The charge-mediated effect of electric field on the perpendicular magnetic anisotropy (PMA) of Fe/MgO interfaces is investigated using first-principles calculations. We present an approach by discussing this effect in relation to the intrinsic dipole field existing at the Fe/MgO interface. A firm correlation between the PMA and the interfacial dipole is established and further verified in the absence of an applied electric field. The on-site projected PMA analysis not only elucidates that the effect of electric field on the PMA extends beyond the interfacial Fe layer, but also shows that the second Fe layer carries the largest contribution to the effect. This observation is interpreted in relation to the orbital hybridization changes induced by applying an electric field.

We investigate the electronic specific heat of superoptimally substituted ${\mathrm{BaFe}}_2{\left({\mathrm{As}}_{1-x}{\mathrm{P}}_x\right)}_2$ single crystals in the superconducting state using high-resolution nanocalorimetry. From the measurements, we extract the substitution dependence of the condensation energy, superconducting gap $\mathrm{\Delta }$, and related microscopic parameters. We find that the anomalous scaling of the specific heat jump $\mathrm{\Delta }C\propto T_{\mathrm{c}}^3$, found in many iron-based superconductors, in this system originates from a $T_{\mathrm{c}}$-dependent ratio $\mathrm{\Delta }/k_{\mathrm{B}}{T}_{\mathrm{c}}$ in combination with a substitution-dependent density of states $N\left({\varepsilon }_{\mathrm{F}}\right)$. A clear enhancement is seen in the effective mass $m^{*}$ as the composition approaches the value that has been associated with a quantum critical point at optimum substitution. However, a simultaneous increase in the superconducting carrier concentration $n_{\mathrm{s}}$ yields a penetration depth $\lambda$ that decreases with increasing $T_{\mathrm{c}}$ without sharp divergence at the quantum critical point. Uemura scaling indicates that $T_{\mathrm{c}}$ is governed by the Fermi temperature $T_{\mathrm{F}}$ for this multiband system.

We analyze the magnetization at the interface between singlet and triplet superconductors and show that its direction and dependence on the phase difference across the junction are strongly tied to the structure of the triplet order parameter as well as to the pairing interactions. We consider equal spin helical, opposite spin chiral, and mixed symmetry pairing on the triplet side and show that the magnetization vanishes at $\varphi =0$ only in the first case, follows approximately a $cos\varphi$ behavior for the second, and shows higher harmonics for the last configuration. We trace the origin of the magnetization to the magnetic structure of the Andreev bound states near the interface, and provide a symmetry-based explanation of the results. Our findings can be used to control the magnetization in superconducting heterostructures and to test symmetries of spin-triplet superconductors.

Recent experiments in optimally hole-doped iron arsenides have revealed a novel magnetically ordered ground state that preserves tetragonal symmetry, consistent with either a charge-spin density wave (CSDW), which displays a nonuniform magnetization, or a spin-vortex crystal (SVC), which displays a noncollinear magnetization. Here we show that, similarly to the partial melting of the usual stripe antiferromagnet into a nematic phase, either of these phases can also melt in two stages. As a result, intermediate paramagnetic phases with vestigial order appears: a checkerboard charge density wave for the CSDW ground state, characterized by an Ising-like order parameter, and a remarkable spin-vorticity density wave for the SVC ground state—a triplet $d$-density wave characterized by a vector chiral order parameter. We propose experimentally detectable signatures of these phases, show that their fluctuations can enhance the superconducting transition temperature, and discuss their relevance to other correlated materials.

In superconductors spin split by an exchange field, thermal effects are coupled to spin transport. We show how an oscillating electromagnetic field in such systems creates spin imbalance, that can be detected with a spin-polarized probe. The sign and magnitude of the probe signal result from a competition between processes converting field-induced spin energy imbalance to spin imbalance, dominant at low frequencies, and microwave-driven pair breaking at high frequencies. In the presence of spin-flip scattering, we show that ac excitation also leads to multistabilities in the superconducting state.

The relationship between the superconducting gap and the pseudogap has been the subject of controversies. In order to clarify this issue, we have studied the superconducting gap and pseudogap of the $\text{high}-T_c$ superconductor ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$ ($x=0.10,14$) by angle-resolved photoemission spectroscopy (ARPES). Through the analysis of the ARPES spectra above and below $T_c$, we have identified a superconducting coherence peak even in the antinodal region on top of the pseudogap of a larger energy scale. The superconducting peak energy nearly follows the pure $d$-wave form. The $d$-wave order parameter ${\mathrm{\Delta }}_0$ [defined by $\mathrm{\Delta }\left(k\right)={\mathrm{\Delta }}_0(cos{k}_{x}a-cos{k}_{y}a)$ ] for $x=0.10$ and 0.14 are nearly the same, ${\mathrm{\Delta }}_0\sim 12–14\mathrm{meV}$, leading to strong coupling $2{\mathrm{\Delta }}_0/k_B{T}_c\sim 10$. The present result indicates that the pseudogap and the superconducting gap are distinct phenomena and can be described by the “two-gap” scenario.

The low-temperature electronic phase diagram of ${\mathrm{Ba}}_{1-x}{\mathrm{Na}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$, obtained using high-resolution thermal-expansion and specific-heat measurements, is shown to be considerably more complex than previously reported, containing nine different phases. Besides the magnetic $C_2$ and reentrant $C_4$ phases, we find evidence for an additional, presumably magnetic, phase below the usual spin-density-wave transition, as well as a possible incommensurate magnetic phase. All these phases coexist and compete with superconductivity, which is particularly strongly suppressed by the $C_4$-magnetic phase due to a strong reduction of the electronic entropy available for pairing in this phase.

Skyrmions are topological spin textures of interest for fundamental science and applications. Previous theoretical studies have usually focused on chiral magnets with broken bulk inversion symmetry, with skyrmions stabilized by easy-axis anisotropy. We investigate here systems that break surface inversion (mirror symmetry) in addition to bulk inversion. This leads to two distinct Dzyaloshinskii-Moriya (DM) terms with strengths $D_{\perp }$, arising from Rashba spin-orbit coupling (SOC), and $D_{\parallel }$ from Dresselhaus SOC. We show that skyrmions become progressively more stable with increasing $D_{\perp }/D_{\parallel }$ and extend into the regime of easy-plane anisotropy. We find that the spin texture and topological charge density of skyrmions develops a nontrivial spatial structure with quantized topological charge in a unit cell given by a Chern number. Our results give a design principle for tuning Rashba SOC and anisotropy to stabilize skyrmions in thin films, surfaces, interfaces, and bulk magnetic materials that break mirror symmetry.

We study three-dimensional systems where the parent metallic state contains a loop of Weyl points. We introduce the minimal $\mathbf{k}·\mathbf{p}$ Hamiltonian, and discuss its symmetries. Guided by this symmetry analysis, we classify the superconducting instabilities that may arise. For a doped Weyl loop material, we argue that—independent of microscopic details—the leading superconducting instability should be to a fully gapped chiral superconductor in three dimensions—an unusual state made possible only by the nontrivial topology of the Fermi surface. This state, which we dub the “meron superconductor,” is neither fully topological nor fully trivial. Meanwhile, at perfect compensation additional states are possible (including some that are fully topological), but the leading instability depends on microscopic details. We discuss the influence of disorder on pairing. In the presence of a spin degeneracy (“Dirac loops”) still more complex superconducting states can arise, including a “skyrmion” superconductor with topological properties similar to superfluid He III-B, which additionally breaks lattice rotation symmetry and exhibits nematic order.

The random energy model (REM) provides a solvable mean-field description of the equilibrium spin-glass transition. Its quantum sibling (the QREM), obtained by adding a transverse field to the REM, has similar properties and shows a spin-glass phase for sufficiently small transverse field and temperature. In a recent work, some of us have shown that the QREM further exhibits a many-body localization-delocalization (MBLD) transition when viewed as a closed quantum system, evolving according to the quantum dynamics. This phase encloses the familiar equilibrium spin-glass phase. In this paper, we study in detail the MBLD transition within the forward-scattering approximation and replica techniques. The predictions for the transition line are in good agreement with the exact diagonalization numerics. We also observe that the structure of the eigenstates at the MBLD critical point changes continuously with the energy density, raising the possibility of a family of critical theories for the MBLD transition.

In the present work, we study the effects of the electronic relaxation of semicore levels on polaron activation energies and dynamics. Within the framework of adiabatic ab initio theory, we utilize both static transition state theory and molecular dynamics methods for an in-depth study of polaronic hopping in delithiated ${\text{LiFePO}}_4 \left({\text{FePO}}_4\right)$. Our results show that electronic relaxation of semicore states is significant in ${\text{FePO}}_4$, resulting in a lower activation barrier and kinetics that is one to two orders faster compared to the result of calculations that do not incorporate semicore states. In general, the results suggest that the relaxation of states far below the Fermi energy could dramatically impact the ab initio polaronic barrier estimates for many transition metal oxides and phosphates.

We investigate the magnetic order of the buckled honeycomb lattice antiferromagnet ${\mathrm{Ba}}_2{\mathrm{NiTeO}}_6$ and its related antiferromagnet ${\mathrm{Ba}}_3{\mathrm{NiTa}}_2{\mathrm{O}}_9$ by neutron diffraction measurements. We observe magnetic Bragg peaks below the transition temperatures, and identify propagation vectors for these oxides. A combination of representation analysis and Rietveld refinement leads to a collinear magnetic order for ${\mathrm{Ba}}_2{\mathrm{NiTeO}}_6$ and a ${120}^{\circ }$ structure for ${\mathrm{Ba}}_3{\mathrm{NiTa}}_2{\mathrm{O}}_9$. We find that the spin model of the bilayer triangular lattice is equivalent to that of the two-dimensional buckled honeycomb lattice having magnetic frustration. We discuss the magnetic interactions and single-ion anisotropy of $\mathrm{Ni}^{2+}$ ions for ${\mathrm{Ba}}_2{\mathrm{NiTeO}}_6$ in order to clarify the origin of the collinear magnetic structures. Our calculation suggests that the collinear magnetic order of ${\mathrm{Ba}}_2{\mathrm{NiTeO}}_6$ is induced by the magnetic frustration and easy-axis anisotropy.

When spin-polarized electrons are reflected at a ferromagnetic surface, their spin-polarization vector exhibits a spin motion, comprised of an azimuthal precession and a polar rotation about the magnetization direction of the ferromagnetic material. It is shown that the angles of precession and of rotation are intimately related by dispersion relations. Comparison with experimental data verifies their applicability. Particular attention has to be paid to the presence of complex zeros of the spin-dependent electron reflectivity.

In this paper, we address the structure and interaction of neighboring asymmetric Néel and Bloch walls in soft ferromagnetic films. First, we review a recent reduced model for the structure of parallel systems of asymmetric walls with potentially interacting tails and provide a derivation via the method of matched asymptotic expansions, starting from the micromagnetic-torque equation. Then, we report on experimentally observed domain-wall transitions under a varying hard-axis field in ${\mathrm{Co}}_{40}{\mathrm{Fe}}_{40}{\mathrm{B}}_{20}$ films. Upon the wall transition, the average hard-axis magnetization in the domains increases significantly, by an amount that depends on the width of the domains. For films of moderate thickness, the hard-axis magnetization jump that the theory predicts excellently agrees with the experimental data. Hence, we conclude that interacting tails of neighboring asymmetric Néel walls cause the observed additional rotation of the magnetization towards large hard-axis fields.

Exact self-consistent soliton dynamics based on the Bogoliubov–de Gennes (BdG) formalism in unconventional Fermi superfluids/superconductors possessing an $\text{SU}\left(d\right)$-symmetric two-body interaction is presented. The derivation is based on the ansatz having the similar form as the Gelfand-Levitan-Marchenko equation in the inverse scattering theory. Our solutions can be regarded as a multicomponent generalization of the solutions recently derived by Dunne and Thies [Phys. Rev. Lett. 111, 121602 (2013)]. We also propose superpositions of occupation states, which make it possible to realize various filling rates even in one-flavor systems, and include Dirac and Majorana fermions. The soliton solutions in the $d=2$ systems, which describe the mixture of singlet $s$-wave and triplet $p$-wave superfluids, exhibit a variety of phenomena such as rotating polar phases by soliton spins, SU(2)-DHN breathers, Majorana triplet states, $s-p$ mixed dynamics, and so on. These solutions are illustrated by animations, where order parameters are visualized by spherical harmonic functions. The full formulation of the BdG theory is also supported, and the double-counting problem of BdG eigenstates and $N$-flavor generalization are discussed.

A theory of spin transport and spin detection in Zeeman-split superconducting films at low temperatures is developed. It is shown that an injection of spin-unpolarized quasiparticles into a Zeeman-split superconductor gives rise to a spin imbalance. The relaxation length of such a spin signal is determined by the energy relaxation length and can be extremely large as compared to the usual spin relaxation length. There can exist two types of signals: due to nonthermalized quasiparticle distribution and due to thermalized overheated electron distribution. They have different decay lengths and can be distinguished by their different dependencies on the applied voltage. The decay length of the nonthermalized signal is determined by the electron-electron scattering rate, renormalized due to superconductivity. The decay length of the thermalized signal is determined by the length on which energy leaves the electronic subsystem and can be very large under special conditions. Applications of the theory to recent experimental data on spin relaxation in Zeeman-split and exchange-split superconductors are discussed. In particular, it can explain the extremely high spin relaxation lengths, experimentally observed in Zeeman-split superconductors, and their growth with the magnetic field and with the applied voltage.

We have illuminated a thin-film superconducting Al lumped-element microwave resonator with 780 nm light and observed the resonator quality factor and resonance frequency as a function of illumination and microwave power in the 20 to 300 mK temperature range. The optically induced microwave loss increases with increasing illumination but decreases with increasing microwave power. Although this behavior may suggest the presence of optically activated two-level systems, we find that the loss is better explained by the presence of nonequilibrium quasiparticles generated by the illumination and excited by the microwave drive. We model the system by assuming that the illumination creates an effective source of phonons with energy higher than double the superconducting gap and solve the coupled quasiparticle-phonon rate equations. We fit the simulation results to our measurements and find good agreement with the observed dependence of the resonator quality factor and frequency shift on temperature, microwave power, and optical illumination. Examination of the model reveals approaches to reducing optically induced loss and improving the relaxation time of superconducting quantum devices.

We have studied magnetic excitations in a field-induced noncollinear commensurate ferrimagnetic phase of ${\mathrm{Ba}}_2{\mathrm{Mg}}_2{\mathrm{Fe}}_{12}{\mathrm{O}}_{22}$ by means of polarized inelastic neutron scattering (PINS) and terahertz (THz) time-domain optical spectroscopy under magnetic field. A previous THz spectroscopy study reported that the field-induced phase exhibits electric-dipole-active excitations with energies of around 5 meV [Kida et al., Phys. Rev. B 83, 064422 (2011)]. In the present PINS measurements, we observed inelastic scattering signals around 5 meV at the zone center in the spin-flip channel. This directly shows that the electric-dipole-active excitations are indeed of magnetic origin, that is, electromagnons. In addition, the present THz spectroscopy confirms that the excitations have oscillating electric polarization parallel to the $c$ axis. In terms of the spin-current model (Katsura-Nagaosa-Balatsky model), the noncollinear magnetic order in the field-induced phase can induce static electric polarization perpendicular to the $c$ axis, but not dynamic electric polarization along the $c$ axis. We suggest that the electromagnon excitations can be explained by applying the magnetostriction model to the out-of-phase oscillations of the magnetic moments, which is deduced from the present experimental results.