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Knez I.,Rice University | Knez I.,IBM | Du R.-R.,Rice University | Sullivan G.,Teledyne Scientific and Imaging
Physical Review Letters | Year: 2012

We present an experimental study of S-N-S junctions, with N being a quantum spin Hall insulator made of InAs/GaSb. A front gate is used to vary the Fermi level into the minigap, where helical edge modes exist. In this regime we observe a ∼2e2/h Andreev conductance peak, consistent with a perfect Andreev reflection on the helical edge modes predicted by theories. The peak diminishes under a small applied magnetic field due to the breaking of time-reversal symmetry. This work thus demonstrates the helical property of the edge modes in a quantum spin Hall insulator. © 2012 American Physical Society. Source


Knez I.,IBM | Rettner C.T.,IBM | Yang S.-H.,IBM | Parkin S.S.P.,IBM | And 3 more authors.
Physical Review Letters | Year: 2014

We observe edge transport in the topologically insulating InAs/GaSb system in the disordered regime. Using asymmetric current paths we show that conduction occurs exclusively along the device edge, exhibiting a large Hall signal at zero magnetic fields, while for symmetric current paths, the conductance between the two mesoscopicly separated probes is quantized to 2e2/h. Both quantized and self-averaged transport show resilience to magnetic fields, and are temperature independent for temperatures between 20 mK and 1 K. © 2014 American Physical Society. Source


Knez I.,Rice University | Du R.-R.,Rice University | Sullivan G.,Teledyne Scientific and Imaging
Physical Review Letters | Year: 2011

We present an experimental study of low temperature electronic transport in the hybridization gap of inverted InAs/GaSb composite quantum wells. An electrostatic gate is used to push the Fermi level into the gap regime, where the conductance as a function of sample length and width is measured. Our analysis shows strong evidence for the existence of helical edge modes proposed by Liu et al. Edge modes persist in spite of sizable bulk conduction and show only a weak magnetic field dependence-a direct consequence of a gap opening away from the zone center. © 2011 American Physical Society. Source


Knez I.,Rice University | Du R.R.,Rice University | Sullivan G.,Teledyne Scientific and Imaging
Physical Review B - Condensed Matter and Materials Physics | Year: 2010

We have studied experimentally the low-temperature conductivity of mesoscopic size InAs/GaSb quantum well Hall bar devices in the inverted regime. Using a pair of electrostatic gates we move the Fermi level into the electron-hole hybridization state, observing a mini gap and Van Hove singularity at its edge. Temperature dependence of the conductivity in the gap shows a residual conductivity, which can be consistently explained by the contributions from the free as well as the hybridized carriers in the presence of impurity scattering, as proposed by Naveh and Laikhtman, [Europhys. Lett. 55, 545 (2001)]. Experimental implications for the stability of proposed quantum spin Hall helical edge states will be discussed. © 2010 The American Physical Society. Source


Daneshgar S.,University of California at Santa Barbara | Daneshgar S.,University of California at San Diego | Griffith Z.,Teledyne Scientific and Imaging | Seo M.,Sungkyunkwan University | Rodwell M.J.W.,University of California at Santa Barbara
IEEE Journal of Solid-State Circuits | Year: 2014

We report 50 GSamples/s track-hold amplifier (THA) and sample-hold amplifier (SHA) designed and fabricated in a 250 nm InP double heterojunction bipolar transistor (DHBT) technology. Because the base-emitter junction reverse breakdown voltage is low in the process technology employed, the circuits use a base-collector junction diode as the switching element in the signal path. Operating with - 5 V and - 2.5 V supplies, the THA achieves > +16 dBm input-referred third-order intercept (IIP3) at signal frequencies below 22 GHz, while the SHA achieves IIP3> +17 dBm for 2-22 GHz inputs. © 1966-2012 IEEE. Source

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