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When diamond anvil cell (DAC) sample chambers are outfitted with both thermal insulation and electrodes, two cutting-edge experimental methods are enabled: Joule heating with spectroradiometric temperature measurement, and electrical resistance measurements of samples heated to thousands of kelvin. The accuracy of temperature and resistance measurements, however, often suffers from poor control of the shape and location of the sample, electrodes, and thermal insulation. Here, we present a recipe for the reproducible and precise fabrication of DAC sample, electrodes, and thermal insulation using a three-layer microassembly. The microassembly contains two potassium chloride thermal insulation layers, four electrical leads, a sample, and a buttressing layer made of polycrystalline alumina. The sample, innermost electrodes, and buttress layer are fabricated by focused-ion-beam milling. Three iron samples are presented as proof of concept. Each is successfully compressed and pulsed Joule heated while maintaining a four-point probe configuration. The highest pressure-temperature condition achieved is $\sim 150$ GPa and $\sim 4000$ K., Comment: 19 pages, 15 figures, 2 tables

Accurate and precise measurements of spectroradiometric temperature are crucial for many high pressure experiments that use diamond anvil cells or shock waves. In experiments with sub-millisecond timescales, specialized detectors such as streak cameras or photomultiplier tubes are required to measure temperature. High accuracy and precision are difficult to attain, especially at temperatures below 3000 K. Here we present a new spectroradiometry system based on multianode photomultiplier tube technology and passive readout circuitry that yields a 0.24 $\mu$s rise-time for each channel. Temperature is measured using five color spectroradiometry. During high pressure pulsed Joule heating experiments in a diamond anvil cell, we document measurement precision to be $\pm 30$ K at temperatures as low as 2000 K during single-shot heating experiments with $0.6$ $\mu$s time-resolution. Ambient pressure melting tests using pulsed Joule heating indicate that the accuracy is $\pm 80$ K in the temperature range 1800-2700 K., Comment: 17 pages, 18 figures

Decades of measurements of the thermophysical properties of hot metals show that pulsed Joule heating is an effective method to heat solid and liquid metals that are chemically reactive or difficult to contain. In order to extend such measurements to megabar pressures, pulsed heating methods must be integrated with diamond anvil cells. We report the design and characterization of a new pulse amplifier that can heat $\sim 5$ to $50$ $\mu$m-diameter metal wires to 1000s of kelvin at megabar pressures using diamond anvil cells. Pulse durations and peak currents can each be varied over 3 orders of magnitude, from 5 $\mu$s to 10 ms and 0.2 to 200 A. The pulse amplifier is integrated with a current probe. Two voltage probes attached to the body of a diamond anvil cell can be used to measure voltage in a four point probe geometry. The accuracy of four point probe resistance measurements for a dummy sample with 0.1 $\Omega$ resistance are typically better than $5 \%$ at all times from 2 $\mu$s to 10 ms after the beginning of the pulse., Comment: 7 pages, 6 figures

By directly altering microscopic interactions, pressure provides a powerful tuning knob for the exploration of condensed phases and geophysical phenomena. The megabar regime represents an exciting frontier, where recent discoveries include novel high-temperature superconductors, as well as structural and valence phase transitions. However, at such high pressures, many conventional measurement techniques fail. Here, we demonstrate the ability to perform local magnetometry inside of a diamond anvil cell with sub-micron spatial resolution at megabar pressures. Our approach utilizes a shallow layer of Nitrogen-Vacancy (NV) color centers implanted directly within the anvil; crucially, we choose a crystal cut compatible with the intrinsic symmetries of the NV center to enable functionality at megabar pressures. We apply our technique to characterize a recently discovered hydride superconductor, CeH$_9$. By performing simultaneous magnetometry and electrical transport measurements, we observe the dual signatures of superconductivity: local diamagnetism characteristic of the Meissner effect and a sharp drop of the resistance to near zero. By locally mapping the Meissner effect and flux trapping, we directly image the geometry of superconducting regions, revealing significant inhomogeneities at the micron scale. Our work brings quantum sensing to the megabar frontier and enables the closed loop optimization of superhydride materials synthesis.

The high-pressure melting curves of metals provide simple and useful tests for theories of melting, as well as important constraints for the modeling of planetary interiors. Here, we present an experimental technique that reveals the latent heat of fusion of a metal sample compressed inside a diamond anvil cell. The technique combines microsecond-timescale pulsed electrical heating with an internally-heated diamond anvil cell for the first time. Further, we use the technique to measure the melting curve of platinum to the highest pressure measured to date. Melting temperature increases from $\sim 3000$ K at 34 GPa to $\sim 4500$ K at 107 GPa, thermodynamic conditions that are between the steep and shallow experimental melting curves reported previously. The melting curve is a linear function of compression over the 0 to 20% range of compression studied here, allowing a good fit to the Kraut-Kennedy empirical model with fit parameter $C=6.0$., Comment: Main text is 30 pages, 8 figures. Supplement is 30 pages, 16 figures

The insulator-to-metal transition in dense fluid hydrogen is an essential phenomenon to understand gas giant planetary interiors and the physical and chemical behavior of highly compressed condensed matter. Using fast laser spectroscopy techniques to probe hydrogen and deuterium precompressed in a diamond anvil cell and laser heated on microsecond timescales, we observe an onset of metal-like reflectivity in the visible spectral range at P>150 GPa and T>3000 K. The reflectance increases rapidly with decreasing photon energy indicating free-electron metallic behavior with a plasma edge in the visible spectral range at high temperatures. The reflectivity spectra also suggest much longer electronic collision time (>1 fs) than previously inferred, implying that metallic hydrogens at the conditions studied are not in the regime of saturated conductivity (Mott-Ioffe-Regel limit). Combined with previously reported data, our results suggest the existence of a semiconducting intermediate fluid hydrogen state en route to metallization., Comment: 23 pages, 15 figures

Hydrogen and helium are the most abundant elements in the universe and they constitute the interiors of gas giant planets. Thus, their equations of states, phase, chemical state, and chemical reactivity at extreme conditions are of great interest. Applying Raman spectroscopy, visual observation, and synchrotron X-ray diffraction in diamond anvil cells (DAC), we performed experiments on H2-He 1:1 and D2-He 1:10 compressed gas mixture up to 100 GPa at 300 K. By comparing with the available data on pure bulk materials, we find no sign of miscibility, chemical reactivity, and new compound formation., Comment: 11 pages, 8 figures, 1 table, supplementary materials

Recent predictions and experimental observations of high Tc superconductivity in hydrogen-rich materials at very high pressures are driving the search for superconductivity in the vicinity of room temperature. We have developed a novel preparation technique that is optimally suited for megabar pressure syntheses of superhydrides using pulsed laser heating while maintaining the integrity of sample-probe contacts for electrical transport measurements to 200 GPa. We detail the synthesis and characterization, including four-probe electrical transport measurements, of lanthanum superhydride samples that display a significant drop in resistivity on cooling beginning around 260 K and pressures of 190 GPa. Additional measurements on two additional samples synthesized the same way show resistance drops beginning as high as 280 K at these pressures. The loss of resistance at these high temperatures is not observed in control experiments on pure La as well as in partially transformed samples at these pressures, and x-ray diffraction as a function of temperature on the superhydride reveal no structural changes on cooling. We infer that the resistance drop is a signature of the predicted room-temperature superconductivity in LaH10, in good agreement with density functional structure search and BCS theory calculations., Comment: 20 pages, 4 figures and Supplementary Materials , added some references, edited and corrected several typos, no change in results

Numerical simulations of heat transport in diamond anvil cells reveal a possibility for absolute measurements of specific heat via high-frequency modulation calorimetry. Such experiments could reveal and help characterize temperature-driven phase transitions at high-pressure, such as melting, the glass transition, magnetic and electric orderings, or superconducting transitions. Specifically, we show that calorimetric information of a sample cannot be directly extracted from measurements at frequencies slower than the timescale of conduction to the diamond anvils (10s to 100s of kHz) since the experiment is far from adiabatic. At higher frequencies, laser-heating experiments allow relative calorimetric measurements, where changes in specific heat of the sample are discriminated from changes in other material properties by scanning the heating frequency from $\sim 1$ MHz to 1 GHz. But laser-heating generates large temperature gradients in metal samples, preventing absolute heat capacities to be inferred. High-frequency Joule heating, on the other hand, allows accurate, absolute specific heat measurements if it can be performed at high-enough frequency: assuming a thin layer of KBr insulation, the specific heat of a 5 $\mu$m-thick metal sample heated at 100 kHz, 1 MHz, or 10 MHz frequency would be measured with 30%, 8% or 2% accuracy, respectively.

Part I shows that quantitative measurements of heat capacity are theoretically possible inside diamond anvil cells via high-frequency Joule heating (100 kHz to 10 MHz), opening up the possibility of new methods to detect and characterize transformations at high-pressure such as the glass transitions, melting, magnetic orderings, or 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 $\sim 1$ MHz frequency, with $< 10%$ accuracy. Second, we test physical models of high-pressure experiments, i.e. diamond-cell mock-ups. Metal foils of 2 to 6 $\mu$m-thickness are clamped between glass insulation inside diamond anvil cells. Fitting data from 10 Hz to $\sim 30$ kHz, we infer the specific heat capacities of Fe, Pt and Ni with $\pm 20$ to $30%$ 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, $\sqrt{\rho c k}$ ($\rho$, $c$ and $k$ being density, specific heat and thermal conductivity), of the insulation surrounding a thin-film heater. Using a $\sim 50$ nm-thick Pt heater surrounded by glass and 10 Hz to 300 kHz frequency, we measure thermal effusivity with $\pm 6%$ accuracy inside the sample chamber of a diamond anvil cell.

A recent paper by Zaghoo et al. presents optical data at high-pressure and high-temperature and interprets the data as evidence for a first-order phase transition to metallic hydrogen during heating. Here we argue that the presented data are contradictory with these claims. Elucidating this issue is important for building a coherent picture that is emerging as the results of theoretical calculations of various levels and experimental investigations employing static and dynamic compression techniques. In this context, the use of adequate probes of the electronic and chemical state is crucial. Optical probes that do not address the energy dependent conductivity while making multiple references to the Drude model are highly speculative. Indeed, the available dynamic and static compression data and theoretical modeling show that study of energy dependent conductivity is important for understanding the nature of hot dense hydrogen. Moreover, the most recent investigations suggest that the critical point, which demarcates the regimes of crossover between insulating and plasma state at low densities to the first order liquid-liquid transition, is above 200 GPa. Below we concentrate on inconsistencies in the interpretation of data in Zaghoo et al, which call for careful examination of their claims and further detailed investigations. We analyze their optical data and use finite element calculations and argue that the high-temperature state studied in Zaghoo et al. is not metallic and that the data cannot discriminate between a first-order phase transition and a continuous phase transition, or even a bandgap drop within one phase, Comment: 1 page, 1 table, 3 figures