Low Density Of Electronic States Research Articles

Topological semimetal interface resistivity scaling for vertical interconnect applications

In this work, we explore the electron scattering characteristics at interfaces between normal metals and topological semimetals in bulk as well as in thin film structures. We consider Cu/Ta and CoSi/Ta as representative metal/metal and topological semimetal/metal interface structures, respectively. For bulk interface structures, we find that metal/topological semimetal interfaces have roughly 20× higher interfacial resistivity than normal metal/metal interfaces primarily due to the low electronic density of states, the Fermi level in bulk topological semimetals. For thin films, we find that normal metal/metal interfacial resistivity shows a weak dependence on film thickness and is generally close to the corresponding bulk value. Interfaces between surface-conduction dominated topological semimetals, such as CoSi and normal metals in thin films, however, show decreasing interfacial resistivity with decreasing film thickness. This apparent reduction in interface resistivity originates from the surface-dominated transport, where the total transmission across the interface varies little with reduced film thickness, yielding an effective increase in interface conductivity at smaller dimensions. These results suggest that topological semimetals may be attractive candidates for next-generation interconnect materials with critically small dimensions where interfaces with other metals are ubiquitous.

A review of the Kondo insulator materials class of strongly correlated electron systems: Selected systems and anomalous behavior

Studies of strongly correlated electron systems have been at the forefront of research in condensed matter physics ever since the discovery of the co-existence of strong Pauli-paramagnetism and superconductivity in the archetypal heavy-fermion compound CeCu2Si2 in 1979. The construct of correlated electron physics typifies the behavior of thermal and electronic properties of a material when the Coulomb interaction between conduction electrons exceeds the electron kinetic energy at a given thermal energy and redefines in remarkable ways our understanding of the behavior of a metal near its ground state. While correlated electron behavior has by now been demonstrated in a variety of different types of materials, Kondo systems in particular are arguably the most intensively studied among these. The Kondo interaction is used to describe the effect that a spin-magnetic ion has on its environment when immersed in the conduction electron sea of a metal. The localized spin of the Kondo ion polarizes nearby conduction electrons to form a so-called Kondo cloud, which acts to screen and magnetically (partially) neutralize the localized spin. In Kondo systems, the low-temperature behavior is prone to the formation of heavy fermions, which is the term given to quasiparticle excitations that define the emergence of effective electron masses that can be up to three orders of magnitude greater than that of a free electron. The Kondo effect presents itself in three guises: first, the single-ion Kondo state which is found in a metal having only a small amount of magnetic ions dissolved into it; second, the incoherent Kondo state in materials where there is a Kondo ion in every crystallographic unit cell of the material, but the Kondo ions remain incoherent or uncoupled from each other; and third, the coherent Kondo lattice state which manifests itself toward low temperatures where the interaction between Kondo ions becomes comparable to the thermal energy of conduction electrons that mediate magnetic exchange between Kondo ions. In a small number of cases, the outcome of a material condensing into the Kondo state turns out to be the peculiar formation of a very narrow energy band gap at the metallic Fermi energy. Such a band gap has significant consequences in practically all of the physical properties of a material that stem from the behavior of conduction electrons in proximity of the Fermi energy. This is most readily seen in electrical resistivity, heat capacity, and magnetic susceptibility. The band gapping gives cause to the term Kondo insulator (also referred to as Kondo semimetal or heavy-fermion semiconductor) that is used to describe this exceptional variety of Kondo systems. The term Kondo insulator is in general use although most Kondo insulators have a small but finite electrical conduction in the low-temperature limit where Kondo screening may be accomplished to its full extent. While the Kondo lattice ground state is exemplified by a very high density of electronic states at the Fermi energy, Kondo insulators, on the other hand, have, by virtue of narrow band gapping, a low density of electronic states. It remains a counter-intuitive observation, therefore, that despite their low density of states, Kondo insulators have curiously strong spin polarization energy scales and accompanying high values of their Kondo temperature, being the defining quantity which acts as an organizing principle in their temperature-dependent physical properties. In this article, we review the fundamentals of the Kondo insulating state, and we discuss the theoretical principles of what is presently understood about the formation of a Kondo insulator. The experimental results of a selected number of examples that have gained prominence in this class of materials are compared to each other in order to seek out similarities that may help deepen our understanding of the Kondo insulating state.

Novel borides of the boron filled β-Mn-type structure

The existence of a boron-filled β-Mn-type compound has been observed for the first time in the Pd-Cu-B system. The crystal structure of this phase, its homogeneity field and phase equilibria in the relevant compositional range have been examined by electron probe microanalysis, and single crystal and powder X-ray diffraction of alloys annealed at 650 °C for 240 h. The B-filled β-Mn-type structure has been found to exist within ∼50 at% - ∼57 at% Pd at 60–80% boron population in [Pd6] octahedral voids (B in 4a). Metal atom ordering (Mo3Al2C-type structure, Pd in 12d, Cu in 8c, B in 4a) has been proved from single crystal data in Pd3Cu2B0.8 at 80% boron population of the 4a boron atom sites. The structures of the B-filled β-Mn-type samples in the Pd-Cu-B system exhibit preferential occupation of the 12d Wyckoff position by the more electronegative element with larger atomic radius, i.e. Pd. Metallic behavior of this phase was ascertained by measurements of electrical resistivity. No superconductivity has been observed from electrical resistivity measurements down to 0.3 K. Ab initio electronic structure calculations indicate a rather low electronic density of states at the Fermi level for Pd3Cu2B, with the palladium d-states contributing almost 50% to the total density. The band structure of Pd3Cu2B exhibits multiple bands intersecting the Fermi energy.

Crystal growth, low-temperature specific heat, and electronic structure of the filled skutterudite compound ThOs4As12

ABSTRACTSingle crystals of the filled skutterudite compound ThOsAs were grown by a Cd:As flux method. Low-temperature specific-heat measurement reveals a very small Sommerfeld coefficient of the electronic term mJ/molK, indicative of a semiconducting or semimetallic character of ThOsAs. A low electronic density of states at the Fermi level is consistent with anticipations based on the Zintl concept for the filled skutterudites as well as with our results of band structure calculations. Investigations of a nonmagnetic material ThOsAs can be relevant to unmask puzzling strong electron correlations in the putative topological Kondo insulator CeOsAs.

On energy accumulation in double layer on the surface of materials with low electron state density

Regularities of energy accumulation in the double layer on the surface of 2D materials with a low density of electronic states are analyzed. It is shown that the limiting voltage on a supercapacitor with electrodes of 2D materials can exceed the voltage of electrolyte decomposition. The experimental data confirming the stated assumptions are presented and discussed.

A reliable and controllable graphene doping method compatible with current CMOS technology and the demonstration of its device applications

The modulation of charge carrier concentration allows us to tune the Fermi level (EF) of graphene thanks to the low electronic density of states near the EF. The introduced metal oxide thin films as well as the modified transfer process can elaborately maneuver the amounts of charge carrier concentration in graphene. The self-encapsulation provides a solution to overcome the stability issues of metal oxide hole dopants. We have manipulated systematic graphene p-n junction structures for electronic or photonic application-compatible doping methods with current semiconducting process technology. We have demonstrated the anticipated transport properties on the designed heterojunction devices with non-destructive doping methods. This mitigates the device architecture limitation imposed in previously known doping methods. Furthermore, we employed EF-modulated graphene source/drain (S/D) electrodes in a low dimensional transition metal dichalcogenide field effect transistor (TMDFET). We have succeeded in fulfilling n-type, ambipolar, or p-type field effect transistors (FETs) by moving around only the graphene work function. Besides, the graphene/transition metal dichalcogenide (TMD) junction in either both p- and n-type transistor reveals linear voltage dependence with the enhanced contact resistance. We accomplished the complete conversion of p-/n-channel transistors with S/D tunable electrodes. The EF modulation using metal oxide facilitates graphene to access state-of-the-art complimentary-metal-oxide-semiconductor (CMOS) technology.

On Field-Effect Photovoltaics: Gate Enhancement of the Power Conversion Efficiency in a Nanotube/Silicon-Nanowire Solar Cell.

Recent years have seen a resurgence of interest in crystalline silicon Schottky junction solar cells distinguished by the use of low density of electronic states (DOS) nanocarbons (nanotubes, graphene) as the metal contacting the Si. Recently, unprecedented modulation of the power conversion efficiency in a single material system has been demonstrated in such cells by the use of electronic gating. The gate field induced Fermi level shift in the low-DOS carbon serves to enhance the junction built-in potential, while a gate field induced inversion layer at the Si surface, in regions remote from the junction, keeps the photocarriers well separated there, avoiding recombination at surface traps and defects (a key loss mechanism). Here, we extend these results into the third dimension of a vertical Si nanowire array solar cell. A single wall carbon nanotube layer engineered to contact virtually each n-Si nanowire tip extracts the minority carriers, while an ionic liquid electrolytic gate drives the nanowire body into inversion. The enhanced light absorption of the vertical forest cell, at 100 mW/cm(2) AM1.5G illumination, results in a short-circuit current density of 35 mA/cm(2) and associated power conversion efficiency of 15%. These results highlight the use of local fields as opposed to surface passivation as a means of avoiding front surface recombination. A deleterious electrochemical reaction of the silicon due to the electrolyte gating is shown to be caused by oxygen/water entrained in the ionic liquid electrolyte. While encapsulation can avoid the issue, a nonencapsulation-based approach is also implemented.

Capacitive charge storage at an electrified interface investigated via direct first-principles simulations

Supercapacitors store energy via the formation of an electric double layer, which generates a strong electric field at the electrode-electrolyte interface. Unlike conventional metallic electrodes, graphene-derived materials suffer from a low electronic density of states (i.e., quantum capacitance), which limits their ability to redistribute charge and efficiently screen this field. To explore these effects, we introduce a first-principles approach based on the effective screening medium framework, which is used to directly simulate the charge storage behavior of single- and multi-layered graphene in a way that more closely approximates operating devices. We demonstrate that the presence of the interfacial field significantly alters the capacitance in electrodes thinner than a few graphene layers, deriving in large part from intrinsic space-charge screening limitations. The capacitance is also found to be highly sensitive to the gap between the electrode and the solvent (contact layer), which offers possibilities for tuning the interfacial capacitance of the electrode by proper engineering of the electrolyte. Our results offer an alternative interpretation of discrepancies between experimental measurements and fixed-band models, and provide specific implications for improving graphene-based devices.

Impact of Graphene Edges on Enhancing the Performance of Electrochemical Double Layer Capacitors

The inherently large surface area and electrical conductivity of graphene-like electrodes have motivated extensive research for their use in supercapacitors. Although these properties are beneficial for the electric double layer (EDL) capacitance, the full utilization of graphene is curtailed by its intrinsically limited quantum capacitance due to the low density of electronic states near the neutrality point. While recent work has demonstrated that modifications to graphene can generally mitigate this limitation, a comprehensive analysis of the impact of graphene edges, which can be created during synthesis and post-treatment, has yet to be reported. Using a theoretical approach, we investigate the influence of graphene edges on both the quantum and EDL capacitances using edge-passivated zigzag graphene nanoribbons (ZGNRs) in [BMIM][PF6] ionic liquid as model systems. Our findings show that the presence of edges improves the quantum capacitance by increasing the electronic density of states, which is further amplified as the ZGNR width decreases. Our analysis also reveals that the EDL microstructure can be noticeably altered by the edges, which in turn increases the EDL capacitance. Through comparisons with pristine graphene electrodes, our study clearly highlights that edge defects in graphene-like electrodes can enhance supercapacitor performance by dramatically augmenting both EDL and quantum capacitances.

Field-effect control of tunneling barrier height by exploiting graphene's low density of states

We exploit the low density of electronic states of graphene to modulate the tunnel current flowing perpendicular to the atomic layers of a multi-layer graphene-boron nitride device. This is achieved by using the electric field effect to raise the Fermi energy of the graphene emitter layer and thereby reduce the effective barrier height for tunneling electrons. We discuss how the electron charge density in the graphene layers and the properties of the boron nitride tunnel barrier determine the device characteristics under operating conditions and derive expressions for carrier tunneling in these highly anisotropic layered heterostructures.

The electric double layer on graphite

The response of the basal plane of graphite towards an external field has been investigated by DFT. Because of the low electronic density of states the field penetrates over a distance of several tenth of a nanometer and exhibits oscillations. These calculations have been combined with a simple model for the electrolyte solution, which is represented as an ensemble of hard spheres. The combined model explains quantitatively the capacity characteristics of graphite. The behavior of graphite is contrasted with that of typical metal surfaces like Ag(1 1 1), Au(1 1 1) and Pt(1 1 1).

Electronic Junction Control in a Nanotube-Semiconductor Schottky Junction Solar Cell

We exploit the low density of electronic states in single wall carbon nanotubes to demonstrate active, electronic modulation of their Fermi level offset relative to n-type silicon in a nanotube-Si (metal-semiconductor) Schottky junction solar cell. Electronic modulation of the Fermi level offset, the junction interface dipole and a field developed across the depletion layer modifies the built-in potential in the device and its power generation characteristics. As produced (before modulation) devices exhibit ∼8.5% power conversion efficiency (PCE). With active modulation the PCE is continuously and reversibly changed from 4 to 11%.

SrPd4B and BaPd4B: New Type of Crystal Structure and Physical Properties

AbstractTwo new intermetallic alkaline‐earth palladium borides, SrPd4B and BaPd4B were synthesised and their physical properties were investigated. The crystal structure of SrPd4B was solved from powder X‐ray diffraction data: new structure type, space group Pnma, a = 6.0014(1) Å, b = 5.5041(1) Å, c = 11.8723(2) Å, RI = 0.065, RP = 0.093. BaPd4B is isostructural with a = 6.0883(1) Å, b = 5.6066(1) Å, c = 12.0050(2) Å, RI = 0.062, RP = 0.097. The relationship of this structure type with the series of derivatives of the CaCu5 type is discussed. Calculated electronic band structures for palladium, Pd3B, SrPd5, SrPd4B and SrPd3B are compared. The role of boron and strontium for the electronic properties is discussed in detail. SrPd4B shows metallic behaviour with a DOS(EF) ≈︁ 1.7 eV–1·f.u.–1 at the Fermi level. Magnetic properties, electrical resistivity and specific heat capacity measurements reveal that the two compounds are diamagnetic metallic conductors with low electronic density of states, in agreement, with the electronic structure calculations.

First principles electronic structure calculations of Co2CrBi Heusler system

First principles calculation results of a new full Heusler system Co2CrBi in stoichiometric composition were presented. The calculations are based on the density functional theory (DFT) within the spin-polarized generalized gradient approximation (σ-GGA) and plane wave pseudopotential method. The system shows nearly half-metallic behavior with very low electronic density of states of minority spins at Fermi level yielding high spin polarization ratio R=0.96. The total magnetic moment of the system was calculated as 5.05μB, which is largely localized at chromium site with μCr=3.35μB. The electronic character of the compound is determined by the 3d electronic states of cobalt and chromium atoms. f-Electronic states of bismuth atom were also included in calculations although it has no distinct effect on electronic structure. The system can be labelled as a new half-metallic material for technological applications.

Possible metal-insulator transition in the filled skutterudite CeFe4As12

Single crystals of filled skutterudite CeFe4As12 were examined with X-ray diffraction and electrical resistivity measurements. We have found two types of crystals with the same structure which differ in morphology, lattice parameter and, most importantly, in the temperature dependence of the electrical resistivity, ρ(T) below 165 K; dρ/dT becomes positive or negative, respectively. The band structure calculations point at the low electronic density of states at the Fermi level and an indirect gap at the Γ point.

Low-density band-gap states in pentacene thin films probed with ultrahigh-sensitivity ultraviolet photoelectron spectroscopy

We detected a very low density of electronic states in the band gap of a nondoped pentacene thin film by using ultraviolet photoelectron spectroscopy with ultrahigh sensitivity and ultralow background. The gap states, which may originate from the highest occupied molecular orbital (HOMO) state in imperfect molecular packing regions, are distributed exponentially up to the Fermi level and control the Fermi level relative to the HOMO band.

Anisotropic magnetization studies ofR2CoGa8single crystals (R=Gd, Tb, Dy, Ho, Er, Tm, Y, and Lu)

Single crystals of ${R}_{2}{\text{CoGa}}_{8}$ series of compounds were grown by high temperature solution growth (flux) method. These compounds crystallize in a tetragonal crystal structure with the space group $P4/mmm$. It has been found that ${R}_{2}{\text{CoGa}}_{8}$ phase only forms with the heavier rare earths, starting from Gd with a relatively large $c/a$ ratio of $\ensuremath{\approx}2.6$. The resultant anisotropic magnetic properties of the compounds were investigated along the two principal crystallographic directions of the crystal, viz., along the [100] and [001] directions. The compounds ${\text{Y}}_{2}{\text{CoGa}}_{8}$ and ${\text{Lu}}_{2}{\text{CoGa}}_{8}$ show diamagnetic behavior down to 5 K, pointing out the nonmagnetic nature of Co in these compounds and a relatively low density of electronic states at the Fermi level. Compounds with the magnetic rare earths antiferromagnetically order at temperatures lower than 30 K. The N\'eel temperature of ${\text{Tb}}_{2}{\text{CoGa}}_{8}$ exceeds that of the Gd analog. The easy axis of magnetization for ${R}_{2}{\text{CoGa}}_{8}$ ($R=\text{Tb}$, Dy, and Ho) is found to be along the [001] direction and it changes to the [100] direction for ${\text{Er}}_{2}{\text{CoGa}}_{8}$ and ${\text{Tm}}_{2}{\text{CoGa}}_{8}$, whereas the magnetization of ${\text{Gd}}_{2}{\text{CoGa}}_{8}$ was found to be almost isotropic. The magnetization behavior is analyzed on the basis of crystalline electric field model. The estimated crystal field parameters explain the magnetocrystalline anisotropy in this series of compounds.

Assessment of the Electrochemical Behavior of Two-Dimensional Networks of Single-Walled Carbon Nanotubes

Scanning electrochemical microscopy (SECM) has been employed in the feedback mode to assess the electrochemical behavior of two-dimensional networks of single-walled carbon nanotubes (SWNTs). It is shown that, even though the network comprises both metallic and semiconducting SWNTs, at high density (well above the percolation threshold for metallic SWNTs) and with approximately millimolar concentrations of redox species the network behaves as a thin metallic film, irrespective of the formal potential of the redox couple. This result is particularly striking since the fractional surface coverage of SWNTs is only approximately 1% and SECM delivers high mass transport rates to the network. Finite element simulations demonstrate that under these conditions diffusional overlap between neighboring SWNTs is significant so that planar diffusion prevails in the gap between the SECM tip and the underlying SWNT substrate. The SECM feedback response diminishes at higher concentrations of the redox species. However, wet gate measurements show that at the solution potentials of interest the conductivity is sufficiently high that lateral conductivity is not expected to be limiting. This suggests that reaction kinetics may be a limiting factor, especially since the low surface coverage of the SWNT network results in large fluxes to the SWNTs, which are characterized by a low density of electronic states. For electroanalytical purposes, significantly, two-dimensional SWNT networks can be considered as metallic films for typical millimolar concentrations employed in amperometry and voltammetry. Moreover, SWNT networks can be inexpensively and easily formed over large scales, opening up the possibility of further electroanalytical applications.

Electrochemistry at Single-Walled Carbon Nanotubes: The Role of Band Structure and Quantum Capacitance

We present a theoretical description of the kinetics of electrochemical charge transfer at single-walled carbon nanotube (SWNT) electrodes, explicitly taking into account the SWNT electronic band structure. SWNTs have a distinct and low density of electronic states (DOS), as expressed by a small value of the quantum capacitance. We show that this greatly affects the alignment and occupation of electronic states in voltammetric experiments and thus the electrode kinetics. We model electrochemistry at metallic and semiconducting SWNTs as well as at graphene by applying the Gerischer-Marcus model of electron transfer kinetics. We predict that the semiconducting or metallic SWNT band structure and its distinct van Hove singularities can be resolved in voltammetry, in a manner analogous to scanning tunneling spectroscopy. Consequently, SWNTs of different atomic structure yield different rate constants due to structure-dependent variations in the DOS. Interestingly, the rate of charge transfer does not necessarily vanish in the band gap of a semiconducting SWNT, due to significant contributions from states which are a few k(B)T away from the Fermi level. The combination of a nanometer critical dimension and the distinct band structure makes SWNTs a model system for studying the effect of the electronic structure of the electrode on electrochemical charge transfer.

Band offsets for the epitaxial TiO2/SrTiO3/Si(001) system

We have used x-ray photoelectron spectroscopy with high energy resolution to determine band discontinuities at the two buried interfaces of the epitaxial TiO2 (anatase)/SrTiO3/Si(001) system. The valence band offsets are −2.1±0.1 eV and +0.2±0.1 eV at the SrTiO3/Si and TiO2/SrTiO3 heterojunctions, respectively. Assuming bulk band gaps for the SrTiO3 and TiO2 epitaxial films, the associated conduction band offsets are +0.1±0.1 eV and +0.1±0.1 eV. Si at the interface is in a flatband state, indicating a very low density of electronic states. These results suggest that spin-polarized electron injection from ferromagnetic Co-doped TiO2 anatase into Si should be facile.