The interplay between local, repulsive interactions and disorder acting only on one spin orientation of lattice fermions (“spin-dependent disorder”) is investigated. The nonmagnetic disorder vs interaction phase diagram is computed using dynamical mean-field theory in combination with the geometric average over disorder. The latter determines the typical local density of states and is therefore sensitive to Anderson localization. The effect of spin-dependent disorder is found to be very different from that of conventional disorder. In particular, it destabilizes the metallic solution and leads to a spin-selective, localized phase at weak interactions and strong disorder.

Dzyaloshinskii-Moriya interaction (DMI) plays a central role in breaking chiral symmetry of magnetic domain wall structure. The recently observed chiral dependence of domain wall structures in ultrathin magnetic films with perpendicular anisotropy indicates the presence of a strong DMI. We calculate the indirect exchange interaction between magnetic ions mediated by spin-polarized conduction electrons with a Rashba spin-orbit coupling. We find the resulting DMI increases with the spin-orbit coupling strength, but decreases with the spin-polarization of the conduction electrons. The estimated DMI magnitude is comparable to the experimental results.

The magnetic ground state in the double perovskite system $\mathrm{S}{\mathrm{r}}_{2-x}\mathrm{C}{\mathrm{a}}_x\mathrm{CoOs}{\mathrm{O}}_6$ changes from an antiferromagnet $(x=0)$, to a spin glass $(x=1)$, to a ferrimagnet $(x=2)$ as the Ca content increases. This crossover is driven by chemical pressure effects that control the relative strength of magnetic exchange interactions. The synthesis, crystal structure, and magnetism of $\mathrm{SrCaCoOs}{\mathrm{O}}_6$ and $\mathrm{C}{\mathrm{a}}_2\mathrm{CoOs}{\mathrm{O}}_6$ are investigated and compared with $\mathrm{S}{\mathrm{r}}_2\mathrm{CoOs}{\mathrm{O}}_6$. Both compounds adopt a monoclinic crystal structure with rock-salt ordering of $\mathrm{C}{\mathrm{o}}^{2+}$ and $\mathrm{O}{\mathrm{s}}^{6+}$ and $a^{-}{a}^{-}{b}^{+}$ octahedral tilting, but the average Co–O–Os bond angle evolves from $158.0{\left(3\right)}^{\circ }$ in $\mathrm{SrCaCoOs}{\mathrm{O}}_6$ to $150.54{\left(9\right)}^{\circ }$ in $\mathrm{C}{\mathrm{a}}_2\mathrm{CoOs}{\mathrm{O}}_6$ as the smaller $\mathrm{C}{\mathrm{a}}^{2+}$ ion replaces $\mathrm{S}{\mathrm{r}}^{2+}$. While this change may seem minor, it has a profound effect on the magnetism, changing the magnetic ground state from antiferromagnetic in $\mathrm{S}{\mathrm{r}}_2\mathrm{CoOs}{\mathrm{O}}_6$ $(T_{\mathrm{N}1}=108\mathrm{K}$, $T_{\mathrm{N}2}=70\mathrm{K})$, to a spin glass in $\mathrm{SrCaCoOs}{\mathrm{O}}_6$ $(T_{\mathrm{f}1}=32\mathrm{K}$, $T_{\mathrm{f}2}=13\mathrm{K})$, to ferrimagnetic in $\mathrm{C}{\mathrm{a}}_2\mathrm{CoOs}{\mathrm{O}}_6$ $(T_{\mathrm{C}}=145\mathrm{K})$. In the first two compounds the observation of two transitions is consistent with weak coupling between the Co and Os sublattices.

An attractive proposal for the hidden order (HO) in the heavy electron compound ${\mathrm{URu}}_2{\mathrm{Si}}_2$ is an itinerant multipole order of high rank. It is due to the pairing of electrons and holes centered on the zone center and boundary, respectively, in states that have maximally different total angular momentum components. Due to the pairing with a commensurate zone boundary ordering vector the translational symmetry is broken and a HO quasiparticle gap opens below the transition temperature $T_{HO}$. Inelastic neutron scattering (INS) has demonstrated that for $T<T_{HO}$ the collective magnetic response is dominated by a sharp spin exciton resonance at the ordering vector Q that is reminiscent of spin exciton modes found inside the gap of unconventional superconductors and Kondo insulators. We use an effective two-orbital tight-binding model incorporating the crystalline-electric-field effect to derive closed expressions for quasiparticle bands reconstructed by the multipolar pairing terms. We show that the magnetic response calculated within that model exhibits the salient features of the resonance found in INS. We also use the calculated dynamical susceptibility to explain the low-temperature NMR relaxation rate.

The local atomic and electronic structures around the dopants in Cr-doped $\left({\mathrm{Bi}}_x{\mathrm{Sb}}_{1-x}\right){}_2{\mathrm{Te}}_3$ are studied by x-ray absorption fine structure (XAFS) measurements and first-principles calculations. Both Cr and Bi are confirmed substituting Sb sites $({\mathrm{Cr}}_{\mathrm{Sb}}$ and ${\mathrm{Bi}}_{\mathrm{Sb}})$. The six nearest Te atoms around Cr move towards Cr and shorten the Cr-Te bond lengths to 2.76 Å and 2.77 Å for $x=0.1$ and $x=0.2$, respectively. Importantly, we reveal the hybridization between the Sb/Te $p$ states and Cr $d$ states by the presence of a pre-edge peak at Cr $K$-absorption edge, which is also supported by our ab initio calculations. These findings provide important clues to understand the mechanism of ferromagnetic order in this system with quantum anomalous Hall effect.

We demonstrate on the basis of ab initio simulations how passivated semiconductor surfaces can be exploited to study bulklike interaction properties and wave functions of magnetic impurities on the atomic scale with conventional and spin-polarized scanning tunneling microscopy. By applying our approach to the case of $3d$ transition metal impurities close to the $\mathrm{H}/\mathrm{Si}\left(111\right)$ surface, we show exemplarily that their wave functions in Si are less extended than for Mn in GaAs, thus obstructing ferromagnetism in Si. Finally, we discuss possible applications of this method to other dilute magnetic semiconductors.

Finding more efficient ways of exploring the doping phase diagrams of high-temperature superconductors as well as probing the fundamental properties of these materials are essential ingredients for driving the discovery of new materials. We use a doping technique involving gating with ionic liquids to systematically and continuously tune the $T_c$ of superconducting ${\mathrm{La}}_2{\mathrm{CuO}}_{4+x}$ thin films. We probe both the transport properties and the penetration depth of these samples and find that Homes scaling ${\lambda }^{-2}\propto \sigma T_c$ is obeyed, consistent with these materials being in the dirty limit. This result is independent of the precise mechanism for the gating process as all of the parameters of the scaling relationship are determined by direct measurements on the films.

Spontaneous breaking of time-reversal symmetry in superconductors with the $p_x+i{p}_y$ symmetry of the order parameter allows for a class of effects which are analogous to the anomalous Hall effect in ferromagnets. These effects exist below the critical temperature, $T<T_c$. We develop a kinetic theory of such effects. In particular, we consider anomalous Hall thermal conductivity, the polar Kerr effect, the anomalous Hall effect, and the anomalous photo- and acousto-galvanic effects.

In this work we demonstrate explicit analytical expressions for both charge and spin currents which constitute the $2\times 2$ spinor in magnetic tunnel junctions with noncollinear magnetizations under applied voltage. The calculations have been performed within the free electron model in the framework of the Keldysh formalism and WKB approximation. We demonstrate that spin/charge currents and spin transfer torques are all explicitly expressed through only three irreducible quantities, without further approximations. The conditions and mechanisms of deviation from the conventional sine angular dependence of both spin currents and torques are shown and discussed. It is shown in the thick barrier approximation that all tunneling transport quantities can be expressed in an extremely simplified form via Slonczewski spin polarizations and our effective spin averaged interfacial transmission probabilities and effective out-of-plane polarizations at both interfaces. It is proven that the latter plays a key role in the emergence of perpendicular spin torque as well as in the angular dependence character of all spin and charge transport considered. It is demonstrated directly also that for any applied voltage, the parallel component of spin current at the FM/I interface is expressed via collinear longitudinal spin current components. Finally, spin transfer torque behavior is analyzed in a view of transverse characteristic length scales for spin transport.

The crystal-field and exchange parameters are determined for the single-crystalline hydride $\mathrm{ErF}{\mathrm{e}}_{11}\mathrm{TiH}$ compound by analyzing the experimental magnetization curves obtained in magnetic fields of up to 60 T. By using the calculated parameters we succeeded in modeling theoretical magnetization curves for $\mathrm{ErF}{\mathrm{e}}_{11}\mathrm{TiH}$ up to 200 Т and to study in detail the transition from ferrimagnetic to a ferromagnetic state in the applied magnetic field.

We report experimental and theoretical studies of spin wave eigenmodes in transversely magnetized thin film Permalloy wires. Using broadband ferromagnetic resonance technique, we measure the spectrum of spin wave eigenmodes in individual wires as a function of magnetic field and wire width. Comparison of the experimental data to our analytical model and micromagnetic simulations shows that the intrinsic dipolar edge pinning of spin waves is negligible in transversely magnetized wires. Our data also quantify the degree of extrinsic edge pinning in Permalloy wires. This work establishes the boundary conditions for dynamic magnetization in transversely magnetized thin film wires for the range of wire widths and thicknesses studied, and provides a quantitative description of the spin wave eigenmode frequencies and spatial profiles in this system as a function of the wire width.

The phase diagram of electron-doped pnictides is studied varying the temperature, electronic density, and isotropic in-plane quenched disorder strength and dilution by means of computational techniques applied to a three-orbital ($xz,yz,xy$) spin-fermion model with lattice degrees of freedom. In experiments, chemical doping introduces disorder but in theoretical studies the relationship between electronic doping and the randomly located dopants, with their associated quenched disorder, is difficult to address. In this publication, the use of computational techniques allows us to study independently the effects of electronic doping, regulated by a global chemical potential, and impurity disorder at randomly selected sites. Surprisingly, our Monte Carlo simulations reveal that the fast reduction with doping of the Néel $T_N$ and the structural $T_S$ transition temperatures, and the concomitant stabilization of a robust nematic state, is primarily controlled in our model by the magnetic dilution associated with the in-plane isotropic disorder introduced by Fe substitution. In the doping range studied, changes in the Fermi surface produced by electron doping affect only slightly both critical temperatures. Our results also suggest that the specific material-dependent phase diagrams experimentally observed could be explained as a consequence of the variation in disorder profiles introduced by the different dopants. Our findings are also compatible with neutron scattering and scanning tunneling microscopy, unveiling a patchy network of locally magnetically ordered clusters with anisotropic shapes, even though the quenched disorder is locally isotropic. This study reveals a remarkable and unexpected degree of complexity in pnictides: the fragile tendency to nematicity intrinsic of translational invariant electronic systems needs to be supplemented by quenched disorder and dilution to stabilize the robust nematic phase experimentally found in electron-doped 122 compounds.

We have made a systematic investigation of charge transport in one-dimensional chains of Josephson junctions where the characteristic Josephson energy is much less than the single-junction Cooper-pair charging energy, $E_J\ll E_{CP}$. Such chains are deep in the insulating state, where superconducting phase coherence across the chain is absent, and a voltage threshold for conduction is observed at the lowest temperatures. We find that Cooper-pair tunneling in such chains is completely suppressed. Instead, charge transport is dominated by tunneling of single electrons, which is very sensitive to the presence of BCS quasiparticles on the superconducting islands of the chain. Consequently, we observe a strong parity effect, where the threshold voltage vanishes sharply at a characteristic parity temperature $T^{*}$, which is significantly lower than the critical temperature $T_c$. A measurable and thermally activated zero-bias conductance appears above $T^{*}$, with an activation energy equal to the superconducting gap, confirming the role of thermally excited quasiparticles. Conduction below $T^{*}$ and above the voltage threshold occurs via injection of single electrons/holes into the Cooper-pair insulator, forming a nonequilibrium steady state with a significantly enhanced effective temperature. Our results explicitly show that single-electron transport dominates deep in the insulating state of Josephson junction arrays. This conduction process has mostly been ignored in previous studies of both superconducting junction arrays and granular superconducting films below the superconductor-insulator quantum phase transition.

We study one-component fermions in chain lattices with proximity-induced superconducting gap and interparticle short-range interaction, capable of hosting Majorana fermions. By systematically tracking various physical quantities, we show that topological states and topological phase transitions in the system can be identified by multiple signatures in thermodynamic quantities and pair-condensate properties, in good agreement with the known signatures in the ground-state energy and entanglement spectrum. We find the disappearance of the topological phase in a largely attractive regime, in which the system undergoes a first-order transition between two topologically trivial states. In addition, the stability of the signatures against finite size, disorder, and inhomogeneity is analyzed. Our results provide additional degrees of freedom for the characterization of topological states with interaction and for the experimental detection of emergent Majorana fermions.

We report on our exact diagonalization studies of the effects of screened and unscreened long-range Coulomb interaction on the topological superconductivity (TS) phase in a quasi-one-dimensional semiconductor wire, proximity-coupled to an $s$-wave superconductor. We found that inclusion of the Coulomb interaction results in an enlargement of the region of parameter values where TS can be observed. This enlargement is proportional to the screening length of the Coulomb interaction. We also found that the interaction decreases the superconducting bulk gap. Some screening of the Coulomb interaction was found to be essential for observing TS in a quasi-one-dimensional semiconductor wire proximity-coupled to an $s$-wave superconductor. The theoretical framework employed here is quite general and can be used to address the issue of orbital effects and multichannel two-dimensional quantum wire for observation of TS in the presence of the Coulomb interaction between electrons.

For a general quantum many-body system, we show that its ground-state entanglement imposes a fundamental constraint on the low-energy excitations. For two-dimensional systems, our result implies that any system that supports anyons must have a nonvanishing topological entanglement entropy. We demonstrate the generality of this argument by applying it to three-dimensional quantum many-body systems and showing that there is a pair of ground-state topological invariants that are associated to their physical boundaries. From the pair, one can determine whether the given boundary can or cannot absorb pointlike or linelike excitations.

The renormalized adiabatic PBE (rAPBE) method has recently been shown to comprise a significant improvement over the random phase approximation (RPA) for total energy calculations of simple solids and molecules. Here we consider the formation energies of 19 group I and II metal oxides and a few transition-metal oxides. The mean absolute error relative to experiments is 0.21 eV and 0.38 eV per oxygen atom for rAPBE and RPA, respectively, and thus the rAPBE method greatly improves the description of metal-oxygen bonds across a wide range of oxides. The failure of the RPA can be partly attributed to the lack of error cancellation between the correlation energy of the oxide on the one hand and the bulk metal and oxygen molecule on the other hand, which are all separately predicted much too negative by the RPA. We ascribe the improved performance of the rAPBE to its significantly better description of absolute correlation energies which reduces the need for error cancellation. The rAPBE is just one out of an entire class of renormalized exchange-correlation kernels which should be further investigated.

We report low-temperature de Haas–van Alphen (dHvA) effect measurements in magnetic fields up to 35 T of the heavy-fermion superconductor ${\mathrm{Ce}}_2{\mathrm{PdIn}}_8$. The comparison of the experimental results with band-structure calculations implies that the $4f$ electrons are itinerant rather than localized. The cyclotron masses estimated at high field are only moderately enhanced, $8m_0$ and $14m_0$, but are substantially larger than the corresponding band masses. The observed angular dependence of the dHvA frequencies suggests quasi-two-dimensional Fermi surfaces in agreement with band-structure calculations. However, the deviation from ideal two-dimensionality is larger than in ${\mathrm{CeCoIn}}_5$, to which ${\mathrm{Ce}}_2{\mathrm{PdIn}}_8$ bears a lot of similarities. This subtle distinction accounts for the different superconducting critical temperatures of the two compounds.

A systematic study of 32 honeycomb monolayer II-VI semiconductors is carried out by first-principles methods. While none of the two-dimensional (2D) structures can be energetically stable, it appears that BeO, MgO, CaO, ZnO, CdO, CaS, SrS, SrSe, BaTe, and HgTe honeycomb monolayers have a good dynamic stability. The stability of the five oxides is consistent with the work published by Zhuang et al. [Appl. Phys. Lett. 103, 212102 (2013)]. The rest of the compounds in the form of honeycomb are dynamically unstable, revealed by phonon calculations. In addition, according to the molecular dynamic (MD) simulation evolution from these unstable candidates, we also find two extra monolayers dynamically stable, which are tetragonal BaS $\left[P4/nmm\left(129\right)\right]$ and orthorhombic HgS $[P{2}_1/m\left(11\right)]$. The honeycomb monolayers exist in the form of either a planar perfect honeycomb or a low-buckled 2D layer, all of which possess a band gap and most of them are in the ultraviolet region. Interestingly, the dynamically stable SrSe has a gap near visible light, and displays exotic electronic properties with a flat top of the valence band, and hence has a strong spin polarization upon hole doping. The honeycomb HgTe has recently been reported to achieve a topological nontrivial phase under appropriate in-plane tensile strain and spin-orbital coupling (SOC) [J. Li et al., arXiv:1412.2528]. Some II-VI partners with less than $5\%$ lattice mismatch may be used to design novel 2D heterojunction devices. If synthesized, potential applications of these 2D II-VI families could include optoelectronics, spintronics, and strong correlated electronics.