Field-induced excitonic insulator in graphite

In graphite, electrons and holes are confined to their lowest Landau levels when magnetic field exceeds 10 T. Between 22 T and 70 T, two insulating states emerge, with critical temperatures each displaying a distinct dome-like field dependence [1]. The summit of the first dome corresponds to a critical temperature of 9.2 K and a critical magnetic field of 47 T. At this critical field, hole and electron Landau sub-bands simultaneously cross the Fermi level allowing exciton formation with infintesimal Coulomb attraction. Quantifying the effective mass and the spatial separation of the excitons in the basal plane, we found that the expected degeneracy temperature of the excitonic fluid is close to the experimentally measured critical temperature. This supports the picture of a metal-insulator transition driven by the Bose-Einstein Condensation (BEC) of excitons [2]. The evolution of this dome under hydrostatic pressure documents an original case of BCS-BEC crossover, which is tunable by both magnetic field and pressure, but its summit remains locked at a fixed temperature [3].

References:

1. Benoît Fauqué et al. Two phase transitions induced by a magnetic field in graphite. Phys. Rev. Lett. 110, 266601 (2013).
2. Jinhua Wang et al. Critical point for Bose-Einstein condensation of excitons in graphite. Proc. Natl. Acad. Sci. 117, 30215 (2020).
3. Yuhao Ye et al. Tuning the BCS-BEC crossover of electron-hole pairing with pressure, Nature Communications 15:9794 (2024)

Indirect excitons

Spatially indirect excitons (IXs), also known as interlayer excitons, are formed by electrons and holes in separated layers in a heterostructure (HS). Due to the layer separation, the IX lifetimes are orders of magnitude longer than lifetimes of spatially direct excitons. The long lifetimes allow IXs to cool below the temperature of quantum degeneracy and form quantum bosonic states. We present recent results in quantum IX systems. In GaAs HS: Cooper-pair-like excitons [1], excitonic Bose polarons [2], and the Mott transition in excitonic Bose polarons [3]. In van der Waals HS: long-distance IX transport [4], IX mediated long- distance spin transport [5], and efficient IX transport with anomalously high diffusivity, orders of magnitude higher than for regular diffusive exciton transport in van der Waals heterostructures, agreeing with long-range ballistic transport [6].

Acknowledgements: The PL and PLE studies were supported by DOE Award DE-FG02- 07ER46449, the device fabrication by NSF Grant 1905478, the GaAs heterostructure growth by Gordon and Betty Moore Foundation Grant GBMF9615 and NSF Grant DMR 2011750.

References:

1. Z. Zhou, W. J. Brunner, E. A. Szwed, H. Henstridge, L. H. Fowler-Gerace, L. V. Butov, Efficient transport of indirect excitons in a van der Waals heterostructure, arXiv:2507.04556 (2025).
2. D. J. Choksy, E. A. Szwed, L. V. Butov, K. W. Baldwin, L. N. Pfeiffer, Fermi edge singularity in neutral electron-hole system, Nat. Phys. 19, 1275 (2023).
3. E. A. Szwed, B. Vermilyea, D. J. Choksy, Z. Zhou, M. M. Fogler, L.V. Butov, D.K. Efimkin, K.W. Baldwin, L.N. Pfeiffer, Excitonic Bose-polarons in electron-hole bilayers, Nano Lett. 24, 13219 (2024).
4. E.A.Szwed,B.Vermilyea,D.J.Choksy,Z.Zhou,M.M.Fogler,L.V.Butov,K.W.Baldwin, L. N. Pfeiffer, Mott transition in excitonic Bose polarons, arXiv:2504.07227 (2025).
5. L. H. Fowler-Gerace, Zhiwen Zhou, E. A. Szwed, D. J. Choksy, L. V. Butov, Transport and localization of indirect excitons in a van der Waals heterostructure, Nat. Photon. 18, 823 (2024).
6. Z. Zhou, E. A. Szwed, D. J. Choksy, L. H. Fowler-Gerace, L. V. Butov, Long-distance decay- less spin transport in indirect excitons in a van der Waals heterostructure, Nat. Commun. 15, 9454 (2024).

Investigations of the possible excitonic insulator state in neutral monolayer WTe₂

When WTe₂ is thinned down to a bilayer or monolayer a small charge gap develops which may well originate from interactions between electrons and holes in what would otherwise be a compensated semimetal. The resulting state (which, seemingly incidentally, is topological) has been proposed to be a two-dimensional excitonic insulator. However, the charge or spin density waves that would be expected in a standard exciton condensation picture have not been detected. I will discuss the results of several experiments investigating the electronic spectrum as a function of doping, disorder, and screening by an adjacent layer. Signatures of strong correlations are indeed present, including non-thermal broadening, negative compressibility, and suppression of the gap when the compensation is broken, but the EI paradigm will probably need to be amended or extended to properly understand the thermodynamics and transport of this unusual electronic system.

Optical and cavity engineering of driven excitonic condensates

Bilayer materials hosting interlayer excitons—comprising electrons in one layer and holes in the other—are a promising experimental platform for realising high-temperature condensates and studying their dynamical properties. Imposing a chemical potential bias through optical pumping or electrical contacts drives exciton condensates into distinct dynamical regimes. We investigate how these regimes manifest in emitted light and how they are influenced by placing the material within an optical cavity.

We show that in a bilayer system where the charge can tunnel between the layers, the chemical potential bias means that an exciton condensate is in the dynamical regime of the Josephson effect. By increasing the bias voltage, the system undergoes a transition from the phase- trapped to phase-delocalized dynamical condensation. Optical spectroscopy can identify these phases, with a strong response to weak fields near the transition due to the instability in the order parameter dynamics [1].

If such a system is placed in an optical cavity within the phase-trapped regime, coupling to photons favours a superradiant state. The phenomenon allows the device to convert DC currents into coherent photons at tunable frequencies determined by the bias and material thickness. These findings highlight mechanisms to control and harness excitonic condensates for optoelectronic applications [2].

References:

1. Alexander Osterkorn, Yuta Murakami, Tatsuya Kaneko, Zhiyuan Sun, Andrew J Millis, Denis Golež, arXiv:2410.22116.
2. Zhiyuan Sun, Yuta Murakami, Fengyuan Xuan, Tatsuya Kaneko, Denis Golež and Andrew J. Millis PRL 133, 217002 (2024).

Manipulating Topological Phases and Correlated States in HfTe₅

Controlling topological phases in quantum materials offers a route to explore emergent quantum states and develop devices with topologically protected carriers. Yet, few materials allow both efficient tunability and in situ electronic measurements. Here, we present our work on HfTe₅, a prototypical van der Waals material with exceptional topological tunability. First, we apply a large, controllable uniaxial strain to induce a topological phase transition from a weak topological insulator (WTI) to a strong topological insulator (STI). This transition leads to a dramatic increase in resistivity of over 190,000% and results in surface-state-dominated transport at cryogenic temperatures. Second, we find that the WTI phase of HfTe5 supports zeroth Landau level physics at moderate magnetic fields. Fields above 10 T drive transitions to 1D Weyl modes and, under low carrier density, stabilize a spin-triplet excitonic insulator phase, enabled by strong electronic instabilities in quasi-1D systems. Notably, in the STI phase, this excitonic phase emerges at even lower fields. Third, we explore thin HfTe₅ devices (<100 nm), where enhanced surface-to-bulk transport and correlated phenomena appear. These observations highlight HfTe₅ as a versatile platform for studying topological transitions and emergent correlated states. Together, these results position HfTe₅ as a key material for advancing quantum device applications, from spintronics to fault-tolerant topological quantum computing.

Counterflow excitonic plasmons on a chip

A defining feature of exciton condensates is the emergence of a Goldstone mode associated with spontaneous interlayer phase coherence, which enables dissipationless counterflow of electrons and holes – analogous to superfluidity in a Bose-Einstein condensate (BEC). In BECs, such superfluid behavior results from the spontaneous breaking of a continuous symmetry (typically global U(1) charge conservation). In contrast, in excitonic insulators, this symmetry is often explicitly broken by coupling to the lattice, leading to a gapped pseudo-Goldstone mode and frequently accompanied by lattice distortions. This complicates both the identification of a true excitonic condensate and the interpretation of spectroscopic signatures. Moreover, the pseudo- Goldstone mode is optically silent in the far field, making its detection especially challenging.

In this talk I will present a theoretical framework demonstrating that on-chip terahertz (THz) spectroscopy provides a direct and linear probe of the pseudo-Goldstone mode in two-dimensional excitonic insulators. This mode—referred to here as a counterflow excitonic plasmon—involves in-phase oscillations of electrons and holes. Although these oscillations produce no net dipole moment and thus evade far-field optical detection, they can couple efficiently to near-field THz pulses guided along on-chip metallic transmission lines. I will show how this coupling enables the excitation and detection of counterflow excitonic plasmons, and discuss the potential of this technique to address long standing questions in this field.

Green’s function zeros in Mott quantum magnets

Even deep in strongly correlated Mott insulating phases, the free, non-interacting energy- momentum relation plays a crucial role for the analytic structure of the single- particle Green’s function G [1]. In particular, the momentum structure of the zero eigenvalues of G is given by an appropriately renormalized form of the bare electronic dispersion, despite the presence of a hard gap which prevents from a straightforward spectroscopic access. After exploring topological classification schemes based on Green’s function zeros and their connection with low-energy excitations in spin liquids [2], I will present setups that have been put forward for an experimental detection [3,4]. In the second part of the talk, I will extend the notion of zeros of G to long-range ordered magnetic phases [5] and discuss the role of temperature. If time allows, I will discuss other types of symmetry breaking and connect to altermagnets with interlayer excitonic order [6].

References:

1. L. Del Re. et al. in preparation
2. N. Wagner, L. Crippa, A. Amaricci, P. Hansmann, M. Klett, E. König, T. Schäfer, D. Di Sante, J. Cano, A. J. Millis, A. Georges and G. Sangiovanni, Nat. Commun. 14, 7531 (2023).
3. N. Wagner, D. Guerci, A. J. Millis and G. Sangiovanni, Phys. Rev. Lett. 133, 126504 (2024).
4. E. Stepanov, M. Chatzieleftheriou, N. Wagner and G. Sangiovanni, Phys. Rev. B 110,L161106 (2024).
5. C. Lehmann, L. Crippa, G. Sangiovanni and J. Budich, arXiv:2502.19479
6. F. Valerio Servilio, et al., in preparation

Signatures of Collective Ordering and Ferroelectric Phases of Moiré Excitons

In this talk, I will present our recent observation of many-body interaction-induced ferroelectric ordering of moiré excitons in H-stacked WSe₂/WS₂ heterobilayer. Strong exciton-exciton repulsion leads to an excitonic Mott state with a large on-site energy U_xx ~35 meV. Due to the interplay of anisotropic nature of dipolar interactions, large U_xx, and spatially indirect in-plane excitons in H-stacking, we observe signatures of ferroelectric ordering of moiré excitons in time-resolved photoluminescence spectra. In particular, we find a reduction in emission lifetime consistent with this ordering, which can be thought of as a novel cooperative phenomenon. Our observations open new avenues to explore a system of correlated moiré electrons and excitons as a rich platform to study and create quantum matter in a driven-dissipative setting and also a many-body quantum open system simulator to uncover novel cooperative phenomena.

Ab initio calculations of excitons in complex 2D van der Waals Heterostructures

I will describe a number of recently developed computational methods to predict the optical properties of van der Waals heterostructures containing hundreds of atoms in a unit cell. To obtain the single-particle band structure, we employ a layer-projected scissors (LAPS) operator that incorporates short and long-range electron self-energy effects and ensures a proper description of the band alignment at the interface [1]. I will show that the LAPS method yields an accuracy comparable to the many-body GW approximation, but at the cost of a standard density functional theory (DFT) calculation. To compute the optical excitations, we solve the Bethe-Salpeter Equation (BSE) using a minimal basis for the electron-hole states. The screened Coulomb interaction is calculated using a mixed quantum-classical electrostatic model – an extension of the previously published QEH model. By using a hierarchical basis comprising dielectric eigenstates of the individual monolayers, we can converge the BSE calculations using only a handful of basis functions. Combining these methods, we perform a high-throughput screening to identify vdW heterobilayers with interesting excitonic properties. The calculated data is made available in the open heterostructure database HetDB [2,3], which is integrated with the C2DB monolayer database [4].

References:

1. Dario A. Leon et al., arXiv:2505.17292 (2025) 2. https://hetdb.fysik.dtu.dk
3. M. O. Sauer et al. arXiv:2504.05754
4. https://c2db.fysik.dtu.dk

Electromagnetic responses of Excitonic Insulators

In this seminar, I will first introduce the main concepts of bilayer exciton insulator, a new type of charge neutral quantum liquid recently realized in 2D materials. Then I will mostly focus on the electromagnetic responses of bilayer excitonic insulators (EIs) and identify two distinct collective modes: (1) Two gapped plasmon modes couple to the layer symmetric gauge field. The transverse mode is nearly dispersionless in the long wavelength limit, while the longitudinal mode, accounting for total charge fluctuations, has a linear dispersion with velocity proportional to 2D electrical polarizability. (2) A gapless phase (Goldstone) mode and a gapped amplitude mode, associated with the fluctuations of EI order parameter, couple to the layer antisymmetric gauge field. In the long wavelength and low frequency limit, the phase mode behaves like an acoustic phonon with speed inversely proportional to the square root of exciton compressibility. Significantly, its linear dispersion yields a cubic frequency dependence of the real admittance in microwave impedance microscopy (MIM), providing a method to detect the Goldstone mode directly.