Session A16: Emerging Superconductors: Field, Strain, and Pressure Effects

High pressure equation of state and phase behavior of YbHx nanoparticles

Hydride compounds are the subject of much investigation due to their relatively high superconducting temperatures, and nanoscale forms of materials are known to have different phase expression relative to their bulk counterparts. Therefore, we investigate whether a superhydride can be formed in ytterbium hydride nanoparticles.  Ytterbium hydride nanoparticles were placed in diamond anvil cells to apply pressure during x-ray diffraction measurements to determine equation of state and crystal structure.  In some cases, nanoparticle samples were placed alongside ammonia borane as a hydrogen source.  The presence of ammonia borane appears to partially convert β-phase (Fmm) YbHx nanoparticles into α-phase (Pnma).  Nanoparticles in the β phase appear to become amorphous at approximately 29 GPa, and α-phase nanoparticles may absorb hydrogen from ammonia borane when it is present at high pressure.  Our results are compared with previously published data on bulk ytterbium hydride.   

Partial levitation, type-II-superconductor characteristic, at room temperature and atmospheric pressure in PCPOSOS

We synthesized materials, Pb_10-xCu_x(P(O_1-yS_y)₄)₆O_1-zS_z (PCPOSOS), called PCPOSOS, which exhibit superconducting behavior at room temperature and atmospheric pressure. These materials displayed characteristics of a superconductor, including zero resistance, the Meissner effect, and partial levitation when placed on a magnet (arXiv: 2307.12037). The partial levitation is caused by an inhomogeneity in the magnetic field of the magnet and occurred within the range of critical magnetic fields, H_c1 and H_c2. That is, the magnetic field of the magnet increased with going from center to the edge of magnet. The magnet had approximately 2000G at the center and approximately 3,000G at its edge. The levitation occurred near center. This indicates the center of the magnet is close to H_c1. It disappeared between center and the edge near H_c2, with H_c1 being much smaller than H_c2, because the magnetic moment at H_c2 is much smaller than that at H_c1. When the magnet is slightly moved, the levitation returns to its original position. This phenomenon is analyzed as flux pinning, which is typical of a type-II superconductor. Moreover, the quantum-locking phenomenon, characteristic of a Type-I superconductor, may appear. However, we interpret PCPOSOS as a Type-II superconductor. We will show two videos of levitations and two videos of magnets.   

Pressure-induced superconductivity in bi-layer nickelate La3Ni2O7+δ

 The recent report of pressure-induced superconductivity in the La₃Ni₂O₇ has garnered considerable attention, where the superconducting transition temperature (T_c) is reported to be as high as 80 K [1] in perovskite Ruddlesden-Popper crystal structure [2]. To explore oxygen content dependence of superconducting properties of La₃Ni₂O₇, we synthesized La₃Ni₂O_7+δ  samples via different routes and performed electrical resistivity measurements under high pressure.

 Parent single-phase La₃Ni₂O₇ were synthesized by solid state reaction and then annealed in air or in high-pressure oxygen. The oxygen contents of samples were evaluated by thermogravimetry. Electrical resistance measurements under high pressure were performed using PPMS and Diamond anvil cell with boron-doped diamond electrodes [3]. Cubic boron nitride was used as a pressure medium.

 In the sample of annealed in high pressure oxygen, the electrical resistance decreased over the whole temperature range with increasing pressure, and superconducting transition appears at 26.1 GPa with T_c = 78 K. Compared to the previous work [1], appearance of superconductivity is about 10 GPa higher, and the decrease in T_c with increasing pressure is more pronounced. This result suggests that superconducting properties of La₃Ni₂O₇ may depend on oxygen content. In the presentation, we will also report the results of resistivity measurements on samples that were annealed in air.   

Pressure evolution of crystal structure, magnetism, and electrical resistivity in bilayer and trilayer nickelates

The recent discovery of superconductivity in bilayer Ruddlesden-Popper nickelate La₃Ni₂O₇, with T_C up to 80 K at pressure above 14 GPa, has generated significant interest in exploring high temperature superconductivity in nickelates. However, considering the structure ambiguity of high pressure phase and the crucial role of the structure-property relationship in understanding the mechanism behind high T_C superconductivity, in this study, we report the high pressure synthesis of bilayer and trilayer Ruddlesden-Popper nickelates, La₃Ni₂O₇ and La₄Ni₃O₁₀, at 20 GPa and 1400 °C. Single crystal X-ray diffraction results reveal lattice compression with subtle structure variations compared to ambient pressure. The evolution of this structural feature under pressure is further explored through in situ high pressure laboratory X-ray single crystal diffraction. We will present our investigation, including the magnetization and electrical resistivity measurements.    

Replication and study of anomalies in LK-99, the alleged ambient-pressure, room-temperature superconductor

The unique simultaneous appearance of zero resistivity and magnetic field expulsion enable superconductors to have tremendous potential for various applications, such as efficient electric power transmission, much smaller or more powerful magnets, magnetic levitation, high-speed computing, etc. The superconducting critical transition temperature  has continuously been enhanced owing to the century-old effort at superconductivity. It appears that all record high T_cs since 1994 have been achieved in compounds under very high pressure. The ultimate goal in the superconductivity field is to find a way to retain a coherent quantum state under ambient conditions, i.e., room temperature of ~ 300 K and atmospheric pressure. We have studied LK-99 [Pb_10-x Cu_x(PO₄)₆O], alleged by Lee et al. to exhibit superconductivity at room temperature and ambient pressure, and have reproduced all the anomalies except for half-levitation they reported as evidence for the claim of LK-99 being an ambient-pressure, room-temperature superconductor. We found that these anomalies are associated with the structural transition of the Cu₂S impurity in their sample and not with superconductivity.   

Strain effects and structural correlation in paraelectric tellurides

PbTe and related tellurides have long been known for their outstanding thermoelectric properties, with a recent resurgence of interest in their complex local structure, quantum paraelectricity, and superconductivity. Despite its simple rocksalt structure, neutron and X-ray diffuse scattering data for PbTe indicate the existence of a Pb off-centering and the appearance of fluctuating local dipoles [1]. This surprising local reduction of symmetry is still not completely understood, and it opens the prospect of relating the peculiar local structure to complex functional properties. Furthermore, alloyed tellurides exhibit superconductivity at low charge-carrier densities. The soft polar modes and the low-density superconductivity are characteristics analogous to the quantum paraelectric SrTiO₃. In addition, the electronic subsystem in quantum paraelectrics is strongly affected by lattice strains; recent work indicates dramatic effects of plastic deformation in SrTiO₃, including evidence for significantly enhanced superconductivity and ferroelectricity [2]. Here, we combine diffuse X-ray scattering, charge transport, and NMR measurements to probe the effects of elastic/plastic strain on structural correlations, quantum paraelectricity, and superconductivity in tellurides.

[1] K.A. Holm et al., Phys. Rev. B, 102, 024112 (2020).

[2] S. Hameed, D. Pelc et al., Nat. Mat. 21, 54 (2022)   

Analysis on unconventional superconductivity in La3Ni2O7 and La4Ni3O10 under high pressure: theory vs. experiment

Recent discovery of high Tc superconductivity in a bilayer nickelate La3Ni2O7 has sparked a vast wave of new interest [1]. In fact, this very material was previously proposed as a good starting point for seeking high Tc superconductivity by one of the present authors [2].  Discussed there was the possibility of realizing a nearly half-filled bilayer Hubbard model with large interlayer hopping utilizing the d3z2-r2 orbitals. In the present study, we construct a four-orbital (d3z2-r2 +dx2-y2, x 2) model of La3Ni2O7 under high pressure through DFT calculations and Wannierization, and apply FLEX to study   superconductivity [3]. We find that the Tc can be comparable to those of the high Tc  cuprates, and the effects of the dx2-y2 orbitals are limited, so that the experimental Tc ~ 80K can be understood as a consequence of realizing the bilayer Hubbard model comprising the d3z2-r2 orbitals. We next study the possibility of superconductivity in a trilayer nickelate La4Ni3O10 [4]. We perform structural optimization calculations at high pressures, which results in tetragonal symmetry of the lattice. Using the obtained structure, the possibility of superconductivity is studied using FLEX, which shows that superconductivity with Tc comparable to that of relatively low Tc cuprates is possible. These results are compared with experiments.

[1] H. Sun et al., Nature 621, 493 (2023). [2] M. Nakata et al., Phys. Rev. B 95, 214509 (2017). [3] H. Sakakibara et al., arXiv: 2306.06039.  [4] H. Sakakibara et al., arXiv: 2309.09462.    

Possibility of Superconductivity in a Trilayer Ruddlesden-Popper Nickelate La4N3O10-δ Under High Pressure

 Recently, La₃Ni₂O₇ was reported to show superconductivity at 80 K under pressure of 15 GPa [1]. La₃Ni₂O₇ is a bilayer Ruddlesden-Popper phase nickelate that have two layers of NiO₂ plane in its unit cell, while there is trilayer family compound: La₄Ni₃O₁₀. As there had been no paper yet that reports superconductivity in La₄Ni₃O₁₀, we examined the possibility of superconductivity in La₄Ni₃O₁₀ under high pressure [2].

 The polycrystalline sample was synthesized via solid-phase reaction and hot isostatic pressing process from La₂O₃ and NiO, and characterized by powder X-ray diffraction and thermogravimetry to find it to be single phase La₄Ni₃O_9.99. We used diamond anvil cell with boron-doped diamond electrodes designed for four-terminal resistance measurement under high pressure [3]. Cubic boron nitride powder was used as a pressure-transmitting medium.

 The synthesized La₄Ni₃O_9.99 displayed metallic behavior around room temperature in all measured pressures. As we applied the pressure, a drop in resistance suddenly appeared below 5 K at 32.8 GPa. The temperature where the drop appears tended to increase by increasing the pressure beyond 46.2 GPa. Measurements of electrical resistance under magnetic fields were also conducted at 69.4 GPa, where the drop in electrical resistance was suppressed by magnetic field. Therefore, the observed decrease in electrical resistance is most likely to be the result of a superconducting transition in La₄Ni₃O_9.99.   

Superconductivity induced by hole-doping in the nodal-line semimetal NaAlGe

 Recently, correlated Dirac electron system have attracted much attention in the material research field. It is predicted that correlation effects in the nodal line bands will lead to the various ground states such as antiferromagnetic, exciton, and superconducting phases, but there are few candidate materials [1,2]. In this context, we focus on the nodal line semimetal NaAlGe which has a nodal line band near Fermi energy and an anomalously large Wilson ratio of 10. Moreover, NaAlGe exhibits semiconductive electrical conductivity at low temperatures, indicating the formation of a pseudogap at approximately100 K; the origin of the pseudogap in NaAlGe may be associated with excitonic instability [3].

In this work, we investigated hole-doping effects on the ground state of the solid solution Na(Al_1–xZn_x)Ge by electrical resistivity, magnetic susceptibility, and heat capacity measurements. It found that the pseudogap is suppressed continuously with increasing Zn content, followed by the appearance of a superconducting dome with the highest transition temperature of 2.8 K. This superconductivity most likely results from excitonic fluctuations.   

Probing the superconductivity of kagome metal CsV3Sb5 with self-field critical current

The kagome metal CsV₃Sb₅ features an interesting interplay between the charge-density-wave (CDW) order and superconductivity. The CDW phase is complex and potentially breaks the time-reversal symmetry, which can result in an exotic superconducting state. Here, we use the self-field critical current (I_c,sf) as a proxy to investigate the superconducting gap [1]. We show that our I_c,sf data can be understood when considering a single s-wave gap. Furthermore, the superconducting critical temperature is insensitive to disorder, leading us to conclude that CsV₃Sb₅ is a conventional s-wave superconductor. Finally, we measure I_c,sf under pressure using a device-integrated diamond anvil cell [2,3]. In particular, we investigate the superconducting state at pressures where the CDW phase can be removed. Our results show that s-wave gap symmetry is retained irrespective of the presence of the CDW order.

[1] Zhang et al., Nano Lett. 23, 872 (2023)

[2] Xie et al., Nano Lett. 21, 9310 (2021)

[3] Ku et al., Rev. Sci. Instrum. 93, 083912 (2022)   

Structural routes to stabilize superconducting La3Ni2O7 at ambient pressure

The recent discovery of an 80~K superconducting state stabilized under pressure in La₃Ni₂O₇ [1] has provided an exciting new opportunity to study and develop high-temperature unconventional superconductors beyond the cuprates. One of the most striking observations of this material is that superconductivity is linked to a subtle change in the space group of the material that occurs under pressure. This raises important questions about how the structure and superconducting state are interconnected and whether it is possible to stabilize the superconducting state at ambient pressure.

 

Using Density functional theory-based structural relaxations, we have explored how the crystal structure of La₃Ni₂O₇ responds to external tuning parameters, such as uniaxial and biaxial strain [2]. Our study reveals two key results. Firstly, the structural transition is captured entirely within the mean-field framework of DFT, without the need for any correlated parameter such as a Hubbard U. And secondly, the structural transition observed under pressure is mediated almost entirely by a reduction of the b-axis lattice constant, which suggests that uniaxial compression along the [010] direction or in-plane biaxial compression are also sufficient as tuning parameters to control this structural transition.

Furthermore, we show that increasing the size of the A-site cations can also induce the structural transition via chemical pressure, and identify Ac₃Ni₂O₇ and Ba₃Ni₂O₇ as potential candidates for a high-temperature superconducting nickelate at ambient pressure.

 

[1] Sun et al. Nature, 621, 493-498 (2023)

[2] LCR and PW, arXiv:2309.15745 (2023)   

Bandgap and Raman Signature of Undoped and Cu-doped Lead Phosphate Apatite Pb10(PO4)6O

Recent claims of a room-temperature ambient-pressure superconductor by Korean scientists have ushered in a worldwide race to replicate the superconductivity of a new compound Pb_10-xCu_x(PO₄)₆O called LK-99. In the meantime, many theory papers have appeared to calculate the band structure of LK-99 and explore its potential superconductivity. Despite these efforts, the basic properties of LK-99 and its parent compound Pb₁₀(PO₄)₆O are still missing. Here we first synthesized Pb₁₀(PO₄)₆O, and obtained its Raman spectrum and electronic bandgap using UV-Vis spectroscopy, confirming that Pb₁₀(PO₄)₆O is a wide bandgap insulator. We then synthesized two types of Cu-doped Pb₁₀(PO₄)₆O and used Raman and UV-Vis to investigate the effect of doping. Raman spectra reveal that LK-99 synthesized using the original Korean recipe is highly inhomogeneous, while Cu-doped Pb₁₀(PO₄)₆O with our own recipe has a relatively pure single phase. Despite the evidence of incorporation of Cu in Pb₁₀(PO₄)₆O from EDX mapping, no Raman shift due to doping is observed in both samples. On the other hand, both Cu-doped samples exhibit slightly lower bandgaps than Pb₁₀(PO₄)₆O, in agreement with reported calculations. Proper doping of Cu is responsible for the possible transition of Pb₁₀(PO₄)₆O from insulating to metallic and even superconducting, Raman signature and bandgap of Pb₁₀(PO₄)₆O pave the way for further investigation and replication of LK-99.   

New pressure induced superconductor Sn3S4 using original diamond anvil cell

We have successfully discovered new superconductor Sn3S4 by transport measurements under high pressure using the newly developed original diamond anvil cell (DAC) with electrode and heater made of boron doped diamond thin films. Superconductivity was theoretically predicted and the expected superconducting transition temperatures are varying from 3.3-4.7@10GPa to 31.9K@30GPa depending on the theory [1,2]. The sample Sn3S4 is stable only under high pressure. When the sample placed in the ambient condition, sample was quickly decomposed. Due to this reason, it is necessary to measure the sample without release of the pressure. Then we have developed original diamond cell with 4 electrodes for resistivity measurements and heater electric circuit. Using this DAC, we can heat the sample and measure the resistivity simultaneously. The starting material is mixture of 2SnS and SnS2 and squeezed in DAC up to 30GPa, and heated by electric current through heater circuit. Obtained sample shows superconductivity and Tc gradually increases with decreasing pressure. The maximum Tc is around 13K at 5.6GPa [3]. Below 5.6GPa the sample decomposes and superconductivity also disappears. Our newly developed DAC can simultaneously perform high-pressure synthesis and resistivity measurements, and it is indispensable tool for exploration of pressure induced superconductors such as metal hydride high-Tc materials.   

Characterization and Strain Measurements of the Half-Heusler Superconductors YPtBi and LaPtBi

Materials with a half-Heusler structure have been investigated for decades, primarily due to their interesting magnetic properties. This class of materials has recently seen a surge of interest due to their non-trivial band structures and the possibility of exotic high-spin superconductivity.  YPtBi [1] and LaPtBi [2] are known to exhibit superconducting transitions below 1 K, and in the case of YPtBi, there is evidence for line nodes in the superconducting gap. Nonlinear magnetic response measurements are particularly suited for probing emergent superconductivity [3], and the application uniaxial strain has been shown to enhance the transition temperature of some materials [4]. We have grown single crystals of both compounds out of bismuth flux under vacuum encapsulation and confirmed the single-phase nature of our samples via X-ray diffraction. Here we report initial charge transport and nonlinear magnetic response measurements of electronic properties, as well as the effect of strain on the superconducting transition.

References:

[1] N. P. Butch et al. Phys. Rev. B 84, 220504(R) (2011)

[2] G. Goll et al. Physica B 403, 1065 (2008)

[3] D. Pelc et al. Nat. Comm. 10, 2729 (2019)

[4] S. Hameed et al. Nat. Mat. 21, 54 (2022)