The effects of oxygen in spinel oxide Li1+xTi2-xO4-δ thin films.

The evolution from superconducting LiTi2O4-δ to insulating Li4Ti5O12 thin films has been studied by precisely tuning the oxygen pressure in the sample fabrication process. In superconducting LiTi2O4-δ films, with the increase of oxygen pressure, the oxygen vacancies are filled gradually and the c-axis lattice constant decreases. When the oxygen pressure increases to a certain critical value, the c-axis lattice constant becomes stable, which implies that the sample has been completely converted to Li4Ti5O12 phase. The two processes can be manifested by the angular bright-field images of the scanning transmission electron microscopy techniques. The transition temperature (T ch ) of magnetoresistance from the positive to the negative shows a nonmonotonic behavior, i.e. first decrease and then increase, with the increase of oxygen pressure. We suggest that the decrease Tch can be attributed to the suppressing of orbital-related state, and the inhomogeneous phase separated regions contribute positive MR and thereby lead to the reverse relation between Tch and oxygen pressure.

Introduction

In the research on oxide superconductors, the oxygen always plays an important role in the superconductivity and their normal state behaviors[1,2]. In copper oxide high- critical temperature (Tc) superconductors, such as Nd2-xCexCuO4±δ[3-5], Pr2−CeCuO4±δ[6-8] and YBa2Cu3O7−δ[9-11], T can be greatly improved in a large range by adjusting the oxygen content during the annealing process, as well as the titanium oxide systems, such as SrTiO3[12,13] and TiO[14,15]. Oxygen has a strong effect not only on superconductivity, but also on many other properties. For instance, the antiferromagnetism[16] and the charge density wave[17] can also be tuned by the oxygen vacancies. Furthermore, the doping and disorder effects induced by oxygen vacancies can cause obviously change on Hall resistance and magnetoresistance (MR) behaviors in the normal state[5,6,18]. Studying the oxygen effects is of great help to understand the mechanism of superconductivity, transport and other properties of the oxide superconductor[9,19,20].

Among hundreds of spinel oxides, the metallic lithium titanate LiTi2O4 is the only known oxide superconductor, which Tc is as high as 13.7 K[21], discovered by Johnston et al. in 1973[22]. Previous studies have disclosed that LiTi2O4 is a BCS s-wave superconductor with intermediate electron-phonon coupling (λel-ph ~ 0.65)[23,24]. Nevertheless, an enhanced density of states has been unveiled by magnetic susceptibility[25] and specific heat measurements[23], indicating that d-d electronic correlations cannot be ignored in this system. Meanwhile, due to the mixed-valence of Ti ions in the frustrated Ti sublattice, LiTi2O4 exhibits complicated spin-orbit fluctuations, which is evidenced by the resonant inelastic soft-x-ray scattering[26], nuclear magnetic resonance[27] and magnetic susceptibility measurements[25]. Very recently, electrical transport and tunneling spectra measurements on high quality epitaxial [001]-oriented LiTi2O4 films have revealed an orbital-related state below ~50 K, confirmed by a twofold in-plane angular dependent MR, positive MR as well as the relation Δ ~−B2 [28].

Interestingly, by tuning the oxygen in the process of sample deposition, the phase of the thin film changes from LiTi2O4-δ to Li4Ti5O12 along with the superconductor-insulator phase transition[29]. However, this transition seems to happen abruptly, which hinders us from understanding the nature of the transition. Previous work on LiTi2O4 polycrystals has disclosed that the existence of oxygen-site distortion induces prominent changes in the electronic states near EF[30]. In addition, tunneling spectra experiments on LiTi2O4 films of different orientations reveal an anisotropic electron-phonon coupling in this system, which is regarded to originate from the Jahn-Teller distortions enhanced by oxygen vacancies[31]. Nevertheless, it is still unclear what happens in the microstructure of the sample during the transition from LiTi2O4 to Li4Ti5O12. Moreover, the mechanism of the oxygen effects on superconductivity of LiTi2O4 has never been investigated, as well as the transport behaviors in the normal state. Therefore, it is worthy of tuning the oxygen pressure (PO2) in the process of film deposition to clarify these questions.

In this work, we carefully manipulated the transition from LiTi2O4-δ to Li4Ti5O12 thin films by adjusting the PO2 in the process of pulsed laser deposition (PLD). First, the high quality LiTi2O4-δ superconducting thin films can be obtained in the high vacuum environment. Tuning the PO2 from 10−7 to 10−4 Torr, the c-axis lattice constant gradually decreases, indicating that the filling of oxygen vacancies dominates in this process. Second, when PO2 is higher than 10−4 Torr, the c-axis lattice constant stops to decrease, indicating the finish of transition from LiTi2O4-δ to Li4Ti5O12 phase. These two processes can be revealed from the angular bright-field images (ABF) of LiTi2O4-δ and Li4Ti5O12 by the scanning transmission electron microscopy (STEM) techniques. In addition, the temperature (T) of MR from the positive to the negative shows a nonmonotonic behavior, i.e. first decrease and then increase, with the increase of PO2. Combined with the electron energy-loss spectroscopy (EELS) measurements, we suggest that the decrease of T under lower PO2 stems from the suppression of orbital-related state via filling the oxygen vacancies, and the increase of T under higher PO2 is due to the phase separation in some regions, which dominates the positive MR (p-MR).

Results and Discussion

The θ–2θ XRD spectra of (001) Li1+xTi2-xO4-δ (0 ≤ × ≤ 1/3) samples grown in different PO2 are shown in Fig. 1(a). The (001)-oriented LiTi2O4-δ thin films are achieved when the films are deposited under PO2 ≤ 10−6 Torr. Instead, the (001)-oriented Li4Ti5O12 thin films are formed at PO2 > 10−4 Torr. The XRD patterns of the samples in different PO2 are quite similar except that the diffraction peaks gradually shift to higher angle in the LiTi2O4-δ films at larger PO2. In order to check the crystallization quality of the thin films, we also perform φ-scan. In Fig. 1(b), the φ-scans of (404) plane of both LiTi2O4-δ and Li4Ti5O12 samples display four-fold symmetry with uniformly distributed peaks. From the θ–2θ XRD spectra, we can extract the value of the out-of-plane lattice constant (c-axis) as a function of the PO2. As seen in Fig. 1(c), when PO2 < 10−4 Torr, c gradually decreases with increasing PO2. However, when PO2 is higher than 10−4 Torr, c is saturated, indicating the complete formation of Li4Ti5O12 phase. As a result, a phase transition from LiTi2O4-δ to Li4Ti5O12 has been successfully achieved by tuning PO2 during the sample deposition.

Figure 1

(a) The θ–2θ XRD spectra of epitaxial Li1+xTi2-xO4-δ (0 ≤ × ≤ 1/3) thin films grown on (001) MAO substrates at different PO2. (b) The φ-scan measurements of LiTi2O4-δ and Li4Ti5O12 thin films on MAO (001) in the (404) reflection. (c) The lattice constant along c axis of the samples versus PO2.

In order to further study the effects of oxygen on superconducting state and normal state, we systematically measured the resistances of various thin films from LiTi2O4-δ to Li4Ti5O12. The R-T curves of the LiTi2O4-δ thin films with different oxygen pressures are shown in Fig. 2(a). Increasing the PO2 during the deposition, the samples undergo a transition from metal to insulator in the normal state. In Fig. 2(b), the residual resistivity ratio (RRR) decreases monotonically with increasing PO2. Here, the RRR is defined as the ratio of room temperature resistivity to the resistivity of T°nset, i.e. R (300 K)/R (T°nset), where the Tonset is the critical temperature at the beginning of superconducting transition. We plot the dependence of Tc0 on PO2 as seen in Fig. 2(b), and the T of the LiTi2O4-δ thin films is quite stable at PO2 < 5.4 × 10−6 Torr, whereas it drops rapidly when PO2 > 5.4 × 10−6 Torr.

Figure 2

(a) The R-T curves of Li1+xTi2-xO4-δ (0 ≤ × ≤ 1/3) thin films grown on (001) MAO substrate with different PO2 during the deposition. (b) The PO2 dependence of RRR and Tc0 of the films in (a) are plotted. The gray and red dashed lines are used to guide eyes. Tc0 is defined as the temperature where resistance is lower than 10−6 Ohm. Inset: zoom the R-T curves in Fig. 2(a) at low temperature range.

To find out the microstructure evolution from LiTi2O4-δ to Li4Ti5O12, we have carried out atomic-resolution STEM measurements on these high-quality samples. Figure 3 shows the ABF images along the [110] direction and the corresponding line profiles in different regions with different types of oxygen vacancies. In Fig. 3(a), the O columns, as indicated by red arrows, are imaged as dark spots due to the absorption nature of the ABF contrast, and the contrast of the Ti columns as indicated by the blue arrows is darker than the O columns based on the ~Z1/3 contrast mechanism where Z is the atomic number. Thus, in the pristine regions the contrast of O1 and O2 is of approximate equal darkness, and oxygen vacancies as shown in Fig. 3(b) and (c), are imaged as light gray spots. Here, we divided the positions of oxygen atoms into two types, i.e. O1 and O2 (see Fig. 3(d)) to describe clearly the distribution of oxygen vacancies, as shown by the red and blue arrows. To visualize the oxygen vacancies clearly, we extracted the line profiles on the oxygen rows as indicated by the yellow and red rectangles.

Figure 3

ABF images of LiTi2O4-δ thin film in (a) pristine, (b) O2 vacancy and (c) O1 & O2 vacancy regions. (d) Structure model of LiTi2O4 projected along [110] direction, where the atomic positions of both O1 and O2 oxygen are labeled by red and blue arrow, respectively. (e) and (f) Line profiles of ABF contrast with filled yellow and red color, obtained from the corresponding yellow and red areas in (b) and (c), respectively. Atomic positions of O1 and O2 are also labeled by red and blue arrows, respectively. Note that the lower panel in (e) exhibits contrast between O1 (red arrows) and O2 (blue arrows) close to the ideal structure.

From the line profile of ABF contrast in Fig. 3(e), we can find that the depth of the ABF contrast valley (darkness) at O2 positions is lower than that at O1 as seen in the [110] direction, which means some vacancies exist in the O2 sites. Similarly, some vacancies at O1 and O2 exist in another region as shown in Fig. 3(f). However, these oxygen vacancies have not been observed in the Li4Ti5O12 samples[32]. It is known that the LiTi2O4-δ exhibits serious aging effects in forms of polycrystal and single crystal[30]. The LiTi2O4-δ thin films, especially the one deposited in the higher vacuum, are much more stable. It is reasonable to speculate that the samples in higher vacuum will contain more oxygen vacancies. Increasing oxygen pressure will fill these oxygen vacancies and finally turn the superconducting phase to insulating Li4Ti5O12.

The phase evolution with PO2 should inevitably make difference in the electronic states. In LiTi2O4-δ, one concern is about the orbital-related state. Normally, the formation of the orbital order results from the band split near the Fermi level. As for LiTi2O4-δ, the distortion of Ti-O octahedron leads the splitting of Ti 3d band to eg and t2g band[33], and the orbital-related state is expected to exist. Although it has been unveiled in previous work, it remains unclear in the origin[25]. One of the evidence is the crossover from the negative MR (n-MR) to the p-MR at T ~ 50 K in the normal state. Entering the superconducting state, the orbital-related state interacts with Cooper pairs and results in an unexpected relation between the superconducting gap and the applied magnetic field, i.e. Δ~ −B2. This relation implies the coexistence of the superconducting state and the orbital-related state. Therefore, it is deserved to clarify how the oxygen makes the influence on these two states.

To clarify this issue, we finely tune the PO2 around 10−6 Torr to avoid the Li4Ti5O12 phase. Then, we focus on the effects of PO2 on R and MR. In the precise tuning process, the vacuum value is not a good scale due to the limitation of the vacuum gauge. Fortunately, the RRR decreases monotonically with the increase of oxygen pressure, which can reflect the trend of PO2 and the oxygen defects as discussed above. Thus, we use RRR to index the samples, named S1 to S8 with RRR in the range between 5.6 and 1.5. As shown in Fig. 4(a), the Tc seems unchanged in the tuning range. For samples S1 to S5, the MR at 35 K changes from positive to negative as seen in Fig. 4(b). By fitting the MR with the Kohler’s formula, i.e. MR ~A0B2 [2], the slope A0 can be obtained for these samples at different temperatures. In Drude model, A0 is proportional to μ2 (i.e. μ = eτ/m) with μ the mobility, τ the relaxation time and m the electron mass. With the increase of temperature, the value of A0 decreases from positive to negative as seen in Fig. 4(c). A negative mobility cannot be understood in this simplified model, and the n-MR is interpreted as the suppression of spin-orbital fluctuations in this system[28]. The Tch from p-MR to n-MR is extracted from Fig. 4(c) and plotted in Fig. 4(d). From S5 to S1, Tch gradually increases with the increase of RRR.

Figure 4

(a) The R-T curves with increase of PO2 are defined as S1–S8 in sequence. The inset is the zoom of R-T curves. (b) The field dependence of MR of S1–S8 grown on (001) MAO substrates at 35 K. (c) The slope value A0 of MR can be obtained for various samples at different temperatures. With temperature increasing, the value of A0 changes from the positive to the negative. (d) The relationship between RRR and Tch. The gray dashed line is used to guide eyes. The shadow areas represent the uncertainties in defining Tch due to the impurities.

However, further reducing the RRR, the relation between Tch and RRR will be broken. For instance, the MR becomes stronger for the samples deposited under higher PO2, e.g. S6–S8, and thus the Tch goes up. In this regime, the formation of Li4Ti5O12 phase may lead to more boundaries in phase separated samples. Such inhomogeneity in the magnetic field usually exhibits strong p-MR[34,35]. For the samples S1–S5, the p-MR below T mainly origins from the orbital-related state since the LiTi2O4-δ phase dominates the transport[25]. We speculate that filling the oxygen vacancies seems to suppress the p-MR but in fact the orbital-related state.

In order to verify this assumption, we should evaluate the effects of oxygen vacancies on Ti valance. Although oxygen vacancies have been detected by STEM, the content of oxygen vacancies cannot be quantified. Therefore, we collected EELS profiles of both LiTi2O4-δ and Li4Ti5O12 films. As seen in Fig. 5, the Ti L2,3 edges, from 2p1/2 and 2p3/2 to 3d orbits respectively, split into two peaks in Li4Ti5O12, but not in LiTi2O4-δ. Usually, the splitting of L2,3 is attributed to the degeneracy lifting of Ti 3d orbits by the crystal field.

Figure 5

(a) The EELS profiles for Ti L2,3 edges of the Li4Ti5O12 thin film. The L2 and L3 edges split into four peaks. (b) The EELS profiles for Ti L2,3 edges of the LiTi2O4-δ thin film. Only two peaks appear of L2,3 edges.

The missing split of peaks in LiTi2O4-δ EELS may origin from two reasons. First, the energy gap between eg and t2g, named Δe–t, is too small to be discernable in EELS. According to the band calculations, Δe-t equals to 2.1 eV and 2.4 eV for the ideal structure LiTi2O4 and Li4Ti5O12, respectively[36]. Moreover, four peaks were also observed in Li7Ti5O12, where Δe-t equals to 1.8 eV[32]. Considering the existence of oxygen vacancies, which may further distort the Ti-O octahedrons, we do not expect a smaller crystal field. Therefore, the change of Δe–t cannot account for the discernable peak splitting in LiTi2O4-δ. Second, the valance of Ti in ideal LiTi2O4 is +3.5. If large numbers of oxygen vacancies exist in LiTi2O4-δ, the Ti3.5+ will transform to Ti3+. In this condition, the electrons on t2g band increase, and thereby the hoping possibility from Ti 2p to t2g is reduced due to the Pauli Exclusion Principle. Consequently, oxygen vacancies will smear out the peaks of Ti 2p to t2g in EELS.

Based on the EELS results, we can give a reasonable explanation for the suppression of the orbital-related state by filling oxygen vacancies. In general, the formation of the orbital order results from the band split near the Fermi level. As for LiTi2O4, crystal field splits Ti 3d bands to eg and t2g bands[33]. The oxygen vacancies in LiTi2O4-δ system, on the one hand, enhance the distortion of Ti-O octahedrons, on the other hand, dope electrons to enhance the electron correlations, which are beneficial to the formation of the orbital-related state. With the filling of oxygen vacancies, the valence of Ti increases and some of Ti sites become empty states, thereby weaken the orbital-related state.

Compared to the obviously suppressed orbital-related state, the T of the LiTi2O4-δ thin films is quite stable at PO2 < 5.4 × 10−6 Torr. Actually, the O 2p bands are far below the Fermi level with weak p-d hybridizations[30,33]. Although the oxygen vacancies induce doping effect and influence on the splitting of Ti 3d bands by the crystal field, the density of states near Fermi surface may not change obviously, and thus the T remains the same.

In conclusion, we studied the evolution from LiTi2O4-δ to Li4Ti5O12 with increasing oxygen pressure during the thin film deposition. By transport and STEM measurements, we have disclosed that there are two processes happened during the evolution, i.e. the filling of oxygen vacancies and the forming of Li4Ti5O12. The EELS results of the LiTi2O4-δ and Li4Ti5O12 samples provide the evidence that the orbital-related state is suppressed by the filling of oxygen vacancies. The evolution of electronic states by adjusting the oxygen content gives an insight into the interaction between the orbital-related state and the superconductivity in LiTi2O4-δ.

Methods

The (00l)-oriented Li1+xTi2-xO4-δ (0 ≤ × ≤ 1/3) thin films are grown on (00 l) MgAl2O4 (MAO) substrates by PLD with a KrF excimer laser (λ = 248 nm). Before the deposition, the MAO substrates are annealed at 1000 °C for 5 hours in the air[37,38] to obtain the smooth surface. The sintered Li4Ti5O12 ceramic target is used to fabricate the films, with pulse frequency of 4 Hz, energy density of 1.5 J/cm2, and deposition temperature of ~700 °C. The deposition rate is determined by measuring the thickness of ultra-thin films using X-ray reflectivity analysis. In this study, we fix the film thickness ~150 nm. After the deposition, all the thin films are quenched to the room temperature in situ.

X-ray diffraction (XRD) is employed to characterize the phase and crystalline quality of Li1+xTi2-xO4-δ (0 ≤ × ≤ 1/3) thin films. The microstructure is detected by the spherical aberration-corrected scanning transmission electron microscopy techniques (Cs-STEM). The transport properties are measured by the Quantum Design Physical Property Measurement System (PPMS) with the temperature down to 2 K and magnetic field up to 9 T. Samples are etched into Hall bar by the UV lithography and Ar plasma etching technology for the measurement of the resistance properties.