Electronic Origin of the Inhomogeneous Pairing Interaction in the High-Tc Superconductor Bi2Sr2CaCu2O8+d

Identifying the mechanism of superconductivity in the high-temperature cuprate superconductors is one of the major outstanding problems in physics. We report local measurements of the onset of superconducting pairing in the high-transition temperature (Tc) superconductor Bi2Sr2CaCu2O8+d using a lattice-tracking spectroscopy technique with a scanning tunneling microscope. We can determine the temperature dependence of the pairing energy gaps, the electronic excitations in the absence of pairing, and the effect of the local coupling of electrons to bosonic excitations. Our measurements reveal that the strength of pairing is determined by the unusual electronic excitations of the normal state, suggesting that strong electron-electron interactions rather than low-energy (<0.1 volts) electron-boson interactions are responsible for superconductivity in the cuprates

Electronic structure in underdoped cuprates due to the emergence of a pseudogap

The phenomenological Green's function developed in the works of Yang, Rice and Zhang has been very successful in understanding many of the anomalous superconducting properties of the deeply underdoped cuprates. It is based on considerations of the resonating valence bond spin liquid approximation and is designed to describe the underdoped regime of the cuprates. Here we emphasize the region of doping, $x$, just below the quantum critical point at which the pseudogap develops. In addition to Luttinger hole pockets centered around the nodal direction, there are electron pockets near the antinodes which are connected to the hole pockets by gapped bridging contours. We determine the contours of nearest approach as would be measured in angular resolved photoemission experiments and emphasize signatures of the Fermi surface reconstruction from the large Fermi contour of Fermi liquid theory (which contains $1+x$ hole states) to the Luttinger pocket (which contains $x$ hole states). We find that the quasiparticle effective mass renormalization increases strongly towards the edge of the Luttinger pockets beyond which it diverges.Comment: 11 pages, 9 figure

Deterministic control of magnetic vortex wall chirality by electric field

Concepts for information storage and logical processing based on magnetic domain walls have great potential for implementation in future information and communications technologies. To date, the need to apply power hungry magnetic fields or heat dissipating spin polarized currents to manipulate magnetic domain walls has limited the development of such technologies. The possibility of controlling magnetic domain walls using voltages offers an energy efficient route to overcome these limitations. Here we show that a voltage-induced uniaxial strain induces reversible deterministic switching of the chirality of a magnetic vortex wall. We discuss how this functionality will be applicable to schemes for information storage and logical processing, making a significant step towards the practical implementation of magnetic domain walls in energy efficient computing

Spectroscopic scanning tunneling microscopy insights into Fe-based superconductors

In the first three years since the discovery of Fe-based high Tc superconductors, scanning tunneling microscopy (STM) and spectroscopy have shed light on three important questions. First, STM has demonstrated the complexity of the pairing symmetry in Fe-based materials. Phase-sensitive quasiparticle interference (QPI) imaging and low temperature spectroscopy have shown that the pairing order parameter varies from nodal to nodeless s\pm within a single family, FeTe1-xSex. Second, STM has imaged C4 -> C2 symmetry breaking in the electronic states of both parent and superconducting materials. As a local probe, STM is in a strong position to understand the interactions between these broken symmetry states and superconductivity. Finally, STM has been used to image the vortex state, giving insights into the technical problem of vortex pinning, and the fundamental problem of the competing states introduced when superconductivity is locally quenched by a magnetic field. Here we give a pedagogical introduction to STM and QPI imaging, discuss the specific challenges associated with extracting bulk properties from the study of surfaces, and report on progress made in understanding Fe-based superconductors using STM techniques.Comment: 36 pages, 23 figures, 229 reference

Bias dependence of spin transfer torque in Co₂MnSi Heusler alloy based magnetic tunnel junctions

Heusler compounds are of interest as electrode materials for use in magnetic tunnel junctions (MTJs) due to their half metallic character, which leads to 100% spin polarization and high tunneling magnetoresistance. Most work to date has focused on the improvements to tunneling magnetoresistance that can stem from the use of Heusler electrodes, while there is much less work investigating the influence of Heusler electrodes on the spin transfer torque properties of MTJs. Here, we investigate the bias dependence of the anti-damping like and field-like spin transfer torque components in both symmetric (Co2MnSi/MgO/Co2MnSi) and asymmetric (Co2MnSi/MgO/CoFe) structure Heusler based MTJs using spin transfer torque ferromagnetic resonance. We find that while the damping like torque is linear with respect to bias for both MTJ structures, the asymmetric MTJ structure has an additional linear component to the ordinarily quadratic field like torque bias dependence and that these results can be accounted for by a free electron tunneling model. Furthermore, our results suggest that the low damping and low saturation magnetization properties of Heusler alloys are more likely to lead significant improvements to spin torque switching efficiency rather than their half metallic character

Massive Dirac fermion observed in lanthanide-doped topological insulator thin films

This publication arises from research funded by the John Fell Oxford University Press (OUP) Research Fund, a DARPA MESO Project (No. N66001-11-1-4105), and funding from the European Union Seventh Framework Programme under Grant Agreement 312483 – ESTEEM2 (Integrated Infrastructure Initiative I3) and transnational access to MPI-IS (WP13). Part of this work was performed at the Stanford Nano Shared Facilities (SNSF). S.E.H. was supported by the VPGE (Stanford University), L.C.M. by EPSRC (UK), and P.S. acknowledges partial funding from EPSRC, Corpus Christi College (Oxford), and the Studienstiftung des deutschen Volkes (Germany).The breaking of time reversal symmetry (TRS) in three-dimensional (3D) topological insulators (TIs), and thus the opening of a 'Dirac-mass gap' in the linearly dispersed Dirac surface state, is a prerequisite for unlocking exotic physical states. Introducing ferromagnetic long-range order by transition metal doping has been shown to break TRS. Here, we present the study of lanthanide (Ln) doped Bi2Te3, where the magnetic doping with high-moment lanthanides promises large energy gaps. Using molecular beam epitaxy, single-crystalline, rhombohedral thin films with Ln concentrations of up to ∼35%, substituting on Bi sites, were achieved for Dy, Gd, and Ho doping. Angle-resolved photoemission spectroscopy shows the characteristic Dirac cone for Gd and Ho doping. In contrast, for Dy doping above a critical doping concentration, a gap opening is observed via the decreased spectral intensity at the Dirac point, indicating a topological quantum phase transition persisting up to room-temperature.Peer reviewe

Study of Ho-doped Bi2Te3 topological insulator thin films

Breaking time-reversal symmetry through magnetic doping of topological insulators has been identified as a key strategy for unlocking exotic physical states. Here, we report the growth of Bi2Te3 thin films doped with the highest magnetic moment element Ho. Diffraction studies demonstrate high quality films for up to 21% Ho incorporation. Superconducting quantum interference device magnetometry reveals paramagnetism down to 2 K with an effective magnetic moment of ∼5 μB/Ho. Angle-resolved photoemission spectroscopy shows that the topological surface state remains intact with Ho doping, consistent with the material's paramagnetic state. The large saturation moment achieved makes these films useful for incorporation into heterostructures, whereby magnetic order can be introduced via interfacial coupling

Comparative assessment of sire evaluation by univariate and bivariate animal model for estimation of breeding values of first lactation traits in HF cross cattle

The aim of the present investigation was to study the superiority of bivariate over univariate sire evaluation. Data were collected on 1,988 first parity Karan Fries cows, spread over 31 years. The (co) variance components estimated by using average information restricted maximum likelihood (AIREML) were fitted into univariate and bivariate animal models for prediction of breeding values. Low heritability estimates were obtained for fertility traits ranging from 0.02 (FDPR) to 0.19 (AFC) indicating lesser role of additive gene action in fertility of dairy cattle. Comparative analysis revealed that the breeding values estimated using bivariate animal model had lower error variance and greater range in comparison to univariate animal models. The mean sire breeding values for production traits estimated by bivariate analysis ranged from 3055.50 to 3063.15 kg and were higher compared to the mean sire breeding values estimated by univariate animal model. The inclusion of fertility traits along with production traits improved the differentiating ability of bivariate animal model with respect to the production performance

Transverse Domain Wall Profile for Spin Logic Applications

Domain wall (DW) based logic and memory devices require precise control and manipulation of DW in nanowire conduits. The topological defects of Transverse DWs (TDW) are of paramount importance as regards to the deterministic pinning and movement of DW within complex networks of conduits. In-situ control of the DW topological defects in nanowire conduits may pave the way for novel DW logic applications. In this work, we present a geometrical modulation along a nanowire conduit, which allows for the topological rectification/inversion of TDW in nanowires. This is achieved by exploiting the controlled relaxation of the TDW within an angled rectangle. Direct evidence of the logical operation is obtained via magnetic force microscopy measurement