Program

(Click on the speaker's name to read the abstract)

  SUNDAY MONDAY TUESDAY WEDNESDAY THURSDAY FRIDAY
8:30   Arne Laucht Annica Black-Schaffer Ingrid D. Barcelos Paulo V. Santos  
8:55   Dominik Zumbuhl
9:20   Sergey A. Dvoretsky Contributed 5 Vladimir Falko Contributed 15
9:45   Contributed 6 Contributed 16 Contributed 23
10:10   Contributed 1 Contributed 7 Contributed 13 Contributed 17 Contributed 24
10:35   Coffee break Coffee break

Coffee break

Coffee break Coffee break
11:00   Felix von Oppen 70 years of the transistor (1947-2017)
Avik Ghosh
Leandro Malard Moreira Gian Salis Andrew Mitchell
11:25  
11:50   Sven Höfling Contributed 8 Contributed 14 Contributed 18 Contributed 25
12:15   Contributed 9 Social Program Contributed 19 Closing
12:40  

Lunch/

Discussion

 

Lunch/

Discussion

 

Lunch/

Discussion

 

Lunch

 

13:05  
13:30  
13:55    
14:20   Tutorial 1: Spintronics
J. Carlos Egues
Tutorial 2: 2D materials
Christiano de Matos
Tutorial 3: MBE growth
Christoph Deneke
 
14:45    
15:10    
15:35   Werner Wegscheider Sergio Ulloa Vanessa Sih  
16:00    
16:25   Coffee break Coffee break Coffee break  
16:50   Contributed 2 Contributed 10 Contributed 20  
17:15   Contributed 3 Contributed 11 Contributed 21  
17:40   Contributed 4 Contributed 12 Contributed 22  
18:05 Opening

 

Poster

 

Poster

 

Poster

 

 
18:30  
18:55  
19:20         Dinner

 

 
19:45          
20:10          
 
  • Day 1 | Sunday  - August 13, 2017
Opening
 
  • Day 2 | Monday  - August 14, 2017
Arne Laucht
Spin Qubits in Silicon – Advantages of Dressed States
 
A single electron spin in silicon is dressed by a microwave field to create a new qubit with tangible advantages for quantum computation and nanoscale research.
Coherent dressing of a quantum two-level system has been demonstrated on a variety of systems, including atoms, self-assembled quantum dots, and superconducting quantum bits. It is used to gain access to a new quantum system with improved properties - a different and tuneable level splitting, faster and easier control, and longer coherence times. Here, we present coherent dressing of a single electron spin bound to a 31P donor in isotopically purified silicon. The electron spin constitutes a two level quantum system with extremely long coherence times of $T_{2}^{\rm CPMG}=0.5 s$ and excellent control fidelities of $99.95 \%$, figures of merit that are on a par with the best solid-state quantum bits realized.
In our work we investigate the properties of the dressed, donor-bound electron spin in silicon, and probe its potential for the use as quantum bit in scalable architectures. Here, the two dressed spin-polariton levels constitute the quantum bit. We observe a Mollow triplet in the excitation spectrum, and demonstrate full two-axis control of the driven qubit in the dressed frame with a number of different control methods. We present coherent control with an oscillating magnetic field, an oscillating electric field, by frequency modulating the driving field, or by a simple detuning pulse. We measure coherence times of $T_{2\rho}^* = 2.4 ms$ and $T_{2\rho}^{\rm Hahn} = 9 ms$, one order of magnitude longer than those of the undressed qubit. Furthermore, we demonstrate that the dressed spin can be driven at Rabi frequencies as high as its transition frequency, making it a model system for the breakdown of the rotating wave approximation.
This research was funded by the Australian Research Council via CQC2T (CE110001027) and the US Army Research Office (W911NF-13-1-0024).
 
Sergey Dvoretskiy
MBE growth of HgCdTe hetero- and nanostructures
 
The modern tendencies of development of infrared detectors based on HgCdTe heterostructures are fabricating photoconductors and photodiodes type focal plane arrays (FPA). For producing of high quality FPA’s it is necessary to grow HgCdTe with high uniformity properties over the large surface area. The growth of HgCdTe hetero- and nanostructures on GaAs and Si substrates allows to decrease the cost of HgCdTe material and essentially to simplify the technological process of FPA fabricating.
The investigations of development of HgCdTe hetero- and nanostructures growth on GaAs substrates are presented. Multi-chamber MBE installation for growth of HgCdTe epilayers on GaAs and Si substrates up to 4” in diameter with precise control of films quality in situ allows to solve many problems connected with the producing high uniformity MCT layer composition over the surface area and control the MCT composition throughout the thickness. The defects formation mechanisms, its nature, the parameters characterization allows are presented.The growth of HgCdTe heterostructures with different composition distribution throughout the thickness allows to prepare material with unique properties that lead to simplification of fabricating high quality IR detectors on their basis.
The new fields of science are connected with different new physical phenomena studied on HgCdTe nanostructures such as single or multilayer quantum wells (QW). The results of HgCdTe based QW mostly HgTe QW growth with ellipsomentric control and parameters measurement are presented. The application of HgCdTe heterostructures for different IR detector type is presented. We presented the results of study of different HgTe QW in field of carrier transport, interaction with THz radiation, for laser radiation, 2D and 3D TI etc.
This work were partially supported by grants RBFR “15-52-16017 NTSIL_a”, “15-52-16008 NTSIL_a” and “Volkswagen Stiftung”.
 
Felix von Oppen
Topological superconductivity and Majorana bound states in chains of magnetic adatoms
 

Recent experiments provide possible evidence for Majorana bound states in chains of magnetic adatoms on a conventional superconductor. The formation of topological superconductivity in this system relies on ferromagnetic order of the magnetic moments and spin-orbit coupling of the substrate superconductor.
In this talk, I will discuss the physical picture underlying these experiments which starts with the physics of individual magnetic adatoms and includes a possible explanation of the unexpectedly strong localization of the observed end states.

Sven Höfling
Interband Cascade Lasers: Current Status and Future Challenges
 
The Interband Cascade Laser (ICL) combines the interband transition as in a conventional diode laser with the cascading scheme of a Quantum Cascade Laser. ICLs allow for an external quantum efficiency greater than which is enabled because of the special band alignment of GaInSb/AlAs/InAs-interfaces that separates hole and electron injector and internally feed each cascade with carriers. This makes ICLs a unique with great design flexibility. By changing the InAs layer thickness of the typically used W-shaped quantum well (W-QW) the emission wavelength can be tuned within the entire mid infrared region which is known as the fingerprint region of a variety of industrially relevant molecules. Absorption spectra of two prominent ones (acetone and nitric oxide) are shown in Fig. 1 together with room temperature spectra of broad area ICLs driven under pulsed condition. Meanwhile GaSb-based ICLs that are operational in continuous wave mode at room temperature have been realized from 2.8 µm to 5.6 µm.
The greatest advantage over the QCL is the low threshold power. Lasing at a temperature of 25 °C could be achieved at an input power as low as 29 mW which is especially beneficial for battery powered sensing systems. In the talk wavelength dependent performance characteristics will be discussed as well as different technologies that enable operation on a single longitudinal mode.
 
J. Carlos Egues.
Mesoscopic spin-orbit interaction and its relevance for novel topological phenomena
 
The spin orbit interaction in semiconductors underlies many topological phenomena such as the quantum spin Hall effect in topological insulators, skyrmions in chiral magnets and crossed persistent spin helices, and Majorana bound states in quantum wires and dots. In this tutorial, I will first (i) present a brief yet rigorous derivation of the spin-orbit Hamiltonian in quantum wells, wires and dots and then (ii) discuss the main role of this interaction in the context of some topological systems. More specifically, I will discuss the Rashba spin-orbit interaction in wires with proximity induced superconductivity and a magnetic field leading to Majorana bound states (‘Kitaev model’), the canonical Bernevig-Hugues-Zhang (BHZ) model for topological insulators and its possible realization in ordinary III-V quantum wells with only electrons, skyrmionic textures in ordinary two-dimensional electron gases and stretchable persistent spin helices. Many of these topics are subjects of ongoing research in my group in São Carlos and in collaboration with colleagues abroad.
 
Werner Wegscheider
Heterostructure designs for qubit realizations providing topological protection
 
An elegant approach to circumvent the decoherence problem, present e.g. for quantum dot spin-qubit realizations, are topologically protected systems. The quantum Hall effect (QHE) provides such a topological protection. It turns out that quasiparticle excitations of certain fractional QHE states, i.e. the 5/2 and 12/5 states, are expected to exhibit anyonic exchange statistics and are, thus, interesting for quantum computing. However, the experimentally observed gap energies in ultrahigh mobility two-dimensional electron systems based on the AlGaAs/GaAs material system, corresponding to these states are still small and inconsistent with theoretical predictions. I will outline current and new heterostructure design concepts, including buried gates fabricated by local ion implantation, in order to solve this problem. Another way to achieve topological protection is by the combination of a two-dimensional topological insulator (2DTI), the combined InAs/GaSb quantum well system in our case, with a superconductor. As a result of the broken band-alignment between InAs and GaSb a hybridization gap forms and for a Fermi level alignment within this gap a 2DTI should result. I will give an overview on the current status of this material system and the experimental signatures for the predicted edge channel transport consistent with the 2DTI state. Finally, I will present results on InAs and InSb quantum well systems. These semiconductors, exhibiting pronounced spin-orbit coupling, could, when interfaced with a superconductor, provide a scalable platform for the formation of Majorana bound states.
In collaboration with: C. Reichl, T. Tschirky, C. Lehner, S. Fält, M. Berl. W. Dietsche, L. Tiemann, K. Ensslin, T Ihn, S. Müller, M. Karalic, C. Mittag, V. Pribiag, L. Kouwenhoven
 
  • Day 3 | Tuesday  - August 15, 2017
Annica Black-Schaffer
Topological superconductors with Majorana fermions
 
Topological superconductors with Majorana fermion quasiparticles form a newly discovered class of matter. The Majorana fermion can be seen as half an electron, or more accurately, the electron wave function has split up into two separate parts. This non-local property is currently been intensively explored for implementing fault-tolerant quantum computation. I will explain where and why Majorana fermions appear, in particular focusing on systems where very standard components are combined to achieve the required non-trivial topology: spin-orbit coupled semiconductors, magnetism, and conventional s-wave superconductivity. I will also present some of our recent results in modeling topological superconductors with Majorana fermions, focusing on a simple mean to detect Majorana fermions, their robustness against disorder, and how they often appear in conjunction with spontaneous currents.
 
Avik Ghosh
Nano-electronics and Moore’s Law - what comes next?
 

With the current slow-down of Moore's law and the abolition of the ITRS roadmap, there is a pressing need to explore various material, architectural and physical solutions for low-power electronics, ranging from spintronics to 2D materials to subthermal switching. I will summarize the opportunities and challenges for various material, device and circuit level solutions in addressing the future of electronics. Digital electronics is based on the control of charge current. Over the last decade or two, we have made enormous progress both in understanding the quantum flow of charge in nano-structures at their molecular scales, as well as translating this understanding into practical, predictive simulation tools. I will start with a multiscale model that couples bandstructure and quantum transport for physics based compact models for charge, spin and heat flow, ranging from ballistic to diffusive, quantum to classical, non-interacting to many-body. I will then show how we can convert these into 'first principles' simulation platforms that take into account details of the interfacial chemistry, bonding, spin and topological indices to provide atomistic insights into non-equilibrium properties. Finally, I will show how we can use these tools to explore emerging low-power devices ranging from nano-magnetic switches for all spin logic, to metal insulator transitions in complex oxides, to 'phase transition switches' such as relays, tunnelFETs and 2D chiral tunneling FETs that attempt to bypass the fundamental Boltzmann tyranny limiting today's silicon devices. I will also outline possible architectural solutions such as neuromorphic and approximate computing schemes that maybe able to utilize existing CMOS hardware in innovative ways.

Christiano J. S. de Matos
2D materials and their application to optoelectronics and photonics
 
Since the isolation of graphene in 2004, a wide range of other atomically thin (2D) materials have been obtained and studied. 2D conductors, insulators, semiconductors and even superconductors have been identified, with properties that are different from their bulk (3D) counterparts. Additionally, 2D materials can be stacked to yield 2D heterostructures, allowing for a new generation of thin and flexible electronic, optoelectronic and photonic devices. This tutorial will review the recent advances in the science and technology of 2D materials, discussing the methods to synthesise and characterise them, as well as some of their applications in optoelectronics and photonics.
 
Sergio E. Ulloa
Long-range exchange interactions between magnetic impurities in Dirac materials
 
Dirac materials, where electronic carriers have non-trivial orbital and spin multicomponent spinor states, have opened a fascinating new area of materials research. The availability of 2D materials with strong spin-orbit effects, such as transition metal dichalcogenides (TMD) or bismuthene, as well as their 3D analogues (such as NaBi), provide for unique playgrounds to study different properties in many labs across the world.  
The presence of strong spin-orbit coupling allows for interesting exchange effects between magnetic impurities (MIs) embedded in these materials. Through the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction, carriers in metallic (or doped semiconducting) hosts mediate long-range interactions between MIs which moreover have strong non-collinear character. Such sizable Dzyaloshinskii-Moriya (DM) interactions between MIs has been shown to exist in 2D [1], and 3D materials [2], and to give rise to strongly anisotropic interactions.

I will describe how states at the edges of crystallites or lateral interfaces of 2D materials result in unusually long-range RKKY and DM interactions between MIs adsorbed or hybridized in these regions. We use a tight-binding description of the materials and study differences between different boundary geometries. The boundary states are shown to mediate interactions between MIs that may give rise to interesting magnetic phases. The combination of long range interactions and DM terms leads to helical and strongly frustrated impurity interactions in chains of MIs, with remarkable phase transitions as the range and relative signs of the different interactions is varied. We show that the magnetic configurations depend on the impurity concentration and doping levels in the host, opening an interesting experimental approach to study these phase transitions.

  • Day 4 | Wednesday - August 16, 2017
Ingrid D. Barcelos
Study of structural properties of heterostructures formed from twodimensional materials
 
Large part of the technological advances that emerged from solid state physics has its origin in the manufacture of semiconductor heterostructures. They currently make up the research object of two-thirds of all research groups working in semiconductor physics. This is due to the fact that new properties arise by changes in the electronic structure of interfaces that occur to put different materials in contact. A natural tendency is the predictable search heterostructure concepts and fabrication methods using new materials. This presentation consists of single/few layer graphene foils produced by chemical vapor deposition (CVD) are rolled with selfpositioned layers of InGaAs/Cr forming compact multi-turn tubular structures that consist on successive graphene/metal/semiconductor heterojunctions on a radial superlattice. Using elasticity theory and Raman spectroscopy, we show that it is possible to produce homogeneously curved graphene with a curvature radius on the 600−1200 nm range. Additionally, the study of tubular structures also allows the extraction of values for the elastic constants of graphene that are in excellent agreement with elastic constants found in the literature. However, our process has the advantage of leading to a well-defined and nonlocal curvature. From the results described in this work, one can assume that curvature effects solely do not modify the Raman signature of graphene and that strain phenomena observed previously may be ascribed to possible stretching due to the formation of local atomic bonds. This implies that the interactions of graphene with additional materials on heterostructures must be investigated in detail prior to the development of applications and devices.
 
Vladimir Falko
New 2D semiconductor: atomically thin crystals of γ-InSe
 

We present the analysis of electronic band structure of InSe and (other III-VI semiconductors) films, from the stoichiometric mono-layer to N-layer films, and we describe the resulting optical properties of these 2D materials [1,2]. This study is based on the ab initio DFT and related multi-orbital tight-binding model analysis of the electronic band structure and wave functions in the two-dimensional N-layer InSe crystals, and it is compared to the results of luminescence spectroscopy of this material. We show [1-3] that the band gap in InSe (and GaSe) strongly depend on the number of layers, with a strong (more than twice) reduction from the monolayer to crystals with N>6. We find that the conduction-band-edge electron mass in few-layer InSe is quite light (comparable to Si), which suggests opportunities for high-mobility devices and the development of nanocircuits. In contrast, the valence band in mono-, bi- and trilayer InSe is flat, opening possibilities for strongly correlated hole gases in p-doped films. We also propose a model to describe electronic properties of misaligned layers of InSe. Using the band structure and wave functions, we analyse optical transitions in thin films of InSe, identify their polarisation and compare the results of modelling to the measurements performed on hBN-encapsulated atomically thin InSe crystals.

[1] D. Bandurin, et al, Nature Nanotechnology (2016); doi:10.1038/nnano.2016.242
[2] Magorrian, S., Zolyomi, V. & Falko V. , Phys. Rev. B 94, 245431 (2016)
[3] Mudd, G. W., Molas, M. R., Chen, X., Zólyomi, V., Nogajewski, K., Kudrynskyi.
 
Leandro Malard
Time resolved and non-linear optics in 2D materials
 

In this work we will show our recent developments on the understanding of electron relaxation pathways and the non-linear optical properties of novel 2D materials. By using two-color pump probe scheme, we have studied the photo-excited electronic cooling in graphene in presence of controlled defect densities. We clearly observe an inverse linear dependence of the electron scattering rate with the mean distance between defects in the samples [1]. This dependence can be explained the defect assisted acoustic phonon supercollision model in graphene. The disorder- assisted scattering process allows for large phonon recoil momentum values and the entire thermal distribution of acoustic phonons can contribute to the scattering process, resulting in efficient carrier energy dissipation. Also we have used both Second Harmonic Generation (SHG) and Coherent Anti Stokes Raman Scattering (CARS) to study the nonlinear optical properties of mono- and few-layers of molybdenum disulfide (MoS_2) and graphene respectively. In the case of the molybdenum disulfide we have observed efficient SHG from odd number of layers due to the absence of inversion symmetry [2]. By using different laser excitation energies, we could probe the resonant effect in the SHG due to the presence of different optical transitions in MoS_2. By analyzing the resonant profile of SHG we can observe the different types of excitons in this material, which are compared with recent theoretical results in the literature. Because graphene have inversion symmetry, the SHG is almost absent, however third order nonlinearities are greatly enhanced in this material due its peculiar band structure. We have studied four wave mixing process in graphene, and in particular we will discuss the CARS process in graphene [3].

We acknowledge FAPEMIG, CNPq, Finep and CAPES.

[1] T. V. Alencar et al., Nano Letters 14, 5621 (2014).
[2] L. M. Malard et al., Phys. Rev. B 87, 201401 (2013).
[3] L. Lafetá L., arXiv1701.09023 (2017).

  • Day 5 | Thursday  - August 17, 2017
 
Paulo V. Santos
Exciton-polariton lattices in semiconductor microcavities
 
Microcavity exciton-polaritons (MPs) are bosonic quasi-particles resulting from the strong coupling between photons in a microcavity (MCs) and excitons in a quantum well (QW) embedded in it. MPs have a very low effective mass (10$^{-4}$-10$^{-5}$ of the electron mass) and, therefore, de Broglie wavelengths $λ_{dB}≫1~\mu$m. As a result, MPs experience strong confinement effects even for $\mu$m-sized potentials.  Furthermore, MPs undergo at high densities a transition to a Bose-Einstein(BE)-like a condensate with extended temporal and spatial coherences.[1]
In this talk, we review recent results on the confinement of MP and their condensates in micro-structured MCs grown by Molecular Beam Epitaxy. Micrometric static confinement potentials for MPs can be produced by structuring the thickness of the MC layers in-between growth steps. Spatially resolved photoluminescence shows confined states with discrete energy levels for MP confined in the  traps. Dynamic lattices for MPs are be produced by modulating the MC using the strain field of a surface acoustic wave (SAW). A SAW propagating on the surface of an (Al,Ga)As polariton MC induces a periodic energy modulation of both the photonic and excitonic polariton components. For SAW wavelengths  $<<\lambda_{dB}$, this lateral modulation forms a one-dimensional (1D) moving MP crystal with period and contrast given by the SAW period and amplitude, respectively. [2] 2D MP moving lattices can be formed by interfering two orthogonal SAW beams.[3,4] The latter are,  solid-state analogs of optical lattices of cold atoms. They thus form a prototype system for the investigation of many body interactions in non-equilibrium quantum phases as well as for the implementation of functionalities for quantum information processing.
[1] J. Kasprzak et al., Nature 443, 409 (2006).
[2] E. A. Cerda-Méndez et al., Phys. Rev. Lett. 105, 116402 (2010), Phys. Rev. Lett. 111, 146401 (2013).
[3]  J. Buller et al., Phys. Rev. B94, 125432 (2016). 
 
Gian Salis
Control of spin precession by drift and diffusion in a 2D electron gas
 

Drift and diffusion of spin polarization in a semiconductor two-dimensional electron gas is strongly influenced by spin precession in the effective spin-orbit magnetic field. The non-commuting spin rotations that occur between subsequent scattering events typically lead to rapid spin dephasing, which can be lifted by engineering the spin-orbit interaction to a persistent spin helix (PSH) symmetry [1], or by laterally confining the electron gas to a length scale smaller than the spin-orbit length [2]. If the spin-orbit interaction is linear in momentum, the average precession angle only depends on the distance the electrons travel, irrespective of whether transport occurs by diffusion or by drift. We show that for cubic Dresselhaus spin-orbit interaction, drift and diffusion by same distances lead to spin precession angles that differ by a factor of two. We have measured spin dynamics in a GaAs-based two-dimensional electron gas tuned to the PSH symmetry using spatially and time-resolved Kerr rotation measurements. Spin polarization is locally injected using a focused circularly polarized laser pulse. In absence of an external magnetic field, the spin polarization measured at a fixed position with respect to the injection point is found to precess with time. The precession frequency depends linearly on the drift velocity and can be explained within a simple model [3,4]. This finding highlights the role of nonlinear SOI in spin transport and is relevant for spintronics applications that require spin manipulation in absence of an external magnetic field.

[1] J. Schliemann, J. C. Egues, and D. Loss, Phys. Rev. Lett. 90, 146801 (2003).
[2] P. Altmann, M. Kohda, C. Reichl, W. Wegscheider and G. Salis, Phys. Rev. B 92, 235304 (2015)
[3] P. Altmann, F. G. G. Hernandez, G. J. Ferreira, M. Kohda, C. Reichl, W. Wegscheider and G. Salis, Phys. Rev. Lett. 116, 196802 (2016).
[4] G. J. Ferreira, F. G. G. Hernandez, P. Altmann, and G. Salis, Phys. Rev. B 95, 125119 (2017)

 
Christoph Deneke
What you need to know about molecular beam epitaxy in 50 min
 

In the last decades, nanotechnology has entered our daily life with one of the representatives being semiconductor technology in its various forms. Beside the ability to scale processes down to nanometer sizes, semiconductor technology and science is driven by the capacity to form semiconductor junctions and combinations. This idea, known as heterostructure, was created in the late 50s, after the invention of the transistor in the late 40s. It took some two decades before its realization by a major technological breakthrough – the invention of molecular beam epitaxy (MBE). By now, heterostructures and heterostructure based devices are the backbone of internet communication, mobile telephones and other advanced electronics – many grown by MBE. In this tutorial, I would like to give an overview of MBE - a technique that is important for basic science providing samples for quantum mechanical studies - like the quantum hall effect, single photon sources or Kondo effect – to device structures including high efficient solar cells, quantum cascade lasers or high mobility transistors embedded in cell phones. I will cover the basics of epitaxial growth, do an introduction, what a heterostructure is and its possibilities for band structure design. To illustrate these possibilities, I will take about photoluminescence from heterostructures like quantum wells and quantum dots and discuss, how a 2D electron gas emerge at heterostructure boundaries. I will provide examples from our research to explain basic growth phenomena from flat layer to pattern substrates and luminescence from semiconductor nanostructures fabricated in our labs.

Vanessa Sih
Electrical generation and manipulation of electron and nuclear spin polarization in semiconductors
 
Current-induced spin polarization is a phenomenon in which carrier spins in semiconductors are oriented when subjected to current flow. However, the mechanism that produces this spin polarization remains an open question. Existing theory predicts that the spin polarization should be proportional to the spin-orbit splitting yet no clear trend had been observed experimentally. We perform experiments on samples designed so that the magnitude and direction of the in-plane current and applied magnetic field can be varied and use optical techniques to measure the electrical spin generation efficiency and spin-orbit splitting [1]. Contrary to expectation, the magnitude of the current-induced spin polarization is shown to be smaller for crystal directions corresponding to larger spin-orbit splitting. Angle-dependent measurements demonstrate that the steady-state in-plane spin polarization is not along the direction of the spin-orbit field, which we attribute to anisotropic spin relaxation. We show that this electrically-generated electron spin polarization can drive dynamic nuclear spin polarization and measure the nuclear spin polarization as a function of laboratory time and applied electric and magnetic field [2]. In order to understand how different parameters affect the electrical spin generation efficiency, we perform measurements on samples with different indium concentrations and doping densities [3].
[1] B. M. Norman, C. J. Trowbridge, D. D. Awschalom, and V. Sih, "Current-Induced Spin Polarization in Anisotropic Spin-Orbit Fields," Phys. Rev. Lett. 112, 056601 (2014).
[2] C. J. Trowbridge, B. M. Norman, Y. K. Kato, D. D. Awschalom, and V. Sih, "Dynamic nuclear polarization from current-induced electron spin polarization," Phys. Rev. B 90, 085122 (2014).
[3] M. Luengo-Kovac et al, in preparation (2017).
 
  • Day 6 | Friday  - August 18, 2017
Dominik Zumbuhl
Hyperfine and Spin-Orbit Spin Relaxation in a GaAs Single Electron Quantum Dot
 
Understanding and controlling spin relaxation is central for spin qubits, setting an upper limit to the coherence time T2. The spin-orbit (SO) and hyperfine interactions are the most important mechanisms, giving rise to spin relaxation by emitting the Zeeman energy into a phonon. In presence of moderate magnetic fields, it has been shown that spin relaxation is primarily caused by the SO interaction.

Here, we present measurements of the spin relaxation rate W in a gate defined single-electron GaAs quantum dot at electron temperatures down to 60 mK as a function of both direction as well as strength of magnetic field, spanning an unprecedented range from 0.6 T to 14 T applied in the plane of the 2D electron gas. Due to the interplay of Rashba and Dresselhaus SO contributions, W shows strong anisotropy when varying the direction of the applied in-plane magnetic field B with a piezoelectric rotator. Along the crystal axis where SOI coupling is weak, a spin relaxation time T1 of 57+/-10 s has been obtained at a magnetic field of 0.6 T. However, quite surprisingly, this is still more than one order of magnitude shorter than the expected value based on SO mediated spin relaxation. Further, W shows a B3 dependence and becomes isotropic at the lowest magnetic fields. These observations thus indicate hyperfine interaction mediated spin relaxation (non flip-flop) via phonons at the lowest magnetic fields used here.
Command of the dot orbitals, control of the B-field direction and low-B-field measurements -- made possible by a low electron temperature -- reveal hyperfine spin relaxation and allow comprehensive modeling, giving excellent agreement between experiment and theory.

Andrew Mitchell
Quantum simulations with semiconductor quantum dots
 
Charge-Kondo quantum dots have recently emerged as a new nanoelectronics paradigm [1,2]. A pseudospin qubit is implemented with quasidegenerate macroscopic charge states of a large semiconductor dot, connected to two or more leads via quantum point contacts. Such devices offer unprecedented control over quantum effects and strongly correlated electron physics on the nanoscale. I this talk I show that charge-Kondo quantum dot devices are essentially perfect quantum simulators of nontrivial quantum impurity models, with Majorana-mediated quantum critical transport in two-channel charge-Kondo experiments [1] agreeing with our theory [2] over a remarkable 9 orders of magnitude. I then discuss more exotic physics in three-lead systems and coupled-dot devices, with finally an outlook towards realizing lattice spin models.

[1] Z. Iftikhar et al, Nature 526, 233 (2015)
[2] A. K. Mitchell et al, Phys. Rev. Lett. 116, 157202 (2016)

 

 

Social program

To be announced