Persistence of fermionic spin excitations through a genuine Mott transition in $\ensuremath{\kappa}$-type organics

Persistence of fermionic spin excitations through a genuine Mott transition in $κ$ -type organics

S. Imajo, N. Kato, R. J. Marckwardt, E. Yesil, H. Akutsu, and Y. Nakazawa

Phys. Rev. B 105, 125130 – Published 22 March 2022

Abstract

We investigate the continuous variation of electronic states from a Fermi liquid to a quantum spin liquid induced by chemical substitution in the organic dimer-Mott system of $κ - {[{(BEDSe- TTF)}_{x} {(BEDT- TTF)}_{1 - x}]}_{2} Cu [N {(CN)}_{2}] Br$ . Electrical transport measurements reveal that the mixing of BEDSe-TTF into the BEDT-TTF layers induces a quantum Mott transition around $x = 0.10$ . Although a charge gap disappears at this point, magnetic susceptibility and heat capacity measurements indicate that fermionic low-energy excitations remain in the insulating salts, suggesting that fermionic spin excitations persist. We propose that the transition can be classified into a genuine Mott transition.

Received 30 March 2021
Revised 8 March 2022
Accepted 9 March 2022

DOI:https://doi.org/10.1103/PhysRevB.105.125130

Physics Subject Headings (PhySH)

Quantum spin liquid Quasiparticles & collective excitations Spin dynamics

Mott insulators Organic superconductors

Resistivity measurements Susceptibility measurements X-ray diffraction

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

S. Imajo ^1,2,*, N. Kato¹, R. J. Marckwardt ¹, E. Yesil¹, H. Akutsu¹, and Y. Nakazawa¹

¹Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan
²Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581, Japan

^*imajo@issp.u-tokyo.ac.jp

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Vol. 105, Iss. 12 — 15 March 2022

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Figure 1
(a), (b) Schematic phase diagrams near a pressure-induced Mott transition, accompanied by (a) an antiferromagnetic transition and (b) no magnetic transition. (c) Arrangement of the donor molecules in $κ$ -type organics. Molecular dimers form a distorted triangular lattice, characterized by the transfer integral ratio $t^{'} / t$ . The right figure shows the chemical structural formulas of BEDT-TTF and BEDSe-TTF. (d) Mixing ratio $x$ dependence of unit-cell volume $V$ and the ratio of in-plane unit-cell length $c / a$ of $κ - {[{(BEDSe- TTF)}_{x} {(BEDT- TTF)}_{1 - x}]}_{2} Cu [N {(CN)}_{2}] Br$ at room temperature. The data point for $x = 1$ is obtained from Ref. [14]. Dotted lines are linear fits to these parameters.
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Figure 2
(a) Temperature dependence of reduced resistivity $R (T) / R (30 K)$ of mixed salts $κ - {[{(BEDSe- TTF)}_{x} {(BEDT- TTF)}_{1 - x}]}_{2} Cu [N {(CN)}_{2}] Br$ ( $x = 0$ , 0.06, 0.08, 0.11, and 0.17). For $x = 0.06$ and 0.08, data at 2 T are also plotted. Arrows denote the superconducting transition temperature $T_{c}$ . The dotted curve on the $x = 0.08$ indicates that data do not show hysteresis depending on heating and cooling. (b) Arrhenius plot of the resistance. Dotted lines are linear fits using the respective charge gap $Δ / k_{B}$ . (c) Magnetic susceptibility $M / H$ at 1 T as a function of temperature. Data for $x = 0$ are obtained from Ref. [4]. (d) $C_{p} / T$ vs $T^{2}$ for each salt in a magnetic field. The dashed line shows a fitting to $C_{p} / T = γ_{N} + β T^{2}$ , where $γ_{N}$ and $β$ are the electronic and lattice heat capacity coefficients, respectively. (e) Zero-field heat capacity for each mixed salt plotted as $C_{p} / T$ vs $T^{2}$ . We additionally show the data for $x = 0$ taken from the literature [29]. (f) Low-temperature heat capacity of the salt $x = 0.17$ at 0, 2, 4, and 6 T.
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Figure 3
Substitution ratio $x$ dependence of physical parameters: (a) $(d R / d T) / R$ , (b) $Δ / k_{B}$ and $χ (15 K)$ , and (c) $γ_{N}$ and $β$ . To clarify superconducting and PSC regions, the dotted curve with $T_{c}$ (black box) and the dashed curve are also plotted in (a). The light blue region in (a) roughly corresponds to the metal-insulator crossover area $(d R / d T) / R = 0$ . Gray shaded areas in (b) and (c) represent the possible position of QMT. Translucent curves superimposed over data points provide a visual guide.
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Figure 4
Suggested electronic phase diagram of dimer-Mott organic system. Each salt is located at symbols (red circle: $κ - {(BEDT- TTF)}_{2} Cu [N {(CN)}_{2}] Br$ ; blue triangle: $κ - {(BEDT- TTF)}_{2} Cu [N {(CN)}_{2}] Cl$ ; green square: $κ - {(BEDT- TTF)}_{2} {Cu}_{2} {(CN)}_{3}$ ). Arrows signify routes for changing parameters (red arrow: our study; green arrow: Refs. [6, 36, 44]; blue arrow: Refs. [4, 34, 38, 50, 51, 52]; violet arrow: Refs. [42, 43]; black arrow: Ref. [32]). The orange area indicates the superconducting phase in the conventional FL region.
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Physical Review B

covering condensed matter and materials physics

Persistence of fermionic spin excitations through a genuine Mott transition in $κ$ -type organics

S. Imajo, N. Kato, R. J. Marckwardt, E. Yesil, H. Akutsu, and Y. Nakazawa

Phys. Rev. B 105, 125130 – Published 22 March 2022

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Physical Review B

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Persistence of fermionic spin excitations through a genuine Mott transition in κ-type organics

S. Imajo, N. Kato, R. J. Marckwardt, E. Yesil, H. Akutsu, and Y. Nakazawa

Phys. Rev. B 105, 125130 – Published 22 March 2022

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Persistence of fermionic spin excitations through a genuine Mott transition in $κ$ -type organics