Coherent heavy charge carriers in an organic conductor near the bandwidth-controlled Mott transition

S. Oberbauer, S. Erkenov, W. Biberacher, N. D. Kushch, R. Gross, and M. V. Kartsovnik

Phys. Rev. B 107, 075139 – Published 17 February 2023

Abstract

The physics of the Mott metal-insulator transition (MIT) has attracted huge interest in the last decades. However, despite broad efforts, some key theoretical predictions are still lacking experimental confirmation. In particular, it is not clear whether the large coherent Fermi surface survives in immediate proximity to the bandwidth-controlled first-order MIT. A quantitative experimental verification of the predicted behavior of the quasiparticle effective mass, renormalized by many-body interactions, is also missing. Here we address these issues by employing organic $κ$ -type salts as exemplary quasi-two-dimensional bandwidth-controlled Mott insulators and gaining direct access to their charge-carrier properties via magnetic quantum oscillations. We trace the evolution of the effective cyclotron mass as the conduction bandwidth is tuned very close to the MIT by means of precisely controlled external pressure. We find that the sensitivity of the mass renormalization to tiny changes of the bandwidth is significantly stronger than theoretically predicted and is even further enhanced upon entering the transition region where the metallic and insulating phases coexist. On the other hand, even on the very edge of its existence, the metallic ground state preserves a large coherent Fermi surface with no significant enhancement of scattering.

Received 21 September 2022
Revised 31 January 2023
Accepted 3 February 2023

DOI:https://doi.org/10.1103/PhysRevB.107.075139

Physics Subject Headings (PhySH)

Metal-insulator transition

Mott insulators Organic superconductors

Shubnikov-de Haas effect

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

S. Oberbauer^1,2,*, S. Erkenov^1,2, W. Biberacher¹, N. D. Kushch ^1,3, R. Gross ^1,2,4, and M. V. Kartsovnik ^1,†

¹Walther-Meißner-Institut, Bayerische Akademie der Wissenschaften, Walther-Meißner-Strasse 8, D-85748 Garching, Germany
²Physik-Department, Technische Universität München, D-85748 Garching, Germany
³Institute of Problems of Chemical Physics, Russian Academy of Sciences, Ac. Semenov avenue 1, Chernogolovka 142432, Russian Federation
⁴Munich Center for Quantum Science and Technology (MCQST), D-80799 Munich, Germany

^*Present address: attocube systems AG, 85540 Haar, Germany.
^†mark.kartsovnik@wmi.badw.de

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Vol. 107, Iss. 7 — 15 February 2023

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Images

Figure 1
Pressure-temperature phase diagram of $κ$ -Cl based on the data from Refs. [6, 7]. Black squares are the SC transition temperatures obtained from our zero-field resistive measurements [see inset in Fig. 2]. Inset: 2D Fermi surface (thick solid lines) of $κ$ -Cl in the NM state; the gray rectangle is the first Brillouin zone. Dashed lines indicate the classical cyclotron orbit $α$ (blue) and the magnetic-breakdown orbit $β$ (red) in a strong magnetic field [21, 22] (see text).
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Figure 2
(a) Crystals of $κ$ -Cl (sample 1) and $κ$ -NCS crystals along with the InSb pressure sensor mounted for resistive measurements in the pressure cell, before loading into the cell. The samples are aligned for measurements in a magnetic field $B$ perpendicular to the layers. (b) Examples of the field-dependent resistivity of $κ$ -Cl sample 1 at different pressures, at $T = 100$ mK. Pressures $P \leq 33$ MPa correspond to the NM-AFI coexistence region of the phase diagram. The curves for $P = 27$ and 21 MPa are scaled down by factors of 6 and 20 000, respectively. The inset shows the temperature dependence of the zero-field resistivity at the same pressures. For $P = 21$ MPa, the solid and dashed lines correspond to the down and up $T$ sweeps, respectively, revealing a strong hysteresis in the phase-coexistence regime. In addition, a $T$ sweep up under a magnetic field of 15 T, suppressing the SC transition, is shown for this pressure (dotted line).
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Figure 3
(a) Oscillatory component of resistivity $ρ_{osc} (B) \equiv ρ (B) - ρ_{bg} (B)$ in $κ$ -Cl, normalized to the nonoscillating resistivity background $ρ_{bg} (B)$ ; data obtained at different pressures, at $T = 100$ mK. The curves are vertically shifted for clarity. (b) FFT spectra for three curves from panel (a). The dominant peak originates from the magnetic-breakdown orbit $β$ encircling the entire Fermi surface (see inset in Fig. 1). Its persistence at the lowest pressure indicates that the large Fermi surface survives, without a significant change, inside the coexistence region, even very close to the purely insulating region of the phase diagram.
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Figure 4
Pressure-dependent effective cyclotron mass of $κ$ -Cl samples 1 (solid circles) and 2 (triangles) along with the mass obtained for the $κ$ -NCS crystal (open circles) measured in parallel with $κ$ -Cl sample 1. Note that $κ$ -NCS remains metallic down to ambient pressure; the corresponding mass value at $P = 0$ (diamond) is taken from Ref. [63]. The left scale is given in units of the free electron mass $m_{0}$ , the right scale in units of the band cyclotron mass $m_{c, band} = 2.8 m_{0}$ [64, 65]. Both materials exhibit the same inverse-linear pressure dependence in the homogeneous NM state; the dashed line is the fit to Eq. (1). The hatched rectangle shows the phase coexistence region for the $κ$ -Cl salt. Inset: Inverse renormalization factor $m_{c, band} / m_{c}$ demonstrating the $P$ -linear dependence in the purely metallic state and the deviation of $κ$ -Cl from this behavior in the coexistence region. The top-axis scale shows the $U / t$ ratio estimated based on the band-structure calculations [66] (see text).
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Figure 5
Dingle temperature of $κ$ -Cl versus pressure. No significant increase of $T_{D}$ and, hence, no enhancement of scattering occurs upon entering the NM-AFI coexistence region (hatched zone). The dashed line is a linear fit to the data between 27 and 83 MPa suggesting a weak gradual decrease of $T_{D}$ at increasing pressure. The right-hand axis shows rescaling of the Dingle temperature to the scattering rate.
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