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 TwoBilayer 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 singleparticle 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 halffilling 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/antiskyrmion 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 pwave 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) halffilled Landau Levels (LLs), the competition between longrange repulsive and shortrange attractive components of Coulomb interaction leads to compressible chargeordered phases commonly known as quantum Hall stripes. It is also known that evendenominator 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 inplane 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 halffilling, 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 twodimensional (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 heavyhole lighthole mixing and spinorbit coupling which result in LLs that are highly nonlinear and exhibit numerous crossings as a function of magnetic field, including one between the two lowestenergy LLs with opposite pseudospins. It was previously demonstrated that such a crossing could be gatetuned 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 gatetuned 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 reentrant insulating phase between ν = 1/3 and 2/5 disappears near the crossing and reappears 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 GuerreroSuarez, 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 twodimensional electron gas (2DEG) resides in a 30 nm quantum well with density n_{ }= 1.05x10^{11 }cm^{2} and mobility μ = 18x10^{6 }cm^{2}/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 [011] 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 [011] 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 Modulationdoped twodimensional (2D) carrier systems hosted in GaAs quantum wells provide nearly ideal testing grounds for exploring exotic lowtemperature manybody phenomena. While 2D electron systems have long been at the forefront for exploration of interactiondriven phenomena, 2D hole systems (2DHSs) offer an attractive alternative, owing to their large effective mass, strong and tunable spinorbit coupling, and complex bandstructure. For the first time, in a 2DHS in any material, we obtain peak mobility ≃10 x 10^{6} cm^{2}/Vs at density of only ≃3.8 x 10^{10} /cm^{2}, at 300 mK which rises to ≃18 x 10^{6} cm^{2}/Vs (implying a meanfreepath ≃57 µm) when measured at 30 mK. We achieve this by optimizing the structure design by systematically adjusting the alloy fraction (x) of the Al_{x}Ga_{1x}As barrier near the quantum well, and the width of the quantum well. Lowtemperature 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 10^{7} cm^{2}/Vs in 2DHSs represents a significant leap, considering that the highest recorded mobilities, until two years ago, were only ≃2 x 10^{6 }cm^{2}/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 twodimensional 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 twodimensional 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 lowdensity and highquality. Here, we report transport measurements in a clean (lowdisorder) twodimensional electron system with anisotropic effective mass and Fermi sea. The data reveal that at extremely low electron densities, when the r_{s} parameter, the ratio of the Coulomb to the Fermi energy, exceeds 38, the currentvoltage 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 depinning 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 onedimensional 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 quasiparticlequasihole excitations and these gaps are typically measured from the temperature (T) dependence of the FQHE R_{xx} minima in the thermally activated regime. Here, we examine the possibility of determining these gaps by measuring the microwave power (P) variation of the R_{xx} 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 10^{11}cm^{2} and μ = 10^{7}cm^{2}/Vs and report associated results. 
Tuesday, March 5, 2024 1:06PM  1:18PM 
G17.00009: rf spectroscopy of Wigner solids in ultralowdisorder GaAs hosted twodimensional 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 highmagneticfield 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 oldergeneration 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 higherorder 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: Highlyanisotropic evendenominator fractional quantum Hall state in an orbitallycoupled halffilled Landau level Chengyu Wang, Mansour Shayegan, Adbhut Gupta, Edwin Y Chung, Loren N Pfeiffer, Kirk Baldwin, Roland Winkler The evendenominator fractional quantum Hall states (FQHSs) in halffilled Landau levels are generally believed to host nonAbelian quasiparticles and be of potential use in topological quantum computing. Of particular interest is the competition and interplay between the evendenominator FQHSs and other ground states, such as anisotropic phases and composite fermion Fermi seas. Here we report the observation of an evendenominator FQHS with highlyanisotropic inplane transport coefficients at Landau level filling factor ν = 3/2. We observe this state in an ultrahighquality GaAs twodimensional hole system when a large inplane magnetic field is applied. By increasing the inplane field, we observe a sharp transition from an isotropic composite fermion Fermi sea to an anisotropic evendenominator FQHS. Our data and calculations suggest that a unique feature of twodimensional holes, namely the coupling between heavyhole and lighthole states, combines different orbital components in the wavefunction of one Landau level, and leads to the emergence of a highlyanisotropic evendenominator FQHS. Our results demonstrate that the GaAs twodimensional hole system is a unique platform for the exploration of exotic, manybody 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, ChiaTse Tai, Casey S Calhoun, Adbhut Gupta, Kirk Baldwin, Loren N Pfeiffer, Mansour Shayegan The evendenominator fractional quantum Hall state (FQHS) at fillingfactor ν = 1/2 is among the most enigmatic manybody phases in twodimensional electron systems as it appears in the groundstate 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 twocomponent (Abelian) or onecomponent (nonAbelian) has been debated since its experimental discovery over 30 years ago. Here, we report strong 1/2 FQHSs in ultrahighquality, wide, GaAs quantum wells, with transport energy gaps up to ≈ 4 K, among the largest gaps reported for any evendenominator FQHS. The 1/2 FQHS is flanked by numerous, Jainsequence 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 highorder FQHSs. Insofar as FQHSs at ν = 8/17 and 7/13 are precisely the theoreticallypredicted, simplest, daughter states of the onecomponent Pfaffian 1/2 FQHS, our data suggest a topological phasetransition 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 ultraclean 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 Lowdisorder twodimensional 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, manybody, 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 lowtemperature conductivity, in recordmobility, ultrahighquality, twodimensional 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 lowestenergy, intrinsic, hypercorelated bubble defects in a WC made of fluxelectron composite fermions, rather than bare electrons [3]. The agreement is particularly noteworthy, given that the calculations are done for disorderfree 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 ultrahighquality 2D electron system Kevin A Villegas Rosales, Pranav Thekke Madathil, Edwin Y Chung, Loren N Pfeiffer, Kirk Baldwin, Kenneth W West, Mansour Shayegan In lowdisorder, twodimensional 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 nonlinear currentvoltage (IV) and noise characteristics [1,2]. We report here measurements on an ultralowdisorder, dilute 2DES, confined to a GaAs quantum well. In the insulating phases, we observe remarkable nonlinear IV 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 welldefined 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].

Tuesday, March 5, 2024 2:06PM  2:18PM 
G17.00014: Correlation, pairing, and crystallization of 2D electronflux 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 twodimensional 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 electronelectron Coulomb interaction can lead to numerous, exotic, manybody phenomena, e.g., fractional quantum Hall states (FQHSs), Wigner crystal, and composite fermion (CF) Fermi sea. Our experiments on ultrahighquality, dilute GaAs 2DESs reveal very rich physics in the extreme quantum limit. Remarkably, we observe a pronounced, sharp, minimum in magnetoresistance at the evendenominator 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 evendenominator FQHS in close competition with Wigner crystal states near ν = 1/6. On the flanks of ν = 1/6, a sequence of odddenominator FQHSs are observed at ν = p/(6p±1) (p = 1, 2, 3). These are the Jainsequence FQHSs of 6flux CFs emanating from ν = 1/6, analogous to the standard Jainsequence FQHSs of 2flux and 4flux CFs observed on the flanks of CF Fermi seas. Our results reveal a close competition between three fundamentally different ground states of 6flux CFs in the extreme quantum limit: a BCStype, paired state (incompressible liquid), a pinned Wigner crystal (insulating, solid), and a correlated Fermi sea (compressible, liquid). 
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