Nodal d + id pairing and topological phases on the triangular lattice of Na(x)CoO(2).yH(2)O: evidence for an unconventional superconducting state

We show that finite angular momentum pairing chiral superconductors on the triangular lattice have point zeroes in the complex gap function. A topological quantum phase transition takes place through a nodal superconducting state at a specific carrier density x(c) where the normal state Fermi surface crosses the isolated zeros. For spin-singlet pairing, we show that the second-nearest-neighbor (d+id)-wave pairing can be the dominant pairing channel. The gapless critical state at x (c) approximately 0.25 has six Dirac points and is topologically nontrivial with a T3 spin relaxation rate below T(c). This picture provides a possible explanation for the unconventional superconducting state of Na(x)Co O(2). yH(2)O. Analyzing a pairing model with strong correlation using the Gutzwiller projection and symmetry arguments, we study these topological phases and phase transitions as a function of Na doping.     

Prospecting for the superfluid transition in electron-hole coupled quantum wells using coulomb drag

I investigate the possibility of using Coulomb drag to detect a precursor of the predicted (but as yet not definitively observed) superfluid transition in electron-hole coupled quantum wells. The drag transresistivity rho(21) is shown to be significantly enhanced above the mean-field transition temperature T(c) and to diverge logarithmically as T-->T(+)(c), due to electron-hole pairing fluctuations which are somewhat analogous to the Maki-Thompson contribution to conductivity in metals above the superconducting T(c). The enhancement in rho(21) is estimated to be detectable at temperatures much higher than T(c).     

Superfluid response in electron-doped cuprate superconductors

We propose a weakly coupled two-band model with dx(2)(-y(2)) pairing symmetry to account for the anomalous temperature dependence of superfluid density rho(s) in electron-doped cuprate superconductors. This model gives a unified explanation to the presence of an upward curvature in rho(s) near T(c) and a weak temperature dependence of rho(s) in low temperatures. Our work resolves a discrepancy in the interpretation of different experimental measurements and suggests that the pairing in electron-doped cuprates has predominately dx(2)(-y(2)) symmetry in the whole doping range.     

Bridging the gap by squeezing superfluid matter

Cooper pairing between fermions in dense matter leads to the formation of a gap in the fermionic excitation spectrum and typically exponentially suppresses transport properties. However, we show here that reactions involving conversion between different fermion species, such as Urca reactions in nuclear matter, become strongly enhanced and approach their ungapped level when the matter undergoes density oscillations of sufficiently large amplitude. We study both the neutrino emissivity and the bulk viscosity due to direct Urca processes in hadronic, hyperonic, and quark matter and discuss different superfluid and superconducting pairing patterns.     

Size-asymmetric primitive model at low temperature: description of ion pairing and location of the critical point

We argue that Bjerrum's approach to ion pairing is inappropriate for the size-asymmetric primitive model in the neighborhood of its critical point, and propose a new approach based on the Stillinger-Lovett pairing procedure. The new approach recursively scales up the ion size until linear approximations are suitable for analyzing such a model. To locate the critical point, a residual van der Waals interaction between pairs is added, with an energy cutoff adjusted to match the critical temperature of the restricted primitive model. The locations and downward trends of T(c) and rho(c) with asymmetry are found to compare favorably with simulations.     

非对称核物质中质子和中子的1S0态超流性

利用Brueckner-Hartree-Fock和BCS理论方法，计算了非对称核物质中处于¹S₀态的质子和中子的对关联能隙，着重研究和讨论了能隙的同位旋依赖性和三体核力的影响.结果表明：随核物质的同位旋非对称度增大，中子¹S₀态超流相存在的密度范围逐渐缩小而且对关联能隙峰值稍有升高；质子¹S₀态超流相存在的密度范围迅速扩大而且对关联能隙峰值显著降低.三体核力对非对称核物质中¹S₀态中子超流性及其同位旋依赖性的影响相对较小，但对¹S₀态质子超流性具有重要影响，而且其效应随核子数密度增大而迅速增强.三体核力的主要作用是强烈地抑制了具有高非对称度的核物质中高密度区域的¹S₀态质子超流性，导致质子超流相存在的密度范围显著缩小. 关键词： 同位旋非对称核物质 质子和中子超流性 三体核力 BCS理论     

Breached pairing superfluidity: possible realization in QCD

We propose a wide universality class of gapless superfluids, and analyze a limit that might be realized in quark matter at intermediate densities. In the breached pairing color superconducting phase heavy s quarks, with a small Fermi surface, pair with light u or d quarks. The ground state has a superfluid and a normal Fermi component simultaneously. We expect a second-order phase transition, as a function of increasing density, from the breached pairing phase to the conventional color-flavor locked phase.     

Topology of electronic charge density and energetics of planar faults in fcc metals

Using ab initio calculations we have studied the energetics and the evolution of the electronic charge density with shear in three fcc metals exhibiting different deformation properties, aluminum, silver, and iridium. The charge redistribution described by the change in character of specific charge density critical points (cps), is ascertained from the values of the charge density, rho(0), and its three principal curvatures, rho( parallel parallel), rho(hh), and rho(vv), respectively. The change in character of cps correlates with the energetics. For all three metals, rho(hh) vanishes near the unstable stacking configuration. The symmetry or asymmetry of the charge redistribution, measured by rho(hh)/rho(vv), may be an important factor determining stacking fault energies.     

Melting pattern of diquark condensates in quark matter

Thermal color superconducting phase transitions in high density three-flavor quark matter are investigated in the Ginzburg-Landau approach. The effects of nonzero strange quark mass, electric and color charge neutrality, and direct instantons are considered. Weak coupling calculations show that an interplay between the mass and electric neutrality effects near the critical temperature gives rise to three successive second-order phase transitions as the temperature increases: a modified color-flavor locked (mCFL) phase (ud, ds, and us pairings) --> a d-quark superconducting (dSC) phase (ud and ds pairings) --> an isoscalar pairing phase (ud pairing) --> a normal phase (no pairing). The dSC phase is novel in the sense that while all eight gluons are Meissner screened as in the mCFL phase, three out of nine quark quasiparticles are always gapless.     

Potential-energy surfaces for asymmetric heavy-ion reactions

We calculate the macroscopic potential energy of deformation as a function of mass asymmetry and distance between mass centers for shape configurations of interest in heavy-ion reactions. For the system³⁰⁰120 we also study the effect of adding microscopic shell and pairing corrections to the macroscopic potential energy. The shape configurations are generated by bringing together two separated spheres of unequal size. After the spheres touch the shapes are constructed by filling in the neck while keeping constant the radii of the end spheres, the nuclear density and the total nuclear volume. The macroscopic energy is calculated as the sum of a Coulomb energy and a nuclear macroscopic energy that takes into account the finite range of the nuclear force. For systems throughout the periodic table we display the calculated energy as a function of distance between mass centers and mass asymmetry in the form of contour maps. Some important features of the contour maps are the stationary points of the potential energy and how they change in character and location as functions of the nuclear system considered. For example, for light systems there is a maximum in the potential energy for symmetric shapes. As we move to heavier systems this peak in the potential-energy surface splits into two asymmetric peaks that are separated by a symmetric saddle point. This occurs when Z²/A ≈ 30 for the total system. As the systems become still heavier the peaks become more and more asymmetric. In heavy-ion reactions for which the asymmetry of the system is smaller than that corresponding to the peak, the smaller nucleus tends to suck up the larger one. For larger asymmetries the larger nucleus tends to suck up the smaller one. For heavy systems the binary fission saddle point is lower than the maximum in the one-dimensional interaction barrier. The penetrability calculated for the multidimensional potential-energy surface is therefore increased relative to that for the one-dimensional barrier. The microscopic shell and pairing corrections lower the potential energy for configurations in which the target and/or projectile are magic or nearly magic. This effect persists to somewhat inside the point of touching. These corrections also lower the energy near the ground state.     

Surface properties of nuclear pairing with the Gogny force in a simplified model

Surface properties of neutron-neutron (T=1) pairing in semi-infinite nuclear matter in a hard wall potential are investigated in BCS approximation using the Gogny force. Surface enhancement of the gap function, pairing tensor and correlation energy density is put into evidence.     

Spin–orbit effects and superconductivity in oxide materials

In a variety of materials superconductivity is associated with the existence of a quantum critical point (QCP). In the case of the hole doped cuprates there is evidence which suggests that the important quantum degrees of freedom near the superconducting critical point are localized charge and spin density fluctuations. We argue that if these degrees of freedom are strongly coupled by spin–orbit interactions, a new type of quantum criticality arises with monopole-like quasi-particles as the important quantum degrees of freedom. In layered material this type of quantum criticality can be modeled using a 2-dimensional non-linear Schrodinger equation with an SU(N) gauge field. We exhibit a pairing wave function for quasi-particles that has topological order and anisotropic properties. The superconducting transition would in some respects resemble a KT transition.     

Thermodynamic properties of superconducting nuclear matter

Phase diagrams of superconducting nuclear matter are calculated by solving a set of finite temperature gap equations, using several Skyrme effective interactions. Our results indicate that nuclear matter may have a superconducting phase in a small region with density near one half of the normal nuclear matter density and temperature k_BT ≲ 1.4 MeV. Our calculation is based on a finite temperature Green's function method with an abnormal pair cutoff approximation. The same approximation is employed in deriving the internal energy, entropy and chemical potential of superconducting nuclear matter. In this way, its equation of state is obtained, and compared with that of normal nuclear matter. The energy gap of superconducting nuclear matter is found to depend rather sensitively on both density and temperature. This dependence is analysed in terms of the Skyrme interaction parameters. The correlation effect on chemical potential is found to be important at high density, and its inclusion is essential in determining the equation of state of superconducting nuclear matter.     

Local energy density functional and density-dependent pairing in nuclear systems

An approach based on the local energy density functional method for describing the ground-state properties of superfluid nuclei is presented. A generalized variational principle is formulated which corresponds, in the weak pairing approximation, to a full treatment of the Hartree-Fock-Bogoliubov problem with an effective contact pairing interaction. The Gor’kov equations for generalized Green’s functions are treated exactly in the coordinate-space representation. The method is used to calculate the differential observables including odd-even mass differences and odd-even effects in charge radii which turn out to be very sensitive to the density dependence of the effective pairing force. A better knowledge of this density dependence allows one to make predictions for the pairing gap at the Fermi surface as a function of nuclear matter density. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 4, 260–265 (25 August 1998)     

Field-induced quantum critical point in CeCoIn5

The resistivity of the heavy-fermion superconductor CeCoIn5 was measured as a function of temperature, down to 25 mK and in magnetic fields of up to 16 T applied perpendicular to the basal plane. With increasing field, we observe a suppression of the non-Fermi liquid behavior, rho approximately T, and the development of a Fermi liquid state, with its characteristic rho=rho(0)+AT2 dependence. The field dependence of the T2 coefficient shows critical behavior with an exponent of 1.37. This is evidence for a field-induced quantum critical point (QCP), occurring at a critical field which coincides, within experimental accuracy, with the superconducting critical field H(c2). We discuss the relation of this field-tuned QCP to a change in the magnetic state, seen as a change in magnetoresistance from positive to negative, at a crossover line that has a common border with the superconducting region below approximately 1 K.     

Doubly ordered superconducting state in a doped antiferromagnet

In a weakly doped quasi-two-dimensional antiferromagnet with a Fermi contour in the form of small pockets, the Coulomb repulsion gives rise to a doubly ordered superconducting state of coexisting condensates with a large pair momentum and a zero one. The pairing with the large momentum determines the superconducting transition temperature, below which the order with zero momentum coexists as an induced order until the temperature corresponding to the initiation of the phonon pairing mechanism is reached. The superconductivity-induced orbital current density wave eliminates the pairing-repulsion-caused zero points from the two-gap quasiparticle spectrum and leads to a deviation of the relative phase of the superconducting order parameter components from π.     

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.     

Superfluid density in the d-density-wave scenario

Recently Chakravarty, Laughlin, Morr, and Nayak [Phys. Rev. B 62, 4880 (2000)] made an interesting proposal that the cuprate superconductors possess a hidden " d-density-wave" (DDW) order. We study the implication of this proposal for the superfluid density rho(s). We find that it predicts a temperature gradient [d rho(s)/dT](T = 0) that is strongly doping dependent near the critical doping at which the superconducting gap vanishes. This demonstrates that the DDW scenario is inconsistent with existing well-established experimental data.     

Self-consistency and search for collective effects in semiclassical pairing theory

A simple model, in which nuclei are represented as homogeneous spheres of symmetric nuclear matter, is used to study the effects of a self-consistent pairing interaction on the isoscalar nuclear response. Effects due to the finite size of nuclei are suitably taken into account. The semiclassical equations of motion derived in a previous paper for the time-dependent Hartree-Fock-Bogoliubov problem are solved in an improved (linear) approximation in which the pairing field is allowed to oscillate and to become complex. The new solutions are in good agreement with the old ones and also with the result of well-known quantum approaches. The role of the Pauli principle in eliminating one possible set of solutions is also discussed. The density response function is explicitly evaluated and it is shown that the energy-weighted sum rule is restored to its correct value by a part of the fluctuations of the imaginary pairing field. The remaining part of these imaginary fluctuations, together with the fluctuations of the real part, could give rise to collective excitations in the density response function. A detailed analysis of the monopole and quadrupole strength functions shows that there are practically no collective effects in these channels at low excitation energy.