arXiv subjects

J. F. Annett

Publications and source records attributed to J. F. Annett.

32 records · Page 2Linked to original sources

arXiv2002

Spin triplet superconductivity in Sr2RuO4

Sr2RuO4 is at present the best candidate for being a superconducting analogue of the triplet superfluidity in ^3He. This material is a good (albeit correlated) Fermi liquid in the normal state and an exotic superconductor below Tc. The mechanism of superconductivity and symmetry of the order parameter are the main puzzling issues of on-going research. Here we present the results of our search for a viable description of the superconducting state realised in this material. Our calculations are based on a three-dimensional effective three-band model with a realistic band structure. We have found a state with non-zero order parameter on each of the three sheets of the Fermi surface. The corresponding gap in the quasi-particle spectrum has line or point nodes on the alpha and beta sheets and is complex with no nodes on the gamma sheet. This state describes remarkably well a number of existing experiments including power low temperature dependence of the specific heat, penetration depth, thermal conductivity etc. The stability of the state with respect to disorder and different interaction parameters are also analyzed briefly.

cond-mat.supr-con↗

arXiv2002

Exotic pairing via a central attraction: from the BCS to the Bose limits

In the context of a simple model featuring an explicit, central interaction potential, and using a standard functional-integral technique, we study superconductivity with angular momentum quantum number l=2 as an emergent property of the many-body system. Our interaction potential is attractive at a finite distance r_0, and the breaking of the rotational symmetry is the result of an interplay between r_0 and the inter-particle distance r_s which we deem generic to interactions of this type. However such interplay, responsible for the preference of a d-wave state for a range of intermediate densities, takes place only in the BCS limit. In contrast, as the Bose-Einstein (BE) limit is approached the internal energy of the "preformed pairs" becomes the dominant contribution and there is a quantum phase transition in which the s-wave symmetry is restored. We also find that the limiting value of the critical temperature is k_B T_c --> 3.315(\hbar^2/2m*)[n/2(2l+1)]^{2/3}, which coincides with the usual result only for l=0; for l>0, it differs in the degeneracy factor 1/(2l+1), which lowers T_c. Our results thus place constraints on exotic pairing in the BE limit, while at the same time indicating a particularly interesting route to pairing with l>0 in a BCS superconductor.

cond-mat.supr-con↗

arXiv2002

Spin Polarized Current in the Ground State of Superconductor - Ferromagnet - Insulator Trilayers

We study the ground state properties of a superconductor - ferromagnet - insulator trilayer on the basis of a Hubbard Model featuring exchange splitting in the ferromagnet and electron - electron attraction in the superconductor. We solve the spin - polarized Hartree - Fock - Gorkov equations together with the Maxwell's equation (Ampere's law) fully self-consistently. For certain values of the exchange splitting we find that a spontaneous spin polarized current is generated in the ground state and is intimately related to Andreev bound states at the Fermi level. Moreover, the polarization of the current strongly depends on the band filling.

cond-mat.mes-hall↗

arXiv2002

P-wave Pairing in superconducting Sr2RuO4

Two models for the superconducting state of Sr2RuO4 have been analyzed. In an "orbital model" the in-plane and out of-plane interactions between electrons on neighboring Ru atoms are assumed to be attractive. In spite of the single particle hybrisation of the orbitals the model shows two superconducting transitions and fine-tuning of parameters is necessary to get a single transition. Otherwise the model correctly describes a number of experimental data. The "orbital proximity model" introduced here assumes the existence of special kind of inter-orbital interaction, which mixes the symmetries of the order parameters. This model naturally shows single temperature transition, but as our calculations indicate, does not lead to correct dependence of the electronic specific heat.

cond-mat.supr-con↗

arXiv2002

Spontaneous Spin Polarized Currents in Superconductor-Ferromagnetic Metal Heterostructures

We study a simple microscopic model for thin, ferromagnetic, metallic layers on semi-infinite bulk superconductor. We find that for certain values of the exchange spliting, on the ferromagnetic side, the ground states of such structures feature spontaneously induced spin polarized currents. Using a mean-field theory, which is selfconsistent with respect to the pairing amplitude $χ$, spin polarization $\vec{m}$ and the spontaneous current $\vec{j}_s$, we show that not only there are Andreev bound states in the ferromagnet but when their energies $E_n$ are near zero they support spontaneous currents parallel to the ferromagnetic-superconducting interface. Moreover, we demonstrate that the spin-polarization of these currents depends sensitively on the band filling.

cond-mat.mes-hall↗

arXiv2001

Three Band Model for Superconductivity in Sr2RuO4

We analyse the pairing symmetry of the order parameter in superconducting Sr2RuO4. Basing on a realistic three dimensional 3 band energy spectrum we have introduced effective attractive ellectron-electron interactions and found a solution which has line nodes in the gap in alpha and beta bands but not in gamma band. This state breaks time reversal symmetry and leads to the proper temperature dependence of the specific heat, in-plane penetration depth and thermal conductivity.

cond-mat.supr-con↗

arXiv2000

Superconductivity in Disordered Sr2RuO4

We discuss the influence of disorder on the critical temperature Tc of a p-wave superconductor. To describe disordered Sr2RuO4 we use the extended Hubbard model with random site energies treated in the Coherent Potential Approximation.

cond-mat.supr-con↗

arXiv2000

Superconducing Alloys with Weak and Strong Scattering: Anderson's Theorem and a Superconductor-Insulator Transition

We have studied the effects of strong impurity scattering on disordered superconductors beyond the low impurity concentration limit. By applying the full CPA to a superconductiong A-B binary alloy, we calculated the fluctuations of the local order parameters $Δ_{A}, Δ_{B}$ and charge densities, $n_{A}, n_{B}$ for weak and strong on site disorder. We find that for narrow band alloy s-wav e superconductors the conditions for Anderson's theorem are satisfied in general only for the case of particle-hole symmetry. In this case it is satisfied regardless whether we are in the weak or strong scattering regimes. Interestingly, we find that strong scattering leads to band splitting and in this regime for any band filling we have a critical concentration where a superconductor-insulator quantum phase transition occurs at T=0.

cond-mat.supr-con↗

arXiv2000

Frustration in d-wave Superconducting Circuits: $π$-Ring Behaviour

A closed superconducting circuit containing an odd number of $π$-junctions, a $π$-ring, has a finite current in the ground state. We explicitly construct such rings for d-wave superconductors and demonstrate the existence of spontaneous currents by direct self-consistent solutions to the Bogoliubov de Gennes equations. We show that the current has a topological origin due to the frustration of the d-wave order parameter, which is only partially explained by the Sigrist-Rice tunneling formula.

cond-mat.supr-con↗

arXiv1999

Van Hove Singularity and Superconductivity in Disordered Sr2RuO4

On the basis of a simple model we analyse the influence of disorder on critical temperature T_C in p-wave superconductors. The disorder is treated by means of the Coherent Potential Approximation (CPA) and we focus our attention on the effect of a van Hove singularity near Fermi energy E_F. For the appropriate values of its parameters our model reproduces the experimentally found behaviour of Sr2RuO4.

cond-mat↗

arXiv1999

Coherent Potential Approximation for `d - wave' Superconductivity in Disordered Systems

A Coherent Potential Approximation is developed for s-wave and d-wave superconductivity in disordered systems. We show that the CPA formalism reproduces the standard pair-breaking formula, the self-consistent Born Approximation and the self-consistent T-matrix approximation in the appropriate limits. We implement the theory and compute T_c for s-wave and d-wave pairing using an attractive nearest neighbor Hubbard model featuring both binary alloy disorder and a uniform distribution of scattering site potentials. We determine the density of states and examine its consequences for low temperature heat capacity. We find that our results are in qualitative agreement with measurements on Zn doped YBCO superconductors.

cond-mat.supr-con↗

arXiv1998

The importance of self-consistency in determining interface properties of S-I-N and D-I-N structures

We develop a method to solve the Bogoliubov de Gennes equation for superconductors self-consistently, using the recursion method. The method allows the pairing interaction to be either local or non-local corresponding to $s$ and $d$--wave superconductivity, respectively. Using this method we examine the properties of various $S-I-N$ and $D-I-N$ interfaces. In particular we self-consistently calculate the spatially varying density of states and the superconducting order parameter. We see that changing the strength of the insulating barrier, at the interface, does not, in the case of an $s$--wave superconductor, dramatically, change the low energy local density of states, in the superconducting region near the interface. This is in stark contrast to what we see in the case of a $D-I-N$ interface where the local particle density of states is changed dramatically. Hence we deduce that in calculating such properties as the conductance of $S-I-N$ and $D-I-N$ structures it is far more important to carry out a self-consistent calculations in the $d$--wave case.

cond-mat.supr-con↗

arXiv1998

Van Hove Singularity and D-Wave Pairing in Disordered Superconductors

We apply the coherent potential approximation (CPA) to a simple model for disordered superconductors with d-wave pairing. We demonstrate that whilst the effectiveness of an electronic Van Hove singularity to enhance the transition temperature T$_c$ is reduced by disorder it is not eliminated. In fact we give a qualitative account of changes in the T$_c$ vs. doping curve with increasing disorder and compare our results with experiments on the Y_{0.8}Ca_{0.2}Ba_2(Cu_{1-c}Zn_c)_{3}O_{7-δ} alloys.

cond-mat.supr-con↗

arXiv1997

Self-consistent interface properties of d and s-wave superconductors

We develop a method to solve the Bogoliubov de Gennes equation for superconductors self-consistently, using the recursion method. The method allows the pairing interaction to be either local or non-local corresponding to s and d-wave superconductivity, respectively. Using this method we examine the properties of various S-N and S-S interfaces. In particular we calculate the spatially varying density of states and order parameter for the following geometries (i) s-wave superconductor to normal metal, (ii) d-wave superconductor to normal metal, (iii) d-wave superconductor to s-wave superconductor. We show that the density of states at the interface has a complex structure including the effects of normal surface Friedel oscillations, the spatially varying gap and Andeev states within the gap, and the subtle effects associated with the interplay of the gap and the normal van Hove peaks in the density of states. In the case of bulk d-wave superconductors the surface leads to mixing of different order parameter symmetries near the interface and substantial local filling in of the gap.

cond-mat.supr-con↗