Fractionalized fermi liquids

In spatial dimensions d>or=2, Kondo lattice models of conduction and local moment electrons can exhibit a fractionalized, nonmagnetic state (FL(*)) with a Fermi surface of sharp electronlike quasiparticles, enclosing a volume quantized by (rho(a)-1)(mod 2), with rho(a) the mean number of all electrons per unit cell of the ground state. Such states have fractionalized excitations linked to the deconfined phase of a gauge theory. Confinement leads to a conventional Fermi liquid state, with a Fermi volume quantized by rho(a)(mod 2), and an intermediate superconducting state for the Z2 gauge case. The FL(*) state permits a second order metamagnetic transition in an applied magnetic field.     

Carrier-concentration dependence of the pseudogap ground state of superconducting Bi₂Sr(₂-x)La(x)CuO(₆+δ) revealed by ⁶³,⁶⁵Cu-nuclear magnetic resonance in very high magnetic fields

We report the results of the Knight shift by ?3,??Cu-NMR measurements on single-layered copper-oxide Bi?Sr(?-x)La(x)CuO(?+δ) conducted under very high magnetic fields up to 44 T. The magnetic field suppresses superconductivity completely, and the pseudogap ground state is revealed. The ?3Cu-NMR Knight shift shows that there remains a finite density of states at the Fermi level in the zero-temperature limit, which indicates that the pseudogap ground state is a metallic state with a finite volume of Fermi surface. The residual density of states in the pseudogap ground state decreases with decreasing doping (increasing x) but remains quite large even at the vicinity of the magnetically ordered phase of x ≥ 0.8, which suggests that the density of states plunges to zero upon approaching the Mott insulating phase.     

Enhancement of laser-induced field emission in tungsten due to a metastable d band

A possible theory explaining the experimentally observed large enhancement of the field emission current from a tungsten needle exposed to a laser beam is presented. Calculations based on the density functional theory indicate a large density of 5d states and less dense 6p states above the Fermi level. The incident laser beam of 2.5 eV pumps the electrons to an unoccupied 6p band, some of which then de-excite to 5d states at lower energies, forming metastable states since the radiative decay from 5d states to the ground state 6s band is forbidden. On the other hand, the tunneling time from the metastable state is shorter than that from the ground state because of a smaller surface barrier, which may lead to the observed enhancement of the emitted current.     

Hartree-Fock ground state of the three-dimensional electron gas

In 1962, Overhauser showed that within Hartree-Fock (HF) the electron gas is unstable to a spin-density wave state. Determining the true HF ground state has remained a challenge. Using numerical calculations for finite systems and analytic techniques, we study the unrestricted HF ground state of the three-dimensional electron gas. At high density, we find broken spin symmetry states with a nearly constant charge density. Unlike previously discussed spin wave states, the observed wave vector of the spin-density wave is smaller than 2k(F). The broken-symmetry state originates from pairing instabilities at the Fermi surface, a model for which is proposed.     

Asymptotic Stability of Ground States in Some Hamiltonian PDEs with Symmetry

We consider a ground state (soliton) of a Hamiltonian PDE. We prove that if the soliton is orbitally stable, then it is also asymptotically stable. The main assumptions are transversal nondegeneracy of the manifold of the ground states, linear dispersion (in the form of Strichartz estimates) and nonlinear Fermi Golden Rule. We allow the linearization of the equation at the soliton to have an arbitrary number of eigenvalues. The theory is tailor made for the application to the translational invariant NLS in space dimension 3. The proof is based on the extension of some tools of the theory of Hamiltonian systems (reduction theory, Darboux theorem, normal form) to the case of systems invariant under a symmetry group with unbounded generators.     

相干态表象中费米交换算符的处理

利用费米相干态和正规乘积内的积分法,研究费米体系下两个态的交换算符.得到了交换算符在相干态表象中和粒子数表象中的表示,同时将其推广到在多状态情况下态之间的循环交换.     

Geometric and electronic structures of sulfur-edge-terminated zigzag edge graphene nanoribbons

The stable geometric and electronic structures of the fully and half sulfur-edge-functionalized ZGNRs at their widths of four zigzag carbon chains (S-4-ZGNRs) have been studied by using the ab initio density-functional method. It is found from our calculations that (1) under the periodic boundary condition, the two-dimensional plane structures are the most stable ground states in all the possible isomers of the S-4-ZGNRs at both 100% and 50% terminations, which are all metallic. (2) A much delocalized characteristic S-px lone-pair electron's band crossing its Fermi level appears in the case of fully S-edge-termination, which is more extended in a large energy range of over 8.0 eV, in contrast to the corresponding oxygen-px (O-px) band of the O-4-ZGNR, covering only a small energy range of 3.2 eV. In the case of half S-edge-termination, however, the S-px band is found to be much more localized, which forms two almost flat bands at about+3.0 eV above its Fermi level. (3) More interestingly, at 50% S-edge-termination, a flat portion of the π-electron edge states is found to lie a little bit below its Fermi level, making its unpolarized ground state unstable. And thus the spin-polarized antiferromagnetic (AFM) state is found to be the real ground state of the half S-4-ZGNR, which is a semiconductor with an indirect energy gap of about 0.16 eV. In the AFM ground state, there exists magnetic moments of about 0.2μ_B on each edge carbon atom, which is FM coupling along the same edge, but AFM coupling between its two edges.     

轨道Feshbach共振附近类碱土金属原子的杂质态问题

近年来,碱土金属原子和类碱土金属原子体系的研究成为冷原子物理的研究热点之一.特别是最近在~(173)Yb原子中发现的轨道Feshbach共振,使得研究有强相互作用的碱土金属和类碱土金属原子系统成为可能,极大扩展了此类原子体系的研究范围.本文介绍了~(173)Yb费米气体在轨道Feshbach共振附近的杂质态问题.在此问题中,位于~3P0态的杂质原子与处于基态的背景费米海相互作用,并在费米海表面产生分子态或极化子态.本文使用试探波函数的研究方法,首先对分子态和吸引极化子态进行介绍,并重点描述了分子态与吸引极化子态间的转变.其次归纳总结了排斥极化子态的相关性质,如有效质量、衰变率等.然后考虑双费米面情况,介绍在闭通道中引入另外一个费米面对系统产生的影响.最后简要介绍二维~(173)Yb费米气体中的杂质态问题.     

Collapse mechanism of the condensate wavefunction in the Bose-Fermi mixture with attraction between the components

The ground state of the mixture of degenerate Bose and Fermi atoms in a trap has been analyzed on the basis of the effective Hamiltonian. The two types of the solutions of the modified Gross-Pitaevskii equation that correspond to the stationary and unstable states of the Bose gas have been found numerically. The chemical potential and energy are found as functions of the number of bosons for these two types of the solutions. The manyvalued character of these functions has been analyzed and the critical number of bosons at which the system collapse occurs has been determined.     

Integrative models for asymmetric fermi superfluids: emergence of a new exotic pairing phase

We introduce an exactly solvable model to study the competition between the Larkin-Ovchinnikov-Fulde-Ferrell (LOFF) and breached-pair superfluid in strongly interacting ultracold asymmetric Fermi gases. One can thus investigate homogeneous and inhomogeneous states on equal footing and establish the quantum phase diagram. For certain values of the filling and the interaction strength, the model exhibits a new stable exotic pairing phase which combines an inhomogeneous state with an interior gap to pair excitations. It is proven that this phase is the exact ground state in the strong-coupling limit, while numerical examples in finite lattices show that also at finite interaction strength it can have lower energy than the breached-pair or LOFF states.     

Polarons in alkaline-earth-like atoms with multiple background Fermi surfaces

We study the impurity problem in a Fermi gas of ¹⁷³Yb atoms near an orbital Feshbach resonance (OFR), where a single moving particle in the ³P₀ state interacts with two background Fermi seas of particles in different nuclear states of the ground ¹S₀ manifold. By employing wave function ansatz to molecule and polaron states, we investigate various properties of the molecule, the attractive polaron, and the repulsive polaron states. In comparison to the case where only one Fermi sea is populated, we find that the presence of an additional Fermi sea acts as an energy shift between the two channels of the OFR. In addition, quantum fluctuations near the Fermi level can also induce sizable effects to various properties of the attractive and repulsive polarons.     

Reconstructing the electron in a fractionalized quantum fluid

The low-energy physics of the fractional Hall liquid is described in terms of quasiparticles that are qualitatively distinct from electrons. We show, however, that a long-lived electronlike quasiparticle also exists in the excitation spectrum: the state obtained by the application of an electron creation operator to a fractional quantum Hall ground state has a nonzero overlap with a complex, high energy bound state containing an odd number of composite-fermion quasiparticles. The electron annihilation operator similarly couples to a bound complex of composite-fermion holes. We predict that these bound states can be observed through a conductance resonance in experiments involving a tunneling of an external electron into the fractional quantum Hall liquid. A comment is made on the origin of the breakdown of the Fermi liquid paradigm in the fractional Hall liquid.     

Influence of an enlargement of the model space on number projected quasiparticle calculations

Results of number projected quasiparticle calculations for Sn isotopes in large and in small model spaces are compared when the force strengths and single-particle energies are determined consistently within each model space. When extending the model space, one observes that the model parameters extracted from the odd nuclei become more satisfactory. For even nuclei the collective states are not lowered in energy although electromagnetic transition rates increase considerably. Spectroscopic factors for one-nucleon transfer reactions change noticeably only for shells close to the Fermi level. Two-nucleon transfer cross-sections are strongly increased for ground state to ground state transitions only. We criticize a usual approximation formula for theL=0 two-nucleon transfer cross-section.     

Influence of an enlargement of the model space on number projected quasiparticle calculations

Results of number projected quasiparticle calculations for Sn isotopes in large and in small model spaces are compared when the force strengths and single-particle energies are determined consistently within each model space. When extending the model space, one observes that the model parameters extracted from the odd nuclei become more satisfactory. For even nuclei the collective states are not lowered in energy although electromagnetic transition rates increase considerably. Spectroscopic factors for one-nucleon transfer reactions change noticeably only for shells close to the Fermi level. Two-nucleon transfer cross-sections are strongly increased for ground state to ground state transitions only. We criticize a usual approximation formula for theL=0 two-nucleon transfer cross-section.     

Evolution of individual quantum Hall edge states in the presence of disorder

By using the Bloch eigenmode matching approach, we numerically study the evolution of individual quantum Hall edge states with respect to disorder. As demonstrated by the two-parameter renormalization group flow of the Hall and Thouless conductances, quantum Hall edge states with high Chern number n are completely different from that of the n = 1 case. Two categories of individual edge modes are evaluated in a quantum Hall system with high Chern number. Edge states from the lowest Landau level have similar eigenfunctions that are well localized at the system edge and independent of the Fermi energy. On the other hand, at fixed Fermi energy, the edge state from higher Landau levels exhibit larger expansion, which results in less stable quantum Hall states at high Fermi energies. By presenting the local current density distribution, the effect of disorder on eigenmode-resolved edge states is distinctly demonstrated.     

BEC-BCS crossover in "magnetized" Feshbach-resonantly paired superfluids

We map out the detuning-magnetization phase diagram for a magnetized (unequal number of atoms in two pairing hyperfine states) gas of fermionic atoms interacting via an s-wave Feshbach resonance (FR). The phase diagram is dominated by the coexistence of a magnetized normal gas and a singlet-paired superfluid with the latter exhibiting a BCS-Bose Einstein condensate crossover with reduced FR detuning. On the BCS side of strongly overlapping Cooper pairs, a sliver of finite-momentum paired Fulde-Ferrell-Larkin-Ovchinnikov magnetized phase intervenes between the phase-separated and normal states. In contrast, for large negative detuning a uniform, polarized superfluid, that is, a coherent mixture of singlet Bose-Einstein-condensed molecules and fully magnetized single-species Fermi sea, is a stable ground state.     

The Gor'kov and Melik-Barkhudarov Correction to the Mean-Field Critical Field Transition to Fulde–Ferrell–Larkin–Ovchinnikov States

The Fulde–Ferrell–Larkin–Ovchinnikov (FFLO) states, characterized by Cooper pairs condensed at finite-momentum are, at the same time, exotic and elusive. It is partially due to the fact that the FFLO states allow superconductivity to survive even in strong magnetic fields at the mean-field level. The effects of induced interactions at zero temperature are calculated in both clean and dirty cases, and it is found that the critical field at which the quantum phase transition to an FFLO state occurs at the mean-field level is strongly suppressed in imbalanced Fermi gases. This strongly shrinks the phase space region where the FFLO state is unstable and more exotic ground state is to be found. In the presence of high level impurities, this shrinkage may destroy the FFLO state completely.     

The non-Fermi-liquid nature of the metallic states of the Hubbard Hamiltonian

It has long since been argued that the metallic states of the single-band Hubbard Hamiltonian ? in two spatial dimensions (i.e. for d = 2) should be non-Fermi liquid, a possibility that would lead to the understanding of the observed anomalous behaviour of the doped copper-oxide-based superconducting compounds in their normal metallic states. Here we present a formalism which enables us to express, for arbitrary d, the behaviour of the momentum-distribution function nσ(k) pertaining to uniform metallic ground states of ? close to S _F; σ (the Fermi surface of the fermions with spin index σ, σ = ↑, ↓) in terms of a small number of constant parameters which are bound to satisfy certain inequalities implied by the requirement of the stability of the ground state of the system. These inequalities restrict the range of variation in nσ(k) for k infinitesimally inside and outside the Fermi sea pertaining to fermions with spin index σ and consequently the range of variation in the zero-temperature limit of nσ(k) for k on S _F; σ On the basis of some available accurate numerical results for nσ(k) pertaining to the Hubbard and the t-J Hamiltonian, we conclude that, at least in the strong-coupling regime, the metallic ground states of ? for d = 2 cannot be Fermi-liquid nor can they in general be purely Luttinger or marginal Fermi liquids. We further rigorously identify the pseudogap phenomenon, or 'truncation' of the Fermi surface, clearly observed in the normal states of underdoped copper-oxide-based superconductors, as corresponding to a line of resonance energies (i.e. these energies strictly do not relate to quasiparticles) located below the Fermi energy, with a concomitant suppression to zero of the jump in nσ(k) over the 'truncated' parts of the Fermi surface. Our analyses make explicit the singular significance of the non-interacting energy dispersion ε _k underlying ? in determining the low-energy spectral and transport properties of the metallic ground states of ?.     

Spin-charge density waves in the Hubbard model

The ground state of the Hubbard model in a square lattice is examined in the Hartree-Fock mean field approximation at zero temperature. At small finite hole doping, the system has periodically distributed soliton like structures whose modulations are incommensurate. In a self-consistent way, the Fermi energy can always be located in a gap. The incommensurate states have lower energies than the commensurate antiferromagnetic states calculated at the same filling. These soliton structures persist even when a sizeable nearest neighbor repulsive interaction is included.     

Calculation of Canonical Properties and Excited States by Path Integral Numerical Methods

Path integral Monte Carlo method in the expanded ensemble is used for calculation of the ratio of partition functions for different classes of permutations treating the problem of several interacting identical particles (fermions) in an external field. Wang‐Landau algorithm is used for adjustment of balancing factors. For systems consisting of greater than two number of particles we propose an advanced variant of our approach which implies calculation of the ratio of positive and negative contributions to the partition function. Densities and energies of the sequence of excited states starting from the ground state for a system of non interacting quantum particles are calculated in turn, one by one, by means of considering systems with artificially excluded lowest energy levels and further obtaining of the ”ground state” of each next system constructed in this way. The idea of evaluation of densities of excited states for a quantum system with interparticle interaction by evaluating density difference between systems of different number of noninteracting Fermi‐copies of the system of interest is realized in terms of cyclic expansion formalism for a simple 2D system of two spinless fermions interacting via Coulomb repulsion (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)