Giant lattice softening at a Lifshitz transition in Sr2RuO4.

The interplay of electronic and structural degrees of freedom in solids is a topic of intense research. More than 60 years ago, Lifshitz discussed a counterintuitive possibility: lattice softening driven by conduction electrons at topological Fermi surface transitions. The effect that he predicted, however, was small and has not been convincingly observed. Using a piezo-based uniaxial pressure cell to tune the ultraclean metal strontium ruthenate while measuring the stress-strain relationship, we reveal a huge softening of the Young's modulus at a Lifshitz transition of a two-dimensional Fermi surface and show that it is indeed driven entirely by the conduction electrons of the relevant energy band.

Suppressed Fluctuations as the Origin of the Static Magnetic Order in Strained Sr_{2}RuO_{4}.

Combining first-principles density-functional calculations and Moriya's self-consistent renormalization theory, we explain the recently reported counterintuitive appearance of an ordered magnetic state in uniaxially strained Sr_{2}RuO_{4} beyond the Lifshitz transition. We show that strain weakens the quantum spin fluctuations, which destroy the static order, more strongly than the tendency to magnetism. A different rate of decrease of the spin fluctuations vs magnetic stabilization energy promotes the onset of a static magnetic order beyond a critical strain.

Effect of alloying in monolayer niobium dichalcogenide superconductors.

When sulfur and silicon are incorporated in monolayer 2H-NbSe2 the superconducting transition temperature, Tc, has been found to vary non-monotonically. This was assumed to be a manifestation of fractal superconductivity. Using first-principles calculations, we show that the nonmonotonic dependence of Tc is insufficient evidence for multifractality. A unifying aspect in our study are selenium vacancies in NbSe2, which are magnetic pair-breaking defects that we propose can be present in considerable concentrations in as-grown NbSe2. We show that sulfur and silicon can occupy the selenium sites and reduce the pair-breaking effect. Furthermore, when sulfur is incorporated in NbSe2, the density of states at the Fermi level and the proximity to magnetism in the alloy are both reduced compared to the parent compound. Based on our results, we propose an alternative explanation of the non-monotonic change in Tc which does not require the conjecture of multifractality.

Magnetization Process of Atacamite: A Case of Weakly Coupled S=1/2 Sawtooth Chains.

We present a combined experimental and theoretical study of the mineral atacamite Cu_{2}Cl(OH)_{3}. Density-functional theory yields a Hamiltonian describing anisotropic sawtooth chains with weak 3D connections. Experimentally, we fully characterize the antiferromagnetically ordered state. Magnetic order shows a complex evolution with the magnetic field, while, starting at 31.5 T, we observe a plateaulike magnetization at about M_{sat}/2. Based on complementary theoretical approaches, we show that the latter is unrelated to the known magnetization plateau of a sawtooth chain. Instead, we provide evidence that the magnetization process in atacamite is a field-driven canting of a 3D network of weakly coupled sawtooth chains that form giant moments.

Competing magnetic phases and fluctuation-driven scalar spin chirality in the kagome metal YMn6Sn6.

Identification, understanding, and manipulation of novel magnetic textures are essential for the discovery of new quantum materials for future spin-based electronic devices. In particular, materials that manifest a large response to external stimuli such as a magnetic field are subject to intense investigation. Here, we study the kagome-net magnet YMn6Sn6 by magnetometry, transport, and neutron diffraction measurements combined with first-principles calculations. We identify a number of nontrivial magnetic phases, explain their microscopic nature, and demonstrate that one of them hosts a large topological Hall effect (THE). We propose a previously unidentified fluctuation-driven mechanism, which leads to the THE at elevated temperatures. This interesting physics comes from parametrically frustrated interplanar exchange interactions that trigger strong magnetic fluctuations. Our results pave a path to chiral spin textures, promising for novel spintronics.

Reduction of the Spin Susceptibility in the Superconducting State of Sr_{2}RuO_{4} Observed by Polarized Neutron Scattering.

Recent observations [A. Pustogow et al., Nature (London) 574, 72 (2019).NATUAS0028-083610.1038/s41586-019-1596-2] of a drop of the ^{17}O nuclear magnetic resonance (NMR) Knight shift in the superconducting state of Sr_{2}RuO_{4} challenged the popular picture of a chiral odd-parity paired state in this compound. Here we use polarized neutron scattering (PNS) to show that there is a 34±6% drop in the magnetic susceptibility at the Ru site below the superconducting transition temperature. We measure at lower fields H∼1/3H_{c2} than a previous PNS study allowing the suppression to be observed. The PNS measurements show a smaller susceptibility suppression than NMR measurements performed at similar field and temperature. Our results rule out the chiral odd-parity d=z[over ^](k_{x}±ik_{y}) state and are consistent with several recent proposals for the order parameter including even-parity B_{1g} and odd-parity helical states.

Spin Fluctuations in Sr_{2}RuO_{4} from Polarized Neutron Scattering: Implications for Superconductivity.

Triplet pairing in Sr_{2}RuO_{4} was initially suggested based on the hypothesis of strong ferromagnetic spin fluctuations. Using polarized inelastic neutron scattering, we accurately determine the full spectrum of spin fluctuations in Sr_{2}RuO_{4}. Besides the well-studied incommensurate magnetic fluctuations, we do find a sizable quasiferromagnetic signal, quantitatively consistent with all macroscopic and microscopic probes. We use this result to address the possibility of magnetically driven triplet superconductivity in Sr_{2}RuO_{4}. We conclude that, even though the quasiferromagnetic signal is stronger and sharper than previously anticipated, spin fluctuations alone are not enough to generate a triplet state strengthening the need for additional interactions or an alternative pairing scenario.

Why have band theorists been so successful in explaining and predicting novel superconductors?

In this contribution to the J. Phys.: Condens. Matter memorial issue in honor of Sandro Massidda I reflect on a phenomenon Sandro had been a part of. While theoretical condensed matter physicists have made, over the years, exciting and most elegant contributions to the theory of superconductivity (which, in and by itself, is one of the most beautiful constructs in theoretical physics), some of them of utmost importance, they have had less success in predicting and explaining superconducting states and mechanisms in specific materials. More down-to-earth computational materials scientists, who often go by the moniker 'band theorists', have been much more successful in applying (usually other people's) ideas in such circumstances. In this essay I give some examples, largely drawn from my own experience, and speculate on their meaning.

Novel Fe-Based Superconductor LaFe_{2}As_{2} in Comparison with Traditional Pnictides.

The recently discovered Fe-based superconductor (FeBS) LaFe_{2}As_{2} seems to break away from an established pattern that doping an FeBS beyond 0.2e/Fe destroys superconductivity. LaFe_{2}As_{2} has an apparent doping of 0.5e, yet superconducts at 12.1 K. Its Fermi surface bears no visual resemblance with the canonical FeBS fermiology. It also exhibits two phases, none magnetic and only one superconducting. We show that the difference between them nonetheless has a magnetic origin, the one featuring disordered moments, and the other locally nonmagnetic. We find that La there assumes an unusual valence of +2.6 to +2.7, so that the effective doping is reduced to 0.30-0.35e. A closer look reveals the same key elements: hole Fermi surfaces near Γ-Z and electron ones near the X-P lines, with the corresponding peak in susceptibility, and a strong tendency to stripe magnetism. The physics of LaFe_{2}As_{2} is thus more similar to the FeBS paradigm than hitherto appreciated.

Uncovering the Mechanism of the Impurity-Selective Mott Transition in Paramagnetic V_{2}O_{3}.

While the phase diagrams of the one- and multiorbital Hubbard model have been well studied, the physics of real Mott insulators is often much richer, material dependent, and poorly understood. In the prototype Mott insulator V_{2}O_{3}, chemical pressure was initially believed to explain why the paramagnetic-metal to antiferromagnetic-insulator transition temperature is lowered by Ti doping while Cr doping strengthens correlations, eventually rendering the high-temperature phase paramagnetic insulating. However, this scenario has been recently shown both experimentally and theoretically to be untenable. Based on full structural optimization, we demonstrate via the charge self-consistent combination of density functional theory and dynamical mean-field theory that changes in the V_{2}O_{3} phase diagram are driven by defect-induced local symmetry breakings resulting from dramatically different couplings of Cr and Ti dopants to the host system. This finding emphasizes the high sensitivity of the Mott metal-insulator transition to the local environment and the importance of accurately accounting for the one-electron Hamiltonian, since correlations crucially respond to it.

Frustration-driven C 4 symmetric order in a naturally-heterostructured superconductor Sr2VO3FeAs.

A subtle balance between competing interactions in iron-based superconductors (FeSCs) can be tipped by additional interfacial interactions in a heterostructure, often inducing exotic phases with unprecedented properties. Particularly when the proximity-coupled layer is magnetically active, rich phase diagrams are expected in FeSCs, but this has not been explored yet. Here, using high-accuracy 75As and 51V nuclear magnetic resonance measurements, we investigate an electronic phase that emerges in the FeAs layer below T 0 ~ 155 K of Sr2VO3FeAs, a naturally assembled heterostructure of an FeSC and a Mott-insulating vanadium oxide. We find that frustration of the otherwise dominant Fe stripe and V Neel fluctuations via interfacial coupling induces a charge/orbital order in the FeAs layers, without either static magnetism or broken C 4 symmetry, while suppressing the Neel antiferromagnetism in the SrVO3 layers. These findings demonstrate that the magnetic proximity coupling stabilizes a hidden order in FeSCs, which may also apply to other strongly correlated heterostructures.

Nontrivial Role of Interlayer Cation States in Iron-Based Superconductors.

Unconventional superconductivity in iron pnictides and chalcogenides has been suggested to be controlled by the interplay of low-energy antiferromagnetic spin fluctuations and the particular topology of the Fermi surface in these materials. Based on this premise, one would also expect the large class of isostructural and isoelectronic iron germanide compounds to be good superconductors. As a matter of fact, they, however, superconduct at very low temperatures or not at all. In this work we establish that superconductivity in iron germanides is suppressed by strong ferromagnetic tendencies, which surprisingly do not originate from changes in bond angles or bond distances with respect to iron pnictides and chalcogenides, but are due to changes in the electronic structure in a wide range of energies happening upon substitution of atom species (As by Ge and the corresponding spacer cations). Our results indicate that superconductivity in iron-based materials may not always be fully understood based on d or d-p model Hamiltonians only.

Structural origin of the anomalous temperature dependence of the local magnetic moments in the CaFe2As2 family of materials.

We report a combination of Fe Kß x-ray emission spectroscopy and density functional reduced Stoner theory calculations to investigate the correlation between structural and magnetic degrees of freedom in CaFe2(As1-xPx)2. The puzzling temperature behavior of the local moment found in rare earth-doped CaFe2As2 [H. Gretarsson et al., Phys. Rev. Lett. 110, 047003 (2013)] is also observed in CaFe2(As1-xPx)2. We explain this phenomenon based on first-principles calculations with scaled magnetic interaction. One scaling parameter is sufficient to describe quantitatively the magnetic moments in both CaFe2(As1-xPx)2 (x=0.055) and Ca0.78La0.22Fe2As2 at all temperatures. The anomalous growth of the local moments with increasing temperature can be understood from the observed large thermal expansion of the c-axis lattice parameter combined with strong magnetoelastic coupling. These effects originate from the strong tendency to form As-As dimers across the Ca layer in the CaFe2As2 family of materials. Our results emphasize the dual local-itinerant character of magnetism in Fe pnictides.

Theoretical prediction of a strongly correlated Dirac metal.

Recently, the most intensely studied objects in the electronic theory of solids have been strongly correlated systems and graphene. However, the fact that the Dirac bands in graphene are made up of sp(2) electrons, which are subject to neither strong Hubbard repulsion U nor strong Hund's rule coupling J, creates certain limitations in terms of novel, interaction-induced physics that could be derived from Dirac points. Here we propose GaCu3(OH)6Cl2 (Ga-substituted herbertsmithite) as a correlated Dirac-Kagome metal combining Dirac electrons, strong interactions and frustrated magnetic interactions. Using density functional theory, we calculate its crystallographic and electronic properties, and observe that it has symmetry-protected Dirac points at the Fermi level. Its many-body physics is diverse, with possible charge, magnetic and superconducting instabilities. Through a combination of various many-body methods we study possible symmetry-lowering phase transitions such as Mott-Hubbard, charge or magnetic ordering, and unconventional superconductivity, which in this compound assumes an f-wave symmetry.

Sign-reversal of the in-plane resistivity anisotropy in hole-doped iron pnictides.

Unconventional superconductivity usually originates from several strongly coupled degrees of freedom, such as magnetic, charge and elastic. A highly anisotropic electronic phase, not driven by lattice degrees of freedom, has been proposed in some of these superconductors, from cuprates to iron-based compounds. In the iron pnictide BaFe2As2, this nematic phase arises in the paramagnetic phase and is present for wide doping and temperature ranges. Here we probe the in-plane electronic anisotropy of electron- and hole-doped BaFe2As2 compounds. Unlike other materials, the resistivity anisotropy behaves very differently for electron- and hole-type dopants and even changes sign on the hole-doped side. This behaviour is explained by Fermi surface reconstruction in the magnetic phase and spin-fluctuation scattering in the paramagnetic phase. This unique transport anisotropy unveils the primary role played by magnetic scattering, demonstrating the close connection between magnetism, nematicity and unconventional superconductivity.

Na2IrO3 as a molecular orbital crystal.

Contrary to previous studies that classify Na(2)IrO(3) as a realization of the Heisenberg-Kitaev model with a dominant spin-orbit coupling, we show that this system represents a highly unusual case in which the electronic structure is dominated by the formation of quasimolecular orbitals (QMOs), with substantial quenching of the orbital moments. The QMOs consist of six atomic orbitals on an Ir hexagon, but each Ir atom belongs to three different QMOs. The concept of such QMOs in solids invokes very different physics compared to the models considered previously. Employing density functional theory calculations and model considerations we find that both the insulating behavior and the experimentally observed zigzag antiferromagnetism in Na(2)IrO(3) naturally follow from the QMO model.

Spin waves and revised crystal structure of honeycomb iridate Na2IrO3.

We report inelastic neutron scattering measurements on Na2IrO3, a candidate for the Kitaev spin model on the honeycomb lattice. We observe spin-wave excitations below 5 meV with a dispersion that can be accounted for by including substantial further-neighbor exchanges that stabilize zigzag magnetic order. The onset of long-range magnetic order below T(N)=15.3 K is confirmed via the observation of oscillations in zero-field muon-spin rotation experiments. Combining single-crystal diffraction and density functional calculations we propose a revised crystal structure model with significant departures from the ideal 90° Ir-O-Ir bonds required for dominant Kitaev exchange.

de Haas-van Alphen study of the Fermi surfaces of superconducting LiFeP and LiFeAs.

We report a de Haas-van Alphen oscillation study of the 111 iron pnictide superconductors LiFeAs with T(c) ≈ 18 K and LiFeP with T(c) ≈ 5 K. We find that for both compounds the Fermi surface topology is in good agreement with density functional band-structure calculations and has almost nested electron and hole bands. The effective masses generally show significant enhancement, up to ~3 for LiFeP and ~5 for LiFeAs. However, one hole Fermi surface in LiFeP shows a very small enhancement, as compared with its other sheets. This difference probably results from k-dependent coupling to spin fluctuations and may be the origin of the different nodal and nodeless superconducting gap structures in LiFeP and LiFeAs, respectively.