Pressure-induced unusual metallic state in EuNiO{}_{3}

Pressure-induced unusual metallic state in EuNiO

Hisao Kobayashi kobayash@sci.u-hyogo.ac.jp Graduate School of Material Science and Center for Novel Material Science under Multi-Extreme Conditions, University of Hyogo, Koto Hyogo 678-1297, JAPAN    Shugo Ikeda Graduate School of Material Science and Center for Novel Material Science under Multi-Extreme Conditions, University of Hyogo, Koto Hyogo 678-1297, JAPAN    Yoshitaka Yoda Japan Synchrotron Radiation Institute, Hyogo 679-5198, JAPAN    Naohisa Hirao Japan Synchrotron Radiation Institute, Hyogo 679-5198, JAPAN    Yasuo Ohishi Japan Synchrotron Radiation Institute, Hyogo 679-5198, JAPAN    J. A. Alonso Instituto de Ciencia de Materiales de Madrid, CSIC, Cantoblanco, E-28049 Madrid, SPAIN    M. J. Martinez-Lope Instituto de Ciencia de Materiales de Madrid, CSIC, Cantoblanco, E-28049 Madrid, SPAIN    R. Lengsdorf II. Physikalisches Institut, Universität zu Köln, Zülpicher Straße 77, 50937 Köln, GERMANY    D. I. Khomskii II. Physikalisches Institut, Universität zu Köln, Zülpicher Straße 77, 50937 Köln, GERMANY    M. M. Abd-Elmeguid II. Physikalisches Institut, Universität zu Köln, Zülpicher Straße 77, 50937 Köln, GERMANY
July 26, 2019
Abstract

The perovskite antiferromagnetic ( 220 K) insulator EuNiO undergoes at ambient pressure a metal-to-insulator transition at = 460 K which is associated with a simultaneous orthorhombic-to-monoclinic distortion, leading to charge disproportionation. We have investigated the change of the structural and magnetic properties of EuNiO with pressure (up to 20 GPa) across its quantum critical point (QCP) using low-temperature synchrotron angle-resolved x-ray diffraction and Eu nuclear forward scattering of synchrotron radiation, respectively. With increasing pressure we find that after a small increase of ( 2 GPa) and the induced magnetic hyperfine field at the Eu nucleus ( 9.7 GPa), both and are strongly reduced and finally disappear at 10.5 GPa, indicating a magnetic QCP at . The analysis of the structural parameters up to 10.5 GPa reveals no change of the lattice symmetry within the experimental resolution. Since the pressure-induced insulator-to-metal transition occurs at 6 GPa, this result implies the existence of an antiferromagnetic metallic state between 6 and 10.5 GPa. We further show from the analysis of the reported high pressure electrical resistance data on EuNiO at low-temperatures that in the vicinity of the QCP the system behaves as non-Fermi-liquid, with the resistance changing as , with n=1.6, whereas it becomes a normal Fermi-liquid, n = 2, for pressures above 15 GPa. On the basis of the obtained data a magnetic phase diagram in the (, ) space is suggested.

PACS numbers

71.30.+h, 62.50.-p, 71.28.+d, 76.80.+y

preprint:

I Introduction

The study of magnetic quantum transitions in strongly correlated electron systems has been the subject of continuous interest due to the observation of novel ground states near/at the magnetic-to-nonmagnetic transition leading to quantum critical point (QCP). Known examples are heavy-fermion metals where non-Fermi-liquid (NFL) phases (e.g. CeCuAu lohneysen96 (); lohneysen98 () and YbRhSi custers03 ()) and unconventional superconductivity (e.g. CePdSi mathur98 () and UGe saxena00 ()) appear.

Another promising but different class of systems for such studies are strongly correlated transition metal oxides khomskii14 () (e.g. O perovskites, = rare earth ion; = transition metal). The key aspect of these materials is that the interplay between spin, charge, and orbital degrees of freedom leads to the existence of several competing phases and in turn to complex and unusual phase diagrams imada98 (); dagotto05 (). Of particular interest are magnetically ordered transition metal oxides, in which a metal-to-insulator (MI) transition as well a magnetic-to-nonmagnetic transition can be tuned by external pressure. In this context, a central issue is what is the impact of those degrees of freedom on the ground state properties when a magnetic insulator is tuned to a nonmagnetic metal across a QCP.

In this respect, the perovskite rare earth nickelates NiO ( La) are excellent candidates for such studies as they exhibit an insulating antiferromagnetic (AF) ground state: The phase diagram of the NiO series is shown in Fig. 1. They display a well-defined MI transition at a temperature which increases with decreasing the size of ion ( = 130 K (Pr), cc, 600 K (Lu) torrance92 (); medarde97 ()). Simultaneously the lattice symmetry changes from orthorhombic () to monoclinic () which contains two nonequivalent Ni-sites (NiO) octahedra with slightly different Ni-O bond lengths, indicating charge disproportionation (CD), 2Ni Ni + Ni alonso99_0 (); alonso99_1 (); mizokawa00 ().

Another though related interpretation of the phenomena occurring at the MI transition is the important role of oxygen holes mizokawa00 (); johnston14 (), which play a very important role in these materials with negative charge-transfer gap zaanen85 (). This picture is actually very close to the picture of disproportionation to more and less covalent Ni sites, proposed by Goodenough goodenough96 () and in Ref. zhou04_0 (). As a matter of fact, in reality both these pictures are different sides for the same phenomenon, with the charges on Ni sites more different for smaller rare earths in the series NiO, and more equivalent for larger . Further on we denote both these pictures as CD.

Recently, resonant x-ray diffraction staub02 (); scagnoli06 (), Raman spectroscopic studies zaghrioui01 () and high resolution x-ray absorption at the Ni -edge medarde09 () indicate the existence of CD in the whole NiO series. Moreover, very recent studies on NiO using soft x-ray magnetic powder diffraction bodenthin11 () demonstrate that the NiO compounds have very similar electronic and magnetic states despite the large variation of the value of their .

At low temperatures, the transition to AF ordered state is also related to the size of the ion, i.e. for large ions ( = Pr and Nd) the MI transition occurs simultaneously with an antiferromagnetic (AF) ordering of the (Ni) sublattice (i.e. ), whereas for smaller ( = Sm Lu) ions, is much lower than (e.g. for EuNiO, = 220 K and = 463 K). The magnetic structure in the Ni sublattice of NiO proposed by neutron-powder diffraction alonso99_0 (); garcia92 (); rodriguez98 () has a magnetic propagation vector =(1/2,0,1/2) and consists of an up-up-down-down stacking of Ni magnetic moments. An alternative non-collinear magnetic structure with the same propagation vector is recently suggested by resonant soft x-ray magnetic diffraction studies on NiO scagnoli06 (); bodenthin11 (); scagnoli08 ().

As mentioned above, according to the magnetic phase diagram of the NiO series at ambient pressure (see Fig. 1), the ground state changes from an antiferromagnetic insulating ( La) to a nonmagnetic metallic state ( = La) through a QCP. Figure 1 shows the phase diagram in terms of the tolerance factor , which reflects the degree of distortion of perovskites and is determined by ratio of the relative -O and Ni-O bond lengths, and , . As the distortion is larger in for small ions, increases as the radius increases. The figure also shows that increases with increasing pressure. Regarding the effect of external pressure on and in NiO, only few compounds have been investigated up to very high pressure until now, in particular the pressure dependence of in them. While the initial change () up to about 2.8 GPa reveals a small increase of with pressure for = Sm, Eu and Gd zhou08 (), for NdNiO and PrNiO ( = ) is strongly reduced with pressure obradors93 (); canfield93 () and even suppressed to zero in PrNiO across a QCP zhou05 (). In the latter case a broad NFL behavior in the vicinity of the QCP has been reported zhou05 ().

In this work we study the pressure effect on the structural, transport and magnetic properties of one of perovskite nickelates, EuNiO. We have selected EuNiO ( = 463 K and = 220 K) which is one of typical NiO compounds with and thereby allows one to investigate the evolution of the magnetic state under high pressure across a QCP. The presence of Eu Mössbauer isotope in EuNiO allows us to realize such an investigation at a microscopic level using the Eu nuclear forward scattering (NFS) of synchrotron radiation - a technique based on the Mössbauer effect. The application of the Eu NFS technique allows one to probe the magnetic state of the Ni sublattice of EuNiO under pressure via the induced magnetic hyperfine (hf) field at the Eu nuclei which results from the ordered Ni magnetic moment. Some preliminary data which demonstrate the applicability of this technique on EuNiO have been published by some of the authors elsewhere (lengsdorf04 (), see below). The present Eu NFS data on EuNiO, in combination with low temperature synchrotron angle-resolved x-ray diffraction measurements up to about 20 GPa reveal a magnetic QCP at 10.5 GPa. Since the high pressure resistance data on EuNiO reveal a pressure-induced insulator-to-metal (IM) transition at 6 GPa lengsdorf04 (), the results suggest that for pressures between 6 and 10.5 GPa the pressure-induced metallic state is magnetically ordered. Similar metallic antiferronagnetic state was proposed in Ref. mazin07 (), and recently observed in strained multilayers of PrNiO/PrAlO hepting14 (). The presence of this state can be possibly explained by the picture of spin density wave lee11 (). As the lattice symmetry in this state in our system remains monoclinic, we suppose that certain CD exist in this case too, although we do not have definite proof of that. We further show from the analysis of the high pressure electrical resistivity data on EuNiO lengsdorf04 () that in different pressure ranges beyond the QCP both NFL and Fermi liquid (FL) regimes are realized.

It is worthwhile to mention that beside NiO bulk samples, rare earths nickelates are very actively studies nowadays as thin films and multilayers frano13 (); chakhalian14 (). In particular, one can effectively control their properties by using epitaxial strain (and spatial confinement) induced by the substrate, which is rather similar, but not identical to the pressure effects studied in this paper.

Ii Experimental details

A polycrystalline sample of EuNiO was prepared under an oxygen pressure of 200 bars. Details of the preparation and characterization were published elsewhere alonso95 (). The Eu NFS experiments were carried out under pressure and at low temperature using a clamp-type diamond-anvil cell (DAC) on beamline BL09XU at SPring-8. The first excited nuclear state of Eu has an energy of 21.541 keV (resonance energy) and a half lifetime of 9.7 ns. The pulsed synchrotron radiation was monochromatized to a bandwidth of 1.8 meV at the resonant excitation energy of Eu nuclei by a high-resolution monochromator. The monochromatized x-ray transmitted through the sample was detected with the stacked Si-avalanche photodiodes. The storage ring was operated in a special bunch mode where the interval between successive single bunches is 165.2 ns which is much longer than the half lifetime of the first excited nuclear state. The powder-samples were loaded with ruby chips into a sample cavity of a 0.5 mm diameter in a 0.2 mm thick Inconel 625 alloy gasket and mixtures of methanol-ethanol as a pressure-transmitting medium to ensure hydrostatic conditions. Pressure was calibrated by measuring the wavelength shift of the luminescence line of ruby chips in the clamp-type DAC at room temperature.

The x-ray diffraction data were collected under pressure up to approximately 20 GPa at 8 K by the angle-dispersive technique and using an image-plate detector on beamline BL10XU at SPring-8. The incident x-ray wavelength was 0.4153 Å, which was calibrated by measuring the x-ray diffraction pattern of CeO at ambient conditions. The powder-samples were loaded into a He-gas driven DAC with ruby chips and He as a pressure-transmitting medium to ensure hydrostatic conditions. Pressure was calibrated by measuring the wavelength shift of the luminescence line of ruby chips in the DAC at 8 K.

Iii Results and Discussion

iii.1 High pressure Eu nuclear forward scattering

As mentioned above, the Eu NFS of synchrotron radiation allows one to probe the magnetic state of the Ni sublattice of EuNiO under pressure via the induced magnetic hf field at the Eu nuclei which results from the ordered Ni magnetic moment. originates from the exchange (transferred) field due the admixture of the magnetic () excited state into the nonmagnetic () ground state. The magnitude of depends both on the size and relative orientation of the Ni moments around the Eu nuclei. The first attempt to demonstrate the applicability of this technique on EuNiO has been reported in Ref. lengsdorf04 (). In this work the authors only measured two pressure points (9.5 and 14.4 GPa) and detected no magnetic signal at 14.4 GPa, and thus these preliminary measurements provided no information about the pressure dependence of or , which is necessary to construct a (, )-magnetic phase diagram. In the present work using the high pressure Eu NFS technique we performed systematic measurements of the pressure dependences of and of EuNiO and were able to locate the magnetic QCP and thus to construct the (, )-magnetic phase diagram of EuNiO.

Figures 2 (a) and (b) show some selected Eu NFS spectra at different pressures and temperatures, both in the paramagnetic stats (7.3 GPa at 300K and 10.7 GPa at 5 K) and magnetically ordered state (4.7, 7.3, and 9.7 GPa at 5 K). In all spectra, we observe quantum beats due to electric quadruple and/or magnetic hf interactions which cause splitting of the nuclear levels and thus lead to a constructive interference of the photons emitted from these energy levels. As seen in Fig. 2 (a), the frequencies of quantum beats in the Eu NFS spectra observed below 9.7 GPa at 5 K are higher than those in the spectrum at 10.7 GPa and 5 K. These high frequencies of the quantum beats come from large energy splitting due to magnetic hf splitting of the Eu nuclear levels in the magnetically ordered state. In comparison, the feature of low frequency in the Eu NFS spectrum on the paramagnetic state at 7.3 GPa and 300 K (Fig. 2 (b)) is similar to that at 10.7 GPa and 5 K shown in Fig. 2 (a). These results suggest that magnetic ordering in EuNiO disappears at 5 K above 10.7 GPa.

The fits to the Eu NFS spectra were performed using the program package MOTIF shvydko00 (), applying the full dynamical theory of nuclear resonant scattering and including the diagonalization of the complete hyperfine Hamiltonian. The spectra at 5 K and 10.7 GPa and at 7.3 GPa and 300 K can be fitted by assuming a pure electric quadrupole interaction, indicating a paramagnetic state in EuNiO at 5 K and 10.7 GPa. However, the Eu NFS spectra in the magnetically ordered state at 5 K below 9.7 GPa were impossible to fit by only quadrupole interaction; therefore they were fitted by assuming a combined quadrupole and magnetic hf interactions. In such a case, we have considered the magnetic structure of EuNiO at ambient pressure defined by the magnetic propagation vector k = (1/2,0,1/2) as determined by neutron-powder diffraction rodriguez98 (). In this magnetic structure, the single Eu site is subdivided into two magnetically nonequivalent Eu sites with the ratio of 1:1. One Eu site is sandwiched between two [111] (in cubic setting) layers with parallel spins, i.e., surrounded by six Ni atoms with spin up and two Ni atoms with spin down, sees a transferred hf field, while the other Eu site is nonmagnetic owing to the cancellation of the Ni antiparallel sublattice magnetic moments at these Eu sites rodriguez98 ().

As shown in Fig. 2 (a), the Eu NFS spectra at 5 K below 9.7 GPa were well fitted by assuming two different Eu sites with the ratio of 1:1, that is, one half of Eu nuclei has with both electric quadrupole and magnetic hf interactions, and the other has a small electric quadrupole interaction only. Consequently, these results suggest that the magnetic structure in the Ni sublattice of EuNiO does not change under pressure up to 9.7 GPa. The values refined at 5 K are shown in Fig. 2 (a) as a function of pressure. As seen in Fig. 3 (a), the refined values of gradually increases with increasing pressure up to 7.3 GPa, passes through a maximum around 8 GPa, and then disappears at 10.7 GPa, indicating a suppression of Ni magnetic moments.

To obtain the pressure dependence of , we have analyzed temperature dependences of the Eu NFS spectra at different pressures on the basis of the fitting procedure (see above). Figure 2 (b) shows that the analytical spectra well reproduce the observed ones within the assumptions given above. The values of were evaluated from the temperature dependence of at different pressures (, ), using the Brillouin function with = 1/2. Figure 3 (b) shows the values of the pressure dependence of . As it is evident from Fig. 2 (b), slightly increases with pressure up to about 2 GPa. At higher pressures decreases and then collapses at about 10.5 GPa, indicating the collapse of magnetic order of the Ni moments in EuNiO. The initial increase of with pressure ( 2.4 GPa) is in a good agreement with the data of EuNiO reported from resistivity in Ref. zhou08 () and reflects the localized character of the Ni 3-states in the insulating phase of EuNiO.

Thus, the Eu NFS results reveal that with increasing pressure both and disappear at 10.5 GPa. Keeping in mind that EuNiO displays the pressure-induced IM transition at 6 GPa lengsdorf04 (), this result implies that under pressure the ground state of EuNiO changes from AF insulator to AF metal at and then to a nonmagnetic metal above . The impact of this finding on the complexity (, )-phase diagram of EuNiO will be discussed in the following sections.

iii.2 High pressure synchrotron angle-resolved x-ray diffraction at 8 K

Figure 4 shows some selected integrated x-ray diffraction patterns of EuNiO under pressure at 8 K. In the inset of Fig. 4 (a), the diffraction line at 18 deg. in the pattern corresponds to the (224) refraction in the orthorhombic structure. This diffraction line may split to two (224) and (22) refractions in the monoclinic structure. But, as it is also known from the structural data on NdNiO staub02 () and PrNiO medarde09 (), the monoclinic distortion in rare earth nickelates with larger rare earths is very small and difficult to detect directly. That is why it took long time, and required the use of novel, more sophisticated techniques, to finally establish that also these systems NiO, with larger ions, have monoclinic structure at low temperatures staub02 (); scagnoli06 (); medarde09 (). The situation is the same in our case: as seen in the inset of Fig. 4 (a), we did not observe the monoclinic distortion in EuNiO within our experimental resolution. All diffraction lines in the x-ray diffraction patterns are labeled with the indices of the orthorhombic structure. But these results indicate at least that there is no other pressure-induced structural symmetry change up to 20 GPa at 8 K, within our experimental resolution.

Integrated x-ray diffraction patterns were analyzed with the Rietveld refinement program RIETAN-2000 izumi00 () using the orthorhombic structure. In each x-ray diffraction pattern, the regions at 2 7.2, 8.4 and 12.2 deg. were excluded from the refinement procedure where very weak diffraction lines from impurity phases were observed. All integrated x-ray diffraction lines in the patterns up to 20 GPa gave good fits as shown in Fig. 4. It should be noted that these refinement procedures were used to derive individual atomic coordination parameters in addition to the lattice parameters.

In Fig. 5 (a), we show the pressure dependence of the refined lattice parameters of EuNiO at 8 K. As shown in Fig. 5 (a), the pressure dependences of = , = , and = 2 exhibit no discontinuity up to 10.5 GPa, indicating within the experimental resolution no structural phase transition at 6 GPa. These pressure variations further reveal that the pressure dependences of , and are quite different. With increasing pressure, the value of decreases more rapidly than those of and and the value of almost saturates at 10.5 GPa. However, no change of the lattice symmetry is observed.

The linear compressibilities of the lattice parameters were estimated to be = 1.11(2), = 2.52(3), and =1.20(1) GPa below 10.5 GPa. The estimated value of is twice larger than those of and . Furthermore, the pressure dependences of and are different from those at room temperature lengsdorf04 (). The bulk moduli below 10 GPa and above 11 GPa were evaluated based on the Murnaghan equation,

(1)

where is the ambient-pressure volume and represents a pressure derivative of . The solid lines in Fig. 5 (b) represent the best-fitting curves obtained and the values were refined to be 195(2) and 232(4) GPa below 10 GPa and above 11 GPa, respectively. The value refined below 10 GPa is comparable with those refined at room temperature lengsdorf04 (); zhou04 ().

Of particular interest is our finding that no change of the lattice symmetry has been observed in metallic magnetic ground state for 6 GPa 10.5 GPa. One possibility is that the monoclinic distortion associated with CD is too weak to be detected by our measurements. But the alternative is that the CD is still preserved to some extent also in the metallic state in EuNiO above 6 GPa. This possibility does not contradict our results of Eu NFS in the AF metallic state. In this respect, we want to mention that such an unusual ground state has been predicted in and observed under high pressure in similar compounds orbitally degenerate compounds (e.g. YNiO mazin07 ()).

Regarding the observed anomalous pressure dependence of the lattice parameter for 10.5 GPa, it is obvious that the anomaly of is not related to the pressure-induced IM transition at 6 GPa but rather corresponds to the transition from the antiferromagnetic metallic to a nonmagnetic metallic state at , i.e. at the magnetic QCP of EuNiO. The origin of this anomaly will be discussed in Section C.

iii.3 Magnetic and electronic transitions versus structural parameters

In the following, we would like discuss the structural response to the pressure-induced IM transition and magnetic QCP in EuNiO. In the inset of Fig. 5 (a), we show the pressure dependence of the effective bandwidth as deduced from the structural parameters, which reveals a significant increase around the pressure-induced IM transition ( 6 GPa). is known to be related to the ligand-to-metal hybridization (effective hopping) and can be described in NiO compounds in terms of the Ni-O bond length and the Ni-O-Ni bond angle by the relation of harrision80 (). Thus an increase of implies a corresponding increase of the effective hopping which is expected as the system is tuned to a metallic state. The averaged and values of EuNiO under pressure were evaluated from the refined lattice and individual atomic coordination parameters to estimate the averaged value under pressure comm (). Obviously the pressure-induced increase of above the pressure-induced IM transition ( 6 GPa) reflects the onset of the metallic state in EuNiO.

On the other hand, is also related to () through the perturbation formula for insulators khomskii14 ():

(2)

where is the on-site - Coulomb interaction energy, the effective hoping matrix element, describes the ligand-to-metal charge-transfer energy, and is the Coulomb repulsion of the two holes at the oxygen site. As shown in Fig. 2 (b), the pressure dependence of in the insulating phase of EuNiO ( 6 GPa) reveals an increase with pressure up to about 2.4 GPa, followed by a decrease upon approaching the pressure-induced IM transition, whereas monotonously increases in the whole insulator phase ( 6 GPa). Such a deviation from a linear correlation between and in magnetic insulators zhou08 () reflects the proximity to a crossover from localized to itinerant electronic behavior in EuNiO above about 2.4 GPa. This is in agreement with the picture proposed by Zhou . zhou03 () to explain the pressure-induced magnetic properties in the NiO series.

Now, we would like to provide an explanation of the observed anomalous behavior of the lattice parameter across the QCP at 10.5 GPa. The pressure-induced change of the lattice parameter is governed by two competing mechanisms that act in opposite ways: (i) the decrease of the average Ni-O bond length due to compression of the NiO octahedra which leads to a gradual with pressure; and (ii) the increase of the average Ni-O-Ni bonding angle which results in a reduction of the tilting of NiO octahedra and thereby an with pressure. We argue that below 10.5 GPa the compression of (i) is the dominating factor and prevails in the pressure dependence of , leading to a decrease of with pressure. However, at and beyond the QCP the compression of becomes weaker and comparable to that due to the increase of (ii). We believe that this could be related to a corresponding change of the magnetoelastic coupling at and beyond the QCP. As a result, the two competing mechanisms become almost equal and compensate each other, causing a nearly pressure independent change of the lattice parameter for 10.5 GPa. In such a case, one would anticipate melting of the charge disproportionation in the nonmagnetic metallic state, caused by the increased bandwidth.

iii.4 Suggested (, )-phase diagram of EuNiO

On the basis of our Eu NFS and low temperature synchrotron x-ray data, we would like to propose the (, )-phase diagram for EuNiO, shown in Fig. 6. The overall features of the phase diagram can be summarized as follows: at ambient pressure below 460 K the system undergoes a transition from the orthorhombic metallic state to a monoclinic CD insulating state. The insulating CD state exhibits antiferromagnetic ordering below 220 K. Under pressure, this AF insulating and CD ground state changes to an AF metallic and CD state at 6 GPa. At higher pressures the system displays a transition to a nonmagnetic state at 10.5 GPa i.e. QCP. By further increasing pressure beyond the QCP, the system reveals NFL and FL behavior for 10.5 GPa 14.8 GPa and 15.9 GPa 17.5 GPa, respectively. The identification of NFL and FL regions are based on the analysis of the original low temperature electrical resistance data on EuNiO reported in Ref. lengsdorf04 (). To analyze the electrical resistance data (), we used the power-law fitting to (, ) of EuNiO in the temperature range between 10 and 45 K with ()(=()-) , where n = 2 and 1 n 2 for FL and NFL behavior, respectively. As shown in Figs. 7 (a) and (b), the power-law with n = 1.6 fits the experimental data fairly well for = 11.5, 13.2 and 14.8 GPa, indicating NFL behavior, whereas n = 2 is the best fitting for 15.9 and 17.5 GPa, corresponding to FL behavior. Our finding of a NFL behavior in EuNiO with n = 1.6 is similar to that reported from high pressure resistivity data on PrNiO zhou05 (). The authors show in addition that the suppression of the insulating state of PrNiO ( = 130K ) above 1.3 GPa is accompanied by a transformation to a NFL phase in which the resistivity varies proportional to with n = 1.33 and 1.6 over a broad pressure range. In EuNiO with , we only observe a NFL behavior with n=1.6.

Iv Summary

We have investigated the pressure effect up to about 20 GPa on the structural and magnetic properties of the antiferromagnetic insulator rare earth nickelate EuNiO using low-temperature synchrotron angle-resolved x-ray diffraction and Eu nuclear forward scattering (NFS) of synchrotron radiation, respectively. The Eu NFS technique allows one to probe the magnetic state of the Ni sublattice of EuNiO under pressure via the induced magnetic hyperfine field at the Eu nuclei which originates from the ordered Ni magnetic moments, and thus to investigate the evolution of the magnetic state under high pressure across a quantum critical point. The experimental results can be summarized as follows.

EuNiO shows two transitions: an insulator-to-metal transition at 6 GPa (already reported) and magnetic-to-nonmagnetic transition with a quantum critical point at 10.5 GPa. In this context, we would to refer to the recent observation of similar metallic antiferromagnetic phase in PrNiO strained multilayers hepting14 ().

The analysis of the pressure dependence of the structural parameters revealed a significant increase of the effective bandwidth , which is related to effective hopping, around the pressure-induced IM transition ( 6 GPa). However, we did not detect, within the resolution of the x-ray measurements, any anomalies in the lattice parameters at the IM transition at 6 GPa, and have seen only slight change of the bulk modulus at the quantum phase transition at 10.5 GPa. This lets us suggest that most probably the charge disproportionation, existing in NiO in the insulating phase, survives to certain extent also in the metallic phase. Furthermore, we have shown from the analysis of reported high pressure resistance data on EuNiO at low-temperatures that in the vicinity of the QCP the system behaves as non-Fermi-liquid, the resistance behaving as , with n=1.6, whereas it becomes a normal Fermi-liquid, n = 2, for pressures above 15 GPa. Based on all obtained data we propose the (, )-phase diagram for EuNiO, shown in Fig. 6. We feel that the properties of other nickelates of this class might be similar to those revealed here, i.e. they may be representative also for other perovskite nickelates.

Acknowledgements.
The Eu NFS and x-ray diffraction experiments under pressure were performed at SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal Nos. 2010A1517, 2008B1460, and 2007A1450). M.M.A. and D.K. would like to thank the Deutsche Forschungsgemeinschaft (DFG) for the support through SFB 608. The work of D.K. was supported by the German project FOR 1346 and by Cologne University within the German Excellence Initiative.

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Figure 1: (Color online) The phase diagram of the NiO series as a function of the tolerance factor (see text) and external pressure, adapted from data in Refs. torrance92 (); medarde97 (); alonso99_1 (). PM and AF stand for paramagnetic and antiferromagnetic, respectively.
Figure 2: (Color online) Selected Eu nuclear forward scattering spectra of EuNiO (a) at 5 K under pressures and (b) at 7.3 GPa as a function of temperature. The closed circles with error bars indicate the observed spectra and the red solid lines represent the best fitted curves obtained using MOTIF shvydko00 ().
Figure 3: (a) Refined magnetic hyperfine field at 5 K and (b) evaluated Néel temperature of EuNiO as a function of pressure. At ambient pressure, was refined from the data at 3 K in Ref. lengsdorf04 () using MOTIF shvydko00 () by assuming two different Eu sites with the ratio of 1:1. Lines through the data points are only a guide to the eye. The broken lines in (a) and (b) indicate the pressure-induced insulator-to-metal transition at 6 GPa which is taken from the high pressure low temperature resistivity data reported in Ref. lengsdorf04 ().
Figure 4: Selected integrated x-ray diffraction patterns of EuNiO under (a) 2.0, (b) 7.6, and (c) 17.4 GPa at 8 K, where the crosses show the integrated x-ray diffraction intensities. The solid lines represent the results of Rietveld refinement fitting izumi00 () and the tick marks show the positions of all reflections allowed by the orthorhombic symmetry. The differences between the integrated and calculated intensities are shown below the tick marks. The inset in (a) shows an enlarged high-angle 2 region in the pattern at 2.0 GPa, where the effect of monoclinic distortion can be expected.
Figure 5: (Color online) (a) Refined lattice parameters, , , and , ( = , = , and = 2) and (b) evaluated volume, , of EuNiO at 8 K as a function of pressure. The broken lines in (a) are visual guides. The solid lines in (b) represent the fitting results based on the Murnaghan equation. The inset in (a) shows the pressure dependence of normalized bandwidth / which was evaluated by the refined lattice and individual atomic coordination parameters under pressure. The broken lines in the inset are visual guides.
Figure 6: (Color online) Pressure vs. temperature phase diagram of EuNiO. The closed red circles represent Néel temperature evaluated by present Eu nuclear forward scattering data. The open squares indicate the pressure dependence metal-to-insulator transition temperature reproduced from Ref. cheng10 (); extrapolated to the critical pressure (cross point at low temperatures) indicates the pressure-induced insulator-to-metal transition at 6 GPa which is taken from the high pressure low temperature resistivity data reported in Ref. lengsdorf04 (). Non Fermi-liquid (NFL) and Fermi-liquid (FL) regions above 10.5 GPa are identified from the analysis of the original low temperature electrical resistance data on EuNiO reported in Ref. lengsdorf04 ().
Figure 7: (Color online) Power-law temperature dependences of electrical resistance in the pressure ranges from 11.5 to 14.8 GPa (a) and from 15.9 to 17.5 GPa (b). The closed symbols indicate the experimental date taken from Ref. lengsdorf04 (). The solid lines represent the fitting results.
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