Current-Induced Metallization and Valence Transition in Black SmS

TL;DR

This study investigates how applying an electric current induces a transition from insulator to metal in SmS, revealing that increased hybridization between Sm 4f and 5d orbitals causes the valence change and metallization.

Contribution

It provides direct experimental evidence linking current-induced metallization in SmS to enhanced Sm 4f-5d hybridization and valence transition, clarifying the origin of CIMT.

Findings

Carrier density rapidly increases with current at low temperatures.

The direct gap size increases with current.

Mean valence shifts from Sm2+ to mixed valence with current.

Abstract

A strongly-correlated insulator, samarium mono-sulfide (SmS), presents not only the pressure-induced insulator-to-metal transition (IMT) with the color change from black to golden-yellow but also current-induced IMT (CIMT) with negative resistance. To clarify the origin of the CIMT of SmS, the electronic structure change has been investigated by optical reflectivity and angle-integrated photoelectron spectra by applying an electric current. At lower temperatures than about 100 K, where the nonlinear $V$-$I$ curve has been observed, the carrier density rapidly increases, accompanied by decreasing relaxation time of carriers with increasing current. Then, the direct gap size increases, and the mean valence changes from Sm$^{2+}$-dominant SmS to the mixed-valent one with increasing current. These results suggest that the CIMT originates from increasing the Sm $4f$-$5d$ hybridization intensity induced by the applied current.

Equations

$$\tilde{\epsilon }(\omega )={\epsilon }_{\infty }$$

$$R_0(\omega )$$

$$R_{expt}(\omega )$$

Full text

††thanks: Present address: Institute of Liberal Arts and Sciences, Nagoya University, Nagoya, Aichi 464-8601, Japan††thanks: Present address: Center for General Education, Aichi Institute of Technology, Toyota, Aichi 470-0392, Japan

Supplementary Material for

Current-Induced Metallization and Valence Transition in Black SmS

Shin-ichi Kimura

kimura.shin-ichi.fbs@osaka-u.ac.jp

Graduate School of Frontier Biosciences, Osaka University, Suita, Osaka 565-0871, Japan

Department of Physics, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan

Institute for Molecular Science, Okazaki, Aichi 444-8585, Japan

  

Hiroshi Watanabe

Graduate School of Frontier Biosciences, Osaka University, Suita, Osaka 565-0871, Japan

Department of Physics, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan

  

Shingo Tatsukawa

Department of Physics, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan

  

Takuto Nakamura

Graduate School of Frontier Biosciences, Osaka University, Suita, Osaka 565-0871, Japan

Department of Physics, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan

  

Keiichiro Imura

Department of Physics, Graduate School of Science, Nagoya University, Nagoya Aichi 464-8602, Japan

  

Hiroyuki S. Suzuki

Institute for Solid State Physics, The University of Tokyo, Kashiwa, Chiba 277-8581, Japan

  

Noriaki K. Sato

Department of Physics, Graduate School of Science, Nagoya University, Nagoya Aichi 464-8602, Japan

S1. Drude and Lorentz fitting for reflectivity spectra

In the current-dependent reflectivity ($R(\omega)$) measurements, the absolute value of the $R(\omega)$ could not be obtained because the conventional in-situ gold evaporation method [1] can not be used. Then, we adopted the following combination of Drude and Lorentz functions [2] for the fitting of experimentally obtained $R(\omega)$ spectra ($R_{expt}(\omega)$), which do not have exact reflectivity, as follows:

[TABLE]

Here, $\tilde{\varepsilon}(\omega)$ is a complex dielectric function as a function of frequency (photon energy). Constant values of $\varepsilon_{\infty}$, $\varepsilon_{0}$, $e$, and $m_{0}$ are the sum of the real part of the dielectric constant at a higher frequency (energy) region, the dielectric constant of vacuum, the elementary charge, and the electron rest mass, respectively. In the Drude function, $N_{eff}$ is the effective electron density, $N_{eff}=\dfrac{N_{0}m_{0}}{m^{*}}$, where $N_{0}$ and $m^{*}$ are the carrier density and the effective carrier mass, and $\tau$ is the relaxation time of carriers. In the Lorentz function, $N_{ph}$ are the effective density of phonons, $N_{ph}=\dfrac{N_{ph}^{0}m_{0}}{m_{ph}}$, where $N_{ph}^{0}$ and $m_{ph}$ are the density and reduced mass of the phonon, respectively, and $\omega_{ph}$ and $\Gamma_{ph}$ are the frequency and the dumping constant of the phonon, respectively. $R_{expt}(\omega)$ spectra can be regarded as a relative $R(\omega)$ spectra, which corresponds to actual $R(\omega)$ spectra ($R_{0}(\omega)$) multiplied by the magnification constant ($c$). We obtained seven parameters ($\varepsilon_{\infty}$, $N_{eff}$, $\tau$, $N_{ph}$, $\omega_{ph}$, $\Gamma_{ph}$, and $c$) in the function fitted by a nonlinear least-square method to $R_{expt}(\omega)$. In these parameters, even though $c$ is changed in different sample settings, $\varepsilon_{\infty}$, and $\omega_{ph}$ are almost constant as 3.0 and 22.5 meV, respectively. Other parameters, $N_{eff}$, $\tau$, $N_{ph}$, and $\Gamma_{ph}$, are changed by temperatures and currents. An example of the fitting results is shown in Fig. S1. $N_{eff}$ and $\tau$ as functions of temperature and current are plotted in Figs. 1(c–f), and $N_{ph}$ and $\Gamma_{ph}$ are shown in the next section.

S2. Temperature and Current dependences of phonon parameters

Current-dependent $\Gamma_{ph}$ and $N_{ph}$ for TO phonons at $T=20$ and 60 K, at which the nonlinear current dependence appears, are shown in Fig. S2. Both parameters are unchanged so much in comparison with $N_{eff}$ and $\tau$ shown in Figs. 1(c–f), especially the damping constant $\Gamma_{ph}$ is almost constant in the whole current region, even though $N_{eff}$ and $\tau$ are strongly variable. This result suggests that the sample temperature, one of the origins of the $\Gamma_{ph}$, did not change so much in the measured current region below 1.6 A.