Mixed Order Parameter Research Articles

Phase-sensitive evidence for a predominant d-wave pairing symmetry in the electron doped superconductor La 2− xCe xCuO 4− y

We have performed a phase-sensitive test of the symmetry of the superconducting order parameter of the electron doped cuprate La 2− x Ce x CuO 4− y using a superconducting quantum interferometer with spatially distributed Josephson junctions. The magnetic field dependence of the critical current gives strong evidence for a predominant d x 2− y 2 (d-wave) symmetry of the superconducting order parameter of the particular sample measured. It also gives upper limits for the s wave component in a mixed order parameter of the type s+ id.

Influence of surface roughness on subdominant pairing ind-wave superconductors

We study a d-wave superconductor with dominant $d_{x^2-y^2}$-wave order parameter and subdominant pairing in either the $s$- or the $d_{xy}$-wave channel near a surface. In particular we analyze the influence of surface roughness on the mixed order parameter which may break the time-reversal symmetry. We find that the subdominant component is suppressed by the roughness independent of its pairing symmetry; for very rough surfaces the subdominant component may even vanish completely. Additionally we discuss a possible real-valued admixture which counteracts the suppression of the $d_{x^2-y^2}$-wave order parameter at the surface.

Influence of surface roughness on subdominant pairing in d‐wave superconductors

AbstractWe study a d‐wave superconductor with dominant d‐wave order parameter and subdominant pairing in either the s‐ or the dxy‐wave channel near a surface. In particular we analyze the influence of surface roughness on the mixed order parameter which may break the time‐reversal symmetry. We find that the subdominant component is suppressed by the roughness independent of its pairing symmetry; for very rough surfaces the subdominant component may even vanish completely. Additionally we discuss a possible real‐valued admixture which counteracts the suppression of the d‐wave order parameter at the surface.

Structural Effects of Self-Organization of Biaxial Molecules with Internal Rotation in Nematic Liquid Crystals

The effect of correlation between the conformational and orientational degrees of freedom of biaxial molecules with internal rotation on the conformational, orientational, and mixed order parameters of the molecules and on the function of the conformational distribution of molecules in nematic liquid crystals was studied in terms of molecular statistical theory. The correlation has a strong effect on polarizability of molecules with π-conjugated fragments. An explanation is offered to the experimental dependences of the mean value and anisotropy of polarizability on the character and degree of orientational ordering of molecules in the nematic phase.

Magnetic-field dependence of the maximum supercurrent of La2-xCexCuO4-y interferometers: evidence for a predominant dx2-y2 superconducting order parameter.

We performed a phase-sensitive test of the symmetry of the superconducting order parameter of the electron doped cuprate La(2-x)Ce(x)CuO(4-y) using a superconducting quantum interferometer with spatially distributed Josephson junctions. The studies were made on a thin film grown on a SrTiO3 tetracrystal substrate. The superconducting transition temperature was about 29 K which indicates that the sample is close to optimal doping. The magnetic field dependence of the critical current gives strong evidence for a predominant dx(2)(-y(2)) order parameter symmetry of the sample measured. It also gives upper limits for the s-wave component in a mixed order parameter of the type s+idx(2)(-y(2)).

D+isversusd+id′time reversal symmetry breaking states in finite size systems

We report self-consistent quasiclassical calculations of spontaneous currents and magnetic moments in finite size unconventional superconducting systems, namely: (i) in isolated d-wave superconductor islands where, in addition to the dominant order parameter (with a d_{x^2-y^2} symmetry), a subdominant order parameter of s or d_{xy} symmetry is added; (ii) in grain boundary junctions between two arbitrarily oriented d-wave superconductors, and between a d-wave and an s-wave superconductor. We show that the profile of the spontaneous current density and the magnetic field distribution depend on the time-reversal symmetry breaking properties of the system. For the d_{x^2-y^2}+id_{xy} state, vortices appear near the edges of the finite size systems. We associate these vortices with the chiral nature of the mixed order parameter. The method developed here is quite general, and can be used for predicting properties of any finite size superconducting system.

Mixing of d x2− y2 and d xy superconducting states for different filling and temperature

We investigate the solution of the gap equation for mixed order parameter symmetry states as a function of filling using a two-dimensional tight-binding model incorporating second-neighbor hopping for tetragonal and orthorhombic lattice. The principal (major) component of the order parameter is taken to be of the d x 2− y 2 type. As suggested in several investigations the minor component of the order parameter is taken to be of the d xy type. Both the permissible mixing angles 0 and π/2 between the two components are considered. As a function of filling pronounced maxima of d x 2− y 2 order parameter is accompanied by minima of the d xy order parameter. At fixed filling, the temperature dependence of the two components of the order parameter is also studied in all cases. The variation of critical temperature T c with filling is also studied and T c is found to increase with second-neighbor hopping.

Mixed-parity superconductivity inSr2RuO4

We show that in Sr2RuO4 the Fermi surface geometry as inferred from angle resolved photoemission experiments has important implications for a pairing interaction dominated by incommensurate, strongly anisotropic, spin fluctuations. For a spin fluctuation spectrum consistent with inelastic neutron scattering experiments the system is close to an accidental degeneracy between even parity spin singlet and odd parity spin triplet channels. This opens the possibility of a mixed parity order parameter state in Sr2RuO4. We determine the stable and metastable order parameter phases at low temperatures and discuss especially phases with order parameter nodes.

Observation of the second harmonic in the phase dependence of a superconducting current in Nb/Au/YBCO heterojunctions

The second harmonic in the dependence of a superconducting current on the phase difference of superconducting wave functions of the electrodes was observed experimentally in Nb/Au/(001)YBa2Cu3Ox heterojunctions. Possible reasons for its appearance were discussed within the framework of a mixed (d±s) symmetry order parameter of YBa2Cu3Ox.

CONFORMATIONAL AND ORIENTATIONAL ORDER OF MOLECULES IN NEMATIC LIQUID CRYSTALS

The statistical properties of the conformational and orientational distribution of molecules with internal rotation in the isotropic and nematic phases of liquid crystal (LC) are investigated in terms of molecular statistical theory. The paper discusses the effect of mutual correlation between the molecular conformational and orientational degrees of freedom on the conformational, orientational, and mixed order parameters of molecules in LC and on the nematic–isotropic liquid transition temperature. For these order parameters, the recurrent relation method is suggested and used to derive the partial functions of the conformational and orientational distributions of molecules in LC.

Clapping modes in unconventional superconductors

We consider a superconducting state with a mixed symmetry order parameter components, e.g. d+ is or d+ id′ with d′ = d xy . We argue for the existence of the new orbital magnetization mode which corresponds to the oscillations of relative phase φ between two components around an equilibrium value of φ = π 2 . It is similar to the so called “clapping” mode in superfluid 3 He - A. We estimate the frequency of this mode ω 0( B, T) depending on the field and temperature for the specific case of magnetic field induced d′ state. We find that this mode is tunable with an applied magnetic field with ω 0( B, T) ∝ BΔ 0, where Δ 0 is the magnitude of the d-wave order parameter. We argue also that similar filed induced clapping mode should be present in an organic p-wave superconductors.

Andreev reflection in point-contact tunneling spectroscopy on Bi 2Sr 1.94La 0.06CuO 6+ δ single crystals

In this paper, we report tunneling spectroscopies on overdoped Bi 2Sr 1.94La 0.06CuO 6+ δ (Bi2201) single crystals taken by point-contact with gold tips. Below the superconducting transition temperature, temperature-dependent Andreev reflection, zero bias conductance peak (ZBCP), a dip feature at |eV|∼2 Δ and the asymmetrical conductance curve have been observed. Quantitative analysis using extended BTK theory results in that d x 2− y 2 and, in some cases, mixed order parameter symmetry can fit well to the experimental data. The fitted s-wave component is varied for different run experiments even with the same crystal, indicating that it is determined by the detailed contact condition between tip and crystal. The observed temperature ( T) dependence of the energy gap deviates from the BCS-type. A dip feature at |eV|∼2 Δ scales with Δ for T below T c, revealing it may relate to the pairing interaction in the high T c superconductor.

Temperature dependence of electronic Raman scattering as a probe of pairing symmetry in high-Tc superconductors

Abstract The temperature dependence of electronic Raman scattering has been studied in Bi2Sr2CaCu2O8+δ crystals. In optimally doped crystals, the temperature dependence of static Raman (ω→0) slope suggests that the gap nodes are shifted from the positions prescribed by a pure d-wave pairing. The low-frequency part of B1g spectra as a function of disorder gives further evidence of a mixed order parameter.

The intrinsic effects of a magnetic field on the microwave properties of s or d order parameter symmetry in high- superconductors

A theoretical discussion of the intrinsic effects of a magnetic field on the real and imaginary parts of the ac electrical characteristics of superconductors is reported. The subsequent phenomenological theory emphasizes only the role of the scattering mechanisms of quasi-particles by static independent vortices. However both s- and d-wave order parameter symmetries are considered. Power laws for the field dependence at low temperature are obtained: , and , for an s and a d gap respectively, and while and . Since the published data do not seem to be (always) in agreement with these laws, this indicates effects of `extrinsic origin', e.g. granularity and weak links. A mixed order parameter symmetry cannot be ruled out in cases where these extrinsic effects occur.

Mixed (s+id)-wave order parameters in the Van Hove scenario

In the Van Hove scenario including orthorhombic distortion effect, we develop a pair of coupled gap equations for the mixed (s+id)-wave order parameter. It is found that a mixed s+id symmetry state is realized in a certain range of relative strength of the s and d interactions, and there are two second-order transitions between the mixed and the pure symmetry states. Particular attention is paid to the temperature dependence of two components in the mixed order parameter as well as their evolution from a pure s to a pure d symmetry state.

A molecular field theory of smectic A liquid crystals

A molecular theory for smectic A liquid crystals is described which starts from an expression for the internal energy in terms of the orientational and translational order parameters. This expression is used to determine the single particle distribution function by invoking the molecular field approximation. The relationship of this model to the McMillan theory of smectic A liquid crystals which contains an additional mixed order parameter is explained. The predictions of our theory have been explored for a wide range of the strength parameters occurring in it. The phase diagram which contains isotropic, nematic, smectic A and plastic crystal phases has been constructed and the properties of the smectic A-nematic transition evaluated in detail; these include the second order transition temperatures and tricritical point, the entropy of transition and the change in the order parameters. Such properties, which have been determined both numerically and analytically, are found to be in good agreement with the...

The influence of band degeneracy and hybridization on correlations in narrow bands

AbstractStarting from a Hubbard Hamiltonian for two bands, including exchange and hybridization, a formal theory of correlations is developed. The possibility of different order parameters is pointed out: magnetic, excitonic, and mixed order parameters. A specific model is discussed and the renormalized band structures of three representative states are calculated: The magnetic insulator, the excitonic insulator, and the correlated metal. A comparison of the energies of these states in given.