{"id":"4cf77667-5b85-415e-9256-2ca3b2fe2ffa","arxiv_id":"2607.11237","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":5.5,"correctness_risk":"medium","formal_verification":"none","parameter_count":4,"one_line_summary":"Hot-electron cooling in epitaxial TiN is anisotropic: [111] films show longer e-ph lifetimes (~16 fs calc, 110 fs exp) than [100] and [110], enabling orientation-based tuning.","lead":"TiN thin films cool hot electrons at different rates depending on crystal face: [111] is slowest, [110] fastest. Theory and pump-probe data agree on the trend, so orientation can be used as a design knob for plasmonic and photothermal devices.","discovery_kind":"extension","skeptic_critique":{"model":"grok-4.5","headline":"Slab-model e-ph lifetimes may not represent bulk-like films, and the TTM/phase-space ordering mismatch undercuts a clean e-ph assignment of the TAS ranking.","rationale":"The Reader correctly isolates the multi-exponential TAS assignment and the TTM vs phase-space discrepancy as the weakest assumption; that is the same soft spot I find load-bearing. I only sharpen it by noting that the theoretical side is itself a slab calculation whose surface/termination sensitivity is not quantified against bulk, so both ends of the claimed consistency rest on models that can introduce orientation-dependent artifacts. Absolute timescale mismatch is already acknowledged by the authors and is not fatal; the ranking itself is the claim. Because the paper already flags the TTM ordering failure and supplies XRD/TEM/XPS evidence of quasi-epitaxy and bulk-like stoichiometry, the concern does not rise to REJECT. It does keep the verdict at CONDITIONAL: the orientation trend is plausible and useful, but a clean demonstration that e-ph phase space (rather than phonon bottlenecks or fit choices) is the common cause still requires the concrete check above. No change to the Reader’s overall posture is needed.","tokens_in":11006,"tokens_out":805,"duration_ms":9274,"concrete_test":"Recompute the orientation-resolved e-ph lifetimes (Eq. 1, same 10 meV EF window, same Monte-Carlo sampling) on bulk TiN supercells or on slabs thickened to ≥4 nm with both terminations, and re-extract τ1 from the raw TAS kinetics after (i) fixing the intermediate/slow components to common values across orientations and (ii) restricting the probe window to a single isosbestic region. If either the calculated order or the experimental order reverses or the [100]–[110] gap collapses below the reported ~10 fs difference, the systematic e-ph ranking claim does not hold.","verdict_should_be":"CONDITIONAL","load_bearing_attack":"The central claim is that crystallographic orientation systematically tunes e-ph-mediated hot-carrier relaxation, with calculated quasiparticle lifetimes near EF ([111]-Ti 15.96 fs > [100] 13.69 fs > [110] 11.12 fs) and measured TAS cooling times (110, 90, 80 fs) sharing the same order. The load-bearing link is that both quantities are governed by the same orientation-dependent e-ph scattering strength (phase space + matrix elements). That link is least secure for two tightly coupled reasons. (1) Lifetimes are obtained from thin slabs (>2 nm, 15 Å vacuum, 16×16×1 k-grid, MLWF of Ti-d/N-p) rather than bulk or thick-film models; surface states, termination choice ([111]-Ti preferred by ZPE), and quantum-well remnants can reshape the density of states and e-ph phase space near EF relative to the 100 nm experimental films. (2) The paper itself reports that a two-temperature-model analysis (Fig. S9) yields the opposite [100] vs [110] ordering (112.9 / 83.9 / 78.6 fs for [111]/[110]/[100]) because TTM omits energy-conserving phase space; yet the experimental assignment of the fastest multi-exponential component (τ1 in the global fit over 520–550 nm) still rests on a TTM interpretation. If the measured ranking is partly set by orientation-dependent optical-phonon bottlenecks, residual surface oxide, or probe-window spectral weight rather than the intrinsic e-ph phase space that the slabs compute, the claimed consistency is only qualitative and the design guideline is weaker than stated.","agreement_with_reader":"agree"},"referee_report":{"model":"grok-4.5","summary":"The manuscript claims that crystallographic orientation systematically tunes electron–phonon-mediated hot-carrier relaxation in TiN thin films. First-principles calculations on oriented slabs (MLWF + Fermi’s golden rule) yield quasiparticle e–ph lifetimes near EF of 15.96 fs ([111]-Ti), 13.69 fs ([100]) and 11.12 fs ([110]), attributed to orientation-dependent energy-conserving phase space. Quasi-epitaxial 100 nm films grown on MgO are characterized by XRD/TEM/XPS and measured by 400 nm pump / white-light probe TAS; global multi-exponential fits of the 520–550 nm kinetics give population cooling times of 110 fs, 90 fs and 80 fs that follow the same order. The authors emphasize that the few-fs and few-hundred-fs scales represent single-event scattering versus collective cooling, yet share a consistent trend, thereby offering orientation as a design handle for refractory plasmonics and energy conversion.","tokens_in":11449,"tokens_out":1171,"duration_ms":22571,"significance":"If the orientation-dependent ranking is robust, the work supplies a concrete materials-design guideline for TiN-based plasmonic and photothermal platforms, where longer-lived hot carriers (or slower cooling) can be selected by choosing the [111] growth direction. Strengths include the independent theory and experiment streams (no mutual fitting), careful film characterization that rules out bulk oxidation, explicit phase-space analysis that rationalizes the lifetime order, and open acknowledgment of the single-event versus collective timescale distinction. The combination of slab e–ph calculations with orientation-controlled epitaxial films is a useful addition to the TMN hot-carrier literature.","major_comments":[{"comment":"Methods §2.1, Eq. (1) and Fig. 2g–i: lifetimes are extracted from thin slabs (>2 nm, 15 Å vacuum, 16\times16\times1 k-mesh, Ti-d/N-p MLWFs) with a preferred [111]-Ti termination chosen by ZPE. Experimental films are 100 nm thick. Surface states, termination, and residual quantum-well character can reshape the near-EF DOS and e–ph phase space relative to bulk-like films. A bulk (or substantially thicker-slab) calculation of the same orientation-resolved lifetimes, or an explicit demonstration that the anisotropy survives bulk Brillouin-zone sampling, is needed to confirm that the computed ranking is not surface-dominated.","section":"Methods 2.1 / Results Fig. 2"},{"comment":"Discussion (paragraph containing Fig. S9) and Methods §2.2: the two-temperature-model analysis yields the reversed ordering [111] > [110] > [100] because TTM omits energy-conserving phase space, yet the experimental assignment of the fastest global-fit component τ1 to e–ph coupling is still justified by reference to the TTM. This internal tension weakens the claim that the measured ranking directly reflects the same phase-space anisotropy computed for the slabs. A clearer, TTM-independent justification for isolating τ1 (e.g., fluence dependence, spectral-weight analysis, or orientation-resolved phonon-bottleneck estimates) is required before the design guideline can be considered quantitative.","section":"Discussion / Methods 2.2"},{"comment":"Table 1 and abstract: absolute timescales differ by a factor of ~7. While the single-event versus collective distinction is stated, the phonon-bottleneck argument (optical-to-acoustic transfer across the calculated band gaps) remains qualitative and is not shown to be orientation-dependent. Without even a semi-quantitative estimate of how many scattering events or how the bottleneck strength varies with orientation, the assertion that crystallographic orientation provides a “practical and powerful route to tune e–ph-governed relaxation” rests only on a qualitative trend.","section":"Table 1 / Discussion"}],"minor_comments":[{"comment":"Abstract: “Pump- probe measurements reveals” should be “reveal”; several other hyphenation and spacing inconsistencies appear throughout.","section":"Abstract"},{"comment":"Figure 2g–i and Table 1: the averaging window (10 meV about EF) and the precise definition of the reported lifetime (mean, median, or Fermi-surface average) should be stated in the caption or methods.","section":"Fig. 2 / Table 1"},{"comment":"Experimental lifetimes lack uncertainty estimates from the global fit; adding standard errors or bootstrap ranges would strengthen Table 1.","section":"Table 1 / Methods 2.2"},{"comment":"Notation for the [111] termination ([111](Ti), [111]-Ti, Ti-terminated) is inconsistent across text, figures and SI.","section":"Throughout"}],"recommendation":"major_revision","confidential_remarks":"The central trend is interesting and the experimental film quality is high, but the load-bearing link between slab quasiparticle lifetimes and TAS cooling times is currently only qualitative. I would not reject; a careful revision that addresses the slab-to-bulk and TTM-assignment issues should make the paper suitable. Novelty is solid for the TiN community but incremental relative to prior orientation or e–ph studies on other metals."},"author_rebuttal":null,"desk_editor":{"model":"grok-4.5","letter":"The new piece is the first side-by-side, orientation-controlled comparison of first-principles e-ph quasiparticle lifetimes and TAS cooling times on the same three faces of quasi-epitaxial TiN. Earlier theory treated bulk rock-salt TiN; this paper shows [111]-Ti > [100] > [110] in both the calculated few-fs scattering lifetimes (15.96 / 13.69 / 11.12 fs) and the measured population cooling (110 / 90 / 80 fs). That ranking is the result worth taking away.\n\nThey do the work carefully. Slabs are thick enough to suppress quantum-well artifacts, phonons are stable, MLWFs reproduce the bands near EF, and the phase-space analysis (Fig. S4) explains why [111] is longest. On the experimental side, XRD/TEM/SAED confirm quasi-epitaxy on MgO, XPS shows bulk oxygen is below detection after sputtering, and the global multi-exponential fit is standard. They explicitly note that single-event scattering and collective cooling are different quantities that only need to share the trend. That honesty helps.\n\nSoft spots are real but limited. Absolute timescales differ by ~7\times, as expected once optical-phonon bottlenecks enter. More awkwardly, their own TTM analysis (Fig. S9) reverses the [100]–[110] order because it drops energy-conserving phase space, yet the assignment of the fastest TAS component still leans on TTM language. The slab-versus-100 nm film mismatch is also present: surface termination and residual surface states could reshape the DOS near EF relative to the thick films. Neither issue invents the anisotropy; both keep the claim at the level of a robust ranking rather than a quantitative lifetime map. Free parameters (Gaussian broadening, Monte-Carlo sampling, fit window) are conventional and do not appear to drive the ordering.\n\nThis is for people who grow or model refractory plasmonic films and want a practical handle on hot-carrier lifetime. It does not open a new subfield, but it is clean enough that a serious editor should send it to referees. I would cite the ranking when I need an orientation lever for TiN, and I would bring the paper to reading group if we are discussing e-ph engineering in TMNs.","headline":"Solid orientation-resolved ranking of e-ph cooling in epitaxial TiN; absolute timescales and TTM ordering mismatch keep it qualitative, but the result is real and usable.","tokens_in":12050,"tokens_out":562,"would_cite":true,"duration_ms":7023,"reading_group":"maybe","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":[],"model":"grok-4.5","headline":"Crystal orientation tunes how fast hot electrons cool in TiN thin films, with [111] lasting longest.","keywords":["hot-carrier lifetime","electron-phonon scattering","crystal orientations","titanium nitride","first-principles calculation","ultrafast transient absorption spectroscopy","refractory plasmonics"],"falsifier":"A measurement or calculation that reverses the lifetime ordering (for example, [110] longer than [111]) under identical film thickness, stoichiometry, and pump–probe conditions would falsify the claimed orientation control of e–ph cooling.","tokens_in":11911,"feed_emoji":"⚡","tokens_out":596,"duration_ms":5776,"temperature":0.7,"pith_summary":"This paper shows that the crystallographic direction of a titanium nitride thin film changes how quickly photoexcited hot electrons dump their energy into the lattice. First-principles calculations of electron–phonon scattering near the Fermi level give longer single-event lifetimes for [111] (about 16 fs) than for [100] or [110]. The same ordering appears in ultrafast pump–probe measurements of population cooling on the films (110 fs versus 90 fs and 80 fs). Because the theory and experiment track the same trend even though they probe different timescales, the authors argue that orientation is a practical handle for engineering hot-carrier lifetime in refractory plasmonic and energy-conversion devices.","feed_headline":"TiN crystal orientation slows or speeds hot-electron cooling","feed_subtitle":"[111] films cool slowest; theory and pump–probe data track the same trend for device design","key_machinery":"Orientation-resolved electron–phonon quasiparticle lifetimes obtained from maximally localized Wannier functions and Fermi’s golden rule on DFT slabs; these microscopic rates are then compared with the fastest component of a global multi-exponential fit to transient-absorption kinetics, interpreted via a two-temperature model as collective hot-electron cooling.","core_discovery":"In orientation-controlled TiN thin films, electron–phonon scattering strength is anisotropic: the Ti-terminated [111] direction has a longer calculated quasiparticle lifetime near the Fermi level (15.96 fs) than [100] (13.69 fs) or [110] (11.12 fs), and the measured hot-electron cooling times follow the same order (110 fs, 90 fs, 80 fs). The anisotropy arises mainly from orientation-dependent energy-conserving phase space for phonon-assisted scattering.","pith_inferences":[],"forward_implications":[],"fun_headline_variants":["TiN orientation tunes hot-electron cooling rates","[111] TiN stretches hot-carrier lifetime via weaker e-ph scattering","Crystal face sets anisotropic e-ph scattering in TiN films","Orientation slows hot-electron cooling most in TiN [111]","TiN [111] cools slowest as calculated lifetimes track measurements"],"cache_read_input_tokens":128,"weakest_assumption_plain":"That the shortest lifetime extracted from the multi-exponential fit of the pump–probe signal can be cleanly assigned to electron–phonon coupling for every crystal orientation.","fun_headline_variants_meta":{"raw":{"variants":["TiN orientation tunes hot-electron cooling rates","[111] TiN stretches hot-carrier lifetime via weaker e-ph scattering","Crystal face sets anisotropic e-ph scattering in TiN films","Orientation slows hot-electron cooling most in TiN [111]","TiN [111] cools slowest as calculated lifetimes track measurements"]},"model":"grok-4.5","effort":"low","cost_usd":0.00582,"raw_usage":{"total_tokens":1548,"prompt_tokens":867,"num_sources_used":0,"completion_tokens":90,"cost_in_usd_ticks":58200000,"prompt_tokens_details":{"text_tokens":867,"audio_tokens":0,"image_tokens":0,"cached_tokens":0},"completion_tokens_details":{"audio_tokens":0,"reasoning_tokens":591,"accepted_prediction_tokens":0,"rejected_prediction_tokens":0}},"tokens_in":867,"tokens_out":90,"duration_ms":6145,"temperature":1.0,"reasoning_tokens":591,"cache_read_input_tokens":0,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-07-14T06:05:14.651702+00:00","model_set":{"reader":"grok-4.5"},"falsifier":"A measurement or calculation that reverses the lifetime ordering (for example, [110] longer than [111]) under identical film thickness, stoichiometry, and pump–probe conditions would falsify the claimed orientation control of e–ph cooling.","supporting_citations":[],"review_version":1}