Epitaxial strain modulated electronic properties of interface controlled nickelate superlattice
Perovskite nickelate heterostructure consisting of single unit cell of EuNiO and LaNiO have been grown on a set of single crystalline substrates by pulsed laser interval deposition to investigate the effect of epitaxial strain on electronic and magnetic properties at the extreme interface limit. Despite the variation of substrate in-plane lattice constants and lattice symmetry, the structural response to heterostructuring is primarily controlled by the presence of EuNiO layer. In sharp contrast to bulk LaNiO or EuNiO, the superlattices grown under tensile strains exhibit metal to insulator transition (MIT) below room temperature. The onset of magnetic and electronic transitions associated with the MIT can be further separated by application of large tensile strain. Furthermore, these transitions can be entirely suppressed by very small compressive strain. X-ray resonant absorption spectroscopy measurements reveal that such strain-controlled MIT is directly linked to strain induced self-doping effect without any chemical doping.
The sudden change in the electrical conductivity across the metal insulator transition (MIT) of complex oxides remains a topic of long-standing interest in condensed matter physics and materials science mit_rmp (). Apart from the fundamental physics aspect of understanding the origin of MIT, a lot of attempts are being made towards the realization of next generation functional devices utilizing MIT ramanathanreview (); tokuranature (); parkinscience (). Practical realization of such devices depends strongly on the ability to maintain sharp metal-insulator transition as the size reduction of the materials towards the nanometer thick device scale and epitaxial strain can significantly modify MIT schlom (); hwang (); jakrmp ().
As a prototypical example having MIT, massive efforts have been made over the last 5 years about the manipulation of the MIT of rare-earth perovskite nickelate (NiO) using external perturbation such as light, strain, electric and magnetic fields etc. (see Ref. ownreview, ; ramanathan1, ; ramanathan2, and references therein). Epitaxial strain i.e. mismatch of lattice constants between the single crystalline substrate and NiO, has been found to be very successful in manipulation of these transitions jian_nc (); triscone111 (); nno_badmetal (); eno_prb (); sno_prb (); sno_aplmaterials (); keimer_raman (); snojap (); lnothoery (); lnoarpes (). For example, the first-order metal to insulator transition (MIT) can be suppressed entirely by compressive strain. Though the MIT is accompanied by spin and charge ordering transitions and structural symmetry lowering in bulk NdNiO nno_co (); scagnoliprb (); lorenzoprb (); noorbitalordering (); keimer_scattering (), the MIT and magnetic transition can be separated by tensile strain, leading to a paramagnetic insulating phase jian_nc (). Surprisingly, charge ordering and symmetry lowering transitions are absent in ultra-thin NdNiO films (thickness 6 nm), grown under tensile strain nno_upton (); derek_nno (). Nickelates being a prototypical strongly correlated system, exhibit highly nontrivial transport properties in the metallic phase. One such frequently discussed phenomenon is the non-Fermi liquid (NFL) behavior of the metallic phase, and epitaxial strain is able to control scaling behavior (power exponents) of the NFL phase jian_nc (); nno_badmetal (). In addition, NiO members have been combined with dielectric materials such as LaAlO, DyScO etc. to study the effect of quantum confinement, and the responses of the orbital and spin degrees of freedom to heterostructuring and epitaxial strain lno_keimer_nm (); lno_jian_prb (); keimer_prl13 (); keimer_xld13 (); lno_epl (); lno_keimer_science (); nno_prmaterials (); keimer_xld15 (). However, study of ultra-thin superlattices consisting of dissimilar nickelate layers is very limited own_enolno () and the response of electronic and magnetic structure to the underlying epitaxial strain is still largely unknown.
The choice of ions determine the structural symmetry of bulk NiO and a very strong connection between the temperature scale of electronic, magnetic transitions and Ni-O-Ni has been observed in bulk NiO series rno_phase1 (); rno_phase (). For example, bulk LaNiO (LNO) with rhombohedral symmetry has the smallest distortion (Ni-O-Ni 165.2) in the NiO series and remains metallic and paramagnetic without any structural transition. On the other hand, bulk EuNiO (ENO) is strongly distorted (Ni-O-Ni 147.9) and undergoes a first order MIT around 460 K with a charge ordering transition and structural transition and well separated magnetic transition (paramagnetic to -antiferromagnetic) at 200 K bulk_eudopedlno (). Since each NiO member has a rather strong propensity for maintaining bulk-like symmetry even in thin film geometry icheng_prb (), a strong structural competition can be anticipated in the ultra-thin limit for the superlatices consisting of dissimilar nickelates layers, and can in turn result in new electronic and magnetic phenomena.
Towards this goal, we have synthesized and investigated the effect of epitaxial strain by growing 1 uc EuNiO/ 1 uc LaNiO superlattices (1ENO/1LNO SL, uc= unit cell in pseudocubic setting, see Fig. 1(a)) on a variety of substrates. To elucidate the microscopic effect of epitaxial strain on the structural, electronic, and magnetic properties of these superlattices, X-ray diffraction (XRD), dc transport, Hall effect, resonant soft X-ray absorption spectroscopy (XAS) and X-ray linear dichroism (XLD) measurements have been performed. Surprisingly, we have found that in-spite of the strong variation of substrate strain and symmetry, the structural response of the SLs in this ultimate interface limit is primarily governed by the ENO layer. The heterostructure grown under tensile strain undergoes a MIT and a magnetic transition below room temperature, emphasizing entire modulation of the electronic properties, sharply contrasted to the bulk ENO and LNO. Moreover, by the judicious application of epitaxial strain these transitions can be made to occur simultaneously or separated with temperature or even entirely suppressed. Oxygen edge XAS measurements revealed that such a drastic change in the electronic behaviour is related to a strain induced self-doping effect selfdoping (); selfdopinglno (). Such manipulation of the electronic and magnetic transitions by the application of epitaxial strain highlights the remarkable power of heteroepitaxy in determining physical properties of perovskite nickelates.
Ii Experimental details
[1EuNiO/1LaNiO]10 superlattices [(1ENO/1LNO) SL], oriented along the pseudo cubic [0 0 1] direction were grown on a variety of single crystal substrates by pulsed laser interval deposition misha_jap (); nnoprl () from polycrystalline stoichiometric EuNiO and LaNiO targets. The substrates used in this work: DyScO (DSO), NdGaO (NGO), LaAlO (LAO), and YAlO (YAO) have been selected to avoid polar discontinuity at the film/substrate interface scirep (). The symmetry of the substrates and the corresponding expected strain values for ENO and LNO are listed in Table-I. Growth of all samples were monitored by in-situ high pressure RHEED (reflection high energy electron diffraction). All films were grown at 620 C and 150 mTorr of oxygen pressure and were post annealed at growth temperature under 650 Torr pressure of pure oxygen. XRD measurements were carried out around the (0 0 2) reflection of the substrate (pseudocubic notation) with a Panalytical XPert Pro materials research diffractometer (MRD). X-ray absorption spectra (XAS) of Ni edges were recorded at the 4-ID-C beam line of Advanced Photon Source (APS). transport measurements, using a four probe Van Der Pauw geometry, were performed in Physical Property Measurement System (PPMS Quantum Design). - measurements confirmed ohmic behavior of all electrical contacts.
|for ENO||for LNO|
Iii Results and discussion
Epitaxial growth: Since the optimal growth conditions for ENO and LNO thin films are different eno_growth (); own_apl (), it is crucial to find out the mutually compatible growth conditions for the layer by layer epitaxial stabilization of both ENO and LNO layers to form high quality 1ENO/1LNO SL. The sequence of layer by layer growth has been shown in Fig. 1(b). The variation of the intensity of the specular spot in the RHEED pattern, recorded during the growth of 1ENO/1LNO superlattice on a NGO substrate, has been plotted in Fig. 1(c). The full recovery of intensity after the deposition of each unit cell confirms the desired layer by layer stabilization of both EuNiO and LaNiO. The RHEED image (Fig. 1(d)), taken after cooling the sample to room temperature, shows the streak patterns of specular (0, 0) and off-specular (0, 1) Bragg reflections, implying atomically smooth surface morphology. The presence of half order reflections (marked by arrows) due to the in-plane doubling of the unit cell eno_growth () (also observed for superlattices grown on other substrates), confirm that superlattices have either orthorhombic or monoclinic symmetry at room temperature.
Following Poisson argument about elasticity, it is generally anticipated that the single crystalline thin film should undergo out-of-plane compression (expansion) to accommodate in-plane tensile (compressive) strain. Experimentally, however, the effects of epitaxial strain on nickelate thin films and heterostructures are more complex and markedly depart from the expected tetragonal distortion eno_prb (); jak_prl (); icheng_prb (); keimer_xld13 (); keimer_xld15 (). To investigate the effect of epitaxial strain on our 1ENO/1LNO SLs, 2- scans have been recorded using Cu radiation (Fig. 1(e)). Each of the diffraction patterns consist of a sharp substrate peak together with a film peak (indicated by a solid triangle) and thickness fringes confirming the growth along the desired pseudo cubic (0 0 1) direction. Out-of-plane pseudocubic lattice constant () for the SL grown on YAO is found to be 3.875 0.005 Å, which is enlarged compared to both bulk ENO and LNO, and is as expected for a tetragonal distortion under high compressive strain. While the close proximity of the substrate and film peaks for the samples grown on LAO and NGO substrates prohibits a reliable estimation of , nevertheless it can be immediately seen that the film peak for the NGO substrate is close to bulk ENO. Interestingly, (3.798 Â± 0.005 Å) of the SL grown on DSO is also very close to the lattice constant of bulk ENO (3.8 Å). The very different orbital responses and electronic properties of the SLs (discussed latter in this paper) suggest that such bulk ENO-like lattice constant of the SLs under tensile strain does not arise from simple strain relaxation. We also note that single layer films of ENO and LNO under tensile strain also show corresponding bulk-like lattice constants jak_prl (); eno_prb (); icheng_prb () and such anomalous behaviors are related to the strain compensation by octahedral tilts, rotations and breathing mode distortions. In contrary to single layer LNO films, LNO layers in the present SLs under tensile strain undergo out-of plane compression so that the resultant of the SL remains consistent with the bulk ENO value. This indicates that the overall symmetry of the SL takes the lower form as in ENO () glazernotation (), likely due to the inability of the rotation system seen in bulk LNO to stabilize in the presence of the smaller Eu ions.
Structural responses of these SLs to the epitaxial strain have been further investigated by X-ray linear dichroism (XLD) measurements at room temperature. In such experiment, absorption at Ni edges are measured with horizontally (H) and vertically (V) polarized X-rays (Fig. 2(a)) and the difference in the energy position and intensity provides information about the splitting between the orbitals and their preferential electronic occupation jak_prl (); lno_epl (); keimer_xld13 (); keimer_xld15 (); nnoprl (); lnoxld (). The absorptions labeled as and after background subtraction of the Ni edge and the difference signal () are shown in Fig. 2(b). As seen, the line shapes of the spectra confirm the expected Ni oxidation state in these superlattices. The ratio, of holes on and orbitals can be obtained from the measured , using the sum rules keimer_xld13 (); keimer_xld15 () where () is the integrated area of (). We note that bulk NiO does not show any preference between these two levels noorbitalordering (); co_mazin (). The variation of and the energy splitting between and orbitals obtained from Fig. 2(b) is plotted in Fig. 2(c) as a function of substrate lattice constant. As immediately seen, the large compressive strain provided by the YAO substrate results in a derivative-like shape of the XLD () spectra confirming the expected orbital splitting with / 1. orbital is higher in energy compared to by 50 meV and is close to unity, emphasizing equal population on both orbitals. On the other end, the SL on DSO with / 1 shows a orbital splitting of around 100 meV and much larger hole density in orbitals. Surprisingly, for intermediate compressive (LAO) and tensile (NGO) strain cases, the holes density is slightly larger in orbitals and the energy separation between two orbitals is below the accuracy of the XLD measurement ( 50 meV). This apparently conflicting observation for the film on NGO substrate can be resolved by including complex octahedral distortions acting to accommodate the moderate amount of strain jak_prl (); own_enolno ().
The summary of temperature dependent resistivity measurements on these SLs is shown in Fig. 3(a). As reported earlier own_enolno (), the samples grown on NGO substrate remains metallic down to 125 K and then undergo a MIT. During heating from low temperature it becomes metallic at 155 K. This hysteresis signifies the first order nature of the transition. This behavior is drastically different from the entirely insulating behavior of single layer ENO or entirely metallic behaviour of LNO films grown on NGO substrates below 300 K eno_prb (). Such a large change emphasizes a complete modulation of the electronic structure by heteroepitaxy. Surprisingly, in the metallic phase the resistivity shows an unconventional linear- dependence (over 190 K - 280 range, shown in Supplemental Materials sup ()) while the Debye temperature of bulk nickelates is around 420 K debyetemp (). Such linear- dependent resistivity has been also observed in high cuprates, pnictide and organic superconductor, ruthenate, heavy fermion metals etc. and has been very often linked to the quantum criticality ruthenates (); cuprates (); pnictides (); hightcresistivity (); nflexponent (). Furthermore, the increase of tensile strain on DSO results in a higher resistivity at room temperature. Also, while the insulator to metal transition temperature during heating remains similar to the SL on NGO, the magnitude of thermal hysteresis becomes much smaller. On the other hand, the superlattices grown on LAO and YAO remains metallic down to low temperature without any hysteric behavior. This suppression of the MIT by compressive strain resembles the behavior of ultrathin films of PrNiO, NdNiO, SmNiO, EuNiO jian_nc (); triscone111 (); nno_badmetal (); eno_prb (); sno_aplmaterials (); keimer_raman (). The dc transport of these metallic samples exhibits a dependence over a large range of temperature and then switches to linear- dependent behavior (see Fig. 3(b) and Supplemental Materials sup ()). dependence of resistivity is a characteristic of NFL phase proximal to a two-dimensional quantum critical point nflexponent (). Such switching of dependence to linear behavior has been also observed in NdNiO thin films under compressive strain and can be accounted by Boltzmann-type transport theory with multiple bands near a quantum critical point (for details see Refs. jian_nc, ; nflexponent, ).
In the past, long range magnetic orderings of bulk NiO, single layer films and superlattice structures consisting of NiO layers have been investigated by neutron diffraction etype () and resonant X-ray scattering nno_prmaterials (); jian_nc (); triscone111 (); keimer_raman (); derek_nno (); keimer_prl13 (); noorbitalordering (); derek_prb2 (); scatteringpolycrystal (); own_enolno (). These investigations showed that the insulating phase of these materials always shows a E-antiferromagnetic ordering with the transition temperature that can be either = or and the magnetic wave vector is (1/2, 0, 1/2) [(1/4, 1/4, 1/4)], [ and denotes orthorhombic and pseudocubic settings respectively]. The signature of this unconventional magnetic ordering can also be identified in dc transport measurement zhou_prl (); pno_hall (); sno_hall () and SQUID magnetometry rno_squid (). Following the analysis of Zhou et al. zhou_prl (), vs. plot (inset of Fig. 3(c)) was used to determine a 145 K for the SL grown on NGO, which is very close to (1555 K) determined from resonant X-ray scattering measurement own_enolno (). Similar analysis for the sample on DSO substrate yields 90 K (Fig. 3(c)). These and are drastically altered compared to bulk compund of formally the same chemical composition EuLaNiO ( = = 190K) bulk_eudopedlno (). This result implies that epitaxial strain and the presence of hetero-interface have tremendous impact in ground state in this class of materials. Further investigations are required to determine any connection between these transition temperatures and orbital polarization of these SLs xldtransitiontem (). To examine the possibility for an antiferromagnetic metallic state keimer_raman (); lnoafm (), Hall effect measurements were carried out on the metallic superlattices. Previous work on nickelates indicates that Hall coefficient () shows a sign change from hole like to electron like behavior around pno_hall (); sno_hall (). This sign switching behaviour is absent in our SL s (see Fig. 3(d)) and this indicates that these metallic SLs do not likely to have E-AFM ordering.
In general, electronic structure of correlated materials is parametrized by hopping strength (), electron-electron correlation (), and charge transfer energy () or effective charge transfer energy () in context of the Zannen-Sawatzky-Allen (ZSA) phase diagram zsaphase () or its’ modified version cuo2_dd (). In relation to nickelates, very early photoemission spectroscopy measurements revealed that NiO has very small charge transfer energy pnoxas (); xps_dd (). The insulating phase has been identified as a covalent insulator by Barman et al xps_dd () with the gap arising from the + + charge fluctuations cuo2_dd (); nco_fujimori () (here denotes a hole on oxygen orbital). The importance of ligand hole states in realizing the insulator to metal transition in NiO has been further emphasized in several recent theoretical and experimental works khomskii_holeordering (); millis_siteselective (); sawatzky_hf (); georges_dmft (); own_enolno (). In addition, a recent RIXS (resonant inelastic x-ray scattering) experiment on Ni has clearly confirmed the presence of negative and the band gap of a O 2-2 type rixs (). To understand the strain induced suppression of the insulating phase, we focus on O -edge resonant X-ray absorption spectra xas1 (); pnoxas (); jian_nc (); nidopedlio (); cupratexas (); john (), where ligand hole states () can be identified as a prepeak around 528.5 eV due to the transition (here is a hole in oxygen 1 core state) The degree of Ni-O bond covalency can be monitored by the intensity, position, and width of this prepeak.
A direct inspection of Figure 4(a) and upper panel of Fig. 4(b) shows the movement of the prepeak towards higher photon energy as strain becomes more compressive (NGO LAO YAO), thus emphasizing a decrease of charge transfer energy with compressive strain. Microscopically, at the first approximation is related to the electron affinity of oxygen [(O)], the ionization potential of Ni [(Ni)], relative Madelung potential between Ni and O, and the nearest-neighbor distance between Ni and O () as = + (O) - (Ni) - / mit_rmp (). This observation implies that strain induced change in originates from the strong modulation in the relative Madelung potential. Most importantly, the FWHM of the pre-peak also increases with compressive strain signifying the enhancement of Ni-O hybridization. The modulation in both charge transfer energy and hybridization (covalency) results in the complete suppression of the insulating phase as schematically illustrated in Fig. 4(c). Such strain-induced ‘self-doping’ of 1ENO/1LNO SL highlights the utility of strain as a means of effective carrier doping without detrimental effects of chemical disorders.
In contrast to the SLs on other substrates, 1ENO/1LNO SL on DSO has a sizable energy splitting between two orbitals with different electronic occupancies (Fig. 2(c)). This implies the dominance of a uniform Jahn-Teller distortion over the other structural distortion modes for this SL (e.g. breathing mode, staggered Jahn-Teller order etc landautheorymillis ()). In this case, the electronic transitions from O 1 Ni -O hybridized states and O 1 Ni -O , hybridized states occur at slightly different energies and with different intensity (intensity number of holes). This in turn can lead to the observed shift of O pre-peak to higher photon energy and corresponding enhancement in FWHM for the SL on DSO (Fig. 3c). Further RIXS and, polarized XAS experiment on O edge will be required to investigate this scenario rixsdm ().
To summarize, correlated metal LaNiO and charge transfer insulator EuNiO have been heterostructured in the form of unit cell superlattices 1 uc EuNiO/1 uc LaNiO and the effects of epitaxial strain have been investigated using XRD, dc transport, Hall effect, resonant XAS and XLD measurements. The electronic and magnetic phases are highly tunable by application of strain and several unusual phases including non-Fermi liquid, paramagnetic insulator, antiferromagnetic insulator phases have been observed. The detailed analysis of XAS spectra on oxygen -edge revealed strain induced strong modulation of charge transfer energy and covalency, resulting insulator to metal transition.
S.M. and J.C. deeply thank D. Khomskii and K. Haule for theoretical discussion. S.M. is supported by IISc start up grant. D.M. is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Early Career Award Program under Award No. 1047478. Work at Brookhaven National Laboratory was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DESC0012704. J.C., X.L. are supported by the Gordon and Betty Moore Foundation EPiQS Initiative through Grant No. GBMF4534. X.L. acknowledges support of DE-SC 00012375 grant for synchrotron work. This research used resources of the Advanced Photon Source, a U.S. Department of Energy Office of Science User Facility operated by Argonne National Laboratory under Contract No. DE-AC02-06CH11357.
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