Showing papers on "Fermi energy published in 2007"
TL;DR: In this article, the authors focus on the origin of the D and G peaks and the second order of D peak and show that the G and 2 D Raman peaks change in shape, position and relative intensity with number of graphene layers.
6,496 citations
TL;DR: A graphene bilayer with a relative small angle rotation between the layers is considered and it is found that the low energy dispersion is linear, as in a single layer, but the Fermi velocity can be significantly smaller than the single-layer value.
Abstract: We consider a graphene bilayer with a relative small angle rotation between the layers--a stacking defect often seen in the surface of graphite--and calculate the electronic structure near zero energy in a continuum approximation. Contrary to what happens in an AB stacked bilayer and in accord with observations in epitaxial graphene, we find: (a) the low energy dispersion is linear, as in a single layer, but the Fermi velocity can be significantly smaller than the single-layer value; (b) an external electric field, perpendicular to the layers, does not open an electronic gap.
1,277 citations
TL;DR: It is shown that ABO fails in graphene, a zero-bandgap semiconductor that becomes a metal if the Fermi energy is tuned applying a gate voltage, Vg, which induces a stiffening of the Raman G peak that cannot be described within ABO.
Abstract: The adiabatic Born-Oppenheimer approximation (ABO) has been the standard ansatz to describe the interaction between electrons and nuclei since the early days of quantum mechanics. ABO assumes that the lighter electrons adjust adiabatically to the motion of the heavier nuclei, remaining at any time in their instantaneous ground state. ABO is well justified when the energy gap between ground and excited electronic states is larger than the energy scale of the nuclear motion. In metals, the gap is zero and phenomena beyond ABO (such as phonon-mediated superconductivity or phonon-induced renormalization of the electronic properties) occur. The use of ABO to describe lattice motion in metals is, therefore, questionable. In spite of this, ABO has proved effective for the accurate determination of chemical reactions, molecular dynamics and phonon frequencies in a wide range of metallic systems. Here, we show that ABO fails in graphene. Graphene, recently discovered in the free state, is a zero-bandgap semiconductor that becomes a metal if the Fermi energy is tuned applying a gate voltage, Vg. This induces a stiffening of the Raman G peak that cannot be described within ABO.
1,276 citations
TL;DR: Gate-modulated low-temperature Raman spectra reveal that the electric field effect (EFE), pervasive in contemporary electronics, has marked impacts on long-wavelength optical phonons of graphene and demonstrate optically the particle-hole symmetry about the charge-neutral Dirac point.
Abstract: Gate-modulated low-temperature Raman spectra reveal that the electric field effect (EFE), pervasive in contemporary electronics, has marked impacts on long-wavelength optical phonons of graphene. The EFE in this two-dimensional honeycomb lattice of carbon atoms creates large density modulations of carriers with linear dispersion (known as Dirac fermions). Our EFE Raman spectra display the interactions of lattice vibrations with these unusual carriers. The changes of phonon frequency and linewidth demonstrate optically the particle-hole symmetry about the charge-neutral Dirac point. The linear dependence of the phonon frequency on the EFE-modulated Fermi energy is explained as the electron-phonon coupling of massless Dirac fermions.
1,061 citations
TL;DR: The results reveal that the occupied states are closest to the gold Fermi energy, indicating that the tunneling transport through these molecules is analogous to hole tunneling through an insulating film.
Abstract: We measure the low bias conductance of a series of substituted benzene diamine molecules while breaking a gold point contact in a solution of the molecules. Transport through these substituted benzenes is by means of nonresonant tunneling or superexchange, with the molecular junction conductance depending on the alignment of the metal Fermi level to the closest molecular level. Electron-donating substituents, which drive the occupied molecular orbitals up, increase the junction conductance, while electron-withdrawing substituents have the opposite effect. Thus for the measured series, conductance varies inversely with the calculated ionization potential of the molecules. These results reveal that the occupied states are closest to the gold Fermi energy, indicating that the tunneling transport through these molecules is analogous to hole tunneling through an insulating film.
302 citations
TL;DR: Evaporative cooling was performed to cool fermionic 173Yb atoms in a crossed optical dipole trap and a plunge of evaporation efficiency was observed as a result of Fermi degeneracy.
Abstract: Evaporative cooling was performed to cool fermionic 173Yb atoms in a crossed optical dipole trap. The large elastic collision rate leads to efficient evaporation and we have successfully cooled the atoms to 0.37+/-0.06 of the Fermi temperature, that is to say, to a quantum degenerate regime. In this regime, a plunge of evaporation efficiency was observed as a result of Fermi degeneracy.
254 citations
TL;DR: In this paper, the authors measured the low bias conductance of a series of substituted benzene diamine molecules while breaking a gold point contact in a solution of the molecules, and showed that the occupied states are closest to the gold Fermi energy.
Abstract: We measure the low bias conductance of a series of substituted benzene diamine molecules while breaking a gold point contact in a solution of the molecules. Transport through these substituted benzenes is by means of nonresonant tunneling or superexchange, with the molecular junction conductance depending on the alignment of the metal Fermi level to the closest molecular level. Electron-donating substituents, which drive the occupied molecular orbitals up, increase the junction conductance, while electron-withdrawing substituents have the opposite effect. Thus for the measured series, conductance varies inversely with the calculated ionization potential of the molecules. These results reveal that the occupied states are closest to the gold Fermi energy, indicating that the tunneling transport through these molecules is analogous to hole tunneling through an insulating film.
243 citations
TL;DR: In this article, the authors derived a formula to calculate the ballistic thermal conductance of a two-dimensional system directly from the dispersion relations of phonons and electrons, and applied the method to a graphene and investigated both the temperature and the Fermi energy dependences of the ballistic conductance.
Abstract: We derive a formula to calculate the ballistic thermal conductance of a two-dimensional system directly from the dispersion relations of phonons and electrons. We apply the method to a graphene and investigate both the temperature and the Fermi energy dependences of the ballistic thermal conductance. The ballistic thermal conductance per unit length of a graphene becomes isotropic from the threefold rotational symmetry. In the intrinsic graphene where the Fermi energy crosses the Dirac point, the thermal conductance of electrons increases in proportion to ${T}^{2}$ with temperature, while the phonon conductance increases in proportion to ${T}^{1.5}$ due to the quadratic dispersion relation of the out-of-plane acoustic mode and prevails over the electron-derived conductance irrespective of temperature. As the Fermi energy is moved from the Dirac point for the gated graphenes, the thermal conductance of electrons increases monotonically and the temperature dependence changes from a ${T}^{2}$ dependence in the intrinsic graphene to a $T$-linear one at low temperatures. The electron thermal conductance of the gated graphenes dominates over the phonon contribution at low temperatures.
241 citations
TL;DR: In this article, the effect of the curvature of free standing graphene samples on the electronic structure of the system has been studied, where the curvatures were modeled as smooth curved bumps and the Green's function of Dirac fermions in the curved surface was computed.
Abstract: We study the effect of the curved ripples observed in the free standing graphene samples on the electronic structure of the system. We model the ripples as smooth curved bumps and compute the Green's function of the Dirac fermions in the curved surface. Curved regions modify the Fermi velocity that becomes a function of the point on the graphene surface and induce energy dependent oscillations in the local density of states around the position of the bump. The corrections are estimated to be of a few percent of the flat density at the typical energies explored in local probes such as scanning tunnel microscopy that should be able to observe the predicted correlation of the morphology with the electronics. We discuss the connection of the present work with the recent observation of charge anisotropy in graphene and propose that it can be used as an experimental test of the curvature effects.
213 citations
TL;DR: In this article, the pairing gap for cold Fermi atoms and low-density neutron matter as a function of the FermI momentum times the scattering length was studied. But the pairing gaps were not as large as those of cold atoms.
Abstract: Experiments with cold Fermi atoms can be tuned to probe strongly-interacting fluids that are very similar to the low-density neutron matter found in the crusts of neutron stars. In contrast to traditional superfluids and superconductors, matter in this regime is very strongly paired, with gaps of the order of the Fermi energy. We compute the $T=0$ equation of state and pairing gap for cold atoms and low-density neutron matter as a function of the Fermi momentum times the scattering length. Results of quantum Monte Carlo calculations show that the equations of state are very similar. The neutron matter pairing gap at low densities is found to be very large but, except at the smallest densities, significantly suppressed relative to cold atoms because of the finite effective range in the neutron-neutron interaction.
179 citations
TL;DR: In this article, sound waves are excited in an optically trapped degenerate Fermi gas of spin-up and spin-down atoms with magnetically tunable interactions, and measured sound velocities test theories of hydrodynamic wave propagation and predictions of the equation of state.
Abstract: Sound waves are excited in an optically trapped degenerate Fermi gas of spin-up and spin-down atoms with magnetically tunable interactions. Measurements are made throughout the crossover region, from a weakly interacting Fermi gas through the resonant Fermi superfluid regime to a Bose condensate of dimer molecules. The measured sound velocities test theories of hydrodynamic wave propagation and predictions of the equation of state. At resonance, the sound velocity exhibits universal scaling with the Fermi velocity, to within 1.8% over a factor of 30 in density.
TL;DR: In this article, the effect of passivation on the electronic properties of graphenes and their impact on nanoelectronic devices based on graphene was studied. But the results of this paper are limited to the case of armchair and zigzag ribbons.
Abstract: The finite size effects on the electronic structure of graphene ribbons are studied using first principles density functional techniques. The energy gap [difference between highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO)] dependence for finite width and length is computed for both armchair and zigzag ribbons and compared to their one-dimensional (infinite length) cases. The results suggest, in addition to quantum confinement along the width of the ribbon, an additional finite size effect emerges along the length of ribbons only for metallic armchair ribbons. The origin of additional quantum confinement in these structures is analyzed based on the energy states near the Fermi energy: both HOMO and LUMO energy levels for metallic armchair ribbons are delocalized entirely on the ribbons while for nonmetallic ribbons, these states are localized at the edges only. The results are discussed in light of effect of passivation on the electronic properties of graphenes and their impact on nanoelectronic devices based on graphenes.
TL;DR: In this paper, the authors compare a number of proposed models for the arcs and find that the one that best models the data is a $d$-wave energy gap with a lifetime broadening whose temperature dependence is suggestive of fluctuating pairs.
Abstract: Angle resolved photoemission data in the pseudogap phase of underdoped cuprates have revealed the presence of a truncated Fermi surface consisting of Fermi arcs. We compare a number of proposed models for the arcs and find that the one that best models the data is a $d$-wave energy gap with a lifetime broadening whose temperature dependence is suggestive of fluctuating pairs.
TL;DR: The spin-flip scattering at the junction interfaces opens the Josephson channel of the odd-frequency spin-triplet Cooper pairs, and the local density of states in a half metal has a large peak at the Fermi energy.
Abstract: Motivated by a recent experiment [Keizer et al, Nature (London) 439, 825 (2006)], we study the Josephson effect in superconductor/diffusive half metal/superconductor junctions using the recursive Green function method The spin-flip scattering at the junction interfaces opens the Josephson channel of the odd-frequency spin-triplet Cooper pairs As a consequence, the local density of states in a half metal has a large peak at the Fermi energy Therefore the odd-frequency pairs can be detected experimentally by using the scanning tunneling spectroscopy
TL;DR: In this paper, the Fermi energy dependence of the (time-averaged) current and shot noise in an impurity-free carbon bilayer (length $L⪡\text{width}$ $W$) was compared with known results for a monolayer.
Abstract: We calculate the Fermi energy dependence of the (time-averaged) current and shot noise in an impurity-free carbon bilayer (length $L⪡\text{width}$ $W$), and compare with known results for a monolayer. At the Dirac point of charge neutrality, the bilayer transmits as two independent monolayers in parallel: Both current and noise are resonant at twice the monolayer value, so that their ratio (the Fano factor) has the same $1∕3$ value as in a monolayer---and the same value as in a diffusive metal. The range of Fermi energies around the Dirac point within which this pseudodiffusive result holds is smaller, however, in a bilayer than in a monolayer (by a factor ${l}_{\ensuremath{\perp}}∕L$, with ${l}_{\ensuremath{\perp}}$ the interlayer coupling length).
TL;DR: A fully self-consistent and still computationally undemanding self-interaction correction scheme that overcomes limitations of density-functional theory calculations of electronic transport based on local exchange and correlation functionals is presented.
Abstract: Density-functional theory calculations of electronic transport based on local exchange and correlation functionals contain self-interaction errors. As a consequence, insulating molecules in weak contact with metallic electrodes erroneously form highly conducting junctions. Here we present a fully self-consistent and still computationally undemanding self-interaction correction scheme that overcomes these limitations. The method is implemented in the transport code SMEAGOL and applied to the prototypical case of benzene molecules and gold electrodes. The Kohn-Sham highest occupied molecular orbital now reproduces closely the negative of the molecular ionization potential and is moved away from the gold Fermi energy. This leads to a drastic reduction of the low-bias current in much better agreement with experiments.
TL;DR: It is shown that this pairing state caused by the "Amperean" attractive interaction between spinons on a Fermi surface mediated by the U(1) gauge field can explain many of the observed low temperature phenomena.
Abstract: Recent experiments on the organic compound kappa-(BEDT-TTF)2Cu2(CN)3 raise the possibility that the system may be described as a quantum spin liquid. Here we propose a pairing state caused by the "Amperean" attractive interaction between spinons on a Fermi surface mediated by the U(1) gauge field. We show that this state can explain many of the observed low temperature phenomena and discuss testable consequences.
TL;DR: Spatially resolved radio-frequency spectroscopy of a trapped Fermi gas with resonant interactions is presented and a spectral gap at low temperatures is observed.
Abstract: We present spatially resolved radio-frequency spectroscopy of a trapped Fermi gas with resonant interactions and observe a spectral gap at low temperatures. The spatial distribution of the spectral response of the trapped gas is obtained using in situ phase-contrast imaging and 3D image reconstruction. At the lowest temperature, the homogeneous rf spectrum shows an asymmetric excitation line shape with a peak at 0.48(4){epsilon}{sub F} with respect to the free atomic line, where {epsilon}{sub F} is the local Fermi energy.
TL;DR: The transport properties of an interacting Fermi gas in a three-dimensional optical lattice is explored and strong attraction between the atoms leads to the formation of local pairs with a reduced tunneling rate.
Abstract: We explore the transport properties of an interacting Fermi gas in a three-dimensional optical lattice. The center of mass dynamics of the atoms after a sudden displacement of the trap minimum is monitored for different interaction strengths and lattice fillings. With increasingly strong attractive interactions the weakly damped oscillation, observed for the noninteracting case, turns into a slow relaxational drift. Tuning the interaction strength during the evolution allows us to dynamically control the transport behavior. Strong attraction between the atoms leads to the formation of local pairs with a reduced tunneling rate. The interpretation in terms of pair formation is supported by a measurement of the number of doubly occupied lattice sites. This quantity also allows us to determine the temperature of the noninteracting gas in the lattice to be as low as $(27\ifmmode\pm\else\textpm\fi{}2)%$ of the Fermi temperature.
TL;DR: In this paper, the electronic structures of α-PbO and β-pbO2 have been investigated by X-ray photoemission, Xray absorption and Xray emission spectroscopies, supported by bandstructure calculations performed within the framework of density functional theory.
Abstract: The electronic structures of α-PbO and β-PbO2 have been investigated by X-ray photoemission, X-ray absorption and X-ray emission spectroscopies, supported by bandstructure calculations performed within the framework of density functional theory. The relative intensity of a peak found at the bottom of the valence band for both oxides changes dramatically between Al Kα X-ray photoemission and O K shell X-ray emission spectra, demonstrating that the states associated with this peak possess dominant Pb 6s character. This finding is in accord with partial densities of states derived from bandstructure calculations but is at variance with the conventional view that the Pb 6s states in PbO are close to the Fermi energy and hybridise with empty 6p states to give a metal based directional 6s–6p lone pair. The photoemission onset of β-PbO2 contains a well-defined metallic Fermi edge. The position of the onset structure suggests that the metallic nature of PbO2 arises from occupation of conduction band states above the main valence band, probably arising from oxygen vacancy defects. The conduction electrons of β-PbO2 are strongly perturbed by ionisation of Pb core levels, giving rise to distinctive satellites in core XPS whose energies correspond to those of the conduction electron plasmon.
TL;DR: In this article, the authors showed that vacuum level alignment is only valid for certain O/O heterojunctions rather than a general rule for organic junctions, and that the mode of energy level alignment depends on the Fermi level position in the organic energy gap.
Abstract: Electronic structures of organic/organic (O/O) heterojunctions have been studied by photoemission spectroscopy We showed that vacuum level alignment is only valid for certain O/O heterojunctions rather than a general rule for organic junctions The mode of energy level alignment is found to depend on the Fermi level position in the organic energy gap In general, when the Fermi level is near the midgap position, vacuum level alignment at the O/O heterojunction is observed, whereas when the Fermi level is close to the edge of the lowest unoccupied or highest occupied molecular orbital level, Fermi level pinning accompanied by molecular orbital level bending is observed at the O/O heterojunction
TL;DR: It is shown that strong pairing correlations in Fermi gases lead to the appearance of a gaplike structure in the rf spectrum, both in the balanced superfluid and in the normal phase above the Clogston-Chandrasekhar limit.
Abstract: We show that strong pairing correlations in Fermi gases lead to the appearance of a gaplike structure in the rf spectrum, both in the balanced superfluid and in the normal phase above the Clogston-Chandrasekhar limit. The average rf shift of a unitary gas is proportional to the ratio of the Fermi velocity and the scattering length with the final state. In the strongly imbalanced case, the rf spectrum measures the binding energy of a minority atom to the Fermi sea of majority atoms. Our results provide a qualitative understanding of recent experiments by Schunck et al.
TL;DR: It is shown how a topological change of the Fermi surface leads to an effective reduction of the dimensionality in the electronic density of states in the low charge density regime and it is shown that the superconducting critical temperature is significantly tuned in this regime by the spin-orbit coupling.
Abstract: In this Letter we show how, for small values of the Fermi energy compared to the spin-orbit splitting of Rashba type, a topological change of the Fermi surface leads to an effective reduction of the dimensionality in the electronic density of states in the low charge density regime. We investigate its consequences on the onset of the superconducting instability. We show that the superconducting critical temperature is significantly tuned in this regime by the spin-orbit coupling. We suggest that materials with strong spin-orbit coupling are good candidates for enhanced superconductivity.
TL;DR: In this paper, the brittle and elastic properties of B2-MgRE intermetallics have been investigated using first-principles density functional calculations, and the calculated equilibrium lattice constants and enthalpies of formation are in overall agreement with the available experiment and theoretical results.
Abstract: The brittle and elastic properties of the B2-MgRE (RE = Sc, Y, Ce, Pr, Nd, Gd, Tb, Dy, Ho, Er) intermetallics have been investigated using first-principles density functional calculations. The calculated equilibrium lattice constants and enthalpies of formation are in overall agreement with the available experiment and theoretical results. The related physical properties of those compounds are compared with that of ductile YCu. The Fermi energy occurs above a peak in the DOS for B2-MgRE intermetallics, whereas for ductile YCu the Fermi energy occurs near a minimum in the DOS. For B2-YCu, the partial density of states of d-states at the Fermi energy is low, while for B2-MgRE the RE d-states are partially occupied, indicating their important roles in the directional bonding for this material. The Cauchy pressure (C12-C44) and the ratio of bulk to shear modulus B/G are used to assess the brittle/ductile behavior of B2-MgRE and YCu compounds. It can be concluded that the B2-MgRE alloys have brittle behavior. MgSc is the most brittle, and MgHo is the least brittle amongst those alloys.
TL;DR: In this paper, the inter-band effects of magnetic field on orbital magnetic susceptibility and Hall effect in weak magnetic field have been studied theoretically at absolute zero for the model of massless Fermions in two dimension described by Weyl equation similar to graphenes.
Abstract: Inter-band effects of magnetic field on orbital magnetic susceptibility and Hall effect in weak magnetic field have been studied theoretically at absolute zero for the model of massless Fermions in two dimension described by Weyl equation similar to graphenes, which are simplified version of newly-found one in molecular solids, α-ET 2 I 3 , described by the “tilted Weyl” equation. The dependences on the Fermi energy of both orbital susceptibility and Hall conductivity near the zero-gap region scale with the elastic scattering time and then are very singular.
TL;DR: In this article, angle-resolved photoemission spectroscopy has been used to identify energy scales of the many-body interactions in correlated materials, focused on binding energies of up to a few hundred meV below the Fermi energy.
Abstract: To date, angle-resolved photoemission spectroscopy has been successful in identifying energy scales of the many-body interactions in correlated materials, focused on binding energies of up to a few hundred meV below the Fermi energy. Here, at higher energy scale, we present improved experimental data from four families of high-T{sub c} superconductors over a wide doping range that reveal a hierarchy of many-body interaction scales focused on: the low energy anomaly ('kink') of 0.03-0.09eV, a high energy anomaly of 0.3-0.5eV, and an anomalous enhancement of the width of the LDA-based CuO{sub 2} band extending to energies of {approx} 2 eV. Besides their universal behavior over the families, we find that all of these three dispersion anomalies also show clear doping dependence over the doping range presented.
TL;DR: In this paper, the BCS-BEC mean field theory was used to analyze strongly interacting Fermi gases in the unitary regime by considering the generalization to an arbitrary number N of spin-1/2 fermion flavors with Sp(2N) symmetry.
Abstract: We analyze strongly interacting Fermi gases in the unitary regime by considering the generalization to an arbitrary number N of spin-1/2 fermion flavors with Sp(2N) symmetry. For N=\infty this problem is exactly solved by the BCS-BEC mean-field theory, with corrections small in the parameter 1/N. The large-N expansion provides a systematic way to determine corrections to mean-field predictions, allowing the calculation of a variety of thermodynamic quantities at (and in the proximity to) unitarity, including the energy, the pairing gap, and upper-critical polarization (in the case of a polarized gas) for the normal to superfluid instability. For the physical case of N=1, among other quantities, we predict in the unitarity regime, the energy of the gas to be \xi=0.28 times that for the non-interacting gas and the pairing gap to be 0.52 times the Fermi energy.
TL;DR: In this paper, the orbital magnetism was studied in graphene monolayer within the effective-mass approximation, in models of short-range and long-range disorders, the magnetization was calculated with self-consistent Born approximation.
Abstract: The orbital magnetism is studied in graphene monolayer within the effective-mass approximation. In models of short-range and long-range disorders, the magnetization is calculated with self-consistent Born approximation. In the zero-field limit, the susceptibility becomes highly diamagnetic around zero energy, while it has a long tail proportional to the inverse of the Fermi energy. We demonstrated how the magnetic oscillation vanishes and converges to the susceptibility, in going from a strong-field regime to a weak-field regime. The behavior at zero energy is shown to be highly singular.
TL;DR: A new intersubband-induced SO term is derived which resembles the functional form of the Rashba SO but is nonzero even in symmetric structures, which gives rise to a nonzero ballistic spin-Hall conductivity and can induce an unusual Zitterbewegung with cycloidal trajectories without magnetic fields.
Abstract: We investigate the spin-orbit (SO) interaction in two-dimensional electron gases in quantum wells with two subbands. From the 8x8 Kane model, we derive a new intersubband-induced SO term which resembles the functional form of the Rashba SO but is nonzero even in symmetric structures. This follows from the distinct parity of the confined states (even or odd) which obliterates the need for asymmetric potentials. We self-consistently calculate the new SO coupling strength for realistic wells and find it comparable to the usual Rashba constant. Our new SO term gives rise to a nonzero ballistic spin-Hall conductivity, which changes sign as a function of the Fermi energy (epsilon(F)) and can induce an unusual Zitterbewegung with cycloidal trajectories without magnetic fields.
TL;DR: In this article, complex density functional theory (DFT) calculations of band energy structure and density of states were performed using the tight-binding, linear muffin-tin orbital (TB-LMTO) and full potential linear augmented plane wave (FP-LAPW) methods to clarify origin of thermoelectricity.