Program VITI School Aachen 2014 - Virtual Institute for Topological

Program of the 2nd Spinograph and VITI school
Monday
(8-12-2014)
Tuesday
12-2014)
(9-
Wednesday
(10-12-2014)
Thursday
(11-12-2014)
9:00 - 10:00
V. Falko I
S. Blügel
I. Brihuega
10:00 - 10:30
Coffee
Coffee
Coffee
10:30 - 11:30
V. Falko II
G. Burkard
B. Beschoten
11:30 - 12:30
J. Folk
O. Rader
M. Ciorga
12:30 - 14:00
Registration
Opening 13:45
Lunch
Lunch
Lunch
14:00 - 15:00
L. Molenkamp
J. I. Aynés
J. Lado
B. Trauzettel
F. Finocchiaro
N. García
G. Tkachov
C. Niu
M. Eschbach
Coffee
Coffee
Coffee
J. L. Sambricio
D. Bandurin
15:00 - 15:30
A. Kis
15:30 - 16:00
16:00 - 16:30
16:30 - 17:00
17:00 - 17:30
Coffee
G. Woltersdorf
R. Zierold
B. Gao
18:00 - 20:30
Poster, Fingerfood
and Beer
20:30 - 21:30
VITI Closed Meeting
LAB TOUR (17-19)
19:45 - 22:30
Workshop - Dinner
Friday
(12-12-2014)
Spinograph - Closed
Meeting
Program
Monday, 8th December 2014
12:30 - 14:00
Registration Opening 13:45
14:00 -15:00
L. Molenkamp
HgTe as a topological insulator
15:00 - 16:00
A. Kis
MoS2 and dichalcogenide based devices and hybrid heterostuctures
16:00 - 16:30
Coffee
16:30 - 17:30
R. Zierold
Synthesis and electrical transport characterization of V-VI nanostructures:
The influence of topological surface states on the thermoelectric
performance
Tuesday, 9th December 2014
09:00 - 10:00
V. Falko
Spin and quantum transport in 2D materials I.
10:00 - 10:30
Coffee
10:30 - 11:30
V. Falko
Spin and quantum transport in 2D materials
11:30 - 12:30
J. Folk
Quantum interference as a probe of spin relaxation in graphene II.
12:30 - 14:00
Lunch
J.I Aynés
J. Lado
Spin transport in double-gated boron nitride encapsulated bilayer graphene
Magnetism and electronics in graphene quantum Hall bars
15:00 - 15:30
G. Tkachov
Proximity and Josephson effects in topological insulator/superconductor
structures
15:30 - 16:00
Coffee
16:00 - 17:00
G. Wolterdorf
Spin Hall Effect in Metallic Multilayers
17:00-17:30
B. Gao
Synthesis and multi method characterizations of BiTeCI
14:00 - 15:00
Poster,
18:00 - 20:30 Fingerfood and
Beer
Program
Wednesday, 10th December 2014
09:00 - 10:00
S. Blügel
10:00 - 10:30
Coffee
10:30 - 11:30
G. Burkard
Spin and valley physics in graphene and transition-metal dichalcogenides
11:30 - 12:30
O. Rader
Introduction into ARPES and ist application to topological insulators
12:30 - 14:00
Lunch
14:00 - 15:00
B. Trauzettel
Transport properties of helical edge states
15:00 - 15:30
C.Niu
Engineering Topological phases in Bi-based 2D topological insulators
15:30 - 16:00
Coffee
16:00-16:30
J.L. Sambricio
16:00 - 18:00
Lab Tour
19:45 - 22:30
First-principles theory applied to topological insulators
TBA
WorkshopDinner
Thursday, 11th December
09:00 - 10:00
I. Brihuega
10:00 - 10:30
Coffee
10:30 - 11:30
B. Beschoten
Spin and charge transport in Co/MgO/graphene nonlocal spin-valve devices
11:30 - 12:30
M. Ciorga
Electrical spin injection into high mobility 2DEG systems
12:30 - 14:00
Lunch
F. Finocciaro
Probing graphene physics at the atomic scale with a scanning tunneling microscope
N. Garcia
Low-energy Models for Transition Metal Dichalcogenides: Scatteriong Therory and
Emergence of Bound States
Quantum Spin Hall phase in multilayer graphene
15:00 - 15:30
M. Eschbach
Epitaxial Sb2Te3 Heterostructures: A New (Route to) Topological p-n Junction
15:30 - 16:00
Coffee
16:00 - 16:30
D. Bandurin
14:00 - 15:00
TBA
Friday, 12th 2014
Spinograph Closed
Meeting
Abstracts of Lectures
HgTe as a Topological Insulator
L.W. Molenkamp
Physics Institute (EP3), Würzburg University,
Am Hubland, Würzburg 97074, Germany
HgTe is a zincblende-type semiconductor with an inverted band structure. While the bulk material is
a semimetal, lowering the crystalline symmetry opens up a gap, turning the compound into a
topological insulator.
The most straightforward way to do so is by growing a quantum well with (Hg,Cd)Te barriers. Such
structures exhibit the quantum spin Hall effect, where a pair of spin polarized helical edge channels
develops when the bulk of the material is insulating.
Our transport data[1-3] provide very direct evidence for the existence of this third quantum Hall
effect, which now is seen as the prime manifestation of a 2-dimensional topological insulator.
To turn the material into a 3-dimensional topological insulator, we utilize growth induced strain in
relatively thick (ca. 100 nm) HgTe epitaxial layers. The high electronic quality of such layers allows a
direct observation of the quantum Hall effect of the 2-dimensional topological surface states[4].
These states appear to be decoupled from the bulk. This allows us to induce a supercurrent is
induced in the surface states by contacting these structures with Nb electrodes[5].
References:
[1] M. König et al., Science 318, 766 (2007).
[2] A. Roth et al., Science 325, 294 (2009).
[3] C. Brüne et al., Nature Physics 8, 486 (2012).
[4] C. Brüne et al., Phys. Rev. Lett. 106, 126803 (2011).
[5] L. Maier et al, Phys. Rev. Lett. 109, 186806 (2012).
J.B. Oostinga et al., PRX 3, 021007 (2013).
MoS2 and dichalcogenide based devices and hybrid
heterostructures
Andras Kis
EPFL, Lausanne, Switzerland
MoS2 and transition metal dichalcogenides have opened numerous research directions and potential
applications for this diverse family of nanomaterials [1,2]. I will start by presenting our work on
achieving epitaxial growth of MoS2 [3] and of modelling the Schottky barriers between MoS2 and
metallic electrodes. I will continue showing our work on heterostructures, oriented towards realizing
combinations of 2D and 3D materials into van der Waals heterostructures [4]. I will report on highperformance photodetectors based on 2D/3D heterostructures that can operate with internal gain
and high sensitivity [4]. Our devices also show very low noise, due to the unique architecture of the
2D/3D heterojunction. Next, I will give an update on our efforts to realize high-performance electrical
circuits based on TMD materials [5].
References:
[1] Q. H. Wang, K. Kalantar-Zadeh, A. Kis, J. N. Coleman, and M. S. Strano, Nat. Nanotechnol. 7, 699
(2012).
[2] A. C. Ferrari, F. Bonaccorso, V. Falko, K. S. Novoselov, S. Roche, P. Bøggild, S. Borini, F. Koppens,
V. Palermo, N. Pugno, J. A. Garrido, R. Sordan, A. Bianco, L. Ballerini, M. Prato, E. Lidorikis, J.
Kivioja, C. Marinelli, T. Ryhänen, A. Morpurgo, J. N. Coleman, V. Nicolosi, L. Colombo, A. Fert, M.
Garcia-Hernandez, A. Bachtold, G. F. Schneider, F. Guinea, C. Dekker, M. Barbone, C. Galiotis, A.
Grigorenko, G. Konstantatos, A. Kis, M. Katsnelson, C. W. J. Beenakker, L. Vandersypen, A.
Loiseau, V. Morandi, D. Neumaier, E. Treossi, V. Pellegrini, M. Polini, A. Tredicucci, G. M.
Williams, B. H. Hong, J. H. Ahn, J. M. Kim, H. Zirath, B. J. van Wees, H. van der Zant, L. Occhipinti,
A. D. Matteo, I. A. Kinloch, T. Seyller, E. Quesnel, X. Feng, K. Teo, N. Rupesinghe, P. Hakonen, S.
R. T. Neil, Q. Tannock, T. Löfwander, and J. Kinaret, Nanoscale (2014).
[3] D. Dumcenco, D. Ovchinnikov, K. Marinov, O. Lopez-Sanchez, D. Krasnozhon, M.-W. Chen, P.
Gillet, A. F. i Morral, A. Radenovic, and A. Kis, ArXiv14050129 Cond-Mat (2014).
[4] O. Lopez-Sanchez, D. Dumcenco, E. Charbon, and A. Kis, ArXiv14113232 Cond-Mat (2014).
[5] D. Krasnozhon, D. Lembke, C. Nyffeler, Y. Leblebici, and A. Kis, Nano Lett. (2014).
Synthesis and Electrical Transport Characterization of V-VI
Nanostructures: The Influence of Topological Surface States on the
Thermoelectric Performance
R. Zierold, B. Hamdou, S. Zastrow, H. Osterhage, J. Gooth, and K. Nielsch
Institute of Nanostructure and Solid State Physics, Universität Hamburg, Hamburg, Germany
[email protected]
On the one hand, there is currently substantial effort being invested into creating efficient
thermoelectric nanowires based on V-VI chalcogenide-type materials. A key premise of these efforts
is the assumption that the generally good thermoelectric properties that these materials exhibit in
bulk form will translate into similarly good or even better thermoelectric performance of the same
materials in nanowire or thin film form. On the other hand, various of these V-VI materials are
topological insulators (TIs) representing a new state of quantum matter with a bulk band gap and
gapless surface states that are protected by time-reversal symmetry. In contrast to the bulk bands,
the charge carriers in the surface states behave like massless Dirac fermions, which carry electrical as
well as spin currents with high mobility. While in bulk samples the surface plays a negligible role in
charge transport, a significant impact of the surface states on the thermoelectric properties is
expected in nanostructures, due to their high surface to volume ratio. In this talk, we present, first,
the synthesis of chalogenide nanowires and thin films by catalytic vapor-liquid-solid growth and
atomic layer deposition, respectively. [1,2] Second, low temperature magnetoresistance
measurements on single crystalline Sb2Te3 and Bi2Te3 nanowires allowed for observation of
Aharonov-Bohm oscillations, weak anti-localization and Shubnikov-de Haas effect indicating the
presence of topological surface states in our nanowires. [3,4] Moreover, data analysis of
measurements on electric field-effect nanowire devices unambiguously revealed the linear
dispersion relation of the surface states, not accessible by (standard) angle-resolved photoemission
spectroscopy (ARPES) due to limitations in lateral resolution of that technique. [5] Third, we calculate
the thermoelectric performance of topological insulator nanowires and thin films based on Bi2Te3,
Sb2Te3, and Bi2Se3 as a function of diameter and film thickness, respectively, as well as versus the
Fermi level. We show that the thermoelectric performance of topological insulator nanostructures
does not derive from the properties of the bulk material in a straightforward way. For all investigated
nanostructure systems the competition between surface states and bulk channel causes a significant
modification of the thermoelectric transport coefficients if the diameter is reduced into the sub-10
μm range. [6,7] Our results show that the interplay between bulk and surface channel limits the
maximum thermoelectric performance of topological insulator V-VI semiconducting nanostructures.
Thus, novel approaches have to be discussed and explored to overcome these limitations and to pave
the way to their possible application in efficient thermoelectric devices. This work was supported by
the German science foundation (DFG) via the German priority program SPP 1386 “Nanostructured
Thermoelectrics”, SPP 1666 “Topological Insulators” as well as within the Graduiertenkolleg 1286
“Functional Metal-Semiconductor Hybrid Systems.”
References:
[1] B. Hamdou et al. Adv. Mater. 25, 239-244 (2013).
[2] S. Zastrow et al. Semicond. Sci. Tech. 28, 035010 (2013).
[3] B. Hamdou et al. Appl. Phys. Lett. 102, 223110 (2013).
[4] B. Hamdou et al. Appl. Phys. Lett. 103, 193107 (2013).
[5] J. Gooth et al. Appl. Phys. Lett. 104, 243115 (2014).
[6] H. Osterhage et al. Appl. Phys. Lett. 105, 123117 (2014).
[7] J. Gooth et al. Semicond. Sci. Tech. in press (2015).
Spin and quantum transport in 2D materials
V. Falko
Department of Physics, University Lancaster,UK
One lecture will be devoted to an overview of quantum transport effects in graphene, and the role
that electron spin may have on weak localisation, in particular, discussing the regimes of decoherence determined by scattering off spin-full defects. The second lecture will discuss spin-orbit
effects in hexagonal transition metal dichalcogenides, in terms of spin relaxation regimes and a
crossover between weak localisation and weak anti-localisation behaviour.
Quantum interference as a probe of spin relaxation in graphene
J. Folk
Department of Physics and Astronomy, University of British Columbia, Canada
This talk will describe how simple magnetoresistance measurements of weak localization and
universal conductance fluctuations in an in-plane magnetic field can be used to determine the rates
of various spin relaxation mechanisms in graphene or other 2D materials. At a generic level the
theoretical grounding of this approach is similar to what was developed for metals two to three
decades ago, but the approach turns out to be especially effective for graphene because of its atomic
thin-ness. Using this technique, spin relaxation rates due to magnetic impurities and spin-orbit
interaction are estimated for exfoliated and for SiC-based graphene.
Spin transport in double-gated boron nitride encapsulated bilayer
graphene
Josep Ingla-Aynés, Marcos H. D. Guimarães, Paul Zomer, Juliana C. Brant, Niko Tombros, Bart J. van
Wees
Physics of Nanodevices, Zernike Institute for Advanced Materials,
University of Groningen, Groningen
Active control of spin information with electric fields is a challenge for the future spintronic devices.
When applying a perpendicular electric field to a bilayer graphene flake, the inversion symmetry of
the system is broken and a Rashba spin-orbit field appears inducing a gap opening. The spin-orbit
field also generates extra spin-relaxation allowing us to achieve the electrical control of spin
information. Using the fast pick-up technique described in [1], we fabricate stacks of boron nitride
encapsulated bilayer graphene to study their spin properties under the action of a perpendicular
electric field applied by a top and a back gate using a lateral spin valve geometry and Hanle
precession measurements.
References:
[1] P. J. Zomer, M. H. D. Guimãraes, J. C. Brant, N. Tombros and B. J. van Wees, Appl. Phys. Lett. 105,
013101 (2014).
Magnetism and electronics in graphene quantum Hall bars
J. L. Lado, J. Fernández-Rossier
International Iberian Nanotechnology Laboratory (INL),
Av. Mestre José Veiga, 4715-330 Braga, Portugal
Application of a perpendicular magnetic field to charge neutral graphene is expected to result in a
variety of broken symmetry phases, including antiferromagnetic, canted and ferromagnetic. All these
phases open a gap in bulk but have very different edge states and non-collinear spin order, recently
confirmed experimentally. Here we provide an integrated description of both edge and bulk for the
various magnetic phases of graphene Hall bars making use of a non-collinear mean field Hubbard
model. Our calculations show that, at the edges, the three types of magnetic order are either
enhanced (zigzag) or suppressed (armchair). Interestingly, we find that preformed local moments in
zigzag edges interact with the quantum Spin Hall like edge states of the ferromagnetic phase and can
induce back-scattering.
References:
1. D. A. Abanin, P. A. Lee, and L. S. Levitov, Phys. Rev. Lett. 96, 176803 (2006).
2. A. F. Young, C. R. Dean, L. Wang, H. Ren, P. Cadden- Zimansky, K. Watanabe, T. Taniguchi, J. Hone,
K. L. Shepard, and P. Kim, Nat Phys 8, 550 (2012).
3. A. F. Young, J. D. Sanchez-Yamagishi, B. Hunt, S. H. Choi, K. Watanabe, T. Taniguchi, R. C. Ashoori
and P. Jarillo-Herrero, Nature 505, 528 (2014).
4. J. L. Lado, J. Fernandez-Rossier, Phys. Rev. B 90, 165429 (2014).
Proximity and Josephson effects in topological
insulator/superconductor structures
Grigory Tkachov
University of Würzburg, Am Hubland, 97074 Würzburg, Germany
Email: [email protected]
There is currently much effort being put into understanding superconducting phenomena in
topological insulator (TI) materials. This has several reasons. First, superconductivity in TIs is
expected to be unconventional. Unlike typical superconductors, e.g., Pb, Al, or Nb, where Cooper
pairs have the ordinary s-wave symmetry, in TIs the pairing scenarios are richer, including the
possibility of spin-triplet p-wave correlations. I will discuss how such p-wave pairing can be induced
through the proximity effect in TI/superconductor bilayers [1, 2]. Special emphasis will be put on
impurity scattering and its effect on the induced p-wave correlations [3]. One more reason for
looking at superconducting TIs is the unusual Josephson physics associated with protected currentcarrying states that are immune to disorder. I will review recent related experimental and theoretical
work [4-7] showing that, apart from an interesting research potential, TI Josephson junctions could
be implemented for designing nano-SQUIDs and engineering macroscopic quantum states such as
flux qubits.
Acknowledgments. This work has been done in collaboration with E. M. Hankiewicz, P. Burset, and B.
Trauzettel (Würzburg University, ITP4), L. Maier, C. Gould, C. Brüne, H. Buhmann, and L. W.
Molenkamp (Würzburg University, EP3), I. Sochnikov, C. A. Watson, J. R. Kirtley, and K. A. Moler
(Stanford University). The financial support of the German Research Foundation (DFG) through grant
No TK60/1-1 is also gratefully acknowledged.
References:
[1] L. Fu and C. L. Kane, Phys. Rev. Lett. 100, 096407 (2008).
[2] T. D. Stanescu, J. D. Sau, R. M. Lutchyn, and S. Das Sarma, Phys. Rev. B 81, 241310(R) (2010)
[3] G. Tkachov, Phys. Rev. B 87, 245422 (2013).
[4] S. Hart, H. Ren, T. Wagner, P. Leubner, M. Mühlbauer, C. Brüne, H. Buhmann, L. W. Molenkamp,
and A. Yacoby, Nat. Phys. 10, 638 (2014).
[5] V. S. Pribiag, A. J. A. Beukman, F. Qu, M. C. Cassidy, C. Charpentier, W. Wegscheider, and L. P.
Kouwenhoven, arXiv: 1408.1701.
[6] I. Sochnikov, L. Maier, C. A. Watson, J. R. Kirtley, C. Gould, G. Tkachov, E. M. Hankiewicz, C. Brüne,
H. Buhmann, L. W. Molenkamp, and K. A. Moler, arXiv: 1410.1111.
[7] G. Tkachov, P. Burset, B. Trauzettel, and E. M. Hankiewicz, arXiv:1409.7301.
Spin Hall Effect in Metallic Multilayers
Georg Woltersdorf 1,2 Martin Obstbaum 2, Dahai Wei 2, Martin Decker 2, and Christian H. Back 2
1
2
Institute of Physics, Martin-Luther-University Halle, 06120 Halle, Germany
Physics Department, University of Regensburg, Regensburg, 93053, Germany
The discovery of the spin pumping effect and the Spin Hall Effect (SHE) has stimulated the research
on dynamics in metallic magnetic nanostructures. Here a comprehensive study of the SHE in metallic
multilayers will be presented. We study the direct as well as the inverse SHE. In the case of the
direct SHE a dc charge current is applied in the plane of a ferromagnet/normal metal layer stack and
the SHE creates a spin polarization at the surface of the normal metal leading to the injection of a
spin current into the ferromagnet [1,2]. This spin current is absorbed in the ferromagnet and causes
a spin transfer torque. Using time and spatially resolved Kerr microscopy we measure the transferred
spin momentum and compute the spin Hall angle. In a second set of experiments using identical
samples pure spin currents are injected by the spin pumping effect from the ferromagnet into the
normal metal [3]. The spin current injected by spin pumping has a large ac component transverse to
the static magnetization direction and a very small dc component parallel to the magnetization
direction. The inverse SHE converts these spin current into charge current [4,5]. The corresponding
inverse SHE voltages induced by spin pumping at ferromagnetic resonance (FMR) are measured in
permalloy/platinum and permalloy/gold multilayers in various excitation geometries and as a
function of frequency in order to separate the contributions of anisotropic magnetoresistance and
SHE. In addition, we present experimental evidence for the ac component of inverse SHE voltages
generated by spin pumping [6,7].
References:
[1]
[2]
[3]
[4]
[5]
[6]
[7]
K. Ando et al., Phys. Rev. Lett. 101, 036601, (2008).
V. E. Demidov et al., Phys. Rev. Lett. 107, 107204 (2011).
Y. Tserkovnyak, A. Brataas, and G.E.W. Bauer, Phys. Rev. Lett. 88, 117601 (2002).
E. Saitoh et al., Appl. Phys. Lett. 88, 182509 (2006).
O. Mosendz, et al., Phys. Rev. Lett. 104, 046601 (2010).
H. Jiao and Gerrit E. W. Bauer, Phys. Rev. Lett. 110, 217602 (2013).
D. Wei et al. Nat. Comm. 5, 3768 (2014).
Synthesis and multi method characterizations of BiTeCl
Bo Gao
Shanghai Institute of microsystem and information technology, Chinese Academy of Sciences,
Shanghai 200050, China
E-mail: [email protected]
Single crystal BiTeCl was synthesized through a two-step flux method. Various characterization
results confirmed the chemical purity and the lattice structure of the sample. Magnetoresistance
measurements revealed weak antilocalization behavior near 6K, and further reducing the
temperature led to the suppression of it. In some of the samples the weak antilocalization was even
turned into weak localization by reducing the temperature. Angle resolved photoemission
spectroscopy measurements did not reveal any Dirac state on either Te- or Cl- terminated surfaces,
while a Rashba-like band was found on the Te surface. The observed weak antilocalization behavior is
possibly related to the Rashba-like band in the material, and the suppression of weak antilocalization
is likely due to the enhanced inelastic scattering at low temperature. [1-2]
* This work has been performed in collaboration with Hui Li, Tao Xu, Zhuojun Li, Wei Li, Mao Ye, Shan
Qiao, Xiaoming Xie (Shanghai Institute of microsystem and information technology, Chinese Academy
of Sciences); Mengyu Yao, K. F. Zhang, Y. R. Song, Dong Qian, Chunlei Gao, Jinfeng Jia ( Shanghai
Jiaotong University), and others.
References:
[1] Jacimovic J, Mettan X, Pisoni A, Gaal R, Katrych S, Demko L, Akrap A, Forro L, Berger H, Bugnon P
and Magrez A 2014 Scripta Mater 76, 69 (2014).
[2] Chen Y L, Kanou M, Liu Z K, Zhang H J, Sobota J A, Leuenberger D, Mo S K, Zhou B, Yang S L,
Kirchmann P S,Lu D H, Moore R G, Hussain Z, Shen Z X, Qi X L and Sasagawa T 2013 Nat.Phys. 9, 704.
[3] Koga T, Nitta J, Akazaki T and Takayanagi H 2002 Phys. Rev. Lett. 89, 046801.
First-principles theory applied to topological insulators
Stefan Blügel
Peter Grünberg Institut and Institute for Advanced Simulation, Forschungszentrum Jülich and JARA,
D-52425 Jülich, Germany
I will start with a brief introduction to density functional theory (DFT), the incorporation of spin-orbit
interaction and discuss limits of applicability of DFT. Then, I will discuss its application to freestanding and deposited graphene and discuss the role of spin-orbit interaction. I discuss applications
to two-dimensional topological insulators (TI) such as a Bi-double layer and related TI, a subject that
will be discussed in more detail by Dr. Chengwang Niu. I may discuss the search for two-dimensional
Chern insulators. I will introduce Wannier functions and discuss for the example of the Chern
number how topological invariants are determined. Since bandgaps in TI are small, as last part of my
lecture I will introduce the many-body perturbation theory in the GW approximation to the selfenergy, a theory that goes beyond the density-functional theory, which is able to relate directly to
photoemission, a tool that proved very powerful in the analysis of the electronic structure of
topological insulators, which will be further discussed in the lecture of Professor Oliver Rader. Most
likely I will discuss Bi, and Bi2Te3 and related systems. I will not be able to discuss the degree of
topological protection at the presence of non-magnetic or magnetic impurities and possible
quasiparticle interference due to scattering that can be observed in scanning tunneling microscopy.
Here, I refer to the poster of Philipp Rüßmann.
The lecture benefitted from collaborations with Irene Aguilera, Gustav Bihlmayer, Christoph
Friedrich, Phivos Mavropoulos, Yuriy Mokrousov and Daniel Wortmann.
Work is supported by the Virtual Institute VITI of the Helmholtz Association and by the Priority
Program on Topological Insulators SPP-1666 of the DFG.
Spin and valley physics in graphene and transition-metal
dichalcogenides
Guido Burkard
Department of Physics, University of Konstanz, Germany
Graphene and other two-dimensional (2D) materials have many interesting physical properties. Here,
we concentrate on the properties of such 2D materials related to the electron spin. The low
concentration of nuclear spins and weak spin-orbit coupling is expected to allow for long-lasting
electron spin coherence in graphene. For the prospect of defining a quantum register consisting of
localized electron spins, one can envision using electrons confined to quantum dots. However, the
absence of a band gap requires new ideas for the localization of electrons. Moreover, an important
role is played by the valley degeneracy in graphene, both for spin coherence and for the exchange
coupling between spins in tunnel-coupled quantum dots. The first part of this talk will consist of an
overview over the theory of spin qubits in graphene quantum dots. In the second part of the talk, I
will introduce another emerging class of 2D materials, the monolayer transition metal
dichalcogenides (TMDCs). The 2D TMDCs share many properties of graphene, but comprise a band
gap and relatively strong spin-orbit coupling, leading to an interesting interplay of spin and valley
degrees of freedom. Using k·p theory combined with parameters from density functional theory
(DFT), a low-energy effective Hamiltonian for the TMDCs can be derived. Using this theory, we can
understand the form of the spin-orbit coupling, as well as other properties, of the TMDCs, and study
quantum dots formed by electrostatic gating.
Introduction into ARPES and its application to topological
insulators
Oliver Rader
Institut für Physik und Astronomie, HZB
In this talk, the method of angle-resolved photoelectron spectroscopy will be introduced. Issues such
as probing depth, element specificity, determination of two- and three dimensional band structures
of surfaces, quantum wells and bulk material will be addressed. We will discuss dipole selection rules,
the appearance of correlation effects, lifetime broadening, sublattice diffraction effects, and the
identificaton of electronic surface states. Spin polarimetry will be explained.
In the second half of the talk, an overview of topological insulators will be given. The aspects of
photoelectron spectroscopy introduced in the first part will be applied to topological insulators
presenting in this way a cross section through the ARPES literature on topological insulators.
Transport properties of helical edge states
Björn Trauzettel
Institute for Theoretical Physics and Astrophysics, Würzburg University
A single pair of helical edge states as realized, for instance, at the boundary of a quantum spin Hall
insulator is known to be robust against elastic single particle backscattering as long as time reversal
symmetry is preserved. However, there is no symmetry preventing inelastic backscattering as
brought about by phonons in the presence of Rashba spin orbit coupling or by electron-electron
interactions. In the first part of the talk, we discuss two possibilities of backscattering off a so called
Rashba impurity in a two-terminal configuration. We show certain robustness against inelastic
backscattering mediated by electron-phonon coupling and unexpected temperature dependence due
to two-particle backscattering in the presence of weak interactions. In the second part of the talk, we
extend the number of terminals from two to four and treat the transport through two helical liquids
that are coupled to each other in a central scattering region. We analyze the Kondo problem in such
a four-terminal configuration with emphasis on the bias dependence and the detectability of the
Kondo cloud. Finally, we discuss a way to measure the total parity of four Majorana bound states in a
topological Josephson junction formed on the basis of two helical liquids coupled in parallel to a loop
of an ordinary s-wave superconductor.
References:
C.-X. Liu, J. C. Budich, P. Recher, and B. Trauzettel, Phys. Rev. B 83, 035407 (2011).
J. C. Budich, F. Dolcini, P. Recher, and B. Trauzettel, Phys. Rev. Lett. 108, 086602 (2012).
F. Crépin, J. C. Budich, F. Dolcini, P. Recher, and B. Trauzettel, Phys. Rev. B 86, 121106(R) (2012).
T. Posske, C.-X. Liu, J. C. Budich, and B. Trauzettel, Phys. Rev. Lett. 110, 016602 (2013).
T. Posske and B. Trauzettel, Phys. Rev. B 89, 075108 (2014).
F. Crépin and B. Trauzettel, Phys. Rev. Lett. 112, 077002 (2014).
F. Geissler, F. Crépin, and B. Trauzettel, Phys. Rev. B 89, 235136 (2014).
Engineering topological phases in Bi-based
2D topological insulators
Chengwang Niu, Hongbin Zhang, Gustav Bihlmayer, Daniel Wortmann,
Stefan Blügel, and Yuriy Mokrousov
Peter Grünberg Institut and Institute for Advanced Simulation, Forschungszentrum Jülich and JARA,
D-52425 Jülich, Germany
E-mail: [email protected]
As the heaviest atom with effectively stable isotope and strong spin-orbit coupling (SOC), bismuth is
an important ingredient for both 2D and 3D topological insulators (TIs)1,2. Here we focus mainly on
Bi thin films to investigate the topological phase transitions and the realization of the giant gap 2D
TIs. We find that a phase transition from normal TI phase to time-reversal broken TI phase and then
to quantum anomalous Hall (QAH) phase can be realized in Bi(111) bilayer as a function of the
exchange field strength3,4. We further investigate H-Bi(111) and H-Bi(110) thin films5. The Hdecorated Bi (111) film exhibits a topological energy gap of 1.01 eV, which is much larger than in
known TIs. For the case of the Bi(1̅10) film, H-decoration induces the realization of the 2D TI phase
with bulk direct energy gap of 0.34 eV. The possibility of observing the quantum anomalous Hall
effect in H-decorated Bi was also explored.
References:
[1] M. Hasan and C. Kane, Rev. Mod. Phys. 82, 3045 (2010).
[2] X.-L. Qi and S.-C. Zhang, Rev. Mod. Phys. 83, 1057 (2011).
[3] H. Zhang, F. Freimuth, G. Bihlmayer, S. Blügel, and Y. Mokrousov, Phys. Rev. B 86, 035104 (2012).
[4] H. Zhang, F. Freimuth, G. Bihlmayer, M. Ležaić, S. Blügel, and Y. Mokrousov, Phys. Rev. B 87,
205132 (2013).
[5] C. Niu, G. Bihlmayer, H. Zhang, D. Wortmann, S. Blügel, and Y. Mokrousov, submitted.
Probing graphene physics at the atomic scale with a scanning
tunneling microscope
Iván Brihuega
Dept. de Física de la Materia Condensada, Universidad Autónoma de Madrid, Madrid, Spain
[email protected], www.ivanbrihuega.com
Scanning tunneling microscopy/spectroscopy operating at low temperatures in ultrahigh-vacuum
environments (UHV-LTSTM) is a unique technique which enables to image, electronically characterize
and manipulate surfaces with atomic precision. In this talk, after briefly introducing the STM
technique, I will show how we use it to explore graphene physics at an atomic level.
I will concentrate on atomic vacancies and hydrogen atoms which are considered as ideal candidates
to induce graphene magnetism. I will mainly focus on our investigations, at the atomic scale, of the
impact that such point defects have in the structural, electronic and magnetic properties of graphene
layers grown on different substrates as SiC, metals or graphite surfaces, where the pure
bidimensionality of graphene gives to these atomic defects a critical role [1-5].
Fig.1 Scanning tunneling microscope and atomic vacancy on graphene
References:
[1] M. Ugeda, I. Brihuega, F. Guinea and J. M. Gómez-Rodríguez, Phys. Rev. Lett 104, 096804 (2010).
[2] M. M. Ugeda, et al. , Phys. Rev. Lett 107, 116803 (2011).
[3] M.M. Ugeda, et al. Phys Rev. B, 85, 121402 (R) (2012).
[4] I. Brihuega, et al., Phys. Rev. Lett. 109, 196802 (2012).
[5] H. González-Herrero, unpublished.
Spin and charge transport in Co/MgO/graphene nonlocal spin-valve
devices
Bernd Beschoten
2nd Institute of Physics and JARA-FIT, RWTH Aachen University, 52074 Aachen, Germany
[email protected]
In the first part of the talk, the influence of MgO barriers on spin- and charge-transport properties is
addressed in both single-layer (SLG) and bilayer graphene (BLG) nonlocal spin-valve devices [1-3]. By
successive oxygen treatments of the MgO barrier we observe a gradual increase of the contactresistance–area products (RcA) of Co/MgO spin injection and detection electrodes and a transition
from linear to nonlinear characteristics in the respective differential dV-dI curves. With this
manipulation of the contacts, both spin lifetime and the amplitude of the spin signal can significantly
be increased by a factor of seven in the same device. This demonstrates that contact-induced spin
dephasing is the bottleneck for spin transport in graphene devices with small RcA values.
In the second part of the talk, a new fabrication method of graphene spin-valve devices is presented
that yields enhanced spin and charge transport properties by improving both the electrode-tographene and graphene-to-substrate interface. In these devices, Co/MgO spin injection electrodes
are first fabricated onto Si++/SiO2. Thereafter, a graphene–hBN heterostructure is mechanically
transferred onto these prepatterned electrodes. Room temperature spin transport in single-, bi-, and
trilayer graphene devices exhibit nanosecond spin lifetimes with spin diffusion lengths reaching
10 μm combined with carrier mobilities exceeding 20 000 cm2/(V s) [4].
[1] T.- Y. Yang, J. Balakrishnan, F. Volmer, A. Avsar, M. Jaiswal, J. Samm, S. R. Ali, A. Pachoud, M. Zeng,
M. Popinciuc, G. Güntherodt, B. Beschoten, and B. Özyilmaz, Phys. Rev. Lett. 107, 047206 (2011).
[2] F. Volmer, M. Drögeler, E. Maynicke, N. von den Driesch, M. L. Boschen, G. Güntherodt, and B.
Beschoten, Phys. Rev. B 88, 161405(R) (2013).
[3] F. Volmer, M. Drögeler, E. Maynicke, N. von den Driesch, M. L. Boschen, G. Güntherodt, C.
Stampfer, and B. Beschoten, Phys. Rev. B 90, 165403 (2014).
[4] M. Drögeler, F. Volmer, M. Wolter, B. Terrés, K. Watanabe, T. Taniguchi, G. Güntherodt, C.
Stampfer, and B. Beschoten, Nano Lett. 14, 6050 (2014).
Electrical spin injection into high mobility 2DEG systems
Mariusz Ciorga
Institute for Experimental and Applied Physics, University of Regensburg, Germany
Electrical generation and control of electron spins in semiconductors is the central theme in
semiconductor spintronics and of a big importance for device prospects. Effective spin injection into
two-dimensional (2D) electron systems is particularly desirable as it is prerequisite for many new
functionalities in future devices, with a Datta-Das spin field effect transistor [1] being a primary
example. Over the last couple of years there has been a real progress in understanding and
realization of spin injection into bulk semiconductors; spin injection into high mobility 2D systems is,
however, still a relatively open matter.
In the first part of the talk I will discuss general issues related to electrical spin injection and
detection in semiconductors, addressed in the so-called standard model of spin injection based on
spin drift-diffusion equations. I will illustrate the discussion mainly with the results of our
experiments on GaAs-based structures with diluted ferromagnetic semiconductor (Ga,Mn)As
employed as a source and a detector of spin-polarized carriers [2].
In the second part I will focus on the problem of spin injection in high mobility 2D electron gases
(2DEGs). I will present results of our latest experiments [3] on 2DEG system confined in an inverted
AlGaAs/GaAs heterojunction, which revealed a spin signal significantly exceeding the prediction of
the standard model of spin injection. A strong correlation of this signal with the width of the contacts
and with the electron mean free path supports the claim that ballistic transport in the 2D region
below ferromagnetic contacts should be taken into account to fully describe the experimental
outcome. These results call for a comprehensive ballistic theory of spin injection, which is urgently
needed in order to guide further experiments on spin injection into high mobility 2DEGs.
References:
[1] S. Datta and B. Das, Appl. Phys. Lett. 56, 665 (1990).
[2] M . Ciorga et al., Phys. Rev. B 79, 165321 (2009).
[3] M. Oltscher et al., to appear in Phys. Rev. Lett.
Low-energy Models for Transition Metal Dichalcogenides:
Scattering Theory
and Emergence of Bound States
F. Finocchiaro1, F. Guinea,1;2
1
Instituto de Ciencia de Materiales de Madrid (CSIC),Sor Juana Inez de la Cruz 3, E-28049 Madrid,
Spain
2
School of Physics and Astronomy,University of Manchester, Oxford Road,
Manchester, M13 9PL, UK
Transition metal dichalcogenides are attracting widespread attention as promising semiconducting
platforms for spintronic and optoelectronic applications. When these systems are exfoliated down to
a single layer they exhibit a transition from an indirect to a direct band gap located at the two
inequivalent corners of the hexagonal Brillouin zone K and K'. A low energy model for describing the
spectrum around these two valleys can be obtained by expanding a 6-bands tight binding model up
to quadratic order in the momentum. Such an expansion produces a quadratically corrected massive
Dirac Hamiltonian which possesses qualitative distinct features from the standard massive Dirac
model. We discuss the topological properties of such a low-energy model in a single-valley
description as a function of the sign of the quadratic correction. We develop scattering theory off a
vacancy, which we model as a circular crack and link the behavior of the scattering-cross sections to
the emergence of bound states. We compare the results obtained within this description to those
obtained for the massive Dirac model, exploring different boundary conditions.
Quantum Spin Hall phase in multilayer graphene
N. A. Garcia-Martinez, J. L. Lado, J. Fernández-Rossier
International Iberian Nanotechnology Laboratory (INL)
Av. Mestre José Veiga, 4715-330 Braga, Portugal
We address the question of whether multilayer graphene systems are Quantum Spin Hall (QSH)
insulators. Since interlayer coupling coples pz orbitals to s orbitals of different layers and Spin-Orbit
(SO) couples pz orbitals with px and py of opposite spins new spins mixing channels appear in the
multilayer scenario that were not present in the monolayer. This new spin-mixing channels cast a
doubt on the validity of the spin-conserving Kane-Mele [1] model for multilayers and motivates our
choice of a four orbital tight-binding model in the Slater-Koster [2] approximation with intrinsic SpinOrbit interaction.
To completely determine if the QSH phase is present we calculate for different number of layers both
the Z2 invariant [3] for different stackings (only for inversion symmetric systems), and the density of
states at the edge of semi-infinite graphene ribbon with armchair termination.
We find that systems with even number of layers are normal insulators while systems with odd
number of layers are QSH insulators, regardless of the stacking.
We acknowledge financial support by Marie-Curie-ITN 607904-SPINOGRAPH
References:
[1] C. L. Kane and E. J. Mele, Phys. Rev. Lett. 95, 226801 (2004).
[2] J. C. Slater, and G. F. Koster, Phys. Rev. 94, 1498.
[3] L. Fu and C. Kane, Phys. Rev. B 76, 045302.
Epitaxial Sb2Te3/Bi2Te3 Heterostructures: A New (Route to)
Topological p-n Junction
M. Eschbach1, E. Mlynczak1,2, J. Kellner³, J. Kampmeier4, M. Lanius4, C. Weyrich4, M. Gehlmann1,
S. Döring1, P. Gospodaric1, G. Mussler4, N. Demarina4, L. Plucinski1, Th. Schäpers4, D. Grützmacher4, M.
Morgenstern3, C.M. Schneider1
1
Forschungszentrum Jülich GmbH, Peter Grünberg Institut (PGI-6), 52425 Jülich, Germany
²Faculty of Physics and Applied Computer Science, AGH University of Science and Technology, al.
Mickiewicza 30, 30-059 Kraków, Poland
³II. Physikalisches Institut B, RWTH Aachen University, 52074 Aachen, Germany
4
Forschungszentrum Jülich GmbH, Peter Grünberg Institut (PGI-9), 52425 Jülich, Germany
Recently, in the field of 3D Topological Insulators various attempts have been carried out to tune the
chemical potential and, more specifically, the Dirac point to a desired energetic position i.e. to
'engineer' the electronic bandstructure for the purpose of designing future spintronic devices. Here
we show the first direct experimental proof, by angle-resolved photoemission, of the realization of a
topological p-n junction made of a heterostructure of two different 3D TI materials Bi2Te3 and
Sb2Te3 grown on Si(111). In the experiment we observe an energetic shift of the entire electronic
structure of about 200 meV when decreasing the upper Sb2Te3 layer from a thickness of 25 QL to 6
QL. On the one hand, we consider surface doping and the creation of a ternary alloy at the surface
and on the other hand the creation of a depletion region and a built-in electric field at the interface
of the two TI materials to be responsible for the shift. The latter contribution is supported by solving
Schrödinger and Poisson equations self-consistently for a 1D model system.
Poster
Kalle Benidas
Ferromagnetic contacts on topological insulators:
Lithographic realization on strained 3-dimensional HgTe
Jan Böttcher
Anomalous Dirac surface Screening vs. Self-Consistent Hartree Band
Structure Calculations for the 3D TI HgTe
Marc Drögeler
Nanosecond Spin Lifetimes in Single- and Few-Layer Graphene-hBN
Heterostructures at Room Temperature
Christopher Franzen
Supression of contact-induced spin dephasing in
graphene/MgO/Co spin-valve devices by successive oxygen treatment
Florian Geißler
Random Rashba spin-orbit coupling at the quantum spin Hall edge
Shaham Jafarpisheh
Vapor Phase Deposition of Bismuth Selenide
Juba Bouaziz
Chiral of magnetism adatoms from Rashba electrons
Stefan Jürgens
Screening properties and plasmons of Hg(Cd)Te quantum wells
Andor Kormanyos
Spin-orbit coupling, quantum dots and qubits in monolayer transition metal
dichalcogenides
Gregor Mussler
Molecular-beam epitaxy of 3d topological insulator thin films
Chengwang Niu
Functionalized Bismuth Films: Giant Gap Quantum Spin Hall and
Valley-Polarized Quantum Anomalous Hall States
Regine Ockelmann
Linear magnetoresistance and weak anti-localization measurements
on vapour phase deposited Bi2Se3 single crystal flakes
Philipp Rüßmann
First principles calculation of quasiparticle spin interference and
time-reversal scattering on surfaces with strong spin-orbit coupling
Flaviano José dos Santos
Charge and spin transport in graphene nanoribbons with adsorbed
impurities and disorder
Christian Weyrich
Magnetotransport on topological insulator thin films deposited
by MBE selective area growth

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