions between Ni 3d to O 2p orbitals, the high energy excitation peaks at 5–5.5 eV come from the transitions between O 2p and La 3d orbitals. In a similar way to optical conductivity, the application of tensile strain shifts the inter-band transition peaks to the lower energy region while compressive strains pull them towards higher energy values. Hence, suppression of metallic nature i.e., the electron excitation is evident in case of tensile strain. However application of compressive strain favours the metallic nature of the system by enhancing the inter-band transitions. Effect of strain on the electronic structure of LaNiO3 can also be explained studying the transport properties of LaNiO3 under different tensile and compressive strain, which is our next endeavour. The understanding of the effect of strain is not straight forward as different possibilities result in different consequences. The enhancement of metallic nature under compressive strain can be understood well with a simple explanation using the change in bond lengths. As hybridization between Ni 3d and O 2p orbitals controls the transport mechanism in LaNiO3, the change in the orbital overlap between Ni and O atoms can affect the conductivity. For example, under application of 2% tensile strain, the Ni–O bond length increases by 0.039 Å, while for 2% compressive strain the Ni–O bond length decreases by the same amount. In other words, as the system gets compressed, the bond length decreases, hence orbital overlap increases. Therefore the conductivity increases and the metallic nature is favoured. However, for tensile strain, as the atoms move far away from each other with increase in the magnitude of tensile strain, chance of orbital overlap decreases considerably, hence conductivity of the system decreases. Although the chemical bonding in LaNiO3 remains predominantly ionic, a substantial amount of covalent bond is also present there. While the ionic part of the chemical bonding in LaNiO3 comes from Ni–O–Ni bonds, covalency comes from both La–O and Ni–O bonds [14]. This mixed character may trigger the observed intriguing features in LaNiO3.

4. Conclusion First-principle calculations were carried out to investigate the presence of strong correlation in the cubic LaNiO3 system and the effect of strain therein. LaNiO3 system without strain shows a distinct Drude peak, typical of metal, and several inter-band transitions. Optical conductivity, plasma frequency, dielectric function, extinction coefficient and refractive index for LaNiO3 were calculated and the effects of both tensile and compressive strain on these properties were discussed in detail in terms of alternation in the electronic structure. Our results are in good agreement with the available reports. While in-plane stretching of the system (tensile strain) forbids electron hopping and tries to localize electrons, compressive strain triggers the inter-band electronic transitions and hence prefers the conducting nature over the electron localization. Our findings clearly indicate that like unstrained system, LaNiO3 under strain also retains metallic nature with strong electron correlation, and system under tensile strain is more strongly correlated compared to the system under compressive strain. Our calculations give a clear insight of the effect of strain on the physical properties of LaNiO3 and may be of immense help in controlling the conducting properties of the system in device purposes by suitable application of strain.

D. Misra, T.K. Kundu / Computational Materials Science 112 (2016) 113–119

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