arXiv subjects

E. Granado

Publications and source records attributed to E. Granado.

At least 19 recordsLinked to original sources

arXiv2025

Magnetostriction as the origin of the magnetodielectric effect in La2CoMnO6

The La2CoMnO6 (LCMO) perovskite has received a lot of attention due to its near room temperature magnetodielectric effect. Despite the recent efforts, the mechanism ruling the correlation between its magnetic and dielectric properties is not yet fully understood. In order to address this issue, we conducted a detailed investigation of the coupling between the structural, electronic and magnetic properties of a polycrystalline LCMO sample. Using magnetic field-dependent x-ray powder diffraction and measurements with a capacitive dilatometer, we show that applying an external magnetic field decreases the unit cell volume, thereby modifying the octahedral distortions. Experiments involving temperature and field-dependent x-ray absorption spectroscopy at the Co-L2,3 edges provide further evidence that the spin-orbit interaction of outermost Co 3d-orbital and the field-induced enhancement of covalence effects are the key contributors to the magnetostrictive effects. From a detailed analysis using multiplet and density functional theory calculations, we propose that the field-induced modulations of the orbital hybridization and the ligand-to-metal charge transfer are responsible for the changes in the dielectric response of LCMO, thus enabling a direct coupling between magnetic, elastic and dielectric properties in this material.

cond-mat.mtrl-sci↗

arXiv2024

Magnetic structure and component-separated transitions of HoNiSi$_{3}$

HoNiSi$_{3}$ is an intermetallic compound characterized by two successive antiferromagnetic transitions at $T_{N1} = 6.3$ K and $T_{N2} = 10.4$ K. Here, its zero-field microscopic magnetic structure is inferred from resonant x-ray magnetic diffraction experiments on a single crystalline sample that complement previous bulk magnetic susceptibility data. For $T < T_{N2}$, the primitive magnetic unit cell matches the chemical cell. The magnetic structure features ferromagnetic {\it ac} planes stacked in an antiferromagnetic $\uparrow \downarrow \uparrow \downarrow$ pattern. For $T_{N1} < T < T_{N2}$, the ordered magnetic moment points along $\vec{a}$, and for $T < T_{N1}$ a component along $\vec{c}$ also orders. A symmetry analysis indicates that the magnetic structure for $T<T_{N1}$ is not compatible with the presumed orthorhombic $Cmmm$ space group of the chemical structure, and therefore a slight lattice distortion is implied. Mean-field calculations using a simplified magnetic Hamiltonian, including a reduced set of three independent exchange coupling parameters determined by density functional theory calculations and two crystal electric field terms taken as free-fitting parameters, are able to reproduce the main experimental observations. An alternative approach using a more complete model including seven exchange coupling and nine crystal electric field terms is also explored, where the search of the ground state magnetic structure compatible with the available anisotropic magnetic susceptibility and magnetization data is carried out with the help of an unsupervised machine learning algorithm. The possible magnetic configurations are grouped into five clusters, and the cluster that yields the best comparison with the experimental macroscopic data contains the parameters previously found with the simplified model and also predicts the correct ground-state magnetic structure.

cond-mat.str-el↗

arXiv2024

Spin-state ordering and intermediate states in the mixed-valence cobalt oxyborate Co$_3$O$_2$BO$_3$ with spin crossover

Spin-state ordering - a periodic pattern of ions with different spin-state configurations along a crystal lattice - is a rare phenomenon, and its possible interrelation with other electronic degrees of freedom remains little explored. Here we perform a structural investigation of the mixed-valence Co homometallic ludwigite Co$_2^{2+}$Co$^{3+}$O$_2$BO$_3$. A superstructure consistent with a long-range Co$^{3+}$ spin-state ordering is observed between $T_{4}=580$ K and $T_{3}=510$ K. Intermediate states with mesoscopic correlations are detected below $T_{3}$ down to $T_{1}=480$ K with a change of dimensionality at $T_{2}=495$ K. The spin-state correlations are connected to the charge sector as revealed by the abrupt changes in the electrical resistance at $T_1$ and $T_2$. The evolution of the structural parameters below $T_{1}$ indicate that the spin crossover is ignited by a moderate degree of thermally-induced Co$^{2+}$/Co$^{3+}$ charge disorder. Charge and spin-state degrees of freedom can be interrelated in mixed-valence spin-crossover materials, leading to sharp transitions involving intermediate spin-state/charge correlated states at the mesoscale.

cond-mat.str-el↗

arXiv2022

Enhanced magnetism and suppressed magnetoelastic coupling induced by electron doping in Ca$_{1-x}$Y$_{x}$MnReO$_6$

The Ca$_2$MnReO$_6$ double perovskite is a spin-orbit-assisted Mott insulator with exotic magnetic properties, including a largely non-collinear Mn$^{2+}$ spin arrangement and nearly orthogonal coupling between such spins and the much smaller Re $5d$ magnetic moments. Here, the electron-doped compound Ca$_{1.7}$Y$_{0.3}$MnReO$_6$ is investigated. Neutron and X-ray powder diffraction confirm that nearly full chemical order is maintained at the Mn and Re sites under the Y substitution at the Ca site. X-ray absorption measurements and an analysis of the Mn-O/Re-O bond distances show that the Mn oxidation state remains stable at +2 whereas Re is reduced upon doping. The electron doping increases the magnetic ordering temperature from $T_c = 121$ to $150$ K and also enhances significantly the ferromagnetic component of the Mn spins at the expense of the antiferromagnetic component at the base temperature ($T=3$ K). The lattice parameter anomalies at $T_c$ observed in the parent compound are suppressed by the electron doping. The possible reasons for the enhanced magnetism and the suppressed magnetoelastic coupling in Ca$_{1.7}$Y$_{0.3}$MnReO$_6$ are discussed.

cond-mat.str-el↗

arXiv2021

Structural and spectroscopic investigation of the charge-ordered, short-range ordered, and disordered phases of the Co$_3$O$_2$BO$_3$ ludwigite

In this work, we investigate the representative case of the homometallic Co ludwigite Co$^{2+}_2$Co$^{3+}$O$_2$BO$_3$ ($Pbam$ space group) with four distinct Co crystallographic sites [$M1$-$M4$] surrounded by oxygen octahedra. The mixed-valent character of the Co ions up to at least $T=873$ K is verified through x-ray absorption near-edge structure (XANES) experiments. Single crystal x-ray diffraction (XRD) and neutron powder diffraction (NPD) confirm that the Co ions at the $M4$ site are much smaller than the others at low temperatures, consistent with a Co$^{3+}$ oxidation state at $M4$ and Co$^{2+}$ at the remaining sites. The size difference between the Co ions in the $M4$ and $M2$ sites is continuously reduced upon warming above $\approx 370$ K, indicating a gradual charge redistribution within the $M4$-$M2$-$M4$ (424) ladder in the average structure. An increasing structural disorder, is noted above $\approx 370$ K, The local Co-O distance distribution, revealed by Co $K$-edge Extended X-Ray Absorption Fine Structure (EXAFS) data and analyzed with an evolutionary algorithm method, is similar to that inferred from the XRD crystal structure below $\approx 370$ K. At higher temperatures, the local Co-O distance distribution remains similar to that found at low temperatures, at variance with the average crystal structure obtained with XRD. We conclude that the oxidation states Co$^{2+}$ and Co$^{3+}$ are instantaneously well defined in a local atomic level at all temperatures, however the thermal energy promotes local defects in the charge-ordered configuration of the 424 ladders upon warming. These defects coalesce into a phase-segregated state within a narrow temperature interval ($475< T < 495$ K). Finally, a transition at $\approx 500$ K revealed by differential scanning calorimetry (DSC) in the iron ludwigite Fe$_3$O$_2$BO$_3$ is discussed.

cond-mat.str-el↗

arXiv2021

Crystal, local atomic and electronic structures of YbFe$_2$Zn$_{20-x}$Cd$_x$ ($0 \leq x \leq 1.4$): a multi-band system with possible coexistence of light and heavy fermions

The partial (up to 7 %) substitution of Cd for Zn in the Yb-based heavy-fermion material YbFe$_2$Zn$_{20}$ is known to induce a slight ($\sim 20$ %) reduction of the Sommerfeld specific heat coefficient $γ$ and a huge (up to two orders of magnitude) reduction of the $T^2$ resistivity coefficient $A$, corresponding to a drastic and unexpected reduction of the Kadowaki-Woods ratio $A/γ^2$. Here, Yb $L_{3}$-edge X-ray absorption spectroscopy shows that the Yb valence state is close to $3+$ for all $x$, whereas X-ray diffraction reveals that Cd replace the Zn ions only at the $16c$ site of the $Fd\bar{3}m$ cubic structure, leaving the $48f$ and $96g$ sites with full Zn occupation. Ab-initio electronic structure calculations in pure and Cd-doped materials, carried out without considering correlations, show multiple conduction bands with only minor modifications of the band dispersions near the Fermi level and therefore do not explain the resistivity drop introduced by Cd substitution. We propose that the site-selective Cd substitution introduces light conduction bands with substantial contribution of Cd($16c$) $5p$ levels that have weak coupling to the Yb$^{3+}$ $4f$ moments. These light fermions coexist with heavy fermions originated from other conduction bands with larger participation of Zn($48f$ and $96g$) $4p$ levels that remain strongly coupled with the Yb$^{3+}$ local moments.

cond-mat.str-el↗

arXiv2020

Electronic and magnetic properties of stoichiometric CeAuBi$_{2}$

We report the electronic and magnetic properties of stoichiometric CeAuBi$_{2}$ single crystals. At ambient pressure, CeAuBi$_{2}$ orders antiferromagnetically below a Néel temperature ($T_{N}$) of 19 K. Neutron diffraction experiments revealed an antiferromagnetic propagation vector $\hatτ = [0, 0, 1/2]$, which doubles the paramagnetic unit cell along the $c$-axis. At low temperatures several metamagnetic transitions are induced by the application of fields parallel to the $c$-axis, suggesting that the magnetic structure of CeAuBi$_{2}$ changes as a function of field. At low temperatures, a linear positive magnetoresistance may indicate the presence of band crossings near the Fermi level. Finally, the application of external pressure favors the antiferromagnetic state, indicating that the 4$f$ electrons become more localized.

cond-mat.str-el↗

arXiv2020

The effect of structural and magnetic disorder on the 3$d$-5$d$ exchange interactions of La$_{2-x}$Ca$_{x}$CoIrO$_{6}$

The delicate balance between spin-orbit coupling, Coulomb repulsion and crystalline electric field interactions observed in Ir-based oxides is usually manifested as exotic magnetic behavior. Here we investigate the evolution of the exchange coupling between Co and Ir for partial La substitution by Ca in La$_{2}$CoIrO$_6$. A great advantage of the use of Ca$^{2+}$ as replacement for La$^{3+}$ is the similarity of its ionic radii. Thus, the observed magnetic changes can more easily be associated to electronic variations. A thorough investigation of the structural, electronic and magnetic properties of the La$_{2-x}$Ca$_{x}$CoIrO$_6$ system was carried out by means of synchrotron x-ray powder diffraction, muon spin rotation and relaxation ($μ$SR), AC and DC magnetization, XAS, XMCD, Raman spectroscopy, electrical resistivity and dielectric permittivity. Our XAS results show that up to 25% of Ca substitution at the La site results in the emergence of Co$^{3+}$, possibly in high spin state, while the introduction of larger amount of Ca leads to the increase of Ir valence. The competing magnetic interactions resulting from the mixed valences lead to a coexistence of a magnetically ordered and an emerging spin glass (SG) state for the doped samples. Our $μ$SR results indicate that for La$_{2}$CoIrO$_6$ a nearly constant fraction of a paramagnetic (PM) phase persists down to low temperature, possibly related to the presence of a small amount of Ir$^{3+}$ and to the anti-site disorder at Co/Ir sites. For the doped compounds the PM phase freezes below 30 K, but there is still some dynamics associated with the SG. The dielectric data obtained for the parent compound and the one with 25% of Ca-doping indicate a possible magnetodielectric effect, which is discussed in terms of the electron hopping between the TM ions, the anti-site disorder and the distorted crystalline structure.

cond-mat.mtrl-sci↗

arXiv2020

Possible quantum fluctuations in the vicinity of the quantum critical point of $\mathbf{(Sr, Ca)_3Ir_4Sn_{13}}$ revealed by high-energy X-ray diffraction study

We explore the evolution of the structural phase transition of $\rm{(Sr, Ca)_3Ir_4Sn_{13}}$, a model system to study the interplay between structural quantum criticality and superconductivity, by means of high-energy x-ray diffraction measurements at high pressures and low temperatures. Our results confirm a rapid suppression of the superlattice transition temperature $T^*$ against pressure, which extrapolates to zero at a critical pressure of $\approx 1.79(4)$ GPa. The temperature evolution of the superlattice Bragg peak in $\rm{Ca_3Ir_4Sn_{13}}$ reveals a drastic decrease of the intensity and an increase of the linewidth when $T \rightarrow 0$ K and $p \rightarrow p_c$. Such anomaly is likely associated to the emergence of quantum fluctuations that disrupt the formation of long-range superlattice modulation. The revisited temperature-pressure phase diagram of $\rm{(Sr, Ca)_3Ir_4Sn_{13}}$ thus highlights the intertwined nature of the distinct order parameters present in this system and demonstrates some similarities between this family and the unconventional superconductors.

cond-mat.str-el↗

arXiv2019

Anisotropic Lattice Compression and Pressure-Induced Electronic Phase Transitions in Sr$_2$IrO$_4$

The crystal lattice of Sr$_2$IrO$_4$ is investigated with synchrotron X-ray powder diffraction under hydrostatic pressures up to $P=43$ GPa and temperatures down to $20$ K. The tetragonal unit cell is maintained over the whole investigated pressure range, within our resolution and sensitivity. The $c$-axis compressibility $κ_c(P,T) \equiv -({1} / {c}) ({d c} / {d P})$ presents an anomaly with pressure at $P_1=17$ GPa at fixed $T=20$ K that is not observed at $T=300$ K, whereas $κ_a(P,T)$ is nearly temperature-independent and shows a linear behavior with $P$. The anomaly in $κ_c(P,T)$ is associated with the onset of long-range magnetic order, as evidenced by an analysis of the temperature-dependence of the lattice parameters at fixed $P=13.7 \pm 0.5$ GPa. At fixed $T=20$ K, the tetragonal elongation $c/a(P,T)$ shows a gradual increment with pressure and a depletion above $P_2=30$ GPa that indicates an orbital transition and possibly marks the collapse of the $J_{eff}=1/2$ spin-orbit-entangled state. Our results support pressure-induced phase transitions or crossovers between electronic ground states that are sensed, and therefore can be probed, by the crystal lattice at low temperatures in this prototype spin-orbit Mott insulator.

cond-mat.str-el↗

arXiv2019

The zero-field-cooled exchange bias effect in phase-segregated La$_{2-x}$A$_{x}$CoMnO$_{6-δ}$ (A = Ba, Ca, Sr; x = 0, 0.5)

In the zero-field-cooled exchange bias (ZEB) effect the unidirectional magnetic anisotropy is set at low temperatures even when the system is cooled in the absence of external magnetic field. La$_{1.5}$Sr$_{0.5}$CoMnO$_{6}$ stands out as presenting the largest ZEB reported so far, while for La$_{1.5}$Ca$_{0.5}$CoMnO$_{6}$ the exchange bias field ($H_{EB}$) is one order of magnitude smaller. Here we show that La$_{1.5}$Ba$_{0.5}$CoMnO$_{6}$ also exhibits a pronounced shift of its magnetic hysteresis loop, with intermediate $H_{EB}$ value in respect to Ca- and Sr-doped samples. In order to figure out the microscopic mechanisms responsible for this phenomena, these compounds were investigated by means of synchrotron X-ray powder diffraction, Raman spectroscopy, muon spin rotation and relaxation, AC and DC magnetization, X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD). The parent compound La$_{2}$CoMnO$_{6}$ was also studied for comparison, as a reference of a non-ZEB material. Our results show that the Ba-, Ca- and Sr-doped samples present a small amount of phase segregation, and that the ZEB effect is strongly correlated to the system's structure. We also observed that mixed valence states Co$^{2+}$/Co$^{3+}$ and Mn$^{4+}$/Mn$^{3+}$ are already present at the La$_{2}$CoMnO$_{6}$ parent compound, and that Ba$^{2+}$/Ca$^{2+}$/Sr$^{2+}$ partial substitution at La$^{3+}$ site leads to a large increase of Co average valence, with a subtle augmentation of Mn formal valence. Estimates of the Co and Mn valences from the $L$-edge XAS indicate the presence of oxygen vacancies in all samples (0.05$\leq δ\leq$0.1). Our XMCD results show a great decrease of Co moment for the doped compounds, and indicate that the shift of the hysteresis curves for these samples is related to uncompensated antiferromagnetic coupling between Co and Mn.

cond-mat.mtrl-sci↗

arXiv2019

Magnetic structure and magnetoelastic coupling of GdNiSi3 and TbNiSi3

The series of intermetallic compounds $R$NiSi$_3$ ($R$ = rare earth) shows interesting magnetic properties evolving with $R$ and metamagnetic transitions under applied magnetic field for some of the compounds. The microscopic magnetic structures must be determined to rationalize such rich behavior. Here, resonant x-ray magnetic diffraction experiments are performed on single crystals of GdNiSi$_{3}$ and TbNiSi$_{3}$ at zero field. The primitive magnetic unit cell matches the chemical cell below the Néel temperatures $T_{N}$ = 22.2 and 33.2 K, respectively. The magnetic structure is determined to be the same for both compounds (magnetic space group $Cmmm'$). It features ferromagnetic {\it ac} planes that are stacked in an antiferromagnetic $+-+-$ pattern, with the rare-earth magnetic moments pointing along the $\vec{a}$ direction, which contrasts with the $+--+$ stacking and moment direction along the $\vec{b}$ axis previously reported for YbNiSi$_3$. This indicates a sign reversal of the coupling constant between second-neighbor $R$ planes as $R$ is varied from Gd and Tb to Yb. The long {\it b} lattice parameter of GdNiSi$_{3}$ and TbNiSi$_{3}$ shows a magnetoelastic expansion upon cooling below $T_N$, pointing to the conclusion that the $+-+-$ stacking is stabilized under lattice expansion. A competition between distinct magnetic stacking patterns with similar exchange energies tuned by the size of $R$ sets the stage for the magnetic ground state instability observed along this series.

cond-mat.str-el↗

arXiv2019

Electronic structure of Fe and magnetism in the $3d/5d$ double perovskites Ca$_2$FeReO$_6$ and Ba$_2$FeReO$_6$

The Fe electronic structure and magnetism in (i) monoclinic Ca$_2$FeReO$_6$ with a metal-insulator transition at $T_{MI} \sim 140$ K and (ii) quasi-cubic half-metallic Ba$_2$FeReO$_6$ ceramic double perovskites are probed by soft x-ray absorption spectroscopy (XAS) and magnetic circular dichroism (XMCD). These materials show distinct Fe $L_{2,3}$ XAS and XMCD spectra, which are primarily associated with their different average Fe oxidation states (close to Fe$^{3+}$ for Ca$_2$FeReO$_6$ and intermediate between Fe$^{2+}$ and Fe$^{3+}$ for Ba$_2$FeReO$_6$) despite being related by an isoelectronic (Ca$^{2+}$/Ba$^{2+}$) substitution. For Ca$_2$FeReO$_6$, the powder-averaged Fe spin moment along the field direction ($B = 5$ T), as probed by the XMCD experiment, is strongly reduced in comparison with the spontaneous Fe moment previously obtained by neutron diffraction, consistent with a scenario where the magnetic moments are constrained to remain within an easy plane. For $B=1$ T, the unsaturated XMCD signal is reduced below $T_{MI}$ consistent with a magnetic transition to an easy-axis state that further reduces the powder-averaged magnetization in the field direction. For Ba$_2$FeReO$_6$, the field-aligned Fe spins are larger than for Ca$_2$FeReO$_6$ ($B=5$ T) and the temperature dependence of the Fe magnetic moment is consistent with the magnetic ordering transition at $T_C^{Ba} = 305$ K. Our results illustrate the dramatic influence of the specific spin-orbital configuration of Re $5d$ electrons on the Fe $3d$ local magnetism of these Fe/Re double perovskites.

cond-mat.str-el↗

arXiv2018

Tetragonal-orthorhombic phase coexistence under magnetic fields in BaFe$_2$As$_2$ and Sr(Fe$_{1-x}$Co$_{x}$)$_2$As$_2$: evidence of magnetically driven structural transition

Synchrotron x-ray diffraction experiments were performed on BaFe$_2$As$_2$ and Sr(Fe$_{1-x}$Co$_{x}$)$_2$As$_2$ single crystals as a function of temperature and applied magnetic field along the tetragonal $[1 \bar{1} 0]$ direction, complemented by electrical resistivity and specific heat experiments. For a BaFe$_2$As$_2$ crystal with spin-density-wave antiferromagnetic ordering temperature $T_{AF}=132.5$ K and onset of the orthorhombic phase at $T_{o}=137$ K, the magnetic field favors the growth of tetragonal domains that compete with orthorhombic ones for $T \gtrsim T_{AF}$. For a Sr(Fe$_{1-x}$Co$_{x}$)$_2$As$_2$ crystal with more separated transitions ($T_{AF} = 132$ K and $T_{o} = 152$ K), the crystal structure also shows significant field-dependence in a narrow temperature interval close to $T_{AF}$. These results favor magnetism as the driver of the structural and nematic transitions in 122 Fe pnictides.

cond-mat.supr-con↗

arXiv2018

First-order structural transition and pressure-induced lattice/phonon anomalies in Sr$_2$IrO$_4$

We investigate the crystal structure and lattice vibrations of Sr$_2$IrO$_4$ by a combined phonon Raman scattering and x-ray powder diffraction experiment under pressures up to $66$ GPa and room temperature. Density functional theory (DFT) and $ab$-initio lattice dynamics calculations were also carried out. A first-order structural phase transition associated with an $8$ % collapse of the $c$-axis is observed at high pressures, with phase coexistence being observed between $\sim 40$ and $55$ GPa. At lower pressures, lattice and phonon anomalies were observed, reflecting crossovers between isostructural competing states. A critical pressure of $P_1=17$ GPa is associated with: (i) a reduction of lattice volume compressibility and a change of behavior of the tetragonal $c/a$ ratio take place above $P_1$; (ii) a four-fold symmetry-breaking lattice strain associated with lattice disorder; (iii) disappearance of two Raman active modes (at $\sim 180$ and $\sim 260$ cm$^{-1}$); and (iv) development of an asymmetric Fano lineshape for the $\sim 390$ cm$^{-1}$ mode. DFT indicates that the phase above $P_1$ is most likely non-magnetic. Exploring the similarities between iridate and cuprate physics, we argue that these observations are consistent with the emergence of a rotational symmetry-breaking electronic instability at $P_1$, providing hints for the avoided metallization under pressure and supporting the hypothesis of possible competing orders that are detrimental to superconductivity in this family. Alternative scenarios for the transition at $P_1$ are also suggested and critically discussed. Additional phonon and lattice anomalies in the tetragonal phase are observed at $P_2=30$ and $P_3=40$ GPa, indicating further competing phases that are stabilized at high pressures.

cond-mat.str-el↗

arXiv2018

Coupling of phonons with orbital dynamics and magnetism in CuSb$_2$O$_6$

Strongly interacting phonons and orbital excitations are observed in the same energy range for CuSb$_2$O$_6$, unlocking a so-far unexplored type of electron-phonon interaction. An orbital wave at $\sim 550$ cm$^{-1}$ softens on warming and strongly interferes with a phonon at $\sim 500$ cm$^{-1}$, giving rise to a merged excitation of mixed character. An electronic continuum grows on warming to the orbital ordering temperature $T_{OO}$=400 K, generating an important phonon decay channel. This direct and simultaneous observation of orbital and vibrational excitations reveals details of their combined dynamics. In addition, phonon frequency anomalies due to magnetic correlations are observed below $\sim 150$ K, much above the three-dimensional magnetic ordering temperature $T_N^{3D}=8.5$ K, confirming one-dimensional magnetic correlations along Cu-O-O-Cu linear chains in the paramagnetic state.

cond-mat.str-el↗

arXiv2018

Exotic Magnetism and Spin-Orbit Assisted Mott Insulating State in a $3d-5d$ Double Perovskite

The magnetic structure of Ca$_2$MnReO$_6$ double perovskite is investigated by neutron powder diffraction and bulk magnetization, showing dominant non-collinear Mn magnetic moments [$4.35(7)$ $μ_B$] that are orthogonally aligned with the small Re moments [$0.22(4)$ $μ_B$]. $Ab$-initio electronic structure calculations show that the strong spin-orbit coupling for Re $5d$ electrons combined with a relatively modest on-site Coulomb repulsion ($U_{eff}^{Re} \gtrsim 0.6$ eV) is sufficient to render this material insulating. This is a rare example of spin-orbit assisted Mott insulator outside the realm of iridates, with remarkable magnetic properties.

cond-mat.str-el↗

arXiv2018

Lattice dynamics of ASb2O6 (A=Cu,Co) with trirutile structure

Raman spectroscopy experiments on single crystals of CuSb2O6 and CoSb2O6 quasi-one-dimensional antiferromagnets with trirutile crystal structure were performed, with a focus on the first material. The observed Raman-active phonon modes and previously reported infrared-active modes were identified with the aid of ab-initio lattice dynamics calculations. The structural transition between monoclinic beta-CuSb2O6 and tetragonal alpha-CuSb2O6 phases at Ts=400 K is manifested in our spectra by a "repulsion" of two accidentally quasi-degenerate symmetric modes below Ts, caused by a phonon mixing effect that is only operative in the monoclinic beta-CuSb2O6 phase due to symmetry restrictions. Also, two specific phonons, associated with CuO6 octahedra rotation and with a Jahn-Teller elongation mode, soften and broaden appreciably as T -> Ts. A crossover from a displacive to an order-disorder transition at Ts is inferred.

cond-mat.str-el↗