Dynamic nonlinear behavior is reported at high currents in the quantum Hall regime of GaAs heterostructures, resulting from breakdown of the dissipationless current flow. It is demonstrated that this breakdown is spatially localized and transient switching is observed on microsecond time scales among a set of distinct dissipative states. A simple macroscopic picture is proposed to account for these novel phenomena.PACS numbers: 73.40.Lq, 72.20.Ht, 72.20.My, 72.70. + m The quantum Hall effect 1 ' 2 is of great import for both many-body physics and fundamental metrology. The extreme accuracy with which the Hall resistance is quantized, despite the presence of disorder in the inversion-layer devices, is now fairly well understood as being due to the nearly complete freedom from dissipation in the quantized Hall regime. However, the nature of the localized states in a high magnetic field, the role of finite electric fields, and the nature of various dissipative effects remain poorly understood. Ebert etal. 3 have recently discovered that there is a critical current density above which the dissipation suddenly rises by several orders of magnitude. We report in this Letter unexpected new phenomena associated with this breakdown. We show that the breakdown is spatially localized and exhibits a rich time-dependent structure. In addition to a strong background of broadband noise we observe transient switching on a microsecond time scale among a discrete set of distinct dissipative states. Our observations demonstrate the significance of this breakdown phenomenon and provide a deeper understanding of the novel transport properties associated with the quantum Hall effect.Two high-quality GaAs-Ga^Al^As (# = 0.29) heterojunction devices [hereafter referred to as GaAs(7) and GaAs (8)] were used in this study. Both devices have zero-magnetic-field mobilities in excess of 10 5 cm 2 /(V s) at 4.2 K, and at 1.1 K yield excellent 6453.2-£2 (i =4) Hall steps that are flat and reproducible to at least 0.02 ppm. Figure 1 gives the sample geometry and displays the current dependence of the Hall resistance R H = (V 3 -V 4 )// SD and the dissipative voltage V x = V 2 -V 4 (at its minimum) for GaAs (7). Table I shows that V x changes by 7 orders of magnitude between / SD =25 and 370 juA and becomes as large as one-tenth of the Hall voltage V H while (as shown in Fig. 1) the value of JR H decreases by only 0.1 ppm! Similarly the other Hall-probe resistance R H f = (^I-^ASD decreases by only 0.6 ppm. These changes in R u are -0.01% of what is expected from the mixing of V x into V H due to the known misalignment of the Hall probes (3 rel-1374

No abstract

Despite considerable research efforts, pancreatic cancer is associated with a dire prognosis and a 5-year survival rate of only 10%. Early symptoms of the disease are mostly nonspecific. The premise of improved survival through early detection is that more individuals will benefit from potentially curative treatment. Artificial intelligence (AI) methodology has emerged as a successful tool for risk stratification and identification in general health care. In response to the maturity of AI, Kenner Family Research Fund conducted the 2020 AI and Early Detection of Pancreatic Cancer Virtual Summit ( www.pdac-virtualsummit.org ) in conjunction with the American Pancreatic Association, with a focus on the potential of AI to advance early detection efforts in this disease. This comprehensive presummit article was prepared based on information provided by each of the interdisciplinary participants on one of the 5 following topics: Progress, Problems, and Prospects for Early Detection; AI and Machine Learning; AI and Pancreatic Cancer—Current Efforts; Collaborative Opportunities; and Moving Forward—Reflections from Government, Industry, and Advocacy. The outcome from the robust Summit conversations, to be presented in a future white paper, indicate that significant progress must be the result of strategic collaboration among investigators and institutions from multidisciplinary backgrounds, supported by committed funders.

The fine-structure constant a has been determined from precision measurements of quantized Hall resistances Ru of three different GaAs-Al^Gai-^As heterostructures. The result, a" 1 = l37.035968(23) (0.17 ppm), isinexeellentagreementwiththeO.ilppm value obtained from the gyromagnetic ratio of the proton, y p ', and 2e/h via the Josephson effect. Our RH value can be combined with y p ' and 2e/k to yield a more accurate value of a' 1 independent of the ohm: a" i = 137.035 965(12) (0.089 ppm).PACS numbers: 06.20. Jr, 06.30.Lz, 72.20.My, 73.40.Lq The work of von Klitzing, Dorda, and Pepper 1 first demonstrated the possibility that measurements of quantized Hall resistances R H of the two-dimensional (2D) electron gas formed in certain solid-state devices could be used to obtain a precise value for the fine-structure constant a. This possibility was subsequently verified to an accuracy of 1.3 ppm by Braun, Staben, and von Klitzing, 2 and to 0.88 ppm by Yamanouchi etal. 3 All these experiments used silicon metal-oxidesemiconductor field-effect transistors (MOSFETs), which require a magnetic field B~ 13 T to reach the high-field quantization regime. Since the effective mass of the electrons in the 2D gas in GaAs formed at the interface of GaAs-Al^Ga^As heterojunctions is three times less than in Si, this regime can be reached with B < 10 T. In this Letter we report the first precision measurements of R H in this 2D electron system. Our result gives the most accurate determination of a from any quantized Hall resistance experiment to date.In GaAs-ALcGa^As heterojunctions, the quantized Hall effect is observed as a series of flat steps in plots of R H as a function of B. 4 At a step, the Fermi energy is between two Landau levels, with all the conducting states filled in the lower level and empty in the upper level. The quantized Hall resistance (the ratio between the Hall voltage across the sample and the current /) is given bywhere /i 0 =47T x 10" 7 H/m is the permeability of vacuum; c =(299 792 458 ± 1.2) m/s (0.004 ppm) is the speed of light in vacuum 5 ; h is Planck's constant, e is the electron charge, and the quantum number i is the number of completely filled Landau levels.The GaAs-Al^Ga^As (pc =0.29) heterostructure 3