13 research outputs found
Nonequilibrium electron spectroscopy of Luttinger liquids
Understanding the effects of nonequilibrium on strongly interacting quantum
systems is a challenging problem in condensed matter physics. In dimensions
greater than one, interacting electrons can often be understood within
Fermi-liquid theory where low-energy excitations are weakly interacting
quasiparticles. On the contrary, electrons in one dimension are known to form a
strongly-correlated phase of matter called a Luttinger liquid (LL), whose
low-energy excitations are collective density waves, or plasmons, of the
electron gas. Here we show that spectroscopy of locally injected high-energy
electrons can be used to probe energy relaxation in the presence of such strong
correlations. For detection energies near the injection energy, the electron
distribution is described by a power law whose exponent depends in a continuous
way on the Luttinger parameter, and energy relaxation can be attributed to
plasmon emission. For a chiral LL as realized at the edge of a fractional
quantum Hall state, the distribution function grows linearly with the distance
to the injection energy, independent of filling fraction.Comment: 4+ pages, 3 figure
Tuning the Spin Hall Effect in a Two-Dimensional Electron Gas
We provide a theoretical framework for the electric field control of the
electron spin in systems with diffusive electron motion. The approach is valid
in the experimentally important case where both intrinsic and extrinsic
spin-orbit interaction in a two-dimensional electron gas are present
simultaneously. Surprisingly, even when the extrinsic mechanism is the dominant
driving force for spin Hall currents, the amplitude of the spin Hall
conductivity may be considerably tuned by varying the intrinsic spin-orbit
coupling via a gate voltage. Furthermore we provide an explanation of the
experimentally observed out-of-plane spin polarization in a (110) GaAs quantum
well
Crossover to the Anomalous Quantum Regime in the Extrinsic Spin Hall Effect of Graphene
Recent reports of spin-orbit coupling enhancement in chemically modified graphene have opened doors to studies of the spin Hall effect with massless chiral fermions. Here, we theoretically investigate the interaction and impurity density dependence of the extrinsic spin Hall effect in spin-orbit coupled graphene. We present a nonperturbative quantum diagrammatic calculation of the spin Hall response function in the strong-coupling regime that incorporates skew scattering and anomalous impurity density-independent contributions on equal footing. The spin Hall conductivity dependence on Fermi energy and electron-impurity interaction strength reveals the existence of experimentally accessible regions where anomalous quantum processes dominate. Our findings suggest that spin-orbit-coupled graphene is an ideal model system for probing the competition between semiclassical and bona fide quantum scattering mechanisms underlying the spin Hall effect
Quantum Diagrammatic Theory of the Extrinsic Spin Hall Effect in Graphene
We present a rigorous microscopic theory of the extrinsic spin Hall effect in disordered graphene based on a nonperturbative quantum diagrammatic treatment incorporating skew scattering and anomalous---impurity concentration-independent---quantum corrections on equal footing. The leading skew scattering contribution to the spin Hall conductivity is shown to quantitatively agree with Boltzmann transport theory over a wide range of parameters. Our self-consistent approach---where all topologically equivalent noncrossing diagrams are resummed---unveils that the skewness generated by spin--orbit-active impurities deeply influences the anomalous component of the spin Hall conductivity, even in the weak scattering regime. This seemingly counterintuitive result is explained by the rich sublattice structure of scattering potentials in graphene, for which traditional Gaussian disorder approximations fail to capture the intricate correlations between skew scattering and side jumps generated through diffusion. Finally, we assess the role of quantum interference corrections by evaluating an important subclass of crossing diagrams recently considered in the context of the anomalous Hall effect, the and diagrams [Ado et al., EPL 111, 37004 (2015)]. We show that diagrams---encoding quantum coherent skew scattering---display a strong Fermi energy dependence, dominating the anomalous spin Hall component away from the Dirac point. Our findings have direct implications for nonlocal transport experiments in spin--orbit-coupled graphene systems
Spin Hall effect in a 2DEG in the presence of magnetic couplings
It is now well established that the peculiar linear-in-momentum dependence of the Rashba (and of the Dresselhaus) spin-orbit coupling leads to the vanishing of the spin Hall conductivity in the bulk of a two-dimensional electron gas (2DEG). In this paper we discuss how generic magnetic couplings change this behaviour providing then a potential handle on the spin Hall effect. In particular we examine the influence of magnetic impurities and an in-plane magnetic field. We find that in both cases there is a finite spin Hall effect and we provide explicit expressions for the spin Hall conductivity. The results can be obtained by means of the quasiclassical Green function approach, that we have recently extended to spin-orbit coupled electron systems
Privacy-Preserving Federated Brain Tumour Segmentation
Due to medical data privacy regulations, it is often infeasible to collect
and share patient data in a centralised data lake. This poses challenges for
training machine learning algorithms, such as deep convolutional networks,
which often require large numbers of diverse training examples. Federated
learning sidesteps this difficulty by bringing code to the patient data owners
and only sharing intermediate model training updates among them. Although a
high-accuracy model could be achieved by appropriately aggregating these model
updates, the model shared could indirectly leak the local training examples. In
this paper, we investigate the feasibility of applying differential-privacy
techniques to protect the patient data in a federated learning setup. We
implement and evaluate practical federated learning systems for brain tumour
segmentation on the BraTS dataset. The experimental results show that there is
a trade-off between model performance and privacy protection costs.Comment: MICCAI MLMI 201