We use resonant x-ray scattering at the nickel L2,3 edges to investigate the interplay between orbital degrees of freedom and charge density waves (CDWs) in the superconductor BaNi2(As1-xPx)2.Both the incommensurate and commensurate CDWs in this system exhibit strong resonant enhancement with distinct energy and polarization dependencies, indicative of orbital ordering. Azimuthal-angle-dependent measurements reveal a lowering of the local Ni site symmetry, consistent with monoclinic or lower point group symmetry. The scattering signatures of both CDWs are dominated by contributions from Ni dxz,yz orbitals, with similar orbital character despite their distinct wave vectors. These findings point to a shared orbital-driven formation mechanism and provide new insight into the symmetry breaking and orbital and nematic fluctuations in the high-temperature regime of the superconductor BaNi2(As1-xPx)2.

We use resonant x-ray scattering at the nickel L_{2,3} edges to investigate the interplay between orbital degrees of freedom and charge density waves (CDWs) in the superconductor BaNi_{2}(As_{1-x}P_{x})_{2}. Both the incommensurate and commensurate CDWs in this system exhibit strong resonant enhancement with distinct energy and polarization dependencies, indicative of orbital ordering. Azimuthal-angle-dependent measurements reveal a lowering of the local Ni site symmetry, consistent with monoclinic or lower point group symmetry. The scattering signatures of both CDWs are dominated by contributions from Ni d_{xz,yz} orbitals, with similar orbital character despite their distinct wave vectors. These findings point to a shared orbital-driven formation mechanism and provide new insight into the symmetry breaking and orbital and nematic fluctuations in the high-temperature regime of the superconductor BaNi_{2}(As_{1-x}P_{x})_{2}.

We use resonant X-ray scattering at the nickel L_2,3 edges to investigate the interplay between orbital degrees of freedom and charge density waves (CDW) in the superconductor BaNi_2(As_1-xP_x)_2. Both the incommensurate and commensurate CDWs in this system exhibit strong resonant enhancement with distinct energy and polarization dependencies, indicative of orbital ordering. Azimuthal-angle-dependent measurements reveal a lowering of the local Ni site symmetry, consistent with monoclinic or lower point group symmetry. The scattering signatures of both CDWs are dominated by contributions from Ni d_xz,yz orbitals, with similar orbital character despite their distinct wave vectors. These findings point to a shared orbital-driven formation mechanism and provide new insight into the symmetry breaking and orbital/nematic fluctuations in the high-temperature regime of the superconductor BaNi_2(As_1-xP_x)_2.

With its potential for drastically reduced operation power of information processing devices, electric field control of magnetism has generated huge research interest. Recently, novel perspectives offered by the inherently large spin–orbit coupling of 5 d transition metals have emerged. Here, nonvolatile electrical control of the proximity‐induced magnetism in SrIrO 3 based back‐gated heterostructures is demonstrated. Up to a 700% variation of the anomalous Hall conductivity (σ AHE ) and Hall angle (Θ AHE ) as function of the applied gate voltage V g is reported. In contrast, the Curie temperature T C ≈ 100 K and magnetic anisotropy of the system remain essentially unaffected by V g indicating a robust ferromagnetic state in SrIrO 3 which strongly hints to gating‐induced changes of the anomalous Berry curvature. The electric‐field induced ferroelectric‐like state of SrTiO 3 enables nonvolatile switching behavior of σ AHE and Θ AHE below 60 K. The large tunability of this system, opens new avenues toward efficient electric‐field manipulation of magnetism.

We discuss the ferromagnetic phase of the planar Kondo lattice model using the bond fermion technique. For sufficiently small Kondo exchange we find a weakly ferromagnetic phase already for the weakly doped Kondo insulator. We present the phase diagram of the model and its quasiparticle band structure in the different phases.

Superconductivity in FeSe has recently attracted a great deal of attention because it emerges out of an electronic nematic state of elusive character. Here we study both the electronic normal state and the superconducting gap structure using heat-capacity measurements on high-quality single crystals. The specific-heat curve, from 0.4 K to T-c = 9.1 K, is found to be consistent with a recent gap determination using Bogoliubov quasiparticle interference [P. O. Sprau et al., Science 357, 75 (2017)]; however, only if nodes are introduced on either the electron or the hole Fermi-surface sheets. Our analysis, which is consistent with quantum-oscillation measurements, relies on the presence of one hole and one electron band only, and thus the fate of the theoretically predicted second electron pocket remains mysterious.

Superconductivity in FeSe has recently attracted a great deal of attention because it emerges out of an electronic nematic state of elusive character. Here we study both the electronic normal state and the superconducting gap structure using heat-capacity measurements on high-quality single crystals. The specific-heat curve, from 0.4 K to Tc = 9.1 K, is found to be consistent with a recent gap determination using Bogoliubov quasiparticle interference [P. O. Sprau et al., Science 357, 75 (2017)], however only if nodes are introduced on either the electron or the hole Fermi-surface sheets. Our analysis, which is consistent with quantum-oscillation measurements, relies on the presence of only two bands, and thus the fate of the theoretically predicted second electron pocket remains mysterious.

We report the development of an ab initio electronic structure method applicable to generic substitutionally disordered real materials. Acharge self-consistent orbital-based extension of the coherent potential approximation due to Blackman, Esterling, and Berk (BEB-CPA) is combined with the mixed-basis pseudopotential density functional theory approach. The general formalism in terms of a nonorthogonal basis set including subtleties of the pseudopotential framework is outlined. The BEB-CPA is validated on a binary tight-binding toy model against exact diagonalization of a randomly occupied cluster. Finally, the developed ab initio approach is benchmarked for a binary CuZn alloy, which confirms the robustness of the charge self-consistent procedure with respect to initial conditions.

The iron-based superconductors AFe_{2}As_{2} with A=K, Rb, Cs exhibit large Sommerfeld coefficients approaching those of heavy-fermion systems. We have investigated the magnetostriction and thermal expansion of this series to shed light on this unusual behavior. Quantum oscillations of the magnetostriction allow identifying the band-specific quasiparticle masses which by far exceed the band-structure derived masses. The divergence of the Grüneisen ratio derived from thermal expansion indicates that with increasing volume along the series a quantum critical point is approached. The critical fluctuations responsible for the enhancement of the quasiparticle masses appear to weaken the superconducting state.

A highly intriguing aspect in iron-pnictide superconductors is the composition-dependent electronic structure, in particular the question if and how charge carriers are introduced to the system upon substitution of Ba by alkali metals or of Fe by other transition metals, TM. We report on a systematic study of spatial structure and electronic states by x-ray diffraction and x-ray absorption on a large number of compositions in the (Ba,K)(Fe,TM)(2)As-2 family. The coherent combination of detailed structural information with an in-depth analysis of the electronic structure allows us to sensitively disentangle (charge-carrier) "doping" effects from "substitutional" effects. Results include a doping character that is site-decoupled, as well as TM 3d energy-level schemes that exhibit non-standard level sequences and even t(2)-e level crossings. Our study indicates that doping per se seems to play a lesser role than expected for pnictide superconductivity and magnetism.

We report a detailed low-temperature thermodynamic investigation (heat capacity and magnetization) of the superconducting state of KFe2As2 for H || c axis. Our measurements reveal that the properties of KFe2As2 are dominated by a relatively large nodeless energy gap (Delta?0 = 1.9 kBTc) which excludes dx2-y2 symmetry. We prove the existence of several additional extremely small gaps (?Delta0 < 1.0 kBTc) that have a profound impact on the low-temperature and low-field behavior, similar to MgB2, CeCoIn5 and PrOs4Sb12. The zero-field heat capacity is analyzed in a realistic self-consistent 4-band BCS model which qualitatively reproduces the recent laser ARPES results of Okazaki et al. (Science 337 (2012) 1314). Our results show that extremely low-temperature measurements, i.e. T < 0.1 K, will be required in order to resolve the question of the existence of line nodes in this compound.

The magnetic properties and Mott transition of the Hubbard model on the square lattice with frustration are studied at half-filling and zero temperature by the variational cluster approximation. When the onsite repulsion U is large, the magnetically disordered state is realized in a highly frustrated region between the Neel and collinear phases, and the magnetic ordering with longer periodicity in space corresponding to the ordering vector Q = (pi,pi/2) is not found there. As for the Mott transition, in addition to the Mott gap and double occupancy, which clarify the nature of the transition, the structure of the self-energy in the spectral representation is studied in detail below and above the Mott transition point. The spectral structure of the self-energy is almost featureless in the metallic phase, but clear single dispersion appears in the Mott insulator phase and this gives rise to the Mott gap.

We analyze the double exchange model with antiferromagnetic background by the book-keeping fermion method. First, we consider the motion of a single conduction electron. We assume that the localized spins form the Néel state and the added conduction electron forms only three types of states under the strong Hund’s-rule coupling, and we regard these states as the book-keeping fermions. Under these assumptions, we first calculate the spectral functions and find a good agreement with the results given by the exact diagonalizations for finite-size clusters. We next consider the situation where there are finite numbers of spin flips occurring in the down-spin sublattice in the Néel ordered state. Considering the effects of the spin defects, we add a new book-keeping fermion and estimate the energy of the spin defects by a perturbation theory. This theory gives the phase diagram comparable with the numerical results.

We study the one-dimensional double-exchange model with L localized spins and one mobile electron. We solve the Schrödinger equation analytically and obtain the energies and wave functions for all the eigenstates with spin S = (L − 1)/2 exactly. As an application, we compute the single-particle Green's function. We show that, for vanishing exchange interactions between localized spins, the single-particle spectrum is entirely incoherent and the lowest band has an infinite band mass, i.e., the single electron is localized due to its interaction with the spin excitations. The analysis of the wave function gives us a particularly simple ground state.