Search

This article discusses the use of magnetic methods in studying new superconducting hydrides in a diamond anvil cell (DAC). The high-pressure experiments have led to the discovery of high-transition-temperature (Tc) hydrides by creating metallic hydrogen 'alloys' with other elements. However, measuring the complete expulsion of the magnetic field, known as the Meissner state, in these hydrides is challenging due to the high pressures required. The article provides a summary of four alternative techniques used to probe the Meissner state in a DAC, including the AC susceptibility method, the double-modulation technique, SQUID magnetometry, and the NV− magnetometry. The authors argue that the growing experimental evidence supports the claims about superhydrides as nearly room-temperature superconductors. [Extracted from the article]

Finding ways to achieve higher values of the transition temperature, [T.sub.c], in superconductors remains a great challenge. The super-conducting phase is often one of several competing types of electronic order, including antiferromagnetism and charge density waves (1-5). An emerging trend documented in heavy-fermion (1) and organic (2) conductors is that the maximum [T.sub.c] for superconductivity occurs under external conditions that cause the critical temperature for a competing order to go to zero. Recently, such competition has been found in multilayer copper oxide high-temperature superconductors (HTSCs (3-5)) that possess two crystallographically inequivalent Cu[O.sub.2] planes in the unit cell. However, whether the competing electronic state can be suppressed to enhance [T.sub.c] in HTSCs remains an open question. Here we show that pressure-driven phase competition leads to an unusual two-step enhancement of [T.sub.c] in optimally doped trilayer [Bi.sub.2][Sr.sub.2][Ca.sub.2][Cu.sub.3][O.sub.10+δ] (Bi2223). We find that [T.sub.c] first increases with pressure and then decreases after passing through a maximum. Unexpectedly, [T.sub.c] increases again when the pressure is further raised above a critical value of around 24 GPa, surpassing the first maximum. The presence of this critical pressure is a manifestation of the crossover from the competing order to superconductivity in the inner of the three Cu[O.sub.2] planes. We suggest that the increase at higher pressures occurs as a result of competition between pairing and phase ordering in different Cu[O.sub.2] planes., We used Bi2223 single crystals in the present investigation. The structure of Bi2223 consists of two insulating Bi-O and Sr-O blocking layers intercalating in the c-axis direction with two structurally [...]

Iron in crustal and mantle minerals adopts several possible oxidation states: this has implications for biogeochemical processes (1), oxygenation of the atmosphere (2) and the oxidation state of the mantle [...]

There is a great interest in electronic transitions in hydrogen-rich materials under extreme conditions. It has been recently suggested that the group IVa hydrides such as methane (C[H.sub.4]), silane (Si[H.sub.4]), and germane (Ge[H.sub.4]) become metallic at far lower pressures than pure hydrogen at equivalent densities because the hydrogen is chemically compressed in group IVa hydride compounds. Here we report measurements of Raman and infrared spectra of silane under pressure. We find that [SiH.sub.4] undergoes three phase transitions before becoming opaque at 27-30 GPa. The vibrational spectra indicate the material transforms to a polymeric (framework) structure in this higher pressure range. Room-temperature infrared reflectivity data reveal that the material exhibits Drude-like metallic behavior above 60 GPa, indicating the onset of pressure-induced metallization. high pressure | hydrogen-rich materials

If successfully developed, calorimetry at tens of GPa of pressure could help characterize phase transitions in materials such as high-pressure minerals, metals, and molecular solids. Here, we extend alternating-current calorimetry to 9 GPa and 300 K in a diamond anvil cell and use it to study phase transitions in H2O. In particular, water is loaded into the sample chambers of diamond-cells, along with thin metal heaters (1 µm-thick platinum or 20 nm-thick gold on a glass substrate) that drive high-frequency temperature oscillations (20 Hz to 600 kHz; 1 to 10 K). The heaters also act as thermometers via the third-harmonic technique, yielding calorimetric data on (1) heat conduction to the diamonds and (2) heat transport into substrate and sample. Using this method during temperature cycles from 300 to 200 K, we document melting, freezing, and proton ordering and disordering transitions of H2O at 0 to 9 GPa, and characterize changes in thermal conductivity and heat capacity across these transitions. The technique and analysis pave the way for calorimetry experiments on any non-metal at pressures up to ~100 GPa, provided a thin layer (several µm-thick) of thermal insulation supports a metallic thin-film (tens of nm thick) Joule-heater attached to low contact resistance leads inside the sample chamber of a diamond-cell. [ABSTRACT FROM AUTHOR]

Part I shows that quantitative measurements of heat capacity are theoretically possible inside diamond anvil cells via high-frequency Joule heating (100 kHz-10MHz), opening up the possibility of new methods to detect and characterize transformations at high-pressure such as the glass transitions, melting, magnetic orderings, and the onset of superconductivity. Here, we test the possibility outlined in Part I, using prototypes and detailed numerical models. First, a coupled electrical-thermal numerical model shows that specific heat of metals inside diamond cells can be measured directly using ~1MHz frequency, with <10% accuracy. Second, we test physical models of high-pressure experiments, i.e., diamond-cell mock-ups. Metal foils of 2-6 μm-thickness are clamped between glass insulation inside diamond anvil cells. Fitting data from 10 Hz to ~30 kHz, we infer the specific heat capacities of Fe, Pt, and Ni with ±20%-30% accuracy. The electrical test equipment generates -80 dBc spurious harmonics, which overwhelm the thermally induced harmonics at higher frequencies, disallowing the high precision expected from numerical models. An alternative Joule-heating calorimetry experiment, on the other hand, does allow absolute measurements with <10% accuracy, despite the -80 dBc spurious harmonics: the measurement of thermal effusivity, √ρck (ρ, c, and k being density, specific heat, and thermal conductivity), of the insulation surrounding a thin-film heater. Using a ~50 nm-thick Pt heater surrounded by glass and 10 Hz-300 kHz frequency, we measure thermal effusivity with ±6% accuracy inside the sample chamber of a diamond anvil cell. [ABSTRACT FROM AUTHOR]

Author(s): Jung-Fu Lin (corresponding author) [1, 4]; Viktor V. Struzhkin [1]; Steven D. Jacobsen [1]; Michael Y. Hu [2]; Paul Chow [2]; Jennifer Kung [3]; Haozhe Liu [2]; Ho-kwang Mao [...]

We report the results of high-pressure Raman spectroscopy studies of alkali metal fluoroargentates (M2AgF4, where M = Na, K, Rb) combined with theoretical and X-ray diffraction studies for the K member of the series. Theoretical density functional calculations predict two structural phase transitions for K2AgF4: one from low-pressure monoclinic P21/c (beta) phase to intermediate-pressure tetragonal I 4 2d structure at 6 GPa, and another to high-pressure triclinic P 1 phase at 58 GPa. However, Raman spectroscopy and X-ray diffraction data indicate that both polymorphic forms of K2AgF4, as well as two other fluoroargentate phases studied here, undergo amorphization at pressures as low as several GPa. [ABSTRACT FROM AUTHOR]

The pressure dependence of electrical resistance of single-crystal magnetite (Fe3O4) was measured under quasi-hydrostatic conditions to 100 GPa using low-temperature, megabar diamond-anvil cell techniques in order to gain insight into the anomalous behavior of this material that has been reported over the years in different high-pressure experiments. The measurements under nearly hydrostatic pressure conditions allowed us to detect the clear Verwey transition and the highpressure structural phase. The appearance of a metallic ground state after the suppression of the Verwey transition around 20 GPa and the concomitant enhancement of the electrical resistance caused by the structural transformation to the high-pressure phase form reentrant semiconductingmetallic- semiconducting behavior, although the appearance of the metallic phase is highly sensitive to stress conditions and details of the measurement technique. [ABSTRACT FROM AUTHOR]