Session K20: Matter at Extreme Conditions: Static Superconductivity Experiments

Probing superconductivity in superhydride materials using nanoscale quantum sensors

Tuning pressure provides an interesting playground for the exploration of novel condensed phases. Recent work probing superconductivity in hydride materials at megabar (~ 100 GPa) pressures in diamond anvil cells represents one facet of this rich landscape. Transport studies of super hydrides have consistently demonstrated a sharp drop in sample resistance at high temperatures. However, constraints on sample size, enormous pressure gradients, and complicated sample geometries are testing the limits of conventional high pressure probes of magnetism. To this end, we introduce a novel platform for magnetometry at megabar pressures with high sensitivity and diffraction-limited sub-micron spatial resolution using Nitrogen Vacancy (NV) color centers incorporated directly into the diamond culet. Through simultaneous transport and magnetic measurements, we probe the dual hallmarks of superconductivity, including the first spatially resolved measurements of the Meissner effect, in a high pressure hydride system.   

Observation of Room Temperature Superconductivity in Hydride at Near Ambient Pressure

Superconductivity, one of most profound phenomenon in nature. However, this elusive quantum state has yet to revolutionize the world due to the low temperatures required. Consequently, ambient conditions superconductivity has become one of the most sought after goals of science since Kamerlingh Onnes’ first observation of superconductivity in elemental mercury at 4.2 kelvin over a century ago. Over the last decade, high pressure compression have dominated the search for high temperature superconductivity. Leading the way has been the “chemical precompression” of hydrogen dominant alloys demonstrating critical superconducting transition temperatures (T_c) approaching the freezing point of water in the rare earth hydrides LaH₁₀ and YH₉ at megabar pressures. Our discovery of room temperature superconductivity in a carbonaceous sulfur hydride highlighted that ternary or greater systems are likely the key to higher T_c’s and ambient conditions superconductivity. Here, we report a recent devolopmemts of new materials that exhibits superconductivity at near ambient conditions. These compounds were synthesized under high pressure-temperature conditions, and then after full recoverability its materials and superconducting properties are examined along compression pathways. With these material, the dawn of ambient superconductivity and applied technologies has arrived with a direct path now open for tailoring extreme science hydrides to “materials by design”. 

    

Investigating the Crystal Structure of Nitrogen-Doped Lutetium Hydride at High Pressure

Hydrogen rich high temperature superconductors discovered recently exhibit conventional superconductivity mediated by strong electron-phonon coupling, under high pressures. These metal hydride compounds have long been investigated in an effort to increase T_c via chemical doping. Currently, there are many efforts to dope metal hydride systems with lighter elements in order to find stable or metastable phases at lower pressures. The intrinsic link between atomic configuration and physical properties demands the most accurate determination of the structure of these materials. Here we investigate the crystal structure of Nitrogen-Doped Lutetium Hydride system. We present high pressure X-ray diffraction, Raman spectroscopy and neutron diffraction measurements and explore the pressure dependence of the structure of Nitrogen-Doped Lutetium Hydride.

    

High-Pressure Magnetic Susceptibility Studies on Carbonaceous Sulfur Hydride

The most significant discovery in reaching high-temperature superconductivity is the pressure-driven disproportionation of hydrogen sulfide (H₂S) to H₃S. As H₂S readily mixes with hydrogen to form guest-host structures at lower pressures, we consider the comparable size of methane (CH₄) to H₂S should allow molecular exchange within a large assemblage of van der Waals solids that are (highly) hydrogen-rich with H₂ inclusions that are then the building blocks for novel superconducting compounds at extreme conditions. A superior test for superconductivity is the search for a strong diamagnetic transition in the magnetic susceptibility. Here, we report superconductivity in a photochemically transformed carbonaceous sulfur hydride system using a novel double-frequency modulation method with a transition temperature as high as 262 K. Double-frequency method displayed significant improvements over the standard susceptibility technique, allowing the detection of the superconducting transition for very small samples such as Carbonaceous Sulfur Hydride

    

Synthesis and superconducting properties of praseodymium superhydride

Recently, many hydrogen-rich superhydrides with interesting superconducting properties have been synthesized at high-pressure temperature conditions. Still, the experimental synthesis of pure, stoichiometric superhydrides at relatively lower pressures remains largely unexplored. We are interested in synthesizing pure, stoichiometric hydrogen-rich materials at the lowest possible pressure. Pure hydrogen (H₂), ammonia borane (NH₃BH₃), and paraffin (C_nH_2n+2) are being used as hydrogen sources for the synthesis of hydrides. However, it is unclear how additional elements such as B, N, and C present in ammonia borane and paraffin affect the structure and superconducting properties of hydrides. Here, we present the synthesis of various Pr-H phases carried out using pure hydrogen and elemental praseodymium. We will also present the superconducting properties calculated for the synthesized Pr-H phases. The structural properties of the superhydride phase will be compared with the literature report where the phase was synthesized using ammonia borane. This study will help to understand the effect of various hydrogen sources on the structural parameters of superconducting hydrides.

    

Pressure-induced phase transitions in high entropy oxide and metal hydrides in DAC at over 100 GPa conditions

In this presentation, several selected crystalline systems were in situ studied under high pressure using synchrotron x-ray diffraction (XRD) technique in diamond anvil cell (DAC). Research topics involve phase transitions, isostructural transition, pressure tune charge transfer and mixed valance modifications behaviors, and negative linear compression at over 100 GPa conditions. One high entropy oxide system was used to investigate the compositions dependence of the phase transition sequence upon compression, and the fast compression rate effect was also studied. For many metal hydrides with the predicted superconducting properties, the responding superconducting phases determination are still quite challenging. Several selected cases such as Ca-H, Hf-H, would be presented based on the recent XRD results and discussed the possible general trend.   

Optical investigations of pressure-induced phases in type-II Weyl semi-metal WTe2

WTe₂ crystalizes in the T_d structure at an ambient pressure, and undergoes a structural phase transition to the 1T’ structure under the application of pressure. We investigated pressure-induced variation of structural and electronic phases by optical methods. Whereas we observed several clear signatures of pressure-induced phase transitions consistently with previous reports, we could characterize the intermediate state existing between two structural phases of T_d and 1T’, which would be related to the pressured-induced superconductivity observed in WTe₂.   

Chemically Tunable High Pressure Phase Transitions in Layered Si2Te3

 

Silicon telluride (Si₂Te₃) is a transparent-red, two-dimensional (2D) layered semiconductor that is of interest for its compatibility with silicon and for its unusual semiconducting opto-electronic and structural properties. Si2Te3 platelets were compressed to 12 GPa in a diamond anvil cell and measured with x-ray diffraction, Brillouin scattering, and Raman scattering. We show that Si₂Te₃ undergoes a semiconductor-to-metal electronic phase transition under pressure at 9.5 +/- 0.5 GPa coincident with a structural transition occurring at 8 +/- 0.5 GPa.  Further, since silicon telluride is layered, we demonstrate a novel strategy to chemically tune the high pressure phase transition (both increasing and decreasing) through intercalation of zero-valent metals (Cu, Mn, Ge).  This work demonstrates chemically tuning of thermodynamic behavior and addresses the high pressure phase behavior of silicon telluride. 

    

Evidence of ferromagnetism collapse and valence instability in EuB6 at high pressures

Rare earth hexaborides host several complex physical systems ranging from low temperature super conductors up to Kondo systems with proposed non-trivial topology. Among them all EuB₆ is singled out for ordering in a ferromagnetic manner with well reported puzzling properties such as a two-step magnetic transition below 15 K ^[1], large negative magnetoresistance^[2] and magnetic polarons up to 40 K ^[3]. Most recently it was shown that EuB₆ may host Weyl points nearby its Fermi level rendering new possibilities for non-trivial electronic behavior with proposed links to magnetization properties^[4]. In this work we structurally compressed EuB₆ crystals utilizing hydrostatic pressures up to 30 GPa and probed magnetic and electronic properties utilizing X-ray spectroscopy techniques such as X-ray Absorption Near Edge Spectroscopy (XANES), X-ray magnetic Circular Dichroism (XMCD) and  Synchrotron Mossbauer Spesctroscopy (SMS). Our findings show evidences for ferromagnetic collapse above 20 GPa with accompanying mean valency increase within applied pressure range and no sign for structural phase transitions shown by X-ray diffraction. This opens possibilities for fine tuning topological properties utilizing external adjusting parameters in this compound.

¹  S. Sullow, I. Prasad, M. C. Aronson J. L. Sarrao, Z. Fisk D. Hristova, A. H. Lacerda, M. F. Hundley A. Vigliante and D. Gibbs PHYSICAL REVIEW B VOLUME 57, NUMBER 10 (1998).

² Manna, Rudra Sekhar, et al. "Lattice strain accompanying the colossal magnetoresistance effect in EuB 6." Physical review letters 113.6 (2014): 067202.

³ Merlin Pohlit, Sahana Rößler, Yuzo Ohno, Hideo Ohno, Stephan von Molnár, Zachary Fisk, Jens Müller, and Steffen Wirth. PHYSICAL REVIEW LETTERS 120, 257201 (2018)

⁴ Yuan, Jian, et al. "Magnetization tunable Weyl states in EuB6." Physical Review B 106.5 (2022): 054411.   

Author not Attending

Effects of pressure on electronic resistance and the critical magnetic field for Ti₅₃Zr₂₇Ni₂₀ quasicrystals were investigated. Superconducting transition temperature, Tc, increased from 1.99 K to 6.47 K with increasing pressure from 0.78 GPa, and 75.63 GPa, respectively. With increasing the magnetic field, the superconductivity was gradually suppressed. The GL fit revealed that the maximum upper critical value was 9.62 T at 75.63 GPa. The synchrotron-based X-ray diffraction measurement results revealed that Ti₅₃Zr₂₇Ni₂₀ quasicrystals maintained the icosahedral structure. Our combined results of structure and transport measurements suggest that increasing the pressure will increase the Tc in Ti₅₃Zr₂₇Ni₂₀ quasicrystals. The results will be compared with the ones for the hydride systems.

    

Strain Induced Superconductivity in Semiconductors

Non-hydrostatic stresses such as those under uniaxial or shear strains have been increasingly introduced in high-pressure study and strain engineering to extend the phase space and enrich the accessible structures and properties of matter. Our recent theoretical studies revealed that diamond, which is insulating in ambient or hydrostatic high-pressure environments, can be driven into a metallic state by biaxial compression-shear strains and even become superconducting. This surprising result indicates that strain engineering with non-hydrostatic stresses may offer an effective tool for generating and tuning superconductivity among covalent solids. Our further work showed that semiconductors like Si and SiC also exhibit intrinsic strain induced superconductivity under distinct uniaxial and multiaxial deformation paths, allowing convenient and robust tuning strategies for exploring the intrinsic connection and reversible transition between the semiconducting and superconducting states of these materials, opening vast untapped structural configurations for rational exploration of tunable emergence and transition of these intricate quantum phenomena in a broad range of materials.   

Second harmonic AC calorimetry technique within a diamond anvil cell

Tuning the energy density of matter at high pressures gives rise to exotic and often unprecedented properties, e.g., structural transitions, insulator–metal transitions, valence fluctuations, topological order, and the emergence of superconductivity. The study of specific heat has long been used to characterize these kinds of transitions, but their application to the diamond anvil cell (DAC) environment has proved challenging. Limited work has been done on the measurement of specific heat within DACs, in part due to the difficult experimental setup. To this end, we have developed a novel method for the measurement of specific heat within a DAC that is independent of the DAC design and is, therefore, readily compatible with any DACs already performing high pressure resistance measurements. As a proof-of-concept, specific heat measurements of the MgB2 superconductor were performed, showing a clear anomaly at the transition temperature (Tc), indicative of bulk superconductivity. This technique allows for specific heat measurements at higher pressures than previously possible.   

Reflectivity Studies on Nitrogen-Doped Lutetium Hydride at High Pressure

The optical studies of materials are a fundamentally important measurement to understand the nature of an insulator to metal transition and superconducting energy gap. Here we report reflectivity studies of a gasket material (rhenium) as a function of pressure by taking silver as a reference inside a diamond anvil cell (DAC) to overcome the diamond absorption. We also show that knowledge of the rhenium reflectivity can be used to calculate reflectivity of a sample inside a DAC. The Nitrogen-doped lutetium hydride is a rare-earth hydride which undergoes a dramatic color change and conductivity properties under pressure. In this work, we measure the absolute reflectivity of N-doped lutetium hydride under pressure and fit Drude-Lorentz mathematical models to these reflections.   

Light and warm superconductors: fact or fiction?

BCS theory predicts that light elements should make high temperature superconductors, and that is the consensus in the physics community. Accordingly, NSF and other funding agencies are devoting important resources to such research [1]. Hydrides under high pressure are expected to be high T_c superconductors, and during the last 7 years 15 different such hydrides, e.g. H₃S, LaH₁₀, CSH, etc, have been claimed to be superconductors with T_c's exceeding those of the high T_c cuprates, up to and above room temperature. Many more hydrides have been claimed to be high T_c superconductors based on theoretical evidence, and many other light-element compounds have been predicted to be high T_csuperconductors by analogy with MgB₂. I will argue that NONE of those claims and predictions have been independently and reproducibly verified, and that in fact to date the experimental evidence that light elements favor high temperature superconductivity is ZERO. If what I am arguing is true, it indicates that there is a.fundamental flaw in the BCS assumption that the electron-phonon interaction causes superconductivity. Instead, the theory of hole superconductivity [2] predicts that high temperature superconductivity results from holes conducting through closely spaced negatively charged anions, as is the case in the cuprates, pnictides, and MgB₂, and that the ionic mass is irrelevant.

 

[1] NSF Funding Opportunity - Light and Warm Superconductors, January 27, 2021.

[2] References in https://jorge.physics.ucsd.edu/hole.html.