Interfacial Magnetism in Manganite Superlattices

We use a two-orbital double-exchange model including Jahn-Teller lattice distortions, superexchange interactions, and long-range Coulomb (LRC) interactions to investigate the origin of magnetically disordered interfaces between ferromagnetic metallic (FM) and antiferromagnetic insulating (AFI) manganites in FM/AFI superlattices. The induced magnetic moment in the AFI layer varies non-monotonically with increasing AFI layer width as seen in the experiment. We provide a framework for understanding this non-monotonic behavior which has a one-to-one correspondence with the magnetization of the FM interface. The obtained insights provide a basis for improving the tunneling magnetoresistance in FM/AFI manganite superlattices by avoiding a magnetic dead layer (MDL) in the FM manganite.Comment: 5 pages, 5 figures. To appear in PR

Mirages, anti-mirages, and further surprises in quantum corrals with non-magnetic impurities

We investigate the local density of states (LDOS) for non-interacting electrons in a hard wall ellipse in the presence of a single non-magnetic scattering center. Using a T-matrix analysis we calculate the local Green's function and observe a variety of quantum mirage effects for different impurity positions. Locating the impurity near positions with LDOS maxima for the impurity free corral can either lead to a reduction or an enhancement of the LDOS at the mirror image point, i.e. a mirage or anti-mirage effect, or even suppress LDOS maxima in the entire area of the corral.Comment: 6 pages, 7 figure

Combined Effect of Bond- and Potential-Disorder in Half-Doped Manganites

We analyze the effects of both bond- and potential-disorder in the vicinity of a first-order metal insulator transition in a two-band model for manganites using a real-space Monte Carlo method. Our results reveal a novel charge-ordered state coexisting with spin-glass behavior. We provide the basis for understanding the phase diagrams of half-doped manganites, and contrast the effects of bond- and potential-disorder and the combination of both.Comment: 4 pages, 3 figures, published versio

Electronic and Magnetic Reconstructions in Manganite Superlattices

We investigate the electronic reconstruction at the interface between ferromagnetic metallic (FM) and antiferromagnetic insulating (AFI) manganites in superlattices using a two-orbital double-exchange model including superexchange interactions, Jahn-Teller lattice distortions, and long range Coulomb interactions. The magnetic and the transport properties critically depend on the thickness of the AFI layers. We focus on superlattices where the constituent parent manganites have the same electron density n = 0.6. The induced ferromagnetic moment in the AFI layers decreases monotonically with increasing layer width, and the electron-density profile and the magnetic structure in the center of the AFI layer gradually return to the bulk limit. The width of the AFI layers and the charge-transfer profile at the interfaces control the magnitude of the magnetoresistance and the metal-insulator transition of the FM/AFI superlattices.Comment: 11 pages, 10 figure

Superconductivity with Finite-Momentum Pairing in Zero Magnetic Field

In the BCS theory of superconductivity, one assumes that all Cooper pairs have the same center of mass momentum. This is indeed enforced by self consistency, if the pairing interaction is momentum independent. Here, we show that for an attractive nearest neighbor interaction, this is different. In this case, stable solutions with pairs with momenta q and -q coexist and, for a sufficiently strong interaction, one of these states becomes the groundstate of the superconductor. This finite-momentum pairing state is accompanied by a charge order with wave vector 2q. For a weak pairing interaction, the groundstate is a d-wave superconductor

Fractional Flux Quantization in Loops of Unconventional Superconductors

The magnetic flux threading a conventional superconducting ring is typically quantized in units of $\Phi_0=hc/2e$. The factor 2 in the denominator of $\Phi_0$ originates from the existence of two different types of pairing states with minima of the free energy at even and odd multiples of $\Phi_0$. Here we show that spatially modulated pairing states exist with energy minima at fractional flux values, in particular at multiples of $\Phi_0/2$. In such states condensates with different center-of-mass momenta of the Cooper pairs coexist. The proposed mechanism for fractional flux quantization is discussed in the context of cuprate superconductors, where $hc/4e$ flux periodicities as well as uniaxially modulated superconducting states were observed.Comment: 5 pages, 3 figure

Flux-Periodicity Crossover from hc/e in Normal Metallic to hc/2e in Superconducting Loops

The periodic response of a metallic or a superconducting ring to an external magnetic flux is one of the most evident manifestations of quantum mechanics. It is generally understood that the oscillation period hc/2e in the superconducting state is half the period hc/e in the metallic state, because the supercurrent is carried by Cooper pairs with a charge 2e. On the basis of the Bardeen-Cooper-Schrieffer theory we discuss, in which cases this simple interpretation is valid and when a more careful analysis is needed. In fact, the knowledge of the oscillation period of the current in the ring provides information on the electron interactions. In particular, we analyze the crossover from the hc/e periodic normal current to the hc/2e periodic supercurrent upon turning on a pairing interaction in a metal ring. Further, we elaborate on the periodicity crossover when cooling a metallic loop through the superconducting transition temperature Tc.Comment: To be bublished in "Superconductors", InTech (Rijeka), 2012 (ISBN 979-953-307-798-6

Supercurrent as a Probe for Topological Superconductivity in Magnetic Adatom Chains

A magnetic adatom chain, proximity coupled to a conventional superconductor with spin-orbit coupling, exhibits locally an odd-parity, spin-triplet pairing amplitude. We show that the singlet-triplet junction, thus formed, leads to a net spin accumulation in the near vicinity of the chain. The accumulated spins are polarized along the direction of the local $\mathbf{d}$-vector for triplet pairing and generate an enhanced persistent current flowing around the chain. The spin polarization and the "supercurrent" reverse their directions beyond a critical exchange coupling strength at which the singlet superconducting order changes its sign on the chain. The current is strongly enhanced in the topological superconducting regime where Majorana bound states appear at the chain ends. The current and the spin profile offer alternative routes to characterize the topological superconducting state in adatom chains and islands.Comment: 5 pages, 3 figures, 5 pages of supplemental material