Energy-dependent Density Of States Research Articles

Elastic, electronic, optical, and thermodynamic properties of M2SeC (M = Hf, Zr) under high pressure

The physical properties of the synthesized MAX phases M2SeC (M = Hf, Zr) have been investigated for the first time using the DFT method in the pressure range 0–25 GPa. The computed lattice parameters are consistent with the earlier findings at ambient pressure. The electronic band structure and energy dependent density of states reveal the metallic nature of the titled compounds. The metallic behavior of M2SeC (M = Hf, Zr) remains unchanged within the studied pressure range. Possible changes in the mechanical properties between Hf2SeC and Zr2SeC are revealed by analyzing the peaks in the density of states (DOS) for both compounds. Partial DOS is also considered under different pressures. The studied compounds remain mechanically stable up to 25 GPa. Both Hf2SeC and Zr2SeC are elastically anisotropic and brittle, but as the pressure increases up to 25 GPa, the brittleness of the materials decreases. At 5 and 10 GPa, however, both compounds exhibit a ductile behavior. The optical properties of Hf2SeC have been computed for the first time and compared with those of Zr2SeC. The investigated compounds are promising as solar heat reflectors due to their higher reflectivity in the low energy state. Different thermal properties, such as Debye temperature, melting temperature, and minimum thermal conductivity, have been explored within the studied pressure range to recognize their suitability for high-temperature applications. Both Hf2SeC and Zr2SeC have the potential to be used as coating materials for thermal barriers, although Hf2SeC is a better option.

Interactions and Mobility Edges: Observing the Generalized Aubry-André Model.

Using synthetic lattices of laser-coupled atomic momentum modes, we experimentally realize a recently proposed family of nearest-neighbor tight-binding models having quasiperiodic site energy modulation that host an exact mobility edge protected by a duality symmetry. These one-dimensional tight-binding models can be viewed as a generalization of the well-known Aubry-André model, with an energy-dependent self-duality condition that constitutes an analytical mobility edge relation. By adiabatically preparing low and high energy eigenstates of this model system and performing microscopic measurements of their participation ratio, we track the evolution of the mobility edge as the energy-dependent density of states is modified by the model's tuning parameter. Our results show strong deviations from single-particle predictions, consistent with attractive interactions causing both enhanced localization of the lowest energy state due to self-trapping and inhibited localization of high energy states due to screening. This study paves the way for quantitative studies of interaction effects on self-duality induced mobility edges.

Experimentally Validated Hopping-Transport Model for Energetically Disordered Organic Semiconductors

Charge transport in disordered organic semiconductors occurs by hopping of charge carriers between localized sites that are randomly distributed in a strongly energy dependent density of states. Extracting disorder and hopping parameters from experimental data like temperature dependent current-voltage characteristics typically relies on parametrized mobility functionals that are integrated in a drift-diffusion solver. Surprisingly, the functional based on the extended Gaussian disorder model (eGDM) has been extremely successful at this, despite it being based on the assumption of nearest neighbor hopping (nnH) on a regular lattice. We here propose a variable range hopping (VRH) model that has been integrated in a freeware drift-diffusion solver. The mobility model has been calibrated using kinetic Monte Carlo calculations and shows good agreement with the Monte Carlo calculations over the experimentally relevant part of the parameter space. The model is applied to temperature-dependent space charge limited current (SCLC) measurements of different systems. In contrast to the eGDM, the VRH model provides a consistent description of both p-type and n-type devices. We find a critical ratio of aNN/$\alpha$ (mean inter-site distance / localization radius) of ~3 below which hopping to non-nearest neighbors becomes important around room temperature and the eGDM cannot be used for parameter extraction. Typical (Gaussian) disorder values in the range 45-120 meV are found, without any clear correlation with photovoltaic performance when the same active layer is used in an organic solar cell.

Trap states extraction of p-channel SnO thin-film transistors based on percolation and multiple trapping carrier conductions

A new method is proposed to extract the trap states in p-channel SnO thin-film transistors (TFTs). In this method, the dominant conduction mechanisms under different temperatures have been taken into account. There are percolation and multiple trapping conduction mechanisms. Combined with the drain current-temperature (Ids-T) and capacitance-voltage (C-V) measurements, the variation of trap states concentrations with the surface potential is derived. Results show that energy-dependent density of states (DOS) is in the order of 1019eV/cm3. And the maximum of the density of states at the interface between the gate dielectric and the SnO channel is Nss=1.77×1014cm−2eV−1.

Evolution of electronic states in n-type copper oxide superconductor via electric double layer gating.

The occurrence of electrons and holes in n-type copper oxides has been achieved by chemical doping, pressure, and/or deoxygenation. However, the observed electronic properties are blurred by the concomitant effects such as change of lattice structure, disorder, etc. Here, we report on successful tuning the electronic band structure of n-type Pr2−xCexCuO4 (x = 0.15) ultrathin films, via the electric double layer transistor technique. Abnormal transport properties, such as multiple sign reversals of Hall resistivity in normal and mixed states, have been revealed within an electrostatic field in range of −2 V to + 2 V, as well as varying the temperature and magnetic field. In the mixed state, the intrinsic anomalous Hall conductivity invokes the contribution of both electron and hole-bands as well as the energy dependent density of states near the Fermi level. The two-band model can also describe the normal state transport properties well, whereas the carrier concentrations of electrons and holes are always enhanced or depressed simultaneously in electric fields. This is in contrast to the scenario of Fermi surface reconstruction by antiferromagnetism, where an anti-correlation is commonly expected.

Infrared properties of cuprates in the pseudogap state: a study of Mitrović-Fiorucci and Sharapov-Carbotte scattering rates

The frequency dependent scattering rate of generalized Drude model contains important information on the electronic structure and on scattering mechanism. In the present investigation, we study the frequency dependent scattering rate of cuprates (Mitrović-Fiorucci/Sharapov-Carbotte scattering rate) in the pseudogap phase using the non-constant energy dependent Yang-Rice-Zhang (YRZ) density of states. First, with the energy dependent density of states, the scattering rate shows a depression at low energy coming from the opening of the pseudogap. Second, the evolution of 1/τ(ω,T) with temperature shows the observed increase in scattering rate with temperature at lower frequencies and the temperature independence of 1 /τ(ω) at higher frequencies. Third, the signature of the thresholds due to the boson density of states and to the electronic density of states are also observed. These signatures are qualitatively in accord with the experiments.

Energy position of the transport path in disordered organic semiconductors

The concept of transport energy is the most transparent theoretical approach to describe hopping transport in disordered systems with steeply energy dependent density of states (DOS), in particular in organic semiconductors with Gaussian DOS. This concept allows one to treat hopping transport in the framework of a simple multiple-trapping model, replacing the mobility edge by a particular energy level called the transport energy. However, there is no consensus among researchers on the position of this transport level. In this article, we suggest a numerical procedure to find out the energy level most significantly contributing to charge transport in organic semiconductors. The procedure is based on studying the effects of DOS modifications on the charge carrier mobility in straightforward computer simulations. We also show why the most frequently visited energy, computed in several numerical studies to determine the transport energy, is not representative for charge transport.

Strong asymmetrical bias dependence of magnetoresistance in organic spin valves: the role of ferromagnetic/organic interfaces

We report a highly asymmetric magnetoresistance (MR) bias dependence, with the inverse MR peaking at a negative bias and a sign reversal occurring at a positive bias in prototypical La0.7Sr0.3MnO3 (LSMO)/Alq3/Co organic spin valve (OSV) with a tunnel barrier between LSMO and Alq3. This behavior is in strong contrast with the commonly found inverse MR in entire bias range for LSMO/Alq3/Co OSVs. The MR bias voltage dependence is independent on the type of the tunnel barrier, either SrTiO3 or Al2O3. Together with first-principle calculations, we demonstrate that the strongly hybridized Co d-states with Alq3 molecules at the interface are responsible for the efficient d-states spin injection and the observed MR bias dependence is originated from the energy dependent density of states of Co d-states. These findings open up new possibilities to engineer interfacial bonding between ferromagnetic materials and a wide variety of molecule selections for the desired spin transport properties.

Finite-Bias Cooper Pair Splitting

In a device with a superconductor coupled to two parallel quantum dots (QDs) the electrical tunability of the QD levels can be used to exploit nonclassical current correlations due to the splitting of Cooper pairs. We experimentally investigate the effect of a finite potential difference across one quantum dot on the conductance through the other completely grounded QD in a Cooper pair splitter fabricated on an InAs nanowire. We demonstrate that the nonlocal electrical transport through the device can be tuned by electrical means and that the energy dependence of the effective density of states in the QDs is relevant for the rates of Cooper pair splitting (CPS) and elastic cotunneling. Such experimental tools are necessary to understand and develop CPS-based sources of entangled electrons in solid-state devices.

Andreev Reflection in Heavy-Fermion Superconductors and Order Parameter Symmetry inCeCoIn5

Differential conductance spectra are obtained from nanoscale junctions on the heavy-fermion superconductor CeCoIn5 along three major crystallographic orientations. Consistency and reproducibility of characteristic features among the junctions ensure their spectroscopic nature. All junctions show a similar conductance asymmetry and Andreev reflectionlike conductance with a reduced signal ( approximately 10%-13%), both commonly observed in heavy-fermion superconductor junctions. Analysis using the extended Blonder-Tinkham-Klapwijk model indicates that our data provide the first spectroscopic evidence for d_{x;{2}-y;{2}} symmetry. To quantify our conductance spectra, we propose a model by considering the general phenomenology in heavy fermions, the two-fluid behavior, and an energy-dependent density of states. Our model fits to the experimental data remarkably well and should invigorate further investigations.

Kondo quantum dot coupled to ferromagnetic leads: Numerical renormalization group study

We systematically study the influence of ferromagnetic leads on the Kondo resonance in a quantum dot tuned to the local moment regime. We employ Wilson's numerical renormalization group method, extended to handle leads with a spin asymmetric density of states, to identify the effects of (i) a finite spin polarization in the leads (at the Fermi-surface), (ii) a Stoner splitting in the bands (governed by the band edges) and (iii) an arbitrary shape of the leads density of states. For a generic lead density of states the quantum dot favors being occupied by a particular spin-species due to exchange interaction with ferromagnetic leads leading to a suppression and splitting of the Kondo resonance. The application of a magnetic field can compensate this asymmetry restoring the Kondo effect. We study both the gate-voltage dependence (for a fixed band structure in the leads) and the spin polarization dependence (for fixed gate voltage) of this compensation field for various types of bands. Interestingly, we find that the full recovery of the Kondo resonance of a quantum dot in presence of leads with an energy dependent density of states is not only possible by an appropriately tuned external magnetic field but also via an appropriately tuned gate voltage. For flat bands simple formulas for the splitting of the local level as a function of the spin polarization and gate voltage are given.

Van Hove Non-Fermi Superconductors and Critical Temperature Degradation

We analyzed the non-Fermi behavior of copper oxides superconductors with an energy-dependent density of states of the van Hove (logarithmic) form, using the model proposed by Anderson and Yin and Chakravarty. The critical temperature degradation, due to a small amount of non-magnetic impurities, is also analyzed. It is shown that the decrease of the critical temperature is not of the Abrikosov–Gorkov type.

Density of states and impurity effects in non-Fermi superconductors

We analyzed the non-Fermi behavior of high-temperature copper oxides, extending the model proposed by Anderson and Yin and Chakravarty. We show that there is an agreement between critical temperatures calculated in two different ways. We discussed also the effect of an energy-dependent density of states (which can be singular), and the critical temperature degradation in the non-Fermi liquid, due to a small amount of impurities.

Influence of measurement on the lifetime and the linewidth of unstable systems

We investigate the quantum Zeno effect in the case of electron tunneling out of a quantum dot in the presence of continuous monitoring by a detector. It is shown that the Schr\"odinger equation for the whole system can be reduced to Bloch-type rate equations describing the combined time-development of the detector and the measured system. Using these equations we find that continuous measurement of the unstable system does not affect its exponential decay to a reservoir with a constant density of states. The width of the energy distribution of the tunneling electron, however, is not equal to the inverse life-time -- it increases due to the decoherence generated by the detector. We extend the analysis to the case of a reservoir described by an energy dependent density of states, and we show that continuous measurement of such quantum systems affects both the exponential decay rate and the energy distribution. The decay does not always slow down, but might be accelerated. The energy distribution of the tunneling electron may reveal the lines invisible before the measurement.

Impurities in d-Wave Superconductors: Effect of Singular Density of States

We analyzed the effect of nonmagnetic impurities in d-wave superconductors with an energy dependent density of states. Two singular forms for the density of states have been considered. The strength of singularity affect the degradation of the critical temperature and reduces the pair-breaking effect.

Fragmentation dynamics of Fe(CO) 5 upon femtosecond excitation: a time-dependent statistical description

We present a time-dependent statistical model for the multiple CO abstraction from iron pentacarbonyl which is initiated by femtosecond excitation. The theory is based on a classical RRKM-model. The short pulse excitation is treated within time-dependent perturbation theory and the fragmentation probability is calculated via the energy dependent density of states of the parent and fragment molecules. We consider sequential and concerted CO loss separately. Application to recent experiments (Ihee et al., Chem. Phys. Lett. 281 (1997) 10) yields good agreement with the experimental results and points towards a sequential dissociation process.

Suppression of superconductivity in Sm and Co substituted Bi 2Sr 2Ca 1Cu 2O 8+ δ system

The present work investigates the effect of Sm and Co substitution in Bi 2Sr 2Ca 1Cu 2O 8+ δ system. The substitution of Sm leads to an initial increase of transition temperature followed by a decrease. No such increase of transition temperature is observed in Sm and Co co-substituted samples, thus reflecting localization of excess holes in these samples. The suppression of superconductivity in both kind of samples occurs due to hole filling and disorder effects. The insulating state of the samples is described by 3D variable range hopping of conduction mechanism with energy dependent density of states.

Low temperature properties of incommensurate ferroelectrics (PbySn1-y)2P2Se6

The complex dielectric susceptibility ϵ = ϵ′ + ϵ′′ of the incommensurate ferroelectrics (PbySn1-y)2P2Se6 (at y ≥0.4) has been studied at various frequencies (0.5 Hz – 100 kHz) in the temperature range from 10 mK to 200 K. Three different types of dielectric behaviour are distinguished at low temperature. Below 500mK typical glassy behaviour is observed according to the two-level system (TLS) model with a constant density of states. In the range from 0.5 K to 7 K the dielectric results are described in the TLS model with an energy dependent density of states and above 7 K the dielectric dispersion and associated maximum of ϵ′ has a thermoactivated relaxation nature. The latter is attributed to the freezing of the relaxation dynamics of the incommensurate modulation.

Critical temperature of a non-Fermi-liquid superconductor with an energy-dependent density of states

We calculate the critical temperature of a superconductor with a non-Fermi-liquid ground state. Assuming BCS-like pairing we obtain the analytical expression of the critical temperature for a constant density of states and for an energy-dependent density of states that has two different forms. {copyright} {ital 1997} {ital The American Physical Society}

The flow-equation method for the single impurity in a 2D superconductor

Following the flow equations given by Wegner, we study the single impurity state in a 2D superconductor with an energy-dependent density of states. The Anderson model withU = 0 has been used to perform analytical calculations of the single impurity energy in a superconductor with different energy spectrum. We showed that the single impurity energy is drastically affected by an energy-dependent density of states. For the 2D van Hove superconductor the correction is of the order of δ2.