Session W61: 5d/4d Transition Metal Systems

Electronic structure and ferroelectricity of multiferroic Lu0.5Sc0.5FeO3

We investigated the electronic structure of a multiferroic Lu_0.5Sc_0.5FeO₃ single crystal using x-ray absoprtion spectroscopy, cluster model calculation, and ab initio analysis. X-ray linear dichroism measurement reveals that the A-site ion hybridizes strongly with O ions, where the hybridization strength of Lu is nearly twice larger than that of Sc. In addition, the anisotropic hybridization of Lu is larger, that results in the wider splitting of a_1g states from e_g^π states. The difference affects the energetic landscape of ferroelectric transition which is captured in ab initio calculation. Interestingly, the Born effective charge of planar O ions exhibits small deviation from a nominal valence instead of the anisotropic bonding, while that of apical O ion shows large value. Our observation suggest that the electronic contribution of e_g^π and e_g^σ bonds to six apical O ions in AO₈ cage should be counted in to describe a ferroelectric instability in h-A(Mn,Fe)O₃
  

Competing magnetic orders and anisotropic transport in quantum critical Sr3Ru2O7

We investigated Sr₃Ru₂O₇, the classic example of a quantum critical metal known to show a metamagnetic quantum phase transition as well as electronic nematicity, through first-principles calculations. The density functional calculations predict a ferromagnetic ground state in contrast to its experimentally observed paramagnetic character. This raises the question whether there exists competing magnetic orders that suppresses its ferromagnetism and if so, then what are those? We performed a thorough search to identify such low energy antiferromagnetically ordered metastable states. We indeed obtain such an (almost) doubly degenerate state with a striped order and calculated its associated transport properties within Boltzmann transport theory. Our results indicate significant transport anisotropy which have been discussed in context of strong spin fluctuations present in this material.   

Dynamical simulation of spin-orbit insulator and application to bilayer iridate

Spin-orbit insulators emerge in 5d electron systems with strong spin-orbit interaction, which can be described by a Hubbard model in the intermediate coupling regime of intra-atomic Coulomb repulsion (U) comparable to the dominant hopping amplitude (t). A reliable calculation in this regime is theoretically challenging because of the lack of a small parameter. We have developed a new numerical approach that enables large-scale numerical simulations (on finite lattices of more than 10,000 sites) of dynamical correlation functions at finite temperatures in a broad range of U/t values. The simulation of the self-consistent density matrix dynamics is combined with sampling from the Boltzmann distribution. As a relevant application of the newly developed method, we have calculated the dynamical spin structure factor of the bilayer iridate Sr₃Ir₂O₇. Our approach gives an excellent account of the experimental observations. The present method can be applied not only to iridates but also to various systems without any restrictions on the hopping amplitudes, lattice geometry, or electron filling fraction.

[1] G.-W. Chern, K. Barros, Z. Wang, H. Suwa, and C. D. Batista, Phys. Rev. B 97, 035120 (2018)   

Infrared Spectroscopic Study on Doping Evolution of the Electronic Response of Sr3(Ir1-xRux)2O7

We investigate the electronic response of the spin-orbit-coupled Sr₃(Ir_1-xRu_x)₂O₇ system over a wide doping range by using infrared spectroscopy. Our data show that the electronic ground state evolves from the effective total angular momentum J_eff = 1/2 Mott state to a correlated metallic state with Ru doping. We observe that the Ru-doping-driven insulator-metal transition occurs only above a critical Ru concentration of x_c ≈ 0.35. The low-energy optical response of the compound located at the insulator-metal transition boundary shows a peculiar behavior that might be associated with the spin-lattice coupling. Our data also suggest that the effect of the spin-orbit coupling in the electronic response may persist even in the metallic compounds with high Ru concentration. We will discuss the changes in the electronic structure associated with the insulator-metal transition.   

Temperautre dependence of the low-energy electronic structure of Ca3Ru2O7

We use angular-resolved photoemission spectroscopy (ARPES) and ab initio calculations to study the low-energy electronic band structure of a bilayer ruthenate system, Ca₃Ru₂O₇, across a wide temperature range. Ca₃Ru₂O₇ is a bad metal which undergoes two magnetic transitions [1]: a Néel ordering at 56 K and a spin-reorientation with a coupled structural transition at 48 K. At low temperatures we find it to be a compensated semimetal, in general agreement with previous ARPES [2] and de Haas-van Alphen [3] experiments. Our measurements across the 48 K transition, however, reveal dramatic changes in the low-energy electronic structure in terms of the shape and size of the Fermi surface. This is accompanied by a significant increase in the scattering rate, and our temperature-dependent ARPES suggests new insights into the unusual transport properties of this system in its “bad metal” phase. We further explore the role of crystal structure, magnetism, and spin-orbit coupling in the evolution of the electronic structure across the 48 K phase transition.

[1] Cao, et al., PRL 78 (1997) 1751
[2] Baumberger et al., PRL 96 (2006) 107601
[3] Kikugawa et al., JPSJ 79 (2010) 024704   

Hydrodynamic electron flow in PdCoO2: in-plane microwave spectroscopy

Hydrodynamic electron flow—in which electronic viscosity affects transport properties—has been observed in the DC resistance of PdCoO₂ [1]. Predictions that hydrodynamic effects also influence the AC electromagnetic response of a metal exist [2, 3], but confirming experimental evidence is lacking. In these proposals, the electronic viscosity affects the gradient of the induced current density, thereby influencing the diffusion of electromagnetic fields into the sample. Here we report results of microwave spectroscopy measurements of PdCoO₂ aimed at testing these predictions.

In the measurements reported here, a microwave magnetic field was applied parallel to the ab plane. The induced response has two components. First, flow along a high-mobility (a/b) direction, with a velocity gradient along the low-mobility c direction. Second, flow along the low-mobility c direction, with a velocity gradient along a high-mobility (a/b) direction. The latter is favorable to observing nonlocal electrodynamics. We see the onset of the anomalous skin effect.

[1] Moll et al., Science 351 6277 (2016)
[2] Gurzhi, Sov. Phys. Usp. 11 255 (1968)
[3] Forcella et al., Phys. Rev. B 90 035142 (2014)   

Hydrodynamic electron flow in PdCoO2: out-of-plane microwave spectroscopy

Hydrodynamic electron flow—in which electronic viscosity affects transport properties—has been observed in the DC resistance of PdCoO₂ [1]. Predictions that hydrodynamic effects also influence the AC electromagnetic response of a metal exist [2,3], but confirming experimental evidence is lacking. In these proposals, the electronic viscosity affects the gradient of the induced current density, thereby influencing the diffusion of electromagnetic fields into the sample. Here we report results of microwave spectroscopy measurements of PdCoO₂ aimed at testing these predictions.

In the measurements reported here, a microwave magnetic field was applied parallel to the c axis. The induced flow is always along a high-mobility (a/b) direction, with a velocity gradient along the other high-mobility (b/a) direction. This geometry is favorable to observing the effect of viscosity. The results appear to be inconsistent with existing predictions.

[1] Moll et al., Science 351 6277 (2016)
[2] Gurzhi, Sov. Phys. Usp. 11 255 (1968)
[3] Forcella et al., Phys. Rev. B 90 035142 (2014)   

Suppression of ferromagnetic spin correlation in the filled skutterudite superconductor SrOs4As12 revealed by 75As NQR-NMR

Recently new filled-skutterudite compounds SrT₄As₁₂ (T = Fe, Ru, Os) have been synthesized, which provides a new opportunity of systematic studies of the effects of different d electron of 3d, 4d and 5d in the systems [1]. Motivated by the recent observation of ferromagnetic spin correlations in the 3d compound SrFe₄As₁₂ [2], we have carried out nuclear quadrupole resonance (NQR) and nuclear magnetic resonance (NMR) measurements to investigate the role of magnetic fluctuations in the newly discovered superconductor 5d-compound SrOs₄As₁₂ which exhibits a superconducting transition at T_c = 4.8 K [3]. From the temperature dependence of Knight shift K and nuclear spin lattice relaxation time T₁, we found that no obvious ferromagnetic spin correlations in SrOs₄As₁₂. Furthermore, the temperature dependence of 1/T₁ in the superconducting state evidences a conventional s-wave superconductivity in SrOs₄As₁₂.

[1] K. Nishine, Y. Kawamura, J. Hayashi, and C. Sekine, Jpn. J.Appl. Phys. 56, 05FB01 (2017).
[2] Q.-P, Ding, et al. Phys. Rev. B 98, 155149 (2018).
[3] Q.-P. Ding et al., Phys. Rev. B 100, 054516 (2019).   

First Principles Studies of Fe-intercalated $\mathrm{NbS_2}$

Layered transition metal dichalcogenides (TMDs) can allow for the intercalation of magnetic ions, resulting in novel magnetic and electronic properties which can be tuned by altering intercalant species and concentration. Recent experiments on $\mathrm{Fe_{1/3}NbS_2}$, a bulk antiferromagnet with a N\'{e}el temperature of 42K, have shown that an applied current can reversibly switch the magnetic order, which is read out in the resistivity*. It is hypothesized that the magnetoelectric response to the current is due to a redistribution of magnetic domains**. To shed light on these findings, we examine the ground state structure and magnetic properties of $\mathrm{Fe_{1/3}NbS_2}$ using density functional theory calculations. We compute energetics of different experimentally proposed magnetic orderings, and the corresponding band structures and Fermi surfaces. Implications of our calculations for the reported switching behavior and resistivity response are discussed.\newline
* Nair et al., arXiv:1907.11698 (2019)\newline
**Little et al., arXiv:1908.00657 (2019)   

Theory of magneto-elastoresistance and application to WTe2: exploring electronic structure and extremely large magnetoresistance under strain

The application of uniaxial stress to is a promising route to probe and control the properties of quantum materials. One crucial step is to quantify the effects of strain on the electronic band structure, carrier density and mobility. Here, we demonstrate that much information can be obtained by exploring a novel experimental observable: magneto-elastoresistance (MER), which refers to magnetic field-driven changes of the elastoresistance. We apply this powerful approach to study the combined effect of strain and magnetic fields on the semi-metallic transition metal dichalcogenide WTe₂, and discover a large and temperature non-monotonic elastoresistance (ER) that can be tuned by magnetic field. We report on our theoretical analysis of these observations based on semi-classical Boltzmann transport theory combined with input from first-principles calculations. We highlight how MER can generally yield new insights beyond the zero field ER. For WTe₂ specifically we derive an effective low-energy three-band model that can account for the salient experimental features.   

Photoelectrical imaging and coherent spin-state readout of single nitrogen-vacancy centers in diamond

Selected by Focus Topic Organizer (Xuedan Ma)   

Nonequilibrium Orbital Transitions via Applied Electrical Current in Calcium Ruthenate

Simultaneous control of structural and physical properties via applied electrical current poses a key, new research topic and technological significance. Studying the spin-orbit-coupled antiferromagnet Ca₂RuO₄, and its derivative with 3% Mn doping to alleviate the violent first-order transition for more robust measurements, we find that a small applied electrical current couples to the lattice by significantly reducing its orthorhombicity and octahedral rotations, concurrently diminishing the 125K antiferromagnetic transition and inducing a new orbital order below 80K. The phase diagram reveals a critical regime near a current density of 0.15A/cm² that separates the vanishing antiferromagnetic order and the new orbital order. Further increasing current density (> 1A/cm²) enhances competitions between relevant interactions in a metastable manner, leading to a peculiar glassy behavior above 80K. The coupling between the lattice and nonequilibrium driven current is interpreted theoretically in terms of t_2g orbital occupancies. The current-controlled lattice is the driving force of the observed novel phenomena. Finally, we note that current-induced diamagnetism is not discerned in pure and slightly doped Ca₂RuO₄.   

Quantum liquid from strange frustration in the trimer magnet Ba4Ir3O10

We present the experimental observation of a new kind of frustrated quantum liquid arising in an unlikely place: the magnetic insulator Ba₄Ir₃O₁₀ where Ir₃O₁₂ trimers form an unfrustrated square lattice. Experimentally we find a quantum liquid state persisting down to 0.2 K that is stabilized by strong antiferromagnetic interaction with Curie-Weiss temperature -766 K. The astonishing frustration parameter of 3800 is beyond any known iridate thus far. Heat capacity and thermal conductivity are both linear at low temperatures, a familiar feature in metals but here in an insulator pointing to an exotic quantum liquid state; a mere 2% Sr substitution for Ba produces long-range order at 130 K and destroys the linear-T features. Although the Ir⁴⁺(5d⁵) ions in Ba₄Ir₃O₁₀ appear to form Ir₃O₁₂ trimers of face-sharing IrO₆ octahedra, we propose that intra-trimer exchange is reduced and the lattice recombines into an array of coupled 1D chains with additional spins. An extreme limit of decoupled 1D chains can explain most but not all of the striking experimental observations, indicating that the inter-chain coupling plays an important role in the novel frustration mechanism leading to this quantum liquid.