Showing papers on "Fermi energy published in 2001"

Observation of Fermi pressure in a gas of trapped atoms.

Abstract: We report the attainment of simultaneous quantum degeneracy in a mixed gas of bosons (lithium-7) and fermions (lithium-6). The Fermi gas has been cooled to a temperature of 0.25 times the Fermi temperature by thermal collisions with the evaporatively cooled bosons. At this temperature, the spatial size of the gas is strongly affected by the Fermi pressure resulting from the Pauli exclusion principle and gives clear experimental evidence for quantum degeneracy.

Quantum Interference and Ballistic Transmission in Nanotube Electron Waveguides

Abstract: The electron transport properties of well-contacted individual single-walled carbon nanotubes are investigated in the ballistic regime. Phase coherent transport and electron interference manifest as conductance fluctuations as a function of Fermi energy. Resonance with standing waves in finite-length tubes and localized states due to imperfections are observed for various Fermi energies. Two units of quantum conductance ${2G}_{0}{\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}4e}^{2}/h$ are measured for the first time, corresponding to the maximum conductance limit for ballistic transport in two channels of a nanotube.

Ballistic spin-polarized transport and Rashba spin precession in semiconductor nanowires

Abstract: We present numerical calculations of the ballistic spin-transport properties of quasi-one-dimensional wires in the presence of the spin-orbit (Rashba) interaction. A tight-binding analog of the Rashba Hamiltonian that models the Rashba effect is used. By varying the robustness of the Rashba coupling and the width of the wire, weak- and strong-coupling regimes are identified. Perfect electron-spin modulation is found for the former regime, regardless of the incident Fermi energy and mode number. In the latter however, the spin conductance has a strong energy dependence due to a nontrivial subband intermixing induced by the strong Rashba coupling. This would imply a strong suppression of the spin modulation at higher temperatures and source-drain voltages. The results may be of relevance for the implementation of quasi-one-dimensional spin-transistor devices.

Quantum theory of a nematic Fermi fluid

Abstract: We develop a microscopic theory of the electronic nematic phase proximate to an isotropic Fermi liquid in both two and three dimensions. Explicit expressions are obtained for the small amplitude collective excitations in the ordered state; remarkably, the nematic Goldstone mode (the director wave) is overdamped except along special directions dictated by symmetry. At the quantum critical point we find a dynamical exponent of $z=3,$ implying stability of the Gaussian fixed point. The leading perturbative effect of the overdamped Goldstone modes leads to a breakdown of Fermi-liquid theory in the nematic phase and to strongly angle-dependent electronic self energies around the Fermi surface. Other metallic liquid-crystal phases, e.g., a quantum hexatic, behave analogously.

Electrochemical carbon nanotube field-effect transistor

Abstract: We explore the electric-field effect of carbon nanotubes (NTs) in electrolytes. Due to the large gate capacitance, Fermi energy (EF) shifts of order ±1 V can be induced, enabling to tune NTs from p to n-type. Consequently, large resistance changes are measured. At zero gate voltage, the NTs are hole-doped in air with |EF|≈0.3–0.5 eV, corresponding to a doping level of ≈1013 cm−2. Hole-doping increases in the electrolyte.

Fermi systems with long scattering lengths

Abstract: Ground-state energies and superfluid gaps are calculated for degenerate Fermi systems interacting via long attractive scattering lengths such as cold atomic gases, neutron, and nuclear matter. In the intermediate region of densities, where the interparticle spacing $(\ensuremath{\sim}{1/k}_{F})$ is longer than the range of the interaction but shorter than the scattering length, the superfluid gaps and the energy per particle are found to be proportional to the Fermi energy and thus differ from the dilute and high-density limits. The attractive potential increase linearly with the spin-isospin or hyperspin statistical factor such that, e.g., symmetric nuclear matter undergoes spinodal decomposition and collapses whereas neutron matter and Fermionic atomic gases with two hyperspin states are mechanically stable in the intermediate density region. The regions of spinodal instabilities in the resulting phase diagram are reduced and do not prevent a superfluid transition.

Anomalously high thermoelectric figure of merit in Bi1−xSbx nanowires by carrier pocket alignment

Abstract: Electronic transport calculations were carried out for Bi1−xSbx nanowires (0⩽x⩽0.30) of diameters 10 nm⩽dW⩽100 nm at 77 K. A band structure phase diagram was generated, showing the dependence of the relative band edge positions on diameter and composition. Calculations of the thermoelectric figure-of-merit (ZT) predict that the performance of Bi1−xSbx nanowires is superior to that of Bi nanowires and to that of the bulk alloy. An exceptionally high value of ZT for p-type nanowires at 77 K was found for dW∼40 nm and x∼0.13, which is explained by the coalescence in energy of up to ten valence subband edges to maximize the density-of-states at the Fermi energy.

Topological Nature of Anomalous Hall Effect in Ferromagnet

Abstract: The anomalous Hall effect in two-dimensional ferromagnets is discussed to be the physical realization of the parity anomaly in (2+1)D, and the band crossing points behave as the topological singularity in the Brillouin zone. This appears as the sharp peaks and the sign changes of the transverse conductance $\sigma_{xy}$ as a function of the Fermi energy and/or the magnetization. The relevance to the experiments including the three dimensional systems is also discussed.

Algebraic Fermi Liquid from Phase Fluctuations: "Topological" Fermions, Vortex "Berryons," and QED3 Theory of Cuprate Superconductors

Abstract: Within the phase fluctuation model for the pseudogap state of cuprate superconductors we identify a novel statistical "Berry phase" interaction between the nodal quasiparticles and fluctuating vortex-antivortex excitations. The effective action describing this model assumes the form of an anisotropic Euclidean quantum electrodynamics in (2+1) dimensions (QED (3)) and naturally generates non-Fermi liquid behavior for its fermionic excitations. The doping axis in the x -T phase diagram emerges as a quantum critical line which regulates the low energy fermiology.

Local spectroscopy of a Kondo impurity: Co on Au(111)

Abstract: We present a detailed study of the local electronic properties of the Kondo system formed from cobalt adatoms deposited onto Au(111) at a temperature of 6.6 K. Cryogenic scanning-tunneling spectroscopy was used to observe impurity-induced resonances at the Fermi energy and at the Au(111) surface-state band edge. The line shape of the Fermi-energy resonance, identified as a Kondo resonance, is observed to vary with lateral position from the impurity center and with impurity binding position on the reconstructed Au(111) surface. Little vertical dependence is seen in the resonance line shape for positions above the center of the impurity. Interaction effects between Kondo impurities are observed to remain small as cobalt coverage is increased up to 1 ML on the gold surface. The Kondo resonance is shown theoretically to be a member of a general class of Fano resonances arising from the interaction of a discrete impurity state with a conduction-electron continuum. The asymmetric line shape of the resonance thus reflects quantum interference between the d orbital and continuum conduction electron channels, as well as their coupling to the STM tip.

Electronic structure of carbon nanocones.

Abstract: Topology related changes in the local density of states near the apex of carbon nanocones are investigated using both tight-binding and ab initio calculations. Sharp resonant states are found to dominate the electronic structure in the region close to the Fermi energy. The strength and the position of these states with respect to the Fermi level depend sensitively on the number and the relative positions of the pentagons constituting the conical tip. Carbon nanocones are thus proposed as good candidates for nanoprobes in scanning probe microscopy.

Fermi surface nesting and nanoscale fluctuating charge/orbital ordering in colossal magnetoresistive oxides.

Abstract: We used high-resolution angle-resolved photoemission spectroscopy to reveal the Fermi surface and key transport parameters of the metallic state of the layered colossal magnetoresistive oxide La1.2Sr1.8Mn2O7. With these parameters, the calculated in-plane conductivity is nearly one order of magnitude larger than the measured direct current conductivity. This discrepancy can be accounted for by including the pseudogap, which removes at least 90% of the spectral weight at the Fermi energy. Key to the pseudogap and to many other properties are the parallel straight Fermi surface sections, which are highly susceptible to nesting instabilities. These nesting instabilities produce nanoscale fluctuating charge/orbital modulations, which cooperate with Jahn-Teller distortions and compete with the electron itinerancy favored by double exchange.

Effect of interface states on electron transport in 4H-SiC inversion layers

Abstract: The effect of trapping in interface states on channel conductance and field-effect mobility in SiC MOSFETs is studied experimentally and theoretically. Hall effect measurements in n-channel MOS devices with varying densities of interface states were used to determine the effect of trapping on carrier mobility. The dependence of electron mobility on immobile interfacial charge density was quantified and was found to be similar to that in silicon, provided that the mobility is normalized to /spl mu//sub 0/, the value in the absence of Coulomb scattering. A relationship has been established between the ratio of field-effect mobility to the actual carrier mobility and the density of interface states at the Fermi energy.

Pauli blocking of collisions in a quantum degenerate atomic fermi gas

Abstract: We have produced an interacting quantum degenerate Fermi gas of atoms composed of two spin states of magnetically trapped ${}^{40}\mathrm{K}$. The relative Fermi energies are adjusted by controlling the population in each spin state. Thermodynamic measurements reveal a resulting imbalance in the mean energy per particle between the two species, which is a factor of 1.4 at our lowest temperature. This imbalance of energy comes from a suppression of collisions between atoms in the gas due to the Pauli exclusion principle. Through measurements of the thermal relaxation rate we have directly observed this Pauli blocking as a factor of 2 reduction in the effective collision cross section in the quantum degenerate regime.

Nuclear stopping as a probe for in-medium nucleon-nucleon cross sections in intermediate energy heavy ion collisions.

Abstract: Using an isospin-dependent quantum molecular dynamics, nuclear stopping in intermediate heavy ion collisions has been studied. The calculation has been done for colliding systems with different neutron-proton ratios in beam energy ranging from 15 MeV/ $u$ to 150 MeV/ $u$. It is found that, in the energy region from above Fermi energy to 150 MeV/ $u$, nuclear stopping is very sensitive to the isospin dependence of in-medium nucleon-nucleon cross section, but insensitive to symmetry potential. From this investigation, we propose that nuclear stopping can be used as a new probe to extract the information on the isospin dependence of in-medium nucleon-nucleon cross section in intermediate energy heavy ion collisions.

Superconductivity and electronic structure of perovskite MgCNi 3

Abstract: The electronic structure, stability, electron phonon coupling and superconductivity of the non-oxide perovskite MgCNi$_3$ are studied using density functional calculations. The band structure is dominated by a Ni $d$ derived density of states peak just below the Fermi energy, which leads to a moderate Stoner enhancement, placing MgCNi$_3$ in the range where spin fluctuations may noticeably affect transport, specific heat and superconductivity, providing a mechanism for reconciling various measures of the coupling $\lambda$. Strong electron phonon interactions are found for the octahedral rotation mode and may exist for other bond angle bending modes. The Fermi surface contains nearly cancelling hole and electron sheets that give unusual behavior of transport quantities particularly the thermopower. The results are discussed in relation to the superconductivity of MgCNi$_3$.

Ideal spin filters: A theoretical study of electron transmission through ordered and disordered interfaces between ferromagnetic metals and semiconductors

Abstract: It is predicted that certain atomically ordered interfaces between some ferromagnetic metals (F) and semiconductors (S) should act as ideal spin filters that transmit electrons only from the majority spin bands or only from the minority spin bands of the F to the S at the Fermi energy, even for F with both majority and minority bands at the Fermi level. Criteria for determining which combinations of F, S and interface should be ideal spin filters are formulated. The criteria depend only on the bulk band structures of the S and F and on the translational symmetries of the S, F and interface. Several examples of systems that meet these criteria to a high degree of precision are identified. Disordered interfaces between F and S are also studied and it is found that intermixing between the S and F can result in interfaces with spin anti-filtering properties, the transmitted electrons being much less spin polarized than those in the ferromagnetic metal at the Fermi energy. A patent application based on this work has been commenced by Simon Fraser University.

Electronic Structure Analyses of Sn-doped In2O3

Abstract: Electronic structures of Sn-doped In2O3 (ITO) have been investigated for the first time by using a first-principles calculation method based on the density functional theory. Calculated partial density of states (PDOS) analyses showed that a Sn atom substituted for an indium one formed three impurity bands with s-like symmetry, the second band of the three bands overlapped the conduction band of In2O3, and the Fermi energy of ITO was captured in this impurity band. The PDOS analyses also revealed that the substitution of a Sn atom did not significantly destroy the shape of density of states around the bottom of the conduction band, which gave a physical foundation for the Burstein-Moss shift model used up to now. Carrier generation mechanism and past experimental results, such as those of X-ray photoelectron spectroscopy, temperature dependency of electrical conductivity and carrier-concentration dependency of optical effective mass of ITO, are discussed based on the present theoretical calculation results.

Atomic and electronic structures of a Boron impurity and its diffusion pathways in crystalline Si

Abstract: We present extensive first-principles total-energy calculations for boron-silicon interstitial complexes with various configurations and charge states within the density functional theory. We find several stable and metastable configurations. We also find that the stability of each configuration is sensitive to its charge state: The most stable configuration is a pair of a substitutional B and an interstitial Si for positively charged state, whereas an interstitialcy configuration is the most stable for negatively charged state. The pair and the interstitialcy configurations have almost same formation energies in their neutral charge states. Examination of electron states induced by the B-Si complexes indicates that the neutral interstitialcy configuration is an active center for electron spin resonance measurements. It is also found that the B-Si complex is a negative-U system in which neutral charge states are only metastable with the Fermi energy at any position in the energy gap, corroborating the earlier experimental finding by Watkins and collaborators. Further, we present diffusion pathways and corresponding activation energies for the B-Si complex. It is found that the pathways and the activation energies are again sensitive to the charge state, opening a possibility of recombination enhanced diffusion. The calculated results are compared quantitatively with experiments and previous calculations available.

Magnetization measurements of magnetic two-dimensional electron gases.

Abstract: We directly measure the magnetization of both the conduction electrons and ${\mathrm{Mn}}^{2+}$ ions in (Zn,Cd,Mn)Se two-dimensional electron gases (2DEGs) by integrating them into ultrasensitive micromechanical magnetometers. The interplay between spin and orbital energy in these magnetic 2DEGs causes Landau level degeneracies at the Fermi energy. These Landau level crossings result in novel features in the de Haas--van Alphen oscillations, which are quantitatively reproduced by a simple model.

Trapping in organic field-effect transistors

Abstract: Current–voltage characteristics of single- and polycrystalline organic field-effect transistors are analyzed. The effect of bulk, interface, and grain boundary traps is investigated. The frequently observed dependence of the field-effect mobility on the gate voltage is ascribed to trapping processes rather than to an intrinsic charge transport mechanism in these organic semiconductors. Furthermore, the thermally activated mobility in polycrystalline devices, frequently observed, is ascribed to the formation of a potential barrier at the grain boundaries of the polycrystalline semiconductor. The barrier height depends significantly on the trap density and the position of the Fermi energy and therefore on the gate voltage.

Pumping at resonant transmission and transferred charge quantization

Abstract: We consider pumping through a small quantum dot separated from the leads by two point contacts, whose conductances, G 1 and G 2 , serve as pumping parameters. When the dot is pinched, i.e., G 1 , G 2 ⪡ e 2 / h , we find that there is a “resonance line” in the parameter plane {G 1 , G 2 } along which the Fermi energy in the leads aligns with the energy of the quasi-bound state in the quantum dot. When G 1 and G 2 are modulated periodically and adiabatically such that the pumping contour defined by G 1 = G 1 ( t ) and G 2 = G 2 ( t ) encircles the resonance line, the current is quantized: the charge pumped through the dot during each period of the modulation is close to a single electronic charge.

Universal Fluctuations in Heavy-Ion Collisions in the Fermi Energy Domain

Abstract: We discuss the scaling laws of both the charged fragments multiplicity n fluctuations and the charge of the largest fragment Zmax fluctuations for Xe+Sn collisions in the range of bombarding energies between 25 A·MeV and 50 A·MeV . We show at Elab � 32MeV/A the transition in the fluctuation regime of Zmax which is compatible with the transition from the ordered to disordered phase of excited nuclear matter. The size (charge) of the largest fragment is closely related to the order parameter characterizing this process. Theoretical description of the fragment production in heavy-ion (HI) collisions depends on whether the equilibrium has been reached before the system starts fragmenting. Possibility of the critical behavior associated with the transition from the particle evaporation regime at low excitation energies to the explosion of the hot source at about 5 - 10 MeV/nucleon cannot be excluded. Unfortunately, this exciting possibility is difficult to study because all standard models and methods of characterizing different phases and transitions of the nuclear matter in HI collisions assume an equilibrium mechanism of the fragment production. In this work, we shall apply new methods of the theory of universal fluctuations of observables in finite systems [1] to examine what can be said in a model independent way about the fragmentation mechanism and the phase changement in HI collisions in the Fermi energy domain. Our analysis, which is independent of the assumption of the equilibrium in the fragments production process, uses the data of the INDRA multidetector system for Xe + Sn collisions at 25 MeV ≤ Elab/A ≤ 50 MeV [2–5]. Several features of finite systems are important if one wants to study either the criticality or the distance to the critical point [1]. These are : (i) The �-scaling of the normalized probability distribution P [m] of the variable m for different ’system sizes’ :

Electronic structure, magnetism, and superconductivity of MgC x Ni 3

Abstract: The electronic structure of the newly discovered superconducting perovskite MgCNi3 is calculated using the LMTO method. The states near the Fermi energy are found to be dominated by Ni-d. The Stoner factor is low while the electron-phonon coupling constant is estimated to be about 0.5, which suggests that the material is a conventional type of superconductor where Tc is not affected by magnetic interactions. However, the proximity of the Fermi energy to a large peak in the density of states in conjunction with the reported nonstoichiometry of the compound has consequences for the stability of the results.

Scanning tunneling spectroscopy of quantum well and surface states of thin Ag films grown on GaAs(110)

Abstract: We investigate the electronic states of thin Ag films grown on GaAs(110) surfaces at low temperatures by scanning tunneling spectroscopy with single-layer thickness resolution. We identify the quantum-well states arising from the z confinement of the two-dimensional Ag films, and find an unoccupied Shockley-type surface state $180\ifmmode\pm\else\textpm\fi{}30\mathrm{meV}$ above the Fermi energy. The s-p electronic band dispersion along the \ensuremath{\Gamma}-L direction is found to be shifted upward by $190\ifmmode\pm\else\textpm\fi{}20\mathrm{meV}$ compared to pure Ag(111) surfaces. This shift, and the fact that the Shockley-type surface state is unoccupied and thus also shifted upward compared to pure Ag(111) surfaces are connected to the lattice strain of the quasiperiodically modulated Ag film. Implications of the results for other Ag thin films are discussed.

Pairing fluctuations and pseudogaps in the attractive Hubbard model

Abstract: The two-dimensional attractive Hubbard model is studied in the weak-to-intermediate-coupling regime by employing a nonperturbative approach. It is shown that this approach is in quantitative agreement with Monte Carlo calculations for both single-particle and two-particle quantities. Both the density of states and the single-particle spectral weight show a pseudogap at the Fermi energy below some characteristic temperature T{sup *}, also in good agreement with quantum Monte Carlo calculations. The pseudogap is caused by critical pairing fluctuations in the low-temperature renormalized classical regime ({Dirac_h}{omega}

Effective s - and p -wave contact interactions in trapped degenerate Fermi gases

Abstract: The structure and stability of dilute degenerate Fermi gases trapped in an external potential is discussed with special emphasis on the influence of s- and p-wave interactions. In a first step an Effective Contact Interaction for all partial waves is derived, which reproduces the energy spectrum of the full potential within a mean-field model space. Using the s- and p-wave part the energy density of the multi-component Fermi gas is calculated in Thomas-Fermi approximation. On this basis the stability of the one- and two-component Fermi gas against mean-field induced collapse is investigated. Explicit stability conditions in terms of density and total particle number are given. For the single-component system attractive p-wave interactions limit the density of the gas. In the two-component case a subtle competition of s- and p-wave interactions occurs and gives rise to a rich variety of phenomena. A repulsive p-wave part, for example, can stabilize a two-component system that would otherwise collapse due to an attractive s-wave interaction. It is concluded that the p-wave interaction may have important influence on the structure of degenerate Fermi gases and should not be discarded from the outset.

Rashba precession in quantum wires with interaction

Abstract: Rashba precession of spins moving along a one-dimensional quantum channel is calculated, accounting for Coulomb interactions. The Tomonaga-Luttinger model is formulated in the presence of spin-orbit scattering and solved by Bosonization. Increasing interaction strength at decreasing carrier density is found to enhance spin precession and the nominal Rashba parameter due to the decreasing spin velocity compared with the Fermi velocity. This result can elucidate the observed pronounced changes of the spin splitting on applied gate voltages that are estimated to influence the interface electric field in heterostructures only a little.