Metallic Regime Research Articles

Effect of asymmetric power distribution in bipolar reactive sputtering on the optoelectronic and microstructure properties of titanium dioxide for solar water splitting

Titanium dioxide is one of the most commonly used materials for solar water splitting, because of its excellent corrosion resistance. However, its optoelectronic properties are not sufficient to achieve high solar to hydrogen conversion. It is known that the choice of deposition method has a great influence on the optoelectronic and structural properties of titanium dioxide.Here we combine two different sputtering configurations and vary the process parameters to obtain thin films of black TiOx. Bipolar magnetron sputtering from two metallic titanium targets is used in combination with an oxygen partial pressure control via a closed-loop residual gas monitoring system. The power distribution between the two targets in the bipolar sputtering mode is thereby varied from sample to sample, while the oxygen partial pressure remains constant. This causes one target to shift into the oxidic regime, while the other target shifts into the metallic regime. Because the metallic target has a higher sputter yield the process leads to off-stochiometric samples. These samples exhibit a broad-band absorption and an increased photoelectrochemical activity (up to five times) compared to samples without the modification of the power distribution. The results are discussed based on the optoelectronic and microstructural analysis of the thin films.

Ultra Long Period Cepheids: Observation and Theory

Ultra Long Period Cepheids are becoming a very interesting and important topic thanks to the contribution that they can give to understanding the current tension existing between the early-universe and local Hubble constant measurements. These bright pulsating variables are observable up to cosmological distances (larger than 100 Mpc) allowing us, in principle, to measure the Hubble constant without the need for secondary indicators, thus reducing the possible systematic errors in the calibration of the extragalactic distance scale. The Ultra Long Period Cepheids also represent a useful tool for obtaining information on the star formation history of the host galaxy and a challenge for the evolutionary and pulsational models, particularly in the very metal poor regime. In this paper, the largest known ULP sample, consisting of 72 objects, including 10 new candidates, is analyzed to give an observational and theoretical overview of their role as distance indicators and of their evolutionary properties.

Asteroseismology of the multiple stellar populations in the globular cluster M4

We present a new asteroseismic analysis of the stars in the globular cluster (GC) M4 based on the data collected by the K2 mission. We report the detection of solar-like oscillation in 37 stars, 32 red giant branch (RGB) and six red horizontal branch (rHB) stars, which is the largest sample for this kind of study in GCs up to date. Combining information from asteroseismology and multi-band photometry, we estimate both the masses and the radii of our targets. Our estimates are in agreement with independent sources, serving as a crucial verification of asteroseismology in the low metallicity regime. As M4 is an old GC, it hosts multiple stellar populations differing in light element abundances and in helium mass fraction. This generates a mass difference between the populations along the RGB, which in the case of M4 is estimated to be 0.017 M⊙. With this wealth of information, we can assign population membership and estimate the average mass of the stellar populations; however, the current uncertainties do not allow us to resolve this mass difference. The population membership and the seismic data of RGB and HB stars allow us, however, to assess the integrated mass loss along the RGB of the first generation stars in the cluster. We obtain ΔM = 0.227 ± 0.028 M⊙, which is in good agreement with independent estimates. Finally, we observe the presence of a statistically significant mass-temperature gradient in the rHB stars. This represents the first direct, model-independent observation of the colour-temperature-mass correlation predicted by the theory.

Diagnostics for PopIII galaxies and direct collapse black holes in the early universe

ABSTRACT Forthcoming observational facilities will make the exploration of the early universe routine, likely probing large populations of galaxies at very low metallicities. It will therefore be important to have diagnostics that can solidly identify and distinguish different classes of objects in such low metallicity regimes. We use new photoionization models to develop diagnostic diagrams involving various nebular lines. We show that combinations of these diagrams allow the identification and discrimination of the following classes of objects in the early universe: PopIII and direct collapse black holes (DCBH) in pristine environments, PopIII and DCBH embedded in slightly enriched interstellar medium (ISM; $\rm Z\sim 10^{-5}-10^{-4}$), and (metal poor) PopII and active galactic nucleus in enriched ISM. Diagnostics involving rest-frame optical lines (that will be accessible by James Webb Space Telescope) have a better discriminatory power but also rest-frame ultraviolet diagnostics can provide very useful information. Interestingly, we find that metal lines such as [O iii]λ5007 and C ivλ1549 can remain relatively strong (about a factor of 0.1–1 relative H β and He iiλ1640, respectively), even in extremely metal poor environments ($\rm Z\sim 10^{-5}-10^{-4}$), which could be embedding PopIII galaxies and DCBH.

Quantum simulator of extended bipartite Hubbard model with broken sublattice symmetry: Magnetism, correlations, and phase transitions

We describe here a quantum simulator of extended bipartite Hubbard model with broken sublattice symmetry. The simulator consists of a structured lateral gate confining two dimensional electrons in a quantum well into artificial minima arranged in a hexagonal lattice. The sublattice symmetry breaking is generated by forming an artificial triangular graphene quantum dot (ATGQD) with zigzag edges. The resulting extended Hubbard model generates tunable ratio of tunneling strength to electron-electron interactions and of sublattice symmetry with control over shape. The validity of the simulator is confirmed for small systems using mean-field and exact diagonalization many-body approaches which show that the ground state changes from a metallic to an antiferromagnetic (AF) phase by varying the distance between sites or depth of the confining potential. The one-electron spectrum of these triangular dots contains a macroscopically degenerate shell at the Fermi level. The shell persists at the mean-field level for weak interactions (metallic phase) but disappears for strong interactions, in the AF phase. We determine the effects of electron-electron interactions on the ground state, the total spin, and the excitation spectrum as a function of filling of the ATGQD. We find that the half-filled charge neutral shell leads to a partially spin polarized state in both metallic and AF regimes in accordance with Liebs theorem. In both regimes a relatively large gap separates the spin polarized ground state to the first excited many-body state at half filling of the degenerate shell. By adding or removing an electron, this gap drops dramatically, and alternate total spin states emerge with energies nearly degenerate to a spin polarized ground state.

Interplay of band geometry and topology in ideal Chern insulators in the presence of external electromagnetic fields

Ideal Chern insulating phases arise in two-dimensional systems with broken time-reversal symmetry. They are characterized by having nearly-flat bands, and a uniform quantum geometry -- which combines the Berry curvature and quantum metric -- and by being incompressible. In this work, we analyze the role of the quantum geometry in ideal Chern insulators aiming to describe transport in presence of external out-of-plane magnetic and electric fields. We firstly show that in the absence of external perturbations, novel Berry connections appear in ideal Chern insulating phases. Secondly, we provide a detailed analysis of the deformation of the quantum geometry once weak out-of-plane magnetic fields are switched on. The perturbed Berry curvature and quantum metric provide an effective quantum geometry, which is analyzed in the insulating regime and provides an application of our novel connections. The conditions under which the Girvin-MacDonald-Platzman algebra is realized in this situation are discussed. Furthermore, an investigation of electrical transport due to the new effective quantum geometry is presented once an electric field is added. Restricting to the case of two bands in the metallic regime the quantum metric appears as measurable quantum mechanical correction in the Hall response. Our findings can be applied, for instance, to rhombohedral trilayer graphene at low energies.

Evolution of magnetic and transport properties in the Cu-doped pyrochlore iridateEu2(Ir1−xCux)2O7

We have investigated the effect of Cu substitution in Eu2(Ir1-xCux)2O7 with the help of magnetic and transport property measurements. XPS measurement reveals that each Cu2+ converts Ir4+ to double amount of Ir5+ ions. The metal-insulator transition temperature (T_MI) is obtained around 120 K. In the insulating phase, at lower temperature below 50 K, the temperature dependent resistivity follows a power law dependence and the magnitude of the exponent increases with Cu concentrations. The temperature dependent thermopower is observed to follow the electrical resistivity down to 50 K, except for a sudden drop in thermopower at temperature below 50 K. We find negligible Hall voltage in the metallic regime of the samples but a sudden Hall voltage is developed below 50 K. We observe bifurcation in zero field cooled and field cooled (ZFC-FC) magnetization below irreversibility temperature, exchange bias and negative magnetoresistance at 3 K and the magnitude of all these properties increases with Cu concentrations. In the insulating region (below 6 K) there exists a linear specific heat and its coefficient decreases with Cu doping which indicates the reduction of spinon contribution with Cu doping.

Non-Fermi liquid phase and linear-in-temperature scattering rate in overdoped two-dimensional Hubbard model

SignificanceMost metals display an electron-scattering rate [Formula: see text] that follows [Formula: see text] at low temperatures, as prescribed by Fermi liquid theory. But there are important exceptions. One of the most prominent examples is the "strange" metal regime in overdoped cuprate supercondcutors, which exhibits a linear T dependence of the scattering rate [Formula: see text] that reaches a putative Planckian limit. Here, using cutting-edge computational approaches, we show that T-linear scattering rate can emerge from the overdoped Hubbard model at low temperatures. Our results agree with cuprate experiments in various aspects but challenge the Planckian limit. Finally, by identifying antiferromagnetic fluctuations as the physical origin of the T-linear scattering rate, we discover the microscopic mechanism of strange metallicity in cuprates.

Large-scale behavior of energy spectra of the quantum random antiferromagnetic Ising chain with mixed transverse and longitudinal fields

In recent years it became clear that the metallic regime of systems that exhibit a many body localization (MBL) behavior show properties which are quite different than the vanilla metallic region of the single particle Anderson regime. Here we show that the large scale energy spectrum of a canonical microscopical model featuring MBL, displays a non-universal behavior at intermediate scales, which is distinct from the deviation from universality seen in the single particle Anderson regime. The crucial step in revealing this behavior is a global unfolding of the spectrum performed using the singular value decomposition (SVD) which takes into account the sample to sample fluctuations of the spectra. The spectrum properties may be observed directly in the singular value amplitudes via the scree plot, or by using the SVD to unfold the spectra and then perform a number of states variance calculation. Both methods reveal an intermediate scale of energies which follow super Posissonian statistics.

Structure and transport properties of the quasi-one-dimensional telluride Ta1.2Os0.8Te4

We report the crystal growth, structural characterization, and physical properties of an off-stoichiometric van der Waals telluride ${\mathrm{Ta}}_{1.2}{\mathrm{Os}}_{0.8}{\mathrm{Te}}_{4}$, which is isostructural with the noncentrosymmetric ${\mathrm{WTe}}_{2}$, a prototypical type-II Weyl semimetal. Transport measurements have been carried out in magnetic fields up to 9 T. A low-temperature resistivity upturn is observed along all three crystallographic directions at zero field. The along-chain ${\ensuremath{\rho}}_{a}$ and interchain ${\ensuremath{\rho}}_{b}$ are found to increase logarithmically at low temperatures, while in the low-$T$ metallic regime, both longitudinal and transverse transports support a three-dimensional Fermi liquid ground state. As temperature is further increased, successive crossovers to electronic states of reduced dimensionality can be seen in both interchain resistivities. The transverse magnetoresistance is weak but anisotropic, the analysis of which indicates the Kohler's rule is severely violated over a large temperature range. The positive linear field dependence of Hall resistivity at selected temperatures indicates the dominance of single-band hole-type charge carriers, consistent with the results of the angle-resolved photoemission spectroscopy. Our results not only establish this transition-metal telluride family as a good system for studying various dimensional crossover phenomena, but also offer the potential for exploring topologically nontrivial phases with reduced dimensionality, given its structural similarity to ${\mathrm{WTe}}_{2}$.

F-electron hybridised Fermi surface in magnetic field-induced metallic YbB12

The nature of the Fermi surface observed in the recently discovered family of unconventional insulators starting with SmB6 is a subject of intense inquiry. Here we shed light on this question by accessing quantum oscillations in the high magnetic field-induced metallic regime above ≈47 T in YbB12, which we compare with the unconventional insulating regime. In the field-induced metallic regime, we find prominent quantum oscillations in the electrical resistivity characterised by multiple frequencies and heavy effective masses. The close similarity in Lifshitz-Kosevich low-temperature growth of quantum oscillation amplitude in insulating YbB12 to field-induced metallic YbB12, points to an origin of quantum oscillations in insulating YbB12 from in-gap neutral low energy excitations. Higher frequency Fermi surface sheets of heavy quasiparticle effective mass emerge in the field-induced metallic regime of YbB12 in addition to multiple heavy Fermi surface sheets observed in both insulating and metallic regimes. f-electron hybridisation is thus observed to persist from the unconventional insulating to the field-induced metallic regime of YbB12, in contrast to the unhybridised conduction electron Fermi surface observed in unconventional insulating SmB6. Our findings thus require an alternative model for YbB12, of neutral in-gap low energy excitations, wherein the f-electron hybridisation is retained.

Exploration of trivial and nontrivial electronic phases and of collinear and noncollinear magnetic phases in low-spin d5 perovskites

The $4d$ and $5d$ transition metal oxides have become important members of the emerging quantum materials family due to competition between onsite Coulomb repulsion ($U$) and spin-orbit coupling (SOC). Specifically, the systems with $d^5$ electronic configuration in an octahedral environment are found to be capable of posessing invariant semimetallic state and perturbations can lead to diverse magnetic phases. In this work, by formulating a multi-band Hubbard model and performing SOC tunable DFT+$U$ calculations on a prototype SrIrO$_3$ and extending the analysis to other iso-structural and isovalent compounds, we present eight possible electronic and magnetic configurations in the $U$-SOC phase diagram that can be observed in the family of low-spin $d^5$ perovskites. They include the protected Dirac semimetal state, metal and insulator regimes, collinear and noncollinear spin ordering. The latter is explained through connecting hopping interactions to the rotation and tilting of the octahedra as observed in GdFeO$_3$. Presence of several soft phase boundaries makes the family of $d^5$ perovskites an ideal platform to study electronic and magnetic phase transitions under external stimuli.

Anomalous Nernst Effect and Giant Magnetostriction in Microwave-Synthesized La0.5Sr0.5CoO3

The anomalous Nernst effect (ANE) is the generation of a thermoelectric voltage transverse to the imposed temperature gradient and applied magnetic field in a ferromagnet. This effect enables magnetic metals and alloys to be exploited for heat to electrical energy conversion. Available reports on the ANE in perovskite oxides are very few compared to those on metallic alloys. Here, we report two independent phenomena, ANE and magnetostriction, in the perovskite oxide La0.5Sr0.5CoO3 synthesized by microwave irradiation. La0.5Sr0.5CoO3 is metallic and ferromagnetic below TC = 247 K with electrons as the majority charge carriers. Magnetic field dependence of the Nernst coefficient (Sxy) at a fixed temperature below TC resembles magnetization. The anomalous contribution to Sxy shows a maximum value of ∼0.21 μV/K at 180 K for H = 50 kOe. Analysis of the data indicates that the mechanism of the ANE in this sample is skew scattering in a bad metallic regime. The magnetostriction has a positive sign, that is, the length of the sample elongates along the field direction, and the fractional change in the length is 500 ppm in a magnetic field of 50 kOe for T ≤ 40 K, which is a giant value among 3d perovskite oxides. It is suggested that in addition to field-induced spin–orbit coupling, structural changes also contribute to the observed giant magnetostriction in this compound. The coexistence of both the ANE and the giant magnetostriction in a single material is interesting for thermal energy harvesting and actuator applications.

Recent Progress in Conducting Polymer Composite/Nanofiber-Based Strain and Pressure Sensors.

The Conducting of polymers belongs to the class of polymers exhibiting excellence in electrical performances because of their intrinsic delocalized π- electrons and their tunability ranges from semi-conductive to metallic conductive regime. Conducting polymers and their composites serve greater functionality in the application of strain and pressure sensors, especially in yielding a better figure of merits, such as improved sensitivity, sensing range, durability, and mechanical robustness. The electrospinning process allows the formation of micro to nano-dimensional fibers with solution-processing attributes and offers an exciting aspect ratio by forming ultra-long fibrous structures. This review comprehensively covers the fundamentals of conducting polymers, sensor fabrication, working modes, and recent trends in achieving the sensitivity, wide-sensing range, reduced hysteresis, and durability of thin film, porous, and nanofibrous sensors. Furthermore, nanofiber and textile-based sensory device importance and its growth towards futuristic wearable electronics in a technological era was systematically reviewed to overcome the existing challenges.

Magnetisation and magneto-transport measurements on CeBi single crystals

ABSTRACT We report the synthesis of CeBi single crystals out of Bi self-flux and a systematic study of the magnetic and transport properties with varying temperature and applied magnetic fields. From these and data, we could assemble the field-temperature (H−T) phase diagram for CeBi and visualise the three-dimensional M−T−H surface. In the phase diagram, we identify regions with well-defined magnetisation values and identify a new phase region. The magnetoresistance (MR) in the low-temperature regime shows, above 6 T a power-law, non-saturated behaviour with large MR ( at 2 K and 13.95 T), along with Shubnikov–de Haas oscillations. With increasing temperatures, MR decreases, and then becomes negative for K. This crossover in MR seems to be unrelated to any specific magnetic or metamagnetic transitions, but rather is associated with changing from a low-temperature normal metal regime with little or no scattering from the Ce moments and an anomalously large MR, to increased scattering from local Ce moments and a negative MR as temperature increases.

Temporal studies of titanium ionised density fraction in reactive HiPIMS with nitrogen admixture

Temporal evolutions of the ground state number densities of titanium atoms and ions and the ionised density fraction during pulse on-time are experimentally investigated for the high-power impulse magnetron sputtering process in an Ar and Ar/N2 atmosphere. For the study, two distinct pulses with the same pulse length at a constant average power but different pulse off-times were selected. In both conditions, four representative points within the hysteresis curve were chosen, and the time evolutions of sputtered species densities were investigated in both the target and the substrate regions. At the pulse beginning, a high density of residual particles sputtered during the previous pulse is present in both studied regions. In the target region, in conditions of longer pulse off-time and thus higher peak discharge current, 95% of sputtered particles are ionised in metallic, transition and compound regimes. With shorter pulse off-time conditions, and thus a lower peak discharge current, the ionised density fraction is the lowest in the metallic regime; in the transition and compound regimes, it exceeds 80%. In the substrate region, the temporal evolution of sputtered species densities differs from that obtained in the target region. After the pulse ignition, titanium atoms and ions remaining near the substrate from the previous pulse are pushed away. In the metallic regime, the sputtered atoms refill the substrate region already during the pulse on-time in both the low- and high-current conditions. In the high-current conditions, the titanium ions arrive at the substrate together with the sputtered atoms; however, in the low-current conditions, the titanium ion density decreases during the pulse on-time, and the titanium ions arrive later during the pulse off-time. In the transition and compound regimes, both densities steadily decrease during the pulse on-time and the titanium atoms and ions arrive at the substrate during the pulse off-time.

Efficient Slave-Boson Approach for Multiorbital Two-Particle Response Functions and Superconductivity

We develop an efficient approach for computing two-particle response functions and interaction vertices for multiorbital strongly correlated systems based on fluctuation around rotationally-invariant slave-boson saddle-point. The method is applied to the degenerate three-orbital Hubbard-Kanamori model for investigating the origin of the s-wave orbital antisymmetric spin-triplet superconductivity in the Hund's metal regime, previously found in the dynamical mean-field theory studies. By computing the pairing interaction considering the particle-particle and the particle-hole scattering channels, we identify the mechanism leading to the pairing instability around Hund's metal crossover arises from the particle-particle channel, containing the local electron pair fluctuation between different particle-number sectors of the atomic Hilbert space. On the other hand, the particle-hole spin fluctuations induce the s-wave pairing instability before entering the Hund's regime. Our approach paves the way for investigating the pairing mechanism in realistic correlated materials.

Skewed non-Fermi liquids and the Seebeck effect

We consider non-Fermi liquids in which the inelastic scattering rate has an intrinsic particle-hole asymmetry and obeys $\omega/T$ scaling. We show that, in contrast to Fermi liquids, this asymmetry influences the low-temperature behaviour of the thermopower even in the presence of impurity scattering. Implications for the unconventional sign and temperature dependence of the thermopower in cuprates in the strange metal (Planckian) regime are emphasized.

Charge density wave breakdown in a heterostructure with electron-phonon coupling

Understanding the influence of vibrational degrees of freedom on transport through a heterostructure poses considerable theoretical and numerical challenges. In this work, we use the density-matrix renormalization group (DMRG) method together with local basis optimization (LBO) to study the half-filled Holstein model in the presence of a linear potential, either isolated or coupled to tight-binding leads. In both cases, we observe a decay of charge-density-wave (CDW) states at a sufficiently strong potential strength. Local basis optimization selects the most important linear combinations of local oscillator states to span the local phonon space. These states are referred to as optimal modes. We show that many of these local optimal modes are needed to capture the dynamics of the decay, that the most significant optimal mode on the initially occupied sites remains well described by a coherent-state typical for small polarons, and that those on the initially empty sites deviate from the coherent-state form. Additionally, we compute the current through the structure in the metallic regime as a function of voltage. For small voltages, we reproduce results for the Luttinger parameters. As the voltage is increased, the effect of larger electron-phonon coupling strengths becomes prominent. Further, the most significant optimal mode remains almost unchanged when going from the ground state to the current-carrying state in the metallic regime.

Nanostructure, compositional and magnetic studies of Poly(aniline)–CoFe[formula omitted]O[formula omitted] nanocomposites

Magnetic and conducting composites of cobalt ferrite nanoparticles and poly(aniline) have been synthesized in acid media by the oxidative polymerization of aniline in the presence of a dispersion of magnetic nanoparticles, using Fe(III) ions as an oxidizing agent. The materials have been characterized by X-ray diffraction studies, X-ray fluorescence, energy dispersive X-ray spectroscopy, thermogravimetric analysis, transmission electron microscopy, spectrum-imaging electron energy loss spectroscopy, conductivity and DC magnetization measurements. The composite composition and morphology depend on the aniline/magnetic nanoparticle molar ratio used during the synthesis; for composites with a low polymer content, poly(aniline) (PANI), growth occurs preferably in the vicinity of the magnetic nanoparticles, while for composites with a higher polymer content, the magnetic nanoparticles are embedded in the PANI matrix. It was also observed that iron is incorporated in the composites during their preparation. The electrical conductivities of all the synthesized materials have values in the range 1–100 S cm−1, i.e., in the metallic regime. Also, the composites feature good magnetic properties, with ferrimagnetic behavior for CoFe2O4 and the composites. The coercivity decreases as the PANI/ferrite ratio increases, indicating that poly(aniline) affects the magnetic anisotropy. All these results suggest that the morphology and the composition affect the magnetic behavior.