Session G20: Matter at Extreme Conditions: High-pressure Superconductivity II and Quantum Phenomena

Novel Hydride Superconductors Under High Pressure

Room-temperature superconductors have long been the ultimate goal of scientists. Superhydrides exhibit abundant crystal structures and electronic structures under high pressure. They are the forefront of physics, materials science and superconductivity. Recently, researchers have discovered binary H₃S and LaH₁₀ with T_c up to 203 K and 260 K, respectively. In this regard, these findings set a new record for superconductivity, a step closer to room-temperature superconductivity.

      We have discovered several new binary hydride superconductors such as Ce-H, Pr-H and Ba-H with atomic and molecular hydrogen configurations by using high pressure experimental techniques. Interestingly, CeH₁₀ has a superconducting transition temperature of 115 K at 95 GPa, which is the highest one of the binary hydrides below 1 Megabar. Based on binary hydrides, we have expanded the targets to the ternary system, and found the record superconducting transition temperature in La-Ce-H system below 1 Megabar (180 K at about 100 GPa). In addition, by introducing Al atoms, we have stabilized the metastable hexagonal phase LaH₁₀ to 146 GPa, with superconducting transition temperature reaching 223 K. These studies have not only yielded new types of hydride superconductors, but also deepened the understanding of high-temperature superconductivity in hydrides, laying a foundation for future experiments to obtain new high-temperature superconductors under mild conditions.   

High-Pressure Structure, Equation of State, and superconductivity of Bi0.5Sb1.5Te3: Observation of a Novel Bi-Sb-Te Alloy

Post-transition metal and metalloid chalcogenides from A₂B₃ (A = Sb, Bi, B = S, Se, Te) is a unique family of materials known to show superconductivity and electronic topological transitions at high pressure. These materials are also of interest due to their thermoelectric properties. Bi_0.5Sb­_1.5Te₃ from the A₂B₃ family is found to show superconductivity with T_c of up to ~9 K under pressure. However, its structural and dynamical properties under pressure have not been fully explored. Here, we investigate the structural and dynamics properties of Bi_0.5Sb­_1.5Te₃ with x-ray diffraction, Raman spectroscopy, and infrared spectroscopy up to 50 GPa. Structural phase transitions observed by X-ray diffraction are consistent with spectroscopic results measured under pressure. An Electronic topological transition inferred at lower pressure can be correlated with Raman measurements. Notably, above ~25 GPa the Bi_0.5Sb­_1.5Te₃ forms a body-centered cubic Bi-Sb-Te alloy with a superconducting T_c near 9 K. Other compounds within the A₂B₃ family are reported to have high-pressure phases with different identified symmetries. These discrepancies are discussed using Raman and infrared measurements.   

Pressure Induced Superconductivity; How it Relates to Structural Changes in Lithium Isotopes

Superconductors are materials characterized by zero electrical resistance and perfect diamagnetism. As the third lightest element, lithium is the simplest of all known superconductors, and as such, can provide unique insights into superconductivity. Pressure induced superconductivity in lithium was shown by several groups (Deemyad et al., Shimizu et al., Struzhkin et al. 2002) and exhibits a very complicated behavior. Although the origin of this complex superconducting phase diagram was attributed to the structural properties of lithium, the structure of lithium within its superconducting region was not measured until now.    

In this study, we attempt to better characterize the structural changes within the superconducting region of lithium isotopes through analysis of synchrotron X-Ray Diffraction (XRD) data we obtained in Argonne National Laboratory. We establish the low temperature and high pressure phase boundaries of lithium between 0 and 55GPa.

In addition, we show that the transition from fcc structure to hR1 phase coincides with sudden drop in the superconducting transition and we show the boundaries of this phase in comparison with what has been previously thought. We further show that lithium does not exhibit isotope effects in its phase boundaries in this region. In this poster I will present the results of these studies and discuss the theoretical calculations of Tc values of the FCC-hR1-CI16 phases that can be directly compared to experimental data.   

Phase Sensitive Detection Applied to Superconducting Transport Measurements

Measurements of superconducting transitions in materials under very high pressures (e.g., in diamond anvil cells) can be challenging due to the very small samples involved. Phase sensitive detection methods are used in both electrical resistance and magnetic susceptibility measurements in order to extract signatures of superconductivity (i.e., T_c) in such samples. The background signals in such measurements are often not well understood, leading to controversies in the literature.  We present a method of extracting evidence for both zero-resistivity and the Meissner effect in a single electrical measurement for small samples within diamond-anvil cells, in particular, through the observation of a transient inductance at T_c . A framework for differentiating a superconducting transition from instrumental or environmental artifacts is developed and data from superconducting samples that have been well characterized using other techniques along with data for proposed near-ambient superconductivity in the Lu-N-H system.   

Pressure Driven Competition between Superconductivity and Charge Density Wave States in BaSbTe2.1S0.9

BaSbTe2.1S0.9 (BSTS) is a newly discovered 2D layered material which exhibits charge density wave (CDW) order under ambient conditions. CDW is a correlated electronic phenomenon in a periodic modulation of charge density of a material. Both CDW and superconductivity are correlated states and often observed in the materials under different conditions. Currently, however, the correlation between the two states is not well understood. Here we present the pressure studies of the magnetic resistivity and XRD analysis of BSTS up to 30 GPa under quasi-hydrostaitc conditions where we show that CDW is suppressed under high pressure and leads to the emergence of superconductivity at around 10 Gpa. Our results support that CDW and superconductivity are competitive electronic states in BSTS.   

Electrical Resistance and Magnetic Susceptibility Evidence for Near Ambient Superconductivity in Nitrogen-Doped Lutetium Hydride

The discovery of superconductivity in nitrogen-doped lutetium hydride at near-ambient conditions has attracted significant attention¹ but also controversy as many experimental groups report no superconductivity at these conditions in Lu-N-H samples they have synthesized (e.g., Refs. ^2-5). We present the electrical resistance⁶ and magnetic susceptibility measurements on nitrogen-doped lutetium hydride samples that are in very good agreement with both previously reported Tc and its pressure dependence for nitrogen-doped lutetium hydride. We also find that the observation of these very high Tc is highly variable and that some samples, including those that have degraded over time, show no evidence for superconductivity   

Structural Phase Transitions in KC8 under High Pressure

KC8 displays intriguing phase transitions under varying pressures, making it an ideal candidate for investigation for possible a novel superconductor under pressure with hydrogen. Our findings provide a robust foundation for a broader exploration of carbon-rich compounds featuring sp3 networks. At ambient conditions, KC8 displays a 2x2 phase. As pressure increases from 2.3 GPa to 21 GPa, phase transitions occur along the a and b axes. At 2.3 GPa, a transition to a square root 3 x square root 3 phase is observed, in line with previous data. At 3.0 GPa, the 2x2 phase disappears, while the square root 3 phase persists until 6 GPa. Between 6 GPa and 21 GPa, new phases emerge, but their identification is pending. At 21 GPa, we observe evidence of changes not only in the a and b axes but also in the c axis. Importantly, the compressibility of the c axis, from ambient conditions up to the point just before these transitions, aligns with previously reported data. We have utilized Raman and IR spectroscopy to investigate the incorporation of hydrogen into KC8.   

Pressure-dependence of the Colossal Magnetoresistance of Europium Cadmium Phosphate (EuCd2P2)

Europium Cadmium Phosphate (EuCd₂P₂) is a newly discovered strongly correlated electron system that exhibits colossal magnetoresistance (CMR) at ambient pressure. Colossal Magnetoresistance is an important material property that can have pleothra of applications in  technology. It is generally assumed that CMR is a property intrinsic to manganese oxides with mixed valence and a cubic perovskite structure. However, the strong CMR  seen in EuCd2P2 without manganese, oxygen or perovskite structures underscores a need for a more comprehensive theory of this phenomenon.

Pressure is an effective tool to tune the interactions in materials and allow testing the theoretical hypothesis. In the case of EuCd₂P₂, it is known that the observed peak in the temperature dependence of the electrical resistivity occurs right above its neel temperature where it transitions from a paramagnetic material to an A-type anti–ferromagnetic (AFM) material. Ambient pressure studies suggest that CMR in EuCd₂P₂ arises from spin fluctuations. By comparing the AC magnetic susceptibility and the electrical resistance of EuCd₂P₂ at the temperatures where the CMR is most prominent, direct evidence of this theory can be obtained. The application of pressure brings the atomic planes closer and their interactions directly contribute to the magnetic state of the EuCd₂P₂.

In this study, we have investigated the AC magnetic susceptibility of EuCd₂P₂ as a function of pressure and temperature in a Diamond Anvil Cell (DAC) device. Measuring magnetic susceptibility provides valuable insights to the magnetic behavior and electron correlation properties of a material.Diamond Anvil Cell (DAC) allows achieving the highest static pressure conditions and is compatible with operation at low temperatures. However, due to very small sample size in a typical DAC experiment magnetic susceptibility experiments are extremely challenging. In this study, we present the method of construction of the experimental setup for such measurements, the difficulties associated with obtaining an acceptable signal-to-background ratio, techniques of calibration and data analysis. Future work will include the findings that will help enhance our current understanding of CMR and the unique properties associated with EuCd₂P₂.   

Exploring Novel Material Properties of Colossal Magnetoresistant Material EuCd2P2 through Transport Measurements at Extreme Conditions

Exotic compounds that display novel quantum phenomena are the foundations of technological innovation. EuCd­­₂P₂ is a newly discovered strongly correlated electron system which displays colossal magnetoresistance (CMR) as an as-grown crystal despite failing to satisfy the accepted paradigm of archetypal CMR materials (i.e. La_0.75Ca_0.25MnO₃,  La_0.67Ca_0.33MnO₃ and other mixed-valence perovskite manganites). Colossal Magnetoresistance is the tendency of a material to experience a dramatic change in electrical resistance as a response to the presence of a magnetic field. This quantum phenomena lends itself to a robust increase in hard disk drive data density.

Because this sample doesn’t adhere to existing CMR theoretical modeling (namely the once standard double-exchange model) high pressure experimentation can be used to investigate the origin of CMR. Ambient pressure studies suggest the spin fluctuations above the material transition into A-type antiferromagnetism (AFM) as attributional phenomena for the CMR mechanism. In this explorative study not only have we sought to establish a better paradigm for CMR, but we have endeavored to discover new potential quantum ground states such as superconductivity in the crystal. To achieve these ends, we take electrical transport measurements of the material in a temperature range of 4-300 K and pressure range of 0-33 GPa through the use of standard high pressure devices and processes such as diamond anvil cells (DACs) and 4-probe electrical resistivity systems.

In this study, to complement supplementary x-ray diffraction (XRD) measurements which fail to associate the sample’s CMR behavior with structural phase transitions, we have taken electrical resistivity measurements which appear to suggest the phenomena’s electrical dependency.   

A simple and accurate extension to the quasiharmonic approximation for the calculation of thermodynamic properties of solids.

The quasiharmonic approximation (QHA) is the main computational method for the prediction of structural and thermodynamic properties of periodic solids, including their phase stability, at arbitrary temperatures and pressures. In a QHA calculation, the static energy and the harmonic phonon density of states are calculated on a volume grid spanning the region of interest. QHA has had great success in the prediction of material properties which are difficult to measure or inaccessible to experiment. The most important drawback of QHA is that, at temperatures higher than a material- and pressure-dependent validity limit, there is a complete breakdown of the theory with spurious thermodynamic predictions across the board. This is caused by the incorrect description of anharmonicity by QHA via the volume dependence of the harmonic vibrational frequencies. In the past, computationally intensive approaches such as ab initio molecular dynamics (AIMD) have been used to calculate thermodynamic properties beyond the QHA limit. To circumvent the use of complex methods like AIMD, we present in this work a simple and efficient way of extending the validity range of QHA. Our method is based on the combination of three ingredients: i) the calculation of force constants to nth order and the subsequent calculation of effective temperature-dependent second-order force constants using the hiphive method (Eriksson et al., Adv. Theory Simul. 2 (2019) 1800184), ii) Allen's quasiparticle theory (Phys. Rev. B 92 (2015) 064106) that posits the calculation of the anharmonic entropy from temperature-dependent frequencies, and iii) a simple fitting method based on the Debye model to obtain the system's free energy as well as the rest of the thermodynamic properties. The proposed method is conceptually and computationally very simple, with a cost similar to that of plain QHA, and reproduces the QHA results below its validity limit by construction. Therefore, our method retains the accuracy of QHA at low temperature while providing a physically grounded and accurate extension to QHA in the high-temperature limit. The implementation of our new method in the gibbs2 program is described, as well as illustrative examples.   

A model-free uncertainty-aware equation of state for gold

This study introduces an advanced Uncertainty-Aware Equation of State (UEOS) toolkit rooted in the error-in-variables Gaussian process. Our approach deviates from traditional methods by generating a gold EOS table (U790) directly from quantum simulation data, bypassing the need for fitting into conventional EOS models like the Mie-Grüneisen equation. The toolkit meticulously accounts for systematic and model-derived uncertainties originating from quantum simulations and experimental data. These uncertainties are seamlessly integrated using Gaussian operations, including additions and derivatives, to obtain the final free energies, specifically the Gibbs free energy.

A critical validation of our methodology involved a comparative analysis between our new EOS table and the existing LEOS 790 table. Remarkably, the disparity between these tables falls well within the 5% margin for the temperature-density grid considered in this study, affirming the robustness of our results.  Furthermore, we have expanded this analysis into the challenging domain of plasma data, including the recent Hugoniot measurements.   

Gravitational Wave Signatures of Particle Physics Models with Extended Gauge Symmetries

The Standard Model of elementary particles describes remarkably well the physics at the most fundamental scale, but it leaves several questions unanswered: What is the nature of dark matter? What is the origin of the matter-antimatter asymmetry of the Universe? How do neutrinos become massive? Finding answers to those questions requires an understanding of what happened when high energy physics effects were prevalent in the Universe, i.e., a small fraction of a second after the Big Bang. Diving this far back into the Universe's history has become possible thanks to the recent first detection of gravitational waves. A primordial stochastic gravitational wave background, although not yet discovered, is expected to carry information precisely about the very early stages of the evolution of the Universe. Those gravitational waves could have been generated through a variety of processes, including first order phase transitions, cosmic strings, and domain walls. Their expected spectrum is within the reach of upcoming gravitational wave experiments, such as LISA, DECIGO, Big Bang Observer, Cosmic Explorer and Einstein Telescope. I will discuss how to use such signals to probe particle physics theories with extended gauge symmetries broken at high energy scales, otherwise completely inaccessible in conventional particle physics experiments.