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## Creator

[Ryo Matsumoto](https://orcid.org/0000-0001-6294-5403), Sayaka Yamamoto, [Kensei Terashima](https://orcid.org/0000-0003-0375-3043), [Kazuki Yamane](https://orcid.org/0000-0002-0162-5411), [Yoshihiko Takano](https://orcid.org/0000-0002-1541-6928)

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©2024 The Physical Society of Japan
Matsumoto, R., Yamamoto, S., Terashima, K., Yamane, K., & Takano, Y. (2024). Electrical Transport Properties of van der Waals Insulator CrGeTe3 under Extremely High Pressure up to 52 GPa. Journal of the Physical Society of Japan, 93(4), 044710,https://doi.org/10.7566/JPSJ.93.044710[In Copyright](http://rightsstatements.org/vocab/InC/1.0/)

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[Electrical Transport Properties of van der Waals Insulator CrGeTe<sub>3</sub> under Extremely High Pressure up to 52 GPa](https://mdr.nims.go.jp/datasets/7ea18332-54b3-47d5-b3fb-770f17235bdd)

## Fulltext

1 Electrical Transport Property in van der Waals Insulator 1 CrGeTe3 under Extremely High Pressure up to 52 GPa 2 *Ryo Matsumoto1, Sayaka Yamamoto1,2, Kensei Terashima1, Kazuki Yamane1,2, Yoshihiko Takano1,2 3 1Research Center for Materials Nanoarchitectonics (MANA), 4 National Institute for Materials Science, Tsukuba, Ibaraki 305-0047, Japan 5 2Graduate School of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, 6 Ibaraki 305-8577, Japan 7 *Corresponding author; Email: matsumoto.ryo@nims.go.jp 8  9 Abstract 10 Layered ternary chalcogenide CrGeTe3 has attracted considerable interest as a two-dimensional van 11 der Waals magnet. The application of high pressures induces unique property modifications, such as 12 a drastic increase in the Curie temperature to a value close to room temperature. In this study, the 13 electrical transport properties of CrGeTe3 were investigated under extremely high pressures up to 52 14 GPa to understand the physical properties of the high-pressure phase further, particularly the 15 appearance of superconductivity. The temperature dependences of the resistance, Hall effect, and 16 magnetoresistance at high pressures were measured using an originally designed diamond anvil cell. 17 The structural phase transitions from the R-3 to R3 structure and amorphization are investigated via in 18 situ Raman spectroscopy and X-ray diffraction at high pressures. Unlike in CrSiTe3, the emergence of 19 superconductivity in CrGeTe3 in the high-pressure phase is not observed up to 52 GPa. Amorphization 20 is the possible main reason for the absence of superconductivity in CrGeTe3.  21  2 1. Introduction 22 Two-dimensional (2D) van der Waals (2D-vdW) materials with layered structures have received 23 considerable attention because they are a promising platform for examining the abundant physical 24 phenomena available based on a tunable film thickness [1-5]. Among these materials, layered 25 Cr(Si.Ge)Te3 is a ferromagnetic insulator that has recently attracted interest as a low-dimensional 26 magnetic material [6,7]. At ambient pressure, CrSiTe3 exhibits a paramagnetic-to-ferromagnetic phase 27 transition at a Curie temperature (TC) of 32 K [8]. Interestingly, TC drastically increases as pressure is 28 applied, reaching 138 K at 7.8 GPa, from 36 K at 4.6 GPa [9]. At higher pressure, CrSiTe3 undergoes 29 a structural phase transition from the R-3 to R3 space group and an insulator-to-metal transition (IMT) 30 [10]. Moreover, superconductivity at the transition temperature of 3 K has been reported for R3 31 structures. Ferromagnetism vanishes during the structural phase transition, and superconductivity 32 appears instead. Further investigation of the physical properties under high pressures is expected in 33 this system because several 2D-vdW materials exhibit more interesting functionalities at higher 34 pressures, as 2H-TaS2 shows a record-high Tc of 16 K above 150 GPa [11].  35 Isostructural CrGeTe3 is also a 2D-vdW ferromagnetic insulator with a TC of 65 K, which is 36 twice that of CrSiTe3 at ambient pressure [12,13]. The higher TC motivated us to reveal several 37 functionalities, such as the drastic enhancement in TC via the intercalation of organic ions into the 38 layered structure [14]. The application of high pressures to CrGeTe3 has also been reported to enhance 39 the magnetic properties, for example, a drastic enhancement in Tc up to 250 K under 9 GPa [15] and 40 the control of magnetic anisotropy [16]. Based on the electrical and structural properties, an IMT at 41 11 GPa [15] and a structural phase transition from R-3 to R3 at a high pressure of 18 GPa have been 42 reported [17]. A unique crystalline-to-amorphous transition occurs at higher pressure [17], as observed 43 in the case of CrSiTe3 [18]. Raman observations and first-principles phonon dispersion calculations 44 indicate that pressure-induced phonon softening occurs in CrGeTe3 [19], which is preferred for 45 superconductivity [20-22]. However, electrical transport properties under pressure have only been 46 reported below 11 GPa [15]. Further investigation under higher pressures will accelerate the 47 understanding of the physical properties of the crystalline R3 and amorphous phases in this system, 48 including pressure-induced superconductivity.  49 In this study, we evaluate the electrical transport properties, including the temperature-50 dependent resistance, Hall effect, and magnetoresistance (MR), of CrGeTe3 single crystals under 51 extremely high pressures up to 52 GPa using a custom-designed diamond anvil cell (DAC). 52 Furthermore, in situ Raman spectroscopy measurements and X-ray diffraction (XRD) analyses are 53 performed under pressure to investigate the structural phase transition and amorphization. In contrast 54 to GeSiTe3, pressure-induced superconductivity is not observed at pressures up to 52 GPa for CrGeTe3. 55  56 2. Experimental Procedures 57 Single-crystalline CrGeTe3 was grown using the following procedure: The starting materials 58 of elemental Cr, Ge, and Te with molar ratio of 1:1:36 were sealed inside an evacuated quartz tube. 59 The ampoule was heated at 700°C for 20 days and then slowly cooled to 500°C over a period of 1.5 60 days, and then subjected to furnace cooling. The crystal structures of the resultant samples were 61 analyzed using XRD using a MiniFlex 600 diffractometer (Rigaku) with Cu Kα radiation (λ = 1.5418 62  3 Å). The VESTA software package was used to visualize crystal structures [23]. The crystal 63 composition was investigated using energy-dispersive X-ray spectroscopy (EDX) with a JSM-6010LA 64 scanning electron microscope (JEOL). The temperature dependence of the magnetization of the 65 obtained crystals was evaluated using a magnetic property measurement system (Quantum Design). 66 Electrical transport measurements were obtained at high pressures via the standard four-probe method 67 using a custom-designed DAC [24-26] set within a physical property measurement system with a 68 superconducting magnet (Quantum Design). Cubic boron nitride powder was used as a pressure-69 transmitting medium. The applied pressures were estimated from the fluorescence emitted by ruby 70 powders [27] and from the Raman spectrum acquired from a culet of the upper diamond anvil [28], 71 which was obtained using an inVia Raman Microscope (Renishaw plc). These measurements were 72 performed simultaneously with high-pressure in situ Raman spectroscopy acquisitions of the sample. 73 The XRD patterns at high pressures were obtained in a DAC by utilizing synchrotron radiation at the 74 AR-NE1A beamline of the Photon Factory (PF) situated at the High Energy Accelerator Research 75 Organization (KEK). The X-ray beam was monochromatized to an energy of 30 keV (λ = 0.41815 Å) 76 and introduced to the sample in the DAC through a collimator with a diameter of 50 μm. The obtained 77 XRD patterns were subsequently integrated into a one-dimensional profile using an IPAnalyzer [29]. 78 Cubic BN powder was also used as the pressure-transmitting medium in the XRD measurements under 79 high pressures. 80  81 3. Results and Discussion 82 Figure 1 (a) shows the XRD pattern of a single crystal of CrGeTe3. The inset displays a typical 83 micrograph and the crystal structure with the definitions of the a, b, and c axes. The well-developed 84 plate-like morphology of CrGeTe3 is apparent in the images. The pattern including only the (003n) 85 diffraction peaks indicates a highly c-axis-oriented crystal. The elemental composition is determined 86 to be Cr:Ge:Te = 1.9:2:6.0 from the EDX analysis by normalizing to Ge = 2. Figures 1 (b) and 1 (c) 87 show the temperature dependence of the magnetization M and resistivity ρ of the CrGeTe3 single 88 crystal at ambient pressure. Using the derivative of the magnetization dM/dT and the Arrhenius 89 relationship, ρ = ρ0exp(Ea/kBT), where ρ0 is the residual resistivity, Ea is the activation energy, kB is 90 the Boltzmann constant, and T is the temperature, the TC and activation energy are determined to be 91 65 K and 0.24 eV, respectively. The observed magnetic and electric properties at ambient pressure are 92 consistent with the previously reported values [30]. 93  94  4 Fig. 1. (Color online) Characterization of the obtained CrGeTe3 single crystal. (a) XRD pattern. 95 Inset: typical micrograph and crystal structure. (b) Temperature dependence of magnetization 96 and the derivative dM/dT as an inset. (c) Temperature dependence of resistivity and Arrhenius 97 plot (inset). 98  99 Figures 2 (a) and (b) show the results of Hall measurements at 300 K with the application of 100 various pressures to the CrGeTe3 single crystal. The slope of the Hall voltage is positive at each 101 pressure, indicating p-type characteristics. The Hall voltage drastically decreases with increasing 102 pressure up to 3.2 GPa. The carrier concentration of the sample is calculated from the slope of the Hall 103 voltage vs. the magnetic field strength using the formula VH/I = H/ned, where VH is the Hall voltage, 104 I is the current, n is the number of carriers, e is the elementary charge, H is the magnetic field strength, 105 and d is the sample thickness. Figure 2 (c) shows the applied pressure dependence of the carrier 106 concentration in the CrGeTe3 single crystal at various pressures. The carrier concentration at the lowest 107 pressure of 0.44 GPa is 1.3×1018 cm-3, indicating that the sample shows a semiconducting character 108 under these conditions. The application of pressure enhances the carrier concentration to values on the 109 order of at least 1021 cm-3. 110 Figure 2(d) shows the magnetic field dependence of the MR ratio (R−R0)/R0 at various 111 pressures, where R0 is the resistance at zero-field of the CrGeTe3 single crystal. The MR ratio decreases 112 as the pressure increased from 0.90 to 1.8 GPa. The sign of the MR ratio switches from negative to 113 positive between 1.8 and 3.2 GPa, corresponding to a spin-reorientation transition from the c axis to 114 the ab plane [31]. In addition, the magnitude of the MR ratio decreases at 3.2 GPa. The sign of the 115 MR again reverses, becoming negative at 4.3 GPa, and its magnitude at this pressure is comparable to 116 that at 1.8 GPa. With compression up to 13 GPa, the magnitude of the MR ratio increases slightly 117 without any further changes in its sign. Recent high-pressure magnetic measurements have indicated 118 that the magnetization drastically decreases as the pressure is increased up to 4.0 GPa, before 119 increasing again with an increase in pressure from 5.0 to 7.3 GPa [15]. Further investigation of the 120 origin of the magnetic transition at approximately 4.0 GPa should prove to be an interesting topic for 121 future studies. 122  123 Fig. 2. (Color online) Hall effect at 300 K in the CrGeTe3 single crystal under various pressures 124 in the ranges of (a) 0.44–3.2 GPa and (b) 4.3–13 GPa. (c) Applied pressure dependence of the 125 carrier concentration. (d) Magnetic field dependence of the MR ratio at pressures in the range 126  5 of 0.90–13 GPa. 127  128 Figure 3 (a) shows the temperature dependence of the resistance of the CrGeTe3 single crystal 129 at pressures ranging from 0.44 to 18 GPa. At 0.90 GPa, the curve of resistance exhibits two hump-like 130 structures at approximately 80 K (hump 1) and 190 K (hump 2). As shown in the logarithmic scale of 131 Fig. 3 (b), hump 1 gradually disappears, and the temperature of hump 2 increases with the applied 132 pressure. According to a previous report [15], the hump in the resistance curve is a possible signature 133 of magnetic transition. At 3.2 GPa, hump 2 appears at approximately 250 K, which is slightly higher 134 than that reported previously [15]. The resistance at 300 K decreases monotonically when the pressure 135 increases, as shown in Fig. 3 (c). In total, the resistance of the CrGeTe3 single crystal at 300 K 136 decreases by more than two orders of magnitude as the pressure increases from ambient pressure to 137 18 GPa. The inset in Fig. 3 (c) shows the temperature dependence of the resistance at 3.2 GPa with a 138 standard power-law fit, R(T) = R0 + ATn, where R0 is the residual resistance, A is a characteristic 139 constant, and T is the temperature. The fitting results yield the parameters of n = 2.58(1), R0 = 0.30(3) 140 Ω, and A = 1.15(5)×10-5 Ω/K2. This well-fitted feature indicates that an IMT occurred at 3.2 GPa, 141 which is slightly lower than the previously reported IMT pressure [15]. The drastic reduction in 142 resistance and IMT with the application of pressure is consistent with the increasing carrier 143 concentration. 144 Figure 3 (d) shows the temperature dependence of the resistance of the CrGeTe3 single crystal 145 at high pressures ranging from 15 to 52 GPa. The decrease in the resistance with increasing pressure 146 continues up to 52 GPa. In the case of CrSiTe3, the original R-3 structure undergo a high-pressure 147 phase of the R3 structure, and pressure-induced superconductivity appears above 9 GPa. Although a 148 similar pathway for the structural phase transition from R-3 to R3 above 18 GPa has been reported for 149 CrGeTe3 [17], pressure-induced superconductivity is absent at the pressure. In addition, 150 superconductivity has been never observed between 23 and 52 GPa, which is the amorphous region 151 of CrGeTe3 [17].  152  153 Fig. 3. (Color online) Temperature dependence of the resistance of the CrGeTe3 single crystal at 154 high pressures in the ranges of (a,b) 0.44–18 GPa and (d) 15–52 GPa. (c) Applied pressure 155 dependence of the resistance at 300 K. The inset is the temperature dependence of the resistance 156 at 3.2 GPa with power-law fitting.  157  158  6 Structural analyses using in situ Raman spectroscopy and XRD under pressure were conducted 159 to determine the absence of superconductivity in CrGeTe3. Figure 4 (a) shows the Raman spectra of 160 the CrGeTe3 single crystal under various pressures. At ambient pressure, five Raman-active modes 161 are identified at different wavenumbers: 109.6 cm-1 (Eg3), 135.7 cm-1 (Ag1), 211.9 cm-1 (Eg4), 232.2 cm-162 1 (Eg5), and 291.0 cm-1 (Ag2). These assignments are based on previously reported spectroscopic studies 163 [32,33]. A small unidentified peak, labeled with an asterisk in Fig. 4 (a), appears between the Ag1 and 164 Eg3 modes with slight compression. All the observed peaks, apart from the Eg4 mode, gradually shifts 165 to higher wavenumbers as the pressure increases to 11 GPa via phonon hardening without a structural 166 phase transition, as shown in Fig. 4 (b). The peak corresponding to the Eg4 mode disappears at 167 approximately 4 GPa. However, all the Raman peaks disappear at pressures higher than 15 GPa, 168 corresponding to a pressure-induced crystalline-to-crystalline transition from the R-3 to R3 structure, 169 as reported in the literature [19]. With a further increase in the pressure to 52 GPa, no new peaks 170 appear in the spectrum. 171 Although an emergence of pressure-induced superconductivity in CrGeTe3 is expected in the 172 R3 structure, considering the case of CrSiTe3, it is absent even above the critical pressure of the 173 structural transition. To answer this question, amorphization was investigated by comparing the 174 Raman spectra before and after compression. Figure 4 (c) shows a comparison of the Raman spectra 175 of ambient CrGeTe3 single crystals and recovered samples from various maximum pressures. The 176 sharp peaks observed at ambient pressure disappear in the recovered sample from 52 and 39 GPa, and 177 broad humps are observed. Peak broadening after decompression is a signature of pressure-induced 178 amorphization, as seen in phase-change memory materials, such as Ge2Sb2Te5, GeSb2Te4, and 179 SnSb2Te4 [34]. The recovered sample from 20 GPa, whose pressure is close to the critical pressure of 180 the R-3 to R3 transition, shows the peak broadening, although the peak positions are almost reproduced. 181 The sample recovered from 5.6 GPa exhibits relatively sharp peaks, although the overall shape of the 182 spectrum is different from that recorded at ambient pressure without compression. The irreversibility 183 of the Raman spectra suggests that pressure-induced amorphization progresses gradually with 184 increasing pressure.  185 Figure 4 (d) shows the XRD patterns of the CrGeTe3 single crystal at 1.6 GPa and the sample 186 recovered from 22.6 GPa. The insets display Debye–Scherrer diffraction peaks. All the observed 187 diffraction peaks at 1.6 GPa correspond to the trigonal R-3 structure, except for the peaks from cubic 188 BN, which is a pressure-transmitting medium. The Debye–Scherrer ring at 1.6 GPa exhibits spot-like 189 diffractions, reflecting a single-crystalline feature. In contrast, the spot-like diffractions disappear, and 190 only broadened rings are observed in the recovered sample from 22.6 GPa. The irreversibility of the 191 XRD pattern also indicates amorphization around the critical pressure of structural transition in 192 CrGeTe3 [17]. As CrGeTe3 requires a higher pressure to undergo the R3 structure than CrSiTe3, the 193 absence of superconductivity could be related to progressive amorphization under high-pressure 194 conditions. 195 One of the possibilities for inducing the absence of superconductivity in CrGeTe3 based on the 196 aforementioned scenario is the deterrence of amorphization under high pressures. In our configuration 197 of DAC, a solid pressure-transmitting medium is used, which provides a nonhydrostatic pressure. The 198 typical signature of the nonhydrostatic pressure is observed a broadened ruby fluorescence as shown 199  7 in Fig. S1 [35]. Nonhydrostatic pressure creates distortion stress and affects the physical properties of 200 the crystal [36]. The differences between the measured resistances and the absence of 201 superconductivity could possibly be the result of the uniaxial nature of the applied pressure, because 202 2D materials are sensitive to strain, as has been observed for graphene [37], transition metal 203 dichalcogenides [38], and black phosphorous [39]. In future study, the application of hydrostatic 204 pressure using a liquid pressure-transmitting medium is expected to induce superconductivity in 205 CrGeTe3. In addition, the irreversible change in crystal structure after compression is interesting 206 because of the possibility of quenching the high-pressure phase under ambient conditions. In particular, 207 a drastic enhancement in TC close to room temperature has been reported with the application of 208 pressure [15]. Physical property measurements of the recovered amorphous sample are important for 209 the practical use of magnetic materials. 210  211 Fig. 4. (Color online) (a) Raman spectra of the CrGeTe3 single crystal at various pressures. (b) 212 Applied pressure dependence of the peak positions of the Raman modes. (c) Comparison of 213 Raman spectra from the original CrGeTe3 and the recovered sample after compression under 214 various pressures. (d) XRD patterns from the CrGeTe3 single crystal at 1.6 GPa and the 215 recovered sample from 22.6 GPa. The insets are Debye–Scherrer diffraction rings. 216  217 4. Conclusion 218 We performed electrical transport measurements and observed the resistance, Hall effect, and 219 MR of CrGeTe3 single crystals at high pressures. The carrier concentration increased from ~1018 cm-220 3 at 0.44 GPa to 1021 cm-3 at 13 GPa. The sign of the MR ratio switched twice: from negative to positive 221 at 3.2 GPa and from positive to negative at 4.3 GPa. A drastic reduction in the resistance with IMT 222 was observed at approximately 3.2 GPa. In transport measurements, superconductivity observed for 223 CrSiTe3 was not observed even in the high-pressure phase of the R3 structure. The in situ Raman 224 analysis of the sample at high pressures revealed a crystalline-to-crystalline transition from the R-3 to 225 R3 space group, similar to superconducting CrSiTe3. The irreversibility of the Raman spectrum and 226 XRD pattern suggests that the CrGeTe3 single crystal gradually transformed into an amorphous phase 227 with increasing pressure. We hypothesized that this amorphization would impede the emergence of 228  8 superconductivity in CrGeTe3. In a future study, the measurement of the physical properties of the 229 recovered sample will be interesting from the viewpoint of the possibility of quenching the high-230 pressure phase. 231  232 Acknowledgment 233 This study was partly supported by the JST-Mirai Program Grant Number JPMJMI17A2 and JSPS 234 KAKENHI Grant Number 23K13549. 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