Phonon Softening and Superconductivity in Tellurium under Pressure

Formation Mechanism of Elemental Te Produced in Tellurite Glass Systems by Femtosecond Laser Irradiation

The formation of elemental trigonal tellurium (t-Te) on tellurite glass surfaces exposed to femtosecond laser pulses is discussed. Specifically, the underlying elemental crystallization phenomenon is investigated by altering laser parameters in common tellurite glass compositions under various ambient conditions. Elemental crystallization of t-Te by a single femtosecond laser pulse is unveiled by high-resolution imaging and analysis. The thermal diffusion model reveals the absence of lattice melting upon a single laser pulse, highlighting the complexity of the phase transformation. The typical cross-section displays three different crystal configurations over its depth, in which the overall thickness increases with each subsequent pulse. The effect of various controlled atmospheres shows the suppressing nature of the elemental crystallization, whereas the substrate temperature shows no significant impact on the nucleation of t-Te nanocrystals. This research gives new insight into the elemental crystallization of glass upon femtosecond laser irradiation and shows the potential to fabricate functional transparent electronic micro/nanodevices.

Superconductivity in Layered van der Waals Hydrogenated Germanene at High Pressure

The emergence of superconductivity in two-dimensional (2D) materials has attracted tremendous research efforts because the origins and mechanisms behind the unexpected and fascinating superconducting phenomena remain unclear. In particular, the superconductivity can survive in 2D systems even with weakened disorder and broken spatial inversion symmetry. Here, structural and superconducting transitions of 2D van der Waals (vdW) hydrogenated germanene (GeH) are observed under compression and decompression processes. GeH possesses a superconducting transition with a critical temperature (T_c) of 5.41 K at 8.39 GPa. A crystalline to amorphous transition occurs at 16.80 GPa, while superconductivity remains. An abnormal increase of T_c up to 6.11 K was observed during the decompression process, while the GeH remained in the 2D amorphous phase. A combination study of in situ high-pressure synchrotron X-ray diffraction, in situ high-pressure Raman spectroscopy, transition electron microscopy, and density functional theory simulations suggests that the superconductivity in 2D vdW GeH is attributed to the increased density of states at the Fermi level as well as the enhanced electron-phonon coupling effect under high pressure even in the form of an amorphous phase. The unique pressure-induced phase transition of GeH from 2D crystalline to 2D amorphous metal hydride provides a promising platform to study the mechanisms of amorphous hydride superconductivity.

High-temperature superconductivity on the verge of a structural instability in lanthanum superhydride

The possibility of high, room-temperature superconductivity was predicted for metallic hydrogen in the 1960s. However, metallization and superconductivity of hydrogen are yet to be unambiguously demonstrated and may require pressures as high as 5 million atmospheres. Rare earth based “superhydrides”, such as LaH₁₀, can be considered as a close approximation of metallic hydrogen even though they form at moderately lower pressures. In superhydrides the predominance of H-H metallic bonds and high superconducting transition temperatures bear the hallmarks of metallic hydrogen. Still, experimental studies revealing the key factors controlling their superconductivity are scarce. Here, we report the pressure and magnetic field dependence of the superconducting order observed in LaH₁₀. We determine that the high-symmetry high-temperature superconducting Fm-3m phase of LaH₁₀ can be stabilized at substantially lower pressures than previously thought. We find a remarkable correlation between superconductivity and a structural instability indicating that lattice vibrations, responsible for the monoclinic structural distortions in LaH₁₀, strongly affect the superconducting coupling.

The experimental studies to understand the superconductivity of superhydrides remain scarce. Here, the authors report pressure and magnetic field dependence of superconductivity in LaH₁₀, and indicate lattice vibrations strongly affect superconducting coupling.

Electronic-structure methods for materials design

The accuracy and efficiency of electronic-structure methods to understand, predict and design the properties of materials has driven a new paradigm in research. Simulations can greatly accelerate the identification, characterization and optimization of materials, with this acceleration driven by continuous progress in theory, algorithms and hardware, and by adaptation of concepts and tools from computer science. Nevertheless, the capability to identify and characterize materials relies on the predictive accuracy of the underlying physical descriptions, and on the ability to capture the complexity of realistic systems. We provide here an overview of electronic-structure methods, of their application to the prediction of materials properties, and of the different strategies employed towards the broader goals of materials design and discovery.

Hot electron relaxation dynamics in semiconductors: assessing the strength of the electron-phonon coupling from the theoretical and experimental viewpoints

The rapid development of the computational methods based on density functional theory, on the one hand, and of time-, energy-, and momentum-resolved spectroscopy, on the other hand, allows today an unprecedently detailed insight into the processes governing hot-electron relaxation dynamics, and, in particular, into the role of the electron-phonon coupling. Instead of focusing on the development of a particular method, theoretical or experimental, this review aims to treat the progress in the understanding of the electron-phonon coupling which can be gained from both, on the basis of recently obtained results. We start by defining several regimes of hot electron relaxation via electron-phonon coupling, with respect to the electron excitation energy. We distinguish between energy and momentum relaxation of hot electrons, and summarize, for several semiconductors of the IV and III-V groups, the orders of magnitude of different relaxation times in different regimes, on the basis of known experimental and numerical data. Momentum relaxation times of hot electrons become very short around 1 eV above the bottom of the conduction band, and such ultrafast relaxation mechanisms are measurable only in the most recent pump-probe experiments. Then, we give an overview of the recent progress in the experimental techniques allowing to obtain detailed information on the hot-electron relaxation dynamics, with the main focus on time-, energy-, and momentum-resolved photoemission experiments. The particularities of the experimental approach developed by one of us, which allows to capture time-, energy-, and momentum-resolved hot-electron distributions, as well as to measure momentum relaxation times of the order of 10 fs, are discussed. We further discuss the main advances in the calculation of the electron-phonon scattering times from first principles over the past ten years, in semiconducting materials. Ab initio techniques and efficient interpolation methods provide the possibility to calculate electron-phonon scattering times with high precision at reasonable numerical cost. We highlight the methods of analysis of the obtained numerical results, which allow to give insight into the details of the electron-phonon scattering mechanisms. Finally, we discuss the concept of hot electron ensemble which has been proposed recently to describe the hot-electron relaxation dynamics in GaAs, the applicability of this concept to other materials, and its limitations. We also mention some open problems.

Allotropes of tellurium from first-principles crystal structure prediction calculations under pressure

We investigated the allotropes of tellurium under hydrostatic pressure based on density functional theory calculations and crystal structure prediction methodology. Our calculated enthalpy-pressure and energy-volume curves unveil the transition sequence from the trigonal semiconducting phase, represented by the space group P3₁21 in the range of 0–6 GPa, to the body centered cubic structure, space group Im3̄m, stable at 28 GPa. In between, the calculations suggest a monoclinic structure, represented by the space group C2/m and stable at 6 GPa, and the β-Po type structure, space group R3̄m, stable at 10 GPa. The face-centered structure is found at pressure as high as 200 GPa. As the pressure is increased, the transition from the semiconducting phase to metallic phases is observed.

Through first-principles simulations, we suggest the phase stability of the allotropic transition sequence of tellurium from the trigonal structure up to the cubic structure.

Superconductivity at the Polar-Nonpolar Phase Boundary of SnP with an Unusual Valence State

Structural, magnetic, and electrical characterizations reveal that SnP with an unusual valence state (nominally Sn^{3+}) undergoes a ferroelectriclike structural transition from a simple NaCl-type structure to a polar tetragonal structure at approximately 250 K at ambient pressure. First-principles calculations indicate that the experimentally observed tetragonal distortion enhances the charge transfer from Sn to P, thereby making the polar tetragonal phase energetically more stable than the nonpolar cubic phase. Hydrostatic pressure is found to promptly suppress the structural phase transition in SnP, leading to the emergence of bulk superconductivity in a phase-competitive manner. These findings suggest that control of ferroelectriclike instability in a metal can be a promising way for creating novel superconductors.

On the critical temperature discontinuity at the theoretical bcc-fcc phase transition in compressed selenium and tellurium superconductors

Recent hydrides-driven advent in the high-pressure phonon-mediated superconductivity motivates research on chemical elements which compound with hydrogen. It is desired that such elements should allow chemical pre-compression of hydrogen to assure the induction of the superconducting phase with the high transition temperature (T _C). Herein, we present detailed theoretical insight into the properties of the superconducting state induced under pressure (p) in two of such component elements, namely selenium (Se) and tellurium (Te) at [Formula: see text] GPa and [Formula: see text] GPa, respectively. The assumed external pressure conditions allow us to conduct our analysis just above previously theoretically predicted bcc-fcc structural phase transition of Se and Te, and identify the possible associated discontinuity effect of the critical temperature. In particular, our numerical analysis is conducted within Migdal-Eliashberg formalism, due to the confirmed electron-phonon pairing mechanism and relatively high electron-phonon coupling constant in the materials of interest. We predict that T _C values in Se and Te equal 8.13 K and 5.96 K, respectively, and mark the highest critical temperature values for these elements within the postulated fcc phase. Additionally, we supplement these results by the estimated maximum values of the superconducting energy band gap and the effective mass of electrons. We predict that all these parameters can be used as a guidelines for experimental observation of the critical temperature discontinuity and the corresponding bcc-fcc phase transition in Se and Te superconductors. Moreover, we show that the thermodynamics of superconducting phase in both elements may exhibit deviations from the conventional estimates of the Bardeen-Cooper-Schrieffer theory, and suggest existence of the strong-coupling and retardation effects. Finally, we note that our results can be also instructive for future screening of chemical elements for applications in superconducting hydrides.

Estimating carrier relaxation times in the Ba8Ga16Ge30 clathrate in the extrinsic regime

We used a semi-empirical method to extract carrier relaxation times at different temperatures (τ(T)) in thermoelectric materials from a combination of experimental results and first-principles calculations. The methodology is based on the Boltzmann transport equation formalism within the relaxation time approximation. It can be applied to single crystals and polycrystalline materials. We applied the method to investigate the electronic transport properties of the clathrate compound Ba₈Ga₁₆Ge₃₀ type-I. The calculations indicate that the carrier relaxation process in single crystals is dominated by electron-phonon scattering (τ ∝ T^-3/2), while in polycrystalline materials scattering at grain boundaries dominates (τ ∼ cte). The Seebeck coefficient, electrical conductivity, and electron heat conduction are in consistent agreement with experiment. Furthermore, the Slack relation for lattice heat conductivity was successfully applied to the material. The calculated figure of merit is in good agreement with experimental results.

Exploration of new superconductors and functional materials, and fabrication of superconducting tapes and wires of iron pnictides

This review shows the highlights of a 4-year-long research project supported by the Japanese Government to explore new superconducting materials and relevant functional materials. The project found several tens of new superconductors by examining ∼1000 materials, each of which was chosen by Japanese experts with a background in solid state chemistry. This review summarizes the major achievements of the project in newly found superconducting materials, and the fabrication wires and tapes of iron-based superconductors; it incorporates a list of ∼700 unsuccessful materials examined for superconductivity in the project. In addition, described are new functional materials and functionalities discovered during the project.

Novel superconducting skutterudite-type phosphorus nitride at high pressure from first-principles calculations

State of the art variable composition structure prediction based on density functional theory demonstrates that two new stoichiometries of PN, PN3 and PN2, become viable at high pressure. PN3 has a skutterudite-like Immm structure and is metastable with positive phonon frequencies at pressures between 10 and 100 GPa. PN3 is metallic and is the first reported nitrogen-based skutterudite. Its metallicity arises from nitrogen p-states which delocalise across N4 rings characteristic of skutterudites, and it becomes a good electron-phonon superconductor at 10 GPa, with a Tc of around 18 K. The superconductivity arises from strongly enhanced electron-phonon coupling at lower pressures, originating primarily from soft collective P-N phonon modes. The PN2 phase is an insulator with P2/m symmetry and is stable at pressures in excess of 200 GPa.

Giant phonon softening and enhancement of superconductivity by phosphorus doping of BaNi2As2

The effects of phosphorus doping on the structural and superconducting phase transitions of BaNi2(As(1-x)P(x))2 were studied. The specific heat, resistivity, and magnetic susceptibility were measured. The results revealed an abrupt increase in the superconducting transition temperature (T(c)) from 0.6 K in the triclinic phase (space group P1¯) with less phosphorus (x≤0.067) to 3.3 K in the tetragonal phase (space group I4/mmm) with more phosphorus (x≥0.067). Our data analysis suggests that a doping-induced softening related to an in-plane Ni and As(P) phonon mode is responsible for the enhanced superconductivity in the tetragonal phase.

High-pressure crystallography

The ability of pressure to change inter-atomic distances strongly leads to a wide range of pressure-induced phenomena at high pressures: for example metallisation, amorphisation, superconductivity and polymerisation. Key to understanding these phenomena is the determination of the crystal structure using x-ray or neutron diffraction. The tools necessary to compress matter above 1 million atmospheres (1 Megabar or 100 GPa) were established by the mid 1970s, but it is only since the early 1990s that we have been able to determine the detailed crystal structures of materials at such pressures. In this chapter I briefly review the history of high-pressure crystallography, and describe the techniques used to obtain and study materials at high pressure. Recent crystallographic studies of elements are then used to illustrate what is now possible using modern detectors and synchrotron sources. Finally, I speculate as to what crystallographic studies might become possible over the next decade.

Ab initio method for calculating electron-phonon scattering times in semiconductors: application to GaAs and GaP

We propose a fully ab initio approach to calculate electron-phonon scattering times for excited electrons interacting with short-wavelength (intervalley) phonons in semiconductors. Our approach is based on density functional perturbation theory and on the direct integration of electronic scattering probabilities over all possible final states with no ad hoc assumptions. We apply it to the deexcitation of hot electrons in GaAs, and calculate the lifetime of the direct exciton in GaP, both in excellent agreement with experiments. Matrix elements of the electron-phonon coupling, and their dependence on the wave vector of the final state and on the phonon modes, are shown to be crucial ingredients of the evaluation of electron-phonon scattering times.

Nonlocal screening, electron-phonon coupling, and phonon renormalization in metals

A method for calculating the phonon self-energy in metals arising from the coupling between phonons and electrons near the Fermi surface is developed. The essence of this scheme is the separation of the inter- and intraband parts of the electron polarizability. The intraband contribution provides extra screenings and is closely related to the electron-phonon coupling and phonon softening in metals. Applications of this scheme to phonons in MgB2 give excellent results when compared with experiments and previous theoretical work. In addition, both electron and hole dopings are found to reduce the renormalization effect of the E(2g) phonon mode, which indicates a weakened electron-phonon coupling in the doped systems.

Large structural modulations in incommensurate Te-III and Se-IV

The high-pressure phase of tellurium, Te-III, is found to have an incommensurate monoclinic structure, superspace group I(')2/m(0q0)s0, of a type previously unknown in the elements. Te-III is stable from 4.5(2) to 29.2(7) GPa; the previously reported transition to a distinct Te-IV phase at 10.6 GPa is not observed. The incommensurate wave vector of Te-III is strongly pressure dependent and varies in a strongly nonlinear way. Se-IV is found to be isostructural with Te-III.