arXiv subjects

Q. Shi

Publications and source records attributed to Q. Shi.

32 records · Page 2Linked to original sources

arXiv2017

Hall field-induced resistance oscillations in MgZnO/ZnO heterostructures

We report on nonlinear magnetotransport in a two-dimensional electron gas hosted in a MgZnO/ZnO heterostructure. Upon application of a direct current, we observe pronounced Hall field-induced resistance oscillations (HIRO) which are well known from experiments on high-mobility GaAs/AlGaAs quantum wells. The unique sensitivity of HIRO to the short-range component of the disorder potential allows us to unambiguously establish that the mobility of our MgZnO/ZnO heterostructure is limited by impurities residing within or near the 2D channel. Demonstration that HIRO can be realized in a system with a much lower mobility, much higher density, and much larger effective mass than in previously studied systems, highlights remarkable universality of the phenomenon and its great promise to be used in studies of a wide variety of emerging 2D materials.

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arXiv2016

Evidence for a new symmetry breaking mechanism reorienting quantum Hall nematics

We report on the effect of in-plane magnetic field $B_\parallel$ on stripe phases in higher ($N=2,3$) Landau levels of a high-mobility 2D electron gas. In accord with previous studies, we find that a modest $B_\parallel$ applied parallel to the native stripes aligns them perpendicular to it. However, upon further increase of $B_\parallel$, stripes are reoriented back to their native direction. Remarkably, applying $B_\parallel$ perpendicular to the native stripes also aligns stripes parallel to it. Thus, regardless of the initial orientation of stripes with respect to $B_\parallel$, stripes are ultimately aligned \emph{parallel} to $B_\parallel$. These findings provide evidence for a $B_\parallel$-induced symmetry breaking mechanism which challenge current understanding of the role of $B_\parallel$ and should be taken into account when determining the strength of the native symmetry breaking potential. Finally, our results might indicate nontrivial coupling between the native and external symmetry breaking fields, which has not yet been theoretically considered.

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arXiv2016

Microwave Photoresistance in an Ultrahigh Quality GaAs Quantum Well

The temperature dependence of microwave-induced resistance oscillations (MIRO), according to the theory, originates from electron-electron scattering. This scattering affects both the quantum lifetime, or the density of states, and the inelastic lifetime, which governs the relaxation of the nonequilibrium distribution function. Here, we report on MIRO in an ultrahigh mobility ($μ> 3 \times 10^7$ cm$^2$/Vs) 2D electron gas at $T$ between $0.3$ K and $1.8$ K. In contrast to theoretical predictions, the quantum lifetime is found to be $T$-independent in the whole temperature range studied. At the same time, the $T$-dependence of the inelastic lifetime is much \emph{stronger} than can be expected from electron-electron interactions.

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arXiv2016

Reorientation of quantum Hall stripes within a partially filled Landau level

We investigate the effect of the filling factor on transport anisotropies, known as stripes, in high Landau levels of a two-dimensional electron gas. We find that at certain in-plane magnetic fields, the stripes orientation is sensitive to the filling factor within a given Landau level. This sensitivity gives rise to the emergence of stripes away from half-filling while an orthogonally-oriented, native stripes reside at half-filling. This switching of the anisotropy axes within a single Landau level can be attributed to a strong dependence of the native symmetry breaking potential on the filling factor.

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arXiv2016

Resistively detected high-order magnetoplasmons in a high-quality 2D electron gas

We report on high-order magnetoplasmon resonances detected in photoresistance in high-mobility GaAs quantum wells. These resonances manifest themselves as a series of photoresistance extrema in the regime of Shubnikov-de Haas oscillations. Extending to orders above 20, the extrema exhibit alternating strength, being less (more) pronounced at even (odd) order magnetoplasmon modes. This experimental technique provides sensitive and elegant means to detect and investigate multiple magnetoplasmon modes and could be applied to other systems.

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arXiv2015

Shubnikov-de Haas oscillations in two-dimensional electron gas under subterahertz radiation

We report on magnetotransport measurements in a two-dimensional (2D) electron gas subject to subterahertz radiation in the regime where Shubnikov-de Haas oscillations (SdHO) and microwave-induced resistance oscillations (MIRO) coexist over a wide magnetic field range, spanning several harmonics of the cyclotron resonance. Surprisingly, we find that the SdHO amplitude is modified by the radiation in a non-trivial way owing to the oscillatory correction which has the same period and phase as MIRO. This finding challenges our current understanding of microwave photoresistance in 2D electron gas, calling for future investigations.

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arXiv2015

Strong Transport Anisotropy in a Ge/SiGe Quantum Well in Tilted Magnetic Fields

We report on a strong transport anisotropy in a 2D hole gas in a Ge/SiGe quantum well, which emerges only when both perpendicular and in-plane magnetic fields are present. The ratio of resistances, measured along and perpendicular to the in-plane field, can exceed $3\times 10^4$. The anisotropy occurs in a wide range of filling factors where it is determined {\em primarily} by the tilt angle. The lack of significant anisotropy without an in-plane field, easy tunability, and persistence to higher temperatures and filling factors set this anisotropy apart from nematic phases in GaAs/AlGaAs.

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arXiv2015

Spinless composite fermions in an ultra-high quality strained Ge quantum well

We report on an observation of a fractional quantum Hall effect in an ultra-high quality two-dimensional hole gas hosted in a strained Ge quantum well. The Hall resistance reveals precisely quantized plateaus and vanishing longitudinal resistance at filling factors $ν= 2/3, 4/3$ and $5/3$. From the temperature dependence around $ν= 3/2$ we obtain the composite fermion mass of $m^\star \approx 0.4\,m_e$, where $m_e$ is the mass of a free electron. Owing to large Zeeman energy, all observed states are spin-polarized and can be described in terms of spinless composite fermions.

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arXiv2015

Transport anisotropy in Ge quantum wells in the absence of quantum oscillations

Recent study of a high-mobility 2D hole gas in a strained Ge quantum well revealed strong transport anisotropy in the quantum Hall regime when the magnetic field was tilted away from the sample normal. In the present study we demonstrate that the anisotropy persists to such high temperatures and filling factors that quantum oscillations are no longer observed. This finding rules out the formation of a stripe phase as a possible origin for the observed anisotropy. However, we also show that the observed anisotropy is not consistent with other known anisotropies, such as those arising from finite thickness effects or surface roughness.

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arXiv2014

Nonlinear transport in 2D electron gas exhibiting colossal negative magnetoresistance

We report on nonlinear transport measurements in a GaAs/AlGaAs quantum well exhibiting a colossal negative magnetoresistance effect. Under applied dc bias, the magnetoresistance becomes nonmonotonic, exhibiting distinct extrema that move to higher magnetic fields with increasing current. In the range of magnetic fields corresponding to the resistivity minimum at zero bias, the resistivity increases linearly with current and the rate of this increase scales with the inverse magnetic field. The latter observation is consistent with the theory, proposed more than 35 years ago, considering classical memory effects in the presence of strong, dilute scatterers.

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arXiv2014

Observation of microwave-induced resistance oscillations in strained Ge/SiGe quantum wells

Microwave-induced resistance oscillations (MIRO) have been extensively studied for more than a decade but, until now, have remained unique to GaAs/AlGaAs-based 2D electron systems. Here, we report on the first observation of MIRO in a 2D hole gas hosted in Ge/SiGe quantum well. Our findings confirm that MIRO is a universal phenomenon and demonstrate that microwave photoresistance can be utilized to probe the energy spectrum and the correlation effects of 2D holes in Ge/SiGe quantum wells.

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arXiv2014

Colossal negative magnetoresistance in a 2D electron gas

We report on a colossal negative magnetoresistance (MR) in GaAs/AlGaAs quantum well which, at low temperatures, is manifested by a drop of the resistivity by more than an order of magnitude at a magnetic field $B \approx 1$ kG. In contrast to MR effects discussed earlier, the MR reported here is not parabolic, even at small $B$, and persists to much higher in-plane magnetic fields and temperatures. Remarkably, the temperature dependence of the resistivity at $B \approx 1$ kG is linear over the entire temperature range studied (from 1 to 30 K) and appears to coincide with the high-temperature limit of the zero-field resistivity, hinting on the important role of acoustic phonons.

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arXiv2014

Hall field-induced resistance oscillations in Ge/SiGe quantum wells

We report on a magnetotransport study in a high-mobility 2D hole gas hosted in a pure Ge/SiGe quantum well subject to dc electric fields and high frequency microwave radiation. We find that under applied dc bias the differential resistivity exhibits a pronounced maximum at a magnetic field which increases linearly with the applied current. We associate this maximum with the fundamental peak of Hall field-induced resistance oscillations (HIRO) which are known to occur in 2D electron gases in GaAs/AlGaAs systems. After taking into account the Dingle factor correction, we find that the position of the HIRO peak is well described by the hole effective mass $m^\star \approx 0.09\,m_0$, obtained from microwave photoresistance in the same sample.

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arXiv2013

Phase-sensitive bichromatic photoresistance in a two-dimensional electron gas

We have studied microwave photoresistance in a two-dimensional electron system subject to two radiation fields (frequencies $ω_1$ and $ω_2$) using quantum kinetic equation. We have found that when $ω_2/ω_1= 1 + 2/N$, where $N$ is an integer, and both waves have the same polarization, the displacement mechanism gives rise to a new, phase-sensitive photoresistance. This photoresistance oscillates with the magnetic field and can be a good fraction of the total photoresistance under typical experimental conditions. The inelastic mechanism, on the other hand, gives zero phase-sensitive photoresistance if the radiation fields are circularly polarized.

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