Disordered Fulde-Ferrel-Larkin-Ovchinnikov State in d-wave Superconductors

We study the Fulde-Ferrel-Larkin-Ovchinnikov (FFLO) superconducting state in the disordered systems. We analyze the microscopic model, in which the d-wave superconductivity is stabilized near the antiferromagnetic quantum critical point, and investigate two kinds of disorder, namely, box disorder and point disorder, on the basis of the Bogoliubov-deGennes (BdG) equation. The spatial structure of modulated superconducting order parameter and the magnetic properties in the disordered FFLO state are investigated. We point out the possibility of "FFLO glass" state in the presence of strong point disorders, which arises from the configurational degree of freedom of FFLO nodal plane. The distribution function of local spin susceptibility is calculated and its relation to the FFLO nodal plane is clarified. We discuss the NMR measurements for CeCoIn_5.Comment: Submitted to New. J. Phys. a focus issue on "Superconductors with Exotic Symmetries

Testing Higgs models via the H±W∓ZH^\pm W^\mp Z vertex by a recoil method at the International Linear Collider

In general, charged Higgs bosons $H^\pm$ appear in non-minimal Higgs models. The $H^\pm W^\mp Z$ vertex is known to be related to the violation of the global symmetry (custodial symmetry) in the Higgs sector. Its magnitude strongly depends on the structure of the exotic Higgs models which contain higher isospin $SU(2)_L$ representations such as triplet Higgs bosons. We study the possibility of measuring the $H^\pm W^\mp Z$ vertex via single charged Higgs boson production associated with the $W^\pm$ boson at the International Linear Collider (ILC) by using the recoil method. The feasibility of the signal $e^+e^-\to H^\pm W^\mp \to \ell \nu jj$ is analyzed assuming the polarized electron and positron beams and the expected detector performance for the resolution of the two-jet system at the ILC. The background events can be reduced to a considerable extent by imposing the kinematic cuts even if we take into account the initial state radiation. For a relatively light charged Higgs boson whose mass $m_{H^\pm}$ is in the region of 120-130 GeV $< m_{H^\pm} < m_W+m_Z$, the $H^\pm W^\mp Z$ vertex would be precisely testable especially when the decay of $H^\pm$ is lepton specific. The exoticness of the extended Higgs sector can be explored by using combined information for this vertex and the rho parameter.Comment: 22 pages, 23 figure

Random Spin-orbit Coupling in Spin Triplet Superconductors: Stacking Faults in Sr_2RuO_4 and CePt_3Si

The random spin-orbit coupling in multicomponent superconductors is investigated focusing on the non-centrosymmetric superconductor CePt_3Si and the spin triplet superconductor Sr_2RuO_4. We find novel manifestations of the random spin-orbit coupling in the multicomponent superconductors with directional disorders, such as stacking faults. The presence of stacking faults is indicated for the disordered phase of CePt_3Si and Sr_2RuO_4. It is shown that the d-vector of spin triplet superconductivity is locked to be d = k_y x - k_x y with the anisotropy \Delta T_c/T_c0 \sim \bar{\alpha}^2/T_c0 W_z, where \bar{\alpha}, T_c0, and W_z are the mean square root of random spin-orbit coupling, the transition temperature in the clean limit, and the kinetic energy along the c-axis, respectively. This anisotropy is much larger (smaller) than that in the clean bulk Sr_2RuO_4 (CePt_3Si). These results indicate that the helical pairing state d = k_y x - k_x y in the eutectic crystal Sr_2RuO_4-Sr_3Ru_2O_7 is stabilized in contrast to the chiral state d = (k_x \pm i k_y) z in the bulk Sr_2RuO_4. The unusual variation of T_c in CePt_3Si is resolved by taking into account the weak pair-breaking effect arising from the uniform and random spin-orbit couplings. These superconductors provide a basis for discussing recent topics on Majorana fermions and non-Abelian statistics.Comment: J. Phys. Soc. Jpn. 79 (2010) 08470

Reduction of Tc due to Impurities in Cuprate Superconductors

In order to explain how impurities affect the unconventional superconductivity, we study non-magnetic impurity effect on the transition temperature using on-site U Hubbard model within a fluctuation exchange (FLEX) approximation. We find that in appearance, the reduction of Tc roughly coincides with the well-known Abrikosov-Gor'kov formula. This coincidence results from the cancellation between two effects; one is the reduction of attractive force due to randomness, and another is the reduction of the damping rate of quasi-particle arising from electron interaction. As another problem, we also study impurity effect on underdoped cuprate as the system showing pseudogap phenomena. To the aim, we adopt the pairing scenario for the pseudogap and discuss how pseudogap phenomena affect the reduction of Tc by impurities. We find that 'pseudogap breaking' by impurities plays the essential role in underdoped cuprate and suppresses the Tc reduction due to the superconducting (SC) fluctuation.Comment: 14 pages, 28 figures To be published in JPS

Bounds on Decoherence and Error

When a confined system interacts with its walls (treated quantum mechanically), there is an intertwining of degrees of freedom. We show that this need not lead to entanglement, hence decoherence. It will generally lead to error. The wave function optimization required to avoid decoherence is also examined.Comment: 10 pages, plain TeX, no figure

Antiferromagnetic Order and \pi-triplet Pairing in the Fulde-Ferrell-Larkin-Ovchinnikov State

The antiferromagnetic Fulde-Ferrell-Larkin-Ovchinnikov (AFM-FFLO) state of coexisting d-wave FFLO superconductivity and incommensurate AFM order is studied on the basis of Bogoliubov-de Gennes (BdG) equations. We show that the incommensurate AFM order is stabilized in the FFLO state by the appearance of the Andreev bound state localized around the zeros of the FFLO order parameter. The AFM-FFLO state is further enhanced by the induced \pi-triplet superconductivity (pair density wave). The AFM order occurs in the FFLO state even when it is neither stable in the normal state nor in the BCS state. The order parameters of the AFM order, d-wave superconductivity, and \pi-triplet pairing are investigated by focusing on their spatial structures. Roles of the spin fluctuations beyond the BdG equations are discussed. Their relevance to the high-field superconducting phase of CeCoIn_5 is discussed.Comment: Typos are fixed. Published versio

Effects of proximity to an electronic topological transition on normal state transport properties of the high-Tc superconductors

Within the time dependent Ginzburg-Landau theory, the effects of the superconducting fluctuations on the transport properties above the critical temperature are characterized by a non-zero imaginary part of the relaxation rate gamma of the order parameter. Here, we evaluate Im gamma for an anisotropic dispersion relation typical of the high-Tc cuprate superconductors (HTS), characterized by a proximity to an electronic topological transition (ETT). We find that Im gamma abruptly changes sign at the ETT as a function of doping, in agreement with the universal behavior of the HTS. We also find that an increase of the in-plane anisotropy, as is given by a non-zero value of the next-nearest to nearest hopping ratio r=t'/t, increases the value of | Im gamma | close to the ETT, as well as its singular behavior at low temperature, therefore enhancing the effect of superconducting fluctuations. Such a result is in qualitative agreement with the available data for the excess Hall conductivity for several cuprates and cuprate superlattices.Comment: to appear in Phys. Rev.

Electronic structure of spinel-type LiV_2O_4

The band structure of the cubic spinel compound LiV_2O_4, which has been reported recently to show heavy Fermion behavior, has been calculated within the local-density approximation using a full-potential version of the linear augmented-plane-wave method. The results show that partially-filled V 3d bands are located about 1.9 eV above the O 2p bands and the V 3d bands are split into a lower partially-filled t_{2g} complex and an upper unoccupied e_{g} manifold. The fact that the conduction electrons originate solely from the t_{2g} bands suggests that the mechanism for the mass enhancement in this system is different from that in the 4f heavy Fermion systems, where these effects are attributed to the hybridization between the localized 4f levels and itinerant spd bands.Comment: 5 pages, revte

Antiferromagnetic Phases in the Fulde-Ferrell-Larkin-Ovchinnikov State of CeCoIn_5

The antiferromagnetic (AFM) order in the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) superconducting state is analyzed on the basis of a Ginzburg-Landau theory. To examine the possible AFM-FFLO state in CeCoIn_5, we focus on the incommensurate AFM order characterized by the wave vector Q = Q_{0} \pm q_inc with Q_0 =(\pi,\pi,\pi) and q_inc \parallel [110] or [1-10] in the tetragonal crystal structure. We formulate the two component Ginzburg-Landau theory and investigate the two degenerate incommensurate AFM order. We show that the pinning of AFM moment due to the FFLO nodal planes leads to multiple phases in magnetic fields along [100] or [010]. The phase diagrams for various coupling constants between the two order parameters are shown for the comparison with CeCoIn_5. Experimental results of the NMR and neutron scattering measurements are discussed.Comment: 6pages, Proceedings of ICHE2010, To appear in J. Phys. Soc. Jpn. Supp

Possibility of Unconventional Pairing Due to Coulomb Interaction in Fe-Based Pnictide Superconductors: Perturbative Analysis of Multi-Band Hubbard Models

Possibility of unconventional pairing due to Coulomb interaction in iron-pnictide superconductors is studied by applying a perturbative approach to realistic 2- and 5-band Hubbard models. The linearized Eliashberg equation is solved by expanding the effective pairing interaction perturbatively up to third order in the on-site Coulomb integrals. The numerical results for the 5-band model suggest that the eigenvalues of the Eliashberg equation are sufficiently large to explain the actual high Tc for realistic values of Coulomb interaction and the most probable pairing state is spin-singlet s-wave without any nodes just on the Fermi surfaces, although the superconducting order parameter changes its sign between the small Fermi pockets. On the other hand the 2-band model is quite insufficient to explain the actual high Tc.Comment: 2 pages, 3 figures. Proceedings of the Intl. Symposium on Fe-Oxypnictide Superconductors (Tokyo, 28-29th June 2008