Abstract Body

Low dimensional materials have been a major subject of interest in recent years. In particular,　the transitions metal dichalcogenides (TMDs), quasi-2D layered materials with weak van der　Waals coupling between layers, received a lot of attention. TMDs exhibit strong electron- electron and electron-phonon interactions that lead to complicated phase diagrams showing a variety of ground states.

A fascinating frontier, largely unexplored, is the stacking of strongly correlated phases of TMDs. We study 4Hb-TaS2, which naturally realizes an alternating stacking of 1T-TaS2 and 1H-TaS2 structures. The former is a well-known Mott insulator, which has recently been proposed to host a gapless spin-liquid ground state. The latter is a superconductor known to also host a competing charge density wave state. This raises the question of how these two components affect each other when stacked together. Using Muon Spin Relaxation, we show that 4Hb-TaS2 is a superconductor that breaks time-reversal symmetry, abruptly at the superconducting transition [1].

Using scanning superconducting quantum interference device (SQUID) microscopy we found a spontaneous vortex phase whose vortex density depends on the magnetic history of the sample above Tc[2].

In addition, using scanning tunneling spectroscopy we find spectroscopic evidence for the existence of topological surface superconductivity. These include edge modes running along the 1H-layer terminations as well as under the 1T-layer terminations, where they separate between superconducting regions of distinct topological nature [3].

While specific heat measurements find a fully gapped superconductor, we show using the Little-Parks experiment that 4Hb-TaS2 is not a s-wave superconductor. Together, all the accumulated data strongly suggests that 4Hb-TaS2 is a chiral superconductor.

References

[1] A. Ribak et al., Sci. Adv. 6, eaax9480 (2020).

[2] E. Persky et al., Nature 607, pages 692–696 (2022).

[3] A. K.Nayak et al., Nature Phys. 17,1413 (2021).

Abstract Body

As one of the classic transition metal carbides, two-dimensional (2D) Mo2C possesses a wealth of controllable electronic properties and exhibits great application potential in the field of photocatalysis, energy storage, and superconductivity due to its variety of charge, orbital and spin degree of freedom. According to the first principles calculations, the superconducting transition temperature value of 2D Mo2C, strongly related to its electronic structure, varies in the range of 2 - 13 K. Therefore, tuning the electronic structure will be the key issue to obtaining the Mo2C with high-quality and superior performance. In this study, we adopted a topochemical method to synthesize 2D Mo2C powders with high quality. Based on that, the F-Mo2C and N, P-Mo2C samples are prepared by fluorine modification and N, P co-doping, respectively. Moreover, the surface morphology, electronic structure, and superconductivity of all samples are systematically studied as well as the superconductivity.

The superconducting transition temperature (Tc) of the Mo2C sample was measured by using the zero-field cooling (ZFC) and field cooling (FC) modes; the value of H was 10 Oe. The values of Tc were determined from the M/H – T plot in ZFC mode. In comparison with 3.2 K of the pure Mo2C sample, the highest Tc value of 5.7 K and 6.1 K can be obtained in F-Mo2C sample and N, P-co-doped Mo2C sample, respectively.

References

[1] Liu H-D, Lu H-Y, Jiao N, et al. Physic. Chem. Chem. Physic., 2023, 25(1): 580-589.

[2] Guo Y, Xu K, Wu C, et al. Chem. Soc. Rev., 2015, 44(3): 637-646.

Acknowledgment

This work was supported by Northwest Institute for Non-ferrous Metal Research (No. YK2110), National Key Research and Development Program of China (No. 2021YFB3800200).

Keywords: molybdenum carbide, topochemical transformation, surface functionalization, superconductivity

Abstract Body

The misfit layer compounds {(SnSe)_1.16}_m(NbSe₂)_n have a crystal structure consisting of monochalcogenide layers of SnSe and dichalcogenide layers of NbSe₂ stacked by van der Waals forces. These materials exhibit a unique stacking structure with partial breaking of the spatial inversion symmetry, resulting in distinctive physical properties. One such property, observed in single crystals with m = 1 and n = 2, is that the in-plane upper critical field (H_c2) reaches almost twice the Pauli limit (H_P), suggesting the presence of an unconventional superconducting state under strong magnetic fields [1]. On the other hand, the superconducting transition temperature (T_c) is about 5 K, which is lower than that of 7 K in pure NbSe₂. The T_c may decrease by electron-carrier doping because the Tc of NbSe₂ decreases with this doping [2]. Therefore, it is expected that hole doping could increase the T_c in {(SnSe)_1.16}₁(NbSe₂)₂, as has been reported in another misfit layer compound [3]. In this study, we have synthesized In-doped {(SnSe)_1.16}_m(NbSe₂)_n single crystals in which the Sn site of the charge-supplying SnSe layer was partially substituted with In and investigated their superconducting properties. The single crystals were prepared by the molten salt flux method. X-ray diffraction (XRD) and energy dispersive X-ray spectroscopy (EDX) confirmed the doped In ions in the grown single crystals. Furthermore, we observed the change in the superconducting properties of these single crystals compared to the parent compound {(SnSe)_1.16}₁(NbSe₂)₂. In this talk, we will report on the effect of In-doping on superconducting properties.

Abstract Body

BiS₂-based superconductor La(O, F)BiS₂ has a layered structure composed of an insulating rare-earth oxide layer and two conducting BiS₂ layers stacked alternately. This material exhibits superconductivity at about 3 K[1]. The application of physical pressure increases the superconducting transition temperature (T_c) to about 10 K, and the application of chemical pressure through elemental substitution also increases T_c to about 5 K [2,3]. Recently, it was reported that the T_c increased by partially substituting Sn for Bi to about 6 K [4]. In this study, in order to further improve the T_c of Sn-substituted LaO_0.5F_0.5BiS₂, we investigated the effect of chemical pressure application on the superconducting properties in LaO_0.5F_0.5(Bi, Sn)S₂ using  the partial substitution of Nd ion for La ion.

Single crystals of La_1-xNd_xO_0.5F_0.5Bi_1-ySn_yS₂ were prepared using the molten flux method. The physical properties of these samples were evaluated by X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX), and electrical resistivity measurements. Figure 1 shows the temperature dependence of electrical resistivity. It was found that T_c^zero reaches a maximum of about 6 K at y = 0.12 and x = 0.05, which is the highest value at ambient pressure among the BiS₂-based compounds. This Nd dependence indicates that the superconducting properties in the Sn-substituted sample do not increase with increasing chemical pressure, suggesting that other factors to control the superconducting properties exist. In this presentation, the effects of Nd and Sn substitutions will be discussed in more detail.

References

[1] Y. Mizuguchi et al, J. Phys. Soc. Jpn., 81, 114725(2012).

[2] Y. Mizuguchi, J. Phys. Soc. Jpn, 88, 041001(2019).

[3] S. Demura, J. Phys. Soc. Jpn, 88, 041002(2019).

[4] S. Kobayashi et al, J. Phys. Soc. Jpn. 93, 024707 (2024).

Figure 1. Temperature dependence of the electrical resistivity for La_1-xNd_xO_0.5F_0.5Bi_1-ySn_yS₂, y = 0.12, x = 0 - 0.30. (The inset shows an enlarged view near the transition temperature.)

Keywords: Superconductivity, Single crystal, Element-substitution, Structural instability

Abstract Body

Superconductivity with T_c ~4.7 K was discovered in Sc₆FeTe₂ with ordered Fe₂P structure in 2023 [1]. One of peculiar feature of Sc₆FeTe₂ is the existence of residual electronic specific heat of ~40% of the normal state value even at 0.5 K. A similar residual electronic specific heat has been reported in Lu₂Fe₃Si₅ with T_c ~6.0 K. However, later specific heat measurements down to 0.4 K demonstrated the existence of second drop of electronic specific heat at lower temperatures [2], and Lu₂Fe₃Si₅ is considered to be a typical two-gap superconductor similar to MgB₂. In order to elucidate the origin of residual specific heat and evolution of superconductivity in Sc₆FeTe₂, we have prepared polycrystalline samples of (Sc_1-xLu_x)₆FeTe₂. X-ray diffraction patterns shown in the Fig. 1 demonstrate that Sc can be fully replaced by Lu, although peaks originated from (Sc,Lu)-alloy become stronger with x. On the other hand, T_c is quickly suppressed with x, and it becomes lower than 2 K at x = 0.2. Contrary to this result, point defects created by 3 MeV proton irradiation do not suppressing T_c appreciably in Sc₆FeTe₂, supporting single-gap s-wave superconductivity. The origin of the residual specific heat in Sc₆FeTe₂ is considered to be originated from the phase-separated Sc impurity phase.

[1] Y. Shinoda et al, J. Phys. Soc. Jpn. 92, 103701 (2023).

[2] Y. Nakajima et al., Phys. Rev. Lett. 100, 1570010 (2008).

Abstract Body

Superconductivity in Zr₆FeSb₂ was investigated through low-temperature specific heat measurements. Polycrystalline samples were synthesized by arc-melting, followed by annealing. Powder X-ray diffraction (PXRD) and electron-probe microanalysis (EPMA) revealed that the obtained samples were phase-pure Zr₆FeSb₂ with secondary phases comprising less than 2%. Superconductivity was observed below 1.3 K, as evidenced by electrical resistivity and AC magnetic susceptibility measurements [1]. Figure 1 shows the electronic specific heat divided by temperature C_e/T as a function of temperature T, measured at magnetic fields of 0 T and 0.6 T. In the superconducting state at 0 T, we observed a broad superconducting transition, indicated by a peak at 0.75 K, and a large residual electronic specific heat coefficient γ_res = 39 mJ/K²mol. This value is approximately 57% of the normal-state electronic specific heat coefficient γ_N = 68 mJ/K²mol, as determined from the 0.6 T data. The large γ_res does not originate from secondary phases, as PXRD and EPMA showed that the sample was nearly single-phase. We suggest that significant pair-breaking occurs due to paramagnetic impurities, as indicated by the low-temperature Curie tail observed in the magnetic susceptibility in the normal state. This system can exhibit mixing between Fe and Sb, expressed as Zr₆Fe_1-xSb_2+x (0.0 ≤ x ≤ 0.3) [2]. Fe ions in the Sb site may possess paramagnetic moments, resulting in magnetic pair-breaking in the superconducting state. Further study is warranted.

References

[1] R. Matsumoto et al., J. Phys. Soc. Jpn. 93, 065001 (2024).

[2] G. Melnyk et al., J. Phase Equilibria 20, 497 (1999).

Figure 1. Electronic specific heat divided by temperature, C_e/T, as a function of temperature T. Closed and open circles represent data measured under magnetic fields of 0 T and 0.6 T, respectively. The solid curve shows the calculated C_e/ in the BCS weak coupling limit with a thermodynamic critical temperature of. T_c = 0.95 K and a residual electronic specific heat coefficient γ_res of 39 mJ/K²mol. The dashed curve represents the calculated Ce/T assuming Gaussian broadening of T_c with a standard deviation of 0.15 K

Keywords:Specific heat, Zr₆FeSb₂, Metal rich compound

Abstract Body

Quasi two-dimensional organic superconductor, λ-(BETS)₂GaCl₄ where BETS stands for bis(ethyleneditio)tetraselenafulvalene, have been actively studied regarding the electronic properties [1] and attracted much attention because they shows interesting superconducting phenomena, such as the existence of FFLO state (Fulde-Ferrel-Larkin-Ovchinnkov) that has been reported experimentally by thermodynamical measurement [2], the distorted of the nodal line compared to the d-wave SC gap symmetry which is similar to that found in cuprates superconductor [3], spin fluctuation that exist close to the superconducting state [4] and the deviation from normal Fermi liquid in the metallic state [5]. Furthermore, by changing the central atom of the tetrahedral GaCl₄, λ-(BETS)₂Fe_1-yGa_yCl₄, and changing the halogen atom ones,λ-(BETS)₂GaBr_xCl_4-x, x< 2, does not have serious effect on the crystal structure, nonetheless, they provide a large variety of electronic properties [6].

As for the Fermi liquidity studied the temperature dependence of the resistivity of unconventional superconductors shows n = 1, i.e., non-Fermi liquid (non-FL), where n defined as the power value of ρ=ρ₀+ATⁿ, while normal Fermi liquid (FL) will show n = 2 [7-8]. The change from n=1 to 2, can be observed by doping in the cuprates system [7] or applying pressure in the organic κ-(ET)₄Hg_2.89Br₈ system [9]. Our recent studied on the metallic state in λ-(BETS)₂GaCl₄ (abbreviated as λ-Ga) under physical pressure showed that just above the superconducting transition, the metallic state has the n value around 1.38(2). Furthermore, by applying pressure, it gradually increases to n = 2.4 at P = 0.28 GPa, then stabilized at n = 2 for the higher-pressure region. This behavior of n differs with that of cuprates and κ-(ET)₄Hg_2.89Br_8, although there is a deviation from n = 2 [5]. Furthermore, we found an upturn anomaly in the resistivity data under pressure of 0.13 and 0.28 GPa as shown in the inset of Fig.1(a). The results of magnetic susceptibility revealed that the superconducting volume fraction decreases by applying small pressure at P = 0.13 GPa. This implies the superconducting phase lost to the magnetic one [5,12]. The results of resistivity and magnetic susceptibility measurements under pressure suggest the formation of a dome of superconducting state. We aim to reveal whether the n value keeps decreasing towards n = 1 in the region of negative pressure. It becomes necessary to see the whole picture of non-Fermi liquidity in λ-(BETS)₂GaCl₄ by checking the chemical pressure effect giving the negative pressure region of the phase diagram of λ-(BETS)₂GaCl₄. It can be achieved by measuring the λ-(BETS)₂GaBr_xCl_4-x, x< 2 system.

Previous research on electrical properties of λ-(BETS)₂GaBr_xCl_4-x at ambient pressure revealed the superconducting transition can be observed when x < 0.75 [10]. The magnetic susceptibility measurement for the compound with x = 0.7 confirmed the anomaly which correspond to metal-to-insulator transition. By further research, it reveals that the insulating state correspond to the spin density wave (SDW) phase [6,11]. Our resistivity result at x = 0.2, shows the upturn toward lower temperatures suggesting the metal-insulating transition, then it vanished at x = 0.4 as shown in Figure 1(b) and (c) in which different with previous study [6]. In correlation with λ-Ga studied under physical pressure, the spin fluctuation which correspond to SDW phase observed at ambient pressure [8] and the metal-insulator transition appear by low pressure, P=0.13 GPa. Thus we suggest this relates to the upturn anomaly in x~0.2 although it not as sharp [12]. Furthermore, the superconducting transition, T_c, increase at x=0.4 followed with the decrease of n value. This behavior is similar to that of λ-Ga under physical pressure below 0.3 GPa, which shows the increase of T_c toward P_c = 0.13 GPa followed with deviation of n value from the normal metal [5]. The existence of insulating state at x=0.75 after vanishing at x=0.4 from resistivity measurement [11], indicates the competition between magnetic and superconducting phase in the λ-(BETS)₂GaBr_xCl_4-x. Furthermore, the superconducting transition temperature decreases under magnetic fields as shown in Fig 1. (a-c). For the compound x= 0, 0.2, and 0.4 under magnetic field, we confirmed that the n value hardly changes compared to the zero field. We will discuss the detail of the chemical pressure dependence of Fermi liquidity and how it behaves under the magnetic field to establish the general phase diagram of λ-(BETS)₂GaBr_xCl_4-x and give a hint in the field-induced superconductivity in this system.

References

[1] H. Aizawa, et al., J. Phys. Soc. Jpn 87, 9 (2018).

[2] S. Imajo et.al., Phys. Rev. B. 103, 22 (2021)

[3] D. P. Sari et al., Phys. Rev. B 104, 224506 (2021).

[4] Kobayashi et al., Phys. Rev. B 96, 12 (2017).

[5] M. Ueno, et al., Interactions 245, 71 (2024).

[6] H.Tanaka et.al., J. Am. Chem. Soc 121, 4 (1999).

[7] C. Proust and L. Taillefer, Annu. Rev. Condens. Matter Phys. 10, 409-429 (2019).

[8] H. Oike et.al., J. Phys. Soc. Jpn. 93, 4 (2024).

[9] H. Taniguchi, et.al., J. Phys. Soc. Jpn. 76, 11379 (2007)

[10] H. Kobayashi, H. Cui, and A. Kobayashi., Chem. Rev. 104 11 (2004).

[11] T. Kobayashi, et al., Phys. Rev. Res. 2, 2 (2020).

[12] A. A. Firdaus, et al., submitted (2024).

Acknowledgment

This work is supported by Grant JSPS KAKENHI Grant Number JP20H04463, JP21K13885.

Figure 1. The normalized resistivity in λ-(BETS)₂GaBr_xCl_4-x at (a) x=0, resulting λ-(BETS)₂GaCl₄ with the range 8-24 K, (b) at x=0.2 with the range 17-30 K, and (c) x=0.4 with the range 10-17 K under various magnetic field including the error bar of n using the fitting function ρ/ρ_rt=ρ₀+ATⁿ.  The dashed line is the fitting result. The inset of the left figure is the physical pressure dependence in the case of λ-(BETS)₂GaCl₄.

Keywords: organic superconductor, negative pressure, insulating phase, non-Fermi liquid, spin fluctuation