Metal-bosonic insulator-superconductor transition in boron-doped granular diamond

Granularity mediated multiple reentrances with negative magnetoresistance in disordered TiN thin films

Granular superconductors are the common examples of experimentally accessible model systems which can be used to explore various fascinating quantum phenomena that are fundamentally important and technologically relevant. One such phenomenon is the occurrence of reentrant resistive states in granular superconductors. Here, we report the observation of multiple reentrant resistive states for a disordered TiN thin film in its temperature and magnetic field dependent resistance measurements, R(T) and R(B), respectively. At each of the peak-temperatures corresponding to the zero-field R(T), a resistance peak appears in the R(B) around zero field which leads to a negative magnetoresistance (MR) region in its surrounding. These low-field negative MR regions appear for both perpendicular and parallel field directions with relatively higher amplitude and larger width for the parallel field. By adopting a granularity-based model, we show that the superconducting fluctuations in granular superconductors may lead to the observed reentrant states and the corresponding negative MR. Here, we propose that the reduction in the density of states in the fermionic channel due to the formation of Cooper pairs leads to the reentrant resistive state and the competition between the conduction processes in the single particle and Cooper channels result into the multiple resistive reentrances.

Unconventional Giant "Magnetoresistance" in Bosonic Semiconducting Diamond Nanorings

The emergence of superconductivity in doped insulators such as cuprates and pnictides coincides with their doping-driven insulator-metal transitions. Above the critical doping threshold, a metallic state sets in at high temperatures, while superconductivity sets in at low temperatures. An unanswered question is whether the formation of Cooper pairsin a well-established metal will inevitably transform the host material into a superconductor, as manifested by a resistance drop. Here, this question is addressed by investigating the electrical transport in nanoscale rings (full loops) and half loops manufactured from heavily boron-doped diamond. It is shown that in contrast to the diamond half-loops (DHLs) exhibiting a metal-superconductor transition, the diamond nanorings (DNRs) demonstrate a sharp resistance increase up to 430% and a giant negative "magnetoresistance" below the superconducting transition temperature of the starting material. The finding of the unconventional giant negative "magnetoresistance", as distinct from existing categories of magnetoresistance, that is, the conventional giant magnetoresistance in magnetic multilayers, the colossal magnetoresistance in perovskites, and the geometric magnetoresistance in semiconductor-metal hybrids, reveals the transformation of the DNRs from metals to bosonic semiconductors upon the formation of Cooper pairs. DNRs like these could be used to manipulate Cooper pairs in superconducting quantum devices.

Effect of Impurity Scattering on Percolation of Bosonic Islands and Superconductivity in Fe Implanted NbN Thin Films

A reentrant temperature dependence of the thermoresistivity ρxx(T) between an onset local superconducting ordering temperature Tloconset and a global superconducting transition at T=Tglooffset has been reported in disordered conventional 3-dimensional (3D) superconductors. The disorder of these superconductors is a result of either an extrinsic granularity due to grain boundaries, or of an intrinsic granularity ascribable to the electronic disorder originating from impurity dopants. Here, the effects of Fe doping on the electronic properties of sputtered NbN layers with a nominal thickness of 100 nm are studied by means of low-T/high-μ0H magnetotransport measurements. The doping of NbN is achieved via implantation of 35 keV Fe ions. In the as-grown NbN films, a local onset of superconductivity at Tloconset=15.72K is found, while the global superconducting ordering is achieved at Tglooffset=15.05K, with a normal state resistivity ρxx=22μΩ·cm. Moreover, upon Fe doping of NbN, ρxx=40μΩ·cm is estimated, while Tloconset and Tglooffset are measured to be 15.1 K and 13.5 K, respectively. In Fe:NbN, the intrinsic granularity leads to the emergence of a bosonic insulator state and the normal-metal-to-superconductor transition is accompanied by six different electronic phases characterized by a N-shaped T dependence of ρxx(T). The bosonic insulator state in a s-wave conventional superconductor doped with dilute magnetic impurities is predicted to represent a workbench for emergent phenomena, such as gapless superconductivity, triplet Cooper pairings and topological odd frequency superconductivity.

High-Pressure Synthesis of Superconducting Sn3S4 Using a Diamond Anvil Cell with a Boron-Doped Diamond Heater

High-pressure techniques open exploration of functional materials in broad research fields. An established diamond anvil cell with a boron-doped diamond heater and transport measurement terminals has performed the high-pressure synthesis of a cubic Sn₃S₄ superconductor. X-ray diffraction and Raman spectroscopy reveal that the Sn₃S₄ phase is stable in the pressure range of P > 5 GPa in a decompression process. Transport measurement terminals in the diamond anvil cell detect a metallic nature and superconductivity in the synthesized Sn₃S₄ with a maximum onset transition temperature (T_c^onset) of 13.3 K at 5.6 GPa. The observed pressure-T_c relationship is consistent with that from the first-principles calculation. The observation of superconductivity in Sn₃S₄ opens further materials exploration under high-temperature and -pressure conditions.

Film-thickness-driven superconductor to insulator transition in cuprate superconductors

The superconductor-insulator transition induced by film thickness control is investigated for the optimally doped cuprate superconductor La_1.85Sr_0.15CuO₄. Epitaxial thin films are grown on an almost exactly matched substrate LaAlO₃ (001). Despite the wide thickness range of 6 nm to 300 nm, all films are grown coherently without significant relaxation of the misfit strain. Electronic transport measurement exhibits systematic suppression of the superconducting phase by reducing the film thickness, thereby inducing a superconductor-insulator transition at a critical thickness of ~10 nm. The emergence of a resistance peak preceding the superconducting transition is discussed based on the weak localization. X-ray photoelectron spectroscopy results show the possibility that oxygen vacancies are present near the interface.

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.

Diamond thin films: giving biomedical applications a new shine

Progress made in the last two decades in chemical vapour deposition technology has enabled the production of inexpensive, high-quality coatings made from diamond to become a scientific and commercial reality. Two properties of diamond make it a highly desirable candidate material for biomedical applications: first, it is bioinert, meaning that there is minimal immune response when diamond is implanted into the body, and second, its electrical conductivity can be altered in a controlled manner, from insulating to near-metallic. In vitro, diamond can be used as a substrate upon which a range of biological cells can be cultured. In vivo, diamond thin films have been proposed as coatings for implants and prostheses. Here, we review a large body of data regarding the use of diamond substrates for in vitro cell culture. We also detail more recent work exploring diamond-coated implants with the main targets being bone and neural tissue. We conclude that diamond emerges as one of the major new biomaterials of the twenty-first century that could shape the way medical treatment will be performed, especially when invasive procedures are required.

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.

Reentrant Resistive Behavior and Dimensional Crossover in Disordered Superconducting TiN Films

A reentrant temperature dependence of the normal state resistance often referred to as the N-shaped temperature dependence, is omnipresent in disordered superconductors - ranging from high-temperature cuprates to ultrathin superconducting films - that experience superconductor-to-insulator transition. Yet, despite the ubiquity of this phenomenon its origin still remains a subject of debate. Here we investigate strongly disordered superconducting TiN films and demonstrate universality of the reentrant behavior. We offer a quantitative description of the N-shaped resistance curve. We show that upon cooling down the resistance first decreases linearly with temperature and then passes through the minimum that marks the 3D-2D crossover in the system. In the 2D temperature range the resistance first grows with decreasing temperature due to quantum contributions and eventually drops to zero as the system falls into a superconducting state. Our findings demonstrate the prime importance of disorder in dimensional crossover effects.

Formation of Boron-Carbon Nanosheets and Bilayers in Boron-Doped Diamond: Origin of Metallicity and Superconductivity

The insufficient data on a structure of the boron-doped diamond (BDD) has frustrated efforts to fully understand the fascinating electronic properties of this material and how they evolve with doping. We have employed X-ray diffraction and Raman scattering for detailed study of the large-sized BDD single crystals. We demonstrate a formation of boron-carbon (B-C) nanosheets and bilayers in BDD with increasing boron concentration. An incorporation of two boron atoms in the diamond unit cell plays a key role for the B-C nanosheets and bilayer formation. Evidence for these B-C bilayers which are parallel to {111} planes is provided by the observation of high-order, super-lattice reflections in X-ray diffraction and Laue patterns. B-C nanosheets and bilayers minimize the strain energy and affect the electronic structure of BDD. A new shallow acceptor level associated with B-C nanosheets at ~37 meV and the spin-orbit splitting of the valence band of ~6 meV are observed in electronic Raman scattering. We identified that the superconducting transitions occur in the (111) BDD surfaces only. We believe that the origin of Mott and superconducting transitions is associated with the two-dimensional (2D) misfit layer structure of BDD. A model for the BDD crystal structure, based on X-ray and Raman data, is proposed and confirmed by density functional theoretical calculation.

Template synthesis of single-phase δ(3)-MoN superconducting nanowires

We demonstrate a new and effective method of producing single-phase superconducting δ3-MoN nanowires from bundled Mo6SyIz (8.2 ≤ y + z ≤ 10) nanowire templates in the presence of ammonia gas. Magnetic susceptibility and electrical resistance measurements confirm single-phase material synthesis. Measurements of four-contact resistance on single wires with diameters above 100 nm in a magnetic field are used to determine the critical field, while diameter dependence and magnetization measurements are used to investigate the homogeneity of the nanowires.