Quantum phase transitions and the role of impurity-substrate hybridization in Yu-Shiba-Rusinov states

Spin-dependent scattering from magnetic impurities inside a superconductor gives rise to Yu-Shiba-Rusinov (YSR) states within the superconducting gap. They can be modeled by the largely equivalent Kondo or Anderson impurity models. The role of the magnetic and nonmagnetic properties of the impurity in relation to the coupling to the substrate is still under debate. Here, we use a scanning tunneling microscope to make a quantitative connection between the energy of a YSR state and the impurity-substrate hybridization. We corroborate the impurity substrate coupling as a key energy scale for surface derived YSR states using the Anderson impurity model. By combining experimental data from YSR state spectra and additional conductance measurements, we can determine on which side of the quantum phase transition the system resides. We thus provide a crucial step towards a more quantitative understanding of the crucial role of impurity substrate coupling utilizing the Anderson model.

Filtering spins by scattering from a lattice of point magnets

Nature creates electrons with two values of the spin projection quantum number. In certain applications, it is important to filter electrons with one spin projection from the rest. Such filtering is not trivial, since spin-dependent interactions are often weak, and cannot lead to any substantial effect. Here we propose an efficient spin filter based upon scattering from a two-dimensional crystal, which is made of aligned point magnets. The polarization of the outgoing electron flux is controlled by the crystal, and reaches maximum at specific values of the parameters. In our scheme, polarization increase is accompanied by higher reflectivity of the crystal. High transmission is feasible in scattering from a quantum cavity made of two crystals. Our findings can be used for studies of low-energy spin-dependent scattering from two-dimensional ordered structures made of magnetic atoms or aligned chiral molecules.

Velocity independent constraints on spin-dependent DM-nucleon interactions from IceCube and PICO

Adopting the Standard Halo Model (SHM) of an isotropic Maxwellian velocity distribution for dark matter (DM) particles in the Galaxy, the most stringent current constraints on their spin-dependent scattering cross-section with nucleons come from the IceCube neutrino observatory and the PICO-60 $$\hbox {C}_3\hbox {F}_8$$ C 3 F 8 superheated bubble chamber experiments. The former is sensitive to high energy neutrinos from the self-annihilation of DM particles captured in the Sun, while the latter looks for nuclear recoil events from DM scattering off nucleons. Although slower DM particles are more likely to be captured by the Sun, the faster ones are more likely to be detected by PICO. Recent N-body simulations suggest significant deviations from the SHM for the smooth halo component of the DM, while observations hint at a dominant fraction of the local DM being in substructures. We use the method of Ferrer et al. (JCAP 1509: 052, 2015) to exploit the complementarity between the two approaches and derive conservative constraints on DM-nucleon scattering. Our results constrain $$\sigma _{\mathrm{SD}} \lesssim 3 \times 10^{-39} \mathrm {cm}^2$$ σ SD ≲ 3 × 10 - 39 cm 2 ($$6 \times 10^{-38} \mathrm {cm}^2$$ 6 × 10 - 38 cm 2 ) at $$\gtrsim 90\%$$ ≳ 90 % C.L. for a DM particle of mass 1 TeV annihilating into $$\tau ^+ \tau ^-$$ τ + τ - ($$b\bar{b}$$ b b ¯ ) with a local density of $$\rho _{\mathrm{DM}} = 0.3~\mathrm {GeV/cm}^3$$ ρ DM = 0.3 GeV / cm 3 . The constraints scale inversely with $$\rho _{\mathrm{DM}}$$ ρ DM and are independent of the DM velocity distribution.

The Interaction Potential

The interaction potentials and their spatial Fourer transforms are derived for nuclear and magnetic scattering, as well as for interactions with atomic electric fields. For the nuclear interaction, the Fermi pseudopotential is introduced and the scattering length operator is defined. The neutron spin dependence of the nuclear and magnetic interaction is calculated, and general expressions for spin-dependent scattering are obtained. The longitudinal and XYZ polarization analysis methods are described, and the technique of spherical neutron polarimetry is explained. The Blume-Maleev equation which gives the final neutron polarization for an arbitrary incident polarization are derived.

Antisymmetric linear magnetoresistance and the planar Hall effect

The phenomena of antisymmetric magnetoresistance and the planar Hall effect are deeply entwined with ferromagnetism. The intrinsic magnetization of the ordered state permits these unusual and rarely observed manifestations of Onsager’s theorem when time reversal symmetry is broken at zero applied field. Here we study two classes of ferromagnetic materials, rare-earth magnets with high intrinsic coercivity and antiferromagnetic pyrochlores with strongly-pinned ferromagnetic domain walls, which both exhibit antisymmetric magnetoresistive behavior. By mapping out the peculiar angular variation of the antisymmetric galvanomagnetic response with respect to the relative alignments of the magnetization, magnetic field, and electrical current, we experimentally distinguish two distinct underlying microscopic mechanisms: namely, spin-dependent scattering of a Zeeman-shifted Fermi surface and anomalous electron velocities. Our work demonstrates that the anomalous electron velocity physics typically associated with the anomalous Hall effect is prevalent beyond the ρxy(Hz) channel, and should be understood as a part of the general galvanomagnetic behavior.