Cooper pair splitting is predicted to occur in hybrid devices where a superconductor is coupled to two ferromagnetic wires placed at a distance less than the superconducting coherence length. The integration of novel compensated halfmetallic ferromagnetic composition in magnetic tunnel junctions is presented as well. Plots of transmission and reflection coefficients as ns interface, where a represents the probability of andreev reflection, b gives the probability of ordinary reflection, c and d give transmission probability without and with branch crossing, respectively. Andreev reflection spectroscopy in niobium point contacts. The andreev spectra show clear and reproducible features that could be related to two nodeless gaps. Lowtemperature differential conductance measurements on the heavyfermion system pros4sb12 revealed multiple spectral features, including. These states are formed at the interface due to the interplay between virtual andreev and normal reflections and provide long range superconducting correlations on the graphene layer. Pointcontact andreev reflection spectroscopy measurements were performed on various unconventional superconducting systems, as a function of temperature and magnetic field, to probe their superconducting order parameter symmetries. The resolution and performance of a model system have been evaluated numerically. Professor david schmeltzer this dissertation is devoted to a study of detecting the majorana fermion induced crossed andreev re. An andreev reflection consists of the reflection of a quasielectron into a quasihole or vice versa at the n side of an ns interface, a process which.
Andreev, is a type of particle scattering which occurs at interfaces between a superconductor s and a normal state material n. Andreev reflection in unitary and nonunitary triplet. Andreev reflection ar, named after the russian physicist alexander f. We find resonance peaks in the andreev reflection amplitude, which are related to surface bound states in the superconductor. Determination of the spin polarization of halfmetallic. Wojtowicz, department of physics, university of notre dame, notre dame, in 46556 and. The quasiparticle reflection and transmission properties at normal conductorsuperconductor interfaces are examined for unitary and nonunitary spin triplet pairing states recently discussed in connection with sr 2 ruo 4. The probability of andreev reflection for an electron with energy defined by eqn. Detecting majorana fermion induced crossed andreev. Interface bound states have been theoretically predicted to appear at isolated graphenesuperconductor junctions. In the optical problem, a disordered medium backed by a phaseconjugating mirror can become completely transparent.
Here we report the observation of perfect andreev reflection in pointcontact spectroscopya clear signature of klein tunnelling and a manifestation of the. The phenomenon of crossed andreev reflection car is known to play a key role in nonlocal electron transport across threeterminal normalsuperconductingnormal nsn devices. Electrical transport in carbon nanotubes and graphene. Probing exotic superconducting order parameters using. Majorana fermions are particles that constitute their own antiparticles. Visualizing pure quantum turbulence in superfluid 3he. Pdf multiple andreev reflection in singleatom niobium. The phenomenon of andreev reflection is introduced as the electronic analogue of optical phaseconjugation. We revisit a problem of andreev reflection from the isolated, rectilinear vortex line. Andreev reflection spectroscopy in transition metal oxides. Sequence of andreev reflections can lead to formation of quasiparticle standing waves, andreev bound states, with energies smaller than the minimum. Schmeltzer physics department, city college of the city university of new york, new york, new york 10031, usa abstract we investigate the e ect of the majorana fermions which are formed at the boundary of a pwave superconductor. For finite majorana energies, it is difficult to obtain the scattering.
Scattering length of andreev reflection from quantized. Pointcontact andreev reflection spectroscopy pcars and planar tunneling spectroscopy are used to investigate the density of states and superconducting gap. Phononassisted andreev reflection in a hybrid junction. It is concluded that the ultimate magnitude for conductance uctuations is doubled to g 2 e 2 h.
Point contact andreev reflection by nanoindentation of. Such a structure may be used as a switching device. Andreev reflection in an stype superconductor proximized. The parameter z characterizes the barrier strength at the interface. Multiple andreev reflections spectroscopy of superconducting lifeas single. Superconducting electron focusing and guiding based on the. Secondly, we explored specular andreev reflection and have observed conductance peaks at the superconducting energy gap in normal metal graphene superconductor ns junctions. How can multiple andreev reflections be explained as a. Andreev reflection 5 and optical phase conju gation. Detecting majorana fermion induced crossed andreev reflection by lei fang adviser. Existing and proposed experiments on josephson junctions and bipolar junctions in graphene are discussed from a unified perspective. Andreev reflection spectroscopy is a versatile tool for studying the properties of various materials in the field of condensed matter physics. With increasing doping, a regular superconductor is ex pected with the andreev conductance of the order of e2 2 h. Pdf imaging andreev reflection in graphene semantic.
The second process, klein tunneling, is the tunneling through a pn junction. In the optical problem, a disordered medium backed. Measurement of spin polarization by andreev reflection in. In addition, we find that the nonlocal conductance is an odd function of bias at the topological phase transition, acting as a current rectifier in the lowbias limit. Box 5046, 2600 ga delft, netherlands, national cri center for semiconductor nanorods, postech. Coherent charge transport along ballistic paths can be introduced into graphene by andreev reflection, for which an electron reflects from a superconducting contact as a hole, while a cooper pair is transmitted. Dependence of andreev reflection and schottky barriers on.
Andreev reflection by experimentally realistic, threedimensional vortex tangles in 3heb, and contrast them with the latest experimental measurements of pure quan. Systematic effort is made to extend the technique of andreev reflection to high magnetic field in order to determine the sign of the spin polarization. Pcar spectroscopy is driven by the interest to find materials with high degree of spin polarization p. This thesis searches for signatures of this process, called crossed andreev reflection car. Andreev reflection and klein tunneling in graphene. I have understood the mechanisms at work in single andreev reflections normalsuperconducter interface and andreev bound states nsn. The scattering matrix s is computed using the continuity equations and the unitarity properties. Nadgornya department of physics and astronomy, wayne state university, detroit, mi 48201 t. Measurement of spin polarization by andreev reflection in ferromagnetic in1xmnxsb epilayers r. Ballistic josephson junctions are predicted to support a number of exotic physics processess, providing an ideal system to inject the supercurrent in the quantum hall regime. In particular, the method has been widely used to study the order parameter in superconductors. This is a tutorial article based on a lecture delivered in june 1999 at the nato advanced study institute in ankara. Kouwenhoven kavli institute of nanoscience, delft university of technology, p. But applying this reasoning to higher order processes such as this 5th order reflection doesnt seem to work.
Visualizing pure quantum turbulence in superfluid he. Muc h earlier 5, it w as also predicted that timerev ersed tra jectories, in v olving scattering from t w o sup erconductors, w ould result in h4e oscillations in the conductance as a function of the sup erconducting. University of groningen coherent andreev reflection in. The phenomena of andreev reflection and crossed andreev reflection can be understood from the general properties of the scattering s 5 11. An electron meeting the interface between a normal conductor and a superconductor produces a cooper pair in the superconductor and a retroreflected electron hole in the normal conductor legend. In structures with small transparent interfaces between the contact material and superconducting samples sinis double barrier structures there is a current deficit on i n v. This technique involves depositing a superconductor lead onto a thin film of the material to be studied through a nanohole formed in a layer of photoresist, using an atomic force microscope as a nanoindenter. Exciton transport and andreev reflection in a bilayer quantum hall system by a. Pdf we have developed a simple method to measure the transport spin polarization of ferromagnetic materials. Our previous pointcontact andreev reflection studies of the heavyfermion superconductor cecoin5 using au tips have shown two clear features. Dependence of andreev reflection and schottky barriers on gamnasnb interface treatment k. More generally, we identify conditions for crossed andreev reflection to dominate the nonlocal conductance and show how to design a cooper pair splitter in the open regime.
However, the intended goal of the project, observation of specular andreev reflection, was not achieved. N normal conductor, s superconductor, red electron, green hole. Crossed andreev reflection car is enhanced at the bias energy of ev. Andreev reflection of thermal quasiparticles from quantized vortices is an important technique to visualize quantum turbulence in low temperature he3. Bilayer graphene supporting information engineering. Arrows indicate the spin band occupied by each particle. We use a liquidhelium cooled scanning gate microscope sgm to image andreev reflection in graphene in the magnetic focusing regime, where carriers move along cyclotron orbits between. If we set i i i 12 and i 3 0, we will find 2 12 12 2 ie v v h v 12, 3 sc 2 vv vv. It is a chargetransfer process by which normal current in n is converted to supercurrent in s. Tu delft is sustaining member of open education global except where otherwise noted, contents on this site are licensed under a creative commons attributionnoncommercialsharealike 4. Perfect andreev reflection due to the klein paradox in a. A versatile technique for performing spin polarization measurements via point contact andreev reflection has been developed. Andreev reflection an overview sciencedirect topics. In the spirit of this symposium, our contnbution has an analogy as its icitmotiv.
The trajectories of an incident particle and the andreev reflected hole. Anisotropy and temperature behavior of the order parameters. For multiple andreev reflections of order 3, the following mechanism makes sense to me. Andreev re ection fabrizio dolcini scuola normale superiore di pisa, nest italy dipartimento di fisica del politecnico di torino italy lecture notes for xxiii physics graddays, heidelberg, 59 october 2009. Herein, we demonstrate electrical transport measurements on ballistic superconductorgraphenesuperconductor junctions by contacting g. Pointcontact andreev reflection pcar spectroscopy measurements have been performed in the febased superconductor lafeaso1. The phenomenon of andreev re ection is introduced as the electronic analogue of optical phaseconjugation.
Pointcontact andreev reflection spectroscopy of heavy. We describe in detail the formation of these states from combining the dirac equation with the. Experimental scanning tunneling microscopy stm and scanning tunneling spectroscopy sts techniques will be described as standard tools for studying this effect. Based on the principle of andreev reflection, we suggest a novel normalmetal superconductor composite structure capable of electron focusing and guiding. Investigations of crossed andreev reflection in hybrid. B phononassisted andreev reflection in a hybrid junction based on a quantum dot kacper bocian 0 wojciech rudzinski 0 0 faculty of physics, adam mickiewicz university, ul. Multiple andreev reflections revealed by the energy distribution of.
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