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arxiv: 2605.01893 · v1 · submitted 2026-05-03 · ❄️ cond-mat.supr-con · cond-mat.mtrl-sci

Recognition: 4 theorem links

Two distinct superconducting regimes in Ti4Co2O under pressures

Lifen Shi , Keyuan Ma , Binbin Ruan , Zhen Wang , Pengtao Yang , Zhian Ren , Jianping Sun , Gang Li
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Fabian O. von Rohr Bosen Wang Jinguang Cheng
Authors on Pith no claims yet

Pith reviewed 2026-05-08 19:28 UTC · model grok-4.3

classification ❄️ cond-mat.supr-con cond-mat.mtrl-sci
keywords superconductivityhigh pressureTi4Co2Oupper critical fieldresistivity exponentdensity of statesFermi liquidphonon scattering
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The pith

Ti4Co2O develops two distinct superconducting regimes under pressure, one with Fermi-liquid transport and peak critical field, the other with rising Tc and phonon scattering.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper measures how the superconducting transition temperature Tc and upper critical field Bc2(0) of Ti4Co2O vary with pressure up to 70 GPa using electrical transport. Tc first increases slowly, drops sharply between 10 and 20 GPa, then rises again to reach 4.31 K at the highest pressure, while Bc2(0) forms a dome that peaks near 5 GPa at nearly twice its ambient value and stays above the Pauli limit. The normal-state resistivity follows a power law with exponent n equal to 2 at low pressure, indicating Fermi-liquid behavior, then switches to n equal to 4 at high pressure, pointing to stronger phonon scattering. These changes occur without any structural phase transition, and calculations tie the non-monotonic Tc to shifts in the electronic density of states. The work shows that even a material with weak spin-orbit coupling can host pressure-tunable superconducting behavior split into two clear regimes.

Core claim

By comparing the normal-state and superconducting properties, we identify two distinct superconducting regimes, with a low-pressure superconducting phase characterized by an enhanced Bc2(0) values and Fermi-liquid normal-state electrical transport (the exponent n = 2), and a high-pressure superconducting phase with a monotonically increased Tc and an enhancement in phonon scatterings (the exponent n = 4).

What carries the argument

The normal-state resistivity power-law exponent n, which switches from 2 to 4 with pressure and is used together with the dome-shaped Bc2(0) and non-monotonic Tc to separate the two regimes.

If this is right

  • Pressure can be used to switch the normal-state scattering from electron-electron (Fermi liquid) to phonon-dominated in this superconductor.
  • The low-pressure dome in Bc2(0) identifies an optimal window near 5 GPa for maximum critical field that exceeds the Pauli limit.
  • Tc continues to rise at the highest pressures, implying that values above 4.31 K are reachable if pressure can be increased further.
  • The absence of a structural transition up to 55.8 GPa means the regime switch and Tc changes arise from electronic density-of-states evolution under compression.
  • The calculated density-of-states changes explain why Ti4Co2O behaves differently from its isostructural compounds under pressure.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • Similar pressure-driven switches between scattering regimes may appear in related compounds if their density-of-states evolution follows the same pattern.
  • The two regimes could host different superconducting gap structures, which specific-heat or tunneling measurements under pressure could test.
  • The large bulk modulus of 192 GPa suggests the material remains stable for sustained high-pressure studies of its superconducting properties.

Load-bearing premise

The resistivity exponent n directly signals the dominant scattering mechanism without major contributions from disorder, magnetic fluctuations, or other effects that can produce similar power laws.

What would settle it

Transport data at pressures between 10 and 20 GPa showing that the exponent n changes gradually or stays near 3 instead of switching cleanly from 2 to 4 while Tc and Bc2 continue their reported trends.

Figures

Figures reproduced from arXiv: 2605.01893 by Binbin Ruan, Bosen Wang, Fabian O. von Rohr, Gang Li, Jianping Sun, Jinguang Cheng, Keyuan Ma, Lifen Shi, Pengtao Yang, Zhen Wang, Zhian Ren.

Figure 1
Figure 1. Figure 1: FIG. 1 view at source ↗
Figure 4
Figure 4. Figure 4: FIG. 4 view at source ↗
read the original abstract

We report on the pressure dependence of superconducting transition temperature Tc and upper critical field Bc2(0) through electrical transport of the Ti4Co2O superconductor (eg.,the superconducting transition temperature Tc = 2.5 K and the Bc2(0)=7.2T=2.9Tc). We find that the Tc exhibits non-monotonic pressure dependence:it rises monotonically at first with a pressure coefficient of dTc/dP=0.034 K/GPa, but rapidly decreases around 10-20 GPa, and then increases with the dTc/dP = 0.023 K/GPa, up to= 4.31 K at 69.7 GPa. Concurrently, the Bc2(0)exhibits a dome shaped pressure dependence, with its maximum at 5 GPa of almost twice the value at ambient pressure, exceeding the weak-coupling Pauli paramagnetic limit Bp throughout the whole pressure range. By comparing the normal-state and superconducting properties, we identify two distinct superconducting regimes, with a low-pressure superconducting phase characterized by an enhanced Bc2(0)values and Fermi-liquid normal-state electrical transport (the exponent n = 2), and a high-pressure superconducting phase with a monotonically increased Tc and an enhancement in phonon scatterings (the exponent n = 4). Room-temperature synchrotron X-ray diffraction indicates that there is no structural transition up to 55.8 GPa, which gives a relatively large bulk modulus of 192 GPa in comparison with other alloy superconductors. First-principles calculations suggest that the nonmonotonic Tc maybe closely related to the evolution of the density of states of Ti4Co2O upon compression, which is different from those of isostructural superconductors Ti4Ir2O and Nb4Rh2C. Our results show that even in the Ti4Co2O with weak spin-orbit coupling, superconductivity remains highly sensitive to the external stimuli such as pressure.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

1 major / 2 minor

Summary. The manuscript reports pressure-dependent electrical transport measurements on the superconductor Ti4Co2O, showing a non-monotonic Tc(P) that rises initially (dTc/dP = 0.034 K/GPa), drops around 10-20 GPa, then rises again (dTc/dP = 0.023 K/GPa) to 4.31 K at 69.7 GPa. Bc2(0) exhibits a dome peaking at ~5 GPa (exceeding the Pauli limit throughout), while room-temperature synchrotron XRD shows no structural transition to 55.8 GPa (bulk modulus 192 GPa). First-principles calculations link the Tc behavior to DOS evolution. The central claim is the existence of two distinct superconducting regimes: low-pressure (enhanced Bc2(0), n=2 Fermi-liquid transport) and high-pressure (monotonically rising Tc, n=4 phonon scattering).

Significance. If the two-regime partition holds, the work demonstrates pressure-tuned superconductivity in a weak-SOC material that differs from isostructural Ti4Ir2O and Nb4Rh2C, with the combination of transport data, structural stability, and DFT providing a coherent picture of how normal-state scattering and DOS changes affect Tc and Bc2. The experimental values (Tc up to 4.31 K, Bc2 exceeding Pauli limit) and absence of structural transition are concrete and falsifiable.

major comments (1)
  1. The partition into two distinct regimes rests on interpreting the resistivity exponent n (from ρ(T) = ρ0 + A T^n) as a direct diagnostic of scattering mechanism (n=2 for Fermi liquid at low P, n=4 for phonon scattering at high P). The manuscript does not provide quantitative analysis or bounds excluding contributions from residual disorder, interband scattering, or weak magnetic fluctuations, which can produce effective exponents in the same range. This assumption is load-bearing for the central claim of two regimes.
minor comments (2)
  1. Reported values such as Tc = 4.31 K, Bc2(0) = 7.2 T at ambient pressure, and the pressure coefficients lack stated uncertainties or error bars; inclusion of these (and raw resistivity curves) would strengthen the transport analysis.
  2. The first-principles DOS calculations are invoked to explain non-monotonic Tc, but the manuscript does not show explicit comparison plots of calculated DOS vs. pressure against the measured Tc(P) curve.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for the careful reading and constructive feedback on our manuscript. We address the major comment below and will revise the text to incorporate additional discussion.

read point-by-point responses

  1. Referee: The partition into two distinct regimes rests on interpreting the resistivity exponent n (from ρ(T) = ρ0 + A T^n) as a direct diagnostic of scattering mechanism (n=2 for Fermi liquid at low P, n=4 for phonon scattering at high P). The manuscript does not provide quantitative analysis or bounds excluding contributions from residual disorder, interband scattering, or weak magnetic fluctuations, which can produce effective exponents in the same range. This assumption is load-bearing for the central claim of two regimes.

    Authors: We agree that the resistivity exponent alone does not rigorously exclude all alternative scattering channels and that a more explicit discussion of this limitation is needed. The two-regime distinction in the manuscript is drawn from the joint evolution of multiple observables: the non-monotonic Tc(P), the dome in Bc2(0) that peaks near 5 GPa, the change in normal-state exponent, and the DFT-computed density-of-states variation. In the revised manuscript we will add a dedicated paragraph in the discussion section that (i) recalls the standard theoretical ranges for n in the presence of disorder, interband scattering, and weak magnetic fluctuations, (ii) notes that the observed switch from n≈2 to n≈4 occurs precisely where Bc2(0) reaches its maximum and Tc begins its second rise, and (iii) contrasts the behavior with the isostructural compounds Ti4Ir2O and Nb4Rh2C where similar exponent changes are discussed in the literature. While a fully quantitative decomposition would require additional modeling or higher-resolution data not available in the present study, the multi-probe consistency provides supporting evidence for the regime assignment. We will also insert appropriate caveats to avoid over-interpretation of n. revision: yes

Circularity Check

0 steps flagged

No circularity: claims rest on independent measurements and calculations

full rationale

The paper derives its central claim of two distinct superconducting regimes directly from measured pressure dependences of Tc, Bc2(0), and the fitted resistivity exponent n, together with XRD data confirming structural stability and separate first-principles DOS calculations. None of these steps reduce by the paper's own equations or self-citations to quantities defined solely by the target result; the regime partition follows from the observed non-monotonic trends rather than being imposed by construction.

Axiom & Free-Parameter Ledger

0 free parameters · 2 axioms · 0 invented entities

The central claim rests on standard condensed-matter interpretations of resistivity power laws and on first-principles electronic-structure calculations; no new free parameters are introduced and no new entities are postulated.

axioms (2)
  • domain assumption Resistivity temperature dependence exponent n=2 indicates Fermi-liquid behavior and n=4 indicates dominant phonon scattering
    Invoked to label the low-pressure and high-pressure normal states as distinct regimes.
  • domain assumption Absence of structural transition in XRD implies that observed changes in superconductivity arise from electronic-structure evolution
    Used to attribute the non-monotonic Tc and regime crossover to DOS changes rather than lattice symmetry breaking.

pith-pipeline@v0.9.0 · 5706 in / 1675 out tokens · 38582 ms · 2026-05-08T19:28:16.093647+00:00 · methodology

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Works this paper leans on

1 extracted references

  1. [1]

    1 Two distinct superconducting regimes in Ti4Co2O under pressuresLifen Shi1, 2, 3 =, Keyuan Ma3, 4 =, Binbin Ruan1, 2=, Zhen Wang1,2, Pengtao Yang1,2,Zhian Ren1,2, Jianping Sun1,2, Gang Li1,2, Fabian O. von Rohr4*, Bosen Wang1,2* andJinguang Cheng1,2*1Beijing National Laboratory for Condensed Matter Physics and Institute of Physics,Chinese Academy of Scie...

    2020