Abstract Body

BiCh₂-based superconductors have a stacking structure with a conducting layer composed of Bi and Ch (Ch=S, Se) ions and an insulating layer consisting of Ln and O ions. LaO_0.5F_0.5BiS₂, belonging to these superconductors, shows superconductivity at about 3 K [1]. Furthermore, Pb substitution for partial Bi ions induces the superconducting transition temperature (T_c) to about 5 K at LaO_0.5F_0.5Bi_0.9Pb_0.1S₂ and a structural phase transition at about 150 K [2,3]. Therefore, the relationship between T_c and the structural phase transition temperature (T_ano) is crucial for understanding the superconducting mechanism. So far, we have synthesized La_1-xSr_xO_0.5F_0.5Bi_0.9Pb_0.1S₂ single crystals, where Sr ions are partially substituted for La ion and characterized their physical properties to investigate this relationship. As a result, T_c decreased while T_ano increased with increasing Sr substitution. On the other hand, details of the crystal structure with Sr substitution have yet to be clarified.

In this study, we performed a single-crystal structure analysis for LaO_0.5F_0.5Bi_0.9Pb_0.1S₂ substituted for Sr ion by using synchrotron X-ray diffraction measurements and determined the detailed structure. In the talk, we will discuss the structural changes at the atomic displacement level by Sr substitution and their relation to T_c and T_ano.

References

[1] Y. Mizuguchi et al. J. Phys. Soc. Jpn 81, 114725 (2012).
[2] S. Otsuki et. al. Solid State Commun. 270, 17-21 (2018).
[3] S.Okada et. al. J. Phys. Soc. Jpn. 93, 064701 (2024).

Keywords: Element Substitution, BiS₂-based Superconductors, Structure analysis, Single crystals growth

Abstract Body

BiS₂-based compound LaOBiS₂ shows superconductivity at about 3 K via the partial elemental substitution of Fluorine (F) for Oxygen (O), corresponding to an electron carrier doping to a conduction BiS₂ layer [1]. The superconducting transition temperature (T_c) increases by partial elemental substitution, indicating this material is sensitive to structural distortion. Recently, it was found that the partial substitution of lead (Pb) for bismuth (Bi) increases T_c to about 5 K [2]. The Pb concentration is below 15 %. Furthermore, this substitution also induces anomalous behavior in the temperature dependence of electrical resistivity. This electrical resistivity anomaly has been reported to be a structural phase transition to a low-symmetry structure [3]. Although the transition relates to the enhancement of superconductivity, the change of detail properties has not been understood. Therefore, it is important to clarify the Pb-substitution effect in order to understand the superconducting mechanism of BiS₂-based compounds.

In this study, we synthesized polycrystals of LaO_0.4F_0.6Bi_1-xPb_xS₂ and investigated their thermoelectric properties using the thermal transport option (TTO) for PPMS to clarify the Pb-substitution effect.

Polycrystalline samples of LaO_0.4F_0.6Bi_1-xPb_xS₂ were prepared by the solid-state reaction.

Pb-substituted samples were successfully synthesized because XRD patterns for all samples could be indexed with the structure of LaOBiS₂. Electrical resistivity measurement revealed that superconductivity appears for x=0.12, not seen for x=0.08, 0.18 (Fig 1(a)). In addition, the anomaly was observed above x=0.12. This result is consistent with previous study [3]. On the other hand, the absolute value of the Seebeck coefficient at room temperature increases, indicating that hole carriers are monotonically doped via Pb-substitution (Fig 1 (b)). Moreover, it was found that the value of thermal conductivity is more significant for x=0.12 and smaller for x=0.18 compared to x=0.08 (Fig 1(c)), suggesting that the disorder of the crystal structure is enhanced at x=0.18 in comparison with x=0.12. These results indicate that the T_c of the Pb-substituted La(O,F)BiS₂ depends not only on the electron carrier concentration but also on the disorder of the crystal structure. In the presentation, we will report the evaluation of physical properties in LaO_0.4F_0.6Bi_1-xPb_xS₂ in detail.

References

[1] Y. Mizuguchi et al, J. Phys. Soc. Jpn. 81, 114725 (2012).
[2] S. Otsuki et al, Solid. State. Commun. 270, 17-21 (2018).
[3] S. Okada et al, J. Phys. Soc. Jpn. 93, 064701 (2024).

Acknowledgment

We performed their thermos electric properties using the thermal transport option (TTO) for PPMS at Nano Frontier Superconducting Materials Group in NIMS. We appreciate the experimental support by Mr. Okabe and Ms. Yamashida.

Figure 1 Temperature dependence of the (a) electrical resistivity, (b) Seebeck coefficient and (c) thermal conductivity

Keywords: BiS₂-based superconductor, Elemental substitution, Structural phase transition, Thermoelectric properties

Abstract Body

Eu_3-xSr_xBi₂S₄F₄ (x = 0, 1 and 2) are classified as a group of BiS₂-based superconductors [1,2]. Eu₃Bi₂S₄F₄ and Eu₂SrBi₂S₄F₄ exhibit superconductivity through self-doping with Eu-valence fluctuation [1, 3]. The superconducting transition temperature (T_c) is observed at 1.5 and 0.8 K for Eu₃Bi₂S₄F₄ and Eu₂SrBi₂S₄F₄ respectively. It has been demonstrated that T_c increases with the application of external pressure and/or chemical pressure [4-7]. The Eu_3-xSr_xBi₂S_4-ySe_yF₄ compound exhibits a tetragonal structure (space group: I4 / mmm) and T_c is nearly 3 K [7]. Upon application of pressure, its T_c increases continuously up to approximately 3 GPa [8]. In contrast, the pressure dependence of T_c in Eu₂SrBi₂S_2.5Se_1.5F₄ is complex accompanied by structural phase transitions up to 12 GPa [9]. In order to confirm the physical properties of Eu₂SrBi₂S₂Se₂F₄, we have measured the electrical resistivity and X-ray diffraction patterns (XRD) of this material above 10 GPa in the present study.

Figure 1 shows the pressure dependence of T_c in Eu₂SrBi₂S₂Se₂F₄. Two peaks were observed in T_c at 2.62 K (at 1.5 GPa) and 2.61 K (at 5 GPa). Figure 2 presents the XRD patterns of Eu₂SrBi₂S₂Se₂F₄ under high-pressure. At 4.88 GPa, the appearance of new peaks indicated by arrows suggests the occurrence of a structural phase transition. Therefore, it can be concluded that Eu₂SrBi₂S₂Se₂F₄ should exhibit multiple superconducting phases up to 5 GPa.

In this paper, the properties of the superconducting phases, focusing on the temperature dependence of resistivity and X-ray diffraction patterns under high-pressure.

References

[1] H. F. Zhai et al. J. Am. Chem. Soc. 136 15386 (2014).
[2] Z. Haque et al. Inorg. Chem. 56 3182 (2017).
[3] Z. Haque et al. Mater. Today Proc. 36 743 (2021).
[4] Y. Luo et al. Phys. Rev. B 90 220510 (2014).
[5] K. Ishigaki et al. JPS Conf. Proc. 30, 011058 (2020).
[6] P. Zhang et al. Europhys. Lett. 111 27002 (2015).
[7] Z. Haque et al. Inorg. Chem. 57 37 (2018).
[8] N. Subbulakshmi et al. J. Supercond. Nov. Magn. 32 2359 (2019).
[9] K. Ishigaki et al. J. Phys. Soc. Jpn, 93, 024706 (2024).

Acknowledgment

This work was supported by JSPS KAKENHI Grant Number JP19H00648. Prof. L. C. Gupta is acknowledged for his initial contributions to the work on these and other similar materials while he was a visiting scientist at the Dept. of Chemistry, IIT, Delhi.

Keywords: BiS₂-based superconductor, High-pressure, Structural phase transition

Abstract Body

The topological superconductivity has attracted much interest over the last few decades [1]. The chiral superconductivity is one of the topological superconducting states with spontaneous time-reversal symmetry breaking (TRSB). SrPtAs (transition temperature T_c=2.4K, point group; D_6h) is one of the candidates for TRSB superconductor and it is the first discovered compound as the series of the honeycomb network superconductors [2,3]. The crystal structure of SrPtAs contains Pt-As honeycomb layers with local inversion symmetry breaking piled up with pi/3 rotation(Fig.1a). It is suggested that the chiral d-wave superconducting state is the most plausible candidate of pairing symmetry [3-6].

Recently the honeycomb pnictide superconductorBaPtAs_1-xSb_x (T_c=1.6-3.2K, point group; D_3h) with broken inversion symmetry was discovered (Fig.1b) [7-9]. T_c shows non-monotonic behavior with increasing x and the TRSB is observed at x=1 [9,10]. A possible way to explain these results is to consider the change of the pairing symmetry with varying x.

In order to investigate the electronic structures of BaPtSb (x=1) and BaPtAs (x=0), we carry out the first principles calculations using the Quantum Espresso package [11,12]. Fig. 1c(d) shows the electronic band structure with and without spin-orbit coupling and the density of states without spin-orbit coupling for BaPtSb (BaPtAs), and we clearly see that they have quite similar band structures. In  BaPtSb (BaPtAs), Pt 5d and Sb 5p (As 4p) orbitals are dominant around the Fermi level and they compose three Fermi surfaces. Therefore, we construct low-energy effective tight-binding models which reproduce their band structure around the Fermi level using Wannier90 package [13]. We solve the gap equation for each pairing state classified by the group theoretical analysis and examine a condensation energy. We found that the spin singlet chiral d-wave state could be a potential candidate for the stable pairing symmetry in BaPtSb [14].

References

[1] N. Read et al., Phys. Rev. B 61, 10267 (2000).
[2] Y. Nishikubo et al., J. Phys. Soc. Jpn. 80, 055002 (2011).
[3] P. K. Biswas et al., Phys. Rev. B 87, 180503 (2013).
[4] J Goryo et al., Phys. Rev. B 86, 100507(R) (2012).
[5] M. H. Fischer et al., Phys. Rev. B 89, 020509(R) (2014).
[6] H. Ueki et al., Phys. Rev. B 99, 144510 (2019).
[7] K. Kudo et al., J. Phys. Soc. Jpn. 87, 063702 (2018).
[8] K. Kudo et al., J. Phys. Soc. Jpn. 87, 073708 (2018).
[9] T. Ogawa et al., J. Phys. Soc. Jpn. 91, 123702 (2022).
[10] T. Adachi et al., submitted.
[11] P. Giannozzi et al., J. Phys.: Condens. Matter 21, 395502 (2009).
[12] P. Giannozzi et al., J. Phys.: Condens. Matter 29, 465901 (2017).
[13] G. Pizzi et al., J. Phys. Cond. Matt. 32, 165902 (2020).
[14] T. Imazu et al., in preparation.

Figure 1. The crystal structure of (a) SrPtAs and (b) BaPtSb, BaPtAs. The electronic band structure and the density of state of (c) BaPtSb and (d) for BaPtAs.

Keywords: chiral superconductivity, time-reversal symmetry breaking

Abstract Body

Layered compounds, such as cuprates, iron-pnictides, and HfNCl, are a showcase of superconductors with high T_c. Among them, ZrP_2-xSe_x is a layered superconductor [1] and is thought to have the potential to exhibit a higher T_c by modifying electron filling, band width and so on.  ZrCuP₂, which has a structure in which a nonmagnetic Cu layer is inserted into ZrPSe, is a promising candidate for a new superconductor [2]. The crystal structure of ZrCuP₂ is identical to that of the existing material ZrCuSiP [3], and is also identical to that of LaOFeAs, which has the highest T_c among iron-pnictide superconductors.

In this study, we performed first-principles electronic structure calculations for ZrCuP₂ and its related compound ZrNiP₂. In ZrNiP₂, a large increase in the density of states just below the Fermi level is observed, which is generally favorable for high-T_c superconductivity [4]. We also calculated the phonon dispersion and the electron-phonon interaction for these compounds and estimated their T_c using the isotropic approximation.

References

[1] H. Kito et al.  J. Phys. Soc. Jpn. 83, 074713 (2014)
[2] I. Hase et al.  PCP-4, 36^th International Symposium on Superconductivity (ISS2023), 29 Nov. (2023), Wellington
[3] H. Abe and K. Yoshii,  J. Solid State Chem. 165, 372 (2002).
[4] V. Stanev et al.  npj Comput. Mater. 4, 29 (2018)

Figure 1. Density of States of ZrNiP₂ and ZrCuP₂.

Keywords: Superconductivity, First-principles calculation, Layered structure, ZrCuP₂, ZrNiP₂

Abstract Body

We have comprehensively searched for new superconductors in the antiperovskite transition metal pnictides A_nM₃X (A = alkali metals, alkaline earth metals, rare earths; M = transition metals; X = pnictogen; n = 1 or 2), a material system that remains underexplored. As a result, we have discovered a variety of new antiperovskites including following superconductors. Mg₂Rh₃P exhibits superconductivity at 3.8 K by introducing about 5 mol% Mg deficiency (Fig. 1a) [1]. Although non-centrosymmetric (NCS) antiperovskites of CaPd₃P and SrPd₃P are non-superconducting, a centrosymmetric (CS) phase appeared in their solid solution (Ca,Sr)Pd₃P exhibits superconductivity at 3.5 K [2,3]. Partial substitution of Pt into SrPd₃As results in the appearance of superconductivity at 3.7 K associated with a structural phase transition (Fig. 1b) [5]. LaPd₃P with a new prototype NCS cubic structure exhibits a superconductivity at 0.38 K (Fig. 1c) [4]. Thus, the antiperovskite transition metal pnictide is a promising material system that still has room to explore new superconductors.

References

[1] A. Iyo et al, Phys. Rev. Materials 3, 124802 (2019).
[2] A. Iyo et al, Inorg. Chem., 59, 12397 (2020).  
[3] I. Hase et al, J. Phys.: Conf. Ser.1975, 012004 (2021).
[4] A. Iyo et al, Inorg. Chem.,60, 18017 (2021).
[5] A. Iyo et al, Inorg. Chem., 61, 12149 (2022).

Acknowledgment

This study was supported by the Japan Society for the Promotion of Science (JSPS) Grants-in-Aid for Scientific Research (KAKENHI) (22K04193).

Figure 1 (a) Crystal structure of Mg2Rh3P (Mo3Al2C type, P4132), (b) Phase diagram of Sr(Pd,Pt)3As, (c) Crystal structure of LaPd3P (new prototype, I-43m).

Keywords: new superconductor, antiperovskite, transition metal pnictide, Mg₂Rh₃P, (Ca,Sr)Pd₃P, Sr(Pd,Pt)₃As, LaPd₃P

Abstract Body

The development of quantum materials is a source of breakthrough new phenomena and innovative functions, and is becoming increasingly important as a starting point and foundation for next-generation industries. Although various methods have been established to predict the functions of quantum materials based on existing theories, one often fails to experimentally synthesize materials as theoretically designed. Because of the difficulty in predicting the phase formation of new materials, the actual development of new materials has been based on empirical knowledge, physical intuition, and exhaustive search, which has been a bottleneck in the development of materials. One of the reasons for the difficulties is the lack of available experimental data necessary to accurately predict the phase formation of new materials. Although the trial-and-error process of materials development has accumulated a large amount of experimental data on the success or failure of phase formation, failed cases are rarely disclosed. Furthermore, the chemical compositions and crystal structures of materials in the open databases are limited to those theoretically calculated or successfully synthesized.

In the simple system of ternary ABX3, the Goldschmidt tolerance factor, which consists of ionic radii, is known as an indicator for the formation of a cubic perovskite phase. However, such an indicator for phase formation is unknown for multinary compounds. In this study, we focus on layered perovskite compounds [Figure 1 (left)] as a typical example of multinary systems to identify indicators for phase formation (phase formation determinants) that would be available for predicting phase formation. We developed a machine learning model to predict phase formation using hundreds of experimental data on phase formation. To predict the formation of new materials in layered perovskite compounds, we developed the Python codes to compute the phase formation determinants for this system using SISSO (Sure Independence Screening and Sparsifying Operator) [1], a type of symbolic regression that is expected to have high extrapolation performance. The experimental data on phase formability in an arsenic system were classified [Figure 1(right)], achieving a classification accuracy of ~ 90%. We also constructed a prediction model by using ~ 300 experimental data on phase formability including non-arsenic systems and evaluated the generalization performance of the phase formation determinants.

Abstract Body

Superconductivity in WS₂ with the superconducting transition temperature T_c=8.8K which is the highest among all transition-metal dichalcogenides has recently been discovered [1] and has attracted considerable attention as a candidate material for topological superconductivity [2,3]. When pressure is applied, T_c decreases monotonically with increasing pressure down to 3.5K at 18GPa above which a broad transition region from the 2M structure to the 3R structure is observed up to 40GPa where the system shows a semiconducting behavior with no superconductivity [4]. At further higher pressures above 45GPa, 3R-WS₂ becomes metallic and shows superconductivity with T_c of 2.5 K almost independent of pressure up to 65GPa [4]. In our previous work [5], we have investigated the pressure dependence of T_c of WS₂ based on the first-principles calculation and have found that, in 2M-WS₂, the obtained T_c decreases with increasing pressure as observed in the experiments although the values of T_c are about 1/3~1/6 of the experimental values. As for 3R-WS₂, T_c is estimated to be zero independent of pressure and then the conventional BCS phonon mechanism is considered to fail to account for the superconductivity in 3R-WS₂.

In the present paper, we then focus especially on 3R-WS₂ to discuss a possible unconventional mechanism of the superconductivity. Figure 1 (a) shows the first-principles results of the Fermi surfaces (FSs) of 3R-WS₂, where there are three small hole FSs centered at the G point (A, B, and C) and six small electron FSs between the G point  and the six K points (D). Based on the band dispersions obtained from the first-principles calculations, the bare susceptibilities in the band representation are calculated and plotted in Figure 1 (b), where the intra-band component of the electron band (D-D) shows a peak at the M point due to the nesting between the six electron FSs in addition to a peak at the G point, while the inter-band components between the hole and electron FSs (A-D, B-D and C-D) show peaks at the incommensurate wave vectors due to the nesting between the three hole FSs and the six electron FSs. Those peaks of the susceptibilities are considered to represent the effects of CDW and/or excitonic fluctuations as have been discussed in many layered transition-metal dichalcogenides [6]. Then, we discuss a possible mechanism of the superconductivity due to the CDW and/or the excitonic fluctuations. Detailed results will be presented on the day of the conference.

References

[1] Y. Fang et al., Adv. Mater. 31, 1901942 (2019)
[2] Y. Yuan et al., Nature Physics 15, 1046 (2019)
[3] Y. W. Li et al., Nature Communications 12, 2874 (2021)
[4] W. Zhang et al., J. Phys. Chem. Lett. 12, 3321 (2021)
[5] Y. Wang, T. Sekikawa, K. Sano and Y. Ōno, The 24th Asian Workshop on First-Principles Electronic Structure Calculations, Oct. 30-Nov. 1, 2023, Fudan University, Shanghai, China
[6] S Manzeli et al., Nat. Rev. Mater. 2, 17033 (2017).

Acknowledgment

This work was partially supported by JSPS KAKENHI Grant Number 21K03399 and the Niigata University Fellowship System. Numerical calculations were performed in part using the facilities of the Center for Computational Sciences, University of Tsukuba and Center for Computational Materials Science, Institute for Materials Research, Tohoku University.

Abstract Body

Intercalation of guest ions into the van der Waals (vdW) gap of layered materials is a powerful route to create novel material phases and functionalities. Ionic gating is a technique to control the motions and configuration of ions, both for intercalation and surface electrostatic doping. The advance of ionic gating enables the in-situ probe of dynamics of ion diffusion, carrier doping and transport properties. Here we performed in-situ resistivity and Raman experiments on the potassium ion (K+) intercalation of single crystal MoS2 and constructed a temperature-carrier density phase diagram. The K+-intercalation induces a structural transition from the prismatically coordinated phase to the octahedrally coordinated phase, where anisotropic three-dimensional superconductivity and possible charge density wave state were observed. The present ionic gating offers a comprehensive view of the intercalated phases and proves that the electrostatically induced superconductivity is distinct from that in the intercalated phase.

Abstract Body

NbS₂, a type of layered transition metal dichalcogenide, has polytypes. The 2H polytype shows a superconducting transition at about 6 K. The interlayers of 2H-NbS₂ are stacked by van der Waals forces, allowing the intercalation of ions [1, 2]. The superconducting transition temperature (T_c) of alkali metal-intercalated NbS₂ increases as the ionic radius of the alkali metal rises [2]. Depending on the increase in the ionic radius, the two-dimensionality of the NbS₂ layer is enhanced, suggesting it affects the superconducting properties. However, the detailed relationship between the ionic radius and superconducting properties has not been revealed.

In this study, single crystals of Cs- or K-intercalated NbS₂ were synthesized by using the flux method with CsCl and/or KCl. We found that these single crystals were grown by changing the combination of the flux. The composition analysis by Energy Dispersive X-ray Spectroscopy shows that the intercalation ratio of Cs or K ion is about 10 % in the prepared samples. These samples exhibit T_c at about 3 K in the electrical resistivity measurements. This resistivity result indicates that the T_c did not depend on the ionic radius, which is contrary to the previous result. In this talk, we will discuss the detailed method of synthesis of ion-intercalated single crystals using the flux method and the effect of ion intercalation of Cs and K ions on the superconducting properties.

References

[1] A. Lerf, et al., Mater. Res. Bull. 14, 797-805 (1979).
[2] M. Nagao, et al Z. Naturforsch. 76, 0123 (2021).

Keywords: flux, single crystal, intercalate, layered materials

Abstract Body

TiSe₂ is a transition-metal dichalcogenide that exhibits a 2a₀ × 2a₀× 2c₀ charge density wave (CDW) at temperatures below 202 K, where a₀ and c₀ are lattice constants. Moreover, superconductivity is induced in TiSe₂ by employing doping or high pressure. Extensive research on TiSe₂ has been conducted and has provided knowledge on the CDW properties and the coexistence or competition of the induced superconductivity with the CDW. However, the driving mechanism for the CDW remains unresolved, though the exciton condensation mechanism and the band-type Jahn–Teller mechanism have been proposed.

One way to address the question of the CDW driving mechanism is to investigate the energy gap of CDW, the CDW gap, which is related to the driving mechanism. Several studies have used scanning tunneling microscopy/scanning tunneling spectroscopy (STM/STS), angle-resolved photoemission spectroscopy (ARPES) to investigate the CDW gap size for TiSe₂ and the energy at which the CDW gap opens. They also discussed the driving mechanism with reference to information about the CDW gap. However, the estimated gap size and the energy at which the gap opens vary among the previous studies. Thus, a consensus understanding of the CDW gap has not been attained.

In the present study, we investigated the CDW gap in TiSe₂ by performing STM/STS measurements. We observed spatial variation in the tunneling spectrum. In addition, in the STM images, we found that the CDW pattern changed depending on the bias voltage. In the symposium, we will discuss the energy where the CDW gap opens in TiSe₂.

Abstract Body

The misfit layered compound (BiSe)_1.10(NbSe₂)_m is a superconductor with a crystal structure in which an insulating BiSe layer and superconducting NbSe₂ layers are alternately stacked via van der Waals forces. The superconducting transition temperature (T_c) varies depending on the number of superconducting layers (m). Additionally, superconductivity has anisotropy in the upper critical field (μ₀H_c2) along the in-plane crystal axes. The μ₀H_c2 along the a-axis limits by the Pauli limit, while the b-axis exceeds the Pauli Limit [1]. Furthermore, the T_c is 3 - 4 K, which is lower than 7 K for NbSe₂. The origin of the difference in the T_c is not known so far. One of the possibilities is the difference in the carrier concentration. Therefore, tuning of the carrier concentration may increase the T_c.

In this study, we investigated the Ag substitution effect for the Bi site for the superconducting properties of single-crystal samples in (BiSe)_1.10(NbSe₂)_m. These single crystals were grown using the flux method. As a result, we successfully grew single crystals with dimensions of 0.5² to 1.5² mm²(Fig.1). The obtained samples were evaluated by the X-ray diffraction (XRD) measurement, elemental composition analysis measurement by using Scanning Electron Microscope – Energy Dispersive X-ray spectrometry (SEM-EDX) and temperature and magnetic field dependent resistivity measurements. The XRD measurements indicated that the single crystals of (BiSe)_1.10(NbSe₂)_m were obtained, and m was changed with increasing Ag substitution. In this presentation, we will discuss the change in physical properties in (BiSe)_1.10(NbSe₂)_m by the Ag substitution.

References

[1] S. Matsuzawa et al., J. Phys. Conf. Ser. 2545 012002 (2023).

Figure 1. Optical image of (Bi1-xAgxSe)1.10(NbSe2)m single crystal (xAg = 0.3).

Keywords: Misfit compound, Element substitution, Superconductivity

Abstract Body

Electron-doped strontium titanate SrTiO₃ is one of the most dilute superconductors studied extensively for more than half a century [1]. The superconductivity is observed for extremely low carrier concentrations n = 10¹⁸~10²¹/cm³ corresponding to about 10^-4~10^-1 electrons per Ti atom. The superconducting transition temperature T_c shows a characteristic dome shape as a function of n with a peak of T_c=0.45K at n~10²⁰/cm³ [2]. In the non-doped case, SrTiO₃ shows a huge dielectric constant of about 2×10⁴ at low temperatures and is considered to be close to a ferroelectric transition [3]. In fact, the ferroelectric transition is induced by replacing ¹⁶O with ¹⁸O [4] and by replacing Sr with Ca [5]. Furthermore, T_c is found to be enhanced towards the quantum critical point (QCP) of the ferroelectric transition [5]. Theoretically, Edge et al. has proposed a model of superconductivity due to quantum ferroelectric fluctuations by taking into account of doping dependence of the ferroelectric soft-mode optical phonons based on the first-principles calculations [6]. Explicit estimates of T_c based on the first-principles calculations, however, were not done there as unphysical imaginary phonon frequencies due to ferroelectric instabilities are obtained at low doping n<10²⁰/cm³.

In this study, we explicitly estimate T_c of electron-doped SrTiO₃ on the basis of the first-principles calculations (Quantum ESPRESSO) in the over-doped regime with n=10²⁰~10²¹/cm³, where the imaginary frequencies were not obtained. When n decreases from 10²¹/cm³ to 10²⁰/cm³, the frequencies of the ferroelectric optical phonons near the Γ-point monotonically decreases while the electron–phonon coupling constant λ monotonically increases, and then estimated T_c monotonically increase as consistent with experiments in the over-doped regime.

References

[1] M. N. Gastiasoro, J. Ruhman, R. M. Fernandes, Ann. Phys. 417, 168107 (2020)
[2] J. F. Schooley et al., Phys. Rev. Lett. 14, 305 (1965)
[3] E. Sawaguchi, A. Kikchi, Y. Kodera, J. Phys. Soc. Jpn. 17, 1666 (1962)
[4] M. Itoh et al., Phys. Rev. Lett. 82, 3540 (1999)
[5] C. W. Rischau et al., Nat. Phys. 13, 643 (2017)
[6] J. M. Edge et al., Phys. Rev. Lett. 115, 247002 (2015)

Acknowledgment

This work was partially supported by JSPS KAKENHI Grant Number 21K03399 and the Niigata University Fellowship System. Numerical calculations were performed in part using the facilities of the Center for Computational Sciences, University of Tsukuba and Center for Computational Materials Science, Institute for Materials Research, Tohoku University.

Keywords: SrTiO3, Superconductivity, First-principles calculation, Density functional theory

Abstract Body

Josephson current has a problem called the cosine-term problem, which has been known since the 1970s. The dissipative Josephson current has a term that depends on the phase difference (cosine term) and a term that does not. The cosine-term problem is an inconsistency between theories and experiments, in which theories show that these two terms have the same sign, but experiments give results with opposite signs.[1] Recently, in connection with the superconducting qubit, a positive-sign cosine term has been obtained in an experiment at the extremely low-temperature and low-frequency limit. However, since the finite value of the original dissipative Josephson current is caused by thermally excited quasiparticles, the author considers that the cosine-term problem in temperature ranges other than extremely low temperatures is still unsolved. Several theories have been suggested so far regarding the cosine-term problem, but all of these considered only the sign of the cosine-term at the limit of zero frequency. In actual experiments, the frequency dependence of the damping rate at the resonant frequency of the Josephson plasmon has been observed, so calculations must be performed at a finite frequency. Furthermore, the frequency dependence does not arise only from the term that depends on the phase difference, but it is necessary to examine the frequency dependence of the entire dissipative current. Recently, the author showed that when the electronic state is in a non-equilibrium steady state due to an external field at the resonant frequency, the effective temperature changes due to the phase difference, and the damping rate of Josephson plasmon behaves differently from previous theories.[2]

In this symposium, in addition to this change in electronic state, we show that collective excitation modes also depend on the phase difference, which results in the dependence of the damping rate of Josephson plasmon on the phase difference that is consistent with experiments. In general, phase fluctuations and amplitude fluctuations exist in superconductors, and in simple cases, these two modes are uncoupled, but when the particle-hole symmetry is broken, these two fluctuations couple. In Josephson junctions, the amplitude mode also exists, but when the phase difference is zero, they do not couple with Josephson plasmons, which is a phase difference fluctuation. On the other hand, when a finite phase difference occurs due to the presence of a dc current, these two modes couple due to the breaking of time-reversal symmetry. As a result, Josephson plasmons are affected by the amplitude mode and has a dependence on the phase difference. Figure 1 shows the frequency dependence of the impedance of a Josephson junction. The numbers shown in the figure are values of cosine of the phase difference; as the phase difference increases, the resonance frequency decreases and the peak position shifts to lower energy. The inset shows the damping rate of the Josephson plasmons at this resonance frequency, which becomes smaller as the phase difference decreases. This result is consistent with that of experiments.

References

[1] D. N. Langenberg, Rev. Phys. Appl. (Paris) 9, 35 (1974).
[2] T. Jujo, J. Phys. Soc. Jpn. 93, 094701 (2024).

Acknowledgment

The numerical computation in this work was carried out at the Yukawa Institute Computer Facility.

Figure 1. The curves show the frequency dependence of impedance. The values of the lines indicate the cosine of the phase difference. The inset shows the value of the damping rate of the Josephson plasmons at the resonance frequency, and the value of the cosine ranges from 0.05 to 0.95. The temperature is at T/Tc=0.6.

Keywords: Josephson plasmons, Amplitude fluctuation, Damping rate, Cosine-term problem

Abstract Body

The recent observation of superconductivity in (Nd,Sr)NiO₂ thin films [1] has revived the long-standing interest in nickel oxides that has persisted since the discovery of high-temperature superconductivity in copper oxides. The superconducting state of (Nd,Sr)NiO₂ manifests itself when the formal 3d electron count of Ni is 8.8, which is equivalent to that of Cu in copper oxides, suggesting an intimate relation between the electron state and superconductivity. In fact, a subsequent study reported the observation of superconductivity in a Nd₆Ni₅O₁₂ thin film, which has the same formal electron count [2]. Stimulated by these observations, we fabricated fluorinated (Nd,Sr)₂NiO₄ and studied their properties. The formal electron configuration of Ni in the parent compound Nd₂NiO₄ is 3d⁸. That means that if one of the four oxygen atoms is replaced by fluorine, the electron configuration would be 3d⁹ and substitution of Sr should lead to a 3d electron count of 8.8.

The fluorinated samples were fabricated via a three-step process based on a report on La₂NiO₃F [3] but with an improvement to reduce the amounts of impurity. Both bulk and thin film samples were studied, starting with the fabrication of (Nd,Sr)₂NiO₄ phase samples. The bulk samples were synthesized via solid-state reaction, while the thin films by pulsed laser deposition (PLD) on SrTiO₃ or LaAlO₃ substrates. These samples were fluorinated to yield (Nd,Sr)₂NiO₃F₂ using ZnF₂ as a fluorinating agent. (Nd,Sr)₂NiO₃F was then obtained by a topochemical reduction of (Nd,Sr)₂NiO₃F₂ using CaH₂ as a reducing agent. The obtained samples were single-phase without apparent impurity phases as was confirmed by X-ray diffraction. The temperature dependence of the magnetic susceptibility of bulk Nd₂NiO₃F revealed a magnetic phase transition at around 160 K, closely resembling the behavior observed in La₂CuO₃F₂, which possesses the same 3d⁹ electron configuration [4]. Therefore, it is plausible to assume that the electronic structure of Nd₂NiO₃F is similar to La₂CuO₃F₂ as expected. However, the resistivity of the Nd₂NiO₃F thin film increased with decreasing the temperature and showed an insulating behavior. Further, as of now, superconductivity has not been observed including in the Sr-doped samples that were fabricated using a similar approach. Further investigation to understand the reason behind the absence of superconductivity in (Nd,Sr)₂NiO₃F is ongoing as clarifying this aspect could provide valuable insights into the mechanism of high temperature superconductivity in copper oxides.

References

[1] D. Li, K. Lee, B. Y. Wang, M. Osada, S. Crossley, H. R. Lee, Y. Cui, Y. Hikita and H. Y. Hwang, Nature 572, 624 (2019).
[2] G. A. Pan, D. Ferenc Segedin, H. LaBollita, Q. Song, E. M. Nica, B. H. Goodge, A. T. Pierce, S. Doyle, S. Novakov, D. Córdova Carrizales, A. T. N’Diaye, P. Shafer, H. Paik, J. T. Heron, J. A. Mason, A. Yacoby, L. F. Kourkoutis, O. Erten, C. M. Brooks, A. S. Botana and J. A. Mundy, Nat. Mater. 21, 160 (2021).
[3] K. Wissel, A. M. Malik, S. Vasala, S. Plana-Ruiz, U. Kolb, P. R. Slater, I. da Silva, L. Alff, J. Rohrer and O. Clemens, Chem. Mater. 32, 3160 (2020).
[4] J. Jacobs, M. A. L. Marques, H.-C. Wang, E. Dieterich and S. G. Ebbinghaus, Inorg. Chem. 60, 13646 (2021).

Keywords: nickel oxide, fluoridization, topochemical reduction

Abstract Body

The quasi-two-dimensional organic conductor λ-(BETS)₂FeCl₄ undergoes an antiferromagnetic state under 8.5 K accompanied by a metal-to-insulator transition. Furthermore, superconductivity appears when an external magnetic field of more than 18 T is applied. This was thought to be due to the so-called Jaccarino-Peter compensation mechanism, in which the 3d electrons of iron form a magnetically ordered state in a zero magnetic field, and the internal magnetic field generated by the 3d electrons suppress the superconductivity of the π electrons. However, this does not explain the reason for the insulating state at 8.5 K.

In 1999, zero-resistivity was also observed under the high-pressure above 3 kbar and below 2.3 K by means of resistivity measurements. However, the Meissner effect had not been observed and this zero-resistivity cannot be attributed to superconductivity. In this study, we performed magnetic susceptibility measurements under precisely controlled pressure on this material to investigate the relationship between the metal-insulator transition and superconductivity.

As a result, we could surmise that superconductivity was achieved at 2.8 K for 1.8 kbar and at 2.3 K for 2.640 kbar. Details of our experiments will be presented.