Fermi Level Density Research Articles

Dual Mode Kelvin Probe: Featuring Ambient Pressure Photoemission Spectroscopy and Contact Potential Difference

We describe a novel dual-mode Kelvin probe featuring ambient pressure Photoemission Spectroscopy (PES), which yields information on the absolute work function (Φ) of a metal and the Ionisation Potential (IP) of a semiconductor, coupled with a high resolution Contact Potential Difference capability which can be extended to Surface Photovoltage measurements. The relative energy resolution are 50 meV for PES and 1-3 meV for CPD. To surmount the limitation of electron scattering in air the incident photon energy is rastered rather than applying a variable retarding electric field as is used UPS. We propose a mechanism of atmospheric ion generation and show that for the metal photoresponse obeys Fowler Theory. The relationship between CPD and photoelectric threshold is a useful tool in characterizing the electrical behavior of materials. We illustrate this with native oxide covered Cu and n-type Si. Further we show that the photoresponse can be used to generate the near Fermi-level Density of States (DOS) in Iron and Nickel-Phthalocyanine.

Partially gapped Fermi surfaces in La3Co4Sn13revealed by nuclear magnetic resonance

We report a study of a single-crystal La${}_{3}$Co${}_{4}$Sn${}_{13}$ by means of the specific heat and ${}^{59}$Co nuclear magnetic resonance (NMR) spectroscopy. A first-order phase transition with a marked peak at ${T}^{*}\ensuremath{\simeq}152$ K has been discerned by the specific heat measurement. The observed transition has been connected to a structural change from a simple cubic to a body-centered-cubic superstructure with crystallographic cell doubling, accompanied by the Fermi surface reconstruction. Indeed, NMR observations clearly indicate a significant change in the local electronic characteristics across this phase transition. The spin-lattice-relaxation rate measurement further provides an estimate of Co 3$d$ Fermi-level density of states ${N}_{d}$(${E}_{F}$), revealing a visible reduction in ${N}_{d}$(${E}_{F}$) in the low-temperature phase. This finding essentially associated with the partially gapped Fermi surfaces would be appropriate for the isostructural analog of Sr${}_{3}$Ir${}_{4}$ Sn${}_{13}$, which has been claimed to possess charge-density-wave (CDW) behavior with a three-dimensional crystallographic structure.

Enhancement of thermoelectric performance driven by Ge substitution in SrSi2 alloy

The effect of partial substitution of Ge in the Si sites of SrSi2 alloy has been investigated by means of electrical and thermal transport studies. Electrical resistivity (ρ), Seebeck coefficient (S) and thermal conductivity (κ) measurements were performed on the SrSi2−xGex alloys with x = 0–0.12. The room-temperature resistivity of these substituted alloys decreases with increasing Ge content up to x = 0.06 and then starts to increase upon further substitution. Seebeck coefficient study shows a substantial increase in S with Ge content, and a maximum of about 280 µV K−1 was obtained for x = 0.03 near 85 K. These observations can be realized as the broadening of band gap and the changes in the Fermi-level density of states (DOS) upon Ge doping. Analysis of the thermal conductivity of the SrSi2−xGex alloys suggests that the heat transport is essentially associated with the lattice phonons. In addition, low-temperature peak in the lattice thermal conductivity of these alloys drops markedly with increasing Ge content, attributed to the point-defect scattering of the phonons. Thermoelectric performance characterized by the figure-of-merit, ZT was evaluated for each alloy, and the largest ZT value of about 0.13 at room temperature was achieved for SrSi1.94Ge0.06 alloy, nearly three times higher than that of pure SrSi2.

Electronic fine structure in the nickel carbide superconductor Th2NiC2

The recently reported nickel carbide superconductor, body centered tetragonal $I4/mmm$ Th$_2$NiC$_2$ with T$_c$ = 8.5 K increasing to 11.2 K upon alloying Th with Sc, is found to have very fine structure in its electronic spectrum, according to density functional based first principles calculations. The filled Ni 3d band complex is hybridized with C $2p$ and Th character to and through the Fermi level ($E_f$), and a sharply structured density of states arises only when spin-orbit coupling is included, which splits a zone-center degeneracy leaving a very flat band edge lying at the Fermi level. The flat part of the band corresponds to an effective mass $m^*_{z} \rightarrow \infty$ with large and negative $m^*_{x}=m^*_{y}$. Although the region over which the effective mass characterization applies is less than 1% of the zone volume, it yet supplies of the order of half the states at (or just above) the Fermi level. The observed increase of T$_c$ by hole-doping is accounted for if the reference as-synthesized sample is minutely hole-doped, which decreases the Fermi level density of states and will provide some stabilization. In this scenario, electron doping will increase the Fermi level density of states and the superconducting critical temperature. Vibrational properties are presented, and enough coupling to the C-Ni-C stretch mode at 70 meV is obtained to imply that superconductivity is electron-phonon mediated.

Separation of the contributions to the magnetization of Tm1 − xYb x B12 solid solutions in steady and pulsed magnetic fields

The magnetization of substitutional Tm1 − xYbxB12 solid solutions is studied in the composition range 0 < x ≤ 0.81. The measurements are performed at low temperatures (1.9–300 K) in steady (up to 11 T) and pulsed (up to 50 T, pulse duration of 20–100 ms) magnetic fields. An analysis of the experimental data allowed the contributions to the magnetization of the paramagnetic phase of the Tm1 − xYbxB12 compounds to be separated. These contributions include a Pauli component, which corresponds to the response of the heavy-fermion manybody states that appears in the energy gap in the vicinity of the Fermi level (density of states (3−4) × 1021 cm−3 meV−1), and a contribution with saturation in high magnetic fields attributed to the localized magnetic moments ((0.8–3.7)μB per unit cell) of the nanoclusters formed by rare-earth ions with an antiferromagnetic interaction.

Electrical characterization of Si(100) surface at p-Si/SiGe/Si structure using low temperature Hall measurement analysis

The pinning position of Fermi level and surface charge density at Si(100) surface of epitaxially grown p-Si/SiGe/Si structure has been evaluated via low temperature Hall experiment analysis. It is explained how the density of two-dimensional hole gas (2DHG) formed in the SiGe quantum well is affected by structural parameters, and proximity of Si surface charges and 2DHG. This approach is based on static charge transfer and Mid-gap Pinning Model (MPM). Finally, the passivation of Si surface by diluted HF etchant has been discussed.

Band Structure and Quantum Oscillations in YBa2Cu3O7: A Local Spin Density Approximation with the On-Site Coulomb Interaction Study

First-principles calculations have been performed on the Bravais lattice, the density of states (DOS), the band structure (BS) and the Fermi Surface (FS) topology for high-Tc superconductor YBa2Cu3O7. We used the method of the Full-Potential non-orthogonal Local-Orbital Minimum-Basis Band-structure Scheme (FPLO) [in the local spin density approximation with the on-site Coulomb interaction (LSDA+U) and the coherent potential approximation (CPA)] coupled with the XFSF program. Our FPLO Fermi surface consists of four large quasi-2D cylinders centered on the Brillouin zone corners with mostly CuO2 plane character and two quasi-1D sheets with mostly Cu–O apex (or apical O) chain character. The main feature of this Fermi surface is that each CuO chain-sheet shows up in its center a small tubular pocket with a funnel-like shape (Fermi pockets) with mainly Ba–O apex character which could give rise to the quantum oscillations observed by Doiron-Leyraud et al. (Nature 447: 565, 2007). Our FPLO band structure (BS) shows that three flat bands cross the Fermi level (FL) which suggests that the two Fermi pockets could be produced by the conjunction near the Fermi level of two flat bands arising from the apex oxygen (BaO layer). This is consistent with the density of states (DOS) where the Fermi level is dominated by the two O apex (BaO) peaks (∼90 %). Moreover, the strong narrow peak exhibited in the DOS just below the Fermi level (FL) corresponds to the van Hove singularity (VHS) and originates from the hybridization of Cu 3d and apex O2p (BaO) orbitals. In other hand, the calculated value of the bare band-structure Sommerfeld constant γ0band=10.60 mJ/(mol K2) corresponds to a very high Fermi level density of states N(EF)=4.72 states/(eV×cell) and the electron–phonon coupling constant λ is found to be about 1.64 which implies that YBa2Cu3O7 compound is a strong-coupling superconductor and the cyclotron effective mass m∗ is enhanced in comparison with the band mass mb [m∗=(1+λ)mb]. Also, the discrepancy between our FPLO calculated Debye temperature (274.15 K) and that obtained experimentally (437 K) may originate from a phonon anisotropy likely generated by the antiferromagnetic spin fluctuations. Our results therefore provide evidence of changes in the topology of the Fermi surface materialized by the Ba–O apex small pockets formed in the center of the Cu–O apex Fermi surface sheets.

Conduction mechanism of resistive switching films in MgO memory devices

In this work, nonpolar resistance switching behavior was demonstrated in Pt/MgO/Pt structure. The resistance ratio of high resistance state (HRS) and low resistance state (LRS) is about on the order of 105 for the compliance current (Icomp) of 1 mA at 300 K. Using enough Icomp (≥0.5 mA) during SET processes, the LRS resistances reach a minimum of about 102–103 Ω and the RESET currents reach a maximum of about 10−4–10−3 A. Experimental results indicate that the conduction mechanism in MgO films is dominated by the hopping conduction and the Ohmic conduction in HRS and LRS, respectively. Therefore, the electrical parameters of trap energy level, trap spacing, Fermi level, electron mobility, and effective density of states in conduction band in MgO films were obtained.

Reliability characteristics and conduction mechanisms in resistive switching memory devices using ZnO thin films.

In this work, bipolar resistive switching characteristics were demonstrated in the Pt/ZnO/Pt structure. Reliability tests show that ac cycling endurance level above 106 can be achieved. However, significant window closure takes place after about 102 dc cycles. Data retention characteristic exhibits no observed degradation after 168 h. Read durability shows stable resistance states after 106 read times. The current transportation in ZnO films is dominated by the hopping conduction and the ohmic conduction in high-resistance and low-resistance states, respectively. Therefore, the electrical parameters of trap energy level, trap spacing, Fermi level, electron mobility, and effective density of states in conduction band in ZnO were identified.

Local charge neutrality condition, Fermi level and majority carrier density of a semiconductor with multiple localized multi-level intrinsic/impurity defects

For semiconductors with localized intrinsic/impurity defects, intentionally doped or unintentionally incorporated, that have multiple transition energy levels among charge states, the general formulation of the local charge neutrality condition is given for the determination of the Fermi level and the majority carrier density. A graphical method is used to illustrate the solution of the problem. Relations among the transition energy levels of the multi-level defect are derived using the graphical method. Numerical examples are given for p-doping of the CdTe thin film used in solar panels and semi-insulating Si to illustrate the relevance and importance of the issues discussed in this work.

Modelling the role of size, edge structure and terminations on the electronic properties of graphene nano-flakes

The addition of graphene nano-flakes to the suite of materials for graphene-based nanotechnology requires a complete understanding of the relationship between shape, structure, properties and property dispersion. Due to the large number of configurational degrees of freedom, this is a very challenging undertaking, particularly if morphological ensembles contain a reasonable array of sizes, shapes (edges and corners) and edge/corner terminations. We report results of density functional tight-binding simulations of zigzag (ZZ) and armchair (AC) hexagonal graphene nano-flakes, with unterminated, monoyhydride or dihydride terminated edges and corners. We find that hexagonal nano-flakes with AC edges are most likely to be achievable experimentally, providing that sufficient H is present during synthesis (or processing) to facilitate dihydride edge and corner passivation, forming a circumference of sp3 hybridized C atoms. This is significant, since the energy of the Fermi level and electronic density of states in the vicinity of the Fermi level are sensitive to the structural and chemical characteristics of the atoms around the circumference, which can be modified post-synthesis.

Electronic structure and transport properties of SrAl2Si2: Effect of yttrium substitution

We report the results of yttrium substitution on the electrical resistivity (ρ), the thermal conductivity (κ), as well as the Seebeck coefficient (S) of the Sr1−xYxAl2Si2 alloys with 0 ≤ x ≤ 0.20. Both ρ(T) and S(T) data suggest that SrAl2Si2 is a semimetallic, low charge carrier density system with a pseudogap at the Fermi level density of states (DOS). Upon substituting Y onto the Sr sites, the electrical resistivity and the absolute value of the Seebeck coefficient decrease significantly. Such an observation can be associated with the modification of the electronic band structure due to electron doping via Y substitution. Analysis of the thermal conductivity reveals the contribution of various thermal scattering mechanisms through chemical substitution. Theoretical studies with density functional theory are also employed to investigate the electronic band structure of Sr1−xYxAl2Si2. It is revealed that SrAl2Si2 possesses a shallow DOS at the Fermi level with both n-type and p-type charge carriers. Upon Y substitution a shift in the Femi level occurs such that the Sr1−xYxAl2Si2 system becomes more metallic with increasing x, being consistent with the experimental findings.

Approximate graphical method of solving Fermi level and majority carrier density of semiconductors with multiple donors and multiple acceptors

We present a generic approximate graphical method for determining the equilibrium Fermi level and majority carrier density of a semiconductor with multiple donors and multiple acceptors compensating each other. Simple and easy-to-follow procedures of the graphical method are described. By graphically plotting two wrapping step functions facing each other, one for the positive hole-ionized donor and one for the negative electron-ionized acceptor, we have the crossing point that renders the Fermi level and majority carrier density. Using the graphical method, new equations are derived, such as the carrier compensation proportional to NA/ND, not the widely quoted NA — ND. Visual insight is offered to view not only the result of graphic determination of Fermi level and majority carrier density but also the dominant and critical pair of donors and acceptors in compensation. The graphical method presented in this work will help to guide the design, adjustment, and improvement of the multiply doped semiconductors. Comparison of this approximate graphical method with previous work on compensation, and with some experimental results, is made. Future work in the field is proposed.

One-dimensional alkali-dopedC60chains encapsulated in BN nanotubes

We study the energetics, electronic structures, and electron-phonon couplings of a one-dimensional potassium-doped ${\mathrm{C}}_{60}$ chain encapsulated in a boron nitride nanotube using the framework of the density-functional theory. We demonstrate that the reaction of potassium doping is exothermic and the resulting material is one-dimensional metal where conducting electrons are only in the ${\mathrm{C}}_{60}$ chain. Interestingly, the Fermi-level density of states has a peculiar pressure dependence and can be larger than those in the three-dimensional alkali-doped fullerene compounds, indicating the possibility of various phase transitions. We also discuss the electron-phonon couplings and the possibility of superconductivity.

Light intensity effects on electrical properties of AgIn 5S 8 thin films

The light illumination effects on the current conduction mechanism in thermally annealed polycrystalline AgIn 5S 8 thin films has been investigated by means of dark and photoexcited conductivity measurements as a function of temperature. The dark electrical conductivity analysis in the temperature region of 30–300 K, reflected the domination of thermionic emission and variable range hopping of charge carriers over the grain boundaries above and below 90 K, respectively. Conductivity activation energies of ~ 155 and 78 meV (in the temperature regions of 230–300 K and 90–220 K, respectively), a density of localized states (evaluated assuming a localization length of 5 A 0 ) of 1.17 × 10 20 cm − 3 eV − 1 , an average hopping distance of 41.51 A 0 (at 60 K) and an average hopping energy of 28.64 meV have been determined from the dark electrical measurements. When the sample was exposed to illumination at specific excitation intensity, the values of the conductivity activation energy, the average hopping energy and the average hopping range were decreased significantly. On the other hand, the density of localized states near the Fermi level increased when the light intensity was increased. Such behavior is attributed to the temporary shift in Fermi level and/or trap density reduction by electron-hole recombination.

Electronic structure of [formula omitted] from photoelectron spectroscopic studies

The valence band electronic structure of the Bi 1− x Pb x FeO 3 ( x=0.02–0.15) system which undergoes a R3c to cubic phase transition with Pb doping has been studied by using X-ray and ultra-violet photoelectron spectroscopy. The cubic composition showed an enhancement of the oxygen 2p character in the near Fermi level density of states, possibly due to the weakening of the Fe 3d–O 2p–Bi 6p hybridization strengths following the changes in the topology of the oxygen octahedra in its structure. The compositions with the R3c structure showed a much larger band gap and band width compared to those reported from LSDA+U calculations. We attribute this to a larger effective Coulomb interaction (U eff).

Electronic structures of AlB2-type superconducting YbGa1+xSi1−x alloys probed by NMR and Seebeck coefficient

We report the electronic properties of the AlB2-type compounds YbGa1+xSi1−x (x=0, 0.15, and 0.3) studied by means of the nuclear magnetic resonance (NMR) and the Seebeck coefficient measurements. These materials are of current interest due to the presence of superconductivity with Yb element. From the analysis of G69a NMR spin-lattice relaxation rates, we deduce the Ga 4s partial Fermi level density of states Ns(EF) for these compounds. The result indicates a gradual increase in Ns(EF) with increasing x in YbGa1+xSi1−x. In addition, the evolution of the Seebeck coefficient feature can be understood well within the band-filling scenario. From the Seebeck coefficient analysis, we find that the variation in the total Fermi level density of states N(EF) is not consistent with the trend of superconducting temperature Tc which shows a gradual decrease with Ga content. These observations support the hypothesis that the electronic Fermi level density of states is not the key factor in determining the superconducting transition temperature of YbGa1+xSi1−x.

Comparative NMR study of copper-based intermetallics with ZrCuSiAs-type structure

The electronic characteristics of ZrCuGe2, ZrCuSi2, and HfCuSi2 are systematically investigated using C63u NMR spectroscopy. The quadrupole splittings, Knight shifts, and spin-lattice relaxation times on each individual compound have been identified. We found that the observed electric field gradient is consistent with the covalent bonding nature within the Cu atomic layers. The Knight shifts together with relaxation rates provide a measure of Cu d partial Fermi-level density of states, Nd(EF). Universally small Nd(EF) was found in all studied materials, suggests that the Cu d states are well below the Fermi energy and therefore the characteristic electronic structure near EF is primarily of sp type. We further pointed out that the low Nd(EF) value is an important factor for the lack of superconductivity in these Cu-based intermetallics within the ZrCuSiAs-type structure.

Defect annealing in neutron and ion damaged silicon: Influence of defect clusters and doping

We have explored defect annealing in radiation damaged silicon in a regime characterized by defect clusters and higher doping. Several types of pnp and npn Si bipolar transistors have been irradiated with ions and neutrons, then isochronally annealed from 300 to 600 K to study the evolution of deep level transient spectroscopy (DLTS) defect signatures. Variations in these data with radiation environment, Fermi level, annealing temperature, and doping density have been used to separate the contributions of three dominant defects to the DLTS defect spectra. We find that the normal Si divacancy and a divacancylike defect with similar properties make similar contributions to a DLTS peak normally associated with transitions from the single minus charge state of the divacancy. However the latter defect is clearly associated with the presence of defect clusters. The vacancy-donor center can also contribute to this high temperature DLTS signature, and its relative importance can be quantitatively assessed by varying doping density and the bias applied to the sample p/n junctions during annealing, and also by the observation that another, donor-related defect grows in as this center anneals. The ratio of vacancy-donor and vacancy-oxygen pairs appears to accurately follow that seen in earlier studies of gamma-irradiated Si. Discussions are presented concerning the effects of defect clustering on the structure, appearance, and evolution of the defects we have identified.

Electrically driven magnetism on a Pd thin film

Using first-principles density functional calculations we demonstrate that ferromagnetism can be induced and modulated on an otherwise paramagnetic Pd metal thin-film surface through application of an external electric field. As free charges are either accumulated or depleted at the Pd surface to screen the applied electric field there is a corresponding change in the surface density of states. This change can be made sufficient for the Fermi-level density of states to satisfy the Stoner criterion, driving a transition locally at the surface from a paramagnetic state to an itinerant ferromagnetic state above a critical applied electric field, Ec. Furthermore, due to the second-order nature of this transition, the surface magnetization of the ferromagnetic state just above the transition exhibits a substantial dependence on electric field, as the result of an enhanced magnetoelectric susceptibility. Using a linearized Stoner model we explain the occurrence of the itinerant ferromagnetism and demonstrate that the magnetic moment on the Pd surface follows a square-root variation with electric field consistent with our first-principles calculations.