Fermi surface evolution and Luttinger theorem in Na(x)CoO2: a systematic photoemission study

We report a systematic angle-resolved photoemission study on Na(x)CoO2 for a wide range of Na concentrations (0.3 < or = x < or = 0.72). In all the metallic samples at different x, we observed (i) only a single holelike Fermi surface centered around gamma and (ii) its area changes with x according to the Luttinger theorem. We also observed a surface state that exhibits a larger Fermi surface area. The e'(g) band and the associated small Fermi surface pockets near the K points predicted by band calculations are found to sink below the Fermi energy in a manner almost independent of the doping and temperature.     

Breakup of the Fermi surface near the mott transition in low-dimensional systems

We investigate the Mott transition in weakly coupled one-dimensional (1D) fermionic chains. Using a generalization of dynamical mean field theory, we show that the Mott gap is suppressed at some critical hopping t{ perpendicular}{c2}. The transition from the 1D insulator to a 2D metal proceeds through an intermediate phase where the Fermi surface is broken into electron and hole pockets. The quasiparticle spectral weight is strongly anisotropic along the Fermi surface, both in the intermediate and metallic phases. We argue that such pockets would look like "arcs" in photoemission experiments.     

Reconstructed Fermi surface of underdoped Bi2Sr2CaCu2O(8+δ) cuprate superconductors

The Fermi surface topologies of underdoped samples of the high-T(c) superconductor Bi2Sr2CaCu2O(8+δ) have been measured with angle resolved photoemission. By examining thermally excited states above the Fermi level, we show that the observed Fermi surfaces in the pseudogap phase are actually components of fully enclosed hole pockets. The spectral weight of these pockets is vanishingly small at the magnetic zone boundary, creating the illusion of Fermi "arcs." The area of the pockets as measured in this study is consistent with the doping level, and hence carrier density, of the samples measured. Furthermore, the shape and area of the pockets is well reproduced by phenomenological models of the pseudogap phase as a spin liquid.     

Weak-localization-like temperature-dependent conductivity of a dilute two-dimensional hole gas in a parallel magnetic field

We have studied the magnetotransport properties of a high mobility two-dimensional hole gas (2DHG) in a 10 nm GaAs quantum well with densities in the range of (0.7-1.6) x 10(10) cm(-2) on the metallic side of the zero-field "metal-insulator transition." In a parallel field well above B(c) that suppresses the metallic conductivity, the 2DHG exhibits a conductivity Delta(g)(T) approximately (1/pi) (e(2)/h)lnT reminiscent of weak localization for Fermi liquids. The experiments are consistent with the coexistence of two phases in our system: a metallic phase and a weakly insulating Fermi liquid phase.     

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.     

On the existence of rapid oscillations in various phases of quasi-one-dimensional (TMTSF)2PF6

Magnetoresistance in (TMTSF)₂PF₆ has been studied. It have been found that rapid oscillations of the magnetoresistance are absent in the metallic state and are present in the spin-ordered states solely, including both the lowest and higher order FISDW states. The spin-ordered state, which had previously been believed to be insulating, is not totally gapped; at least, at a finite temperature, there remains a vestigial Fermi surface comprising 2D metallic “pockets.” Our data agree qualitatively with the theory that considers the coexistence of two spin-density waves with two respective nesting vectors. The text was submitted by the authors in English.     

Evolution of the Fermi surface across a magnetic order-disorder transition in the two-dimensional Kondo lattice model: a dynamical cluster approach

We use the dynamical cluster approximation, with a quantum Monte Carlo cluster solver on clusters of up to 16 orbitals, to investigate the evolution of the Fermi surface across the magnetic order-disorder transition in the two-dimensional doped Kondo lattice model. In the paramagnetic phase, we observe the generic hybridized heavy-fermion band structure with large Luttinger volume. In the antiferromagnetic phase, the heavy-fermion band drops below the Fermi surface giving way to hole pockets centered around k=(pi/2,pi/2) and equivalent points. In this phase Kondo screening does not break down, but the topology of the resulting Fermi surface is that of a spin-density wave approximation in which the localized spins are frozen.     

Complete Fermi surface in BaFe2As2 observed via Shubnikov-de Haas oscillation measurements on detwinned single crystals

We show that the Fermi surface (FS) in the antiferromagnetic phase of BaFe(2)As(2) is composed of one hole and two electron pockets, all of which are three dimensional and closed, in sharp contrast to the FS observed by angle-resolved photoemission spectroscopy. Considerations on the carrier compensation and Sommerfeld coefficient rule out existence of unobserved FS pockets of significant sizes. A standard band structure calculation reasonably accounts for the observed FS, despite the overestimated ordered moment. The mass enhancement, the ratio of the effective mass to the band mass, is 2-3.     

Characterisation of the Fermi surface and phase transitions of (BEDO-TTF)2 ReO4·(H2O) by physical property measurements and electronic band structure calculations

The electronic properties of the organic superconductor (BEDO-TTF)₂ ReO₄·(H₂O) were investigated by temperature dependent resistivity, ESR, Hall effect and magnetoresistance measurements. Shubnikov-de Haas (SdH) oscillations were observed in magnetic fields up to 24 T in the temperature range 0.5 K to 4.2 K. The electronic band structure of (BEDO-TTF)₂ ReO₄·(H₂O) was calculated by employing the extended Hückel tight binding method on the basis of its room temperature crystal structure. The two observed SdH frequencies of 75 T and 37 T correspond very well with two cross-sectional areas of the hole and electron Fermi surface pockets obtained from the tight binding calculation. From the temperature dependence of the SdH oscillation amplitudes, the cyclotron effective mass (m_c) belonging to the larger and smaller pockets were found to be 0.9 m₀ and m_c=1.15 m₀ respectively. Measurements of the angular dependence of the SdH frequencies show no deviation from that expected for a cylindrical Fermi surface. In terms of our tight binding calculations and experimental measurements, probable causes for the 213 K and 35 K phase transitions are discussed. The calculations show that (BEDO-TTF)₂ ReO₄·(H₂O) is a two dimensional semimetal but possesses a hidden nesting. The latter is likely to cause an SDW instability leading to the 35 K transition. The resistivity drop associated with the 213 K transition is likely to be induced by an abrupt increase in the relaxation time. The excellent agreement between the calculated and experimentally observed Fermi surface implies that, with decreasing temperature below 35 K, (BEDO-TTF)₂ ReO₄·(H₂O) gradually gets out of the SDW state and re-enters the original metallic state, in which it becomes superconducting below 2.4 K.Reported at the 13th Genral Conference of the Condensed Matter Division of the European Physical Society, Regensburg, March 1993     

Fermi surface of NaxCoO2

Doping evolution of the Fermi surface topology of Na(x)CoO(2) is studied systematically. Both local density approximation (LDA) and local spin density approximation (LSDA) predict a large Fermi surface as well as small hole pockets for doping levels x approximately 0.5. In contrast, the hole pockets are completely absent for all doping levels within LSDA+U. More importantly, we find no violation of Luttinger's rule in this system. The measured Fermi surface of Na(0.7)CoO(2) can be explained by its half-metallic behavior and agrees with our LSDA+U calculations.     

ARPES on Na0.6CoO2: Fermi surface and unusual band dispersion

The electronic structure of single crystals Na0.6CoO2, which are closely related to the superconducting Na0.3CoO2.yH(2)O (T(c) approximately 5 K), is studied by angle-resolved photoelectron spectroscopy. While the measured Fermi surface (FS) is consistent with the large FS enclosing the Gamma point from the band theory, the predicted small FS pockets near the K points are absent. In addition, the band dispersion is found to be highly renormalized, and anisotropic along the two principal axes (Gamma-K, Gamma-M). Our measurements also indicate that an extended flatband is formed slightly above E(F) along Gamma-K.     

Charge-density-wave instabilities and quantum transport in the monophosphate tungsten bronzes with m = 5 alternate structure

Resistivity, thermoelectric power and magnetotransport measurements have been performed on single crystals of the quasi two-dimensional monophosphate tungsten bronzes (PO₂)₄(WO₃)_2m for m =5 with alternate structure, between 0.4 K and 500 K, in magnetic fields of up to 36 T. These compounds show one charge density instability (CDW) at 160 K and a possible second one at 30 K. Large positive magnetoresistance in the CDW state is observed. The anisotropic Shubnikov-de Haas and de Haas-van Alphen oscillations detected at low temperatures are attributed to the existence of small electron and hole pockets left by the CDW gap openings. Angular dependent magnetoresistance oscillations (AMRO) have been found at temperatures below 30 K. The results are discussed in terms of a weakly corrugated cylindrical Fermi surface. They are shown to be consistent with a change of the Fermi surface below 30 K. Received 23 November 1999 and Received in final form 23 March 2000     

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.     

Novel superconductivity in metallic SnH(4) under high pressure

From first-principles calculations, a high-pressure metallic phase of SnH(4) with a novel layered structure intercalated by "H(2)" units is revealed. This structure is stable at pressure between 70 and 160 GPa. A remarkable feature of this structure is the presence of soft modes in the phonon band structure induced by Fermi surface nesting and Kohn anomalies that lead to very strong electron-phonon coupling. The application of the Allen-Dynes modified McMillan equation with the calculated electron-phonon coupling parameter lambda shows that a superconducting critical temperature close to 80 K can be achieved at 120 GPa.     

A metallic surface state with uniaxial spin polarization on Tl/Ge(111)-(1 × 1)

We show that a metallic surface state is formed on Tl/Ge(111)-(1 × 1). The surface state forms electron pockets around K of the surface Brillouin zone. A first-principles calculation reveals that the electron pockets are composed of a single branch of a spin-split surface-state band. The spin quantization axis is along the surface normal and inverts according to the time-reversal symmetry. Since this spin-split branch is the unique metallic band on this surface, the surface conductivity should be governed by this spin-split branch, suggesting a possible spin-polarized electric current.     

Reconstruction of bands in metallic hydrogen

The generalized theory of normal properties of a metal for the case of the properties of the electronic band of electron–phonon systems with a variable electron density of states is used to study the normal phase of metallic hydrogen at a pressure of 500 GPa and a temperature of 200 K. We calculated the frequency dependence of the real ReΣ(ω) and imaginary ImΣ(ω) parts of the self-energy part of the electron Green’s function Σ(ω), as well as the electron density of states N(ε) of the stable phase of metallic hydrogen with the I41/amd symmetry at a pressure of 500 GPa, renormalized by the strong electron–phonon coupling. It is found that the electron conduction band of the I41/amd phase of metallic hydrogen undergoes insignificant reconstruction near the Fermi level because of the renormalization by the electron–phonon coupling.     

Effects of gas adsorption and collisions on electrical transport in single-walled carbon nanotubes

Degassing of bundles of single-walled carbon nanotubes in vacuum at 500 K is found to drive the thermoelectricpower (TEP) strongly negative, indicating that the degassed metallic tubes in a bundle are n type. The magnitude of the negative TEP indicates that important asymmetry in the electronic carbon pi bands exists near the Fermi energy. Easily measurable increases in the TEP ( approximately 5-10 &mgr;V/K) and resistivity ( 2%-10%) are observed at 500 K upon exposure to N2 and He, suggesting that even gas collisions with the nanotube wall can contribute significantly to the transport properties.     

Reconstruction of the Fermi surface of HTSC cuprates in a high magnetic field

The effect of a high magnetic field on the electronic structure of HTSC cuprates is considered. The study is performed in the t-t′-t″-J* model, and the high magnetic field effect is taken into account not only as the Zeeman splitting of the one-electron levels, but also in the occupation numbers of the states with different spin projections and in the formation of the spin correlation functions. The field is assumed to be high enough to align all of the spins along the field. As a result, the Fermi surface reconstruction is obtained from four hole pockets about the nodal point (π/2, π/2) in the paramagnetic phase to a large hole pocket about the point (π, π) in the ferromagnetic phase. As the magnetic field strength decreases, a number of quantum phase transitions are revealed; they are manifested in the changed Fermi surface topology. The Fermi surface reconstruction with a decreasing field is qualitatively the same as that with an increasing doping degree in the absence of a magnetic field.     

铁基超导体系基于电子关联强度的统一相图

铁基超导和铜基超导具有诸多相似性,这为建立统一的高温超导机理图像提供了可能性.然而,对铁基超导体系中无论是进行电荷掺杂、还是等价掺杂来改变化学压力,都能产生定性上类似、而细节上纷繁复杂的相图,这对建立统一的图像造成了困难.研究化学掺杂效应如何在微观上影响电子结构和超导电性,区分主导超导电性演化的主要因素和次要因素,对建立统一图像和揭示高温超导机理至关重要.本文综述了对铁基超导体系中化学掺杂效应的一系列角分辨光电子能谱研究,涵盖了基于FeAs和FeSe面的多种代表性铁基超导体系,包括异价掺杂、等价掺杂、在元胞不同位置的化学掺杂,及其对电子体系在费米面结构、杂质散射、电子关联强度等方面的影响.实验结果表明：电子关联性或能带宽度是多个铁基超导相图背后的普适参数,不同的晶格和杂质散射效应导致了并不重要的复杂细节,而费米面拓扑结构与超导电性的关联并不强.这些结果对弱耦合机理图像提出了挑战,并促使人们通过局域反铁磁交换作用配对图像在带宽演化层面上统一地理解铁基超导.