Registration Room: 132:028

Welcome and introduction to the programme Room: 132:028

Introduction to Quantum Magnets - I

• Pinaki Sengupta

Room: 132:028 A short course on Quantum Magnets.

Introduction to Quantum Magnets - II

• Pinaki Sengupta

Room: 132:028

Introduction to Quantum Magnets - III

• Pinaki Sengupta

Room: 132:028

Introduction to Quantum Magnets - IV

• Pinaki Sengupta

Room: 132:028

General aspects of mesoscopic transport - I

• Jonathan Bird

Room: 132:028 This broad set of lectures starts from an introductory level and proceeds to the more advanced one. Tentative outline: <br>(A) Density of States 2DEG density of states, Quantum-wire density of states Quantum-dot density of states, Density of states of actual nanostructures, <br>(B) Transmission in Nanostructures I: 1D conductance quantization, Quantum point contacts (QPCs), Electron channels in quantum wires, Quantum-wire conductance, <br>(C) Transmission in Nanostructures II (More on 1D Conductance Quantization): Temperature dependence, Spin & other degeneracies, Si & carbon nanotubes, Energy relaxation in ballistic transport, Quantum contact resistance, Non-linear transport in QPCs, <br>(D) Transmission in Nanostructures III: Disordered quantum wires, Breakdown of 1D conductance quantization, The Landauer formula, The Landauer formula & scaled CMOS.

Spin-orbit interaction in quantum wires - I

• Almas Sadreev

Room: 132:028 In this set of lectures basic concepts such as Rashba and Dresselhaus spin-orbit interactions will be described. The example of In doped GaAs 2DEG structures will be discussed, how to solve the problem of electron transmission in electronic devices based on these structures, spin-polarized transmission in curvilinear wires and spin evolution for the process of transmission, and finally about ac effects. In a more detailed part of the lectures we consider electron transmission in 1d quantum wire with the Rashba spin-orbit interaction(SOI). The wire is subjected by dc and ac potentials of finger gate that results in local time-periodic Rashba SOI. We calculate the conductance of the 1d wire as dependent on incident energy, frequency of ac potential and a value of dc potential. A conductance, spin polarization and spin flip transmission are calculated as dependent on external constant magnetic field, the ac frequency and energy of incident electrons which show an analog of electron spin resonance for dynamical Rashba SOI.

General aspects of mesoscopic transport - II

• Jonathan Bird

Room: 132:028

Spin-orbit interaction in quantum wires - II

• Almas Sadreev

Room: 132:028

General aspects of mesoscopic transport - III

• Jonathan Bird

Room: 132:028

Electronic transport across a helical edge state exposed to a uniform magnetic field

• Abhiram Soori

Room: 132:028 We study electronic transport across a helical edge state exposed to a uniform magnetic field over a finite length. We show that this system exhibits Fabry-Pérot type resonances in electronic transport. The intrinsic spin anisotropy of the helical edge states allows us to tune these resonances by changing the direction of the magnetic field while keeping its magnitude constant. These resonances provide a unique way to probe the helical nature of the theory. We also study the effect of a static impurity which can induce back-scattering in the presence of a magnetic field. <br><br>Ref: arXiv:1112.5400

The 1D g-factor and 0.7 anomaly - I

• Adam Micolich

Room: 132:028 Overview of ecent results on the 1D g-factor and 0.7 anomaly in QPCs with independent control over density.

The 1D g-factor and 0.7 anomaly - II

• Adam Micolich

Room: 132:028

The 1D g-factor and 0.7 anomaly - III

• Adam Micolich

Room: 132:028

Spin relaxation in low dimension - I

• Mikhail Glazov

Room: 132:028

Spin currents - I

• Yuli Lyanda-Geller

Room: 132:028 A new spin-electric state arising due to spin current under the circumstances of weak spin relaxation is presented In this state, two opposite boundary regions of a two-dimensional conductor carry oppositely directed electric fields perpendicular to flowing electric current, and close to 100% oppositely directed spin polarizations. Conditions for experimental observation of this new state are discussed.

The 1D g-factor and 0.7 anomaly - IV

• Adam Micolich

Room: 132:028

Spin relaxation in low dimension - II

• Mikhail Glazov

Room: 132:028

Spin currents - II

• Yuli Lyanda-Geller

Room: 132:028

Unitarity of scattering and edge spin accumulation in ballistic and quasiballistic regime

• Alexander Khaetskii

Room: 132:028 The phenomenon of edge spin accumulation, which appears due to spin-orbit interaction in 2D mesoscopic structures in the presence of a charge current, is considered. We consider the case of a strong spin-orbit-related splitting of the electron spectrum, i.e. a spin precession length is small compared to the mean free path l. The structure can be either in a ballistic regime (when the mean free path is the largest scale in the problem) or quasiballistic regime (when l is much smaller than the sample size). We showed how physics of edge spin accumulation in different situations should be understood from the point of view of unitarity of boundary scattering. Using transparent method of scattering states, we were able to explain some previous puzzling theoretical results. In particular, we clarified the important role of the form of the spin-orbit Hamiltonian, the role of the boundary conditions, etc., and revealed the wrong results obtained in this field. The detailed comparison with the existing theoretical works is presented. In particular, the relation between the edge spin density and the bulk spin current in different regimes is discussed. For example, for the cubic Hamiltonian the "edge" contribution to the spin density can be larger than the "bulk" one which appears as a result of the spin flux from the bulk. This demands the reinterpretation of the experimental results [1]. <br><br>[1]. J. Wunderlich et al., PRL 94, 047204 (2005).

Spin currents - III

• Yuli Lyanda-Geller

Room: 132:028

Spin dynamics of cold exciton condensates

• Alexey Kavokin

Room: 132:028

Mid-term Meeting of IRSES SPINMET Project Room: 132:028 Midterm meeting Friday afternoon, September 14, there is a mid- term meeting of the project "IRSES SPINMET", in which the participants of this network will make presentations. The meeting, that will take place at Nordita, is open for all the participants of the Nordita/Spin 2012 meeting: 13.00- 14.00, A. Khaetskii, Unitarity of scattering and spin accomulation in ballistic and quasi- ballistic regimes 14.00- 15.00 ßN.T. Bagraev, Fractional quantum conductance and magnetic resonance phenomena in silicon nanostructures 15.00-15.45 I. Levkivskyi, Dephasing in electronic Mach-Zehnder interferometers at filling factor 2 15.45- 16.30 G. Pavlovic, Mesoscopic optical Berry phase interferometer 16.30- 17.15 O. Kyriienko, Spin- orbit coupled cold exciton condensates

Opening of workshop part Room: 132:028

Spin dynamics in semiconductors and graphene

• Ming-Wei Wu

Room: 132:028 In this talk we are going to present our theoretical investigations on spin dynamics of semiconductors and graphene under various conditions. It is shown from a fully microscopic kinetic-spin-Bloch-equation (KSBE) approach that the single-particle approach is inadequate in accounting for the spin relaxation/dephasing (R/D) both in the time domain and the spacial domain. The momentum dependence of the effective magnetic field (the Dresselhaus and the Rashba terms) and the momentum dependence of the spin diffusion rate along the special gradient all serve as inhomogeneous broadenings. It is pointed out that in the presence of inhomogeneous broadening, any scattering, including the carrier-carrier Coulomb scattering, can cause irreversible spin R/D. Moreover, besides the spin R/D channel the scattering provides, it also gives rise to the counter effect to the inhomogeneous broadening. The scattering tends to drive carriers to a more homogeneous states and therefore suppresses the inhomogeneous broadening. Finally, this approach is valid in both strong and weak scattering regime and can be used to study systems far away from the equilibrium such as electrons of high spin polarization and/or electrons with strong electric field (hot electrons). Many novel effects are predicted from our theory and some have been realized experimentally very recently.

Spins and pseudospins in graphene

• Roland Winkler

Room: 132:028 Recently atomically flat layers of carbon known as graphene have become the rising star in spintronics as their electrons carry notonly the ordinary spin degree of freedom, but they also have a pseudo-spin degree of freedom tied to the electrons' orbital motion which could enable new routes for spintronics.Here we focus on bilayer graphene (BLG). Using group theory we have established a complete description of how electrons in BLG interactwith electric and magnetic fields. We show that electrons in BLG experience an unusual type of matter-field interactions where magnetic and electric fields are virtually equivalent: every coupling of an electron's degrees of freedom to a magnetic field is matched by an analogous coupling of the same degrees of freedom to an electric field. This counter-intuitive duality of matter-field interactions allows novel ways to create and manipulate spin and pseudo-spin polarizations via external fields that are not available in other materials.

Spin geometric phase in InGaAs spin interferometers

• Junsaku Nitta

Room: 132:028 We have investigated a geometric phase, also known as a Berry phase, of electron spin in gate-fitted InGaAs ring arrays subject to Rashba spin-orbit (SO) coupling. Studying the spin geometric phase is important because it is associated with various spin-dependent phenomena such as persistent spin currents, and it may be utilized for robust control of a spin phase. In this study, we electrically observe the spin geometric phase by measuring the radius dependence of the Aharonov-Casher (AC) spin-interference effect. An InGaAs/InAlAs heterostructure was employed to fabricate ring arrays which consist of a huge number of rings. Five ring arrays with different radii were fabricated by electron beam lithography and reactive-ion etching. A gate electrode on top of the ring arrays was attached with a 200-nm-thick Al2O3 insulation layer in order to control the Rashba SO strength. Magnetoresistances of the ring arrays were measured as a function of the Rashba SO strength at 1.7 K. By applying a perpendicular magnetic field, clear Al’tshuler-Aronov-Spivak (AAS) oscillations with h/2e period were observed due to the ensemble averaging. Since the AAS effect stems from the interference between time-reversal symmetric paths, the orbital phase always makes constructive interference at zero magnetic field. Therefore, the gate voltage dependence of the AAS oscillations’ amplitude along B = 0 corresponds to the time-reversal symmetric AC oscillations. By increasing the radius of the ring, the AC oscillations are shifted towards weaker Rashba SO regions with their shortened period. The phase shift in the AC phase can be attributed to the modulation of the spin geometric phase. The spin geometric phase tends to increase with a larger radius, which is in good agreement with the theoretical prediction [1]. <br><br>[1] F. Nagasawa, J. Takagi, Y. Kunihashi, M. Kohda, and J. Nitta, Phys. Rev. Lett. 108, 086801 (2012).

Engineering geometrical spin phase

• Diego Frustaglia

Room: 132:028 Research on spin geometric (Berry) phases in mesoscopic systems has been active for 20 years already. During this time, several proposals were put forward for the detection of topological effects in spin interferometers subject to magnetic textures, accompanied by several experimental attempts of different success. However, incontrovertible evidence of these effects was found only recently in mesoscopic rings under the action to spin-orbit coupling (Rashba rings), providing a new impulse to the field. Here, after a brief account of previous achievements, we shall discuss some new possibilities for electronic manipulation based on the control of the spin geometric phases in nanodevices as Rashba loops (rings and polygons) subject to additional magnetic fields.

Coherent spin dynamics in semiconductor low-dimensional systems

• Mikhail Glazov

Room: 132:028 The studies of spin phenomena in semiconductor low-dimensional systems have grown into the spintronics – rapidly developing area of the condensed matter physics. The most important problems in this area are the creation of charge carrier spin polarization and its detection, as well as electron spin control by nonmagnetic methods, e.g. optically. Here, we present a review of recent achievements in the studies of spin dynamics of electrons, holes, and their complexes in the pump-probe method. The main attention is paid to singly charged quantum dots and to quantum wells with low electron denisty. The microscopic mechanisms of spin orientation of charge carriers and their complexes by short circularly polarized optical pulses and the processes of the spin Faraday and Kerr rotation signals formation are analyzed. Specific features of temporal behavior of different spin signals are addressed. Theoretical models and experimental manifestations of the resonant spin amplification, spin precession mode locking and nuclei induced electron spin precession frequency focusing effects are discussed. <br><br>Tentative plan of my talk: <br>1. Optical transitions in quantum dots: two-level model, transformation of electron wavefunction by the optical pulse, Rabi oscillations. Impact of selection rules: quantum disks vs. spherical nanocrystals. <br>2. Optical control of electron spins in quantum dots. <br>3. Spin Faraday, Kerr and ellipticity effects in quantum dots. Role of ensemble inhomogeneity. <br>4. Electron spin precession in the external magnetic field. <br>5. Spin accumulation caused by the train of pump pulses. Resonant spin amplification and mode-locking of electron spin coherence. Nuclei-induced electron spin precession frequency focusing. <br><br>References: <br>[1] M.M. Glazov, Coherent spin dynamics of electrons and excitons in nanostructures (a review), Physics of the Solid State 54, 1 (2012). <br>[2] I.A. Yugova, M.M. Glazov, E.L. Ivchenko, and Al.L. Efros, Pump-probe Faraday rotation and ellipticity in an ensemble of singly charged quantum dots, Phys. Rev. B 80, 104436 (2009). <br>[3] A. Greilich, D.R. Yakovlev, A. Shabaev, Al.L. Efros, I.A. Yugova, R. Oulton, V. Stavarache, D. Reuter, A. Wieck, and M. Bayer, Mode Locking of Electron Spin Coherences in Singly Charged Quantum Dots, Science 313, 341 (2006). <br>[4] M.M. Glazov, I.A. Yugova, S. Spatzek, A. Schwan, S. Varwig, D.R. Yakovlev, D. Reuter, A.D. Wieck, and M. Bayer, Effect of pump-probe detuning on the Faraday rotation and ellipticity signals of mode-locked spins in (In,Ga)As/GaAs quantum dots, Phys. Rev. B 82, 155325 (2010). <br>[5] A. Greilich, Sophia E. Economou, S. Spatzek, D.R. Yakovlev, D. Reuter, A.D. Wieck, T.L. Reinecke, and M. Bayer, Ultrafast optical rotations of electron spins in quantum dots, Nat. Phys. 5, 262 (2009). <br>[6] A. Greilich, A. Shabaev, D.R. Yakovlev, Al.L. Efros, I.A. Yugova, D. Reuter, A.D. Wieck, and M. Bayer, Nuclei-Induced Frequency Focusing of Electron Spin Coherence, Science 317, 1896 (2007). <br>[7] M. M. Glazov, I. A. Yugova, and Al. L. Efros, Electron spin synchronization induced by optical nuclear magnetic resonance feedback, Phys. Rev. B 85, 041303(R) (2012).

Transport of electrons through a photon cavity

• Vidar Gudmundsson

Room: 132:028 We demonstrate that with a stepwise introduction of complexity to a model of an electron system embedded in a photonic cavity and a carefully controlled stepwise truncation of the ensuing many-body space it is possible to describe the time-dependent transport of electrons through the system with a non-Markovian generalized quantum master equation. We show how this approach retains effects of the geometry of an anisotropic electronic system. The Coulomb interaction between the electrons and the full electromagnetic coupling between the electrons and the photons are treated in a non-perturbative way using "exact numerical diagonalization". We will elucidate how the sequential tunneling of electrons in and out of the system is influenced by the symmetry of their wavefunctions and spin structure, i.e. how correlation effects shape the dynamic transport properties of these few electron systems.

Polariton spin effects: from spinor condensates to optoelectronic devices

• Alberto Bramati

Room: 132:028 Exciton-polaritons are mixed light-matter quasi-particles arising from the strong coupling between photons and excitons in a micrometer sized cavity with embedded quantum wells. They have been studied extensively since the discovery of strong light–matter coupling in these systems in 1992. Polaritons are bi-dimensional composite bosons that can exhibit macroscopic quantum coherence effects at high temperatures (5-300K) due to their very low mass (~10-4 times that of the electron, inherited from their photonic component). In particular, polaritons behave as a quantum fluid with specific properties coming from their out of equilibrium nature, determined by their short lifetime (few picoseconds) [1-3]. At the same time, microcavity polaritons, due to the strong spin dependent non-linear polariton-polariton interactions (inherited from their excitonic component), are very promising for the realization of integrated optoelectronic devices opening the way both to operation at the quantum level and to spin based architectures. <br> <br>In this talk I will first discuss our recent observation of oblique dark half-solitons in a spinor polariton quantum fluid diretly created by resonant laser excitation in an InGaAs semiconductor microcavity. In the second part of the talk I will discuss the perspectives opened by semiconductor microcavities as tools for all-optical information processing, due to their low-threshold, polarization-dependent nonlinear emission, fast operation and integrability. In particular I will show how the spin-dependent polariton-polariton interactions can be exploited to implement a polarization controlled optical gate [4] as well as a non-local, all-optical spin switch [5], potentially working at high rates due to the large polariton velocity and reduced lifetime. <br> <br>[1] A. Amo, J. Lefrère, S. Pigeon, C. Adrados, C. Ciuti, I. Carusotto, R. Houdré, E. Giacobino, A. Bramati, Nature Physics 5, 805 (2009). <br>[2] A. Amo, S. Pigeon, D. Sanvitto, V. G. Sala, R. Hivet, I. Carusotto, F. Pisanello, G. Leménager, R. Houdré, E. Giacobino, C. Ciuti and A. Bramati, Science, 332,1167 (2011). <br>[3] D. Sanvitto, S. Pigeon, A. Amo, D. Ballarini, M. De Giorgi, I. Carusotto, R. Hivet, F. Pisanello, V. G. Sala, P. S. S. Guimaraes, R. Houdré, E. Giacobino, C. Ciuti, A. Bramati & G. Gigli, Nature Photonics, 5, 610 (2011). <br>[4] C. Leyder, T. C. H. Liew, A. V. Kavokin, I. A. Shelykh, M. Romanelli, J.-Ph. Karr, E. Giacobino and A. Bramati, Phys. Rev. Lett. 99, 196402 (2007). <br>[5] A. Amo, T.H.C Liew, C. Adrados, A.V. Kavokin, R. Houdré, E. Giacobino and A. Bramati, Nature Photonics, 4, 361 (2010).

Optical spin Hall effect in microcavities

• Timothy C. H. Liew

Room: 132:028

Magnetic monopoles behavior of half solitons hand half vortices in a polariton quantum fluid

• Guillaume Malpuech

Room: 132:028 Monopoles are magnetic charges accelerating along a constant magnetic field. They are absent in Maxwells equations (contrary to electric charges) and have never been observed as fundamental particles. The seminal work of Dirac [1] showed nonetheless that monopoles are allowed by the laws of quantum mechanics. Their non-observation has motivated the search for monopole analogues in the form of quasiparticles in solid-state systems. An alternative system where monopole analogues have been predicted is a quantum fluid such as a spinor Bose-Einstein condensate. In this case, monopoles take the form of topological excitations combining phase and spin geometries [3]. Condensates of microcavity polaritons – quasiparticles arising from the strong coupling between photons and excitons in a semiconductor structure, are an ideal system to explore this phenomenon due to their unique spin structure and the easy control of the polariton wavefunction using optical techniques. Polariton condensates have already provided experimental reports of fascinating quantum fluid phenomena like superfluidity [4], oblique dark solitons [5], or vortices and half-vortices [6]. <br><br>In this work we show that the half-integer topological defects (half-solitons [7] or half-vortices) characterized by the presence of the topological defect in only one of the two spin components of a condensate behave as magnetic monopoles [8]. These defects show divergent spin textures, analogous to the field of a point electric charge, and behave like magnetic charges [3]. We provide a clear experimental observation of one of these excitations, half-solitons in a GaAs-based microcavity. Half-solitons are formed in the wake of a point-like barrier in the flow path of a polariton condensate created by resonant excitation with linearly polarized light. Studying the polarization of emission we provide the full spin tomography of these excitations and, by tracking their trajectory, we demonstrate that they behave as magnetic charges accelerated along an effective magnetic field due to the TE-TM splitting. These observations show that polariton condensates are an ideal system to study analogue physics. <br><br>[1] Dirac, Proc. Roy. Soc. A 133, 60 (1931). <br>[2] D. Solnyshkov et al., Phys. Rev. B 85, 073105 (2012). <br>[3] A. Amo et al., Nature Phys. 5, 805 (2009). <br>[4] A. Amo et al., Science 332, 1167 (2011). <br>[5] K. G. Lagoudakis et al., Nature Phys. 4, 706 (2008); K. G. Lagoudakis et al., Science 326, 974 (2009). <br>[6] H. Flayac et al., Phys. Rev. B 83, 193305 (2011).

Manipulating quantum fluids

• Daniele Sanvitto

Room: 132:028 Polaritons are light-matter quasi-particles which are easily formed in semiconductor microcavities under strong coupling between cavity photons and quantum well excitons. They have recently shown very surprising phenomena, typical of Bose-Einstein condensates (BEC), despite their intrinsic dissipative nature. Some of these phenomena have paved the way for the study of new properties which are unique for such a system formed by non-equilibrium quaiparticles. Indeed polaritons, compared to their atomic-BEC counterpart, due to their hybrid lightmatter nature, offer strong advantages, amongst which a much higher temperature of condensation, the ability to be easily manipulated/observed and the possibility of straightforwardly integrate with present semiconductor technology. We will review some of the fundamental properties of polaritons under condensation regime [1-4] and we will show how it is possible, by using a mixture of different spin population, to form, guide and disrupt polariton quantum fluids running along the plane of a semiconductor microcavity [5,6]. Finally, by using controlled fluid dynamics experiment, we will demonstrate the first building block in the use of polariton condensates for the possible realisation of the next generation all-optical devices. <br><br>[1] J. Kasprzak, M. Richard, S. Kundermann, et al., Nature 443, 409 (2006). <br>[2] E. Del Valle, D. Sanvitto, F. P. Laussy et al. Phys. Rev. Lett. 103, 096404 (2009) <br>[3] A. Amo, D. Sanvitto, F. P. Laussy, et al. Nature 457, 291 (2009). <br>[4] A. Amo, J. Lefrère, S. Pigeon, et al., Nat. Phys. 5, 805 (2009). <br>[5] D. Sanvitto, F. M. Marchetti, M. H. Szymanska, et al., Nat. Phys. 6, 527 (2010). [6] D. Sanvitto, S. Pigeon, A. Amo et al. Nat. Phot. 5, 610 (2011).

Dynamics of inhomogeneous spinor polariton system

• Ivan Savenko

Room: 132:028 We present a formalism suitable for the description of the dynamics of an inhomogeneous spinor polariton system in real space and time accounting for all relevent types of the processes. Namely, we take into account polariton-polariton and polariton phonon interactions and effective TE-TM magnetic field acting on polariton spin. Our consideration is based on the Lindblad approach for the density matrix dynamics, where we model the spin dynamics of the polaritons in 1D channels, investigating the role played

Intersubband polaritonics with spin-orbit interaction

• Oleksandr Kyriienko

Room: 132:028 We investigate the intersubband polaritons formed in the asymmetric quantum well (AQW) embedded into the semiconductor microcavity and study the effects of spin-orbit interaction. The Rashba and Dresselhaus spin-orbit interaction remove the spin degeneracy of electrons and allow the four types of intersubband excitations. While optical spin-flip transitions are suppressed, the spectrum of elementary excitations showed the appearance of upper, lower and middle polaritonic branches based on spin-conserving transitions. We analyze the influence of spin-orbit interaction (SOI) and show that accounting of non-zero photon momentum leads to the non-zero spin polarization in the system. We predict the possibility of creation of pure spin currents in the considered systems with long coherence length by using the external laser source.

Josephson effect for spinor excitons and cavity polaritons

• Goran Pavlovic

Room: 132:028

Equilibration of electrons and conductance of quantum wires

• Konstantin Matveev

Room: 132:028 I will discuss corrections to the conductance of long uniform quantum wires caused by the processes of equilibration of the electron system. In the most interesting case of strongly interacting electrons the equilibration is dominated by the spin excitations. This gives rise to the corrections to conductance that reach values comparable with the conductance quantum even at temperatures small compared to the Fermi energy.

Role of spin in quantum wire transport

• Kalarikad Thomas

Room: 132:028 Transport in quantum wires has attracted wide interest since the discovery of conductance quantization in 1988. Apart from this single-particle phenomenon, interest in quantum wires has steadily grown due to theoretical predictions that at low densities electron-electron interactions may lead to many different effects such as Luttinger liquid phenomenon, Wigner crystallization, spin polarization gas etc. While direct observation of a Luttinger liquid was found to be elusive in transport experiments, spin-related effects such as the 0.7 structure and zero bias conductance anomaly in ballistic quantum wires are believed to be due to interactions. A lot of theoretical models were proposed to explain the 0.7 structure [1] including spin polarization, Kondo effect, spin-incoherent transport, and Wigner crystallization. One of the important experimental breakthroughs came after predictions [2] that a tunable confinement potential which competes with the interaction strength may shine light on the origin of the 0.7 structure. Quantum wires were fabricated with an additional top gate to examine the effect of confinement strength and density. We have reported a bifurcation of the 1D electron gas in to two spatial rows as a possible consequence of a weakening confinement and increasing interaction strength [3,4]. Our recent experiments indicate that a zig-zag configuration is formed in the wire before the onset of the double-row formation. In this talk we shall present some of the recent experiments in this regime of transport with emphasis on the implications of lowering confinement strength on the 0.7 structure. <br><br>1. K. J. Thomas, J. T. Nicholls, M. Y. Simmons, M. Pepper, D. R. Mace, and D. A. Ritchie. “Possible Spin Polarization in a One-Dimensional Electron Gas”, Phys. Rev. Lett. 77,135 (1996) <br>2 .A. D. Klironomos, J. S. Meyer, T. Hikihara and K. A. Matveev, ‘Spin coupling in zigzag Wigner crystals’, Phys. Rev. B 76, 075302 (2007). <br>3. W. K. Hew, K. J. Thomas, M. Pepper, I. Farrer, D. Anderson, G. A. C. Jones, and D. A. Ritchie, “Incipient Formation of an Electron Lattice in a Weakly Confined Quantum Wire”, Phys. Rev. Lett., 102, 056804 (2009). <br>4. L. W. Smith, W. K. Hew, K. J. Thomas, M. Pepper, I. Farrer, D. Anderson, G. A. C. Jones, and D. A. Ritchie, “Row coupling in an interacting quasi-one-dimensional quantum wire”, Phys. Rev. B., 80, 041306® (2009).

Spin magnetization and electron localization in semiconductor quantum wires and quantum point contacts

• Irina Yakimenko

Room: 132:028 The physics of low-dimensional semiconductor structures such as quantum wires (QW) and quantum point contacts (QPCs) has developed into an important part of nanotechnology, specially in connection with spintronics and quantum information. In our current studies we focus on electronic states, effects of electron interactions and conductance anomalies in semiconductor quantum point contacts and wires with different geometries to a model GaAs/AlGaAs device [1]. Using the local spin density approximation (LSDA) we recover spin-polarized states in the middle of a conventional QPC results in the 0.7 (2e2/h) conductance anomaly as well as spin-split states near the two edges of the QPC that may be associated with the conductance anomaly in the vicinity of 0.25 (2e2/h). Effects of spontaneous magnetization might be also used for spintronics applications such as spin filters and all-electric QPC spin-polarizer. These systems are physically rich and constantly bring new experimental data such as the unusual conductance behavior in the case of shallow confinement potential in a low electron density regime, the anomalous properties of the 2DEG around the pinch-off regime and the formation of bound states with Kondo physics involved. This leads to the study of ballistic transport in low-dimensional semiconductor-based nanostructures in the ultralow electron density domain where the interaction-induced localization of electrons takes place. By means of LSDA we have shown that the localization of electrons within the barrier embedded in the wire and that the localization is highly dependent on the sharpness and length of the potential barrier. For a shallow barrier-free wire we retraced the structural transitions at low densities from a single chain of localized states to double and triple chains (Wigner spin lattices). We have found that the double chain appears as a double zig-zag configuration [2]. Localized electrons may have interesting applications for semiconductor nanodevices. Indeed the formation of bound state and spin binding within QPC confinement potential opens possibilities to store information and to realize qubits in quantum circuits. We have studied the electron transport in a quantum wire in the presence of bias between source and drain [3]. Our work has confirmed that spontaneous spin splitting does occur within the wire and it is responsible for both the 0.25 and 0.85 plateaus. We have also shown that the 0.25 plateau consists of two regions, one that is spin polarized, and another that is degenerate with a conductance that remains essentially the same at both sides of the transition. This result is of potential interest for semiconductor spintronics since it opens a new possibility for spin manipulation by an electric field. <br><br>[1] K.-F. Berggren and I. I. Yakimenko. J. Phys.: Condens. Matter 20, 164203 (2008). <br>[2] E. Welander, K.-F. Berggren, I. I. Yakimenko. Phys. Rev. B 82, 073307 (2010). <br>[3] H. Lind, I. I. Yakimenko, K.-F. Berggren. Phys. Rev. B 83, 075308 (2011).

Observation of fractional ac Josephson effect: the signature of Majorana particles

• Leonid Rokhinson

Room: 132:028 In 1928 Dirac reconciled quantum mechanics and special relativity in a set of coupled equations which became the cornerstone of quantum mechanics. Its main prediction that every elementary particle has a complex conjugate counterpart – an antiparticle – has been confirmed by numerous experiments. A decade later Majorana showed that Dirac's equation for spin-1/2 particles can be modified to permit real wavefunctions. The complex conjugate of a real number is the number itself, which means that such particles are their own antiparticles. The most intriguing feature of Majorana particles is that in low dimensions they obey non-Abelian statistics and can be used to realize quantum gates that are topologically protected from local sources of decoherence. While the search for Majorana fermions among elementary particles is still ongoing, excitations sharing their properties may emerge in electronic systems. Specifically, it has been predicted that Majorana excitations may be formed in some unconventional states of matter. I will report the observation of the fractional ac Josephson effect in a hybrid semiconductor/superconductor InSb/Nb nanowire junction, a hallmark of topological matter. When the junction is irradiated with rf frequency f at zero external magnetic field, quantized voltage steps (Shapiro steps) with a height ΔV=hf/2e are observed, as is expected for conventional superconductor junctions where the supercurrent is carried by charge-2e Cooper pairs. At high fields the height of the first Shapiro step is doubled to hf/e, suggesting that the supercurrent is carried by charge-e quasiparticles. This is a unique signature of Majorana fermions, elusive particles predicted ca. 80 years ago.

Andrew current induced by ferromagnetic resonance

• Julia Meyer

Room: 132:028 We study the proximity effect in ferromagnetic/superconducting hybrid junctions under non-equilibrium conditions. In particular, we are interested in the charge current that may be induced at ferromagnetic resonance in the Andreev subgap regime. To this end, we study charge transport through a metallic dot coupled to a superconducting and a ferromagnetic lead with a precessing magnetization due to ferromagnetic resonance. Using the quasiclassical theory, we find that the magnetization precession induces a dc current in the subgap regime even in the absence of a bias voltage. This effect is due to the rectification of the ac spin currents at the interface with the ferromagnet; it exists in the absence of spin current in the superconductor. Interestingly, when the dot is strongly coupled to the superconductor, the induced current is strongly enhanced as compared to the induced current in the normal state in a wide range of parameters.

Theory of STM spectroscopy of magnetic impurities on GaAs surfaces and subsurfaces

• Carlo Canali

Room: 132:028 State-of-the-art STM techniques have made it possible to substitute transition metal impurities for individual atoms in semiconductor crystals and have provided detailed information on the nature of the bound acceptor or donor states. Individual coupled acceptor (or donor)-impurity centers represent a new class of nanomagnets which we refer to as acceptor or donor magnets. In this talk I will present results of the electronic and magnetic properties of these nanomagnets, based on a recently introduced tight-binding method [1] that is able to capture the salient features of Mn atoms near the (110) GaAs surface. The effective total spin J of local moments formed from acceptor states bound to Mn ions in GaAs is identified with a topological Chern number, given by the integral of a Berry curvature functional over all possible magnetic moment directions. When individual Mn atoms are close to the sample surface, the total spin changes from J=1 to J =2, due to quenching of the acceptor orbital moment. For Mn pairs in bulk, the total J depends on pair orientation in the GaAs lattice and on the separation between the Mn atoms. We point out that Berry curvature variations as a function of local moment orientation can profoundly influence the quantum-spin dynamics of these magnetic entities. <br><br>Work was done in collaboration with T. O. Strandberg and A. H. MacDonald. <br><br>[1] T. O. Strandberg, C. M. Canali, A. H. MacDonald, Phys. Rev. B 80, 024425, (2009).

Pure spin decoherence in quantum dots

• Yuli Lyanda-Geller

Room: 132:028

Spin and momentum in 1D semiconductor nanostructures

• Michael Pepper

Room: 132:028 In this lecture a review of work performed with K.J. Thomas, T.M. Chen, A.C.Graham,D.A. Ritchie, I. Farrer will be presented on the role of momentum in determining the transport properties of electrons in 1D nanostructures formed from the GaAs-AlGaAs heterostructure. It will be shown that by applying a source-drain voltage the energy bands corresponding to the two directions of momentum can be manipulated, (1). This technique gives considerable understanding of the spin splitting in the channel which seems to occur spontaneously, allowing measurements of such properties as an interaction enhanced g value, (2-4). When electrons move in one direction only they adopt a ferromagnetic configuration, a variety of experiments will be described which illustrate this effect, including the coherence length of the spin polarisation, (5). <br><br>1. “Nonlinear transport in a single-mode one-dimensional electron gas.” Thomas K. J., Nicholls J. T., Simmons M. Y., Pepper M., Mace D. R., Ritchie D. A., Phil. Mag. B, 77,1213-1218. 1998. <br>2. “Direct observation of non-equilibrium spin populations in quasi-one-dimensional nanostructures” Chen T. -M., Graham A. C., Pepper M., Farrer I., Ritchie D. A., Anderson D. and Jones G. A. C., 2010, Nano Lett., 10, 2330–2334. <br>3. “Odd-even spin effects and variation of g factor in a quasi-one-dimensional subband”, Chen T-M., Graham A. C., Pepper M., Sfigakis F., Farrer I. and Ritchie D. A., 2009, Phys. Rev. B, 79, 081301 – 4 <br>4. “Bias-controlled spin polarization in quantum wires” Chen T-M., Graham A. C., Pepper M., Farrer I. and Ritchie D. A., Appl. Phys. Lett., 93, 032101, 2008 <br>5. “All-electrical injection and detection of a spin polarized current using a 1D conductor”, T.-M. Chen, M. Pepper, I. Farrer, G. A. C. Jones, D. A. Ritchie, arXiv:1202.5606, 2012

Local Rashba spin-orbit in dc and ac biased quantum wires

• Almas Sadreev

Room: 132:028

Rashba spin-orbit interaction in a quantum wire superlattice

• Sigurdur Erlingson

Room: 132:028 We study the effects of a longitudinal periodic potential on a parabolic quantum wire defined in a two-dimensional electron gas with Rashba spin-orbit interaction. For an infinite wire superlattice we find that the energy gaps are shifted away from the usual Bragg planes due to the Rashba spin-orbit interaction. We have also calculated the charge conductance through a periodic potential of a finite length via the nonequilibrium Green's function method combined with the Landauer formalism. We find dips in the conductance that correspond well to the energy gaps of the infinite wire superlattice. From the infinite wire energy dispersion, we derive an equation relating the location of the conductance dips as a function of the Fermi energy to the Rashba spin-orbit coupling strength. We propose that the strength of the Rashba spin-orbit interaction can be extracted via a charge conductance measurement.

Spin currents in a cold exciton gas

• Leonid Butov

Room: 132:028 An indirect exciton is a bound pair of an electron and a hole confined in spatially separated layers. Due to their long lifetimes, indirect excitons can cool down below the temperature of quantum degeneracy. This gives an opportunity to study cold exciton gases. We will present spontaneous coherence and condensation, phase singularities, spatial ordering, and spin currents in a cold exciton gas.

Optical transitions of excitons in a quantum well with Rashba spin-orbit coupling

• Vadim Kovalev

Room: 132:028 Interband and intraband optical transitions in a quantum well (QW) with structure inversion asymmetry (SIA) is theoretically studied. We pay main attention to the effect of simultaneously accounting for Coulomb interaction between electrons and holes created by the light and their spin-orbit (SO) coupling due to SIA of the QW. Such a spin-orbit interaction is referred in the literature as a Rashba spin-orbit term. We found the probability transitions both for light exciton and heavy exciton. It is shown that their behavior under the influence of the light is different due to the distinction of Rashba spin-orbit Hamiltonians of heavy and light holes. We also found that Rashba SO coupling strongly modifies the selection rules for intraband transitions .

Magnetic semiconductors

• Jose Pablo Baltanas

Room: 132:028 Magnetic semiconductors are hybrid systems in which magnetic impurities couple indirectly via a bath of itinerant spin carriers. The density of magnetic impurities can be modulated along the growth direction of the semiconductor to build periodic magnetic structures at the nanoscale, where quantum effects are relevant. Here, we present a theory of collective spin excitations in low-dimensional magnetic-semiconductor superlattices with a view to spin-wave band design for magnonics, and the possible development of interfaces with (spin-) electronic systems.

Universality in Kondo effect

• Luiz Oliveira

Room: 132:028 The last decade saw remarkable advances in (i) the fabrication of such elementary nanostructured devices as the single-electron transistor and (ii) the measurement of their transport properties. Parallel theoretical progress having offered clearer understanding of the universal functions describing such properties in the Kondo regime, insight can be gained from detailed quantitative comparisons between laboratory data and theoretical analyses. The talk will compare the experimental results of Grobis et al. [1], who systematically studied the temperature and gate-voltage dependences of the conductance of a single-electron transistor, with theoretical predictions based on the spin-degenerate Anderson model. It will be shown that a universal curve exactly derived under the assumption that the dot magnetic moment is partially screened at high temperatures fits all the data within the experimental accuracy. The physical implications of this assumption, which suggests that the dot moments couples anistropically to the neighboring electron gases, will be discussed. <br><br>[1] M. Grobis, I. G. Rau, R. M. Potok, H. Shtrikman, and D. Goldhaber-Gordon, Phys. Rev. Lett. 100, 246601 (2008).

Kondo physics in the presence of Rashba spin-orbit coupling

• Kevin Ingersent

Room: 132:028 The interplay between Kondo physics and spin-orbit interactions has puzzled physicists since the 1960s. Several recent studies have reached conflicting conclusions concerning the properties of a magnetic impurity in a two-dimensional electron gas with Rashba spin-orbit interactions. Subjects of controversy include (1) whether the Kondo physics is intrinsically "one-channel" or "two-channel" in character, with implications for the low-temperature properties; and (2) whether or not the Rashba coupling significantly alters the Kondo temperature scale below which the impurity magnetic moment becomes collectively screened by delocalized electrons. This talk will resolve the above issues, differentiating conditions under which the Kondo scale is exponentially enhanced due to the presence of spin-orbit interactions from cases where the Rashba coupling has negligible effect. Possible experimental probes of spin-orbit effects on Kondo physics will be discussed. <br><br>Work conducted with Arturo Wong with support from the NSF Materials World Network progam

Tunable spin and charge Seebek effects in magnetic molecular junctions

• Pablo S. Cornaglia

Room: 132:028 The increasing interest in the thermoelectric properties of materials and the quest for high Seebeck coefficients is motivated by the promise of more efficient solid state refrigerators and the conversion of waste heat into electricity. The Seebeck effect refers to the generation of a charge current (or a voltage drop) by a temperature gradient applied across a metal, and the spin-Seebeck effect, concerns the thermal generation of pure spin currents. The recent experimental observation of the Seebeck effect in different nano-structures, in particular in molecular junctions and quantum dots, opened new routes to study these phenomena.<br><br> On the one hand, the high sensitivity of these systems to external fields, their scalability and tunability make them potential candidates for a variety of technological applications. On the other hand, thermoelectric and thermomagnetic effects provide a unique probe of electron correlation effects and are a useful tool to gain further insight on fundamental problems like the Kondo regime where the energy transfer is dominated by spin fluctuations. <br><br>I'll present results for the charge and spin Seebeck effects of a spin-1 molecular junction as a function of temperature (T), applied magnetic field (H), and molecular magnetic anisotropy (D) obtained using Wilson's numerical renormalization group [1]. A hard-axis magnetic anisotropy produces a large enhancement of the charge Seebeck coefficient Sc (~ kB/|e|) whose value only depends on the residual interaction between quasiparticles in the low temperature Fermi-liquid regime. In the underscreened spin-1 Kondo regime, the high sensitivity of the system to magnetic fields makes it possible to obtain a sizable value for the spin Seebeck coefficient even for magnetic fields much smaller than the Kondo temperature. Similar effects can be obtained in C60 junctions where the control parameter is the gap between a singlet and a triplet molecular state. <br><br>I'll also discuss the thermoelectric properties of an SU(4) Kondo resonance, that describes the low temperature transport through clean C nanotubes [2]. <br><br>[1] Pablo S. Cornaglia, G. Usaj, and C. A. Balseiro, Phys Rev. B Rapid Communications (to appear). <br>[2] P. Roura-Bas, L. Tossi, A. A. Aligia, and Pablo S. Cornaglia (submitted).

Optical orientation and spin dephasing in high-mobility quantum wells

• Sergey Tarasenko

Room: 132:028 An overview of recent theoretical and experimental studies of optical orientation and spin dephasing of electrons in n-type quantum well structures is presented. It is shown that the electron spin dynamics in high-mobility quantum wells drastically differs from that in low-mobility structures and is highly sensitive to the quantum well crystallographic orientation, details of scattering, and external fields. We discuss the dynamic coupling of the in-plane and out-of-plane spin components caused by scattering asymmetry [1], the anomalous behavior of the Hanle effect in high-mobility quantum wells [2], spin orientation by linearly polarized optical pulses [3] and spin dephasing and diffusion [4] in (110)-grown quantum wells. <br><br>[1] A.V. Poshakinskiy and S.A. Tarasenko, Phys. Rev. B 84, 155326 (2011). <br>[2] A.V. Poshakinskiy and S.A. Tarasenko, Phys. Rev. 84, 073301 (2011). <br>[3] V.A. Gorelov, S.A. Tarasenko, and N.S. Averkiev, JETP 113, 873 (2011). <br>[4] R. Völkl, M. Griesbeck, S.A. Tarasenko et al., Phys. Rev. B 83, 241306(R) (2011).

Electron g-factor anisotropy in semiconductor quantum wells

• Antonio Ferreira da SIlva

Room: 132:028 In view of the g-factor tuning for spin manipulation in semiconductor nanostructures, the electron effective g-factor in GaAs and other III-V semiconductor quantum wells (QWs) (g*QW) have been much investigated; however, for example, the basic anisotropy Δg*QW (i.e. the difference between in-plane and perpendicular g*QW) is still not well understood. Such anisotropy was predicted by Ivchenko and Kiselev [1] and observed with spin quantum beat measurements [2, 3]; but in the well width dependence there is not yet a very good agreement between theory and experiment. <br><br>We present a simple and accurate expression and show different results for the effective electron g-factor in III-V symmetric QWs in good agreement with the experiments. The theory uses standard envelope function approximation based on [4] and represents an alternative solution with respect to that by Ivchenko and Kiselev [1]. The quantum confinement effects in g*QW are easily understood and in particular the anisotropy Δg*QW is shown to be given by a mesoscopic interface spin-orbit (SO) contribution, as the Rashba one. <br><br>An effective Hamiltonian for the conducting electrons in the QW plus an external magnetic field is introduced in which the Rashba and the effective Zeeman terms appear on an equal footing. The expression for the band-edge g*QW (as a function of the well width, plus the band off-set and the bulk parameters in the Kane model) is then easily obtained with first-order perturbation theory. <br>The extension of the theory to general structures is also discussed. The obtained effective Hamiltonian and expression for g*QW should be of help in the electron g-factor tuning, and in other spintronic problems. <br><br>[1] E. L. Ivchenko and A. A. Kiselev, Sov. Phys. Semicond. 26, 827 (1992). <br>[2] P. Le Jeune et al., Semicond. Sci. Technol. 12, 380 (1997). <br>[3] A. Malinowski and R. T. Harley, Phys. Rev. B 62, 2051 (2000). <br>[4] E. de Andrada e Silva, G. La Rocca, and F. Bassani, Phys. Rev. B 55, 16293 (1997).

Closing of workshop Room: 132:028

Kondo effect in mesoscopic and nanoscopic devices - I

• Pablo S. Cornaglia

Room: 132:028

Spin-related phenomena in quantum microcavities - I

• Timothy C. H. Liew

Room: 132:028

Kondo effect in mesoscopic and nanoscopic devices - II

• Pablo S. Cornaglia

Room: 132:028

Spin-related phenomena in quantum microcavities - II

• Timothy C. H. Liew

Room: 132:028

Kondo effect in mesoscopic and nanoscopic devices - III

• Pablo S. Cornaglia

Room: 132:028

Spin-related phenomena in quantum microcavities - III

• Timothy C. H. Liew

Room: 132:028

Kondo effect in mesoscopic and nanoscopic devices - IV

• Pablo S. Cornaglia

Room: 132:028

Spin-related phenomena in quantum microcavities - IV

• Timothy C. H. Liew

Room: 132:028

Spintronic materials

• Olle Eriksson

Room: 132:028 These introductory lectures wi1 tentavely cover; <br>(A) Magnetic interactions in general,magnetisk (super exchnge, direct exchange, RKKY, double exchange) (super-exchange, direkt exchange, RKKY, double exchange), <br>(B) DFT with applications, <br>(C) Spin-orbit interaction abd magnetic anistotropy, <br>(D) Atomistic spin-dynamics with application nano-systems.

Closing of entire programme Room: 132:028