Browsing by Author "Zhang, Chi"
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Item 5/2 state in high election density gallium arsenide/aluminum gallium arsenide quantum well(2010) Zhang, Chi; Du, Rui-RuiThis Master of Science Thesis is concerned with electronic transport in the higher Landau levels (LL) in a two-dimensional electron system, where novel many-body electronic phases have been observed. Particular attention is paid to the even-denominator fractional quantum Hall states at LL filling factors 5/2 and 7/2, and the anisotropic states at 9/2 and 11/2. In a high electron density (n = 6.3 x 10 11cm-2), high mobility (mu = 1 x 107cm2/Vs) modulation-doped GaAs/Al0.24Ga 0.76As quantum well, we observed the nu = 5/2 quantum Hall plateau at a high magnetic field B = 10 T. In contrast to previous findings in a lower density system, electronic transport at nu = 9/2 and nu = 11/2 is essentially isotropic. Anisotropic transport at 9/2 and 11/2 can be induced by an in-plane magnetic field, B//. Depending on the B// direction, the nu = 5/2 diagonal resistances in a high B// either remain isotropic or become strongly anisotropic. Our data suggest a new regime for electronic transport in higher LLs.Item BEAST 2.5: An advanced software platform for Bayesian evolutionary analysis(Public Library of Science, 2019) Bouckaert, Remco; Vaughan, Timothy G.; Barido-Sottani, Joëlle; Duchêne, Sebastián; Fourment, Mathieu; Gavryushkina, Alexandra; Heled, Joseph; Jones, Graham; Kühnert, Denise; De Maio, Nicola; Matschiner, Michael; Mendes, Fábio K.; Müller, Nicola F.; Ogilvie, Huw A.; du Plessis, Louis; Popinga, Alex; Rambaut, Andrew; Rasmussen, David; Siveroni, Igor; Suchard, Marc A.; Wu, Chieh-His; Xie, Dong; Zhang, Chi; Stadler, Tanja; Drummond, Alexei J.Elaboration of Bayesian phylogenetic inference methods has continued at pace in recent years with major new advances in nearly all aspects of the joint modelling of evolutionary data. It is increasingly appreciated that some evolutionary questions can only be adequately answered by combining evidence from multiple independent sources of data, including genome sequences, sampling dates, phenotypic data, radiocarbon dates, fossil occurrences, and biogeographic range information among others. Including all relevant data into a single joint model is very challenging both conceptually and computationally. Advanced computational software packages that allow robust development of compatible (sub-)models which can be composed into a full model hierarchy have played a key role in these developments. Developing such software frameworks is increasingly a major scientific activity in its own right, and comes with specific challenges, from practical software design, development and engineering challenges to statistical and conceptual modelling challenges. BEAST 2 is one such computational software platform, and was first announced over 4 years ago. Here we describe a series of major new developments in the BEAST 2 core platform and model hierarchy that have occurred since the first release of the software, culminating in the recent 2.5 release.Item Enhancement of the v=5/2 Fractional Quantum Hall State in a Small In-Plane Magnetic Field(American Physical Society, 2012) Liu, Guangtong; Zhang, Chi; Tsui, D.C.; Knez, Ivan; Levine, Aaron; Du, R.R.; Pfeiffer, L.N.; West, K.W.Using a 50-nm-width ultraclean GaAs=AlGaAs quantum well, we have studied the Landau level filling factor v=5/2 fractional quantum Hall effect in a perpendicular magnetic field B 1:7 Tand determined its dependence on tilted magnetic fields. Contrary to all previous results, the 5=2 resistance minimum and the Hall plateau are found to strengthen continuously under an increasing tilt angle 0< <25 (corresponding to an in-plane magnetic field 060 , and the composite fermion series [2 þ p=ð2p 1Þ, p ¼ 1; 2] can be identified. Based on our results, we discuss the relevance of a Skyrmion spin texture at v= 5/2 associated with small Zeeman energy in wide quantum wells, as proposed by Wo´js et alItem Experimental studies of the fractional quantum Hall effect and the Wigner solids(2010) Zhang, Chi; Du, Rui-RuiThis Ph.D. thesis is composed of two parts: the first part is concerned with electron transport in the higher Landau levels (LL) in a two-dimensional electron system (2DES); the second part is focused on the Wigner Crystal in two-dimensional hole system (2DHS). We report on the high Landau-level (LL) magnetotransport (including tilt-fields) in a high purity modulation-doped GaAs/AlGaAs quantum Well (QW) with twice the electron density of standard samples. A quantized nu = 5/2 Hall plateau is observed at B ∼ 10 T, with an activation gap Delta 5/2 ∼ 125 mK; the plateau can persist up to a ∼ 25° tilt-field. This finding is discussed in the context of proposed Moore-Read Pfaffian (Pf) wave function (or Anti-Pfaffian (APf)) being possible ground states at 5/2. The tilted-fields induce background resistance at 5/2 that could be either isotropic or anisotropic, depending simply on in-plane magnetic field orientation with respect to the GaAs crystalline axis. Such data indicate a substantial coupling between the 5/2 collective phases and the GaAs crystal. In a high hole density (p = 2.0x1011 cm-2), high mobility (micro = lx106 cm2/V s) 20 nm wide GaAs/AlGaAs quantum well, we observed reentrant insulating phases around Landau-level filling factor nu = 1/5 and nu = 2/9 at very high magnetic fields. Previous experiments reported the reentrant insulating phases around the nu = 1/3 FQHE state in dilute 2DHS and around the nu = 1/5 FQH liquid in 2DES, respectively. It is rather interesting that our experimental results in the hole system look like the former electron results at low fields. Our T-dependent conductance measurements exhibit rather intriguing behaviors: the Arrhenius plot of the conductance (vs. 1/T) suggests a certain energy scale in the melting procedure of the solid phase. We also observed a large threshold for electric field in the differential conductance measurements, which is an indication of the sliding motion of Wigner solid driven by an external field. These phenomena are related to the depinning of Wigner solids under various magnetic fields. A possible explanation is that the effective pinning potential increases with the magnetic fields.Item Landau level crossing in a spin-orbit coupled two-dimensional electron gas(AIP Publishing, 2015) Wu, Xing-Jun; Li, Ting-Xin; Zhang, Chi; Du, Rui-RuiWe have studied experimentally theﾠLandau levelﾠ(LL) spectrum of a two-dimensionalﾠelectron gasﾠ(2DEG)ﾠin an In0.53Ga0.47As/InPﾠquantum wellﾠstructure by means of low-temperature magneto-transport coincidence measurement in vectorﾠmagnetic fields.ﾠIt is well known that LL crossing occurs in tiltedﾠmagnetic fieldsﾠdue to a competition between cyclotron energy andﾠZeeman effect.ﾠRemarkably, here we observe an additional type of level-crossing resulting from a competition between Rashba andﾠZeeman splittingﾠin a smallﾠmagnetic field,ﾠconsistent with theﾠtheoreticalﾠprediction for strongly spin-orbit coupledﾠ2DEG.Item Shifted Landau ladders and low field magneto-oscillations in high-mobility GaAs 2D hole systems(AIP Publishing, 2017) Zhang, Po; Wang, Jianli; Zhang, Chi; Du, Rui-Rui; Pfeiffer, L.N.; West, K.W.We present well-developed low-field magneto-resistance oscillations originating from zero-field spin splitting (ZFSS) of heavy holes in high mobility GaAs/AlGaAs quantum wells. This low field oscillation is 1/B-periodic and emerges before the onset of Shubnikov–de Haas oscillations. The effect can be explained by resonant scattering between two Landau ladders shifted by the ZFSS gap, which in turn can be measured by comparing with the hole cyclotron energy. A front gate is fabricated to tune the ZFSS and hence the oscillation period.Item Thermopower and Nernst measurements in a half-filled lowest Landau level(American Physical Society, 2018) Liu, Xiaoxue; Li, Tingxin; Zhang, Po; Pfeiffer, L.N.; West, K.W.; Zhang, Chi; Du, Rui-RuiMotivated by the recent proposal by Potter et al. [Phys. Rev. X 6, 031026 (2016)] concerning possible thermoelectric signatures of Dirac composite fermions, we perform a systematic experimental study of thermoelectric transport of an ultrahigh-mobility GaAs/AlxGa1−xAs two-dimensional electron system at filling factor v=1/2. We demonstrate that the thermopower Sxx and Nernst Sxy are symmetric and antisymmetric with respect to B=0 T, respectively. The measured properties of thermopower Sxx at v=1/2 are consistent with previous experimental results. The Nernst signals Sxy of v=1/2, which have not been reported previously, are nonzero and show a power-law relation with temperature in the phonon-drag dominant region. In the electron-diffusion dominant region, the Nernst signals Sxy of v=1/2 are found to be significantly smaller than the linear temperature dependent values predicted by Potter et al., and decreasing with temperature faster than linear dependence.Item Transport of a sliding Wigner crystal in the four flux composite fermion regime(American Physical Society, 2015) Zhang, Chi; Du, Rui-Rui; Manfra, M.J.; Pfeiffer, L.N.; West, K.W.In two-dimensional (2D) electron systems, Wigner crystals (WC) and fractional quantum Hall effect (FQHE) liquids are competing ground states under low temperatures (T) and high magnetic fields (B). Here we report differential conductivity results demonstrating the reentrant insulating phase around ν=1/5 in a 2D hole system in AlGaAs/GaAs quantum wells and unexpected features in the solid-liquid phase transition between WC and FQHE liquids in ultrahigh magnetic fields up to 45 T. Remarkably, the electric field (E) plays an equivalent role as the temperature does in our phase diagram. From the E−T “duality” analysis, a characteristic length of 450 nm is derived, which can be understood as the phase-coherent domain size of WC. Moreover, evidence shows that with weak disorder the insulating phase and composite fermion liquid could be coexisting around ν= 1/5, pointing to the possibility that the insulating phase is the four flux quantum Wigner crystal, as proposed by theories.Item Tuning Edge States in Strained-Layer InAs/GaInSb Quantum Spin Hall Insulators(American Physical Society, 2017) Du, Lingjie; Li, Tingxin; Lou, Wenkai; Wu, Xingjun; Liu, Xiaoxue; Han, Zhongdong; Zhang, Chi; Sullivan, Gerard; Ikhlassi, Amal; Chang, Kai; Du, Rui-RuiWe report on a class of quantum spin Hall insulators (QSHIs) in strained-layer InAs / GaInSb quantum wells, in which the bulk gaps are enhanced up to fivefold as compared to the binary InAs / GaSb QSHI. Remarkably, with consequently increasing edge velocity, the edge conductance at zero and applied magnetic fields manifests time reversal symmetry-protected properties consistent with the Z 2 topological insulator. The InAs / GaInSb bilayers offer a much sought-after platform for future studies and applications of the QSHI.Item Tuning the charge states in InAs/GaSb or InAs/GaInSb composite quantum wells by persistent photoconductivity(AIP Publishing, 2017) Tong, Bingbing; Han, Zhongdong; Li, Tingxin; Zhang, Chi; Sullivan, Gerard; Du, Rui-RuiWe have experimentally studied the persistent photoconductivity (PPC) in inverted InAs/GaSb and InAs/GaInSb quantum wells, which can be tuned into a bulk-insulating state by electron-hole hybridization. Specifically we tune the bulk band structure and carriers with light-emitting diode (LED) illuminations. The persistent photoconductivity could be negative or positive, depending on the specific doping structure and the illuminating photon energy. Compared to the widely-used electro-statically gating method, our findings provide a more flexible and non-invasive way to control the band structures and charge states in InAs/GaSb and InAs/GaInSb quantum wells (QWs).