Strange-metal behavior in correlated molecular conductors

arXiv:2610.12044 · cond-mat.str-el · Submitted 2026-10-08 · Read on arXiv

Listen

Radio episode about this paper

Transcript

Introduction to the show: ident: Quantum Radio. Generated commentary on the latest quantum physics and condensed matter papers.

Kai: Today's paper: "Strange-metal behavior in correlated molecular conductors".

Mira: The gist The κ-BEDT-TTF salts are prototypical Mott systems, realizing a correlation-driven electronic transition from a Fermi-liquid state to a Mott insulator,

Kai: First, who's behind it and why it matters.

Title and authors: Kai: The title itself, "Strange-metal behavior in correlated molecular conductors," tells you right away that this isn't just another simple metal study; they are looking at something more fundamental.

Mira: It suggests they're investigating how strong electronic correlations and structure combine to create these strange states, moving beyond the standard pictures of how these materials should behave.

Lev: From a hardware standpoint, seeing what the paper actually built and measured is important—it tells us if these weird behaviors are something we can even try to observe in a lab.

Kai: Exactly. The authors are focusing on specific salts, like kappa-Hg3−δBr8 and kappa-Hg3−δCl8, which they found have this unusual transport signature.

Mira: And what they're highlighting is that these systems show linear temperature dependence in resistivity up to pretty high temperatures, which usually flags a strange metal situation.

Lev: That linear temperature dependence is the first big signal we look for when trying to identify something outside of the usual Fermi liquid behavior, and it's a key indicator here.

The paper's summary: Kai: The paper summarizes how these salts, which have incommensurate anion layers, show a temperature-dependent dc resistivity that follows ρ(T) proportional to T across a wide range.

Mira: That linear relationship is what sets them apart from the other compounds they tested, like kappa-CuBr and kappa-CuNCS, which show a quadratic dependence on temperature before superconductivity starts.

Lev: So, the summary is basically showing that in these specific molecular conductors, you get this linear resistivity scaling over a huge temperature range.

Kai: And it goes deeper than just the DC measurement; they look at frequency-dependent complex conductivity to see how the carriers respond optically and electronically.

Mira: That's where things get interesting because they find that for the strange metals, the absolute value of that conductivity follows a power law, σˆ(ω) proportional to omega minus zero point seven.

Lev: That power law is a specific signature; it’s not just some random noise but points toward a breakdown in the conventional description of how these carriers move through the material.

The paper's improvements: Kai: The authors point out that this research provides better insight into the microscopic origins by linking this power-law optical response to universal Planckian dissipation.

Mira: They show that for these strange metals, the ratio of a certain scattering rate to temperature scales with a factor of alpha about one point two, which is a universal scaling constant.

Lev: That scaling factor alpha is useful because it connects the system's behavior back to general principles of dissipation in strongly correlated systems.

Kai: They also suggest that the key difference between the Fermi liquids and these strange metals lies in the crystal structure itself, specifically those large-scale spatial modulations in the anion layer.

Mira: That structural modulation is what they argue causes local chemical potential variations, which feeds into these electronic correlations to produce the strange metal features.

Lev: So, they're making a connection between how things are physically built—the crystal structure—and the resulting electronic state we measure with transport and light.

Conclusion: Kai: To wrap up, this paper on "Strange-metal behavior in correlated molecular conductors" establishes that these specific kappa-BEDT-TTF salts fit the criteria for strange metals based on their linear temperature dependence and unique optical response.

Mira: The main implication is that the structure of the anion layers plays a crucial role in driving this behavior, showing that strong correlation isn't enough on its own.

Lev: And from what I see, Lev thinks that linking this linear-in-frequency scattering rate to universal Planckian dissipation is a very strong argument for how these systems operate at this critical state.

Kai: Yeah, it really solidifies the picture that disorder and structural modulation are important factors in pushing these materials into the strange metal phase we observe.

Mira: So, even though the Fermi liquid state is expected in many strongly correlated metals, this study shows how structural randomness can suppress it and lead to these strange metal features.

Lev: It suggests that both strong correlation and those large-scale spatial modulations are necessary ingredients to get that strange metal behavior seen in kappa-Hg3−δBr8 and kappa-Hg3−δCl8.

Physikalisches Institut, Universitat Stuttgart · Institut f¨ur Anorganische Chemie, Universitat Stuttgart

cond-mat.str-el

Submitted: 2026-10-08

Updated: 2026-10-08

Comments: 7 Pages (3 Figures) + Supplementary

License: http://creativecommons.org/licenses/by/4.0/

Importance score: 74/100

The gist: The gist The κ-BEDT-TTF salts are prototypical Mott systems, realizing a correlation-driven electronic transition from a Fermi-liquid state to a Mott insulator, and their transport and optical

Key concepts

Strange Metal
A state where charge transport violates the usual temperature-dependent power laws of normal metals, specifically showing linear resistivity ($ ho(T) \propto T$). It suggests a breakdown of conventional Fermi-liquid quasiparticle transport.
Planckian Dissipation
This refers to a specific type of electronic response in strange metals where the ratio $\hbar\Gamma/k_BT$ approaches a universal scaling factor ($\alpha \approx 1.2$). It indicates that the system's energy relaxation is governed by universal quantum limits rather than simple temperature-dependent scattering.
Stretched Drude Behavior
This describes the frequency dependence of optical conductivity, where the response follows a power law $|\sigma(\omega)| \propto \omega^{-0.7}$. This behavior is characteristic of strange metals and resembles features seen in high-temperature cuprate superconductors.
Incommensurate Anion Layers
The specific crystal structure of $\kappa$-Hg3−\deltaX8 salts features anion layers that are not perfectly periodic. These large-scale spatial modulations cause local variations in the chemical potential, which is a key structural feature driving the strange metal behavior.

Terminology

Summary

The gist The κ-BEDT-TTF salts are prototypical Mott systems, realizing a correlation-driven electronic transition from a Fermi-liquid state to a Mott insulator, and their transport and optical investigations reveal hallmarks of “strange metals” such as linear resistivity and Planckian dissipation

Strange Metal Identification

The label “strange metal” marks the breakdown of Fermi-liquid quasiparticle transport It is assigned to compounds whose charge transport violates the temperature-dependent power laws of conventional metals and records linear resistivity ρ(T) ∝ T [1–3] Theoretical studies have linked it to various effects such as electronic correlations, disorder, magnetic/spin fluctuations, quantum criticality and entanglement without reaching consensus but agree on the observation of Planckian dissipation [1, 4–13] The presence of electronic inhomogeneities could be a decisive factor in the emergence of strange-metal behavior in doped transition-metal superconductors [5, 14]

Key Experimental Findings

The study investigates the electrodynamics of two charge transfer salts, κ-(BEDT-TTF)4Hg3−δX8 [21, 22] (X = Br, and Cl, abbreviated as κ-Hg3−δBr8 and κ-Hg3−δCl8) which are composed of incommensurate anion layers The results show that both systems exhibit metallic behavior with a linear-in-T increase up to elevated temperatures and comparable slopes Most importantly, a ρ(T) ∝ T2 dependence is absent for both κ-Hg3−δX8 compounds Deeper insight into the microscopic mechanisms can be gained from the frequency-dependent complex conductivity ˆσ(ω) = σ1(ω) + iσ2(ω) [28] For strange metals, the ω-dependence strongly deviates from the conventional Drude roll-off, approaching a strongly reduced slope σˆ(ω) ∝ ω −0.7 at low T

Optical Response and Scaling

The frequency-dependent optical response reveals that the itinerant carriers follow a power law σˆ(ω) ∝ ω −0.7 This behavior resembles the power-law reported for high-Tc cuprates [9, 29] and can be referred to as “stretched Drude behavior” The linear-in-frequency response scales with universal Planckian dissipation, following ħΓ = αkBT The ratio ħΓ/kBT approaches the scaling factor α ≈ 1.2 for both κ-Hg3−δBr8 and κ-Hg3−δCl8

Structural Origin of Behavior

The key difference lies in the crystal structure: only regular anion layers allow for archetypal Fermi-liquid behavior at low temperatures The incommensurate compounds κ-Hg3−δX8 possess largescale spatial modulations in the anion layer, which cause local chemical potential variations This structural modulation along with strong electronic correlations inherent to κ-BEDT-TTF salts leads to “strange-metal” features observed in the electrical and optical response

Comparison with Fermi Liquids

The Fermi-liquid compounds κ-CuBr and κ-CuNCS exhibit a quadratic increase of scattering rate with energy, Γ(T, ω) ∝ [(2πkBT)2 + (ħω)2] In contrast, the strange metals κ-Hg3−δBr8 and κ-Hg3−δCl8 show a scattering rate that increases linearly with frequency, Γ(ω) ∝ ω This linear increase of resistivity over a large temperature range is characteristic of the strange metal state The comparison shows that while Fermi-liquid compounds approach Γ(ω) ∝ ω2 at low T, the strange metals acquire Γ(ω) ∝ ω

Conclusion on Mechanism

The key difference lies in the crystal structure: only regular anion layers allow for archetypal Fermi-liquid behavior at low temperatures The incommensurate compounds κ-Hg3−δX8 possess largescale spatial modulations in the anion layer, which cause local chemical potential variations Our study implies that the structural modulation along with strong electronic correlations inherent to κ-BEDT-TTF salts leads to “strange-metal” features observed in the electrical and optical response The results elucidate the link between the presence of randomness and the occurrence of the strange metal phase in strongly correlated electron systems

Final Summary

Our comprehensive study establishes the molecular conductors κ-(BEDT-TTF)4Hg3−δX8 as “strange metals” because (i) the dc resistivity rises as ρ(T) ∝ T to ambient temperature, (ii) the complex conductivity reveals a stretched Drude decay σˆ(ω) ∝ ω −0.7, (iii) the scattering rate obtained from broad-range infrared measurements increases linearly with frequency, and (iv) follows the Planckian dissipation These observations differ greatly from the Fermi-liquid state confirmed in organic superconductors κ-CuBr and κ-CuNCS by transport and optical properties The key difference lies in the crystal structure: only regular anion layers allow for archetypal Fermi-liquid behavior at low temperatures The incommensurate compounds κ-Hg3−δX8 possess largescale spatial modulations in the anion layer, which cause local chemical potential variations Our study implies that the structural modulation along with strong electronic correlations inherent to κ-BEDT-TTF salts leads to “strange-metal” features observed in the electrical and optical response Even though the Fermi-liquid state is generally one of the fundamental properties of strongly correlated metals, it is often suppressed by disorder, temperature etc. revealing the strange-metal behavior

How it works

The linear-in-frequency scattering rate of these strange metals scales to the Planckian dissipation This response only occurs in systems with incommensurate anion layers and establishes a direct link to large-scale spatial modulations The local potential becomes uniform and – in addition – the effective correlation strength decreases when pressure is applied, implying that both strong correlation and spatial modulation are necessary to establish the strange metal behavior observed in κ-Hg3−δX8

Comparison of Transport and Optical Data

The comparison of dc resistivity ρdc obtained from transport studies with the optical resistivity in the low-frequency limit validates that our frequency-dependent measurements capture the electronic ground state and that the relaxation of itinerant carriers is not affected by any other factors For strange metals κ-Hg3−δBr8 and κ-Hg3−δCl8, ρωc exhibits comparable absolute values and the same anisotropy as obtained from dc measurements

Extended Drude Analysis

The extended Drude model extracts a frequency-dependent scattering rate Γ(ω) and effective mass m∗ from Eq. (S3) without any underlying assumptions For the strange-metal compounds, as illustrated in Figs.

Improvements for AI systems

  1. textbfIncreased Predictive Capability for Strange Metal Signatures: The improved system can now predict strange metal behavior based on specific structural features rather than just electronic correlations alone, by recognizing that This response only occurs in systems with incommensurate anion layers and establish a direct link to large-scale spatial modulations.

  2. textbfEnhanced Material Screening for Charge Transport: The system can accurately classify materials based on transport metrics, differentiating between Fermi liquids and strange metals using the specific quantitative criteria mentioned, such as comparing the quadratic increase of scattering rate with energy in Fermi liquids versus the linear increase with frequency in strange metals.

  3. textbfImproved Analysis of Inhomogeneous Systems: The system can model and interpret the role of structural disorder in electronic behavior, specifically linking the effect mimics spatial disorder resulting in local variations in charge transfer to the emergence of strange metal features.

  4. textbfRefinement of Optical Response Modeling: The improved system can accurately predict the low-frequency optical conductivity dependence, recognizing that for strange metals, σˆ(ω) ∝ ω −0.7, which is much flatter than that observed in the Fermi liquid compounds.

  5. textbfContextual Scaling of Dissipation: The system can apply universal scaling relations to characterize dissipation in novel systems by calculating the scaling factor α, as demonstrated by finding that for strange metals, the ratio ħΓ/kBT, which is plotted as a function of normalized frequency ħω/kBT in Fig. 3(f,h), approaches the scaling factor α ≈ 1.2.

Abstract

The κ-BEDT-TTF salts are prototypical Mott systems, realizing a correlation-driven electronic transition from a Fermi-liquid state to a Mott insulator. The metallic compounds with spatially modulated Hg-containing anions, κ -(BEDT-TTF) 4 Hg 3-δX 8 (X = Br, and Cl), however, fall outside this classification. Our temperature-dependent transport and broad-range optical investigations reveal a ρ(T) proportional to T dependence of the resistivity up to elevated temperatures and a power-law behavior of the frequency-dependent conductivity σ (ω) proportional to ω-0.7; well-known hallmarks of ``strange metals''. The linear-in-frequency scattering rate of these strange metals scales to the Planckian dissipation. We demonstrate that this response only occurs in systems with incommensurate anion layers and establish a direct link to large-scale spatial modulations. Our conclusions can be extended to other strongly correlated strange-metal compounds

Sources

Related papers