arXiv subjects

Ronny Sutarto

Publications and source records attributed to Ronny Sutarto.

32 records · Page 2Linked to original sources

arXiv2020

Strain induced stabilization of a static Jahn-Teller distortion in the O$^*$-phase of La$_{7/8}$Sr$_{1/8}$MnO$_3$

At room temperature, bulk La$_{7/8}$Sr$_{1/8}$MnO$_3$ is in the dynamic Jahn--Teller O$^*$ phase, but undergoes a transition to a static, magnetically ordered Jahn--Teller phase at lower temperatures. Here we study a $6$ unit cells thin film of this compound grown on SrTiO$_3$, resulting in small compressive strain due to a lattice mismatch of $\lesssim 0.2\%$. We combine X-ray absorption spectroscopy with multiplet ligand field theory to study the local electronic and magnetic properties of Mn in the film. We determine the Mn $d_{3z^2-r^2}$ orbital to be $0.13\;\text{eV}$ lower in energy than the $d_{x^2-y^2}$, which is a disproportionately large splitting given the small degree of compressive strain. We interpret this as resulting from the strain providing a preferential orientation for the MnO$_6$ octahedra, which are strongly susceptible to such a deformation in the vicinity of the phase transition. Hence, they collectively elongate along the $c$ axis into a static Jahn--Teller arrangement. Furthermore, we demonstrate the strongly covalent character of La$_{7/8}$Sr$_{1/8}$MnO$_3$, with a contribution of nearly $50\%$ of the one-ligand-hole configuration $d^{5} \underline{L}^1$ to the ground state wavefunction. Finally, we find the system to be in a high-spin configuration, with the projection of the local magnetic moment on the quantization axis being about $3.7\;μ_{\text{B}}/\text{Mn}$. We show, however, that the system is close to a high-spin--low-spin transition, which might be triggered by crystal field effects.

cond-mat.str-el↗

arXiv2020

Sudden collapse of magnetic order in oxygen deficient nickelate films

Oxygen vacancies play a crucial role in the control of the electronic, magnetic, ionic, and transport properties of functional oxide perovskites. Rare earth nickelates (RENiO$_{3-x}$) have emerged over the years as a rich platform to study the interplay between the lattice, the electronic structure, and ordered magnetism. In this study, we investigate the evolution of the electronic and magnetic structure in thin films of RENiO$_{3-x}$, using a combination of X-ray absorption spectroscopy and imaging, resonant X-ray scattering, and extended multiplet ligand field theory modeling. We find that oxygen vacancies modify the electronic configuration within the Ni-O orbital manifolds, leading to a dramatic evolution of long-range electronic transport pathways despite the absence of nanoscale phase separation. Remarkably, magnetism is robust to substantial levels of carrier doping, and only a moderate weakening of the $(1/4, 1/4, 1/4)_{pc}$ antiferromagnetic order parameter is observed, whereas the magnetic transition temperature is largely unchanged. Only at a certain point long-range magnetism is abruptly erased without an accompanying structural transition. We propose the progressive disruption of the 3D magnetic superexchange pathways upon introduction of point defects as the mechanism behind the sudden collapse of magnetic order in oxygen-deficient nickelates. Our work demonstrates that, unlike most other oxides, ordered magnetism in RENiO$_{3-x}$ is mostly insensitive to carrier doping. The sudden collapse of ordered magnetism upon oxygen removal may provide a new mechanism for solid-state magneto-ionic switching and new applications in antiferromagnetic spintronics.

cond-mat.str-el↗

arXiv2020

V 3$d$ charge and orbital states in V$_2$OPO$_4$ probed by x-ray absorption spectroscopy

V 3$d$ charge and orbital states in V$_2$OPO$_4$ have been investigated by means of x-ray absorption spectroscopy (XAS). The electronic structure of V$_2$OPO$_4$ is very unique in that the charge transfer between V$^{2+}$ and V$^{3+}$ in face sharing VO$_6$ chains provides negative thermal expansion as reported by Pachoud {\it et al.} [J. Am. Chem. Soc. {\bf 140}, 636 (2018).] The near edge region of O 1$s$ XAS exhibits the three features which can be assigned to transitions to O 2$p$ mixed into the unoccupied V 3$d$ $t_{2g}$ and $e_{g}$ orbitals of V$^{2+}$ and V$^{3+}$. The V 2$p$ XAS line shape can be reproduced by multiplet calculations for a mixed valence state with V$^{2+}$ and V$^{3+}$. The polarization dependence of the O 1$s$ and V 2$p$ XAS spectra indicates V 3$d$ orbital order in which $xy$ and $yz$ (or $zx$) orbitals are occupied at the V$^{3+}$ site in the face sharing chains. The occupied $xy$ orbital is essential for the antiferromagnetic coupling between the V$^{2+}$ and V$^{3+}$ sites along the chains while the occupied $yz$ (or $zx$) orbital provides the antiferromagnetic coupling between the V$^{2+}$ and V$^{3+}$ sites between the chains.

cond-mat.str-el↗

arXiv2020

Controlling the electrical and magnetic ground states by doping in the complete phase diagram of titanate Eu1-xLaxTiO3 thin films

EuTiO3, a band insulator, and LaTiO3, a Mott insulator, are both antiferromagnetic with transition temperatures ~ 5.5 K and ~ 160 K, respectively. Here, we report the synthesis of Eu1-xLaxTiO3 thin films with x = 0 to 1 by oxide molecular beam epitaxy. The films in the full range have high crystalline quality and show no phase segregation, allowing us carry out transport measurements to study their electrical and magnetic properties. From x = 0.03 to 0.95, Eu1-xLaxTiO3 films show conduction by electrons as charge carriers, with differences in carrier densities and mobilities, contrary to the insulating nature of pure EuTiO3 and LaTiO3. Following a rich phase diagram, the magnetic ground states of the films vary with increasing La-doping level, changing Eu1-xLaxTiO3 from an antiferromagnetic insulator to an antiferromagnetic metal, a ferromagnetic metal, a paramagnetic metal, and back to an antiferromagnetic insulator. These emergent properties reflect the evolutions of the band structure, mainly at the Ti t2g bands near the Fermi level, when Eu2+ are gradually replaced by La3+. This work sheds light on this method for designing the electrical and magnetic properties in strongly-correlated oxides and completes the phase diagram of the titanate Eu1-xLaxTiO3.

cond-mat.str-el↗

arXiv2019

Doping-dependent phonon anomaly and charge-order phenomena in the HgBa$_{2}$CuO$_{4+δ}$ and HgBa$_{2}$CaCu$_{2}$O$_{6+δ}$

Using resonant X-ray diffraction and Raman spectroscopy, we study charge correlations and lattice dynamics in two model cuprates, HgBa$_{2}$CuO$_{4+δ}$ and HgBa$_{2}$CaCu$_{2}$O$_{6+δ}$. We observe a maximum of the characteristic charge order temperature around the same hole concentration ($p \approx 0.09$) in both compounds, and concomitant pronounced anomalies in the lattice dynamics that involve the motion of atoms in and/or adjacent to the CuO$_2$ layers. These anomalies are already present at room temperature, and therefore precede the formation of the static charge correlations, and we attribute them to an instability of the CuO$_2$ layers. Our finding implies that the charge order in the cuprates is an emergent phenomenon, driven by a fundamental variation in both lattice and electronic properties as a function of doping.

cond-mat.supr-con↗

arXiv2019

Epitaxial Growth of Perovskite SrBiO$_3$ Film on SrTiO$_3$ by Oxide Molecular Beam Epitaxy

Hole-doped perovskite bismuthates such as Ba$_{1-x}$K$_x$BiO$_3$ and Sr$_{1-x}$K$_x$BiO$_3$ are well-known bismuth-based oxide high-transition-temperature superconductors. Reported thin bismuthate films show relatively low quality, likely due to their large lattice mismatch with the substrate and a low sticking coefficient of Bi at high temperatures. Here, we report the successful epitaxial thin film growth of the parent compound strontium bismuthate SrBiO$_3$ on SrO-terminated SrTiO$_3$ (001) substrates by molecular beam epitaxy. Two different growth methods, high-temperature co-deposition or recrystallization cycles of low-temperature deposition plus high-temperature annealing, are developed to improve the epitaxial growth. SrBiO$_3$ has a pseudocubic lattice constant $\sim$4.25 Å, an $\sim$8.8\% lattice mismatch on SrTiO$_3$ substrate, leading to a large strain in the first few unit cells. Films thicker than 6 unit cells prepared by both methods are fully relaxed to bulk lattice constant and have similar quality. Compared to high-temperature co-deposition, the recrystallization method can produce higher quality 1-6 unit cell films that are coherently or partially strained. Photoemission experiments reveal the bonding and antibonding states close to the Fermi level due to Bi and O hybridization, in good agreement with density functional theory calculations. This work provides general guidance to the synthesis of high-quality perovskite bismuthate films.

cond-mat.supr-con↗

arXiv2019

Itinerancy-dependent non-collinear spin textures in SrFeO3, CaFeO3, and CaFeO3/SrFeO3 heterostructures probed via resonant x-ray scattering

Non-collinear, multi-q spin textures can give rise to exotic, topologically protected spin structures such as skyrmions, but the reason for their formation over simple single-q structures is not well understood. While lattice frustration and the Dzyaloshinskii-Moriya interaction are known to produce non-collinear spin textures, the role of electron itinerancy in multi-q formation is much less studied. Here we investigated the non-collinear, helical spin structures in epitaxial films of the perovskite oxides SrFeO3 and CaFeO3 using magnetotransport and resonant soft x-ray magnetic diffraction. Metallic SrFeO3 exhibits features in its magnetoresistance that are consistent with its recently proposed multi-q structure. Additionally, the magnetic Bragg peak of SrFeO3 measured at the Fe L edge resonance energy asymmetrically broadens with decreasing temperature in its multi-q state. In contrast, insulating CaFeO3 has a symmetric scattering peak with an intensity 10x weaker than SrFeO3. Enhanced magnetic scattering at O K edge prepeak energies demonstrates the role of a negative charge transfer energy and the resulting oxygen ligand holes in the magnetic ordering of these ferrates. By measuring magnetic diffraction of CaFeO3/SrFeO3 superlattices with thick CaFeO3 layers, we find that the CaFeO3 helical ordering is coherent across 1 unit cell-thick SrFeO3 layers but not 6 unit cell-thick layers. We conclude that insulating CaFeO3 supports only a simple single-q helical structure in contrast to metallic SrFeO3 that hosts multi-q structures. Our results provide important insight into the role of electron itinerancy in the formation of multi-q spin structures.

cond-mat.mtrl-sci↗

arXiv2019

Evolution of charge order topology across a magnetic phase transition in cuprate superconductors

Charge order is now accepted as an integral constituent of cuprate high-temperature superconductors, one that is intimately related to other instabilities in the phase diagram including antiferromagnetism and superconductivity. Unlike nesting-induced Peierls-like density waves, the charge correlations in the CuO2 planes have been predicted to display a rich momentum space topology depending on the detailed fermiology of the system. However, to date charge order has only been observed along the high-symmetry Cu-O bond directions. Here, using resonant soft X-ray scattering, we investigate the evolution of the full momentum space topology of charge correlations in Ln2CuO4 (Ln=Nd, Pr) as a function of intrinsic electron doping. We report that, upon electron doping the parent Mott insulator, charge correlations first emerge in a hitherto-unobserved form, with full (Cinf) rotational symmetry in momentum-space. At higher doping levels, the orientation of charge correlations is sharply locked to the Cu-O bond high-symmetry directions, restoring a more conventional bidirectional charge order with enhanced correlation lengths. Through charge susceptibility calculations, we closely reproduce the drastic evolution in the topology of charge correlations across an antiferromagnetic quantum phase transition, highlighting the interplay between spin and charge degrees of freedom in electron-doped cuprates. Finally, using the established link between charge correlations and the underlying fermiology, we propose a revised phase diagram of Ln2CuO4 with a superconducting region extending toward the Mott limit.

cond-mat.str-el↗

arXiv2019

Complex magnetic order in nickelate slabs

Magnetic ordering phenomena have a profound influence on the macroscopic properties of correlated-electron materials, but their realistic prediction remains a formidable challenge. An archetypical example is the ternary nickel oxide system RNiO3 (R = rare earth), where the period-four magnetic order with proposals of collinear and non-collinear structures and the amplitude of magnetic moments on different Ni sublattices have been subjects of debate for decades. Here we introduce an elementary model system - NdNiO3 slabs embedded in a non-magnetic NdGaO3 matrix - and use polarized resonant x-ray scattering (RXS) to show that both collinear and non-collinear magnetic structures can be realized, depending on the slab thickness. The crossover between both spin structures is correctly predicted by density functional theory and can be qualitatively understood in a low-energy spin model. We further demonstrate that the amplitude ratio of magnetic moments in neighboring NiO6 octahedra can be accurately determined by RXS in combination with a correlated double cluster model. Targeted synthesis of model systems with controlled thickness and synergistic application of polarized RXS and ab-initio theory thus provide new perspectives for research on complex magnetism, in analogy to two-dimensional materials created by exfoliation.

cond-mat.str-el↗

arXiv2018

Intertwined Spin and Orbital Density Waves in MnP Uncovered by Resonant Soft X-ray Scattering

Unconventional superconductors are often characterized by numerous competing and even intertwined orders in their phase diagrams. In particular, the electronic nematic phases, which spontaneously break rotational symmetry and often simultaneously involve spin, charge and/or orbital orders, appear conspicuously in both the cuprate and iron-based superconductors. The fluctuations associated with these phases may provide the exotic pairing glue that underlies their high-temperature superconductivity. Helimagnet MnP, the first Mn-based superconductor under pressure, lacks high rotational symmetry. However our resonant soft X-ray scattering (RSXS) experiment discovers novel helical orbital density wave (ODW) orders in this three-dimensional, low-symmetry system, and reveals intertwined ordering phenomena in unprecedented detail. In particular, a ODW forms with half the period of the spin order and fully develops slightly above the spin ordering temperature, their domains develop simultaneously, yet the spin order domains are larger than those of the ODW, and they cooperatively produce another ODW with 1/3 the period of the spin order. These observations provide a comprehensive picture of the intricate interplay between spin and orbital orders in correlated materials, and they suggest that nematic-like physics ubiquitously exists beyond two-dimensional and high-symmetry systems, and the superconducting mechanism of MnP is likely analogous to those of cuprate and iron-based superconductors.

cond-mat.supr-con↗

arXiv2017

Orbital order and fluctuations in the two-leg ladder materials BaFe$_2X_3$ ($X$ = S and Se) and CsFe$_2$Se$_3$

The electronic structure of BaFe$_2X_3$ ($X$ = S and Se) and CsFe$_2$Se$_3$ in which two-leg ladders are formed by the Fe sites are studied by means of x-ray absorption and resonant inelastic x-ray scattering spectroscopy. The x-ray absorption spectra at the Fe L edges for BaFe$_2X_3$ exhibit two components, indicating that itinerant and localized Fe 3$d$ sites coexist. Substantial x-ray linear dichroism (XLD) is observed in polarization dependent spectra, indicating the existence of orbital order or fluctuation in the Fe-ladder even above the Néel temperature $T_N$. Direct exchange interaction along the legs of the Fe-ladder stabilizes the orbital and antiferromagnetic orders in BaFe$_2$S$_3$, while the ferromagnetic molecular orbitals are realized between the rungs in CsFe$_2$Se$_3$.

cond-mat.supr-con↗

arXiv2016

Quasiparticle interference of heavy fermions in resonant X-ray scattering

Resonant X-ray scattering (RXS) has recently become an increasingly important tool for the study of ordering phenomena in correlated electron systems. Yet, the interpretation of the RXS experiments remains theoretically challenging due to the complexity of the RXS cross-section. Central to this debate is the recent proposal that impurity-induced Friedel oscillations, akin to quasiparticle interference signals observed with the scanning tunneling microscope (STM), can lead to scattering peaks in the RXS experiments. The possibility that quasiparticle properties can be probed in RXS measurements opens up a new avenue to study the bulk band structure of materials with the orbital and element selectivity provided by RXS. Here, we test these ideas by combining RXS and STM measurements of the heavy fermion compound CeMIn$_5$ (M = Co, Rh). Temperature and doping dependent RXS measurements at the Ce-M$_4$ edge show a broad scattering enhancement that correlates with the appearance of heavy f-electron bands in these compounds. The scattering enhancement is consistent with the measured quasiparticle interference signal in the STM measurements, indicating that quasiparticle interference can be probed through the momentum distribution of RXS signals. Overall, our experiments demonstrate new opportunities for studies of correlated electronic systems using the RXS technique.

cond-mat.str-el↗

arXiv2014

Charge ordering in the electron-doped superconductor Nd2-xCexCuO4

In cuprate high-temperature superconductors, an antiferromagnetic Mott insulating state can be destabilized toward unconventional superconductivity by either hole- or electron-doping. In addition to these two electronic phases there is now a copious amount of evidence that supports the presence of a charge ordering (CO) instability competing with superconductivity inside the pseudogap state of the hole-doped (p-type) cuprates, but so far there has been no evidence of a similar CO in their electron-doped (n-type) counterparts. Here we report resonant x-ray scattering (RXS) measurements which demonstrate the presence of charge ordering in the n-type cuprate Nd2-xCexCuO4 near optimal doping. Remarkably we find that the CO in Nd2-xCexCuO4 occurs with similar periodicity, and along the same direction, as the CO in p-type cuprates. However, in contrast to the latter, the CO onset in Nd2-xCexCuO4 is higher than the pseudogap temperature, and is actually in the same temperature range where antiferromagnetic fluctuations are first detected -- thereby showing that CO and antiferromagnetic fluctuations are likely coupled in n-type cuprates. Overall our discovery uncovers a missing piece of the cuprate phase diagram and opens a parallel path to the study of CO and its relationship to other phenomena, such as antiferromagnetism (AF) and high-temperature superconductivity.

cond-mat.supr-con↗

arXiv2012

Strain-Induced Spin States in Atomically Ordered Cobaltites

Epitaxial strain imposed in complex oxide thin films by heteroepitaxy is recognized as a powerful tool for identifying new properties and exploring the vast potential of materials performance. A particular example is LaCoO3, a zero spin, nonmagnetic material in the bulk, whose strong ferromagnetism in a thin film remains enigmatic despite a decade of intense research. Here, we use scanning transmission electron microscopy complemented by X-ray and optical spectroscopy to study LaCoO3 epitaxial thin films under different strain states. We observed an unconventional strain relaxation behavior resulting in stripe-like, lattice modulated patterns, which did not involve uncontrolled misfit dislocations or other defects. The modulation entails the formation of ferromagnetically ordered sheets comprising intermediate or high spin Co3+, thus offering an unambiguous description for the exotic magnetism found in epitaxially strained LaCoO3 films. This observation provides a novel route to tailoring the electronic and magnetic properties of functional oxide heterostructures.

cond-mat.mtrl-sci↗