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Nematic fluctuations and phase transitions in LaFeAsO: A Raman scattering study
Raman scattering experiments on LaFeAsO with distinct antiferromagnetic ( T AFM =140 K) and tetragonal-orthorhombic ( T S =155 K) transitions show a quasielastic peak (QEP) in B 2g symmetry (2 Fe tetragonal cell) that fades away below ~ T AFM and is ascribed to electronic nematic fluctuations. A scaling of the reported shear modulus with the T dependence of the QEP height rather than the QEP area indicates that magnetic degrees of freedom drive the structural transition. As a result, the large separation between T S and T AFM in LaFeAsO compared to BaFe 2 As 2 manifests itself in slower dynamics of nematic fluctuations in the former.
Dynamic electronic correlation effects in NbO 2 as compared to VO 2
In this study we present a comparative investigation of the electronic structures of NbO 2 and VO 2 obtained within a combination of density functional theory and cluster-dynamical mean-field theory calculations. We investigate the role of dynamic electronic correlations on the electronic structure of the metallic and insulating phases of NbO 2 and VO 2 , with a focus on the mechanism responsible for the gap opening in the insulating phases. For the rutile metallic phases of both oxides, we obtain that electronic correlations lead to a strong renormalization of the t 2g subbands, as well as the emergence of incoherent Hubbard subbands, signaling that electronic correlations are also important in the metallic phase of NbO 2 . Interestingly, we find that nonlocal dynamic correlations do play a role in the gap formation of the [body-centered-tetragonal (bct)] insulating phase of NbO 2 , by a similar physical mechanism as that recently proposed by us in the case of the monoclinic (M 1 ) dimerized phase of VO 2 . Finally, although the effect of nonlocal dynamic correlations in the gap opening of bct phase is less important than in the (M 1 and M 2 ) monoclinic phases of VO 2 , their presence indicates that the former is not a purely Peierls-type insulator, as it was recently proposed.
Search for multipolar instability in URu 2 Si 2 studied by ultrasonic measurements under pulsed magnetic field
In this study, the elastic properties of URu 2 Si 2 in the high magnetic field region above 40 T, over a wide temperature range from 1.5 to 120 K, were systematically investigated by means of high-frequency ultrasonic measurements. The investigation was performed at high magnetic fields to better investigate the innate bare 5f-electron properties, since the unidentified electronic thermodynamic phase of unknown origin, the so-called “hidden order” (HO), and associated hybridization of conduction and f-electrons (c-f hybridization) are suppressed at high magnetic fields. From the three different transverse modes we find contrasting results; both the Γ 4 (B 2g ) and Γ 5 (E g ) symmetry modes C 66 and C 44 show elastic softening that is enhanced above 30 T, while the characteristic softening of the Γ 3 (B 1g ) symmetry mode (C 11 – C 12 )/2 is suppressed in high magnetic fields. These results underscore the presence of a hybridization-driven Γ 3 (B 1g ) lattice instability in URu 2 Si 2 . However, the results from this work cannot be explained by using existing crystalline electric field schemes applied to the quadrupolar susceptibility in a local 5f 2 configuration. Instead, we present an analysis based on a band Jahn-Teller effect.
Interplay of lattice, electronic, and spin degrees of freedom in detwinned BaFe 2 As 2 : A Raman scattering study
We report results of Raman scattering experiments on twin-free BaFe 2 As 2 with the main focus placed on understanding the influence of electronic and spin degrees of freedom on the lattice dynamics. In particular, we scrutinize the Eg modes and the As A 1g mode. Each of the two Eg phonons in the tetragonal phase is observed to split into a B 2g and a B 3g mode upon entering the orthorhombic stripe-magnetic phase. The splitting amounts to approximately 10 cm -1 and less than 5 cm -1 for the low- and the high-energy E g mode, respectively. The detailed study of the fully symmetric As mode using parallel incident and outgoing photon polarizations along either the antiferromagnetic or the ferromagnetic Fe-Fe direction reveals an anisotropic variation of the spectral weight with the energy of the exciting laser indicating a polarization-dependent resonance effect. Along with the experiments we present results from density functional theory calculations of the phonon eigenvectors, the dielectric function, and the Raman tensor elements. The comparison of theory and experiment indicates that (i) orbital-selective electronic correlations are crucial to understand the lattice dynamics and (ii) all phonon anomalies originate predominantly from the magnetic ordering and the corresponding reconstruction of the electronic bands at all energies.
Spin gaps in the ordered states of La 2 Li X O 6 ( X = Ru , Os ) and their relation to the distortion of the cubic double perovskite structure in 4 d 3 and 5 d 3 magnets
Time-of-flight inelastic neutron scattering measurements have been carried out on polycrystalline samples of the 4d 3 and 5d 3 double pervoskite antiferromagnets La 2 LiRuO 6 and La 2 LiOsO 6 . These reveal the development of an inelastic spin gap in La 2 LiRuO 6 and La 2 LiOsO 6 of ~1.8(8) and 6(1) meV, below their respective ordering temperatures TN, ~23.8 and 30 K. The bandwidths of the spin excitations are shown to be ~5.7(9)and12(1)meV, respectively, at low temperatures. Spin gaps are surprising in such magnets as the t 2g levels of Ru 5+ or Os 5+ are expected to be half-filled, resulting in an anticipated orbital singlet for both materials. We compare these results in monoclinic double perovskites La 2 LiRuO 6 and La 2 LiOsO 6 with those in cubic Ba 2 YRuO 6 and Ba 2 YOsO 6 as well as with those in other monoclinic La 2 NaRuO 6 ,La 2 NaOsO 6 , and Sr 2 ScOsO 6 and model the inelastic magnetic scattering with linear spin-wave theory using minimal anisotropic exchange interactions. We discuss the possible role of the distortion of the face-centered-cubic double perovskite structure on the spin gap formation and geometric frustration in these materials and show that T N scales with the top of the spin-wave band in all members of these families that display long-range order.
High- T c superconductivity in CaKFe 4 As 4 in absence of nematic fluctuations
Here, we employ polarization-resolved Raman spectroscopy to study the multiband stoichiometric T c =35 K superconductor CaKFe 4 As 4 . We do not detect Pomeranchuk-like electronic nematic fluctuations which were universally observed in the XY (B 2g ) symmetry Raman response for most families of the Fe-based superconductors. In the superconducting state we observe, consistent with a nodeless order parameter, a full spectral weight suppression at low energies and the emergence of a composite pair-breaking coherence feature at energies between 12 and 20 meV. We analyze the superconductivity-induced phonon self-energy effects and give an estimation for the electron-phonon coupling constant λ Γ =0.0015 which is insufficient to provide attraction for high- T c pairing.
Origin of magnetic excitation gap in double perovskite Sr 2 FeOsO 6
Sr 2 FeOsO 6 is an insulating double perovskite compound which undergoes antiferromagnetic transitions at 140 ( T N1 ) and 67 K ( T N2 ). To study the underlying electronic and magnetic interactions giving rise to this behavior we have performed inelastic neutron scattering (INS) and resonant inelastic x-ray scattering (RIXS) experiments on polycrystalline samples of Sr 2 FeOsO 6 . The INS data reveal that the spectrum of spin excitations remains ungapped below T N1 , however below T N2 a gap of 6.8 meV develops. The RIXS data reveals splitting of the T 2g multiplet consistent with that seen in other 5d 3 osmium based double perovskites. Altogether these results suggest that spin-orbit coupling is important for ground state selection in 3d–5d 3 double perovskite materials.
Bicollinear antiferromagnetic order, monoclinic distortion, and reversed resistivity anisotropy in FeTe as a result of spin-lattice coupling
The bicollinear antiferromagnetic order experimentally observed in FeTe is shown to be stabilized by the coupling g ~ 12 between monoclinic lattice distortions and the spin-nematic order parameter with B 2g symmetry, within a three-orbital spin-fermion model studied with Monte Carlo techniques. A finite but small value of g ~ 12 is required, with a concomitant lattice distortion compatible with experiments, and a tetragonal-monoclinic transition strongly first order. Remarkably, the bicollinear state found here displays a planar resistivity with the reversed puzzling anisotropy discovered in transport experiments. Orthorhombic distortions are also incorporated, and phase diagrams interpolating between pnictides and chalcogenides are presented. Here, we conclude that the spin-lattice coupling we introduce is sufficient to explain the challenging properties of FeTe.
Spin-Orbit Coupling Controlled J = 3 / 2 Electronic Ground State in 5 d 3 Oxides
Spin-orbit entanglement in 5d-based transition metal oxides (TMOs) has been identified as a route to a host of unconventional physical states including quantum spin liquids, Weyl semimetals, and axion insulators. Yet despite intense interest, no clear rules have emerged for the treatment of spin-orbit coupling (SOC) in 5d TMOs outside of idealised LS or jj coupling paradigms. This is exemplified in 5d 3 oxides in which an orbitally-quenched singlet ground state is anticipated, yet SOC is manifest in the observed magnetic properties. Here we solve this long-outstanding puzzle by revealing that the electronic ground state of Os5+ 5d 3 ions is an unquenched J = 3/2 state. Resonant inelastic x-ray scattering (RIXS) in Ca3LiOsO6 and Ba 2 YOsO 6 exposes a SOC-controlled splitting of the t 2g manifold. The results are successfully described using an intermediate-coupling framework in which oxygen hybridisation promotes the breakdown of the orbital singlet. This framework opens the door to realistic treatment of SOC across a range of 5d TMOs beyond the 5d 3 case.
Orbital Domain Dynamics in Magnetite below the Verwey Transition
Here, the metal-insulator phase transition in magnetite, known as the Verwey transition, is characterized by a charge-orbital ordering and a lattice transformation from a cubic to monoclinic structure. We use x-ray photon correlation spectroscopy to investigate the dynamics of this charge-orbitally ordered insulating phase undergoing the insulator-to-metal transition. By tuning to the Fe L 3 edge at the (001/2) superlattice peak, we probe the evolution of the Fe t 2g orbitally ordered domains present in the low temperature insulating phase and forbidden in the high temperature metallic phase. We observe two distinct regimes below the Verwey transition. In the first regime, magnetite follows an Arrhenius behavior and the characteristic timescale for orbital fluctuations decreases as the temperature increases. In the second regime, magnetite phase separates into metallic and insulating domains, and the kinetics of the phase transition is dictated by metallic-insulating interfacial boundary conditions.
Effect of d electrons on defect properties in equiatomic NiCoCr and NiCoFeCr concentrated solid solution alloys
Here, the role of d electrons in determining distributions of formation and migration energies for point defects in equiatomic NiCoCr and NiCoFeCr concentrated solid solution alloys (CSAs) are studied regarding electron density deformation flexibility based on first-principles calculations. The disordered state is taken into account by constructing special quasirandom structures. The migration barriers are determined by directly optimizing the saddle point. It is found that the formation energies of interstitials in CSAs are lower than those in pure Ni, whereas the formation energies of vacancies are higher. In both NiCoCr and NiCoFeCr, Co-related dumbbell interstitials exhibit lower formation energies. Notably, the distributions of migration energies for Cr interstitials and vacancies exhibit a remarkable overlap region. A detailed analysis of electronic properties reveals that the electronic charge deformation flexibility regarding e g to t 2g transition has a dominant effect on defect energetics for different elements in CSAs. Thus the electron deformation ability is suggested as a key factor in understanding the peculiar defect behavior in CSAs.
Optical response of BiFe O 3 films subjected to uniaxial strain
The impact of single-axis lattice expansion on the optical response of BiFeO 3 films is examined. Low-energy He implantation is used to tailor morphotropic phases of BiFeO 3 films and study changes in their optical spectra with continuously increasing lattice expansion. He ion implantation of epitaxial rhombohedral (R)- and tetragonal (T)-like BiFeO 3 films induces uniaxial out-of-plane strain that, on R-like films, eventually leads to a complete R-T phase transition. This approach allows us to provide insights into the optical response of BiFeO 3 films. Strain doping of T-like films leads to a significant redshift of the optical absorption spectra that is theoretically explained by a lowering of Fe 3dt 2g states. R-like films, on the other hand, show a less-pronounced sensitivity to uniaxial strain and a blueshift of about 250 meV at the strain-induced R-T transition. Here, the results demonstrate that strain doping allows a deeper examination of the optical properties of epitaxial phases that are otherwise impossible to access by standard epitaxy.
Strain-modulated anisotropic electronic structure in superconducting RuO 2 films
The binary ruthenate, RuO 2 , has been the subject of intense interest due to its itinerant antiferromagnetism and strain-induced superconductivity. The strain mechanism and its effect on the microscopic electronic states leading to the normal and superconducting state, however, remain undisclosed. Here, we investigate highly strained epitaxial (110) RuO 2 films using polarization-dependent oxygen K-edge x-ray absorption spectroscopy (XAS). Through the detection of pre-edge peaks arising from O:2p - Ru:4d hybridization, we uncover the effects of epitaxial strain on the orbital/electronic structure near the Fermi level. Our data show robust strain-induced shifts of orbital levels and a reduction of hybridization strength. Furthermore, we reveal a pronounced in-plane anisotropy of the electronic structure along the [110] / [1$\bar{1}$0] directions naturally stemming from the symmetry-breaking epitaxial strain of the substrate. The B 2g symmetry component of the epitaxially enforced strain breaks a sublattice degeneracy, resulting in an increase of the density of states at the Fermi level (E F ), possibly paving the way to superconductivity. Finally, these results underscore the importance of the effective reduction from tetragonal to orthorhombic lattice symmetry in (110) RuO 2 films and its relevance towards the superconducting and magnetic properties.
Discerning element and site-specific fluctuations of the charge-orbital order in Fe 3 O 4 below the Verwey transition
Despite countless experimental probes into magnetite's electronic structure across the Verwey transition Fe 3 O 4 , the exact origin of this archetypical metal-insulator transition remains a puzzle. Advanced x-ray diffraction techniques have mostly resolved the monoclinic structure of the insulating phase, including interatomic bond lengths, but the complexity of the charge-orbitally ordered state is difficult to disentangle. Here we combined resonant elastic x-ray scattering and x-ray photon correlation spectroscopy to probe charge-orbital fluctuations in the insulating state of magnetite. By accessing the Bragg forbidden $(00\frac{1}{2})_{c}$ peak at the oxygen K-edge, we complement our previous study on the iron L 3 – edge to reveal the dynamics of the iron 3d and oxygen 2p orbital domains. Our new results reveal a decoupling of the orbital correlation lengths between the oxygen 2p states and site-specific iron 3d states, and we further show charge-orbital domain fluctuations at the iron t 2g orbital sites of trimeron chains. These results also demonstrate an experimental method capable of distinguishing electronic dynamics between the oxygen ligands and the transition metal that underpins emergent behaviors in complex oxides.
Orbital degree of freedom in high entropy oxides
The spin, charge, and lattice degrees of freedom and their interplay in high entropy oxides were intensively investigated in recent years. However, how the orbital degree of freedom is affected by the extreme disorder in high entropy oxides has not been studied. In this work, using perovskite structured RVO 3 as a materials playground, we report how the disorder arising from mixing different rare earth ions at the rare earth site affects the orbital ordering of V 3+ t 2g -electrons. Since each member of RVO 3 (R = rare earth and Y) crystallizes into the same orthorhombic Pbnm structure, the configurational entropy should not be critical for the stability of (R 1,..., Rn)VO 3 . The spin and orbital ordering was studied by measuring magnetic properties and specific heat of single crystals. Rather than the number and type of rare earth ions, the average and variance of ionic radius are the key factors determining the spin and orbital order in (R 1,..., Rn)VO 3 . When the size variance is small, the average ionic radius takes precedence in dictating spin and orbital order. Increasing size variance suppresses the G-type orbital order (G-OO) and C-type antiferromagnetic order (C-AF) but favors the C-OO/G-AF state and spin-orbital entanglement. In conclusion, these findings suggest that the extreme disorder introduced by mixing multiple rare earth ions in high entropy perovskites might be employed to preserve the orbital degree of freedom to near the magnetic order, which is necessary for the electronic driven orbital ordering in a Kugel-Khomskii compound.
The Mechanical Design Optimization of a High Field HTS Solenoid
This paper describes the conceptual design optimization of a large aperture, high field (24 T at 4 K) solenoid for a 1.7 MJ superconducting magnetic energy storage device. The magnet is designed to be built entirely of second generation (2G) high temperature superconductor tape with excellent electrical and mechanical properties at the cryogenic temperatures. The critical parameters that govern the magnet performance are examined in detail through a multiphysics approach using ANSYS software. The analysis results formed the basis for the performance specification as well as the construction of the magnet.
Two-Fold Reduction of J c Anisotropy in FeSe 0.5 Te 0.5 Films Using Low-Energy Proton Irradiation
High in-field performance and low anisotropy of critical current density J c are important for superconducting wires/tapes in rotating machine and magnet applications. Here, we report measurements of angular dependence of critical current density J c in pro-ton irradiated FeSe 0.5 Te 0.5 (FST) films at magnetic field up to 35 T. Here, we observed two-fold reduction in the J c anisotropy of FST films at 4.2 K and 15 Tesla magnetic field by irradiation with low-energy (190 keV) proton. In light of recent demonstration of roll-to-roll irradiation process successfully incorporated in the standard 2G HTS long length production wire, low-energy proton irradiation can be a practical solution to enhance the performance of iron-based super-conducting wires and tapes, as low-energy ion sources are inexpensive to operate and are readily available commercially.