Electromagnetic Meissner effect in spin-one color superconductors

It is shown that color-superconducting quark matter, where quarks of the same flavor form Cooper pairs with spin one, exhibits an electromagnetic Meissner effect. This is in contrast to spin-zero color superconductors where Cooper pairs consist of quarks with different flavors.     

Final-state effects in the radio frequency spectrum of strongly interacting fermions

We model the impact of final-state interactions on the radio frequency spectrum of a strongly interacting two-component superfluid Fermi gas. In addition to a broad asymmetric peak coming from the breakup of Cooper pairs, we find that, for appropriate parameters, one can observe a sharp symmetric "bound-bound" spectral line coming from the conversion of Cooper pairs in one channel to pairs or molecules in another.     

Towards a theory of superconductivity based on collective Cooper pairs

Superconductivity could be seen as a Bose-Einstein condensation (BEC) of Cooper pairs. However, the creation and annihilation operators of Cooper pairs do not satisfy the bosonic commutation relations and then, the mentioned viewpoint has a weakness in its foundation. In this work, we introduce the concept of collective Cooper pairs (CCP) as linear combinations of Cooper pairs and prove their bosonic nature at the dilute limit. This bosonic nature is given rise from their diffuse character on the Cooper pairs, which permits the accumulation of many collective pairs at a single quantum state. Moreover, the superconducting ground state proposed by Bardeen, Cooper and Schrieffer (BCS) can be written in terms of these collective Cooper pairs, which means that the BCS theory is consistent with a possible BEC theory of superconductivity based on collective Cooper pairs. Finally, we calculate the energy spectra and the BEC critical temperature of CCP.     

Bosonic nature of collective Cooper pairs

Superconductivity is not considered as a Bose-Einstein condensation, because the creation and annihilation operators of Cooper pairs do not satisfy bosonic commutation relations. However, collective pairs can be constructed by a linear combination of Cooper pairs and we demonstrate in this Letter that these collective Cooper pairs have bosonic nature. In addition, the Bardeen-Cooper-Schrieffer (BCS) superconducting ground state can be built by means of these pairs and in consequence, could be treated as a Bose-Einstein condensate.     

Single Cooper pair pump

We have operated a Cooper pair pump, a linear array of superconducting tunnel junctions in which single Cooper pairs are moved under the influence of ac signals applied to two gate electrodes. The pump is based on the Coulomb blockade of charge tunneling. Because of the small junction capacitance precisely one Cooper pair is transferred per ac cycle. The current-voltage characteristics of this device show current plateaus close to 2ef, wheref is the frequency of the ac voltages. Deviations are explained in terms of Zener tunneling, Cooper pair co-tunneling, and sporadic quasiparticle tunneling.     

Long-range triplet Josephson current driven by the bias voltage

We study the long-range triplet Josephson current in a clean junction composed of two s-wave superconductors and a normal-metal/ferromagnet/normal-metal trilayer. Through applying the bias voltages on the metal regions by two antiparallel half-metal electrodes, we show that the amplitude and direction of this long-range current can be controlled flexibly. Such current arises from the fact that the applied voltage can produce a nonequilibrium spin-dependent quasiparticle distribution in the metal regions so that the Cooper pairs entering these regions acquire extra momenta, which will lead to a spin-transition process in the metal regions. This process can produce the parallel spin-triplet pairs in the central ferromagnet layer. In particular, if the voltage is applied only to one metal region, we further find that the recently discovered long-range superharmonic Josephson current will appear because of the transport of an even number of parallel spin-triplet pairs.     

Crossover from a pseudogap state to a superconducting state

This paper deduces that the particular electronic structure of cuprate superconductors confines Cooper pairs to be first formed in the antinodal region which is far from the Fermi surface,and these pairs are incoherent and result in the pseudogap state.With the change of doping or temperature,some pairs are formed in the nodal region which locates the Fermi surface,and these pairs are coherent and lead to superconductivity.Thus the coexistence of the pseudogap and the superconducting gap is explained when the two kinds of gaps are not all on the Fermi surface.It also shows that the symmetry of the pseudogap and the superconducting gap are determined by the electronic structure,and non-s wave symmetry gap favours the high-temperature superconductivity.Why the high-temperature superconductivity occurs in the metal region near the Mott metal-insulator transition is also explained.     

Cooper Molecules: Second Pairing of Cooper Pairs in Gapless Superconductor CeCoIn_5

We establish a quantum field theory of phase transitions in gapless superconductor CeCoIn5.It is found that uniform Cooper pair gases with pure gradient interactions with negative coefficient can undergo a BardeenCooper-Schrieffer (BCS) condensation below a critical temperature.In the BCS condensation state,bare Cooper pairs with opposite wave vectors are bound into Cooper molecules,and uncoupled bare Cooper pairs are transformed into a new kind of quasiparticle,i.e.,the dressed particles.The Cooper molecule s.ystem is a condensate or a superfluid,and the dressed particle s.ystem is a normal fluid.The critical temperature is derived anal.yticall.y.The critical temperature of the superconductor CeCoIn5 is obtained to be T_c = 2.289 K,which approaches the experimental data.The transition from the BCS condensation state to the normal state is a first-order phase transition.     

两库柏对相互作用问题的研究

在文中给出了库柏对的两种不同表达形式。发现用由对算符组成的哈密顿量来计算更方便,用这些对算符对易性质,而不是把它们看成费米子算符的组合产物。这样一来,研究的库柏对相互作用问题也把库柏对看成对算符。文中解决了两个库柏对相互作用问题,并讨论了推广到多库柏系统的情况。     

约瑟夫森器件中的宏观量子现象及超导量子计算

超导体中的电子结成库珀对，凝聚到可以用一个宏观波函数来描绘的能量基态，该波函数的位相是代表了成百万库珀对集体运动的宏观变量．以约瑟夫森结为基础元件的超导约瑟夫森器件，使人们能够控制并测量一个超导体的位相和库珀对数目，因此是研究宏观量子现象的理想系统．文章回顾了约瑟夫森器件中的宏观量子现象研究的发展历程，介绍了当前超导约瑟夫森器件在量子计算中的重要应用，并对它们的未来作了简要的展望．     

电-声超导电性的对称理论

利用新的库珀对平均汤近似，导出了电－声系统中声子－库珀对有效作用，证明了声子通过交换虚的库珀对能够产生一个有效的吸引力，导致两动量相反的声子配对并构成稳定的声超导态。发现电子超导态与声子超导态的直积能构成稳定的电－声对称的高温超导基态。在声子－库珀对弱耦合极限下，基态对称破缺成为传统的ＢＣＳ基态。     

电—声超导电性的对称理论

利用新的库珀对平均汤近似，导出了电－声系统中声子－库珀对有效作用，证明了声子通过交换虚的库珀对能够产生一个有效的吸引力，导致两动量相反的声子配对并构成稳定的声超导态。发现电子超导态与声子超导态的直积能构成稳定的电－声对称的高温超导基态。在声子－库珀对弱耦合极限下，基态对称破缺成为传统的ＢＣＳ基态。     

Finite-bias Cooper pair splitting

In a device with a superconductor coupled to two parallel quantum dots (QDs) the electrical tunability of the QD levels can be used to exploit nonclassical current correlations due to the splitting of Cooper pairs. We experimentally investigate the effect of a finite potential difference across one quantum dot on the conductance through the other completely grounded QD in a Cooper pair splitter fabricated on an InAs nanowire. We demonstrate that the nonlocal electrical transport through the device can be tuned by electrical means and that the energy dependence of the effective density of states in the QDs is relevant for the rates of Cooper pair splitting (CPS) and elastic cotunneling. Such experimental tools are necessary to understand and develop CPS-based sources of entangled electrons in solid-state devices.     

Feshbach-resonant interactions in 40K and 6Li degenerate Fermi gases

We theoretically examine a system of Fermi degenerate atoms coupled to bosonic molecules by a Feshbach resonance, focusing on the superfluid transition to a molecular Bose-Einstein condensate dressed by Cooper pairs of atoms. This problem raises interest because it is unclear at present whether bimodal density distributions observed recently in 40K and 6Li are due to a condensate of bosonic molecules or fermionic atom pairs. As opposed to 40K, we find that any measurable fraction of above-threshold bosonic molecules is necessarily absent for the 6Li system in question, which strongly implicates Cooper pairs as the culprit behind its bimodal distributions.     

Coboson formalism for Cooper pairs and its application to Richardson’s equations

We propose a many-body formalism for Cooper pairs which has similarities to the one we recently developed for composite boson excitons (coboson in short). Its Shiva diagram representation evidences that N Cooper pairs differ from N single pairs through electron exchange only: no direct coupling exists due to the very peculiar form of the reduced BCS potential. As a first application, we here use this formalism to derive Richardson’s equations for the exact eigenstates of N Cooper pairs. This derivation gives hints on why the N(N-1)N(N-1) dependence of the N-pair ground state energy we recently obtained by solving Richardson’s equations analytically in the low density limit, stays valid up to the dense regime. No higher order dependence exists under large overlap, a surprising result hard to accept at first. We also briefly question the BCS wave function ansatz compared to Richardson’s exact form, in the light of our understanding of coboson many-body effects.     

Physical Mechanism of Photon Pairing in a Kerr Nonlinear Medium

We establish that Cooper instability of the ground state of a normal blackbody triggers the attractive photons to form the pairs. We next find that however small the effective interphoton attraction V ₀ is, Cooper pairs of photon always exist in the ground state of a Kerr nonlinear blackbody. It has been pointed out that there are the three fundamental properties of the Cooper pair of photon: the extremely large binding energy, the extremely small radius, and the extremely high critical temperature. These properties are reflected in the many-photon bound state described by the optical superfluid theory.     

Resonance tunneling of cooper pairs in a superconductor-polymer-superconductor josephson junction

It is shown that the superconducting current flowing though a polymer in a superconductor-polymer-superconductor Josephson structure is due to resonant tunneling of Cooper pairs. The critical current and the thickness of the polymer in which the superconducting current is observed depend on the coherence length of a Cooper pair in the superconductor contacting the polymer.     

A Non-unitary Mapping from Cooper Pairs to Bosons

We derive a non-hermitian boson-fermion Hamiltonian, that is equivalent to the entirely fermionic Richardson Hamiltonian which describes the dynamics of conduction electrons in a superconductor. This is done using a generalized Dyson mapping, that replaces Cooper pairs with bosons. We show that the calculation of some physical quantities is simpler when one uses the boson-fermion Hamiltonian rather than the original Richardson Hamiltonian.     

Conductance spectroscopy of spin-triplet superconductors

We propose a novel experiment to identify the symmetry of superconductivity on the basis of theoretical results for differential conductance of a normal metal connected to a superconductor. The proximity effect from the superconductor modifies the conductance of the remote current depending remarkably on the pairing symmetry: spin singlet or spin triplet. The clear-cut difference in the conductance is explained by symmetry of Cooper pairs in a normal metal with respect to frequency. In the spin-triplet case, the anomalous transport is realized due to an odd-frequency symmetry of Cooper pairs.     

Superconducting nanofilms: molecule-like pairing induced by quantum confinement

Quantum confinement of the perpendicular motion of electrons in single-crystalline metallic superconducting nanofilms splits the conduction band into a series of single-electron subbands. A distinctive feature of such a nanoscale multi-band superconductor is that the energetic position of each subband can vary significantly with changing nanofilm thickness, substrate material, protective cover and other details of the fabrication process. It can occur that the bottom of one of the available subbands is situated in the vicinity of the Fermi level. We demonstrate that the character of the superconducting pairing in such a subband changes dramatically and exhibits a clear molecule-like trend, which is very similar to the well-known crossover from the Bardeen-Cooper-Schrieffer regime to Bose-Einstein condensation (BCS-BEC) observed in trapped ultracold fermions. For Pb nanofilms with thicknesses of 4 and 5 monolayers (MLs) this will lead to a spectacular scenario: up to half of all the Cooper pairs nearly collapse, shrinking in the lateral size (parallel to the nanofilm) down to a few nanometers. As a result, the superconducting condensate will be a coherent mixture of almost molecule-like fermionic pairs with ordinary, extended Cooper pairs.