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We have created a spatially homogeneous polariton condensate in thermal equilibrium, up to very high condensate fraction. Under these conditions, we have measured the coherence as a function of momentum and determined the total coherent fraction of this boson system from very low density up to density well above the condensation transition. These measurements reveal a consistent power law for the coherent fraction as a function of the total density over nearly three orders of its magnitude. The same power law is seen in numerical simulations solving the twodimensional GrossPitaevskii equation for the equilibrium coherence.more » « lessFree, publiclyaccessible full text available May 3, 2025

Phase fluctuations determine the lowenergy properties of quantum condensates. However, at the condensation threshold, both density and phase fluctuations are relevant. While strong emphasis has been given to the investigation of phase fluctuations, which dominate the physics of the quantum system away from the critical point, number fluctuations have been much less explored even in thermal equilibrium. In this work, we report experimental observation and theoretical description of fluctuations in a circularly confined nonequilibrium BoseEinstein condensate of polaritons near the condensation threshold. We observe critical fluctuations, which combine the number fluctuations of a singlemode condensate state and competition between different states. The latter is analogous to mode hopping in photon lasers. Our theoretical analysis indicates that this phenomenon is of a quantum character, while classical noise of the pump is not sufficient to explain the experiments. The manifestation of a critical quantum state competition unlocks possibilities for the study of condensate formation while linking to practical realizations in photonic lasers.more » « lessFree, publiclyaccessible full text available March 22, 2025

The discovery of the fractional quantum Hall state (FQHS) in 1982 ushered a new era of research in manybody condensed matter physics. Among the numerous FQHSs, those observed at evendenominator Landau level filling factors are of particular interest as they may host quasiparticles obeying nonAbelian statistics and be of potential use in topological quantum computing. The evendenominator FQHSs, however, are scarce and have been observed predominantly in lowdisorder twodimensional (2D) systems when an excited electron Landau level is half filled. An example is the wellstudied FQHS at filling factor
$\mathit{\nu}\mathbf{=}$ 5/2 which is believed to be a BardeenCooperSchrieffertype, paired state of fluxparticle composite fermions (CFs). Here, we report the observation of evendenominator FQHSs at$\mathit{\nu}\mathbf{=}$ 3/10, 3/8, and 3/4 in the lowest Landau level of an ultrahighquality GaAs 2D hole system, evinced by deep minima in longitudinal resistance and developing quantized Hall plateaus. Quite remarkably, these states can be interpreted as evendenominator FQHSs of CFs, emerging from pairing of higherorder CFs when a CF Landau level, rather than an electron or a hole Landau level, is halffilled. Our results affirm enhanced interaction between CFs in a hole system with significant Landau level mixing and, more generally, the pairing of CFs as a valid mechanism for evendenominator FQHSs, and suggest the realization of FQHSs with nonAbelian anyons.Free, publiclyaccessible full text available December 26, 2024 
Strong interactions and topology drive a wide variety of correlated ground states. Some of the most interesting of these ground states, such as fractional quantum Hall states and fractional Chern insulators, have fractionally charged quasiparticles. Correlations in these phases are captured by the binding of electrons and vortices into emergent particles called composite fermions. Composite fermion quasiparticles are randomly localized at high levels of disorder and may exhibit charge order when there is not too much disorder in the system. However, more complex correlations are predicted when composite fermion quasiparticles cluster into a bubble, and then these bubbles order on a lattice. Such a highly correlated ground state is termed the bubble phase of composite fermions. Here we report the observation of such a bubble phase of composite fermions, evidenced by the reentrance of the fractional quantum Hall effect. We associate this reentrance with a bubble phase with two composite fermion quasiparticles per bubble. Our results demonstrate the existence of a new class of strongly correlated topological phases driven by clustering and charge ordering of emergent quasiparticles.more » « less

Abstract The interplay of strong Coulomb interactions and of topology is currently under intense scrutiny in various condensed matter and atomic systems. One example of this interplay is the phase competition of fractional quantum Hall states and the Wigner solid in the twodimensional electron gas. Here we report a Wigner solid at ν = 1.79 and its melting due to fractional correlations occurring at ν = 9/5. This Wigner solid, that we call the reentrant integer quantum Hall Wigner solid, develops in a range of Landau level filling factors that is related by particlehole symmetry to the so called reentrant Wigner solid. We thus find that the Wigner solid in the GaAs/AlGaAs system straddles the partial filling factor 1/5 not only at the lowest filling factors, but also near ν = 9/5. Our results highlight the particlehole symmetry as a fundamental symmetry of the extended family of Wigner solids and paint a complex picture of the competition of the Wigner solid with fractional quantum Hall states.more » « less

Abstract Domain walls in fractional quantum Hall ferromagnets are gapless helical onedimensional channels formed at the boundaries of topologically distinct quantum Hall (QH) liquids. Naïvely, these helical domain walls (hDWs) constitute two counterpropagating chiral states with opposite spins. Coupled to an swave superconductor, helical channels are expected to lead to topological superconductivity with high order nonAbelian excitations^{1–3}. Here we investigate transport properties of hDWs in the
ν = 2/3 fractional QH regime. Experimentally we found that current carried by hDWs is substantially smaller than the prediction of the naïve model. Luttinger liquid theory of the system reveals redistribution of currents between quasiparticle charge, spin and neutral modes, and predicts the reduction of the hDW current. Inclusion of spinnonconserving tunneling processes reconciles theory with experiment. The theory confirms emergence of spin modes required for the formation of fractional topological superconductivity.