Superconducting group-IV semiconductors

Discovery of the final primitive Frank-Kasper phase of clathrate hydrates

In weakly bound materials such as water, one of the three primitive Frank-Kasper (FK) phases, the Z phase, is long absent due to the relatively unstable framework. The Z phase in clathrate hydrate, which is known as the HS-I structure, has now been found by precise tuning of the molecular guest structure. In the crystal structure, the never stabilized combination water cage of two 15-hedra and two 14-hedra formed with its original symmetries, providing sufficient gas capacity to the 12-hedral cages. With the discovery of the final FK clathrate hydrate, guest design now enables engineering of weak interactions in any mix of the three, illuminating how to leverage properties of clathrates in the broadest sense.

Superhard and Superconducting Bilayer Borophene

Two-dimensional superconductors, especially the covalent metals such as borophene, have received significant attention due to their new fundamental physics, as well as potential applications. Furthermore, the bilayer borophene has recently ignited interest due to its high stability and versatile properties. Here, the mechanical and superconducting properties of bilayer-δ6 borophene are explored by means of first-principles computations and anisotropic Migdal-Eliashberg analytics. We find that the coexistence of strong covalent bonds and delocalized metallic bonds endows this structure with remarkable mechanical properties (maximum 2D-Young's modulus of ~570 N/m) and superconductivity with a critical temperature of ~20 K. Moreover, the superconducting critical temperature of this structure can be further boosted to ~46 K by applied strain, which is the highest value known among all borophenes or two-dimensional elemental materials.

Computational Screening and Stabilization of Boron-Substituted Type-I and Type-II Carbon Clathrates

Boron substitution represents a promising approach to stabilize carbon clathrate structures, but no thermodynamically stable substitution schemes have been identified for frameworks other than the type-VII (sodalite) structure type. To investigate the possibility for additional tetrahedral carbon-based clathrate networks, more than 5000 unique boron decoration schemes were investigated computationally for type-I and type-II carbon clathrates with a range of guest elements including Li, Na, K, Rb, Cs, Mg, Ca, Sr, and Ba. Density functional theory calculations were performed at 10 and 50 GPa, and the stability and impact of boron substitution were evaluated. The results indicate that the boron-substituted carbon clathrates are stabilized under high-pressure conditions. Full cage occupancies of intermediate-sized guest atoms (e.g., Na, Ca, and Sr) are the most favorable energetically. Clathrate stability is maximized when the boron atoms are substituted within the hexagonal rings of the large [51262]/[51264] cages. Several structures with favorable formation enthalpies <-200 meV/atom were predicted, and type-I Ca8B16C30 is on the convex hull at 50 GPa. This structure represents the first thermodynamically stable type-I clathrate identified and suggests that boron-substituted carbon clathrates may represent a large family of diamond-like framework materials with a range of structure types and guest/framework substitutions.

Sluggish Electron Transfer of Oxygen-Terminated Moderately Boron-Doped Diamond Electrode Induced by Large Interfacial Capacitance between a Diamond and Silicon Interface

Boron-doped diamond (BDD) has tremendous potential for use as an electrode material with outstanding characteristics. The substrate material of BDD can affect the electrochemical properties of BDD electrodes due to the different junction structures of BDD and the substrate materials. However, the BDD/substrate interfacial properties have not been clarified. In this study, the electrochemical behavior of BDD electrodes with different boron-doping levels (0.1% and 1.0% B/C ratios) synthesized on Si, W, Nb, and Mo substrates was investigated. Potential band diagrams of the BDD/substrate interface were proposed to explain different junction structures and electrochemical behaviors. Oxygen-terminated BDD with moderate boron-doping levels exhibited sluggish electron transfer induced by the large capacitance generated at the BDD/Si interface. These findings provide a fundamental understanding of diamond electrochemistry and insight into the selection of suitable substrate materials for practical applications of BDD electrodes.

Structural stability and electronic and mechanical properties of nitrogen- and boron-doped fluorinated diamane

In this work, the structural, electronic and mechanical properties of fluorinated diamane (F-diamane) with N and B dopants are systemically investigated using first-principles calculation. The N atom tends to stay in the external substituted site without F saturation, while the B-doped structure of the substituted external site with F saturation is the most stable. Ab initio molecular dynamics simulations confirm the thermal stability. The band structures of stable doped structures are similar to that of pristine F-diamane, due to the slight contribution of the dopant to the band edges. In addition, after the introduction of the B dopant, the formation energy reduces, and the transition barrier of graphene bilayers into diamane is smaller, indicating the feasibility of graphene bilayer fluoridation. Furthermore, we find that these doped structures have mechanical stability with isotropic elastic constants, Young's modulus, shear modulus and Poisson's ratio. Our work would promote the synthesis and development of two-dimensional diamane.

Prediction of potential hard sodium carbaboride compounds assuming sp3-bonded covalent clathrates

Boron-carbon clathrates have attracted great attention due to their unique sp3-bonded structure and excellent electronic properties. Here, by performing first-principles calculations, we predicted six stoichiometric Na-B-C clathrates (NaBC11, Na2B2C10, NaB2C10, Na2B4C8, NaB4C8, and Na2B6C6) based on Na-doped boron-carbon clathrates. As a result, NaBC11, Na2B2C10, and NaB2C10 were found to become energetically favorable. Under ambient conditions, the electronic structure calculations show that NaBC11 and Na2B2C10 are indirect band gap semiconductors, and NaB2C10, Na2B4C8, and NaB4C8 exhibit metallic features. Na2B2C10 and Na2B4C8 are found to be synthesized at 22.7 and 14.2 GPa, respectively. Interestingly, the formation enthalpies of NaxB2C10 and NaxB4C8 (x = 0, 1, and 2) clathrates decrease in turn with the increased number of Na atoms in the same synthetic paths. Moreover, the ideal indentation strengths of NaBC11, Na2B2C10, and NaB2C10 approach 40 GPa, indicating that they are hard materials with superior hardness. These findings offer valuable insights for advancing the synthesis of boron-carbon clathrates.

Prediction ofπ-electrons mediated high-temperature superconductivity in monolayer LiC12

Prediction and synthesis of two-dimensional high transition temperature (T_C) superconductors is an area of extensive research. Based on calculations of the electronic structures and lattice dynamics, we predict that graphene-like layered monolayer LiC₁₂is aπ-electrons mediated Bardeen-Cooper-Schrieffer-type superconductor. Monolayer LiC₁₂is theoretically stable and expected to be synthesized experimentally. From the band structures and the phonon dispersion spectrum, it is found that the saddle point ofπ-bonding bands induces large density of states at the Fermi energy level. There is strongly coupled between the vibration mode in the in-plane direction of the lithium atoms and theπ-electrons of carbon atoms, which induces the high-T_Csuperconductivity in LiC₁₂. TheT_Ccan reach to 41 K under an applied 10% biaxial tensile strain based on the anisotropic Eliashberg equation. Our results show that monolayer LiC₁₂is a good candidate asπ-electrons mediated electron-phonon coupling high-T_Csuperconductor.

Conventional High-Temperature Superconductivity in Metallic, Covalently Bonded, Binary-Guest C-B Clathrates

Inspired by the synthesis of XB₃C₃ (X = Sr, La) compounds in the bipartite sodalite clathrate structure, density functional theory (DFT) calculations are performed on members of this family containing up to two different metal atoms. A DFT-chemical pressure analysis on systems with X = Mg, Ca, Sr, Ba reveals that the size of the metal cation, which can be tuned to stabilize the B-C framework, is key for their ambient-pressure dynamic stability. High-throughput density functional theory calculations on 105 Pm3̅ symmetry XYB₆C₆ binary-guest compounds (where X, Y are electropositive metal atoms) find 22 that are dynamically stable at 1 atm, expanding the number of potentially synthesizable phases by 19 (18 metals and 1 insulator). The density of states at the Fermi level and superconducting critical temperature, T_c, can be tuned by changing the average oxidation state of the metal atoms, with T_c being highest for an average valence of +1.5. KPbB₆C₆, with an ambient-pressure Eliashberg T_c of 88 K, is predicted to possess the highest T_c among the studied Pm3̅n XB₃C₃ or Pm3̅ XYB₆C₆ phases, and calculations suggest it may be synthesized using high-pressure high-temperature techniques and then quenched to ambient conditions.

Recent Developments of High-Pressure Spark Plasma Sintering: An Overview of Current Applications, Challenges and Future Directions

Spark plasma sintering (SPS), also called pulsed electric current sintering (PECS) or field-assisted sintering technique (FAST) is a technique for sintering powder under moderate uniaxial pressure (max. 0.15 GPa) and high temperature (up to 2500 °C). It has been widely used over the last few years as it can achieve full densification of ceramic or metal powders with lower sintering temperature and shorter processing time compared to conventional processes, opening up new possibilities for nanomaterials densification. More recently, new frontiers of opportunities are emerging by coupling SPS with high pressure (up to ~10 GPa). A vast exciting field of academic research is now using high-pressure SPS (HP-SPS) in order to play with various parameters of sintering, like grain growth, structural stability and chemical reactivity, allowing the full densification of metastable or hard-to-sinter materials. This review summarizes the various benefits of HP-SPS for the sintering of many classes of advanced functional materials. It presents the latest research findings on various HP-SPS technologies with particular emphasis on their associated metrologies and their main outstanding results obtained. Finally, in the last section, this review lists some perspectives regarding the current challenges and future directions in which the HP-SPS field may have great breakthroughs in the coming years.

Silicon-Aluminum Phase-Transformation-Induced Superconducting Rings

The development of devices that exhibit both superconducting and semiconducting properties is an important endeavor for emerging quantum technologies. We investigate superconducting nanowires fabricated on a silicon-on-insulator (SOI) platform. Aluminum from deposited contact electrodes is found to interdiffuse with Si along the entire length of the nanowire, over micrometer length scales and at temperatures well below the Al-Si eutectic. The phase-transformed material is conformal with the predefined device patterns. The superconducting properties of a transformed mesoscopic ring formed on a SOI platform are investigated. Low-temperature magnetoresistance oscillations, quantized in units of the fluxoid, h/2e, are observed.

Valence-skipping and quasi-two-dimensionality of superconductivity in a van der Waals insulator

Valence fluctuation of interacting electrons plays a crucial role in emergent quantum phenomena in correlated electron systems. The theoretical rationale is that this effect can drive a band insulator into a superconductor through charge redistribution around the Fermi level. However, the root cause of such a fluctuating leap in the ionic valency remains elusive. Here, we demonstrate a valence-skipping-driven insulator-to-superconductor transition and realize quasi-two-dimensional superconductivity in a van der Waals insulator GeP under pressure. This is shown to result from valence skipping of the Ge cation, altering its average valency from 3+ to 4+, turning GeP from a layered compound to a three-dimensional covalent system with superconducting critical temperature reaching its maximum of 10 K. Such a valence-skipping-induced superconductivity with a quasi-two-dimensional nature in thin samples, showing a Berezinskii-Kosterlitz-Thouless-like character, is further confirmed by angle-dependent upper-critical-field measurements. These findings provide a model system to examine competing order parameters in valence-skipping systems.

An electrically conductive and ferromagnetic nano-structure manganese mono-boride with high Vickers hardness

The combination of various desired physical properties greatly extends the applicability of materials. Magnetic materials are generally mechanically soft, yet the combination of high mechanical hardness and ferromagnetic properties is highly sought after. Here, we report the synthesis and characterization of nanocrystalline manganese boride, CrB-type MnB, using the high-pressure and high-temperature method in a large volume press. CrB-type MnB shares the specificity of large numbers of unpaired electrons of manganese ions and strong covalent boron zigzag chains. Thus, manganese mono-boride exhibits "strong" ferromagnetic, magnetocaloric behavior, and possesses high Vickers hardness. We demonstrate that zigzag boron chains in this structure not only play a pivotal role in strengthening mechanical properties but also tuning the exchange correlations between manganese atoms. Nontoxic and Earth-abundant CrB-type MnB is much more incompressible and tougher than traditional ferromagnetic materials. The unique combination of high mechanical hardness, magnetism, and electrical conductivity properties makes it a particularly promising candidate for a wide range of applications.

B-Doped δ-Layers and Nanowires from Area-Selective Deposition of BCl3 on Si(100)

Atomically precise, δ-doped structures forming electronic devices in Si have been routinely fabricated in recent years by using depassivation lithography in a scanning tunneling microscope (STM). While H-based precursor/monatomic resist chemistries for incorporation of donor atoms have dominated these efforts, the use of halogen-based chemistries offers a promising path toward atomic-scale manufacturing of acceptor-based devices. Here, B-doped δ-layers were fabricated in Si(100) by using BCl₃ as an acceptor dopant precursor in ultrahigh vacuum. Additionally, we demonstrate compatibility of BCl₃ with both H and Cl monatomic resists to achieve area-selective deposition on Si. In comparison to bare Si, BCl₃ adsorption selectivity ratios for H- and Cl-passivated Si were determined by secondary ion mass spectrometry depth profiling (SIMS) to be 310(10):1 and 1529(5):1, respectively. STM imaging revealed that BCl₃ adsorbed readily on bare Si at room temperature, with SIMS measurements indicating a peak B concentration greater than 1.2(1) × 10²¹ cm^-3 with a total areal dose of 1.85(1) × 10¹⁴ cm^-2 resulting from a 30 langmuir BCl₃ dose at 150 °C. In addition, SIMS showed a δ-layer thickness of ∼0.5 nm. Hall bar measurements of a similar sample were performed at 3.0 K, revealing a sheet resistance of ρ_□ = 1.9099(4) kΩ □^-1, a hole carrier concentration of p = 1.90(2) × 10¹⁴ cm^-2, and a hole mobility of μ = 38.0(4) cm² V^-1 s^-1 without performing an incorporation anneal. Finally, 15 nm wide B δ-doped nanowires were fabricated from BCl₃ and were found to exhibit ohmic conduction. This validates the use of BCl₃ as a dopant precursor for atomic-precision fabrication of acceptor-doped devices in Si and enables development of simultaneous n- and p-type doped bipolar devices.

In Situ High-Pressure Synthesis of New Outstanding Light-Element Materials under Industrial P-T Range

High-pressure synthesis (which refers to pressure synthesis in the range of 1 to several GPa) adds a promising additional dimension for exploration of compounds that are inaccessible to traditional chemical methods and can lead to new industrially outstanding materials. It is nowadays a vast exciting field of industrial and academic research opening up new frontiers. In this context, an emerging and important methodology for the rapid exploration of composition-pressure-temperature-time space is the in situ method by synchrotron X-ray diffraction. This review introduces the latest advances of high-pressure devices that are adapted to X-ray diffraction in synchrotrons. It focuses particularly on the "large volume" presses (able to compress the volume above several mm3 to pressure higher than several GPa) designed for in situ exploration and that are suitable for discovering and scaling the stable or metastable compounds under "traditional" industrial pressure range (3-8 GPa). We illustrated the power of such methodology by (i) two classical examples of "reference" superhard high-pressure materials, diamond and cubic boron nitride c-BN; and (ii) recent successful in situ high-pressure syntheses of light-element compounds that allowed expanding the domain of possible application high-pressure materials toward solar optoelectronic and infra-red photonics. Finally, in the last section, we summarize some perspectives regarding the current challenges and future directions in which the field of in situ high-pressure synthesis in industrial pressure scale may have great breakthroughs in the next years.

Effect of disorder on superconductivity of NbN thin films studied using x-ray absorption spectroscopy

The superconducting transition temperature (T_C) of rock-salt type niobium nitride (δ- NbN) typically varies between 9 to 17 K and the theoretically predicted value of 18 K has not been achieved hitherto. The lowT_Cinδ- NbN has been assigned to some structural disorder which is always present irrespective of the microstructure (polycrystalline or epitaxial), methods or conditions adopted during the growth of NbN thin films. In this work, we investigate the atomic origin of such suppression of theT_Cinδ- NbN thin films by employing combined methods of experiments andab initiosimulations. Sputteredδ- NbN thin films with different disorder were studied using the synchrotron-based N and Nb K-edge x-ray absorption spectroscopy techniques. A strong correlation between the superconductivity and the electronic structure reconstruction was observed. The theoretical analysis revealed that under N-rich growth conditions, atomic and molecular N-interstitial defects assisted by cation vacancies form spontaneously and results into a smeared electronic structure around Fermi-level. The role of electronic smearing on theT_Cis thoroughly discussed.

Nanoscale Reactivity Mapping of a Single-Crystal Boron-Doped Diamond Particle

Boron-doped diamond (BDD) is most often grown by chemical vapor deposition (CVD) in polycrystalline form, where the electrochemical response is averaged over the whole surface. Deconvoluting the impact of crystal orientation, surface termination, and boron-doped concentration on the electrochemical response is extremely challenging. To tackle this problem, we use CVD to grow isolated single-crystal microparticles of BDD with the crystal facets (100, square-shaped) and (111, triangle-shaped) exposed and combine with hopping mode scanning electrochemical cell microscopy (HM-SECCM) for electrochemical interrogation of the individual crystal faces (planar and nonplanar). Measurements are made on both hydrogen- (H-) and oxygen (O-)-terminated single-crystal facets with two different redox mediators, [Ru(NH₃)₆]^3+/2+ and Fe(CN)₆^4-/3-. Extraction of the half-wave potential from linear sweep and cyclic voltammetric experiments at all measurement (pixel) points shows unequivocally that electron transfer is faster at the H-terminated (111) surface than at the H-terminated (100) face, attributed to boron dopant differences. The most dramatic differences were seen for [Ru(NH₃)₆]^3+/2+ when comparing the O-terminated (100) surface to the H-terminated (100) face. Removal of the H-surface conductivity layer and a potential-dependent density of states were thought to be responsible for the behavior observed. Finally, a bimodal distribution in the electrochemical activity on the as-grown H-terminated polycrystalline BDD electrode is attributed to the dominance of differently doped (100) and (111) facets in the material.

Surface-Functionalized Boron Nanoparticles with Reduced Oxide Content by Nonthermal Plasma Processing for Nanoenergetic Applications

The development of an in situ nonthermal plasma technology improved the oxidation and energy release of boron nanoparticles. We reduced the native oxide layer on the surface of boron nanoparticles (70 nm) by treatment in a nonthermal hydrogen plasma, followed by the formation of a passivation barrier by argon plasma-enhanced chemical vapor deposition (PECVD) using perfluorodecalin (C₁₀F₁₈). Both processes occur near room temperature, thus avoiding aggregation and sintering of the nanoparticles. High-resolution transmission electron microscopy (HRTEM), high-angular annular dark-field imaging (HAADF)-scanning TEM (STEM)-energy dispersive spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS) demonstrated a significant reduction in surface oxide concentration due to hydrogen plasma treatment and the formation of a 2.5 nm thick passivation coating on the surface due to PECVD treatment. These results correlated with the thermal analysis results, which demonstrated a 19% increase in energy release and an increase in metallic boron content after 120 min of hydrogen plasma treatment and 15 min of PECVD of perfluorodecalin. The PECVD coating provided excellent passivation against air and humidity for 60 days. We conclude in situ nonthermal plasma reduction and passivation lead to the amelioration of energy release characteristics and the storage life of boron nanoparticles, benefits conducive for nanoenergetic applications.

Engineering of the spin on dopant process on silicon on insulator substrate

We report on a systematic analysis of phosphorus diffusion in silicon on insulator thin film via spin-on-dopant process (SOD). This method is used to provide an impurity source for semiconductor junction fabrication. The dopant is first spread into the substrate via SOD and then diffused by a rapid thermal annealing process. The dopant concentration and electron mobility were characterized at room and low temperature by four-probe and Hall bar electrical measurements. Time-of-flight-secondary ion mass spectroscopy was performed to estimate the diffusion profile of phosphorus for different annealing treatments. We find that a high phosphorous concentration (greater than 10²⁰ atoms cm^-3) with a limited diffusion of other chemical species and allowing to tune the electrical properties via annealing at high temperature for short time. The ease of implementation of the process, the low cost of the technique, the possibility to dope selectively and the uniform doping manufactured with statistical process control show that the methodology applied is very promising as an alternative to the conventional doping methods for the implementation of optoelectronic devices.

Phonon softening and higher-order anharmonic effect in the superconducting topological insulator SrxBi2Se3

We report the anharmonic effect of the Raman-scattering spectrum in the low-carrier density superconductor SrxBi2Se3, which is dominated by the quartic term. Compared to the parent Bi2Se3, the superconducting SrxBi2Se3crystals show obvious phonon softening in the three optical phonon modesA1g1,Eg2, andA1g2. Based on the simulations by the Fano function, we present compelling evidence of the enhanced electron-phonon coupling in SrxBi2Se3for its smaller asymmetric parameterq. Moreover, an anomalous broadening and intensity antiresonance in the characteristic Raman peaks were observed as the temperature decreased to around 160 K. Thus, the superconducting topological material SrxBi2Se3represents a counterintuitive example where the electron-phonon interaction is strengthened by the accumulation of electron carriers.

Superconductivity in Compression-Shear Deformed Diamond

Diamond is a prototypical ultrawide band gap semiconductor, but turns into a superconductor with a critical temperature T_{c}≈4  K near 3% boron doping [E. A. Ekimov et al., Nature (London) 428, 542 (2004)NATUAS0028-083610.1038/nature02449]. Here we unveil a surprising new route to superconductivity in undoped diamond by compression-shear deformation that induces increasing metallization and lattice softening with rising strain, producing phonon mediated T_{c} up to 2.4-12.4 K for a wide range of Coulomb pseudopotential μ^{*}=0.15-0.05. This finding raises intriguing prospects of generating robust superconductivity in strained diamond crystal, showcasing a distinct and hitherto little explored approach to driving materials into superconducting states via strain engineering. These results hold promise for discovering superconductivity in normally nonsuperconductive materials, thereby expanding the landscape of viable nontraditional superconductors and offering actionable insights for experimental exploration.

Carbon-boron clathrates as a new class of sp3-bonded framework materials

Carbon-based frameworks composed of sp³ bonding represent a class of extremely lightweight strong materials, but only diamond and a handful of other compounds exist despite numerous predictions. Thus, there remains a large gap between the number of plausible structures predicted and those synthesized. We used a chemical design principle based on boron substitution to predict and synthesize a three-dimensional carbon-boron framework in a host/guest clathrate structure. The clathrate, with composition 2Sr@B₆C₆, exhibits the cubic bipartite sodalite structure (type VII clathrate) composed of sp³-bonded truncated octahedral C₁₂B₁₂ host cages that trap Sr²⁺ guest cations. The clathrate not only maintains the robust nature of diamond-like sp³ bonding but also offers potential for a broad range of compounds with tunable properties through substitution of guest atoms within the cages.

Metal-Organic Frameworks in Modern Physics: Highlights and Perspectives

Owing to the synergistic combination of a hybrid organic-inorganic nature and a chemically active porous structure, metal-organic frameworks have emerged as a new class of crystalline materials. The current trend in the chemical industry is to utilize such crystals as flexible hosting elements for applications as diverse as gas and energy storage, filtration, catalysis, and sensing. From the physical point of view, metal-organic frameworks are considered molecular crystals with hierarchical structures providing the structure-related physical properties crucial for future applications of energy transfer, data processing and storage, high-energy physics, and light manipulation. Here, the perspectives of metal-organic frameworks as a new family of functional materials in modern physics are discussed: from porous metals and superconductors, topological insulators, and classical and quantum memory elements, to optical superstructures, materials for particle physics, and even molecular scale mechanical metamaterials. Based on complementary properties of crystallinity, softness, organic-inorganic nature, and complex hierarchy, a description of how such artificial materials have extended their impact on applied physics to become the mainstream in material science is offered.

InN superconducting phase transition

InN superconductivity is very special among III-V semiconductors, as other III-V semiconductors (such as GaAs, GaN, InP, InAs, etc.) usually lack strong covalent bonding and thus seldom show superconductivity at low temperatures. Here, we probe the different superconducting phase transitions in InN highlighted by its microstructure. Those chemical-unstable phase-separated inclusions, such as metallic indium or In2O3, are intentionally removed by HCl acid etching. The quasi-two-dimensional vortex liquid-glass transition is observed in the sample with a large InN grain size. In contrast, the superconducting properties of InN with a small grain size are sensitive to acid etching, showing a transition into a nonzero resistance state when the temperature approaches zero. Since the value of ξ0 (the zero-temperature-limit superconducting coherence length) is close to the grain size, it is suggested that individual InN grains and intergrain coupling should be responsible for the sample-dependent InN superconducting phase transition. Our work establishes a guideline for engineering superconductivity in III-nitride.

Viewpoint: the road to room-temperature conventional superconductivity

It is a honor to write a contribution on this memorial for Sandro Massidda. For both of us, at different stages in our lives, Sandro was first and foremost a friend. We both admired his humble, playful and profound approach to life and physics. In this contribution we describe the route which permitted to meet a long-standing challenge in solid state physics, i.e. room temperature superconductivity. In less than 20 years the critical temperature of conventional superconductors, which in the last century had been widely believed to be limited to 25 K, was raised from 40 K in MgB₂ to 265 K in LaH₁₀. This discovery was enabled by the development and application of computational methods for superconductors, a field in which Sandro Massidda played a major role.

Superconductivity with high hardness in Mo3C2

This work synthesized a high hardness and superconductive polycrystalline Mo₃C₂ material by the HPHT method. Mo₃C₂ exhibits superconductivity below 8.2 K and its hardness is far higher than that of the traditionally used superconductive materials.

Semiconductor SERS of diamond

In this work, we report a favorable diamond substrate to realize semiconductor surface-enhanced Raman spectroscopy (SERS) for trace molecular probes with high sensitivity, stability, reproducibility, recyclability and universality. The boron-doped diamond (BDD) with surface hydrogenation or oxygenation has matched energy levels corresponding to the target molecules and plays a critical role in achieving SERS. The enhancement factor based on BDD substrates can reach 104-105, which approaches those obtained with most nanostructured compound semiconductors and is nearly 3-4 orders of magnitude higher than those of state-of-the-art single-element semiconductors (silicon, germanium, and graphene). The mechanism of SERS is determined to be charge transfer with vibronic coupling, which could enhance the molecular polarizability tensor. Because of its unique properties such as chemical inertness, wide bandgap, modulated doping, surface functionalization, biocompatibility, and high thermal conductivity, the single-element semiconductor diamond can serve a high-performance semiconductor SERS platform with applications in broad fields.

Bosonic Confinement and Coherence in Disordered Nanodiamond Arrays

In the presence of disorder, superconductivity exhibits short-range characteristics linked to localized Cooper pairs which are responsible for anomalous phase transitions and the emergence of quantum states such as the bosonic insulating state. Complementary to well-studied homogeneously disordered superconductors, superconductor-normal hybrid arrays provide tunable realizations of the degree of granular disorder for studying anomalous quantum phase transitions. Here, we investigate the superconductor-bosonic dirty metal transition in disordered nanodiamond arrays as a function of the dispersion of intergrain spacing, which ranges from angstroms to micrometers. By monitoring the evolved superconducting gaps and diminished coherence peaks in the single-quasiparticle density of states, we link the destruction of the superconducting state and the emergence of bosonic dirty metallic state to breaking of the global phase coherence and persistence of the localized Cooper pairs. The observed resistive bosonic phase transitions are well modeled using a series-parallel circuit in the framework of bosonic confinement and coherence.

Superconducting Ferromagnetic Nanodiamond

Superconductivity and ferromagnetism are two mutually antagonistic states in condensed matter. Research on the interplay between these two competing orderings sheds light not only on the cause of various quantum phenomena in strongly correlated systems but also on the general mechanism of superconductivity. Here we report on the observation of the electronic entanglement between superconducting and ferromagnetic states in hydrogenated boron-doped nanodiamond films, which have a superconducting transition temperature T_c ∼ 3 K and a Curie temperature T_Curie > 400 K. In spite of the high T_Curie, our nanodiamond films demonstrate a decrease in the temperature dependence of magnetization below 100 K, in correspondence to an increase in the temperature dependence of resistivity. These anomalous magnetic and electrical transport properties reveal the presence of an intriguing precursor phase, in which spin fluctuations intervene as a result of the interplay between the two antagonistic states. Furthermore, the observations of high-temperature ferromagnetism, giant positive magnetoresistance, and anomalous Hall effect bring attention to the potential applications of our superconducting ferromagnetic nanodiamond films in magnetoelectronics, spintronics, and magnetic field sensing.

Discovery of Hexagonal Structured Pd-B Nanocrystals

We report on hexagonal close-packed (hcp) palladium (Pd)-boron (B) nanocrystals (NCs) by heavy B doping into face-centered cubic (fcc) Pd NCs. Scanning transmission electron microscopy-electron energy loss spectroscopy and synchrotron powder X-ray diffraction measurements demonstrated that the B atoms are homogeneously distributed inside the hcp Pd lattice. The large paramagnetic susceptibility of Pd is significantly suppressed in Pd-B NCs in good agreement with the reduction of density of states at Fermi energy suggested by X-ray absorption near-edge structure and theoretical calculations.

Phonon-mediated high-T c superconductivity in hole-doped diamond-like crystalline hydrocarbon

We here predict by ab initio calculations phonon-mediated high-T_c superconductivity in hole-doped diamond-like cubic crystalline hydrocarbon K₄-CH (space group I2₁/3). This material possesses three key properties: (i) an all-sp³ covalent carbon framework that produces high-frequency phonon modes, (ii) a steep-rising electronic density of states near the top of the valence band, and (iii) a Fermi level that lies in the σ-band, allowing for a strong coupling with the C-C bond-stretching modes. The simultaneous presence of these properties generates remarkably high superconducting transition temperatures above 80 K at an experimentally accessible hole doping level of only a few percent. These results identify a new extraordinary electron-phonon superconductor and pave the way for further exploration of this novel superconducting covalent metal.

Comparison of fast electron transfer kinetics at platinum, gold, glassy carbon and diamond electrodes using Fourier-transformed AC voltammetry and scanning electrochemical microscopy

Complementary techniques reveal new insights on electron transfer rates at different electrode materials.

Recent progress on carbon-based superconductors

This article reviews new superconducting phases of carbon-based materials. During the past decade, new carbon-based superconductors have been extensively developed through the use of intercalation chemistry, electrostatic carrier doping, and surface-proving techniques. The superconducting transition temperature T c of these materials has been rapidly elevated, and the variety of superconductors has been increased. This review fully introduces graphite, graphene, and hydrocarbon superconductors and future perspectives of high-T c superconductors based on these materials, including present problems. Carbon-based superconductors show various types of interesting behavior, such as a positive pressure dependence of T c. At present, experimental information on superconductors is still insufficient, and theoretical treatment is also incomplete. In particular, experimental results are still lacking for graphene and hydrocarbon superconductors. Therefore, it is very important to review experimental results in detail and introduce theoretical approaches, for the sake of advances in condensed matter physics. Furthermore, the recent experimental results on hydrocarbon superconductors obtained by our group are also included in this article. Consequently, this review article may provide a hint to designing new carbon-based superconductors exhibiting higher T c and interesting physical features.

Semiconductor-inspired design principles for superconducting quantum computing

Superconducting circuits offer tremendous design flexibility in the quantum regime culminating most recently in the demonstration of few qubit systems supposedly approaching the threshold for fault-tolerant quantum information processing. Competition in the solid-state comes from semiconductor qubits, where nature has bestowed some very useful properties which can be utilized for spin qubit-based quantum computing. Here we begin to explore how selective design principles deduced from spin-based systems could be used to advance superconducting qubit science. We take an initial step along this path proposing an encoded qubit approach realizable with state-of-the-art tunable Josephson junction qubits. Our results show that this design philosophy holds promise, enables microwave-free control, and offers a pathway to future qubit designs with new capabilities such as with higher fidelity or, perhaps, operation at higher temperature. The approach is also especially suited to qubits on the basis of variable super-semi junctions.

Superconducting circuits offer great promise for quantum computing, but implementations require careful shielding from control electronics. Here, the authors take inspirations from semiconductor spin-based qubits to design Josephson junctions quantum circuits whose qubits do not require microwave control.

Ambient-Pressure Polymerization of Carbon Anions in the High-Pressure Phase Mg2C

Experimental and theoretical methods were employed to investigate the ambient-pressure, metastable phase transition pathways for Mg2C, which was recovered after high-pressure synthesis. We demonstrate that at temperatures above 600 K isolated C(4-) anions within the Mg2C structure polymerize into longer-chain carbon polyanions, resulting in the formation of the α-Mg2C3 (Pnnm) structure, which is another local energy minimum for the carbon-magnesium system. Access to the thermodynamic ground state (decomposition into graphite) was achieved at temperatures above ∼1000 K. These results indicate that recoverable high-pressure materials can serve as useful high-energy precursors for ambient-pressure materials synthesis, and they show a novel mechanism for the formation of carbon chains from methanide structures.

The Hardest Superconducting Metal Nitride

Transition–metal (TM) nitrides are a class of compounds with a wide range of properties and applications. Hard superconducting nitrides are of particular interest for electronic applications under working conditions such as coating and high stress (e.g., electromechanical systems). However, most of the known TM nitrides crystallize in the rock–salt structure, a structure that is unfavorable to resist shear strain, and they exhibit relatively low indentation hardness, typically in the range of 10–20 GPa. Here, we report high–pressure synthesis of hexagonal δ–MoN and cubic γ–MoN through an ion–exchange reaction at 3.5 GPa. The final products are in the bulk form with crystallite sizes of 50 – 80 μm. Based on indentation testing on single crystals, hexagonal δ–MoN exhibits excellent hardness of ~30 GPa, which is 30% higher than cubic γ–MoN (~23 GPa) and is so far the hardest among the known metal nitrides. The hardness enhancement in hexagonal phase is attributed to extended covalently bonded Mo–N network than that in cubic phase. The measured superconducting transition temperatures for δ–MoN and cubic γ–MoN are 13.8 and 5.5 K, respectively, in good agreement with previous measurements.

A practical guide to using boron doped diamond in electrochemical research

This article serves as a guide to those working with boron doped diamond electrodes, especially the first time user. It outlines the key material properties required when interpretating electrochemical data and provides a summary of experimental approaches to determining electrode quality.

BaC: a thermodynamically stable layered superconductor

To predict all stable compounds in the Ba-C system, we perform a comprehensive study using first-principles variable-composition evolutionary algorithm USPEX. We find that at 0 K the well-known compound BaC2 is metastable in the whole pressure range 0-40 GPa, while intercalated graphite phase BaC6 is stable at 0-19 GPa. A hitherto unknown layered orthorhombic Pbam phase of BaC has structure consisting of alternating layers of Ba atoms and layers of stoichiometry Ba2C3 containing linear C3 groups and is predicted to be stable in the pressure range 3-32 GPa. From our electron-phonon coupling calculations, the newly found BaC compound is a phonon-mediated superconductor and has a critical superconductivity temperature Tc of 4.32 K at 5 GPa. This compound is dynamically stable at 0 GPa and therefore may be quenchable under normal conditions.