Chiral p-wave pairing of ultracold fermionic atoms due to a quadratic band touching

We study the superfuild ground state of ultracold fermions in optical lattices with a quadratic band touching. Examples are a checkerboard lattice around half filling and a kagome lattice above one third filling. Instead of pairing between spin states, here we focus on pairing interactions between different orbital states. We find that our systems have only odd-parity(orbital) pairing instability while the singlet(orbital) pairing instability vanishes thanks to the quadratic band touching. In the mean field level, the ground state is found to be a chiral p-wave pairing superfluid(mixed with finite f-wave pairing order-parameters) which supports Majorana fermions.     

Stability of superflow for ultracold fermions in optical lattices

We investigate the stability of superflow of paired fermions in an optical lattice. We show that there are two distinct dynamical instabilities which limit the superflow in this system. One dynamical instability occurs when the superfluid stiffness becomes negative; this evolves, with increasing pairing interaction, from the fermion pair breaking instability to the well-known dynamical instability of lattice bosons. The second, more interesting, dynamical instability is marked by the emergence of a transient atom density wave. Both dynamical instabilities can be experimentally accessed by tuning the pairing interaction and the fermion density.     

Pairing in doped inversion-symmetric Weyl semimetals: a finite temperature analysis from Thouless criterion

In doped Weyl semimetal with inversion symmetry, the two pairing states, i.e., the zero momentum BCS pairing and the finite momentum Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) pairing are possible in principle. In this paper we use the standard Thouless criterion for the onset of pairings to investigate the leading pairing instability at the finite temperature. Our results suggest that both BCS and FFLO instabilities are possible depending on the on-site attractive interaction. The competition between the BCS pairing and FFLO pairing is driven by the mutual suppression between density of state near the Fermi surface and finite energy band structure in the whole Brillouin zone. For small and intermediate interaction, the former dominates and supports BCS pairing, while for strong interaction, the latter wins and favors FFLO pairing. We expect our results at the finite temperature can provide some important message to identify the true ground state.     

d-wave superconductivity and pomeranchuk instability in the two-dimensional hubbard model

We present a systematic stability analysis for the two-dimensional Hubbard model, which is based on a new renormalization group method for interacting Fermi systems. The flow of effective interactions and susceptibilities confirms the expected existence of a d-wave pairing instability driven by antiferromagnetic spin fluctuations. More unexpectedly, we find that strong forward scattering interactions develop which may lead to a Pomeranchuk instability breaking the tetragonal symmetry of the Fermi surface.     

Generalizing the Cooper-pair instability to doped Mott insulators

Copper oxides become superconductors rapidly upon doping with electron holes, suggesting a fundamental pairing instability. The Cooper mechanism explains normal superconductivity as an instability of a fermi-liquid state, but high-temperature superconductors derive from a Mott-insulator normal state, not a fermi liquid. We show that precocity to pair condensation with doping is a natural property of competing antiferromagnetism and d-wave superconductivity on a singly-occupied lattice, thus generalizing the Cooper instability to doped Mott insulators, with significant implications for the high-temperature superconducting mechanism.     

Generalizing the Cooper-pair instability to doped Mott insulators

Copper oxides become superconductors rapidly upon doping with electron holes, suggesting a fundamental pairing instability. The Cooper mechanism explains normal superconductivity as an instability of a fermi-liquid state, but high-temperature superconductors derive from a Mott-insulator normal state, not a fermi liquid. We show that precocity to pair condensation with doping is a natural property of competing antiferromagnetism and d-wave superconductivity on a singly-occupied lattice, thus generalizing the Cooper instability to doped Mott insulators, with significant implications for the high-temperature superconducting mechanism.     

Coexistence of ferromagnetism and singlet superconductivity via kinetic exchange

We propose a novel mechanism for the coexistence of metallic ferromagnetism and singlet superconductivity assuming that the magnetic instability is due to kinetic exchange. Within this scenario, the unpaired electrons which contribute to the magnetization have a positive feedback on the gain of the kinetic energy in the coexisting phase by undressing the effective mass of the carriers involved in the pairing. The evolution of the magnetization and pairing amplitude and the phase diagram are first analyzed for a generic kinetic exchange model and then are determined within a specific case with spin dependent bond-charge occupation.     

Kinetic step pairing

We report on the theoretical and experimental discovery of pairing of identical crystal steps. We first show that step bunching always occurs at long wavelength in the vicinity of an instability threshold when step dynamics is local. But an instability towards a stable train of pairs can be obtained when steps dynamics is nonlocal. This instability is shown to occur for transparent steps under electromigration. Observations on Si(111) under electromigration around 1230 degrees C show stable trains of pairs. By controlling both supersaturation and electromigration, we establish an experimental morphology diagram, from which we conclude that the transparency kinetic coefficient is negative.     

Non-fermi liquid and pairing in electron-doped cuprates

We study the normal state and pairing instability in electron-doped cuprates in a model with long-ranged antiferromagnetic spin fluctuations close to an antiferromagnetic quantum-critical point. We show that the fermionic self-energy has a non-Fermi-liquid form leading to peculiar frequency dependencies of the conductivity and the Raman response. We solve the pairing problem and demonstrate that T(c) is determined by the curvature of the Fermi surface, and the pairing gap delta (kappa, omega) is strongly nonmonotonic along the Fermi surface. The normal state frequency dependencies, the value of T(c) is approximately 10 K, and the kappa dependence of the gap agree with the experiment.     

Non-Abelian anyon superconductivity

Non-Abelian anyons exist in certain spin models and may exist in quantum Hall systems at certain filling fractions. In this work, we studied the ground state of dynamical SU(2) level-kappa Chern-Simons non-Abelian anyons at finite density and no external magnetic field. We find that, in the large-kappa limit, the topological interaction induces a pairing instability and the ground state is a superconductor with d+id gap symmetry. We also develop a picture of pairing for the special value kappa=2 and argue that the ground state is a superfluid of pairs for all values of kappa.     

BCS instability of a two-layer system of composite fermions

Pairing instability is considered for a two-layer electron system in a strong magnetic field with an even-fractional filling ν=1/(2m) (m is an integer) of the lowest Landau level in each of the layers. The limit of large distance d between the layers is analyzed. Microscopic analysis is carried out in the eikonal approximation in the composite-fermion formalism. It is found that the condition for pairing instability in this model is independent of d. Due to the marginal character of the composite-fermion system, pairing instability in the particle-particle (BCS) channel arises only for η<2 or η=2, but Mv ₀>43, where η and v ₀ are the parameters of the assumed electron-electron interaction, v∝v ₀/r ^η, and M is the band electron mass. In the particle-hole (isospin density wave) channel, instability is not observed.     

Spin-triplet pairing instability of the spinon fermi surface in a U(1) spin liquid

Recent experiments on the organic compound kappa-(ET)2Cu2(CN)3 have provided a promising example of a two-dimensional spin liquid state. This phase is described by a two-dimensional spinon Fermi sea coupled to a U(1) gauge field. We study Kohn-Luttinger-like pairing instabilities of the spinon Fermi surface due to singular interaction processes with twice-the-Fermi-momentum transfer. We find that under certain circumstances the pairing instability occurs in odd-orbital-angular momentum or spin-triplet channels. Implications to experiments are discussed.     

Phenomenological theory of the s-wave state in superconductors without an inversion center

In materials without an inversion center of symmetry the spin degeneracy of the conducting band is lifted by an antisymmetric spin orbit coupling (ASOC). Under such circumstances, spin and parity cannot be separately used to classify the Cooper pairing states. Consequently, the superconducting order parameter is generally a mixture of spin singlet and triplet pairing states. In this paper we investigate the structure of the order parameter and its response to disorder for the most symmetric pairing state (A₁). Using the example of the heavy Fermion superconductor CePt₃Si, we determine characteristic properties of the superconducting instability. Depending on the type of the pairing interaction, the gap function is characterized by the presence of line nodes. We show that this line nodes move in general upon temperature. Such nodes would be essential to explain recent low-temperature data of thermodynamic quantities such as the NMR-T₁ ^-1, London penetration depth, and heat conductance. Moreover, we study the effect of (non-magnetic) impurity on the superconducting state.     

Ultrarelativistic electron-hole pairing in graphene bilayer

We consider the ground state of an electron-hole graphene bilayer composed of two independently-doped graphene layers when a condensate of spatially separated electron-hole pairs is formed. In the weak coupling regime the pairing affects only the conduction band of the electron-doped layer and the valence band of the hole-doped layer, thus the ground state is similar to an ordinary BCS condensate. At strong coupling, an ultrarelativistic character of the electron dynamics reveals itself and the bands which are remote from Fermi surfaces (valence band of electron-doped layer and conduction band of hole-doped layer) are also affected by the pairing. Analysis of the instability of the unpaired state shows that s-wave pairing with band-diagonal condensate structure, described by two gaps, is preferable. The relative phase of the gaps is fixed, however at weak coupling this fixation diminishes allowing gapped and soliton-like excitations. The coupled self-consistent gap equations for these two gaps are solved at zero temperature in the constant-gap approximation and in the approximation of a separable potential. It is shown that, if the characteristic width of the pairing region is of the order of magnitude of the chemical potential, then the value of the gap in the spectrum is not much different from the BCS estimation. However if the pairing region is wider, then the gap value can be much larger and depends exponentially on its energy width.     

量子色动力学在有限温度密度区间的相结构

本文综述量子色动力学在有限温度密度区间的相结构,包括高密条件下的色超导态和在高温条件下相对论重离子碰撞实验中产生的强耦合夸克-胶子等离子体。我们简要地介绍在相对论重离子碰撞实验中发现强耦合夸克-胶子等离子体的历史,并且讨论用体粘滞系数与熵的比值确定QCD临界点的可能性。对于在高密区域的相结构,我们着重讨论错位配对情况下的非标准色超导态,解释无能隙色超导态的色磁不稳定性、Sarma不稳定性和Higgs不稳定性.     

Vertical pairing of identical particles suspended in the plasma sheath

It is shown experimentally that vertical pairing of two identical microspheres suspended in the sheath of a radio-frequency (rf) discharge at low gas pressures (a few Pa) appears at a well-defined instability threshold of the rf power. The transition is reversible, but with significant hysteresis on the second stage. A simple model which uses measured microsphere resonance frequencies and takes into account, in addition to the Coulomb interaction between negatively charged microspheres, their interaction with positive-ion-wake charges, seems to explain the instability threshold quite well.     

量子色动力学在有限温度密度区间的相结构(英文)

本文综述量子色动力学在有限温度密度区间的相结构,包括高密条件下的色超导态和在高温条件下相对论重离子碰撞实验中产生的强耦合夸克-胶子等离子体。我们简要地介绍在相对论重离子碰撞实验中发现强耦合夸克-胶子等离子体的历史,并且讨论用体粘滞系数与熵的比值确定QCD临界点的可能性。对于在高密区域的相结构,我们着重讨论错位配对情况下的非标准色超导态,解释无能隙色超导态的色磁不稳定性、Sarma不稳定性和Higgs不稳定性.     

Competing mechanisms of plasma transport in inhomogeneous configurations with velocity shear: the solar-wind interaction with earth's magnetosphere

Two-dimensional simulations of the Kelvin-Helmholtz instability in an inhomogeneous compressible plasma with a density gradient show that, in a transverse magnetic field configuration, the vortex pairing process and the Rayleigh-Taylor secondary instability compete during the nonlinear evolution of the vortices. Two different regimes exist depending on the value of the density jump across the velocity shear layer. These regimes have different physical signatures that can be crucial for the interpretation of satellite data of the interaction of the solar wind with the magnetospheric plasma.     

Stability of nuclear rotational states

The stability of rotational states described by using a procedure of the Hartree-Fock-Bogoliubov method is discussed. The stability condition is obtained in a general form and some of its properties are examined with the aid of simplifying approximations. It is pointed out in conclusion that the Coriolis force might lead to the instability of rotational states for sufficiently large angular momenta. The instability of rotational states is caused by the instability of the pairing density, and also by that of the nuclear density distribution. In the former case it is shown that below the critical angular momentum, a first-order phase transition takes place and in some cases the rotational band breaks off. In the latter case, it might be considered that the assumption of an axially symmetric deformation is broken or large change of the equilibrium deformation is brought suddenly. Then discontinuous change of the rotational level might be seen at the unstable point.