Quantum transport in 3D Weyl semimetals: Is there a metal-insulator transition?

We calculate the transport properties of three-dimensional Weyl fermions in a disorderedenvironment. The resulting conductivity depends only on the Fermi energy and thescattering rate. First we study the conductivity at the spectral node for a fixedscattering rate and obtain a continuous transition from an insulator at weak disorder to ametal at stronger disorder. Within the self-consistent Born approximation the scatteringrate depends on the Fermi energy. Then it is crucial that the limits of the conductivityfor a vanishing Fermi energy and a vanishing scattering rate do not commute. As a result,there is also metallic behavior in the phase with vanishing scattering rate and only aquantum critical point remains as an insulating phase. The latter turns out to be acritical fixed point in terms of a renormalization-group flow.     

Temperature and magnetic-field-enhanced hall slope of a dilute 2D hole system in the ballistic regime

We report the temperature (T) and perpendicular magnetic-field (B) dependence of the Hall resistivity rho(xy)(B) of dilute metallic 2D holes in GaAs over a broad range of temperature (0.02-1.25 K). The low B Hall coefficient, R(H), is found to be enhanced when T decreases. Strong magnetic fields further enhance the slope of rho(xy)(B) at all temperatures studied. Coulomb interaction corrections of a Fermi liquid (FL) in the ballistic regime can not explain the enhancement of rho(xy) which occurs in the same regime as the anomalous metallic longitudinal conductivity. In particular, although the metallic conductivity in 2D systems has been attributed to electron interactions in a FL, these same interactions should reduce, not enhance, the slope of rho(xy)(B) as T decreases and/or B increases.     

Magnetoconductance of a two-dimensional metal in the presence of spin-orbit coupling

We show that in the metallic phase of a two dimensional electron gas the spin-orbit coupling due to structure inversion asymmetry leads to a characteristic anisotropy in the magnetoconductance. Within the assumption that the metallic phase can be described by a Fermi liquid, we compute the conductivity in the presence of an in-plane magnetic field. Both the spin-orbit coupling and the Zeeman coupling with the magnetic field give rise to two spin subbands, in terms of which most of the transport properties can be discussed. The strongest conductivity anisotropy occurs for Zeeman energies of the order of the Fermi energy corresponding to the depopulation of the upper spin subband. The energy scale associated with the spin-orbit coupling controls the strength of the effect. More in particular, we find that the detailed behavior and the sign of the anisotropy depends on the underlying scattering mechanism. Assuming small angle scattering to be the dominant scattering mechanism our results agree with recent measurement on Si-MOSFET's in the vicinity of the metal-insulator transition. Received 11 July 2001     

Kinetic phenomena in amorphous metal-insulator nanocomposites

The experimental dependences of the kinetic properties (conductivity, thermopower, and magnetoresistance) of amorphous metal-insulator nanocomposites on the metallic phase concentration and temperature have been analyzed. A correlation is established between the magnetoresistance and electron density of states at the Fermi level in composite metallic grains.     

金属薄膜的量子输运理论

运用格林函数方法和久保公式研究金属薄膜中的电子输运．考虑了量子尺寸效应及来自杂质和两个粗糙表面的散射，计算单粒子格林函数和平行电导率．计算结果表明：（1）在薄膜系统中，电子数密度、平均自由程以及来自杂质和表面散射的电导率都以π/k_F（k_F为费密波矢的数值）为周期随厚度d振荡；（2）在薄膜和厚膜的两种极限、以及取表面粗糙度的最低阶近似下的结果可以推出用半经典和量子方法所得的金属薄膜的电导公式 关键词：     

Variable-range hopping in a silicon inversion layer

The low temperature conductance in the inversion layer of an MNOSFET shows a transition from activated to metallic conduction as the Fermi level is raised. The temperature dependence in the activated region is consistent with two-dimensional variable-range hopping and the minimum metallic conductivity, 4500 Ω^-1m^-1, agrees with a theoretical estimate based on Mott's formula.     

Coexistence of Coulomb blockade and zero bias anomaly in a strongly coupled nanodot

The current-voltage characteristics through a metallic nanoparticle which is well coupled to a metallic lead are measured. It is shown that the I-V curves are composed of two contributions. One is a suppression of the tunneling conductivity at the Fermi level, and the second is an oscillating feature which shifts with gate voltage. The results indicate that zero-bias anomaly and Coulomb blockade phenomena coexist in an asymmetric strongly coupled zero-dimensional system.     

Destruction and appearance of surface metallization in the system K/Si(111) 7×37

Qualitative changes are observed in the character of the surface electronic structure accompanying the adsorption of potassium on a Si(111) 7×7 surface. The metallic conductivity of the Si(111)7×7 surface is destroyed at the very early stages of adsorption. A new band induced by the adsorption of potassium is observed below the Fermi level. It is found that the K/Si(111)7×7 interface is semiconducting right up to saturating coverage. A surface transition from an insulating into a metallic state, accompanied by pinning of the Fermi level, is observed in the region of saturating coverage. Metallic conductivity arises in the adsorbed potassium layer as a result of the development of an induced surface band at the Fermi level. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 1, 27–30 (10 July 1997)     

Nested fermi surface and electronic instability in Ca3Ru2O7

High-resolution angular resolved photoemission data reveal well-defined quasiparticle bands of unusually low weight, emerging in line with the metallic phase of Ca(3)Ru(2)O(7) below approximately 30 K . At the bulk structural phase transition temperature of 48 K, we find clear evidence for an electronic instability, gapping large parts of the underlying Fermi surface that appears to be nested. Metallic pockets are found to survive in the small, non-nested sections, constituting a low-temperature Fermi surface with 2 orders of magnitude smaller volume than in all other metallic ruthenates. The Fermi velocities and volumes of these pockets are in agreement with the results of complementary quantum oscillation measurements on the same crystal batches.     

SU(2) gauge theory of the Hubbard model: emergence of an anomalous metallic phase near the Mott critical point

We propose one possible mechanism for an anomalous metallic phase appearing frequently in two spatial dimensions, that is, local pairing fluctuations. Introducing a pair-rotor representation to decompose bare electrons into collective pairing excitations and renormalized electrons, we derive an SU(2) gauge theory of the Hubbard model as an extended version of its U(1) gauge theory. Since our effective SU(2) gauge theory admits two kinds of collective bosons corresponding to pair excitations and density fluctuations, respectively, an intermediate phase appears naturally between the spin liquid Mott insulator and Fermi liquid metal of the U(1) gauge theory, characterized by softening of density-fluctuation modes as the Fermi liquid, but gapping of pair-excitation modes. We show that this intermediate phase is identified with an anomalous metallic phase because there are no electronlike quasiparticles although it is metallic.     

Intrinsic spin Hall effect in platinum: first-principles calculations

Spin Hall effect (SHE) is studied with first-principles relativistic band calculations for platinum, which is one of the most important materials for metallic SHE and spintronics. We find that intrinsic spin Hall conductivity (SHC) is as large as approximately 2000(variant Planck's over 2 pi/e)(Omega cm)(-1) at low temperature and decreases down to approximately 200(variant Planck's over 2 pi/e)(Omega cm)(-1) at room temperature. It is due to the resonant contribution from the spin-orbit splitting of the doubly degenerated d bands at high-symmetry L and X points near the Fermi level. By modeling these near degeneracies by an effective Hamiltonian, we show that SHC has a peak near the Fermi energy and that the vertex correction due to impurity scattering vanishes. We therefore argue that the large SHE observed experimentally in platinum is of intrinsic nature.     

Pseudogap formation in the intermetallic compounds (Fe1-xVx)3Al

Optical conductivity data of the intermetallic compounds (Fe1-xVx)3Al ( 0相似文献   

Insight into the physical properties of two niobium based compounds,Nb3Be and Nb3Be2, via a first principles calculation

We investigate the structural, electronic, mechanical and elastic properties of two niobium based intermetallic compounds, Nb₃Be and Nb₃Be_2, by using the density functional theory (DFT) based theoretical method. A good agreement is found among the structural parameters of both the phases with the experimentally evaluated parameters. For both the phases metallic conductivity is observed, while the Nb₃Be phase is more conducting than the Nb₃Be₂ phase. The evaluated DOS at the Fermi level indicates that the Nb₃Be₂ phase is electrically more stable than the Nb₃Be phase. For both phases the Nb-4d state is mostly responsible for metallic conductivity. A study of the total charge density and Mulliken atomic population reveals the existence of covalent, metallic and ionic bonds in both intermetallics. Both the phases are mechanically stable in nature, while the Nb₃Be phase is more ductile than the Nb₃Be₂ phase. A study of the Vickers hardness shows that the Nb₃Be₂ phase is harder than the Nb₃Be phase. Both compounds are anisotropic in nature, while the Nb₃Be phase possesses larger anisotropic characteristics than the Nb₃Be₂ phase. The Debye temperature of both the compounds are also calculated and discussed.     

One-electron dispersion relations in metallic VO2

One-electron dispersion relations are presented for those bands in VO₂ involved in the metallic conduction which characterises the rutile phase. The density of states at the Fermi energy (0.56 eV) is found to be 2.89 eV^-1 per cation, and the metallic state to be stable against Mott-insulation.     

Electron-electron interactions in the 2D electron system

In this paper, we report studies of the electron-electron interaction effects in 2D electron systems. The interaction manifests in renormalization of the effective spin susceptibility, effective mass, g^∗-factor, conductivity etc. By applying in-plane magnetic field, we tuned the effective interaction between the electrons and compared with theory the temperature dependence of the conductivity. We find a good agreement with interaction corrections calculated within the Fermi liquid theory. To address the question on the origin of the metal-insulator transition (MIT) in 2D, we explored transport and magnetotransport properties in the vicinity of the MIT and compared our data with solutions of two equations of the renormalization group (RG) theory, which describes temperature evolutions of the resistivity and interaction parameters for 2D electron system. We found a good agreement between the ρ(T,B_∥) data and the RG-theory in a wide range of the in-plane fields. These results support the Fermi liquid type origin of the metallic state and the interpretation of the observed 2D MIT as the true quantum phase transition.     

Metallic behavior of lightly doped La2-xSrxCuO4 with a Fermi surface forming an arc

Lightly doped La2-xSrxCuO4 in the so-called "insulating" spin-glass phase has been studied by angle-resolved photoemission spectroscopy. We have observed that a "quasiparticle" (QP) peak crosses the Fermi level in the node direction of the d-wave superconducting gap, forming an "arc" of Fermi surface, which explains the metallic behavior at high temperatures of the lightly doped materials. The QP spectral weight of the arc smoothly increases with hole doping, which we attribute to the n approximately x behavior of the carrier number in the underdoped and lightly doped regions.     

Superconductivity and pseudogap states studied by microwave conductivity measurements of λ-(BEDT-TSF)2GaCl4

We have investigated the microwave response at 45 GHz in an organic superconductor λ-(BEDT-TSF)₂GaCl₄ with T_c = 4.8 K. We determine the μ₀H_c2–T phase diagram from microwave loss and find that the superconducting state is in the pure limit (l/ξ_GL  10). Although the real part of the complex conductivity (=σ₁ + iσ₂) does not show a coherence peak just below T_c, the London penetration depth completely saturates at low temperatures down to T/T_c = 0.2, which may provide an evidence for a conventional s-wave pairing. In the metallic state below about 50 K, (parallel to the c-axis) deviates downward from , while σ₂, which should be zero in a conventional metal, increases exponentially toward T_c. In spite of the fact that the Hagen–Rubens limit is well satisfied as far as the dc conductivity is concerned, a Drude model is unable to explain the large positive σ₂. In order to explain such anomalies in the metallic state, we propose a possible existence of so-called a pseudogap near a Fermi level. The anomalous increase of the positive σ₂ may be attributed to an appearance of pre-formed electron pairs in the pseudogap state. This appearance can be regarded as a precursor to the superconducting transition. Such a precursory phenomenon has been observed also in the isostructural FeCl₄ salt with the anomalous metallic states, which shows a negative σ₂ in contrast to the GaCl₄ salt. Just the opposite of ground states in between the GaCl₄ and FeCl₄ salts may result in the contrasting anomalous metallic states with different precursory phenomena with opposite signs of σ₂.     

Spin freezing transition and non-Fermi-liquid self-energy in a three-orbital model

A single-site dynamical mean-field study of a three band model with the rotationally invariant interactions appropriate to the t_(2g) levels of a transition metal oxide reveals a quantum phase transition between a paramagnetic metallic phase and an incoherent metallic phase with frozen moments. The Mott transitions occurring at electron densities n=2, 3 per site take place inside the frozen moment phase. The critical line separating the two phases is characterized by a self-energy with the frequency dependence Sigma(omega) approximately sqrt[omega] and a broad quantum critical regime. The findings are discussed in the context of the power law observed in the optical conductivity of SrRuO3.