Fermi Energy

We present a detailed theoretical study of the ultrafast quasiparticle relaxation dynamics observed in normal metals and heavy fermion materials with femtosecond time-resolved optical pump-probe spectroscopy. For normal metals, a nonthermal electron distribution gives rise to a temperature (T ) independent electron-phonon relaxation time at low temperatures, in contrast to the T -3 -divergent behavior predicted by the two-temperature model. For heavy fermion compounds, we find that the blocking of electron-phonon scattering for heavy electrons within the density-of-states peak near the Fermi energy is crucial to explain the rapid increase of the electron-phonon relaxation time below the Kondo temperature. We propose the hypothesis that the slower Fermi velocity compared to the sound velocity provides a natural blocking mechanism due to energy and momentum conservation laws.

Electrons in quantum materials exhibiting coexistence of dispersionless (flat) bands piercing dispersive (steep) bands can give rise to strongly correlated phenomena, and are associated with unconventional superconductivity. It is known that in twisted trilayer graphene steep Dirac cones can coexist with band flattening, but the phenomenon is not stable under layer misalignments. Here we show that such a twisted sandwiched graphene (TSWG) -a threelayer van der Waals heterostructure with a twisted middle layer -can have very stable flat bands coexisting with Dirac cones near the Fermi energy when twisted to 1.5 • . These flat bands require a specific high-symmetry stacking order, and our atomistic calculations predict that TSWG always relaxes to it. Additionally, with external fields, we can control the relative energy offset between the Dirac cone vertex and the flat bands. Our work establishes twisted sandwiched graphene as a new platform for research into strongly interacting phases, and topological transport beyond Dirac and Weyl semimetals. Graphene, an atomically thin crystal of carbon, provides an experimentally favorable platform for two dimensional (2D) Dirac physics as it exhibits ultrarelativistic Dirac cones in its band structure, described with massless quasiparticles when weak spin-orbit coupling is neglected.

The spatial and energy-dependent fluctuations of the local density of states (LDOS) are calculated by means of a transfer matrix formalism for a model point contact (PC). It is shown that decreasing the size of the orifice the LDOS fluctuations become more and more pronounced in the region near to the orifice. As the Kondo temperature of a magnetic impurity is determined by the LDOS at the Fermi energy a broadening of the Kondo resonance is predicted for small metallic PC's in agreement with the results of recent measurements.

We employ the Green's function technique to investigate the vacancy-induced quasi-localized magnetic moment formation in monolayer graphene starting with the Dirac Hamiltonian, which focuses on the πorbitals only, involving the nearest neighbor(NN)(t) and moderate second neighbor(SN)(t′ < t/3) hopping integrals. The vacancy defect is modeled by the addition of the on-site perturbation potential to the Hamiltonian. We find that, when (t′/t) << 1, the vacancy induced π-state at the zero of energy(zero-mode state(ZMS)) does not inhabit the minority sub-lattice due to the strong scalar potential induced by the vacancy(the ZMSs get lodged in the majority sub-lattice) whereas, when (t′/t) is increased, the ZMS is somewhat suppressed. This shows that, not only the shift of the Fermi energy away from the linearly-dispersive Dirac points, the issue of this topological localization is also hinged on the ratio (t′/t). Furthermore, when a vacancy is present, the three sp 2hybridized σ states of each of the three nearest-neighbor carbon atoms, forming a carbon triangle surrounding the vacancy, are close to the Fermi energy (E F ). The Hund's coupling between these σ electrons and the remaining electron which occupies the π state spin polarizes the π state leading to local moment formation close to E F . Since the system at the Fermi level has low electronic density, there is poor screening of such magnetic moments. This may lead to a high Curie temperature for such vacancy-induced moments.

As a two-dimensional semi-metal, graphene offers clear advantages for plasmonic applications over conventional metals, such as stronger optical field confinement, in situ tunability and relatively low intrinsic losses. However, the operational frequencies at which plasmons can be excited in graphene are limited by the Fermi energy EF, which in practice can be controlled electrostatically only up to a few tenths of an electronvolt. Higher Fermi energies open the door to novel plasmonic devices with unprecedented capabilities, particularly at mid-infrared and shorter-wave infrared frequencies. In addition, this grants us a better understanding of the interaction physics of intrinsic graphene phonons with graphene plasmons. Here, we present FeCl3-intercalated graphene as a new plasmonic material with high stability under environmental conditions and carrier concentrations corresponding to EF &gt; 1 eV. Near-field imaging of this highly-doped form of graphene allows us to characterize p...

The energy density state are the powerful factor for evaluate the validity of a material in any application. This research focused on examining the electrical properties of the Se6Te4- xSbx glass semiconductor with x=1, 2 and 3, using the thermal evaporation technique. D.C electrical conductivity was used by determine the current, voltage and temperatures, where the electrical conductivity was studied as a function of temperature and the mechanical electrical conduction were determined in the different conduction regions (the extended and localized area and at the Fermi level). In addition, the density of the energy states in these regions is calculated using the mathematical equations. The constants of energy density states are determined, namely the electron hopping distance, the width of the tails, and pre - exponential factor. The densities of the energetic states (extended N (Eext), localize N (Eloc) and at the Fermi states N (Ef) will be calculated in each of the regions. More...

Interplay of magnetism and electronic band topology in unconventional magnets enables the creation and fine control of novel electronic phenomena. In this work, we use scanning tunneling microscopy and spectroscopy to study thin films of a prototypical kagome magnet Fe3Sn2. Our experiments reveal an unusually large number of densely-spaced spectroscopic features straddling the Fermi level. These are consistent with signatures of low-energy Weyl fermions and associated topological Fermi arc surface states predicted by theory. By measuring their response as a function of magnetic field, we discover a pronounced evolution in energy tied to the magnetization direction. Electron scattering and interference imaging further demonstrates the tunable nature of a subset of related electronic states. Our experiments provide a direct visualization of how in-situ spin reorientation drives changes in the electronic density of states of the Weyl fermion band structure. Combined with previous repor...

Weyl semimetals were experimentally discovered as a new quantum phase of matter that exhibits topologically protected states characterized by separated Weyl points. Similar to other topological materials, the research of Landau level transitions can provide abundant information on Fermi surfaces. Extensive experimental efforts on low-temperature transport and optical properties have been dedicated to investigating the nontrivial topology structure. However, there are some theoretically predicted unconventional Landau level transitions in Weyl semimetals that have not been observed experimentally. And, their contribution to the electromagnetic response of Weyl semimetal remains elusive. Here, we report the magneto-optical study of Landau quantization and selection rules in Weyl semimetal NbP under Voigt geometry. By changing the direction of the electric vector of the incoming light under Voigt and Faraday geometry, abundant Landau level transition modes which contain = 0, ±1, ±2 selection rules are obtained. The richness of the optical spectra, particularly the ones from the Voigt geometry, allows us to determine the location of Fermi energy and its evolution with magnetic fields. We further extract the transitions between Landau levels and Fermi energy and deduce the Landau index. Our results reveal the track of Fermi energy under varying magnetic fields based on the Landau level transitions under Voigt geometry and suggest that a combination of the magneto-optical spectra under Voigt and Faraday geometry can effectively determine the key properties of topological materials that are otherwise inaccessible.

The transition metal dichalcogenide 1T-TaS 2 is a layered material exhibiting charge density waves. Based on angleresolved photoemission experiments mapping spectral weight at the Fermi surface and density functional theory calculations we discuss possible mechanisms involved with the creation of charge density waves. At first the flat parts of the elliptically shaped Fermi surface appear to play an important role via Fermi surface nesting. A closer analysis of the charge density wave induced new Brillouin zones and the possible energy balance between elastic deformation energy and electronic energy points to a more complicated scenario.

As a possible way of modifying the intrinsic properties of graphene we study the doping of graphene by embedded boron clusters with density functional theory. Cluster doping is technologically relevant as the cluster implantation technique can be readily applied to graphene. We find that B7 clusters embedded into graphene and graphene nanoribbons are structurally stable and locally metallize the system. This is done both by the reduction of the Fermi energy and by the introduction of boron states near the Fermi level. A linear chain of boron clusters forms a metallic "wire" inside the graphene matrix. In a zigzag edge graphene nanoribbon the cluster-related states tend to hybridize with the edge and bulk states. The magnetism in boron doped graphene systems is generally very weak. The presence of boron clusters weakens the edge magnetism in zigzag edge graphene nanoribbon, rather than making the system appropriate for spintronics. Thus the doping of graphene with the cluster implantation technique might be a viable technique to locally metallize graphene without destroying its attractive bulk properties.

After the publication of the Letter, we became aware of related work simultaneously published by Bhalla et al. [Phys. Rev. B 105, 125407 (2022)]. Using Kubo's formalism, the authors theoretically studied "acoustogalvanic effects" in two-dimensional Dirac materials that arise from sound-induced pseudogauge fields. Despite the differences between our two models, the authors predicted acoustoelectric responses that qualitatively resemble our experimental observations.

Fano resonance is a quantum effect particularly useful for determining the optical spectra of semiconductor heterostructures and radiation enhancement of semiconductor-based devices. We deal with the nonlinear amplitude equation to find the nonlinear plasmon modes and breather solutions in a magnetic impurities-added graphene-plasmonic waveguide at near and mid-IR frequency range. The results show that the coupling degree between the plamon modes and breather solutions in order to form plasmon-solitons is intensely influenced by the effective nonlinear refraction assigned to the waveguide and also by the Fermi energy; tunable plasmonsolitons can be formed with a lowly required bias voltage. We also deduce that the stable plasmon-solitons and subsequently the most condensed radiative feature with the largest effective propagation length can be obtained at interband transition edge if the resonance of the effective nonlinear refraction is of Fano type. The reason is inferred as the presence of a reversible mechanism caused by the optical doping which in turn bears a successive competition between the diffraction and nonlinearity. Accordingly, we indicate that the Fano resonance-induced plasmon-soliton modes supported by the Co-added graphene-plasmonic waveguide experience a frequency shift up to Δω ¼ 60 THz in contrast to the plasmon-soliton modes in the pristine graphene-based waveguide. This in turn, proposes a novel technique for the spectroscopy of magnetic impurities-added graphene-dielectric heterostructures. On the other hand, very sensitive modulation of the nonlinear response tunable with a low Fermi energy as presented in this study can delineate modern schemes for the next generation graphene plasmonic devices including the nanoscale radiation sources, modulators, biosensors and couplers.

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We describe very fast electron dynamics for a graphene nanoribbon driven by a control electromagnetic field in the terahertz frequency regime. The mobility as a function of bias field has been found to possess a large threshold value when entering a nonlinear transport regime. This value depends on the lattice temperature, electron density, impurity scattering strength, nanoribbon width and correlation length for the line-edge roughness. An enhanced electron mobility beyond this threshold has been observed, which is related to the initially-heated electrons in high energy states with a larger group velocity. However, this mobility enhancement quickly reaches a maximum governed by the Fermi velocity in graphene and the dramatically increased phonon scattering. Super-linear and sub-linear temperature dependences of the mobility are seen in the linear and nonlinear transport regimes, which is attributed separately to the results of sweeping electrons from the right Fermi edge to the left one through elastic scattering and moving electrons from lowenergy states to high-energy ones through field-induced electron heating. The threshold field is pushed up by a decreased correlation length in the high field regime, and is further accompanied by a reduced magnitude in the mobility enhancement. This implies an anomalous high-field increase of the line-edge roughness scattering with decreasing correlation length due to the occupation of high-energy states by field-induced electron heating. Additionally, a self-consistent device modeling has been proposed for graphene transistors under an optical modulation on its gate, which employs Boltzmann moment equations up to the third order for describing fast carrier dynamics and full wave electromagnetics coupled to the Boltzmann equation for describing spatial-temporal dependence of the total field. Finally, a detailed comparison of the derived Maxwell-Boltzmann moment equations in this paper with the well known Vlasov-Maxwell equations is also included.

Here, we comprehensively investigate the atomic structures and electronic properties of different antiphase boundaries in III-V semiconductors with different orientations and stoichiometries, including {110}, {100}, {111}, {112} and {113} ones, based on first-principle calculations. Especially, we demonstrate how the ladder or zigzag chemical bond configuration can lead for the different cases to a gapped semiconducting band structure, to a gapped metallic band structure or to a non-gapped metallic band structure. Besides, we evidence that the ladder APB configurations impact more significantly the Fermi energy levels than the zigzag APB configurations. We finally discuss how these different band structures can have some consequences on the operation of monolithic III-V/Si devices for photonics or energy harvesting.

We investigated negative photoconductivity in graphene using ultrafast terahertz techniques. Infrared transmission was used to determine the Fermi energy, carrier density and mobility of p-type CVD graphene samples. Time-resolved terahertz photoconductivity measurements using a tunable mid-infrared pump probed these samples at photon energies between 0.35eV-1.55eV, approximately one-half to three times the Fermi energy of the samples. Although interband optical transitions in graphene are blocked for pump photon energies less than twice the Fermi energy, we observe negative photoconductivity at all pump photon energies investigated, indicating that interband excitation is not required to observe this effect. Our results are consistent with a thermalized free carrier population that cools by electron-phonon scattering, but inconsistent with models of negative photoconductivity based on population inversion. I.

Graphene has proven to be a promising candidate for nanotechnological applications, in particular, design Veselago's lenses where the use of pn interfaces could allow to focus electrons without need for magnetic fields. These lenses can study in graphene-superconducting-graphene structures, where it is possible to focus electrons in electrons or electrons in holes. In this work, we study how tuning the Fermi energy or using imperfect interfaces affects the focusing of electrons or holes allowing to realize conditions that favor hole focusing over electron focusing.

In a quantum wire with ideal helical modes, the conductance is quantized in units of e 2 /h, provided the wire is connected to Fermi liquid leads. We show that this universality does not hold in partially gapped quasi-helical systems such as Rashba nanowires subject to a magnetic field, which are commonly used to mimic helical Luttinger liquids. Instead, their conductance takes a non-universal value that depends on the interactions in the wire, even in the presence of Fermi liquid leads. The non-universal conductance is rooted in a non-trivial mixing of spin and charge degrees of freedom, which in turn defines a non-helical low-energy Hamiltonian.

We explore the collective electronic excitations of bilayer molybdenum disulfide (MoS2) using the density functional theory together with the random phase approximation. The many-body dielectric function and electron energy-loss spectra are calculated using an ab initio based model involving material-realistic physical properties. The electron energy-loss function of bilayer MoS2 system is found to be sensitive to either electron or hole doping and it is owing to the fact that the Kohn-Sham band dispersions are not symmetric for energies above and below the zero Fermi level. Three plasmon modes are predicted. A damped high-energy mode, one optical mode (in-phase mode) for which the plasmon dispersion exhibits √ q in the long wavelength limit originating from low-energy electron scattering and finally a highly damped acoustic mode (out-of-phase mode).

We have measured the dependence of the Fermi energy on carrier concentration in Sn doped InGaAs at 4.2 K and 300 K. At 4.2 K the Fermi energy was measured by photoluminescence spectroscopy, and at 300 K it was deduced from transport measurements of thermionic emission. In both cases the dependence of the Fermi energy on the mobile electron concentration, measured by Hall effect, strongly deviates from standard theoretical predictions, and the deviation increases with concentration. The most striking observed anomaly is the near saturation of the Fermi level when the Hall concentration exceeds 10 19 cm -3 .

We describe a method of Fermi energy measurement, based on the analysis of thermionic emission and diffusion over a barrier with a built-in charge. The method can be applied to a variety of semiconductors and has been successfully tested by measuring the Fermi energy in GaAs.

The combination of photonic crystal fiber (PCF) and graphene-supporting surface plasmon polaritons (SPP) presents a new approach to achieving a plasmonic sensor with adjustable properties in the terahertz (THz) frequency range. In this study, we investigate a liquid-core PCF-based graphene plasmonic sensor, where the analyte to be detected is located on both the sensing layer surface and the fiber core. As a result, the dispersion relations of both graphene plasmon (GP) and core-guide mode can be influenced by the analyte, leading to a negative refractive index (RI) wavelength sensitivity. This unique performance is attributed to the higher modulation degree of the core mode on the analyte RI (Δneff.core) compared to that of the GP mode (Δneff.GP). By reducing the graphene Fermi energy, a positive sensibility is achieved with the modulation relationship of Δneff.core < Δneff.GP. Subsequently, the geometry dependence is explored to optimize the sensing capabilities. Furthermore, we demonstrate the sensor's tunability by dynamically varying the graphene Fermi energy (E f ). By adjusting the E f from 0.6 to 0.9 eV, the detection range can be artificially shifted from 0.554-0.574 THz to 0.686-0.724 THz, obtaining a tunability of 0.44 THz/eV and a higher sensitivity of 1.2667 THz/RIU. This design facilitates the efficient utilization of the limited bandwidth to detect various RIs and provides a flexible approach to constructing multiple sensing channels. To the best of our knowledge, this is the first report of graphene plasmonic sensing based on core-filled PCF in the THz frequency range. The novel analysis method of modulation degree and dispersion matching has the potential to be widely applied in THz plasmonic sensing and could lead to various nanoscience applications.

All-electrical writing and reading of spin states attract considerable attention for their promising applications in energy-efficient spintronics devices. Here we show, based on rigorous first-principles calculations, that the spin properties can be manipulated and detected in molecular spinterfaces, where an iron tetraphenyl porphyrin (FeTPP) molecule is deposited on boron-substituted graphene (BG). Notably, a reversible spin switching between the S ¼ 1 and S ¼ 3=2 states is achieved by a gate electrode. We can trace the origin to a strong hybridization between the Fed z 2 and B-p z orbitals. Combining density functional theory with nonequilibrium Green's function formalism, we propose an experimentally feasible three-terminal setup to probe the spin state. Furthermore, we show how the in-plane quantum transport for the BG, which is non-spin polarized, can be modified by FeTPP, yielding a significant transport spin polarization near the Fermi energy (> 10% for typical coverage). Our work paves the way to realize allelectrical spintronics devices using molecular spinterfaces.

All-electrical writing and reading of spin states attract considerable attention for their promising applications in energy-efficient spintronics devices. Here we show, based on rigorous first-principles calculations, that the spin properties can be manipulated and detected in molecular spinterfaces, where an iron tetraphenyl porphyrin (FeTPP) molecule is deposited on boron-substituted graphene (BG). Notably, a reversible spin switching between the S ¼ 1 and S ¼ 3=2 states is achieved by a gate electrode. We can trace the origin to a strong hybridization between the Fed z 2 and B-p z orbitals. Combining density functional theory with nonequilibrium Green's function formalism, we propose an experimentally feasible three-terminal setup to probe the spin state. Furthermore, we show how the in-plane quantum transport for the BG, which is non-spin polarized, can be modified by FeTPP, yielding a significant transport spin polarization near the Fermi energy (> 10% for typical coverage). Our work paves the way to realize allelectrical spintronics devices using molecular spinterfaces.

We present a theoretical model for the calculation of the energy loss rate (ELR) of hot electrons in a monolayer graphene due to their coupling with acoustic phonons at high perpendicular magnetic fields. Electrons interact with both transverse acoustic (TA) and longitudinal acoustic (LA) phonons. Numerical simulations of the ELR are performed as a function of the magnetic field, the electron temperature, the electron density, and the Landau level broadening. We find robust oscillations of the ELR as a function of the filling factor ν that originate from the oscillating density of states at the Fermi level. Screening effects on the deformation potential coupling are taken into account, and it is found that they lead to a significant reduction of ELR, especially, at low electron temperatures, Te, and high magnetic fields. At temperatures much lower than the Bloch–Grüneisen temperature, the ELR shows a Te4 dependence that is related to the unscreened electron interaction with TA acous...

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We have calculated the conductivities of the 3d, 4d, and 5d transition metals by using a quasiclassical Boltzmann approach which utilizes self-consistent energy bands. Electron scattering is characterized by a constant scattering strength which treats all sources of resistivity on an equal footing. This results in a single parameter theory with which we are able to reproduce, qualitatively, trends and anomalies in conductivity throughout the transition metal series. By the choice of an appropriate scattering strength at each temperature, we are able to reproduce these trends over the experimentally observable temperature range of 10-1000 K. In particular, the well known dip in conductivity at the middle of the series, especially pronounced for Mn, is understood in terms of the host electronic structure.

We present a novel technique for measuring the lifetime of quasiparticle excitations of a 2DES by investigating the tunnelling into a quantum dot from a 2DES over an extended range of energy from the Fermi energy to the sub-band edge. We ÿnd that the lifetime qp, of a quasihole excitation, caused by removing an electron from a 2DES state with energy below the Fermi energy EF, has the form qp = ˜= , where is a constant of order unity.

We use the framework of supersymmetric transformations in the construction of coupled systems of Dirac fermions. Its energy operator is a composite of the generators of the associated superalgebra, and the two coupled Dirac fermions acquire different Fermi velocities. We discuss in detail the peculiar spectral properties of the new system. The emergent phenomena like level crossing or generation of bound states in the continuum (BICs) are illustrated in two explicit examples.

In a clean Fermi liquid, due to spin up/spin down symmetry, the dc spin current driven by a magnetic field gradient is finite even in the absence of impurities. Hence, the spin conductivity σ s assumes a well-defined collisiondominated value in the disorder-free limit, providing a direct measure of the inverse strength of electron-electron interactions. In neutral graphene, with Fermi energy at the Dirac point, the Coulomb interactions remain unusually strong, such that the inelastic scattering rate comes close to a conjectured upper bound τ −1 inel k B T /h, similar to the case of strongly coupled quantum critical systems. The strong scattering is reflected by a minimum of spin conductivity at the Dirac point, where it reaches σ s = 0.121 α 2 µ 2 s h at weak Coulomb coupling α, µ s ≈ µ B being the magnetic moment of the electronic spins. Up to the replacement of quantum units, e 2 /h → µ 2 s /h, this result equals the collisiondominated electrical conductivity obtained previously. This accidental symmetry is, however, broken to higher orders in the interaction strength. For gated graphene and two-dimensional metals in general, we show that the transport time is parametrically smaller than the collision time. We exploit this fact to compute the collision-limited σ s analytically as σ s = 1 C µ T 2 µ 2 s h , with C = 4π 2 α 2 2 3 ln(1/2α) − 1 for weak Coulomb coupling α.

We propose a natural way to create quantum-confined regions in graphene in a system that allows large-scale device integration. We show, using first-principles calculations, that a single graphene layer on a trenched region of [0001] SiC mimics i)the energy bands around the Fermi level and ii) the magnetic properties of free-standing graphene nanoribbons. Depending on the trench direction, either zigzag or armchair nanoribbons are mimicked. This behavior occurs because a single graphene layer over a SiC surface loses the graphene-like properties, which are restored solely over the trenches, providing in this way a confined strip region.

We study the tunneling conductance of a silicene-based ferromagnet/insulator/superconductor (FIS) junction by the use of the spin-dependent Dirac-Bogoliubov de-Gennes equation. We demonstrate that the conductance spectra are strongly affected by exchange energy h, Fermi energy EF, and external perpendicular electric field Ez. In the thin barrier limit of insulator silicene IS, the zero-bias charge conductance of the FIS silicene junction oscillates as a function of barrier strength χG. It is shown that the period of oscillations changes from π/2 to π corresponding to undoped and doped silicene. Remarkably, in contrast to that of the graphene FIS junction where the conductance only vanishes at the exchange energy h=EF, here due to the buckled structure of silicene, there is a transport gap region for the range of h values and the magnitude of such a gap region can be controlled by Ez. Moreover, it is found that by appropriate choice of h and Ez, it is possible to achieve a fully spin...

We investigate the optical conductivity and far-infrared magneto-optical response of BaNiS 2 , a simple square-lattice semimetal characterized by Dirac nodal lines that disperse exclusively along the out-of-plane direction. With the magnetic field aligned along the nodal line the in-plane Landau level spectra show a nearly √ B behavior, the hallmark of a conical-band dispersion with a small spin-orbit coupling gap. The optical conductivity exhibits an unusual temperature-independent isosbestic line, ending at a Van Hove singularity. First-principles calculations unambiguously assign the isosbestic line to transitions across Dirac nodal states. Our work suggests a universal topology of the electronic structure of Dirac nodal lines.

Using a mode-matching method, we investigate the ballistic transport properties of noninteracting electrons in two types of semiconductor nanostructures in which the confining potential has smooth, rounded corners. The rounded corners are simulated by dividing the structures into narrow strips and varying the width of each strip. The two structures we have investigated are a straight channel connecting two reservoirs of two-dimensional electrons and a crossbar structure consisting of two perpendicular channels, one of which connects to the reservoirs. In the case of the straight channel we find that the rounded corners decreases oscillations in the conductance. For the crossbar structure we have compared with results from Berggren et al. (K.-F. Berggren, C. Besev and Z.-L. Ji, Physica Scripta T 42 (1992) 141) who used a model with sharp corners. For low values of the Fermi energy our results are in good agreement with theirs, which validates the simplification of using sharp corners. As the energy increases differences appear, and at certain energies, the rounded corners drastically changes the conductance, transforming a resonance peak to an anti-resonance dip.

Using a mode-matching method, we investigate the ballistic transport properties of noninteracting electrons in two types of semiconductor nanostructures in which the confining potential has smooth, rounded corners. The rounded corners are simulated by dividing the structures into narrow strips and varying the width of each strip. The two structures we have investigated are a straight channel connecting two reservoirs of two-dimensional electrons and a cross-bar structure consisting of two perpendicular channels, one of which connects to the reservoirs. In the case of the straight channel we find that the rounded corners decreases oscillations in the conductance. For the cross-bar structure we have compared with results from Berggren et al. (K.-F. Berggren, C. Besev and Z.-L. Ji, Physica Scripta T 42 (1992) 141) who used a model with sharp corners. For low values of the Fermi energy our results are in good agreement with theirs, which validates the simplification of using sharp corners. As the energy increases differences appear, and at certain energies, the rounded corners drastically changes the conductance, transforming a resonance peak to an anti-resonance dip.

Graphene has proven to be a promising candidate for nanotechnological applications, in particular, design Veselago's lenses where the use of pn interfaces could allow to focus electrons without need for magnetic fields. These lenses can study in graphene-superconducting-graphene structures, where it is possible to focus electrons in electrons or electrons in holes. In this work, we study how tuning the Fermi energy or using imperfect interfaces affects the focusing of electrons or holes allowing to realize conditions that favor hole focusing over electron focusing.

We perform a comprehensive analysis of the spectrum of graphene plasmons which arise when a pair of sheets are confined between thick conducting materials. The associated enhanced local fields may be employed in the manipulation of light on the nanoscale by adjusting the separation between the graphene layers, the energy band gap as well as the concentration of charge carriers in the conducting media surrounding the two-dimensional (2D) layers. We present a theoretical formulation for calculating the plasmon spectrum of an encapsulated pair of 2D layer and apply it to graphene. This calculation is relevant to studies of energy transfer via plasmon excitations when graphene is confined by a pair of thick conducting materials. We employ the random-phase approximation (RPA) integral equation for a system composed of two identical semi-infinite conducting plasmas with planar bounding surfaces at $z = \pm a/2$ enclosing a pair of 2D semiconductor plasma at $z=\pm b/2$ in the narrow gap r...

Profiting from previous works done with the INDRA multidetector [1] on the description of the light response L of the CsI(Tl) crystals to different impinging nuclei [2, 3], we propose an improved ∆E − L identification-calibration procedure for Silicon-Cesium Iodide (Si-CsI) telescopes, namely an Advanced Mass Estimate (AME) method. AME is compared to the usual, simple visual analysis of the corresponding two-dimensional map of ∆E − E type, by using INDRA experimental data from nuclear reactions induced by heavy ions in the Fermi energy regime. We show that the capability of such telescopes to identify both the atomic Z and the mass A numbers of light and heavy reaction products, can be quantitatively improved thanks to the proposed approach. This conclusion opens new possibilities to use INDRA for studying these reactions especially with radioactive beams. Indeed, the determination of the mass for charged reaction products becomes of paramount importance to shed light on the role of the isospin degree of freedom in the nuclear equation of state [4, 5] .

We analyze the out of plane hopping in models of layered systems where the in-plane properties deviate from Landau's theory of a Fermi liquid. We show that the hopping term acquires a non trivial energy dependence, due to the coupling to in plane excitations, and can be either relevant or irrelevant at low energies or temperatures. The latter is always the case if the Fermi level lies close to a saddle point in the dispersion relation.

We study the existence and topological stability of Fermi points in a graphene layer and stacks with many layers. We show that the discrete symmetries (spacetime inversion) stabilize the Fermi points in monolayer, bilayer and multilayer graphene with orthorhombic stacking. The bands near k = 0 and ǫ = 0 in multilayers with the Bernal stacking depend on the parity of the number of layers, and Fermi points are unstable when the number of layers is odd. The low energy changes in the electronic structure induced by commensurate perturbations which mix the two Dirac points are also investigated.

We study the transport properties of a neutral graphene sheet with curved regions induced or stabilized by topological defects. The proposed model gives rise to Dirac fermions in a random magnetic field and in the random space dependent Fermi velocity induced by the curvature. This last term leads to singular long range correlated disorder with special characteristics. The Drude minimal conductivity at zero energy is found to be inversely proportional to the density of topological disorder, a signature of diffusive behavior.

We have produced a single quantum dot using a gate fabricated by electron beam lithography on a high mobility heterostructure. A 0.5pm square of two dimensional electron gas is defined between four 0.18pm wide metal lines. When the current is restricted to pass through the box, structure is seen in the conductance versus gate voltage curves associated with resonances. The device behaves in a similar way to a one dimensional ballistic point contact with two narrow barriers in it. Putting a D.C. voltage on the device reveals two resonant peaks enabling the level separation at the Fermi energy within the box to be measured as 1.4meV. A magnetic fields destroys the resonances when the level separations are similar to the magnetic level broadening

When a carbon nanotube is truncated with certain type of edges, boundary states localized near the edges appear at the Fermi level. Starting from lattice models, low-energy effective theories are constructed which describe electron correlation effects on the boundary states. We then focus on a thin metallic carbon nanotube which supports one or two boundary states and discuss physical consequences of the interaction between the boundary states and bulk collective excitations. By the renormalization group analyses together with the open boundary bosonization, we show that the repulsive bulk interactions suppress the charge fluctuations at boundaries and assist the spin polarization.

Two-dimensional graphite sheets with a certain type of edges are known to support boundary states localized near the edges. Forming a flat band with a sharp peak in the density of states at the Fermi energy, they can trigger a magnetic instability or a distortion of the lattice in the presence of electron-electron or electron-phonon interactions. We shall discuss a relationship between chiral symmetry, which is the origin of the zero-energy edge states, and several types of induced orders such as spin density waves or lattice distortions. We also investigate electron correlation effects on the edge states for a wrapped quasi one-dimensional geometry, i.e., carbon nanotube, by means of the renormalization group and the open boundary bosonization.

Large efforts in improving thermoelectric energy conversion are devoted to energy filtering by nanometer size potential barriers. In this work, we perform an analysis and optimization of such barriers for improved energy filtering. We merge semiclassical with quantum mechanical simulations to capture tunneling and reflections due to the barrier and analyze the influence of the width W, the height Vb, and the shape of the barrier, and the position of the Fermi level (EF) above the band edge, ηF. We show that for an optimized design, ∼40% improvement in the thermoelectric power factor can be achieved if the following conditions are met: ηF is large; Vb−EF is somewhat higher but comparable to kBT; and W is large enough to suppress tunneling. Finally, we show that a smooth energy barrier is beneficial compared to a sharp (square) barrier for increasing the thermoelectric power factor.

Using four-terminal Landauer-Büttiker formalism and Green’s function technique, in this present paper, we calculate numerically spin Hall conductance (SHC) and longitudinal conductance of a finite size kagome lattice with Rashba spin-orbit (SO) interaction both in the presence and absence of external magnetic flux in clean limit. In the absence of magnetic flux, we observe that depending on the Fermi surface topology of the system SHC changes its sign at certain values of Fermi energy. Unlike the infinite system (where SHC is a universal constant ±e8π), here SHC depends on the external parameters like SO coupling strength, Fermi energy, etc. We show that in the presence of any arbitrary magnetic flux, periodicity of the system is lost and the features of SHC tend to get reduced because of elastic scattering. But again at some typical values of flux (ϕ=12, 14, 34…, etc.) the system retains its periodicity depending on its size and the features of spin Hall effect (SHE) reappears. Our...

In this work we report for the first time, to the best of our knowledge, the tunable electronic properties of beta-borophene (BB) on the polar substrate ( Zr O 2 ). We provide an analytical prescription for the calculation of ground state energy in the presence of the electron–phonon (e-ph) interaction, within the framework of the Lee-Low-Pines theory. In the theoretical investigation of the polaron formation in BB, we describe its effective masses, polaronic band-gap, mobility, and Fermi velocities, which are different in each coordinate due to the out-of-plane buckling structure. We also analyze how the average effective mass and Fermi velocities of the charge carriers in the buckling structure are affected by the external magnetic field. It is shown that the polaronic energy becomes more effective in presence of a magnetic field, which we confirm through an analytical prescription. The characteristics of the evolution of average effective mass and average effective Fermi velocity...