Browsing by Author "Zhang, Jie"

Now showing 1 - 6 of 6
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    Microwave photocurrent from the edge states of InAs/GaInSb bilayers
    (American Physical Society, 2018) Zhang, Jie; Li, Tingxin; Du, Rui-Rui; Sullivan, Gerard
    Motivated by the recent low-temperature experiments on bulk FeSe, we study the electron correlation effects in a multiorbital model for this compound in the nematic phase using the U(1) slave-spin theory. We find that a finite nematic order helps to stabilize an orbital selective Mott phase. Moreover, we propose that when the d- and s-wave bond nematic orders are combined with the ferro-orbital order, there exists a surprisingly large orbital selectivity between the xz and yz orbitals even though the associated band splitting is relatively small. Our results explain the seemingly unusual observation of strong orbital selectivity in the nematic phase of FeSe, uncover new clues on the nature of the nematic order, and set the stage to elucidate the interplay between superconductivity and nematicity in iron-based superconductors.
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    Microwave Spectroscopy on Two Dimensional Electron/Hole Gases
    (2018-06-04) Zhang, Jie; Du, Rui-Rui
    We develop new techniques to explore how electrons and holes in 2D semiconductors behave under microwave radiation in a low temperature regime. To observe cyclotron resonance (CR) in GaAs/AlGaAs quantum wells, thermal methods with the sample and thermometer sealed in a vacuum can in a 3He environment and reflection spectroscopy via power sensing are developed. This configuration has high sensitivity and can detect CR with only 10nW of microwave power. It obviates the need for sampling thinning/wedging which is required in conventional transmission spectroscopy and hence preserves sample quality. With these advantages, we are able to detect narrow CR peaks with a small full width at half maximum (FWHM). Carrier effective masses are measured in various two dimensional electron gases (2DEGs) and two dimensional hole gases (2DHGs). Transport scattering time and single particle relaxation time are two important time scales that can be extracted from this measurement. The ratio of these times can be used as an indication of scattering angle scale limited by carrier mobility. The most exciting feature of this measurement is that a multi-photon phenomenon is observed for the first time in this system other than when using photoresistance measurements. This is made possible due to the use of millimeter waves, as opposed to the more traditional terahertz and infrared regimes used in earlier configurations. We further investigate the density dependence of the observable order of the multi-photon transitions and find that it is more prominent in higher density samples. By simultaneously measuring microwave reflectance and electrical resistance, we find that the plasmon-coupled CR mode is only present in the optical signal, not in the electrical. We attribute this discrepancy to the difference in the ability to pick up the corresponding signal in the scattering process. Though experimental data is very convincing, theoretical explanation is still needed to account for these phenomena. We also analyzed the photoresistance of the InAs/GaSb inverted bilayer. This system is known for its marvelous helical edge state in the quantum spin Hall effect (QSHE). By tuning the back gate, the Fermi level can be positioned in the bulk gap where the conductivity only comes from the edge state. This is verified by taking measurements on a Corbino disk where the edge state is shunted. We find that the photocurrent is prohibited in the charge neutral point (CNP) due to the lack of spin flipping mechanisms. A finite magnetic field provides a source for this process from which we see an enhancement of the photocurrent. Further effort in observing gap opening and resonance with the Zeeman energy needs to be made to understand this interesting system. Therefore, better wafer quality is strongly desired.
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    Numerical Modeling of the Formation and Evolution of Basement-Involved Structures in Wyoming
    (2014-01-21) Zhang, Jie; Morgan, Julia K.; Anderson, John B.; Akin, John Edward.; Dugan, Brandon; Sawyer, Dale S.
    The Wyoming foreland is composed of basement-involved structures and intermontane basins formed during the Laramide Orogeny. Based on their sizes, structures in this area can be categorized into primary uplifts and secondary folds. Tectonic models suggest the primary uplifts form by sliding the crustal slabs along a deep-seated, large-scale regional detachment in the lower crust, and rotating the basement wedges along listric primary faults. The secondary folds are located close to and trend sub-parallel to the adjacent primary structures, suggesting a causative or correlative relationship between the two, although this connection has not been firmly established through field and seismic investigations. I carry out numerical simulations using both the finite element method (FEM) and discrete element method (DEM) to explore the structural evolution of these secondary basement-involved structures. The first study investigates the Laramide-age Sheep Mountain anticline, located in the eastern Bighorn Basin of Wyoming, using comparative FEM and DEM simulations. The kinematic and mechanical results of the two simulations are similar, thus verifying the methodological comparison. Differences in the geometric details, however, provide important perspectives on the capabilities of the two methods. The mechanical properties defined through this comparative study are then employed in DEM simulations that investigate the relationships between primary and secondary structures during the displacement of large crustal slabs along primary thrust faults. My results show that the displacements and geometries of the primary faults have great impact on the distributions and throw values of the secondary faults. For shallow primary faults with limited regional shortening, the numbers and the displacements of secondary faults are evenly distributed across the basin, with no preference in dip direction. For steep primary faults with significant regional shortening, conjugate faults form early and subsequently cluster into groups. I also explore the influences of initial sedimentary thickness, sedimentary mechanical stratigraphy, and syn-tectonic sedimentation on the distribution of secondary faults. Thicker Pre-Laramide deposits allow more secondary faults to form early during deformation, absorbing the horizontal shortening within the sedimentary layer. The presence of weak shale layers in the sedimentary section allows numerous small faults to form, and limits the depth of all the faults. Syn-tectonic sedimentation reduces the number of secondary faults that form in the basinal area, and displacements along those faults are very small. In this case, most of the deformation is accommodated by the faults located above the ramp take-off location, at the edge of the syn-tectonic deposits.
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    Production, properties and purification of carbon nanotubes
    (1995) Zhang, Jie; Smalley, Richard E.
    The production, properties and purification of the novel carbon nanotubes have been systematically studied. A fully automatic electric arc carbon nanotube generator is designed and constructed. The state of the electric arc is monitored and adjusted by a computer to maintain a stable growth condition. The optimum conditions for the high yield and quality of carbon nanotubes were found in this apparatus. Nanotubes grown in the arc are found to be highly defective. These defects are ascribed to the tube-tube sintering due to the excessive heat in the arc. This sintering effect can be reduced through the use of a better water-cooled cathode but can't be eliminated. A method of purification of arc grown nanotubes was developed and was found to be highly effective. A growth model is also proposed based on properties of carbon nanotubes and the electric arc in which they were grown.
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    Pure and binary associating fluids near active surfaces
    (2002) Zhang, Jie; Nordlander, Peter J.
    The properties of associating fluids in contact with adsorbing surfaces are essential for the control of many processes of current industrial and scientific interest. Potential applications are diverse. Despite the need for a molecular understanding of interfacial properties, only in the past a few years have simple, accurate theories been developed for even simple fluids. Our group has developed a new density functional theory which applies the weighting from Tarazona's hard sphere density functional theory to Whertheim's bulk first-order perturbation theory to investigate inhomogeneous pure associating fluids confined between hard walls. This theory has been shown to be in good agreement with computer simulation results. In this work, we extend this promising theory to binary mixtures of hard spheres and associating fluids, and apply the mixture theory to non-additive hard spheres. We have accurately predicted phase separation, critical temperature and structural properties for this model. We then extend this theory to describe the properties of associating fluids near active surfaces. Metropolis Monte Carlo computer simulations are performed for one-sited (dimerizing), two-sited (linear chain forming) and four-sited (cluster forming) molecules near active surfaces. Our theory compares favorably with simulation results for a wide range of fluid density and bonding energy for surface coverage, density profiles and faction of monomers.
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    Simultaneous measurements of microwave photoresistance and cyclotron reflection in the multiphoton regime
    (American Physical Society, 2018) Zhang, Jie; Du, Rui-Rui; Pfeiffer, L.N.; West, K.W.
    We simultaneously measure photoresistance with electrical transport and coupled plasmon-cyclotron resonance using microwave reflection spectroscopy in high-mobility GaAs/AlGaAs quantum wells under a perpendicular magnetic field. Multiphoton transitions are revealed as sharp peaks in the resistance and the cyclotron reflection on samples with various carrier densities. Our main finding is that plasmon coupling is relevant in the cyclotron reflection spectrum but has not been observed in the electrical conductivity signal. We discuss possible mechanisms relevant to reflection or dc conductivity signal to explain this discrepancy. We further confirm a trend that more multiphoton features can be observed using higher carrier density samples.