arXiv subjects

A. S. Botana

Publications and source records attributed to A. S. Botana.

At least 19 recordsLinked to original sources

arXiv2026

Layer Architecture Shapes Electronic, Magnetic, and Lattice Interactions in Ruddlesden-Popper Nickelates

The discovery of superconductivity in Ruddlesden-Popper nickelates has raised a central question: how does layer architecture shape the electronic, magnetic, and lattice interactions relevant to pairing? Here, we report a detailed comparative study of the two polymorphs of La3Ni2O7--the alternating monolayer-trilayer (LNO-1313) and bilayer (LNO-2222) structures--and the related trilayer compound La4Ni3O10, using both Ni L3- and O K-edge RIXS. We find that LNO-1313 and La4Ni3O10 share strikingly similar electronic, magnetic, and lattice excitations, whereas bilayer LNO-2222 exhibits distinct features. Compared to LNO-2222, LNO-1313 and La4Ni3O10 have weaker orbital polarization, enhanced 3d8L character, a reduced out-of-plane magnetic-exchange scale, and stronger EPC. Within an effective local-moment framework, an entangled-dimer scenario provides a natural description of the spin excitations generated by strong antiferromagnetic interlayer coupling. Its advantage over conventional spin-wave theory is clearest in bilayer LNO-2222, where the interlayer coupling dominates the intralayer interactions. These findings provide critical experimental constraints for future theoretical models for the low-energy physics relevant to superconductivity in these layered nickelates.

cond-mat.str-el↗

arXiv2024

Capping effects on spin and charge excitations in parent and superconducting Nd1-xSrxNiO2

Superconductivity in infinite layer nickelates Nd1-xSrxNiO2 has so far been achieved only in thin films raising questions on the role of substrates and interfaces. Given the challenges associated with their synthesis it is imperative to identify their intrinsic properties. We use Resonant Inelastic X-ray Scattering (RIXS) to investigate the influence of the SrTiO3 capping layer on the excitations of Nd1-xSrxNiO2 (x = 0 and 0.2). Spin excitations are observed in parent and 20% doped Nd1-xSrxNiO2 regardless of capping, proving that magnetism is intrinsic to infinite-layer nickelates and appears in a significant fraction of their phase diagram. In parent and superconducting Nd1-xSrxNiO2, the spin excitations are slightly hardened in capped samples compared to the non-capped ones. Additionally, a weaker Ni - Nd charge transfer peak at ~ 0.6 eV suggests that the hybridization between Ni 3d and Nd 5d orbitals is reduced in capped samples. From our data, capping induces only minimal differences in Nd1-xSrxNiO2 and we phenomenologically discuss these differences based on the reconstruction of the SrTiO3 - NdNiO2 interface and other mechanisms such as crystalline disorder.

cond-mat.supr-con↗

arXiv2024

Electronic band structure of a superconducting nickelate probed by the Seebeck coefficient in the disordered limit

Superconducting nickelates are a new family of strongly correlated electron materials with a phase diagram closely resembling that of superconducting cuprates. While analogy with the cuprates is natural, very little is known about the metallic state of the nickelates, making these comparisons difficult. We probe the electronic dispersion of thin-film superconducting 5-layer ($n=5$) and metallic 3-layer ($n=3$) nickelates by measuring the Seebeck coefficient, $S$. We find a temperature-independent and negative $S/T$ for both $n=5$ and $n=3$ nickelates. These results are in stark contrast to the strongly temperature-dependent $S/T$ measured at similar electron filling in the cuprate La$_{1.36}$Nd$_{0.4}$Sr$_{0.24}$CuO$_4$. The electronic structure calculated from density functional theory can reproduce the temperature dependence, sign, and amplitude of $S/T$ in the nickelates using Boltzmann transport theory. This demonstrates that the electronic structure obtained from first-principles calculations provides a reliable description of the Fermiology of superconducting nickelates, and suggests that, despite indications of strong electronic correlations, there are well-defined quasiparticles in the metallic state. Finally, we explain the differences in the Seebeck coefficient between nickelates and cuprates as originating in strong dissimilarities in impurity concentrations. Our study demonstrates that the high elastic scattering limit of the Seebeck coefficient reflects only the underlying band structure of a metal, analogous to the high magnetic field limit of the Hall coefficient. This opens a new avenue for Seebeck measurements to probe the electronic band structures of relatively disordered quantum materials.

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arXiv2024

Preempted phonon-mediated superconductivity in the infinite-layer nickelates

Nickelate superconductors are outstanding materials with intriguing analogies with the cuprates. These analogies suggest that their superconducting mechanism may be unconventional, although this fundamental question is currently under debate. Here, we scrutinize the role played by electronic correlations in enhancing the electron-phonon coupling in the infinite-layer nickelates and the extent to which this may promote superconductivity. Specifically, we use $ab$ $initio$ many-body perturbation theory to perform state-of-the-art $GW$ and Eliashberg-theory calculations. We find that the electron-phonon coupling is effectively enhanced compared to density-functional-theory calculations. This enhancement may lead to low-$T_c$ superconductivity in the parent compounds already. However, it remains marginal in the sense that it cannot explain the record $T_c$s obtained with doping. This circumstance implies that conventional superconductivity is preempted by another pairing mechanism in the infinite-layer nickelates.

cond-mat.supr-con↗

arXiv2023

Polymorphism in Ruddlesden-Popper $La_{3}Ni_{2}O_{7}$: Discovery of a Hidden Phase with Distinctive Layer Stacking

We report the discovery of a novel form of Ruddlesden-Popper (RP) oxide, which stands as the first example of long-range, coherent polymorphism in this class of inorganic solids. Rather than the well-known, uniform stacking of perovskite blocks ubiquitously found in RP phases, this newly discovered polymorph of the bilayer RP phase $La_{3}Ni_{2}O_{7}$adopts a novel stacking sequence in which single and trilayer blocks of $NiO_{6}$ octahedra alternate in a 1313 sequence. Crystals of this new polymorph are described in space group Cmmm, although we note evidence for a competing Imcm variant. Transport measurements at ambient pressure reveal metallic character with evidence of a charge density wave transition with onset at T = 134 K, which lies intermediate between that of the standard 2222 polymorph of $La_{3}Ni_{2}O_{7}$ (space group Amam) and the trilayer RP phase, $La_{4}Ni_{3}O_{10}$. The discovery of such polymorphism could reverberate to the expansive range of science and applications that rely on RP materials, particularly the recently reported signatures of superconductivity with $T_{c}$ as high as 80 K above 14 GPa in bilayer $La_{3}Ni_{2}O_{7}$.

cond-mat.supr-con↗

arXiv2023

Orbital polarization, charge transfer, and fluorescence in reduced valence nickelates

This paper presents a simple formalism for calculating X-ray absorption (XAS) and resonant inelastic x-ray scattering (RIXS) that has as input orbital-resolved density of states from a single-particle or many-body \textit{ab initio} calculation and is designed to capture itinerant-like features. We use this formalism to calculate both the XAS and RIXS with input from DFT and DFT+DMFT for the recently studied reduced valence nickelates $R_4$Ni$_3$O$_8$ and $R$NiO$_2$ ($R$ a rare earth), and these results are then contrasted with those for the cuprate CaCuO$_2$ and the unreduced nickelate $R_4$Ni$_3$O$_{10}$. In contrast to the unreduced $R_4$Ni$_3$O$_{10}$, the reduced valence nickelates as well as the cuprate show strong orbital polarization due to the dominance of $x^2-y^2$ orbitals for the unoccupied $3d$ states. We also reproduce two key aspects of a recent RIXS experiment for $R_4$Ni$_3$O$_8$: (i) a charge transfer feature between $3d$ and oxygen $2p$ states whose energy we find to decrease as one goes from $R$NiO$_2$ to $R_4$Ni$_3$O$_8$ to the cuprate, and (ii) an energy-dependent polarization reversal of the fluorescence line that arises from hybridization of the unoccupied $z^2$ states with $R$ 5d states. We end with some implications of our results for the nature of the $3d$ electrons in reduced valence nickelates.

cond-mat.str-el↗

arXiv2022

Role of Oxygen States in the Low Valence Nickelate La$_4$Ni$_3$O$_8$

The discovery of superconductivity in square-planar low valence nickelates has ignited a vigorous debate regarding their essential electronic properties: Do these materials have appreciable oxygen charge-transfer character akin to the cuprates, or are they in a distinct Mott-Hubbard regime where oxygen plays a minimal role? Here, we resolve this question using O $K$-edge resonant inelastic x-ray scattering (RIXS) measurements of the low valence nickelate La$_{4}$Ni$_{3}$O$_{8}$ and a prototypical cuprate La$_{2-x}$Sr$_{x}$CuO$_{4}$ ($x=0.35$). As expected, the cuprate lies deep in the charge-transfer regime of the Zaanen-Sawatzky-Allen scheme. The nickelate, however, is not well described by either limit of the ZSA scheme and is found to be of mixed charge-transfer/Mott-Hubbard character with the Coulomb repulsion $U$ of similar size to the charge-transfer energy $Δ$. Nevertheless, the transition-metal-oxygen hopping is larger in La$_{4}$Ni$_{3}$O$_{8}$ than in La$_{2-x}$Sr$_{x}$CuO$_{4}$, leading to a significant superexchange interaction and an appreciable hole occupation of the ligand O orbitals in La$_{4}$Ni$_{3}$O$_{8}$ despite its larger $Δ$. Our results clarify the essential characteristics of low valence nickelates and put strong constraints on theoretical interpretations of superconductivity in these materials.

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arXiv2021

Nickelate superconductors: an ongoing dialog between theory and experiments

After decades of fundamental research, unconventional superconductivity has recently been demonstrated in rare-earth infinite-layer nickelates. The current view depicts these systems as a new category of superconducting materials, as they appear to be correlated metals with distinct multiband features in their phase diagram. Here, we provide an overview of the state of the art in this rapidly evolving topic.

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arXiv2021

Strong Superexchange in a $d^{9-δ}$ Nickelate Revealed by Resonant Inelastic X-Ray Scattering

The discovery of superconductivity in a $d^{9-δ}$ nickelate has inspired disparate theoretical perspectives regarding the essential physics of this class of materials. A key issue is the magnitude of the magnetic superexchange, which relates to whether cuprate-like high-temperature nickelate superconductivity could be realized. We address this question using Ni L-edge and O K-edge spectroscopy of the reduced trilayer nickelate $d^{9-1/3}$ La4Ni3O8 and associated theoretical modeling. A magnon energy scale of ~80 meV resulting from a nearest-neighbor magnetic exchange of $J = 69(4)4$ meV is observed, proving that $d^{9-δ}$ nickelates can host a large superexchange. This value, along with that of the Ni-O hybridization estimated from our O K-edge data, implies that trilayer nickelates represent an intermediate case between the infinite-layer nickelates and the cuprates, and suggests that they represent a promising route towards higher-temperature nickelate superconductivity.

cond-mat.supr-con↗

arXiv2020

Synthesis, Engineering, and Theory of 2D van der Waals Magnets

Spontaneous magnetic order is a routine instance in three-dimensional (3D) materials but for a long time, it remained elusive in the 2D world. Recently, the first examples of (stand-alone) 2D van der Waals (vdW) crystals with magnetic order, either antiferromagnetic or ferromagnetic, have been reported. In this review, we describe the state of the art of the nascent field of magnetic 2D materials focusing on synthesis, engineering, and theory aspects. We also discuss challenges and some of the many different promising directions for future work.

cond-mat.mtrl-sci↗

arXiv2020

Intertwined density waves in a metallic nickelate

Nickelates are a rich class of materials, ranging from insulating magnets to superconductors. But for stoichiometric materials, insulating behavior is the norm, as for most late transition metal oxides. Notable exceptions are the 3D perovskite LaNiO$_3$, an unconventional paramagnetic metal, and the layered Ruddlesden-Popper phases R$_4$Ni$_3$O$_{10}$, (R=La, Pr, Nd). The latter are particularly intriguing because they exhibit an unusual metal-to-metal transition. Here, we demonstrate that this transition results from an incommensurate density wave with both charge and magnetic character that lies intermediate in behavior between the metallic density wave seen in chromium metal and the insulating stripes typically found in layered nickelates. As such, R$_4$Ni$_3$O$_{10}$, which appears to be the first known example of an itinerant spin density wave in a 3d transition metal oxide, represents an important bridge between the paramagnetism of 3D metallic LaNiO$_3$ at higher nickel valence and the polaronic behavior of quasi-2D R$_{2-x}$Sr$_x$NiO$_4$ at lower nickel valence.

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arXiv2019

Spin stripe order in a square planar trilayer nickelate

Trilayer nickelates, which exhibit a high degree of orbital polarization combined with an electron count (d8.67) corresponding to overdoped cuprates, have been identified as a promising candidate platform for achieving high-Tc superconductivity. One such material, La4Ni3O8, undergoes a semiconductor-insulator transition at ~105 K, which was recently shown to arise from the formation of charge stripes. However, an outstanding issue has been the origin of an anomaly in the magnetic susceptibility at the transition and whether it signifies formation of spin stripes akin to single layer nickelates. Here we report single crystal neutron diffraction measurements (both polarized and unpolarized) that establish that the ground state is indeed magnetic. The ordering is modeled as antiferromagnetic spin stripes that are commensurate with the charge stripes, the magnetic ordering occurring in individual trilayers that are essentially uncorrelated along the crystallographic c-axis. Comparison of the charge and spin stripe order parameters reveals that, in contrast to single-layer nickelates such as La2-xSrxNiO4 as well as related quasi-2D oxides including manganites, cobaltates, and cuprates, these orders uniquely appear simultaneously, thus demonstrating a stronger coupling between spin and charge than in these related low-dimensional correlated oxides.

cond-mat.str-el↗

arXiv2018

Averievite: a copper oxide kagome antiferromagnet

Averievite, Cu$_5$V$_2$O$_{10}$(CsCl), is an oxide mineral composed of Cu$^{2+}$ kagome layers sandwiched by Cu$^{2+}$-V$^{5+}$ honeycomb layers. We have synthesized this oxide and investigated its properties from ab initio calculations along with susceptibility and specific heat measurements. The data indicate a Curie-Weiss temperature of 185 K as well as long-range magnetic order at 24 K due to the significant interlayer coupling from the honeycomb copper ions. This order is suppressed by substituting these coppers by isoelectronic zinc, suggesting that Zn-substituted averievite is a promising spin liquid candidate. A further proposed substitution that replaces V$^{5+}$ by Ti$^{4+}$ not only dopes the material, but is predicted to give rise to a two-dimensional electronic structure featuring Dirac crossings. As such, averievite is an attractive platform for S=1/2 kagome physics with the potential for realizing novel electronic states.

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arXiv2018

Layered palladates and their relation to nickelates and cuprates

We explore the layered palladium oxides La$_2$PdO$_4$, LaPdO$_2$ and La$_4$Pd$_3$O$_8$ via ab initio calculations. La$_2$PdO$_4$, being low spin $d^8$, is quite different from its high spin nickel analog. Hypothetical LaPdO$_2$, despite its $d^9$ configuration, has a paramagnetic electronic structure very different from cuprates. On the other hand, the hypothetical trilayer compound La$_4$Pd$_3$O$_8$ ($d^{8.67}$) is more promising in that its paramagnetic electronic structure is very similar to that of overdoped cuprates. But even in the $d^9$ limit (achieved by partial substitution of La with a 4+ ion), we find that an antiferromagnetic insulating state cannot be stabilized due to the less correlated nature of Pd ions. Therefore, this material, if it could be synthesized, would provide an ideal platform for testing the validity of magnetic theories for high-temperature superconductivity.

cond-mat.supr-con↗

arXiv2017

Separation of Electron and Hole Dynamics in the Semimetal LaSb

We report investigations on the magnetotransport in LaSb, which exhibits extremely large magnetoresistance (XMR). Foremost, we demonstrate that the resistivity plateau can be explained without invoking topological protection. We then determine the Fermi surface from Shubnikov - de Haas (SdH) quantum oscillation measurements and find good agreement with the bulk Fermi pockets derived from first principle calculations. Using a semiclassical theory and the experimentally determined Fermi pocket anisotropies, we quantitatively describe the orbital magnetoresistance, including its angle dependence. We show that the origin of XMR in LaSb lies in its high mobility with diminishing Hall effect, where the high mobility leads to a strong magnetic field dependence of the longitudinal magnetoconductance. Unlike a one-band material, when a system has two or more bands (Fermi pockets) with electron and hole carriers, the added conductance arising from the Hall effect is reduced, hence revealing the latent XMR enabled by the longitudinal magnetoconductance. With diminishing Hall effect, the magnetoresistivity is simply the inverse of the longitudinal magnetoconductivity, enabling the differentiation of the electron and hole contributions to the XMR, which varies with the strength and orientation of the magnetic field. This work demonstrates a convenient way to separate the dynamics of the charge carriers and to uncover the origin of XMR in multi-band materials with anisotropic Fermi surfaces. Our approach can be readily applied to other XMR materials.

cond-mat.mtrl-sci↗

arXiv2017

Origin of the Extremely Large Magnetoresistance in the Semimetal YSb

Electron-hole (e-h) compensation is a hallmark of multi-band semimetals with extremely large magnetoresistance (XMR) and has been considered to be the basis for XMR. Recent spectroscopic experiments, however, reveal that YSb with non-saturating magnetoresistance is uncompensated, questioning the e-h compensation scenario for XMR. Here we demonstrate with magnetoresistivity and angle dependent Shubnikov - de Haas (SdH) quantum oscillation measurements that YSb does have nearly perfect e-h compensation, with a density ratio of $0.95$ for electrons and holes. The density and mobility anisotropy of the charge carriers revealed in the SdH experiments allow us to quantitatively describe the magnetoresistance with an anisotropic multi-band model that includes contributions from all Fermi pockets. We elucidate the role of compensated multi-bands in the occurrence of XMR by demonstrating the evolution of calculated magnetoresistances for a single band and for various combinations of electron and hole Fermi pockets.

cond-mat.mtrl-sci↗

arXiv2017

Mimicking cuprates: large orbital polarization in a metallic square-planar nickelate

High temperature cuprate superconductivity remains a defining problem in condensed matter physics. Among myriad approaches to addressing this problem has been the study of alternative transition metal oxides with similar structures and 3d electron count that are suggested as proxies for cuprate physics. None of these analogs has been superconducting, and few are even metallic. Here, we report that the low-valent, quasi-two-dimensional trilayer compound, Pr4Ni3O8 avoids a charge-stripe ordered phase previously reported for La4Ni3O8, leading to a metallic ground state. By combining x-ray absorption spectroscopy and density functional theory calculations, we further find that metallic Pr4Ni3O8 exhibits a low-spin configuration and significant orbital polarization of the unoccupied eg states with pronounced dx2-y2 character near the Fermi energy, both hallmarks of the cuprate superconductors. Belonging to a regime of 3d electron count found for hole-doped cuprates, Pr4Ni3O8 thus represents one of the closest analogies to cuprates yet reported and a singularly promising candidate for high-Tc superconductivity if appropriately doped.

cond-mat.str-el↗

arXiv2016

Charge ordering in Ni$^{1+}$/Ni$^{2+}$ nickelates: La$_4$Ni$_3$O$_8$ and La$_3$Ni$_2$O$_6$

Ab initio calculations have been performed to unravel the origin of the recently found superlattice peaks in the trilayer nickelate La$_4$Ni$_3$O$_8$. These peaks arise from static charge ordering of Ni$^{2+}$/ Ni$^{1+}$ stripes oriented at 45$^{\circ}$ to the Ni-O bonds. An insulating state originates from a combination of structural distortions and magnetic order, with the gap being formed solely within the d$_{x^2-y^2}$ manifold of states. When doped, electrons or holes would go into these states, in a similar fashion to what occurs in the cuprates. Analogous calculations suggest that checkerboard charge order should occur in the bilayer nickelate La$_3$Ni$_2$O$_6$. These results reveal a close connection between La$_4$Ni$_3$O$_8$ and La$_3$Ni$_2$O$_6$ with La$_{2-x}$Sr$_x$NiO$_4$ for x=1/3 and x=1/2, respectively.

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