Abstract Body

The hexagonal pnictide CaAgP is a promising candidate for a topological line-node semimetal. This compound was synthesized to be the ZrNiAl-type crystal structure. Subsequently, it was demonstrated that the material possesses a topologically nontrivial band structure, by using first-principles calculations [1]. It is also reported that surface states are expected to exist in the nodal ring, resulting in drumhead dispersion along the (001) plane. The bottom of the drumhead surface can be regarded as a flat band.

Recently, an experimental study has revealed that the surface states of Pd-doped CaAgP (CaAg_1-xPd_xP) exhibit unconventional superconductivity around 1.5 K [2]. They also carried out the soft point contact experimental study, that is known to be surface-sensitive measurement, and observed that the so-called bell-shaped conductance spectra, that is a manifestation of the unconventional SC.

In contrast, a recent angle-resolved photoemission spectroscopy (ARPES) measurement has asserted that the bulk band gap and surface states in CaAgP are topologically trivial, as evidenced by a comparison with first-principles calculations [3]. Furthermore, they pointed out that the conventional LDA/GGA approach is incapable of reproducing the ARPES results, while a DFT calculation based on the Heyd-Scuseria-Ernzerhof (HSE) hybrid exchange-correlation functional [4] may give an accurate description of the topological characteristics of the bands in proximity to E_F within this system. This observation implies that there is still room for consideration in the theoretical predictions for topological materials. In addition, the nature of superconductivity and the bell-shaped conductance spectra observed in CaAg_1-xPd_xP should be re-examined in light of these findings.

Motivated by these backgrounds, we theoretically study the superconducting state of CaAg_1-xPd_xP systems based on first-principles calculations and the Eliashberg theory. First, we obtain the electronic state consistent with the ARPES measurements by using the so-called Yukawa-screened Perdew-Burke-Ernzerhof (YS-PBE) exchange functional [5] implemented in the WIEN2k code [6], which is known to yield similar results to the HSE. Then we construct an effective tight-binding Hamiltonian for the system using the Wannier 90 code [7]. We solve the linearized Eliashberg equation for several superconducting pairing states to elucidate the nature of the superconducting state realized in CaAg_1-xPd_xP. We will also present the theoretical tunneling spectra assuming the obtained plausible superconducting state.

References

[1] A. Yamakage et al., J. Phys. Soc. Jpn. 85, 013708 (2016).
[2] R. Yano et al., Nat. Commun. 14, 6817 (2023).
[3] N. Xu et al., Phys. Rev. B 97, 161111(R) (2018).
[4] J. Heyd et al., J. Chem. Phys. 118, 8207 (2003).
[4] F. Tran and P. Blaha, Phys. Rev. B 83, 235118 (2011).
[5] P. Blaha et al., WIEN2k, An Augmented Plane Wave + Local Orbitals Program for Calculating Crystal Properties (Tech. Univ. Wien, Vienna, 2001).
[6] G. Pizzi et al., J. Phys.: Condens. Matter 32, 165902 (2020).

Keywords: Topological superconductivity, First-principles calculations, Eliashberg theory

Abstract Body

Using the break junction (BJ) technique, we have investigated temperature dependences of both the maximum Josephson current I_Jc and the gap value D of the superconducting Bi-based cuprate Pb_xBi_2-xSr₂Ca₂Cu₃O_10+y (Pb-Bi2223) micro-crystals that can form superconductor- insulator-superconductor (SIS) structures. To investigate the Josephson-current dependence on the electrode superconducting properties, we additionally prepared samples with different critical temperature T_c obtained by nitrogen gas reduction annealing (T_c(zero) ~90 K) and compared with the as-grown samples^[1] (T_c(zero)~105 K).

According to the I(V) curve of the annealed Pb-Bi2223 sample, the magnitude of I_Jc was approximately 0.17 mA, and the resistance outside the gap R_N was 16 W, thus leading to I_JcR_N ~ 2.6 mV at T = 7.9 K. The temperature dependence of I_JcR_N showed a non-monotonic decrease with increasing T, which is different from that of the conventional BCS behaviour. The current I_Jc completely disappeared at approximately 89 K, roughly matching the T_c(zero), while the gap D was continuously observed above T_c(zero) of 90 K. The gap value D  was 30~ 40 meV, so that the calculated value (p/2)D/e on the basis of the Ambegaokar Baratoff theory with s-wave-symmetry of the BCS order parameter is ≈ I_JcR_{N (AB)} = 50~ 60 mV, whereas the observed I_JcR_N (~ 2.6 mV) is one-order smaller. These features were roughly the same as our previous results of those of the as-grown samples with T_c(zero)~105 K, indicating that the I_JcR_N did not change significantly for reduced T_c but approximately the same doping level.

References

[1] A. Sugimoto et al, J. Phys.: Conf. Ser. 1975, 012005 (2021).

Keywords: Bi2223, Break Junction, Josephson current, IjcRN

Abstract Body

For application of type Ⅱ superconductors, controlling vortices is important. For example, pinning of vortices is needed for higher critical current, and manipulation of the vortex is needed for a quantum computer. For the pinning of vortices, slanted or splayed columnar defects are effective.[1] Also, the splayed columnar defects cause an anomalous peak effect.

In order to investigate these properties of vortices in the superconductor with splayed columnar defects, we use three-dimensional molecular dynamics (3D-MD) method[2].

In 3D-MD, three dimensional superconductors are divided into two-dimensional layers. We solve equations of motion of each vortex in each layer.

We include the force by the external current \( f \begin{subarray}{rl} d \\ i,l \end{subarray} \) , the interaction force between the vortices \( f \begin{subarray}{rl} VV \\ i,l \end{subarray} \) , the fluctuation force due to heat \( f \begin{subarray}{rl} f \\ i,l \end{subarray} \) , the pinning force due to defects \( f \begin{subarray}{rl} imp \\ i,l \end{subarray} \) , the bending force of the vortices \( f \begin{subarray}{rl} b \\ i,l \end{subarray} \) , and the force from the external magnetic field \( f \begin{subarray}{rl} M \\ i,l \end{subarray} \) in the equations of motion. In this model, we have introduced a three-dimensional pinning force. Figure shows a examples of columnar defects, and distribution of vortices obtained by this simulation using this method. We investigate field dependence of the critical current and the origin of the peak effect.

References

[1] A. Park, S. Pyon, K. Ohara, N. Ito, and T. Tamegai, Phys. Rev. B 97,064516(2018)
[2]C. J. Olson, R. T. Scalettar, G. T. Zimányi, and N. Grønbech-Jensen, Phys. Rev. B 62, R3612(R) (2000)

Acknowledgment

We would like to thank T. Tamegai for giving us useful advice related to experimental results of critical current.

Figure. Columnar defects (a) and distribution of vortices (b)

Keywords: Vortex, 3D-Molecular dynamics, Critical current, Peak effect

Abstract Body

Active research on superconductivity in locally noncentrosymmetric systems has continued over the last decade. In these systems, exotic superconducting (SC) phases are predicted to stabilize in magnetic fields [1, 2] due to spatially inhomogeneous antisymmetric spin-orbit coupling (ASOC) caused by local spatial asymmetry and paramagnetic pair-breaking effects. The vortex state is of interest because exotic SC phases stabilize in a magnetic field. We are particularly interested in quasiparticle (QP) states. A previous study in the Pauli limit [2], neglecting vortices, reported the formation of the QP spectral gap at the central conduction layer in the pair density wave (PDW) state (π-phase difference between layers without local inversion symmetry). This phenomenon occurs due to the QP transitions from the outer conduction layers and the magnetic field induced shift of the density of states, despite the disappearance of the SC order parameter in the central layer. The QP states in the vortex core in the central layer could also be affected by perturbations due to QP transitions from the outer layers. We reported numerical results based on the quasiclassical method for the BCS state (0-phase difference), which is the low-field phase of a trilayer Rashba system [4]. The structure of the local density of states around the vortex core differs significantly from layer to layer, reflecting the inhomogeneous ASOC. In the PDW state, however, we failed to obtain the QP spectral gap at the central layer, in contrast to the result in Ref. [2], and instead used the Bogoliubov-de Gennes equation to calculate the QP states in the trilayer Rashba system to report the spectral gap in the excitation energy spectra in the PDW state [4]. In this study, we report the local QP density of states layer by layer in depth.

[1]  D. Maruyama et al.,  J. Phys. Soc. Jpn. 81, 034702 (2012).
[2]  T. Yoshida et al., Phys. Rev. B 86, 134514 (2012).
[3]  Y. Higashi et al., Phys. Rev. B 93, 104529 (2016).
[4]  Y. Higashi, Meeting Abstracts of the Physical Society of Japan 76.2 (2021).

Abstract Body

High-T_c superconductivity in cuprates is caused by carrier doping into a Mott insulator. It is important to reveal the nature of the strong on-site Coulomb interaction for understanding the electronic structure of high-T_c cuprates. Many studies have been conducted to experimentally determine the intra-atomic on-site Coulomb energy U_dd (U_pp) between Cu 3d electrons (O 2p electrons) [1-7]. Recently, the on-site Coulomb energies have been estimated using the Cini-Sawatzky method [8,9] for resonant photoemission spectra [6,7]. For single layer cuprates, the on-site Coulomb energy strongly depends on the presence or absence of apical oxygens for Cu sites [4,5]. Actually, the on-site Coulomb energy has been estimated to be U_pp ~ 3.3 eV for T'-type Pr_1.3-xLa_0.7Ce_xCuO₄ without apical oxygens [7].

In this study, we have investigated the on-site Coulomb energies U_dd and U_pp for the single-layer T-type La_2-xSr_xCuO₄ (x = 0.03, 0.15, and 0.22) by means of resonant photoemission spectroscopy. Figure 1 shows the valence-band spectra as a function of the incident photon energy for La_2-xSr_xCuO₄ (x = 0.22). With increasing photon energy, the resonant structures shift toward higher binding energy as shown by bars in Fig. 1. These structures suggest the Auger two-hole satellite (O KVV auger structure). In order to estimate U_pp, we have subtracted the spectrum at hn = 529.25 eV to that at hn = 526 eV and compared the subtracted spectrum with the O 2p valence-band spectrum at hn = 55 eV as shown in Fig. 2. Here, the contribution of the O 2p states was expected to be dominant in the spectrum at hn = 55 eV. Based on the Cini-Sawatzky method, we have conducted a numerical self-convolution for the O 2p one-hole spectrum at hn = 55 eV and obtained the two-hole spectrum. The energy difference between the Auger peak and the self-convoluted spectrum indicates U_pp. Thus, U_pp is estimated to be ~5.4 eV, which is in good agreement with U_pp ~ 5 eV in the result of previous Auger spectroscopy measurement (x = 0.15) [1].

In this presentation, we will show the detail of the resonant photoemission spectra and the doping dependence of the on-site Coulomb energy for La_2-xSr_xCuO₄.

References

[1] R. Bar-Deroma et al., Phys. Rev. B 45, 2361 (1992).
[2] A. Fujimori et al., Phys. Rev. B 35, 8814(R) (1987).
[3] Z.-X. Shen et al., Phys. Rev B 36, 8414 (1987).
[4] H. Das et al., Phys. Rev. B 79, 134522 (2009).
[5] C. Weber et al., Phys. Rev. B 82, 125107 (2010).
[6] A. Chainani et al., Phys. Rev. Lett. 119, 057001 (2017).
[7] A. Chainani et al., Phys. Rev. B 107, 195152 (2023).
[8] M. Cini, Solid State Communications 20, 605 (1976); 24,
681-684 (1977); Phys. Rev. B 17, 2788 (1978).
[9] G. A. Sawatzky, Phys. Rev. Lett. 39, 504 (1977).

Keywords: resonant photoemission spectra, High-T_c superconductivity, cuprates, on-site Coulomb interaction, Mott insulator

Abstract Body

High-T_c cuprate superconductors exhibit various quantum phases such as superconductivity, antiferromagnetism, pseudogap, charge order, and strange metal. The strange metal is characterized by a linear-in-temperature resistivity and the link to the high-T_c superconductivity has been expected [1]. By using angle-resolved photoemission spectroscopy (ARPES), it has been reported that the temperature-dependent quasiparticle structure in Bi-family cuprates is associated with the superfluid density which characterizes the phase coherence of the Cooper pairs [2]. Furthermore, the coherent quasiparticle peak disappears in the high-temperature region where the in-plane electrical resistivity shows the linear temperature dependence [3]. These evidences indicate the importance for investigating the temperature dependence of the quasiparticle structure. As for La_2-xSr_xCuO₄, the temperature dependence of the quasiparticle structure has been reported only by angle-integrated photoemission spectroscopy [4] where a distinct coherent quasiparticle peak is hidden due to the integration of angle information. Thus, the fate of coherent quasiparticles and its relationship to the linear-in-temperature resistivity have not been clarified in La_2-xSr_xCuO₄.

In this work, we have performed ARPES measurements to investigate the temperature dependence of the quasiparticle structure in the nodal region of La_2-xSr_xCuO₄. Figure 1 shows the temperature dependence of the energy distribution curve at the Fermi wave number in La_2-xSr_xCuO₄ (x = 0.22). One can see a distinct quasiparticle peak at T = 50 K, while the quasiparticle peak completely disappears at T = 300 K. This result agrees with the temperature dependence of the electrical resistivity, indicating the loss of the coherent quasiparticles at high temperature [5].

In this presentation, we will show the detailed result of the temperature dependence of quasiparticle structure and the doping dependence in La_2-xSr_xCuO₄.

References

[1] R. A. Cooper et al., Science 323, 603-607 (2009).
[2] D. L. Feng et al., Science 289, 277-281 (2000).
[3] A. Kaminski et al., Phys. Rev. Lett. 90, 207003 (2003).
[4] M. Hashimoto et al., Phys. Rev. B 79, 140502(R) (2009).
[5] N. E. Hussey et al., Philos. Trans. R. Soc. A 369, 1626 (2011).

Figure 1. Energy distribution curves on the Fermi wave number in La_2-xSr_xCuO₄ (x = 0.22) at T = 50 K and 300 K

Keywords: cuprate, ARPES, quasiparticle, temperature dependence, doping dependence

Abstract Body

In multilayered cuprate superconductors, calcium is a vital constituent, because it plays a key role in separating CuO₂ planes and enhancement of critical temperatures (T_c) above 100 K^1,2). We demonstrated the synthesis of calcium-free double-layered cuprates and developed multilayered cupurate, such as Sr₂SrCu₂O₄(X,O)₂ (X = F, Cl, and Br) and M(Sr,Ba)₂SrCu₂O_y (M = Hg/Re, Tl, and B/C). We revealed that the Sr metal could be employed to partition two CuO₂ planes and the oxyfluoride and mercury-based materials show a T_c of 107 K and 110 K^3,4). Moerover, we found that the length of the a-axis of developed cuprates closed to that of the infinite-layered SrCuO₂ by increasing a number of CuO₂ planes, n. It is comparable to known cuprate with Ca element. Furthermore, the superconducting properties, including critical current density was investigated through magnetic properties measutements⁵⁾. In Hg-based cuprate superconductors, a comparison of the irreversibility lines between Ca-free and representative Ca-containing Hg-based compounds highlighted that the replacement of Ca by Sr in the CuO₂ planes does not significantly impact the temperature dependence of B_irr. The development of Ca-free cuprate superconductors and their critical current properties will be reported in detail.

References

[1] E. Takayama-Muromachi, Chem. Mater. 10, 2686–2698 (1998).
[2] A. Iyo, et al., J. Phys. Soc. Jpn. 76, 094711 (2007).
[3] H. Ninomiya, et al., Commun Mater 2, 13 (2021).
[4] H. Ninomiya et al., Chem. Mater. 33, 24, 9690–9697 (2021).
[5] H. Ninomiya et al., Supercond. Sci. Technol. 36 115014 (2023).

Keywords: Ca-free cuprate, cuprate superconductor

Abstract Body

The Y-Ba-Cu-O family of high-T_c superconductors, including YBa₂Cu₃O_y (Y123), Y₂Ba₄Cu₇O_y (Y247), and YBa₂Cu₄O₈ (Y124), presents significant potential for the advancement of second-generation coated conductors. Among these, Y124 shows a stoichiometric composition and possesses a twin-free micro structure. Although T_c of Y124 is approximately 81 K, slightly lower than that of Y123, it can be substantially increased to around 90 K through Ca doping at the Y site [1]. However, in high-T_c cuprate superconductors, misorientation between grains can significantly reduce intergrain critical current density (J_c), making tri-axial grain alignment along the a, b, and c axes essential for optimizing transport properties. Our previous work has demonstrated a magnetic grain-orientation technique utilizing a modulated rotating magnetic field (MRF) [2], without using the epitaxial growth methods. This technique has proven effective for materials exhibiting tri-axial magnetic anisotropy. To advance this magneto-scientific technique towards practical applications, a comprehensive understanding of factors influencing magnetization axes and magnetic anisotropies is essential. Investigating twin-free Y124 offers valuable insights into the quantitative relationships between magnetic alignment techniques and superconducting properties. This study focuses on synthesizing and aligning (Y_1-xDy_x)Ba₂Cu₄O₈ [(Y_1-xDy_x)124], exploring the impact of Dysprosium (Dy) doping on magnetic anisotropy and grain orientation under various magnetic fields. Such investigations are crucial for enhancing our understanding of superconducting materials and advancing their potential applications in technological innovations.

Single crystals of (Y_1-xDy_x)124 with varying nominal Dy concentration levels, x=0,0.05, 0.1, 0.25, 0.50, 0.75, and 1, were grown using the flux method in ambient pressure, employing KOH as the flux medium [1]. Powder XRD measurements were carried out for synthesized (Y_1-xDy_x)124 samples. Nearly all diffraction peaks observed in the XRD patterns were recognized as reflections of the Y124 structure. This result indicates the use of the KOH flux method facilitates crystal growth in the (Y_1-xDy_x)124 phase, even in an ambient pressure. Pulverized (Y_1-xDy_x)124 microcrystals were mixed with Araldite Standard (η_init ~ 40 Pa s) epoxy resin in a weight ratio of 1:10 and aligned under MRFs of 1-10 T. The magnetization axes and degrees of orientation of the magnetically aligned (MA) powder samples of (Y_1-xDy_x)124 were determined from XRD and (017) pole figure measurements.

The XRD patterns of MA-(Y_1-xDy_x)124 revealed a notable enhancement in the (00l), (h00), and (0k0) peaks corresponding to the α, β, and γ planes under the MRF of 1-10 T, respectively. For (Y_1-xDy_x)124 (x = 0), the magnetization axes followed the order χ_c > χ_a > χ_b, and remained unchanged with change in x. Fig. 1 shows XRD patterns of α and γ planes of MA-(Y1-xDyx)124 samples under the MRF of 1T. As shown in Fig. 1, for (Y_1-xDy_x)124 (x = 0) at MRF of 1 T, alignment was achieved only along the c-axis, suggesting that the static component of the magnetic field was sufficient on its own, whereas the rotating component did not contribute adequately. This insufficiency might be attributed to either a lack of sufficient magnetic orientation energy or the necessity for an extended magnetic alignment period. Pole figure measurements at 10 T showed two-fold symmetric spots. MA-Y124 achieved strong in-plane orientation with a full width at half maximum (FWHM) of less than 4°. Even at 5 T, the sample maintained a high degree of in-plane alignment, with an FWHM under 5°, indicating that 5 T is adequate to generate the required magnetic orientation energy in Y124. Additionally, the degree of in-plane orientation increased progressively with higher x values. The changes in the in-plane orientation degree of MA-(Y_1-xDy_x)124 will be explored further in the presentation.

References

[1] Horii, S. et al., Supercond. Sci. Technol., 28, 105003 (2015).
[2] Horii, S. et al., Physica C supercond., 470, 1056 (2010).

Figure 1. XRD patterns at the (a) α and (b) γ planes for the (Y_1-xDy_x)124 powder samples aligned under MRF of 1 T

Keywords: Magnetic anisotropy, Tri-axial magnetic alignment, YBa₂Cu₄O₈