PROJECTS AT SOIS: MATERIALS FOR SPINTRONICS APPLICATIONS

Spin-Orbit Interaction Spectroscopy (SOIS) Institute of Physics, Ecole Polytechnique Fédérale de Lausanne Swiss Light Source, Paul Scherrer Institut

Hugo Dil

sois.epfl.ch Dil

SpintronicsNovel electronics design to use the spin of the electron and not the charge

Main advantage: No movement of charge necessary

• Lower power consumption • Faster • Smaller • Flexibility (not only 0 or 1) • Spin is quantum state: quantum computing

Common approach based on magnetics

Giant Magneto Resistance (GMR): Nobel prize 2007 Albert Fert, Peter Grünberg

sois.epfl.ch Dil

Taking the magnet out

Physics 2, 50 (2009)

Spintronics without magnetismDavid Awschalom

Department of Physics, University of California, Santa Barbara, CA 93106, USA

Nitin Samarth

Department of Physics, The Pennsylvania State University, University Park, PA 16802,

USA

Published June 15, 2009

The spin-orbit e�ect is at the heart of e�orts to merge spintronics—where information is carried

and stored by spin, rather than by charge—with semiconductor technology.

Subject Areas: Spintronics

The 2007 Nobel Prize in Physics (awarded to AlbertFert and Peter Grünberg) highlighted the remarkablyrapid transition of “spintronics” from fundamental stud-ies of spin-dependent transport in metallic ferromagneticmultilayers [1] to a device technology critical to the mag-netic storage industry. While the mainstream of spin-tronics continues to expand the scientific and technologi-cal frontiers of ferromagnetic metal devices [2], a parallele�ort in semiconductor spintronics is growing vigorously[3]. This incarnation of spintronics has the same gen-eral motivation as metallic spintronics: to understandand control the transport of spin-polarized currents andto eventually apply this knowledge in information tech-nologies. However, semiconductor spintronics also bringswith it the promise of integrating the best qualities oftwo disparate worlds: the unparalleled storage capac-ity of magnetic memory and the impressive computingpower of semiconductor logic. Add to this the possibilityof exploiting the relatively long-lived quantum coherenceof spin states in semiconductors [4] and one can evenimagine unleashing the full power of the quantum worldin truly revolutionary devices that exploit both the am-plitude and phase of wave functions. Needless to say,the realization of this potential requires concerted e�ortsaimed at both understanding the mechanisms for spin-dependent transport in semiconductors as well as criti-cally comparing spin-based device schemes with existingtechnologies [5].

At first glance, it seems almost a given that ferromag-netism would be a necessary and integral component ofany scheme for semiconductor spintronic devices. For in-stance, a semiconductor spintronic device generically re-quires an imbalance between spin “up” and “down” pop-ulations of electrons (or holes). We can imagine this im-balance being created by the injection of spin-polarizedcharge carriers from a ferromagnet, which acts as a spinpolarizer. Alternatively, we could build devices from fer-romagnetic semiconductors that have an intrinsic spinimbalance. Indeed, important advances have been madein semiconductor spintronics by using these very notions,with a number of interesting proof-of-concept semicon-ductor spintronic device demonstrations that incorpo-rate ferromagnetic elements for injecting, detecting, and

manipulating spins [6]. But, discoveries in recent yearshave inspired a completely di�erent avenue to semicon-ductor spintronics—one that does not involve any ferro-magnetism whatsoever [7–9].

The development of this alternate track—“spintronicswithout magnetism”—relies on our ability to manipulatecarrier spins in semiconductors through the spin-orbit in-teraction. This is an attractive pathway for designingsemiconductor spintronic devices because spin-orbit cou-pling enables the generation and manipulation of spinssolely by electric fields. This is easy to understand in aqualitative way by recalling that spin-orbit coupling isthe natural outcome of incorporating special relativitywithin quantum mechanics (the Dirac and Pauli equa-tions). In the rest frame of an electron moving througha lattice, the external electric field (along with that fromthe atomic cores) is Lorentz transformed into a magneticfield that can act upon the spin of the electron. Usingthe spin-orbit interaction for manipulating electron spinobviates the design complexities that are often associ-ated with incorporating local magnetic fields into devicearchitectures. As we discuss below, the basic conceptualframework for the influence of the spin-orbit interactionon mobile electron spins has deep and old roots, but theexperimental harnessing of these concepts is very con-temporary and still at an early—and exciting—stage ofdevelopment and discovery.

The spin-orbit coupling generates spin polarizationthrough two conceptually di�erent processes: spin-dependent scattering and the acquisition of a geomet-ric phase. The former idea harks back to the heydayof quantum mechanics, when Sir Neville Mott first usedthe Dirac equation to calculate the spin-dependent skewscattering of relativistic electrons by a Coulomb poten-tial, in which electrons with spin up and down are scat-tered in opposite trajectories [10]. Mott’s argument hasnow resurfaced within condensed matter physics in theanomalous [11] and spin Hall e�ects [12]. Further, band-structure e�ects provide variations on this interplay be-tween scattering and the spin-orbit interaction by theremoval of spin degeneracy in momentum space [13–15].This removal of spin degeneracy acts like an “e�ectivemagnetic field” that can be engineered into a semicon-

DOI: 10.1103/Physics.2.50URL: http://link.aps.org/doi/10.1103/Physics.2.50

c• 2009 American Physical Society

8

SPG Mitteilungen Nr. 30

Progress in Physics (17)

Spintronics without magnetism?

J. Hugo DilPhysik-Institut, Universität Zürich, 8057 Zürich, and Swiss Light Source, Paul Scherrer Institute, 5232 Villigen PSI

jan-hugo.dil@psi.ch

Introduction

Spintronics is the vision of using the spin of the electron rather than its charge to control information flows, which can be exploited in a future quantum computer. In the field of data storage, spintronics has already become reality; the giant magnetoresistance effect, for the discovery of which Fert and Grünberg received the 2008 Nobel prize, is used in modern memory devices. Currently many researchers are working on systems where small magnetic structures are used to design a complete spintronics ensemble. The magnetic moments of the electrons, i.e. the spin, lead to a net magnetization in these structures. As we know from our basic physics lectures the spin of an electron can in the simplest approach be manipulated by magnetic fields. Here we will discuss a possible alternative which is fully based on non-magnetic materials and utilizes the spin-orbit inter-action in low-dimensional structures to control the electron spin. The advantage of this approach is that it can lead to reduced information processing time at even lower energy consumption and is as such highly interesting and promi-sing for future applications.For any useful spin computer one needs at least the follow-ing components:

(i) The manipulation of the electron spin in a controlled way.

(ii) The creation of a spin-polarized current from a non-polarized source of electrons; i.e. a spin filter.

(iii) The coherent transport of spin-polarized electrons over macroscopic distances without the loss of infor-mation.

(iv) The storage of information on macroscopic time sca-les.

Because spin-orbit interaction is a dynamic process it is diffcult to envisage spin-storage based on such an effect, as data storage requires a steady state solution and thus will most likely remain based on magnetic structures. The possible realization of the other three components without magnetic materials is discussed in the next sections.

Spin manipulation; the Rashba effect

Spin manipulation without magnetic fields relies on spin-orbit coupling. A promising candidate to achieve a con-trolled manipulation is the Rashba effect, which relies on the breaking of space inversion symmetrie, as will be ex-plained below.In many cases the surface of a metal (or semiconductor) crystal is not just a truncation of the bulk. For instance at semiconductor surfaces the dangling bonds which are for-

med due to the sudden truncation of the crystal, induce a rearrangement of the atoms to lift these free bonds. Fur-thermore due to the confinement between a projected band gap and the image potential, surface states can form, which have totally different properties as the electronic states in the bulk material. In both cases the main cause of the dif-ferences is that at the surface the translational symmetry is broken along the surface normal, or, more general, that the space inversion symmetry is broken.For an electron with a given momentum k and spin or( )- . space inversion symmetry means that it is equivalent whether the electron moves in one direction or the other; i.e. E ( E (k k, ) , )=- -- . In the absence of a magnetic field, time inversion symmetry holds; i.e. E ( E (k k, ) , )= .- - . In the bulk of a non-magnetic, centrosymmetric metal both time and space inversion symmetry are observed, resulting in the formation of spin-degenerate states E ( E (k k, ) , ) .= .-

For states located at a surface or interface (e.g surface states or quantum well states) the space inversion symme-try is actually broken, which means that the spin degenera-cy does not necessarily hold for these states.

That the spin degeneracy is actually lifted for surface states can be understood by the following simple relativistic argu-ment. The sudden termination of the crystal at the surface creates a potential gradient perpendicular to the surface, which can also be regarded as a local electric field. In the rest frame of a moving valence electron this electric field becomes a magnetic field through a Lorentz transforma-tion. This magnetic field causes a Zeeman splitting of the electronic states and thus an energy difference between the states with different spin orientations. The magnitude of this magnetic field and thus also the energy splitting de-pends on the momentum of the electron and changes sign for opposite momenta. At zero momentum the splitting dis-appears and the bands are degenerate.This momentum dependent energy splitting of surface or interface states is typically referred to as the Rashba (or Rashba-Bychkov) effect (see Ref. [1] and references the-

Rashba effect

The Rashba-Bychkov effect is a spin-splitting of bands in a 2D electron gas due to a net electric field and spin-orbit coupling. Although initially described for semi-conductor heterostructures it was first observed on the surface of gold. The spin splitting does not result in a net magnetization of the system.

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Spintronics: magnetics vs. SOI

Magnetics Spin-orbit interaction

Transistor

Memorywriting

Magnetic fields are non-local and slow Spin-orbit interaction is local and instantaneous

B. Slomski, et al. Nature Scientific Reports (2013)Novel materials needed! Rashba systems and topological materials

sois.epfl.ch Dil

Relevance of the Rashba effect

We use InSb nanowires (15), which areknown to have strong spin-orbit interaction anda large g factor (16). From our earlier quantum-dot experiments, we extract a spin-orbit lengthlso ≈ 200 nm corresponding to a Rashba param-eter a ≈ 0.2 eV·Å (17). This translates to a spin-orbit energy scale a2m*/(2ħ2) ≈ 50 meV (m* =0.015me is the effective electron mass in InSb,me is the bare electron mass, and ħ is Planck’sconstant h divided by 2p). Importantly, the gfactor in bulk InSb is very large (g ≈ 50), yield-ing EZ/B ≈ 1.5 meV/T. As shown below, we findan induced superconducting gap D ≈ 250 meV.Thus, for m = 0, we expect to enter the topo-logical phase for B ~ 0.15 T where EZ starts toexceed D. The energy gap of the topologicalsuperconductor is estimated to be a few kelvin(17), if we assume a ballistic nanowire. Thetopological gap is substantially reduced in a dis-ordered wire (18, 19). We have measured meanfree paths of ~300 nm in our wires (15), implyinga quasi-ballistic regime in micrometer-long wires.With these numbers, we expect Majorana zero-energy states to become observable below 1 Kand around 0.15 T.

A typical sample is shown in Fig. 1B.We firstfabricate a pattern of narrow (50-nm) and wider(300-nm) gates on a silicon substrate (20). Thegates are covered by a thin Si3N4 dielectric be-fore we randomly deposit InSb nanowires. Next,we electrically contact those nanowires thathave landed properly relative to the gates. Thelower contact in Fig. 1B fully covers the bottompart of the nanowire. We have designed the up-per contact to only cover half of the top part ofthe nanowire, avoiding complete screening ofthe underlying gates. This allows us to changethe Fermi energy in the section of the nanowire(NW) with induced superconductivity. We haveused either a normal (N) or superconducting (S)material for the lower and upper contacts, re-sulting in three sample variations: (i) N-NW-S,(ii) N-NW-N, and (iii) S-NW-S. Here, we dis-cuss our main results on the N-NW-S devices,whereas the other two types, serving as controldevices, are described in (20).

To perform spectroscopy on the induced su-perconductor, we created a tunnel barrier in thenanowire by applying a negative voltage to anarrow gate (dark green area in Fig. 1, B and C).A bias voltage applied externally between the Nand S contacts drops almost completely acrossthe tunnel barrier. In this setup, the differentialconductance dI/dV at voltage V and current I isproportional to the density of states at energy E =eV (where e is the charge on the electron) relativeto the zero-energy dashed line in Fig. 1C. Figure1D shows an example taken at B = 0. The twopeaks at T250 meV correspond to the peaks in thequasi-particle density of states of the inducedsuperconductor, providing a value for the in-duced gap, D ≈ 250 meV. We generally find afinite dI/dV in between these gap edges. We ob-serve pairs of resonances with energies symmetricaround zero bias superimposed on nonresonant

currents throughout the gap region. Symmetricresonances likely originate from Andreev boundstates (21, 22), whereas nonresonant current in-dicates that the proximity gap has not fully de-veloped (23).

Figure 2 summarizes our main result. Figure2A shows a set of dI/dV-versus-V traces taken at

increasingB fields in 10-mTsteps from 0 (bottomtrace) to 490 mT (top trace), offset for clarity. Weagain observe the gap edges at T250 meV. Whenwe apply a B field between ~100 and ~400 mTalong the nanowire axis, we observe a peak atV= 0. The peak has an amplitude up to ~0.05·2e2/hand is clearly discernible from the background

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Fig. 1. (A) Outline of theoretical proposals. (Top) Conceptual device layout with a semiconductingnanowire in proximity to an s-wave superconductor. An external B field is aligned parallel to the wire.The Rashba spin-orbit interaction is indicated as an effective magnetic field, Bso, pointing perpendicularto the nanowire. The red stars indicate the expected locations of a Majorana pair. (Bottom) Energy, E,versus momentum, k, for a 1D wire with Rashba spin-orbit interaction, which shifts the spin-down band(blue) to the left and the spin-up band (red) to the right. Blue and red parabolas are for B = 0; blackcurves are for B ≠ 0, illustrating the formation of a gap near k = 0 of size Ez (m is the Fermi energy withm = 0 defined at the crossing of parabolas at k = 0). The superconductor induces pairing between statesof opposite momentum and opposite spin, creating a gap of size D. (B) Implemented version of the-oretical proposals. Scanning electron microscope image of the device with normal (N) and super-conducting (S) contacts. The S contact only covers the right part of the nanowire. The underlying gates,numbered 1 to 4, are covered with a dielectric. [Note that gate 1 connects two gates, and gate 4connects four narrow gates; see (C).] (C) (Top) Schematic of our device. (Bottom) illustration of energystates. The green rectangle indicates the tunnel barrier separating the normal part of the nanowire onthe left from the wire section with induced superconducting gap, D. [In (B), the barrier gate is alsoshown in green.] An external voltage, V, applied between N and S drops across the tunnel barrier. Redstars again indicate the idealized locations of the Majorana pair. Only the left Majorana is probed inthis experiment. (D) Example of differential conductance, dI/dV, versus V at B = 0 and 65 mK, servingas a spectroscopic measurement on the density of states in the nanowire region below thesuperconductor. Data are from device 1. The two large peaks, separated by 2D, correspond to the quasi-particle singularities above the induced gap. Two smaller subgap peaks, indicated by arrows, likelycorrespond to Andreev bound states located symmetrically around zero energy. Measurements areperformed in dilution refrigerators with the use of the standard low-frequency lock-in technique(frequency = 77 Hz, excitation = 3 mV) in the four-terminal (devices 1 and 3) or two-terminal (device 2)current-voltage geometry.

25 MAY 2012 VOL 336 SCIENCE www.sciencemag.org1004

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! /me=0.397 andE!̄,Ag=−63 meV on the Ag!111" side21 and mx

! /me=−0.25,E0,x=94 meV, E!̄,x=0, on the RB side.15 The Ag!111" andRB dispersions are shown in Fig. 4!b".

We plot in Fig. 4!c" PAg←x, and in Fig. 4!d" PAg→x fordifferent band fillings as described by the value of EF !seeRef. 22 for computational details". In the absence of RBcoupling, the spin current across the boundary vanishes. Thebreaking of the spin-rotation symmetry by the RB couplinginduces a spin current on the Ag!111" side in Figs. 4!c" and4!d". This induced spin current is strongly enhanced by theonset of an unconventional FSST when the Fermi level trig-gers a topological transition of the RB Fermi surface and "ivanishes. Thus, the RB metal acts as a spin injector or a spinacceptor depending on the polarity of the applied voltagedifference across the boundary. Finally, even for nonidealsystems, such spin currents might lead to local spin accumu-lation that could be detected with magnetic scanning tunnel-ing microscopy.

In conclusion, we have measured with SARPES two dif-ferent types of Fermi surface spin textures in theBixPb1−x /Ag!111" surface alloys. They are separated at x#0.5 by a topological transition in the Fermi surface. In theconventional situation x#0.5, the spin polarizations on thetwo RB-split Fermi contours rotate in opposite directions. Inthe unconventional situation 0.5#x#0.7, they rotate in thesame direction. Despite the random intermixing of Bi andPb, the measured momentum-resolved spin polarizations arewell reproduced by a phenomenological disorder-free nearly-free-electron RB model in which x merely tunes the Fermienergy and the curvature of the Fermi sea. At last, we haveargued that RB-like systems with strong spin-orbit splittingand EF#E!̄ can be used as a spin filter and thus can lead toa finite spin accumulation in nonmagnetic materials withoutthe need of any magnetic field.

Fruitful discussions with M. Grioni and G. Bihlmayer aregratefully acknowledged. We thank C. Hess, F. Dubi, and M.Klöckner for technical support. This work is supported bythe Swiss National Science Foundation.

1 S. A. Wolf et al., Science 294, 1488 !2001".2 Y. A. Bychkov and E. I. Rashba, JETP Lett. 39, 78 !1984".3 S. Datta and B. Das, Appl. Phys. Lett. 56, 665 !1990".4 D. S. Saraga and D. Loss, Phys. Rev. B 72, 195319 !2005".5 E. Cappelluti, C. Grimaldi, and F. Marsiglio, Phys. Rev. B 76,

085334 !2007".6 E. Cappelluti, C. Grimaldi, and F. Marsiglio, Phys. Rev. Lett. 98,

167002 !2007".7 J. Sinova et al., Phys. Rev. Lett. 92, 126603 !2004".8 J. Wunderlich, B. Kaestner, J. Sinova, and T. Jungwirth, Phys.

Rev. Lett. 94, 047204 !2005".9 M.-H. Liu, G. Bihlmayer, S. Blügel, and C.-R. Chang, Phys.

Rev. B 76, 121301!R" !2007".10 C. R. Ast et al., Phys. Rev. Lett. 98, 186807 !2007".11 D. Pacilé et al., Phys. Rev. B 73, 245429 !2006".12 F. Meier, H. Dil, J. Lobo-Checa, L. Patthey, and J. Osterwalder,

Phys. Rev. B 77, 165431 !2008".13 G. Bihlmayer, S. Blügel, and E. V. Chulkov, Phys. Rev. B 75,

195414 !2007".

14 J. Premper, M. Trautmann, J. Henk, and P. Bruno, Phys. Rev. B76, 073310 !2007".

15 C. R. Ast et al., Phys. Rev. B 77, 081407!R" !2008".16 C. R. Ast, G. Wittich, P. Wahl, R. Vogelgesang, D. Pacilé, M. C.

Falub, L. Moreschini, M. Papagno, M. Grioni, and K. Kern,Phys. Rev. B 75, 201401!R" !2007".

17 J. Nitta, T. Akazaki, H. Takayanagi, and T. Enoki, Phys. Rev.Lett. 78, 1335 !1997".

18 J. H. Dil et al., Phys. Rev. Lett. 101, 266802 !2008".19 R. Winkler, Spin-Orbit Coupling Effects in Two-Dimensional

Electron and Hole Systems, Springer Tracts in Modern Physics!Springer, Berlin, 2003", Vol. 191.

20 M. Hoesch, T. Greber, V. N. Petrov, M. Muntwiler, M. Hengs-berger, W. Auwaerter, and J. Osterwalder, J. Electron Spectrosc.Relat. Phenom. 124, 263 !2002".

21 F. Reinert, G. Nicolay, S. Schmidt, D. Ehm, and S. Hüfner, Phys.Rev. B 63, 115415 !2001".

22 B. Srisongmuang, P. Pairor, and M. Berciu, Phys. Rev. B 78,155317 !2008".

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MEIER et al. PHYSICAL REVIEW B 79, 241408!R" !2009"

RAPID COMMUNICATIONS

241408-4

Spin injection Spin charge conversion

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Photoemission spectroscopy

“In sum, one can say there is hardly one among the great problems in which modern physics is so rich to which Einstein has not made a remarkable contribution. That he may sometimes have missed the target in his speculations, as, for example, in his hypothesis of light quanta, cannot really be held too much against him, for it is not possible to introduce really new ideas even in the most exact sciences without sometimes taking a risk.”

In his 1913 letter nominating Einstein for the membership of Prussian Academy,

Max Planck wrote:

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ARPES: angle-resolved photoemission spectroscopy

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Swiss Light Source groupEmbedded in larger unit

(5 senior scientists, ~6 post docs, ~4 PhD students, 3 technicians)2 beamlines

4 end stations: -Spin-resolved ARPES (COPHEE)

-High-resolution ARPES -Soft X-ray ARPES

-Resonant inelastic X-ray scattering

Many other collaborations and possibilities

Typical Master projects: -Study (spin-resolved) electronic structure of novel spintronic materials -Study change of electronic/spin structure under operando conditions

-Develop and test new measurement possibilities (sample environment)

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Eating pudding

The proof of the pudding is in the eating

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In-situ transport measurements

3

Photo of system exterior appearance Unit top view

Unit side view Tip holder Sample holder

Approach stage Sample stage STM measurement ：HOPG sample

Micro 4 point probe (M4PP)

Master projects: -Setting and testing up novel M4PP equipment

-Combined M4PP and ARPES experiments