Femtosecond X-ray Snapshots Reveal Hidden Excitons in a Quantum Crystal

In the layered crystal titanium diselenide, one of the most contested questions in condensed matter physics has been quietly simmering for more than four decades: is the material an excitonic insulator, or is something more ordinary going on? A new study published in Nature Physics has now brought the sharpest answer yet, using pulses of extreme-ultraviolet light lasting just a few femtoseconds to catch fleeting electron–hole pairs in the act. The work, led by Alfred Zong and Michael W. Zuerch of the University of California, Berkeley, together with colleagues in the United States, Japan, and China, reveals short-range excitonic correlations surviving well above the material’s phase transition temperature, and a dramatically enhanced excitonic susceptibility right at the transition itself.

Excitons are bound states formed when an electron, promoted out of its usual place in a crystal, pairs up with the positively charged hole it leaves behind. Like other bosons, excitons can, in principle, condense into a single macroscopic quantum state when cooled sufficiently, producing an exotic phase of matter known as an excitonic insulator. Physicists have made striking progress observing this condensation in engineered low-dimensional systems, such as moiré heterostructures and atomically thin layers, where signatures like perfect Coulomb drag have been reported. But in bulk crystals the story has been far murkier, because the ideal superfluidity of a Bose condensate is nearly impossible when quasiparticles scatter between the bands hosting the electrons and holes.

Instead of long-range quantum coherence, theorists expect a bulk excitonic condensate to manifest as a spatial density order, a superlattice whose period is set by the inverse momentum separation of the bound electrons and holes. The trouble is that such a superlattice looks almost identical to an ordinary charge-density wave, a more mundane ordering of charge driven largely by lattice forces. This ambiguity sits at the heart of the long-running debate over 1T-TiSe2, a quasi-two-dimensional transition metal dichalcogenide that undergoes a phase transition at a critical temperature of roughly 200 kelvin. Earlier momentum-resolved electron energy-loss spectroscopy measurements reported a softening of an electronic collective mode consistent with an exciton condensate’s amplitude mode, but subsequent higher-resolution measurements found no such softening, because Landau damping at finite momentum blurs the signal.

Ultrafast spectroscopy has offered a way around this impasse, since the microscopic interactions underlying excitonic and charge-density-wave orders unfold on distinct timescales. Time-resolved angle-resolved photoemission can track how quickly the spectral gap and folded bands collapse after a laser pulse, and the collapse times depend strongly on the photoexcited carrier density, a hallmark of transient electron–hole dissociation. Yet this technique loses its power above the transition temperature, where long-range order vanishes and only short-range fluctuations remain in the quasi-two-dimensional system. What was needed was a probe sensitive to local electronic structure, one that works even without long-range order.

The Berkeley-led team turned to ultrafast broadband extreme-ultraviolet absorption spectroscopy, a pump–probe technique with element and orbital specificity. The excitonic instability in 1T-TiSe2 originates from the Coulomb attraction between selenium 4p holes and titanium 3d electrons near the Fermi level, so the researchers focused on absorption features involving precisely those states. Guided by first-principles calculations of the orbital-projected density of states, they assigned the observed selenium absorption peaks to transitions from spin–orbit-split selenium 3d core levels into unoccupied selenium 4p states, while the titanium M2,3 edges showed anomalies, including a blueshift of more than two electronvolts and a broad hump spanning 38 to 53 electronvolts, that only time-dependent density functional theory incorporating electron–electron interactions could reproduce.

Among these spectral features, one selenium peak stood out as an ideal reporter. When the team excited the crystal with a 3.4-femtosecond pulse, all the absorption peaks responded within 100 femtoseconds, and coherent oscillations at 6.0 terahertz, corresponding to the A1g phonon in which selenium ions displace out of plane relative to stationary titanium ions, rippled across most of the spectrum. But one selenium peak, labelled peak 2, showed no phonon-induced modulation at all, because its coupling to the A1g mode is negligible. That silence made it a clean window onto the local electronic and excitonic dynamics, free from the interfering rattling of the lattice.

The first surprise came when the researchers lowered the pump fluence nearly tenfold, from 1.7 to 0.2 millijoules per square centimetre, to limit the number of photoexcited carriers. While most transient features scaled down proportionally, the titanium many-body continuum displayed a sign reversal of its transient signal at 200 femtoseconds, a qualitatively different behaviour implying an additional dynamical process beyond simple carrier excitation. Sweeping the sample temperature in roughly 5-kelvin steps across the transition at this low fluence revealed an order-parameter-like onset, showing that this spectral window is sensitive to the establishment of long-range order, likely because the valence charge excitations producing the broad hump are modified by the changing titanium 3d states across the transition.

The selenium peak told an equally compelling story. From a state-filling perspective, its transient transmission decrease reflects an increased number of selenium 4p holes below the Fermi level, holes that could otherwise bind titanium 3d electrons into excitons. At 25 kelvin the decrease was larger than at 310 kelvin, pointing to additional holes released by the break-up of condensed excitons in the ordered state, and temperature-dependent measurements showed a clear onset of this suppression at the critical temperature. Crucially, because this peak is dominated by local dipole transitions, its time-domain signature is agnostic about whether long-range phase coherence exists among the excitons, meaning the team could hunt for excitonic correlations even above the transition.

What they found there is the study’s headline result. The pump fluence controls the transient mobile carrier density, and hence the plasmon frequency, which governs how quickly screening breaks the bound electron–hole pairs; the initial response time should therefore scale as one over the square root of the fluence. Below the transition temperature, the initial suppression indeed became markedly faster as fluence increased, consistent with earlier photoemission measurements. But remarkably, at 300 kelvin, far above the transition, the same trend held: the response visibly slowed at lower fluence, following the same inverse-square-root scaling with a constant offset attributable to the time needed to create mobile holes in the absence of excitonic interactions. The team considered alternative microscopic processes that depend on carrier density, but none could account for the observed fluence- and temperature-dependent trends, leaving short-range excitonic correlations persisting in the normal state as the natural interpretation.

Then came a second surprise at the transition itself. At 190 kelvin, close to the critical temperature, the initial response was consistently fast and roughly independent of pump fluence, breaking the scaling seen everywhere else. Measuring across temperatures from 23 to 350 kelvin at both low and high fluences showed that at low fluence the response time decreases gradually from both sides, reaching a minimum near the transition, while at high fluence it stays short and temperature-independent. The researchers formalized this with a simple model in which the carrier density decomposes into a temperature-independent susceptibility plus a diverging excitonic susceptibility that grows as a power of the inverse distance from the transition. The accelerated hole production near the transition reflects an enhanced excitonic susceptibility, where local excitonic correlations are so fragile that even a weak laser pulse breaks them, analogous to how enhanced phonon fluctuations in diffuse scattering expose a lattice instability. The authors also ruled out a dominant role for charge-density-wave phonons, noting that transient phonon renormalization of about 20 percent could only account for a 20 percent change in response time, dwarfed by the threefold reduction they measured. Because the material is quasi-two-dimensional, the excitonic correlations above the transition are likely confined in-plane, contributing to short-range fluctuations that lack three-dimensional phase coherence, while previous work suggests excitons also maintain interplane coherence in the ordered state. Together, the findings offer a unified picture in which excitonic correlations shape both the amplitude and the phase of the ordered state, and they provide a template, built on element- and orbital-resolved femtosecond spectroscopy, for hunting excitonic instabilities in other bulk candidates such as Ta2NiSe5 and Ta2Pd3Te5.

Subject of Research: Short-range excitonic correlations and enhanced excitonic susceptibility in the excitonic insulator candidate 1T-TiSe2 probed by few-femtosecond extreme-ultraviolet absorption spectroscopy

Article Title: Short-range excitonic correlations and enhanced excitonic susceptibility in 1T-TiSe2

Article References: Zong, A., Lin, S.-C., Sato, S. A., Berger, E., Nebgen, B. R., Hui, M., Lv, B. Q., Cheng, Y., Xia, W., Guo, Y., Xiang, D., & Zuerch, M. W. (2026). Short-range excitonic correlations and enhanced excitonic susceptibility in 1T-TiSe2. Nature Physics. https://doi.org/10.1038/s41567-026-03423-z

Image Credits: AI Generated

DOI: 10.1038/s41567-026-03423-z

Keywords: excitons, excitonic insulator, 1T-TiSe2, charge-density wave, ultrafast spectroscopy, extreme ultraviolet, phase transition, quantum materials, transition metal dichalcogenide, excitonic susceptibility, femtosecond pump-probe, condensed matter physics