arXiv subjects

Loren N. Pfeiffer

Publications and source records attributed to Loren N. Pfeiffer.

At least 19 recordsLinked to original sources

arXiv2026

Controlled Switching of Bose-Einstein Condensation in a Mixture of Two Species of Polaritons

We report temperature-dependent switching between lower and upper polariton condensation in a GaAs/AlGaAs microcavity when both of these species have comparable populations in a mixture. Using angle-resolved photoluminescence, we observe that at low temperatures, condensation occurs in the lower polariton branch, while at elevated temperatures, the upper polariton branch can become favored. At an intermediate temperature, we observe instability in the condensate formation, characterized by metastable correlations of the fluctuations in intensity and linewidth of the lower and upper polariton branches.

cond-mat.quant-gas↗

arXiv2026

Geometric control of hyperbolic exciton-polariton condensate dimers

Coupled many-body quantum systems exhibit rich emergent physics with diverse stationary and dynamical behaviors. By engineering platforms with tunable and distinct coupling mechanisms, new insights emerge into the collective behavior of coupled many body systems. Particles can be exchanged via evanescent or ballistic coupling: the former, based on proximity, yields large spectral splitting, while the latter requires strict phase-matching, analogous to phase-coupled harmonic oscillators, and has a smaller impact on the energy landscape. We demonstrate an all-optically tunable quantum fluid dimer based on exciton-polariton condensates in a photonic crystal waveguide with hyperbolic (saddle-like) dispersion. Varying the dimer's angle relative to the grating tunes the coupling from evanescent to ballistic. We directly observe spectral features and mass flow shaped by the saddle dispersion. This work highlights photonic crystals as powerful platforms to explore condensed matter phenomena lying at the interface between delay-coupled nonlinear oscillators and tight binding physics.

physics.optics↗

arXiv2026

Dynamical development of long-range spatial coherence in non-equilibrium bosonic condensation

Development of spontaneous coherence is one of the hallmarks of bosonic condensation in a variety of physical systems, such as cold atoms, confined photons, and hybrid light-matter quasiparticles like exciton polaritons in semiconductors. While spatial coherence is well understood once a steady-state condensate has been established, its temporal evolution as the condensate forms is largely unexplored. Here, we explore the dynamical formation of a non-equilibrium, driven-dissipative exciton-polariton condensate through both time-resolved experiments and numerical modeling. Our study reveals that the spatial coherence is established through two distinct stages. The early-time stage is interaction-driven and features transient oscillations in spatial coherence. This stage is followed by a steady-state regime characterized by a spatially-uniform high degree of coherence that extends over the entire size of the system and persists over time. These stages of spatial coherence development occur in both free flowing and confined exciton-polariton systems that undergo a quench - rapid growth of the condensate starting from two different initial settings. Our study offers a deep insight into the process by which long-range spatial coherence is established in a non-equilibrium bosonic condensate.

cond-mat.quant-gas↗

arXiv2026

Electron Temperature and Electron-Electron Scattering Length in GaAs/AlGaAs Using Mesoscopic Multiparallel Aperture Geometries

Quantifying electron temperatures and electron-electron scattering lengths is essential for understanding electron transport regimes in two-dimensional systems. To that end, this work experimentally investigates low-temperature ballistic transport using semiclassical cyclotron orbits in a multiparallel aperture geometry. The mesoscopic geometry is fabricated on a GaAs/AlGaAs heterostructure of very high electron mobility. The amplitudes of maxima in magnetoresistance due to ballistic transport are observed to decay exponentially with temperature, allowing determination of the electron-electron scattering length from the data. A quantitative relation between electron temperature and an applied dc bias heating current is established by comparing magnetoresistance peak amplitudes measured as a function of temperature and dc bias, reflecting bias-induced Joule heating. In addition, the resistance measured near zero magnetic field decreases with increasing temperature, providing signatures of superballistic conductance.

cond-mat.mes-hall↗

arXiv2026

Discrete states and ballistic interference in quantum wires approaching macroscopic lengths

Increasing the size of a system showing quantum effects is a difficult task limited by decoherence, a diminishing quantum level spacing, and the effects of disorder spoiling the quantum behavior when growing in size. Systems in 1D offer very strong confinement in the transverse directions, thus generally enhancing quantum effects, but are notoriously sensitive to disorder. In this work, we present a system of 1D electrons exhibiting discrete quantum levels and fully ballistic coherent quantum interference with lengths of up to 18\,$μ$m. Tunneling spectroscopy between two parallel quantum wires with a central gated segment shows intricate interference patterns exhibiting several different periods in magnetic field and density. An analysis over three different wire lengths and a comparison with single particle numerical simulations without any free parameters remarkably explains the full pattern including the observed periods. Therefore, these wires are essentially ideal 1D systems with aspect ratios approaching 1'000. In addition, at low bias, we also observe not only the Coulomb charging energies but can clearly resolve the discrete orbital and spin states in up to 10\,$μ$m long wires when filling 100 electrons with the center gate. This is made visible by a state-of-the-art low temperature and low noise measurement system. The spin filling sequence is completely regular, strictly alternating spin up and down, avoiding high spin states, while the peak conductance is modulated in accordance with the previously discussed interference patterns. These striking results show that single particle Schrödinger quantum mechanics such as ballistic quantum interference and discrete quantum states may be observed, under the right conditions, in systems of up to 18\,$μ$m length, thus approaching macroscopic sizes.

cond-mat.mes-hall↗

arXiv2026

Mirror-mediated long-range coupling and robust phase locking of spatially separated exciton-polariton condensates

Lattice arrays are valuable simulators for complex mathematical problems, but physical systems typically allow only short-range coupling. We demonstrate a method for independently tunable, long-range interactions between polariton condensates in two-dimensional lattices by using vertical emission and external imaging to couple arbitrary sites. Two geometrically isolated condensates are phase-locked without planar coupling, verified via phase-resolved interferometry. Analytical modeling reveals mechanisms for robust coherence. The mirror-based scheme, free of cameras or modulators, offers a pure, high-bandwidth analogt element. Extension to dense graphs via segmented micro-mirrors is limited only by imaging optics, enabling scalable, energy-efficient polaritonic hardware for neuromorphic computation.

physics.optics↗

arXiv2026

Observation of even-denominator fractional quantum Hall states at $ν= 3/4$ and 5/4 in the lowest Landau level

Two-dimensional electron systems (2DESs) confined to wide GaAs quantum wells provide a unique platform to study exotic fractional quantum Hall states (FQHSs) because the 2DES has a bilayer charge distribution with significant interlayer tunneling. Precise control over the 2DES density allows the tuning of the interlayer tunneling over a wide range. Here, we present our discovery of new even-denominator FQHSs in the lowest Landau level (orbital index \textit{N} = 0) at filling factors $ν= 3/4$ and 5/4 in an ultrahigh-quality 2DES confined to a 72.5-nm-wide GaAs quantum well. The ground states at $ν= 3/4$ and 5/4 both evolve from composite fermion Fermi seas to FQHSs as the density is raised so that interlayer tunneling is sufficiently reduced and the 2DES becomes two-component, signaled by the behavior of the FQHSs flanking $ν= 3/4$ and 5/4. The two-component nature of the $ν=3/4$ and 5/4 FQHSs is also evident from their extreme sensitivity to the bilayer charge distribution symmetry: both states disappear quickly when the charge distribution is made asymmetric by only $\simeq 2\%$. We find a natural explanation for the 3/4 and 5/4 FQHSs in terms of two states linked by particle-hole symmetry, and using the Scarola-Jain bilayer composite fermion framework which is a generalization of the well-known, two-component, Halperin state ($Ψ_{331}$ state). Our observations elucidate the crucial role of competing energy and length scales in wide quantum wells in stabilizing new ground states.

cond-mat.mes-hall↗

arXiv2026

Cascade of fractional quantum Hall states in 2D system

The observation of the fractional quantum Hall (FQH) effect in 2D electron gases ushered in investigations of topological phases driven by strong electron correlations. Their remarkable features include fractionalized elementary excitations, gapless boundary states, and non-trivial quantum entanglement patterns. Thanks to persistent efforts in the building of new platforms and making higher-quality samples, a diverse plethora of FQH states have been unveiled in experiments. We report a systematic study of ultrahigh-quality GaAs/AlGaAs quantum wells with mobility up to 3.7*10^7 cm^2/V/s using quantum transport measurements in nuclear adiabatic demagnetization and dilution refrigerators down to 1 mK. In addition to many FQH states that have already been identified in previous work, new longitudinal resistance dips are observed at filling factors 17/33 and 15/31. The application of an in-plane magnetic field causes disparate variations of the FQH states. The theoretical foundation of these states is discussed in the framework of composite fermion theory. While most fractions can be explained as non-interacting composite fermions forming integer quantum Hall states, a few states correspond to FQH states of composite fermions that arise from residual interaction between them. We summarize the observed fractions in the range of 0 < ν < 2 and propose a pattern to account for their experimental appearance that provides an intuitive picture about the relative strengths of different FQH states.

cond-mat.mes-hall↗

arXiv2026

Effective-Hamiltonian reconstruction through Bloch-wave interferometry in bulk GaAs driven by strong THz fields

Reconstructing effective Hamiltonians of condensed matter systems directly from experimental data is challenging because of the intricate relationship between Hamiltonian parameters and observables. Here, we reconstruct an effective three-band electron-hole (e-h) Hamiltonian in bulk GaAs based on high-order sideband generation (HSG) induced by quasi-continuous NIR and THz lasers. We perform polarimetry of high-order sidebands while varying the wavelength and polarization of the NIR laser, as well as the strength of the THz field. An analytic model is derived to incorporate the effects of both dephasing and quantum fluctuations around the semiclassical e-h recollision pathways. Surprisingly, the contribution of quantum fluctuations to the decay of sideband intensity with increasing sideband order is comparable to the contribution of dephasing. We simultaneously and unambiguously determine through Bloch-wave interferometry the effective Hamiltonian parameter that determines the e-h reduced masses, the bandgap of GaAs, and two dephasing constants associated with two e-h species. We demonstrate that full Hamiltonian reconstruction can be achieved by combining HSG measurements with absorbance spectroscopy. Unexpectedly, we find that the extracted bandgap of GaAs is about 10 meV larger than the value inferred from previous absorbance measurements. Quantum-kinetic analysis suggests that, in the HSG experiments, the e-h energy may be renormalized through Fröhlich interaction that is modulated by the strong THz fields. We also show that the energy threshold for optical-phonon emission can be suppressed by applying a strong THz field, leading to nearly constant dephasing rates.

cond-mat.mtrl-sci↗

arXiv2026

Temporal hopping dynamics in exciton-polariton condensation

Polariton condensates provide a versatile platform for exploring non-equilibrium phase transitions and collective phenomena in open quantum systems. Near the condensation threshold, these systems are particularly sensitive to fluctuations and instabilities, which can strongly influence the condensate formation. Using optical trapping and homodyne detection, we directly access the photon statistics and second-order correlation function $g^{(2)}(0)$ of the condensate. We show that polariton condensation near the threshold is not a purely static transition, but instead undergoes a dynamical regime characterized by stochastic hopping between condensed and non-condensed states. These intermittent dynamics are accompanied by a gradual reduction of $g^{(2)}(0)$ towards unity, revealing the progressive build-up of coherence even in the presence of strong temporal fluctuations. Numerical simulations, based on a stochastic Truncated Wigner description of the driven-dissipative polariton field, reproduce these dynamics and capture the essential role of noise and reservoir interactions. This work demonstrates that the observed temporal hopping is an intrinsic feature of polariton condensation, providing a dynamical perspective that goes beyond static descriptions of the condensation phase transition.

cond-mat.mes-hall↗

arXiv2026

Observation of e/4 charge at $ν=1/2$ in GaAs

Even-denominator fractional quantum Hall states (FQHSs) fall outside the standard Laughlin's and Jain's odd-denominator hierarchy. In this work, we study the FQHS $ν=1/2$ in the lowest Landau level. The state is confined within a 70 nm-wide GaAs quantum well, where the electrons exhibit a bilayer-like charge distribution. Inter-layer interactions stabilize the $ν=1/2$ FQHS, which is predicted to host quasiparticles with charge e/4 - with either Abelian or non-Abelian topological order. Here, we report on shot-noise measurements of partitioned quasiparticles at $ν=1/2$, where charge partitioning is generated by a unique etch-defined quantum point contact. Our measurements were performed on two nominally identical devices, at two independent experimental setups. Analysis of shot noise in the weak-backscattering regime in each device reveals quasiparticles with charge e/4. These observations provide a clear benchmark for future studies aimed at probing the topological order of the $ν=1/2$ FQHS and its quasiparticles' exchange statistics.

cond-mat.mes-hall↗

arXiv2025

Observation of a Stripe/nematic Phase of Composite Fermions

Electronic stripe/nematic phases are fascinating strongly-correlated states characterized by spontaneous rotational symmetry breaking. In the quantum Hall regime, such phases typically emerge at half-filled, high-orbital-index ($N\geq2$) Landau levels (LLs) where the short-range Coulomb interaction is softened by the nodes of electron wave functions. In the lowest ($N=0$) LLs, these phases are not expected. Instead, composite fermion (CF) liquids and fractional quantum Hall states, which are well explained in the picture of weakly interacting CF quasiparticles, are favored. Here we report the observation of an unexpected stripe/nematic phase in the \textit{lowest} LL at filling factor $ν=5/8$ in ultrahigh-quality GaAs two-dimensional \textit{hole} systems, evinced by a pronounced in-plane transport anisotropy. Remarkably, $ν=5/8$ can be mapped to a half-filled, high-index CF LL ($N_{\text{CF}}=2$), analogous to the $N=2$ hole LL. Our finding signals a novel stripe/nematic phase of CFs, driven by the residual long-range interaction among these emergent quasiparticles. This phase is surprisingly robust, surviving up to $\sim$100 mK. Its absence in electron-type systems suggests that severe LL mixing stemming from the large hole effective mass and non-linear LL fan diagram plays a crucial role in modifying the CF-CF interaction.

cond-mat.mes-hall↗

arXiv2025

Topological Protection in a Landau Flat Band at $ν=7/11$, a Candidate Filling Factor for Unconventional Correlations

Strong interactions in Landau flat bands are known to stabilize correlated states that do not form in other types of flat bands. We report hallmarks of topological protection at the Landau level filling factor v=7/11 in a two-dimensional electron system. The $ν=7/11$ filling factor is the particle-hole conjugate of $ν=4/11$, a filling factor intensely studied for the possibility of realizing unconventional electronic correlations. Our data establishes a new instance for an unusual fractional quantum Hall state and opens up possibilities for the study of unconventional correlations in an enlarged parameter space. We report and discuss transport signatures developing at other filling factors of interest $ν= 11/17$, $5/8$, and $8/13$, which however in our sample do not exhibit topological protection.

cond-mat.mes-hall↗

arXiv2025

Fractional quantum Hall state at $ν= 1/2$ with energy gap up to 6 K, and possible transition from one- to two-component state

The fractional quantum Hall state (FQHS) observed in the lowest Landau level at filling factor $ν=1/2$ in wide quantum wells has been enigmatic for decades because the two-dimensional electron system (2DES) has a bilayer charge distribution but with significant interlayer tunneling. Of particular interest is whether the 1/2 FQHS in this system has a one-component (1C) or two-component (2C) origin; these are typically identified as the Pfaffian (non-Abelian) or the $Ψ_{331}$ (Abelian) FQHSs, respectively. We report here our experimental study of the evolution of the correlated states of an ultrahigh-quality 2DES confined to a 72.5-nm-wide GaAs quantum well. At the lowest densities, the 2DES displays only odd-denominator FQHSs, and the ground state at $ν= 1/2$ is a composite fermion Fermi sea. As the density is increased, a FQHS emerges at $ν= 1/2$, and becomes very strong. In a finite density range where the 1/2 FQHS is strongest, we also observe its daughter FQHSs at $ν= 8/17$ and 7/13, consistent with the theoretically expected daughter states of a Pfaffian 1/2 FQHS. At the highest densities, the 2DES becomes 2C, signaled by the emergence of a bilayer Wigner crystal state and the transitions of FQHSs flanking $ν=1/2$. The 1/2 FQHS remains robust near this transition and, notably, its charge transport energy gap exhibits an \textit{upward} cusp with a maximum value of about 6 K on the 1C side of the transition; this is the largest gap reported for any even-denominator FQHS. Our observation of the transition of the 2DES ground states near $ν=1/2$ to 2C states at high densities, and our measurements of the robustness of the 1/2 FQHS against charge distribution asymmetry, suggest that the 1/2 FQHS also makes a transition from 1C to 2C. Such a transition from a non-Abelian to Abelian state can open avenues for topological quantum information and quantum criticality.

cond-mat.mes-hall↗

arXiv2025

Total Faraday rotation by the Hall effect in a 2D electron gas

We report the realization of near total Faraday rotation of $θ_F$=1.43 rad (82 degrees) on a single pass through a 2D electron gas (2DEG), approaching the ideal limit of $π/2$ rad (90 degrees). The corresponding Verdet constant V = $9.5\times10^{8}$ rad T$^{-1}$m$^{-1}$, exceeds by approximately one order of magnitude that reported in other material systems. Our measurements were conducted at microwave frequencies (f=9.2-11.2 GHz) in a 2DEG with a high dc mobility $μ$ = $7\times10^6$ cm$^2$V$^{-1}$s$^{-1}$, in a hollow waveguide at low-magnetic field (B < 200 mT). Near-total Faraday rotation is attributed to the Hall effect with weak radiative coupling to the 2DEG in the inertial, collisionless regime, $ωτ\gg 1$, where $τ$ is the charge transport scattering time. A conducting iris was used to realize weak radiative coupling. Under these conditions, Faraday rotation is strongly enhanced away from the dissipation peak at cyclotron resonance. Our work demonstrates that the classical Hall effect could be ideally suited for the implementation of ideal non-reciprocal devices.

cond-mat.mes-hall↗

arXiv2025

Edge State Selective Measurement of Quantum Hall Dispersions

Edge states reflect the key physical properties yet are difficult to probe individually, particularly when several states are present at an edge. We present momentum resolved tunneling spectroscopy between a quantum well and a quantum wire to extract the dispersions of the quantum Hall edge states. Momentum and energy selective tunneling allows to separately address the different states even if they are spatially overlapping. This delivers the edge state velocities over broad ranges of magnetic field and density, in excellent agreement with a hard-wall model. This technique provides a basis for future edge state selective spectroscopy on quantum materials.

cond-mat.mes-hall↗

arXiv2025

Nonlinear transport of Wigner solid phase surrounding the two-flux composite fermion liquid

We have investigated the low temperature (T) transport properties of fractional quantum Hall (FQH) states in a high-mobility two-dimensional hole gas. According to the composite fermion (CF) model, FQH states stemming from a half-filled Landau level, specifically at filling factors $ν=p/(2p+1) (p=\pm 1,\pm 2,\pm 3,...)$, can be associated with two-flux-attached CFs at the corresponding Lambda filling factor p. The zero-resistance minima and Hall plateaus of these states exhibit unusual temperature dependencies, characterized by rapid increases in width below a threshold temperature around 100 mK. Differential conductivity measurements from Corbino samples reveal that the regimes surrounding the CF liquid display clear nonlinear transport characteristics. This nonlinearity implies that each CF liquid is surrounded by CF solid phase composed of dilute CF excitations. Quantitatively, the applied electric field E influences the motion of CF solid in a way analogous to T, which is dubbed the "E-T duality". Our analysis indicates that this E-T duality is consistent with the Berezinskii-Kosterlitz-Thouless theory in two-dimensional phase transitions.

cond-mat.mes-hall↗

arXiv2025

Strong coupling of polaritons at room temperature in a GaAs/AlGaAs structure

We report direct measurement of the dispersion relation of polaritons in GaAs/AlGaAs microcavity structures at room temperature, which clearly shows that the polaritons are in the strong coupling limit. The Rabi splitting of the polariton states decreases as the polariton gas increases in density, but even when the polariton gas becomes a coherent, Bose-condensate-like state, the polaritons retain a strong exciton component, as seen in the nonlinear energy shift of the light emission. This opens up the possibility of polaritonic devices at room temperature in a material system which can be grown with very high quality and uniformity.

cond-mat.mes-hall↗