Bulletin of the American Physical Society
2024 APS March Meeting
Monday–Friday, March 4–8, 2024; Minneapolis & Virtual
Session G17: Experimental Progress on the Fractional Quantum Hall Effect |
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Sponsoring Units: DCMP Chair: Ramesh Mani, Georgia State University Room: M100H |
Tuesday, March 5, 2024 11:30AM - 11:42AM |
G17.00001: Orbitally Controlled Quantum Hall State in Decoupled Two-Bilayer Graphene Sheets Soyun Kim, Dohun Kim, Kenji Watanabe, Takashi Taniguchi, Jurgen H Smet, Youngwook Kim We report on integer and fractional quantum Hall states in a stack of two twisted Bernal bilayer graphene sheets. By exploiting the momentum mismatch in reciprocal space, the single-particle tunneling between both bilayers is suppressed. Since the bilayers are spatially separated by only 0.34 nm, the stack benefits from strong interlayer Coulombic interactions. These interactions can cause the formation of a Bose–Einstein condensate. Indeed, such a condensate is observed for half-filling in each bilayer sheet. However, only when the partially filled levels have orbital index 1. It is absent for partially filled levels with orbital index 0. This discrepancy is tentatively attributed to the role of skyrmion/anti-skyrmion pair excitations and the dependence of the energy of these excitations on the orbital index. The application of asymmetric top and bottom gate voltages enables to influence the orbital nature of the electronic states of the graphene bilayers at the chemical potential and to navigate in orbital mixed space. The latter hosts an even denominator fractional quantum Hall state at total filling of −3/2. These observations suggest a unique edge reconstruction involving both electrons and chiral p-wave composite fermions. |
Tuesday, March 5, 2024 11:42AM - 11:54AM |
G17.00002: Emergent nematic phase in GaAs quantum wells Elliot Bell, Yoon Jang Chung, Kirk Baldwin, Kenneth W West, Loren N Pfeiffer, Michael A Zudov In high (N ≥ 2) half-filled Landau Levels (LLs), the competition between long-range repulsive and short-range attractive components of Coulomb interaction leads to compressible charge-ordered phases commonly known as quantum Hall stripes. It is also known that even-denominator fractional quantum Hall states in the N = 1 LL (ν = 5/2 and 7/2) can transform into stripe phases upon the application of an in-plane magnetic field. Here, we report on experimental evidence of another nematic phase which is stabilized in a very narrow range of tilt angles. While the width of this phase (in terms of filling factor) is similar to that of conventional stripes, about 0.2, it can be realized in a rather wide range of filling factors (between about 2 and 5/2). Interestingly, the phase is the most robust not near half-filling, but near ν = 7/3, where it competes with fractional quantum Hall effect. |
Tuesday, March 5, 2024 11:54AM - 12:06PM |
G17.00003: Fractional quantum Hall ferromagnetism and disappearing insulating phases near a Landau level crossing in the extreme quantum limit Casey S Calhoun, Chengyu Wang, Pranav Thekke Madathil, Loren N Pfeiffer, Kirk Baldwin, Mansour Shayegan When the two lowest Landau levels (LLs) in a two-dimensional (2D) carrier system cross, the resulting dominance of the exchange energy over the Zeeman energy can produce a ferromagnetic quantum Hall state (QHS). In the past, such crossings have been studied in 2D electrons in GaAs and AlAs by tuning the g factor or the valley splitting by applying hydrostatic pressure or uniaxial strain, respectively. GaAs 2D holes are unique thanks to their strong heavy-hole light-hole mixing and spin-orbit coupling which result in LLs that are highly nonlinear and exhibit numerous crossings as a function of magnetic field, including one between the two lowest-energy LLs with opposite pseudospins. It was previously demonstrated that such a crossing could be gate-tuned to occur at LL filling factor ν = 1 where a robust ferromagnetic QHS was observed [1]. In this work, we study the same crossing in the extreme quantum limit near ν = 1/3. When the crossing is gate-tuned to occur at ν = 1/3, we observe a clear and sharp dip in the measured energy gap for the ν = 1/3 fractional QHS. The 1/3 gap does not close at the crossing, signaling ferromagnetism of the fractional QHS. Additionally, we find that the re-entrant insulating phase between ν = 1/3 and 2/5 disappears near the crossing and re-appears when the crossing is tuned away. |
Tuesday, March 5, 2024 12:06PM - 12:18PM |
G17.00004: Metastable nematicity at ν = 7/2 in GaAs quantum wells mediated by nuclear spin polarization Ramon Guerrero-Suarez, James R Nakamura, John Watson, Saeed Fallahi, Geoffrey C Gardner, Michael J Manfra Isotropic fractional quantum Hall states at ν=5/2 and ν=7/2 in the N=1 Landau level may undergo a transition to compressible anisotropic nematic states as Hamiltonian parameters are varied. Here, we report the observation of a metastable anisotropic state at ν=7/2 that relaxes to an isotropic state depending on the degree of nuclear spin polarization in the GaAs quantum well. The two-dimensional electron gas (2DEG) resides in a 30 nm quantum well with density n = 1.05x1011 cm-2 and mobility μ = 18x106 cm2/Vs. For all temperatures down to T=10 mK, transport remains isotropic at ν=7/2 when sweeping the magnetic field from ν=4 to ν=3. However transport at ν=7/2 becomes highly anisotropic when sweeping from high magnetic field to lower values. With currently flowing along the [01-1] direction a strong resistance peak is observed while a local minima is observed for current flowing along the [011] direction. The anisotropy becomes stronger as the starting magnetic field for the sweep down is increased. The resistance peak along the [01-1] is also metastable. Once the field sweep is terminated, the resistance peak at ν=7/2 begins to decay with time. The field, temperature, and temporal dependence of this behavior indicate the impact of the nuclear spin polarization state. |
Tuesday, March 5, 2024 12:18PM - 12:30PM |
G17.00005: Setting new standards for mobility of 2D holes in GaAs quantum wells Adbhut Gupta, Chengyu Wang, Siddharth Kumar Singh, Kirk Baldwin, Roland Winkler, Mansour Shayegan, Loren N Pfeiffer Modulation-doped two-dimensional (2D) carrier systems hosted in GaAs quantum wells provide nearly ideal testing grounds for exploring exotic low-temperature many-body phenomena. While 2D electron systems have long been at the forefront for exploration of interaction-driven phenomena, 2D hole systems (2DHSs) offer an attractive alternative, owing to their large effective mass, strong and tunable spin-orbit coupling, and complex band-structure. For the first time, in a 2DHS in any material, we obtain peak mobility ≃10 x 106 cm2/Vs at density of only ≃3.8 x 1010 /cm2, at 300 mK which rises to ≃18 x 106 cm2/Vs (implying a mean-free-path ≃57 µm) when measured at 30 mK. We achieve this by optimizing the structure design by systematically adjusting the alloy fraction (x) of the AlxGa1-xAs barrier near the quantum well, and the width of the quantum well. Low-temperature magnetotransport data exhibit numerous delicate fractional quantum Hall states which have never been seen before at such low densities in a 2DHS. The achievement of mobilities exceeding 107 cm2/Vs in 2DHSs represents a significant leap, considering that the highest recorded mobilities, until two years ago, were only ≃2 x 106 cm2/Vs. Using transport calculations specific to our structures, we discern the scattering mechanisms that limit the mobility in our new samples. |
Tuesday, March 5, 2024 12:30PM - 12:42PM |
G17.00006: Anisotropic disordered Wigner solid in a two-dimensional electron system Md. Shafayat Hossain, Meng Ma, Kevin Villegas Rosales, Edwin Y Chung, Loren N Pfeiffer, Kenneth W West, Kirk Baldwin, Mansour Shayegan In a two-dimensional electron system, when Coulomb energy dominates over the kinetic energy, electrons should freeze into an ordered array known as Wigner solid. Its realization is challenging as it requires simultaneous low-density and high-quality. Here, we report transport measurements in a clean (low-disorder) two-dimensional electron system with anisotropic effective mass and Fermi sea. The data reveal that at extremely low electron densities, when the rs parameter, the ratio of the Coulomb to the Fermi energy, exceeds 38, the current-voltage characteristics become strongly nonlinear at small DC biases [1]. Several features of the nonlinear characteristics, including the voltage thresholds, are consistent with the formation of a Wigner solid pinned by the ubiquitous disorder potential. Notably, our data show the transference of effective mass anisotropy of the Fermi liquid to the pinned Wigner solid: we observe anisotropic resistance, activation gap, and de-pinning voltage in the Wigner solid phase. These observations point to an elusive state of matter, namely anisotropic Wigner solid. |
Tuesday, March 5, 2024 12:42PM - 12:54PM |
G17.00007: Tunneling spectroscopy in the weak backscattering limit at the ν = 1/3 FQHE Tanmay Maiti, James R Nakamura, Shuang Liang, Geoffrey C Gardner, Michael J Manfra Fractional quantum Hall edge modes are theoretically described as one-dimensional Luttinger liquids. Here we report measurements of tunneling spectroscopy in the weak backscattering regime at the Laughlin ν = 1/3 state. The experiment utilizes a quantum point contact (qpc) fabricated on AlGaAs/GaAs screening well heterostructures that have previously been shown to promote sharp edge confining potential. Differential conductance has been measured over a range of qpc transmission ranging from weak backscattering to strong backscattering. The temperature dependence of differential conductance was measured for several temperatures ranging from T=10mK to 250mK. Comparison of the data with theoretical predications for Luttinger liquid behavior at n=1/3 will be presented. |
Tuesday, March 5, 2024 12:54PM - 1:06PM |
G17.00008: Microwave activation studies of fractional quantum Hall effect Ramesh Mani, Annika Kriisa, Christian Reichl, Werner Wegscheider Fractional quantum Hall states are characterised by gap energies for quasiparticle-quasihole excitations and these gaps are typically measured from the temperature (T) dependence of the FQHE Rxx minima in the thermally activated regime. Here, we examine the possibility of determining these gaps by measuring the microwave power (P) variation of the Rxx minima with the specimen at base temperature, followed by a calibrated conversion of P to T. We note, that from the experimental point of view, there are some definite experimental advantages to such microwave based measurements over the conventional temperature dependent measurements: i) The applied microwave power can be controlled with great precision at the microwave source and, therefore, very small incremental changes in temperature appear possible at the sample with small power changes at the source, ii) The source microwave power (and therefore, in principle, the temperature) can be varied smoothly at the desired rate. iii) Since microwave induced heating occurs very locally at the specimen, rapid heating and cooling with small time constants can be realized very easily. In sum, it is a lot easier to measure, change, and control the microwave power than it is to change, actively control, and measure the temperature inside a dilution refrigerator. Thus, we examine the activation characteristics for a number of readily observable electronic FQHE states in GaAa/AlGaAs heterostructures characterized by n = 2 x 1011cm-2 and μ = 107cm2/Vs and report associated results. |
Tuesday, March 5, 2024 1:06PM - 1:18PM |
G17.00009: rf spectroscopy of Wigner solids in ultralow-disorder GaAs hosted two-dimensional hole systems in high magnetic fields alexander roubos, Adbhut Gupta, Lloyd W Engel, Chengyu Wang, Kirk Baldwin, Loren N Pfeiffer, Mansour Shayegan We study the high-magnetic-field Wigner solid (WS) phase in a new generation [1,2] of 2D hole systems (2DHS) in GaAs. The WS are invariably pinned by disorder in the semiconductor host and exhibit rf pinning modes, whose frequency measures the strength of disorder. In one sample, a 320MHz resonance is observed, significantly lower than in older-generation 2DHS with nearly identical quantum well width and density. Previous DC transport studies [3] demonstrated that 2DHS of similar density show a WS phase reentrant in the narrow region, 1/3 < ν < 2/5. We observe a pinning mode in this reentrant range, a feature not seen in the older, more disordered samples. Effects of higher-order fractional quantum Hall effect (FQHE) states on the WS are striking, with the pinning mode strongly depressed in frequency and amplitude at ν = 2/7. Partial suppression of the pinning mode due to a FQHE state is seen also for ν = 1/5, though only at elevated temperature. |
Tuesday, March 5, 2024 1:18PM - 1:30PM |
G17.00010: Highly-anisotropic even-denominator fractional quantum Hall state in an orbitally-coupled half-filled Landau level Chengyu Wang, Mansour Shayegan, Adbhut Gupta, Edwin Y Chung, Loren N Pfeiffer, Kirk Baldwin, Roland Winkler The even-denominator fractional quantum Hall states (FQHSs) in half-filled Landau levels are generally believed to host non-Abelian quasiparticles and be of potential use in topological quantum computing. Of particular interest is the competition and interplay between the even-denominator FQHSs and other ground states, such as anisotropic phases and composite fermion Fermi seas. Here we report the observation of an even-denominator FQHS with highly-anisotropic in-plane transport coefficients at Landau level filling factor ν = 3/2. We observe this state in an ultra-high-quality GaAs two-dimensional hole system when a large in-plane magnetic field is applied. By increasing the in-plane field, we observe a sharp transition from an isotropic composite fermion Fermi sea to an anisotropic even-denominator FQHS. Our data and calculations suggest that a unique feature of two-dimensional holes, namely the coupling between heavy-hole and light-hole states, combines different orbital components in the wavefunction of one Landau level, and leads to the emergence of a highly-anisotropic even-denominator FQHS. Our results demonstrate that the GaAs two-dimensional hole system is a unique platform for the exploration of exotic, many-body ground states. |
Tuesday, March 5, 2024 1:30PM - 1:42PM |
G17.00011: Topological phase transition between composite fermion and Pfaffian daughter states near ν = 1/2 fractional quantum Hall state Siddharth Kumar Singh, Chengyu Wang, Chia-Tse Tai, Casey S Calhoun, Adbhut Gupta, Kirk Baldwin, Loren N Pfeiffer, Mansour Shayegan The even-denominator fractional quantum Hall state (FQHS) at filling-factor ν = 1/2 is among the most enigmatic many-body phases in two-dimensional electron systems as it appears in the ground-state rather than an excited Landau level. It is observed in wide quantum wells where the electrons have a bilayer charge distribution with finite tunneling. Whether this 1/2 FQHS is two-component (Abelian) or one-component (non-Abelian) has been debated since its experimental discovery over 30 years ago. Here, we report strong 1/2 FQHSs in ultrahigh-quality, wide, GaAs quantum wells, with transport energy gaps up to ≈ 4 K, among the largest gaps reported for any even-denominator FQHS. The 1/2 FQHS is flanked by numerous, Jain-sequence FQHSs at ν = p/(2p±1) up to 8/17 and 9/17. Remarkably, as we raise the density while keeping the charge distribution in the quantum well symmetric, and strengthen the 1/2 FQHS, the 8/17 and 7/13 FQHSs suddenly become strong, much stronger than their neighboring high-order FQHSs. Insofar as FQHSs at ν = 8/17 and 7/13 are precisely the theoretically-predicted, simplest, daughter states of the one-component Pfaffian 1/2 FQHS, our data suggest a topological phase-transition of 8/17 and 7/13 FQHSs from the Jain- states to the daughter states of the Pfaffian, and that the parent 1/2 FQHS we observe is the Pfaffian state. |
Tuesday, March 5, 2024 1:42PM - 1:54PM |
G17.00012: Evidence for correlated defects in an ultra-clean Wigner crystal in the extreme quantum limit Pranav Thekke Madathil, Chengyu Wang, Siddharth Kumar Singh, Adbhut Gupta, Kevin Villegas Rosales, Yoon Jang Chung, Kenneth W West, Kirk Baldwin, Loren N Pfeiffer, Lloyd W Engel, Mansour Shayegan Low-disorder two-dimensional electron systems in the presence of a strong, perpendicular magnetic field terminate at very small Landau level filling factors in a Wigner crystal (WC), where the electrons form an ordered array to minimize the Coulomb repulsion. The nature of this exotic, many-body, quantum phase is yet to be fully understood and experimentally revealed. Here we probe one of WC’s most fundamental macroscopic parameters, namely the energy gap that determines its low-temperature conductivity, in record-mobility, ultra-high-quality, two-dimensional electrons confined to GaAs quantum wells. The WC domains in these samples contain ≃ 1000 electrons. The measured gaps are a factor of three larger than previously reported for lower quality samples [1,2], and agree remarkably well with values predicted for the lowest-energy, intrinsic, hyper-corelated bubble defects in a WC made of flux-electron composite fermions, rather than bare electrons [3]. The agreement is particularly noteworthy, given that the calculations are done for disorder-free composite fermion WCs, and there are no adjustable parameters. The results attest to the exceptionally high quality of the samples and provide evidence for composite fermion WCs indeed being more stable compared to their electron counterparts. |
Tuesday, March 5, 2024 1:54PM - 2:06PM |
G17.00013: Moving crystal phases of a quantum Wigner solid in an ultra-high-quality 2D electron system Kevin A Villegas Rosales, Pranav Thekke Madathil, Edwin Y Chung, Loren N Pfeiffer, Kirk Baldwin, Kenneth W West, Mansour Shayegan In low-disorder, two-dimensional electron systems (2DESs), the fractional quantum Hall states at very small Landau level fillings (ν) terminate in a Wigner solid (WS) phase, where electrons arrange themselves in a periodic array. The WS is typically pinned by the residual disorder sites and manifests an insulating behavior, with non-linear current-voltage (I-V) and noise characteristics [1,2]. We report here measurements on an ultra-low-disorder, dilute 2DES, confined to a GaAs quantum well. In the insulating phases, we observe remarkable non-linear I-V and noise characteristics as a function of increasing current, with current thresholds delineating three distinct phases of the WS: a pinned phase (P1) with very small noise, a second phase (P2) in which dV /dI fluctuates between positive and negative values and is accompanied by very high noise, and a third phase (P3) where dV /dI is nearly constant and small, and noise is about an order of magnitude lower than in P2. In the depinned (P2 and P3) phases, the noise spectrum also reveals well-defined peaks at frequencies that vary linearly with the applied current, suggestive of washboard frequencies. We discuss the data in light of a recent theory that proposes different dynamic phases for a driven WS [3].
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Tuesday, March 5, 2024 2:06PM - 2:18PM |
G17.00014: Correlation, pairing, and crystallization of 2D electron-flux composite fermions near ν = 1/6 filling Chengyu Wang, Pranav Thekke Madathil, Siddharth Kumar Singh, Adbhut Gupta, Edwin Y Chung, Loren N Pfeiffer, Kirk Baldwin, Mansour Shayegan The fate of two-dimensional electron systems (2DESs) at very low Landau level filling factors (ν << 1) has been of fundamental interest and under extensive debate for decades. The dominant electron-electron Coulomb interaction can lead to numerous, exotic, many-body phenomena, e.g., fractional quantum Hall states (FQHSs), Wigner crystal, and composite fermion (CF) Fermi sea. Our experiments on ultrahigh-quality, dilute GaAs 2DESs reveal very rich physics in the extreme quantum limit. Remarkably, we observe a pronounced, sharp, minimum in magnetoresistance at the even-denominator filling ν = 1/6 superimposed on an insulating background. The activation energies determined from the temperature dependence of the longitudinal resistance also reveal a clear minimum at ν = 1/6, qualitatively similar to what we observe for the ν = 1/7 FQHS. Our data signal a developing even-denominator FQHS in close competition with Wigner crystal states near ν = 1/6. On the flanks of ν = 1/6, a sequence of odd-denominator FQHSs are observed at ν = p/(6p±1) (p = 1, 2, 3). These are the Jain-sequence FQHSs of 6-flux CFs emanating from ν = 1/6, analogous to the standard Jain-sequence FQHSs of 2-flux and 4-flux CFs observed on the flanks of CF Fermi seas. Our results reveal a close competition between three fundamentally different ground states of 6-flux CFs in the extreme quantum limit: a BCS-type, paired state (incompressible liquid), a pinned Wigner crystal (insulating, solid), and a correlated Fermi sea (compressible, liquid). |
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