Density of states in the bilayer graphene with the excitonic pairing interaction

In the present paper, we consider the excitonic effects on the single particle normal density of states (DOS) in the bilayer graphene (BLG). The local interlayer Coulomb interaction is considered between the particles on the non-equivalent sublattice sites in different layers of the BLG. We show the presence of the excitonic shift of the neutrality point, even for the noninteracting layers. Furthermore, for the interacting layers, a very large asymmetry in the DOS structure is shown between the particle and hole channels. At the large values of the interlayer hopping amplitude, a large number of DOS at the Dirac’s point indicates the existence of the strong excitonic coherence effects between the layers in the BLG and the enhancement of the excitonic condensation. We have found different competing orders in the interacting BLG. Particularly, a phase transition from the hybridized excitonic insulator phase to the coherent condensate state is shown at the small values of the local interlayer Coulomb interaction.     

Subgap density of states in superconductor-normal metal bilayers in the cooper limit

We present transport and tunneling measurements of Pb-Ag bilayers with thicknesses, d(Pb) and d(Ag), that are much less than the superconducting coherence length. The transition temperature, T(c), and energy gap, Delta, in the tunneling density of states (DOS) decrease exponentially with d(Ag) at fixed d(Pb). Simultaneously, a DOS that increases linearly from the Fermi energy grows and introduces states within the gap. The integrated subgap DOS approaches 40% of the normal state value in the lowest T(c) film investigated (T(c) approximately 0.1 T(Pb)(c,bulk)). This behavior suggests that a growing fraction of quasiparticles decouple from the superconductor as T(c)-->0. The linear dependence is consistent with the quasiparticles becoming trapped on integrable trajectories in the metal layer.     

Sensitivity analysis of the modulated photocurrent method

This study presents a simulation that is used to determine the sensitivity of the phase shift analysis of the modulated photocurrent method to the differences in the fine scale structures in the density of states (DOS) distributions. Four DOS distributions are considered and the expected data are obtained. The results show that the modulated photocurrent method is very sensitive to such fine features in the DOS distributions. A comparison is also made with the sensitivity of other techniques commonly used in the determination of the DOS profiles.     

A general derivation of the density of states function for quantum wells and superlattices

The intent of this paper is to provide the reader with a detailed summary of the development of the density of states (DOS) functions for two-dimensional systems. Specifically, the DOS is derived for an infinite quantum well, a finite well, and a periodic array of coupled wells (a superlattice). Many authors state that the DOS is “simply …” without references, yet many who are new to the subject of two-dimensional systems may not see the “simplicity,” for instance, of the derivation of the DOS for a superlattice. We also show the relationships between the expressions for each case when the appropriate limits are taken. This comparison shows the consistency that such a general derivation furnishes to each expression.     

Quantum dot in the pseudogap Kondo state

We investigate the transport properties of a (small) quantum dot connected to Fermi liquid leads with a power-law density of states (DOS). Such a system, if experimentally realizable, will have interesting physical properties including: (i) non-saturating Coulomb blockade peak widths; (ii) a non-unitary Kondo peak symmetrically placed between Coulomb blockade peaks; (iii) an absence of conductance away from particle-hole symmetry at sufficiently low temperatures; and (iv) evidence of a quantum critical point as a function of dot-lead hopping. These properties are compared and contrasted with one dimensional Luttinger systems exhibiting a power-law “tunneling-DOS”.     

Structural relaxation and aging scaling in the Coulomb and Bose glass models

We employ Monte Carlo simulations to study the relaxation properties of the two-dimensional Coulomb glass in disordered semiconductors and the three-dimensional Bose glass in type-II superconductors in the presence of extended linear defects. We investigate the effects of adding non-zero random on-site energies from different distributions on the properties of the correlation-induced Coulomb gap in the density of states (DOS) and on the non-equilibrium aging kinetics highlighted by the density autocorrelation functions. We also probe the sensitivity of the system’s equilibrium and non-equilibrium relaxation properties to instantaneous changes in the density of charge carriers in the Coulomb glass or flux lines in the Bose glass.     

Density of states in a superconductor carrying a supercurrent

We have measured the tunneling density of states (DOS) in a superconductor carrying a supercurrent or exposed to an external magnetic field. The pair correlations are weakened by the supercurrent, leading to a modification of the DOS and to a reduction of the gap. As predicted by the theory of superconductivity in diffusive metals, we find that this effect is similar to that of an external magnetic field.     

Isobaric spin mixing from self-consistent treatment of coulomb interaction

From a self-consistent independent-particle model of the nucleus, isobaric spin mixing in the ground state of nuclei is calculated for the mass region 4 < A < 208. From this model, allowed Fermi β-decay matrix elements are calculated. The Coulomb energy differences calculated from our model turn out to be about the same as those obtained from the (p, n) reaction data. A relation of the direct Coulomb energy of a nucleus to its elastic form factor is pointed out.     

Anisotropic property in interacting quantum wires embedded in (110) plane

We investigate the theoretically combined effect of spin-orbit interactions and Coulomb interaction on the ground state and transport property of a quantum wire oriented along different crystallographic directions in the (110) plane. We find that the electron’s ground state exhibits phase transition among spin density wave, charge density wave, singlet superconductivity and metamagnetism, which can be controlled by changing the crystallographic orientation, the strengths of the spin-orbit interactions and the Coulomb interaction. The ac conductance exhibits a significant anisotropic behavior and a out-of-plane spin polarization which can be tuned by an in-plane electric field.     

Zero bias anomaly in the density of states of low-dimensional metals

We consider the effect of Coulomb interactions on the average density of states (DOS) of disordered low-dimensional metals for temperatures T and frequencies ω smaller than the inverse elastic life-time 1/τ. Using the fact that long-range Coulomb interactions in two dimensions (2d) generate ln²-singularities in the DOS ν(ω) but only ln-singularities in the conductivity σ(ω), we can re-sum the most singular contributions to the average DOS via a simple gauge-transformation. If σ(ω) > 0, then a metallic Coulomb gapν(ω) ∝ |ω|/e ⁴ appears in the DOS at T = 0 for frequencies below a certain crossover frequency Ω ₂ which depends on the value of the DC conductivity σ(0). Here, - e is the charge of the electron. Naively adopting the same procedure to calculate the DOS in quasi 1d metals, we find ν(ω) ∝ (|ω|/Ω ₁)^1/2exp(- Ω ₁/|ω|) at T = 0, where Ω ₁ is some interaction-dependent frequency scale. However, we argue that in quasi 1d the above gauge-transformation method is on less firm grounds than in 2d. We also discuss the behavior of the DOS at finite temperatures and give numerical results for the expected tunneling conductance that can be compared with experiments. Received 28 August 2001 / Received in final form 28 January 2002 Published online 9 July 2002     

Coulomb gap: how a metal film becomes an insulator

Electron tunneling measurements of the density of states (DOS) in ultrathin Be films reveal that a correlation gap mediates their insulating behavior. In films with sheet resistance R<5000 Omega the correlation singularity appears as the usual perturbative ln(V) zero bias anomaly (ZBA) in the DOS. As R is increased further, however, the ZBA grows and begins to dominate the DOS spectrum. This evolution continues until a nonperturbative |V| Efros-Shklovskii Coulomb gap spectrum finally emerges in the highest R films. Transport measurements of films which display this gap are well described by a universal variable range hopping law R(T) = (h/2e(2))exp(T0/T)(1/2).     

Electronic state on the surface in the strongly correlated electron systems

In order to clarify the tunneling spectroscopy in high-T_c cuprates, we study electronic state of the surface in the strongly correlated electron systems. First, we obtain Green's function of strongly correlated normal bulk system using the fluctuation exchange (FLEX) approximation. Next, we insert infinite potential into the bulk system and obtain Green's function of surface. We find that the density of states (DOS) in strongly correlated bulk systems are different from that on the surface, and the difference decreases as the magnitude of Coulomb interaction (U) increases.     

Two regimes in the magnetic field response of superconducting MgB<Subscript>2</Subscript>

Using Scanning Tunneling Microscope at low temperature we explore the superconducting phase diagram in the π-band of the two-band superconductor MgB₂. In this band the peculiar shape of the local tunneling spectra and their dynamics in the magnetic field reveal the complex character of the quasiparticle density of states (DOS). The gap in the DOS is first rapidly filled with states in raising the magnetic field up to 0.5 T and then slowly approaches the normal state value: the gap is observed up to 2 T. Such a change in the DOS dynamics suggests the existence of two terms in the DOS of the π-band: a first one, reflecting an intrinsic superconductivity in the band and a second one, originating from an inter-band coupling to the σ-band. Our findings allow a deeper understanding of the unique phase diagram of MgB₂.     

Time scale for energy equipartition in a two-dimensional FPU model

The FPU problem, i.e., the problem of energy equipartition among normal modes in a weakly nonlinear lattice, is here studied in dimension two, more precisely in a model with triangular cell and nearest-neighbors Lennard-Jones interaction. The number n of degrees of freedom ranges from 182 to 6338. Energy is initially equidistributed among a small number n(0) of low frequency modes, with n(0) proportional to n. We study numerically the time evolution of the so-called spectral entropy and the related "effective number" n(eff) of degrees of freedom involved in the dynamics; in this (rather typical) way we can estimate, for each n and each specific energy (energy per degree of freedom) epsilon, the time scale T(n)(epsilon) for energy equipartition. Numerical results indicate that in the thermodynamic limit the equipartition times are short: more precisely, for large n at fixed epsilon we find a limit curve T(infinity)(epsilon), and T(infinity) grows only as epsilon(-1) for small epsilon. Larger equipartition times are obtained by lowering epsilon, at fixed n, below a crossover value epsilon(c)(n). However, epsilon(c) appears to vanish by increasing n (faster than 1n), and the total energy E=nepsilon, rather than epsilon, appears to be the relevant variable when n is large and epsilon相似文献   

p + ip-wave pairing symmetry at type-II van Hove singularities

Based on the random phase approximation calculation in two-orbital honeycomb lattice model, we investigate the pairing symmetry of Ni-based transition-metal trichalcogenides by electron doping access to type-II van Hove singularities (vHs). We find that chiral even-parity d + id-wave (E_g) state is suppressed by odd-parity p + ip-wave (E_u) state when electron doping approaches the type-II vHs. The type-II vHs peak in density of states (DOS) enables to strengthen the ferromagnetic fluctuation, which is responsible for triplet pairing. The competition between antiferromagnetic and ferromagnetic fluctuation results in pairing phase transition from singlet to triplet pairing. The Ni-based transitionmetal trichalcogenides provide a promising platform to unconventional superconductor emerging from electronic DOS.     

Nonequilibrium critical behavior in unidirectionally coupled stochastic processes

Phase transitions from an active into an absorbing, inactive state are generically described by the critical exponents of directed percolation (DP), with upper critical dimension d(c)=4. In the framework of single-species reaction-diffusion systems, this universality class is realized by the combined processes A-->A+A, A+A-->A, and A-->0. We study a hierarchy of such DP processes for particle species A,B,..., unidirectionally coupled via the reactions A-->B, ...(with rates mu(AB),...). When the DP critical points at all levels coincide, multicritical behavior emerges, with density exponents beta(i) which are markedly reduced at each hierarchy level i> or =2. This scenario can be understood on the basis of the mean-field rate equations, which yield beta(i)=1/2(i-1) at the multicritical point. Using field-theoretic renormalization-group techniques in d=4-epsilon dimensions, we identify a new crossover exponent phi, and compute phi=1+O(epsilon(2)) in the multicritical regime (for small mu(AB)) of the second hierarchy level. In the active phase, we calculate the fluctuation correction to the density exponent on the second hierarchy level, beta(2)=1/2-epsilon/8+O(epsilon(2)). Outside the multicritical region, we discuss the crossover to ordinary DP behavior, with the density exponent beta(1)=1-epsilon/6+O(epsilon(2)). Monte Carlo simulations are then employed to confirm the crossover scenario, and to determine the values for the new scaling exponents in dimensions d< or =3, including the critical initial slip exponent. Our theory is connected to specific classes of growth processes and to certain cellular automata, and the above ideas are also applied to unidirectionally coupled pair annihilation processes. We also discuss some technical as well as conceptual problems of the loop expansion, and suggest some possible interpretations of these difficulties.     

Low-energy linear structures in dense oxygen: implications for the epsilon phase

Using density functional theory implemented within the generalized gradient approximation, a new nonmagnetic insulating ground state of solid oxygen is proposed and found to be energetically favored at pressures corresponding to the epsilon phase. The newly predicted static ground state is composed of linear herringbone-type chains of O2 molecules and has Cmcm symmetry (with an alternative monoclinic cell). Importantly, this phase supports IR-active zone-center phonons, and their computed frequencies are found to be in broad agreement with recent infrared absorption experiments.     

Delocalized quasiparticles in the vortex state of an overdoped high-T(c) superconductor probed by 63Cu NMR

We report the spin Knight shift (K(s)) and the nuclear spin-lattice relaxation rate (1/T1) in the vortex state as a function of magnetic field (H) up to 28 T in the high-Tc superconductor TlSr2CaCu2O6.8 (Tc = 68 K). At low temperatures well below Tc, both K(s) and 1/T1 measured around the middle point between the two nearest vortices (saddle point) increase substantially with increasing field, which indicate that the quasiparticle states with an ungapped spectrum are extended outside the vortex cores in a d-wave superconductor. The density of states (DOS) around the saddle point is found to be kappaN(0)square root[H/H(c2)], with kappa = 0.5-0.7 and N0 being the normal-state DOS.     

Half-Heusler合金Cu1-xFexMnSb的电子结构和反铁磁-铁磁相变

基于密度泛函理论（DFT）,使用局域密度近似（LDA）研究了Heusler合金Cu1-xFexMnSb的电子结构和反铁磁-铁磁相变。研究发现,两种磁状态下的合金晶格常数随掺杂浓度x变化很好地满足Vegard定理。当x>0.5时,铁磁态合金的总磁矩很好地符合SP规律,然而当x<0.5时,却发生了明显的偏离。由于整个体系存在RKKY和超交换磁耦合的竞争,因而在x=0.25时,我们观察到了独特的反铁磁—铁磁相变。进一步的态密度分析发现,Cu的掺杂浓度可以有效调整铁磁态合金的费米面位置,并且反铁磁态合金由于不同自旋方向的Mn原子的分波态密度相互补偿,总态密度形成了几乎完全对称的自旋向上带和自旋向下带。